arXiv subjects

Chao-Xing Liu

Publications and source records attributed to Chao-Xing Liu.

At least 109 records · Page 6Linked to original sources

arXiv2016

Electrically tunable spin polarization of chiral edge modes in a quantum anomalous Hall insulator

In the quantum anomalous Hall effect, chiral edge modes are expected to conduct spin polarized current without dissipation and thus hold great promise for future electronics and spintronics with low energy consumption. However, spin polarization of chiral edge modes has never been established in experiments. In this work, we theoretically study spin polarization of chiral edge modes in the quantum anomalous Hall effect, based on both the effective model and more realistic tight-binding model constructed from the first principles calculations. We find that spin polarization can be manipulated by tuning either a local gate voltage or the Fermi energy. We also propose to extract spin information of chiral edge modes by contacting the quantum anomalous Hall insulator to a ferromagnetic (FM) lead. The establishment of spin polarization of chiral edge modes, as well as the manipulation and detection in a fully electrical manner, will pave the way to the applications of the quantum anomalous Hall effect in spintronics.

cond-mat.mes-hall↗

arXiv2016

Electric control of inverted gap and hybridization gap in type II InAs/GaSb quantum wells

The quantum spin Hall effect has been predicted theoretically and observed experimentally in InAs/GaSb quantum wells as a result of inverted band structures, for which electron bands in InAs layers are below heavy hole bands in GaSb layers in energy. The hybridization between electron bands and heavy hole bands leads to a hybridization gap away from k=0. A recent puzzling observation in experiments is that when the system is tuned to more inverted regime by a gate voltage (a larger inverted gap at k=0), the hybridization gap decreases. Motivated by this experiment, we explore the dependence of hybridization gap as a function of external electric fields based on the eight-band Kane model. We identify two regimes when varying electric fields: (1) both inverted and hybridization gaps increase and (2) inverted gap increases while hybridization gap decreases. Based on the effective model, we find that light-hole bands in GaSb layers play an important role in determining hybridization gap. In addition, a large external electric field can induce a strong Rashba splitting and also influence hybridization gap.

cond-mat.mes-hall↗

arXiv2016

Heavy Dirac fermions in a graphene/topological insulator hetero-junction

The low energy physics of both graphene and surface states of three-dimensional topological insulators is described by gapless Dirac fermions with linear dispersion. In this work, we predict the emergence of a "heavy" Dirac fermion in a graphene/topological insulator hetero-junction, where the linear term almost vanishes and the corresponding energy dispersion becomes highly non-linear. By combining {\it ab initio} calculations and an effective low-energy model, we show explicitly how strong hybridization between Dirac fermions in graphene and the surface states of topological insulators can reduce the Fermi velocity of Dirac fermions. Due to the negligible linear term, interaction effects will be greatly enhanced and can drive "heavy" Dirac fermion states into the half quantum Hall state with non-zero Hall conductance.

cond-mat.mes-hall↗

arXiv2016

Classification of topological crystalline insulators based on representation theory

Topological crystalline insulators define a new class of topological insulator phases with gapless surface states protected by crystalline symmetries. In this work, we present a general theory to classify topological crystalline insulator phases based on the representation theory of space groups. Our approach is to directly identify possible nontrivial surface states in a semi-infinite system with a specific surface, of which the symmetry property can be described by 17 two-dimensional space groups. We reproduce the existing results of topological crystalline insulators, such as mirror Chern insulators in the $pm$ or $pmm$ groups, $C_{nv}$ topological insulators in the $p4m$, $p31m$ and $p6m$ groups, and topological nonsymmorphic crystalline insulators in the $pg$ and $pmg$ groups. Aside from these existing results, we also obtain the following new results: (1) there are two integer mirror Chern numbers ($\mathbb{Z}^2$) in the $pm$ group but only one ($\mathbb{Z}$) in the $cm$ or $p3m1$ group for both the spinless and spinful cases; (2) for the $pmm$ ($cmm$) groups, there is no topological classification in the spinless case but $\mathbb{Z}^4$ ($\mathbb{Z}^2$) classifications in the spinful case; (3) we show how topological crystalline insulator phase in the $pg$ group is related to that in the $pm$ group; (4) we identify topological classification of the $p4m$, $p31m$, and $p6m$ for the spinful case; (5) we find topological non-symmorphic crystalline insulators also existing in $pgg$ and $p4g$ groups, which exhibit new features compared to those in $pg$ and $pmg$ groups. We emphasize the importance of the irreducible representations for the states at some specific high-symmetry momenta in the classification of topological crystalline phases. Our theory can serve as a guide for the search of topological crystalline insulator phases in realistic materials.

