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.     

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.     

Electronic thermal conductivity of superconducting thin films of lead-indium alloy

The thermal conductivity of thin films of superconducting Pb-3 at. % In alloy is measured in the normal and the superconducting states. The ratio of the electronic thermal conductivity in the superconducting state K_es to that in the normal state K_en is found to be in good agreement with the Bardeen, Rickayzen and Tewordt theory with 2?_O = 4.36 kT_c.     

Material characterization of superconducting T′-Nd2CuO4 films synthesized by metal organic decomposition

Recently we have achieved superconductivity in T′-RE₂CuO₄ (RE = Pr, Nd, Sm, Eu, and Gd), films by metal organic decomposition (MOD). In this article, we first report the cation off-stoichiometry effect, which aims at screening out the possibility of hole doping by cation deficiencies. We also investigated the structure and microstructure of superconducting T′-Nd₂CuO₄ films synthesized by MOD. This investigation aims at elucidating why MOD-grown Nd₂CuO₄ films become superconducting whereas our previous Nd₂CuO₄ films grown by molecular beam epitaxy were not superconducting.     

Übergangstemperatur zur Supraleitung,Energielücke und Widerstandsverlauf amorpher und kristallisierter Thallium-Tellur-Schichten

Thin Tl/Te-films are condensed onto a cooled substrate at 4 K. An amorphous phase is obtained. The maximum superconducting transition temperature of the amorphous Tl/Te-films is 4.2 K. Measurements of the energy gap by superconducting tunneling yield a ratio 2Δ/k T _c =4.6 for amorphous Tl/Te-films and 2Δ/k T _c =3.8 for disordered, pure Tl-films.     

Evolution of a hotspot in a thin BSCCO structured film

Two voltage jumps were found in the current–voltage characteristic of a thin superconducting film of Bi₂Sr₂CaCu₂O_x at temperature 58 K. The fluorescent thermal imaging technique revealed a localized heat dissipation (hotspot) nucleated at the current causing the second jump, and the hotspots’ temperature is close to the critical temperature of the superconducting film.     

On the superconductivity in T1BiTe2

The temperature dependence of the magnetic susceptibility of T1BiTe₂ and T1Te, in the range 0.050 to 1.5K, has been investigated. The superconducting transition in the T1Te samples at T = 0.190K is discovered. In T1BiTe₂ superconductivity has not been seen down to 0.050 K. All this shows that the superconducting transition discovered earlier by Hein and Swiggard at 0.140 K in T1BiTe₂ is characteristic of the T1Te foreign phase.     

Structure of high-T c superconducting tape of the second generation with high current-carrying capacity

Commercial high-T _c superconducting tape produced by Superpower Inc. (the U.S.A.) is studied by scanning and transmission electron microscopy. The superconductor structure is shown to consist of a superconducting layer of average composition GdBa_1.5Cu_2.5–3.0O _y coated by a silver layer and buffer layers of LaMnO₃, MgO,Y₂O₃, and Al₂O₃ deposited on a metallic tape of nickel alloy (Ni-Cr-Mo-Fe-W-Co-Mn). The superconducting layer is formed by the superconducting oxide GdBa₂Cu₃O _y containing lamellar inclusions Gd₂O₃.     

Phase diagram of YBa2Cu3O7−y at T<T c according to transverse nuclear relaxation data for Cu(2)

Two peaks are observed at T=35 and 47 K in the transverse relaxation rate for Cu(2) nuclei in YBa₂Cu₃O_7?y . A comparison of the relaxation rates for isotopes ⁶³Cu(2) and ⁶⁵Cu(2) at T=47 K indicates the magnetic nature of relaxation. The enhancement of local magnetic field fluctuations perpendicular to CuO₂ planes at T=47 K is associated with critical fluctuations of orbital currents. The peak at T=35 K is attributed to the emergence of an inhomogeneous superconducting phase. The obtained experimental results and the available data from the literature made it possible to propose a qualitatively new phase diagram of the superconducting state.     

