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Jacob Linder

Publications and source records attributed to Jacob Linder.

At least 55 records · Page 3Linked to original sources
arXiv2024

Thermoelectric Effect in Altermagnet-Superconductor Junctions

We propose altermagnet-superconductor junctions as a way to achieve a thermoelectric response in metals free of external or stray magnetic fields. We combine qualitative analysis in a simplified model with a more rigorous approach based on the inverse proximity effect in the functional-integral formulation. We show that coupling an altermagnet to a superconductor in a bilayer induces a momentum-dependent spin-splitting in the superconductor. When tunneling occurs between this bilayer and a different altermagnet, a spin-dependent particle-hole symmetry breakdown arises in the transport, which leads to a thermoelectric response. Our results show that the altermagnet-superconductor junctions may achieve comparable thermoelectric performance to ferromagnet-superconductor junctions, featuring a nonmonotonic dependence of the figure of merit on the strength of the altermagnetic splitting. We also point out an often overlooked fact regarding the inverse proximity effect in superconductors, namely that even in a normal metal-superconductor junction there is a minigap in the superconductor, which gives rise to a four-peak structure in the DOS reminiscent of spin-split superconductors. Our results show that altermagnetic metals, unlike conventional antiferromagnets, can be used for efficient cryogenic thermoelectricity.

cond-mat.supr-con↗
arXiv2024

Superconductor-altermagnet memory functionality without stray fields

A novel class of antiferromagnets, dubbed altermagnets, exhibit a non-relativistically spin-split band structure reminiscent of $d$-wave superconductors, despite the absence of net magnetization. This unique characteristic enables utilization in cryogenic stray-field-free memory devices, offering the possibility of achieving high storage densities. In this Letter, we determine how a proximate altermagnet influences the critical temperature $T_c$ of a conventional $s$-wave singlet superconductor. Considering both a bilayer and trilayer, we show that such hybrid structures may serve as stray-field free memory devices where the critical temperature is controlled by rotating the Néel vector of one altermagnet, providing infinite magnetoresistance. Furthermore, our study reveals that altermagnetism can coexist with superconductivity up to a critical strength of the altermagnetic order as well as robustness of the altermagnetic influence on the conduction electrons against non-magnetic impurities, ensuring the persistence of the proximity effect under realistic experimental conditions.

cond-mat.supr-con↗
arXiv2024

Critical temperature of triplet superconductor-ferromagnet bilayers as a probe for pairing symmetry

Identifying superconducting materials with spin-polarized Cooper pairs is an important objective both for exploration of new fundamental physics and for cryogenic applications in spintronics and quantum sensing. We here compute the critical temperature $T_c$ of the superconducting transition in a bilayer comprised of a superconductor with an intrinsic spin-triplet order parameter and a ferromagnet. We determine how $T_c$ varies both with the thickness of the ferromagnet and its magnetization direction. We show that both the orbital and spin part of the triplet superconducting order parameter leave clear signatures in $T_c$ which do not appear in a bilayer of a conventional s-wave superconductor and a ferromagnet. In particular, the dependence of $T_c$ on these variables changes depending on whether or not the superconducting order parameter features Andreev bound-states and also changes qualitatively when the magnetization is rotated in the plane of the ferromagnetic film. Measurements of $T_c$ in such bilayers are therefore useful to identify the pairing symmetry of intrinsic triplet superconductors.

cond-mat.supr-con↗
arXiv2024

RKKY interaction in triplet superconductors: Dzyaloshinskii-Moriya-type interaction mediated by spin-polarized Cooper pairs

The Ruderman--Kittel--Kasuya--Yosida (RKKY) interaction governs the coupling between localized spins and is strongly affected by the environment in which these spins reside. In superconductors, this interaction becomes long-ranged and provides information about the orbital symmetry of the superconducting order parameter. In this work, we consider the RKKY interaction between localized spins mediated by $p$-wave triplet superconductors. In contrast to the well-studied RKKY interaction in $d$-wave superconductors, we find that the spin of the Cooper pair in a triplet state also modulates the spin--spin coupling. We consider several different types of $p$-wave triplet states, and find that the form of the RKKY interaction changes significantly with the symmetries of the order parameter. For non-unitary superconducting states, two new terms appear in the RKKY interaction: a background spin magnetization coupling to the individual spins and, more interestingly, an effective Dzyaloshinskii--Moriya term. The latter term oscillates with the separation distance between the impurity spins. Finally, we find that the finite spin expectation value in non-unitary superconductors in concert with the conventional RKKY interaction can lead to non-collinear magnetic ground states even when the Dzyaloshinskii--Moriya term is negligible. The RKKY interaction in $p$-wave triplet superconductors thus offers a way to achieve new ground state spin configurations of impurity spins and simultaneously provides information about the underlying superconducting state.

