arXiv subjects

Cheol-Hwan Park

Publications and source records attributed to Cheol-Hwan Park.

At least 19 recordsLinked to original sources

arXiv2026

Accurate calculation of Wannier centers, position matrix, and composite operators using translationally equivariant and higher-order finite differences

The momentum-space derivatives of Bloch wavefunctions are essential for studying quantum geometry and the equilibrium and response properties of solids. In practical first-principles calculations, these derivatives are obtained via Wannier interpolation of position and related composite matrices. These matrices are initially evaluated on a coarse k-point grid using finite-difference approximations and then interpolated to a dense grid. The accuracy of the finite-difference approximation directly impacts the convergence and reliability of the result. In this work, we present two key improvements to the finite-difference calculation of position and composite operators for Wannier interpolation. First, we formulate a translationally equivariant scheme that preserves the underlying symmetries of the system and significantly reduces finite-difference errors. Second, we introduce a higher-order finite-difference approach that yields a more accurate approximation of the k-space derivatives by systematically increasing the convergence rate. From a real-space perspective, these improvements correspond to better approximations of the position operator at the locations of the Wannier functions. We also present a generalization of the finite-difference scheme, which may reduce the number of finite-difference points while maintaining accuracy. We demonstrate the effectiveness of our methods by applying them to the calculation of Wannier centers and spreads, electric polarization, off-diagonal position matrix elements, orbital magnetization, and spin Hall conductivity. Our results demonstrate significant reductions in finite-difference errors, elimination of symmetry-violating errors, and improved convergence with respect to k-point sampling. These methods have been implemented in the open-source packages and can be readily adopted in other Wannier-based codes with minimal computational overhead.

cond-mat.mtrl-sci↗

arXiv2026

First-principles study of bulk stacking, $J_{\rm eff}$ picture, magnetic Hamiltonian, $g$ factors, and structural distortions of $\alpha$-RuCl$_3$

$\alpha$-RuCl$_3$ is a candidate Kitaev material that exhibits zigzag antiferromagnetic ordering below 7 K. One contentious issue regarding this material is its bulk structure in the low-temperature phase. Recently, it has become generally accepted from experiments that the low- and high-temperature structures belong to the $R\bar{3}$ and $C2/m$ space groups, respectively. However, there was no theoretical study supporting the $R\bar{3}$-type structure as the low-temperature structure. In this study, we use constrained density functional theory to show that the $R\bar{3}$ structure is lower in energy than the $C2/m$ structure, in agreement with experimental observations. Then, we show that the conduction band minimum states are almost of the $J_\textrm{eff}=1/2$ and $m_\textrm{eff}=-1/2$ character, if we set the angular momentum quantization axis to be parallel to the N\'eel vector; this is the first analysis of the $J_\textrm{eff}$ picture for $\alpha$-RuCl$_3$ from this perspective. In addition, we compute the anisotropic magnetic exchange parameters and $g$ factors of monolayer $\alpha$-RuCl$_3$, thereby providing a comprehensive understanding of its magnetism. Our results demonstrate that both second-nearest-neighbor exchange interactions and magnetic moments not captured by the conventional atomic orbital projection method are necessary for accurate description of the magnetism in $\alpha$-RuCl$_3$. Moreover, the calculated $g$ factors are in fairly good agreement with experimental measurements, especially the small anisotropy between their in-plane and out-of-plane components. Finally, we examine the effects of structural distortions from a perfect RuCl$_6$ octahedron, already present in bulk $\alpha$-RuCl$_3$ without any external perturbation, on the magnetic properties. (The abstract is cut here due to the word limit; see the pdf file for the full abstract.)

cond-mat.mtrl-sci↗

arXiv2025

Non-adiabatic phonon self-energy due to electrons with finite linewidths

We develop the theory of the non-adiabatic phonon self-energy arising from coupling to electrons with finite linewidths using the spectral representation of Green's functions. Our formalism naturally includes the contribution from the intra-band electronic transitions (as well as the inter-band ones) at all electron wavevectors to the phonon linewidths, which is forbidden for zone-center optical phonons if infinitesimal electron linewidths are used. As a proof of principle, we use the theory to calculate the linewidth of the double-degenerate, zone-center optical $E_{\rm 2g}$ phonons of graphene as a function of the chemical potential. The calculated phonon linewidths obtained with finite electron linewidths differ significantly from those obtained with infinitesimal electron linewidths even at low temperatures. Intra-band electronic transitions play an important role in making this difference. Moreover, only the results accounting for the finite electron linewidths are in quantitative agreement with the available experimental data. The presented formalism is suitable for first-principles calculations.

