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![Page 1: Berry phase driven Hall effects Naoto Nagaosa Department of Applied Physics The University of Tokyo June 22, 2011 @ Beijing.](https://reader035.fdocuments.in/reader035/viewer/2022062314/56649ef45503460f94c07403/html5/thumbnails/1.jpg)
Berry phase driven Hall effects
Naoto Nagaosa
Department of Applied PhysicsThe University of Tokyo
June 22, 2011 @ Beijing
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Collaborators
TheoryH. Katsura, J. H. Han, J. Zang, J. H. Park, K. Nomura, M. Mostovoy, B.J.Yang
ExperimentX. Z. Yu, Y. Onose, N. Kanazawa, Y. Matsui,
Y. Shiomi, Y. Tokura
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Berry phase M.V.Berry, Proc. R.Soc. Lond. A392, 45(1984)
t
))(( tXEn
Transitions between eigen-states are forbidden during the adiabatic change
Projection to the sub-space of Hilbert space and constrained quantum system
Connection of the wave-function in sub-space of Hilbert space
Berry phase, gauge connection
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Path integral and Aharonov-Bohm effect
1C
2C
1C2C
NC
1a2a
Na
Amplitude from A to B
N
jja
1
1a
2a
cieeaaaa /02121 ||
0x 0x
1x 1x
nXXXkrr ,,,,, 21 Generalized space
Berry Phase
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Electrons with ”constraint”
Projection onto positive energy stateSpin-orbit interaction
as SU(2) gauge connection
Dirac electrons
doublydegenerate
positive energy states.
E
k
Bloch electrons
Projection onto each bandBerry phase
of Bloch wavefunction
k
E
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Solid angle by spins acting as a gauge field
gauge flux
Si
Sj
Sk
|ci > |cj >
Fictitious flux (in a continuum limit)
conductionelectron
acquire a phase factor
scalar spin chiralityscalar spin chirality
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k-space
e-
Fermi surface
r-space
e-
kBk
x
dt
dk
dt
d
rdt
dre
dt
dB
r
k
Equation of motion
BrBk
1c1c
llB llB
Luttinger, Blout, Niu
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Issues to be discussed
1. Hall effects of uncharged particles -- photons and magnons
2. r-space vs. k-space Berry phase
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Can neutral particle show Hall effect ?
Thermal Hall effect by phonon : Tb3Ga5O12
Strohm, Rikken, & Wyder, PRL 95 (‘05).
Thermal Hall angle: at 5K.
Hall effect of photon
M. Onoda et al, Phys. Rev. Lett. 93, 083901 (2004).K.Y. Bliokh and Y.P. Bliokh Phys. Rev. Lett. 96, 073903 (2006).F. D. M. Haldane and S. Raghu, Phys. Rev. Lett. 100, 013904 (2008) O. Hosten, P. Kwiat, Science 319, 787 (2008).
Hall effect of magnons in insulating magnets ? Yes ! [ H.Katsura-N.N.-P.A.Lee (PRL09)]
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Metals
Wiedemann-Franz law
Righi-Leduc effect
F.D.M. Haldane, PRL 93 (‘04).
How about Mott insulators ?
Spins can carry thermal Hall current ?
cf.) Magnon spin Hall effect (S. Fujimoto, arXiv: 0811.2263)
Thermal Hall effect by phonon : Tb3Ga5O12
Strohm, Rikken, & Wyder, PRL 95 (‘05).
Thermal Hall angle: at 5K.
Thermal Hall effect in solids
applicable to AHE also
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• Hubbard model with complex hopping ( )
Ring-exchange:
Scalar spin chirality
D. Sen & R. Chitra, PRB (‘95) O.I. Motrunich, PRB (‘06).
Coupling between spin chirality and magnetic field
Second-order: ijie
ijie
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Spin Chirality due to Spin Wave
Ferromagnet: Antiferromagnet 120°structure
Scalar chirality:
Collinear spin structure:
i
j k
i
• Geometric Cancellation
Exact cancellation
∵
i
j k
i
1-magnon term also cancels
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i) Lattice structure: square(□), triangular ( ), kagome, …△
ii) Magnetic structure: FM ( ), AFM ( ), 120°, spiral, …
iii) Anisotropy of hopping → non-uniform
• No-go theorem: FM order with an edge-sharing geometry → ×
Kagome FM Kagome AFM q=0
Corner sharing geometry, e.g., Kagome !!
classical AFM kagome
i) q=0 g.s. ⇔χ FM
ii) g.s. ⇔χ AFM
NO-GO Theorem applicable to many cases !
