We have studied far-infrared charge dynamics of Sr- and O- co-doped La1.985Sr0.015CuO4+δ with δ = 0.024 (p = 0.063 per Cu) and δ = 0.032(p = 0.07). We found that two-dimensional Wigner lattice order is the ground state of cuprates away from half-filling. We found that the presence of 2D Wigner lattice and the pinned Goldstone mode is essential for cuprate physics and superconductivity. We propose that all high Tc physics are based on the existence of these peculiar 2D electron lattices.

WIGNER LATTICE ORDER, COLLECTIVE MODE, AND SUPERCONDUCTIVITY IN THE La1.985Sr0.015CuO4+δ SYSTEM

We have studied far-infrared charge dynamics of Sr- and O- co-doped La1.985Sr0.015CuO4+delta with delta = 0.024 (p = 0.063 per Cu) and delta = 0.032 (p = 0.07). We found that two-dimensional Wigner lattice order is the ground state of cuprates away from half-filling. We found that the presence of 2D Wigner lattice and the pinned Goldstone mode is essential for the cuprate physics and superconductivity. We propose that all the high Tc physics are based on the existence of these peculiar 2D electron lattices.

Wigner Lattice Order, Collective Mode, and Superconductivity in La1.985Sr0.015CuO4+delta System

We present infrared $ab$-plane conductivity data for the layered cobaltate ${\mathrm{Na}}_{x}\mathrm{Co}{\mathrm{O}}_{2}$ at three different doping levels ($x=0.25$, 0.50, and 0.75). The Drude weight increases monotonically with hole doping, $1\ensuremath{-}x$. At the lowest hole doping level $x=0.75$ the system resembles the normal state of underdoped cuprate superconductors with a scattering rate that varies linearly with frequency and temperature and there is an onset of scattering by a bosonic mode at $600\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. Two higher hole-doped samples ($x=0.50$ and 0.25) show two different-size gaps ($110\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ and $200\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, respectively) in the optical conductivities at low temperatures and become insulators. The spectral weights lost in the gap region of 0.50 and 0.25 samples are shifted to prominent peaks at $200\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ and $800\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, respectively. We propose that the two gapped states of the two higher hole-doped samples ($x=0.50$ and 0.25) are pinned charge-ordered states.

Infrared conductivity of Na x Co O 2 : Evidence of gapped states

The reflectance of high-quality single crystals of La 2− x Sr x CuO 4 , with x =0.184 and 0.22, has been measured with the electromagnetic radiation polarized along the ab -plane for frequencies between 30 cm −1 and 9000 cm −1 (0.004–1.1 eV) and for temperatures varying from 10 K to 300 K. In the normal state, the overdoped samples develop a gap-like depression below T * in both the conductivity and the scattering rate. A signature of the pseudogap state is a depression of 1/ τ ( ω , T ) below the high-frequency straight-line extrapolation. This feature can be seen for temperatures below the superconducting transition all the way up to room temperature for the material with the nominal Sr concentration of 0.184. In the case of La x Sr 2− x CuO 4+ δ with x =0.22 the pseudogap state exists below 200 K.

The ab-plane optical conductivity of overdoped La2−xSrxCuO4 for x=0.184 and 0.22: evidence of a pseudogap

To demonstrate the first part of the titled statement we discuss examples of large effects at or very near T c associated with lattice instabilities. We then verify, again with examples, how such models are indeed misleading. The examples discussed include structural changes near T c as measured by neutron diffraction and ion channeling, capacitance changes as measured for a superconductive electrode/electrolyte double layer, phonon softening measured by Raman scattering and neutron diffraction, thermal difference reflectance spectra, persistent photoconductivity, isotope shifts, pressure-induced oxygen ordering effects, NMR studies of paramagnetic center densities, reflectance and Raman studies of sub-gap continua, photoemission studies of states near E F , linearly temperature-dependent normal-state transport properties, broadening of the resistive transition by a magnetic field, and finally we address the question of why T c is so high in some cuprates.

Effective-medium models for cuprate high-temperature superconductivity are incomplete and misleading

Recent infrared reflectance spectroscopy on high quality crystals of a number of HTSC systems shows thatall have finite conductivity in the frequency region of the superconducting gap. Results on untwinned YBCO from a number of laboratories show that this absorption is not due to experimental problems or sample-to-sample variations. Other materials also show absorption features in the gap region in the form of peaks. We discuss these results in terms of recent ideas of the effect of impurities in d-wave superconductors.

M. Reedyk

Papers

Gap states in HTSC by infrared spectroscopy