Magnetotransport in modulation-doped In0.53Ga0.47As/In0.52Al0.48As heterojunctions: in-plane effective mass,quantum and transport mobilities of 2D electrons

Shubnikov–de Haas (SdH) and Hall effect measurements, performed in the temperature range between 3.3 and 20 K and at magnetic fields up to 2.3 T, have been used to investigate the electronic transport properties of lattice-matched In_0.53Ga_0.47As/In_0.52Al_0.48As heterojunctions. The spacer layer thickness (t_S) in modulation-doped samples was in the range between 0 and 400 Å. SdH oscillations indicate that two subbands are already occupied for all samples except for that witht_S =  400 Å. The carrier density in each subband, Fermi energy and subband separation have been determined from the periods of the SdH oscillations. The in-plane effective mass (m ^* ) and the quantum lifetime (τ_q) of 2D electrons in each subband have been obtained from the temperature and magnetic field dependences of the amplitude of SdH oscillations, respectively. The 2D carrier density (N₁) in the first subband decreases rapidly with increasing spacer thickness, while that (N₂) in the second subband, which is much smaller thanN₁ , decreases slightly with increasing spacer thickness from 0 to 200 Å. The in-plane effective mass of 2D electrons is similar to that of electrons in bulk In_0.53Ga_0.47As and show no dependence on spacer thickness. The quantum mobility of 2D electrons is essentially independent of the thickness of the spacer layer in the range between 0 and 200 Å. It is, however, markedly higher for the samples with a 400 Å thick spacer layer. The quantum mobility of 2D electrons is substantially smaller than the transport mobility which is obtained from the Hall effect measurements at low magnetic fields. The transport mobility of 2D electrons in the first subband is substantially higher than that of electrons in the second subband for all samples with double subband occupancy. The results obtained for transport-to-quantum lifetime ratios suggest that the scattering of electrons in the first subband is, on average, forward displaced in momentum space, while the electrons in the second subband undergo mainly large-angle scattering.     

Investigation of the chemical state of copper in Cu/SiO2 composite films by x-ray photoelectron spectroscopy

The concentration and chemical state of copper in the subsurface region of Cu/SiO₂ composite films obtained by simultaneous magnetron sputtering from two sources (Cu and SiO₂) are determined by x-ray photoelectron spectroscopy (XPS). It is established that copper in the as-grown film is primarily in the form of unoxidized atoms dispersed in a SiO₂ matrix. Annealing of the film results in practically no oxidation, but about 70% of the copper atoms condense into metallic clusters with sizes below 10 Å in the subsurface region and about 50 Å in the bulk of the film. The changes in the binding energy of core electrons, and especially in the energies of Auger electrons, are so large in this situation that photoelectron and Auger spectroscopy are efficient methods for monitoring the chemical state of this composite material.     

Giant Rashba spin splitting in Bi2Se3:Tl

First‐principles calculations are employed to demonstrate a giant Rashba spin splitting in Bi₂Se₃:Tl. Biaxial tensile and compressive strain is used to tune the splitting by modifying the potential gradient. The band gap is found to increase under compression and decreases under tension, whereas the dependence of the Rashba spin splitting on the strain is the opposite. Large values of α_R = 1.57 eV Å at the bottom of the conduction band (electrons) and α_R = 3.34 eV Å at the top of the valence band (holes) are obtained without strain. These values can be further enhanced to α_R = 1.83 eV Å and α_R = 3.64 eV Å, respectively, by 2% tensile strain. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Mössbauer determination of the E2/M1 mixing ratio of the 77 keV transition in197Au and of the sign of the electric field gradient in KAu(CN)2

A Mössbauer measurement of the E2/M1 mixing ratio of the 77 KeV transition in¹⁹⁷Au yielded δ=?0.352±0.005. With this result, Mössbauer absorption spectra of KAu(CN)₂ single crystals show that the electric field gradient at¹⁹⁷Au in this compound is negative. This implies that the field gradient is mainly produced by 6p_z electrons. The data also indicate a large vibrational anisotropy of Au in KAu(CN)₂.     

