Charge density wave phase of ${\mathrm{VSe}}_{2}$ revisited

Charge density wave phase of ${VSe}_{2}$ revisited

Wouter Jolie, Timo Knispel, Niels Ehlen, Konstantin Nikonov, Carsten Busse, Alexander Grüneis, and Thomas Michely

Phys. Rev. B 99, 115417 – Published 12 March 2019

Abstract

Scanning tunneling microscopy and spectroscopy are used to image the charge density wave at the surface of cleaved ${VSe}_{2}$ and to probe its local density of states at 5 K. The main features in the spectrum are linked to the contributions of the $p$ -like and $d$ -like bands of ${VSe}_{2}$ found in angle-resolved photoemission spectroscopy and tight-binding calculations. Different from previous tunneling spectroscopy work, we find a narrow partial gap at the Fermi level that we associate with the charge density wave phase. The energy scale of the gap found in the experiment is in good agreement with the charge density wave transition temperature of ${VSe}_{2}$ , under the assumption of weak electron-phonon coupling, consistent with the Peierls model of Fermi surface nesting. The role of defects is investigated, which reveals that the partial gap in the density of states and hence the charge density wave itself is extremely stable, though the order, phase, and amplitude of the charge density waves on the surface are strongly perturbed by defects.

Received 2 October 2018
Revised 21 January 2019

DOI:https://doi.org/10.1103/PhysRevB.99.115417

Physics Subject Headings (PhySH)

Charge density waves

Transition metal dichalcogenides

Angle-resolved photoemission spectroscopy Scanning tunneling spectroscopy Tight-binding model

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Wouter Jolie^1,2,*, Timo Knispel¹, Niels Ehlen¹, Konstantin Nikonov^1,3, Carsten Busse^1,2,4, Alexander Grüneis¹, and Thomas Michely¹

¹II. Physikalisches Institut, Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany
²Institut für Materialphysik, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Straße 10, 48149 Münster, Germany
³Kurnakov Institute of General and Inorganic Chemistry of the Russian Academy of Sciences, 119991 Moscow, Russia
⁴Department Physik, Universität Siegen, 57068 Siegen, Germany

^*wjolie@ph2.uni-koeln.de

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Vol. 99, Iss. 11 — 15 March 2019

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Figure 1
In-plane structure of ${VSe}_{2}$ : (a) STM image of ${VSe}_{2}$ ( $U = - 0.75 V$ , $I = 0.2$ nA, image size $6.2 \times 6.2 {nm}^{2}$ ). (b) STM image recorded with a different tip showing another atomic contrast ( $U = - 0.2 V$ , $I = 0.5$ nA, image size $5.3 \times 5.3 {nm}^{2}$ ). (c) FT of the STM image shown in (a). The large arrow represents a reciprocal-lattice vector of ${VSe}_{2}$ , while the small arrow represents a reciprocal-lattice vector of the CDW. (d) Ball model of the $(4 \times 4)$ structure, in which the balls denote atoms and the different colors visualize the superstructure. The primitive unit cell of the lattice and the superstructure are displayed with a continuous and dashed rhomboid, respectively.
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Figure 2
Influence of the tip on the CDW modulation: (a) STM image of ${VSe}_{2}$ ( $U = - 0.2 V$ , $I = 0.2$ nA, image size $8.5 \times 8.5 {nm}^{2}$ ). Two pronounced tip changes are marked with arrows. (b) FT of region I shown in (a). (c) FT of region II. (d) Radial average of both FTs. The CDW and atomic peaks are labeled.
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Figure 3
LDOS of ${VSe}_{2}$ : (a) STS spectrum of ${VSe}_{2}$ ( $U_{stab} = - 1 V$ , $I_{stab} = 1$ nA). (b) Normalized STS spectrum. We find peaks around $- 0.1$ and 0.75 eV, dips around the Fermi level and 0.45 eV, and shoulders at $- 0.8$ and $- 0.5 eV$ .
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Figure 4
Tight-binding calculation of the bands and DOS of ${VSe}_{2}$ : (a) Band structure of ${VSe}_{2}$ in high-symmetry directions. The color scale encodes the orbital character of the bands ranging from pure $d$ - (yellow) to pure $p$ -orbital character (dark blue). ARPES intensity maxima used for the tight-binding fit are denoted by red colored dots. (b) Comparison of the tight-binding calculated DOS (medium blue line) and the STS spectrum (green dots). The contribution of $d$ -states to the tight-binding DOS is shown as a yellow line, while the contribution of the $p$ -states is shown as a dark blue line.
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Figure 5
CDW gap in STS spectra: High-resolution STS spectrum of ${VSe}_{2}$ ( $U_{stab} = 0.2 V$ , $I_{stab} = 3.2$ nA). We find again the features mentioned above, with a pronounced gap around the Fermi level. The green dots represent a third-order polynomial fit to the data in the vicinity of the Fermi energy, but neglecting the gap. The inset shows the difference between the STS spectrum and the polynomial fit, ${DOS}_{CDW}$ , which is then fitted with a Gaussian (red) to extract the CDW gap $2 Δ$ from its FWHM.
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Figure 6
Defects in ${VSe}_{2}$ : (a) STM image containing both point defects and the CDW ( $U = - 0.3 V$ , $I = 0.21$ nA, image size $20 \times 20 {nm}^{2}$ ). The number of defects per $(1 \times 1)$ unit cell amounts to 0.8%. (b) FT of (a). The spot corresponding to the atomic lattice and the CDW are well visible. (c) Corresponding STS map recorded at $U = - 0.05 V$ . (d) STM image from a ${VSe}_{2}$ crystal with a large defect density amounting to 2.0% ( $U = 0.355 V$ , $I = 0.1$ nA, image size $20 \times 20 {nm}^{2}$ ). (e) FT of (d), showing six sharp spots corresponding to the atomic lattice. Six diffuse features rotated by $30^{\circ}$ become visible when enhancing the contrast; see the inset. (f) Normalized STS spectrum measured at the position marked by a blue dot in (d), showing a pronounced CDW gap, although the CDW is barely visible at the location of the spectrum acquisition ( $U_{stab} = 0.5 V$ , $I_{stab} = 0.2$ nA).
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Physical Review B

covering condensed matter and materials physics

Charge density wave phase of ${VSe}_{2}$ revisited

Wouter Jolie, Timo Knispel, Niels Ehlen, Konstantin Nikonov, Carsten Busse, Alexander Grüneis, and Thomas Michely

Phys. Rev. B 99, 115417 – Published 12 March 2019

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Physical Review B

covering condensed matter and materials physics

Charge density wave phase of VSe2 revisited

Wouter Jolie, Timo Knispel, Niels Ehlen, Konstantin Nikonov, Carsten Busse, Alexander Grüneis, and Thomas Michely

Phys. Rev. B 99, 115417 – Published 12 March 2019

Abstract

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Charge density wave phase of ${VSe}_{2}$ revisited