arXiv subjects

Maciej M. Maska

Publications and source records attributed to Maciej M. Maska.

26 records · Page 2Linked to original sources

arXiv2003

Critical field in a model with local pairs

We analyze the role of Zeeman and orbital pair breaking mechanisms in models appropriate for short coherence length superconductors. In particular, we investigate the attractive Hubbard and the pair hoping models. The orbital pair breaking mechanism dominates in the majority of models with $s$--wave and $d$--wave superconducting order parameters. On the other, the repulsive pair hopping interaction leads to $η$--type pairing, that is stable against the orbital pair breaking. External magnetic field reduces this type of pairing predominantly due to the Zeeman coupling. According to the recent experiments this mechanism is responsible for closing of the pseudogap. Moreover, the temperature dependence of the gap closing field in $η$--phase fits the experimental data very well. We discuss whether the preformed pairs in the $η$--phase could be responsible for the pseudogap phenomenon.

cond-mat.supr-con↗

arXiv2003

Vortex structure in d-density wave scenario of pseudogap

We investigate the vortex structure assuming the d-density wave scenario of the pseudogap. We discuss the profiles of the order parameters in the vicinity of the vortex, effective vortex charge and the local density of states. We find a pronounced modification of these quantities when compared to a purely superconducting case. Results have been obtained for a clean system as well as in the presence of a nonmagnetic impurity. We show that the competition between superconductivity and the density wave may explain some experimental data recently obtained for high-temperature superconductors. In particular, we show that the d-density wave scenario explains the asymmetry of the gap observed in the vicinity of the vortex core.

cond-mat.supr-con↗

arXiv2002

Upward curvature of the upper critical field in the Boson--Fermion model

We report on a non-conventional temperature behavior of the upper critical field ($H_{c2}(T)$) which is found for the Boson-Fermion (BF) model. We show that the BF model properly reproduces two crucial features of the experimental data obtained for high-$T_c$ superconductors: $H_{c2}(T)$ does not saturate at low temperatures and has an upward curvature. Moreover, the calculated upper critical field fits very well the experimental results. This agreement holds also for overdoped compounds, where a purely bosonic approach is not applicable.

cond-mat.supr-con↗

arXiv2002

Reentrant superconductivity in a strong applied field within the tight-binding model

It was suggested by Rasolt and Tešanović that the Landau level quantization in isotropic superconductors could enhance superconductivity in a very strong magnetic field, above the upper critical field ($H_{c2}$). We derive a generalized Harper equation for superconducting systems, and show that a similar reentrant behavior appears in a lattice model, even though the Landau--level--structure is destroyed by the periodic potential in that case. Both the orbital and the Zeeman field--induced effects are taken into account.

cond-mat.supr-con↗

arXiv2001

Upper critical field for underdoped high-T_c superconductors. Pseudogap and stripe--phase

We investigate the upper critical field in a stripe--phase and in the presence of a phenomenological pseudogap. Our results indicate that the formation of stripes affects the Landau orbits and results in an enhancement of $H_{c2}$. On the other hand, phenomenologically introduced pseudogap leads to a reduction of the upper critical field. This effect is of particular importance when the magnitude of the gap is of the order of the superconducting transition temperature. We have found that a suppression of the upper critical field takes place also for the gap that originates from the charge--density waves.

cond-mat.supr-con↗

arXiv2000

Upper critical field for anisotropic superconductivity. A tight-binding approach

We study the problem of the upper critical field ($H_{c2}$) for tight-binding electrons in a two-dimensional lattice. The external magnetic field is introduced into the model Hamiltonian both via the Peierls substitution and the Zeeman term. Carrying out calculations for finite systems we analyze the influence of the external field in the commensurable and incommensurable case on an equal footing. The upper critical field has been discussed for intrasite as well as anisotropic intersite pairing that, in the absence of magnetic field, has a $d_{x^2-y^2}$ symmetry. A comparison of $H_{c2}$ determined for different symmetries shows that the on-site pairing is more affected by the external field i.e., the critical temperature for the on-site pairing decreases with the increase of the magnetic field faster than in the anisotropic case. Moreover, we have shown that the tight-binding form of the Bloch energy can lead to the upward curvature of $H_{c2}$, provided that the Fermi level is close enough to the van Hove singularity.

cond-mat.supr-con↗

arXiv1998

Upper critical field for electrons in two-dimensional lattice

We address a problem of the upper critical field in a lattice described by a two-dimensional tight-binding model with the on-site pairing. We develop a finite-system-approach which enables investigation of magnetic and superconducting properties of electrons on clusters, consisting of a few thousand sites. We discuss how the quasiparticle density of states changes with the applied external magnetic field and present the temperature dependence of the upper critical field. We also briefly discuss possible extension of the model to account for the properties of high-temperature superconductors.

cond-mat.supr-con↗

arXiv1998

Ground state energy of the Hubbard model: cluster-perturbative results

When electron correlations are important it is often necessary to use numerical methods to solve the Hamiltonian for a finite system (cluster) "exactly". Unfortunately, such methods are restricted to small systems. We propose to combine the "exact" numerical diagonalization for small clusters with the perturbative calculations to take into account the intracluster as well as intercluster interactions.

cond-mat.str-el↗