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.     

Dilation analyticity in constant electric field

The resolvent of the operatorH ₀(ε, θ)=?Δe ^-20+εx ₁ e ^θ is not analytic in θ for θ in a neighborhood of a real point, if the electric field ε is non-zero. (One manifestation of this singular behavior is that for 0<|Im θ|<π/3,H ₀(ε, θ) has no spectrum in the finite plane.) Nevertheless it is shown that the techniques of dilation analyticity still can be used to discuss the long-lived states (resonances) of a system described by a Hamiltonian of the formH=?Δ+εx ₁+V(x).     

Searching for effects of neutrino magnetic moments at reactors and accelerators

The sensitivity of the νe elastic scattering experiments to possible effects of nonzero neutrino magnetic moment (μ) is analyzed and the optimal experimental condition under which such effects are relatively enhanced are found. New, somewhat more restrictive than the previously derived bounds on μ, are obtained on the basis of existing data by the use of selfconsistent statistical analyses: μ_νe < 1.52 × 10^?10e/2m_e and μ_νμ < 1.19 × 10^?9e/2m_e. These bounds are expected to be improved approximately by an order of magnitude in experiments planned to be performed in the near future.     

155Gd and 161Dy Mössbauer study of R1+εFe4B4 alloys (R = Gd,Dy)

Tetragonal R_1+εFe₄B₄ alloys with R = Gd and Dy have been investigated by Mössbauer spectroscopy, using the ¹⁵⁵Gd and ¹⁶¹Dy resonances, respectively. The Gd quadrupolar interaction e²qQ = 12.65(5)mm/s is the largest observed to date in metallic compounds of Gd. For Dy this interaction is e²qQ = 74(2) mm/s, a value rather small for a Dy compound. Both results imply a strong lattice contribution to the electric field gradient. A crystal-field term A⁰₂ = -2450(50) K/a²_o is inferred. Our data are consistent with a point-charge calculation, provided charges of opposite signs are assumed for Fe and B atoms. Hyperfine parameters show some dispersion, reflecting the quasi incommensurate nature of the R and Fe+B sublattices in the R_1+εFe₄B₄ structure.     

πℓ2, Kℓ3and K0−K0 constraints on leptoquarks and supersymmetric particles

Mass bounds from different processes for charged Higgs particles, for pseudoscalar leptoquarks occurring in dynamically broken theories, for vector leptoquarks occurring in the Pati-Salam type of grand unified theories and for the exotic particles of supersymmetric theories are studied. The processes considered are the π → eν to π → μν branching ratio, the CP-violating K?3 decay parameters, and the CP-violating part of the K_L ? K_S mass difference. The π → eν to π → μν branching ratio sets the most stringent bounds, 0.5 TeV for charged Higgs particles, 1 TeV for pseudoscalar leptoquarks, 125 TeV for vector leptoquarks and 0.1–1 TeV for the supersymmetric partners of ordinary particles. The π?2 branching ratio should be sensitive at the present experimental accuracy to the effects of pseudoscalar leptoquarks.     

Lower and upper variational bounds in calculations of Coulomb and nuclear systems

All formulas that are necessary for deriving not only upper (E _U) but also lower (E _L) variational bounds on the energy of systems featuring a few nonrelativistic particles are obtained with trial functions in the form of expansions in multidimensional Gaussian functions or exponentials. For potentials that are used most widely, all matrix elements are expressed in terms of known functions, a circumstance that simplifies considerably relevant numerical calculations. This is so for systems featuring an arbitrary number of particles in the case of a Gaussian basis and for three-particle systems in the case of an exponential basis. Numerical results for E _U and E _L, which are characterized by record accuracies, are presented for some Coulomb and nuclear systems such as the He atom; the e ⁺ e ^? e ^?, ppμ^?, 3α, and 4α systems; and hypertritium (pnΛ). Lower bounds with exponential trial functions are obtained for the first time (the corresponding formulas are presented for the first time as well); for a Gaussian basis, lower bounds for Coulomb systems have not been known either. Given E _L and E _U, limits within which the exact value of energy, E ₀, lies can be indicated with confidence. Moreover, an analysis of the correlation between E _L and E _U with increasing number of terms in the expansion of the trial function makes it possible to improve the accuracy (at least by one order of magnitude) of the value E _∞ extrapolated to infinity. By considering specific examples, it is shown that the exponential basis is advantageous in relation to the Gaussian one.     