cond-mat.mes-hall↗

arXiv2015

Topological crystalline protection in a photonic system

Topological crystalline insulators are a class of materials with a bulk energy gap and edge or surface modes, which are protected by crystalline symmetry, at their boundaries. They have been realized in electronic systems: in particular, in SnTe. In this work, we propose a mechanism to realize photonic boundary states topologically protected by crystalline symmetry. We map this one-dimensional system to a two-dimensional lattice model with opposite magnetic fields, as well as opposite Chern numbers in its even and odd mirror parity subspaces, thus corresponding to a topological mirror insulator. Furthermore, we test how sensitive and robust edge modes depend on their mirror parity by performing time dependent evolution simulation of edge modes in a photonic setting with realistic experimental parameters.

cond-mat.mes-hall↗

arXiv2015

Topological Nonsymmorphic Crystalline Superconductors

Topological superconductors possess a nodeless superconducting gap in the bulk and gapless zero energy modes, known as "Majorana zero modes", at the boundary of a finite system. In this work, we introduce a new class of topological superconductors, which are protected by nonsymmorphic crystalline symmetry and thus dubbed "topological nonsymmorphic crystalline superconductors". We construct an explicit Bogoliubov-de Gennes type of model for this superconducting phase in the D class and show how Majorana zero modes in this model are protected by glide symmetry. Furthermore, we generalize the classification of topological nonsymmorphic crystalline superconductors to the classes with time reversal symmetry, including the DIII and BDI classes, in two dimensions. Our theory provides a guidance to search for new topological superconducting materials with nonsymmorphic crystal structures.

cond-mat.supr-con↗

arXiv2015

Electrically tunable multiple Dirac cones in thin films of (LaO)2(SbSe2)2 family of materials

Two-dimensional Dirac physics has aroused great interests in condensed matter physics ever since the discovery of graphene and topological insulators due to its importance in both fundamental physics and device applications. The ability to control the properties of Dirac cones, such as bandgap and Fermi velocity, is essential for the occurrence of various new phenomena and the development of next-generation electronic devices. Based on first-principles calculations and an analytical effective model, we propose a new Dirac system with eight Dirac cones in thin films of the (LaO)2(SbSe2)2 family of materials with an external gate voltage. The advantage of this system lies in its tunability: the existence of gapless Dirac cones, their positions, Fermi velocities and anisotropy all can be controlled by an experimentally feasible gate voltage. We identify the layer dependent spin texture induced by spin-orbit coupling as the underlying physical reason for the tunability of Dirac cones in this system. As a consequence, we show that the electrically tunable quantum anomalous Hall effect with a high Chern number can be induced by introducing magnetization into this system.

cond-mat.mtrl-sci↗

arXiv2015

The quantum anomalous Hall effect

The quantum anomalous Hall effect is defined as a quantized Hall effect realized in a system without external magnetic field. Quantum anomalous Hall effect is a novel manifestation of topological structure in many-electron systems, and may have potential applications in future electronic devices. In recent years, quantum anomalous Hall effect has been proposed theoretically and realized experimentally. In this review article, we provide a systematic overview of the theoretical and experimental developments in this field.

cond-mat.mes-hall↗

arXiv2015

Two different topological insulator phases in NaBaBi

By breaking the inversion symmetry of the three-dimensional Dirac metal Na$_3$Bi, we realize topological insulator (TI) phases in the known compound NaBaBi using $ab~initio$ calculations. Two distinct TI phases emerge: one phase is due to band inversion between the Bi $p$ and Na $s$ bands, and the other phase (under pressure) is induced by inversion of the Bi $p$ and Ba $d$ bands. Both phases exhibit Dirac-cone-type surface states, but they have opposite spin textures. In the upper cone, a left-handed spin texture exists for the $s$-$p$ inverted phase (similar to a common TI, e.g., Bi$_2$Se$_3$), whereas a right-handed spin texture appears for the $p$-$d$ inverted phase. NaBaBi presents a prototype model of a TI that exhibits different spin textures in the same material. In addition, NaBaBi may exhibit the introduction of correlation effects to the topological state owing to the existence of $d$ states in the band inversion.

