Superfluidity and spin superfluidity (in spinor Bose gases and - - PowerPoint PPT Presentation

Superfluidity and spin superfluidity

(in spinor Bose gases and magnetic insulators)

Rembert Duine

Institute for Theoretical Physics, Utrecht University Department of Applied Physics, Eindhoven University of Technology

Nature Physics 11, 1022–1026 (2015), Phys. Rev. Lett. 116, 117201 (2016), arXiv:1603.01996 [cond-mat.quant-gas], arXiv:1604.03706 [cond-mat.mes-hall]

Collaborators

Utrecht:

Benedetta Flebus Kevin Peters Gerrit Bauer (also Delft/Sendai)

Other:

Jogundas Armaitis (Vilnius) Ludo Cornelissen, J. Liu, Bart van Wees (Groningen) Yaroslav Tserkovnyak, Scott Bender (UCLA->UU)

• J. Ben Youssef (Brest), Arne Brataas (NTNU)

Long-term motivation

superconductor

lead lead L I=V/R with R independent of L below room T

spin superfluid

lead lead L ? above room T?

Outline

• Introduction superfluidity and spin superfluidity
• Spinor gases: combining superfluidity and spin

superfluidity (theory)

• Spin transport through magnetic insulators (theory

and experiment)

Superfluid (mass) transport

c c

j t      

c c c

j v t           

=

i ce 

 

+

supercurrent: Superfluid U(1) order parameter:

 

c

J k      

stiffness interactions Wave-like excitations:

c

g 

If zero then

2 s

J k  

(ballistic transport)

c



c c

v J m       

Im Re

carried by condensate chemical potential 

 

2 neigbours

; , ,

i xc i j xc j i xc

dS J S S J S S j dt j J S S x y z

 





            



           

, xc i j i j

H J S S   



 

(ballistic) spin transport

Magnetic insulator (exchange only) Landau-Lifschitz equation: spin current:

2 xc

J k  

particle-like excitations: spin waves/magnons

Halperin/Hohenberg (1969)

 

 

2 1 1 z z xc i i i i i i i i

H J S S S K S B S

 

     

  

  

Spin superfluidity (I)

Easy-plane magnetic insulator [SO(3) -> U(1)]

Halperin/Hohenberg (1969); review: Sonin (2010)

exchange anisotropy field

 

2 3

1

BEC B xc

T k J B K  

Review: Giamarchi (2008)

   

†

ˆ 2 Re ˆ ~ 2 Im ˆ ˆ 1 a S a a a              

Holstein- Primakoff trafo:

Spin superfluidity (II)

2

ˆ ˆ

z xc

dS J S S KS z Bz dt             

Zero T dynamics governed by Landau-Lifshitz equation: Write:

2 cos ~ 2 sin 1

c c c

n S n n                

2 c xc c s c

n J n j t B K Kn t                 

Same as equations for mass superfluid!

Halperin/Hohenberg (1969); Koenig et al. (2001); review: Sonin (2010)

xc c

J Kn k  

Spin superfluidity (III)

,

; polarization in -direction

s xc c

j J n z

 

 

• mass current jc
• phase of superfluid
• rder parameter 
• superfluid stiffness Js
• interparticle

interactions g

• spin current js
• in-plane angle of

magnetization 

• exchange interactions

Jxc

• easy-plane anisotropy K

Mass vs. spin superfluidity

Halperin/Hohenberg (1969); review: Sonin (2010)

i ce 

  

Outline

• Introduction superfluidity and spin superfluidity
• Spinor gases: combining superfluidity and spin

superfluidity (theory)

• Spin transport through magnetic insulators (theory

and experiment)

Related work

• Our work: hydrodynamic description treating spin

and mass superfluidity on equal footing

• Spin currents in spinor gases:
• K. Kudo and Y. Kawaguchi, Physical Review A 84, 043607 (2011).
• H. Flayac, et al., Physical Review B 88, 184503 (2013).
• Q. Zhu, Q.-f. Sun, and B. Wu, Phys. Rev. A 91, 023633 (2015).
• Stability of spirals:
• R. W. Cherng, et al., Phys. Rev. Lett. 100, 180404 (2008).
• Magnon condensation:
• Fang, et al., Phys. Rev. Lett. 116, 095301 (2016) – tonight’s evening talk (?).
• Reviews:
• Y. Kawaguchi and M. Ueda, Physics Reports 520, 253 (2012).
• D. M. Stamper-Kurn and M. Ueda, Rev. Mod. Phys. 85, 1191 (2013).

• Which system is both a mass and spin

superfluid?

• What is the interaction between mass and

spin superfluidity in such a system? NB: a ferromagnetic mass superfluid

Questions



spin superfluid

Ferromagnetic spinor Bose with quadratic Zeeman effect

     

2 2 2 2 2 † † † 1

ˆ ˆ ˆ ˆ ˆ ˆ ˆ 2

z z

H dx BF K F g g F m                   



  

We have hamiltonian: Deep in ferromagnetic regime, zero T linearized mean-field equations reduce to equations for uncoupled mass and spin superfluid with:

s xc

J J m   

field Superfluid and spin stiffness: Collective modes: g1<0, leads to ferromagnetism

 

1 mass c g

g mk     

 

spin

K K B k m    

Armaitis and RD, arXiv (2016)

easy-plane anisotropy leads to Bose condensation

Collective (Goldstone) modes

T TFM TBEC Tmagnon BEC

quadratic spin modes linear density + quadratic spin modes linear density & spin modes

ferromagnet superfluid ferromagnet superfluid & spin superfluid

Coupling between spin and mass superfluid

 

with

z

v v n t t m                

Occurs via hydrodynamic derivative:

Influence of spin polarized mass current on magnetization dynamics: “spin transfer” Current driven by magnetic texture

Armaitis and RD, arXiv (2016)

Stationary solutions

' ' 2 '

z s

n K J           

Stationary solutions possible if:

Armaitis and RD, arXiv (2016)

• Dipolar interactions (lower critical current)
• Upper critical current
• Quasi-equilbrium magnon condensation
• Magnon kinetics
• Coupling to nematic/antiferromagnetic
• rder

Discussion

Outline

• Introduction superfluidity and spin superfluidity
• Spinor gases: combining superfluidity and spin

superfluidity (theory)

• Spin transport through magnetic insulators (theory

and experiment)

Long-term motivation

superconductor

lead lead L I=V/R with R independent of L below room T

spin superfluid

lead lead L ? above room T?

Experiment (I)

• Pt strips on magnetic insulator Yttrium-Iron-Garnett (YIG)
• Non-local (“drag”) resistance RD=V/I
• Room T

Experiment (II)

• Short distance: 1/distance
• Long distance: exponential decay (length scale 10 m)

Idea

Electron charge current Electron spin current Magnon spin current Electron spin current Electron charge current Magnon spintronics: electical control over magnon spin currents Magnonic many-body physics Route to spin currents with low dissipation

Image courtesy Ludo Cornelissen and Bart van Wees

Interfacial spin current

R=V/I

2

cos  cos cos

= *

Model

2 2 s m m

j           

magnon diffusion & relaxation: signal ~

/

m

L

e 



For spin superfluid this would be ~

1 1 C L 

Takei and Tserkovnyak (2014); Flebus, Bender, Tserkovnyak, RD (2016)

Take home messages

• Spin superfluidity is not ferromagnetic superfluidity
• Spinor Bose gas can be spin and/or mass superfluid
• Recent developments of spintronics pave the way

for integration electronic and magnonic (eventually superfluid) low-dissipation spin currents & bosonic many-body physics