arXiv subjects

Valentin Yu. Irkhin

Publications and source records attributed to Valentin Yu. Irkhin.

8 recordsLinked to original sources

arXiv2026

Is it possible to determine unambiguously the Berry phase solely from quantum oscillations?

The Berry phase, a fundamental geometric phase in quantum systems, has become a crucial tool for probing the topological properties of materials. Quantum oscillations, such as Shubnikov-de Haas (SdH) oscillations, are widely used to extract this phase, but its unambiguous determination remains challenging. This work highlights the inherent ambiguities in interpreting the oscillation phase solely from SdH data, primarily due to the influence of the spin factor $R_S$, which depends on the Land\'e $g$-factor and effective mass. While the Lifshitz-Kosevich (LK) theory provides a framework for analyzing oscillations, the unknown g-factor introduces significant uncertainty. For instance, a zero oscillation phase could arise either from a nontrivial Berry phase or a negative $R_S$. We demonstrate that neglecting $R_S$ in modern studies, especially for topological materials with strong spin-orbit coupling, can lead to doubtful conclusions. Through theoretical analysis and numerical examples, we show how the interplay between the Berry phase and Zeeman effect complicates phase determination. Additionally, we also discuss another underappreciated mechanism - the magnetic field dependence of the Fermi level. Our discussion underscores the need for complementary experimental techniques to resolve these ambiguities and calls for further research to refine the interpretation of quantum oscillations in topological systems.

cond-mat.mtrl-sci↗

arXiv2025

Co$_2$MnZ (Z = Al, Si, Ga, Ge, Sn) Heusler alloys as candidate materials for spintronic and microelectronic applications: Electronic structure, transport, and magnetism

Magnetic and electronic transport properties of Co$_2$MnZ (Z = Al, Ga, Ge, Si, Sn) Heusler alloys were experimentally investigated. Electrical resistivity, in the temperature range from 4.2 to 300 K, as well as field dependences of the Hall effect and magnetization at T = 4.2 K in magnetic fields up to 100 kOe and 70 kOe, respectively, were measured. Experimental data are in good agreement with the results of the theoretical DFT calculations of the electronic structure and magnetic moments. In the band structure of Co$_2$MnSi, half-metallicity is formed with the full spin polarization and the half-metallic gap of about 0.6 eV. In Co$_2$MnZ (Z = Al, Ge, Sn), it is shifted from the Fermi energy by the hole pockets at the point $\Gamma$, preventing thereby the formation of the half-metallic state. In a peculiar case of Co$_2$MnGa, the antisite defects are expected to determine structural and electronic properties. For the Co$_2$MnAl and Co$_2$MnGa topological semimetals, Weyl topological points are found at the Fermi energy; however, for Z = Si, Ge, Si, these features are located deeper within to the valence band. The results show that Co$_2$MnGe and Co$_2$MnSn are usual ferromagnets, Co$_2$MnAl and Co$_2$MnGa alloys are topological semimetals that can find application in microelectronics, while Co$_2$MnSi is a half-metallic ferromagnet that is in high demand in spintronics.

cond-mat.mtrl-sci↗

arXiv2025

Anomalous lattice specific heat and rattling phonon modes in quadruple perovskites

Experimental data on the specific heat $C_p$ of quadruple perovskites ACu$_3$Fe$_2$Re$_2$O$_{12}$ (A = Mn, Cu, La, Ce, Dy) are presented, demonstrating an anomalous concave-down $C_p/T$ vs. $T^2$ curve and a bell-shaped feature in $\beta(T) = (C_p - \gamma T)/T^3$ plotted against $T$ on a logarithmic scale. This feature is most pronounced for A = Cu and Mn. These findings can be explained by the rattling phenomenon, previously identified in other systems such as filled skutterudites and $\beta$-pyrochlores. Using first-principles DFT+U calculations, the presence of a rattling mode in A = Mn system is directly confirmed. A qualitative interpretation of the rattling mechanism in terms of a pseudo-Jahn-Teller effect is proposed.

