Spin- and valley-coupled electronic states in monolayer WSe2 on bilayer graphene

Abstract

We have fabricated a high-quality monolayer WSe2 film on bilayer graphene by epitaxial growth, and revealed the electronic states by spin-and angle-resolved photoemission spectroscopy. We observed a direct energy gap at the Brillounin-zone corner in contrast to the indirect nature of gap in bulk WSe2, which is attributed to the lack of interlayer interaction and the breaking of spaceinversion symmetry in monolayer film. A giant spin splitting of ∼0.5 eV, which is the largest among known monolayer transition-metal dichalcogenides, is observed in the energy band around the zone corner. The present results suggest a high potential applicability of WSe2 to develop advanced devices based with the coupling of spin-and valley-degrees of freedom.Transition-metal dichalchogenides (TMDs) MX 2 (M = transition metal, X = chalcogen) have been a target of intensive studies because they show a variety of physical properties such as superconductivity and charge density wave.[1] Recently, monolayer MX 2 (the thinnest limit of multilayer 2H-MX 2 ) has attracted special attention since it provides a platform to realize advanced electronic devices utilizing the spin-and valley-degrees of freedom. Moreover, owing to the valley degree of freedom, electrons at the K (K') point are robust against the phonon scattering connecting the K and K' points.[2] It has been predicted that such spin-split bands host anomalous quantum phenomena like spin-valley-filter effect, anomalous quantum Hall effect,[2] and magnetic-field-controlled spin current, [7] owing essentially to the "real-spin" nature of valley-coupled electronic states in monolayer MX 2 [2] in contrast to the "pseudo-spin" nature in graphene. [8] It is thus of great importance to experimentally establish the spin-dependent band structure of MX 2 to understand the origin of exotic physical properties and design electronic devices based on the band-structure engineering.Recently, it has been reported that monolayer MoS 2 and WSe 2 obtained by exfoliating a bulk crystal have a direct band gap at the K (K') point while no clear band splitting was observed. [9,10] In contrast, monolayer MoSe 2 grown epitaxially on bilayer graphene [11] and monolayer MoS 2 on HOPG [12] exhibit the band splitting of ∼180 meV. There is no consensus on whether the energy bands at the K (K') point in bulk WSe 2 are spin-polarized or not [13,14]. These facts have left an experimental ambiguity regarding the spin-split/polarized nature of the energy bands and its relationship to the exotic physical properties in MX 2 [15,16]. In this context, monolayer WSe 2 is a suitable candidate to access such an issue since WSe 2 is expected to have a large spinorbit coupling due to the heavy atomic mass and hence the spin splitting should be large enough to be experimentally detected. [3,13,14] It is also noted that the large spin splitting would be a great advantage to realize more effective electronic devices. While angle-resolved photoemission spectroscopy (ARPES) has been applied to both an exfoliated monolayer film a...

Large quantum-spin-Hall gap in single-layer 1T′ WSe2

“…The pure 1 H phase shows a sizable gap below the Fermi level; it is therefore a semiconductor similar to the bulk case 21 . The top valence band at splits into two branches toward because of the strong spin–orbit coupling of W. The valence band maximum is at , consistent with prior studies of this phase 24,26 . For the mixed sample, the 1 T ′ phase gives rise to additional valence bands of very different dispersion relations, and the topmost valence band reaches near the Fermi level to form a fairly flat portion around the zone center.…”

Monolayer 1T-NbSe2 as a Mott insulator

“…This behavior is characteristic of a TMD ultrathin film, as has been observed in monolayer WSe 2 and TiSe 2 on bilayer graphene. 23,24 STM revealed the formation of monolayer NbSe 2 islands (Figure 1d), whose height (~0.7 nm; see line profile in Figure 1e) was nearly equal to the distance between adjacent NbSe 2 layers in bulk 2H-NbSe 2 , 0.63 Å. 10 We found that this film was composed of two different types of islands.…”

“…Figure 1b shows the reflection high-energy electron diffraction pattern of bilayer graphene on 6H-SiC(0001), which signified a 1 × 1 streak pattern and 6 ffiffi ffi 3 p 6 ffiffi ffi 3 p R30 3 spots originating from the bilayer graphene and the buffer layer beneath it, respectively. 23,24 After co-evaporation of Nb and Se atoms onto a substrate kept at 530°C in ultrahigh vacuum, the reflection highenergy electron diffraction intensity of 6 ffiffi ffi 3 p 6 ffiffi ffi 3 p R30 3 spots was reduced and a new 1 × 1 pattern appeared (Figure 1c). This behavior is characteristic of a TMD ultrathin film, as has been observed in monolayer WSe 2 and TiSe 2 on bilayer graphene.…”

“…Bilayer graphene was prepared by annealing an n-type Si-rich 6H-SiC(0001) single-crystal wafer, 23,24 by resistive heating at 1100°C in high vacuum-better than 1.0 × 10 − 9 Torr-for 20 min. A monolayer NbSe 2 film was grown by evaporating Nb on the bilayer graphene substrate in a Se atmosphere.…”

Nearly-free-electron system of monolayer Na on the surface of single-crystal HfSe2

“…94.201121 Understanding and controlling electrons in reduced dimensions, for example, at the interfaces between disparate semiconductors, underpins modern electronic devices [1][2][3][4]. In recent years, this has found renewed prominence through the study of electrons naturally confined in atomically thin layers, such as in graphene or monolayer transition-metal dichalcogenides, opening prospects to achieve novel functionality such as ultrafast electronic [5], spintronic, or valleytronic devices [6][7][8][9][10][11]. To progress towards these goals, it is critical to understand the behavior of electrons in two-dimensional (2D) solids, and the influence of many-body interactions between them.…”