Highly Anisotropic Magnetism and Nearly Isotropic Magnetocaloric Effect in Mn_(3)Sn_(2)Single Crystals

Mn_(3)Sn_(2)has been proposed as an ideal material for magnetic refrigeration.It undergoes two successive ferromagnetic transitions(T_(C1)=262 K and T_(C2)=227 K)and one antiferromagnetic transition(TN=192 K).Herein we report,for the first time,the preparation of single crystals of Mn_(3)Sn_(2)from Bi flux.The resultant anisotropic magnetic properties and magnetocaloric effect are investigated along the three principal crystallographic directions of the crystal.Significant anisotropy of magnetic susceptibility and multiple field-induced metamagnetic transitions were found at low fields,whereas the magnetocaloric effect was found to be almost isotropic and larger than that of the polycrystalline one.The maximum magnetic entropy change amounts to-ΔSM=4.01 J·kg^(-1)·K^(-1)near T_(C1)under a magnetic field change of μ_(0)ΔH=5 T along the c-axis,with the corresponding refrigerant capacity of 1750 mJ·cm^(-3).Combined with a much wider cooling temperature span(~80 K),our results demonstrate Mn_(3)Sn_(2)single crystal to be an attractive candidate working material for active magnetic refrigeration at low temperatures.

Hybridization Effects Revealed by Angle-Resolved Photoemission Spectroscopy in Heavy-Fermion Ce2IrIn8

We utilize high-resolution resonant angle-resolved photoemission spectroscopy(ARPES)to study the band structure and hybridization effect of the heavy-fermion compound Ce2 IrIn8.We observe a nearly flat band at the binding energy of 7 meV below the coherent temperature Tcoh^40 K,which characterizes the electrical resistance maximum and indicates the onset temperature of hybridization.However,the Fermi vector and the Fermi surface volume have little change around Tcoh,which challenges the widely believed evolution from a hightemperature small Fermi surface to a low-temperature large Fermi surface.Our experimental results of the band structure fit well with the density functional theory plus dynamic mean-field theory calculations.

Pressure effect in the Kondo semimetal CeRu4Sn6 with nontrivial topology

Kondo semimetal CeRu_4Sn_6 is attracting renewed attention due to the theoretically predicted nontrivial topology in its electronic band structure. We report hydrostatic and chemical pressure effects on the transport properties of single-and poly-crystalline samples. The electrical resistivity ρ(T) is gradually enhanced by applying pressure over a wide temperature range from room temperature down to 25 mK. Two thermal activation gaps estimated from high-and low-temperature windows are found to increase with pressure. A flat ρ(T) observed at the lowest temperatures below 300 mK appears to be robust against both pressure and field. This feature as well as the increase of the energy gaps calls for more intensive investigations with respect to electron correlations and band topology.

Unusual electronic structure of Dirac material BaMnSb_(2) revealed by angle-resolved photoemission spectroscopy

High resolution angle resolved photoemission measurements and band structure calculations are carried out to study the electronic structure of BaMnSb_(2). All the observed bands are nearly linear that extend to a wide energy range. The measured Fermi surface mainly consists of one hole pocket around Γ and a strong spot at Y which are formed from the crossing points of the linear bands. The measured electronic structure of BaMnSb_(2) is unusual and deviates strongly from the band structure calculations. These results will stimulate further efforts to theoretically understand the electronic structure of BaMnSb_(2) and search for novel properties in this Dirac material.

Unusual thermodynamics of low-energy phonons in the Dirac semimetal Cd_(3)As_(2)

By studying the thermal conductivity,specific heat,elastic modulus,and thermal expansion as a function of temperature for Cd_(3)As_(2),we have unveiled a couple of important thermodynamic features of the low-energy phonons strongly interacting with Dirac electrons.The existence of soft optical phonons,as inferred from the extremely low thermal conductivity,is unambiguously confirmed by low-temperature specific heat revealing significant deviation from Debye's description.The estimated Debye temperature is small in the range of 100-200 K and varies significantly depending upon the measurement used in its experimental determination.The thermodynamic Gr¨uneisen ratioγreveals a remarkable reduction below about 100 K,an energy scale that is highly relevant to the Dirac states,towards negative values below about 10 K that are indicative of lattice instability.

