Wendy Xu
286G
5/28/10
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Electrical resistivity goes to zero
Meissner effect: magnetic field is
excluded from superconductor below
critical temperature
Type I: abrupt scnon-sc transition with
field
◦ Pure metals
◦ low temperatures and small magnetic fields
◦ BCS Theory: Cooper pairs
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Type II: scmixed statenon-sc
◦ Alloys, intermetallics, ceramics, cuprates
◦ Higher temperatures and fields higher
currents
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AM2X2
◦ A: alkaline earth or lanthanide
◦ M: transition metal
◦ X: group 3-6
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Variety of bonding & properties
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Mixed valency e.g. EuNi2P2
Heavy fermion behavior e.g. CeCu2Si2
Magnetism e.g. BaFe2As2
Superconductivity e.g. BaFe2As2
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Johrendt et al.
J. Solid St. Chem. 130
(1997) 254-265
AM2X2 Tetragonal I4/mmm
Layers of edge sharing MX4
tetrahedra separated by planes of A
atoms
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MX4 almost undistorted w/ strong
M-X bonds
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X-X interlayer distances varies
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A is an electron donor, and maintains
geometry
◦ Changing M from left to right, M-M
distance increases, X-X distance
decreases
◦ Changing A from small to big, X-X
distance increases
◦ Alkaline earth—almost completely
ionized
◦ Ln—d shells partially occupied, not
completely ionized
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I4/mmm
a=3.464A, c=10.631A (2.3C)
LuC NaCl layers alternate w/ Ni2B2 layers
B-C:1.47A, short
B-B: 2.94A
Lu-C: 2.499A, strong
c expands, a contracts
Ni-Ni (planar): 2.449A, strong
shorter than metallic metal (2.5A)
Ni-B: 2.10A
B-Ni-B: 108.75, 110.9
Rigid Ni2B2 layers, nearly ideal NiB4
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Ln contraction: a axis contracts as size of Ln ion decreases
c axis expands, volume contraction small
Siegrist et al.
Nature 367 (1994)
254-256
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Contribution of all atoms present
All five Ni(3d) orbital contributions
roughly equal
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Lu(5d) contribution non-negligible
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L. F. Mattheiss
Phys. Rev. B 49 (1994)
13279
◦ doping at this site less favorable than in typical
cuprate sc’s
Q. Huang et al.
arXiv:0806.2776v2
9 Jul 2008
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At 142K, NMAFM transition accompanies
tetragonalorthorhombic structural transition
Q. Huang et al.
arXiv:0806.2776v2
9 Jul 2008
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(Ba0.6K0.4)Fe2As2 Tc=38K
◦ Ideal FeAs4
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KFe2As2 exists
◦ r(Ba2+)=1.42A
◦ r(K+)=1.51A
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As x=01
◦ As-Fe-As gets smaller
 Fe(3dx2-y2) and As(3sp)
overlap increases
◦ Fe-Fe gets shorter
◦ FeAs4 stretched along c
Rotter et al.
DOI: 10.1002/anie.200
S. Kimber et al. Nature
Mat. 8 (2009) 471-475
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P=4GPa Tc=35K
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Similarities to doping
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◦ Lower Tc than doping due to slightly smaller
N(EF)
◦ a lattice parameter trend
◦ As-Fe-As converge to 109.5 towards sc region
Modification of Fermi surface by
structural distortions more important
than charge doping for sc
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Ba(Fe1.9Pt0.1)As2 Tc=25K
All sc structures are
tetragonal
Ba(Fe2-xMx)As2
◦ M=Co, Ni(3d), Rh(4d), Pt(5d)
◦ a increases, c decreases
◦ Similar Tc’s
 Regardless of mass,
bandwidth, and spin orbit
coupling
Xiyu Zhu et al.
arXiv:1001.4913v3
1 Apr 2010
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SC’s w/ ThCr2Si2 structure
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LuNi2B2C Tc=23K
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BaFe2As2
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◦ Intermediate Tc values bridging gap btw pure metal sc’s
and high Tc cuprates
◦ Multiband 3D sc
◦ K doped Tc=38K
◦ High pressure Tc=35K
◦ Pt doped Tc=25K
Fermi surface very important for sc, but what
exactly what leads to sc in these materials are not
clear