TDPAC studies on181Hf implanted into diamond

¹⁸¹Hf ions were implanted at (900±10)K into (100) cleaved single crystal diamonds. After subsequent annealing residence site parameters were determined for the implanted ions employing the (133 keV)–(482 keV) cascade in¹⁸¹Ta in TDPAC technique. Fractions of 5% each were determined that experience axially symmetric electric field gradients (EFG) V_zz(1)=5.5×10¹⁷ V cm^–2 and V_zz(2)=9.6×10¹⁷ V cm^–2, respectively. The bulk of the ions are strongly disturbed by still higher electric field gradients.     

Static electric quadrupole interaction of Ta- and Hf-ions in barium and lead titanate

The static electric quadrupole interaction of¹⁸¹Ta and¹⁷⁸Hf in polycrystalline barium and lead titanate at the site of titanium has been measured using time differential PAC and the Mössbauer effect. The electric field gradients (EFG) at room temperature at the¹⁸¹Ta nucleus are ¦V _zz¦=(3.6±0.2)·10¹⁷V/cm² in BaTiO₃ and ¦V _zz¦=(14.6±0.6)·10¹⁷ V/cm² in PbTiO₃. The temperature dependence of the quadrupole interaction has been studied giving the following EFG values: ¦V _zz¦=(2.4±0.2)·10¹⁷ V/cm² in the monoclinic and ¦V _zz¦=(1.1±0.3)·10¹⁷ V/cm² in the rhomboedral phase of BaTiO₃, and ¦V _zz¦=(15.7±0.6)·10¹⁷ V/cm² for¹⁸¹Ta/PbTiO ₃ at 77 °K. The EFG of¹⁷⁸Hf in PbTiO₃ has been derived from a Mössbauer effect experiment to beV _zz=+(10.7±0.5)·10¹⁷ V/cm². The results are compared with EFG's calculated in a point charge model and with experimental EFG's measured at⁴⁴Sc and⁵⁷Fe in the same titanates by other authors. Contributions of covalent bonds to the effective EFG's in perovskit crystals are discussed.     

Study of the electric quadrupole interaction at Ta impurities in rare earth metal hosts

The time differential perturbed angular correlation technique has been used to measure the electric fieldgradient (EFG) at the site of¹⁸¹Ta impurities in the heavy Rare Earth metals Gd, Tb, Dy, Ho and Er at room temperature. It is found that the ratio α ≡ ¦V _zz ^eff /V _zz ^lat ¦ between the measured EFGV _zz ^eff and the lattice EFGV _zz ^lat , which is known from lattice sum calculations, is in the order of α?300, suggesting that an important contribution to the EFG is due to electrons localized at the impurity. The ratio α is not constant throughout the Rare Earth series. It decreases from Gd to Tb and increases between Tb and Er. This behaviour is compared to the results of a previous investigation with the impurity Cd in the same hosts.     

Magnetic and electric hyperfine interactions of181Ta implanted in thulium

The magnetic and electric hyperfine interaction at the site of dilute¹⁸¹Ta impurities in the rare earth metal Tm has been investigated as a function of temperature by TDPAC measurements. The samples were prepared by ion implantation of radioactive¹⁸¹Hf. In the paramagnetic phase between 100 K and 700 K the electric fieldgradient is a linear function of temperature: V_zz(T)=V_zz(O)·(1-A·T) with A=4.6·10^?4K^?1 and V_zz(293K)=6.4 (4)·10¹⁷v/cm². The TDPAC spectrum observed at 4.2 K reflects the 4 magnetically non-equivalent sites for an impurity in magnetically ordered Tm. The relative values and amplitudes of the corresponding 4 magnetic hyperfine fields are consistent with the predictions of the RKKY theory.     

The magnetic hyperfine field and the electric field gradient at181Ta impurities in Gd metal

The magnetic hyperfine fieldH _hf and the electric field gradientV _zz at¹⁸¹Ta impurties in metallic Gd were determined by time differential perturbed angular correlation measurements with the 133 keV K-conversion electron 482 keV -cascade of¹⁸¹Ta. The sources for these measurements were prepared by implantation of radioactive¹⁸¹Hf ions into Gd. The results are: |H _hf(TaGd; 77 K)|=285(14)kG, and |V _zz(TaGd; 330 K)|=5.32(15)·10¹⁷V/cm². The value ofH _hf fits well into the systematics for 5d impurities in Gd and indicates a positive core polarisation contribution, which is expected if the conduction electrons of Gd have to a large extent d-character. The electric field gradients of the 5d impurities in Gd are not consistent with a proportionality between the ionic and the electronic contribution.     

