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S. Mironov

Publications and source records attributed to S. Mironov.

At least 55 records · Page 3Linked to original sources
arXiv2017

Perturbations in generalized Galileon theories

We discuss the approaches by Deffayet et al. (DPSV) and Kobayashi et al. (KYY) to the analysis of linearized scalar perturbations about a spatially flat FLRW background in Horndeski theory. We identify additional, potentially important terms in the DPSV approach. However, these terms vanish upon a judicious gauge choice. We derive a gauge invariant quadratic action for metric and Galileon perturbations in $\mathcal{L}_3$ and $\mathcal{L}_3+\mathcal{L}_4$ theories and show that actions obtained in the DPSV and KYY approaches follow from this gauge invariant action in particular gauges.

hep-th↗
arXiv2017

Cosmological bounce and Genesis beyond Horndeski

We study a "classical" bouncing scenario in beyond Horndeski theory. We give an example of spatially flat bouncing solution that is non-singular and stable throughout the whole evolution. The model is arranged in such a way that the scalar field driving the cosmological evolution initially behaves like full-fledged beyond Horndeski, whereas at late times it becomes a massless scalar field minimally coupled to gravity.

hep-th↗
arXiv2017

Spin-valve effect in S/F/F and F/S/F structures of atomic thickness

We show that the peculiarities of the electron band structure strongly affect the spin-valve effect in heterostructures consisting of a superconductor (S) and two ferromagnetic layers F1 and F2. For the S/F1/F2 systems the energy shift between the electron bands in the S and F2 layers determines wether the critical temperature $T_c$ of the superconductor increases or decreases as a function of the angle $θ$ between the magnetic moments in the ferromagnets. In the case of half-metallic F2 layer the type of the spin-valve effect becomes dependent on the position of the only occupied spin-band and the minimum of $T_c$ may correspond to the anti-parallel, parallel or non-collinear orientations of magnetic moments. Also, for the first time, we analytically demonstrate the possibility of the triplet spin-valve effect in the F/S/half-metal structures.

cond-mat.supr-con↗
arXiv2017

Spontaneous currents in superconducting systems with strong spin-orbit coupling

We show that Rashba spin-orbit coupling at the interface between a superconductor and a ferromagnet should produce a spontaneous current in the atomic thickness region near the interface. This current is counter-balanced by the superconducting screening current flowing in the region of the width of the London penetration depth near the interface. Such current carrying state creates a magnetic field near the superconductor surface, generates a stray magnetic field outside the sample edges, changes the slope of the temperature dependence of the critical field $H_{c3}$ and may generate the spontaneous Abrikosov vortices near the interface.

cond-mat.supr-con↗
arXiv2017

Towards topological quantum computer

One of the principal obstacles on the way to quantum computers is the lack of distinguished basis in the space of unitary evolutions and thus the lack of the commonly accepted set of basic operations (universal gates). A natural choice, however, is at hand: it is provided by the quantum R-matrices, the entangling deformations of non-entangling (classical) permutations, distinguished from the points of view of group theory, integrable systems and modern theory of non-perturbative calculations in quantum field and string theory. Observables in this case are (square modules of) the knot polynomials, and their pronounced integrality properties could provide a key to error correction. We suggest to use R-matrices acting in the space of irreducible representations, which are unitary for the real-valued couplings in Chern-Simons theory, to build a topological version of quantum computing.

hep-th↗
arXiv2016

Generalized Galileons: instabilities of bouncing and Genesis cosmologies and modified Genesis

We study spatially flat bouncing cosmologies and models with the early-time Genesis epoch in a popular class of generalized Galileon theories. We ask whether there exist solutions of these types which are free of gradient and ghost instabilities. We find that irrespectively of the forms of the Lagrangian functions, the bouncing models either are plagued with these instabilities or have singularities. The same result holds for the original Genesis model and its variants in which the scale factor tends to a constant as $t\to -\infty$. The result remains valid in theories with additional matter that obeys the Null Energy Condition and interacts with the Galileon only gravitationally. We propose a modified Genesis model which evades our no-go argument and give an explicit example of healthy cosmology that connects the modified Genesis epoch with kination (the epoch still driven by the Galileon field, which is a conventional massless scalar field at that stage).

