arXiv subjects

S. V. Eremeev

Publications and source records attributed to S. V. Eremeev.

At least 19 recordsLinked to original sources

arXiv2026

Chromium chalcohalide Janus monolayer ferromagnets with perpendicular magnetic anisotropy and high Curie temperature

Using density functional theory, we revisit the magnetic properties of a recently proposed family of noncentrosymmetric two-dimensional magnetic materials, chromium chalcohalide monolayers, CrXY (X=S, Se, Te; Y=Cl, Br, I). These systems consist of three atomic planes stacked in the X-Cr-Y sequence, which breaks inversion symmetry, giving rise to their designation as "Janus" monolayers. We consider both 1T and 1H structural polymorphs of CrXY. Among the two polymorphs, the 1T phase is consistently more favorable, with energy gains exceeding 0.55 eV per formula unit. Our total-energy calculations reveal that all dynamically stable CrXY monolayers exhibit ferromagnetic coupling. However, robust out-of-plane magnetic anisotropy is observed only in the CrSI and CrSeI compositions, for both 1T and 1H structures. The perpendicular magnetic anisotropy results from a constructive interplay between single-ion and anisotropic exchange contributions that overcome the dipole-dipole interaction. We further quantify the Dzyaloshinskii-Moriya interaction (DMI) in CrSI and CrSeI for both polymorphs, and reveal a weak-to-moderate DMI strength as compared to the isotropic exchange interaction term. Finally, for systems exhibiting ferromagnetic coupling and perpendicular magnetic anisotropy, the exchange and anisotropy parameters derived from density functional theory calculations are employed as inputs for large-scale atomistic spin dynamics simulations to probe the temperature evolution of real-space magnetic structures. The calculated Curie temperatures are at least 210 K for 1T-CrSI, 235-260 K for 1H-CrSeI, and 370-410 K for 1H-CrSI. In contrast, the sizable DMI in 1T-CrSeI results in a worm-like domain ground state at zero external field and enables the stabilization of skyrmions under a perpendicular magnetic field.

cond-mat.mtrl-sci↗

arXiv2026

Anomalous phase shift and superconducting diode effect in Josephson junctions via thin films of rare-earth intermetallic magnets

The superconductor/ferromagnet/superconductor (S/F/S) Josephson junctions (JJs) with an anomalous ground state phase shift $φ_0 \neq 0,π$ ($φ_0$-S/F/S JJs) enable the implementation of the zero-field Josephson diode effect with the possibility to control the diode efficiency and polarity. It is just as important that in this case $φ_0$ provides a coupling between the superconducting phase and the magnetization of the interlayer. Such $φ_0$-S/F/S JJs can be used for superconducting memory and logic circuit applications. Here we present the results of theoretical calculation of the current-phase relationship (CPR), exhibiting the Josephson diode effect and $φ_0\neq 0,π$, for a JJ through a specific magnetic material. As the interlayer of the JJ we consider an ultra-thin film of intermetallic lanthanide ($Ln$)-based compound $\mathrm{GdIr_2Si_2}$. Using the density functional theory (DFT) methods, we study the electronic structure and magnetic properties of the film. Then the effective tight-binding Hamiltonian (TBH), demonstrating high quantitative consistency with the electronic properties obtained from DFT calculations, is constructed. The TBH is used to calculate CPR in the framework of the Bogolubov-de Gennes approach. The CPRs demonstrate a pronounced $φ_0$ of the order of unity and a pronounced Josephson diode effect with the diode efficiency $ \lesssim 0.3$. Moreover, the efficiency can be controlled via rotation of in-plane magnetization in the interlayer. The prospects for utilizing alternative magnetic $Ln$-based materials of the $LnT_2X_2$ family ($T$ is a transition metal and $X$ is a $p$-element from groups III-V) for the implementation in $φ_0$-S/F/S JJs are also discussed.

cond-mat.supr-con↗

arXiv2026

Unveiling the superconducting scenario in multiphase superconductor CeRh$_2$As$_2$ from space-group symmetry analysis and DFT calculations

