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 $J_{\rm c}$. 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 $1/\tau$.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 $d \simeq \tilde\Delta$ (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 $\Delta$ breaks down for $d \simeq \tilde\Delta$.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 Na0.35_{0.35}CoO2⋅1.3_{2}{\cdot}1.3H2_{2}O Single Crystal

We report the in-plane resistivity and magnetic susceptibility of the layered cobalt oxide Na$_{0.35}$CoO$_{2}{\cdot}1.3$H$_{2}$O single crystal. The temperature dependence of the resistivity shows metallic behavior from room temperature to the superconducting transition temperature $T_{c}$ of 4.5 K. Sharp resistive transition, zero resistivity and almost perfect superconducting volume fraction below $T_{c}$ indicate the good quality and the bulk superconductivity of the single crystal. The upper critical field $H_{c2}$ and the coherence length $\xi$ are obtained from the resistive transitions in magnetic field parallel to the c-axis and the $ab$-plane. The anisotropy of $\xi$, $\xi_{ab} / \xi_{c} =$ 12 nm/1.3 nm $\simeq$ 9.2, suggests that this material is considered to be an anisotropic three dimensional superconductor. In the field parallel to the $ab$-plane, $H_{c2}$ 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 $\lambda$-(BETS)$_2$FeCl$_4$. 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$^{3+}$ ions. We further argue that the Fe$^{3+}$ moments are essential to stabilize the resulting singlet, two-dimensional superconducting stateComment: 9 pages 3 figure