Pragalv Karki | University of Oregon (original) (raw)

Papers by Pragalv Karki

arXiv (Cornell University), Nov 10, 2022

Dispersionless flat bands can be classified into two types: (1) non-singular flat bands whose eig... more Dispersionless flat bands can be classified into two types: (1) non-singular flat bands whose eigenmodes are completely characterized by compact localized states; and (2) singular flat bands that have a discontinuity in their Bloch eigenfunctions at a band touching point with an adjacent dispersive band, thereby requiring additional extended states to span their eigenmode space. In this study, we design and numerically demonstrate two-dimensional thin-plate acoustic metamaterials in which tunable flat bands of both kinds can be achieved. Non-singular flat bands are achieved by fine-tuning the ratio of the global tension and the bending stiffness in triangular and honeycomb lattices of plate resonators. A singular flat band arises in a kagome lattice due to the underlying lattice geometry, which can be made degenerate with two additional flat bands by tuning the plate tension. A discrete model of the continuum thin-plate system reveals the interplay of geometric and mechanical factors in determining the existence of flat bands of both types. The singular nature of the kagome lattice flat band is established via a metric called the Hilbert-Schmidt distance calculated between a pair of eigenstates infinitesimally close to the quadratic band touching point. We also simulate an acoustic manifestation of a robust boundary mode arising from the singular flat band and protected by real-space topology in a finite system. Our theoretical and computational study establishes a framework for exploring flat-band physics in a tunable classical system, and for designing acoustic metamaterials with potentially useful sound manipulation capabilities.

In presenting this dissertation in partial fulfillment of the requirements for a graduate degree ... more In presenting this dissertation in partial fulfillment of the requirements for a graduate degree from the University of North Dakota, I agree that the library of this University shall make it freely available for inspection. I further agree that permission for extensive copying for scholarly purposes may be granted by the professor who supervised my dissertation work or, in his absence, by the chairperson of the department or the dean of the Graduate School. It is understood that any copying or publication or other use of this dissertation or part thereof for financial gain shall not be allowed without my written permission. It is also understood that due recognition shall be given to me and to the University of North Dakota in any scholarly use which may be made of any material in my dissertation.

Bulletin of the American Physical Society, Mar 3, 2020

Bulletin of the American Physical Society, Mar 18, 2021

Journal of Physics: Condensed Matter, 2018

In this study, we simulated a quantum rotor model describing a Josephson junction array (JJA) in ... more In this study, we simulated a quantum rotor model describing a Josephson junction array (JJA) in the phase representation at zero temperature in a perpendicular magnetic field [Formula: see text] (in units of [Formula: see text]) on a [Formula: see text] square lattice with spacing a for [Formula: see text]. The superconductor-insulator transition (SIT) is tuned by the ratio of charging energy to Josephson coupling, U/J. Abrupt drops in the magnetization values were observed in the bigger lattices at certain values of B and U/J caused by the formation of vortices. Increasing U/J at a fixed B field causes quantum vortex melting. The magnetization drops to zero around [Formula: see text] indicating SIT. For B = 0.1 the SIT occurs without an intermediate vortex state and the magnetization scales as [Formula: see text], whereas for B = 0.4 the scaling is [Formula: see text] during the vortex melting. For B between 0.1 and 0.4 the scaling is not clear. We used the diffusion Monte Carlo (DMC) method with a guiding wavefunction optimized using the variational Monte Carlo (VMC) method. The ground state energy is calculated easily in DMC and its error estimates were generally smaller than [Formula: see text], both with and without the guiding wavefunction. Quantities like magnetization and vorticity that do not commute with the Hamiltonian were calculated using an efficient forward walking algorithm. Their estimates are affected severely in absence of the guiding wavefunction. With the guiding wavefunction, errors for the magnetization were generally less than [Formula: see text] and going up to [Formula: see text] percent around the phase transition from the Meissner to the vortex state, and without the guiding wavefunction errors were generally higher than [Formula: see text] and going up to [Formula: see text] around the critical point.

Physical Review Applied, 2021

Slowing down, stopping, and reversing a signal is a core functionality for information processing... more Slowing down, stopping, and reversing a signal is a core functionality for information processing. Here, we show that this functionality can be realized by tuning the dispersion of a periodic system through a dispersionless, or flat, band. Specifically, we propose a phononic metamaterial based on plate resonators, in which the phonon band dispersion can be modified from an acoustic-like to an optical character by modulating a uniform prestress. The switch is enabled by the change in sign of an effective coupling between fundamental modes, which generically leads to a nearly dispersionfree band at the transition point. We demonstrate how adiabatic tuning of the band dispersion can immobilize and reverse the propagation of a sound pulse in simulations of a one-dimensional resonator chain. Our study relies on the basic principles of thin-plate elasticity independently of any specific material, making our results applicable across varied length scales and experimental platforms. More broadly, our approach could be replicated for signal manipulation in photonic metamaterials and electronic heterostructures.