一种在室温附近具有高热电性能的准一维块体材料

在低维材料体系中探索高热电性能是热电材料研究领域的一个重要方向.实际上,近年来所发现的热电材料中的很大一部分,比如Bi_(2)Te_(3),SnSe,Mg_(3)Sb_(2)等,都具有准二维晶体结构.但是,到目前为止对准一维材料的热电研究还非常少见.本文报道了一种在室温附近具有高热电性能的准一维块体材料:TLCu_(3)Te_(2).在室温下,该材料沿单晶针状方向具有高达1.3的热电优值;在400 K时,其热电优值甚至可以达到1.5,一定程度超越了目前该温区的商用材料热电性能.通过详细的热电输运实验并结合第一性原理计算,作者发现TlCu_(3)Te_(2)的高热电优值主要起源于饼状费米口袋的高度各向异性准一维电子色散关系带来的大热电功率因子,以及准一维重元素复杂晶格导致的极低热导率.该研究表明准一维块体材料中的单轴性热电效应是一个值得深入探索的热电材料新领地.

Revisit of the anisotropic vortex states of 2H-NbSe_(2) towards the zero-field limit

We revisited the vortex states of 2H-Nb Se2towards zero fields by a low-temperature scanning tunneling microscope.Fine structures of the anisotropic vortex states were distinguished, one is a spatially non-splitting zero bias peak, and the other is an in-gap conductance anomaly resembling evolved crossing features around the center of the three nearest vortices.Both of them distribute solely along the next nearest neighboring direction of the vortex lattice and become unresolved in much higher magnetic fields, implying an important role played by the vortex–vortex interactions. To clarify these issues,we have studied the intrinsic vortex states of the isolated trapped vortex in zero fields at 0.45 K. It is concluded that the anisotropic zero bias peak is attributed to the superconducting gap anisotropy, and the spatially evolved crossing features are related to the vortex–vortex interaction. The vortex core size under the zero-field limit is determined. These results provide a paradigm for studying the inherent vortex states of type-II superconductors especially based on an isolated vortex.

Temperature-induced Lifshitz transition in topological insulator candidate HfTe_5

The ongoing discoveries and studies of novel topological quantum materials have become an emergent and important field of condensed matter physics.Recently,HfTe_5 ignited renewed interest as a candidate of a novel topological material.The single-layer HfTe_5 is predicted to be a two-dimensional large band gap topological insulator and can be stacked into a bulk that may host a temperature-driven topological phase transition.Historically,HfTe_5 attracted considerable interest for its anomalous transport properties characterized by a peculiar resistivity peak accompanied by a sign reversal carrier type.The origin of the transport anomaly remains under a hot debate.Here we report the first high-resolution laser-based angle-resolved photoemission measurements on the temperature-dependent electronic structure in HfTe_5.Our results indicated that a temperature-induced Lifshitz transition occurs in HfTe_5,which provides a natural understanding on the origin of the transport anomaly in HfTe_5.In addition,our observations suggest that HfTe_5 is a weak topological insulator that is located at the phase boundary between weak and strong topological insulators at very low temperature.

PbTaSe2拓扑表面态诱导的向列超导电性

通过平面内磁场转角的电阻和点接触谱的测量,作者发现拥有Dirac节点线的拓扑材料PbTaSe2中超导能隙存在明显的二重对称行为,低于PbTaSe2六角晶格结构的对称性,表明向列超导电性的存在.这种二重对称性会一直持续到表面超导的上临界场Hc2R,尽管体超导在体上临界场c2RH<<Hc2R以上已经被抑制.该结果表明二重对称行为起源于表面的向列超导电性.同时,在测量的精度范围内,磁场转角比热的数据也并没有观测到体超导的各向异性.值得注意的是, PbTaSe2的向列超导电性与拓扑超导候选材料CuxBi2Se3有所不同,后者的二重对称行为来自于体态超导.结合理论分析,本文认为PbTaSe2的向列超导电性可能起源于其拓扑表面态形成的拓扑超导态而不是体态超导的近邻效应,该理论预言PbTaSe2的表面超导会破坏时间反演对称.PbTaSe2的系统研究有助于进一步探索向列超导电性与拓扑超导之间的关系.

