arXiv subjects

S. Mironov

Publications and source records attributed to S. Mironov.

At least 37 records · Page 2Linked to original sources

arXiv2022

Stable non-singular cosmologies in beyond Horndeski theory and disformal transformations

In this note we collect, systemise and generalise the existing results for relations between general Horndeski theories and beyond Horndeski theories via disformal transformations of metric. We derive additional disformal transformation rules relating Lagrangian functions of beyond Horndeski theory and corresponding Horndeski theory and demonstrate that some of them become singular at some moments(s) once one constructs a non-singular cosmological solution in beyond Horndeski theory that is free from ghost, gradient instabilities and strong gravity regime during the entire evolution of the system. The key issue here is that such solutions are banned in Horndeski theory due to existing no-go theorem. The proof of singular behaviour of disformal relations in this case resolves the apparent contradiction between the fact that Horndeski and beyond Horndeski theories appear related by field redefinition but describe different physics in the context of non-singular cosmologies.

hep-th↗

arXiv2021

Phase transitions in superconductor/ferromagnet bilayer driven by spontaneous supercurrents

We investigate superconducting phase transition in superconductor(S)/ferromagnet(F) bilayer with Rasba spin-orbit interaction at S/F interface. This spin-orbit coupling produces spontaneous supercurrents flowing inside the atomic-thickness region near the interface, which are compensated by the screening Meissner currents [S. Mironov and A. Buzdin, Phys. Rev. Lett \textbf{118}, 077001 (2017)]. In the case of thin superconducting film the emergence of the spontaneous surface currents causes the increase of the superconducting critical temperature and we calculate the actual value of the critical temperature shift. We also show that in the case of type-I superconducting film this phase transition can be of the first order. In the external magnetic field the critical temperature depends on the relative orientation of the external magnetic field and the exchange field in the ferromagnet. Also we predict the in-plane anisotropy of the critical current which may open an alternative way for the experimental observation of the spontaneous supercurrents generated by the SOC.

cond-mat.supr-con↗

arXiv2020

Superluminality in DHOST theory with extra scalar

We consider DHOST Ia theory interacting gravitationally with an additional conventional scalar field minimally coupled to gravity. At the linearized level of perturbations about cosmological background, we find that in the presence of a slowly rolling extra scalar field, one of the modes generically propagates at superluminal speed. This result is valid for any stable cosmological background. We identify a subclass of DHOST Ia theories in which this superluminality property is absent, and all modes may propagate (sub)luminally. We discuss possible implications for the interacting DHOST Ia theories.

hep-th↗

arXiv2020

Superluminality in beyond Horndeski theory with extra scalar field

We study the superluminality issue in beyond Horndeski theory with additional scalar field, which is minimally coupled to gravity and has no second derivatives in the Lagrangian. We present the quadratic action for perturbations in cosmological backgrounds, stability conditions and expressions for sound speeds. We find that in the case of conventional additional scalar whose flat-space propagation speed is that of light, one of the modes in interacting theory is necessarily superluminal when this scalar rolls, even arbitrarily slowly. This result holds in any theory of the beyond Horndeski class (with 6 arbitrary functions in the Lagrangian) and for any stable rolling background. More generally, the requirement of the absence of superluminality imposes non-trivial constraints on the structure of the theory.

hep-th↗

arXiv2020

Magnetic Flux Pumping in Superconducting Loop Containing a Josephson $ψ$ Junction

We demonstrate that a Josephson junction with a half-metallic weak link integrated into the superconducting loop enables the pumping of magnetic flux piercing the loop. In such junctions, the ground state phase $ψ$ is determined by the mutual orientation of magnetic moments in two ferromagnets surrounding the half-metal. Thus, the precession of magnetic moment in one of two ferromagnets controlled, e.g., by the microwave radiation, results in the accumulation of the phase $ψ$ and subsequent switching between the states with different vorticities. The proposed flux pumping mechanism does not require the application of voltage or external magnetic field which enables the design of electrically decoupled memory cells in superconducting spintronics.

cond-mat.supr-con↗

arXiv2020

Subluminal cosmological bounce beyond Horndeski

We address the issue of potential superluminal propagation of gravitational waves in backgrounds neighboring the previously suggested bounce [arXiv:1807.08361] in beyond Horndeski theory. We find that the bouncing solution lies right at the boundary of the region where the gravitational waves propagate at speed exceeding that of light, i.e. that solution suffers superluminality problem. We suggest a novel version of a completely stable bouncing model where both scalar and tensor perturbations remain safely subluminal not only on the solution itself but also in its neighbourhood. The model remains free of superluminality when extra matter in the form of radiation or, more generally, ideal fluid with equation of state parameter $w\leq 1/3$ (and also somewhat higher) is added. Superluminality reappears when extra matter is added whose sound velocity is equal or close to 1 in flat space; an example is scalar field minimally coupled to metric. The latter property is characteristic of all beyond Horndeski cosmologies; we briefly discuss its significance.

