Superconductors In Mixed State Research Articles

The sign reversal of the acoustoelectric effect in anisotropic superconductors (abstract)

Experimental and theoretical investigations of high-temperature superconductors have revealed that the Fermi surface contains both electron- and hole-like parts, and the superconducting gap has large anisotropy in the a-b plane. In the present paper we show that these two findings are sufficient to produce the sign reversal of the acoustoelectric effect observed in films of YBa1Cu3O7−δ in transition from the normal to superconducting state. For the purpose of studying the effect we derive a diagram method and calculate the acoustoelectric current produced by an acoustic wave in the normal state of an anisotropic metal. Then, we find that superconducting fluctuations decrease the current. Finally, we investigate the acoustoelectric effect in the mixed superconducting state. In this state the normal acoustoelectric current arises due to the quasiparticle excitation drift produced by the acoustic wave. It is compensated by the superconducting current which forces vortices to move. It is the motion that produces the acoustoelectric voltage in the superconducting state. We conclude that if the superconducting gap takes different values on the electron- and hole-like parts of the Fermi surface, then the acoustoelectric voltage can sharply change the sign near the superconducting transition. This effect is caused by the change of relative concentrations of the electron- and hole-like excitations.

Phase transitions in the mixed state of superconductors with anisotropic pairing

The symmetry approach is used to classify the superconducting lattices near H c2 and to show that for the unconventional superconductors the phase transition from one-quantum hexagonal lattice to the lattice with double or triplet magnetic flux per unit sell or with reduced number of rotational elements is quite characteristic. In the context of present theory the H T phase diagram of UPt 3 with two close superconducting phase transitions is analyzed.

Quasi-particle tunnelling between vortex cores in a mixed state superconductor

We examine the low lying quasi-particle states of a chain of vortices contained within a thin slab of superconductor and demonstrate that bound states of individual vortices hybridize to form bands of extended states. As a consequence of the phase gradient around a vortex, one such band is centred on zero energy and will therefore contribute to quasi-particle transport at energies far below T c .

Thermodynamics of vortex motion in type II superconductors

We re-examine the calculations of the transport entropy of vortices and find that the old definitions of the heat current available in the literature are unsatisfactory. Based on the microscopic theory of nonstationary superconductivity, we obtain the prescription how to calculate the heat current for the mixed state of superconductors. We re-calculate the transport entropy of vortices in the high-field limit to demonstrate the difference between the new and old definitions of the heat current.

Anomalous Hall effect in the mixed state of high-temperature superconductors

The Hall effect in the mixed state of high- T c superconductors (HTSC) is of an anomalous nature: near the transition there is a range of temperatures and of magnetic fields where the sign of the Hall effect is opposite to that in the normal state. The universality of the phenomenon in question is indicative of its connection with some general properties of the mixed state of type-II superconductors, namely, with peculiarities of motion of magnetic flux vortex lines (vortices) in these superconductors. This work puts forward a model accounting for a number of vortex motion specific features and providing a possibility to obtain the characteristics of the anomalous Hall effect. The work is based on the phenomenologically generalized results of Bardeen-Stephen and Nozieres-Vinen, supplemented with an allowance for a new mechanism of vortex “friction” associated with Andreev electron reflection on the interface between the normal core and the superconducting periphery of a vortex. Within the framework of the model suggested, magnetic field (and temperature) dependences of the longitudinal and Hall resistances of a mixed state superconductor have been calculated at temperatures nearing T c. At certain quite realistic parameters which define the forces acting on the vortices, there is a range of magnetic fields and temperatures where the sign of the Hall effect is opposite to that in the normal state. The lower limit of this range is the irreversibility line and the upper critical field.

Modulated microwave absorption in superconductors

Modulated microwave absorption in conventional superconductors at low magnetic field is discussed. The microwave losses in the mixed state of superconductors are described in terms of viscous motion of vortices depinned by the critical state magnetization current. Modulation of this current yields hysteretic effects similar to those observed in high-temperature superconductors. The considered model of the microwave absorption is generalized to involve both the case of conventional and high-temperature superconductors.

Microwave absorption in superconducting loops formed by touching lead pieces

New data for microwave absorption in artificial superconducting loops with Josephson junctions formed by pressing irregular lead pieces together are presented. Two simultaneous series of microwave absorption lines with periods of 0.54 and 7.0 Oe for the case formed of three irregular pieces of lead are presented. Areas of the superconducting loops with Josephson junctions are 38.3 and 2.96 μm2, respectively. The separation of the first derivative of the absorption lines indicates a band of microwave absorption which is proportional to the square root of the microwave power. Irregular, reproducible lines of microwave absorption were detected for the sample made by 500 irregular pieces of lead at low magnetic field and at high magnetic field, in mixed superconducting and normal state for very low microwave power. A simple model for the microwave absorption in the superconducting loop with Josephson junction is presented.

