arXiv subjects

Xi Dai

Publications and source records attributed to Xi Dai.

At least 91 records · Page 5Linked to original sources

arXiv2018

CT-X: an efficient continuous-time quantum Monte Carlo impurity solver in Kondo Regime

In the present paper, we present an efficient continuous-time quantum Monte Carlo impurity solver with high acceptance rate at low temperature for multi-orbital quantum impurity models with general interaction. In this hybridization expansion impurity solver, the imaginary time evolution operator for the high energy multiplets, which decays very rapidly with the imaginary time, is approximated by a probability normalized $δ$-function. As the result, the virtual charge fluctuations of $f^{n}\rightarrow f^{n\pm1}$ are well included on the same footing without applying Schrieffer-Wolff transformation explicitly. As benchmarks, our algorithm perfectly reproduces the results for both Coqblin-Schriffeer and Kondo lattice models obtained by CT-J method developed by Otsuki {\it et al}. Furthermore, it allows capturing low energy physics of heavy-fermion materials directly without fitting the exchange coupling $J$ in the Kondo model.

cond-mat.str-el↗

arXiv2018

Antiferromagnetic Chern insulators in non-centrosymmetric systems

We investigate a new class of topological antiferromagnetic (AF) Chern insulators driven by electronic interactions in two-dimensional systems without inversion symmetry. Despite the absence of a net magnetization, AF Chern insulators (AFCI) possess a nonzero Chern number $C$ and exhibit the quantum anomalous Hall effect (QAHE). Their existence is guaranteed by the bifurcation of the boundary line of Weyl points between a quantum spin Hall insulator and a topologically trivial phase with the emergence of AF long-range order. As a concrete example, we study the phase structure of the honeycomb lattice Kane-Mele model as a function of the inversion-breaking ionic potential and the Hubbard interaction. We find an easy $z$-axis $C=1$ AFCI phase and a spin-flop transition to a topologically trivial $xy$-plane collinear antiferromagnet. We propose experimental realizations of the AFCI and QAHE in correlated electron materials and cold atom systems.

cond-mat.str-el↗

arXiv2018

Visualization of electronic topology in ZrSiSe by scanning tunneling microscopy

As emerging topological nodal-line semimetals, the family of ZrSiX (X = O, S, Se, Te) has attracted broad interests in condensed matter physics due to their future applications in spintonics. Here, we apply a scanning tunneling microscopy (STM) to study the structural symmetry and electronic topology of ZrSiSe. The glide mirror symmetry is verified by quantifying the lattice structure of the ZrSe bilayer based on bias selective topographies. The quasiparticle interference analysis is used to identify the band structure of ZrSiSe. The nodal line is experimentally determined at $\sim$ 250 meV above the Fermi level. An extra surface state Dirac point at $\sim$ 400 meV below the Fermi level is also determined. Our STM measurement provides a direct experimental evidence of the nodal-line state in the family of ZrSiX.

cond-mat.mtrl-sci↗

arXiv2017

Non-collinear magnetic structure and multipolar order in Eu$_2$Ir$_2$O$_7$

The magnetic properties of the pyrochlore iridate material Eu$_2$Ir$_2$O$_7$ (5$d^5$) have been studied based on the first principle calculations, where the crystal field splitting $Δ$, spin-orbit coupling (SOC) $λ$ and Coulomb interaction $U$ within Ir 5$d$ orbitals are all playing significant roles. The ground state phase diagram has been obtained with respect to the strength of SOC and Coulomb interaction $U$, where a stable anti-ferromagnetic ground state with all-in/all-out (AIAO) spin structure has been found. Besides, another anti-ferromagnetic states with close energy to AIAO have also been found to be stable. The calculated nonlinear magnetization of the two stable states both have the d-wave pattern but with a $π/4$ phase difference, which can perfectly explain the experimentally observed nonlinear magnetization pattern. Compared with the results of the non-distorted structure, it turns out that the trigonal lattice distortion is crucial for stabilizing the AIAO state in Eu$_2$Ir$_2$O$_7$. Furthermore, besides large dipolar moments, we also find considerable octupolar moments in the magnetic states.

