arXiv subjects

Jacob Linder

Publications and source records attributed to Jacob Linder.

At least 109 records · Page 6Linked to original sources

arXiv2018

Induced unconventional superconductivity on the surface states of Bi$_2$Te$_3$ topological insulator

Topological superconductivity is central to a variety of novel phenomena involving the interplay between topologically ordered phases and broken-symmetry states. The key ingredient is an unconventional order parameter, with an orbital component containing a chiral $p_x$ + i$p_y$ wave term. Here we present phase-sensitive measurements, based on the quantum interference in nanoscale Josephson junctions, realized by using Bi$_2$Te$_3$ topological insulator. We demonstrate that the induced superconductivity is unconventional and consistent with a sign-changing order parameter, such as a chiral $p_x$ + i$p_y$ component. The magnetic field pattern of the junctions shows a dip at zero externally applied magnetic field, which is an incontrovertible signature of the simultaneous existence of 0 and $π$ coupling within the junction, inherent to a non trivial order parameter phase. The nano-textured morphology of the Bi$_2$Te$_3$ flakes, and the dramatic role played by thermal strain are the surprising key factors for the display of an unconventional induced order parameter.

cond-mat.supr-con↗

arXiv2018

Tunable superconducting critical temperature in ballistic hybrid structures with strong spin-orbit coupling

We present a theoretical description and numerical simulations of the superconducting transition in hybrid structures including strong spin-orbit interactions. The spin-orbit coupling is taken to be of Rashba type for concreteness, and we allow for an arbitrary magnitude of the spin-orbit strength as well as an arbitrary thickness of the spin-orbit coupled layer. This allows us to make contact with the experimentally relevant case of enhanced interfacial spin-orbit coupling via atomically thin heavy metal layers. We consider both interfacial spin-orbit coupling induced by inversion asymmetry in an S/F-junction, as well as in-plane spin-orbit coupling in the ferromagnetic region of an S/F/S- and an S/F-structure. Both the pair amplitudes, local density of states and critical temperature show dependency on the Rashba strength and, importantly, the orientation of the exchange field. In general, spin-orbit coupling increases the critical temperature of a proximity system where a magnetic field is present, and enhances the superconducting gap in the density of states. We perform a theoretical derivation which explains these results by the appearance of long-ranged singlet correlations. Our results suggest that $T_c$ in ballistic spin-orbit coupled superconducting structures may be tuned by using only a single ferromagnetic layer.

cond-mat.supr-con↗

arXiv2018

Field-free nucleation of antivortices and giant vortices in non-superconducting materials

Giant vortices with higher phase-winding than $2π$ are usually energetically unfavorable, but geometric symmetry constraints on a superconductor in a magnetic field are known to stabilize such objects. Here, we show via microscopic calculations that giant vortices can appear in intrinsically non-superconducting materials, even without any applied magnetic field. The enabling mechanism is the proximity effect to a host superconductor where a current flows, and we also demonstrate that antivortices can appear in this setup. Our results open the possibility to study electrically controllable topological defects in unusual environments, which do not have to be exposed to magnetic fields or intrinsically superconducting, but instead display other types of order.

cond-mat.supr-con↗

arXiv2017

Equations of Motion and Frequency Dependence of Magnon-Induced Domain Wall Motion

Spin waves can induce domain wall motion in ferromagnets. We derive the equations of motion for a transverse domain wall driven by spin waves. Our calculations show that the magnonic spin-transfer torque does not cause rotation-induced Walker breakdown. The amplitude of spin waves that are excited by a localized microwave field depends on the spatial profile of the field and the excitation frequency. By taking this frequency dependence into account, we show that a simple one-dimensional model may reproduce much of the puzzling frequency dependence observed in early numerical studies.

cond-mat.mes-hall↗

arXiv2017

Tunability of Andreev levels via spin-orbit coupling in Zeeman-split Josephson junctions

