73Ge NMR spectra in germanium single crystals with different isotopic composition

We have studied the influence of isotopic disorder on the local deformations in Ge single crystals from both experimental and calculation points of view. The nuclear magnetic resonance (NMR) spectra of⁷³Ge nuclei (the nuclear spin equals 9/2) in perfect single crystals of germanium with different isotopic content were measured at temperatures 80, 300 and 450 K. Abnormal broadening of the spectrum was found to occur when the magnetic field was aligned along the [111] axis of a crystal. The observed specific angular dependence of the quadrupole broadening was attributed to isotopic disorder among atoms of germanium sited around the⁷³Ge NMR probe. Local lattice deformations in germanium crystal lattice due to isotopic impurity atoms were calculated in the framework of the adiabatic bond charge model. The results obtained were applied to study random noncubic crystal field interactions with the nuclear quadrupole moments and corresponding effects in NMR spectra. Simulated second and fourth moments of resonance frequency distributions caused by the magnetic dipole-dipole and electric quadrupole interactions are used to analyze the lineshapes, theoretical predictions agree qualitatively with the experimental data.     

Einfluß der anisotropen Abbremsung von Mg24-Rückstoßkernen in Na- und NaCl-Einkristallen auf die Kernresonanzfluoreszenzausbeute

The relative nuclear resonance fluorescence (NRF) yield of 1.37 MeV quantas, emitted from Mg²⁴-recoils, was determined in a scattering experiment. In single crystals of Na and NaCl, which were doped with Mg²⁴, a dependence of the NRF yield on the direction of the emitted gamma quantas relative to the orientation of the crystals, was observed. The effect can be explained by taking into account the anisotropic deceleration of the Mg²⁴-recoils in the single crystals. The deceleration was calculated by using Born-Mayer potentials. Results of the calculations of the anisotropic NRF yields are presented and discussed.     

Non-linear soliton excitations in antiferromagnetic chains

Topological excitations play a crucial role in antiferromagnetic chains. In the present review, we focus on the dynamical fluctuations induced by these quasi-particles, trying to show how they can be observed experimentally. In particular the pulsed nuclear magnetic resonance (NMR) technique can probe these fluctuations at very low frequency. In the same context, neutron spin echo (NSE) measurements are briefly mentioned. A discussion of soliton magnetic resonance (SMR) measurements in also presented: they reveal the existence of internal precessions in the solitons (Dyons) and they show that the uniform (q=0) soliton modes can be detected directly. The experimental data to be discussed were obtained on three compounds: TMMC which provides good examples for “broad” solitons and CsCoCl₃ for “narrow” solitons. A discussion of NMR data obtained with NENP is given in the context of the Haldane's conjecture. Member of Equipe de Recherche CNRS no. 216.     

Low-lying dipole excitations in the stable Cd isotopes: A systematics

Low-lying dipole excitations in the medium-weight vibrational nuclei of the Cd isotopic chain were investigated by means of nuclear resonance fluorescence experiments performed at the bremsstrahlung beam of the Stuttgart Dynamitron accelerator (endpoint energy 4.1 MeV). Detailed information has been obtained on excitation energies, spins, decay widths, and transition probabilities of numerous excited states in ^{110–114,116}Cd. Additionally, the use of two Compton polarimeters enabled model-independent parity assignments for excitations in the even-even isotopes. Strongly excited J ^π = 1^? states are found in all even-even Cd nuclei at excitation energies near the sumof the energies of the first 2⁺ and 3^? states. These excitations are interpreted as the 1^? member of the quadrupole-octupole coupled quintuplet (2+?3^?). The fragmented strength observed in the odd isotopes ^111,113Cd is compared with the strength distributions in the neighboring even-even Cd isotopes.     

Phonons in iron: from the bulk to an epitaxial monolayer

The confinement of materials in low-dimensional structures has significant impact on propagating excitations like phonons. Using the isotope-specific 57Fe nuclear resonant vibrational spectroscopy we were able to determine elastic and thermodynamic properties of ultrathin Fe films on W(110). With decreasing thickness one observes a significant increase of the mean atomic displacement that goes along with an enhancement of vibrational modes at low energies as compared to the bulk. The analysis reveals that these deviations result from atomic vibrations of the single atomic layers at the two boundaries of the film, while the atoms inside the films vibrate almost bulklike.     

