Asle Sudbø - Academia.edu (original) (raw)

Papers by Asle Sudbø

Physical Review B, Jan 16, 2007

A system exhibiting multiple simultaneously broken symmetries offers the opportunity to influence... more A system exhibiting multiple simultaneously broken symmetries offers the opportunity to influence physical phenomena such as tunneling currents by means of external control parameters. In this paper, we consider the broken SU (2) (internal spin) symmetry of ferromagnetic systems coexisting with i) the broken U (1) symmetry of superconductors and ii) the broken spatial inversion symmetry induced by a Rashba term in a spin-orbit coupling Hamiltonian. In order to study the effect of these broken symmetries, we consider tunneling currents that arise in two different systems; tunneling junctions consisting of non-unitary spin-triplet ferromagnetic superconductors and junctions consisting of ferromagnets with spin-orbit coupling. In the former case, we consider different pairing symmetries in a model where ferromagnetism and superconductivity coexist uniformly. An interplay between the relative magnetization orientation on each side of the junction and the superconducting phase difference is found, similarly to that found in earlier studies on spin-singlet superconductivity coexisting with spiral magnetism. This interplay gives rise to persistent spin-and charge-currents in the absence of an electrostatic voltage that can be controlled by adjusting the relative magnetization orientation on each side of the junction. In the second system, we study transport of spin in a system consisting of two ferromagnets with spin-orbit coupling separated by an insulating tunneling junction. A persistent spin-current across the junction is found, which can be controlled in a well-defined manner by external magnetic and electric fields. The behavior of this spin-current for important geometries and limits is studied.

Physical Review Letters, Oct 3, 2006

We study tunneling currents in a model consisting of two non-unitary ferromagnetic spin-triplet s... more We study tunneling currents in a model consisting of two non-unitary ferromagnetic spin-triplet superconductors separated by a thin insulating layer. We find a novel interplay between ferromagnetism and superconductivity, manifested in the Josephson effect. This offers the possibility of tuning dissipationless currents of charge and spin in a well-defined manner by adjusting the magnetization direction on either side of the junction.

Physical Review B, Apr 12, 2007

We present a theoretical study of a ferromagnet/s-wave superconductor junction to investigate the... more We present a theoretical study of a ferromagnet/s-wave superconductor junction to investigate the signatures of induced triplet correlations in the system. We apply the extended BTK-formalism and allow for an arbitrary magnetization strength/direction of the ferromagnet, a spin-active barrier, Fermi-vector mismatch, and different effective masses in the two systems. It is found that the phase associated with the xy-components of the magnetization in the ferromagnet couples with the superconducting phase and induces spin-triplet pairing correlations in the superconductor, if the tunneling barrier acts as a spin-filter. This feature leads to an induced spin-triplet pairing correlation in the ferromagnet, along with a spin-triplet electron-hole coherence due to an interplay between the ferromagnetic and superconducting phase. As our main result, we investigate the experimental signatures of retrorelection, manifested in the tunneling conductance of a ferromagnet/s-wave superconductor junction with a spin-active interface.

Physical Review Letters, Dec 2, 2020

Motivated by recent progress on synthesizing two-dimensional magnetic van der Waals systems, we p... more Motivated by recent progress on synthesizing two-dimensional magnetic van der Waals systems, we propose a setup for detecting the topological Berezinskii-Kosterlitz-Thouless (BKT) phase transition in spin-transport experiments on such structures. We demonstrate that the spatial correlations of injected spin-currents into a pair of metallic leads can be used to measure the predicted universal jump of 2/π in the ferromagnet spin-stiffness as well as its predicted universal square root dependence on temperature as the transition is approached from below. Our setup provides a simple route to measuring this topological phase transition in two-dimensional magnetic systems, something which up to now has proven elusive. It is hoped that this will encourage experimental efforts to investigate critical phenomena beyond the standard Ginzburg-Landau paradigm in low-dimensional magnetic systems with no local order parameter.

Physical review, Oct 5, 2018

Spin-valve structures are usually associated with the ability to modify the resistance of electri... more Spin-valve structures are usually associated with the ability to modify the resistance of electrical currents. We here demonstrate a profoundly different effect of a spin-valve. In combination with a topological insulator and superconducting materials, we show that a spin-valve can be used to toggle quantum vortices in and out of existence. In the antiparallel configuration, the spin-valve causes superconducting vortex nucleation. In the parallel configuration, however, no vortices appear. This switching effect suggests a new way to control quantum vortices.

