arXiv subjects

Chee Kwan Gan

Publications and source records attributed to Chee Kwan Gan.

At least 19 recordsLinked to original sources

arXiv2026

Coupling between CaWO$_4$ phonons and Er$^{3+}$ dopants

We investigate the lattice dynamics of CaWO$_4$, a promising host crystal for erbium-based quantum memories, using inelastic neutron scattering together with density-functional perturbation theory. The measured phonon dispersion along the (100), (001), and (101) reciprocal space direction reveals phonon bands extending up to 130 meV, with a gap between 60 and 80 meV, in good agreement with our calculations. From a symmetry analysis of the phonon eigenmodes, we identify eight Raman-active modes that can couple directly to the Er$^{3+}$ crystal-field operators, including a low-energy $B_g$ mode at 9.1 meV that is expected to play a dominant role in phonon-assisted spin-lattice relaxation. These results provide a microscopic description of the phonon bath in CaWO$_4$ and establish a basis for engineering phononic environments to mitigate the loss of stored quantum states and optimize Er-doped CaWO$_4$ for quantum-memory applications.

cond-mat.mtrl-sci↗

arXiv2024

Benchmarking Quantum Convolutional Neural Networks for Classification and Data Compression Tasks

Quantum Convolutional Neural Networks (QCNNs) have emerged as promising models for quantum machine learning tasks, including classification and data compression. This paper investigates the performance of QCNNs in comparison to the hardware-efficient ansatz (HEA) for classifying the phases of quantum ground states of the transverse field Ising model and the XXZ model. Various system sizes, including 4, 8, and 16 qubits, through simulation were examined. Additionally, QCNN and HEA-based autoencoders were implemented to assess their capabilities in compressing quantum states. The results show that QCNN with RY gates can be trained faster due to fewer trainable parameters while matching the performance of HEAs.

quant-ph↗

arXiv2023

In-plane dominant anisotropy stochastic magnetic tunnel junction for probabilistic computing: A Fokker-Planck study

Recently there is considerable interest to realize efficient and low-cost true random number generators (RNGs) for practical applications. One important way is through the use of bistable magnetic tunnel junctions (MTJs). Here we study the magnetization dynamics of an MTJ, with a focus to realize efficient random bit generation under the assumption that the orientation dependence of the energy of the nanomagnet is described by two perpendicular in-plane anisotropies. We find that a high rate of random bit generation is achievable away from the pure easy-axis situation by tuning a single parameter $H_z$ so that it is either (a) toward a barrierless-like single easy plane situation when $H_z$ reduces to zero, or (b) toward a stronger easy plane situation when $H_z$ becomes increasingly negative where transitions between low energy states are confined in the stronger easy plane that contains the saddle points. We find that the MTJs maintain their fast magnetization dynamical characteristics even in the presence of a magnetic field. Our findings provide a valuable guide to achieving efficient generation of probabilistic bits for applications in probabilistic computing.

cond-mat.other↗

arXiv2022

A size-consistent Grüneisen-quasiharmonic approach for lattice thermal conductivity

We propose a size-consistent Grüneisen-quasiharmonic approach (GQA) to calculate the lattice thermal conductivity $κ_l$ where the Grüneisen parameters that measure the degree of phonon anharmonicity are calculated directly using first-principles calculations. This is achieved by identifying and modifying two existing equations related to the Slack formulae for $κ_l$ that suffer from the size-inconsistency problem when dealing with non-monoatomic primitive cells (where the number of atoms in the primitive cell $n$ is greater than one). In conjunction with other thermal parameters such as the acoustic Debye temperature $θ_a$ that can also be obtained within the GQA, we predict $κ_l$ for a range of materials taken from the diamond, zincblende, rocksalt, and wurtzite compounds. The results are compared with that from the experiment and the quasiharmonic Debye model (QDM). We find that in general the prediction of $θ_a$ is rather consistent among the GQA, experiment, and QDM. However, while the QDM somewhat overestimates the Grüneisen parameters and hence underestimates $κ_l$ for most materials, the GQA predicts the experimental trends of Grüneisen parameters and $κ_l$ more closely. We expect the GQA with the modified Slack formulae could be used as an effective and practical predictor for $κ_l$, especially for crystals with large $n$.

