arXiv subjects

Congjun Wu

Publications and source records attributed to Congjun Wu.

At least 73 records · Page 4Linked to original sources

arXiv2022

Symmetry Constraints on Direct-Current Josephson Diodes

It is necessary to break both time-reversal and parity symmetries to realize a Josephson, or superconducting, diode exhibiting nonreciprocal critical direct-currents (DC). In fact, these conditions are still insufficient. The dependencies of the free energy on the phase difference across the junction and the magnetic field are classified, exhibiting the current-reversion (JR), field-reversion, and field-current reversion conditions, respectively. To exhibit the DC Josephson diode effect, all symmetries satisfying the JR condition need to be broken. The relations of critical currents with respect to the magnetic field are classified into five classes, including three exhibiting the diode effect. These symmetry considerations are applied to concrete examples. Our work reveals that the DC Josephson diode effect is a natural consequence of the JR symmetry breaking, hence, providing a guiding principle to understand or design a DC Josephson diode.

cond-mat.supr-con↗

arXiv2022

The Symmetry Principle in Condensed Matter Physics (I)

Symmetry distills the simplicity of natural laws from the complexity of physical phenomena. The symmetry principle is of vital importance in various aspects of modern physics, including analyzing atomic spectra, determining fundamental interactions in the Standard Model, and unifying physics at different energy scales. In this chapter, novel applications of this principle are reviewed in condensed matter physics and cold atom physics for exploring new states of matter.

cond-mat.str-el↗

arXiv2021

Two-orbital model for possible superconductivity pairing mechanism in nickelates

The newly synthesized strontium doped RNiO$_2$ (R=Nd, La) superconductors have stimulated extensive interests in understanding their pairing mechanism and pairing nature. Here we study the pairing mechanism in this family from a two-orbital model comprising the Ni- $3d_{x^2-y^2}$- and $3d_{xy}$- orbitals, equipped with extended Hubbard interactions and induced low-energy effective superexchange interactions. We then study the pairing symmetry in this system by using large scale variational Monte Carlo approach. Our results yield the intraorbital $d_{x^2-y^2}$-wave singlet pairing as the leading pairing symmetry in the nickelates, which is analogous to the cuprates. However, there exist two important differences between the physical properties of the two families due to the fact that at the low Sr-doping regime, while the Ni-$3d_{x^2-y^2}$ orbitals remain half-filled and singly-occupied to form a Mott-insulating background, the Ni-$3d_{xy}$ orbitals accommodate nearly all the extra doped holes, which move freely on this background. The first difference lies in the single-particle aspect: while the $3d_{x^2-y^2}$ degree of freedom remains Mott insulating with spectra weight pinned down at zero at low dopings, the $3d_{xy}$ one behaves as Fermi liquid with spectra weight near 1. The second difference lies in the pairing aspect: while the huge intra-$3d_{x^2-y^2}$-orbital pairing gap is actually a pseudo gap which has nothing to do with the SC, the small intra-$3d_{xy}$-orbital pairing gap serves as the true superconducting pairing gap, which is related to the $T_c$ via the BCS relation. Both differences can be verified by the angle-resolved photo-emission spectrum.

cond-mat.str-el↗

arXiv2021

Emergent Weyl Fermions in an Orbital Multipolar Ordering Phase

Multipolar orderings in degenerate orbital systems offer unique opportunities for emergent topological phases. The phase diagram of interacting spinless fermions in a $p$-band diamond lattice at unit filling is first studied to elucidate the essential role of orbital multipolar orderings in the evolution of multifold degenerate band nodes. The free band structure around the Brillouin zone center is described by two quadratic band nodes each with a threefold degeneracy, which are spanned by the bonding and anti-bonding $p$-orbital multiplets, respectively. Upon switching on interactions, the triply degenerate band node is split into a pair of Weyl fermions with opposite chirality due to the onset of orbital multipolar orderings. Further raising interactions ultimately drives the system into an insulating phase with the orbital quadrupolar ordering. Our study is then generalized to spin-$1/2$ fermions, which has direct relevance with solid-state materials. The system develops full spin polarization through a ferromagnetic transition at tiny interactions, leaving the remaining orbital sector activated. The ensuing transitions take place in the orbital sector as a natural consequence, qualitatively recovering the phase diagram of spinless fermions. Our findings shed new light on the realization of emergent novel fermions with a prospect being a frontier at the confluence of topology, orbital physics and strong correlation.

