arXiv subjects

Talat S. Rahman

Publications and source records attributed to Talat S. Rahman.

At least 19 recordsLinked to original sources

arXiv2025

Machine learned potential for defected single layer hexagonal boron nitride

Development of machine learned interatomic potentials (MLIP) is critical for performing reliable simulations of materials at length and time scales that are comparable to those in the laboratory. We present here a MLIP suitable for simulations of the temperature dependent structure and dynamics of single layer hexagonal boron nitride (h-BN) with defects and grain boundaries, developed using a strictly local equivariant deep neural network as formulated in the Allegro code. The training dataset consisted of about 30,000 images of h-BN with and without point defects generated with ab-initio molecular dynamics simulations, based on density functional theory (DFT), at 500, 1000, and 1500K. The developed MLIP predicts potential energies and forces with a mean absolute error (MAE) of 4 meV/atom and 60 meV/Angstrom , respectively. It also reproduces phonon dispersion curves and density of vibrational states of pristine bulk h-BN that are comparable with that obtained from density functional theory-based calculations. Molecular dynamics simulations of the motion of the 4|8 grain boundary unit in h-BN shows the first step to have an activation barrier ~2.2 eV, indicating immobility of the grain boundary. Moving the grain boundary units past the first shows much lower activation barriers of ~0.42eV, suggesting a facile motion of the grain boundary once the first movement is stimulated. These simulations yield a scaled mobility of 1.739*10^(-11) m^3/Js for a temperature of 1500K which, given the inherent differences in the set-ups, is not too far from the experimental value of 1.36*10^(-9) m^3/Js. The ability to predict grain boundary mobility within reasonable agreement with experiment demonstrates the robustness of the MLIP and its suitability for reliable simulations of defect structures and dynamics in single layer h-BN.

cond-mat.mtrl-sci↗

arXiv2025

Unraveling spin entanglement using quantum gates with scanning tunneling microscopy-driven electron spin resonance

Quantum entanglement is a fundamental resource for quantum information processing, and its controlled generation and detection remain key challenges in scalable quantum architectures. Here, we numerically demonstrate the deterministic generation of entangled spin states in a solid-state platform by implementing quantum gates via electron spin resonance combined with scanning tunneling microscopy (ESR-STM). Using two titanium atoms on a MgO/Ag(100) substrate as a model, we construct a two-qubit system whose dynamics are coherently manipulated through tailored microwave pulse sequences. We generate Bell states by implementing a Hadamard gate followed by a controlled-NOT gate, and evaluate its fidelity and concurrence using the quantum-master equation-based code TimeESR. Our results demonstrate that ESR-STM can create entangled states with significant fidelity. This study paves the way for the realization of atom-based quantum circuits and highlights ESR-STM as a powerful tool for probing and engineering entangled states on surfaces.

quant-ph↗

arXiv2025

Unveiling Coverage Dependent Interactions of N-Methylaniline with the Pt(111) Surface

This study aims to elucidate the adsorption and surface chemistry of N-methylaniline (NMA) on Pt(111), using it as a model molecule to probe the activation mechanisms of aromatic amines on catalytic surfaces. Through a combination of density functional theory (DFT) calculations and experimental techniques such as temperature programmed X-ray photoelectron spectroscopy (TP-XPS), temperature programmed desorption (TPD), and Fourier transform infrared reflection absorption spectroscopy(FT-IRRAS),we explored the coverage-dependent behaviour of NMA on Pt(111) to identify key steps in the activation process. The population of certain reaction paths is driven by a coverage dependent balance between molecule surface charge transfer and intermolecular interactions, dictating the selective activation of specific bonds. Our findings reveal how coverage influences the orientation and bonding of NMA on the Pt(111)surface. At lower coverages, the molecule binds to the surface through the phenyl ring and activation, facilitating C-N bond cleavage to the ring under HCN formation. In comparison, at higher coverages, the molecule binds only through the nitrogen atom and desorbs intact. These insights into variable bond activation lay the ground work for understanding the fundamental processes involved in potential heterogeneously catalyzed reactions of aromatic amines, contributing to the development of new catalytic strategies.

cond-mat.mtrl-sci↗

arXiv2025

Realization of Two-dimensional Discrete Time Crystals with Anisotropic Heisenberg Coupling

