arXiv subjects

A. Buzdin

Publications and source records attributed to A. Buzdin.

At least 37 records · Page 2Linked to original sources

arXiv2014

Standard, inverse and triplet spin-valve effects in F1/S/F2 systems

We demonstrate that contrary to the common belief the critical temperature Tc of clean F1/S/F2 spin valves can depend non-monotonically on the angle between the magnetic moments of the ferromagnetic F1 and F2 layers. Depending on the system parameters the minimum of Tc may correspond to parallel, antiparallel or non-collinear mutual orientation of magnetic moments. Such anomalous behavior can reveal itself only provided the ferromagnetic layers differ from each other and it completely disappears in the dirty limit.

cond-mat.supr-con↗

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↗

arXiv2012

On the theory of the vortex state in the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase

We demonstrate that the vortex state in the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase may be very different depending on the field orientation relative to the crystalline axes. We calculate numerically the upper critical field near the tricritical point taking into account the modulation of the order parameter along the magnetic field as well as the higher Landau levels. For s-wave superconductors with the anisotropy described by an elliptical Fermi surface we propose a general scheme of the analysis of the angular dependence of upper critical field at all temperatures on the basis of the exact solution for the order parameter. Our results show that the transitions (with tilting magnetic field) between different types of mixed states may be a salient feature of the FFLO phase. Moreover we discuss the reasons for the first-order phase transition into the FFLO state in the case of CeCoIn5 compound.

cond-mat.supr-con↗

arXiv2012

Penetration of the magnetic field into the twinning plane in the type I and II superconductors

It is demonstrated that in the type I and II superconductors with low-transparent twinning planes (TP) the penetration of external parallel magnetic field into the region of the twinning plane can be energetically favorable. In the type I superconductors the twinning planes become similar to Josephson junctions and the magnetic field penetrates into the center of the TP in the form of soft Josephson-like vortices. This leads to increase in the critical magnetic field values. The corresponding phase diagram in the parameter plane "temperature - magnetic field" essentially differs from the one obtained without taking the finite value of the magnetic field near the TP into account. Comparison between obtained phase diagrams and experimental data for different type I superconductors can allow to estimate the value of the TP transparency, which is the only fitting parameter in our theory.

cond-mat.supr-con↗

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

Long ranged singlet proximity effect in ferromagnetic nanowires

Recently a long ranged superconductor/ferromagnet (S/F) proximity effect has been reported in Co crystalline nanowires [1, Nature, 6 389 (2010)]. Since the authors of [1] take care to avoid the existence of magnetic domains, the triplet character of the long ranged proximity effect is improbable. Here we demonstrate that in the one-dimensional ballistic regime the standard singlet S/F proximity effect becomes long ranged. We provide an exact solution for the decay of the superconducting correlations near critical temperature ($T_{c}$) and for arbitrary impurities concentration. In particular we find a specific regime, between the diffusive and ballistic ones, where the decay length is simply the electronic mean-free path. Finally possible experiments which could permit to elucidate the nature of the observed long ranged proximity effect in Co nanowires are discussed.

cond-mat.mes-hall↗

arXiv2009

Different types of the Fulde-Ferrell-Larkin-Ovchinnikov states induced by anisotropy effects

The crystal structure determines both the Fermi surface and pairing symmetry of the superconducting metals. It is demonstrated in the framework of the general phenomenological approach that this is of the primary importance for the determination of the structure of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase in the magnetic field. The FFLO modulation of the superconducting order parameter may be revealed in the form of the higher Landau level states or/and modulation along the magnetic field. The transition between different FFLO states could occur with the temperature variation or with the magnetic field rotation.

cond-mat.supr-con↗

arXiv2008

Direct coupling between magnetism and superconducting current in Josephson Phi junction

We study the proximity effect between conventional superconductor and magnetic normal metal with a spin-orbit interaction of the Rashba type. Using the phenomenological Ginzburg-Landau theory and the quasiclassical Eilenberger approach it is demonstrated that the Josephson junction with such a metal as a weak link has a special non-sinusoidal current-phase relation. The ground state of this junction is caracterized by the finite phase difference Phi, which is proportional to the strength of the spin-orbit interaction and the exchange field in the normal metal. The proposed mechanism of the Phi junction formation gives a direct coupling between the superconducting current and the magnetic moment in the weak link. Therefore the Phi junctions open interesting perspectives for the superconducting spintronics.

