Energy gap,transition temperature and electron-electron interaction of the weak coupling superconductors tin and indium as a function of the mean free path of the electrons

The energy gap and the transition temperature of the weak coupling superconductors tin and indium are measured as a function of the mean free path of the electrons. Both of them increase with decreasing mean free path. The ratioα=2Δ ₀/kT_c increases proportionally to the reciprocal mean free path from 3.6 to 4.0 for tin and from 3.6 to 3.95 for indium. A theoretical consideration is presented showing that the electron-electron interaction is increased in dirty superconductors. Without any fit we find from the theoretical calculation an increase inNV for indium ofδNV=5 · 10^?10 m/l _tr and for tinδNV=2 · 10^?10 m/l _tr (l_tr is the transport mean free path of the electrons). The experiments give an increase inNV for indium ofδNV=4.2 · 10^?10 m/l_tr and for tinδNV=1.7 · 10^?10 m/l _tr. The agreement is surprisingly good.     

Radiative decay and autoionization in the4 D-states of the 3d 94s5s configuration in CuI

The CuI 3d ⁹4s5s ⁴ D-states, which lie above the ionization limit, were populated by stepwise collisional and laser excitation. With respect to their total angular momentum, the⁴ D-states have completely different decay modes. Whereas the lifetimesτ(⁴ D _7/2)=8.2(4)·10^?9s andτ(⁴ D _1/2)=7.9(6)·10^?9 s are determinrd by radiative transitions and were measured by time-resolved spectroscopy, the lifetimesτ(⁴ D _5/2)=1.59(6)·10^?12s andτ(⁴ D _3/2)=1.28(5)·10^?12s are due to autoionization and were deduced from the widths of the excitation signals observed by ion detection.     

Nuclear magnetic relaxation by self-diffusion in solid lithium:T 1-frequency dependence

We report on measurements of the⁷Li nuclear spin relaxation timeT ₁ in solid lithium as a function of temperature (?170°C≦T≦+180°C) and Larmor frequency (450kHz≦v _Li≦31.5 MHz). Using a relaxation model developed by Wolf and Cavelius and combining it with Seeger's diffusion formalism, the diffusion parameters for mono-and divacancy migration were evaluated by a least squares fit to the newly obtainedT ₁ data as well as to previousT _1? measurements. The result for the self-diffusion coefficientD ^SD is given byD ^SD=D ₁₀·exp(?Q ₁/RT)·[1+D ₂₁·exp(?Q ₂₁/RT)], withD ₁₀=0.038 cm²s^?1,Q ₁=12.0 kcal mol^?1,D ₂₁=250,Q ₂₁=4 kcal mol^?1 andR=1.98₅·10^?3 kcal mol^?1 degree^?1. Due to the flexibility of Seeger's formula, which contrasts with the standard Arrhenius interpretation of diffusion, discrepancies between earlier high- and low-frequency NMR investigations were eliminated. Furthermore, an excellent agreement with available results from tracer experiments was achieved by taking into account the theoretical predictions of the isotope effect and the vacancy correlation factor.     

Lifetimes of charged and neutralD mesons

We have measured the lifetimes of hadronically produced charged and neutralD mesons using silicon microstrip detectors and an active silicon target in the NA32 spectrometer at the CERN SPS. We obtainτ _D± = (10.9± _1.5 ^1.9 )·10^?13s andτ _D ^(?)10 = (4.2±0.5)·10^?13s based on 59 and 90 fully reconstructed decays respectively, giving a ratioτ _D±/τ _D ^(?)10 of 2.6 ±0.5.     

