Session H19: Invited Session: Current-Driven Spin Textures

Doppler velocimetry of a current driven spin helix

We present direct observation of the translational motion of spin helices in GaAs quantum wells under the influence of applied electric fields. Previously, the lifetime of such helices was observed by time-resolving the amplitude of light diffracted from the periodic spin polarization [1]. This technique cannot be applied to tracking the motion of current-driven spin helices because diffraction amplitude is insensitive to translation of the center of mass of a periodic structure. In this talk, we describe a new experimental technique, Doppler spin velocimetry, capable of resolving displacements of spin polarization at the level of 1 nm on a picosecond time scale [2]. This is accomplished through the use of heterodyne detection to measure the optical phase of the diffracted light. We discuss experiments in which this technique is used to measure the motion of spin helices as a function of temperature, in-plane electric field, and photoinduced spin polarization amplitude. Several striking observations will be reported -- for example, the spin helix velocity changes sign as a function of wavevector and is zero at the wavevector that yields the largest spin lifetime. Another important observation is that the velocity of spin polarization packets becomes equal to the drift velocity of the high-mobility electron gas in the limit of small spin helix amplitude. Finally, we show that spin helices continue propagate at the same speed as the Fermi sea even when the electron drift velocity exceeds the Fermi velocity of 10$^{7}$ cm-s$^{-1}$. In collaboration with J. D. Koralek and J. Orenstein, UC Berkeley and LBNL, D. R. Tibbetts, J. L. Reno, and M. P. Lilly, SNL. Supported by DOE under Contract No. DE-AC02-05CH11231 and DE-AC04-94AL85000. \\[4pt] [1] J. D. Koralek et al., ``Emergency of the persistent spin helix in semiconductor quantum wells,'' Nature 458, 610-613 (2009). \\[0pt] [2] L. Yang et al, ``Doppler velocimetry of spin propagation in a two-dimensional electron gas,'' to appear in Nature Physics.   

Experimental studies of skyrmion textures and spin torque effects in chiral magnets

Small angle neutron scattering and measurements of a topological Hall signal identify the formation of skyrmion lattices in the non-centrosymmetric B20 compounds MnSi [1], Mn$_{1-x}$Fe$_{x}$Si, Mn$_{1-x}$Co$_{x}$Si and the strongly doped semiconductor Fe$_{1-x}$Co$_{x}$Si [2]. This observation has been confirmed by Lorentz force microscopy in thin samples of Fe$_{1-x}$Co$_{x}$Si, FeGe and, most recently, MnSi, where even individual skyrmions have been spotted [3]. Because the skyrmion lattices are exceptionally weakly pinned to the crystal lattice, extreme care has to be exercised when studying the precise intrinsic morphology of related spin textures in bulk samples. As a particularly striking property each skyrmion supports precisely one quantum of emergent magnetic flux. This permits a highly efficient coupling between skyrmions and conduction electrons which results in spin torque effects at ultra-low current densities as seen in small angle neutron scattering [4] and the emergent electric field when the skyrmions move [5].\\[4pt] Work in collaboration with: T. Adams, A. Bauer, B. Binz, P. B\"oni, G. Brandl, R. A. Duine, K. Everschor, C. Franz, M. Garst, R. Georgii, S. Gottlieb-Sch\"onmeyer, W. Heusler, M. Janoschek, F. Jonietz, T. Keller, K. Mitterm\"uller, S. M\"uhlbauer, W. M\"unzer, A. Neubauer, P.G. Niklowitz, C. Pfleiderer, A. Rosch, T. Schulz, A. Tischendorf, M. Wagner.\\[4pt] [1] S. M\"uhlbauer et al., Science {\bf 323}, 915 (2009); A. Neubauer et al., Phys. Rev. Lett. {\bf 102}, 186602 (2010); C. Pfleiderer et al., J. Phys. Cond. Matter {\bf 22}, 164207 (2010); T. Adams et al., Phys. Rev. Lett., in press, arXiv/1107.0993. \\[0pt] [2] W. M\"unzer et al., Phys. Rev. B {\bf 81}, 041203(R) (2010). \\[0pt] [3] X. Z. Yu et al., Nature {\bf 465}, 901 (2010); X. Z. Yu et al., Nature Materials {\bf 10}, 106 (2010). \\[0pt] [4] F. Jonietz et al., Science, {\bf 330}, 1648 (2010). \\[0pt] [5] {\it Emergent electrodynamics of skyrmions in a chiral magnet}, T. Schulz, R. Ritz, A. Bauer, M. Halder, M. Wagner, C. Franz, and C. Pfleiderer, K. Everschor, M. Garst, and A. Rosch, preprint 2011.   

