arXiv subjects

Durga Paudyal

Publications and source records attributed to Durga Paudyal.

At least 19 recordsLinked to original sources

arXiv2026

Electronic structure, magnetic interactions, and magnonics of 2D trichloride materials

Using advanced density functional theory, this study demonstrates the robust thermodynamic, structural, and dynamical stability of pristine, 3d-doped (Ti, Cr), and 4f-doped (Ce) transition metal trichlorides (MCl3). While standard generalized gradient approximation (GGA) incorrectly predicts metallic behavior, hybrid functional calculations successfully capture their semiconducting nature, yielding accurate band gaps of 2.54 eV (VCl3), 4.01 eV (CrCl3), and 2.61 eV (TiCl3). These materials exhibit intrinsic ferromagnetism, with CrCl3 displaying overlapping topological features in both its magnon and phonon dispersions along the high-symmetry K direction, which induces magnon-phonon coupling. Incorporating 3d dopants into VCl3 successfully tunes the bandgap (1.32 eV for Ti and 2.74 eV for Cr) without destroying ferromagnetism, while 4f Ce-doping in CrCl3 introduces localized states below the Fermi level that yield strong nearest-neighbor exchange coupling. Ultimately, this electronic and magnetic tunability highlights the potential of these 2D trichlorides in advanced spintronic functionalities.

cond-mat.mtrl-sci↗

arXiv2026

Ultrafast Light-Induced Magnetoelectric Effect in van der Waals Magnetic Semiconductor Heterostructures

Atomic-scale heterostructures of van der Waals (vdW) magnets and semiconductors provide a unique environment for exploring magnetic dynamics. In contrast to typical photothermal excitation of precessional magnetization dynamics by a pump laser pulse, we find that ultrafast optical excitation of a WS$_2$/CrGeTe$_3$ (CGT) bilayer produces an opposite sign of magnetic torque compared to an isolated CGT film. Experimental observations by time-resolved magneto-optic Kerr effect (TR-MOKE) and theoretical analysis by density functional theory (DFT) and Landau-Lifshitz-Gilbert (LLG) simulations support a mechanism in which charge transfer of photoexcited carriers across the interface alters the perpendicular magnetic anisotropy, which in turn generates a torque on the magnetic layer to trigger precessional magnetization dynamics. These results provide new avenues for ultrafast manipulation of magnetization in vdW heterostructures with type-II band alignments. Lastly, we show that optically-generated spin currents from WS$_2$ into CGT can also trigger precessional dynamics via angular momentum transfer.

cond-mat.mes-hall↗

arXiv2026

Cut-and-Project Density Functional Theory for Quasicrystals

Cut-and-project from a symmetric structure in a higher-dimensional space is a standard method for describing the structure of a large class of quasicrystals. By means of a novel localization procedure, we now show how local physical interactions within these quasicrystals are also accurately described by cut-and-project, from corresponding physical interactions in the higher-dimensional space. A density functional theory (DFT++) formulation allows the cut-and-project method to handle the Schroedinger equation for interactions in quasicrystals. The theory is both rigorous and computationally tractable. The resulting ab initio approach specifies quasicrystalline quantum states, in contrast to previous approaches which only worked with crystalline approximants of the quasi-periodic structures.

cond-mat.mtrl-sci↗

arXiv2025

Quantum Coherence of Rare-Earth Ions in Heterogeneous Photonic Interfaces

Harnessing rare-earth ions in oxides for quantum networks requires integration with bright emitters in III-V semiconductors, but local disorder and interfacial noise limit their optical coherence. Here, we investigate the microscopic origins of the ensemble spectrum in Er$^{3+}$:TiO$_2$ epitaxial thin films on GaAs and GaSb substrates. Ab initio calculations combined with noise-Hamiltonian modeling and Monte Carlo simulations quantify the effects of interfacial and bulk spin noise and local strain on erbium crystal-field energies and inhomogeneous linewidths. Photoluminescence excitation spectroscopy reveals that Er$^{3+}$ ions positioned at increasing distances from the III-V/oxide interface produce a systematic blue shift of the $Y_1\rightarrow Z_1$ transition, consistent with strain relaxation predicted by theory. Thermal annealing produces a compensating redshift and linewidth narrowing, isolating the roles of oxygen-vacancy and gallium-diffusion noise. These results provide microscopic insight into disorder-driven decoherence, offering pathways for precise control of hybrid quantum systems for scalable quantum technologies.

