arXiv subjects

Congjun Wu

Publications and source records attributed to Congjun Wu.

At least 55 records · Page 3Linked to original sources

arXiv2024

Quantum Phonon Dynamics Induced Spontaneous Spin-Orbit Coupling

Spin-orbit coupling in solids is typically a single-body effect arising from relativity. In this work, we propose a spontaneous generation of spin-orbit coupling from symmetry breaking. A spin-dependent electron-phonon coupling model is investigated on a half-filled square lattice, which is solved by sign-problem-free quantum Monte Carlo simulations. The phase diagram as function of phonon frequency $ω$ and coupling constant $λ$ is fully investigated. The spin-orbit coupling emerges as an order in the ground state for any $λ$ in the adiabatic limit, accompanied by a breathing mode of lattice distortion and a staggered loop spin-current. This phase dominates in the entire range of $ω$ with $λ< λ_{\infty}$, a critical value in the $ω\to \infty$ limit. With increasing $ω$ and $λ> λ_{\infty}$, the emergent spin-orbit coupling is suppressed and a phase transition occurs leading to charge-density-wave degenerate with superconductivity order. Our work opens up the possibility of hidden spin-orbit coupling in materials where it is otherwise forbidden by lattice symmetry and paves the way to explore new usable materials or devices in spintronics.

cond-mat.str-el↗

arXiv2024

Electric Space-time Translation and Floquet-Bloch Wavefunction

As for the study of Landau level wavefunctions for the quantum Hall effect, the magnetic Bloch wavefunctions based on the magnetic translation symmetry have been extensively investigated in the past few decades. In this article, the electric Floquet-Bloch wavefunctions based on the electric translation symmetry are studied as well as the momentum-frequency Brillouin zone, which is applied to the problem of one dimensional tight-binding model under an external electric field. The spectrum of electric Floquet-Bloch states can be generated by the projective representation of electric translation group, and the topological properties of these states are investigated.

cond-mat.mes-hall↗

arXiv2024

Parameters dependent superconducting transition temperature in high temperature superconductors

Understanding the evolution of the superconducting transition temperature in relation to doping and interaction strengths is one of the most challenging problems in high temperature superconductivity. By refining determinant quantum Monte Carlo algorithm, we characterize the parameter dependence of the superconducting transition temperature within a bilayer Hubbard model, which is sign-problem-free at arbitrary filling. A striking feature of this model is its similarities to the bilayer nickelate-based superconductor $\mathrm{La}_{3}\mathrm{Ni}_{2}\mathrm{O}_{7}$, where superconductivity emerge from the bilayer $\mathrm{Ni}\mathrm{O}_{2}$ planes. We find that interlayer spin-exchange $J$ is critical to interlayer pairing, ant that on-site interaction $U$ contributes negatively to superconductivity at low doping levels but positively to it at high doping levels. Our findings identify the key parameter dependent superconducting transition temperature in nickelate-based superconductors and provide a new understanding of the high temperature superconductivity.

cond-mat.supr-con↗

arXiv2024

Superconducting diode effect and interference patterns in Kagome CsV3Sb5

The interplay among frustrated lattice geometry, nontrivial band topology and correlation yields rich quantum states of matter in Kagome systems. A series of recent members in this family, AV3Sb5 (A= K, Rb, Cs), exhibit a cascade of symmetry-breaking transitions, involving the 3Q chiral charge ordering, electronic nematicity, roton pair-density-wave and superconductivity. The nature of the superconducting order is yet to be resolved. Here, we report an indication of chiral superconducting domains with boundary supercurrents in intrinsic CsV3Sb5 flakes. Magnetic field-free superconducting diode effect is observed with polarity modulated by thermal histories, suggesting dynamical superconducting order domains in a spontaneous time-reversal symmetry breaking background. Strikingly, the critical current exhibits the double-slit superconducting interference patterns when subjected to an external magnetic field. Characteristics of the patterns are modulated by thermal cycling. These phenomena are proposed as a consequence of periodically modulated supercurrents flowing along certain domain boundaries constrained by fluxoid quantization. Our results imply a chiral superconducting order, opening a potential for exploring exotic physics, e.g. Majorana zero modes, in this intriguing topological Kagome system.

