Heat Capacity and Magnetic Phase Diagram of the Low-Dimensional Antiferromagnet Y2_2BaCuO5_5

A study by specific heat of a polycrystalline sample of the low-dimensional magnetic system Y$_2$BaCuO$_5$ is presented. Magnetic fields up to 14 T are applied and permit to extract the ($T$,$H$) phase diagram. Below $\mu_0H^*\simeq2$ T, the N\'eel temperature, associated with a three-dimensional antiferromagnetic long-range ordering, is constant and equals $T_N=15.6$ K. Above $H^*$, $T_N$ increases linearly with $H$ and a field-induced increase of the entropy at $T_N$ is related to the presence of an isosbestic point at $T_X\simeq20$ K, where all the specific heat curves cross. A comparison is made between Y$_2$BaCuO$_5$ and the quasi-two-dimensional magnetic systems BaNi$_{2}$V$_{2}$O$_{8}$, Sr$_2$CuO$_2$Cl$_2$, and Pr$_2$CuO$_4$, for which very similar phase diagrams have been reported. An effective field-induced magnetic anisotropy is proposed to explain these phase diagrams.Comment: 14 pages, 7 figure

Chlamydia psittaci: a relevant cause of community-acquired pneumonia in two Dutch hospitals.

Of all hospitalised community-acquired pneumonias (CAPs) only a few are known to be caused by Chlamydia psittaci. Most likely the reported incidence, ranging from of 0% to 2.1%, is an underestimation of the real incidence, since detection of psittacosis is frequently not incorporated in the routine microbiological diagnostics in CAP or serological methods are used

Shot noise in ferromagnetic single electron tunneling devices

Frequency dependent current noise in ferromagnetic double junctions with Coulomb blockade is studied theoretically in the limit of sequential tunneling. Two different relaxation processes are found in the correlations between spin polarized tunneling currents; low frequency spin fluctuations and high frequency charge fluctuations. Spin accumulation in strongly asymmetric junctions is shown to lead to a negative differential resistance. We also show that large spin noise activated in the range of negative differential resistance gives rise to a significant enhancement of the current noise.Comment: 8 pages, 13 eps-figures include

Free energy barrier for melittin reorientation from a membrane-bound state to a transmembrane state

An important step in a phospholipid membrane pore formation by melittin antimicrobial peptide is a reorientation of the peptide from a surface into a transmembrane conformation. In this work we perform umbrella sampling simulations to calculate the potential of mean force (PMF) for the reorientation of melittin from a surface-bound state to a transmembrane state and provide a molecular level insight into understanding peptide and lipid properties that influence the existence of the free energy barrier. The PMFs were calculated for a peptide to lipid (P/L) ratio of 1/128 and 4/128. We observe that the free energy barrier is reduced when the P/L ratio increased. In addition, we study the cooperative effect; specifically we investigate if the barrier is smaller for a second melittin reorientation, given that another neighboring melittin was already in the transmembrane state. We observe that indeed the barrier of the PMF curve is reduced in this case, thus confirming the presence of a cooperative effect

Wang-Landau simulation for the quasi-one-dimensional Ising model

We revisit the nature of the quasi-one-dimensional Ising model on the basis of Wang-Landau simulation. We introduce the density of states in the two-dimensional energy space corresponding to the intra- and inter-chain directions. We then analyze the interchain coupling dependence of the specific heat of the anistropic two-dimensional Ising model in the context of the density of states, and further discuss the size dependences of the peak temperature. We also discuss the feature of the phase transition in the three-dimensional case.Comment: 7 pages, 8 figures, to appear in J. Phys. Soc. Jp

Superconductivity in an exactly solvable Hubbard model with bond-charge interaction

The Hubbard model with an additional bond-charge interaction $X$ is solved exactly in one dimension for the case $t=X$ where $t$ is the hopping amplitude. In this case the number of doubly occupied sites is conserved. In the sector with no double occupations the model reduces to the $U=\infty$ Hubbard model. In arbitrary dimensions the qualitative form of the phase diagram is obtained. It is shown that for moderate Hubbard interactions $U$ the model has superconducting ground states.Comment: Revtex, 14 pages, 1 figure (uuencoded compressed tar-file

Ga NMR study of the local susceptibility in SrCr8Ga4O19: pseudogap and paramagnetic defects

We present the first Ga(4f) NMR study of the Cr susceptibility in the archetype of Kagome based frustrated antiferromagnets, SrCr$_{8}$Ga$_{4}$O$_{19}$. Our major finding is that the susceptibility of the frustrated lattice goes through a maximum around 50 K. Our data also supports the existence of paramagnetic ``clusters'' of spins, responsible for the Curie behavior observed in the macroscopic susceptibility at low T. These results set novel features for the constantly debated physics of geometrically frustrated magnets.Comment: 4 pages, 5 figures Submitted to Phys. Rev. Let

Specific heat and magnetization study on single crystals of a frustrated, quasi one-dimensional oxide: Ca3Co2O6

Specific heat and magnetization measurements have been carried out under a range of magnetic fields on single crystals of Ca3Co2O6. This compound is composed of Ising magnetic chains that are arranged on a triangular lattice. The intrachain and interchain couplings are ferromagnetic and antiferromagnetic, respectively. This situation gives rise to geometrical frustration, that bears some similarity to the classical problem of a two-dimensional Ising triangular antiferromagnet. This paper reports on the ordering process at low-T and the possibility of one-dimensional features at high-T.Comment: 7 pages, 6 figures, accepted for publication in PR

Quantum disorder in the two-dimensional pyrochlore Heisenberg antiferromagnet

We present the results of an exact diagonalization study of the spin-1/2 Heisenberg antiferromagnet on a two-dimensional version of the pyrochlore lattice, also known as the square lattice with crossings or the checkerboard lattice. Examining the low energy spectra for systems of up to 24 spins, we find that all clusters studied have non-degenerate ground states with total spin zero, and big energy gaps to states with higher total spin. We also find a large number of non-magnetic excitations at energies within this spin gap. Spin-spin and spin-Peierls correlation functions appear to be short-ranged, and we suggest that the ground state is a spin liquid.Comment: 7 pages, 11 figures, RevTeX minor changes made, Figure 6 correcte