Superconductivity in self-flux-synthesized single crystalline R_2Pt_3Ge_5(R=La,Ce,Pr)

In order to study the basic superconductivity properties of R_2Pt_3Ge_5, we synthesized the single crystalline samples by the Pt–Ge self-flux method. R_2Pt_3Ge_5(R = La, Ce) were also grown for a systematic study. Zero-resistivity was observed in both the La-and Pr-based samples below the reported superconducting transition temperatures. However, magnetic susceptibility measurements showed low superconductivity volume fractions in both La_2Pt_3Ge_5 and R_2Pt_3Ge_5(less than2%). Ce_2Pt_3Ge_5 did not show any signature of superconductivity. From the specific heat measurements, we did not observe a superconducting transition peak in R_2Pt_3Ge_5, suggesting that it is not a bulk superconductor. The magnetic susceptibility and heat capacity measurements revealed two antiferromagnetic(AFM) orders in R_2Pt_3Ge_5 at T_(N1)= 4.2 K and T_(N2)= 3.5K, as well as a single AFM transition at TN= 3.8 K in Ce_2Pt_3Ge_5.     

Superconductivity in an intermetallic oxide Hf_3Pt_4Ge_2O

Discovery of a new superconductor with distinct crystal structure and chemistry often provides great opportunity for further expanding superconductor material base, and also leads to better understanding of superconductivity mechanisms. Here, we report the discovery of superconductivity in a new intermetallic oxide Hf_3Pt_ Ge_2O synthesized through a solid-state reaction. The Hf_3Pt_ Ge_2O crystallizes in a cubic structure(space group Fm-3 m) with a lattice constant of a = 1.241 nm, whose stoichiometry and atomic structure are determined by electron microscopy and x-ray diffraction techniques. The superconductivity at 4.1 K and type-II superconducting nature are evidenced by the electrical resistivity, magnetic susceptibility, and specific heat measurements. The intermetallic oxide Hf_3Pt_ Ge_2O system demonstrates an intriguing structural feature that foreign oxygen atoms can be accommodated in the interstitial sites of the ternary intermetallic framework. We also successfully synthesized a series of Hf_3Pt_ Ge_2O1+δ(-0.25 ≤δ≤ 0.5), and found theδ-dependent superconducting transition temperature Tc. The atomic structure and the electronic structure are also substantiated by first-principles calculations. Our results present an entirely new family of superconductors with distinct structural and chemical characteristics, and could attract research interest in further finding new superconductors and exploring novel physics pertaining to the 5 d-electron in these intermetallic compound systems.     

Superconducting Properties and Absence of Time Reversal Symmetry Breaking in the Noncentrosymmetric Superconducting Compounds TaxRe1-x(0.1≤x≤0.25)

Unconventional superconductivity,in particular,in noncentrosymmetric systems,has been a long-sought topic in condensed matter physics.Recently,Re-based superconductors have attracted great attention owing to the potential time-reversal symmetry breaking in their superconducting states.We report the superconducting properties of noncentrosymmetric compounds Ta_xRe_1-x with 0.1 ≤x≤0.25,and find that the superconducting transition temperature reaches a maximum of ~8 K at the optimal level x=0.15.Nevertheless,muon-spin rotation and relaxation measurements reveal no time-reversal symmetry breaking existing in its superconducting state,which is in sharp contrast to both centrosymmetric Re metal and many other noncentrosymmetric Re-based superconductors.     

Molecular beam epitaxy and superconductivity of stoichiometric FeSe and KxFe2_ySe2 crystalline films

Our recent progress in the fabrication of FeSe and KxFe2_ySe2 ultra thin films and the understanding of their superconductivity properties is reviewed. The growth of high-quality FeSe and KxFe2_ySe2 films is achieved in a well controlled manner by molecular beam epitaxy. The high-quality stoichiometric and superconducting crystalline thin films allow us to investigate the intrinsic superconductivity properties and the interplay between the superconductivity and the film thickness, the local structure, the substrate, and magnetism. In situ low-temperature scanning tunneling spectra reveal the nodes and the twofold symmetry in FeSe, high-temperature superconductivity at the FeSe/SrTiO3 interface, phase separation and magnetic order in KxFe2_ySe2, and the suppression of superconductivity by twin boundaries and Fe vacancies. Our findings not only provide fundamental information for understanding the mechanism of unconventional superconductivity, but also demonstrate a powerful way of engineering superconductors and raising the transition temperature.     

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.     