Hydrogen-induced reduction of electric field gradient in La2Fe14BHx studied by NMR on La

The ¹³⁹La NMR spin-echo spectra of La₂Fe₁₄BH_x are reported. Since lanthanum has no 4f electrons, the observed quadrupole splittings are directly proportional to the lattice part of the electric field gradient, V_zz^lat. This quantity decreases as the content of interstitial hydrogen x grows. To describe the fall-off of V_zz^lat, a dimensionless function f₂(x) is introduced, normalised so that f₂(0)=1. Analysis of the new and earlier published data obtained by several experimental techniques demonstrates that the reduction of both the second-order crystal field parameter A₂₀ and the lattice electric field gradient V_zz^lat is described by the same function f₂(x) in all R₂Fe₁₄BH_x. The only distinction that needs to be made is between R being a light and a heavy rare earth.     

The low temperature quadrupole orientation of103Ru in ruthenium

The quadrupole frequencyv _Q =e ² qQ/h of¹⁰³Ru (Z=44,N=59) in a ruthenium single crystal has been measured using the technique of low temperature quadrupole orientation to bev _Q (¹⁰³RuRu)=−14.7(5) MHz. Temperatures below 2 mK were reached in this experiment using a PrNi₅ demagnetization stage attached to a³He−⁴He dilution refrigerator. Using the measured magnitude of the RuRu electric field gradient (EFG) at low temperatures |eq(RuRu)|=1.02(3)×10¹⁷ V.cm⁻² [1] and adopting the sign ofeq(RuRu) to be negative from systematics, this result yields a value for the ground-state electric quadrupole moment of¹⁰³Ru ofQ(¹⁰³Ru)=+0.59(2) b. This moment may be interpreted using the weak coupling model of de-Shalit [2]. A Korringa constant for¹⁰³RuRu ofC _K=39(6) Ks was measured in this experiment. Taking advantage of a small iridium contamination of the ruthenium single crystal, the quadrupole moment of the¹⁹²Ir ground state was determined to beQ(¹⁹²Ir)=+2.12(25) b. The sign of the IrRu electric field gradient was found to be negative as a result of this work.     

The electric field gradient and its temperature dependence for ruthenium in scandium and yttrium

The electric field gradients (EFG) for ruthenium in scandium and yttrium metal were determined by TDPAC measurements to be 19(4) × 10¹⁷ V/cm² and 5.5(12) × 10¹⁷ V/cm² respectively at room temperature. The EFG for Ru in Sc was found to vary considerably in the temperature range from 14 to 700 K, whereas for Ru in Y only an extremely small temperature dependence of the EFG was observed.     

The electric field gradient at the nuclear site of177Lu in Zn and In

The electric quadrupole interaction frequencyν _Q =eQV _zz /h of¹⁷⁷Lu in single crystals of Zn and In has been measured by the method of low temperature nuclear orientation. The results are $$\begin{gathered} v_Q ({}^{177}Lu\underline {Zn} ) = - 180(5)MHz \hfill \\ v_Q ({}^{177}Lu\underline {In} ) = - 19(5)MHz. \hfill \\ \end{gathered} $$ With the known quadrupole moment of¹⁷⁷LuQ=3.39 (2) b we derive for the electric field gradientV _zz (Lu Zn)=?2.20 (5)×10¹⁷ V/cm² andV _zz (Lu In)=?0.23 (6)×10¹⁷ V/cm². The results are compared with magnetostriction measurements of silver single crystals doped with rare earth atoms.     

The electric field gradients for Hg at different lattice sites in Be

¹⁹⁹Tl ions were implanted into a Be single crystal. From time differential perturbed e^– angular correlation experiments with the 5/2^– state of¹⁹⁹Hg the effective electric field gradientV _zz ^eff =±15.2(1.8)×10¹⁷ V · cm^–2 was deduced for Hg on an octahedral interstitial site in Be. From a nuclear orientation experiment, the sign ofV _zz ^eff for^197mHg implanted in Be is determined. In time differential perturbed angular correlation experiments with the 5/2 state of¹⁹⁹Hg populated in the -decay of¹⁹⁹Au implanted into Be, the effective electric field gradient for Hg on a substitutional site in Be was determined asV _zz ^eff =–1.2(2)×10¹⁷V · cm^–2. The dependence of the site population of Tl implanted in Be on annealing was investigated by use of the channeling technique. In a comparison of the derived effective field gradients with the corresponding lattice field gradient contributions different electronic enhancement factors are derived for different sites of Hg in Be.     

