355 research outputs found
Heat Capacity and Magnetic Phase Diagram of the Low-Dimensional Antiferromagnet YBaCuO
A study by specific heat of a polycrystalline sample of the low-dimensional
magnetic system YBaCuO is presented. Magnetic fields up to 14 T are
applied and permit to extract the (,) phase diagram. Below
T, the N\'eel temperature, associated with a
three-dimensional antiferromagnetic long-range ordering, is constant and equals
K. Above , increases linearly with and a
field-induced increase of the entropy at is related to the presence of an
isosbestic point at K, where all the specific heat curves cross.
A comparison is made between YBaCuO and the quasi-two-dimensional
magnetic systems BaNiVO, SrCuOCl, and
PrCuO, for which very similar phase diagrams have been reported. An
effective field-induced magnetic anisotropy is proposed to explain these phase
diagrams.Comment: 14 pages, 7 figure
Size Dependence In The Disordered Kondo Problem
We study here the role randomly-placed non-magnetic scatterers play on the
Kondo effect. We show that spin relaxation effects (with time )in the
vertex corrections to the Kondo self-energy lead to an exact cancellation of
the singular temperature dependence arising from the diffusion poles. For a
thin film of thickness and a mean-free path , disorder provides a
correction to the Kondo resistivity of the form
that explains both the disorder and sample-size depression of the Kondo effect
observed by Blachly and Giordano (PRB {\bf 51}, 12537 (1995)).Comment: 11 pages, LaTeX, 2 Postscript figure
Methane emissions from a flooded rice field in the south of Brazil.
The State of Rio Grande do Sul in Brazil cultivates about 1Mha of rice in paddy fields. The soils are prepared using either conventional tillage (CT, 41% of the area) or no tillage (NT, 14% of area), the remaining falling in a mixed soil-preparation category. The outcomes of the current study represent the first evaluation of CH4 emissions from flooded rice fields in the south of Brazil. This information will feed the Brazilian greenhouse gas inventory. The study was carried out from January through March 2003 at the IRGA experimental station located in the municipality of Cachoerinha, Rio Grande do Sul, Brazil. Rice has been cultivated in this Gleisol area since 1994 using either the CT or NT system. The closed chamber method was used to collect air samples from 9:00 AM to 12:00 Noon on a weekly basis or in 24-hour campaigns; samples were analyzed using gas chromatography. Soil and plant parameters were also measured in order to determine which ecosystem factors affect CH4 emissions from the soil into the atmosphere. Along the period, CH4 emission rates varied from 24 to 703 mg m-2 day-1. NT plot emissions were initially greater than those from the CT plot, probably due to having maintained the crop residues on the surface of soil in the NT system. Nevertheless, CH4 emission rates in the CT plot were higher than in the NT plot 14 days after flooding, probably due to the higher root mass in the deeper soil layer in the NT system. The close relationship (P<0.01) found between CH4 emissions and soil temperature in both systems explains 60% of CH4 emissions. Total CH4 emissions were 33 and 22 g m-2 in the CT and NT systems, respectively. The emission variation between the soil preparation systems corresponds to 2,860 kg ha-1 CO2 equivalents. Moreover, this reduction represents 0.8 Mg ha-1 yr-1 C equivalents, greater than the average value of 0.58 Mg C ha-1 year-1 for C sequestration in agricultural soils in the subtropical region of Brazil. The 24-hour campaign emissions produced a sigmoid curve into both the atmosphere and the chamber, albeit with an inverse relationship. The 24-hour emissions were controlled by the soil and flood-water temperatures
Electron-phonon anomaly related to charge stripes: static stripe phase versus optimally-doped superconducting La1.85Sr0.15CuO4
Inelastic neutron scattering was used to study the Cu-O bond-stretching
vibrations in optimally doped La1.85Sr0.15CuO4 (Tc = 35 K) and in two other
cuprates showing static stripe order at low temperatures, i.e.
