Growth Dynamics of Photoinduced Domains in Two-Dimensional Charge-Ordered Conductors Depending on Stabilization Mechanisms

Photoinduced melting of horizontal-stripe charge orders in quasi-two-dimensional organic conductors \theta-(BEDT-TTF)2RbZn(SCN)4[BEDT-TTF=bis(ethylenedithio)tetrathiafulvalene] and \alpha-(BEDT-TTF)2I3 is investigated theoretically. By numerically solving the time-dependent Schr\"odinger equation, we study the photoinduced dynamics in extended Peierls-Hubbard models on anisotropic triangular lattices within the Hartree-Fock approximation. The melting of the charge order needs more energy for \theta-(BEDT-TTF)2RbZn(SCN)4 than for \alpha-(BEDT-TTF)2I3, which is a consequence of the larger stabilization energy in \theta-(BEDT-TTF)2RbZn(SCN)4. After local photoexcitation in the charge ordered states, the growth of a photoinduced domain shows anisotropy. In \theta-(BEDT-TTF)2RbZn(SCN)4, the domain hardly expands to the direction perpendicular to the horizontal-stripes. This is because all the molecules on the hole-rich stripe are rotated in one direction and those on the hole-poor stripe in the other direction. They modulate horizontally connected transfer integrals homogeneously, stabilizing the charge order stripe by stripe. In \alpha-(BEDT-TTF)2I3, lattice distortions locally stabilize the charge order so that it is easily weakened by local photoexcitation. The photoinduced domain indeed expands in the plane. These results are consistent with recent observation by femtosecond reflection spectroscopy.Comment: 9 pages, 8 figures, to appear in J. Phys. Soc. Jpn. Vol. 79 (2010) No.

Finite-Temperature Properties across the Charge Ordering Transition -- Combined Bosonization, Renormalization Group, and Numerical Methods

We theoretically describe the charge ordering (CO) metal-insulator transition based on a quasi-one-dimensional extended Hubbard model, and investigate the finite temperature ($T$) properties across the transition temperature, $T_{\rm CO}$. In order to calculate $T$ dependence of physical quantities such as the spin susceptibility and the electrical resistivity, both above and below $T_{\rm CO}$, a theoretical scheme is developed which combines analytical methods with numerical calculations. We take advantage of the renormalization group equations derived from the effective bosonized Hamiltonian, where Lanczos exact diagonalization data are chosen as initial parameters, while the CO order parameter at finite-$T$ is determined by quantum Monte Carlo simulations. The results show that the spin susceptibility does not show a steep singularity at $T_{\rm CO}$, and it slightly increases compared to the case without CO because of the suppression of the spin velocity. In contrast, the resistivity exhibits a sudden increase at $T_{\rm CO}$, below which a characteristic $T$ dependence is observed. We also compare our results with experiments on molecular conductors as well as transition metal oxides showing CO.Comment: 9 pages, 8 figure

Pressure-Induced Zero-Gap Semiconducting State in Organic Conductor α\alpha-(BEDT-TTF)2_2I3_3 Salt

We show a zero-gap semiconducting (ZGS) state in the quasi-two-dimensional organic conductor $\alpha$-(BEDT-TTF)$_2$I$_3$ salt, which emerges under uniaxial pressure along the a-axis (the stacking axis of the BEDT-TTF molecule). The ZGS state is the state in which a Dirac cone with the band spectrum of a linear dispersion exists around the Fermi point connecting an unoccupied (electron) band with an occupied (hole) band. The spectrum exhibits a large anisotropy in velocity, which depends on the direction from the Fermi point. By varying the magnitude of several transfer energies of a tight-binding model with four sites per unit cell, it is shown that the ZGS state exists in a wide pressure range, and is attributable to the large anisotropy of the transfer energies along the stacking axis.Comment: 6 pages, 8 figures, to be published in J. Phys. Soc. Jp

