Anomalous temperature dependence of the 1.7T magnetic hyperfine field of fluorine in nickel

By means of time-differential perturbed angular distribution (TDPAD) techniques, the magnetic hyperfine field ofF atoms implanted in a ferromagnetic Ni lattice has been measured at temperatures from 80 K to 650 K obtaining, for example, a fieldH ₁(295 K)=+1.72(3)T at room temperature. The temperature dependence of the reduced fieldH ₁(T)/H ₁(0) has been found to deviate up to 38% from that of the reduced magnetization of the Ni host. Comparing this result for the halogenF with other nontransitional impurities in nickel, it seems that such deviations become more pronounced with increasing number of excessp-electrons of the impurity. With a simple two-zone model distinguishing between the hyperfine fields originating in the local and in the more distant undisturbed surrounding of theF impurity, the observed temperature dependence can be explained. In this approach the disturbed magnetization of the inner zone is described by localized Ni moments in the molecular field approximation.A satellite magnetic field of + 9.4T was observed at 80 K only.Supported by the Bundesministerium für Forschung und Technologie     

Measurement of the spin-depairing interaction in Sn0.75Eu0.25Mo6S8

Mössbauer effect measurements of the ¹⁵¹ Eu resonance in the Chevrel phase superconductor Sn_0.75Eu_0.25Mo₆S₈ have been used to obtain the temperature dependence of the Eu paramagnetic relaxation rate. This consists of a temperature independent part arising from spin-spin interactions and a part linear in T due to the Korringa process. From the slope we obtain $\text{I}$ = 0.0033/Eu atom-spin, where $\text{I}$ is the exchange coupling between the rare-earth spin and the conduction electron spin, and N(E_F) is the local density of states. This value is roughly one order of magnetude lower than that measured in binary superconductors, and accounts for the very weak dependence of the transition temperature on magnetic impurity concentration.     

Infrared absorption on impurity excitations near the upper edge of spin-wave band of the antiferromagnet

The dependence of infrared (IR) light absorption on external magnetic field is considered when the impurity level approaches the upper edge of spin-wave band of antiferromagnet. It is explained a strongly nonlinear field dependence and rapid decrease of corresponding line intensity been observed experimentally in Fe_1?cCo_cF₂. Such behaviour is due to existence of some subthreshold range near the band edge and corresponds to the incoherent collective rearrangement (ICR) of the system spectrum.     

Experimental studies of electrical conduction mechanism of H2-reduced BaTiO3

ESR, resistivity and Seeback coefficient measurements have been performed on both ceramics and single crystals of reduced semiconducting BaTiO₃.From the results that the observed temperature dependence of the Seeback coefficient can be explained by the temperature dependence of carrier concentration estimated from the electric resistivity making use of the data of electron mobility, it is concluded that the electric conduction in reduced BaTiO₃ is due to the band conduction rather than to the hopping process. From the measurement of the temperature dependence of the ESR intensity of the F-center, the number of electrons trapped at the F-centers decreases exponentially with temperature, while the number of conduction electrons increases. This temperature dependence can not be simply explained as that of the unionized donors in semiconductor. Therefore, the ESR signal considered as that of the F-center may not be due to simple donors, even though some of the conduction electrons may be originated in them.     

Screening effects in two-dimensional electron gas

The potential produced by a charged impurity at the interface of a highly doped GaAlAs and GaAs is calculated at a finite temperature. The electron gas formed at the interface is described as a two dimensional gas in which the impurity is assumed to be dipped. Temperature dependence of the impurity potential is calculated in the random phase approximation (R.P.A.) as well as in the modified temperature dependent Thomas-Fermi (M.T.T.F.) approximation which is defined to include temperature effects and to reduce to Thomas-Fermi result at zero temperature. The binding energy of the impurity for the ground state is calculated in R.P.A. and in M.T.T.F.. It is shown that at temperature T, much larger than the Fermi temperature, T_F, M.T.T.F. gives binding energies close to R.P.A. results.     

