Abstract
Topologically nontrivial materials host protected edge states associated with the bulk band inversion through the bulkedge correspondence. Manipulating such edge states is highly desired for developing new functions and devices practically using their dissipationless nature and spinmomentum locking. Here we introduce a transitionmetal dichalcogenide VTe_{2}, that hosts a charge density wave (CDW) coupled with the band inversion involving V3d and Te5p orbitals. Spin and angleresolved photoemission spectroscopy with firstprinciples calculations reveal the huge anisotropic modification of the bulk electronic structure by the CDW formation, accompanying the selective disappearance of Diractype spinpolarized topological surface states that exist in the normal state. Thorough three dimensional investigation of bulk states indicates that the corresponding band inversion at the Brillouin zone boundary dissolves upon the CDW formation, by transforming into anomalous flat bands. Our finding provides a new insight to the topological manipulation of matters by utilizing CDWs’ flexible characters to external stimuli.
Introduction
Since the discovery of topological insulators, a wide variety of topological phases have been intensively developed and established in realistic materials^{1,2}. The upcoming target is to explore the guiding principles for the manipulation of these topological states. A key parameter to characterize the topological nature of materials is the band inversion realized by the crossing and anticrossing of energy bands with opposite parities. In bulk materials, the control of band inversion has been mostly done through the tuning of spinorbit coupling (SOC) by element substitution^{3,4}, with some exceptions related to the topological crystalline insulators^{5}. In the present work, we focus on charge density wave (CDW), i.e., the spontaneous modulation of charge density and lattice that modifies the periodicity and symmetry of the host crystal. In TaS_{2}, for example, the StarofDavid type CDW superstructure induces the effective narrowing of valence bands thus causing the Mott transition^{6,7,8}. From this viewpoint, CDW could also modify the band structures that involve the band inversion, and induce topological change. Moreover, it is worth noting that CDW can be flexibly controlled by external stimuli.
The transition metal dichalcogenides (TMDCs) are a wellknown family of layered materials that host a variety of CDWs reflecting their quasitwo dimensionality^{9}. Manipulations of CDW states have been intensively investigated and realized, especially in the aforementioned archetypal material TaS_{2}, by various stimuli, such as pressure^{7}, electric field^{10}, and optical pulse^{11,12}. The feasibility of CDW thus paves the way toward triggering the exotic phase transitions and generating new functions. More recently, there has been increasing interest in SOC effect in TMDCs^{13,14,15,16,17,18}. Especially in the tellurides, (Mo,W)Te_{2}^{14,15,16} and (Ta,Nb)IrTe_{4}^{17,18} are reported to be topological Weyl semimetals. Here, the peculiar quasionedimensional chainlike structures inherent to tellurides, as well as their stronger SOC compared with selenides and sulfides, are essential in realizing the topologically nontrivial states. In this stream, we focus on the telluride material VTe_{2}, to investigate the interplay of band topology and CDW instability.
VTe_{2} has CdI_{2} structure in the hightemperature normal 1T phase, consisting of trigonal layers formed by edgesharing VTe_{6} octahedra (Fig. 1a). With cooling, it undergoes a phase transition to the CDW phase at around 475 K, appearing as a jump in the temperaturedependent resistivity^{19}. The resulting CDW state exhibits a (3 × 1 × 3) superstructure characterized by double zigzag chains of vanadium atoms (Fig. 1d; hereafter we refer it as 1T″ phase, see also Supplementary Note 1)^{19,20,21}. This superstructure commonly appears in groupV transition metal ditellurides MTe_{2} (M = V, Nb, Ta)^{20,22}. Its relation with CDW has been discussed to originate from the peculiar chemical σbonding among t_{2g} dorbitals connecting three adjacent M sites, in, for example, other TMDCs including TaS_{2}^{23,24} and purple bronze AMo_{6}O_{17} (A = Na, K)^{25,26,27}. Previous band calculation on NbTe_{2}, on the other hand, implies the importance of Fermi surface nesting and electronphonon coupling as the origin of CDW with 1T″ lattice distortion^{28}. However, the modifications of electronic structure by this CDW transition have not been systematically investigated so far. It is worth noting that the contraction of metalmetal bond length (Δa_{max}/a) via the CDW transition in VTe_{2} is fairly large (~9.1%)^{20} and comparable with that in TaS_{2} (~7.0%)^{29}. We also note that the CDW in VTe_{2} can be optically manipulated, as reported by recent timeresolved diffraction studies^{30,31}.
