arXiv subjects

Sergey I. Nikitin

Publications and source records attributed to Sergey I. Nikitin.

3 recordsLinked to original sources

arXiv2026

Wrinkles and Magnetic Flux Trapping in Graphite Nanoflakes: A Possible Source and Manifestation of Room-Temperature Superconductivity

The paper reports on a possible manifestation of local high-temperature superconductivity in graphite nanoflakes obtained from the bulk pyrolytic graphite by an extended grinding and subsequent annealing at 670 K in the air. Analysis of transmission electron microscopy (TEM) images shows that such treatment leads to a formation of high-density wrinkle-type defects at the basal-plane surface. The sample reveals the magnetic flux trapping - one of the key signatures of superconductivity - that persists up to 390 K and above. At the same time, both the as-ground (unannealed) and annealed in the vacuum samples manifest negligible flux trapping, and no wrinkles in their TEM images were found. The direct correlation between the appearance of high-density wrinkle arrays and the detection of the trapped magnetic flux above the room temperature strongly suggests that wrinkled regions serve a source of local high-temperature superconductivity in graphite.

cond-mat.supr-con↗

arXiv2023

Double-loop hysteresis of multisite dilute Sr(Y$_{1-x}$Dy$_x$)$_2$O$_4$ single crystal Kramers paramagnets: electron-phonon interaction, quantum tunneling and cross-relaxation

Experimental and theoretical studies of the dynamic magnetization in swept magnetic fields of the orthorhombic SrY$_2$O$_4$ single-crystals doped with the Dy$^{3+}$ Kramers ions (0.01 and 0.5 at.%) with natural abundances of even and odd Dy isotopes are presented. Impurity ions substitute for Y$^{3+}$ ions at two nonequivalent crystallographic sites with the same local $C_s$ symmetry but strongly different crystal fields. Well pronounced double-loop hysteresis is observed at temperatures 2, 4, 5 and 6 K for sweeping rates of 5 and 1 mT/s. The microscopic model of spectral, magnetic and kinetic properties of Dy$^{3+}$ ions is developed based on the results of EPR, site selective optical spectra and magnetic relaxation measurements. The derived approach to the dynamic magnetization in the sweeping field based on the numerical solution of generalized master equations with time-dependent transition probabilities induced by the electron-phonon interaction, quantum tunneling and cross-relaxation allowed us to reproduce successfully the evolution of the hysteresis loop shape with temperature, sweeping rate and concentration of paramagnetic ions.

cond-mat.mtrl-sci↗

arXiv2020

Symmetry breaking in single crystal SrTiO$_3$ plates: EPR manifestations

Electron paramagnetic resonance studies of the impurity Fe$^{3+}$ and Mn$^{4+}$ centers indicate that the symmetry of the crystal structure of millimeter-scale strontium titanate plates above 105 K lowers in the bulk from cubic to tetragonal, different from that of the conventional low-temperature antiferrodistortive phase. It is shown that the effect does not originate from the residual stress; its magnitude (observed tetragonal distortion) and particular manifestations depend on plate orientation, surface quality and the geometry of a sample. Peculiarities of the observed phenomenon are presented and possible scenario of its realization is discussed.

cond-mat.mtrl-sci↗