Despite of the low transition temperature, the recently identified superconductor CeRh$_2$As$_2$ has garnered significant interest due to its unique symmetry and magnetic characteristics, particularly the existence of two superconducting (SC) phases under a magnetic field, one of which exceeds the Pauli-Clogston limit. The field-induced transition from a low-field even-parity state to a high-field odd-parity state is usually described as a singlet-triplet transition. However, it is uncommon for a single compound to exhibit both triplet and singlet SC scenarios. The aim of this paper is to investigate the possibilities of symmetry changes in the SC state without a change of spin multiplicity. To this end, we construct the SC order parameter based on Anderson pair functions, considering the phase winding within the symmetry of the point group $D_{4h}$ and the magnetic group $4/mm^{\prime }m^{\prime}$. It was found that two triplets with opposite-spin and equal-spin pairing states of symmetry $E_{1u}^{\prime +}$, are nodeless but exhibit distinct internal structures and may be associated with low-and high-field phases. Additionally, nontrivial Cooper pairing resulting from the non-symmorphic structure of the space group was examined, particularly in the case where the Fermi surface intersects with the boundaries of a Brillouin zone (BZ). It was determined that at the X point, triplet pairs are even, while singlet pairs can be either even or odd. Furthermore, at the X point, pair density waves that alter phase by $π$ at the atomic centers linked by lattice translations are also feasible. To explore the possibility of such scenarios, precise DFT calculations of the band structure were performed, revealing the contribution of Ce $4f$ electrons to the states at the Fermi level. Thus, the even-odd transition can take place in a triplet scenario at symmetry points of a BZ.

cond-mat.supr-con↗

arXiv2026

Elucidating Na$_2$KSb band structure: near-band-gap photoemission spectroscopy and DFT calculations

The electronic band structure of Na$_{2}$KSb was studied by a combination of low-energy photoemission spectroscopy and density functional theory (DFT) calculations. The optical and photoemission quantum efficiency (QE) spectra, along with longitudinal energy distribution curves (EDCs) of multialkali Na$_{2}$KSb(Cs,Sb) photocathodes were measured in the temperature range of 80--295 K. The thresholds of various band-to-band transition in Na$_{2}$KSb were observed in the optical and QE spectra of Na$_{2}$KSb(Cs,Sb) photocathodes. The evolution of EDC derivatives with varying photon energy reveals a fine structure related to the emission of two types of electrons: (i) ballistic electrons, which are excited from heavy hole, light hole and split-off valence bands, and (ii) photoelectrons, that are captured in the side valleys of Na$_{2}$KSb conduction band. The analysis of EDCs and QE spectra allowed us to determine the band structure parameters of Na$_{2}$KSb at $T = 80$ K, including the band gap $E_{\text{g}} = 1.52 \pm 0.02$ eV, spin-orbit splitting $Δ_{\text{SO}} = 0.59 \pm 0.04$ eV and the energy separations between $Γ$ and side valleys of the conduction band: $Δ_{Γ-\text{X}1} = 0.41 \pm 0.05$ eV and $Δ_{Γ-\text{X}2} = 0.65 \pm 0.05$ eV. The experimentally determined band gaps and side valley positions, as well as the energies of the final electronic states of optical transitions are in good agreement with the DFT calculations. The obtained data on the hot electron dynamics and electronic band structure of Na$_{2}$KSb are crucial to improve the understanding of the photoemission processes in this material and will contribute to the development of the robust spin-polarized electron sources with multialkali photocathodes.

cond-mat.mtrl-sci↗

arXiv2026

Revealing the (111) surface electronic structure of epitaxially grown Na$_2$KSb photocathode

