Splitting Of Valence Band Research Articles

Ultrafast spin transfer and its impact on the electronic structure.

Optically induced intersite spin transfer (OISTR) promises manipulation of spin systems within the ultimate time limit of laser excitation. Following its prediction, signatures of ultrafast spin transfer between oppositely aligned spin sublattices have been observed in magnetic alloys and multilayers. However, it is known neither from theory nor from experiment whether the band structure immediately follows the ultrafast change in spin polarization or whether the exchange split bands remain rigid. We show that ultrafast spin transfer occurs even in ferromagnetic gadolinium metal. Charge transfer between localized surface and extended valence-band states leads to a decrease of the surface spin polarization. This synchronously alters the exchange splitting of the bulk valence bands during laser excitation. Moreover, the onset of demagnetization can be tuned by over 200 fs by changing the temperature-dependent spin mixing. Our results show a promising route to ultrafast control of the magnetization, widening the impact and applicability of OISTR.

Orthogonally and linearly polarized green emission from a semipolar InGaN based microcavity

Abstract Polarized light has promising applications in biological inspections, displays, and precise measurements. Direct emission of polarized light from a semiconductor device is highly desired in order to reduce the size and energy-consumption of the whole system. In this study, we demonstrate a semipolar GaN-based microcavity light-emitting diode (MCLED) that could simultaneously produce green light with perpendicular and parallel polarizations to the c*-axis. Orthogonally polarized emission with a narrow linewidth (∼0.2 nm) arises from the valence band splitting and birefringent nature of the semipolar GaN material, as well as the mode selection of the resonant cavity. By modulating the cavity length, the device is capable of switching between single- and multi-mode emission spectra. We believe that the approach of employing a cavity structure and semipolar GaN can be extended to produce orthogonally and linearly polarized blue, red, and violet light by adjusting the material compositions.

Lattice vibrational modes in changchengite from Raman spectroscopy and first principles electronic structure

AbstractWe measured room‐temperature phonon Raman spectra of changchengite (IrBiS) and compared the experimental phonon wavenumbers to the theoretical ones obtained by means of the ab initio density‐functional‐theory calculations in the presence and absence of the spin‐orbit coupling effects. Combining two different excitation photon energies all the symmetry predicted Raman modes are experimentally observed. The electronic properties of IrBiS are found to be similar to the recently studied isostructural compound IrBiSe showing a large Dresselhaus spin‐orbit valence band splitting. A good agreement between the experimental and theoretically predicted Raman phonon wavenumbers is found only when the lattice parameter is constrained to the experimental value. The inclusion of the spin orbit coupling does not significantly affect the phonon wavenumbers.

Odd-even layer-number effect of valence-band spin splitting in WTe2

When a crystal becomes thin to the atomic level, peculiar phenomena discretely depending on its layer numbers ($n$) start to appear. Here, we investigate the electronic band dispersions of multilayer ${\mathrm{WTe}}_{2}$ (2--5 layers), by performing laser-based microfocused angle-resolved photoelectron spectroscopy on exfoliated flakes sorted by $n$. We observe that the holelike valence bands start to cross the Fermi level when the number of layers is increased from 2- to 3 layers, which should be related to the insulator-semimetal transition, as well as the 30--70-meV spin splitting of valence bands manifesting in even $n$ as a signature of stronger structural asymmetry. Our result fully demonstrates the possibility of the large energy-scale band and spin manipulation through the finite-$n$ stacking procedure.

Deep transfer learning correlation study of electronic and spin properties in buckled III–V monolayers

The correlation study of the electronic and spin properties in the buckled III–V monolayers using the transfer learning network approach is performed. Band engineering based on such approach has been applied to the target parameters of bandgap without spin–orbit coupling and with spin–orbit coupling, spin splitting at K-point in either of valence and conduction bands, and band splitting at Γ-point in the valence band. It is also found that some special nonlinear combination of Group-III and V atomic numbers together with their corresponding atomic mass can accurately describe the mentioned band engineering parameters of III–V monolayers. In this regard, a nonlinear descriptor relation as a function of atomic number and atomic mass is reported for every parameter. The maximum decoupling of Eg, WOS from other compounds is observed for InN, BSb and GaN. For this parameter, heavier compounds tend to have more correlation with the other ones. A similar pattern is observed for EgWS, although the correlation for heavier compounds is more evident. In addition, more compounds demonstrate discriminatory behavior for EgWS. Furthermore, the maximum correlating parameter among III–V compounds is valence band splitting. For heavier compounds, valence and conduction spin-splitting demonstrate sensible discrimination between the band engineering predictions and DFT calculations. Interestingly, for AlSb, a clear independent behavior for conduction band spin-splitting is also reported.

