Superconductivity of MgB2

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15 Νοε 2013 (πριν από 3 χρόνια και 11 μήνες)

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Superconductivity of MgB2

Zhiping Yin

Mar. 14, 2007

Final project of Advanced electronic structure course PHY250
-
6

Outline


Experiment


Electronic structure


Phonons


Summary

Akimitsu’s Discovery: 2001

MgB2:
an intercalated graphite
-
like system



Searching for ferromagnetism,


superconductivity near
40K
was discovered



in 2001



Quickly reproduced and synthesis techniques


were extended by several groups



Crystal Structure is simple: quasi
-
2D



Electronic structure is simple: s
-
p electrons,
intermetallic compound.



CENTRAL QUESTION: What is the origin

of high T
C

in MgB2?

J. Nagamatsu,
et al
., Nature (London)
410
, 63 (2001)

T. Yildirim,
et al
., Phys. Rev. Lett.
87
, 37001 (2001)


Top papers

1. Superconductivity at 39 K in magnesium diboride


J. Nagamatsu,
et al
., Nature (London)
410
, 63 (2001).

2. Boron isotope effect in superconducting MgB2


S. L. Bud'ko,
et al
., Phys. Rev. Lett.
86
, 1877 (2001).

3. Superconductivity of metallic boron in MgB2


J. Kortus,
et al
., Phys. Rev. Lett.
86
, 4656 (2001).

4. Thermodynamic and Transport Properties of Superconducting Mg(^10)B2


D. K. Finnemore,
et al
., Phys. Rev. Lett.
86
, 2420 (2001).

5. Superconductivity in Dense MgB2 Wires


P. C. Canfield ,
et al
., Phys. Rev. Lett.
86
, 2423 (2001).

6. Superconductivity of MgB2: Covalent Bonds Driven Metallic


J. M. An and W. E. Pickett, Phys. Rev. Lett.
86
, 4366 (2001).

7. Giant Anharmonicity and Nonlinear Electron
-
Phonon Coupling in MgB2: A Combined First
-

Principles
Calculation and Neutron Scattering Study


T. Yildirim,
et al
., Phys. Rev. Lett.
87
, 37001 (2001).

8. Electron
-
phonon interaction in the normal and superconducting states of MgB2


Y. Kong,
et al
., Phys. Rev. B.
64
, 020501(R) (2001).



Electronic structure


Near Fermi level almost B p character, other
contributions are very small.


All bands are highly dispersive (light).


p
z
bands are quite isotropic, 3D, cross Ef


p
x,y

bands are 2D, only two (bonding) of them cross
Ef.


2D character contribute > 30% to N(0).


p
z

bands hybridize with the empty Mg s band,
increasing the effective ionicity.


Bands can be perfectly described by tight binding
model with




Hole
-
type conduction bands like the high
-
Tc
cuprates. (0.28,0.59)


Multiple gaps.

J. Kortus,
et al
., Phys. Rev. Lett.
86
, 4656 (2001).

J. M. An and W. E. Pickett, Phys. Rev. Lett.
86
, 4366 (2001).

T. Yildirim,
et al
., Phys. Rev. Lett.
87
, 37001 (2001).

phonons

Deformation potential
D
=13 eV/A (amazingly large for a metal)

B
-
B boning sigma bands couple rather strongly to optical B
-
B bond
-
stretching modes with wave
numbers around 600 cm^(
-
1) (74meV)

El
-
ph coupling strength for s
-
wave pairing yields lambda~0.8

E2g mode is hihgly anharmonic, nonlinear contributions to EPC.



Y. Kong,
et al
., Phys. Rev. B.
64
, 020501(R) (2001).



A. Y. Liu,
et al
., Phys. Rev. Lett.
87
, 87005 (2001).

In
-
plane boron modes


T. Yildirim,
et al
., Phys. Rev. Lett.
87
, 37001 (2001).

Summary


covalent bonds become metallic.


Large d
eformation potential
D
=13 eV/A


2D (cylinder) Fermi surfaces focus strength.


Yet structure remains stable: intrinsic covalency.


Phonon mediated pairing s
-
wave.


Mediate el
-
ph coupling constant ~ 0.8. (Holes in the B
-
B bonding
sigma bands, relative softness of the optical bond
-
stretching modes.)


Strongest coupling of the in
-
plane B motion, which is anharmonic.


High frequency contributes most to Tc.


Boron isotope effect gives Delta Tc=1.0 K, which further confirms
that boron phonon modes are playing an important role in the
superconductivity of MgB2.