S. B. OTA
1488
current was given and the voltage was measured. Then
the current was made negative with the same value and
the voltage was measured again. A systematic difference
in voltage (dV) was found in the measurement.
3. Conclusion
Figure 1 shows dV as a function of temperature for
YBa2Cu3O7. It is seen that the difference in voltage or the
directionality is enhanced near Tc. The directionality also
increases as the current is increased from 1 mA to 100
mA. The observed directionality should be present in
conventional superconducting materials but is enhanced
in case of YBa2Cu3O7. The theoretical understanding of
the observed directionality is as follows. The total current
through the superconductor can be written as follows:
diss disp
J J
con st
s
JA
s
JJ (1)
where Js is the superconducting current, Jdisp is the dis-
placement current and Jdiss is the dissipation current. Jdisp
and Jdiss becomes negligible with time and therefore only
Js need to be considered for the equilibrium I-V charac-
teristic. Moreover, it is known that:
(2)
This gives rise to flux quantization and near the transi-
tion trapped quantized magnetic flux. This can give rise
to observed directionality. Such directionality can arise
possibly due to unconventional pairing [22]. The most
discussed unconventional pairing is in the context of high
temperature superconductors in which the pairing chan-
nel is l = 2 or d-wave. Such pairing states occur in various
Figure 1. The temperature dependence of difference in vol-
tage (dV) for YBa2Cu3O7. The transition temperature Tc is
nearly 90 K. dV is shown for two current values; 1 mA (■)
and 100 mA (◊).
theories based on microscopic models which can show
such unusual properties in the superconducting state.
The measured dc voltage also showed increased noise
near Tc. This can possibly arise due to the motion of Ab-
rikosov flux lines and has the nature similar to 1/f noise
[23,24].
The HTSC YBa2Cu3O7 has been studied by d.c. four
probe electrical resistance measurement near the super-
conducting transition. A directionality in the d.c. I-V
characteristic was observed. The observed directionality
can be understood theoretically as due to trapped quan-
tized magnetic flux. Theoretical understanding of such
observation is also consistent with microscopic models
based on d-wave superconducting state.
4. Acknowledgements
The author is benefited from his visit to Europe in 1988-
1992 for HTSC research, Xiamen, China during 1995 for
statistical physics conference and New Orleans and Dal-
las, USA during 2008 and 2011 respectively for APS
March meeting. The author acknowledges Dr. C. Ian-
nicello and others of American Institute of Physics for
providing access to the URL of AIP UniPHY.
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