Search results for "Condensed Matter::Superconductivity"
showing 10 items of 632 documents
Temperature and magnetic-field dependence of quantum creep in various high-Tcsuperconductors
1999
To investigate the quantum tunneling of almost macroscopic vortex segments we measured the normalized relaxation rate Q of superconducting currents for various high-T{sub c} superconductors (HTS's) down to 100 mK in magnetic fields up to 7 T. At fields below {approx_equal}0.5 T, Q is essentially independent of T in the temperature regime between thermally activated and quantum motion as theoretically expected. However, at higher fields, we find an unexpected linear T dependence of Q persisting down to the lowest temperatures in all investigated samples. Since these compounds were chosen to represent the distinct classes of dirty and clean HTS's, the extrapolated Q(T=0) values are used to di…
Sub-kelvin current amplifier using DC-SQUID
2000
Abstract We have set up a system where a low-noise DC-SQUID is used as a current amplifier. The SQUID output is read using a wide band electronics unit based on the noise cancellation scheme. The SQUID has been installed in a compact Nanoway PDR50 dilution refrigerator, and superconducting transitions of Ti/Au thermometer strips for X-ray calorimeter applications have been measured. We can operate at 100 mK using a SQUID with Pd shunt resistors. Noise and bandwidth results of the setup are presented.
Nature of Heavy Quasiparticles in Magnetically Ordered Heavy FermionsUPd2Al3andUPt3
2002
The optical conductivity of the heavy fermions $\mathrm{UPd}{}_{2}{\mathrm{Al}}_{3}$ and $\mathrm{UPt}{}_{3}$ has been measured in the energy range from 0.04 to 5 meV. In both compounds a well pronounced pseudogap of less than 1 meV develops in the optical response at low temperatures; we relate this to the antiferromagnetic ordering. From the energy dependence of the effective mass and scattering rate we conclude that the enhancement of the mass mainly occurs below the energy which is related to magnetic correlations between the local magnetic moments and the itinerant electrons. This implies that the magnetic order in these compounds is the prerequisite to the formation of the heavy quasi…
Origin of the plateau in the temperature dependence of the normalized magnetization relaxation rate in disordered high-temperature superconductors
2008
The temperature $T$ dependence of the normalized magnetization relaxation rate $S$ in optimally doped ${\text{YBa}}_{2}{\text{Cu}}_{3}{\text{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ films with the external dc magnetic field $H$ oriented along the $c$ axis exhibits the well-known plateau in the intermediate $T$ range, associated with the presence of elastic (collective) vortex creep. The disappearance of the $S(T)$ plateau in the high-$H$ domain $(H\ensuremath{\ge}20\text{ }\text{kOe})$ is not completely understood. We show that in the case of high-temperature superconductors with significant quenched disorder the $S(T)$ plateau is directly related to a crossover in the vortex-creep process…
Fluxons in Josephson Transmission Lines
1996
The superconducting Josephson junctions (Barone and Paterno 1982; Parmentier 1978; Lomdahl 1985; Likharev 1986; Pedersen 1986) have proven to be one of the most successful testing grounds for nonlinear wave theory; their use for information processing and storage is quite attractive. In the long Josephson junction or transmission line, the physical quantity of interest is a quantum of magnetic flux, or a fluxon, which has a soliton behavior. It is a remarkably robust and stable object, which can be easily manipulated at high speed and stored electronically. Consequently it should be used as a basic bit in information processing systems.
Evidence for Multiband Superconductivity in the Heavy Fermion CompoundUNi2Al3
2004
Epitaxial thin films of the heavy fermion superconductor UNi2Al3 with Tc(max)=0.98 K were investigated. The transition temperature Tc depends on the current direction which can be related to superconducting gaps opening at different temperatures. Also the influence of the magnetic ordering at TN approximately 5 K on R(T) is strongly anisotropic, indicating different coupling between the magnetic moments and itinerant charge carriers on the multisheeted Fermi surface. The upper critical field Hc2(T) suggests an unconventional spin-singlet superconducting state.
Guided vortex motion in Nb films on facetted substrate surfaces
2003
Abstract Anisotropy of the pinning force in a superconductor can cause a guiding effect on the vortices, which leads to the appearance of new components in the galvanomagnetic quantities of the sample. In this case one can observe an additional odd magnetoresistive component with respect to magnetic field reversal. Furthermore, an even contribution to the Hall voltage is observed. Guided motion of vortices in Nb films on facetted α-Al2O3 (1 0 1 0) was found by measuring the longitudinal and transversal resistivities of three films with transport current directed parallel, perpendicular and at an angle of 45° with respect to the facet ridges. Field inversion was used to separate the even and…
Microscopic model for multiple flux transitions in mesoscopic superconducting loops
2006
A microscopic model is constructed which is able to describe multiple magnetic flux transitions as observed in recent ultra-low temperature tunnel experiments on an aluminum superconducting ring with normal metal - insulator - superconductor junctions [Phys. Rev. B \textbf{70}, 064514 (2004)]. The unusual multiple flux quantum transitions are explained by the formation of metastable states with large vorticity. Essential in our description is the modification of the pairing potential and the superconducting density of states by a sub-critical value of the persistent current which modulates the measured tunnel current. We also speculate on the importance of the injected non-equilibrium quasi…
THE OPEN BCS MODEL, ITS STOCHASTIC LIMIT AND SOME GENERALIZATIONS
2005
In this paper we use the stochastic limit approach as a tool to discuss the open BCS model of low temperature superconductivity. We also briefly discuss the role of a second reservoir interacting with the first one (but not with the system) in the computation of the critical temperature corresponding to the transition from a normal to a superconducting phase.
Density of Phonon States in Superconducting FeSe as a Function of Temperature and Pressure
2010
The temperature and pressure dependence of the partial density of phonon states (phonon-DOS) of iron atoms in superconducting ${\text{Fe}}_{1.01}\text{Se}$ was studied by $^{57}\text{F}\text{e}$ nuclear inelastic scattering. The high-energy resolution allows for a detailed observation of spectral properties. A sharpening of the optical phonon modes and shift of all spectral features toward higher energies by $\ensuremath{\sim}4\mathrm{%}$ with decreasing temperature from 296 to 10 K was found. However, no detectable change at the tetragonal--orthorhombic phase transition around 100 K was observed. Application of a pressure of 6.7 GPa, connected with an increase in the superconducting temper…