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Polarons and confinement of electronic motion to two dimensions in a layered manganite

Nature volume 440, pages 1025–1028 (2006)Cite this article

Abstract

A remarkable feature of layered transition-metal oxides—most famously, the high-temperature superconductors—is that they can display hugely anisotropic electrical and optical properties (for example, seeming to be insulating perpendicular to the layers and metallic within them), even when prepared as bulk three-dimensional single crystals. This is the phenomenon of ‘confinement’, a concept at odds with the conventional theory of solids, and recognized1 as due to magnetic and electron–lattice interactions within the layers that must be overcome at a substantial energy cost if electrons are to be transferred between layers. The associated energy gap, or ‘pseudogap’, is particularly obvious in experiments where charge is moved perpendicular to the planes, most notably scanning tunnelling microscopy2 and polarized infrared spectroscopy3. Here, using the same experimental tools, we show that there is a second family of transition-metal oxides—the layered manganites La2-2xSr1+2xMn2O7—with even more extreme confinement and pseudogap effects. The data demonstrate quantitatively that because the charge carriers are attached to polarons (lattice- and spin-textures within the planes), it is as difficult to remove them from the planes through vacuum-tunnelling into a conventional metallic tip, as it is for them to move between Mn-rich layers within the material itself.

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Figure 1: Structure and bulk transport properties of La1.4Sr1.6Mn2O7.
Figure 2: Unprocessed STM micrographs of La1.4Sr1.6Mn2O7.
Figure 3: Temperature-dependent STM spectroscopy of La1.4Sr1.6Mn2O7.
Figure 4: Optical conductivity deduced from near-normal-incidence reflectivity measured on single crystal samples (sharp peaks at 20–80 meV are due to phonons)19.

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Acknowledgements

We thank T. Ishikawa for providing the optical conductivity data shown in Fig. 4, D. McPhail and R. Chater for the static SIMS measurements, and A. Fisher and J. Mesot for discussions. We acknowledge support from a Wolfson–Royal Society Research Merit Award, the NEC corporation, and the European Commission through a FW6 STREP programme. H.M.R. thanks T. F. Rosenbaum for support through an NSF Materials Research Science and Engineering Centers grant.

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Authors and Affiliations

  1. Laboratory for Neutron Scattering, ETH-Zürich and Paul Scherrer Institut, 5232, Villigen, Switzerland

    H. M. Rønnow

  2. London Centre for Nanotechnology & Department of Physics and Astronomy, University College London, Gower Street, WC1E 6BT, London, UK

    Ch. Renner & G. Aeppli

  3. Department of Physics and Astronomy, University College London, Gower Street

    Ch. Renner & G. Aeppli

  4. Bell Laboratories, Lucent Technologies, 600 Mountain Avenue, New Jersey, 07974, Murray Hill, USA

    T. Kimura

  5. Department of Applied Physics, University of Tokyo, Bunkyo-ku, Tokyo, 113-8656, Japan

    Y. Tokura

  6. Spin Superstructure Project (SSS), ERATO, Japan Science and Technology Agency (JST), 305-0046, Tsukuba, Japan

    Y. Tokura

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  1. H. M. Rønnow
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Correspondence to Ch. Renner.

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Rønnow, H., Renner, C., Aeppli, G. et al. Polarons and confinement of electronic motion to two dimensions in a layered manganite. Nature 440, 1025–1028 (2006). https://doi.org/10.1038/nature04650

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