Semiconductor Epitaxy in Superconducting Templates.

Integration of high-quality semiconductor-superconductor devices into scalable and complementary metal-oxide-semiconductor compatible architectures remains an outstanding challenge, currently hindering their practical implementation. Here, we demonstrate growth of InAs nanowires monolithically integrated on Si inside lateral cavities containing superconducting TiN elements. This technique allows growth of hybrid devices characterized by sharp semiconductor-superconductor interfaces and with alignment along arbitrary crystallographic directions. Electrical characterization at low temperature reveals proximity induced superconductivity in InAs via a transparent interface.

Highly Asymmetric Chiral Domain-Wall Velocities in Y-Shaped Junctions.

Recent developments in spin-orbit torques allow for highly efficient current-driven domain wall (DW) motion in nanowires with perpendicular magnetic anisotropy. Here, we show that chiral DWs can be driven into nonequilibrium states that can persist over tens of nanoseconds in Y-shaped magnetic nanowire junctions that have an input and two symmetric outputs. A single DW that is injected into the input splits and travels at very different velocities in the two output branches until it reaches its steady-state velocity. We find that this is due to the disparity between the fast temporal evolution of the spin current derived spin-orbit torque and a much-slower temporal evolution of the DMI-derived torque. Changing the DW polarity inverts the velocity asymmetry in the two output branches, a property that we use to demonstrate the sorting of domains.

Dramatic influence of curvature of nanowire on chiral domain wall velocity.

The use of current pulses to move domain walls along nanowires is one of the most exciting developments in spintronics over the past decade. We show that changing the sign of the curvature of a nanowire changes the speed of chiral Néel domain walls in perpendicularly magnetized nanowires by up to a factor of 10. The domain walls have an increased or decreased velocity in wires of a given curvature, independent of the domain wall chirality and the sign of the current-induced spin-orbit torques. Thus, adjacent domain walls move at different speeds. For steady motion of domain walls along the curved nanowire, the torque must increase linearly with the radius, which thereby results in a width-dependent tilting of the domain wall. We show that by using synthetic antiferromagnetic nanowires, the influence of the curvature on the domain wall's velocity is eliminated, and all domain walls move together, emphasizing the use of such structures for spintronic applications.

Enhanced spin-orbit torques by oxygen incorporation in tungsten films.

The origin of spin-orbit torques, which are generated by the conversion of charge-to-spin currents in non-magnetic materials, is of considerable debate. One of the most interesting materials is tungsten, for which large spin-orbit torques have been found in thin films that are stabilized in the A15 (ß-phase) structure. Here we report large spin Hall angles of up to approximately -0.5 by incorporating oxygen into tungsten. While the incorporation of oxygen into the tungsten films leads to significant changes in their microstructure and electrical resistivity, the large spin Hall angles measured are found to be remarkably insensitive to the oxygen-doping level (12-44%). The invariance of the spin Hall angle for higher oxygen concentrations with the bulk properties of the films suggests that the spin-orbit torques in this system may originate dominantly from the interface rather than from the interior of the films.

Giant thermal spin-torque-assisted magnetic tunnel junction switching.

Spin-polarized charge currents induce magnetic tunnel junction (MTJ) switching by virtue of spin-transfer torque (STT). Recently, by taking advantage of the spin-dependent thermoelectric properties of magnetic materials, novel means of generating spin currents from temperature gradients, and their associated thermal-spin torques (TSTs), have been proposed, but so far these TSTs have not been large enough to influence MTJ switching. Here we demonstrate significant TSTs in MTJs by generating large temperature gradients across ultrathin MgO tunnel barriers that considerably affect the switching fields of the MTJ. We attribute the origin of the TST to an asymmetry of the tunneling conductance across the zero-bias voltage of the MTJ. Remarkably, we estimate through magneto-Seebeck voltage measurements that the charge currents that would be generated due to the temperature gradient would give rise to STT that is a thousand times too small to account for the changes in switching fields that we observe.

Highly efficient in-line magnetic domain wall injector.

We demonstrate a highly efficient and simple scheme for injecting domain walls into magnetic nanowires. The spin transfer torque from nanosecond long, unipolar, current pulses that cross a 90° magnetization boundary together with the fringing magnetic fields inherently prevalent at the boundary, allow for the injection of single or a continual stream of domain walls. Remarkably, the currents needed for this "in-line" domain wall injection scheme are at least one hundred times smaller than conventional methods.

Fluctuating stripes at the onset of the pseudogap in the high-T(c) superconductor Bi(2)Sr(2)CaCu(2)O(8+x).

