arXiv subjects

Jacob Linder

Publications and source records attributed to Jacob Linder.

At least 37 records · Page 2Linked to original sources

arXiv2025

The fate of $p$-wave spin polarization in helimagnets with Rashba spin-orbit coupling

It has recently been realized that magnetic systems with coplanar magnetic order that are invariant under the combined operation of time-reversal and translation with half a unit cell feature energy bands with a symmetry-protected p-wave spin polarization. Such $p$-wave magnets are a sought-after spin analogy of unconventional triplet superconducting pairing and show promise for use in spintronics. Metallic helimagnets provide a realization of $p$-wave magnetism, but such order often occurs in systems lacking inversion symmetry so that Rashba spin-orbit interactions can be prominent. An important question is therefore how the magnitude and the existence of $p$-wave spin polarization is affected by Rashba spin-orbit interaction. Here, we prove that while the $p$-wave symmetry of the spin-polarized bands is strictly speaking removed by such spin-orbit interactions in helimagnets unless the period of the helix is fine-tuned, the actual quantitative deviation from $p$-wave symmetry is extremely weak unless the period of the helix is only a few lattice sites. Thereafter, we show that the $p$-wave magnetism becomes completely robust in pairs of antiferromagnetically coupled helices. More precisely, the $p$-wave spin-polarization of the bands then appears regardless of the periodicity and regardless of the strength of the spin-orbit interactions. This shows that antiferromagnetically coupled helimagnetic chains produce robust $p$-wave spin polarization free of fine-tuning requirements, making them attractive for potential spintronic applications.

cond-mat.mes-hall↗

arXiv2025

Anisotropic light-tailored RKKY interaction in two-dimensional $d$-wave altermagnets

Altermagnets are known in spintronics for their intrinsic spin-splitting and unconventional magnetic responses, particularly to magnetic impurities. However, effectively controlling the magnetic exchange interactions in altermagnets is challenging for practical applications. Here, we propose using circularly polarized light to tune the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction in two-dimensional $d$-wave altermagnets. Using the real-space retarded Green's functions approach, our results show that while the Heisenberg and Ising exchanges dominate, a notable Dzyaloshinskii-Moriya (DM) interaction also plays a key role. Furthermore, the inherent strength of altermagnetism imprints chirp-like signatures into the magnetic responses, which can be dynamically tuned via light. We mainly demonstrate that gate-induced Rashba spin-orbit coupling is essential in response to light -- light selectively and anisotropically adjusts the DM interaction without affecting the other exchanges. Our findings further indicate that rotating the altermagnet by $45^\circ$ relative to the light's polarization direction generates a Dirac-like dispersion and different DM interactions. We finally extract critical thresholds where light reverses DM interactions along one axis or balances both in-plane components. The anisotropic light-driven control of RKKY interactions in 2D altermagnets not only highlights their unique properties but also opens new avenues for engineering tailored magnetic characteristics in spintronic applications.

cond-mat.mes-hall↗

arXiv2025

Interfacial spin-orbit-coupling-induced strong spin-to-charge conversion at an all-oxide ferromagnetic /quasi-two-dimensional electron gas interface

Functional oxides and hybrid structures with interfacial spin orbit coupling and the Rashba-Edelsterin effect (REE) are promising materials systems for thermal tolerance spintronic device applications. Here, we demonstrate efficient spin-to-charge conversion through enhanced interfacial spin orbit coupling at the all-oxide interface of La1-xCaxMnO3 with quasi-two-dimensional (quasi-2D) SrTiO3 (LCMO/STO). The quasi-2D interface is generated via oxygen vacancies at the STO surface. We obtain a spin-to-charge conversion efficiency of ~ 2.32 +- 1.3 nm, most likely originating from the inverse REE, which is relatively large versus all-metallic spin-to-charge conversion materials systems. The results highlight that the LCMO/STO 2D electron gas is a potential platform for spin-based memory and transistor applications.

cond-mat.mes-hall↗

arXiv2024

Dzyaloshinskii-Moriya-type spin-spin interaction from mixed-parity superconductivity

