Protection of the surface states of a topological insulator ......In WTI, topological non-triviality...
Transcript of Protection of the surface states of a topological insulator ......In WTI, topological non-triviality...
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Hiroshima University
Protection of the surface states of a topological insulator: Berry phase perspective
Ken-Ichiro Imura
collaborators: Yositake TakaneSophia UniversityTomi Ohtsuki
Igor HerbutTakahiro FukuiYasuhiro HatsugaiAkihiro Tanaka
Simon-Fraser UniversityIbaraki UniversityTsukuba UniversityNIMS, Tsukuba
Hiroshima University
Koji Kobayashi Sophia University
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The surface states of a topological insulator has a dual personality:extrovert and introvert
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The topological insulator has a dual personality.
Its metallic (gapless) surface state also has a dual personality.
bulksurface
introvert:
extrovert:
staying on the surface; not penetrating into the bulk
inducing a fictitious magnetic field on curved surfaces; solenoid or magnetic monopole type (spin) Berry phase
insulatingmetallic
topologically protected
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What are the “defining properties” of the topological insulator?
- Existence of a protected gapless surface state
- The bulk-surface correspondence
Emergence of the protected gapless state on its surface
Existence of a non-trivial topological invariant characterizing the bulk state
one-to-one correspondence
Bulk:
Surface:
cf. (a pair of) Dirac cones in graphene
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Introduction totopological insulators
A chronological viewpoint:spin Hall effect topological insulator
spatial dimension: (of the bulk)
2D 3D
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can be modeled by a Wilson-Dirac type effective (bulk) Hamiltonian
The two bands get inverted by changing the sign of a mass term.
“inverted” gap
Bernevig et al., Science ’07
Band “inversion” due to spin-orbit coupling
p-type orbital
s-type orbital
gap massEinstein Newtonian
magnitude of the SOC
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• The winding number (Chern number)
2D example: how to characterize the bulk
mapping:
stereographic projection
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-1.0 -0.5 0.5 1.0kêp
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bulk
edge
winding number
The bulk-edge correspondence
protected gapless edge mode
non-trivial
trivial
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The Wilson-Dirac type (bulk) effective Hamiltonian
Dirac equation in 3+1 D
• gamma matrices:
•Wilson term: a “spice” for the mass term
• Two types of Pauli matrices:
spin:orbital:
Time-reversal symmetry preserved
Zhang et al., Nature Phys. ’09; Liu et al., PRB ’10
matrix
topological insulators
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3D generalization
-10 -5m0êm2
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trivial trivialSTI STIWTI
spectrum flattening
winding number
matrix
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bulk
non-trivial
winding number
trivial
slab geometrya single helical Dirac cone
spectrum: gapless spectrum: gappedno surface state
surface
The bulk-surface correspondence: 3D version
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Bulkenergy gap
(band strcture)
Surfacestate
None
Yes
normal gap
inverted gap gapless, protected
Bulk-surface correspondence
Zhang et al., Nature Phys. ’09
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The noninvasive metallic state
Inducing a fictitious magnetic filed on curved surfaces
Finite-size energy gap
The topological protection does not
help.- sensitivity to the geometry of the sample
Act I: The extrovert
Act II:The surface stays introvert as a consequence of its extroverted character.
Noninvasiveness protected by the Berry phase
cylindrical sample(TI nanowire)
spherical sample(TI nanoparticle)
imaginary solenoid
effective monopole
spin connection
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Which is the genuine surface?
• Consider a lattice (tight-binding) realization of topological insulator • A lattice model is “sparse”, existing
only on sites and links
Why is the surface state noninvasive into the bulk?
Yet, in reality protected surface states appear only on macroscopic surfaces.
KI & Takane, arXiv:1211.2088
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Act I: Extrovert
The property of inducing a fictitious magnetic field on curved surfaces
Protection of the surface states in topological insulators: Berry phase perspective
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bulk effective Hamiltonian:
The cylindrical topological insulator
KI, Takane & Tanaka, Phys. Rev. B 84, 195406 (2011)
Effective Dirac Hamiltonian on the cylindrical surface:
Surface eigenstates:
spin Berry phase
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orbital part
If one chooses the base double-valued,Possible interpretation
Spin-to-surface locking
Half-odd integral quantization of the orbital angular momentum
single-valued
anti-periodic
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Gapped surface states on the cylinder
• spin Berry phase
• spin-to-surface locking
Half-odd integral quantization
Finite-size energy gap
anti-periodic
Finite-size energy gap decays slowly (only algebraically)
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prism:
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a system of infinitely large slab
cf. case of a slab-shaped sample
• The surfaces are flat.But there are two of them (two Dirac cones); one on the top and the other at the bottom.