A recent study has established the Na$_2$KSb(Cs) photocathode as a highly efficient emitter of spin-polarized electrons. However, the electronic structure of alkali antimonides remains poorly understood. In this work, we report the first crystalline epitaxial growth of Na$_2$KSb films, achieved via chemical vapor deposition (CVD) on a graphene-coated SiC(0001) substrate. The high crystalline quality of these films enabled a direct investigation of the material's electronic structure using angle-resolved photoemission spectroscopy (ARPES). By comparing the experimental results with density functional theory (DFT) calculations, we have identified dispersive surface states originating from different terminations of the Na$_2$KSb(111) surface. Furthermore, we demonstrate that the crystalline order of the film is preserved following its activation via the deposition of Cs and Sb. This finding opens a pathway for investigating the electronic structure of multialkali Na$_2$KSb(Cs) photocathodes and for rationally improving their properties.

cond-mat.mtrl-sci↗

arXiv2024

Interplay between surface Dirac and Rashba states specific for topologically nontrivial van der Waals superlattices

Here we show that, in contrast to the observed surface states in well studied pnictogen chalcogenide van der Waals (vdW) topological insulators (TIs) with quintuple layer (QL) or septuple layer~(SL) structure, in superlattices, comprising the alternating QL and SL vdW blocks, the Dirac state becomes accompanied by emergent spin-polarized states of the Rashba type. This specific feature is caused by an inequivalence of the surface and subsurface structural blocks and an electrostatic potential bending near the surface. Within density functional theory~(DFT) and $\emph{ab-initio}$ tight-binding~(TB) calculations we analyze peculiarities of these states depending on the surface termination, structural parameters and chemical composition. It is found that their possible hybridization with the Dirac state significantly affects its dispersion and spatial localization. We analyze the influence of intrinsic magnetism on behavior of the termination-dependent surface states for magnetic QL/SL superlattices. These findings provide a better understanding of the existing experimental observations of such QL/SL alternating superlattices.

cond-mat.mtrl-sci↗

arXiv2024

Interfacing Quantum Spin Hall and Quantum Anomalous Hall insulators: Bi bilayer on MnBi$_2$Te$_4$-family materials

Meeting of non-trivial topology with magnetism results in novel phases of matter, such as Quantum Anomalous Hall (QAH) or axion insulator phases. Even more exotic states with high and tunable Chern numbers are expected at the contact of intrinsic magnetic topological insulators (IMTIs) and 2D topological insulators (TIs).Here we synthesize a heterostructures composed of 2D TI and 3D IMTIs, specifically of bismuth bilayer on top of MnBi$_2$Te$_4$-family of compounds and study their electronic properties by means of angle-resolved photoelectron spectroscopy (ARPES) and density functional theory (DFT). The epitaxial interface is characterized by hybridized Bi and IMTI electronic states. The Bi bilayer-derived states on different members of MnBi$_2$Te$_4$-family of materials are similar, except in the region of mixing with the topological surface states of the substrate. In that region, the new, substrate dependent interface Dirac state is observed. Our \emph{ab initio} calculations show rich interface phases with emergence of exchange split 1D edge states, making the Bi/IMTI heterostructures promising playground for observation of novel members in the family of quantum Hall effects.

cond-mat.other↗

arXiv2023

Insight into the electronic structure of the centrosymmetric skyrmion magnet GdRu$_2$Si$_2$

The discovery of a square magnetic-skyrmion lattice in GdRu$_2$Si$_2$, with the smallest so far found skyrmion diameter and without a geometrically frustrated lattice, has attracted significant attention, particularly for potential applications in memory devices and quantum computing. In this work, we present a comprehensive study of surface and bulk electronic structures of GdRu$_2$Si$_2$ by utilizing momentum-resolved photoemission (ARPES) measurements and first-principles calculations. We show how the electronic structure evolves during the antiferromagnetic transition when a peculiar helical order of 4$f$ magnetic moments within the Gd layers sets in. A nice agreement of the ARPES-derived electronic structure with the calculated one has allowed us to characterize the features of the Fermi surface (FS), unveil the nested region along the $k_z$ at the corner of the 3D FS, and reveal their orbital compositions. Our findings suggest that the Ruderman-Kittel-Kasuya-Yosida interaction plays a decisive role in stabilizing the spiral-like order of Gd 4$f$ moments responsible for the skyrmion physics in GdRu$_2$Si$_2$. Our results provide a deeper understanding of electronic and magnetic properties of this material, which is crucial for predicting and developing novel skyrmion-based devices.