A discrete time crystal (DTC) is the paradigmatic example of a phase of matter that occurs exclusively in systems out of equilibrium. This phenomenon is characterized by the spontaneous symmetry breaking of discrete time-translation and provides a rich playground to study a fundamental question in statistical physics: what mechanism allows for driven quantum systems to exhibit emergent behavior that deviates from their counterparts with time-independent evolution? Unlike equilibrium phases, DTCs exhibit macroscopic manifestations of coherent quantum dynamics, challenging the conventional narrative that thermodynamic behavior universally erases quantum signatures. However, due to the difficulty of simulating these systems with either classical or quantum computers, previous studies have been limited to a set of models with Ising-like couplings -- and mostly only in one dimension -- thus precluding our understanding of the existence (or not) of DTCs in models with interactions that closely align with what occurs in nature. In this work, by combining the latest generation of IBM quantum processors with state-of-the-art tensor network methods, we are able to demonstrate the existence of a DTC in a two-dimensional system governed by anisotropic Heisenberg interactions. Our comprehensive analysis reveals a rich phase diagram encompassing spin-glass, ergodic, and time-crystalline phases, highlighting the tunability of these phases through multiple control parameters. Crucially, our results emphasize the interplay of initialization, interaction anisotropy, and driving protocols in stabilizing the DTC phase. By extending the study of Floquet matter beyond simplified models, we lay the groundwork for exploring how driven systems bridge the gap between quantum coherence and emergent non-equilibrium thermodynamics.

quant-ph↗

arXiv2023

Mapping Spin Interactions from Conductance Peak Splitting in Coulomb Blockade

We investigate the transport properties of a quantum dot coupled to leads interacting with a multi-spin system using the generalized master equation within the Coulomb blockade regime. We find that if two states for each scattering region electron manifold are included, several signatures of the interacting spin system appear in steady-state transport properties. We provide a theoretical mapping of differential conductance peak signatures and all spin Hamiltonian parameters related to the inclusion of excited state transitions between uncharged and charged electron manifolds. Our predictions describe a scheme of only using a quantum dot and differential conductance to measure magnetic anisotropy, inter-spin exchange coupling, exchange coupling between the spin system and itinerant electron, and applied magnetic field response.

cond-mat.mes-hall↗

arXiv2023

Thickness dependence of superconductivity in FeSe films

The films of FeSe on substrates have attracted attention because of their unusually high-temperature (Tc) superconducting properties whose origins continue to be debated. To disentangle the competing effects of the substrate and interlayer and intralayer processes, we present here results of density functional theory (DFT)-based analysis of the electronic structure of unsupported FeSe films consisting of 1 to 5 layers (1L-5L). Furthermore, by solving the Bardeen-Schrieffer-Cooper (BCS) equation with spin-wave exchange attraction derived from the Hubbard model, we find the superconducting critical temperature Tc for 1L-5L and bulk FeSe systems in reasonable agreement with experimental data. Our results point to the importance of correlation effects in superconducting properties of single- and multi-layer FeSe films, independently of the role of substrate.

cond-mat.supr-con↗

arXiv2023

Dark exciton energy splitting in monolayer WSe2: insights from time-dependent density-functional theory

We present here a formalism based on time-dependent density-functional theory (TDDFT) to describe characteristics of both intra- and inter-valley excitons in semiconductors, the latter of which had remained a challenge. Through the usage of an appropriate exchange-correlation kernel (nanoquanta), we trace the energy difference between the intra- and inter-valley dark excitons in monolayer (1L) WSe2 to the domination of the exchange part in the exchange-correlation energies of these states. Furthermore, our calculated transition contribution maps establish the momentum resolved weights of the electron-hole excitations in both bright and dark excitons thereby providing a comprehensive understanding of excitonic properties of 1L WSe2. We find that the states consist of hybridized excitations around the corresponding valleys which leads to brightening of the dark excitons, i.e., significantly decreasing their lifetime which is reflected in the PL spectrum. Using many-body perturbation theory, we calculate the phonon contribution to the energy bandgap and the linewidths of the excited electrons, holes and (bright) exciton to find that as the temperature increases the bandgap significantly decreases, while the linewidths increase. Our work paves for describing the ultrafast charge dynamics of transition metal dichalcogenide within an ab initio framework.

cond-mat.mtrl-sci↗

arXiv2022

Electronic control and switching of entangled spin state using anisotropy and exchange in the three-particle paradigm

