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Metal to Orthogonal Metal Transition

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Chinese Physics Letters 2020年第4期37卷 114-120页

作者： Chuang Chen Xiao Yan Xu yang Qi zi yang meng Beijing National Laboratory for Condensed Matter Physics and Institute of Physics Chinese Academy of SciencesBeijing 100190 Department of Physics Hong Kong University of Science and TechnologyClear Water BayHong Kong Department of Physics University of California at San DiegoLa JollaCalifornia 92093USA Center for Field Theory and Particle Physics Department of PhysicsFudan UniversityShanghai 200433 State Key Laboratory of Surface Physics Fudan UniversityShanghai 200433 Collaborative Innovation Center of Advanced Microstructures Nanjing 210093 Department of Physics and HKU-UCAS Joint Institute of Theoretical and Computational Physics The University of Hong KongPokfulam RoadHong Kong Songshan Lake Materials Laboratory Dongguan 523808

Orthogonal metal is a new quantum metallic state that conducts electricity but acquires no Fermi surface(FS)or quasiparticles, and hence orthogonal to the established paradigm of Landau’s Fermi-liquid(FL). Such a state may hold the key of understanding the perplexing experimental observations of quantum metals that are beyond FL, i.e., dubbed non-Fermi-liquid(nFL), ranging from the Cu-and Fe-based oxides, heavy fermion compounds to the recently discovered twisted graphene heterostructures. However, to fully understand such an exotic state of matter, at least theoretically, one would like to construct a lattice model and to solve it with unbiased quantum many-body machinery. Here we achieve this goal by designing a 2D lattice model comprised of fermionic and bosonic matter fields coupled with dynamic Z2 gauge fields, and obtain its exact properties with sign-free quantum Monte Carlo simulations. We find that as the bosonic matter fields become disordered, with the help of deconfinement of the Z2 gauge fields, the system reacts with changing its nature from the conventional normal metal with an FS to an orthogonal metal of n FL without FS and quasiparticles and yet still responds to magnetic probe like an FL. Such a quantum phase transition from a normal metal to an orthogonal metal, with its electronic and magnetic spectral properties revealed, is calling for the establishment of new paradigm of quantum metals and their transition with conventional ones.

关键词： quantum Fermi gauge

Quantum simulation of lattice gauge theories on superconducting circuits:Quantum phase transition and quench dynamics

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Chinese Physics B 2022年第2期31卷 217-222页

作者： zi-Yong Ge Rui-Zhen Huang zi-yang meng Heng Fan Beijing Nntional Laboratory for Condensed Matter Physics Institute of PhysicsChinese Academy of SciencesBeijing 100190China School of Physical Sciences University of Chinese Academy of SciencesBeijing 100190China Kuvli Institute for Theoretical Sciences University of Chinese Academy of SciencesBeijing 100190China Songshan Lake Materials Laboratory Dongguan 523808China Department of Physics and HKU-UCAS Joint Institute of Theoretical and Computational Physics The University of Hong Kong.Hong Kong SARChina CAS Center for Excellence in Topological Quantum Computation.University of Chinese Academy of Sciences.Beijing 100190 China

Recently,quantum simulation of low-dimensional lattice gauge theories(LGTs)has attracted many interests,which may improve our understanding of strongly correlated quantum many-body ***,we propose an implementation to approximate Z;LGT on superconducting quantum circuits,where the effective theory is a mixture of a LGT and a gauge-broken *** using matrix product state based methods,both the ground state properties and quench dynamics are systematically *** an increase of the transverse(electric)field,the system displays a quantum phase transition from a disordered phase to a translational symmetry breaking *** the ordered phase,an approximate Gauss law of the Z;LGT emerges in the ground ***,to shed light on the experiments,we also study the quench dynamics,where there is a dynamical signature of the spontaneous translational symmetry *** spreading of the single particle of matter degree is diffusive under the weak transverse field,while it is ballistic with small velocity for the strong ***,due to the emergent Gauss law under the strong transverse field,the matter degree can also exhibit confinement dynamics which leads to a strong suppression of the nearest-neighbor *** results pave the way for simulating the LGT on superconducting circuits,including the quantum phase transition and quench dynamics.

