arXiv subjects

Selçuk Parlak

Publications and source records attributed to Selçuk Parlak.

5 recordsLinked to original sources

arXiv2026

Tight-binding and density-functional study of the Raman tensor in two-dimensional massive Dirac fermion systems

Recently, two unusual features were theoretically predicted for the Raman response of out-of-plane phonons in magnetic two-dimensional materials hosting massive Dirac fermions. First, the phase difference between certain Raman tensor elements was found to be quantized to $\pm π/2$, sensitive only to the sign of the Dirac fermion mass. Second, a selection rule was identified in the Raman intensity under circularly polarized light, which generalizes the well-known optical valley selection rule. These predictions were based on a low-energy effective model in the continuum approximation. Here, we test the robustness of those results for more realistic theoretical approaches. First, we calculate the Raman tensor for an electronic tight-binding model on a honeycomb lattice with broken time-reversal and inversion symmetries. Second, we compute the Raman tensor from density-functional theory for a monolayer of ferromagnetic 2H-RuCl$_2$. Both calculations corroborate the analytical results found in the continuum model, thereby theoretically confirming the peculiar behavior of the Raman tensor for two dimensional massive Dirac fermion systems.

cond-mat.mtrl-sci↗

arXiv2025

Inherited Berry curvature of phonons in Dirac materials with time-reversal symmetry

The Berry curvature of phonons is an active subject of research in condensed matter physics. Here, we present a model in which phonons acquire a Berry curvature through their coupling to electrons in crystals with time-reversal symmetry. We illustrate this effect for BaMnSb$_2$, a quasi two-dimensional Dirac insulator, whose low-energy massive Dirac fermions generate a phonon Berry curvature that is proportional to the electronic valley Chern number.

cond-mat.mes-hall↗

arXiv2025

Raman tensor for two-dimensional massive Dirac fermions

Raman spectroscopy is a valuable characterization tool for two-dimensional materials. Starting from model Hamiltonians for Chern insulators and magnetized monolayers of transition metal dichalcogenides, we theoretically predict two unconventional features of Raman spectroscopy. First, a selection rule emerges in the Raman tensor when the incident and scattered photons are circularly polarized. This rule generalizes the well-known valley selection rule of optical conductivity in Dirac insulators. Second, for an electronic model with single massive Dirac fermion, the phase difference between Raman tensor elements is quantized to $\pmπ/2$ for any frequency of the incident light. The quantization is robust under perturbations and the sign of the phase difference is reversed when the mass term of the Hamiltonian is inverted.

cond-mat.mes-hall↗

arXiv2023

Detection of phonon helicity in nonchiral crystals with Raman scattering

Recently, it has been predicted that the Berry curvature of electrons can produce an angular momentum for phonons. In systems with time-reversal symmetry, the direction of the phonon angular momentum is locked to the phonon wave vector. Accordingly, this phenomenon has received the name of ``phonon helicity". Here, we present a theory to unveil the signatures of such phonon helicity using Raman scattering. We show that the intensity of Raman scattering for circularly polarized light in BaMnSb$_2$ (a prototypical nonchiral Dirac insulator) changes under a reversal of the phonon wave vector, and that the phonon helicity can be inferred from that change. We compare our results to recent reports of Raman-based detection of phonon angular momentum in chiral crystals.

cond-mat.mes-hall↗

arXiv2021

Scaling and renormalization in the modern theory of polarization: application to disordered systems

We develop a scaling theory and a renormalization technique in the context of the modern theory of polarization. The central idea is to use the characteristic function (also known as the polarization amplitude) in place of the free energy in the scaling theory and in place of the Boltzmann probability in a position-space renormalization scheme. We derive a scaling relation between critical exponents which we test in a variety of models in one and two dimensions. We then apply the renormalization to disordered systems. In one dimension the renormalized disorder strength tends to infinity indicating the entire absence of extended states. Zero(infinite) disorder is a repulsive(attractive) fixed point. In two and three dimensions, at small system sizes, two additional fixed points appear, both at finite disorder, $W_a$($W_r$) is attractive(repulsive) such that $W_a<W_r$. In three dimensions $W_a$ tends to zero, $W_r$ remains finite, indicating metal-insulator transition at finite disorder. In two dimensions we are limited by system size, but we find that both $W_a$ and $W_r$ decrease significantly as system size is increased.

cond-mat.dis-nn↗