quant-ph↗

arXiv2025

Intrinsic Berry phase contribution to Hall conductivity in CoS$_2$

In Weyl semi-metals, the conduction and valence bands intersect at distinct points on the Brillouin zone (Weyl points), which act as monopoles of Berry curvature in momentum space. This nontrivial band topology, identified from electronic structure calculations, gives rise to various exotic magneto-transport properties. Hybrid functional calculations that incorporate a portion of exact exchange, magneto-transport measurements, and temperature-dependent resistivity confirm nontrivial band topology and half-metallicity in CoS$_2$ of magnetic ordering temperature $T_{\rm C} \approx 120~\mathrm{K}$. However, electronic structure calculations also show that application of small strain transforms this half metallic character to the metallic. Interestingly, the magnetoresistance (MR) of the CoS$_2$ films is characterized by a reentrant weak localization above a critical field at $T \leq 60\,\mathrm{K}$ and a negative to positive transition in MR as the $T$ goes from $ T_{\rm C}$. Experimental observation of anomalous Hall resistivity and $ab~initio$ computed band structure, Berry curvature, and Hall conductivity ($\sigma_{xy}$) demonstrate that the $\sigma_{xy}$ in CoS$_2$ is primarily driven by the intrinsic Karplus-Luttinger contribution, often linked to Berry phase physics.

cond-mat.mtrl-sci↗

arXiv2025

Functionalized Cr$_2$C MXenes: Novel Magnetic Semiconductors

We report an \textit{ab initio} investigation of functionalized and 3$d$-electrons doped Cr$_2$C MXenes. Upon functionalization, the Cr$_2$C becomes chemically, dynamically, and mechanically stable, and it exhibits magnetic semiconducting behavior. Cr$_2$CF$_2$ stands out as a wide band gap semiconductor, possessing super exchange interaction mediated by F atoms within the layer, however, the applied strain transforms it from an indirect to a direct band gap semiconductor. Strong spin-phonon coupling found in Cr$_2$CH$_2$ is supported by the distorted Cr spin density due to hydrogen environment. Two magnon branches, associated with two sub-lattice spins, are found in the ferromagnetic Cr$_2$CO$_2$ and antiferromagnetic Cr$_2$CF$_2$. Depending on the types of 3$d$-electron dopants and functionalization, Cr$_2$C MXenes (except for Cr$_2$CO$_2$) change from the indirect band gap magnetic semiconductor to different states of electronic and magnetic matter including exotic direct band gap magnetic bipolar semiconductor. In addition, we reveal a band inversion between the two highest valence bands in the Fe-doped Cr$_2$CCl$_2$.