cond-mat.mtrl-sci↗

arXiv2022

New spin-polarized electron source based on alkali-antimonide photocathode

New spin-dependent photoemission properties of alkali antimonide semiconductor cathodes are predicted based on the detected optical spin orientation effect and DFT band structure calculations. Using these results, the Na$_2$KSb/Cs$_3$Sb heterostructure is designed as a spin-polarized electron source in combination with the Al$_{0.11}$Ga$_{0.89}$As target as a spin-detector with spatial resolution. In the Na$_2$KSb/Cs$_3$Sb photocathode, spin-dependent photoemission properties were established through detection of high degree of photoluminescence polarization and high polarization of the photoemitted electrons. It was found that the multi-alkali photocathode can provide electron beams with emittance very close to the limits imposed by the electron thermal energy. The vacuum tablet-type sources of spin-polarized electrons have been proposed for accelerators, that can exclude the construction of the photocathode growth chambers for photoinjectors.

cond-mat.mtrl-sci↗

arXiv2021

Novel magnetic stoichiometric superconductor EuRbFe4As4

In the novel stoichiometric iron-based material RbEuFe4As4 superconductivity coexists with a peculiar long-range magnetic order of Eu 4f states; their coexistance is puzzling and represents a challenge for both experiment and theory. Using angle-resolved photoemission spectroscopy, resonant photoemission spectroscopy, Andreev reflection spectroscopy and scanning tunneling spectroscopy we have addressed this puzzle and unambigously shown that Fe- and Eu-derived states are largely decoupled and that superconducting and a long range magnetic orders exist almost independently from each other.

cond-mat.supr-con↗

arXiv2021

Topological magnetic materials of the (MnSb$_2$Te$_4$)$\cdot$(Sb$_2$Te$_3$)$_n$ van der Waals compounds family

Combining robust magnetism, strong spin-orbit coupling and unique thickness-dependent properties of van der Waals crystals could enable new spintronics applications. Here, using density functional theory, we propose the (MnSb$_2$Te$_4$)$\cdot$(Sb$_2$Te$_3$)$_n$ family of stoichiometric van der Waals compounds that harbour multiple topologically-nontrivial magnetic phases. In the groundstate, the first three members of the family, i.e. MnSb$_2$Te$_4$, ($n=0$), MnSb$_4$Te$_7$, ($n=1$), and MnSb$_6$Te$_{10}$, ($n=2$), are 3D antiferromagnetic topological insulators (AFMTIs), while for $n \geq 3$ a special phase is formed, in which a nontrivial topological order coexists with a partial magnetic disorder in the system of the decoupled 2D ferromagnets, whose magnetizations point randomly along the third direction. Furthermore, due to a weak interlayer exchange coupling, these materials can be field-driven into the FM Weyl semimetal ($n=0$) or FM axion insulator states ($n \geq 1$). Finally, in two dimensions we reveal these systems to show intrinsic quantum anomalous Hall and AFM axion insulator states, as well as quantum Hall state, achieved under external magnetic field, but without Landau levels. Our results provide a solid computational proof that MnSb$_2$Te$_4$, is not topologically trivial as was previously believed that opens possibilities of realization of a wealth of topologically-nontrivial states in the (MnSb$_2$Te$_4$)$\cdot$(Sb$_2$Te$_3$)$_n$ family.

cond-mat.mtrl-sci↗

arXiv2020

When superconductivity does not fear magnetism: Insight into electronic structure of RbEuFe$_{4}$As$_{4}$

