Evolution of Superconducting-Transition Temperature with Superfluid Density and Conductivity in Pressurized Cuprate Superconductors

What factors fundamentally determine the value of superconducting transition temperature Tc in high temperature superconductors has been the subject of intense debate.Following the establishment of an empirical law known as Homes'law,there is a growing consensus in the community that the Tc value of the cuprate superconductors is closely linked to the superfluid density(ρ_(s))of its ground state and the conductivity(σ)of its normal state.However,all the data supporting this empirical law(ρ_(s)=AσT_(c))have been obtained from the ambientpressure superconductors.In this study,we present the first high-pressure results about the connection of the quantities of ρ_(s) and σ with T_(c),through the studies on the Bi_(1.74)Pb_(0.38)Sr_(1.88)CuO_(6+δ)and Bi_(2)Sr_(2)CaCu_(2)O_(8+δ),in which the value of their high-pressure resistivity(ρ=1/σ)is achieved by adopting our newly established method,while the quantity ofρs is extracted using Homes'law.We highlight that the Tc values are strongly linked to the joint response factors of magnetic field and electric field,i.e.,ρ_(s) and σ,respectively,implying that the physics determining T_(c) is governed by the intrinsic electromagnetic fields of the system.

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.

NMR and NQR studies on transition-metal arsenide superconductors LaRu2As2,KCa2Fe4As4F2,and A2Cr3As3

We report 75As-nuclear magnetic resonance(NMR)and nuclear quadrupole resonance(NQR)measurements on transition-metal arsenides LaRu2As2,KCa2Fe4As4F2,and A2Cr3As3.In the superconducting state of LaRu2As2,a Hebel–Slichter coherence peak is found in the temperature dependence of the spin-lattice relaxation rate 1/T1 just below Tc,which indicates that LaRu2As2 is a full-gap superperconducor.For KCa2Fe4As4F2,antiferromagnetic spin fluctuations are observed in the normal state.We further find that the anisotropy rate RAF=Tc 1/Tab 1 is small and temperature independent,implying that the low energy spin fluctuations are isotropic in spin space.Our results indicate that KCa2Fe4As4F2 is a moderately overdoped iron-arsenide high-temperature superconductor with a stoichiometric composition.For A2Cr3As3(A=Na,K,Rb,Cs),we calculate the electric field gradient by first-principle method and assign the 75As-NQR peaks to two crystallographically different As sites,paving the way for further NMR investigation.

Pressure-Dependent Point-Contact Spectroscopy of Superconducting PbTaSe2 Single Crystals

We develop an experimental tool to investigate the order parameter of superconductors by combining pointcontact spectroscopy measurement with high-pressure technique.It is demonstrated for the first time that planar point-contact spectroscopy measurement on noncentrosymmetric superconducting PbTaSe2 single crystals is systematically subjected to hydrostatic pressures up to 12.1 kbar.Under such a high pressure,the normal-state contact resistance is sensitive to the applied pressure,reflecting the underlying variation of contact transparency upon pressures.In a superconducting state,the pressure dependence of the energy gap△0 and the critical temperature Tc for gap opening/closing are extracted based on a generalized Blond-Tinkham-Klapwijk model.The gap ratio 2△0/kB Tc indicates a crossover from weak coupling to strong coupling in electron pairing strength upon pressure for PbTaSe2.Our experimental results show the accessibility and validity of high-pressure pointcontact spectroscopy,offering rich information about high-pressure superconductivity.

Magnetotransport Properties of a Nodal Line Semimetal TiSi

We report the magnetoresistance（MR）, de Haas-van Alphen（dHvA） effect and Hall effect measurements on a single crystal of TiSi, which is predicted to be a nodal line semimetal. With application of a magnetic field, a metal-to-insulator-like transition in ρ（T） and a nonsaturating MR are observed at low temperatures. The dHvA oscillations reveal a small Fermi-surface pocket with a nontrivial Berry phase. The analysis of the nonlinear Hall resistivity shows that TiSi is a multiband system with low carrier densities and high mobilities. All these results unambiguously prove the existence of Dirac fermions in TiSi.

Superconductivity at the Normal Metal/Dirac Semimetal Cd3As2 Interface

We investigate the interface between a three-dimensional Dirac semimetal Cd3As2 and a normal metal via softpoint contact spectroscopy measurement.The superconducting gap features were detected below 3.8 K and 7.1 K in the case of Cd3As2 single crystals sputter-coated with the Pt and Au films,respectively,in the differential conductance dI/dV-V plots of the point contacts.As the applied magnetic field increased,the drop in the zerobias contact resistance shifted toward lower temperatures.The topologically non-trivial band structure of Cd3As2 is considered to play a crucial role in inducing the superconductivity.Apart from realizing superconductivity in topological materials,our creative approach can be used to investigate possible topological superconductivity and exhibits a high application potential in electronic devices.

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.

Co-doping effects on magnetism and superconductivity in the 112-type EuFeAs2 system

The discovery of EuFeAs_(2),currently the only charge-neutral parent phase of the 112-type iron-pnictide system,provides a new platform for the study of elemental doping effects on magnetism and superconductivity(SC).In this study,a series of polycrystalline EuFe_(1-y)Co_(y)As_(2)and Eu_(0.9)Pr_(0.1)Fe_(1-y)Co_(y)As_(2)samples are synthesized through solid-state reaction,and the evolutions of SC and magnetism with Co doping in EuFeAs_(2)and Eu_(0.9)Pr_(0.1)FeAs_(2)are investigated by electrical transport and magnetic susceptibility measurements.For EuFe_(1-y)Co_(y)As_(2),the Eu-related antiferromagnetic(AFM)transition around 40 K is barely affected by Co doping,while the Fe-related spin density wave(SDW)transition temperature drops rapidly.Meanwhile,SC is induced by a trace amount of Co doping,with a highest transition temperature T_(c)~28 K found in EuFe0.9Co0.1As_(2).For the Eu_(0.9)Pr_(0.1)Fe1-yCo_(y)As_(2)series,the magnetism and superconductivity show similar evolutions upon Co doping,and the highest T_(c)is enhanced to 30.6 K with an optimum doping level y~0.07.Our results shed light on the competition between SC and SDW with Co doping in the 112-type EuFeAs_(2)system.

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].