HKU Scholars Hubhttp://hub.hku.hkThe DSpace digital repository system captures, stores, indexes, preserves, and distributes digital research material.Wed, 16 Jun 2021 12:53:43 GMT2021-06-16T12:53:43Z50871- Coulomb Liquid Phases of Bosonic Cluster Mott Insulators on a Pyrochlore Latticehttp://hdl.handle.net/10722/266130Title: Coulomb Liquid Phases of Bosonic Cluster Mott Insulators on a Pyrochlore Lattice
Authors: Lv, Jian Ping; Chen, Gang; Deng, Youjin; Meng, Zi Yang
Abstract: © 2015 American Physical Society. © 2015 American Physical Society. Employing large-scale quantum Monte Carlo simulations, we reveal the full phase diagram of the extended Hubbard model of hard-core bosons on the pyrochlore lattice with partial fillings. When the intersite repulsion is dominant, the system is in a cluster Mott insulator phase with an integer number of bosons localized inside the tetrahedral units of the pyrochlore lattice. We show that the full phase diagram contains three cluster Mott insulator phases with 1/4, 1/2, and 3/4 boson fillings, respectively. We further demonstrate that all three cluster Mott insulators are Coulomb liquid phases and its low-energy property is described by the emergent compact U(1) quantum electrodynamics. In addition to measuring the specific heat and entropy of the cluster Mott insulators, we investigate the correlation function of the emergent electric field and verify it is consistent with the compact U(1) quantum electrodynamics description. Our result sheds light on the magnetic properties of various pyrochlore systems, as well as the charge physics of the cluster magnets.
Thu, 01 Jan 2015 00:00:00 GMThttp://hdl.handle.net/10722/2661302015-01-01T00:00:00Z
- Monte Carlo Study of Lattice Compact Quantum Electrodynamics with Fermionic Matter: The Parent State of Quantum Phaseshttp://hdl.handle.net/10722/270102Title: Monte Carlo Study of Lattice Compact Quantum Electrodynamics with Fermionic Matter: The Parent State of Quantum Phases
Authors: Xu, XY; Qi, Y; Zhang, L; Assaad, FF; Xu, CK; Meng, Z
Abstract: The interplay between lattice gauge theories and fermionic matter accounts for fundamental physical phenomena ranging from the deconfinement of quarks in particle physics to quantum spin liquid with fractionalized anyons and emergent gauge structures in condensedmatter physics. However, except for certain limits (for instance, a large number of flavors of matter fields), analytical methods can provide few concrete results. Here we show that the problem of compact U(1) lattice gauge theory coupled to fermionic matter in (2+1) D is possible to access via sign-problem-free quantum Monte Carlo simulations. One can hence map out the phase diagramas a function of fermion flavors and the strength of gauge fluctuations. By increasing the coupling constant of the gauge field, gauge confinement in the form of various spontaneous-symmetry-breaking phases such as the valence-bond solid (VBS) and Neel antiferromagnet emerge. Deconfined phases with algebraic spin and VBS correlation functions are also observed. Such deconfined phases are incarnations of exotic states of matter, i.e., the algebraic spin liquid, which is generally viewed as the parent state of various quantum phases. The phase transitions between the deconfined and confined phases, as well as that between the different confined phases provide various manifestations of deconfined quantum criticality. In particular, for four flavors N-f = 4, our data suggest a continuous quantum phase transition between the VBS and Neel order. We also provide preliminary theoretical analysis for these quantum phase transitions.
Tue, 01 Jan 2019 00:00:00 GMThttp://hdl.handle.net/10722/2701022019-01-01T00:00:00Z
- Correlated states in twisted double bilayer graphenehttp://hdl.handle.net/10722/285491Title: Correlated states in twisted double bilayer graphene
Authors: Shen, C; Chu, Y; Wu, Q; Li, N; Wang, S; Zhao, Y; Tang, J; Liu, J; Tian, J; Watanabe, K; Taniguchi, T; Yang, R; Meng, ZY; Shi, D; Yazyev, OV; Zhang, G
Abstract: Electron–electron interactions play an important role in graphene and related systems and can induce exotic quantum states, especially in a stacked bilayer with a small twist angle1,2,3,4,5,6,7. For bilayer graphene where the two layers are twisted by the ‘magic angle’, flat band and strong many-body effects lead to correlated insulating states and superconductivity4,5,6,7. In contrast to monolayer graphene, the band structure of untwisted bilayer graphene can be further tuned by a displacement field8,9,10, providing an extra degree of freedom to control the flat band that should appear when two bilayers are stacked on top of each other. Here, we report the discovery and characterization of displacement field-tunable electronic phases in twisted double bilayer graphene. We observe insulating states at a half-filled conduction band in an intermediate range of displacement fields. Furthermore, the resistance gap in the correlated insulator increases with respect to the in-plane magnetic fields and we find that the g factor, according to the spin Zeeman effect, is ~2, indicating spin polarization at half-filling. These results establish twisted double bilayer graphene as an easily tunable platform for exploring quantum many-body states.
Wed, 01 Jan 2020 00:00:00 GMThttp://hdl.handle.net/10722/2854912020-01-01T00:00:00Z
- Quantum Phases of SrCu2(BO3)2 from High-Pressure Thermodynamicshttp://hdl.handle.net/10722/285492Title: Quantum Phases of SrCu2(BO3)2 from High-Pressure Thermodynamics
Authors: Guo, J; Sun, G; Zhao, B; Wang, L; Hong, W; Sidorov, VA; Ma, N; Wu, Q; Li, SL; Meng, ZY; Sandvik, AW; Sun, L
Abstract: We report heat capacity measurements of SrCu2(BO3)2 under high pressure along with simulations of relevant quantum spin models and map out the (P,T) phase diagram of the material. We find a first-order quantum phase transition between the low-pressure quantum dimer paramagnet and a phase with signatures of a plaquette-singlet state below T=2 K. At higher pressures, we observe a transition into a previously unknown antiferromagnetic state below 4 K. Our findings can be explained within the two-dimensional Shastry-Sutherland quantum spin model supplemented by weak interlayer couplings. The possibility to tune SrCu2(BO3)2 between the plaquette-singlet and antiferromagnetic states opens opportunities for experimental tests of quantum field theories and lattice models involving fractionalized excitations, emergent symmetries, and gauge fluctuations.
Wed, 01 Jan 2020 00:00:00 GMThttp://hdl.handle.net/10722/2854922020-01-01T00:00:00Z
- Confinement transition in the QED3-Gross-Neveu-XY universality classhttp://hdl.handle.net/10722/285489Title: Confinement transition in the QED3-Gross-Neveu-XY universality class
Authors: Janssen, L; Wang, W; Scherer, MM; Meng, ZY; Xu, XY
Abstract: The coupling between fermionic matter and gauge fields plays a fundamental role in our understanding of nature, while at the same time posing a challenging problem for theoretical modeling. In this situation, controlled information can be gained by combining different complementary approaches. Here, we study a confinement transition in a system of Nf flavors of interacting Dirac fermions charged under a U(1) gauge field in 2+1 dimensions. Using quantum Monte Carlo simulations, we investigate a lattice model that exhibits a continuous transition at zero temperature between a gapless deconfined phase, described by three-dimensional quantum electrodynamics, and a gapped confined phase, in which the system develops valence-bond-solid order. We argue that the quantum critical point is in the universality class of the QED3-Gross-Neveu-XY model. We study this field theory within a 1/Nf expansion in fixed dimension as well as a renormalization group analysis in 4-ϵ space-time dimensions. The consistency between numerical and analytical results is revealed from large to intermediate flavor number.
Wed, 01 Jan 2020 00:00:00 GMThttp://hdl.handle.net/10722/2854892020-01-01T00:00:00Z
- Designer Monte Carlo simulation for the Gross-Neveu-Yukawa transitionhttp://hdl.handle.net/10722/286294Title: Designer Monte Carlo simulation for the Gross-Neveu-Yukawa transition
Authors: Liu, Y; Wang, W; Sun, K; Meng, ZY
Abstract: In this paper, we study the quantum criticality of Dirac fermions via large-scale numerical simulations, focusing on the Gross-Neveu-Yukawa chiral-Ising quantum critical point (QCP) with critical bosonic modes coupled with Dirac fermions. We show that finite-size effects at this QCP can be efficiently minimized via model design, which maximizes the ultraviolet cutoff and at the same time places the bare control parameters closer to the nontrivial fixed point to better expose the critical region. Combined with the efficient self-learning quantum Monte Carlo algorithm, which enables a nonlocal update of the bosonic field, we find that moderately large system size (up to 16×16) is already sufficient to produce robust scaling behavior and critical exponents. The conductance of free Dirac fermions is also calculated, and its frequency dependence is found to be consistent with the scaling behavior predicted by the conformal field theory. The methods and model-design principles developed for this study can be generalized to other fermionic QCPs, and thus provide a promising direction for controlled studies of strongly correlated itinerant systems.
