Mean residence time of potassium ions on tungsten as measured with reference to ionization efficiency

The mean residence time, τ_i, of potassium ions on “clean” and oxygenated tungsten has been measured, together with the ionization efficiency, as a function of surface temperature T by using incident K beams of low intensity (10⁹–10¹² atoms cm^?2 s^?1). For T higher than ~900 K the observed τ_i followed Frenkel's equation $\text{τ}{}_{\mathrm{<mtext>i</mtext>}}\text{= τ}{}_{\mathrm{<mtext>i</mtext>}}{}^0\text{exp}\text{(}\text{Q}{}_{\mathrm{<mtext>i</mtext>}}\text{kT}\text{)}$ as usual and the agreement of the ionic desorption energy Q_i and of the pre-exponential factor τ_i⁰ with the corresponding values of previous experiments was quite satisfactory. Below 830 ~ 910 K, where a steep drop of ionization efficiency began to be noticeable, Arrhenius plots of τ_i deviated considerably from linearity. The apparent increment of the desorption energy was shown to be nearly equal to the decrement of thermionic work function of tungsten as obtained from the ionization efficiency and Saha-Langmuir equation. The increase of surface coverage by potassium was accordingly taken as the main cause of the departure of Arrhenius plots from linearity. Under certain conditions of incident beam intensity and surface temperature τ_i was observed to make an abrupt change from a higher to a lower value — a difference expressed as 100–140 meV in terms of the difference in ionic desorption energy. This peculiar phenomenon was attributed to the phase change of adsorbed potassium on tungsten.     

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.     

Formation of Fe i –B pairs in silicon at high temperatures

We report on the detection of Fe _i –B pairs in heavily B doped silicon using ⁵⁷Fe emission Mössbauer spectroscopy following implantation of radioactive ⁵⁷Mn⁺ parent ions (T _1/2?=?1.5 min) at elevated temperatures >?850 K. The Fe _i –B pairs are formed upon the dissociation of Fe _i –V pairs during the lifetime of the Mössbauer state (T _1/2?=?100 ns). The resulting free interstitial Fe_i diffuses over sufficiently large distances during the lifetime of the Mössbauer state to encounter a substitutional B impurity atom, forming Fe _i –B pairs, which are stable up to ～1,050 K on that time scale.     

Quasiparticle lifetime of electron–electron interactions for two-dimensional electron gases with magnetic field

We theoretically study the electron–electron scattering rate τ_ee⁻¹for electrons in a two-dimensional electron gas with a perpendicular magnetic field, within theGWand plasmon-pole approximations, as functions of temperatureT, impurity scattering rate Γ and magnetic fieldB. The τ_ee⁻¹increases with increasingTand increasing Γ, and shows the structure of the Landau levels asBis changed.     

Carrier concentration dependence and temperature dependence of mobility in silicon (100) N-channel inversion layers at low temperatures

The carrier concentration (N_s) dependence of electron mobility in Si (100) inversion layers has been measured at temperatures T = 1.5?70K for high- and low-mobility MOSFETs. An extrinsic term is observed in the T-dependent part of the scattering probability, τ^?1_T. At T = 4.4 K, τ^?1_T depends on N_s as N^?1.9_s in low mobility samples. In high-mobility samples, τ^?1_T increases with increasing N_s in high N_s region while τ^?1_T ∝ N^?1.6_s in low N_s region. The N_s-dependence of τ^?1_T becomes weaker with increasing T in both of low- and high-mobility samples. At N_s = 3 × 10¹² cm^?2, τ^?1_T depends on T as T^1.8 in low-mobility samples and τ^?1_T ∝ T^2.0 in high- mobility samples at T 5 K.     

