arXiv subjects

Mitali Banerjee

Publications and source records attributed to Mitali Banerjee.

26 records · Page 2Linked to original sources

arXiv2023

Excitonic interplay between surface polar III-nitride quantum wells and MoS$_2$ monolayer

III-nitride wide bandgap semiconductors exhibit large exciton binding energies, preserving strong excitonic effects at room temperature. On the other hand, semiconducting two-dimensional (2D) materials, including MoS$_2$, also exhibit strong excitonic effects, attributed to enhanced Coulomb interactions. This study investigates excitonic interactions between surface GaN quantum well (QW) and 2D MoS$_2$ in van der Waals heterostructures by varying the spacing between these two excitonic systems. Optical property investigation first demonstrates the effective passivation of defect states at the GaN surface through MoS$_2$ coating. Furthermore, a strong interplay is observed between MoS$_2$ monolayers and GaN QW excitonic transitions. This highlights the interest of the 2D material/III-nitride QW system to study near-field interactions, such as Förster resonance energy transfer, which could open up novel optoelectronic devices based on such hybrid excitonic structures.

cond-mat.mes-hall↗

arXiv2021

Isolated Ballistic Non-Abelian Interface Channel

Non-abelian anyons are prospective candidates for fault-tolerant topological quantum computation due to their long-range entanglement. Curiously these quasiparticles are charge-neutral, hence elusive to most conventional measurement techniques. A proposed host of such quasiparticles is the $ν$=5/2 quantum Hall state. The gapless edge modes can provide the topological order of the state, which in turn identifies the chirality of the non-abelian mode. Since the $ν$=5/2 state hosts a variety of edge modes (integer, fractional, neutral), a robust technique is needed to isolate the fractional channel while retaining its original non-abelian character. Moreover, a single non-abelian channel can be easily manipulated to interfere, thus revealing the state's immunity to decoherence. In this work, we exploit a novel approach to gap-out the integer modes of the $ν$=5/2 state by interfacing the state with integer states, $ν$=2 & $ν$=3 (1). The electrical conductance of the isolated interface channel was 0.5e$^2$/h, as expected. More importantly, we find a thermal conductance of 0.5$κ_0$T (with $κ_0$=$π^2k_B^2$/3h), confirming unambiguously the non-abelian nature of the $ν$=1/2 interface channel and its Particle-Hole Pfaffian topological order. Our result opens new avenues to manipulate and test other exotic QHE states and braid, via interference, the isolated fractional channels.

cond-mat.mes-hall↗

arXiv2019

Giant violation of Wiedemann-Franz law in doping layers of modern AlGaAs heterostructures

We analyze the data of the recent paper Nature 559, 205 (2018) and show that it contains an observation of thermal and electric conductivities of the doping layers in GaAs/AlGaAs heterostructures which violates the Wiedemann-Franz law. Namely, the measured thermal conductivity of the doping layers is similar to that of a metal while the electrical conductivity is exponentially small. We find that these results may be related to the exciton contribution to thermal conductivity calculated in several recent theoretical works for metallic samples.

cond-mat.mes-hall↗

arXiv2019

Low temperature saturation of phase coherence length in topological insulators

Implementing topological insulators as elementary units in quantum technologies requires a comprehensive understanding of the dephasing mechanisms governing the surface carriers in these materials, which impose a practical limit to the applicability of these materials in such technologies requiring phase coherent transport. To investigate this, we have performed magneto-resistance (MR) and conductance fluctuations\ (CF) measurements in both exfoliated and molecular beam epitaxy grown samples. The phase breaking length ($l_ϕ$) obtained from MR shows a saturation below sample dependent characteristic temperatures, consistent with that obtained from CF measurements. We have systematically eliminated several factors that may lead to such behavior of $l_ϕ$ in the context of TIs, such as finite size effect, thermalization, spin-orbit coupling length, spin-flip scattering, and surface-bulk coupling. Our work indicates the need to identify an alternative source of dephasing that dominates at low $T$ in topological insulators, causing saturation in the phase breaking length and time.

cond-mat.mes-hall↗

arXiv2018

Melting of interference in the fractional quantum Hall effect: Appearance of neutral modes