Wed, 01 Jan 2020 00:00:00 GMThttp://hdl.handle.net/10722/2862942020-01-01T00:00:00Z
- Revealing fermionic quantum criticality from new Monte Carlo techniqueshttp://hdl.handle.net/10722/276341Title: Revealing fermionic quantum criticality from new Monte Carlo techniques
Authors: Xu, XY; Liu, ZH; Pan, GP; Qi, Y; Sun, K; Meng, Z
Abstract: This review summarizes recent developments in the study of fermionic quantum criticality, focusing on new progress in numerical methodologies, especially quantum Monte Carlo methods, and insights that emerged from recently large-scale numerical simulations. Quantum critical phenomena in fermionic systems have attracted decades of extensive research efforts, partially lured by their exotic properties and potential technology applications, and partially awakened by the profound and universal fundamental principles that govern these quantum critical systems. Due to the complex and non-perturbative nature, these systems face the most difficult and challenging problems in the study of modern condensed matter physics, and many important fundamental problems remain open. Recently, new developments in model design and algorithm improvements enabled unbiased large-scale numerical solutions to be achieved in the close vicinity of these quantum critical points, which paves a new pathway towards achieving controlled conclusions through combined efforts of theoretical and numerical studies, as well as possible theoretical guidance for experiments in heavy-fermion compounds, Cu-based and Fe-based superconductors, ultra-cold fermionic atomic gas, twisted graphene layers, etc, where signatures of fermionic quantum criticality exist.
Tue, 01 Jan 2019 00:00:00 GMThttp://hdl.handle.net/10722/2763412019-01-01T00:00:00Z
- Quantum Monte Carlo study of strange correlator in interacting topological insulatorshttp://hdl.handle.net/10722/268475Title: Quantum Monte Carlo study of strange correlator in interacting topological insulators
Authors: Wu, Han Qing; He, Yuan Yao; You, Yi Zhuang; Xu, Cenke; Meng, Zi Yang; Lu, Zhong Yi
Abstract: © 2015 American Physical Society. Distinguishing the nontrivial symmetry-protected topological (SPT) phase from the trivial insulator phase in the presence of electron-electron interaction is an urgent question to the study of topological insulators, due to the fact that most of the topological indices defined for free electron systems are very likely unsuitable for interacting cases. In this work, we demonstrate that the strange correlator is a sensitive diagnosis to detect SPT states in interacting systems. Employing large-scale quantum Monte Carlo (QMC) simulations, we investigate the interaction-driven quantum phase transition in the Kane-Mele-Hubbard model. The transition from the quantum spin Hall insulator at weak interaction to an antiferromagnetic Mott insulator at strong interaction can be readily detected by the momentum space behavior of the strange correlator in single-particle, spin, and pairing sectors. The interaction effects on the symmetry-protected edge states in various sectors, i.e., the helical Luttinger liquid behavior, are well captured in the QMC measurements of strange correlators. Moreover, we demonstrate that the strange correlator is technically easier to implement in QMC and more robust in performance than other proposed numerical diagnoses for interacting topological states, as only static correlations are needed. The attempt in this work paves the way for using the strange correlator to study interaction-driven topological phase transitions in fermionic as well as bosonic systems.
Thu, 01 Jan 2015 00:00:00 GMThttp://hdl.handle.net/10722/2684752015-01-01T00:00:00Z
- Mott insulating states and quantum phase transitions of correlated SU(2N) Dirac fermionshttp://hdl.handle.net/10722/268581Title: Mott insulating states and quantum phase transitions of correlated SU(2N) Dirac fermions
Authors: Zhou, Zhichao; Wang, Da; Meng, Zi Yang; Wang, Yu; Wu, Congjun
Abstract: © 2016 American Physical Society. The interplay between charge and spin degrees of freedom in strongly correlated fermionic systems, in particular of Dirac fermions, is a long-standing problem in condensed matter physics. We investigate the competing orders in the half-filled SU(2N) Hubbard model on a honeycomb lattice, which can be accurately realized in optical lattices with ultracold large-spin alkaline-earth fermions. Employing large-scale projector determinant quantum Monte Carlo simulations, we have explored quantum phase transitions from the gapless Dirac semimetals to the gapped Mott insulating phases in the SU(4) and SU(6) cases. Both of these Mott insulating states are found to be columnar valence bond solid (cVBS) and to be absent of the antiferromagnetic Néel ordering and the loop current ordering. Inside the cVBS phases, the dimer ordering is enhanced by increasing fermion components and behaves nonmonotonically as the interaction strength increases. Although the transitions generally should be of first order due to a cubic invariance possessed by the cVBS order, the coupling to gapless Dirac fermions can soften the transitions to second order through a nonanalytic term in the free energy. Our simulations provide important guidance for the experimental explorations of novel states of matter with ultracold alkaline-earth fermions.
Fri, 01 Jan 2016 00:00:00 GMThttp://hdl.handle.net/10722/2685812016-01-01T00:00:00Z
- Diagnosis of Interaction-driven Topological Phase via Exact Diagonalizationhttp://hdl.handle.net/10722/268583Title: Diagnosis of Interaction-driven Topological Phase via Exact Diagonalization
Authors: Wu, Han Qing; He, Yuan Yao; Fang, Chen; Meng, Zi Yang; Lu, Zhong Yi
Abstract: © 2016 American Physical Society. We propose a general scheme for diagnosing interaction-driven topological phases in the weak interaction regime using exact diagonalization (ED). The scheme comprises the analysis of eigenvalues of the point-group operators for the many-body eigenstates and the correlation functions for physical observables to extract the symmetries of the order parameters and the topological numbers of the underlying ground states at the thermodynamic limit from a relatively small size system afforded by ED. As a concrete example, we investigate the interaction effects on the half-filled spinless fermions on the checkerboard lattice with a quadratic band crossing point. Numerical results support the existence of a spontaneous quantum anomalous Hall phase purely driven by a nearest-neighbor weak repulsive interaction, separated from a nematic Mott insulator phase at strong repulsive interaction by a first-order phase transition.
Fri, 01 Jan 2016 00:00:00 GMThttp://hdl.handle.net/10722/2685832016-01-01T00:00:00Z
- Competing pairing channels in the doped honeycomb lattice Hubbard modelhttp://hdl.handle.net/10722/268584Title: Competing pairing channels in the doped honeycomb lattice Hubbard model
Authors: Xu, Xiao Yan; Wessel, Stefan; Meng, Zi Yang
Abstract: © 2016 American Physical Society. Proposals for superconductivity emerging from correlated electrons in the doped Hubbard model on the honeycomb lattice range from chiral d+id singlet to p+IP trIPlet pairing, depending on the considered range of doping and interaction strength, as well as the approach used to analyze the pairing instabilities. Here, we consider these scenarios using large-scale dynamic cluster approximation (DCA) calculations to examine the evolution in the leading pairing symmetry from weak to intermediate coupling strength. These calculations focus on doping levels around the van Hove singularity (VHS) and are performed using DCA simulations with an interaction-expansion continuous-time quantum Monte Carlo cluster solver. We calculated explicitly the temperature dependence of different uniform superconducting pairing susceptibilities and found a consistent picture emerging upon gradually increasing the cluster size: while at weak coupling the d+id singlet pairing dominates close to the VHS filling, an enhanced tendency towards p-wave trIPlet pairing upon further increasing the interaction strength is observed. The relevance of these systematic results for existing proposals and ongoing pursuits of odd-parity topological superconductivity are also discussed.
Fri, 01 Jan 2016 00:00:00 GMThttp://hdl.handle.net/10722/2685842016-01-01T00:00:00Z
- Quantum critical point of Dirac fermion mass generation without spontaneous symmetry breakinghttp://hdl.handle.net/10722/268632Title: Quantum critical point of Dirac fermion mass generation without spontaneous symmetry breaking
Authors: He, Yuan Yao; Wu, Han Qing; You, Yi Zhuang; Xu, Cenke; Meng, Zi Yang; Lu, Zhong Yi
Abstract: © 2016 American Physical Society. We study a lattice model of interacting Dirac fermions in (2+1) dimensions space-time with an SU(4) symmetry. While increasing the interaction strength, this model undergoes a continuous quantum phase transition from a weakly interacting Dirac semimetal to a fully gapped and nondegenerate phase without condensing any Dirac fermion bilinear mass operator. This unusual mechanism for mass generation is consistent with recent studies of interacting topological insulators/superconductors, and also consistent with recent progress in the lattice QCD community.
