arXiv subjects

Asle Sudbø

Publications and source records attributed to Asle Sudbø.

At least 109 records · Page 6Linked to original sources

arXiv2009

Spin-polarized Josephson current in SFS junctions with inhomogeneous magnetization

SHORTENED ABSTRACT: We study numerically the properties of spin- and charge-transport in a nanoscale diffusive superconductor$\mid$ferromagnet$\mid$superconductor junction when the magnetization texture is non-uniform. Specifically, we incorporate the presence of a Bloch/Neel domain walls and conical ferromagnetism, including spin-active interfaces. The superconducting leads are assumed to be of s-wave type. We investigate how the 0-$π$ transition is influenced by the inhomogeneous magnetization texture and focus on the particular case where the charge-current vanishes while the spin-current is non-zero. In the case of a Bloch/Neel domain-wall, the spin-current can be seen only for one component of the spin polarization, whereas in the case of conical ferromagnetism the spin-current has the three components. We explain all of these results in terms of the interplay between the triplet anomalous Green's function induced in the ferromagnetic region and the local direction of the magnetization vector in the ferromagnet. Interestingly, we find that the spin-current exhibits discontinuous jumps at the 0--$π$ transition points of the critical charge-current. We explain this result in terms of the different symmetry obeyed by the current-phase relation when comparing the charge- and spin-current. Specifically, we find that whereas the charge-current obeys the well-known relation $I_c(ϕ) = -I_c(2π-ϕ)$, the spin-current satisfies $I_s(ϕ) = I_s(2π-ϕ)$, where $ϕ$ is the superconducting phase difference.

cond-mat.supr-con↗

arXiv2009

Phase-controlled proximity-effect in ferromagnetic Josephson junctions: calculation of DOS and electronic specific heat

We study the thermodynamic properties of a dirty ferromagnetic S$\mid$F$\mid$S Josephson junction with s-wave superconducting leads in the low-temperature regime. We employ a full numerical solution with a set of realistic parameters and boundary conditions, considering both a uniform and non-uniform exchange field in the form of a Bloch domain wall ferromagnetic layer. The influence of spin-active interfaces is incorporated via a microscopic approach. We mainly focus on how the electronic specific heat and density of states (DOS) of such a system is affected by the \textit{proximity effect}, which may be tuned via the superconducting phase difference. Our main result is that it is possible to \textit{strongly modify the electronic specific heat} of the system by changing the phase difference between the two superconducting leads from 0 up to nearly $π$ at low temperatures. An enhancement of the specific heat will occur for small values $h\simeqΔ$ of the exchange field, while for large values of $h$ the specific heat is suppressed by increasing the phase difference between the superconducting leads. These results are all explained in terms of the proximity-altered DOS in the ferromagnetic region, and we discuss possible methods for experimental detection of the predicted effect.

cond-mat.supr-con↗

arXiv2009

Probing phase-separation in Bose-Fermi mixtures by the critical superfluid velocity

We investigate the effect exerted by spin-polarized fermions on the interaction between superfluid bosons for a Bose-Fermi mixture residing on an optical lattice, with particular emphasis on the possibility of an induced phase-separation. Using a set of microscopic parameters relevant to a $^{40}$K-$^{87}$Rb mixture, we show how the phase-separation criterion may be directly probed by means of the critical superfluid velocity of the bosonic condensate. We report quantitative results for the magnitude of the superfluid velocity and its dependence on the trap depth, the boson-fermion interaction, and the fermionic filling fraction. All of these parameters can be controlled experimentally in a well-defined manner. We propose an experimental setup for probing the critical superfluid velocity.

cond-mat.quant-gas↗

arXiv2009

Josephson current in graphene: the role of unconventional pairing symmetries

We investigate the Josephson current in a graphene superconductor/normal/superconductor junction, where superconductivity is induced by means of the proximity effect from external contacts. We take into account the possibility of anisotropic pairing by also including singlet nearest-neighbor interactions, and investigate how the transport properties are affected by the symmetry of the superconducting order parameter. This corresponds to an extension of the usual on-site interaction assumption, which yields an isotropic s-wave order parameter near the Dirac points. Here, we employ a full numerical solution as well as an analytical treatment, and show how the proximity effect may induce exotic types of superconducting states near the Dirac points, e.g. $p_x$- and $p_y$-wave pairing or a combination of s-wave and $p+ıp$-wave pairing. We find that the Josephson current exhibits a weakly-damped, oscillatory dependence on the length of the junction when the graphene sheet is strongly doped. The analytical and numerical treatments are found to agree well with each other in the s-wave case when calculating the critical current and current-phase relationship. For the scenarios with anisotropic superconducting pairing, there is a deviation between the two treatments, especially for the effective $p_x$-wave order parameter near the Dirac cones which features zero-energy states at the interfaces. This indicates that a numerical, self-consistent approach becomes necessary when treating anisotropic superconducting pairing in graphene.