Untersuchung der Zustandsdichte und der charakteristischen Energieverluste von Ir,Rh, Pt und Pd mit Isochromatenmessungen

X-ray isochromats of the above metals measured with high resolution are presented. Assuming a rigid band model for the four metals they give — with some usual approximations — the qualitative form of the density of states. It turns out that the densities of statesZ(ε) at the Fermi limits decrease in the succession Ir, Rh, Pt, Pd. This is in contradiction to low temperature measurements of the electronic specific heatc _e which for the same succession give an appreciable raising inZ(ε). It is assumed that this discrepancy is caused by a special coupling of the electrons to the transverse phonons. In consequence of this,Z(ε) of the renormalized electron system measured byc _e can be very different from that of the bare electrons measured by the isochromats. Assuming that the transition temperature of elemental superconductors is related according to BCS-theory to the latter density of states the absence of superconductivity of Rh, Pt and Pd can be understood. In addition the isochromats give values for the characteristic energy losses of the four metals.     

Surface-induced magnetism of the solids with impurities and vacancies

Using the quantum-mechanical approach combined with the image charge method we calculated the lowest energy levels of the impurities and neutral vacancies with two electrons or holes located in the vicinity of flat surface of different solids. Unexpectedly we obtained that the magnetic triplet state is the ground state of the impurities and neutral vacancies in the vicinity of surface, while the nonmagnetic singlet is the ground state in the bulk, for e.g. He atom, Li⁺, Be⁺⁺ ions, etc. The energy difference between the lowest triplet and singlet states strongly depends on the electron (hole) effective mass μ, dielectric permittivity of the solid ε₂ and the distance from the surface z₀. For z₀=0 and defect charge ∣Z∣=2 the energy difference is more than several hundreds of Kelvins at μ=(0.5−1)m_e and ε₂=2-10, more than several tens of Kelvins at μ=(0.1−0.2)m_e and ε₂=5-10, and not more than several Kelvins at μ<0.1m_e and ε₂>15 (m_e is the mass of a free electron). Pair interaction of the identical surface defects (two doubly charged impurities or vacancies with two electrons or holes) reveals the ferromagnetic spin state with the maximal exchange energy at the definite distance between the defects (∼5-25 nm). We estimated the critical concentration of surface defects and transition temperature of ferromagnetic long-range order appearance in the framework of percolation and mean field theories, and RKKY approach for semiconductors like ZnO. We obtained that the nonmagnetic singlet state is the lowest one for a molecule with two electrons formed by a pair of identical surface impurities (like surface hydrogen), while its next state with deep enough negative energy minimum is the magnetic triplet. The metastable magnetic triplet state appeared for such molecule at the surface indicates the possibility of metastable ortho-states of the hydrogen-like molecules, while they are absent in the bulk of material. The two series of spectral lines are expected due to the coexistence of ortho- and para-states of the molecules at the surface. We hope that obtained results could provide an alternative mechanism of the room temperature ferromagnetism observed in TiO₂, HfO₂, and In₂O₃ thin films with contribution of the oxygen vacancies. We expect that both anion and cation vacancies near the flat surface act as magnetic defects because of their triplet ground state and Hund's rule. The theoretical forecasts are waiting for experimental justification allowing for the number of the defects in the vicinity of surface is much larger than in the bulk of as-grown samples.     

Full piezoelectric tensors of wurtzite and zinc blende ZnO and ZnS by first-principles calculations

The complete piezoelectric tensors of both the wurtzite and zinc blende polymorphs of ZnO and ZnS have been computed by ab initio periodic linear combination of atomic orbitals (LCAO) methods, based mainly on the Hartree-Fock Hamiltonian, with an all-electron Gaussian-type basis set. The computational scheme was based on the Berry phases theory, yielding directly the proper piezoelectric stress coefficients e_ik=(∂P_i/∂ε_k)_E; also the strain coefficients d_ik=(∂ε_k/∂E_i)_τ were obtained, by intermediate calculation of the full elasticity tensors of all four crystals. In particular, the e₁₅ wurtzite shear constants were included for the first time in such calculations. A careful study of the clamped-ion and internal-strain piezoelectric components shows that the latter ones are well simulated by classical point-charge calculations including quantum-mechanical structural relaxation. The much larger piezoelectric response of ZnO with respect to ZnS is explained by analysing signs and ratios of the respective clamped-ion and internal-strain components.     