Structural phase transition, antiferromagnetism and two superconducting domes in LaFeAsO_(1-x)F_x(0

We report^(75) As nuclear magnetic resonance(NMR)/nuclear quadrupole resonance(NQR) and transmission electron microscopy(TEM) studies on LaFeAsO_(1-x)F_x. There are two superconducting domes in this material. The first one appears at 0.03 ≤ x ≤0.2 with T_c^(max) = 27 K, and the second one at 0.25 ≤x≤0.75 with T_c^(max) = 30 K. By NMR and TEM, we demonstrate that a C4-to-C2 structural phase transition(SPT) takes place above both domes, with the transition temperature T_s varying strongly with x. In the first dome, the SPT is followed by an antiferromagnetic(AF) transition, but neither AF order nor low-energy spin fluctuations are found in the second dome. By ^(75) As nuclear spin-lattice relaxation rate(1/T_1) measurements, we find that AF order and superconductivity coexist microscopically in LaFeAsO_(0.97) F_(0.03). In the coexisting region, 1/T_1 decreases at T_c but becomes proportional to T below 0.6 T_c, indicating gapless excitations. Therefore, in contrast to the early reports, the obtained phase diagram for x ≤ 0.2 is quite similar to the doped BaFe_2As_2 system. The electrical resistivity p in the second dome can be fitted by ρ = ρ0 + AT^n with n = 1 and a maximal coefficient A at around xopt = 0.5-0.55 at which T_s extrapolates to zero and Tc is the maximal, which suggests the importance of quantum critical fluctuations associated with the SPT. We have constructed a complete phase diagram of LaFeAsO_(1-x)F_x, which provides insight into the relationship between SPT, antiferromagnetism and superconductivity.

Large transverse thermoelectric figure of merit in a topological Dirac semimetal

The Seebeck effect encounters a few fundamental constraints hindering its thermoelectric(TE)conversion efficiency.Most notably,there are the charge compensation of electrons and holes that diminishes this effect,and the Wiedemann-Franz(WF)law that makes independent optimization of the corresponding electrical and thermal conductivities impossible.Here,we demonstrate that in the topological Dirac semimetal Cd3As2 the Nernst effect,i.e.,the transverse counterpart of the Seebeck effect,can generate a large TE figure of merit zNT.At room temperature,zNT≈0.5 in a small field of 2 T and it significantly surmounts its longitudinal counterpart for any field.A large Nernst effect is generically expected in topological semimetals,benefiting from both the bipolar transport of compensated electrons and holes and their high mobilities.In this case,heat and charge transport are orthogonal,i.e.,not intertwined by the WF law anymore.More importantly,further optimization of zNT by tuning the Fermi level to the Dirac node can be anticipated due to not only the enhanced bipolar transport,but also the anomalous Nernst effect arising from a pronounced Berry curvature.A combination of the topologically trivial and nontrivial advantages promises to open a new avenue towards high-efficient transverse thermoelectricity.

通过微量锰掺杂直接合成体超导的Fe(Te,Se)单晶

无砷的铁硒基超导体Fe(Te,Se)在高场应用方面具有广阔的前景,但至今仍缺乏制备体超导块材的有效方法.本文通过微量的Mn掺杂,一步法直接合成了无间隙铁的厘米尺寸高性能体超导单晶.通过电阻、磁化率与比热表征,发现Mn掺杂浓度为1%时,样品具有最好的体超导电性,且上临界场和临界电流密度相比未掺杂时显著提高.最佳掺杂样品在2 K时自场下的临界电流密度高达4.5×10^(5)A cm^(-2),且在高场下衰减很小.利用扫描隧道显微镜观测,证实了微量锰掺杂有效消除了层间的间隙铁原子.本工作首次提供了一种大规模制备高性能Fe(Te,Se)超导块材的实用方法,为其未来高场应用奠定了基础.

Superconductivity in LaPd2Bi2 with CaBe2Ge2-type structure

Since the discovery of superconductivity in LaFeAsO1-xFx, the high-Tc iron-based superconductors have been extensively studied from both experimental and theoretical viewpoints [1-8].However,the mechanism of the unconventional superconductivity is still to be resolved.To address such issues,numerous 3d,4d,5d transition metal pnietide,silicide,germanide,chalcogenide materials crystalizing in the similar crystal structure with iron pnictide/ selenide suoerconductors were studied [9-24].