TDPAC studies of the intercalation compound Li x TiS2 in the region 0≤x≤1

The electrochemical intercalation of Li⁺ into hafnium-doped TiS₂ was studied using time-differential perturbed angular correlations (TDPAC). The¹⁸¹Ta nuclear quadrupole interaction was monitored as a function of the charge transfer,n _F. For low uptakes, 0≤n _F≤0.03, a two-phase region was found with an empty host interaction, characterized byV _q=458(4) MHz, η=0.19(3) and δ-0.083(5), and a second interaction corresponding to lithiated material withv _q=597(5) MHz, η=0.13(4) and δ=0.061(5). No evidence was found for intermediate phases.     

Electric field gradients at Ta in Zr and Hf inter-metallic compounds

Here we calculate the electric field gradient (EFG) at the nucleus of the substitutional Ta impurity site in Zr₂T and Hf₂T (T=Cu, Ag, Au, and Pd) C11_b inter-metallic compounds. We use the ab initio FP-LAPW method as embodied in the Wien97 code in a super-cell approach and include lattice relaxations around the impurity. Our results are compared with EFG values inferred from measurements of the quadrupole coupling constants at the ¹¹¹Ta probe in these compounds performed with the time differential perturbed angular correlation (TDPAC) technique. We also performed EFG calculations for the pure inter-metallic compounds. Through the comparison of theoretical and experimental EFGs in these cases, we elucidate the role played by the Ta probe in the TDPAC measurements of Hf and Zr compounds. Our results show that, although the EFGs at the Hf site are very similar to the EFGs at the Ta impurity, there is no direct correlation between the Zr and Ta EFGs.     

A local moment approach to the gapped Anderson model

We develop a non-perturbative local moment approach (LMA) for the gapped Anderson impurity model (GAIM), in which a locally correlated orbital is coupled to a host with a gapped density of states. Two distinct phases arise, separated by a level-crossing quantum phase transition: a screened singlet phase, adiabatically connected to the non-interacting limit and as such a generalized Fermi liquid (GFL); and an incompletely screened, doubly degenerate local moment (LM) phase. On opening a gap (δ) in the host, the transition occurs at a critical gap δ_c, the GFL [LM] phase occurring for δ<δ_c [ δ>δ_c] . In agreement with numerical renormalization group (NRG) calculations, the critical δ_c = 0 at the particle-hole symmetric point of the model, where the LM phase arises immediately on opening the gap. In the generic case by contrast δ_c > 0, and the resultant LMA phase boundary is in good quantitative agreement with NRG results. Local single-particle dynamics are considered in some detail. The major difference between the two phases resides in bound states within the gap: the GFL phase is found to be characterised by one bound state only, while the LM phase contains two such states straddling the chemical potential. Particular emphasis is naturally given to the strongly correlated, Kondo regime of the model. Here, single-particle dynamics for both phases are found to exhibit universal scaling as a function of scaled frequency ω/ω_m ⁰ for fixed gaps δ/ω_m ⁰, where ω_m ⁰ is the characteristic Kondo scale for the gapless (metallic) AIM; at particle-hole symmetry in particular, the scaling spectra are obtained in closed form. For frequencies |ω|/ω_m ⁰ ≫δ/ω_m ⁰, the scaling spectra are found generally to reduce to those of the gapless, metallic Anderson model; such that for small gaps δ/ω_m ⁰≪ 1 in particular, the Kondo resonance that is the spectral hallmark of the usual metallic Anderson model persists more or less in its entirety in the GAIM.     

Fe-Hydroxysulphates from Bacterial Fe2+ Oxidation

The perturbed angular correlation method (PAC) was applied to investigate the lattice location of implanted ¹¹¹In probe ions in Hf₂Ni and Zr₂Ni intermetallic compounds. It is concluded that the ¹¹¹In/¹¹¹Cd probe nuclei experiencing the highly asymmetric electric field gradient (EFG) occupy the unique hafnium or zirconium 8(h) sites in the investigated phases. Above room temperature, the EFGs decrease linearly with temperature. The results are compared with that of previous PAC measurements with ¹⁸¹Ta probes.     

Influence of the stress on magnetoelectric effect in magnetostrictive-PZT bilayers

In this letter, we investigate the influence of the stress on magnetoelectric (ME) effect in a magnetostrictive-PZT bilayer. ME voltage coefficient α*_E = δE / δH, where δE is the induced electric field for an applied alternating current (ac) magnetic field δH, is obtained by solving the stress-related piezoelectric constitutive equation and the conventional magnetostrictive equation with appropriate boundary condition. Based on the free-energy density function of the PZT film in stress state, we get the stress-related piezoelectric charge coefficient ^p d*₃₁ and dielectric permittivity ^pε*₃₃. After taking the cobalt ferrite (CFO) as magnetostrictive phase, it is found that α*_E increases with decreasing 2-d compressive stress for CFO-PZT, which not only is qualitatively consistent with previous experimental measurements, but also provides a possible route to improve the ME effect.      

