Superconductivity in Superconductivity in MgB2MgB2

Superconductivity in Superconductivity in MgB2MgB2

Khalil Ziq Khalil Ziq Physics DepartmentPhysics Department

KFUPMKFUPMApril 12 2003April 12 2003

New Superconductor Developments

-Fullerenes: Tc engineered to 117K.-Iron and Sulfur become SC under

pressure.-Plastic SC: polythiophene.

-DNA can be made superconducting.

-MgB2 changes our thinking (again).

The Borides WorldMgB2 Jan 10 2001

Jun AkimitsuAPS March 2001

Ferromagnetism:EuB6 with S=7/2 4f-electron local moment and TC=16 KCa1-xLaxB6 with 0.1 B/La and TC=600-1000 KCaB2C2 with 0.0001 B /f.u. and TC=770 K

Superconductivity:LNi2B2C with L=Lu, Tm, Er, Ho, Dy, Y) and Tc=6-17 KMgB2 with Tc=39 KBoron: From the Arabic Buraq.

Magnesium: District of Magnesia Greece,

MgB2

MgB2 Production

•Mg : B = 1:2 molar ratioGround mixture for 15 minutesPressed into 1g pellets

•Heat @800-950 C for 2hours .

Structure

The boron atoms form graphite-like sheets separated by hexagonal layers of Mg atoms.

Equilibrium Magnetization

Eqlm. Magnetization

H[Oe]

0 10000 20000 30000 40000

Meq

[G]

-100

-75

-50

-25

0

4K20K30K

M=-Moln(Hc2/H)Mo~λ^2

London model

H[Oe]

1000 10000

M[G

]

-15

-10

-5

0

Evaluate Hc2, Hc and Mo

T[K]

0 10 20 30 40

M0[

G]

0

20

40

60

T [K]

0 10 20 30 40

Hc2

[O

e]

0

10000

20000

30000

40000

Perucchi et al., Phys. Rev. Lett. 89, 097001 (2002)

Upper Critical Field Hc2 Along the c-Axis

0 10 20 30 400

1

2

3

4

5

6

T(K)

H c2 (T)

Tc

Sologubenko et al.

Helfand-Werthamer

Welp et al.

Angst et al.

Ziq

Scaling of Meq

H/Hc1 10

Meq

/Hc

-0.025

-0.020

-0.015

-0.010

-0.005

0.000

Hysteresis loops

h (H/Hc)

0 2 4 6 8 10

M/H

c

-0.050

-0.025

0.000

0.025

0.0504K15K25K30K35K

a

T(K)

0 10 20 30 40

Jn=

J/H

c (A

/cm

2 /Oe

)

0

25

50

75

100

H=2HcH=4Hc

b

Pinning force=H*Jc

H (Oe)0 20000 40000

Pf

(A O

e/c

m2 )

0e+0

2e+9

4e+9

6e+94K15K25K30K

Max. Pinning Force

H(Oe)

1e+2 1e+3 1e+4 1e+5

Fp(

max

) (O

e A

/cm

2 )

1e+7

1e+8

1e+9

1e+10

Hc HirrHc2

Thermodynamic results supports clean limit SC, unlike optical results which support a dirty-limit scenario

The superconducting gap with 2/kBTc~1.2 is substantially below the estimate of the weak-coupling BCS theory.

The optical conductivity rises in an s-wave like manner.

Conclusion