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A. J. Zaleski

Publications and source records attributed to A. J. Zaleski.

11 recordsLinked to original sources
arXiv2012

Increase of superconducting order parameter in the magnetic superconductor Dy0.6Y0.4Rh3.85Ru0.15B4 in a wide magnetic field range. Is it a further candidate in p-wave superconductors?

The Andreev reflection spectra were studied for the first time in the magnetic superconductor Dy0.6Y0.4Rh3.85Ru0.15B4. It is found that an external magnetic field (up to 0.7 Hc2) is a strong stimulator of superconductivity in contrast with traditional superconductors with a singlet pairing. The ratio 2Δ/kBTc \approx 4.0 obtained for some contacts is higher than the value 3.52 which is typical of conventional superconductors with a weak electron-phonon interaction. It has been proposed that in Dy0.6Y0.4Rh3.85Ru0.15B4 a triplet mechanism of superconducting pairing is realized.

cond-mat.supr-con↗
arXiv2011

Synthesis, Crystal Growth and Epitaxial Layer Deposition of FeSe0.88 Superconductor and Other Poison Materials by Use of High Gas Pressure Trap System

The FeSe samples in the form of polycrystals, single crystals and thin films have been prepared and characterized. The synthesized material has been hot isostatically pressed under pressure of 0.45 GPa of 5N purity argon with the use of the high gas pressure trap system (HGPTS). Thin films have been fabricated by the mixed procedures with the use of DC sputtering from various types of targets and processed employing the HGPTS. The used HGPTS assures a full separation of the active volume for synthesis or crystal growth of material and the inert gas medium. The obtained FeSe0.88 samples have Tc between 8 and 12 K. The samples have been characterized by SEM, EDX, XRD, magnetic susceptibility and resistivity measurements.

cond-mat.supr-con↗
arXiv2010

Andreev reflection spectroscopy of the new Fe-based superconductor EuAsFeO0.85F0.15: evidence for the strong order parameter anisotropy

Andreev reflection spectra have been measured in a new superconductor EuAsFeO0.85F0.15 having an unexpectedly low superconducting transition temperature Tc~11.3 K among related FeAs compounds on a base Sm and Gd surrounding Eu in the series of lanthanides. The nearly fivefold lower Tc, as against the expected value, is attributed to the divalent properties of Eu ions when in the compound investigated along with the weakly magnetic Eu3+ ions may be present and the strongly magnetic Eu2+ ones that is a strong destructive factor for superconductivity. Most of the spectra measured showed features that corresponds to two energy gaps whose values varied from contact to contact within 2Δs/kTc=2.2{\pm}4.7 and 2Δ1/kTc=5.1{\pm}11.7 for small and large gap, respectively. The corresponding variations for single-gap spectra are 2Δ/kTc = 2.6{\pm}6.4. The relatively large size of crystallites (no less than ~25 μ{m}) and the large number of contacts measured (several tens) suggest with a high degree of probability that the spectra obtained account quite fully for the gap distribution practically in all crystallographic directions. The data obtained and the absence of zero gaps in the measured spectra evidence in favor of the anisotropic s- or s\pm-symmetry of the order parameter in EuAsFeO0.85F0.15 that was revealed in other similar compounds with higher Tc. Thus, the character of the gap function Δ(k) in this compound is inconsistent with the d-wave superconductivity observed in some low-Tc pnictides.

cond-mat.supr-con↗
arXiv2009

Superconducting and magnetic properties of a new EuAsFeO0.85F0.15 superconductor

Polycrystalline samples of a new superconducting EuAsFeO0.85F0.15 compound with critical temperature Tc=11K were prepared by solid state synthesis. Its electric and magnetic properties have been investigated in magnetic fields from 0.1 to 140000 Oe. Critical magnetic fields Hc1, and Hc2 were measured and hence the magnetic penetration depths and the coherence length have been estimated. The temperature dependence Hc2 (T) exhibits clear hyperbolic - type behavior starting with the lowest fields. The data derived were used to estimate probable high Tc and Hc2 in compounds doped with rare-earths having small atomic radii.

cond-mat.supr-con↗
arXiv2008

Intergranular flux pinning in underdoped and overdoped R(1-x)CaxBa2Cu3Oz (R=Y,Gd; x=0,0.2) samples

