ESR study of the undoped heavy-fermion compound YbRh2Si2

The electron spin resonance (ESR) of the heavy-fermion metal YbRh₂Si₂ has been studied. The angular variation and the temperature dependence of the ESR line width have been measured in YbRh₂Si₂ single crystals in the temperature range of 4–25 K. The characteristic spin-fluctuation temperatureT ^* ～ 17 K estimated from these studies coincides very well with other experimental data. A well-behaved ESR signal due to local Yb³⁺ moments strongly supports the localized moment scenario for heavy-fermion quantum critical points.     

Temperature-dependent 29Si NMR resonance shifts in lightly- and heavily-doped Si:P

Careful NMR measurements on a very lightly-doped reference silicon sample provide a convenient highly precise and accurate secondary chemical shift reference standard for ²⁹Si MAS-NMR applicable over a wide temperature range. The linear temperature-dependence of the ²⁹Si chemical shift measured in this sample is used to refine an earlier presentation of the paramagnetic (high-frequency) ²⁹Si resonance shifts in heavily-doped n-type silicon samples near the metal–nonmetal transition. The data show systematic decreases of the local magnetic fields with increasing temperature in the range 100–470 K for all samples in the carrier concentration range from 2×10¹⁸ cm⁻³ to 8×10¹⁹ cm⁻³. This trend is qualitatively similar to that previously observed for the two-orders of magnitude larger ³¹P impurity NMR resonance shifts in the same temperature and concentration ranges. The ²⁹Si and ³¹P resonance shifts are not related by a simple scaling factor, however, indicating that impurity and host nuclei are affected by different subsets of partially-localized extrinsic electrons at all temperatures.     

Perylene hexafluoroantimonate: Suppression of the Peierls transition by anion-chain disorder

Crystal growth and X-ray structural analysis are reported for the new quasi-one-dimensional organic conductor perylene hexafluoroantimonate. Due to deviation from exact 4:3 stoichiometry and anion-chain disorder the three-dimensional Peierls transition is suppressed to temperatures below 30 K in spite of a molecular field transition temperature of 330 K. Anisotropy of the microwave conductivity exceeds 3000:1 at room temperature. Curie paramagnetic defects with comparatively small electron spin resonance line width predominate at low temperature. Thermally activated paramagnetic defects, giving rise to pronounced Overhauser shift, are separated in the 30-100 K range. They are explained by neutral Perylene intra-stack defects, whose concentration can be reduced by both aging and annealing. The angular and temperature dependence of the conduction electron spin resonance line is analysed in detail, exhibiting the influence of intra-stack dipole interaction growing with temperature. Restricted diffusion of the conduction electron spins with diffusion coefficient of about 6 cm²/s parallel to the stacking direction is detected by fixed gradient pulsed electron spin resonance at room temperature. Received 17 July 1998     

Temperature behaviour of optical properties of Si+-implanted SiO2

Silicon nanocrystals were prepared by Si⁺-ion implantation and subsequent annealing of SiO₂ films thermally grown on a c-Si wafer. Different implantation energies (20-150 keV) and doses - cm ^-2 ) were used in order to achieve flat implantation profiles (through the thickness of about 100 nm) with a peak concentration of Si atoms of 5, 7, 10 and 15 atomic%. The presence of Si nanocrystals was verified by transmission electron microscopy. The samples exhibit strong visible/IR photoluminescence (PL) with decay time of the order of tens of μs at room temperature. The changes of PL in the range 70-300 K can be well explained by the exciton singlet-triplet splitting model. We show that all PL characteristics (efficiency, dynamics, temperature dependence, excitation spectra) of our Si⁺-implanted SiO₂ films bear close resemblance to those of a light-emitting porous Si and therefore we suppose similar PL origin in both materials. Received 1st September 1998 and Received in final form 7 September 1999     

