17,334 research outputs found
Quantum Melting of Charge Order due to Frustration in Two-Dimensional Quarter-Filled Systems
The effect of geometrical frustration in a two-dimensional 1/4-filled
strongly correlated electron system is studied theoretically, motivated by
layered organic molecular crystals. An extended Hubbard model on the square
lattice is considered, with competing nearest neighbor Coulomb interaction, V,
and that of next-nearest neighbor along one of the diagonals, V', which favor
different charge ordered states. Based on exact diagonalization calculations,
we find a metallic phase stabilized over a broad window at V' ~ V even for
large Coulomb repulsion strengths as a result of frustrating the charge ordered
states. Slightly modifying the lattice geometry relevant to the actual organic
compounds does not alter the results, suggesting that this `quantum melting' of
charge order is a robust feature of frustrated strongly correlated 1/4-filled
systems.Comment: 5 pages, 4 figures, to be published in Phys. Rev.
Formulating the Net Gain of MISO-SFN in the Presence of Self-Interferences
In this study, an analytical formula for multiple-input single-output single frequency network gain (MISO-SFNG) is investigated. To formulate the net MISO-SFNG, we derived the average signal to interference plus noise ratio (SINR) where the gain achieved by the distributed MISO diversity as a function of power imbalance is curve-fitted. Further, we analyzed the losses owing to self-interferences resulting from the delay spread and imperfect channel estimation. We verified the accuracy and effectiveness of the derived formula by comparing the measurement results with the analytical results. The derived formula helps to understand how various system factors affect the gain under a given condition. The formula can be used to evaluate the MISO-SFNG and to predict the MISO-SFN coverage in various system configurations
Non-monotonic temperature dependent transport in graphene grown by Chemical Vapor Deposition
Temperature-dependent resistivity of graphene grown by chemical vapor
deposition (CVD) is investigated. We observe in low mobility CVD graphene
device a strong insulating behavior at low temperatures and a metallic behavior
at high temperatures manifesting a non-monotonic in the temperature dependent
resistivity.This feature is strongly affected by carrier density modulation. To
understand this anomalous temperature dependence, we introduce thermal
activation of charge carriers in electron-hole puddles induced by randomly
distributed charged impurities. Observed temperature evolution of resistivity
is then understood from the competition among thermal activation of charge
carriers, temperature-dependent screening and phonon scattering effects. Our
results imply that the transport property of transferred CVD-grown graphene is
strongly influenced by the details of the environmentComment: 7 pages, 3 figure
Frustrated Spin System in theta-(BEDT-TTF)_2RbZn(SCN)_4
The origin of the spin gap behavior in the low-temperature dimerized phase of
theta-(BEDT-TTF)_2RbZn(SCN)_4 has been theoretically studied based on the
Hartree-Fock approximation for the on-site Coulomb interaction at absolute
zero. Calculations show that, in the parameter region considered to be relevant
to this compound, antiferromagnetic ordering is stabilized between dimers
consisting of pairs of molecules coupled with the largest transfer integral.
Based on this result an effective localized spin 1/2 model is constructed which
indicates the existence of the frustration among spins. This frustration may
result in the formation of spin gap.Comment: 4 pages, 5 figures, to be published in J. Phys. Soc. Jpn. 67 (1998)
no.
Journey of water in pine cones
Pine cones fold their scales when it rains to prevent seeds from short-distance dispersal. Given that the scales of pine cones consist of nothing but dead cells, this folding motion is evidently related to structural changes. In this study, the structural characteristics of pine cones are studied on micro-/macro-scale using various imaging instruments. Raindrops fall along the outer scales to the three layers (bract scales, fibers and innermost lignified structure) of inner pine cones. However, not all the layers but only the bract scales get wet and then, most raindrops move to the inner scales. These systems reduce the amount of water used and minimize the time spent on structural changes. The result shows that the pine cones have structural advantages that could influence the efficient motion of pine cones. This study provides new insights to understand the motion of pine cones and would be used to design a novel water transport system.119Ysciescopu
Incommensurate Mott Insulator in One-Dimensional Electron Systems close to Quarter Filling
A possibility of a metal-insulator transition in molecular conductors has
been studied for systems composed of donor molecules and fully ionized anions
with an incommensurate ratio close to 2:1 based on a one-dimensional extended
Hubbard model, where the donor carriers are slightly deviated from quarter
filling and under an incommensurate periodic potential from the anions. By use
of the renormalization group method, interplay between commensurability energy
on the donor lattice and that from the anion potential has been studied and it
has been found that an "incommensurate Mott insulator" can be generated. This
theoretical finding will explain the metal-insulator transition observed in
(MDT-TS)(AuI).Comment: 4 pages, 4 figures, submitted to J. Phys. Soc. Jpn. at December 24
200
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 () properties across the transition temperature, . In order to calculate dependence of physical quantities such as the
spin susceptibility and the electrical resistivity, both above and below
, 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- is determined by quantum Monte Carlo simulations. The
results show that the spin susceptibility does not show a steep singularity at
, 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 , below which a characteristic 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
Charge Ordering in Organic ET Compounds
The charge ordering phenomena in quasi two-dimensional 1/4-filled organic
compounds (ET)_2X (ET=BEDT-TTF) are investigated theoretically for the
and -type structures, based on the Hartree approximation for the
extended Hubbard models with both on-site and intersite Coulomb interactions.
It is found that charge ordered states of stripe-type are stabilized for the
relevant values of Coulomb energies, while the spatial pattern of the stripes
sensitively depends on the anisotropy of the models. By comparing the results
of calculations with the experimental facts, where the effects of quantum
fluctuation is incorporated by mapping the stripe-type charge ordered states to
the S=1/2 Heisenberg Hamiltonians, the actual charge patterns in the insulating
phases of -(ET)_2MM'(SCN)_4 and -(ET)_2I_3 are deduced.
Furthermore, to obtain a unified view among the , and
-(ET)_2X families, the stability of the charge ordered state in
competition with the dimeric antiferromagnetic state viewed as the Mott
insulating state, which is typically realized in -type compounds, and
with the paramagnetic metallic state, is also pursued by extracting essential
parameters.Comment: 35 pages, 27 figures, submitted to J. Phys. Soc. Jp
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