Hydrothermally Obtaining Superconductor Single Crystal of FeSe_(0.2)Te_(0.8)without Interstitial Fe

We report a hydrothermal route to remove interstitial excess Fe in non-superconducting iron chalcogenide Fe_(1+δ)Se_(1-x)Te_(x) single crystals.The extra-Fe-free(δ~0)FeSe_(0.2)Te_(0.8) single crystal thus obtained shows bulk superconductivity at T_c~13.8 K,which is about 2 K higher than the FeSe_(0.2)Te_(0.8) sample obtained by usual postannealing process.The upper critical fieldμ_0H_(c2) is estimated to be~42.5 T,similar to the annealed FeSe_(0.2)Te_(0.8).It is surprising to find that the hydrothermal FeSe_(0.2)Te_(0.8)exhibits a remarkably small isothermal magnetization hysteresis loop at T=3 K.This yields an extremely low critical current density J_(c)~1.1×10^(2)A·cm^(-2)(over 100times smaller than the annealed FeSe_(0.2)Te_(0.8)) and indicates more free vortices in the hydrothermal FeSe_(0.2)Te_(0.8).

Fe1+yTexSe1-x:A Delicate and Tunable Majorana Material

We report the observation for the pz electron band and the band inversion in Fe1+yTexSe1-xwith angleresolved photoemission spectroscopy. Furthermore, we found that excess Fe(y>0) inhibits the topological band inversion in Fe1+yTexSe1-x,which explains the absence of Majorana zero modes in previous reports for Fe1+yTexSe1-xwith excess Fe. Based on our analysis of different amounts of Te doping and excess Fe, we propose a delicate topological phase in this material. Thanks to this delicate phase, one may be able to tune the topological transition via applying lattice strain or carrier doping.

Quasi-Two-Dimensional Nature of High-Tc Superconductivity in Iron-Based(Li,Fe)OHFeSe

The intercalated iron selenide(Li,Fe)OHFeSe has a strongly layered structure analogous to the quasi-twodimensional(2D)bismuth cuprate superconductors,and exhibits both high-temperature(Tc)and topological superconductivity.However,the issue of its superconductivity dimensionality has not yet been fully investigated so far.Here we report that the quasi-2D superconductivity features,including the high anisotropyγ=151 and the associated quasi-2D vortices,are also revealed for(Li,Fe)OHFeSe,based on systematic experiments of the electrical transport and magnetization and model fittings.Thus,we establish a new vortex phase diagram for(Li,Fe)OHFeSe,which delineates an emergent quasi-2D vortex-liquid state,and a subsequent vortex-solid dimensional crossover from a pancake-like to a three-dimensional state with decreasing temperature and magnetic field.Furthermore,we find that all the quasi-2D characteristics revealed here for the high-Tciron selenide superconductor are very similar to those reported for high-Tcbismuth cuprate superconductors.

Unusual Normal and Superconducting State Properties Observed in Hydrothermal Fe_(1-δ)Se Flakes

The electronic and superconducting properties of Fe_(1-δ)Se single-crystal flakes grown hydrothermally are studied by the transport measurements under zero and high magnetic fields up to 38.5 T.The results contrast sharply with those previously reported for nematically ordered Fe Se by chemical-vapor-transport(CVT)growth.No signature of the electronic nematicity,but an evident metal-to-nonmetal crossover with increasing temperature,is detected in the normal state of the present hydrothermal samples.Interestingly,a higher superconducting critical temperature T_c of 13.2 K is observed compared to a suppressed T_c of 9 K in the presence of the nematicity in the CVT Fe Se.Moreover,the upper critical field in the zero-temperature limit is found to be isotropic with respect to the field direction and to reach a higher value of-42 T,which breaks the Pauli limit by a factor of 1.8.

Magnetic-Field-Induced Spin Nematicity in FeSe_(1-x)S_(x)and FeSe_(1-y)Te_(y)Superconductor Systems

The angular-dependent magnetoresistance(AMR)of the ab plane is measured on the single crystals of ironchalcogenide FeSe_(1-x)S_(x)(x=0,0.07,0.13 and 1)and FeSe_(1-y)Te_(y)(y=0.06,0.61 and 1)at various temperatures under fields up to 9 T.A pronounced twofold-anisotropic carrier-scattering effect is identified by AMR,and attributed to a magnetic-field-induced spin nematicity that emerges from the tetragonal normal-state regime below a characteristic temperature Tsn.This magnetically polarized spin nematicity is found to be ubiquitous in the isoelectronic FeSe_(1-x)S_(x)and FeSe_(1-y)Te_(y)systems,no matter whether the sample shows an electronic nematic order at T_(s)≤T_(sn),or an antiferromagnetic order at TN<T_(sn),or neither order.Importantly,we find that the induced spin nematicity shows a very different response to sulfur substitution from the spontaneous electronic nematicity:The spin-nematic T_(sn)is not suppressed but even enhanced by the substitution,whereas the electronicnematic T_(s)is rapidly suppressed,in the FeSe_(1-x)S_(x)system.Furthermore,we find that the superconductivity is significantly suppressed with the enhancement of the induced spin nematicity in both FeSe_(1-x)S_(x)and FeSe_(1-y)Te_(y)samples.

Anisotropic Superconducting Properties of Kagome Metal CsV_3Sb_5

We systematically measure the superconducting(SC)and mixed state properties of high-quality CsV_3 Sb_5 single crystals with T_c-3.5 K.We find that the upper critical field H_(c2)(T)exhibits a large anisotropic ratio of H_(c2)^(ab)/H_(c2)^c^9 at zero temperature and fitting its temperature dependence requires a minimum two-band effective model.Moreover,the ratio of the lower critical field,H_(c1)^(ab)/H_(c1)^c,is also found to be larger than 1,which indicates that the in-plane energy dispersion is strongly renormalized near Fermi energy.Both H_(c1)(T)and SC diamagnetic signal are found to change little initially below T_c-3.5 K and then to increase abruptly upon cooling to a characteristic temperature of-2.8 K.Furthermore,we identify a two-fold anisotropy of in-plane angular-dependent magnetoresistance in the mixed state.Interestingly,we find that,below the same characteristic T-2.8 K,the orientation of this two-fold anisotropy displays a peculiar twist by an angle of 60°characteristic of the Kagome geometry.Our results suggest an intriguing superconducting state emerging in the complex environment of Kagome lattice,which,at least,is partially driven by electron-electron correlation.

化学掺杂笼目结构超导体CsV_(3-x)Ti_(x)Sb_(5)及其两个显著物相

钒基笼目结构(kagome)超导体CsV_(3)Sb_(5)展现出反常霍尔效应(AHE)、电荷密度波(CDW),相列序和配对密度波(PDW)等新奇物性而得到了广泛关注.在kagome层进行化学取代掺杂是直接揭示内禀物性相互关联机制的最有效途径之一,但目前尚未见报道.本文成功制备出钛(Ti)化学掺杂的CsV_(3)Sb_(5)单晶,其中Ti原子直接取代kagome层中的部分V原子.CsV_(3-x)Ti_(x)Sb_(5)展现出2个显著不同的物相:少取代相展示出一个非常规V形的超导能隙,同时与减弱的长程CDW、PDW、AHE和相列序共存;多取代相则为常规U形的超导能隙并与短程CDW共存,同时长程CDW、AHE、PDW和具有二重对称性的面内磁电阻均消失.除Ti元素外,作者还成功制备了Cr和Nb掺杂的CsV_(3)Sb_(5)单晶.该研究成果开辟了制备化学掺杂笼目超导体的新方法以及研究不同电子关联态和超导配对精确调控的新平台.