Physical Review B, Dec 14, 2015

We demonstrate that the Higgs mechanism in three-dimensional topological superconductors exhibits... more We demonstrate that the Higgs mechanism in three-dimensional topological superconductors exhibits unique features with experimentally observable consequences. The Higgs model we discuss has two superconducting components and an axion-like magnetoelectric term with the phase difference of the superconducting order parameters playing the role of the axion field. Due to this additional term, quantum electromagnetic and phase fluctuations lead to a robust topologically non-trivial state that holds also in the presence of interactions. In this sense, we show that the renormalization flow of the topologically nontrivial phase cannot be continuously deformed into a topologically non-trivial one. One consequence of our analysis of quantum critical fluctuations, is the possibility of having a first-order phase transition in the bulk and a second-order phase transition on the surface. We also explore another consequence of the axionic Higgs electrodynamics, namely, the anomalous Hall effect. In the low frequency London regime an anomalous Hall effect is induced in the presence of an applied electric field parallel to the surface. This anomalous Hall current is induced by a Lorentz-like force arising from the axion term, and it involves the relative superfluid velocity of the superconducting components. The anomalous Hall current has a negative sign, a situation reminiscent of, but quite distinct in physical origin from the anomalous Hall effect observed in high-T c superconductors. In contrast to the latter, the anomalous Hall effect in topological superconductors is non-dissipative and occurs in the absence of vortices.

Physical Review Letters, Aug 1, 2004

The critical properties of N-color London model are studied in d = 2 + 1 dimensions. The model is... more The critical properties of N-color London model are studied in d = 2 + 1 dimensions. The model is dualized to a theory of N vortex fields interacting through a Coulomb and a screened potential. The model with N = 2 shows two anomalies in the specific heat. From the critical exponents α and ν, the mass of the gauge field, and the vortex correlation functions, we conclude that one anomaly corresponds to an inverted 3Dxy fixed point, while the other corresponds to a 3Dxy fixed point. There are N fixed points, namely one corresponding to an inverted 3Dxy fixed point, and N − 1 corresponding to neutral 3Dxy fixed points. This represents a novel type of quantum fluid, where superfluid modes arise out of charged condensates.

Physical Review B, Feb 14, 2008

We investigate the conductance spectra of a normal/superconductor graphene junction using the ext... more We investigate the conductance spectra of a normal/superconductor graphene junction using the extended Blonder-Tinkham-Klapwijk formalism, considering pairing potentials that are both conventional (isotropic swave) and unconventional (anisotropic d-wave). In particular, we study the full crossover from normal to specular Andreev reflection without restricting ourselves to special limits and approximations, thus expanding results obtained in previous work. In addition, we investigate in detail how the conductance spectra are affected if it is possible to induce an unconventional pairing symmetry in graphene, for instance a d-wave order parameter. We also discuss the recently reported conductance-oscillations that take place in normal/superconductor graphene junctions, providing both analytical and numerical results.

Physical review, Aug 11, 2022

Motivated by the recent experimental observation of an intermediate bosonic metallic state in the... more Motivated by the recent experimental observation of an intermediate bosonic metallic state in the two-dimensional superconductor-insulator transition at T = 0, we study an extended Bose Hubbard model in the limit of large number of particles per site. Using a representation of this in terms of two coupled XY models, we find, in addition to an insulating phase and a (2+ 1)D superfluid phase, two other phases. One phase is a 2D superfluid phase where a crossover from (2 + 1)D to 2D has taken place as a result of incipient charge ordering, signalled by θ disordering, and which is closely related to a supersolid phase. The other new phase is an edge metal state characterized by zero superfluid stiffness, zero charge ordering, and zero bulk compressibility. However, the edge compressibility of the system is nonzero. While we do not find any intermediate state with 2D metallic conductivity, we are able to connect these results to STM experiments on MoS 2 showing brims of finite density of states around the entire edge of 2D MoS 2 samples.

arXiv (Cornell University), Mar 15, 2007

Physical review, Dec 4, 2020

We study superconductivity in a normal metal, arising from effective electron-electron interactio... more We study superconductivity in a normal metal, arising from effective electron-electron interactions mediated by spin-fluctuations in a neighboring antiferromagnetic insulator. Introducing a frustrating next-nearest neighbor interaction in a Néel antiferromagnet with an uncompensated interface, the superconducting critical temperature is found to be enhanced as the frustration is increased. Further, for sufficiently large next-nearest neighbor interaction, the antiferromagnet is driven into a stripe phase, which can also give rise to attractive electron-electron interactions. For the stripe phase, as previously reported for the Néel phase, the superconducting critical temperature is found to be amplified for an uncompensated interface where the normal metal conduction electrons are coupled to only one of the two sublattices of the magnet. The superconducting critical temperature arising from fluctuations in the stripe phase antiferromagnet can be further enhanced by approaching the transition back to the Néel phase.