Structure and magnetic properties of FeCoN films

We obtained crystals of RuS₂ doped with ⁵⁷Fe from a Bi melt and determined the EPR hyperfine structure corresponding to ⁵⁷Fe³⁺ in low-spin configuration. In crystals that were doped with both Fe and Cr an increase of the Fe³⁺ resonance and a simultaneous decrease of the Cr³⁺ resonance occurred by IR irradiation and revealed the same wavelength dependence. Compared with as-grown crystals the iron-doped crystals turned out to have a rather high electrical resistivity of about 10⁴Omega cm at room temperature. For these iron-doped crystals two different activation energies of 0.04 eV and 0.35 eV of the free charge carriers (electrons) were determined from measurements of the electrical conductivity in the range of 94 K and 294 K. Received: 29 July 1996/Accepted: 13 November 1996     

Detailed spectroscopy in the superdeformed second minimum of 240Pu

Spectroscopic studies in the superdeformed shape isomer of ²⁴⁰Pu using γ-spectroscopy, conversion electron spectroscopy and transmission resonance spectroscopy have been performed. In a high-resolution and high-efficiency γ-spectroscopy experiment the out-of-band decays of several excited superdeformed rotational sequences with K=2^? and 1^? could be identified together with evidence for a weakly populated 0^? octupole band. Surprisingly, no low-lying collective quadrupole excitations could be observed. Complementary information could be obtained in conversion electron measurements in coincidence with isomeric fission, resulting in the first identification of the lowest ß-vibrational K=0⁺ band. For all rotational bands the variation of the moment of inertia with spin could be studied. A predominant population of negative parity states in the second well could be observed, which can be explained by the selective population and depopulation of the second minimum. Complementary transmission resonance measurements have been performed, yielding new information on the fine structure of high-lying (ß-)vibrational multi-phonon states. A new method could be established to determine the excitation energy of the fission isomer ground state from measured level densities.     

Sound velocities of compressed Fe3C from simultaneous synchrotron X‐ray diffraction and nuclear resonant scattering measurements

The applications of nuclear resonant scattering in laser‐heated diamond anvil cells have provided an important probe for the magnetic and vibrational properties of ⁵⁷Fe‐bearing materials under high pressure and high temperature. Synchrotron X‐ray diffraction is one of the most powerful tools for studying phase stability and equation of state over a wide range of pressure and temperature conditions. Recently an experimental capability has been developed for simultaneous nuclear resonant scattering and X‐ray diffraction measurements using synchrotron radiation. Here the application of this method to determine the sound velocities of compressed Fe₃C is shown. The X‐ray diffraction measurements allow detection of microscale impurities, phase transitions and chemical reactions upon compression or heating. They also provide information on sample pressure, grain size distribution and unit cell volume. By combining the Debye velocity extracted from the nuclear resonant inelastic X‐ray scattering measurements and the structure, density and elasticity data from the X‐ray diffraction measurements simultaneously obtained, more accurate sound velocity data can be derived. Our results on few‐crystal and powder samples indicate strong anisotropy in the sound velocities of Fe₃C under ambient conditions.     

Electron paramagnetic resonance study of Cu-Co interactions in Ba2Co(HCOO)64 H2O

Electron paramagnetic resonance studies of Cu²⁺ doped in single crystals of dibarium cobalt formate tetrahydrate are carried out. Unlike in other cobalt salts, resolved hyperfine spectra are observed at room temperature itself. At 77K, the hyperfine linewidth shows an angular and nuclear spin dependence. Further, the linewidth increases as the temperature is lowered. The various possibilities of Cu-Co couplings like spin-flip and cross relaxation etc. are discussed. The spin-Hamiltonian parameters change conspicuously as the temperature is lowered which is ascribed to a vibrational mixing of the electronic levels in the ground state of Cu²⁺.     

