arXiv subjects

A. M. Bobkov

Publications and source records attributed to A. M. Bobkov.

70 records · Page 4Linked to original sources

arXiv2012

Recovering of superconductivity in exchange fields exceeding Pauli limiting field under spin-dependent quasiparticle distribution

We study theoretically the simultaneous influence of spin accumulation potential $eV_\uparrow-eV_\downarrow$ and the Zeeman exchange field on singlet superconductivity. It is shown that the pair-breaking effect of the Zeeman field can be fully compensated by creation of the appropriate spin accumulation potential in the superconductor. Moreover, superconductivity can be recovered for exchange fields well exceeding the Pauli limiting field. It is proposed that the effect can be experimentally realized on the basis of voltage biased junction consisting of a thin superconducting film sandwiched between two half metals.

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arXiv2012

Long-range proximity effect for opposite-spin pairs in S/F heterostructures under non-equilibrium quasiparticle distribution

By now it is known that in a singlet superconductor/ferromagnet (S/F) structure the superconducting correlations carried by opposite-spin pairs penetrate into the ferromagnet over a short distance of the order of magnetic coherence length. The long-range proximity effect (LRPE), taking place on the length scale of the normal metal coherence length, can only be maintained by equal-spin pairs, which can be generated by magnetic inhomogeneities in the system. In this work we have predicted a new type of LRPE, which can take place in S/F heterostructures under non-equilibrium conditions. The superconducting correlations in the F region are generated by opposite-spin Cooper pairs and equal-spin pairs are not involved. The possibility for an opposite-spin pair to penetrate into the ferromagnet over a large distance is provided by creation of the proper non-equilibrium quasiparticle distribution there. This leads to a sharp increase (up to a few orders of magnitude) of the critical Josephson current through a S/F/S junction at some values of the voltage controlling the nonequilibrium distribution in the F interlayer.

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arXiv2011

Time-reversal symmetry breaking state near the surface of $s_{\pm}$-superconductor

The structure of superconducting order parameter near the surface of a two-band superconductor with $s_{\pm}$ order parameter in the bulk is theoretically investigated. The main parameter of the surface, which determines the appropriate physics is the coefficient of the interband scattering $R_{12}$. For small $R_{12}$ the superconducting order parameter is only suppressed to some extent near the surface for the both bands. For intermediate and strong interband scattering there are two possible non-trivial surface states of the order parameter: (i) purely real solution, where the symmetry of the superconducting state near the surface is changed from $s_{\pm}$ to conventional $s_{++}$ and (ii) time-reversal symmetry breaking (TRB) state. In this state the order parameters in the two bands acquire phases $ϕ_{1,2}(x) \neq (0,π)$ upon approaching the surface. We argue that at low temperatures the TRB surface state can be more energetically favorable than the $s_{\pm} \to s_{++}$ time reversal symmetry conserving state (TR). For higher temperatures up to $T_c$ only the TR state can exist. The transition between the two temperature regions is rather sharp. Signatures of the transition between the TRB and the TR surface states can be detected by the measurements of the local density of states and the angle-resolved density of states.

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arXiv2011

Influence of the spin-dependent quasiparticle distribution on the Josephson current through a ferromagnetic weak link

The Josephson current flowing through weak links containing ferromagnetic elements is studied theoretically under the condition that the quasiparticle distribution over energy states in the interlayer is spin-dependent. It is shown that the interplay between the spin-dependent quasiparticle distribution and the triplet superconducting correlations induced by the proximity effect between the superconducting leads and ferromagnetic elements of the interlayer, leads to the appearence of an additional contribution to the Josephson current. This additional contribution $j_t$ can be extracted from the full Josephson current in experiment. The features of the additional supercurrent $j_t$, which are of main physical interest are the following: (i) We propose the experimental setup, where the contributions given by the short-range (SRTC) and long-range (LRTC) components of triplet superconducting correlations in the interlayer can be measured separately. It can be realized on the basis of S/N/F/N/S junction, where the interlayer is composed of two normal metal regions with a spiral ferromagnet layer sandwiched between them. For the case of tunnel junctions the measurement of $j_t$ in such a system can provide direct information about the energy-resolved anomalous Green's function components describing SRTC and LRTC. (ii) In some cases the exchange field-suppressed supercurrent can be not only recovered but also enhanced with respect to its value for non-magnetic junction with the same interface resistances by the presence of spin-dependent quasiparticle distribution. This effect is demonstrated for S/N/S junction with magnetic S/N interfaces. In addition, it is also found that under the considered conditions the dependence of the Josephson current on temperature can be nontrivial: at first the current rises upon temperature increasing and only after that starts to decline.

