815 research outputs found
Isotope effect on the superfluid density in conventional and high-temperature superconductors
We investigate the isotope effect on the London penetration depth of a
superconductor which measures , the ratio of superfluid density to
effective mass. We use a simplified model of electrons weakly coupled to a
single phonon frequency , but assume that the energy gap
does not have any isotope effect. Nevertheless we find an isotope effect for
which is significant if is sufficiently large that it
becomes comparable to , a regime of interest to high cuprate
superconductors and possibly other families of unconventional superconductors
with relatively high . Our model is too simple to describe the cuprates
and it gives the wrong sign of the isotope effect when compared with
experiment, but it is a proof of principle that the isotope effect exists for
in materials where the pairing gap and is not of phonon origin
and has no isotope effect.Comment: 9 pages, 6 figure
Calculating critical temperatures of superconductivity from a renormalized Hamiltonian
It is shown that one can obtain quantitatively accurate values for the
superconducting critical temperature within a Hamiltonian framework. This is
possible if one uses a renormalized Hamiltonian that contains an attractive
electron-electron interaction and renormalized single particle energies. It can
be obtained by similarity renormalization or using flow equations for
Hamiltonians. We calculate the critical temperature as a function of the
coupling using the standard BCS-theory. For small coupling we rederive the
McMillan formula for Tc. We compare our results with Eliashberg theory and with
experimental data from various materials. The theoretical results agree with
the experimental data within 10%. Renormalization theory of Hamiltonians
provides a promising way to investigate electron-phonon interactions in
strongly correlated systems.Comment: 6 pages, LaTeX, using EuroPhys.sty, one eps figure included, accepted
for publication in Europhys. Let
Suppression of superconductivity by Neel-type magnetic fluctuations in the iron pnictides
Motivated by recent experimental detection of Neel-type ()
magnetic fluctuations in some iron pnictides, we study the impact of competing
and spin fluctuations on the superconductivity of these
materials. We show that, counter-intuitively, even short-range, weak Neel
fluctuations strongly suppress the state, with the main effect arising
from a repulsive contribution to the pairing interaction, complemented
by low frequency inelastic scattering. Further increasing the strength of the
Neel fluctuations leads to a low- d-wave state, with a possible
intermediate phase. The results suggest that the absence of
superconductivity in a series of hole-doped pnictides is due to the combination
of short-range Neel fluctuations and pair-breaking impurity scattering, and
also that of optimally doped pnictides could be further increased if
residual fluctuations were reduced.Comment: revised version accepted for publication in PR
The electron-phonon coupling strength at metal surfaces directly determined from the Helium atom scattering Debye-Waller factor
A new quantum-theoretical derivation of the elastic and inelastic scattering
probability of He atoms from a metal surface, where the energy and momentum
exchange with the phonon gas can only occur through the mediation of the
surface free-electron density, shows that the Debye-Waller exponent is directly
proportional to the electron-phonon mass coupling constant . The
comparison between the values of extracted from existing data on the
Debye-Waller factor for various metal surfaces and the values known
from literature indicates a substantial agreement, which opens the possibility
of directly extracting the electron-phonon coupling strength in quasi-2D
conducting systems from the temperature or incident energy dependence of the
elastic Helium atom scattering intensities.Comment: 14 pages, 2 figures, 1 tabl
Sum Rules and Ward Identities in the Kondo Lattice
We derive a generalized Luttinger-Ward expression for the Free energy of a
many body system involving a constrained Hilbert space. In the large limit,
we are able to explicity write the entropy as a functional of the Green's
functions. Using this method we obtain a Luttinger sum rule for the Kondo
lattice. One of the fascinating aspects of the sum rule, is that it contains
two components, one describing the heavy electron Fermi surface, the other, a
sea of oppositely charged, spinless fermions. In the heavy electron state, this
sea of spinless fermions is completely filled and the electron Fermi surface
expands by one electron per unit cell to compensate the positively charged
background, forming a ``large'' Fermi surface. Arbitrarily weak magnetism
causes the spinless Fermi sea to annihilate with part of the Fermi sea of the
conduction electrons, leading to a small Fermi surface. Our results thus enable
us to show that the Fermi surface volume contracts from a large, to a small
volume at a quantum critical point. However, the sum rules also permit the
possible formation of a new phase, sandwiched between the antiferromagnet and
the heavy electron phase, where the charged spinless fermions develop a true
Fermi surface.Comment: 24 pages, 4 figures. Version two contains a proof of the "Entropy
formula" which connects the entropy directly to the Green's functions.
