Superconductivity of In-doped Sn0.62Pb0.33Ge0.05Te alloys

A study of the Hall and Seebeck coefficients and of resistivity has been carried out on an Sn_0.62Pb_0.33Ge_0.05Te alloy doped by 5 and 10 at. % In. A superconducting transition with the maximum critical temperature T _C～4 K has been discovered in samples with hole concentrations p≥1×10²¹ cm^?3. The dependence of T _C on hole concentration has been established to be of a threshold nature. The onset of superconductivity is accompanied practically simultaneously by a growth of the resistivity and a sharp drop of the Seebeck coefficient. These features in the experimental data indicate the existence of a band of In resonance states within the allowed valence-band spectrum and strong resonance hole scattering to impurity states. The threshold character of the T _C(p) dependence is connected with the holes filling the resonance states. A positive correlation between the resonance scattering intensity and the critical temperature is observed.     

Resistivity superconducting transition in single-crystalline Cd0.95Ni0.05Sb system consisting of non-superconducting CdSb and NiSb phases

Resistivity superconducting transition has been for the first time found in single crystal of two-component 0.95(CdSb)–0.05(NiSb) system. End members of the system are not superconductors under normal conditions. Insulating behavior in temperature dependence of the electrical resistivity, which is due to hopping conductivity, precedes the transition. The resistivity superconducting transition is rather broad, since at cooling down the electrical resistivity starts to fall at 10.5 K, whereas zero resistivity is reached only at ~2.3 K. Longitudinal magnetic field gradually depresses superconductivity and shifts the superconducting transition to lower temperatures. Under magnetic field above 0.5 T, superconductivity is totally destroyed. Main features observed in the resistivity superconducting transition, including its unusually big width and insulating electrical behavior above the transition, can be related to inhomogeneity of the single crystal studied. According to XRD and SEM examinations, the single crystal consists of major CdSb phase and minor NiSb phase. The NiSb phase forms inhomogeneities in the CdSb matrix. Micro-sized needle-like NiSb crystals and nano-sized Ni_1-xSb_x clusters can be considered as typical inhomogeneities.     

Effect of Te doping on superconductivity and charge-density wave in dichalcogenides 2H-NbSe2-xTex （x = 0, 0.1, 0.2）

Single crystals of Te-doped dichalcogenides 2H-NbSe2-xTex （x ---- 0, 0.10, 0.20） were grown by vapour transport method. The effect of Te doping on the superconducting and charge-density wave （CDW） transitions has been investigated. The sharp decrease of residual resistance ratio, RRR = R（3OOK）/R（SK）, with increasing Te content was observed, indicating that the disorder in the conducting plane is induced by Te doping. Meanwhile the superconducting transition temperature, Tc, decreases monotonically with Te content. However, the CDW transition temperature, TCDW, shown by a small jump in the temperature dependence of the resistivity near 30 K, increases slightly. The results show that the suppression of superconductivity might be caused by the enhancement of CDW ordering. The disorder has little influence on the CDW ordering.     

Study of the superconducting gap in a Fe0.5TiSe2 single crystal by ultrahigh-resolution photoemission spectroscopy

The 1T-Fe_0.5TiSe₂ compound has been investigated by ultrahigh-resolution photoemission spectroscopy in order to reveal changes in the density of states at a possible superconducting transition. The experimental results showed the presence of a superconducting gap with a width of about 2 meV in the spectrum measured at 4.5 K.     

Magnetic susceptibility and superconductivity of (Pb<Subscript>0.2</Subscript>Sn<Subscript>0.8</Subscript>)<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>In<Subscript>x</Subscript>Te alloys as a function of in content

Experimental data on the superconductivity of In-doped Pb_zSn_1?zTe alloys (z=0.2) are discussed. The superconducting transition was detected from simultaneous measurements of the resistivity and magnetic susceptibility of a series of samples with different indium contents (2–12 mol % InTe). The superconducting transition detected by the magnetic susceptibility was observed at a temperature which was, on the average, 0.1 K below that determined from the resistivity. The increase in the superconducting transition temperature T _c with increasing indium content is of a threshold character, with T _c being proportional to the inverse electronic density of states at the Fermi level. The observed features in the experimental data are accounted for in terms of indium impurity resonance states in the material.     

