arXiv subjects

Chao-Yang Tan

Publications and source records attributed to Chao-Yang Tan.

10 recordsLinked to original sources

arXiv2026

Interband optical conductivities in two-dimensional tilted Dirac bands revisited within the tight-binding model

Within the framework of linear response theory, we theoretically investigated the interband longitudinal optical conductivities (LOCs) in two-dimensional (2D) tilted Dirac bands using a tight-binding (TB) model, incorporating the effects of band tilting and Dirac-point shifting. We identified three characteristic critical frequencies in the interband LOCs of the TB model: the partner frequencies, the sharp- peak frequency, and the cutoff frequency. In contrast to conventional critical frequencies, these three types are consistently absent in the corresponding linearized $k\cdot p$ model. Notably, the sharp-peak frequency and cutoff frequency remain robust against variations in band tilting and Dirac-point shifting. By employing analytical expressions derived via the Lagrange multiplier method, we elucidate the origins of the conventional critical frequencies and their partner counterparts. In contrast, the sharp-peak frequency and cutoff frequency are associated with interband optical transitions at high-symmetry points of the energy bands, arising from the Pauli exclusion principle and the finite boundaries of the Brillouin zone. Our theoretical predictions are intended to guide future experimental studies on tilt-dependent optical phenomena in 2D tilted Dirac systems.

cond-mat.mes-hall↗

arXiv2025

Stacking-induced type-II quantum spin Hall insulators with high spin Chern number in unconventional magnetism

While stacking two type-I quantum spin Hall insulators typically results in a trivial insulator, the behavior of the assembly of type-II quantum spin Hall insulators remains unexplored. In this article, based on calculations of a lattice model, we demonstrate that stacking two type-II quantum spin Hall insulators does not yield a trivial insulator but instead forms a nontrivial quantum spin Hall insulator with high spin Chern number. In this phase, two pairs of topological edge states with opposite chirality and polarization coexist at the boundary. Our calculations further reveal that the quantized spin Hall conductivity of the bilayer is twice that of the monolayer. When U(1) symmetry is present, the high spin Chern number phase remains stable; when U(1) symmetry is broken, it persists over a broad parameter range. Furthermore, based on first-principles electronic structure calculations, we demonstrate that bilayer Nb$_2$SeTeO is a type-II quantum spin Hall insulator with a high spin Chern number in altermagnetism and unconventional compensated magnetism. Moreover, extending this strategy to multilayer stacks naturally leads to a quantum spin Hall insulator with a higher spin Chern number. Our work not only deepens the distinction between type-I and type-II quantum spin Hall insulators, but also offers a route toward realizing highly quantized spin Hall conductivity.

cond-mat.mes-hall↗

arXiv2025

Type-II quantum spin Hall insulator

Quantum spin Hall effect is usually realized in two-dimensional materials with time-reversal symmetry, but whether it can be realized without symmetry protection remains unexplored. Here, we propose type-II quantum spin Hall insulator with quantized spin Hall conductivity, whose edge states with opposite chirality and polarization, distributed in different Brillouin zone regions, connect the conduction and valence bands at the boundary. Thus, the type-II quantum spin Hall insulator does not require any symmetry protection other than translational symmetry. Then, based on symmetry analysis and the first-principles electronic structure calculations, we demonstrate that type-II quantum spin Hall insulator can be realized in both altermagnetic materials and Luttinger compensated magnetic materials. Furthermore, based on lattice model, we find that as long as $U(1)$ symmetry exists, type-II quantum spin Hall insulator phase can always exist stably. However, if $U(1)$ symmetry is broken, type-II quantum spin Hall insulator phase transforms into an obstructed atomic insulator phase as spin-orbit coupling effect is enhanced. Therefore, our work not only proposes a new mechanism for realizing the quantum spin Hall effect, but also enriches the types of unconventional magnetic topological phases.

cond-mat.mes-hall↗

arXiv2025

Crystal valley Hall effect

The time-reversal symmetry is thought to be a necessary condition for realizing valley Hall effect. If the time-reversal symmetry is broken, whether the valley Hall effect can be realized has not been explored. In this letter, based on symmetry analysis and the first-principles electronic structure calculations, we demonstrate that the vally Hall effect without time-reversal symmetry can be realized in two-dimensional altermagnetic materials Fe$_2$WSe$_4$ and Fe$_2$WS$_4$. Due to crystal symmetry required, the vally Hall effect without time-reversal symmetry is called crystal vally Hall effect. In addition, under uniaxial strain, both monolayer Fe$_2$WSe$_4$ and Fe$_2$WS$_4$ can realize piezomagnetic effect. Under biaxial compressive stress, both monolayer Fe$_2$WSe$_4$ and Fe$_2$WS$_4$ will transform from altermagnetic semiconductor phase to bipolarized topological Weyl semimetal phase. Our work not only provides a new direction for exploring the novel valley Hall effect, but also provides a good platform for exploring altermagnetic semiconductors and altermagnetic topological phase transitions.

