R. A. Hughes

Intrinsic picosecond response times of Y–Ba–Cu–O superconducting photodetectors

We report our femtosecond time-resolved measurements on the photoresponse of an epitaxial YBa2Cu3O7−x (YBCO) thin-film photodetector, patterned into a microbridge geometry. By varying the current–voltage biasing conditions between the superconducting and resistive (hot spot) states, we observed transients that correspond to the nonequilibrium kinetic-inductance and the nonequilibrium electron-heating response mechanisms, respectively. The two-temperature model and the Rothwarf–Taylor theory have been used to simulate the measured wave forms and to extract the temporal parameters. The electron thermalization time and the electron–phonon energy relaxation time were determined by the electron temperature rise and decay times, which were found to be 0.56 and 1.1 ps, respectively, in the resistive state. We have also measured the ratio between the phonon and electron specific heats to be 38, which corresponds to a phonon–electron scattering time of 42 ps. No phonon-trapping effect (typical for low-temperature ...

Vertically aligned wurtzite CdTe nanowires derived from a catalytically driven growth mode

Substrate-based catalytic growth modes have been widely used to fabricate vertically aligned nanowires for most technologically relevant semiconducting systems, with CdTe being a notable exception. The catalysts that promote a one-dimensional nanoscale growth mode in other systems seem to fail, creating the need for an alternative approach. Here, we demonstrate how nanowire structures can be derived from a newly developed catalytically driven process. The vertically aligned nanowires produced are highly faceted and share an epitaxial relationship with the underlying substrate. The nanowire structures could also be described as nanorods; they show a high degree of size uniformity over large areas with heights of 300 nm imposed by growth dynamics. Two-dimensional x-ray diffraction techniques indicate that the CdTe exists in the wurtzite crystal structure instead of the zinc blende structure normally associated with the bulk material. The work presented here adds these substrate-based wurtzite CdTe nanowires to the growing list of potential building blocks for nano-based devices.

Electro‐optic sampling of 1.5‐ps photoresponse signal from YBa2Cu3O7−δ thin films

Photoresponse signals with widths as short as 1.5 ps are observed from epitaxial YBa2Cu3O7−δ  thin films using electro‐optic sampling techniques. Voltage transients less than 2 ps wide are seen in 100‐ and 200‐nm films exposed to 150‐fs laser pulses and cooled to 79 K. At low bias currents, the amplitude of the fast response varies linearly with the bias current, suggesting a kinetic inductive mechanism. A negative transient about 15‐ps long is also seen that may provide evidence for nonequilibrium recombination of excited quasiparticles into Cooper pairs. At high bias currents or large laser fluences, a fast tail with a decay time of about 10 ps appears in the response followed by a slow, resistive bolometric component due to sample heating. Nonequilibrium aspects of the photoresponse and the origin of the fast tail are discussed.

Magnetocaloric effect in Ni-Mn-Ga thin films under concurrent magnetostructural and Curie transitions

An investigation of the magnetocaloric effect for Ni-Mn-Ga films with a composition chosen to yield the highly advantageous magnetostructural phase transition between the paramagnetic austenitic and the ferromagnetic martensitic phases is presented. The observed effect is particularly strong at low magnetic fields, yielding a maximum negative entropy change of −1.4 J/kg K for a field change of only 0.5 T. It is also observed that the cooling process yields a 40% larger entropy change compared to the heating process. Temperature dependent magnetic, structural, and transport measurements indicate that the entropy peak difference between cooling and heating cycles is associated with a stronger overlap of the Curie transition of the austenitic phase with the magnetostructural phase transition upon cooling. The observed behavior is significant to micro-length-scale spot cooling applications utilizing thin films and large-scale magnetic refrigeration applications where low magnetic fields are favorable.

Evaluation of LaSrGaO4 as a substrate for YBa2Cu3O7-δ

Abstract We present a comparison study between YBa2Cu3O7−δ thin films deposited on LaSrGaO4 substrates. The LaSrGaO4 crystals were grown, cut and polished in-house while the LaAlO3 substrates were prepared commercially. The deposited films were characterized by a variety of techniques including DC resistivity, critical current density, X-ray diffraction including rocking-curve analysis and scanning electron microscopy. Growth temperatures above 780°C on both substrates yielded highly oriented c-axis material with excellent transport properties. Because LaSrGaO4 has the advantage of being untwinned, a desirable property for microwave-device applications, we conclude that it has much promise as a potential substitute for LaAlO3.

