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Berry curvature and topological modes for magnons
Shuichi Murakami Department of Physics, Tokyo Institute of Technology
KITP, UCSB
Nov.14, 2013
Collaborators: R. Shindou (Tokyo Tech. Peking Univ.)
R. Matsumoto (Tokyo Tech.) J. Ohe (Toho Univ.) E. Saitoh (Tohoku Univ. )
Magnon thermal Hall effect
• Matsumoto, Murakami, Phys. Rev. Lett. 106, 197202 (2011)
• Matsumoto, Murakami, Phys. Rev. B 84, 184406 (2011)
Topological magnonic crystals
• Shindou, Matsumoto, Ohe, Murakami, Phys. Rev. B 87,174427 (2013)
• Shindou, Ohe, Matsumoto, Murakami, Saitoh, Phys. Rev. B87, 174402 (2013)
Phenomena due to Berry curvature in momentum space
• Hall effect Quantum Hall effect
chiral edge modes
• Spin Hall effect (of electrons) Topological insulators
helical edge/surface modes
•Spin Hall effect of light one-way waveguide in photonic crystal
• Magnon thermal Hall effect topological magnonic
crystal
Fermions
Bosons
Present work
Gapped Topological edge/surface
modes in gapped systems Gapless Various Hall effects
semiclassical eq. of motion for
wavepackets
( )1( )n
n
E kx k k
k
k eE
n( : band index)
: periodic part of the Bloch wf. knu
xki
knknexux
)()(
: Berry curvature ( ) n nn
u uk i
k k
- SM, Nagaosa, Zhang, Science (2003)
- Sinova et al., Phys. Rev. Lett. (2004)
Adams, Blount; Sundaram,Niu, …
Intrinsic spin Hall effect in metals& semiconductors
Spin-orbit coupling Berry curvature depends on spin
Force // electric field
2
2
,
( ) zBxy nk n
n k
k Tc k
V
Magnon Thermal Hall conductivity
22
2
2 20
1 1log (1 ) log (log ) 2Li ( )
tc dt
t
Q xy yxj T
T. Qin, Q. Niu and J. Shi,Phys. Rev. Lett. 107, 236601 (2011) R. Matsumoto, S. Murakami, Phys. Rev. Lett. 106, 197202 (2011)
Berry curvature
( )1( )n
n
E kx k k
k
k U
( ) n nn
u uk i
k k
: Berry curvature
equilibrium
Density matrix
Current
deviation by
external field
(1) Semiclassical theory ( 2) Linear response theory (Kubo formula)
Eq. of motion
: Bose distribution
Cf: different from previous works
Katsura, Nagaosa, and Lee, PRL.104, 066403 (2010).
Onose, et al., Science 329, 297 (2010);
Magnetic dipole interaction
• Dominant in long length scale (microns) • Similar to spin-orbit int. Berry curvature • Long-ranged nontrivial, controlled by shape
Magnetic domains
Generalized eigenvalue eq. : MSFVW mode
• Landau-Lifshitz (LL) equation
• Maxwell equation
• Boundary conditions
00 HB ,
//2//121 HHBB ,
0 0
0 0
1 0, , : thickness of the film, , m(z)
0 1
: saturation magnetization, : static magnetic field, film,
ˆ : 2 2 matrix of the Green's func
x y
H M z
x y
m imH M L
m im
M H z
G tion, : frequency of the spin wave
B. A. Kalinikos and A. N. Slavin, J. Phys. C 19, 7013 (1986)
)( HMM
dt
d
Magnetostatic modes
in ferromagnetic films (YIG)
/2
/2
ˆˆ ˆ( ) ( ) ( ) ( ) ( , ) ( )L
z H ML
H z z H z z dz G z z zm m m m m
H
Berry curvature
for magnetostatic forward volume wave mode
: Band structure for magnetostatic
forward volume-wave mode : Berry curvature
Berry curvature is zero for backward volume wave and surface wave
• R. Matsumoto, S. Murakami, PRL 106,197202 (2011), PRB84, 184406 (2011)
T: paraunitary matrix
Bosonic BdG eq. and Berry curvature
Generalized eigenvalue eq.
bosonic Bogoliubov-de Gennes Hamiltonian
Diagonalization
Berry curvature for n-th band
k k zH
k z k zT T
k z k zT T
z z
Linear response theory
Berry curvature
T: paraunitary matrix
Bosonic BdG eq. and Berry curvature
Generalized eigenvalue eq.
