Field modulation effects induced by sample spinning: application to high-resolution magic angle spinning NMR

High-resolution magic angle spinning (HRMAS) has become an extremely versatile tool to study heterogeneous systems. HRMAS relies on magic angle spinning of the sample and on pulse sequences originally developed for liquid state NMR. In most cases the outcome of the experiment is conform to what is expected from high-resolution liquid state NMR spectroscopy. However in some instances, experiments run under MAS can produce some very puzzling results. After reviewing the basic hardware which is at the heart of HRMAS spectroscopy, we show that the origin of this behavior lies in the natural time-dependence of some physical quantities imparted by the rotation. We focus in particular on the effects of B1 inhomogeneities on the nutation, the (90 degrees)+x-t-(90 degrees )-x and the MLEV16 experiments. Different models of radiofrequency distribution of B1 fields in a solenoidal coil are derived from simple geometrical considerations. These models are shown by NMR spin dynamics calculations to reproduce the experimental NMR results. They are also consistent with electromagnetic simulations of the B1 field distribution inside a solenoidal coil.     

Antiferroelectric phase transitions in single crystals PbZrO3:Sn revisited

This paper is a review of earlier results which have been completed with recent investigations on the nature and sequence of phase transitions evolving in the antiferroelectric PbZr_1–xSn_xO₃ single crystals (0 < x < 0.35). It was found that these crystals undergo several first-order phase transitions. To investigate these transitions, five experimental methods were used to characterize the crystal properties: birefringence, Raman light scattering, dielectric, thermodynamic and electromechanical methods, and the atomic force microscopy-piezoresponse force microscopy (AFM-PFM) method to focus also on nanoscale properties. Temperature dependencies have been tentatively measured in a wide range, including a region above T_C, where precursor dynamics is observed in the form of non-centrosymmetric regions existing locally in crystal lattices.

New experimental data (Raman light scattering and PFM) are presented mainly to revise the phase diagram and, at the same time, to complement other studies on the problem of pre-transitional effects. The phase transition mechanism in PbZr_1–xSn_xO₃ is discussed taking into account the incommensurability of crystal lattices, ferroelastic behavior of multicell cubic phases, electric field influence on antiferroelectric to ferroelectric transformation, and flexoelectric effect.     

Multi-dimensional 1H NMR nuclear Overhauser spectroscopy under magic angle spinning: theory and application to elastomers

The process of cross-relaxation between different protons (nuclear Overhauser effect) is investigated in soft solids by 2- and 3-dimensional NMR under the conditions of fast magic-angle spinning. The cross-relaxation rates are found to depend weakly on fast motions in the Larmor frequency range and strongly on slow motions of the order of the spinning frequency W _R. Explicit expressions for the W _R dependent cross-relaxation rates are derived for different motional models. These findings were tested experimentally on elastomers, i.e., on a cross-linking series of styrene-butadiene rubbers where the cross-relaxation was studied as a function of W _R. Short mixing times as are required for extracting the relaxation rates could be realized conveniently using a pulsed magnetic-field gradient for coherence pathway selection. As in solution NMR, relative couplings between chemically resolved spins can be determined from the peak intensities. By combining cross-relaxation measurements with T ₁ measurements, the distribution of correlation times can be probed for slow and fast timescales, respectively. Only the former were found to depend on the crosslink density.     

The effects of a laser field on phase transitions in liquid crystals

We investigate theoretically some phase transitions in liquid crystals in the presence of a laser beam. We found, in non-absorbing nematics, a laser-induced one-way transition from a paranematic to a nematic phase. In absorbing nematics we found, in addition to this transition, a one-way transition from a nematic to a paranematic phase with increasing laser intensity. Further, we found a reentrant nematic or a reentrant paranematic via paranematic or nematic phase respectively. In the case of smectic A, laser absorption results in a coupling between the positional and orientational orders. As a result, the smectic A to nematic transition can change from second order to first order and the smectic C to smectic A transition can become first-order in the field of a laser.     

A study on new phase transitions in Cs2CaCl4·2H2O single crystals by observation of Cs NMR

The ¹³³Cs 1/2→−1/2 spin-lattice relaxation rate, , and the spin-spin relaxation rate, , for a Cs₂CaCl₄·2H₂O single crystal have been measured in function of temperature. The dominant relaxation mechanism of this crystal over the whole temperature range investigated here proceeds via quadrupole interaction. The changes in the ¹³³Cs spin-lattice relaxation rate near 325 K (=T_c1) and 360 K (=T_c2) correspond to phase transitions in the crystal. The change in the spin-lattice relaxation rate at T_c1 is small because the crystal lattice does not change very much during this phase transition. The change in near T_c2 is due to the critical slowing down of the soft mode that typically occurs in structural phase transitions. The temperature dependence of the spin-lattice relaxation rate for this crystal has maximum values at about 240 K, which is attributable to the effect of molecular motion as described by Bloembergen-Purcell-Pound theory. The phase transition temperatures T_c1 and T_c2 obtained from the temperature dependence of the relaxation rate is also clear from data obtained using differential scanning calorimetry. Therefore, we know that previously unreported phase transitions occur at 325 and 360 K.     