Superconducting and normal state properties of dilute alloys of LaSn3 containing Ce impurities

The dependence of the superconducting transition temperature T_c on Ce impurity concentration and the specific heat jump at T_c as a function of T_c are reported for the system ($\text{La}$Ce)Sn₃. The experimental results are analyzed using a theory due to Kaiser concerning the effect on superconductivity of nonmagnetic localized resonant impurity states This analysis yields values for the intraatomic Coulomb repulsion parameter U_eff and the Ce local density of states at the Fermi level N_? (E_F). The results of low temperature normal state heat capacity and magnetic susceptibility measurements which give independent estimates of N_? (E_F) are also reported. A large pressure dependence of the T_c of ($\text{La}$Ce)Sn₃ alloys was observed for pressures up to 20 kbar. This behavior is similar to that previously observed in several other superconducting matrix-Ce impurity systems in which the Ce solute 4f electron shell undergoes a continuous-pressure induced demagnetization.     

Untersuchung gestörter Winkelverteilungen an F19 in ferromagnetischen Gittern

The perturbation of the 197 keV transition angular distribution in F¹⁹ was investigated by time-dependent spin rotation measurements following excitation with a pulsed beam. The recoil implantation technique was used to determine the internal magnetic fields for F¹⁹ in Fe, Co and Ni lattices. The results are:H _HF(F¹⁹ in Fe)=+(95700±500) Oe,H _HF(F¹⁹ in Co)=+(59500±1500) Oe,H _HF(F¹⁹ in Ni)=?(21830±350)Oe. The temperature and field dependence of the effective fields was studied. Strong satellite fields due to lattice perturbations were detected. The half life and the gyromagnetic ratio of the 197 keV 5/2⁺ state in F¹⁹ were redetermined asT _1/2=(80.2±0.5) nsec andg=+1.436±0.007.     

Abnormal electronic transport in disordered graphene nanoribbon

We investigate the conductivity σ of graphene nanoribbons with zigzag edges as a function of Fermi energy E_F in the presence of the impurities with different potential range. The dependence of σ(E_F) displays four different types of behavior, classified to different regimes of length scales decided by the impurity potential range and its density. Particularly, low density of long range impurities results in an extremely low conductance compared to the ballistic value, a linear dependence of σ(E_F) and a wide dip near the Dirac point, due to the special properties of long range potential and edge states. These behaviors agree well with the results from a recent experiment by Miao et al. [Science 317 (2007) 1530 (SOM)].     

Magneto-optical properties of Dy<Superscript>3+</Superscript> in oxide glasses: The origin of the magneto-optical activity of <Emphasis Type="Italic">f-f</Emphasis> transitions and its anomalous temperature dependence

The temperature dependence of the absorption spectra and magnetic circular dichroism due to f-f transitions from the ⁶ H _15/2 to ⁶ F _5/2 and ⁶(F _7/2 + H _5/2) states in the Dy³⁺ ion in (Dy₂O₃-P₂O₅-SiO₂-GeO₂) and (Dy₂O₃-La₂O₃-Al₂O₃-B₂O₃-SiO₂-GeO₂) glasses and the temperature dependence of the Faraday effect were studied. The temperature dependence of the Faraday effect caused by f-d transitions was found to differ from that of the magnetic circular dichroism due to f-f transitions. It was shown that f-f transitions occur preferentially in Dy³⁺ ions associated into clusters. The origin of the paramagnetic magneto-optical activity of f-f transitions was analyzed. It was shown that the contributions to this activity can differ in value and sign and that the ratio between these contributions depends on the transition type. In some cases, this difference results in an anomalous temperature dependence of the magneto-optical activity.     