cond-mat.mtrl-sci↗

arXiv2022

A first-principles investigation of the linear thermal expansion coefficients of BeF$_2$: Giant thermal expansion

We present the results of a theoretical investigation of the linear thermal expansion coefficients (TECs) of BeF$_2$, within a direct Gruneisen formalism where symmetry-preserving deformations are employed. The required physical quantities such as the optimized crystal structures, elastic constants, mode Gruneisen parameters, and phonon density of states are calculated from first-principles. BeF$_2$ shows an extensive polymorphism at low pressures, and the lowest energy phases [$α$-cristobalite with space group (SG) P$4_1 2_1 2$ and its similar phase with SG P$4_3 2_1 2$] are considered in addition to the experimentally observed $α$-quartz phase. For benchmarking purposes, similar calculations are performed for the rutile phase of ZnF$_2$, where the volumetric TEC ($α_v$), derived from the calculated linear TECs along the $a$ ($α_a$) and $c$ ($α_c$) directions, is in very good agreement with experimental data and previous theoretical results. For the considered phases of BeF$_2$, we do not find any negative thermal expansion (NTE). However we observe diverse thermal properties for the distinct phases. The linear TECs are very large, especially $α_c$ of the $α$-cristobalite phase and its similar phase, leading to giant $α_v$ ($\sim 175 \times 10^{-6} {\rm K}^{-1}$ at 300 K). The giant $α_v$ arises from large Gruneisen parameters of low-frequency phonon modes, and the C13 elastic constant that is negatively signed and large in magnitude for the $α$-cristobalite phase. The elastic constants, high-frequency dielectric constants, Born effective charge tensors, and thermal properties of the above phases of BeF$_2$ are reported for the first time and hence serve as predictions.

cond-mat.mtrl-sci↗

arXiv2021

Path-Integral Treatment of Quantum Bouncers

The one-sided bouncer and the symmetric bouncer involve a one-dimensional particle in a piecewise linear potential. For such problems, the time-dependent quantum mechanical propagator cannot be found in closed form. The semiclassical Feynman path integral is a very appealing approach, as it approximates the propagator by a closed-form expression (a sum over a finite number of classical paths). In this paper we solve the classical path enumeration problem. We obtain closed-form expressions for the initial velocity, bounce times, focal times, action, van Vleck determinant, and Morse index for each classical path. We calculate the propagator within the semiclassical approximation. The numerical results agree with eigenfunction expansion results away from caustics. We derive mappings between the one-sided bouncer and symmetric bouncer, which explains why each bounce of the one-sided bouncer increases the Morse index by 2 and results in a phase change of $π$. We interpret the semiclassical Feynman path integral to obtain visualizations of matter wave propagation based on interference between classical paths, in analogy with the traditional visualization of light wave propagation as interference between classical ray paths.

quant-ph↗

arXiv2021

Complementary local-global approach for phonon mode connectivities

Sorting and assigning phonon branches (e.g., longitudinal acoustic) of phonon modes is important for characterizing the phonon bands of a crystal and the determination of phonon properties such as the Grüneisan parameter and group velocity. To do this, the phonon band indices (including the longitudinal and transverse acoustic) have to be assigned correctly to all phonon modes across a path or paths in the Brillouin zone. As our solution to this challenging problem, we propose a computationally efficient and robust two-stage hybrid method that combines two approaches with their own merits. The first is the perturbative approach in which we connect the modes using degenerate perturbation theory. In the second approach, we use numerical fitting based on least-squares fits to circumvent local connectivity errors at or near exact degenerate modes. The method can be easily generalized to other condensed matter problems involving Hermitian matrix operators such as electronic bands in tight-binding Hamiltonians or in a standard density-functional calculation, and photonic bands in photonic crystals.

cond-mat.mtrl-sci↗

arXiv2020

Efficacious symmetry-adapted atomic displacement method for lattice dynamical studies