cond-mat.str-el↗

arXiv2017

Instability of Dirac semimetal phase under strong magnetic field

The quantum limit can be easily reached in the Dirac semimetals under the magnetic field, which will lead to some exotic many-body physics due to the high degeneracy of the topological zeroth Landau bands (LBs). By solving the effective Hamiltonian, which is derived by tracing out the high energy degrees of freedom, at the self-consistent mean field level, we have systematically studied the instability of Dirac semimetal under a strong magnetic field. A charge density wave (CDW) phase and a polarized nematic phase formed by "exciton condensation" are predicted as the ground state for the tilted and untilted bands, respectively. Furthermore, we propose that, distinguished from the CDW phase, the polarized nematic phase can be identified in experiments by anisotropic transport and Raman scattering.

cond-mat.mtrl-sci↗

arXiv2017

Lifshitz transition mediated electronic transport anomaly in bulk ZrTe5

Zirconium pentatelluride ZrTe$_5$, a fascinating topological material platform, hosts exotic chiral fermions in its highly anisotropic three-dimensional Dirac band and holds great promise advancing the next-generation information technology. However, the origin underlying its anomalous resistivity peak has been under debate for decades. Here we provide transport evidence substantiating the anomaly to be a direct manifestation of a Lifshitz transition in the Dirac band with an ultrahigh carrier mobility exceeding 3$\times$10$^5$ cm$^2$ V$^{-1}$ s$^{-1}$. We demonstrate that the Lifshitz transition is readily controllable by means of carrier doping, which sets the anomaly peak temperature $T_p$. $T_p$ is found to scale approximately as $n_H^{0.27}$, where the Hall carrier concentration $n_H$ is linked with the Fermi level by $ε_F$ $\propto$ $n_H^{1/3}$ in a linearly dispersed Dirac band. This relation indicates $T_p$ monotonically increases with $ε_F$, which serves as an effective knob for fine tuning transport properties in pentatelluride-based Dirac semimetals.

cond-mat.mtrl-sci↗

arXiv2016

Topological nodal line semimetals

We review the recent, mainly theoretical, progress in the study of topological nodal line semimetals in three dimensions. In these semimetals, the conduction and the valence bands cross each other along a one-dimensional curve in the three-dimensional Brillouin zone, and any perturbation that preserves a certain symmetry group (generated by either spatial symmetries or time-reversal symmetry) cannot remove this crossing line and open a full direct gap between the two bands. The nodal line(s) is hence topologically protected by the symmetry group, and can be associated with a topological invariant. In this Review, (i) we enumerate the symmetry groups that may protect a topological nodal line; (ii) we write down the explicit form of the topological invariant for each of these symmetry groups in terms of the wave functions on the Fermi surface, establishing a topological classification; (iii) for certain classes, we review the proposals for the realization of these semimetals in real materials and (iv) we discuss different scenarios that when the protecting symmetry is broken, how a topological nodal line semimetal becomes Weyl semimetals, Dirac semimetals and other topological phases and (v) we discuss the possible physical effects accessible to experimental probes in these materials.

cond-mat.mes-hall↗

arXiv2016

Detecting Chiral Magnetic Effect by Lattice Dynamics

In the present paper, we propose that the chiral magnetic effect, the direct consequence of the presence of Weyl points in the band structure, can be detected by its coupling to certain phonon modes, which behave like pseudo scalars under point group transformations. Such coupling can generate resonance between intrinsic plasmon oscillation and the corresponding phonon modes, leading to dramatic modification of the optical response by the external magnetic field, which provides a new way to study chiral magnetic effect by optical measurements.

cond-mat.mtrl-sci↗

arXiv2016

Topological Phase Transition and Chiral-Anomaly Driven Negative Magneto-Resistance in Bulk Black Phosphorus

We report the anisotropic magneto-transport measurement on a non-compound band semiconductor black phosphorus (BP) with magnetic field B up to 16 Tesla applied in both perpendicular and parallel to electric current I under hydrostatic pressures. The BP undergoes a topological Lifshitz transition from band semiconductor to a zero-gap Dirac semimetal state, characterized by a weak localization-weak antilocaliation transition at low magnetic fields and the emergence of a nontrivial Berry Phase of detected by SdH magneto-oscillations in magnetoresistance curves. In the transition region, we observe a pressure-dependent negative MR only in the B//I configuration. This negative longitudinal MR is attributed to the Adler-Bell-Jackiw anomaly (topological E$\cdot$B term) in the presence of weak antilocalization corrections.