The temperature dependences of AC magnetic susceptibility at different magnetic field amplitudes and frequencies are investigated for underdoped and overdoped R(1-x)Ca(x)Ba(2)Cu(3)O(7-delta)(R=Y; Gd and x=0; 0.2) polycrystalline samples. The activation energy, Ea, for thermally assisted flux flow (TAFF) in intergranular region is determined. It was established that a correlation exists between the intergranular critical current and flux pinning activation energy. In underdoped samples the intergranular current shows S-I-S behaviour and the activation energy is small, while in overdoped samples the intergranular current is changed to S-N-S type and the activation energy increases. 2D pancake vortices are characteristic of underdoped samples, while 3D vortex system exists in overdoped samples. In fact we demonstrate that Ca substitution not only increases carrier concentration, but improves intergranular activation energy for TAFF.

cond-mat.supr-con↗
arXiv2006

Magnetic studies of GaN nanoceramics

The synthesis, morphology and magnetization measurements of GaN nanoceramics obtained under high pressure are reported. In particular the effect of grain size on magnetic properties of GaN nanopowders and nanoceramics was investigated. It was found that for the GaN nanoceramic characterized by the stronger deformation of nanocrystalline grains the diamagnetic signal changes with external magnetic field similarly to the Meissner effect in superconductors.

cond-mat.supr-con↗
arXiv2005

Resistivity memory effect in La(1-x)Sr(x)MnO(3)

During the study of magnetoresistivity in La(1-x)Ca(x)MnO(3) it was found that after cycling of the magnetic field, some kind of magnetic field memory effect was observed. For La0.5Ca0.5MnO3 after cycling of the magnetic field to 13T and back to zero, "frozen" magnetoresistivity decreases about 20 times comparing to zero field value, while for La(0.47)Ca(0.53)MnO(3) it is already about four orders of magnitude. This effect can be observed only for concentration region 0.45 < x < 0.55. In zero magnetic field, temperature dependence of resistivity ro(T) shows semiconducting-like behavior, while after magnetic field cycling it becomes metal-like. So it looks as we are dealing with magnetic field induced semiconductor (or dielectric) to metal transition. Such effect can be explained within phase-separation picture. In zero magnetic field material consists of antiferromagnetic matrix (insulating phase) and coexisting ferromagnetic, conducting phase. Magnetic field application causes ferromagnetic phase to form some kind of conducting channels which shunts semiconducting matrix phase. Such structure is preserved after reduction of magnetic field, leaving the material conducting.

cond-mat.str-el↗
arXiv2005

Magnetic Moment Peculiarities in Bismuth Bicrystals with Large Crystallite Disorientation Angles

Magnetization measurements prove that the magnetic properties of large-angle (theta > 30o) bismuth bicrystals with crystallite interface (CI) of twisting type essentially differ from well-known results on single-crystalline specimens. Two superconducting phases were observed at CI of bicrystals (ordinary rhombohedral Bi is not a superconductor). It was shown that one of them (phase with Tc ~ 8,4 K) is localized in central part of crystallite interface. It was also found that in adjacent layers (width of layer d2 ~ 20 nm) of CI the proximity effect is significant.

cond-mat.supr-con↗
arXiv2004

Effects of Substituting Calcium for Yttrium on the Superconducting Properties of YBa2Cu3Oz Bulk Samples

We report systematic studies of AC magnetic susceptibility and transport properties of Y(1-x)Ca(x)Ba2Cu3Oz bulk samples with 0\leq x\leq 0.4. Single phase materials, reduction of carrier concentration and decrease of Tc to 85 K were obtained at doping levels up to 20%. For Y0.7Ca0.3Ba2Cu3Oz sample the improvement of grains boundary transport and screening capabilities has been observed as a result of the optimal ratio between carrier concentration and impurity phase BaCuO2 presence. The appearence of bulk pinning and nonlinear effects starting at the highest temperature were detected also.

cond-mat.supr-con↗
arXiv2001

Incipient superconductivity in TaB$_{2}$

Magnetic properties of TaB$_{2}$ were studied by means of DC and AC susceptibility measurements. It has been found that the compound becomes superconducting at T$_{c}$ = 9.5 K. From upper critical field measurements the superconducting parameters have been evaluated, which indicated that TaB$_{2}$ is a hard type-II superconductor. Possible reasons why superconductivity has not been discovered in previous studies are briefly discussed.

cond-mat.supr-con↗