La1.48Nd0.4Sr0.12CuO4 and La1.875Ba0.125CuO4. All three compounds exhibit a
very similar phonon anomaly, which is not predicted by conventional band
theory. It is argued that the phonon anomaly reflects a coupling to charge
inhomogeneities in the form of stripes, which remain dynamic in superconducting
La1.85Sr0.15CuO4 down to the lowest temperatures. These results show that the
phonon effect indicating stripe formation is not restricted to a narrow region
of the phase diagram around the so-called 1/8 anomaly but occurs in optimally
doped samples as well.Comment: to appear in J. Low Temp. Phy
Giant phonon anomalies and central peak due to charge density wave formation in YBaCuO
The electron-phonon interaction is a major factor influencing the competition
between collective instabilities in correlated-electron materials, but its role
in driving high-temperature superconductivity in the cuprates remains poorly
understood. We have used high-resolution inelastic x-ray scattering to monitor
low-energy phonons in YBaCuO (superconducting
K), which is close to a charge density wave (CDW) instability. Phonons in a
narrow range of momentum space around the CDW ordering vector exhibit extremely
large superconductivity-induced lineshape renormalizations. These results imply
that the electron-phonon interaction has sufficient strength to generate
various anomalies in electronic spectra, but does not contribute significantly
to Cooper pairing. In addition, a quasi-elastic "central peak" due to CDW
nanodomains is observed in a wide temperature range above and below ,
suggesting that the gradual onset of a spatially inhomogeneous CDW domain state
with decreasing temperature is a generic feature of the underdoped cuprates
Theory for phonon-induced superconductivity in MgB
We analyze superonductivity in MgB observed below K resulting
from electron-phonon coupling involving a mode at meV and
most importantly the in-plane B-B vibration at
meV. The quasiparticles originating from - and -states couple
strongly to the low-frequency mode and the -vibrations respectively.
Using two-band Eliashberg theory, and , we calculate the gap functions (,
).
Our results provide an explanation of recent tunneling experiments.
We get .Comment: revised version, accepted for publication in PR
Dielectric functions and collective excitations in MgB_2
The frequency- and momentum-dependent dielectric function as well as the energy loss function Im[-\protect{]} are calculated for intermetallic superconductor
by using two {\it ab initio} methods: the plane-wave pseudopotential method and
the tight-binding version of the LMTO method. We find two plasmon modes
dispersing at energies -8 eV and -22 eV. The high energy
plasmon results from a free electron like plasmon mode while the low energy
collective excitation has its origin in a peculiar character of the band
structure. Both plasmon modes demonstrate clearly anisotropic behaviour of both
the peak position and the peak width. In particular, the low energy collective
excitation has practically zero width in the direction perpendicular to boron
layers and broadens in other directions.Comment: 3 pages with 10 postscript figures. Submitted to PRB on May 14 200
Can one extract the electron-phonon-interaction from tunneling data in case of the multigap superconductor MgB?
In the present work we calculate the tunneling density of states (DOS) of
MgB% for different tunneling directions by directly solving the two-band
Eliashberg equations (EE) in the real-axis formulation. This procedure reveals
the fine structures of the DOS due to the optical phonons. Then we show that
the numeric inversion of the standard \emph{single-band} EE (the only available
method), when applied to the \emph{two-band} DOS of MgB, may lead to
wrong estimates of the strength of certain phonon branches (e.g. the )
in the extracted electron-phonon spectral function . The
fine structures produced by the two-band interaction at energies between 20 and
100 meV turn out to be clearly observable only for tunneling along the
planes, when the extracted contains the combination
\textbf{+}, together with a minor \textbf{+} component. Only in this case
it is possible to extract information on the -band contribution to the
spectral functions. For any other tunneling direction, the -band
contribution (which does not determine the superconducting properties of
MgB) is dominant and almost coincides with the whole
for tunneling along the c axis. Our results are compared with recent
experimental tunneling and point-contact data.Comment: 5 pages, 3 figures. Submitted to Phys. Rev. B (Brief Reports
Interplay between electron-phonon and Coulomb interactions in cuprates
Evidence for strong electron-phonon coupling in high-Tc cuprates is reviewed,
with emphasis on the electron and phonon spectral functions. Effects due to the
interplay between the Coulomb and electron-phonon interactions are studied. For
weakly doped cuprates, the phonon self-energy is strongly reduced due to
correlation effects, while there is no corresponding strong reduction for the
electron self-energy. Polaron formation is studied, focusing on effects of
Coulomb interaction and antiferromagnetic correlations. It is argued that
experimental indications of polaron formation in undoped cuprates are due to a
strong electron-phonon interaction for these systems.Comment: 43 pages and 22 figure
- …