14,471 research outputs found
Metallic characteristics in superlattices composed of insulators, NdMnO3/SrMnO3/LaMnO3
We report on the electronic properties of superlattices composed of three
different antiferromagnetic insulators, NdMnO3/SrMnO3/LaMnO3 grown on SrTiO3
substrates. Photoemission spectra obtained by tuning the x-ray energy at the Mn
2p -> 3d edge show a Fermi cut-off, indicating metallic behavior mainly
originating from Mn e_g electrons. Furthermore, the density of states near the
Fermi energy and the magnetization obey a similar temperature dependence,
suggesting a correlation between the spin and charge degrees of freedom at the
interfaces of these oxides
Self-tuning of threshold for a two-state system
A two-state system (TSS) under time-periodic perturbations (to be regarded as
input signals) is studied in connection with self-tuning (ST) of threshold and
stochastic resonance (SR). By ST, we observe the improvement of signal-to-noise
ratio (SNR) in a weak noise region. Analytic approach to a tuning equation
reveals that SNR improvement is possible also for a large noise region and this
is demonstrated by Monte Carlo simulations of hopping processes in a TSS. ST
and SR are discussed from a little more physical point of energy transfer
(dissipation) rate, which behaves in a similar way as SNR. Finally ST is
considered briefly for a double-well potential system (DWPS), which is closely
related to the TSS
20 K superconductivity in heavily electron doped surface layer of FeSe bulk crystal
A superconducting transition temperature Tc as high as 100 K was recently
discovered in 1 monolayer (1ML) FeSe grown on SrTiO3 (STO). The discovery
immediately ignited efforts to identify the mechanism for the dramatically
enhanced Tc from its bulk value of 7 K. Currently, there are two main views on
the origin of the enhanced Tc; in the first view, the enhancement comes from an
interfacial effect while in the other it is from excess electrons with strong
correlation strength. The issue is controversial and there are evidences that
support each view. Finding the origin of the Tc enhancement could be the key to
achieving even higher Tc and to identifying the microscopic mechanism for the
superconductivity in iron-based materials. Here, we report the observation of
20 K superconductivity in the electron doped surface layer of FeSe. The
electronic state of the surface layer possesses all the key spectroscopic
aspects of the 1ML FeSe on STO. Without any interface effect, the surface layer
state is found to have a moderate Tc of 20 K with a smaller gap opening of 4
meV. Our results clearly show that excess electrons with strong correlation
strength alone cannot induce the maximum Tc, which in turn strongly suggests
need for an interfacial effect to reach the enhanced Tc found in 1ML FeSe/STO.Comment: 5 pages, 4 figure
A performance comparison of the contiguous allocation strategies in 3D mesh connected multicomputers
The performance of contiguous allocation strategies can be significantly affected by the distribution of job execution times. In this paper, the performance of the existing contiguous allocation strategies for 3D mesh multicomputers is re-visited in the context of heavy-tailed distributions (e.g., a Bounded Pareto distribution). The strategies are evaluated and compared using simulation experiments for both First-Come-First-Served (FCFS) and Shortest-Service-Demand (SSD) scheduling strategies under a variety of system loads and system sizes. The results show that the performance of the allocation strategies degrades considerably when job execution times follow a heavy-tailed distribution. Moreover, SSD copes much better than FCFS scheduling strategy in the presence of heavy-tailed job execution times. The results also show that the strategies that depend on a list of allocated sub-meshes for both allocation and deallocation have lower allocation overhead and deliver good system performance in terms of average turnaround time and mean system utilization
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.
Signatures of Electronic Correlations in Optical Properties of LaFeAsOF
Spectroscopic ellipsometry is used to determine the dielectric function of
the superconducting LaFeAsOF ( = 27 K) and undoped LaFeAsO
polycrystalline samples in the wide range 0.01-6.5 eV at temperatures 10 350 K. The free charge carrier response in both samples is heavily
damped with the effective carrier density as low as 0.0400.005 electrons
per unit cell. The spectral weight transfer in the undoped LaFeAsO associated
with opening of the pseudogap at about 0.65 eV is restricted at energies below
2 eV. The spectra of superconducting LaFeAsOF reveal a
significant transfer of the spectral weight to a broad optical band above 4 eV
with increasing temperature. Our data may imply that the electronic states near
the Fermi surface are strongly renormalized due to electron-phonon and/or
electron-electron interactions.Comment: 4 pages, 4 figures, units in Fig.2 adde
Quasiparticle Interference on the Surface of Topological Crystalline Insulator Pb(1-x)Sn(x)Se
Topological crystalline insulators represent a novel topological phase of
matter in which the surface states are protected by discrete point
group-symmetries of the underlying lattice. Rock-salt lead-tin-selenide alloy
is one possible realization of this phase which undergoes a topological phase
transition upon changing the lead content. We used scanning tunneling
microscopy (STM) and angle resolved photoemission spectroscopy (ARPES) to probe
the surface states on (001) PbSnSe in the topologically
non-trivial (x=0.23) and topologically trivial (x=0) phases. We observed
quasiparticle interference with STM on the surface of the topological
crystalline insulator and demonstrated that the measured interference can be
understood from ARPES studies and a simple band structure model. Furthermore,
our findings support the fact that PbSnSe and PbSe have
different topological nature.Comment: 5 pages, 4 figure
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
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