Biswajit Karmakar

Delocalized-localized transition in a semiconductor two-dimensional honeycomb lattice

We report the magnetotransport properties of a two-dimensional electron gas in a modulation-doped AlGaAs/GaAs heterostructure subjected to a lateral potential with honeycomb geometry. Periodic oscillations of the magnetoresistance and a delocalized-localized transition are shown by applying a gate voltage. We argue that electrons in such artificial-graphene lattices offer a promising approach for the simulation of quantum phases dictated by Coulomb interactions.

Controlled coupling of spin-resolved quantum Hall edge states

We introduce and experimentally demonstrate a new method that allows us to controllably couple copropagating spin-resolved edge states of a two-dimensional electron gas (2DEG) in the integer quantum Hall regime. The scheme exploits a spatially periodic in-plane magnetic field that is created by an array of Cobalt nanomagnets placed at the boundary of the 2DEG. A maximum charge or spin transfer of 28±1% is achieved at 250 mK.

Metamorphosis of a quantum Hall bilayer state into a composite fermion metal

Abstract Composite fermion (CF) metal states emerge in quantum Hall bilayers at total Landau level filling factor ν T = 1 when the tunneling gap collapses by application of in-plane component of the magnetic field. Evidence of this transformation is found in the continua of spin excitations observed by inelastic light scattering below the spin-wave mode at the Zeeman energy. The low-lying spin modes are interpreted as quasiparticle excitations with simultaneous changes in spin orientation and composite fermion Landau level index. The results highlight significant differences of bilayer CF quasiparticles from those in single layers.

Nanoscale Mach-Zehnder interferometer with spin-resolved quantum Hall edge states

We realize a nanoscale-area Mach-Zehnder interferometer with co-propagating quantum Hall spin-resolved edge states and demonstrate the persistence of gate-controlled quantum interference oscillations, as a function of an applied magnetic field, at relatively large temperatures. Arrays of top-gate magnetic nanofingers are used to induce a resonant charge transfer between the pair of spin-resolved edge states. To account for the pattern of oscillations measured as a function of magnetic field and gate voltage, we have developed a simple theoretical model which satisfactorily reproduces the data.

Optical Anisotropy of Electronic Excitations in Elliptical Quantum Dots

The authors report that anisotropic confining potentials in laterally coupled semiconductor quantum dots (QDs) have large impacts in optical transitions and energies of intershell collective electronic excitations. The observed anisotropies are revealed by inelastic light scattering as a function of the in-plane direction of light polarization and can be finely controlled by modifying the geometrical shape of the QDs. These experiments show that the tuning of the QD confinement potential offers a powerful method to manipulate electronic states and far-infrared intershell optical transitions in QDs.

Towards an Electronic Interferometer based on Spin-Resolved Quantum Hall Edge States

Spin resolved edge states are ideal candidates for the implementation of dual-rail quantum computation architectures by encoding the qubit in the spin degree of freedom of the co-propagating edge states. An important element for the realization of such architectures is a coherent beam splitter that controllably mixes the two co-propagating spin-resolved edge channels. Coupling of the spin resolved edge states is demonstrated recently by spin-flip scattering event that is induced by in-plane spatially-dependent periodic magnetic field of the nano-magnet array placed at the boundary of the mesa. In this paper we discuss the nanofabrication and our preliminary transport analysis of an electronic interferometer device made of two nano-magnetic arrays placed in close proximity. The impact of temperature in the coherent properties of the devices is addressed.

Evidence of interlayer interaction in magnetoluminescence spectra of electron bilayers

Magneto-luminescence studies in electron bilayers reveal the hallmarks of the even-denominator and other quantum Hall states in the intensities and energies of the inter-band optical recombination lines. In the presence of a small tunneling gap between the layers the magneto-optical emission from the lowest anti-symmetric subband, not populated in a single-electron picture, displays maxima at filling factors 1 and 2/3. These findings uncover a loss of pseudospin polarization, where the pseudospin describes the layer index degree of freedom, that is linked to an anomalous population of the anti-symmetric level due to excitonic correlations. The results demonstrate a new realm to probe the impact of inter-layer Coulomb interaction in quantum Hall bilayers.

QUANTUM PHASE TRANSITION OF A QUANTUM HALL EXCITONIC STATE IN ELECTRON BILAYERS

We review recent inelastic light scattering results that shed light on the interplay between incompressible and compressible quantum phases of electron bilayers at total filling factor νT = 1. Our analysis and previous data suggests a first-order transition and phase coexistence of incompressible and compressible regions close to the phase boundary. We focus here on the origin of phase coexistence and argue that density fluctuations due to residual disorder in the quantum Hall regime, previously invoked as source of compressible puddles, play a marginal role. The sharpness of the phase boundary in the presence of inhomogeneous broadening and finite temperature effects remains to be explained.

Seeing emergent phases in quantum Hall double layers

We review recent studies by optics methods of emergent phases in the quantum Hall (QH) regimes of double layers with finite tunneling at Landau level filling factor ν=1. In measurements of spin excitations by inelastic light scattering and of elastically scattered Rayleigh light under the application of in-plane magnetic fields, we uncovered evidence of a quantum phase transition that occurs when a many-body tunneling gap collapses. The transformation can be regarded as a transition from an incompressible highly correlated QH state to a compressible composite-fermion bilayer system. The correlated QH state is characterized by the presence of populations of bound electron-hole pairs across the tunneling gap. Quantitative determinations of the density of such excitonic pairs are obtained from inelastic light scattering spectra of spin excitations. The correlated QH state displays resonant Rayleigh scattering with unusual temperature dependence.