K. W. Becker

Magnetic properties and spin waves of bilayer magnets in a uniform field

Abstract:The two-layer square lattice quantum antiferromagnet with spins 12 shows a zero-field magnetic order-disorder transition at a critical ratio of the inter-plane to intra-plane couplings. Adding a uniform magnetic field tunes the system to canted antiferromagnetism and eventually to a fully polarized state; similar behavior occurs for ferromagnetic intra-plane coupling. Based on a bond operator spin representation, we propose an approximate ground state wavefunction which consistently covers all phases by means of a unitary transformation. The excitations can be efficiently described as independent bosons; in the antiferromagnetic phase these reduce to the well-known spin waves, whereas they describe gapped spin-1 excitations in the singlet phase. We compute the spectra of these excitations as well as the magnetizations throughout the whole phase diagram.

Magnetism of a tetrahedral cluster spin chain

We discuss the magnetic properties of a dimerized and completely frustrated tetrahedral spin-1/2 chain. Using a combination of exact diagonalization and bond-operator theory the quantum phase diagram is shown to incorporate a singlet-product, a dimer, and a Haldane phase. In addition we consider one-, and two-triplet excitations in the dimer phase and evaluate the magnetic Raman cross section which is found to be strongly renormalized by the presence of a two-triplet bound state. The link to a novel tellurate materials is clarified.

Renormalization approach to many-particle systems

This paper presents a renormalization approach to many-particle systems. By starting from a bare Hamiltonian H=H 0 +H 1 with an unperturbed part H 0 and a perturbation H 1 , we define an effective Hamiltonian which has a band-diagonal shape with respect to the eigenbasis of H 0 . This means that all transition matrix elements are suppressed which have energy differences larger than a given cutoff λ that is smaller than the cutoff A of the original Hamiltonian. This property resembles a recent flow equation approach on the basis of continuous unitary transformations. For demonstration of the method we discuss an exact solvable model, as well as the Anderson-lattice model where the well-known quasiparticle behavior of heavy fermions is derived.

Spectral signatures of the BCS-BEC crossover in the excitonic insulator phase of the extended Falicov-Kimball model

We explore the spontaneous formation of an excitonic insulator state at the semimetalsemiconductor transition of mixed-valence materials in the framework of the spinless Falicov-Kimball model with direct f-f electron hopping. Adapting the projector-based renormalization method, we obtain a set of renormalization differential equations for the extended Falicov-Kimball model parameters and finally derive analytical expressions for the order parameter, as well as for the renormalized c- and f-electron dispersions, momentum distributions, and wave-vector resolved single-particle spectral functions. Our numerical results proved the valence transition picture, related to the appearance of the excitonic insulator phase, in the case of overlapping c and f bands. Thereby the photoemission spectra show significant differences between the weak-to-intermediate and intermediateto-strong Coulomb attraction regimes, indicating a BCS-BEC transition of the excitonic condensate.

The dynamics of a hole in a CuO4 plaquette: electron energy-loss spectroscopy of Li2CuO2

We have measured the energy- and momentum-dependent loss function of

Exciton condensation due to electron-phonon interaction

{\mathrm{Li}}_{2}{\mathrm{CuO}}_{2}

Excitonic resonances in the 2D extended Falicov-Kimball model

single crystals by means of electron energy-loss spectroscopy in transmission. Using the same values for the model parameters, the low-energy features of the spectrum as well as published Cu

Single-particle excitations and phonon softening in the one-dimensional spinless Holstein model

{2p}_{3/2}

Valence transition in the periodic Anderson model

x-ray photoemission data of

Dominant particle-hole contributions to the phonon dynamics in the spinless one-dimensional Holstein model

{\mathrm{Li}}_{2}{\mathrm{CuO}}_{2}