24 research outputs found
Temperature dependence of the spectral weight in p- and n-type cuprates: a study of normal state partial gaps and electronic kinetic energy
The optical conductivity of CuO2 (copper-oxygen) planes in p- and n-type
cuprates thin films at various doping levels is deduced from highly accurate
reflectivity data. The temperature dependence of the real part sigma1(omega) of
this optical conductivity and the corresponding spectral weight allow to track
the opening of a partial gap in the normal state of n-type Pr{2-x}Ce(x)CuO4
(PCCO), but not of p-type Bi2Sr2CaCu2O(8+delta} (BSCCO) cuprates. This is a
clear difference between these two families of cuprates, which we briefly
discuss. In BSCCO, the change of the electronic kinetic energy Ekin - deduced
from the spectral weight- at the superconducting transition is found to cross
over from a conventional BCS behavior (increase of Ekin below Tc to an
unconventional behavior (decrease of Ekin below Tc) as the free carrier density
decreases. This behavior appears to be linked to the energy scale over which
spectral weight is lost and goes into the superfluid condensate, hence may be
related to Mott physics
Optical Sum Rule anomalies in the High-Tc Cuprates
We provide a brief summary of the observed sum rule anomalies in the
high-T cuprate materials. A recent issue has been the impact of a
non-infinite frequency cutoff in the experiment. In the normal state, the
observed anomalously high temperature dependence can be explained as a `cutoff
effect'. The anomalous rise in the optical spectral weight below the
superconducting transition, however, remains as a solid experimental
observation, even with the use of a cutoff frequency.Comment: 4 pages, 2 figures, very brief review of optical sum rule anomal
Optical Sum Rule in Finite Bands
In a single finite electronic band the total optical spectral weight or
optical sum carries information on the interactions involved between the charge
carriers as well as on their band structure. It varies with temperature as well
as with impurity scattering. The single band optical sum also bears some
relationship to the charge carrier kinetic energy and, thus, can potentially
provide useful information, particularly on its change as the charge carriers
go from normal to superconducting state. Here we review the considerable
advances that have recently been made in the context of high oxides, both
theoretical and experimental.Comment: Review article accepted for publication in J. Low Temp. Phys. 29
pages, 33 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
Electromotive forces and the Meissner effect puzzle
In a voltaic cell, positive (negative) ions flow from the low (high)
potential electrode to the high (low) potential electrode, driven by an
`electromotive force' which points in opposite direction and overcomes the
electric force. Similarly in a superconductor charge flows in direction
opposite to that dictated by the Faraday electric field as the magnetic field
is expelled in the Meissner effect. The puzzle is the same in both cases: what
drives electric charges against electromagnetic forces? I propose that the
answer is also the same in both cases: kinetic energy lowering, or `quantum
pressure'
Respuesta radiación infrarroja y visible de pelìculas delgadas de Bi2Sr2CaC2O8+d
Para una película delgada epitaxial superconductora de Bi2Sr2CaC2O8+d crecida sobre un substra-to de LaAlO3, por el método de pulverización catódica a altas presiones de oxígeno, se comparan las mediciones de la resistividad dc en el plano ab con los valores encontrados al calcular la con-ductividad infraroja, cuando la frecuencia tiende a cero, a través del cálculo del peso espectral, después de aplicar la regla de la suma de Ferrell-Glover-Tinkham (FGT), considerando el modelo de Drude-Lorentz-London. La conductividad óptica fue obtenido de la medida de la reflectividad entre 30-25000 cm-1 y temperaturas entre 20 y 30K
Imaging of room-temperature ferromagnetic nano-domains at the surface of a non-magnetic oxide
International audienceTwo-dimensional electron gases at oxide surfaces or interfaces show exotic ordered states of matter, like superconductivity, magnetism or spin-polarized states, and are a promising platform for alternative oxide-based electronics. Here we directly image a dense population of randomly distributed ferromagnetic domains of similar to 40 nm typical sizes at room temperature at the oxygen-deficient surface of SrTiO3, a non-magnetic transparent insulator in the bulk. We use laser-based photoemission electron microscopy, an experimental technique that gives selective spin detection of the surface carriers, even in bulk insulators, with a high spatial resolution of 2.6 nm. We furthermore find that the Curie temperature in this system is as high as 900 K. These findings open perspectives for applications in nano-domain magnetism and spintronics using oxide-based devices, for instance through the nano-engineering of oxygen vacancies at surfaces or interfaces of transition-metal oxides
The surface state of URu2Si2
International audienceThe 'hidden-order' (HO) transition of URu2Si2 remains a puzzle after 25 years of research. Using high-resolution angle-resolved photoemission spectroscopy (ARPES) we found that a hole-like band around Γ having its band maximum at E = −35 meV, and previously thought to be a bulk band of the system, is indeed a surface state not related to the HO phase transition. Here we present our detailed investigations to assign that state to a surface feature, and discuss on the possible origins of this surface band