We study Andreev reflection and Andreev levels $\varepsilon$ in Zeeman-split superconductor/Rashba wire/Zeeman-split superconductor junctions by solving the Bogoliubov de-Gennes equation. We theoretically demonstrate that the Andreev levels $\varepsilon$ can be controlled by tuning either the strength of Rashba spin-orbit interaction or the relative direction of the Rashba spin-orbit interaction and the Zeeman field. In particular, it is found that the magnitude of the band splitting is tunable by the strength of the Rashba spin-orbit interaction and the rength of the wire, which can be interpreted by a spin precession in the Rashba wire. We also find that if the Zeeman field in the superconductor has the component parallel to the direction of the junction, the $\varepsilon$-$ϕ$ curve becomes asymmetric with respect to the superconducting phase difference $ϕ$. Whereas the Andreev reflection processes associated with each pseudospin band are sensitive to the relative orientation of the spin-orbit field and the exchange field, the total electric conductance interestingly remains invariant.

cond-mat.supr-con↗

arXiv2017

Quantum kinetic equations and anomalous non-equilibrium Cooper pair spin accumulation in Rashba wires with Zeeman splitting

We derive the theoretical and numerical framework for investigating nonequilibrium properties of spin-orbit coupled wires with Zeeman splitting proximized by a superconductor in the non-linear diffusive regime. We demonstrate that the anisotropic behaviour of triplet Cooper pairs in this system leads to novel spin accumulation profiles tunable by the magnetic field and strength of applied voltage bias. This paves the way for enhanced manipulation of superconducting spintronic devices, and enables further investigation of nonequilibrium effects in proximity-coupled superconducting structures more generally.

cond-mat.supr-con↗

arXiv2017

Intrinsic Superspin Hall Current

We discover an intrinsic superspin Hall current: an injected charge supercurrent in a Josephson junction containing heavy normal metals and a ferromagnet generates a transverse spin supercurrent. There is no accompanying dissipation of energy, in contrast to the conventional spin Hall effect. The physical origin of the effect is an antisymmetric spin density induced among transverse modes $k_y$ near the interface of the superconductor arising due to the coexistence of $p$-wave and conventional $s$-wave superconducting correlations with a belonging phase mismatch. Our predictions can be tested in hybrid structures including thin heavy metal layers combined with strong ferromagnets and ordinary $s$-wave superconductors.

cond-mat.mes-hall↗

arXiv2017

Supercurrent Vortex Pinball via a Triplet Cooper Pair Inverse Edelstein Effect

We consider the Josephson effect through a thin spin-orbit coupled layer in the presence of an exchange field, and discover a set of supercurrent vortices appearing in the system which can be controllably moved around in the system by varying either the direction of the exchange field, its strength, or the spin-orbit coupling magnitude via a gate voltage. We refer to this phenomenon as a supercurrent vortex pinball effect and show that its origin is the spin polarization of the triplet Cooper pairs induced in the system. The supercurrent vortices thus arise from what resembles a Cooper pair-induced inverse Edelstein effect. Our results highlight the importance of considering higher-dimensional models for superconducting hybrid structures which unveils novel phenomena that are hidden in commonly used effective 1D models.

cond-mat.supr-con↗

arXiv2017

Conservation of spin supercurrents in superconductors

We demonstrate that spin supercurrents are conserved upon transmission through a conventional superconductor, even in the presence of spin-dependent scattering by impurities with magnetic moments or spin-orbit coupling. This is fundamentally different from conventional spin currents, which decay in the presence of such scattering, and this has important implications for the usage of superconducting materials in spintronic hybrid structures.

cond-mat.supr-con↗

arXiv2017

Non-equilibrium quantum transport and analytical conductance formula in spin-textured diffusive superconducting heterostructures

We study non-equilibrium quantum transport of spin, heat, and charge in diffusive heterostructures including both superconductors and materials with spin-dependent fields, such as textured ferromagnets and spin-orbit coupled materials. Using the quasiclassical theory of superconductivity, we derive a general analytical formula for the zero-temperature charge conductance valid in the non-linear regime. The analytical results are in excellent agreement with previous numerical calculations at small but finite temperatures. We predict a strong deviation between the conductance spectra and local density of states in certain junctions, which is important to correctly identify spectroscopic fingerprints of unconventional superconductivity.

cond-mat.supr-con↗

arXiv2017

Quasiclassical theory for the superconducting proximity effect in Dirac materials

