arXiv subjects

Klaus Halterman

Publications and source records attributed to Klaus Halterman.

At least 37 records · Page 2Linked to original sources

arXiv2015

Charge and Spin Currents in Ferromagnetic Josephson junctions

We determine, using a self consistent method, the charge and spin currents in ballistic Josephson junctions consisting of ferromagnetic ($F$) layers sandwiched between superconducting ($S$) electrodes ($SFS$-type junctions). When there are two $F$ layers, we also consider the experimentally relevant configuration where a normal ($N$) nonmagnetic spacer layer separates them. We study the current-phase relationships as functions of geometrical parameters that are accessible experimentally including the angles that characterize the relative orientation of the magnetization in the $F$ layers. Our self-consistent method ensures that the proper charge conservation laws are satisfied. As we vary the phase difference $Δφ$ between the two outer $S$ electrodes, multiple harmonics in the current phase relations emerge, their extent depends on the interface scattering strength and the relative $F$ layer widths and magnetization orientations. By manipulating the relative $F$ layer magnetization orientations, we find that the charge supercurrent can reverse directions or vanish altogether. These findings are discussed in the context of the generation and long-range nature of triplet pair correlation. We also investigate the spin currents and associated spin transfer torques throughout the junction. For noncollinear relative magnetizations, the non-conserved spin currents in a given $F$ region gives rise to net torques that can switch directions at particular magnetic configurations or $Δφ$ values. The details of the spin current behavior are shown to depend strongly on the degree of magnetic inhomogeneity in the system.

cond-mat.supr-con↗

arXiv2015

Josephson Currents and Spin Transfer Torques in Ballistic SFSFS Nanojunctions

Utilizing a full microscopic Bogoliubov-de Gennes (BdG) approach, we study the equilibrium charge and spin currents in ballistic $SFSFS$ Josephson systems, where $F$ is a uniformly magnetized ferromagnet and $S$ is a conventional $s$-wave superconductor. From the spatially varying spin currents, we also calculate the associated equilibrium spin transfer torques. Through variations in the relative phase differences between the three $S$ regions, and magnetization orientations of the ferromagnets, our study demonstrates tunability and controllability of the spin and charge supercurrents. The spin transfer torques are shown to reveal details of the proximity effects that play a crucial role in these types of hybrid systems. The proposed $SFSFS$ nanostructure is discussed within the context of a superconducting magnetic torque transistor.

cond-mat.supr-con↗

arXiv2015

Long-Range Spin-Triplet Correlations and Edge Spin Currents in Diffusive Spin-Orbit Coupled SNS Hybrids with a Single Spin-Active Interface

Utilizing a SU(2) gauge symmetry technique in the quasiclassical diffusive regime, we theoretically study finite-sized two-dimensional intrinsic spin-orbit coupled superconductor/normal-metal/superconductor (S/N/S) hybrid structures with a single spin-active interface. We consider intrinsic spin-orbit interactions (ISOIs) that are confined within the N wire and absent in the s-wave superconducting electrodes (S). Using experimentally feasible parameters, we demonstrate that the coupling of the ISOIs and spin moment of the spin-active interface results in maximum singlet-triplet conversion and accumulation of spin current density at the corners of the N wire nearest the spin-active interface. By solely modulating the superconducting phase difference, we show how the opposing parities of the charge and spin currents provide an effective venue to experimentally examine pure edge spin currents not accompanied by charge currents. These effects occur in the absence of externally imposed fields, and moreover are insensitive to the arbitrary orientations of the interface spin moment. The experimental implementation of these robust edge phenomena are also discussed.

cond-mat.supr-con↗

arXiv2015

Spontaneous Edge Accumulation of Spin Currents in Finite-Size Two-Dimensional Diffusive Spin-Orbit Coupled SFS Heterostructures