Electrons living in a two-dimensional world under a strong magnetic field - the so-called fractional quantum Hall effect (FQHE) - often manifest themselves as fractionally charged quasiparticles (anyons). Moreover, being under special conditions they are expected to be immune to the environment, thus may serve as building blocks for future quantum computers. Interference of such anyons is the very first step towards understanding their anyonic statistics. However, the complex edge-modes structure of the fractional quantum Hall states, combined with upstream neutral modes, have been suspected to prevent an observation of the much sought after interference of anyons. Here, we report of finding a direct correlation between the appearance of neutral modes and the gradual disappearance of interference in a Mach-Zehnder interferometer (MZI), as the bulk filling factor is lowered towards Landau filling ν_B =1; followed by a complete interference quench at ν_B=1. Specifically, the interference was found to start diminishing at ν_B~1.5 with a growing upstream neutral mode, which was detected by a born upstream shot noise in the input quantum point contact (QPC) to the MZI. Moreover, at the same time a ν_QPC =1/3 conductance plateau, carrying shot-noise, appeared in the transmission of the QPC - persisting until bulk filling ν_B =1/2. We identified this conductance plateau to result from edge reconstruction, which leads to an upstream neutral mode. Here, we also show that even the particle-like quasiparticles are accompanied by upstream neutral modes, therefore suppressing interference in the FQHE regime.

cond-mat.mes-hall↗

arXiv2017

Observation of half-integer thermal Hall conductance

Topological states of matter are characterized by topological invariant, which are physical quantities whose values are quantized and do not depend on details of the measured system. Of these, the easiest to probe in experiments is the electrical Hall conductance, which is expressed in units of $e^2/h$ ($e$ the electron charge, $h$ the Planck's constant). In the fractional quantum Hall effect (FQHE), fractional quantized values of the electrical Hall conductance attest to topologically ordered states, which are states that carry quasi particles with fractional charge and anyonic statistics. Another topological invariant, which is much harder to measure, is the thermal Hall conductance, expressed in units of $κ_0T=(π^2kB^2/3h)T$ ($kB$ the Boltzmann constant, $T$ the temperature). For the quantized thermal Hall conductance, a fractional value attests that the probed state of matter is non-abelian. Quasi particles in non-abelian states lead to a ground state degeneracy and perform topological unitary transformations among ground states when braided. As such, they may be useful for topological quantum computation. In this paper, we report our measurements of the thermal Hall conductance for several quantum Hall states in the first excited Landau level. Remarkably, we find the thermal Hall conductance of the $ν=5/2$ state to be fractional, and to equal $2.5κ_0T$

cond-mat.mes-hall↗

arXiv2016

Observed Quantization of Anyonic Heat Flow

The quantum of heat conductance of ballistic one-dimensional (1D) channels, being gQ=k0T with k0=pi^2*2kB^2/3h (T - temperature, kB - Boltzmann's constant, h - Planck's constant), is an important fundamental constant. While the quantization of the electrical conductance of 1D ballistic conductors has long been experimentally established, a demonstration of the quantization of thermal conductance proved to be much harder. It has already been accomplished for weakly interacting systems of phonons, photons, and electronic Fermi-liquids. Theoretically, however, the quantization must also hold in strongly interacting systems, such as the Fractional Quantum Hall effect (FQHE), where electrons fractionalize into anyons and chargeless quasiparticles such as neutral Majorana fermions. Since the bulk in the FQHE is incompressible, it is not expected to contribute significantly to the heat conductance, which is determined by chiral 1D edge modes. The thermal conductance reflects topological properties of the FQH electronic systems to which the electrical conductance gives no access. Here, we present results of extensive measurements of the heat conductance in 'particle-like' (Laughlin's) and 'hole-like' fractional states. We verify the universal value of the quantum of thermal conductance of the charged fractional modes as well as for chargeless neutral modes. We also prove the validity of the theoretical predictions for the more complex (and less studied) 'hole-like' states. Heat transport measurements open a door to ample information, not easily accessible by conductance measurements.

cond-mat.mes-hall↗

arXiv2011

Signature effects of spin clustering and distribution of spin couplings on magnetization behaviour in Ni-Fe-Mo and Ni-Fe-W alloys

The spontaneous magnetization as a function of temperature is investigated for a number of disordered Ni-Fe-Mo and Ni-Fe-W alloys using superconducting quantum interference device magnetometry, with a focus on the low-T behavior as well as the critical exponents associated with the magnetic phase transition. While the low-T magnetization is found to be well described by Bloch's T^{3/2} law, an extraordinary enhancement of the spin-wave parameter B and the reduced coefficient B_{3/2}=BT_C ^{3/2} are observed with increasing Fe dilution as compared to conventional 3d ferromagnets, whereas the critical amplitudes are found to decrease systematically. Recent locally self-consistent calculations of finite-temperature spin dynamics in a generic diluted magnet provide an understanding in terms of two distinct energy scales associated with weakly coupled bulk spins in the FM matrix and strongly coupled cluster spins. In view of similar behaviour observed in diluted magnetic semiconductors and other ferromagnetic alloys, it is proposed that these distinctive features corresponding to the three important temperature regimes provide macroscopic indicators of signature effects of spin clustering on magnetization behaviour in disordered ferromagnets.

cond-mat.mtrl-sci↗