244 research outputs found
Pressure-tuning of the c-f hybridization in Yb metal detected by infrared spectroscopy up to 18 GPa
It has been known that the elemental Yb, a divalent metal at mbient pressure,
becomes a mixed-valent metal under external pressure, with its valence reaching
~2.6 at 30 GPa. In this work, infrared spectroscopy has been used to probe the
evolution of microscopic electronic states associated with the valence
crossover in Yb at external pressures up to 18 GPa. The measured infrared
reflectivity spectrum R(w) of Yb has shown large variations with pressure. In
particular, R(w) develops a deep minimum in the mid-infrared, which shifts to
lower energy with increasing pressure. The dip is attributed to optical
absorption due to a conduction c-f electron hybridization state, similarly to
those previously observed for heavy fermion compounds. The red shift of the dip
indicates that the - hybridization decreases with pressure, which is
consistent with the increase of valence.Comment: 2 pages, to appear in J. Phys. Soc. Jpn. Supp
Searching for the Slater Transition in the Pyrochlore CdOsO with Infrared Spectroscopy
Infrared reflectance measurements were made on the single crystal pyrochlore
CdOsO in order to examine the transformations of the
electronic structure and crystal lattice across the boundary of the metal
insulator transition at . All predicted IR active phonons are
observed in the conductivity over all temperatures and the oscillator strength
is found to be temperature independent. These results indicate that charge
ordering plays only a minor role in the MIT and that the transition is strictly
electronic in nature. The conductivity shows the clear opening of a gap with
. The gap opens continuously, with a temperature
dependence similar to that of BCS superconductors, and the gap edge having a
distinct dependence. All of these
observables support the suggestion of a Slater transition in CdOsO.Comment: 4 pages, 4 figure
Optical investigation of the charge-density-wave phase transitions in
We have measured the optical reflectivity of the quasi
one-dimensional conductor from the far infrared up to the
ultraviolet between 10 and 300 using light polarized along and normal to
the chain axis. We find a depletion of the optical conductivity with decreasing
temperature for both polarizations in the mid to far-infrared region. This
leads to a redistribution of spectral weight from low to high energies due to
partial gapping of the Fermi surface below the charge-density-wave transitions
at 145 K and 59 K. We deduce the bulk magnitudes of the CDW gaps and discuss
the scattering of ungapped free charge carriers and the role of fluctuations
effects
Sum rules and energy scales in the high-temperature superconductor YBa2Cu3O6+x
The Ferrell-Glover-Tinkham (FGT) sum rule has been applied to the temperature
dependence of the in-plane optical conductivity of optimally-doped
YBa_2Cu_3O_{6.95} and underdoped YBa_2Cu_3O_{6.60}. Within the accuracy of the
experiment, the sum rule is obeyed in both materials. However, the energy scale
\omega_c required to recover the full strength of the superfluid \rho_s in the
two materials is dramatically different; \omega_c \simeq 800 cm^{-1} in the
optimally doped system (close to twice the maximum of the superconducting gap,
2\Delta_0), but \omega_c \gtrsim 5000 cm^{-1} in the underdoped system. In both
materials, the normal-state scattering rate close to the critical temperature
is small, \Gamma < 2\Delta_0, so that the materials are not in the dirty limit
and the relevant energy scale for \rho_s in a BCS material should be twice the
energy gap. The FGT sum rule in the optimally-doped material suggests that the
majority of the spectral weight of the condensate comes from energies below
2\Delta_0, which is consistent with a BCS material in which the condensate
originates from a Fermi liquid normal state. In the underdoped material the
larger energy scale may be a result of the non-Fermi liquid nature of the
normal state. The dramatically different energy scales suggest that the nature
of the normal state creates specific conditions for observing the different
aspects of what is presumably a central mechanism for superconductivity in
these materials.Comment: RevTeX 4 file, 9 pages with 7 embedded eps figure