Effect of MeV electron irradiation on the free volume of polyimide

The free volume of the microvoids in the polyimide samples, irradiated with 6 MeV electrons, was measured by the positron annihilation technique. The free volume initially decreased the virgin value from ～13.70 to ～10.98 Å³ and then increased to ～18.11 Å³ with increasing the electron fluence, over the range of 5?×?10¹⁴ – 5?×?10¹⁵ e/cm². The evolution of gaseous species from the polyimide during electron irradiation was confirmed by the residual gas analysis technique. The polyimide samples irradiated with 6 MeV electrons in AgNO₃ solution were studied with the Rutherford back scattering technique. The diffusion of silver in these polyimide samples was observed for fluences >2?×?10¹⁵ e/cm², at which microvoids of size ≥3 Å are produced. Silver atoms did not diffuse in the polyimide samples, which were first irradiated with electrons and then immersed in AgNO₃ solution. These results indicate that during electron irradiation, the microvoids with size ≥3 Å were retained in the surface region through which silver atoms of size ～2.88 Å could diffuse into the polyimide. The average depth of diffusion of silver atoms in the polyimide was ～2.5 μm.     

Half-metallic properties of the CuHg2Ti-type Mn2ZnSi full-Heusler compound

The half-metallic properties of novel CuHg₂Ti-type Mn₂ZnSi full-Heusler compound were examined by density functional theory (DFT) calculations. The electronic band structures and density of states of the Mn₂ZnSi compound show that spin-up electrons are metallic, but the spin-down bands are semiconductor with a gap of 0.48 eV, and the spin-flip gap is of 0.28 eV. The Mn₂ZnSi Heusler compound has a magnetic moment of 2 μ_B at the equilibrium lattice constant a = 5.80 Å. The Mn₂ZnSi full-Heusler compound is ferrimagnetic and maintains the half-metallic character having 100% polarization for lattice constants ranging between 5.62 and 6.91 Å.     

Relative escape depth of Auger and photoelectrons from a solid surface

A new method of determining the escape depth of electrons in the energy range 0–1000 eV is described. Values obtained were 6.1 Å and 6.8 Å at 234 and 266 eV, respectively, in GeO₂.     

Formation of the Co/Si(110) interface: Phase composition and magnetic properties

The formation of the Co/Si(110)16 × 2 interface and its magnetic properties are studied by high-energy-resolution photoelectron spectroscopy using synchrotron radiation and magnetic linear dichroism in the photoemission of core electrons. It is shown that a cobalt coating less than 7 Å thick deposited on the silicon surface at room temperature results in the formation of an ultrathin (1.7 Å) interfacial cobalt silicide layer and a layer of silicon-cobalt solid solution. The ferromagnetic ordering of the interface is observed at an evaporation dose corresponding to 6–7 Å in which case a cobalt metal film begins to grow on the solid solution layer. During 300°C-annealing of the sample covered by a nanometer-thick cobalt layer, the metal film gradually disappears and four silicide phases arise: metastable ferromagnetic silicide Co₃Si and three stable nonmagnetic silicides (Co₂Si, CoSi, and CoSi₂).     

Tunneling in ultrathin SiO2 layers on silicon: Comments on dispersion relations for electrons and holes

A fundamental asymmetry between the tunneling probabilities for electrons and holes has been observed in ultrathin SiO₂ layers (20–30 Å). These probabilities have been measured on the same MOS samples using a new experimental technique combining dark characteristics with measurements of photocurrent suppression by the SiO₂ layer. Interpretation by Franz dispersion relations using conduction and valence band energies appropriate to thick SiO₂ films is not possible even assuming different effective masses in the two bands. This may be attributed to the distortion of the energy bands in the SiO₂ caused by the presence of positive oxide charge.     

Reactive scattering of alkali dimers: K2 + HgCl2, HgBr2, HgI2

Angular distribution measurements of KX reactive scattering of a potassium dimer K₂ beam by mercuric halide molecules HgX₂ are reported. All the reactions exhibit strong forward peaking in the centre of mass differential cross sections and large total reaction cross sections Q_r ～ 150 Ų. However, there is substantial backward peaking ( forward peak) for HgBr₂, HgI₂. Despite the direct stripping dynamics, both alkali atoms of the K₂ dimer become bound alkali halide molecules in most (perhaps all) reactive collisions. A major fraction of the reaction exoergicity is disposed into vibrational excitation of the product KX molecules. A mechanism involving a first electron jump in the entrance valley and a second electron jump in the exit valley of the potential surface is suggested to explain the rapid transfer of both K₂ valence electrons.     

