MuSR/µ + SR studies in KDP and DKDP

Transcription

1 Hyperfine Interactions 106 (1997) MuSR/µ + SR studies in KDP and DKDP K. Nishiyama a,w.k.dawson a,b,s.ohira a and S. Ikeda c a Meson Science Laboratory, Faculty of Science, University of Tokyo, Bunkyo-ku, Tokyo 113, Japan b Institute of Physical and Chemical Research (RIKEN), Wako, Saitama , Japan c National Laboratory of High Energy Physics, Oho, Tsukuba 305, Japan The temperature dependence of muon interactions has been studied in ferroelectric KDP (H 2KPO 4) and DKDP (D 2KPO 4) using conventional µsr and muon spin resonance spectroscopy. In longitudinal field measurements, a fast relaxing component and a slow relaxing component were observed. The slow relaxing component is attributed to diamagnetic muons. The muon spin resonance measurements indicate that the fast relaxing component results from some muonium like species: either normal or anomalous. In zero field and weak longitudinal field µsr (0 100 G), a remarkable peak in the fast relaxing component is observed around 220 K in both KDP and DKDP. An additional feature is also seen around 300 K. The amplitude of the resonance measurement has a broad minimum around 200 K which corresponds to the maximum in the relaxation rate in longitudinal field (100 G). The temperature dependence of the muonium relaxation rate in KDP is almost identical to that of DKDP. The diamagnetic fraction also shows almost no difference in relaxation rate or asymmetry for DKDP and KDP. 1. Introduction KDP (H 2 KPO 4 ), and DKDP (D 2 KPO 4 a deuterated version of KDP) are ferroelectric (FE) materials which are commonly used as optical parametric oscillators in laser physics. They are also essential components in many non-linear optics studies. The ferroelectric-transition temperature (T FE ) of KDP is 123 K, and DKDP is 214 K. A considerable amount of research has been devoted to explain the difference in T FE between KDP and DKDP. Over the course of the last 55 years, numerous mechanisms have been proposed to explain this effect; however, they essentially fall between two extreme camps of thought: on the one end a tunneling effect [1], and on the other end an order disorder effect [2]. Almost all of the models currently proposed utilize phonon interactions at some level in the theory. In a previous paper, we reported the first longitudinal field (LF) repolarization measurements on KDP and DKDP [3]. In this paper, we have extended the temperature dependence measurement for both materials. In that report [3], DKDP had been examined in the temperature range around 220 K, but KDP had not. The relaxation J.C. Baltzer AG, Science Publishers

2 112 K. Nishiyama et al. / MuSR/µ + SR studies in KDP and DKDP rate (λ) for DKDP in 100 G longitudinal magnetic field (LF) shows a large increase at roughly 225 K. The T FE of DKDP is also located around 220 K; hence this maximum appeared to correlate with the ferroelectric phase transition. However, in our recent measurements of KDP, we observe the same peak in the LF relaxation rates (λ LF ) in KDP at about 220 K. Moreover, there also appears to be a secondary increase in relaxation rate around 300 K in both KDP and DKDP. The relaxation maximum at 300 K is not explained by any theory. The only distinguishable feature between the DKDP and KDP data is a factor of two difference in magnitude of the Mu spin relaxation rate. Added to these extended LF measurements, we report the first Mu spin resonance measurements in KDP and DKDP. These measurements are helpful in establishing whether the λ LF effects we observed originate from a muonium signal. These resonance measurements show a temperature dependence in the resonance amplitude which parallels the LF repolarization measurements. We also studied the diamagnetic species in 100 G and 30 G TF. In this paper, we summarize the measurements we have made on KDP and DKDP. A more detailed discussion on the interpretation will be reported elsewhere. 2. Experiment All these measurements were carried out using the pulsed beam at the University of Tokyo Meson Science Laboratory. The LF measurements were carried out at the surface muon channel, and the resonance measurements were carried out at the backward muon channel. Transverse magnetic field (TF) measurements were carried out at both the surface and the backward muon channels. Fields between 0 and 100 G were used in all these measurements (TF and LF). Single crystal samples of KDP and DKDP with the dimensions of approximately mm were mounted on a glass fiber sample holder and placed inside a helium flow cryostat. Resonance measurements were carried out using a specially designed resonance circuit and coaxial cabling to transfer the rf inside the He flow cryostat. The circuit was tuned for a 60 MHz resonance frequency. The muons were injected into the sample with the muon spin polarization parallel to the c-axis. 3. Transverse field measurements: the diamagnetic muon The transverse-magnetic field (TF) measurements of the diamagnetic fraction (asymmetry) of KDP and DKDP are seen in figs. 1(a), (b). At lowest temperatures, the fractions of µ + to Mu are about 50%. A distinct temperature dependence in the asymmetry can be seen in the diamagnetic fraction of KDP whereas no such dependence is seen in DKDP. In DKDP, there is no significant change in the asymmetry.

