arXiv subjects

Simin Nie

Publications and source records attributed to Simin Nie.

32 records · Page 2Linked to original sources

arXiv2020

Mapping the Dirac fermions in intrinsic antiferromagnetic topological insulators (MnBi$_2$Te$_4$)(Bi$_2$Te$_3$)$_n$ (n=0, 1)

Topological surface states with intrinsic magnetic ordering in the MnBi$_2$Te$_4$(Bi$_2$Te$_3$)$_n$ compounds have been predicted to host rich topological phenomena including quantized anomalous Hall effect and axion insulator state. Here we use scanning tunneling microscopy to image the surface Dirac fermions in MnBi$_2$Te$_4$ and MnBi$_4$Te$_7$. We have determined the energy dispersion and helical spin texture of the surface states through quasiparticle interference patterns far above Dirac energy, which confirms its topological nature. Approaching the Dirac point, the native defects in the MnBi$_2$Te$_4$ septuple layer give rise to resonance states which extend spatially and potentially hinder the detection of a mass gap in the spectra. Our results demonstrate that regulating defects is essential to realize exotic topological states at higher temperatures in these compounds.

cond-mat.mtrl-sci↗

arXiv2019

Weyl Semimetals with $S_4$ symmetry

In the time-reversal-breaking centrosymmetric systems, the appearance of Weyl points can be guaranteed by an odd number of all the even/odd parity occupied bands at eight inversion-symmetry-invariant momenta. Here, based on symmetry analysis and first-principles calculations, we demonstrate that for the time-reversal-invariant systems with $S_4$ symmetry, the Weyl semimetal phase can be characterized by the inequality between a well-defined invariant $η$ and an $S_4$ indicator $z_2$. By applying this criterion, we find that some candidates, previously predicted to be topological insulators, are actually Weyl semimetals in the noncentrosymmetric space group with $S_4$ symmetry. Our first-principles calculations show that four pairs of Weyl points are located in the $k_{x,y}$ = 0 planes, with each plane containing four same-chirality Weyl points. An effective model has been built and captures the nontrivial topology in these materials. Our strategy to find the Weyl points by using symmetry indicators and invariants opens a new route to search for Weyl semimetals in the time-reversal-invariant systems.

cond-mat.mtrl-sci↗

arXiv2019

Topological Electronic States in HfRuP Family Superconductors

Based on the first-principles calculations and experimental measurements, we report that the hexagonal phase of ternary transition metal pnictides TT'X (T=Zr, Hf; T'=Ru; X=P, As), which are well-known noncentrosymmetric superconductors with relatively high transition temperatures, host nontrivial bulk topology. Before the superconducting phase transition, we find that HfRuP belongs to a Weyl semimetal phase with 12 pairs of type-II Weyl points, while ZrRuAs, ZrRuP and HfRuAs belong to a topological crystalline insulating phase with trivial Fu-Kane $\mathbb Z_2$ indices but $nontrivial$ mirror Chern numbers. High-quality single crystal samples of the noncentrosymmetric superconductors with these two different topological states have been obtained and the superconductivity is verified experimentally. The wide-range band structures of ZrRuAs have been identified by ARPES and reproduced by theoretical calculations. Combined with intrinsic superconductivity, the nontrivial topology of the normal state may generate unconventional superconductivity in both bulk and surfaces. Our findings could largely inspire the experimental searching for possible topological superconductivity in these compounds.

cond-mat.mtrl-sci↗

arXiv2019

Topological Crystalline Insulators with $C_2$ Rotation Anomaly

Based on first-principles calculations and symmetry-based indicator analysis, we find a class of topological crystalline insulators (TCIs) with $C_2$ rotation anomaly in a family of Zintl compounds, including $\mathrm{Ba}_{3}\mathrm{Cd}_{2}\mathrm{As}_{4}$, $\mathrm{Ba}_{3}\mathrm{Zn}_{2}\mathrm{As}_{4}$ and $\mathrm{Ba}_{3}\mathrm{Cd}_{2}\mathrm{Sb}_{4}$. The nontrivial band topology protected by coexistence of $C_2$ rotation symmetry and time-reversal symmetry $T$ leads to two surface Dirac cones at generic momenta on both top and bottom surfaces perpendicular to the rotation axis. In addition, ($d-2$)-dimensional helical hinge states are also protected along the hinge formed by two side surfaces parallel with the rotation axis. We develop a method based on Wilson loop technique to prove the existence of these surface Dirac cones due to $C_2$ anomaly and precisely locate them as demonstrated in studying these TCIs. The helical hinge states are also calculated. Finally, we show that external strain can be used to tune topological phase transitions among TCIs, strong Z$_2$ topological insulators and trivial insulators.

