Fluctuations in the Diamagnetic Response of Disordered Metals (original) (raw)
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A RIGOROUS APPROACH TO THE MAGNETIC RESPONSE IN DISORDERED SYSTEMS
Reviews in Mathematical Physics, 2012
This paper is a part of an ongoing study on the diamagnetic behavior of a quantum gas of non-interacting charged particles subjected to an external uniform magnetic field together with a random electric potential. We prove the existence of an almost-sure non-random thermodynamic limit for the grand-canonical pressure, magnetization and zero-field orbital magnetic susceptibility. We also give an explicit formulation of these thermodynamic limits. Our results cover a wide class of physically relevant random potentials which model not only crystalline disordered solids, but also amorphous solids.
Free magnetic moments in disordered metals
Journal of Experimental and Theoretical Physics Letters, 2006
The screening of magnetic moments in metals, the Kondo effect, is found to be quenched with a finite probability in the presence of nonmagnetic disorder. Numerical results for a disordered electron system show that the distribution of Kondo temperatures deviates strongly from the result expected from random matrix theory. A pronounced second peak emerges for small Kondo temperatures, showing that the probability that magnetic moments remain unscreened at low temperatures increases with disorder. Analytical calculations, taking into account correlations between eigenfunction intensities yield a finite width for the distribution in the thermodynamic limit. Experimental consequences for disordered mesoscopic metals are discussed. PACS: 72.10.Fk, 72.15.Qm, 75.20.Hr, 72.15.Rn In a metal with antiferromagnetic exchange interaction between a local magnetic moment and the conduction electrons, correlations cause a change in the Fermi liquid ground state. The screening of the localized spin by the formation of a Kondo singlet below the Kondo temperature, T K , is affected by disorder in various ways. Fluctuations in the exchange coupling due to random positioning of magnetic moments results in a dispersion of T K [1]. Since T K is defined by an integral equation similar to the BCS equation for the critical temperature of a superconductor, one could expect, by analogy, the Anderson theorem [2] to be valid. In that case, the leading correction to T K would be of order T
Journal de Physique, 1989
2014 Des fluctuations universelles de conductance (UCF) de l'ordre de e2/h, ont été récemment observées à basse température dans des métaux désordonnés mésoscopiques, indépendamment de la taille de l'échantillon et du degré de désordre. L'explication théorique qui suivit supposait que les impuretés, source du désordre, étaient indépendantes les unes des autres. Nous réexaminons ce problème avec l'hypothèse plus réaliste que les impuretés sont en fait corrélées et d'autant plus que leur concentration est plus élevée. Nous montrons ainsi que les UCF sont modifiées par ces interactions : le résultat e2/h est multiplié par un facteur numérique plus grand ou plus petit que un, suivant que les impuretés se repoussent ou s'attirent. Les modifications dépendent, en particulier, de la concentration en impuretés et donc du degré de désordre. Par conséquent le caractère universal des fluctuations de conductance n'est plus vrai dans les systèmes réalistes. De nouvelles expériences seraient utiles pour tester cette théorie en comparant des cas où les interactions sont répulsives ou attractives. Abstract. 2014 Low temperature universal conductance fluctuations (UCF) of the order of e2/h have been recently observed in mesoscopic disordered metals, independent of sample size and degree of disorder. The theoretical explanation which followed assumed that the impurities, at the source of the disorder, were independent of each other. We reexamine this problem using the more realistic assumption that the impurities are, in fact, correlated and more and more so for increasing concentration. We show that the UCF are modified by these interactions : the e2/h result is multiplied by a numerical factor larger or smaller than one, depending on whether the impurities repel or attract each other. The modifications do depend, in particular, on the impurity concentration and thus on the degree of disorder. Therefore the universal character of the conductance fluctuations breaks down in realistic systems. Further experiments would be useful to test the present theory by comparing cases where the interactions are repulsive or attractive.
Scaling Theory of Magnetoresistance in Disordered Local Moment Ferromagnets
Physical Review Letters, 2005
We present a scaling theory of magneto-transport in Anderson-localized disordered ferromagnets. Within our framework a pronounced magnetic-field-sensitive resistance peak emerges naturally for temperatures near the magnetic phase transition. We find that the resistance anomaly is a direct consequence of the change in localization length caused by the magnetic transition. For increasing values of the external magnetic field, the resistance peak is gradually depleted and pushed towards higher temperatures. Our results are in good agreement with magneto-resistance measurements on a variety of disordered magnets.
Quantum magnetoresistance fluctuations in an amorphous metal
Physical review. B, Condensed matter, 1991
We report an observation of aperiodic magnetoresistance fluctuations in an amorphous system. We have observed fluctuations in amorphous Pd80Si20 wires with macroscopic lengths, up to L=1 mm (L/L~=12000, where L" is the phase-coherence length). The root-mean-square amplitude of the resistance fluctuations is proportional to L', in agreement with theory. We observe a reduction in the fluctuation amplitude at low magnetic fields, which we attribute to magnetic scattering, possibly in the Kondo regime. Two simple models are presented to describe this behavior, one based on a single population of magnetic scatterers with temperature-dependent magnetic moment, and the other based on two scatterer populations, each with fixed moment. Both models agree with the data. We use the high-field amplitude of the magnetoresistance-fluctuation structure to predict the characteristic field scale; the prediction is larger than the observed field scale by a factor of 2.
Strength and scales of itinerant spin fluctuations in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML">mml:mrowmml:mn3mml:midparamagnetic metals
Physical review, 2016
The full spin density fluctuations (SDF) spectra in 3d paramagnetic metals are analyzed from first principles using the linear response technique. Using the calculated complete wavevector and energy dependence of the dynamic spin susceptibility, we obtain the most important, but elusive, characteristic of SDF in solids: on-site spin correlator (SC). We demonstrate that the SDF have a mixed character consisting of interacting collective and single-particle excitations of similar strength spreading continuously over the entire Brillouin zone and a wide energy range up to femtosecond time scales. These excitations cannot be adiabatically separated and their intrinsically multiscale nature should be always taken into account for a proper description of metallic systems. Overall, in all studied systems, despite the lack of local moment, we found a very large SC resulting in an effective fluctuating moment of the order of several Bohr magnetons.
Impurity interactions in disordered metals. II. Conductance fluctuations in mesoscopic systems
Physical review. B, Condensed matter, 1992
We reexamine the theory of conductance fluctuations in mesoscopic systems, in particular, the sensitivity of such fluctuations to the motion of one single impurity. In contrast with all previous theories on the subject, we do not assume the impurities to be independent. Instead, we take into account the existence of a local atomic order between the impurities which results from their mutual interactions. We show that, to the lowest order in this local ordering, its overall effect cancels out from the sum of all contributing diagrams, so that the universal character of the conductance fluctuations is still preserved. In contrast, it does show up when a single impurity is moved, in which case the resulting conductance fluctuations become dependent on the degree of electronic disorder and dimensionality. It is conjectured that, as the amount of impurity local ordering increases, it will also modify the universal character of the conductance fluctuations.
Disordered magnetic systems in two dimensions
Journal of Magnetism and Magnetic Materials, 1991
Recent theoretical results for the two-dimensional Ising model with random bonds are applied to dilute and mixed magnetic systems, The rounding effect on the quenched specific heat peak, observed in experiments with strong disorder, is explained by the the appearance of a new thermodynamic phase between the ferro-and paramagnetic phase. The relations of this result with computer simulations are discussed. In particular, finite-size effects on the disorder-induced phase are estimated.