Research Activities– Off-line Experiments –17AU012

Laser-ARPES

Tracking charge-density wave formation and detangling
dichroic signatures in VSe2
O. J. Clark1, J. M. Riley1,2, P. D. C. King1
1

School of Physics & Astronomy, University of St Andrews, St Andrews KY1 69SS, United Kingdom
2
Diamond Light Source, Harwell Campus, Didcot OX11 ODE, United Kingdom

Keywords: Transition-metal dichalcogenides (TMD), Charge Density Wave (CDW), Laser-ARPES

Group V transition-metal dichalcogenides (TMDs) have the chemical formula MX2, where M=V, Nb,
Ta and X = S, Se, Te. The metallic variants are well-known systems for supporting charge density wave
(CDW) formation [1].
The effects of the CDW on the electronic structure of 2H-NbSe2 and 2H-TaSe2 are well understood[2,3], with clear signatures in spectroscopic techniques. VSe2 is not as well studied. Its CDW is known
largely from transport measurements, with only weak potential signatures visible in ARPES thus far. Its
thermodynamic preference for the 1T-structure as opposed to the 2H-structure, as well as its complex
three-dimensional Fermi surface means that the resulting CDW phase could be rather different from that ofits 2H-structured sister compounds.

a

LH-LV
0.5

b
E-EF (eV)

i.

0.0

LH

LV

0.5

ii.

i.

-0.0

i.

-0.5

ii.

0.0

-1.0

iii.

-0.5

0.5

ii.

-1.5

0.0

0.5
0.0

-2.0

-0.5

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CL-CR

CL+CR

-0.5

-2.5

iii.

iii.

-3.0

0.5

Max

0.0
-0.5

-0.5

Min

kx (Å-1)

CL

0.0

-3.5

-1.0 -0.5 0.0 0.5 1.0

ky (Å-1)ky (Å-1)

-0.5

c

-0.8

-0.4

0.0

0.4

ky (Å-1)

0.8

CR-1.0 -0.5 0.0 0.5 1.0

kx (Å-1)

FIGURE 1. Previous APRES data taken in the CDW phase of 1T-VSe2. The data was collected at the CASIOPEE beamlineof SOLEIL, Paris, with 110eV light in linear horizontal (LH), linear vertical (LV), circular left (CL) and circular right (CR)polarized light.

Our recent ARPES measurements (Figure 1) additionally show extremely complex dichroic signatureswith both linearly and circularly polarized light of both the flat transition metal derived conduction bands
and the dispersive chalcogen derived valence bands, further motivating the need for a higher resolution- 147 -

spectroscopic study.
A dataset obtained with the 6eV laser source at HiSOR is shown in Fig. 2, probing the chalcogen p-valence
states only. This is qualitatively similar to the p-bands measured in synchrotron ARPES (Fig. 1). However,
the measured linewidth of the bands shown here is on the order of 100meV, much larger than the intrinsic
instrumental resolution [4]. We attribute this to a non-uniform cleaved surface, with a potentially curved
surface or spatially-dependent local workfunction variations leading to substantial extrinsic bandbroadening within our probing light spot area.

(a)

(b)

E-EF(eV)

0.0
-0.1
-0.2
-0.3
-0.4
-0.1

0.0

0.1

-0.1

kII (Å-1)

0.0

0.1

kII (Å-1)

FIGURE 2. Low temperature phase of VSe2 probed with 6eV light in (a) horizontal and (b) linear polarization.

Moreover, matrix elements lead to a consistent suppression of the spectral weight at the Γ point, unlike at
the higher photon energies shown in Fig. 1 (but similar to features observed in some other transition-metal
dichalcogenides [5]). This is true irrespective of the probing light polarization (Fig. 2). Ultimately, together,
these two effects hindered our efforts to track any changes in the band structure through the charge density
wave transition (TC = 110K).
In conclusion, a change of band dispersion was observed between linearly vertical and linearly horizontal
polarized light, but poor linewidths intrinsic to our samples coupled with a matrix element effectsuppressing weight at the VBM meant that neither the subtleties of the mechanism behind complex
dichroic signatures, nor that of the charge density wave formation could be addressed.

