arXiv subjects

Hai-Jun Zhang

Publications and source records attributed to Hai-Jun Zhang.

18 recordsLinked to original sources

arXiv2022

Reply to "Comment on 'Anisotropic Scattering Caused by Apical Oxygen Vacancies in Thin Films of Overdoped High Temperature Cuprate Superconductors'"

In our recent work [Phys. Rev. Lett. 128, 137001 (2002)], we proposed that the apical oxygen vacancies act as anisotropic scattering impurities. Within the Born approximation, this leads to a quasi-particle scattering rate that is maximal (zero) in the antinodal (nodal) direction. This unique angular dependence provides a straightforward mechanism for some puzzling experimental results in overdoped La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) films regarding the superfluid density $ρ_s$ and optical conductivity $σ_1$. In a recent comment by H. U. Ozdemir et al. [arXiv:2206.01301], the importance of the nature of the impurity scattering is re-emphasized, but some challenges to our picture are raised: that we did not consider the change of the electrostatic potential for in-plane electrons once the apical oxygen is missing, the change of Fermi surface topology as the van Hove point is passed, self-energy corrections caused by d-wave pairing, and vertex corrections caused by forward scattering. These concerns are interesting but are either irrelevant or further enhances our conclusions. We discuss these points one by one in this reply.

cond-mat.supr-con↗

arXiv2022

Anisotropic scattering caused by apical oxygen vacancies in thin films of overdoped high-temperature cuprate superconductors

There is a hot debate on the anomalous behavior of superfluid density $ρ_s$ in overdoped La$_{2-x}$Sr$_x$CuO$_4$ films in recent years. The linear drop of $ρ_s$ at low temperatures implies the superconductors are clean, but the linear scaling between $ρ_s$ (in the zero temperature limit) and the transition temperature $T_c$ is a hallmark of the dirty limit in the Bardeen-Cooper-Schrieffer (BCS) framework [I. Bozovic et al., Nature 536, 309 (2016)]. This dichotomy motivated exotic theories beyond the standard BCS theory. We show, however, that such a dichotomy can be reconciled naturally by the role of increasing anisotropic scattering caused by the apical oxygen vacancies. Furthermore, the anisotropic scattering also explains the "missing" Drude weight upon doping in the optical conductivity, as reported in the THz experiment [F. Mahmood et al., Phys. Rev. Lett. 122, 027003 (2019)]. Therefore, the overdoped cuprates can actually be described consistently by the $d$-wave BCS theory with the unique anisotropic scattering.

cond-mat.supr-con↗

arXiv2017

Accessible Coherence and Coherence Distribution

The definition of accessible coherence is proposed. Through local measurement on the other subsystem and one way classical communication, a subsystem can access more coherence than the coherence of its density matrix. Based on the local accessible coherence, the part that can not be locally accessed is also studied, which we call it remaining coherence. We study how the bipartite coherence is distributed by partition for both l1 norm coherence and relative entropy coherence, and the expressions for local accessible coherence and remaining coherence are derived. we also study some examples to illustrate the distribution.

quant-ph↗

arXiv2017

Estimation on geometric measure of quantum coherence

We study the geometric measure of quantum coherence recently proposed in [Phys. Rev. Lett. 115, 020403 (2015)]. Both lower and upper bounds of this measure are provided. These bounds are shown to be tight for a class of important coherent states -- maximally coherent mixed states. The trade-off relation between quantum coherence and mixedness for this measure is also discussed.

quant-ph↗

arXiv2017

One-way quantum deficit and quantum coherence in the anisotropic $XY$ chain

In this study, we investigate pairwise non-classical correlations measured using a one-way quantum deficit as well as quantum coherence in the $XY$ spin-1/2 chain in a transverse magnetic field for both zero and finite temperatures. The analytical and numerical results of our investigations are presented. In the case when the temperature is zero, it is shown that the one-way quantum deficit can characterize quantum phase transitions as well as quantum coherence. We find that these measures have a clear critical point at $λ=1$. When $λ\le1$, the one-way quantum deficit has an analytical expression that coincides with the relative entropy of coherence. We also study an $XX$ model and an Ising chain at the finite temperatures.

quant-ph↗

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

Large-gap quantum spin Hall insulators in tin films

The search of large-gap quantum spin Hall (QSH) insulators and effective approaches to tune QSH states is important for both fundamental and practical interests. Based on first-principles calculations we find two-dimensional tin films are QSH insulators with sizable bulk gaps of 0.3 eV, sufficiently large for practical applications at room temperature. These QSH states can be effectively tuned by chemical functionalization and by external strain. The mechanism for the QSH effect in this system is band inversion at the Γpoint, similar to the case of HgTe quantum well. With surface doping of magnetic elements, the quantum anomalous Hall effect could also be realized.