We theoretically study spin and charge currents through finite-size two-dimensional $s$-wave superconductor/uniform ferromagnet/s-wave superconductor (SFS) junctions with intrinsic spin-orbit interactions (ISOIs) using a quasiclassical approach. Considering experimentally realistic parameters, we demonstrate that the combination of spontaneously broken time-reversal symmetry and lack of inversion symmetry can result in spontaneously accumulated spin currents at the edges of finite-size two-dimensional magnetic SF hybrids. Due to the spontaneous edge spin accumulation, the corners of the F wire host the maximum spin current density. We further reveal that this type edge phenomena are robust and independent of either the actual type of ISOIs or exchange field orientation. Moreover, we study spin current-phase relations in these diffusive spin-orbit coupled SFS junctions. Our results unveil net spin currents, not accompanied by charge supercurrent, that spontaneously accumulate at the sample edges through a modulating superconducting phase difference. Finally, we discuss possible experimental implementations to observe these edge phenomena.

cond-mat.mes-hall↗

arXiv2015

Proximity Induced Vortices and Long-Range Triplet Supercurrents in Ferromagnetic Josephson Junctions and Spin Valves

Using a Keldysh formalism, we study supercurrent (SC) transport in several types of diffusive ferromagnet(F)/superconductor(S) configurations. We separate out the even- and odd-frequency components and identify the relative contributions from the singlet and triplet channels. We first consider 1D Josephson structures consisting of a uniform SFFS structure and a trilayer SFFFS configuration. Our results demonstrate that for SCs flowing perpendicular to the FF interfaces, incorporating a conical texture yields the most effective way to observe the signatures of the long-ranged SC. We also consider three different types of finite-sized 2D structures subjected to an applied magnetic field: a uniform SFS junction, and two SFFS configurations with differing FF bilayer arrangements. In one case, the FF interface is parallel with the SF interfaces while in the other case, the FF junction is oriented perpendicular to the SF interfaces. For the uniform SFS junction, we analytically derive the magnetic field induced SC and pair potential, providing insight into the variations of the critical SCs and proximity vortices. Our extensive computations demonstrate that the induced long-range SCs can penetrate deeply into uniform FF bilayers when singlet SCs flow parallel to the FF interfaces which is in stark contrast to configurations where SC flows perpendicular to the FF interfaces. We pinpoint the origin of the induced triplet and singlet correlations through spatial profiles of the decomposed total SCs. We find that the penetration of the long-range SCs associated with SCs flowing parallel to the FF interfaces, are more pronounced when the thickness of the F strips are unequal. Lastly, we show that the corresponding experimentally accessible SFFN spin valve presents an effective platform in which the predicted long-range effects can be effectively generated and probed.

cond-mat.mes-hall↗

arXiv2015

Long-Range Triplet Supercurrents Induced by Singlet Supercurrents Parallel to Magnetic Interfaces

Employing a spin-parameterized Kleldysh-Usadel technique for the diffusive regime, we demonstrate that even in the low proximity limit, considerable long-ranged triplet supercurrents can be effectively generated by spin-singlet supercurrents flowing \textit{parallel} to the interfaces of uniform double ferromagnet interlayers with noncollinear exchange fields "{\it independent}" of actual junction geometry. The triplet supercurrents are found to be most pronounced when the thicknesses of the ferromagnet strips are unequal. To experimentally verify this generic phenomenon, we propose an accessible and well-controlled structure that can fully isolate the long-range triplet effects.

cond-mat.supr-con↗

arXiv2014

Tunneling Conductance and Spin Transport in Clean Ferromagnet-Ferromagnet-Superconductor Heterostructures