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arXiv2010

Triplet contribution to the Josephson current in the nonequilibrium SFS junction

The Josephson current through a long s-wave superconductor/weak ferromagnet/s-wave superconductor weak link is studied theoretically in the regime of nonequilibrium spin-dependent occupation of electron states in the ferromagnetic intelayer. While under the considered nonequilibrium conditions the standard supercurrent, carried by the singlet part of current-carrying density of states, practically is not modified, the additional supercurrent flowing via the triplet part of the current-carrying density of states appears. Depending on voltage, controlling the particular form of spin-dependent nonequilibrium in the interlayer, this additional current can enhance or reduce the usual current of the singlet component and also switch the junction between $0$- and $π$-states.

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arXiv2010

Order parameter sign-reversal near $s_\pm$-superconductor surface

The superconducting order parameter and LDOS spectra near an impenetrable surface are studied on the basis of selfconsistent calculations for a two band superconductor with nodeless extended s-+wave order parameter symmetry, as possibly realized in Fe-based high-temperature superconductors. It is found that for a wide range of parameters the spatial behavior of the order parameter at a surface is not reduced to a trivial suppression. If the interband scattering at a surface is of the order of the intraband one or dominates it, it can be energetically favorable to change the symmetry of the superconducting state near the surface from $s_{\pm}$ to conventional s-wave. The range of existing this surface conventional superconductivity is very sensitive to the relative values of interband and intraband pairing potentials. It is shown that the LDOS spectra near the surface can qualitatively differ upon calculating with and without taking into account the selfconsistency of the order parameter.

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arXiv2009

Triplet contribution to the Josephson current in the nonequilibrium superconductor/ferromagnet/superconductor junction

The Josephson current through a long s-wave superconductor/weak ferromagnet/s-wave superconductor weak link is studied theoretically in the regime of nonequilibrium spin-dependent occupation of electron states in the ferromagnetic intelayer. While under the considered nonequilibrium condition the standard supercurrent, carried by the singlet part of current-carrying density of states, is not modified, the additional supercurrent flowing via the triplet part of the current-carrying density of states appears. Depending on voltage, controlling the particular form of spin-dependent nonequilibrium in the interlayer, this additional current can enhance or reduce the usual current of the singlet component and also switch the junction between 0- and $π$-states.

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arXiv2008

Proximity-driven source of highly spin-polarized ac current on the basis of superconductor/weak ferromagnet/superconductor voltage-biased Josephson junction

We theoretically investigate an opportunity to implement a source of highly spin-polarized ac current on the basis of superconductor/weak ferromagnet/superconductor (SFS) voltage-biased junction in the regime of essential proximity effect and calculate the current flowing through the probe electrode tunnel coupled to the ferromagnetic interlayer region. It is shown that while the polarization of the dc current component is generally small in case of weak exchange field of the ferromagnet, there is an ac component of the current in the system. This ac current is highly spin-polarized and entirely originated from the non-equilibrium proximity effect in the interlayer. The frequency of the current is controlled by the voltage applied to SFS junction. We discuss a possibility to obtain a source of coherent ac currents with a certain phase shift between them by tunnel coupling two probe electrodes at different locations of the interlayer region.

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arXiv2008

Influence of the atomic-scale inhomogeneity of the pair interaction on extracted from the STM spectra characteristics of high-$T_c$ superconductors

The influence of the atomic-scale inhomogeneities of the pairing interaction strength on the superconducting order parameter and the conductance spectra measurable by STM is studied in the framework of weak-coupling BCS-like theory for two-dimensional lattice model. First of all, it is found that the inhomogeneity having the form of atomic-scale regions of enhanced pair interaction increases the ratio of the local low-temperature gap in differential conductance spectra to the local temperature of vanishing the gap $2Δ_g/T_p$. Even in the framework of mean-field treatment this ratio is shown to be larger than the one corresponding to the homogeneous case. It is shown that the effect of thermal phase fluctuations of the superconducting order parameter can further increase this ratio. Taking them into account in the framework of a toy model we obtained the ratio $2Δ_g/T_p$ to be $\sim 7-8$. It is found that the additional atomic-scale hopping element disorder and weak potential scatterers, which can also take place in cuprate materials, have no considerable effect on the statistical properties of the system, including the distribution of the gaps, $T_p$ and the ratio $2Δ_g/T_p$. The second consequence of the atomic-scale order parameter inhomogeneity is the anticorrelation between the low-temperature gap and the high-temperature zero-bias conductance. The obtained results could bear a relation to recent STM measurements.

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arXiv2007

Influence of the atomic scale inhomogeneity of the pair interaction on the local pair formation and density of states in high-T_c superconductors

The influence of the atomic-scale inhomogeneities of the pairing interaction on the superconducting order parameter distribution and the LDOS is studied in the framework of mean-field BCS theory for two-dimensional lattice model. It is found that the ratio of the local low-temperature gap in differential conductance to the local temperature of vanishing the gap $2Δ_g/T_p$ can take large enough values compared to the homogeneous case. This ratio practically does not depend on the location in the sample and is independent on the concentration of local pair interaction perturbations in wide range of concentrations. The obtained results could bear a relation to the recent measurements by Gomes {\it et. al} Nature, 447, 569 (2007).

