arXiv subjects

Subhasis Panda

Publications and source records attributed to Subhasis Panda.

15 recordsLinked to original sources

arXiv2026

Quantum Geometry Driven Optical Responses in 1T-MX$_2$ Monolayers: A Symmetry-Constrained Slater-Koster Tight-Binding Approach

Centrosymmetric 1T-MX$_2$ monolayers(MLs) have attracted considerable attention owing to their intriguing transport properties and potential technological applications arising from the interplay among quantum geometry, electronic band structure, and band-gap characteristics. Despite this rich physics, a comprehensive microscopic tight-binding(TB) description that simultaneously captures these properties remains insufficiently established, while first-principles approaches can be computationally demanding for systematic investigations across different materials and perturbations. Here, we develop a transferable eleven-band Slater-Koster TB description of ML 1T-MX$_2$ TMDs and use it to establish a connection among their microscopic electronic structure, quantum geometry, and optical response. The model is constructed in an orthogonal orbital basis from the crystal geometry and symmetry-constrained SK parameters, with material-specific parametrizations obtained from DFT calculations for ML ZrS$_2$ and HfS$_2$. The resulting geometry-based Hamiltonian accurately describes the low-energy electronic structures and provides a natural framework for extending the analysis to the broader isostructural 1T-MX$_2$ family. We find that the pristine MLs possess a finite quantum metric, with the dominant contribution concentrated in the two highest occupied bands owing to the small near-gap energy separation and strong metal-chalcogen $p$-$d$ hybridization. Furthermore, we verify the interband $f$-sum rule, which directly relates the integrated optical spectral weight to the Brillouin-zone-averaged quantum metric. Our results establish optical spectral weight as an experimentally accessible probe of the quantum geometry of occupied Bloch states and provide a unified microscopic framework for connecting electronic structure, quantum geometry, and measurable optical responses across the 1T-MX$_2$ family.

cond-mat.mes-hall↗

arXiv2026

Euler characteristic of the Selmer group attached to an Artin representation

Longo and Vigni extended a result of Greenberg by giving a relation between the cardinality of the Selmer group and the characteristic power series of its Pontryagin dual over the cyclotomic $\mathbb{Z}_p$-extension for a $p$-adic Galois representation. In this paper, we extend this result to the case of Artin representations, following the framework of Greenberg and Vatsal.

math.NT↗

arXiv2026

On the Diophantine Equation $p^x+ (2p+1)^y =z^2$ with Consecutive Exponents

We study the Diophantine equation $p^x+ (2p+1)^y =z^2$ over positive integers $x$, $y$ and $z$ for every odd prime $p$. We prove that $(x,y,z)=(2,1,p+1)$ is the unique solution except possibly when $2p+1$ is composite. In that case, it reduces to a family depending on one parameter, and we show that the parameter must be odd and satisfies an explicit upper bound, reducing the problem to finitely many cases.

math.NT↗

arXiv2026

Magnetic field Controlled Anderson Delocalization in a Spinful Non-Hermitian Chain

Anderson localization (AL) and the non-Hermitian skin effect (NHSE) represent two paradigmatic localization phenomena driven, respectively, by disorder and non-Hermiticity. In one-dimensional (1D) non-Hermitian systems, these factors are known to compete and provide a smooth crossover between AL and NHSE upon parameter tuning. Here, we show that this interplay is fundamentally enriched in spinful systems, where an external magnetic field acts as an additional degree to manipulate the localization behavior. By investigating a disordered 1D spinful non-Hermitian chain, we demonstrate that under appropriately correlated disorder configurations across spin sectors, the magnetic field enhances the AL $\rightarrow$ NHSE crossover. Interestingly, this facilitates the Anderson delocalization transition even in strongly disordered systems where states would otherwise be Anderson localized. By analyzing the inverse participation ratio and the mean center of mass, we map the resulting triple interplay between disorder, non-Hermiticity, and the magnetic field strength, identifying regimes of Anderson localization and skin accumulation. We further reveal that this magnetic field driven delocalization phenomenon originates from an effective suppression of disorder strength via Zeeman-induced inter-chain coupling across the spin sectors.

cond-mat.mes-hall↗

arXiv2025

Optoelectronic properties and performance optimization for photovoltaic applications of R3m-RbGeX$_3$ (X = Cl, Br, I) perovskites: A combined DFT and SCAPS-1D study

