Bulk Inversion Asymmetry Research Articles

Spin orientation and spin precession in inversion-asymmetric quasi-two-dimensional electron systems

Inversion asymmetry induced spin splitting of the electron states in quasi two-dimensional (2D) systems can be attributed to an effective magnetic field B which varies in magnitude and orientation as a function of the in-plane wave vector k||. Using a realistic 8x8 Kane model that fully takes into account spin splitting because of both bulk inversion asymmetry and structure inversion asymmetry we investigate the spin orientation and the effective field B for different configurations of a quasi 2D electron system. It is shown that these quantities depend sensitively on the crystallographic direction in which the quasi 2D system was grown as well as on the magnitude and orientation of the in-plane wave vector k||. These results are used to discuss how spin-polarized electrons can precess in the field B(k||). As a specific example we consider GaInAs-InP quantum wells.

Theory of Electrically Controlled Resonant Tunneling Spin Devices

AbstractWe report device concepts that exploit spin-orbit coupling for creating spin polarized current sources using nonmagnetic semiconductor resonant tunneling heterostructures, without external magnetic fields. The resonant interband tunneling spin filter exploits large valence band spin-orbit interaction to provide strong spin selectivity. The bi-directional spin pump induces the simultaneous flow of oppositely spin-polarized current components in opposite directions through spin-dependent resonant tunneling. The efficiency of resonant tunneling spin devices can be improved when the effects of structural inversion asymmetry (SIA) and bulk inversion asymmetry (BIA) are combined properly, and incorporated into device design. The current spin polarizations of the proposed devices are electrically controllable, and potentially amenable to high-speed modulation. In principle, the electrically modulated spin-polarized current source could be integrated in optoelectronic devices for added functionality.

Magnetoelectric subbands and eigenstates in the presence of Rashba and Dresselhaus spin–orbit interactions in a quantum wire

We derive the eigenenergies and spin-dependent eigenstates of electrons in a quantum wire subjected to an external magnetic field. These are calculated in the presence of spin–orbit interactions arising from the Rashba (structural inversion asymmetry) and Dresselhaus (bulk inversion asymmetry) effects. We consider three cases: the external magnetic field is oriented (i) along the axis of the wire, (ii) perpendicular to the axis but parallel to the electric field associated with structural inversion asymmetry (Rashba effect), and (iii) perpendicular to the axis as well as this electric field. In all cases, the dispersions of the eigenenergies are non-parabolic and the subbands do not have a fixed spin quantization axis (meaning that the spin polarization of the electron is wavevector dependent). Except in the second case, the dispersion diagrams are also, in general, asymmetric about the energy axis.

Spin-orbit coupling and magnetic spin states in cylindrical quantum dots

We make a detailed analysis of each possible spin-orbit coupling of zincblende narrow-gap cylindrical quantum dots built in a two-dimensional electron gas. These couplings are related to both bulk (Dresselhaus) and structure (Rashba) inversion asymmetries. We study the competition between electron-electron and spin-orbit interactions on electronic properties of 2-electron quantum dots.

Spin Lifetime Tuning in Zincblende Heterostructures and Tions to Spin Devices

AbstractWe present analytical expressions for the D'yakonov-Perel' spin relaxation rates under the combined action of bulk and structural inversion asymmetry for [111] zincblende heterostructures when terms up to linear and third order in k are included in the Hamiltonian. We see for [111] heterostructures that, under the right conditions, the lowest-order-in-k component of the spin relaxation tensor can be made to vanish for all spin components at the same time. We study how the inclusion of terms of higher order in k affects these results. We finally discuss a proposal for a resonant spin lifetime transistor (RSLT) using the spin lifetime tuning concepts presented above, where the characteristics of the [111] device give the designer an added degree of freedom on the direction of the injected spins.