关键词： quantum simulation superconducting circuits lattice gauge theories

Superconductivity near the(2+1)-Dimensional Ferromagnetic Quantum Critical Point

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Chinese Physics Letters 2022年第9期39卷 47-52页

作者： Yunchao Hao Gaopei Pan Kai Sun zi yang meng yang Qi State Key Laboratory of Surface Physics and Department of Physics Fudan UniversityShanghai 200433China Beijing National Laboratory for Condensed Matter Physics and Institute of Physics Chinese Academy of SciencesBeijing 100190China School of Physical Sciences University of Chinese Academy of SciencesBeijing 100190China Department of Physics University of MichiganAnn ArborMI 48109USA Department of Physics and HKU-UCAS Joint Institute of Theoretical and Computational Physics The University of Hong KongPokfulam RoadHong KongChina Collaborative Innovation Center of Advanced Microstructures Nanjing 210093China

We utilize both analytical and numerical methods to study the superconducting transition temperature Tnear a fermionic quantum critical point(QCP) using a model constructed by Xu et al. [Phys. Rev. X 7, 031059(2017)] as an example. In this model, the bosonic critical fluctuation plays the role of pairing glue for the Cooper pairs, and we use a Bardeen–Cooper–Schrieffer-type mean-field theory to estimate T. We further argue that the Tc computed from the BCS theory approximates a pseudogap temperature TPG, instead of the Berezinskii–Kosterlitz–Thouless transition temperature T, which is confirmed by our determinant quantum Monte Carlo simulation. Moreover, due to the fact that electron density of state starts to deplete at T, the critical scaling of the underlying QCP is also affected below TPG. Thus, when studying the critical behavior of fermionic QCPs, we need to monitor that the temperature is above TPG instead of T. This was often ignored in previous studies.

关键词： critical quantum utilize

Solving quantum rotor model with different Monte Carlo techniques

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Chinese Physics B 2022年第4期31卷 208-218页

作者： Weilun Jiang Gaopei Pan Yuzhi Liu zi-yang meng Beijing National Laboratory for Condensed Matter Physics and Institute of Physics Chinese Academy of SciencesBeijing 100190China School of Physical Sciences University of Chinese Academy of SciencesBeijing 100190China Department of Physics and HKU-UCAS Joint Institute of Theoretical and Computational Physics The University of Hong KongPokfulam RoadHong KongChina Songshan Lake Materials Laboratory Dongguan 523808China

We systematically test the performance of several Monte Carlo update schemes for the(2+1)d XY phase transition of quantum rotor *** comparing the local Metropolis(LM),LM plus over-relaxation(OR),Wolff-cluster(WC),hybrid Monte Carlo(HM),hybrid Monte Carlo with Fourier acceleration(FA)schemes,it is clear that among the five different update schemes,at the quantum critical point,the WC and FA schemes acquire the smallest autocorrelation time and cost the least amount of CPU hours in achieving the same level of relative error,and FA enjoys a further advantage of easily implementable for more complicated interactions such as the long-range *** results bestow one with the necessary knowledge of extending the quantum rotor model,which plays the role of ferromagnetic/antiferromagnetic critical bosons or Z_(2)topological order,to more realistic and yet challenging models such as Fermi surface Yukawa-coupled to quantum rotor models.

关键词： Monte Carlo methods

Momentum Space Quantum Monte Carlo on Twisted Bilayer Graphene

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Chinese Physics Letters 2021年第7期38卷 130-135页

作者： Xu Zhang Gaopei Pan Yi Zhang Jian Kang zi yang meng Department of Physics and HKU-UCAS Joint Institute of Theoretical and Computational Physics The University of Hong KongPokfulam RoadHong Kong SARChina Beijing National Laboratory for Condensed Matter Physics and Institute of Physics Chinese Academy of SciencesBeijing 100190China School of Physical Sciences University of Chinese Academy of SciencesBeijing 100190China Kavli Institute for Theoretical Sciences University of Chinese Academy of SciencesBeijing 100190China School of Physical Science and Technology and Institute for Advanced Study Soochow UniversitySuzhou 215006China

