arXiv subjects

Xi Dai

Publications and source records attributed to Xi Dai.

At least 127 records · Page 7Linked to original sources

arXiv2015

Electronic Structure of Transition Metal Dichalcogenides PdTe2 and Cu0.05PdTe2 Superconductors Obtained by Angle-Resolved Photoemission Spectroscopy

The layered transition metal chalcogenides have been a fertile land in solid state physics for many decades. Various MX2-type transition metal dichalcogenides, such as WTe2, IrTe2, and MoS2, have triggered great attention recently, either for the discovery of novel phenomena or some extreme or exotic physical properties, or for their potential applications. PdTe2 is a superconductor in the class of transition metal dichalcogenides, and superconductivity is enhanced in its Cu-intercalated form, Cu0.05PdTe2. It is important to study the electronic structures of PdTe2 and its intercalated form in order to explore for new phenomena and physical properties and understand the related superconductivity enhancement mechanism. Here we report systematic high resolution angle-resolved photoemission (ARPES) studies on PdTe2 and Cu0.05$PdTe2 single crystals, combined with the band structure calculations. We present for the first time in detail the complex multi-band Fermi surface topology and densely-arranged band structure of these compounds. By carefully examining the electronic structures of the two systems, we find that Cu-intercalation in PdTe2 results in electron-doping, which causes the band structure to shift downwards by nearly 16 meV in Cu0.05PdTe2. Our results lay a foundation for further exploration and investigation on PdTe2 and related superconductors.

cond-mat.supr-con↗

arXiv2015

Topological Node-Line Semimetal in Three Dimensional Graphene Networks

Graphene, a two dimensional (2D) carbon sheet, acquires many of its amazing properties from the Dirac point nature of its electronic structures with negligible spin-orbit coupling. Extending to 3D space, graphene networks with negative curvature, called Mackay-Terrones crystals (MTC), have been proposed and experimentally explored, yet their topological properties remain to be discovered. Based on the first-principle calculations, we report an all-carbon MTC with topologically non-trivial electronic states by exhibiting node-lines in bulk. When the node-lines are projected on to surfaces to form circles, "drumhead" like flat surface bands nestled inside of the circles are formed. The bulk node-line can evolve into 3D Dirac point in the absence of inversion symmetry, which has shown its plausible existence in recent experiments.

cond-mat.mes-hall↗

arXiv2015

Topological Nodal Line Semimetal and Dirac Semimetal State in Antiperovskite Cu$_3$PdN

Based on first-principles calculation and effective model analysis, we propose that the cubic antiperovskite material Cu$_3$PdN can host a three-dimensional (3D) topological nodal line semimetal state when spin-orbit coupling (SOC) is ignored, which is protected by coexistence of time-reversal and inversion symmetry. There are three nodal line circles in total due to the cubic symmetry. "Drumhead"-like surface flat bands are also derived. When SOC is included, each nodal line evolves into a pair of stable 3D Dirac points as protected by C$_4$ crystal symmetry. This is remarkably distinguished from the Dirac semimetals known so far, such as Na$_3$Bi and Cd$_3$As$_2$, both having only one pair of Dirac points. Once C$_4$ symmetry is broken, the Dirac points are gapped and the system becomes a strong topological insulator with (1;111) Z$_2$ indices.

cond-mat.mtrl-sci↗

arXiv2015

Interaction-induced quantum anomalous Hall phase in (111) bilayer of LaCoO$_3$

In the present paper, the Gutzwiller density functional theory (LDA+G) has been applied to study a bilayer system of LaCoO$_3$ grown along the $(111)$ direction on SrTiO$_3$. The LDA calculations show that there are two nearly flat bands located at the top and bottom of $e_{g}$ bands of Co atoms with the Fermi level crossing the lower one, which is almost half-filled. After including both the spin-orbit coupling and the Coulomb interaction in the LDA+G method, we find that the interplay between spin-orbit coupling and Coulomb interaction stabilizes a very robust ferromagnetic insulator phase with non-zero Chern number, which indicates the possibility to realize quantum anomalous Hall effect in this system.

cond-mat.str-el↗

arXiv2015

$i$QIST: An open source continuous-time quantum Monte Carlo impurity solver toolkit