Superconducting Abrikosov transition in the large-N limit

Abstract The continuous transition from a normal metal to a mixed superconducting state, the Abrikosov flux lattice, is known to be driven by fluctuations to a first-order transition immediately below six dimensions. We consider here a fictitious U( N ) internal symmetry for the order parameter and analyze the large- N limit. It leads to a classical theory which differs from the Landau-Ginzburg one by a non-local potential due to fluctuations. The analysis of the non-linear equations and of the full phase diagram is difficult, but we can prove that while the Abrikosov solution remains valid above six dimensions for some range of the parameters, in all dimensions lower or equal to six the transition becomes discontinuous. A separate analysis is needed for the cases 4 d ⩽ 6 and 2 ⩽ 4, but the conclusions are the same.

Surface studies using stem

Quantum oscillations were first observed in the mixed state of superconductor, NbSe2, by Graebner and Robbins in 1976. More recently, the investigation of a number of other materials has shown that de Haas-van Alphen effect oscillations persist below Bc2 in many systems, so that quantum oscillations in the vortex state appear to be the rule rather than the exception. Such measurements demonstrate that the effects of Landau quantization persist in the mixed state of a clean type II superconductor. In this paper we review work on NbSe2, V3Si and Nb3Sn, for which detailed measurements exist. We find that the principal effect of the superconducting order parameter is to reduce the amplitude of the oscillatory magnetisation. In principle, quantum oscilltory measurements of this type will give detailed information on the anisotropies of the superconducting state.

N.M.R. Study on 31P in an amorphous superconducting (Mo 0.5Ru 0.5) 80P 20 alloy

Knight-shift and spin-lattice relaxation time have been measured on 31P in an amorphous (Mo 0.5Ru 0.5) 80P 20 alloy obtained by splat-cooling. Measurements were carried out in applied fields of 5.2 and 8.7 kG in the normal state and in the mixed superconducting state for temperatures down to 1.4°K (T co = 5.4°K). Just below T c (T c/T = 1.1), the reduced relaxation rate peaks at a value R s/R n of 1.5; then, it decreases, for 1.2 ⩽ T c/T ⩽ 2.5, according to an exponential law yielding an energy gap value in excellent agreement with BCS prediction for weak-coupled superconductors.

On the transport entropy of vortices in the mixed superconducting state

Serious objections have been raised by Hu against Maki's derivation of the transport entropy of a vortex in the mixed state of a dirty type II superconductor. In this letter it is shown that Maki's expression, though derived on the basis of a wrong argument, is nevertheless correct.

Correlation between Flux-Line Lattice and Crystal Axis in a Mixed State of Superconductors

Correlation between Flux-Line Lattice and Crystal Axis in a Mixed State of Superconductors

Study of the Distribution of N2 in Ta and V Using Rotational Hysteresis in the Superconducting Mixed State

Rotational hysteresis curves of selectively oriented single crystals of tantalum and vanadium were determined in the mixed superconducting state at 2°K, as a function of nitrogen content. Enhanced flux pinning, which varied in magnitude with the nitrogen content, was observed when the magnetic field was parallel to the 〈110〉 directions in vanadium and the 〈100〉 directions in tantalum. These results are consistent with the hypothesis that the nitrogen forms clusters on specific crystallographic planes, determined by the respective elastic anisotropies of the solvent metal.

Magnetic contribution to the low temperature specific heat of V3Si in the mixed superconducting state

Abstract In V3Si it was found that there is a magnetic contribution to the specific heat in the mixed superconducting state of the form Cm = γmH T + amH T3. It is shown that the field and temperature dependence of Cm and the measured values of γm and am can be accounted for in terms of flux line penetration at fields slightly higher than Hc1.

Resistivity measurement of V 3Si single crystal in the mixed superconducting state

Measurements of the resistivity of a V 3Si single crystal in the mixed superconducting state, near the critical temperature T c indicate a sharp peak in the critical field H ¢ for the onset of resistivity. In the (100) direction, H ¢ is approximately five times larger than in the vicinity of this direction, and the width of the peak in H ¢ is approximately 2.5°. The crystal had been subjected to stress of order 50 kg/cm 2 in the (100) direction. A tentative explanation is suggested which takes into account the domain structure caused by the martensitic transformation. The sharp peak and the large anisotropy are attributed to the possibility that the domain walls act as potential barriers to the flux flow.

On Electron Tunneling from a Hard Superconductor in the Mixed State

Gorkov's derivation of the Ginzburg-Landau equation from BCS theory is reformulated in such a form that one can apply it to dynamical processes involving quasiparticle excitations. As the simplest example, the method is applied to electron tunneling from an intrinsic hard superconductor in Abrikosov's mixed state. It is shown that near the transition point one obtains the usual tunneling current simply averaged over Abrikosov's distribution of energy gap. Some numerical estimates are made of the dependence of I-V characteristics on magnetic field in the case of Nb.