In this Article, we investigate the electronic structures of VTe_{2}, by employing spin and angleresolved photoemission spectroscopy (ARPES) and firstprinciples calculations. We focus on the modifications of the bulk bands through the CDW formation, and its relevance to the Dirac surface states stemming from the topological band inversion. Experimentally it is not easy to perform highly precise ARPES measurement on the normal 1T phase of pristine VTe_{2} (>475 K), since such a high temperature may cause the sample degradation. Therefore we start from TiTe_{2} showing a simple 1T structure down to the lowest temperature^{32}, and develop the single crystalline V_{1−x}Ti_{x}Te_{2} to access both 1T and 1T″ phases at appropriate temperatures. Figure 1f displays the schematic electronic phase diagram of V_{x}Ti_{1−x}Te_{2}, based on the temperaturedependent ARPES measurements. With increasing Ti, the CDW phase transition becomes gradually suppressed to the lower temperature region (see Supplementary Note 2). For investigating both the normal and CDW phases within a single sample, we synthesized the minimally Tidoped single crystals (0.10 ≤ x ≤ 0.13) showing the transition just below room temperature (280–250 K). To discuss the electronic structures in momentum space, we introduce the following notation of the Brillouin zones (BZ). In 1T, the 2dimensional (3dimensional) BZ is represented by the hexagonal plane (prism) reflecting the trigonal symmetry (Fig. 1b, c). In 1T″, the BZ changes into a smaller one of lower symmetry as indicated in Fig. 1e. In this paper, we use the 1T BZ notation to present the band structures in 1T and 1T″ in a common fashion. To account for the inplane anisotropy of vanadium chains that is essential in 1T″, we define \({\bar{\mathrm{K}}}_1\), \({\bar{\mathrm{K}}}_2\) and \({\bar{\mathrm{M}}}_1\), \({\bar{\mathrm{M}}}_2\) as depicted in Fig. 1e (see Supplementary Note 1 for the details of BZ).
Results
Overviewing the electronic structures and modification by CDW
Let us start by briefly overviewing the anisotropic modification of electronic structures from 1T to 1T″ by presenting the ARPES data successfully focused on a CDW singledomain region. Figure 2a shows the ARPES image for the normalstate 1TV_{0.87}Ti_{0.13}Te_{2} taken at 300 K with a Hedischarge lamp (photon energy hν = 21.2 eV). We find Vshaped band dispersions along \({\bar{\mathrm{K}}}  {\bar{\mathrm{M}}}  {\bar{\mathrm{K}}}\) that clearly cross the Fermi level (E_{F}). Looking at the higher binding energy (E_{B}) region, these Vshaped bands are connected to the Diracconelike bands reminiscent of surface states in topological insulators, with the band crossing (Dirac points) at \({\bar{\mathrm{M}}}\). The topological character of these Dirac bands will be discussed later. On the other hand, Fig. 2b displays the ARPES results on a singledomain CDW state in 1T″VTe_{2} (200 K, synchrotron light hν = 90 eV). Because of the zigzag type CDW formation, the system now loses the threefold rotational symmetry, and the three equivalent \({\bar{\mathrm{M}}}\) points in 1T turn into one \({\bar{\mathrm{M}}}_1\) and two \({\bar{\mathrm{M}}}_2\). Here, the Vshaped band and Diraclike bands remain at \({\bar{\mathrm{M}}}_1\), whereas at \({\bar{\mathrm{M}}}_2\) side the unusual flat band is observed and the Diraclike state is vanished. Thus, the 1T1T″ CDW transition induces the huge directional change of electronic structure accompanying the selective disappearance of Diraclike states. In the following, we discuss these band structures in detail, by comparing with band calculations.