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M. Chhowalla et al., The chemistry of two-dimensional layered transition metal dichalcogenide nanosheets, Nature
Chemistry 5, 263-275 (2013)
K. Rossnagel et al., Fermi surface, charge-density-wave gap, and kinks in 2H-TaSe2, Physical Review B 72,
121103(R) (2005)
C. J. Arguello et al., Visualizing the charge density wave transition in 2H-NbSe2 in real space, Physical Review B
89, 235115 (2014)
H. Iwasawa et al., Development of laser-based scanning µ-ARPES system with ultimate energy and momentum
resolutions, Ultramicroscopy 182, 85-91 (2017)
H. Cercellier et al. Evidence for an Excitonic Insulator Phase in 1T−TiSe2, Physical Review Letters 9, 146403(2007)- 148 -

17AU013

Laser-ARPES

Observation of d electron quantum-well states in Pd(100)ultrathin films
Hidetake Tanabea , Sunsuke Sakuragia , Eike F. Schwierb ,Kenya Shimadab , and Tetsuya Satoaa

Department of Applied Physics and Physico-Informatics, Keio University, Hiyoshi, Yokohama 223-0061,
Japanb
Hiroshima Synchrotron Radiation Center, Hiroshima University, Higashi-Hiroshima 739-0046, Japan

Keywords: Pd, ultrathin film, quantum well states.

When the film thickness of the metal corresponds to the Fermi wave number, the quantum well (QW)
states are formed in the thin film[1,2]. It is reported that the QW states formed in metal thin films bring
peculiar physical properties periodically as a function of the film thickness [3]. It was also confirmed thatin the Pd (100) ultrathin films, ferromagnetism appears periodically depending on the film thickness[4,5].
In terms of the Stoner model, which is the simplest model of the ferromagnetic expression in metals, it isbelieved that density of states around εF in the Pd(100) thin film increase by the quantum confinement of
the d electron[6], and the thickness dependent ferromagnetism appears. Due to the possibility ofdeveloping new magnetic nanoscale materials whose magnetism can be controlled, this topic is of great
technical concern.
ARPES is an ideal tool for the study of QW by which it may be able to directly observe subbands
caused by QW states. In particular, it is expected that high resolution measurement can be performed in a
Laser ARPES apparatus using a laser with high coherency as a light source. In this research, we aim to
directly discuss the relation between magnetism and QW state by observing the QW state of d electron
formed in Pd (100) ultrathin film by ARPES.
All experiments were performed at the HiSOR Laser ARPES apparatus. We used Cu(100) single crystal
as a substrate of Pd ultrathin films to avoid charging. To get a clean and flat surface, we repeated cycle ofAr+ sputtering subsequent annealing at 900 K in the ultra-high vacuum chamber until clean Auger electron
spectroscopy (AES) and sharp low energy electron diffraction (LEED) spots are obtained. Pd was
deposited on the Cu(100) substrate at the rate of 0.5 monolayers(ML) per minutes at room temperature. We
estimated thickness of Pd ultrathin films by means of a reflected high energy electron diffraction (RHEED)
oscillation technique [Fig. 1(a)].
Figure 1(b) shows LEED image of Pd film on Cu(100) substrate. Four-fold sharp spots were observed,
indicating the epitaxial grew of Pd on Cu(100).

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(b)Intensity(a.u.)

(a)

0

100

200

300

400

500

600

Time(sec)

FIGURE 1. (a) RHHED oscillation during the deposition of the Pd on Cu(100) substrate. The oscillatory change in the
RHHED intensity was observed, and the film thickness was estimated from the period of oscillation. (b) LEED image of thePd film of Cu (100) substrate. Sharp spots were observed.

Figure 2 shows image plots of the obesrved band disperisons along the ΓM line of the 5.2 ML Pd film
taken with hν = 1.4 eV at 30 K. Clear band dispersion was not observed. It may be due to that thephotoionization cross section became small by using the low photon energy light hν = 1.4 eV. Therefore, It
is expected that clear band dispersion can be observed using higher photon energy light.

Binding energy (eV)

0.0

-0.2
-0.4
-0.6

-10

-5

0 -1 510k (Å )FIGURE 2. Band dispersion of the 5.2ML Pd film taken with hν = 1.4 eV at 30 K. Clear band dispersion was notobserved. This nay be due to that the photoionization cross section became small by using the low photon energy.

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A. Shikin, O.Reder, G. Prudnikova et al., Phys. Rev B 65, 075403 (2002).
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J. Li, M. Przybylski, Y. He, and Y. Z. Wu, Phys. Rev B 82, 214406 (2010).
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S. Aihara, H. Kageshima, T.Sakai, and T. Sato, J. Appl. Phys. 112, 073910 (2012).
S.K. Saha, S. Manna, M. Przybylski, V.S. Stepamyuk, and J. Kirschner, Phys. Rev. B 90, 081402(R) (2014).

