arXiv subjects

Jacob Linder

Publications and source records attributed to Jacob Linder.

At least 73 records · Page 4Linked to original sources

arXiv2022

Quasiclassical boundary conditions for spin-orbit coupled interfaces with spin-charge conversion

The quasiclassical theory of superconductivity provides a methodology to study emergent phenomena in hybrid structures comprised of superconductors interfaced with other materials. A key component in this theory is the boundary condition that the Green functions describing the materials must satisfy. Recently, progress has been made toward formulating such a boundary condition for interfaces with spin-orbit coupling, the latter playing an important role for several phenomena in spintronics. Here, we derive a boundary condition for spin-orbit coupled interfaces that includes gradient terms which enables the description of spin-Hall like effects with superconductors due to such interfaces. As an example, we show that the boundary conditions predict that a supercurrent flowing through a superconductor that is coupled to a normal metal via a spin-orbit interface can induce a non-local magnetization in the normal metal.

cond-mat.supr-con↗

arXiv2022

Threefold enhancement of superconductivity and the role of field-induced odd-frequency pairing in epitaxial aluminum films near the 2D limit

BCS theory has been widely successful at describing elemental bulk superconductors. Yet, as the length scales of such superconductors approach the atomic limit, dimensionality as well as the environment of the superconductor can lead to drastically different and unpredictable superconducting behavior. Here, we report a threefold enhancement of the superconducting critical temperature and gap size in ultrathin epitaxial Al films on Si(111), when approaching the 2D limit, based on high-resolution scanning tunneling microscopy/spectroscopy (STM/STS) measurements. In magnetic field, the Al films show type II behavior and the Meservey-Tedrow-Fulde (MTF) effect for in-plane magnetic fields. Using spatially resolved spectroscopy, we characterize the vortex structure in the MTF regime and find strong deviations from the typical Abrikosov vortex. We corroborate these findings with calculations that unveil the role of odd-frequency pairing and a paramagnetic Meissner effect. These results illustrate two striking influences of reduced dimensionality on a BCS superconductor and present a new platform to study BCS superconductivity in large magnetic fields.

cond-mat.supr-con↗

arXiv2022

Going beyond the Chandrasekhar-Clogston limit in a flat-band superconductor

The Chandrasekhar-Clogston limit normally places stringent conditions on the magnitude of the magnetic field that can coexist with spin-singlet superconductivity, restricting the critical induced Zeeman shift to a fraction of the superconducting gap. Here, we consider a model system where the spin-singlet Cooper pairing in a dispersive band crossing the Fermi level is boosted by an additional flat-band located away from the Fermi level. The boosting of the pairing in the dispersive band allows for nontrivial solutions to the coupled gap equations for spin-splitting fields considerably larger than the superconducting gaps at zero field. Further, the additional Cooper pairing in the flat-band, away from the Fermi level, increases the superconducting condensation energy without affecting the paramagnetic susceptibility of the system, making the free energy favor the superconducting state. This opens up the possibility for spin-singlet superconductivity beyond the standard Chandrasekhar-Clogston limit.

cond-mat.supr-con↗

arXiv2022

Complete magnetic control over the superconducting thermoelectric effect

Giant thermoelectric effects are known to arise at the interface between superconductors and strongly polarized ferromagnets, enabling the construction of efficient thermoelectric generators. We predict that the thermopower of such a generator can be completely controlled by a magnetic input signal: Not only can the thermopower be toggled on and off by rotating a magnet, but it can even be entirely reversed. This in situ control diverges from conventional thermoelectrics, where the thermopower is usually fixed by the device design.

cond-mat.supr-con↗

arXiv2022

RKKY interaction in a spin-split superconductor

We determine theoretically the interaction between two magnetic impurities embedded in a spin-split $s$-wave superconductor. The spin-splitting in the superconductor gives rise to two different interaction types between the impurity spins, depending on whether their spins lie in the plane perpendicular to the spin-splitting field (Heisenberg) or not (Ising). For impurity separation distances exceeding $ξ_S$, we find that the magnitude of the spin-splitting can determine whether an antiferromagnetic or ferromagnetic alignment of the impurity spins is preferred by the RKKY interaction. Moreover, the Ising and Heisenberg terms of the RKKY interaction alternate on being the dominant term and their magnitudes oscillate as a function of distance between the impurities.