In the novel stoichiometric iron-based material RbEuFe$_{4}$As$_{4}$ superconductivity coexists with a peculiar long-range magnetic order of Eu 4f states. Using angle-resolved photoemission spectroscopy, we reveal a complex three dimensional electronic structure and compare it with density functional theory calculations. Multiple superconducting gaps were measured on various sheets of the Fermi surface. High resolution resonant photoemission spectroscopy reveals magnetic order of the Eu 4f states deep into the superconducting phase. Both the absolute values and the anisotropy of the superconducting gaps are remarkably similar to the sibling compound without Eu, indicating that Eu magnetism does not affect the pairing of electrons. A complete decoupling between Fe- and Eu-derived states was established from their evolution with temperature, thus unambiguously demonstrating that superconducting and a long range magnetic orders exist independently from each other. The established electronic structure of RbEuFe$_{4}$As$_{4}$ opens opportunities for the future studies of the highly unorthodox electron pairing and phase competition in this family of iron-based superconductors with doping.

cond-mat.supr-con↗

arXiv2020

Nature of the Dirac gap modulation and surface magnetic interaction in axion antiferromagnetic topological insulator MnBi$_2$Te$_4$

Modification of the gap at the Dirac point (DP) in antiferromagnetic (AFM) axion topological insulator MnBi$_2$Te$_4$ and its electronic and spin structure has been studied by angle- and spin-resolved photoemission spectroscopy (ARPES) under laser excitation with variation of temperature (9-35~K), light polarization and photon energy. We have distinguished both a large (62-67~meV) and a reduced (15-18~meV) gap at the DP in the ARPES dispersions, which remains open above the Néel temperature ($T_\mathrm{N}=24.5$~K). We propose that the gap above $T_\mathrm{N}$ remains open due to short-range magnetic field generated by chiral spin fluctuations. Spin-resolved ARPES, XMCD and circular dichroism ARPES measurements show a surface ferromagnetic ordering for large-gap sample and significantly reduced effective magnetic moment for the reduced-gap sample. These effects can be associated with a shift of the topological DC state towards the second Mn layer due to structural defects and mechanical disturbance, where it is influenced by a compensated effect of opposite magnetic moments.

cond-mat.mtrl-sci↗

arXiv2019

Variety of magnetic topological phases in the (MnBi$_2$Te$_4$)(Bi$_2$Te$_3$)$_m$ family

Quantum states of matter combining non-trivial topology and magnetism attract a lot of attention nowadays; the special focus is on magnetic topological insulators (MTIs) featuring quantum anomalous Hall and axion insulator phases. Feasibility of many novel phenomena that \emph{intrinsic} magnetic TIs may host depends crucially on our ability to engineer and efficiently tune their electronic and magnetic structures. Here, using angle- and spin-resolved photoemission spectroscopy along with \emph{ab initio} calculations we report on a large family of intrinsic magnetic TIs in the homologous series of the van der Waals compounds (MnBi$_2$Te$_4$)(Bi$_2$Te$_3$)$_m$ with $m=0, ..., 6$. Magnetic, electronic and, consequently, topological properties of these materials depend strongly on the $m$ value and are thus highly tunable. The antiferromagnetic (AFM) coupling between the neighboring Mn layers strongly weakens on moving from MnBi2Te4 (m=0) to MnBi4Te7 (m=1), changes to ferromagnetic (FM) one in MnBi6Te10 (m=2) and disappears with further increase in m. In this way, the AFM and FM TI states are respectively realized in the $m=0,1$ and $m=2$ cases, while for $m \ge 3$ a novel and hitherto-unknown topologically-nontrivial phase arises, in which below the corresponding critical temperature the magnetizations of the non-interacting 2D ferromagnets, formed by the \MBT\, building blocks, are disordered along the third direction. The variety of intrinsic magnetic TI phases in (MnBi$_2$Te$_4$)(Bi$_2$Te$_3$)$_m$ allows efficient engineering of functional van der Waals heterostructures for topological quantum computation, as well as antiferromagnetic and 2D spintronics.

cond-mat.mtrl-sci↗

arXiv2018

Competing rhombohedral and monoclinic crystal structures in Mn$Pn_2Ch_4$ compounds: an {\em ab-initio} study

