Complex wave function, Chiral spin order parameter and Phase Problem

We study the two dimensional Hubbard model by use of the ground state algorithm in the Monte Carlo simulation. We employ complex wave functions as trial function in order to have a close look at properties such as chiral spin order ($\chi$SO) and flux phase. For half filling, a particle-hole transformation leads to sum rules with respect to the Green's functions for a certain choice of a set of wave functions. It is then analytically shown that the sum rules lead to the absence of the $\chi$SO. Upon doping, we are confronted with the sign problem, which in our case %ch appears as a ``phase problem" due to the phase of the Monte Carlo weights. The average of the phase shows an exponential decay as a function of inverse temperature similarly to that of sign by Loh Jr. et. al. . We compare the numerical results with those of exact numerical calculations.Comment: 28 pages, 9 figures(hard copy will be available upon request

Imaging phase separation near the Mott boundary in the correlated organic superconductors κ\kappa-(BEDT-TTF)2_{2}X

Electronic phase separation consisting of the metallic and insulating domains with 50 -- 100 $\mu$m in diameter is found in the organic Mott system $\kappa$-[($h$8-BEDT-TTF)$_{1-x}$($d$8-BEDT-TTF)$_{x}$]$_{2}$Cu[N(CN)$_{2}$]Br by means of scanning micro-region infrared spectroscopy using the synchrotron radiation. The phase separation appears below the critical end temperature 35 -- 40 K of the first order Mott transition. The observation of the macroscopic size of the domains indicates a different class of the intrinsic electronic inhomogeneity from the nano-scale one reported in the inorganic Mott systems such as High-$T_{c}$ copper and manganese oxides.Comment: 4 pages, 3 figure

Optical probe of carrier doping by X-ray irradiation in organic dimer Mott insulator κ\kappa-(BEDT-TTF)2_{2}Cu[N(CN)2]_{2}]Cl

We investigated the infrared optical spectra of an organic dimer Mott insulator $\kappa$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Cl, which was irradiated with X-rays. We observed that the irradiation caused a large spectral weight transfer from the mid-infrared region, where interband transitions in the dimer and Mott-Hubbard bands take place, to a Drude part in a low-energy region; this caused the Mott gap to collapse. The increase of the Drude part indicates a carrier doping into the Mott insulator due to irradiation defects. The strong redistribution of the spectral weight demonstrates that the organic Mott insulator is very close to the phase border of the bandwidth-controlled Mott transition.Comment: 4 pages, 4 figure

Formation of Primordial Protostars

The evolution of collapsing metal free protostellar clouds is investigated for various masses and initial conditions. We perform hydrodynamical calculations for spherically symmetric clouds taking account of radiative transfer of the molecular hydrogen lines and the continuum, as well as of chemistry of the molecular hydrogen. The collapse is found to proceed almost self-similarly like Larson-Penston similarity solution. In the course of the collapse, efficient three-body processes transform atomic hydrogen in an inner region of \sim 1 M_{\sun} entirely into molecular form. However, hydrogen in the outer part remains totally atomic although there is an intervening transitional layer of several solar masses, where hydrogen is in partially molecular form. No opaque transient core is formed although clouds become optically thick to H$_{2}$ collision-induced absorption continuum, since H$_{2}$ dissociation follows successively. When the central part of the cloud reaches stellar densities ($\sim 10^{-2} {\rm g cm^{-3}}$), a very small hydrostatic core (\sim 5 \times 10^{-3} M_{\sun}) is formed and subsequently grows in mass as the ambient gas accretes onto it. The mass accretion rate is estimated to be 3.7 \times 10^{-2} M_{\sun} {\rm yr^{-1}} (M_{\ast}/M_{\sun})^{-0.37}, where $M_{\ast}$ is instantaneous mass of the central core, by using a similarity solution which reproduces the evolution of the cloud before the core formation.Comment: 20 pages, 5 Postscript figures, uses AAS LaTe

Mott transition and superconductivity in the strongly correlated organic superconductor κ\kappa-(BEDT-TTF)2_{2}Cu[N(CN)2]_{2}]Br

The magnetic field effect on the phase diagram of the organic Mott system $\kappa$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Br in which the bandwidth was tuned by the substitution of deuterated molecules was studied by means of the resistivity measurements performed in magnetic fields. The lower critical point of the first-order Mott transition, which ended on the upper critical field $H_{\rm c2}$-temperature plane of the superconductivity, was determined experimentally in addition to the previously observed upper critical end point. The lower critical end point moved to a lower temperature with the suppression of $T_{\rm c}$ in magnetic fields and the Mott transition recognized so far as the $S$-shaped curve reached $T =$ 0 when $H > H_{\rm c2}$ in the end.Comment: 5 pages, 4 figure

Disorder Effect on the Vortex Pinning by the Cooling Process Control in the Organic Superconductor κ\kappa-(BEDT-TTF)2_2Cu[N(CN)2_2]Br

We investigate the influence of disorders in terminal ethylene groups of BEDT-TTF molecules (ethylene-disorders) on the vortex pinning of the organic superconductor $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Br. Magnetization measurements are performed under different cooling-processes. The second peak in the magnetization hysteresis curve is observed for all samples studied, and the hysteresis width of the magnetization becomes narrower by cooling faster. In contradiction to the simple pinning effect of disorder, this result shows the suppression of the vortex pinning force by introducing more ethylene-disorders. The ethylene-disorder domain model is proposed for explaining the observed result. In the case of the system containing a moderate number of the ethylene-disorders, the disordered molecules form a domain structure and it works as an effective pinning site. On the contrary, an excess number of the ethylene-disorders may weaken the effect of the domain structure, which results in the less effective pinning force on the vortices.Comment: 6 pages, 6 figure

From ballistic to Brownian vortex motion in complex oscillatory media

We show that the breaking of the rotation symmetry of spiral waves in two-dimensional complex (period-doubled or chaotic) oscillatory media by synchronization defect lines (SDL) is accompanied by an intrinsic drift of the pattern. Single vortex motion changes from ballistic flights at a well-defined angle from the SDL to Brownian-like diffusion when the turbulent character of the medium increases. It gives rise, in non-turbulent multi-spiral regimes, to a novel ``vortex liquid''.Comment: 5 pages, 4 figure