R. Scheuermann

Chiral Induction in Lyotropic Liquid Crystals: Insights into the Role of Dopant Location and Dopant Dynamics†

The pitch P of the helix can be directly observed in the polarizing optical microscope as the periodic pattern of the “fingerprint texture”. Chiral induction in liquid crystals (LCs) is one of the most sensitive methods for the detection of chirality. The unique chirality effects in LCs have been studied widely, including the molecular induction mechanism in thermotropic LCs and in a self-assembled twodimensional model system. For LLCs, however, the molecular induction mechanism in the N* phase has been a matter of discussion for more than 20 years. There are two proposed mechanisms: a) a dispersive chiral interaction between dopants in adjacent micelles (the dopant should preferentially be located at the micellar surface), and b) a steric dopant–amphiphile interaction yielding distorted micelles (in this case the solubilization of the chiral dopant within the micelle should be favorable). 10, 11] The temperature dependence of the pitch P(T) is expected to differ for the two mechanisms: in (a), P increases linearly with T, whilst in (b), P may decrease hyperbolically (with T ; see Supporting Information). Experimental studies on the chiral induction mechanism include pitch measurements of varied guest–host systems 7] and X-ray diffraction, however, the latter has not provided clear evidence of distorted micelles. The pitch was found to depend on the chemical composition of the LLC host phase, the temperature, the dopant concentration and, in particular, the chemical nature of the dopant. A general correlation between properties of the chiral dopant and its chiral induction power in a host phase has not yet been established for LLCs, in contrast to the molecular concepts developed for thermotropic LCs (see, for example, Ref. [1] and Ref. [12]), which are also important for LLCs, as we will discuss later. A crucial point of discussion regarding LLCs, especially in view of the suggested mechanisms, is the actual location of the chiral dopants in the N* phase: within the apolar core of the micelle or at the micellar surface. The dopant location is proposed to play an important role for the chiral induction power, 13] but locating the dopants experimentally has not yet been successful. Recently, a magnetic resonance method suitable for studying dopants present at low concentrations, avoidedlevel-crossing muon spin resonance (ALC-mSR), was used to reveal the dopant location in lamellar LLC phases. The ALCmSR technique involves the formation of a radical by the addition of muonium, Mu, a light hydrogen isotope (mH = 9mMu) with a positive muon m + as the nucleus, to an unsaturated bond (Scheme 1). The time-integrated muon spin polarization, which is proportional to the muon decay asymmetry A, is measured as a function of an external magnetic field. Resonances occur when there is coupling between eigenstates of the three-spin-=2 system composed of the radical electron, the muon, and the proton bound to the same carbon as the muon. The resonance field Bres is determined by the hyperfine coupling constants of the radical; these coupling constants are, among other factors, sensitive to the polarity of the surroundings, and Bres is shifted to higher values with increasing polarity. The polarity of the local environment and thus the location in the LLC is determined by comparing Bres in the LLC with the values in a Figure 1. Schlieren texture and model of the nematic (N) LLC host phase with disk-like micelles (left). Fingerprint texture and model of the chiral nematic (N*) phase; micelles represent the helical modulation of the director n with pitch P induced by doping the host phase with 4.37% R-MA (right).

Direct spectroscopic observation of a shallow hydrogenlike donor state in insulating SrTiO3.

We present a direct spectroscopic observation of a shallow hydrogenlike muonium state in SrTiO(3) which confirms the theoretical prediction that interstitial hydrogen may act as a shallow donor in this material. The formation of this muonium state is temperature dependent and appears below ∼ 70K. From the temperature dependence we estimate an activation energy of ∼ 50 meV in the bulk and ∼ 23 meV near the free surface. The field and directional dependence of the muonium precession frequencies further supports the shallow impurity state with a rare example of a fully anisotropic hyperfine tensor. From these measurements we determine the strength of the hyperfine interaction and propose that the muon occupies an interstitial site near the face of the oxygen octahedron in SrTiO(3). The observed shallow donor state provides new insight for tailoring the electronic and optical properties of SrTiO(3)-based oxide interface systems.