cond-mat.supr-con↗
arXiv2023

dc Josephson Effect in Altermagnets

The ability of magnetic materials to modify superconducting systems is an active research area for possible applications in thermoelectricity, quantum sensing, and spintronics. We consider the fundamental properties of the Josephson effect in a third class of magnetic materials beyond ferromagnets and antiferromagnets: altermagnets. We show that despite having no net magnetization, altermagnets induce $0$-$π$ oscillations. The decay length and oscillation period of the Josephson coupling are qualitatively different from ferromagnetic junctions and depend on the crystallographic orientation of the altermagnet. The Josephson effect in altermagnets thus serves a dual purpose: it acts as a signature that distinguishes altermagnetism from conventional (anti)ferromagnetism and offers a way to tune the supercurrent via flow direction anisotropy.

cond-mat.supr-con↗
arXiv2023

Supercurrent-induced spin switching via indirect exchange interaction

Localized spins of single atoms adsorbed on surfaces have been proposed as building blocks for spintronics and quantum computation devices. However, identifying a way to achieve current-induced switching of spins with very low dissipation is an outstanding challenge with regard to practical applications. Here, we show that the indirect exchange interaction between spin impurities can be controlled by a dissipationless supercurrent. All that is required is a conventional superconductor and two spin impurities placed on its surface. No triplet Cooper pairs or exotic material choices are needed. This finding provides a new and accessible way to achieve the long-standing goal of supercurrent-induced spin switching.

cond-mat.mes-hall↗
arXiv2023

Spin pumping from a ferromagnetic insulator into an altermagnet

A class of antiferromagnets with spin-polarized electron bands, yet zero net magnetization, called altermagnets is attracting increasing attention due to their potential use in spintronics. Here, we study spin injection into an altermagnet via spin pumping from a ferromagnetic insulator. We find that the spin pumping behaves qualitatively different depending on how the altermagnet is crystallographically oriented relative the interface to the ferromagnetic insulator. The altermagnetic state can enhance or suppress spin pumping, which we explain in terms of spin-split altermagnetic band structure and the spin-flip probability for the incident modes. Including the effect of interfacial Rashba spin-orbit coupling, we find that the spin-pumping effect is in general magnified, but that it can display a non-monotonic behavior as a function of the spin-orbit coupling strength. We show that there exists an optimal value of the spin-orbit coupling strength which causes an order of magnitude increase in the pumped spin current, even for the crystallographic orientation of the altermagnet which suppresses the spin pumping.

cond-mat.mes-hall↗
arXiv2023

Barrier and finite size effects on the extension of topological surface-states into magnetic insulators

The interplay between magnetic and topological order can give rise to phenomena such as the quantum anomalous Hall effect. The extension of topological surface states into magnetic insulators (MIs) has been proposed as an alternative to using intrinsically magnetic topological insulators (TIs). Here, we theoretically study how this extension of surface states into a magnetic insulator are influenced both by the interface barrier potential separating a topological insulator and a magnetic insulator and by finite size effects in such structures. We find that the the gap in the surface states depends non-monotonically on the barrier strength. A small, but finite, barrier potential turns out to be advantageous as it permits the surface states to penetrate even further into the MI. Moreover, we find that due to finite size effects in thin samples, increasing the spin-splitting in the MI can actually decrease the gap of the surface states, in contrast to the usual expectation that the gap opens as the spin-splitting increases.

cond-mat.mes-hall↗
arXiv2023

Converting a triplet Cooper pair supercurrent into a spin-signal

Superconductivity with spin-polarized Cooper pairs is known to emerge by combining conventional spinless superconductors with materials that have spin-dependent interactions, such as magnetism and spin-orbit coupling. This enables a dissipationless and conserved flow of spin. However, actually utilizing the spin-polarization of such supercurrents have proven challenging. Here, we predict an experimental signature of current-carrying triplet Cooper pairs in the form of an induced spin-signal. We show that a supercurrent carried only by triplet Cooper pairs induces a non-local magnetization that is controlled by the polarization direction of the triplet Cooper pairs. This provides a measurement protocol to directly use the spin-polarization of the triplet Cooper pairs in supercurrents to transfer spin information in a dissipationless manner.

