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Igor Bogush

Publications and source records attributed to Igor Bogush.

At least 19 recordsLinked to original sources
arXiv2026

Geometry-induced current decrowding in superconducting 3D constrictions

Current crowding is a ubiquitous limitation in nanoscale devices, where confinement and sharp features distort current flow, generating localized current-density hotspots and premature failure. In superconductors, this effect suppresses the order parameter and promotes vortex nucleation, reducing the critical current below the intrinsic depairing limit. Here, we demonstrate that 3D shaping of superconducting nanoarchitectures overcomes geometric current crowding through spatial current redistribution, establishing a geometry-induced decrowding effect. Using finite-element modeling based on the time-dependent Ginzburg--Landau equation, we reveal that 3D constrictions exhibit a pronounced and tunable response to moderate in-plane magnetic fields---a functionality absent in planar geometries. This field-controlled geometry enables critical-current modulation and unveils new vortex-dynamics regimes, including a non-reciprocal critical current. Furthermore, curvature and finite thickness fundamentally alter vortex nucleation by enabling apex-mediated entry of single vortex lines, followed by their 3D splitting into two vortex filaments---a mechanism that does not occur in 2D manifolds. Our findings demonstrate 3D geometric engineering as a design paradigm for superconducting nanoarchitectures, offering control over current distribution and vortex dynamics in devices approaching the depairing limit.

cond-mat.supr-con↗
arXiv2025

Vortex frequency locking and Shapiro steps in superconductor open nanotubes

The movement of magnetic flux quanta (Abrikosov vortices) in superconductors leads to dissipation and is influenced by various ordering effects arising from vortex-vortex, vortex-defect, and vortex-edge interactions. Under combined dc and ac stimuli, when the distance traveled by fluxons during an ac cycle corresponds to an integer multiple of the vortex lattice period, the superconductor's current-voltage (I-V) curve displays synchronization (Shapiro) steps. However, in planar constrictions, frequency-locking effects rely on a perfectly ordered vortex lattice and are typically observed when periodic vortex pinning arrays dominate over intrinsic uncorrelated disorder. Here, we propose 3D superconducting open nanotubes as systems free of periodic disorder, where the I-V curves are expected to display pronounced Shapiro steps. Using the time-dependent Ginzburg-Landau equation, we attribute the predicted effect to a reduction in the dimensionality of vortex motion. Namely, rolling a planar film into a tube causes the 2D vortex array, which initially moves throughout the film, to evolve into quasi-1D vortex chains that are restricted to areas where the normal component of the magnetic field is near its maximum. The discussed effects are relevant for superconducting devices, where vortex nucleation frequency and voltage stabilization by an external ac stimulus can enhance their operation.

cond-mat.supr-con↗
arXiv2024

Conformal approach to physics simulations for thin curved 3D membranes

Three-dimensional nanoarchitectures are widely used across various areas of physics, including spintronics, photonics, and superconductivity. In this regard, thin curved 3D membranes are especially interesting for applications in nano- and optoelectronics, sensorics, and information processing, making physics simulations in complex 3D geometries indispensable for unveiling new physical phenomena and the development of devices. Here, we present a general-purpose approach to physics simulations for thin curved 3D membranes, that allows for performing simulations using finite difference methods instead of meshless methods or methods with irregular meshes. The approach utilizes a numerical conformal mapping of the initial surface to a flat domain and is based on the uniformization theorem stating that any simply-connected Riemann surface is conformally equivalent to an open unit disk, a complex plane, or a Riemann sphere. We reveal that for many physical problems involving the Laplace operator and divergence, a flat-domain formulation of the initial problem only requires a modification of the equations of motion and the boundary conditions by including a conformal factor and the mean/Gaussian curvatures. We demonstrate the method's capabilities for case studies of the Schr\"{o}dinger equation for a charged particle in static electric and magnetic fields for 3D geometries, including C-shaped and ring-shaped structures, as well as for the time-dependent Ginzburg-Landau equation.