Commensurability oscillations and a new phase transition in YBa2Cu3Oy single crystals

Magnetization oscillations due to the commensurability of the vortex and crystal lattice periods in YBa₂Cu₃O_y (y=6.97±0.02) single crystals are investigated using a high angular resolution magnetometer. A sharp peak in the temperature dependence of the oscillation amplitude as well as other features in the behavior of the oscillation amplitude and of the irreversible magnetization are observed at T _f ∼60 K. It is inferred that T _f is the temperature of a transition of the solid vortex state to a smectic phase. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 11, 832–837 (10 June 1999)     

Phase transitions in 8CB liquid crystal confined to a controlled-pore glass: Deuteron NMR and small angle X-ray scattering studies

We present results of the deuteron nuclear magnetic resonance (NMR) and small-angle X-ray scattering (SAXS) study of ordering and phase transition behavior of octylcyanobiphenyl (8CB) liquid crystal confined to a controlled-pore glass (CPG) with nontreated and silanes-treated pore surfaces. The deuteron NMR spectra allowed to determine the degree of nematic liquid crystal ordering and also provided an indirect information on the confined 8CB smectic ordering via its influence on the nematic ordering. For the smectic phase these data are supplemented with measurements of the temperature dependence of the first-order SAXS diffraction pattern. The NMR results indicate that the average nematic and smectic order parameters of 8CB in the nontreated CPG are only weakly perturbed by the confinement. The SAXS data further suggest that in confined 8CB for both nontreated and silane-treated CPG a domainlike pattern appears in accordance with the Imry-Ma theorem.     

Method for analyzing second-order phase transitions: application to the ferromagnetic transition of a polaronic system

A new method for analyzing second-order phase transitions is presented and applied to the polaronic system La(0.7)Ca(0.3)MnO3. It utilizes heat capacity and thermal expansion data simultaneously to correctly predict the critical temperature's pressure dependence. Analysis of the critical phenomena reveals second-order behavior and an unusually large heat capacity exponent.     

Employment of a novel ultrasonic method to investigate high pressure phase transitions in oleic acid

In this work, the variation of sound velocity with hydrostatic pressure for oleic acid is evaluated up to 350 MPa. During the measurement, we identified the phase transformation of oleic acid and the presence of the hysteresis of the dependence of sound velocity on pressure. From the performed measurements, it can be seen that the dependence of sound velocity on pressure can be used to investigate phase transformations in natural oils. Ultrasonic waves were excited and detected using piezoelectric LiNbO₃(Y-36 cut) 5 MHz transducers. The phase velocity of the longitudinal ultrasonic waves was measured using a cross-correlation method to evaluate the time of flight.     

Comparative study of the effect of low-dose gamma irradiation on phase transitions in single crystals of triglycine sulfate and its deuterated analog

The effect of low γ-radiation doses (< 0.5 MR) on the specific heat C _p of crystals of triglycine sulfate and its deuterated analog was studied using precision vacuum adiabatic calorimetry in the temperature range 80–350 K. Appreciable changes in the behavior of C _p of nominally pure and γ-irradiated crystals were detected near the phase transition temperature (T _c ), which decreased as the dose was lowered.     

Emf induced by a change in the samarium ion valence as a result of a phase transition in SmS single crystals

The mechanism of the emf appearing in a semiconductor under heating in the absence of external temperature gradients, an effect revealed by the present authors, is considered. The experiments were performed on samarium sulfide (SmS) single crystals. It is shown that the emf is generated by an abrupt change in the samarium ion valence, which results from the ion screening by the electrons activated into the conduction band. We succeeded in obtaining emf pulses 1.3 s long with an amplitude of up to 2.5 V at T ～ 460 K, as well as CW emf generation within the 375-to 405-K temperature interval with a maximum value of ～50 mV.     

London theory for superconducting phase transitions in external magnetic fields: application to UPt3

For multicomponent superconductors, it is known that the presence of symmetry breaking fields can lead to multiple superconducting phase transitions. Motivated by recent small angle neutron scattering experiments on the vortex state of UPt3, the London theory in the vicinity of such phase transitions is determined. It is found that the form of this London theory is in general quite different than that for conventional superconductors. This is due to the existence of a diverging correlation length associated with these phase transitions. One striking consequence is that nontrivial vortex lattices exist arbitrarily close to H(c1). Applications to UPt3, CeIn3, U(1-x)Th(x)Be(13), electron doped cuprate superconductors, Sr(2)RuO(4), and MgCNi(3) are discussed.     