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Berry curvatureBose distribution function
Kubo formula for thermal Hall conductivity
c.f. Matsumoto- Murakami
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Magnon dispersion
Around k=0
Spin Wave Hamiltonian
TKNN-like formula:
T-linear & B-linear!
Thermal Hall effect in Kagome ferromagnet
Skew scattering ? Small in the scattering of low energy limit (s-wave).
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(constraint: )
RVB ( resonating valence bond ) state, P. W. Anderson(‘87)
quantum liquid of singlets
Mean field theory of RVB state
U(1) (internal) gauge-field
Candidate materials κ-(BEDT-TTF)Cu2(CN)3, ZnCu3(OH)6Cl2, Na4Ir3O8(3d, strong SO), …
Spinon (charge=0, spin 1/2)(S. Frorens & A. Georges, PRB 70
(‘04))
Quantum spin liquid
ija : gauge field spin chirality
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RVB theory under magnetic field
• spin model( )△ :
Slave rotor rep.:
κ-(BEDT-TTF)Cu2(CN)3
Ring exchange term
Scalar chirality
Lee and Lee, O. I. Motrunich
Ioffe-Larkin, Nagaosa-Lee
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Deconfined spinon ( gauge dependent object)
coupled to ( via ring exchange term )
Lorentz force
Magnon (gauge invariant object)
coupled to
intrinsic Hall effectThermal Hall effect due to spinons spinon metal ・ Fermi surface ( gapless spinon picture )
spinon current conductivity :Wiedemann-Franz law
Spinon v.s. Magnon
AB
A
Thermal Hall angle
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Thermal conductivity in κ-(BEDT-TTF)Cu2(CN)3 M. Yamashita et al., Nature Phys. 5 (‘09)
T-linear
0.02 W/Km ⇔@0.3 K
Spinon lifetime
Spinon effective mass
Thermal Hall angle @ B [T]
M. Yamashita et al., Science 328 (‘10)
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Target material -Lu2V2O7
Pyrochlore Lattice
(111) Plane is Kagome
Collinear ferromagnet
insulator
0
0.2
0.4
0.6
0.8
1Lu2V2O7
H || [111]H=0.1T
M(
B/V
))
100
101
102
103
104
Res
isti
vity
(cm
)
0 50 100 1500
0.5
1
1.5 x
x(W
/Km
)
T(K)
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Thermal Hall conductivity for Lu2V2O7
(=Tc)
-5 0 5
20K
Magnetic Field (T)-5 0 5
30K
-5 0 5
-2
-1
0
1
2 40K
50K
-5 0 5
10K
60K 70K
-2
-1
0
1
2 80K x
y (1
0-3 W
/Km
)Lu2V2O7 H||[100]
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-10 0 10
-1
0
1
2T=50K
(1
0-3 W
/Km
)
0H (T)
“spontaneous” component
Emergent at Tc
Almost isotropic
Temperature dependence, anisotropy
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Discussion
Origin of thermal Hall conductivity?Possibility of electronic origin can be ruled out by Wiedemann Franz law.
xxe<10-5 W/Km below 100K
xy decreases with H at low T. Opening of magnon gap
xy is observed only below TC.
Coherent magnon transport is crucial for the xy.
xy is almost proportional to M.External Hirrelevant
23
50 100
0.5
1
1.5
0
0.2
0.4
0.6
0.8
1
0
T (K)
xy
(10
-3 W
/ K
m)
M (
B / V4
+)
0 H = 0.1 T 0 H = 7 T
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1
2
3
4
D12
D23D31
Theory of magnon Hall effect based on DM interaction Katsura & Nagaosa
ijijiijji )e/J()|iSS(DSSJj| 2
~
nDiJeJ iji ij ~
Magnons acquire Berry phase owing to DM interaction.
(isotropic)
,expLi22
2),(B
B5/2
B
2
B2/3
2B
Tk
Hg
JS
Tk
JS
Hg
a
TkTH
-10 0 10-1
0
1
T=20K
Magnetic Field (T)
xy
(10-3
W/K
m)
H||[100]
xycalc(H)
C
D/J=0.32Cf. D/J=0.19 for CdCr2O4
i|i site
c.f. Matsumoto -Murakami
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Gauge field of spin textures in insulating magnets M.Mostovoy, K.Nomura and N.N. PRL2011
Spin dynamics in the intermediate virtual states of the exchange int.Coupling between gauge field e and E Multi-orbital Mott insulator
Finite even without inversion asymmetry or spin-orbit interaction
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k-space
e-
Fermi surface
r-space
e-
kBk
x
dt
dk
dt
d
rdt
dre
dt
dB
r
k
Equation of motion
20 , xyxy
Bk induced AHE
“dissipationless” nature
02 , xyxy
Br induced AHE
Cf. normal HEneBneB xxxyxy /,/ 2
BrBk
one flux quantum/(nm)2~4000T !
