Effects of domain walls on hole motion in the two-dimensional t-J model at finite temperature

The t-J model on the square lattice, close to the t-J_z limit, is studied by quantum Monte Carlo techniques at finite temperature and in the underdoped regime. A variant of the Hoshen-Koppelman algorithm was implemented to identify the antiferromagnetic domains on each Trotter slice. The results show that the model presents at high enough temperature finite antiferromagnetic (AF) domains which collapse at lower temperatures into a single ordered AF state. While there are domains, holes would tend to preferentially move along the domain walls. In this case, there are indications of hole pairing starting at a relatively high temperature. At lower temperatures, when the whole system becomes essentially fully AF ordered, at least in finite clusters, holes would likely tend to move within phase separated regions. The crossover between both states moves down in temperature as doping increases and/or as the off-diagonal exchange increases. The possibility of hole motion along AF domain walls at zero temperature in the fully isotropic t-J is discussed.Comment: final version, to appear in Physical Review

Detection of pairing correlation in the two-dimensional Hubbard model

Quantum Monte Carlo method is used to re-examine superconductivity in the single-band Hubbard model in two dimensions. Instead of the conventional pairing, we consider a `correlated pairing', \langle \tilde{c}_{i\uparrow} \tilde{c}_{i'\downarrow} %\tilde{c}_{j'\downarrow}^\dagger \tilde{c}_{j \uparrow}^\dagger \rangle with $\tilde{c}_{i\sigma} \equiv c_{i\sigma}(1-n_{i-\sigma})$, which is inferred from the $t$-$J$ model, the strong-coupling limit of the Hubbard model. The pairing in the $d$-wave channel is found to possess both a divergence like $1/T$ in the pairing susceptibility and a growth of the ground-state pairing correlation with sample size, indicating an off-diagonal long-range order near (but not exactly at) half-filling.Comment: 3 pages, revtex, 6 figures available on request from [email protected]

Quantum lattice fluctuations in a frustrated Heisenberg spin-Peierls chain

As a simple model for spin-Peierls systems we study a frustrated Heisenberg chain coupled to optical phonons. In view of the anorganic spin-Peierls compound CuGeO3 we consider two different mechanisms of spin-phonon coupling. Combining variational concepts in the adiabatic regime and perturbation theory in the anti-adiabatic regime we derive effective spin Hamiltonians which cover the dynamical effect of phonons in an approximate way. Ground-state phase diagrams of these models are determined, and the effect of frustration is discussed. Comparing the properties of the ground state and of low-lying excitations with exact diagonalization data for the full quantum spin phonon models, good agreement is found especially in the anti-adiabatic regime.Comment: 9 pages, 7 figures included, submitted to Phys. Rev.

Effect of nearest neighbor repulsion on the low frequency phase diagram of a quarter-filled Hubbard-Holstein chain

We have studied the influence of nearest-neighbor (NN) repulsion on the low frequency phase diagram of a quarter-filled Hubbard-Holstein chain. The NN repulsion term induces the apparition of two new long range ordered phases (one $4k_F$ CDW for positive $U_{eff} = U-2g^2/\omega$ and one $2k_F$ CDW for negative $U_{eff}$) that did not exist in the V=0 phase diagram. These results are put into perspective with the newly observed charge ordered phases in organic conductors and an interpretation of their origin in terms of electron-molecular vibration coupling is suggested.Comment: 10 pages, 10 figure

Ferromagnetic Polarons in Manganites

Using the Lanczos method in linear chains we study the double exchange model in the low concentration limit, including an antiferromagnetic super-exchange K. In the strong coupling limit we find that the ground state contains ferromagnetic polarons whose length is very sensitive to the value of K/t. We investigate the dispersion relation, the trapping by impurities, and the interaction between these polarons. As the overlap between polarons increases, by decreasing K/t, the effective interaction between them changes from antiferromagnetic to ferromagnetic. The scaling to the thermodynamic limit suggests an attractive interaction in the strong coupling regime (J_h > t) and no binding in the weak limit (J_h \simeq t).Comment: 12 pages, accepted in PRB, to be published in Novembe

