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

superconductorsBraunen E. Smith

K. Cho, I. Mihut and C. C. AgostaDepartment of Physics, Clark University

S. W. Tozer, T. P. Murphy and E. C. PalmThe National High Magnetic Field Laboratory, Tallahassee

J. A. SchlueterMaterials Science Division, Argonne National Laboratory

We acknowledge support from DOE grant #ER46214.

• Introduction– Organic Superconductor -(ET)2Cu(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)2Cu(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 Tc (~10 K) and Hc2 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):

Birr ()sin()

BirrBirr()cos()

Birr||

2

1

Solid Line: Birr=3.6 T Birr||=32 T Dashed Line: Birr=3.6 T Birr||=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 (Bc2)

•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 Bc2 not seen in CeCoIn5

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?