arXiv subjects

Wayne Zheng

Publications and source records attributed to Wayne Zheng.

11 recordsLinked to original sources

arXiv2026

Pressure-Induced Superconducting-like Transition in the $\it d$-wave Altermagnet Candidate CsV$_2$Se$_2$O

Altermagnetism generates exchange-type spin splitting without net magnetization and, in its $\it d$-wave form, resembles the angular symmetry of unconventional $\it d$-wave superconductivity. Whether this correspondence bears directly on superconducting instabilities in real correlated materials remains open. Here we study the quasi-two-dimensional vanadium oxychalcogenide CsV$_2$Se$_2$O (CVSO), a square-net $\it d$-wave altermagnet candidate, through combined experimental and theoretical investigation of its lattice structure, electronic structure and transport properties. At ambient pressure, CVSO is a weakly insulating parent state with a density-wave-like anomaly near 100 K, and its bulk properties are most consistent with a G-type compensated antiferromagnetic background. Under compression, the density-wave-like feature is suppressed, the magnetoresistance evolves from predominantly negative to positive, and a superconducting-like resistive downturn emerges below about 3 K. This low-temperature anomaly is reproducible across samples and pressure media, and is suppressed by magnetic field. Room-temperature X-ray diffraction reveals no symmetry lowering, whereas does show a pronounced compressibility anomaly over the same pressure range. CVSO thus reveals a pressure-tuned phase diagram in which a reconstructed weakly insulating parent state gives way to strange-metal-like transport and superconducting-like behavior, echoing broader phenomenology associated with unconventional superconductors, including cuprates and nickelates.

cond-mat.supr-con↗

arXiv2025

Competing $s$-wave pairing in overdoped $t$-$J$ model

The $d$-wave pairing symmetry has long been considered a defining feature of high-temperature superconductivity in cuprates. In this work, we reveal that $s$-wave pairing states exhibit variational energies comparable to the $d$-wave state in a square $t$-$J$ model, particularly at high doping levels ($\delta\gtrsim 15\%$) by using the state-of-the-art tensor network simulation. This surprising result suggests that $s$-wave pairing may play an important role in the cuprate phase diagram, especially for the overdoped region. Our findings provide a potential resolution to discrepancies in recent Josephson tunneling experiments on twisted bilayer cuprates and offer new insights into the evolution of pairing symmetry with doping.

cond-mat.str-el↗

arXiv2025

Revealing quantum phase string effect in doped Mott-insulator: a tensor network state approach

We apply the fermionic tensor network (TN) state method to understand the strongly correlated nature in a doped Mott insulator. We conduct a comparative study of the $\sigma t$-$J$ model, in which the no-double-occupancy constraint remains unchanged but the quantum phase string effect associated with doped holes is precisely switched off. Thus, the ground state of the $\sigma t$-$J$ model can serve as a well-controlled reference state of the standard $t$-$J$ model. In the absence of phase string, the spin long-range antiferromagnetic (AFM) order is found to be essentially decoupled from the doped holes, and the latter contribute to a Fermi-liquid-like compressibility and a coherent single-particle propagation with a markedly reduced pairing tendency. In contrast, our TN calculations of the $t$-$J$ model indicate that the AFM order decreases much faster with doping and the single-particle propagation of doped holes gets substantially suppressed, concurrently with a much stronger charge compressibility at small doping and a significantly amplified Cooper pairing tendencies. These findings demonstrate that quantum many-body interference from phase strings plays a pivotal role in the $t$-$J$ model, mediating long-range entanglement between spin and charge degrees of freedom.

cond-mat.str-el↗

arXiv2024

Competing pair density wave orders in the square lattice $t$-$J$ model

Over the last two decades, the competing orders in high-$T_{c}$ cuprates have been intensely studied, such as pseudogap phase, charge density waves (CDW), and pair density waves (PDW), which are thought to play a crucial role in high-temperature superconductivity. Using the $t$-$J$ model on a square lattice as the simplest model for high-$T_{c}$ cuprates, we employed the fermionic tensor product state (fTPS) method for numerical investigations. Our study revealed new types of PDW states alongside the well-known $d$-wave state and the recently discovered fluctuating PDW state within the low-energy subspace of the $t$-$J$ model. We believe that the competition among these states in the underdoped region suggests the potential existence of a fluctuating quantum liquid of PDW states, providing direct evidence for the pseudogap phase's "cheap vortex" scenario. Furthermore, we discuss the potential experimental implication of our discovery.