cond-mat.supr-con↗

arXiv2022

Spin-orbit pumping

We study theoretically the effect of a rotating electric field on a diffusive nanowire and find an effect that is analogous to spin pumping, which refers to the generation of spin through a rotating magnetic field. The electron spin couples to the electric field because the particle motion induces an effective magnetic field in its rest frame. In a diffusive system the velocity of the particle, and therefore also its effective magnetic field, rapidly and randomly changes direction. Nevertheless, we demonstrate analytically and via a physical argument why the combination of the two effects described above produces a finite magnetization along the axis of rotation. This manifests as a measurable spin-voltage in the range of tens of microvolts.

cond-mat.mes-hall↗

arXiv2021

Unconventional Meissner screening induced by chiral molecules in a conventional superconductor

The coupling of a superconductor to a different material often results in a system with unconventional superconducting properties. A conventional superconductor is a perfect diamagnet expelling magnetic fields out of its volume, a phenomenon known as Meissner effect. Here, we show that the simple adsorption of a monolayer of chiral molecules, which are non-magnetic in solution, onto the surface of a conventional superconductor can markedly change its diamagnetic Meissner response. By measuring the internal magnetic field profile in superconducting Nb thin films under an applied transverse field by low-energy muon spin rotation spectroscopy, we demonstrate that the local field profile inside Nb is considerably modified upon molecular adsorption in a way that also depends on the applied field direction. The modification is not limited to the chiral molecules/Nb interface, but it is long ranged and occurs over a length scale comparable to the superconducting coherence length. Zero-field muon spin spectroscopy measurements in combination with our theoretical analysis show that odd-frequency spin-triplet states induced by the chiral molecules are responsible for the modification of the Meissner response observed inside Nb. These results indicate that a chiral molecules/superconductor system supports odd-frequency spin-triplet pairs due to the molecules acting as a spin active layer and therefore they imply that such system can be used as a simpler alternative to superconductor/ferromagnet or superconductor/topological insulator hybrids for the generation and manipulation of unconventional spin-triplet superconducting states.

cond-mat.supr-con↗

arXiv2021

Magnon spin current induced by triplet Cooper pair supercurrents

At the interface between a ferromagnetic insulator and a superconductor there is a coupling between the spins of the two materials. We show that when a supercurrent carried by triplet Cooper pairs flows through the superconductor, the coupling induces a magnon spin current in the adjacent ferromagnetic insulator. The effect is dominated by Cooper pairs polarized in the same direction as the ferromagnetic insulator, so that charge and spin supercurrents produce similar results. Our findings demonstrate a way of converting Cooper pair supercurrents to magnon spin currents.

cond-mat.mes-hall↗

arXiv2021

Spin injection and spin relaxation in odd-frequency superconductors

The spin transport inside an odd-frequency spin-triplet superconductor differs from that of a conventional superconductor due to its distinct symmetry properties. We study spin transport inside an emergent odd-frequency superconductor by replacing the spin-singlet gap matrix in the Usadel equation with a matrix representing spin-triplet pairing that is odd under inversion of energy. We show that the peculiar nature of the density of states allows for an even larger spin injection than in the normal-state. Moreover, when the odd-frequency pairing inherits its temperature dependence from a conventional superconductor through the proximity effect, the density of states can transition from gapless to gapped as the temperature decreases. At the transition point, the spin accumulation inside the odd-frequency superconductor is peaked and larger than in the normal-state. While the spin-flip scattering time is known to decrease below the superconducting transition temperature in conventional superconductors, we find that the same is true for the spin-orbit scattering time in odd-frequency superconductors. This renormalization is particularly large for energies close to the gap edge, if such a gap is present.

cond-mat.supr-con↗

arXiv2021

Superconductivity assisted change of the perpendicular magnetic anisotropy in V/MgO/Fe junctions