cond-mat.supr-con↗

arXiv2024

Frustrated superconductivity and sextetting order

The superconducting state typically favors a uniform spatial distribution akin to ferromagnetism. Nevertheless, the pair-density-wave state exhibits sign changes in the pairing order, leading to potential frustrations in phase coherence.We propose a mechanism to the sextetting order stemming from the frustrations in the phase coherence of a pair-density-wave state, whose spatial modulation manifests a vortex-antivortex honeycomb lattice. The classical ground state configurations are mapped to Baxter's three-coloring model, revealing a macroscopic degeneracy accompanied by extensive entropy. The phase coherence problem intertwines the U(1) phases and the vorticity variables. While the resultant color and phase fluctuations suppress the pair-density-wave order, they maintain the sextetting order above the superconducting transition temperature ($T_{\text{c}}$). The $1/3$-fractional vortex emerges as the fundamental topological defect in the sextetting order. This novel mechanism of frustrated superconductivity provides an alternative explanation for the experimental observed fractional oscillations in CsV$_3$Sb$_5$.

cond-mat.supr-con↗

arXiv2024

Mott insulating states of the anisotropic SU(4) Dirac fermions

We employ the large-scale quantum Monte-Carlo simulations to investigate the Mott-insulating states of the half-filled SU(4) Hubbard model on the square lattice with a staggered-flux pattern. The noninteracting band structure that evolves from a nested Fermi surface at zero flux to isotropic Dirac cones at $π$ flux, exhibits anisotropic Dirac cones as the flux varies in between. Our simulations show transitions between the three phases of Dirac semimetal, antiferromagnet and valence-bond solid. A direct continuous transition between the antiferromagnetic phase and the valence-bond-solid phase is realized via varying the flux in the Mott regime. The simulated critical exponents remarkably agree with those of SU(4) $J$-$Q$ model. Inside the valence-bond-solid phase induced by the flux, the plaquette valence-bond state with vanishing single-particle gap is identified. At strong coupling, the valence-bond-solid phase disappears and the Mott-insulating state is always accompanied by antiferromagnetic ordering, regardless of the magnitude of the flux.

cond-mat.quant-gas↗

arXiv2024

Interlayer Coupling Driven High-Temperature Superconductivity in La$_3$Ni$_2$O$_7$ Under Pressure

The newly discovered high-temperature superconductivity in La$_3$Ni$_2$O$_7$ under pressure has attracted a great deal of attentions. The essential ingredient characterizing the electronic properties is the bilayer NiO$_2$ planes coupled by the interlayer bonding of $3d_{z^2}$ orbitals through the intermediate oxygen-atoms. In the strong coupling limit, the low energy physics is described by an intralayer antiferromagnetic spin-exchange interaction $J_{\parallel}$ between $3d_{x^2-y^2}$ orbitals and an interlayer one $J_{\perp}$ between $3d_{z^2}$ orbitals. Taking into account Hund's rule on each site and integrating out the $3d_{z^2}$ spin degree of freedom, the system reduces to a single-orbital bilayer $t$-$J$ model based on the $3d_{x^2-y^2}$ orbital. By employing the slave-boson approach, the self-consistent equations for the bonding and pairing order parameters are solved. Near the physically relevant $\frac{1}{4}$-filling regime (doping $δ=0.3\sim 0.5$), the interlayer coupling $J_{\perp}$ tunes the conventional single-layer $d$-wave superconducting state to the $s$-wave one. A strong $J_{\perp}$ could enhance the inter-layer superconducting order, leading to a dramatically increased $T_c$. Interestingly, there could exist a finite regime in which an $s+id$ state emerges.

cond-mat.supr-con↗

arXiv2024

Bilayer $t$-$J$-$J_\perp$ Model and Magnetically Mediated Pairing in the Pressurized Nickelate La$_3$Ni$_2$O$_7$

The recently discovered nickelate superconductor La$_3$Ni$_2$O$_7$ has a high transition temperature near 80 K under pressure, which offers additional avenues of unconventional superconductivity. Here with state-of-the-art tensor-network methods, we study a bilayer $t$-$J$-$J_\perp$ model for La$_3$Ni$_2$O$_7$ and find a robust $s$-wave superconductive (SC) order mediated by interlayer magnetic couplings. Large-scale density matrix renormalization group calculations find algebraic pairing correlations with Luttinger parameter of $K_{\rm SC} \simeq 1$. Infinite projected entangled-pair state method obtains a nonzero SC order directly in the thermodynamic limit, and estimates a strong pairing strength $\barΔ_z \sim \mathcal{O}(0.1)$. Tangent-space tensor renormalization group simulations further determine a high SC temperature $T_c^*/J \sim \mathcal{O}(0.1)$ and clarify the temperature evolution of SC order. Because of the intriguing orbital selective behaviors and strong Hund's rule coupling in the compound, $t$-$J$-$J_\perp$ model has strong interlayer spin exchange (while negligible interlayer hopping), which greatly enhances the SC pairing in the bilayer system. Such a magnetically mediated strong pairing has also been observed recently in the optical lattice of ultracold atoms. Our accurate and comprehensive tensor-network calculations reveal robust SC order in the bilayer $t$-$J$-$J_\perp$ model and shed light on the high-$T_c$ superconductivity in the pressurized nickelate La$_3$Ni$_2$O$_7$.