High-pressure synthesis of β-W-type Nb3Te

High-pressure, high-temperature techniques were used to synthesize β-W-type Nb₃Te with a lattice parameter a = 5·261. This compound represents the first reported β-W-type chalcogenide. The superconducting transition is no higher than 2·5°K.     

Field dependence of the superconducting transition and of the onset of magnetism in Y4Co3

Measurements of a.c.-susceptibility in the temperature range from 0.1–12 K superimposed over d.c.-magnetic fields up to 1 T are reported. The results in zero d.c.-field have revealed the onset of magnetic order at 5.6 K and indicated a superconducting transition at 2 K. This transition was determined from the center of susceptibility decrease, in agreement with the result published recently by Kolodziejczyk et al. The superconducting transition is suppressed by an applied field of about 1 T. Down to the lowest obtainable temperature no reappearance of magnetic order has been found even in the highest applied field.     

A stable superconducting state at 8 K and ambient pressure in\alpha _t - (BEDT - TTF^ \star )_2 I_3 ^1

We report bulk superconductivity at 8 K and ambient pressure in crystals of α _t (BEDT-TTF)₂I₃. In contrast to the earlier observed metastable superconducting state at 8 K in crystals of β-(BEDT-TTF)₂I₃ here the superconducting state is stable and the crystals can be prepared by tempering α-(BEDT-TTF)₂I₃ above 70 °C for several days. ac-susceptibility measurements show that the observed superconducting state at 8 K is a bulk property of the crystals. Resistivity measurements indicate a sharp superconducting transition at 8 K with an onset temperature of about 9 K. The upper critical fields H_c2 at 1.3 K lie between 3 and 11 T depending on the direction of the magnetic field with respect to the crystal axes. ESR- as well as NMR-measurements indicate a total transformation of the α-phase crystals into the new superconducting α _t -crystals after tempering.     

Specific heat anomaly of single phase YBa2Cu3O7-δ at superconducting transition temperature

The specific heat of single phase YBa₂Cu₃O_7-δ has been measured using non-adiabatic method between 4.2K and 120K. There is a specific heat anomaly Δc at 90K (about 3.2% of total specific heat) approximately, due to superconducting transition. From the measured value of ΔC and transition temperature T_c, the electronic density of state at Fermi level N(E_F) and Sommerfeld parameter γ calculated are 2.55±0.30states/eV.Cu-atom and 2.77±0.30 mJ/mole.K², respectively. The experimental result of N(E_F) is consistent with that of the band calculation by Mattheiss. The Debye temperature above T_c in this material deduced from Debye function is about 340K. Below 20K, the relation C=γ'T+βT³ is satisfied. But the value of γ' is smaller. That means, most of the electrons have formed superconducting Cooper pairs which give no contribution to specific heat below 20K.     

Local features of the crystal structure of superconducting iron chalcogenides Fe(TeSe)<Subscript>1 – δ</Subscript>

The local crystal structure of superconducting powders of iron chalcogenides FeTe_xSe_1–x (x = 0.1, 0.22, 0.49, 0.8, 0.9) prepared by dry synthesis (without mineralizer) has been studied by EXAFS spectroscopy above the K Se and K Fe absorption edges in the temperature range of 80–300 K. The dependences of Se–Fe, Fe–Te, and Fe–Fe interatomic bond lengths and degrees of their local disordering (Debye–Waller factors) on the tellurium content and temperature have been obtained. Einstein temperatures characterizing the stiffness of each bond have been determined. The correlation of the Se–Fe bond stiffness with the dependence of the critical temperature of the superconducting transition T_c on the composition of the samples under study have been established, which indicates the specific role of the Se–Fe bond in the superconducting state formation in iron chalcogenides FeTe_xSe_1–x.     

