Zero temperature coefficient of resistivity induced by photovoltaic effect in Y Ba2Cu3O6.96 ceramics

Feng Yang, Mengyuan Han, and Fanggao Chang

Citation: AIP Advances 5, 017126 (2015); doi: 10.1063/1.4906563 View online: http://dx.doi.Org/10.1063/1.4906563

View Table of Contents: http://scitation.aip.org/content/aip/journal/adva/5/1?ver=pdfcov Published by the AIP Publishing

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Zero temperature coefficient of resistivity induced by photovoltaic effect in YBa2Cu3O696 ceramics

Feng Yang, Mengyuan Han, and Fanggao Changa

Henan Key Laboratory of Photovoltaic Materials, College of Physics and Electronic Engineering, Henan Normal University, Xinxiang 453007, P. R. China

(Received 22 December 2014; accepted 13 January 2015; published online 22 January 2015)

I-V characteristics of YBCO-Ag system under blue laser (X=450 nm) illumination were studied from 100 to 300 K and obvious photovoltaic effects were observed. All the I-V curves in the temperature range intersect at a point in the first quadrant while the laser points to the cathode electrode, indicating a zero temperature coefficient of resistivity. This implies that the outputting voltage keeps constant in a broad temperature range when a critical bias current is assigned. The intersection points of different laser intensities fall in a straight line, the slope of which (Rc) is independent of temperature and laser intensity. © 2015 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License. [http://dx.doi.org/10.1063/L4906563]

INTRODUCTION

Photovoltaic effect (PV) of high temperature superconductors have been widely studied for over twenty years.1-6 Photo-induced voltage in YBa2Cu3O7-s (YBCO), in particular, has been reported in the early 1990's and extensively investigated ever since.7-11 Yet its nature and mechanism remain unsettled. We have reported a large PV effect of YBCO thin films deported on N-type semiconductor substrate.12 However, the high resistance of the substrate always masks the primary electrical properties of YBCO.13,14 Recently, remarkable PV effect induced by blue-laser (X=450 nm) illumination was also discovered in YBa2Cu3O6 96 (YBCO) ceramics.15 It was found that all the I-V curves measured at different temperatures intersect at one point in the first quadrant when YBCO was illuminated around the cathode electrode above Tc. Recently, many researches focus on the functional materials with tunable, low and zero temperature coefficient of resistivity (TCR) which is of great importance not only in precision measurement but also in hybrid electronic device applications.16-19 The results in our experiment indicate that if a critical bias current is assigned, the outputting voltage remains constant in a very wide temperature range. This paper presents further investigations on the detailed electrical characteristics induced by photovoltaic effect of YBCO ceramics, in order to widen the practical applications of this important material.

EXPERIMENT

The I-V characteristics experiments were performed on an YBa2Cu3O6.96 ceramic sample of 0.52 mm thickness and 8.64 x 2.26 mm2 rectangular shape and illuminated by continuous wave blue-laser (X=450 nm). To remove the complex influence of the substrate, bulk superconductor is used in the study rather than thin film sample.12-14 Moreover, the bulk material could be conducive for its simple preparation procedure and relatively low cost for applications. The copper lead wires are cohered on the YBCO sample with silver paste forming four circular electrodes of about 0.5 mm in radius, aligned in a straight line on the sample (as shown in the inset of Fig. ). All the electronic properties experiments about laser illumination in this paper were carried out by the laser

aElectronic mail: fanggaochang@hotmail.com

2158-3226/2015/5(1 )/017126/6 5,017126-1 ©Author(s) 2015 I^B

FIG. 1. (a) Temperature dependence of the resistance of YBCO ceramic with different laser intensity irradiation. (b) I-V curves at 300 K in dark (blue diamond) and under 502 mW/cm2 blue laser illumination (red circle). The real picture of the sample mounted on the sample holder is shown in the inset of (a). The inset of (b) shows schematically Ag-YBCO junction.

beam points around the cathode electrodes. I-V properties of the sample were measured using the vibration sample magnetometer (VersaLab, Quantum Design) with a quartz crystal window. The crystalline structure of the YBCO samples was characterized by x-ray diffraction (XRD) using a diffractometer with Cu Ka1 radiation.

