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Physica C 468 (2008) 872–875 www.elsevier.com/locate/physc

Temperature dependence of conduction noise of MgB2 superconductor D.P. Singh a, Neeraj Khare b,*, Chandra Shekhar c, O.N. Srivastava c b

a School of Physics and Materials Science, Thapar University, Patiala 147004, India Department of Physics, Indian Institute of Technology Delhi, New Delhi 110016, India c Department of Physics, Banaras Hindu University, Varanasi 221005, India

Received 31 October 2007; received in revised form 20 January 2008; accepted 14 February 2008 Available online 10 March 2008

Abstract Temperature dependent behavior of conduction noise of MgB2 superconductor has been studied. The frequency spectrum of conduction noise shows 1/fa type of behavior, with a = 0.7–0.8. Temperature dependence of conduction noise is found to exhibit two peaks at 26 K and 38 K respectively. The peak at 38 K is attributed to movement of vortices and due to fluctuation in resistance from change of state from superconducting to normal state. The peak in the conduction noise at 26 K is ascribed to the movement of vortices due to enhanced thermally activated vortex hopping. The analysis of enhanced conduction noise in the low temperature region indicates the presence large density of fluctuators with activation energies around 0.048 eV. Ó 2008 Elsevier B.V. All rights reserved. PACS: 74.70.Ad; 72.70.+m; 74.25.Qt; 74.40.+k Keywords: MgB2 superconductor; Conduction noise; Vortex dynamics

1. Introduction Since the discovery of superconductivity in MgB2 at transition temperature, Tc 39 K [1], this binary intermetallic superconductor has become most studied superconductor. The studied aspects include critical current density [2,3], nature of grain boundaries [4–6], energy gaps [7] and vortex dynamics [8–14]. The relatively high upper critical field (18T) [8] of polycrystalline MgB2 superconductor is a very interesting feature of this superconducting material. The vortex dynamics in the MgB2 is very interesting and has been attracting lot of attention [8–14]. An unusual behavior in magnetic relaxation was found in MgB2 superconductor [9]. Fluctuation in M–H loop at temperature below Tc was also observed, which was attributed to flux jumps [10]. The occurrence of vortex avalanche effect has also been noticed at 15 K in MgB2, which lowers

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Corresponding author. Tel./fax: +91 11 26591352. E-mail address: [email protected] (N. Khare).

0921-4534/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2008.02.002

the critical current density [11]. The deposition of Au film on the MgB2 films has been found to improve critical current density by avoiding the occurrence of avalanche effect [11]. Magneto optical studies on MgB2 samples have shown the appearance of turbulence structure occurring in MgB2 at the abrupt dendritic flux penetration well below Tc [12]. Mumtaz et al. [15] have noted unusual noise in the magnetization relaxation in MgB2 superconductor, which was attributed to presence of highly unstable vortex pattern at low temperature and low field. Conduction noise is another versatile technique to explore the kinetics of vortices in superconductors and it has been used by several workers for investigating highTc superconductors [16,17]. Recently, conduction noise measurement has been used for studying the vortex avalanche phenomena in MgB2 at 4.2 K [18,19], where a peak in frequency spectrum was found in the presence of magnetic field that was attributed to vortex–antivortex annihilation. Kim et al. [20] have used voltage noise characteristics along with current–voltage characteristics to investigate dynamic vortex property of MgB2 near the

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peak effect region. Temperature dependence of conduction noise in MgB2 superconductor has not yet been reported, which can provide more information on the dynamics of trapped vortices and the superconducting transition in MgB2. The present paper reports the temperature dependence of conduction noise for investigating the vortex dynamics of polycrystalline MgB2 superconductor. Temperature dependence of conduction noise shows two peaks at 26 K and 38 K, which are attributed to enhanced thermally activated vortex hopping and change in superconducting to normal state transition respectively. 2. Experimental MgB2 bulk sample was prepared using commercially available MgB2 powder (99% Alfa Aesar, Johnson Mathey) and sintering it in Rf induction furnace. The MgB2 powder was ground thoroughly in a dry box and the powder was palletized at a pressure 3.0 tons/inch2. The MgB2 pellets along with a pellet of pure Mg powder was kept in graphite crucible and inserted into silica tube. Argon gas was flown in the silica tube during the sintering and cool down duration. The sintering was done in Rf induction furnace (12 kW) at 950 °C for 10 min and then Rf power was switched off. The sample was left in silica tube in the flowing Argon to cool down to room temperature. The details of sample preparation are described elsewhere [21]. The X-ray diffraction studies confirmed the phase purity of MgB2. The lattice parameter of the sample ˚ and b = 3.515 A ˚ . The resulwas found to be a = 3.085 A tant MgB2 sample was polycrystalline with grain size 1 lm. The superconducting transition temperature was determined by the temperature dependence of resistance (R–T) and by the ac susceptibility measurements. Standard four-probe technique was used for the R–T measurement. For ac susceptibility measurement, an ac signal of 491 Hz was applied. Conduction noise of the sample was measured by fourprobe technique. Fig. 1 shows the schematic diagram of the conduction noise measurement. A 1 mA current was passed through the sample by a battery operated low noise current source. The voltage signal was dc filtered and amplified by a low noise preamplifier and detected by a dynamic signal analyzer (DSA), which performs rms averages of fast Fourier transform (FFT) of the input signal.

