Journal of Modern Physics, 2012, 3, 1487-1489
http://dx.doi.org/10.4236/jmp.2012.310183 Published Online October 2012 (http://www.SciRP.org/journal/jmp)
Observation of Vortex Lattice Related Anomalies in
Polycrystalline YBa2Cu3O7−x near the
S. B. Ota
Institute of Physics, Bhubaneswar, India
Received July 30, 2012; revised August 28, 2012; accepted September 7, 2012
The d.c. I-V characteristic of polycrystalline YBa2Cu 3O7−x high temperature superconductors (HTSC) is measured near
the transition temperature (Tc). The Tc was found to be 90 K with a width of 2 K. The voltage was measured at various
current values and with reversing the current. A difference in voltage was found for forward and reverse current direc-
tions near Tc. The observed directionality of the I-V characteristic can be understood in terms of quantized magnetic
flux by the self-field of the current and the proximity junctions in these materials. This can also be understood qualita-
tively as due to the d-wave superconductivity. The measured dc voltage showed increased noise near Tc which is possi-
bly related to 1/f noise due to the motion of Abrikosov flux lines.
Keywords: HTSC; Transition Temperature; 1/f Noise
Superconducting materials have attracted interest in re-
cent years [1-17]. Such materials show negligible elec-
trical resistance and magnetic flux exclusion. Electrical
and thermal properties of these materials have been stud-
ied below 100 K. These materials have technological
applications which involve operation across or below the
superconducting transition. Superconducting magnets,
superconducting switches, temperature standards and
switching devices are some examples. The transition
temperature of YBa2Cu3O7 which is a high temperature
superconductor is about 90 K [8,9,12-16]. The effect of
magnetic field on the specific heat anomaly near the su-
perconducting transition is considerably different from
the effect on conventional superconductors. It has been
found that the specific heat peak associated with the su-
perconducting transition mainly reduces in amplitude and
broadens by the application of magnetic field. This is
unlike the type-II conventional superconductors in which
case the application of magnetic field shifts the specific
heat jump at Tc to lower temperatures with practically
very little change in the shape of the anomaly. There are
also several other unusual behavior in the HTSC which is
not yet understood . Other studies to understand the
microscopic filed distribution concerning the supercon-
ducting state has been carried out with high-field μSR
. Here we note that other materials such as CuO has
also been studied in the context of theoretical under-
standing of HTSC [20,21].
Electrical resistance measurement on polycrystalline
YBa2Cu3O7 was carried by an automated d.c. four termi-
nal technique . This setup is built around a closed
cycle refrigerator. The YBa2Cu3O7 sample was in the
form of rectangular bar having dimension ≈ 1 × 2 × 11
mm3. The sample was characterized as as been discussed
elsewhere [8,9]. The electrical contacts made using silver
paste had a resistance of 30 Ohms for the two current
leads. The electrical resistance of the sample at 100 K
was nearly 0.1 Ohms. The current was varied from 1 mA
to 100 mA in steps using a Keithley model 224/2243
programmable current source. The dc voltage was meas-
ured using a Keithley model 182 sensitive digital volt-
meter. A calibrated type D silicon diode thermometer
was used in conjunction with a Leybold model LTC60
temperature controller to control and monitor the tem-
perature of the sample site. The calibrated diode has a
standard measurement accuracy of about 1 percent. The
measurement was done as the sample was warmed from
about 70 K. The HTSC has a superconducting transition
(Tc) of 90 K and a width of about 2 K. At each tempera-
ture the current was increased from 1 mA to 100 mA in
steps. For each current value, first the positive polarity of
opyright © 2012 SciRes. JMP
S. B. OTA
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.
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:
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:
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 . 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
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.
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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