arXiv subjects

Sergey A. Nikolaev

Publications and source records attributed to Sergey A. Nikolaev.

8 recordsLinked to original sources

arXiv2024

Large Chiral Orbital Texture and Orbital Edelstein Effect in Co/Al Heterostructure

Recent experiments by S. Krishnia et al., Nano Lett. 23, 6785 (2023) reported an unprecedentedly large enhancement of torques upon inserting thin Al layer in Co/Pt heterostructure that suggested the presence of a Rashba-like interaction at the metallic Co/Al interface. Based on first-principles calculations, we reveal the emergence of a large helical orbital texture in reciprocal space at the interfacial Co layer, whose origin is attributed to the orbital Rashba effect due to the formation of the surface states at the Co/Al interface and where spin-orbit coupling is found to produce smaller contributions with a higher-order winding of the orbital momentum. Our results unveil that the orbital texture gives rise to a non-equilibrium orbital accumulation producing large current-induced torques, thus providing an essential theoretical background for the experimental data and advancing the use of orbital transport phenomena in all-metallic magnetic systems with light elements.

cond-mat.mtrl-sci↗

arXiv2022

Quantum Dynamics of Magnetic Skyrmions: Consistent Path Integral Formulation

We present a path integral formalism for the intrinsic quantum dynamics of magnetic skyrmions coupled to a thermal background of magnetic fluctuations. Upon promoting the skyrmion's collective coordinate $\boldsymbol{R}$ to a dynamic variable and integrating out the magnonic heat bath, we derive the generalized equation of motion for $\boldsymbol{R}$ with a non-local damping term that describes a steady-state skyrmion dynamics at finite temperatures. Being essentially temperature dependent, the intrinsic damping is shown to originate from the coupling of thermally activated magnon modes to the adiabatic potential driven by a rigid skyrmion motion, which can be regarded as another manifestation of emergent electrodynamics inherent to topological magnetic textures. We further argue that the diagonal components of the damping term act as the source of dissipation and inertia, while its off-diagonal components modify the gyrotropic motion of a magnetic skyrmion. By means of numerical calculations for the lattice spin model of chiral ferromagnets, we study the temperature behavior of the intrinsic damping as a function of magnetic field in periodic and confined geometries. The intrinsic damping is demonstrated to be highly non-local, revealing its quantum-mechanical nature, that becomes more pronounced with increasing temperature. At high temperatures when the magnon occupation factors are large, the intrinsic damping is shown to yield a modified Thiele's equation with the additional non-local dissipative and mass terms that exhibit an almost linear temperature behavior. Our results provide a microscopic background for semiclassical magnetization dynamics and establish a framework for understanding spin caloritronics effects in topological magnetic textures.

cond-mat.mes-hall↗

arXiv2021

High-mobility two-dimensional carriers from surface Fermi arcs in magnetic Weyl semimetal films

High-mobility two-dimensional carriers originating from surface Fermi arcs in magnetic Weyl semimetals are highly desired for accessing exotic quantum transport phenomena and for topological electronics applications. Here, we demonstrate high-mobility two-dimensional carriers that show quantum oscillations in magnetic Weyl semimetal SrRuO3 epitaxial films by systematic angle-dependent, high-magnetic field magnetotransport experiments. The exceptionally high-quality SrRuO3 films were grown by state-of-the-art oxide thin film growth technologies driven by machine learning algorithm. The quantum oscillations for the 10-nm SrRuO3 film show a high quantum mobility of 3500 cm2/Vs, a light cyclotron mass, and two-dimensional angular dependence, which can be attributed to the surface Fermi arcs. The linear thickness dependence of the phase shift of the quantum oscillations provides evidence for the non-trivial nature of the quantum oscillations mediated by the surface Fermi arcs. In addition, at low temperatures and under magnetic fields of up to 52 T, the quantum limit of SrRuO3 manifests the chiral anomaly of the Weyl nodes. Emergence of the hitherto hidden two-dimensional Weyl states in a ferromagnetic oxide pave the way to explore novel quantum transport phenomena for topological oxide electronics.

cond-mat.mtrl-sci↗

arXiv2021

Tunable vertical ferroelectricity and domain walls by interlayer sliding in $β$-ZrI$_{2}$

Vertical ferroelectricity where a net dipole moment appears as a result of in-plane ionic displacements has gained enormous attention following its discovery in transition metal dichalcogenides. Based on first-principles calculations, we report on the evidence of robust vertical ferroelectricity upon interlayer sliding in layered semiconducting $β$-ZrI$_{2}$, a sister material of polar semimetals MoTe$_{2}$ and WTe$_{2}$. The microscopic origin of ferroelectricity in ZrI$_{2}$ is attributed to asymmetric shifts of electronic charges within a trilayer, revealing a subtle interplay of rigid sliding displacements and charge redistribution down to ultrathin thicknesses. We further investigate the variety of ferroelectric domain boundaries and predict a stable charged domain wall with a quasi-two-dimensional electron gas and a high built-in electric field that can increase electron mobility and electromechanical response in multifunctional devices. Semiconducting behaviour and a small switching barrier of ZrI$_{2}$ hold promise for novel ferroelectric applications, and our results provide important insights for further development of slidetronics ferroelectricity.