We report an implementation of the momentum space quantum Monte Carlo(QMC)method on the interaction model for the twisted bilayer graphene(TBG).The long-range Coulomb repulsion is treated exactly with the flat bands,spin and valley degrees of freedom of electrons taking into *** prove the absence of the minus sign problem for QMC simulation when either the two valleys or the two spin degrees of freedom are *** taking the realistic parameters of the twist angle and interlayer tunnelings into the simulation,we benchmark the QMC data with the exact band gap obtained at the chiral limit,to reveal the insulating ground states at the charge neutrality point(CNP).Then,with the exact Green's functions from QMC,we perform stochastic analytic continuation to obtain the first set of single-particle spectral function for the TBG model at *** momentum space QMC scheme therefore offers the controlled computation pathway for systematic investigation of the electronic states in realistic TBG model at various electron fillings.

关键词： momentum exact twisted

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Chinese Physics B 2021年第1期30卷 1-13页

作者： Yuan-Da Liao Xiao-Yan Xu zi-yang meng Jian Kang Beijing National Laboratory for Condensed Matter Physics and Institute of Physics Chinese Academy of SciencesBeijing 100190China School of Physical Sciences University of Chinese Academy of SciencesBeijing 100190China Department of Physics University of California at San DiegoLa JollaCalifornia 92093USA Department of Physics and HKU-UCAS Joint Institute of Theoretical and Computational Physics The University of Hong KongPokfulam RoadHong Kong SARChina Songshan Lake Materials Laboratory Dongguan 523808China School of Physical Science and Technology&Institute for Advanced Study Soochow UniversitySuzhou 215006China

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.

关键词： twisted bilayer graphene correlated insulator quantum Monte Carlo simulation

Network-Initialized Monte Carlo Based on Generative Neural Networks

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Chinese Physics Letters 2022年第5期39卷 23-28页

作者： Hongyu Lu Chuhao Li Bin-Bin Chen Wei Li yang Qi zi yang meng Department of Physics and HKU-UCAS Joint Institute of Theoretical and Computational Physics The University of Hong KongHong KongChina Beijing National Laboratory for Condensed Matter Physics and Institute of PhysicsChinese Academy of SciencesBeijing 100190China School of Physical Sciences University of Chinese Academy of SciencesBeijing 100190China Institute of Theoretical Physics Chinese Academy of SciencesBeijing 100190China School of Physics Beihang UniversityBeijing 100191China State Key Laboratory of Surface Physics Fudan UniversityShanghai 200438China Center for Field Theory and Particle Physics Department of PhysicsFudan UniversityShanghai 200433China

We design generative neural networks that generate Monte Carlo configurations with complete absence of autocorrelation from which only short Markov chains are needed before making measurements for physical observables,irrespective of the system locating at the classical critical point,fermionic Mott insulator,Dirac semimetal,or quantum critical *** further propose a network-initialized Monte Carlo scheme based on such neural networks,which provides independent samplings and can accelerate the Monte Carlo simulations by significantly reducing the thermalization *** demonstrate the performance of our approach on the two-dimensional Ising and fermion Hubbard models,expect that it can systematically speed up the Monte Carlo simulations especially for the very challenging many-electron problems.

关键词： process critical chains

Emergent O(4) symmetry at the phase transition from plaquette-singlet to antiferromagnetic order in quasi-two-dimensional quantum magnets

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Chinese Physics B 2021年第6期30卷 570-582页

作者： Guangyu Sun Nvsen Ma Bowen Zhao Anders WSandvik zi yang meng Beijing National Laboratory for Condensed Matter Physics and Institute of Physics Chinese Academy of SciencesBeijing 100190China School of Physical Sciences University of Chinese Academy of SciencesBeijing 100190China School of Physics Key Laboratory of Micro-Nano Measurement-Manipulation and PhysicsBeihang UniversityBeijing 100191China Department of Physics Boston University590 Commonwealth AvenueBostonMassachusetts 02215USA Department of Physics and HKU-UCAS Joint Institute of Theoretical and Computational Physics The University of Hong KongHong KongChina