Thermal dependence of impurity induced fluorescence in the system RbMgxMn1-xF3

The absorption spectrum of RbMnF₃ and the excitation spectra of the system RbMg_xMn_1-xF₃ at 10 K as well as the fluorescence spectra and lifetimes of Mn²⁺ in the systems RbMg_xMn_1-xF₃ and KMg_xMn_1-xF₃ in the region 10–300 K were measured. The lifetime and fluorescence temperature dependence suggest that the origin of the fluorescence occurs at Mn²⁺ sites slightly perturbed by impurity ions and that a non-radiative energy transfer mechanism is responsible for the observed thermal quenching. By using different Mn²⁺ concentrations in the above systems the dependence of the energy transfer on the Mn²⁺ concentration is shown. Finally, a preliminary observation on laser stimulated Mn²⁺ luminescence in the system RbMg_xMn_1-xF₃ is reported.     

Ground state properties of the two impurity Anderson model in a 1/N expansion

The intersite contribution to the ground state energy for the two impurity Anderson model is calculated to first order in 1/N. The dependence of this contribution on the interimpurity separation has an R^?1 or R^?3 envelope for distances smaller or larger than a length ν_F/T_A ranges from a value equal to the bare ?-level position (in the weak valent regime) to the Kondo temperature (in the local moment limit).     

Enhancement of superconductivity in disordered films by parallel magnetic field

We show that the superconducting transition temperature T _c (H) of a very thin highly disordered film with strong spin-orbital scattering can be increased by a parallel magnetic field H. This effect is due to the polarization of magnetic impurity spins, which reduces the full exchange scattering rate of electrons; the largest effect is predicted for spin-1/2 impurities. Moreover, for some range of magnetic impurity concentrations, the phenomenon of superconductivity induced by magnetic field is predicted: the superconducting transition temperature T _c (H) is found to be nonzero in the range of magnetic fields 0 < H* ≤ H ≤ H _c .     

Generation of color centers when annealing LiF crystals irradiated by Kr ions with an energy of 150 MeV

The effect thermal annealing has on F and F _n centers in LiF crystals irradiated by Kr ions with an energy of 150 MeV is studied with allowance for fluence and ionic currents. It is found that annealing at a temperature of 400 K using crystals irradiated at a fluence of ≥10¹³ iom/cm² reduces the concentration of F centers (due to annihilation with H centers) and raises the concentration of complex F _n centers.     

Temperature dependence of the lower critical field Hc1 in SmFeASO0.9F0.1 and Ba0.6K0.4Fe2As2 iron-arsenide superconductors

We report detailed measurements of the temperature dependence of the lower critical field H_c1 of the FeAs-based superconductor SmFeAsO_0.9F_0.1 (Sm-1111) and Ba_0.6K_{0. 4}Fe₂As₂ (BaK-122) by global and local magnetization measurements. It is found that the obtained H_c1 for both kinds of samples show a weak temperature dependence in low temperature region. We argue that this weak T-dependence of H_c1 of Sm-1111 does not indicate a conventional s-wave state, instead it satisfies a T² dependence up to a temperature of (0.5–0.6) T_c. In contrast, the data of H_c1(T) of BaK-122 superconductor clearly show a multigap behavior. Excellent fitting to the data can be reached with two s-wave superconducting gaps. Comparison of the absolute values of H_c1(0) between Sm-1111 and BaK-122 shows a relatively large superfluid density for the latter. This may indicate the distinction between the electron doped and hole-doped FeAs-based superconductors.     

Anderson localization and superconducting transition temperature in two-dimensional systems

Effects of the Anderson localization on superconducting transition temperature T_c are examined by calculating a two-electron propagator K rigorously up to 0[(?_Fτ₀)^?1ln(Tτ₀)], where τ₀ is the electron life time due to impurity scattering and ?_F the Fermi energy. The results show that in K the pair-breaking terms cancel out among themselves exactly and the remaining terms which contribute to the correction to the density of states and to the renormalization of electron-electron interaction by impurity scattering lead to the changes in T_c of 0[{ln(Tτ₀)}²] and of 0[{ln(Tτ₀)}³], respectively.     