Dipolar studies in CaF2 with Ce3+

Dielectric relaxation in CaF₂ doped with various amounts of Ce³⁺ (0·01 to 1·0 mol%) was measured. The value of the activation energy for orientation of the dipoles {Ce³⁺-F^? interstitial} was determined to be H = (0·46 ± 0·01) eV. The frequency factor was found to have the value τ_o = (5 ± 1) × 10^?15 sec, giving for the vibrational frequency of the interstitial the value ν_o = (5 ± 1) × 10¹³ sec^?1.The number of dipoles contributing to the dielectric loss peak was determined to be between 10¹⁷ and 8 × 10¹⁷ cm^?3 for the different doping amounts of Ce³⁺. Optical absorption measurements showed the existence of large aggregate bands. We could verify that there exists a second-order reaction of aggregation, which is responsible for the non-linearity found between optical absorption at 305 nm and the nominal concentration of Ce³⁺ in the samples. On the other hand, if we assume that the centers which contribute to optical absorption at 305 nm are those also responsible for the relaxation peak, we find that the number contributing to each process is not the same. We can define an interaction radius R as the minimum separation between two dipoles allowing them to contribute to the relaxation peak. From our experimental data R ? 3·8 × 10^?7 cm.     

Stochastic model of angular distributions of fragments originating from the fission of excited compound nuclei

The anisotropy of angular distributions of fission fragments and the average multiplicity of prescission neutrons were calculated within a stochastic approach to fission dynamics on the basis of three-dimensional Langevin equations. This approach was combined with a Monte Carlo algorithm for the degree of freedom K (projection of the total angular momentum I onto the fission axis). The relaxation time τ _K in the coordinate K was considered as a free parameter of the model; it was estimated on the basis of a fit to experimental data on the anisotropy of angular distributions. Specifically, the relaxation time τ _K was estimated at 2 × 10^?21 s for the compound nuclei ²²⁴Th and ²²⁵Pa and at 4 × 10^?21 s for the heavier nuclei ²⁴⁸Cf, ²⁵⁴Fm, and ²⁶⁴Rf. The potential energy was calculated on the basis of the liquid-drop model with allowance for finiteness of the range of nuclear forces and for the diffuseness of the nuclear surface. A modified one-body viscosity mechanism featuring a coefficient k _s that takes into account the reduction of the contribution from the wall formula was used to describe collective-energy dissipation. The coefficient k _s was also treated as a free parameter and was estimated at 0.5 on the basis of a fit to experimental data on the average prescission multiplicity of neutrons.     

Investigation of the decay of charmed particles produced in neutrino interactions

The decay of charmed particles produced by high-energy neutrinos has been studied by an experiment using simultaneously emulsion, bubble chamber and counter techniques. Eight charmed particle candidates, 5 positively charged and 3 neutral, have been found in the emulsion, where their production and decay have been directly observed. One of these events is identified as a Λ_c⁺ baryon of mass 2.26±0.02 GeV/c² which undergoes the decay Λ_c⁺→pK^?π⁺ after a proper time of (7.3±0.1) · 10^?13 s.A statistical analysis of the other observed decays leads to the mean-life values τ⁺=(2.5_?1.1^+2.2) · 10^?13 s, τ⁰=(0.53_?0.25^+0.57) · 10^?13 s, for the sample of charged particles enriched by a similar event found in a previous experiment, and for the sample of 3 neutral particles, respectively. The former value is only slightly affected by including in the sample the Λ_c⁺ event or excluding that of the previous experiment.     

Fluorescence lifetimes and radiative rate constants of alkanes at 25°C

Fluorescence lifetimes of 33 alkanes were measured at 25°C using a 0.7 ns pulse radiolysis system and the relationship between molecular structure, fluorescence lifetime (τ), quantum yield (Φ_f) and radiative rate constant (R = $\text{Φ}{}_{\mathrm{f}}\text{τ}$) is discussed. The τ's showed a large dependence on the molecular structure, however, the R-values were approximately constant for groups of alkanes: R = 1.3×10⁶s^?1 for n-alkanes, R ≈ 4×10⁶ s^?1 for aliphatic vicinally substituted dimethyl alkanes, R = 7×10⁶s^-1 for cyclohexane and alkylcyclohexanes, and R = 10×10⁶s^?1 for di- and trimethylcyclohexanes. The excited state lifetimes of some non-fluorescing alkanes were also determined from steady-state photolytic quenching data and found to be very low (? 0.3 ns).     