Fri, 01 Jan 2016 00:00:00 GMThttp://hdl.handle.net/10722/2686322016-01-01T00:00:00Z
- Gapped Spin-1/2 Spinon Excitations in a New Kagome Quantum Spin Liquid Compound Cu3Zn(OH)6FBrhttp://hdl.handle.net/10722/268647Title: Gapped Spin-1/2 Spinon Excitations in a New Kagome Quantum Spin Liquid Compound Cu3Zn(OH)6FBr
Authors: Feng, Zili; Li, Zheng; Meng, Xin; Yi, Wei; Wei, Yuan; Zhang, Jun; Wang, Yan Cheng; Jiang, Wei; Liu, Zheng; Li, Shiyan; Liu, Feng; Luo, Jianlin; Li, Shiliang; Zheng, Guo Qing; Meng, Zi Yang; Mei, Jia Wei; Shi, Youguo
Abstract: © 2017 Chinese Physical Society and IOP Publishing Ltd. We report a new kagome quantum spin liquid candidate Cu 3 Zn(OH) 6 FBr, which does not experience any phase transition down to 50 mK, more than three orders lower than the antiferromagnetic Curie-Weiss temperature (∼200 K). A clear gap opening at low temperature is observed in the uniform spin susceptibility obtained from 19 F nuclear magnetic resonance measurements. We observe the characteristic magnetic field dependence of the gap as expected for fractionalized spin-1/2 spinon excitations. Our experimental results provide firm evidence for spin fractionalization in a topologically ordered spin system, resembling charge fractionalization in the fractional quantum Hall state.
Sun, 01 Jan 2017 00:00:00 GMThttp://hdl.handle.net/10722/2686472017-01-01T00:00:00Z
- Non-fermi liquid at (2 + 1)D ferromagnetic quantum critical pointhttp://hdl.handle.net/10722/268597Title: Non-fermi liquid at (2 + 1)D ferromagnetic quantum critical point
Authors: Xu, Xiao Yan; Sun, Kai; Schattner, Yoni; Berg, Erez; Meng, Zi Yang
Abstract: We construct a two-dimensional lattice model of fermions coupled to Ising ferromagnetic critical fluctuations. Using extensive sign-problem-free quantum Monte Carlo simulations, we show that the model realizes a continuous itinerant quantum phase transition. In comparison with other similar itinerant quantum critical points (QCPs), our QCP shows a much weaker superconductivity tendency with no superconducting state down to the lowest temperature investigated, hence making the system a good platform for the exploration of quantum critical fluctuations. Remarkably, clear signatures of non-Fermi liquid behavior in the fermion propagators are observed at the QCP. The critical fluctuations at the QCP partially resemble Hertz-Millis-Moriya behavior. However, careful scaling analysis reveals that the QCP belongs to a different universality class, deviating from both ð2 þ 1ÞD Ising and Hertz-Millis-Moriya predictions.
Sun, 01 Jan 2017 00:00:00 GMThttp://hdl.handle.net/10722/2685972017-01-01T00:00:00Z
- Quantum spin liquid emerging in two-dimensional correlated Dirac fermionshttp://hdl.handle.net/10722/268517Title: Quantum spin liquid emerging in two-dimensional correlated Dirac fermions
Authors: Meng, Z. Y.; Lang, T. C.; Wessel, S.; Assaad, F. F.; Muramatsu, A.
Abstract: At sufficiently low temperatures, condensed-matter systems tend to develop order. A notable exception to this behaviour is the case of quantum spin liquids, in which quantum fluctuations prevent a transition to an ordered state down to the lowest temperatures. There have now been tentative observations of such states in some two-dimensional organic compounds, yet quantum spin liquids remain elusive in microscopic two-dimensional models that are relevant to experiments. Here we show, by means of large-scale quantum Monte Carlo simulations of correlated fermions on a honeycomb lattice (a structure realized in, for example, graphene), that a quantum spin liquid emerges between the state described by massless Dirac fermions and an antiferromagnetically ordered Mott insulator. This unexpected quantum-disordered state is found to be a short-range resonating valence-bond liquid, akin to the one proposed for high-temperature superconductors: the possibility of unconventional superconductivity through doping therefore arises in our system. We foresee the experimental realization of this model system using ultra-cold atoms, or group IV elements arranged in honeycomb lattices. © 2010 Macmillan Publishers Limited. All rights reserved.
Fri, 01 Jan 2010 00:00:00 GMThttp://hdl.handle.net/10722/2685172010-01-01T00:00:00Z
- A quantum spin-liquid in correlated relativistic electronshttp://hdl.handle.net/10722/268637Title: A quantum spin-liquid in correlated relativistic electrons
Authors: Meng, Z. Y.; Lang, T. C.; Wessel, S.; Assaad, F. F.; Muramatsu, A.
Abstract: In recent years, an increasing number of systems displaying exotic quantum states like unconventional superconductivity, quantum spin-liquids, or topological states were experimentally found. Here we summarize findings in quantum Monte Carlo simulations of correlated electrons on a honeycomb lattice, the structure of graphene, that revealed an unexpected spin-liquid emerging between a state described by massless Dirac fermions and an antiferromagnetically ordered Mott insulator. Moreover, we found that this quantum-disordered state is a resonating valence-bond (RVB) liquid, akin to the one proposed for high temperature superconductors. This was the first unbiased determination of a RVB-liquid in an electronic system. © 2012 by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Sun, 01 Jan 2012 00:00:00 GMThttp://hdl.handle.net/10722/2686372012-01-01T00:00:00Z
- Dual fermion method for disordered electronic systemshttp://hdl.handle.net/10722/268654Title: Dual fermion method for disordered electronic systems
Authors: Terletska, H.; Yang, S. X.; Meng, Z. Y.; Moreno, J.; Jarrell, M.
Abstract: While the coherent potential approximation (CPA) is the prevalent method for the study of disordered electronic systems, it fails to capture nonlocal correlations and Anderson localization. To incorporate such effects, we extend the dual fermion approach to disordered systems using the replica method. The developed method utilizes the exact mapping to the dual fermion variables, and includes intersite scattering via diagrammatic perturbation theory in the dual variables. The CPA is recovered as a zeroth-order approximation. Results for single- and two-particle quantities show good agreement with a cluster extension of the CPA; moreover, weak localization is captured. As a natural extension of the CPA, our method presents an alternative to existing nonlocal cluster theories for disordered systems, and has potential applications in the study of disordered systems with electronic interactions. © 2013 American Physical Society.
Tue, 01 Jan 2013 00:00:00 GMThttp://hdl.handle.net/10722/2686542013-01-01T00:00:00Z
- Spin-liquid phase in the hubbard model on the honeycomb latticehttp://hdl.handle.net/10722/268547Title: Spin-liquid phase in the hubbard model on the honeycomb lattice
Authors: Meng, Z. Y.; Lang, T. C.; Wessel, S.; Assaad, F. F.; Muramatsu, A.
Abstract: The Hubbard model encapsulates the physics of strongly correlated quantum systems in its most basic form. It has been studied intensively in the context of the high-temperature superconductivity. A number of novel phases were recently proposed for Hubbard-like models on the honeycomb lattice, the structure of graphene. We analyzed the Hubbard model of spin-1/2 fermions on the honeycomb lattice at half-filling using large-scale quantum Monte Carlo simulations. We find that the weak coupling semimetal and the antiferromagnetic Mott insulator at strong interaction are separated by an extended gapped phase in an intermediate coupling regime. Exploring excitation gaps, various correlation functions as well as probing for flux quantization, we conclude that a quantum spin liquid, lacking any conventional order, emerges with local charge and spin correlations, best described by a resonating valence bonds state. © Springer-Verlag Berlin Heidelberg 2011.
Sat, 01 Jan 2011 00:00:00 GMThttp://hdl.handle.net/10722/2685472011-01-01T00:00:00Z
- Typical medium dynamical cluster approximation for the study of Anderson localization in three dimensionshttp://hdl.handle.net/10722/268472Title: Typical medium dynamical cluster approximation for the study of Anderson localization in three dimensions
Authors: Ekuma, C. E.; Terletska, H.; Tam, K. M.; Meng, Z. Y.; Moreno, J.; Jarrell, M.
Abstract: We develop a systematic typical medium dynamical cluster approximation that provides a proper description of the Anderson localization transition in three dimensions (3D). Our method successfully captures the localization phenomenon both in the low and large disorder regimes, and allows us to study the localization in different momenta cells, which renders the discovery that the Anderson localization transition occurs in a cell-selective fashion. As a function of cluster size, our method systematically recovers the reentrance behavior of the mobility edge and obtains the correct critical disorder strength for Anderson localization in 3D. © 2014 American Physical Society.