Ion beam induced luminescence studies of LiAlO2 using negative ions

Lithium aluminate (LiAlO₂) is the candidate material for solid tritium breeder applied in the developing fusion reactors. The research of its defect behavior under ion irradiation was proceeded in the negative ions induced luminescence setup of the GIC4117 Tandem accelerator in Beijing Normal University. The ion beam induced luminescence (IBIL) measurement was performed by 20 keV H⁻ ions at room temperature. The luminescence spectra showed seven emission bands: the 4.55 eV may due to a self-trapped exciton (STE), the 4.06 eV and the 1.72 eV may due to impurity or intrinsic defect, the 3.54 eV due to F center, the 3.20 eV due to F⁺ center, the 2.93 eV due to F₂ center, the 2.30 eV due to F-center aggregates (F_n center), respectively. The intensity evolutions of each band with fluence were presented and the corresponding mechanisms were discussed.     

Effect of pressure on the distribution of hyperfine field about A Sn impurity in iron

The distribution of hyperfine field at the iron atoms in an FeSn alloy has been measured as a function of pressure to 15 kbar. The frequency of the main NMR line was found to change such that (? ln H_m/?P)_T = ? 1.7 × 10^?4 kbar^?1, which is close to the value in pure iron. The value for the satellite line, due to iron atoms which are third nearest neighbours of the impurity, was (? ln H₃/?P)_T = ? 2.4 × 10^?4 kbar^?1. The magnitude of the difference of the pressure derivatives of H_m and H₃ is not consistent with models in which the Sn atom is screened by conduction electrons but may be understood in terms of a perturbation of the 3d band of iron. The discrepancy between spin wave theory and the measured temperature dependence of the hyperfine fields at constant pressure is not removed by correction to constant volume.     

Isotope and temperature effects on nuclear magnetic shieldings and spin-rotation constants calculated at the coupled-cluster level

The temperature dependence of nuclear shieldings as well as isotope effects on shieldings and spin-rotation constants have been computationally investigated for H₂, HF, F₂, CO, and N₂ employing the coupled-cluster singles and doubles (CCSD) method augmented by a perturbative treatment of triple excitations (CCSD(T)) for the calculation of potential curves, shieldings and spin–rotation functions together with finite-element techniques for the solution of the rovibrational problem. Calculated and measured temperature dependence of the isotropic shieldings agrees for N₂, while for CO and F₂ the computed temperature dependence is smaller than the experimental result. Isotropic shieldings have been deduced on the basis of our calculations from the measured spin-rotation constants for four isotopomers of H₂ and agree, as required by theory. However, calculated and measured temperature dependence of the isotope shifts between HD and D₂ differ by up to 10% which is larger than the estimated error bars for the experimental values. For HF and CO, calculated and measured isotope shifts agree, while for N₂ no experimental data for comparison are available. In case of spin–rotation constants, the calculated dependence on the rotational angular momentum quantum number are for both H₂ and F₂ in good agreement with the dependence deduced from measurements, while for HF not enough experimental data are available for a comparison.     

Superconductivity of (Sn1−z Pbz)1−x InxTe alloys

The temperature dependences of the electrical resistivity of (Sn_1?z Pb_z)_1?x In_xTe alloys with different concentrations of lead (z=0–0.60) and indium (x=0.03–0.20) were studied at temperatures T=0.4–4.2 K in magnetic fields from zero to H=15 kOe. A resistivity drop of no less than three-four orders of magnitude was observed in this range of alloy compositions. Application of a magnetic field above a critical level resulted in a recovery of the sample resistivity to the original value. The observed resistivity drop is identified with a superconducting transition. The critical parameters of the superconducting transition (T _c and H _c2) were determined at the drop to one half the normal resistivity level. Experimental dependences of the critical supercon-ducting-transition temperature T _c and of the second critical magnetic field H _c2 on the contents of lead (z) and indium (x) were measured. The data obtained confirm a strong localization of the In impurity states and are evidence of the extrinsic nature of superconductivity in the class of materials under study. It was established that as the Pb content in (Sn_1?z Pb_z)_1?x In_xTe increases, T _c and H _c2 decrease as the Fermi level E _F (fixed in the In impurity resonance band) leaves the Δ extremum and the superconductivity breaks down when E _F leaves the LΣ saddle point in the valence-band energy spectrum.