hep-th↗

arXiv2019

Genesis with general relativity asymptotics in beyond Horndeski theory

We suggest a novel version of a cosmological Genesis model within beyond Horndeski theory. It combines the initial Genesis behavior of Creminelli et al. [arXiv:1007.0027, arXiv:1209.3768] with complete stability property of the previous beyond Horndeski construction [arXiv:1705.06626]. The specific features of the model are that space-time rapidly tends to Minkowski in asymptotic past and that both asymptotic past and future are described by General Relativity (GR).

hep-th↗

arXiv2019

Crossover between standard and inverse spin-valve effect in atomically thin superconductor/half-metal structures

Spin-singlet Cooper pairs consisting of two electrons with opposite spins cannot directly penetrate from a superconductor to a half-metal (fully spin polarized ferromagnets) which blocks the superconducting proximity effect between these materials. In this paper we demonstrate that, nevertheless, two half-metallic layers electrically coupled to the superconducting film substantially affect its critical temperature and produce the spin valve effect. Within the tight-binding model for the atomically thin multilayered spin valves we show that depending on the details of the electron energy spectra in half-metals the critical temperature as a function of the angle between the spin quantization axes in half-metals can be either monotonically increasing or decreasing. This finding highlights the crucial role of the band structure details in the proximity effect with half-metals which cannot be adequately treated in the quasiclassical theories.

cond-mat.supr-con↗

arXiv2019

Cosmological scenarios with bounce and Genesis in Horndeski theory and beyond: An essay in honor of I.M. Khalatnikov on the occasion of his 100th birthday

This essay is a brief review of the recent studies of non-singular cosmological scenarios with bounce and Genesis and their stability in a subclass of scalar-tensor theories with higher derivatives -- beyond Horndeski theories. We discuss the general results of stability analysis of the non-singular cosmological solutions in beyond Horndeski theories, as well as other closely related topics: 1) the no-go theorem, which is valid in the general Horndeski theories but not in their extensions, 2) singularities in disformal transformations relating beyond Horndeski theories with general ones, 3) healthy behaviour of the scalar sector in the unitary gauge despite divergencies of coefficients in the quadratic action for perturbations ("$γ$-crossing"). We describe several specific examples of bouncing cosmologies and models with Genesis epoch which have neither ghosts nor gradient instabilities among the linearized perturbations about the homogeneous isotropic background during entire evolution.

hep-th↗

arXiv2019

More about stable wormholes in beyond Horndeski theory

It is known that Horndeski theories, like many other scalar-tensor gravities, do not support static, spherically symmetric wormholes: they always have either ghosts or gradient instabilities among parity-even linearized perturbations. Here we address the issue of whether or not this no-go theorem is valid in "beyond Horndeski" theories. We derive, in the latter class of theories, the conditions for the absence of ghost and gradient instabilities for non-spherical parity even perturbations propagating in the radial direction. We find, in agreement with existing arguments, that the proof of the above no-go theorem does not go through beyond Horndeski. We also obtain conditions ensuring the absence of ghosts and gradient instabilities for all parity odd modes. We give an example of beyond Horndeski Lagrangian which admits a wormhole solution obeying our (incomplete set of) stability conditions. Even though our stability analysis is incomplete, as we do not consider spherically symmetric parity even modes and parity even perturbations propagating in angular directions, as well as "slow" tachyonic instabilities, our findings indicate that beyond Horndeski theories may be viable candidates to support traversable wormholes.

hep-th↗

arXiv2019

In-situ creation and control of Josephson junctions with a laser beam

We propose the use of a laser beam tightly focused on a superconducting strip to create a Josephson junction by photothermal effect. The critical current of this junction can be easily controlled by the laser intensity. We show that a periodic modulation of the intensity substantially changes the dynamic properties of the junction and results in the appearance of Shapiro steps without microwave radiation. The experimental realization of optically driven Josephson junctions may open a way for the ultra-fast creation and switching of complex patterns of superconducting devices with tunable geometry and current-phase relations.

