Superfluidity and spin superfluidity (in spinor Bose gases and magnetic insulators)

Transcription

1 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, (2015), Phys. Rev. Lett. 116, (2016), arxiv: [cond-mat.quant-gas], arxiv: [cond-mat.mes-hall]

2 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)

3 Long-term motivation lead superconductor lead I=V/R with R independent of L below room T L lead spin superfluid lead? above room T? L

4 Outline Introduction superfluidity and spin superfluidity Spinor gases: combining superfluidity and spin superfluidity (theory) Spin transport through magnetic insulators (theory and experiment)

5 Superfluid (mass) transport Superfluid U(1) order parameter: Im = i e c Re c t c Wave-like excitations: J k stiffness v J m c c j v t supercurrent: c c c j c interactions + g c carried by condensate chemical potential Jk If 2 c zero then s (ballistic transport)

6 Halperin/Hohenberg (1969) (ballistic) spin transport Magnetic insulator (exchange only) H J S S xc i j i, j Landau-Lifschitz equation: spin current: ds i 2 J xcsi Sj JxcS S j dt j neigbours i j J S S; x, y, z xc particle-like excitations: Jxck 2 spin waves/magnons

7 Halperin/Hohenberg (1969); review: Sonin (2010) Spin superfluidity (I) Easy-plane magnetic insulator [SO(3) -> U(1)] H J S S S K S B S BEC B xc z z xc i i i i i i i i exchange anisotropy field 1 23 T k J B K Holstein- Primakoff trafo: S 2 Re aˆ ~ 2 Im aˆ aa ˆ ˆ 1 Review: Giamarchi (2008)

8 Halperin/Hohenberg (1969); Koenig et al. (2001); review: Sonin (2010) Spin superfluidity (II) Zero T dynamics governed by Landau-Lifshitz equation: ds dt ˆ 2 z J xcs S KS z Bz ˆ Write: S 2nc cos ~ 2nc sin nc 1 Same as equations for mass superfluid! J Kn k xc c n c 2 Jxcnc js t BK Knc t

9 Spin superfluidity (III) js, Jxcnc; polarization in z-direction

10 Halperin/Hohenberg (1969); review: Sonin (2010) Mass vs. spin superfluidity mass current j c phase of superfluid order parameter superfluid stiffness J s interparticle interactions g spin current j s in-plane angle of magnetization exchange interactions J xc easy-plane anisotropy K e i c

11 Outline Introduction superfluidity and spin superfluidity Spinor gases: combining superfluidity and spin superfluidity (theory) Spin transport through magnetic insulators (theory and experiment)

12 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, (2011). - H. Flayac, et al., Physical Review B 88, (2013). - Q. Zhu, Q.-f. Sun, and B. Wu, Phys. Rev. A 91, (2015). Stability of spirals: - R. W. Cherng, et al., Phys. Rev. Lett. 100, (2008). Magnon condensation: - Fang, et al., Phys. Rev. Lett. 116, (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).

13 Questions 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 spin superfluid

14 Ferromagnetic spinor Bose with quadratic Zeeman effect We have hamiltonian: field easy-plane anisotropy leads to Bose condensation g 1 <0, leads to ferromagnetism ˆ z H dxˆ BF K F ˆ g ˆ ˆ g ˆ Fˆ 2m z 0 1 Deep in ferromagnetic regime, zero T linearized mean-field equations reduce to equations for uncoupled mass and spin superfluid with: 2 Superfluid and spin stiffness: J J m s xc Collective modes: mass c g0 g1 mk spin k K K m B Armaitis and RD, arxiv (2016)

15 Collective (Goldstone) modes T T FM ferromagnet quadratic spin modes T BEC superfluid ferromagnet linear density + quadratic spin modes T magnon BEC superfluid & spin superfluid linear density & spin modes

16 Coupling between spin and mass superfluid Occurs via hydrodynamic derivative: v with v n z t t m Influence of spin polarized mass current on magnetization dynamics: spin transfer Current driven by magnetic texture Armaitis and RD, arxiv (2016)

17 Stationary solutions Stationary solutions possible if: ' n z K ' 2 J s ' Armaitis and RD, arxiv (2016)

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

19 Outline Introduction superfluidity and spin superfluidity Spinor gases: combining superfluidity and spin superfluidity (theory) Spin transport through magnetic insulators (theory and experiment)

20 Long-term motivation lead superconductor lead I=V/R with R independent of L below room T L lead spin superfluid lead? above room T? L

21 Experiment (I) - Pt strips on magnetic insulator Yttrium-Iron-Garnett (YIG) - Non-local ( drag ) resistance R D =V/I - Room T

22 Experiment (II) - Short distance: 1/distance - Long distance: exponential decay (length scale 10 m)

23 Idea Electron charge current Magnon spintronics: electical control over magnon Electron spin currents Magnonic spin many-body current physics Image courtesy Ludo Cornelissen and Bart van Wees Electron spin current Route to spin currents with low dissipation Electron charge current

24 Interfacial spin current R=V/I 2 cos cos cos = *

25 Model magnon diffusion & relaxation: js m 2 For spin superfluid this would be ~ 2 m signal ~ 1 1 CL e L / m Takei and Tserkovnyak (2014); Flebus, Bender, Tserkovnyak, RD (2016)

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27 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