Peculiarities of the radiospectroscopy line shape in nanomaterials

The theory of the radiospectroscopy (nuclear magnetic resonance [NMR] and electron spin resonance [ESR]) line shape for nanomaterials is developed. The consideration was performed in the core and shell models which are, respectively, the nanoparticle regions unperturbed and perturbed by the surface influence. The shift of the resonance frequency by the surface tension was taken into account. The homogeneously broadened line shape was supposed to be Gaussian or Lorentzian. Inhomogeneous broadening of lines via the distribution of nanoparticle sizes was calculated for several forms of the size distribution function. The splitting of radiospectroscopy spectra into two lines decreases with particle sizes, which looks like that in the bulk and on the surface. It was shown to be the characteristic feature of nanomaterial spectra. The changing of these lines’ intensity and width with the change of the distribution function parameters and the particle size decrease was considered. The comparison of the theory with NMR spectra of¹⁷O and²⁵Mg observed recently in nanocrystalline MgO is performed. The calculations fit pretty good the observed size dependence of the line shape, intensity and width.     

NMR line shift of gallium in GaAs crystals in the temperature range 160–360 K

The shift in the nuclear magnetic resonance (NMR) line of ⁷¹Ga isotopes in GaAs crystals is investigated in the temperature range 160–360 K. It is observed that the position of the resonance line is temperature-independent. The value of the shift relative to an isolated nucleus is 440 and 420 ppm for the three samples investigated. The roles of various contributions to the NMR line shift are discussed. Fiz. Tverd. Tela (St. Petersburg) 40, 1418–1419 (August 1998)     

Structural study of the Lu2Si2O7-Sc2Si2O7 system

Different compositions in the Lu₂Si₂O₇-Sc₂Si₂O₇ system have been synthesized following the ceramic method. All XRD patterns are compatible with the thortveitite structure (β-RE₂Si₂O₇ polymorph). Unit cell parameters change linearly with composition, which indicates a complete solid solubility of Sc₂Si₂O₇ in Lu₂Si₂O₇. ²⁹Si MAS NMR spectra show a decrease of the ²⁹Si chemical shift with increasing Sc content. A correlation reported in the literature to predict ²⁹Si chemical shifts in silicates is applied here to obtain the theoretical variation in ²⁹Si chemical shift values in the system Lu₂Si₂O₇-Sc₂Si₂O₇ and the results compare favourably with the values obtained experimentally. The FWHM values of the ²⁹Si MAS NMR curves indicate a random distribution of Lu and Sc in the structure of the intermediate members. Finally, the IR study of the system confirms the solubility of Sc₂Si₂O₇ in Lu₂Si₂O₇, showing the splitting of several modes in the intermediate members and a linear shift of the frequency on going from one end-member to the other.     

Solid-state NMR investigations of Si-29 and N-15 enriched silicon nitride

We compare ²⁹Si magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectra from the two modifications of silicon nitride, α-Si₃N₄ and β-Si₃N₄, with that of a fully (²⁹Si, ¹⁵N)-enriched sample ²⁹Si₃¹⁵N₄, as well as ¹⁵N NMR spectra of Si₃¹⁵N₄ (having ²⁹Si at natural abundance) and ²⁹Si₃¹⁵N₄. We show that the ¹⁵N NMR peak-widths from the latter are dominated by J(²⁹Si–¹⁵N) through-bond interactions, leading to significantly broader NMR signals compared to those of Si₃¹⁵N₄. By fitting calculated ²⁹Si NMR spectra to experimental ones, we obtained an estimated coupling constant J(²⁹Si–¹⁵N) of 20 Hz. We provide ²⁹Si spin-lattice (T₁) relaxation data for the ²⁹Si₃¹⁵N₄ sample and chemical shift anisotropy results for the ²⁹Si site of β-Si₃N₄. Various factors potentially contributing to the ²⁹Si and ¹⁵N NMR peak-widths of the various silicon nitride specimens are discussed. We also provide powder X-ray diffraction (XRD) and mass spectrometry data of the samples.     