Superconductivity and normal state magnetoresistance in superconducting FeSe:Sb

We prepared a series of β-FeSe samples with a nominal composition of Fe1.11Se1-xSbx(0≤x≤0.5).The X-ray diffraction,transport and magnetic measurements were performed on these samples to investigate the structure,the superconducting properties and the normal state transport and magnetic properties.Although the X-ray diffraction data suggested that Sb atoms were not incorporated into the β-FeSe phase,the transport data showed observable changes of superconductivity,normal state resistivity and magnetoresistance.This was represented by the increase in the superconducting transition temperature and the upper critical field.Also,for the samples with a low level of Sb content,a clear decrease of the normal state resistivity and a substantial increase of the residual resistance ratio were observed.Furthermore,the samples showed a significant increase of the normal state magnetoresistance that appeared not to follow the Kohler's rule.The results were discussed in the frame of reduction of excess Fe at interstitial sites of β-FeSe.     

Multiband nodeless superconductivity near the charge-density-wave quantum critical point in ZrTe_(3-x)Se_x

It was found that selenium doping can suppress the charge-density-wave(CDW) order and induce bulk superconductivity in ZrTe_3. The observed superconducting dome suggests the existence of a CDW quantum critical point(QCP) in ZrTe_3-xSex near x ≈ 0.04. To elucidate the superconducting state near the CDW QCP, we measure the thermal conductivity of two ZrTe_(3-x)Se_x single crystals(x = 0.044 and 0.051) down to 80 m K. For both samples, the residual linear term κ_0/T at zero field is negligible, which is a clear evidence for nodeless superconducting gap. Furthermore, the field dependence of κ_0/T manifests a multigap behavior. These results demonstrate multiple nodeless superconducting gaps in ZrTe_(3-x)Se_x,which indicates conventional superconductivity despite of the existence of a CDW QCP.     

Superconductivity in Mg-Doped Layered Intermetallic Compound NbB2

We have performed low temperature resistivity p（T） and specific heat C（T） measurements on a superconducting polycrystalline Nb0.75Mg0.25B2 sample. The results indicate that the superconducting transition temperature is -4.6 K. The zero temperature upper critical field determined from the resistivity and specific heat is 3123 Oe. The electronic coefficient of specific heat γn=4.51 mJmol^-1K^2 and the Debye temperature θD=419 K are obtained by fitting the zero-field specific heat data in the normal state. At low temperatures, the electronic specific heat in the superconducting state follows Ces/γnTc = 2.84 exp（-1.21Tc/T）. This indicates that the superconducting pairing in Nb0.75Mg0.25B2 has s-wave symmetry.     

Physical Properties Revealed by Transport Measurements for Superconducting Nd_(0.8)Sr_(0.2)NiO_2 Thin Films

The newly discovered superconductivity in infinite-layer nickelate superconducting films has attracted much attention,largely because their crystalline and electronic structures are similar to those of high-T_c cuprate superconductors.The upper critical field can provide a great deal of information on the subject of superconductivity,but detailed experimental data are still lacking for these films.We present the temperature-and angle-dependence of resistivity,measured under different magnetic fields H in Nd_(0.8)Sr_(0.2)NiO_2 thin films.The onset superconducting transition occurs at about 16.2 K at 0 T.Temperature-dependent upper critical fields,determined using a criterion very close to the onset transition,show a clear negative curvature near the critical transition temperature,which can be explained as a consequence of the paramagnetically limited effect on superconductivity.The temperaturedependent anisotropy of the upper critical field is obtained from resistivity data,which yields a value decreasing from 3 to 1.2 with a reduction in temperature.This can be explained in terms of the variable contribution from the orbital limit effect on the upper critical field.The angle-dependence of resistivity at a fixed temperature,and at different magnetic fields,cannot be scaled to a curve,which deviates from the prediction of the anisotropic Ginzburg-Landau theory.However,at low temperatures,the resistance difference can be scaled via the parameter Hβ| cos θ|(β=6-1),with θ being the angle enclosed between the c-axis and the applied magnetic field.As the first detailed study of the upper critical field of nickelate thin films,our results clearly indicate a small anisotropy,and a paramagnetically limited effect,in terms of superconductivity,in nickelate superconductors.     

Single Crystal Growth and Physical Property Characterization of Non-centrosymmetric Superconductor PbTaSe_2

We report on the single crystal growth and superconducting properties of PbTaSe_2 with the non-centrosymmetric crystal structure.By using the chemical vapor transport technique,centimeter-size single crystals are successfully obtained.The measurement of temperature dependence of electrical resistivity p(T) in both normal and superconducting states indicates a quasi-two-dimensional electronic state in contrast to that of polycrystalline samples.Specific heat C(T) measurement reveals a bulk superconductivity with T_c≌3.75 K and a specific heat jump ratio of 1.42.All these results are in agreement with a moderately electron-phonon coupled,type-ⅡBardeen-Cooper-Schrieffer superconductor.     

