arXiv subjects

Wei-Feng Tsai

Publications and source records attributed to Wei-Feng Tsai.

At least 19 recordsLinked to original sources

arXiv2017

Stable charge density wave phase in a 1T-TiSe$_2$ monolayer

Charge density wave (CDW) phases are symmetry-reduced states of matter in which a periodic modulation of the electronic charge frequently leads to drastic changes of the electronic spectrum, including the emergence of energy gaps. We analyze the CDW state in a 1T-TiSe$_2$ monolayer within a density functional theory framework and show that, similarly to its bulk counterpart, the monolayer is unstable towards a commensurate $2{\times}2$ periodic lattice distortion (PLD) and CDW at low temperatures. Analysis of the electron and phonon spectrum establishes the PLD as the stable $T=0$ K configuration with a narrow bandgap, whereas the undistorted and semi-metalic state is stable only above a threshold temperature. The lattice distortions as well as the unfolded and reconstructed band structure in the CDW phase agree well with experimental results. We also address evidence in our results for the role of electron-electron interactions in the CDW instability of 1T-TiSe$_2$ monolayers.

cond-mat.mes-hall↗

arXiv2017

Modulation of pairing symmetry with bond disorder in unconventional superconductors

We study a two-orbital $t$-$J_1$-$J_2$ model, originally developed to describe iron-based superconductors at low energies, in the presence of bond disorder (via next-nearest-neighbor $J_2$-bond dilution). By using the Bogoliubov--de Gennes approach, we self-consistently calculate the local pairing amplitudes and the corresponding density of states, which demonstrate a change of dominant pairing symmetry from $s_\pm$ wave to $d$ wave when increasing disorder strength as long as $J_1\lesssim J_2$. Moreover, the combined pairing interaction and strong bond disorder lead to the formation of $s_\pm$ wave "islands" with length scale of the superconducting coherence length embedded in a $d$ wave "sea." This picture is further complemented by the disorder-averaged pair-pair correlation functions, distinct from the case with potential disorder, where the "sea" is insulating. Due to this inevitable formation of spatial inhomogeneity, the superconducting $T_c$ determined by the superfluid density $ρ_s(T)$ obviously deviates from the value predicted by the conventional Abrikosov-Gorkov theory, where the pairing amplitudes are viewed as uniformly suppressed as the disorder increases.

cond-mat.supr-con↗

arXiv2016

Chiral p-wave superconductivity in Sb(111) thin films close to Van Hove singularities

We theoretically investigate the development of unconventional superconductivity in the Sb(111) thin film when its Fermi level is tuned to near type-II Van Hove singularities (VHS), which locate at non-time-reversal invariant momenta. Via patch renormalization group analysis, we show that the leading instability is a chiral p + ip-wave superconducting order. The origin of such pairing relies on the hexagonal structure of the VHS and strong spin-orbit coupling, resulting in the anisotropy of the electron-electron scattering to provide an attractive channel. Our study hence suggests that superconducting Sb thin films originated from VHS physics may host Majorana zero modes in the magnetic vortices and provides another application perspective to such material.

cond-mat.str-el↗

arXiv2016

Duality in topological superconductors and topological ferromagnetic insulators in a honeycomb lattice

The ground state of large Hubbard $U$ limit of a honeycomb lattice near half-filling is known to be a singlet $d+id$-wave superconductor. It is also known that this $d+id$ superconductor exhibits a chiral $p+ip$ pairing locally at the Dirac cone, characterized by a $2\mathbb{Z}$ topological invariant. By constructing a dual transformation, we demonstrate that this $2\mathbb{Z}$ topological superconductor is equivalent to a collection of two topological ferromagnetic insulators. As a result of the duality, the topology of the electronic structures for a $d+id$ superconductor is controllable via the change of the chemical potential by tuning the gate voltage. In particular, instead of being always a chiral superconductor, we find that the $d+id$ superconductor undergoes a topological phase transition from a chiral superconductor to a quasi-helical superconductor as the gap amplitude or the chemical potential decreases. The quasi-helical superconducting phase is found to be characterized by a topological invariant in the pseudo-spin charge sector with vanishing both the Chern number and the spin Chern number. We further elucidate the topological phase transition by analyzing the relationship between the topological invariant and the rotation symmetry. Due to the angular momentum carried by the gap function and spin-orbit interactions, we show that by placing $d+id$ superconductors in proximity to ferromagnets, varieties of chiral superconducting phases characterized by higher Chern numbers can be accessed, providing a new platform for hosting large numbers of Majorana modes at edges.