Wed, 01 Jan 2014 00:00:00 GMThttp://hdl.handle.net/10722/2684722014-01-01T00:00:00Z
- Odd-Parity triplet superconducting phase in multiorbital materials with a strong spin-orbit coupling: Application to doped Sr2IrO4http://hdl.handle.net/10722/268560Title: Odd-Parity triplet superconducting phase in multiorbital materials with a strong spin-orbit coupling: Application to doped Sr2IrO4
Authors: Meng, Zi Yang; Kim, Yong Baek; Kee, Hae Young
Abstract: © 2014 American Physical Society. We explore possible superconducting states in t2g multiorbital correlated electron systems with strong spin-orbit coupling (SOC). In order to study such systems in a controlled manner, we employ large-scale dynamical mean-field theory (DMFT) simulations with the hybridization expansion continuous-time quantum Monte Carlo (CTQMC) impurity solver. To determine the pairing symmetry, we go beyond the local DMFT formalism using parquet equations to introduce the momentum dependence in the two-particle vertex and correlation functions. In the strong SOC limit, a singlet, d-wave pairing state in the electron-doped side of the phase diagram is observed at weak Hund's coupling, which is triggered by antiferromagnetic fluctuations. When the Hund's coupling is comparable to SOC, a twofold degenerate, triplet p-wave pairing state with relatively high transition temperature emerges in the hole-doped side of the phase diagram, which is associated with enhanced charge fluctuations. Experimental implications to doped Sr2IrO4 are discussed.
Wed, 01 Jan 2014 00:00:00 GMThttp://hdl.handle.net/10722/2685602014-01-01T00:00:00Z
- iQIST: An open source continuous-time quantum Monte Carlo impurity solver toolkithttp://hdl.handle.net/10722/268572Title: iQIST: An open source continuous-time quantum Monte Carlo impurity solver toolkit
Authors: Huang, Li; Wang, Yilin; Meng, Zi Yang; Du, Liang; Werner, Philipp; Dai, Xi
Abstract: © 2015 Elsevier B.V. All rights reserved. Quantum impurity solvers have a broad range of applications in theoretical studies of strongly correlated electron systems. Especially, they play a key role in dynamical mean-field theory calculations of correlated lattice models and realistic materials. Therefore, the development and implementation of efficient quantum impurity solvers is an important task. In this paper, we present an open source interacting quantum impurity solver toolkit (dubbed iQIST). This package contains several highly optimized quantum impurity solvers which are based on the hybridization expansion continuous-time quantum Monte Carlo algorithm, as well as some essential pre- and post-processing tools. We first introduce the basic principle of continuous-time quantum Monte Carlo algorithm and then discuss the implementation details and optimization strategies. The software framework, major features, and installation procedure for iQIST are also explained. Finally, several simple tutorials are presented in order to demonstrate the usage and power of iQIST.
Thu, 01 Jan 2015 00:00:00 GMThttp://hdl.handle.net/10722/2685722015-01-01T00:00:00Z
- Dynamical generation of topological masses in Dirac fermionshttp://hdl.handle.net/10722/268486Title: Dynamical generation of topological masses in Dirac fermions
Authors: He, Yuan Yao; Xu, Xiao Yan; Sun, Kai; Assaad, Fakher F.; Meng, Zi Yang; Lu, Zhong Yi
Abstract: © 2018 American Physical Society. We report the discovery of a topological Mott insulator in strongly correlated Dirac semimetals. Such an interaction-driven topological state has been theoretically proposed and is observed here with unbiased large-scale numerical simulations. In our model, interactions between electrons are mediated by Ising spins in a transverse field. The results indicate that the topological mass term is dynamically generated and the resulting quantum phase transition belongs to the (two-plus-one-dimensional) N=8 chiral Ising universality class. These conclusions stem from large-scale sign-free quantum Monte Carlo simulations.
Mon, 01 Jan 2018 00:00:00 GMThttp://hdl.handle.net/10722/2684862018-01-01T00:00:00Z
- Itinerant quantum critical point with frustration and a non-Fermi liquidhttp://hdl.handle.net/10722/268601Title: Itinerant quantum critical point with frustration and a non-Fermi liquid
Authors: Liu, Zi Hong; Xu, Xiao Yan; Qi, Yang; Sun, Kai; Meng, Zi Yang
Abstract: © 2018 American Physical Society. Employing the self-learning quantum Monte Carlo algorithm, we investigate the frustrated transverse-field triangle-lattice Ising model coupled to a Fermi surface. Without fermions, the spin degrees of freedom undergo a second-order quantum phase transition between paramagnetic and clock-ordered phases. This quantum critical point (QCP) has an emergent U(1) symmetry and thus belongs to the (2+1)D XY universality class. In the presence of fermions, spin fluctuations introduce effective interactions among fermions and distort the bare Fermi surface towards an interacting one with hot spots and Fermi pockets. Near the QCP, non-Fermi-liquid behaviors are observed at the hot spots, and the QCP is rendered into a different universality with Hertz-Millis-type exponents. The detailed properties of this QCP and possibly related experimental systems are also discussed.
Mon, 01 Jan 2018 00:00:00 GMThttp://hdl.handle.net/10722/2686012018-01-01T00:00:00Z
- Effect of Zn doping on the antiferromagnetism in kagome Cu4-xZnx(OH)6FBrhttp://hdl.handle.net/10722/268608Title: Effect of Zn doping on the antiferromagnetism in kagome Cu4-xZnx(OH)6FBr
Authors: Feng, Zili; Wei, Yuan; Liu, Ran; Yan, Dayu; Wang, Yan Cheng; Luo, Jianlin; Senyshyn, Anatoliy; Cruz, Clarina Dela; Yi, Wei; Mei, Jia Wei; Meng, Zi Yang; Shi, Youguo; Li, Shiliang
Abstract: © 2018 American Physical Society. Barlowite Cu4(OH)6FBr shows three-dimensional (3D) long-range antiferromagnetism, which is fully suppressed in Cu3Zn(OH)6FBr with a kagome quantum spin liquid ground state. Here we report systematic studies on the evolution of magnetism in the Cu4-xZnx(OH)6FBr system as a function of x to bridge the two limits of Cu4(OH)6FBr(x=0) and Cu3Zn(OH)6FBr(x=1). Neutron-diffraction measurements reveal a hexagonal-to-orthorhombic structural change with decreasing temperature in the x=0 sample. While confirming the 3D antiferromagnetic nature of low-temperature magnetism, the magnetic moments on some Cu2+ sites on the kagome planes are found to be vanishingly small, suggesting strong frustration already exists in barlowite. Substitution of interlayer Cu2+ with Zn2+ with gradually increasing x completely suppresses the bulk magnetic order at around x=0.4 but leaves a local secondary magnetic order up to x∼0.8 with a slight decrease in its transition temperature. The high-temperature magnetic susceptibility and specific-heat measurements further suggest that the intrinsic magnetic properties of kagome spin liquid planes may already appear from x>0.3 samples. Our results reveal that the Cu4-xZnx(OH)6FBr may be the long-thought experimental playground for the systematic investigations of the quantum phase transition from a long-range antiferromagnet to a topologically ordered quantum spin liquid.
Mon, 01 Jan 2018 00:00:00 GMThttp://hdl.handle.net/10722/2686082018-01-01T00:00:00Z
- Emergent symmetry and conserved current at a one-dimensional incarnation of deconfined quantum critical pointhttp://hdl.handle.net/10722/278599Title: Emergent symmetry and conserved current at a one-dimensional incarnation of deconfined quantum critical point
Authors: Huang, RZ; Lu, DC; You, YZ; Meng, ZY; Xiang, T
Abstract: The deconfined quantum critical point (DQCP) was originally proposed as a continuous transition between two spontaneous symmetry breaking phases in 2D spin-1/2 systems. While great efforts have been spent on the DQCP for 2D systems, both theoretically and numerically, ambiguities among the nature of the transition are still not completely clarified. Here we shift the focus to a recently proposed 1D incarnation of DQCP in a spin-1/2 chain. By solving it with the variational matrix product state in the thermodynamic limit, a continuous transition between a valence-bond solid phase and a ferromagnetic phase is discovered. The scaling dimensions of various operators are calculated and compared with those from field theoretical description. At the critical point, two emergent O(2) symmetries are revealed, and the associated conserved current operators with exact integer scaling dimensions are determined with scrutiny. Our findings provide the low-dimensional analog of DQCP where unbiased numerical results are in perfect agreement with the controlled field theoretical predictions and have extended the realm of the unconventional phase transition as well as its identification with the advanced numerical methodology.