Figure 3a, b respectively displays the bulk band calculation for the normalstate V_{0.87}Ti_{0.13}Te_{2} along high symmetry lines (ΓKMΓ) and the Fermi surface at k_{z} = 0 (see Supplementary Note 3). They are characterized by the circular and triangular hole Fermi surfaces respectively around Γ and K. The ARPES results on the normalstate V_{0.90}Ti_{0.10}Te_{2} (350 K, marked as “#1” in Fig. 1f) are shown in Fig. 3c, d. Here a Hedischarge lamp (hν = 21.2 eV) is used as the light source. The black broken curves in Fig. 3c are the guides for the bands plotted by tracking the peaks of the energy/momentum distribution curves (EDCs/MDCs) (see Supplementary Note 4). We find holelike bands centered at around \(\bar \Gamma\) and \({\bar{\mathrm{K}}}\) points, in a qualitative agreement with the calculation in Fig. 3a (We note that the tiny electron pocket at Γ is not clearly detected, probably reflecting the finite energy mismatch of the bands compared with the calculation.). In the EDCs along the \({\bar{\mathrm{M}}}  {\bar{\mathrm{K}}}\) line (Fig. 3d), we can clearly trace a dispersive band that crosses E_{F}.
To grasp the essential electronic modification via the CDW formation, we survey the temperature and dopingdependent ARPES results (Hedischarge lamp, hν = 21.2 eV). Note that the 1T″ phase inevitably contains the inplane 120degree CDW domains reflecting the threefold symmetry of 1T, and usually ARPES measurements include the signals of multiple domains (see Supplementary Note 5). Figure 3e, f respectively shows the ARPES image and corresponding EDCs of V_{0.90}Ti_{0.10}Te_{2} in the CDW 1T″ phase (multidomain, 20 K, #2). Comparing with the hightemperature 1T phase (#1), we notice a very unusual flat band appearing at E_{B} ~ 0.1–0.25 eV as denoted by the red curve in Fig. 3e, spreading over the measured momentum region. This is most well recognized in Fig. 3f as the series of EDC peaks at E_{B} ~ 0.2 eV around the \({\bar{\mathrm{K}}}_1/{\bar{\mathrm{K}}}_2\) point. The blue broken curve in Fig. 3e, on the other hand, indicates the dispersive band crossing E_{F} resembling the hightemperature normal phase (see Supplementary Note 6 for the detailed temperaturedependence). Such coexistence of flat/dispersive bands results from the mixing of CDW domains. In the pristine 1T″VTe_{2} (multidomain, 15 K, #3) as shown in Fig. 3g, h, the similar flat band with slightly different energy and dispersion is more clearly observed (the red broken curve), together with the 1Tlike dispersive band (the blue broken curve). The appearance of this anomalous flat band is thus the common signature of the CDW 1T″ phase, which however is seemingly beyond the simple band folding and gap opening in the Fermi surface nesting scenario. The localized nature of this electronic structure will be discussed later.
Bulk and surface band structures in 1T normal phase
Here we introduce the topological aspect that can be relevant to the 1T1T″ CDW transition. In the normal 1T phase, the band calculation suggests the band inversion involving V3d and Te5p orbitals at around the M and L points. The calculations of 1TV_{0.87}Ti_{0.13}Te_{2} at several k_{z}, plotted along the direction parallel to ΓM (k_{ΓM}, see Fig. 4a) are displayed in Fig. 4b. The colors of curves show the weight of atomic orbitals depicted by a false colorscale (see Supplementary Note 3 for detailed orbital components), whereas the black broken curves are the results without SOC. Focusing on the topmost two bands at M and L, labelled as A and B, we find that their orbital characters of mainly V3d (bluelike, even parity) and Te5p_{x} + p_{y} (redlike, odd parity) get inverted at a finite k_{z} (~0.76 π/c). In the corresponding (0 0 1) slab calculation (Fig. 4c), a new band dispersing around E − E_{F} = −0.6 eV near \({\bar{\mathrm{M}}}\) can be recognized, that does not exist in the bulk calculations. It roughly follows the trajectory of the virtual crossing points of bulk bands A and B for no SOC. This is a surface state topologically protected by the band inversion at (a*/2, 0, k_{z}) occurring due to the moderate k_{z} dispersions of V3d and Te5p bands. Indeed, in the \({\bar{\mathrm{K}}}  {\bar{\mathrm{M}}}  {\bar{\mathrm{K}}}\) direction (Fig. 4d), this surface state shows a Dirac conelike dispersion connecting bands A and B. Figure 4e shows the ARPES image of normalstate V_{0.90}Ti_{0.10}Te_{2} along \({\bar{\mathrm{K}}}  {\bar{\mathrm{M}}}  {\bar{\mathrm{K}}}\) (350 K, hν = 21.2 eV). By carefully analyzing the EDC/MDC (see Supplementary Note 7), we can indeed quantify the bottom of bulk band A (E_{B} ~ 0.30 eV), the top of bulk band B (~0.90 eV), and the crossing point of the surface Dirac cone (DP, ~0.66 eV). We note that this bulk band A forms the triangular Fermi surfaces centered at \({\bar{\mathrm{K}}}\) points in the k_{z} = 0 plane, as shown in the Fermi surface image in Fig. 4f (V_{0.87}Ti_{0.13}Te_{2}, 300 K, hν = 83 eV).
