arXiv subjects

V. A. Horielyi

Publications and source records attributed to V. A. Horielyi.

7 recordsLinked to original sources

arXiv2026

Reconfigurable Current Distributions in Percolating Au Films and CrO2 Powder Compacts - Beyond Geometrical Connectivity

Despite fundamental differences in morphology, dimensionality, and microscopic transport, semicontinuous Au films and compacted CrO2 powders converge on a common network-level principle: the electrically relevant network is a condition-dependent subset of the physical contact network. In Au films near the geometrical percolation threshold, increasing bias reversibly shifts the low-temperature response from activated nearest-neighbour hopping with negative magnetoresistance to a weak-localization-like regime with positive magnetoresistance, consistent with broader electronic participation of the fixed island network. In CrO2 powder compacts, magnetic field mainly changes the spin-dependent resistances of existing intergranular junctions and thereby reorders competing current paths; temperature and measuring current can additionally expand or deplete the active network. Cohn's theorem provides a compact sensitivity principle for both cases: local resistance changes influence the measured response in proportion to the current carried by the affected links. The comparison identifies two limiting but interconvertible modes of reconfiguration - link enabling and link reweighting - and shows why geometrical connectivity alone is insufficient for interpreting transport in strongly inhomogeneous conductors.

cond-mat.dis-nn↗

arXiv2026

Low-Temperature Magnetoresistance Hysteresis in Granular Cr1-xFexO2: Slow Relaxation of Transport-Active Magnetic Configurations

Low-temperature magnetoresistance loops of compacted CrO2 powders become strongly nonmonotonic when electrical transport is dominated by a small number of spin-dependent intergranular tunnelling paths. We reanalyse previously reported data for undoped CrO2 and Fe-containing Cr1-xFexO2, focusing on additional branch crossings beyond the conventional low-field hysteresis. In the Fe-containing specimen, the resistance decreases, passes through a minimum, rises over an extended field interval, and then decreases again at higher field. This nonmonotonic hysteretic feature is strongly reduced when the magnetic-field sweep is slowed. Digitization of the three fastest sweeps shows that the field at which the resistance starts to recover shifts approximately linearly with sweep rate, corresponding to an effective magnetotransport relaxation time of about 3 s. This scale is attributed not to microscopic spin flips but to the slow evolution of local moment and domain configurations at the intergranular contacts dominating the current. The observations are consistent with conventional negative tunnelling magnetoresistance superimposed on a slower resistance-increasing contribution associated with transient local magnetic disorder. Fe increases coercivity and interfacial magnetic heterogeneity while reducing the overall negative magnetoresistance. The results support a transport-weighted magnetic-reconfiguration mechanism for the unusual hysteresis shape.

cond-mat.dis-nn↗

arXiv2026

Transport-Weighted Coercivity in Granular CrO2: Why Magnetoresistance and Magnetization Can Disagree

Magnetoresistance hysteresis is often used to estimate the coercive field of a magnetic material. We reanalyse our published magnetotransport and magnetometry data on compacted powders of the half-metallic ferromagnet CrO2 and show that this identification can fail in a granular conductor. The field Hp at the resistance maximum may be close to, above, or below the bulk coercive field Hc, depending on temperature and measuring current. For three samples with different particle shapes and insulating-shell thicknesses, the digitized ratio Hp/Hc follows the same qualitative sequence and suggests a common crossover near 15 K. We interpret this behaviour as a change in which interparticle junctions control the electrical response. Bulk magnetometry averages over the entire magnetic volume, whereas tunnelling resistance gives the largest weight to the small set of links that carries the current. Cohn's exact network-sensitivity theorem provides a rigorous linear-response definition of these transport weights. In simple terms, Cohn showed that the influence of a given contact on the total resistance is proportional to the square of the current flowing through that contact. Combined with known magnetic pair-correlation effects, this leads to the concept of transport-weighted coercivity. In granular sensors and spin-dependent composites, the resistance peak cannot automatically be treated as the bulk coercive field. Conversely, the difference Hp - Hc may serve as a diagnostic of current-path localization, barrier evolution, and device-to-device variability.

