arXiv subjects

J. W. A. Robinson

Publications and source records attributed to J. W. A. Robinson.

31 records · Page 2Linked to original sources

arXiv2017

Structural properties of thin-film ferromagnetic topological insulators

We present a comprehensive study of the crystal structure of the thin-film, ferromagnetic topological insulator (Bi, Sb)$_{2-x}$V$_x$Te$_3$. The dissipationless quantum anomalous Hall edge states it manifests are of particular interest for spintronics, as a natural spin filter or pure spin source, and as qubits for topological quantum computing. For ranges typically used in experiments, we investigate the effect of doping, substrate choice and film thickness on the (Bi, Sb)$_2$Te$_3$ unit cell using high-resolution X-ray diffractometry. Scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy measurements provide local structural and interfacial information. We find that the unit cell is unaffected in-plane by vanadium doping changes, and remains unchanged over a thickness range of 4--10 quintuple layers (1 QL $\approx$ 1 nm). The in-plane lattice parameter ($a$) also remains the same in films grown on different substrate materials. However, out-of-plane the $c$-axis is reduced in films grown on less closely lattice-matched substrates, and increases with the doping level.

cond-mat.mtrl-sci↗

arXiv2016

Andreev bound states in superconductor/ferromagnet point contact Andreev reflection spectra

As charge carriers traverse a single superconductor ferromagnet interface they experience an additional spin-dependent phase angle which results in spin mixing and the formation of a bound state called the Andreev Bound State. This state is an essential component in the generation of long range spin triplet proximity induced superconductivity and yet the factors controlling the degree of spin mixing and the formation of the bound state remain elusive. Here we demonstrate that point contact Andreev reflection can be used to detect the bound state and extract the resulting spin mixing angle. By examining spectra taken from La1.15Sr1.85Mn2O7 single crystal - Pb junctions, together with a compilation of literature data on highly spin polarised systems, we show that the existence of the Andreev Bound State both resolves a number of long standing controversies in the Andreev literature as well as defining a route to quantify the strength of spin mixing at superconductor-ferromagnet interfaces. Intriguingly we find that for these high transparency junctions, the spin mixing angle appears to take a relatively narrow range of values across all the samples studied. The ferromagnets we have chosen to study share a common property in terms of their spin arrangement, and our observations may point to the importance of this property in determining the spin mixing angle under these circumstances.

cond-mat.supr-con↗

arXiv2015

Strong odd-frequency correlations in fully gapped Zeeman-split superconductors

It is now well established that at a superconductor/ferromagnet (S/F) interface an unconventional superconducting state arises in which the pairing state is odd-frequency. The hallmark signature of this superconducting state is generally understood to be an enhancement of the electronic density of states (DoS) at subgap energies close to the S/F interface. However, here we show that an odd frequency state can be present even if the DoS is fully gapped. As an example, we show that this is the case in the pioneering S/FI (where FI is a insulating ferromagnet) tunneling experiments of Meservey and Tedrow, and we derive a generalized analytical criterium to describe the effect of odd-frequency pairing on the DoS. Finally, we propose a simple experiment in which odd-frequency pairing in a Zeeman-split superconductor can be unambiguously detected via the application of an external magnetic field.

cond-mat.supr-con↗

arXiv2015

Reversible control of spin-polarised supercurrents in ferromagnetic Josephson junctions

Magnetic inhomogeneity at a superconductor (S)/ferromagnet (F) interface converts spin-singlet Cooper pairs into spin-one triplet pairs. These pairs are immune to the pair-breaking exchange field in F and support a long-range proximity effect. Although recent experiments have confirmed the existence of spin-polarised triplet supercurrents in S-F-S Josephson junctions, reversible control of the supercurrent has been impossible because of the robust pre-configured nature of the inhomogeneity. Here we use a barrier comprising three F layers whose relative magnetic orientation, and hence the interfacial inhomogeneity, can be controlled by small magnetic fields; we show that this enables full control of the triplet supercurrent and, by using finite element micromagnetic simulations, we can directly relate the experimental data to the theoretical models which provide a general framework to understand the role played by magnetic states in long-range supercurrent modulation.