Bulk inversion asymmetry enhancement of polarization efficiency in nonmagnetic resonant-tunneling spin filters

We analyze the effect of bulk inversion asymmetry (BIA) on the efficiency of nonmagnetic heterostructure resonant tunneling spin filters grown along $[001].$ We show that when only structural inversion asymmetry (SIA) is taken into account, the theoretical upper limit of current spin polarization for the Rashba effect resonant tunneling spin filter with a one-sided collector geometry is $2/\ensuremath{\pi}(\ensuremath{\approx}63.7%),$ independent of the direction of the one-sided collector. When both SIA and BIA effects are included, we can significantly enhance or degrade current spin polarization from the SIA-only value by varying the collection angle, with the best and worst directions, respectively, being $[110]$ and $[11\ifmmode\bar\else\textasciimacron\fi{}0]$ (or vice versa, depending on the relative sign between the SIA and BIA coefficients). We can similarly improve the spin filtering efficiency of Rashba effect resonant tunneling spin filters that rely on the use of a lateral electric field in the emitter region to induce spin polarization, simply by applying the field along $[110]$ (or $[11\ifmmode\bar\else\textasciimacron\fi{}0]).$

Description of bulk inversion asymmetry in the effective-bond-orbital model

We have extended the effective-bond-orbital model (EBOM) method [Y. C. Chang, Phys. Rev. B 37, 8215 (1988)] to include the effects of the bulk inversion asymmetry (BIA) present in zinc blendes. This is accomplished without adding to the number of basis states or extending the range of interaction. We have also investigated a variant form of the EBOM proposed in the original formulation that offers improved zone-center behavior, but may also generate spurious solutions in heterostructure calculations due to poor description of bulk zone-boundary band structure. We offer suggestions for avoiding this problem so that this variant form of EBOM may be used safely. In general, we find that the addition of BIA effects in EBOM results in improved descriptions of zone-center band structure, but also in a loss of accuracy far from the Brillouin-zone center. We illustrate the use of the BIA extension with band-structure calculations for bulk GaSb. We show that the spin splitting predicted by the extended EBOM method for an AlSb/GaSb superlattice is in good agreement with $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ calculations that include BIA effects.

Anisotropic splitting of intersubband spin plasmons in quantum wells with bulk and structural inversion asymmetry

In semiconductor heterostructures, bulk and structural inversion asymmetry and spin-orbit coupling induce a $\mathbf{k}$-dependent spin splitting of valence and conduction subbands, which can be viewed as being caused by momentum-dependent crystal magnetic fields. This paper studies the influence of these effective magnetic fields on the intersubband spin dynamics in an asymmetric n-type ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}$ quantum well. We calculate the dispersions of intersubband spin plasmons using linear-response theory. The so-called D'yakonov-Perel' decoherence mechanism is inactive for collective intersubband excitations, i.e., crystal magnetic fields do not lead to decoherence of spin plasmons. Instead, we predict that the main signature of bulk and structural inversion asymmetry in intersubband spin dynamics is a threefold, anisotropic splitting of the spin plasmon dispersion. The importance of many-body effects is pointed out, and conditions for experimental observation with inelastic light scattering are discussed.

InAs/GaSb/AlSb resonant tunneling spin device concepts

We discuss device concepts for creating spin-polarized current sources without external magnetic fields, using non-magnetic 6.1 A ̊ semiconductor resonant tunneling structures. Spin filters, spin pumps, and spin transistors that exploit structural and bulk inversion asymmetries will be examined.

Optical spin resonances due to structure and bulk inversion asymmetry in heterostructures

Optical spin–flip excitations in the conduction band of III–V semiconductor heterostructures are considered theoretically taking into account structure inversion asymmetry (SIA) and bulk inversion asymmetry (BIA) of such systems. Possible spin transitions both in the absence of a magnetic field ( B=0) as well as in the presence of a magnetic field B parallel to the growth direction [0 0 1] are investigated. The theory is based on the three-level model of the narrow-gap band structure including the BIA [Phys. Rev. 100 (1955) 580] and SIA [J. Phys. C. 17 (1984) 6039] contributions. We show in particular that the SIA mechanism not only results in the Bychkov–Rashba spin splitting at B=0 but it also gives rise to the possibility of optical transitions between the two spin-split energy branches.

