Rareearth doped Topological Insulators 4 September 2018 Jinsu
Rare-earth doped Topological Insulators 4. September, 2018 Jinsu Kim Myung-Hwa Jung Department of Physics Sogang University, Seoul, Korea
Contents q 3 D topological insulators (TIs) • Nonmagnetic doping effect • Magnetic doping effect q Rare-earth doped TIs Antiferromagnetic • Gd-doped Bi 2 Se 3 • Gd-doped Bi 2 Te 3 Gd doping effect Vacuum annealing effect Tuning effect of EF Possible Weyl semimetal (Magnetic dopants)
Periodic table 3 D Topological insulators (TIs) A 2 B 3 (A = Sb or Bi & B = Se or Te) : Sb 2 Se 3, Sb 2 Te 3, Bi 2 Se 3, Bi 2 Te 3 ex) Bi 2 Se 3 Bi : 6 s 26 p 3 Bi 3+ : 6 s 26 p 0 (or Bi 3 - : 6 s 26 p 6) : Insulator Se : 4 s 24 p 4 Se 2 - : 4 s 24 p 6
Fermi level issue Exotic topological surface properties EF at the Dirac point But, difficult to locate the Fermi level at the Dirac point difficult to distinguish the surface state from the bulk band E EF BCB SS BVB ARPES Δ Dirac point EF k Bulk electron is measured… (n 3 D~1019/cm 3 vs. n 2 D~1012/cm 2)
EF tuning by annealing (Bi 2 Te 3+ ) Antisite defects EB (e. V) S 4 Te. Bi (n-type) S 3 S 2 S 1 EF Bi. Te (p-type) kx (Å-1) Fine tuning of Bi 2 Te 3+ (by Te annealing)
Fine tuning of EF by doping (Bi 2 -x. Cax. Se 3) x=0 Se vacancies (n-type) Metallic x = 0. 025 x > 0. 01 Ca 2+ instead of Bi 3+ (p-type) Non-metallic PRL 103, 246601 (2009) Metallic
Magnetic doping (Bi 2 -x. Trx. Se 3) nonmagnetic No gap opening (nonmagnetic Tl) Science 329, 5992 (2010) Crx(Bi, Sb)2 -x. Te 3 FM Gap opening (magnetic Fe) FM Spin-related applications Quantum phenomena QAHE EF Science 340, 167 (2013)
Periodic table Bi 3+ Tr : 4 s 23 dn Tr 2+ : 4 s 03 dn Transition metals (Tr 2+) • Bi 2 -x. Trx. Te 3: Trivial topological insulator Tr 2+ ( 4 s 04 p 03 dn ) Doping of charge carriers (hole doping) Tuning magnetism (only FM ordering)
Periodic table Bi 3+ Rare-earth metals (Ln 3+) Gd : 6 s 24 f 8 Gd 3+ : 6 s 04 f 7 (~7μB) • Bi 2 -x. Lnx. Te 3: Another type of magnetic TIs? Ln 3+ ( 5 s 25 p 64 fn ) No doping of charge carriers? Tuning only magnetism (AFM ordering? )
Antiferromagnetism in Gdx. Bi 2 -x. Se 3 θP (K) TN (K) n -0. 70 - 0. 20 n -1. 85 - 0. 30 n -6. 83 6. 2 0. 40 n -7. 98 7. 0 x Type 0. 15 n ( cm-3 ) Weak antiferromagnetic signal at x 0. 3 PM AFM
Competing TSS with AFM in Gdx. Bi 2 -x. Se 3 Landau level fan diagram 1/2 (Berry phase factor) 1/2 2 D surface state 0 3 D bulk band
Gap opening in Gdx. Bi 2 -x. Se 3 No gap Gap Bi 0. 85 Gd 0. 14 Se 3 ~ 60 me. V AFM TRS breaking Gap opening
Phase transition in Gdx. Bi 2 -x. Te 3 = 12 K x < 0. 09: PM (weak FM) x > 0. 09, AFM
2 D Fermi surface at x = 0. 09 1/2 At x = 0. 09 (MCP), = 1/2 for B//c 2 D TSS x < 0. 09: PM (weak FM) x > 0. 09, AFM
Band structure of Gdx. Bi 2 -x. Te 3 , x=0 NM PM AFM 0. 208Å-1
Gap scale by magnetic dopant Gd 15% Bi 0. 85 Gd 0. 15 Te 3 ~ 60 me. V cf) Bi 0. 86 Gd 0. 14 Se 3 ~ 60 me. V
Gap scale by magnetic dopant Gd 15% 24% Science 329, 5992 (2010) Our results Bi 0. 85 Gd 0. 15 Te 3 Bi 0. 76 Fe 0. 24 Te 3 ~ 60 me. V ~ 44 me. V Gd 3+ ~ 7 B Fe 2+ ~ 4 B
Annealing effect in Gdx. Bi 2 -x. Te 3 Near edge x-ray absorption fine structure (NEXAFS) & XPS : sensitive to surface • Gd peak increases with x • Annealed at 250 o. C for 2 h under UHV (1 x 10 -10 Torr) Annealed As-grown After annealing, • Gd peak ↑ more populated Gd Before annealing, • Fitted only with bound Bi After annealing, • Bound Bi ↓ + Unbound Bi out-diffusion metallic Bi at surface
