Electric Field Gradient

By combining first-principles electronic structure calculations and existing time-differential γ -γ perturbedangular-correlation experiments we studied the site localization, the local environment, and the electronic structure of Cd impurities in sapphire (α-Al 2 O 3 ) single crystals in different charged states. The ab initio calculations were performed with the full-potential augmented plane wave plus local orbitals method and the projector augmented wave method. Comparing the calculated electric-field-gradient tensor at the Cd nuclei in the α-Al 2 O 3 host lattice and the corresponding available experimental values, we have seen that it is equally possible for Cd to replace an Al atom (in a negative charge state) or to be placed in an interstitial site (in a neutral charge state). To finally address the issue of the Cd impurity localization, we performed formation energy calculations. These results have shown that Cd placed in the substitutional Al site, in the negatively charged state, is the most probable configuration.

We present the first theoretical study of the electronic properties of superlattices formed from semimagnetic semiconductors. We explicitly consider the Cd0.8Mn0.2Te/Cd0.7Mn0.3Te superlattice system. We show magnetic field splittings both including (zero spin temperature) and neglecting (high spin temperature) the exchange interaction. We find that the exchange interaction dominates the magnetic effects. We present calculations of the derivative of the superlattice band gap with magnetic field as a function of the superlattice layer thickness. We show calculations of the change in superlattice band gap with magnetic field for several magnetic fields as a function of temperature.

C14 Laves phase Nb 0.975 Fe 2.025 compound was investigated by means of the Mössbauer spectroscopy. Spectra were recorded in the temperature range of 5-300K. Their analysis in terms of three sub spectra yielded information on magnetic and lattice dynamical properties of Fe atoms regularly occupying 2a and 6h lattice sites and, excessively, 4f sites. No indication of magnetism was observed down to the temperature of T50K, and spectral parameters viz. center shift, CS, and the main component of the electric field gradient, V zz , behave regularly. In particular, analysis of CS(T) in terms of the Debye model yielded the following values of the Debye temperature, T D : 453(5) K for the site 6h, 544(10) K for the site 2a, 479(4) K for the weighted average over 6h and 2a sites, and 363(35) K for the site 4f. Below 50K anomalous behavior was observed: a broadening of the spectrum appeared indicating thereby a transition into a magnetic phase. Analysis of a temperature dependence of the hyperfine field, B, associated with the 6h and 2a sub spectra yielded the magnetic ordering temperature T C1 50K for the former and T C2 31K for the latter. The maximum values of the hyperfine field at 5K, B o , were 8.2 kGs and 3.3 kGs, respectively. The B o -values were used to estimate values the underlying magnetic moments,  Fe (6h)=0.055-0.065  B and  Fe (2a)=0.02-0.025  B . Noteworthy, they are over one order of magnitude lower than those theoretically calculated. The CS and V zz parameters showed anomalies in the magnetic phase, in particular the former exhibited a strong departure from the Debye model prediction testifying to a significant effect of magnetism on the lattice vibrations.

Self-diffusion and related short-time dynamic and structural properties were investigated for mixtures of carbon dioxide and the ionic liquid 1-n-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide [bmim] + [Tf2N] -for a broad range of carbon dioxide molar fractions and at different temperatures. The studies were performed by a novel multinuclear pulsed field gradient (PFG) NMR technique, which combines the advantages of a high magnetic field (17.6 T) and a high magnetic field gradient (up to 30 T/m), in combination with molecular dynamics simulations. In general, a satisfactory agreement was observed between the experimental and simulation diffusion data. Under all conditions examined, the self-diffusion coefficients of carbon dioxide were found to be approximately an order of magnitude larger than the corresponding self-diffusion coefficients of the ions. It was observed that an increase in temperature and in the amount of carbon dioxide in the ionic liquid led to an increase in the ion self-diffusivities without changing the relationship between the self-diffusion coefficients of the cations and anions. An observation of a slightly higher diffusivity of the cations in comparison to that of the anions is attributed to the preferential mobility of the cations in the direction of the ring plane. The diffusion activation energies of the ions were found to decrease gradually with an increase of the carbon dioxide content in the ionic liquid. The activation energy of the carbon dioxide diffusion in all cases was found to be smaller than those of the ions.

