Transcript Recent* Heat Capacity Measurements

Flux Lattice Melting in
Niobium
Charlotte Bowell
28th February 2005
Talk Plan
Introduction to Melting
 Evidence for melting in HTS
 Evidence for melting in niobium
 Heat capacity measurements on
niobium

Vortex Lattice Melting
In High-Tc
materials,
Hm (T)<<Hc2 (T)
Observation of vortex lattice melting in BSCCO by SANS
Nature 1993
1.5 K
56 K
62 K
<< Tc = 85 K
Diffraction pattern from the hexagonal flux line
lattice vs. temperature at B = 50 mT
Observation by magnetisation measurements
Local
magnetisation
measurements
on BSCCO
E. Zeldov et al.
Nature 1995
Jump in magnetisation - melting transition is first order
Observation by Heat Capacity Measurements
B = 0 T
Heat capacity
measurements on
YBCO
A. Schilling et al.
PRL 1997
B = 9 T
Entropy jumps
at first order
flux melting
Low Tc superconductors near Hc2
Abrikosov Predictions -No melting
Magnetisation vs. H or T
-M
Field vs. Position
Hc2(T) (or Tc2(H))
H (or T )
F10 Fourier component of
spatial variation of magnetic
field
F10  M
Expected SANS intensity near Hc2
no melting
I10 - intensity of a (1,0)
diffraction spot
I10  (F10)2
 (Tc2 – T )2
 for no melting
I10  Tc2 -T
near Hc2
(1,0)
spot
SANS results for Niobium
50mK
Abrikosov
behaviour
confirmed;
flux lattice
melting not
seen...
Forgan et al.
Phys. Rev. Lett.
88, 167003 (2002)
I10  Tc2 -T
… but melting only expected ~ 7 mK
below Tc2(B )
Does not
look like
first order
melting but
something
is going on
I
T
Magnetisation measurements on Niobium
Samples magnetically
reversible
Deviation from
Abrikosov (M  T)
behaviour is seen
Differentiate the results
dBc 2
Cs  Cn H   T
dT
 M 
. 
 T  H
 do heat capacity
measurements
Sulivan and Siedel Measurement
Technique Phys. Rev. 173 (1968)
AC heat into a heat capacity gives AC
temperature signal inversely proportional to
heat capacity
Use idea of RC circuits
to make sure thermal
time constants OK
 = RC
R = thermal resistance
C = heat capacity
Earlier heat capacity measurements
Farrant and
Gough PRL
1974
Done at
constant
magnetic
induction
What happens
at constant H?
Probe
1K Pot
Control
Heater
Sample
goes here
Mixing
Chamber
Copper Block
Sample Holder
Sample (Titania)
Copper Block
Titania
Alan Bradley
Resistor
Sample Heater
Nylon Wire