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A. Zawadowski

Publications and source records attributed to A. Zawadowski.

At least 19 recordsLinked to original sources
arXiv2011

Light scattering study of low-energy vibrational excitations in the metallic glass Ni$_{67}$Zr$_{33}$ using electronic Raman scattering

The Raman response of the metallic glass Ni$_{67}$Zr$_{33}$ is measured as a function of polarization and temperature and analyzed theoretically. Unexpectedly, the intensity in the range up to 300\wn increases upon cooling, which is counterintuitive when the response originates from vibrations alone as in insulators. The increase finds a natural explanation if the conduction electrons are assumed to scatter on localized vibrations with a scattering probability proportional to the Debye-Waller factor. None of our assumptions is material specific, and the results are expected to be relevant for disordered systems in general.

cond-mat.dis-nn↗
arXiv2008

Comment on "Twofold Advance in the Theoretical Understanding of Far-From-Equilibrium Properties of Interacting Nanostructures"

Boulat, Saleur and Schmitteckert (BSS) reported results on the full $I-V$ characteristics of the interacting resonant level model (IRLM) exhibiting region with unexpected negative differential conductance (NDC). Using time-dependent density matrix renormalization group complemented with the exact solution performed at a special point (the self-dual point) in the parameter space BSS have shown that at nonzero Coulomb interaction $U$ the current flowing through the impurity level (IL) exhibits a power-law asymptotics as a function of large applied bias voltage. Similar conclusion was earlier reached by Doyon. Even though their results are solid and supported by both analytic and numeric arguments, BSS concluded that "the NDC at large voltage seems a truly nonperturbative behavior, with unclear physical origin". On the contrary, the remarkable physics of NDC can be explained by simple physical arguments and, as we shall show in this Comment, in certain circumstances can be calculated in the framework of perturbation theory.

cond-mat.str-el↗
arXiv2008

On the applicability of bosonization and the Anderson-Yuval methods at the strong-coupling limit of quantum impurity problems

The applicability of bosonization and the Anderson-Yuval (AY) approach at strong coupling is investigated by considering two generic impurity models: the multichannel interacting resonant-level and the anisotropic Kondo models. The two methods differ in the renormalization of the conduction-electron density of states (DoS) near the impurity site. Reduction of the DoS, absent in bosonization but accounted for in the AY approach, is shown to be vital in some models yet redundant in others. The criterion being the stability of the strong-coupling fixed point. Renormalization of the DoS is essential for an unstable fixed point, but redundant when a decoupled entity with local dynamics is formed. This rule can be used to boost the accuracy of both methods at strong coupling.

cond-mat.str-el↗
arXiv2007

Coupling of conduction electrons to two-level systems formed by hydrogen: A scattering approach

An effective Hamiltonian which could model the interaction between a tunneling proton and the conduction electrons of a metal is investigated. A remarkably simple correlation between the motion of the $TLS$-atom and an angular-momentum change of scattering electron is deduced, at the first-order Born level, by using a momentum-space representation with plane waves for initial and final states. It is shown that the angular average of the scattering amplitude-change at the Fermi surface depends solely on the difference of the first two phase shifts, for small-distance displacements of the heavy particle. For such a limit of displacement, and within a distorted-wave Born approximation for initial and final states, the change in the scattering amplitude is expressed via trigonometric functions of scattering phase shifts at the Fermi energy. The numerical value of this change is analyzed in the framework of a self-consistent screening description for impurity-embedding in a paramagnetic electron gas. In order to discuss the so-called antiabatic limit on the same footing, a comparison with matrix elements obtained by the potential-gradient of an unscreened Coulomb field is given as well. The coupling of the tunneling proton to a free-electron-like electron gas is in the typical range obtained, by ultrasound experiments for different metallic glasses, from scattering rates for a Korringa-type relaxation process. That coupling is too weak to be in the range required for realization of the two-channel Kondo effect.

cond-mat.str-el↗
arXiv2007

Revised theory of the magnetic surface anisotropy of impurities in metallic mesoscopic samples

In several experiments the magnitude of the contribution of magnetic impurities to the Kondo resistivity shows size dependence in mesoscopic samples. It was suggested ten years ago that magnetic surface anisotropy can be responsible for the size dependence in cases where there is strong spin-orbit interaction in the metallic host. The anisotropy energy has the form $ΔE=K_d ({\bf n}{\bf S})^2$ where ${\bf n}$ is the vector perpendicular to the plane surface, ${\bf S}$ is the spin of the magnetic impurity and $K_d>0$ is inversely proportional to distance $d$ measured from the surface. It has been realized that in the tedious calculation an unjustified approximation was applied for the hybridizations of the host atom orbitals with the conduction electrons which depend on the position of the host atoms. Namely, the momenta of the electrons were replaced by the Fermi momentum $k_F$. That is reinvestigated considering the $k$-dependence which leads to singular energy integrals and in contrary to the previous result $K_d$ is oscillating like $\sin (2 k_F d)$ and the distance dependence goes like $1/d^3$ in the asymptotic region. As the anisotropy is oscillating, for integer spin the ground state is either a singlet or a doublet depending on distance $d$, but in the case of the doublet there is no direct electron induced transition between those two states at zero temperature. Furthermore, for half-integer ($S > 1/2$) spin it is always a doublet with direct transition only in half of the cases.

