1,454 research outputs found
Doping evolution of the electronic specific heat coefficient in slightly-doped La2-xSrxCuO4 single crystals
Detailed doping dependence of the electronic specific heat coefficient gamma
is studied for La2-xSrxCuO4 (LSCO) single crystals in the slightly-doped
regime. We find that gamma systematically increases with doping, and
furthermore, even for the samples in the antiferromagnetic (AF) regime, gamma
already acquires finite value and grows with x. This suggests that finite
electronic density of states (DOS) is created in the AF regime where the
transport shows strong localization at low temperatures, and this means the
system is not a real insulator with a clear gap even though it still keeps long
range AF order.Comment: 4 pages, 4 figures, accepted for publication in Journal of Physics:
Conference Series (LT25 proceeding
Magic Doping Fractions in High-Temperature Superconductors
We report hole-doping dependence of the in-plane resistivity \rho_{ab} in a
cuprate superconductor La_{2-x}Sr_{x}CuO_{4}, carefully examined using a series
of high-quality single crystals. Our detailed measurements find a tendency
towards charge ordering at particular rational hole doping fractions of 1/16,
3/32, 1/8, and 3/16. This observation appears to suggest a specific form of
charge order and is most consistent with the recent theoretical prediction of
the checkerboard-type ordering of the Cooper pairs at rational doping fractions
x = (2m+1)/2^n, with integers m and n.Comment: 5 pages, 3 figure, resubmitted to Phys. Rev. Lett. The Tc vs. x
diagram has been added and the discussions have been modified to focus more
on the experimental result
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
Precision Search for Magnetic Order in the Pseudogap Regime of La2-xSrxCuO4 by Muon Spin Relaxation
We report a high precision search for orbital-like magnetic order in the
pseudogap region of La2-xSrxCuO4 single crystals using zero-field muon spin
relaxation (ZF-muSR). In contrast to previous studies of this kind, the effects
of the dipolar and quadrupolar interactions of the muon with nearby nuclei are
calculated. ZF-muSR spectra with a high number of counts were also recorded to
determine whether a magnetically ordered phase exists in dilute regions of the
sample. Despite these efforts, we find no evidence for static magnetic order of
any kind in the pseudogap region above the hole-doping concentration p = 0.13.Comment: 8 pages, 7 figure
A comparative study on three modes of s-process nucleosynthesis in extremely metal-poor AGB stars
Carbon-enhanced metal-poor (CEMP) stars in the Galactic halo have a wide
range of neutron-capture element abundance patterns. To identify their origin,
we investigated three modes of -process nucleosynthesis that have been
proposed to operate in extremely metal-poor (EMP) Asymptotic Giant Branch (AGB)
stars: the convective 13C burning, which occurs when hydrogen is engulfed by
the helium flash convection in low-mass AGB stars, the convective 22Ne burning,
which occurs in the helium flash convection of intermediate-mass AGB stars, and
the radiative 13C burning, which occurs in the C pocket that is formed
during the inter-pulse periods. We show that the production of -process
elements per iron seed (-process efficiency) does not depend on metallicity
below , because 16O in the helium zone dominates the
neutron poison. The convective 13C mode can produce a variety of -process
efficiencies for Sr, Ba and Pb, including the maxima observed among CEMP stars.
The 22Ne mode only produce the lowest end of -process efficiencies among
CEMP models. We show that the combination of these two modes can explain the
full range of observed enrichment of -process elements in CEMP stars. In
contrast, the 13C pocket mode can hardly explain the high level of enrichment
observed in some CEMP stars, even if considering star-to-star variations of the
mass of the 13C pocket. These results provide a basis for discussing the binary
mass transfer origin of CEMP stars and their subgroups.Comment: 22 pages, 13 figures, 1 table. Accepted for publication in
Publications of the Astronomical Society of Japa
Evolution of Low- and Intermediate-Mass Stars with [Fe/H] <= -2.5
We present extensive sets of stellar models for 0.8-9.0Msun in mass and -5 <=
[Fe/H] <= -2 and Z = 0 in metallicity. The present work focuses on the
evolutionary characteristics of hydrogen mixing into the He-flash convective
zones during the core and shell He flashes which occurs for the models with
[Fe/H] <~ -2.5. Evolution is followed from the zero age MS to the TPAGB phase
including the hydrogen engulfment by the He-flash convection during the RGB or
AGB phase. There exist various types of mixing episodes of how the H mixing
sets in and how it affects the final abundances at the surface. In particular,
we find H ingestion events without dredge-ups that enables repeated
neutron-capture nucleosynthesis in the He flash convective zones with 13
C(a,n)16 O as neutron source. For Z = 0, the mixing and dredge-up processes
vary with the initial mass, which results in different final abundances in the
surface. We investigate the occurrence of these events for various initial mass
and metallicity to find the metallicity dependence for the He-flash driven deep
mixing (He-FDDM) and also for the third dredge-up (TDU) events. In our models,
we find He-FDDM for M <= 3Msun for Z = 0 and for M <~ 2Msun for -5 <~ [Fe/H] <~
-3. On the other hand, the occurrence of the TDU is limited to the mass range
of ~1.5Msun to ~5Msun for [Fe/H] = -3, which narrows with decreasing
metallicity. The paper also discusses the implications of the results of model
computations for observations. We compared the abundance pattern of CNO
abundances with observed metal-poor stars. The origins of most iron-deficient
stars are discussed by assuming that these stars are affected by binary mass
transfer. We also point out the existence of a blue horizontal branch for -4 <~
[Fe/H] <~ -2.5.Comment: 19 pages, 12 figures, accepted by MNRA
Microscopic analysis of the chemical reaction between Fe(Te,Se) thin films and underlying CaF
To understand the chemical reaction at the interface of materials, we
performed a transmission electron microscopy (TEM) observation in four types of
Fe(Te,Se) superconducting thin films prepared on different types of substrates:
CaF2 substrate, CaF2 substrate with a CaF2 buffer layer, CaF2 substrate with a
FeSe buffer layer, and a LaAlO3 substrate with a CaF2 buffer layer. Based on
the energy-dispersive X-ray spectrometer (EDX) analysis, we found possible
interdiffusion between fluorine and selenium that has a strong influence on the
superconductivity in Fe(Te,Se) films. The chemical interdiffusion also plays a
significant role in the variation of the lattice parameters. The lattice
parameters of the Fe(Te,Se) thin films are primarily determined by the chemical
substitution of anions, and the lattice mismatch only plays a secondary role.Comment: 30 pages, 9 figur
Electronic inhomogeneity and competing phases in electron-doped superconducting Pr0.88LaCe0.12CuO4
We use neutron scattering to demonstrate that electron-doped superconducting
Pr0.88LaCe0.12CuO4 in the underdoped regime is electronically phase separated
in the ground state, showing the coexistence of a superconducting phase with a
three-dimensional antiferromagnetically ordered phase and a
quasi-two-dimensional spin density wave modulation. The Neel temperature of
both antiferromagnetic phases decreases linearly with increasing
superconducting transition temperature (Tc) and vanishes when optimal
superconductivity is achieved. These results indicate that the electron-doped
copper oxides are close to a quantum critical point, where the delicate
energetic balance between different competing states leads to microscopic
heterogeneity.Comment: 14 pages, 4 figures, accepted to Phys. Rev. B as a rapid
communicatio
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