M A T E R I A L S F O R T O M O R R O W ' S I N N O V A T I V E A N D S U S T A I N A B L E I N D U S T R Y

S C I E N T I F I C H I G H L I G H T S

6 0 H I G H L I G H T S 2 0 2 4 I

Synchrotron radiation unveils disorder-driven charge correlations in 6R-NbSeTe

A combination of photo-emission spectroscopy, diffuse X-ray scattering and electron microscopy experiments reveals a complex band structure and temperature-dependent short-range charge fluctuations in self-stacked 1T-1H layers in 6R-NbSeTe, highlighting the disorder-driven quenching of the charge density wave. The results demonstrate how self-stacked bulk heterostructures can be utilized to engineer emergent phases of matter.

Transition metal dichalcogenides (TMDCs) offer exciting possibilities for developing advanced materials with tuneable electronic properties, crucial for applications in quantum computing, energy storage, and beyond. Investigating new stacking configurations and phases can unlock novel correlated states, paving the way for breakthroughs in these fields. TMDCs exhibit chemical flexibility, enabling atomic intercalation and the tuning of competing electronic orders. Typically, van der Waals heterostructures are constructed by stacking monolayers with varying twist angles and symmetry-breaking configurations to achieve correlated electronic phases. However, controlling these features in bulk TMDCs remains challenging, with notable success primarily in the metastable 4Hb [1] and 6R phases of TaS2. These phases exhibit intriguing properties such as chiral superconductivity and coexisting charge density waves (CDW). Expanding similar stacking approaches to other TMDCs could unlock new opportunities for exploring complex correlated phases.

In this work, growth parameters were optimized to chemically integrate 2H-NbSe2 and 1T-NbTe2, forming the 6R-phase bulk NbSeTe – a perfect 3× stacking of c-axis-oriented 1T−1H heterobilayers. This rhombohedral 6R structure decouples the van der Waals layers, diminishing the parent compounds’ charge correlations. Figures 47a and 47b present the conventional unit cell of the 6R-NbSeTe structure, with stacked 1T and 1H layers confirmed through atomic-resolution transmission electron microscopy (TEM), which precisely identifies Nb atoms positioned between chalcogen layers in both trigonal (1H) and octahedral (1T) coordination. The surface electronic structure of 6R-NbSeTe features two hole-like bands near Γ (Nb character in 1T geometry) and one electron-like band centred at M (Nb character in 1T geometry), as shown by angle-resolved photo-emission spectroscopy (ARPES) results in Figure 47c, confirmed by the matching simulated band structure. The ARPES results reveal no gap opening at the Fermi level, and Fermi surface nesting appears weak at qCDW = 2/3ΓM.

Notably, the lack of clear translation vectors to nest parts of the Fermi surface does not rule out charge modulations – a common feature in TMDCs where CDW formation is driven primarily by electron-phonon interactions [2,3]. To confirm the presence of any charge modulation, diffuse X-ray scattering experiments were conducted at beamline ID28. Figure 48 illustrates the scattering data, highlighting two key observations. First, diffuse intensity rods appear between Bragg peaks along the L direction, indicating coherent 2D layered growth (Figure 48b). Second, scattering maps in the HK0 plane show a diffuse signal, characteristic of a precursor to

Fig. 47: a) Top view of the 6R-NbSeTe unit cell. b) TEM image showing

the lateral view of the conventional unit cell

in 6R-NbSeTe. c) Angle- resolved photo-emission

spectra of NbSeTe along the Γ-M direction, compared with the calculated band

structure.