arXiv subjects

R. Micnas

Publications and source records attributed to R. Micnas.

12 recordsLinked to original sources

arXiv2019

On the Boson-Fermion resonant model on a lattice

We review briefly the properties of a mixture of mutually interacting bosons (bound electron pairs) and itinerant fermions on a lattice (the boson-fermion model). The calculations of the superconducting phase transition temperature ($T_{c}$) and the phase diagram are the main concern. The self-consistent $T$-matrix method is applied to determine the superconducting critical temperature from a pseudogap phase. The method takes into account the pairing fluctuations effects. The $T$-matrix results for $T_{c}$ are given for a 3D cubic lattice with tight-binding dispersion of electrons and standard bosons, and they are also compared with those of the BCS- mean-field approximation (MFA). Our results describe the BCS-Bose-Einstein condensation (BEC) crossover in the boson-fermion mixture with resonant interaction. The energy scales involved in the pseudogap formation are also analysed. PACS 74.20.-z,74.20.Mn,71.28.+d

cond-mat.supr-con↗

arXiv2018

Effect of boson on-site repulsion on the superfluidity in the boson-fermion-Hubbard model

We analyze the finite-temperature phase diagram of the boson-fermion-Hubbard model with Feshbach converting interaction, using the coherent-state path-integral method. We show that depending on the position of the bosonic band, this type of interaction, even if weak, can drive the system into the resonant superfluid phase in the strong bosonic interaction limit. It turns out that this phase can exist for an arbitrary number of fermions (i.e., fermionic concentration between 0 and 2) but with the bosonic particle number very close to an integer value. We point out that the standard time-of-flight method in optical lattice experiments can be an adequate technique to confirm the existence of this resonant phase. Moreover, in the non-resonant regime, the enhancement of the critical temperature of the superfluid phase due to Feshbach interaction is also observed. We account for this interesting phenomena for a hole- or particlelike pairing mechanism depending on the system density and mutual location of the fermionic and bosonic bands.

cond-mat.quant-gas↗

arXiv2015

Conductivity of strongly correlated bosons in optical lattices in an Abelian synthetic magnetic field

Topological phase engineering of neutral bosons loaded in an optical lattice opens a new window for manipulating of transport phenomena in such systems. Exploiting the Bose Hubbard model and using the magnetic Kubo formula proposed in this paper we show that the optical conductivity abruptly changes for different flux densities in the Mott phase. Especially, when the frequency of the applied field corresponds to the on-site boson interaction energy, we observe insulator or metallic behavior for a given Hofstadter spectrum. We also prove, that for different synthetic magnetic field configurations, the critical conductivity at the tip of the lobe is non-universal and depends on the energy minima of the spectrum. In the case of $1/2$ and $1/3$ flux per plaquette, our results are in good agreement with those of the previous Monte Carlo (MC) study. Moreover, we show that for half magnetic-flux through the cell the critical conductivity suddenly changes in the presence of a superlattice potential with uniaxial periodicity.

cond-mat.quant-gas↗

arXiv2010

Real space inhomogeneities in high temperature superconductors: the perspective of two-component model

The two-component model of high temperature superconductors in its real space version has been solved using Bogoliubov-de Gennes equations. The disorder in the electron and boson subsystem has been taken into account. It strongly modifies the superconducting properties and leads to local variations of the gap parameter and density of states. The assumption that the impurities mainly modify boson energies offers natural explanation of the puzzling positive correlation between the positions of impurities and the values of the order parameter found in the scanning tunnelling microscopy experiments.

cond-mat.supr-con↗

arXiv2010

Effects of Disorder on Superconductivity of Systems with Coexisting Itinerant Electrons and Local Pairs

We study the influence of diagonal disorder (random site energy) of local pair (LP) site energies on the superconducting properties of a system of coexisting local pairs and itinerant electrons described by the (hard-core) boson-fermion model. Our analysis shows that the properties of such a model with s-wave pairing can be very strongly affected by the diagonal disorder in LP subsystem (the randomness of the LP site energies). This is in contrast with the conventional s-wave BCS superconductors, which according to the Anderson's theorem are rather insensitive to the diagonal disorder (i.e. to nonmagnetic impurities). It has been found that the disorder effects depend in a crucial way on the total particle concentration n and the LP level position DELTA_o and depending on the parameters the system can exhibit various types of superconducting behaviour, including the LP-like, intermediate (MIXED)and the 'BCS'-like. In the extended range of {n,DELTA_o} the superconducting ordering is suppressed by the randomness of the LP site energies and the increasing disorder induces a changeover from the MIXEDlike behaviour to the BCS-like one, connected with abrupt reduction of T_c and energy gap to zero. However, there also exist a definite range of {n,DELTA_o} in which the increasing disorder has a quite different effect: namely it can substantially enhance T_c or even lead to the phenomenon which can be called disorder induced superconductivity. Another interesting effect is a possibility of a disorder induced bound pair formation of itinerant electrons, connected with the change-over to the LP-like regime.

