Magnon Bose Einstein Condensation a brief introduction Dec. 21 st - PowerPoint PPT Presentation

Introduction to Spintronics --- 2018 Fall Magnon Bose Einstein Condensation a brief introduction Dec. 21 st 2018 何梦云 李思衡 李娜 孙慧敏 陈文杰 1

After this presentation, you are supposed to have some basic understanding of: • What is magnon BEC? • Why is it so special? • How to achieve it? • Several experiments . 2

Bose Einstein Condensation 3

Bose Einstein Condensation If the total particle number N is fixed, the chemical potential has a finite value. BEC 4

Bose Einstein Condensation If the total particle number N is NOT fixed, the chemical potential is always zero! Why? 5

Bose Einstein Condensation 6

Bose Einstein Condensation If the total particle number N is NOT fixed, the chemical potential is always zero! no BEC 7

Magnon magnon spin wave (boson s = 1) no BEC 8

R.I.P. magnon BEC 9

Magnon BEC Last hope: raise the chemical potential. But how? 10

Magnon BEC Defined at thermal equilibrium ， But at thermal equilibrium … What happens beyond thermal equilibrium? 11

Magnon create magnons destroy magnons scattering heating cooling magnon’s life oscillating dissipation magnetic field into lattice 12

Magnon scattering into equilibrium : dissipation into lattice : �� �� quasi-equilibrium 13

Magnon BEC YIG film V. E. Demidov et al ., Phys. Rev. Lett. 100 , 047205 (2008). V. E. Demidov et al ., Phys. Rev. Lett. 101 , 257201 (2008). 14

Magnon BEC YIG film O. Dzyapko et al ., Appl. Phys. Lett. 92 , 162510 (2008). 15

Magnon BEC YIG film O. Dzyapko et al ., Appl. Phys. Lett. 92 , 162510 (2008). 16

First experimental discovery of mBEC 17

Magnon BEC  magnons FM material + � + pumping � � ��� = � ��� → BEC ① microwave electromagnetic field—density of magnon ② � �� > 1�� , � �� < 100�� — quasi-equilibrium ③ transparent for visible light 18

Experimental set-up Detection: Brillouin light scattering (BLS) Demokritov S O. et al. Nature, 2006, 443(7110):430-433. Dzyapko O. et al. New Journal of Physics, 2007, 9(3):64. 19

Dispersion curve � ��� −� � � � Dzyapko O. et al. New Journal of Physics, 2007, 9(3):64. 20

BLS spectrum without pumping ~ v D ( ) � � � � � ��� � � = � � � � = ���� �� � − 1 � − 1 � � � � � � � � Demokritov S O. et al. Nature, 2006, 443(7110):430-433. 21

BLS spectrum with pumping Dzyapko O. et al. New Journal of Physics, 2007, 9(3):64. 22

BLS spectrum with pumping  m 300ns Dzyapko O. et al. New Journal of Physics, 2007, 9(3):64. 23

BLS spectrum with pumping 0.24 0.7 P=5.9w P=4.0w 2.4 1.75 Demokritov S O. et al. Nature, 2006, 443(7110):430-433. 24

BLS spectrum after pumping switch-off Dzyapko O. et al. New Journal of Physics, 2007, 9(3):64. 25

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Research about magnon-magnon interaction 2017 • Parametric pumping • Doubly degenerate mBEC   2 4 a           W ( r r ) ( r r ) g ( r r ) int m g>0, repulsive interaction; g<0, attractive interaction * * b b b b      12 i  b (  g 1 1 2 g 2 2 ) b 1 0 1 V V * * b b b b      12   2 2 1 1 i b ( g 2 g ) b 2 0 2 V V          12 ( g /  ) ( 2 g /  ) 0 27

Research about magnon-magnon interaction • MOKE • Wavelength: 490nm • Duration: 100 fs • Repetition frequency ωL/(2π)=82.379MHz k BEC 28

Research about magnon-magnon interaction • H 0 =113.91kA/m • A threshold • P<P th , before the condensation of magnons 29

Spin current induced magnons 2017 • Spin-Hall effect • Ferromagnetic Permalloy (Py) strip+Pt • Magnetization of Py: 10.2KG • Spin current//M • Microfocus Brillouin light scattering (BLS) technique      ( ) D ( ) n ( ) ρ: spectral density of magnons • k y =πm/w 30

Spin current induced magnons • H 0 =200Oe      ( ) D ( ) n ( ) thermal equilibrium 0 0      ( ) D ( ) n ( ) I I Oersted field of the current 31

Spin current induced magnons    n ( ) k T / h 0 B 0      n ( ) k T /( h ） I B eff   T n ( )    eff R ( ) I     n ( ) T / h 0 0 • T eff : frequency independent scaling of R • μ: a comprehensive modulation on R(ν) 32

Current induced and detected mBEC • Dissipationless magnon transport • I dc = 0, nac from the injector decays exponentially • I dc = I BEC , formation of magnon BEC • I dc = I SW , magnon damping is completely compensated 33

Current induced and detected mBEC • µ 0 H = 50mT, T = 280K • � = ± 180 ◦ , magnon accumulation underneath the modulator • � =0 ◦ , magnon depletion obtained in this configuration 34

Current induced and detected mBEC • Linear dependence: SHE induced injection effects • Quadratic dependence: thermal activation 35

Thank you for your attentions! 36