In this work, authors report an investigation that reveals the possibility of dosing "Portland-Alkaline" hybrid cements by mixing small amounts of Clinker with different cementitious materials (in the form of binary mixtures, ternary, etc.). Specifically, one binary and three ternary cements are described in the paper: B2-FA [20% Clinker + 80% Flying Ash (FA)]; B3-FA [20% Clinker +50% FA + 30% Ground Granulated Blast Furnace Slag (GBBFS)]; B3-MK [20% Clinker + 40% Blast Furnace Slag (BFS) + 40% Metakaolin]; B3-BT [20% Clinker + 40% Blast Furnace Slag (BFS) + 40% Dehydroxylated Bentonite]. These cements (in the absence and presence of a solid activator) have been hydrated with water. All of them have the same Clinker content (20%) and all of them have technological properties (mechanical strength development) similar or even better than the commercial cements used as reference. Finally, and the most relevant is that all of them, in their hydration process, there is a generation of more than one type of cementitious gel.

We present a 27 Al NMR study of three giant-unit-cell complex metallic compounds, ␤-Al 3 Mg 2 , the "Bergman-phase" Mg 32 ͑Al, Zn͒ 49 , and Ј-Al-Pd-Mn, which contain some hundreds up to more than a thousand atoms in the unit cell. The NMR spectra of monocrystalline samples are strongly inhomogeneously broadened by the electric quadrupole interaction and the line shapes are featureless and powderlike, but still exhibit significant orientation-dependent variation of the intensity on the satellite part of the spectrum in the magnetic field. Measuring orientation-dependent satellite intensity in appropriate frequency windows yields rotation patterns that can be related to the structure and symmetry of the giant unit cells. For a theoretical reproduction of the rotation patterns, we derived a distribution of the electric-field-gradient ͑EFG͒ tensors for each of the investigated compounds from existing structural models using point-charge and ab initio calculations. The EFG distribution yields important structural information on the manifold of different local atomic environments in the unit cell and distinguishes crystallographically inequivalent lattice sites from the equivalent ones. The distribution of the EFGs for the 1168-atom unit cell of ␤-Al 3 Mg 2 was successfully determined by a pointcharge calculation, whereas the ab initio approach was successful for the 160-atom unit cell of the Mg 32 ͑Al, Zn͒ 49 Bergman phase. For the 258-atom unit cell of Ј-Al-Pd-Mn, the experimental rotation patterns revealed a pseudotenfold symmetry, whereas the theoretical point-charge calculation revealed predominant twofold symmetry with traces of tenfold symmetry, so that no quantitative matching between the theory and experiment could be obtained.

In single crystals of malonic acid grown from heavy water, the methylene protons have been partially exchanged with deuterons. Upon X irradiation at room temperature, the • CD(COOD) 2 radical is formed in an amount comparable to the • CH(COOD) 2 radical species. In the present work, EPR and ENDOR analyses of the R-deuteron hyperfine coupling (hfc) and nuclear quadrupolar coupling (nqc) tensors at room temperature have been performed. The hyperfine coupling tensor is, when scaled with the differences in the nuclear g-factor, almost identical to the R-proton coupling of the • CH(COOH) 2 radical at room temperature. The quadrupolar coupling tensor was found to be virtually coaxial with the hyperfine coupling tensor. The quadrupolar coupling constant is 149.8 ( 1 kHz, and the asymmetry factor η ) 0.092 ( 0.020. It is known that, at room temperature, the malonic acid radical exhibits thermal motion between two potential energy minima separated by about (12°. Assuming that the observed hfc and nqc tensors are the result of thermal averaging between these two conformations of the radical, a simple two-site jump model was used to estimate the rigid-limit tensors. The most significant result obtained was for the nqc tensor, for which the calculations resulted in a quadrupolar coupling constant of 160 kHz and an asymmetry factor η ) 0.026. These values are fairly close to the nqc parameters for the methylene deuterons in malonic acid at low temperature. The quadrupolar coupling tensor has been theoretically modeled using Slater orbitals and formal electronic populations, as well as electron populations obtained from RHF/CI INDO-type calculations. The simple model to compute the electric field gradient at the R-deuteron caused by the charge distribution at the sp 2 -hybridized R-carbon was found to be as successful as more advanced methods. Furthermore, density functional theoretical (DFT) calculations for both the malonic acid radical and the native malonic acid molecule have been performed. Field gradients calculated by the DFT method significantly overestimate the quadrupolar tensors for both the R-deuteron of the radical and the methylene deuterons of the malonic acid molecule. Calculations using electron populations from the RHF/CI INDO calculations show that contributions to the quadrupolar coupling tensor from electrons and nuclei beyond the nearest-neighbor atom of the deuteron are significant.

