arXiv subjects

Shouvik Datta

Publications and source records attributed to Shouvik Datta.

At least 55 records · Page 3Linked to original sources

arXiv2015

Conformal perturbation theory and higher spin entanglement entropy on the torus

We study the free fermion theory in 1+1 dimensions deformed by chemical potentials for holomorphic, conserved currents at finite temperature and on a spatial circle. For a spin-three chemical potential μ, the deformation is related at high temperatures to a higher spin black hole in hs[0] theory on AdS_3 spacetime. We calculate the order μ^2 corrections to the single interval Renyi and entanglement entropies on the torus using the bosonized formulation. A consistent result, satisfying all checks, emerges upon carefully accounting for both perturbative and winding mode contributions in the bosonized language. The order μ^2 corrections involve integrals that are finite but potentially sensitive to contact term singularities. We propose and apply a prescription for defining such integrals which matches the Hamiltonian picture and passes several non-trivial checks for both thermal corrections and the Renyi entropies at this order. The thermal corrections are given by a weight six quasi-modular form, whilst the Renyi entropies are controlled by quasi-elliptic functions of the interval length with modular weight six. We also point out the well known connection between the perturbative expansion of the partition function in powers of the spin-three chemical potential and the Gross-Taylor genus expansion of large-N Yang-Mills theory on the torus. We note the absence of winding mode contributions in this connection, which suggests qualitatively different entanglement entropies for the two systems.

hep-th↗

arXiv2014

Relative entropy in higher spin holography

We examine relative entropy in the context of the higher-spin/CFT duality. We consider 3$d$ bulk configurations in higher spin gravity which are dual to the vacuum and a high temperature state of a CFT with $\mathcal{W}$-algebra symmetries in presence of a chemical potential for a higher spin current. The relative entropy between these states is then evaluated using the Wilson line functional for holographic entanglement entropy. In the limit of small entangling intervals, the relative entropy should vanish for a generic quantum system. We confirm this behaviour by showing that the difference in the expectation values of the modular Hamiltonian between the states matches with the difference in the entanglement entropy in the short-distance regime. Additionally, we compute the relative entropy of states corresponding to smooth solutions in the $SL(2,\mathbb{Z})$ family with respect to the vacuum.

hep-th↗

arXiv2014

A universal correction to higher spin entanglement entropy

We consider conformal field theories in 1+1 dimensions with W-algebra symmetries, deformed by a chemical potential μfor the spin-three current. We show that the order μ^2 correction to the Re'nyi 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 μ^2 correction across distinct free field CFTs and higher spin black hole solutions holographically dual to CFTs with W-symmetry.

hep-th↗

arXiv2014

Dynamic frequency dependence of bias activated negative capacitance in semiconductor diodes under high forward bias

We observed qualitatively dissimilar frequency dependence of negative capacitive response under high charge injection in two sets of junction diodes which are functionally different from each other i.e. electroluminescent diodes and non-luminescent Si-based diodes. Using the technique of bias-activated differential capacitance response, we investigated the mutual dynamics of different rate processes in different diodes. We explain these observations as the mutual competition of fast and slow electronic transition rates albeit differently. This study provides a better understanding of the physics of junction diodes operating under high charge carrier injection and may lead to superior device functionalities.

cond-mat.mes-hall↗

arXiv2014

Electrical Signature of Excitonic Electroluminescence and Mott Transition at Room Temperature

Mostly optical spectroscopies are used to investigate existence of excitons, Mott transitions and other exquisite excitonic condensed phases of matter. On the other hand, electrical signatures of excitons are hardly explored. Here we examine steady state, small signal, electrical response of AlGaInP based multi-quantum well laser diodes to identify and investigate the presence of excitons in electroluminescence. This frequency dependent response shows bias activated capacitance following a phenomenological rate equation. Dynamic dependence of this response on frequency reverses after light emission. This results in negative activation energy which we explain with the formation of a stable, intermediate transition state whose average energy matches well with excitonic binding energy of these III-V semiconductors. Hence we identify the presence of excitons which are also responsible for observed electroluminescence at low charge injections by electrical measurements alone. Further increase in charge injection suppresses the electrical signature of excitons representing their Mott transition into electron-hole plasma which is supported by standard optical measurements. This kind of correlation between electrical and optical properties of excitons was not systematically investigated in the past. Therefore, this study demonstrates a fresh experimental approach to electrically probe the rich and complex physics of excitons.

cond-mat.mes-hall↗

arXiv2014

Higher spin entanglement entropy from CFT

We 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) x SL(3, R) Chern-Simons theory. Our result suggests that the order $μ^2$ correction to the entanglement entropy may be universal for W-algebra CFTs with spin-three chemical potential, and constitutes a check of the holographic entanglement entropy proposal for higher spin theories of gravity in AdS_3.