Controlling the perpendicular magnetic anisotropy (PMA) in thin films has received considerable attention in recent years due to its technological importance. PMA based devices usually involve heavy-metal (oxide)/ferromagnetic-metal bilayers, where, thanks to interfacial spin-orbit coupling (SOC), the in-plane (IP) stability of the magnetization is broken. Here we show that in V/MgO/Fe(001) epitaxial junctions with competing in-plane and out-of-plane (OOP) magnetic anisotropies, the SOC mediated interaction between a ferromagnet (FM) and a superconductor (SC) enhances the effective PMA below the superconducting transition. This produces a partial magnetization reorientation without any applied field for all but the largest junctions, where the IP anisotropy is more robust; for the smallest junctions there is a reduction of the field required to induce a complete OOP transition ($H_\text{OOP}$) due to the stronger competition between the IP and OOP anisotropies. Our results suggest that the degree of effective PMA could be controlled by the junction lateral size in the presence of superconductivity and an applied electric field. We also discuss how the $H_\text{OOP}$ field could be affected by the interaction between magnetic stray fields and superconducting vortices. Our experimental findings, supported by numerical modelling of the ferromagnet-superconductor interaction, open pathways to active control of magnetic anisotropy in the emerging dissipation-free superconducting spin electronics.

cond-mat.supr-con↗

arXiv2021

The effect of midgap states on the magnetic exchange interaction mediated by a $d$-wave superconductor

We theoretically study the indirect interaction between two ferromagnetic contacts located on the surface of a $d$-wave superconductor. When the magnets are connected to a $\{010\}$ edge of the superconductor we find an oscillating RKKY interaction that varies in sign as the distance between the magnetic contacts is varied. However, when coupling the magnets to an $\{110\}$ edge of the superconductor, we find that the presence of midgap states qualitatively changes the results. The ground state of the system is then found to always favor alignment of the magnets as this configuration most strongly suppresses the midgap states, leading to a larger condensation energy which dominates over the intrinsic RKKY interaction.

cond-mat.supr-con↗

arXiv2021

Controllable enhancement of $p$-wave superconductivity via magnetic coupling to a conventional superconductor

Unconventional superconductors are of high interest due to their rich physics, a topical example being topological edge-states associated with $p$-wave superconductivity. A practical obstacle in studying such systems is the very low critical temperature $T_\text{c}$ that is required to realize a $p$-wave superconducting phase in a material. We predict that the $T_\text{c}$ of an intrinsic $p$-wave superconductor can be significantly enhanced by coupling it via an atomically thin ferromagnetic layer (F) to a conventional $s$-wave or a $d$-wave superconductor with a higher critical temperature. We show that this $T_\text{c}$-boost is tunable via the direction of the magnetization in F. Moreover, we show that the enhancement in $T_\text{c}$ can also be achieved using the Zeeman-effect of an external magnetic field. Our findings provide a way to increase $T_\text{c}$ in $p$-wave superconductors in a controllable way and make the exotic physics associated with such materials more easily accessible experimentally.

cond-mat.supr-con↗

arXiv2021

Planar graphene-NbSe$_2$ Josephson junctions in a parallel magnetic field

Thin transition metal dichalcogenides sustain superconductivity at large in-plane magnetic fields due to Ising spin-orbit protection, which locks their spins in an out-of-plane orientation. Here we use thin NbSe$_2$ as superconducting electrodes laterally coupled to graphene, making a planar, all van der Waals two-dimensional Josephson junction (2DJJ). We map out the behavior of these novel devices with respect to temperature, gate voltage, and both out-of-plane and in-plane magnetic fields. Notably, the 2DJJs sustain supercurrent up to $H_\parallel$ as high as 8.5 T, where the Zeeman energy $E_Z$ rivals the Thouless energy $E_{Th}$, a regime hitherto inaccessible in graphene. As the parallel magnetic field $H_\parallel$ increases, the 2DJJ's critical current is suppressed and in a few cases undergoes suppression and recovery. We explore the behavior in $H_\parallel$ by considering theoretically two effects: a 0-$π$ transition induced by tuning of the Zeeman energy and the unique effect of ripples in an atomically thin layer which create a small spatially varying perpendicular component of the field. The 2DJJs have potential utility as flexible probes for two-dimensional superconductivity in a variety of materials and introduce high $H_\parallel$ as a newly accessible experimental knob.

