1,401 research outputs found
Observational Constraints on Phantom Crossing DGP Gravity
We study the observational constraints on the Phantom Crossing DGP model. We
demonstrate that the crossing of the phantom divide does not occur within the
framework of the original Dvali-Gabadadze-Porrati (DGP) model or the DGP model
developed by Dvali and Turner. By extending their model in the framework of an
extra dimension scenario, we study a model that realizes crossing of the
phantom divide. We investigate the cosmological constraints obtained from the
recent observational data of Type Ia Supernovae, Cosmic Microwave Background
anisotropies, and Baryon Acoustic Oscillations. The best fit values of the
parameters with 1 (68%) errors for the Phantom Crossing DGP model are
, . We find that
the Phantom Crossing DGP model is more compatible with the observations than
the original DGP model or the DGP model developed by Dvali and Turner. Our
model can realize late-time acceleration of the universe, similar to that of
CDM model, without dark energy due to the effect of DGP gravity. In
our model, crossing of the phantom divide occurs at a redshift of .Comment: 17 pages, 9 figures, 1 table, Accepted for publication in
International Journal of Modern Physics
Electronic Phase Diagram of High-T_c Cuprate Superconductors from a Mapping of the In-Plane Resistivity Curvature
We propose that Resistivity Curvature Mapping (RCM) based on the in-plane
resistivity data is a useful way to objectively draw an electronic phase
diagrams of high-T_c cuprates, where various crossovers are important. In
particular, the pseudogap crossover line can be conveniently determined by RCM.
We show experimental phase diagrams obtained by RCM for
Bi_{2}Sr_{2-z}La_{z}CuO_{6+\delta}, La_{2-x}Sr_{x}CuO_{4}, and
YBa_{2}Cu_{3}O_{y}, and demonstrate the universal nature of the pseudogap
crossover. Intriguingly, the electronic crossover near optimum doping depicted
by RCM appears to occur rather abruptly, suggesting that the quantum critical
regime, if exists, must be very narrow.Comment: 4 pages, 3 color figures, final version accepted for publication in
Phys. Rev. Let
Inhomogeneous CuO_{6} Tilt Distribution and Charge/Spin Correlations in La_{2-x-y}Nd_{y}Sr_{x}CuO$_{4} around commensurate hole concentration
Phononic and magnetic Raman scattering are studied in
LaNdSrCuO with three doping concentrations: x ~
1/8, y = 0; x ~ 1/8, y = 0.4; and x = 0.01, y = 0. We observe strong disorder
in the tilt pattern of the CuO_{6} octahedra in both the orthorhombic and
tetragonal phases which persist down to 10 K and are coupled to bond disorder
in the cation layers around 1/8 doping independent of Nd concentration. The
weak magnitude of existing charge/spin modulations in the Nd doped structure
does not allow us to detect the specific Raman signatures on lattice dynamics
or two-magnon scattering around 2200 cm-1.Comment: to be published in Phys. Rev.