We further perform the hνdependent ARPES to confirm the twodimensionality of the Dirac surface states and to clarify the k_{z}dependent bulk electronic structure that is relevant to the band inversion. Figure 4h–j displays the ARPES images of 1TV_{0.90}Ti_{0.10}Te_{2} near the \({\bar{\mathrm{M}}}\) point (see the red arrow in Fig. 4g), recorded at 320 K with different photon energies, hν = 63, 69, and 78 eV, respectively corresponding to k_{z} ~ 0, π/2c, and π/c. Circle markers with vertical (horizontal) error bars represent the peak positions of EDCs (MDCs) (see Supplementary Note 7). Figure 4k displays the schematic band dispersions overlaid with the experimental peak plots extracted from Fig. 4h–j. Here we find that the bulk bands A and B clearly show the finite k_{z}dispersions (respectively larger than 0.1 and 0.4 eV at \({\bar{\mathrm{M}}}\)). As can be seen in Fig. 4b, this k_{z}dispersion is essential for the band inversion at (a*/2, 0, k_{z}), the origin of the topological surface state appearing around the \({\bar{\mathrm{M}}}\) point. On the other hand, the Dirac surface state, that is highlighted by the overlaid orange curves in Fig. 4k, is almost independent of hν, indicating the twodimensional nature of the topological surface state.
Bulk and surface band structures in 1T″ CDW phase
To unambiguously elucidate the anisotropic electronic structures in the CDW 1T″ phase, here we utilize the small spot size (typically 300 μm) of the synchrotron light beam and separately measure the inplane CDW domains of VTe_{2} (see Supplementary Note 5). Figure 5a shows the Fermi surface image of the singledomain 1T″VTe_{2} (200 K, hν = 90 eV). We find that the two sides of the triangular Fermi surface around \({\bar{\mathrm{K}}}\) observed in the normal 1T phase (Fig. 4f) are completely absent in the CDW state, and the remaining one forms the quasionedimensional Fermi surface marked by the red broken curves. Figure 5b, d displays the ARPES images along representative two cuts (cut #1 and #2 as denoted in Fig. 5a), nearly along \({\bar{\mathrm{K}}}_1  {\bar{\mathrm{M}}}_1  {\bar{\mathrm{K}}}_1\) and \({\bar{\mathrm{K}}}_1  {\bar{\mathrm{M}}}_2  {\bar{\mathrm{K}}}_2\), respectively. Though they are originally the equivalent momentum cuts in the normal 1T phase, distinctive features are clearly observed. Along \({\bar{\mathrm{K}}}_1  {\bar{\mathrm{M}}}_1  {\bar{\mathrm{K}}}_1\), there are several bands crossing E_{F} including the 1Tlike dispersion (the black broken curve), together with the Diracconelike band (the white broken curve) as clearly seen in the MDCs (Fig. 5c). On the other hand, along \({\bar{\mathrm{K}}}_1  {\bar{\mathrm{M}}}_2  {\bar{\mathrm{K}}}_2\), the bands crossing E_{F} as well as the Dirac band completely disappear (see the MDCs in Fig. 5e), and the peculiar flat band (the black broken curve in Fig. 5d) appears around E_{B} ~ 0.2–0.3 eV. These indicate that the CDW induces the drastic directional modification of electronic structure.
