Spin-conserving Transitions Research Articles

Is the Bethe-Salpeter Formalism Accurate for Excitation Energies? Comparisons with TD-DFT, CASPT2, and EOM-CCSD.

Developing ab initio approaches able to provide accurate excited-state energies at a reasonable computational cost is one of the biggest challenges in theoretical chemistry. In that framework, the Bethe–Salpeter equation approach, combined with the GW exchange-correlation self-energy, which maintains the same scaling with system size as TD-DFT, has recently been the focus of a rapidly increasing number of applications in molecular chemistry. Using a recently proposed set encompassing excitation energies of many kinds [J. Phys. Chem. Lett.2016, 7, 586–591], we investigate here the performances of BSE/GW. We compare these results to CASPT2, EOM-CCSD, and TD-DFT data and show that BSE/GW provides an accuracy comparable to the two wave function methods. It is particularly remarkable that the BSE/GW is equally efficient for valence, Rydberg, and charge-transfer excitations. In contrast, it provides a poor description of triplet excited states, for which EOM-CCSD and CASPT2 clearly outperform BSE/GW. This contribution therefore supports the use of the Bethe–Salpeter approach for spin-conserving transitions.

Second-Order Perturbation Formula for Magnetocrystalline Anisotropy Using Orbital Angular Momentum Matrix

We derive a second-order perturbation formula for an electronic system subject to spin–orbit interactions (SOI). The energy correction originates in the spin-conserving and the spin-flip transitions. The former are represented by the orbital angular momentum (OAM) acquired via the SOI. The latter come from the quantum fluctuation effect. By using our formula, we examine the relativistic electronic structures of a d orbital chain and L10 alloys. The appearance of OAM in the chain is understood by using a parabolic-bands model and the exact expressions of the single-particle states. The total energy is found to be accurately reproduced by the formula. The self-consistent fully relativistic first-principles calculations based on the density functional theory are performed for five L10 alloys. It is found that the formula reproduces qualitatively the behavior of their exact magnetocrystalline anisotropy (MCA) energies. While the MCA of FePt, CoPt, and FePd originates in the spin-conserving transitions, that in MnAl and MnGa originates in the spin-flip contributions. For FePt, CoPt, and FePd, the tendency of the MCA energy with the variation in the lattice constant obeys basically that of the spin-flip contributions. These results indicate that not only the anisotropy of OAM, but also that of spin-flip contributions must be taken into account for the understanding of the MCA of the L10 alloys.

Intersubband polaritons with spin-orbit interaction

We investigate intersubband polaritons formed in the asymmetric quantum well (AQW) embedded into the semiconductor microcavity and study the effects of spin-orbit interaction (SOI) acting on intersubband excitations. The spin-orbit interaction of Rashba and Dresselhaus type remove the spin degeneracy of electrons with finite value of in-plane momentum and allow four types of intersubband excitations. While optical spin-flip transitions are suppressed, the spectrum of elementary excitations shows the appearance of upper, lower and middle polaritonic branches based on spin-conserving transitions. The accounting of finite photon momentum leads to non-zero average spin projection of electronic ensemble in the first excited subband under cw excitation for both isotropic (Rashba) and anisotropic (Rashba and Dresselhaus) SOI. We predict the possibility of spin current generation in the considered systems with long coherence length.

Cyclotron resonance in P-doped InSb quantum wells

A combined experimental and theoretical study of cyclotron resonance was conducted on a two-dimensional hole system in an InSb quantum well, which was 9 nm thick and compressively strained by ∼0.65%. An effective mass of 0.065mo and a g-factor |g∗|≈23 were deduced from cyclotron resonance features at magnetic fields of 2.5 T and 6 T, respectively, when the InSb quantum well had a hole density of 3.5×1011 cm−2. At higher magnetic fields, separate cyclotron resonances are observed for different spin-conserving transitions. The magnetic field dependences of the effective mass and g-factor for holes are well explained by an 8-band Pidgeon-Brown model generalized to include the effects of the confinement potential and pseudomorphic strain.