c-Axis Transport and Resistivity Anisotropy of Lightly- to Moderately-Doped La_{2-x}Sr_{x}CuO_{4} Single Crystals: Implications on the Charge Transport Mechanism
Both the in-plane and the out-of-plane resistivities (\rho_{ab} and \rho_{c})
are measured in high-quality La_{2-x}Sr_{x}CuO_{4} (LSCO) single crystals in
the lightly- to moderately-doped region, x = 0.01 to 0.10, and the resistivity
anisotropy is determined. In all the samples studied, the anisotropy ratio \rho
_{c}/\rho_{ab} quickly increases with decreasing temperature, although in
non-superconducting samples the strong localization effect causes \rho
_{c}/\rho_{ab} to decrease at low temperatures. Most notably, it is found that
\rho_{c}/\rho_{ab} at moderate temperatures (100 - 300 K) is almost completely
independent of doping in the non-superconducting regime (x = 0.01 to 0.05);
this indicates that the same charge confinement mechanism that renormalizes the
c-axis hopping rate is at work down to x = 0.01. It is discussed that this
striking x-independence of \rho_{c}/\rho_{ab} is consistent with the idea that
holes form a self-organized network of hole-rich regions, which also explains
the unusually metallic in-plane transport of the holes in the lightly-doped
region. Furthermore, the data for x > 0.05 suggest that the emergence of the
superconductivity is related to an increase in the c-axis coupling.Comment: 7 pages, 5 figures, submitted to Phys. Rev.
Correlation gap in the heavy-fermion antiferromagnet UPd_2Al_3
The optical properties of the heavy-fermion compound UPdAl have been
measured in the frequency range from 0.04 meV to 5 meV (0.3 to 40 cm) at
temperatures K. Below the coherence temperature K, the hybridization gap opens around 10 meV. As the temperature decreases
further ( K), a well pronounced pseudogap of approximately 0.2 meV
develops in the optical response; we relate this to the antiferromagnetic
ordering which occurs below K. The frequency dependent mass and
scattering rate give evidence that the enhancement of the effective mass mainly
occurs below the energy which is associated to the magnetic correlations
between the itinerant and localized 5f electrons. In addition to this
correlation gap, we observe a narrow zero-frequency conductivity peak which at
2 K is less than 0.1 meV wide, and which contains only a fraction of the
delocalized carriers. The analysis of the spectral weight infers a loss of
kinetic energy associated with the superconducting transition.Comment: RevTex, 15 pages, 7 figure
Doping dependence of the gap anisotropy in LCCO studied by millimeter-wave spectroscopy
We measure the penetration depth of optimally doped and underdoped
La2-xCexCuO4 in the millimeter frequency domain (4 - 7 cm-1) and for
temperatures 2 K < T < 300 K. The penetration depth as function of temperature
reveals significant changes on electron doping. It shows quadratic temperature
dependence in underdoped samples, but increases almost exponentially at optimal
doping. Significant changes in the gap anisotropy (or even in the gap symmetry)
may account for this transition.Comment: 4 pages, 4 figure
Optical symmetries and anisotropic transport in high-Tc superconductors
A simple symmetry analysis of in-plane and out-of-plane transport in a family
of high temperature superconductors is presented. It is shown that generalized
scaling relations exist between the low frequency electronic Raman response and
the low frequency in-plane and out-of-plane conductivities in both the normal
and superconducting states of the cuprates. Specifically, for both the normal
and superconducting state, the temperature dependence of the low frequency
Raman slope scales with the axis conductivity, while the
Raman slope scales with the in-plane conductivity. Comparison with experiments
in the normal state of Bi-2212 and Y-123 imply that the nodal transport is
largely doping independent and metallic, while transport near the BZ axes is
governed by a quantum critical point near doping holes per
CuO plaquette. Important differences for La-214 are discussed. It is also
shown that the axis conductivity rise for is a consequence of
partial conservation of in-plane momentum for out-of-plane transport.Comment: 16 pages, 8 Figures (3 pages added, new discussion on pseudogap and
charge ordering in La214
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