arXiv subjects

Filipp N. Rybakov

Publications and source records attributed to Filipp N. Rybakov.

28 records · Page 2Linked to original sources

arXiv2021

The absence of superconductivity in the next-to-leading order Ginzburg-Landau functional for Bardeen-Cooper-Schrieffer superconductor

Shortly after the Gor'kov microscopic derivation of the Ginzburg-Landau (GL) model via a small order parameter expansion in Bardeen-Cooper-Schrieffer theory of superconductivity, the derivation was carried to next-to-leading order in that parameter and its spatial derivatives. The aim was to obtain a generalized GL free energy that approximates the microscopic model better. Since 1960s, multiple works have claimed or implicitly assumed that this extended GL model corresponds to the free energy and has solutions in the form of local minima describing superconductivity, such as vortex solutions. In contrast to this, we prove that this extended GL functional does not represent free energy since it does not have any solutions in the form of minima. Accordingly, it cannot be used to describe superconducting states.

cond-mat.supr-con↗

arXiv2021

Geometry and Symmetry in Skyrmion Dynamics

The uniform motion of chiral magnetic skyrmions induced by a spin-transfer torque displays an intricate dependence on the skyrmions' topological charge and shape. We reveal surprising patterns in this dependence through simulations of the Landau-Lifshitz-Gilbert equation with Zhang-Li torque and explain them through a geometric analysis of Thiele's equation. In particular, we show that the velocity distribution of topologically non-trivial skyrmions depends on their symmetry: it is a single circle for skyrmions of high symmetry and a family of circles for low-symmetry configurations. We also show that the velocity of the topologically trivial skyrmions, previously believed to be the fastest objects, can be surpassed, for instance, by antiskyrmions. The generality of our approach suggests the validity of our results for exchange frustrated magnets, bubble materials, and others.

cond-mat.mes-hall↗

arXiv2021

Magnetic skyrmion braids

Filamentary textures can take the form of braided, rope-like superstructures in nonlinear media such as plasmas and superfluids. The formation of similar superstructures in solids has been predicted, for example from flux lines in superconductors. However, their observation has proved challenging. Here, we use electron microscopy and numerical methods to reveal braided superstructures of magnetic skyrmions in thin crystals of B20-type FeGe. Their discovery opens the door to applications of rich topological landscapes of geometric braids in magnetic solids.

cond-mat.str-el↗

arXiv2020

Antichiral Ferromagnetism

Here by combining a symmetry-based analysis with numerical computations we predict a new kind of magnetic ordering - antichiral ferromagnetism. The relationship between chiral and antichiral magnetic order is conceptually similar to the relationship between ferromagnetic and antiferromagnetic order. Without loss of generality, we focus our investigation on crystals with full tetrahedral symmetry where chiral interaction terms - Lifshitz invariants - are forbidden by symmetry. However, we demonstrate that leading chirality-related term leads to nontrivial smooth magnetic textures in the form of helix-like segments of alternating opposite chiralities. The unconventional order manifests itself beyond the ground state by stabilizing excitations such as domains and skyrmions in an antichiral form.

cond-mat.str-el↗

arXiv2020

Magnetic skyrmions, chiral kinks and holomorphic functions

We present a novel approach to understanding the extraordinary diversity of magnetic skyrmion solutions. Our approach combines a new classification scheme with efficient analytical and numerical methods. We introduce the concept of chiral kinks to account for regions of disfavoured chirality in spin textures, and classify two-dimensional magnetic skyrmions in terms of closed domain walls carrying such chiral kinks. In particular, we show that the topological charge of magnetic skyrmions can be expressed in terms of the constituent closed domain walls and chiral kinks. Guided by our classification scheme, we propose a method for creating hitherto unknown magnetic skyrmions which involves initial spin configurations formulated in terms of holomorphic functions and subsequent numerical energy minimization. We numerically study the stability of the resulting magnetic skyrmions for a range of external fields and anisotropy parameters, and provide quantitative estimates of the stability range for the whole variety of skyrmions with kinks. We show that the parameters limiting this range can be well described in terms of the relative energies of particular skyrmion solutions and isolated stripes with and without chiral kinks.

cond-mat.str-el↗

arXiv2020

Synthetic nuclear Skyrme matter in imbalanced Fermi superfluids with a multicomponent order parameter