Epitaxial growth of GaAsBi on thin step-graded InGaAs buffer layers

Molecular beam epitaxy growth and analysis of GaAsBi on compositional step-graded InGaAs buffer layers are presented in this study. The developed buffer is only 240 nm thick, exhibits very low surface roughness while reaching up to 0.46% lattice-mismatch with a GaAs substrate. Reciprocal-space mappings showed that 500 nm thick GaAsBi layers with 2.7%–5.3% Bi remain pseudomorphic with the InGaAs buffer, in contrast to GaAsBi grown on GaAs that were found to incur up to 50% lattice relaxation. CuPtB-type ordering and associated polarized photoluminescence were also found in the bismide layers grown on the InGaAs buffers. Optical anisotropy of a strain-free 2.7% Bi GaAsBi was further analysed by a suite of optical techniques indicating that the valence band splitting is ∼40 meV. This study advances synthesis techniques of thick GaAsBi layers for optoelectronic device applications.

Influence of the spin-orbit split-off valence band on the hole g factor in semiconductor nanocrystals

We present results of $\mathbit{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbit{p}$ calculations of the effective $g$ factor of holes confined in spherical, cube, and planar semiconductor nanocrystals (NCs). We use the six-band Luttinger model for semiconductors with the zinc-blende crystal structure and study the size dependence of the ${\mathrm{\ensuremath{\Gamma}}}_{8}$ top valence subband hole $g$ factor caused by the admixture of the spin-orbit split-off valence subband ${\mathrm{\ensuremath{\Gamma}}}_{7}$. We present semianalytical expressions for the hole $g$ factor which depends on the light- to heavy-hole effective mass ratio $\ensuremath{\beta}$ and on the ratio between spin-orbit energy splitting of valence band ${\mathrm{\ensuremath{\Delta}}}_{\text{SO}}$ and the hole quantization energy ${E}_{h}$. The admixture of ${\mathrm{\ensuremath{\Gamma}}}_{7}$ states is significant for small ${\mathrm{\ensuremath{\Delta}}}_{\text{SO}}/{E}_{h}$ and, in spherical and cube NCs, leads to a strong size dependence of the hole $g$ factor. In thin planar nanoplatelets (NPLs) with infinite or large lateral sizes, the dependence of the heavy-hole $g$ factor on NPL thickness is relatively weak. It is drastically enhanced and may become nonmonotonic in NPLs with finite in-plane sizes due to the additional hole states mixing. We discuss our results in comparison with published experimental data for CdSe- and InP-based spherical NCs and NPLs and point out the specificity of extracting hole $g$ factor from the data measured on excitons.

Cation-size mismatch as a predictive descriptor for structural distortion, configurational disorder, and valence-band splitting in II-IV-N2 semiconductors

The II-IV-N2 class of heterovalent ternary nitrides has gained significant interest as alternatives to the III-nitrides for electronic and optoelectronic applications. In this study, we apply first-principles calculations based on density functional theory to systematically investigate the effects of structural distortions due to cation size mismatch on the configurational disorder of the cation sublattice and the valence band structure in this class of materials. We find that larger size mismatch between the group-II and the group-IV cations results in stronger lattice distortions from the ideal hexagonal ratio, which in turn inhibits the propensity of these materials toward octet-rule violating cation disorder. We also demonstrate that the formation energy of a single cation antisite pair, which is fast and simple to calculate, is a strong indicator of a material's propensity toward disorder. Furthermore, the breaking of in-plane symmetry leads to a splitting of the top three valence bands at Γ, which is also directly related to the magnitude of structural distortions. Our work demonstrates that the structural and functional properties of the II-IV-N2 materials can be finely tuned through controllable structural distortions that stem from the choice of cations.

