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Unusual Vortex Interactions and Dynamics in SNS Arrays Nadya Mason
University of Illinois at Urbana-Champaign
500nm
Vortices in superconductors New device structures: Strong
competition between pinning and interactions
Current-driven: Interesting non-equilibrium behavior
Focus on De-pinning dynamics: 1) Low fields: New understanding of dissipation 2) High fields: Unusual de-pinning due to interactions
H > 0
T c 0
H c 2
H
T
Lattice
Meissner
Abrikosov
Vortices in Type II Superconductors
-Vortex interactions over scale of penetration depth λ ~ 1/sqrt(Tc) (~ 2µm, our sample)
Equilibrium vortex phases — can be interacting, many-body systems
Experimental phase diagram of LaSCO (Rev. Mod. Phys. 82, 109, 2010)
Φ0 =h/2e
-Dominate transport below Tc
Non-equilibrium vortex phases
Thermally-driven transitions: de-pinning & melting
Current-driven de-pinning & critical phenomena *pinning & interactions stronger than thermal fluctuations
Science (2015)
500nm
SNS Island arrays as model 2D superconductors
Standard long-junction behavior at Tc (array transition)
- Undergoes Berezinskii-Kosterlitz-Thouless transition at T2 (jumps in a for V = Ia ): signifies 2D superconducting
behavior
- Ic vs T fits well for single, diffusive SNS junction
- TBKT ~ EThouless ~ 1/d2
(J. Phys.: Cond. Matt. 25, 445701 (2013)
I+
I-
V+ V-
• Strong pinning potential • Interactions controlled by vortex
density, island spacing
Nb on Au
Vortex Pinning Behavior
• At finite field, vortices populate array • Periodic potential • Lowest energy at center of triangle • Highest energy between islands
f = Φ/Φ0 or Φ0 per plaquette
Filling characterized by frustration parameter, f
Nb island
f=1/4 1/4 filled
Rich features due to competition between periodic potential and vortex repulsion
I-V Measurements to focus on De-pinning
• Current provides Lorentz Force, driving vortices
• Vortex motion measured as voltage across sample
Vortex Motion Dominant
dV/dI vs I IV in vortex-dominant region
Pinned vortices
Vortex flow Ic
RN
Transition from pinned to flux flow
Flux Flow Resistance
Rff
Flux flow: Vortices move at terminal velocity (linear IV, flat dV/dI)
Why isn’t there a peak in the data?
Predicted dV/dI from Molecular Model
Increasing T N. Poccia et al., Science 349, 1202 (2015).
C. D. Chen, P. Delsing, D. B. Haviland, Y. Harada, and T. Claeson, Phys. Rev. B 54, 9449 (1996).
M. S. Rzchowski, S. P. Benz, M. Tinkham, and C. J. Lobb, Phys. Rev. B 42, 2041. (1990).
Commonly absent in SNS array studies.
Dynamic Vortex Models
Mass term ( 0 for small R, C, i.e. overdamped)
Effective potential (array geometry)
Vortex-vortex interactions Thermal fluctuations
Dissipation (1/R)
RCSJ Array
W. Yu, K. H. Lee, and D. Stroud. Phys. Rev. B 47, 5906 (1993).
Equations of motion of a Vortex
Molecular Vortex Models
Driving Force
C. J. Olson, C. Reichhardt, and F. Nori, Phys. Rev. Lett. 81, 3757 (1998).
Potential used by Mondragon, Hughes
Models produce similar dynamic behavior in low filling regime…
Dynamic Vortex Models
Dilute Vortex Population Predictions
• Extrapolate V=0 necessitates dV/dI peak
V I∝
2 2dV I I∝ −
V I∝
Nonzero I Intercept
• Linear IV • Nonzero I intercept • Does not converge with V I∝
Rapid increase in V is damped: consider modifying damping term …
Only velocity-dependent forces can change concavity of dV/dI Need to add “delayed” drag term
Correlates system at different times
Data fit by including history dependent dissipation
Example: Exponential Response Effects of history dependent dissipation
Long relaxation time limit is similar to our data.
2 2dV I I∝ −
dV I I∝ −
Only velocity-dependent forces can change concavity of dV/dI Need to add “delayed” drag term
Simplest approximation:
Possible origin: Time-dependent quasiparticle scattering from moving vortices Microscopic origin: ?? Re-forming of superfluid around vortex cores? Quasiparticle diffusion? Time-dependent dissipation relevant to understanding typical signatures of vortex depinning dynamics
Agreement between theory and experiment
Correlates system at different times
Data fit by including history dependent dissipation
Mason Group, Univ. of Illinois - Malcolm Durkin, Rita Garrido-Menacho
- Ian Mondragon-Shem (UIUC/Yale), Taylor Hughes (UIUC)
- Sarang Gopalakrishnan (Caltech/CUNY)
Work supported by the DOE under DE-FG02-07ER46453 through the Frederick Seitz Materials Research Laboratory
Acknowledgements
Conclusions Current-driven vortices with strong competition between pinning and interactions Interesting non-equilibrium behavior
- For vortex de-pinning & phase transitions in low fields, need to consider history-dependent dissipation to obtain correct dynamics - For vortex de-pinning in high fields, unusual incommensurate commensurate transition
Future Work: Scaling of transitions, effects of point disorder