Normal state resistivity and Tç studies of superconducting Ti1−xSbx system

The superconducting transition temperature (T_c) and the temperature dependence of the normal state resistivity of the Ti_1?xSb_x system between T_c and 300 K have been studied. The T_c values are found to depend on the heat treatment of the samples. Below 40 K, all alloys show a T² dependence of the resistivity. However, the sample with x = 0.53 is not superconducting and shows a different behaviour of the resistivity.     

Superconductivities and structural properties of ternary Tiben Zrben Ta alloys

Superconductivities and structural properties of Ti-Zr-Ta ternary alloys are extensively investigated. The TiZrTa sample has a cubic structure (β -phase) and shows a sharp superconducting transition at a critical temperature (T_c) of about 7.3 K. In addition, two series of Ti-Zr-Ta alloys, with nominal compositions of Ti_65-xZr₃₅Ta_x and Ti_xZr_65-xTa₃₅ respectively, are prepared, and their superconductivities and crystal structures change regularly with the chemical composition. Our experimental study also indicates that the annealing processing of this kind of material can cause the transition temperature to increase and the highest T_c is observed to be about 8.3 K in annealed samples.     

High superconducting transition temperatures in the palladium-noble metal-hydrogen system

High superconducting transition temperatures, T_c, of 16.6, 15.6 and 13.6 K have been observed in Pd-(Cu, Ag and Au) alloys, charged with large amounts of H by means of ion implantation at liquid Helium-temperatures. A peculiar phase-transition indicates that weak phonon modes might be responsible for the high T_c-values. The difference between the maximum T_c-values can be described as a type of isotope effect T_c∞M ^?1/2.     

Evidence for a three Tc behavior of the Kondo superconductor (La,Y)Ce

The dependence of the superconducting transition temperature T_c on Ce impurity concentration is reported for the (La_0.80Y_0.20)Ce system. Susceptibility and resistivity measurements on samples with about 0.85 at. % Ce show unique behavior which must be interpreted as a superposition of three transition curves: a first transition at 0.55 K from the normal to the superconducting state, a second one at 0.27 K back to the normal state, and finally a third one at about 0.05 K again to the superconducting state. The experimental evidence of this third transition (T_c3) is of special importance for the decision about the relevant physical mechanism of pair-breaking at low temperature (T ? T_K).     

Influence of lattice instability on the superconducting properties of “La3S4”

The normal state properties (the electronic specific heat constant, Debye temperature and electrical resistivity) and superconducting state properties [the superconducting transition temperature, T_c, and the upper critical field at 0 K, H_c²(0)] have been studied in the La₃S₄-La₂S₃ system. The superconducting properties and the electronic specific heat constant exhibit the maximum values in the alloy with the lowest sulfur content that does not undergo a low temperature crystallographic transformation. At lower sulfur contents the alloys exhibit a cubic to tetragonal transformation at ～80 K with a serious degradation in their superconducting properties, especially H_c² (0). These alloys clearly illustrate that materials which are almost but not quite unstable are good superconductors, relative to the more stable compositions.     

Superconductivity of vapour quenched beryllium and beryllium-based alloys

The properties of Be films, quench-condensed upon a³He cooled substrate, have been investigated by resistance and tunneling measurements. The superconducting transition temperature,T _c , of Be films increased with thickness and a thick film limit of 9.95 K could be estimated. Alloying with Al or Pb decreasedT _c. The ratios between energy gaps andT _c 's indicated that Be is a weak coupling superconductor, and no phonon induced structure could be traced in tunneling curves neither in pure Be nor in the Be based alloys. Resistance change during annealing as well as superconducting data indicated that the vapour quenched Be films were amorphous as deposited.     

Superconductivity in PdAlx hydrides

Hydrides of PdA$\text{l}$_x alloys prepared by electron beam co-evaporation and subsequent implantation of hydrogen ions are found to exhibit anomalous changes in the superconducting transition temperature with increasing A$\text{l}$ content. At large hydrogen concentrations, H/Pd ～ 1, the addition of aluminum increases T_c only slightly (from 8.1 to 8.6°K for 7 at. % A$\text{l}$), and then yields an abrupt decrease in T_c at 10% A$\text{l}$. This behavior is in sharp contrast to PdAg_xH_y alloys which exhibit an enhanced T_c ～ 16°K at relatively low silver content.     