cond-mat.supr-con↗

arXiv2009

Crossed Andreev reflection in superconducting graphene spin-valves: Spin-switch effect

We consider the non-local quantum transport properties of a graphene superconducting spin-valve. It is shown that one may create a spin-switch effect between perfect elastic co-tunneling (CT) and perfect crossed Andreev-reflection (CAR) for all bias voltages in the low-energy regime by reversing the magnetization direction in one of the ferromagnetic layers. This opportunity arises due the possibility of tuning the local Fermi-level in graphene to values equivalent to a weak, magnetic exchange splitting, thus reducing the Fermi surface for minority spins to a single point and rendering graphene to be half-metallic. Such an effect is not attainable in a conventional metallic spin-valve setup, where the contributions from CT and CAR tend to cancel each other and noise-measurements are necessary to distinguish these processes.

cond-mat.supr-con↗

arXiv2009

0-$π$ phase shifts in Josephson junctions as a signature for the $s_{\pm}$-wave pairing state

We investigate Josephson junctions with superconducting ferropnictides, both in the diffusive and ballistic limit. We focus on the proposed $s_\pm$-wave state, and find that the relative phase shift intrinsic to the $s_\pm$-wave state may provide 0-$π$ oscillations in the Josephson current. This feature can be used to discriminate this pairing state from the conventional s-wave symmetry. The 0-$π$ oscillations appear both as a function of the ratio of the interface resistances for each band and, more importantly, as a function of temperature, which greatly aids in their detection.

cond-mat.supr-con↗

arXiv2009

Calculation of Drag and Superfluid Velocity from the Microscopic Parameters and Excitation Energies of a Two-Component Bose-Einstein Condensate on an Optical Lattice

We investigate a model of a two-component Bose-Einstein condensate residing on an optical lattice. Within a Bogolioubov-approach at the mean-field level, we derive exact analytical expressions for the excitation spectrum of the two-component condensate when taking into account hopping and interactions between arbitrary sites. Our results thus constitute a basis for works that seek to clarify the effects of higher-order interactions in the system. We investigate the excitation spectrum and the two branches of superfluid velocity in more detail for two limiting cases of particular relevance. Moreover, we relate the hopping and interaction parameters in the effective Bose-Hubbard model to microscopic parameters in the system, such as the laserlight wavelength and atomic masses of the components in the condensate. These results are then used to calculate analytically and numerically the drag coefficient between the components of the condensate. We find that the drag is most effective close to the symmetric case of equal masses between the components, regardless of the strength of the intercomponent interaction and the lattice well depth.

cond-mat.quant-gas↗

arXiv2009

Pairing symmetry conversion by spin-active interfaces in superconducting junctions

We study the proximity-induced superconducting correlations in a normal metal connected to a superconductor when the interface between them is spin-active and the normal metal is ballistic or diffusive. Remarkably, for any interface spin polarization there is a critical interface resistance, above which the conventional even-frequency proximity component vanishes completely at the chemical potential, while the odd-frequency component remains finite. We propose a way to unambiguously observe the odd-frequency component.

cond-mat.supr-con↗

arXiv2009

Proximity effect in ferromagnet/superconductor hybrids: from diffusive to ballistic motion

We present an analytical study of the proximity effect in ferromagnet/superconductor (F/S) heterostructures, allowing for an arbitrary magnetic exchange energy as well as arbitrary impurity and spin-flip scattering rates within a quasiclassical approach. While previous studies mainly have focused on the clean or dirty limits, our results grant access to the regime of intermediate impurity concentrations, thus allowing us to probe the crossover from the clean to dirty limit. We find that in the crossover regime, all possible symmetry correlations of the proximity-induced anomalous Green's function are induced in the ferromagnet. We also point out that the local density of states oscillates spatially, not only for an F/S bilayer, but also for a normal/superconductor (N/S) bilayer in the diffusive limit, a fact which appears to have gone unnoticed in the literature. Within the weak-proximity effect regime, we present compact analytical expressions valid for arbitrary exchange fields and impurity scattering rates for \textit{i)} the local density of states in an F/S bilayer, \textit{ii)} the Josephson current in an S/F/S junction, and \textit{iii)} the critical temperature in an F/S/F multilayer. For all cases, we study in particular the crossover regime between diffusive and ballistic motion. Our results may be useful for analyzing experimental data in cases when the dirty limit is not fully reached, thus invalidating the use of the Usadel equation.

