274 research outputs found
The Pseudogap in La(2-x)Sr(x)CuO(4): A Raman Viewpoint
We report the results of Raman scattering experiments on single crystals of
La(2-x)Sr(x)CuO(4) [La214] as a function of temperature and doping. In
underdoped compounds low-energy B1g spectral weight is depleted in association
with the opening of a pseudogap on regions of the Fermi surface located near
(pi, 0) and (0, pi). The magnitude of the depletion increases with decreasing
doping, and in the most underdoped samples, with decreasing temperature. The
spectral weight that is lost at low-energies (omega < 800 cm-1) is transferred
to the higher energy region normally occupied by multi-magnon scattering. From
the normal state B2g spectra we have determined the scattering rate
Gamma(omega, T) of qausiparticles located near the diagonal directions in
k-space, (pi/2, pi/2) regions. In underdoped compounds, Gamma(omega, T) is
suppressed at low temperatures for energies less than Eg(x) ~ 800 cm-1. The
observed doping dependence of the two-magnon scattering and the scattering rate
suppression thus suggest that the pseudogap is characterized by an energy scale
Eg ~ J, where J is the antiferromagnetic super-exchange energy. Comparison with
the results from other techniques provides a consistent picture of the
pseudogap in La214.Comment: 6 pages, 5 figures, minor revisions include correct form of the B2g
Raman response function and new figures of the recalculated B2g scattering
rate. Presented at the APS March99 Meeting, accepted for publication in the
Canadian Journal of Physic
Assessment of risks to public water supply from low flows and harmful water quality in a changing climate
Water resources planning and management by water utilities have traditionally been based on consideration of water availability. However, the reliability of public water supplies can also be influenced by the quality of water bodies. In this study, we proposed a framework that integrates the analysis of risks of inadequate water quality and risks of insufficient water availability. We have developed a coupled modeling system that combines hydrological modeling of river water quantity and quality, rules for water withdrawals from rivers into storage reservoirs, and dynamical simulation of harmful algal blooms in storage reservoirs. We use this framework to assess the impact of climate change, demand growth, and landâuse change on the reliability of public water supplies. The proposed method is tested on the River Thames catchment in the south of England. The results show that alongside the wellâknown risks of rising water demand in the south of England and uncertain impacts of climate change, diffuse pollution from agriculture and effluent from upstream waste water treatment works potentially represent a threat to the reliability of public water supplies in London. We quantify the steps that could be taken to ameliorate these threats, though even a vigorous pollutionâprevention strategy would not be sufficient to offset the projected effects of climate change on water quality and the reliability of public water supplies. The proposed method can help water utilities to recognize their system vulnerability and evaluate the potential solutions to achieve more reliable water supplies. supplie
Evidence for Two Superconducting Gaps in
We have measured the Raman spectra of polycrystalline MgB from 25 {\cm}
to 1200 {\cm}. When the temperature was decreased below the superconducting
transition temperature , we observed a superconductivity-induced
redistribution in the electronic Raman continuum. Two pair-breaking peaks
appear in the spectra, suggesting the presence of two superconducting gaps.
Furthermore, we have analyzed the measured spectra using a quasi
two-dimensional model in which two s-wave superconducting gaps open on two
sheets of Fermi surface. For the gap values we have obtained (2.7 meV) and (6.2 meV). Our results suggest
that a conventional phonon-mediated pairing mechanism occurs in the planar
boron bands and is responsible for the superconductivity of MgB.Comment: 3 figure
Comment on "Superconducting gap anisotropy vs. doping level in high-T_c cuprates" by C. Kendziora et al, PRL 77, 727 (1996)
In a recent paper Kendziora et al concluded that the superconducting gap in
overdoped Bi-2212 is isotropic. From data obtained from electronic Raman
scattering measurements, their conclusion was based on the observation that
pair breaking peaks occured at approximately the same frequency in different
scattering geometries and that the normalized scattering intensity at low
energies was strongly depleted. We discuss a different interpretation of the
raw data and present new data which is consistent with a strongly anisotropic
gap with nodes. The spectra can be successfully described by a model for Raman
scattering in a d_{x^{2}-y^{2}} superconductor with spin fluctuations and
impurity scattering included.Comment: 1 page revtex plus 1 postscript figur
Strong diamagnetic response and specific heat anomaly above T_c in underdoped La_(2-x)Sr_xCuO_4
By measuring AC susceptibility using a very low amplitude of the AC field (<1
mG) it is shown that underdoped samples of La_(2-x)Sr_xCuO_4 (LASCO), are
diamagnetic in a temperature region above T_c up to a temperature T^*. This
behavior is only observed with AC fields along the c-axis whereas for fields in
the ab-plane no diamagnetism above Tc was detected. The diamagnetism is almost