cond-mat.mtrl-sci↗

arXiv2025

Exchange interaction in ACu3Fe2Re2O12 quadruple perovskites

Quadruple perovskites ACu$_3$Fe$_2$Re$_2$O$_{12}$ attract considerable interest due to their high Curie temperatures (up to $710$K), which strongly depend on the A-site cation. In this work, we employ first-principles calculations to investigate their electronic structure and magnetic exchange interactions. A band mechanism of magnetism that explains the antiferromagnetic character of the exchange interactions and their strong dependence on the filling of the Re $t_{2g}$ states is proposed. These antiferromagnetic interactions stabilize ferrimagnetic ground state. The calculated Curie temperatures, obtained within the Onsager reaction field theory, are in a good agreement with experimental data.

cond-mat.mtrl-sci↗

arXiv2024

Spin-orbit coupling induced orbital entanglement in a three-band Hubbard model

The effect of the spin-orbit coupling on the ground state properties of the square-lattice three-band Hubbard model with a single electron per site is studied by a generalized Hartree-Fock approximation. We calculate the full phase diagram and show that there appear additional orbital-entangled phases brought about by competition of various exchange channels or by the spin-orbit coupling in addition to conventional states stabilized by the Kugel-Khomskii mechanism. One of these phases previously proposed to explain magnetic properties of Sr$_2$VO$_4$ is characterized by vanishing dipolar magnetic moments and antiferro-octupolar ordering. We calculated microscopic parameters for this material and demonstrate that it is located near a phase boundary of two orbital-entangled and two conventional antiferromagnetic ferro-orbital states.

cond-mat.str-el↗

arXiv2023

Hubbard Bands and Exotic States in Doped and Undoped Mott Systems: The Kotliar-Ruckenstein Representation

The slave-particle representation is a promising method to treat the properties of exotic strongly correlated systems. We develop a unified approach to describe both the paramagnetic state with possible spin-liquid features and states with strong long-range or short-range magnetic order. Combining the Kotliar-Ruckenstein representation and fractionalized spin-liquid deconfinement picture, the Mott transition and Hubbard subbands are considered. The spectrum in the insulating state is significantly affected by the presence of the spinon spin-liquid spectrum and a hidden Fermi surface. Presenting a modification of the Kotliar-Ruckenstein representation in the spin-wave region, we treat the case of magnetic order, with special attention being paid to the half-metallic ferromagnetic state. The formation of small and large Fermi surfaces for doped current carriers in the antiferromagnetic state is also discussed.

cond-mat.str-el↗

arXiv2020

Unconventional magnetism and electronic state in frustrated layered system PdCrO$_2$

First-principles calculations and a model consideration of magnetically frustrated layered material PdCrO$_2$ are performed. The results on the exchange parameters are in agreement with the experimental data on the Curie-Weiss temperature ($θ$). We show that experimentally observed strong suppression of the Néel temperature ($T_N$) in comparison with the Curie-Weiss temperature is due to three main factors. First, as expected, this is connected with the layered structure and relatively small exchange interaction along the $c$ axis. Second, deformation of the ideal in-plane 120$^{\circ}$ magnetic structure is crucial to provide finite $T_N$ value. However, these two factors are still insufficient to explain low $T_N$ and the large frustration factor $|θ|/T_N$. Thus, we suggest a scenario of an exotic non-Fermi-liquid state in PdCrO$_2$ above $T_N$ within the frameworks of the Anderson lattice model, which seems to explain qualitatively all its main peculiarities.

cond-mat.str-el↗

arXiv2011

A Modern Model Description of Magnetism

A comparison is given of the fundamental works of S. V. Vonsovsky on the many-electron polar and s-d(f) exchange models with the subsequent development of the theory of magnetism of transition and rare earth metals and their compounds. Special attention is given to the derivation of different many-electron models (by Heisenberg, Hubbard, Anderson) and to the interrelation between them. Among the problems considered in this review, of most importance are the many-electron approaches used in the description of systems of d and f electrons, the atomic representation of X-operators, and the problem of strong itinerant magnetism and formation of local moments. The application of these concepts to highly correlated systems, in particular, to the half-metallic ferromagnets and the Kondo lattices is discussed.

cond-mat.str-el↗