Interacting impurity spins adsorbed on surfaces have been suggested as basic components for applications in quantum computation and spintronics. Such spins usually prefer a parallel or antiparallel configuration, but weakly non-collinear alignments are possible due to the Dzyaloshinskii-Moriya interaction (DMI) that arises in the presence of relativistic spin-orbit coupling. Here, we show that an effective Dzyaloshinskii-Moriya-type interaction (DMTI) can emerge purely from superconducting correlations without any spin-orbit interaction. We give an analytical proof and provide a numerical study which shows that DMTI arises in mixed-parity superconductors solely from the superconducting pairing. Moreover, we show that the same effect can be realized in Josephson junctions between s-wave and p-wave superconductors, where a phase bias toggles the DMTI entirely on and off. These results enable a way to engineer spin textures using superconducting order.

cond-mat.supr-con↗

arXiv2024

Minimal Models and Transport Properties of Unconventional $p$-Wave Magnets

New unconventional compensated magnets with a $p$-wave spin polarization protected by a composite time-reversal translation symmetry have been proposed in the wake of altermagnets. To facilitate the experimental discovery and applications of these unconventional magnets, we construct an effective analytical model. The effective model is based on a minimal tight-binding model for unconventional $p$-wave magnets that clarifies the relation to other magnets with $p$-wave spin-polarized bands. One of the most prominent advantages of our analytical model is the possibility to employ various analytical approaches while capturing essential features of $p$-wave magnets. We illustrate the effective model by evaluating the tunneling conductance in junctions with $p$-wave magnets, revealing a large magnetoresistance, spin filtering, and anisotropic bulk spin conductivity beyond linear response despite the absence of a net magnetization. These results show that unconventional $p$-wave magnets offer several useful functionalities, broadening the material selection for spintronics devices.

cond-mat.mes-hall↗

arXiv2024

Crossed Andreev reflection revealed by self-consistent Keldysh-Usadel formalism

Crossed Andreev reflection (CAR) is a process that creates entanglement between spatially separated electrons and holes. Such entangled pairs have potential applications in quantum information processing, and it is therefore relevant to determine how the probability for CAR can be increased. CAR competes with another non-local process called elastic cotunneling (EC), which does not create entanglement. In conventional normal metal/superconductor/normal metal heterostructures, earlier theoretical work predicted that EC dominates over CAR. Nevertheless, we show numerically that when the Keldysh-Usadel equations are solved self-consistently in the superconductor, CAR can dominate over EC. Self-consistency is necessary both for the conversion from a quasiparticle current to a supercurrent and to describe the spatial variation of the order parameter correctly. A requirement for the CAR probability to surpass the EC probability is that the inverse proximity effect is small. Otherwise, the subvoltage density of states becomes large and EC is strengthened by quasiparticles flowing through the superconductor. Therefore, CAR becomes dominant in the non-local transport with increasing interface resistance and length of the superconducting region. Our results show that even the simplest possible experimental setup with easily accessible normal metals and superconductors can provide dominant CAR by designing the experimental parameters correctly. We also find that spin-splitting in the superconductor increases the subvoltage density of states, and thus always favors EC over CAR. Finally, we tune the chemical potential in the leads such that transport is governed by electrons of one spin type. This can increase the CAR probability at finite values of the spin-splitting compared to using a spin-degenerate voltage bias, and provides a way to control the spin of the conduction electrons electrically.

cond-mat.mes-hall↗

arXiv2024

Josephson transistor and robust supercurrent enhancement with spin-split superconductors

We theoretically investigate the supercurrent flow in a Josephson junction consisting of two spin-split superconductors combined by a normal metal weak link. The normal metal may be driven out of equilibrium, thus modifying the electron and hole occupation and consequently the supercurrent through the system. Considering first an equilibrium normal metal, we find that increasing the spin-splitting field can enhance the supercurrent strongly for long junctions at low temperatures. In contrast to previous work, this is a much larger enhancement (over 100%) and it is achieved for both parallel and antiparallel spin-splitting field configurations, making the effect robust. On the other hand, when a gate voltage is applied to drive the system out of equilibrium, we demonstrate a more efficient $π$-transition of the supercurrent in terms of a lower transition voltage by tuning the spin-splitting. Moreover, we find the application of temperature bias strongly suppresses the supercurrent, resulting in very sharp supercurrent jumps as outputs.