cond-mat.str-el↗
arXiv2005

Energy relaxation due to magnetic impurities in mesoscopic wires: Logarithmic approach

The transport in mesoscopic wires with large applied bias voltage has recently attracted great interest by measuring the energy distribution of the electrons at a given point of the wire, in Saclay. In the diffusive limit with negligible energy relaxation that shows two sharp steps at the Fermi energies of the two contacts, which are broadened due to the energy relaxation. In some of the experiments the broadening is reflecting an anomalous energy relaxation rate proportional to $E^{-2}$ instead of $E^{-3/2}$ valid for Coulomb electron-electron interaction, where $E$ is the energy transfer. Later it has been suggested that such relaxation rate can be due to electron-electron interaction mediated by Kondo impurities. In the present paper the latter is systematically studied in the logarithmic approximation valid above the Kondo temperature. In the case of large applied bias voltage Kondo resonances are formed at the steps of the distribution function and they are narrowed by increasing the bias. An additional Korringa energy broadening occurs for the spins which smears the Kondo resonances, and the renormalized coupling can be replaced by a smooth but essentially enhanced average coupling (factor of 8-10). Thus the experimental data can be described by formulas without logarithmic Kondo corrections, but with enhanced coupling. In certain regions of large bias, that averaged coupling depends weakly on the bias. In those cases the distribution function depends only on the ratio of the electron energy and the bias, showing scaling behavior. The impurity concentrations estimated from those experiments and other dephasing experiments can be very different, and a possible explanation considering the surface spin anisotropy due to strong spin-orbit interaction is the subject of our earlier paper.

cond-mat.str-el↗
arXiv2004

Role of surface anisotropy for magnetic impurities in electron dephasing and energy relaxation and their size effect

Recently the electron dephasing and energy relaxation due to different magnetic impurities have been extensively investigated experimentally in thin wires and in this Letter these quantities are theoretically studied. It was shown earlier that a magnetic impurity in a metallic host with strong spin-orbit interaction experiences a surface anisotropy of the form $H=K_d ({\bf n}{\bf S})^2$ which causes size effects for impurities with integer spin. Here we show that the dephasing and the energy relaxation are influenced by the surface anisotropy in very different ways for integer spin having a singlet ground state. That must result also in strong size effects and may resolve the puzzle between the concentrations estimated from the two kind of experiments.

cond-mat.str-el↗
arXiv2004

Kondo Effect on Mesoscopic Scale (Review)

Following the discovery of the Kondo effect the bulk transport and magnetic behavior of the dilute magnetic alloys have been successfully described. In the last fifteen years new directions have been developed as the study of the systems of reduced dimensions and the artificial atoms so called quantum dots. In this review the first subject is reviewed starting with the scanning tunneling microscope (STM) study of a single magnetic impurity. The next subject is the reduction of the amplitude of the Kondo effect in samples of reduced dimension which was explained by the surface magnetic anisotropy which blocks the motion of the integer spin nearby the surface. The electron dephasing and energy relaxation experiments are discussed with the possible explanation including the surface anisotropy, where the situation in cases of integer and half-integer spins is very different. Finally, the present situation of the theory of dynamical structural defects is briefly presented which may lead to two-channel Kondo behavior.

cond-mat.str-el↗
arXiv2004

Slow two-level systems in point contacts

A great variety of experiments, like heat release measurements, acoustic measurements, and transport measurements on mesoscopic samples have proved that two level systems (TLSs) have a crucial role in the low temperature thermal and electric properties of disordered systems. This paper is aimed at reviewing the role of slow TLSs in point contacts. First the theory of point contacts is summarized, concentrating on the discussion of different point contact models, and on the different regimes of electron flow in the contact, mainly focusing on the ballistic and diffusive limit. The Boltzmann equation is solved in both regimes, and the position dependence of the electrical potential is determined. Then the scattering processes in point contacts are investigated, particularly concentrating on the scattering on slow TLSs. If the the electron assisted transitions between the two states are negligible the electron-two level system interaction can be treated with a simplified Hamiltonian. The scattering on such slow TLSs causes nonlinearity in the current-voltage characteristics of the point contact, which can be determined using Fermi's golden role. These calculations are presented showing both the contribution of elastic and inelastic scattering, and including the dependence on the position of the TLS, and on the effect of high frequency irradiation. These results are used to discuss the differences between these slow TLSs and the fast centers which may be described by the two channel Kondo model. The available experimental results are analyzed, distinguishing between the effects due to the different types of TLSs.

