arXiv subjects

S. Bhattarai

Publications and source records attributed to S. Bhattarai.

At least 19 recordsLinked to original sources

arXiv2026

Performance Analysis of Double Perovskite-Based Solar Cells Using SCAPS-1D Simulation: A brief review

Lead-free double perovskites are among the rapidly developing next-generation solar cell technologies, providing the required low toxicity, stability, as well as high optoelectronic potential. So far, experimentally prepared lead-free perovskite solar cell devices are reported to have low power conversion efficiency (PCE) for practical application as compared to the lead-based perovskites. In recent years, numerical simulations have emerged as a cost-effective approach that plays a crucial role in expediting scientific research, can bridge the gap between experiment and theory, and provide predictive information regarding the preparation of solar cells and their PCEs without undergoing real-time experiments. The tools, such as 1D numerical simulation software SCAPS-1D, are now needed to test newer architectures and determine what exactly is holding them back. So far in the field of solar cell research, SCAPS-1D has been extensively used and looks like a powerful software due to its user-friendliness and simulation of results in a few seconds. The speed and ease of simulation make SCAPS-1D a very popular tool; as a result, it enables rapid optimization of a large number of photovoltaic devices and their performances without undergoing any experimental work, which can save time and money. However, one serious drawback is that the SCAPS-1D simulator works only for 1D configurations. It is ineffective in incorporating atomistic interactions and 3D effects. Hence, the efficacy of the SCAPS-1D simulator solely relies on the accuracy of the input parameters that the user provides, failing which may give wrong results and large deviations from accuracy.

cond-mat.mtrl-sci↗

arXiv2026

Detector-Grade Germanium as a Low-Disorder Host for Indium-Acceptor Spin Qubits: A Five-Qubit Materials-to-Architecture Design Study

Acceptor-bound hole spins in germanium (Ge) offer a promising but underexplored route to semiconductor quantum information processing. We present a theory-guided design study of a detector-grade Ge acceptor-spin platform based on intentionally incorporated indium (In) acceptors in ultra-high-purity Ge. The proposed materials strategy combines a residual impurity background near $10^{10} \mathrm{cm^{-3}}$ with a target In density of approximately $2\times10^{14} \mathrm{cm^{-3}}$, corresponding to an acceptor spacing of about 170 nanometer. A 1 $\mu$m-long active channel with a suitable transverse mode volume can contain about five acceptors on average, enabling a statistically selected post-fabrication register rather than a deterministically placed chain. We analyze the physical basis, device architecture, strain and disorder limits, coupling hierarchy, modeling workflow, fabrication pathway, and scaling prospects. Our results indicate that detector-grade Ge can suppress uncontrolled bulk electrostatic and strain disorder to levels compatible with acceptor-hole qubits, while the spin--orbit-active valence-band manifold supports all-electrical control and dipolar or phonon-mediated coupling. Direct exchange is treated as a close-pair or gate-enhanced interaction rather than the generic mean-spacing coupling. Phononic crystal engineering is identified as a second-stage enhancement for suppressing unwanted acoustic modes and enabling selected cavity-mediated interactions after baseline control, readout, and nearest-neighbor coupling are validated. Remaining challenges include statistical acceptor placement, interface disorder, charge noise, readout integration, and experimental validation. This work identifies detector-grade Ge In-acceptor qubits as a credible intermediate architecture between donor-based impurity qubits and fully gate-defined Ge hole-spin hardware.

quant-ph↗

arXiv2026

AIMBio-Mat: An AI-Native FAIR Platform for Closed-Loop Materials Discovery and Biomedical Translation

Materials discovery and biomedical translation increasingly require models that can reason across composition, processing, structure, biological response, manufacturability, safety, and governance constraints. Existing materials and biomedical data ecosystems are powerful but remain poorly coupled for AI-guided discovery. Here we present AIMBio, a conceptual framework for an AI-native, FAIR, and governance-aware decision layer that links materials provenance, biomedical context, knowledge graphs, uncertainty-aware machine learning, and human-in-the-loop active learning. The framework formulates biomedical-materials discovery as constrained multi-objective optimization under uncertainty and introduces practical requirements for metadata, model documentation, risk-tiered governance, evaluation metrics, and phased implementation. To make the roadmap testable, we add a minimum viable prototype specification and a worked pilot for AI-guided nanomaterials for drug delivery. AIMBio is positioned as exploratory and preclinical discovery infrastructure, not as clinical decision-support software; any clinical or regulated-device use would require separate validation, change control, and regulatory review. The central contribution is a publishable platform blueprint for converting fragmented materials and biomedical records into auditable, experimentally actionable, and translationally responsible discovery workflows.