cond-mat.supr-con↗
arXiv2023

Observation of magnetic state dependent thermoelectricity in superconducting spin valves

Superconductor-ferromagnet tunnel junctions demonstrate giant thermoelectric effects which are being exploited to engineer ultra-sensitive terahertz radiation detectors. Here, we experimentally observe the recently predicted complete magnetic control over thermoelectric effects in a superconducting spin valve, including the dependence of its sign on the magnetic state of the spin valve. The description of the experimental results is improved by the introduction of an interfacial domain wall in the spin filter layer interfacing the superconductor. Surprisingly, the application of high in-plane magnetic fields induces a double sign inversion of the thermoelectric effect, which exhibits large values even at applied fields twice the superconducting critical field.

cond-mat.supr-con↗
arXiv2023

Spin-pumping from a ferromagnetic insulator to an unconventional superconductor with interfacial Andreev bound-states

Spin-pumping from a ferromagnetic insulator into a high-$T_c$ superconductor with a $d$-wave superconducting order parameter has recently been experimentally observed. Such unconventional superconducting order is known to produce interfacial bound-states for certain crystallographic orientations. Here, we present a methodology which can be used to study spin-pumping into unconventional superconductors, including the role of interfacial bound-states. As an example, we determine how the crystallographic orientation of the $d$-wave order parameter relative the interface changes the spin-pumping effect. We find that the spin-pumping effect is slightly enhanced at low temperatures for orientations hosting interfacial bound-states compared to other superconducting states. However, the spin-pumping effect does not show a coherence peak close to $T_c$ for such orientations, and instead remains smaller than the normal state value for all $T$. For orientations not hosting interfacial bound-states, we find that the pumped spin current can be increased to several times the normal-state spin current at frequencies that are small compared to the superconducting gap. Our results show that the spin-pumping dependency on frequency and temperature changes qualitatively depending on the crystallographic orientation of unconventional superconducting order parameters relative the interface.

cond-mat.supr-con↗
arXiv2023

Andreev reflection in altermagnets

Recent works have predicted materials featuring bands with a large spin-splitting distinct from ferromagnetic and relativistically spin-orbit coupled systems. Materials displaying this property are known as altermagnets and feature a spin-polarized band structure reminiscent of a $d$-wave superconducting order parameter. We here consider the contact between an altermagnet and a superconductor and determine how the altermagnetism affects the fundamental process of Andreev reflection. We show that the resulting charge conductance depends strongly on the interfacial orientation of the altermagnet relative to the superconductor, displaying features similar to normal metals or ferromagnets. The zero-bias conductance peak present at the interface in the $d$-wave case are robust toward the presence of an altermagnetic interaction. Moreover, the spin conductance strongly depends on the orientation of the altermagnet relative the interface. These results show how the anisotropic altermagnetic state can be probed by conductance spectroscopy and how it offers voltage control over charge and spin currents that are modulated due to superconductivity.

cond-mat.supr-con↗
arXiv2023

Transient dynamics and quantum phase diagram for the square lattice Rashba-Hubbard model at arbitrary hole doping

Adding a Rashba term to the Hubbard Hamiltonian produces a model which can be used to learn how spin-orbit interactions impact correlated electrons on a lattice. Previous works have studied such a model using a variety of theoretical frameworks, mainly close to half-filling. In this work, we determine the magnetic phase-diagram for the Rashba-Hubbard model for arbitrary hole doping using a sine square deformed lattice mean-field model with an unrestricted ansatz, thus suppressing finite size effects and allowing for inhomogeneous order. We find that the introduction of Rashba spin-orbit coupling significantly alters the ground state properties of the Hubbard model and we observe an increasing complexity of the ground state phase composition for increasing spin-orbit strength. We also introduce a gradual deformed envelope (GDE) technique building on the sine square methodology to facilitate convergence towards ordered and defect-free ground state configurations which is a challenge with the unrestricted ansatz at high interaction strengths. We observe that the use of the GDE technique significantly lowers the free energy of the obtained configurations. Moreover, we consider transient dynamics in the Rashba-Hubbard model by quenching the interaction strength. We find that the quench dynamics within a sine-square methodology allows for the simulation of quasi-open systems by using the zero-energy edge states as a particle reservoir. Interaction quenches at half-filling show a tendency towards quench-induced spatial spin-magnitude inhomogeneity and a non-equilibrium system magnetization lower than equilibrium predictions, possibly related to a build-up of non-local correlations on the lattice.

