Defect study of proton-irradiated liquid-encapsulated Czochralski GaAs using the positron-annihilation technique

The positron lifetime of undoped Liquid-Encapsulated Czochralski (LEC)-GaAs and Si-doped (1.3×10¹⁸ cm^–3) LEC-GaAs was measured before and after irradiation with protons (dose 1×10¹⁵/cm², 15 MeV). In Si-doped GaAs, the decrease of positron lifetime at temperatures between 10 and 300 K are due to the decrease of the positron-diffusion length and the increase of the effective shallow traps such as antisite Ga_As. The annealing stage of the proton-irradiation-induced defects which show the different behavior from that of electron-irradiation-induced defects suggests that proton irradiation creates more complicated defect complexes, containing vacancies rather than isolated vacancy-type defects or simple complexes which have been observed during electron-irradiation processes. Above 700 K, proton-irradiation-induced defects such as vacancy-type defects and simple vacancy complexes are almost annealed out, while Si-induced defects such as Si_Ga-V_Ga complexes cannot be annealed out above 973 K.     

Influence of crystal defects on the irreversibility line of YBa2Cu3O7-x single crystals

High quality flux grown and defect enriched peritectically grown YBa₂Cu₃O_7–x single crystals have been investigated by an ac-susceptibility technique. This method allows to determine an irreversibility line from the temperature and field dependence of the peaked imaginary part of the susceptibility, which is due to magnetic losses. For magnetic fieldsH _ac perpendicular to thec-axis of the crystal, the irreversibility line of the defect enriched crystal shows a shift to higher field values as compared to the perfect crystal, a sign that crystal defects like Y₂BaCuO₅(211)-precipitates and microcracks act as strong pinning centers. ForH _ac parallel to thec-axis no clear evidence for a stronger pinning is found. From these results we conclude that different pinning mechanisms are dominating at different field orientations.     

Low field irreversibility in twinned and untwinned YBaCuO crystals

An abrupt collapse of the irreversibility line is observed in both twinned and untwinned YBaCuO crystals within a critical regime. The exponent of the power law describing the irreversibility line H_irr changes from at high fields to larger values (between 1.5 and 2) below the collapse. We propose that the observed low field anomaly, which extends down to temperatures well below the irreversibility line, is associated with the thermal softening boundary which increases as T² and crosses H_irr shifting it to lower fields. Below this crossover, which occurs when the mean-square thermal displacement √ u² of the vortices exceeds coherence length ξ, the strength of the pinning is strongly reduced.     

A thermodynamic description of the irreversibility line in strongly type-II superconductors

The one-component dense vortex plasma theory, which describes thermal motion of the magnetic vortices in strongly type-II superconductors in a magnetic field, is used to study the irreversibility line in high-T _c superconductors. We propose a possible universal form for the irreversibility line, and examine some recent experiments.     

Positron states and nanoobjects in proton-irradiated quartz single crystals: Positronium atom in quartz

The influence of proton bombardment and metal atom impurities on the structure of quartz single crystals has been studied. The related defects have been studied using positron annihilation spectroscopy (angular correlation of positron-annihilation photons), acoustic absorption, and optical absorption measurements. It is shown that the presence of a narrow component f in the angular distribution of annihilation photons (ADAP), which is related to the formation of parapositronium, determines a high sensitivity of this method with respect to features of the crystal structure of quartz. It is established that the defectness of the structure of irradiated quartz crystals can be characterized by the ratio f/f ₀ of the relative intensities of narrow components in the ADAP curves measured before (f ₀) and after (f) irradiation. Any process leading to a decrease in the probability of positronium formation (e.g., positron loss as a result of the trapping on defects and the interaction with impurity atoms and lattice distortions) decreases the intensity of the narrow component. Based on the ADAP data, estimates of the radii and concentrations of nanodefects in quartz have been obtained and their variation upon annealing at temperatures up to T = 873 K has been studied.     

Why an ac magnetic field shifts the irreversibility line in type-II superconductors

We show that for a thin superconducting strip placed in a transverse dc magnetic field--the typical geometry of experiments with high-T(c) superconductors--the application of a weak ac magnetic field perpendicular to the dc field generates a dc voltage in the strip. This voltage leads to the decay of the critical currents circulating in the strip, and eventually the equilibrium state of the superconductor is established. This relaxation is not due to thermally activated flux creep but to the "walking" motion of vortices in the two-dimensional critical state of the strip with in-plane ac field. Our theory explains the shaking effect that was used for detecting phase transitions of the vortex lattice in superconductors.     

Defect centers in antimony doped NaCl crystals

The existence of both Sb²⁺ and Sb³⁺ in NaCl has been established by a series of correlated experiments like optical absorption, EPR, Electrical conductivity and dielectric loss. Sb²⁺ gives an optical band at 212 nm and in an arbitraryorientation gives an EPR signal whose direction dependence studies indicate three magnetically inequivalent sites. A vacancy occupying a second neighbour site with respect to the impurity ion satisfactorily, explains the observed spectra. The association energy of the impurity ion has been found to be 0·56 eV from ionic conductivity studies. Dielectric loss measurements give two prominent loss peaks which have been assigned to Sb²⁺ and Sb³⁺ on the basis of their temperature variation and other correlated experiments. Radiation damage reduces the Sb³⁺ content while Sb²⁺ is not affected.     

Resistivity,fluctuation-enhanced conductivity,and irreversibility line of high-quality Tl2Ba2Ca2Cu3O10 single crystals

We present measurements of the resistivity tensor components ρ_a (T) and ρ_c (T) of high-quality Tl₂Ba₂Ca₂Cu₃O₁₀ single crystals wit T_c = 118 … 121 K. The in-plane resistivity ρ_a as well as the out-of-plane resistivity ρ_c show a metal-like temperature dependence with an anisotropy ratio ρ_c/ρ_a of up to 10³. The coherence length ξ_c (0) = 0.15 nm could be determined from an analysis of the fluctuation conductivity above T_c. From measurements of the ac-susceptibility in magnetic fields up to 10 Tesla the temperature dependence of the depinning line could be obtained. The diffusion coefficient of the flux lines obtained from the frequency dependence of the depinning temperature shows a thermally activated behaviour of the flux motion in the field range 0.1 T ? B ? 10 T.