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SZFI SzemináriumBalla PéterMTA Wigner FK SZFIGround-state manifolds of 
frustrated Heisenberg magnets and magnetooptical selection rules in 
multiferroics (PhD házivédés)2022. június 16. csütörtök, 10.00, 1. ép., 
tanácsterem,  
https://wigner-hu.zoom.us/j/82814345310?pwd=b2cxeW94VFFiUy8xczI1TUVLQ05Edz09  
 Meeting ID: 828 1434 5310  Passcode: 973614In 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. Minden érdeklődőt szívesen látunk! - Everyone is welcome to 
attend.Juhász Róbertsem-admin AT szfki.hu