cond-mat.str-el↗

arXiv2021

Planar magnetic texture on the surface of a topological insulator

We study the planar magnetic textures in an insulating magnetic film coupled to the Dirac surface state of a topological insulator. It is shown that the radial vortex with winding number $w=\pm1$ leads to the confinement of Dirac states, where an exact mapping to the Schrödinger equation of a two-dimensional hydrogen atom is found. The fully spin polarized zero energy bound state resembles the zeroth Landau level of Dirac electrons in a uniform out-of-plane magnetic field. Interestingly, when the hybrid system is proximity coupled to an $s$-wave superconductor, the existence of Majorana zero modes at Abrikosov vortex depends only on the relative value of the magnetic exchange coupling and the pairing strength. We conclude with a brief discussion on the physical realization with such magnetic textures.

cond-mat.mes-hall↗

arXiv2021

Observation of Superconductivity Induced Ferromagnetism in an Fe-Chalcogenide Superconductor

The interplay among topology, superconductivity, and magnetism promises to bring a plethora of exotic and unintuitive behaviors in emergent quantum materials. The family of Fe-chalcogenide superconductors FeTexSe1-x are directly relevant in this context due to their intrinsic topological band structure, high-temperature superconductivity, and unconventional pairing symmetry. Despite enormous promise and expectation, the local magnetic properties of FeTexSe1-x remain largely unexplored, which prevents a comprehensive understanding of their underlying material properties. Exploiting nitrogen vacancy (NV) centers in diamond, here we report nanoscale quantum sensing and imaging of magnetic flux generated by exfoliated FeTexSe1-x flakes, providing clear evidence of superconductivity-induced ferromagnetism in FeTexSe1-x. The coexistence of superconductivity and ferromagnetism in an established topological superconductor opens up new opportunities for exploring exotic spin and charge transport phenomena in quantum materials. The demonstrated coupling between NV centers and FeTexSe1-x may also find applications in developing hybrid architectures for next-generation, solid-state-based quantum information technologies.

cond-mat.supr-con↗

arXiv2021

Unification of valley and anomalous Hall effects in a strained lattice

Two dimensional lattices are an important stage for studying many aspects of quantum physics, in particular the topological phases. The valley Hall and anomalous Hall effects are two representative topological phenomena. Here we show that they can be unified in a strained honeycomb lattice, where the hopping strengths between neighboring sites are designed by mimicking those between the Fock states in a three-mode Jaynes-Cummings model. Such a strain induces an effective magnetic field which results in quantized Landau levels. The eigenstates in the zeroth Landau level can be represented by the eigenstates of a large pseudo-spin. We find that the valley Hall current and the chiral edge current in the Haldane model correspond to the spin precession around different axes. Our study sheds light on connection between seemingly unrelated topological phases in condensed matter physics.

cond-mat.mes-hall↗

arXiv2021

Superconducting junction with tri-component pairing gap functions

We study a superconducting hetro-junction with one side characterized by the unconventional chiral $p$-wave gap function $p_x\pm ip_y$ and the other side the conventional $s$-wave one. Though a relative phase of $\pm \fracπ{2}$ between any two components of gap functions is favored in the junction region, mutual phase differences cannot achieve $\pm \fracπ{2}$ simultaneously, which results in frustration. Based on a Ginzburg-Landau free energy analysis, the frustrated pattern is determined to be $s+ iη_1 (e^{ iη_2 φ/2}p_x +η_3 e^{- iη_2 φ/2}p_y)$ with $η_j=\pm 1$ ($j=1,2,3$), where $φ$ is the phase difference between the $p_x$- and $p_y$-wave gap functions. Furthermore, we find that the junction exhibits an anisotropic magnetoelectric effect, manifesting itself as an anisotropic spin magnetization along the edge of the junction.

cond-mat.supr-con↗

arXiv2021

Pairing symmetry and topological surface state in iron-chalcogenide superconductors

The symmetries of superconducting gap functions remain an important question of iron-based superconductivity. Motivated by the recent angle-resolved photoemission spectroscopic measurements on iron-chalcogenide superconductors, we investigate the influence of pairing symmetries on the topological surface state. If the surface Dirac cone becomes gapped in the superconducting phase, it implies magnetization induced from time-reversal symmetry breaking pairing via spin-orbit coupling. Based on the crystalline symmetry constraints on the Ginzburg-Landau free energy, the gap function symmetries are among the possibilities of $A_{1g(u)}\pm iA_{2g(u)}$, $B_{1g(u)}\pm iB_{2g(u)}$, or, $E_{g(u)}\pm i E_{g(u)}$. This time-reversal symmetry breaking effect can exist in the normal state very close to $T_c$ with the relative phase between two gap functions locked at $\pm \fracπ{2}$. The coupling between magnetization and superconducting gap functions is calculated based on a three-orbital model for the band structure of iron-chalcogenides. This study provides the connection between the gap function symmetries and topological properties of the surface state.

