Topological crystalline insulators. Tomasz Story Institute of Physics, Polish Academy of Sciences, Warsaw

Transcription

1 Topological crystalline insulators Tomasz Story Institute of Physics, Polish Academy of Sciences, Warsaw

2 Topological crystalline insulators - outline 1. Introduction topological crystalline insulators (TCI) vs topological insulators (TI) 2. IV-VI semiconductors as topological insulators 3. Pb 1-x Sn x Se monocrystals growth, structural, chemical and electric characterization 4. Angle- and spin-resolved photoemission experiments 5. Electronic band structure calculations 6. Magneto-transport studies 7. Summary and future outlook

3 Pb 1-x Sn x Se as topological crystalline insulator Crystal growth: A. Szczerbakow (IP PAS) Structural and chemical characterization: W. Domuchowski, E. Łusakowska, A. Reszka (IP PAS) Magneto-transport studies K. Dybko, M. Szot (IP PAS) Band structure calculations R. Buczko (IP PAS) Photoemission measurements at Lund University (synchrotron facility) and KTH Stockholm (laser facility): P. Dziawa, B.J. Kowalski (IP PAS), T. Balasubramanian (Lund), M.H. Berntsen, O. Tjernberg, B.M. Wojek (KTH) TS

4 Topological insulators: key physical factors Inverted c-band and v-band symmetry Odd number of Dirac cones Strong spin-orbit coupling E so E G Metallic, helical Dirac electronic surface states Topological protection

5 Topological insulators - materials 3D Bi 1-x Sb x Bi 2 Se 3 Bi 2 Te 3 2D HgTe/CdTe QW M.Z. Hasan, C.L. Kane, Rev. Modern Phys. 82, 3045 (2010)

6 Topological crystalline insulators: theoretical idea Theoretical analysis of electronic band structure of tetragonal crystal with 4-fold surface symmetry. Topologically protected surface TCI states (red) are expected even in materials with no spin-orbit coupling. L. Fu, Phys. Rev. Lett. 106, (2011).

7 Topological crystalline insulators SnTe - theoretical analysis SnTe - TCI states with 4 Dirac cones nearby X-points of the surface Brillouin zone Lifshitz transition - Topological changes of Fermi surface T.H. Hsieh et al, Nature Commun. 3, 982 (2012).

8 Topological crystalline insulators: SnTe vs PbTe theoretical analysis PbTe trivial band insulator E G >0 SnTe topological insulator (TCI) E G <0 T.H. Hsieh et al, Nature Commun. 3, 982 (2012).

9 Band inversion in IV-VI semiconductors P. Barone et al., Phys. Rev. B 88, (2013)

10 IV-VI semiconductor family Binary compounds: PbTe, PbSe, PbS, SnTe, GeTe Substitutional solid solutions: Pb 1-x Sn x Te, Pb 1-x Sn x Se Diluted magnetic semiconductors: Sn 1-x Mn x Te, Ge 1-x Mn x Te

11 IV-VI semiconductors Rock-salt crystal structure. Narrow-gap materials (0-0.3 ev) with a direct gap at 4 equivalent L-points. Strong (1 ev) relativistic interactions (spin-orbit and Darwin terms). Small effective masses and high mobilities of electrons and holes. Materials for thermoelectric generators and mid-infrared lasers and detectors.

12 Electron band structure of IV-VI semiconductors Relativistic interactions in PbTe and Pb 1-x Sn x Te

13 E g (ev) 300 K Pb 1-x Sn x Te substitutional solid solutions 0,4 0,3 R. Dornhaus, G. Nimtz, and B. Schlicht, Springer Tracts in Modern Physics vol. 98, Narrow-Gap Semiconductors (Springer, Berlin, 1983) Pb 1-x Sn x Te L 6 0,2 0,1 L K 0,0 12 K -0,1-0,2 L + 6-0,3 L 6 TCI -0,4 0,0 0,2 0,4 0,6 0,8 1,0 Sn content, x

14 E g (ev) Pb 1-x Sn x Se substitutional solid solutions 0,3 0,2 300 K 195 K Pb 1-x Sn x Se L 6 77 K L + 6 0,1 0,0 4 K ü exp. data ý ţ A.J. Strauss x=0.23 x=0.27 x=0.30 L + 6-0,1 L 6 TCI -0,2 0 0,1 0,2 0,3 0,4 Sn content, x

15 Idea of our TCI project Pb 1-x Sn x Se monocrystals (0.18<x<0.3) to observe temperature-driven topological phase transition from trivial insulator to the TCI state n-type crystals required for x>x c (TCI states occupied) cleaving at UHV conditions as a versatile method of preparing high crystal quality, atomically clean (001) surfaces no surface reconstruction expected only Pb 1-x Sn x Se crystals fulfil all these conditions Pb 1-x Sn x Te (x>x c ) and SnTe crystals are heavily p-type doped by electrically active native defects (metal vacancies).

16 Growth of Pb 1-x Sn x Se and Pb 1-x Sn x Te bulk monocrystals Self-selecting vapor growth (SSVG) A. Szczerbakow Natural (001) crystal facets cleavage planes Stoichiometry control of n and p-type conductivity Highly homogeneous chemical composition of solid solutions A. Szczerbakow et al.: J. Cryst. Growth 139, 172 (1994); Prog. Cryst. Growth Charact. Mater. 51, 81 (2005).