Probing QCD via four-jet decays of theZ boson

For a quantitative analysis of the non-Abelian three-gluon vertex a continuous parameterisation of possible deviations from QCD is highly desirable. An almost unique extension of the three-gluon vertex is provided by the dimension-6 operator Tr(F _μ ^ν F _ν ^ρ F _ρ ^μ ). The effects of this interaction term on various observables ine ⁺ e ^?→γ,Z→4 jet production are studied in detail. Presente ⁺ e ^? data are expected to give rather weak bounds on such anomalous contributions to the triple gluon vertex.     

Vector resonances from a strong electroweak sector at linear colliders

We explore the usefulness of very energetic lineare ⁺ e ^? colliders in the TeV range in studying an alternative scheme of electroweak symmetry breaking based on a strong interacting sector. The calculations are performed within the BESS model which contains new vector resonances. If the massM _V of the new boson multiplet lies not far from the maximum machine energy, or if it is lower, such a resonant contribution would be quite manifest. A result of our analysis is that also virtual effects are important. It appears that annihilation into a fermion pair in such machines, at the considered luminosities, would improve only marginally on existing limits if polarized beams are available and left-right asymmetries are measured. On the other hand, the process ofW-pair production bye ⁺ e ^? annihilation would allow for sensitive tests of the hypothesized strong sector, especially if theW polarizations are reconstructed from their decay distributions, and the more so the higher the energy of the machine. Ane ⁺ e ^? collider with c.m. energy \(\sqrt s = 500\) GeV could improve the limits on the model for the range 500<M _V (GeV)<1000 whenW polarization is not reconstructed. IfW polarizations are reconstructed, then the bounds improve for the entire expected range ofM _V . These bounds become more stringent for larger energy of the collider. We have also studied the detectability of the new resonances through the fusion subprocesses, but this channel does not seem to be interesting even for a collider with a c.m. energy \(\sqrt s = 2\) TeV.     

Tests of quantum chromodynamics in the decay of the τ-lepton and in electron positron annihilation into hadrons

We present a method to derive upper and lower bounds for a large class of quantities measurable ine ⁺ e ^? annihilation into hadrons and in τ-decay. The theoretical input are the asymptotic expansions for the vacuum polarization functions of vector and axial vector currents in the euclidean region as given by QCD. Since our method allows straightforward inclusion of (as yet uncalculated) higher order terms in α _s , there is the potential of making high precision tests of QCD even with experiments at rather moderate energies. We argue, therefore, that it may be very worthwhile to pursue experiments ate ⁺ e ^? storage rings below theJ/Ψ resonance and to measure the branching ratios for the decays of the τ-lepton with high accuracy. In the latter case we can also get information on the behaviour of the weak axial vector current which should be very interesting in connection with chiral symmetry breaking.     

Optimal bounds for the pion form factor from analyticity and experimental data

Optimal bounds for the pion electromagnetic form factor F(t) below threshold and on the pion mean-square charge radius 〈r_π²〉 = 6F'(0) are derived. Use is made of analyticity arguments and of experimental data on F(t) from e⁺e^? → π⁺π^? as well as e^?p → e^?nπ⁺. The method accounts in an approximate way for the statistical errors of the experimental information. Numerical results for F(t) are calculated for the CEA as well as the DESY electroproduction data.     

A light neutral intermediate vector boson without large e+e−→ μ+μ− asymmetries

An effective SU(2)×U(1)×U(1)' electroweak theory is shown to permit the occurence of a pair of neutral intermediate vector bosons with masses 40 GeV?m_light?70 GeV, m^W.S._Z⁰<m_heavy?100 GeV. Neutrino neutral current interactions are shown to be the same as in the standard electroweak model, and e⁺e^?→μ⁺μ^? forward-backward asymmetries are within experimental bounds for m_light?40 GeV.     

