arXiv subjects

A. Sudbø

Publications and source records attributed to A. Sudbø.

16 recordsLinked to original sources

arXiv2014

Pressure-Induced $0-π$ Transitions and Supercurrent Crossover in Antiferromagnetic Weak Links

We compute self-consistently the Josephson current in a superconductor-antiferromagnet-superconductor junction using a lattice model, focusing on $0-π$ transitions occurring when the width of the antiferromagnetic region changes from an even to an odd number of lattice sites. Previous studies predicted $0-π$ transitions when alternating between an even and an odd number of sites for sufficiently strong antiferromagnetic order. We study numerically the magnitude of the threshold value for this to occur, and also explain the physics behind its existence in terms of the phase-shifts picked up by the quasiparticles constituting the supercurrent in the antiferromagnet. Moreover, we show that this threshold value allows for \textit{pressure-induced $0-π$ transitions} by destroying the antiferromagnetic nesting properties of the Fermi surface, a phenomenon which has no counterpart in ferromagnetic Josephson junctions, {offering a new way to tune the quantum ground state of a Josephson junction without the need of multiple samples.

cond-mat.supr-con↗

arXiv2009

Spin-Transfer Torque and Magnetoresistance in Superconducting Spin-Valves

We study the spin-transfer torque and magnetoresistance of a ferromagnet$\mid$superconductor$\mid$ferromagnet spin-valve, allowing for an arbitrary magnetization misorientation and treating both s-wave and d-wave symmetries of the superconductor. We take fully into account Andreev reflection and also the spin-triplet correlations that are generated when the magnetizations are non-collinear. It is found that the torque and magnetoresistance are both strongly enhanced when topological zero-energy states are present at the interfaces, which is the case for d-wave superconductors with a crystallographic orientation of [110] relative to the interface ($d_{xy}$-wave symmetry). Moreover, we find that the magnetoresistance displays a strong oscillatory and non-monotonous behavior as a function of $d_S/ξ$ where $d_S$ and $ξ$ are the interlayer width of the superconducting region and the superconducting coherence length, respectively. This feature is also attributed to the crossover from layers of size $d_S\sim 2ξ$ to layers of size $d_S\gg 2ξ$, where the contribution to transport from zero-energy states gradually vanishes.

cond-mat.supr-con↗

arXiv2008

Quantum transport in noncentrosymmetric superconductors and thermodynamics of ferromagnetic superconductors

We consider a general Hamiltonian describing coexistence of itinerant ferromagnetism, spin-orbit coupling and mixed spin-singlet/triplet superconducting pairing in the context of mean-field theory. The Hamiltonian is diagonalized and exact eigenvalues are obtained, thus allowing us to write down the coupled gap equations for the different order parameters. Our results may then be applied to any model describing coexistence of any combination of these three phenomena. As a specific application of our results, we consider tunneling between a normal metal and a noncentrosymmetric superconductor with mixed singlet and triplet gaps. The conductance spectrum reveals information about these gaps in addition to how the influence of spin-orbit coupling is manifested. We also consider the coexistence of itinerant ferromagnetism and triplet superconductivity as a model for recently discovered ferromagnetic superconductors. The coupled gap equations are solved self-consistently, and we study the conditions necessary to obtain the coexistent regime of ferromagnetism and superconductivity. Analytical expressions are presented for the order parameters, and we provide an analysis of the free energy to identify the preferred system state. Moreover, we make specific predictions concerning the heat capacity for a ferromagnetic superconductor. In particular, we report a nonuniversal relative jump in the specific heat, depending on the magnetization of the system, at the uppermost superconducting phase transition. [Shortened abstract due to arXiv submission.]

cond-mat.supr-con↗

arXiv2007

Josephson effect in thin-film superconductor/insulator/superconductor junctions with misaligned in-plane magnetic fields

We study a tunnel junction consisting of two thin-film s-wave superconductors separated by a thin, insulating barrier in the presence of misaligned in-plane exchange fields. We find an interesting interplay between the superconducting phase difference and the relative orientation of the exchange fields, manifested in the Josephson current across the junction. Specifically, this may be written $I_\text{J}^\text{C} = (I_0+I_m ~ \cosϕ) \sinΔθ$, where I_0 and I_m are constants, and $ϕ$ is the relative orientation of the exchange fields while $Δθ$ is the superconducting phase difference. Similar results have recently been obtained in other S/I/S junctions coexisting with helimagnetic or ferromagnetic order. We calculate the superconducting order parameter self-consistently, and investigate quantitatively the effect which the misaligned exchange fields constitute on the Josephson current, to see if I_m may have an appreciable effect on the Josephson current. It is found that I_0 and I_m become comparable in magnitude at sufficiently low temperatures and fields close to the critical value, in agreement with previous work. From our analytical results, it then follows that the Josephson current in the present system may be controlled in a well-defined manner by a rotation of the exchange fields on both sides of the junction. We discuss a possible experimental realization of this proposition.

