275 research outputs found
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
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
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
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
The c axis optical conductivity of layered systems in the superconducting state
In this paper, we discuss the c axis optical conductivity Re [sigma_c(omega)]
in the high T_c superconductors, in the superconducting state. The basic
premise of this work is that electrons travelling along the c axis between
adjacent CuO_2 layers must pass through several intervening layers. In earlier
work we found that, for weak inter-layer coupling, it is preferable for
electrons to travel along the c axis by making a series of interband
transitions rather than to stay within a single (and very narrow) band.
Moreover, we found that many of the properties of the normal state optical
conductivity, including the pseudogap could be explained by interband
transitions. In this work we examine the effect of superconductivity on the
interband conductivity. We find that, while the onset of superconductivity is
clearly evident in the spectrum, there is no clear signature of the symmetry of
the superconducting order parameter.Comment: 6 pages, 4 figure
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
Marginal Fermi liquid analysis of 300 K reflectance of Bi2Sr2CaCu2O8+x
We use 300 K reflectance data to investigate the normal-state electrodynamics
of the high temperature superconductor BiSrCaCuO
over a wide range of doping levels. The data show that at this temperature the
free carriers are coupled to a continuous spectrum of fluctuations. Assuming
the Marginal Fermi Liquid (MFL) form as a first approximation for the
fluctuation spectrum, the doping-dependent coupling constant can
be estimated directly from the slope of the reflectance spectrum. We find that
decreases smoothly with the hole doping level, from underdoped
samples with ( K) where to overdoped
samples with , ( K) where . An analysis of
the intercept and curvature of the reflectance spectrum shows deviations from
the MFL spectrum symmetrically placed at the optimal doping point . The
Kubo formula for the conductivity gives a better fit to the experiments with
the MFL spectrum up to 2000 cm and with an additional Drude component or
an additional Lorentz component up to 7000 cm. By comparing three
different model fits we conclude that the MFL channel is necessary for a good
fit to the reflectance data. Finally, we note that the monotonic variation of
the reflectance slope with doping provides us with an independent measure of
the doping level for the Bi-2212 system.Comment: 11 pages, 11 figure
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