cond-mat.supr-con↗

arXiv2024

Impurity-induced Friedel oscillations in altermagnets and $p$-wave magnets

We investigate the Friedel oscillations of the local density of states (LDOS) induced by a single impurity with both a spin-independent potential and an exchange coupling to the electrons in altermagnets and unconventional $p$-wave magnets. We identify features that make the Friedel oscillations and magnetization distinct from other materials with nontrivial spin texture such as Rashba metals. Because time-reversal symmetry is broken in altermagnets, both magnetic and nonmagnetic impurities lead to local magnetization with the spatial pattern that reflects the symmetry of the altermagnetic splitting. The period of the corresponding oscillations provides an alternative way to quantify the altermagnetic spin splitting and the shape of the altermagnetic bands. The LDOS pattern in $p$-wave magnets, which respect combined time-reversal and translation symmetries, is rich. It reveals anisotropy related directly to the spin splitting, but surprisingly also features LDOS oscillations with a doubled period in the proximity of the impurity. The latter effect is also observed in a Rashba metal with an exchange field and originates from the interplay of propagating and evanescent waves. The obtained results are instrumental for investigating altermagnets and unconventional $p$-wave magnets via tunneling probes.

cond-mat.mes-hall↗

arXiv2024

Optical conductivity of the Majorana mode at the s- and d-wave topological superconductor edge

The Majorana fermion offers fascinating possibilities such as non-Abelian statistics and non-local robust qubits, and hunting it is one of the most important topics in current condensed matter physics. Most of the efforts have been focused on the Majorana bound state at zero energy in terms of scanning tunneling spectroscopy searching for the quantized conductance. On the other hand, a chiral Majorana edge channel appears at the surface of a three-dimensional topological insulator when engineering an interface between proximity-induced superconductivity and ferromagnetism. Recent advances in microwave spectroscopy of topological edge states open a new avenue for observing signatures of such Majorana edge states through the local optical conductivity. As a guide to future experiments, we show how the local optical conductivity and density of states present distinct qualitative features depending on the symmetry of the superconductivity, that can be tuned via the magnetization and temperature. In particular, the presence of the Majorana edge state leads to a characteristic non-monotonic temperature dependence achieved by tuning the magnetization.

cond-mat.mes-hall↗

arXiv2024

Orbital currents in lattice multiorbital systems: Continuity equation, torques, and RKKY interaction

Utilizing the electron orbital degree of freedom in heterostructures is attracting increasing attention due to the possibility of achieving much larger conversion rates between charge and orbital angular momentum flow compared to the intrinsic electron spin. Here, we consider orbital angular momentum currents in a tight-binding multiorbital lattice model and derive their continuity equation. From it, we observe that the current is not conserved and apply similar considerations to the recently discovered altermagnets. We find nonzero orbital torque terms, elucidate their physical mechanism, and show numerically that they contribute to dampening the orbital angular momentum current flowing in multiorbital heterostructures. Moreover, we compute the orbital RKKY interaction and find it exhibits similar characteristics as the orbital angular momentum current mediating it, thus serving as a direct experimental probe of such currents.

cond-mat.mes-hall↗

arXiv2024

Interface-induced magnetization in altermagnets and antiferromagnets

Altermagnets is a class of antiferromagnetic materials which has electron bands with lifted spin degeneracy in momentum space but vanishing net magnetization and no stray magnetic fields. Because of these properties, altermagnets have attracted much attention for potential use in spintronics. We here show that despite the absence of bulk magnetization, the itinerant electrons in altermagnets can generate a magnetization close to edges and vacuum interfaces. We find that surface-induced magnetization can also occur for conventional antiferromagnets with spin-degenerate bands, where the magnetization from the itinerant electrons originates from a subtle yet nonvanishing redistribution of the probability density on the unit-cell level. An intuitive explanation of this effect in a phenomenological model is provided. In the altermagnetic case, the induced magnetization has a different spatial dependence than in the antiferromagnetic case due to the anisotropy of the spin-polarized Fermi surfaces, causing the edge-induced Friedel oscillations of the spin-up and -down electron densities to have different periods. We employ both a low-energy effective continuum model and lattice tight-binding calculations. Our results have implications for the usage of altermagnets and antiferromagnets in nanoscale spintronic applications.