physics.app-ph↗

arXiv2026

Building a Regional Data-Centric Materials Science Ecosystem for Processing-Rich Materials Innovation in the Great Plains

Data-centric materials science is changing how materials are discovered, optimized, manufactured, and qualified, yet many deployment-limiting materials problems still depend on experimental, processing-rich, device-level, and field-relevant data that are difficult to capture in conventional materials databases. This perspective argues that the Great Plains and adjacent interior research corridor can make a distinctive national contribution by organizing distributed experimental assets into a trusted regional materials-data ecosystem. The proposed model emphasizes FAIR metadata, provenance, persistent sample identifiers, uncertainty-aware modeling, semi-closed-loop workflows, stackable workforce training, and tiered governance for academic, public, controlled-access, and industry-protected data. We identify five coupled barriers -- fragmented data, weak algorithm--laboratory translation, uneven access to cyberinfrastructure and technical staff, workforce gaps at the materials--data interface, and insufficient incentives for sharing and reuse -- and propose a staged roadmap for addressing them. A high-purity germanium pilot illustrates how regional strengths can be converted into reusable datasets, benchmark models, trained personnel, and decision-improving workflows. The broader message is that regional leadership in data-centric materials science will depend less on geographic concentration than on trustworthy data practices, interoperable infrastructure, cross-trained people, and application-driven materials challenges.

cond-mat.mtrl-sci↗

arXiv2026

Comparative assessment of germanium-based spin-qubit modalities: donor, acceptor, gate-defined hole, and gate-defined electron platforms

High-purity germanium (Ge) has re-emerged as a versatile semiconductor platform for spin-based quantum information processing because it combines mature materials processing, access to spin-free isotopes, high mobilities, small effective masses, and strong but engineerable spin--orbit coupling. However, ``Ge qubits'' are not a single technology. Donor spin qubits, acceptor spin qubits, gate-defined hole spin qubits, and gate-defined electron spin qubits exploit different parts of the Ge band structure and therefore make distinct trade-offs among coherence, controllability, fabrication complexity, and scalability. Here we compare these four Ge-based spin-qubit modalities on a common physical and architectural footing. We review the shared Ge materials physics, including isotopic purification, the multivalley $L$-point conduction band, the spin-$3/2$ valence band, heavy-hole/light-hole mixing, strain, interfaces, disorder, and phonons. We also introduce a common framework for estimating phononic-crystal-modified $T_1$ using a calibrated reference relaxation rate, a geometry-dependent strain-density-of-states suppression factor, and parasitic relaxation channels. The comparison shows that gate-defined Ge hole-spin qubits currently offer the strongest combination of all-electrical control, demonstrated multiqubit operation, and scalability. Donor, acceptor, and gate-defined electron qubits remain important complementary directions for memory, hybrid, and exploratory architectures. Overall, Ge supports a diverse qubit ecosystem, with gate-defined hole-spin qubits presently providing the clearest path toward scalable Ge-based quantum processors.

quant-ph↗

arXiv2026

Two-Qubit Module Based on Phonon-Coupled Ge Hole-Spin Qubits: Design, Fabrication, and Readout at 1-4 K