cond-mat.supr-con↗
arXiv2024

Vortex ratchet effect in superconductor open nanotubes and nanopetals

Advancements in the fabrication of superconducting 3D nanostructures and the creation of artificial pinning sites pave the way to novel applications and enhancement of nanosensors, bolometers, and quantum interferometers. The dynamics of magnetic flux quanta (Abrikosov vortices) in 3D nanoarchitectures reveal a rich palette of phenomena unseen in planar counterparts. Here, we consider two types of superconductor 3D nanostructures -- open nanotubes and nanopetals -- carrying an azimuthal transport current in a homogeneous external magnetic field. The complex 3D geometry of the structures induces an inhomogeneity of the normal magnetic field and makes the vortices move along preferred paths. By introducing a series of asymmetric pinning sites along these paths, we demonstrate non-reciprocity in the flux transport, which, in the 3D nanostructures, is stronger than in the planar membranes. The enhancement of the vortex ratchet effect manifests via a difference in the vortex depinning current under current reversal in a wider range of magnetic fields. The revealed effect is attributed to the inhomogeneous field-induced vortex channeling through the areas containing the asymmetric pinning sites. Our results demonstrate that the ratchet effect can persist up to higher magnetic fields via extending a superconducting film into the third dimension, without an increase in the number of asymmetric pinning sites.

cond-mat.supr-con↗
arXiv2024

Uniqueness of the static vacuum asymptotically flat spacetimes with massive particle spheres

In this paper, we establish that a four-dimensional static vacuum asymptotically flat spacetime containing a massive particle sphere is isometric to the Schwarzschild spacetime. Our results expand upon existing uniqueness theorems for static vacuum asymptotically flat spacetimes, which focus on scenarios featuring event horizons or photon spheres. Similarly to the uniqueness theorems concerning photon spheres or event horizons, only a single massive particle sphere is sufficient to obtain a unique solution. However, in contrast to previous theorems, our result leads to the existence of an entire spacetime foliation sliced by a set of massive particle spheres spanning various energies.

gr-qc↗
arXiv2024

Mass formulas for supergravity black holes with string singularities

We extend the derivation of mass formulas for stationary axisymmetric asymptotically locally flat solutions with string singularities on the polar axis to general supergravity actions containing vector and scalar fields. It is based on the rod structure of the solutions in Weyl coordinates and is applicable to black holes with Dirac and Misner strings. The obtained formulas differ from the corresponding ones in Einstein-Maxwell theory only by summation over all independent electric charges.

gr-qc↗
arXiv2024

Steering of vortices by magnetic-field tilting in superconductor nanotubes

In planar superconductor thin films, the places of nucleation and arrangements of moving vortices are determined by structural defects. However, various applications of superconductors require reconfigurable steering of fluxons, which is hard to realize with geometrically predefined vortex pinning landscapes. Here, on the basis of the time-dependent Ginzburg-Landau equation, we present an approach for steering of vortex chains and vortex jets in superconductor nanotubes containing a slit. The idea is based on tilting of the magnetic field $\mathbf{B}$ at an angle $\alpha$ in the plane perpendicular to the axis of a nanotube carrying an azimuthal transport current. Namely, while at $\alpha=0^\circ$ vortices move paraxially in opposite directions within each half-tube, an increase of $\alpha$ displaces the areas with the close-to-maximum normal component $|B_\mathrm{n}|$ to the close(opposite)-to-slit regions, giving rise to descending (ascending) branches in the induced-voltage frequency spectrum $f_\mathrm{U}(\alpha)$. At lower $B$, upon reaching the critical angle $\alpha_\mathrm{c}$, close-to-slit vortex chains disappear, yielding $f_\mathrm{U}$ of the $nf_1$-type ($n\geq1$: an integer; $f_1$: vortex nucleation frequency). At higher $B$, $f_\mathrm{U}$ is largely blurry because of multifurcations of vortex trajectories, leading to the coexistence of a vortex jet with two vortex chains at $\alpha=90^\circ$. In addition to prospects for tuning of GHz-frequency spectra and steering of vortices as information bits, our findings lay foundations for on-demand tuning of vortex arrangements in 3D superconductor membranes in tilted magnetic fields.

cond-mat.supr-con↗
arXiv2023

Black hole shadows of massive particles and photons in plasma

Explicitly covariant analytical expressions are derived that describe the boundaries of shadows cast by massive particles scattered by a gravitating object. This covers scenarios with particles having effectively variable mass, such as photons in plasma, geodesics in higher dimensions, and particles interacting with a scalar field. The derived formula takes advantage of recent advances in understanding the relationship between slice-reducible Killing tensors and massive particle surfaces that generalize photon surfaces. The formula allows us to obtain simple approximations of scaling as the particle energy changes. We illustrate this structure using Kerr-NUT and EMD black holes for both massive particles and photons in plasma. The versatility of this framework extends beyond astrophysics and has potential applications in analog models of gravity and condensed matter physics.

