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L.Besombes Y.Leger H. Boukari D.Ferrand H.Mariette J. Fernandez-Rossier

Optical control of an individual spin. L.Besombes Y.Leger H. Boukari D.Ferrand H.Mariette J. Fernandez-Rossier. CEA-CNRS team « Nanophysique et Semi-conducteurs » Institut Néel, CNRS Grenoble, FRANCE. Department of applied physics, University of Alicante, SPAIN. Introduction.

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L.Besombes Y.Leger H. Boukari D.Ferrand H.Mariette J. Fernandez-Rossier

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  1. Optical control of an individual spin L.BesombesY.Leger H. Boukari D.FerrandH.MarietteJ. Fernandez-Rossier CEA-CNRS team « Nanophysique et Semi-conducteurs » Institut Néel, CNRS Grenoble, FRANCE Department of applied physics, University of Alicante, SPAIN

  2. Introduction • Ultimate semiconductor spintronic device: Single magnetic ion / individual carriers -Control of the interaction between a single magnetic atom and an individual carrier. (spin injection, spin transfer) -Manipulation of an individual spin (memory, quantum computing) II-VI Semi-Magnetic semiconductor QDs Magnetic doping (Mn: S=5/2) Localizedcarriers …Towards a single spin memory.

  3. Theoretical proposals Transport: A single QD containing a Mn atom could be use as a spin filter Qu et al. Phys. Rev. B74, 25308 (2006) Nano-magnetism : electrical control of the magnetism. Hawrylak et al. Phys. Rev. Lett. 95, 217206 (2005) Memories : writing and reading of the spin state of a single Mn atom. A.O. Govorov et al., Phys. Rev. B 71, 035338 (2005)

  4. Outline 1. Probing the spin state of a single Magnetic atom - II-VI magnetic self assembled QDs - Carriers-Mn exchange interaction - Importance of QD structural parameters on the spin detection (Shape anisotropy, valence band mixing) 2. Carrier controlled Mn spin splitting - Anisotropy of the hole-Mn interaction - Charge tunable Mn-doped QDs 3. Carriers and Mn spin dynamics

  5. 100mm Individual CdTe/ZnTe QDs • UHV-AFM image of CdTe QDs on ZnTe. • Micro-spectroscopy. QDs density: 5.109 cm-2 • TEM image of CdTe QDs on ZnTe. Size: d=15nm, h=3nm (Lz<<Lx,Ly)

  6. Optical transitions in an individual QD B=0 B=0 e: spin 1/2 h: anisotropic (Jz=3/2) e h Jz=+1 Jz= -1 e h Jz= - 2 Jz= +2 s- s+ G.S. Optical selection rules: Sz= -1/2 Sz= +1/2 e s+ s- z hh Jz= +3/2 Jz= -3/2 lh Jz= +1/2 Jz= -1/2

  7. V p-ZnTe CdTe Gated charged quantum dots • Transfer of holes from the surface states: p type doping of the QDs. • Electrical control of the charge.

  8. Cd: 3d10 4s2 Mn: 3d5 4s2 Cd Te Mn • Mn remplace Cd: Mn2+ • Mn2+ S=5/2, 2S+1=6 • Exchange interaction: • Mn - electron • Mn - hole Mn doped II-VI QDs hn 2-3 nm 10-15 nm Electron:σ = 1/2 Hole:jZ = ±3/2 Mn atom: S = 5/2

  9. Emission of Mn-doped individual QDs • The presence of a single magnetic atom completely control the emission structure. Measurement of the exchange interaction energy of the electron, hole, Mn Phys Rev Lett. 93, 207403 (2004)

  10. Heavy-hole exciton / Mn exchange coupling Exchange constant: s-d, a>0 p-d, b<0 Mn2+ e h e h -5/2 +5/2 Jz = -1 Jz = +1 X+Mn2+ X -3/2 +3/2 -1/2 +1/2 +1/2 -1/2 Heavy hole exciton +3/2 -3/2 e h e h +5/2 -5/2 Jz = -1 Jz = +1 Sz = ±5/2, ±3/2, ±1/2 Mn2+

  11. -5/2 +5/2 … … +5/2 -5/2 s- s+ Heavy-hole exciton / Mn exchange coupling 1 photon (energy, polar) = 1 Mn spin projection X+Mn2+ X Heavy hole exciton Mn2+ • Overall splitting controlled by Ie-Mn and Ih-Mn .