Optical pumping of ions in buffer gases

Transient signals measured with a pulsed rf-optical pumping method are used to determine longitudinal relaxation rates for Sr⁺ ions (even isotopes) in noble gas buffers. Depolarization cross sections of the electronic spin in the Sr⁺5² S _1/2 ground state for binary collisions with rare gas atoms are deduced. The results for σ(Sr⁺5² S _1/2) in Ų are (at temperatures between 374 and 449 °K): 2·10^?5(He),4·10^?5(Ne), 5.7·10^?3(Ar), 1.8·10^?2(Kr), and 4.0·10^?2(Xe). These cross sections for the Sr⁺ ion are about two to three orders of magnitude larger than the corresponding ones for the isoelectronic neutral Rb atom. The large increase of the Sr⁺ relaxation rates is explained with the relaxation mechanism of spin-orbit coupling, taking into account two “indirect” effects of the ionic charge: the increase in the gas kinetic cross sections and the more intimate collisions of the Sr⁺ ion with the noble gas atoms. The depolarization is shown to be predominantly due to short-range interactions. A contribution to the relaxation of the Sr⁺ ion from Sr⁺-noble gas molecule formation, induced by three-body or resonant two-body collisions, could not be established for applied pressuresp between 1.5 and 15 Torr of Ar, Kr, and Xe.     

Relaxation des Natrium-Grundzustandes bei Stößen mit den stabilen Helium- und Wasserstoff-Isotopen

The spin-polarization of the optically pumped Na-groundstate is investigated in presence of the stable He- and H₂-isotopes. The following disorientation cross sections are derived from the pressure dependence of the relaxation rate:σ=(16.2±2.0) · 10^?26 cm² for He³ σ=(2.4±0.5) · 10^?26 cm² for He⁴ σ=(3.9±1.5) · 10^?26 cm² for H₂ σ=(2.3±1.0) · 10^?26 cm² for D₂. These values can be compared with theoretical cross sections based on two relaxation models and indicate the existence of a relaxation mechanism involving the exchange of the electronic alkali-spin with the nuclear spin of the foreign gas.     

Self-diffusion in a series of plastic organic solids studied by NMR

Proton nuclear magnetic resonance relaxation times and linewidth measurements have been made on five polycrystalline organic compounds, triethylenediamine, 3-azabicyclononane, norbornane, norbornylene and norbornadiene. Measurements for each sample were made throughout the plastic crystal phase. The results are analysed in terms of molecular motion. Correlation times τ and activation enthalpies for translational self-diffusion of molecules are evaluated: triethylenediamine τ=7·6×10^?19 exp (96·4/RT)s, 3-azabicyclononane τ=1·7×10^?16 exp (83·6/RT)s, norbornane for 131K<T<306 K, τ=4·6×10^?15 exp (54·5/RT)s for 306K<T<360K, τ=1·1×10^?16 exp (64·8/RT)s, norbornylene, τ=4·×10^?15 exp (48·6/RT)s and norbornadiene τ=6·8×10^?15 exp (39·9/RT)s, where R is the gas constant in units of kJ K^?1mol^?1. The results and mechanism of diffusion are discussed in relation to the thermodynamic properties of the materials.     

The approach to equilibrium in a pure superconductor the relaxation of the Cooper pair density

Electron-phonon and electron-electron collisions are the processes which determine the relaxation timeτ _R of the Cooper pair density. The case is considered for which the deviation of the pair density from equilibrium is small and where the equilibrium state is homogeneous. Starting from the Eliashberg equation, one is able to reduce the problem to a quadrature once the equilibrium Green functions are known. If the deviation is constant in space, andT/T _c?0.5, we find \(\tau _R \approx \sqrt {T_c /(T_c - T)} \tau _c\) , whereτ _c is the collision time (≈10^?8 sec). For a deviation modulated in space,τ _R very quickly reaches the limiting valueτ _c.     