Nodeless superconductivity in the kagome metal CsV_3Sb_5

The recently discovered kagome metal series AV_3Sb_5(A=K, Rb, Cs) exhibits topologically nontrivial band structures, chiral charge order and superconductivity, presenting a unique platform for realizing exotic electronic states. The nature of the superconducting state and the corresponding pairing symmetry are key questions that demand experimental clarification. Here, using a technique based on the tunneling diode oscillator, the magnetic penetration depth ?λ(T) of CsV_3Sb_5 was measured down to 0.07 K. A clear exponential behavior in ?λ(T) with marked deviations from a T or T2 temperature dependence was observed at low temperatures, indicating an absence of nodal quasiparticles. Temperature dependence of the superfiuid density and electronic specific heat can be described by two-gap s-wave superconductivity, consistent with the presence of multiple Fermi surfaces in CsV_3Sb_5. These results evidence nodeless superconductivity in CsV_3Sb_5 under ambient pressure, and constrain the allowed pairing symmetry.     

Structural phase transition,precursory electronic anomaly,and strong-coupling superconductivity in quasi-skutterudite(Sr_(1-x)Ca_x)_3Ir_4Sn_(13) and Ca_3Rh_4Sn_(13)

The interplay between superconductivity and structural phase transition has attracted enormous interest in recent years. For example, in Fe-pnictide high temperature superconductors, quantum fluctuations in association with structural phase transition have been proposed to lead to many novel physical properties and even the superconductivity itself. Here we report a finding that the quasi-skutterudite superconductors(Sr_(1-x)Ca_x)_3Ir_4Sn_(13)(x = 0, 0.5, 1) and Ca_3Rh_4Sn_(13) show some unusual properties similar to the Fe-pnictides, through ~(119)Sn nuclear magnetic resonance(NMR) measurements. In(Sr_(1-x)Ca_x)_3Ir_4Sn_(13), the NMR linewidth increases below a temperature T*that is higher than the structural phase transition temperature Ts. The spin-lattice relaxation rate(1/T1) divided by temperature(T), 1/T1 T and the Knight shift K increase with decreasing T down to T*, but start to decrease below T*, and followed by more distinct changes at Ts. In contrast,none of the anomalies is observed in Ca_3Rh_4Sn_(13) that does not undergo a structural phase transition. The precursory phenomenon above the structural phase transition resembles that occurring in Fe-pnictides. In the superconducting state of Ca_3Ir_4Sn_(13), 1/T1 decays as exp(-?/kBT) with a large gap ? = 2.21 k_BT_c, yet without a Hebel–Slichter coherence peak, which indicates strong-coupling superconductivity. Our results provide new insight into the relationship between superconductivity and the electronic-structure change associated with structural phase transition.     

Crossover from a pseudogap state to a superconducting state

This paper deduces that the particular electronic structure of cuprate superconductors confines Cooper pairs to be first formed in the antinodal region which is far from the Fermi surface,and these pairs are incoherent and result in the pseudogap state.With the change of doping or temperature,some pairs are formed in the nodal region which locates the Fermi surface,and these pairs are coherent and lead to superconductivity.Thus the coexistence of the pseudogap and the superconducting gap is explained when the two kinds of gaps are not all on the Fermi surface.It also shows that the symmetry of the pseudogap and the superconducting gap are determined by the electronic structure,and non-s wave symmetry gap favours the high-temperature superconductivity.Why the high-temperature superconductivity occurs in the metal region near the Mott metal-insulator transition is also explained.     

Observation of Coulomb Gap and Enhanced Superconducting Gap in Nano-Sized Pb Islands Grown on SrTiO_3

We report high-resolution scanning tunneling microscopy(STM)study of nano-sized Pb islands grown on SrTiO3,where three distinct types of gaps with different energy scales are revealed.At low temperature,we find that the superconducting gap(△s)in nano-sized Pb islands is significantly enhanced from the one in bulk Pb,while there is no essential change in superconducting transition temperature Tc,giving rise to a larger BCS ratio 2△s/kBTc^8.31 and implying stronger electron-phonon coupling.The stronger coupling can originate from the interface electron-phonon interactions between Pb islands and SrTiO3.As the superconducting gap is totally suppressed under applied magnetic field,the Coulomb gap with apparent V-shape emerges.Moreover,the size of Coulomb gap(Ac)depends on the lateral size of Pb islands(R)with △c-1/R^035,indicating that quantum size effect can significantly influence electronic correlations.Our experimental results shall shed important light on the interplay among superconductivity,quantum size effect and correlations in nano-sized strong-coupling superconductors.     

