Large Dynamical Axion Field in Topological Antiferromagnetic Insulator Mn_2Bi_2Te_5

The dynamical axion field is a new state of quantum matter where the magnetoelectric response couples strongly to its low-energy magnetic fluctuations.It is fundamentally different from an axion insulator with a static quantized magnetoelectric response.The dynamical axion field exhibits many exotic phenomena such as axionic polariton and axion instability.However,these effects have not been experimentally confirmed due to the lack of proper topological magnetic materials.Combining analytic mode...     

Topological superconductivity in a Bi_2Te_3/NbSe_2 heterostructure:A review

Topological superconductors carry globally protected gapless boundary excitations, which are robust under local perturbations, and thus exhibit both fundamental and applicational importance. An unconventional pairing with p-wave symmetry, such as a nontrivial topology in the superconductor's wavefunction, is required to generate a non-zero Berry phase. Until now, the Bi_2 Te_3/2 H–NbSe_2 heterostructure has proven to be a practical way to realize a topological superconductor in real materials. This complex system, where odd numbers of spin-momentum locked Dirac cone surface states on the topological insulator Bi_2 Te_3 become superconducting, induced by the proximate effect from the underlying s-wave superconductor 2 H–NbSe_2, realizes an effective two-dimensional(2 D) spinless p_x + i p_y topological superconductor. In this review article, we summarize the recent experimental progress of the successful synthesis of Bi_2 Te_3/2 H–NbSe_2 heterostructures using molecular beam epitaxy, determining the thickness limit of the heterostructure, detecting the long thought Majorana quasiparticle inside a magnetic vortex core state by means of scanning tunneling microscopy and demonstration of the unique spatial and spin properties of a Majorana zero mode.     

Modulation of the Dirac Point Band Gap in the Antiferromagnetic Topological Insulator MnBi2Te4 due to the Surface Potential Gradient Change

Journal of Experimental and Theoretical Physics - The possibility of significant modulation of the band gap open at the Dirac point (DP) in the range from 15 to 55 meV for different samples of the...     

Anomalous Hall Effect in Layered Ferrimagnet MnSb_2Te_4

We report on low-temperature electron transport properties of MnSb_2Te_4,a candidate of ferrimagnetic Weyl semimetal.Long-range magnetic order is manifested as a nearly square-shaped hysteresis loop in the anomalous Hall resistance,as well as sharp jumps in the magnetoresistance.At temperatures below 4K,a lnT-type upturn appears in the temperature dependence of longitudinal resistance,which can be attributed to the electron-electron interaction(EEI),since the weak localization can be excluded by the temperature dependence of magnetoresistance.Although the anomalous Hall resistance exhibits a similar lnT-type upturn in the same temperature range,such correction is absent in the anomalous Hall conductivity.Our work demonstrates that MnSb_2Te_4 microflakes provide an ideal system to test the theory of EEI correction to the anomalous Hall effect.     

Gate Tunable Supercurrent in Josephson Junctions Based on Bi_2Te_3 Topological Insulator Thin Films

We report transport measurements on Josephson junctions consisting of Bi_2Te_3 topological insulator(TI) thin films contacted by superconducting Nb electrodes.For a device with junction length L=134 nm,the critical supercurrent I_c can be modulated by an electrical gate which tunes the carrier type and density of the TI film.I_c can reach a minimum when the TI is near the charge neutrality regime with the Fermi energy lying close to the Dirac point of the surface state.In the p-type regime the Josephson current can be well described by a short ballistic junction model.In the n-type regime the junction is ballistic at 0.7 K T 3.8 K while for T 0.7 K the diffusive bulk modes emerge and contribute a larger I_c than the ballistic model.We attribute the lack of diffusive bulk modes in the p-type regime to the formation of p-n junctions.Our work provides new clues for search of Majorana zero mode in TI-based superconducting devices.     

Experimental Evidence of Topological Surface States in Mg_3Bi_2 Films Grown by Molecular Beam Epitaxy

Nodal line semimetal(NLS) is a new quantum state hosting one-dimensional closed loops formed by the crossing of two bands. The so-called type-Ⅱ NLS means that these two crossing bands have the same sign in their slopes along the radial direction of the loop, which requires that the crossing bands are either right-tilted or left-tilted at the same time. According to the theoretical prediction, Mg_3Bi_2 is an ideal candidate for studying the type-Ⅱ NLS by tuning its spin-orbit coupling(SOC). High-quality Mg3 Bi2 films are grown by molecular beam epitaxy(MBE). By in-situ angle resolved photoemission spectroscopy(ARPES), a pair of surface resonance bands around the■ point are clearly seen. This shows that Mg_3Bi_2 films grown by MBE are Mg(1)-terminated by comparing the ARPES spectra with the first principles calculations results. Moreover, the temperature dependent weak anti-localization effect in Mg_3Bi_2 films is observed under magneto-transport measurements, which shows clear two-dimensional(2 D) e-e scattering characteristics by fitting with the Hikami–Larkin–Nagaoka model. Therefore, by combining with ARPES, magneto-transport measurements and the first principles calculations, this work proves that Mg_3Bi_2 is a semimetal with topological surface states. This paves the way for Mg_3Bi_2 to be used as an ideal material platform to study the exotic features of type-Ⅱ nodal line semimetals and the topological phase transition by tuning its SOC.     

