Ipa Instituto Superior Autonomo Of Estudos Politecnicos

Lisbon, Portugal

Ipa Instituto Superior Autonomo Of Estudos Politecnicos

Lisbon, Portugal
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Vovk R.V.,University of Kharkiv | Khadzhai G.Y.,University of Kharkiv | Dobrovolskiy O.V.,University of Kharkiv | Dobrovolskiy O.V.,Goethe University Frankfurt | And 2 more authors.
Physica B: Condensed Matter | Year: 2017

Temperature dependences of the basal-plane electrical resistance of underdoped YBa2Cu3Ot−δ single crystals are investigated in a broad temperature range down to the superconducting transition, after a fast cooldown from 650° C as well as after a room-temperature annealing. A stepped behavior of the superconducting transition has been observed, pointing to an inhomogeneity of the samples. Annealing has been revealed to lead to a narrowing of the superconducting transition and a reduction of both, the residual resistivity and the phonon scattering coefficient. This is accompanied by a slight increase of the Debye temperature attributed to a decrease of the lattice parameter caused, in turn, by a redistribution of the labile oxygen. The revealed changes of the parameters of the temperature dependence of the electrical resistance point to an equalization of the concentration of the labile oxygen in the layers and its ordering. © 2017 Elsevier B.V.


Zavgorodniy A.A.,University of Kharkiv | Vovk R.V.,University of Kharkiv | Obolenskii M.A.,University of Kharkiv | Samoilov A.V.,University of Kharkiv | And 3 more authors.
Functional Materials | Year: 2010

The effect of high hydrostatic pressure on the electric resistance in the ab plane of the oxygendeficient YBa2Cu3O7-δ single crystals has been investigated. It is shown that the pressure-induced redistribution of the labile oxygen results in an intensified phase layering accompanied by structural relaxation processes within the experimental sample volume. A significant displacement of the temperature ranges that correspond to metal-dielectric type transitions and the pseudo-gap realization regime. The description of the excess conductivity as Δσ~(1 - T/T*)exp(Δ*ab/T) can be interpreted in terms of mean field theory where T* is the mean-field temperature of the superconducting transition and the temperature dependence of the pseudo-gap is well described in the framework of the BCS-BEC crossover theory. Increasing the applied pressure results in the narrowing of the temperature range where the PG regime is realized, thus extending the area of linear ρ(T) dependence in the ab plane. © 2010 - STC "Institute fot Scientific Crystals".


Vovk R.V.,University of Kharkiv | Nazyrov Z.F.,University of Kharkiv | Obolenskii M.A.,University of Kharkiv | Goulatis I.L.,University of Kharkiv | And 2 more authors.
Journal of Alloys and Compounds | Year: 2011

We investigate the influence of the disorientation angle between the direction of a constant magnetic field (up to 15 kOe) and the ab-plane α ≡ ∠(H,ab) to the temperature dependence of the excess conductivity in the temperature interval of the transition to the superconducting state in untwinned YBa2Cu3O7-δ single crystals. We discuss the appearance of low temperature "tails" (para-coherent transitions) on the resistive transitions, corresponding to different phase regimes of the vortex-matter state. At temperatures above the critical temperature, the temperature dependence of the excess para-conductivity is interpreted within the Hikami-Larkin theoretical model of the fluctuation conductivity for layered superconductors. © 2011 Elsevier B.V. All rights reserved.


Vovk R.V.,University of Kharkiv | Obolenskii M.A.,University of Kharkiv | Nazyrov Z.F.,University of Kharkiv | Goulatis I.L.,University of Kharkiv | And 2 more authors.
Journal of Materials Science: Materials in Electronics | Year: 2012

In this work we study the structural and resistivity parameters of the 1-2-3 system HTSC single crystals by substituting yttrium to holmium and with a low (5%) aluminum doping. It was determined that in the case of nonstoichiometric samples, the structure of crystals, directly after the solution-melt growth, is close to tetragonal. In the process of additional oxygen saturation we observed a ferroelastic transition to orthorhombic phase, accompanied by the formation of a domain twin structure. The twin boundaries and the aluminum impurities are efficient scattering centers of normal carriers. At aluminum concentration y B 0.5 the tweed structure is not formed. © 2011 Springer Science+Business Media, LLC.


Vovk R.V.,University of Kharkiv | Nazyrov Z.F.,University of Kharkiv | Goulatis I.L.,University of Kharkiv | Chroneos A.,Imperial College London | And 2 more authors.
Modern Physics Letters B | Year: 2013

The influence of praseodymium doping on the electrical resistivity in the ab-plane of Y1-zPrzBa2Cu3O 7-δ single crystals, is investigated. It is determined that as the concentration of praseodymium (0.0 ≤ z ≤ 0.5) is rising there occurs a significant shift of the temperature regions, corresponding to the metal-insulator transitions, as well as to the regime of the implementation of the pseudogap anomaly. The part of the curves related to the metal-insulator transition are well described by means of an asymptotic dependence that corresponds to the implementation of a quantum critical regime in the system, the so-called law of the "one third". © 2013 World Scientific Publishing Company.


