arXiv subjects

Hari Paudyal

Publications and source records attributed to Hari Paudyal.

At least 19 recordsLinked to original sources

arXiv2026

Coherent antiferromagnetic resonance in MnO driven by impulsive terahertz excitation

Antiferromagnets (AFMs) offer a promising platform for ultrafast information processing owing to their intrinsically fast spin dynamics and vanishing net magnetization. Realizing this potential, however, requires understanding how terahertz fields excite coherent magnons and how the resulting spin motion is transduced into an optical signal. Here we report impulsive terahertz (THz) excitation and time-domain detection of the antiferromagnetic resonance (AFMR) in single-crystal manganese(II) oxide. Time-resolved birefringence measurements reveal long-lived coherent spin oscillations in the AFM phase. The resonance softens and becomes strongly damped upon warming toward the Néel temperature. Despite this similar excitation behavior, the detected birefringence in MnO is markedly weaker than in NiO. We associate this suppressed optical visibility with the weak spin--orbit-mediated magneto-optical coupling of orbital-singlet, high-spin Mn$^{2+}$. These results demonstrate that coherent magnon excitation and its optical detection are governed by distinct microscopic interactions.

cond-mat.mtrl-sci↗

arXiv2026

Spin-Lattice Dynamics and Interactions in Magnonic Spinels

Minimizing magnetic damping while understanding spin-lattice interactions remains a key challenge for magnonics. We show site-specific Al/Li ordering in spinel ferrites drives a ferrimagnetic insulating state, quenching the Fermi-level density of states. \textit{Ab initio} calculations reveal collective acoustic modes alongside sub-lattice-selective optical modes on tetrahedral and octahedral networks, providing a microscopic understanding of substitution-driven spin dynamics. Crucially, the low-frequency magnon and acoustic phonon modes intersect, driving strong hybridization for low-loss technologies.

cond-mat.mtrl-sci↗

arXiv2026

Divergence between long- and short-wavelength magnon damping in spinel ferrites

The realization of practical, high-speed magnonic devices requires engineering magnetic materials with low dissipation over wide frequency ranges. While optical and microwave probes are used to infer the damping of low energy/long wavelength modes, the degree to which these $q\sim0$ properties translate into higher-energy, finite-momentum modes remains an important open question. Here, we utilize a combination of ferromagnetic resonance (FMR) and resonant inelastic x-ray scattering on spinel ferrites Li$_{0.5}$Al$_x$Fe$_{2.5-x}$O$_4$ to probe magnons in both the short- and long-wavelength limits. We observe that aluminum substitution both markedly reduces the magnon bandwidth and drastically shortens the high-$q$ magnon lifetimes, in sharp contrast to the ultralow magnon damping inferred from FMR. These findings demonstrate a disparity between how non-magnetic substituents impact magnon damping in the long- and short-wavelength limits, providing a new perspective for assessing candidate materials for magnonic devices.

cond-mat.mtrl-sci↗

arXiv2026

Origin of High-Temperature Antiferromagnetic Order in a van der Waals Material

While Van der Waals (vdW) itinerant antiferromagnets with high Neel temperatures (TN) are highly desirable for spintronics, they remain relatively scarce. Here, we unravel the physical origin of the unusually high TN (= 250 K) in the newly identified vdW compound (Fe0.65Co0.35)4GeTe2. The substitution of Co in Fe4GeTe2 induces layer-selective Fe-Co ordering and stabilizes a robust antiferromagnetic (AFM) state primarily driven by Co moments. The AFM order is further strengthened by enhanced electronic correlations of quasi-localized Co 3d-states at the Fermi level, giving rise to an itinerant-localized duality of the 3d electrons. This interplay generates strong magnetic correlations well above TN and stabilizes low-temperature spin canting with a possible nontrivial Berry curvature. Our results establish (Fe0.65Co0.35)4GeTe2 as a rare material bridging fundamental magnetic interactions with potential applications in AFM spintronics.