Here, to confirm the topological nature of the Dirac surface state in VTe_{2}, we performed spinresolved ARPES on a multidomain sample (see Supplementary Note 8 for the experimental setup). Figure 5f depicts the schematic 2D BZ with CDW multidomains, together with the measurement region (the red arrow). Figure 5g shows the spinintegrated ARPES image with the peak plots obtained from spinresolved spectra (15 K, hν = 21 eV, s polarization). By comparing with the results from the singledomain measurement (Fig. 5b–e), these peaks can be easily assigned to either \({\bar{\mathrm{M}}}_1\) or \({\bar{\mathrm{M}}}_2\) side. The red (blue) and purple (cyan) triangle markers respectively represent the peak positions of spinup (down) spectra at \({\bar{\mathrm{M}}}_1\) and \({\bar{\mathrm{M}}}_2\) sides. As shown in the spinresolved spectra in Fig. 5h, the lower branch of the Dirac cone band clearly shows the spin polarization with sign reversal at \({\bar{\mathrm{M}}}_1\) point (i.e., Dirac crossing point), similarly to the topological surface state in the topological insulators. On the other hand, the bulk flat bands around E_{B} ~ 0.25 eV at \({\bar{\mathrm{M}}}_2\) side indeed show the spin degenerate character. Figure 5i is an expanded viewgraph of spinresolved spectra near the Fermi level at emission angle θ = ±4°. There are slight spinup/down intensity contrasts just below E_{F} (E_{B} ~ 0.05 eV), which should be corresponding to the upper branch of the surface Dirac cone band at \({\bar{\mathrm{M}}}_1\) side.
Similarly to the normal phase, hνdependent ARPES measurement is performed on the CDW state VTe_{2} (15 K, multidomain) to clarify the k_{z}dependence of electronic structures. Figure 5j–l displays the ARPES image near the \({\bar{\mathrm{M}}}_1/{\bar{\mathrm{M}}}_2\) points (see the red arrow in Fig. 5f), recorded with hν = 54, 61.5, and 69 eV, respectively. Again by comparing with the singledomain measurement, we can assign the peaks to either \({\bar{\mathrm{M}}}_1\) or \({\bar{\mathrm{M}}}_2\) side (see Supplementary Note 9). Figure 5m, n respectively shows the schematic band dispersions along \({\bar{\mathrm{K}}}_1  {\bar{\mathrm{M}}}_1  {\bar{\mathrm{K}}}_1\) and \({\bar{\mathrm{K}}}_1  {\bar{\mathrm{M}}}_2  {\bar{\mathrm{K}}}_2\), overlaid with the experimental peak plots extracted from Fig. 5j–l. At the \({\bar{\mathrm{M}}}_1\) side, the Vshaped band with high intensity clearly shows the finite k_{z}dispersion of >0.2 eV, together with the hνindependent Dirac surface state. These are similar to the case of normal 1T phase along \({\bar{\mathrm{K}}}  {\bar{\mathrm{M}}}  {\bar{\mathrm{K}}}\) (Fig. 4k). On the other hand, at the \({\bar{\mathrm{M}}}_2\) side, the emergent flat band shows negligible k_{z}dependence, in contrast to the Vshaped band observed at the \({\bar{\mathrm{M}}}_1\) side. With this we can conclude that the electronic state at the \({\bar{\mathrm{M}}}_2\) side has unusually localized nature, which should give rise to the dissolution of the band inversion along k_{z} direction.
Picture of CDW state based on orbital bonding
To grasp the essential feature of the CDW state, here we introduce the local orbital picture. For simplicity, we adopt an orthogonal octahedral XYZ coordination by considering the VTe_{6} octahedron as shown in Fig. 6a, and focus on V3d t_{2g} (d_{XY}, d_{YZ}, d_{ZX}) orbitals that dominate the density of states near E_{F} (see Supplementary Note 10). Note that this is different from the global xyz setting adopted in Fig. 4b, where z corresponds to the stacking direction. Here we choose Z as the VTe bond direction that is perpendicular to the vanadium’s chain direction b_{m}.
Figure 6b shows the calculated partial density of states (PDOS) for vanadium d_{XY}, d_{YZ}, and d_{ZX} orbitals. In 1T (Fig. 6b left), they are naturally degenerate reflecting the trigonal symmetry. In 1T″ (Fig. 6b right), on the other hand, d_{YZ}/d_{ZX} and d_{XY} orbitals have strikingly different distributions. We can classify the PDOS for 1T″ into three major parts, lower, middle, and upper, respectively lying around E_{B} ~ 0.4, −0.5, and −1.3 eV. They are indicative of bonding (B), nonbonding (NB), and antibonding (AB) bands, arising from the trimerizationlike displacements of three adjacent vanadium atoms formed by the σbonding of d_{YZ}/d_{ZX} orbitals^{23,33}, as marked by the pink oval in Fig. 6a for d_{YZ}. Through the d_{YZ}/d_{ZX} trimerization, the remaining d_{XY} stays relatively intact, thus its PDOS mostly contributes to the middle band. Here, the d_{YZ}/d_{ZX} bonding band makes a peak in PDOS at around E_{B} ~ 0.3 eV, corresponding to the flat bands lying around \({\bar{\mathrm{\Gamma }}}  {\bar{\mathrm{K}}}_2  {\bar{\mathrm{M}}}_2\) (see Supplementary Note 10 for the band dispersions). The experimentally observed flat band in Fig. 5d should be thus reflecting the localized nature of d_{YZ}/d_{ZX} trimers.
