Tailoring the Mesoscopic Structure of Superconductors
to Boost Tc beyond the Present Limits
Jochen Mannhart
Center for Electronic Correlations and Magnetism
University of Augsburg
Loen, June 20, 2007
Augsburg University German
Hammerl
Arno Kampf
Thilo Kopp
Frank Lichtenberg
Christof Schneider
Acknowledgements:
University of Geneva
A.D. Caviglia
N. Reyren
J.-M. Triscone
Pohang University
S.I. Lee
The Road to Room-Temperature Superconductivity
For Fortune:
➢ Tc > 500 K
➢ Je (350 K) > 104
A/cm2
in 5 T
➢ ductile, robust, good thermal properties
➢ good Josephson junctions
➢ environmentally friendly compound
➢ available in large quantities
➢ < 20 € kA/m
For Fame:
➢ Tc = 300 K
➢ no layered cuprate
coated conductors
Two Breakthrough Developments in Materials Fabrication
1) Bulk can be fabricated with film techniques
link to Gurevich, Grant
2) Fabrication of new materials that are designed on the unit cell level
Y. Zhu (2000)
Models and theories usually focus on single phase materials
the limits and problems that have been identified predominantly refer to single phases
Two Breakthrough Developments in Materials Fabrication
1) Bulk can be fabricated with film techniques
D.G. Schlom et al., (2001)
SrTiO3
BaTiO3
SrTiO3
BaTiO3
SrTiO3
BaTiO3
SrTiO3
BaTiO3
BaTiO3
Bi2Sr2CaCu2O8 BaTiO3 / SrTiO3 - superlattice
link to Bozovich
I) New phases
Putting atoms in place by nano-fabrication (e.g. by AFM) or epitaxial procedures
Design and Fabrication of New Superconducting Materials
link to Grant, Uchida, Bozovic
II) Boosting Tc by Optimizing the Mesoscopic Structure
Granular Al:
Design and Fabrication of New Superconducting Materials
G. Deutscher:
“New Superconductors: From Granular to High-Tc” (2005)
link to Geballe
reduction of grain size
enhances Tc by factor 3
1) Kresin Effect: nanoclusters with number of electrons close to magic
Design and Fabrication of New Superconducting Materials
II) Boosting Tc by Optimizing the Mesoscopic Structure
Flux Periodicity of Small Superconducting Rings
BdG calculations of superconducting rings with diameter D at T=0:
➢ magnetic flux may Doppler-shift states beyond EF
➢ state population of the condensate may change with flux
flux periodicity of superconducting rings:
s-wave: h/2e for D ≳ ξ, h/e for D ≲ ξ
d-wave: h/e
the 2 of h/2e is a consequence of the s-wave symmetry (no nodes)
F. Loder, A. Kampf, T. Kopp, J.M., C.W. Schneider, Y.S. Barash, submitted
1) Kresin Effect: nanoclusters with number of electrons close to magic
II) Boosting Tc by Optimizing the Mesoscopic Structure
Design and Fabrication of New Superconducting Materials
1) Kresin Effect: nanoclusters with number of electrons close to magic
2) For superconductors with self-organized real space structure:
support this internal structure by starting with an artificial microstructure,
reduction of the energy costs of phase separation
3) High-Tc at interfaces
Design and Fabrication of New Superconducting Materials
links to Rice, Scalapino, Geballe, Kresin, Kivelson, Bozovic
II) Boosting Tc by Optimizing the Mesoscopic Structure
Using Interfaces to Enhance Tc
Interfaces to:
1) stabilize superconducting phase / suppress phase transitions
2) optimize doping
spatially separate doping layer from layer with pair interaction (see HTS)
link to Uchida
Field Effect Doping of a-axis Oriented Infinite Layer Compounds
GDS
collaboration with S.I. Lee, Pohang University
Doping of Compounds with Large number of CuO2 planes
doping
block
collaboration with S.I. Lee, Pohang University
collaboration with S.I. Lee, Pohang University
Photodoping of HgBa2Ca3Cu4Ox
0
T (K)
100 200 300
0
4
8
R (
mΩ
)
darkXe-lamp 2 h
Xe-lamp 4 h
Doping of Compounds with Large number of CuO2 planes
Using Interfaces to Enhance Tc
Interfaces to:
1) stabilize superconducting phase / suppress phase transitions
2) optimize doping
spatially separate doping layer from layer with pair interaction (see HTS)
3) create novel electronic phases:
― correlation parameters at interfaces different from those of bulk ―
U
W
U, W
Ui
metal Mott-insulator
x
E
EF
CTBCTB
UHB
UHB
LHB
LHB
U
W
DOS
metal
Ub
standard metal Mott-insulator
Interfaces to Correlated Electron Systems
J. M. in “Thin Films and
Heterostructures for Oxide
Electronics” Springer (2005)
G. Sawatzky,
A. Millis,
J.M.