cond-mat.mtrl-sci↗

arXiv2026

Broken site symmetry of Fe adatoms on Bi$_2$Te$_3$

We report a combined scanning tunneling microscopy and atomistic theoretical study of Fe adatoms on the Bi$_2$Te$_3$(111) surface. Topographic imaging at $4.5$~K shows that Fe adatoms in fcc and hcp hollow sites exhibit a threefold-symmetric contrast, consistent with the $C_{3v}$ symmetry of the adsorption site. However, simultaneously acquired differential conductance ($dI/dV$) maps reveal a pronounced reduction in symmetry, evidenced by differential contrast observed at nearest-neighbor Te sites. Density functional theory calculations show that the Fe/Bi$_2$Te$_3$ system undergoes a static Jahn--Teller distortion, reducing the adsorption symmetry from $C_{3v}$ to $C_{1v}$, with the distorted configuration favored by $72.5$~meV. Orbital-projected density of states calculations show that the occupied states near the Fermi level are dominated by $d_{xz}$ and $d_{yz}$ orbitals, whereas the unoccupied states are primarily of $d_{z^2}$, $d_{x^2-y^2}$, and $d_{xy}$ character. The local density of states from these orbitals is in good qualitative agreement with experimental $dI/dV$ spectra. Furthermore, simulated local-density-of-states maps using a tight-binding Green's function approach are consistent with experimental $dI/dV$ maps, confirming that the reduced symmetry originates from the $C_{1v}$ structural distortion.

cond-mat.mes-hall↗

arXiv2026

Kinetics of electron-phonon scattering in silicon resolved by Rydberg transitions of donors

Rydberg states of atoms in vacuum are now well recognized as a resource for quantum technologies. Donors in semiconductors also display analogous states, which have been proposed for similar applications. While they benefit from permanent locations in their host crystals, electron-lattice coupling leads to much shorter excited-state lifetimes than for neutral atoms in vacuum. Here we provide a quantitative description of donor-phonon kinetics, creating a basis for engineering donor systems in realistic material stacks for quantum devices. Our theory incorporates both form factors for the Rydberg states, which given their large extents in real space provide strong selectivity in momentum space, and tabulated deformation potentials for all six phonon branches throughout the Brillouin zone. By confronting this framework with carefully controlled time-resolved free electron laser measurements, we show that the widely quoted position of the silicon conduction band minimum, $k_0$, is inconsistent with observed donor relaxation rates and that quantitative agreement is obtained for a value further from the X-point than commonly assumed. This stringent experiment-theory comparison establishes donor relaxation as a precision metrology for conduction band parameters and scattering processes in silicon, with consequences spanning from quantum devices to classical electronics.

quant-ph↗

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↗

arXiv2025

Suppression of charge-density wave and superconductivity in a lithiated NbSe$_2$ monolayer

We present an \textit{ab initio} investigation of the long-range charge density wave (CDW) order and superconducting properties of the pristine and lithiated NbSe$_2$ monolayer. Stable CDW structures are obtained through atomic reconstruction driven by soft-mode distortions and lithiation, respectively, lead to significant electronic modifications that suppress the CDW order. This suppression is attributed to anisotropic atomic distortions, along with a reduction in the electronic density of states at the Fermi level. As a result, the electron--phonon coupling strength is suppressed, particularly in the lithiated structure, due to reduced contributions from low-frequency phonons, primarily associated with in-plane Nb vibrations. Finally, we observe a sizable anisotropy in the superconducting gap on the Fermi surface, with a superconducting transition temperature of approximately 8~K in the distorted, and 4~K in the lithiated, CDW NbSe$_2$ monolayer.

cond-mat.supr-con↗

arXiv2025

Microscopic Scattering Approach to In-Gap States: Cr Adatoms on Superconducting β-Bi2Pd

We develop a microscopic scattering formalism to describe Yu-Shiba-Rusinov (YSR) states due to a single Cr adatom on the Bi-terminated surface of $β-Bi_2Pd$, by combining ab initio Wannier functions with a real-space Green's function approach in the Bogoliubov-de Gennes formalism. Our framework reproduces key scanning tunneling spectroscopy features, including a single particle-hole asymmetric YSR peak and isotropic dIdV maps around the impurity. Decomposing the YSR states reveals contributions from four nearly degenerate C4v representations, with energy broadening masking their individual signatures. Spin-orbit coupling induces partial spin polarization, while the spatial asymmetry between particle and hole components arises from Cr d-Bi p hybridization. These results highlight the importance of realistic band structures and microscopic modeling for interpreting STM data and provide a foundation for studying impurity chains hosting topological excitations.

cond-mat.supr-con↗

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 ($σ_{xy}$) demonstrate that the $σ_{xy}$ 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 $α$- and $β$-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 $α$-phase, while they vary in the $β$-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 $α$ and positive for $β$, 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 $α$ and along the $y$ direction in the $β$. 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 $β$ than those in $α$, which is due to the involvement of lower symmetry in $β$. 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~$μ$m, which is in a good agreement with available experiment and it falls within the quantum telecommunication wavelength range.

cond-mat.mtrl-sci↗

arXiv2024

Full-bandwidth anisotropic Migdal-Eliashberg theory and its application to superhydrides