Study of the structure and electric quadrupole interaction in the intermetallic compound Hf2Fe

The electric quadrupole interactions at¹⁸¹Ta probe nuclei in a cubic Hf₂Fe lattice were studied by the TDPAC method. In addition, the crystalline structure study of the Hf₂Fe compound was performed. The results of the EQI measurement show the presence of two independent interactions, one at low frequency, characterized by ω _Q ⁽¹⁾ =33 Mrad/s and δ=30%, and the other at high frequency described by ω _Q ⁽²⁾ =207 ± 2 Mrad/s and δ=4%. Both interactions are found to be compatible with the crystalline structure established in this investigation. The large temperature dependence of the electric field gradient of the second interaction in the range from 78K to 1183K was determined. The change in the EFG follows the empiricalT ^3/2 -relation.     

Investigation of phase transitions and thermal expansion of some complex tungsten-containing oxides

The phase transitions have been investigated and the thermal expansion coefficients have been determined in compounds with a structure of brannerite, defect pyrochlore, and tungsten bronze of the general formula M ^I A ^VWO_{6 − δ} (M ^I = □, H₃O, Li, Na, K, Rb, and Cs; A ^V = V, Nb, Sb, and Ta; δ = 0, 0.5) by high-temperature X-ray diffraction and differential thermal analysis. It has been shown that brannerites and tungsten bronze do not undergo phase transitions and the compounds are characterized by a significant anisotropy of the thermal expansion, whereas pyrochlores undergo phase transitions of three types, and thermal deformations in them are determined by the size of M ^I atoms.     

Systematics of perturbative semiclassical quantum defect expansions probed by RKR-QDTand a Fisher-information-based criterion

The systematics of perturbative semiclassical quantum defect expansions corresponding to a hydrogenic potential plus a perturbing term of the form -A/2r^κ, k\geqslant 2\kappa \geqslant 2, are studied as a function of expansion order N. Towards this task the expansions μ _Nare first used as input for constructing associated N-dependent atomic RKR-QDT potential curves. Subsequently the coordinate Fisher information for the energy levels supported by those curves as well as its rate ε with respect to N is semiclassically computed. Then, the plot of relative quantum defect error between successive orders, δμ _N+1,N, with respect to ε serves as convergence indicator for both approximate potentials and quantum defects. For a given κ and when the quantum defect expansion proves to be of limited accuracy the plot reveals an A- and N-dependent scatter of points and “saturation” (the relative error remains almost constant with respect to ε). More importantly, when ε is equal to or lower than the value of ε (N=1) for which πμ ₁\leqslant 1/2_{1}\leqslant 1/2 the relative error exhibits a κ-, A- and N-independent power-law dependence, δμ _N+1,N∝ ε ^m, clearly distinguishing the N=1 order (m=1/2) from all other N>1 orders (m=1). These power-laws may be employed for setting-up confidence level bounds on perturbative expansions.     

Observation of relaxation modes in room temperature ferroelectric liquid crystal mixtures

The dielectric measurements in SmC* and SmA phases of a room temperature ferroelectric liquid crystal mixture FLC-6980 in the cells of different thickness in planer alignment have been carried out in the frequency range 100 Hz to 1 MHz. A relaxation mode (called NRM) whose dielectric increment is less than the Goldstone mode has been observed in the SmC* phase. This mode appears due to the surface effect. Goldstone mode and the soft mode was observable in the vicinity of SmC*-SmA transition temperature (T _C*A). The dielectric parameters of the Goldstone mode, new mode and the soft mode have been studied as a function of frequency and temperature. The calculated values for f_NRM, δε^NRM and distribution parameter α_NRM are found to be 325 kHz, 6 and 0.156 for 5μm thick planer cell at 37°C. It is seen that in the vicinity of theT _C*A, soft mode obeys the Curie-Weiss law given by mean field theory. The results have been compared with materials of large spontaneous polarization.     