cond-mat.str-el↗

arXiv2021

Unconventional quantum phase transitions in a one-dimensional Lieb-Schultz-Mattis system

We study quantum phases and phase transitions in a one-dimensional interacting fermion system with a Lieb-Schultz-Mattis (LSM) type anomaly. Specifically, the inversion symmetry enforces any symmetry-preserving gapped ground state of the system to be a Kitaev chain, following a Lieb-Schultz-Mattis type theorem that we prove. Alternatively, via the Jordan-Wigner transformation, this system describes a spin system whose gapped ground states must break either the inversion or the Ising symmetry associated with fermion parity. We obtain a phase diagram using analytical methods and variational matrix product state simulations, and study the critical behaviors of the quantum phase transitions therein using entanglement entropy, energy variance and finite size scaling of order parameters. In particular, we observe continuous phase transitions between different ordered phases that are beyond the Ginzburg-Landau-Wilson paradigm, in analogy to the deconfined quantum critical points in two spatial dimensions. We show this type of 1D deconfined quantum critical point is described by the Tomonaga-Luttinger liquid theory, and extract the Luttinger parameter and critical exponents. We also identify a gapless phase between two ordered phases, which cannot be described by a U(1) Luttinger liquid.

cond-mat.str-el↗

arXiv2020

Fermi-Hubbard model on non-bipartite lattices: flux problem and emergent chirality

On several one-dimensional (1D) and 2D nonbipartite lattices, we study both free and Hubbard interacting lattice fermions when some magnetic fluxes are threaded or gauge fields coupled. First, we focus on finding out the optimal flux which minimizes the energy of fermions at specific fillings. For spin-$1/2$ fermions at half-filling on a ring lattice consisting of odd-numbered sites, the optimal flux turns out to be $\pmπ/2$. We prove this conclusion for Hubbard interacting fermions utilizing a generalized reflection positivity technique, which can lead to further applications on 2D nonbipartite lattices such as triangular and Kagome. At half-filling the optimal flux patterns on the triangular and Kagome lattice are ascertained to be $\pm[π/2,π/2]$, $\pm[π/2,π/2,0]$, respectively (see the meaning of these notations in the main text). We also find that chirality emerges in these optimal flux states. Then, we verify these exact conclusions and further study some other fillings with the numerical exact diagonalization method. It is found that when it deviates from half-filling, Hubbard interactions can alter the optimal flux patterns on these lattices. Moreover, numerically observed emergent flux singularities driven by strong Hubbard interactions in the ground states, both in 1D and 2D, are discussed and interpreted as some kind of non-Fermi liquid feature.

cond-mat.str-el↗

arXiv2020

Quantum entanglement in the t-J chain: From charge-spin separation to recombination

In contrast to the conventional von Neumann bipartite entanglement entropy (bEE), we show that a more appropriate description of the one-dimensional doped Mott insulator is a new kind of mutual entanglement entropy (mEE) between the charge and spin degrees of freedom. Such a charge-spin mEE can clearly distinguish the important and distinct features between the $t$-$J$ model and the so-called $σ\cdot{t}$-$J$ model. In the latter, the phase string sign structure is switched off such that a single doped hole always behaves like a Bloch wave in the whole regime of $J/t$, whereas in the former it exhibits a series of level crossing with the total momentum jumps in the single-hole ground state from spin-charge separation at $J/t\rightarrow{0}$ to spin-charge recombination at large $J/t$, which are failed to be detected by bEE. We further show that the distinctions between the two models persist to finite energy density, which can be similarly well characterized by mEE but not by bEE. By studying the dynamic time evolution of the states set out of equilibrium at the beginning, we show that mEE indeed always increases with the time, satisfying the common characteristic of entropy.

cond-mat.str-el↗

arXiv2019

Chiral spin liquid with spinon Fermi surfaces in spin-$1/2$ triangular Heisenberg model

