TaS2

Intrinsic resistivity changes associated with charge density wave (CDW) phase transitions in 1$T$-TaS$_2$ hold promise for non-volatile memory and computing devices based on the principle of phase change memory. Intermediate resistance states, which offer distinctive opportunities for neuromorphic computing, have been observed in 1$T$-TaS$_2$ but the metastability responsible for this behavior makes the nature of multistate switching unpredictable. Here, we demonstrate the synthesis of nanothick verti-lateral 1$H$-TaS$_2$/1$T$-TaS$_2$ heterostructures in which the number of endotaxial metallic 1$H$-TaS$_2$ monolayers dictates the number of high-temperature resistance transitions in 1$T$-TaS$_2$ lamellae. Further, we also observe heterochirality in the CDW superlattice structure, which is also modulated in concert with the resistivity steps. This thermally-induced polytype conversion nucleates at folds and kinks where interlayer translations that relax local strain favorably align 1$H$ and 1$T$ layers. This work positions endotaxial TaS2 heterostructures as prime candidates for non-volatile device schemes implementing coupled switching of structure, chirality, and resistance.

Published in: "arXiv Material Science".

We report on electrical gating of the charge-density-wave phases and current in h-BN capped three-terminal 1T-TaS$_2$ heterostructure devices. It is demonstrated that the application of a gate bias can shift the source-drain current-voltage hysteresis associated with the transition between the nearly commensurate and incommensurate charge-density wave phases. The evolution of the hysteresis and the presence of abrupt spikes in the current while sweeping the gate voltage suggest that the effect is electrical rather than self-heating. We attribute the gating to an electric-field effect on the commensurate charge-density-wave domains in the atomic planes near the gate dielectric. The transition between the nearly commensurate and incommensurate charge-density-wave phases can be induced by both the source-drain current and the electrostatic gate. Since the charge-density-wave phases are persistent in 1T-TaS2 at room temperature, one can envision memory applications of such devices when scaled down to the dimensions of individual commensurate domains and few-atomic plane thicknesses.

Published in: "arXiv Material Science".

Van der Waals (vdW) heterostructures continue to attract intense interest as a route of designing materials with novel properties that cannot be found in naturally occurring materials. Unfortunately, this approach is currently limited to only a few layers that can be stacked on top of each other. Here we report a bulk material consisting of superconducting monolayers interlayered with monolayers displaying charge density waves (CDW). This bulk vdW heterostructure is created by phase transition of 1T-TaS2 to 6R at 800 {deg}C in an inert atmosphere. Electron microscopy analysis directly shows the presence of alternating 1T and 1H monolayers within the resulting bulk 6R phase. Its superconducting transition (Tc) is found at 2.6 K, exceeding the Tc of the bulk 2H phase of TaS2. The superconducting temperature can be further increased to 3.6 K by exfoliating 6R-TaS2 and then restacking its layers. Using first-principles calculations, we argue that the coexistence of superconductivity and CDW within 6R-TaS2 stems from amalgamation of the properties of adjacent 1H and 1T monolayers, where the former dominates the superconducting state and the latter the CDW behavior.

Published in: "arXiv Material Science".

Doped Mott insulators exhibit some of the most intriguing quantum phases of matter, including quantum spin-liquids, unconventional superconductors, and non-Fermi liquid metals. Such phases often arise when itinerant electrons are close to a Mott insulating state, and thus experience strong spatial correlations. Proximity between different layers of van der Waals heterostructures naturally realizes a platform for experimentally studying the relationship between localized, correlated electrons and itinerant electrons. Here, we explore this relationship by studying the magnetic landscape of 4Hb-TaS2, which realizes an alternate stack of a candidate spin liquid and a superconductor. We report on a spontaneous vortex phase whose vortex density can be trained in the normal state. We show that time reversal symmetry is broken above Tc, indicating the presence of a magnetic phase independent of the superconductor. Strikingly, this phase does not generate detectable magnetic signals. We use scanning superconducting quantum interference device (SQUID) microscopy to show that it is incompatible with ferromagnetic ordering. The discovery of this new form of hidden magnetism illustrates how combining superconductivity with a strongly correlated system can lead to new, unexpected physics.

Published in: "arXiv Material Science".

Strongly correlated materials exhibit exotic electronic states arising from the strong correlation between electrons. Dimensionality provides a tuning knob because thinning down to atomic thickness reduces screening effects and enhances electron correlations. In this work, a 2D Kondo lattice has been created by stacking a layer of 1T-TaS2 on a 2H-TaS2 crystal, which are bound by weak van der Waals interactions. By using high-resolution scanning tunnelling spectroscopy and density functional theory, we unambiguously demonstrate the formation of a 2D Kondo lattice from an ensemble of independent Kondo impurities present at higher temperatures. The possibility to create 2D Kondo lattices in van der Waals heterostructures paves the way for the exploration of unconventional metallic, magnetic and superconducting states not present in more standard correlated materials.

Published in: "arXiv Material Science".