cond-mat.mtrl-sci↗

arXiv2014

Long-Range Spin-Triplet Helix in Proximity Induced Superconductivity in Spin-Orbit-Coupled Systems

We study proximity induced triplet superconductivity in a spin-orbit-coupled system, and show that the \textbf{d} vector of the induced triplet superconductivity undergoes precession that can be controlled by varying the relative strengths of Rashba and Dresselhaus spin-orbit couplings. In particular, a long-range spin-triplet helix is predicted when these two spin-orbit couplings have equal strengths. We also study the Josephson junction geometry and show that a transition between 0 and $π$ junctions can be induced by controlling the spin-orbit coupling with a gate voltage. An experimental setup is proposed to verify these effects. Conversely, the observation of these effects can serve as a direct confirmation of triplet superconductivity.

cond-mat.mes-hall↗

arXiv2014

Weyl fermions induced Magnon electrodynamics in Weyl semimetal

Weyl fermions, which are fermions with definite chiralities, can give rise to anomalous breaking of the symmetry of the physical system which they are a part of. In their (3+1)-dimensional realizations in condensed matter systems, i.e., the so-called Weyl semimetals, this anomaly gives rise to topological electromagnetic response of magnetic fluctuations, which takes the form of non-local interaction between magnetic fluctuations and electromagnetic fields. We study the physical consequences of this non-local interaction, including electric field assisted magnetization dynamics, an extra gapless magnon dispersion, and polariton behaviors that feature "sibling" bands in small magnetic fields.

cond-mat.mes-hall↗

arXiv2014

Dirac-screening stabilized surface-state transport in a topological insulator

We report magnetotransport studies on a gated strained HgTe device. This material is a threedimensional topological insulator and exclusively shows surface state transport. Remarkably, the Landau level dispersion and the accuracy of the Hall quantization remain unchanged over a wide density range ($3 \times 10^{11} cm^{-2} < n < 1 \times 10^{12} cm^{-2}$). This implies that even at large carrier densities the transport is surface state dominated, where bulk transport would have been expected to coexist already. Moreover, the density dependence of the Dirac-type quantum Hall effect allows to identify the contributions from the individual surfaces. A $k \cdot p$ model can describe the experiments, but only when assuming a steep band bending across the regions where the topological surface states are contained. This steep potential originates from the specific screening properties of Dirac systems and causes the gate voltage to influence the position of the Dirac points rather than that of the Fermi level.

cond-mat.mes-hall↗

arXiv2014

Prediction of a Weyl Semimetal in Hg$_{1-x-y}$Cd$_x$Mn$_y$Te

We study strained Hg$_{1-x-y}$Cd$_x$Mn$_y$Te in a magnetic field using a $\bm{k}\cdot\bm{p}$ model and predict that the system is a Weyl semimetal with two nodes in an experimentally reasonable region of the phase diagram. We also predict two signatures of the Weyl semimetal phase which arise from tunability of the Weyl node splitting. First, we find that the Hall conductivity is proportional to the average Mn ion spin and thus is strongly temperature dependent. Second, we find an unusual magnetic field angle dependence of the Hall conductivity; in particular, we predict a peak in $σ_{xy}$ as a function of field angle in the $xz$-plane and a finite $σ_{yz}$ as the $x$-component of the field goes to 0.