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arXiv2007

Dirac-fermions and conductance-oscillations in (s,d)-wave superconductor/normal graphene junctions

We investigate quantum transport in a normal/superconductor graphene heterostructure, including the possibility of an anisotropic pairing potential in the superconducting region. We find that under certain circumstances, the conductance displays an undamped, oscillatory behaviour as a function of applied bias voltage. Also, we investigate how the conductance spectra are affected by a d-wave pairing symmetry. These results combine unusual features of the electronic structure of graphene with the unconventional pairing symmetry found for instance in high-T_c superconductors.

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arXiv2007

Signatures of retroreflection and induced triplet electron-hole correlations in ferromagnet/s-wave superconductor structures

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.

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arXiv2006

Conductance spectra of ferromagnetic superconductors: Quantum transport in a ferromagnetic metal/non-unitary ferromagnetic superconductor junction

Recent findings of superconductors that simultaneously exhibit multiple spontaneously broken symmetries, such as ferromagnetic order or lack of an inversion center and even combinations of such broken symmetries, have led to much theoretical and experimental research. We consider quantum transport in a junction consisting of a ferromagnetic metal and a non-unitary ferromagnetic superconductor. It is shown that the conductance spectra provides detailed information about the superconducting gaps, and is thus helpful in determining the pairing symmetry of the Cooper pairs in ferromagnetic superconductor.

cond-mat.str-el↗

arXiv2006

The two dimensional Antiferromagnetic Heisenberg model with next nearest neighbour Ising exchange

We have considered the $S=1/2$ antiferromagnetic Heisenberg model in two dimensions, with an additional Ising \nnn interaction. Antiferromagnetic \nnn interactions will lead to frustration, and the system responds with flipping the spins down in the $xy$ plane. For large next nearest neighbour coupling the system will order in a striped phase along the z axis, this phase is reached through a first order transition. We have considered two generalizations of this model, one with random \nnn interactions, and one with an enlarged unit cell, where only half of the atoms have \nnn interactions. In both cases the transition is softened to a second order transition separating two ordered states. In the latter case we have estimated the quantum critical exponent $β\approx 0.25$. These two cases then represent candidate examples of deconfined quantum criticality.

cond-mat.str-el↗

arXiv2006

Tunnelling between non-centrosymmetric superconductors with significant spin-orbit splitting studied theoretically within a two-band treatment

Tunnelling between non-centrosymmetric superconductors with significant spin-orbit splitting is studied theoretically in a two-band treatment of the problem. We find that the critical Josephson current may be modulated by changing the relative angle between the vectors describing absence of inversion symmetry on each side of the junction. The presence of two gaps also results in multiple steps in the quasiparticle current-voltage characteristics. We argue that both these effects may help to determine the pairing states in materials like CePt$_3$Si, UIr and Cd$_2$Re$_2$O$_7$. We propose experimental tests of these ideas, including scanning tunnelling microscopy.

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arXiv2002

Criticality in the 2+1-dimensional compact Higgs model and fractionalized insulators

We use a novel method of computing the third moment M_3 of the action of the 2+1-dimensional compact Higgs model in the adjoint representation with q=2 to extract correlation length and specific heat exponents nu and alpha, without invoking hyperscaling. Finite-size scaling analysis of M_3 yields the ratio (1+alpha)/nu and 1/nu separately. We find that alpha and nu vary along the critical line of the theory, which however exhibits a remarkable resilience of Z_2 criticality. We propose this novel universality class to be that of the quantum phase transition from a Mott-Hubbard insulator to a charge-fractionalized insulator in two spatial dimensions.

cond-mat.str-el↗

arXiv1999

Anomalous scaling dimensions and critical points in type-II superconductors

The existence of a {\it stable critical point}, separate from the Gaussian and XY critical points, of the Ginzburg-Landau theory for superconductors, is demonstrated by direct extraction via Monte-Carlo simulations, of a negative anomalous dimension $η_ϕ$ of a complex scalar field $ϕ$ forming a dual description of a neutral superfluid. The dual of the neutral superfluid is isomorphic to a charged superfluid coupled to a massless gauge-field. The anomalous scaling dimension of the superfluid order-field is positive, while we find that the anomalous dimension of the dual field is negative. The dual gauge-field does not decouple from the dual complex matter-field at the critical point. {\it These two critical theories represent separate fixed points.} The physical meaning of a negative $η_ϕ$ is that the vortex-loop tangle of the superfluid at the critical point fills space {\it more} efficiently than random walkers, {\it without collapsing}. This is due to the presence of the massless dual gauge-field, and the resulting long-ranged {\it vectorial} Biot-Savart interaction between vortex-loop segments, which is a relevant perturbation to the steric $|ψ|^4$ repulsion term. Hence, the critical dual theory is not in the universality class of the $|ψ|^4$-theory.