cond-mat.mtrl-sci↗

arXiv2025

Singly occupied 4$f$ antiferromagnetic insulators: CePO$_4$ and CeVO$_4$

Rare-earth containing wide band gap oxides, which provide spin-photon interface and narrow linewidth optical emission, are getting significant attention as the most promising candidate materials in advancing quantum transduction and memories. Here, from $ab~initio$ calculations, we identify antiferromagnetic ground states in structurally preferred monoclinic CePO$_4$ and tetragonal CeVO$_4$ exhibiting localized occupied and unoccupied Ce $4f$ states with $4f-4f$ transition characteristics. Interestingly, in CePO$_4$, O $2p$ and P $3p$ states hybridize negligibly with Ce $4f$ states, while in CeVO$_4$, V $3d$ and O $2p$ states hybridize and appear as extended states in between the occupied and unoccupied Ce $4f$ states. Here, phonon calculations and analysis identify and differentiate Raman active phonon modes along with the spin phonon coupling of Ce in both CePO$_4$ and CeVO$_4$ that ultimately lead to different $4f$ ground state crystal field multiplets, which are critical to accurately describe electronic transitions for foundational quantum transduction and memories. Further, the identified $C_1$ site symmetry of Ce, lacking inversion symmetry in CePO$_4$, is relevant for quantum memories and $D_{2d}$ site symmetry of Ce exhibiting inversion symmetry in CeVO$_4$ is relevant for quantum transduction.

cond-mat.str-el↗

arXiv2025

Stability, electronic quantum states, and magnetic interactions of Er$^{3+}$ ions in Ga$_2$O$_3$

The chemical, structural, mechanical, and dynamical stabilities of the $\alpha$- and $\beta$-Ga$_2$O$_3$ are confirmed from respective negative formation energies, negative cohesive energies, favorable elastic constants, and positive phonon frequencies. The phonon dispersions indicate that the Ga-O bonds are uniform in the $\alpha$-phase, while they vary in the $\beta$-phase due to the anisotropic polyhedral movement. The defect formation energy analysis confirms that both Er-doped phases prefer Er$^{3+}$ state. The underestimated band gaps of the pristine phases from $ab~initio$ calculations are corrected by employing the hybrid functional calculations. The site preference energy analysis indicates partial occupation of Er in the octahedral site of Ga. Anisotropic nature of hyperfine tensor coefficients of Er are similar in both phases. Calculated magnetic exchange interaction between two Er dopants is negative for $\alpha$ and positive for $\beta$, indicating antiferromagnetic ground state in the former and the ferromagnetic ground state in the latter. A large values of Dzyaloshinskii-Moriya interactions (DMIs) are obtained along the $x$ direction in the $\alpha$ and along the $y$ direction in the $\beta$. The analysis of dielectric constants and refractive indices of both pristine and Er doped phases shows a good agreement with available experimental values. The calculated optical anisotropy is slightly higher in $\beta$ than those in $\alpha$, which is due to the involvement of lower symmetry in $\beta$. The crystal field coefficients (CFCs) calculated from DFT are used to analyze 4$f$ multiplets and 4$f$ - 4$f$ transitions. Thus calculated lowest energy level of the first excited state to the lowest energy level of the ground state is about 1.53~$\mu$m, which is in a good agreement with available experiment and it falls within the quantum telecommunication wavelength range.

cond-mat.mtrl-sci↗

arXiv2025

Ab initio calculations of erbium crystal field splittings in oxide hosts

We present an effective ab initio method to calculate the crystal field coefficients of an erbium (Er3+) ion experiencing different local site symmetries in several wide-band-gap oxides, and then evaluate crystal field splittings of these Er3+ ions for their ground and excited states. The optical transitions between the ground state (Z) and excited state (Y) manifolds of the environmentally shielded 4f states of these Er3+ ions have wavelengths ~1.5 microns and thus have potential applications to quantum communications and quantum memories. These results are in excellent agreement with recent low-temperature measurements, provided the inadequate calculation of the 4f shell screening is adjusted by reducing the radial extent of the 4f wavefunctions by approximately a factor of 2.

cond-mat.mtrl-sci↗

arXiv2025

Ab initio calculations of erbium crystal field splittings in oxide hosts: role of the 4f radial wave function

