arXiv subjects

A. Isaeva

Publications and source records attributed to A. Isaeva.

15 recordsLinked to original sources

arXiv2024

Ubiquitous order-disorder transition in the Mn antisite sublattice of the (MnBi$_2$Te$_4$)(Bi$_2$Te$_3$)$_n$ magnetic topological insulators

Magnetic topological insulators (TIs) herald a wealth of applications in spin-based technologies, relying on the novel quantum phenomena provided by their topological properties. Particularly promising is the (MnBi$_2$Te$_4$)(Bi$_2$Te$_3$)$_n$ layered family of established intrinsic magnetic TIs that can flexibly realize various magnetic orders and topological states. High tunability of this material platform is enabled by manganese-pnictogen intermixing, whose amounts and distribution patterns are controlled by synthetic conditions. Positive implication of the strong intermixing in MnSb$_2$Te$_4$ is the interlayer exchange coupling switching from antiferromagnetic to ferromagnetic, and the increasing magnetic critical temperature. On the other side, intermixing also implies atomic disorder which may be detrimental for applications. Here, we employ nuclear magnetic resonance and muon spin spectroscopy, sensitive local probe techniques, to scrutinize the impact of the intermixing on the magnetic properties of (MnBi$_2$Te$_4$)(Bi$_2$Te$_3$)$_n$ and MnSb$_2$Te$_4$. Our measurements not only confirm the opposite alignment between the Mn magnetic moments on native sites and antisites in the ground state of MnSb$_2$Te$_4$, but for the first time directly show the same alignment in (MnBi$_2$Te$_4$)(Bi$_2$Te$_3$)$_n$ with n = 0, 1 and 2. Moreover, for all compounds, we find the static magnetic moment of the Mn antisite sublattice to disappear well below the intrinsic magnetic transition temperature, leaving a homogeneous magnetic structure undisturbed by the intermixing. Our findings provide a microscopic understanding of the crucial role played by Mn-Bi intermixing in (MnBi$_2$Te$_4$)(Bi$_2$Te$_3$)$_n$ and offer pathways to optimizing the magnetic gap in its surface states.

cond-mat.mtrl-sci↗

arXiv2022

Dilution of the magnetic lattice in the Kitaev candidate $\alpha$-RuCl$_3$ by Rh$^{3+}$ doping

Magnetic dilution of a well-established Kitaev candidate system is realized in the substitutional Ru$_{1-x}$Rh$_x$Cl$_3$ series ($x = 0.02-0.6$). Optimized syntheses protocols yield uniformly-doped single crystals and polycrystalline powders that are isostructural to the parental $\alpha$-RuCl$_3$ as per X-ray diffraction. The Rh content $x$ is accurately determined by the quantitative energy-dispersive X-ray spectroscopy technique with standards. We determine the magnetic phase diagram of Ru$_{1-x}$Rh$_x$Cl$_3$ for in-plane magnetic fields from magnetization and specific-heat measurements as a function of $x$ and stacking periodicity, and identify the suppression of the magnetic order at $x \approx 0.2$ towards a disordered phase, which does not show any clear signature of freezing into a spin glass. Comparing with previous studies on the substitution series Ru$_{1-x}$Ir$_x$Cl$_3$, we propose that chemical pressure would contribute to the suppression of magnetic order especially in Ru$_{1-x}$Ir$_x$Cl$_3$ and that the zigzag magnetic ground state appears to be relatively robust with respect to the dilution of the Kitaev--$\Gamma$--Heisenberg magnetic lattice. We also discovered a slight dependence of the magnetic properties on thermal cycling, which would be due to an incomplete structural transition.