We studied the hyperfine interactions of 181 Ta in In 2 O 3 by means of perturbed-angular-correlation ͑PAC͒ measurements. We prepared thin films of indium sesquioxide with different degrees of initial amorphism and implanted them with 181 Hf. Chemically prepared indium-sesquioxide powder samples were also made starting from neutron-irradiated HfCl 4 , which provides the 181 Hf PAC probes. PAC experiments were performed on each sample at room temperature, after each step of annealing programs at increasing temperatures up to the full crystallization of the samples. The results indicate that the PAC probe occupies preferentially the axially symmetric cation site. Point-charge-model calculations were performed. The calculated asymmetry parameters were compared with those obtained in 181 Hf PAC experiments performed also on other binary oxides, showing that the symmetry of the electric-field-gradient ͑EFG͒ tensor at 181 Ta cation sites in binary oxides is mainly determined by the nearest-neighbor oxygen-ion distribution around the probe. Comparisons of the experimental results in bixbyites obtained for both PAC probes, 111 Cd and 181 Ta, show that the local EFG in bixbyites, are strongly dependent on the geometry of the sites and the electronic configuration of the probes.

With the crystal spectrometer at muon channel I at SIN we measured the 3ds/2-2p,,2 transition in muonic ?a. The h.f.s. of the 2~3,: level was resolved and the spectroscopic quadrupole moment of the nuclear ground state determined: Using a collective particle-rotor model description of *"Na which includes small AK = 1 admixtures in the wave functions, we find agreement between our measured spectroscopic quadrupole moment and the one derived from measured R(E2) values. Combining the quadrupole moment Q with the quadrupole coupling constant measured in the electronic atom of *"Na, we derive the electric field gradient at the nucleus for two different electronic states: (dE/dz) = 0.1 185(32)eao" for the 3'P3,* state, (dE/dz) = 0.0410(27)eai3 for the 42P3,2 state. The field gradient for the 32P3i2 state is compared with recent many-body calculations of the hyperfine interaction in alkali atoms.

69,71 Ga NMR spectroscopy and magnetic susceptibility experiments. The electron transport is predominantly mediated by the Ga-Ga p x-and p y-like electrons in the (001) plane. The specific heat capacity of BaGa 2 was determined and indicated the absence of phase transitions between 1.8 and 320 K.

Nitrogen 14 quadrupole coupling constants for H 2 CN+ and HCN are predicted via ab initio selfconsistent-field and configuration interaction theory. Effects of electron correlation, basis set completeness, and geometrical structure on the predicted electric field gradients are analyzed. The quadrupole coupling constant obtained for H 2 CN+ is one order of magnitude less than in HCN, providing an explanation for the experimental fact that the fine structure of the microwave spectrum of H 2 CN+ has not been resolved. This research also allows a reliable prediction of the nuclear quadrupole moment of 14N, namely Qe 4 N) = 2.00X 10-26 cm 2 •

Isotropic powders and magnetically aligned crystallites of EuBa2Cu307 z (1:2:3)and europium-doped Bi&Sr2CaCu20s (2:2:1:2)were studied by means of x-ray diffraction and "'Eu Mossbauer spectroscopy. The degree of crystallite orientation of the samples and the values of the lattice constants were deter- mined by x-ray diffraction. The Mossbauer spectra were analyzed considering the full hyperfine Hamil- tonian of the nuclear states of the 21.5-keV y transition. The Mossbauer hyperfine parameters obtained from the superconducting and semiconducting phases are presented. A small change is seen in the "'Eu isomer shift when the oxygen deficiency 5 of the I:2:3compound is varied. The shift can be explained by a decrease in the s-electron density due to lattice expansion. The changes in the oxidation state of the copper atoms with varying 5 were determined from the Mossbauer data: The Cu(2) atoms retain their oxidation state, whereas the Cu(1) atoms adjust their valence according to the value of 5. In the 2:2:1:2 samples, the Eu concentration clearly affected the value of the electric-field gradient at the Eu nucleus. Using a standard procedure, magnetically aligned 2:2:1:2samples were prepared. The preferred direc- tion of the crystal c axis changed from parallel to perpendicular alignment with the external magnetic field, when the Eu concentration exceeded 20% of the Ca atoms.

