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1
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Phase Separation in the Ground State of the Model 2D Spin-Pseudospin System

100%
Chikov A., Panov Y., Moskvin A., Budrin K.
Acta Physica Polonica A
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2018
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vol. 133
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issue 3
432-434
EN
We addressed a simplified static 2D spin-pseudospin model which takes into account both conventional Heisenberg spin exchange coupling and the on-site and inter-site charge correlations. Classical Monte-Carlo calculations for large square lattices show that homogeneous ground state solutions found in a mean-field approximation are unstable with respect to phase separation with the charge and spin subsystems behaving like immiscible quantum liquids. For instance, with lowering the temperature one can observe two sequential phase transitions: first, antiferromagnetic ordering in the spin subsystem diluted by randomly distributed charges, then, the charge condensation in the charge droplets. Thermodynamic properties and phase diagram of the 2D spin-pseudospin system are studied by Monte-Carlo simulation with a special attention given to the role played by the on-site correlation.
2
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Zero Temperature Phase Diagram of a d-Wave Superconductor with Anderson Impurities

100%
Borkowski L.
Acta Physica Polonica A
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2009
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vol. 115
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issue 1
120-122
EN
We study the model of a d-wave superconductor interacting with finite concentration of the Anderson impurities at zero temperature. The interaction between impurity and conduction electrons is taken into account within the large-N approximation. We discuss the obtained phase diagram and its dependence on the main energy scales.
3
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Structure and Transport Characteristics of Tunnel Junctions with Hybrid Semiconductor Barriers with Quantum Dots

100%
Shaternik V., Shapovalov A., Prikhna T., Suvorov O., Skorik M., Shaternik A., Bondarchuk V., Zubov E.
Acta Physica Polonica A
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2018
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vol. 133
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issue 4
1060-1064
EN
We propose to realize MoRe/SiO_x(W)/MoRe hybrid junctions by using self-organization effects for the creation of quantum dots (tungsten clusters) in the semiconductor barriers consisting of a mixture of silicon and silicon oxide. Current-voltage characteristics of the MoRe/SiO_x(W)/MoRe samples have been measured in a wide voltage range from -900 to 900 mV at temperatures from 4.2 to 77 K. At low temperatures and for a comparatively small W content in the hybrid barrier, the heterostructures exhibited current-voltage curves of an unusual shape. Single or several current peaks caused by electron tunneling through the allowed states in the barrier have been observed in the transport characteristics. With increasing temperature, superconducting fluctuations in the MoRe electrodes become unimportant, and the current-voltage curve of a heterostructure follows the Ohm law. At last, we present theoretical description of the charge transport in such inhomogeneous systems with account of many-electron processes.
4
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Charge-Density-Wave Origin of Dip-Hump Structures in the Tunnel Spectra of Bi_2Sr_2CaCu_2O_{8+δ}

80%
Ekino T., Gabovich A., Li M., Pękała M., Szymczak H., Voitenko A.
Acta Physica Polonica A
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2007
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vol. 111
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issue 4
573-580
EN
We calculated the differential conductance G as the function of the bias voltage V across the tunnel junction between a normal metal or a conventional superconductor and an inhomogeneous superconductor with charge density waves. Spatial averaging over random domains with varying superconducting and normal state properties was carried out. For high-T_c oxides, irregularly distorted charge density wave patterns with spatially scattered values of various parameters were earlier shown to manifest themselves in a great body of experimental data. The results of calculation were applied to explain the well-known dip-hump structure in the G(V) dependence for Bi_2Sr_2CaCu_2O_{8+δ} and other cuprates.
5
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d-Wave Superconductivity, Orbital Magnetism, and Unidirectional Charge Order in the t-J Model