Current-driven spin dynamics in spin-orbit coupled superconductors

The study of the interplay between spin-orbit coupling (SOC) and superconductivity in two-dimensional electron gases (2DEG) has recently gained impetus following the discovery of i) 2DEGs in InAs or GaAs semiconductor heterostructures that are proximized by ordinary s-wave superconducting leads -- a class of systems which plays a key role in the quest for Majorana fermions -- and ii) 2DEGs that form at interfaces between complex oxides such as ${\rm LaAlO}_3$ and ${\rm SrTiO}_3$, which display tunable SOC and, under appropriate conditions, superconductivity. Motivated by this body of experimental and theoretical literature, we investigate the collective spin dynamics of an archetypical 2DEG model Hamiltonian with Rashba SOC in the presence of {\it repulsive} electron-electron (e-e) interactions. In the absence of superconductivity a Rashba 2DEG exhibits spin oscillations, which, at long wavelength and for weak repulsive interactions, have a frequency $\approx 2 \alpha k_{\rm F}$, $\alpha$ being the strength of SOC and $k_{\rm F}$ the usual 2D Fermi wavenumber in the absence of SOC. These oscillations, however, are damped and quickly decay due to the emission of (double) electron-hole pairs, which, in the normal phase, are present at arbitrary low energies. In the presence of superconductivity, collective spin oscillations continue to exist in a wide range of parameters, because the Cooper pairs are mixtures of singlet and triplet components. Further, these excitations are undamped because they lie inside the superconducting gap where no other excitation exists. These spin oscillations can be excited by the application of a magnetic field or a supercurrent and can be used to realize persistent spin oscillators operating in the frequency range of $10~{\rm GHz} - 1~{\rm THz}$.\\[4pt] Work supported by EU FP7 Programme Grant No. 215368-SEMISPINNET, No. 234970- NANOCTM and No. 248629-SOLID, and by NSF DMR-0705460.   

Emergent electrodynamics from moving magnetic whirls in chiral magnets

In chiral magnets a lattice of magnetic whirls -- so-called skyrmions -- is stabilized in a small temperature and field range by thermal fluctuations [1]. We discuss how electric and spin currents couple to these skyrmions. As the spin of the electrons locally adjusts to the magnetic texture, the electron picks up a Berry phase. The effects of these time-dependent Berry phases are best described by ``artificial'' electric and magnetic fields of an emergent electrodynamics which couple to the spin and the spin currents. The efficient Berry phase coupling together with a partial cancellation of pinning forces due to the stiffness of the skyrmion lattice allows to explain theoretically experiments [2], which show that skyrmion lattices can be controlled by ultrasmall current densities. Using tiny gradients of temperature or magnetic field it is also possible to induce rotations of the skyrmion lattice. The topologically quantized winding number of the skyrmions induces exactly one quantum of emergent magnetic flux per skyrmion. Therefore one can also determine quantitatively the emergent electric field induced by a moving skyrmion following Faraday's law of induction as has been measured in recent experiments [3].\\[4pt] [1] {\it Skyrmion Lattice in a Chiral Magnet}, S. M\"uhlbauer, B. Binz, F. Jonietz, C. Pfleiderer, A. Rosch, A. Neubauer, R. Georgii, P. B\"oni, Science {\bf 323}, 915 (2009). \\[0pt] [2] {\it Spin Transfer Torques in MnSi at Ultralow Current Densities}, F. Jonietz, S. M\"hlbauer, C. Pfleiderer, A. Neubauer, W. M\"unzer, A. Bauer, T. Adams, R. Georgii, P. B?ni, R. A. Duine, K. Everschor, M. Garst, and A. Rosch, Science {\bf 330}, 1648 (2010).\\[0pt] [3] {\it Emergent electrodynamics of skyrmions in a chiral magnet}, T. Schulz, R. Ritz, A. Bauer, M. Halder, M. Wagner, C. Franz, and C. Pfleiderer, K. Everschor, M. Garst, and A. Rosch, preprint 2011.