RESULTS AND DISCUSSION

Persistent photoconductivity (PPC) effect has been well studied many years ago.20-23 This phenomena prove that illumination of oxygen-deficient YBCO samples by visible light leads to a decrease in resistivity. But to the oxygen-rich sample, the resistance was observed to increase when the sample was illuminated by laser. Fig. 1(a) presents the temperature dependence of resistance for YBCO ceramic illuminated by blue laser (X=450 nm). Without light illumination, the resistance decreases with temperature and falls to zero at 88 K. Three striking effects are observed when laser beam is directed around the cathode leads: firstly, the resistance at a fixed temperature increases with laser intensity; secondly, the slope of R-T curves between 100 K and 300 K decreases slightly with laser illumination; and finally, the superconducting transition temperature Tc (onset point) shifts to lower temperatures with increasing laser intensity. These observations are contrary to those obtained from an oxygen-deficient YBCO sample, where light irradiation reduces the resistivity at normal states and enhances the transition temperature Tc.

Remarkable photovoltaic effect in YBCO ceramics has been reported previously.15 Fig. 1(b) shows I-V curves measured at 300 K in dark and laser irradiated at the negative voltage lead by blue laser (502 mW/cm2), respectively. Without light illumination, the I-V curve is a straight line crossing the origin. This straight line moves upwards parallel to the original one with increasing laser intensity irradiating at the cathode leads, while the line moves downwards with the same offset when the sample is illuminated at the other electrode with the same laser intensity.15 There are two limiting cases of interest for a photovoltaic device. The short-circuit condition occurs when V=0. The current in this case is referred to as the short circuit current (Isc). The second limiting case is the open-circuit condition (Voc) which occurs when R^<x> or the current is zero. Figure 1(b) clearly shows that there is a negative photo-induced voltage (Voc) even without any current, while Isc is negative with light illumination, a typical behavior of normal solar cells.

Oxygen-rich YBCO with small energy gap (Eg) can absorb almost full spectrum of sunlight, thereby creating electron-hole pairs (e-h). To produce an open circuit voltage Voc by absorption of photons, it is necessary to spatially separate photo-generated e-h pairs before recombination occurs. YBCO in normal state is a p-type material with holes as charge carrier, while metallic Ag-paste has characteristics of an n-type material. Similar to p-n junctions, the diffusion of electrons in the silver paste and holes in YBCO ceramic will form an internal electrical field pointing to the YBCO

ceramic at the interface, which provides the separation force and leads to a positive Voc and negative Isc for the YBCO-Ag paste system at room temperature (as shown in the inset of Fig. 1(b)).

These observations cannot be ascribed to a thermalelectric effect. The Seebeck coefficient of Cu is positive and that of YBCO is negative above Tc. Both of them would contribute a negative thermal voltage to the Voc if the cathode is irradiated. The positive Voc as the laser illuminated around the cathode electrode indicates that the photo-induced voltage should be a photovoltaic rather than a thermal effect. In particular, the Seebeck coefficient of YBa2Cu3O6.96 is about -0.1 ^V/K at 300K, and that of copper wire is 0.34-1.15 ^V/K.9,24 The temperature of the copper wire at the laser spot can be raised up by a small amount of 0.02 K with maximum laser intensity available at 300 K. This could produce a thermoelectric potential of 2.5x10-8 V which is four orders magnitude smaller than the Voc obtained in Fig 2. It is evident that thermoelectric effect is too small to explain the experimental results. In fact, the temperature variation due to laser irradiation would disappear in less than one minute so that the contribution from thermal effect can be safely ignored.

The increase in resistance with light illumination can be ascribed to the photovoltaic effect of YBCO. When charges flow from anode leads to cathode leads through the sample, they will be blocked by this opposite potential, resulting in an extra resistance of Voc/I. Therefore, the overall resistance under laser irradiation is R = y = + Rd where Rd is the original resistance in dark. It is evident that the resistance increase is a result of the photovoltaic effect associated with the metal-superconductor interface.15

Figure 2(a) shows the current-voltage characteristics curves at selected temperatures from 100 to 300 K under laser illumination of 502 mW/cm2. It can be seen that all the I-V characteristic curves rotate around a point C (Ic = 2.97 mA, Vc = 0.33 mV) in the first quadrant, in contrast with those obtained without light illumination where all the I-V curves pass through the origin. The output voltage Vc remains relatively constant with variation of temperature with bias current Ic, indicating a zero temperature coefficient of resistivity (TCR) (see the orange symbols of Fig. 2(b)).

