Maxim Dzero Kent State University (USA) and MPI-PKS ... · Topological insulators driven by...
Transcript of Maxim Dzero Kent State University (USA) and MPI-PKS ... · Topological insulators driven by...
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Maxim Dzero
Topological insulators driven by electron spin
Kent State University (USA) and
MPI-PKS, Dresden (Germany)
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Idea
• superfluid 3He: B-phase G. E. Volovik (2010)!
H =
✓⇠p�0
~�(p) · ~�~�(p) · ~� �⇠p�0
◆
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Idea
• Spin Hall effect (2D)
Jan Werner and Fakher F. Assaad, PRB 88, 035113 (2013)
A. Bernevig, T. Hughes & S.-C. Zhang, Science 314, 1757 (2006)
M. Konig et al. Science 318, 766 (2007)
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Ø Special case will be discussed:
Minimal model(s) for topological insulators
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Minimal model(s) for topological insulators
• Anderson lattice model: U=0
Ø basis
Ø Hamiltonian (2D)
Equivalent to Bernevig-Hughes-Zhang (BHZ) model
c-f hybridization:
electrons
holes
Philip W. Anderson
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Finite-U: local moment formation P. W. Anderson, Phys. Rev 124, 41, (1961)
Ø local d- or f-electron resonance splits to form a local moment
Philip W. Anderson
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Electron sea
Heavy Fermion Primer: Kondo impurity
Spin (4f,5f): basic!fabric of heavy !electron physics !
2J+1
χ
χ ∼ 1/TCurie
T
Pauli
1
TK
Spin is screened by!conduction electrons!
TK
Kondo Temperature
Ce or Sm impurity total angular momentum J=5/2
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Heavy Fermion Primer
Kondo lattice!
Ø coherent heavy fermions!
Ø insulator!
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Γ X Γ
+1
+1
-1
How do we know if an insulator is topological?
For strong TI one needs an odd number of band inversions!
d2f0
Band inversion near X
even and odd !parity bands:!
: ν = 0
f1d1
: ν = 1
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Two lines out of each Kramers point either connect with the same Kramers point (even number of nodes) or different Kramers points (odd number of nodes)
Ø “Bulk + boundary” correspondence
Topological insulator: Z2 invariant
M. Z. Hasan & C. L. Kane, RMP 82, 3045 (2010)!
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f-orbital insulators: Anderson lattice model
2J+1 2J+1
tetragonal crystal field cubic crystal field
conduction electrons (s,p,d orbitals)
f-electrons
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f-orbital insulators: Anderson lattice model
conduction electrons (s,p,d orbitals)
f-electrons
Non-local hybridization
kσkσ Γ6α
Φ(k) Φ†(k)
Strong spin-orbit coupling is encoded in hybridization
• hybridization: matrix element
odd functions of k
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Interacting electrons: Kondo insulators
• Anderson model: infinite-U limit
f1
f0
f2
Constraint:
Projection (slave-boson) operators:
• infinite-U limit: hamiltonian
mean-field approximation:
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Kondo insulators: mean-field theory
• mean-field Hamiltonian:
Hmf (k) =
!
ξk1 V Φ†Γk
V ΦΓk εf1
"
renormalized position of the f-level
• self-consistency equations:
0.02 0.04 0.06 0.08 0.1 0.12T/�f0
0.1
0.2
0.3
0.4
a
Vdf = 0.55�f0Vdf = 0.65�f0Vdf = 0.75�f0
0.01 0.02 0.030
0.05
0.1
� f /�f0
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Kondo insulators: mean-field theory
• mean-field Hamiltonian:
Hmf (k) =
!
ξk1 V Φ†Γk
V ΦΓk εf1
"
renormalized position of the f-level
P =
!
1 0
0 −1
"
• parity
• time-reversal T =
!
iσy 0
0 iσy
"
Hmf (k) = PHmf (−k)P−1 [Hmf (k)]T = T Hmf (−k)T −1ü ü
P-inversion odd form factor vanishes @ high symmetry points of the Brillouin zone
Hmf (km) =1
2(ξkm
+ εf )1 +1
2(ξkm
− εf )P
L. Fu & C. Kane, PRB 76, 045302 (2007)!
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Z2 invariants:
!
!"#$%&'()'(*(%(+!,&%
++
ky
kx
+
++
++
!"#
-./.0
"#$#!$# !
#')("+
kz
ky
kx
+
12314!5
#')("+
!"#$%&'()'(*(%(+!,&%
67&8
!"#$%&'()'(*(%(+!,&%
1931:4!51
2314!5
kz
ky
kx
++
+
+
kz
jour-ref: Phys. Rev. Lett. 104, 106408 (2010)
tetragonal symmetry: Kramers doublet
1 0.87 0.58
ν = (0; 111)ν = (1; 111)ν = (0; 000)
WTI STI BI
nf
Strong mixed valence favors strong topological insulator!