cond-mat.supr-con↗

arXiv2016

Quantum anomalous Hall effect with field-tunable Chern number near Z2 topological critical point

We study the practicability of achieving quantum anomalous Hall (QAH) effect with field-tunable Chern number in a magnetically doped, topologically trivial insulating thin film. Specifically in a candidate material, TlBi(S$_{1-δ}$Se$_δ$)$_2$, we demonstrate that the QAH phases with different Chern numbers can be achieved by means of tuning the exchange field strength or the sample thickness near the Z2 topological critical point. Our physics scenario successfully reduces the necessary exchange coupling strength for a targeted Chern number. This QAH mechanism differs from the traditional QAH picture with a magnetic topological insulating thin film, where the "surface" states must involve and sometimes complicate the realization issue. Furthermore, we find that a given Chern number can also be tuned by a perpendicular electric field, which naturally occurs when a substrate is present.

cond-mat.mes-hall↗

arXiv2015

Hedgehog spin texture and competing orders associated with strains on the surface of a topological crystalline insulator

We have investigated spin reorientation phenomena and interaction driven effects under the presence of applied strains on the (001) surface of Pb$_{1-x}$Sn$_x$(Te, Se) topological crystalline insulators, which host multiple Dirac cones. Our analysis is based on a four-band $k\cdot p$ model, which captures the spin and orbital textures of the surface states at low energies around the $\bar{X}$ and $\bar{Y}$ points, including the Lifshitz transition. Even without breaking the time-reversal symmetry, we find that certain strains which break the mirror symmetry can induce hedgehog-like spin texture associated with gap formation at the Dirac points. The Chern number of the gapped surface ground state is shown to be tunable through the interplay of strains and a perpendicular Zeeman field. We also consider effects of strain in the presence of interactions in driving competing orders, and obtain the associated phase diagram at the mean-field level. Potential applications of our results for low power consuming electronics are discussed.

cond-mat.mes-hall↗

arXiv2015

Helical Majorana fermions in d_{x^2-y^2} + i d_{xy}-wave topological superconductivity of doped correlated quantum spin Hall insulators

Large Hubbard U limit of the Kane-Mele model on a zigzag ribbon of honeycomb lattice near half-filling is studied via a renormalized mean-field theory. The ground state exhibits time-reversal symmetry (TRS) breaking d + i d'-wave superconductivity. At large spin-orbit coupling, the Z2 phase with non-trivial spin Chern number in the pure Kane-Mele model is persistent into the TRS broken state (called spin-Chern phase), and has two pairs of counter-propagating helical Majorana modes at the edges. As the spin-orbit coupling is reduced, the system undergoes a topological quantum phase transition from the spin-Chern to chiral superconducting states. Possible relevance of our results to adatom-doped graphene and irridate compounds is discussed.

cond-mat.supr-con↗

arXiv2015

Interaction-driven topological and nematic phases on the Lieb lattice

We show that topological states are often developed in two dimensional semimetals with quadratic band crossing points (BCPs) by electron-electron interactions. To illustrate this, we construct a concrete model with the BCP on an extended Lieb lattice and investigate the interaction-driven topological instabilities. We find that the BCP is marginally unstable against infinitesimal repulsions. Depending on the interaction strengths, topological quantum anomalous/spin Hall, charge nematic, and nematic-spin-nematic phases, develop separately. Possible physical realizations of quadratic BCPs are provided.

cond-mat.str-el↗

arXiv2015

Observation of Fermi Arc Surface States in a Topological Metal: A New Type of 2D Electron Gas beyond Z2 Topological Insulators

In a topological insulator, it is the electrons on the surface or edge that carry the signature of topology. Recently, a novel topological state has been proposed in metals or semimetals (gapless) whose band-structure is similar to that of a three-dimensional analog of graphene. However, to this date the signature of its topology remains an open question. We report the experimental discovery of a pair of polarized Fermi arc surface state modes in the form of a new type of two-dimensional polarized electron gas on the surfaces of Dirac semimetals. These Fermi arc surface states (FASS) are observed to connect across an even number of bulk band gapless nodes and found to have their spin uniquely locked to their momentum. We show that these states are distinctly different from the topological surface states (TSS) seen in all known topological insulators. Our observed exotic two-dimensional states not only uncover the novel topology of gapless Dirac metals (such as sodium tribismuth Na$_3$Bi) but also opens new research frontiers for the utilization of topological Fermi arc electron gases for a wide range of fundamental physics and spintronic studies.