Tue, 01 Jan 2019 00:00:00 GMThttp://hdl.handle.net/10722/2785992019-01-01T00:00:00Z
- Antiferromagnetism in the kagome-lattice compound α-Cu3Mg(OH)(6)Br-2http://hdl.handle.net/10722/279476Title: Antiferromagnetism in the kagome-lattice compound α-Cu3Mg(OH)(6)Br-2
Authors: Wei, Y; Feng, Z; dela Cruz, CD; Yi, W; Meng, ZY; Mei, JW; Shi, Y; Li, S
Abstract: The antiferromagnetism in α-Cu3Mg(OH)6Br2 was studied by magnetic-susceptibility, specific-heat, and neutron-diffraction measurements. The crystal structure consists of Cu2+ kagome layers with Mg2+ ions occupying the centers of the hexagons, separated by Br1-ions. The magnetic system orders antiferromagnetically at 5.4 K with the magnetic moments aligned ferromagnetically within the kagome planes. The ordered moment is 0.94μB, suggesting little quantum and geometrical fluctuations. By comparing the magnetic and specific-heat properties with those of the haydeeite, we suggest that α-Cu3Mg(OH)6Br2 may be described by the two-dimensional spin-1/2 Heisenberg kagome model and is in the region of the ferromagnetic-order side of the phase diagram. © 2019 American Physical Society.
Tue, 01 Jan 2019 00:00:00 GMThttp://hdl.handle.net/10722/2794762019-01-01T00:00:00Z
- Valence Bond Orders at Charge Neutrality in a Possible Two-Orbital Extended Hubbard Model for Twisted Bilayer Graphenehttp://hdl.handle.net/10722/278606Title: Valence Bond Orders at Charge Neutrality in a Possible Two-Orbital Extended Hubbard Model for Twisted Bilayer Graphene
Authors: Da Liao, Y; Meng, ZY; Xu, XY
Abstract: An extended Hubbard model on a honeycomb lattice with two orbitals per site at charge neutrality is investigated with unbiased large-scale quantum Monte Carlo simulations. The Fermi velocity of the Dirac fermions is renormalized as the cluster charge interaction increases, until a mass term emerges and a quantum phase transition from Dirac semimetal to valence bond solid (VBS) insulator is established. The quantum critical point is discovered to belong to the 3D N=4 Gross-Neveu chiral XY universality with the critical exponents obtained at high precision. Further enhancement of the interaction drives the system into two different VBS phases, the properties and transition between them are also revealed. Since the model is related to magic-angle twisted bilayer graphene, our results may have relevance towards the symmetry breaking order at the charge neutrality point of the material, and associate the wide range of universal strange metal behavior around it with quantum critical fluctuations.
Tue, 01 Jan 2019 00:00:00 GMThttp://hdl.handle.net/10722/2786062019-01-01T00:00:00Z
- Role of Noether’s Theorem at the Deconfined Quantum Critical Pointhttp://hdl.handle.net/10722/271431Title: Role of Noether’s Theorem at the Deconfined Quantum Critical Point
Authors: Ma, N; You, YZ; Meng, ZY
Abstract: Noether’s theorem is one of the fundamental laws of physics, relating continuous symmetries and conserved currents. Here we explore the role of Noether’s theorem at the deconfined quantum critical point (DQCP), which is a quantum phase transition beyond the Landau-Ginzburg-Wilson paradigm. It was expected that a larger continuous symmetry could emerge at the DQCP, which, if true, should lead to conserved current at low energy. By identifying the emergent current fluctuation in the spin excitation spectra, we can quantitatively study the current-current correlation in large-scale quantum Monte Carlo simulations. Our results reveal the conservation of the emergent current, as signified by the vanishing anomalous dimension of the current operator, and hence provide supporting evidence for the emergent symmetry at the DQCP. Our study demonstrates an elegant yet practical approach to detect emergent symmetry by probing the spin excitation, which could potentially guide the ongoing experimental search for the DQCP in quantum magnets.
Tue, 01 Jan 2019 00:00:00 GMThttp://hdl.handle.net/10722/2714312019-01-01T00:00:00Z
- Dynamics of compact quantum electrodynamics at large fermion flavorhttp://hdl.handle.net/10722/275744Title: Dynamics of compact quantum electrodynamics at large fermion flavor
Authors: Wang, W; Lu, DC; Xu, XY; You, YZ; Meng, ZY
Abstract: Thanks to the development in quantum Monte Carlo technique, the compact U(1) lattice gauge theory coupled to fermionic matter at (2+1)D is now accessible with large-scale numerical simulations, and the ground state phase diagram as a function of fermion flavor (Nf) and the strength of gauge fluctuations is mapped out. Here we focus on the large fermion flavor case (Nf=8) to investigate the dynamic properties across the deconfinement-to-confinement phase transition. In the deconfined phase, fermions coupled to the fluctuating gauge field to form U(1) spin liquid with continua in both spin and dimer spectral functions, and in the confined phase fermions are gapped out into valence bond solid phase with translational symmetry breaking and gapped spectra. The dynamical behaviors provide supporting evidence for the existence of the U(1) deconfined phase and could shine light on the nature of the U(1)-to-VBS phase transition which is of the QED3-Gross-Neveu chiral O(2) universality whose properties are still largely unknown. © 2019 American Physical Society.
Tue, 01 Jan 2019 00:00:00 GMThttp://hdl.handle.net/10722/2757442019-01-01T00:00:00Z
- Itinerant quantum critical point with fermion pockets and hotspotshttp://hdl.handle.net/10722/274002Title: Itinerant quantum critical point with fermion pockets and hotspots
Authors: Liu, ZH; Pan, G; Xu, XY; Sun, K; Meng, ZY
Abstract: Metallic quantum criticality is among the central themes in the understanding of correlated electronic systems, and converging results between analytical and numerical approaches are still under review. In this work, we develop a state-of-the-art large-scale quantum Monte Carlo simulation technique and systematically investigate the itinerant quantum critical point on a 2D square lattice with antiferromagnetic spin fluctuations at wavevector Q=(π,π)—a problem that resembles the Fermi surface setup and low-energy antiferromagnetic fluctuations in high-Tc cuprates and other critical metals, which might be relevant to their non–Fermi-liquid behaviors. System sizes of 60×60×320 (L×L×Lτ) are comfortably accessed, and the quantum critical scaling behaviors are revealed with unprecedented high precision. We found that the antiferromagnetic spin fluctuations introduce effective interactions among fermions and the fermions in return render the bare bosonic critical point into a different universality, different from both the bare Ising universality class and the Hertz–Mills–Moriya RPA prediction. At the quantum critical point, a finite anomalous dimension η∼0.125 is observed in the bosonic propagator, and fermions at hotspots evolve into a non-Fermi liquid. In the antiferromagnetically ordered metallic phase, fermion pockets are observed as the energy gap opens up at the hotspots. These results bridge the recent theoretical and numerical developments in metallic quantum criticality and can serve as the stepping stone toward final understanding of the 2D correlated fermions interacting with gapless critical excitations.
Tue, 01 Jan 2019 00:00:00 GMThttp://hdl.handle.net/10722/2740022019-01-01T00:00:00Z
- Phases and magnetization process of an anisotropic Shastry-Sutherland modelhttp://hdl.handle.net/10722/268469Title: Phases and magnetization process of an anisotropic Shastry-Sutherland model
Authors: Meng, Zi Yang; Wessel, Stefan
Abstract: We examine ground-state properties of the spin-1/2 easy-axis Heisenberg model on the Shastry-Sutherland lattice with ferromagnetic transverse spin exchange using quantum Monte Carlo and degenerate perturbation theory. For vanishing transverse exchange, the model reduces to an antiferromagnetic Ising model that, besides Néel order, harbors regions of extensive ground-state degeneracy. In the quantum regime, we find a dimerized phase of triplet states separated from the Néel ordered phase by a superfluid. The quantum phase transitions between these phases are characterized. The magnetization process shows a magnetization plateau at 1/3 of the saturation value, which persists down to the Ising limit, and a further plateau at 1/2 only in the quantum regime. For both plateaus, we determine the crystalline patterns of the localized triplet excitations. No further plateaus or supersolid phases are found in this model. © 2008 The American Physical Society.
