Aveek Bid

Observation of neutral modes in the fractional quantum Hall regime

The quantum Hall effect takes place in a two-dimensional electron gas under a strong magnetic field and involves current flow along the edges of the sample. For some particle–hole conjugate states of the fractional regime (for example, with fillings between 1/2 and 1 of the lowest Landau level), early predictions suggested the presence of counter-propagating edge currents in addition to the expected ones. When this did not agree with the measured conductance, it was suggested that disorder and interactions will lead to counter-propagating modes that carry only energy—the so called neutral modes. In addition, a neutral upstream mode (the Majorana mode) was expected for selected wavefunctions proposed for the even-denominator filling 5/2. Here we report the direct observation of counter-propagating neutral modes for fillings of 2/3, 3/5 and 5/2. The basis of our approach is that, if such modes impinge on a narrow constriction, the neutral quasiparticles will be partly reflected and fragmented into charge carriers, which can be detected through shot noise measurements. We find that the resultant shot noise is proportional to the injected current. Moreover, when we simultaneously inject a charge mode, the presence of the neutral mode was found to significantly affect the Fano factor and the temperature of the backscattered charge mode. In particular, such observations for filling 5/2 may single out the non-Abelian wavefunctions for the state.

Role of interactions in an electronic Fabry–Perot interferometer operating in the quantum Hall effect regime

Interference of edge channels is expected to be a prominent tool for studying statistics of charged quasiparticles in the quantum Hall effect (QHE). We present here a detailed study of an electronic Fabry–Perot interferometer (FPI) operating in the QHE regime [C. Chamon, et al. (1997) Phys Rev B 55:2331–2334], with the phase of the interfering quasiparticles controlled by the Aharonov–Bohm effect. Our main finding is that Coulomb interactions among the electrons dominate the interference, even in a relatively large area FPI, leading to a strong dependence of the area enclosed by the interference loop on the magnetic field. In particular, for a composite edge structure, with a few independent edge channels propagating along the edge, interference of the outmost edge channel (belonging to the lowest Landau level) was insensitive to magnetic field—suggesting a constant enclosed flux. However, when any of the inner edge channels interfered, the enclosed flux decreased when the magnetic field increased. By intentionally varying the enclosed area with a biased metallic gate and observing the periodicity of the interference pattern, charges e (for integer filling factors) and e/3 (for a fractional filling factor) were found to be expelled from the FPI. Moreover, these observations provided also a novel way of detecting the charge of the interfering quasiparticles.

Observation of large low-frequency resistance fluctuations in metallic nanowires: Implications on its stability

We have measured the low frequency (1mHz<f<10Hz) resistance fluctuations in metallic nanowires (diameter 15nm to 200nm) in the temperature range 77K to 400K. The nanowires were grown electrochemically in polycarbonate membranes and the measurements were carried out in arrays of nanowires by retaining them in the membrane. A large fluctuation in excess of conventional 1/f noise which peaks beyond a certain temperature was found. The fluctuations with a significant low frequency component (~1/f^{3/2}) arise when the diameter of the wire ~15nm and vanishes rapidly as the diameter is increased. We argue that Rayleigh-Plateau instability is the likely cause of this excess noise.

Transmission Phase of a Singly Occupied Quantum Dot in the Kondo Regime

We report on the phase measurements on a quantum dot containing a single electron in the Kondo regime. Transport takes place through a single orbital state. Although the conductance is far from the unitary limit, we measure directly, for the first time, a transmission phase as theoretically predicted of pi/2. As the dots coupling to the leads is decreased, with the dot entering the Coulomb blockade regime, the phase reaches a value of pi. Temperature shows little effect on the phase behavior in the range 30-600 mK, even though both the two-terminal conductance and amplitude of the Aharonov-Bohm oscillations are strongly affected. These results also suggest that previous phase measurements involved transport through more than a single level.

1/f noise in nanowires

We have measured the low-frequency resistance fluctuations (1 mHz < f < 10 Hz) in Ag nanowires of diameter 15 nm

Effect of ambient on the resistance fluctuations of graphene

{\leq}d{\leq}

High-Performance Sensors Based on Resistance Fluctuations of Single-Layer-Graphene Transistors

200 nm at room temperature. The power spectral density (PSD) of the fluctuations has a

Low-frequency random telegraphic noise and 1/f noise in the rare-earth manganite Pr0.63Ca0.37MnO3 near the charge-ordering transition

(1/f{^\alpha})

Correlated conductance fluctuations close to the Berezinskii-Kosterlitz-Thouless transition in ultrathin NbN films.

character as seen in metallic films and wires of larger dimension.Additionally, the PSD has a significant low-frequency component and the value of α increases from the usual 1 to\simeq 3/2 as the diameter d is reduced.The value of the normalized fluctuations

Role of different scattering mechanisms on the temperature dependence of transport in graphene

({\Delta}{R^{2}})/({R^{2}})