We expand here our description of a newly developed simple and effective ab initio method of calculating crystal field coefficients (CFCs) of rare-earth atoms in insulating hosts, focusing on Er in wide band gap oxide hosts MgO, ZnO, TiO$_2$, CaWO$_4$, and PbWO$_4$, which exemplify different local site symmetries. These hybrid functional calculations, which incorporate a portion of exact exchange from Hartree-Fock theory, reproduce the experimentally identified insulating band gaps of these oxides. The negative values of cohesive and formation energies confirm the structural and chemical stability of these oxides, whereas the defect formation energies indicate that Er doped ZnO, CaWO$_4$, and PbWO$_4$ are easier to form compared to Er doped MgO and TiO$_2$. Er doped in these oxide hosts exhibits a spin magnetic moment of $\sim$ 3 $\mu_B$ confirming the $3+$ valence state. The CFCs of these hosted Er are determined from charge densities and potentials obtained from non-spin-polarized calculations, involving a 4\textit{f} core approximation. These CFCs are subsequently used to solve an effective Hamiltonian and generate the crystal field splitting of Er 4f. These calculated Er 4f energy levels are in good agreement with available experiments.

cond-mat.mtrl-sci↗

arXiv2024

Enhancing Stability, Magnetic Anisotropy, and Coercivity of $τ$-L$1_0$ MnAl: Machine Learning, $\textit{Ab Initio}$, and Micromagnetic Modeling

The binary manganese aluminium (MnAl) alloy with L$1_0$ crystal structure is a promising rare earth element-free permanent magnetic material because of its exceptional magnetic properties. However, experimentally synthesizing it in a stable bulk form is extremely challenging. Here, an alternative method of stabilizing the material is proposed and theoretically verified by partially substituting Mn and Al sites with Fe and Ni and identifying its enhanced phase stability, magnetic anisotropy, and coercivity from density functional theory (DFT), machine learning (ML) crystal graph convolution neural network (CGCNN), and micro-magnetic modeling. When considering a fixed ($50\%$)-Ni, the magnetic anisotropy increases with the increasing Fe content but decreases the formation energy. The calculated formation energies, elastic constants, and phonon frequencies demonstrate that all the binary and quaternary compositions are stable. Most importantly, the magnetic moment and magnetic anisotropy constants in $50\%$-Fe substituted composition (equiatomic phase) increase significantly compared to the MnAl. The predicted coercivity of the equiatomic phase is larger than the parent compound calculated by combining DFT computed parameters with micromagnetic simulations.

cond-mat.mtrl-sci↗

arXiv2024

Accurate Machine Learning Predictions of Coercivity in High-Performance Permanent Magnets

Increased demand for high-performance permanent magnets in the electric vehicle and wind turbine industries has prompted the search for cost-effective alternatives.Discovering new magnetic materials with the desired intrinsic and extrinsic permanent magnet properties presents a significant challenge to researchers because of issues with the global supply of rare-earth elements, material stability, and a low maximum magnetic energy product BH$_{max}$.While first-principle density functional theory (DFT) predicts materials' magnetic moments, magneto-crystalline anisotropy constants, and exchange interactions, it cannot compute coercivity ($H_c$).Although it is possible to calculate $H_c$ theoretically with micromagnetic simulations, the predicted value is larger than the experiment by almost an order of magnitude, due to the Brown paradox.To circumvent these, we employ machine learning (ML) methods on an extensive database obtained from experiments, DFT calculations, and micromagnetic modeling.The use of a large dataset enables realistic $H_c$ predictions for materials such as Ce-doped Nd$_2$Fe$_{14}$B, comparing favorably against micromagnetically simulated coercivities.Remarkably, our ML model accurately identifies uniaxial magneto-crystalline anisotropy as the primary contributor to $H_c$. With DFT calculations, we predict the Nd-site dependent magnetic anisotropy behavior in Nd$_2$Fe$_{14}$B, confirming that Nd $4g$-sites mainly contribute to uniaxial magneto-crystalline anisotropy, and also calculate Curie temperature (T$_{C}$).Both calculated results are in good agreement with experiment.The coupled experimental dataset and ML modeling with DFT input predict $H_c$ with far greater accuracy and speed than was previously possible using micromagnetic modeling.Further, we reverse-engineer the inter-grain exchange coupling with micromagnetic simulations by employing the ML predictions.