cond-mat.mtrl-sci↗

arXiv2025

Lippmann-Schwinger Approach for Accurate Photoelectron Wavefunctions and Angle-Resolved Photoemission Spectra from First Principles

We present a conceptually simple and technically straightforward method for calculating photoelectron wavefunctions that is easily integrable with standard wavefunction-based density-functional-theory packages. Our method is based on the Lippmann-Schwinger equation, naturally incorporating the boundary condition that the final photoelectron state must satisfy. The calculated results are in good agreement with the measured photon-energy- and polarization-dependence of the angle-resolved photoemission spectroscopy (ARPES) of graphene, the photon-energy-dependent evolution of the so-called dark corridor arising from the pseudospin, and WSe\textsubscript{2}, the circular dichroism reflecting the hidden orbital polarization. Our study opens doors to do-it-yourself simulations of ARPES with standard density-functional-theory packages, of crucial importance in the era of ``quantum materials,'' whose key experimental tool is ARPES.

cond-mat.mtrl-sci↗

arXiv2025

Theory of infrared magneto-optical effects from chiral phonons in solids

In crystals with broken time-reversal symmetry, zone-center phonons can acquire a finite angular momentum via velocity-dependent forces on the nuclei. Despite having the same order of magnitude as the electron spin angular momentum, the phonon angular momentum can be hard to detect because the frequency splitting is small. Here, by developing a theory of lattice magneto-optical effects in reflection and transmission, we show that infrared magnetic circular dichroism is a sensitive probe of zone-center phonon chirality. We evaluate the infrared magneto-optical Faraday, Kerr, and circular-dichroism spectra of CrI$_3$ from time-dependent density-functional theory in the adiabatic local-density approximation. We find sizeable circular dichroism from the infrared-active E$_u$ mode at $\approx 214$ cm$^{-1}$, even though the calculated splitting is only 0.22 cm$^{-1}$.

cond-mat.mtrl-sci↗

arXiv2025

Verification and Validation of zero-point electron-phonon renormalization of the bandgap, mass enhancement, and spectral functions

Verification and validation of methods and first-principles software are at the core of computational solid-state physics but are too rarely addressed. We compare four first-principles codes: Abinit, Quantum ESPRESSO, EPW, ZG, and three methods: (i) the Allen-Heine-Cardona theory using density functional perturbation theory (DFPT), (ii) the Allen-Heine-Cardona theory using Wannier function perturbation theory (WFPT), and (iii) an adiabatic non-perturbative frozen-phonon method. For these cases, we compute the real and imaginary parts of the electron-phonon self-energy in diamond and BAs, including dipoles and quadrupoles when interpolating. We find excellent agreement between software that implements the same formalism as well as good agreement between the DFPT and WFPT methods. Importantly, we find that the Deybe-Waller term is momentum dependent which impacts the mass enhancement, yielding approximate results when using the Luttinger approximations. Finally, we compare the electron-phonon spectral functions between Abinit and EPW and find excellent agreement even away from the band edges.

cond-mat.mtrl-sci↗

arXiv2024

Phonon-induced renormalization of electron wave functions

The Allen-Heine-Cardona theory allows us to calculate phonon-induced electron self-energies from first principles without resorting to the adiabatic approximation. However, this theory has not been able to account for the change of the electron wave function, which is crucial if interband energy differences are comparable to the phonon-induced electron self-energy as in temperature-driven topological transitions. Furthermore, for materials without inversion symmetry, even the existence of such topological transitions cannot be investigated using the Allen-Heine-Cardona theory. Here, we generalize this theory to the renormalization of both the electron energies and wave functions. Our theory can describe both the diagonal and off-diagonal components of the Debye-Waller self-energy in a simple, unified framework. For demonstration, we calculate the electron-phonon coupling contribution to the temperature-dependent band structure and hidden spin polarization of BiTlSe2 across a topological transition. These quantities can be directly measured. Our theory opens a door for studying temperature-induced topological phase transitions in materials both with and without inversion symmetry.

cond-mat.mtrl-sci↗

arXiv2024

Self-consistent electron lifetimes for electron-phonon scattering

Acoustic phonons in piezoelectric materials strongly couple to electrons through a macroscopic electric field. We show that this coupling leads to a momentum-dependent divergence of the Fan-Migdal electron linewidth. We then develop a self-consistent theory for calculating electron linewidths, which not only removes this piezoelectric divergence but also considerably modifies the linewidth in nonpiezoelectric, polar materials. Our predictions await immediate experimental confirmation, and this self-consistent method should be broadly used in interpreting various experiments on the electronic properties of real materials.

