Locally enhanced conductivity due to the tetragonal domain structure in LaAlO3/SrTiO3 heterointerfaces

Journal name:
Nature Materials
Volume:
12,
Pages:
1091–1095
Year published:
DOI:
doi:10.1038/nmat3753
Received
Accepted
Published online

The ability to control materials properties through interface engineering is demonstrated by the appearance of conductivity at the interface of certain insulators, most famously the {001} interface of the band insulators LaAlO3 and TiO2-terminated SrTiO3 (STO; refs 1, 2). Transport and other measurements in this system show a plethora of diverse physical phenomena3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14. To better understand the interface conductivity, we used scanning superconducting quantum interference device microscopy to image the magnetic field locally generated by current in an interface. At low temperature, we found that the current flowed in conductive narrow paths oriented along the crystallographic axes, embedded in a less conductive background. The configuration of these paths changed on thermal cycling above the STO cubic-to-tetragonal structural transition temperature, implying that the local conductivity is strongly modified by the STO tetragonal domain structure. The interplay between substrate domains and the interface provides an additional mechanism for understanding and controlling the behaviour of heterostructures.

At a glance

Figures

  1. Scanning SQUID measurements of current in LAO/STO heterostructures.
    Figure 1: Scanning SQUID measurements of current in LAO/STO heterostructures.

    a, In our measurement set-up, the magnetic flux through the pickup loop (red) from current flowing in the sample is measured as a function of position. b, Simulated flux image for the dimensions of sample H1 with uniform conductivity. In a.c. flux images, positive flux is flux measured out of the page and negative flux is into the page. c, Magnetic flux image of current in patterned sample H1. Green outline indicates the dimensions of the patterned LAO/STO. d, Reconstructed current densities along the long dimension of the device are obtained from the raw flux image. e, Line cuts through the current densities (dashed lines in d reveal a large modulation of the amplitude of the current density with position). Scale bars, 30 μm.

  2. Current in unpatterned LAO/STO flows in narrow paths.
    Figure 2: Current in unpatterned LAO/STO flows in narrow paths.

    ad, Top: magnetic flux images from current flowing through four 250 μm×85 μm regions of a 5×5 mm sample. The dark and light lines next to each other correspond to a dipolar feature expected for the z component of a magnetic field from a wire-like current. Blue arrows indicate the expected direction of current flowing in a homogeneous sample, showing that more current is flowing in the narrow paths. Middle, bottom: variation in the 2D current density reconstructed from the a.c. flux parallel (red) and antiparallel (blue) to the direction indicated with a black arrow. The background current density cannot be determined from the present data and analysis, so red and blue indicate more and less current flowing with respect to an unknown 2D sheet current. All directions of narrow paths observed in sample M1 are shown: ac, [010]p, b, [100]p and d, [110]p. Scale bars, 30 μm.

  3. Tetragonal domain structure of STO is the origin of narrow paths of enhanced conductivity.
    Figure 3: Tetragonal domain structure of STO is the origin of narrow paths of enhanced conductivity.

    a, Thermal history of sample M1 with points indicating when the a.c. flux was imaged. be, The same area of sample M1 was scanned after initially cooling down from room temperature (b) and after cycling to 125 K (c), 91 K (d) and 117 K (e). Reconfiguration of the pattern of a.c. flux due to narrow paths of higher current occurred only when the sample was cycled above the STO transition at 105 K, but not below. fh, Polarized light microscopy images of different areas in sample M1 were taken at 20 K, at near-crossed polarizers (f) and at near-parallel polarizers (g,h). We found domains aligned along the crystallographic directions [100]p, [010]p, [110]p and . Scale bars, 30 μm.

  4. Schematic twin boundaries and domains in the STO crystal structure in the tetragonal phase.
    Figure 4: Schematic twin boundaries and domains in the STO crystal structure in the tetragonal phase.

    A view of STO domain structure looking along the growth direction, [001]p. a,b, The twin planes (110)p (a) and (101)p (b) result in four possible geometries (the two shown here and their rotations by 90°). The twin boundary extends in the growth direction in a whereas the boundary shown in b extends at approximately 45° to the growth axis. TiO6 octahedral rotations (circular arrows, ) along the lengthened c axis (black arrows) are exaggerated, and the exact structure of TiO6 octahedra close to the twin boundary and interface will be more complicated.

