MuSR/μ+SR studies in KDP and DKDP

The temperature dependence of muon interactions has been studied in ferroelectric KDP ( H₂KPO₄) and DKDP ( D₂KPO₄) 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. This revised version was published online in August 2006 with corrections to the Cover Date.     

Modified strong dipole-proton coupling model and local properties of EPR probe in KDP-type ferroelectrics

The modified strong dipole-proton coupling (MSDPC) model, which predicts several static and dynamic dielectric properties of KDP-type ferroelectrics, is used to investigate the properties of the paramagnetic center SeO ₄ ^3? introduced in the KH₂PO₄ (KDP) and KD₂PO₄ (DKDP) lattice as a local probe in an electron paramagnetic resonance experiment. Paramagnetic center is treated within the MSDPC model as a soft impurity, with anA _probe value of the elastic constant for the dipolec-component induction lower than theA value for the original PO ₄ ^3? group. ForA _probe=0.45A, the molecular dynamics simulations in the paraelectric phase of KDP and DKDP show the effective local double-well potential of the probe, and the temperature dependence of the calculated correlation time τ for the dipole reorientational jumps over the barrier can be fitted by the Arrhenius law. The obtained Arrhenius parameters for KDP are close to the experimental ones, and the experimentally detected higher activation energy in DKDP is reproduced. Also, the high-temperature line broadening and the temperature dependence of the⁷⁷Se hyperfine coupling observed in experiment are discussed within the MSDPC model.     

μ–H Interaction in Hydrogen Bonding Systems

Muon spin rotation (μSR) experiments were performed on single crystal samples of KH₂PO₄(KDP) and KD₂PO₄(dKDP) to study the dynamics of hydrogen in hydrogen bonding systems. At low temperature, the nuclear dipole interaction of muon and proton was confirmed from the angular dependence of precession frequency of the muon spin under zero magnetic field. The muon occupation site was also determined. A clear change in μSR spectra was observed at the antiferroelectric transition temperature (123 K). At 90 K well below the transition temperature, the muon spin starts to relax, possibly due to muon dynamics. This revised version was published online in September 2006 with corrections to the Cover Date.     

Microwave dielectric tangent losses in KDP and DKDP crystals

By adding cubic and quartic phonon anharmonic interactions in the pseudospin lattice coupled mode (PLCM) model for KDP-type crystals and using double-time temperature dependent Green's function method, expressions for soft mode frequency, dielectric constant and dielectric tangent loss are obtained. Using model parameters given by Ganguliet al [9] the dielectric losses are calculated for KDP and DKDP crystals. In the microwave frequency range an increase in frequency (1–35 GHz) is followed by an increase in dielectric tangent loss (1–35) at 98 K and (1–15) × 10⁻² at 333 K for KDP and DKDP crystals respectively. The dielectric tangent loss decreases from 0.052 to 0.042 for KDP crystals with increase in temperature from 130 to 170 K and for DKDP crystals it decreases from 0.0166 to 0.0074 with an increase in temperature from 230–343 K in their paraelectric phases at 10 GHz. This shows Curie-Weiss behavior of the dielectric tangent loss     

The electro-optic kerr effect in noncentrosymmetric KH2PO4 and KD2PO4 monocrystals

Experimental and theoretical results of the determination of the quadratic electro-optic coefficients in noncentrosymmetric KH₂PO₄(KDP) and KD₂PO₄(DKDP) monocrystals are presented. The coefficientR _c=R ₁₁₂₂-R ₁₁₁₁ for the X-cut crystals has been measured by a dynamic method in the a.c. electric field. The theoretical value of the coefficient has been calculated by the Kurtz-Robinson model. It has been found that the coefficientR _c was less than 10×10^–20m²V^–2.     

Spin fluctuations in the vicinity of a charged particle in solid3He

At low temperature the positively charged muon (μ⁺) in solid³He is localized in a polaron which, unlike a chemically bound complex, is held together with polarizational attraction (van der Waals force). The dynamic effects in the muon relaxation are determined by the spin exchange of the³He atoms in the polaron. In crystals with large molar volumes (melting pressureP _m<60 bar) the rate of magnetic field fluctuations at the muon site is at least one order of magnitude lower than in an unperturbed crystal. In an external electric field the μ⁺ produces an anisotropic local distortion which reduces the rate of the local³He spin exchange and leads to an increase of the muon spin relaxation rate.     

Thermal hopping ofμ + between “FμF” centres in NaF

Strongly hydrogen-bonded diamagnetic “FμF” centres are formed by theμ ⁺ in a wide variety of fluoride crystals. Hydrogen atoms are expected to form similar “FHF” complexes. Through the “motional narrowing” of the zero-field muon relaxation function in NaF, we have observed an Arrhenius temperature dependence of the dissociation rate of theFμF complex, yielding a binding energy of 1700 (200) K for theμ ⁺ in theFμF centre.     