  12. Mn-doped individual QDs under magnetic field • Magnetic field dependent PL intensity distribution. NMn=0 NMn=1

  13. Polarization of the Mn spin distribution e h Mn2+ e h Mn2+ Mn spin polarization Jz = +1 Jz = -1 e h e h Mn2+ Mn2+ Teff=12K s- s+ Mn2+ Mn spin conservation • Boltzmann distribution of the Mn-Exciton system: gMn=2 B B

  14. Statistic Mn spin distribution • Resonant excitation B=0T Complex excited states fine structure Selection of Mn spin distribution and spin conservation during the lifetime of the exciton.

  15. 0 -1 2 1 Energy (meV) Carriers-Mn exchange coupling Exp. Th. Effective spin Hamiltonian: • X-Mn Overlap • QD shape • Strain distribution

  16. Detection condition: Exciton-Mn overlap 1.3 meV • Ie-Mn in a flat parabolic potential: Decrease of X-Mn overlap Exchange integrals controlled by the overlap with the Mn atom.

  17. Detection condition:Structural parameters QD1 QD2 QD3 Influence of the QD shape Influence of the valence band mixing Heavy-hole + Mn e Sz= +- 1/2 hh Jz=+ - 3/2 lh Jz=+ - 1/2 Phys Rev B. 72, 241309(R) (2005) Phys Rev Lett. 95, 047403 (2005)

  18. ~ ~ <3/2| j - |-3/2> = 0 Valence band mixing in strained induced QDs • Inhomogeneous relaxation of strain in a strained induced QD (Bir & Pikus Hamiltonian): |3/2> |1/2> |-1/2> |-3/2> E +1/2 -1/2 -3/2 +3/2 +1/2 -1/2 k ~ |3/2> = c1 |3/2>+ c2 |-1/2> c1>>c2 |-3/2> = c3 |-3/2>+ c4 |1/2> c3>>c4 ~ via cross components because

  19. Influence of valence band mixing elh : Heavy-light hole mixing efficiency X X+Mn2+ Possibility to flip from jz= +3/2 to -/3/2 via light holes ~ ~ Effective h-Mn interaction term in the Heavy hole Subspace e h e h Allows simultaneous hole-Mn spin flip

  20. Influence of valence band mixing Exp. elh : Heavy-light hole mixing efficiency Possibility to flip from jz= +3/2 to -/3/2 via light holes ~ ~ Effective h-Mn interaction term in the Heavy hole Subspace Th. e h e h Allows simultaneous hole-Mn spin flip Emission of “non-radiative” exciton states Phys Rev B. 72, 241309(R) (2005)

  21. X-Mn in transverse B field «0» 001 B┴ «-1 » «+1 » Voigt 001 B// Voigt: Complex fine structure… Suppression of the hole Mn exchange interaction Faraday: Zero field structure is conserved Faraday Phys Rev B. 72, 241309(R) (2005)

  22. 1. Probing the spin state of a single Magnetic atom - II-VI magnetic self assembled QDs - Carriers-Mn exchange interaction - Importance of QD structural parameters on the spin detection (Shape anisotropy, valence band mixing) 2. Carrier controlled Mn spin splitting - Anisotropy of the hole-Mn interaction - Charge tunable Mn-doped QDs 3. Carriers and Mn spin dynamics

  23. Biexciton in a Mn-doped QD e h X e h Increase of the excitation density X2 Increase of the number of carriers in the QD. Formation of the biexciton (binding energy 11meV) Similar fine structure for the exciton and the biexciton . . .