The measurements of correlation times of molecules adsorbed on silica gel,alumina and charcoal using NOESY and MAS NMR

The correlation times (τ_c) and cross relaxation rates of toluene, dimethylformamide, tetrahydrofuran, water-acetone and water-dioxane adsorbed on silica gel, alumina and charcoal were obtained by measurements of the integrated intensities of cross and diagonal peaks in their NOESY spectra. The (τ_c) of the above mentioned systems is in the range of 10^?6?10^?9 s, much longer than that in the liquid (10^?12?10^?14 s). It was found that intramolecular rotation of toluene adsorbed on charcoal is slower than that on SiO₂, Al₂O₃ and the inversion of α and β protons in tetrahydrofuran is very fast (τ_c = 1.76×10^?9 s). The cross relaxation plays an important role in the relaxation of molecules adsorbed on solid surfaces with low electron densities.     

Bestimmung von Oszillatorenstärken durch Lebensdauermessungen der ersten angeregten Niveaus für die Elemente Ba,Sr, Ca,In und Na

The lifetimes of the first excited state of the elements Ca, Sr, Ba, In and Na were measured with the method ofOsberghaus ¹. Oscillator strengthes (f) of resonance lines were computed from the measured lifetimes (τ). The measurements were corrected for imprisonment of radiation in the investigated vapor. The error of the τ-values was estimated to be approximately three percent. We got following resultslifetimes for Ca 4¹ P ₁ (4·67±0·11)·10^?9sec, for Sr 5¹ p ₁ (4·56±0·21)·10^?9 sec, for Ba 6¹ p ₁ (8·36±0·25)·10^?9sec, for In 6² S _1/2 (8·531±0·085)·10^?9sec and for Na 3² P-dublett (1·59±0·039)·10^?8sec. For the Na 3² P-dublett the dependency of the lifetime on the pressure of foreign gases (N₂, C₃H₈, C₆H₅?CH₃) was studied.     

Dielectric relaxation of KBr doped with KOH and KOD

The complex dielectric constant of KBr single crystals doped with KOH and with KOD has been measured in the temperature range from 0.32°K to 300°K using a small a. c. signal. A relaxation timeτ ₀ was determined by means of Cole-Cole plots. ForT<4°K the temperature dependence ofτ ₀ can be approximated byAT ^?n , wheren is between 1.2 and 1.3 for the dilute samples, and between 0.8 and 1.0 for two samples with large hydroxyl concentrations.n has the same value for OH^? and OD^? dipoles. The constantA roughly doubles upon substitution ofH byD. The relaxation behavior was found to be independent of concentration in the range between 3×10¹⁸ cm^?3 dipoles and 10¹⁹ dipoles cm^?3.     

Longitudinal ultrasonic attenuation in the spin spiral state of single crystal holmium

Ultrasonic attenuation experiments have been performed using 15–255 MHz longitudinal sound waves along the c-axis of single crystal holmium. Without an external magnetic field, the attenuation increases in the spin-spiral state. This anomalous increase originates, at least in part, from the spin-phonon relaxation mechanism we have proposed, which states that the attenuation coefficient Δα₁ ∞ ω²/(1+ω²τ²) where τ is the longitudinal spin phonon relaxation time. The frequency dependence of the attenuation varies from ω^1·0 to ω^1·5 which suggests a frequency-dependent character for the spin-phonon relaxation time τ. A broad longitudinal attenuation maximum, which presumably is due to the competing processes between $\overline{S}$, J and τ, is observed in the spin-spiral state, where $\overline{S}$ is the thermal average of the spin angular momentum per trivalent ion and J the Fourier transform of the exchange integral. An anomalous suppression of the longitudinal attenuation spike at T_N for frequencies higher than 165 MHz is unexplainable at present. The longitudinal attenuation spike at T_N for frequencies higher than 165 MHz is unexplainable at present. The longitudinal spin-phonon relaxation time for Ho has been determined using experimental data. It has a T^?3 temperature dependence. In the presence of an external magnetic filed in the basal plane the attenuation is decreased and a new peak and a plateau appear at the intermediate phase transitions.     