cond-mat.mtrl-sci↗

arXiv2019

Magnetic semimetals and quantized anomalous Hall effect in EuB6

Exploration of the novel relationship between magnetic order and topological semimetals has received enormous interest in a wide range of both fundamental and applied research. Here we predict that soft ferromagnetic (FM) material EuB6 can achieve multiple topological semimetal phases by simply tuning the direction of the magnetic moment. Explicitly, EuB6 is a topological nodal-line semimetal when the moment is aligned along the [001] direction, and it evolves into a Weyl semimetal with three pairs of Weyl nodes by rotating the moment to the [111] direction. Interestingly, we identify a novel semimetal phase featuring the coexistence of a nodal line and Weyl nodes with the moment in the [110] direction. Topological surface states and anomalous Hall conductivity, which is sensitive to the magnetic order, have been computed and are expected to be experimentally observable. Large-Chern-number quantum anomalous Hall effect can be realized in its [111]-oriented quantum-well structure.

cond-mat.mtrl-sci↗

arXiv2019

Topological nodal-line semimetals in ferromagnetic rare-earth-metal monohalides

Topological semimetals, extending the topological classification from insulators to metals, have greatly enriched our understanding of topological states in condensed matter. This is particularly true for topological nodal-line semimetals (TNLSs). In the present paper, we identify layered materials as promising candidates for hosting TNLSs. Based on first-principles calculations and effective model analysis, we propose that layered ferromagnetic rare-earth-metal monohalides LnX (Ln=La, Gd; X=Cl, Br) exhibit long pursued topological phases. Specifically, single-layer LaX and single-layer GdX are ideal two-dimensional (2D) Weyl semimetals and large-gap 2D quantum anomalous Hall insulators (QAHIs), with band gaps up to 61 meV, respectively. In addition, 3D LaX and 3D GdX are TNLSs with a pair of mirror-symmetry protected nodal lines and 3D weak QAHIs, respectively. The nodal lines in 3D LaX extending through the whole Brillouin zone (BZ) are fairly robust against strong spin-orbit coupling (SOC) and located close to the Fermi level, providing a novel platform toward exploring the exotic properties in nodal-line fermions as well as related device designs.

cond-mat.mtrl-sci↗

arXiv2018

Dirac semimetal in type IV magnetic space group

Analogues of the elementary particles, Dirac fermions in condensed matter have received extensive attention for both scientific interest and device applications. In this work, we generalize the concept of Dirac semimetal (DSM) to the magnetic space groups (MSGs), and define a new category of DSM in type IV MSGs, which is protected by the antiunitarity of the product of the inversion symmetry and the nonsymmophic time-reversal symmetry. Moreover, we propose the interlayer antiferromagnetic (AFM) EuCd2As2 as a promising candidate with only one pair of Dirac points at the Fermi level. Many exotic topological states, such as the triple point semimetal and the AFM topological insulator holding of the half-quantum Hall effect, can be derived from such AFM DSMs by breaking certain symmetry, providing an ideal platform to study topological phase transitions. Our results extend the range of DSM, and open a new way to study the interplay between the DSM and other exotic topological states.

cond-mat.mtrl-sci↗

arXiv2018

Topological phases in the TaSe3 compound

Based on first-principles calculations, we show that stoichiometric TaSe3, synthesized in space group P21/m, belongs to a three-dimensional (3D) strong topological insulator (TI) phase with Z2 invariants (1;100). The calculated surface spectrum shows clearly a single Dirac cone on surfaces, with helical spin texture at a constant energy contour. To check the stability of the topological phase, strain effects have been systematically investigated, showing that many topological phases survive in a wide range of the strains along both the a- and c-axes, such as strong TI (STI), weak TI (WTI) and Dirac semimetal phases. TaSe3 provides us an ideal platform for experimental study of topological phase transitions. More interestingly, since superconductivity in TaSe3 has been reported for a long time, the co-existence of topological phases and superconducting phase suggests that TaSe3 is a realistic system to study the interplay between topological and superconducting phases in the future.