We present a device-level design study for a two-qubit module based on phonon-coupled germanium (Ge) hole-spin qubits targeted for operation at $1$--$4~\mathrm{K}$. Building on prior theoretical modeling of phonon-engineered Ge qubits and phononic-crystal (PnC) cavities, we translate those modeling results into a fabrication-oriented two-qubit layout that integrates two gate-defined hole-spin qubits in a strained Ge quantum well with a GHz PnC defect mode intended to mediate a coherent phonon-based interaction. We specify the SiGe/Ge heterostructure, electrostatic gate layout, PnC cavity geometry, and a compatible nanofabrication pathway, including gate-stack formation, membrane patterning and release, RF/DC wiring, and process-risk mitigation. We further develop a readout architecture combining spin-to-charge conversion with RF reflectometry on a proximal charge sensor, and we provide a link-budget estimate that states the assumed system noise temperature, RF signal contrast, and integration-time requirements for single-shot readout at elevated cryogenic temperatures. Finally, we outline a stepwise benchmarking program for charge stability, single-qubit control, phonon-bandgap modification of relaxation, and resolvable phonon-mediated two-qubit coupling. The manuscript does not report experimental device data; rather, it provides an experimentally actionable bridge from prior modeling to future fabrication and measurement of phonon-coupled Ge hole-spin modules.

cond-mat.mes-hall↗

arXiv2025

Evaluating the Effective Segregation Coefficient in High-Purity Germanium (HPGe) Crystals for Ge Detector Development in Rare-Event Searches

The performance and scalability of rare-event physics experiments depend on large-volume, detector-grade high-purity germanium (HPGe) crystals with precise control of impurity segregation during growth. We report a detailed study of impurity distribution in a single Czochralski-grown HPGe crystal produced at University of South Dakota (USD). The crystal was sectioned longitudinally into 37 segments, enabling the first high-resolution and systematic mapping of dopant profiles along the length of a detector-grade HPGe boule. Hall-effect measurements were used to extract impurity concentrations for boron (B), aluminum (Al), gallium (Ga), and phosphorus (P) in each segment. From these data, we determine effective segregation coefficients ($K_{eff}$) and initial melt concentrations ($C_0$) for the dominant dopants and compare them with classical Burton-Prim-Slichter expectations. The results provide quantitative insight into impurity transport and melt-solid partitioning under realistic detector growth conditions. These findings inform process-optimization strategies for HPGe crystal pulling, improve impurity control along the boule, and support the reliable fabrication of large, low-background HPGe detectors for next-generation rare-event searches.

physics.ins-det↗

arXiv2025

Probing low-mass dark matter from sub-MeV to sub-GeV with germanium-based quantum phononic spectroscopy

We present a germanium phonon-to-charge transducer that integrates a slow-phonon phononic-crystal (PnC) region with radio-frequency quantum point-contact (RF-QPC) readout at 4 K, and we evaluate its dark-sector reach. A calibrated signal-collection model, which combines geometric guiding, propagation survival, and multiplicity-assisted primary-phonon detection, provides selection-corrected thresholds in the $10^{-3}$-$10^{-2}$~eV range and a background model informed by nanosecond timing gates and GHz-band power-spectral-density windows. Under standard halo assumptions, a 100 g module achieves projected sensitivity to DM-electron and DM-nucleon scattering at recoil energies below $10^{-2}$~eV, probing cross sections below $10^{-43}\,\mathrm{cm}^{2}$ for $m_\chi \in [0.01,100]~\mathrm{MeV}/c^{2}$ (with efficiencies and thresholds folded in). We present sensitivities for both heavy-mediator ($F_{\rm DM}=1$) and light-mediator ($F_{\rm DM}\propto 1/q^{2}$) benchmarks, quantify dominant systematics (threshold, phonon quality factor $Q$, and bulk defect densities), and outline a staged program toward kg$\cdot$yr exposures that begins to test models approaching the solar CE$\nu$NS background.

