Előadó: Balla Péter (Wigner FK SZFI)
Előadás címe: Ground-state manifolds of frustrated Heisenberg magnets and magnetooptical selection rules in multiferroics (PhD házivédés)
Dátum: 2022. június 16. csütörtök, 10.00
Helyszín: 1. ép., tanácsterem, illetve online: https://wigner-hu.zoom.us/j/82814345310?pwd=b2cxeW94VFFiUy8xczI1TUVLQ05… Meeting ID: 828 1434 5310 Passcode: 973614
In the first part of the talk, I consider classical Heisenberg magnets on frustrated lattices, where not all the bond energies can be simultaneously minimized. This leads to a large number of energetically equivalent ground states and extended ground-state manifolds in Fourier space, a pattern markedly different from the usual Bragg peaks. I analyze the ground-state (zero-temperature) phase diagram of the Heisenberg model on the face-centered cubic lattice up to third neighbor interactions. I identify different classes of ground states, with a particular focus on triple points and phase boundaries, where the extended ground-state manifolds emerge. I also give a general recipe for constructing Heisenberg models on any Bravais lattice having extended ground-state manifolds (curves in 2D and surfaces in 3D). In magnetoelectric multiferroics, the magnetization and electric polarization are coupled, meaning that we can manipulate the spins with electric fields or the electric polarization with magnetic fields. This coupling also manifests itself in the finite-frequency properties, leading to electromagnons and one-way transparency. In the second part, I consider the magnetooptical selection rules in multiferroics and apply them to the absorption measurements of Sr2CoSi2O7 in its paramagnetic state in high external magnetic fields. I show that the antiunitary symmetries (time-reversed elements of the magnetic symmetry group of a crystal) relate the imaginary and real parts of matrix elements of perturbing operators, leading to a new type of selection rules.