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Substitution of and Li in MgB It Effect on and


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Substitution of C and Li in MgB2: It's Effect on TC and Hc2
 MgB2 is a recently discovered superconductor having a moderately high TC of 39K. Significant increase in the critical
 magnetic field tolerated by this superconductor has been achieved by ~5 at% C substitution at the B sub-lattice. In our
 laboratory we have synthesized C substituted MgB2 for a wide variety of C compositions. We have measured TC as a
 function of C content and external magnetic field. Unlike in conventional superconductors -dHC2/dTC shows an increase
 with decrease in temperature for a carbon substitution of 5 at%. Raman measurements have been carried out in the C
 substituted samples. These measurements highlight the importance of the enhanced electron-phonon interaction in
 determining the TC variation with C substitution.

 Five years after its discovery the study of superconductivity MgB2
 is still being actively pursued in many laboratories. Unlike the (A)                                                                                                                                                                                                  Studies on MgB 2
 high TC cuprates MgB2 does not tolerate chemical substitution. C
 on the B sub-lattice and Al at the Mg sub-lattice are the only                                                                                                                                                                                                                                             high pressure
 substitutions that have been extensively investigated. Of the two                                                                                                                                                                                                                                          valve

 substitutions the former has made more impact, since in spite of
 depleting critical temperature TC, C substitution is seen to                             300                                                                                                                                                                                                               gauge
 enhance the critical magnetic field HC2(0) from 15 Tesla to 39

                                                                                                                                                                (1 01 )
                                                                                                                                                                                                                                                                  MgB2 -2

                                                                                                In te ns ity (a rb. un its)
                                                                                                                                                                                                                                                                                                            Swage lock
 Tesla for C substitution of 5 at%. Fig. 1 shows the structure of                                                                                                                                       * MgO
                                                                                                                                                                                                        + MgB 4
                                                                                                                                                                                                                                                                                                            Wilson seal

                                                                                                                                                 (1 00)
 MgB2 that consists of layers of Mg on a hexagonal lattice                                200                                                                                                                                                                                                               evacuation

                                                                                                                                                                                               (110 )
                                                                                                                                                                                     (002 )

                                                                                                                                                                                                             (1 02)
                                                                                                                                                                                                                      (1 1 1 )
                                                                                                                                  (0 01 )
                                                                             r (mW -cm)
 interleaved with graphitic layers of B.                                                  150
                                                                                                                                                                              +                          *
                                                                                                                                            30                            45                  60                                                                                                            sample space
                                                                                                                                                          2 q (degrees)                                                                                                (b)
 Following the announcement of 39 K TC in MgB2 our lab could

                                                                                                                                                                                                                                 s i gn a l ( a r b. u n i ts )

                                                                                                                                                                                                                                     Di a m a g n e t ic
                                                                                                                                                                                                                                                                                                            quartz tube
 quickly reproduce this result on phase pure samples. It was
 realized that some form of high pressure synthesis was necessary                                                      (c)
 to contain the weight loss due to highly volatile Mg. We employed a                        0
                                                                                                                                                                                                                                                                  10     20        30
                                                                                                                                                                                                                                                                       Temperature (K)
                                                                                                                                                                                                                                                                                        40    50

 sample reaction chamber in which Ar can be pressure locked at 50                                                             0                  50                           100             150                                         200                                250             300

 Bar and the reaction vessel could be placed in a furnace, where a                                                                                                                Temperature (K)

 heat treatment could be performed at 9500C (see schematic in
 Fig.1) The crystal structure of the reacted MgB2, the resistivity of       Fig. 1 : Clockwise: Structure of MgB2, superconducting
 the pellet and the diamagnetic signal showing superconductivity                     transition by resistivity, inset diamagnetic signal and
 are also displayed in Fig.1 Using this apparatus for synthesis we                   XRD patterns, schematic of synthesis assembly
 could obtain substitutions of C in MgB2-xCx with out significant Mg
 loss. However we soon found that the temperature limitation of
 9500C was seriously limiting the extent of C substitution in the
 sample. We then resorted to sealing the reaction mixtures in Ta                                60

 tubes, which we could implement with minor modifications in our                                                                                                                                                                                                                                     MgB 2-xC X
 arc furnace. Having sealed the reactants in Ta tubes, heat
 treatments could be carried out at 12000C. We could then obtain a                              50                                                                                                                                                                                                   T C (Raman)
 stoichiometric substitution of C by this procedure. In Fig.2 we
                                                                                                                                                                                                                                                                                                     T C (measured)
 show the variation of TC obtained by these C substitutions.
 MgB2 is a text book example of a system in which                                               40

