Superfluidity and spin superfluidity
Transcript of Superfluidity and spin superfluidity
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Superfluidity and spin superfluidity
(in spinor Bose gases and magnetic insulators)Rembert Duine
Institute for Theoretical Physics, Utrecht UniversityDepartment 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]
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Collaborators
Utrecht:Benedetta FlebusKevin PetersGerrit 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)
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Long-term motivation
superconductorlead lead
L
I=V/R with R independent of L
below room T
spin superfluid lead lead
L
?
above room T?
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Outline• Introduction superfluidity and spin superfluidity
• Spinor gases: combining superfluidity and spin superfluidity (theory)
• Spin transport through magnetic insulators (theory and experiment)
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Superfluid (mass) transport
ccjt
c c cj v
t
= ic e
+supercurrent:
Superfluid U(1) order parameter:
cJ k stiffness interactions
Wave-like excitations:
cg
If zero then 2sJ k
(ballistic transport)c
c cv J m
Im
Re
carried bycondensate
chemicalpotential
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2
neigbours
; , ,
ixc i j xc
j i
xc
dS J S S J S S jdt
j J S S x y z
,xc i j
i j
H J S S
(ballistic) spin transportMagnetic insulator(exchange only)
Landau-Lifschitzequation:
spin current:
2xcJ k particle-like excitations: spin waves/magnons
Halperin/Hohenberg (1969)
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2
1 1z z
xc i i i i ii 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 31BEC B xcT k J B K
Review: Giamarchi (2008)
†
ˆ2 Re
ˆ~ 2 Im
ˆ ˆ 1
a
S a
a a
Holstein-Primakofftrafo:
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Spin superfluidity (II)
2 ˆ ˆzxc
dS J S S KS z Bzdt
Zero T dynamics governed by Landau-Lifshitz equation:
Write: 2 cos
~ 2 sin1
c
c
c
n
S nn
2cxc c s
c
n J n jt
B K Knt
Same as equationsfor mass superfluid!
Halperin/Hohenberg (1969); Koenig et al. (2001); review: Sonin (2010)
xc cJ Kn k
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Spin superfluidity (III)
, ; polarization in -directions xc cj J n z
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• mass current jc• phase of superfluid
order parameter • superfluid stiffness Js
• interparticleinteractions g
• spin current js• in-plane angle of
magnetization • exchange interactions
Jxc
• easy-plane anisotropy K
Mass vs. spin superfluidityHalperin/Hohenberg (1969); review: Sonin (2010)
ic e
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Outline• Introduction superfluidity and spin superfluidity
• Spinor gases: combining superfluidity and spin superfluidity (theory)
• Spin transport through magnetic insulators (theory and experiment)
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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).
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• 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
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Ferromagnetic spinor Bose with quadratic Zeeman effect
2 2 22 2† † †
0 1ˆ ˆ ˆ ˆ ˆ ˆ ˆ
2z zH 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 xcJ J m
field
Superfluid and spin stiffness:
Collective modes:
g1<0, leads toferromagnetism
0 1mass c g g mk
spin
K K Bk
m
Armaitis and RD, arXiv (2016)
easy-plane anisotropy
leads to Bosecondensation
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Collective (Goldstone) modesT
TFM
TBEC
Tmagnon BEC
quadratic spin modes
linear density + quadratic spin modes
linear density & spin modes
ferromagnet
superfluid ferromagnet
superfluid & spin superfluid
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Coupling between spin and mass superfluid
with zv v nt t m
Occurs via hydrodynamic derivative:
Influence of spin polarizedmass current on magnetizationdynamics: “spin transfer” Current driven by
magnetic texture
Armaitis and RD, arXiv (2016)
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Stationary solutions
' '2 '
z
s
n KJ
Stationary solutionspossible if:
Armaitis and RD, arXiv (2016)
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• Dipolar interactions (lower critical current)• Upper critical current• Quasi-equilbrium magnon condensation• Magnon kinetics• Coupling to nematic/antiferromagnetic
order
Discussion
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Outline• Introduction superfluidity and spin superfluidity
• Spinor gases: combining superfluidity and spin superfluidity (theory)
• Spin transport through magnetic insulators (theory and experiment)
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Long-term motivation
superconductorlead lead
L
I=V/R with R independent of L
below room T
spin superfluid lead lead
L
?
above room T?
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Experiment (I)
- Pt strips on magnetic insulator Yttrium-Iron-Garnett (YIG)- Non-local (“drag”) resistance RD=V/I- Room T
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Experiment (II)
- Short distance: 1/distance- Long distance: exponential decay (length scale 10 m)
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IdeaElectron 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 Cornelissenand Bart van Wees
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Interfacial spin current
R=V/I 2cos cos cos= *
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Model
22
s m
m
j
magnon diffusion & relaxation: signal ~
/ mLe
For spin superfluid this would be ~ 1
1 C LTakei and Tserkovnyak (2014); Flebus, Bender, Tserkovnyak, RD (2016)
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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