cond-mat.mtrl-sci↗

arXiv2024

Predicting quantum materials properties using novel faithful machine learning embeddings

Machine Learning (ML) is accelerating the progress of materials prediction and classification, with particular success in CGNN designs. While classical ML methods remain accessible, advanced deep networks are still challenging to build and train. We introduce two new adaptations and refine two existing ML networks for generic crystalline quantum materials properties prediction and optimization. These new models achieve state-of-the-art performance in predicting TQC classification and strong performance in predicting band gaps, magnetic classifications, formation energies, and symmetry group. All networks easily generalize to all quantum crystalline materials property predictions. To support this, full implementations and automated methods for data handling and materials predictions are provided, facilitating the use of deep ML methods in quantum materials science. Finally, dataset error rates are analyzed using an ensemble model to identify and highlight highly atypical materials for further investigations.

cond-mat.other↗

arXiv2024

Bilayer Ion Trap Design for 2D Arrays

Junctions are fundamental elements that support qubit locomotion in two-dimensional ion trap arrays and enhance connectivity in emerging trapped-ion quantum computers. In surface ion traps they have typically been implemented by shaping radio frequency (RF) electrodes in a single plane to minimize the disturbance to the pseudopotential. However, this method introduces issues related to RF lead routing that can increase power dissipation and the likelihood of voltage breakdown. Here, we propose and simulate a novel two-layer junction design incorporating two perpendicularly rotoreflected (rotated, then reflected) linear ion traps. The traps are vertically separated, and create a trapping potential between their respective planes. The orthogonal orientation of the RF electrodes of each trap relative to the other provides perpendicular axes of confinement that can be used to realize transport in two dimensions. While this design introduces manufacturing and operating challenges, as now two separate structures have to be precisely positioned relative to each other in the vertical direction and optical access from the top is obscured, it obviates the need to route RF leads below the top surface of the trap and eliminates the pseudopotential bumps that occur in typical junctions. In this paper the stability of idealized ion transfer in the new configuration is demonstrated, both by solving the Mathieu equation analytically to identify the stable regions and by numerically modeling ion dynamics. Our novel junction layout has the potential to enhance the flexibility of microfabricated ion trap control to enable large-scale trapped-ion quantum computing.

quant-ph↗

arXiv2024

Implications of the electron-phonon coupling in CuPb$_9$(PO$_4$)$_6$O for superconductivity: an \textit{ab initio} study

We report $ab~initio$ calculations of the electronic and vibrational properties in CuPb$_9$(PO$_4$)$_6$O, including the electron-phonon coupling strength via strong-coupling Migdal-Eliashberg theory. We verify the presence of appealing flat electronic bands near the Fermi level, a strong hybridization between the Cu $3d$ and O $2p$ states, and soft low-energy phonons, which can suggest high-temperature superconducting behavior. However, the electron-phonon coupling strength appears insufficient to overcome the Coulomb repulsion between an electron pair and thus does does not support high-temperature superconductivity in CuPb$_9$(PO$_4$)$_6$O via the conventional electron-phonon Migdal-Eliashberg mechanism. Even neglecting Coulomb repulsion of the electron pair we find this electron-phonon coupling suggests a superconducting transition temperature less than 2~K.

cond-mat.supr-con↗

arXiv2023

Distinguishing erbium dopants in Y$_2$O$_3$ by site symmetry: \textit{ ab initio} theory of two spin-photon interfaces