cond-mat.mtrl-sci↗

arXiv2011

Quantum Anomalous Hall Effect in Magnetic Topological Insulator GdBiTe3

The quantum anomalous Hall (QAH) state is a two-dimensional bulk insulator with a non-zero Chern number in absence of external magnetic fields. Protected gapless chiral edge states enable dissipationless current transport in electronic devices. Doping topological insulators with random magnetic impurities could realize the QAH state, but magnetic order is difficult to establish experimentally in the bulk insulating limit. Here we predict that the single quintuple layer of GdBiTe3 film could be a stoichiometric QAH insulator based on ab-initio calculations, which explicitly demonstrate ferromagnetic order and chiral edge states inside the bulk gap. We further investigate the topological quantum phase transition by tuning the lattice constant and interactions. A simple low-energy effective model is presented to capture the salient physical feature of this topological material.

cond-mat.mes-hall↗

arXiv2011

Rapid Surface Oxidation as a Source of Surface Degradation Factor for Bi2Se3

Bi2Se3 is a topological insulator with metallic surface states residing in a large bulk bandgap. It is believed that Bi2Se3 gets additional n-type doping after exposure to atmosphere, thereby reducing the relative contribution of surface states in total conductivity. In this letter, transport measurements on Bi2Se3 nanoribbons provide additional evidence of such environmental doping process. Systematic surface composition analyses by X-ray photoelectron spectroscopy reveal fast formation and continuous growth of native oxide on Bi2Se3 under ambient conditions. In addition to n-type doping at the surface, such surface oxidation is likely the material origin of the degradation of topological surface states. Appropriate surface passivation or encapsulation may be required to probe topological surface states of Bi2Se3 by transport measurements.

cond-mat.mtrl-sci↗

arXiv2011

Topological superconductivity and Majorana fermions in half-metal / superconductor heterostructure

As a half-metal is spin-polarized at its Fermi level by definition, it was conventionally thought to have little proximity effect to an s-wave superconductor. Here we show that, with interface spin-orbit coupling, $p_x +ip_y$ superconductivity without spin degeneracy is induced on the half-metal, and we give an estimate of its bulk energy gap. Therefore a single-band half-metal can give us a topological superconductor with a single chiral Majorana edge state. Our band calculation shows that two atomic layers of VTe or CrO$_2$ is a single-band half-metal for a wide range ($\sim$0.1eV) of Fermi energy and thus is a suitable candidate material.

cond-mat.supr-con↗

arXiv2010

Topological Insulators in Ternary Compounds with a Honeycomb Lattice

One of the most exciting subjects in solid state physics is a single layer of graphite which exhibits a variety of unconventional novel properties. The key feature of its electronic structure are linear dispersive bands which cross in a single point at the Fermi energy. This so-called Dirac cone is closely related to the surface states of the recently discovered topological insulators. The ternary compounds, such as LiAuSe and KHgSb with a honeycomb structure of their Au-Se and Hg-Sb layers feature band inversion very similar to HgTe which is a strong precondition for existence of the topological surface states. In contrast to graphene with two Dirac cones at K and K' points, these materials exhibit the surface states formed by only a single Dirac cone at the Γ-point together with the small direct band gap opened by a strong spin-orbit coupling (SOC) in the bulk. These materials are centro-symmetric, therefore, it is possible to determine the parity of their wave functions, and hence, their topological character. Surprisingly, the compound KHgSb with the strong SOC is topologically trivial, whereas LiAuSe is found to be a topological non-trivial insulator.

cond-mat.mtrl-sci↗

arXiv2010

Theoretical prediction of topological insulator in ternary rare earth chalcogenides

A new class of three-dimensional topological insulator, ternary rare earth chalcogenides, is theoretically investigated with ab initio calculations. Based on both bulk band structure analysis and the direct calculation of topological surface states, we demonstrate that LaBiTe3 is a topological insulator. La can be substituted by other rare earth elements, which provide candidates for novel topological states such as quantum anomalous Hall insulator, axionic insulator and topological Kondo insulator. Moreover, YBiTe3 and YSbTe3 are found to be normal insulators. They can be used as protecting barrier materials for both LaBiTe3 and Bi2Te3 families of topological insulators for their well matched lattice constants and chemical composition.