Optical anisotropy of CuPt-ordered GaAsBi alloys

The molecular beam epitaxy-grown epitaxial, partially relaxed, GaAsBi x bismide layers of thickness and x ≈ 0.04 composition are examined. The atomic-structure analysis by x-ray diffraction and transmission electron microscopy shows the bismides to be CuPt-type atomic-ordered in both and subvariants. The ordering induces an optical anisotropy, which manifests at normal incidence light-beam propagation. The anisotropy is revealed by various optical spectroscopy techniques—polarized photoluminescence, photo-modulated transmittance and reflectance, polarized transmittance, and spectroscopic ellipsometry. The ordering-induced valence band splitting, determined from modulation spectroscopy measurements, is of about . The splitting is comparable to that in CuPt-ordered conventional III–V semiconductor alloys even though the maximal ordering parameter in investigated bismides is one order of magnitude lower.

One-dimensional ferromagnetic semiconductor CrSbSe3 with high Curie temperature and large magnetic anisotropy

In recent years, low-dimensional ferromagnetic semiconductors have attracted tremendous interest due to their great significance for both fundamental physics and device applications. In this work, a one dimensional (1D) ferromagnetic semiconductor $\mathrm{CrSb}{\mathrm{Se}}_{3}$ with high Curie temperature $({T}_{\mathrm{C}})$ and sizable magnetic anisotropy is proposed based on the first-principle calculations and theoretical model. Quasi-1D chain structure with a moderate cleavage energy of $0.49\phantom{\rule{0.16em}{0ex}}\mathrm{J}/{\mathrm{m}}^{2}$ suggests the possibility to exfoliate a 1D $\mathrm{CrSb}{\mathrm{Se}}_{3}$ ladder (a ladder structure formed by two rows of 1D monoatomic Cr chain) from the bulk. Monte Carlo simulation based on anisotropic Heisenberg model predicts ${T}_{\mathrm{C}}$ to be up to 170 K, which is much higher than that of the bulk. The high ${T}_{\mathrm{C}}$ of 1D $\mathrm{CrSb}{\mathrm{Se}}_{3}$ ladder originates from the combined effect of the strong intraladder ferromagnetic exchange interaction and large magnetic anisotropy, while the lower ${T}_{\mathrm{C}}$ of the bulk results from the weak interladder indirect exchange interaction. In addition, we find that the splitting of top valence bands depends on the direction of magnetization due to the reduced structural symmetry and spin-orbit coupling, indicating a strong magnetoband structure effect. Our theoretical prediction not only provides a material platform for understanding the fundamental physics of 1D magnetism, but also provides a strategy for the search for the 1D ferromagnetic semiconductor with high Curie temperature.

Phonons and Valence-band Splitting in Strained GaAs<SUB>1-x</SUB>N<SUB>x</SUB>/GaAs Epilayers

The strain effects in strained GaAs<SUB>1-x</SUB>N<SUB>x</SUB> epilayers are characterized by Raman spectroscopy and photocurrent spectra at various nitrogen composition. In addition, the nitrogen composition and the strain were determined by using high-resolution X-ray diffraction (HR-XRD). The Raman spectra are observed to be dominated by the GaAs-like longitudinal optical (LO) phonon mode as the strongest peaks show up around 289~294 cm<SUP>-1</SUP>. Moreover, the weak and broad peaks features in the range of 255~276 cm<SUP>-1</SUP> originate from the peaks of GaAs-like transverse optical (TO) phonon mode and disorder induced, GaN-like LO. And the Raman peak shifts toward lower wave number with increasing nitrogen compositions, which is indicates the presence of tensile strain in the strained GaAs<SUB>1-x</SUB>N<SUB>x</SUB> epilayers. The valence-band splitting of GaAs<SUB>1-x</SUB>N<SUB>x</SUB> are obtained from photocurrent spectra. As the nitrogen concentration increases, the tensile strain in strained GaAs<SUB>1-x</SUB>N<SUB>x</SUB> epilayers increases while the valence-band splitting increases.

