12 research outputs found
Wide-band current preamplifier for conductance measurements with large input capacitance
A wide-band current preamplifier based on a composite operational amplifier
is proposed. It has been shown that the bandwidth of the preamplifier can be
significantly increased by enhancing the effective open-loop gain of the
composite preamplifier. The described preamplifier with current gain 10 V/A
showed the bandwidth of about 100 kHz with 1 nF input shunt capacitance. The
current noise of the amplifier was measured to be about 46 fA/
at 1 kHz, close to the design noise minimum. The voltage noise was found to be
about 2.9 nV/ at 1 kHz, which is in a good agreement with the
value expected for the operational amplifier used in the input stage. By
analysing the total noise produced by the preamplifier we found the optimal
frequency range suitable for the fast lock-in measurements to be from 1 kHz to
2 kHz. To get the same signal-to-noise ratio, the reported preamplifier
requires roughly 10% of the integration time used in measurements made with a
conventional preamplifier.Comment: 5 pages, 4 figure
An Electronic Mach-Zehnder Interferometer
Double-slit electron interferometers, fabricated in high mobility
two-dimensional electron gas (2DEG), proved to be very powerful tools in
studying coherent wave-like phenomena in mesoscopic systems. However, they
suffer from small fringe visibility due to the many channels in each slit and
poor sensitivity to small currents due to their open geometry. Moreover, the
interferometers do not function in a high magnetic field, namely, in the
quantum Hall effect (QHE) regime, since it destroys the symmetry between left
and right slits. Here, we report on the fabrication and operation of a novel,
single channel, two-path electron interferometer that functions in a high
magnetic field. It is the first electronic analog of the well-known optical
Mach-Zehnder (MZ) interferometer. Based on single edge state and closed
geometry transport in the QHE regime the interferometer is highly sensitive and
exhibits very high visibility (62%). However, the interference pattern decays
precipitously with increasing electron temperature or energy. While we do not
understand the reason for the dephasing we show, via shot noise measurement,
that it is not a decoherence process that results from inelastic scattering
events.Comment: to appear in Natur
Beating of Aharonov-Bohm oscillations in a closed-loop interferometer
One of the points at issue with closed-loop-type interferometers is beating
in the Aharonov-Bohm (AB) oscillations. Recent observations suggest the
possibility that the beating results from the Berry-phase pickup by the
conducting electrons in materials with the strong spin-orbit interaction (SOI).
In this study, we also observed beats in the AB oscillations in a gate-defined
closed-loop interferometer fabricated on a GaAs/AlGaAs two-dimensional
electron-gas heterostructure. Since this heterostructure has very small SOI,
the picture of the Berry-phase pickup is ruled out. The observation of beats in
this study, with the controllability of forming a single transverse subband
mode in both arms of our gate-defined interferometer, also rules out the
often-claimed multiple transverse subband effect. It is observed that nodes of
the beats with an h/2e period exhibit a parabolic distribution for varying the
side gate. These results are shown to be well interpreted, without resorting to
the SOI effect, by the existence of two-dimensional multiple longitudinal modes
in a single transverse subband. The Fourier spectrum of measured conductance,
despite showing multiple h/e peaks with the magnetic-field dependence that are
very similar to that from strong-SOI materials, can also be interpreted as the
two-dimensional multiple-longitudinal-modes effect