Anisotropic magnetoelastic response in the magnetic Weyl semimetal Co_3Sn_2S_2

Co_3Sn_2S_2 is a recently identified magnetic Weyl semimetal in Shandite compounds. Upon cooling, Co_3Sn_2S_2 undergoes a ferromagnetic transition with c-axis polarized moments(~0.3 μB/Co) around T_C= 175 K, followed by another magnetic anomaly around T_A≈ 140 K. A large intrinsic anomalous Hall effect is observed in the magnetic state below T_C with a maximum of anomalous Hall angle near T_A. Here, we report an elastic neutron scattering on the crystalline lattice of Co_3Sn_2S_2 in a magnetic field up to 10 T. A strongly anisotropic magnetoelastic response is observed, while only a slight enhancement of the Bragg peaks is observed when B//c. The in-plane magnetic field(B//ab) dramatically suppresses the Bragg peak intensity probably by tilting the moments and lattice toward the external field direction. The in-plane magnetoelastic response commences from T_C, and as it is further strengthened below T_A, it becomes nonmonotonic against the field between T_A and T_C because of the competition from another in-plane magnetic order. These results suggest that a magnetic field can be employed to tune the Co_3Sn_2S_2 lattice and its related topological states.     

Spin excitations and spin wave gap in the ferromagnetic Weyl semimetal Co_3Sn_2S_2

We report a comprehensive neutron scattering study on the spin excitations in the magnetic Weyl semimetal Co3Sn2S2 with a quasi-two-dimensional structure.Both in-plane and out-of-plane dispersions of the spin waves were revealed in the ferromagnetic state.Similarly,dispersive but damped spin excitations were found in the paramagnetic state.The effective exchange interactions were estimated using a semi-classical Heisenberg model to consistently reproduce the experimental TCand spin stiffness.However,a full spin wave gap below Eg=2.3 meV was observed at T=4 K.This value was considerably larger than the estimated magnetic anisotropy energy(~0.6 meV),and its temperature dependence indicated a significant contribution from the Weyl fermions.These results suggest that Co3Sn2S2 is a three-dimensional correlated system with a large spin stiffness,and the low-energy spin dynamics can interplay with the topological electron states.     

Electronic structures of vacancies in Co_3Sn_2S_2

Co_3Sn_2S_2 has attracted a lot of attention for its multiple novel physical properties, including topological nontrivial surface states, anomalous Hall effect, and anomalous Nernst effect. Vacancies, which play important roles in functional materials, have attracted increasing research attention. In this paper, by using density functional theory calculations, we first obtain band structures and magnetic moments of Co_3Sn_2S_2 with exchange–correlation functionals at different levels. It is found that the generalized gradient approximation gives the positions of Weyl points consistent with experiments in bulk Co_3Sn_2S_2. We then investigate the electronic structures of defects on surfaces with S and Sn terminations which have been observed in experiments. The results show that the single sulfur vacancy on the S-terminated surface introduces localized bond states inside the bandgap near the Fermi level. For di-and tri-sulfur vacancies, the localized defect states hybridize with neighboring ones, forming bonding states as well as anti-bonding states. The Sn vacancy on the Sn-terminated surface also introduces localized bond states, which are merged with the valence bands. These results provide a reference for future experimental investigations of vacancies in Co_3Sn_2S_2.     

Critical behavior and magnetocaloric effect in magnetic Weyl semimetal candidate Co_(2-x)ZrSn

We investigate the critical exponents and magnetocaloric effect of Co_(2-x)ZrSn polycrystal. The Co_(2-x)ZrSn undergoes a second-order ferromagnetism phase transition around the Curie temperature of Tc ～280 K. The critical behavior in the vicinity of the magnetic phase transition has been investigated by using modified Arrott plot and Kouvel–Fisher methods.The obtained critical exponents, β, γ, and δ can be well described by the scaling theory. The determined exponents of Co_(2-x)ZrSn obey the mean-field model with a long-range magnetic interaction. In addition, the maximum magnetic entropy change-?Sm ax M of Co_(2-x)ZrSn is about 0.57 J·kg~(-1)·K~(-1) and the relative cooling power(RCP) is 14.68 J·kg~(-1) at 50 kOe(1 Oe = 79.5775 A·m~(-1)).     