cond-mat.mes-hall↗

arXiv2024

RKKY interaction in Rashba altermagnets

The interaction between two impurity spins provides vital information about the host system and has been suggested to form a building block in quantum computation and spintronic devices. We here determine this Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction in recently discovered altermagnetic materials, including the presence of a Zeeman field, in a two- and three-dimensional altermagnet. In two dimensions, we also study the effect of Rashba spin-orbit coupling. Our results reveal that the momentum-resolved spin-polarization of the itinerant carriers in the altermagnet changes the RKKY interaction qualitatively from the isotropic spin-splitting in a ferromagnet. The Ising-contribution directed parallel with the altermagnetism is found to exhibit a beating pattern reflecting the shape of the Fermi surface of the system and may thus be further influenced by an out-of-plane Zeeman field. However, the exchange interaction for in-plane impurity spins deviates only slightly from that of a normal metal. On shorter length scales, we find that the Ising, the Dzyaloshinskii-Moriya terms, and other non-collinear interaction terms in the RKKY interaction acquire a rapidly oscillating behavior as a function of the relative angle between the impurity spins, which is not present in the ferromagnetic case. We determine how this new length scale depends on the system parameters analytically. Our results show that the RKKY interaction in altermagnets is qualitatively different from that of ferromagnets despite both breaking time-reversal symmetry.

cond-mat.mes-hall↗

arXiv2024

Colloquium: Spin-orbit effects in superconducting hybrid structures

Spin-orbit coupling (SOC) relates to the interaction between an electron's motion and its spin, and is ubiquitous in solid-state systems. Although the effect of SOC in normal-state phenomena has been extensively studied, its role in superconducting hybrid structures and devices opens many unexplored questions. In conjunction with broken symmetries and material inhomogeneities within superconducting hybrid structures, SOC may have additional contributions, beyond its effects in homogenous materials. Remarkably, even with well-established magnetic or nonmagnetic materials and conventional s-wave spin-singlet superconductors, SOC leads to emergent phenomena including equal-spin triplet pairing and topological superconductivity (hosting Majorana states), a modified current-phase relationship in Josephson junctions, and nonreciprocal transport. SOC is also responsible for transforming quasiparticles in superconducting structures which enhances the spin Hall effect and changes spin dynamics. Taken together, SOC in superconducting hybrid structures and the potential for electric tuning of the SOC strength, creates fascinating possibilities to advance superconducting spintronic devices for energy-efficient computing, and enable topological fault-tolerant quantum computing. By providing a description of experimental techniques and theoretical methods to study SOC, this Colloquium describes the current understanding of resulting phenomena in superconducting structures and offers a framework to select and design a growing class of materials systems where SOC plays an important role.

cond-mat.supr-con↗

arXiv2024

Quasiclassical theory of superconducting spin-splitter effects and spin-filtering via altermagnets

Conducting altermagnets have recently emerged as intriguing materials supporting strongly spin-polarized currents without magnetic stray fields. We demonstrate that altermagnets enable three key functionalities, merging superconductivity and spintronics. The first prediction is a controllable supercurrent-induced edge magnetization, which acts like a dissipationless spin-splitter effect. The second and third predictions are a Cooper pair spin-splitter and a filtering effect, respectively. These effects allow for spatial separation of triplet pairs with opposite spin-polarizations and spin-selective tunneling of Cooper pairs. We derive a quasiclassical theory with associated boundary conditions that describe these phenomena and explain how they can be experimentally verified. Our results open a new path for spatial control of spin signals via triplet Cooper pairs using hybrid superconductor-altermagnet devices.