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arXiv1999

Topological phase-fluctuations, amplitude fluctuations, and criticality in extreme type-II superconductors

We study the effect of critical fluctuations on the $(B,T)$ phase diagram in extreme type-II superconductors in zero and finite magnetic field using large-scale Monte Carlo simulations on the Ginzburg-Landau model in a frozen gauge approximation. We show that a vortex-loop unbinding gives a correct picture of the zero field superconducting-normal transition even in the presence of amplitude fluctuations, which are far from being critical at $T_c$. We extract critical exponents of the dual model by studying the topological excitations of the original model. From the vortex-loop distribution function we extract the anomalous dimension of the dual field $η\simeq -0.18$, and conclude that the charged Ginzburg-Landau model and the neutral 3DXY model belong to different universality classes. We find are two distinct scaling regimes for the vortex-line lattice melting line: a high-field scaling regime and a distinct low-field 3DXY critical scaling regime. We also find indications of an abrupt change in the connectivity of the vortex-tangle in the vortex liquid along a line $T_L \geq T_M$. This is the finite field counter-part of the zero-field vortex-loop blowout. Which at low enough fields appears to coincide with $T_M$. Here, a description of the vortex system only in terms of field induced vortex lines is inadequate at and above the VLL melting temperature.

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arXiv1999

A new broken U(1)-symmetry in extreme type-II superconductors

A phase transition within the molten phase of the Abrikosov vortex system without disorder in extreme type-II superconductors is found via large-scale Monte-Carlo simulations. It involves breaking a U(1)-symmetry, and has a zero-field counterpart, unlike vortex lattice melting. Its hallmark is the loss of number-conservation of connected vortex paths threading the entire system {\it in any direction}, driving the vortex line tension to zero. This tension plays the role of a generalized ``stiffness'' of the vortex liquid, and serves as a probe of the loss of order at the transition, where a weak specific heat anomaly is found.

cond-mat.supr-con↗

arXiv1998

Phase-coherence and the boson-analogy of vortex liquids

The statistical mechanics of the flux-line lattice in extreme type-II super- conductors is studied within the framework of the uniformly frustrated anisotropic 3D XY-model. A finite-field counterpart of an Onsager vortex-loop transition in extreme type-II superconductors renders the vortex liquid phase- incoherent when the Abrikosov vortex lattice undergoes a first order melting transition. For the filling fractions considered in this paper $f$ given by $1/f=12,14,16,20,25,32,48,64,72,84,96,112$, and 128, the vortex liquid phase is not describable as a liquid of well-defined field induced vortex lines. This is due to the proliferation of thermally induced closed vortex-loops with dia- meters of order the magnetic length in the problem, resulting in a "percolation transition" driven by non-field induced vortices also transverse to the direc- tion of the applied magnetic field. This immediately triggers flux-line lattice melting and loss of phase-coherence in all direction. In a non-relativistic 2D boson-analogy picture, this latter feature would correspond to a vanishing mass of the bosons. Scaling functions for the specific heat are calculated in zero and finite magnetic field. From this we conclude that the critical region is of order 10% of $T_c$ for a mass anisotropy $\sqrt{M_z/M}=3$, and increases with increasing mass anisotropy. The entropy jump at the melting transition is cal- culated as a function of magnetic field for a mass-ansitropy slightly lower than that in $YBCO$, and found to be $ΔS=0.1k_B$ per vortex panncake, independent of the magnetic field for the filling fractions considered here. This is slightly lower than experimental values of $ΔS \approx 0.4k_B$ found experimentally for $YBCO$. We attribute this to the slightly lower mass anisotropy used in our simulations.

cond-mat.supr-con↗

arXiv1997

Onsager Loop-Transition and First Order Flux-Line Lattice Melting in High-$T_c$ Superconductors

Monte-Carlo simulations in conjunction with finite-size scaling analysis are used to investigate the $(H,T)$-phase diagram in uniaxial anisotropic high- $T_c$ superconductors, both in zero magnetic field and in intermediate magnetic fields for various mass-anisotropies. The model we consider is the uniformly frustrated anisotropic Villain Model. In zero magnetic field, and for all anisotropies considered, we find one single second order phase transition, mediated by an Onsager vortex-loop 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 $T_m$ is associated with the melting transition of the flux-line lattice. The second anomaly at a temperature $T_z$ is one where phase coherence along the field direction is destroyed. We argue that $T_m=T_z$ 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. 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.

cond-mat↗

arXiv1997

Thermodynamics of superconducting lattice fermions

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 $C_{4v}$ 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 $T_c$. Below $T_c$, 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.

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