cond-mat.supr-con↗

arXiv2009

Theory of superconducting and magnetic proximity effect in S$\mid$F structures with inhomogeneous magnetization textures and spin-active interfaces

We present a study of the proximity effect and the inverse proximity effect in a superconductor$\mid$ferromagnet bilayer, taking into account several important factors which mostly have been ignored in the literature so far. These include spin-dependent interfacial phase shifts (spin-DIPS) and inhomogeneous textures of the magnetization in the ferromagnetic layer, both of which are expected to be present in real experimental samples. Our approach is numerical, allowing us to access the full proximity effect regime. In Part I of this work, we study the superconducting proximity effect and the resulting local density of states in an inhomogeneous ferromagnet with a non-trivial magnetic texture. Our two main results in Part I are a study of how Bloch and Néel domain walls affect the proximity-induced superconducting correlations and a study of the superconducting proximity effect in a conical ferromagnet. The latter topic should be relevant for the ferromagnet Ho, which was recently used in an experiment to demonstrate the possibility to generate and sustain long-range triplet superconducting correlations. In Part II of this work, we investigate the inverse proximity effect with emphasis on the induced magnetization in the superconducting region as a result of the "leakage" from the ferromagnetic region. It is shown that the presence of spin-DIPS modify conclusions obtained previously in the literature with regard to the induced magnetization in the superconducting region. In particular, we find that the spin-DIPS can trigger an anti-screening effect of the magnetization, leading to an induced magnetization in the superconducting region with \textit{the same sign} as in the proximity ferromagnet.

cond-mat.supr-con↗

arXiv2009

Tunneling conductance and local density of states in time-reversal symmetry breaking superconductors under the influence of an external magnetic field

We consider different effects that arise when time-reversal symmetry breaking superconductors are subjected to an external magnetic field, thus rendering the superconductor to be in the mixed state. We focus in particular on two time-reversal symmetry breaking order parameters which are believed to be realized in actual materials: $p+ıp'$-wave and $d+ıs$- or $d+ıd'$-wave. The first order parameter is relevant for Sr$_2$RuO$_4$, while the latter order parameters have been suggested to exist near surfaces in some of the high-$T_c$ cuprates. We investigate the interplay between surface states and vortex states in the presence of an external magnetic field and their influence on both the tunneling conductance and the local density of states. Our findings may be helpful to experimentally identify the symmetry of unconventional time-reversal symmetry breaking superconducting states.

cond-mat.supr-con↗

arXiv2008

Theory of Andreev reflection in junctions with iron-based High-$T_c$ superconductors

We construct a theory for low-energy quantum transport in normal$\mid$superconductor junctions involving the recently discovered iron-based high-$T_c$ superconductors. We properly take into account both Andreev bound surface states and the complex Fermi surface topology in our approach, and investigate the signatures of the possible order parameter symmetries for the FeAs-lattice. Our results could be helpful in determining the symmetry of the superconducting state in the iron-pnicitide superconductors.

cond-mat.supr-con↗

arXiv2008

Nontrivial interplay between superconductivity and spin-orbit coupling in non-centrosymmetric ferromagnets

Motivated by the recent discoveries of ferromagnetic and non-centrosymmetric superconductors, we present a mean-field theory for a superconductor that \textit{both} lacks inversion symmetry and displays ferromagnetism, a scenario which is believed to be realized in UIr. We study the interplay between the order parameters to clarify how superconductivity is affected by the presence of ferromagnetism and spin-orbit coupling. One of our key findings is that the spin-orbit coupling seems to enhance both ferromagnetism and superconductivity in all spin channels. We discuss our results in the context of the heavy fermion superconductor UIr and analyze possible symmetries of the order parameter by the group theory method.

cond-mat.supr-con↗

arXiv2008

Josephson current in diffusive multilayer superconductor/ferromagnet/superconductor junctions