Based on the relativistic spin-polarized density functional theory calculations we investigate the crystal structure, electronic and magnetic properties of a family MnPn2Ch4 compounds, where pnictogen metal atoms (Pn) are Sb and Bi; chalcogens (Ch) are Se, Te. We show that in the series the compounds of this family with heavier elements prefer to adopt rhombohedral crystal structure composed of weakly bonded septuple monoatomic layers while those with lighter elements tend to be in the monoclinic structure. Irrespective of the crystal structure all compounds of the MnPn2Ch4 series demonstrate a weak energy gain (of a few meV per formula unit or even smaller than meV) for antiferromagnetic (AFM) coupling for magnetic moments on Mn atoms with respect to their ferromagnetic (FM) state. For rhombohedral structures the interlayer AFM coupling is preferable while in monoclinic phases intralayer AFM configuration with ferromagnetic ordering along the Mn chain and antiferromagnetic ordering between the chains has a minimum energy. Over the series the monoclinic compounds are characterized by substantially wider bandgap than compounds with rhombohedral structure.

cond-mat.mtrl-sci↗

arXiv2017

2D and 3D topological phases in BiTe$X$ compounds

Recently, it was shown that quantum spin Hall insulator (QSHI) phase with a gap wide enough for practical applications can be realized in the ultra thin films constructed from two inversely stacked structural elements of trivial band insulator BiTeI. Here, we study the edge states in the free-standing Bi$_2$Te$_2$I$_2$ sextuple layer (SL) and the electronic structure of the Bi$_2$Te$_2$I$_2$ SL on the natural BiTeI substrate. We show that the topological properties of the Bi$_2$Te$_2$I$_2$ SL on this substrate keep $\mathbb Z_2$ invariant. We also demonstrate that ultra thin centrosymmetric films constructed in the similar manner but from related material BiTeBr are trivial band insulators up to five-SL film thickness. In contrast to Bi$_2$Te$_2$I$_2$ for which the stacking of nontrivial SLs in 3D limit gives a strong topological insulator (TI) phase, strong TI is realized in 3D Bi$_2$Te$_2$Br$_2$ in spite of the SL is trivial. For the last material of the BiTe$X$ ($X$=I,Br,Cl) series, BiTeCl, both 2D and 3D centrosymmetric phases are characterized by topologically trivial band structure.

cond-mat.mtrl-sci↗

arXiv2017

Quantum spin Hall insulators in centrosymmetric thin films composed from topologically trivial BiTeI trilayers

The quantum spin Hall insulators predicted ten years ago and now experimentally observed are instrumental for a breakthrough in nanoelectronics due to non-dissipative spin-polarized electron transport through their edges. For this transport to persist at normal conditions, the insulators should possess a sufficiently large band gap in a stable topological phase. Here, we theoretically show that quantum spin Hall insulators can be realized in ultra-thin films constructed from a trivial band insulator with strong spin-orbit coupling. The thinnest film with an inverted gap large enough for practical applications is a centrosymmetric sextuple layer built out of two inversely stacked non-centrosymmetric BiTeI trilayers. This nontrivial sextuple layer turns out to be the structure element of an artificially designed strong three-dimensional topological insulator Bi$_2$Te$_2$I$_2$. We reveal general principles of how a topological insulator can be composed from the structure elements of the BiTeX family (X=I, Br, Cl), which opens new perspectives towards engineering of topological phases.

cond-mat.mtrl-sci↗

arXiv2015

Non-Dirac topological surface states in (SnTe)$_{n\geq2}$(Bi$_2$Te$_3$)$_{m=1}$

A new type of topological spin-helical surface states was discovered in layered van der Waals bonded (SnTe)$_{n=2,3}$(Bi$_2$Te$_3$)$_{m=1}$ compounds which comprise two covalently bonded band inverted subsystems, SnTe and Bi$_2$Te$_3$, within a building block. This novel topological states demonstrate non-Dirac dispersion within the band gap. The dispersion of the surface state has two linear sections of different slope with shoulder feature between them. Such a dispersion of the topological surface state enables effective switch of the velocity of topological carriers by means of applying an external electric field.

cond-mat.mtrl-sci↗