Tue, 01 Jan 2008 00:00:00 GMThttp://hdl.handle.net/10722/2684692008-01-01T00:00:00Z
- Antiferromagnetism in the Hubbard model on the Bernal-stacked honeycomb bilayerhttp://hdl.handle.net/10722/268535Title: Antiferromagnetism in the Hubbard model on the Bernal-stacked honeycomb bilayer
Authors: Lang, Thomas C.; Meng, Zi Yang; Scherer, Michael M.; Uebelacker, Stefan; Assaad, Fakher F.; Muramatsu, Alejandro; Honerkamp, Carsten; Wessel, Stefan
Abstract: Using a combination of quantum Monte Carlo simulations, functional renormalization group calculations and mean-field theory, we study the Hubbard model on the Bernal-stacked honeycomb bilayer at half-filling as a model system for bilayer graphene. The free bands consisting of two Fermi points with quadratic dispersions lead to a finite density of states at the Fermi level, which triggers an antiferromagnetic instability that spontaneously breaks sublattice and spin rotational symmetry once local Coulomb repulsions are introduced. Our results reveal an inhomogeneous participation of the spin moments in the ordered ground state, with enhanced moments at the threefold coordinated sites. Furthermore, we find the antiferromagnetic ground state to be robust with respect to enhanced interlayer couplings and extended Coulomb interactions. © 2012 American Physical Society.
Sun, 01 Jan 2012 00:00:00 GMThttp://hdl.handle.net/10722/2685352012-01-01T00:00:00Z
- Lifshitz transition in the two-dimensional Hubbard modelhttp://hdl.handle.net/10722/268537Title: Lifshitz transition in the two-dimensional Hubbard model
Authors: Chen, K. S.; Meng, Z. Y.; Pruschke, T.; Moreno, J.; Jarrell, M.
Abstract: Using large-scale dynamical cluster quantum Monte Carlo simulations, we study the Lifshitz transition of the two-dimensional Hubbard model with next-nearest-neighbor hopping (t ′), chemical potential, and temperature as control parameters. At t ′≤0, we identify a line of Lifshitz transition points associated with a change in the Fermi surface topology at zero temperature. In the overdoped region, the Fermi surface is complete and electron-like; across the Lifshitz transition, the Fermi surface becomes hole-like and develops a pseudogap. At (or very close to) the Lifshitz transition points, a van Hove singularity in the density of states crosses the Fermi level. The van Hove singularity occurs at finite doping due to correlation effects and becomes more singular when t ′ becomes more negative. The resulting temperature dependence on the bare d-wave pairing susceptibility close to the Lifshitz points is significantly different from that found in the traditional van Hove scenarios. Such unambiguous numerical observation of the Lifshitz transition at t ′≤0 extends our understanding of the quantum critical region in the phase diagram and shines lights on future investigations of the nature of the quantum critical point in the two-dimensional Hubbard model. © 2012 American Physical Society.
Sun, 01 Jan 2012 00:00:00 GMThttp://hdl.handle.net/10722/2685372012-01-01T00:00:00Z
- Unconventional superconductivity on the triangular lattice Hubbard modelhttp://hdl.handle.net/10722/268543Title: Unconventional superconductivity on the triangular lattice Hubbard model
Authors: Chen, Kuang Shing; Meng, Zi Yang; Yu, Unjong; Yang, Shuxiang; Jarrell, Mark; Moreno, Juana
Abstract: Using large-scale dynamical cluster quantum Monte Carlo simulations, we explore the unconventional superconductivity in the hole-doped Hubbard model on the triangular lattice. Due to the interplay of electronic correlations, geometric frustration, and Fermi surface topology, we find a doubly degenerate singlet pairing state at an interaction strength close to the bare bandwidth. Such an unconventional superconducting state is mediated by antiferromagnetic spin fluctuations along the Γ-K direction, where the Fermi surface is nested. An exact decomposition of the irreducible particle-particle vertex further confirms the dominant component of the effective pairing interaction comes from the spin channel. Our findings suggest the existence of chiral d+id superconductivity in a hole-doped Hubbard triangular lattice in a strongly correlated regime, and provide insight into the superconducting phases of the water-intercalated sodium cobaltates NaxCoO2· yH2O, as well as the organic compounds κ-(ET)2X and Pd(dmit)2. © 2013 American Physical Society.
Tue, 01 Jan 2013 00:00:00 GMThttp://hdl.handle.net/10722/2685432013-01-01T00:00:00Z
- The characterization of topological properties in quantum Monte Carlo simulations of the Kane-Mele-Hubbard modelhttp://hdl.handle.net/10722/268555Title: The characterization of topological properties in quantum Monte Carlo simulations of the Kane-Mele-Hubbard model
Authors: Meng, Zi Yang; Hung, Hsiang Hsuan; Lang, Thomas C.
Abstract: Topological insulators present a bulk gap, but allow for dissipationless spin transport along the edges. These exotic states are characterized by the Z2 topological invariant and are protected by time-reversal symmetry. The Kane-Mele model is one model to realize this topological class in two dimensions, also called the quantum spin Hall state. In this brief review article, we provide a pedagogical introduction to the influence of correlation effects in the quantum spin Hall states, with special focus on the half-filled Kane-Mele-Hubbard model, solved by means of unbiased determinant quantum Monte Carlo (QMC) simulations. We explain the idea of identifying the topological insulator via π-flux insertion, the Z2 invariant and the associated behavior of the zero-frequency Green's function, as well as the spin Chern number in parameter-driven topological phase transitions. The examples considered are two descendants of the Kane-Mele-Hubbard model, the generalized and dimerized Kane-Mele-Hubbard model. From the Z2 index, spin Chern numbers and the Green's function behavior, one can observe that correlation effects induce shifts of the topological phase boundaries. Although the implementation of these topological quantities has been successfully employed in QMC simulations to describe the topological phase transition, we also point out their limitations as well as suggest possible future directions in using numerical methods to characterize topological properties of strongly correlated condensed matter systems. © World Scientific Publishing Company.
Wed, 01 Jan 2014 00:00:00 GMThttp://hdl.handle.net/10722/2685552014-01-01T00:00:00Z
- Multiplicative logarithmic corrections to quantum criticality in three-dimensional dimerized antiferromagnetshttp://hdl.handle.net/10722/268626Title: Multiplicative logarithmic corrections to quantum criticality in three-dimensional dimerized antiferromagnets
Authors: Qin, Yan Qi; Normand, B.; Sandvik, Anders W.; Meng, Zi Yang
Abstract: © 2015 American Physical Society. We investigate the quantum phase transition in an S=1/2 dimerized Heisenberg antiferromagnet in three spatial dimensions. By performing large-scale quantum Monte Carlo simulations and detailed finite-size scaling analyses, we obtain high-precision results for the quantum critical properties at the transition from the magnetically disordered dimer-singlet phase to the antiferromagnetically ordered Néel phase. This transition breaks O(N) symmetry with N=3 in D=3+1 dimensions. This is the upper critical dimension, where multiplicative logarithmic corrections to the leading mean-field critical properties are expected; we extract these corrections, establishing their precise forms for both the zero-temperature staggered magnetization ms and the Néel temperature TN. We present a scaling ansatz for TN, including logarithmic corrections, which agrees with our data and indicates exact linearity with ms, implying a complete decoupling of quantum and thermal fluctuation effects even arbitrarily close to the quantum critical point. We also demonstrate the predicted N-independent leading and subleading logarithmic corrections in the size dependence of the staggered magnetic susceptibility. These logarithmic scaling forms have not previously been identified or verified by unbiased numerical methods, and we discuss their relevance to experimental studies of dimerized quantum antiferromagnets such as TlCuCl3.
Thu, 01 Jan 2015 00:00:00 GMThttp://hdl.handle.net/10722/2686262015-01-01T00:00:00Z
- Anisotropic softening of magnetic excitations in lightly electron-doped Sr2IrO4http://hdl.handle.net/10722/268579Title: Anisotropic softening of magnetic excitations in lightly electron-doped Sr2IrO4
Authors: Liu, X.; Dean, M. P.M.; Meng, Z. Y.; Upton, M. H.; Qi, T.; Gog, T.; Cao, Y.; Lin, J. Q.; Meyers, D.; Ding, H.; Cao, G.; Hill, J. P.
Abstract: © 2016 American Physical Society. The magnetic excitations in electron-doped (Sr1-xLax)2IrO4 with x=0.03 were measured using resonant inelastic x-ray scattering at the Ir L3 edge. Although much broadened, well defined dispersive magnetic excitations were observed. Comparing with the magnetic dispersion from the undoped compound, the evolution of the magnetic excitations upon doping is highly anisotropic. Along the antinodal direction, the dispersion is almost intact. On the other hand, the magnetic excitations along the nodal direction show significant softening. These results establish the presence of strong magnetic correlations in electron-doped (Sr1-xLax)2IrO4 with close analogies to the hole-doped cuprates, further motivating the search for high temperature superconductivity in this system.
