Softening of Magnetic Excitations Leading to Pressure-Induced Quantum Phase Transition in Gapped Spin System KCuCl₃

Abstract

KCuCl 3 is a three dimensionally coupled spin dimer system, which undergoes a pressure-induced quantum phase transition from a gapped ground state to an antiferromagnetic state at a critical pressure of P c ' 8:2 kbar. Magnetic excitations in KCuCl 3 at a hydrostatic pressure of 4.7 kbar have been investigated by conducting neutron inelastic scattering experiments using a newly designed cylindrical high-pressure clamp cell. A well-defined single excitation mode is observed. The softening of the excitation mode due to the applied pressure is clearly observed. From the analysis of the dispersion relations, it is found that an intradimer interaction decreases under hydrostatic pressure, while most interdimer interactions increase. For a system of isolated S ¼ 1=2 spin dimers with an antiferromagnetic (AF) intradimer exchange interaction J > 0, the triplet excitation is dispersionless and the excitation energy is given by h " !ðqÞ ¼ J. When a threedimensional interdimer exchange interaction J 0 is switched on, excited triplets can hop to neighboring dimers due to the transverse component of the interdimer interactionÞ so that the magnetic excitations become dispersive. With increasing ðJ 0 =JÞ, the dispersion range increases and the lowest excitation energy corresponding to the energy gap of the system decreases. At a quantum critical point ðJ 0 =JÞ c , the gap closes and the disordered ground state changes to the AF-ordered state. 1,2)The AF state stabilized for ðJ 0 =JÞ > ðJ 0 =JÞ c is described by the coherent superposition of the singlet j0; 0i and two triplet components j1; AE1i. Such a quantum phase transition (QPT) was first realized in TlCuCl 3 3-5) by the application of hydrostatic pressure. The critical pressures determined by magnetization measurement and neutron scattering experiment are P c ¼ 0:42 kbar 3) and 1.07 kbar, 5) respectively. There is a discrepancy between the critical pressures obtained from macroscopic and microscopic measurements.In this study, we investigate magnetic excitations in KCuCl 3 under hydrostatic pressure. The crystal structure (space group P2 1 =c) of KCuCl 3 is the same as that of TlCuCl 3 6,7) and is composed of planar dimers of Cu 2 Cl 6 . The dimers are stacked to form infinite double chains parallel to the crystallographic a-axis. These double chains are located at the corners and center of the unit cell in the bc-plane. A strong AF exchange interaction with J ¼ 4:34 meV in the planar dimer of Cu 2 Cl 6 dimerizes the spins of Cu 2þ . As shown in Fig. 1, neighboring spin dimers couple along the a-axis and in the ð1; 0; " 2 2Þ plane, in which the hole orbitals of Cu 2þ spread. [8][9][10][11][12] The lowest triplet excitation occurs at Q ¼ ð0; 0; 1Þ and its equivalent reciprocal points. The magnitude of the energy gap is Á ¼ 2:67 meV. 8,13,14) Consequently, KCuCl 3 is magnetically characterized as an interacting spin dimer system. KCuCl 3 undergoes a QPT under hydrostatic pressure, as observed in TlCuCl 3 .15) The critical pressure obtained through magnetization measurement ...

Width of the longitudinal magnon in the vicinity of the O(3) quantum critical point

“…The decay matrix element shown in Fig. 2 is determined after the operators (16) are substituted in the cubic Lagrangian L 3 ,…”

“…An analogous material KCuCl 3 has similar properties. 15,16 It has been understood that TlCuCl 3 consists of a 3D set of spin-dimer sand, and the QPT in the magnetic field can be considered as a Bose condensation of the spin-dimer triplet excitations (triplons). 1,[17][18][19][20][21][22] The pressure-induced QPT can be also described in terms of triplons.…”

Development of multi-frequency ESR system for high-pressure measurements up to 2.5 GPa

“…For the spin gap system, several attempts have been made to collapse the spin gap by applying pressure to explore the novel phase. It has been suggested or found experimentally that the spin gap was reduced in a spin dimer system KCuCl 3 [1,2], a two-dimensional spin dimer system (C 4 H 12 N 2 )Cu 2 Cl 6 [3,4], two-leg spin ladder systems IPA-CuCl 3 [5], Cu 2 (C 5 H 12 N 2 ) 2 Br 4 [6], and so on. In the well-known two-dimensional orthogonal spin dimer system SrCu 2 (BO 3 ) 2 , the suppression of the gap energy with pressure and the existence of a novel magnetic phase were suggested [7].…”

Development of high-pressure and high-field ESR system using SQUID magnetometer

“…From Eqs. (2) and (3) we obtained D values at various pressures. The D value has very large pressure dependence, as shown in Fig.…”

“…In other words, the pressure can tune the parameters of the Hamiltonian without changing its form, and it may lead to a change in the ground state at zero temperature, or a quantum critical transition. In the strongly correlated spin systems, novel phenomena related to the pressure-induced quantum phase transition have attracted considerable attention recently from both the theoretical and experimental points of view [1][2][3][4][5].…”