361 research outputs found
Nonperturbative Scaling Theory of Free Magnetic Moment Phases in Disordered Metals
The crossover between a free magnetic moment phase and a Kondo phase in low
dimensional disordered metals with dilute magnetic impurities is studied.
We perform a finite size scaling analysis of the distribution of the Kondo
temperature as obtained from a numerical renormalization group calculation of
the local magnetic susceptibility and from the solution of the self-consistent
Nagaoka-Suhl equation. We find a sizable fraction of free (unscreened) magnetic
moments when the exchange coupling falls below a disorder-dependent critical
value . Our numerical results show that between the free moment
phase due to Anderson localization and the Kondo screened phase there is a
phase where free moments occur due to the appearance of random local pseudogaps
at the Fermi energy whose width and power scale with the elastic scattering
rate .Comment: 4 pages, 6 figure
Nucleation of superconducting pairing states at mesoscopic scales at zero temperature
We find the spin polarized disordered Fermi liquids are unstable to the
nucleation of superconducting pairing states at mesoscopic scales even when
magnetic fields which polarize the spins are substantially higher than the
critical one. We study the probability of finding superconducting pairing
states at mesoscopic scales in this limit. We find that the distribution
function depends only on the film conductance. The typical length scale at
which pairing takes place is universal, and decreases when the magnetic field
is increased. The number density of these states determines the strength of the
random exchange interactions between mesoscopic pairing states.Comment: 11 pages, no figure
Quantum replica approach to the under-screened Kondo model
We extend the Schwinger boson large N treatment of the underscreened Kondo
model in a way that correctly captures the finite elastic phase shift in the
singular Fermi liquid. The new feature of the approach, is the introduction of
a flavor quantum number with K possible values, associated with the Schwinger
boson representation. The large N limit is taken maintaining the ratio k=K/N
fixed. This approach differs from previous approaches, in that we do not
explicitly enforce a constraint on the spin representation of the Schwinger
bosons. Instead, the energetics of the Kondo model cause the bosonic degrees of
freedom to ``self assemble'' into a ground-state in which the spins of K bosons
and N-K conduction electrons are antisymmetrically arranged into a Kondo
singlet. With this device, the large N limit can be taken, in such a way that a
fraction K/N of the Abrikosov Suhl resonance is immersed inside the Fermi sea.
We show how this method can be used to model the full energy dependence of the
singular Abrikosov Suhl resonance in the underscreened Kondo model and the
field-dependent magnetization.Comment: Revised draft, with plots explicitly showing logarithmic scaling of
inverse coupling constant. Small corrections prior to submission to journa
Andreev magnetotransport in low-dimensional proximity structures: Spin-dependent conductance enhancement
We study the excess conductance due to the superconducting proximity effect
in a ballistic two-dimensional electron system subject to an in-plane magnetic
field. We show that under certain conditions the interplay of the Zeeman spin
splitting and the effect of a screening supercurrent gives rise to a
spin-selective Andreev enhancement of the conductance and anomalies in its
voltage, temperature and magnetic field characteristics. The magnetic-field
influence on Andreev reflection is discussed in the context of using
superconducting hybrid junctions for spin detection.Comment: 4 pages, 5 figure
Gapless Color Superconductivity
We present the dispersion relations for quasiparticle excitations about the
color-flavor locked ground state of QCD at high baryon density. In the presence
of condensates which pair light and strange quarks there need not be an energy
gap in the quasiparticle spectrum. This raises the possibility of gapless color
superconductivity, with a Meissner effect but no minimum excitation energy.
