Neutron Scattering Study of the Field-Dependent Ground State and the Spin Dynamics in Spin-One-HalfNH4CuCl3

Abstract

Elastic and inelastic neutron scattering experiments have been performed on the dimer spin system NH4CuCl3, which shows plateaus in the magnetization curve at m=1/4 and m=3/4 of the saturation value. Two structural phase transitions at T1 approximately 156 K and at T(2)=70 K lead to a doubling of the crystallographic unit cell along the b direction and as a consequence a segregation into different dimer subsystems. Long-range magnetic ordering is reported below T(N)=1.3 K. The magnetic field dependence of the excitation spectrum identifies successive quantum phase transitions of the dimer subsystems as the driving mechanism for the unconventional magnetization process in agreement with a recent theoretical model.

Ferroelectricity in the gapless quantum antiferromagnetNH4CuCl3

“…These results corroborate the presence of that transition, though it gives no clear indication of the which phase to which it belongs. Below T 1 resides a second transition, T 2 ≈ 70 K, which is very close to the position of the dielectric anomaly found at 67 K. Since previous evidence suggests that T 2 is structural in nature 17,18 , while dielectric results that suggest ordering occurs due to the NH 4 + ions, it seemed appropriate to find the mechanism responsible for the phase change and thus the ferroelectricity.…”

“…Figure 2(b) shows the temperature dependence of ǫ ′ , the real part of the dielectric constant over 1.8-120 K for a pellet sample of ACC, more specifically a thin disk with 3/16" outer diameter and thickness of about 0.2 mm. A clear λ-shaped peak is discernible at 67 ± 0.5 K, meaning that the earlier reported [15][16][17][18] SPT at this temperature involves a dielectric anomaly. The inset in fig.…”

“…In addition to the unusual magnetic behavior, ACC is also known to exhibit a structural phase transition (SPT) at about 70 K, as seen via NMR 15,16 , ultrasonic attenuation 17 and neutron scattering 18 . The latter method detected a change in lattice dimensions in addition to a space group transformation from (P2 1 or P1) to (P1 or P1).…”

Magnetic Cluster Excitations

“…56(a). Since a finite number of triplet states |1, +1 is created at B c , the Rüegg et al, 2003Rüegg et al, , 2004b system undergoes a phase transition to a magnetically ordered state. This condensation of triplet states at T ≈ 0 is therefore a prototype of a quantum phase transition.…”

“…Typical realizations of the category (iii) are weakly coupled dimer-based compounds including KCuCl 3 (Cavadini et al, 1999;Kato et al, 1998), TlCuCl 3 (Oosawa et al, 2002;Rüegg et al, 2003), NH 4 CuCl 3 (Kurniawan et al, 1999;Rüegg et al, 2004b), BaCuSi 2 O 6 (Jaime Rüegg et al, 2007;Sebastian et al, 2005), SrCu 2 (BO 3 ) 2 (Kageyama et al, 2000;Kodama et al, 2002), Cs 3 Cr 2 Br 9 (Grenier et al, 2004;Leuenberger et al, 1985), Quintero-Castro et al, 2010;Wang et al, 2011), and Ba 3 Mn 2 O 8 (Stone et al, 2008;Uchida et al, 2002). Trimer-based compounds such as La 4 Cu 3 MoO 12 as well as the compound SrCu 2 (BO 3 ) 2 (characterized by an array of mutually perpendicular dimers) fulfill in addition the criterion (iv).…”