Critical fields, vortex melting and the irreversibility line in quasi 2D organic superconductors

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15 Νοε 2013 (πριν από 3 χρόνια και 4 μήνες)

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Critical fields, vortex melting and the
irreversibility line in quasi 2D organic
superconductors

Braunen E. Smith

K. Cho, I. Mihut and C. C. Agosta

Department of Physics, Clark University


S. W. Tozer, T. P. Murphy and E. C. Palm

The National High Magnetic Field Laboratory, Tallahassee


J. A. Schlueter

Materials Science Division, Argonne National Laboratory

We acknowledge support from DOE grant #ER46214.


Introduction


Organic Superconductor

-
(ET)
2
Cu(NCS)
2


Experimental Technique


Vortex Melting Point


Smoothing of Melting Point in Parallel Orientation


Irreversibility Line


Comparison to Data from
M.M. Mola, S. Hill, J.S.
Brooks, and J.S. Qualls, Phys. Rev. Lett.
86
, 2130 (2001)
.


Upper Critical Field


DC versus Pulsed Field


Conclusions



-
(ET)
2
Cu(NCS)
2


Layered type
-
II superconductor with BEDT
-
TTF molecules packed in planes separated by
the smaller poorly conducting Cu(NCS)
2

anion
layers.


This structure is similar to HTS (high
temperature superconductors), but samples are
intrinsically cleaner.


Lower T
c

(~10 K) and H
c2

than HTS which
allows for more of the temperature/field space to
be examined.


M.M. Mola, S. Hill, J.S. Brooks, and J.S. Qualls, Phys. Rev. Lett.
86
, 2130 (2001).


Experimental Technique


The National High Magnetic Field
Laboratory, Tallahassee



Portable Dilution Refrigerator



33 T DC Magnet


TDO Technique to probe change in rf
penetration depth as magnet field is
changed



The change in frequency is
proportional to the change in
penetration depth



Resonant frequency ~200MHz



Vortex Melting Point

As the sample orientation approaches parallel (90
°
) the melting
transition becomes less obvious and changes field.

Irreversibility Line

Tinkham Eq. (2D Ginzburg
-
Landau):



B
irr
(

)
sin(

)
B
irr


B
irr
(

)
cos(

)
B
irr
||






2

1
Solid Line: B
irr

=3.6 T
B
irr||
=32 T


Dashed Line: B
irr

=3.6 T
B
irr||
=26 T

M.M. Mola, S. Hill, J.S. Brooks, and J.S. Qualls, Phys. Rev. Lett.
86
, 2130 (2001).


Irreversibility Line

M.M. Mola, S. Hill, J.S. Brooks, and J.S. Qualls, Phys. Rev. Lett.
86
, 2130 (2001).

> 0.5
°

angle resolution

Upper Critical Field (B
c2
)


All pulsed field data taken at Clark University Pulsed Field Lab


Originally thought to be related to time constant of vortices.


Mystery: there should only be small vortex effects in parallel orientation.


Difference in B
c2

not seen in CeCoIn
5

Conclusions


TDO technique can be used to examine vortex melting


As the sample nears the parallel orientation vortex melting transition
smoothes out.


Irreversibility point for parallel sample orientation found to
be 6T lower than previously predicted.


This can be explained with a slight correction to the angle reported
by Mola and Hill.


Difference between pulsed field and DC field data possibly
due to time constant of moving vortices.


The difference in critical field for parallel orientation is a
mystery. The melting transition is smoothed, the hysteresis
in the irreversibility line is very small and the flux jumps are
no longer present, i.e., all the characteristics of the vortex
dynamics have disappeared. Why is the difference in
critical field still present?