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 opposed-anvil-type high pressure cell designed for NMR experiments developed in our laboratory is installed on a double axis goniometer. In spite of the compact size, the cell is capable of generating more than one hundred thousand atm. Good hydrostaticity is obtained by using sealed liquid argon as the pressure transmitting medium. Single crystal samples in the cell can be directed along arbitrary directions in a superconducting magnet, allowing us to obtain precise angle-resolved NMR spectra.
75As-NMR spectrum in the iron-pnictide compound SrFe2As2 under high pressure on 57000 atm. In the paramagnetic and normal-conducting state (28K), one central and two quadrupole-split resonance lines are observed. At the low temperature (4.2K), the whole spectrum consists of two parts. In one of them (AF), each of the three lines splits into two lines by antiferromagnetic order. In the other part (SC), they do not split by antiferromagnetic internal field. The latter is shown to belong to a superconducting state from the measurements of nuclear relaxation rate.
*Incomplete Devil`s Staircase in the Magnetization Curve of SrCu2(BO3)2: M. Takigawa, M. Horvatic, T. Waki, S. Kramer, C. Berthier, F. L. Bertrand, I. Sheikin, H. Kageyama, Y. Ueda and F. Mila, Phys. Rev. Lett.110 (2013) 067210/1-5.
*High-Field Phase Diagram and Spin Structure of Volborthite Cu3V2O7(OH)2⋅2H2O: M. Yoshida, M. Takigawa, S. Krämer, S. Mukhopadhyay, M. Horvati´c, C. Berthier, H. Yoshida, Y. Okamoto and Z. Hiroi, J. Phys. Soc. Jpn.81 (2012) 024703 (1-9).
Magnetic Coulomb Fields of Monopoles in Spin Ice and Their Signatures in the Internal Field Distribution: G. Sala, C. Castelnovo, R. Moessner, S. Sondhi, K. Kitagawa, M. Takigawa, R. Higashinaka and Y. Maeno, Phys. Rev. Lett.108 (2012) 217203 (1-5).
*Crossover from Commensurate to Incommensurate Antiferromagnetism in Stoichiometric NaFeAs Revealed by Single-Crystal 23Na,75As-NMR Experiments: K. Kitagawa, Y. Mezaki, K. Matsubayashi, Y. Uwatoko and M. Takigawa, J. Phys. Soc. Jpn.80 (2011) 033705 (1-4).