We calculate the Josephson current in a diffusive superconductor/ferromagnet/superconductor junction, where the ferromagnetic region contains multiple layers (or domains). In particular, we study a configuration where there are two layers with an arbitrary relative in-plane magnetization orientation, and also include non-ideal interfaces and arbitrary spin-flip scattering. We study the 0-$π$ oscillations of the critical current for varying junction width $d$, and find that the $π$ state vanishes entirely when the magnetic misorientation angle of the two layers exceeds a critical angle $ϕ_c$. While $ϕ_c \to π/2$ in the limit of high temperatures, we find that $ϕ_c$ becomes smaller than $π/2$ at low temperatures compared to $T_c$. 0-$π$ oscillations are also found when varying the temperature or the misorientation angle for fixed values of $d$, and we present phase diagrams that show qualitatively the conditions for the appearance of such oscillations. We also point out how one may obtain significant enhancement of the critical current in such a system by switching the magnetization for selected values of the junction width $d$, and comment on the necessary conditions for establishing a long range triplet Josephson effect.

cond-mat.supr-con↗

arXiv2008

Density of states near a vortex core in ferromagnetic superconductors: Application to STM measurements

We investigate numerically the local density of states (LDOS) in the vicinity of a vortex core in a ferromagnetic superconductor. Specifically, we investigate how the LDOS is affected by the relative weight of the spin bands in terms of the superconducting pairing, and we also examine the effect of different pairing symmetries for the superconducting order parameter. Our findings are directly related to scanning tunneling microscopy measurements and may thus be highly useful to clarify details of the superconducting pairing in recently discovered ferromagnetic superconductors.

cond-mat.supr-con↗

arXiv2008

Strongly spin-polarized current generated in Zeeman-split unconventional superconductors

We consider a thin-film normal metal/superconductor junction in the presence of an externally applied in-plane magnetic field for several symmetries of the superconducting order parameter. For p-wave superconductors, a strongly spin-polarized current emerges due to an interplay between the nodal structure of the superconducting order parameter, the existence or non-existence of zero-energy surface states, and the Zeeman-splitting of the bands which form superconductivity. Thus, the polarization depends strongly on the orbital symmetry of the superconducting state. Our findings suggest a mechanism for obtaining fully spin-polarized currents crucially involving zero-energy surface states, not present in s-wave superconductors.

cond-mat.supr-con↗

arXiv2008

The role of interface transparency and spin-dependent scattering in diffusive ferromagnet/superconductor heterostructures

We present a numerical study of the density of states in a ferromagnet/superconductor junction and the Josephson current in a superconductor/ferromagnet/superconductor junction in the diffusive limit by solving the Usadel equation with Nazarov's boundary conditions. Our calculations are valid for an arbitrary interface transparency and arbitrary spin-dependent scattering rate, which allows us to explore the entire proximity-effect regime. We first investigate how the proximity-induced anomalous Green's function affects the density of states in the ferromagnet for several values of the exchange field. In each case, we consider the effect of the barrier transparency and allow for various concentrations of magnetic impurities. In particular, we address how the zero-energy peak and minigap observed in experiments may be understood in terms of the interplay between the singlet and triplet anomalous Green's function and their dependence on the concentration of magnetic impurities. We also investigate the role of the barrier transparency and spin-flip scattering in a superconductor/ferromagnet/superconductor junction. We suggest that such diffusive Josephson junctions with large residual values of the supercurrent at the 0-$π$ transition, where the first harmonic term in the current vanishes, may be used as efficient supercurrent-switching devices. It is also found that uniaxial spin-flip scattering has very different effect on the 0-$π$ transition points depending on whether one regards the width- or temperature-dependence of the current.

cond-mat.supr-con↗

arXiv2008

Coexistence of itinerant ferromagnetism and a non-unitary superconducting state with line nodes: possible application to UGe$_2$

We construct a mean-field theory for itinerant ferromagnetism coexisting with a non-unitary superconducting state, where only the majority-spin band is gapped and contains line nodes, while the minority-spin band is gapless at the Fermi level. Our study is motivated by recent experimental results indicating that this may be the physical situation realized in the heavy-fermion compound UGe$_2$. We investigate the stability of the mean-field solution of the magnetic and superconducting order parameters. Also, we provide theoretical predictions for experimentally measurable properties of such a non-unitary superconductor: the specific heat capacity, the Knight shift, and the tunneling conductance spectra. Our study should be useful for direct comparison with experimental results and also for further predictions of the physics that may be expected in ferromagnetic superconductors.

cond-mat.supr-con↗