cond-mat.mtrl-sci↗

arXiv2020

Quantum transport evidence of Weyl fermions in an epitaxial ferromagnetic oxide

Magnetic Weyl fermions, which occur in magnets, have novel transport phenomena related to pairs of Weyl nodes, and they are, of both, scientific and technological interest, with the potential for use in high-performance electronics, spintronics and quantum computing. Although magnetic Weyl fermions have been predicted to exist in various oxides, evidence for their existence in oxide materials remains elusive. SrRuO3, a 4d ferromagnetic metal often used as an epitaxial conducting layer in oxide heterostructures, provides a promising opportunity to seek for the existence of magnetic Weyl fermions. Advanced oxide thin film preparation techniques, driven by machine learning technologies, may allow access to such topological matter. Here we show direct quantum transport evidence of magnetic Weyl fermions in an epitaxial ferromagnetic oxide SrRuO3: unsaturated linear positive magnetoresistance (MR), chiral-anomaly-induced negative MR, Pi Berry phase accumulated along cyclotron orbits, light cyclotron masses and high quantum mobility of about 10000 cm2/Vs. We employed machine-learning-assisted molecular beam epitaxy (MBE) to synthesize SrRuO3 films whose quality is sufficiently high to probe their intrinsic quantum transport properties. We also clarified the disorder dependence of the transport of the magnetic Weyl fermions, and provided a brand-new diagram for the Weyl transport, which gives a clear guideline for accessing the topologically nontrivial transport phenomena. Our results establish SrRuO3 as a magnetic Weyl semimetal and topological oxide electronics as a new research field.

cond-mat.mtrl-sci↗

arXiv2019

Localised Magnetism in 2D Electrides

In this work, we investigate intrinsic magnetic properties of monolayer electrides LaBr$_{2}$ and La$_{2}$Br$_{5}$, where excess electrons do not reside at any atomic orbital and act as anions located at interstitial regions. Having demonstrated that conventional first-principles approaches are incapable of treating such non-atomic magnetic orbitals largely underestimating insulating band gaps, we construct effective electronic models in the basis of Wannier functions associated with the anionic states to unveil the microscopic mechanism underlying magnetism in these systems. Being confined at zero-dimensional cavities in the crystal structure, the anionic electrons will be shown to reveal an exotic duality of strong localisation like in $d$- and $f$-electron systems and large spatial extension inherent to delocalised atomic orbitals. While the former tends to stabilise a Mott-insulating state with localised magnetic moments, the latter results in direct exchange between neighbouring anionic electrons that dominates over antiferromagnetic superexchange interactions. On the basis of the derived spin models, we argue that any long-range magnetic order is prohibited in LaBr$_{2}$ by Mermin-Wagner theorem, while intersite anisotropy stabilises weakly coupled ferromagnetic chains along the monoclinic $\mathbf{b}$ axis in La$_{2}$Br$_{5}$. Our study shows that electride materials combining peculiar features of both localised and delocalised atomic states constitute a unique class of strongly correlated materials.

cond-mat.str-el↗

arXiv2018

Nanoskyrmion engineering with $sp$-electron materials: Sn monolayer on SiC(0001) surface

Materials with $sp$-magnetism demonstrate strongly nonlocal Coulomb interactions, which opens a way to probe correlations in the regimes not achievable in transition metal compounds. By the example of Sn monolayer on SiC(0001) surface, we show that such systems exhibit unusual but intriguing magnetic properties at the nanoscale. Physically, this is attributed to the presence of a significant ferromagnetic coupling, the so-called direct exchange, which fully compensates ubiquitous antiferromagnetic interactions of the superexchange origin. Having a nonlocal nature, the direct exchange was previously ignored because it cannot be captured within the conventional density functional methods and significantly challenges ground state models earlier proposed for Sn/SiC(0001). Furthermore, heavy adatoms induce strong spin-orbit coupling, which leads to a highly anisotropic form of the spin Hamiltonian, in which the Dzyaloshinskii-Moriya interaction is dominant. The latter is suggested to be responsible for the formation of a nanoskyrmion state at realistic magnetic fields and temperatures.

cond-mat.str-el↗

arXiv2017

Significant Dzyaloshinskii-Moriya Interaction at Graphene-Ferromagnet Interfaces due to Rashba-effect

The possibility of utilizing the rich spin-dependent properties of graphene has attracted great attention in pursuit of spintronics advances. The promise of high-speed and low-energy consumption devices motivates a search for layered structures that stabilize chiral spin textures such as topologically protected skyrmions. Here we demonstrate that chiral spin textures are induced at graphene/ferromagnetic metal interfaces. This is unexpected because graphene is a weak spin-orbit coupling material and is generally not expected to induce sufficient Dzyaloshinskii-Moriya interaction to affect magnetic chirality. We demonstrate that graphene induces a new type of Dzyaloshinskii-Moriya interaction due to a Rashba effect. First-principles calculations and experiments using spin-polarized electron microscopy show that this graphene-induced Dzyaloshinskii-Moriya interaction can have similar magnitude as at interfaces with heavy metals. This work paves a new path towards two-dimensional material based spin orbitronics.

cond-mat.mtrl-sci↗