cond-mat.supr-con↗

arXiv2021

Magnetotransport in overdoped La$_{2-x}$Sr$_x$CuO$_4$: a Fermi liquid approach

Recently, several experiments on La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) challenged the Fermi liquid picture for overdoped cuprates, and stimulated intensive debates [1]. In this work, we study the magnetotransport phenomena in such systems based on the Fermi liquid assumption. The Hall coefficient $R_H$ and magnetoresistivity $ρ_{xx}$ are investigated near the van Hove singularity $x_{\tiny\text{VHS}}\approx0.2$ across which the Fermi surface topology changes from hole- to electron-like. Our main findings are: (1) $R_H$ depends on the magnetic field $B$ and drops from positive to negative values with increasing $B$ in the doping regime $x_{\tiny\text{VHS}}<x\lesssim0.3$; (2) $ρ_{xx}$ grows up as $B^2$ at small $B$ and saturates at large $B$, while in the transition regime a "nearly linear" behavior shows up. Our results can be further tested by future magnetotransport experiments in the overdoped LSCO.

cond-mat.supr-con↗

arXiv2021

Doping-driven Antiferromagnetic Insulator -- Superconductor Transition: a Quantum Monte Carlo Study

How superconductivity emerges in the vicinity of an antiferromagnetic insulating state is a long-standing issue of strong correlation physics. We study the transition from an antiferromagnetic insulator to a superconductor by hole-doping based on a bilayer generalization of a Hubbard-like model. The projector quantum Monte-Carlo simulations are employed, which are sign-problem-free both at and away from half-filling. An anisotropic Ising antiferromagnetic Mott insulating phase occurs at half-filling, which is weakened by hole-doping. Below a critical doping value, antiferromagnetism coexists with the singlet superconductivity, which is a pairing across each rung with an extended $s$-wave symmetry. As further increasing doping, the antiferromagnetic order vanishes, leaving only a superconducting phase. These results provide important information on how superconductivity appears upon doping the parent Mott-insulating state.

cond-mat.str-el↗

arXiv2020

Single branch of chiral Majorana modes from doubly degenerate Fermi surfaces

Majorana fermions are often proposed to be realized by first singling out one Fermi surface without spin degeneracy via spin-orbit coupling, and then imposing boundaries or defects. In this work, we take a different route starting with two degenerate Fermi surfaces without spin-orbit coupling, and show that by the method of "kink on boundary", the dispersive chiral Majorana fermions can be realized in superconducting systems with $p\pm is$ pairings. The surfaces of these systems develop spontaneous magnetizations whose directions are determined by the boundary orientations and the phase difference between the $p$ and $s$-component gap functions. Along the magnetic domain walls on the surface, there exist chiral Majorana fermions propagating unidirectionally, which can be conveniently dragged and controlled by external magnetic fields. Furthermore, the surface magnetization is shown to be a magnetoelectric effect based on a Ginzburg-Landau free energy analysis. We also discuss how to use the proximity effects to realize chiral Majorana fermions by performing the "kink on boundary" method.

cond-mat.supr-con↗

arXiv2020

Interacting topological mirror excitonic insulator in one dimension

We introduce the topological mirror excitonic insulator as a new type of interacting topological crystalline phase in one dimension. Its mirror-symmetry-protected topological properties are driven by exciton physics, and it manifests in the quantized bulk polarization and half-charge modes on the boundary. And the bosonization analysis is performed to demonstrate its robustness against strong correlation effects in one dimension. Besides, we also show that Rashba nanowires and Dirac semimetal nanowires could provide ideal experimental platforms to realize this new topological mirror excitonic insulating state. Its experimental consequences, such as quantized tunneling conductance in the tunneling measurement, are also discussed.

cond-mat.str-el↗

arXiv2020

Symplectic ferromagnetism and phase transitions in multi-component fermionic systems

In this paper, we study the itinerant ferromagnetic phase in multi-component fermionic systems with symplectic (Sp(4), or isomorphically SO(5)) symmetry. Two different microscopic models have been considered and an effective field theory has been proposed to study the critical behavior of the nonmagnetism-magnetism phase transition. It has been shown that such systems exhibit intriguing ferromagnetism and critical behavior that different from those in spin-$\frac 12$ fermionic systems, or in high-spin systems with SU(N) symmetry. An extension of our results to higher spin systems with Sp(2N) symmetry has also been discussed.