cond-mat.mtrl-sci↗

arXiv2024

Bipolarized Weyl semimetals and quantum crystal valley Hall effect in two-dimensional altermagnetic materials

Magnetism and topology are two major areas of condensed matter physics. The combination of magnetism and topology gives rise to more novel physical effects, which have attracted strongly theoretical and experimental attention. Recently, the concept of altermagnetism has been introduced, characterized by a dual nature: real-space antiparallel spins with zero total magnetic moment and reciprocal-space anisotropic spin polarization. The amalgamation of altermagnetism with topology may lead to the emergence of previously unobserved topological phases and the associated physical effects. In this study, utilizing a four-band lattice model that incorporates altermagnetism and spin group symmetry, we demonstrate that type-I, type-II, and type-III bipolarized Weyl semimetals can exist in altermagnetic systems. Through the first-principles electronic structure calculations, we predict four ideal two-dimensional type-I altermagnetic bipolarized Weyl semimetals Fe$_2$WTe$_4$ and Fe$_2$MoZ$_4$ (Z=S, Se, Te). More significantly, we introduce the quantum crystal valley Hall effect, a phenomenon achievable in three of these materials namely Fe$_2$WTe$_4$, Fe$_2$MoS$_4$, and Fe$_2$MoSe$_4$, when spin-orbit coupling is considered. Therefore, our work not only enriches the topological phases but also provides a material platform for studying novel topological phases in altermagnetism.

cond-mat.mtrl-sci↗

arXiv2023

Effects of spatial dimensionality and band tilting on the longitudinal optical conductivities in Dirac bands

We report a unified theory based on linear response, for analyzing the longitudinal optical conductivity (LOC) of materials with tilted Dirac cones. Depending on the tilt parameter $t$, the Dirac electrons have four phases: untilted, type-I, type-II, and type-III; the Dirac dispersion can be isotropic or anisotropic; the spatial dimension of the material can be one-, two-, or three-dimensions (1D, 2D and 3D). The interband LOCs and intraband LOCs in $d$ dimension (with $d\ge2$) are found to scale as $σ_{0}ω^{d-2}$ and $σ_{0}μ^{d-1}δ(ω)$, respectively, where $ω$ is the frequency and $μ$ the chemical potential. The interband LOC vanishes in 1D due to lack of extra spatial dimension. In contrast, the interband LOCs in 2D and 3D are nonvanishing and share many similar properties. A universal and robust fixed point of interband LOCs appears at $ω=2μ$ no matter $d=2$ or $d=3$, which can be intuitively understood by the geometric structures of Fermi surface and energy resonance contour. The intraband LOCs and the carrier density for 2D and 3D tilted Dirac bands are both closely related to the geometric structure of Fermi surface and the cutoff of integration. The angular dependence of LOCs is found to characterize both spatial dimensionality and band tilting and the constant asymptotic background values of LOC reflect features of Dirac bands. The LOCs in the anisotropic tilted Dirac cone can be connected to its isotropic counterpart by a ratio that consists of Fermi velocities for both 2D and 3D. Most of the findings are universal for tilted Dirac materials and hence valid for a great many Dirac materials in the spatial dimensions of physical interest.

cond-mat.mes-hall↗

arXiv2023

Highly anisotropic optical conductivities in two-dimensional tilted semi-Dirac bands

Within linear response theory, the absorptive part of highly anisotropic optical conductivities are analytically calculated for distinct tilts in two-dimensional (2D) tilted semi-Dirac bands (SDBs). The transverse optical conductivities always vanish. The interband longitudinal optical conductivities (LOCs) in 2D tilted SDBs differ qualitatively in the power-law scaling of $ω$ as $\mathrm{Re}σ_{\perp}^{\mathrm{IB}}(ω)\proptoσ_0\sqrtω$ and $\mathrm{Re}σ_{\parallel}^{\mathrm{IB}}(ω)\proptoσ_0/\sqrtω$. By contrast, the intraband LOCs in 2D tilted SDBs depend on $μ$ in the power-law scaling as $\mathrm{Re}σ_{\perp}^{\mathrm{D}}(ω)\proptoσ_0μ\sqrtμ$ and $\mathrm{Re}σ_{\parallel}^{\mathrm{D}}(ω)\proptoσ_0μ/\sqrtμ$. The tilt-dependent behaviors of LOCs could qualitatively characterize distinct impact of band tilting in 2D tilted SDBs. In particular, for arbitrary tilt $t$ satisfying $0<t\le 2$, the interband LOCs always possess a robust fixed point at $ω=2μ$. The power-law scalings and tilt-dependent behaviors further dictate significant differences in the asymptotic background values and angular dependence of LOCs. Our theoretical predictions should be valid for a broad class of 2D tilted SDB materials, and can also be used to fingerprint 2D tilted SDB from 2D untilted SDB as well as tilted Dirac bands.

