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Interfaces with High Temperature Superconductors Relevance of Interfacial Degrees of Freedom

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. Why consider interfaces ?. most devices are interface driven.

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Interfaces with High Temperature Superconductors Relevance of Interfacial Degrees of Freedom

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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 ? VG = 34 V 0 V - 2.8 V 5 2 1 0.5 RDS (W) 0.2 insulator-superconductor transition observed in a Nd1.2Ba1.8Cu3O7 epitaxial film on SrTiO3 substrate (A. Cassinese et al., 2004) 0.1 0.05 0.02 0.01 35 40 45 50 T (K) ● YBa2Cu3O7-d , electric field across Kapton foils: fractional shifts in RNof 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 single particle processes: interaction between charge excitations in L1 and L2: interaction between charge carriers in L2: electric field energy: electrostatic gate field two-level systems: polarization of dielectric accumulation of charge at interface 2D band:

  6. with (virtual) transitions driven by field of nearest charge carrier interaction of field induced dipoles with the 2D charge carriers induce pairing repulsive term in pairing channel interactionbetween metallic charge carriers and (polarized) two-level systems

  7. Field dependence of TC (at U/4t = 0.1) not strongly dependent on other parameters like CT excitations in SrTiO3 limited by carrier doping repulsive Vz limits Tc saturation of dipole moment 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 major correction band renormalization at coupling to excitons: delocalization with increasing field field energy / 4t renormalization of nearest neighbor spin exchange through charge transfer excitons insignificant

  9. Inclusion of phonon modes coupling to polar phonons at the interface SrTiO3 : soft TO1-mode at 50─80 cm-1 where is the hole-phonon coupling is the polaron binding energy (N. Pavlenko, T.K., cond-mat/0505714) closer to realistic modelling, a further step in complexity:

  10. Strong coupling: superconductor-insulator transition ω ω ●slave-boson evaluation (with d-wave pairing): Ep/t = 0 Ep/t = 1.07 Ep/t = 1.2 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 x(εg) ● the phase diagram now depends on doping at zero field x0 and the field doping x(εg) field-induced reentrant behavior: ● observed (field-induced) Tc shift in HTSC cuprate films depends on doping: inunderdoped films sizable shift whereas in overdoped films (nearly) no shift

  13. BKT transition ε 2D systems: Berezinskii-Kosterlitz-Thouless transition (BKT) TBKT TBKT ● always smaller than TBKT ● increases nonlinearly with doping, due to interface coupling (cf. with experiments by Walkenhorst et al., PRL,1992) [evaluation similar to Kim & Carbotte, 2002]

  14. Nanomagnetism at Interfaces ? Jochen Mannhart Christian Laschinger (theory) Christof Schneider (exp) Alexander Weber (exp)

  15. Epitaxial Film 300 YBa2Cu3O7-d Rg (Ω) 150 0 0 100 200 300 T (K) Measured R(T)-Characteristics (001)/(110)-tilt Grain Boundary 15 YBa2Cu3O7-d Rgb (Ω) Rgb A (Ωcm2) ? 10 5 5×10-9 0 0 0 100 200 300 T (K) C.W. Schneider et al., Phys. Rev. Lett. 92, 257003 (2004)

  16. Y0.8Ca0.2Ba2Cu3O7-δ

  17. Grain Boundary Mechanism R R R Glazman-Matveev exponential power-law dR/dT > 0 Eb T T T 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 ^ R(T) decreases linearly with T, 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 Si:P 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 < TK: pronounced Kondo-resonance Kondo-assisted tunneling magnetic states scatter charges T < TK: strong Kondo-scattering R decreases with T, how?

  22. Kondo resonance ? Magnetic Scattering Centers at Grain Boundaries? localized Cu spins at interface strong potential fluctuations local moment formation varying coupling

  23. ^ R(T) decreases linearly with T 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, Dobrosavljević, and Kotliar PRL 78, 290 (1997) range of linearity is given by width of TK distribution

  24. 15 Rgb (Ω) 10 5 0 0 100 200 300 T (K) 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-δ, 5 K25° [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. (001)/(110) tilt boundary 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 ? charge profile studied by Wehrli, Poilblanc & Rice (2001) and Pavlenko (unpublished) ● charge confined to surface layer when field doping the insulating state ~ underdoped ~80 % , overdoping ~100 % in surface layer electrostrostatic interface tuning is feasible no fundamental objection to higher charge densities achieved areal carrier densities:0.01 ─ 0.05 carriers per unit cell ● limited by dielectric constant εand breakdown field forSrTiO3 films:ε~ 100 and breakdown ~108 V/m

  28. Theoretical design of the interface

  29. Steps towards an approximate solution 2. generalized Lang-Firsov transformation purpose of unitary transformation: 1. bosonization (Holstein-Primakoff) not exact but correct for negligible inversion:

  30. Induced pairing (at U=0) Possibility of Synthesizing an Organic Superconductor (W. A. Little, 1964) Vspine-sc spine: metallic half-filled band ek (polyene chain) side-chains: charge oscillation with low-lying excited state Dsc side-chains (sc) spine 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 x(εg) ∆ ∆ Ep(exp)/t ● the phase diagram now depends on doping at zero field x0 and the field doping x(εg) field-induced reentrant behavior: ● observed (field-induced) Tc shift in HTSC cuprate films depends on doping: inunderdoped films sizable shift whereas in overdoped films (nearly) no shift

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