cond-mat.supr-con↗

arXiv2003

Superconductivity in a two-component model with local electron pairs

Superconductivity in the two component model of coexisting local electron pairs (hard-core charged bosons) and itinerant fermions coupled via charge exchange mechanism is discussed. The cases of isotropic s-wave and anisotropic pairing of extended s-wave and d_{x^2-y^2} symmetries are analyzed for a 2D square lattice within the BCS-mean field approximation and the Kosterlitz-Thouless theory. The phase diagrams and superconducting characteristics of this induced pairing model as a function of the position of the local pair (LP) level and the total carrier concentration are determined. The model exhibits several types of interesting crossovers between the BCS like behavior and that of LP's. In addition, the Uemura plots are obtained for extended s and d_{x^2-y^2} pairing symmetries. Finally, we analyze the pairing fluctuation effects (in 3D) within a generalized T-matrix approach. Some of our results are discussed in connection with a two-component scenario of preformed pairs and unpaired electrons for high temperature superconductors.

cond-mat.supr-con↗

arXiv2003

On the Crossover from BCS Superconductivity to Bose Condensation

We outline a microscopic approach to the superconducting fluctuations and pairing correlations in the attractive Hubbard model above Tc, using the functional integral method. A crossover from BCS superconductivity to Bose condensation of preformed pairs is studied by constructing the appropriate Ginzburg-Landau functionals. The differences between the lattice and the continuum models are discussed. The case of quasi-two-dimensional superconductors as well as the model with non-local pairing interaction are also examined. The effects of gaussian fluctuations of the order parameter are analyzed in the T-matrix approach, the self-consistent Hartree approach and finally within the Ginzburg-Landau theory. Simlarities with a paramagnon theory of itinerant-electron magnetism are pointed out.

cond-mat.supr-con↗

arXiv2002

On the Superconductivity in the Induced Pairing Model

The two component model of coexisting local electron pairs and itinerant fermions coupled via charge exchange mechanism, which mutually induces superconductivity in both subsystems, is discussed. The cases of isotropic s-wave and anisotropic pairing of extended s and d_{x^2-y^2} -wave symmetries are analyzed for a 2D square lattice within the BCS-mean field approximation and the Kosterlitz-Thouless theory. We determined the phase diagrams and superconducting characteristics as a function of the position of the local pair (LP) level and the total electron concentration. The model exhibits several types of interesting crossovers from BCS like behavior to that of LP's. Some of our results are discussed in connection with a two-component scenario of preformed pairs and unpaired electrons for exotic superconductors.

cond-mat.supr-con↗

arXiv2002

Kinetic energy driven superconductivity and pseugogap phase in weakly doped antiferromagnets

We derive an effective Hamiltonian for spin polarons forming in weakly doped antiferromagnets and demonstrate that the system becomes superconducting at finite doping. We argue that the driving mechanism which gives rise to superconductivity is lowering of the kinetic energy by formation of mobile antiferromagnetic spin bipolarons. That source of attraction between holes is by definition effective if the antiferromagnetic correlation length is longer than the radius of forming polarons. Notwithstanding that the attraction is strongest in the undoped system with long range order, the superconducting order parameter vanishes when the doping parameter decreases which should be attributed to emptying the spin polaron band and approaching the Mott insulator phase. Since the hypothetical normal phase of low density gas of fermions is unstable against formation of bound hole pairs the intensity of low energy excitations is suppressed and the pseudogap forms in the underdoped region.

cond-mat.supr-con↗

arXiv2001

Anisotropic Superconductivity in the Induced Pairing Model

The model of local electron pairs and itinerant fermions coupled via charge exchange mechanism, which mutually induces superconductivity in both subsystems is studied for anisotropic pairing symmetry. The phase diagram is presented and the phase fluctuations effects are analyzed within the Kosterlitz-Thouless scenario.

cond-mat.supr-con↗

arXiv1994

Pair fluctuation effects above $T_{\rm c}$

We explore the occurrence of pairing effects above $T_{\rm c}$ in the 2D attractive Hubbard model. The presence of pairs above $T_{\rm c}$ goes beyond the BCS approximation, in which pair formation and condensation occur at the same temperature. Using the fully self-conserving $T$-matrix formalism, which is valid above $T_{\rm c}$, we find that (1) the distribution function, $n(k)$, shows a 10\% change of the weight from ``below'' to ``above'' $k_{\rm F}$ with respect to the free case, and (2) the phase shift, $δϕ(ω,k)$, shows $Θ$-like behavior as a function of $ω$ for large momentum $k$ along the diagonal of the Brillouin zone. Our calculations have been carried out for an interaction of $U/t = - 4.0$ and a temperature of $T/t = 0.125$, where $t$ is the hopping matrix element between nearest neighbors. We conclude that for such an interaction value the Fermi surface is not a well-defined quantity.

cond-mat↗

arXiv1994

Evidence for a Pseudogap Above the Critical Temperature in the Attractive Hubbard Model

We explore the effect of fluctuations for $T \geq T_c$ in the 2-D negative Hubbard model within the framework of the selfconsistent T-matrix formalism, which goes beyond the BCS approximation and includes pair fluctuations. We enter the regime where correlations are important, namely, $U/t = -4.0$, where $t$ denotes the hopping matrix element between nearest neighboors and $U$ is the strength of the on-site interaction. Our results include: 1) In comparison to the free case, the distribution function, $n({\bf k})$, reveals considerable renormalization. This fact makes us conclude that the Fermi surface is blurred for correlated electron systems; 2) There is a pseudogap in the density of states, $N(ω)$, around the chemical potential, $μ$; and 3) The real part of the T-matrix, at zero frequency and zero momentum, vs $T/t$ shows a divergence at a particular temperature. This last result shows that more approximate T-matrix calculations, extended to $T~=~0$ are meaningless.

cond-mat↗