403 research outputs found
Scaling of the superfluid density in high-temperature superconductors
A scaling relation \rho_s \simeq 35\sigma_{dc}T_c has been observed in the
copper-oxide superconductors, where \rho_s is the strength of the
superconducting condensate, T_c is the critical temperature, and \sigma_{dc} is
the normal-state dc conductivity close to T_c. This scaling relation is
examined within the context of a clean and dirty-limit BCS superconductor.
These limits are well established for an isotropic BCS gap 2\Delta and a
normal-state scattering rate 1/\tau; in the clean limit 1/\tau \ll 2\Delta, and
in the dirty limit 1/\tau > 2\Delta. The dirty limit may also be defined
operationally as the regime where \rho_s varies with 1/\tau. It is shown that
the scaling relation \rho_s \propto \sigma_{dc}T_c is the hallmark of a BCS
system in the dirty-limit. While the gap in the copper-oxide superconductors is
considered to be d-wave with nodes and a gap maximum \Delta_0, if 1/\tau >
2\Delta_0 then the dirty-limit case is preserved. The scaling relation implies
that the copper-oxide superconductors are likely to be in the dirty limit, and
that as a result the energy scale associated with the formation of the
condensate is scaling linearly with T_c. The a-b planes and the c axis also
follow the same scaling relation. It is observed that the scaling behavior for
the dirty limit and the Josephson effect (assuming a BCS formalism) are
essentially identical, suggesting that in some regime these two effects may be
viewed as equivalent. This raises the possibility that electronic
inhomogeneities in the copper-oxygen planes may play an important role in the
nature of the superconductivity in the copper-oxide materials.Comment: 8 pages with 5 figures and 1 tabl
On the optical conductivity of Electron-Doped Cuprates I: Mott Physics
The doping and temperature dependent conductivity of electron-doped cuprates
is analysed. The variation of kinetic energy with doping is shown to imply that
the materials are approximately as strongly correlated as the hole-doped
materials. The optical spectrum is fit to a quasiparticle scattering model;
while the model fits the optical data well, gross inconsistencies with
photoemission data are found, implying the presence of a large, strongly doping
dependent Landau parameter
Infrared Signature of the Superconducting State in Pr(2-x)Ce(x)CuO(4)
We measured the far infrared reflectivity of two superconducting
Pr(2-x)Ce(x)CuO(4) films above and below Tc. The reflectivity in the
superconducting state increases and the optical conductivity drops at low
energies, in agreement with the opening of a (possibly) anisotropic
superconducting gap. The maximum energy of the gap scales roughly with Tc as 2
Delta_{max} / kB Tc ~ 4.7. We determined absolute values of the penetration
depth at 5 K as lambda_{ab} = (3300 +/- 700) A for x = 0.15 and lambda_{ab} =
(2000 +/- 300) A for x = 0.17. A spectral weight analysis shows that the
Ferrell-Glover-Tinkham sum rule is satisfied at conventional low energy scales
\~ 4 Delta_{max}.Comment: 4 pages, 4 figure
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
Optical conductivity of URuSi in the Kondo Liquid and Hidden-Order Phases
We measured the polarized optical conductivity of URuSi from room
temperature down to 5 K, covering the Kondo state, the coherent Kondo liquid
regime, and the hidden-order phase. The normal state is characterized by an
anisotropic behavior between the ab plane and c axis responses. The ab plane
optical conductivity is strongly influenced by the formation of the coherent
Kondo liquid: a sharp Drude peak develops and a hybridization gap at 12 meV
leads to a spectral weight transfer to mid-infrared energies. The c axis
conductivity has a different behavior: the Drude peak already exists at 300 K
and no particular anomaly or gap signature appears in the coherent Kondo liquid
regime. When entering the hidden-order state, both polarizations see a dramatic
decrease in the Drude spectral weight and scattering rate, compatible with a
loss of about 50 % of the carriers at the Fermi level. At the same time a
density-wave like gap appears along both polarizations at about 6.5 meV at 5 K.
This gap closes respecting a mean field thermal evolution in the ab plane.
Along the c axis it remains roughly constant and it "fills up" rather than
closing.Comment: 10 pages, 7 figure
Infrared phonon dynamics of multiferroic BiFeO3 single crystal
We discuss the first infrared reflectivity measurement on a BiFeO3 single
crystal between 5 K and room temperature. The 9 predicted ab-plane E phonon
modes are fully and unambiguously determined. The frequencies of the 4 A1
c-axis phonons are found. These results settle issues between theory and data
on ceramics. Our findings show that the softening of the lowest frequency E
mode is responsible for the temperature dependence of the dielectric constant,
indicating that the ferroelectric transition in BiFeO3 is soft-mode driven.Comment: 5 pages (figures included
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