Equation of State of Asymmetric Nuclear Matter at Finite Temperature with a Three-body Force

In the present work, the equation of state of hot asymmetric nuclear matter nas Been investigated in the framework of the finite temperature Brueckner-Hartree-Fock (FTBHF) approach with a microscopic three-body force (TBF). The temperature dependence and the isospin dependence of the single particle properties, such as the proton and neutron single-particle potentials and effective masses have been studied. It is shown that the TBF gives a repulsive contribution to the proton or neutron single particle potential. The energy per nucleon versus asymmetry parameter is found to fulfill a parabolic relation as in the zero temperature case[I]. This means that the symmetry energy at finite temperature Esym can be extracted from‘ the energy difference between pure neutron matter and symmetric matter. The calculated symmetry energy is plotted in Fig.1. It     

Nodeless superconducting gaps in Ca_(10)(Pt_(4-δ)As_8)((Fe_(1-x)Pt_x)_2As_2)_5 probed by quasiparticle heat transport

The in-plane thermal conductivity of the iron-based superconductor Ca10(Pt_(4-δ)As_8)((Fe_(1-x)Pt_x)_2As_2)_5 single crystal("10-4-8", T c= 22 K) was measured down to 80 m K. In a zero field, the residual linear term κ_0/T is negligible, suggesting the nodeless superconducting gaps in this multiband compound. In the magnetic fields, κ_0/T increases rapidly, which mimics the multiband superconductor Nb Se_2 and Lu Ni_2B_2 C with highly anisotropic gap. Such a field dependence of κ_0/T is an evidence for the multiple superconducting gaps with quite different magnitudes or highly anisotropic gap. Compared with the London penetration depth results of the Ca10(Pt_(4-δ)As_8)((Fe_(1-x)Pt_x)_2As_2)_5("10-3-8") compound, the 10-4-8 and 10-3-8 compounds may have a similar superconducting gap structure.     

Synthesis, structure and antiferromagnetic behaviour of brannerite MnV2O6

Brannerite MnV 2 O 6 with plate-like shape is successfully synthesized by hydrothermal method.Its crystal structure and morphology are investigated by x-ray diffraction (XRD),scanning electron microscopy (SEM),transmission electron microscope (TEM),high resolution transmission electron microscopy (HRTEM) and select area electronic diffraction (SAED).The results show that the brannerite MnV 2 O 6 with monoclinic structure has a uniform plate-like shape with a diameter of about 5-8 μm and a thickness of about 500 nm.SAED patterns further confirm the structure of the brannerite MnV 2 O 6 and the single crystalline character of the plate crystal.Magnetic properties are measured by superconducting quantum interference device (SQUID) in a temperature range of 2-300 K under a magnetic field of 1 T.The magnetic measurement results indicate that the material undergoes an antiferromagnetic transition with a N’eel temperature of 17 K.Above 50 K,the inverse susceptibility is fitted well to the Curie-Weiss law with a calculated moment of 5.98 μ B.Finally,the origin of antiferromagnetic behaviour in the brannerite MnV 2 O 6 is explained by means of Anderson model.     

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.     

Nodal superconducting gap in LiFeP revealed by NMR:Contrast with LiFeAs

Identifying the uniqueness of FeP-based superconductors may shed new lights on the mechanism of superconductivity in iron-pnictides.Here,we report nuclear magnetic resonance(NMR) studies on LiFeP and LiFeAs which have the same crystal structure but different pnictogen atoms.The NMR spectrum is sensitive to inhomogeneous magnetic fields in the vortex state and can provide the information on the superconducting pairing symmetry through the temperature dependence of London penetration depth λ_L.We find that λ_L saturates below T～0.2 T_C in LiFeAs,where T_C is the superconducting transition temperature,indicating nodeless superconducting gaps.Furthermore,by using a two-gaps model,we simulate the temperature dependence of λ_L and obtain the superconducting gaps of LiFeAs,as Δ_1=1.2 k_B T_C and Δ_2=2.8 k_BT_C,in agreement with previous result from spin-lattice relaxation.For LiFeP,in contrast,λ_L does not show any saturation down to T～0.03 T_C,indicating nodes in the superconducting gap function.Finally,we demonstrate that strong spin fluctuations with diffusive characteristics exist in LiFeP,as in some cuprate high temperature superconductors.     

Effect of annealing on superconductivity in Fe_(1+y)(Te_(1-x)S_x) system

We have synthesized polycrystalline samples of Fe1.11(Te1-xSx) and single crystals of Fe1+y(Te0.88S0.12),and characterized their properties.Our results show that the solid solution of S in the Fe1.11Te tetragonal lattice is limited,~10%.We observed superconductivity at ~8 K in both polycrystalline samples and single crystals.Magnetization measurements reveal that the volume fraction is small for this superconducting phase in both polycrystalline samples as-synthesized and single crystals as-grown.It is found that annealing in air enhances the superconducting fraction;the maximum fraction is almost 100% in the single crystals annealed in air at 300°C.We discuss the effect of annealing on superconductivity and transport properties at the normal state in the Fe1+y(Te1-xSx) system in terms of decrease of the excess Fe.