Small displacement methods have been successfully used to calculate the lattice dynamical properties of crystals. It involves displacing atoms by a small amount in order to calculate the induced forces on all atoms in a supercell for the computation of force constants. Even though these methods are widely in use, to our knowledge, there is no systematic discussion of optimal displacement directions from the crystal's symmetry point of view nor a rigorous error analysis of such methods. Based on the group theory and point group symmetry of a crystal, we propose displacement directions, with an equivalent concept of the group of $k$, deduced directly in the Cartesian coordinates rather than the usual fractional coordinates, that maintain the theoretical maximum for the triple product $V$ spanned by the three displacements to avoid possible severe roundoff errors. The proposed displacement directions are generated from a minimal set of irreducible atomic displacements that keep the required independent force calculations to a minimum. We find the error in the calculated force constant explicitly depends on the inverse of $V$ and inaccuracy of the forces. Test systems such as Si, graphene, and orthorhombic Sb2S3 are used to illustrate the method. Our displacement method is shown to be very robust in treating low-symmetry cells with a large `aspect ratio' due to huge differences in lattice parameters, use of a large vacuum height, or a very oblique unit cell due to unconventional choice of primitive lattice vectors. It is expected that our displacement strategy can be used to address higher-order interatomic interactions to achieve good accuracy and efficiency.

cond-mat.mtrl-sci↗

arXiv2019

Large thermal anisotropy in monoclinic niobium trisulfide: A thermal expansion tensor study

We present a method based on the Gruneisen formalism to calculate the thermal expansion coefficient (TEC) tensor that is applicable to any crystal system, where the number of phonon calculations associated with different deformations scales linearly with the number of lattice parameters. Compared to simple high-symmetry systems such as cubic or hexagonal systems, a proper consideration of low-symmetry systems such as monoclinic or triclinic crystals demands a clear distinction between the TEC tensor and the lattice-parameter TECs along the crystallographic directions. The latter is more complicated and it involves integrating the equations of motion for the primitive lattice vectors, with input from the TEC tensor. A first-principles study of the TEC is carried out for the first time on a monoclinc crystal, where we unveil high TEC anisotropies in a recently reported monoclinic phase of niobium trisulfide (NbS3) crystal with a relatively large primitive cell (32 atoms per cell) using density-functional theory. We find the occurrence of a negative TEC tensor component is largely due to the mechanical property rather than the anharmonic effect, contrary to the common belief. Our theoretical treatment of the monoclinic system with a single off-diagonal tensor element could be routinely generalized to any crystal system, including the lowest-symmetry triclinic system with three off-diagonal tensor elements.

cond-mat.mtrl-sci↗

arXiv2018

First-principles density-functional calculations using localized spherical-wave basis sets

We present a detailed study of the use of localized spherical-wave basis sets, first introduced in the context of linear-scaling, in first-principles density-functional calculations. Several parameters that control the completeness of this basis set are fully investigated on systems such as molecules and bulk crystalline silicon. We find that the results are in good agreement with those obtained using the extended plane-wave basis set. Since the spherical-wave basis set is accurate, easy to handle, relatively small, and can be systematically improved, we expect it to be of use in other applications.

physics.chem-ph↗

arXiv2018

Anharmonic phonon effects on linear thermal expansion of trigonal bismuth selenide and antimony telluride crystals

We adopted and extended an efficient Grüneisen formalism to study the phonon anharmonicity and linear thermal expansion coefficients (TECs) of trigonal bismuth selenide (Bi$_2$Se$_3$) and antimony telluride (Sb$_2$Te$_3$). Anharmonicity of the systems is studied via extensive calculation of Grüneisen parameters that exploit symmetry-preserving deformations. Consistent with experimental findings, a large anisotropy between the TECs in the $a$ and $c$ directions is found. The larger anharmonicity inherent in Sb$_2$Te$_3$, as compared to Bi$_2$Se$_3$ is offset by the volumetric effect, resulting in comparable temperature dependence of their linear TECs. The Debye temperatures deduced from our first-principles data also agree very well with the existing tabulated values. The highly efficient methodology developed in this work, applied for the first time to study the linear TECs of two trigonal thermoelectric systems, opens up exciting opportunities to address the anharmonic effects in other thermoelectrics and other low-symmetry materials.