Pressure and field dependence of the magnetic transition in GdBa2Cu3O7-x

The influence of hydrostatic pressure and of magnetic field strenght is presented for the low temperature antiferromagnetic ordering temperature (T_N=2.3 K) of GdBa₂Cu₃O_7-x. Data are presented for both superconducting and normal samples, the superconducting sample having a sharp 95 K transition and the oxygen-depleted normal sample being a semiconductor. For both systems the Néel temperatures, extrapolated to zero measuring field, are identical: T_N = (2.33±0.03) K. The effect of pressure is to raise the transition temperature slightly for both samples, dT_N/dP=+0.03 K/kbar for the superconducting sample and +0.04 K/kbar for the normal sample. The temperature dependence of the heat capacity made in several fixed external magnetic fields and the isothermal magnetization for T<T_N provide a measure of the antiferromagnetic-paramagnetic phase boundary, which shows T_N approaching T=0 K at about 2.5 T.     

Tellurium Solubility in TlGaTe<Subscript>2</Subscript> and TlInTe<Subscript>2</Subscript> and the Electrophysical Properties of Solid Solutions

The complex methods of the physicochemical analysis are used to study TlGaTe₂–Te and TlInTe₂–Te alloys in which the tellurium solubility region up to 5.0 at % is observed. The temperature dependences of the lattice parameters and the electrical conductivity of TlGaTe_{2 + x} and TlInTe_{2 + x} have been studied in different crystallographic directions. The TlGaTe_{2 + x} and TlInTe_{2 + x} solid solutions undergo a phase transition at a temperature of 498 K. The transition nature is interpreted.     

Neutron studies of the iron-based family of high TC magnetic superconductors

We review neutron scattering investigations of the crystal structures, magnetic structures, and spin dynamics of the iron-based RFe(As, P)(O, F) (R = La, Ce, Pr, Nd), (Ba,Sr,Ca)Fe₂As₂, and Fe_1+x(Te–Se) systems. On cooling from room temperature all the undoped materials exhibit universal behavior, where a tetragonal-to-orthorhombic/monoclinic structural transition occurs, below which the systems become antiferromagnets. For the first two classes of materials the magnetic structure within the a–b plane consists of chains of parallel Fe spins that are coupled antiferromagnetically in the orthogonal direction, with an ordered moment typically less than one Bohr magneton. Hence these are itinerant electron magnets, with a spin structure that is consistent with Fermi-surface nesting and a very energetic spin wave bandwidth ～0.2 eV. With doping, the structural and magnetic transitions are suppressed in favor of superconductivity, with superconducting transition temperatures up to ≈55 K. Magnetic correlations are observed in the superconducting regime, with a magnetic resonance that follows the superconducting order parameter just like the cuprates. The rare earth moments order antiferromagnetically at low T like ‘conventional’ magnetic superconductors, while the Ce crystal field linewidths are affected when superconductivity sets in. The application of pressure in CaFe₂As₂ transforms the system from a magnetically ordered orthorhombic material to a ‘collapsed’ non-magnetic tetragonal system. Tetragonal Fe_1+xTe transforms to a low T monoclinic structure at small x that changes to orthorhombic at larger x, which is accompanied by a crossover from commensurate to incommensurate magnetic order. Se doping suppresses the magnetic order, while incommensurate magnetic correlations are observed in the superconducting regime.     

Exponential Increase in the Resistivity upon Cooling and Superconductivity in Indium-Doped Pb<Subscript>0.45</Subscript>Sn<Subscript>0.55</Subscript>Te

We have analyzed the temperature and magnetic-field dependences of resistivity ρ(T, H) of semiconducting compound Pb_0.45Sn_0.55Te doped with 5 at % In under a hydrostatic compression at P < 12 kbar. It is found that the temperature dependence ρ(T) at all pressures at T < 100 K is exponential with the activation energy decreasing upon an increase in pressure; this is accompanied with a superconducting transition on the ρ(T) and ρ(H) dependences at P > 4.8 kbar at T > 1 K (T _c = 1.72 K at a level of 0.5ρ _N at P = 6.8 kbar). We consider the model describing the low-temperature “dielectrization” of the semiconducting solid solution and the formation of the superconducting state upon an increase in the hydrostatic compression P > 4 kbar.     

