Flux dynamics and vortex phase diagram of the new superconductor MgB2
Introduction
The recently discovered new superconductor MgB2 generates enormous interests in the field of superconductivity [1]. Many important thermodynamic parameters have already been derived, such as the upper critical field Hc2(0)=13–20.4 T [2], [3], [4], [5], the Ginzburg–Landau parameter κ≈26 [5], and the bulk critical superconducting current density jc≈8×104 A/cm2 at 4.2 K and 12 T [6] in thin films. One big issue concerns however how fast the critical current will decay under a magnetic field and in which region on the field-temperature (H–T) phase diagram the superconductor can carry a large critical current density (jc). This jc is controlled by the mobility of the magnetic vortices, and vanishes at the melting point between the vortex solid and liquid. A finite linear resistivity ρlin=(E/j)j→0 will appear and the relaxation rate will reach 100% at this melting point showing the starting of the reversible flux motion. In this paper we present an extensive investigation on the flux dynamics by magnetic relaxation and transport measurement. A vortex phase diagram will be depicted based on these measurements.
Section snippets
Experimental
Samples investigated here were fabricated by both high pressure (HP) (P=6 GPa at 950°C for 0.5 h) and ambient pressure (AP) synthesis which was described very clearly in a recent publication [7]. HP synthesis is a good technique for producing the MgB2 superconductor since it can make the sample more dense and uniform (in sub-micron scale) and also prevent the oxidization of Mg element during the solid reaction. Our HP samples are very dense and look like metals with shiny surfaces after
Results
Fig. 1 shows the diamagnetic transition of one of the HP samples measured in the field-cooled (FC) and zero-field-cooled (ZFC) process. All other samples show almost similar quality. In the FC process, the temperature was lowered from above Tc to a desired temperature below Tc under a magnetic field, and the data are collected in warming up process with field. Its signal generally describes the surface shielding current and the internal frozen magnetic flux profile. In the ZFC process, the
Vortex phase diagram of MgB2
The phase lines of Hirr(T) and Hc2(T) determined by following the different methods mentioned above are shown in Fig. 6. It is clear that the bulk irreversibility line Hirrbulk(T) determined from MHL measurement at the point just before the appearance of the small tail shown in Fig. 3 terminates at about 8 T at 2 K, which is very close to that determined by resistive measurement and from the M–T measurement (Fig. 4). This strongly indicates that the Hirrbulk(T) is a vortex melting line which
Conclusion
In rather pure samples of MgB2 the irreversibility field is rather low comparing to the upper critical field in low temperature region. This effect has been attributed to either the possible existence of the quantum vortex liquid due to strong quantum fluctuation of vortices, or the easy flux flow through the weak link channels. The temperature and field dependence of the relaxation rate may further suggest that the vortex melting at a finite temperature is also induced by strong quantum
Acknowledgments
This work is supported by the National Science Foundation of China (NSFC 19825111) and the Ministry of Science and Technology of China (project: NKBRSF-G1999064602). HHW gratefully acknowledges Prof. B. Ivlev and Dr. A.F.Th. Hoekstra for fruitful discussions, and continuing financial support from the Alexander von Humboldt foundation, Germany.
References (31)
- et al.
Physica C
(1992) - et al.
Physica C
(1993) - et al.
Physica C
(1990) - et al.
Physica C
(1991) - et al.
Nature
(2001) - S.L. Bud'ko, C. Petrovic, G. Lapertot, C.E. Cunningham, P.C. Canfield, cond-mat/0102413...
- P.C. Canfield, D.K. Finnemore, S.L. Bud'ko, J.E. Ostenson, G. Lapertot, C.E. Cunningham, C. Petrovic, cond-mat/0102289...
- Y. Takano, H. Takeya, H. Fujii, H. Kumakura, T. Hatano, K. Togano, H. Kito, H. Ihara, cond-mat/0102167...
- D.K. Finnemore, J.E. Ostenson, S.L. Bud'ko, G. Lapertot, P.C. Canfield, cond-mat/0102114...
- C.B. Eom, M.K. Lee, J.H. Choi, L. Belenky, X. Song, L.D. Cooley, M.T. Naus, S. Patnaik, J. Jiang, M. Rikel, A....
Chin. Phys. Lett.
Nature
Chin. Phys. Lett.
Phys. Rev.
Cited by (30)
Effect of anisotropic superconductivity on flux pinning in polycrystalline MgB<inf>2</inf>
2006, Physica C: Superconductivity and its ApplicationsComparison of physical properties for carbon nanotube doped MgB<inf>2</inf> superconductors synthesized with different process
2005, Physica C: Superconductivity and its ApplicationsThird harmonic ac susceptibility measurements on MgB<inf>2</inf> bulk: Irreversibility line and frequency dynamic behaviour
2003, Physica C: Superconductivity and its ApplicationsCitation Excerpt :The recent discovery of the superconductivity in MgB2[1], has stimulated large interest as this material shows various peculiar behaviours [2]: (a) an electron–phonon superconducting behaviour, like conventional BCS superconductors; (b) grain alignment does not play a crucial role in superconducting current transport [3]. Thus, from the viewpoint of practical applications, it is important to understand the magnetic irreversibility line (IL) as a function of the temperature [4,5]. IL is generally defined in a resistive way by the appearance of a resistive state at each field [2,3], or by VSM magnetic measurements [3].
Pinning effects and current density broadening of resistance on MgB<inf>2</inf>
2003, Physica C: Superconductivity and its ApplicationsPinning effects and magnetization hysteresis loops in MgB<inf>2</inf>
2003, Physica C: Superconductivity and its ApplicationsDoping effect and flux pinning mechanism of nano-SiC additions in MgB <inf>2</inf> strands
2011, Superconductor Science and Technology