cond-mat.str-el↗

arXiv2023

Interplay of two $E_g$ orbitals in Superconducting La$_3$Ni$_2$O$_7$ Under Pressure

The discovery of high-$T_c$ superconductivity (SC) in La$_3$Ni$_2$O$_7$ (LNO) has aroused a great deal of interests. Previously, it was proposed that the Ni-$3d_{z^2}$ orbital is crucial to realize the high-$T_c$ SC in LNO: The preformed Cooper pairs therein acquire coherence via hybridization with the $3d_{x^2-y^2}$ orbital to form the SC. However, we held a different viewpoint that the interlayer pairing $s$-wave SC is induced by the $3d_{x^2-y^2}$ orbital, driven by the strong interlayer superexchange interaction. To include effects from both $E_g$-orbitals , we establish a two-orbital bilayer $t$-$J$ model. Our calculations reveal that due to the no-double-occupancy constraint, the $3d_{x^2-y^2}$ band and the $3d_{z^2}$ bonding band are flattened by a factor of about 2 and 10, respectively, which is consistent with recent angle-resolved-photo-emission-spectroscopy measurements. Consequently, a high temperature SC can be hardly induced in the $3d_{z^2}$-orbital due to the difficulty to develop phase coherence. However, it can be easily achieved by the $3d_{x^2-y^2}$ orbital under realistic interaction strength. With electron doping, the $3d_{z^2}$-band gradually dives below the Fermi level, but $T_c$ continues to enhance, suggesting that it is not necessary for the high-$T_c$ SC in LNO. With hole doping, $T_c$ initially drops and then rises, accompanied by the crossover from the BCS to BEC-type superconducting transitions.

cond-mat.supr-con↗

arXiv2023

Multiflavor Mott insulators in quantum materials and ultracold atoms

Mott insulators with large and active (or multiflavor) local Hilbert spaces widely occur in quantum materials and ultracold atomic systems, and are dubbed "multiflavor Mott insulators". For these multiflavored Mott insulating materials, the spin-only description with the quadratic spin interactions is often insufficient to capture the major physical processes. In the situation with active orbitals, the Kugel-Khomskii superexchange model was then proposed. We briefly review this historical model and discuss the modern developments beyond the original spin-orbital context. These include and are not restricted to the $4d$/$5d$ transition metal compounds with the spin-orbit-entangled $J=3/2$ quadruplets, the rare-earth magnets with two weakly-separated crystal field doublets, breathing magnets and/or the cluster and molecular magnets, et al. We explain the microscopic origin of the emergent Kugel-Khomskii physics in each realization with some emphasis on the $J=3/2$ quadruplets, and refer the candidate multiflavor Mott insulators as "$J=3/2$ Mott insulators". For the ultracold atoms, we review the multiflavor Mott insulator realization with the ultracold alkaline and alkaline-earth atoms on the optical lattices. Despite a large local Hilbert space from the atomic hyperfine spin states, the system could naturally realize a large symmetry group such as the Sp($N$) and SU($N$) symmetries. These ultracold atomic systems lie in the large-$N$ regime of these symmetry groups and are characterized by strong quantum fluctuations. The Kugel-Khomskii physics and the exotic quantum ground states with the "baryon-like" physics can appear in various limits. We conclude with our vision and outlook on this subject.