80%
Raczkowski M.
Acta Physica Polonica A
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2008
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vol. 114
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issue 1
107-114
EN
Recent scanning tunneling microscopy in the superconducting regime of two different cuprate families revealed unidirectional bond-centered modulation in the local electronic density of states. Motivated by this result we investigate the emergence of modulated d-wave superconductivity coexisting with charge domains that form along one of the crystal axes. While detailed stripe profiles depend on the used form of the Gutzwiller factors, the tendency towards a valence bond crystal remains robust. We also find closely related stripe phase originating from the staggered flux phase, a candidate for the pseudogap phase of lightly doped cuprates.
6
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Pairing Correlations around Negative-U Centre in the Two-Band Model

80%
Ciechan A., Wysokiński K.
Acta Physica Polonica A
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2008
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vol. 114
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issue 1
123-127
EN
The electron energy spectrum of many superconducting materials is characterised by the presence of few bands at the Fermi level. In some cases the superconducting properties seem to be dominated by single band but in others the many-band approach is essential ingredient of their description. In this paper we shall study the properties of superconducting impurity placed in a non-superconducting medium. We are interested in the evolution of superconducting correlations within non-superconducting 1- or 2-dimensional system, their dependence on the distance from impurity and changes induced by the presence of the second band and interband scattering. We use real space description of the material and Bogolyubov-de Gennes approach to superconductivity.
7
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Unidirectional Charge Instability of the d-Wave Resonating Valence Bond Superconductor

80%
Raczkowski M., Capello M., Poilblanc D.
Acta Physica Polonica A
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2009
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vol. 115
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issue 1
77-79
EN
Starting from a uniform d-wave superconducting phase we study the energy cost due to imposed unidirectional defects with a vanishing pairing amplitude. Both renormalized mean-field theory and variational Monte Carlo calculations within the t-J model yield that the energies of inhomogeneous and uniform phases are very close to each other. This suggests that small perturbations in the microscopic Hamiltonian might lead to inhomogeneous superconducting phases in real materials as observed in recent scanning tunneling microscopy on Ca_{2-x}Na_xCuO_2Cl_2.
8
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d-Wave Superconductivity in a System with Open Boundary Conditions

80%
Pisarski P., Harań G.
Acta Physica Polonica A
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2004
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vol. 106
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issue 5
721-725
EN
We discuss the d-wave superconductivity in confined geometry by imposing open boundary conditions on the attractive Hubbard model. Within the Bogoliubov-de Gennes approach we evaluate the order parameter and charge density distribution in the system.
9
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Superconductivity Near Transition to Insulating State in MoC Ultrathin Films Studied by Subkelvin STM

80%
Szabó P., Neilinger P., Trgala M., Grajcar M., Samuely P.
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vol. 126
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issue 1
368-369
EN
We study the homogeneously disordered MoC thin films with thicknesses of 10 and 5 nm and the superconducting transition temperatures near 6 and 4 K, significantly decreased as compared to the bulk T_{c}=8.32 K due to a disorder. The scanning tunnelling spectroscopy reveals in the thicker sample a BCS superconducting energy gap Δ with a broadening parameter Γ equal to about 10 per cent of Δ. Remarkably, Γ increases with temperature. The thinner, more disordered sample shows a gapped superconducting density of states but without any coherence peaks at the gap edge, which could not be approximated by the BCS DOS. Moreover, the reduced DOS around the Fermi level persists above the resistive transition temperature reminding the pseudo-gap known from high-T_{c} cuprates.
10
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Theoretical Analysis of the Short Coherence Length Superconductors

80%
Wysokiński K., Ciechan A., Krzyszczak J.
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issue 2
232-237
EN
Properties of the short coherence length ξ superconductors are markedly different from those with long ξ. The main difference is connected with their response to impurities. Even the s-wave superconductors have been predicted not to obey the so-called Anderson theorem and to show appreciable fluctuations of order parameter. This observation has been recently confirmed in a number of beautiful scanning tunnelling experiments on high temperature cuprate and pnictide superconductors. In this work we study clean two-band systems within BCS-type approach and impure superconductors within mean field Bogolubov-de Gennes approach. In the weak disorder limit both s-wave and d-wave superconductors only weakly react to impurities. Presence of the Van Hove singularities in the density of states of the two-band clean systems strongly increases their superconducting transition temperature if the intra-band pair scattering is the only pairing interaction. The effect of impurities on such superconductors does depend on the sign of the inter-orbital interaction.
11
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Pseudogap-Like Phenomena in Cuprates as a Manifestation of Charge-Density Waves

