A cool spin on supercomputers

Niladri Banerjee explains how the emerging field of "superconducting spintronics" could lead to a new generation of green supercomputers that use far less energy than previous devices<br

Magnetization reorientation due to the superconducting transition in heavy-metal heterostructures

© 2019 American Physical Society. UK. Recent theoretical and experimental work has demonstrated how the superconducting critical temperature (Tc) can be modified by rotating the magnetization of a single homogeneous ferromagnet proximity-coupled to the superconducting layer. This occurs when the superconductor and ferromagnet are separated by a thin heavy normal metal that provides an enhanced interfacial Rashba spin-orbit interaction. In the present work, we consider the reciprocal effect: magnetization reorientation driven by the superconducting phase transition. We solve the tight-binding Bogoliubov-de Gennes equations on a lattice self-consistently and compute the free energy of the system. We find that the relative angle between the spin-orbit field and the magnetization gives rise to a contribution in the free energy even in the normal state, T>Tc, due to band-structure effects. For temperatures below Tc, superconductivity gives rise to a competing contribution. We demonstrate that by lowering the temperature, in addition to reorientation of the favored magnetization direction from in-plane to out-of-plane, a π/4 in-plane rotation for thicker ferromagnetic layers is possible. Furthermore, computation of Tc of the structure in the ballistic limit shows a dependence on the in-plane orientation of the magnetization, in contrast to our previous result on the diffusive limit. This finding is relevant with respect to thin-film heterostructures since these are likely to be in the ballistic regime of transport rather than in the diffusive regime. Finally, we discuss the experimental feasibility of observing the magnetic anisotropy induced by the superconducting transition when other magnetic anisotropies, such as the shape anisotropy for a ferromagnetic film, are taken into account. Our work suggests that the superconducting condensation energy in principle can trigger a reorientation of the magnetization of a thin-film ferromagnet upon lowering the temperature below Tc, in particular for ferromagnets with weak magnetic anisotropies

Electrodynamics of Josephson junctions containing strong ferromagnets

Triplet supercurrents in multilayer ferromagnetic Josephson junctions with misaligned magnetization survive longer barrier thicknesses when compared with singlet supercurrents. The distinctive feature of triplet supercurrents is the scaling of the characteristic voltage of the junction with increasing ferromagnetic barrier thickness - an algebraic decay in contrast to an exponential decay for singlet supercurrents. Although the static properties of these junctions have been extensively studied, the dynamic characteristics remain largely unexplored. Here we report a comprehensive electrodynamic characterization of multilayer ferromagnetic Josephson junctions composed of Co and Ho. By measuring the temperature-dependent current-voltage characteristics and the switching current distributions down to 0.3 K, we show that phase dynamics of junctions with triplet supercurrents exhibits long (in terms of proximity) junction behavior and moderately damped dynamics with renormalized capacitance and resistance. This unconventional behavior possibly provides a different way to dynamically detect triplets. Our results show that new theoretical models are required to fully understand the phase dynamics of triplet Josephson junctions for applications in superconducting spintronics

Controlling the superconducting transition by spin-orbit coupling

Whereas there exists considerable evidence for the conversion of singlet Cooper pairs into triplet Cooper pairs in the presence of inhomogeneous magnetic fields, recent theoretical proposals have suggested an alternative way to exert control over triplet generation: intrinsic spin-orbit coupling in a homogeneous ferromagnet coupled to a superconductor. Here, we proximity-couple Nb to an asymmetric Pt/Co/Pt trilayer, which acts as an effective spin-orbit coupled ferromagnet owing to structural inversion asymmetry. Unconventional modulation of the superconducting critical temperature as a function of in-plane and out-of-plane applied magnetic fields suggests the presence of triplets that can be controlled by the magnetic orientation of a single homogeneous ferromagnet. Our studies demonstrate for the first time an active role of spin-orbit coupling in controlling the triplets – an important step towards the realization of novel superconducting spintronic devices

Superconductivity-induced change in magnetic anisotropy in epitaxial ferromagnet-superconductor hybrids with spin-orbit interaction

The interaction between superconductivity and ferromagnetism in thin film superconductor/ferromagnet heterostructures is usually reflected by a change in superconductivity of the S layer set by the magnetic state of the F layers. Here we report the converse effect: transformation of the magnetocrystalline anisotropy of a single Fe(001) layer, and thus its preferred magnetization orientation, driven by the superconductivity of an underlying V layer through a spin-orbit coupled MgO interface. We attribute this to an additional contribution to the free energy of the ferromagnet arising from the controlled generation of triplet Cooper pairs, which depends on the relative angle between the exchange field of the ferromagnet and the spin-orbit field. This is fundamentally different from the commonly observed magnetic domain modification by Meissner screening or domain wall-vortex interaction, and it offers the ability to fundamentally tune magnetic anisotropies using superconductivity—a key step in designing future cryogenic magnetic memories

Structural and transport properties of Y_1-x(Dy)_xPdBi (0 ≤ x ≤ 1) topological semi-metallic thin films

We report the effect of 4f electron doping on structural, electrical, and magneto-transport properties of Dy doped half Heusler Y1-x(Dy)xPdBi (x = 0, 0.2, 0.5, and 1) thin films grown by pulsed laser deposition. The electrical transport measurements show a typical semi-metallic behavior in the temperature range of 3 K ≤ T ≤ 300 K and a sharp drop in resistivity at low temperatures (1-x(Dy)xPdBi show surface dominated relativistic carrier transport at low temperatures

