Latest Seminar

Spin-Orbit Engineering, by Henri-Jean Drouhin ( Ecole Polytechnique, France)

April 23rd, 2014: Henri-Jean Drouhin ( Ecole Polytechnique, LSI, CNRS and CEA/DSM/IRAMIS, Palaiseau, France)

Spin-Orbit Engineering of Semiconductor Heterostructures

We have proposed new tools which are suited when dealing with semiconductor heterostructures where the spin-orbit interaction plays a significant role.(1) A systematic construction of the probability- and spin-current operators, based on a momentum power expansion of effective Hamiltonians, will be presented. The results, valid whatever the momentum power and including as well the linear (Bychkov-Rashba) term as the cubic (Dresselhaus or D'yakonov-Perel) term, are of special importance for spintronics. It will be shown that the usual boundary conditions, namely the continuity of the envelope function and of a velocity at the interface, according to the BenDaniel-Duke approach, comply with the conservation of the probability current only when first- (Rashba-like) and second-order (free-electron-like) terms are taken into account in the Hamiltonian. This formalism will be applied to a paradigmatic system where the D'yakonov-Perel'term is present: tunneling through the [110] oriented GaAs barrier, a case where the envelope function may be discontinuous at the interface.

(1)Spin–Orbit Engineering of Semiconductor Heterostructures, Henri-Jean Drouhin, Federico Bottegoni, Alberto Ferrari, T. L. Hoai Nguyen, Jean-Eric Wegrowe, and Guy Fishman, in The Wonder of Nanotechnology: Quantum Optoelectronic Devices and Applications, edited by Manijeh Razeghi, Leo Esaki, and Klaus von Klitzing, SPIE (Bellingham, Washington, USA, 2013).

Previous speakers
October 31st, 2013: Giovanni Vignale (Department of Physics and Astronomy, The University of Missouri, Columbia, USA)

Coupled Spin-Charge Dynamics in Semiconductors: Coulomb Drag to Spin Hall Effect
The capability to create spin and electron-hole density gratings in semiconductors quantum wells and to probe their evolution - diffusion and drift - in real time, opens new avenues to the study of coupled spin-charge dynamics of electrons in semiconductors. In this talk I describe some of the physical effects that have been recently observed by this technique: the spin Coulomb drag, the electron-hole Coulomb drag, and the anomalous drift and diffusion caused by spin-orbit interactions. I then discuss a recent theoretical prediction* that an electric field parallel to the wavefronts of an electron-hole grating in a GaAs quantum well generates, via the electronic spin Hall effect, a spin grating of the same wave vector. I refer to this phenomenon as “collective spin Hall effect”. A detailed study of the coupled-spin charge dynamics for quantum wells grown in different directions reveals rich features in the time evolution of the induced spin density, including the possibility of generating a helical spin grating from a density grating.
*Ka Shen and G. Vignale, Phys. Rev. Lett. 111, 136602 (2013)
September 17, 2013: Mathias Klaui (Institute of Physics, The Johannes Gutenberg University Mainz)

Ultra-Fast Domain Wall Dynamics

Since conventional approaches of manipulating magnetization using magnetic fields exhibit unfavourable scaling and results in limited switching speed, alternative approaches based on spin-currents have emerged. The transfer of angular momentum (“spin transfer torque effect”) leads for instance to current-induced domain wall motion (CIDM), which has become the focus of intense research in the last few years. We have comprehensively investigated CIDM and determined the acting adiabatic and non-adiabatic torque terms [1]. We find that the previously neglected diffusive torque term can play an important role for vortex core displacement [2]. By direct imaging we can determine the domain wall spin structure and correlate it with the velocity [3]. By separating charge and spin transport we can generate pure diffusive spin currents with no associated net charge current. Using such diffusive spin currents we find large efficiencies for domain wall displacement due to strong spin accumulation absorption [4]. To increase the spin diffusion length, we use robust turbostratic graphene and find spin injection across transparent contacts. Beyond generating spin currents by spin injection, we have also used the Spin Seebeck Effect, where temperature gradients generate spin currents that then affect for instance domain wall propagation [5] and using Free Electron Laser sources we measure the ultra-fast magnetization dynamics due to superdiffusive spin currents [6].
[1] L. Heyne et al., Phys. Rev. Lett. 105, 187203 (2010); M. Eltschka et al., Phys. Rev. Lett. 105, 056601 (2010).
[2] A. Manchon, W.-S. Kim, and K.-J. Lee, arXiv:1110.3487; A. Bisig, A. Manchon, M. Kläui et al. (in preparation)
[3] A. Bisig, et al., Nature Comm. 4, 2328 (2013).
[4] D. Ilgaz et al., Phys. Rev. Lett. 105, 076601 (2010).
[5] P. Möhrke et al., Sol. Stat. Com. 150, 489 (2010); J. Franken et al., Appl. Phys. Lett. 95, 212502 (2009).
[6] B. Pfau et al., Nature Comm. 3, 1100 (2012)

March 28, 2013: Julie Grollier (CNRS/Thales lab, Palaiseau, France)
Nanodevices for bio-inspired computing: memristors and more

Thanks to the progress in Nanotechnologies and Material Science, physicists and condensed matter scientists have recently been able to build smart nano-devices with enhanced capabilities. Some of these new devices show functionalities that could be extremely interesting for bio-inspired computing. It has been demonstrated for example that some analog and tunable nano-resistors called Memristors can mimic synapses on silicon. The industry is already developing dense networks of these nano-devices for classical digital memories. It is therefore no longer a dream to envisage building bio-inspired chips based on large-scale, high density parallel networks of these advanced devices, and taking advantage of their full functionalities. What's more, the inherent qualities of massively parallel architectures : the speed, the tolerance to defects and the low power consumption are more and more appreciated these days when computer processors are heating so much that they cannot be used at all times, and when transistors are shrinking so much that they will no longer be reliable. It is becoming a common thesis that bio-inspired chips such as Artificial Neural Networks will soon enter the market as a back-up or accelerator of more traditional computing architectures.
In this talk I will review the state of the art of memristors nano-devices and their applications. I will then focus on our work: the development of a new generation of memristors, based on purely electronic effects, the ferroelectric and spin torque memristors. I will show that, by tuning interface properties and finely engineering the dynamics of ferroelectric polarization or magnetization, we can control the response of these memristors. In particular, I will expose how, thanks to the versatility of our spintronic nano-devices, we can implement at the nano-scale a variety of functions that could be the future building blocks of high performance, low power, bio-inspired hardware.