cond-mat.supr-con↗

arXiv2015

Magnetic coupling at ferromagnetic rare earth / transition-metal interfaces: A comprehensive study

Thin film magnetic heterostructures with competing interfacial coupling and Zeeman energy pro- vide a fertile ground to study phase transition between different equilibrium states as a function of external magnetic field and temperature. A rare-earth (RE) / transition metal (TM) ferro- magnetic multilayer is a classic example where the magnetic state is determined by a competition between the Zeeman energy and antiferromagnetic interfacial exchange coupling energy. Techno- logically, such structures offer the possibility to engineer the macroscopic magnetic response by tuning the microscopic interactions between the layers. We have performed an exhaustive study of a nickel/gadolinium system by using the element-specific measurement technique x-ray magnetic circular dichroism, and determined the full magnetic state diagrams as a function of temperature and magnetic layer thickness. We explain our result based on a modified Stoner-Wohlfarth formal- ism and provide evidence of a thickness-dependent phase transition to a magnetic fan state which is critical in understanding magnetoresistance effects in RE/TM systems. The results provide im- portant insight for spintronics and superconducting spintronics where engineering tunable magnetic inhomogeneity is key for certain applications.

cond-mat.mtrl-sci↗

arXiv2014

Proposal for a phase-coherent thermoelectric transistor

Identifying materials and devices which offer efficient thermoelectric effects at low temperature is a major obstacle for the development of thermal management strategies for low-temperature electronic systems. Superconductors cannot offer a solution since their near perfect electron-hole symmetry leads to a negligible thermoelectric response; however, here we demonstrate theoretically a superconducting thermoelectric transistor which offers unparalleled figures of merit of up to $\sim 45$ and Seebeck coefficients as large as a few mV/K at sub-Kelvin temperatures. The device is also phase-tunable meaning its thermoelectric response for power generation can be precisely controlled with a small magnetic field. Our concept is based on a superconductor-normal metal-superconductor interferometer in which the normal metal weak-link is tunnel coupled to a ferromagnetic insulator and a Zeeman split superconductor. Upon application of an external magnetic flux, the interferometer enables phase-coherent manipulation of thermoelectric properties whilst offering efficiencies which approach the Carnot limit.

cond-mat.mes-hall↗

arXiv2014

Pure 2nd harmonic current phase relation in Spin filter Josephson junctions

Higher harmonics in current phase relations of Josephson Junctions (JJs) are predicted to be observed when the first harmonic is suppressed. Conventional theoretical models predict higher harmonics to be extremely sensitive to changes in barrier thickness, temperature, etc. In contrast, experiments with JJs incorporating a spin dependent tunnelling barrier reported here reveal a current phase relation for highly spin polarized barriers which is purely 2nd harmonic in nature, and this is insensitive to changes in barrier thickness. This observation is consistent with recent theoretical predictions of a robust 2nd harmonic current phase relation for certain JJs with ferromagnetic barriers and implies that the standard theory of Cooper pair transport through tunnelling barriers is not applicable for spin dependent tunnelling barriers.

cond-mat.supr-con↗

arXiv2012

Supra-oscillatory critical temperature dependence of Nb-Ho bilayers

We investigate the critical temperature Tc of a thin s-wave superconductor (Nb) proximity coupled to a helical rare earth ferromagnet (Ho). As a function of the Ho layer thickness, we observe multiple oscillations of Tc superimposed on a slow decay, that we attribute to the influence of the Ho on the Nb proximity effect. Because of Ho inhomogeneous magnetization, singlet and triplet pair correlations are present in the bilayers. We take both into consideration when solving the self consistent Bogoliubov-de Gennes equations, and we observe a reasonable agreement. We also observe non-trivial transitions into the superconducting state, the zero resistance state being attained after two successive transitions which appear to be associated with the magnetic structure of Ho.

cond-mat.supr-con↗

arXiv2012

Magnetic-coupling-dependent spin-triplet supercurrents in helimagnet/ferromagnet Josephson junctions