Migdal-Eliashberg theory is one of the state-of-the-art methods for describing conventional superconductors from first principles. However, widely used implementations assume a constant density of states around the Fermi level, which hinders a proper description of materials with distinct features in its vicinity. Here, we present an implementation of the Migdal-Eliashberg theory within the EPW code that considers the full electronic structure and accommodates scattering processes beyond the Fermi surface. To significantly reduce computational costs, we introduce a non-uniform sampling scheme along the imaginary axis. We demonstrate the power of our implementation by applying it to the sodalite-like clathrates YH$_6$ and CaH$_6$, and to the covalently-bonded H$_3$S and D$_3$S. Furthermore, we investigate the effect of maximizing the density of states at the Fermi level in doped H$_3$S and BaSiH$_8$ within the full-bandwidth treatment compared to the constant-density-of-states approximation. Our findings highlight the importance of this advanced treatment in such complex materials.

cond-mat.supr-con↗

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

Electron-phonon physics from first principles using the EPW code

EPW is an open-source software for $\textit{ab initio}$ calculations of electron-phonon interactions and related materials properties. The code combines density functional perturbation theory and maximally-localized Wannier functions to efficiently compute electron-phonon coupling matrix elements on ultra-fine Brillouin zone grids. This data is employed for predictive calculations of temperature-dependent properties and phonon-assisted quantum processes in bulk solids and low-dimensional materials. Here, we report on significant new developments in the code that occurred during the period 2016-2022, namely: a transport module for the calculation of charge carrier mobility and conductivity under electric and magnetic fields within the $\textit{ab initio}$ Boltzmann transport equation; a superconductivity module for the calculation of critical temperature and gap structure in phonon-mediated superconductors within the $\textit{ab initio}$ anisotropic multi-band Eliashberg theory; an optics module for calculations of phonon-assisted indirect transitions; a module for the calculation of small and large polarons without supercells using the $\textit{ab initio}$ polaron equations; and a module for calculating electron-phonon couplings, band structure renormalization, and temperature-dependent optical spectra using the special displacement method. For each capability, we outline the methodology and implementation, and provide example calculations. We describe recent code refactoring to prepare EPW for exascale architectures, we discuss efficient parallelization strategies, and report on extreme parallel scaling tests.

cond-mat.mtrl-sci↗

arXiv2021

Superconducting properties of MoTe$_2$ from the $ab~initio$ anisotropic Migdal-Eliashberg theory

Molybdenum ditelluride (MoTe$_2$) is attracting considerable interest since it is the archetypal type-II Weyl semimetal and a candidate for topological superconductivity. We investigate the superconducting phase diagram of two MoTe$_2$ polymorphs using the $ab~initio$ anisotropic Migdal-Eliashberg theory, and we show that the superconducting dome originates from the synergistic contribution of the density of states at the Fermi level and the transverse acoustic Te modes in the 1T$^\prime$ phase. We find that the electron and hole pockets carry trivial $s$-wave order parameters of slightly different magnitude, reminiscent of a two-gap structure as suggested by recent experiments. We suggest that a possible route for enhancing the superconducting critical temperature, and realizing $s_{+-}$ pairing, in the T$_d$ phase is to exploit its non-trivial band topology via electron doping.

cond-mat.supr-con↗

arXiv2020

Electronic, vibrational, and electron-phonon coupling properties in SnSe$_2$ and SnS$_2$ under pressure

The tin-selenide and tin-sulfide classes of materials undergo multiple structural transitions under high pressure leading to periodic lattice distortions, superconductivity, and topologically non-trivial phases, yet a number of controversies exist regarding the structural transformations in these systems. We perform first-principles calculations within the framework of density functional theory and a careful comparison of our results with available experiments on SnSe$_2$ reveals that the apparent contradictions among high-pressure results can be attributed to differences in experimental conditions. We further demonstrate that under hydrostatic pressure a $\sqrt{3} \times \sqrt{3} \times 1$ superstructure can be stabilized above 20 GPa in SnS$_2$ via a periodic lattice distortion as found recently in the case of SnSe$_2$, and that this pressure-induced phase transition is due to the combined effect of Fermi surface nesting and electron-phonon coupling at a momentum wave vector $\mathbf{q}$ = $(1/3, 1/3, 0)$. In addition, we investigate the contribution of nonadiabatic corrections on the calculated phonon frequencies, and show that the quantitative agreement between theory and experiment for the high-energy $A_{1g}$ phonon mode is improved when these effects are taken into account. Finally, we examine the nature of the superconducting state recently observed in SnSe$_2$ under nonhydrostatic pressure and predict the emergence of superconductivity with a comparable critical temperature in SnS$_2$ under similar experimental conditions. Interestingly, in the periodic lattice distorted phases, the critical temperature is found to be reduced by an order of magnitude due to the restructuring of the Fermi surface.

cond-mat.mtrl-sci↗