References

  1. Ohtomo, A. & Hwang, H. Y. A high-mobility electron gas at the LaAlO3/SrTiO3 heterointerface. Nature 427, 423426 (2004).
  2. Nakagawa, N., Hwang, H. Y. & Muller, D. A. Why some interfaces cannot be sharp. Nature Mater. 5, 204209 (2006).
  3. Thiel, S., Hammerl, G., Schmehl, A., Schneider, C. W. & Mannhart, J. Tunable quasi-two-dimensional electron gases in oxide heterostructures. Science 313, 19421945 (2006).
  4. Reyren, N. et al. Superconducting interfaces between insulating oxides. Science 317, 11961199 (2007).
  5. Brinkman, A. et al. Magnetic effects at the interface between non-magnetic oxides. Nature Mater. 6, 493496 (2007).
  6. Caviglia, A. D. et al. Electric field control of the LaAlO3/SrTiO3 interface ground state. Nature 456, 624627 (2008).
  7. Bell, C. et al. Dominant mobility modulation by the electric field effect at the LaAlO3/SrTiO3 interface. Phys. Rev. Lett. 103, 226802 (2009).
  8. Seri, S. & Klein, L. Antisymmetric magnetoresistance of the SrTiO3/LaAlO3 interface. Phys. Rev. B 80, 180410 (2009).
  9. Ben Shalom, M., Ron, A., Palevski, A. & Dagan, Y. Shubnikov De Haas oscillations in SrTiO3/LaAlO3 interface. Phys. Rev. Lett. 105, 206401 (2010).
  10. Ariando, et al. Electronic phase separation at the LaAlO3/SrTiO3 interface. Nature Commun. 2, 188 (2010).
  11. Dikin, D. A. et al. Coexistence of superconductivity and ferromagnetism in two dimensions. Phys. Rev. Lett. 107, 056802 (2011).
  12. Li, L., Richter, C., Mannhart, J. & Ashoori, R. C. Coexistence of magnetic order and two-dimensional superconductivity at LaAlO3/SrTiO3 interfaces. Nature Phys. 7, 762766 (2011).
  13. Bert, J. A. et al. Direct imaging of the coexistence of ferromagnetism and superconductivity at the LaAlO3/SrTiO3 interface. Nature Phys. 7, 767771 (2011).
  14. Kalisky, B. et al. Critical thickness for ferromagnetism in LaAlO3/SrTiO3 heterostructures. Nature Commun. 3, 922 (2012).
  15. Mannhart, J. & Schlom, D. G. Oxide interfaces—An opportunity for electronics. Science 327, 16071611 (2010).
  16. Cancellieri, C. et al. Electrostriction at the LaAlO3/SrTiO3 interface. Phys. Rev. Lett. 107, 056102 (2011).
  17. Ben Shalom, M., Sachs, M., Rakhmilevitch, D., Palevski, A. & Dagan, Y. Tuning spin-orbit coupling and superconductivity at the SrTiO3/LaAlO3 interface: A magnetotransport study. Phys. Rev. Lett. 104, 126802 (2010).
  18. Joshua, A., Ruhman, J., Pecker, S., Altman, E. & Ilani, S. Gate-tunable polarized phase of two-dimensional electrons at the LaAlO3/SrTiO3 interface. Proc. Natl Acad. Sci. USA 110, 96339638 (2012).
  19. Caviglia, A. D. et al. Tunable Rashba spin-orbit interaction at oxide interfaces. Phys. Rev. Lett. 104, 126803 (2010).
  20. Fête, A., Gariglio, S., Caviglia, A. D., Triscone, J. M. & Gabay, M. Rashba induced magnetoconductance oscillations in the LaAlO3−SrTiO3 heterostructure. Phys. Rev. B 86, 201105 (2012).
  21. Gardner, B. W. et al. Scanning superconducting quantum interference device susceptometry. Rev. Sci. Instrum. 72, 23612364 (2001).
  22. Huber, M. E. et al. Gradiometric micro-SQUID susceptometer for scanning measurements of mesoscopic samples. Rev. Sci. Instrum. 79, 053704 (2008).
  23. Nowack, K. C. et al. Imaging currents in HgTe quantum wells in the quantum spin Hall regime. Nature Mater. advance online publication (2013).
  24. Bert, J. A. et al. Gate-tuned superfluid density at the superconducting LaAlO3/SrTiO3 interface. Phys. Rev. B 86, 060503 (2012).
  25. Bristowe, N. C., Fix, T., Blamire, M. G., Littlewood, P. B. & Artacho, E. Proposal of a one-dimensional electron gas in the steps at the LaAlO3–SrTiO3 interface. Phys. Rev. Lett. 108, 166802 (2012).
  26. Cowley, R. A. Lattice dynamics and phase transitions of strontium titanate. Phys. Rev. 134, A981A997 (1964).
  27. Unoki, H. & Sakudo, T. Electron spin resonance of Fe3+ in SrTiO3 with special reference to the 110°K phase transition. J. Phys. Soc. Jpn 23, 546552 (1967).
  28. Cao, W. & Barsch, G. R. Landau–Ginzburg model of interphase boundaries in improper ferroelastic perovskites of D4h18 symmetry. Phys. Rev. B 41, 43344348 (1990).
  29. Schwingenschlögl, U. & Schuster, C. Interface relaxation and electrostatic charge depletion in the oxide heterostructure LaAlO3/SrTiO3. Europhys. Lett. 86, 27005 (2009).
  30. Pauli, S. A. et al. Evolution of the interfacial structure of LaAlO3 on SrTiO3. Phys. Rev. Lett. 106, 036101 (2011).
  31. Jia, C. L. et al. Oxygen octahedron reconstruction in the SrTiO3/LaAlO3 heterointerfaces investigated using aberration-corrected ultrahigh-resolution transmission electron microscopy. Phys. Rev. B 79, 081405 (2009).
  32. Stengel, M. First-principles modeling of electrostatically doped perovskite systems. Phys. Rev. Lett. 106, 136803 (2011).
  33. Seidel, J. et al. Conduction at domain walls in oxide multiferroics. Nature Mater. 8, 229234 (2009).
  34. Guyonnet, J., Gaponenko, I., Gariglio, S. & Paruch, P. Conduction at domain walls in insulating Pb(Zr0.2Ti0.8)O3 thin films. Adv. Mater. 23, 53775382 (2011).
  35. Scott, J. F., Salje, E. K. H. & Carpenter, M. A. Domain wall damping and elastic softening in SrTiO3: Evidence for polar twin walls. Phys. Rev. Lett. 109, 187601 (2012).
  36. Morozovska, A. N. et al. Impact of free charges on polarization and pyroelectricity in antiferrodistortive structures and surfaces induced by a flexoelectric effect. Ferroelectrics 438, 3244 (2012).
  37. Jalan, B., Allen, S. J., Beltz, G. E., Moetakef, P. & Stemmer, S. Enhancing the electron mobility of SrTiO3 with strain. Appl. Phys. Lett. 98, 132102132103 (2011).

