We study the extended Hubbard model with both on-site and nearest neighbor Coulomb repulsion (U and V , respectively) in the Dynamical Mean Field theory. At quarter filling, the model shows a transition to a charge ordered phase with different sublattice occupancies nA = nB. The effective mass increases drastically at the critical V and a pseudo-gap opens in the single-particle spectral function for higher values of V . The Vc(T )-curve has a negative slope for small temperatures, i.e. the charge ordering transition can be driven by increasing the temperature. This is due to the higher spin-entropy of the charge ordered phase.PACS 71.10.Fd, 71.27.+a, 71.45.Lr The possibility of crystallization of electrons due to their long-range Coulomb repulsion was first proposed by Wigner [1]. He considered an electron system in a uniform positive background at sufficiently low densities. The Wigner lattice is formed when the gain in Coulomb energy due to the localization of the electrons exceeds the gain in kinetic energy for the homogeneous electron distribution. It is experimentally realized in the two dimensional electron gas in a GaAs/AlGaAs heterostructure [2]. Due to the reduced dimensionality, the effect of the Coulomb interaction is enhanced so that the transition to the ordered state occurs at experimentally accessible electron densities.Crystallization of charge carriers (charge ordering) can also be observed in three dimensional systems, even at very high densities [3]. Here the kinetic energy of the electrons or holes has to be reduced drastically for the charge ordered state to become possible. In 4f-electron systems it is the small hybridization of the well localized 4f-orbitals that leads to a reduced kinetic energy. An example is Yb 4 As 3 where a first order charge ordering transition occurs at T c ≈ 295K [4,5]. The carrier concentration in Yb 4 As 3 (approximately one hole per four Yb ions) is considerably larger than typical values for a Wigner lattice. The kinetic energy of the electrons can also be reduced by the interaction with lattice and spin degrees of freedom. An interplay of these mechanisms is responsible for the charge order transition occurring in a variety of rare earth manganites (e.g., in La 1−x Ca x MnO 3 for x ≥ 0.5 [6]).In all examples mentioned so far, the charge ordered phase is the ground state. However, a melting of the charge ordered state on decreasing the temperature (i.e. a reentrant transition) has been found recently in In this Letter, we investigate the simplest model which allows for a charge ordering transition due to the competition between kinetic and Coulomb energy. The extended Hubbard model [9]describes fermions on a lattice with an on-site Coulomb repulsion U , a nearest neighbor Coulomb repulsion V and a hopping matrix element t. The c † iσ (c iσ ) denote creation (annihilation) operators for a fermion at site i with spin σ, the n i are defined as n i = n i↑ + n i↓ where n iσ = c † iσ c iσ and indicates the sum over nearest neighbors.In the following we study ...

We prove the existence of an ordered low temperature phase in a model of soft-self-avoiding closed random surfaces on a cubic lattice by a suitable extension of Peierls contour method. The statistical weight of each surface configuration depends only on the mean extrinsic curvature and on an interaction term arising when two surfaces touch each other along some contour. The model was introduced by F.J. Wegner and G.K. Savvidy as a lattice version of the gonihedric string, which is an action for triangulated random surfaces. * email: pietig@hybrid.tphys.uni-heidelberg.de †

We investigate a model of closed (d − 1)-dimensional soft-self-avoiding random surfaces on a d-dimensional cubic lattice. The energy of a surface configuration is given by E = J(n 2 + 4k n 4 ), where n 2 is the number of edges, where two plaquettes meet at a right angle and n 4 is the number of edges, where 4 plaquettes meet. This model can be represented as a Z 2 -spin system with ferromagnetic nearest-neighbour-, antiferromagnetic next-nearest-neighbour-and plaquette-interaction. It corresponds to a special case of a general class of spin systems introduced by Wegner and Savvidy. Since there is no term proportional to the surface area, the bare surface tension of the model vanishes, in contrast to the ordinary Ising model. By a suitable adaption of Peierls argument, we prove the existence of infinitely many ordered low temperature phases for the case k = 0. A low temperature expansion of the free energy in 3 dimensions up to order x 38 (x = e −βJ ) shows, that for k > 0 only the ferromagnetic low temperature phases remain stable. An analysis of low temperature expansions up to order x 44 for the magnetization, susceptibility and specific heat in 3 dimensions yields critical exponents, which are in agreement with previous results.

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