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

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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. Evidence for microwave stimulation in the presence of vortex
Lara, et al., Scientific Reports, 5, 9187 (2015)

13. TDGL: average vortex radius reduces at high mw frequencies
(analogy to DC LO effect)
Lara, et al., Scientific Reports, 5, 9187 (2015)

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. Normal vs. kBT inhibited avalanches
Lara, et al.,
Phys. Rev. Appl,
8, 034027 (2017)

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. 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. 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. 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. 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. 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. -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. 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