Ground state and finite temperature behavior of 1/4-filled band zigzag ladders

We consider the simplest example of lattice frustration in the 1/4-filled band, a one-dimensional chain with next-nearest neighbor interactions. For this zigzag ladder with electron-electron as well as electron-phonon interactions we present numerical results for ground state as well as thermodynamic properties. In this system the ground state bond distortion pattern is independent of electron-electron interaction strength. The spin gap in the ground state of the zigzag ladder increases with the degree of frustration. Unlike in one-dimension, where the spin-gap and charge ordering transitions can be distinct, we show that in the ladder they occur simultaneously. We discuss spin gap and charge ordering transitions in 1/4-filled materials with one, two, or three dimensional crystal structures. We show empirically that regardless of dimensionality the occurrence of simultaneous or distinct charge and magnetic transitions can be correlated with the ground state bond distortion pattern.Comment: 12 pages, 8 eps figure

Pressure effect on the magnetic susceptibility of low dimensional organic conductors alpha, beta- (BEDT-TTF)2I3 and (TMTSF)2X (X = ClO4, PF6, ReO4)

The pressure dependence of the static susceptibility of the α and β phases of (BEDT-TTF)2I3 and of (TMTSF)2X for X = ClO4−, PF6−, ReO4− was measured using a Faraday balance up to a pressure of 10 kbar. The fractional pressure dependence δ 1nξs/δP is -2.0%/kbar and −3.3%/kbar for the α- and β-(BEDT-TTF)2I3 respectively and it is temperature independent. For (TMTSF)2X δ1nξs/δP is about −3.0%/kbar at room temperature and increases to −4.8%/kbar on cooling. Both the magnitude and temperature dependence of the fractional pressure dependence in those two families are in contrast with the behaviour in TTF-TCNQ, where the magnitude is −8.%/kbar and decreases with decreasing temperature

Magnetic susceptibility of α and β phases of di[bis(ethylenediothiolo)tetrathiafulvalene] tri-iodide [(BEDT−TTF)2I3] under pressure

We report static magnetic susceptibility data, χ(T), for the α and β phases of the organic conductor di[bis(ethylenediothiolo)tetrathiafulvalene] tri-iodide [(BEDT−TTF)2I3] both at ambient pressure and under applied pressures of up to 10 kbar. Ambient pressure ESR data for both phases below 300 K are also presented. The temperature dependence of the spin susceptibility χβs(T) for the superconductor β-(BEDT−TTF)2 I3 is unusually small for organic conductors; the pressure derivative ∂lnχβs∂P is found to be only moderate in magnitude (-3.3% per kbar) and independent of temperature below 300 K. For the αq phase, ∂lnχαs∂P≃−2.0%Kbar at ambient temperature

Possibilities and limitations of forensic DNA analyses considering alternative propositions for the origin of biological traces A statement of the German Stain Commission - a joint Commission of the Institutes of Legal Medicine and Forensic Science Institutes

Over the last decades forensic DNA analysis has become an important tool in criminal investigations, aided by increasing sensitivity and a high level of technological standardization. Due to the continuing methodological improvements the successful genetic analysis of smaller and smaller latent DNA traces has been taken for granted. This leads to a shift in tasks for the forensic DNA expert regarding the evaluation of complex trace scenarios during a court trial. Rather than discussing an individual's DNA as the potential source of a DNA trace, questions put before the expert are now focussed on activities leading to potentially innocent depositions of biological material. The German Stain Commission recognizes the need to assess alternative scenarios of trace deposition. For the evaluation of forensic DNA findings within the context of further evidence in a given case scenario, we introduce the hierarchy of propositions, a well-established concept in the international literature of forensic science. Furthermore, the scientific basis of evaluating alternative DNA transfer scenarios and the possibilities and limitations of expressing an expert opinion are described

Joint recommendations of the project group Statistical analysis of DNA and the German Stain Commission on the statistical analysis of YaEurochromosomal DNA typing results

The analysis of YaEurochromosomal short tandem repeats (Y-STRs) can be an important supplement to the established autosomal DNA analysis. For example, it is often possible to obtain information on the male component of a DNA mixture, even if there is an excess of female DNA. A Y-STR typing result can prove that male DNA is present at a trace and can be compared with reference Y-STR typing results of male suspects. The recommendations presented here by the project group Statistical analysis of DNA of the state criminal investigation offices in cooperation with the German Stain Commission of the German Society of Legal Medicine are intended to support experts in the evaluation of YaEuroSTR typing results. Approaches to estimate the frequency of Y-STR haplotypes are presented as well as approaches to assess the match probability between Y-STR results. From this, recommendations are derived with the aim of a standardized framework to evaluate Y-chromosomal DNA typing results in forensic casework