Transcript スライド 1 - Tata Institute of Fundamental Research

Quasi One-Dimensional
Vortex Flow Driven Through
Mesoscopic Channels
Nobuhito Kokubo
Institute of Materials Science, University of Tsukuba
R. Besseling,
T. Sorop,
P. H. Kes
Kamerlingh Onnes Laboratory, Leiden University
Vortex flow
E
J
Driving force for vortices
  
F  J B
Pinning force for vortices
H
Fp = Jc B
Baarle et al APL 2003
Electric field due to vortex motion
  
E  Bv
velocity
E
hv = F
Jｃ
J
Driving force
Dissipations
in normal core(~px2)
1D Bardeen Stephen(BS) Formula
BS Formula for flux flow resistivity
f 
B 0
Vortex density B:
0 ab  B
h
b
0 1
 n
B
Bc 2 ab
a
b
l
Flow
BS Formula for 1D chain
R f 1D  b f
0 1
ld   n
 B
Bc 2ld a
1D Vortex Flow in Twin Boundaries
A. Gurevich PRL, PRB 2002
b
a
Rf  H
Abrikosov Josephson vortex
IV Curves in Twin Boundaries
Outline of This Talk
Vortex flow channel device
A short summary of previous results
New results
•A kink anomaly in IV characteristics
•ML experiments
Summary of this talk
Mesoscopic Vortex Flow Channels
0.2 – 1mm
Strong pinning NbN layer
J
Weak pinning a-NbGe layer
H
J
SEM picture (w=650nm)
w<l
Matching Effects
The shear modulus
of vortex lattice c66
w
Fp  c66 /w
Fp (10 N/m )
2.0
w=230 nm
6
3
~c66(B)
1.0
a
experimental
data
0
0
0.4
0.8
m0H (T)
b
1.2
f
J
Matching
condition
Mismatch
condition
Mode Locking Experiments : Model
Coherent flow,
average velocity ‘v’
in pinning potential
Lattice Mode :
fint = v/a
Flow direction
a: particle spacing // v
Simplified picture
ML occurs :
fint = p f
Velocity
I= Idc + Irf sin(2pft )
vML
(vML = p a f)
Force
Mode Locking Experiments: Result
w=230nm p=3
E  v B
Large Irf
p=2
f  fint  v / a
B  0 / ab
Irf=0
p=1
f=6MHz
T<<Tc(NbGe)
weff
a
b
V1 pc  0 fn
w eff
n
b
Field Evolution of n and Fc
Vortex density
Oscillation in Fc is closely related with
the flow configurations in channels
PRL 88,247004 (2002)
Field History in Channels
NbN
Field down (FD) mode
H is ramped down after applying
a large field (>Hc2 of NbGe)
Field up (FU) mode
H is ramped up after ZFC
• Field Focusing in channels
NbN
Quasi 1D flow properties
Conventional 2D FF behavior
A decoration image in channels in a field of 50mT
taken by N. Saha,
Field History of Ic & IV Curves
H*
Flow Resistance
High I
a  0.5
Low I
Rd  H
H < H* 1D like vortex flow
Dynamic Change in Flow Structure
f = fint = v/a
at p=1
n=5
DC
n=3
f (MHz) (= v/a)
A kink anomaly mark a dynamic change in flow configuration
Quasi 1D flow Properties
H < H*
Rd  H
n=5
High RFB
n=4
constant n
Quasi 1D flow properties
Low RFB
H*
H > H*
Rd  H
Conventional (2D) Flux Flow
Lower R.F. branch : n
Higher R.F. branch: n+2 H* : 1D - 2D flow transition
H*
Field profile in a channel
FD
FU
Mobile
Mobile
Summary
• Mesoscopic channel system provides very
rich physical properties
• Field history changes the vortex dynamics in
channels
• Quasi-1D motion (square root dependence
on field with constant flow configurations)
• Dynamic change in flow configurations
• Transition from quasi1D to 2D flow properties