hep-ph↗

arXiv2025

Threshold $J/\psi$ Photoproduction as a Probe of Nuclear Gluon Structure

The nuclear EMC effect is the observation that quark distributions in bound nucleons experience significant modification at large $x$ relative to free nucleons. Despite decades of measurements verifying the presence of this effect in quarks across a wide range of nuclei, behavior of large-$x$ gluons in nuclei remains almost completely unknown. As the nuclear physics community seeks out new observables to try to elucidate the mechanisms behind the EMC effect, it becomes striking that we remain ignorant regarding the impact of nuclear effects on gluonic behavior. Recent photonuclear data using the Hall D photon beam have enabled the first measurement of $J/\psi$ photoproduction from nuclei near and below the energy threshold, with the results highlighted in Physical Review Letters as an Editors' Suggestion. These data have placed the first, and currently only, constraints on the behavior of large-$x$ gluons within bound nucleons. However, compared to the quantity of data which currently informs our knowledge of the quark-sector EMC effect, these data are extremely limited, and remain unable to conclusively observe or exclude large modification of gluon distributions. A high-luminosity photonuclear experiment will enable a precision measurement of incoherent $J/\psi$ photoproduction at and below the threshold region. This data will provide the first stringent constraints on nuclear modification of gluon structure or other exotic effects which could impact the production of $J/\psi$ from nuclei. We request 85 PAC days at Hall D using the GlueX detector with a 12 GeV electron beam energy and a coherent photon peak energy of $8$ GeV, split into 80 days using a $^4$He target and 5 calibration days using a $^2$H target.

nucl-ex↗

arXiv2025

Cu2XSiS4 (X = Ge, Sn, and Pb) materials for solar-cell applications: A DFT+SCAPS-1D simulation

By means of the first-principles density functional theory (DFT), I2-II-IV-VI4 type Cu-based quaternary chalcogenides Cu 2 XSiS 4 (X = Ge, Sn, and Pb) have been thoroughly investigated. We report the study of Ge and Sn substitution in the divalent cation site for their potential applications in photovoltaics for the first time. The structural, electronic, optical, and mechanical properties have been calculated. The structural and thermal stability is verified by calculating the elastic constants, formation energy and total potential energy at 300 K from the ab-initio molecular dynamics (MD) simulation. The compounds under our investigation exhibited an indirect band gap in the range of 1.0--1.56 eV, suitable for energy harvesting by trapping the sunlight. The presence of absorption peaks within the visible region complements their potential in photovoltaic applications. For further validation, we have designed a model of a heterostructure (FTO/TiO2/Cu2XSiS4/CuO/Au) solar cell, and a numerical simulation has been performed by solving the Poisson equation and continuity equations to obtain the I-V characteristic by using SCAPS-1D. All the inputs needed for solar- cell simulation in SCAPS-1D have been taken from the DFT results. The corresponding Power Conversion Efficiency (PCE) is denoted by {\eta}% and their respective values for X=Ge, Sn and Pb are 23.46%, 23.29% and 22.60%, at room temperature. The Ge-based system exhibits the highest {\eta}%, owing to its band gap value in the visible range of the solar spectrum. Thus, we report that Ge-based compounds may act as a promising absorber layer in heterostructure solar-cell applications.

cond-mat.mtrl-sci↗

arXiv2025

Ge-based Quantum Sensors for Low-Energy Physics

We present \textbf{GeQuLEP} (Germanium-based Quantum Sensors for Low-Energy Physics), a conceptual design for an advanced quantum sensing platform integrating high-purity germanium (Ge) crystals with engineered phononic crystal cavities. At cryogenic temperatures, these cavities naturally host dipole-bound states, effectively forming quantum dots coupled to radio-frequency quantum point contact (RF-QPC) readout systems. This innovative coupling approach promises ultra-sensitive phonon-mediated charge detection through phonon-induced charge displacement. GeQuLEP is specifically designed to achieve exceptionally low detection thresholds, theoretically enabling single primary phonon sensitivity with anticipated energy depositions as low as \textbf{0.00745~eV}. This unprecedented sensitivity, if realized experimentally, would provide unique access to searches for low-mass dark matter down to the keV/$c^2$ mass range via nuclear and electronic recoils. Additionally, GeQuLEP aims to facilitate the real-time detection of solar \textit{pp} neutrinos through coherent elastic neutrino--nucleus scattering (CE$\nu$NS). By combining phonon-based quantum transduction with quantum-classical hybrid readout schemes, the GeQuLEP architecture represents a scalable, contact-free phonon spectroscopy design that could significantly advance the capabilities of ultra-low-energy rare-event detection at the quantum limit.

astro-ph.IM↗

arXiv2025

Phonon-Coupled Hole-Spin Qubits in High-Purity Germanium: Design and Modeling of a Scalable Architecture