Silicon and transition metal-silicides implanted with57Fe

Si(111) single crystals were implanted with⁵⁷Fe in a broad dose range in order to overlap the concentration range of bulk amorphous Fe _x Si_1−x samples. At high (≥10¹⁶ atoms/cm²) doses the measured hyperfine interaction values were found to be the same as in the bulk amorphous samples, suggesting the same Fe−Si bonding and a very similar structure for the two amorphous phases produced by different methods. A comparison of the isomer shift (δ) and quadrupole splitting (ΔE) values with the values of the stoichiometric crystalliine phases showed the same δ but different δE values indicating similar Fe−Si bonding but different atomic arrangement around the iron atom.     

Phase equilibria in random multiblock copolymers

A mean-field theory for domain structures in random multiblock copolymer melts is developed. We focus on the finite molecular weight effects resulting in a competition between macroscopic phase separation and microdomain formation in the system. We identify an essential parameter N ε controlling the phase behavior of the system, where N is the number of blocks per chain and ε is the composition asymmetry parameter (= the difference between the mean copolymer composition and its critical value). The phase diagram involving N ε and the reduced temperature as variables is obtained. The regions of coexistence of two or more phases are identified. We show that a superstructure formation on cooling is always pre-empted by a macroscopic phase separation of the macroscopically homogeneous (disordered) system yielding two homogeneous phases: H ₀↦H ₁ + H ₂. The third (lamellar) phase separates on further cooling. Then hexagonal and body-centred-cubic phases take over if N ε 1. As the Flory interaction parameter χ increases further, the standard transitions BCC↦HEX↦LAM take place. Received 13 July 2001     

Formation mechanism of the primary faceted phase and complex eutectic structure within an undercooled Ag–Cu–Ge alloy

The solidified microstructure of bulk undercooled Ag₄₀Cu₃₀Ge₃₀ alloy consists of three parts: primary (Ge) phase, the complex structure of (Ag + Ge) and (Ag + ε ₂) pseudobinary eutectics, and (Ag + Ge + ε ₂) ternary eutectic. In comparison, the pseudobinary eutectic no longer appears in an alloy droplet solidified in a drop tube. Once the undercooling exceeds 225 K and the cooling rate is greater than 2×10³ K s⁻¹, the microstructure of the solidified droplet is totally composed of anomalous ternary eutectic. In both cases, the primary (Ge) phase exhibits various faceted growth morphologies at different undercoolings, such as columnar block, long dendrite, equiaxed dendrite and rod-like crystal. Some refined side branches grow from the equiaxed (Ge) dendritic branches composed of {111} twins, which is ascribed to the rapid epitaxial growth of (Ag + Ge) pseudobinary eutectic from the (Ge) dendritic branches. Moreover, both the primary (Ge) phase and the (Ge) phase in the (Ag + Ge) pseudobinary eutectic are effective heterogeneous nuclei for the (Ag+ε ₂) pseudobinary eutectic. As undercooling increases, the (Ge) phase in the (Ag + Ge+ε ₂) ternary eutectic transforms from faceted to non-faceted phase, while the independent nucleation and growth of the (Ag) and ε ₂ phases in the ternary eutectic displaces their previous cooperative growth. These growth kinetics transitions result in the formation of anomalous ternary eutectic.     

PAC studies of hyperfine interactions of181Ta in YBa2Cu3O7-δ and of140Ce in La2CuO4 and La1.85Sr0.15CuO4

Results of TDPAC studies of hyperfine interaction in high temperature superconducting ceramics are reported. The γ-ray cascade of 329–487 keV in¹⁴⁰Ce excited in the decay of¹⁴⁰La in La_2-xSr_xCuO₄ samples (x=0 and 0.15), and 133–482 keV cascade in¹⁸¹Ta excited in the decay of¹⁸¹Hf in YBa₂Cu₃O_7-δ samples were used. The procedure of introducing radioactive¹⁸¹Hf into the ceramics is described and indirect evidence for the occupation of Cu sites by the¹⁸¹Hf-¹⁸¹Ta probe is presented.     

Transport and dielectric properties of thin polycrystalline CoO films

Transport and dielectric properties of polycrystalline CoO films were studied as functions of the applied field, frequency and temperature. TheI–V plots showed that the Poole-Frenkel field emission mechanism is responsible for conduction at fields>10⁵ V/cm. The ac conductivity σ(ω), the imaginary part of the dielectric constantε ₂, and tan δ plots as functions of frequency revealed three dispersion regions. The σ(ω) andε ₂ frequency dependence indicates a non-adiabatic hopping of charge carriers at low frequencies and adiabatic hopping at high frequencies. The activation energy of a dielectric oscillator is 0.15 eV. Work supported by the Office of Naval Research.     

