Rashba spin splitting in the Al0.3Ga0.7N/GaN heterostructure under uniaxial strain

By solving the Schrödinger and Poisson equations self-consistently, changes of the Rashba spin splitting for the Al_0.3Ga_0.7N/GaN heterostructure under uniaxial strain are calculated, and electrons are found to take up the first two subbands. The additional polarization induced by the uniaxial strain leads to a great enhancement of the built-in electric field and the 2DEG concentration. The Rashba spin splitting almost increases linearly with the uniaxial strain, and its amplitude increases by 36% with a strain of 4×10⁻³. The effect of electrons occupying more than one subband on the Rashba spin splitting is discussed. Results show the internal electric field caused by the polarization is crucial for the considerable Rashba spin splitting in the Al_0.3Ga_0.7N/GaN heterostructure and the magnitude of the Rashba spin splitting can be greatly modulated by the uniaxial strain, which would benefit further research and application of spintronics.     

PbTe/PbSrTe半导体非对称量子阱中的Rashba效应

窄带隙半导体异质结构的自旋效应最近受到了国际上的很大关注.Ⅳ-Ⅵ族半导体具有各向异性和多能谷的特征,因此可以预期Rashba自旋效应在不同取向的Ⅳ-Ⅵ族半导体量子阱结构中存在显著差异.计算了多个取向的Pb_1-ySr_yTe/PbTe/Pb_1-xSr_xTe非对称量子阱中的Rashba分裂能,结果表明［100］取向的PbTe量子阱的Rashba分裂能在阱宽为5.0nm时 关键词： Ⅳ-Ⅵ族半导体 非对称量子阱 Rashba效应 自旋-轨道耦合分裂     

Electronic structure of paramagnetic In<Subscript>1-x</Subscript>Mn<Subscript>x</Subscript> As nanowires

The electronic structure, spin splitting energies, and g factors of paramagnetic In_1-xMn_xAs nanowires under magnetic and electric fields are investigated theoretically including the sp-d exchange interaction between the carriers and the magnetic ion. We find that the effective g factor changes dramatically with the magnetic field. The spin splitting due to the sp-d exchange interaction counteracts the Zeeman spin splitting. The effective g factor can be tuned to zero by the external magnetic field. There is also spin splitting under an electric field due to the Rashba spin-orbit coupling which is a relativistic effect. The spin-degenerated bands split at nonzero k_z (k_z is the wave vector in the wire direction), and the spin-splitting bands cross at k_z = 0, whose k_z-positive part and negative part are symmetrical. A proper magnetic field makes the k_z-positive part and negative part of the bands asymmetrical, and the bands cross at nonzero k_z. In the absence of magnetic field, the electron Rashba coefficient increases almost linearly with the electric field, while the hole Rashba coefficient increases at first and then decreases as the electric field increases. The hole Rashba coefficient can be tuned to zero by the electric field.     

Theoretical investigations of spin splittings in asymmetric AlSb/InAs/GaSb heterostructures and the possibility of electric field induced magnetization

We use the effective bond orbital model method to examine the spin splitting due to the Rashba effect in AlSb/InAs/GaSb asymmetric heterostructures. We find for the resulting two-dimensional electron gas (2DEG) under study that large theoretical values of the Bychkov–Rashba coefficients in the range of 30 × 10 ^{− 10}to 50 × 10 ^{− 10}eV · cm can be achieved. Finally, we present a phenomenon that might lead to a direct observation of the Rashba effect. We derive an expression, valid in the diffusive limit, for the spin polarization of the current resulting from a bias parallel to the plane of the quantum well.     

Rashba splitting in MIS structures HgCdTe

The measured parameters of spin-orbit spectral splitting in HgCdTe-based MIS structures with positive and negative Kane gap E _g are compared with the parameters calculated using the three-and four-band Kane model. The disregard of the finite spin-orbit splitting Δ of the valence band in calculations leads to exaggerated values of Rashba splitting (especially for E _g < 0) even for small ratios |E _g|/Δ, although the subband parameters averaged over two spin branches of the spectrum in the two-, three-, and four-band Kane approximations for the same concentrations are practically identical. In the zero-gap HgCdTe, the measured as well as calculated values are noticeably higher, but the four-band approximation leads to values of splitting for both materials which are 20–40% lower than the experimental value. The inclusion of the interband interaction reduces these discrepancies, but does not eliminate them completely. It is shown that the approximations of the 2D spectrum with spin-orbit splitting linear in quasimomentum, which are conventionally used in the analysis, may lower the effective Rashba parameter by a factor of 2–4.     