Coulomb repulsion-induced hyperbolic pairing as a mechanism of high-T c superconductivity

The hyperbolic metric of the dispersion law (the effective mass tensor components of carriers are opposite in sign) in the vicinity of the Fermi contour in high-T _c superconducting cuprates in the case of repulsive interaction gives rise to a superconducting state characterized by the condensate of pairs with a large total momentum (hyperbolic pairing). The gain in the energy of the superconducting state over the normal state is due to the fact that a change in the kinetic energy of pairs (because of the negative light component of the effective mass) dominates over the change in the potential energy (corresponding to energy loss). The shift of the chemical potential upon the transition to the superconducting phase is substantial in this case. With increasing repulsive interaction, the superconducting gap δ_K increases and the resulting gain in energy changes to an energy loss at a certain critical value of the repulsive potential. The low temperature T _c of the superconducting transition and the large value of δ _K in this region of potential values are the reasons for the high value of the 2δ_K/T _c ratio and for the developed quantum fluctuations that are observed in underdoped cuprate superconductors.     

Magnetically dependent superconducting transport in oxide heterostructures with an antiferromagnetic layer

The superconducting current in hybrid superconducting structures Nb/Au/Ca_1?x Sr _x CuO₂/YBa₂Cu₃O_7?δ with an antiferromagnetic layer is experimentally shown to have a Josephson nature, and the deviation from the sinusoidal dependence of the superconducting current on the phase difference between superconducting electrodes is about 20% of the second harmonic. These heterostructures are found to have sensitivity to an applied magnetic field that is much higher than that of conventional Josephson junctions. The experimental shape of the magnetic-field dependence of the critical current in the heterostructures differs from the usual Fraunhofer shape by oscillation with a significantly smaller period along a magnetic field.     

Generation of coherent electromagnetic radiation by superconducting films at liquid-nitrogen temperatures

Generation of coherent electromagnetic radiation by GdBa₂Cu₃O_7?x superconducting films is observed in the frequency range 1–10 MHz at the temperature of boiling liquid nitrogen. The generation is caused by the feedback synchronization of the jumps of Abrikosov vortices produced by a low-frequency external magnetic field. This offers new possibilities for the application of high-T _c superconductors in superconducting electronics.     

Development of the method for low-temperature investigations of HTSC systems on an X-ray electron magnetic spectrometer

Peculiarities of the chemical structure of bulk polycrystalline samples of the high-temperature superconductors Bi₂Sr₂CaCu₂O₈, BiSrCaCu₂O_5.5, BiSrCaCu₃O₈, and YBa₂Cu₃O_{7 ? δ} have been investigated in detail at room and superconducting temperatures on an X-ray electron magnetic spectrometer equipped with an attachment for low-temperature studies. It is shown that covalent bonding is formed at a superconducting temperature between copper and oxygen due to Cu²⁺ ions. Due to the enhancement of the d(Cu)–p(O) hybridization of copper and oxygen electrons in the superconducting state, the d-electron density increases near E _F. The occurrence of additional peaks in the O1s and Sr3d (Ba3d) spectra after transition of the system to the superconducting state indicates changes in the nearest environment of O and Sr (Ba) atoms, in particular, the transition of Sr atoms to a higher oxidation state.     

Superconductivity in a correlated two-dimensional Hubbard model: A theoretical study

In this work we study the superconductivity within an attractive two-dimensional one-band Hubbard model. We consider a d-wave superconducting gap and a Hubbard-I approximation to describe the strongly correlated superconducting regime. We use Green's function method to obtain the order parameter Δ and the superconducting critical temperature T_c. The results show that for fixed values of the superconducting attractive potential U (U<0), the gap increases for low temperatures, whereas diminishes abruptly as the temperature increases. The effect of pressure can be discussed, varying the next-nearest-neighbors hopping t₂, yielding a change in T_c, and also in Δ₀.     