Tetragonal Topological Kondo Insulators
(ν0; ν1, ν2, ν3)
“strong”: (−1)ν0 =8!
m=1
δm = ±1 3 “weak”: (−1)νj =!
km∈Pj
δm = ±1
δ(Γm) = sign(ξkm− εf )
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Q: What factors are important for extending strong topological insulating state to the local moment regime (nf ≈ 1)?
Strong Topological Kondo Insulators
A: Degeneracy = high symmetry (cubic!)
1 0.87 0.58
ν = (0; 111)ν = (1; 111)ν = (0; 000)
WTI STI BI
nf
M. Dzero, Europhys. Jour. B 85, 297 (2012)!
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T. Takimoto, Jour. Phys. Soc. Jpn. 80, 123710 (2011)!V. Alexandrov, M. Dzero, P. Coleman, Phys. Rev. Lett. 111, 206403 (2013)!
Cubic Topological Kondo Insulators
4d 5f
|Γ8(1)⟩ = ±
!
5
6| ± 5/2⟩±
!
1
6|∓
3
2⟩
|Γ8(2)⟩ = ±| ± 1/2⟩
Antonov,Harmon,Yaresko, PRB (2006)
Cubic symmetry (quartet)!
Bands must invert either @ X or M high symmetry points
V. Alexandrov, M. Dzero, P. Coleman, Phys. Rev. Lett. 111, 206403 (2013)!
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Cubic Topological Kondo Insulators Cubic symmetry (quartet)!
Cubic symmetry protects strong topological insulator!
0.56
ν = (0; 000)
STI
1
BI
nf
4d 5f
|Γ8(1)⟩ = ±
!
5
6| ± 5/2⟩±
!
1
6|∓
3
2⟩
|Γ8(2)⟩ = ±| ± 1/2⟩
ν = (1; 000)
V. Alexandrov, M. Dzero, P. Coleman, Phys. Rev. Lett. 111, 206403 (2013))!
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Cubic Topological Kondo Insulators: surface states
• Bulk Hamiltonian:
Assumption: boundary has little effect on the bulk parameters, i.e. mean-field theory in the bulk still holds with open boundaries
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Cubic Topological Kondo Insulators: surface states
Conduction Band
Valence Band
Γ
X
EΓ
EX
!"
X"
X"
Y"
Y"
kx
ky
• effective surface Hamiltonian:
Ø Fermi velocities are small: surface electrons are heavy
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“The world of imagination is boundless. The world of reality has its limits.”
J. J. Rousseau
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Quest for ideal topological insulators
4f-orbitals
∆CF
U
αSO
3d & 4d-orbitals
U
∆CF
αSO
5d-orbitals
αSO
∆CF
U
5f-orbitals
∆CF
UαSO
Ø complex materials with d- & f-orbitals
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Quest for ideal topological insulators
Ø complex materials with d- & f-orbitals
Ø candidates for f-orbital topological insulators
FeSb2, SmB6, YbB12, YbB6 & Ce3Bi4Pt3
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Semiconductors with f-electrons
M. Bat’kova et. al, Physica B 378-380, 618 (2006)!
Ø canonical examples: SmB6 & YbB12
P. Canfield et. al, (2003)!
Ce3Bi4Pt3
Conductivity remains finite!
exponential growth
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Mott’s Hybridization picture N. Mott, Phil. Mag. 30, 403 (1974)!
Formation of heavy-fermion insulator!
nc+nf=2×integer Ø Focus of this talk: SmB6
J. W. Allen, B. Batlogg and P. Wachter, Phys. Rev. B 20, 4807(1979).
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SmB6: potential candidate for correlated TI
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SmB6: experiments
Q: Can we establish that SmB6 hosts helical surface states with Dirac spectrum while relying on experimental data only?
(1) transport is limited to the surface (2) time-reversal symmetry breaking leads to localization (3) strong spin-orbit coupling = helicity (4) Dirac spectrum
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Ø @ T < 5K transport comes from the surface ONLY!
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D. J. Kim, J. Xia & Z. Fisk, Nat. Comm. (2014)!
