By Christian E. Matt Swiss Light Source, Paul Scherrer Institute & Laboratory for Solid State Physics, ETHZ
Claudia G. Fatuzzo, Institute for Condensed Matter Physics, EPFL
Johan Chang, Physik-Institut, Universität Zürich
based on an article published in Physical Review B
For the past 30 years, high-temperature superconductivity presents itself as one of the most important problems for physicists in the field of condensed matter physics. Layered copper-oxide compounds, which still hold the ambient pressure record of the maximum achievable transition temperature (Tc), exhibit a rich phase diagram including Mott and pseudogap physics along with charge-density wave (CDW) and spin-density wave (SDW) orders. An out standing question is to understand how these phenomena are related. There exists compelling evidence for superconductivity (SC) and charge-wave-order co-existing through an intertwined competing relation [1-3]. How this composite order (SC + CDW) relates to the pseudogap phase is, however, much less clear. For this reason, we have investigated the charge stripe ordered system La1.6-xNd0.4SrxCuO4 (Nd-LSCO) by angle-resolved photoemission spectroscopy – one of the best probes for pseudogap physics.
In the system La1.6-xNd0.4SrxCuO4 (Nd-LSCO), charge-density-wave order around the special 1/8 doping suppresses strongly the superconducting transition temperature – see Fig. 1. This allows a low-temperature spectroscopy study of the relation between pseudogap and CDW order . These two effects are difficult to disentangle, as they both manifest themselves by a spectral gap .
Fig. 1: Temperature-doping phase diagram of La1.6−xNd0.4SrxCuO4 (Nd-LSCO) as established by diffraction and resistivity experiments [1,4].
By varying doping concentration p = x, photoemission-spectra were recorded in the overdoped metallic phase (p ≥ 0.25), just inside the pseudogap phase (p = 0.15, 0.2) and at the so-called p = 1/8 doping where stripe order is the strongest (see Fig. 2). In the metallic phase, gapless excitations have been observed all around the Fermi surface manifesting themselves as a single peak in the symmetrized energy distribution curve (EDC). By slightly reducing the doping just into the pseudogap phase a partial gap opens in the so-called anti-nodal region around the zone boundary. In this process spectral weight is conserved but shifted by the presence of the pseudogap. At p = 1/8 doping, a particularly strong enhancement of non-conservative, anti-nodal spectral-weight suppression is found inside the CDW (stripe ordered) phase. The suppression of spectral weight also extends up to much larger energies (~ 100 meV).
It is thus a possibility that CDW order manifests itself in the antinodal spectra at the 1/8 doping. If so, the implication is that in Nd-LSCO, charge-stripe order and the pseudogap phase contributes differently to the suppression of anti-nodal spectral-weight. These results suggest that CDW order, which recently has been identified as a universal property of copper oxide compounds, is not directly linked to the pseudogap phase.
Fig 2: (a) – (d) Antinodal angle-resolved photoemission spectra, taken in the normal state of La1.6−xNd0.4SrxCuO4 for different dopings p = x as indicated. Top panels schematically show the Fermi-surface topology for each of the doping concentrations. (e) Raw symmetrized normal-state energy distribution curves (EDCs) of La1.6-xNd0.4SrxCuO4 (Nd-LSCO) taken in the antinodal region for doping concentrations as indicated in the panel.
 J.M. Tranquada et al., Evidence for stripe correlations of spins and holes in copper oxide superconductors, Nature 375, 561 (1995).
 G. Ghiringhelli et al., Long-Range Incommensurate Charge Fluctuations in (Y,Nd)Ba2Cu3O6+x, Science 337, 821–825 (2012).
 J. Chang et al., Direct observation of competition between superconductivity and charge density wave order in YBa2Cu3O6.67, Nature Phys. 8, 871–876 (2012).
 R. Daou et al., Linear temperature dependence of resistivity and change in the Fermi surface at the pseudogap critical point of a high-Tc superconductor, Nat. Phys. 5, 31 (2009).
 G. Gruner, The dynamics of charge-density waves, Rev. Mod. Phys. 60, 1129 (1988).