gr-qc↗
arXiv2023

Microwave generation and vortex jets in superconductor nanotubes

The dynamics of magnetic flux quanta (Abrikosov vortices) determine the resistive response of superconductors. In pinning-free planar thin films, the penetration and motion of vortices are controlled by edge defects, leading to such arrangements as vortex chains, vortex jets, and phase-slip regimes. Here, relying upon the time-dependent Ginzburg-Landau equation, we predict that these vortex patterns should appear in superconductor open nanotubes even without edge defects, due to the inhomogeneity of the normal magnetic induction component $B_\mathrm{n}$, caused by the 3D tube geometry. The crossing of the half-tubes by dc-driven vortices induces GHz-frequency voltage $U$ oscillations with spectra $U_\mathrm{f}(B)$ evolving between $nf_1$ and $\frac{n}{m}f_1$ [$f_1$: vortex nucleation frequency; $n,m\geq 2$] and blurred in certain ranges of currents and fields. An $nf_1$-spectrum corresponds to a single vortex-chain regime typical for low $B$ and for tubes of small radii. At higher fields, an $\frac{n}{m}f_1$-spectrum points to the presence of $m$ vortex chains in the vortex jets which, in contrast to planar thin films, are not diverging because of constraint to the tube areas where $B_\mathrm{n}$ is close to maximum. A blurry spectrum implies complex arrangements of vortices because of multifurcations of their trajectories. Finally, due to a stronger confinement of single vortex chains in tubes of small radii, we reveal peaks in $dU/dB$ and jumps in the frequency of microwave generation, which occur when the number of fluxons moving in the half-tubes increases by one. In all, our findings are essential for novel 3D superconductor devices which can operate in few- and multi-fluxon regimes.

cond-mat.supr-con↗
arXiv2023

Glued massive particles surfaces

A novel generalization of photon surfaces to the case of massive charged particles is given for spacetimes with at least one isometry, including stationary ones. A related notion of glued massive particle surfaces is also defined. These surfaces join worldlines parametrized by a family of independent conserved quantities and naturally arise in integrable spacetimes. We describe the basic geometric properties of such surfaces and their relationship to slice-reducible Killing tensors, illustrating all concepts with a number of examples. Massive particle surfaces have potential applications in the context of uniqueness theorems, Penrose inequalities, integrability, and the description of black-hole shadows in streams of massive charged particles or photons in a medium with an effective mass and charge.

gr-qc↗
arXiv2022

Supergravity $p$-branes with scalar charge

Standard dilatonic supergravity $p$-branes have scalar charges that are not independent parameters, but are determined by the brane tension and Page charges. This feature can be traced to the no-hair theorem in the four-dimensional Einstein-scalar gravity, implying that more general solutions with independent scalar charges can have naked singularities. Since singular branes are also of interest as tentative classical counterparts of unstable tachyonic branes and/or brane-antibrane systems, it is worth investigating branes with independent scalar charges in more detail. Here we study singular branes associated with the Fisher-Janis-Newman-Winicour solution of four-dimensional gravity. In the case of codimension three, we also construct singular branes endowed with a Zipoy-Voorhees-type oblateness parameter. It is expected that such branes will not be supersymmetric in the string theory. We demonstrate this in the special case of NS5-branes of type II theory. We analyze geodesics and test scalar perturbations of new solutions focusing on possible quantum healing of classical singularities.

gr-qc↗
arXiv2022

The geometry of massive particle surfaces

We propose a generalization of Claudel, Virbhadra, and Ellis photon surfaces to the case of massive charged particles, considering a timelike hypersurface such that any worldline of a particle with mass $m$, electric charge $q$ and fixed total energy $\mathcal{E}$, initially touching it, will remain in this hypersurface forever. This definition does not directly appeal to the equations of motion, but instead make use of partially umbilic nature of the surface geometry. Such an approach should be especially useful in the case of non-integrable equations of motion. It may be applied in the theory of non-thin accretion discs, and also may serve a new tool for some general problems, such as uniqueness theorems, Penrose inequalities and hidden symmetries. The condition for the stability of the worldlines is derived, which reduces to differentiation along the flow of surfaces of a certain energy. We consider a number of examples of electrovacuum and dilaton solutions, find conditions for marginally stable orbits, regions of stable or unstable spherical orbits, stable and unstable photon surfaces, and solutions satisfying the no-force condition.