Muon spin relaxation and knight shift in the heavy-fermion superconductor UPt3

Positive muon spin relaxation experiments have been conducted on the heavy-fermion superconductor UPt₃ in both the normal and superconducting states for zero, transverse, and longitudinally applied magnetic fields. Below 6 K in zero applied field, the μ⁺ relaxation rate is approximately twice that expected from¹⁹⁵Pt nuclear dipolar relaxation alone. Transverse- and longitudinal-field measurements show that the observe relaxation rate depends on magnetic field and is quasistatic in origin. It is suggested that the onset of very weak (≈10⁻³ μ_B/U atom) magnetic ordering below approximately 6 K is responsible for the observed increase in the relaxation rate. μ⁺ Knight shift measurements in the normal state of UPt₃ show a temperature dependent shift K_μ which tracks the bulk susceptibility X. From the K_μ vs. X plot, a μ⁺ hyperfine field of approximately 100 Oe/μ_B is extracted.     

Diffusion,trapping and detrapping of positive muon in Al-0.047% Mg

Positive muon spin relaxation was measured in Al-0.047% Mg quenched from 873 K under zero external magnetic field from 6.3 K to 270 K. The observed spectra were analysed with the calculated muon spin relaxation function which included the static relaxation rates, the trapping rates, the detrapping rates and thet=0 initial trapping fractions. Due to the precise measurements and the realistic expression of spin relaxation function, above four parameters could be determined distinctly. The trapping rates and the distortions around a muon were determined from the values of static relaxation rates at a trapping site. The diffusion features were clearly described by the trapping and detrapping rates. The diffusion process was determined by the temperature dependence of these parameters.     

Microscopic state of the positive muon in 2H-NbSe2

The microscopic state of the positive muon in the transition metal dichalcogenide 2H-NbSe₂ was studied using the muon spin relaxation method (μ⁺SR). We found that the μ⁺SR spectra consist of two components. The ratio of the two components and the dynamics of the muon change at 140 K, at the charge-density-wave transition temperature (32 K), and again at the superconducting transition temperature (7 K). We discuss the relation between conduction electron properties and the muon's behavior. This revised version was published online in August 2006 with corrections to the Cover Date.     

Displacive and order-disorder behavior of KDP-type ferroelectrics on the local scale

The modified strong dipole-proton coupling (MSDPC) model, which predicted several static and dynamic dielectric properties of KH₂PO₄ or KDP-type ferroelectrics, was used to investigate the properties of these crystals on the local scale. Results calculated by molecular dynamics (MD) simulation show that both order-disorder and displacive characteristics of one PO₄ dipole are present in KDP and KD₂PO₄ (DKDP). These results correlate with experimental data from NMR and neutron scattering studies of local properties.     

Anomalous frequency shift of negative muon spin precession in n‐type silicon

The dependence of the residual polarization of negative muons in n‐type Si with impurity concentration (1.6\pm 0.2)\times 10¹³\ cm^-3 on temperature in the 10–300 K range has been investigated. Measurements were carried out in external magnetic field of 0.08 T transverse to the muon spin. Muon spin relaxation and frequency shift were observed at temperatures below 30 K. The relaxation rate at 30 K is equal to 0.25\pm 0.08\,μ s^-1. The frequency shift at 20 K is equal to 7\times 10^-3. Both the relaxation rate and the frequency shift grow with decrease of temperature. Below 30 K the relaxation rate is well described by the dependence \varLambda=bT^-q, where q=2.8. An analysis of present and earlier published data on behavior of negative muon polarization in silicon is given. A possible mechanism of relaxation and frequency shift of muon spin precession in silicon is considered. This revised version was published online in August 2006 with corrections to the Cover Date.     

KDP晶体固-液界面吸附行为的分子模拟研究

通过构建晶体表面-KDP分子界面吸附结构模型, 采用分子动力学和密度泛函计算方法研究KDP分子在(001)和(010)面吸附的物理化学过程, 考察了温度对物理吸附行为的影响. 研究表明: KDP晶体表面的吸附过程和生长习性主要由化学吸附主导, 化学吸附能的计算表明[K-O₈]基元在(001)界面的结合能是(010)界面结合能的2.86倍; 在饱和温度附近, [H₂PO₄]^-阴离子在KDP界面的物理结合能随温度的变化呈现振荡特征, 溶液中有较多的离子团簇形成, 溶液变得很不稳定; 当温度从323 K降低至308 K时, 水分子在界面的结合能总体呈下降趋势, 而KDP分子在界面的吸附能总体呈上升趋势, 脱水过程是水分子和[H₂PO₄]^-阴离子在固液界面边界层竞争吸附的结果. 研究结果对确足晶体生长界面动力学过程发展和完善晶体生长理论有重要意义. 关键词： 分子动力学 双层结构模型 结合能     

Kinetics of electron tunneling transfer in KH<Subscript>2</Subscript>PO<Subscript>4</Subscript> and NH<Subscript>4</Subscript>H<Subscript>2</Subscript>PO<Subscript>4</Subscript> crystals with hydrogen bonds

A model of electron transfer by tunneling between trapped electron and hole centers in crystals with hydrogen bonds under the conditions of thermostimulated mobility of one carrier type in the recombination process has been developed. The proposed model describes all features in the kinetics of induced optical density relaxation observed in nonlinear optical crystals of KH₂PO₄ (KDP) and NH₄H₂PO₄ (ADP) on a wide temporal scale (10⁻⁸–10 s) under pulsed irradiation. The results of model calculations have been compared with experimental data on the photoinduced transient optical absorption (TOA) in KDP and ADP crystals in the visible and UV ranges. The nature of the radiation-induced defects, which account for the TOA, and the dependence of the TOA decay kinetics on the temperature, excitation power, and other experimental conditions have been considered.     