bosonic Bogoliubov-de Gennes Hamiltonian
Diagonalization
Berry curvature for n-th band
k k zH
k z k zT T
k z k zT T
z z
Linear response theory
Berry curvature
2.8MHz/Oe, 1750gauss, 300K
3000Oe, 17.2nm for YIG,
s
ex ex
M T
H l
85.8 10 W/Kmxy
(Example):
Topological chiral modes
in magnonic crystals
• R. Shindou, R. Matsumoto, J. Ohe, S. Murakami, Phys. Rev. B 87,174427
(2013)
• R. Shindou, J. Ohe, R. Matsumoto, S. Murakami, E. Saitoh,Phys. Rev. B87,
174402 (2013)
Phenomena due to Berry curvature in momentum space
• Hall effect Quantum Hall effect
chiral edge modes
• Spin Hall effect (of electrons) Topological insulators
helical edge/surface modes
•Spin Hall effect of light one-way waveguide in photonic crystal
• Magnon thermal Hall effect topological magnonic
crystal
Fermions
Bosons
Present work
Gapped Topological edge/surface
modes in gapped systems Gapless Various Hall effects
Chern number & topological chiral modes
Band gap Chern number for n-th band = integer
topological chiral edge modes
• Analogous to chiral edge states of quantum Hall effect.
• N>0 cw, N<0: ccw mode
bands below
Ch #(clockwise chiraledgestates in the gap at )n
n E
N E
2
Ch2
n nBZ
d kk
Berry curvature
k
bulk mode: Chern number= Ch1
Ch1 topological edge modes
bulk mode: Chern number= Ch2
bulk mode: Chern number= Ch3
(Ch1+Ch2) topological edge modes
( ) n nn
u uk i
k k
Chern number & topological chiral modes
Band gap Chern number for n-th band = integer
topological chiral edge modes
• Analogous to chiral edge states of quantum Hall effect.
• N>0 cw, N<0: ccw mode
bands below
Ch #(clockwise chiraledgestates in the gap at )n
n E
N E
2
Ch2
n nBZ
d kk
Berry curvature
k
bulk mode: Chern number= Ch1
Ch1 topological edge modes
bulk mode: Chern number= Ch2
bulk mode: Chern number= Ch3
(Ch1+Ch2) topological edge modes
( ) n nn
u uk i
k k
Why the Chern number is related
with the number of edge states within the gap?
Mathematics : Index theorem
Physics : Analogy to quantum Hall effect
(in electrons)
Quantum Hall effect of electrons:
-- Chern number & edge states --
Electric field //y
2
( )Fnk
xy nz
E E
eB k
( ) nk nkn
u uB k i
k k: Berry curvature
Hall conductivity for 2D systems (of electrons)
( )
eEy
E E
2 2( )
( )1 1 x
x x
ev eEyj f E j f E
L L E E
Current along x
2
22( )
1 y x
xy
v vie h f E
L E E
Hall conductivity (Kubo formula)
2
:filledband
Chxy n
n
e
h
2
Ch ( )2
n nzBZ
d kB k : Chern number = integer
integer QHE
Example: insulator
Hall conductivity is expressed as a sum of Chern numbers
at T=0
0
0at 0
at
t
t T
Laughlin gedanken experiment: Chern number & edge states
y
y
AL
0
2
0 0Ch Chx xy
y
y
y y y
e ej
TL h TL TLE
TL
ChQ e
Number of electron carried from left end to right end = Ch
Increase the flux charge transport along x
This charge transport is
between the edge states
on the left and right ends.
gapless edge modes exist.
E
j
Gradual change of vector pot. = change of wavenumber
Edge states at the right end
Integer quantum Hall effect
Ch=1
Ch=1
Ch=1 2 chiral
edge states
1 chiral
edge mode
0 edge mode
B
Topological photonic crystals Theory: Haldane, Raghu, PRL100, 013904 (2008) Experiment: Wang et al., Nature 461, 772 (2009)
Ch=0
Ch=1
1 Chiral
edge mode
Photonic gap
Photonic gap
Landau-Lifshitz equation
Maxwell equation (magnetostatic approx.)
Linearized EOM
exchange field (quantum mechanical short-range)
Dipolar field (classical, long range)
External field
• Saturation magnetization Ms • exchange interaction length Q
2D Magnonic Crystal : periodically modulated magnetic materials
YIG (host) Iron (substitute)
ax
ay
H// z modulated
bosonic Bogoliubov – de Gennes eq.