Scanning tunneling microscopic investigation on morphology of magnetic Weyl semimetal YbMnBi_2

YbMnBi_2 is a recently discovered time-reversal-symmetry breaking type-Ⅱ Weyl semimetal.However, as a representation of the new category of topological matters, the scanning tunneling microcopy(STM) results on such important material are still absent.Here, we report the STM investigations on the morphology of vacuum cleaved single crystalline YbMnBi_2 samples.A hill and valley type of topography is observed on the YbMnBi_2 surface, which is consistent with the non-layer nature of its crystal structure.Analysis of STM images yields the information of the index of the vicinal surface.Our results here lay a playground of future atomic scale research on YbMnBi_2.     

Electronic properties of a Weyl semimetal in crossed magnetic and electric fields

The study of Weyl semimetals is one of the most challenging problems of condensed matter physics. These materials exhibit interesting properties in a magnetic field. In this work, we investigate the Landau bands and the density of states (DOS) oscillations in a Weyl semimetal in crossed magnetic and electric fields. An expression is obtained for the energy spectrum of the system using the following three different methods: an algebraic approach, a Lorentz shift-based approach, and a quasi-classical approach. It is interesting that the energy spectrum calculated in terms of the quasi-classical approach coincides with the spectrum obtained using the microscopic approaches. An electric field is shown to change the Landau bands radically. In addition, the classical motion of a three-dimensional Dirac fermion in crossed fields is studied. In the case of a Dirac spectrum, the longitudinal (with respect to magnetic field) component of momentum (p _z ∥ H) is shown to be an oscillating function of the magnetic field. When the electric field is v⊥H/c, the Landau levels collapse and the motion becomes fully linear in an unusual manner. In this case, the wavefunction of bulk states vanishes and only states with p _z = 0 are retained. An electric field affects the character of DOS oscillations. An analytical expression is obtained for the quantum capacitance in crossed fields in the cases of strong and weak electric fields. Thus, an electric field is an additional parameter for adjusting the diamagnetic properties of Weyl semimetals.     

Distinction between critical current effects and intrinsic anomalies in the point-contact Andreev reflection spectra of unconventional superconductors

In this work,we discuss the origin of several anomalies present in the point-contact Andreev reflection spectra of(Li_(1-x)Fe_x)OHFeSe,LiTi_2O_4,and La_(2-x)Ce_xCuO_4.While these features are similar to those stemming from intrinsic superconducting properties,such as Andreev reflection,electron-boson coupling,multigap superconductivity,d-wave and p-wave pairing symmetry,they cannot be accounted for by the modified Blonder–Tinkham–Klapwijk(BTK) model,but require to consider critical current effects arising from the junction geometry.Our results point to the importance of tracking the evolution of the dips and peaks in the differential conductance as a function of the bias voltage,in order to correctly deduce the properties of the superconducting state.     

Impurity-induced states on the surface of a three-dimensional Weyl semimetal

We study theoretically the features of impurity-induced states on the surface of a three-dimensional Weyl semimetal in this work. For calculating the impurity-induced local density of states based on T-matrix formulation, we found that for different Weyl semimetal phases the behaviors of a local impurity exhibit distinguishable prominent features for the surface Fermi arc states. Due to two opposite-directional and -chirality surface currents for a surface, a bound state appears at the unitary limit of scattering intensity near the impurity site. Then the resonance condition for different Weyl semimetal phases and scattering intensity is investigated. Our results can be used to identify distinctive topological phases of Weyl semimetal. Furthermore, the relevance of topological nodal-point and -line systems is discussed. Some relation between our theoretical results and current experimental scheme are also discussed.     

