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تسجيل مستخدم جديدPolarization-dependent infrared reflectivity study of Sr$_{2.5}$Ca$_{11.5}$Cu$_{24}$O$_{41}$ under pressure: Charge dynamics, charge distribution, and anisotropy
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We present a polarization-dependent infrared reflectivity study of the spin-ladder compound Sr$_{2.5}$Ca$_{11.5}$Cu$_{24}$O$_{41}$ under pressure. The optical response is strongly anisotropic, with the highest reflectivity along the ladders/chains (textbf{E}$|$c) revealing a metallic character. For the polarization direction perpendicular to the ladder plane, an insulating behavior is observed. With increasing pressure the optical conductivity for textbf{E}$|$c shows a strong increase, which is most pronounced below 2000~cm$^{-1}$. According to the spectral weight analysis of the textbf{E}$|$c optical conductivity the hole concentration in the ladders increases with increasing pressure and tends to saturate at high pressure. At $sim$7.5~GPa the number of holes per Cu atom in the ladders has increased by $Delta delta$=0.09 ($pm$0.01), and the Cu valence in the ladders has reached the value +2.33. The optical data suggest that Sr$_{2.5}$Ca$_{11.5}$Cu$_{24}$O$_{41}$ remains electronically highly anisotropic up to high pressure, also at low temperatures.
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Within the two-leg $t$-J ladder, the spin dynamics of the pressure-induced two-leg ladder cuprate superconductor Sr$_{14-x}$Ca$_{x}$Cu$_{24}$O$_{41}$ is studied based on the kinetic energy driven superconducting mechanism. It is shown that in the pre
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Using far-infrared spectroscopy we have studied the magnetic field and temperature dependence of the spin gap modes in the chains of Sr$_{14}$Cu$_{24}$O$_{41}$. Two triplet modes T$_1$ and T$_2$ were found in the center of the Brillouin zone at $Delt
a_1=9.65$ meV and $Delta_2=10.86$ meV in zero magnetic field. The T$_1$ mode was excited when the electric field vector ${bf E}$ of the light was polarized along the b axis (perpendicular to the planes of chains and ladders) and T$_2$ was excited for ${bf E}parallel {bf a}$ (perpendicular to the chains and along the rungs). Up to the maximum magnetic field of 18 T, applied along the chains, the electron $g$ factors of these two modes were similar, $g_{1c}=2.049$ and $g_{2c}=2.044$. Full linewidth at half maximum for both modes was 1 cm$^{-1}$ (0.12 meV) at 4K and increased with $T$. The temperature dependence of mode energies and line intensities was in agreement with the inelastic neutron scattering results from two groups [Matsuda {it et al.}, Phys. Rev. B {bf 59}, 1060 (1999) and Regnault {it et al.}, Phys. Rev. B {bf 59}, 1055 (1999)]. The T$_1$ mode has not been observed by inelastic neutron scattering in the points of the $k$-space equivalent to the center of the Brillouin zone. Our study indicates that the zone structure model of magnetic excitations of Sr$_{14}$Cu$_{24}$O$_{41}$ must be modified to include a triplet mode at 9.65 meV in the center of the magnetic Brillouin zone.
The knowledge of the charge carrier distribution among the different orbitals of Cu and O is a precondition for the understanding of the physical properties of various Cu-O frameworks. We employ electron energy-loss spectroscopy to elucidate the char
ge carrier plasmon dispersion in (La, Ca)$_x$Sr$_{14-x}$Cu$_{24}$O$_{41}$ in dependency of $x$ as well as temperature. We observe that the energy of the plasmon increases upon increasing Ca content, which signals an internal charge redistribution between the two Cu-O subsystems. Moreover, contrary to an uncorrelated model we come to the conclusion that the holes transferred to the Cu$_2$O$_3$ ladders are mainly located in the bonding and not in the anti-bonding band. This is caused by an orbital dependent Mott transition.
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