In pursuit of an all-inorganic non-toxic perovskite solar cell (PSC) with enhanced performance, we have investigated the rhombohedral phase of the germanium-based rubidium halide perovskites RbGeX$_3$ (X = Cl, Br, I). The structural analysis followed by an in-depth study of the electronic and optical properties of these materials is performed within the framework of density functional theory (DFT). A detailed investigation of the electronic properties is carried out by examining the band structure and partial density of states (PDOS). PBE and TB-mBJ exchange-correlation functionals are used with and without the spin-orbit coupling (SOC), thereby obtaining accurate predictions of the band gaps. The key optical properties such as the real and imaginary parts of the dielectric function, absorption coefficient and refractive index are studied using PBE functional and compared among the three halides. RbGeI$_3$ exhibits the lowest band gap of 0.96 eV with the TB-mBJ + SOC functional, along with the most favorable optical properties, hence, it was identified as the most suitable candidate for the absorber layer (AL) of the PSC. SCAPS-1D simulation is performed using various input parameters for the AL such as the band gap, the effective densities of states for the conduction and valence bands and the electron and hole mobilities extracted from the DFT calculation. Performance optimization is done by exploring the impact of different inorganic hole transport layers (HTLs) and electron transport layers (ETLs). The impact of different layer thicknesses, doping densities, defect densities at the AL, ETL/AL, and AL/HTL interfaces, various back contacts, and the influence of series and shunt resistance on the overall device performance are studied. The optimized all-inorganic device demonstrated a remarkable power conversion efficiency (PCE) of $25.76\%$ with a fill factor (FF) of $79.81\%$.

cond-mat.mtrl-sci↗

arXiv2025

Gauge Field Induced Unconventional Skin Effect in Spinful Non-Hermitian Systems

The non-Hermitian skin effect (NHSE), a hallmark of non-Hermitian systems, stems from the topological nature of complex energy spectra, typically characterized by a non-zero spectral winding number. Beyond the spinless frameworks considered so far, here we realize a generic, tunable spinful NHSE in a 1D tight-binding lattice endowed with spin-dependent Abelian gauge fields. With proper tuning of the gauge parameter, we uncover an emergence of bidirected, spin-polarized zero-winding skin states, appearing in the absence of transpose-type time-reversal symmetry (T RS{\dag}) and featuring scale-restricted localization with non-Bloch spectral stability. While clearly distinct from the known Z2 and Critical NHSEs, these unconventional skin states evolve into them upon enforcing TRS{\dag} and introducing inter-spin coupling via magnetic fields, respectively. The magnetic field further drives a transition from a bidirectional to a unidirectional skin configuration. Our work unifies the previously known zero-winding NHSEs within a broader framework and provides experimentally accessible routes for realization in photonic and ultracold atomic systems with synthetic gauge fields.

cond-mat.mes-hall↗

arXiv2024

Fingerprint of Non-Hermiticity in d-wave Altermagnet

We develop the non-Hermitian counterpart of a new class of collinear magnets, dubbed altermagnets, delineated by net zero magnetization with momentum-dependent spin-splitting bands. The application of an imaginary gauge field in a two-dimensional $d$-wave altermagnet injects a non-reciprocal intercell hopping without hosting exceptional points (EPs). This non-Hermitian phase hosts a point gap which remains robust in the presence and absence of Rashba spin-orbit coupling. By attaching a ferromagnetic lead with the altermagnet, we uncover the emergence of two pairs of second-order EPs. We establish that each of the EPs is associated with a half-integer quantized topological charge, a hallmark signature of the non-Hermitian topology. The existence of this non-Hermitian exceptional phase has been further confirmed by linear variation with the respective momentum and coalescence of the spin expectation value at the EPs. Finally, we demonstrate that the application of a planar magnetic field to the junction not only tunes the location of the EPs but also can annihilate a single pair or even all pairs of EPs with opposite topological charges depending upon the field strength and direction.

cond-mat.mes-hall↗

arXiv2022

Ab-initio study of the effect of bromide mixing into RbPbI$_3$ on the structural, electronic and optical properties