Nonmagnetic semiconductor spin transistor

We propose a spin transistor using only nonmagnetic materials that exploits the characteristics of bulk inversion asymmetry (BIA) in (110) symmetric quantum wells. We show that extremely large spin splittings due to BIA are possible in (110) InAs/GaSb/AlSb heterostructures, which together with the enhanced spin decay times in (110) quantum wells demonstrates the potential for exploitation of BIA effects in semiconductor spintronics devices. Spin injection and detection is achieved using spin-dependent resonant interband tunneling and spin transistor action is realized through control of the electron spin lifetime in an InAs lateral transport channel using an applied electric field (Rashba effect). This device may also be used as a spin valve, or a magnetic field sensor.

Spin relaxation in (110) and (001) InAs/GaSb superlattices

We report an enhancement of the electron spin relaxation time (T1) in a (110) InAs/GaSb superlattice by more than an order of magnitude (25 times) relative to the corresponding (001) structure. The spin dynamics were measured using polarization sensitive pump probe techniques and a mid-infrared, subpicosecond PPLN OPO. Longer T1 times in (110) superlattices are attributed to the suppression of the native interface asymmetry and bulk inversion asymmetry contributions to the precessional D'yakonov Perel spin relaxation process. Calculations using a nonperturbative 14-band nanostructure model give good agreement with experiment and indicate that possible structural inversion asymmetry contributions to T1 associated with compositional mixing at the superlattice interfaces may limit the observed spin lifetime in (110) superlattices. Our findings have implications for potential spintronics applications using InAs/GaSb heterostructures.

Spin dephasing in quantum wires

We study high-field spin transport in a quantum wire using a semiclassical approach. Spin dephasing (or spin depolarization) in the wire is caused by D'yakonov-Perel' relaxation associated with bulk inversion asymmetry (Dresselhaus spin-orbit coupling) and structural inversion asymmetry (Rashba spin-orbit coupling). The depolarization rate is found to depend strongly on the initial polarization of the spin. If the initial polarization is along the axis of the wire, the spin depolarizes \ensuremath{\sim}100 times slower compared to the case when the initial polarization is transverse to the wire axis. We also find that in the range 4.2\char21{}50 K, temperature has a weak influence and the driving electric field has a strong influence on the depolarization rate. The steady state distribution of the spin components parallel and transverse to the wire axis also depend on the initial polarization. If the initial polarization is along the wire axis, then the steady state distribution of both components is a flat-topped uniform distribution, whereas if the initial polarization is transverse to the wire axis, then the distribution of the longitudinal component resembles a Gaussian, and the distribution of the transverse component is U shaped.

Effect of bulk inversion asymmetry on the Datta-Das transistor

A model of the Datta-Das spin field-effect transistor is presented which, in addition to the Rashba interaction, takes into account the influence of bulk inversion asymmetry of zinc-blende semiconductors. In the presence of bulk inversion asymmetry, the conductance is found to depend significantly on the crystallographic orientation of the channel. We determine the channel direction optimal for the observation of the Datta-Das effect in GaAs and InAs-based devices.

Electron-spin polarization in symmetric type-II quantum wells from bulk inversion asymmetry

The effect of the bulk inversion asymmetry on electron-spin polarization by resonant tunneling in symmetric, nonmagnetic, type-II semiconductor quantum wells is investigated within the envelope function approximation. A $14\ifmmode\times\else\texttimes\fi{}14\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ matrix Hamiltonian is used which includes the nonparabolicity and spin-split nature of the energy-band structure. Spin-dependent boundary conditions are derived and the transfer-matrix method is applied to obtain analytical expressions for the electron-spin polarization in single- and double-quantum wells. Numerical calculations of the polarization are performed for quantum well systems made from InAs and GaSb. It is shown that the inversion asymmetry of the bulk materials can produce electron-spin polarizations of up to $90%$ for oblique tunneling.

