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A. Buzdin

Publications and source records attributed to A. Buzdin.

At least 19 recordsLinked to original sources
arXiv2024

Magnetoelectric effect in the helical state of a superconductor/ferromagnet bilayer

We study the microscopic mechanism of nucleation of the helical superconducting state maintained by a spin-splitting field and weak Rashba spin-orbit coupling in a 2D superconductor/ferromagnet system and present an exact solution for the Gor'kov equations, which provides a full description of the thermodynamic properties of the system under consideration. This permits us to obtain the Ginzburg-Landau expansion and use it to analyze the possibility of controlling magnetization using the transport supercurrent in both dc and quasistatic ac regimes. We show that these properties are related to the manifestation of the diode effect in superconducting systems with spin-orbit interaction.

cond-mat.supr-con↗
arXiv2023

Delocalization of vortex magnetic field near a planar defect

In this letter we discuss physical mechanisms for delocalization of the magnetic field of Abrikosov vortex in superconductors in the presence of planar defects of various electronic transparency. The resulting strong perturbation of the supercurrent can significantly affect the local flux measurements and mimic the vortices that carried only part of a flux quantum.

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arXiv2020

Non uniform superconductivity in wires with strong spin-orbit coupling

We study theoretically the onset of nonuniform superconductivity in a one-dimensional single wire in presence of Zeeman (or exchange field) and spin-orbit coupling. Using the Green's function formalism, we show that the spin-orbit coupling stabilizes modulated superconductivity in a broad range of temperatures and Zeeman fields. We investigate the anisotropy of the temperature-Zeeman field phase diagram, which is related to the orientation of the Zeeman field. In particular, the inhomogeneous superconducting state disappears if this latter field is aligned or perpendicular to the wire direction. We identify two regimes corresponding to weak and strong spin-orbit coupling respectively. The wave-vector of the modulated phase is evaluated in both regimes. The results also pertain for quasi-1D superconductors made of weakly coupled 1D chains.

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arXiv2020

Interplay between superconductivity and spin-dependent fields in nanowire-based systems

The interplay between superconductivity, spin-orbit coupling, and Zeeman or exchange field, is studied theoretically in two different setups: a single wire in which all these fields coexist, and a double wire system in which superconducting pairing and the spin-dependent fields are spatially separated. We first explore a magnetoelectric effect, namely the appearance of anomalous charge supercurrents. We determine the conditions under which such currents are allowed by symmetry and express them in terms of the SU(2) electric and magnetic fields. In leading order in the strength of the fields we find that in the single wire setup such currents may appear only when the Zeeman field has both, a longitudinal and transverse component with respect to the spin-orbit field. In contrast, in the two wire setup a parallel component to the SOC can generate the anomalous current, which is allowed by symmetry. We confirm these findings by calculating explicitly the current in both setups together with the self-consistent superconducting order parameter. The latter shows in the ground-state a spatial modulation of the phase that leads to currents that compensate the anomalous current, such that in both cases the ground state corresponds to a total zero-current state. However, in the two wire setup this zero-current state consists of two finite currents flowing in each of the wires in opposite direction.

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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.

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arXiv2019

Superconductivity driven helical magnetic structure in EuRbFe$_4$As$_4$ ferromagnetic superconductor

Recently the evidence of the helical magnetic structure modulated along $c$-axis with the period of four lattice parameters was obtained in easy $ab$ plane ferromagnetic superconductor EuRbFe$_4$As$_4$ [K. Iida et al., Phys. Rev. B 100, 014506 (2019)]. We argue that such structure may appear due to the presence of superconductivity. In spite of the very small value of the exchange field acting on the superconducting electrons in EuRbFe$_4$As$_4$, the exchange mechanism of interaction between superconductivity and ferromagnetism could dominate over the electromagnetic one and this circumstance could favor the emergence of the short-period magnetic structure (with the period less than the superconducting coherence length). Such a situation differs from one in the similar compound P-doped EuFe$_2$As$_2$, where the electromagnetic mechanism dominates and results in the magnetic structure with significantly larger period (of the order of London penetration depth). We also analyze the effect of the external magnetic field on the onset temperature of the modulated magnetic structure.

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arXiv2019

Electromagnetic proximity effect controlled by spin-triplet correlations in superconducting spin-valve structures

The spin-triplet correlations in superconducting spin valve structures arising in the presence of noncollinear textures of magnetic moment are shown to enhance strongly the electromagnetic proximity effect, i. e. the long-range leakage of the magnetic field from the ferromagnet (F) to the superconducting (S) layer. Both the dirty and clean limits are studied on the basis of the Usadel and Eilenberger theory, correspondingly. Our results suggest a natural explanation for the puzzling enhancement of the spontaneous magnetic fields induced by the noncollinear magnetic structures observed by the muon spin rotation techniques in a wide class of layered S/F systems. We show that the electromagnetic proximity effect causes the shift of the Fraunhofer dependence of the critical current on the external magnetic field in the Josephson junction with one superconducting electrode covered by the ferromagnetic layer. This provides an alternative way to measure both the magnitude and the direction of the spontaneous magnetic field induced in the superconductor. We also demonstrate the possibility of the long ranged superconductivity control of the magnetic state in F$_1$/S/F$_2$ structures.

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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.

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arXiv2018

Generation of a superconducting vortex via Néel skyrmions

We consider a type-II superconducting thin film in contact with a Néel skyrmion. The skyrmion induces spontaneous currents in the superconducting layer, which under the right condition generate a superconducting vortex in the absence of an external magnetic field. We compute the magnetic field and current distributions in the superconducting layer in the presence of Néel skyrmion.

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arXiv2018

Temperature controlled FFLO instability in superconductor-ferromagnet hybrids

We show that a wide class of layered superconductor-ferromagnet (S/F) hybrids demonstrate the emergence of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase well below the superconducting transition temperature. Decreasing the temperature one can switch the system from uniform to the FFLO state which is accompanied by the damping of the diamagnetic Meissner response down to zero and also by the sign change in the curvature of the current-velocity dependence. Our estimates show that an additional layer of the normal metal (N) covering the ferromagnet substantially soften the conditions required for the predicted FFLO instability and for existing S/F/N systems the temperature of the transition into the FFLO phase can reach several kelvins.

cond-mat.supr-con↗
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

Re-orientation of easy axis in $φ_0$ junction

We study theoretically a dynamics of $φ_0$ junction with direct coupling between magnetic moment and Josephson current which shows features close to Kapitza pendulum. We have found that starting with oscillations along $z$-axis, the character of magnetization dynamics changes crucially and stable position of magnetic moment $\vec m$ is realized between $z-$ and $y$-axes depending on parameters of the system. Changes in critical current and spin-orbit interaction lead to the different stability regions for magnetization. An excellent agreement between analytical and numerical results is obtained for low values of the Josephson to magnetic energy ratio.

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arXiv2017

Magnetization reversal by superconducting current in $φ_0$ Josephson junctions

We study magnetization reversal in a $φ_0$ Josephson junction with direct coupling between magnetic moment and Josephson current. Our simulations of magnetic moment dynamics show that by applying an electric current pulse, we can realize the full magnetization reversal. We propose different protocols of full magnetization reversal based on the variation of the Josephson junction and pulse parameters, particularly, electric current pulse amplitude, damping of magnetization and spin-orbit interaction. We discuss experiments which can probe the magnetization reversal in $φ_0$-junctions.

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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.

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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)].

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