We present a design and modeling of a scalable quantum processor architecture utilizing hole-spin qubits defined in gate-controlled germanium (Ge) quantum dots, where coherent spin-phonon coupling is predicted to facilitate qubit manipulation and long-range interactions. The architecture exploits the strong, electrically tunable spin-orbit interactions intrinsic to hole states in Ge, integrated with high-quality phononic crystal cavities (PnCCs) to enable fully electrical qubit control and phonon-mediated coupling. Employing a streamlined simulation framework built upon multiband $\mathbf{k}\cdot\mathbf{p}$ modeling and finite-element methods, we quantify key performance metrics, including electrically tunable $ g $-factors ranging from $1.3$ to $2.0$, spin-phonon coupling strengths up to $6.3\,\mathrm{MHz}$, phononic cavity quality factors exceeding $10^4$, and phonon-mediated spin relaxation times ($T_1$) reaching milliseconds. The proposed architecture concurrently achieves extended spin coherence and rapid gate operations through strategic electric field modulation and engineered phononic bandgap environments. Furthermore, isotopically enriched high-purity Ge crystals grown in-house at the University of South Dakota, significantly enhance device coherence by minimizing disorder and hyperfine interactions. This integrated approach, merging advanced materials engineering, precise spin-orbit coupling, and phononic cavity design, establishes a promising CMOS-compatible pathway toward scalable, high-fidelity quantum computing.

quant-ph↗

arXiv2022

Evidence of cluster dipole states in germanium detectors operating at temperatures below 10 K

By studying charge trapping in germanium (Ge) detectors operating at temperatures below 10 K, we demonstrate for the first time that the formation of cluster dipole states from residual impurities is responsible for charge trapping. Two planar detectors with different impurity levels and types are used in this study. When drifting the localized charge carriers created by $α$ particles from the top surface across a detector under a lower bias voltage, significant charge trapping is observed when compared to operating at a higher bias voltage. The amount of charge trapping shows a strong dependence on the type of charge carriers. Electrons are trapped more than holes in a p-type detector while holes are trapped more than electrons in a n-type detector. When both electrons and holes are drifted simultaneously using the widespread charge carriers created by $γ$ rays inside the detector, the amount of charge trapping shows no dependence on the polarity of bias voltage.

physics.ins-det↗

arXiv2021

Low-Energy Solar Neutrino Detection Utilizing Advanced Germanium Detectors

We explore the possibility to use advanced germanium (Ge) detectors as a low-energy solar neutrino observatory by means of neutrino-nucleus elastic scattering. A Ge detector utilizing internal charge amplification for the charge carriers created by the ionization of impurities is a novel technology with experimental sensitivity for detecting low-energy solar neutrinos. Ge internal charge amplification (GeICA) will amplify the charge carriers induced by neutrino interacting with Ge atoms through emission of phonons. It is those phonons that will create charge carriers through the ionization of impurities to achieve an extremely low energy threshold of $\sim$0.01 eV. We demonstrate the phonon absorption, excitation, and ionization probability of impurities in a Ge detector with impurity levels of 3$\times$10$^{10}$ cm$^{-3}$, 9$\times$10$^{10}$ cm$^{-3}$, and 2$\times$10$^{11}$ cm$^{-3}$. We present the sensitivity of such a Ge experiment for detecting solar neutrinos in the low-energy region. We show that, if GeICA technology becomes available, then a new opportunity arises to observe $pp$ and 7Be solar neutrinos. Such a novel detector with only 1 kg of high-purity Ge will give $\sim$ 10 events per year for $pp$ neutrinos and $\sim$ 5 events per year for 7Be neutrinos with a detection energy threshold of 0.01 eV.

physics.ins-det↗

arXiv2021

Implication of the Temperature-Dependent Charge Barrier Height of Amorphous Germanium Contact Detector in Searching for Rare Event Physics