Superconductivity and Charge Density Wave in Iodine-Doped CuIr_2Te_4

We report a systematic investigation on the evolution of the structural and physical properties,including the charge density wave(CDW) and superconductivity of the polycrystalline CuIr_2Te_(4-x)I_x for 0.0 ≤x ≤ 1.0.Xray diffraction results indicate that both of a and c lattice parameters increase linearly when 0.0 ≤ x ≤ 1.0.The resistivity measurements indicate that the CDW is destabilized with slight x but reappears at x≥0.9 with very high T_(CDW).Meanwhile,the superconducting transition temperature T_c enhances as x increases and reaches a maximum value of around 2.95 K for the optimal composition CuIr_2Te_(1.9)I_(0.1) followed by a slight decrease with higher iodine doping content.The specific heat jump(ΔC/γT_c) for the optimal composition CuIr_2Te_(3.9)I_(0.1) is approximately 1.46,which is close to the Bardeen-Cooper-Schrieffer value of 1.43,indicating that it is a bulk superconductor.The results of thermodynamic heat capacity measurements under different magnetic fields |C_p(T,H)],magnetization M(T,H) and magneto-transport ρ(T,H) measurements further suggest that CuIr_2Te_(4-x)I_x bulks are type-Ⅱ superconductors.Finally,an electronic phase diagram for this CuIr_2Te_(4-x)I_x system has been constructed.The present study provides a suitable material platform for further investigation of the interplay of the CDW and superconductivity.     

Mixed Type of the Magnetic Order in Intrinsic Magnetic Topological Insulators Mn(Bi,Sb)2Te4

JETP Letters - It is known that Mn(Bi1 – xSbx)2Te4 is an intrinsic magnetic topological insulator, where the Dirac point can be localized at the Fermi level by substituting Bi atoms for Sb...     

Long-Time Magnetic Relaxation in Antiferromagnetic Topological Material EuCd_2As_2

Magnetic topological materials have attracted much attention due to the correlation between topology and magnetism. Recent studies suggest that EuCd_2As_2 is an antiferromagnetic topological material. Here by carrying out thorough magnetic, electrical and thermodynamic property measurements, we discover a long-time relaxation of the magnetic susceptibility in EuCd_2As_2. The(001) in-plane magnetic susceptibility at 5 K is found to continuously increase up to ~10% over the time of ~14 hours. The magnetic relaxation is anisotropic and strongly depends on the temperature and the applied magnetic field. These results will stimulate further theoretical and experimental studies to understand the origin of the relaxation process and its effect on the electronic structure and physical properties of the magnetic topological materials.     

Coalescence of Andreev Bound States on the Surface of a Chiral Topological Semimetal

JETP Letters - We experimentally investigate the magnetic field dependence of Andreev transport through a region of proximity-induced superconductivity in the CoSi topological chiral semimetal....     

Dynamics of Electronic States and Magnetoabsorption in 3D Topological Insulators in a Quantizing Magnetic Field

Quantum states have been calculated analytically; the dynamics of a wave packet in a magnetic field has been investigated, and the optical absorption coefficient has been calculated for surface states in 3D topological insulators of the Bi₂Te₃ family. We have detected a qualitative effect of the hexagonal warping of the spectrum on the structure of wavefunctions at the Landau levels, its manifestation in the features of the wave packet dynamics in a quantizing magnetic field, as well as in the frequency dependence of the optical absorption coefficient, in which new peaks that are absent in the isotropic model of the spectrum appear depending on the polarization of the incident wave. The effects considered here can be manifested in the optical and transport experiments with topological insulators, which makes it possible to determine the parameters of their band structure.     

凝聚体中的拓扑量子态

探寻拓扑上非平庸的凝聚体物质状态,特别是其电子结构和输运性质,是当前凝聚体物理 学领域非常重要的前沿研究方向。本文讨论的大多数主题都与电子波函数的拓扑性质有关。全文 除简短的引言外,包括拓扑量子现象、各种拓扑相、拓扑性准粒子的异常输运性质、拓扑性集 体激发和耦合激发,以及继续发展的拓扑量子态研究五个章节。这些章节着重反映拓扑量子态研 究的各个侧面,汇总起来方可以凸显凝聚体中拓扑量子态的全貌。     

相变记录介质GeSb2Te4薄膜的光学性质,晶体结构和晶化特性

采用高频磁控溅射制备了GeSb_2Te_4薄膜.系统地研究了真空热退火对GeSb_2Te_4薄膜光学性质和晶体结构的影响.对非晶态GeSb_2Te_4薄膜进行了热分析,给出了其结晶过程的动力学参数.     

Topological surface States in three-dimensional magnetic insulators

An electron moving in a magnetically ordered background feels an effective magnetic field that can be both stronger and more rapidly varying than typical externally applied fields. One consequence is that insulating magnetic materials in three dimensions can have topologically nontrivial properties of the effective band structure. For the simplest case of two bands, these "Hopf insulators" are characterized by a topological invariant as in quantum Hall states and Z2 topological insulators, but instead of a Chern number or parity, the underlying invariant is the Hopf invariant that classifies maps from the three-sphere to the two-sphere. This Letter gives an efficient algorithm to compute whether a given magnetic band structure has nontrivial Hopf invariant, a double-exchange-like tight-binding model that realizes the nontrivial case, and a numerical study of the surface states of this model.     

Topological Quantization of the Magnetic Flux

The quantization of the magnetic flux in superconducting rings is studied in the frame of a topological model of electromagnetism that gives a topological formulation of electric charge quantization. It turns out that the model also embodies a topological mechanism for the quantization of the magnetic flux with the same relation between the fundamental units of magnetic charge and flux as there is between the Dirac monopole and the fluxoid.     

Topological Josephson Junction in Transverse Magnetic Field

JETP Letters - We consider Majorana zero modes in a Josephson junction on top of a topological insulator in transverse magnetic field. Majorana zero modes reside at periodically located nodes of...     

Magnetic Properties of Iron-Doped Bi2Se3, a Topological Insulator

Physics of the Solid State - We study the dc magnetization of a Bi1.94Fe0.06Se3 single crystal in the temperature range of 1.9 to 400 K in applied magnetic fields up to 70 kOe for two...