Metallic phase transition metal dichalcogenide quantum dots showing different optical charge excitation and decay pathways

The charge excitation and decay pathways of two-dimensional heteroatomic quantum dots (QDs) are affected by the quantum confinement effect, bandgap structure and strong exciton binding energy. Recently, semiconducting transition metal dichalcogenides (TMDs) have been intensively studied; however, the charge dynamics of metallic phase QDs (mQDs) of TMDs remain relatively unknown. Herein, we investigate the photophysical properties of TMD-mQDs of two sizes, where the TMD-mQDs show different charge excitation and decay pathways that are mainly ascribed to the defect states and valence band splitting, resulting in a large Stokes shift and two excitation bands for maximum photoluminescence (PL). Interestingly, the dominant excitation band redshifts as the size increases, and the time-resolved PL peak redshifts at an excitation wavelength of 266 nm in the smaller QDs. Additionally, the lifetime is shortened in the larger QDs. From the structural and theoretical analysis, we discuss that the charge decay pathway in the smaller QDs is predominantly affected by edge oxidation, whereas the vacancies play an important role in the larger QDs.

Spin relaxation of holes in In0.53Ga0.47As/InP quantum wells

Polarization-resolved photoluminescence was used to study spin relaxation of photoexcited holes in In0.53Ga0.47As/InP quantum wells in a quantizing magnetic field as a function of temperature. At a temperature below 10 K, the circular polarization of the photoluminescence due to the spin-split valence band Landau levels was found temperature-independent. In this temperature range fast hole spin relaxation as compared to their lifetime leads to the photoluminescence circular polarization determined by the ratio of these times. Increasing temperature resulted in efficient hole spin thermalization in the Zeeman split valence band Landau levels and as a consequence, in vanishing photoluminescence polarization. Fits of the experimental data by the theory allowed a determination of the hole spin relaxation times related to different Landau levels and the corresponding hole effective g-factor. Direct measurements of the hole spin relaxation times prove the obtained results.

Identification of Excitons and Biexcitons in Sb2Se3 under High Photoluminescence Excitation Density

AbstractThe near‐band‐edge emission of Sb2Se3 microcrystals is studied in detail under high photoluminescence excitation density using a pulsed UV laser (λ = 266 nm, pulse width 0.6 ns). Based on the peak energy positions and the excitation power density and temperature dependencies (T = 3–110 K) of the photoluminescence spectra, the emission is interpreted as a recombination of two pairs (A and B) of free excitons and biexcitons appearing because of valence band splitting. The magnitude of the valence band splitting due to the crystal field (Δcr = 20–22 meV) is estimated in the Brillouin zone center. At T = 3 K, the A and B biexciton emission at 1.302 and 1.322 eV, respectively, and the A and B free exciton emission at 1.311 and 1.333 eV, respectively, are observed. The binding energy of A and B biexcitons is 9 and 11 meV, respectively, and the binding energy of A free exciton is 6 meV. The activation energy of thermal quenching for all observed peaks is 20 meV.

The electrical and spin properties of monolayer and bilayer Janus HfSSe under vertical electrical field

In this paper, the electrical and spin properties of mono- and bilayer HfSSe in the presence of a vertical electric field are studied. The density functional theory is used to investigate their properties. Fifteen different stacking orders of bilayer HfSSe are considered. The mono- and bilayer demonstrate an indirect bandgap, whereas the bandgap of bilayer can be effectively controlled by the electric field. While the bandgap of bilayer closes at large electric fields and a semiconductor to metal transition occurs, the effect of a normal electric field on the bandgap of the monolayer HfSSe is quite weak. Spin–orbit coupling causes band splitting in the valence band and Rashba spin splitting in the conduction band of both mono- and bilayer structures. The band splitting in the valence band of the bilayer is smaller than a monolayer, however, the vertical electric field increases the band splitting in bilayer one. The stacking configurations without mirror symmetry exhibit Rashba spin splitting which is enhanced with the electric field.

Observation of unoccupied states of SnTe(111) using pump-probe ARPES measurement

The authors perform pump-probe ARPES measurement on a SnTe (111) surface to reveal the position of the Dirac point, and the Rashba splitting of the bulk valence band.

