Acoustic attenuation in a type-II superconductor at high magnetic fields

Abstract

We have calculated the longitudinal acoustic attenuation in a type II superconductor in high magnetic fields within a mean-field BCS theory. We predict two new features in the corresponding attenuation signal as compared to that of the Meissner state. Our analytical calculations predict the existence of oscillations in the attenuation as the external magnetic field is varied-this effect is associated with the Landau level structure of the electron states and is analogous to the well-known de Haas van Alphen oscillations in the mixed state. The attenuation directly probes the quasiparticle energies; the presence of gapless points in the quasiparticle spectrum, which is characteristic of type-II superconductors at high magnetic fields, shows up in the frequency ω and temperature T dependence of the attenuation in the limit of low ω and and low T respectively. At low T there is no analogue to the discontinuity in the attenuation observed in the Meissner state whenhω = 2∆, where ∆ is the quasiparticle energy gap. This result opens up the possiblity of experimentally determining the existence and nature of the gapless points in the quasiparticle spectrum of a type-II superconductor in high magnetic fields.

Hall conductance of a pinned vortex lattice in a high magnetic field

“…Note that as grows and the DOS spreads over to higher energies, there remains a high DOS at low energies due to the low gap states that retain the characteristic spectrum of the DA. Therefore, it is expected that the DA will be appropriate at low [3,5] (see however reference [8] for a thorough discussion).…”

“…A good description of the de Haas-van Alphen oscillations has been obtained taking into account the Landau quantization [4]. The gapless nature of the spectrum also leads to an enhanced accoustic attenuation in contrast to usual gapped superconductors [5] and it survives even in the presence of disorder [6].…”

“…Close to the upper critical field, and at small values of ∆, a diagonal approximation (DA), where the coupling between the LL is neglected, is appropriate [3,5]. In this regime the quasiparticles propagate coherently throughout the vortex lattice.…”

Exploring induced mechanical vibrations in superconducting magnets

“…In any case attenuation is much smaller for a superconductor well below its critical temperature Tc and smaller still than copper or aluminum conductor stabilizers. The attenuation coefficient in superconductors increases with increased magnetic field reaching normal state level at critical field [15] but there are non-trivial dependencies according to [16]. Multi-material entities, like Nb3Sn + Cu, have their own features but in first order those are ignored.…”

STM differential conductance of a disordered extreme type-II superconductor at high magnetic fields

“…19 Low-temperature gapless quasiparticle excitations strongly influence the phenomenology of superconductors at high fields, including thermodynamics, 20 thermal transport, 21 and acoustic attenuation. 22 Furthermore, the persistence of the dHvA signal deep within the mixed state of threedimensional extreme type-II systems ͑A-15's, 2 nonmagnetic borocarbides, 7 and MgB 2 superconductors 9 ͒ can be attributed to the presence of a small portion of the Fermi surface containing gapless and near gapless quasiparticle excitations, surrounded by regions where the gap is large. 23,24 The purpose of this work is to address yet another probe of lowtemperature quasiparticle excitations-the tunneling current.…”