The rare-earth site of samples having the compositions (Fe,Cu)Sr2(Eu, Ce)nCu204+~+ z (n = 2 and 3) were studied by tSIEu Mrssbauer spectroscopy. The results obtained are compared with measurements of EuBa2Cu307_ 8 (1:2:3) and Bi2Sr2(Cal_yEuy)Cu2Os § ~ (2:2:1:2) samples. The full quadrupole Hamiltonian of the 21.5-keV T-transition was applied in analyzing the measured spectra. Simultaneous fitting of spectra recorded from crystallites oriented in various directions was used to determine the hyperfine parameters of the n = 2 and 3 samples. Two different electric field gradients, not discernible in simple random crystallite measurements, were obtained for the n = 3 series.

Stress induced quadrupole interaction at the probe nucleus ( 111 Cd) in silicon has been studied using the perturbed g2g angular correlation (PAC) method. The extra nuclear field, at the sites of the nuclei, is produced via the disturbances of the surrounding charges by the action of a uniaxial compressive stress on the samples. However, the probes situated at various lattice locations in the sample showed different responses for the same value of stress. The various lattice environments are mainly caused by the involvement of either tellurium or antimony donor atoms in the samples. As a result, the donor free substitutional probe atoms experience a finite nuclear quadrupole interaction due to the broken symmetry of the charge distribution upon uniaxial compression; those probe atoms, which form pairs with donors, exhibit a strong electric-field gradient (EFG) that appears to be pressure independent.

Owing to the Coulomb attraction, oppositely charged impurities in semiconductors tend to form pairs when they become mobile. We employed the perturbed angular correlation probe 111 In as an acceptor impurity and studied its interaction with the group VI elements S, Se and Te in Si. They were implanted at room temperature and subsequently the samples were annealed at increasing temperatures in order to remove the implantation-induced damage and increase the mobility of the impurities. After the S and Se implantations, no pair formation was observed. For Te however, a new quadrupole interaction frequency of n Q =444(1) MHz (Z=0) was found, which was attributed to the trapping of a Te atom at the In probe. The formation probability of the In-Te pair and the orientation of the corresponding electric field gradient were studied. The observed properties are compatible with the assumption that a complex between a substitutional In acceptor and a Te atom in the adjacent octahedral interstitial site is formed.

The perturbed γ -γ angular correlation method has been employed to study indium-impurity pairs in silicon, consisting of the probe atom ( 111 In/ 111 Cd) and several group-VI donors. Such pairs can be identified via the interaction between the quadrupole moments of the probe nucleus and the electric field gradient (EFG) associated with the formed defect complex. A new quadrupole interaction frequency (QIF) of ν Q = 444 (1) MHz (η = 0) is measured at T = 293 K in Te implanted silicon after annealing the sample above 700 K. The complex is attributed to the In-Te pair along 100 -crystal axis in silicon. In addition, the temperature dependence of the QIF characterizing the pair has also been studied. The implantations of S and Se could not lead to the formation of observable complexes despite similar treatment of all samples.

Recent results on the study of high-TC superconductors using the e --γ perturbed angular correlation technique are presented. The basic features of the experimental equipment and its installation at the ISOLDE facility are briefly described. Results obtained from 197m Hg implanted into high quality Y1Ba2Cu3O 6+δ epitaxy thin films are presented and discussed.

Lattice sites and collective ordering of fluorine atoms in oxygen-reduced samples of HgBa 2 CuO 4 were investigated with the perturbed angular correlation technique by measuring the electric field gradients induced at 199m Hg nuclei. The experimental data were interpreted with the help of ab initio calculations of charge distributions for different fluorine configurations in Hg m Ba 2m Cu m O 4m F n , supercells. Internal parameters were allowed to relax, to cancel residual atomic forces due to the dopant. The experimental results show clearly that fluorine occupies only the center of the mercury mesh. For a fluorine content ␦F Ͼ ϳ 0.35 the best agreement with theoretical data is obtained under the assumption that fluorine shows a tendency toward ordering along interstitial rows parallel to a , b. In conformity with experimental data from diffraction techniques the calculations show elongations of the O͑2͒-Hg-O͑2͒ dumbbell and barium shifts towards the fluorine atoms as effects of the dopant. No strong ordering of the transferred charge at the copper planes is present in the computed charge density, even when the fluorine atoms order at high concentrations.

