Polyakov-Mellin Bootstrap for AdS loops

We consider holographic CFTs and study their large $N$ expansion. We use Polyakov-Mellin bootstrap to extract the CFT data of all operators, including scalars, till $O(1/N^4)$. We add a contact term in Mellin space, which corresponds to an effective $\phi^4$ theory in AdS and leads to anomalous dimensions for scalars at $O(1/N^2)$. Using this we fix $O(1/N^4)$ anomalous dimensions for double trace operators finding perfect agreement with \cite{loopal} (for $\Delta_{\phi}=2$). Our approach generalizes this to any dimensions and any value of conformal dimensions of external scalar field. In the second part of the paper, we compute the loop amplitude in AdS which corresponds to non-planar correlators of in CFT. More precisely, using CFT data at $O(1/N^4)$ we fix the AdS bubble diagram and the triangle diagram for the general case.Comment: 22 pages, 4 figures, version published in JHE

Fundamental exciton linewidth broadening in monolayer transition metal dichalcogenides

Monolayer Transition Metal Dichalcogenides (TMDS) are highly luminescent materials despite being sub-nanometer thick due to the ultra-short ($<1$ ps) radiative lifetime of the strongly bound bright excitons hosted by these materials. The intrinsically short radiative lifetime results in a large broadening in the exciton band with a magnitude that is about two orders greater than the spread of the light cone itself. The situation calls for a need to revisit the conventional light cone picture. We present a modified light cone concept which places the light line $(\hbar cQ)$ as the generalized lower bound for allowed radiative recombination. A self-consistent methodology, which becomes crucial upon inclusion of large radiative broadening in the exciton band, is proposed to segregate the radiative and the non-radiative components of the homogeneous exciton linewidth. We estimate a fundamental radiative linewidth of $1.54\pm0.17\$meV, owing purely to finite radiative lifetime in the absence of non-radiative dephasing processes. As a direct consequence of the large radiative limit, we find a surprisingly large ($\sim 0.27$ meV) linewidth broadening due to zero-point energy of acoustic phonons. This obscures the precise experimental determination of the intrinsic radiative linewidth and sets a fundamental limit on the non-radiative linewidth broadening at $T = 0$ K

Bandstructure Effects in Ultra-Thin-Body DGFET: A Fullband Analysis

This paper discusses a few unique effects of ultra-thin-body double-gate NMOSFET that are arising from the bandstructure of the thin film Si channel. The bandstructure has been calculated using 10-orbital $sp^3d^5s^*$ tight-binding method. A number of intrinsic properties including band gap, density of states, intrinsic carrier concentration and parabolic effective mass have been derived from the calculated bandstructure. The spatial distributions of intrinsic carrier concentration and $$ effective mass, arising from the wavefunction of different contributing subbands are analyzed. A self-consistent solution of Poisson-Schrodinger coupled equation is obtained taking the full bandstructure into account, which is then applied to an insightful analysis of volume inversion. The spatial distribution of carriers over the channel of a DGFET has been calculated and its effects on effective mass and channel capacitance are discussed.Comment: 13 pages, 21 figure

Self-Powered, Highly Sensitive, High Speed Photodetection Using ITO/WSe2/SnSe2 Vertical Heterojunction

Two dimensional transition metal di-chalcogenides (TMDCs) are promising candidates for ultra-low intensity photodetection. However, the performance of these photodetectors is usually limited by ambience induced rapid performance degradation and long lived charge trapping induced slow response with a large persistent photocurrent when the light source is switched off. Here we demonstrate an indium tin oxide (ITO)/WSe$_2$/SnSe$_2$ based vertical double heterojunction photoconductive device where the photo-excited hole is confined in the double barrier quantum well, whereas the photo-excited electron can be transferred to either the ITO or the SnSe$_2$ layer in a controlled manner. The intrinsically short transit time of the photoelectrons in the vertical double heterojunction helps us to achieve high responsivity in excess of $1100$ A/W and fast transient response time on the order of $10$ $\mu$s. A large built-in field in the WSe$_2$ sandwich layer results in photodetection at zero external bias allowing a self-powered operation mode. The encapsulation from top and bottom protects the photo-active WSe$_2$ layer from ambience induced detrimental effects and substrate induced trapping effects helping us to achieve repeatable characteristics over many cycles