The experimental achievements during the past year in demonstrating the existence of long-ranged spin-triplet supercurrents in ferromagnets proximity coupled to singlet superconductors open up the possibility for new interesting physics and applications [for a review, see M. Eschrig, Phys. Today 64(1), 43 (2011)]. Our group reported the injection of triplet supercurrents into a magnetically uniform ferromagnet (Co) by sandwiching it between two helimagnet/superconductor (Ho/Nb) bilayers to form a Nb/Ho/Co/Ho/Nb-type Josephson device. In the function of the Ho layer thicknesses, the supercurrent was found to modulate in a complex way that seemed to depend on the magnetic structure of Ho. To understand this unusual behavior, we have theoretically studied the properties of an ideal Josephson device with a helimagnet/ferromagnet/helimagnet (HM/F/HM) barrier in the clean limit using the Eilenberger equation; we show, in particular, that the maximum triplet supercurrent that can pass across the barrier will depend non-monotonically on the thicknesses of the HM layers if the HM and F layers are magnetically exchange coupled at their interface.

cond-mat.supr-con↗

arXiv2009

Critical current of a Josephson junction containing a conical magnet

We calculate the critical current of a superconductor/ferromagnetic/superconductor (S/FM/S) Josephson junction in which the FM layer has a conical magnetic structure composed of an in-plane rotating antiferromagnetic phase and an out-of-plane ferromagnetic component. In view of the realistic electronic properties and magnetic structures that can be formed when conical magnets such as Ho are grown with a polycrystalline structure in thin-film form by methods such as direct current sputtering and evaporation, we have modeled this situation in the dirty limit with a large magnetic coherence length ($ξ_f$). This means that the electron mean free path is much smaller than the normalized spiral length $λ/2π$ which in turn is much smaller than $ξ_f$ (with $λ$ as the length a complete spiral makes along the growth direction of the FM). In this physically reasonable limit we have employed the linearized Usadel equations: we find that the triplet correlations are short ranged and manifested in the critical current as a rapid oscillation on the scale of $λ/2π$. These rapid oscillations in the critical current are superimposed on a slower oscillation which is related to the singlet correlations. Both oscillations decay on the scale of $ξ_f$. We derive an analytical solution and also describe a computational method for obtaining the critical current as a function of the conical magnetic layer thickness.

cond-mat.supr-con↗

arXiv2008

Parity-dependent proximity effect in superconductor/antiferromagnet heterostructures

We report the effect on the superconducting transition temperature (Tc) of a Nb film proximity coupled to the synthetic antiferromagnet Fe/{Cr/Fe}_(N-1). We find that there is a parity dependence of Tc on the total number of Fe layers, N; locally Tc is always a maximum when N is even, and a minimum when N is odd. The Fe electron mean free path and coherence length are indicative of dirty limit behavior; as such, we numerically model our data using the linearized Usadel equations with good correlation.

cond-mat.supr-con↗

arXiv2007

0-pi oscillations in nanostructured Nb/Fe/Nb Josephson junctions

The physics of the $π$ phase shift in ferromagnetic Josephson junctions may enable a range of applications for spin-electronic devices and quantum computing. We investigate transitions from ``0'' to ``$π$'' states in Nb/Fe/Nb Josephson junctions by varying the Fe barrier thickness from 0.5 nm to 5.5 nm. From magnetic measurements we estimate for Fe a magnetic dead layer of about 1.1 nm. By fitting the characteristic voltage oscillations with existing theoretical models we extrapolate an exchange energy of 256 meV, a Fermi velocity of $1.98 \times 10^5$ m/s and an electron mean free path of 6.2 nm, in agreement with other reported values. From the temperature dependence of the $I_CR_N$ product we show that its decay rate exhibits a nonmonotonic oscillatory behavior with the Fe barrier thickness.

cond-mat.supr-con↗

arXiv2006

Critical Current Oscillations in Strong Ferromagnetic Pi-Junctions

We report magnetic and electrical measurements of Nb Josephson junctions with strongly ferromagnetic barriers of Co, Ni and Ni80Fe20 (Py). All these materials show multiple oscillations of critical current with barrier thickness implying repeated 0-pi phase-transitions in the superconducting order parameter. We show in particular that the Co barrier devices can be accurately modelled using existing clean limit theories and so that, despite the high exchange energy (309 meV), the large IcRN value in the pi-state means Co barriers are ideally suited to the practical development of superconducting pi-shift devices.

cond-mat.supr-con↗