cond-mat.str-el↗
arXiv2023

On-off switch and sign change for non-local Josephson diode in spin-valve Andreev molecules

Andreev molecules consist of two coherently coupled Josephson junctions and permit non-local control over supercurrents. By making the barriers magnetic and thus creating a spin-valve, we predict that a non-local Josephson diode effect occurs that is switchable via the magnetic configuration of the barriers. The diode effect is turned on, off, or changes its sign depending on whether the spin-valve is in a parallel, normal, or antiparallel configuration. These results offer a way to exert complete control over a non-local Josephson diode effect via the spin degree of freedom rather than varying a global magnetic flux which affects the entire system and likely neighbouring components in a device architecture.

cond-mat.supr-con↗
arXiv2023

Superconducting proximity effect and long-ranged triplets in dirty metallic antiferromagnets

Antiferromagnets have no net spin-splitting on the scale of the superconducting coherence length. Despite this, antiferromagnets have been observed to suppress superconductivity in a similar way as ferromagnets, a phenomenon that still lacks a clear understanding. We find that this effect can be explained by the role of impurities in antiferromagnets. Using quasiclassical Green's functions, we study the proximity effect and critical temperature in diffusive superconductor-metallic antiferromagnet bilayers. The non-magnetic impurities acquire an effective magnetic component in the antiferromagnet. This not only reduces the critical temperature but also separates the superconducting correlations into short-ranged and long-ranged components, similar to ferromagnetic proximity systems.

cond-mat.supr-con↗
arXiv2023

Quasiclassical theory for antiferromagnetic metals

Unlike ferromagnetism, antiferromagnetism cannot readily be included in the quasiclassical Keldysh theory because of the rapid spatial variation in the directions of the magnetic moments. The quasiclassical framework is useful because it separates the quantum effects occurring at length scales comparable to the Fermi wavelength from other length scales, and has successfully been used to study a wide range of phenomena involving both superconductivity and ferromagnetism. Starting from a tight-binding Hamiltonian, we develop general quasiclassical equations of motion and boundary conditions, which can be used to describe two-sublattice metallic antiferromagnets in the dirty limit. The boundary conditions are applicable also for spin-active boundaries that can be either compensated or uncompensated. Additionally, we show how nonuniform or dynamic magnetic textures influence the equations and we derive a general expression for observables within this framework.

cond-mat.supr-con↗
arXiv2022

Giant magnetoanisotropy in the Josephson effect and switching of staggered order in antiferromagnets

We predict that the amplitude of the Josephson current through an antiferromagnetic weak link changes by several orders of magnitude upon rotation of the Néel order parameter characterizing the staggered magnetic order. This occurs due the presence of spin-orbit coupling arising from structural inversion asymmetry which makes the band gap in the antiferromagnet highly sensitive to the staggered order parameter direction. We also demonstrate that when phase-biasing the junction, magnetization dynamics is induced which switches the direction of the Néel vector in the antiferromagnet. These results reveal an interesting versatility of antiferromagnetic Josephson junctions as they offer both a large tunability of the supercurrent magnitude via the staggerered magnetization and phase-coherent control over the Néel order parameter.

cond-mat.supr-con↗
arXiv2022

Effective quasiclassical models for odd-frequency superconductivity: energy-symmetry, preserved spectral weight, and Meissner response

The odd-frequency superconducting state appears generally in hybrid structures consisting of conventional superconductors and other materials, and features electrons that form temporally non-local Cooper pairs. The quasiclassical theory of superconductivity has been extensively used to model such systems, finding in many cases excellent agreement with experimental measurements. Therefore, it is of interest to study effective models of odd-frequency superconductivity to predict new phenomena associated with this form of unconventional pairing. We establish necessary criteria that the quasiclassical Green functions in odd-frequency superconducting systems in the dirty limit must satisfy in order to be physically reasonable, including conservation of spectral weight. We show that it is possible to write down effective models which satisfy all the abovementioned criteria, but which still predict different behavior when it comes to the density of states and the magnetic response of the superconductor. For instance, an effective model for the odd-frequency anomalous Green function that gives a conserved spectral weight can yield either a peaked or gapped density of states at the Fermi energy, and exhibit conventional, zero, or unconventional Meissner response. This finding demonstrates the importance of carefully considering the properties of effective models describing odd-frequency superconductivity in order to obtain physically reasonable results.

cond-mat.supr-con↗