Bridget Kane

QUANTIZED CONDUCTANCE IN QUANTUM WIRES WITH GATE-CONTROLLED WIDTH AND ELECTRON DENSITY

We describe quantum wires and point contacts fabricated in GaAs/AlxGa1−xAs heterostructures that are free of the disorder introduced by modulation doping and in which the electron density and the confining potential are separately adjustable by lithographically defined gates. We observe conductance plateaus quantized near even multiples of e2/h in 2 μm wires and up to 15 conductance steps in 5 μm wires at temperatures below 1 K. Near the conductance threshold the quantum point contact and the 2 μm wire both show additional structure below 2e2/h.

Measurement instrumentation for electrical transport experiments in extreme pulsed magnetic fields generated by flux compression

We describe purpose-built instrumentation that has proved successful in making transport measurements of materials in ultrahigh magnetic fields generated by explosive-driven flux compression. The experimental arrangement minimizes severe problems of heating and pick-up associated with the microsecond pulsed field, for which dB/dt can be as large as 109 T/s. Electrical connection to multiple samples in the high field region is via long, lithographically-defined, nested coplanar transmission lines. Contactless measurements by capacitive coupling, as well as resistively coupled measurements, are made at frequencies of order 1 GHz so that pick-up from the microsecond pulse can be effectively filtered. We demonstrate our technique with data on semiconductor samples taken to 450 T using Russian MC-1 type flux compression generators combined with U.S. explosives at the Ancho Canyon firing point, Los Alamos National Laboratory. We discuss the obstacles encountered during these experiments, and outline improvement...

Experimental determination of theB−Tphase diagram ofYBa2Cu3O7−δto 150 T forB⊥c

The B-T phase diagram for thin film YBa_2Cu_3O_7-d with B parallel to the superconducting layers has been constructed from GHz transport measurements to 150T. Evidence for a transition from a high T regime dominated by orbital effects, to a low T regime where paramagnetic limiting drives the quenching of superconductivity, is seen. Up to 110T the upper critical field is found to be linear in T and in remarkable agreement with extrapolation of the longstanding result of Welp et al arising from magnetisation measurements to 6T. Beyond this a departure from linear behaviour occurs at T=74K, where a 3D-2D crossover is expected to occur.

Quantum point contact in a magnetic field: Far-infrared resonant heating observed in photoconductivity

We report on the far-infrared photoresponse of a quantum point contact device fabricated on a top-gated GaAs/AlGaAs heterostructure. The top-gated architecture avoids the disorder built into conventional modulation-doped structures. We observe a distinctive far-infrared magnetophotoresponse. This depends on the wavelength of the radiation and on the carrier density, which is controlled by the gate voltage. We conclude by comparison with transport data that the oscillations observed in photoconductivity and which are centred around the cyclotron energy arise from the resonant heating of electrons by the far-infrared radiation.

Experimental determination of the B-T phase diagram of YBa{sub 2}Cu{sub 3}O{sub 7-{delta}} to 150 T for B(perpendicular sign)c

The B-T phase diagram for thin film YBa_2Cu_3O_7-d with B parallel to the superconducting layers has been constructed from GHz transport measurements to 150T. Evidence for a transition from a high T regime dominated by orbital effects, to a low T regime where paramagnetic limiting drives the quenching of superconductivity, is seen. Up to 110T the upper critical field is found to be linear in T and in remarkable agreement with extrapolation of the longstanding result of Welp et al arising from magnetisation measurements to 6T. Beyond this a departure from linear behaviour occurs at T=74K, where a 3D-2D crossover is expected to occur.

Experimental determination of the B-T phase diagram of YBCO to 150 T for B⊥T

The B-T phase diagram for thin film YBa_2Cu_3O_7-d with B parallel to the superconducting layers has been constructed from GHz transport measurements to 150T. Evidence for a transition from a high T regime dominated by orbital effects, to a low T regime where paramagnetic limiting drives the quenching of superconductivity, is seen. Up to 110T the upper critical field is found to be linear in T and in remarkable agreement with extrapolation of the longstanding result of Welp et al arising from magnetisation measurements to 6T. Beyond this a departure from linear behaviour occurs at T=74K, where a 3D-2D crossover is expected to occur.

Experimental determination of B-T phase diagram of YBa_2Cu_3O_7-d to 150T for B perpendicular to c

The B-T phase diagram for thin film YBa_2Cu_3O_7-d with B parallel to the superconducting layers has been constructed from GHz transport measurements to 150T. Evidence for a transition from a high T regime dominated by orbital effects, to a low T regime where paramagnetic limiting drives the quenching of superconductivity, is seen. Up to 110T the upper critical field is found to be linear in T and in remarkable agreement with extrapolation of the longstanding result of Welp et al arising from magnetisation measurements to 6T. Beyond this a departure from linear behaviour occurs at T=74K, where a 3D-2D crossover is expected to occur.

Correlated electron phenomena in ultra-low-disorder quantum point contacts and quantum wires

The study of electron interaction effects in one dimensional transport requires heterostructures with ultra-low disorder. We have developed a novel GaAs/AlGaAs structure which avoids the random impurity potential present in conventional surface-gated HEMT devices by using epitaxially grown gates to produce an enhancement mode FET. Our quantum point contacts (QPCs) exhibit almost ideal conductance quantisation, however, below 2e/sup 2//h additional structure is observed. Such structure has also been seen in QPCs fabricated from ultra-high-mobility surface gated HEMTs and has been interpreted as evidence for a spin-correlated state. In our quantum wire devices this additional structure is further enhanced, indicating that the effective length of the 1D region may be a significant factor in determining the strength of correlation.

Low-disorder quantum wire with gate-tunable width and electron density

We have fabricated quasi-one dimensional (1D) quantum wires based upon a novel GaAs/AlGaAs heterostructure in which carriers are induced by an epitaxially-grown n/sup +/-GaAs gate rather than by modulation doping, thus minimising disorder. By using side-gates to further constrain the electrons at the centre of the wires, we observe at low temperatures a quantisation of the resistance, indicative of ballistic transport through the constriction. Motivated by theoretical predictions concerning electron correlation effects in 1D, we report initial studies of the dependence of the observed conductance plateaux upon temperature and electron density.

Fabrication and characterization of precisely graded parabolic quantum wells using monolayer deposition

We describe the fabrication and low-temperature characterization of high quality parabolic quantum wells, grown with GaAs/Al/sub x/Ga/sub 1-x/As molecular beam epitaxy using the novel method of depositing single monolayers of either GaAs or Al/sub x/Ga/sub 1-x/As so as to produce an average parabolic potential. The high quality of these wells is attested to by the appearance of magnetoresistance oscillations associated with the population of the second subband, observable at low temperatures when the well is oriented nearly parallel to the magnetic field, a phenomenon not previously observed in parabolic quantum wells.