cond-mat.mes-hall↗
arXiv2003

Orbital Kondo behavior from dynamical structural defects

The interaction between an atom moving in a model double-well potential and the conduction electrons is treated using renormalization group methods in next-to-leading logarithmic order. A large number of excited states is taken into account and the Kondo temperature $T_K$ is computed as a function of barrier parameters. We find that for special parameters $T_K$ can be close to $1 {\rm K}$ and it can be of the same order of magnitude as the renormalized splitting $Δ$. However, in the perturbative regime we always find that $T_K \alt Δ$ with a $T_K \alt 1 {\rm K}$ [Aleiner {\em et al.}, Phys. Rev. Lett. {\bf 86}, 2629 (2001)]. We also find that $Δ$ remains unrenormalized at energies above the Debye frequency, $ω_{\rm Debye}$.

cond-mat.str-el↗
arXiv2002

Can the Spin-Orbit Interaction Break the Channel Degeneracy of the Two-Channel Orbital Kondo Problem?

Two-level systems (TLS) interacting with conduction electrons are possibly described by the two-channel Kondo Hamiltonian. In this case the channel degeneracy is due to the real spin of the electrons. The possibility of breaking that degeneracy has interest on his own. In fact, we show that the interaction of the conduction electrons with a spin-orbit scatterer nearby the TLS leads to the breaking of the channel degeneracy only in the case of electron-hole symmetry breaking. The generated channel symmetry breaking TLS-electron couplings are, however, too weak to result in any observable effects. Our analysis is also relevant for heavy fermion systems.

cond-mat.str-el↗
arXiv2001

The Role of Electron-Hole Symmetry Breaking in the Kondo Problems

In studying the different Kondo problems it is generally assumed that breaking the electron-hole symmetry does not affect the perturbative infrared divergencies. It is shown here that, in contrast, breaking that symmetry may in some cases lead to observable modifications while in other cases it does not.

cond-mat.str-el↗
arXiv2001

Comparison between one- and two-channel Kondo effects in mesoscopic systems

There are strong recent indications that the Kondo phenomena play important role in the electronic transport and dephasing mechanism for some of the experimentally investigated metallic samples. The spin and the orbital Kondo problem with two-level systems provide very similar behavior above the Kondo temperature. Below the Kondo temperature the behavior is drastically different as in the latter case the entropy remains non zero. In the following several results derived earlier by different methods are simply reproduced by the time-ordered diagram technique in the leading logarithmic approximation valid above the Kondo temperature.

cond-mat.str-el↗
arXiv2001

Theory of the non-equilibrium quasiparticle distribution induced by Kondo defects

It is shown that in resistive nanowires out of equilibrium containing either single- or two-channel Kondo impurities the distribution function $f(E,U)$ obeys scaling behavior in terms of the quasiparticle energy $E$ and the bias voltage $U$. The numerically calculated $f(E,U)$ curves explain quantitatively recent experiments on Cu and Au nanowires. The systematics of the impurity concentration c_{imp} extracted from the comparison between theory and results on various Cu and Au samples strongly suggests that in these systems the scaling arises from magnetic Kondo impurities.

cond-mat.mes-hall↗
arXiv2001

Kondo effect in mesoscopic systems

The Kondo effect may develop in those cases where there are non-commuting operators describing the interaction between the conduction electrons and impurities or defects with internal degrees of freedom. This interaction may involve spin or orbital variables. There are cases where the conduction electrons have conserved quantum numbers that do not appear in the coupling. An example is where the Kondo effect involves orbital degrees of freedom, and the interaction is independent of the real spins of the electrons, which are conserved and lead to the two-channel Kondo (2CK) problem. The low temperature behavior is very different for the one- and two-channel cases, as in the first case a Fermi liquid is formed while in the second one strong deviations appear and a non-Fermi liquid state is realized. Mesoscopic samples provide a unique possibility to study a few or even a single Kondo impurities. In this paper we first review how the original spin Kondo problem is affected by surface anisotropy and the fluctuations of the density of states in point contacts (PCs). We discuss the physics of a single magnetic impurity on the surface of a sample. The orbital 2CK effect due to dynamical defects is considered. The nature of these defects is not known but they are excellent candidates to describe the zero-bias anomalies with non-Fermi liquid character in PCs, and the dephasing time and transport in short wires. There are two main concerns regarding this interpretation: First, the tunneling centers formed by heavy impurities may produce a Kondo temperature that is too low, and secondly, the splittings seen in the experiments are much smaller than expected from this model. It would be therefore extremely important to identify the microstructure of these two-level systems and find new realizations for them.

cond-mat.str-el↗