Version three contains corrections to typos and a more extensive discussion
of the physics at finite
The Complex Gap in Color Superconductivity
We solve the gap equation for color-superconducting quark matter in the 2SC
phase, including both the energy and the momentum dependence of the gap,
\phi=\phi(k_0,\vk). For that purpose a complex Ansatz for \phi is made. The
calculations are performed within an effective theory for cold and dense quark
matter. The solution of the complex gap equation is valid to subleading order
in the strong coupling constant g and in the limit of zero temperature. We find
that, for momenta sufficiently close to the Fermi surface and for small
energies, the dominant contribution to the imaginary part of arises from
Landau-damped magnetic gluons. Further away from the Fermi surface and for
larger energies the other gluon sectors have to be included into Im\phi. We
confirm that Im contributes a correction of order g to the prefactor of
\phi for on-shell quasiquarks sufficiently close to the Fermi surface, whereas
further away from the Fermi surface Im\phi and Re\phi are of the same order.
Finally, we discuss the relevance of Im\phi for the damping of quasiquark
excitations.Comment: 23 pages, 3 figures, 8 tables. Typos corrected, minor corrections to
the text. Accepted for publication in PR
Dynamics of spin transport in voltage-biased Josephson junctions
We investigate spin transport in voltage-biased spin-active Josephson
junctions. The interplay of spin filtering, spin mixing, and multiple Andreev
reflection leads to nonlinear voltage dependence of the dc and ac spin current.
We compute the voltage characteristics of the spin current (I_S) for
superconductor-ferromagnet-superconductor (SFS) Josephson junctions. The
sub-harmonic gap structure of I_S(V) is shown to be sensitive to the degree of
spin mixing generated by the ferromagnetic interface, and exhibits a pronounced
even-odd effect associated with spin transport during multiple Andreev
reflection processes. For strong spin mixing both the magnitude and the
direction of the dc spin current can be sensitively controlled by the bias
voltage.Comment: 4 pages, 3 figure
Antiferromagnetism and singlet formation in underdoped high-Tc cuprates: Implications for superconducting pairing
The extended model is theoretically studied, in the context of hole
underdoped cuprates. Based on results obtained by recent numerical studies, we
identify the mean field state having both the antiferromagnetic and staggered
flux resonating valence bond orders. The random-phase approximation is employed
to analyze all the possible collective modes in this mean field state. In the
static (Bardeen Cooper Schrieffer) limit justified in the weak coupling regime,
we obtain the effective superconducting interaction between the doped holes at
the small pockets located around . In contrast
to the spin-bag theory, which takes into acccount only the antiferromagnetic
order, this effective force is pair breaking for the pairing without the nodes
in each of the small hole pocket, and is canceled out to be very small for the
pairing with nodes which is realized in the real cuprates.
Therefore we conclude that no superconducting instability can occur when only
the magnetic mechanism is considered. The relations of our work with other
approaches are also discussed.Comment: 20 pages, 7 figures, REVTeX; final version accepted for publicatio
The internal Josephson effect in a Fermi gas near a Feshbach resonance
We consider a two-component system of Fermi atoms and molecular bosons in the
vicinity of a Feshbash resonance. We derive an effective action for the system,
which has a term describing coherent tunneling of the molecular bosons into
Cooper pairs and vice versa. In the equilibrium state, global phase coherence
may be destroyed by thermal or quantum phase fluctuations. In the
non-equilibrium regime, the system may show an internal AC Josephson effect
leading to real time oscillations in the number of molecular bosons.Comment: 5 pages, 2 figure
Quasiparticle scattering time in superconducting films: from dirty to clean limit
We study the quasiparticle energy relaxation processes in superconducting Nb
films of different thicknesses corresponding to different electron mean free
paths in a state far from equilibrium, that is the highly dissipative flux-flow
state driven up to the instability point. From the measured current-voltage
curves we derive the vortex critical velocity for several temperatures.
From the values, the quasiparticle energy relaxation time
is evaluated within the Larkin-Ovchinnikov model and
numerical calculations of the quasiparticle energy relaxation rates are carried
out to support the experimental findings. Besides the expected constant
behavior of for the dirty samples, we observe a strong
temperature dependence of the quasiparticle energy relaxation time in the clean
samples. This feature is associated with the increasing contribution from the
electron-phonon scattering process as the dirty limit is approached from the
clean regime
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