Zero-doping state and electron–hole asymmetry in an ambipolar cuprate

Journal name:
Nature Physics
Volume:
6,
Pages:
579–583
Year published:
DOI:
doi:10.1038/nphys1717
Received
Accepted
Published online

A Mott insulator is a material that is insulating because of strong Coulomb repulsions between electrons. Doping charge carriers, electrons or holes into a Mott insulator can induce high-temperature superconductivity. Thus, what exactly happens when a charge carrier is doped into a Mott insulator is a key question in many-body physics1, 2, 3, 4. To address this issue, ideally one should start from a zero-doping state5, 6, 7 and be able to introduce both holes and electrons in the dilute limit. However, such an idealized experiment has been impossible because of the lack of suitable materials. Here we show that a new ‘ambipolar’ cuprate makes it possible for the first time to cross the zero-doping state in the same material, which in turn allows us to address the physics of the extremely low-doping region. Surprisingly, we found that the antiferromagnetic ground state sharply changes between electron- and hole-doped sides, and this change is dictated by the existence of only 0.1ppm of charge carriers. Moreover, we observed that the Néel temperature TN shows an unexpected reduction in a narrow range centred at the zero-doping state, across which the system exhibits asymmetric behaviours in transport measurements. Our findings reveal the inherently different nature of electron and hole doping in the dilute limit of a Mott-insulating cuprate.

At a glance

Figures

  1. Schematics of YLBLCO.
    Figure 1: Schematics of YLBLCO.

    a, Crystal structure of YLBLCO and the sites for chemical doping. Note that in actual YLBLCO the Cu–O chains are fragmented and randomly oriented, leading to a macroscopically tetragonal structure. b, Generalized phase diagram of cuprate materials, where AF and SC denote antiferromagnetic and superconducting regions, respectively. Attainable doping ranges are shown for several cuprate materials including YLBLCO.

  2. Temperature dependence of [rho]c.
    Figure 2: Temperature dependence of ρc.

    ac, ρc(T) of YLBLCO in semi-log plots, where 4W (four-wire) and 2W (two-wire) denote the methods for resistivity measurements. df, Temperature derivative of ρc(T) normalized by ρc. The arrows indicate the position of the peak or the dip, at which the Néel transition is supposed to take place.

  3. Magnetic ground states of YLBLCO studied by neutron scattering.
    Figure 3: Magnetic ground states of YLBLCO studied by neutron scattering.

    af, Neutron scattering intensities obtained by [0.5,0.5,L] scans for y=6.22 (electron doped) (a,d), y=6.32 (nearly zero doping) (b,e) and y=6.69 (hole doped) (c,f) at different temperatures. The insets show schematics of the magnetic structures of YLBLCO for respective regimes. The red and blue spheres at the Cu(2) sites stand for spins with opposite directions (the exact direction of the spin easy axis is unknown), and grey spheres are the Cu(1) atoms. The small (or negligible) moments of the Cu(1) atoms are not plotted. The letters A and B denote the difference (π-phase shift) in spin arrangements in the plane. gi, Temperature dependencies of the integrated intensity of the magnetic reflections for y=6.22 (g), 6.32 (h) and 6.69 (i). The inset to h magnifies the low-temperature data for y=6.32. The error bars represent one standard deviation.

  4. Evolution of key parameters in YLBLCO.
    Figure 4: Evolution of key parameters in YLBLCO.

    a, ρc shows a peak at y=6.32, signifying the zero-doping point. b, The carrier concentration decreases exponentially towards the zero-doping point as y is varied; here, the RH data at 300K are used for the calculations, except for y=6.36 and 6.49 for which 350K data are used. Light red and blue backgrounds denote the n- and p-type regimes, respectively. The carrier mobility along the c axis calculated from these data is 10−3–10−2cm2V−1s−1 and is nearly independent of both the concentration and the type of carriers. c, TN as a function of the concentration of both p- and n-type carriers (solid lines are guides to the eye), together with the anticipated phase boundary for the regimes not covered in this study (dashed lines). Note that the horizontal axis is plotted in a logarithmic scale, and the region studied here is highlighted with light colours. TN is extracted from the ρc(T) data shown in Fig. 2d–f. The carrier concentrations for y=6.30, 6.31 and 6.32 are estimated from ρc by extrapolating the relationship between ρc and RH.

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Affiliations

  1. Institute of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka 567-0047, Japan

    • Kouji Segawa &
    • Yoichi Ando
  2. Department of Physics, University of Virginia, Charlottesville, Virginia 22904-4714, USA

    • M. Kofu &
    • S-H. Lee
  3. Central Research Institute of Electric Power Industry, Yokosuka, Kanagawa 240-0196, Japan

    • I. Tsukada
  4. Institute for Materials Research, Tohoku University, Sendai, 980-8577, Japan

    • H. Hiraka &
    • M. Fujita
  5. NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA

    • S. Chang
  6. Advanced Institute for Materials Research, Tohoku University, Sendai, 980-8577, Japan

    • K. Yamada

Contributions

All authors made critical comments on the manuscript. Y.A., K.S., S-H.L. and K.Y. contributed to planning of the experiments. K.S. synthesized the sample and carried out the transport measurements. K.S. and Y.A. analysed and interpreted the transport data. M.K., S-H.L., H.H., M.F., S.C. and K.Y. contributed to data collection of the neutron scattering. M.K., S-H.L. and K.Y. contributed to analysing the neutron data.

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The authors declare no competing financial interests.

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