Optical oxygen sensing materials based on trinuclear starburst ruthenium(II) complexes assembled in mesoporous silica

The preparation and oxygen sensing properties of optical materials based on two trinuclear starburst ruthenium(II) complexes: [Ru₃(bpy)₆(TMMB)]⁶⁺ (1) and [Ru₃(phen)₆(TMMB)]⁶⁺ (2) (bpy=2,2′-bpyridine, phen=1,10-phenathroline, TMMB=1,3,5-tris[2-(2′-pyridyl)benzimidazoyl]methylbenzene) assembled in two mesoporous silicate (MS) are described in this paper. The luminescence of Ru complexes/silicate assemble materials can be quenched by molecular oxygen with good sensitivity (I₀/I₁>5 for 2/MS and I₀/I₁>3 for 1/MS), indicating that trinuclear starburst Ru(II) complexes/MS systems are sensitive to oxygen molecules.     

Structural phase transitions in Pd0.8 Si0.2 and Pd0.75 Si0.20 Ag0.05 alloys observed by pac

The phase transition in the alloys Pd_0.8 Si_0.2 and Pd_0.75 Si_0.20 Ag_0.05 have been investigated through the quadrupole interaction of¹¹¹Cd impurities. The quadrupole interactions were measured by means of the TDPAC technique from room temperature up to about 870 K. The variation of the quadrupole interaction with temperature in the alloy PdSiAg shows aT ^3/2 dependence below and above 629 K, with coefficientsB=5.43(25)·10⁻⁵ K^−3/2 andB=3.70(15)·10⁻⁵ K^−3/2, respectively. This demonstrates that the alloy undergoes a phase transition around 629 K. The existence of two electric field gradients observed in the alloy PdSi,V _zz (1)=3.47(54)·10¹⁷ V/cm² andV _zz (2)=2.29(36)·10¹⁷ V/cm², indicates that there are two different¹¹¹Cd sites. The corresponding fractionsf ₁ andf ₂ strongly depend on temperature. Below 520 K, most¹¹¹Cd nuclei are subject to the higher EFGV _zz (1) (f ₁≈70%), whereas above 520 Kf ₁ falls rapidly to zero andV _zz (2) becomes dominant. The temperature dependences of thef ₁ andf ₂ reveal a picture of the phase transition between the two crystal structures.     

NMR Study on Coexistence of Magnetic Order and Superconductivity in RuSr2GdCu2O8

Electronic and magnetic properties of RuSr₂GdCu₂O₈ have been investigated by resistivity, magnetization and NMR measurements. Magnetic order (T _C=133 K) and superconducting transition (T _S(onset) 52 K) have been confirmed in RuSr₂GdCu₂O₈. We observed two kinds of Ru-NMR signals (the hyperfine fields of ¹⁰¹Ru are 590 kOe and 290 kOe), suggesting a possible charge segregation of Ru⁴⁺(S=1) and Ru⁵⁺(S=3/2) in the RuO₂ layers. Holes can be inherently doped in the CuO₂ layers from the (Ru⁴⁺, Ru⁵⁺)O₂ layers, and the superconductivity can occur under weak magnetic interactions between Ru and Cu spins in RuSr₂GdCu₂O₈.     

Maslov index for power-law potentials

The Maslov index in the semiclassical Bohr–Sommerfeld quantization rule is calculated for one-dimensional power-law potentials V (x) = ?V ₀/x ^s, x > 0, 0 < s < 2 The result for the potential V(x)=-V ₀/x ^1/2 is compared with the recently reported exact solution. The case of a spherically symmetric power-law potential is also considered.     

The temperature dependence of the nuclear quadrupole interaction of 44Ti(EC)44Sc in rutile

The temperature dependence of the Nuclear Quadrupole Interaction on ⁴⁴Sc in rutile was measured by Time Differential Perturbed Angular Correlation in the temperature range from 300 K to 945 K. Whereas $\upomega _{\rm Q} = \mathrm{eQV}_{\rm zz}/4\rm \hbar$ with V_zz denoting the largest component of the electric field gradient tensor in magnitude increases with increasing temperature, the asymmetry parameter η remains essentially constant. This observation fits into the systematic with other probes provided the sign of V_zz is negative.     