cond-mat.mtrl-sci↗

arXiv2017

Topological semimetal in honeycomb lattice LnSI

Recognized as elementary particles in the standard model,Weyl fermions in condensed matter have received growing attention. However, most of the previously reportedWeyl semimetals exhibit rather complicated electronic structures that, in turn, may have raised questions regarding the underlying physics. Here, we report for the first time promising topological phases that can be realized in specific honeycomb lattices, including ideal Weyl semimetal structures, 3D strong topological insulators, and nodal-line semimetal configurations. In particular, we highlight a novel semimetal featuring both Weyl nodes and nodal lines. Guided by this model, we demonstrated that GdSI the long perceived ideal Weyl semimetal has two pairs ofWeyl nodes residing at the Fermi level, and that LuSI (YSI) is a 3D strong topological insulator with the right-handed helical surface states. Our work provides a new mechanism to study topological semimetals, and proposes a platform towards exploring the physics of Weyl semimetals as well as related device designs.

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↗

arXiv2016

Electronic Evidence of Temperature-Induced Lifshitz Transition and Topological Nature in ZrTe5

The topological materials have attracted much attention recently. While three-dimensional topological insulators are becoming abundant, two-dimensional topological insulators remain rare, particularly in natural materials. ZrTe5 has host a long-standing puzzle on its anomalous transport properties; its underlying origin remains elusive. Lately, ZrTe5 has ignited renewed interest because it is predicted that single-layer ZrTe5 is a two-dimensional topological insulator and there is possibly a topological phase transition in bulk ZrTe5. However, the topological nature of ZrTe5 is under debate as some experiments point to its being a three-dimensional or quasi-two-dimensional Dirac semimetal. Here we report high-resolution laser-based angle-resolved photoemission measurements on ZrTe5. The electronic property of ZrTe5 is dominated by two branches of nearly-linear-dispersion bands at the Brillouin zone center. These two bands are separated by an energy gap that decreases with decreasing temperature but persists down to the lowest temperature we measured (~2 K). The overall electronic structure exhibits a dramatic temperature dependence; it evolves from a p-type semimetal with a hole-like Fermi pocket at high temperature, to a semiconductor around ~135 K where its resistivity exhibits a peak, to an n-type semimetal with an electron-like Fermi pocket at low temperature. These results indicate a clear electronic evidence of the temperature-induced Lifshitz transition in ZrTe5. They provide a natural understanding on the underlying origin of the resistivity anomaly at ~135 K and its associated reversal of the charge carrier type. Our observations also provide key information on deciphering the topological nature of ZrTe5 and possible temperature-induced topological phase transition.

cond-mat.mtrl-sci↗

arXiv2015

Two-Dimensional Oxide Topological Insulator With Iron-Pnictide Superconductor LiFeAs Structure

By using first-principles calculations, we propose that ZrSiO can be looked as a three-dimensional (3D) oxide weak topological insulator (TI) and its single layer is a long-sought-after 2D oxide TI with a band gap up to 30 meV. Calculated phonon spectrum of the single layer ZrSiO indicates it is dynamically stable and the experimental achievements in growing oxides with atomic precision ensure that it can be readily synthesized. This will lead to novel devices based on TIs, the so called "topotronic" devices, operating under room-temperature and stable when exposed in the air. Thus, a new field of "topotronics" will arise. Another intriguing thing is this oxide 2D TI has the similar crystal structure as the well-known iron-pnictide superconductor LiFeAs. This brings great promise in realizing the combination of superconductor and TI, paving the way to various extraordinary quantum phenomena, such as topological superconductor and Majorana modes. We further find that there are many other isostructural compounds hosting the similar electronic structure and forming a $WHM$-family with $W$ being Zr, Hf or La, $H$ being group IV or group V element, and $M$ being group VI one.

cond-mat.mtrl-sci↗

arXiv2015

Band gap anomaly and topological properties in lead chalcogenides

Band gap anomaly is a well-known issue in lead chalcogenides PbX (X=S, Se, Te, Po). Combining ab initio calculations and tight-binding (TB) method, we have studied the band evolution in PbX, and found that the band gap anomaly in PbTe is mainly related to the high onsite energy of Te 5s orbital and the large s-p hopping originated from the irregular extended distribution of Te 5s electrons. Furthermore, our calculations show that PbPo is an indirect band gap (6.5 meV) semiconductor with band inversion at L point, which clearly indicates that PbPo is a topological crystalline insulator (TCI). The calculated mirror Chern number and surface states double confirm this conclusion.

cond-mat.mtrl-sci↗