Photoemission results of intermetallic superconductors: Nb3Al and MgB2

We study the superconducting electronic structures of Nb₃Al and MgB₂ using high-resolution spectroscopy. The obtained spectrum of Nb₃Al measured below T_c shows clear opening of the superconducting gap with a sharp pile up in the density of states and a shift of the leading edge. In addition, the spectrum shows a peak-dip-hump line shape expected from the strong-coupling theory. On the other hand, for MgB₂, the superconducting-state spectrum measured at 5.4 K shows a coherent peak with a shoulder structure, in sharp contrast to that expected from a single isotropic gap. The superconducting spectral shape of MgB₂ can be explained in terms of a multicomponent gap.     

Superconductivity and calorimetric studies of Pr1.85Ce0.15CuO4+δ under different annealing conditions

Polycrystalline samples of electron-doped Pr_1.85Ce_0.15CuO_4+δ have been prepared under different annealing conditions and investigated by means of X-ray-diffraction, oxygen content analysis, electrical resistivity, magnetic susceptibility and low temperature specific heat measurements. X-ray-diffraction patterns show that samples contain a single T′ phase. The superconducting transition temperatures T_cm taken with the onset of diamagnetism in magnetic-susceptibility measurements are 20 and 19.5 K for sample annealed in flowing Ar gas and in vacuum (∼10⁻³ torr), respectively. The data of the samples, which are annealed in flowing Ar gas, show clear evidence for an αT² term at zero magnetic field in superconducting electronic specific heat, and are consistent with d-wave superconductivity. However, this behavior is not observed in the other sample, which is annealed in vacuum. These results indicate that different heat treatments affect the oxygen content, homogeneity, superconducting transition temperature T_c, superconducting volume fraction, and the superconducting pairing symmetry of Pr_1.85Ce_0.15CuO_4+δ.     

Spectroscopic studies of the high-Tc superconducting cuprate perovskites

We present XRD, ESR, IR, DR(UV, VIS) as well as resistivity and magnetic susceptibility data of single phase superconducting perovskites YBa₂Cu₃O_x and EuBa₂Cu₃O_x. A rich phonon structure for both oxide compounds has been observed with a peak at: 72, 108, 135, 148, 172, 570 and 620 cm^-1. The edge of the low energy side was observed at 55 cm^-1. The estimated carrier density for YBa₂Cu₃O_x is (1–2) × 10²¹ cm^-3. A weak ESR signal has also been observed for superconducting phases which is associated with the lattice defects.We also report ESR and optical measurements of constitutent materials as well as not superconducting phases Y₂Cu₂O₅ and BaCuO₂.     

Specific heat of the (La,Gd) Al2 system in the superconducting and normal state

Specific heat measurements between 0.5 and 4.2°K are reported for the system ($\text{La}$, Gd) Al₂ in both the superconducting and normal state. The observed specific heat jump at the superconducting transition temperature T_c is in excellent agreement with the Abrikosov-Gor'kov (AG) theory. This is in accordance with the previously reported close correspondence of the T_c vs. Gd concentration curve with the AG theory. Two very interesting features occur in the normal state specific heat. First, the Gd impurities cause a surprisingly strong enhancement of the electronic specific heat coefficient. Second, there is a large magnetic field dependent Schottky-like anomaly at low temperatures. This anomaly persists even in the superconducting state.     

Magnetic properties of the high Tc superconductor YBa2Cu3O7−δ

Magnetic measurements have been performed on the YBa₂Cu₆O_7−δ superconducting system in the temperature range 4.2–320 K. A hystereris loop indicating a coexistence of the superconducting and magnetic ordering below H_c1 at 4.2 K has been recorded.     

Superconducting properties of the pyrochlore oxide Cd2Re2O7

We report the superconducting properties of the pyrochlore oxide Cd₂Re₂O₇. The bulk superconducting transition temperature T_c is about 1.0 K, and the upper critical field H_c2 determined by the measurement of specific heat under magnetic fields is 0.29 T. The superconducting coherence length is estimated to be 34 nm. Specific heat data measured on single crystals suggest that the superconducting gap of Cd₂Re₂O₇ is nodeless.