The high-resolution infrared spectrum of Si2H6: rotation–torsion analysis of the ν5 and ν7 fundamentals,and torsional splittings in the degenerate vibrational states

The infrared active ν₇ and ν₅ fundamentals of disilane, coupled by an x,y Coriolis interaction, have been analysed on a Fourier transform spectrum between 2120 and 2225?cm^?1, at the Doppler limited spectral resolution. A Fermi resonance with 2ν ₂?+?ν₉ affects the Δ K?=?1 side of ν₇, and both ν₇ and ν₅ show the effects of several additional localized perturbations. Line splittings in the ν₅ transitions are not observed, showing that the torsional splitting in the ν₅ excited state and in the vibrational ground state are almost equal. The intrinsic torsional splitting of ν₇ is found to be smaller than in the ground vibrational state by 0.0085?cm^?1. This splitting value and those found for the other two infrared active degenerate fundamentals, ν₈ and ν₉, follow the trend expected from our theoretical predictions. Exploratory numerical calculations show that the decrease of the torsional splittings, observed in the fundamental degenerate vibrational states of disilane, can actually be accounted for by the head–tail and torsional Coriolis coupling of all the degenerate vibrational fundamentals, in several torsional states.     

Second Order IR Spectrum of the K4Fe(CN)6.3H2 O Crystal

The investigation of second order vibrational spectra of solid hydrates is a complicated experimental and theoretical problem. Up to new, there is a small number of IR-absorption studies and a few Raman ones, which consider multiphonon processes. Recently, the theoretical works of Agranovich^1,2 and Ruvaldis and Zawadowski³ have appear which treat some aspects of two-phonon processes in molecular crystals. The emphasis of these works is placed on some effects characteristic only for the second order spectra - Fermi resonance, two-bound states, simultaneous excitations of two molecules etc.     

Study of （Ga,Mn）N prepared by Mn-ion implantation using optical techniques

This paper reports that （Ga, Mn）N is prepared using implantation of 3at.% Mn Ions into undoped GaN. Structural characterization of the crystals was performed using x-ray diffraetion（XRD）. Detailed XRD measurements have revealed the characteristic of Mn-ion implanted GaN with a small contribution of other compounds. With Raman spectroscopy measurements, the spectra corresponding to the intrinsic GaN layers demonstrate three Raman active excitations at 747, 733 and 566 cm-1 identified as EI（LO）, A1 （LO） and E~, respectively. The Mn-doped GaN layers exhibit additional excitations at 182, 288, 650 725, 363, 506cm^-1 and the vicinity of E~ mode. The modes observed at 182, 288, 650 725em 1 are assigned to macroscopic disorder or vacancy-related defects caused by Mn-ion implantation. Other new phonon modes are assigned to Mnx-Ny, Gax-Mny modes and the local vibrational mode of Mn atoms in the （Ga, Mn）N, which are in fair agreement with the standard theoretical results.     

The theory of fission of a singlet frenkel exciton into two localised triplet excitations

The non-radiative rate of singlet exciton fission into two localised triplet excitations is calculated for molecular crystals. An energy gap law is obtained. Strong coupling limit between the singlet-triplet and the ground state-triplet vibrational states of the molecules is found more suitable for such reactions in molecular crystals. The rate of decay (W) calculated for tetracene lies in the range 10⁹–10¹¹ sec^?1. W is also calculated for naphthalene and anthracene crystals.     

Mössbauer spectroscopy of ferrocene-containing thermotropic liquid crystals

⁵⁷Fe Mössbauer spectroscopy has been used to study some liquid crystalline disubstituted derivatives of ferrocene (FLCn). The characteristic Debye temperatures for these compounds were found to be in the range of 25–31 K. The order parameters, and intramolecular- and lattice-contributions to the nuclear vibrational anisotropy have been obtained from the analysis of the angular dependence of the Mössbauer parameters.     

Untersuchung über den Einfluß von Einteilchenanregungen auf die Myonenspektren schwerer deformierterug-Kerne

The effect of nuclear rotations and single particle excitations on the X-ray spectrum of deformed heavyμ-mesic atoms is discussed. The effect is calculated for ₆₃ ¹⁵³ Eu, it is found negligible small and strong dependent on the quantum numbers of nuclear single particle states. A test nucleus is considered with appropriate chosen nuclear states to make predictions on other nuclei.     