cond-mat.supr-con↗

arXiv2024

Spin pumping in an altermagnet/normal metal bilayer

Altermagnetism is a subclass of antiferromagnetism that features spin-polarized electron bands of a non-relativistic origin despite the absence of a net magnetiation in the material. We here theoretically study spin pumping from an altermagnetic insulator into a normal metal. The symmetry properties of the lattice and spin order of the altermagnet alters the magnon dispersion compared to a conventional square lattice antiferromagnet. We find that for a homogeneous magnetic field, the spin pumping current is the same as that of a regular antiferromagnet. If, however the magnetic field becomes spatially dependent, we predict that the altermagnetic order leaves a unique fingerprint on the spin pumping behaviour when the orientation of the spatial modulation does not align with the high-symmetry paths of magnon degeneracy in the altermagnet. This demonstrates that altermagnets can be used for THz spin pumping purposes with novel behaviour, distinguishing them from their regular antiferromagnet counterparts.

cond-mat.mes-hall↗

arXiv2024

Inverse spin-Hall effect and spin-swapping in spin-split superconductors

When a spin-splitting field is introduced to a thin film superconductor, the spin currents polarized along the field couples to energy currents that can only decay via inelastic scattering. We study spin and energy injection into such a superconductor where spin-orbit impurity scattering yields inverse spin-Hall and spin-swapping currents. We show that the combined presence of a spin-splitting field, superconductivity, and inelastic scattering gives rise to a strong enhancement of the ordinary inverse spin-Hall effect, as well as unique inverse spin-Hall and spin-swapping signals orders of magnitude stronger than the ordinary inverse spin-Hall signal. These can be completely controlled by the orientation of the spin-splitting field, resulting in a long-range charge and spin accumulations detectable much further from the injector than in the normal-state. While the enhanced inverse spin-Hall signals offer a major improvement in spin detection sensitivity, the unique spin-swap signals can be utilized for designing devices where both the spin and current directions are controlled and altered throughout the geometry.

cond-mat.mes-hall↗

arXiv2024

Superconducting phase diagram and spin diode effect via spin accumulation

Spin-split superconductors offer new functionality compared to conventional superconductors such as diode-effects and efficient thermoelectricity. The superconducting state can nevertheless only withstand a small amount of spin-splitting. Here, we self-consistently determine the spin transport properties and the phase diagram of a spin-split superconductor in the presence of an injected spin accumulation. Energy and spin relaxation are accounted for in the relaxation time approximation via a single effective inelastic scattering parameter. We find that the spin-splitting field in the superconductor enables a spin diode effect. Moreover, we consider the superconducting phase diagram of a system in contact with a spin accumulation and in the presence of spin relaxation, and find that the inclusion of energy and spin relaxation alters the phase diagram qualitatively. In particular, these mechanisms turn out to induce a superconducting state in large parts of the phase diagram where a normal state would otherwise be the ground-state. We identify an FFLO-like state even in the presence of impurity scattering which can be controllably tuned on and off via the electrically induced spin accumulation. We explain the underlying physics from how the superconducting order parameter depends on the non-equilibrium modes in the system as well as the behavior of these modes in the presence of energy and spin relaxation when a spin-splitting field is present.

cond-mat.supr-con↗

arXiv2024

Enhanced controllable triplet proximity effect in superconducting spin-orbit coupled spin valves with modified superconductor/ferromagnet interfaces

In a superconductor/ferromagnet hybrid, a magnetically controlled singlet-to-triplet Cooper pair conversion can modulate the superconducting critical temperature. In these triplet superconducting spin valves, such control usually requires inhomogeneous magnetism. However, in the presence of spin-orbit coupling from an interfacial heavy-metal layer, the singlet/triplet conversion rate and thus the critical temperature, can be controlled via the magnetization direction of a single homogeneous ferromagnet. Here, we report significantly enhanced controllable pair conversion to a triplet state in a Nb/Pt/Co/Pt superconducting spin valve in which Pt/Co/Pt is homogeneously magnetized and proximity-coupled to a superconducting layer of Nb. The Co/Pt interface furthest away from Nb is modified by a sub-nanometer-thick layer of Cu or Au. We argue that the enhancement is most likely associated from an improvement of the Co/Pt interface due to the insertion of Cu and Au layers. Additionally, the higher normalized orbital moments in Au measured using X-ray magnetic circular dichroism shows that increasing spin-orbit coupling enhances the triplet proximity effect - an observation supported by our theoretical calculations. Our results provide a pathway to enhancing triplet pair creation by interface engineering for device development in superspintronics.

cond-mat.supr-con↗