August 10
Aug. 10 - 4a
10:00 - 10:30
11:00 - 11:30
11:30 - 12:00
Magnetic Quantum Phase Transitions and Critical Behavior in TlCuCl3 and KCuCl3
Hidekazu Tanaka
TlCuCl3 and KCuCl3 are magnetically characterized as three-dimensionally coupled spin-dimer system. Their ground states are spin singlets with excitation gaps Δ/kB of 7.5 K and 31 K, respectively. The origin of the gap is the strong antiferromagnetic exchange interaction in the chemical dimer to form a spin dimer. In a magnetic field, which is higher than the critical field Hc corresponding to the gap, these systems can undergoes magnetic ordering with the transverse-ordered moments [1,2]. The field-induced magnetic quantum phase transition (QPT) in TlCuCl3 has been extensively studied by various techniques. The results obtained were in accordance with theory which describes the field-induced magnetic QPT as the Bose-Einstein condensation of spin triplets [3]. For example, the critical exponent phi of the phase boundary defined by T(H) ~ (H-Hc)1/φ is close to φBEC =3/2 derived from the magnon BEC theory [3,4]. Under a hydrostatic pressure P, the gap decreases and closes completely at Pc = 0.42 kbar and 8.2 kbar for TlCuCl3 and KCuCl3, respectively [5]. For P > Pc, these systems undergoes antiferromagnetic ordering, which is characterized by the same ordering vector as that for the field-induced magnetic ordering. The gap and N\'{e}el temperature are presented as functions of pressure. The occurrence of this pressure-induced magnetic QPT is attributed to the decrease of the intradimer interaction and the increase of interdimer interaction with applied pressure. The present results are discussed in connection with recent theory [6,7].

[1] H. Tanaka et al.: J. Phys. Soc. Jpn. 70 (2001) 939.
[2] A. Oosawa et al.: Phys. Rev. B 66 (2002) 104405.
[3] T. Nikuni et al.: Phys. Rev. Lett. 84 (2000) 5868.
[4] N. Kawashima: J. Phys. Soc. Jpn. 73 (2004) 3219.
[5] K. Goto it et al.: J. Phys. Soc. Jpn. 73 (2004) 3254, cond-mat/0602540.
[6] M. Matsumoto et al.}: Phys. Rev. B 69 (2004) 054423.
[7] O. Nohadani et al.: Phys. Rev. B 72 (2005) 024440