Quantum impurity solvers have a broad range of applications in theoretical studies of strongly correlated electron systems. Especially, they play a key role in dynamical mean-field theory calculations of correlated lattice models and realistic materials. Therefore, the development and implementation of efficient quantum impurity solvers is an important task. In this paper, we present an open source interacting quantum impurity solver toolkit (dubbed $i$QIST). This package contains several highly optimized quantum impurity solvers which are based on the hybridization expansion continuous-time quantum Monte Carlo algorithm, as well as some essential pre- and post-processing tools. We first introduce the basic principle of continuous-time quantum Monte Carlo algorithm and then discuss the implementation details and optimization strategies. The software framework, major features, and installation procedure for $i$QIST are also explained. Finally, several simple tutorials are presented in order to demonstrate the usage and power of $i$QIST.

cond-mat.str-el↗

arXiv2015

Observation of the chiral anomaly induced negative magneto-resistance in 3D Weyl semi-metal TaAs

Weyl semi-metal is the three dimensional analog of graphene. According to the quantum field theory, the appearance of Weyl points near the Fermi level will cause novel transport phenomena related to chiral anomaly. In the present paper, we report the first experimental evidence for the long-anticipated negative magneto-resistance generated by the chiral anomaly in a newly predicted time-reversal invariant Weyl semi-metal material TaAs. Clear Shubnikov de Haas oscillations (SdH) have been detected starting from very weak magnetic field. Analysis of the SdH peaks gives the Berry phase accumulated along the cyclotron orbits to be π, indicating the existence of Weyl points.

cond-mat.mtrl-sci↗

arXiv2015

Anomalous High-Energy Waterfall-Like Electronic Structure in 5d Transition Metal Oxide Sr2IrO4 with a Strong Spin-Orbit Coupling

The layered 5d transition metal oxides like Sr2IrO4 have attracted significant interest recently due to a number of exotic and new phenomena induced by the interplay between the spin-orbit coupling, bandwidth W and on-site Coulomb correlation U. In contrast to a metallic behavior expected from the Mott-Hubbard model due to more spatially extended 5d orbitals and moderate U, an insulating ground state has been observed in Sr2IrO4. Such an insulating behavior can be understood by an effective J_eff=1/2 Mott insulator model by incorporating both electron correlation and strong spin-orbital coupling, although its validity remains under debate at present. In particular, Sr2IrO4 exhibits a number of similarities to the high temperature cuprate superconductors in the crystal structure, electronic structure, magnetic structure, and even possible high temperature superconductivity. Here we report a new observation of the anomalous high energy electronic structure in Sr2IrO4. By taking high-resolution angle-resolved photoemission measurements on Sr2IrO4 over a wide energy range, we have revealed that the high energy electronic structures show unusual nearly-vertical bands that extend over a large energy range. Such anomalous high energy behaviors resemble the high energy waterfall features observed in the cuprate superconductors, adding one more important similarity between these two systems. While strong electron correlation plays an important role in producing high energy waterfall features in the cuprate superconductors, the revelation of the high energy anomalies in Sr2IrO4 points to a novel route in generating exotic electronic excitations from the strong spin-orbit coupling and a moderate electron correlation.

cond-mat.str-el↗

arXiv2015

Weyl semimetal phase in non-centrosymmetric transition metal monophosphides

Based on first principle calculations, we show that a family of nonmagnetic materials including TaAs, TaP, NbAs and NbP are Weyl semimetal (WSM) without inversion center. We find twelve pairs of Weyl points in the whole Brillouin zone (BZ) for each of them. In the absence of spin-orbit coupling (SOC), band inversions in mirror invariant planes lead to gapless nodal rings in the energy-momentum dispersion. The strong SOC in these materials then opens full gaps in the mirror planes, generating nonzero mirror Chern numbers and Weyl points off the mirror planes. The resulting surface state Fermi arc structures on both (001) and (100) surfaces are also obtained and show interesting shapes, pointing to fascinating playgrounds for future experimental studies.

cond-mat.mtrl-sci↗

arXiv2014

Thermodynamics of the $α$-$γ$ transition in cerium studied by an LDA + Gutzwiller method

The $α$-$γ$ transition in cerium has been studied in both zero and finite temperature by Gutzwiller density functional theory. We find that the first order transition between $α$ and $γ$ phases persists to the zero temperature with negative pressure. By further including the entropy contributed by both electronic quasi-particles and lattice vibration, we obtain the total free energy at given volume and temperature, from which we obtain the $α$-$γ$ transition from the first principle calculation. We also computed the phase diagram and pressure versus volume isotherms of cerium at finite temperature and pressure, finding excellent agreement with the experiments. Our calculation indicate that both the electronic entropy and lattice vibration entropy plays important role in the $α$-$γ$ transition.

