arXiv subjects

A. Wiśniewski

Publications and source records attributed to A. Wiśniewski.

6 recordsLinked to original sources

arXiv2024

Temperature and excitation energy dependence of Raman scattering in nodal line Dirac semimetal ZrAs_{2}

We present a Raman study of ZrAs_{2} single crystals, a nodal line semimetal with symmetry-enforced Dirac-like band crossings. We identified the symmetry of phonon modes by polarized light measurements and comparison with calculated phonon frequencies. Significant dependence of peak intensities on the excitation wavelength was observed, indicating quantum interference effects. Phonon peaks in the spectra are superimposed on the electronic background, with quasi-elastic scattering observed for the 785 nm excitation. We identified the Fano shape of the 171 cm^{-1} Ag mode due to interference of the phonon state with the electronic continuum. The temperature dependence of phonon peaks linewidth indicates that the electron-phonon coupling plays an essential role in phonon decay.

cond-mat.mtrl-sci↗

arXiv2023

The impact of hydrostatic pressure, nonstoichiometry, and doping on trimeron lattice excitations in magnetite during axis switching

Trimeron lattice excitations in single crystalline magnetite, in the form of the $c$ axis switching (i.e. the reorganization of the lattice caused by external magnetic field) at temperatures below the Verwey temperature $T_V$ are observed by magnetization experiments. These excitations exhibit strong sensitivity to doping (with Zn, Al, and Ti), nonstoichiometry and hydrostatic pressure ($p < 1.2$ GPa). The considered indicators of the axis switching (AS) are: the switching field $B_{sw}$, the energy density needed to switch the axis $E_{sw}$ and the activation energy $U$. Our results show that hydrostatic pressure $p$ weakens the low$-T$ magnetite structure (decreases $T_V$) and has roughly similar effects on AS in Zn-doped Fe$_3$O$_4$ and, in much less extent, in stoichiometric magnetite. We have, however, found that while doping/nonstoichiometry also lowers $T_V$, making it more prone to temperature chaos, it drastically increases the switching field, activation and switching energies suggesting that the trimeron order, subject to change while AS occurs, is more robust. Consequently, we conclude that the manipulation of trimerons in the process of axis switching and the mechanisms leading to the Verwey transition are distinct phenomena.

cond-mat.str-el↗

arXiv2022

Electronic properties of TaAs2 topological semimetal investigated by transport and ARPES

We have performed electron transport and ARPES measurements on single crystals of transition metal dipnictide TaAs2 cleaved along the ($\overline{2}$ 0 1) surface which has the lowest cleavage energy. A Fourier transform of the Shubnikov-de Haas oscillations shows four different peaks whose angular dependence was studied with respect to the angle between the magnetic field and the [$\overline{2}$ 0 1] direction. The results indicate the elliptical shape of the Fermi surface cross-sections. Additionally, a mobility spectrum analysis was carried out, which also reveals at least four types of carriers contributing to the conductance (two kinds of electrons and two kinds of holes). ARPES spectra were taken on freshly cleaved ($\overline{2}$ 0 1) surface and it was found that bulk states pockets at the constant energy surface are elliptical, which confirms the magnetotransport angle dependent studies. First-principles calculations support the interpretation of the experimental results. The theoretical calculations better reproduce the ARPES data if the theoretical Fermi level is increased, which is due to a small n-doping of the samples. This shifts the Fermi level closer to the Dirac point, allowing to investigate the physics of the Dirac and Weyl points, making this compound a platform for the investigation of the Dirac and Weyl points in three-dimensional materials.

cond-mat.other↗

arXiv2021

Enhancement of the pinning potential in a single-crystal superconductor FeTe0.65Se0.35 under the influence of hydrogen sorption

As a result of experimental studies of the single-crystal iron-chalcogenide compound FeTe0.65Se0.35, the effect of structural transitions caused by hydrogen sorption on the magnetic and current-carrying properties of a superconductor has been established. An increase in the volume-averaged effective pinning potential (and the associated critical current density) after the process of hydrogen sorption at temperatures up to 150 °C - 200 °C can be explained by the appearance of additional pinning centers due to the local action of implanted H ions on its crystal structure and electronic states. It was confirmed that hydrogenation is an efficient tool for increasing flux pining properties of superconductors.

cond-mat.supr-con↗

arXiv2020

Conductance spectra of (Nb, Pb, In)/NbP -- superconductor/Weyl semimetal junctions

The possibility of inducing superconductivity in type-I Weyl semimetal through coupling its surface to a superconductor was investigated. A single crystal of NbP, grown by chemical vapor transport method, was carefully characterized by XRD, EDX, SEM, ARPES techniques and by electron transport measurements. The mobility spectrum of the carriers was determined. For the studies of interface transmission, the (001) surface of the crystal was covered by several hundred nm thick metallic layers of either Pb, or Nb, or In. DC current-voltage characteristics and AC differential conductance through the interfaces as a function of the DC bias were investigated. When the metals become superconducting, all three types of junctions show conductance increase, pointing out the Andreev reflection as a prevalent contribution to the subgap conductance. In the case of Pb-NbP and Nb-NbP junctions, the effect is satisfactorily described by modified Blonder-Tinkham-Klapwijk model. The absolute value of the conductance is much smaller than that for the bulk crystal, indicating that the transmission occurs through only a small part of the contact area. An opposite situation occurs in In-NbP junction, where the conductance at the peak reaches the bulk value indicating that almost whole contact area is transmitting and, additionally, a superconducting proximity phase is formed in the material. We interpret this as a result of indium diffusion into NbP, where the metal atoms penetrate the surface barrier and form very transparent superconductor-Weyl semimetal contact inside. However, further diffusion occurring already at room temperature leads to degradation of the effect, so it is observed only in the pristine structures. Despite of this, our observation directly demonstrates possibility of inducing superconductivity in a type-I Weyl semimetal.

cond-mat.mes-hall↗

arXiv2011

Mössbauer studies of powdered single crystals of FeTe0.5Se0.5

The Mössbauer measurements performed on powdered single crystals of FeTe0.5Se0.5 (Tc ~ 14.7 K) reveal minor content of two impurity phases, identified as Fe3O4 and Fe7Se8, among the major tetragonal phase. From the shape of impurity subspectra it follows that Fe7Se8 behaves in superparamagnetic manner, most likely because of randomly distributed Fe vacancies in the lattice structure of Fe Te Se. The magnetite content in the powdered absorber exposed to ambient air conditions remains unchanged during period of 16 months. Ageing effects were observed for the samples stored under argon atmosphere and small increase of the isomer shift of the doublet was detected. Presented temperature dependence of the hyperfine parameters can be explained as due to possible orthorhombic distortion or to temperature behaviour of impurity phases Fe3O4 and Fe7Se8. Strong tendency to formation of crystalline texture of powdered sample is observed.

cond-mat.supr-con↗