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17BU019

Micro-ARPES

Micro-ARPES study of a Weyl semimetal
candidate MoTe2
Yoshitaka Nakataa, Kentaro Kasaia, Eike F. Schwier b, Akihiro Inob, Keiji Uenoc,
Nobuyuki Aokia and Kazuyuki SakamotoaaDepartment of Materials Science, Chiba University, Chiba 263-8522, Japan
Hiroshima Synchrotron Radiation Center, Hiroshima University, Higashi-Hiroshima 739-0047, Japanc
Department of Chemistry, Saitama University, Saitama 338-8570, Japanb

Keywords: Weyl semimetal, Fermi arc, Topological surface states, MoTe2, Micro-ARPES.

Three-dimensional (3D) topological Weyl semimetals are a new class of quantum materials that show
peculiar electronic structures, unique quantum phenomena such as a chiral anomaly, anomalous Hall effectand negative magnetoresistance, and have a broad potential for device applications [1-7]. The electronic
states of Weyl semimetals are characterized by the topology of the bulk bund structure, which leads to thepresence of bands with liner dispersion, like graphene, around the nodes called Weyl points. Together withthis graphene like property, Weyl semimetals show unique spin texture originating from the Weyl points
that act as monopoles in the momentum space. These monopoles-like Weyl points always show up in pair,and the plane between a pair of Weyl points has Chern number C=1. Owing to this unique bulk electronic
structure, the surface states of Weyl semimetals, which are topologically protected, show unclosed Fermi
surface called Fermi arc at the Fermi level (EF). This Fermi arc is a hallmark of 3D Weyl semimetals. The
Weyl semimetals can be further classified into two types, the type-I in which the Lorentz symmetry isapproximately preserved and the type-II in which this symmetry is broken and have tilted Weyl cones withboth electron and hole pockets at the EF. Due to the difference in these two types, the Weyl Fermions oftype-II leads to further intriguing physical properties [8].

FIGURE 1. Crystal structures of MoTe2 crystals. The most stable 2H has a semiconducting electronic states and the 1T’ hasa semimetal band structure. Td appears by cooling the 1T’ phase.

Recently, type-II Weyl semimetals has been proposed to exist in layered transition metal
dichalcogenides, such as MoTe2 [8-12]. The most stable crystal structure of MoTe2 is the hexagonal 2H
phase that shows a semiconducting electronic property, and the next most stable structure is the monoclinic
1T’ phase that shows a semimetal electronic states. The orthorhombic noncentrosymmetric Td phase,
which appears by cooling the centrosymmetric 1T’ down below 250 K, is proposed to be a Weyl semimetal.
The Td-MoTe2 would have 8 Weyl points that are located at binding energies of 6 meV and 50 meV above
EF, and the Fermi arc is predicted to be looked between the hole and electron pockets of the Fermi surface.

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However, also several angle-resolved photoelectron spectroscopy (ARPES) measurements have been
performed on the Td-MoTe2, no strong evidence on the Fermi arc has been reported so far. The main
reason for the lack of observation would be come from the quality of the sample. That is, it is difficult togrow 1T’- MoTe2 that is large enough for normal ARPES measurements (ARPES using light whose spot
size is in the order of 100 m).

FIGURE 2. The Fermi surface of the Td-MoTe2 single crystal obtained using the laser-based micro-ARPES apparatus at
HiSOR.

In order to solve this problem and to observe the Fermi arc and/or the topological surface states, wehave performed laser-based micro-ARPES measurements at HiSOR. The light spot of this apparatus isapproximately 5 m. By checking the ARPES spectra at different sample positions, we notice that ourTd-MoTe2 sample contains a lot of domains, and that the size of one single crystal domain isapproximately 30x30 m2. Figure 2 shows the Fermi surface of the Td-MoTe2 sample, measured aftercleaving in ultra high vacuum. We clearly observed the electron and hole pockets, and trace of the Fermiarc (indicated by the yellow circle in Fig. 2). Although the Fermi arc is not clear (its intensity comparable
to those reported in the literature), we conclude that MoTe2 would be a type-II Weyl semimetal based on
the band dispersion of this “Fermi arc”. In this paper, we will first compare the present results and thosereported previously, and will discuss this “Fermi arc” and the topological surface state in more details.

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