To investigate the detailed evolution process of the intersection point, temperature dependence of Voc is plotted in Fig. 2(b) (green circles). It can be easily seen that Voc decreases linearly with increasing temperature: Voc = Vmoc - a(T - Tc), where Tc is the onset superconducting transition temperature of YBCO in dark, Vmoc is the Voc at Tc and a is a parameter related to laser intensity. It is well known that, the resistivity of oxygen-rich YBCO in normal state increases linearly with temperature: R = ¡3T + R0, where R0 is the residual resistance. Since V = Voc + RdI (see Fig. 1(b)), we have

V = (ßI - a)T + Vm

It is obvious that the voltage is independent of temperature if I = Ic = a. The values of a and p can be determined by fitting the data presented in Fig. 2(b) and Fig. 1(a) respectively and thus Ic can be calculated to be 2.88 mA which is fairly consistent with the value of 2.96 mA obtained directly from

FIG. 2. (a) I-V characteristics scanning temperature between 100 K and 300 K with 502 mW/cm2 blue laser illumination. (b) Open circuit voltage (Voc) and outputting voltage (Vc) obtained from I-V curves when Ic = 2.97 mA as a function of temperature from 100 to 300 K.

FIG. 3. Critical voltage (Vc) dependence of laser intensity (a) and critical current (b) obtained from I-V curves when the YBCO-Ag paste system illuminated by the blue laser around the cathode electrode.

the I-V curves in Fig. 2(a). Therefore the non-zero intersection point of I-V curves is in fact a direct consequence of the temperature dependences of Voc and resistivity of YBCO at normal states.

It is interesting to note that, according to equation (1), the on-set superconducting transition temperature can be expressed as Tc = Rc~R° - Vo^. The first item in the right hand side of the equation is independent of laser power, while the second term is positive and related to laser intensity. This equation can be used, at least quantitatively, to explain the decrease of the superconducting transition temperature under light irradiation (Fig. 1(a)).

To gain further insight into the potential applications of this intersection point, laser intensity and critical current are presented in Fig. 3 as function of outputting voltage Vc. It can be seen that Vc increases linearly with both the laser intensity (Fig. 3(a)) and the critical current Ic (Fig. 3(b)). That is to say that there is a corresponding laser intensity and a bias current for a required output voltage. The resistance calculated by Rc = V is independent of temperature and laser intensity but arises solely from the photovoltaic effect. To the application end, although one can obtain YBCO material with zero TCR by controlling the oxygen content, such a preparation procedure is much more difficult to realize in practice. Therefore each intersection point at different laser intensity appears on a straight line C-O in Fig. 2(a) and can be conveniently utilized to construct a voltage regulator.

Figure 4 summarizes temperature dependence of Voc under different laser power. For all the selected laser intensities Voc decreases linearly with increasing temperature. The absolute value of the line slope, however, increases with increasing laser intensity. Extend these lines, we find that all the lines intersect the temperature axis at a critical point T0 (~450 K) at which Voc equals zero, no matter how strong the laser is. Therefore, T0 is the threshold temperature that the PV effect in YBCO-Ag system disappears, and the upper temperature limit of the zero temperature coefficient of resistivity.

As mentioned previously, the resistance at a fixed temperature can be written as R = Vs + Rd. Taking the derivative with respect to temperature T, dR = 1dVTc + ^Td, we find that the slope of R-T is a function of laser intensity. According to Fig. 4, dVoc/dT is negative and decreases with

100 150 200 250 300 350 400 450 Temperature (K)

FIG. 4. Temperature dependence of Voc with different laser intensity illumination.

increasing laser intensity. This explains why the slopes of the R-T curves decrease slightly with the increasing laser power when YBCO is in the normal state, as shown in Fig. 1(a).

CONCLUSION

In summary, obvious photovoltaic effects were observed in YBCO-Ag paste system illuminated by blue laser (X=450nm) of different intensities. In contrast with the PPC effect reported for oxygen-deficient YBCO, the resistance of the oxygen-rich sample at a fixed temperature increases when the sample is irradiated by laser. The slope of the R-T curves decreases slightly with increasing laser intensity. For fixed laser intensity, all the I-V curves obtained between Tc and T0 intersect at one point in the first quadrant, indicating a zero TCR. The intersection points obtained with various laser intensities fall in a straight line. All these observed features can be understood in terms of the photovoltaic effect associated with the metal-superconductor interface. The findings presented here could lead to new applications such as passive optical devices, optical sensor and voltage regulators with a wide operating temperature range, especially at low temperatures.

ACKNOWLEDGMENTS

This work has been supported by the National Natural Science Foundation of China (Grant No. 60571063), the Fundamental Research Projects of Henan Province, China (Grant No. 122300410231).

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