Energy relaxation of electrons in the (100) n-channel of a Si-MOSFET: II. Surface phonon treatment

The electron energy relaxation is investigated as a function of the “electron temperature” T_e in the n-channel of a (100) surface silicon MOSFET device by inspecting the phenomenological energy relaxation time τ_ε(T_e). τ_ε is determined theoretically and compared to experimental results in order to identify the energy relaxation mechanism(s) present at the interface. Two dimensional electron transport is assumed. Single activation temperature (θ) Rayleigh wave scattering and acoustic Rayleigh wave scattering are studied as possible energy loss processes. The effects of electric subbanding near the surface are included. τ_ε is calculated for T_e ? 15 K in the electric quantum limit. We find that a single θ = 12.0 K Rayleigh phonon fits theory to experiment for a single electron inversion density (N_inv) case, but can not provide a fit simultaneously for more than one N_inv value. Theory and experiment disagree when Rayleigh wave acoustic scattering is assumed.     

Energy relaxation of hot electrons and inelastic collision time in thin metal films at low temperatures

In a sandwich consisting of a thin Au film and a thin Bi film insulated by a 500 Å SiO layer the electrons of one film are heated above the lattice temperature while in the other film they are kept in thermal equilibrium temperature while in the other film they are kept in thermal equilibrium with the phonons. By measuring the magnetoresistance of both films we obtain the temperature difference between electrons and phonons from which we imply the energy-relaxation time τ_ϵ from 0.3 K to 2 K. We find that electron-phonon scattering determines τ_ϵ and, above 5 K, also the inelastic collision time τ_i obtained from weak localization theory in thermal equilibrium.     

Dynamic phenomena in layer superconducting cuprate and organic metals

Dynamic electron spin resonance (ESR) and extended x-ray absorption edge fine structure (XAFS) measurements suggest that layered organic metals and cuprate superconductors behave similarly. The response to microwave radiation in a modulated external magnetic field indicates that: (i) triplet state, T * ESR is observed below T_c for both; (ii) the condensation of free spin doublet D to T* occurs above the transition temperature to superconductivity T_c (10 ± 1 K for the organic metal (BEDT-TTF)₃Ta₂F₁₁ and 92 to 12 K for YBa₂Cu₃O_7-δ and its rare earth derivatives); (iii) antiferomagnetic (AF) resonance is detected above T_c for the organic metal. Here the exchange field between the aligned AF domains: J_AF(150 K) = 130.7 mT (153 mK) is greater than the exchange term J(150 K) ≈ 15 mT (20 mK) between free spins (S = 1/2) leading to T* states; the lifetime of AF domains τ_AF decreases below 150 K and resonance is not detected below 44 K (i.e. τ_AF < 10^-10 s) allowing a superconducting transition to appear below 10 K; (iv) the relaxation time τ₁ for the half field, triplet state ESR absorption increases fourfold near 10 K for the organic metal and, (v) the onset of superconductivity is detected in all superconductors by the appearance of an energy loss at exactly H=0 and, magnetization oscillations observed versus H below T_c when the samples are cooled in a non-zero field H. The spin-lattice relaxation time for the organic metal triplet state, half field ESR near 10 K is interpreted using the Gorter phenomenological relation τ₁ = C_H /α_H, C_H and α_H are respectively the heat capacity and the thermal contact coefficient to the lattice by the spin system, at constant field H . Complementary changes in x-ray edge widths near T_c are correlated to electron-phonon interactions.     

NMR spin lattice relaxation in dilute CuFe alloys with impurity interactions

Previous measurements by Wilkening and Hesse have shown, that the excess relaxation rate ΔT^-1₁ of the matrix nuclei in $\text{CuF}$e dilute alloys can be explained in terms of the LD-model with rapid spin diffusion. Measurements reported in this paper confirm the existence of an electric quadrupole diffusion barrier. It could be shown that the influence of the quadrupole barrier is coupled to large clusters within the alloy. The electron spin lattice relaxation time τ₁ behaves temperature independent in the range 30 K ? T ? 300 K. This can be understood if an effective correlation time τ is introduced, which results from a distrubution of temperature dependent times τ^cl(T) belonging to clusters of different size.     

Spin relaxation of conduction electrons in GaAs

We have used optical spin orientation techniques to measure T₁ of conduction electrons in GaAs (NinA ≈ 10¹⁷ cm^-3) for 4.7 K ? T ? 200 K. From Hall effect measurements we estimated the electron momentum relaxation time τ_p. For 50 K ? T ? 200 K, the product T₁τ_p agrees with our earlier order of magnitude estimate of the D'yakonov-Perel' mechanism, in which band structure induced precession is strongly narrowed by momentum relaxation. The Elliott mechanism is one to two orders of magnitude weaker.     