In this work, we report an atomic scale study on the ferromagnetic insulator manganite LaMnO3.12 using γ − γ PAC spectroscopy. Data analysis reveals a nanoscopic transition from an undistorted to a Jahn-Teller-distorted local environment upon cooling. The percolation thresholds of the two local environments enclose a macroscopic structural transition (Rhombohedric-Orthorhombic). Two distinct regimes of JT-distortions were found: a high temperature regime where uncorrelated polaron clusters with severe distortions of the Mn 3+ O6 octahedra survive up to T ≈ 800 K and a low temperature regime where correlated regions have a weaker JT-distorted symmetry.

High quality YBa 2 Cu 3 O 6x (YBCO) superconducting thin ®lms were implanted with the radioactive 197m Hg (T 1a2 24 h) isotope to low ¯uences of 10 13 atoms/cm 2 and 60 keV energy. The lattice location and stability of the implanted Hg were studied combining the Perturbed Angular Correlation (PAC) and Emission Channeling (EC) techniques. We show that Hg can be introduced into the YBCO lattice by ion implantation into a unique regular site. The EC data show that Hg is located on a highly symmetric site on the YBCO lattice, while the PAC data suggests that Hg occupies the Cu(1) site. Annealing studies were performed under vacuum and O 2 atmosphere and show that Hg starts to diuse only above 653 K.

The Perturbed Angular Correlation (PAC) technique was applied to study the yttrium local environment in YMnO 3 multiferroic manganite. The electric field gradients (EFG) at the Y site have been measured as function of temperature, covering both ferroelectric and magnetic transitions. The results were compared with point charge model (PCM) calculations. The experimental results show two different EFG distributions for all temperatures. Only one can be directly attributed to the yttrium crystalline site in the hexagonal structure.

The energy levels of the quantum dots are evaluated by using Quantum Genetic Algorithm procedure and Hartree-Fock Roothaan method. Based on the calculated energies and wave functions, static dipole polarizability, oscillator strength and electric field gradient of helium/helium-like quantum dots are calculated as a perturbative calculation. It is worth to note that impurity charge and dot radius have a strong effect on the dipole polarizability, the oscillator strength and the electric field gradient. In small dot radii, since the spatial confinement effect is dominant, the polarizability of system is very weak. The polarizability increases monotonically with the increase of dot radius and then reaches a saturation value in large dot radii. On the other hand, the electric field gradient increases as dot radius decreases.

Pulsed laser deposition (PLD) has been proposed several years ago as a suitable technique to deposit a pure Mg film over a radio frequency (rf ) gun Cu backflange in order to obtain a high efficiency photocathode surface for the generation of high brightness electron beams. In this paper we report preliminary experimental results on the emission properties of a PLD grown Mg film within the high electric field gradients of a rf gun showing the effects of the rf conditioning process on the cathode surface. Even though a laser cleaning process should be performed on the sample surface in order to remove contaminated layers, the results presented here are very promising for the realization of a final Mg-based photocathode.

The electric field gradient (efg) at the Nb site in the intermetallic compounds Nb3X (X = Al, In, Si, Ge, Sn) was measured by the Perturbed Angular Correlation (PAC) method using the well-known gamma-gamma cascade of 133 -482 keV in 181Ta from the β- decay of 181Hf, substituting approximately 0.1 atom percent of Nb. The PAC results show that Vzz drops by nearly 40% when X changes from Al to In, and by about 25% when X changes from Si to Ge and Sn. This behavior is most probably related to the change in the degree of sp hybridization in these compounds. The Vzz values of the studied compounds do not follow the well known universal correlation for the efg's in non-cubic metals but the observed trend is well reproduced by results of ab-initio electronic structure calculations. In the case of Nb3 Al a linear temperature dependence of the quadrupole frequencies was observed in the temperature range of 6.5 to 1210 K.