cond-mat.str-el↗

arXiv2022

Two-dimensional ferromagnetic extension of a topological insulator

Inducing a magnetic gap at the Dirac point of the topological surface state (TSS) in a 3D topological insulator (TI) is a route to dissipationless charge and spin currents. Ideally, magnetic order is present only at the surface and not in the bulk, e.g. through proximity of a ferromagnetic (FM) layer. However, such a proximity-induced Dirac mass gap has not been observed, likely due to insufficient overlap of TSS and the FM subsystem. Here, we take a different approach, namely FM extension, using a thin film of the 3D TI Bi$_2$Te$_3$, interfaced with a monolayer of the lattice-matched van der Waals ferromagnet MnBi$_2$Te$_4$. Robust 2D ferromagnetism with out-of-plane anisotropy and a critical temperature of $\text{T}_\text{c}\approx$~15 K is demonstrated by X-ray magnetic dichroism and electrical transport measurements. Using angle-resolved photoelectron spectroscopy, we observe the opening of a sizable magnetic gap in the 2D FM phase, while the surface remains gapless in the paramagnetic phase above T$_c$. This sizable gap indicates a relocation of the TSS to the FM ordered Mn moments near the surface, which leads to a large mutual overlap.

cond-mat.mtrl-sci↗

arXiv2022

Combined experimental and theoretical study of hydrostatic (He-gas) pressure effects in $\alpha$-RuCl$_3$

We report a detailed experimental and theoretical study on the effect of hydrostatic pressure on the structural and magnetic aspects of the layered honeycomb antiferromagent $\alpha$-RuCl$_{3}$. Magnetic susceptibility measurements performed under almost ideal hydrostatic-pressure conditions yield that the phase transition to zigzag-type antiferromagnetic order at $T_N$ = 7.3 K can be rapidly suppressed to about 6.1 K. A further suppression with increasing pressure is impeded due to the occurrence of a pressure-induced structural transition at $p \geq$ 104 MPa, accompanied by a strong dimerization of Ru-Ru bonds, which gives rise to a collapse of the magnetic susceptibility. Whereas the dimerization transition is strongly first order, as reflected by large discontinuous changes in $\chi$ and pronounced hysteresis effects, the magnetic transition under varying pressure and magnetic field also reveals indications for a weakly first-order transition. We assign this observation to a strong magnetoelastic coupling in this system. Measurements of $\chi$ under varying pressure in the paramagnetic regime ($T > T_N$) and before dimerization ($p <$ 100 MPa) reveal a considerable increase of $\chi$ with pressure. These experimental observations are consistent with the results of ab-initio Density Functional Theory (DFT) calculations on the pressure-dependent structure and the corresponding pressure-dependent magnetic model. Comparative susceptibility measurements on a second crystal showing two consecutive magnetic transitions instead of one, indicating the influence of stacking faults. Using different temperature-pressure protocols the effect of these stacking faults can be temporarily overcome, transforming the magnetic state from a multiple-$T_N$ into a single-$T_N$ state.

cond-mat.str-el↗

arXiv2021

Magnetic field tuning of the spin dynamics in the magnetic topological insulators (MnBi$_{2}$Te$_{4}$)(Bi$_{2}$Te$_{3}$)$_{n}$

We report a high frequency/high magnetic field electron spin resonance (HF-ESR) spectroscopy study in the sub-THz frequency domain of the two representatives of the family of magnetic topological insulators (MnBi$_{2}$Te$_{4}$)(Bi$_{2}$Te$_{3}$)$_{n}$ with $n = 0$ and 1. The HF-ESR measurements in the magnetically ordered state at a low temperature of $T = 4$ K combined with the calculations of the resonance modes showed that the spin dynamics in MnBi$_{\text{4}}$Te$_{\text{7}}$ is typical for an anisotropic easy-axis type ferromagnet (FM) whereas MnBi$_{\text{2}}$Te$_{\text{4}}$ demonstrates excitations of an anisotropic easy-axis type antiferromagnet (AFM). However, by applying the field stronger than a threshold value $\sim 6$ T we observed in MnBi$_{\text{2}}$Te$_{\text{4}}$ a crossover from the AFM resonance modes to the FM modes which properties are very similar to the ferromagnetic response of MnBi$_{\text{4}}$Te$_{\text{7}}$. We attribute this remarkably unusual effect unexpected for a canonical easy-axis AFM, which, additionally, can be accurately reproduced by numerical calculations of the excitation modes, to the closeness of the strength of the AFM exchange and magnetic anisotropy energies which appears to be a very specific feature of this compound. Our experimental data evidences that the spin dynamics of the magnetic building blocks of these compounds, the Mn-based septuple layers (SLs), is inherently ferromagnetic featuring persisting short-range FM correlations far above the magnetic ordering temperature as soon as the SLs get decoupled either by introducing a nonmagnetic quintuple interlayer, as in MnBi$_{\text{4}}$Te$_{\text{7}}$, or by applying a moderate magnetic field, as in MnBi$_{\text{2}}$Te$_{\text{4}}$, which may have an effect on the surface topological band structure of these compounds.

