Layered transition metal chalcogenides are attracting general interest as very versatile and multifunctional quasi-two-dimensional materials displaying competing charge density wave order (CDW), orbital order, superconductivity, and in some cases, magnetic order. 1T-TaS2 is of particular interest, as it is thought to satisfy the conditions for an unusual low-temperature Mott insulating state in which the electronic charge density within each layer is modulated by a three-directional IP CDW. This state may be described as a hexagonal array of polarons in the form of the star of David (Fig. 1a,c), defined by the Ta displacements towards the central charged Ta atom(Fig. 1a,b)1. The resulting low temperature (C) structure is commensurate with the underlying lattice (Fig. 1b,c), but spin ordering of the localized electrons at the polaron center is frustrated within the hexagonal lattice2, and no magnetic ordering has been reported so far.
On heating above 220 K3 the commensurate state first breaks up into a striped polaron state around 220 K, and eventually to a patchy state of nearly commensurate (NC) domains separated by domain walls above 280 K. The material becomes superconducting under pressure4 or upon doping5,6,7. The electronic ordering perpendicular to the TaS2 layers and its role in determining out-of-plane (OP) transport and phase stability is currently highly controversial. Mechanically the material behaves as a 2-dimensional Van der Waals solid, with exfoliation properties similar to graphene, which at first sight suggests the inter-layer coupling to be weak. However, band structure calculations so far8,9,10,11,12,13 consistently predict insulating in-plane (IP) transport, with metallic out-of-plane (OP) transport.
The latter is attributed to a highly dispersed band crossing the Fermi level along the OP, Γ–A direction in the Brillouin zone. The coherent OP transport in this band may be disrupted either by random interlayer stacking disorder possibly in combination with pair-wise layer stacking10 or helical disorder14, leading to Anderson localization and insulating resistivity behavior at low temperatures. In addition, the Coulomb repulsion between electrons on polaron sites are expected to lead to a Mott gap in the C state.
The unusual duality in the predicted electronic transport and mechanical properties could be partly reconciled if we consider that transport relies on the overlap of z2 orbitals of Ta atoms on adjacent planes, while the mechanical properties depend on weak bonding by orbitals far from the Fermi level and Coulomb interactions between Ta and S layers12. However, in the absence of systematic transport measurements and electronic structure along the c axis, the issue of OP interactions, and indeed the importance of the IP correlations remain unresolved.