Stephan Haas

Studying Quantum Spin Systems through Entanglement Estimators

We study the field dependence of the entanglement of formation in anisotropic S=1/2 antiferromagnetic chains displaying a T=0 field-driven quantum phase transition. The analysis is carried out via quantum Monte Carlo simulations. At zero temperature the entanglement estimators show abrupt changes at and around criticality, vanishing below the critical field, in correspondence with an exactly factorized state, and then immediately recovering a finite value upon passing through the quantum phase transition. At the quantum-critical point, a deep minimum in the pairwise-to-global entanglement ratio shows that multispin entanglement is strongly enhanced; moreover this signature represents a novel way of detecting the quantum phase transition of the system, relying entirely on entanglement estimators.

Entanglement and factorized ground states in two-dimensional quantum antiferromagnets.

Making use of exact results and quantum Monte Carlo data for the entanglement of formation, we show that the ground state of anisotropic two-dimensional S=1/2 antiferromagnets in a uniform field takes the classical-like form of a product state for a particular value and orientation of the field, at which the purely quantum correlations due to entanglement disappear. Analytical expressions for the energy and the form of such states are given, and a novel type of exactly solvable two-dimensional quantum models is therefore singled out. Moreover, we show that the field-induced quantum phase transition present in the models is unambiguously characterized by a cusp minimum in the pairwise-to-global entanglement ratio R, marking the quantum-critical enhancement of multipartite entanglement.

Anisotropics-wave superconductivity inMgB2

It has recently been observed that

Field-Induced Magnetic Order in Quantum Spin Liquids

{\mathrm{MgB}}_{2}

Universal scaling at field-induced magnetic phase transitions

is a superconductor with a high transition temperature. Here we propose a model of anisotropic s-wave superconductivity which consistently describes the observed properties of this compound, including the thermodynamic and optical response in sintered

Phase diagram of magnetization reversal processes in nanorings

{\mathrm{MgB}}_{2}

Upper critical field and Fulde-Ferrell-Larkin-Ovchinnikov state in CeCoIn5

wires. We also determine the shape of the quasiparticle density of states, the tunneling conductance, and the anisotropy of the upper critical field and the superfluid density which should be detectable once single-crystal samples become more generally available.

Quantum antiferromagnetism in quasicrystals

We study magnetic-field-induced three-dimensional ordering transitions in low-dimensional quantum spin liquids, such as weakly coupled, antiferromagnetic spin- 1/2 Heisenberg dimers and ladders. Using stochastic series expansion quantum Monte Carlo simulations, we obtain the critical scaling exponents which dictate the power-law dependence of the transition temperature on the magnetic field. These are compared with recent experiments on candidate materials and with predictions for the Bose-Einstein condensation of magnons. The critical exponents deviate from isotropic mean-field theory and exhibit different scaling behavior at the lower and upper critical magnetic fields.

Aperiodic nanophotonic design

We study field-induced magnetic order in cubic lattices of dimers with antiferromagnetic Heisenberg interactions. The thermal critical exponents at the quantum phase transition from a spin liquid to a magnetically ordered phase are determined from stochastic series expansion quantum Monte Carlo simulations. These exponents are independent of the interdimer coupling ratios, and converge to the value obtained by considering the transition as a Bose-Einstein condensation of magnons,

Quasiparticle bound states around impurities in d(x(2)-y(2))-wave superconductors

{\ensuremath{\alpha}}_{\mathrm{BEC}}=\frac{3}{2}.