ISSP - The institute for Solid State Physics

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Takigawa Group

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We use nuclear magnetic resonance (NMR) as the major experimental tool to investigate exotic phenomena caused by strong electronic correlation in solids. A remarkable feature of strongly correlated electron systems is the competition among various kinds of ordering such as superconductivity, ferro- or antiferromagnetism, charge and orbital order. Quantum phase transitions between these ground states can be caused by changing the external parameters such as magnetic field or pressure. Nuclei have their own magnetic dipole and electric quadrupole moments, which couple to the magnetic field or electric field gradient produced by surrounding electrons. This makes NMR a powerful local probe for microscopic investigation of the exotic order and fluctuations of multiple degrees of freedom of electrons, i.e., spin, charge and orbital. We use various NMR spectrometers in different environment (low temperature, high magnetic field and high pressures) to investigate transition metal compounds, rare earth compounds, and organic solids.

The boron NMR spectrum at a very low temperature and a high magnetic field obtained from SrCu2(BO3)2, a quantum magnet in which spin-dimers have a planer orthogonal configuration. The sample used in the measurement was intentionally doped with impurities by replacing 0.5 % of magnetic Cu by non-magnetic Zn. The spin-dimers far from impurities form singlet, however, those in the neighborhood of Zn impurity generate local staggered magnetization (spin polaron). While the former corresponds to the intense resonance lines with nearly zero internal field (shown by arrows), additional 15 resonance lines provide a fingerprint of the spin polaron generated by a Zn impurity.
Distribution of Cu-spin moments in the neighborhood of a Zn impurity obtained from analysis of the NMR spectrum The location of the Zn impurity is indicated by × and red circles show Cu atoms. The solid (open) red circles indicate Cu spins directed along (opposite to) the external magnetic field. The Black circles show boron sites, where the numbers show correspondence to the resonance lines in the NMR spectrum. In the NMR spectrum, not only the boron sites on the same layer as the Zn impurity but also the boron sites on the neighboring layers are observed.

Research Subjects

  1. Dynamics and quantum phase transitions in low dimensional or frustrated spin systems
  2. Exotic superconductors
  3. Ordering and fluctuations of charge, orbital, and multipoles in strongly correlated electron systems
  4. Spin and charge dynamics in magnetic organic conductors