cond-mat.mes-hall↗

arXiv2022

Anomalous acoustic plasmons in two-dimensional over-tilted Dirac bands

The over-tilting of Dirac cones has led to various fascinating quantum phenomena. Here we find that two anomalous acoustic plasmons (AAPs) are dictated by the distinct geometry of two-dimensional (2D) type-II Dirac cones, far beyond the conventional $\sqrt{q}$ plasmon. One AAP originates from the strong hybridization of two pockets with large velocity anisotropy at one Dirac point, whereas the other is attributed to the significant enhancement of the band correlation around the open Fermi surface. Remarkably, the plasmons exhibit valley-dependent chirality along the tilting direction due to the chiral electron dispersion. Meanwhile, we discuss the tunability of plasmon dispersion and lifetime by tuning the gap and dielectric substrate. Our work provides a promising way to generate the novel plasmons in Dirac materials.

cond-mat.mes-hall↗

arXiv2022

Signatures of Lifshitz transition in the optical conductivity of two-dimensional tilted Dirac materials

Lifshitz transition is a kind of topological phase transition in which the Fermi surface is reconstructed. It can occur in the two-dimensional (2D) tilted Dirac materials when the energy bands change between the type-I phase ($0 1$) through the type-III phase ($t=1$), where different tilts are parametrized by the values of $t$. In order to characterize the Lifshitz transition therein, we theoretically investigate the longitudinal optical conductivities (LOCs) in type-I, type-II, and type-III Dirac materials within linear response theory. In the undoped case, the LOCs are constants either independent of the tilt parameter in both type-I and type-III phases or determined by the tilt parameter in the type-II phase. In the doped case, the LOCs are anisotropic and possess two critical frequencies determined by $ω=ω_1(t)$ and $ω=ω_2(t)$, which are also confirmed by the joint density of state. The tilt parameter and chemical potential can be extracted from optical experiments by measuring the positions of these two critical boundaries and their separation $Δω(t)=ω_2(t)-ω_1(t)$. With increasing the tilting, the separation becomes larger in the type-I phase whereas smaller in the type-II phase. The LOCs in the regime of large photon energy are exactly the same as that in the undoped case. The type of 2D tilted Dirac bands can be determined by the asymptotic background values, critical boundaries and their separation in the LOCs. These can therefore be taken as signatures of Lifshitz transition therein. The results of this work are expected to be qualitatively valid for a large number of 2D tilted Dirac materials, such as 8-\emph{Pmmn} borophene monolayer, $α$-SnS$_2$, TaCoTe$_2$, TaIrTe$_4$, and $1T^\prime$ transition metal dichalcogenides, due to the underlying intrinsic similarities of 2D tilted Dirac bands.

cond-mat.mes-hall↗

arXiv2021

Anisotropic longitudinal optical conductivities of tilted Dirac bands in 1T$^\prime$-MoS$_2$

1T$^\prime$-MoS$_2$ exhibits valley-spin-polarized tilted Dirac bands in the presence of external vertical electric field and undergoes a topological phase transition between the topological insulator and band insulator around the critical value of the electric field. Within the linear response theory, we theoretically investigate the anisotropic longitudinal optical conductivities of tilted Dirac bands in both undoped and doped 1T$^\prime$-MoS$_2$, including the effects of the vertical electric field. The influence of the spin-orbit coupling gap, band tilting, and vertical electric field on the optical conductivities of tilted Dirac bands is revealed. A theoretical scheme for probing the topological phase transition in 1T$^\prime$-MoS$_2$ via exotic behaviors of longitudinal optical conductivities is proposed. The results for 1T$^\prime$-MoS$_2$ are expected to be qualitatively valid for other monolayer tilted gapped Dirac materials, such as $α$-SnS$_2$, TaCoTe$_2$, and TaIrTe$_4$, due to the similarity in their band structures.

cond-mat.mes-hall↗