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2018.06.12 antonio lara uam NanoFrontMag

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II Jornada científica NanoFrontMag - 12 de junio de 2018 - IMDEA Nanociencia
Antonio Lara
Grupo Magnetrans UAM

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2018.06.12 antonio lara uam NanoFrontMag

  1. 1. Microwave stimulation of superconductivity in the mixed state Antonio Lara, César Gómez-Ruano and Farkhad G. Aliev MAGNETRANS, Universidad Autónoma de Madrid, Spain II Jornada científica de NanoFrontMag june 12 (2018) Victor Moshchlkov Katholieke Universiteit Leuven, Belgium Yuri Galperin University of Oslo, Norway Oleksandr V. Dobrovolskiy Physikalisches Institut, Goethe University, Germany Acknowledgements: Ahmad Awad, Alejandro Silhanek, Valeri Vinokur, Konstantin Ilin
  2. 2. Type I superconductors → no magnetic field inside Type II superconductors → field as vortices. They dissipate energy when they move First image of a vortex lattice. U. Essmann and H. Trauble, Physics Letters 24A, 526 (1967) MO image of avalanches in Pb films. Menghini et al., PRB71, 104506 (2005)
  3. 3. The condition for considering SC vortex to be in linear AC regime is low speed (low frequency/low power drive) non-deformed vortex S21 trace at overlapping down to lower powers, measured at f = 4 GHz and T /Tc = 0.991 K. The sample is a plain Pb film without dots with in freezing field (less than 10 Oe). Linear vortex response Nonlinear response (deformed) vortex) Microwave heating… Lara, et al., Scientific Reports, 5, 9187 (2015)
  4. 4. mw stimulated SC in the type I superconductors (1966-1972 and 2012-) Effective vortex «cooling» under DC drive Increasing effective SC gap under persistent subgap mw (AC) drive ω>>(τE)-1 G.M. Eliashberg et al, J. Low Temp. Phys. 10, 449 (1973) NOTE: deeply subgap (f<<1000 GHz) mw radiation is used (RF on) (RF off)
  5. 5. VNA-FMR, UAM (<40 GHz,>1.5K, <9T) Effective microwave permeability (neglecting reflections) U’’ ~ Losses, U’~screening E field H field Current Helium cryostat with 9T magnet D. Chumakov et al. Phys.Rev. B71,1 (2005) P=1dBm, 20μm separation
  6. 6. Samples Pb (60nm)/Ge(20nm) deposited on top of square 2x2µm2 array of circular Py 35nm /1000nm dotsPb (60nm)/Ge(20nm) film Avalanches mainly in: Plain 70nm thick Pb films MBE, KU Leuven J del Valle et al, Superconductor Science and Tech. 30 2, (2016)
  7. 7. Tc with 99% decrease of U’ U’ H=0 Oe H=150 Oe f=5 GHz 60nm Pb film over PPCs, f=6GHz, small H (inclined field) Tc (K) Influence of mw on Tc
  8. 8. Influence of microwave power on Hc2 Optimum power ( Po ) 60 nm thick Pb film + 35 nm thick Py dots, close to parallel field, f=6 GHz U’ U’ U’
  9. 9. Pb film 60 nm thick without dots, 6 GHz, parallel field (~10% increase of HC2) Analysis: HC2 vs. T and vs. microwave power Pb film 60 nm thick with PPC, 6 GHz, parallel field (~20% increase of HC2) 0,975 0,980 0,985 0,990 0,995 1,000 1,00 1,05 1,10 1,15 1,20 with PPC without PPC NormalizedHC2 (P=-17dBm) T/Tc Method to find HC2 Lara, et al., Scientific Reports, 5, 9187 (2015)
  10. 10. Larkin and Ovchinnikov (LO) in 1975 considered for the first time nonlinear response of a DC driven vortex DC-LO effect plays role in abrupt transition from SC into normal state in the DC driven vortex system near critical velocity Larkin, A. I. & Ovchinnikov, Y. N. Nonlinear conductivity of superconductors in the mixed state. Sov. Phys. JETP. 41, 960 (1976). Al plain film: Leo et al., Physica C470 904 (2010) Anomalous reduction of the vortex viscosity with DC velocity Gurevich, et al., PRB 77, 104501 (2008) B