Such orbital bonding picture is also useful for the intuitive understanding of Fermi surfaces at BZ boundary. Figure 6c depicts the schematics of the Fermi surfaces with the locations of the Dirac crossing points (yellow circle makers at \({\bar{\mathrm{M}}}\) points). According to the calculation in 1T, the three sides of the triangular Fermi surface mainly consist of d_{YZ}, d_{ZX}, and d_{XY} orbitals, respectively (see Supplementary Note 10). This can be regarded as the virtual combination of onedimensional Fermi surfaces formed by the σbonding of d_{YZ}, d_{ZX} and d_{XY} orbitals, raised as the basic concept of the hidden nesting scenario in CdI_{2}type TMDCs^{23} and purple bronzes AMo_{6}O_{17} (A = Na, K)^{25,26,27}. In the CDW 1T″ phase, the two sides of the Fermi surfaces composed of d_{YZ}/d_{ZX} orbitals (\({\bar{\mathrm{M}}}_2\) sides) turn into the localized flat bands as a consequence of the vanadium trimerization, while the one composed of d_{XY} remains intact. This explains the quasi2D to quasi1D change of the triangular Fermi surface at BZ boundary as obtained in the present result.
Discussion
Figure 6d summarizes the observed anisotropic reconstruction of the bulk and topological surface states occurring around the BZ boundaries by the CDW formation. As discussed, the band structures observed in the normal 1T phase are basically in a good agreement with our calculations, indicating the triangular hole Fermi surfaces around the \({\bar{\mathrm{K}}}\) points. At \({\bar{\mathrm{M}}}\) points, the Diractype topological surface states exist in addition to the Vshaped bulk bands. We attribute this to the band inversion of V3d and Te5p orbitals occurring at (a*/2, 0, k_{z}), which can be traced experimentally (Fig. 4k). In the CDW state, on the other hand, drastic changes show up in the electronic structure. The originally triangular hole Fermi surface around the \({\bar{\mathrm{K}}}\) point loses its two sides, and the remaining one forms the onedimensionallike Fermi surface at the \({\bar{\mathrm{M}}}_1\) side. We note that this corresponds to the vanadium zigzag chain direction. At the \({\bar{\mathrm{M}}}_2\) side, the Vshaped bulk band transforms into the flat band at E_{B} ~ 0.25 eV, reflecting the vanadium trimerization. The original Dirac surface state survives at \({\bar{\mathrm{M}}}_1\), but can be no longer seen at \({\bar{\mathrm{M}}}_2\). The disappearance of the Dirac surface state can be ascribed to the absence of the band inversion at \({\bar{\mathrm{M}}}_2\), where the k_{z}dependence of the bulk state is lost due to the flat band formation (Fig. 5n), in contrast to the \({\bar{\mathrm{M}}}_1\) side (Fig. 5m). Thus, in this system, the CDW accompanying the metal trimerization plays a crucial role in selectively switching off the band inversion and corresponding topological surface state.