International Journal of Modern Physics B, May 30, 1999

We consider the Cooper-problem on a lattice model including onsite and near-neighbor interactions... more We consider the Cooper-problem on a lattice model including onsite and near-neighbor interactions. Expanding the interaction in basis functions for the irreducible representation for the point group C4v yields a classification of the symmetry of the Cooper-pair wave function, which we calculate in real-space. A change of symmetry upon doping, from s-wave at low filling fractions, to d x 2 −y 2 at higher filling fractions, is found. Fermi-surface details are thus important for the symmetry of the superconducting wave function. Symmetry forbids mixing of s-wave and d-wave symmetry in the Cooper-pair wavefunction on a square lattice, unless accidental degeneracies occur. This conclusion also holds for the selfconsistent treatment of the many-body problem, at the critical temperature Tc. Below Tc, we find temperatures which are not critical points, where new superconducting channels open up in the order parameter due to bifurcations in the solutions of the nonlinear gap-equation. We calculate the free energy, entropy, coherence length, critical magnetic fields, and Ginzburg-Landau parameter κ. The model is of the extreme type-II variety. At the temperatures where subdominant channels condense, we find cusps in the internal energy and entropy, as well as as BCS-like discontinuities in the specific heat. The specific heat anomalies are however weaker than at the true superconducting critical point, and argued to be of a different nature.

Physical review, Jul 20, 2018

The superfluid drag-coefficient of a weakly interacting three-component Bose-Einstein condensate ... more The superfluid drag-coefficient of a weakly interacting three-component Bose-Einstein condensate is computed on a square optical lattice deep in the superfluid phase, starting from a Bose-Hubbard model with component-conserving, on-site interactions and nearest-neighbor hopping. At the meanfield level, Rayleigh-Schrödinger perturbation theory is employed to provide an analytic expression for the drag density. In addition, the Hamiltonian is diagonalized numerically to compute the drag within mean-field theory at both zero and finite temperatures to all orders in inter-component interactions. Moreover, path integral Monte Carlo simulations, providing results beyond mean-field theory, have been performed to support the mean-field results. In the two-component case the drag increases monotonically with the magnitude of the inter-component interaction γAB between the two components A and B. The increase is independent of the sign of the inter-component interaction. This no longer holds when an additional third component C is included. Instead of increasing monotonically, the drag can either be strengthened or weakened depending on the details of the interaction strengths, for weak and moderately strong interactions. The general picture is that the drag-coefficient between component A and B is a non-monotonic function of the intercomponent interaction strength γAC between A and a third component C. For weak γAC compared to the direct interaction γAB between A and B, the drag-coefficient between A and B can decrease, contrary to what one naively would expect. When γAC is strong compared to γAB, the drag between A and B increases with increasing γAC , as one would naively expect. We attribute the subtle reduction of ρ d,AB with increasing γAC , which has no counterpart in the two-component case, to a renormalization of the inter-component scattering vertex γAB via intermediate excited states of the third condensate C. We briefly comment on how this generalizes to systems with more than three components.

Physical Review B, May 28, 2008

We present a numerical study of the density of states in a ferromagnet/superconductor junction an... more 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 three magnitudes of the exchange field h compared to the superconducting gap ∆: i) h < ∼ ∆, ii) h ∆, iii) h ≫ ∆. In each case, we consider the effect of the barrier transparency and allow for various concentrations of magnetic impurities. We clarify features that may be expected in the various parameter regimes accessible for the ferromagnetic film, with regard to thickness and exchange field. 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. Our results should serve as a useful tool for quantitative analysis of experimental data. 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 supercurrentswitching devices. We numerically solve for the Josephson current in such a junction to clarify to what extent this idea may be realized in an experimental setup. 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 temperaturedependence of the current. Our theory takes into account vital elements that are necessary to obtain quantitative predictions of the supercurrent in such junctions.