80%
Ekino T., Gabovich A., Voitenko A., Suan Li M., Szymczak H., Pękała M.
Acta Physica Polonica A
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2008
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vol. 114
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issue 1
59-66
EN
Differential conductance G as a function of the bias voltage V was measured for break-junctions of superconducting Bi_2Sr_2CaCu_2O_{8+δ} and YBa_2Cu_3O_{7-δ}. The dependences G(V) for both materials clearly demonstrate the so-called dip-hump structures outside the gap region. A theory, which suggests the charge-density-wave origin of the dip-hump structures and explains its specific form by intrinsic inhomogeneity of cuprate materials, was developed. The well-known pseudogap features in the tunnel spectra of high-T_c oxides found both below and above the superconducting critical temperature are also described by the theory, which testifies that both the pseudogap and the dip-hump structures have the same origin. Competing theories and various G(V) peculiarities found for a number of superconducting oxides are briefly discussed.
12
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The Multiple Phase Slip Phenomena in the Narrow Superconducting Channels

80%
Baranov V., Balanov A., Kabanov V.
Acta Physica Polonica A
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2012
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vol. 121
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issue 5-6
1038-1041
EN
The current-voltage characteristic of the narrow superconducting channel is investigated by direct numerical integration of the time-dependent Ginzburg-Landau equations. We have demonstrated that the steps in the current-voltage characteristic correspond to a number of different bifurcation points of the time-dependent Ginzburg-Landau equations. We have analytically estimated the period and the averaged voltage of the oscillating solution for the relatively small currents. We have also found the range of currents where the system transforms to the chaos.
13
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Inter-Orbital Effects in the Impure Pnictide Superconductors

80%
Ciechan A., Wysokiński K.
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issue 2
356-359
EN
We study the local properties of the iron oxypnictides by means of the real space Bogolubov-de Gennes equations. Starting with the realistic energy spectrum and assuming small amount of impurities we calculate the energy dependence of the local density of states and spatial distribution of the local values of the superconducting energy gap. We pay particular attention to the role of the inter-orbital scattering of pairs and the effect of impurity scattering on the superconducting state. The effect of inter-orbital impurities depends on the relative signs of the order parameters related to two orbitals in question. For opposite signs impurities produce bound states inside the gap, while for the same sign of the order parameters they hardly affect the superconductor. The results obtained for impure systems have been shown as maps of the order parameters and local density of states. They well compare with the existing scanning tunnelling microscopy spectra of the iron pnictides.
14
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s-Wave Order Parameter in the Presence of a Momentum-Dependent Impurity Scattering Potential

80%
Rudzińska D., Pisarski P., Harań G.
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EN
Momentum dependence of the impurity potential leads to the anisotropy of a local density of states in isotropic s-wave superconductors. We complete this result by discussing the influence of a momentum-dependent impurity potential on the s-wave order parameter. The scattering process is considered in the T-matrix approximation, and the model impurity potential consists of two separate terms representing isotropic (on-site) and anisotropic (momentum-dependent) parts of the potential. We show that the effect of the isotropic part on the order parameter is local whereas the influence of the momentum-dependent term is nonlocal.
15
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Inhomogeneities in Superconductors Described by the Two-Component Model

80%
Krzyszczak J., Domański T., Wysokiński K.
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issue 2
360-363
EN
We study fluctuations of the superconducting order parameter of s-wave symmetry caused by nonmagnetic disorder present both in the boson and fermion subsystems using the Bogoliubov-de Gennes equations. In particular, we are interested if some features of the d-wave model persist for s-wave symmetry as well. These are: positive correlations between positions of impurities and the magnitude of energy gap, the homogeneity of the local density of states at low energies, nanoscale inhomogeneity of the coherence peak positions and their non-uniform height. We have found that spatial variations of s-wave order parameter are positively correlated with the positions of impurities. Such behavior has been observed in scanning tunneling measurements on d-wave symmetry BSCCO-2122 superconductors.
16
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Real Space Bogoliubov-de Gennes Equations Study of the Boson-Fermion Model