cond-mat.mtrl-sci↗

arXiv2010

Investigation of Magnetic Transport Properties by Wannier Interpolation

We present an efficient {\it ab initio} approach for the study of magnetic transport properties based on the Boltzmann equation with the Wannier interpolation scheme. Within the relaxation time approximation, band-resolved electric conductivity under a finite magnetic field is obtained and the historical motion of the electron wave packet in reciprocal space is determined. As a typical application of this method, we have calculated the electric conductivities of MgB$_2$ under finite magnetic fields. Multiband characters for the individual bands are revealed, and the field dependence of the conductivity tensor is studied systematically with the field orientated parallel and normal to the $c$-axis, respectively. The obtained historical motion is employed to simulate directly the cyclotron motion in the extremal orbit and determine the corresponding effective mass. Moreover, This approach is further exploited to calculate the Hall coefficient in the low-field limit, without the complicated computation for the second ${\mathbf k}$ derivative of the band.

cond-mat.mtrl-sci↗

arXiv2010

First Principles Studies on 3-Dimentional Strong Topological Insulators: Bi2Te3, Bi2Se3 and Sb2Te3

Bi2Se3, Bi2Te3 and Sb2Te3 compounds are recently predicted to be 3-dimentional (3D) strong topological insulators. In this paper, based on ab-initio calculations, we study in detail the topological nature and the surface states of this family compounds. The penetration depth and the spin-resolved Fermi surfaces of the surface states will be analyzed. We will also present an procedure, from which highly accurate effective Hamiltonian can be constructed, based on projected atomic Wannier functions (which keep the symmetries of the systems). Such Hamiltonian can be used to study the semi-infinite systems or slab type supercells efficiently. Finally, we discuss the 3D topological phase transition in Sb2(Te1-xSex)3 alloy system.

cond-mat.mtrl-sci↗

arXiv2010

Theoretical Prediction of Topological Insulators in Thallium-based III-V-VI$_2$ Ternary Chalcogenides

We predict a new class of three dimensional topological insulators in thallium-based III-V-VI$_2$ ternary chalcogenides, including TlBiQ$_2$ and TlSbQ$_2$ (Q = Te, Se and S). These topological insulators have robust and simple surface states consisting of a single Dirac cone at the $Γ$ point. The mechanism for topological insulating behavior is elucidated using both first principle calculations and effective field theory models. Remarkably, one topological insulator in this class, TlBiTe$_2$ is also a superconductor when doped with $p$-type carriers. We discuss the possibility that this material could be a topological superconductor. Another material TlSbS$_2$ is on the border between topological insulator and trivial insulator phases, in which a topological phase transition can be driven by pressure.

cond-mat.mtrl-sci↗

arXiv2009

Growth dynamics and thickness-dependent electronic structure of topological insulator Bi2Te3 thin films on Si

We use real-time reflection high energy electron diffraction intensity oscillation to establish the Te-rich growth dynamics of topological insulator thin films of Bi2Te3 on Si(111) substrate by molecular beam epitaxy. In situ angle resolved photoemission spectroscopy (ARPES), scanning tunneling microscopy and ex situ transport measurements reveal that the as-grown Bi2Te3 films without any doping are an intrinsic topological insulator with its Fermi level intersecting only the metallic surface states. Experimentally, we find that the single-Dirac-cone surface state develops at a thickness of two quintuple layers (2 QL). Theoretically, we show that the interaction between the surface states from both sides of the film, which is determined by the penetration depth of the topological surface state wavefunctions, sets this lower thickness limit.

cond-mat.mes-hall↗

arXiv2009

Electronic Structures and Surface States of Topological Insulator Bi$_{1-x}$Sb$_{x}$

We investigate the electronic structures of the alloyed Bi$_{1-x}$Sb$_x$ compounds based on first-principle calculations including spin-orbit coupling (SOC), and calculate the surface states of semi-infinite systems using maximally localized Wannier function (MLWF). From the calculated results, we analyze the topological nature of Bi$_{1-x}$Sb$_x$, and found the followings: (1) pure Bi crystal is topologically trivial; (2) topologically non-trivial phase can be realized by reducing the strength of SOC via Sb doping; (3) the indirect bulk band gap, which is crucial to realize the true bulk insulating phase, can be enhanced by uniaxial pressure along c axis. (4) The calculated surface states can be compared with experimental results, which confirms the topological nature; (5) We predict the spin-resolved Fermi surfaces and showed the vortex structures, which should be examined by future experiments.

cond-mat.mes-hall↗

arXiv2008

Enhanced Orbital Degeneracy in Momentum Space for LaOFeAs

The Fermi surfaces (FS) of LaOFeAs (in $k_z$=0 plane) consist of two hole-type circles around $Γ$ point, which do not touch each other, and two electron-type co-centered ellipses around M point, which are degenerate along the M-X line. By first-principles calculations, here we show that additional degeneracy exists for the two electron-type FS, and the crucial role of F-doping and pressure is to enhance this orbital degeneracy. It is suggested that the inter-orbital fluctuation is the key point to understand the unconventional superconductivity in these materials.

cond-mat.supr-con↗