cond-mat.str-el↗

arXiv2014

Exploration and prediction of topological electronic materials based on first-principles calculations

The class of topological insulator materials is one of the frontier topics of condensed matter physics. The great success of this field is due to the conceptual breakthroughs in theories for topological electronic states and is strongly motivated by the rich variety of material realizations, thus making the theories testable, the experiments operable, and the applications possible. First-principles calculations have demonstrated unprecedented predictive power for material selection and design. In this article, we review recent progress in this field with a focus on the role of first-principles calculations. In particular, we introduce the Wilson loop method for the determination of topological invariants and discuss the band inversion mechanism for the selection of topological materials. Recent progress in quantum anomalous Hall insulators, large-gap quantum spin Hall insulators, and correlated topological insulators are also covered.

cond-mat.mes-hall↗

arXiv2014

Topological Insulator to Dirac Semimetal Transition Driven by Sign Change of Spin-Orbit Coupling in Thallium Nitride

Based on the first-principles calculations, we reveal that TlN, a simple binary compound with Wurtzite structure, is a three-dimensional (3D) topological insulator (TI) with effectively negative spin-orbit coupling $λ_{eff} < 0$, which makes it distinguished from other TIs by showing opposite spin-momentum locking effect in its surface states. The sign of $λ_{eff}$ depends on the hybridization between N-$2p$ and Tl-$5d$ states, and can be tuned from negative to postive by lattice strain or chemical substitution, which drive the system into a Dirac semimetal with 3D Dirac cones in its bulk states. Such topological phase transition can be realized by electronic mechanism without breaking any crystal symmetry.

cond-mat.mtrl-sci↗

arXiv2014

Strong Anisotropy of Dirac Cone in SrMnBi2 and CaMnBi2 Revealed by Angle-Resolved Photoemission Spectroscopy

The Dirac materials, such as graphene and three-dimensional topological insulators, have attracted much attention because they exhibit novel quantum phenomena with their low energy electrons governed by the relativistic Dirac equations. One particular interest is to generate Dirac cone anisotropy so that the electrons can propagate differently from one direction to the other, creating an additional tunability for new properties and applications. While various theoretical approaches have been proposed to make the isotropic Dirac cones of graphene into anisotropic ones, it has not yet been met with success. There are also some theoretical predictions and/or experimental indications of anisotropic Dirac cone in novel topological insulators and AMnBi2 (A=Sr and Ca) but more experimental investigations are needed. Here we report systematic high resolution angle-resolved photoemission measurements that have provided direct evidence on the existence of strongly anisotropic Dirac cones in SrMnBi2 and CaMnBi2. Distinct behaviors of the Dirac cones between SrMnBi2 and CaMnBi2 are also observed. These results have provided important information on the strong anisotropy of the Dirac cones in AMnBi2 system that can be governed by the spin-orbital coupling and the local environment surrounding the Bi square net.

cond-mat.str-el↗

arXiv2014

Model Hamiltonian for topological Kondo insulator SmB6

Starting from the kp method in combination with first-principles calculations, we systematically derive the effective Hamiltonians that capture the low energy band structures of recently discovered topological Kondo insulator SmB6. Using these effective Hamiltonians we can obtain both the energy dispersion and the spin texture of the topological surface states, which can be detected by further experiments.

cond-mat.mtrl-sci↗

arXiv2014

Large linear magnetoresistance in Dirac semi-metal Cd3As2 with Fermi surfaces close to the Dirac points

We have investigated the magnetoresistive behavior of Dirac semi-metal Cd3As2 down to low temperatures and in high magnetic fields. A positive and linear magnetoresistance (LMR) as large as 3100% is observed in a magnetic field of 14 T, on high-quality single crystals of Cd3As2 with ultra-low electron density and large Lande g factor. Such a large LMR occurs when the magnetic field is applied perpendicular to both the current and the (100) surface, and when the temperature is low such that the thermal energy is smaller than the Zeeman splitting energy. Tilting the magnetic field or raising the temperature all degrade the LMR, leading to a less pronounced quadratic behavior. We propose that the phenomenon of LMR is related to the peculiar field-induced shifting/distortion of the helical electrons' Fermi surfaces in momentum space.