cond-mat.mtrl-sci↗

arXiv2016

Heavy Weyl fermion state in CeRu$_4$Sn$_6$

A new type of topological state in strongly corrected condensed matter systems, heavy Weyl fermion state, has been found in a heavy fermion material CeRu$_4$Sn$_6$, which has no inversion symmetry. Both two different types of Weyl points, type I and II, can be found in the quasi-particle band structure obtained by the LDA+Guztwiller calculations, which can treat the strong correlation effects among the f-electrons from Cerium atoms. The surface calculations indicate that the topologically protected Fermi arc states exist on the (010) but not on the (001) surfaces.

cond-mat.str-el↗

arXiv2016

Topological Nodal Line Semimetals in CaP3 family of materials

We propose that CaP3 family of materials, which include CaP3, CaAs3, SrP3, SrAs3 and BaAs3 can host a three-dimensional topological nodal line semimetal states. Based on first-principle calculations and kp model analysis, we show that a closed topological nodal line exists near the Fermi energy, which is protected by the coexistence of time-reversal and spatial inversion symmetry when the band inversion happens. A drumhead-like surface states are also obtained on the c-direction surface of these materials.

cond-mat.mtrl-sci↗

arXiv2016

Hybrid Weyl Semimetal

We construct a tight-binding model realizing one pair of Weyl nodes and three distinct Weyl semimetals. In the type-I (type-II) Weyl semimetal, both nodes belong to type-I (type-II) Weyl nodes. In addition, there exists a novel type, dubbed "hybrid Weyl semimetal", in which one Weyl node is of type-I while the other is of type-II. For the hybrid Weyl semimetal, we further demonstrate the bulk Fermi surfaces and the topologically protected surface states, analyze the unique Landau level structure and quantum oscillation, and discuss the material realization.

cond-mat.mes-hall↗

arXiv2016

MoTe2: A Type-II Weyl Topological Metal

Based on the ab initio calculations, we show that MoTe2, in its low-temperature orthorhombic structure characterized by an X-ray diffraction study at 100 K, realizes 4 type-II Weyl points between the N-th and N+1-th bands, where N is the total number of valence electrons per unit cell. Other WPs and nodal lines between different other bands also appear close to the Fermi level due to a complex topological band structure. We predict a series of strain-driven topological phase transitions in this compound, opening a wide range of possible experimental realizations of different topological semimetal phases. Crucially, with no strain, the number of observable surface Fermi arcs in this material is 2 - the smallest number of arcs consistent with time-reversal symmetry.

cond-mat.mtrl-sci↗

arXiv2016

Experimental Observation of the Quantum Anomalous Hall Effect in a Magnetic Topological Insulator

The quantized version of the anomalous Hall effect has been predicted to occur in magnetic topological insulators, but the experimental realization has been challenging. Here, we report the observation of the quantum anomalous Hall (QAH) effect in thin films of Cr-doped (Bi,Sb)2Te3, a magnetic topological insulator. At zero magnetic field, the gate-tuned anomalous Hall resistance reaches the predicted quantized value of h/e^2,accompanied by a considerable drop of the longitudinal resistance. Under a strong magnetic field, the longitudinal resistance vanishes whereas the Hall resistance remains at the quantized value. The realization of the QAH effect may lead to the development of low-power-consumption electronics.

cond-mat.mes-hall↗

arXiv2016

Co-existence of Weyl Fermion and Massless Triply Degenerate Nodal Points

By using first-principles calculations, we propose that WC-type ZrTe is a new type of topological semimetal (TSM). It has six pairs of chiral Weyl nodes in its first Brillouin zone, but it is distinguished from other existing TSMs by having additional two paris of massless fermions with triply degenerate nodal points as proposed in the isostructural compounds TaN and NbN. The mirror symmetry, three-fold rotational symmetry and time-reversal symmetry require all of the Weyl nodes to have the same velocity vectors and locate at the same energy level. The Fermi arcs on different surfaces are shown, which may be measured by future experiments. It demonstrates that the "material universe" can support more intriguing particles simultaneously.