Intensity anomalies in the spectra of 2p-3d multiplets of A1XI and A1X observed on the rear surface of laser-irradiated foils

The relative intensities of the 2p-3p multiplets of A1XI (λ ≈ 52 Å) and A1X (λ ≈ 55 Å) emitted from the plasma produced by the irradiation of 9 μm thick aluminium foils with a neodymiun laser focussed to peak intensities ≈ × 10₁₅ Wcm^-2 have been recorded with spatial resolution of ≈ 30 μm. The measured relative intensities indicate that there is partial intermixing of the quantum states near the 3d states of A1XI and A1X in plasma expanding from the rear of the laser-irrdiated foils. The spectra recorded are consistent with the rear plasma being produced by superthermal electrons travelling around the foil edge and striking the back of the foil.     

Electronic-excitation transfer collisions in flames—II temperature dependence of the quenching of Na(32P)-doublet by H2 and O2

Effective cross sections for quenching of the Na(3²P)-doublet by H₂ and O₂ molecules have been measured in flames in the temperature range 1500–2500 K. The H₂-cross section decreases from (9.3±1.0) Ų at 1500 K to (6.8±1.0) Ų at 2500 K. The O₂-cross section decreases from (39±2) Ų at 1720 K to (31±2) Ų at 2500 K. A critical comparison of the flame values with previous literature data on the H₂-cross section at lower temperatures shows that it decreases systematically when the temperature rises from about 400 to 2500 K.     

Dechannelling of 2 MeV He+ at a circular Frank dislocation loop in gold

The dechannelling cross section of a circular Frank dislocation loop of radius 19 Å, and of Burgers vector (a/3)(111) in gold crystal is calculated for (001) channeling of 2 MeV He⁺. The calculated value σ_c = 210 ± 10 Ų is satisfactory compared with the experimental value σ_e = 235 Ų by Merkel et al.     

Structural transformation in Mg2NiH4

X-Ray diffraction measurements show that on heating Mg₂NiH₄ in a 1 atm pressure H₂ atmosphere, above ～250°C it transforms into a cubic structure, metal atoms in CaF₂ arrangement, a = 6.525 Å. It is concluded that the H atoms are in tetrahedral clusters, and that the structure is only weakly ionic. This conclusion is also supported by NMR measurements. The 20°C structure of Mg₂NiH₄ is shown to be describable primarily as a slight monoclinic distortion of the cubic unit cell; a = 6.594 Å, b = 6.412 Å, c = 6.490 Å and β = 93.1°. However, weak small angle lines show that a longer range order exists and that the true unit cell, which we have not determined, must be very large. To what extent the cubic phase should be considered a high temperature and/or low concentration (Mg₂NiH_4??) phase is not resolved.     

Neutron diffraction determination of the magnetic structures of NpCo2Si2 and NpCu2Si2

A small polycrystalline ingot sample of NpCo₂Si₂ (weight ≈ 1.5 g) has been studied by neutron diffration between 2 and 160 K on the multi-detector D1B of ILL, Grenoble. At 100 K, the crystal structure is body-centered tetragonal (space group 14/mmm) with a = 3.886 Å and c =9.649 Å. Below T_N = (44 ± 2) K, seven superlattice lines are observed which correspond to a simple tetragonal lattice with lattice constants as above. They are consistent with a type I antiferromagnetic structure of the Np (2a) sublattice, with (001) ferromagnetic sheets coupled antiferromagnetically according to the sequence +-+-. At 6 K, the neptunium moment obtained from the diffracted intensities is: (1.48 ± 0.20)_μuB, and makes an angle 52° ± 15° with the c axis. The cobalt moment is certainly smallet than 0.3_μuB. The Np moment correlates well with the ²³⁷Np hyperfine field deduced from Mos?sbauer spectroscopy; the sublattice magnetization-temoperature curve follows very well the $\text{J=}\text{1}\text{2}$ brillouin curve. The magnetism is therefore probably of lovalized character in this compound. An isomorphous sample of NpCu₂Si₂ (a = 3.990 Å c = 9.920 Å) was shown to be ferromagnetic below (41 ± 2) K, with the Np moment [1.5 ± 0.2)_μuB] aligned along the c axis.     