Mössbauer study of Fe‐doped CuGeO3 system Cu0.9Ge0.9Fe0.2O3

The Fe‐doped system Cu_0.9Ge_0.9Fe_0.2O₃ has been investigated by means of X‐ray diffractometry, Mössbauer spectroscopy and superconducting quantum interference device. The structure of this system is orthorhombic and the lattice constants are a=4.784 Å, b=8.472 Å and c=2.904 Å, respectively. Magnetic measurements confirm that the spin‐Peierls transition appears in our sample at about 12 K, which is near to the spin‐Peierls transition temperature (T _sp) 14 K of pure CuGeO₃ system. The Mössbauer spectrum shows the superposition of two Zeeman sextets and a broad central line due to Fe³⁺ ions from room temperature to 4.2 K. The Mössbauer parameters show a discontinuity near T _sp. The jump of the magnetic hyperfine field at temperatures lower than T _sp means increasing of the superexchange interaction among the magnetic ions. The jump of the quadrupole splitting and the isomer shift values could be interpreted as due to decrement in symmetry of lattice sites and spontaneous thermal contraction.     

Experimental energy bands,exchange splittings,and lifetimes for Ni in the vicinity of the x-point

Angle-resolved photoelectron spectroscopy using synchrotron radiation has been used to determine energy band dispersions along the ΓKX and ΓX directions in nickel. A detailed picture of spin-dependent energy levels and band topology around the symmetry point X has been derived. We have measured the exchange splitting for different band symmetries and find the splitting for the X₂-S₄ band along (110) to be 0.17 eV. For the X₅-S₃ band we find a splitting of 0.33 eV which is in close agreement with the value found earlier for the Σ₂ band. This can be explained by different self-energy corrections for t_2g and e_g-type states respectively. Our values for the energy positions (inverse lifetimes) are the following: X_2↓ = ?0.04 eV (0.08 eV, X_2↑ = ?0.24 eV (0.19 eV), X_5↑ = ?0.11 eV.     

Piezo-optical constants of gold

The changes induced in the reflectance of single crystal gold due to strain have been determined in piezoreflectance measurements covering the spectral range from 1.7 to 6.5 eV. On the basis of the measurements the band separations L₁ ? L₂′ = 4.6 and X₄ ? X₅ = 3.4 have been determined and a spin orbit splitting of the X₅ level of 0.5–0.7 eV has been resolved.     

Shubnikov-de haas effect in n-CdSnAs2

Shubnikov-de Haas oscillations in the transverse magnetoresistance of single-crystalline n-type CdSnAs₂ have been recorded at temperatures between 2 and 25 K in magnetic fields up to 5T. The electron concentration of the samples ranged from 2 × 10¹⁷ to 2 × 10¹⁸ cm^?3. The angular dependences of the oscillation periods and cyclotron effective masses showed that the conduction band exhibits an energy dependent anisotropy, obeying the Kildal band structure model. For the low-temperature values of the band parameters we found: a band gap E_g = 0.30 eV, a spin-orbit splitting Δ = 0.50 eV, a crystal field splitting parameter δ = ?0.09 eV, and an interband matrix element P = 8.5 × 10^?8eV cm. This simple four-level model was found to be not adequate to describe quantitatively the observed electronic effective g-factor for a sample with low electron concentration.     

Strain effect on the electronic properties of 1T-HfS2 monolayer

We perform first-principles based on the density function theory to investigate electronic and magnetic properties of 1T-HfS₂ monolayer with biaxial tensile strain and compressive strain. The results show that HfS₂ monolayer under strains doesn’t display magnetic properties. When the strain is 0%, the HfS₂ monolayer presents an indirect band gap semiconductor with the band gap is about 1.252 eV. The band gap of HfS₂ monolayer decreases quickly with increasing compressive strain and comes to zero when the compressive strain is above −7%, the HfS₂ monolayer system turns from semiconductor to metal. While the band gap increases slowly with increasing tensile strain and comes to 1.814 eV when the tensile strain is 10%. By comparison, we find that the compressive strain is more effective in band engineering of pristine 1T-HfS₂ monolayer than the tensile strain. And we notice that the extent of band gap variation is different under tensile strain. The change of band gap with strain from 1% to 5% is faster than that of the strain 6–10%. To speak of, the conduction band minimum (CBM) is all located at M point with different strains. While the valence band maximum (VBM) turns from Γ point to K point when the strain is equal to and more than 6%.     