 superconductivity arises mainly due to phonon mediation and
 conventional BCS theory works well in this system. Extensive
 band structure calculations with the state of the art description of
 the electron-phonon interaction have helped in understanding the
 two-gap behaviour and its consequences on the HC2 behaviour in
 this system. Band structure calculations done on C substitution
 indicate that electrons doped by C substitution deplete the hole                               20
 density of states at the Fermi level, which in turn should deplete TC
 much faster with C substitution than is observed in Fig.2. It was
 suggested that the slower variation of the TC with C substitution                                                                               0.00                                                    0.05                                                                           0.10       0.15        0.20
 could arise as a consequence of an increased electron-phonon                                                                                                                                                                    Carbon Fraction, x
 coupling strength with C doping. In order to verify if this was
 indeed true, we performed Raman measurements on the entire set             Fig. 2 : Variation of TC with C substitution, measured and
 of C substituted samples, synthesized in our laboratory. From a                     determined from Raman data
 detailed analysis of E2g phonon, B-B vibration, Raman line-width,
 and g line position w, we calculated the electron-phonon coupling
 strength l2g using Allens formula
                     g = 2pl2 g N (0)w 2
 Where N(0) is the calculated variation of the density of states at EF.
 l2g and N(0) were then used in McMillan equation to calculate TC. A
 good agreement with the experimentally observed variation in TC is
 seen as shown in Fig.2. This implied that the electron-phonon
 coupling strength has indeed increased due to C substitution.                                                                                                                    Fig. 3 : E2g B-B vibrational mode


MgB2 is a binary intermetallic with an intermediate TC. The raw materials are cheap and the synthesis relatively easy and
therefore can be made in large quantities at lower cost. Its intermediate TC makes 20K operation using closed cycle
refrigeration a possibility, cutting down heavily on operational costs of refrigeration for potential applications. Thus MgB2
could be a viable alternative to the low TC magnet wire materials viz., Nb3Sn and Nb-Ti which are routinely employed in the
construction of superconducting magnets. The serious drawback of MgB2 is however is the strong magnetic field
dependence of the critical current density, JC. Research is now on to improve Jc in this material by introducing pinning
centers by neutron irradiation, doping nanoparticles etc.

Recently Hc2 measurements have been carried
out in our magneto resistance set up in                                                     MgB2-x Cx (x=0.1)
external magnetic fields of upto 12 Tesla. The                                 -4
resistive transition temperatures were traced                1x10                                                                                   0T
for MgB2-xCx and Li co-doped MgB1.8C0.2 as a                                                                                                        2T

                                                                   Resistance (W)
function of applied magnetic field in the range                                                                                                     3T
of 0-12 Tesla. The transitions for the samples                                                                                                      4T
with a carbon fraction of x=0.1 is shown in                                                                                                         5T
Fig.4. The onset of the transition, defined as TC                                                                                                   6T
is plotted as a function of magnetic field for                                                                                                      7T
several of the carbon substituted samples in                                                                                                        9T
Fig.5. It can be seen from the figure that with                                                                                                     10T
carbon substitution –dHc2/dTC increases with                                                                                                        11T
decrease in temperature. This is in contrast                                                                                                        12T
with that seen in pristine MgB2 and in                                        0
conventional superconductors, in which it is
observed that -dHc2/dTC decreases with                                                      10        15    20    25    30     35        40         45         50
decrease in temperature. The vertical bar line                                                                   Temperature (K)
shown in Fig.5 emphasises the fact that even
                                                           Fig. 4 : Resistance versus temperature in MgB2-xCx, for x=0.1 under different
though TC at zero field has decreased with
                                                                    external magnetic fields indicated
Carbon substitution of x=0.10, at 20K the
sample is superconducting in fields upto 12
Tesla, showing the remarkable improvement                                  14
in the critical field due to C substitution. The                                                                                              x=0.10
highest -dHc2/dTC is seen for the x=0.1 and is                             12                                                                 x=0.20
1.75 T/K                                                                                                                                      x=0.40
It is well known that the H C2 of a                                        10                                                                 x=0.0

superconducting sample increases with

increase in TC. It will be beneficial to increase                                   8
the TC of the C substituted sample in order to
enhance the HC2 further. To achieve this goal
we have co-doped Li in MgB1.8C0.2 since Li                                          4
would provide compensating holes for electron                                                MgB 2-x Cx
substituted with C. We find a systematic                                            2
                                                                                             1200 C 24 Hours
enhancement in TC with increase in Li content.
Further we see that the curvature of dHC2/dTC                                       0
of the C substituted sample is remarkably
altered due to Li co-doping.                                                            0        5         10    15   20       25   30         35         40
                                                                                                                      TC (K)

                                                           Fig. 5 : The variation of critical field with temperature determined for MgB2-xCx
                                                                    for carbon fractions, x, indicated

1. S. Jemima Balaselvi, N. Gayathri, A. Bharathi, V. S. Sastry and Y. Hariharan, Physica C, 31 (2004) 4072.
2. S. Jemima Balaselvi, N. Gayathri, A. Bharathi, V. S. Sastry and Y. Hariharan, Supercond. Sci. Tech., 17 (2004) 1401.
3. T.Sakuntala, A. Bharathi, S. K. Deb, N.Gayathri, C.S. Sundar and Y.Hariharan J. Phys. Cond. Matl., 17 (2005) 3285.

                                                                                                      Further inquiries:
                                                                              Dr. A. Bharathi, Materials Science Division
                                                    Metallurgy and Materials Group, IGCAR, e-mail:bharathi@igcar.gov.in


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