17 Growth of bulk PbSnSe crystals by SSVG method A. Szczerbakow, Pb 0.76 Sn 0.24 Se monocrystal

18 Pb 0.77 Sn 0.23 Se monocrystal grown by self-selecting vapor growth

19 Structural and chemical characterization X-ray diffraction (XRD) EDX chemical analysis Surface morphology analysis by AFM microscopy

20 Intensity DOS Core level Vacuum level Photoemission electron spectroscopy Valence band hn Energy Energy analyzer hn e - Sample Electron detector Secondary electrons Kinetic energy Binding energy E F

21 Photoemission crystal surface sensitive technique photons electrons nm nm W. Mönch Semiconductor surfaces and interfaces 1993

22 Brillouin zone for (001) surface

23 Electron band structure ARPES studies of Pb 0.77 Sn 0.23 Se E G vs. T T=144 K T=199 K E T=284 K T=126 K T=111 K k

24 Electron band structure of Pb 0.77 Sn 0.23 Se ARPES experimental studies Energy dispersion relation for temperature varying across band inversion point

25 Electron band structure of Pb 0.77 Sn 0.23 Se ARPES experimental studies Energy dispersion E(k) for X- and X-M directions in surface Brillouin zone

26 Electron band structure of Pb 0.77 Sn 0.23 Se ARPES experimental studies Fermi surface cuts E(k x, k y ) for varying binding energy E b

27 Trivial insulator (PbSe) vs topological crystalline insulator (Pb 1-x Sn x Se) PbSe (x=0) x=0.23 x=0.30 B.M. Wojek et al. 2012

28 B.E. (ev) Dirac point Dirac point Pb 0.67 Sn 0.33 Se, T=87 K, hn=18.5 ev B.E. (ev) X 0.4 M X M X X Theory- R. Buczko, P. Kacman, S. Safaei X

29 Band structure of Pb 1-x Sn x Te: tight binding calculations PbSnTe in band inversion region: A) band insulator B) zero bulk band gap C) inverted gap - TCI D) SnTe TCI Yellow p-type cation orbitals Blue p-type anion orbitals

30 Spin polarization of TCI states: tight binding model Pb 0.76 Sn 0.24 Se B.M. Wojek, R. Buczko et al. Phys. Rev. B 87, (2013)

31 Spin polarization of TCI states in SnTe S. Safaei, P. Kacman, R. Buczko, Phys. Rev. B 88, (2013) tight binding calculations

32 Spin polarization of TCI states: SRPES experiment Pb 0.76 Sn 0.24 Se B.M. Wojek, R. Buczko et al. Phys. Rev. B 87, (2013)

33 Spin polarization of TCI states: SRPES experiment Pb 0.6 Sn 0.4 Te S-Y Xu, M.Z. Hasan, Nat. Commun. 3, 1192 (2012).

34 Pb 0.77 Sn 0.23 Se: magneto-transport 2x10-3 a) 3x10-3 2x10-3 b) 250K 1x10-3 xx ( cm) 1x K 130K 58K B (T) xy ( cm) 0-1x x x B (T) 4x10-4 T=4.2K = 0 deg T=4.2K = 41deg 1x10-4 c) 250K 130K 58K xx ( cm) 3x10-4 xy ( cm) 0 2x x B (T) B (T)

35 Pb 0.77 Sn 0.23 Se: magneto-transport Drude model for magneto-conductivity Two parrallel conduction channels Fitting of electron transport parameters BS, BS, SS, SS for bulk crystal and surface channels K. Dybko et al. 2012

36 STM Pb 1-x Sn x Se Y. Okada et al., Science 2013

37 TCI - materials

38 Summary Topological crystalline insulators (TCIs) are a new class of quantum materials in which the topological protection of metallic surface states is warranted not by time-reversal symmetry as in TI but by specific crystalline symmetries. Pb 1-x Sn x Se IV-VI narrow gap semiconductor is a TCI, in which a temperature-driven topological phase transition is observed from a trivial insulator to a TCI due to band inversion. We discussed growth of monocrystals, electronic band structure investigations by angle- and spin-resolved photoemission spectroscopy (ARPES, SRPES), magneto-transport studies, and electronic band structure calculations of Pb 1-x Sn x Se (x 0.4).

39 E g (ev) Future outlook K 195 K 77 K 4 K ü exp. data ý A.J. Strauss ţ P=2 GPa L + 6 L 6 Pb 1-x Sn x Se x=0.15 P=1 GPa x=0.23 TCI x=0.27 x= Sn content, x L 6 L + 6 TCI at P=0 Topological phase diagram study: Pb 1-x Sn x Se i Pb 1-x Sn x Te (T, P) Controlling electrical and optical properties Other surface sensitive experimentai methods - STM Co-existence of ferromagnetism and TCI state in Sn 1-x Mn x Te? TCI/ferromagnet and TCI/superconductor heterostructures Influence of lattice distorsion on TCI