cond-mat.supr-con↗

arXiv2008

Nonsinusoidal current-phase relation in strongly ferromagnetic and moderately disordered SFS junctions

We study the Josephson current in a junction comprising two superconductors linked by a strong ferromagnet in presence of impurities. We focus on a regime where the electron (and hole) motion is ballistic over the exchange length and diffusive on the scale of the weak link length. The current-phase relation is obtained for both two- and three dimensional ferromagnetic weak links. In the clean limit, the possibility of temperature-induced 0- transitions is demonstrated while the corresponding critical current versus temperature dependences are also studied.

cond-mat.mes-hall↗

arXiv2007

FFLO state in thin superconducting films

We present the analysis of the inhomogeneous Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconducting state in thin superconducting films in the parallel magnetic field. For the tetragonal crystal symmetry (relevant to CeCoIn$_{5}$ - the most probable candidate for the FFLO state formation) we predict a very peculiar in-plane angular dependence of the FFLO critical field due to the orbital effect. In the uniform superconducting state the critical field should be isotropic. The magnetic field pins also the direction of the FFLO modulation permitting thus to study the critical current anisotropy. Our calculations reveal a strong critical current anisotropy in the FFLO state in sharp contrast with the usual superconducting state. The predicted characteristic anisotropies of the critical field and critical current may provide an unambiguous probe of the FFLO phase formation.

cond-mat.supr-con↗

arXiv2007

Josephson junctions with negative second harmonic in the current-phase relation: properties of novel varphi-junctions

Several recent experiments revealed a change of the sign of the first harmonic in the current-phase relation of Josephson junctions (JJ) based on novel superconductors, e.g., d-wave based or JJ with ferromagnetic barrier. In this situation the role of the second harmonic becomes dominant and it determines the scenario of a 0-pi transition. We discuss different mechanisms of the second harmonic generation and its sign. If the second harmonic is negative the 0-pi transition becomes continuous and the realization of the so-called varphi junction is possible. We study the unusual properties of such a novel JJ and analyze the possible experimental techniques for their observation.

cond-mat.supr-con↗

arXiv2006

High quality ferromagnetic 0 and pi Josephson tunnel junctions

We fabricated high quality $\Nb/\Al_2Ø_3/\Ni_{0.6}\Cu_{0.4}/\Nb$ superconductor-insulator-ferromagnet-superconductor Josephson tunnel junctions. Depending on the thickness of the ferromagnetic $\Ni_{0.6}\Cu_{0.4}$ layer and on the ambient temperature, the junctions were in the 0 or $π$ ground state. All junctions have homogeneous interfaces showing almost perfect Fraunhofer patterns. The $\Al_2Ø_3$ tunnel barrier allows to achieve rather low damping, which is desired for many experiments especially in the quantum domain. The McCumber parameter $β_c$ increases exponentially with decreasing temperature and reaches $β_c\approx700$ at $T=2.1 {\rm K}$. The critical current density in the $π$ state was up to $5\:\rm{A/cm^2}$ at $T=2.1 {\rm K}$, resulting in a Josephson penetration depth $λ_J$ as low as $160\:\rm{μm}$. Experimentally determined junction parameters are well described by theory taking into account spin-flip scattering in the $\Ni_{0.6}\Cu_{0.4}$ layer and different transparencies of the interfaces.

cond-mat.supr-con↗

arXiv2006

Josephson coupling through ferromagnetic heterojunctions with noncollinear magnetizations