Physical properties,crystal and magnetic structure of layered Fe<Subscript>1.11</Subscript>Te<Subscript>1-</Subscript><Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> superconductors

The physical and structural properties of Fe_1.11Te and Fe_1.11Te_0.5Se_0.5 have been investigated by means of X-ray and neutron diffraction as well as physical property measurements. For the Fe_1.11Te compound, the structure distortion from a tetragonal to monoclinic phase takes place at 64 K accompanied with the onset of antiferromagnetic order upon cooling. The magnetic structure of the monoclinic phase was confirmed to be of antiferromagnetic configuration with a propagation vector k = (1/2, 0, 1/2) based on Rietveld refinement of neutron powder diffraction data. The structural/magnetic transitions are also clearly visible in magnetic, electronic and thermodynamic measurements. For superconducting Fe_1.11Te_0.5Se_0.5 compound, the superconducting transition with T _c = 13.4 K is observed in the resistivity and ac susceptibility measurements. The upper critical field H _c2 is obtained by measuring the resistivity under different magnetic fields. The Kim’s critical state model is adopted to analyze the temperature dependence of the ac susceptibility and the intergranular critical current density is calculated as a function of both field amplitude and temperature. Neutron diffraction results show that Fe_1.11Te_0.5Se_0.5 crystalizes in tetragonal structure at 300 K as in the parent compound Fe_1.11Te and no structural distortion is detected upon cooling to 2 K. However an anisotropic thermal expansion anomaly is observed around 100 K.     

Superconducting properties of tin embedded in nanometer-sized pores of glass

The electrical resistance of tin embedded from a melt in porous glasses with an average pore diameter of ??7 nm has been investigated at low temperatures in magnetic fields up to 2 T. The temperatures of the transition to the superconducting state for nanocrystalline tin have been determined in magnetic fields of 0, 0.3, 0.5, 1.0, 1.5, and 2.0 T. It has been found that the temperature and magnetic-field dependences of the electrical resistance of the nanocomposite under investigation exhibit two transitions to the superconducting state. The nature of the double superconducting transitions has been discussed. The H _c -T _c phase diagram has been constructed using the entire set of data on the magnetic-field and temperature dependences of the electrical resistance of nanostructured tin. This phase diagram indicates that the upper critical magnetic field H _c2(0) for nanostructured tin is almost two orders of magnitude higher than the corresponding field for bulk tin.     

Microwave studies of Bi(2212) HTSC single crystals at nitrogen temperatures

Absorption of microwave power by single-phase Bi(2212) crystals with a broad (15 K) phase transition has been studied. A temperature-localized drop of microwave power absorption at the beginning of the superconducting transition at 80 K has been established. A possible mechanism of this absorption feature based on the concept of synchronization of Josephson junctions near T _c is discussed.     

The low field magnetisation of Y4Co3

The onset of magnetic order at 6 K and a superconducting transition at 2 K was previously observed in Y₄Co₃. In this paper we investigate the magnetisation in very low fields, in the vicinity of the magnetic and superconducting transition (T_c). Below T_c the M-H curves are characterised by broad hysteresis loops. This type of the behaviour indicates that the sample consists of different regions. Some of these regions are superconducting below T_c and others are magnetically ordered below 6 K.     

Low temperature magneto-resistance,logarithmic resistivity rise and anisotropic superconductivity in TaS2(pyridine)12

The superconducting intercalation complex TaS₂(pyridine)${}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ has received considerable attention as a prototype of two dimensional superconductivity. In this material we have detected a substantial, anisotropic magneto-resistance 11 K° above the superconducting transition and an anisotropic logarithmic resistivity rise below 20°K. The resistively measured superconducting transition temperature is also anisotropic. T_c is the lowest when the current is perpendicular to the layers.     

Specific heat of a transforming V3Si crystal

The specific heat of a transforming V₃Si crystal in the normal, mixed, and superconducting states has been measured from 6 to 30K in zero and 52.3 kG magnetic fields. An analysis has been carried out in a self-consistent way based on the second-order phase transition from the normal to the superconducting state in zero magnetic field. Various physical parameters characterizing the superconducting and normal states are derived from the thermodynamics and the BCS weak coupling theory. The most important parameter obtained in this analysis is 2Δ(0)/kT_c = 3.46, which indicates a weak coupling in our V₃Si sample.