Using Interfaces to Enhance Tc
Interfaces to:
1) stabilize superconducting phase / suppress phase transitions
2) optimize doping
spatially separate doping layer from layer with pair interaction (see HTS)
3) create novel electronic phases:
― correlation parameters at interfaces different from those of bulk ―
4) use interface chemistry / induce defects
5) create E and B - fields, break inversion symmetry
6) spatially separate pairing interaction from flow of carriers
interface,
quantum well
pairing
layer
2-DEG
Spatial Separation of Carriers and Pairing Interaction
mobile charge carriers at the interface,
pairing by virtual polarizations of the adjacent layer
Spatial Separation of Carriers and Pairing Interaction
Model System:
mobile electrons
(TiO2)
polarizable dipoles
(SrTiO3)
V. Koerting et al., PRB 71, 104510 (2005)
t = 100 meV
dpd = 2 Å
a = 4 Å
ε = 100
V. Koerting et al., PRB 71, 104510 (2005)
Spatial Separation of Carriers and Pairing Interaction
Model System:
mobile electrons
(TiO2)
polarizable dipoles
(SrTiO3)
Tc
/ 4
t
E-field energy / 4t
0
0.1
0.2
0 0.2 0.4
0.20.40.60.81.0
Spatial Separation of Carriers and Pairing Interaction
Model System:
V. Koerting et al., PRB 71, 104510 (2005)
t = 100 meVband filling
0 10.5
U/4t = 0.0
Idealized
(i.e. non-distorted)
crystal structure of
AnBnO3n+2 = ABOx
= BO6 octahedra (top view)
b
c || [110]perovskite
n = �
ABO3 perovskite
ABO3
n=
5n
= 4
n=
4n
= 5
n = 4.5
A4.5B4.5O15.5
ABO3.444
n = 5
A5B5O17
ABO3.40
n = 4
A4B4O14
ABO3.50
SrNbO3.4
F. Lichtenberg
F. Lichtenberg, D. Widmer, J.G. Bednorz, T. Williams, A. Reller, Z. Phys. B 84, 369 (1991)
SrNbO3.45
5
4
4
5
5
AnBnO3n+2 = ABOx Niobates and Titanates
Electric Susceptibility of SrNbO3.41 along c-axis
V. Bobnar et al., Phys. Rev. B (2002)
T (K)
0 20 40 6080
90
100
110
ε c∞
F. Lichtenberg et al., Prog. Solid State Chem. 29, 1 (2001)
LaTiO3.41 3d0.18
T (K)100
ρ(Ω
cm
)
200 300010-4
100
104
108
The LaAlO3 / SrTiO3 Interface
SrO
TiO2
LaO
AlO2
(001)
……
LaAlO3:
band insulator
SrTiO3:
band insulator
quantum paraelectric
A. Ohtomo, H. Hwang, Nature 427, 423 (2004) link to Bozovic
AFM Images of LaAlO3 (5 uc) / SrTiO3 Heterostructure
step height: 0.4 nm
STEM: Cross Section
5 uc
LaAlO3
SrTiO3
Substrate
(001)
L. Fitting-Kourkoutis, D.A. Muller (Cornell)
HAADF LAADF
SrO0
TiO20
LaO+
AlO2-
……
Interface Conductance vs Number of LaAlO3 Unit Cells
critical thickness
dc = 4 uc
S. Thiel et al., Science 313, 1942 (2006)
Pattern Containing q2-DEG
Optical:
AFM:
Au
5 uc epi LaAlO3
2 uc LaAlO3
+
10 nm poly
LaAlO3
100 μm
1 μm
5 uc epi-LaAlO32 uc LaAlO3
+
10 nm poly LaAlO3
et al. 89
T (mK)
4000 300200100
R s
heet(Ω
/⫽)
0
100
200
300
400
500
8 T
4 T1 T
0.5 T 180 mT
54 mT
36 mT
18 mT
3 mT
0 mT
H ⊥ surface
8 uc
Low Temperature Transport of the Electron Gas
10 mT
I = 100 nA
N. Reyren, J.-M. Triscone et al., submitted
Upper Limit to the Thickness of the Superconducting Sheet
➢ Hall measurements: nS ≃ 3×1013
