Superconducting Diamagnetic Fluctuations in MgB2

The fluctuating diamagnetic magnetization Mfl at constant field H as a function of temperature and the isothermal magnetization Mfl vs H are measured in MgB2, above the superconducting transition temperature. The expressions for Mfl in randomly oriented powders are derived in the Gaussian approximation of local Ginzburg-Landau theory and used for the analysis of the data. The scaled magnetization Mfl/H^{1/2}*T is found to be field dependent. In the limit of evanescent field the behaviour for Gaussian fluctuations is obeyed while for H>~ 100 Oe the field tends to suppress the fluctuating pairs, with a field dependence of Mfl close to the one expected when short wavelength fluctuations and non-local electrodynamic effects are taken into account. Our data, besides providing the isothermal magnetization curves for T>Tc(0) in a BCS-type superconductor such as MgB2, evidence an enhancement of the fluctuating diamagnetism which is related to the occurrence in this new superconductor of an anisotropic spectrum of the superconducting fluctuations.Comment: Tex file, 4 pages, 3 ps figures, submitted to Phys. Rev. Let

On the distinguishability of random quantum states

We develop two analytic lower bounds on the probability of success p of identifying a state picked from a known ensemble of pure states: a bound based on the pairwise inner products of the states, and a bound based on the eigenvalues of their Gram matrix. We use the latter to lower bound the asymptotic distinguishability of ensembles of n random quantum states in d dimensions, where n/d approaches a constant. In particular, for almost all ensembles of n states in n dimensions, p>0.72. An application to distinguishing Boolean functions (the "oracle identification problem") in quantum computation is given.Comment: 20 pages, 2 figures; v2 fixes typos and an error in an appendi

The 2-Channel Kondo Model I: Review of Experimental Evidence for its Realization in Metal Nanoconstrictions

Certain zero-bias anomalies (ZBAs) in the voltage, temperature and magnetic field dependence of the conductance $G(V,T,H)$ of quenched Cu point contacts have previously been interpreted to be due to non-magnetic 2-channel Kondo (2CK) scattering from near-degenerate atomic two-level tunneling systems (Ralph and Buhrman, 1992; Ralph et al. 1994), and hence to represent an experimental realization of the non-Fermi-liquid physics of the T=0 fixed point of the 2-channel Kondo model. In this, the first in a series of three papers (I,II,III) devoted to 2-channel Kondo physics, we present a comprehensive review of the quenched Cu ZBA experiments and their 2CK interpretation, including new results on ZBAs in constrictions made from Ti or from metallic glasses. We first review the evidence that the ZBAs are due to electron scattering from stuctural defects that are not static, but possess internal dynamics. In order to distinguish between several mechanisms proposed to explain the experiments, we then analyze the scaling properties of the conductance at low temperature and voltage and extract from the data a universal scaling function $\Gamma(v)$. The theoretical calculation of the corresponding scaling function within the 2CK model is the subject of papers II and III. The main conclusion of our work is that the properties of the ZBAs, and most notably their scaling behavior, are in good agreement with the 2CK model and clearly different from several other proposed mechanisms.Comment: 35 pages RevTeX, 19 encapsulated postscript figures; this final published version features two additional authors, an additional section reviewing recent experiments on Ti nanoconstrictions that agree very well with the 2-channel Kondo model, 6 new figures (and is much shorter the previous 53 page version, due to reformatting

Unbounded violation of tripartite Bell inequalities

We prove that there are tripartite quantum states (constructed from random unitaries) that can lead to arbitrarily large violations of Bell inequalities for dichotomic observables. As a consequence these states can withstand an arbitrary amount of white noise before they admit a description within a local hidden variable model. This is in sharp contrast with the bipartite case, where all violations are bounded by Grothendieck's constant. We will discuss the possibility of determining the Hilbert space dimension from the obtained violation and comment on implications for communication complexity theory. Moreover, we show that the violation obtained from generalized GHZ states is always bounded so that, in contrast to many other contexts, GHZ states do in this case not lead to extremal quantum correlations. The results are based on tools from the theories of operator spaces and tensor norms which we exploit to prove the existence of bounded but not completely bounded trilinear forms from commutative C*-algebras.Comment: Substantial changes in the presentation to make the paper more accessible for a non-specialized reade

CARBON POWDER MAGNETIZATION THERMOMETRY FOR VERY LOW TEMPERATURES

Nous avons étudié l'aimantation statique d'un noir de carbone amorphe (Carbolac I) en fonction de la température en vue d' utiliser ce matériau comme thermomètre à basse température. Nous trouvons que l'aimantation suit une loi de Curie au-dessus de 10 mK. Les mesures, effectuées dans une série de champs magnétiques, suggèrent un système paramagnétique de spin 1 ayant un facteur g égal à 2.We have investigated the temperature dependent static magnetization of an amorphous carbon black (Carbolac I) for use as a low temperature secondary thermometer. We find that the magnetization obeys Curie's law above 10 mK. Measurements performed in a number of magnetic fields are consistent with a spin I paramagnetic system with a g-factor of 2