Bounds on the masses of neutral generation-changing gauge bosons

The existence of several generations of quarks and leptons suggests the possibility of a gauge symmetry connecting the different generations. The neutral gauge bosons of such a scheme would mediate rare processes such as K_L⁰→μ^±, K⁺→π⁺e^?π⁺, μN→eN and would contribute to ΔM(K_S⁰?K_L⁰). We study these and other processes within a simple theoretical framework and derive bounds involving the masses and coupling constants of the generation-changing gauge bosons and various generation-mixing angles. The lower bounds for the relevant masses lie in the 10–100 TeV region. Various remarks concerning the relevance of these bounds to currently popular theoretical ideas and to future experiments are presented.     

Pionic contribution to the muon magnetic moment

A procedure is developed to derive an optimal lower bounds for the pionic contribution to the muon magnetic moment from analyticity of the pion form factor F(t), its normalization F(0)=1 and from experimental information from both the processes e^?p → e^?π⁺n and e⁺e^? → π⁺π^?. It represents essentially the solution of a certain kind of optimization problem in Hilbert space. Numerical results are presented and compared to the recent data for the muon magnetic moment; we find a_μ(π⁺π^?) ? 42 × 10^?9.     

Electronic,magnetic and elastic properties of ε-phases Fe3X(X=B,C, N) from density-functional theory calculations

First principles calculations have been performed for ε-phases Fe₃X(X=B/C/N) with hexagonal close-packed Fe sublattices. The calculated equilibrium structural parameters of ε-Fe₃C and ε-Fe₃N are in agreement with experimental results. The crystal structure of the ε-Fe₃B is predicted. In the view of the formation energy, the phase stabilities grow up from ε-Fe₃B, θ-Fe₃N, ε-Fe₃C, θ-Fe₃C, θ-Fe₃B to ε-Fe₃N. The bonding nature in ε-Fe₃X (X=B/C/N) is described as a complex mixture of covalent, ionic and metallic characters. The average magnetic moments (Ms) of the ε-Fe₃B, ε-Fe₃C and ε-Fe₃N are 1.53, 1.61 and 1.66 μ_B/atom, respectively. The Ms of Fe are 2.17, 2.23 and 2.25 μ_B in ε-Fe₃B, ε-Fe₃C and ε-Fe₃N, respectively. The Ms of B/C/N are −0.39/−0.26/−0.08 μ_B.The Debye temperatures of ε-Fe₃X (X=B/C/N) are predicted as 427, 565 and 555 K.     

Crack propagation and initiation in low cycle strain controlled fatigue

Experimental techniques for crack extension measurements in cylindrical specimens, subjected to tension-compression strain cycling, are described. Both electrical resistivity and compliance techniques are applicable and yield information on crack growth as well as crack initiation. Crack propagation follows the relationship $$dc/dN = A.c.(\varepsilon _{TR} /\varepsilon _F )^{n + 1} - \frac{{\rho ^* }}{2}$$ withn close to unity. (Herec is the crack length,A a constant andε _F a critical strain characteristic of the material;ε _TR is the total strain range.) The Manson-Coffin equation could be regarded as a consequence of this crack propagation relationship, though the value of the strain hardening exponentn 1 will require further verification and clarification. From preliminary tests it appears that crack initiation in notched specimens is also governed by a Manson-Coffin type relationship, especially if the effective plastic strain at the notch root can be properly computed from $$\varepsilon _{TR,effective} = K_\varepsilon .\varepsilon _{TR} - \varepsilon _e^ + $$ whereK _? is the proper strain concentration factor,ε _TR the total nominal strain range andε _e ⁺ the elastic component under consideration of the plastic constraint at the notch root.     

Low temperature resonances in the electron heat capacity of finite systems

Temperature variations of the heat capacity (C) are studied in a low temperature regime T<δ_F∼ε_F/N for 2D and 3D systems with N∼10²-10⁴ treated as a canonical ensemble of N-noninteracting fermions. The analysis of C is performed by introducing the function φ(ε), the spectral distribution of C, that gives the contribution of each single-particle state to C. This function has two peaks divided by the energy interval . If at some temperature T_res a resonance takes place i.e. the positions of these peaks coincide with energies of two levels nearest to ε_F then C vs T can show a local maximum at T_res. This gives us the possibility to assess the single-particle level spacings near the Fermi level.