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Register a new userSuppressed decay of a laterally confined persistent spin helix
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Added by
Patrick Altmann Mr.
Publication date
2014
fields
Physics
and research's language is
English
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We experimentally investigate the dynamics of a persistent spin helix in etched GaAs wire structures of 2 to 80 um width. Using magneto-optical Kerr rotation with high spatial resolution, we determine the lifetime of the spin helix. A few nanoseconds after locally injecting spin polarization into the wire, the polarization is strongly enhanced as compared to the two-dimensional case. This is mostly attributed to a transition to one-dimensional diffusion, strongly suppressing diffusive dilution of spin polarization. The intrinsic lifetime of the helical mode is only weakly increased, which indicates that the channel confinement can only partially suppress the cubic Dresselhaus spin-orbit interaction.
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We study the lifetime of the persistent spin helix in semiconductor quantum wells with equal Rashba- and linear Dresselhaus spin-orbit interactions. In order to address the temperature dependence of the relevant spin relaxation mechanisms we derive and solve semiclassical spin diffusion equations taking into account spin-dependent impurity scattering, cubic Dresselhaus spin-orbit interactions and the effect of electron-electron interactions. For the experimentally relevant regime we find that the lifetime of the persistent spin helix is mainly determined by the interplay of cubic Dresselhaus spin-orbit interaction and electron-electron interactions. We propose that even longer lifetimes can be achieved by generating a spatially damped spin profile instead of the persistent spin helix state.
We study the phase diagram of the interacting two-dimensional electron gas (2DEG) with equal Rashba and Dresselhaus spin-orbit coupling, which for weak coupling gives rise to the well-known persistent spin-helix phase. We construct the full Hartree-Fock phase diagram using a classical Monte-Carlo method analogous to that used in Phys.Rev.B 96, 235425 (2017). For the 2DEG with only Rashba spin-orbit coupling, it was found that at intermediate values of the Wigner-Seitz radius rs the system is characterized by a single Fermi surface with an out-of-plane spin polarization, while at slightly larger values of rs it undergoes a transition to a state with a shifted Fermi surface and an in-plane spin polarization. The various phase transitions are first-order, and this shows up in discontinuities in the conductivity and the appearance of anisotropic resistance in the in-plane polarized phase. In this work, we show that the out-of-plane spin-polarized region shrinks as the strength of the Dresselhaus spin-orbit interaction increases, and entirely vanishes when the Rashba and Dresselhaus spin-orbit coupling strengths are equal. At this point, the system can be mapped onto a 2DEG without spin-orbit coupling, and this transformation reveals the existence of an in-plane spin-polarized phase with a single, displaced Fermi surface beyond rs > 2.01. This is confirmed by classical Monte-Carlo simulations. We discuss experimental observation and useful applications of the novel phase, as well as caveats of using the classical Monte-Carlo method.
The spin-orbit interaction (SOI) in zincblende semiconductor quantum wells can be set to a symmetry point, in which spin decay is strongly suppressed for a helical spin mode. Signatures of such a persistent spin helix (PSH) have been probed using the transient spin grating technique, but it has not yet been possible to observe the formation and the helical nature of a PSH. Here we directly map the diffusive evolution of a local spin excitation into a helical spin mode by a time- and spatially resolved magneto-optical Kerr rotation technique. Depending on its in-plane direction, an external magnetic field interacts differently with the spin mode and either highlights its helical nature or destroys the SU(2) symmetry of the SOI and thus decreases the spin lifetime. All relevant SOI parameters are experimentally determined and confirmed with a numerical simulation of spin diffusion in the presence of SOI.
Time-resolved Kerr-rotation microscopy explores the influence of optical doping on the persistent spin helix in a [001]-grown CdTe quantum well at cryogenic temperatures. Electron spin diffusion dynamics reveal a momentum-dependent effective magnetic field providing SU(2) spin-rotation symmetry, consistent with kinetic theory. The Dresselhaus and Rashba spin-orbit coupling parameters are extracted independently from rotating the spin helix with external magnetic fields applied parallel and perpendicular to the effective magnetic field. Most importantly, a non-uniform spatiotemporal precession pattern is observed. The kinetic theory framework of spin diffusion allows for modeling of this finding by incorporating the photocarrier density into the Rashba ($alpha$) and the Dresselhaus ($beta_3$) parameters. Corresponding calculations are further validated by an excitation-density dependent measurement. This work shows universality of the persistent spin helix by its observation in a II-VI compound and the ability to fine-tune it by optical doping.
We theoretically study the profile of a supercurrent in two-dimensional Josephson junctions with Rashba-Dresselhaus spin-orbit interaction (RDSOI) in the presence of a Zeeman field. Through investigating self-biased supercurrent (so called $varphi_0$-Josephson state), we obtain explicit expressions for the functionality of the $varphi_0$ state with respect to RDSOI parameters ($alpha,beta$) and in-plane Zeeman field components ($h_x,h_y$). Our findings reveal that, when the chemical potential ($mu$) is high enough compared to the energy gap ($Delta$) in superconducting electrodes, i.e., $mu gg Delta$, RSOI and DSOI with equal strengths ($|alpha|=|beta|$) cause vanishing $varphi_0$ state independent of magnetization and the type of RDSOI. A Zeeman field with unequal components, i.e., $|h_x| eq |h_y|$, however, can counteract and nullify the destructive impact of equal-strength RDSOIs (for one type only), where $musimDelta$, although $|h_x|= |h_y|$ can still eliminate the $varphi_0$ state. Remarkably, in the $musimDelta$ limit, the $varphi_0$ state is proportional to the multiplication of both components of an in-plane Zeeman field, i.e., $h_xh_y$, which is absent in the $mu gg Delta$ limit. Furthermore, our results of critical supercurrents demonstrate that the persistent spin helices can be revealed in a high enough chemical potential regime $mugg Delta$, while an opposite regime, i.e., $musimDelta$, introduces an adverse effect. In the ballistic regime, the maximum of the critical supercurrent occurs at $|alpha|=|beta|$ and the Zeeman field can boost this feature. The presence of disorder and nonmagnetic impurities change this picture drastically so the minimum of the critical supercurrent occurs at and around the symmetry lines $|alpha|=|beta|$.
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