Pressure-Induced Successive Magnetic Phase Transitions in the Spin Gap System TlCuCl₃

Abstract

Under the hydrostatic pressure P ¼ 1:48 GPa, polarized neutron elastic scattering experiments have been carried out on the coupled spin dimer system TlCuCl 3 with the gapped ground state at ambient pressure. Pressure-induced magnetic ordering occurs at the transition temperature T N ¼ 16:9 K. As in the field-induced and impurity-induced magnetic ordered phases, the ordered moments lie in the a-c plane just below T N . An additional spin reorientation (SR) phase transition was observed at T SR ¼ 10 K, where the ordered moments start to incline toward the b-axis. The temperature variations of the direction and the magnitude of the ordered moments were also investigated. The spin gap in interacting spin systems is a macroscopic quantum phenomenon and has been attracting considerable attention both theoretically and experimentally. Recently, unconventional magnetic orderings induced by impurity ions, applied magnetic field and pressure in the spin gap systems have been energetically investigated. The impurityinduced magnetic ordering due to the breaking of the local spin singlet pair has been well studied through the wellknown inorganic spin-Peierls system CuGeO 3 . The coexistence of the lattice dimerization associated with the spin gap and antiferromagnetic ordering have been observed. 1-4)Field-induced and pressure-induced magnetic orderings are quantum phase transitions realized by the vanishing of the spin gap. The field-induced magnetic ordering can be interpreted as the Bose-Einstein condensation of triplet magnons. [5][6][7][8] The title spin gap system TlCuCl 3 undergoes all of these magnetic orderings. [9][10][11][12][13][14][15][16][17] TlCuCl 3 has a monoclinic structure (space group P2 1 =c).18) The crystal structure of TlCuCl 3 consists of planar dimers of Cu 2 Cl 6 . The dimers form infinite double chains along the crystallographic a-axis. These chains are located at the corners and the center of the unit cell in the bc plane, and are separated by Tl þ ions. The magnetic ground state of TlCuCl 3 is the spin singlet 18) with the excitation gap Á ¼ 7:7 K. 9,19) The origin of the spin gap is the strong antiferromagnetic spin dimer in the chemical dimer Cu 2 Cl 6 , and the neighboring spin dimers couple via strong threedimensional interdimer interactions along the double chain and in the ð1; 0; À2Þ plane, in which the hole orbitals of Cu 2þ spread. 20,21) TlCuCl 3 undergoes pressure-induced magnetic ordering above P c $ 0:2 GPa.16) Unpolarized neutron elastic scattering experiments under the hydrostatic pressure P ¼ 1:48 GPa were carried out in order to investigate the spin structure of the pressure-induced magnetic ordering in TlCuCl 3 . 17) Below the ordering temperature T N ¼ 16:9 K, the magnetic Bragg reflections were observed at reciprocal lattice points Q ¼ ðh; 0; lÞ with integer h and odd l, which are equivalent to those points with the lowest magnetic excitation energy at ambient pressure. The spin structure of the pressure-induced ordered phase in TlCuCl 3 for P ¼ 1:48 GPa was determined as shown...

Pressure-induced Magnetic Quantum Phase Transition from Gapped Ground State in TlCuCl3

“…For the present pressure range and H H sf ' 0:7 T, sin 2 ( 1 and cos 2' ( 1, e.g., sin 2 ¼ 0:13 at P ¼ 14:8 kbar. 19) In the antiferromagnetic state, ' ¼ %=4, cos 2 ¼ J=ð2jJ JjÞ and hS ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi 4ÁJJ=3 p . Here, we used the relation J ' 2jJ Jj in the present pressure range.…”

Effect of disorder on a pressure-inducedz=1magnetic quantum phase transition

“…In these transitions, the spectrum is expected to be linear at the quantum-critical point, and hence z = 1. To date, the only known experimental realizations of such a pressure-induced QCP are TlCuCl 3 [3][4][5] and (C 4 H 12 N 2 )Cu 2 Cl 6 (PHCC) [6][7][8].…”

“…Here, disorder is created by randomly modifying the magnetic exchange pathways, leaving the magnetic sites otherwise unperturbed. It has been applied in materials such as TlCu(Cl 1−x Br x ) 3 [13], (NiCl 1−x Br x ) 2 · 4SC(NH 2 ) 2 [14,15], Sul-Cu 2 (Cl 1−x Br x ) 4 [16], and IPA-Cu(Cl 1−x Br x ) 3 [17,18], where in the pure cases excellent experimental realizations of various field-induced QPTs have been found. To date, the pressure-induced z = 1 QCP in the presence of disorder has not been studied experimentally in gapped quantum magnets.…”

Spin Fluctuations in the Ground State of Doped Quantum Spin System TlCu1-xMgxCl3withx=0.0047 Probed by Muon-Spin-Relaxation Technique

“…[12][13][14][15][16][17][18][19][20][21][22] Pressure-induced magnetic ordering was also reported in two systems in which the gap closes owing to the enhancement of interdimer interactions by applyng pressure. 24,25) Recently, impurity-introduced Tl(Cu 1Àx Mg x )Cl 3 single crystals have been grown, and new intriguing phenomena have been reported. [26][27][28][29] The magnetic phase transition to an ordered state is observed by magnetization and specific heat measurements in the zero-field limit, and neutron elasticscattering measurements identified this impurity-induced ordered state as an antiferromagnetically ordered state, of which the magnetic structure is the same as that for the fieldinduced phase in TlCuCl 3 .…”