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Electron dynamics in copper oxide superconductors unveiled by three inelastic-scattering techniques (Press Release)

Release Date
25 Apr, 2014
  • BL11XU (JAEA Quantum Dynamics)
　

Japan Atomic Energy Agency
Tohoku University
Politecnico di Milano
European Synchrotron Radiation Facility
Kyoto University
J-PARC Center
Comprehensive Research Organization for Science and Society
High Energy Accelerator Research Organization
Kwansei Gakuin University

A joint research group of Japan Atomic Energy Agency, Tohoku University, Politecnico di Milano, European Synchrotron Radiation Facility, Kyoto University, J-PARC Center, Comprehensive Research Organization for Science and Society, High Energy Accelerator Research Organization, and Kwansei Gakuin University succeeded in clarifying comprehensive electron dynamics in copper oxide superconductors1).
They performed inelastic scattering2) using three quantum beams3): soft x-ray (at ID08 of ESRF), neutron (at BL01 of J-PARC) and hard x-ray (at BL11XU of SPring-8), each of which has recently achieved technical progress, and observed spin and charge excitations in electron-doped4) copper oxide superconductors.
Although high-Tc superconductivity in the copper oxides shows up upon both electron and hole dopings, the research group found that the excitations in the electron-doped superconductors differ substantially from those in hole-doped ones and have highly itinerant character5). These findings impose constraints on theoretical models and an adequate description of the electronic excitations in the electron- and hole-doped copper oxides is a prerequisite for complete understanding of the superconductivity. Incidentally, this work demonstrates that complementary use of quantum beams is very effective in inelastic scattering for the first time.

Publication of this work：
"High-energy spin and charge excitations in electron-doped copper oxide superconductors"
K. Ishii, M. Fujita, T. Sasaki, M. Minola, G. Dellea, C. Mazzoli, K. Kummer, G. Ghiringhelli, L. Braicovich, T. Tohyama, K. Tsutsumi, K. Sato, R. Kajimoto, K. Ikeuchi, K. Yamada, M. Yoshida, M. Kurooka, and J. Mizuki
Nature Communications 5, 3714 (2014), published 25 April 2014
Figure 1: Schematic figure of inelastic scattering
Figure 1: Schematic figure of inelastic scattering

In the inelastic scattering experiment, incident probes, such as x-rays and neutrons, are irradiated to a sample and then scattered ones are detected. One can observe electronic excitations from the difference of energy between incident and scattered probes. We used inelastic scattering of neutron, hard x-ray, and soft x-ray, in the present work and investigated the excitations of charge (red circles) and spin (blue arrows) of the electron. The background figure shows the crystal structure of the electro-doped copper oxides superconductor (Nd,Pr,La)2-xCexCuO4.

Figure 2: Inelastic scattering spectra of neutron, soft x-ray, and hard x-ray
Figure 2: Inelastic scattering spectra of neutron, soft x-ray, and hard x-ray

Blue marks show peak positions of spin excitations while red ones indicate those of charge excitations. Circles, squares, and triangles are obtained from the data analysis of neutron, soft x-ray, and hard x-ray, respectively. The superconductor has a continuum of excitations between 0 and 2 eV and we complementarily used neutrons for low-energy spin excitations (approximately below 0.3 eV), soft x-rays for high-energy spin and low-energy charge excitations (between 0.3 and 1.0 eV), and hard x-rays for high-energy charge excitations (above 0.8 eV).

Figure 3: Schematic figures of spin and charge excitations in copper oxide superconductors and their doping dependence
Figure 3: Schematic figures of spin and charge excitations
in copper oxide superconductors and their doping dependence

 

《Explanation of technical terms》
1)　Copper oxide superconductor
Superconductivity in copper oxides has the highest critical temperature found so far and it is called high-Tc superconductivity.

2)　Inelastic scattering
Inelastic scattering is a process where irradiated probes transfer their energy to the sample. When the energy is transferred to an electron in the sample, one can investigate dynamics of the electron through the measurement of the scattered intensity of the probe.

3)　Quantum beam
The beams irradiated from artificial devices or sources have notable characteristics, for example, high brilliances and high directionality, which are distinct from radiations from natural sources. Such a well-controlled radiation is referred to as a “quantum beam”. Nowadays it is widely recognized that the quantum beams are useful for clarifying the properties of materials. Development of the beam sources and related experimental techniques has enabled one to obtain unexplored information of materials. Success of the inelastic scattering of the quantum beams in this work is exactly due to the development.

4)　Electron doping
Element substitution can dope mobile carriers into a material. There are　two types of carrier doping: one is electron removal (hole doping) and the other is electron addition (electron doping). Parent compounds of the copper oxide superconductor, where the mobile carriers are undoped, are insulators due to the strong electric repulsion between electrons and therefore are not superconductors. When mobile carriers are doped to the parent compounds, superconductivity emerges. Either electron or hole doping can induce the superconductivity in the copper oxides. In Figure 4, we show a phase diagram of the representative hole-doped system La2-xSrxCuO4 and the electron-doped system Nd2-xCexCuO4.

Figure 4: Phase diagram of copper oxide superconductors as a function of concentration of doped carrier and temperature
Figure 4: Phase diagram of copper oxide superconductors
as a function of concentration of doped carrier and temperature

Either electron or hole doping induces the superconductivity in the copper oxides.

5)　Itinerant character
Electrons which determine the properties of materials have either itinerant or localized character. In the copper oxides, localized electrons in the parent compounds gradually become itinerant as carrier concentration increases. While the localized character remains in the electronic excitations in hole-doped copper oxides, the electrons in electron-doped copper oxides are found to become itinerant even in relatively low carrier concentration.


For more information, please contact:
　Kenji Ishii (Japan Atomic Energy Agency)
　　TEL：0791-58-2643

　Masaki Fujita (Tohoku University)
　　TEL：022-215-2035
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