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

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

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]

Association between life skills and academic performance in adolescents in the autonomous community of aragon (Spain)

Background: Learning and socio-emotional development is promoted through the creation and nurturing of an optimal school climate. This study aims to analyze the relationship between life skills and academic performance in a large sample of adolescents from the autonomous community of Aragón (Spain). Methods: A cross-sectional study was conducted on the life skills and academic performance of a sample of 7th and 8th grade middle school students during the academic year 2018–2019. A sample of 43 middle schools were randomly selected; the final sample comprised 1745 students. The following data were collected through an anonymized, previously validated questionnaire: sociodemographic variables, social skills, self-efficacy, affective balance, and academic performance. Results: We found a statistically significant association between life skills and academic performance (p < 0.001) in our sample. We also observed significant gender differences in life skills, with boys obtaining higher scores in cognitive skills and affective balance, and with girls achieving higher scores in social skills. Conclusion: We argue that life skills should be integrated into educational policies in order to improve the academic performance and health outcomes of students. © 2021 by the authors. Licensee MDPI, Basel, Switzerland

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

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.

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