Thickness-dependent electronic structure in ultrathin ${\mathrm{LaNiO}}_{3}$ films under tensile strain

Thickness-dependent electronic structure in ultrathin ${LaNiO}_{3}$ films under tensile strain

Hyang Keun Yoo, Seung Ill Hyun, Young Jun Chang, Luca Moreschini, Chang Hee Sohn, Hyeong-Do Kim, Aaron Bostwick, Eli Rotenberg, Ji Hoon Shim, and Tae Won Noh

Phys. Rev. B 93, 035141 – Published 29 January 2016

Abstract

We investigated electronic-structure changes of tensile-strained ultrathin $LaNi O_{3}$ (LNO) films from ten to one unit cells (UCs) using angle-resolved photoemission spectroscopy (ARPES). We found that there is a critical thickness $t_{c}$ between four and three UCs below which Ni $e_{g}$ electrons are confined in two-dimensional space. Furthermore, the Fermi surfaces (FSs) of LNO films below $t_{c}$ consist of two orthogonal pairs of one-dimensional (1D) straight parallel lines. Such a feature is not accidental as observed in constant-energy surfaces at all binding energies, which is not explained by first-principles calculations or the dynamical mean-field theory. The ARPES spectra also show anomalous spectral behaviors, such as no quasiparticle peak at the Fermi momentum but fast band dispersion comparable to the bare-band one, which is typical in a 1D system. As its possible origin, we propose 1D FS nesting, which also accounts for FS superstructures observed in ARPES.

Received 27 October 2014
Revised 4 November 2015

DOI:https://doi.org/10.1103/PhysRevB.93.035141

Physics Subject Headings (PhySH)

Electronic structure Surface states

Oxides

Angle-resolved photoemission spectroscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Hyang Keun Yoo^1,2, Seung Ill Hyun³, Young Jun Chang^4,5, Luca Moreschini⁵, Chang Hee Sohn^1,2, Hyeong-Do Kim^1,2,*, Aaron Bostwick⁵, Eli Rotenberg⁵, Ji Hoon Shim^3,6, and Tae Won Noh^1,2

¹Center for Correlated Electron Systems, Institute for Basic Science (IBS), Seoul 151-747, Republic of Korea
²Department of Physics and Astronomy, Seoul National University, Seoul 151-747, Republic of Korea
³Department of Chemistry, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea
⁴Department of Physics, University of Seoul, Seoul 130-743, Republic of Korea
⁵Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
⁶Division of Advanced Nuclear Engineering, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea

^*hdkim6612@snu.ac.kr

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Vol. 93, Iss. 3 — 15 January 2016

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Figure 1
(a)–(f) FSs in the ΓXM (upper panel) and the ZRA (lower panel) planes of tensile-strained LNO films from ten to one UCs obtained by ARPES. As the thickness changes from four to three UCs, we can see an abrupt fermiology change, i.e., the FSs on both symmetry planes become nearly identical and consist of two parallel straight lines along the $k_{x}$ and the $k_{y}$ axes. (g)–(l) ARPES images along the ΓXM (upper panel) and the ZRA (lower panel) lines. To show the changes in band dispersions by thickness control more clearly, the relative intensity maxima in momentum distribution curves (MDCs) or energy distribution curves (EDCs) are plotted with empty circles and triangles, respectively, in (g) and (k). As the thickness is reduced from four to three UCs, the band dispersions change dramatically from those of the ten-UC film. Additionally, they become nearly identical along the ΓXM and the ZRA lines. Thus, a critical thickness $t_{c}$ between four and three UCs is identified at which the electronic structures of the LNO films under tensile strain qualitatively show a different nature.
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Figure 2
(a) MDC stack plots of the band dispersions for two UCs along the XΓX (upper panel) and the MXM (lower panel) lines. The blue circles indicate the peak positions in the MDCs. (b) The same as in (a) but for ten UCs. The dashed lines are guidelines for the comparison of the band dispersions along the XΓX and the MXM lines. In two UCs, they are almost identical, whereas they are quite different in ten UCs. Constant-energy-surface (CES) maps for two UCs (c) on the ΓXM and (d) on the ZRA planes at a binding energy ranging from 0 to 0.4 eV. All CES maps show pairs of parallel straight lines along the $k_{x}$ and the $k_{y}$ axes, irrespective of binding energy. These results strongly suggest the quasi-1D nature of the electronic structures in the LNO films below $t_{c}$ . CES maps for one UC (e) from the generalized gradient approximation (GGA) and (f) from the dynamical mean-field theory combined with the GGA (GGA + DMFT). The $e_{g}$ -orbital contributions are shown by rainbow image scales in (e). The CES maps from the GGA calculations show a clear 2D nature. The fermiology from the GGA + DMFT calculations looks similar to that from the ARPES data; however, a slight increase in the binding energy reveals two-dimensional features. The similar cross shapes of the CESs at higher binding energies $(\geq 0.2 eV)$ originate from incoherent excitations.
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Figure 3
(a) Band dispersion and (b) EDC stack plot along the MXM line for ten UCs. The band structure and a quasiparticle peak at $E_{F}$ are well defined. (c) and (d) The same as in (a) and (b) but for two UCs. (e) Symmetrized EDC plots with respect to the Fermi level. Below $t_{c}$ , the band dispersion is much faster but more diffuse than that above $t_{c}$ . No well-defined quasiparticle peak is observed, and the ARPES spectral weight at the Fermi momentum is strongly suppressed. The black solid lines and the red empty circles represent the dispersion from the GGA and the GGA + DMFT calculations, respectively. For the ten-UC film, the dispersion is well described by the GGA + DMFT calculations with strong electron renormalization. In contrast, for the two-UC film, it is well fit by the GGA calculations.
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Figure 4
(a) FS map for three UCs with a nesting vector Q ∼ (1/4,0,0) which shows a corresponding FS superstructure. The map was symmetrized with respect to the mirror planes and plotted using a logarithmic scale. Note that a $90^{\circ}$ -rotated domain would equally populate in the film. (b) Schematics of a 1D FS and its superstructure. Because the FS is not fully gapped, it is represented by thick dashed lines, and its superstructure is represented by thin ones. If we consider equivalent domains, then the final FS and its superstructure will correspond to the rightmost one. (c) The schematic 1D spin-ordering model with $Q = (1 / 4, 0, 0)$ to explain the diffuse but fast 1D band dispersion. In this diagram, a hole produced in an ARPES process is free to hop along the $y$ axis, but it costs significant energy to move along the $x$ axis due to spin-order breaking.
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Physical Review B

covering condensed matter and materials physics

Thickness-dependent electronic structure in ultrathin ${LaNiO}_{3}$ films under tensile strain

Hyang Keun Yoo, Seung Ill Hyun, Young Jun Chang, Luca Moreschini, Chang Hee Sohn, Hyeong-Do Kim, Aaron Bostwick, Eli Rotenberg, Ji Hoon Shim, and Tae Won Noh

Phys. Rev. B 93, 035141 – Published 29 January 2016

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Physical Review B

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Thickness-dependent electronic structure in ultrathin LaNiO3 films under tensile strain

Hyang Keun Yoo, Seung Ill Hyun, Young Jun Chang, Luca Moreschini, Chang Hee Sohn, Hyeong-Do Kim, Aaron Bostwick, Eli Rotenberg, Ji Hoon Shim, and Tae Won Noh

Phys. Rev. B 93, 035141 – Published 29 January 2016

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Thickness-dependent electronic structure in ultrathin ${LaNiO}_{3}$ films under tensile strain