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0 50 100 1500
0.5
1
0
0.5
1
1.5
Temperature (K)
Resi
stivi
ty (m
Ωcm
)
M( H
= 0.
5 T)
( μ
B/M
o)
0
10
20
30
I (
μB2
/2N
d2
Mo 2
O7) Nd 2Mo 2O 7
I (200) I(111)
T*
TCT*
RMo
Pyrochlore Nd2Mo2O7
Y. Taguchi, Y. Oohara, H. Yoshizawa, N. Nagoasa, and Y. T., Science 2001
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Skyrmion configuration
From Senthil et al.
Skyrmion and spin Berry phase in real space
Solid angle acts as a fictitious magnetic field for carriers
aSSS kji )(
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Pfleiderer, Rosch, Lonzarich et al
Quantum Phase Transition in MnSi
Non-Fermi liquid charge transport
Spin fluctuation on a sphere in momentum space
DM magnet
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MnSiMnSi
S. Mühlbauer et. al., Science 323 915 (2009)
Small angle neutron scattering for Skyrmion Xtal
c.f. early theoretical prediction by A.N.Bogdanov et al.
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Skyrmion Crystal
3-flod-Q
Superposition of three Helix without phase shift
Skyrmion Skyrmion crystal
0321 QQQ
S. Muhlbauer et al. Science 323, 915 (2009).
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Monte Carlo simulation for 2D helimagnet
J. H. Park, J. H. Han, S. Onoda and N.N.
)( kjixy SSS
anisotropy
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Lorentz TEM observation of Skyrmion crystal in (Fe,Co)Si
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Nature (2010)
X. Z. Yu, Y. Onose, N. Kanazawa2, J. H. Park, J. H. Han, Y. Matsui, N. N. Y. Tokura
TheoryExperiment
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Coupled dynamics of conduction electrons and SkX
Effective EMF due to spin texture acting on conduction electrons
Coupling term
Boltzmann equation
LLG equation
Lorentz force
J.D.Zang, J.H. Han, M.Mostovoy, and N.N.
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Skyrmion-induced AHE (MnSi)
Finite but quite small
A. Neubauer et al, PRL 102 186602 (2009)
Relation to the magnetic structure??
M. Lee, W. Kang, Y. Onose, Y. Tokura, and N. P. Ong, PRL (2009).
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Fictitious magnetic flux
one flux quantum/(nm)2~4000T !(double-excahnge model)
MnGe
Nd2Mo2O7
(reference)
FeGe
MnSi
(magnetic) [nm]
yx(topological) [ncm]
(cal.)[T]
70
18
3.0
~0.5
1
1100
~40000
28 5
200
6000
indiscernible
yx ∝ ( Sk density)
©Y. Tokura
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V
j
mc
he ztopxy
Moving magnetic flux produces the transverse electric field
c.f.
xConduction electron number per site
SSpin quantum number
||V
jcj
xy
jcj
xS
S
2
2
Topological Hall effect
“Electromagnetic induction”
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New dissipative mechanism for spin texture
moving flux electric field induced current dissipation
2)/)(( alkF
lmean free path size of Skyrmion
’ does not require spin-orbit int. and can be as large as ~0.1But is determined by DM interaction.
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V
j
))('( || zeVQV
Transverse motion of the Skyrmion as a back-action to the “electromagnetic induction”
Skyrmion Hall effect
1Q Skyrmion charge determined by the direction of the external magnetic field
“Hall angle” 'tan H
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Hall Effect of Light
)|)|
)]([
)||()(
ckcc
ccc
ckccc
ccc
zkizonpolarizati
krvkforce
zzkk
krvrvelocity
c
c
:
:
:
M.Onoda,S.Murakami,N.N. (PRL2004)
Generalized equation of geometrical optics
Photon also has “spin”
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Giant X-ray shift in deformed crystal
Sawada-Murakami-Nagaosa PRL06
Berry curvature in r-k space
610 enhancement
PRL2010
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Berry phase in r-k space
(Real) Space dependent Berry curvature
• Semiclassical equation of motion
D. Xiao et al., PRL (2009)
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Inhomogeneity-induced polarization
Inhomogeneity-induced topological charge polarization !
(Second Chern form)P
r
D. Xiao et al., PRL (2009)
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1. Berry phases in r- and k-spaces,
and (r,k)-space
2. Hall effects of uncharged particles
photons and magnons
3. Hall effect and charge pumping in spin textures
C
Conclusions