Cooper-pair formation in a system of imbalanced fermions leads to the well-studied Fulde-Ferrell or Larkin-Ovchinnikov superfluid state. In the former case the system forms spontaneous phase gradients while in the latter case it forms a stripelike or a crystal-like density gradient. We show that in multicomponent imbalanced mixtures, the superfluid states can be very different from the Fulde-Ferrell-Larkin-Ovchinnikov states. The system generates gradients in both densities and phases by forming three-dimensional vortex-antivortex lattices or lattices of linked vortex loops. The solutions share some properties with the ostensibly unrelated Skyrme model of densely packed baryons and can be viewed as synthetic realization of nuclear Skyrme matter.

cond-mat.supr-con↗

arXiv2019

Variational principles of micromagnetics revisited

We revisit the basic variational formulation of the minimization problem associated with the micromagnetic energy, with an emphasis on the treatment of the stray field contribution to the energy, which is intrinsically non-local. Under minimal assumptions, we establish three distinct variational principles for the stray field energy: a minimax principle involving magnetic scalar potential and two minimization principles involving magnetic vector potential. We then apply our formulations to the dimension reduction problem for thin ferromagnetic shells of arbitrary shapes.

math.AP↗

arXiv2019

Spin-Orbit Protection of Induced Superconductivity in Majorana Nanowires

Spin-orbit interaction (SOI) plays a key role in creating Majorana zero modes in semiconductor nanowires proximity coupled to a superconductor. We track the evolution of the induced superconducting gap in InSb nanowires coupled to a NbTiN superconductor in a large range of magnetic field strengths and orientations. Based on realistic simulations of our devices, we reveal SOI with a strength of 0.15-0.35 eV$\require{mediawiki-texvc}Å$. Our approach identifies the direction of the spin-orbit field, which is strongly affected by the superconductor geometry and electrostatic gates.

cond-mat.mes-hall↗

arXiv2018

Kelvin knots in superconducting state

The failed "vortex-atoms" theory of matter by Kelvin and Tait had a profound impact on mathematics and physics. Building on the understanding of vorticity by Helmholtz, and observing stability of smoke rings, they hypothesised that elementary particles (at that time atoms) are indestructible knotted vortices in luminiferous aether: the hypothetical ideal fluid filling the universe. The vortex-atoms theory identified chemical elements as topologically different vortex knots, and matter was interpreted as bound states of these knotted vortices. This work initiated the field of knot theory in mathematics. It also influenced modern physics, where a close although incomplete analogy exists with the theory of superfluidity, which started with Onsager's and Feynman's introduction of quantum vortices. Indeed many macroscopic properties of superconductors and superfluids are indeed determined by vortex lines forming different "aggregate states", such as vortex crystals and liquids. While crucial importance of knots was understood for many physical systems in the recent years, there is no known physical realization of the central element of Kelvin theory: the stable particle-like vortex knot. Indeed, vortex loops and knots in superfluids and ordinary superconductors form as dynamical excitations and are unstable by Derrick theorem. This instability in fact dictates many of the universal macroscopic properties of superfluids. Here we show that there are superconducting states with principally different properties of the vorticity: where vortex knots are intrinsically stable. We demonstrate that such features should be realised near certain critical points, where the hydro-magneto-statics of superconducting states yields stables vortex knots which behave similar to those envisaged in Kelvin and Tait's theory of vortex-atoms in luminiferous aether.

cond-mat.supr-con↗

arXiv2017

Experimental observation of magnetic bobbers for a new concept of magnetic solid-state memory

The use of chiral skyrmions, which are nanoscale vortex-like spin textures, as movable data bit carriers forms the basis of a recently proposed concept for magnetic solid-state memory. In this concept, skyrmions are considered to be unique localized spin textures, which are used to encode data through the quantization of different distances between identical skyrmions on a guiding nanostripe. However, the conservation of distances between highly mobile and interacting skyrmions is difficult to implement in practice. Here, we report the direct observation of another type of theoretically-predicted localized magnetic state, which is referred to as a chiral bobber (ChB), using quantitative off-axis electron holography. We show that ChBs can coexist together with skyrmions. Our results suggest a novel approach for data encoding, whereby a stream of binary data representing a sequence of ones and zeros can be encoded via a sequence of skyrmions and bobbers. The need to maintain defined distances between data bit carriers is then not required. The proposed concept of data encoding promises to expedite the realization of a new generation of magnetic solid-state memory.

cond-mat.str-el↗