cond-mat.mes-hall↗

arXiv2014

Novel anisotropic spin singlet pairings in Cu$_x$Bi$_2$Se$_3$ and Bi$_2$Te$_3$

Possible anisotropic spin singlet pairings in Bi$_2$X$_3$ (X is Se or Te) are studied. Among six pairings compatible with the crystal symmetry, two novel pairings show nontrivial surface Andreev bound states, which form flat bands and could produce zero bias conductance peak in measurements like point contact spectroscopy. By considering purely repulsive short range Coulomb interaction as the pairing mechanism, the dominant superexchange terms are all antiferromagnetic, which would usually favor spin singlet pairing in Bi$_2$X$_3$. Mean field analyses show that the interorbital pairing interaction favors a mixed spatial-parity anisotropic pairing state, and one pairing channel with zero energy surface states has a sizable component. The results provide important new information for future experiments.

cond-mat.supr-con↗

arXiv2014

Pairing Symmetry in Layered BiS$_2$ Compounds Driven by Electron-Electron Correlation

We investigate the pairing symmetry of layered BiS$_2$ compounds by assuming that electron-electron correlation is still important so that the pairing is rather short range. We find that the extended s-wave pairing symmetry always wins over d-wave when the pairing is confined between two short range sites up to next nearest neighbors. The pairing strength is peaked around the doping level x=0.5, which is consistent with experimental observation. The extended s-wave pairing symmetry is very robust against spin-orbital coupling because it is mainly determined by the structure of Fermi surfaces. Moreover, the extended s-wave pairing can be distinguished from conventional s-wave pairing by measuring and comparing superconducting gaps of different Fermi surfaces.

cond-mat.supr-con↗

arXiv2013

Mapping the unconventional orbital texture in topological crystalline insulators

The newly discovered topological crystalline insulators (TCIs) harbor a complex band structure involving multiple Dirac cones. These materials are potentially highly tunable by external electric field, temperature or strain and could find future applications in field-effect transistors, photodetectors, and nano-mechanical systems. Theoretically, it has been predicted that different Dirac cones, offset in energy and momentum-space, might harbor vastly different orbital character, a unique property which if experimentally realized, would present an ideal platform for accomplishing new spintronic devices. However, the orbital texture of the Dirac cones, which is of immense importance in determining a variety of materials properties, still remains elusive in TCIs. Here, we unveil the orbital texture in a prototypical TCI Pb$_{1-x}$Sn$_x$Se. By using Fourier-transform (FT) scanning tunneling spectroscopy (STS) we measure the interference patterns produced by the scattering of surface state electrons. We discover that the intensity and energy dependences of FTs show distinct characteristics, which can directly be attributed to orbital effects. Our experiments reveal the complex band topology involving two Lifshitz transitions and establish the orbital nature of the Dirac bands in this new class of topological materials, which could provide a different pathway towards future quantum applications.

cond-mat.str-el↗

arXiv2013

Electron Delocalization in Gate-Tunable Gapless Silicene

The application of a perpendicular electric field can drive silicene into a gapless state, characterized by two nearly fully spin-polarized Dirac cones owing to both relatively large spin-orbital interactions and inversion symmetry breaking. Here we argue that since inter-valley scattering from non-magnetic impurities is highly suppressed by time reversal symmetry, the physics should be effectively single-Dirac-cone like. Through numerical calculations, we demonstrate that there is no significant backscattering from a single impurity that is non-magnetic and unit-cell uniform, indicating a stable delocalized state. This conjecture is then further confirmed from a scaling of conductance for disordered systems using the same type of impurities.

cond-mat.mes-hall↗

arXiv2013

Non-trivial spin-texture of the coaxial Dirac cones on the surface of topological crystalline insulator SnTe

We present first principles calculations of the nontrivial surface states and their spin-textures in the topological crystalline insulator SnTe. The surface state dispersion on the [001] surface exhibits four Dirac-cones centered along the intersection of the mirror plane and the surface plane. We propose a simple model of two interacting coaxial Dirac cones to describe both the surface state dispersion and the associated spin-texture. While the out-of-the-plane spin polarization is zero due to the crystalline and time-reversal symmetries, the in-plane spin texture shows helicity with some distortion due to the interaction of the two coaxial Dirac cones, indicating a nontrivial mirror Chern number of -2, distinct from the value of -1 in $Z_{2}$ topological insulator such as Bi/Sb alloys or Bi$_2$Se$_3$. The surface state dispersion and its spin-texture would provide an experimentally accessible way to determine the nontrivial mirror Chern number.