We explore the control and switching of the entangled spin states of multi-spin particle qubit coupled to an electron using a three-particle spin model described by $S_i$ ($i=1,2,3$), in which $S_1=\tfrac{1}{2}$ is an electron and $S_{2,3}$ can have any spin with both exchange coupling and magnetic anisotropy. We derive a general formula for the existence of a switching (DJ) resonance for any spin $S_{2,3}$. We further contrast the entanglement switching mechanisms for the $S_{2,3}=\tfrac{1}{2}$ and $S_{2,3}=1$ spin models. We find that while the onsite magnetic anisotropy in the case of $S_{2,3}>\tfrac{1}{2}$ allows full control of their spin states via interaction with $S_1$, in order to achieve acceptable control of a Bloch vector within the $S_{2,3}=\tfrac{1}{2}$ model, additional mechanisms, such as anisotropic exchange coupling, are required.

quant-ph↗

arXiv2022

Thickness-dependent spin bistable transitions in single-crystalline molecular 2D material

The advent of two-dimensional (2D) crystals has led to numerous scientific breakthroughs. Conventional 2D systems have in-plane covalent bonds and a weak out-of-plane van-der-Waals bond. Here we report a new type of 2D material composed of discrete magnetic molecules, where anisotropic van-der-Waals interactions bond the molecules into a 2D packing. Through mechanical exfoliation, we can obtain single-crystalline molecular monolayers, which can be readily integrated into other 2D systems. Optical spectroscopy suggests the few-layered molecules preserve the temperature-induced spin-crossover switching observed in the bulk form but show a drastic increase in thermal hysteresis unique to these thin 2D molecule assemblies. The trapping of spin bistability with decreasing layer number can arise from domain wall dynamics in reduced dimensions. Our results establish molecular solids with strong anisotropy of intermolecular interactions as precursors to a novel class of 2D materials, affording new possibilities to control functionalities through substrate and interlayer interactions.

cond-mat.mes-hall↗

arXiv2021

Anisotropy-Exchange Resonance as a Mechanism for Entangled State Switching

We explore the three-particle spin model of an $S_{1}=\frac{1}{2}$ particle (e.g. a stationary electron) interacting with two spin-coupled $S_{\text{2,3}}$ particles with exchange coupling and magnetic anisotropy. We find that in the case of $S_{2,3}=1$ particles, the coupled particle entanglement states can be prepared, controlled, and read by the $S_{1}$ particle. We also find that for particular resonance conditions of the magnetic anisotropy strength $D$ and exchange coupling strength $J$, the entanglement state switching behavior is maximized and is robust against a range of anisotropic application of the exchange coupling.

quant-ph↗

arXiv2021

Electron Thermalization and Relaxation in Laser-Heated Nickel by Few-Femtosecond Core-Level Transient Absorption Spectroscopy

Direct measurements of photoexcited carrier dynamics in nickel are made using few-femtosecond extreme ultraviolet (XUV) transient absorption spectroscopy at the nickel M$_{2,3}$ edge. It is observed that the core-level absorption lineshape of photoexcited nickel can be described by a Gaussian broadening ($σ$) and a red shift ($ω_{s}$) of the ground state absorption spectrum. Theory predicts, and the experimental results verify that after initial rapid carrier thermalization, the electron temperature increase ($ΔT$) is linearly proportional to the Gaussian broadening factor $σ$, providing quantitative real-time tracking of the relaxation of the electron temperature. Measurements reveal an electron cooling time for 50 nm thick polycrystalline nickel films of 640$\pm$80 fs. With hot thermalized carriers, the spectral red shift exhibits a power-law relationship with the change in electron temperature of $ω_{s}\proptoΔT^{1.5}$. Rapid electron thermalization via carrier-carrier scattering accompanies and follows the nominal 4 fs photoexcitation pulse until the carriers reach a quasi-thermal equilibrium. Entwined with a <6 fs instrument response function, carrier thermalization times ranging from 34 fs to 13 fs are estimated from experimental data acquired at different pump fluences and it is observed that the electron thermalization time decreases with increasing pump fluence. The study provides an initial example of measuring electron temperature and thermalization in metals in real time with XUV light, and it lays a foundation for further investigation of photoinduced phase transitions and carrier transport in metals with core-level absorption spectroscopy.