Such a drastic modification of band inversion by CDW can be further explained based on the change in crystal symmetry. In the hightemperature 1T phase \(\left( {{\it{P}}{\bar{\mathrm{3}}}{\it{m}}1} \right)\), as mentioned earlier, the Dirac cones appear at the \({\bar{\mathrm{M}}}\) points, due to the band inversions involving the Te5p and V3d orbitals occurring along ML lines. Here, the ML line resides in a mirror plane, which prohibits the hybridization between the band consisting of Te5p antibonding orbitals (odd with respect to the mirror plane) and that derived from the predominantly V3d with finite Te5p bonding orbitals (even with respect to the mirror plane), without the help of SOC. Eventually, the nonrelativistic bands cross at k_{z} ~ 0.76 π/c along ML depicted as the broken curves in Fig. 4b, thus causing the band inversion, due to the k_{z} dependent energy eigenvalues. Only by the SOC, they get mixed and make a gap (~200 meV) around the crossing point. In the lowtemperature 1T” phase (C2/m), on the other hand, while the \({\bar{\mathrm{M}}}_1\) point keeps the similar situation with the \({\bar{\mathrm{M}}}\) point in hightemperature phase, \({\bar{\mathrm{M}}}_2\) no longer resides in the mirror plane. At \({\bar{\mathrm{M}}}_2\), the V3d and Te5p (bonding, antibonding) can now significantly mix and form the well hybridized flat band with no k_{z} dependence, leading to the disappearance of the band inversion. This can be also confirmed by evaluating the orbital components in our bulk calculation (see Supplementary note 11). It indicates the significant d_{YZ}/d_{ZX}p_{Z} hybridization (here X, Y, Z represent the octahedral setting, see Fig. 6a) forming the flat band at E_{B} ~ 0.3–0.4 eV. In contrast, the mirror plane remains for \({\bar{\mathrm{\Gamma }}}  {\bar{\mathrm{M}}}_1\), thus the band inversion and the related Dirac surface state can sustain.
Finally, we would like to mention that the topological character of materials should be determined by considering the detailed band structures for the whole system and quantifying the Berry curvature and topological invariants (Z_{2} index, (spin) Chern number, etc). The difficulty in the present case, however, is that the accuracy of the band calculation on 1T″ phase is still insufficient. Though the qualitative features (such as the quasionedimensional Fermi surface and the flat band formation) are quite well reproduced (see Supplementary Note 12 for the band unfolding calculation), the detailed band structures including their relative energy positions are not in good enough agreement. This makes it difficult to fully discuss the topology which sharply depends on the crossings of band energy levels, particularly on its modification across the CDW transition. In this viewpoint, we would rather like to stress that our present results and microscopic analysis are offering one way to pursue the topological character of a complex material that is still difficult to predict.
In conclusion, we systematically clarified the bulk and surface electronic structures of (V,Ti)Te_{2} by utilizing ARPES and firstprinciples calculations. We revealed that the strongly orbitaldependent reconstruction of the electronic structures occurs through the CDW formation, giving rise to the topological change in particular bands. Considering that CDWs are often flexible to external stimuli^{7,10,11,12}, we can also expect to manipulate the CDWcoupled topological state. The effect of thinning the crystals down to atomic thickness is also worth investigating, which may give rise to hidden CDW states^{10} and electronic phase transitions^{34,35}. The combination of CDW ordering and topological aspects may lead to the new stage of manipulating the quantum materials.
Methods
Sample preparation
Highquality single crystals of V_{1−x}Ti_{x}Te_{2} were grown by the chemical vapor transport method with TeCl_{4} as a transport agent. Temperatures for source and growth zones are respectively set up as 600 °C and 550 °C. The Ti concentrations (x) were characterized by energy dispersive xray spectrometry (EDX) measurements. To access both 1T and 1T″ phases, we used V_{0.90}Ti_{0.10}Te_{2}, V_{0.87}Ti_{0.13}Te_{2}, and VTe_{2} samples with the transition temperature of ~280 K, ~250 K and ~475 K, respectively.
ARPES measurements
ARPES measurements were made at the Department of Applied Physics, The University of Tokyo, using a VUV5000 Hedischarge lamp and a VGScienta R4000 electron analyzer. The photon energy and the energy resolution are set to 21.2 eV (HeIα) and 15 meV. The spot size of measurements was ~2 × 2 mm^{2}. The data in Figs. 2a, 3c–h and 4e were obtained with this condition.
Photonenergydependent and domainselective ARPES measurements were conducted at BL28A in Photon Factory (KEK) by using a system equipped with a Scienta SES2002 electron analyzer. Photonenergydependent measurements on 1TV_{0.90}Ti_{0.10}Te_{2} and 1T″VTe_{2} were performed respectively using circular and spolarized light (hν = 45–90 eV). The energy resolution was set to 30 meV. The relation between the incident light energy and the detected k_{z} value is estimated from the experimentally obtained work function (~3.7 eV) and inner potential (~8.8 eV). The data in Figs. 4h–j and 5j–l were obtained in this condition. Domainselective measurements on 1T″VTe_{2} were performed at 200 K using a 90 eV circularly polarized light with a spot size of ~100 × 300 μm^{2}. The energy resolution was set to 50 meV. The data in Fig. 5b, d were obtained from the different inplane domains of a single sample, by translating the sample without changing its orientation. The Fermi surface images in Fig. 4f, 5a were also obtained with this instrument.