We present a first-principles study of defect formation and electronic structure of erbium (Er)-doped yttria (Y$_2$O$_3$). This is an emerging material for spin-photon interfaces in quantum information science due to the narrow linewidth optical emission from Er dopants at standard telecommunication wavelengths and their potential for quantum memories and transducers. We calculate formation energies of neutral, negatively, and positively charged Er dopants and find the charge neutral configuration to be the most stable, consistent with experiment. Of the two substitutional sites of Er for Y, the $C_2$ (more relevant for quantum memories) and $C_{3i}$ (more relevant for quantum transduction), we identify the former as possessing the lowest formation energy. The electronic properties are calculated using the Perdew-Burke-Ernzerhof (PBE) functional along with the Hubbard $U$ parameter and spin-orbit coupling (SOC), which yields a $\sim$ 6 $μ_B$ orbital and a $\sim$ 3 $μ_B$ spin magnetic moment, and 11 electrons in the Er $4f$ shell, confirming the formation of charge-neutral Er$^{3+}$. This standard density functional theory (DFT) approach underestimates the band gap of the host and lacks a first-principles justification for $U$. To overcome these issues, we performed screened hybrid functional (HSE) calculations, including a negative $U$ for the $4f$ orbitals, with mixing ($α$) and screening ($w$) parameters. These produced robust electronic features with slight modifications in the band gap and the $4f$ splittings depending on the choice of tuning parameters. We also computed the many-particle electronic excitation energies and compared them with experimental values from photoluminescence.

cond-mat.mtrl-sci↗

arXiv2023

Giant magnetic and optical anisotropy in cerium-substituted M-type strontium hexaferrite driven by 4$f$ electrons

By performing density functional calculations, we find a giant magnetocrystalline anisotropy (MCA) constant in abundant element cerium (Ce) substituted M-type hexaferrite, in the energetically favorable strontium site, assisted by a quantum confined electron transfer from Ce to specific iron (2a) site. Remarkably, the calculated electronic structure shows that the electron transfer leads to the formation of Ce$^{3+}$ and Fe$^{2+}$ at the $2a$ site producing an occupied Ce($4f^1$) state below the Fermi level that adds a significant contribution to MCA and magnetic moment. A half Ce-substitution forms a metallic state, while a full substitution retains the semiconducting state of the strontium-hexaferrite (host). In the latter, the band gap is reduced due to the formation of charge transferred states in the gap region of the host. The optical absorption coefficient shows an enhanced anisotropy between light polarization in parallel and perpendicular directions. Calculated formation energies, including the analysis of probable competing phases, and elastic constants confirm that both compositions are chemically and mechanically stable. With successful synthesis, the Ce-hexaferrite can be a new high-performing critical-element-free permanent magnet material adapted for use in devices such as automotive traction drive motors.

cond-mat.mtrl-sci↗

arXiv2022

Unravelling the stability, electronic and physical properties in bulk and (001)-surfacesof newlysynthesized Ti2ZnX (X=C, N)MAX phases

MAX phase family has been extended by the addition of late transition metals at the A-site with the expectation of diverse functional properties, such as magnetism and catalysis. Here, we present our systematic density functional investigation on the phase stability and physical properties of newly synthesized Ti2ZnX (X = C, N) phasesin comparison with conventional Ti2AlX (X = C, N).Due to smaller size of N as compared to C, the unit cell dimensionis reduced when C atoms are replaced by N atoms atthe X-site. The thermodynamic, mechanical and dynamical stabilities are validatedby estimating the formation energies, elastic constants and phonon dispersions, respectively. The elastic properties of Ti2ZnN are nearly isotropic while those of Ti2ZnC are completely anisotropic. To understand the thin-film characteristicsin Ti2ZnX, the surface properties with (001)-terminated slabs are investigated. Both Ti2ZnX bulk and (001)-surfaces exhibit metal-like electronic structure. There is a strong covalent bonding between Ti-X and Ti-Zn atoms.Additional states are generated at the Fermi level (EF) due to the unusual d-pstates hybridization between Ti and Zn atoms.The anisotropy in chemical bonding is confirmed by the cleavage energy difference between Ti-X and Ti-Zn atoms. Here,Ti(X)-001 and Zn-001 terminations are stable surfaces, however, in terms of chemical potentials, Zn-001 termination is the most favorable.

cond-mat.mtrl-sci↗