cond-mat.quant-gas↗

arXiv2019

Two-band model for magnetism and superconductivity in nickelates

The recently discovered superconductivity in Nd$_{1-x}$Sr$_x$NiO$_2$ provides a new opportunity for studying strongly correlated unconventional superconductivity. The single-hole Ni$^+$ ($3d^9$) configuration in the parent compound NdNiO$_2$ is similar to that of Cu$^{2+}$ in cuprates. We suggest that after doping, the intra-orbital spin-singlet and inter-orbital spin-triplet double-hole (doublon) configurations of Ni$^{2+}$ are competing, and we construct a two-band Hubbard model by including both the $3d_{x^2-y^2}$ and $3d_{xy}$-orbitals. The effective spin-orbital super-exchange model in the undoped case is a variant of the $SU(4)$ Kugel-Khomskii model augmented by symmetry breaking terms. Upon doping, the effective exchange interactions between spin-$\frac{1}{2}$ single-holes, spin-1 (triplet) doublons, and singlet doublons are derived. Possible superconducting pairing symmetries are classified in accordance to the $D_{4h}$ crystalline symmetry, and their connections to the superexchange interactions are analyzed.

cond-mat.supr-con↗

arXiv2019

Quaternion, harmonic oscillator, and high-dimensional topological states

Quaternion, an extension of complex number, is the first discovered non-commutative division algebra by William Rowan Hamilton in 1843. In this article, we review the recent progress on building up the connection between the mathematical concept of quaternoinic analyticity and the physics of high-dimensional topological states. Three- and four-dimensional harmonic oscillator wavefunctions are reorganized by the SU(2) Aharanov-Casher gauge potential to yield high-dimensional Landau levels possessing the full rotational symmetries and flat energy dispersions. The lowest Landau level wavefunctions exhibit quaternionic analyticity, satisfying the {\it Cauchy-Riemann-Fueter} condition, which generalizes the two-dimensional complex analyticity to three and four dimensions. It is also the Euclidean version of the helical Dirac and the chiral Weyl equations. After dimensional reductions, these states become two- and three-dimensional topological states maintaining time-reversal symmetry but exhibiting broken parity. We speculate that quaternionic analyticity can provide a guiding principle for future researches on high-dimensional interacting topological states. Other progresses including high-dimensional Landau levels of Dirac fermions, their connections to high energy physics, and high-dimensional Landau levels in the Landau-type gauges, are also reviewed. This research is also an important application of the mathematical subject of quaternion analysis in theoretical physics, and provides useful guidance for the experimental explorations on novel topological states of matter.

cond-mat.str-el↗

arXiv2019

Interference dislocations in condensate of indirect excitons

Phase singularities in quantum states play a significant role both in the state properties and in the transition between the states. For instance, a transition to two-dimensional superfluid state is governed by pairing of vortices and, in turn, unpaired vortices can cause dissipations for particle fluxes. Vortices and other phase defects can be revealed by characteristic features in interference patterns produced by the quantum system. We present dislocation-like phase singularities in interference patterns in a condensate of indirect excitons measured by shift-interferometry. We show that the observed dislocations in interference patterns are not associated with conventional phase defects: neither with vortices, nor with polarization vortices, nor with half-vortices, nor with skyrmions, nor with half-skyrmions. We present the origin of these new phase singularities in condensate interference patterns: the observed interference dislocations originate from converging of the condensate matter waves propagating from different sources.

cond-mat.mes-hall↗

arXiv2019

One-dimensional Quantum Spin Dynamics of Bethe String States

Quantum dynamics of strongly correlated systems is a challenging problem. Although the low energy fractional excitations of one dimensional integrable models are often well-understood, exploring quantum dynamics in these systems remains challenging in the gapless regime, especially at intermediate and high energies. Based on the algebraic Bethe ansatz formalism, we study spin dynamics in a representative one dimensional strongly correlated model, {\it i.e. }, the antiferromagnetic spin-$\frac{1}{2}$ XXZ chain with the Ising anisotropy, via the form-factor formulae. Various excitations at different energy scales are identified crucial to the dynamic spin structure factors under the guidance of sum rules. At small magnetic polarizations, gapless excitations dominate the low energy spin dynamics arising from the magnetic-field-induced incommensurability. In contrast, spin dynamics at intermediate and high energies is characterized by the two- and three-string states, which are multi-particle excitations based on the commensurate Néel ordered background. Our work is helpful for experimental studies on spin dynamics in both condensed matter and cold atom systems beyond the low energy effective Luttinger liquid theory. Based on an intuitive physical picture, we speculate that the dynamic feature at high energies due to the multi-particle anti-bound state excitations can be generalized to non-integrable spin systems.

cond-mat.str-el↗