unusual vortex interactions and dynamics in sns arrays · standard long-junction behavior at t c...

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