Spinresolved ARPES measurements were performed at Efficient SPin Resolved SpectroScOpy end station attached to the APPLEIItype variable polarization undulator beamline (BL9B) at the Hiroshima Synchrotron Radiation Center (HSRC)^{36}. The analyzer of this system consists of two sets of verylowenergy electron diffraction spin detectors, combined with a hemispherical electron analyzer (VGScienta R4000). Measurements of 1T″VTe_{2} were performed using a spolarized 21 eV light at 15 K. In the spinresolved measurements, we set the energy and angle resolutions to 120 meV and ±1.5 deg, respectively. We adopted the Sherman function S_{eff} = 0.25 for analyzing the obtained data. The data in Fig. 5g–i were obtained with this condition.
In all the ARPES experiments, samples were cleaved at room temperature insitu, and the vacuum level was better than 5 × 10^{−10} Torr through the measurements. The Fermi level of the samples were referenced to that of polycrystalline golds electrically connected to the samples.
Band calculations
The relativistic electronic structure of 1TV_{0.87}Ti_{0.13}Te_{2} was calculated within the density functional theory (DFT) using the PerdewBurkeErnzerhof (PBE) exchangecorrelation functional corrected by the semilocal TranBlahamodified BeckeJohnson potential, as implemented in the WIEN2k package^{37}. The effect of Ti doping was treated within the virtual crystal approximation^{38}. The BZ was sampled by a 20 × 20 × 20 kmesh and the muffintin radius R_{MT} for all atoms was chosen such that its product with the maximum modulus of reciprocal vectors K_{max} becomes R_{MT}K_{max} = 7.0. To describe the surface electronic structure, the bulk DFT calculations were downfolded using maximally localized Wannier functions^{39,40} composed of V3d and Te5p orbitals, and the resulting 22band tightbinding transfer integrals implemented within a 200unit supercell. For the pristine 1T and 1T″VTe_{2}, the DFT electronic structure calculations were carried out by OpenMX code (http://www.openmxsquare.org/), using the PBE exchangecorrelation functional and a fully relativistic jdependent pseudopotentials. We adopted and fixed the crystalline structures of 1T and 1T″VTe_{2} reported in ref. ^{20,41} and sampled the corresponding BZ by a 8 × 8 × 8 kmesh.
Data availability
The datasets that support the findings of the current study are available from the corresponding author on reasonable request.
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Acknowledgements
The authors thank N. Katayama for fruitful discussions. We also acknowledge H. Masuda and A. H. Mayo for their assistance with EDX measurements. N.M. acknowledges the support by the Program for Leading Graduate Schools (ALPS). Y.S., M.K., and T.So acknowledge the supports by the Program for Leading Graduate Schools (MERIT). Y.S. and T.So acknowledge the supports by Japan Society for the Promotion of Science through a research fellowship for young scientists. The spinresolved ARPES experiments ware performed under HSRC Proposals Nos. 16AG050 and 16BG040. This work was partly supported by CREST, JST (No. JPMJCR16F1, No. JPMJCR16F2) and the JSPS KAKENHI (No. JP17H01195, No. JP19H05826).
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N.M., T.So., M.S., T.Sh., and K.I. carried out (S)ARPES measurements. M.K., H.T., H.S., and S.I. carried out the crystal growth and characterization. M.S.B., Y.S., and Y.M. carried out the calculations. K.H. and H.K. shared the ARPES infrastructure at Photon Factory, KEK, and assisted with measurements. K.T., K.M., and T.O. shared the SARPES infrastructure at the Hiroshima Synchrotron Radiation Center and assisted with measurements. N.M. and K.I. analyzed (S)ARPES data and wrote the paper with inputs from Y.S., M.S.B., S.I., and Y.M. K.I. conceived and coordinated the research.
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Mitsuishi, N., Sugita, Y., Bahramy, M.S. et al. Switching of band inversion and topological surface states by charge density wave. Nat Commun 11, 2466 (2020). https://doi.org/10.1038/s4146702016290w
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DOI: https://doi.org/10.1038/s4146702016290w
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