Interface contributions to the spin-orbit interaction parameters of electrons at the (001) GaAs/AlGaAs interface

One-body mechanisms of spin splitting of the energy spectrum of 2D electrons in a one-side doped (001) GaAs/Al _x Ga_{1 ? x} As quantum well have been studied theoretically and experimentally. The interfacial spin splitting has been shown to compensate (enhance) considerably the contribution of the bulk Dresselhaus (Bychkov-Rashba) mechanism. The theoretical approach is based on the solution of the effective mass equation in a quasi-triangular well supplemented by a new boundary condition at a high and atomically sharp hetero-barrier. The model takes into account the spin-orbit interaction of electrons with both bulk and interfacial crystal potential having C _2v symmetry, as well as the lack of inversion symmetry and nonparabolicity of the conduction band in GaAs. The effective 2D spin Hamiltonian including both bulk and interface contributions to the Dresselhaus (α_BIA) and Rashba (α_SIA) constants has been derived. The analytical relation between these constants and the components of the anisotropic nonlinear g-factor tensor in an oblique quantizing magnetic field has been found. The experimental approach is based, on one hand, on the detection of electron spin resonance in the microwave range and, on the other hand, on photoluminescence measurements of the nonparabolicity parameter. The interface contributions to α_BIA and α_SIA have been found from comparison with the theory.     

LaBiTe3: An unusual thermoelectric material

Using first‐principles calculations and semi‐classical Boltzmann transport theory, the thermoelectric properties of LaBiTe₃ are studied. The band gap and, hence, the thermoelectric response are found to be easily tailored by application of strain. Independent of the temperature, the figure of merit turns out to be maximal at a doping of about 1.6 × 10²¹ cm^–3. At room temperature we obtain values of 0.4 and 0.5 for unstrained and moderately strained LaBiTe₃, which increases to 1.1 and 1.3 at 800 K. A large spin splitting is observed in the conduction band at the T point. Therefore, LaBiTe₃ merges characteristics that are interesting for thermoelectric as well as spintronic devices. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Thermoelectric effects in a double-dot Aharonov-Bohm interferometer with Rashba spin-orbit interaction

We investigate the thermoelectric effects in a double-dot Aharonov-Bohm interferometer coupled to ferromagnetic leads held at different temperatures. The interplay of Rashba spin-orbit interaction (RSOI) and magnetic flux ϕ induces various interesting spin-dependent interference phenomena. The thermoelectric transport oscillates with ϕ. The peak of the thermopower S and figure of merit ZT splits into two new peaks and its splitting increases with the Rashba induced phase factor φ _R . With increasing φ _R S and ZT at ϕ = ± 2nπ (n = 0,1,2,...) exhibit a conversion from a peak to a valley. In the presence of the interplay of RSOI and ϕ by increasing spin polarization the splitting peaks of S (ZT) become asymmetric and ZT is greatly enhanced. The influence of the quantum dot levels on thermoelectric effects is also analyzed.     

The photoelectron and absorption spectra of allyl halides

The ultraviolet photoelectron spectra (5–20 eV) absorption spectra (1400–2500 Å) of allyl C₃H₅X (X = F, Cl, Br, I) have been measured. The ionisation potentials between 5–16 eV are determined and the type of orbital is interpreted with the aid of the vibration pattern and the band form.From these spectra several conclusions can be drawn concerning the iodide: (1) the ionisation potential of the π_C_C system of allyl iodide is lower than expected when compared to the ionisation potential of the corresponding band of the other halides. (2) The splitting of its np lone pair photoelectron bands is not in accordance with the expected splitting associated with spin-orbit coupling. (3) One of the two components of the lone pair bands has been broadened. These observations can be explained by a through-space interaction of one of the iodine “lone pairs” with the π-system.     

Magnetooptical,optical, and magnetotransport properties of Co/Cu superlattices with ultrathin cobalt layers

We investigated the field dependences of the magnetization and magnetoresistance of superlattices [Co(t _x, Å)/Cu(9.6 Å)]30 prepared by magnetron sputtering, differing in the thickness of cobalt layers (0.3 Å ≤ t _Co ≤ 15 Å). The optical and magnetooptical properties of these objects were studied by ellipsometry in the spectral region of hω= 0.09–6.2 eV and with the help of the transverse Kerr effect (hω= 0.5–6.2 eV). In the curves of an off-diagonal component of the tensor of the optical conductivity of superlattices with t _Co = 3–15 Å, a structure of oscillatory type (“loop”) was detected in the ultraviolet region, resulting from the exchange splitting of the 3d band in the energy spectrum of the face-centered cubic structure of cobalt (fcc Co). Based on magnetic experiments and measurements of the transverse Kerr effect, we found the presence of a superparamagnetic phase in Co/Cu superlattices with a thickness of the cobalt layers of 3 and 2 Å. The transition from superlattices with solid ferromagnetic layers to superparamagnetic cluster-layered nanostructures and further to the structures based on Co and Cu (t _Co = 0.3–1 Å) with a Kondo-like characteristics of the electrical resistivity at low temperatures is analyzed.     