High-Field μSR Instrument at PSI: Detector Solutions

The High-Field μSR instrument is a highly challenging project under realization at the Swiss Muon Source [1] of the Paul Scherrer Institut (PSI, Switzerland). The detector system of the new spectrometer has to satisfy strict requirements on the time resolution and compactness. Muon-spin precession signals with frequencies of up to 1.3GHz in magnetic fields up to 9.5T have to be detected, the reduction of the signal amplitude should not exceed 50%. This requires an accuracy of better than 140 ps (sigma) in measuring the muon-positron time correlations – a time resolution unprecedented for such high fields. The small spiraling radius of the decay positrons in high fields (∼ 1 cm in 9.5T) sets restrictions on the maximal radial dimension of the detector. Preservation of the 10 ppm uniformity of the magnetic field at the sample position requires all detector components located in the vicinity of the sample to be non-magnetic. R&D work on the detector development for the High-Field project at PSI has started in 2004. It was realized that the required time resolution can hardly be achieved within the “standard” detector technology using photomultiplier tubes (PMTs), the limiting factors being attenuation and broadening of the light pulses in the indispensable light guides. Other potentially promising photosensors have been evaluated and the choice was made in favor of Geiger-mode Avalanche Photodiodes (G-APDs) [2]. These novel solid-state photodetectors deliver performance similar to that of PMTs, being at the same time insensitive to magnetic fields, compact, and non-magnetic (when choosing an appropriate packaging). The potential of the G-APD based detector technology for μSR and its reliability have been proven in [3, 4, 5]. The found technical solutions and the gained experience constituted an essential ground for working out the concept of the detector system of the High-Field μSR instrument.

Solvation of a hydrogen isotope in aqueous methanol, NaCl, and KCl solutions.

The muon hyperfine coupling constant (hfc) of the light hydrogen isotope muonium (Mu) was measured in aqueous methanol, NaCl, and KCl solutions with varying concentrations, in deuterated water, and in deuterated methanol. The muon hfc is shown to be sensitive to the size and composition of the primary solvation shell, and the three-dimensional harmonic oscillator model of Roduner et al. (J. Chem. Phys. 1995, 102, 5989) has been modified to account for dependence of the muon hfc on the methanol or salt concentration. The muon hfc of Mu in the aqueous methanol solutions decreases with increasing methanol concentration up to a mole fraction (chiMeOH) of approximately 0.4, above which the muon hfc is approximately constant. The concentration dependence of the muon hfc is due to hydrophobic nature of Mu. It is preferentially solvated by the methyl group of methanol, and the proportion of methanol molecules in the primary solvation shell is greater than that in the bulk solution. Above chiMeOH approximately 0.4, Mu is completely surrounded by methanol. The muon hfc decreases with increasing methanol concentration because more unpaired electron spin density is transferred from Mu to methanol than to water. The unpaired electron spin density is transferred from Mu to the solvent by collisions that stretch one of the solvents bonds. The amount of spin density transferred is likely inversely related to the activation barrier for abstraction from the solvent, which accounts for the larger muon hfc in the deuterated solvents. The muon hfc of Mu in electrolyte solution decreases with increasing concentration of NaCl or KCl. We suggest that the decrease of the muon hfc is due to the amount of spin density transferred from Mu to its surroundings being dependent on the average orientation of the water molecules in the primary solvation shell, which is influenced by both Mu and the ions in solution, and spin density transfer to the ions themselves.

Muon spin spectroscopy of the discotic liquid crystal HAT6.

Avoided level crossing muon spin resonance (ALC-muSR) has been used to study the cyclohexadienyl-type radicals produced by the addition of muonium (Mu) to the discotic liquid crystal HAT6 (2,3,6,7,10,11-hexahexyloxytriphenylene) in the crystalline (Cr) phase, the hexagonal columnar mesophase (Col(h)) and isotropic (I) phase. In the Cr phase unpaired electron spin density can be transferred from the radical to neighboring HAT6 molecules depending on the overlap of their pi-systems and hence on the relative orientation of the triphenylene rings. The two Delta(1) resonances in the ALC-muSR spectra of the Cr phase indicate that the neighboring HAT6 molecules have two preferred orientations with respect to the radical: one which results in negligible spin density transfer and a second where 17% of the unpaired spin density is transferred. The ALC-muSR spectra in Col(h) and I phases are substantially different from those of the Cr phase in that there are two narrow resonances superimposed on an extremely broad and intense resonance. The narrow resonances are due to highly mobile radicals located in the aliphatic region between the columns and the broad resonance is due to radicals incorporated within the columns of HAT6 molecules. The large width and amplitude of this resonance indicates that the radicals within the columns are undergoing rapid electron spin relaxation but the mechanism that causes this relaxation is unknown.

Defect levels and hyperfine constants of hydrogen in beryllium oxide from hybrid-functional calculations and muonium spectroscopy

Abstract The atomistic and electronic structures of isolated hydrogen states in BeO were studied by ab initio calculations and muonium spectroscopy (SR). Whereas standard density-functional theory with a semi-local GGA functional led to a detailed probing of all possible minimum-energy configurations of hydrogen further calculations with the hybrid HSE06 functional provided improved properties avoiding band-gap and self-interaction errors. Similarly to earlier findings for the other wide-gap alkaline-earth oxide, MgO, hydrogen in BeO is also predicted to be an amphoteric defect with the pinning level, E(), positioned in the mid-gap region. Both donor and acceptor levels were found too deep in the gap to allow for hydrogen to act as a source of free carriers. Whereas, hydrogen in its positively-charged state, , adopts exclusively hydroxide-bond OH configurations, and instead show a preference to occupy cage-like interstitial sites in the lattice. in particular displays a multitude of minimum-energy configurations: its lowest-energy ground state resembles closely a trapped-atom state with a nearly spherical spin-density profile. In contrast to the strongly ionic MgO, in BeO was further found to stabilise in additional higher-energy elongated-bond OH configurations whose existence should be traced to a partial covalent character of the Be–O bonding. Calculations of the proton-electron hyperfine coupling for all states showed that the ground-state interstitial configuration is dominated by an isotropic hyperfine interaction with a magnitude very close to the vacuum value, a finding corroborated by the SR-spectroscopy data.