hep-th↗

arXiv2014

Renyi entropies of free bosons on the torus and holography

We analytically evaluate the Renyi entropies for the two dimensional free boson CFT. The CFT is considered to be compactified on a circle and at finite temperature. The Renyi entropies S_n 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 A_{n-1} lattice. We then study the Renyi 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 Renyi 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.

hep-th↗

arXiv2013

Black holes in higher spin supergravity

We study black hole solutions in Chern-Simons higher spin supergravity based on the superalgebra sl(3|2). These black hole solutions have a U(1) gauge field and a spin 2 hair in addition to the spin 3 hair. These additional fields correspond to the R-symmetry charges of the supergroup sl(3|2). Using the relation between the bulk field equations and the Ward identities of a CFT with N=2 super-W_3 symmetry, we identify the bulk charges and chemical potentials with those of the boundary CFT. From these identifications we see that a suitable set of variables to study this black hole is in terms of the charges present in three decoupled bosonic sub-algebras of the N=2 super-W_3 algebra. The entropy and the partition function of these R-charged black holes are then evaluated in terms of the charges of the bulk theory as well as in terms of its chemical potentials. We then compute the partition function in the dual CFT and find exact agreement with the bulk partition function.

hep-th↗

arXiv2012

Supersymmetry of classical solutions in Chern-Simons higher spin supergravity

We 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 W_3 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\geq 4, the sl(N|N-1) theory admits smooth supersymmetric conical defects.

hep-th↗

arXiv2012

Direct evidence of photo-ionization causing luminescence "off" state in CdTe Quantum dots

We present a direct experimental evidence of photo-ionized CdTe quantum dot core having luminescence "off" state at band-edge photo-excitation. We use the autocorrelation of fluorescence fluctuations from dilute colloidal solution of CdTe quantum dots in a diffraction limited detection volume to determine their photo-ionization probability. The ionization probability is in the same order of magnitude (\approx10-6) as the CdSe and CdSe/ZnS core/shell quantum dots. Further, we measured the ionization/ neutralization rates (\approx104 s-1) after addition of β-mercaptanol and found that ionization rate is enhanced by two orders of magnitude after the addition. However, a comparable neutralization rate enhances the light emission quantum yield. The ionization/ neutralization at microsecond time scales puts an upper limit on achieving better quantum yield by thiol addition. The results also indicate that the dominant ionization mechanism at band-edge excitation is tunneling.

cond-mat.mes-hall↗

arXiv2012

Do we need to revisit the Bohr exciton radius of hot excitons?

We report collisional broadening of the E3 excitonic resonances in optical absorption spectra of PbS nanocrystallites of widely varying sizes. Significance of the underlying extended band structure of bulk solids to understand the physics of exciton scattering in semiconductor nanocrystallites of this size range is discussed. We propose an empirical notion of 'effective Bohr exciton radius' as a direct consequence of significant departure from usual static screening limits of coulomb interactions of 'hot' excitons in the region of strong dispersion at energies much above the fundamental band gap. Temperature variation of excitonic resonance reveals how non-phonon energy relaxation processes affects the collisional broadening of E3 resonance over this size range. We argue that ballistic transport of hot excitons can suppress exciton-exciton scattering necessary for photo-induced impact ionization in very small quantum dots. This evidently indicates that impact ionization of hot excitons may be efficient only inside an intermediate nanoscale 'size window'.

cond-mat.mes-hall↗

arXiv2012

Temperature dependent reversal of voltage modulated light emission and negative capacitance in AlGaInP based multi quantum well light emitting devices

We report a reversal in negative capacitance and voltage modulated light emission from AlGaInP based multi-quantum well electroluminescent diodes under temperature variation. Unlike monotonically increasing CW light emission with decreasing temperature, modulated electroluminescence and negative capacitance first increase to a maximum and then decrease while cooling down from room temperature. Interdependence of such electronic and optical properties is understood as a competition between defect participation in radiative recombination and field assisted carrier escape from the quantum well region during temperature variation. The temperature of maximum light emission must coincide with the operating temperature of a device for better efficiency.

cond-mat.mes-hall↗

arXiv2012

Directionally asymmetric self-assembly of cadmium sulfide nanotubes using porous alumina nanoreactors: Need for chemohydrodynamic instability at the nanoscale