gr-qc↗
arXiv2022

Photon surfaces, shadows and accretion disks in gravity with minimally coupled scalar field

In this article, we conduct a sequential study of possible observable images of black hole simulators described by two recently obtained rotating geometries in Einstein gravity, minimally coupled to a scalar field. One of them, "Kerr-like" (KL), can be seen as a legitimate alternative to the rotating Fisher-Janis-Newman-Winicour (FJNW) solution, and the other (TSL) is a scalar generalization of the Tomimatsu-Sato solution. Unlike the previous version of the rotating FJNW, these solutions do indeed satisfy the system's equations of motion. Our study includes both analytical and numerical calculations of equatorial circular orbits, photon regions, gravitational shadows, and radiation from thin accretion disks for various values of the object's angular momentum and scalar charge. The TSL solution was found to simulate Kerr for all valid parameter values with high accuracy. The maximum difference between the deviations of shadows from a circle for the Kerr and TSL cases does not exceed 1% and fits into the experimental observational data M87*. However, near-extreme objects show two times smaller peak values of the observed outflow luminosity of the accretion disk than for the Kerr black hole. The KL solution cannot be ruled out by the experimental data for small values of the scalar charge either. As the scalar charge increases, the optical properties change dramatically. The shadow can become multiply connected, strongly oblate, and the photon region does not hide the singularity, so it should be classified as a strong singularity.

gr-qc↗
arXiv2022

Topological defects in superconducting open nanotubes under gradual and abrupt switch-on of the transport current and magnetic field

We analyze the dynamics of the order parameter in superconducting open nanotubes under a strong transport current in an external homogeneous magnetic field using the time-dependent Ginzburg-Landau equation. Near the critical transport current, the dissipation processes are driven by vortex and phase slip dynamics. The transition between the vortex and phase-slip regimes is found to depend on the external magnetic field only weakly if the magnetic field and/or the transport current are switched on gradually. In the case of an abrupt switch-on of the magnetic field or transport current, the system can be triggered to the stable phase-slip regime, within a certain window of parameters. Finally, a hysteresis effect in the current-voltage characteristics is predicted in superconducting open nanotubes.

cond-mat.supr-con↗
arXiv2022

Slice-reducible conformal Killing tensors, photon surfaces and shadows

We generalize our recent method for constructing Killing tensors of the second rank to conformal Killing tensors. The method is intended for foliated spacetimes of arbitrary dimension $m$, which have a set of conformal Killing vectors. It applies to foliations of a more general structure than in previous literature. The basic idea is to start with reducible Killing tensors in slices constructed from a set of conformal Killing vectors and the induced metric, and then lift them to the whole manifold. Integrability conditions are derived that ensure this, and a constructive lifting procedure is presented. The resulting conformal Killing tensor may be irreducible. It is shown that subdomains of foliation slices suitable for the method are fundamental photon surfaces if some additional photon region inequality is satisfied. Thus our procedure also opens the way to obtain a simple general analytical expression for the boundary of the gravitational shadow. We apply this technique to electrovacuum, and ${\cal N}=2,\,4,\,8$ supergravity black holes, providing a new easy way to establish the existence of exact and conformal Killing tensors.

gr-qc↗
arXiv2021

Einstein-Maxwell-Dilaton-Axion mass formulas for black holes with struts and strings

Mass formulas are obtained for stationary axisymmetric solutions of the Einstein-Maxwell dilaton-axion theory, which have a regular rod structure on the axis of symmetry. Asymptotic mass, angular momentum and charge are expressed as the sums of masses, angular momenta and charges of rods dressed with field contributions. The calculation is based on a three-dimensional sigma model representation of the stationary EMDA system and the Tomimatsu approach proposed for the Einstein-Maxwell system. Our results provide an alternative interpretation of mass formulas and thermodynamics for black holes with Dirac and Misner strings. It is also applicable to aligned multiple black holes with struts.

gr-qc↗
arXiv2021

Killing tensors in foliated spacetimes and photon surfaces

We discuss a recently proposed geometric method for constructing a nontrivial Killing tensor of rank two in a foliated spacetime of codimension one that lifts trivial Killing tensors from slices to the entire manifold. The existence of nontrivial Killing tensor is closely related to generalized photon surfaces. The method is illustrated in some known cases and used to construct the hitherto unknown Killing tensor for the Nutty dyon in dilaton-axion gravity.

gr-qc↗