Shouvik Datta

Electroreflectance and surface photovoltage spectroscopies of semiconductor structures using an indium–tin–oxide-coated glass electrode in soft contact mode

Measurements of electroreflectance and surface photovoltage spectroscopy of semiconductor structures are described using a transparent indium–tin–oxide-coated glass electrode in soft contact mode on the semiconductor surface. This improvisation (simplification) reduces the magnitude of the ac modulation voltage necessary for the electroreflectance measurement to less than a volt from about a kV (∼103 V) as required in the conventional contactless setup. This soft contact mode also enhances the sensitivity of the surface photovoltage signal by three orders of magnitude. We also formulate an analytical criterion to extract the transition energies of a quantum well from the surface photovoltage spectrum.

Higher spin entanglement entropy from CFT

A bstractWe consider free fermion and free boson CFTs in two dimensions, deformed by a chemical potential μ for the spin-three current. For the CFT on the infinite spatial line, we calculate the finite temperature entanglement entropy of a single interval perturbatively to second order in μ in each of the theories. We find that the result in each case is given by the same non-trivial function of temperature and interval length. Remarkably, we further obtain the same formula using a recent Wilson line proposal for the holographic entanglement entropy, in holomorphically factorized form, associated to the spin-three black hole in SL(3, ℝ) × SL(3, ℝ) Chern-Simons theory. Our result suggests that the order μ2 correction to the entanglement entropy may be universal for

Rényi entropies of free bosons on the torus and holography

\mathcal{W}

Universal correction to higher spin entanglement entropy

-algebra CFTs with spinthree chemical potential, and constitutes a check of the holographic entanglement entropy proposal for higher spin theories of gravity in AdS3.

Supersymmetry of classical solutions in Chern-Simons higher spin supergravity

A bstractWe analytically evaluate the Rényi entropies for the two dimensional free boson CFT. The CFT is considered to be compactified on a circle and at finite temperature. The Rényi entropies Sn are evaluated for a single interval using the two point function of bosonic twist fields on a torus. For the case of the compact boson, the sum over the classical saddle points results in the Riemann-Siegel theta function associated with the An−1 lattice. We then study the Rényi entropies in the decompactification regime. We show that in the limit when the size of the interval becomes the size of the spatial circle, the entanglement entropy reduces to the thermal entropy of free bosons on a circle. We then set up a systematic high temperature expansion of the Rényi entropies and evaluate the finite size corrections for free bosons. Finally we compare these finite size corrections both for the free boson CFT and the free fermion CFT with the one-loop corrections obtained from bulk three dimensional handlebody spacetimes which have higher genus Riemann surfaces as its boundary. One-loop corrections in these geometries are entirely determined by quantum numbers of the excitations present in the bulk. This implies that the leading finite size corrections contributions from one-loop determinants of the Chern-Simons gauge field and the Dirac field in the dual geometry should reproduce that of the free boson and the free fermion CFT respectively. By evaluating these corrections both in the bulk and in the CFT explicitly we show that this expectation is indeed true.

Luminescence and solar cell from ligand-free colloidal AgInS2 nanocrystals

We consider conformal field theories in 1 + 1 dimensions with W-algebra symmetries, deformed by a chemical potential mu for the spin-three current. We show that the order mu(2) correction to the Renyi and entanglement entropies of a single interval in the deformed theory, on the infinite spatial line and at finite temperature, is universal. The correction is completely determined by the operator product expansion of two spin-three currents, and by the expectation values of the stress tensor, its descendants and its composites, evaluated on the n-sheeted Riemann surface branched along the interval. This explains the recently found agreement of the order mu(2) correction across distinct free field CFTs and higher spin black hole solutions holographically dual to CFTs with W symmetry.

Higher spin quasinormal modes and one-loop determinants in the BTZ black hole

A bstractWe construct and study classical solutions in Chern-Simons supergravity based on the superalgebra sl(N|N − 1). The algebra for the N = 3 case is written down explicitly using the fact that it arises as the global part of the super conformal

Model for optical absorption in porous silicon

{{\mathcal{W}}_3}

Conformal perturbation theory and higher spin entanglement entropy on the torus

superalgebra. For this case we construct new classical solutions and study their supersymmetry. Using the algebra we write down the Killing spinor equations and explicitly construct the Killing spinor for conical defects and black holes in this theory. We show that for the general sl(N|N − 1) theory the condition for the periodicity of the Killing spinor can be written in terms of the products of the odd roots of the super algebra and the eigenvalues of the holonomy matrix of the background. Thus the supersymmetry of a given background can be stated in terms of gauge invariant and well defined physical observables of the Chern-Simons theory. We then show that for N ≥ 4, the sl(N|N − 1) theory admits smooth supersymmetric conical defects.

Black holes in higher spin supergravity

Ligand-free AgInS2 nanocrystals display significant defect-related sub-band gap photo-absorption in addition to the excitonic absorption. The post-synthesis annealing of the nanocrystal dispersion at 150 °C reduces the intrinsic defect density, and also increases the nanocrystal size (5.8 nm) thereby reducing surface related defects. The AgInS2 nanocrystals exhibit a broad emission spectra, with a large Stokes shift compared to the excitonic absorption, which is attributed to two radiative electron–hole recombination processes: the first one involves the localized defect-state and the delocalized valence/conduction band with a lifetime of ~20 ns, and the second one involves two localized donor and acceptor states with a lifetime >250 ns. The faster decay dominates the higher energy end of the emission spectrum. Quantum dot sensitized solar cells using these ligand-free AgInS2 nanocrystals exhibit a maximum energy conversion efficiency of 0.8% and an open circuit voltage of 0.45 V. The device performance is better compared to previous AgInS2 nanocrystal based solar cells, due to both the use of the ligand-free nanocrystals that improve the charge transport and the post-synthesis annealing which partially removes the mid-gap defect states.