cond-mat.str-el↗

arXiv2021

Strongly anisotropic spin dynamics in magnetic topological insulators

The recent discovery of magnetic topological insulators has opened new avenues to explore exotic states of matter that can emerge from the interplay between topological electronic states and magnetic degrees of freedom, be it ordered or strongly fluctuating. Motivated by the effects that the dynamics of the magnetic moments can have on the topological surface states, we investigate the magnetic fluctuations across the (MnBi$_{\text{2}}$Te$_{\text{4}}$)(Bi$_{\text{2}}$Te$_{\text{3}}$)$_{\text{n}}$ family. Our paramagnetic electron spin resonance experiments reveal contrasting Mn spin dynamics in different compounds, which manifests in a strongly anisotropic Mn spin relaxation in MnBi$_{\text{2}}$Te$_{\text{4}}$ while being almost isotropic in MnBi$_{\text{4}}$Te$_{\text{7}}$. Our density-functional calculations explain these striking observations in terms of the sensitivity of the local electronic structure to the Mn spin-orientation, and indicate that the anisotropy of the magnetic fluctuations can be controlled by the carrier density, which may directly affect the electronic topological surface states.

cond-mat.str-el↗

arXiv2020

Orbital Complexity in Intrinsic Magnetic Topological Insulators MnBi$_4$Te$_7$ and MnBi$_6$Te$_{10}$

Using angle-resolved photoelectron spectroscopy (ARPES), we investigate the surface electronic structure of the magnetic van der Waals compounds MnBi$_4$Te$_7$ and MnBi$_6$Te$_{10}$, the $n=$~1 and 2 members of a modular (Bi$_2$Te$_3$)$_n$(MnBi$_2$Te$_4$) series, which have attracted recent interest as intrinsic magnetic topological insulators. Combining circular dichroic, spin-resolved and photon-energy-dependent ARPES measurements with calculations based on density functional theory, we unveil complex momentum-dependent orbital and spin textures in the surface electronic structure and disentangle topological from trivial surface bands. We find that the Dirac-cone dispersion of the topologial surface state is strongly perturbed by hybridization with valence-band states for Bi$_2$Te$_3$-terminated surfaces but remains preserved for MnBi$_2$Te$_4$-terminated surfaces. Our results firmly establish the topologically non-trivial nature of these magnetic van der Waals materials and indicate that the possibility of realizing a quantized anomalous Hall conductivity depends on surface termination.

cond-mat.str-el↗

arXiv2020

Electron spin resonance and ferromagnetic resonance spectroscopy in the high-field phase of the van der Waals magnet CrCl$_3$

We report a comprehensive high-field/high-frequency electron spin resonance (ESR) study on single crystals of the van der Waals magnet CrCl$_3$. This material, although being known for quite a while, has received recent significant attention in a context of the use of van der Waals magnets in novel spintronic devices. Temperature-dependent measurements of the resonance fields were performed between 4 and 175 K and with the external magnetic field applied parallel and perpendicular to the honeycomb planes of the crystal structure. These investigations reveal that the resonance line shifts from the paramagnetic resonance position already at temperatures well above the transition into a magnetically ordered state. Thereby the existence of ferromagnetic short-range correlations above the transition is established and the intrinsically two-dimensional nature of the magnetism in the title compound is proven. To study details of the magnetic anisotropies in the field-induced effectively ferromagnetic state at low temperatures, frequency-dependent ferromagnetic resonance (FMR) measurements were conducted at 4 K. The observed anisotropy between the two magnetic-field orientations is analyzed by means of numerical simulations based on a phenomenological theory of FMR. These simulations are in excellent agreement with measured data if the shape anisotropy of the studied crystal is taken into account, while the magnetocrystalline anisotropy is found to be negligible in CrCl$_3$. The absence of a significant intrinsic anisotropy thus renders this material as a practically ideal isotropic Heisenberg magnet.