Charge transport through functionalized graphene quantum dots embedded in a polyaniline matrix

Nitrogen-functionalized graphene quantum dots embedded in a polyaniline matrix (NGQD−PANI) are extremely promising candidates for the development of next-generation sensors and for thermoelectric materials design with the distinct advantage of tunability of electronic properties by controlled doping and/or by controlling the inherent disorder in the microstructure. While their application is increasing in photovoltaics, energy storage, and sensing technologies, a clear understanding of conduction in these hybrid systems is lacking. Here, we report a comprehensive study of NGQD−PANI composites with varying NGQD doping levels over a wide range of temperature. We show distinct regimes of conduction as a function of temperature, which include: a transition from Efros− Shklovskii and Larkin−Khmelnitskii variable range hopping at low temperatures to thermally driven electron transport at higher temperatures. Importantly, we find a remarkable 50-fold enhancement in conductivity for 10% NGQD-doped samples and tunability of the crossover temperature between different regimes as a function of the applied voltage bias and doping. Our work provides a general framework to understand the interplay of extrinsic parameters like temperature and voltage bias with intrinsic material properties like doping, which drives the electronic properties in these hybrid systems of technological importance

Supporting Information files for Charge transport through functionalized graphene quantum dots embedded in polyaniline matrix

Supporting Information files for Charge transport through functionalized graphene quantum dots embedded in polyaniline matrix.Nitrogen-functionalized graphene quantum dots embedded in polyaniline matrix (NGQD-PANI) are extremely promising candidates for the development of next-generation sensors and for thermoelectric materials design with the distinct advantage of tunability of electronic properties by controlled doping and/or by controlling the inherent disorder in the microstructure. While their application is increasing in photovoltaics, energy storage and sensing technologies, a clear understanding of conduction in these hybrid systems is lacking. Here, we report a comprehensive study of NGQD-PANI composites with varying NGQD doping levels over a wide range of temperature. We show distinct regimes of conduction as a function of temperature which include: a transition from Efros-Shklovskii and Larkin-Khmelnitskii variable range hopping at low temperatures to thermally driven electron transport at higher temperatures. Importantly, we find a remarkable 50-fold enhancement in conductivity for 10% NGQD doped samples and tunability of the crossover temperature between different regimes as a function of the applied voltage bias and doping. Our work provides a general framework to understand the interplay of extrinsic parameters like temperature and voltage bias with intrinsic material properties like doping which drives the electronic properties in these hybrid systems of technological importance.<br

Large electrocaloric effect in lead-free ferroelectric Ba_0.85Ca_0.15Ti_0.9Zr_0.1O₃ thin film heterostructure

A large electrocaloric effect is reported in a strain-engineered Ba0.85Ca0.15Ti0.9Zr0.1O3 (BCZT) thin film heterostructure driven by the near room-temperature electro-structural phase transition. An epitaxial BCZT/La0.7Sr0.3MnO3 (BCZT/LSMO) heterostructure was grown on single-crystal SrTiO3 (100) substrate using pulsed laser deposition. In-depth x-ray diffraction and x-ray spectroscopic analyses revealed the single-crystalline nature and stoichiometric growth of the heterostructure. Both temperature dependent x-ray diffraction and dielectric measurements revealed a broad second-order-type phase transition near 430 K in the BCZT/LSMO heterostructure. From detailed theoretical analyses of the experimental data it was confirmed that the phase transition around 430 K is second-order in nature, unlike the first order transition observed in bulk BCZT materials. Thermodynamic analyses of polarization revealed unprecedently large adiabatic temperature change of 13.5 K at 430 K under a field change of 1000 kVcm-1 ; hitherto unobserved in a lead-free material. Extremely broad adiabatic temperature change ΔT(T) curves over a wide working range of temperatures (330 K < T < 480 K) resulted in enhanced relative cooling powers which are higher than those reported so far in most electrocaloric materials. We propose that an interfacial strain-induced enhanced tetragonal distortion of the BCZT layer gives rise to these large electrocaloric effects in the BCZT/LSMO heterostructure system. The demonstration of large electrocaloric effect in the lead-free BCZT thin film may open up new pathways towards the design of artificial heterostructures for eco-friendly solid-state cooling applications

Strain driven emergence of topological non-triviality in YPdBi thin films

Half-Heusler compounds exhibit a remarkable variety of emergent properties such as heavy-fermion behaviour, unconventional superconductivity and magnetism. Several of these compounds have been predicted to host topologically non-trivial electronic structures. Remarkably, recent theoretical studies have indicated the possibility to induce non-trivial topological surface states in an otherwise trivial half-Heusler system by strain engineering. Here, using magneto-transport measurements and first principles DFT-based simulations, we demonstrate topological surface states on strained [110] oriented thin films of YPdBi grown on (100) MgO. These topological surface states arise in an otherwise trivial semi-metal purely driven by strain. Furthermore, we observe the onset of superconductivity in these strained films highlighting the possibility of engineering a topological superconducting state. Our results demonstrate the critical role played by strain in engineering novel topological states in thin film systems for developing next-generation spintronic devices