Search for heavy quasistable leptons

Negative results of a search for heavy quasistable charged leptons at the IHEP accelerator are reported. For the selection of heavy leptons a system of scintillation and gas ?erenkov counters was used. The upper limit estimations of the differential cross sections for heavy lepton production in pN collisions at E_p = 70 GeV are, for example, d²σ/dpω (p = 30 GeV/c, θ = 2 mrad, 1 ? M_λ ? 4.8 GeV) = 4.10^?38 cm²/sr · GeV. The results of the present work, together with the data on muon pair production in nucleon-nucleon interactions, show that there are no heavy charged leptons with masses from 0.55 GeV (τ_λ > 7 · 10^?10 sec) up to 4.5 GeV (τ_λ > 3 · 10^?8 sec).     

Multivariate pressure effects on an electron hopping process in ferroelectric KTa<Subscript>1−x</Subscript>Nb<Subscript>x</Subscript>O<Subscript>3</Subscript>

The effect induced by the presence of a polaron related relaxation process on the dielectric properties of a ferroelectric KTa_1?x Nb _x O₃ (KTN) crystal was investigated (10^-2?10⁶ Hz, at 300?375 K) using broadband dielectric spectroscopy. Characterization of the process using just the standard frequency domain dielectric parameters can nonetheless provide penetrating insight into its nature and origins. The three parameters, namely: relaxation time (τ), Cole-Cole loss broadening (α), and dielectric strength (Δ?) provide each one in its own way, much useful and often overlooked information. The Activation Energy along with the Meyer-Neldel dependance, both extracted from τ serve to illuminate the dynamic properties. At the same time, α and especially the combined α(lnτ) relationship, expose the fractal structure of the underlying landscape. Finally, the static parameter Δ?, enables quantification of the dipolar correlations. Hydrostatic pressure (up to 7.5 kbar) was applied to gently perturb the system and observe the outcome on all of the various parameters. This additional degree of freedom allows for a much more comprehensive exploration of the phase space behavior of the system.     

Derg R -Faktor des 2+-Rotationsniveaus von Er168

The rotation of the angular correlation between theγ-group at about 820 keV and the 80 keV radiation in the decay of Tm¹⁶⁸ has been observed in an external magnetic field of 20300 gauss. The result:ω _R·τ=0,485±0,051 yields for theg _R -factor of the 80 keV state of Er¹⁶⁸:g _R =+0,25±0,03. The evaluation includes a paramagnetic correction factor of:β=7,26, (B _eff=β·B _ext). The angular correlation is slightly attenuated by internal fields. For a liquid source of Tm (NO₃)₃ solved in 3 n HNO₃, a measurement of the differential angular correlation as a function of the delay time gave:λ ₂=(5,8±2,9)·10⁷sec^?1, assumingA ₂(t)=A ₂(0)·e ^?λ ₂·t. The half life of the 80 keV state was found to be:T _1/2=(1,92±0,04)·10^?9sec in agreement with earlier measurements.     

Investigation of the decay rate for71Ge in chemical compounds and calibration of the Mössbauer isomer shift of the 67 keV γ-transition of73Ge

Changes of the radioactive decay rate of⁷¹Ge (T _1/2=11.43 days) have been studied experimentally for⁷¹Ge in some compounds of bivalent and quadrivalent germanium. Relative changes Δλ/λ of the electron capture probability have been measured, and the chemical changes Δρ(0) of the electron density at the germanium nuclei have been determined from these measurements. Values for the Mössbauer isomer shift calibration constants for the 67 keV γ-transition of⁷³Ge have been estimated:C=δ/Δρ(0)=(0.061±0.020) mm·s^?1/a.u., Δ〈r ²〉=(21±7)·10^?3 fm², ΔR/R=(6.9±2.3)·10^?4.