313 research outputs found
Geomorphic processes active in the Southwestern Louisiana Canal, Lafourche Parish, Louisiana
The geomorphological changes causing the destruction of the banks of the Southwestern Louisiana Canal are studied by means of field work, laboratory analyses, and infrared color imagery interpretation. Turbulence and flow patterns are mapped, and related to erosion and sediment deposition processes. The accelerated erosion rate of the last decade is discussed, with two causative factors cited: (1) development of faster boats, increasing bank and bottom erosion, and (2) a subsequently larger tidal influx, with greater erosive ability. The physical properties of the canal bank materials are also analyzed. It is concluded that channel erosion progressively increases, with no indications of stabilization, until they merge with other waterways and become indistinguishable from natural water bodies
The use of color infrared imagery for the study of marsh buggy tracks
Color infrared imagery is used to determine the location of buggy routes and to quantify the extent of tracks in a selected area where the marsh is seriously dissected. The imagery is used to show successive stages of destruction. It is recommended that alternate routes be identified in the operating area to eliminate continuous use of the same route and facilitate faster revegetation
Fermi-surface reconstruction and two-carrier model for the Hall effect in YBa2Cu4O8
Pulsed field measurements of the Hall resistivity and magnetoresistance of
underdoped YBa2Cu4O8 are analyzed self-consistently using a simple model based
on coexisting electron and hole carriers. The resultant mobilities and Hall
numbers are found to vary markedly with temperature. The conductivity of the
hole carriers drops by one order of magnitude below 30 K, explaining the
absence of quantum oscillations from these particular pockets. Meanwhile the
Hall coefficient of the electron carriers becomes strongly negative below 50 K.
The overall quality of the fits not only provides strong evidence for
Fermi-surface reconstruction in Y-based cuprates, it also strongly constrains
the type of reconstruction that might be occurring.Comment: 5 pages, 4 figures, updated after publication in Physical Review B
(Rapid Communication
Nodes in the gap structure of the iron-arsenide superconductor Ba(Fe_{1-x}Co_x)_2As_2 from c-axis heat transport measurements
The thermal conductivity k of the iron-arsenide superconductor
Ba(Fe_{1-x}Co_x)_2As_2 was measured down to 50 mK for a heat current parallel
(k_c) and perpendicular (k_a) to the tetragonal c axis, for seven Co
concentrations from underdoped to overdoped regions of the phase diagram (0.038
< x < 0.127). A residual linear term k_c0/T is observed in the T = 0 limit when
the current is along the c axis, revealing the presence of nodes in the gap.
Because the nodes appear as x moves away from the concentration of maximal T_c,
they must be accidental, not imposed by symmetry, and are therefore compatible
with an s_{+/-} state, for example. The fact that the in-plane residual linear
term k_a0/T is negligible at all x implies that the nodes are located in
regions of the Fermi surface that contribute strongly to c-axis conduction and
very little to in-plane conduction. Application of a moderate magnetic field
(e.g. H_c2/4) excites quasiparticles that conduct heat along the a axis just as
well as the nodal quasiparticles conduct along the c axis. This shows that the
gap must be very small (but non-zero) in regions of the Fermi surface which
contribute significantly to in-plane conduction. These findings can be
understood in terms of a strong k dependence of the gap Delta(k) which produces
nodes on a Fermi surface sheet with pronounced c-axis dispersion and deep
minima on the remaining, quasi-two-dimensional sheets.Comment: 12 pages, 13 figures
Hall, Seebeck, and Nernst Coefficients of Underdoped HgBa2CuO4+d: Fermi-Surface Reconstruction in an Archetypal Cuprate Superconductor
Charge density-wave order has been observed in cuprate superconductors whose
crystal structure breaks the square symmetry of the CuO2 planes, such as
orthorhombic YBa2Cu3Oy (YBCO), but not so far in cuprates that preserve that
symmetry, such as tetragonal HgBa2CuO4+d (Hg1201). We have measured the Hall
(R_H), Seebeck (S), and Nernst coefficients of underdoped Hg1201 in magnetic
fields large enough to suppress superconductivity. The high-field R_H(T) and
S(T) are found to drop with decreasing temperature and become negative, as also
observed in YBCO at comparable doping. In YBCO, the negative R_H and S are
signatures of a small electron pocket caused by Fermi-surface reconstruction,
attributed to charge density-wave modulations observed in the same range of
doping and temperature. We deduce that a similar Fermi-surface reconstruction
takes place in Hg1201, evidence that density-wave order exists in this
material. A striking similarity is also found in the normal-state Nernst
coefficient, further supporting this interpretation. Given the model nature of
Hg1201, Fermi-surface reconstruction appears to be common to all hole-doped
cuprates, suggesting that density-wave order is a fundamental property of these
materials
Quantum critical scaling at the edge of Fermi liquid stability in a cuprate superconductor
In the high temperature cuprate superconductors, the pervasiveness of
anomalous electronic transport properties suggests that violation of
conventional Fermi liquid behavior is closely tied to superconductivity. In
other classes of unconventional superconductors, atypical transport is well
correlated with proximity to a quantum critical point, but the relative
importance of quantum criticality in the cuprates remains uncertain. Here we
identify quantum critical scaling in the electron-doped cuprate material
La2-xCexCuO4 with a line of quantum critical points that surrounds the
superconducting phase as a function of magnetic field and charge doping. This
zero-temperature phase boundary, which delineates a metallic Fermi liquid
regime from an extended non-Fermi liquid ground state, closely follows the
upper critical field of the overdoped superconducting phase and gives rise to
an expanse of distinct non Fermi liquid behavior at finite temperatures.