We study the Josephson effect in clean heterojunctions that consist of superconductors connected through two metallic ferromagnets with insulating interfaces. We solve the scattering problem based on the Bogoliubov--de Gennes equation for any relative orientation of in-plane magnetizations, arbitrary transparency of interfaces, and mismatch of Fermi wave vectors. Both spin singlet and triplet superconducting correlations are taken into account, and the Josephson current is calculated as a function of the ferromagnetic layers thicknesses and of the angle $α$ between their magnetizations. We find that the critical Josephson current $I_c$ is a monotonic function of $α$ when the junction is far enough from $0-π$ transitions. This holds when ferromagnets are relatively weak. For stronger ferromagnets, variation of $α$ induces switching between 0 and $π$ states and $I_c(α)$ is non-monotonic function, displaying characteristic dips at the transitions. However, the non-monotonicity is the effect of a weaker influence of the exchange potential in the case of non-parallel magnetizations. No substantial impact of spin-triplet superconducting correlations on the Josephson current has been found in the clean limit. Experimental control of the critical current and $0-π$ transitions by varying the angle between magnetizations is suggested.

cond-mat.supr-con↗

arXiv2006

Density of states in SF bilayers with arbitrary strength of magnetic scattering

We developed the self-consistent method for the calculation of the density of states $N(ε)$ in the SF bilayers. It based on the quasi-classical Usadel equations and takes into account the suppression of superconductivity in the S layer due to the proximity effect with the F metal, as well as existing mechanisms of the spin dependent electron scattering. We demonstrate that the increase of the spin orbit or spin flip electron scattering rates results in completely different transformations of $N(ε)$ at the free F layer interface. The developed formalism has been applied for the interpretation of the available experimental data.

cond-mat.supr-con↗

arXiv2005

Competition between Pi-coupling and FFLO modulation in SF/SF atomic thickness bilayers

We present the detailed theoretical study of a heterostructure comprising of two coupled ferromagnetic superconducting layers. Our model may be also applicable to the layered superconductors with alternating interlayer coupling in a parallel magnetic field. It is demonstrated that such systems exhibit a competition between the nonuniform Larkin-Ovchinnikov-Fulde-Ferrel (FFLO) state and the superconducting state where the sign of the superconducting order parameter is opposite in adjacent layers. We determine the complete temperature-field phase diagram both in the cases of low and strong interlayer coupling. In the former we obtain a new Pi-phase inserted within the FFLO phase and located close to the usual tricritical point, whereas in the latter the bilayer in the state reveals a very high paramagnetic limit and the phenomenon of field-induced superconductivity.

cond-mat.supr-con↗

arXiv2005

Scanning Tunneling Spectroscopy of the superconducting proximity effect in a diluted ferromagnetic alloy

We studied the proximity effect between a superconductor (Nb) and a diluted ferromagnetic alloy (CuNi) in a bilayer geometry. We measured the local density of states on top of the ferromagnetic layer, which thickness varies on each sample, with a very low temperature Scanning Tunneling Microscope. The measured spectra display a very high homogeneity. The analysis of the experimental data shows the need to take into account an additional scattering mechanism. By including in the Usadel equations the effect of the spin relaxation in the ferromagnetic alloy, we obtain a good description of the experimental data.

cond-mat.mes-hall↗

arXiv2005

Peculiar properties of the Josephson junction at the transition from 0 to Pi state

It is demonstrated that in the diffusive superconductor-ferromagnet-superconductor (S/F/S) junctions the current-phase relation is practically sinusoidal everywhere except in a narrow region near the 0-Pi transition. In this region the second harmonic dominates the scenario of the 0-Pi transition. We predict a first order transition for the S/F/S junctions with a homogeneous F barrier. However, in real junctions a small modulation of the thickness of the barrier may favor the continious 0-Pi transition and the realisation of the Josephson junction with an arbitrary ground state phase difference. The performed calculations of the second harmonic amplitude provide a natural explanation of the recent contradictory results on the second harmonic measurements.

cond-mat.supr-con↗

arXiv2005

Field induced superconductivity with enhanced and tunable paramagnetic limit

We demonstrate that in a superconducting multilayered system with alternating interlayer coupling a new type of nonuniform superconducting state can be realized under in-plane magnetic field. The Zeeman effect in this state is compensated by the energy splitting between bonding and antibonding levels. Such compensation mechanism at low temperature leads to the field-induced superconductivity. We discuss the conditions for the experimental observation of the predicted phenomena.

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