Rate of molecular transfer of allyl alcohol across an AOT surfactant layer using muon spin spectroscopy

The transfer rate of a probe molecule across the interfacial layer of a water-in-oil (w/o) microemulsion was investigated using a combination of transverse field muon spin rotation (TF-μSR), avoided level crossing muon spin resonance (ALC-μSR), and Monte Carlo simulations. Reverse microemulsions consist of nanometer-sized water droplets dispersed in an apolar solvent separated by a surfactant monolayer. Although the thermodynamic, static model of these systems has been well described, our understanding of their dynamics is currently incomplete. For example, what is the rate of solute transfer between the aqueous and apolar solvents, and how this is influenced by the structure of the interface? With an appropriate choice of system and probe molecule, μSR offers a unique opportunity to directly probe these interfacial transfer dynamics. Here, we have employed a well characterized w/o microemulsion stabilized by bis(2-ethylhexyl) sodium sulfosuccinate (Aerosol OT), with allyl alcohol (CH2═CH-CH2-OH, AA) as the probe. Resonances due to both muoniated radicals, CMuH2-C*H-CH2-OH and C*H2-CHMu-CH2-OH, were observed with the former being the dominant species. All resonances displayed solvent dependence, with those in the microemulsion observed as a single resonance located at intermediate magnetic fields to those present in either of the pure solvents. Observation of a single resonance is strong evidence for interfacial transfer being in the fast exchange limit. Monte Carlo calculations of the ΔM = 0 ALC resonances are consistent with the experimental data, indicating exchange rates greater than 10(9) s(-1), placing the rate of interfacial transfer at the diffusion limit.

Hyperfine coupling constants of the cyclohexadienyl radical in benzene and dilute aqueous solution.

The muon hyperfine coupling constant (Aμ) of the muoniated cyclohexadienyl radical (C6H6Mu) has been directly measured in a 5 mM solution of benzene in water by the radio-frequency muon spin resonance (RF-μSR) technique. The relative shift of Aμ in aqueous solution compared with the value in neat benzene (ΔAμ/Aμ = +0.98(5)% at 293 K) can now be compared directly with theoretical predictions. Application of the RF-μSR method to other dilute systems will provide extremely important information on understanding solvent effects.

Geant4 Simulation of the New ALC

Monte Carlo simulation programs based on the Geant4 package have become indispensable tools in modern particle physics. Following the advances in muon-spin rotation (μSR) instrumentation we have written a simulation code, MUSRSIM, currently in use in the development and optimisation of contemporary μSR spectrometers. MUSRSIM was employed for optimising the design of a new type of avoided level crossing (ALC) instrument, which is characterised by the exclusive use of Geiger-mode avalanche photodiodes instead of the standard photomultiplier tubes. The simulation code allowed us to study in detail the influence of the detector geometry, of the initial muon beam properties, and (to a limited extent) of the electronic signal processing on the predicted spectra. The good agreement between the simulation results and the measured data, validates MUSRSIM as a reliable software tool also for future instrument development work.

{\mu }{\rm SR}

Muon spin rotation ( μ SR) spectra recorded for manganese silicide MnSi and interpreted in terms of a quantitative analysis constrained by symmetry arguments were recently published. The magnetic structures of MnSi in zero-field at low temperature and in the conical phase near the magnetic phase transition were shown to substantially deviate from the expected helical and conical structures. Here, we present material backing the previous results obtained in zero-field. First, from simulations of the field distributions experienced by the muons as a function of relevant parameters, we confirm the uniqueness of the initial interpretation and illustrate the remarkable complementarity of neutron scattering and μ SR for the MnSi magnetic structure determination. Second, we present the result of a μ SR experiment performed on MnSi crystallites grown in a Zn-flux and compare it with the previous data recorded with a crystal obtained from Czochralski pulling. We find the magnetic structure for the two types of crystals to be identical within experimental uncertainties. We finally address the question of a possible muon-induced effect by presenting transverse field μ SR spectra recorded in a wide range of temperature and field intensity. The field distribution parameters perfectly scale with the macroscopic magnetization, ruling out a muon-induced effect.