• If the thickness is finite, the two Dirac cones communicate each other through the bulk
Finite-size energy gapThe magnitude of the gap is determined by the overlap of the two wave functions.
exponential decay
Linder, Yokoyama & Sudbo, PRB ’09; Lu et al., PRB ’10
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Effective Dirac Hamiltonian on a cylindrical surface spin Berry phase
Physical interpretation of the result
Electrons on a curved surface of topological insulator exhibits an “active” property reflecting its geometry.
An electron on the cylindrical surface behaves as if an imaginary solenoid pierces the cylinder.
Effects of the Berry phase
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The spherical topological insulator
Effective Dirac Hamiltonian on a spherical surface
Bulk effective Hamiltonian:Surface states on the sphere:
KI, Yoshimura,Takane & Fukui, Phys. Rev. B 86, 235119 (2012)
Half-odd integral quantization:
Act I-B:
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A (fictitious) magnetic monopole is induced at the center of the sphere!Physical interpretation:
Effective Dirac Hamiltonian in the single-valued basis
vector potential associated with a magnetic monopole (+ Dirac string)
induced Berry phase
The surface Dirac equation:
forfor
Dirac string on
Dirac string onA pair of magnetic monopole with an opposite magnetic charge: is induced at the origin
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How about the spectrum?
• comparison with the tight-binding spectrum
The wave function:-30 -20 -10 0 10 20 30
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E @JD
reminiscent of Landau levels
monopole harmonics
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“The noninvasive metallic state”
Act II: Unification of the two aspects: the extrovert and the introvert
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Where is the genuine surface?
• Consider a lattice (tight-binding) realization of topological insulator • A lattice model is “sparse”, existing
only on sites and links
Why is the surface state noninvasive into the bulk?
Yet, in reality protected surface states appear only on macroscopic surfaces.
KI & Takane, arXiv:1211.2088
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Why is the surface state not penetrating into the bulk?Answer: because of the Berry phase
A flux tube penetrating the system may awaken the invasive nature of surface state.
Recall that the cylindrical surface state is gapped by the spin-to-surface locking.
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Invasion of the metallic surface statedeeply into the bulk along a flux tube
invasiveness of the surface state
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x
z
0
r
Analytic treatments: hyperbolic “drain” geometry
KI & Takane, arXiv:1211.2088
The derivatives are rewritten as
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Derivation of the surface effective HamiltonianThe zero-energy (degenerate) basis eigenstates:
where
The surface effective Hamiltonian:
spin Berry phaseThe velocities are renormalized by the curvature
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Invasive vs. noninvasive surface state
where&
Yes!
Is there a surface solution?
where
• contrasting behaviors in the two asymptotic regimes
1) deep in the aperture: exponential decay
2) on the xy-plane, far away from the aperture
Berry phase inactivated extended E=0 state appears
constant
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• The noninvasiveness of the surface state is protected by the Berry phase
(After a long period of slavery to the bulk) the surface state is now freed.
Conclusions
• (Reclassified as a noninvasive metallic state) the surface state is shown to be, by itself, topologically protected.
• The introvert surface state sometimes wants to show off: e.g., on cylindrical and spherical surfaces.
• The “gaplessness” is not immune to finite-site size corrections associated with the spin connection.
Such a finite-size correction does not decay exponentially.
role of the system’s geometry: slab vs. cylinder, sphereflat vs. curved surfaces
spin Berry phase
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Other perspectivesSo far, we have considered only “strong” topological insulators.
headed for seek of the diversity? Latest tendency of the field:
- WTI is actually not that weak !
In WTI, topological non-triviality is not weak but only hidden.
Stability of WTI against disorder: the global phase diagram
A possible direction: exploring the nature of weak topological insulators (WTI)
KI, Okamoto, Yoshimura,Takane & Ohtsuki, Phys. Rev. B 86, 245436 (2012)
- Specific features associated with the anisotropic character
KI, Takane & Tanaka, Phys. Rev. B 84, 035443 (2011)
= isotropic
= anisotropic
Kobayashi, Ohtsuki & KI, arXiv:1210.4656, PRL in press.
Ringel, Kraus & Stern, Phys. Rev. B 86, 045112 (2012).
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Experiments on finite-size effects1) Slab (this film) geometry
2) Prism (nanowire) geometryZhang et al., Nature. Phys. (2010)
Peng et al., Nature. Mat. (2009)