Fri, 01 Jan 2016 00:00:00 GMThttp://hdl.handle.net/10722/2685792016-01-01T00:00:00Z
- Amplitude Mode in Three-Dimensional Dimerized Antiferromagnetshttp://hdl.handle.net/10722/268590Title: Amplitude Mode in Three-Dimensional Dimerized Antiferromagnets
Authors: Qin, Yan Qi; Normand, B.; Sandvik, Anders W.; Meng, Zi Yang
Abstract: © 2017 American Physical Society. The amplitude ("Higgs") mode is a ubiquitous collective excitation related to spontaneous breaking of a continuous symmetry. We combine quantum Monte Carlo (QMC) simulations with stochastic analytic continuation to investigate the dynamics of the amplitude mode in a three-dimensional dimerized quantum spin system. We characterize this mode by calculating the spin and dimer spectral functions on both sides of the quantum critical point, finding that both the energies and the intrinsic widths of the excitations satisfy field-theoretical scaling predictions. While the line width of the spin response is close to that observed in neutron scattering experiments on TlCuCl3, the dimer response is significantly broader. Our results demonstrate that highly nontrivial dynamical properties are accessible by modern QMC and analytic continuation methods.
Sun, 01 Jan 2017 00:00:00 GMThttp://hdl.handle.net/10722/2685902017-01-01T00:00:00Z
- Analytic Continuation with Padé Decompositionhttp://hdl.handle.net/10722/268594Title: Analytic Continuation with Padé Decomposition
Authors: Han, Xing Jie; Liao, Hai Jun; Xie, Hai Dong; Huang, Rui Zhen; Meng, Zi Yang; Xiang, Tao
Abstract: © 2017 Chinese Physical Society and IOP Publishing Ltd. The ill-posed analytic continuation problem for Green's functions or self-energies can be carried out using the Padé rational polynomial approximation. However, to extract accurate results from this approximation, high precision input data of the Matsubara Green function are needed. The calculation of the Matsubara Green function generally involves a Matsubara frequency summation, which cannot be evaluated analytically. Numerical summation is requisite but it converges slowly with the increase of the Matsubara frequency. Here we show that this slow convergence problem can be signifcantly improved by utilizing the Padé decomposition approach to replace the Matsubara frequency summation by a Padé frequency summation, and high precision input data can be obtained to successfully perform the Padé analytic continuation.
Sun, 01 Jan 2017 00:00:00 GMThttp://hdl.handle.net/10722/2685942017-01-01T00:00:00Z
- Self-learning quantum Monte Carlo method in interacting fermion systemshttp://hdl.handle.net/10722/268648Title: Self-learning quantum Monte Carlo method in interacting fermion systems
Authors: Xu, Xiao Yan; Qi, Yang; Liu, Junwei; Fu, Liang; Meng, Zi Yang
Abstract: © 2017 American Physical Society. The self-learning Monte Carlo method is a powerful general-purpose numerical method recently introduced to simulate many-body systems. In this work, we extend it to an interacting fermion quantum system in the framework of the widely used determinant quantum Monte Carlo. This method can generally reduce the computational complexity and moreover can greatly suppress the autocorrelation time near a critical point. This enables us to simulate an interacting fermion system on a 100×100 lattice even at the critical point and obtain critical exponents with high precision.
Sun, 01 Jan 2017 00:00:00 GMThttp://hdl.handle.net/10722/2686482017-01-01T00:00:00Z
- Unifying static and dynamic properties in three-dimensional quantum antiferromagnetshttp://hdl.handle.net/10722/268658Title: Unifying static and dynamic properties in three-dimensional quantum antiferromagnets
Authors: Scammell, H. D.; Kharkov, Y.; Qin, Yan Qi; Meng, Zi Yang; Normand, B.; Sushkov, O. P.
Abstract: © 2017 American Physical Society. Quantum Monte Carlo simulations offer an unbiased means to study the static and dynamic properties of quantum critical systems, while quantum field theory provides direct analytical results. We study three-dimensional, critical quantum antiferromagnets by performing a combined analysis using both quantum field theory calculations and quantum Monte Carlo data. Explicitly, we analyze the order parameter (staggered magnetization), Néel temperature, quasiparticle gaps, and the susceptibilities in the scalar and vector channels. We connect the two approaches by deriving descriptions of the quantum Monte Carlo observables in terms of the quasiparticle excitations of the field theory. The remarkable agreement not only unifies the description of the static and dynamic properties of the system but also constitutes a thorough test of perturbative O(3) quantum field theory and opens new avenues for the analytical guidance of detailed numerical studies.
Sun, 01 Jan 2017 00:00:00 GMThttp://hdl.handle.net/10722/2686582017-01-01T00:00:00Z
- Symmetry-enforced self-learning Monte Carlo method applied to the Holstein modelhttp://hdl.handle.net/10722/268600Title: Symmetry-enforced self-learning Monte Carlo method applied to the Holstein model
Authors: Chen, Chuang; Xu, Xiao Yan; Liu, Junwei; Batrouni, George; Scalettar, Richard; Meng, Zi Yang
Abstract: © 2018 American Physical Society. The self-learning Monte Carlo method (SLMC), using a trained effective model to guide Monte Carlo sampling processes, is a powerful general-purpose numerical method recently introduced to speed up simulations in (quantum) many-body systems. In this Rapid Communication, we further improve the efficiency of SLMC by enforcing physical symmetries on the effective model. We demonstrate its effectiveness in the Holstein Hamiltonian, one of the most fundamental many-body descriptions of electron-phonon coupling. Simulations of the Holstein model are notoriously difficult due to a combination of the typical cubic scaling of fermionic Monte Carlo and the presence of extremely long autocorrelation times. Our method addresses both bottlenecks. This enables simulations on large lattices in the most difficult parameter regions, and an evaluation of the critical point for the charge density wave transition at half filling with high precision. We argue that our work opens a research area of quantum Monte Carlo, providing a general procedure to deal with ergodicity in situations involving Hamiltonians with multiple, distinct low-energy states.
Mon, 01 Jan 2018 00:00:00 GMThttp://hdl.handle.net/10722/2686002018-01-01T00:00:00Z
- Correlated insulating phases in the twisted bilayer graphenehttp://hdl.handle.net/10722/296331Title: Correlated insulating phases in the twisted bilayer graphene
Authors: Liao, Y; Xu, XY; Meng, ZY; Kang, J
Abstract: We review analytical and numerical studies of correlated insulating states in twisted bilayer graphene, focusing on real-space lattice models constructions and their unbiased quantum many-body solutions. We show that by constructing localized Wannier states for the narrow bands, the projected Coulomb interactions can be approximated by interactions of cluster charges with assisted nearest neighbor hopping terms. With the interaction part only, the Hamiltonian is SU(4) symmetric considering both spin and valley degrees of freedom. In the strong coupling limit where the kinetic terms are neglected, the ground states are found to be in the SU(4) manifold with degeneracy. The kinetic terms, treated as perturbation, break this large SU(4) symmetry and propel the appearance of intervalley coherent state, quantum topological insulators, and other symmetry-breaking insulating states. We first present the theoretical analysis of moiré lattice model construction and then show how to solve the model with large-scale quantum Monte Carlo simulations in an unbiased manner. We further provide potential directions such that from the real-space model construction and its quantum many-body solutions how the perplexing yet exciting experimental discoveries in the correlation physics of twisted bilayer graphene can be gradually understood. This review will be helpful for the readers to grasp the fast growing field of the model study of twisted bilayer graphene.
Fri, 01 Jan 2021 00:00:00 GMThttp://hdl.handle.net/10722/2963312021-01-01T00:00:00Z
- Evidence of the Berezinskii-Kosterlitz-Thouless phase in a frustrated magnethttp://hdl.handle.net/10722/295272Title: Evidence of the Berezinskii-Kosterlitz-Thouless phase in a frustrated magnet
Authors: Hu, Z; Ma, Z; Liao, YD; Li, H; Ma, C; Cui, Y; Shangguan, Y; Huang, Z; Qi, Y; Li, W; Meng, Z; Wen, J; Yu, W
Abstract: The Berezinskii-Kosterlitz-Thouless (BKT) mechanism, building upon proliferation of topological defects in 2D systems, is the first example of phase transition beyond the Landau-Ginzburg paradigm of symmetry breaking. Such a topological phase transition has long been sought yet undiscovered directly in magnetic materials. Here, we pin down two transitions that bound a BKT phase in an ideal 2D frustrated magnet TmMgGaO4, via nuclear magnetic resonance under in-plane magnetic fields, which do not disturb the low-energy electronic states and allow BKT fluctuations to be detected sensitively. Moreover, by applying out-of-plane fields, we find a critical scaling behavior of the magnetic susceptibility expected for the BKT transition. The experimental findings can be explained by quantum Monte Carlo simulations applied on an accurate triangular-lattice Ising model of the compound which hosts a BKT phase. These results provide a concrete example for the BKT phase and offer an ideal platform for future investigations on the BKT physics in magnetic materials.