3 K. Nishiyama et al. / MuSR/µ + SR studies in KDP and DKDP 113 Fig. 1. Temperature dependence of the TF asymmetry and relaxation rate of the diamagnetic muon fraction (µ + ): (a) KDP asymmetry, (b) DKDP asymmetry, (c) KDP TF relaxation rate, and (d) TF relaxation rate DKDP. In contrast to the asymmetry, the TF-relaxation rates (λ TF ) of the diamagnetic fraction (fig. 1(c), (d)) in KDP and DKDP are identical for all temperatures (at least within the error bars). Moreover, the temperature dependence is virtually indistinguishable. This means that the influence of the 1 H + and 2 H + on the µ + is negligible. The observed values for λ TF increase continuously from room temperature (about 0.04 µs 1 ) to a saturation value at 140 K which extends to lowest temperatures (0.20 µs 1 ) (fig. 1(c), (d)). The best fit for λ TF appears to be an exponential depolarization throughout the entire temperature range studied. 4. Longitudinal field measurements In the LF measurements (fig. 2), the primary features of the temperature dependence are identical except for a factor of two difference in the magnitude of the LF relaxation rate (λ TF ). In both KDP and DKDP, the λ TF is considerably enhanced around the Curie temperature of DKDP. In fig. 2(d), a peak in the λ TF is observed in weak LF (30 G, 100 G) around 210 K for DKDP. A similar peak is seen for KDP in fig. 2(c). The temperature of the maximum λ LF shifts slightly with respect to the applied LF. There

4 114 K. Nishiyama et al. / MuSR/µ + SR studies in KDP and DKDP Fig. 2. Temperature dependence of the repolarization asymmetry and exponential relaxation rates of the muonium fraction in ZF (E), 30 G LF (Greek cross), and 100 G LF (!): (a) KDP asymmetry (b) DKDP asymmetry, (c) KDP relaxation rates, and (d) DKDP relaxation rates. is virtually no evidence of the T EF in the observed relaxation rates of KDP (fig. 2(c)). Moreover, there is even less difference in the asymmetry between KDP and DKDP (figs. 2(a), (b)). Aside from the factor of 2 difference in λ LF,theonly hint of T FE in KDP is seen in the ZF asymmetry where a slight decrease is observed (fig. 2(a) E). However, even this is similar in both KDP and DKDP (cf., fig. 2(a) and fig. 2(b)). The strong similarity in the λ LF and the same temperature dependence (particularly at 220 K) suggest that there is no direct link between the FE transition and the temperature dependence of the Mu relaxation rates. 5. Mu spin resonance In the resonance measurements, the primary objective was to determine whether the measured λ LF corresponded to some Mu species, or, if there was some other conceivable explanation. The low temperature (5 K) resonance spectra of KDP and DKDP are shown in fig. 3. The resonance amplitude for KDP (snow flakes) is less than half the value seen in DKDP (E). However, there are clear similarities in the shape of these two

5 K. Nishiyama et al. / MuSR/µ + SR studies in KDP and DKDP 115 Fig. 3. Mu spin resonance spectra at 5 K for KDP (snow flakes) and DKDP (E). resonance spectra. The central peak at 43 G (60 MHz) corresponds to the resonance frequency of Mu. The origin of the side bands at 20 G (30 MHz) and 65 G (90 MHz) are not entirely established at this point. At t = 0, the side bands appear to be smaller. However, from about 0.7 µs (aftert=0), the intensity of the side bands becomes quite significant. The central peak also increases during this time interval. This may indicate that some initial metastable Mu state is converting to some other more stable Mu state or possibly a radical state. The peak at 20 G is more intense than the one at 65 G and the center appears to be a little offset; however, the peak appears to be real. At room temperature, a more complex spectrum is observed and the broad splitting width is not so visible (fig. 4). Only a weak signal is seen in KDP (snowflakes) whereas a more complex signal is seen for DKDP (E). This may reflect that the Mu is becoming mobile and therefore averaging out the net electric field gradient. It is likely that the majority of the signal from KDP is lost due to the vast quantities of H +. At this point, the measurement is not sufficient to say more about the state of muonium. The temperature dependence of the resonance maximum (43 G) is shown in fig. 5 (bottom). The amplitude of the resonance frequency is seen to decrease to zero from lowest temperatures to about 220 K in both KDP ( and!) and DKDP (E). The sharpest decrease begins somewhere around 100 K (near T EF of KDP) and ends near the T FE of DKDP. The amplitude again recovers above 220 K. Another ledge is suggested at around 300 K. The temperature dependence of the resonance data

6 116 K. Nishiyama et al. / MuSR/µ + SR studies in KDP and DKDP Fig. 4. Mu spin resonance at room temperature for KDP (snow flakes) and DKDP (E). Fig. 5. Temperature dependence of the resonance maximum shown in figs. 3 and 4. The signal appears to vanish around 220 K but then recovers again: (bottom) resonance amplitude in DKDP (E), and KDP (! and for different rf powers); (top) LF-relaxation rate in 100 G field for DKDP (!+) and KDP (!).

7 K. Nishiyama et al. / MuSR/µ + SR studies in KDP and DKDP 117 is strongly correlated with the LF-relaxation measurements (fig. 5 (top)). When λ LF reaction is a maximum, the resonance intensity is a minimum and when λ LF is smallest, the resonance intensity is the largest. 6. Conclusion KDP and DKDP have been studied using muonium spin resonance, longitudinal field repolarization, and transverse field, but direct evidence of the ferroelectric transition in these materials could not be confirmed. In the LF repolarization and resonance measurements, a large and yet unexplained relaxation is observed in both KDP and DKDP at 200 K (the ferroelectric transition of DKDP). A secondary peak at about 300 K is also seen in the relaxation rate of both materials. The diamagnetic muon exhibits the same relaxation rates in both KDP and DKDP. References [1] R. Blinc, J. Phys. Chem. Solids 13 (1960) 204; M. Tokunaga and T. Matsubara, Prog. Theor. Phys. 35 (1966) 581. [2] Y. Tominaga, M. Tokunaga and I. Tatsuzaki, Solid State Commun. 54 (1985) 979; J.C. Slater, J. Chem. Phys. 9 (1941) 16. [3] K. Nishiyama, W.K. Dawson and S. Ikeda, Hyp. Int. 85 (1994) 85.