cond-mat.mes-hall↗

arXiv2014

Evidence of Topological Surface State in Three-Dimensional Dirac Semimetal Cd3As2

The three-dimensional topological semimetals represent a new quantum state of matter. Distinct from the surface state in the topological insulators that exhibits linear dispersion in two-dimensional momentum plane, the three-dimensional semimetals host bulk band dispersions linearly along all directions, forming discrete Dirac cones in three-dimensional momentum space. In addition to the gapless points (Weyl/Dirac nodes) in the bulk, the three-dimensional Weyl/Dirac semimetals are also characterized by "topologically protected" surface state with Fermi arcs on their specific surface. The Weyl/Dirac semimetals have attracted much attention recently they provide a venue not only to explore unique quantum phenomena but also to show potential applications. While Cd3As2 is proposed to be a viable candidate of a Dirac semimetal, more experimental evidence and theoretical investigation are necessary to pin down its nature. In particular, the topological surface state, the hallmark of the three-dimensional semimetal, has not been observed in Cd3As2. Here we report the electronic structure of Cd3As2 investigated by angle-resolved photoemission measurements on the (112) crystal surface and detailed band structure calculations. The measured Fermi surface and band structure show a good agreement with the band structure calculations with two bulk Dirac-like bands approaching the Fermi level and forming Dirac points near the Brillouin zone center. Moreover, the topological surface state with a linear dispersion approaching the Fermi level is identified for the first time. These results provide strong experimental evidence on the nature of topologically non-trivial three-dimensional Dirac cones in Cd3As2.

cond-mat.mtrl-sci↗

arXiv2014

Time-reversal invariant topological superconductivity in doped Weyl semimetals

Time-reversal invariant topological superconductors are a new state of matter which have a bulk superconducting gap and robust Majorana fermion surface states. These have not yet been realized in solid state systems. In this paper, we propose that this state can be realized in doped Weyl semimetals or Weyl metals. The Fermi surfaces of a Weyl metal carry Chern numbers, which is a required ingredient for such a topological superconductor. By applying the fluctuation-exchange approach to a generic model of time-reversal invariant Dirac and Weyl semimetals, we investigate what microscopic interactions can supply the other ingredient, viz., sign changing of the superconducting gap function between Fermi surfaces with opposite Chern numbers. We find that if the normal state is inversion symmetric, onsite repulsive and exchange interactions induce various nodal phases as well as a small region of topological superconductivity on the phase diagram. Unlike the He$^{3}$B topological superconductor, the phase here does not rely on any special momentum dependence of the pairing amplitude. Breaking inversion symmetry precludes some of the nodal phases and the topological superconductor becomes much more prominent, especially at large ferromagnetic interaction. Our approach can be extended to generic Dirac or Weyl metals.

cond-mat.str-el↗

arXiv2014

Three Dimensional Dirac Semimetal and Quantum Transports in Cd3As2

Based on the first-principles calculations, we recover the silent topological nature of Cd3As2, a well known semiconductor with high carrier mobility. We find that it is a symmetry-protected topological semimetal with a single pair of three-dimensional (3D) Dirac points in the bulk and non-trivial Fermi arcs on the surfaces. It can be driven into a topological insulator and a Weyl semi-metal state by symmetry breaking, or into a quantum spin Hall insulator with gap more than 100meV by reducing dimensionality. We propose that the 3D Dirac cones in the bulk of Cd3As2 can support sizable linear quantum magnetoresistance even up to room temperature.

cond-mat.mtrl-sci↗

arXiv2014

Wilson-Loop Characterization of Inversion-Symmetric Topological Insulators

The ground state of translationally-invariant insulators comprise bands which can assume topologically distinct structures. There are few known examples where this distinction is enforced by a point-group symmetry alone. In this paper we show that 1D and 2D insulators with the simplest point-group symmetry - inversion - have a $Z^{\geq}$ classification. In 2D, we identify a relative winding number that is solely protected by inversion symmetry. By analysis of Berry phases, we show that this invariant has similarities with the first Chern class (of time-reversal breaking insulators), but is more closely analogous to the $Z_2$ invariant (of time-reversal invariant insulators). Implications of our work are discussed in holonomy, the geometric-phase theory of polarization, the theory of maximally-localized Wannier functions, and in the entanglement spectrum.

cond-mat.str-el↗