arXiv subjects

Rajarshi Bhattacharyya

Publications and source records attributed to Rajarshi Bhattacharyya.

6 recordsLinked to original sources

arXiv2023

MilliKelvin microwave impedance microscopy in a dry dilution refrigerator

Microwave impedance microscopy (MIM) is a near-field imaging technique that has been used to visualize the local conductivity of materials with nanoscale resolution across the GHz regime. In recent years, MIM has shown great promise for the investigation of topological states of matter, correlated electronic states and emergent phenomena in quantum materials. To explore these low-energy phenomena, many of which are only detectable in the milliKelvin regime, we have developed a novel low-temperature MIM incorporated into a dilution refrigerator. This setup which consists of a tuning-fork-based atomic force microscope with microwave reflectometry capabilities, is capable of reaching temperatures down to 70 mK during imaging and magnetic fields up to 9 T. To test the performance of this microscope, we demonstrate microwave imaging of the conductivity contrast between graphite and silicon dioxide at cryogenic temperatures and discuss the resolution and noise observed in these results. We extend this methodology to visualize edge conduction in Dirac semimetal cadmium arsenide in the quantum Hall regime

physics.app-ph↗

arXiv2023

Anomalous Aharonov-Bohm Interference in the Presence of Edge Reconstruction

Interferometry is a vital tool for studying fundamental features in the quantum Hall effect (QHE). For instance, Aharonov-Bohm (AB) interference in a quantum Hall interferometer can probe the wave-particle duality of electrons and quasiparticles. Here, we report an unusual AB interference in a quantum Hall Fabry-Pérot interferometer (FPI), whose Coulomb interactions were suppressed with a grounded drain in the interior bulk of the FPI. In a descending filling factor from $ν=3$ to $ν\approx5/3$, the magnetic field periodicity, which corresponded to a single 'flux quantum,' agreed accurately with the enclosed area of the FPI. However, in the filling range, $ν\approx5/3$ to $ν=1$, the field periodicity increased markedly, apriori suggesting a drastic shrinkage of the AB area. Moreover, the modulation gate voltage periodicity decreased abruptly at this range. We attribute these unexpected observations to a ubiquitous edge reconstruction, leading to dynamical area changing with the field and a modified modulation gate-edge capacitance. These results are reproducible and support future interference experiments with a QHE-FPI.

cond-mat.mes-hall↗

arXiv2021

Does shot noise always provide the quasiparticle charge?

The fractional charge of quasiparticles is a fundamental feature of quantum Hall effect (QHE) States. The charge has long been measured via shot-noise at moderate temperatures ($T$>30mK), with the Fano factor $F=e^*/e$ revealing the charge $e^*$ of the quasiparticles. However, at sufficiently low temperatures ($T\approx$10mK), we consistently find $F$ being equal to the bulk filling factor, $ν_b$. Surprisingly, noise with $F=ν_b$ is also observed on intermediate conductance plateaus in the transmission of the quantum point contact (QPC), where shot noise is not expected. We attribute the unexpected Fano factor to upstream neutral modes, which proliferate at the lowest spinless Landau level. The universality of the Fano factor is also confirmed when the edge modes do not conform to the bulk. For this, the ubiquitous edge modes at the periphery of the sample are replaced by artificially constructed 'interface modes', propagating at the interface between two adjoined QHE states: the tested state and a different state. We present a new theoretical paradigm based on an interplay between charge and neutral modes, explaining the origin of the universal Fano factor.

cond-mat.mes-hall↗

arXiv2020

QFlow: A Learning Approach to High QoE Video Streaming at the Wireless Edge

The predominant use of wireless access networks is for media streaming applications, which are only gaining popularity as ever more devices become available for this purpose. However, current access networks treat all packets identically, and lack the agility to determine which clients are most in need of service at a given time. Software reconfigurability of networking devices has seen wide adoption, and this in turn implies that agile control policies can be now instantiated on access networks. The goal of this work is to design, develop and demonstrate QFlow, a learning approach to create a value chain from the application on one side, to algorithms operating over reconfigurable infrastructure on the other, so that applications are able to obtain necessary resources for optimal performance. Using YouTube video streaming as an example, we illustrate how QFlow is able to adaptively provide such resources and attain a high QoE for all clients at a wireless access point.

cs.LG↗

arXiv2019

FlowBazaar: A Market-Mediated Software Defined Communications Ecosystem at the Wireless Edge

The predominant use of wireless access networks is for media streaming applications, which are only gaining popularity as ever more devices become available for this purpose. However, current access networks treat all packets identically, and lack the agility to determine which clients are most in need of service at a given time. Software reconfigurability of networking devices has seen wide adoption, and this in turn implies that agile control policies can be now instantiated on access networks. The goal of this work is to design, develop and demonstrate FlowBazaar, an market-based approach to create a value chain from the application on one side, to algorithms operating over reconfigurable infrastructure on the other, so that applications are able to obtain necessary resources for optimal performance. Using YouTube video streaming as an example, we illustrate how FlowBazaar is able to adaptively provide such resources and attain a high QoE for all clients at a wireless access point.

cs.NI↗

arXiv2016

Incentivizing Sharing in Realtime D2D Streaming Networks: A Mean Field Game Perspective

We consider the problem of streaming live content to a cluster of co-located wireless devices that have both an expensive unicast base-station-to-device (B2D) interface, as well as an inexpensive broadcast device-to-device (D2D) interface, which can be used simultaneously. Our setting is a streaming system that uses a block-by-block random linear coding approach to achieve a target percentage of on-time deliveries with minimal B2D usage. Our goal is to design an incentive framework that would promote such cooperation across devices, while ensuring good quality of service. Based on ideas drawn from truth-telling auctions, we design a mechanism that achieves this goal via appropriate transfers (monetary payments or rebates) in a setting with a large number of devices, and with peer arrivals and departures. Here, we show that a Mean Field Game can be used to accurately approximate our system. Furthermore, the complexity of calculating the best responses under this regime is low. We implement the proposed system on an Android testbed, and illustrate its efficient performance using real world experiments.

cs.GT↗