We present a transfer matrix approach that combines the Blonder-Tinkham-Klapwijk (BTK) formalism and self-consistent solutions to the Bogolibuov-de Gennes (BdG) equations and use it to study the tunneling conductance and spin transport in ferromagnet (${\rm F}$)-superconductor (${\rm S}$) trilayers (${\rm F_1F_2 S}$) as functions of bias voltage. The self-consistency ensures that the spin and charge conservation laws are properly satisfied. We consider forward and angularly averaged conductances over a broad range of the strength of the exchange fields and ${\rm F}$ thicknesses, as the relative in-plane magnetization angle, $ϕ$, between the two ferromagnets varies. The $ϕ$-dependence of the self-consistent conductance curves in the trilayers can differ substantially from that obtained via a non-self-consistent approach. The zero bias forward conductance peak exhibits, as $ϕ$ varies, resonance effects intricately associated with particular combinations of the geometrical and material parameters. We find, when the magnetizations are non-collinear, signatures of the anomalous Andreev reflections in the subgap regions of the angularly averaged conductances. When ${\rm F_1}$ is half-metallic, the angularly averaged subgap conductance chiefly arises from anomalous Andreev reflection. The in-plane components of the spin current are strongly bias dependent, while the out-of-plane spin current component is only weakly dependent upon voltage. The components of the spin current aligned with the local exchange field of one of the F layers are conserved in that layer and in the S region, while they oscillate in the other layer. We compute the spin transfer torques, in connection with the oscillatory behavior of spin currents, and verify that the spin continuity equation is strictly obeyed in our method.

cond-mat.supr-con↗

arXiv2014

Angular dependence of superconductivity in superconductor / spin valve heterostructures

We report measurements of the superconducting transition temperature, $T_c$, in CoO/Co/Cu/Co/Nb multilayers as a function of the angle $α$ between the magnetic moments of the Co layers. Our measurements reveal that $T_c(α)$ is a nonmonotonic function, with a minimum near $α=π/{2}$. Numerical self-consistent solutions of the Bogoliubov - de Gennes equations quantitatively and accurately describe the behavior of $T_c$ as a function of $α$ and layer thicknesses in these superconductor / spin-valve heterostructures. We show that experimental data and theoretical evidence agree in relating $T_c(α)$ to enhanced penetration of the triplet component of the condensate into the Co/Cu/Co spin valve in the maximally noncollinear magnetic configuration.

cond-mat.supr-con↗

arXiv2014

Spin Controlled Coexistence of 0 and π States in SFSFS Josephson Junctions

Using the Keldysh-Usadel formalism, we theoretically study the $0$-$π$ transition profiles and current-phase relations of magnetic $SFSFS$ and $SFSFFS$ Josephson nanojunctions in the diffusive regime. By allowing the magnetizations of the ferromagnetic layers to take arbitrary orientations, the strength and direction of the charge supercurrent flowing through the ferromagnetic regions can be controlled via the magnetization rotation in one of the ferromagnetic layers. Depending on the junction parameters, we find opposite current flow in the ferromagnetic layers, revealing that remarkably such configurations possess well-controlled $0$- and $π$-states simultaneously, creating a three-terminal $0$-$π$ spin switch. We demonstrate that the spin-controlled $0$-$π$ profiles trace back to the proximity induced odd-frequency superconducting correlations generated by the ferromagnetic layers. It is also shown that the spin-switching effect can be more pronounced in $SFSFFS$ structures. The current-phase relations reveal the important role of the middle $S$ electrode, where the spin controlled supercurrent depends crucially on its thickness and phase differences with the outer $S$ terminals.

cond-mat.supr-con↗

arXiv2014

Near-perfect absorption in epsilon-near-zero structures with hyperbolic dispersion

We investigate the interaction of polarized electromagnetic waves with hyperbolic metamaterial structures, whereby the in-plane permittivity component $ε_x$ is opposite in sign to the normal component $ε_z$. We find that when the thickness of the metamaterial is smaller than the wavelength of the incident wave, hyperbolic metamaterials can absorb significantly higher amounts of electromagnetic energy compared to their conventional counterparts. We also demonstrate that for wavelengths leading to $\Re(ε_z) \approx 0$, near-perfect absorption arises and persists over a range of frequencies and subwavelength structure thicknesses.

physics.optics↗

arXiv2014

Meissner Effect Probing of Odd-Frequency Triplet Pairing in Superconducting Spin Valves

Superconducting correlations which are long-ranged in magnetic systems have attracted much attention due to their spin-polarization properties and potential use in spintronic devices. Whereas experiments have demonstrated the slow decay of such correlations, it has proven more difficult to obtain a smoking gun signature of their odd-frequency character which is responsible, e.g. for their gapless behavior. We here demonstrate that the magnetic susceptibility response of a normal metal in contact with a superconducting spin-valve provides precisely this signature, namely in form of an anomalous "positive" Meissner effect which may be tuned back to a conventional "negative" Meissner response simply by altering the magnetization configuration of the spin-valve.