cond-mat.mtrl-sci↗

arXiv2013

Gated Silicene as a tunable source of nearly 100% spin-polarized electrons

Silicene is a one-atom-thick 2D crystal of silicon with a hexagonal lattice structure that is related to that of graphene but with atomic bonds that are buckled rather than flat. This buckling confers advantages on silicene over graphene, because it should, in principle, generate both a band gap and polarized spin-states that can be controlled with a perpendicular electric field. Here we use first-principles calculations to show that field-gated silicene possesses two gapped Dirac cones exhibiting nearly 100% spin-polarization, situated at the corners of the Brillouin zone. Using this fact, we propose a design for a silicene-based spin-filter that should enable the spin-polarization of an output current to be switched electrically, without switching external magnetic fields. Our quantum transport calculations indicate that the proposed designs will be highly efficient (nearly 100% spin polarization) and robust against weak disorder and edge imperfections. We also propose a Y-shaped spin/valley separator that produces spin-polarized current at two output terminals with opposite spins.

cond-mat.mtrl-sci↗

arXiv2010

S-wave superconductivity with orbital dependent sign change in the checkerboard models of iron-based superconductors

We study three different multi-orbital models for iron-based superconductors (iron-SCs) in the solvable limit of weakly coupled square plaquettes. The strongest superconducting (SC) pairing is in the $A_{1g}$ $s$-wave channel and its development is correlated with the emergence of the next-nearest-neighbour antiferromagnetism (NNN-AFM). For the models with more than three orbitals, this study suggests that the signs of the intra-orbital pairing order parameters of the $d_{xy}$ and the $d_{xz}$ (or $d_{yz}$) orbitals must be {\it opposite}. Such sign difference stems from the intrinsic symmetry properties of inter-orbital hoppings and might, ultimately, lead to the sign-change of the SC orders between the hole Fermi pockets at the $Γ$ point and produce anisotropic or even gapless SC gaps in the electron Fermi pockets around the $M$ point in reciprocal space, as restoring back to the homogeneous limit.

cond-mat.supr-con↗

arXiv2009

Quasiparticle scattering in two dimensional helical liquid

We study the quasiparticle interference (QPI) patterns caused by scattering off nonmagnetic, magnetic point impurities, and edge impurities, separately, in a two dimensional helical liquid, which describes the surface states of a topological insulator. The unique features associated with hexagonal warping effects are identified in the QPI patterns of charge density with nonmagnetic impurities and spin density with magnetic impurities. The symmetry properties of the QPI patterns can be used to determine the symmetry of microscopic models. The Friedel oscillation is calculated for edge impurities and the decay of the oscillation is not universal, strongly depending on Fermi energy. Some discrepancies between our theoretical results and current experimental observations are discussed.

cond-mat.mes-hall↗

arXiv2009

Impurity-induced bound states in iron-based superconductors with s-wave cos(kx)cos(ky) pairing symmetry

Using both the self-consistent Bogoliubov-de Gennes formulation and non-self-consistent T-matrix approach, we perform a comprehensive investigation of the in-gap bound states induced by a localized single impurity in iron-based superconductors. We focus on studying signatures associated with the unconventional sign-changed s-wave pairing symmetry. For a non-magnetic impurity, we find that there are two in-gap bounds, symmetric with respect to zero energy, only in the sign changed s-wave pairing state, not in the sign-unchanged s-wave state, due to the existence of non-trivial Andreev bound states caused by the sign change. For a magnetic impurity, we find that due to the breakdown of the local time-reversal symmetry, there exist only bound state solutions (with orbital degeneracy) carrying one of the electron-spin polarizations around the impurity. As increasing the scattering strength, the system undergoes a quantum phase transition (level crossing) from a spin-unpolarized ground state to a spin-polarized one. While the results for the magnetic impurity are qualitatively similar in both the sign-changed and sign-unchanged s-wave superconducting states, the bound states in the first case are more robust and there is no $π$ phase shift of the SC gap near the impurity in the strong scattering regime.

cond-mat.supr-con↗