Chroneos A.I.,Imperial College London | Vovk R.V.,University of Kharkiv | Goulatis I.L.,University of Kharkiv | Nazyrov Z.F.,University of Kharkiv | And 3 more authors.
Acta Physica Polonica A | Year: 2011

Fluorine (F) doping and the formation of F-vacancy (FnV m) clusters have been extensively studied in silicon (Si) as they can suppress the transient self-interstitial mediated diffusion of boron (B). Recent experimental studies by Bernardi et al. revealed that there is no significant concentration of FnVm clusters (for n ≥ 4, m ≥ 1) in disagreement with a number of density functional theory studies. In the present study we use electronic structure calculations to evaluate the binding energies of FnVm clusters and Vn clusters. The significant binding energies of the Vn clusters reveals that the concentration of the large FnVm clusters (n ≥ 4; m ≥ 1) will be limited compared to the Vn clusters or even smaller clusters.


Vovk R.V.,University of Kharkiv | Nazyrov Z.F.,University of Kharkiv | Obolenskii M.A.,University of Kharkiv | Zavgorodniy A.A.,University of Kharkiv | And 2 more authors.
Functional Materials | Year: 2011

The temperature dependence of longitudinal and transverse conductivity is investigated in Y 1-zPr zBa 2Cu 3O 7-δ single crystals with different praseodymium concentrations. It is shown, that the increasing of praseodymium concentration in samples of Y 1-zPr zBa 2Cu 3O 7-δleads to enhancement of the localization effects and suppression of the superconducting state. For Yy 1- zPr zBa 2CU3O 7-δ (in contrast to YBa 2CU3O 7-δ, compounds) the anisotropy of the normal resistivity P c/p ab(T) is described well in the framework of a universal "1/2 law" for thermally activated hopping conductivity. © 2011 - STC.


Vovk R.V.,University of Kharkiv | Zavgorodniy A.A.,University of Kharkiv | Obolenskii M.A.,University of Kharkiv | Goulatis I.L.,University of Kharkiv | Pinto Simoes V.M.,Ipa Instituto Superior Autonomo Of Estudos Politecnicos
Journal of Materials Science: Materials in Electronics | Year: 2011

The effect of high hydrostatic pressure on the electric resistance in the ab-plane of the oxygen deficient YBa 2Cu 3O 7-δ single crystals is investigated. It is shown that the pressure induced redistribution of the labile oxygen results in the strengthening of the phase immiscibility in layers. This process is accompanied with structural relaxation and ascending diffusion processes within the volume of the experimental sample. We observe a significant displacement of the temperature range that corresponds to metal-dielectric type transitions and the pseudo-gap realization regime. © 2010 Springer Science+Business Media, LLC.


Vovk R.V.,University of Kharkiv | Nazyrpv Z.F.,University of Kharkiv | Khadzhai G.Y.,University of Kharkiv | Pinto Simoes V.M.,Dom Afonso III Higher Institute | And 2 more authors.
Functional Materials | Year: 2013

The effect of a constant magnetic field to 12.7 kOe on the temperature dependences of electric conductivity of aluminum-doped YBaCuO single crystals with a system of unidirectional twinning boundaries has been investigated. It is determined that the twinning boundaries are effective centers of scattering of fluctuation carriers. Directly near Tc the temperature dependences of the excess paraconductivity are satisfactorily described by the Hikami-Larkin theoretical model of fluctuation conductivity for layered superconducting systems. The reasons for the suppression of three-dimensional superconducting fluctuations and the nonmonotonic dependence of ξc(0,H) in weak magnetic fields with the magnetic field vector oriented along c axis were discussed. It was shown that the lack of fan-shaped expansion of the resistive transitions in the magnetic field in these samples is conditioned by the lack of the no pinning vortex liquid phase due to increased pinning of the vortex lattice at the twin boundaries. © 2013 - STC "Institute for Single Crystals".


Vovk R.V.,University of Kharkiv | Nazyrov Z.F.,University of Kharkiv | Obolenskii M.A.,University of Kharkiv | Simoes V.M.P.,Ipa Instituto Superior Autonomo Of Estudos Politecnicos | And 2 more authors.
Acta Physica Polonica A | Year: 2011

The effect of annealing at room temperature on the excess conductivity of oxygen deficient HoBa2Cu3O7-δ single crystal is studied. It is shown that there is substantial extension of the interval of linear dependence ρab(T) and narrowing of the temperature area of the pseudo-gap regime as a result of annealing. The excess conductivity satisfies the exponential temperature dependence in a wide temperature interval. The temperature dependence of the pseudogap is satisfactorily described by the BCS-BEC theoretical model.

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