Hebel-Slichter peak and superconducting energy gap in Rb3C60 observed by muon spin relaxation

Muon spin relaxation has been observed in both the normal and superconducting states of Rb₃C₆₀ (T _c=29.3K). The field dependence of theT ₁ spin relaxation rate is due to muonium undergoing spin-exchange scattering with conduction electrons, making this the first observation of muonium in a metal. The temperature dependence ofT ₁ ^–1 shows a Hebel-Slichter coherence peak just belowT _c which is not seen in¹³C spin relaxation. The peak can be fit assuming spin relaxation due to interaction with the quasiparticle excitations of a BCS superconductor provided the density of states is broadened relative to that of BCS. Such fits yield a value for the zero temperature energy gap, ₀/k _B , of 53(4)K, consistent with weak-coupling BCS.     

Anomalous anisotropies and field dependencies of the muon Knight shift and the relaxation rate in monocrystalline Bi

The angular dependence of the muon Knight shift,K _μ, and the muon relaxation rate in Bi at 11 K were measured in external magnetic fields up to 1 T. BothK _μ and the second moment,M ₂, are field dependent and involveP ₄ ⁰(cos θ) andP ₄ ³(cos θ) terms in the angular dependence. The Knight shift behaviour is discussed in terms of the dipole-dipole interaction and the de Haas-van Alphen effect, a consistent interpretation was not achieved in either case. The field dependence ofM ₂ is in complete contrast to the second moment calculations and points to a field dependent redistribution of the charge distribution around the interstitial site.     

Probing oxygen in high Tc superconductor LaSrCuO by negative muons

A novel microscopic method of muon spin rotation for negative muon (μ⁻) attached to oxygen is applied on highT _c materials LaSrCuO. The paramagnetic shift and relaxation rate are measured as a function of Sr concentration and temperature. Analysis shows the large difference in shift and relaxation between two different sites. A large anisotropy was also observed by preliminary measurements along different crystalline axes.     

Relaxation and shift of the precession frequency of the spin of a negative muon in n-type silicon

The residual polarization of negative muons in n-type silicon with impurity density (1.6±0.2) · 10¹³ cm⁻³ is investigated as a function of temperature in the range 10–300 K. The measurements are performed in an external magnetic field of 0.08 T oriented transversely to the spin of the muons. Relaxation of the muon spin and a shift of the precession frequency are observed at temperatures below 30 K. The relaxation rate at 30 K equals 0.25±0.08 μs⁻¹. The shift of the precession frequency at 20 K equals 7 · 10⁻³. Both the relaxation rate and the shift of the precession frequency increase as the temperature decreases. At temperatures below 30 K the relaxation rate is described well by the relation Λ=bT ^−q , where q=2.8±0.2. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 7, 539–543 (10 April 1996)     

Dielectric and conduction mechanism studies of PVA-orthophosphoric acid polymer electrolyte

Polyvinyl alcohol (PVA)-based proton conducting polymer electrolytes have been prepared by the solution cast technique. The conductivity is observed to increase from 10⁻⁹ to 10⁻⁴ S cm⁻¹ as a result of orthophosphoric acid (H₃PO₄) addition. The plot of conductivity vs temperature shows that a phase transition occurred at 343 K in the sample PVA-33 wt% H₃PO₄. The β-relaxation peak is observed at 313 K. The glass transition temperature of PVA-33 wt% H₃PO₄ is 343 K. Orthophosphoric acid seems to play a dual role, i.e., as a proton source and as a plasticizer. The ac conductivity σ _ac = Aω ^s was also calculated in the temperature range from 303 to 353 K. The conduction mechanism was inferred by plotting the graph of s vs T from which the conduction mechanism for sample PVA-17 wt% H₃PO₄ was inferred to occur by way of the overlapping large polaron tunneling (OLPT) model and the conduction mechanism for the sample PVA-33 wt% H₃PO₄ by way of the correlated barrier height (CBH) model.     

Irradiation-induced processes and electronic excitations in nonlinear optical materials

The original results obtained under the supervision of the authors and the literature data on optoirradiative stability, irradiation-induced processes, and electronic excitations in nonlinear optical materials, using a comparatively new wide-gap nonlinear LiB₃O₅ crystal as well as KDP (KH₂PO₄) and DKDP (KD₂PO₄) crystals are reviewed. Urals State Technical University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 66–84, March, 2000.