An Infrared study of the Josephson vortex state in high-Tc cuprates
We report the results of the c-axis infrared spectroscopy of La_{2-x} Sr_x
CuO_4 in high magnetic field oriented parallel to the CuO_2 planes. A
significant suppression of the superfluid density with magnetic field rho_s(H)
is observed for both underdoped (x=0.125) and overdoped (x=0.17) samples. We
show that the existing theoretical models of the Josephson vortex state fail to
consistently describe the observed effects and discuss possible reasons for the
discrepancies
Evolution of the Hall Coefficient and the Peculiar Electronic Structure of the Cuprate Superconductors
Although the Hall coefficient R_H is an informative transport property of
metals and semiconductors, its meaning in the cuprate superconductors has been
ambiguous because of its unusual characteristics. Here we show that a
systematic study of R_H in La_{2-x}Sr_{x}CuO_{4} single crystals over a wide
doping range establishes a qualitative understanding of its peculiar evolution,
which turns out to reflect a two-component nature of the electronic structure
caused by an unusual development of the Fermi surface recently uncovered by
photoemission experiments.Comment: 4 pages, 5 figures, final version appeared in Phys. Rev. Let
Hall effect in superconducting Fe(Se0.5Te0.5) thin films
The Hall effect is investigated for eight superconducting Fe(Se_0.5_Te_0.5_)
thin films grown on MgO and LaSrAlO_4_ substrates with different transition
temperatures (T_c_). The normal Hall coefficients (R_H_) have positive values
with magnitude of 1 - 1.5 x 10^-3^ cm^3^/C at room temperature for the all
samples. With decreasing temperature, we find two characteristic types of
behavior in R_H_(T) depending on T_c_. For thin films with lower T_c_
(typically T_c_ < 5 K), R_H_ start decreasing approximately below T = 250 K
toward a negative side, some of which shows sign reversal at T = 50 - 60 K, but
turns positive toward T = 0 K. On the other hand for the films with higher T_c_
(typically T_c_ > 9 K), R_ H_ leaves almost unchanged down to T = 100 K, and
then starts decreasing toward a negative side. Around the temperatures when
R_H_ changes its sign from positive to negative, obvious nonlinearity is
observed in the field-dependence of Hall resistance as to keep the low-field
R_H_ positive while the high-field R_H_ negative. Thus the electronic state
just above T_c_ is characterized by n_e_ (electron density) > n_h_ (hole
density) with keeping \mu_e_ < \mu_h_. These results suggest the dominance of
electron density to the hole density is an essential factor for the occurence
of superconductivity in Fe-chalcogenide superconductors.Comment: 11 pages, 4 figures, revised version for Physical Review B. accepted
for publication in Physical Review
Strong charge fluctuations manifested in the high-temperature Hall coefficient of high-T_c cuprates
By measuring the Hall coefficient R_H up to 1000 K in La_2CuO_4,
Pr_{1.3}La_{0.7}CuO_4, and La_{2-x}Sr_xCuO_4 (LSCO), we found that the
temperature (T) dependence of R_H in LSCO for x = 0 - 0.05 at high temperature
undoubtedly signifies a gap over which the charge carriers are thermally
activated, which in turn indicates that in lightly-doped cuprates strong charge
fluctuations are present at high temperature and the carrier number is not a
constant. At higher doping (x = 0.08 - 0.21), the high-temperature R_H(T)
behavior is found to be qualitatively the same, albeit with a weakened
temperature dependence, and we attempt to understand its behavior in terms of a
phenomenological two-carrier model where the thermal activation is considered
for one of the two species. Despite the crude nature of the model, our analysis
gives a reasonable account of R_H both at high temperature and at 0 K for a
wide range of doping, suggesting that charge fluctuations over a gap remain
important at high temperature in LSCO deep into the superconducting doping
regime. Moreover, our model gives a perspective to understand the seemingly
contradicting high-temperature behavior of R_H and the in-plane resistivity
near optimum doping in a consistent manner. Finally, we discuss possible
implications of our results on such issues as the scattering-time separation
and the large pseudogap.Comment: 9 pages, 8 figures; final version, to appear in Phys. Rev.
Magnetic order in lightly doped La_{2-x}Sr_{x}CuO_{4}
We study long wavelength magnetic excitations in lightly doped
La_{2-x}Sr_{x}CuO_{4} (x < 0.03) detwinned crystals. The lowest energy magnetic
anisotropy induced gap can be understood in terms of the antisymmetric spin
interaction inside the antiferromagnetic (AF) phase. The second magnetic
resonace, analyzed in terms of in-plane spin anisotropy, shows unconventional
behavior within the AF state and led to the discovery of collective spin
excitations pertaining to a field induced magnetically ordered state. This
state persists in a 9 T field to more than 100 K above the N\'{e}el temperature
in x = 0.01.Comment: 5 pages, 5 figure
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