ESR of Gd-impurities in the heavy fermion compounds CeCu2Si2 and CeAl3

The heavy fermion systems CeCu₂Si₂ and CeAl₃ are characterized by a huge quasiparticle density of states responsible for the large electronic specific heat. The observation of a Gd³⁺ electron spin resonance (ESR) in single crystals CeCu₂Si₂ and in polycrystalline CeAl₃ clearly demonstrates the local character of these quasiparticles. Nevertheless, the Gd-spin relaxation shows remarkable anomalies with respect to the isostructural compounds LaCu₂Si₂ and LaAl₃: Probably via RKKY coupling, Ce 4f-spin fluctuations give rise to an enhanced Gd-spin relaxation resulting in an unusual non-linear thermal broadening around the Kondo temperature. From this we obtain information about the temperature dependence of the Ce 4f-spin correlation time.     

Magnetooptical microwave spectroscopy of the coherent magnetic state in the mixed valence compound SmB6 in the frequency range 40–120 GHz

In undoped pure single crystals of the mixed valence compound SmB₆ anomalous ESR absorption is observed in the frequency range v=40–120 GHz at temperatures of 1.8–4.2 K. The ESR for the case of the coherent ground state consists of two components corresponding to g-factors g ₁=1.907±0.003 and g ₂=1.890±0.003. The amplitude of both ESR lines strongly depends on temperature in the temperature range studied: the amplitude of the first line with g=g ₁ increases and the amplitude of the second line decreases with temperature. A model based on consideration of intrinsic defects in the SmB₆ crystalline lattice, with a densit ∼10¹⁵−10¹⁶ cm⁻³, is suggested as an explanation for the anomalous ESR-behavior. In the frequency range v>70 GHz at T=4.2 K, in addition to the main ESR lines, a new magnetic resonance with a hysteretic field dependence is discovered. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 10, 707–712 (25 November 1996) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Nuclear magnetic resonance study of the heavy-fermion system URu2Si2

Nuclear magnetic resonance (NMR) and relaxation studies on ²⁹Si have been carried out on the heavy Fermion system URu₂Si₂. Above the Kondo temperature of about 60 K, the nuclear relaxation time T₁ is nearly temperature independent, which is consistent with the occurrence of fluctuations of localized U moments. Below about 60 K T₁ is inversely proportional to temperature suggesting that the system behaves like a Fermi liquid. A sharp increase in T₁ occurs below 17 K which is probably associated with the opening of an energy gap at the Fermi surface due to the formation of a spin density wave state. Below about 10 K, T₁ reacquires the inverse temperature dependence observed in the 17 K ∼ 60 K temperature range.     

Combined Use of 2-D NMR Correlation Experiments, GIAO DFT 13C Chemical Shifts and 1-D NOESY Methods in Regioisomeric and Conformational Structure Determination of Cyclophanes in Solution

The combined application of two-dimensional nuclear magnetic resonance (2-D NMR) correlation experiments and gauge-including atomic orbital (GIAO) ¹³C NMR chemical shift calculations allowed reliable and simple determination of regioisomeric structure of heterocyclic substituents on the calix[4]arene lower rim. Moreover, the 1-D double pulsed field gradient nuclear Overhauser effect technique allows quick and efficient measurement of small nuclear Overhauser effects and, in doing so, establishes a 3-D structure of calix[4]arene simply and unequivocally. In general, these methods may find application in the regio- and stereoisomeric structure determination of complicated macrocyclic compounds.     

The time evolution of the sol-gel process:29Si NMR study of the hydrolysis and condensation reactions of tetraethoxysilane

The structure and properties of the glass products prepared by the sol-gel process are strongly dependent on the experimental conditions of their preparation. The time dependence of the structural intermediate formation obtained during hydrolysis and condensation reactions in tetraethoxysilane, ethanol, H₂O, HCl (molar ratio 1:3:1.5:0.01) sol-gel reaction mixture was investigated with²⁹Si nuclear magnetic resonance spectroscopy at −75°C. It is clear that the substitution of the Si ethoxy group (OCH₂CH₃) by a hydroxyl (OH) shifts the resonance lines downfield. The dimer peaks are located about 9–10 ppm upfield from the corresponding monomers, and Si located in the middle of the trimers appeared at the chemical shift of 18–19 ppm from the corresponding monomers. The peaks of cyclic trimers, cyclic tetramers and the other higher oligomers are relatively broad and weak and exhibit a multiplet structure. The time dependence of the concentrations of the individual types of silicon in the reaction mixture was evaluated from the integral intensity of the corresponding Si atoms from the²⁹Si NMR spectra. The results obtained represent an input data to the various kinetic models of the complicated hydrolysis and condensation reactions.     