Mössbauer effect study of SrEu2Fe2O7

The antiferromagnetic ferrite SrEu₂Fe₂O₇ was studied by Mössbauer effect of⁵⁷Fe in the range 77–650 K. Analysis of the data gives a Neel temperature of 544 K±2 K and an extrapolated effective magnetic fieldH(OK) of 545 kG. Good fit to the spectrum at 290 K is obtained withH _eff=475 kG, δ=0.37 mm/s, 1/2e ² qQ(1+η ²/3)=0.70mm/s, θ and ?=90°. The Brillouin function forS=5/2 poorly fits the experimental data of hyperfine magnetic fields. In the critical rangeH _eff can be described byH(T)=H(0)D(1?T/T _N )^β, where β nearly 0.29. The quadrupole interaction parameter is quite temperature independent, of the order of 0.69 mm/s in the paramagnetic state. Agreement with the value obtained by a point charge calculation is good. The largest axis of the EFG tensor,V _zz , lies in the plane (110), making an angle of 22.6° with thec axis. The direction of magnetization is nearly perpendicular toV _zz in [110] direction.     

Mössbauer study of the magnetic and electric hyperfine interaction of dilute57Fe impurities in rare earth metal hosts

The hyperfine interaction of dilute⁵⁷Fe in the rare earth (RE)metals Gd to Lu was investigated by Mössbauer measurements with⁵⁷Co doped RE sources. In all hosts well split, 2-lines spectra were observed at room temperature, with slight asymmetries of the line intensities in some cases. The quadrupole splitting eQV_zz/2 increases from 0.29 mm/sec for Gd to 0.50 mm/sec for Tb, and decreases by less than 10 % between Tb and Lu. Only about 10 % of the corresponding electric fieldgradient (EFG) can be accounted for by the ionic EFG on a substitutional RE site. The temperature dependence of the EFG was measured in the case of Tb. No variation within 3 percent was found between 300 K and 700 K. Measurements of the magnetic hyperfine interaction at low temperatures were carried out in Tb. The saturation field of⁵⁷Fe in this host is H_hf(FeTb;4.2 K)=25(2) KOe. The temperature dependence of the magnetic hyperfine field does not follow the host magnetization (T_c=220K) but vanishes at about 80 K. Similar anomalies of H_hf(T) have previously been observed for other transition element impurities in the RE ferromagnets.     

The effect of temperature and concentration on the electric field gradient in binary indium alloys

The concentration and temperature dependence of the quadrupole hyperfine interaction of¹¹¹Cd in InTl (hcp),InPb (fct),InTl (fct) andInCd (fct and fcc) alloys were studied using the perturbed angular correlation technique. The change in the observed quadrupole interaction frequency with concentration can be described by a linear dependence on the axial ratioc/a in all cases. In the alloys with identical crystal structures the strength of thec/a dependence is independent of the solute, in contrast to the strength of the concentration dependence. In all cases where no phase transition occurs, the change in the electric field gradient with temperature follows the empirical relationV _zz (T)=V _zz(0) · (1–B·T ^3/2), where the coefficientB depends on the lattice structure, on the solute-solvent combination and on the concentration. The phase transitions ofInCd alloys at 293 K could clearly be seen as discontinuities in the temperature curves. A similar series of discontinuities observed around 116 K suggests the existence of a cubic low temperature phase.     

Hyperfine interactions measurements on 181Ta probes in HfV2Hx compounds: (0⩽x≲4)

The electric field gradient (EFG) has been measured on ¹⁸¹Ta probe sites in HfV₂H_x alloys (0?x?4) using the time differential perturbed angular correlation technique. One observes that the main component V_zz of the EFG decreases by a factor of 2 for hydrogen concentrations increasing from x = 0 to x = 4 hydrogen atoms/formula. The opposite behavior was observed on ⁵¹V by using the NMR technique. At T = 4.2 K the EFG tensor is non-axially symmetric. The observed asymmetry factor for x = 0 is η = 0.58(3) and it slightly increases to η = 0.70 for x?4. The system is orthorhombic at low temperatures for all x and the experiments suggest that hydrogen would stabilize a tetragonal phase at higher temperatures. The relaxation effects are absent for x = 0 and vanishingly small for x ? 4 but they are observed at intermediate concentrations supporting the idea of a hydrogen ordering for x = 4.