cond-mat.mtrl-sci↗

arXiv2020

Spatially non-homogeneous metallization of VO2: a TDDFT+DMFT analysis

We provide insights into the atomistic details of the ultrafast spatially-resolved breakdown of the insulating M1 phase in bulk VO2 employing an ab initio technique based on time-dependent density-functional theory and dynamical mean-field theory (DMFT-TDDFT). We find that the system is initially metallized preferentially along the vanadium-dimer chains (CR axis), with a subsequent growth of CR-elongated metallic bubbles. Moreover, we trace the breakdown of the insulating phase to two types of oxygen atoms, resulting from vanadium dimerization, which produce an unusual charge-density modulation in the oxygen-atom chains with significant charge transfer to the inter-dimer distance. These results are in qualitative agreement with experimental data and shed light on the interplay between valence charge and lattice structure and its role in the ultrafast response of strongly correlated insulators.

cond-mat.str-el↗

arXiv2020

A closer look at how symmetry constraints and the spin-orbit coupling shape the electronic structure of Bi(111)

Relativistic density-functional-theory calculations of Bi(111) thin films are performed to revisit their band structure and that of macroscopic samples. The band structure of a our 39-bilayer film ($\sim$~15~nm) shows that (1) $\sim$9-nm films are enough to describe that of Bi(111), (2) The two split surface-state metallic branches along the $\overline{ΓM}$ direction do not overlap with the bulk band at the zone boundary but lie within the A7-distortion-induced conduction-valence band gap, and (3) Neither the existence of the metallic surface states nor their observed splitting is related to inversion \emph{asymmetry}. Thus, the spin texture observed in such states is not caused by the lifting of the Kramers degeneracy and their splitting is not of the Rashba-type. We instead propose that (1) the large splitting of the metallic branches is a $m_j=\pm1/2$-$m_j=\pm3/2$ splitting and (2) the spin texture observed for the metallic branches may only occur because the almost unaltered strong covalent bonds retained by Bi(111) surface atoms cannot afford magnetic polarization. We emphasize that degeneracy at the $M$-point of the SBZ of Bi(111) -- implied by the translational symmetry of the surface -- is satisfied irrespectively of the presence of inversion symmetry centers. We show that the magnetic-moment discontinuity at $M$ does not exist, which also explains why the measured spin-polarization of the metallic branches vanishes near $M$. We induce the Rashba effect on the band structure of Bi(111) via different structural/electronic perturbations to reveal the actual lifting of the Kramers degeneracy and find that the magnitude of the perturbation imposed on a film correlates with the magnitude of the splitting and the localization of the Rashba-split states.

cond-mat.mtrl-sci↗

arXiv2020

Excited states in hydrogenated single-layer MoS$_2$

We calculate the excitation spectrum of single-layer MoS$_2$ at several hydrogen coverages by using a method based on first-principles Density-Matrix Time-Dependent Density-Functional Theory (TDDFT). Our results show that the fully hydrogenated system is metallic, while in the low-coverage limit the spectrum of single-layer MoS$_2$ includes spin-polarized partially filled localized mid-gap states. These states arise from s-orbitals of H atoms which make a tilted bond with the surface S atoms. The calculated absorption spectrum of the system reveals standard excitonic peaks, which correspond to the bound valence-band hole and conduction-band electron, as well as excitonic peaks that involve the mid-gap charges. As in the case of pristine single-layer MoS$_2$, binding energies of the excitons of the hydrogenated system are found to be relatively large (few tens of meV), making their experimental detection facile and suggesting hydrogenation as a knob for tuning the optical properties of single-layer MoS$_2$. Importantly, we find hydrogenation to suppress visible light photoluminescence, in agreement with experimental observations. As an aside, we contrast the effects of hydrogen coverage to that of the next two elements in the same column of the periodic table (the lightest metals), Li and Na, on the spectral properties of single-layer MoS$_2$ which lead instead to the formation of n-doped non-magnetic semiconductors that do not allow excitonic states.

cond-mat.mtrl-sci↗

arXiv2019

Sub-monolayer structures of Ag overlayers on Ge(111): experimental observations and first-principles study

