FIELD-INDUCED NEW QUANTUM PHENOMENA IN MAGNETIC SYSTEMS

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We have developed many quantum magnets, which have various crystal structures, and have investigated their magnetic properties in high magnetic fields. Followings are representative results. Coupled S=1/2 spin dimer systems undergo novel quantum phase transitions in high magnetic fields. These systems often have singlet ground states with excitation gaps. In high magnetic field, one of the triplets whose energy decreases with magnetic field plays an important role. The triplet can hop to neighboring site as a quasi-particle and interact due to the transverse and longitudinal components of the interdimer exchange interaction. The quasi-particle has characteristics of boson and is called magnon or triplon. Consequently, the system can be described as interaction boson system, where magnetic field corresponds to the chemical potential. We found that the magnetization plateaus in SrCu_2(BO_3)_2 and other systems. The magnetization plateau can occur when the hopping of magnons are strongly … More suppressed due to spin frustration or parity and the periodic arrangement of magnons (Wigner crystal) are stabilized due to the repulsive interaction. This is a typical quantum many-body effect By means of NMR in high magnetic fields on SrCu_2(B0_3)_2, we verify the realization of Wigner crystal of magnons. On the other hand, the field-induced antiferromagnetic (AF) ordering occurs when the hopping of magnons is dominant The ordering can be described as the Bose-Einstein condensation (BEC) of magnons. We have performed various measurements on the field-induced AF ordering in TlCuCl_3. Together with theoretical analysis, we verify that the field-induced AF ordering in TlCuCl_3 is the BEC of magnons. The present study demonstrated that there are cases where the quantum magnets exhibit the nature characteristic of ensemble of quantum particles. This provides a new concept of magnetism.To encourage our research project, we organized three open symposiums inviting researchers from abroad and International Symposium on Quantum Spin Systems QSS04. We also supported three international meetings. Less