Shuichi Murakami Department of Physics, Tokyo Institute of Technology

Transcription

1 EQPCM, ISSP, U. Tokyo June, 2013 Berry curvature and topological phases for magnons Shuichi Murakami Department of Physics, Tokyo Institute of Technology Collaborators: R. Shindou (Tokyo Tech. Peking Univ.) R. Matsumoto (Tokyo Tech.) J. Ohe (Toho Univ.) E. Saitoh (IMR, Tohoku Univ. ) Magnon thermal Hall effect for magnetostatic modes Matsumoto, Murakami, Phys. Rev. Lett. 106, (2011). Matsumoto, Murakami, Phys. Rev. B 84, (2011) Topological Magnonic crystals Shindou, Matsumoto, Ohe, Murakami, Phys. Rev. B 87, (2013), Shindou, Ohe, Matsumoto, Murakami, Saitoh, Phys. Rev. B 87, (2013),

2 Phenomena due to Berry curvature of band structure Gapless Gapped Hall effect Spin Hall effect (of electrons) Topological edge/surface modes in gapped systems Quantum Hall effect chiral edge modes Topological insulators helical edge/surface modes Fermions Bosons Spin Hall effect of light one-way waveguide in photonic crystal Magnon thermal Hall effect topological magnonic crystal

3 Intrinsic spin Hall effect in metals& semiconductors semiclassical eq. of motion for wavepackets Force - SM, Nagaosa, Zhang, Science (2003) - Sinova et al., Phys. Rev. Lett. (2004) Adams, Blount; Sundaram,Niu, : Berry curvature : periodic part of the Bloch wf. ( : band index) It represents geometric structure of bands in k-space

4 Magnon thermal Hall effect

5 Magnon thermal Hall effect by Berry curvature previous works -- Theory: S. Fujimoto, Phys. Rev. Lett. 103, (2009). H. Katsura, N. Nagaosa, and P. A. Lee, Phys. Rev. Lett.104, (2010). Experiment & theory: Y. Onose, et al., Science 329, 297 (2010); Lu 2 V 2 O 7 : Ferromagnet Dyaloshinskii-Moriya interaction Berry phase Thermal Hall conductivity Correction term! R. Matsumoto, S. Murakami, Phys. Rev. Lett. 106, (2011). R. Matsumoto, S. Murakami, Phys. Rev. B 84, (2011)

6 Magnon Thermal Hall conduc1vity (Righi- Leduc effect) Berry curvature R. Matsumoto, S. Murakami, Phys. Rev. Lett. 106, (2011) T. Qin, Q. Niu and J. Shi,Phys. Rev. Lett. 107, (2011) (1) Semiclassical theory ( 2) Linear response theory Eq. of motion Density matrix ρ: Bose distribution Current equilibrium deviation by external field : Berry curvature

7 Magnetostatic modes in ferromagnetic films (e.g. in YIG) MSBVW (magnetostatic backward volume mode) Magnon ( exchange) No anisotropy in YIG Quantum mechnical, short ranged Wavenumber (cm -1 ) Magnetostatic mode ( dipolar interaction film) Classical, long-ranged

8 Magne1c dipole interac1on Dominant in long length scale (microns) Similar to spin- orbit int. Berry curvature Long- ranged nontrivial, controlled by shape Magnetic domains

9 Magnetostatic modes in ferromagnetic films M: magnetization, γ: gyromagnetic ratio, H: external magnetic field Landau-Lifshitz (LL) equation Maxwell equation Boundary conditions (magnetostatic limit) Generalized eigenvalue eq. B. A. Kalinikos and A. N. Slavin, J. Phys. C 19, 7013 (1986)

10 Magnetostatic modes in ferromagnetic films Berry curvature (a) MagnetoStatic Surface Wave (MSSW) (b) MagnetoStatic Backward Volume Wave (MSBVW) Zero Berry curvature symmetry (2-fold in-plane rotation + time reversal ) (c) MagnetoStatic Forward Volume Wave (MSFVW) We can expect the Berry curvature to be nonzero!

11 Berry curvature for MSFVW mode Dispersion for n=0 5. (H 0 /M 0 =1.0) Berry curvature R. Matsumoto, S. Murakami, PRL 106, (2011), PRB84, (2011)

12 Bosonic BdG eq. and Berry curvature Generalized eigenvalue eq. Cf: Phonons: Qin, Zhou, Shi, PRB 86, (2012) Electrons: Sumiyoshi, Fujimoto, JPSJ 82, (2013) Bogoliubov-de Gennes Hamiltonian Diagonalization T: paraunitary matrix Berry curvature for n-th band

13 Thermal Hall conductivity for bosonic BdG eq. Linear response theory Berry curvature (e.g.) MSFVW(Magnetostatic forward volume wave) mode higher T (room temp.) T-indep. (Example): universal curve