Correlation effects in the resonant and nonresonant manganese3s→2pphoton emission inMnF2

Experimental results for manganese $3\stackrel{\ensuremath{\rightarrow}}{s}2p$ photon emission in ${\mathrm{MnF}}_{2}$ that occurs after resonant and nonresonant production of a $2p$ hole are presented. Transitions that involve a change in the spin of the ${3d}^{5}$ subshell are observed and identified. Results of a free-ion calculation that includes the effect of the interaction between the configurations with a single $3s$ hole and two $3p$ holes in the final state is also given. Good overall agreement is found between the experiment and the calculation. This comparison allows a direct interpretation of the absorption spectrum, the Raman emission spectra, and the normal or nondispersive fluorescence spectrum in terms of well-defined ionic states. Spin-changing transitions in both absorption and emission are identified, and their intensities relative to the spin-conserving transitions are found to be well predicted by theory.

Spin phenomena in intersubband transitions

Intersubband transitions have been studied in InAs/AlSb multi quantum wells subjected to a parallel magnetic field. In this Voigt configuration we observe two resonances whose oscillator strengths increase with magnetic field. The high energy line is interpreted as a superposition of two spin-conserving intersubband transitions while the low energy resonance corresponds to two combined spin-flip intersubband transitions. The lines are separated in energy by the depolarization shift which influences only the spin-conserving excitations. The spin-conserving transitions are induced by the parallel magnetic field although the incident light is polarized in the plane of the quantum well. The combined resonances, on the other hand, are made possible by a combination of the InAs bulk inversion asymmetry in conjunction with its narrow band gap and the high carrier density as present in our samples. In the multiple reflection path geometry the spin-flip lines can be resolved and we observe a zero-field spin-splitting of approximately Δ E=17 meV which is also a result of the bulk inversion asymmetry.

Magnetic-Field-Induced Spin-Conserving and Spin-Flip Intersubband Transitions in InAs Quantum Wells.

Intersubband optical resonances in InAs/AlSb quantum wells have been studied in the Voigt configuration. Both spin-conserving and spin-flip transitions in the midinfrared spectral range were observed simultaneously. We demonstrate that the in-plane magnetic field mediates the spin-conserving transitions, whereas the bulk inversion asymmetry of InAs induces spin-flip excitations. The difference in the observed resonance energies allows us to deduce directly the depolarization shift. Moreover, we show that this depolarization strongly reduces the linewidth of the spin-conserving resonance.

The effect of Landau quantization on cyclotron resonance in a non-parabolic quantum wells

Electron cyclotron resonance is studied in very deep quantum wells consisting of InAs between AlSb barriers. High electron mobility and strong conduction band non-parabolicity together with the small effective mass and the large effective g factor of this material enables the authors to observe simultaneous cyclotron transitions between adjacent sets of spin-split Landau states. Their experiments resolve spin-conserving transitions involving two or three different Landau levels depending on the filling factor. The results are compatible with a single-particle model.

Magnetophonon effect in a nonparabolic band:n-type InSb

Measurements of the transverse Ohmic magnetophonon effect have been performed at 77 K in a set of samples of $n$-InSb having carrier concentrations in the range from $n=5\ifmmode\times\else\texttimes\fi{}{10}^{13}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ to $n=7\ifmmode\times\else\texttimes\fi{}{10}^{16}$ ${\mathrm{cm}}^{\ensuremath{-}3}$. With increasing doping, the minima in the second derivative of the resistance with respect to the magnetic field are shifted to higher magnetic fields. Even in the purest samples the values of the resonant magnetic fields for harmonic numbers up to $N=12$ can only be explained if the contributions of spin-conserving transitions involving both $L=0$ spin levels and spin-split Landau levels with $L>0$ are taken into account. These transitions occur at magnetic fields which are higher than the fields for the $L=0$ lower-spin-level transition because of the nonparabolicity of the InSb conduction band. A superposition of Lorentzian lines with an empiricall determined half-width $\ensuremath{\Delta}(N)$ proportional to the harmonic number $N$ and weighted with the value of the Fermi distribution function at the energy of the lower level is shown to give a good fit to the data yielding a bard-edge effective mass of ${{m}^{*}}_{0}=0.0138 {m}_{0}$. In more highly doped samples, the shift of the extrema to higher magnetic fields is partially caused by the larger value of the damping parameter $\ensuremath{\gamma}$ because of the lower mobility. After applying an appropriate correction to the extremal positions of the high-concentration samples, a shift remains which qualitatively can be explained by the increasing contribution of higher-order transitions because of the higher population of these levels. Finally, the shifts in extremal position as a consequence of an increased electron temperature, e.g., induced by application of electric fields or photoexcitation, are discussed within the framework of this model.