cond-mat.mtrl-sci↗

arXiv2017

Anharmonic interatomic force constants and thermal conductivity from Grüneisen parameters: an application to graphene

Phonon-mediated thermal conductivity, which is of great technological relevance, fundamentally arises due to anharmonic scattering from interatomic potentials. Despite its prevalence, accurate first-principles calculations of thermal conductivity remain challenging, primarily due to the high computational cost of anharmonic interatomic force constant (IFCs) calculations. Meanwhile, the related anharmonic phenomenon of thermal expansion is much more tractable, being computable from the Grüneisen parameters associated with phonon frequency shifts due to crystal deformations. In this work, we propose a novel approach for computing the largest cubic IFCs from the Grüneisen parameter data. This allows an approximate determination of the thermal conductivity via a much less expensive route. The key insight is that although the Grüneisen parameters cannot possibly contain all the information on the cubic IFCs, being derivable from spatially uniform deformations, they can still unambiguously and accurately determine the largest and most physically relevant ones. By fitting the anisotropic Grüneisen parameter data along judiciously designed deformations, we can deduce (i.e., reverse engineer) the dominant cubic IFCs and estimate three-phonon scattering amplitudes. We illustrate our approach by explicitly computing the largest cubic IFCs and thermal conductivity of graphene, especially for its out-of-plane (flexural) modes that exhibit anomalously large anharmonic shifts and thermal conductivity contributions. Our calculations on graphene not only exhibits reasonable agreement with established DFT results, but also presents a pedagogical opportunity for introducing an elegant analytic treatment of the Grüneisen parameters of generic two-band models. Our approach can be readily extended to more complicated crystalline materials with nontrivial anharmonic lattice effects.

cond-mat.mtrl-sci↗

arXiv2016

Direct calculation of the linear thermal expansion coefficients of MoS2 via symmetry-preserving deformations

Using density-functional perturbation theory and the Grüneisen formalism, we directly calculate the linear thermal expansion coefficients (TECs) of a hexagonal bulk system MoS$_2$ in the crystallographic $a$ and $c$ directions. The TEC calculation depends critically on the evaluation of a temperature-dependent quantity $I_i(T)$, which is the integral of the product of heat capacity and $Γ_i(ν)$, of frequency $ν$ and strain type $i$, where $Γ_i(ν)$ is the phonon density of states weighted by the Grüneisen parameters. We show that to determine the linear TECs we may use minimally two uniaxial strains in the $z$ direction, and either the $x$ or $y$ direction. However, a uniaxial strain in either the $x$ or $y$ direction drastically reduces the symmetry of the crystal from a hexagonal one to a base-centered orthorhombic one. We propose to use an efficient and accurate symmetry-preserving biaxial strain in the $xy$ plane to derive the same result for $Γ(ν)$. We highlight that the Grüneisen parameter associated with a biaxial strain may not be the same as the average of Grüneisen parameters associated with two separate uniaxial strains in the $x$ and $y$ directions due to possible preservation of degeneracies of the phonon modes under a biaxial deformation. Large anisotropy of TECs is observed where the linear TEC in the $c$ direction is about $1.8$ times larger than that in the $a$ or $b$ direction at high temperatures. Our theoretical TEC results are compared with experiment. The symmetry-preserving approach adopted here may be applied to a broad class of two lattice-parameter systems such as hexagonal, trigonal, and tetragonal systems, which allows many complicated systems to be treated on a first-principles level.

cond-mat.mtrl-sci↗

arXiv2015

Large anharmonic effect and thermal expansion anisotropy of metal chalcogenides: The case of antimony sulfide