We derive the quasiclassical non-equilibrium Eilenberger and Usadel equations to first order in quantities small compared to the Fermi energy, valid for Dirac edge and surface electrons with spin-momentum locking, as relevant for topological insulators. We discuss in detail several of the key technical points and assumptions of the derivation, and provide a Riccati-parametrization of the equations. Solving first the equilibrium equations for S/N and S/F bilayers and Josephson junctions, we study the superconducting proximity effect in Dirac materials. Similarly to related works, we find that the effect of an exchange field depends strongly on the direction of the field. Only components normal to the transport direction lead to attenuation of the Cooper pair wavefunction inside the F. Fields parallel to the transport direction lead to phase-shifts in the dependence on the superconducting phase difference for both the charge current and density of states in an S/F/S-junction. Moreover, we compute the differential conductance in S/N and S/F bilayers with an applied voltage bias, and determine the dependence on the length of the N and F regions and the exchange field.

cond-mat.mes-hall↗

arXiv2017

Spin-switch Josephson junctions with magnetically tunable $\sin(δφ/n)$ shape

With a combination of simple analytical arguments and extensive numerical simulations, we theoretically propose a Josephson junction with $n+1$ superconductors where the current-phase relation can be toggled in situ between a $\sin(δφ)$ and $\sin(δφ/n)$ shape using an applied magnetic field. Focusing in particular on the case $n=2$, we show that by using realistic system parameters such as unequal interface transparencies, the $\sin(δφ/2)$-shaped solution retains its $2π$-periodicity due to discontinuities at $δφ= \pm π$. Moreover, we demonstrate that as one toggles between the $\sin(δφ)$- and $\sin(δφ/2)$-shaped solutions, the system acts as an on--off switch, and can acheive more than two orders of magnitude difference between the supercurrent in the on and off states. Finally, we argue that the same approach can be generalized to switchable $\sin(δφ/n)$ junctions for arbitrary integers~$n$, which we motivate by analytically solving the Josephson equations for double- and triple-barrier junctions.

cond-mat.supr-con↗

arXiv2017

Controlling supercurrents and their spatial distribution in ferromagnets

Spin-triplet Cooper pairs induced in ferromagnets form the centrepiece of the emerging field of superconducting spintronics [1,2]. Usually the focus of research is on the spin polarization of the triplets, potentially enabling low-dissipation magnetization switching and domain wall motion. However, the fundamental mechanism for generating triplet pairs [3,4] also permits control over a parameter which has not been addressed before, namely the spatial distribution of the supercurrent. Here we demonstrate this control by tailoring distinct supercurrent pathways in the ferromagnetic weak link of a Josephson junction. Combining micromagnetic simulations with three-dimensional critical current calculations, based on the Usadel description of mesoscopic superconductivity [5], we designed a disk-shaped structure with a magnetic vortex, which induces two distinct supercurrent channels across the junction. The design was successfully tested with superconducting quantum interferometry (SQI). Moreover, we show how the position of the pathways can be controlled by moving the vortex with a magnetic field. This novel approach allows adaptable supercurrent paths to be dynamically reconfigured to switch between different functionalities in the same device.

cond-mat.mes-hall↗

arXiv2017

Triplet Cooper pairs induced in diffusive s-wave superconductors interfaced with strongly spin-polarized magnetic insulators or half-metallic ferromagnets

Interfacing superconductors with strongly spin-polarized magnetic materials opens the possibility to discover new spintronic devices in which spin-triplet Cooper pairs play a key role. Motivated by the recent derivation of spin-polarized quasiclassical boundary conditions capable of describing such a scenario in the diffusive limit, we consider the emergent physics in hybrid structures comprised of a conventional s-wave superconductor (e.g. Nb, Al) and either strongly spin-polarized ferromagnetic insulators (e.g. EuO, GdN) or halfmetallic ferromagnets (e.g. CrO2, LCMO). In contrast to most previous works, we focus on how the superconductor itself is influenced by the proximity effect, and how the generated triplet Cooper pairs manifest themselves in the self-consistently computed density of states (DOS) and the superconducting critical temperature Tc. We provide a comprehensive treatment of how the superconductor and its properties are affected by the triplet pairs, demonstrating that our theory can reproduce the recent observation of an unusually large zero-energy peak in a superconductor interfaced with a half-metal, which even exceeds the normal-state DOS. We also discuss the recent observation of a large superconducting spin-valve effect with a Tc change ~1K in superconductor/half-metal structures, in which case our results indicate that the experiment cannot be explained fully by a long-ranged triplet proximity effect.