hep-th↗
arXiv2016

Magnetic mapping of defects in type-II superconductors

Recently it was discovered that the non-uniform Meissner current flowing around the pinning sites in the type-II superconductor induces the unconventional vortex-antivortex pairs with the non-quantized magnetic flux [J.-Y. Ge, et al., Nat. Commun. 6, 6573 (2015)]. Here we provide the theory of this phenomenon showing that the vortex-like structures originate from the perturbation of the current streamlines by the non-superconducting defect, which results in the generation of the localized magnetic field. The position and the shape of such vortex dipoles are shown to be very sensitive to the defect form. Thus, applying the external magnetic field or current to the superconductor and using, e.g., the high-resolution scanning Hall microscope to measure the stray magnetic field one can plot the map containing the information about the position of the defects and their shape.

cond-mat.supr-con↗
arXiv2015

Effective model for a short Josephson junction with a phase discontinuity

We consider a short Josephson junction with a phase discontinuity $κ$ created, e.g., by a pair of tiny current injectors, at some point $x_0$ along the length of the junction. We derive the effective current-phase relation (CPR) for the system as a whole, i.e., reduce it to an effective point-like junction. From the effective CPR we obtain the ground state of the system and predict the dependence of its critical current on $κ$. We show that in a large range of $κ$ values the effective junction behaves as a $φ_0$ Josephson junction, i.e., has a unique ground state phase $φ_0$ within each $2π$ interval. For $κ\approxπ$ and $x_0$ near the middle of the junction one obtains a $φ_0\pmφ$ junction, i.e., the Josephson junction with degenerate ground state phase $φ_0\pmφ$ within each $2π$ interval. Further, in view of possible escape experiments especially in the quantum domain, we investigate the scaling of the energy barrier and eigenfrequency close to the critical currents and predict the behavior of the escape histogram width $σ(κ)$ in the regime of the macroscopic quantum tunneling.

cond-mat.supr-con↗
arXiv2015

Triplet proximity effect in superconducting heterostructures with a half-metallic layer

We present the Usadel theory describing the superconducting proximity effect in heterostructures with a half-metallic layer. It is shown that the full spin polarization inside the half-metals gives rise to the giant triplet spin-valve effect in superconductor (S) - ferromagnet (F) - half-metal (HM) trilayers as well as to the $φ_0$-junction formation in the S/F/HM/F/S systems. In addition, we consider the exactly solvable model of the S/F/HM trilayers of atomic thickness and demonstrate that it reproduces the main features of the spin-valve effect found within the Usadel approach. Our results are shown to be in a qualitative agreement with the recent experimental data on the spin-valve effect in ${\rm MoGe/Cu/Ni/CrO_2}$ hybrids [A. Singh et al., Phys. Rev. X 5, 021019 (2015)].

cond-mat.supr-con↗
arXiv2014

Single Cycle Thin Film Compressor Opening the door to Zeptosecond-Exawatt Physics

This article demonstrates a new compression scheme that has the potential to compress a high energy pulse as high as a few hundred Joules in a pulse as short as one optical cycle at 0.8μm making a true ultra-relativistic λ^3 pulse. This pulse could have a focused intensity of 10^24W/cm2 or a0 of 1000. It could form an efficient, 10%, relativistic mirror that could compress the pulse to the atto-zeptosecond regime, with an upshifted wavelength of 1-10keV. This technique could be a watershed making the entry of petawatt pulses into the exawatt and zeptosecond regime possible.

physics.optics↗
arXiv2012

Vanishing Meissner effect as a hallmark of in-plane FFLO instability in superconductor - ferromagnet layered systems

