c-Axis Optical Sum Rule and a Possible New Collective Mode inLa2−xS

Abstract

We present the c-axis optical conductivity σ1c(ω, T ) of underdoped (x = 0.12) and optimally doped (x = 0.15) La2−xSrxCuO4 from 4 meV to 1.8 eV obtained by a combination of reflectivity and transmission spectra. In addition to the opening of the superconducting gap, we observe an increase of conductivity above the gap up to 270 meV with a maximal effect at about 120 meV. This may indicate a new collective mode at a surprisingly large energy scale. The Ferrell-Glover-Tinkham sum rule is violated for both doping levels. Although the relative value of the violation is much larger for the underdoped sample, the absolute increase of the low-frequency spectral weight, including that of the condensate, is higher in the optimally doped regime. Our results resemble in many respects the observations in YBa2Cu3O 7−δ .PACS numbers: 74.25. Gz, 78.20.Ci The charge transport between the CuO 2 planes in the high-T c cuprates forms one of the most intriguing puzzles of these materials. On the one hand the essential common physics seems to lie in the collective behavior of holes doped into a 2D Mott insulator, while the interplane conductivity strongly depends on the interlayer chemistry, which varies dramatically among different members of the high-T c family. On the other hand, it is the in-plane 'confinement' that preserves a possibility of significant lowering of the c-axis kinetic energy (KE) as the 3D coherent movement of the Cooper pairs is restored below T c [1,2], which, however, was shown to be a small effect in some single-layer compounds [3,4]. The models based on the KE lowering in the superconducting (SC) state predict the violation of the Ferrell-Glover-Tinkham (FGT) optical sum rule [5], which means that the spectral weight (SW) of the SC condensate is collected not only from energies of the order of 2∆ but also from higher frequencies. For the c-axis the violation has been experimentally tested in YBa 2 Cu 3 O 7−δ (YBCO) over a broad doping range and a significant increase of the relative violation value was found in the underdoped regime [6]. In the case of La 2−x Sr x CuO 4 (LSCO) a value of 50% has been reported for a slightly underdoped sample [7], although the doping dependence is still not known. Thus, the c-axis KE lowers in the SC state, which, however, leaves room for debate whether this is a by-product or the very reason for superconductivity [1,2,8,9].

Microscopic gauge-invariant theory of thec-axis infrared response of bilayer cuprate superconductors and the origin of the superconductivity-induced absorption bands

“…The calculated c-axis conductivity of the single-layer superconductor is qualitatively similar to the measured conductivity of La 2−x Sr x CuO 4 reported in Ref. 46. Figure 15(c) shows the c-axis conductivities of the same bilayer systems as in (a), but calculated using the approach proposed by SM, where the nondiagonal components of the conductivity σ bl/int and σ int/bl are neglected, and the diagonal component σ bl/bl (σ int/int ) is approximated by the conductivity of the single-layer superconductor with the hopping parameter equal to t ⊥ (t ′ ⊥ ).…”

Optical probes of electron correlations in solids

“…Another interesting application of this is to superconductors. In figure 17 we show the transmission spectrum of LSCO at a temperature slightly above T c and far below [8]. One can see that the position of the maxima has changed and this shows up in the polariton dispersion in an interesting way (see right panel figure 17).…”

In-plane optical spectral weight transfer in optimally dopedBi2Sr2Ca2Cu3O10

“…5͒ enables an alternative way to quantify the superconductivity-induced spectral weight changes which gives perhaps a better feeling of the error bars involved. 30 We recently applied this procedure, which is essentially a temperature-modulation method, to a similar set of data on optimally doped Bi2212. In order to separate the superconductivity-induced effect from the large temperature dependence in the normal state not related to the onset of superconductivity we apply the "slope-difference operator" ⌬ s defined as…”