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arXiv2007

Influence of spin filtering and spin mixing on the subgap structure of I-V characteristics in superconducting quantum point contact

The effect of spin filtering and spin mixing on the dc electric current for voltage biased magnetic quantum point contact with superconducting leads is theoretically studied. The I-V characteristics are calculated for the whole range of spin filtering and spin mixing parameters. It is found that with increasing of spin filtering the subharmonic step structure of the dc electric current, typical for low-transparency junction and junction without considerable spin filtering qualitatively changes. In the lower voltage region and for small enough spin mixing the peak structure arises. When spin mixing increases the peak subgap structure evolves to the step structure. The voltages where subharmonic gap features are located are found to be sensitive to the value of spin filtering. The positions of peaks and steps are calculated analytically and the evolution of the subgap structure from well-known tunnel limit to the large spin filtering case is explained in terms of multiple Andreev reflection (MAR) processes. In particular, it is found that for large spin filtering the subgap feature at $eV_k$ arises from $2k^{\rm th}$ and $(2k\pm 1)^{\rm th}$ order MAR processes, while in the tunnel limit the step at $eV_n$ is known to result from $n^{\rm th}$ order MAR process.

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arXiv2006

Nonequilibrium Transport in Superconductor/Ferromagnet/Superconductor Diffusive Junctions: Interplay between Proximity Effect and Ferromagnetism

The theory of the I-V characteristics in diffusive superconductor/weak ferromagnet/superconductor (SFS) junction is developed. We show that the exchange field $h$ of the ferromagnet manifests itself as an additional conductance peak at $eV \sim Δ+h$ in the phase-coherent regime, when the Thouless energy is of the order of superconducting order parameter. The excess current exhibits non-monotonous dependence on the exchange field and non-trivial temperature behavior, which is strongly influenced by the temperature dependence of the exchange field.

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arXiv2005

Discrete-lattice model approach to the tunneling between d-wave superconductors: interference of tunnel bonds

The Josephson current between d-wave superconductors is investigated in the framework of tight-binding lattice model. The junction is modelled by a small number of connecting bonds. It is obtained that the Josephson current through one bond vanishes when at least one of the superconductors has $(110)$ interface-to-crystal orientation. Interference between the nearest bonds is appeared to be very important. In particular, it is the interference term that leads to the nonzero Josephson current for $(110)$ orientation. Also, interference of two connecting bonds manifests itself in non-monotonic behavior of the critical Josephson current in dependence on the distance between the bonds.

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arXiv2005

Electronic structure of d-wave superconducting quantum wires

We present analytical and numerical results for the electronic spectra of wires of a d-wave superconductor on a square lattice. The spectra of Andreev and other quasiparticle states, as well as the spatial and particle-hole structures of their wave functions, depend on interference effects caused by the presence of the surfaces and are qualitatively different for half-filled wires with even or odd number of chains. For half-filled wires with an odd number of chains N at (110) orientation, spectra consist of N doubly degenerate branches. By contrast, for even N wires, these levels are split, and all quasiparticle states, even the ones lying above the maximal gap, have the characteristic properties of Andreev bound states. These Andreev states above the gap can be interpreted as a consequence of an infinite sequence of Andreev reflections experienced by quasiparticles along their trajectories bounded by the surfaces of the wire. Our microscopic results for the local density of states display atomic-scale Friedel oscillations due to the presence of the surfaces, which should be observable by scanning tunneling microscopy. For narrow wires the self-consistent treatment of the order parameter is found to play a crucial role. In particular, we find that for small wire widths the finite geometry may drive strong fluctuations or even stablilize exotic quasi-1D pair states with spin triplet character.

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arXiv2004

Zero-energy Andreev surface bound states in the lattice model

The conditions for zero-energy Andreev surface bound states to exist are found for the lattice model of d-wave superconductor with arbitrary surface orientation. Both nearest neighbors and next nearest neighbors models are considered. It is shown that the results are very sensitive to the surface orientation. In particular, for half-filled $(hl0)$-surface zero-energy Andreev surface states only appear under the condition that $h$ and $l$ are odd simultaneously

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arXiv2001

Josephson current between chiral superconductors

We study chiral interface Andreev bound states and their influence on the Josephson current between clean superconductors. Possible examples are superconducting Sr2RuO4 and the B-phase of the heavy-fermion superconductor UPt3. We show that, under certain conditions, the low-energy chiral surface states enhance the critical current of symmetric tunnel junctions at low temperatures. The enhancement is substantially more pronounced in quantum point contacts. In classical junctions dispersive chiral states result in a logarithmic dependence of the critical current. This logarithmic behavior contains the temperature, the barrier transparency and the broadening of the bound states, and depends on the detailed relation between these parameters. The Josephson current through the domain wall doesn't acquire this logarithmic enhancement, although the contribution from the bound states is important in this case as well.

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