Cheol-Hwan Park

In this article, we provide an overview of theoretical and computational methods for studying magnetic van der Waals materials with special emphasis on first-principles computations and their application. First, we discuss the issues, such as how to properly consider electronic interactions via weak interlayer couplings, the exchange-correlation functional used for density-functional theory calculations, Hund physics, and dynamical mean-field theory methods. We then switch gears to the optical properties of magnetic van der Waals materials with specific emphasis on intriguing excitons and the interplay between spin and orbital degrees of freedom. Finally, we discuss chiral phonons and magnon-phonon interactions.

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.

We present a conceptually simple and technically straightforward method for calculating photoelectron wave functions that is easily integrable with standard wave-function-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_{2}, the circular dichroism reflecting the hidden orbital polarization. Our Letter 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.

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.

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.

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.