Dynamics of electrons in surface states with large spin-orbit splitting. L. Perfetti, Laboratoire des Solides Irradiés

Transcription

1 Dynamics of electrons in surface states with large spin-orbit splitting L. Perfetti, Laboratoire des Solides Irradiés

2 Outline Topology of surface states on the Bi(111) surface Spectroscopy of electronic states above the Fermi level Coherent phonon coupling to individual Bloch waves Charge flow in the Dirac cone of Topological Insulators Circular dichroism in BiTeI Ultrafast currents at the surface of photoexcited Tis

3 Motivation: Spin charge conversion at interfaces with Rashba states J. C. Rojas Sànchez, Nature Comm. (2013)

4 Gold (111) Spin Orbit Splitting

5 Soleil Synchrotron, Cassiopée Beamline Principles of Angle Resolved Photoelectron Spectroscopy

6 Outline Topology of surface states on the Bi(111) surface Spectroscopy of electronic states above the Fermi level Coherent phonon coupling to individual Bloch waves Charge flow in the Dirac cone of Topological Insulators Circular dichroism in BiTeI Ultrafast currents at the surface of photoexcited Tis

7 Surface states with large spin-orbit splitting on Bi(111) Yoshiyuki Ohtsubo Amina Taleb Cassiopée Beamline

8 Dispersion of surface states along high symmetry directions K Γ M K Γ M

9 + - Non trivial topology of surface states on the Bi(111) H. Ohtsubo, New Journal of Physics (2013) M - + Γ

10 No continuous and time resversal perturbation can changes the topology and preserves tha bandgap M Γ Γ M Ab-initio calculations are not accurate enough in order to discriminate among the two scenarios

11 Outline Topology of surface state on the Bi(111) surface Spectroscopy of electronic states above the Fermi level Coherent phonon coupling to individual Bloch waves Charge flow in the Dirac cone of Topological Insulators Circular dichroism in BiTeI Ultrafast currents at the surface of photoexcited Tis

12 FemtoARPES Laboratory

13 Angle Resolved Photoelectron Spectroscopy Julien Mauchain Evangelos Papalazarou

14 Laser System Center 1.5 ev Energy 6 µj Repetition rate 250 KHz

15 Generation of 4th Harmonic: 6.3 ev J. Faure, Rev. Sci. Instrum. (2012) Jérôme Faure 78 fs cross correlation 80 fs

16 Observation of states above Fermi fs 100 fs

17 Giant anisotropy of the spin-orbit splitting Y. Ohtsubo, Phys. Rev. Lett. (2012) Cassiopée Polarization P Polarization S Azimuthal angle Azimuthal angle High order corrections to the Rashba coupling

18 First principle calculations of the surface states Y. Ohtsubo, Phys. Rev. Lett. (2012)

19 Out-of plane component of the spin orientation A. Takayama, Phys. Rev. Lett. (2011) Unquenching of orbital momentum

20 Outline Topology of surface state on the Bi(111) surface Spectroscopy of electronic states above the Fermi level Coherent phonon coupling to individual Bloch waves Charge flow in the Dirac cone of Topological Insulators Circular dichroism in BiTeI Surface currents emitted by photoexcited Tis

21 Coupling to individual Bloch states to coherent phonons A. Papalazarou, Phys. Rev. Lett. (20012)

22 Coherent excitation of the A 1g mode f i

23 Brice Arnaud Y. Giret, Phys. Rev. Lett. (2011) Spring

24 D. M. Fritz, Science (2007) S. L. Johnson, Phys. Rev. Lett. (2008) A 1g mode Frequency 2.96 THz Damping 2.6 ps Amplitude mev Shift 1.6 pm D = 0.91 ev/å

25 Wavevector dependence of A 1g phonon to bulk states Weak coupling of surface states to totally symmetric phonons A. Papalazarou, Phys. Rev. Lett. (20012)

26 Outline Topology of surface state on the Bi(111) surface Spectroscopy of electronic states above the Fermi level Coherent phonon coupling to individual Bloch waves Charge flow in the Dirac cone of Topological Insulators Circular dichroism in BiTeI Surface currents emitted by photoexcited Tis