Wed, 01 Jan 2020 00:00:00 GMThttp://hdl.handle.net/10722/2952722020-01-01T00:00:00Z
- Vestigial anyon condensation in kagome quantum spin liquidshttp://hdl.handle.net/10722/296396Title: Vestigial anyon condensation in kagome quantum spin liquids
Authors: Wang, YC; Yan, Z; Wang, C; Qi, Y; Meng, ZY
Abstract: We construct a lattice model of topological order (kagome quantum spin liquids) and solve it with unbiased quantum Monte Carlo simulations. A three-stage anyon condensation with two transitions from a Z(2) boxed times Z(2) topological order to a Z(2) topological order and eventually to a trivial symmetric phase is revealed. These results provide concrete examples of phase transitions between topological orders in quantum magnets. The designed quantum spin liquid model and its numerical solution offer a playground for further investigations on vestigial anyon condensation.
Fri, 01 Jan 2021 00:00:00 GMThttp://hdl.handle.net/10722/2963962021-01-01T00:00:00Z
- Correlation-Induced Insulating Topological Phases at Charge Neutrality in Twisted Bilayer Graphenehttp://hdl.handle.net/10722/297234Title: Correlation-Induced Insulating Topological Phases at Charge Neutrality in Twisted Bilayer Graphene
Authors: Da Liao, Y; Kang, J; Breiø, CN; Xu, XY; Wu, HQ; Andersen, BM; Fernandes, RM; Meng, ZY
Abstract: Twisted bilayer graphene (TBG) provides a unique framework to elucidate the interplay between strong correlations and topological phenomena in two-dimensional systems. The existence of multiple electronic degrees of freedom-charge, spin, and valley-gives rise to a plethora of possible ordered states and instabilities. Identifying which of them are realized in the regime of strong correlations is fundamental to shed light on the nature of the superconducting and correlated insulating states observed in the TBG experiments. Here, we use unbiased, sign-problem-free quantum Monte Carlo simulations to solve an effective interacting lattice model for TBG at charge neutrality. Besides the usual cluster Hubbard-like repulsion, this model also contains an assisted-hopping interaction that emerges due to the nontrivial topological properties of TBG. Such a nonlocal interaction fundamentally alters the phase diagram at charge neutrality, gapping the Dirac cones even for infinitesimally small interactions. As the interaction strength increases, a sequence of different correlated insulating phases emerge, including a quantum valley Hall state with topological edge states, an intervalley-coherent insulator, and a valence bond solid. The charge-neutrality correlated insulating phases discovered here provide the sought-after reference states needed for a comprehensive understanding of the insulating states at integer fillings and the proximate superconducting states of TBG.
Fri, 01 Jan 2021 00:00:00 GMThttp://hdl.handle.net/10722/2972342021-01-01T00:00:00Z
- Topological invariants for interacting topological insulators. II. Breakdown of single-particle Green's function formalismhttp://hdl.handle.net/10722/268478Title: Topological invariants for interacting topological insulators. II. Breakdown of single-particle Green's function formalism
Authors: He, Yuan Yao; Wu, Han Qing; Meng, Zi Yang; Lu, Zhong Yi
Abstract: © 2016 American Physical Society. Topological phase transitions in free fermion systems can be characterized by the closing of single-particle gap and the change in topological invariants. However, in the presence of electronic interactions, topological phase transitions can be more complicated. In paper I of this series [Phys. Rev. B 93, 195163 (2016)10.1103/PhysRevB.93.195163], we have proposed an efficient scheme to evaluate the topological invariants based on the single-particle Green's function formalism. Here, in paper II, we demonstrate several interaction-driven topological phase transitions (TPTs) in two-dimensional (2D) interacting topological insulators (TIs) via large-scale quantum Monte Carlo (QMC) simulations, based on the scheme of evaluating topological invariants presented in paper I. Across these transitions, the defining symmetries of the TIs have been neither explicitly nor spontaneously broken. In the first two models, the topological invariants calculated from the Green's function formalism succeed in characterizing the topologically distinct phases and identifying interaction-driven TPTs. However, in the other two models, we find that the single-particle gap does not close and the topological invariants constructed from the single-particle Green's function acquire no change across the TPTs. Unexpected breakdown of the Green's function formalism in constructing the topological invariants is thus discovered. We thence classify the topological phase transitions in interacting TIs into two categories in practical computation: Those that have noninteracting correspondence can be characterized successfully by the topological invariants constructed from the Green's functions, while for the others that do not have noninteracting correspondence, the Green's function formalism experiences a breakdown, but more interesting and exciting phenomena, such as emergent collective critical modes at the transition, arise. Discussion on the success and breakdown of topological invariants constructed from the Green's function formalism in the context of symmetry protected topological (SPT) states is presented.
Fri, 01 Jan 2016 00:00:00 GMThttp://hdl.handle.net/10722/2684782016-01-01T00:00:00Z
- Visualizing a bosonic symmetry protected topological phase in an interacting fermion modelhttp://hdl.handle.net/10722/268629Title: Visualizing a bosonic symmetry protected topological phase in an interacting fermion model
Authors: Wu, Han Qing; He, Yuan Yao; You, Yi Zhuang; Yoshida, Tsuneya; Kawakami, Norio; Xu, Cenke; Meng, Zi Yang; Lu, Zhong Yi
Abstract: © 2016 American Physical Society. Symmetry protected topological (SPT) phases in free fermion and interacting bosonic systems have been classified, but the physical phenomena of interacting fermionic SPT phases have not been fully explored. Here, employing large-scale quantum Monte Carlo simulation, we investigate the edge physics of a bilayer Kane-Mele-Hubbard model with zigzag ribbon geometry. Our unbiased numerical results show that the fermion edge modes are gapped out by interaction, while the bosonic edge modes remain gapless at the (1+1)d boundary, before the bulk quantum phase transition to a topologically trivial phase. Therefore, finite fermion gaps both in the bulk and on the edge, together with the robust gapless bosonic edge modes, prove that our system becomes an emergent bosonic SPT phase at low energy, which is directly observed in an interacting fermion lattice model.
Fri, 01 Jan 2016 00:00:00 GMThttp://hdl.handle.net/10722/2686292016-01-01T00:00:00Z
- Effect of Nematic Order on the Low-Energy Spin Fluctuations in Detwinned BaFe1.935Ni0.065As2http://hdl.handle.net/10722/268630Title: Effect of Nematic Order on the Low-Energy Spin Fluctuations in Detwinned BaFe1.935Ni0.065As2
Authors: Zhang, Wenliang; Park, J. T.; Lu, Xingye; Wei, Yuan; Ma, Xiaoyan; Hao, Lijie; Dai, Pengcheng; Meng, Zi Yang; Yang, Yi Feng; Luo, Huiqian; Li, Shiliang
Abstract: © 2016 American Physical Society. The origin of nematic order remains one of the major debates in iron-based superconductors. In theories based on spin nematicity, one major prediction is that the spin-spin correlation length at (0,π) should decrease with decreasing temperature below the structural transition temperature Ts. Here, we report inelastic neutron scattering studies on the low-energy spin fluctuations in BaFe1.935Ni0.065As2 under uniaxial pressure. Both intensity and spin-spin correlation start to show anisotropic behavior at high temperature, while the reduction of the spin-spin correlation length at (0,π) happens just below Ts, suggesting the strong effect of nematic order on low-energy spin fluctuations. Our results favor the idea that treats the spin degree of freedom as the driving force of the electronic nematic order.
Fri, 01 Jan 2016 00:00:00 GMThttp://hdl.handle.net/10722/2686302016-01-01T00:00:00Z
- Self-learning Monte Carlo methodhttp://hdl.handle.net/10722/268479Title: Self-learning Monte Carlo method
Authors: Liu, Junwei; Qi, Yang; Meng, Zi Yang; Fu, Liang
Abstract: © 2017 American Physical Society. Monte Carlo simulation is an unbiased numerical tool for studying classical and quantum many-body systems. One of its bottlenecks is the lack of a general and efficient update algorithm for large size systems close to the phase transition, for which local updates perform badly. In this Rapid Communication, we propose a general-purpose Monte Carlo method, dubbed self-learning Monte Carlo (SLMC), in which an efficient update algorithm is first learned from the training data generated in trial simulations and then used to speed up the actual simulation. We demonstrate the efficiency of SLMC in a spin model at the phase transition point, achieving a 10-20 times speedup.
Sun, 01 Jan 2017 00:00:00 GMThttp://hdl.handle.net/10722/2684792017-01-01T00:00:00Z