Superconductivity in Au(10Å)/Ge(13Å) alternating ultra-thin layered films

We report the observation of a unique tricritical superconducting transition behaviour (a peak in the R-T curve below T_c) in Au(10Å)/ Ge(13Å) alternating ultra-thin layered sandwich films. A first transition from the normal to the superconducting state occurs at around 2K, a second (back to the normal state) at about 1.8K, and finally a third at about 1.7K (again to the superconducting state). The tricritical transition appearance is accompanied by a resistance anomaly and an electromotive force anomaly in the superconducting transition regime. The H_c2 behaviour is strongly anisotropic. From an analysis of the H_c2 anisotropy, the system seems to be a three dimensional coupled-layer superconductor.     

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.     

Effect of external pressure on Tc of as‐grown and thermally treated superconducting Rbx Fe2–ySe2 single crystals

We report on the effect of external pressure on the superconducting transition temperature (T_c) of as‐grown and thermally treated single crystals of superconducting iron chalcogenide Rb_0.85Fe_1.9Se₂. The superconducting transition temperature of 27.1 K at ambient pressure for the as‐grown sample was found to increase up to 33.2 K for the sample annealed for 3 h at 215 °C in vacuum. An increase of T_c up to 28.2 K was observed for the as‐grown sample at a pressure of 0.83 GPa. For all the studied crystals, annealed in the temperature range between 215 °C and 290 °C, the external pressure seems to decrease the superconducting transition temperature and a negative pressure coefficient of T_c was observed. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Electron-phonon interaction and coulomb pseudopotential in AuGa alloys from resistivity and superconductivity measurements

Measurements of the electrical resistivity as a function of temperature from 250 to 270 K for Au and f.c.c. phase AuGa alloys are presented. The linear dependence of the resistivity with temperature and previous results of the superconducting transition temperature of the alloys are used to obtain a value for T_c of pure Au. From these results a new method is introduced to estimate the Coulomb pseudopotential μ¹ for gold.     

First-principles study on superconductivity of the gold–indium alloy under high pressure

The superconductivity of gold–indium alloys has been investigated using first-principles calculations based on the density functional theory. At ambient pressure, the calculated superconducting transition temperature (T _c) is 0.04 μ K in pure gold, but T _c dramatically increases by substituting indium atoms for gold atoms. The gold–indium alloy having 12.5 atomic percent indium (Au_0.875In_0.125) shows T _c of 0.1 K, and Au_0.75In_0.25 marks 1.7 K. The dramatic increase in T _c owing to the alloying effect is caused by the enhancement of the electron–phonon coupling. The superconductivity of gold is predicted to be drastically weakened with increasing pressure and virtually disappear at 10 GPa, but it continues up to at least 30 GPa by the inclusion of indium atoms.     

Pressure-induced phase transformation in In2Bi

We have made measurements of the pressure dependence of the superconducting transition temperature, T_c, for In₂Bi and related alloys. For In₂Bi- phase alloys, a large discontinuity in T_c is seen at 15–20 kbar, which we associate with a phase transformation first seen by Bridgman [1]. Our measurements suggest that this transformation is produced by the decomposition of In₂Bi into In₅Bi₃ and an In-rich phase. In the low pressure phase, T_c shows a minimum at 9–15 kbar whereas it depends linearly on pressure in the high pressure phase with ?T_c/?P equal to -4.9 × 10^-5 K bar^-1.     