27 Bi 2 Se 3 Chiral states connecting Time reversal symmetry points with opposite parity Y. Xia, Nature Physics (2009)

28 Fermi level above the conduction band minimum n-type Bi 2 Te 3 M. Hajlaoui Nature Comm. (2014)

29 Bi 2 Te 2 Se: Fermi level just below the conduction band minimum Spatial separation of photoexcited holes and electrons

30 Bi 2.2 Te 3 : Fermi level just above the valence band maximum Bi 2.2 Te 3 Bi 2 Te 2 Se

31 Photo-Dember effect and surface band bending

32 Thermalization of the 2DEG M. Hajlaoui Nature Comm. (2014)

33 +4 µs Bi 2 Te 2 Se Transient filling of the dirac cone persisting for microseconds M. Neupane, Phys. Rev. Lett. (2016) Delays (fs)

34 Outline Topology of surface state on the Bi(111) surface Spectroscopy of electronic states above the Fermi level Coherent phonon coupling to individual Bloch waves Charge flow in the Dirac cone of Topological Insulators Circular dichroism in BiTeI Ultrafast currents at the surface of photoexcited Tis

35 Electronic structure of BiTeI G. Landot, Phys. Rev. Lett. (2012) large spin-orbit interaction lack of inversion symmetry

36 J. Mauchain, Phys. Rev. Lett. (2013) Electronic thermalization 300 fs Electronic cooling 10 ps

37 Optical orientation upon photoexcitation with circular polarized light S. Pfalz, Phys. Rev. B (2005) Bulk GaAs and QW structures Circular dichroism in monolayer MoS 2 K. F. Mak Nature Nanotech. (2012) T. Cao, Nature Comm. (2012) Polarized photoluminescence Strong excitonic effects

38 Dichroism generated by the dipole matrix element Within the sudden approximation no relaxation is possible J. Osterwalder, Lect. Notes Phys. (2006) + -

39 Circular probe dichroism Nearly odd with respect to the inplane wavevector Following C 3v symmetry Maximal along Γ-K

40 Circular pump map probes the dichroic properties of the intermediate state Possible relaxation of spin and orbital polarization in the intermediate state Ultrafast relaxation expected in absence of charge gap and excitons

41 Large difference between circular pump and circular probe dichroism Even like Odd like

42 In absence of magnetic field, left and right polarized light deposit the same amount of energy L. Demko, Phys. Rev. Lett. (2012) The helicity affect the spatial distribution of electronic energy density Energy Density Superdiffusive transport within the duration of the pump pulse J. Mauchain, Phys. Rev. Lett. (2013) Distance

43 Outline Topology of surface state on the Bi(111) surface Spectroscopy of electronic states above the Fermi level Coherent phonon coupling to individual Bloch waves Charge flow in the Dirac cone of Topological Insulators Circular dichroism in BiTeI Ultrafast currents at the surface of photoexcited TIs

44 Motivation: Generation of circular photocurrents via asimmetric distributions in Topological states J. W. McIver, Nature nanotech. (2011)

45 Lucas Braun Tobias Kampfrath Fritz Haber Institute Principles of THz emission spectroscopy Pockels effect E 800 nm 1.5 ev 100 nm

46 Topological surface states are protected against perturbations preserving time reversal symmetry. The Dirac cone survives upoon exposure to 1 atmosfere of Nitrogen

47 THz generation is a second order process Broken inversion symmetry is necessary Surface sensitivity

48 Symmetry restrain the azimuthal dependence of the detected signal

49 corrections for the transmittion function and detector response electric field in front of the surface

50 Maxwell equations dictate the relation between the emitted field and the surface current

51 Generation mechanism of injection currents Generation mechanism of shift currents

52 Conclusions Bi(111) surface is topologically non-trivial The spin-orbit splitting can be highy anisotropic. Weak coupling of surface states to totally symmetric modes

53 Conclusions Charge separation hinders electron-hole recombination in topological insulators Superdiffusive transport dependent on photon helicity Observation of shift currents at the surface of TIs Angle (deg)

54 Aknowledments Time resolved ARPES E. Papalazarou, J. Mauchain, M. Marsi Laboratoire des Solides Irradiés, Ecole Polytechniquew Time resolved THz L. Broun, T. Kamprath Fritz Haber institute High resolution ARPES H. Othsubo, A. Taleb Synchrotron Soleil Theory Samples E. Chulckov, B. Arnaud R. J. Cava, M. Konczykowski, O. E. Tereshchenko