Hybrid Weyl semimetal under crossed electric and magnetic fields: Field tuning of spectrum type

There are two types (WSM-I and WSM-II) of the WSMs. The WSMs of different types have various topological and transport properties. Besides pure WSM-I and WSM-II, there exists a novel type, dubbed “hybrid Weyl semimetal”, which contains the Weyl points of both types. In this Letter we consider the hybrid WSM under crossed magnetic and electric fields. The electromagnetic field induces transition between different types of spectrum in Weyl point (WP). Thus, hybrid phase of the WSM can be tunable using the electromagnetic field. Finally, we proposed a new field-induced type of hybrid WSM in which two different regimes of spectrum coexist. In this case, the spectrum near the first WP corresponds to electric regime (no Landau levels) and the spectrum in the second WP with opposite chirality corresponds to magnetic regime (there are Landau levels).     

Spin–lattice relaxation phenomena in the magnetic state of a suggested Weyl semimetal CeAlGe

ABSTRACT

In this work, we report the results of DC susceptibility, AC susceptibility and related technique, resistivity, transverse and longitudinal magnetoresistance and heat capacity on polycrystalline magnetic semimetal CeAlGe. This compound undergoes antiferromagnetic type ordering around 5.2 K (T¹). Under the application of external magnetic fields, parallel alignment of magnetic moments is favoured above 0.5?T. At low field and temperature, frequency and AC field amplitude response of AC susceptibility indicate the presence of spin–lattice relaxation phenomena. The observation of spin–lattice interaction suggests the presence of the Rashba–Dresselhaus spin–orbit interaction which is associated with inversion and time-reversal symmetry breaking. Additionally, the presence of negative and asymmetric longitudinal magnetoresistance indicates anomalous velocity contribution to the magnetoresistance due to the Rashba–Dresselhaus spin–orbit interaction which is further studied by heat capacity.     

Re_3W的点接触安德烈夫反射谱研究

本文通过对不同晶体结构Re_3W样品的点接触测量和对比研究,证实具有中心对称结构和非中心对称结构的Re_3W都是弱耦合Bardeen-Cooper-Schrieffer超导体,同时发现在两个相表面都可以形成很理想的点接触结,即电子通过界面时受到的非弹性散射很弱.将Re_3W样品置于大气环境近六个月后重新进行测量,仍然能够得到类似的结果,表明Re_3W具有很好的稳定性. Re_3W的这种优良特性,不仅可通过点接触实验得到的参数推算出Re_3W两个相的费米速度,而且提供了一种简单的方法,可以在点接触实验中利用Re_3W来印证针尖材料的费米速度和测量其自旋极化率等.作为尝试,本文用Re_3W/Ni点接触结测量了铁磁性金属Ni的自旋极化率,得到了与前人报道一致的结果.     

新型GeS_2-Ga_2S_3-KI玻璃性能的研究

用传统熔融淬冷法制备了新型硫卤玻璃(100-x)(80GeS2-20Ga2S3)-xKI(x=0,10,20mol%).利用差热分析、可见/近红外吸收光谱、红外透射光谱等技术对准三元硫卤玻璃体系GeS2-Ga2S3-KI的组成、结构和性能关系进行了研究.分析结果表明:GeS2-Ga2S3-KI三元系统玻璃具有较宽的玻璃形成区;当KI的含量为10mol%时,玻璃热稳定性最好;随着KI的加入,玻璃的红外截止波长无明显变化,皆为12.5μm;然而随着局域电位场的增大,玻璃的短波吸收限向短波方向发生了移动,光学带隙存在增大趋势.     