The exploration of germanium (Ge) detectors with amorphous Ge (a-Ge) contacts has drawn attention to the searches for rare-event physics such as dark matter and neutrinoless double-beta decay. The charge barrier height (CBH) of the a-Ge contacts deposited on the detector surface is crucial to suppress the leakage current of the detector in order to achieve la ow-energy detection threshold and high-energy resolution. The temperature-dependent CBH of a-Ge contacts for three Ge detectors is analyzed to study the bulk leakage current (BLC) characteristics. The detectors were fabricated at the University of South Dakota using homegrown crystals. The CBH is determined from the BLC when the detectors are operated in the reverse bias mode with a guard-ring structure, which separates the BLC from the surface leakage current (SLC). The results show that CBH is temperature dependent. The direct relation of the CBH variation to temperature is related to the barrier inhomogeneities created on the interface of a-Ge and crystalline Ge. The inhomogeneities that occur at the interface were analyzed using the Gaussian distribution model for three detectors. The CBH of a-Ge contact is projected to zero temperature. The implication of the CBH at zero temperature is discussed for Ge detectors with a-Ge contacts in searching for rare-event physics.

physics.ins-det↗

arXiv2020

Investigation of the Electrical Conduction Mechanisms in P-type Amorphous Germanium (a-Ge) Used as a-Ge Contacts for Ge Detectors

Electrical conduction mechanisms in the disordered material system is experimentally studied for p-type amorphous germanium (a-Ge) used for high-purity Ge detector contacts. The localization length and the hopping parameters in a-Ge are determined using the surface leakage current measured from three high-purity planar Ge detectors. The temperature-dependent hopping distance and hopping energy are obtained for a-Ge fabricated as the electrical contact materials for high-purity Ge planar detectors. As a result, we find that the hopping energy in a-Ge increases as temperature increases while the hopping distance in a-Ge decreases as temperature increases. The localization length of a-Ge is on the order of $2.13^{-0.05}_{+0.07} A^\circ$ to $5.07^{-0.83}_{+2.58}A^\circ$, depending on the density of states near the Fermi energy level within bandgap. Using these parameters, we predict that the surface leakage current from a Ge detector with a-Ge contacts can be much smaller than one yocto amp (yA) at helium temperature, suitable for rare-event physics searches.

physics.ins-det↗

arXiv2020

Characterization of High-Purity Germanium (Ge) Crystals for Developing Novel Ge Detectors

High-purity germanium (HPGe) crystals are required to be well-characterized before being fabricated into Ge detectors. The characterization of HPGe crystals is often performed with the Hall Effect system, which measures the carrier concentration, the Hall mobility, and the electrical resistivity. The reported values have a strong dependence on the size of the ohmic contacts and the geometry of the samples used in conducting the Hall Effect measurements. We conduct a systematic study using four samples cut from the same location in a HPGe crystal made into different sized ohmic contacts or different geometries to study the variation of the measured parameters from the Hall Effect system. The results are compared to the C-V measurements provided by the Ge detector made from the same crystal. We report the systematic errors involved with the Hall Effect system and find a reliable technique that minimizes the systematic error to be only a few percent from the Hall Effect measurements.

physics.ins-det↗

arXiv2020

Impact of Charge Trapping on the Energy Resolution of Ge Detectors for Rare-Event Physics Searches

Charge trapping degrades the energy resolution of germanium (Ge) detectors, which require to have increased experimental sensitivity in searching for dark matter and neutrinoless double-beta decay. We investigate the charge trapping processes utilizing nine planar detectors fabricated from USD-grown crystals with well-known net impurity levels. The charge collection efficiency as a function of charge trapping length is derived from the Shockley-Ramo theorem. Furthermore, we develop a model that correlates the energy resolution with the charge collection efficiency. This model is then applied to the experimental data. As a result, charge collection efficiency and charge trapping length are determined accordingly. Utilizing the Lax model (further developed by CDMS collaborators), the absolute impurity levels are determined for nine detectors. The knowledge of these parameters when combined with other traits such as the Fano factor serve as a reliable indicator of the intrinsic nature of charge trapping within the crystals. We demonstrate that electron trapping is more severe than hole trapping in a p-type detector and the charge collection efficiency depends on the absolute impurity level of the Ge crystal when an adequate bias voltage is applied to the detector. Negligible charge trapping is found when the absolute impurity level is less than 1.0$\times$10$^{11}/$cm$^{3}$ for collecting electrons and 2.0$\times$10$^{11}/$cm$^{3}$ for collecting holes.

physics.ins-det↗