Bulletin of the American Physical Society, 2018

Journal of Physics: Condensed Matter, 2020

Two-dimensional (2D) materials have been experimentally proven to manifest almost all types of ma... more Two-dimensional (2D) materials have been experimentally proven to manifest almost all types of material properties observed in bulk materials. However, 2D magnetism was elusive until recently. In this work, we used an approach that synergistically uses density functional theory, and Monte Carlo methods to investigate the magnetic and electronic properties of magnetic double transition metal MXene alloys (Hf2MnC2O2 and Hf2VC2O2) by exploiting realistic surface terminations via creating surface defects including oxygen vacancies and H adatoms. We found that introducing surface oxygen vacancies or hydrogen adatoms is able to modify the electronic structures, magnetic anisotropies, and exchange couplings. Depending on the defect concentration, a ferromagnetic half-metallic state can be realized for both Hf2VC2O2 and Hf2MnC2O2. Bare Hf2VC2O2 exhibits easy-axis anisotropy, whereas bare Hf2MnC2O2 exhibits easy-plane anisotropy; however, defects can change the latter to easy-axis anisotropy...

The Journal of Physical Chemistry C, 2019

Next-generation spintronic nanoscale devices require two-dimensional (2D) materials with robust f... more Next-generation spintronic nanoscale devices require two-dimensional (2D) materials with robust ferromagnetism. Among 2D materials, MXenes are favorable for spintronic applications due to their high electron conductivity and mobility. A recently reported MXene, Hf 2 MnC 2 O 2 , possesses a high Curie temperature (greater than 800 K) and a high magnetic moment per formula unit (3 µ B). Since 2D materials have greater elastic strain limits than their bulk counterparts, their properties can be tuned effectively using strain engineering. Here, we investigate modifications in the structural, electronic, and magnetic properties produced by uniaxial strain on a Hf 2 MnC 2 O 2 monolayer. The strain-free Hf 2 MnC 2 O 2 nanosheet is an indirect-band-gap semiconductor. Our calculations predict that an indirect-to-direct band gap transition occurs at about 1%-3% tensile strain applied in the armchair direction. At 7% strain applied in the zigzag direction and 9% strain applied in the armchair direction, this semiconductor material becomes a half-metal which is favorable for spintronic applications. Under 4% compressive strain in either direction, a semiconductor-to-metal transition is predicted.

Applied Surface Science, 2018

Electronic and mechanical properties of stiff rhenium carbide monolayers: a first-principles inve... more Electronic and mechanical properties of stiff rhenium carbide monolayers: a first-principles investigation,

Journal of Physics: Condensed Matter, 2016

(LC) networks on 4000x4000 lattices. We calculate the dynamical conductivity using an equation-of... more (LC) networks on 4000x4000 lattices. We calculate the dynamical conductivity using an equation-of-motion method in which timestep error is eliminated and windowing error is minimized [1]. We extract the critical exponent a such that σ(ω) ∝ ω −a at low frequencies. The results suggest that there are three different universality classes. The L ij C i model, with capacitances from each site to ground, has a = 0.32. The L ij C ij model, with capacitances along bonds, has a = 0. The L ij C i C ij model, with both types of capacitances, has a = 0.30. This implies that classical percolative 2D superconductor-insulator transitions (SITs) generically have σ(ω) → ∞ as ω → 0. Therefore, experiments that give a constant conductivity as ω → 0 must be explained in terms of quantum effects.

Journal of Physics: Condensed Matter, 2017

Coarse-grained superconductor-insulator composites exhibit a superconductor-insulator transition ... more Coarse-grained superconductor-insulator composites exhibit a superconductor-insulator transition governed by classical percolation, which should be describable by networks of inductors and capacitors. We study several classes of random inductor-capacitor networks on square lattices. We present a unifying framework for defining electric and magnetic response functions, and we extend the Frank-Lobb bond-propagation algorithm to compute these quantities by network reduction. We confirm that the superfluid stiffness scales approximately as [Formula: see text] as the superconducting bond fraction p approaches the percolation threshold p c . We find that the diamagnetic susceptibility scales as [Formula: see text] below percolation, and as [Formula: see text] above percolation. For models lacking self-capacitances, the electric susceptibility scales as [Formula: see text]. Including a self-capacitance on each node changes the critical behavior to approximately [Formula: see text].