Superconductivity and incommensurate spin fluctuations in a generalized t-J model for the cuprates

We consider the slave-fermion Schwinger-boson decomposition of an effective model obtained through a systematic low-energy reduction of the three-band Hubbard Hamiltonian. The model includes a three-site term t'' similar to that obtained in the large-U limit of the Hubbard model but of opposite sign for realistic or large O-O hopping. For parameters close to the most realistic ones for the cuprates, the mean-field solution exhibits d+s superconductivity (predominantly d_{x^2-y^2}) with a dependence on doping x very similar to the experimentally observed. We also obtained incommensurate peaks at wave vectors near $\pi (1,1 +(-) 2x)$ in the spin structure factor, which also agree with experiment.Comment: 9 pages, latex, 2 figures, to appear in Europhys. Let

Magnetocaloric effect in Gd/W thin film heterostructures

In an effort to understand the impact of nanostructuring on the magnetocaloric effect, we have grown and studied gadolinium in MgO/W(50 $\textrm{\AA}$)/[Gd(400 $\textrm{\AA}$)/W(50 $\textrm{\AA}$)]$_8$ heterostructures. The entropy change associated with the second order magnetic phase transition was determined from the isothermal magnetization for numerous temperatures and the appropriate Maxwell relation. The entropy change peaks at a temperature of 284 K with a value of approximately 3.4 J/kg-K for a 0-30 kOe field change; the full width at half max of the entropy change peak is about 70 K, which is significantly wider than that of bulk Gd under similar conditions. The relative cooling power of this nanoscale system is about 240 J/kg, somewhat lower than that of bulk Gd (410 J/kg). An iterative Kovel-Fisher method was used to determine the critical exponents governing the phase transition to be $\beta=0.51$, and $\gamma=1.75$. Along with a suppressed Curie temperature relative to the bulk, the fact that the convergent value of $\gamma$ is that predicted by the 2-D Ising model may suggest that finite size effects play an important role in this system. Together, these observations suggest that nanostructuring may be a promising route to tailoring the magnetocaloric response of materials

Effect of nonmagnetic impurities on stripes in high-Tc cuprates

We perform the numerically exact diagonalization study of the t-J model with nonmagnetic impurities to clarify the relation between Zn impurities and the stripes. By examining the hole-hole correlation function for a two-hole \sqrt{18}x\sqrt{18} cluster with a single impurity, we find that the impurity has a tendency to stabilize vertical charge stripes. This tendency is caused by the gain of the kinetic energy of holes moving along the stripes that are formed avoiding the impurity.Comment: 3 pages including 2 figures. Proceedings for ISS2000 (Tokyo, October 2000). To be published in Physica

Temperature Derivative of the Superfluid Density in the Attractive Hubbard model

Based on extensions of the grand-canonical Quantum Monte-Carlo algorithm to incorporate magnetic fields, we provide numerical data confirming the existence of a Kosterlitz-Thouless transition in the attractive Hubbard model. Here, we calculate the temperature derivative of the superfluid density, to pin down the transition. Away from half-band filling, the above quantity, shows a response which increases with lattice size at the transition temperature. In contrast, such a signal is not observed for the case of a half-band filling.Comment: Latex 8 pages, 3 figures (in postscript format) appendded at the end of the fil

Comment on "Why is the DNA denaturation transition first order?"

In this comment we argue that while the conclusions in the original paper (Y. Kafri, D. Mukamel and L. Peliti, Phys. Rev. Lett. 85, 4988 (2000)) are correct for asymptotically long DNA chains, they do not apply to the chains used in typical experiments. In the added last paragraph, we point out that for real DNA the average distance between denatured loops is not of the order of the persistence length of a single-stranded chain but much larger. This corroborates our reasoning that the double helix between loops is quite rigid, and thereby our conclusion.Comment: 1 page, REVTeX. Last paragraph adde