cond-mat.mtrl-sci↗

arXiv2024

Plasmon-Phonon Hybridization in Doped Semiconductors from First Principles

Although plasmons and phonons are the collective excitations that govern the low-energy physics of doped semiconductors, their nonadiabatic hybridization and mutual screening have not been studied from first principles. We achieve this goal by transforming the Dyson equation to the frequency-independent dynamical matrix of an equivalent damped oscillator. Calculations on doped GaAs and TiO2 agree well with available Raman data and await immediate experimental confirmation from infrared, neutron, electron-energy-loss, and angle-resolved photoemission spectroscopies.

cond-mat.mtrl-sci↗

arXiv2024

Nonlinear Hall effect from long-lived valley-polarizing relaxons

The nonlinear Hall effect has attracted much attention due to the famous, widely adopted interpretation in terms of the Berry curvature dipole in momentum space. Using ab initio Boltzmann transport equations, we find a 60% enhancement in the nonlinear Hall effect of p-doped GeTe and its noticeable frequency dependence, qualitatively different from the predictions based on the Berry curvature dipole. The origin of these differences is long-lived valley polarization in the electron distribution arising from electron-phonon scattering. Our findings await immediate experimental confirmation.

cond-mat.mes-hall↗

arXiv2022

Strain-induced collapse of Landau Levels in real Weyl semimetals

The collapse of Landau levels under an electric field perpendicular to the magnetic field is one of the distinctive features of Dirac materials. So is the coupling of lattice deformations to the electronic degrees of freedom in the form of gauge fields which allows the formation of pseudo-Landau levels from strain. We analyze the collapse of Landau levels induced by strain on realistic Weyl semimetals hosting anisotropic, tilted Weyl cones in momentum space. We perform first-principles calculations, to establish the conditions on the external strain for the collapse of Landau levels in TaAs which can be experimentally accessed.

cond-mat.mes-hall↗

arXiv2022

Converging tetrahedron method calculations for the nondissipative parts of spectral functions

Many physical quantities in solid-state physics are calculated from k-space summation. For spectral functions, the frequency-dependent factor can be decomposed into the energy-conserving delta function part and the nondissipative principal value part. A very useful scheme for this k-space summation is the tetrahedron method. Tetrahedron methods have been widely used to calculate the summation of the energy-conserving delta function part such as the imaginary part of the dielectric function. On the other hand, the corresponding tetrahedron method for the nondissipative part such as the real part of the dielectric function has not been used much. In this paper, we address the technical difficulties in the tetrahedron method for the nondissipative part and present an easy-to-implement, stable method to overcome those difficulties. We demonstrate our method by calculating the static and dynamical spin Hall conductivity of platinum. Our method can be widely applied to calculate linear static or dynamical conductivity, self-energy of an electron, and electric polarizability, to name a few.

cond-mat.mtrl-sci↗

arXiv2022

A comprehensive theory of second-order spin photocurrents

The spin photocurrents, direct currents induced by light, hold great promise for introducing new elements to spintronics. However, a general theory for spin photocurrents in real materials which is applicable to systems with spin-orbit coupling or noncollinear magnetism is absent. Here, we develop such a general theory of second-order spin photocurrents. We find that the secondorder spin photocurrents can be classified into Drude, Berry curvature dipole, shift, injection, and rectification currents, which have different physical origins and symmetry properties. Surprisingly, our theory predicts a direct pure spin rectification current in an insulator induced by photons with energies lower than the material band gap. This phenomenon is absent in the case of the charge photocurrent. We find that the pure spin current of BiTeI induced by subgap light is large enough to be observable in experiments. Moreover, the subgap pure spin photocurrent is highly tunable with the polarization of light and the flowing direction of the spin photocurrent. This study lays the groundwork for the study of nonlinear spin photocurrents in real materials and provides a route to engineer light-controlled spin currents.

cond-mat.mtrl-sci↗

arXiv2021

Wannier Function Perturbation Theory: Localized Representation and Interpolation of Wavefunction Perturbation

Thanks to the nearsightedness principle, the low-energy electronic structure of solids can be represented by localized states such as the Wannier functions. Wannier functions are actively being applied to a wide range of phenomena in condensed matter systems. However, the Wannier-functionbased representation is limited to a small number of bands and thus cannot describe the change of wavefunctions due to various kinds of perturbations, which require sums over an infinite number of bands. Here, we introduce the concept of the Wannier function perturbation, which provides a localized representation of wavefunction perturbations. Wannier function perturbation theory allows efficient calculation of numerous quantities involving wavefunction perturbation, among which we provide three applications. First, we calculate the temperature-dependent indirect optical absorption spectra of silicon near the absorption edge nonadiabatically, i.e., differentiating phonon-absorption and phonon-emission processes, and without arbitrary temperature-dependent shifts in energy. Second, we establish a theory to calculate the shift spin conductivity without any band-truncation error. Unlike the shift charge conductivity, an exact calculation of the shift spin conductivity is not possible within the conventional Wannier function methods because it cannot be obtained from geometric quantities for low-energy bands. We apply the theory to monolayer WTe$_2$. Third, we calculate the spin Hall conductivity of the same material again without any band-truncation error. Wannier function perturbation theory is a versatile method that can be readily applied to calculate a wide range of quantities related to various kinds of perturbations.