Spin-orbit effects in carbon nanotubes – Analytical results

We report a study of the normal and superconducting state properties of the Ti _x V_1?x alloys for x = 0.4, 0.6, 0.7 and 0.8 with the help of dc magnetization, electrical resistivity and heat capacity measurements along with the electronic structure calculation. The superconducting transition temperature T _c of these alloys is higher than that of elemental Ti and is also higher than elemental V for x ≤ 0.7. The roles of electron density of states, electron-phonon coupling and spin fluctuations in the normal and superconducting state properties of these alloys have been investigated in detail. The experimentally observed value of T _c is found to be considerably lower than that estimated on the basis of electron density of states and electron-phonon coupling in the x = 0.4, 0.6 and 0.7 alloys. There is some evidence as well for the preformed Cooper pair in all these Ti-V alloys in the temperature regime well above T _c . Similar to x = 0.6 [Md. Matin, L.S. Sharath Chandra, R.K. Meena, M.K. Chattopadhyay, A.K. Sinha, M.N. Singh, S.B. Roy, Physica B 436, 20 (2014)], the normal state properties of the x = 0.4 alloy showed the signature of the presence of spin fluctuations. The difference between the experimentally observed T _c and that estimated by considering electron density of states and electron-phonon coupling in the x = 0.4, 0.6 and 0.7 alloys is attributed to the possible influence of these spin fluctuations. We show that the non-monotonous variation of T _c as a function of x in the Ti _x V_1?x alloys is due to the combined effects of the electron-phonon coupling and the spin fluctuations.     

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.     

改进铜铌溅射型QWR超导腔性能的探讨

通过直流偏压二级溅射方法，在无氧铜腔体表面溅射一层铌膜，研制了铜铌溅射型射频超导1/4波长谐振腔(quarter wave resonator,QWR)，该腔主要用于重离子的加速，是北京放射性核束装置中后加速部分的预研项目.目前国际上很多实验室都在研究进一步提高铜铌溅射型QWR超导腔的性能，通过多种方法的实验研究，发现在无氧铜衬底与铌膜之间加入一层氮 化铌（NbN）薄膜，可以使得表面铌膜的超导温度转变点由原来的8.8K提高到了接近9.6K ，该方法有可能成为提高QWR腔加速性能的重要途径，目前进一步研究正 关键词： 溅射 QWR超导腔 氮化铌NbN 超导温度     

FeTe as a candidate material for new iron-based superconductor

Tetragonal FeSe is a superconductor with a transition temperature T_c of 8 K and shows a huge enhancement of T_c with applying pressure. Tetragonal FeTe has a structure very analogous to superconducting FeSe, but does not show superconducting transition. We investigated the pressure effect of resistivity on FeTe. The resistivity at room temperature decreased with increasing pressure. An anomaly in resistivity around 80 K shifted towards a lower temperature with increasing pressure.     

Superconductivity at disordered interfaces

The increase of the superconducting transition temperature T_c due to the tunneling of conduction electrons into negative-U centers at a disordered metal-semiconductor interface is calculated. The strong dependence of the experimental increase of T_c on the Fermi energy of the metal is accounted for by the polaronic reduction of the tunneling matrix elements. The latter reduction is dynamically suppressed by the decreasing lifetime of the localized state as E_F increases. The theoretical enhancement is sufficiently strong to explain the increase of T_c observed in eutectic alloys.     

Formation of superconductive supersaturated TaZr solid solutions by pulsed electron beam treatment

Amorphous TaZr films were prepared by simultaneous evaporation onto sapphire substrates. Pulsed electron beam treatment with deposited energy densities above a threshold of about 2 J cm^?2 led to a crystallization of the films and to the formation of supersaturated TaZr solid solutions with a bcc structure up to Zr concentrations of 54 at %. The crystalline alloys were superconducting with a maximum transition temperature T_c of 7.7 K. The films decomposed totally at a temperature of 1100°C during furnace annealing.     

Metastable phases in YBCO films created by short-term annealings

The effect of short-term low-temperature annealing in air and in vacuum on the properties of HTSC films of YBCO is studied. It is shown that, under certain conditions of preparation of initial samples, a transition from the HTSC phase with the superconducting transition temperature T_c=90 K to a phase with T_c=60 K occurs without a noticeable change in the oxygen content. It is found that, as a result of short-term annealings, a transition from the HTSC phase with T_c=60 K to the phase with T_c=90 K can occur only through the vacuum annealing stage, which converts the sample into the superconducting state. Short-term annealings lead to multiple reversible “switching” of the films from one phase to another. The obtained results are of practical interest, since the proposed method can be used to quickly obtain superconducting YBCO films in various phase states. It is shown, in addition, that the annealing procedure makes it possible not only to increase the oxygen concentration but also to produce a structural rearrangement of a YBCO film.