In this paper, we investigate the effects of doping in the local structure of SnO2 by measuring the hyperfine interactions at impurity nuclei using the Time Differential Perturbed Gamma-Gamma Angular Correlation (TDPAC) method in addition to density functional theory simulations. The hyperfine field parameters have been probed as a function of the temperature in thin film samples. The experimental results reveal that 117Cd/In and 111In/Cd are incorporated and stabilized in the SnO2 lattice replacing the cationic site. Significant differences in the electric field gradient were observed from TDPAC measurements with both the probe nuclei. Furthermore, the absence of strongly damped spectra further indicates that implanted Cd atoms (for 117Cd/In probe nuclei measurements) easily occupy regular substitutional Sn sites with good stability. The simulated value for the electric field gradient obtained with the first oxygen neighbor removed is closer to the experimental value observed for 1...

Standard approximations for the exchange-correlation functional are known to deviate from linear dependence of the energy on the electron and spin numbers (in -space). Violation of this flat-plane condition underlies the failure of all known approximate functionals to describe band gaps in strongly correlated systems. Then crucial for further development of functionals is recognition of the behavior of the energy as a function of, and its derivatives (derivatives discontinuity pattern at 0K limit). In this Letter, the energy derivatives are analyzed thoroughly. It is shown that apart from the well-known type of discontinuity pattern, three other patterns are possible in the vicinity of the singlet state point. The zero temperature limits of energy derivatives at this point are derived, found different for various directions. Existence of all discontinuity patterns is illustrated on the example of diatomic molecules set.

We have carried out an accurate determination of the quadrupole moment of the deuteron nucleus. The evaluation of the constant is achieved by combining high accuracy Born-Oppenheimer calculations of the electric field gradient at the nucleus in the H 2 molecule with spectroscopic measurements of the quadrupolar splitting in D 2 and HD. The derived value is Q = 0.285783(30) fm 2 .

Jump frequencies of 111 In/Cd tracer atoms were measured for a series of layered phases La n CoIn 3n+2 using the technique of perturbed angular correlation of gamma rays (PAC). The frequencies were determined by analysis of nuclear quadrupole relaxation produced by fluctuating electric field gradients. Samples were synthesized having nominal values n= 1, 2, 3, 5 and ∞, with n=∞ corresponding to the L1 2 phase LaIn 3. The phases form heuristically from LaIn 3 by replacing every (n+1) th (100) mixed plane of La and In atoms with a plane of Co-atoms. For the n=1 phase, LaCoIn 5 , jump frequencies were too small to detect. Two signals were observed, one for indium atoms next to the Co-planes and the other for more distant indium atoms. No relaxation was observed for atoms next to the Co-planes, indicating that there is no diffusion across the Co-planes. With increasing n, jump rates for the other In-atoms increased toward values observed for LaIn 3. Jump frequency activation enthalpies for n= 3 and 5 were observed to be the same as for n=∞, suggesting the same diffusion mechanism. However, the jump-frequency prefactors were found to be smaller for small n, which is attributed to reductions in the connectivity of the diffusion sublattice. We conclude that diffusion in the layered phases is remarkably similar to diffusion in LaIn 3 once the reduced connectivity is taken into account.

5'V Magic-angle-spinning (MAS) NMR has been applied to V,O, at two different magnetic field strengths (4.7 and 7.1 T). Both the magnitude and relative orientation of the quadrupole and chemical shift (CS) tensors have been determined by iterative fitting of the 5'V MAS NMR lineshapes at the two magnetic field strengths. The reliability of the results is discussed. Moreover, it is shown that previous low-field single-crystal data are fully consistent with the high-field powder-sample MAS NMR results provided that a slight noncoincidence between the CS tensor and the crystal frame axes is considered. The electric field gradient tensor at the vanadium and lithium sites is subsequently used to test several electronic structure calculation at an ab initio Hartree-Fock level in V,O, and y-LiV,O, crystals. It is shown that a wide distribution of oxygen charges must be considered to describe the particular environment of each type of oxygen atoms. Furthermore, this analysis supports the fact that the vanadyl bond is likely a short ionic bond. NMR is found to be a valuable experimental tool to get insight into the nature of chemical bonds in vanadium oxides.

