Correlated Partial Disorder in a Weakly Frustrated Quantum Antiferromagnet.

Partial disorder-the microscopic coexistence of long-range magnetic order and disorder-is a rare phenomenon that has been experimentally and theoretically reported in some Ising- or easy plane-spin systems, driven by entropic effects at finite temperatures. Here, we present an analytical and numerical analysis of the S=1/2 Heisenberg antiferromagnet on the sqrt[3]×sqrt[3]-distorted triangular lattice, which shows that its quantum ground state has partial disorder in the weakly frustrated regime. This state has a 180° Néel ordered honeycomb subsystem coexisting with disordered spins at the hexagon center sites. These central spins are ferromagnetically aligned at short distances, as a consequence of a Casimir-like effect originated by the zero-point quantum fluctuations of the honeycomb lattice.

Scopolamine for uterine involution of dairy cows.

The objective of this study was to evaluate the effect of administration of scopolamine (a parasympatholytic drug) immediately after delivery, on uterine involution in dairy cows. The cows were divided into two groups: group T (treated with scopolamine butylbromide 40 mg/100 kg P.V., within 24 h of delivery) and group C (treated with 2 ml/q of saline solution). The cows were monitored at T0, within 24 h of delivery, T7, T14, T28 and T40 respectively 7th, 14th, 28th and 40th day postpartum, by measuring the levels of hydroxyproline (HYP), an important marker of uterine involution; by ultrasound examination for the measurement of the diameter of uterine horns and of blood flow in the middle uterine artery; and by evaluation of reproductive indices. HYP showed higher concentrations in the T group than in the C group at all times, but the difference was significant at T7 and T40. Uterine diameters were reduced from T7 to T40, with lower values in the T group, as compared to the C group; the pulsatility index (PI) of the middle uterine artery increased in the T group compared to the C group, with a statistically significant difference at T7. There was a better conception-delivery interval and higher pregnancy rate in the T group compared to the C group. It is likely that scopolamine blocked the uterine contractions in postpartum, for the duration of its half-life. After cessation of its pharmacological effect, the uterus may have started to contract more effectively. These results suggested that scopolamine could be useful as a pharmacological approach to postpartum management.

Classical antiferromagnetism in kinetically frustrated electronic models.

We study, by means of the density matrix renormalization group, the infinite U Hubbard model--with one hole doped away from half filling--in triangular and square lattices with frustrated hoppings, which invalidate Nagaoka's theorem. We find that these kinetically frustrated models have antiferromagnetic ground states with classical local magnetization in the thermodynamic limit. We identify the mechanism of this kinetic antiferromagnetism with the release of the kinetic energy frustration, as the hole moves in the established antiferromagnetic background. This release can occur in two different ways: by a nontrivial spin Berry phase acquired by the hole, or by the effective vanishing of the hopping amplitude along the frustrating loops.

Incommmensurability and unconventional superconductor to insulator transition in the hubbard model with bond-charge interaction.

We determine the quantum phase diagram of the one-dimensional Hubbard model with bond-charge interaction X in addition to the usual Coulomb repulsion U>0 at half-filling. For large enough X

Magnetization plateaus induced by a coupling to the lattice.

We present a novel mechanism for the appearance of magnetization plateaus in quasi-one-dimensional quantum spin systems, which is induced by the coupling to the underlying lattice. We investigate in detail a simple model of a frustrated spin-1/2 Heisenberg chain coupled to adiabatic phonons under an external magnetic field, but the present mechanism is expected to be more general. Using field theoretic methods complemented by extensive density matrix renormalization group techniques, we show that magnetization plateaus at nontrivial rational values of the magnetization can be stabilized by the lattice coupling. We suggest that such a scenario could be relevant for some low dimensional frustrated spin-Peierls compounds.

Superconductivity in the two-dimensional t-J model.

Using computational techniques, it is shown that pairing is a robust property of hole-doped antiferromagnetic insulators. In one dimension and for two-leg ladder systems, a BCS-like variational wave function with long-bond spin singlets and a Jastrow factor provides an accurate representation of the ground state of the t-J model, even though strong quantum fluctuations destroy the off-diagonal superconducting long-range order in this case. However, in two dimensions it is argued-and numerically confirmed using several techniques, especially quantum Monte Carlo-that quantum fluctuations are not strong enough to suppress superconductivity.

Doped stripes in models for the cuprates emerging from the one-hole properties of the insulator.

The extended and standard t-J models are computationally studied on ladders and planes, with emphasis on the small J/t region. At couplings compatible with photoemission results for undoped cuprates, half-doped stripes separating pi-shifted antiferromagnetic (AF) domains are found, as in Tranquada's interpretation of neutron experiments. Our main result is that the elementary stripe "building block" resembles the properties of one hole at small J/t, with robust AF correlations across the hole induced by the local tendency of the charge to separate from the spin. This suggests that the seed of half-doped stripes already exists in the unusual properties of the insulating parent compound.