Recent experiments [Guo et al., Phys. Rev. Lett. 124 206602(2020)] on thermodynamic properties of the frustrated layered quantum magnet SrCu_(2)(BO_(3))_(2)-the Shastry–Sutherland material-have provided strong evidence for a lowtemperature phase transition between plaquette-singlet and antiferromagnetic order as a function of pressure. Further motivated by the recently discovered unusual first-order quantum phase transition with an apparent emergent O(4) symmetry of the antiferromagnetic and plaquette-singlet order parameters in a two-dimensional "checkerboard J-Q" quantum spin model[Zhao et al., Nat. Phys. 15 678(2019)], we here study the same model in the presence of weak inter-layer couplings. Our focus is on the evolution of the emergent symmetry as the system crosses over from two to three dimensions and the phase transition extends from strictly zero temperature in two dimensions up to finite temperature as expected in SrCu_(2)(BO_(3))_(2).Using quantum Monte Carlo simulations, we map out the phase boundaries of the plaquette-singlet and antiferromagnetic phases, with particular focus on the triple point where these two ordered phases meet the paramagnetic phase for given strength of the inter-layer coupling. All transitions are first-order in the neighborhood of the triple point. We show that the emergent O(4) symmetry of the coexistence state breaks down clearly when the interlayer coupling becomes sufficiently large, but for a weak coupling, of the magnitude expected experimentally, the enlarged symmetry can still be observed at the triple point up to significant length scales. Thus, it is likely that the plaquette-singlet to antiferromagnetic transition in SrCu_(2)(BO_(3))_(2) exhibits remnants of emergent O(4) symmetry, which should be observable due to additional weakly gapped Goldstone modes.

关键词： quantum phase transitions quantum spin systems emergent symmetry quantum Monte Carlo simulations

Nonlocal Effects of Low-Energy Excitations in Quantum-Spin-Liquid Candidate Cu_(3)Zn(OH)_(6)FBr

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Chinese Physics Letters 2021年第9期38卷 96-100页

作者： Yuan Wei Xiaoyan Ma zili Feng Yongchao Zhang Lu Zhang Huaixin yang yang Qi zi yang meng Yan-Cheng Wang Youguo Shi Shiliang Li Beijing National Laboratory for Condensed Matter Physics and Institute of PhysicsChinese Academy of SciencesBeijing 100190China School of Physical Sciences University of Chinese Academy of SciencesBeijing 100190China Institute for Solid State Physics University of TokyoKashiwa 277-8581Japan yangtze River Delta Physics Research Center Co. Ltd.Liyang 213300China State Key Laboratory of Surface Physics Department of PhysicsFudan UniversityShanghai 200433China Department of Physics and HKU-UCAS Joint Institute of Theoretical and Computational Physics The University of Hong KongPokfulam RoadHong KongChina School of Materials Science and Physics China University of Mining and TechnologyXuzhou 221116China Songshan Lake Materials Laboratory Dongguan 523808China

We systematically study the low-temperature specific heats for the two-dimensional kagome antiferromagnet,Cu_(3)Zn(OH)_(6)*** specific heat exhibits a T1.7 dependence at low temperatures and a shoulder-like feature above *** construct a microscopic lattice model of Z_(2) quantum spin liquid and perform large-scale quantum Monte Carlo simulations to show that the above behaviors come from the contributions from gapped anyons and magnetic ***,we find the entropy associated with the shoulder decreases quickly with grain size d,although the system is paramagnetic to the lowest *** this can be simply explained by a core-shell picture in that the contribution from the interior state disappears near the surface,the 5.9-nm shell width precludes any trivial *** a large length scale signifies the coherence length of the nonlocality of the quantum entangled excitations in quantum spin liquid candidate,similar to Pippard’s coherence length in *** approach therefore offers a new experimental probe of the intangible quantum state of matter with topological order.

关键词： temperature quantum trivial