39 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
Flux-flow induced giant magnetoresistance in all-amorphous superconductor-ferromagnet hybrids
We present magnetoresistance measurements on all-amorphous ferromagnet (F) /
superconductor (S) heterostructures. The F/S/F trilayers show large
magnetoresistance peaks in a small field range around the coercive field of the
F layers, at temperatures within and below the superconducting transition. This
is interpreted as flux flow of weakly pinned vortices induced by the stray
field of Bloch magnetic domains in the F layers. Bilayers show much smaller
effects, implying that the Bloch walls of the F-layers in the trilayer line up
and focus the stray fields. The data are used to discuss the expected minimum
F-layer thickness needed to nucleate vortices.Comment: 6 pages, 6 figure
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
Increasing the performance of the superconducting spin valve using a Heusler alloy
We have studied superconducting properties of the spin-valve thin layer
heterostructures CoO/F1/Cu/F2/Cu/Pb where the ferromagnetic F1 layer was
standardly made of Permalloy whereas for the F2 layer we have taken a specially
prepared film of the Heusler alloy CoCrFeAl with a small degree
of spin polarization of the conduction band. The heterostructures demonstrate a
significant superconducting spin-valve effect, i.e. a complete switching on and
off of the superconducting current flowing through the system by manipulating
the mutual orientations of the magnetization of the F1 and F2 layers. The
magnitude of the effect is doubled in comparison with the previously studied
analogous multilayers with the F2 layer made of the strong ferromagnet Fe.
Theoretical analysis shows that a drastic enhancement of the switching effect
is due to a smaller exchange field in the heterostructure coming from the
Heusler film as compared to Fe. This enables to approach almost ideal
theoretical magnitude of the switching in the Heusler-based multilayer with the
F2 layer thickness of \,nm
Odd-frequency superconducting states with different types of Meissner response:Problem of coexistence
We consider physical properties of a superconductor with a recently proposed
type of odd-frequency pairing that exhibits diamagnetic Meissner response
("odd-dia state"). Such a state was suggested in order to address stability
issues arising in an odd-frequency superconducting state with paramagnetic
Meissner response ("odd-para state"). Assuming the existence of an odd-dia
state (due to a proper retarded interaction), we study its coexistence with an
odd-para state. The latter is known to be generated as an induced
superconducting component in, e.g., singlet superconductor/ferromagnet
proximity structures or triplet superconductor/normal metal systems.
Calculating the superfluid density of the mixed odd-para/odd-dia state and the
Josephson current between the odd-para and odd-dia states, we find that the
expressions for the currents in both cases have non-vanishing imaginary
contributions and are therefore unphysical. We show that a realization of the
odd-dia state implies the absence of a Hamiltonian description of the system,
and suggest that there exists no physically realizable perturbation that could
give rise to the spontaneous symmetry breaking necessary for an actual
realization of the odd-dia superconducting state.Comment: 16 pages. Version 2: updated references; final version as published
in PR
Local impedance on a rough surface of a chiral -wave superconductor
We develop a self-consistent approach for calculating the local impedance at
a rough surface of a chiral -wave superconductor. Using the quasiclassical
Eilenberger-Larkin-Ovchinnikov formalism, we numerically find the pair
potential, pairing functions, and the surface density of states taking into
account diffusive electronic scattering at the surface. The obtained solutions
are then employed for studying the local complex conductivity and surface
impedance in the broad range of microwave frequencies (ranging from subgap to
above-gap values). We identify anomalous features of the surface impedance
caused by generation of odd-frequency superconductivity at the surface. The
results are compared with experimental data for SrRuO and provide a
microscopic explanation of the phenomenological two-fluid model suggested
earlier to explain anomalous features of the microwave response in this
material.Comment: 19 pages, 10 figures. Version 2: final version as published in PR
Resonant proximity effect in normal metal / diffusive ferromagnet / superconductor junctions
Resonant proximity effect in the normal metal / insulator / diffusive
ferromagnet / insulator / - and d-wave superconductor (N/I/DF/I/S) junctions
is studied for various regimes by solving the Usadel equation with the
generalized boundary conditions. It is shown that the resonant proximity effect
originating from the exchange field in DF layer strongly modifies the tunneling
conductance and density of states.Comment: 14 pages, 11 figures, sequel to Phys. Rev. B 72, 052512
(2005)(cond-mat/0510657
Theory of two-dimensional macroscopic quantum tunneling in YBa Cu O Josephson junctions coupled to an LC circuit
We investigate classical thermal activation (TA) and macroscopic quantum
tunneling (MQT) for a YBaCuO (YBCO) Josephson junction
coupled to an LC circuit theoretically. Due to the coupling between the
junction and the LC circuit, the macroscopic phase dynamics can be described as
the escape process of a fictitious particle with an anisotropic mass moving in
a two-dimensional potential. We analytically calculate the escape rate
including both the TA and MQT regime by taking into account the peculiar
dynamical nature of the system. In addtion to large suppression of the MQT rate
at zero temperature, we study details of the temperature dependece of the
escape rate across a crossover region. These results are in an excellent
agreement with recent experimental data for the MQT and TA rate in a YBCO
biepitaxial Josephson junction. Therefore the coupling to the LC circuit is
essential in understanding the macroscopic quantum dynamics and the qubit
operation based on the YBCO biepitaxial Josephson junctions.Comment: 13pages, 7 figures, 1 table, to appear in Phys. Rev. B 80 (2009
On the Meissner Effect of the Odd-Frequency Superconductivity with Critical Spin Fluctuations: Possibility of Zero Field FFLO pairing
We investigate the influence of critical spin fluctuations on electromagnetic
responses in the odd-frequency superconductivity. It is shown that the Meissner
kernel of the odd-frequency superconductivity is strongly reduced by the
critical spin fluctuation or the massless spin wave mode in the
antiferromagnetic phase. These results imply that the superfluid density is
reduced, and the London penetration depth is lengthened for the odd-frequency
pairing. It is also shown that the zero field Flude-Ferrell-Larkin-Ovchinnikov
pairing is spontaneously realized both for even- and odd-frequency in the case
of sufficiently strong coupling with low lying spin-modes.Comment: 10 pages, 7 figure
Proximity effects and characteristic lengths in ferromagnet-superconductor structures
We present an extensive theoretical investigation of the proximity effects
that occur in Ferromagnet/Superconductor () systems. We use a numerical
method to solve self consistently the Bogoliubov-de Gennes equations in the
continuum. We obtain the pair amplitude and the local density of states (DOS),
and use these results to extract the relevant lengths characterizing the
leakage of superconductivity into the magnet and to study spin splitting into
the superconductor. These phenomena are investigated as a function of
parameters such as temperature, magnet polarization, interfacial scattering,
sample size and Fermi wavevector mismatch, all of which turn out to have
important influence on the results. These comprehensive results should help
characterize and analyze future data and are shown to be in agreement with
existing experiments.Comment: 24 pages, including 26 figure