Electric-field-induced orientation of iron tetragonal centers in KTaO3

Fe _K ³⁺ -O _i ^2? impurity centers in a KTaO₃ sample to which a dc electric field E=75 kV/cm is applied are shown to be oriented at temperatures T≥120 K. In these conditions, the effective local field acting on the electric dipole moment of a center exceeds the applied field by a factor 7.6.     

High field effects in insulating CdF2: Gd crystals

CdF₂ crystals doped with Gd, before any conversion to semiconductor, exhibit at low temperatures electrical current and electroluminescence with d.c. voltage applied.The electrical conduction, attributed to electrons, is bulk limited and the i = i (V, T) characteristics are interpreted as a Poole-Frenkel conduction.The electroluminescence, due to intrinsic and impurity emission, is attributed to the impact process of field accelerated electrons, which give rise to electron-hole pair formation and impurity centre excitation.     

Desorption kinetics and surface diffusion of potassium,rubidium and cesium on a silicon(111)7 × 7-surface

Using pulsed atomic beam technique and a surface ionization ion microscope, the desorption kinetics and the surface diffusion of the alkalis potassium, rubidium and cesium were investigated on a Si(111)7 × 7-surface at extremely low alkali coverages. In the temperature range 1120 … 800 K, the mean adsorption lifetime τ(T) = τ₀ · exp(E_desⁱ/kT) and the mean diffusion length x(T) - defined in the equilibrium between adsorption, diffusion and desorption - were measured. From these data the diffusion constant D(T) = D₀ · exp(-E_diff/kT) was obtained as D = x^?2/τ. For temperatures T ? 750 K, the diffusion constant was calculated from nonstationary alkali concentration profiles using the Boltzmann-Matano method. From the temperature dependence of these quantities the parameters of desorption (E_des,ⁱ τ₀) and surface diffusion (E_diff, D₀) for K, Rb and Cs on Si(111) were obtained. The values of E_diff and D₀ are comparably high and may be interpreted by non-localized diffusion according to a model proposed by Bonzel (Surf. Sci. 21 (1970) 45).     

Pulsed EPR hole-burning experiment on dangling bond centers in a-Si:H aerosol particles formed by thermal decomposition of silane

Si dangling bond centers in aerosol particles of amorphous hydrogenated silicon formed by thermal decomposition of SiH₄ in Ar were studied by pulsed electron paramagnetic resonance. The hole-burning and inversion-recovery experiments demonstrate that large-scale rapid spectral diffusion takes place in the samples with high spin concentration. Correlation times τ_c of the spectral diffusion and spin-lattice relaxation timesT ₁ were obtained in the temperature range between 77 and 290 K. Above 130 K, τ_c andT ₁ are proportional one to the other. The unusual feature of this spectral diffusion is that the shape of the central part of the spectral hole does not change when the delay time increases. The other paramagnetic centers previously investigated showed a remarkable change of the hole shape which was induced by modulation of dipolar interaction due to spin flips. It is suggested that the observed anomaly in the Si dangling bond centers arises due to cooperative spin flips.     

The effect ofs-d exchange interaction on the nuclear spin relaxation in dilute alloys

The effect ofs-d exchange interaction on the relaxation rate of host nuclear spins has been investigated in dilute magnetic alloys. The leading logarithmic corrections to the Korringa relaxation rate have been calculated together with some lower order logarithmic terms to avoid a divergence at the Kondo temperature (T _k ). The obtained formulae are valid at and aboveT _k and in weak or strong magnetic fields (ω _i ?T or ω _i ?T, respectively, where ω _i is the Larmor frequency of the impurity spin). In second order of the perturbation theory our result reproduces that obtained by Giovannini and Heeger apart from a numerical factor of the order of unity and from some contribution of order (J/? _f )².     