cond-mat.supr-con↗

arXiv2014

Superconducting Spintronics with Magnetic Domain Walls

The recent experimental demonstration of spin-polarized supercurrents offer a venue for establishment of a superconducting analogue to conventional spintronics. Whereas domain wall motion in purely magnetic structures is a well-studied topic, it is not clear how domain wall dynamics may influence superconductivity and if some functional property can be harnessed from such a scenario. Here, we demonstrate that domain wall motion in superconducting systems offers a unique way of controlling the quantum state of the superconductor. Considering both the diffusive and ballistic limits, we show that moving the domain wall to different locations in a Josephson junction will change the quantum ground state from being in a 0 state to a $π$ state. Remarkably, we also show that domain wall motion can be used to turn on and off superconductivity: the position of the domain wall determines the critical temperature $T_c$ and thus if the system is in a resistive state or not, causing even a quantum phase transition between the dissipationless and normal state at $T=0$. In this way, one achieves dynamical control over the superconducting state within a single sample by utilizing magnetic domain wall motion.

cond-mat.supr-con↗

arXiv2013

Singlet-Triplet Superconducting Quantum Magnetometer

Motivated by the recent experimental realization of a quantum interference transistor based on the superconducting proximity effect, we here demonstrate that the inclusion of a textured ferromagnet both strongly enhances the flux sensitivity of such a device and additionally allows for singlet-triplet switching by tuning a bias voltage. This functionality makes explicit use of the induced spin-triplet correlations due to the magnetic texture. Whereas the existence of such triplet correlations is well-known, our finding demonstrates how spin-triplet superconductivity may be utilized for concrete technology, namely to improve the functionality of ultra-sensitive magnetometers.

cond-mat.supr-con↗

arXiv2013

Spin-controlled superconductivity and tunable triplet correlations in graphene nanostructures

We study graphene ferromagnet/superconductor/ferromagnet (F/S/F) nanostructures via a microscopic self-consistent Dirac Bogoliubov-de Gennes formalism. We show that as a result of proximity effects, experimentally accessible spin switching phenomena can occur as one tunes the Fermi level $μ_F$ of the F regions or varies the angle $θ$ between exchange field orientations. Superconductivity can then be switched on and off by varying either $θ$ or $μ_F$ (a spin-controlled superconducting graphene switch). The induced equal-spin triplet correlations in S can be controlled by tuning $μ_F$, effectively making a graphene based two-dimensional spin-triplet valve.

cond-mat.supr-con↗

arXiv2012

Proximity Effects in Conical-Ferromagnet/Superconductor bilayers

We study various aspects of proximity effects in $F/S$ (Ferromagnet/Superconductor) bilayers, where $F$ has a spiral magnetic texture such as that found in Holmium, Erbium and other materials, and $S$ is a conventional s-wave superconductor. We numerically solve the Bogoliubov-de Gennes (BdG) equations self-consistently and use the solutions to compute quantities relevant to the proximity effects in these bilayers. We obtain the relation between the superconducting transition temperature $T_c$ and the thicknesses $d_F$ of the magnetic layer by solving the linearized BdG equations. We find that the $T_c(d_F)$ curves include multiple oscillations. Moreover, the system may be reentrant not only with $d_F$, as is the case when the magnet is uniform, but with temperature $T$: the superconductivity disappears in certain ranges of $d_F$ or $T$. The $T$ reentrance occurs when $d_F$ is larger than the spatial period of the conical exchange field. We compute the condensation free energies and entropies from the full BdG equations and find the results are in agreement with $T_c$ values obtained by linearization. The inhomogeneous nature of the magnet makes it possible for all odd triplet pairing components to be induced. We have investigated their properties and found that, as compared to the singlet amplitude, both the $m=0$ and $m=\pm 1$ triplet components exhibit long range penetration. For nanoscale bilayers, the proximity lengths for both layers are also obtained. These lengths oscillate with $d_F$ and they are found to be long range on both sides. These results are shown to be consistent with recent experiments. We also calculate the reverse proximity effect described by the three dimensional local magnetization, and the local DOS, which reveals important energy resolved signatures associated with the proximity effects.