cond-mat.str-el↗

arXiv2023

High-order Time-Reversal Symmetry Breaking Normal State

Spontaneous time-reversal symmetry breaking plays an important role in studying strongly correlated unconventional superconductors. When two superconducting gap functions with different symmetries compete, the relative phase channel ($θ_-\equiv θ_1-θ_2$) exhibits an Ising-type $Z_2$ symmetry due to the second order Josephson coupling, where $θ_{1,2}$ are the phases of two gap functions respectively. In contrast, the $U(1)$ symmetry in the channel of $θ_+\equiv \frac{θ_1+θ_2}{2}$ is intact. The phase locking, i.e., ordering of $θ_-$, can take place in the phase fluctuation regime before the onset of superconductivity, i.e. when $θ_+$ is disordered. If $θ_-$ is pinned at $\pm\fracπ{2}$, then time-reversal symmetry is broken in the normal state, otherwise, if $θ_-=0$, or, $π$, rotational symmetry is broken, leading to a nematic normal state. In both cases, the order parameters possess a 4-fermion structure beyond the scope of mean-field theory, which can be viewed as a high order symmetry breaking. We employ an effective two-component $XY$-model assisted by a renormalization group analysis to address this problem. As a natural by-product, we also find the other interesting intermediate phase corresponds to ordering of $θ_+$ but with $θ_-$ disordered. This is the quartetting, or, charge-4e, superconductivity, which occurs above the low temperature $Z_2$-breaking charge-2e superconducting phase. Our results provide useful guidance for studying novel symmetry breaking phases in strongly correlated superconductors.

cond-mat.supr-con↗

arXiv2023

Evidence for Quantum Stripe Ordering in a Triangular Optical Lattice

Understanding strongly correlated quantum materials, such as high $T_\textrm{c}$ superconductors, iron-based superconductors, and twisted bilayer graphene systems, remains to be one of the outstanding challenges in condensed matter physics. Quantum simulation with ultra-cold atoms in particular optical lattices, which provide orbital degrees of freedom, is a powerful tool to contribute new insights to this endeavor. Here, we report the experimental realization of an unconventional Bose-Einstein condensate of $^{87}$Rb atoms populating degenerate $p$-orbitals in a triangular optical lattice, exhibiting remarkably long coherence times. Using time-of-flight spectroscopy, we observe that this state spontaneously breaks the rotational symmetry and its momentum spectrum agrees with the theoretically predicted coexistence of exotic stripe and loop current orders. Like certain strongly correlated electronic systems with intertwined orders, as high-$T_\textrm{c}$ cuprate superconductors, twisted bilayer graphene, and the recently discovered chiral density-wave state in kagome superconductors $\textrm{AV}_3 \textrm{Sb}_5$ (A=K, Rb, Cs), the newly demonstrated quantum state, in spite of its markedly different energy scale and the bosonic quantum statistics, exhibits multiple symmetry breakings at ultralow temperatures. These findings hold the potential to enhance our comprehension of the fundamental physics governing these intricate quantum materials.

cond-mat.quant-gas↗

arXiv2023

Charge 4e superconductivity and chiral metal in the $45^\circ$-twisted bilayer cuprates and similar bilayers

The material realization of the charge-4e/6e superconductivity (SC) is a big challenge. Here we propose realization of the charge-4e SC and chiral metal through stacking a homo-bilayer with the largest twist angle, forming the twist-bilayer quasi-crystal (TB-QC), exampled by the 45$^\circ$-twisted bilayer cuprates and 30$^\circ$-twisted bilayer graphene. When each mononlayer hosts a pairing state with the largest pairing angular momentum, previous studies yield that the second-order interlayer Josephson coupling would drive chiral topological SC (TSC) in the TB-QC. Here we propose that, above the $T_c$ of the chiral TSC, either the total- or relative- pairing phase of the two layers can be unilateral quasi-ordered or ordered, leading to the charge-4e SC or the chiral metal phase. Based on a thorough symmetry analysis to get the low-energy effective Hamiltonian, we conduct a combined renormalization-group and Monte-Carlo study and obtain the phase diagram, which includes the charge-4e SC and chiral metal phases.

cond-mat.supr-con↗

arXiv2023

Effect of Rare-earth Element Substitution in Superconducting R$_3$Ni$_2$O$_7$ Under Pressure

Recently, high temperature ($T_c\approx 80$K) superconductivity (SC) has been discovered in La$_3$Ni$_2$O$_7$ (LNO) under pressure. Question arises whether the transition temperature $T_c$ could be further enhanced under suitable conditions. A possible route for realizing higher $T_c$ is element substitution. Similar SC could appear in rare-earth (RE) R$_3$Ni$_2$O$_7$ (RNO, R=RE element) material series under pressure. The electronic properties in the RNO materials are dominated by the Ni $3d$ orbitals in the bilayer NiO$_2$ plane. In the strong coupling limit, the SC could be fully characterized by a bilayer single $3d_{x^2-y^2}$-orbital $t$-$J_{\parallel}$-$J_{\perp}$ model. Under RE element substitution from La to RE element, the lattice constant decreases and the electronic hopping increases, leading to stronger superexchanges between the $3d_{x^2-y^2}$ orbitals. Based on the slave-boson mean-field theory, we explore the pairing nature and the evolution of $T_c$ in RNO materials. Consequently, it is found that the element substitution does not alter the pairing nature, i.e. the inter-layer $s$-wave pairing is always favored in RNO. However, the $T_c$ increases from La to Sm and a nearly doubled $T_c$ is achieved for SmNO. This work provides evidence for possible higher $T_c$ R$_3$Ni$_2$O$_7$ materials, which may be realized in further experiments.