Fig. 1. Schematic of set up for conduction noise measurement.

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The measurement was carried out in the temperature range of 4.2–45 K and frequency range of 1.5–40 Hz. The background noise of experimental set up was 1  1015 V2/Hz. 3. Results and discussion Fig. 2 shows variation of resistance of MgB2 sample with temperature. The figure shows metallic behavior of the sample before the superconducting transition. The onset and end point transition are 39 K and 38.5 K respectively that indicates the transition temperature of 39 K. AC susceptibility of MgB2 as a function of temperature is shown in Fig. 3. The curve also indicates the transition temperature 39 K. Fig. 4 shows the p frequency spectrum of noise power spectral density ( Sv) at different temperatures of MgB2 superconductor in the frequency range of 1.5 Hz to 40 Hz. The conduction noise typically shows 1/fa type of behavior, with a = 0.7–0.8. The temperature dependence of conduction noise with temperature is also evident from the frequency spectrum. In order to see the temperature dependence of conduction noise more clearly, we have p plotted the variation of noise power spectral density ( Sv) at

Fig. 2. Temperature dependence of resistance of MgB2 sample.

Fig. 3. AC susceptibility versus temperature curve for MgB2 sample.

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Fig. 4. Frequency spectrum of conduction noise power spectral density (Sv) of MgB2 sample taken at different temperature.

The peak in conduction noise around 26 K is singular for MgB2 superconductor. This type of enhanced conduction noise at temperatures below than Tc has not been noticed in high-Tc superconductors. Studies of magneto optical imaging of MgB2 [12], magnetic relaxation effect [15] and observation of noise in magnetization also indicated the occurrence of movement of vortices at low temperatures. Our observation of the peak in conduction noise at 26 K seems to have similar origin and indicates movement of vortices at that temperature. We have cooled the MgB2 sample in earth’s magnetic field (200 mG) and the vortex density would be very low. However thermally activated vortex hopping can take place, which can contribute in the enhanced conduction noise. The inset of fig. 5 shows bias current dependence of the Sv (9 Hz) at 26 K. It shows that the noise is 1  1015 V2/Hz (background noise) for the bias current smaller than 90 lA. The Sv shows an increase when the bias current is increased from 100 lA to 10 mA. The increase of Sv with the bias current indicates that the enhanced noise at 26 K is due to movement of vortices. The pinned vortices in superconductors can be considered as having different pinning energies and acting as fluctuators. With the increase in the temperature, when thermal energy is comparable to the activation energy, it can lead to enhanced fluctuations in vortex density and to the observed large noise. The noise due to thermally activated vortex hopping can be analyzed in the framework of DDH model [23]. If D(E), the distribution of activation energy of the fluctuators varies slowly compared to k T, then it may be deduced directly from temperature dependence of the noise spectra; DðEÞ ¼

2pfS v kT

ð1Þ

with Fig. 5. Temperature dependence of noise spectral density (Sv) at 9 Hz of MgB2 sample. Circular points are the experimental observed values and solid line is guide to eye. The inset shows bias current dependence of Sv (9 Hz) at temperature 26 K.

frequency of 9 Hz with temperature (Fig. 5). Temperature dependence of conduction noise shows two peaks at 26 K and 38 K respectively. The observed peak value in conduction noise around 38 K is near to Tc and may be attributed to the fluctuation in resistivity of the sample due to change from superconducting to normal state. This type of maxima in conduction noise near Tc has been observed in high Tc superconductor [16,22]. However it is to note that superconducting transition of MgB2 superconductor is sharp (DT 0.5 K), whereas the observed peak in conduction noise is broad. When the temperature approaches to Tc, the trapped vortices will start moving due to lowering of pinning potential and this phenomenon can start at temperature 1–2 K lower than Tc. Thus, in the present case the enhanced conduction noise near Tc seems to be due to mixed effect of movement of vortices and also due to resistivity fluctuations.

E  kT lnð2pf s0 Þ

ð2Þ

where E is the activation energy, k is Boltzman’s constant and s0 ð 1012 sÞ is attempt frequency. From the Sv vs T curve shown in Fig. 5 and using Eqs. (1) and (2), we found that the enhanced noise peak in Fig. 5 in the low temperature region indicates the presence of large distribution of activation energies around 0.048 eV. It is to point out that the magnetic relaxation measurement on MgB2 superconductor also showed that the pinning energy distribution peaked around 0.040.07 eV [24]. Therefore, the presence of large density of fluctuators having activation energies 0.048 eV seems to be responsible for the enhanced noise in the low temperature region. 4. Conclusion Conduction noise in MgB2 superconductor shows 1/fa type of behavior. Temperature dependence of conduction noise exhibits a peak at 26 K and another peak at 38 K. The peak in the conduction noise at 26 K is attributed to movement of vortices due to enhanced

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