arXiv (Cornell University), Jul 2, 2014

We compare the phase-diagrams of an effective theory of a three-dimensional multi-band supercondu... more We compare the phase-diagrams of an effective theory of a three-dimensional multi-band superconductor obtained within standard and cluster mean-field theories, and in large-scale Monte Carlo simulations. In three dimensions, mean field theory fails in locating correctly the positions of the phase transitions, as well as the character of the transitions between the different states. A cluster mean-field calculations taking into account order-parameter fluctuations in a local environment improves the results considerably for the case of extreme type-II superconductors where gauge-field fluctuations are negligible. The large fluctuations in the multi-component superconducting order parameter originate with strong frustration due to interband Josephson-couplings. A novel chiral metallic phase found in previous works using large scale Monte-Carlo computations, is not obtained either within the single-site mean-field theory or the improved cluster mean-field theory of order parameter fluctuations. In three-dimensional superconductors, this unusual metallic phase originates with gauge-field fluctuations.

Physical Review B, Aug 28, 2008

We investigate numerically the local density of states (LDOS) in the vicinity of a vortex core in... more 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.

Physical review, Jun 9, 2020

Physical Review B, Aug 12, 2010

Motivated by recent proposals for the generation of Majorana fermions in semiconducting hybrid st... more Motivated by recent proposals for the generation of Majorana fermions in semiconducting hybrid structures, we examine possible experimental fingerprints of such excitations. Whereas previous works mainly have focused on zero-energy states in vortex cores in this context, we demonstrate analytically an alternative route to detection of Majorana excitations in semiconducting hybrid structures: interface-bound states that may be probed directly via conductance spectroscopy or STM-measurements. We estimate the necessary experimental parameters required for observation of our predictions.

Physical Review Letters, Apr 10, 2000

The critical properties of a type-II superconductor model are investigated using a dual vortex re... more The critical properties of a type-II superconductor model are investigated using a dual vortex representation. Computing the propagators of gauge field A and dual gauge field h in terms of a vortex correlation function, we obtain the values ηA = 1 and η h = 1 for their anomalous dimensions. This provides support for a dual description of the Ginzburg-Landau theory of type-II superconductors in the continuum limit, as well as for the existence of a stable charged fixed point of the theory, not in the 3DXY universality class.

Physical review, Feb 1, 1998

Monte-Carlo simulations in conjunction with finite-size scaling analysis are used to investigate ... more Monte-Carlo simulations in conjunction with finite-size scaling analysis are used to investigate the (H, T)-phase diagram in uniaxial anisotropic high-Tc superconductors, both in zero magnetic field (B = 0) and in intermediate magnetic fields (0 ≪ B ≪ Bc2) for various mass-anisotropies. The model we consider is the uniformly frustrated anisotropic Villain Model, which is dual to the Lattice London Model with an infinite London penetration length. The quantities we consider are various helicity moduli, the structure function, the specific heat, and the distribution of closed non-field induced vortex loops as a function of the loop-size. In zero magnetic field, and for all anisotropies considered, we find one single second order phase transition, mediated by an Onsager vortex-loop unbinding transition, or blowout. This is the superconductor-normal metal transition. A comparison with numerical simulations and a critical scaling analysis of the zero-field loop-transition yields the same exponent of the loop-distribution function at the critical point. In the intermediate magnetic field regime, we find two anomalies in the specific heat. The first anomaly at a temperature Tm is associated with the melting transition of the flux-line lattice. The Lindemann-ratio at the melting is given by cL ≈ 0.24. The second anomaly at a temperature Tz is one where phase coherence in the BCS order parameter across the sample along the field direction is destroyed. We argue that Tm = Tz in the thermodynamic and continuum limit. Hence, there is no regime where the flux-line lattice melts into a disentangled flux-line liquid. The loss of phase coherence parallel to the magnetic field in the sample is argued to be due to the proliferation of closed non-field induced vortex loops on the scale of the magnetic length in the problem, resulting in flux-line cutting and recombination. In the flux-line liquid phase, therefore, flux-lines appear no longer to be well defined entities. Above the melting temperature, the system always exhibits an incoherent vortex-liquid phase characterized by lack of phase coherence in the BCS order parameter parallel to the magnetic field. For increasing anisotropy, we resolve a delta-function peak in the specific heat. A finite-size scaling analysis of the delta-function peak specific heat anomaly at the melting transition is used to extract the discontinuity of the entropy at the melting transition. This entropy discontinuity is found to increase rapidly with mass-anisotropy, at least for not too layered compounds.