Probing Superconductors using Point Contact Andreev Reflection Pratap Raychaudhuri Pratap Raychaudhuri Tata Institute of Fundamental Research Tata Institute of Fundamental Research Mumbai Mumbai Collaborators: Gap anisotropy in YNi 2 B 2 C G. Sheet, S. Mukhopadhyay, D. Jaiswal, S. Ramakrishnan, H. Takeya (Japan) Phys. Rev. Lett. 93, 156802 (2004). Nanostructured Nb S. Bose, P. Vasa, P. Ayyub, R. Bannerjee (Ohio)
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Probing Superconductors using Point Contact Andreev Reflection Pratap Raychaudhuri Tata Institute of Fundamental Research Mumbai Collaborators: Gap anisotropy.
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Probing Superconductors using Point Contact Andreev Reflection
Pratap RaychaudhuriPratap Raychaudhuri
Tata Institute of Fundamental ResearchTata Institute of Fundamental Research
MumbaiMumbai
Collaborators:
Gap anisotropy in YNi2B2C
G. Sheet, S. Mukhopadhyay, D. Jaiswal, S. Ramakrishnan, H. Takeya (Japan)
Phys. Rev. Lett. 93, 156802 (2004).
Nanostructured Nb
S. Bose, P. Vasa, P. Ayyub, R. Bannerjee (Ohio)
Free path: the electron accelerates
V
Scattering Centre (elementary excitation, defects): the electron loses energy
K.E imparted to the electron=
Mean free path
Sample size eV
Lattice
a<<l
e V= (1/2)mv2
T
Ballistic Transport
• Statistically no scattering
• Electrons kinetic energy=eV
• Spectroscopic Probe
Experiment
L He
I=Idc
+Iac
sint
V=Vdc
+Vac
sint
I
Iac
<<Idc
Vdc
-dc bias voltage on the junction
Iac
/Vac
~dI/dV: the differential conductance of the junction
Spectroscopy using Point Contact
Electron phonon interaction
(Au foil/ Au tip)
-6 -4 -2 0 2 4 60.95
1.00
1.05
1.10
1.15
1.20
1.25
1.30
1.35
G(V
)/G
n
V(mV)
Nb film / Pt-Ir Tip
T= 4.2K Z=0.6 delta=0.9meV Superconducting energy gap
Superconductor
Normal Metal
1
0ikx
inc e
0
0
ikx
refl ikx
eb a
e
iqx iqxtrans
u vc e d e
v u
Normal Reflection
Andreev reflection 0( ) ( )V x V x
Fitting parameters:
superconducting gap
Z-barrier height parameter
-broadening parameter
N(E)
E(meV)
BCS density of states
2 2(0) Re
E iN
E i
Superconducting Energy Gap
Angle resolved probe capable of probing different k directions on the Fermi surface
-6 -4 -2 0 2 4 60.95
1.00
1.05
1.10
1.15
1.20
1.25
1.30
1.35
G(V
)/G
n
V(mV)
Nb film / Pt-Ir Tip
T= 4.2K Z=0.6 delta=0.9meV
Fitting parameters:
superconducting gap
Z-barrier height parameter
-broadening parameter
Superconducting gap anisotropy in YNi2B2C
Discovered in TIFR in 1994
Tc~14.6K
Type II Supserconductor:
BCS Superconductor with conventional electron phonon coupling
Unusual Vortex Symmetries evolving with temperatureThermal Conductivity
Izawa et al., PRL 89, 137006 (2002)
Specific Heat
Park et al., PRL 92, 237002 (2004)
Angular variation in magnetic field
Unusual gap function symmetry
s+g(mixed angular
momentum symmetry)
)4sin(cos12
1)( 4
0
k
K Maki, P Thalmeier and others
Purely Geometrical with no microscopic origin
k of YNi2B2C
S-wave superconductivity
s+g symmetry of the order parameter
Multiband superconductivty???
Crystal used for this study
Susceptibility
Primary Secondary
Sample
Intervortex spacing: 1500ÅÅ
Very low defect density
-8 -6 -4 -2 0 2 4 6 8
0.98
1.05
1.12
1.19
1.26
14.5 K
1.75 KI || c-axis
YNi2B
2C/Ag tip
G(V
)/Gn
V (mV)
-3 -2 -1 0 1 2 30.98
1.00
1.02
1.04
1.06
1.08
1.10
1.12
6.0 K
2.32 KYNi
2B
2C/Ag tip
I || a-axis
G (
V)
/ Gn
V (mV)
Gap anisotropy
||2~ 3.6I c
B ck T
c
a
I||a
I||c/I||a ~ 7
at 1.75 K
c
a
I||c
Temperature dependence
0 2 4 6 8 10 12 140.00
0.05
0.10
0.15
0.20
0.25
0.30
I || c (meV)
I |
| a (m
eV)
T (K)
0.0
0.3
0.6
0.9
1.2
1.5
1.8
)4sin(cos2
1)( 4
00 gsk
)4sin(cos1
2
1)( 4
0
k
Two Band SuperconductorTwo Band Superconductor
Suhl et al, PRL 3, 552 (1959)
No Interband scattering
Weak Interband Scattering
TT
T
sg
g
Tc
Temperature dependent Temperature dependent s+gs+g