Multiple-quantum NMR spectra of partially oriented indene: a new approach to estimating order in a nematic phase

Cyclic J cross polarisation (CYCLCROP) is a sensitive method for the noninvasive monitoring of (13)C distributions and fluxes. The PRAWN rotating frame Hartmann-Hahn mixing sequence ameliorates problems associated with sensitivity to Hartmann-Hahn mismatch and reduces RF power deposition. The combination of CYCLCROP with echo planar imaging (EPI) for spatial encoding of the proton detected carbon signal allows efficient use of the available signal to be made, permitting a significant improvement in the temporal resolution of any study. We report here on some initial experiments to demonstrate the feasibility of echo planar proton detected (13)C imaging using CYCLCROP based upon the PRAWN module, including the application of the technique to the measurement of transport and accumulation of (13)C-labelled sucrose in a castor bean seedling. Two methods that can be used to eliminate the effect of the J-splitting in the EP images are presented. In addition, a fast, image-based B(1) field-mapping method which may be used to quantitatively map the low frequency RF field in a dual resonant ((13)C/(1)H) probe is presented. The technique utilises the above described imaging method, permitting fully quantitative, 64x64 axial field maps to be generated in about a minute.     

A study of the solid state-gaseous phase equilibrium in CdSe and its application to the growing of single crystals from the gaseous phase by Frerich's method

The equilibrium between the solid state and the gaseous phase of the semi-conducting compound CdSe is investigated. This is a study of two equilibrium reactions CdSe(s)Cd(g) + + (1/2) Se₂(g) and Se₆(g)3 Se₂(g) as a function of the temperature. The results were used to estimate the optimum conditions for the growing of CdSe single crystals from the gaseous phase by Frerichs' method. The basic electrical and optical properties were then measured on the crystals. In conclusion the results of the experiments are evaluated and compared with those of other authors.In conclusion, the authors thank Dr. E. Klier and other members of the department for much valuable advice and remarks, and laboratory assistants O. Kundrátová and J. Rejna for help in the work.     

The complement: a solution to liquid drop finite size effects in phase transitions

The effects of the finite size of a liquid drop undergoing a phase transition are described in terms of the complement, the largest (but mesoscopic) drop representing the liquid in equilibrium with the vapor. Vapor cluster concentrations, pressure, and density from fixed mean density lattice gas (Ising) calculations are explained in terms of the complement generalization of Fisher's model. Accounting for this finite size effect is important for extracting the infinite nuclear matter phase diagram from experimental data.     

Refusing to twist: demonstration of a line hexatic phase in DNA liquid crystals

We report conclusive high resolution small angle x-ray scattering evidence that long DNA fragments form an untwisted line hexatic phase between the cholesteric and the crystalline phases. The line hexatic phase is a liquid-crystalline phase with long-range hexagonal bond-orientational order, long-range nematic order, but liquidlike, i.e., short-range, positional order. So far, it has not been seen in any other three dimensional system. By line-shape analysis of x-ray scattering data we found that positional order decreases when the line hexatic phase is compressed. We suggest that such anomalous behavior is a result of the chiral nature of DNA molecules.     

Erratum to: Multiple magnetic transitions in single crystals of Ce12Fe57.5As41and La12Fe57.5As41

Commercial computers based on electronic logic devices have brought great changes to the world. However, traditional electronic devices are suffering from numerous technical challenges in their attempts to continue to satisfy Moore's law. Alloptical logic devices, as promising successors to their electronic counterparts, have become a major focus of optics research. In this paper, we provide a review of current all-optical logic devices. The logic gates in these devices, which are described in the first part of the review, are divided into five categories based on the different principles used in their realization. Complex optical devices with various functions and reconfigurable devices are summarized in the next section. In the final part of this paper, we discuss some of the previous works on all-optical integrated chips with specific functions. This review will provide a complete technological roadmap for all-optical devices and aims to be helpful in possible future developments in this growing field.     

Very fast magic angle spinning (1)H-(14)N 2D solid-state NMR: sub-micro-liter sample data collection in a few minutes

Substantial resolution and sensitivity enhancements of solid-state (1)H detected (14)N HMQC NMR spectra at very fast MAS rates up to 80 kHz, in a 1mm MAS rotor, are presented. Very fast MAS enhances the (1)H transverse relaxation time and efficiently decouples the (1)H-(14)N interactions, both effects leading to resolution enhancement. The micro-coil contributes to the sensitivity increase via strong (14)N rf fields and high sensitivity per unit volume. (1)H-(14)N HMQC 2D spectra of glycine and glycyl-L-alanine at 70 kHz MAS at 11.7 T are observed in a few minutes with a sample volume of 0.8 μL.