cond-mat.mes-hall↗

arXiv2014

Topological magnetic crystalline insulators and co-representation theory

Gapless surface states of time reversal invariant topological insulators are protected by the anti-unitary nature of the time reversal operation. Very recently, this idea was generalized to magnetic structures, in which time reversal symmetry is explicitly broken, but there is still an anti-unitary symmetry operation combining time reversal symmetry and crystalline symmetry. These topological phases in magnetic structures are dubbed "topological magnetic crystalline insulators". In this work, we present a general theory of topological magnetic crystalline insulators in different types of magnetic crystals based on the co-representation theory of magnetic crystalline symmetry groups. We construct two concrete tight-binding models of topological magnetic crystalline insulators, the $\hat{C}_4Θ$ model and the $\hat{\bf τ}Θ$ model, in which topological surface states and topological invariants are calculated explicitly. Moreover, we check different types of anti-unitary operators in magnetic systems and find that the systems with $\hat{C}_4Θ$, $\hat{C}_6Θ$ and $\hat{\bf τ}Θ$ symmetry are able to protect gapless surface states. Our work will pave the way to search for topological magnetic crystalline insulators in realistic magnetic materials.

cond-mat.mes-hall↗

arXiv2014

Localized States and Quantum Spin Hall Effect in Si-Doped InAs/GaSb Quantum Wells

We study localized in-gap states and quantum spin Hall effect in Si-doped InAs/GaSb quantum wells. We propose a model describing donor and/or acceptor impurities to describe Si dopants. This model shows in-gap bound states and wide conductance plateau with the quantized value $2e^2/h$ in light dopant concentration, consistent with recent experiments by Du et al. We predict a conductance dip structure due to backward scattering in the region where the localization length $ξ$ is comparable with the sample width $L_y$ but much smaller than the sample length $L_x$.

cond-mat.mes-hall↗

arXiv2013

Quantum Anomalous Hall Effect in Magnetically Doped InAs/GaSb Quantum Wells

The quantum anomalous Hall effect has recently been observed experimentally in thin films of Cr doped (Bi,Sb)$_2$Te$_3$ at a low temperature ($\sim$ 30mK). In this work, we propose realizing the quantum anomalous Hall effect in more conventional diluted magnetic semiconductors with doped InAs/GaSb type II quantum wells. Based on a four band model, we find an enhancement of the Curie temperature of ferromagnetism due to band edge singularities in the inverted regime of InAs/GaSb quantum wells. Below the Curie temperature, the quantum anomalous Hall effect is confirmed by the direct calculation of Hall conductance. The parameter regime for the quantum anomalous Hall phase is identified based on the eight-band Kane model. The high sample quality and strong exchange coupling make magnetically doped InAs/GaSb quantum wells good candidates for realizing the quantum anomalous Hall insulator at a high temperature.

cond-mat.mes-hall↗

arXiv2013

Topological non-symmorphic crystalline insulators

In this work, we identify a new class of Z2 topological insulator protected by non-symmorphic crystalline symmetry, dubbed a "topological non-symmorphic crystalline insulator". We construct a concrete tight-binding model with the non-symmorphic space group pmg and confirm the topological nature of this model by calculating topological surface states and defining a Z2 topological invariant. Based on the projective representation theory, we extend our discussion to other non-symmorphic space groups that allows to host topological non-symmorphic crystalline insulators.

cond-mat.mes-hall↗

arXiv2013

Reentrant quantum anomalous Hall effect with in-plane magnetic fields in HgMnTe quantum wells

Quantum anomalous Hall effect has been predicted in HgMnTe quantum wells with an out-of-plane magnetization of Mn atoms. However, since HgMnTe quantum wells are paramagnetic, an out-of-plane magnetic field is required to polarize magnetic moments of Mn atoms, which inevitably induces Landau levels and makes it difficult to identify the origin of the quantized Hall conductance experimentally. In this work, we study the quantum anomalous Hall effect in the presence of an in-plane magnetic field in Mn doped HgTe quantum wells. For a small out-of-plane magnetic field, the in-plane magnetic field can drive the system from a normal insulating state to a quantum anomalous Hall state. When the out-of-plane magnetic field is slightly above the transition point, the system shows a reentrant behavior of Hall conductance, varying from $-e^2/h$ to 0 and back to $-e^2/h$, with increasing in-plane magnetic fields. The reentrant quantum anomalous Hall effect originates from the interplay between the exchange coupling of magnetic moments and the direct Zeeman coupling of magnetic fields. The calculation incorporating Landau levels shows that there is no qualitative change of the reentrant behavior.

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