We explore nanoscale hydrodynamical effects on synthesis and self-assembly of cadmium sulfide nanotubes oriented along one direction. These nanotubes are synthesized by horizontal capillary flow of two different chemical reagents from opposite directions through nanochannels of porous anodic alumina which are used primarily as nanoreactors. We show that uneven flow of different chemical precursors is responsible for directionally asymmetric growth of these nanotubes. On the basis of structural observations using scanning electron microscopy, we argue that chemohydrodynamic convective interfacial instability of multicomponent liquid-liquid reactive interface is necessary for sustained nucleation of these CdS nanotubes at the edges of these porous nanochannels over several hours. However, our estimates clearly suggest that classical hydrodynamics cannot account for the occurrence of such instabilities at these small length scales. Therefore, we present a case which necessitates further investigation and understanding of chemohydrodynamic fluid flow through nanoconfined channels in order to explain the occurrence of such interfacial instabilities at nanometer length scales.

cond-mat.mes-hall↗

arXiv2012

Higher spin fermions in the BTZ black hole

Recently it has been shown that the wave equations of bosonic higher spin fields in the BTZ background can be solved exactly. In this work we extend this analysis to fermionic higher spin fields. We solve the wave equations for arbitrary half-integer spin fields in the BTZ black hole background and obtain exact expressions for their quasinormal modes. These quasinormal modes are shown to agree precisely with the poles of the corresponding two point function in the dual conformal field theory as predicted by the AdS/CFT correspondence. We also obtain an expression for the 1-loop determinant in terms of the quasinormal modes and show it agrees with that obtained by integrating the heat kernel found by group theoretic methods.

hep-th↗

arXiv2011

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

We solve the wave equations of arbitrary integer spin fields in the BTZ black hole background and obtain exact expressions for their quasinormal modes. We show that these quasinormal modes precisely agree with the location of the poles of the corresponding two point function in the dual conformal field theory as predicted by the AdS/CFT correspondence. We then use these quasinormal modes to construct the one-loop determinant of the higher spin field in the thermal BTZ background. This is shown to agree with that obtained from the corresponding heat kernel constructed recently by group theoretic methods.

hep-th↗

arXiv2011

Voltage modulated electro-luminescence spectroscopy and negative capacitance - the role of sub-bandgap states in light emitting devices

Voltage modulated electroluminescence spectra and low frequency ({\leq} 100 kHz) impedance characteristics of electroluminescent diodes are studied. Voltage modulated light emission tracks the onset of observed negative capacitance at a forward bias level for each modulation frequency. Active participation of sub-bandgap defect states in minority carrier recombination dynamics is sought to explain the results. Negative capacitance is understood as a necessary dielectric response to compensate any irreversible transient changes in the minority carrier reservoir due to radiative recombinations mediated by slowly responding sub-bandgap defects. Experimentally measured variations of the in-phase component of modulated electroluminescence spectra with forward bias levels and modulation frequencies support the dynamic influence of these states in the radiative recombination process. Predominant negative sign of the in-phase component of voltage modulated electroluminescence signal further confirms the bi-molecular nature of light emission. We also discuss how these states can actually affect the net density of minority carriers available for radiative recombination. Results indicate that these sub-bandgap states can suppress external quantum efficiency of such devices under high frequency operation commonly used in optical communication.

cond-mat.mtrl-sci↗

arXiv1999

A Model for the Optical Absorption in Porous Silicon Quantum Wires

In this paper, we analyse the optical absorption in porous silicon . This is the first attempt to explicitly demonstrate that it is not possible to extract the band gap of such low dimensional nanostructures like porous silicon from a Tauc plot of Square Root(alpha h nu) vs (h nu) . These objections are also valid for other reduced dimensional systems like quantum wires and quantum dots etc. So we model the absorption process assuming that porous silicon is a pseudo 1D material system having a distribution of band gaps. We show that in order to explain the absorption we specifically need to invoke - (a) k is not conserved in optical transitions,(b) the oscillator strength of these transitions depends on the size of the nanostructure in which the absorption takes place and (c) the distribution of band gaps significantly influences the optical absorption. A natural explanation of the temperature dependence of absorption in porous silicon also follows from our model . We have also shown that porosity can be inferred non-destructively from transmission measurements in the region of low absorption. One can easily generalize our equations and use them to analyse the absorption process of other low dimensional materials too .

cond-mat.soft↗

arXiv1999

Observation of Type I/II Transition in GaAs/InGaP Heterostructure by C-V Profiling

It is known in the literature that InGaP alloy synthesized within a certain range of growth temperature (520 - 720 degree Centrigrade) shows Cu-Pt crystal ordering. This ordering reduces the band gap energy . It was predicted that the ordering induced modification of energy levels of InGaP lowers its conduction band edge, which could change a type I band alignment at a GaAs/InGaP heterojunction to type II . Here we report the first observation of this ordering related type I/II transition for a GaAs/InGaP heterojunction by C-V profiling experiments done at room temperature .

cond-mat.mtrl-sci↗