  11. 11. Direct observation of LO effect through MSSC For interstitial vortices moving at high speed core shrinks and reduces dissipation (Larkin and Ovchinnikov 1975) -200 -100 0 100 200 -27 -26 -25 -24 -23 -H2 -H1 H2 H (Oe) 7.14K 7.13K 7.12K 7.11K 7.10K 7.09K 7.08K Optimumpower(dBm) H1 f=6 GHz Matching conditions: (pinned vortex) Relatively enhanced dissipation (smaller OP) Interstitial vortices (higher velocities) Reduced dissipation (higher PO)
  12. 12. Evidence for microwave stimulation in the presence of vortex Lara, et al., Scientific Reports, 5, 9187 (2015)
  13. 13. TDGL: average vortex radius reduces at high mw frequencies (analogy to DC LO effect) Lara, et al., Scientific Reports, 5, 9187 (2015)
  14. 14. Vortex depinning frequency Less mw power is required to trigger avalanches near vortex depinning frequencies Lara, et al., Phys. Rev. Appl, 8, 034027 (2017)
  15. 15. Normal vs. kBT inhibited avalanches Lara, et al., Phys. Rev. Appl, 8, 034027 (2017)
  16. 16. f=6.3 GHz Affected by losses Unaffected by losses U’’ Thermally driven avalanch inhibition (TDAI) observed close to depinning frequencies Çiçek et al, Cryogenics, 63, 143 (2014) Peak in losses Lara, et al., Phys. Rev. Appl, 8, 034027 (2017)
  17. 17. dU’’/dH Avalanche inhibition coincides with peaks in losses (high vortex mobility) Avalanche inhibition close to Hc2 Lara, et al., Phys. Rev. Appl, 8, 034027 (2017)
  18. 18. MODEL Due to mw driven LO effect close to Tc and close to fDP relative variation of vortex core at high mw powers is reduced: avalanches are inhibited There can be situations with overlap depending on P and f. Overlap would facilitate avalanches
  19. 19. Stimulation effect on vortices under DC+mw current ( Nb films) In collaboration with Dr. O. Dobrovolskiy Vortices enter the sample Vortices enter faster than leave Antivortices start to enter through the left, and annihilate with vortices 0 2 4 6 8 10 0,28 0,30 0,32 0,34 Aspect Ratio 3.6 Aspect Ratio 5 IDC f / f0 0 2 4 6 8 10 0 2 4 6 8 10 12 f / f0 IDC (%overIDC (0.1f0 )) T=0.56 Tc (4 K) T=0.75 Tc (5.4 K) T=4K T=5.4K
  20. 20. Conclusions •We present evidence for microwave stimulated superconductivity in the presence of vortex Among direct consequences: a)Reduced dissipation of mw driven vortex b) Thermally driven inhibition of superconducting vortex avalanches c) Enhanced critical current in the presence of mw excitation
  21. 21. Differential losses Im{U(f,H)} vs. external field Sample CWG Hrf =0.1 Oe; T=6.1K; Perpendicular component of RF field drives SC vortex Perpendicular HDC in steps of 5 Oe Awad, et al, Phys.Rev. B84, 224511 (2011) PREVIOUS WORK: microwave vortex depinning detected via avalanches (T<0.8Tc).
  22. 22. -30 -20 -10 0 10 -2 -1 0 d|S21 |/dP P (dBm) 14.7 GHz (derivative) -30 -20 -10 0 10 -15.60 -15.55 -15.50 -15.45 -15.40 |S21 |(dB) P (dBm) f=14.7 GHz -20 -10 0 -12.48 -12.46 -12.44 -12.42 |S21 |(dB) P (dBm) f=11 GHz These results are very reproducible “Histeretic” behavior in power d/dP f=11 GHz f=14 GHz Thermally driven inhibition of vortex avalanches Lara, et al., Phys. Rev. Appl, 8, 034027 (2017)
  23. 23. MW stimulated avalanches in NbN: effect of MSSC Similar but harder to observe effects in NbN lilms (deposited by by Ilin, KUT, Karlsruhe) 180 nm thick NbN films on sapphire