Evidence for pseudogap and phase-coherence gap separation by Andreev reflection experiments in Au/ La 2 - x Sr x CuO 4 point-contact junctions

We present new Au/La_2-xSr_xCuO₄ (LSCO) point-contact conductance measures as a function of voltage and temperature in samples with 0.08 ? x ? 0.2. Andreev reflection features disappear at about the bulk T _c , giving no evidence of gap for T > T _c . The fit of the normalized conductance at any T < T _c supports a (s + d)-wave symmetry of the gap, whose dominant low-T s component follows the T _c ( x ) curve in contrast with recent angle-resolved photoemission spectroscopy and quasiparticle tunneling data. These results prove the separation between pseudogap and phase-coherence superconducting gap in LSCO at x 0.2. Received 14 June 2001     

Effect of a transverse electric field on the Landau bands in a Weyl semimetal

The Landau bands in crossed magnetic and electric fields are studied for the case of a Weyl semimetal. The expression for the energy spectrum of such a system is obtained using an approach based on the Lorentz shift. It is shown that the electric field leads to a substantial transformation of the Landau bands. At the electric field equal to v_F H/c, the collapse of the Landau levels occurs and the motion becomes completely linear. Under this condition, the wavefunction is nonzero only for the states with p _z = 0. This significantly affects the phenomena related to the unusual surface states, which are characteristic of such materials.     

Investigation of 3D Dirac semimetal supported terahertz dielectric-loaded plasmonic waveguides

The tunable propagation properties of 3D Dirac semimetal (DSM)-supported dielectric-loaded surface plasmons structures have been investigated in the THz regime, including the influences of the Fermi level of 3D DSM layer, the fiber shape and operation frequencies. The results indicate that the shape of dielectric fiber affects the hybrid mode significantly, on the condition that if a_x (the semi-minor axis length of the dielectric semi-ellipse) is relatively small, the fiber shows good mode confinement and low loss simultaneously, and the figure of merit reaches more than 200. The propagation property can be manipulated in a wide range by changing the Fermi level of 3D DSM, e.g. if the Fermi level varies in the range of 0.05 eV–0.15 eV, the propagation length changes in the range of 9.073×10³–2.715×10⁴ μm, and the corresponding modulation depth is 66.5%. These results are very helpful to understand the tunable mechanisms of the 3D DSM plasmonic devices, such as switchers, modulators, and sensors.     

Temperature and junction-type dependency of Andreev reflection in MgB2

We studied the voltage and temperature dependency of the dynamic conductance of normal metal-MgB₂ junctions obtained either with the point-contact technique (with Au and Pt tips) or by making Ag-paint spots on the surface of MgB₂ samples. The fit of the conductance curves with the generalized BTK model gives evidence of pure s-wave gap symmetry. The temperature dependency of the gap, measured in Ag-paint junctions (dirty limit), follows the standard BCS curve with 2Δ/k_BT_c=3.3. In out-of-plane, high-pressure point-contacts we obtained almost ideal Andreev reflection characteristics showing a single small s-wave gap Δ=2.6±0.2 meV (clean limit).     

GeS_2-Ga_2S_3-Sb_2S_3-AgCl玻璃的三阶非线性光学特性

采用熔融-急冷法制备了(100-x)Ge20Ga5Sb10S65-xAgCl(x=0,5,10,15,20mol%)系列硫卤玻璃,测试了样品的密度、从可见到中远红外的透过性能以及折射率参量,根据Z-扫描测试原理用钛宝石飞秒激光器测试了样品的三阶非线性特性随波长变化的特性.结果表明:非线性折射率n2和非线性吸收系数β随着波长的增加而增加,AgCl含量为20mol%的样品在800nm处的非线性折射率n2=1.581×10-10esu,非线性吸收系数β=4.707cm/GW.与传统硫系玻璃相比,三阶非线性性能明显提高.     

Doping dependence of the superconducting gap by Andreev reflection in Au/La2−xSrxCuO4 point-contact junctions

We studied the doping dependence of the superconducting gap in La_2−xSr_xCuO₄ (LSCO) by means of Andreev reflection measurements in Au/LSCO point-contact junctions. The Andreev reflection features were found to disappear at T_c^A close to the bulk T_c. The fit of the conductance curves with the BTK-Tanaka-Kashiwaya model gives good results if a (s+d)-wave gap symmetry is used. The low-temperature dominant isotropic gap component Δ_s follows very well the T_c vs. x curve, while the gap-like features observed by angle-resolved photoemission spectroscopy and tunneling scale with T^∗. This confirms the different origin of these two energy scales at T<T_c.