Anisotropic colloids through non-trivial buckling

We present a study on buckling of colloidal particles, including experimental, theoretical and numerical developments. Oil-filled thin shells prepared by emulsion templating show buckling in mixtures of water and ethanol, due to dissolution of the core in the external medium. This leads to conformations with a single depression, either axisymmetric or polygonal depending on the geometrical features of the shells. These conformations could be theoretically and/or numerically reproduced in a model of homogeneous spherical thin shells with bending and stretching elasticity, submitted to an isotropic external pressure.Comment: submitted to EPJ

Stripes and holes in a two-dimensional model of spinless fermions and hardcore bosons

We consider a Hubbard-like model of strongly-interacting spinless fermions and hardcore bosons on a square lattice, such that nearest neighbor occupation is forbidden. Stripes (lines of holes across the lattice forming antiphase walls between ordered domains) are a favorable way to dope this system below half-filling. The problem of a single stripe can be mapped to a spin-1/2 chain, which allows understanding of its elementary excitations and calculation of the stripe's effective mass for transverse vibrations. Using Lanczos exact diagonalization, we investigate the excitation gap and dispersion of a hole on a stripe, and the interaction of two holes. We also study the interaction of two, three, and four stripes, finding that they repel, and the interaction energy decays with stripe separation as if they are hardcore particles moving in one (transverse) direction. To determine the stability of an array of stripes against phase separation into particle-rich phase and hole-rich liquid, we evaluate the liquid's equation of state, finding the stripe-array is not stable for bosons but is possibly stable for fermions.Comment: 24 pages, 18 figure

Present distribution of the populations of Patella candei D´Orbigny, 1840 (Mollusca, Gastropoda) in the Canary Islands. A species in danger of extinction

A report on the conditions and the spatial distribution of the assemblages of an intertidal limpet Patella candei D´Orbigny, 1840 is presented. The study of this species, classified as a species in danger of extinction, was carried out in order to take steps about its recovery and preservation. This dangerous situation, due to the shellfish exceedings of the canarian limpet species, has caused an important decrease in their assemblages. A total of 3 617 specimens of Patella candei were found around the littoral of Fuerteventura and Lobos islet. The best structured assemblages were located in areas far away from centres of population, Monumento Natural de los Cuchillos de Vigán in the east, Parque Natural de Jandía in the south.Se realiza un estudio sobre el estado y la distribución espacial de las poblaciones de la lapa intermareal Patella candei D'Orbigny, 1840, catalogada como especie en peligro de extinción, para aportar información y establecer medidas adecuadas para su recuperación y conservación. Esta crítica situación, consecuencia de la sobreexplotación marisquera de las especies de lapas en Canarias, ha provocado una disminución alarmante de sus efectivos. Se cartografiaron 3 617 ejemplares de P. candei en todo el litoral de Fuerteventura y del islote de Lobos. Las poblaciones mejor estructuradas se encontraron en áreas situadas lejos de núcleos urbanos, como son el Monumento Natural de los Cuchillos de Vigán, en la vertiente este, y el Parque Natural de Jandía, en el sur.Instituto Español de Oceanografí

Communication skills training in undergraduate nursing programs in Spain

Objectives of the present cross sectional study were to analyze the offer of subjects with communication skills in the nursing degree in Spain and to describe the distribution of these subjects. Documentary, systematic and independent search of web pages from Spanish universities was carried out to collect the following variables: subjects with communication skills, course in which the subject is taught, type of teaching (exclusive or combined), type of subject (compulsory or optional) and number of credits on communication skills. Although an average of 3.6 subjects per center was offered, most of the subjects were combined and with little communication content load. In one third of the centers, the offer was below 2.3 credits. Only 1 in 6 centers had exclusive communication skills subjects, and a quarter of them were optional. The teaching load of communication contents was highest in optional subjects. The offer of communication skills contents in Spanish Nursing Schools was scarce and very heterogeneous between centers and between courses in a center, with excessive presence of combined and optional subjects. Our results may be useful when developing the teaching guides for subjects with communication skills, as well as when defining communication competencies in the different Nursing Schools