Spin polarization in monolayer MoS2 in the presence of proximity-induced interactions

When monolayer (ML) MoS2 is placed on a substrate, the proximity-induced interactions such as the Rashba spin-orbit coupling (RSOC) and exchange interaction (EI) can be introduced. Thus, the electronic system can behave like a spintronic device. In this study, we present a theoretical study on how the presence of the RSCO and EI can lead to the band splitting, the lifting of the valley degeneracy and to the spin polarization in [Formula: see text]- and [Formula: see text]-type ML MoS2. We find that the maxima of the in-plane spin orientation in the conduction and valence bands in ML MoS2 depend on the Rashba parameter and the effective Zeeman field factor. At a fixed Rashba parameter, the minima of the split conduction band and the maxima of the split valence band along with the spin polarization in ML MoS2 can be tuned effectively by varying the effective Zeeman field factor. On the basis that the EI can be induced by placing the ML MoS2 on a ferromagnetic substrate or by magnetic doping in ML MoS2, we predict that the interesting spintronic effects can be observed in [Formula: see text]- and [Formula: see text]-type ML MoS2. This work can be helpful to gain an in-depth understanding of the basic physical properties of ML MoS2 for application in advanced electronic and optoelectronic devices.

Near-room-temperature rhombohedral Ge1-xPbxTe thermoelectrics

Thermoelectric GeTe alloys have recently attracted renewed interests due to the extraordinary performance. It is particularly interesting in the low-symmetry structure, where the symmetry-breaking induced diversity in the arrangement in energy of split valence bands might enable an optimal band convergence for thermoelectric enhancements. This in principle leads to an extra degree of freedom for advancing GeTe thermoelectrics though a control of the degree of rhombohedral-distortion in the structure. Here we demonstrate this concept and realize its extraordinariness near room temperature, an extremely important temperature range for thermoelectric power generation/cooling applications in which very few options on materials other than conventional Bi2Te3 are currently available. This is enabled by a continuous manipulation of the rhombohedral-distortion in simple Ge1-xPbxTe solid solutions quenched from the high-temperature single phase region, a simultaneous optimization of carrier concentration and a reduction in lattice thermal conductivity due to PbTe-alloying. The resultant figure of merit, zT, at 200–350 K, is highly competitive to that of commercially available p-type Bi2Te3 alloys and higher than that of other known p-type thermoelectrics.

Temperature-dependent optical and electrical characterization of Cu-Ga-S thin films and their diode characteristics on n-Si

In this paper, optical and electrical properties of thermally deposited Cu-Ga-S thin films were investigated using temperature-dependent optical transmission and electrical conductivity measurements. The analysis of the transmission spectra resulted in formation of three direct optical transitions due to the possible valence band splitting in the structure. The band gap values were calculated by means of absorption coefficient and incident photon energy was found in decreasing behavior as the temperature rises. The measured current-voltage values were used to extract the conductivity values which stand in the range of 1.73–2.62 (×104 Ω−1 cm−1) depending on the ambient temperature. These dark conductivity values were modeled by thermionic emission mechanism. The conductivity activation energies in the structures were calculated as 6.4, 14.5 and 40.7 meV according to the effects of grain boundary potentials. In addition, the films deposited on n-Si wafer showed a diode characteristic under the applied bias voltage between indium (In) front and silver (Ag) back contacts. From current-voltage measurements across the Si-based diode, about four orders of magnitude rectification was observed and the results were analyzed to determine the main diode parameters at dark and room temperature conditions.

The carbon and oxygen K-edge NEXAFS spectra of CO.

We present and analyze high resolution near edge X-ray absorption fine structure (NEXAFS) spectra of CO+ at the carbon and oxygen K-edges. The spectra show a wealth of features that appear very differently at the two K-edges. The analysis of these features can be divided into three parts; (i) repopulation transition to the open shell orbital - here the C(1s) or O(1s) to 5σ transition, where the normal core hole state is reached from a different initial state and different interaction than in X-ray photoelectron spectroscopy; (ii) spin coupled split valence bands corresponding to C(1s) or O(1s) to π* transitions; (iii) remainder weak and long progressions towards the double ionization potentials containing a manifold of peaks. These parts, none of which has correspondence in NEXAFS spectra of neutral molecules, are dictated by the localization of the singly occupied 5σ orbital, adding a dimension of chemistry to the ionic NEXAFS technique.