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1
Interfaces with High Temperature
Superconductors Relevance of Interfacial Degrees
of Freedom Thilo Kopp, Universität Augsburg
(1) electrostatic interface tuning (SuFETs)
(2) nanomagnetism at interfaces of HTSCs
2
Why consider interfaces ?

• most devices are interface driven

• HTSC cables are not single crystals
• - grain boundaries may control the
  transport

• interfaces of correlated electronic systems may
  provide a new type of complexity
• reconstruction of electronic states (?)

3
Electrostatic interface tuning (SUFETs)
theory Natalia Pavlenko Verena
Koerting Qingshan Yuan Peter Hirschfeld
field doping ? instead of ? chemical
doping
tune phase transitions electrostatically ?
experiment Jochen Mannhart Gennadij
Logvenov Christof Schneider
4
Is electrostatic interface tuning feasible ?
?
YBa2Cu3O7-d , electric field across Kapton foils
fractional shifts in RN of O(10-5)
(Fiory et al., 1990)
YBa2Cu3O7-d , electric field across SrTiO3
barriers
?
with 4 x106 V/cm major Tc shift
(J. Mannhart, 1991, 96)
Tc shifts of 10 K YBCO film on SrTiO3
?
with 10 mC/cm2 gate polarization
(G. Logvenov, 2003)
DS-channel 8 nm polycrystalline YBa2Cu3O7-d
gate barrier 300 nm epitaxial Ba0.15Sr0.85TiO3
5
Theoretical design of the interface
electric field energy
electrostatic gate field
two-level systems
polarization of dielectric
accumulation of charge at interface
2D band
6
interaction between metallic charge carriers and
(polarized) two-level systems
7
Field dependence of TC (at U/4t 0.1)
maximum in Tc for intermediate fields
field energy / 4t
(V. Koerting, Q. Yuan, P. Hirschfeld, T.K., and
J. Mannhart, PRB 71, 104510 (2005))
8
Strong coupling mapping onto a t-J model
renormalization of nearest neighbor spin exchange
through charge transfer excitons
insignificant
9
Inclusion of phonon modes
(N. Pavlenko, T.K., cond-mat/0505714)
closer to realistic modelling, a further step in
complexity
10
Strong coupling superconductor-insulator
transition
coupling to phonons
localization with increasing doping
similar evaluation for the CMR-manganites compare
Röder, Zang, and Bishop (PRL 1996)
double exchange ? excitonic narrowing
JT phonon ? soft phonon mode
doping x
11
Strong coupling superconductor-insulator
transition
coupling to excitons
coupling to phonons
localization with increasing doping
delocalization with increasing field
transition not only depends on the overall
dopping but also on the details of chemical
versus field doping
12
Strong coupling reentrant behavior
the phase diagram now depends on
doping at zero field x0 and
the field doping x(eg)
?
field-induced reentrant behavior
observed (field-induced) Tc shift in HTSC cuprate
films depends on doping in
underdoped films sizable shift
whereas in overdoped films (nearly) no
shift
?
13
BKT transition
2D systems Berezinskii-Kosterlitz-Thouless
transition (BKT)
?
always smaller than
TBKT
TBKT
?
increases nonlinearly with doping, due to
interface coupling (cf. with experiments
by Walkenhorst et al., PRL,1992)
TBKT
evaluation similar to Kim Carbotte, 2002
14
Nanomagnetism at Interfaces ?
Jochen Mannhart Christian Laschinger
(theory) Christof Schneider (exp) Alexander
Weber (exp)
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Measured R(T)-Characteristics
(001)/(110)-tilt Grain Boundary
15
YBa2Cu3O7-d
Rgb (O)
Rgb A (Ocm2)
?
10
5
510-9
0
0
0
100
200
300
T (K)
C.W. Schneider et al., Phys. Rev. Lett. 92,
257003 (2004)
16
Y0.8Ca0.2Ba2Cu3O7-d
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Grain Boundary Mechanism
Tunneling
Nanobridges
Resonant Tunneling
tunnel barrier
Eb
18
TEM image of a 30º 001 YBCO tilt grain boundary
atomic reconstruction at a large
angle grain boundary
Cu/O partially occpuied
N.D. Browning et al., Physica C 294, 183 (1998)
19
Grain Boundary Mechanism
Phenomenology
(1)
if transport scattering rate depends, besides
, on a single energy scale
with a pronounced increase for
(2)
if is randomly distributed with assuming
that is wide and has no structure
up to
then
range of linearity given by width of T?
distribution
20
Grain Boundary Mechanism
potential fluctuations and distribution of
bonds in a nanobridge
? formation of local moments
compare formation of localized moments in SiP
Lakner, von Löhneysen, Langenfeld,
and Wölfle (1994)
? distribution of Kondo temperatures
21
Magnetic States at Grain Boundaries
Nanobridges
Tunneling
Kondo- resonance
insulating barrier
tunnel barrier
Kondo-resonance
magnetic states assist tunneling T lt TK
pronounced Kondo-resonance
Kondo-assisted tunneling
magnetic states scatter charges T lt TK strong
Kondo-scattering
R decreases with T, how?
22
Magnetic Scattering Centers at Grain Boundaries?
localized Cu spins at interface
strong potential fluctuations local moment
formation varying coupling
23
Kondo Disorder at Grain Boundaries
1) Single Kondo impurity
2) Kondo impurities with distribution P(TK)
(disordered interface)
compare with R(T) of certain Kondo alloys
Miranda, Dobrosavljevic, and Kotliar
PRL 78, 290 (1997)
range of linearity is given by width of TK
distribution
24
Summary
Challenge Interfaces in Correlated Electron
Systems
example grain boundaries in HTSC
new states at the interface
anomalous transport through interface
example SuFET with HTSC
25
Nanobridges across Grain Boundaries?
YBa2Cu3O7-d, 5 K 25 001-tilt 100 µm wide
M. Däumling et al., Appl. Phys. Lett. 61, 1355
(1992)B.H. Moeckly et al., Phys. Rev. B 47, 400
(1993)
26
Measured I (V)-Characteristic (23 Junctions in
Series)
4.2 K
115 K
207 K
C.W. Schneider et al., Phys. Rev. Lett. 92,
257003 (2004)
27
Is electrostatic interface tuning feasible ?
achieved areal carrier densities
0.01 - 0.05 carriers per unit cell
?
limited by dielectric constant e and breakdown
field for SrTiO3 films e 100
and breakdown 108 V/m
28
Theoretical design of the interface
29
Steps towards an approximate solution
1. bosonization (Holstein-Primakoff)
not exact but correct for negligible inversion
30
Induced pairing (at U0)
second order perturbation theory for zero field
exciton
positive attractive interaction
31
Including a repulsive interaction in the metallic
layer
field energy / 4t
(V. Koerting, Q. Yuan, P. Hirschfeld, T.K., and
J. Mannhart, PRB 71, 104510 (2005))
32
Strong coupling reentrant behavior
the phase diagram now depends on
doping at zero field x0 and
the field doping x(eg)
?
field-induced reentrant behavior
observed (field-induced) Tc shift in HTSC cuprate
films depends on doping in
underdoped films sizable shift
whereas in overdoped films (nearly) no
shift
?