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D. Foerster

Publications and source records attributed to D. Foerster.

9 recordsLinked to original sources
arXiv2001

The emergence of charged collective modes from a large N extrapolation of the Hubbard model

We consider a symplectic extrapolation of the Hubbard model of N fold replicated electrons and solve this model exactly in two special cases, at N=infinity in the bosonic sector and for any N on a dimer of two points. At N=infinity we find a multiplet of collective modes that contains neutral spin fluctuations and charged pair fluctuations that are degenerate with each other at zero doping. Our solution of the symplectic model on a dimer of two points for any N interpolates smoothly between N=1 and N=infinity without any visible discontinuity. These results suggest that the inclusion of charged pairing modes in weakly doped antiferromagnets is essential and that an expansion about the N=infinity limit is appropriate in this context.

cond-mat.str-el↗
arXiv2000

Metal insulator transition in TlSr2CoO5 from orbital degeneracy and spin disproportionation

To describe the metal insulator transition in the new oxide TlSr2CoO5 we investigate its electronic structure by LDA and model Hartree-Fock calculations. Within LDA we find a homogeneous metallic and ferromagnetic ground state, but when including the Coulomb interaction more explicitly within the Hartree-Fock approximation, we find an insulating state of lower energy with both spin and orbital order. We also interpret our results in terms of a simple model.

cond-mat.str-el↗
arXiv2000

A planar extrapolation of the correlation problem that permits pairing

It was observed previously that an SU(N) extension of the Hubbard model is dominated, at large N, by planar diagrams in the sense of 't Hooft, but the possibility of superconducting pairing got lost in this extrapolation. To allow for this possibility, we replace SU(N) by U(N,q), the unitary group in a vector space of quaternions. At the level of the free energy, the difference between the SU(N)and U(N,q) extrapolations appears only to first nonleading order in N.

cond-mat.str-el↗
arXiv1999

A planar diagram approach to the correlation problem

We transpose an idea of 't Hooft from its context of Yang and Mills' theory of strongly interacting quarks to that of strongly correlated electrons in transition metal oxides and show that a Hubbard model of N interacting electron species reduces, to leading orders in N, to a sum of almost planar diagrams. The resulting generating functional and integral equations are very similar to those of the FLEX approximation of Bickers and Scalapino. This adds the Hubbard model at large N to the list of solvable models of strongly correlated electrons. PACS Numbers: 71.27.+a 71.10.-w 71.10.Fd

cond-mat.str-el↗
arXiv1998

Magnetic Structure and NMR signal of Spin Peierls Solitons

We compute the magnetic profile of spin Peierls solitons in a simple Heisenberg model with magneto elastic couplings, using independently the DMRG method and the Hartree Fock approximation. Our results agree qualitatively with published NMR data provided we average over the spin profiles. We conclude that he dynamics of the spin plus lattice system must be included in a more detailed theory of the spin Peierls transition in CuGeO.

cond-mat.str-el↗
arXiv1997

Can Majorana Fermions correctly describe the ordered state of an antiferromagnetic Heisenberg chain?

To check the reliability of the Majorana representation of quantum spin systems, we use it to compute the ground state energy of an antiferromagnetic spin 1/2 chain to one loop order. We find a very small one loop correction of the mean field energy and a discrepancy of >100% compared to the Bethe Ansatz result. We conclude that a careful handling of the gauge degrees of freedom of this representation is crucial to get correct results.

cond-mat.str-el↗
arXiv1996

The Quantum O(N) Heisenberg Antiferromagnet for N -> infinityy

We study the O(N) quantum Heisenberg antiferromagnet using a parametrisation in terms of real fermions. The N->infinity limit of the model is controlled by a saddle point that is infinitely degenerate in many cases and which represents singlet states on dimers that cover the lattice. Our results are similar to results reported previously on an SU(N) generalisation of the quantum Heisenberg antiferromagnet for N->infinity.

cond-mat.str-el↗
arXiv1996

Two-dimensional S=1/2 antiferromagnet on a plaquette lattice

We consider a simplified model of the magnetic structure of the two-dimensional compound $CaV_4O_9$ in terms of interacting square plaquettes of spins with two distinct antiferromagnetic exchange constants. We analyze the competition between two types of singlet ground states and the Neel ordered one in terms of respectively, numerical cluster expansion and nonlinear spin wave theory. The resulting phase diagram agrees well with known Quantum Monte Carlo results and suggests a first order transition between ordered and singlet ground states as a function of the exchange constants.

cond-mat↗