Growth of Co-Silicides from single crystal and evaporated Si

We have investigated the reaction of a thin Co film with a (100) Si (Si ^c ) or an evaporated Si (Si ^e , which is amorphous) substrate during thermal annealing. On either substrate, Co₂Si and CoSi form simultaneously and the growth of each phase has a square root of time dependence. Both silicides grow faster on Si ^c than on Si ^e . A model is proposed to calculate the effective diffusion constant in each silicide from the growth data of the silicides. The activation energies of the effective diffusion constants in Co₂Si and CoSi grown on Si ^c are 1.7±0.1 eV and 1.8±0.1 eV, respectively; while those on Si ^e are 1.85±0.1 eV and 1.9 ±0.1 eV, respectively. The differences observed for the two substrates are tentatively attributed to the presence of impurities in Si^e and to the microstructural differences of the silicides formed on either substrate.     

Isotope shifts and hyperfine structure in the transitions of gadolinium

High-resolution resonance ionization mass spectrometry has been used to measure isotope shifts and hyperfine structure in all (J = 2-6) and the transitions of gadolinium (Gd I). Gadolinium atoms in an atomic beam were excited with a tunable single-frequency laser in the wavelength range of 422-429 nm. Resonant excitation was followed by photoionization with the 363.8 nm line of an argon ion laser and resulting ions were mass separated and detected with a quadrupole mass spectrometer. Isotope shifts for all stable gadolinium isotopes in these transitions have been measured for the first time. Additionally, the hyperfine structure constants of the upper states have been derived for the isotopes ^{155, 157} Gd and are compared with previous work. Using prior experimental values for the mean nuclear charge radii, derived from the combination of muonic atoms and electron scattering data, field shift and specific mass shift coefficients for the investigated transitions have been determined and nuclear charge parameters for the minor isotopes ^{152, 154} Gd have been calculated. Received 18 November 1999     

An ab initio calculation of O and Si NMR parameters for SiO2 polymorphs

Ab initio molecular orbital calculations (Hartree–Fock, HF and density functional theories, DFTs) have been carried out for SiO₂ polymorphs coesite, low cristobalite, and α-quartz, in order to investigate the reliability of this method for predicting ²⁹Si and ¹⁷O nuclear magnetic resonance (NMR) properties of silicates. Oxygen- and silicon-centered clusters consisting of one (1T) to three tetrahedral (3T) shells (one to four atomic shells), taken from real crystal structure, have been investigated. It is found that for reasonable predication of both the ²⁹Si and ¹⁷O chemical shifts (δ_i^Si and δ_i^O), the minimum cluster is one that gives the correct second neighbors to the nucleus of interest. Both the δ_i^Si and δ_i^O have reached convergence with respect to cluster size at the OH-terminated two tetrahedral (2T) shell (three atomic shells around Si and four atomic shells around O) model. At convergence, the calculated δ_i^Si values agree well (within ±1 ppm) with experimental data. The calculated ¹⁷O electric field gradient (EFG)-related parameters also agree with experimental data within experimental uncertainties. The calculation also reproduces small differences in δ_i^O for O sites with similar tetrahedral connectivities, but shows deviations up to about 10 ppm in relative difference for O sites with different tetrahedral connectivities. The poor performance for the latter is mainly due to the approximations of the HF method. Our study thus suggests that the ab initio calculation method is a reliable mean for predicting ²⁹Si and ¹⁷O NMR parameters for silicates. Such an approach should find application not only to well-ordered crystalline phases, but also to disordered materials, by combining with other techniques, such as the molecular dynamics simulation method.     