We derive a compact matrix expression for the linear thermal expansion coefficients (TECs) for a general orthorhombic system which relates the elastic properties and the integrated quantities based on deformation and mode dependent Gruneisen parameters and mode dependent heat capacities. The density of Gruneisen parameters $Γ(ν)$ as a function of frequency $ν$, weighted by the number of phonon modes, is introduced and found to be insightful in interpreting the TEC results. Using density-functional perturbation theory and Gruneisen formalism for thermal expansion, we illustrate the general usefulness of this method by calculating the linear and volumetric TECs of a low-symmetry orthorhombic compound antimony sulfide (Sb2S3), a compound belonging to a large class of technologically and fundamentally important materials. Even though negative Gruneisen parameters are found for deformations in all three crystal directions, the $Γ(ν)$ data rule out the occurrences of negative TECs at all temperatures. Sb2S3 exhibits a large thermal expansion anisotropy where the TEC in the $b$ direction can reach as high as $13\times 10^{-6}$~(1/K) at high temperatures, about two and seven times larger than the TECs in the $c$ and $a$ direction, respectively. Our work suggests a general and practical first-principles approach to calculate the thermal properties of other complicated low-symmetry systems.

cond-mat.mtrl-sci↗

arXiv2014

Low-bias Negative Differential Resistance effect in armchair graphene nanoribbon junctions

Graphene nanoribbons with armchair edges (AGNRs) have bandgaps that can be flexibly tuned via the ribbon width. A junction made of a narrower AGNR sandwiched between two wider AGNR leads was recently reported to possess two perfect transmission channels close to the Fermi level. Here, we report that by using a bias voltage to drive these transmission channels into the gap of the wider AGNR lead, we can obtain a negative differential resistance (NDR) effect. Owing to the intrinsic properties of the AGNR junctions, the on-set bias reaches as low as ~ 0.2 V and the valley current almost vanishes. We further show that such NDR effect is robust against details of the atomic structure of the junction, substrate and whether the junction is made by etching or by hydrogenation.

cond-mat.mes-hall↗

arXiv2014

Current-induced switching of magnetic tunnel junctions: Effects of field-like spin-transfer torque, pinned-layer magnetization orientation and temperature

We study current-induced switching in magnetic tunnel junctions (MTJs) in the presence of a field-like spin-transfer torque and titled pinned-layer magnetization in the high current limit at finite temperature. We consider both the Slonczewski and field-like torques with coefficients $a_J$ and $b_J $, respectively. At finite temperatures, $σ= b_J/a_J = \pm1$ leads to a smaller mean switching time compared that with $σ= 0$. The reduction of switching time in the presence of the field-like term is due to the alignment effect (for $σ> 0$) and the initial torque effect.

cond-mat.mes-hall↗

arXiv2014

First-principles study of the lattice dynamics of Sb2S3

We present a lattice dynamics study of orthorhombic antimony sulphide (Sb2S3) obtained using density-functional calculations in conjunction with the supercell force-constant method. The effect of Born effective charges is taken into account using a mixed-space approach, resulting in the splitting of longitudinal and transverse optical (LO-TO) phonon branches near the zone center. Zone-center frequencies agree well with Raman scattering experiments. Due to the slow decay of the interatomic force constants (IFC), a minimal 2x4x2 supercell (Pnma setting) with 320 atoms is crucial for an accurate determination of the dispersion relations. Smaller supercells result in artificial acoustic phonon softening and unphysical lifting of degeneracies along high symmetry directions. We propose a scheme to investigate the convergence of the IFC with respect to the supercell sizes. The phonon softening can be attributed to the periodic images that affect the accuracy of the force constants, and the truncation of long-ranged forces. The commensuration of the q-vectors with the supercell size is crucial to preserve degeneracies in Sb2S3 crystals.

cond-mat.mtrl-sci↗

arXiv2013

First-principles study of the thermoelectric properties of strained graphene nanoribbons

We study the transport properties, in particular, the thermoelectric figure of merit ZT of armchair graphene nanoribbons, AGNR-N (for N=4-12, with widths ranging from 3.7 to 13.6~Å) through strain engineering, where N is the number of carbon dimer lines across the AGNR width. We find that the tensile strain applied to AGNR-$N$ changes the transport properties by modifying the electronic structures and phonon dispersion relations. The tensile strain increases the ZT value of the AGNR-$N$ families with N=3p and N=3p+2, where $p$ is an integer. Our analysis based on accurate density-functional theory calculations suggests a possible route to increase the ZT values of AGNR-$N$ for potential thermoelectric applications.

cond-mat.mtrl-sci↗