arXiv subjects

Ajit Srivastava

Publications and source records attributed to Ajit Srivastava.

24 records · Page 2Linked to original sources

arXiv2015

Signatures of Bloch-band geometry on excitons: non-hydrogenic spectra in transition metal dichalcogenides

The geometry of electronic bands in a solid can drastically alter single-particle charge and spin transport. We show here that collective optical excitations arising from Coulomb interactions also exhibit unique signatures of Berry curvature and quantum geometric tensor. A non-zero Berry curvature mixes and lifts the degeneracy of $l \neq 0$ states, leading to a time-reversal-symmetric analog of the orbital Zeeman effect. The quantum geometric tensor, on the other hand, leads to $l$-dependent shifts of exciton states that is analogous to the Lamb shift. Our results provide an explanation of the non-hydrogenic exciton spectrum recently calculated for transition metal dichalcogenides. Numerically, we find a Berry curvature induced splitting of $\sim 10$ meV between the $2p_x \pm i2p_y$ states of WSe$_2$.

cond-mat.mes-hall↗

arXiv2014

Optically Active Quantum Dots in Monolayer WSe$_2$

Semiconductor quantum dots have emerged as promising candidates for implementation of quantum information processing since they allow for a quantum interface between stationary spin qubits and propagating single photons. In the meanwhile, transition metal dichalcogenide (TMD) monolayers have moved to the forefront of solid-state research due to their unique band structure featuring a large band gap with degenerate valleys and non-zero Berry curvature. Here we report the observation of quantum dots in monolayer tungsten-diselenide with an energy that is 20 to 100 meV lower than that of two dimensional excitons. Photon antibunching in second-order photon correlations unequivocally demonstrates the zero-dimensional anharmonic nature of these quantum emitters. The strong anisotropic magnetic response of the spatially localized emission peaks strongly indicates that radiative recombination stems from localized excitons that inherit their electronic properties from the host TMD. The large $\sim$ 1 meV zero-field splitting shows that the quantum dots have singlet ground states and an anisotropic confinement most likely induced by impurities or defects in the host TMD. Electrical control in van der Waals heterostructures and robust spin-valley degree of freedom render TMD quantum dots promising for quantum information processing.

cond-mat.mes-hall↗

arXiv2014

Valley Zeeman Effect in Elementary Optical Excitations of a Monolayer WSe2

A monolayer of a transition metal dichalcogenide (TMD) such as WSe$_2$ is a two-dimensional (2D) direct band-gap valley-semiconductor having an effective Honeycomb lattice structure with broken inversion symmetry. The inequivalent valleys in the Brillouin zone could be selectively addressed using circularly-polarized light fields, suggesting the possibility for magneto-optical measurement and manipulation of the valley pseudospin degree of freedom. Here we report such experiments that demonstrate the valley Zeeman effect -- strongly anisotropic lifting of the degeneracy of the valley pseudospin degree of freedom using an external magnetic field. While the valley-splitting measured using the exciton transition is consistent with the difference of the conduction and valence band orbital magnetic moments, the trion transition exhibits an unexpectedly large valley Zeeman effect which cannot be understood using an independent electron-hole picture. Instead, we find an explanation using the recently predicted large Berry curvature and the associated magnetic moment for the electron-hole exchange interaction modified trion dispersion. Our results raise the possibility of observing optical excitation induced valley Hall effect in monolayer TMDs or topological states of photons strongly coupled to trion excitations in a microcavity.

cond-mat.mes-hall↗

arXiv2011

Hyperfine interaction dominated dynamics of nuclear spins in self-assembled quantum dots

We measure the dynamics of nuclear spins in a self-assembled quantum dot at a magnetic field of 5 Tesla and identify two distinct mechanisms responsible for the decay of the Overhauser field. We attribute a temperature-independent decay which lasts ~100 seconds to intra-dot diffusion induced by hyperfine-mediated indirect nuclear spin interaction. In addition, we observe a gate-voltage and temperature dependent decay stemming from co-tunneling mediated nuclear spin flip processes. By adjusting the gate-voltage and lowering the electron temperature to ~200 milliKelvin, we prolong the corresponding decay time to ~30 hours. Our measurements indicate possibilities for exploring quantum dynamics of the central spin model using a single self-assembled quantum dot.

cond-mat.mes-hall↗

arXiv2008

Diffusion-limited exciton-exciton annihilation in single-walled carbon nanotubes: A time-dependent analysis

To provide physical insight into the recently observed photoluminescence saturation behaviors in single-walled carbon nanotubes implying the existence of an upper limit of exciton densities, we have performed a time-dependent theoretical study of diffusion-limited exciton-exciton annihilation in the general context of reaction-diffusion processes, for which exact treatments exist. By including the radiative recombination decay as a Poissonian process in the exactly-solvable problem of one-dimensional diffusion-driven two-particle annihilation, we were able to correctly model the dynamics of excitons as a function of time with different initial densities, which in turn allowed us to reproduce the experimentally observed photoluminescence saturation behavior at high exciton densities. We also performed Monte Carlo simulations of the purely stochastic, Brownian diffusive motion of one-dimensional excitons, which validated our analytical results. Finally, we consider the temperature-dependence of this diffusion-limited exciton-exciton annihilation and point out that high excitonic densities in SWNTs could be achieved at low temperature in an external magnetic field.

cond-mat.mes-hall↗

arXiv1995

"Classical" Vortex Nucleation in Superflow Through Small Orifices

We report on numerical solutions of the Gross-Pitaevskii equation for two-dimensional flow of a superfluid condensate through a small orifice. Above a critical velocity of about $30\%$ of the speed of sound, cavitation occurs in the throat of the orifice. The cavitating bubbles form the cores of singly quantized vortices which detach from the boundary and are convected downstream.

cond-mat↗