cond-mat.supr-con↗

arXiv2023

Trion states and quantum criticality of attractive SU(3) Dirac fermions

We perform the projector quantum Monte Carlo (QMC) simulation to study the trion formation and quantum phase transition in the half-filled attractive SU(3) Hubbard model on a honeycomb lattice. With increasing attractive Hubbard interaction, our simulations demonstrate a continuous quantum phase transition from the semimetal to charge density wave (CDW) at the critical coupling $U_c/t=-1.52(2)$. The critical exponents $ν=0.82(3)$ and $η=0.58(4)$ determined by the QMC simulation remarkably disagree with those of the $N=3$ chiral Ising universality class suggested by the effective Gross-Neveu-Yukawa (GNY) theory, but coincide with the $N=1$ chiral Ising universality class. In the CDW phase, we show that on-site and off-site trions coexist and the off-site trion forms a local bond state. Our work not only illustrates the formation of off-site trions in two-dimensional Hubbard model, but also raises doubts about the extent of applicability of GNY model on the attractive SU(3) Dirac fermions.

cond-mat.quant-gas↗

arXiv2023

Nonzero angular momentum density wave phases in SU($N$) fermions with singlet-bond and triplet-current interactions

We employ the sign-problem-free projector determinant quantum Monte Carlo method to study a microscopic model of SU($N$) fermions with singlet-bond and triplet-current interactions on the square lattice. We find the gapped singlet $p_x$ and gapless triplet $d_{x^2-y^2}$ density wave states in the half-filled $N=4$ model. Specifically, the triplet $d_{x^2-y^2}$ density wave order is observed in the weak triplet-current interaction regime. As the triplet-current interaction strength is further increased, our simulations demonstrate a transition to the singlet $p_x$ density wave state, accompanied by a gapped mixed-ordered area where the two orders coexist. With increasing singlet-bond interaction strength, the triplet $d_{x^2-y^2}$-wave order persists up to a critical point after which the singlet $p_x$ density wave state is stabilized, while the ground state is disordered in between the two ordered phases. The analytical continuation is then performed to derive the single-particle spectrum. In the spectra of triplet $d_{x^2-y^2}$ and singlet $p_x$ density waves, the anisotropic Dirac cone and the parabolic shape around the Dirac point are observed, respectively. As for the mixed-ordered area, a single-particle gap opens and the velocities remain anisotropic at the Dirac point.

cond-mat.str-el↗

arXiv2023

An electromagnetic way to derive basic relativistic transformations

We derive the relativistic velocity addition law, the transformations of electromagnetic fields and space-time intervals by examining the drift velocities in a crossed electromagnetic field configuration. The postulate of the light velocity invariance is not taken as a priori, but is derived as the universal upper limit of physical drift velocities. The key is that a physical drift of either an electric charge or a magnetic charge remains a drift motion by inertial reference frame transformations. Such a simple fact is incompatible with the Galilean velocity addition. This derivation provides a way to introduce relativity via elementary electromagnetism.

physics.gen-ph↗

arXiv2022

Orbital-Active Dirac Materials from the Symmetry Principle

Dirac materials, starting with graphene, have drawn tremendous research interest in the past decade. Instead of focusing on the $p_z$ orbital as in graphene, we move a step further and study orbital-active Dirac materials, where the orbital degrees of freedom transform as a two-dimensional irreducible representation of the lattice point group. Examples of orbital-active Dirac materials occur in a broad class of systems, including transition-metal-oxide heterostructures, transition-metal dichalcogenide monolayers, germanene, stanene, and optical lattices. Different systems are unified based on symmetry principles. The band structure of orbital-active Dirac materials features Dirac cones at $K(K')$ and quadratic band touching points at $Γ$, regardless of the origin of the orbital degrees of freedom. In the strong anisotropy limit, i.e., when the $π$-bonding can be neglected, flat bands appear due to the destructive interference. These features make orbital-active Dirac materials an even wider playground for searching for exotic states of matter, such as the Dirac semi-metal, ferromagnetism, Wigner crystallization, quantum spin Hall state, and quantum anomalous Hall state.

cond-mat.str-el↗