Universität Karlsruhe

Institut für Theoretische Festkörperphysik

Spin electronics

Mikhail Pletyukhov, Pablo San-Jose, Elsa Prada, Alexander Shnirman, Gerd Schön
(Former group members: Michele Governale, Jürgen König, Jan Martinek, Yasuhiro Utsumi, Ulrich Zülicke)

Conventional electronics, as it is used for example in nowadays information technology, is based on the manipulation of electric currents, i.e., it makes use of the charge of the electrons. Electrons possess, however, a spin degree of freedom, associated with a magnetic moment. Spin electronics, or magneto electronics, deals with phenomena in which both the charge and the spin degree of freedom of electrons (or holes) are exploited at the same time. A large variety of spin-dependent transport effects appears in different regimes of condensed matter physics. Some of them have already proven technologically relevant and are used in commercial devices, such as the giant-magneto-resistance effect in hard-drive read heads. Others are currently subject to basic research. In our group, we investigate several different aspects of spin electronics:

The spin of electrons and holes in semiconductors is influenced by spin-orbit coupling, which can be tuned in a controlled way by external gates in nanostructured devices such as quantum wells and quantum wires. This gives rise to the so-called Rashba effect, which results in the precession of the electron's spin in the ballistic transport regime. We studied the effect of Rashba spin-orbit coupling on transport through quantum wires and proposed a scheme to perform spin filtering without external magnetic fields. Universal properties of electron and hole spin precession in realistic field-effect-transistor devices were discussed by us recently. We studied the interplay of spin-orbit coupling and correlation effects in quantum wires and in quantum rings. In particular, persistent currents in quantum rings were considered in the framework of the multicomponent Luttinger liquid model [1]. We also established the range of parameters of the quantum wire where the spin precession is observable, in contrast to the regime where spin-orbit coupling and backscattering interaction acting together generate a spin-gap [2]. The effect of Rashba coupling on the polarization and screening properties of 2DEG as well as the spin-orbit-induced damping of collective modes have been investigated [3].

The interplay of ferromagnetism and charging effects gives rise to spin-dependent transport through quantum dots or metallic single-electron transistors that are coupled to ferromagnetic leads. We addressed spin fluctuations and spin accumulation in metallic single-electron transistors [4]. For quantum-dot systems, we investigated spin accumulation and spin precession in the weak-coupling regimes [5] as well as Kondo physics in the presence of ferromagnetic leads in the strong-coupling regime [6-8].

One important task inside the field of Spintronics in that of detection of spin entanglement due to its connection to the theory of Quantum Information. To do that we analyse shot-noise measurements (or higher order cumulants) and violation of Bell-type inequalities in mesoscopic multiterminal conductors. In particular, we consider the effect of decoherence in such detection schemes and provide a generalization of the conserving voltage probe model for inelastic scattering to obtain the full counting statistics of non-locally entangled incoming states [9-11]

We studied spin-orbit mediated relaxation and dephasing of electron spins in quantum dots. Higher order contributions provide a relaxation mechanism that dominates for low magnetic fields and is of geometrical origin. In the low-field limit relaxation is dominated by coupling to electron-hole excitations and possibly 1/f noise rather than phonons [12].

Kondo effect in electron transport
The zero bias anomalies are split by the presence of ferromagnetic electrodes and by an applied magnetic field [6,7]

Some selected publications
  1.  Persistent currents in a multicomponent Tomonaga-Luttinger liquid: Application to a mesoscopic semiconductor ring with spin-orbit interaction
    M. Pletyukhov and V. Gritsev
    Phys. Rev. B 70, 165316 (2004).

  2.  Competing Effects of Interactions and Spin-Orbit Coupling in a Quantum Wire
    V. Gritsev, G. Japaridze, M. Pletyukhov, and D. Baeriswyl
    Phys. Rev. Lett. 94, 137207 (2005).

  3. Screening in the two-dimensional electron gas with spin-orbit coupling
    M. Pletyukhov and V. Gritsev
    Phys. Rev. B 74, 045307 (2006)

  4. Tunneling spectroscopy of two-level systems inside a Josephson junction
    I. Martin, L. Bulaevskii, and A. Shnirman
    Phys. Rev. Lett. 95, 127002 (2005)

  5. Spin accumulation in ferromagnetic single-electron transistors in the cotunneling regime
    J. Martinek, J. Barnas, S. Maekawa, H. Schoeller, and G. Schön
    Phys. Rev. B 66, 014402 (2002).

  6. Kondo effect in quantum dots coupled to ferromagnetic leads
    J. Martinek, Y. Utsumi, H. Imamura, J. Barnas, S. Maekawa, J. König, and G. Schön
    Phys. Rev. Lett. 91, 127203 (2003).

  7. Nonequilibrium Kondo effect in a quantum dot coupled to ferromagnetic leads
    Y. Utsumi, J. Martinek, G. Schön, H. Imamura, and S. Maekawa
    Phys. Rev. B 71, 245116 (2005)
  8. Quantum dots attached to ferromagnetic leads: Exchange field, spin precession, and Kondo effect
    J. König, J. Martinek, J. Barnas, G. Schön
    Lecture Notes in Physics, Vol. 658 (2005), "CFN Lecture Notes on Functional Nanostructures Vol. 1",
    pp. 145-164, eds. K. Busch et al.

  9. Clauser-Horne inequality and decoherence in mesoscopic conductors
    E. Prada, F. Taddei, and R. Fazio
    Phys. Rev. B 72, 125333 (2005)

  10. Clauser-Horne inequality for the full counting statistics
    F. Taddei, L. Faoro, E. Prada and R. Fazio
    New J. Phys. 7, 183 (2005).

  11. Effect of inelastic scattering on spin entanglement detection through current noise
    P. San-Jose and E. Prada
    Phys. Rev. B 74, 045305 (2006)

  12. Geometrical spin dephasing in quantum dots
    P. San-Jose, G. Zarand, A. Shnirman, and G. Schön
    Phys. Rev. Lett. 97, 076803 (2006)

Relevant grants and some links

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