cond-mat.supr-con↗

arXiv2012

Proximity effects and triplet correlations in Ferromagnet/Ferromagnet/Superconductor nanostructures

We report the results of a study of superconducting proximity effects in clean Ferromagnet/Ferromagnet/Superconductor (${\rm F_1F_2S}$) heterostructures, where the pairing state in S is a conventional singlet s-wave. We numerically find the self-consistent solutions of the Bogoliubov-de Gennes (BdG) equations and use these solutions to calculate the relevant physical quantities. By linearizing the BdG equations, we obtain the superconducting transition temperatures $T_c$ as a function of the angle $α$ between the exchange fields in $\rm F_1$ and $\rm F_2$. We find that the results for $T_c(α)$ in ${\rm F_1F_2S}$ systems are clearly different from those in ${\rm F_1 S F_2}$ systems, where $T_c$ monotonically increases with $α$ and is highest for antiparallel magnetizations. Here, $T_c(α)$ is in general a non-monotonic function, and often has a minimum near $α\approx 80^{\circ}$. For certain values of the exchange field and layer thicknesses, the system exhibits reentrant superconductivity with $α$: it transitions from superconducting to normal, and then returns to a superconducting state again with increasing $α$. This phenomenon is substantiated by a calculation of the condensation energy. We compute, in addition to the ordinary singlet pair amplitude, the induced odd triplet pairing amplitudes. The results indicate a connection between equal-spin triplet pairing and the singlet pairing state that characterizes $T_c$. We find also that the induced triplet amplitudes can be very long-ranged in both the S and F sides and characterize their range. We discuss the average density of states for both the magnetic and the S regions, and its relation to the pairing amplitudes and $T_c$. The local magnetization vector, which exhibits reverse proximity effects, is also investigated.

cond-mat.supr-con↗

arXiv2011

Perfect Absorption in Ultrathin Epsilon-Near-Zero Metamaterials Induced by Fast-Wave Non-Radiative Modes

Above-light-line surface plasmon polaritons can arise at the interface between a metal and epsilon-near-zero metamaterial. This unique feature induces unusual fast-wave non-radiative modes in a epsilon-near-zero material/metal bilayer. Excitation of this peculiar mode leads to wide-angle perfect absorption in low-loss ultrathin metamaterials. The ratio of the perfect absorption wavelength to the thickness of the epsilon-near-zero metamaterial can be as high as 10^4; the electromagnetic energy can be confined in a layer as thin as λ/10000. Unlike conventional fast-wave leaky modes, these fast-wave non-radiative modes have quasi-static capacitive features that naturally match with the space-wave field, and thus are easily accessible from free space. The perfect absorption wavelength can be tuned from mid- to far-infrared by tuning the epsilon = 0 wavelength while keeping the thickness of the structure unchanged.

physics.optics↗

arXiv2011

Reentrant superconductivity in conical-ferromagnet/superconductor nanostructures

We study a bilayer consisting of an ordinary superconductor and a magnet with a spiral magnetic structure of the {\rm Ho} type. We use a self consistent solution of the Bogolioubov-de Gennes equations to evaluate the pair amplitude, the transition temperature, and the thermodynamic functions, namely, the free energy and entropy. We find that for a range of thicknesses of the magnetic layer the superconductivity is reentrant with {\it temperature} $T$: as one lowers $T$ the system turns superconducting, and when $T$ is further lowered it turns normal again. This behavior is reflected in the condensation free energy and the pair potential, which vanish both above the upper transition and below the lower one. The transition is strictly reentrant: the low and high temperature phases are the same. The entropy further reveals a range of temperatures where the superconducting state is {\it less ordered} than the normal one.

cond-mat.supr-con↗