Interplay of electron-electron and electron-phonon interactions in the low-temperature phase of $1T\ensuremath{-}{\mathrm{TaS}}_{2}$

Interplay of electron-electron and electron-phonon interactions in the low-temperature phase of $1 T - {TaS}_{2}$

Doohee Cho, Yong-Heum Cho, Sang-Wook Cheong, Ki-Seok Kim, and Han Woong Yeom

Phys. Rev. B 92, 085132 – Published 18 August 2015

Abstract

We investigate the interplay of the electron-electron and electron-phonon interactions in the electronic structure of an exotic insulating state in the layered dichalcogenide $1 T − {TaS}_{2}$ , where the charge-density-wave (CDW) order coexists with a Mott correlation gap. Scanning tunneling microscopy and spectroscopy measurements with high spatial and energy resolution determine unambiguously the CDW and the Mott gap as 0.20–0.24 eV and 0.32 eV, respectively, through the real space electron phases measured across the multiply formed energy gaps. An unusual local reduction of the Mott gap is observed on the defect site, which indicates the renormalization of the on-site Coulomb interaction by the electron-phonon coupling as predicted by the Hubbard-Holstein model. The Mott-gap renormalization provides insight into the disorder-induced quasimetallic phases of $1 T − {TaS}_{2}$ .

Received 2 December 2014

DOI:https://doi.org/10.1103/PhysRevB.92.085132

Authors & Affiliations

Doohee Cho¹, Yong-Heum Cho^2,3, Sang-Wook Cheong^3,4, Ki-Seok Kim², and Han Woong Yeom^1,2,*

¹Center for Artificial Low Dimensional Electronic Systems, Institute for Basic Science (IBS), 77 Cheongam-Ro, Pohang 790-784, Republic of Korea
²Department of Physics, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Republic of Korea
³Laboratory for Pohang Emergent Materials, Pohang University of Science and Technology, Pohang 790-784, Korea
⁴Rutgers Center for Emergent Materials and Department of Physics and Astronomy, Rutgers University, Piscataway, New Jersey 08854, USA

^*yeom@postech.ac.kr

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Vol. 92, Iss. 8 — 15 August 2015

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Figure 1
(a) STM image (tunneling current $I_{t} = 100$ pA, sample bias $V_{s} = + 0.2 V$ and $50 \times 50 {nm}^{2})$ of $1 T − {TaS}_{2}$ at 78 K displaying long-range ordered CDWs. The inset shows its corresponding 2D Fourier transform. Orange and purple hexagons [parallelograms in Fig. 1] indicate the first Brillouin zone (unit cells in real space) of $p (\sqrt{13} \times \sqrt{13}) R 13.9^{\circ}$ C-CDW and $1 \times 1$ normal states, respectively. (b) STM image ( $3.5 \times 3.5 {nm}^{2}$ and $30 \times 30$ pixel) acquired during $d I / d V$ measurements ( $I_{t} = 100$ pA and $V_{S} = - 1.20$ V). The superimposed sketches highlight the units of the CDW.
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Figure 2
(a) Normalized $d I / d V$ spectrum acquired along the green arrow in Fig. 1. (b) A collection of 900 normalized $d I / d V$ (upper part) and raw $d I / d V$ (lower part) spectra over the area shown in Fig. 1. The highlighted spectra (red, orange, green, and black) are obtained at the marked positions in Fig. 2. The black dashed curve was acquired at 273 K. (c) Previously reported spectra, acquired by STS [7], ARPES, and tr-ARPES [23] for the C-CDW phase. The red (blue) triangle indicates the U(L)HS.
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Figure 3
(a) Spatially averaged STS spectrum. The color scale indicates the normalized correlation coefficient (NCC) as referenced by the image at +0.22 eV. The red and blue curves are also averaged spectra which are acquired in the enhanced and suppressed region in the LDOS map of the UHS, respectively. The black dots denote the reductions of the LDOS intensity due to the gap opening. The black arrows mark the subband onsets (shoulders or peaks) close to the dips. The green arrows indicate the phase flip between the onsets. (b) LDOS maps of the states marked with arrows in Fig. 3 on a $3.5 \times 3.5 {nm}^{2}$ area. The frame colors of the maps represent their NCC.
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Figure 4
Filled (a) and empty (b) state STM images ( $I_{t} = 100$ pA, $V_{s} = \mp$ 0.20 eV, and $52 \times 52 {nm}^{2}$ ) of the $1 T − {TaS}_{2}$ surface with point defects marked by the circles. (c) The enlarged STM images of two types of defects marked by dashed boxes in Figs. 4 and 4. (d) Normalized $d I / d V$ spectra of two different defect types. The colors of the circles in the STM image and the spectra indicate their types.
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Figure 5
Filled (a) and empty (b) state STM images ( $I_{t} = 100$ pA, $V_{s} = \mp$ 0.20 eV and $3.5 \times 5.5 {nm}^{2}$ ) of type-A defect on the $1 T − {TaS}_{2}$ surface, marked by the triangles. (c) Normalized differential conductance map measured along the white dashed arrow in Figs. 5 and 5. The tunneling conditions are $I_{t} = 300$ pA and $V_{S}$ = –0.80 V. (d) Representative normalized $d I / d V$ spectra acquired at the CDW crests denoted by triangles in Fig. 5. The black arrows indicate the subband splittings which are similar to the pristine CDW clusters.
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Interplay of electron-electron and electron-phonon interactions in the low-temperature phase of $1 T - {TaS}_{2}$

Doohee Cho, Yong-Heum Cho, Sang-Wook Cheong, Ki-Seok Kim, and Han Woong Yeom

Phys. Rev. B 92, 085132 – Published 18 August 2015

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Physical Review B

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Interplay of electron-electron and electron-phonon interactions in the low-temperature phase of 1T−TaS2

Doohee Cho, Yong-Heum Cho, Sang-Wook Cheong, Ki-Seok Kim, and Han Woong Yeom

Phys. Rev. B 92, 085132 – Published 18 August 2015

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Interplay of electron-electron and electron-phonon interactions in the low-temperature phase of $1 T - {TaS}_{2}$