14 Topological chiral modes in magnonic crystals Shindou, Matsumoto, Ohe, Murakami, Phys. Rev. B 87, (2013), Shindou, Ohe, Matsumoto, Murakami, Saitoh, Phys. Rev. B 87, (2013),

15 Phenomena due to Berry curvature of band structure Gapless Gapped Hall effect Spin Hall effect (of electrons) Topological edge/surface modes in gapped systems Quantum Hall effect chiral edge modes Topological insulators helical edge/surface modes Fermions Bosons Spin Hall effect of light one-way waveguide in photonic crystal Magnon thermal Hall effect magnonic crystal

16 Chern number & topological chiral modes Band gap Chern number for n-th band = integer Berry curvature tooological chiral edge modes Analogous to chiral edge states of quantum Hall effect. bulk mode: Chern number= Ch 3 (Ch 1 +Ch 2 ) topological edge modes bulk mode: Chern number= Ch 2 Ch 1 topological edge modes bulk mode: Chern number= Ch 1

17 2D Magnonic Crystal : periodically modulated magne1c materials Landau- Lifshitz equa1on Maxwell equa1on (magnetosta1c approx.) YIG (host) Iron (subs1tute) Satura1on magne1za1on M s exchange interac1on length Q Linearized EOM modulated H// z a x a y External field exchange field (quantum mechanical short- range) Dipolar field (classical, long range) bosonic Bogoliubov de Gennes eq.

18 magnonic crystal Chern number for the 1 st band λ=0.35um, r=1 `dipolar regime 2 nd Lowest band Lowest magnon band `exchange regime YIG (host) Iron (subs1tute) : unit cell size : aspect ra1o of unit cell Larger lauce const. dipolar interac1on is dominant non- trivial Chern integer (like spin- orbit interac1on) H// z a x a y

19 Simula1on (by Dr. Ohe) External AC magne1c field applied f=4.5ghz bulk bulk f=4.4ghz edge bulk f=4.2ghz bulk External field: dc field: out- of- plane ac field: in- plane

20 Magnonic crystals with ferromagnetic dot array R. Shindou, J. Ohe, R. Matsumoto, S. Murakami, E. Saitoh, arxiv: 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

21 Magnonic crystals: decorated square lattice Equilibrium spin configuration H ext < H c =1.71 H ext H ext Magnetostatic energy Tilted along H ext H ext > H c Collinear // H ext

22 Magnonic crystals: calculation of spin-wave bands Magnetostatic energy Landau-Lifshitz eq. Rotated frame (equilibrium spin direction z axis) Generalized eigenvalue eq. where

23 Magnonic crystals: calculation of spin-wave bands H=0 H=0.47H c H=1.01H c H=1.1H c Red: Ch=-1 Blue: Ch=+1 Time- reversal symmetry H=0.76H c H=0.82H c H=1.4H c H=2.35H c Small H<<H c Large H>>H c Topologically trivial Weak dipolar interac1on compared with H

24 Magnonic crystals: edge states and Chern numbers (1) bulk Strip geometry (bulk+edge) Edge states +1 chiral mode H=0.47H c -1 chiral mode -1 chiral mode H=0.76H c +1 chiral mode H=0.82H c Red: Ch=-1 Blue: Ch=+1

25 atomic orbitals One cluster = atom Equilibrium configuration Spin wave excitations: atomic orbitals relative phase for precessions H<H c : noncollinear H>H c : collinear // z n J =0 (s-orbital) n J =+1 (p x +ip y -orbital) n J =2 (d-orbital) n J =3 (p x -ip y -orbital) n J =0 softens at H=H c n J =1 and n J =3 degenerate at H=0 n J =2 is lowest at H=0: favorable for dipolar int.

26 Magnonic crystals: tight-binding model with atomic orbitals (example) : H=0.47H c H=0.82H c gap between 3rd and 4 th bands retain only n J =0 and n J =1 orbitals tight binding model Gap closes at M Hamiltonian (parameters dependent on H ext ) Gap closing + topological transition Gap closes at Γ

27 complex phase for hopping p x +ip y orbitals +i n J =0 (s-orbital) n J =+1 (p x +ip y -orbital) = Model for quantum anomalous Hall effect e.g. Bernevig et al., Science 314, 1757 (2006);

28 Summary Magnon thermal Hall effect (Righi-Leduc effect) Topological chiral modes in magnonic crystals magnonic crystal with dipolar int. bosonic BdG Berry curvature & Chern number Thin film 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, (2011) Matsumoto, Murakami, Phys. Rev. B 84, (2011) Shindou, Matsumoto, Ohe, Murakami, Phys. Rev. B 87, (2013), Shindou, Ohe, Matsumoto, Murakami, Saitoh, Phys. Rev. B 87, (2013),