Pac studies of the electric quadrupole interaction of181Ta in yttrium and copper oxides and in super conducting ceramics YBa2Cu3O7−δ

The electric quadrupole interaction of¹⁸¹Ta in Y₂O₃, CuO, and in the YBa₂Cu₃O_7−δ high temperature superconductors, doped with¹⁸¹Hf, has been studied by the time-differential perturbed angular correlation method (TDPAC). In Y₂O₃, two distinct fractions of the probe nuclei were detected, which were attributed to the substitutional positions at the two cation sites in the lattice. In CuO, and in the ceramic samples, broad distributions of quadrupole frequencies were observed. Possible configurations of the probe nuclei in these substances are discussed.     

M?ssbauer and X-ray studies of the structural phase transformations and suppression of polymorphism in La1 ? xSrxMn0.98Fe0.02O3 + δ (x = 0.05?0.30)

The specific features of the structural transformations in La_{1 − x} Sr _x Mn_0.98Fe_0.02O_{3 + δ} (x = 0.05−0.30) as functions of the Sr content have been investigated using M?ssbauer spectroscopy and X-ray diffraction. It has been shown that, when the Sr content is lower than 10%, three phases are formed: PnmaII, PnmaI, and R-3c. In the Sr concentration range from 10 to 20%, the PnmaI and R-3c phases are formed. At a strontium content of 20%, the rhombohedral phase is formed. When the strontium content increases to 30%, the presumably monoclinic (distorted cubic) phase is formed in addition to the rhombohedral phase. The specific features of the formation and suppression of the phases in the LaMn_0.98Fe_0.02O_{3 + δ} compound are compared with those observed in the strontium-doped La_{1 − x} Sr _x Mn_0.98Fe_0.02O_{3 + δ} compound. The general regularities of the structural transformations and substantial differences between them have been revealed.     

Influence of thermal activation on dislocation inelasticity of LiF crystals with different impurity compositions

The method of double piezoelectric oscillator is used to investigate the influence of thermal activation (293–573 K) on the dislocation inelasticity and dislocation structure of the annealed LiF crystals having different impurity compositions. On curves illustrating the dependence of the internal friction on the strain amplitude δ(ε₀), a peak having a hysteresis nature is established; it is displaced toward smaller ε₀ values with increasing temperature. A subsequent increase in δ(ε₀) after passage of the peak is caused by the processes of dislocation multiplication upon exposure to ultrasound. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 90–96, August, 2008.     

Effect of electric field on the rheological and dielectric properties of a liquid crystal exhibiting nematic-to-smectic-A phase transition

We report simultaneous measurements of shear viscosity (η) and dielectric constant (ε) of octyloxy cyanobiphenyl (8OCB) in the nematic (N) and smectic-A (SmA) phases as functions of temperature and electric field. With increasing electric field η increases in the N phase whereas it decreases in the SmA phase and saturates beyond a particular field in both the phases. The flow curves in the intermediate-field range show two Newtonian regimes in the N phase. The temperature-dependent behavior of η and ε at zero or at small electric field suggests the occurrence of several structures that results from precessional motion of the director along the neutral direction as reported in similar other system. We show that the precessional motions are gradually suppressed with increasing electric field and the effective viscosity resembles with the Miesowicz viscosity η ₁ at high enough electric field. In the intermediate field range the temperature-dependent η exhibits anomalous behavior across the N-SmA phase transition which is attributed to the large contribution of Leslie coefficient α ₁.     

Effect of iron doping and oxygen stoichiometry on infrared absorption in Y1Ba2Cu3O7−δ

We report infrared absorption of Y₁Ba₂Cu₃O_7−δ as a function of oxygen stoichiometry (0<δ<1) and copper substitution by iron in the spectral range of 450–700 cm⁻¹. The strong bands associated with Cu-O vibrations undergo significant changes in their frequencies and intensities asδ is varied across the orthorhombic to tetragonal phase. These changes coupled with those arising as a result of doping with iron has helped in identifying the nature of the vibrational modes.     

TDPAC characterization of tin oxides using181Ta

In connection with a general study of the evolution of tin-oxygen thin films, we report here on the hyperfine interactions of¹⁸¹Ta substitutionally replacting tin in the isolated phases SnO and SnO₂. For this purpose, pure SnO pressed powder and a thin SnO₂ film were implanted with¹⁸¹Hf. In both cases, unique quadrupole frequencies were found after thermal annealing treatments. The results indicate that the following hyperfine parameters: υ _Q = 740.6(2.1) MHz, η=0.07(2) and υ _Q = 971.5(1.9) MHz, η=0.72(1) characterize¹⁸¹Ta in SnO and SnO₂, respectively.