We study the interplay of competing interactions in spin-$1/2$ triangular Heisenberg model through tuning the first- ($J_1$), second- ($J_2$), and third-neighbor ($J_3$) couplings. Based on large-scale density matrix renormalization group calculation, we identify a quantum phase diagram of the system and discover a new {\it gapless} chiral spin liquid (CSL) phase in the intermediate $J_2$ and $J_3$ regime. This CSL state spontaneously breaks time-reversal symmetry with finite scalar chiral order, and it has gapless excitations implied by a vanishing spin triplet gap and a finite central charge on the cylinder. Moreover, the central charge grows rapidly with the cylinder circumference, indicating emergent spinon Fermi surfaces. To understand the numerical results we propose a parton mean-field spin liquid state, the $U(1)$ staggered flux state, which breaks time-reversal symmetry with chiral edge modes by adding a Chern insulator mass to Dirac spinons in the $U(1)$ Dirac spin liquid. This state also breaks lattice rotational symmetries and possesses two spinon Fermi surfaces driven by nonzero $J_2$ and $J_3$, which naturally explains the numerical results. To our knowledge, this is the first example of a gapless CSL state with coexisting spinon Fermi surfaces and chiral edge states, demonstrating the rich family of novel phases emergent from competing interactions in triangular-lattice magnets.

cond-mat.str-el↗

arXiv2018

Hidden spin current in doped Mott antiferromagnets

We investigate the nature of doped Mott insulators using exact diagonalization and density matrix renormalization group methods. Persistent spin currents are revealed in the ground state, which are concomitant with a nonzero total momentum or angular momentum associated with the doped hole. The latter determines a nontrivial ground state degeneracy. By further making superpositions of the degenerate ground states with zero or unidirectional spin currents, we show that different patterns of spatial charge and spin modulations will emerge. Such anomaly persists for the odd numbers of holes, but the spin current, ground state degeneracy, and charge/spin modulations completely disappear for even numbers of holes, with the two-hole ground state exhibiting a d-wave symmetry. An understanding of the spin current due to a many-body Berry-like phase and its impact on the momentum distribution of the doped holes will be discussed.

cond-mat.str-el↗

arXiv2017

Charge-spin mutual entanglement: A case study by exact diagonalization of the one hole doped $t$-$J$ loop

A doped Mott insulator exhibits peculiar properties associated with its singular sign structure. As a case study, we investigate the ground state and excitations of finite-size Heisenberg loops doped with one hole by exact diagonalization. We find that there appear a series of quantum critical points (QCPs), which separate regimes by distinct total momenta along the axis of the ratio $J/t$ ($J$ and $t$ denote the superexchange coupling and hopping integral, respectively). Each QCP involves a crystal momentum jump with level crossing or merging of lowest energy levels. In contrast to the conserved total momentum, however, a broad momentum distribution of \emph{individual} electrons is also found, indicating charge incoherence/translational symmetry breaking in violation of the one-to-one correspondence. Such a charge incoherence is further related to quantum fluctuations or the transverse part of ${\bf S}^2=3/4$ with $S^z=\pm 1/2$ in the one-hole ground state. Turning off the phase-string sign structure, by contrast, we show that the total momentum of the ground state reduces to null in the whole regime of $J/t$ with no more QCP or incoherence. We introduce the so-called charge-spin mutual entanglement to characterize these novel properties, with the entanglement spectrum providing additional information on the charge incoherence, which capture the nature of strong correlation due to the many-body quantum interference.

cond-mat.str-el↗

arXiv2015

Classification and Monte Carlo study of symmetric $Z_{2}$ spin liquids on the triangular lattice

We study different ways of symmetry fractionalization in $Z_{2}$ spin liquids on the triangular lattice. Our classification can be used to identify the symmetry fractionalization in the $Z_{2}$ spin liquid reported in recent density-matrix-renormalization-group simulations for $J_{1}-J_{2}$ spin model on the triangular lattice. We find 64 types of symmetry enriched $Z_{2}$ spin liquid states on triangular lattice. Besides 8 states constructed in Schwinger-boson parton wavefunctions, 12 more states can be realized in Abrikosov-fermion parton construction. Within a larger gauge group than SU(2), the rest 44 states are also found in a spin-3/2 system. Among 20 types of Abrikosov-fermion Gutzwillerprojected wavefunctions, No.B5 state is a promising candidate for the $Z_{2}$ spin liquid for $J_{1}-J_{2}$ spin model on the triangular lattice. No.B5 lies close to Dirac spin liquid (DSL). However, variational Monte Carlo simulation find that DSL has a good variational energy and $J_{1}-J_{2}$ spin model cannot open a gap for spinons on top of DSL to stabilize No.B5 state.

cond-mat.str-el↗