Ø Idea: Ohm’s law
in ideal topo insultor resistivity is independent of sample’s thickness: surface transport
Ø Resistivity ratio
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SmB6: transport experiments Ø Non-magnetic ions (Y) on the surface: time-reversal symmetry is preserved
Kim, Xia & Fisk, arXiv:1307.0448
e e
X
Localization e e
No backscattering
Ø Magnetic ions (Gd) on the surface: time-reversal symmetry is broken
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Quantum correction to conductivity (2D)
k�k
k1
k2k3
k01
k02
k03
Ø incoming wave k is split into two complimentary waves
�G = ��R
R2= � e2
2⇡2~ log
✓⌧�(T )
⌧tr
◆
⌧�(T ) ⇠ T�p Inelastic scattering time
Interference correction to conductance:
[G. Bergmann (1975)] !
[E. Abrahams et al. (1979); L. P. Gor’kov et al (1979)] !
waves propagate independently and interfere in the final state -k
Weak localizaGon
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Quantum correction to conductivity (2D) Ø quantum interference in topological insulators
[Qi & Zhang (2010)] !
In topological insulators two waves always interfere destructively: weak anti-localization
Electron spin rotates adiabatically by or ⇡ �⇡
�G
�T< 0
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Quantum correction to conductivity (2D)
Hikami-Larkin-Nagaoka formula
�G(B) = �↵ e2
2⇡2~F (B)
F (B) =
1
2+
~c4eBL2
�
!+ ln
4eBL2
�
~c
!
Ø weak anti-localization (WAL) in a perpendicular magnetic field.
No effect in a parallel field
I~B
Interference correction in a field is negative: WAL
↵ > 0-0.4 -0.2 0 0.2 0.4
B/B0 x 10-3
-4
-3
-2
-1
0
δσ(B
)(e2 /h
)
α = 0.42 α = 0.47 α = 0.49
[MD et al. (2015)]!
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SmB6: transport experiments Ø weak anti-localization (WAL) in SmB6
S. Thomas et al. arXiv:1307.4133*diffusion (classical) conductivity is field-independent
k�k
k1
k2k3
k01
k02
k03
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SmB6: quantum oscillations experiments
G. Li et al. (Li Lu group Ann Arbor) Science (2015)
Ø very light effective mass: 0.07-0.1me
Strong surface potential!
Idea: zero-energy Landau level exists for Dirac electrons:
Experimental consequence: shift in Landau index n must be observed – only E0=0 contributes at infinite magnetic field!
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SmB6: quantum oscillations experiments … BUT Ø Key results:
• Rapid quantum oscillations corresponding to 50% the volume of the BZ
• Samples are bulk insulators
• Large mean-free path (~ few microns)
Contradicts earlier quantum oscillation
experiments (Science’14)!
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Collaborators:
Piers Coleman, Rutgers U. Victor Galitski, U. of Maryland
Kai Sun, U. Michigan
Victor Alexandrov, Rutgers U.
Bitan Roy, U. of Maryland Jay Deep Sau, U. of Maryland Maxim Vavilov, U. of Wisconsin
Kostya Kechedzhi, NASA-Ames
Piers
Victor G. Victor A. Kai
Jay Deep
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Conclusion & open questions
• Many-body instabilities on the surface of TKI driven by both long-range ! Coulomb and short range interaction.!
Ø Role of correlations?!
• Why all Kondo insulators have cubic symmetry?![Kondo semimetals have tetragonal symmetry]
• surface conductance & insulating bulk below 5K! SmB6 is a correlatedtopological insulator • quantum oscillations experiments confirm Dirac!
Dirac spectrum of surface electrons!• weak anti-localization: strong SO coupling
• effect magnetic vs. non-magnetic doping on ! the magnitude of surface conductivity
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What makes topological Kondo insulators special? Q: Are topological Kondo insulators adiabatically connected to topological band insulators? A: NO! Gap closes as the strength of U gradually increases
Jan Werner and Fakher F. Assaad, PRB 88, 035113 (2013)
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Mott’s Hybridization picture N. Mott, Phil. Mag. 30, 403 (1974)!
Mo(, 1973
Formation of Heavy f-bands: electrons and localized f doublets hybridize, possibly due to Kondo effect!
|kσ⟩
H = (|kσ⟩Vσα(k)⟨α| + h.c.) |α⟩ ≡ |±⟩
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Mean-field theory for SmB6: is N=1/4 small enough? • integrated spectral weight of the gap: T-dependence
Nyhus, Cooper, Fisk, Sarrao, ! PRB 55, 12488 (1997)!
Mean-field-like onset of the insulating gap!
• full insulating gap: dependence on pressure
Derr et al. PRB 77, 193107 (2008)!
0.02 0.04 0.06 0.08 0.1 0.12T/�f0
0.1
0.2
0.3
0.4
a
Vdf = 0.55�f0Vdf = 0.65�f0Vdf = 0.75�f0
0.01 0.02 0.030
0.05
0.1
� f /�f0