Spin splitting induced circular photocurrent in quantum wells

A theory of the circular photogalvanic effect caused by spin splitting in quantum wells is developed. Direct interband transitions between the hole and electron size-quantized subbands are considered. The photocurrent excitation spectrum is shown to depend strongly on the form of the spin–orbit interaction. In the case of structure inversion asymmetry induced (Rashba) spin splitting, it is a linear function of light frequency near the absorption edge. In contrast, when the bulk inversion asymmetry (Dresselhaus splitting) dominates, the photocurrent edge behavior is parabolic.

An efficient multiband envelope function approximation method for spintronics

We have developed an eight-band finite-difference envelope functionapproximation model capable of reproducing in almost all situations the true D2d orC2vsymmetry of [001] grown zinc-blende heterostructures. We have used our model tostudy the relative contributions of the bulk inversion asymmetry (BIA) andstructural inversion asymmetry to the spin splitting in the conduction band ofasymmetric AlSb/GaSb/InAs/AlSb quantum wells, and clarify apparentlycontradictory statements about the relative magnitude of the two contributions.We show that, in the system under study, the inclusion of BIA effects changesconsiderably the angular dependence and the magnitude of the splitting. We alsoinvestigate how BIA changes the transmission properties of a resonant tunnellingstructure.

Intersubband spin-density excitations in quantum wells with Rashba spin splitting

In inversion-asymmetric semiconductors, spin-orbit coupling induces a $\mathbf{k}$-dependent spin splitting of valence and conduction bands, which is a well-known cause for spin decoherence in bulk and heterostructures. Manipulating nonequilibrium spin coherence in device applications thus requires understanding how valence and conduction band spin splitting affects carrier spin dynamics. This paper studies the relevance of this decoherence mechanism for collective intersubband spin-density excitations (SDE's) in quantum wells. A density-functional formalism for the linear spin-density matrix response is presented that describes SDE's in the conduction band of quantum wells with subbands that may be nonparabolic and spin split due to bulk or structural inversion asymmetry (Rashba effect). As an example, we consider a 40 nm ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}$ quantum well, including Rashba spin splitting of the conduction subbands. We find a coupling and wave-vector-dependent splitting of the longitudinal and transverse SDE's. However, decoherence of the SDE's is not determined by subband spin splitting, due to collective effects arising from dynamical exchange and correlation.

Microscopic interface asymmetry and spin-splitting of electron subbands in semiconductor quantum structures

The microscopic interface asymmetry of (001)-grown semiconductor heterostructures that gives rise to heavy-light hole coupling even at zero in-plane wave vector k ‖, modifies also the subband dispersion of confined electrons. Starting from a multiband envelope formulation we apply matrix perturbation theory to derive explicit expressions caused by this interface asymmetry, which in the 2×2 conduction band Hamiltonian appear as a warping and a spin-splitting term. The warping term results in an inequivalence of the dispersion along [110] and [1 1 0] as required by the microscopic C 2v symmetry, while the spin-splitting term has the same structure as the corresponding term derived from the zinc-blende bulk inversion asymmetry. Implications with respect to spin-relaxation will be discussed.

Spin-orbit splitting in semiconductor quantum dots with a parabolic confinement potential

We present a theoretical study of the effect of spin-orbit interaction on the electron energy spectrum of cylindrical semiconductor quantum dots. The study is based on a simple effective one-band approximation. The dependence of energy levels on parameters of the dots and the applied external magnetic field is studied. Contributions of the bulk inversion asymmetry (the Dresselhaus term) and the system inversion asymmetry (the Rashba term) to the spin splitting of the electron energy states are discussed. The spin splitting of electron states with nonzero angular momentum is demonstrated theoretically for InSb and InAs small quantum dots at zero magnetic field. We find a ``crossing'' of the electron energy states with the same angular momentum and different spin polarizations in a nonzero magnetic field.