dipolar interaction
non-trivial Chern integer
topological chiral edge modes
(like in quantum Hall effect)
x y
y
x
a a
ar
a
: unit cell size
: aspect ratio of unit cell
YIG (host) Iron (substitute)
ax
ay
H// z `exchange’ regime
`dipolar’ regime
• Phase diagram
Magnonic gap between
1st and 2nd bands
2
1 1( )2BZ
d kC k
C1 (Chern number)
for 1st band
=Number of
topological chiral
modes within the gap
2nd Lowest band
Lowest magnon band
[10G
Hz]
chiral magnonic band in magnonic crystal
bulk
bulk f=4.2GHz
bulk
f=4.5GHz
bulk
f=4.4GHz
edge
Simulation (by Dr. Ohe) DC magnetic field : out-of-plane AC magnetic field : in-plane
Magnonic crystals with ferromagnetic dot array R. Shindou, J. Ohe, R. Matsumoto, S. Murakami, E. Saitoh, PRB (2013)
dot (=thin magnetic disc) cluster: forming “atomic orbitals”
convenient for (1) understanding how the topological phases appear
(2) designing topological phases
decorated square lattice decorated honeycomb lattice
Each island is assumed to behave as monodomain H//z
H//z
Equilibrium spin configuration
2.4, 2 1.2, 1.70, 1.0x y se e r V M
Magnetostatic energy
Magnonic crystals: decorated square lattice
Hext
Hext
Hext < Hc=1.71
Hext > Hc
Collinear // Hext
Tilted along Hext
• Spin-wave bands and Chern numbers
H=0
H=0.47Hc
H=0.82Hc
H=1.01Hc
H=1.1Hc
H=1.4Hc
Red: Ch=-1
Blue: Ch=+1
Time-reversal
symmetry
Topologically
trivial
•Small H<<Hc
•Large H>>Hc
Dipolar interaction is
weak
Nontrivial phases (i.e. nonzero Chern number )
in the intermediate magnetic field strength
topological chiral edge modes
Topologically nontrivial
= chiral edge modes
+1 chiral mode -1 chiral mode
Change of Chern number at gap closing event
parameter
Chi
Chj
Ch’i
Ch’j
( Chi +ChI =Ch’i +Ch’I: sum is conserved)
DChi = -DChj = ±1
: Dirac cone at gap closing
volume mode (=bulk)
gap closes
magnonic
(volume-mode)
bands
H=0.47Hc
H=0.76Hc
H=0.82Hc
Magnonic crystals: edge states and Chern numbers (1)
Red: Ch=-1
Blue: Ch=+1
+1 chiral mode
-1 chiral mode
-1 chiral mode
+1 chiral mode
Edge states
bulk Strip geometry
(bulk+edge)
“atomic orbitals” within a single cluster
Spin wave excitations: “atomic orbitals”
relative phase for precessions
H<Hc: noncollinear H>Hc:
collinear // z
nJ=1 and nJ=3 degenerate at H=0
nJ=0 softens
at H=Hc
nJ=2 is lowest at H=0: favorable for dipolar int.
nJ=0
(s-orbital)
nJ=+1
(px+ipy-orbital)
nJ=3
(px-ipy-orbital) nJ=2
(dx2-y2-orbital)
Equilibrium configuration
Energy levels of atomic orbitals
Magnonic crystals: tight-binding model with atomic orbitals
Gap closes at M
(example) :
H=0.47Hc H=0.82Hc
gap between 3rd and 4th bands
retain only nJ=0 and nJ=1 orbitals
tight binding model
complex phase for hopping
px+ipy orbitals
nJ=0
(s-orbital)
nJ=+1
(px+ipy-orbital)
+i
= Model for quantum anomalous Hall effect
(e.g. Bernevig et al., Science 314, 1757 (2006))
Design of topological magnonic crystals
• Metal or insulator ? either is OK
(for metal: damping might be a problem for observation of edge modes.)
• Symmetry restrictions:
• Usually, the magnonic crystals with in-plane magnetic field
is non-topological by symmetry reasons.
• Out-of-plane magnetic field is OK.
(cf.: Thin-film with out-of-plane magnetic field has nonzero
Berry curvature.)
• Magnonic gap is required
Strong contrast within unit cell is usually required.
1 ferromagnet and vacuum
2 ferromagnets with very different saturation magnetization
• Multiband & sublattice structure:
nonzero Berry curvature requires multiband structure
Sublattice structure is helpful for designing topological magnonic crystals.
Design of topological magnonic crystals
• Metal or insulator ? either is OK
(for metal: damping might be a problem for observation of edge modes.)
• Symmetry restrictions:
• Usually, the magnonic crystals with in-plane magnetic field
is non-topological by symmetry reasons.
• Out-of-plane magnetic field is OK.
(cf.: Thin-film with out-of-plane magnetic field has nonzero
Berry curvature.)
• Magnonic gap is required
Strong contrast within unit cell is usually required.
1 ferromagnet and vacuum
2 ferromagnets with very different saturation magnetization
• Multiband & sublattice structure:
nonzero Berry curvature requires multiband structure
Sublattice structure is helpful for designing topological magnonic crystals.
Summary • Magnon thermal Hall effect (Righi-Leduc effect)
• Topological chiral modes in magnonic crystals magnonic crystal with dipolar int.
Berry curvature & Chern number
Array of columnar magnet in other magnet phases with different Chern numbers by changing lattice constant
Array of disks non-zero Chern numbers
atomic orbitals tight-binding model reproduce spin-wave bands
• Matsumoto, Murakami, Phys. Rev. Lett. 106,197202 (2011)
• Matsumoto, Murakami, Phys. Rev. B 84, 184406 (2011)
• Shindou, Matsumoto, Ohe, Murakami, PRB87, 174427 (2013)
• Shindou, Ohe, Matsumoto, Murakami, Saitoh, PRB87, 174402 (2013)