/cm2
➙ t ≲ 10 nm
➢ Tc = 200 mK ➙ n ≳ 3×1019
/cm3
C.S. Koonce, M.L. Cohen et al.,
PRL 163, 380 (1967)
link to Cohen, Geballe
Tuning Tc with Gate Voltages
back gate, 5 uc
- 80 V
- 60 V
- 40 V
- 20 V
0 V
0 V
20 V
40 V
80 V
120 V
0
200
400
600VG
R D
S(Ω
/⫽)
T (K)
0 0.1 0.2 0.3 0.4 0.5
N. Reyren et al., to be published
Upper Limit to the Thickness of the Superconducting Sheet
➢ Hall measurements: nS ≃ 3×1013
/cm2
➙ t ≲ 4 nm
overdoped ➙ n ≳ 9×1019
/cm3
C.S. Koonce, M.L. Cohen et al.,
PRL 163, 380 (1967)
link to Cohen, Geballe
≲ 4 nm SrTiO3
LaAlO3
J. Mannhart and H. Hilgenkamp, Supercond. Sci. Technol. 10, 880 (1997)
J.G. Wen
Δ
Insulating Layers at High-Tc Grain Boundaries
300
carriers / Cu
0 0.1 0.2 0.3 0.4 0.5
T(K
)
0
100
200
super-
conductor
AFM-
insulat.
link to Gurevich
300
carriers / Cu
0 0.1 0.2 0.3 0.4 0.5
T(K
)
0
100
200
super-
conductor
AFM-
insulat.
I(m
A)
V (mV)
I(m
A)
V (mV)
24° YBa2Cu3O7-x grain boundary
T = 4.2 K
Insulating Layers at High-Tc Grain Boundaries
J. Mannhart and H. Hilgenkamp, Supercond. Sci. Technol. 10, 880 (1997)
Proposal: Boosting Tc with second phases:
➢ inducing correlations at interfaces
➢ providing pairing interactions
➢ doping the interfaces
➢ generating electric and magnetic fields
➢ generating stress/strain at interface
➢ extending stability range of superconducting phase
standard low Tc superconductor
Tc-booster phase
interface with high Tc
link to Chu, Bozovich
Searching for the Preferable Mesoscopic Structure
links to Beasley, Gurevich
small c-axis coupling
Searching for the Preferable Mesoscopic Structure
cross-sections:
problematic for anisotropic
superconductors
length scales: correlation lengths, λel, ξ, ...
mesoscopic structure required
(superconducting) matrix: HTS, organics, whatever ...
interfaces with boosted Tc
effectively percolating
structure of some
early HTS USOs?
Tc booster phase
Searching for the Preferable Mesoscopic Structure
How to Grow the Hybrid-Structures?
Bulk and film methods
The growth problem of interest for a spectrum of applications
first industrial solutions for the growth of nano-scale inclusions in coated conductors
A. Catana et al., Appl. Phys. Lett. 60, 1016 (1992)
Y2O3 in epitaxial YBa2Cu3O7-x film
link to Gurevich
Analogon: III-V Heterostructures
AlGaAs/GaAs-HEMT
S.M. Sze: „High Speed Semiconductor Devices“
Mobile Electrons at Interfaces of Complex Materials
Use Mesoscopic Structure to Enhance Tc
➢ Kresin Effect: nanoclusters with number of electrons close to magic
➢ For superconductors that like a self-organized internal structure:
add matching microstructure
➢ “Hybrid” superconductors to gain a high-Tc at interfaces
Conclusion:
Conclusion:
“Hybrid” Superconductors:
standard low Tc superconductor
Tc-booster phase
interface with high Tc
➢ superconducting matrix
➢ second phase to enhance Tc of interface layers
➢ optimization of the mesoscopic structure
Road to higher Tc’s?
