Neutron Diffraction Study of the Pressure-Induced Magnetic Ordering in the Spin Gap System TlCuCl₃

Abstract

Neutron elastic scattering measurements have been performed under a hydrostatic pressure in order to investigate the spin structure of the pressure-induced magnetic ordering in the spin gap system TlCuCl 3 . Below the ordering temperature T N ¼ 16:9 K for the hydrostatic pressure P ¼ 1:48 GPa, 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. This indicates that the spin gap closes due to the applied pressure. The spin structure of the pressure-induced magnetic ordered state for P ¼ 1:48 GPa was determined.KEYWORDS: TlCuCl 3 , spin gap, pressure-induced magnetic ordering, spin structure, neutron elastic scattering DOI: 10.1143/JPSJ.72.1026The spin gap system is a magnetic system having the singlet spin liquid ground state with a finite excitation gap. 1)Recently, the magnetic ordering induced by modifying the gapped ground state by an external field or impurity doping has been energetically investigated. When a magnetic field, which is higher than the gap field H g corresponding to the energy gap Á ¼ g B H g , is applied in a spin gap system, the energy gap vanishes, and the system can undergo the magnetic ordering with the help of three-dimensional (3D) interactions. On the other hand, when nonmagnetic ions are substituted for magnetic ions in a spin gap system, the singlet ground state is disturbed, so that staggered moments are induced around the impurities. If the induced moments interact through effective exchange interactions, which are mediated by intermediate singlet spins, the 3D long-range order can arise. Such field-induced and impurity-induced magnetic orderings were observed in many spin gap systems. 2-7)The application of pressure is another method of controlling the quantum magnetism including the spin gap and the spin-Peierls transition. Some significant pressure effects have been observed in some quantum spin systems. The pressure-induced magnetic ordering was observed in Cu 2 (C 5 H 12 N 2 ) 2 Cl 4 , 8) which was first assumed to be an S ¼ 1=2 Heisenberg antiferromagnetic two-leg ladder 9) and was later characterized as a frustrated 3D spin gap system. 10) In Cu 2 (C 5 H 12 N 2 ) 2 Cl 4 , some of the singlet spin pairs are broken by the applied pressure to become paramagnetic spins, and the magnetic ordering similar to the impurity-induced antiferromagnetic ordering occurs in the rest of the spins. 8)The spin gap remains even in the ordered state.In a well-known inorganic spin-Peierls (SP) material CuGeO 3 , the SP phase was enhanced by the applied pressure, 11) so that the revival of the SP phase was observed for highly Mg-doped CuGeO 3 , in which the SP transition does not occur and the impurity-induced antiferromagnetic ordering only occurs at the low temperature for ambient pressure. 12)This paper is concerned with the pressure-induced magnetic ordering in the spin gap system TlCuCl 3 .We summarize the physical properties of TlCuCl ...

Quantum phases and phase transitions of Mott insulators

“…Similar results apply to S Φ for the parameter regime in which it exhibits a second order transition [31]. Experimentally, the results below are relevant to materials that can be tuned across the magnetic ordering transition by applied pressure (such as TlCuCl 3 [8]), or to materials which happen to be near a critical point at ambient pressure (such as LaCuO 2.5 [32]). …”

“…While a quantitative synthesis of theory and experiment in the superconducting materials remains elusive, much progress has been made in describing a number of antiferromagnetic Mott insulators. A number of such insulators have been studied extensively in the past decade, with a few prominent examples being CaV 4 O 9 [1], (C 5 H 12 N 2 ) 2 Cu 2 Cl 4 [2,3,4], SrCu 2 (BO 3 ) 2 [5,6], TlCuCl 3 [7,8,9,10], and Cs 2 CuCl 4 [11,12]. In some cases, it has even been possible to tune these insulators across quantum phase transitions by applied pressure [8] or by an applied magnetic field [3,4,7,9].…”

Uniaxial pressure dependencies of the phase boundary of TlCuCl3

“…[1] The ground state of TlCuCl3 is very sensitive to pressure, hydrostatic pressure of order 0.5 GPa is sufficient to induce the Néel order already in zero magnetic field. [3] In order to obtain more information about this drastic influence of pressure we performed high-resolution measurements of the thermal expansion αi = ∂ ln Li/∂T and the magnetostriction ǫi = (Li(H) − Li(0))/Li(0) and derive the uniaxial pressure dependencies of the transition temperatures and fields. Fig.…”

High-order study of the quantum critical behavior of a frustrated spin- 12 antiferromagnet on a stacked honeycomb bilayer

“…In principle, of course, such bosons may undergo Bose-Einstein condensation (BEC) under suitable conditions and become superfluid. Indeed, such a superfluid state has been observed experimentally in the magnetic insulator TlCuCl 3 [4][5][6][7], which is a physical realization of such a coupled-dimer magnet, in which pairs of Cu 2+ ions are antiferromagnetically coupled to form a crystalline network of dimers in a specific pattern. The BEC is induced by placing the compound in an external magnetic field, which thereby Zeeman splits the otherwise degenerate three magnetic triplet sub-states.…”