cond-mat.mes-hall↗

arXiv2016

Topological Semimetals with Triply Degenerate Nodal Points in θ-phase Tantalum Nitride

Using first-principles calculation and symmetry analysis, we propose that θ-TaN is a topological semimetal having a new type of point nodes, i.e., triply degenerate nodal points. Each node is a band crossing between degenerate and non-degenerate bands along the high-symmetry line in the Brillouin zone, and is protected by crystalline symmetries. Such new type of nodes will always generate singular touching points between different Fermi surfaces and 3D spin texture around them. Breaking the crystalline symmetry by external magnetic field or strain leads to various of topological phases. By studying the Landau levels under a small field along $c$-axis, we demonstrate that the system has a new quantum anomaly that we call "helical anomaly".

cond-mat.mtrl-sci↗

arXiv2016

Spectroscopic Evidence of Type II Weyl Semimetal State in WTe2

Quantum topological materials, exemplified by topological insulators, three-dimensional Dirac semimetals and Weyl semimetals, have attracted much attention recently because of their unique electronic structure and physical properties. Very lately it is proposed that the three-dimensional Weyl semimetals can be further classified into two types. In the type I Weyl semimetals, a topologically protected linear crossing of two bands, i.e., a Weyl point, occurs at the Fermi level resulting in a point-like Fermi surface. In the type II Weyl semimetals, the Weyl point emerges from a contact of an electron and a hole pocket at the boundary resulting in a highly tilted Weyl cone. In type II Weyl semimetals, the Lorentz invariance is violated and a fundamentally new kind of Weyl Fermions is produced that leads to new physical properties. WTe2 is interesting because it exhibits anomalously large magnetoresistance. It has ignited a new excitement because it is proposed to be the first candidate of realizing type II Weyl Fermions. Here we report our angle-resolved photoemission (ARPES) evidence on identifying the type II Weyl Fermion state in WTe2. By utilizing our latest generation laser-based ARPES system with superior energy and momentum resolutions, we have revealed a full picture on the electronic structure of WTe2. Clear surface state has been identified and its connection with the bulk electronic states in the momentum and energy space shows a good agreement with the calculated band structures with the type II Weyl states. Our results provide spectroscopic evidence on the observation of type II Weyl states in WTe2. It has laid a foundation for further exploration of novel phenomena and physical properties in the type II Weyl semimetals.

cond-mat.mes-hall↗

arXiv2016

Electronic Evidence for Type II Weyl Semimetal State in MoTe2

Topological quantum materials, including topological insulators and superconductors, Dirac semimetals and Weyl semimetals, have attracted much attention recently for their unique electronic structure, spin texture and physical properties. Very lately, a new type of Weyl semimetals has been proposed where the Weyl Fermions emerge at the boundary between electron and hole pockets in a new phase of matter, which is distinct from the standard type I Weyl semimetals with a point-like Fermi surface. The Weyl cone in this type II semimetals is strongly tilted and the related Fermi surface undergos a Lifshitz transition, giving rise to a new kind of chiral anomaly and other new physics. MoTe2 is proposed to be a candidate of a type II Weyl semimetal; the sensitivity of its topological state to lattice constants and correlation also makes it an ideal platform to explore possible topological phase transitions. By performing laser-based angle-resolved photoemission (ARPES) measurements with unprecedentedly high resolution, we have uncovered electronic evidence of type II semimetal state in MoTe2. We have established a full picture of the bulk electronic states and surface state for MoTe2 that are consistent with the band structure calculations. A single branch of surface state is identified that connects bulk hole pockets and bulk electron pockets. Detailed temperature-dependent ARPES measurements show high intensity spot-like features that is ~40 meV above the Fermi level and is close to the momentum space consistent with the theoretical expectation of the type II Weyl points. Our results constitute electronic evidence on the nature of the Weyl semimetal state that favors the presence of two sets of type II Weyl points in MoTe2.

cond-mat.mtrl-sci↗