Electron and nuclear spin dynamics in plastically deformed silicon crystals enriched in isotope 29Si

Paramagnetic defects of a new type with a concentration of about 10¹⁵ cm^?3 are shown to be generated during the plastic deformation of isotope-rich (72%, 76% ²⁹Si) silicon crystals at a temperature of 950°C. The electron paramagnetic resonance (EPR) spectra of these defects are anisotropic and have a significant width (up to 1 kOe). The nonuniform broadening of the EPR lines is caused by the variation of the internal magnetic field in correlated defect clusters. The nuclear magnetic resonance (NMR) spectra of the deformed crystals consist of Pake doublets split by nuclear spin-spin interaction. The broadening of the NMR spectra is caused by nuclear dipole-dipole relaxation.     

Electron spin resonance investigation of the Heavy-Fermion compound CeCu2(Si1−x Ge x )2

The Kondo-lattice system CeCu₂(Si_1?x Ge _x )₂ exhibits an alloying induced transition from a coherent Fermi-liquid (x=0) with strongly enhanced effective masses to an antiferromagnetically ordered heavy-fermion system (x=1). This transition is studied by Gd³⁺ ESR in oriented powder samples. The temperature dependence of the ESR line width follows a characteristic pattern which allows one to distinguish between the different ground states. The results obtained in polycrystalline CeCu₂Si₂ are in good agreement with the measurements performed in single crystals. Finally we compare our results with⁶³Cu-NMR data obtained from CeCu₂Si₂ and CeCu₂Ge₂.     

Magic-angle-spinning NMR on solid biological systems. Analysis Of the origin of the spectral linewidths

Magic-angle-spinning (MAS) high-power ¹H-decoupled ¹³C and ³¹P NMR has been applied to solid biological materials to obtain information about the mechanisms that determine the spectral linewidths. The line broadening in MAS ³¹P NMR spectra of solid tobacco mosaic virus (TMV) has been investigated by selective saturation and T₂ measurements. About 90 Hz stems from homogeneous effects, whereas the inhomogeneous contribution is approximately 100 Hz. The inhomogeneous line broadening is assigned to macroscopic inhomogeneities in the sample and not to variations in the nucleotide bases along the RNA strand in TMV. It is concluded that sample preparation is of vital importance for obtaining well-resolved spectra. Under optimal preparation techniques the isotropic values of the chemical shift of the different ³¹P sites have been determined to obtain information about the secondary structure of the viral RNA. The chemical shift anisotropy has been determined from the relative intensities of the spinning side bands in the spectra. The chemical shift information is used to make a tentative assignment of the resonance in terms of the three structurally distinguishable phosphate groups in TMV. The origin of the linewidths in MAS NMR has been examined further by ¹³C NMR of approximately 10% ¹³C-enriched coat protein of cowpea chlorotic mottle virus, using selective excitation and saturation techniques, as well as measurements of the relaxation times T_1γ and T₂. The CO resonance in the spectrum is composed of an inhomogeneous and homogeneous part with a total linewidth of 700 Hz. The homogeneous linewidth, contributing with 200 Hz, is found to arise from slow molecular motions in the solid on a millisecond timescale.     

Magnetic resonance of intrinsic defects in the spin-Peierls magnet CuGeO3

Magnetic resonance in pure single-crystal CuGeO₃ at frequencies 9–75 GHz in the temperature range 1.2–25 K is investigated. Splitting of the magnetic-resonance line into several spectral components is observed at temperatures below 5 K, where spin-Peierls dimerization suppresses the magnetic susceptibility and the ESR signal intensity. Analysis of the magnetic resonance spectra over a wide frequency range with different directions of the magnetic field at different temperatures makes it possible to identify among these components the ESR signals due to defects, having effective spin S=1/2 and spin S=1, in the spin-Peierls phase. The g factor corresponding to these ESR signals is the same and close to the value characteristic for the ion Cu²⁺. Another magnetic-resonance line is characterized by a strongly anisotropic g factor and an increase (at a threshold in the excitation power) in the susceptibility both at resonance and in the line wings. These signals are tentatively attributed to two possible types of planar defects arising on the walls of domains of the spin-Peierls state with different values of the dimerization phase. Zh. éksp. Teor. Fiz. 114, 1876–1896 (November 1998)