Structural phase transitions in Fe3+ -doped ferroelectric TlGaSe2 crystal

This paper presents the results of dielectric constant and Electron Paramagnetic Resonance (EPR) investigations of Fe³⁺-doped TlGaSe₂ single crystals in the temperature range of 15–300 K. The influence of Fe impurities on dielectric properties and phase transitions of TlGaSe₂ crystal has been studied. The results were considered in comparison with earlier observed results from pure TlGaSe₂ compounds. We observed the considerable decrease of the dielectric constant as well as the change of the shape of the temperature dependence of the dielectric constant in doped crystals. Some certain significant changes of EPR spectra, which are associated with a strong splitting and appearance of additional resonance lines, were observed at the temperatures below 110 K. Such transformations are considered as the result of non-equivalent displacements of different groups of Tl atoms during the structural phase transitions.     

Inelastic nuclear resonant absorption of synchrotron radiation in thin films and multilayers

Inelastic nuclear resonant absorption of synchrotron radiation is an efficient and unique method for the direct measurement of vibrational density of states (VDOS) of thin films and interfaces that contain Mössbauer isotopes. This is demonstrated for the ⁵⁷Fe nuclear resonance in the case of amorphous and crystalline Tb–Fe alloy thin films and buried Fe/Cr interfaces in epitaxial α-Fe(0 0 1)/Cr(0 0 1) superlattices.

     

Energy–density functional plus quasiparticle–phonon model theory as a powerful tool for nuclear structure and astrophysics

During the last decade, a theoretical method based on the energy–density functional theory and quasiparticle–phonon model, including up to three-phonon configurations was developed. The main advantages of themethod are that it incorporates a self-consistentmean-field and multi-configuration mixing which are found of crucial importance for systematic investigations of nuclear low-energy excitations, pygmy and giant resonances in an unified way. In particular, the theoretical approach has been proven to be very successful in predictions of new modes of excitations, namely pygmy quadrupole resonance which is also lately experimentally observed. Recently, our microscopically obtained dipole strength functions are implemented in predictions of nucleon-capture reaction rates of astrophysical importance. A comparison to available experimental data is discussed.     

Nitric oxide concentration measurements in atmospheric pressure flames using electronic-resonance-enhanced coherent anti-Stokes Raman scattering

We report the application of electronic-resonance-enhanced coherent anti-Stokes Raman scattering (ERE-CARS) for measurements of nitric oxide concentration ([NO]) in three different atmospheric pressure flames. Visible pump (532 nm) and Stokes (591 nm) beams are used to probe the Q-branch of the Raman transition. A significant resonance enhancement is obtained by tuning an ultraviolet probe beam (236 nm) into resonance with specific rotational transitions in the (v’=0, v”=1) vibrational band of the A²Σ⁺–X²Π electronic system of NO. ERE-CARS spectra are recorded at various heights within a hydrogen-air flame producing relatively low concentrations of NO over a Hencken burner. Good agreement is obtained between NO ERE-CARS measurements and the results of flame computations using UNICORN, a two-dimensional flame code. Excellent agreement between measured and calculated NO spectra is also obtained when using a modified version of the Sandia CARSFT code for heavily sooting acetylene-air flames (φ=0.8 to φ=1.6) on the same Hencken burner. Finally, NO concentration profiles are measured using ERE-CARS in a laminar, counter-flow, non-premixed hydrogen-air flame. Spectral scans are recorded by probing the Q₁ (9.5), Q₁ (13.5) and Q₁ (17.5) Raman transitions. The measured shape of the [NO] profile is in good agreement with that predicted using the OPPDIF code, even without correcting for collisional effects. These comparisons between [NO] measurements and predictions establish the utility of ERE-CARS for detection of NO in flames with large temperature and concentration gradients as well as in sooting environments. PACS 07.88.+y; 42.62.Fi; 42.65.Dr     

Single crystal magnetic susceptibility of the quasi-one-dimensional organic conductor (fluoranthene) 2PF6

Magnetic susceptibility was measured on a bundle of single crystals of difluoranthene phosphor hexafluoride [(FA)₂^·+PF₆^?] with external field parallel or perpendicular to the molecular stacking direction. Except for lowest temperatures, where the paramagnetic contribution of sample defects is of importance, magnetic susceptibility is dominated by the strongly anisotropic molecular diamagnetism. Spin paramagnetism is separated in the metallic and semiconducting phase and compared with magnetic resonance results and predictions of available models.