arXiv subjects

Aseem Rajan Kshirsagar

Publications and source records attributed to Aseem Rajan Kshirsagar.

4 recordsLinked to original sources

arXiv2026

Empirical approaches to Fröhlich excitonic polarons in polar semiconductors

The excitonic correlation between electron and hole and these carriers' interactions with underlying crystal lattice govern the opto-electronic response of semiconductors. The latter is dominated in ionic crystals by the Fröhlich interaction corresponding to the long-range electric field generated by polar phonons. Theoretical description of complex interplay of these two effects, excitonic and polaronic, has been a formidable challenge. The present paper reviews the physics of Fröhlich excitonic polarons from the point of view of empirical approaches and supplements it with a few original developments. At first, we review the models for excitonic polarons in ionic semiconductors built analogously to Lee-Low-Pines (LLP) model for free polarons, and later extended by Pollman and Büttner (PB). These models have been applied in past to the case of weakly interacting polarons (e.g. GaAs). We consider their applicability to ionic solids such as TlCl or 3D lead halide perovskites, where electron-hole correlations are relatively stronger. In these compounds, electrons and holes have almost equal effective masses, which allows us to derive a new analytical expressions of PB effective interaction potential. The refined Kane approach to PB's model is shown to (i) bridge the regime between weakly interacting polarons and excitonic polarons with strong electron-hole correlations and (ii) recover the LLP model for free polarons in the limit of vanishing correlation. Various developments carried out in this paper also include extension of Kane and PB's semi-empirical models to incorporate Fröhlich-like interaction with multiple polar phonons, essential for reconciliation of experimental observations in multi-atom systems. In the end, we discuss the relation of the empirical approaches with ab initio methods and provide an outlook of their application to lower-dimensional systems.

cond-mat.mtrl-sci↗

arXiv2025

Flipping of electronic spins in BiFeO$_3$ via chiral $d-d$ excitations

BiFeO$_3$ is a multiferroic material featuring ferroelectricity and noncollinear antiferromagnetism. Definitive and efficient control of the characteristic spin texture of BiFeO$_3$ is attractive for emerging quantum devices. In this regard, crystal-field $d\rightarrow d$ excitations localized on Fe atomic sites in BiFeO$_3$ provide an avenue for manipulation of the spin texture as they induce a complex interplay among the spin, charge, and lattice degrees of freedom. In this work, the \textit{ab initio} \textit{GW}-BSE method is used to characterize these excitations within an excitonic picture. We find that the $d-d$ transitions appear as strongly bound, chiral, spin-flip excitons deep within the electronic band gap as a result of the intricate competition between the lattice potential, the antiferromagnetic ordering, the spin-orbit coupling, and the electron-hole interaction. Most crucially, these excitons are composed of electron-hole pairs with opposite spins that constitute almost all of their $\pm \hbar$ total angular momentum. These excitons of specific angular momentum can be selectively excited using circularly polarized light, consequently modulating the local magnetic moment.

cond-mat.mtrl-sci↗

arXiv2024

Understanding electronic excited states in BiFeO$_3$ via ab initio calculations and symmetry analysis

BiFeO$_3$ is a technologically relevant multiferroic perovskite featuring ferroelectricity and antiferromagnetism. Its lattice, magnetic, and ferroelectric degrees of freedoms are coupled to its optically active excitations and thus hold the potential to be reversible probed and controlled by light. In this work, we combine ab initio density functional and many-body perturbation theory methods with an extensive symmetry and atomic-orbital analysis to describe and understand the electronic excited states spectrum and its imprint on the optical absorption spectrum with quantitative accuracy and qualitative insights. We find that the optical absorption spectrum of BiFeO$_3$ contain several strongly bound and spatially localized electronic transitions in which the spin-degree of freedom is almost fully flipped. With our analysis we thoroughly characterize these localized spin-flip transitions in terms of the unusual crystal field splitting of Fe-$3d$ single-electron orbitals. Our symmetry analysis further allows us to thoroughly explain how the spin content and the energetic fine structure of these strongly bound excitons are dictated by the interplay between crystal symmetry, electron-hole attraction, and the spin-orbit coupling.

cond-mat.mtrl-sci↗

arXiv2023

Reconfigurable Multifunctional van der Waals Ferroelectric Devices and Logic Circuits

In this work, we demonstrate the suitability of Reconfigurable Ferroelectric Field-Effect- Transistors (Re-FeFET) for designing non-volatile reconfigurable logic-in-memory circuits with multifunctional capabilities. Modulation of the energy landscape within a homojunction of a 2D tungsten diselenide (WSe$_2$) layer is achieved by independently controlling two split-gate electrodes made of a ferroelectric 2D copper indium thiophosphate (CuInP$_2$S$_6$) layer. Controlling the state encoded in the Program Gate enables switching between p, n and ambipolar FeFET operating modes. The transistors exhibit on-off ratios exceeding 10$^6$ and hysteresis windows of up to 10 V width. The homojunction can change from ohmic-like to diode behavior, with a large rectification ratio of 10$^4$. When programmed in the diode mode, the large built-in p-n junction electric field enables efficient separation of photogenerated carriers, making the device attractive for energy harvesting applications. The implementation of the Re-FeFET for reconfigurable logic functions shows how a circuit can be reconfigured to emulate either polymorphic ferroelectric NAND/AND logic-in-memory or electronic XNOR logic with long retention time exceeding 10$^4$ seconds. We also illustrate how a circuit design made of just two Re-FeFETs exhibits high logic expressivity with reconfigurability at runtime to implement several key non-volatile 2-input logic functions. Moreover, the Re-FeFET circuit demonstrates remarkable compactness, with an up to 80% reduction in transistor count compared to standard CMOS design. The 2D van de Waals Re-FeFET devices therefore exhibit groundbreaking potential for both More-than-Moore and beyond-Moore future of electronics, in particular for an energy-efficient implementation of in-memory computing and machine learning hardware, due to their multifunctionality and design compactness.

cond-mat.mes-hall↗