228 research outputs found
Resistivity phase diagram of cuprates revisited
The phase diagram of the cuprate superconductors has posed a formidable
scientific challenge for more than three decades. This challenge is perhaps
best exemplified by the need to understand the normal-state charge transport as
the system evolves from Mott insulator to Fermi-liquid metal with doping. Here
we report a detailed analysis of the temperature (T) and doping (p) dependence
of the planar resistivity of simple-tetragonal HgBaCuO
(Hg1201), the single-CuO-layer cuprate with the highest optimal . The
data allow us to test a recently proposed phenomenological model for the
cuprate phase diagram that combines a universal transport scattering rate with
spatially inhomogeneous (de)localization of the Mott-localized hole. We find
that the model provides an excellent description of the data. We then extend
this analysis to prior transport results for several other cuprates, including
the Hall number in the overdoped part of the phase diagram, and find little
compound-to-compound variation in (de)localization gap scale. The results point
to a robust, universal structural origin of the inherent gap inhomogeneity that
is unrelated to doping-related disorder. They are inconsistent with the notion
that much of the phase diagram is controlled by a quantum critical point, and
instead indicate that the unusual electronic properties exhibited by the
cuprates are fundamentally related to strong nonlinearities associated with
subtle nanoscale inhomogeneity.Comment: 22 pages, 5 figure
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
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
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
Indirect excitons in a potential energy landscape created by a perforated electrode
We report on the principle and realization of an excitonic device: a ramp that directs the transport of indirect excitons down a potential energy gradient created by a perforated electrode at a constant voltage. The device provides an experimental proof of principle for controlling exciton transport with electrode density gradients. We observed that the exciton transport distance along the ramp increases with increasing exciton density. This effect is explained in terms of disorder screening by repulsive exciton-exciton interactions
Transport of indirect excitons in high magnetic fields
We present spatially and spectrally resolved photoluminescence measurements of indirect excitons in high magnetic fields. Long indirect exciton lifetimes give the opportunity to measure magnetoexciton transport by optical imaging. Indirect excitons formed from electrons and holes at zeroth Landau levels (0e−0h indirect magnetoexcitons) travel over large distances and form a ring emission pattern around the excitation spot. In contrast, the spatial profiles of 1e−1h and 2e−2h indirect magnetoexciton emission closely follow the laser excitation profile. The 0e−0h indirect magnetoexciton transport distance reduces with increasing magnetic field. These effects are explained in terms of magnetoexciton energy relaxation and effective mass enhancement
- …