Tunable electronic structure in stained two dimensional van der Waals g-C2N/XSe2 (X = Mo,W) heterostructures

The electronic structure of the strained g-C₂N/XSe₂ (X=Mo, W) van der Waals heterostructures are investigated by first-principles calculations. The g-C₂N/MoSe₂ heterostructure is an indirect band gap semiconductor at a strain from 0% to 8%, where its band gap is 0.66, 0.61, 0.73, 0.60 and 0.33 eV. At K point, the spin splitting is 186, 181, 39, 13 and 9 meV, respectively. For g-C₂N/WSe₂ heterostructures, the band gap is 0.32, 0.37, 0.42, 0.45 and 0.36 eV, and the conduction band minimum is shifted from Г-M region to K-Г region as the strain increases from 0% to 8%. Its spin splitting monotonically decreases as a strain raises to 8%, which is 445, 424, 261, 111 and 96 meV, respectively. Moreover, at a strain less than 4%, the conduction band mainly comes from g-C₂N, but it comes from XSe₂ (X=Mo, W) above 6%. Our results show that the g-C₂N/XSe₂ heterostructures have tunable electronic structures, which makes it a potential candidate for novel electronic devices.     

Momentenanalyse derR 2-Nullphononen-Linie in NaF unter einaxialem Druck

The R₂ zero-phonon absorption line 4480 Å in NaF at 4.2 °K and 23 °K is studied under uniaxial stress along 〈100〉 and 〈100〉. According to the F₃ model of theR center the R₂ line is found to be due to a transition between a degenerate ground state (²E) and a nondegenerate excited state (²A) of this center. Instead of a splitting into single components under stress, changes in line shape are observed which are analyzed by means of the method of moments. The zeroth and first moment of the line are calculated as functions of the magnitude of stress and temperature. The theoretical moments are found to be in good agreement with those determined from the experimental data. From a comparison of the measured moments with the theoretical expressions the stress splitting parameters are obtained which describe the removal of both the orientational and the orbital degeneracy of theR center under uniaxial stress. The corresponding strain parameters of the R² line in NaF are compared with those in other alkali halides.     

Growth and studies of Hg1−xCdxTe based low dimensional structures

The band structure of HgTe quantum wells (QWs) has been determined from absorption experiments on superlattices in conjunction with calculations based on an 8×8 k·p model. The band structure combined with self-consistent Hartree calculations has enabled transport results to be quantitatively explained.Rashba spin–orbit, (SO) splitting has been investigated in n-type modulation doped HgTe QWs by means of Shubnikov–de Haas oscillations (SdH) in gated Hall bars. The heavy hole nature of the H1 conduction subband in QWs with an inverted band structure greatly enhances the Rashba SO splitting, with values up to 17 meV.By analyzing the SdH oscillations of a magnetic two-dimensional electron gas (2DEG) in modulation-doped n-type Hg_1−xMn_xTe QWs, we have been able to separate the gate voltage-dependent Rashba SO splitting from the temperature-dependent giant Zeeman splitting, which are of comparable magnitudes. In addition, hot electrons and Mn ions in a magnetic 2DEG have been investigated as a function of current.Nano-scale structures of lower dimensions are planned and experiments on sub-micrometer magneto-transport structures have resulted in the first evidence for ballistic transport in quasi-1D HgTe QW structures.     

Electron–hole symmetry and spin–orbit splitting in IV–VI asymmetric quantum wells

It is shown that, due to the electron–hole symmetry of the fundamental gap of the lead–salts (PbTe, PbSe and PbS), the Rashba spin splitting in their flat band asymmetric quantum wells is much reduced with the usual equal conduction and valence band-offsets. Different from the III–V case, we find that the important structure inversion asymmetry for the Rashba splitting in IV–VI quantum wells with different left and right barriers is not a material property (i.e., barrier height, effective mass or band gap) but results from the band alignment. This is shown by specific envelope function calculations of the spin-dependent subband structure of Pb_1−xEu_xTe/PbTe/Pb_1−yEu_yTe asymmetric quantum wells (x≠y), based on a simple but accurate four-band kp model for the bulk band structure near the gap, which takes into account band anisotropy, nonparabolicity and multi-valley effects.     

Structural and phase transition of α-Al2O3 powders obtained by co-precipitation method

Aluminium oxide has been synthesized by co-precipitation technique at different annealing temperature. Powder XRD confirms the formation of α-Al₂O₃ with rhombohedral crystal structure having lattice constant a = 4.76 Å and b = 12.99 Å by the Scherer formula, the average crystallite size is estimated to be 66 nm. The scanning electron microscope results expose the fact that the α-Al₂O₃ nanomaterials are seemingly porous in nature and highly agglomerated. Chemical composition of aluminium oxide is confirmed by energy dispersive spectroscopy. The molecular functional group is confirmed by FTIR. Optical absorption of α-Al₂O₃ has been studied in the UV–vis region and its direct band gap is estimated to be 5.97 eV. This study involves the structural and phase transition of Al₂O₃ and also indicates that α-Al₂O₃ has considerable properties, deserving further investigation for the energetic materials with excellent properties for the possibility of using thin-layer α-Al₂O₃ as a thermo luminescence material.