cond-mat.mtrl-sci↗

arXiv2021

Magnetic anisotropy and magnetic ordering of transition-metal phosphorus trisulfides

Here, a magnetic model with an unprecedentedly large number of parameters was determined from first-principles calculations for transition-metal phosphorus trisulfides (TMPS$_3$'s), which reproduced the measured magnetic ground states of bulk TMPS$_3$'s. Our Monte Carlo simulations for the critical temperature, magnetic susceptibility, and specific heat of bulk and few-layer TMPS$_3$'s agree well with available experimental data and show that the antiferromagnetic order of FePS$_3$ and NiPS$_3$ persists down to monolayers. Remarkably, the orbital polarization, which was neglected in recent first-principles studies, dramatically enhances the magnetic anisotropy of FePS$_3$ by almost two orders of magnitude. A recent Raman study [K. Kim et al., Nat. Commun. 10, 345 (2019)] claimed that magnetic ordering is absent in monolayer NiPS$_3$ but simultaneously reported a strong two-magnon continuum; we show that the criterion used to judge magnetic ordering there is invalid in monolayer NiPS$_3$, thus providing an understanding of the two seemingly contradictory experimental results. The rich predictions on the magnetic susceptibility and specific heat of few-layer FePS$_3$ and NiPS$_3$ await immediate experimental verifications.

cond-mat.mtrl-sci↗

arXiv2021

Gaussian time-dependent variational principle for the finite-temperature anharmonic lattice dynamics

The anharmonic lattice is a representative example of an interacting bosonic many-body system. The self-consistent harmonic approximation has proven versatile for the study of the equilibrium properties of anharmonic lattices. However, the study of dynamical properties therewithin resorts to an ansatz, whose validity has not yet been theoretically proven. Here, we apply the time-dependent variational principle, a recently emerging useful tool for studying the dynamic properties of interacting many-body systems, to the anharmonic lattice Hamiltonian at finite temperature using the Gaussian states as the variational manifold. We derive an analytic formula for the position-position correlation function and the phonon self-energy, proving the dynamical ansatz of the self-consistent harmonic approximation. We establish a fruitful connection between time-dependent variational principle and the anharmonic lattice Hamiltonian, providing insights in both fields. Our work expands the range of applicability of time-dependent variational principle to first-principles lattice Hamiltonians and lays the groundwork for the study of dynamical properties of the anharmonic lattice using a fully variational framework.

cond-mat.mtrl-sci↗

arXiv2021

General, Strong Impurity-Strength Dependence of Quasiparticle Interference

Quasiparticle interference (QPI) patterns in momentum space are often assumed to be independent of the strength of the impurity potential when compared with other quantities, such as the joint density of states. Here, using the $T$-matrix theory, we show that this assumption breaks down completely even in the simplest case of a single-site impurity on the square lattice with an $s$ orbital per site. Then, we predict from first-principles, a very rich, impurity-strength-dependent structure in the QPI pattern of TaAs, an archetype Weyl semimetal. This study thus demonstrates that the consideration of the details of the scattering impurity including the impurity strength is essential for interpreting Fourier-transform scanning tunneling spectroscopy experiments in general.

cond-mat.mtrl-sci↗

arXiv2020

Kagome van-der-Waals Pd3P2S8 with flat band

With the advanced investigations into low-dimensional systems, it has become essential to find materials having interesting lattices that can be exfoliated down to monolayer. One particular important structure is a kagome lattice with its potentially diverse and vibrant physics. We report a van-der-Waals kagome lattice material, Pd3P2S8, with several unique properties such as an intriguing flat band. The flat band is shown to arise from a possible compact-localized state of all five 4d orbitals of Pd. The diamagnetic susceptibility is precisely measured to support the calculated susceptibility obtained from the band structure. We further demonstrate that Pd3P2S8 can be exfoliated down to monolayer, which ultimately will allow the possible control of the localized states in this two-dimensional kagome lattice using the electric field gating.

cond-mat.mtrl-sci↗