We demonstrate that in a wide class of multilayered superconductor - ferromagnet structures (e.g., S/F, S/F/N and S/F/F') the vanishing Meissner effect signals the appearance of the in-plane Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) modulated superconducting phase. In contrast to the bulk superconductors the FFLO instability in these systems can emerge at temperatures close to the critical one and is effectively controlled by the S layer thickness and the angle between magnetization vectors in the F/F' bilayers. The predicted FFLO state reveals through the critical temperature oscillations vs the perpendicular magnetic field component.

cond-mat.supr-con↗
arXiv2011

Non-Gaussianity of scalar perturbations generated by conformal mechanisms

We consider theories which explain the flatness of the power spectrum of scalar perturbations in the Universe by conformal invariance, such as conformal rolling model and Galilean Genesis. We show that to the leading {\it non-linear} order, perturbations in all models from this class behave in one and the same way, at least if the energy density of the relevant fields is small compared to the total energy density (spectator approximation). We then turn to the intrinsic non-Gaussianities in these models (as opposed to non-Gaussianities that may be generated during subsequent evolution). The intrinsic bispectrum vanishes, so we perform the complete calculation of the trispectrum and compare it with the trispecta of local forms in various limits. The most peculiar feature of our trispectrum is a (fairly mild) singularity in the limit where two momenta are equal in absolute value and opposite in direction (folded limit). Generically, the intrinsic non-Gaussianity can be of detectable size.

astro-ph.CO↗
arXiv2011

Fluctuations in a mesoscopic superconducting ring: resonant behavior of conductivity and specific heat in two mode critical regime

The critical fluctuations in a mesoscopic superconducting ring are studied within the Ginzburg-Landau approach. The nonlocal conductivity as well as the specific heat are calculated as functions of the magnetic flux $Φ$ through the ring. At $Φ=Φ_0/2$ two low-energy eigenstates become degenerate and near this point the behavior of fluctuations-dependent quantities change dramatically: both the zero Fourier component $σ_0$ of the fluctuation conductivity and the specific heat become non-monotonic functions of $|Φ-Φ_0/2|$ with rather special resonant structure.

cond-mat.mes-hall↗
arXiv2010

Properties of scalar perturbations generated by conformal scalar field

Primordial scalar perturbations may be generated when complex conformal scalar field rolls down its negative quartic potential. We begin with the discussion of peculiar infrared properties of this scenario. We then consider the statistical anisotropy inherent in the model. Finally, we discuss the non-Gaussianity of scalar perturbations. Because of symmetries, the bispectrum vanishes identically. We present a general expression for the trispectrum and give its explicit form in the folded limit.

hep-th↗
arXiv2009

Ultracold-neutron infrastructure for the gravitational spectrometer GRANIT

The gravitational spectrometer GRANIT will be set up at the Institut Laue Langevin. It will profit from the high ultracold neutron density produced by a dedicated source. A monochromator made of crystals from graphite intercalated with potassium will provide a neutron beam with 0.89 nm incident on the source. The source employs superthermal conversion of cold neutrons in superfluid helium, in a vessel made from BeO ceramics with Be windows. A special extraction technique has been tested which feeds the spectrometer only with neutrons with a vertical velocity component v < 20 cm/s, thus keeping the density in the source high. This new source is expected to provide a density of up to 800 1/cm3 for the spectrometer.

nucl-ex↗
arXiv2008

Accumulation and extraction of ultracold neutrons from a superfluid helium converter coated with fluorinated grease

We report experiments on the production of ultracold neutrons (UCN) in a converter of superfluid helium coated with fluorinated grease. We employed our technique of window-free extraction of accumulated UCN from the helium, in which they were produced by downscattering neutrons of a cold beam from the Munich research reactor. The time constant for UCN passage through the same extraction aperture as in a previous experiment was a factor two shorter, despite a lower mean velocity of the accumulated UCN in the present experiments. A time-of-flight measurement of the cold neutron spectrum incident on the converter allowed us to estimate the multi-phonon contribution to the UCN production. The UCN production rate inferred from two methods agrees with the theoretical expectation.

nucl-ex↗