194 research outputs found
Evidence for Triplet Superconductivity in a Superconductor-Ferromagnet Spin Valve
We have studied the dependence of the superconducting (SC) transition
temperature on the mutual orientation of magnetizations of Fe1 and Fe2 layers
in the spin valve system CoO_x/Fe1/Cu/Fe2/Pb. We find that this dependence is
nonmonotonic when passing from the parallel to the antiparallel case and
reveals a distinct minimum near the orthogonal configuration. The analysis of
the data in the framework of the SC triplet spin valve theory gives direct
evidence for the long-range triplet superconductivity arising due to
noncollinearity of the two magnetizations.Comment: 5 pages (including 4 EPS figures). Version 2: final version as
published in PR
Decoherence due to nodal quasiparticles in d-wave qubits
We study the Josephson junction between two d-wave superconductors, which is
discussed as an implementation of a qubit. We propose an approach that allows
to calculate the decoherence time due to an intrinsic dissipative process:
quantum tunneling between the two minima of the double-well potential excites
nodal quasiparticles which lead to incoherent damping of quantum oscillations.
The decoherence is weakest in the mirror junction, where the contribution of
nodal quasiparticles corresponds to the superohmic dissipation and becomes
small at small tunnel splitting of the energy level in the double-well
potential. For available experimental data, we estimate the quality factor.Comment: 5 pages, 3 EPS figures; the style file jetpl.cls is included. Version
2: minor correction
Density of states in d-wave superconductors of finite size
We consider the effect of the finite size in the ab-plane on the surface
density of states (DoS) in clean d-wave superconductors. In the bulk, the DoS
is gapless along the nodal directions, while the presence of a surface leads to
formation of another type of the low-energy states, the midgap states with zero
energy. We demonstrate that finiteness of the superconductor in one of
dimensions provides the energy gap for all directions of quasiparticle motion
except for \theta=45 degrees (\theta is the angle between the trajectory and
the surface normal); then the angle-averaged DoS behaves linearly at small
energies. This result is valid unless the crystal is 0- or 45-oriented (\alpha
\ne 0 or 45 degrees, where \alpha is the angle between the a-axis and the
surface normal). In the special case of \alpha=0, the spectrum is gapped for
all trajectories \theta; the angle-averaged DoS is also gapped. In the special
case of \alpha=45, the spectrum is gapless for all trajectories \theta; the
angle-averaged DoS is then large at low energies. In all the cases, the
angle-resolved DoS consists of energy bands that are formed similarly to the
Kronig-Penney model. The analytical results are confirmed by a self-consistent
numerical calculation.Comment: 9 pages (including 5 EPS figures), REVTeX
Anomalous Surface Impedance due to Odd-frequency Cooper Pairs
We discuss dynamical response of odd-frequency Cooper pairs to
electromagnetic field. By using the quasiclassical Green function method, we
calculate the surface impedance (Z=R-iX) of a normal metal thin film which
covers a superconductor. In contrast to the standard relation (i.e., R << X),
the surface impedance in spin-triplet proximity structures shows anomalous
behavior (i.e., R>X) at low temperatures. This unusual relation is a result of
the penetration of odd-frequency pairs into the normal metal and reflects the
negative Cooper pair density.Comment: 5 pages, 3 figures embede
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