We present a joint experimental and theoretical determination of structures of Ag adatoms on the Ge(111) surface using low energy electron diffraction, low energy electron microscopy, scanning tunneling microscopy, and density functional theory-based calculations, as functions of coverages and temperature. Experimentally for clean Ge(111), c(2X8) and (2X1) phases occur, while Ag overlayers cause (4X4), (V3XV3)R30 and (3X1) surface structural phases. The dependence of the growth behavior of these different phases was examined as a function of temperature, Ag deposition rate and coverage, substrate step density, and history of temperature cycling. First-principles calculations of the electronic and geometric structures and vibrational dynamics show the Ge(111)-c(2X8) configuration with Ge adatoms adsorbed on three-fold hollow (T4) sites to be the energetically most favored phase of the Ge(111) surface, among unreconstructed Ge(111), reconstructed Ge(111)-2X1, and Ge(111)-c(2X8) structures. The Ge(111)-Ag(3X1) overlayer of the system has Ge atoms forming a honeycomb chain on a missing top layer reconstructed surface, with metal at 1/3 ML coverage in channel. The Ge (111)-Ag(V3XV3)R30 overlayer contains one monolayer Ag forming inequivalent Ag triangles in a surface unit cell on the missing top layer reconstructed Ge(111) surface. The Ge(111)-Ag(4X4) overlayer formed at low Ag coverage contains two triangular subunits at different heights: one with six Ag adatoms and the other with three Ge adatoms on the intact double layer Ge(111) surface. The temperature and coverage dependent surface phase diagram, obtained by minimizing the surface free energy, captures the main features of the experimental phase diagram.

cond-mat.mtrl-sci↗

arXiv2019

On the validity of the Arrhenius picture in two-dimensional submonolayer growth

For surface-mediated processes, such as on-surface synthesis, epitaxial growth and heterogeneous catalysis, a constant slope in the Arrhenius diagram of the corresponding rate of interest against inverse temperature, $\log R$ {\it vs} $1/k_B T$, is traditionally interpreted as the existence of a bottleneck elementary reaction (or rate-determining step), whereby the constant slope (or apparent activation energy, $E_{app}^{R}$) reflects the value of the energy barrier for that reaction. Here, we show that a constant value of $E_{app}^{R}$ can be obtained even if control shifts from one elementary reaction to another. In fact, we show that $E_{app}^{R}$ is a weighted average and the leading elementary reaction will change with temperature while the actual energy contribution for every elementary reaction will contain, in addition to the traditional energy barrier, a configurational term directly related to the number of local configurations where that reaction can be performed. For this purpose, we consider kinetic Monte Carlo simulations of two-dimensional submonolayer growth at constant deposition flux, where the rate of interest is the tracer diffusivity. In particular, we focus on the study of the morphology, island density and diffusivity by including a large variety of single-atom, multi-atom and complete-island diffusion events for two specific metallic heteroepitaxial systems, namely, Cu on Ni(111) and Ni on Cu(111), as a function of coverage and temperature.

cond-mat.mtrl-sci↗

arXiv2019

Prediction of activation energy barrier of island diffusion processes using data-driven approaches

We present models for prediction of activation energy barrier of diffusion process of adatom (1-4) islands obtained by using data-driven techniques. A set of easily accessible features, geometric and energetic, that are extracted by analyzing the variation of the energy barriers of a large number of processes on homo-epitaxial metallic systems of Cu, Ni, Pd, and Ag are used along with the activation energy barriers to train and test linear and non-linear statistical models. A multivariate linear regression model trained with energy barriers for Cu, Pd, and Ag systems explains 92% of the variation of energy barriers of the Ni system, whereas the non-linear model using artificial neural network slightly enhances the success to 93%. Next mode of calculation that uses barriers of all four systems in training, predicts barriers of randomly picked processes of those systems with significantly high correlation coefficient: 94.4% in linear regression model and 97.7% in artificial neural network model. Calculated kinetics parameters such as the type of frequently executed processes and effective energy barrier for Ni dimer and trimer diffusion on the Ni(111) surface obtained from KMC simulation using the predicted (data-enabled) energy barriers are in close agreement with those obtained by using energy barriers calculated from interatomic interaction potential.

cond-mat.mtrl-sci↗

arXiv2018

Plasmon excitations in mixed metallic nanoarrays

We study the plasmonic properties of arrays of atomic chains which comprise noble (Cu, Ag, and Au) and transition (Pd, Pt) metal atoms using time-dependent density-functional theory. We show that the response to the electromagnetic radiation is related to both physics, the geometry-dependent confinement of sp-valence electrons, and chemistry, the energy position of d-electrons in the different atomic species and the hybridization between d and sp electrons. As a result it is possible to tune the position of the surface plasmon resonance, split it to several peaks, and eventually achieve broadband absorption of radiation. Mixing the arrays with transition metals can strongly attenuate the plasmonic behaviour. We analyze the origin of these phenomena and show that they arise from rich interactions between single-particle electron-hole and collective electron excitations. The tunability of the plasmonic response of arrays of atomic chains, which can be realized on solid surfaces, opens wide possibilities for their applications. In the present study we obtain guidelines how the desired properties can be achieved.

cond-mat.mes-hall↗