  24. σ + σ - Carrier controlled Mn spin splitting X2 (J=0) X, J=±1 G.S. • Optical control of the magnetization: - One exciton splits the Mn spin levels - With two excitons, the exchange interaction vanishes… Phys Rev B. 71, 161307(R) (2005)

  25. Gated charged Mn-doped quantum dots e h e h Charge tunable sungle Mn-doped QDs allow us to probe independantly the interactions between electron and Mn or hole and Mn Phys Rev Lett. 97, 107401 (2006)

  26. Variation of hole-Mn exchange interaction e h e h Ie-Mn = 40 μeV Ih-Mn(X+) = 95 μeV Ih-Mn(X) = 150 μeV Ih-Mn(X-) = 170 μeV • The hole confinement is influenced by the Coulomb attraction X+, Mn X, Mn X-, Mn Increasing the hole-Mn overlap by injecting electrons in the QD e X+, Mn hardly resolved Mn h

  27. Negatively charged exciton in a Mn doped QD • Isotropic e-Mn interaction • Anisotropic h-Mn interaction û e h Initial state: 1 h + 1 Mn e h J=2 Final state: 1 e + 1 Mn J=3

  28. Optical recombination of the charged exciton • Optical transitions between: Jz=-1 Proportional to the overlap: J=2 J=3 Eigenstates of He-Mn

  29. Optical recombination of the charged exciton 1 Probability Energy J=2 J=3

  30. Optical recombination of the charged exciton 1 Probability Energy û J=2 J=3

  31. Optical recombination of the charged exciton 1 Probability Energy û J=2 e-Mn: isotropic h-Mn: anisotropic J=3

  32. Charged exciton in a single QD: Influence of VBM û e h (+3/2,-1/2) Initial state: 1 h + 1 Mn (-3/2,+1/2) J=2 Final state: 1 e + 1 Mn J=3 Phys Rev Lett. 97, 107401 (2006)

  33. Charged exciton in a single QD: Influence of VBM û e h (+3/2,-1/2) Initial state: 1 h + 1 Mn (-3/2,+1/2) J=2 Final state: 1 e + 1 Mn J=3

  34. Negatively / Positively charged Mn-doped QDs • X-, Mn • X+, Mn • Reversed initial and final states e, Mn h, Mn h, Mn J=2 e, Mn J=3

  35. Gated controlled magnetic anisotropy Energy ST Mz Heisenberg Ising Free hh Q=-1 Q=0 Q=+1 Mn+1h= Nano-Magnet

  36. 1. Probing the spin state of a single Magnetic atom - II-VI magnetic self assembled QDs - Carriers-Mn exchange interaction - Importance of QD structural parameters on the spin detection (Shape anisotropy, valence band mixing) 2. Carrier controlled Mn spin splitting - Anisotropy of the hole-Mn interaction - Charge tunable Mn-doped QDs 3. Carriers and Mn spin dynamics

  37. 1 photon (σ, E) 1 Mn spin state Spin dynamics vs photon statistics 1 Mn atom Sz -5/2 +5/2 If Sz(t=0) = -5/2 … … P (Sz = -5/2) 1 +5/2 -5/2 -5/2 s- s+ ? ~1/6 0 t Photon statistics ?

  38. Correlation measurement on single QDs Whole PL autocorrelation Single emitter statistics : Select a QD with a large splitting to spectrally isolate a Mn spin state Antibunching: The QDs cannot emit two photons with a given energy at the same time Use of a SIL to increase the signal

  39. E X X+Mn2+ t Single Mn spin dynamics Auto Correlation on one line in one polarization (s+, -5/2) 8 ns τX-Mn One Mn spin projection Photon bunching at short delay

  40. E X X+Mn2+ Single Mn spin dynamics Power dependence Auto Correlation on one line in one polarization σ + P0 2 x P0 τX-Mn 3 x P0 Mixing between Mn spin relaxation time and X-Mn spin relaxation time One Mn spin projection

  41. -5/2 +5/2 … … +5/2 -5/2 s- s+ Single Mn spin dynamics Polarization Cross-Correlation σ + σ - Direct evidence of the spin transfer One Mn spin projection Influence of magnetic field?...To be continued…

  42. Summary • Optical probing of a single carrier/single magnetic atom interaction. - The exchange coupling is controlled by the carrier / Mn overlap. - BUT, real self assembled QDs: - Shape anisotropy - Valence band mixing • Hole-Mn complex is highly anisotropic but non-negligeable effects of heavy-light hole mixing • Charged single Mn-doped QDs: Change the magnetic properties of the Mn with a single carrier. • Photon statistics reveals a complex spin dynamics. …. Store information on a single spin?

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