cond-mat.mtrl-sci↗

arXiv2019

Surface states and Rashba-type spin polarization in antiferromagnetic MnBi$_2$Te$_4$

The layered van der Waals antiferromagnet MnBi$_2$Te$_4$ has been predicted to combine the band ordering of archetypical topological insulators such as Bi$_2$Te$_3$ with the magnetism of Mn, making this material a viable candidate for the realization of various magnetic topological states. We have systematically investigated the surface electronic structure of MnBi$_2$Te$_4$(0001) single crystals by use of spin- and angle-resolved photoelectron spectroscopy experiments. In line with theoretical predictions, the results reveal a surface state in the bulk band gap and they provide evidence for the influence of exchange interaction and spin-orbit coupling on the surface electronic structure.

cond-mat.str-el↗

arXiv2019

Spin-glass state and reversed magnetic anisotropy induced by Cr doping in the Kitaev magnet $\alpha$-RuCl$_3$

Magnetic properties of the substitution series Ru$_{1-x}$Cr$_x$Cl$_3$ were investigated to determine the evolution from the anisotropic Kitaev magnet $\alpha$-RuCl$_3$ with $J_{\rm eff} = 1/2$ magnetic Ru$^{3+}$ ions to the isotropic Heisenberg magnet CrCl$_3$ with $S = 3/2$ magnetic Cr$^{3+}$ ions. Magnetization measurements on single crystals revealed a reversal of the magnetic anisotropy under doping, which we argue to arise from the competition between anisotropic Kitaev and off-diagonal interactions on the Ru-Ru links and approximately isotropic Cr-Ru and isotropic Cr-Cr interactions. In addition, combined magnetization, ac susceptibility and specific-heat measurements clearly show the destabilization of the long-range magnetic order of $\alpha$-RuCl$_3$ in favor of a spin-glass state of Ru$_{1-x}$Cr$_x$Cl$_3$ for a low doping of $x\backsimeq0.1$. The corresponding freezing temperature as a function of Cr content shows a broad maximum around $x\backsimeq0.45$.

cond-mat.str-el↗

arXiv2018

Pressure-induced dimerization and valence bond crystal formation in the Kitaev-Heisenberg magnet alpha-RuCl3

Magnetization and high-resolution x-ray diffraction measurements of the Kitaev-Heisenberg material alpha-RuCl3 reveal a pressure-induced crystallographic and magnetic phase transition at a hydrostatic pressure of p=0.2 GPa. This structural transition into a triclinic phase is characterized by a very strong dimerization of the Ru-Ru bonds, accompanied by a collapse of the magnetic susceptibility. Ab initio quantum-chemistry calculations disclose a pressure-induced enhancement of the direct 4d-4d bonding on particular Ru-Ru links, causing a sharp increase of the antiferromagnetic exchange interactions. These combined experimental and computational data show that the Kitaev spin liquid phase in alpha-RuCl3 strongly competes with the crystallization of spin singlets into a valence bond solid.

cond-mat.str-el↗

arXiv2017

Nearest-neighbor Kitaev exchange blocked by charge order in electron doped $α$-RuCl$_{3}$