80%
Krzyszczak J., Domański T., Wysokiński K.
Acta Physica Polonica A
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2008
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vol. 114
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issue 1
165-169
EN
The boson-fermion model was proposed to describe superconductivity in short coherence length superconductors. In this work we use it to study impurity induced inhomogeneities in high temperature superconductors as found in numerous scanning tunnelling measurements. The model was formulated in real space and solved with help of Bogoliubov-de Gennes approach. Disorder in the boson or fermion subsystem is directly coupled to the superconducting order parameter and leads to severe changes of superconducting properties like local order parameter and density of states. We present the results for many impurities randomly distributed over otherwise clean and periodic two-dimensional square lattice.
17
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The Fulde-Ferrell-Larkin-Ovchinnikov Superconductivity in Disordered Systems

70%
Ptok A.
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issue 2
420-422
EN
The Fulde-Ferrell-Larkin-Ovchinnikov phase, with a spatially oscillating order parameter, may be induced by strongly magnetic field at low temperature. It is believed that the Fulde-Ferrell-Larkin-Ovchinnikov phase can exist only in homogeneous superconductors, and even weak impurity potential can lead to its destruction. The analysis of the Fulde-Ferrell-Larkin-Ovchinnikov phase in the Bogoliubov-de Gennes equation shows however, that this phase can exist in the presence of weak disorder. Using Bogoliubov-de Gennes equations, we discussed the influence of diagonal and off-diagonal disorder on the Fulde-Ferrell-Larkin-Ovchinnikov phase.
18
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Some Properties of the Model of a~Superconductor with Pair Hopping and Magnetic Interactions at Half-Filling

70%
Kapcia K.
Acta Physica Polonica A
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2012
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vol. 121
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issue 4
733-737
EN
We present our preliminary studies of an effective model of a superconductor with short coherence length involving magnetic interactions. The Hamiltonian considered consists of (i) the effective on-site interaction U, (ii) the intersite magnetic exchange interactions (J^z, J^{xy}) between nearest-neighbors and (iii) the intersite charge exchange term I, determining the hopping of electron pairs between nearest-neighbor sites. In the analysis of the phase diagrams and thermodynamic properties of this model for half-filling (n=1) we have adopted the variational approach, which treats the on-site interaction term exactly and the intersite interactions within the mean-field approximation. One finds that the system considered can exhibit very interesting multicritical behaviors (including tricritical, critical-end and bicritical points) caused by the competition between magnetism and superconductivity, even for n=1. Our investigations show that, depending on the values of interaction parameters, the system at half-filling can exhibit three homogeneous phases: superconducting (SS), (anti-)ferromagnetic (F) and nonordered (NO). The transitions between ordered phases (SS, F) and the NO phase can be first order as well as second order ones, whereas SS-F transition is first order one. Temperature dependencies of the order parameters and thermodynamic properties of the system at the sequence of transitions: SS→F→NO with increasing temperature for J/I=0.3, U/I_0 = 0.69 and n=1 are also presented.
19
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Obtaining a Rough Flux Front in Type-II Superconductors Using a Critical State Model

51%
Romero-Salazar C., Hernández-Flores O., Chabanenko V., Kuchuk E., Abaloszewa I., Nabialek A., Pérez-Rodríguez F.
Acta Physica Polonica A
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2016
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vol. 130
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issue 2
645-648
EN
In this work we numerically modelled a periodic magnetic flux pattern which qualitatively reproduces the so-called sand avalanches scenario in type-II superconductors. To model these sand-pile patterns we consider a perturbation on the critical current which, as a first approximation, follows a periodic function which depends on the position.
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