519 research outputs found
The Pseudogap in La(2-x)Sr(x)CuO(4): A Raman Viewpoint
We report the results of Raman scattering experiments on single crystals of
La(2-x)Sr(x)CuO(4) [La214] as a function of temperature and doping. In
underdoped compounds low-energy B1g spectral weight is depleted in association
with the opening of a pseudogap on regions of the Fermi surface located near
(pi, 0) and (0, pi). The magnitude of the depletion increases with decreasing
doping, and in the most underdoped samples, with decreasing temperature. The
spectral weight that is lost at low-energies (omega < 800 cm-1) is transferred
to the higher energy region normally occupied by multi-magnon scattering. From
the normal state B2g spectra we have determined the scattering rate
Gamma(omega, T) of qausiparticles located near the diagonal directions in
k-space, (pi/2, pi/2) regions. In underdoped compounds, Gamma(omega, T) is
suppressed at low temperatures for energies less than Eg(x) ~ 800 cm-1. The
observed doping dependence of the two-magnon scattering and the scattering rate
suppression thus suggest that the pseudogap is characterized by an energy scale
Eg ~ J, where J is the antiferromagnetic super-exchange energy. Comparison with
the results from other techniques provides a consistent picture of the
pseudogap in La214.Comment: 6 pages, 5 figures, minor revisions include correct form of the B2g
Raman response function and new figures of the recalculated B2g scattering
rate. Presented at the APS March99 Meeting, accepted for publication in the
Canadian Journal of Physic
Impact of lithium composition on the thermoelectric properties of the layered cobalt oxide system LixCoO2
Thermoelectric properties of the layered cobalt oxide system LixCoO2 were
investigated in a wide range of Li composition, 0.98 >= x >= 0.35. Single-phase
bulk samples of LixCoO2 were successfully obtained through electrochemical
deintercalation of Li from the pristine LiCoO2 phase. While LixCoO2 with x >=
0.94 is semiconductive, the highly Li-deficient phase (0.75 >= x >= 0.35)
exhibits metallic conductivity. The magnitude of Seebeck coefficient at 293 K
(S293K) significantly depends on the Li content (x). The S293K value is as
large as +70 ~ +100 uV/K for x >= 0.94, and it rapidly decreases from +90 uV/K
to +10 uV/K as x is lowered within a Li composition range of 0.75 >= x >= 0.50.
This behavior is in sharp contrast to the results of x <= 0.40 for which the
S293K value is small and independent of x (+10 uV/K), indicating that a
discontinuous change in the thermoelectric characteristics takes place at x =
0.40 ~ 0.50. The unusually large Seebeck coefficient and metallic conductivity
are found to coexist in a narrow range of Li composition at about x = 0.75. The
coexistence, which leads to an enhanced thermoelectric power factor, may be
attributed to unusual electronic structure of the two-dimensional CoO2 block.Comment: 29 pages, 1 table, 8 figure
Anisotropic softening of collective charge modes in the vicinity of critical doping in a doped Mott insulator
Momentum resolved inelastic resonant x-ray scattering is used to map the
evolution of charge excitations over a large range of energies, momenta and
doping levels in the electron doped Mott insulator class
NdCeCuO. As the doping induced AFM-SC
(antiferromagnetic-superconducting) transition is approached, we observe an
anisotropic softening of collective charge modes over a large energy scale
along the Gamma to (\pi,\pi)-direction, whereas the modes exhibit broadening
( 1 eV) with relatively little softening along Gamma to (\pi,0) with
respect to the parent Mott insulator (x=0). Our study indicates a systematic
collapse of the gap consistent with the scenario that the system dopes
uniformly with electrons even though the softening of the modes involves an
unusually large energy scale.Comment: 5 pages + 5 Figure
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