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Unconventional ordering phenomena in cuprates

High-temperature superconductivity (HTSC) represents one of the most fascinating yet puzzling phenomena in contemporary condensed matter physics, with a convincing microscopic theory of the pairing mechanism still lacking. Among the various materials exhibiting unconventional superconductivity, the family of copper-oxides -- or cuprates -- has so far dominated the scene, drawing considerable efforts both on the experimental and theoretical side. A particularly relevant question regards the nature of the normal state, which characterizes the CuO2 planes above the critical temperature. In the normal state of the underdoped cuprates (around 12% hole-doping) there is evidence for various lattice instabilities, which include different phenomena, such as stripes, checkerboard order, and charge-density waves. Using complementary surface and bulk sensitive synchrotron based techniques these phenomena in a family of single-layered cuprates, Bi2Sr2-xLaxCuO6, or Bi2201, as a function of doping and temperature, have been studied.
Bi-based cuprates are in general characterized by a well-known corrugation of the lattice, also known as supermodulation: in Bi2201 this structural feature, labeled Q1, has a period of 4 unit cells, and runs along the [110] tetragonal direction (b* axis). We discovered that electron-lattice coupling in these materials triggers a second modulation, but only in a narrow doping range. Figure 1(a) shows the angle-resolved photoemission spectroscopy (ARPES) data measured at the BaDElPh beamline on a 11% doped samples, where a new set of bands (see yellow circles) is found, providing evidence for a new structural feature (henceforth denominated Q2), running parallel to Q1, i.e. along b*. Interestingly, these bands show an intriguing temperature evolution, highlighted in Fig. 1(b,c). The temperature-dependent separation between the bands is controlled by the wavevector of the new supermodulation Q2. The latter can be tracked with both ARPES and low-energy electron diffraction (LEED) [see inset of Fig. 1(e) and linecut in Fig. 1(d)], which consistently reveal its quasi-linear drift from high (Q~1/12) to low (Q~1/8) temperature, starting around 150K [see Fig. 1(e)], while Q1 stays fixed at Q=0.25.

Figure 1: ARPES/LEED results on Bi2201 UD15K. (a) ARPES momentum-energy map along the nodal direction Γ→ (π,π). (b,c) Zoom-in of (a) showing the temperature evolution of the supermodulated bandstructure. (d) LEED cuts along b* [see also inset of (e)]. (e) Temperature evolution of the Q1 and Q2 wavevectors.
 

The ARPES and LEED results characterize the surface structure of Bi2201 samples. The next crucial question is whether the same features are present in the bulk of the material. In order to investigate this phenomenology deeper in the bulk, we resorted to photon-based techniques, which possess a larger probing depth. Hard x-ray diffraction (XRD) and scattering (REXS) maps as a function of temperature have been acquired at =17 keV (Mo-Kα emission) and ~9 keV (Cu-K edge), respectively. These results, shown in Fig. 2(a-d), provide evidence that the new supermodulation Q2 is also found in the bulk of the material: its fingerprint in XRD/REXS is revealed through the presence of elongated rods [see legend in Fig.2(b)]. They correspond to a charge modulation well-defined in the CuO2 planes (i.e., along Qb*) but with poor coherence perpendicular to them (along Qc). Most interestingly, the extension of Q2 in the bulk does not show any temperature dependence as can be seen, comparing the REXS maps in Figs. 2(c,d), taken at 6 K and 120 K, respectively.

 

Figure 2:  (a,b) X-ray diffraction map and pictorial legend of visible structural reflections, respectively. (c,d) Resonant scattering maps (Cu-K edge) at low and high temperature, respectively. (e) Supermodulated Fermi surface, with nesting mechanisms (AN/N connectors) highlighted.


Analysis of possible electronic instabilities revealed that nesting channels of the Q1-modulated Fermi surface could explain the extremal wavevectors observed in ARPES and LEED [see Fig. 2(e)]. In this sense, the interplay between nodal (N) and antinodal (AN) nesting mechanisms, together with reduced-dimensionality at the surface, explain the observed behavior as originating from an incipient surface-enhanced charge-density wave (see original paper for more details). Ultimately, we have discovered another manifestation of strong electron-lattice coupling in cuprates, which is found to be an important mechanism even in a material characterized by a complex structure to start with.


This research was conducted by the following research team:

  • Riccardo Comin, Jonathan Rosen, Giorgio Levy, David Fournier, Zhi-Huai Zhu, Bart Ludbrook, Christian N. Veenstra, Alessandro Nicolaou, Doug Wong, Pinder Dosanijh, George A. Sawatzky, Andrea Damascelli, Department of Physics & Astronomy, University of British Columbia, Vancouver, Canada.
  • Yoshiyuki Yoshida, Hiroshi Eisaki, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Japan.
  • Graeme R. Blake, Thomas T.M. Palstra, Materials Science Centre, University of Groningen, Groningen, The Netherlands.
  • Ronny Sutarto, Feizhou He, Canadian Light Source, Saskatoon, Canada
  • Alex F. Pereira, Yi Lu, Bernhard Keimer, Max Planck Institute for Solid State Research, Stuttgart, Germany.
  • Luca Petaccia, Elettra Sincrotrone Trieste, Trieste, Italy.


Contact persons:
Riccardo Comin: rcomin@physics.ubc.ca
Luca Petaccia: luca.petaccia@elettra.eu
Andrea Damascelli: damascelli@physics.ubc.ca

Reference

J.A. Rosen, R. Comin, et al., “Surface-enhanced charge-density-wave instability in underdoped Bi2Sr2-xLaxCuO6+δ”, Nature Communications 4, 1977 (2013); DOI: 10.1038/ncomms2977
 
 
Last Updated on Friday, 22 November 2013 10:24