Doped Mott insulators have a strong propensity to form patterns of holes and spins often referred to as stripes. In copper oxides, doping also gives rise to the pseudogap state, which can be transformed into a high-temperature superconducting state with sufficient doping or by reducing the temperature. A long-standing issue has been the interplay between the pseudogap, which is generic to all hole-doped copper oxide superconductors, and stripes, whose static form occurs in only one family of copper oxides over a narrow range of the phase diagram. Here we report observations of the spatial reorganization of electronic states with the onset of the pseudogap state in the high-temperature superconductor Bi(2)Sr(2)CaCu(2)O(8+x), using spectroscopic mapping with a scanning tunnelling microscope. We find that the onset of the pseudogap phase coincides with the appearance of electronic patterns that have the predicted characteristics of fluctuating stripes. As expected, the stripe patterns are strongest when the hole concentration in the CuO(2) planes is close to 1/8 (per copper atom). Although they demonstrate that the fluctuating stripes emerge with the onset of the pseudogap state and occur over a large part of the phase diagram, our experiments indicate that the stripes are a consequence of pseudogap behaviour rather than its cause.

Nanoscale proximity effect in the high-temperature superconductor Bi{2}Sr{2}CaCu{2}O{8+delta} using a scanning tunneling microscope.

High-temperature cuprate superconductors exhibit extremely local nanoscale phenomena and strong sensitivity to doping. While other experiments have looked at nanoscale interfaces between layers of different dopings, we focus on the interplay between naturally inhomogeneous nanoscale regions. Using scanning tunneling microscopy to carefully track the same region of the sample as a function of temperature, we show that regions with weak superconductivity can persist to elevated temperatures if bordered by regions of strong superconductivity. This suggests that it may be possible to increase the maximum possible transition temperature by controlling the distribution of dopants.

Extending universal nodal excitations optimizes superconductivity in Bi2Sr2CaCu2O8+delta.

Understanding the mechanism by which d wave superconductivity in the cuprates emerges and is optimized by doping the Mott insulator is one of the major outstanding problems in condensed-matter physics. Our high-resolution scanning tunneling microscopy measurements of the high-transition temperature (Tc) superconductor Bi2Sr2CaCu2O8+delta show that samples with different Tc values in the low doping regime follow a remarkably universal d wave low-energy excitation spectrum, indicating a doping-independent nodal gap. We demonstrate that Tc instead correlates with the fraction of the Fermi surface over which the samples exhibit the universal spectrum. Optimal Tc is achieved when all parts of the Fermi surface follow this universal behavior. Increasing the temperature above Tc turns the universal spectrum into an arc of gapless excitations, whereas overdoping breaks down the universal nodal behavior.

Electronic origin of the inhomogeneous pairing interaction in the high-Tc superconductor Bi2Sr2CaCu2O8+delta.

Identifying the mechanism of superconductivity in the high-temperature cuprate superconductors is one of the major outstanding problems in physics. We report local measurements of the onset of superconducting pairing in the high-transition temperature (Tc) superconductor Bi2Sr2CaCu2O8+delta using a lattice-tracking spectroscopy technique with a scanning tunneling microscope. We can determine the temperature dependence of the pairing energy gaps, the electronic excitations in the absence of pairing, and the effect of the local coupling of electrons to bosonic excitations. Our measurements reveal that the strength of pairing is determined by the unusual electronic excitations of the normal state, suggesting that strong electron-electron interactions rather than low-energy (<0.1 volts) electron-boson interactions are responsible for superconductivity in the cuprates.

Visualizing pair formation on the atomic scale in the high-Tc superconductor Bi2Sr2CaCu2O8+delta.

Pairing of electrons in conventional superconductors occurs at the superconducting transition temperature T(c), creating an energy gap Delta in the electronic density of states (DOS). In the high-T(c) superconductors, a partial gap in the DOS exists for a range of temperatures above T(c) (ref. 2). A key question is whether the gap in the DOS above T(c) is associated with pairing, and what determines the temperature at which incoherent pairs form. Here we report the first spatially resolved measurements of gap formation in a high-T(c) superconductor, measured on Bi2Sr2CaCu2O8+delta samples with different T(c) values (hole concentration of 0.12 to 0.22) using scanning tunnelling microscopy. Over a wide range of doping from 0.16 to 0.22 we find that pairing gaps nucleate in nanoscale regions above T(c). These regions proliferate as the temperature is lowered, resulting in a spatial distribution of gap sizes in the superconducting state. Despite the inhomogeneity, we find that every pairing gap develops locally at a temperature T(p), following the relation 2Delta/k(B)T(p) = 7.9 +/- 0.5. At very low doping (< or =0.14), systematic changes in the DOS indicate the presence of another phenomenon, which is unrelated and perhaps competes with electron pairing. Our observation of nanometre-sized pairing regions provides the missing microscopic basis for understanding recent reports of fluctuating superconducting response above T(c) in hole-doped high-T(c) copper oxide superconductors.