cond-mat.supr-con↗

arXiv2021

Spin-pumping in superconductor-antiferromagnetic insulator bilayers

We study theoretically spin pumping in bilayers consisting of superconductors and antiferromagnetic insulators. We consider both compensated and uncompensated interfaces and include both the regular scattering channel and the Umklapp scattering channel. We find that at temperatures close to the critical temperatures and precession frequencies much lower than the gap, the spin-current is enhanced in superconductors as compared to normal metals. Otherwise, the spin-current is suppressed. The relevant precession frequencies where the spin-current in SC/AFI is enhanced compared to NM/AFI is much lower than the typical resonance frequencies of antiferromagnets, which makes the detection of this effect experimentally challenging. A possible solution lies in the shifting of the resonance frequency by a static magnetic field.

cond-mat.supr-con↗

arXiv2021

Self-consistent solution for the magnetic exchange interaction mediated by a superconductor

We theoretically determine the magnetic exchange interaction between two ferromagnets coupled by a superconductor using a tight-binding lattice model. The main purpose of this study is to determine how the self-consistently determined superconducting state influences the exchange interaction and the preferred ground-state of the system, including the role of impurity scattering. We find that the superconducting state eliminates RKKY-like oscillations for a sufficiently large superconducting gap, making the anti-parallel orientation the ground state of the system. Interestingly, the superconducting gap is larger in the parallel configuration than in the anti-parallel configuration, giving a larger superconducting condensation energy, even when the preferred ground state is anti-parallel. We also show that increasing the impurity concentration in the superconductor causes the exchange interaction to decrease, likely due to an increasing localization of the mediating quasiparticles in the superconductor.

cond-mat.mes-hall↗

arXiv2021

Magnetic control of superconducting heterostructures using compensated antiferromagnets

Due to the lack of a net magnetization both at the interface and in the bulk, antiferromagnets with compensated interfaces may appear incapable of influencing the phase transition in an adjacent superconductor via the spin degree of freedom. We here demonstrate that such an assertion is incorrect by showing that proximity-coupling a compensated antiferromagnetic layer to a superconductor-ferromagnet heterostructure introduces the possibility of controlling the superconducting phase transition. The superconducting critical temperature can in fact be modulated by rotating the magnetization of the single ferromagnetic layer within the plane of the interface, although the system is invariant under rotations of the magnetization in the absence of the antiferromagnetic layer. Moreover, we predict that the superconducting phase transition can trigger a reorientation of the ground state magnetization. Our results show that a compensated antiferromagnetic interface is in fact able to distinguish between different spin-polarizations of triplet Cooper pairs.

cond-mat.supr-con↗

arXiv2021

Temporarily enhanced superconductivity from magnetic fields

Contrary to the expected detrimental influence on superconductivity when applying a magnetic field, we predict that the abrupt onset of such a field can temporarily strongly enhance the superconducting order parameter. Specifically, we find that the supercurrent in a Josephson junction with a normal metal weak link can increase more than twentyfold in this way. The effect can be understood from the interplay between the energy-dependence of Andreev reflection and the abrupt spin-dependent shift in the distribution functions for excitations in the system. The duration of the increase depends on the inelastic scattering rate in the system and is estimated to be in the range of nanoseconds. We demonstrate this by developing a method which solves the Usadel equation for an arbitrary time-dependence. This enables the study of ultrafast time-dependent physics in heterostructures combining superconductors with different types of materials.

cond-mat.supr-con↗

arXiv2021

Spin pumping between noncollinear ferromagnetic insulators through thin superconductors

Dynamical magnets can pump spin currents into superconductors. To understand such a phenomenon, we develop a method utilizing the generalized Usadel equation to describe time-dependent situations in superconductors in contact with dynamical ferromagnets. Our proof-of-concept theory is valid when there is sufficient dephasing at finite temperatures, and when the ferromagnetic insulators are weakly polarized. We derive the effective equation of motion for the Keldysh Green's function focusing on a thin film superconductor sandwiched between two noncollinear ferromagnetic insulators of which one is dynamical. In turn, we compute the spin currents in the system as a function of the temperature and the magnetizations' relative orientations. When the induced Zeeman splitting is weak, we find that the spin accumulation in the superconducting state is smaller than in the normal states due to the lack of quasiparticle states inside the gap. This feature gives a lower backflow spin current from the superconductor as compared to a normal metal. Furthermore, in superconductors, we find that the ratio between the backflow spin current in the parallel and anti-parallel magnetization configuration depends strongly on temperature, in contrast to the constant ratio in normal metals.

cond-mat.supr-con↗