New pulse sequences for T1− and T1/T2−contrast enhancing in NMR imaging

Improved pulse sequences DIFN (abbreviation of the words: DIFferentiation by N pulses), 90° − τ₁ − 180° − τ₁ − … 180° − τ_n, with optimised time intervals τ₁ for T₁ measurement and contrast enhancing in NMR imaging are presented. The pulse sequences DIFN have a better sensitivity to T₁ than the well-known pulse sequence SR. In contrast to the IR pulse sequence, the information given by the DIFN pulse sequence is more reliable, because the NMR signal does not change its sign. For a given time interval τ₀ ≤ (0.1 − 0.3) T₁′ the DIFN pulse sequences serve as T₁-filters. They pass the signal components with relatively short T₁ < T₁′ and suppress the components with relatively long T₁ < T₁′. The effects of the radiofrequency field inhomogeneity and inaccurate adjusting of pulse lengths are also considered. It is also proposed in this work to use the joint T₁T₂-contrast in NMR imaging obtained as a result of applying the DIFN pulse sequences in combination with the well-known Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence. The region of interest, where the contrast should be especially enhanced, is specified by the two times at which measurements are performed, which allow the amplitudes of pixels to reach some defined levels by spin-lattice and spin-spin relaxation.     

New insights in the T dependence of the electrical resistivity in size-effect dominated pure metal foils from measurements on high-purity Au at low T

Analysis of Soffer's size-effect theory for electrical resistivity shows, for measurements in such a T range for which the temperature dependent portion of the resistivity, ρ_i, is always much smaller than the residual bulk resistivity ρ_∞(0) of the metal studied, that while size-effects leave the essential T dependence of ρ_i unchanged, it may increase its absolute value and the observed residual resistivity ρ(0), thus explaining recent results of Caplin et al. This also corrects the general conclusion arrived at by the latter authors, i.e. that the T dependence of ρ of a metal foil of given residual resistivity is the same as that of a bulk sample of the same residual resistivity provided that the latter is governed by impurity scattering, as being true for a narrow T range only, i.e. for which ρ_i(T) ? ρ_∞(0). However, for this T range a procedure is outlined which allows one to extract values of the surface specularity parameter p_S and also ρ_∞ of the metal foils studied.     

Magnetoelectric effect in samarium molybdate

This paper reports on the nonlinear magnetoelectric effect (MEE) in the orthorhombic ferroelectric ferroelastic β′ phase of samarium molybdate Sm₂(MoO₄)₃ observed in magnetic fields up to 20 T and temperatures from 4.4 to 0.43 K. The magnetic-field-induced electric polarization in Sm₂(MoO₄)₃ is an order of magnitude larger than that in isomorphic Gd₂(MoO₄)₃. This provides support for the magnetostriction mechanism proposed by us for the MEE in rare-earth molybdates. The polarization in Sm₂(MoO₄)₃ was found to fall off with time. The relaxation time constant τ increases with decreasing temperature from τ=10² s at T=4.4 K to τ≈10³ s at T=0.43 K.     

Absence of CDW transition (Td=200K) in dehydrogenized 1TTaS2

The CDW transition (T_d=200K) is absent in dehydrogenized 1TTaS₂. A small amount of hydrogen impurity induces the lock-in transition. A new transition is observed in dehydrogenized 1TTaS₂ at 6K. Hydrogen impurity suppresses this transition.     

Biexponential intersubband relaxation in n-modulation-doped quantum-well structures

Intersubband scattering in an n-modulation-doped GaAs/Al_0.31Ga_0.69As quantum-well structure is systematically investigated as a function of temperature and pump intensity. For the first time biexponential relaxation is observed during infrared bleaching experiments. The fast component with a time constant of ∼1 ps, which represents the depopulation of the first excited well subband, is found to dominate the signal more and more with decreasing temperature and pump intensity. The decay time of the slower component rises with decreasing temperature from τ₂=8 ps atT=300 K to τ₂=23 ps atT=10 K. This component is believed to be connected with a carrier transfer to the potential minima in the barrier layers. The intensity dependent excitation mechanism and the relaxation processes are discussed with the help of detailed numerical simulations.