Rajdeep Sensarma

Plasmon-phonon coupling in graphene

Collective excitations of coupled electron-phonon systems are calculated for both monolayer and bilayer graphene, taking into account the non-perturbative Coulomb coupling between electronic excitations in graphene and the substrate longitudinal optical phonon modes. We find that the plasmon-phonon coupling in monolayer graphene is strong at all densities, but in bilayer graphene the coupling is significant only at high densities satisfying the resonant condition

Observation of Elastic Doublon Decay in the Fermi-Hubbard Model

\omega_{pl} \approx \omega_{ph}

Dynamic screening and low-energy collective modes in bilayer graphene

. The difference arises from the peculiar screening properties associated with chirality of graphene. Plasmon-phonon coupling explains the measured quasi-linear plasmon dispersion in the long wavelength limit, thus resolving a puzzle in the experimental observations.

Competition between pairing and ferromagnetic instabilities in ultracold Fermi gases near Feshbach resonances.

We investigate the decay of highly excited states of ultracold fermions in a three-dimensional optical lattice. Starting from a repulsive Fermi-Hubbard system near half filling, we generate additional doubly occupied sites (doublons) by lattice modulation. The subsequent relaxation back to thermal equilibrium is monitored over time. The measured absolute doublon lifetime covers 2 orders of magnitude. In units of the tunneling time h/J it is found to depend exponentially on the ratio of on-site interaction energy U to kinetic energy J. We argue that the dominant mechanism for the relaxation is a simultaneous many-body process involving several single fermions as scattering partners. A many-body calculation is carried out using diagrammatic methods, yielding fair agreement with the data.

Chiral Rashba spin textures in ultracold Fermi gases

We theoretically study the dynamic screening properties of bilayer graphene within the random phase approximation assuming quadratic band dispersion and zero gap for the single-particle spectrum. We calculate the frequency dependent dielectric function of the system and obtain the low energy plasmon dispersion and broadening of the plasmon modes from the dielectric function. We also calculate the optical spectral weight (i.e. the dynamical structure factor) for the system. We find that the leading order long wavelength limit of the plasmon dispersion matches with the classical result for 2D electron gas. However, contrary to electron gas systems, the non-local plasmon dispersion corrections decrease the plasmon frequency. The non-local corrections are also different from the single layer graphene. Finally, we also compare our results with the double layer graphene system (i.e. a system of two independent graphene monolayers).

Particle-hole asymmetry in doped mott insulators: implications for tunneling and photoemission spectroscopies.

We study the quench dynamics of a two-component ultracold Fermi gas from the weak into the strong interaction regime, where the short time dynamics are governed by the exponential growth rate of unstable collective modes. We obtain an effective interaction that takes into account both Pauli blocking and the energy dependence of the scattering amplitude near a Feshbach resonance. Using this interaction we analyze the competing instabilities towards Stoner ferromagnetism and pairing.

Lifetime of double occupancies in the Fermi-Hubbard model

Spin-orbit (SO) coupling is an important ingredient in many recently discovered phenomena such as the spin-Hall effect and topological insulators. Of particular interest is topological superconductivity, with its potential application in topological quantum computation. The absence of disorder in ultracold atomic systems makes them ideal for quantum computation applications; however, the SO coupling schemes proposed thus far are experimentally impractical owing to large spontaneous emission rates in the alkali fermions. In this paper, we develop a scheme to generate Rashba SO coupling with a low spontaneous emission extension to a recent experiment. We show that this scheme generates a Fermi surface spin texture for

Density fluctuation effects on the exciton condensate in double-layer graphene

^{40}\mathrm{K}

Modulation spectroscopy and dynamics of double occupancies in a fermionic Mott insulator.

atoms, which is observable in time-of-flight measurements. The chiral spin texture, together with conventional

Momentum-resolved optical lattice modulation spectroscopy for bosons in optical lattices

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