Olga Sikora

Spin excitations used to probe the nature of exchange coupling in the magnetically ordered ground state of Pr 0.5 Ca 0.5 MnO 3

We have used time-of-flight inelastic neutron scattering to measure the spin wave spectrum of the canonical half-doped manganite

Variational Monte Carlo simulations using tensor-product projected states

{\mathrm{Pr}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{MnO}}_{3}

Magnetic excitations in vanadium spinels

in its magnetic and orbitally ordered phase. The data, which cover multiple Brillouin zones and the entire energy range of the excitations, are compared with several different models that are all consistent with CE-type magnetic order, but arise through different exchange coupling schemes. The Goodenough model, i.e., an ordered state comprising strong nearest-neighbor ferromagnetic interactions along zigzag chains with antiferromagnetic interchain coupling, provides the best description of the data, provided that further neighbor interactions along the chains are included. We are able to rule out a coupling scheme involving formation of strongly bound ferromagnetic dimers, i.e., Zener polarons, on the basis of gross features of the observed spin wave spectrum. A model with weaker dimerization reproduces the observed dispersion but can be ruled out on the basis of discrepancies between the calculated and observed structure factors at certain positions in reciprocal space. Adding further neighbor interactions results in almost no dimerization, i.e., recovery of the Goodenough model. These results are consistent with theoretical analysis of the degenerate double exchange model for half-doping, and provide a recipe for how to interpret future measurements away from half-doping, where degenerate double exchange models predict more complex ground states.

Seeing the light: Experimental signatures of emergent electromagnetism in a quantum spin ice

We propose an efficient numerical method, which combines the advantages of recently developed tensor-network based methods and standard trial wave functions, to study the ground-state properties of quantum many-body systems. In this approach, we apply a projector in the form of a tensor-product operator to an input wave function, such as a Jastrow-type or Hartree-Fock wave function, and optimize the tensor elements via variational Monte Carlo. The entanglement already contained in the input wave function can considerably reduce the bond dimensions compared to the regular tensor-product state representation. In particular, this allows us to also represent states that do not obey the area law of entanglement entropy. In addition, for fermionic systems, the fermion sign structure can be encoded in the input wave function. We show that the optimized states provide good approximations of the ground-state energy and correlation functions in the cases of two-dimensional bosonic and fermonic systems.

Quantum Ice: A Quantum Monte Carlo Study

We study magnetic excitations in vanadium spinel oxides

Chain-based order and quantum spin liquids in dipolar spin ice

A{\mathrm{V}}_{2}{\mathrm{O}}_{4}

Quantum Liquid with Deconfined Fractional Excitations in Three Dimensions

(A=\mathrm{Zn},\mathrm{Mg},\mathrm{Cd})

Extended quantum U(1)-liquid phase in a three-dimensional quantum dimer model

using two models: the first one is a superexchange model for vanadium

Electromagnetism on ice: classical and quantum theories of proton disorder in hexagonal water ice

S=1