The ultra-high efficiency and cost-effective photovoltaics based on halide preovskites have brought a revolution to ongoing photovoltaic research, surpassing the expectations of the scientific community. However, structural stability is a severe issue that hinders their wide-scale integration at the device level. Compositional engineering with the halide mixing has become an efficient way to deal with this issue without compromising device efficiency. Herein, the structural, electronic and optical properties of the bromide mixed orthorhombic $\rm{{RbPb(I_{1-x}Br_x)_3}}$ (where, $\rm{x}=0.25$, $0.50$ and $0.75$) are calculated using the density functional theory. The electronic bandstructure and density of states (DOS) are calculated using both PBE (Perdew-Burke-Ernzerhof) and TB-mBJ (Tran Blaha modified Becke Johnson) potential. The lowest energy bandgaps of $2.288$ and $2.986$ eV for bromide mixing of $\rm{x}=0.50$ are obtained using PBE and TB-mBJ, respectively. In contrast, the mixed bromide phases possess a smaller effective mass, facilitating a better carrier transport through the mixed halide. Using PBE, the excitons appear to be the Mott-Wannier type. However, the TB-mBJ predicts the exciton to be Frenkel type for bromide mixing of $\rm{x}=0.75$ and a Mott-Wannier type for all other mixing under study. The spectroscopic limited maximum efficiency (SLME) is observed to be at the highest values of $14.0$\% and $4.1$\% for the equal admixture of I and Br using PBE and TB-mBJ, respectively. The calculated properties are consistent with the reported data of the similar structures.

cond-mat.mtrl-sci↗

arXiv2022

On characteristic ideal of Selmer group associated to Artin representations

Selmer group for an Artin representation over totally real fields was studied by Greenberg and Vatsal. In this paper we study the Selmer groups for an Artin representation over a totally complex field. We establish an algebraic function of the characteristic ideal of the Selmer group associated to Artin representation over the cyclotomic $\Z_p$- extension of the rational numbers under certain mild hypotheses and construct several examples to illustrate our result. We also prove that in this situation $\mu$-invariant of the dual Selmer group is independent of the choice of the lattice.

math.NT↗

arXiv2022

The effect of B-site alloying on the electronic and opto-electronic properties of RbPbI3: A DFT study

Divalent cations mixed lead halide perovskites with enhanced performances, high stabilities, and reduced toxicity are requisite to make persistent progress in perovskite solar cells. However, the mixing strategy is not reported extensively in search of a lead reduced structure. Herein, we report the structural, electronic and optical properties of RbPb{1-x}MxI3 (where, M={Sn,Ge} and x={0.25, 0.50, 0.75}) by alloying the B-site with Sn and Ge, using the density functional theory. The formation enthalpy is estimated for all RbPb{1-x}MxI3 (with x= 0.25, 0.50, 0.75), which confirms stability for all the structures. The energy bandgap and density of states (DOS) have been thoroughly investigated. The energy bandgap decreases with the increasing Sn/Ge contents, the lowest bandgap of 1.850 eV is observed at x = 0.50 in the case of RbPb{1-x}GexI3 systems. Further, the effective masses and the binding energy of excitons and spectroscopic limited maximum efficiency (SLME) are also estimated for all the mixed systems. The exciton type is observed to change from Mott-Wannier to Frenkel type with increasing the contents of both Sn and Ge at the B-site. The maximum efficiency of 23% is achieved using an active layer containing an equal admixture of Sn/Ge and Pb. The estimated parameters of both the mixed systems are consistent with the available literature of similar types.

cond-mat.mtrl-sci↗

arXiv2019

First principle studies on the optoelectronic properties of rubidium lead halides

Entirely inorganic perovskites have attracted enormous attention of late owing to their outstanding applications in optoelectronics including highly stable perovskite solar cells. In-depth understanding of the optoelectronic and transport properties of such materials are vital for practical implementation of the same. The carrier transport properties of the electronic devices based on perovskite materials significantly depend on the effective mass of the respective charge carriers. Here, we have performed first principle calculations with FP-LAPW method for the orthorhombic rubidium lead halide structures (\ch{RbPbX_3}, where \ch{X=I,Br,Cl}) to study the optoelectronic and transport properties. The effective mass of electron (hole) is found to be minimum for \ch{RbPbBr_3} (\ch{RbPbI_3}), suggesting an efficient transport of electrons (holes) in the corresponding materials. Our calculated values such as the dielectric constants, refractive indices, absorption coefficients and reflectivities show good agreement with reported experimental data. To the best of our knowledge, ab-initio study of electronic and optical properties of \ch{RbPbBr_3} \& \ch{RbPbCl_3} in orthorhombic phase (\ch{NH_4CdCl_3} type structure) is reported for the first time.

