298 research outputs found
Measurement of exciton correlations using electrostatic lattices
We present a method for determining correlations in a gas of indirect
excitons in a semiconductor quantum well structure. The method involves
subjecting the excitons to a periodic electrostatic potential that causes
modulations of the exciton density and photoluminescence (PL). Experimentally
measured amplitudes of energy and intensity modulations of exciton PL serve as
an input to a theoretical estimate of the exciton correlation parameter and
temperature. We also present a proof-of-principle demonstration of the method
for determining the correlation parameter and discuss how its accuracy can be
improved.Comment: 10 pages, 11 figure
Exciton gas transport through nano-constrictions
An indirect exciton is a bound state of an electron and a hole in spatially
separated layers. Two-dimensional indirect excitons can be created optically in
heterostructures containing double quantum wells or atomically thin
semiconductors. We study theoretically transmission of such bosonic
quasiparticles through nano-constrictions. We show that quantum transport
phenomena, e.g., conductance quantization, single-slit diffraction, two-slit
interference, and the Talbot effect, are experimentally realizable in systems
of indirect excitons. We discuss similarities and differences between these
phenomena and their counterparts in electronic devices.Comment: (v2) Updated title, text, and references; 12 pages, 9 figure
Imaging of tumor hypoxia with [124I]IAZA in comparison with [18F]FMISO and [18F]FAZA – first small animal PET results
PURPOSE: This study was performed to compare the 2-nitroimidazole derivatives [124I]IAZA, [18F]FAZA and well known [18F]FMISO in visualization of tumor hypoxia in a mouse model of human cancer using small animal PET. METHODS: PET imaging of female Balb/c nude mice bearing A431 tumors on a Phillips Mosaic small animal PET scanner was performed 3 h p.i. for all three tracers. Mice injected with [124I]IAZA were scanned again after 24 h and 48 h. In addition to the mice breathing air, in the case of [18F]FAZA and [124I]IAZA a second group of mice for each tracer was kept in an atmosphere of carbogen gas (5% of CO2 + 95 % of O2; from 1 h before to 3 h after injection) to evaluate the oxygenation dependency on uptake (all experiments n = 4). After the final PET scan animals were sacrificed and biodistribution was studied. RESULTS: Mice injected with [18F]FAZA displayed significantly higher tumor-to background (T/B) ratios (5.19 +/- 0.73) compared to those injected with [18F]FMISO (3.98 +/- 0.66; P lt; 0.001) 3 h p.i. Carbogen breathing mice showed lower ratios ([18F]FAZA: 4.06 +/- 0.59; [124I]IAZA: 2.02 +/- 0.36). The T/B ratios increased for [124I]IAZA with time (24 h: 3.83 +/- 0.61; 48 h: 4.20 +/- 0.80), but after these late time points the absolute whole body activity was very low, as could be seen from the biodistribution data (< 0.1 %ID/g for each investigated organ) and ratios were still lower than for [18F]FAZA 3 h p.i. Due to de-iodination uptake in thyroid was high. Biodistribution data were in good agreement with the PET results. CONCLUSIONS: [18F]FAZA showed superior biokinetics compared to [18F]FMISO and [124I]IAZA in this study. Imaging at later time points that are not possible with the short lived 18F labeled tracers resulted in no advantage for [124I]IAZA, i. e. tumor to normal tissue ratios could not be improved. © 1999 Canadian Society for Pharmaceutical Sciences
Doping-Dependent Raman Resonance in the Model High-Temperature Superconductor HgBa2CuO4+d
We study the model high-temperature superconductor HgBa2CuO4+d with
electronic Raman scattering and optical ellipsometry over a wide doping range.
The resonant Raman condition which enhances the scattering cross section of
"two-magnon" excitations is found to change strongly with doping, and it
corresponds to a rearrangement of inter-band optical transitions in the 1-3 eV
range seen by ellipsometry. This unexpected change of the resonance condition
allows us to reconcile the apparent discrepancy between Raman and x-ray
detection of magnetic fluctuations in superconducting cuprates. Intriguingly,
the strongest variation occurs across the doping level where the antinodal
superconducting gap reaches its maximum.Comment: 4 pages, 4 figures, contact authors for Supplemental Materia
The rate of quasiparticle recombination probes the onset of coherence in cuprate superconductors
The condensation of an electron superfluid from a conventional metallic state
at a critical temperature is described well by the BCS theory. In the
underdoped copper-oxides, high-temperature superconductivity condenses instead
from a nonconventional metallic "pseudogap" phase that exhibits a variety of
non-Fermi liquid properties. Recently, it has become clear that a charge
density wave (CDW) phase exists within the pseudogap regime, appearing at a
temperature just above . The near coincidence of and
, as well the coexistence and competition of CDW and superconducting
order below , suggests that they are intimately related. Here we show that
the condensation of the superfluid from this unconventional precursor is
reflected in deviations from the predictions of BSC theory regarding the
recombination rate of quasiparticles. We report a detailed investigation of the
quasiparticle (QP) recombination lifetime, , as a function of
temperature and magnetic field in underdoped HgBaCuO
(Hg-1201) and YBaCuO (YBCO) single crystals by ultrafast
time-resolved reflectivity. We find that exhibits a local
maximum in a small temperature window near that is prominent in
underdoped samples with coexisting charge order and vanishes with application
of a small magnetic field. We explain this unusual, non-BCS behavior by
positing that marks a transition from phase-fluctuating SC/CDW composite
order above to a SC/CDW condensate below. Our results suggest that the
superfluid in underdoped cuprates is a condensate of coherently-mixed
particle-particle and particle-hole pairs
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