cond-mat.dis-nn↗

arXiv2026

Emergent Coherence at the Edge of Magnetism: Low-Doped La2-xSrxCuO4+delta Revisited

The La2-xSrxCuO4+delta (LSCO) system provides a unique experimental setting for exploring how magnetism, superconductivity, and disorder jointly shape charge transport in a doped Mott insulator. Transport measurements in lightly doped and oxygen-enriched LSCO reveal a strongly insulating normal state governed by variable-range hopping, accompanied by pronounced nonlinear current-voltage characteristics and, at low temperatures, current-induced negative differential resistance. With increasing carrier concentration, these features evolve into regimes characterized by granular and percolative superconductivity near the threshold of bulk superconductivity and, eventually, into a homogeneous strange-metal state close to optimal doping. Throughout this evolution, the transport response shows marked sensitivity to disorder, electronic inhomogeneity, and external control parameters, such as bias current and magnetic field. Rather than reflecting a sequence of sharply distinct phases, the observed transport regimes form a continuous crossover from a localization-dominated insulating state to granular superconductivity and further to a coherent metallic state. This crossover is driven primarily by the progressive enhancement of electronic screening, inter-region coupling, and superconducting connectivity, rather than by abrupt changes in the underlying microscopic scattering mechanisms. Taken together, the available transport data provide a coherent experimental basis for understanding how disorder and mesoscale electronic inhomogeneity organize charge transport and superconductivity across the LSCO phase diagram, underscoring the central role of percolation and nonequilibrium effects in underdoped cuprates.

cond-mat.str-el↗

arXiv2025

Electron Transport in Compacted Powders of VO2 Nanoparticles: Variable Range Hopping vs Percolation Behavior

Electron transport properties in compacted VO2 nanopowders were studied. While VO2 usually exhibits a first-order metal-insulator transition (MIT) at ~340K, in our compressed nanopowder samples the MIT was significantly broadened due to structural disorder, interparticle barriers, and phase coexistence. Resistivity measurements in the temperature range of 78 - 682 K initially suggested a variable range hopping (VRH) transport mechanism, but further analysis indicates that the observed temperature dependence is governed by percolative conductivity, modified by activation-assisted tunneling effects. Suppression of the expected resistance jump at the MIT is attributed to dynamic intergranular barrier restructuring, residual localized states, and percolative electron transport. These findings highlight the necessity of considering percolation effects when analyzing transport mechanisms in granular VO2-based systems.

cond-mat.str-el↗

arXiv2025

Changes in the coercivity fields of magnetoresistance hysteresis loops under the influence of a spin-polarized current

Using the example of a pressed sample consisting of chromium dioxide nanoparticles coated with insulating shells, we study the relationship between the electronic transport system and magnetic subsystem in granular spin-polarized metals. It is shown that the spin-polarized tunneling transport current can affect the coercivity fields of the percolation cluster formed in the sample with decreasing temperature.

cond-mat.mes-hall↗

arXiv2016

Features of Low Temperature Tunnel Magnetoresistance in Pressed Powder of Chromium Dioxide

Resistive and magnetoresistive properties of two samples of compacted powders of ferromagnetic half-metal CrO_{2} with shape anisotropy of nanoparticles were studied. The powders were prepared by the method of hydrothermal synthesis and consisted of needle-like particles with average diameters 24-34 nm and mean length of 300 nm. One of the samples has been made of compacted powder of pure CrO_{2}, while another sample has been prepared from a substitution solid solution Cr_{1-x}Fe_{x}O_{2}. The aim of this work was to study the effect of Fe impurity on the value of tunnel resistance and tunnel magnetoresistance for compacted CrO_{2} powders. It was found that the addition of Fe impurity leads to an increase in the coercive force of the powder and reduce the tunnel magnetoresistance. We assume resonant tunneling mechanism on the Fe impurities. We found a strong dependence of the magnetoresistance on the spin relaxation rate in the process of magnetization reversal. The possible reasons for such dependence are discussed.

cond-mat.mes-hall↗