arXiv subjects

Bishal Thapa

Publications and source records attributed to Bishal Thapa.

8 recordsLinked to original sources

arXiv2026

Raman spectroscopy of the van der Waals altermagnet Co$_{1/4}$NbSe$_2$

We investigate the influence of Co intercalation and altermagnetic order on the lattice dynamics of the layered compound Co$_{1/4}$NbSe$_2$. Polarization-resolved Raman spectroscopy, supported by density-functional theory, enables identification of six Raman-active phonons. Co intercalation drives a substantial reconstruction of the vibrational spectrum through zone folding of NbSe$_2$ phonons, producing hybridized modes with mixed zone-center and zone-boundary character. Despite this, Co atoms do not participate in any Raman-active modes by symmetry, which is in marked contrast to related 1/3 compounds where intercalant modes do contribute to the Raman spectrum. Temperature-dependent Raman measurements across the altermagnetic transition show no discontinuities, which is consistent with short-range spin correlations in the quasi-one-dimensional Co chains. However, we find evidence for spin-phonon coupling in A$_{1g}$ symmetry modes owing to their out-of-plane Se displacements. Our work demonstrates the substantial impact of intercalation on the vibrational properties of transition metal dichalcogenides and the presence of spin-phonon interactions in a newly discovered altermagnetic material.

cond-mat.mtrl-sci↗

arXiv2026

Assessing the suitability of the Thomas-Fermi-von Weizsäcker density functional for itinerant magnetism

We assess the ability of the Thomas--Fermi--von Weizsacker (TFW) functional within orbital-free density functional theory (DFT) to describe itinerant magnetism. Magnetic stability is evaluated through the susceptibility obtained from the second derivative of the total energy with respect to the net magnetization. Calculations are performed for the paramagnetic metals Al and Pd and the canonical ferromagnets Fe, Co, and Ni, with the results benchmarked against Kohn--Sham DFT. The orbital-free results show poor agreement with the Kohn--Sham predictions, failing to capture even the qualitative trends. Using the orbital-free ground-state density with the Kohn--Sham functional in a non-self-consistent calculation yields reasonable qualitative agreement, although the quantitative agreement remains limited. These results highlight fundamental limitations of the TFW functional for describing itinerant magnetism.

cond-mat.mtrl-sci↗

arXiv2026

Is altermagnetism in vanadium oxychalcogenides a lost cause?

Vanadium-based oxychalcogenide compounds with the inverse Lieb-lattice (ILL) structural pattern have recently been proposed as candidate altermagnets (AM). However, early studies postulated ferromagnetic interlayer coupling, a critical requirement for preserving the bulk AM state. Here we present a systematic survey of the complete AV2Q2O family (A = K, Rb, Cs; Q = S, Se, Te) in terms of their magnetic ordering and interlayer coupling. While intralayer exchange interaction favors AM ordering in a single ILL layer across the entire family, the relatively weak interlayer coupling in most cases favors Kramers-degenerate antiferromagnetic order with a doubled magnetic unit cell. This means that most stoichiometric bulk materials, including the previously proposed candidate KV2Se2O, are not altermagnetic, with CsV2Te2O being the only exception. Using hole doping to simulate alkali vacancies, we show that realistic deviations from stoichiometry do not change the magnetic ground state in these compounds.

cond-mat.str-el↗

arXiv2026

Mystery of the 175 cm$^{-1}$ Raman Mode in MnTe Altermagnet

MnTe has recently attracted exceptional attention due to its well-established altermagnetism, prompting a thorough reexamination of its properties. In particular, it was found that a Raman-active excitation at ~175 cm$^{-1}$, routinely assigned to the E2g phonon, is incompatible with this interpretation. It was further hypothesized that this mode is a "leakage", due to symmetry lowering, of an otherwise forbidden phonon. Here, using first-principles calculations, we decisively rule out this hypothesis and propose an alternative interpretation that the "mystery mode" is an electronic excitation, i.e., a plasmon, enabled by hole self-doping. The resolution of this mystery will require additional experiments and shed new light on the nature of electronic transport in MnTe.