frequency independent in the frequency range 0.1-10 kHz. At T* a broad step
anomaly in the specific heat is inferred through measurements of the elastic
constant c33. We suggest that the observed diamagnetism and the anomaly in the
elastic constant are associated with the existence of phase incoherent Cooper
pairs between Tc and T*.Comment: 5 pages 7 figures, to appear in Phys. rev
Doping Dependence of the Pseudogap in La(2-x)Sr(x)CuO(4)
We report the results of Raman scattering experiments on single crystals of
La(2-x)Sr(x)CuO(4) that span the range from underdoped (x = 0.10) to overdoped
(x =0.22). The spectra are consistent with the existence of a strong
anisotropic quasiparticle interaction that results in a normal state depletion
of spectral weight from regions of the Fermi surface located near the zone
axes. The strength of the interaction decreases rapidly with increasing hole
concentration and the spectral evidence for the pseudogap vanishes when the
optimum doping level is reached. The results suggest that the pseudogap and
superconducting gap arise from different mechanisms.Comment: 7 pages, 6 eps figures, added new sections, figures, reference
Raman scattering through a metal-insulator transition
The exact solution for nonresonant A1g and B1g Raman scattering is presented
for the simplest model that has a correlated metal-insulator transition--the
Falicov-Kimball model, by employing dynamical mean field theory. In the general
case, the A1g response includes nonresonant, resonant, and mixed contributions,
the B1g response includes nonresonant and resonant contributions (we prove the
Shastry-Shraiman relation for the nonresonant B1g response) while the B2g
response is purely resonant. Three main features are seen in the nonresonant
B1g channel: (i) the rapid appearance of low-energy spectral weight at the
expense of higher-energy weight; (b) the frequency range for this low-energy
spectral weight is much larger than the onset temperature, where the response
first appears; and (iii) the occurrence of an isosbestic point, which is a
characteristic frequency where the Raman response is independent of temperature
for low temperatures. Vertex corrections renormalize away all of these
anomalous features in the nonresonant A1g channel. The calculated results
compare favorably to the Raman response of a number of correlated systems on
the insulating side of the quantum-critical point (ranging from Kondo
insulators, to mixed-valence materials, to underdoped high-temperature
superconductors). We also show why the nonresonant B1g Raman response is
``universal'' on the insulating side of the metal-insulator transition.Comment: 12 pages, 11 figures, ReVTe
Evidence for Magnetic Pseudoscaling in Overdoped La(2-x)Sr(x)CuO(4)
We report the results of electronic Raman scattering experiments on an
overdoped La(1.78)Sr(0.22)CuO(4) single crystal as a function of temperature.
The scattering rate Gamma(w->0,T) has been determined from the normal state
B(1g) spectra in the range 50 K < T < 300 K. Gamma(T) decreases linearly from
300 K to about 175 K and then undergoes a reduction with respect to the
expected mean-field behavior. This trend suggests a crossover to pseudoscaling
regime at about T(cr)=160 K. The results are in good agreement with the
prediction of the nearly antiferromagnetic Fermi liquid model. There is no
evidence of a pseudogap in the spectra obtained from this overdoped sample.Comment: RevTex, 4 pages with 3 PS figures included, replaced with minor
changes in the text and reference
Phonon number measurements using single photon opto-mechanics
We describe a system composed of two coupled optical cavity modes with a
coupling modulated by a bulk mechanical resonator. In addition, one of the
cavity modes is irreversibly coupled to a single photon source. Our scheme is
an opto-mechanical realisation of the Jaynes-Cummings model where the qubit is
a dual rail optical qubit while the bosonic degree of freedom is a matter
degree of freedom realised as the bulk mechanical excitation. We show the
possibility of engineering phonon number states of the mechanical oscillator in
such a system by computing the conditional state of the mechanics after
successive photon counting measurements
Coherent coupling between radio frequency, optical, and acoustic waves in piezo-optomechanical circuits
The interaction of optical and mechanical modes in nanoscale optomechanical
systems has been widely studied for applications ranging from sensing to
quantum information science. Here, we develop a platform for cavity
optomechanical circuits in which localized and interacting 1550 nm photons and
2.4 GHz phonons are combined with photonic and phononic waveguides. Working in
GaAs facilitates manipulation of the localized mechanical mode either with a
radio frequency field through the piezo-electric effect, or optically through
the strong photoelastic effect. We use this to demonstrate a novel acoustic
wave interference effect, analogous to coherent population trapping in atomic
systems, in which the coherent mechanical motion induced by the electrical
drive can be completely cancelled out by the optically-driven motion. The
ability to manipulate cavity optomechanical systems with equal facility through
either photonic or phononic channels enables new device and system
architectures for signal transduction between the optical, electrical, and
mechanical domains
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