Download references

Author information

  1. These authors contributed equally to this work

    • Beena Kalisky &
    • Eric M. Spanton

Affiliations

  1. Department of Applied Physics, Stanford University, Stanford, California 94305, USA

    • Beena Kalisky,
    • Hilary Noad,
    • John R. Kirtley,
    • Katja C. Nowack,
    • Yanwu Xie,
    • Harold Y. Hwang &
    • Kathryn A. Moler
  2. Department of Physics, Nano-magnetism Research Center, Institute of Nanotechnology and Advanced Materials, Bar-Ilan University, Ramat-Gan 52900, Israel

    • Beena Kalisky
  3. Department of Physics, Stanford University, Stanford, California 94305, USA

    • Eric M. Spanton,
    • Katja C. Nowack &
    • Kathryn A. Moler
  4. Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA

    • Eric M. Spanton,
    • Hilary Noad,
    • Christopher Bell,
    • Hiroki K. Sato,
    • Masayuki Hosoda,
    • Yasuyuki Hikita,
    • Harold Y. Hwang &
    • Kathryn A. Moler
  5. Department of Advanced Materials Science, The University of Tokyo, Kashiwa, Chiba 277-8561, Japan

    • Hiroki K. Sato &
    • Masayuki Hosoda
  6. Max Planck Institute for Solid State Research, D-70569 Stuttgart, Germany

    • Carsten Woltmann,
    • Georg Pfanzelt &
    • Jochen Mannhart
  7. Experimental Physics VI, Center for Electronic Correlations and Magnetism, Institute of Physics, University of Augsburg, D-86135 Augsburg, Germany

    • Rainer Jany &
    • Christoph Richter

Contributions

B.K., E.M.S., H.N., J.R.K. and K.C.N. performed the SQUID measurements. B.K. performed polarized-light measurements. B.K., E.M.S. and J.R.K. analysed the data with input from K.A.M. C.B., H.K.S., Y.X., M.H. and Y.H. grew samples H1–H5. C.W., G.P. and R.J. grew samples M1 and M2. E.M.S., B.K. and K.A.M. prepared the manuscript with input from all co-authors. H.Y.H., J.M. and K.A.M. guided the work.

Competing financial interests

The authors declare no competing financial interests.

Corresponding authors

Correspondence to:

Author details

Supplementary information

PDF files

  1. Supplementary Information (1.53MB)

    Supplementary Information

Additional data