A Michelson interferometer using electron waves

An electron interferometer of the Michelson type is realized. Monoenergetic 25 keV electrons emitted from a line source 1000 Å in width are deflected by 90 ° in a magnetic Castaing prism and reflected on two mirrorsM ₁ andM ₂ held at a potential ΔU _m negative with respect to the cathode. In the experiment the mirrors are represented by height differences on the surface of an silvered glass plate. The reflected electrons are once more deflected by the magnetic prism behind which the coherent partial beams 1 and 2 reflected on the mirrorsM ₁ andM ₂, respectively, are superimposed using an electrostatic biprism to form two-beam interferences. The observed fringe shift indicates a phase shift due to differences in height between the equipotentials reflecting the partial beams 1 and 2. It is estimated that path differences less than the electron wavelength of 0.08 Å can be observed.     

Precious Metals - Part 2 - The Spectroscopic Characterisation and X-Ray Crystal Structure of Pd2(Pph3)3Cl5O

This work is on the synthesis and characterisation of a new phosphine stabilised palladium compound. the compound was first obtained from the rejects of cluster syntheses stored in the laboratory. Later on, it was prepared from PdCl₂ and triphenyl phosphine. the compound was characterised by ³¹P {¹H} NMR, UV/visible spectroscopy and elemental analysis. the crystal and molecular structure of Pd₂(PPh₃)₃Cl₅O was determined by X-ray analysis. the compound crystallizes in orthorhombic space group Pbca, N° 61, a = 19.009(2)Å, b=22.283(2)Å, c=23.726(2)Å, V=10050(20)ų, Z=8 residuals R[I>2σ(I)]=0.0457 and R(all) = 0.0636, MoKα radiation, 20 °C.     

Surface sensitivity of low energy electron Mössbauer spectroscopy (LEEMS) using57Fe

Very low energy electrons (LEE) (E _e ≤15 eV) are produced with high intensity directly by Mössbauerabsorption and conversion in the case of⁵⁷Fe [1, 4, 5]. These electrons should be very surface sensitive due to their very low attenuation length compared to the 7.3 keV K-Conversion electrons of⁵⁷Fe [5, 11]. We have examined the surface sensitivity of these resonant LEE, using nonresonant⁵⁶Fe metal and⁵⁶Fe stainless steel foils coated with about 20 Å and 50 Å⁵⁷Fe, respectively. They were exposed to air after evaporation: The 20 Å samples are found to be fully oxidized [5]. Depth Selective Conversion Electron Mössbauer Spectroscopy (DCEMS), performed with a high transmission orange type magnetic spectrometer [5, 6, 13] reveals a two layer structure of the 50 Å samples. Low Energy Electron Mössbauer Spectroscopy (LEEMS) [5] is found to be significantly more surface sensitive than conventional DCEMS, but not as surface sensitive as Auger Electron Mössbauer Spectroscopy (AEMS) using LMM-Auger electrons of 500–600 eV, as expected due to the different mean free path. But because of the very low intensity of these Auger electrons this mode appears to be not very useful for practical application.     

First-principles study on the electronic and magnetic properties of InN nanosheets doped with 2p elements

The electronic structure of InN nanosheets doped by light elements (Be, B, C, and O) is studied based on spin-polarized density functional theory within the generalized gradient approximation. The results show that the Be and C dopants in InN nanosheets induce spin polarized states in the band gap, or near the valence band, which generates local magnetic moments of 1.0 µ_B with one dopant atom. Due to the exchange spin-splitting, the three 2p electrons of Be atom are all in p_x and p_y orbitals (↑↑↓). So Be will coordinate with host atoms by σ coordination bond. The long-range ferromagnetic order above room temperature is attributed to p–p coupling. For C atom, the configuration of the five 2p electrons is (↑↑↑↓↓), and the unpaired electron is in p_z(↑) orbital. So the π bond will be formed between C atom and other atoms. Due the weak π bond cannot support long-range coupling, no stable magnetism is sustained if two C dopants are separated by longer than 3.58 Å.