Recent NMR measurements yielded the electric field gradient (EFG) at the nucleus of alkalimetal atoms adsorbed on a W'(110) single-crystal surface. This paper compares these experimental results with the results of a model calculation for alkali-metal atoms adsorbed on jellium. The model predicts the correct signs of the measured EFG. The absolute values of the EFG's are in reasonable agreement with measurements. Calculated temperature dependences of the EFG's were found to be rather small.

The electric field gradients at 199m Hg nuclei have been measured via the perturbed angular correlation (PAC) technique, allowing a full characterization of the Hg neighborhood charge distribution at high oxygen doping on the Hg planes. The PAC technique has been applied to investigate the effect of high oxygen pressure during the measurement. Polycrystalline HgBa 2 CaCu 2 O 6+δ (Hg-1212) samples have been annealed at 152 bar pressurized oxygen. The influence of oxygen pressure during the experiment was then investigated by measuring the samples at atmospheric pressure and under 152 bar oxygen pressure. The present set of PAC experiments shows that at high oxygen concentrations there is a non-uniform oxygen distribution. Moreover, the Hg environment is not free from oxygen and the results hint to a new type of ordering.

A sample of nanocrystalline YI. S EUO.203 was examined by 151Eu Mossbauer spectroscopy. The degree of covalency of the Eu-O bond has been studied. The spectrum of the cubic Y1.8EUO.203 sample has been resolved into 2 contributions due to europium in the C 3i and C 2 sites, for the first time in 151Eu Mossbauer spectroscopy. The degree of covalency and the electric field gradient of

The shape of the ␥ resonance absorption peak of the Eu 3ϩ ion in a disordered structure was investigated in some phosphate, borate, and silicate glasses by using 151 Eu Mössbauer spectroscopy. The quality of the fits was tested by using the Durbin-Watson d statistics. The observed full width at half maximum of the peak was resolved in a contribution of the broadening and a contribution of the quadrupole splitting, due to the distortion of the Eu site compared to a cubic symmetry. The Eu-O bond was found to have a covalent admixture with 6s character. The axial component of the electric-field gradient at the Eu site was found to be correlated with the optical basicity of the glass.

The confluence of quantum mechanics and complexity, which leads to the emergence of rich, exotic states of matter, motivates the extension of our concepts of quantum ordering. The twin concepts of spontaneously broken symmetry, described in terms of a Landau order parameter, and of off-diagonal long-range order (ODLRO), are fundamental to our understanding of phases of matter. In electronic matter it has long been assumed that Landau order parameters involve an even number of electron fields, with integer spin and even charge, that are bosons. On the other hand, in low-dimensional magnetism, operators are known to fractionalize so that the excitations carry spin-1/2. Motivated by experiment, mean-field theory and computational results, we extend the concept of ODLRO into the time domain, proposing that in a broken symmetry state, quantum operators can fractionalize into half-integer order parameters. Using numerical renormalization group studies we show how such fractionalized order can be induced in quantum impurity models. We then conjecture that such order develops spontaneously in lattice quantum systems, due to positive feedback, leading to a new family of phases, manifested by a coincidence of broken symmetry and fractionalized excitations that can be detected by experiment.

In this review we compile and discuss the published plethora of cell biological effects which are ascribed to electric fields (EF), magnetic fields (MF) and electromagnetic fields (EMF). In recent years, a change in paradigm took place concerning the endogenously produced static EF of cells and tissues. Here, modern molecular biology could link the action of ion transporters and ion channels to the ''electric'' action of cells and tissues. Also, sensing of these mainly EF could be demonstrated in studies of cell migration and wound healing. The triggers exerted by ion concentrations and concomitant electric field gradients have been traced along signaling cascades till gene expression changes in the nucleus. Far more enigmatic is the way of action of static MF which come in most cases from outside (e.g. earth magnetic field). All systems in an organism from the molecular to the organ level are more or less in motion. Thus, in living tissue we mostly find alternating fields as well as combination of EF and MF normally in the range of extremely low-frequency EMF. Because a bewildering array of model systems and clinical devices exits in the EMF field we concentrate on cell biological findings and look for basic principles in the EF, MF and EMF action.

