Title:	Phonons and Spin Dynamics in the Novel Superconductor LaPt2Si2
DNr:	SNIC 2020/5-183
Project Type:	SNIC Medium Compute
Principal Investigator:	Martin Månsson <condmat@kth.se>
Affiliation:	Kungliga Tekniska högskolan
Duration:	2020-04-01 – 2021-04-01
Classification:	10304 10306
Homepage:	https://www.kth.se/profile/condmat
Keywords:

Abstract

The coexistence of superconductivity (SC) and charge- and/or spin-density waves (CDW and/or SDW) is fundamental to our understanding behind the mechanism of high-Tc SC. Materials like Fe-based pnictides, transition metal dichalcogenides or cuprate superconductors have gained significant interest as the fluctuations associated with CDW and /or SDW are believed to be a key factor in inducing SC in these systems. Recently, the quasi-2D Pt-based rare earth intermetallic material LaPt2Si2 has attracted a lot of attention as it exhibits strong interplay between CDW and SC. The material LaPt2Si2 has a structural transition from a high temperature tetragonal structure to a low temperature orthorombic structure. For a polycrystalline sample a transition temperature 112 K has been reported [1]. At the structural transition the onset of a charge density wave with periodicity of three multiples of the lattice constant a has been observed. Cooling below Tsc=1.22 K the material becomes a superconductor. We have recently (March 2020) performed an inelastic neutron scattering experiment on a single crystal of LaPt2Si2 with the HRC instrument at J-PARC, Japan, to investigate the phonon excitation spectra at temperatures below and above the structural transition. For inclusion in the neutron instrument beamtime application, nonmagnetic preliminary calculations of the phonon spectra were performed with the commercial VASP software [2,3] and Phonopy [4]. We are now about to pursue more extensive and accurate modelling of the electronic structure and phonon spectra of LaPt2Si2. To this end we use the open source density functional theory (DFT) Elk software version 6.3.2 [5]. Elk has functionality for calculation of the electron-phonon coupling, the Eliashberg function and superconducting gap, and superconducting phase transition temperature from the McMillan-Allen-Dynes equation. We are going to investigate how the electronic structure differ comparing nonmagnetic, spin-polarized, and in spin-polarized with spin orbit coupling (SOC) calculations. The first step in the DFT calculations will be to relax the internal coordinates of the ions in the chemical unit cells while keeping the lattice vectors fixed. This relaxation will affect all calculated observables to lesser or greater extent. We will calculate the phonon dispersion curves for the tetragonal and orthorombic phases in order to unravel the role of the electron-phonon coupling in the structural transition. We will perform calculations of electronic and phononic states for the primitive cell and for the 3a superstructure. Moreover, we will investigate the magnetic susceptibility and the possible tendency to spin density wave formation. [1] R. Gupta, et al. J. Phys.: Cond. Mat. 28 195702 (2016). [2] G. Kresse and J. Furthmuller PRB, 54 11169, (1996). [3] G. Kresse and J. Furthmuller, Comp. Mater. Sci. 6, 15 (1996). [4] A. Togo and I. Tanaka Scripta Materialia, 108, 1, (2015). [5] The Elk FP-LAPW code. http://elk.sourceforge.net/