arXiv subjects

G. Zarand

Publications and source records attributed to G. Zarand.

42 records · Page 3Linked to original sources

arXiv2001

Hysteretic Optimization

We propose a new optimization method based on a demagnetization procedure well known in magnetism. We show how this procedure can be applied as a general tool to search for optimal solutions in any system where the configuration space is endowed with a suitable `distance'. We test the new algorithm on frustrated magnetic models and the traveling salesman problem. We find that the new method successfully competes with similar basic algorithms such as simulated annealing.

cond-mat.dis-nn↗

arXiv2001

Dynamics of a tunneling magnetic impurity: Kondo effect induced incoherence

We study how the formation of the Kondo compensation cloud influences the dynamical properties of a magnetic impurity that tunnels between two positions in a metal. The Kondo effect dynamically generates a strong tunneling impurity-conduction electron coupling, changes the temperature dependence of the tunneling rate, and may ultimately result in the destruction of the coherent motion of the particle at zero temperature. We find an interesting two-channel Kondo fixed point as well for a vanishing overlap between the electronic states that screen the magnetic impurity. We propose a number of systems where the predicted features could be observed.

cond-mat.str-el↗

arXiv1999

Thermodynamics of the dissipative two-state system: a Bethe Ansatz study

The thermodynamics of the dissipative two-state system is calculated exactly for all temperatures and level asymmetries for the case of Ohmic dissipation. We exploit the equivalence of the two-state system to the anisotropic Kondo model and extract the thermodynamics of the former by solving the thermodynamic Bethe Ansatz equations of the latter. The universal scaling functions for the specific heat $C_α(T)$ and static dielectric susceptibility $χ_α(T)$ are extracted for all dissipation strengths $0<α<1$ for both symmetric and asymmetric two-state systems. The logarithmic corrections to these quantities at high temperatures are found in the Kondo limit $α\to 1^{-}$, whereas for $α< 1$ we find the expected power law temperature dependences with the powers being functions of the dissipative coupling $α$. The low temperature behaviour is always that of a Fermi liquid.

cond-mat.str-el↗

arXiv1998

Comment on ``Point-Contact Study of Fast and Slow Two-Level Fluctuators in Metallic Glasses'' by Keijsers, Shklyarevskii and van Kempen

We point out that a recent experiment by Keijsers, Shklyarevskii and van Kempen [Phys. Rev. Lett. 77, 3411 (1996)] on metallic glass point contacts containing two-level systems (TLS) effectively measures for the first time the conductance contributions of individual TLSs (previous experiments averaged over a distribution of TLSs). For this case the predictions of the Kozub-Kulik theory for the TLS-electron interaction differ from those of the Vladar-Zawadowski theory, the latter giving much better agreement with experiment.

cond-mat.str-el↗

arXiv1997

Instability of the marginal commutative model of tunneling centers interacting with metallic environment: Role of the electron-hole symmetry breaking

The role of the electron-hole symmetry breaking is investigated for a symmetrical commutative two-level system in a metal using the multiplicative renormalization group in a straightforward way. The role of the symmetries of the model and the path integral technique are also discussed in detail. It is shown that the electron-hole symmetry breaking may make the model non-commutative and generate the assisted tunneling process which is, however, too small itself to drive the system into the vicinity of the two-channel Kondo fixed point. While these results are in qualitative agreement with those of Moustakas and Fisher (Phys. Rev. B 51, 6908 (1995), ibid 53, 4300 (1996)) the scaling equations turn out to be essentially different. We show that the main reason for this difference is that the procedure for the elimination of the high energy degrees of freedom used by Moustakas and Fisher leaves only the free energy invariant, however, the couplings generated are not connected to the dynamical properties in a straightforward way and should be interpreted with care. These latter results might have important consequences in other cases where the path integral technique is used to produce the scaling equations and calculate physical quantities.

cond-mat.str-el↗

arXiv1997

Orbital Kondo-effect from tunneling impurities

The article reviews recent results for the low energy physics of fast tunneling centers in metallic environments. For strong enough couplings to the environment these tunneling centers display an orbital Kondo effect and give rise to a non-Fermi-liquid behavior. This latter property is explained by establishing a mapping of the tunneling center model to the multichannel Kondo model via the renormalization group transformation combined with a $1/N_f$ expansion. The case of M-state systems, the role of the splittings and the present experimental situation are also discussed.

cond-mat.str-el↗