Research ArticleCONDENSED MATTER PHYSICS

Atomic-scale visualization of surface-assisted orbital order

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Science Advances  22 Sep 2017:
Vol. 3, no. 9, eaao0362
DOI: 10.1126/sciadv.aao0362
  • Fig. 1 The crystal structure and topography of cleaved surfaces of CeCoIn5.

    (A) Schematic of the tetragonal crystal structure of CeCoIn5 with dxz-dyz OOs at the topmost cobalt plane. (B and C) Overviews of two typical cleaved surfaces in this study. Narrow terraces (10 to 50 nm) are separated by a step of ~0.76 nm. The topographic images are colorized with their derivatives to emphasize the atomically resolved lattice structures. Red arrows in (C) indicate line defects [(A): tunneling current IT = 50 pA, sample bias voltage VS = 50 mV; (B): IT = 15 pA, VS = 10 mV]. (D) Typical atomically resolved STM image taken on the surface of (B) (IT = 1 nA, VS = 50 mV). We observed an almost identical atomic lattice image on the surface of (C), as shown in Fig. 2B.

  • Fig. 2 Current-dependent topographies of the terraces in Fig. 1 (B and C).

    The white box in the images indicates the unit cell. (A) Atomically resolved topographies of the terraces from Fig. 1B (VS = 10 mV, IT = 5 and 100 nA at T = 500 mK). By increasing the current set point, new atoms appear in between original atomic sites with an interval of ~0.5 nm. (B) Atomically resolved topographies of terraces in Fig. 1C (VS = 10 mV, IT = 1, 30, and 100 nA at T = 1.7 K and at B = 5 T). An external out-of-plane magnetic field is applied merely to have better stability for STM measurement. With IT ≳ 10 nA, the shape of atoms gradually changes to a dumbbell with two lobes. The angle between the dumbbells at adjacent sites is 90°. (C) Current trace as a function of tip approaching distance Δz, measured at an atomic site in (B) with VS = 10 mV. IT shows an exponential dependence up to ~160 nA, indicating the tunneling regime. We note that Δz is the relative distance between the tip and surface. Δz = 0 corresponds to the position of the tip, where atomic contact occurs in the Iz spectrum.

  • Fig. 3 Current-dependent topographies of the surface in Fig. 1C with a line defect.

    (A) Topography of the terrace at 1.7 K with a line defect along the vertical axis of the image (IT = 10 nA, VS = 10 mV). (B) Line profiles indicated as lines a, b, and c in (A). Atomic positions in profiles a and b match very well with those in profile c in both sides of the line defect, indicating that no lattice distortion is present across the line defect. (C) Topography in the same field of view with (A) but with a shorter TSD (IT = 100 nA, VS = 10 mV). Dumbbell shapes are now visible. (D) Magnified image of the rectangle area indicated with a sky blue rectangle in (C). Ellipses help to visualize the change in the dumbbell arrangement across the line defect.

  • Fig. 4 First-principles electronic structures.

    (A and B) DOS of d orbitals of bulk (A) and surface (B) Co atoms of CeCoIn5. Positive (negative) values show the majority-spin (minority-spin) contribution. The inset of (B) shows the DOS of dxz and dyz orbitals around the Fermi energy, demonstrating unbalanced orbital occupations. (C to E) Integrated charge density at the slab surface between −0.2 and 0.0 eV in the xz and xy planes. The xy planes are chosen at 0.10 nm (D) and 0.25 nm (E) above the topmost Co plane. The maximum value to present the color-coded plot is chosen as 10, 5, and 1 e/nm3 for (C), (D), and (E), respectively.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/9/eaao0362/DC1

    fig. S1. Tunneling spectra taken on CeIn- and Co-terminated surfaces and their locations.

    fig. S2. Current-dependent cross-sectional profiles of Ce-In and Co planes.

    fig. S3. Charge density profiles of the Co atom orbitals on the Co termination.

    note S1. Excluding the possibility of tip-induced artifacts in dumbbell formation.

    note S2. Topographic similarity between our dumbbell ordered structure and the images in the study of Takahashi et al. (14) and their difference in underlying physics.

    movie S1. Evolution of dumbbell structure by decreasing the TSD.

  • Supplementary Materials

    This PDF file includes:

    • fig. S1. Tunneling spectra taken on CeIn- and Co-terminated surfaces and their locations.
    • fig. S2. Current-dependent cross-sectional profiles of Ce-In and Co planes.
    • fig. S3. Charge density profiles of the Co atom orbitals on the Co termination.
    • note S1. Excluding the possibility of tip-induced artifacts in dumbbell formation.
    • note S2. Topographic similarity between our dumbbell ordered structure and the images in the study of Takahashi et al. (14) and their difference in underlying physics.
    • Legend for movie S1

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    Other Supplementary Material for this manuscript includes the following:

    • movie S1 (.mov format). Evolution of dumbbell structure by decreasing the TSD.
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