We have carried out 75 As NMR experiments on a single 5 μm thick epitaxial layer of metal organic chemical vapor deposition (MOCVD) Al x Ga 1−x As, with x = 0.522, using a novel stripline based NMR setup. Different arsenic surroundings in the lattice give rise to five As[Al n Ga 4−n ] sites with (n = 0−4). By mounting a thin film of Al 0.522 Ga 0.478 As in a home-built NMR probe with rotation stage and stripline detector, we could successfully distinguish different arsenic nearest neighbors coordinations. Furthermore, we were able to observe various 75 As quadrupolar tensor orientations for each As[Al n Ga 4−n ] coordination, which gives us a unique insight into the structure of this material. The individual resonances for each of these coordinations appear to be very broad, as a result of the variation in the local symmetry due to the distribution of aluminum and gallium over the lattice. In particular, the NMR resonance of one orientation of the As[Al 2 Ga 2 ] site is strongly broadened. The line widths prove to be larger than predicted on the basis of the (distribution in) quadrupolar interaction parameters obtained in a previous study of powdered Al x Ga 1−x As films (P. J. Knijn et al. Phys. Chem. Chem. Phys. 2010, 12, 11517−11535). In Density Functional Theory (DFT) based calculations, these wider distributions in quadrupolar interaction parameters can be explained by local variations in the first coordination sphere of arsenic, which become apparent when the internal structure is relaxed before quantum mechanical calculations of the Electric Field Gradient Tensors (EFGs). A detailed simulation of the NMR line intensities, taking the actual NMR excitation parameters into account, prove the absence of any kind of local or long-range order in the occupation of the Al/Ga sites in the lattice.

The residual shallow donor in GaN is investigated by electron spin resonance and the Overhauser shift double resonanCe technique. From the resolved quadrupolar splitting, we determine the electric field gradients at the Ga and the N site. The hyperfine interaction, as determined simultaneously from the Overhauser shift and the Knight shift, supports a delocalized donor wavefunction with an effective Bohr radius of 20-30A.

The residual shallow donor in GaN is investigated by electron spin resonance and the Overhauser shift double resonanCe technique. From the resolved quadrupolar splitting, we determine the electric field gradients at the Ga and the N site. The hyperfine interaction, as determined simultaneously from the Overhauser shift and the Knight shift, supports a delocalized donor wavefunction with an effective Bohr radius of 20-30A.

Room temperature MSssbauer spectra of Crl-xFex alloys (x = 0.05-0.26) have been fitted using a pseudo-lorentzian lineshape; this leads to the widths of gaussian processes which contribute to the line broadening. The variation of isomer shift with composition can he described well by an extension to the cellular model of Miedema and co-workers. Estimations of the contributions of the s electron charge transfer and the s-d electron conversions to the isomer shift are presented. i.

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2 7~1 and 6 3 , 6 5~~ NMR measurements are reported for powder samples of stable Al-Cu-Fe and Al-Cu-Ru icosahedral quasicrystals and their crystalline approximants, and for an Al-Pd-Mn singlegrain quasicrystal. Furthermore, ' '~1 NQR spectra at 4.2 K have been observed in the Al-Cu-Fe and Al-Cu-Ru samples. From the quadrupole-perturbed NMR spectra at different magnetic fields, and from the zero-field NQR spectra, a wide distribution of local electric-field gradient (EFG) tensor components and principal-axis-system orientations was found at the Al site. A model EFG calculation based on a 1/1 Al-Cu-Fe approximant was successful in explaining the observed NQR spectra. The average local gradient is largely determined by the p-electron wave function at the A1 site, while the width of the distribution is due to the lattice contribution to the EFG. Comparison of "CU NMR with 27Al NMR shows the EFG distribution at the two sites is similar, but the electronic contribution to the EFG is considerably smaller at the Cu site, in agreement with a more s-type wave function of the conduction electrons. The overall spread of EFG values is well reproduced by the calculation based on the approximant. However, the experimental spectra indicate a much larger number of nonequivalent sites when compared with the simulated NQR spectra based on the 1/1 approximant. The short-range, local chemical order is well represented by the approximant, but differences in coordination must be included at intermediate range in the quasicrystal. Measurements of 27Al Knight shift, magnetic susceptibility, and nuclear spin-lattice relaxation time as a function of temperature yield results which indicate a reduction of the density of states at the Fermi level by a factor of 7 or 8 from the value in Al metal, consistent with the notion of a pseudogap for these quasicrystals. No differences in the measured parameters were detected as a function of composition of the quasicrystatline alloys, arguing against a fine structure in the density of states at the Fermi level.