cond-mat.supr-con↗

arXiv2017

Universal Absence of Walker Breakdown and Linear Current-Velocity Relation via Spin-Orbit Torques in Coupled and Single Domain Wall Motion

We consider theoretically domain wall motion driven by spin-orbit and spin Hall torques. We find that it is possible to achieve universal absence of Walker breakdown for all spin-orbit torques using experimentally relevant spin-orbit coupling strengths. For spin-orbit torques other than the pure Rashba spin-orbit torque, this gives a linear current-velocity relation instead of a saturation of the velocity at high current densities. The effect is very robust and is found in both soft and hard magnetic materials, as well as in the presence of the Dzyaloshinskii-Moriya interaction and in coupled domain walls in synthetic antiferromagnets, where it leads to very high domain wall velocities. Moreover, recent experiments have demonstrated that the switching of a synthetic antiferromagnet does not obey the usual spin Hall angle-dependence, but that domain expansion and contraction can be selectively controlled toggling only the applied in-plane magnetic field magnitude and not its sign. We show for the first time that the combination of spin Hall torques and interlayer exchange coupling produces the necessary relative velocities for this switching to occur.

cond-mat.mes-hall↗

arXiv2017

Anisotropic Andreev Reflection and Josephson Effect in Ballistic Phosphorene

We study Andreev reflection and the Josephson effect in a ballistic monolayer of black phosphorous, known as phosphorene. Due to the anisotropic band structure of this system, the supercurrent changes with an order of magnitude when comparing tunneling along two perpendicular directions in the monolayer. We show that the main reason for this effect is a large difference in the number of transverse modes in Andreev bound states. The oscillatory behavior of the supercurrent as a function of the length and chemical potential of the junction also differs substantially depending on the orientation of the superconducting electrodes deposited on the phosphorene sheet. For Andreev reflection, we show that a gate voltage controls the probability of this process and that the anisotropic behavior found in the supercurrent case is also present for conductance spectra.

cond-mat.supr-con↗

arXiv2017

p-wave triggered superconductivity in single layer graphene on an electron-doped oxide superconductor

Electron pairing in the vast majority of superconductors follows the Bardeen-Cooper-Schrieffer theory of superconductivity, which describes the condensation of electrons into pairs with antiparallel spins in a singlet state with an s-wave symmetry. Unconventional superconductivity is predicted in single layer graphene where the electrons pair with a p-wave or chiral d-wave symmetry, depending on the position of the Fermi energy with respect to the Dirac point. By placing single layer graphene on an electron-doped (non-chiral) d-wave superconductor and performing local scanning tunnelling microscopy and spectroscopy, here we show evidence for a p-wave triggered superconducting density of states in single layer graphene. The realization of unconventional superconductivity in single layer graphene offers an exciting new route for the development of p-wave superconductivity using two-dimensional materials with transition temperatures above 4.2 K.

cond-mat.supr-con↗

arXiv2017

Spin Seebeck effect and thermoelectric phenomena in superconducting hybrids with magnetic textures or spin-orbit coupling

We theoretically consider the spin Seebeck effect, the charge Seebeck coefficient, and the thermoelectric figure of merit in superconducting hybrid structures including either magnetic textures or intrinsic spin-orbit coupling. We demonstrate that large magnitudes for all these quantities are obtainable in Josephson-based systems with either zero or a small externally applied magnetic field. This provides an alternative to the thermoelectric effects generated in high-field ($\sim 1$ T) superconducting hybrid systems, which were recently experimentally demonstrated. The systems studied contain either textured ferromagnets, spin-active interfaces, or spin-orbit coupling. We present a framework for calculating the linear thermoelectric response for both spin and charge of a system upon applying temperature and voltage gradients based on quasiclassical theory which allows for arbitrary spin-dependent textures and fields to be conveniently incorporated.

cond-mat.mes-hall↗