cond-mat.supr-con↗

arXiv2019

Theory of the magnetic domains phases in ferromagnetic superconductors

Recently discovered superconducting P-doped EuFe$_2$As$_2$ compounds reveal the situation when the superconducting critical temperature substantially exceeds the ferromagnetic transition temperature. The main mechanism of the interplay between magnetism and superconductivity occurs to be an electromagnetic one and a short period magnetic domain structure was observed just below Curie temperature [Stolyarov et al., Sci. Adv. \textbf{4}, eaat1061 (2018)]. We elaborate a theory of such transition and demonstrate how the initial sinusoidal magnetic structure gradually transforms into a soliton-like domain one. Further cooling may trigger a first-order transition from the short-period domain Meissner phase to the self-induced ferromagnetic vortex state and we calculate the parameters of this transition. The size of the domains in the vortex state is basically the same as in the normal ferromagnet, but with the domain walls which should generate the set of vortices perpendicular to the vortices in the domains.

cond-mat.supr-con↗

arXiv2018

Towards wormhole beyond Horndeski

We address the issue of whether a no-go theorem for static, spherically symmetric wormholes, proven in Horndeski theories, can be circumvented by going beyond Horndeski. We show that the ghost instabilities which are at the heart of the no-go theorem, can indeed be avoided. The wormhole solutions with the latter property are, however, strongly fine tuned, and hence it is likely that they are unstable. Furthermore, it remains unclear whether these solutions have other pathologies, like gradient instabilities along angular and radial directions.

hep-th↗

arXiv2018

Bounce beyond Horndeski with GR asymptotics and $γ$-crossing

It is known that beyond Horndeski theory admits healthy bouncing cosmological solutions. However, the constructions proposed so far do not reduce to General Relativity (GR) in either infinite past or infinite future or both. The obstacle is so called $γ$-crossing, which off hand appears pathological. By working in the unitary gauge, we confirm the recent observation by Ijjas [arXiv:1710.05990] that $γ$-crossing is, in fact, healthy. On this basis we construct a spatially flat, stable bouncing Universe solution whose asymptotic past and future are described by GR with conventional massless scalar field.

hep-th↗

arXiv2018

Properties of perturbations in beyond Horndeski theories

We study whether the approach of Deffayet et al. (DPSV) can be adopted for obtaining a derivative part of quadratic action for scalar perturbations in beyond Horndeski theories about homogeneous and isotropic backgrounds. We find that even though the method does remove the second and higher derivatives of metric perturbations from the linearized Galileon equation, in the same manner as in the general Horndeski theory, it gives incorrect result for the quadratic action. We analyse the reasons behind this property and suggest the way of modifying the approach, so that it gives valid results.

hep-th↗

arXiv2018

From Topological to Quantum Entanglement

Entanglement is a special feature of the quantum world that reflects the existence of subtle, often non-local, correlations between local degrees of freedom. In topological theories such non-local correlations can be given a very intuitive interpretation: quantum entanglement of subsystems means that there are "strings" connecting them. More generally, an entangled state, or similarly, the density matrix of a mixed state, can be represented by cobordisms of topological spaces. Using a formal mathematical definition of TQFT we construct basic examples of entangled states and compute their von Neumann entropy.

hep-th↗

arXiv2018

Electromagnetic proximity effect in planar superconductor-ferromagnet structures

The spread of the Cooper pairs into the ferromagnet in proximity coupled superconductor - ferromagnet (SF) structures is shown to cause a strong inverse electromagnetic phenomenon, namely, the long-range transfer of the magnetic field from the ferromagnet to the superconductor. Contrary to the previously investigated inverse proximity effect resulting from the spin polarization of superconducting surface layer, the characteristic length of the above inverse electrodynamic effect is of the order of the London penetration depth, which usually much larger than the superconducting coherence length. The corresponding spontaneous currents appear even in the absence of the stray field of the ferromagnet and are generated by the vector-potential of magnetization near the S/F interface and they should be taken into account at the design of the nanoscale S/F devices. Similarly to the well-known Aharonov-Bohm effect, the discussed phenomenon can be viewed as a manifestation of the role of vector potential in quantum physics.

cond-mat.supr-con↗

arXiv2018

Colored Alexander polynomials and KP hierarchy

We discuss the relation between knot polynomials and the KP hierarchy. Mainly, we study the scaling 1-hook property of the coloured Alexander polynomial: $\mathcal{A}^\mathcal{K}_R(q)=\mathcal{A}^\mathcal{K}_{[1]}(q^{\vert R\vert})$ for all 1-hook Young diagrams $R$. Via the Kontsevich construction, it is reformulated as a system of linear equations. It appears that the solutions of this system induce the KP equations in the Hirota form. The Alexander polynomial is a specialization of the HOMFLY polynomial, and it is a kind of a dual to the double scaling limit, which gives the special polynomial, in the sense that, while the special polynomials provide solutions to the KP hierarchy, the Alexander polynomials provide the equations of this hierarchy. This gives a new connection with integrable properties of knot polynomials and puts an interesting question about the way the KP hierarchy is encoded in the full HOMFLY polynomial.

hep-th↗