A quantum spin-liquid might be realized in $α$-RuCl$_{3}$, a honeycomb-lattice magnetic material with substantial spin-orbit coupling. Moreover, $α$-RuCl$_{3}$ is a Mott insulator, which implies the possibility that novel exotic phases occur upon doping. Here, we study the electronic structure of this material when intercalated with potassium by photoemission spectroscopy, electron energy loss spectroscopy, and density functional theory calculations. We obtain a stable stoichiometry at K$_{0.5}$RuCl$_3$. This gives rise to a peculiar charge disproportionation into formally Ru$^{2+}$ (4$d^6$) and Ru$^{3+}$ (4$d^5$). Every Ru 4$d^5$ site with one hole in the $t_{2g}$ shell is surrounded by nearest neighbors of 4$d^6$ character, where the $t_{2g}$ level is full and magnetically inert. Thus, each type of Ru sites forms a triangular lattice and nearest-neighbor interactions of the original honeycomb are blocked.

cond-mat.str-el↗

arXiv2017

Signatures of low-energy fractionalized excitations in $\alpha$-RuCl$_3$ from field-dependent microwave absorption

Topologically ordered states of matter are generically characterized by excitations with quantum number fractionalization. A prime example is the spin liquid realized in Kitaev's honeycomb-lattice compass model where spin-flip excitations fractionalize into Majorana fermions and Ising gauge fluxes. While numerous compounds have been proposed to be proximate to such a spin-liquid phase, clear-cut evidence for fractionalized excitations is lacking. Here we employ microwave absorption measurements to study the low-energy excitations in $\alpha$-RuCl$_3$ over a wide range of frequencies, magnetic fields, and temperatures, covering in particular the vicinity of the field-driven quantum phase transition where long-range magnetic order disappears. In addition to conventional gapped magnon modes we find a highly unusual broad continuum characteristic of fractionalization which -- most remarkably -- extends to energies below the lowest sharp mode and to temperatures significantly higher than the ordering temperature, and develops a gap of a nontrivial origin in strong magnetic fields. Our results unravel the signatures of fractionalized excitations in $\alpha$-RuCl$_3$ and pave the way to a more complete understanding of the Kitaev spin liquid and its instabilities.

cond-mat.str-el↗

arXiv2017

Pressure effect and Superconductivity in $β$-Bi$_4$I$_4$ Topological Insulator

We report a detailed study of the transport coefficients of $β$-Bi$_4$I$_4$ quasi-one dimensional topological insulator. Electrical resistivity, thermoelectric power, thermal conductivity and Hall coefficient measurements are consistent with the possible appearance of a charge density wave order at low temperatures. Both electrons and holes contribute to the conduction in $β$-Bi$_4$I$_4$ and the dominant type of charge carrier changes with temperature as a consequence of temperature-dependent carrier densities and mobilities. Measurements of resistivity and Seebeck coefficient under hydrostatic pressure up to 2 GPa show a shift of the charge density wave order to higher temperatures suggesting a strongly one-dimensional character at ambient pressure. Surprisingly, superconductivity is induced in $β$-Bi$_4$I$_4$ above 10 GPa with of 4.0 K which is slightly decreasing upon increasing the pressure up to 20 GPa. Chemical characterisation of the pressure-treated samples shows amorphization of $β$-Bi$_4$I$_4$ under pressure and rules out decomposition into Bi and BiI$_3$ at room-temperature conditions.

cond-mat.str-el↗

arXiv2016

J$_{eff}$ description of the honeycomb Mott insulator $α$-RuCl$_3$

Novel ground states might be realized in honeycomb lattices with strong spin-orbit coupling. Here we study the electronic structure of $α$-RuCl$_3$, in which the Ru ions are in a d5 configuration and form a honeycomb lattice, by angle-resolved photoemission, x-ray photoemission and electron energy loss spectroscopy supported by density functional theory and multiplet calculations. We find that $α$-RuCl$_3$ is a Mott insulator with significant spin-orbit coupling, whose low energy electronic structure is naturally mapped onto Jeff states. This makes $α$-RuCl$_3$ a promising candidate for the realization of Kitaev physics. Relevant electronic parameters such as the Hubbard energy U, the crystal field splitting 10Dq and the charge transfer energy are evaluated. Furthermore, we observe significant Cl photodesorption with time, which must be taken into account when interpreting photoemission and other surface sensitive experiments.

cond-mat.str-el↗