Analysis within a toy model suggests that gapless color superconductivity may
occur only as a metastable phase.Comment: 4 pages, Revtex, eps figures include
Two tricritical lines from a Ginzburg-Landau expansion: application to the LOFF phase
We study the behavior of the two plane waves configuration in the LOFF phase
close to T=0. The study is performed by using a Landau-Ginzburg expansion up to
the eighth order in the gap. The general study of the corresponding grand
potential shows, under the assumption that the eighth term in the expansion is
strictly positive, the existence of two tricritical lines. This allows to
understand the existence of a second tricritical point for two antipodal plane
waves in the LOFF phase and justifies why the transition becomes second order
at zero temperature. The general analysis done in this paper can be applied to
other cases.Comment: LaTex file, 15 pages, 6 figure
Paramagnetic Breakdown of Superconductivity in Ultrasmall Metallic Grains
We study the magnetic-field-induced breakdown of superconductivity in
nm-scale metal grains having a mean electron level spacing (bulk gap). Using a generalized variational BCS approach that
yields good qualitative agreement with measured spectra, we argue that Pauli
paramagnetism dominates orbital diamagnetism, as in the case of thin films in a
parallel magnetic field. However, the first-order transition observed for the
latter can be made continuous by finite size effects. The mean-field procedure
of describing the system by a single pairing parameter breaks down for
.Comment: 4 pages of revtex, 3 postscript figures, uses psfrag.sty, epsfig.sty.
Slightly revised and improved version, matching published versio
Localized versus itinerant magnetic moments in Na0.72CoO2
Based on experimental 59Co-NMR data in the temperature range between 0.1 and
300 K, we address the problem of the character of the Co 3d-electron based
magnetism in Na0.7CoO2. Temperature dependent 59Co-NMR spectra reveal different
Co environments below 300 K and their differentiation increases with decreasing
temperature. We show that the 23Na- and 59Co-NMR data may consistently be
interpreted by assuming that below room temperature the Co 3d-electrons are
itinerant. Their magnetic interaction appears to favor an antiferromagnetic
coupling, and we identify a substantial orbital contribution corb to the
d-electron susceptibility. At low temperatures corb seems to acquire some
temperature dependence, suggesting an increasing influence of spin-orbit
coupling. The temperature dependence of the spin-lattice relaxation rate
T1-1(T) confirms significant variations in the dynamics of this electronic
subsystem between 200 and 300K, as previously suggested. Below 200 K, Na0.7CoO2
may be viewed as a weak antiferromagnet with TN below 1 K but this scenario
still leaves a number of open questions.Comment: 8.7 pages, 6 Figures, submitted to Phys. Rev.
Superconducting Properties under Magnetic Field in NaCoOHO Single Crystal
We report the in-plane resistivity and magnetic susceptibility of the layered
cobalt oxide NaCoOHO single crystal. The
temperature dependence of the resistivity shows metallic behavior from room
temperature to the superconducting transition temperature of 4.5 K.
Sharp resistive transition, zero resistivity and almost perfect superconducting
volume fraction below indicate the good quality and the bulk
superconductivity of the single crystal. The upper critical field and
the coherence length are obtained from the resistive transitions in
magnetic field parallel to the c-axis and the -plane. The anisotropy of
, 12 nm/1.3 nm 9.2, suggests that this
material is considered to be an anisotropic three dimensional superconductor.
In the field parallel to the -plane, seems to be suppressed to the
value of Pauli paramagnetic limit. It may indicate the spin singlet
superconductivity in the cobalt oxide.Comment: 4 pages, 4 figure
Superconductivity in an organic insulator at very high magnetic fields
We investigate by electrical transport the field-induced superconducting
state (FISC) in the organic conductor -(BETS)FeCl. Below 4 K,
antiferromagnetic-insulator, metallic, and eventually superconducting (FISC)
ground states are observed with increasing in-plane magnetic field. The FISC
state survives between 18 and 41 T, and can be interpreted in terms of the
Jaccarino-Peter effect, where the external magnetic field {\em compensates} the
exchange field of aligned Fe ions. We further argue that the Fe
moments are essential to stabilize the resulting singlet, two-dimensional
superconducting stateComment: 9 pages 3 figure
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