Restored TSS after annealing Before annealing, ( x = 0. 15 ) • TN = 11. 5 K • MR = 360% • β=0 After annealing, • TN disappears • MR = 1400% • β = 1/2 • Two-band Hall Restored TSS • C-W fit x = 0. 10 (MCP)
Gap closing after annealing Bi 2 Te 3 Gd 0. 15 Bi 1. 85 Te 3 As-grown Annealed No gap Gap opening Gap closing (NM) (AFM) Pristine (PM)
Microscopic analysis of annealing effect After Before After • Pristine A Te. Bi 1 B Te. Bi 2 C VBi 2 • Gd doping α Quintuple layer of Bi 2 Te 3 β γ Gd. Bi 2 Bii Gd. Bi 1 After annealing disappears. increases. appears. Bi. Te 1
Schematic picture upon annealing Evaporation of top surface Te VTe 1 (volatile Te) Migration of adjacent Bi Bi. Te 1 VBi 1 pairs Isolated Bi. Te 1 defect Liberation of VBi 1 Gd. Bi 2 1 : Occupied by out-diffused Gd Gd. Bi 1 ( defect), Gd. Bi 2 ( defect) : increment : removal Restored TSS
Tuning effect in Gdx. Bi 2 -x. Te 3 -y. Sey (y=0. 2) (x = 0. 1) LMR TMR LMR TN = 9. 2 K LMR y = 0. 2 Carrier type n (cm-3) (cm 2/Vs) 2 K p 5. 22 1018 7120 300 K p 9. 93 1018 178 cf) y = 0 ; TN = 9. 2 K, p = 29. 4 1018 cm-3 y =0 EF
Tuning effect in Gdx. Bi 2 -x. Te 3 -y. Sey (y=0. 6) (x = 0. 1) TMR LMR TN = 9. 1 K y = 0. 6 Carrier type n (cm-3) (cm 2/Vs) 2 K n 0. 58 1018 211 300 K p 7. 70 1018 89 cf) y = 0. 2 ; TN = 9. 2 K, p = 5. 22 1018 y =0. 6 cm-3 y =0. 2 y =0 EF
Tuning effect in Gdx. Bi 2 -x. Te 3 -y. Sey (y=1. 5) (x = 0. 1) TMR LMR TN = 9. 0 K TMR LMR y = 1. 5 Carrier type n (cm-3) (cm 2/Vs) 2 K n 14. 1 1018 894 300 K n 14. 5 1018 59 y =1. 5 y =0. 6 cf) y = 0. 6 ; TN = 9. 1 K, n = 0. 58 1018 cm-3 y =0. 2 y =0 EF
Summary for tuning effect by Se y 1. 5 1. 0 0. 7 0. 6 0. 5 0. 2 0. 1 T (K) ρ Carrier n μ (mΩcm) type (1018 cm-3) (cm 2/Vs) 2 K 6. 6 14. 1 894 300 K 7. 3 14. 5 59. 4 3 K 42 5. 36 27. 5 300 K 17 3. 31 111 2 K 3800 n 0. 42 3. 91 300 K 25 p 5. 57 45. 0 2 K 57 n 0. 58 211 300 K 9. 1 p 7. 70 88. 9 3 K 7. 8 0. 75 1060 300 K 1. 0 4. 53 136 2 K 0. 2 5. 22 7120 300 K 3. 5 9. 93 178 2 K 0. 06 13. 3 7850 300 K 1. 3 22. 1 215 n n p p p TN = 9. 0 ~ 9. 2 K n-type Crossover point y ~ 0. 7 p-type
Evolution of electrical transport y = 0. 6 y = 1. 5 n-type y = 0. 2 Crossover point y ~ 0. 7 y = 0. 1 p-type y = 1. 0 y = 0. 7
Possible Weyl state Inversion symmetry breaking E·B term Charge pumping Ta. As, Nb. P, Ta. P Time reversal symmetry breaking Zr. Te 5, Na 3 Bi, Cd 3 As 2, Bi 1 -x. Sbx, Y 2 Ir 2 O 7, Hg. Cr 2 Se 4, Hg 1 -x-y. Cdx. Mny. Te Gdx. Bi 2 -x. Te 3 -x. Sey ● Gd (antiferromagnetic ordering) : acts as effective magnetic field Zeeman splitting Reduction of bulk band gap ● Se (n-type carrier doping) : tunes the Fermi level Weyl point at EF Negative LMR WAL
New Weyl materials Inversion symmetry breaking Magnetic field (magnetic order) Ta. As, Nb. P, Ta. P Time reversal symmetry breaking Zr. Te 5, Na 3 Bi, Cd 3 As 2, Bi 1 -x. Sbx, Y 2 Ir 2 O 7, Hg. Cr 2 Se 4, Hg 1 -x-y. Cdx. Mny. Te Gdx. Bi 2 -x. Te 3 -x. Sey ● Gd (antiferromagnetic ordering) : acts as effective magnetic field Zeeman splitting Reduction of bulk band gap ● Se (n-type carrier doping) : tunes the Fermi level Weyl point at EF EFF Fermi Magnetic Zero-gap level material dopants tuning x = xx 3== , yx 0123= y 321 x = x 3 , y = y 2 New Weyl material
Weyl metal state in Gdx. Bi 2 -x. Te 3 -y. Sey Renormalization group analysis Effective field theory
Acknowledgements TI and Weyl bulk group Extreme Quantum Materials Laboratory (EQML) http: //eqml. sogang. ac. kr/ssmc/ e-mail: mhjung@sogang. ac. kr Magnetic thin Thank you film group
- Slides: 31