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Fingerprints of Pseudogap and Charge ordering : A photoemission spectroscopy investigation

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.

Claudia Fatuzzo110x150

By Christian E. Matt Swiss Light Source, Paul Scherrer Institute & Laboratory for Solid State Physics, ETHZClaudia G. Fatuzzo, Institute for Condensed Matter Physics, EPFLJohan Chang, Physik-Institut, Universität Zürich

Based on article published in Physical Review B

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 [4]. These two effects are difficult to disentangle, as they both manifest themselves by a spectral gap [5].

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.

[1] J.M. Tranquada et al., Evidence for stripe correlations of spins and holes in copper oxide superconductors, Nature 375, 561 (1995).

[2] G. Ghiringhelli et al., Long-Range Incommensurate Charge Fluctuations in (Y,Nd)Ba2Cu3O6+x, Science 337, 821–825 (2012).

[3] J. Chang et al., Direct observation of competition between superconductivity and charge density wave order in YBa2Cu3O6.67, Nature Phys. 8, 871–876 (2012).

[4] 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).

[5] G. Gruner, The dynamics of charge-density waves, Rev. Mod. Phys. 60, 1129 (1988).

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