The role of substrate surface alteration in the fabrication of vertically aligned CdTe nanowires

Previously we have described the deposition of vertically aligned wurtzite CdTe nanowires derived from an unusual catalytically driven growth mode. This growth mode could only proceed when the surface of the substrate was corrupted with an alcohol layer, although the role of the corruption was not fully understood. Here, we present a study detailing the remarkable role that this substrate surface alteration plays in the development of CdTe nanowires; it dramatically improves the size uniformity and largely eliminates lateral growth. These effects are demonstrated to arise from the altered surfaces ability to limit Ostwald ripening of the catalytic seed material and by providing a surface unable to promote the epitaxial relationship needed to sustain a lateral growth mode. The axial growth of the CdTe nanowires is found to be exclusively driven through the direct impingement of adatoms onto the catalytic seeds leading to a self-limiting wire height associated with the sublimation of material from the sidewall facets. The work presented furthers the development of the mechanisms needed to promote high quality substrate-based vertically aligned CdTe nanowires. With our present understanding of the growth mechanism being a combination of selective area epitaxy and a catalytically driven vapour-liquid-solid growth mode, these results also raise the intriguing possibility of employing this growth mode in other material systems in an effort to produce superior nanowires.

In situ growth of PbSrYCaCuO films by laser ablation

We report the successful preparation of 1212 PbSrYCaCuO thin films by laser ablation. The films are grown in situ on (100)LaAlO3 at the relatively low substrate temperature of 610 °C. The starting composition of the target is not that of the superconductor produced. Its composition was choosen to yield the 2213 PbSrYCaCuO phase, but the structural analysis shows a predominantly 1212 film with a small impurity phase. The films are highly oriented and show superconducting transitions with onsets at 90 K and zero resistance by 75 K.

The role of vicinal silicon surfaces in the formation of epitaxial twins during the growth of III-V thin films

We examine the role of vicinal surface steps in the formation and propagation of twins during the growth of epitaxial III-V thin films (GaAs, InP, GaSb, AlSb) on silicon substrates. This is achieved through the combined use of two-dimensional X-ray diffraction and conventional transmission electron microscopy techniques, which allow for both a macro and nano/micro characterization of the material systems. Observed is a systematic suppression of twins formed opposite to the tilt direction of vicinal substrates through a process of step-flow overgrowth of nucleated twins, and an enhancement of twins toward the tilt direction when the fastest growth planes are aligned with the step-flow. These results indicate a probable path to the enhancement of the electronic mobility of lateral devices based on III-V semiconductors on silicon.

Picosecond photoresponse of epitaxial YBa2Cu3O7−δ thin films

Photoresponse signals as fast as 16 ps (full width at half‐maximum) have been observed from current‐biased bridge structures of epitaxial YBa2Cu3O7−δ thin films on LaAlO3 using 5 ps, 820 nm laser pulses. Operating at liquid‐nitrogen temperature (77.4 K), the amplitude of the fast response was found to be linear with current at low bias currents. At higher bias currents, a slow component appeared in the signal with a decay over several nanoseconds which could be attributed to a resistive bolometric response. Fast transients about 20 ps wide have been observed in films with thicknesses ranging from 47 to 200 nm. We believe the fast response is primarily due to a kinetic inductive bolometric mechanism associated with heating of the film by the laser pulse. There is some evidence of a nonbolometric contribution but it is not yet conclusive.

Structural and transport properties of epitaxial niobium-doped BaTiO3 films

BaTi1−xNbxO3 films, spanning the entire range of x, have been deposited on MgAl2O4 substrates. X-ray diffraction measurements indicate single phase films for all values of x, contrary to a previously reported niobium solubility limit. Films show extreme sensitivity to high temperature oxygen exposure, which destroys conductivity and severely disrupts crystallinity. Under optimum growth conditions increasing x gives rise to a Ti4+ to Ti3+ transformation in the oxidation state accompanied by increased conductivity with a semiconductor-metal transition near x=0.2. Temperature dependent magnetic measurements show an anomalous rise in the spin moment, a feature supportive of the small singlet bipolaron model.