cond-mat.mtrl-sci↗

arXiv2024

Altermagnetism in the layered intercalated transition metal dichalcogenide CoNb$_4$Se$_8$

Altermagnets (AMs) are a new class of magnetic materials that combine the beneficial spintronics properties of ferromagnets and antiferromagnets, garnering significant attention recently. Here, we have identified altermagnetism in a layered intercalated transition metal diselenide, CoNb$_4$Se$_8$, which crystallizes with an ordered sublattice of intercalated Co atoms between NbSe$_2$ layers. Single crystals are synthesized, and the structural characterizations are performed using single crystal diffraction and scanning tunneling microscopy. Magnetic measurements reveal easy-axis antiferromagnetism below 168 K. Density functional theory (DFT) calculations indicate that A-type antiferromagnetic ordering with easy-axis spin direction is the ground state, which is verified through single crystal neutron diffraction experiments. Electronic band structure calculations in this magnetic state display spin-split bands, confirming altermagnetism in this compound. The layered structure of CoNb$_4$Se$_8$ presents a promising platform for testing various predicted properties associated with altermagnetism.

cond-mat.str-el↗

arXiv2023

Assessing the source of error in the Thomas-Fermi-von Weizsäcker density functional

We investigate the source of error in the Thomas-Fermi-von Weizsäcker (TFW) density functional relative to Kohn-Sham density functional theory (DFT). In particular, through numerical studies on a range of materials, for a variety of crystal structures subject to strain and atomic displacements, we find that while the ground state electron density in TFW orbital-free DFT is close to the Kohn-Sham density, the corresponding energy deviates significantly from the Kohn-Sham value. We show that these differences are a consequence of the poor representation of the linear response within the TFW approximation for the electronic kinetic energy, confirming conjectures in the literature. In so doing, we find that the energy computed from a non-self-consistent Kohn-Sham calculation using the TFW electronic ground state density is in very good agreement with that obtained from the fully self-consistent Kohn-Sham solution.

cond-mat.mtrl-sci↗

arXiv2020

GymFG: A Framework with a Gym Interface for FlightGear

Over the past decades, progress in deployable autonomous flight systems has slowly stagnated. This is reflected in today's production air-crafts, where pilots only enable simple physics-based systems such as autopilot for takeoff, landing, navigation, and terrain/traffic avoidance. Evidently, autonomy has not gained the trust of the community where higher problem complexity and cognitive workload are required. To address trust, we must revisit the process for developing autonomous capabilities: modeling and simulation. Given the prohibitive costs for live tests, we need to prototype and evaluate autonomous aerial agents in a high fidelity flight simulator with autonomous learning capabilities applicable to flight systems: such a open-source development platform is not available. As a result, we have developed GymFG: GymFG couples and extends a high fidelity, open-source flight simulator and a robust agent learning framework to facilitate learning of more complex tasks. Furthermore, we have demonstrated the use of GymFG to train an autonomous aerial agent using Imitation Learning. With GymFG, we can now deploy innovative ideas to address complex problems and build the trust necessary to move prototypes to the real-world.

cs.AI↗

arXiv2018

Network Signatures from Image Representation of Adjacency Matrices: Deep/Transfer Learning for Subgraph Classification

We propose a novel subgraph image representation for classification of network fragments with the targets being their parent networks. The graph image representation is based on 2D image embeddings of adjacency matrices. We use this image representation in two modes. First, as the input to a machine learning algorithm. Second, as the input to a pure transfer learner. Our conclusions from several datasets are that (a) deep learning using our structured image features performs the best compared to benchmark graph kernel and classical features based methods; and, (b) pure transfer learning works effectively with minimum interference from the user and is robust against small data.

cs.CV↗