μSR and NMR study on quantum critical point in quantum spin ladder (CH₃)₂CHNH₃Cu(Cl_xBr_1−x)₃

Abstract

CH 3 ) 2 CHNH 3 Cu(Cl x Br 1-x ) 3 abbreviated IPACu(Cl x Br 1-x ) 3 is a mixed system of spin-gap compounds IPACuCl 3 and IPACuBr 3 . This mixed system is reported by macroscopic measurements to show a magnetic order when the value of x is between the quantum critical points (QCPs) 0.44 and 0.87. We have investigated microscopically the ground state of the samples x = 0.40, 0.45 and 0.50 which are located near the QCP of x c = 0.44, by Muon Spin Relaxation (mSR) and NMR measurements. For x = 0.40, mSR measurements in zero and longitudinal fields have shown that the spin fluctuation shows a significant slowing down with decreasing temperature, although the magnetic order does not appear down to T = 15 mK, indicating that the ground state is an exotic one rather than a gapped one. For x = 0.45 and 0.50, mSR and NMR measurements do not show any magnetic order at low temperatures, indicating that the QCP is larger than the reported value of x c = 0.44.

Ground State of Bond-Disordered Quasi-One-Dimensional Spin System (CH3)2CHNH3Cu(ClxBr1−x)3 with x = 0, 0.25, and 0.3

“…1. No muon spin rotation due to a static hyperfine field was observed for all the samples investigated down to 0.3 K. This is consistent with the results of macroscopic quantities showing no indication of the magnetic order [13,14]. As the temperature was lowered, one can see that the depolarization becomes faster and that the shape of the short-time region of the curves changes from Gaussian-type to Lorentzian-type.…”

“…The volume fraction of  2 phase, that can be written as A 2 starts from 0.2 at 6.5 K to increase and reaches 0.4 at 0.3 K for both the samples of x = 0.25 and 0.30. This behavior is commonly observed irrespective of x in the region between 0.25 and 0.40 [13,14]. The temperature dependence of the two depolarization rates  1 and  2 under various LFs are shown in Fig.…”

“…In determining these parameters, we first set all of them free and performed fitting to find that the quasi static nuclear part width  stays nearly constant to be 0.24(2) for x = 0.25 and 0.23(2) for x = 0.30. Next, we fixed  to be the averaged value 0.23(s −1 ) and performed all the fitting again to obtain the other parameters [13,14].…”

“…So far, we have been studied the disordered system by microscopic tools of LF-SR and NMR [10,11,12] to reveal the existence of soft-mode toward absolute zero in the Fourier spectrum of spin fluctuation in the sample of x=0.35 [13] and 0.40 [14]. For the latter, in particular, we have shown that no magnetic order takes place in temperatures down to 15 mK indicating the realization of the exotic ground state with zero critical temperature.…”

Low-energy spin dynamics in randomness-introduced spin-gap systemsTl1−xKxCuCl3Suzuki1, Yamada2, Yamada3 et al. 2010Phys. Rev. B

“…The LF-SR measurements are suitable for this purpose. 33,[38][39][40] Thus, to obtain the microscopic information about the appearance of the BG phase and of other magnetic phases, we carried out ZFand LF-SR measurements in the wide region of x from 0.40 to 0.65.…”