Together with signatures of two distinct flavors of quantum fluctuations, this
suggests that quantum criticality plays a significant role in shaping the
anomalous properties of the cuprate phase diagram.Comment: 16 pages, 3 figures + supplementary materia
Quasiparticle Heat Transport in BaKFeAs: Evidence for a k-dependent Superconducting Gap without Nodes
The thermal conductivity of the iron-arsenide superconductor
BaKFeAs ( 30 K) was measured in single crystals
at temperatures down to mK (/600) and in magnetic
fields up to T (/4). A negligible residual linear term
in as shows that there are no zero-energy quasiparticles
in the superconducting state. This rules out the existence of line and in-plane
point nodes in the superconducting gap, imposing strong constraints on the
symmetry of the order parameter. It excludes d-wave symmetry, drawing a clear
distinction between these superconductors and the high- cuprates. However,
the fact that a magnetic field much smaller than can induce a residual
linear term indicates that the gap must be very small on part of the Fermi
surface, whether from strong anisotropy or band dependence, or both
A Phenomenological Description of the Non-Fermi-Liquid Phase of MnSi
In order to understand the non-Fermi-liquid behavior of MnSi under pressure
we propose a scenario on the basis of the multispiral state of the magnetic
moment.
This state can describe the recent critical experiment of the Bragg sphere in
the neutron scattering which is the key ingredient of the non-Fermi-liquid
behavior.Comment: 3 page
Shubnikov-de Haas oscillations in YBa_2Cu_4O_8
We report the observation of Shubnikov-de Haas oscillations in the underdoped
cuprate superconductor YBaCuO (Y124). For field aligned along the
c-axis, the frequency of the oscillations is T, which corresponds
to % of the total area of the first Brillouin zone. The effective
mass of the quasiparticles on this orbit is measured to be times
the free electron mass. Both the frequency and mass are comparable to those
recently observed for ortho-II YBaCuO (Y123-II). We show that
although small Fermi surface pockets may be expected from band structure
calculations in Y123-II, no such pockets are predicted for Y124. Our results
therefore imply that these small pockets are a generic feature of the copper
oxide plane in underdoped cuprates.Comment: v2: Version of paper accepted for publication in Physical Review
Letters. Only minor changes to the text and reference
Evidence for a small hole pocket in the Fermi surface of underdoped YBa2Cu3Oy
The Fermi surface of a metal is the fundamental basis from which its
properties can be understood. In underdoped cuprate superconductors, the Fermi
surface undergoes a reconstruction that produces a small electron pocket, but
whether there is another, as yet undetected portion to the Fermi surface is
unknown. Establishing the complete topology of the Fermi surface is key to
identifying the mechanism responsible for its reconstruction. Here we report
the discovery of a second Fermi pocket in underdoped YBa2Cu3Oy, detected as a
small quantum oscillation frequency in the thermoelectric response and in the
c-axis resistance. The field-angle dependence of the frequency demonstrates
that it is a distinct Fermi surface and the normal-state thermopower requires
it to be a hole pocket. A Fermi surface consisting of one electron pocket and
two hole pockets with the measured areas and masses is consistent with a
Fermi-surface reconstruction caused by the charge-density-wave order observed
in YBa2Cu3Oy, provided other parts of the reconstructed Fermi surface are
removed by a separate mechanism, possibly the pseudogap.Comment: 23 pages, 5 figure
- …