372 research outputs found
Scaling of the superfluid density in severely underdoped YBa2Cu3O6+y
Recent measurements on extremely-underdoped YBa2Cu3O6+y [Phys. Rev. Lett. 99,
237003 (2007)] have allowed the critical temperature (T_c), superfluid density
[rho_0 (T << T_c)] and dc conductivity [sigma_dc (T ~ T_c)] to be determined
for a series of electronic dopings for T_c ~ 3 - 17 K. The general scaling
relation rho_0/8 ~ 4.4 sigma_dc T_c is observed, extending the validity of both
the ab-plane and c-axis scaling an order of magnitude and creating a region of
overlap. This suggests that severely underdoped materials may constitute a
Josephson phase; as the electronic doping is increased a more uniform
superconducting state emerges.Comment: 5 pages, 3 figure
Doping of a One-Dimensional Mott Insulator: Photoemision and Optical Studies of SrCuO
The spectral properties of a one-dimensional (1D) single-chain Mott insulator
SrCuO have been studied in angle-resolved photoemission and optical
spectroscopy, at half filling and with small concentrations of extra charge
doped into the chains via high oxygen pressure growth. The single- particle gap
is reduced with oxygen doping, but the metallic state is not reached. The
bandwidth of the charge-transfer band increases with doping, while the state
becomes narrower, allowing unambiguous observation of separated spinon and
holon branches in the doped system. The optical gap is not changed upon doping,
indicating that a shift of chemical potential rather than decrease of
corelation gap is responsible for the apparent reduction of the photoemission
gap.Comment: 4 pages, 2 figure
Coherence, incoherence and scaling along the c axis of YBa_2Cu_3O_{6+x}
The optical properties of single crystals of YBa_2Cu_3O_{6+x} have been
examined along the c axis above and below the critical temperature (T_c) for a
wide range of oxygen dopings. The temperature dependence of the
optically-determined value of the dc conductivity (\sigma_{dc}) in the normal
state suggests a crossover from incoherent (hopping-type) transport at lower
oxygen dopings (x \lesssim 0.9) to more coherent anisotropic three-dimensional
behavior in the overdoped (x \approx 0.99) material at temperatures close to
T_c. The assumption that superconductivity occurs along the c axis through the
Josephson effect yields a scaling relation between the strength of the
superconducting condensate (\rho_{s,c}, a measure of the number of
superconducting carriers), the critical temperature, and the normal-state
c-axis value for \sigma_{dc} just above T_c; \rho_{s,c} \propto \sigma_{dc}
T_c. This scaling relation is observed along the c axis for all oxygen dopings,
as well as several other cuprate materials. However, the agreement with the
Josephson coupling model does not necessarily imply incoherent transport,
suggesting that these materials may indeed be tending towards coherent behavior
at the higher oxygen dopings.Comment: Six pages with four figures and one tabl
Systematic review of antimicrobial drug prescribing in hospitals.
Prudent antibiotic prescribing to hospital inpatients has the potential to reduce the incidences of antimicrobial resistance and healthcare-associated infection. We reviewed the literature from January 1980 to November 2003 to identify rigorous evaluations of interventions to improve hospital antibiotic prescribing. We identified 66 studies with interpretable data of which 16 reported 20 microbiological outcomes: Gram negative resistant bacteria (GNRB), 10 studies; Clostridium difficile associated diarrhoea (CDAD), 5 studies; vancomycin resistant enterococci (VRE), 3 studies and methicillin resistant Staphylococcus aureus (MRSA), 2 studies. Four studies provide good evidence that the intervention changed microbial outcomes with low risk of alternative explanations, eight studies provide less convincing evidence and four studies were negative. The strongest and most consistent evidence was for CDAD but we were able to analyse only the immediate impact of interventions because of nonstandardised durations of follow up. The ability to compare results of studies could be substantially improved by standardising methodology and reporting
Infrared properties of MgAlBC) single crystals in the normal and superconducting state
The reflectivity of -oriented MgAl(BC) single crystals has been measured by means of infrared
microspectroscopy for cm. An increase with doping of
the scattering rates in the and bands is observed, being more
pronounced in the C doped crystals. The -band plasma frequency also
changes with doping due to the electron doping, while the -band one is
almost unchanged. Moreover, a interband excitation, predicted
by theory, is observed at eV in the undoped sample,
and shifts to lower energies with doping. By performing theoretical calculation
of the doping dependence , the experimental observations can be
explained with the increase with electron doping of the Fermi energy of the
holes in the -band. On the other hand, the band density of
states seems not to change substantially. This points towards a reduction
driven mainly by disorder, at least for the doping level studied here. The
superconducting state has been also probed by infrared synchrotron radiation
for cm in one pure and one C-doped sample. In the
undoped sample ( = 38.5 K) a signature of the -gap only is observed.
At = 0.08 ( = 31.9 K), the presence of the contribution of the
-gap indicates dirty-limit superconductivity in both bands.Comment: 12 pages, 9 figure
Two-dimensional conical dispersion in ZrTe5 evidenced by optical spectroscopy
Zirconium pentatelluride was recently reported to be a 3D Dirac semimetal,
with a single conical band, located at the center of the Brillouin zone. The
cone's lack of protection by the lattice symmetry immediately sparked vast
discussions about the size and topological/trivial nature of a possible gap
opening. Here we report on a combined optical and transport study of ZrTe5,
which reveals an alternative view of electronic bands in this material. We
conclude that the dispersion is approximately linear only in the a-c plane,
while remaining relatively flat and parabolic in the third direction (along the
b axis). Therefore, the electronic states in ZrTe5 cannot be described using
the model of 3D Dirac massless electrons, even when staying at energies well
above the band gap 6 meV found in our experiments at low temperatures.Comment: Physical Review Letters 122, 217402 (2019). Corrected acknowledgment
LEC GaAs for integrated circuit applications
Recent developments in liquid encapsulated Czochralski techniques for the growth of semiinsulating GaAs for integrated circuit applications have resulted in significant improvements in the quality and quantity of GaAs material suitable for device processing. The emergence of high performance GaAs integrated circuit technologies has accelerated the demand for high quality, large diameter semiinsulating GaAs substrates. The new device technologies, including digital integrated circuits, monolithic microwave integrated circuits and charge coupled devices have largely adopted direct ion implantation for the formation of doped layers. Ion implantation lends itself to good uniformity and reproducibility, high yield and low cost; however, this technique also places stringent demands on the quality of the semiinsulating GaAs substrates. Although significant progress was made in developing a viable planar ion implantation technology, the variability and poor quality of GaAs substrates have hindered progress in process development
Non-uniform carrier density in CdAs evidenced by optical spectroscopy
We report the detailed optical properties of CdAs crystals in a wide
parameter space: temperature, magnetic field, carrier concentration and crystal
orientation. We investigate high-quality crystals synthesized by three
different techniques. In all the studied samples, independently of how they
were prepared and how they were treated before the optical experiments, our
data indicate conspicuous fluctuations in the carrier density (up to 30%).
These charge puddles have a characteristic scale of 100 m, they become
more pronounced at low temperatures, and possibly, they become enhanced by the
presence of crystal twinning. The Drude response is characterized by very small
scattering rates ( meV) for as-grown samples. Mechanical treatment,
such as cutting or polishing, influences the optical properties of single
crystals, by increasing the Drude scattering rate and also modifying the high
frequency optical response. Magneto-reflectivity and Kerr rotation are
consistent with electron-like charge carriers and a spatially non-uniform
carrier density.Comment: Accepted in Physical Review
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