cond-mat.mtrl-sci↗

arXiv2018

Fuzzy expert system for prediction of prostate cancer

A fuzzy expert system (FES) for the prediction of prostate cancer (PC) is prescribed in this article. Age, prostate-specific antigen (PSA), prostate volume (PV) and $\%$ Free PSA ($\%$FPSA) are fed as inputs into the FES and prostate cancer risk (PCR) is obtained as the output. Using knowledge based rules in Mamdani type inference method the output is calculated. If PCR $\ge 50\%$, then the patient shall be advised to go for a biopsy test for confirmation. The efficacy of the designed FES is tested against a clinical data set. The true prediction for all the patients turns out to be $68.91\%$ whereas only for positive biopsy cases it rises to $73.77\%$. This simple yet effective FES can be used as supportive tool for decision making in medical diagnosis.

q-bio.QM↗

arXiv2018

From Classical Periodic Orbits in Integrable $\pi$-Rational Billiards to Quantum Energy Spectrum

In the present note, we uncover a remarkable connection between the length of periodic orbit of a classical particle enclosed in a class of 2-dimensional planar billiards and the energy of a quantum particle confined to move in an identical region with infinitely high potential wall on the boundary. We observe that the quantum energy spectrum of the particle is in exact one-to-one correspondence with the spectrum of the amplitude squares of the periodic orbits of a classical particle for the class of integrable billiards considered. We have established the results by geometric constructions and exploiting the method of reflective tiling and folding of classical trajectories. We have further extended the method to 3-dimensional billiards for which exact analytical results are scarcely available - exploiting the geometric construction, we determine the exact energy spectra of two new tetrahedral domains which we believe are integrable. We test the veracity of our results by comparing them with numerical results.

quant-ph↗

arXiv2013

Metric deformation and boundary value problems in 3D

A novel perturbative method, proposed by Panda {\it et al.} [1] to solve the Helmholtz equation in two dimensions, is extended to three dimensions for general boundary surfaces. Although a few numerical works are available in the literature for specific domains in three dimensions such a general analytical prescription is presented for the first time. An appropriate transformation is used to get rid of the asymmetries in the domain boundary by mapping the boundary into an equivalent sphere with a deformed interior metric. The deformed metric produces new source terms in the original homogeneous equation. A deformation parameter measuring the deviation of the boundary from a spherical one is introduced as a perturbative parameter. With the help of standard Rayleigh-Schr{\"o}dinger perturbative technique the transformed equation is solved and the general solution is written down in a closed form at each order of perturbation. The solutions are boundary condition free and which make them widely applicable for various situations. Once the boundary conditions are applied to these general solutions the eigenvalues and the wavefunctions are obtained order by order. The efficacy of the method has been tested by comparing the analytic values against the numerical ones for three dimensional enclosures of various shapes. The method seems to work quite well for these shapes for both, Dirichlet as well as Neumann boundary conditions. The usage of spherical harmonics to express the asymmetries in the boundary surfaces helps us to consider a wide class of domains in three dimensions and also their fast convergence guarantees the convergence of the perturbative series for the energy. Direct applications of this method can be found in the field of quantum dots, nuclear physics, acoustical and electromagnetic cavities.

math-ph↗

arXiv2011

Metric deformation and boundary value problems in 2D

A new analytical formulation is prescribed to solve the Helmholtz equation in 2D with arbitrary boundary. A suitable diffeomorphism is used to annul the asymmetries in the boundary by mapping it into an equivalent circle. This results in a modification of the metric in the interior of the region and manifests itself in the appearance of new source terms in the original homogeneous equation. The modified equation is then solved perturbatively. At each order the general solution is written in a closed form irrespective of boundary conditions. This method allows one to retain the simple form of the boundary condition at the cost of complicating the original equation. When compared with numerical results the formulation is seen to work reasonably well even for boundaries with large deviations from a circle. The Fourier representation of the boundary ensures the convergence of the perturbation series.

quant-ph↗