arXiv subjects

G. Litak

Publications and source records attributed to G. Litak.

31 records · Page 2Linked to original sources

arXiv2001

Spatial Fluctuations of the Order Parameter in P-Wave Superconductors

The effect of nonmagnetic disorder on the pairing amplitude is studied by means of a perturbation method. Using an extended one band Hubbard model with an intersite attraction we analyze various solutions of p-wave pairing symmetry and discuss their instability calculating fluctuations of order parameter. The model is applied to describe the effect of disorder in Sr2RuO4. The results shows that the real solution with line nodes can be favoured by disorder.

cond-mat.supr-con↗

arXiv2001

Van Hove Singularity and Superconductivity in Disordered Hubbard Model

We apply the Coherent Potential Approximation (CPA) to a simple extended Hubbard model with a nearest and next nearest neighbour hopping for disordered superconductors with s-, d- and p-wave pairing. We show how the Van Hove singularities in the electron density of states enhance the transition temperature Tc for exotic superconductors in a clean and weakly disordered system. The Anderson theorem and pair-breaking effects in presence of Van Hove singularity caused by non-magnetic disorder are also discussed.

cond-mat.supr-con↗

arXiv2001

Surface Quality of a Work Material Influence on Vibrations in a Cutting Process

The problem of stability in the machining processes is an important task. It is strictly connected with the final quality of a product. In this paper we consider vibrations of a tool-workpiece system in a straight turning process induced by random disturbances and their effect on a product surface. Basing on experimentally obtained system parameters we have done the simulations using one degree of freedom model. The noise has been introduced to the model by the Langevin equation. We have also analyzed the product surface shape and its dependence on the level of noise.

nlin.CD↗

arXiv2001

Interlayer Coupling and p-wave Pairing in Strontium Ruthenate

On the basis of a three orbital model and an effective attractive interaction between electrons we investigate the possible superconducting states, with $p$ and $f$-wave internal symmetry, of Sr$_2$RuO$_4$. For an orbital dependent interaction which acts between in plane and out of plane nearest neighbour Ruthenium atoms we find a state for which the gap in the quasi-particle spectra has a line node on the $α$ and $β$ sheets of the Fermi Surface, but it is complex with no nodes on the $γ$-sheet. We show that this state is consistent with all the available experimental data. In particular, we present the results of our calculations of the specific heat and penetration depth as functions of the temperature.

cond-mat.supr-con↗

arXiv2001

Disorder Induced Fluctuations of the Pairing Parameter in P-Wave Superconductors

We study the effect of site diagonal disorder on the pairing amplitude by a perturbation method. Using an extended Hubbard model with the intersite attraction we analyze fluctuations of order parameter in presence of non-magnetic disorder and discuss the instability of various solutions with p-wave paring.

cond-mat.supr-con↗

arXiv2001

Spatial fluctuations of the pairing potential in disordered superconductors

We study the effect of site diagonal, non-magnetic, disorder on the a pairing amplitude in an extended Hubbard model with the intersite attraction. Analyzing fluctuations of a pairing potential we discuss the instability of mixed solutions, 's+d' and 's+id', in presence of disorder. The influence of disorder on extended s- and d-wave superconductors appear to be comparable but in certain regions of the phase diagram, even weak disorder can change the symmetry of the order parameter.

cond-mat.supr-con↗

arXiv2001

Three Band Model for Superconductivity in Sr2RuO4

We analyse the pairing symmetry of the order parameter in superconducting Sr2RuO4. Basing on a realistic three dimensional 3 band energy spectrum we have introduced effective attractive ellectron-electron interactions and found a solution which has line nodes in the gap in alpha and beta bands but not in gamma band. This state breaks time reversal symmetry and leads to the proper temperature dependence of the specific heat, in-plane penetration depth and thermal conductivity.

cond-mat.supr-con↗

arXiv2000

Superconductivity in Disordered Sr2RuO4

We discuss the influence of disorder on the critical temperature Tc of a p-wave superconductor. To describe disordered Sr2RuO4 we use the extended Hubbard model with random site energies treated in the Coherent Potential Approximation.

cond-mat.supr-con↗

arXiv2000

Superconducing Alloys with Weak and Strong Scattering: Anderson's Theorem and a Superconductor-Insulator Transition

We have studied the effects of strong impurity scattering on disordered superconductors beyond the low impurity concentration limit. By applying the full CPA to a superconductiong A-B binary alloy, we calculated the fluctuations of the local order parameters $Δ_{A}, Δ_{B}$ and charge densities, $n_{A}, n_{B}$ for weak and strong on site disorder. We find that for narrow band alloy s-wav e superconductors the conditions for Anderson's theorem are satisfied in general only for the case of particle-hole symmetry. In this case it is satisfied regardless whether we are in the weak or strong scattering regimes. Interestingly, we find that strong scattering leads to band splitting and in this regime for any band filling we have a critical concentration where a superconductor-insulator quantum phase transition occurs at T=0.

cond-mat.supr-con↗

arXiv1999

Van Hove Singularity and Superconductivity in Disordered Sr2RuO4

On the basis of a simple model we analyse the influence of disorder on critical temperature T_C in p-wave superconductors. The disorder is treated by means of the Coherent Potential Approximation (CPA) and we focus our attention on the effect of a van Hove singularity near Fermi energy E_F. For the appropriate values of its parameters our model reproduces the experimentally found behaviour of Sr2RuO4.

cond-mat↗

arXiv1999

Percolation of Superconductivity

In case of superconductors whose electrons attract each other only if they are near certain centers, the question arises 'How many such centers are needed to make the ground state superconducting?' We shall examine it in the context of a random U Hubbard model. In short we study the case where U_i is -|U| and 0 with probability c and 1-c respectively on a lattice whose sites are labelled i using the Gorkov decoupling and the Coherent Potential Approximation (CPA). We argue that for this model there is a critical concentration c_0 below which the system is not a superconductor.

cond-mat.supr-con↗

arXiv1999

Coherent Potential Approximation for `d - wave' Superconductivity in Disordered Systems

A Coherent Potential Approximation is developed for s-wave and d-wave superconductivity in disordered systems. We show that the CPA formalism reproduces the standard pair-breaking formula, the self-consistent Born Approximation and the self-consistent T-matrix approximation in the appropriate limits. We implement the theory and compute T_c for s-wave and d-wave pairing using an attractive nearest neighbor Hubbard model featuring both binary alloy disorder and a uniform distribution of scattering site potentials. We determine the density of states and examine its consequences for low temperature heat capacity. We find that our results are in qualitative agreement with measurements on Zn doped YBCO superconductors.

cond-mat.supr-con↗

arXiv1998

Van Hove Singularity and D-Wave Pairing in Disordered Superconductors

We apply the coherent potential approximation (CPA) to a simple model for disordered superconductors with d-wave pairing. We demonstrate that whilst the effectiveness of an electronic Van Hove singularity to enhance the transition temperature T$_c$ is reduced by disorder it is not eliminated. In fact we give a qualitative account of changes in the T$_c$ vs. doping curve with increasing disorder and compare our results with experiments on the Y_{0.8}Ca_{0.2}Ba_2(Cu_{1-c}Zn_c)_{3}O_{7-δ} alloys.

cond-mat.supr-con↗