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  • Letter

Current-driven insulator-to-metal transition without Mott breakdown in Ca2RuO4

Davide Curcio, Charlotte E. Sanders, Alla Chikina, Henriette E. Lund, Marco Bianchi, Veronica Granata, Marco Cannavacciuolo, Giuseppe Cuono, Carmine Autieri, Filomena Forte, Guerino Avallone, Alfonso Romano, Mario Cuoco, Pavel Dudin, Jose Avila, Craig Polley, Thiagarajan Balasubramanian, Rosalba Fittipaldi, Antonio Vecchione, and Philip Hofmann
Phys. Rev. B 108, L161105 – Published 16 October 2023
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    Abstract

    The electrical control of a material's conductivity is at the heart of modern electronics. Conventionally, this control is achieved by tuning the density of mobile charge carriers. A completely different approach is possible in Mott insulators such as Ca2RuO4, where an insulator-to-metal transition (IMT) can be induced by a weak electric field or current. While the driving force of the IMT is poorly understood, it has been thought to be a breakdown of the Mott state. Using in operando angle-resolved photoemission spectroscopy, we show that this is not the case: The current-induced conductivity is caused by the formation of in-gap states with only a minor reorganization of the Mott state. Electronic structure calculations show that these in-gap states form at the boundaries of structural domains that emerge during the IMT. At such boundaries, the overall gap is drastically reduced, even if the structural difference between the domains is small and the individual domains retain their Mott character. The inhomogeneity of the sample is thus key to understanding the IMT, as it leads to a nonequilibrium semimetallic state that forms at the interface of Mott domains.

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    • Received 13 February 2023
    • Revised 9 June 2023
    • Accepted 2 October 2023

    DOI:https://doi.org/10.1103/PhysRevB.108.L161105

    ©2023 American Physical Society

    Physics Subject Headings (PhySH)

    1. Research Areas
    Metal-insulator transition
    1. Physical Systems
    Mott insulators
    1. Techniques
    Angle-resolved photoemission spectroscopyDensity functional theory
    Condensed Matter, Materials & Applied Physics

    Authors & Affiliations

    Davide Curcio1, Charlotte E. Sanders2, Alla Chikina1, Henriette E. Lund1, Marco Bianchi1, Veronica Granata3, Marco Cannavacciuolo3, Giuseppe Cuono4, Carmine Autieri4, Filomena Forte5, Guerino Avallone3, Alfonso Romano3, Mario Cuoco5, Pavel Dudin6, Jose Avila6, Craig Polley7, Thiagarajan Balasubramanian7, Rosalba Fittipaldi5, Antonio Vecchione5, and Philip Hofmann1,*

    • 1Department of Physics and Astronomy, Aarhus University, DK-8000 Aarhus C, Denmark
    • 2Central Laser Facility, STFC Rutherford Appleton Laboratory, Harwell OX11 0QX, United Kingdom
    • 3Dipartimento di Fisica “E. R. Caianiello,” Universitá degli Studi di Salerno, via Giovanni Paolo II 132, I-84084 Fisciano (Sa), Italy
    • 4International Research Centre Magtop, Institute of Physics, Polish Academy of Sciences, Aleja Lotników 32/46, PL-02668 Warsaw, Poland
    • 5CNR-SPIN, via Giovanni Paolo II 132, I-84084 Fisciano, Italy
    • 6Synchrotron SOLEIL, FR-91192 Gif-sur-Yvette, France
    • 7MAX IV Laboratory, Lund University, SE-211 00 Lund, Sweden

    • *philip@phys.au.dk

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    Issue

    Vol. 108, Iss. 16 — 15 October 2023

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    • Figure 1
      Figure 1

      (a) Sketch of the experiment. The current-induced voltage drop results in local photoemission spectra that are displaced in energy, following the local potential landscape across the sample. (b) Optical microscope image of the sample. The contacts are seen on the left- and right-hand side. The area explored by ARPES is marked by an orange square. The inset shows a map of the photoemission intensity inside this area. (c) Externally measured I/V curve and (d) local I/V curve obtained from averaging the observed energy shift between the spectra within the black square marked in the inset of (b) [26]. The red markers indicate the current values for which data are displayed in Figs. 2 and 3.

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    • Figure 2
      Figure 2

      Photoemission spectra across the IMT: (a)–(d) Photoemission intensity at a binding energy of 2.37 eV for I0, I1, I2, I4 [see markers in Fig. 1]. The surface Brillouin zone is superimposed. The green line marks the direction and range of the spectra in Fig. 3. The red arrow marks a circular constant energy contour stemming from the dxy states. (e)–(h) Intensity as a function of energy along the Y¯S¯ direction. (i)–(l) Angle-integrated energy distribution curves for no current and the three currents, integrated along the green line in (a). The photoemission intensity at low binding energy has been fitted by two peaks, A and B, in the same way as in Ref. [28]. The peak positions are also marked in (i)–(l).

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    • Figure 3
      Figure 3

      Mapping the IMT throughout the data set. (a) In-gap photoemission intensity across the sample for selected currents marked in Fig. 1. The integration region is given by the red rectangles in (b). (b) Corresponding photoemission spectra at the location marked by a white rectangles in (a). (c) Angle-integrated energy distribution curves obtained from the spectra in (b).

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    • Figure 4
      Figure 4

      (a) Sketch of the Ca2RuO4 bulk crystal structures and corresponding density of states (DOS). S and S′ have unit cells with a different c-axis lengths. S and S′ are insulating with the indicated gaps but the band alignment leads to a smaller effective gap at the interface between S and S′. (b) Superlattice formed by the S and S′ unit cells separated by an interface unit cell with intermediate properties (gray octahedra). The corresponding DOS shows a substantial reduction of the charge gap. The zero of the energy scale is fixed to the valence band maximum of the superlattice throughout this Letter.

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