No abstract

Surface-acoustic-wave propagation on high-quality AlGaAs/GaAs heterostructures containing a twodimensional electron system has been examined in the fractional quantum Hall effect (FQHE) regime. The response of the electron system to the sound wave is found to be similar in the FQHE states to that previously studied in the integral quantum Hall states. However, a striking disparity is observed at Landau-level filling v** j, where sound propagation is distinctly different from that in the neighboring filling-factor range. We propose that phase separation of the 2DEG may be responsible for the feature at v-j.

Tunneling and conductivity measurements through the metal-insulator transition in amorphous Nb^Si^ are reported. The authors observe the correlation gap A which varies with resistivity and have related this to the metal-insulator transition as observed in the conductivity. The samples were prepared by a process which allows precise control of the Nb concentration. The results as a function of voltage, temperature, and concentration are compared with current theoretical predictions.PACS numbers: 71.30.+h, 71.50.+t, 72.15.Cz, 73.40.Gk The metal-insulator transition in disordered materials has been recently studied in several transport measurements. 1 " 4 All experiments indicate that there is no minimum metallic conductivity. 3 ' 4 In addition, tunneling measurements have shown a strong anomaly in the density of states at the Fermi level indicative of very strong many-body contributions to the metal-insulator transition. 1 ' 2 Altshuler and Aronov 5 had earlier shown that many-body effects were important. In a phenomenological scaling theory McMillan 6 incorporated both localization and correlation effects on equal footing. He predicted a squareroot singularity N(p) = N(0)[l + (E/A) 1/2 ] in the single-particle density of states at the Fermi level on the metallic side of the transition. This was later verified by the tunneling experiments. The parameter A was interpreted as a correlation gap-the precursor of the energy gap in the single-particle density of states which opens up on the insulating side of the metal-insulator transition. At the metal-insulator (M-I) transition, A goes to zero, N(0) goes to zero, and the energy gap begins to open up.In this Letter we report high-precision tunneling and conductivity measurements through the metal-insulator transition in amorphous Nb x Si xx . Our samples were prepared by simultaneously cosputtering Nb and Si onto a substrate producing a shallow concentration gradient. Unlike previous measurements which had difficulty in producing controlled, uniform concentration changes, our method allowed us to measure the properties of the amorphous material in a controlled and systematic way as we moved through the M-I transition in small, uniform increments. As in previous experiments we have observed a correlation gap A which varies with resistivity. However unlike previous experiments we have made low-temperature transport measurements to correlate our tunneling results directly with the metal-insultor transition. In addition we have observed the metal-insulator transition by the onset of the breakdown of the single-step tunneling process when the energy gap appears at the Fermi energy and localized states appear in the Nb^Sii^. By controlling concentration as easily as voltage and temperature we can obtain experimental relationships among them and in this Letter these are presented and compared with current theoretical predictions.Our samples were prepared by cosputtering Nb and Si from separate targets onto sapphire substrates (see inset in Fig. 1). The films had a thickness...

Experiments on small superconducting tunnel junctions in a Giaever-Zeller-plus-SQUID geometry clearly show an I(V) feature due to Josephson super-current in a regime where single-electron charging effects are dominant. The I(V) data exhibit marked oscillations with gate voltage characteristic of single-electron charging. The Josephson feature shows both this effect and SQUID-related oscillations with applied magnetic field. The behavior can be understood through the usual model for charging effects in such circuits, extended to include two-electron transitions.

No abstract