Two-dimensional materials have proven to be a prolific breeding ground of new and unstudied forms of magnetism and unusual metallic states, particularly when tuned between their insulating and metallic phases. In this paper we present work on a new metal to insulator transition system FePS3 . This compound is a two-dimensional van-der-Waals antiferromagnetic Mott insulator. Here we report the discovery of an insulator-metal transition in FePS3, as evidenced by x-ray diffraction and electrical transport measurements, using high pressure as a tuning parameter. Two structural phase transitions are observed in the x-ray diffraction data as a function of pressure and resistivity measurements show evidence of the onset of a metallic state at high pressures. We propose models for the two new structures that can successfully explain the x-ray diffraction patterns.
Recently there has been a renewed interest in the charge density wave transition of TiSe2, fuelled by the possibility that this transition may be driven by the formation of an excitonic insulator or even an excitonic condensate. We show here that the recent ARPES measurements on TiSe2 can also be interpreted in terms of an alternative scenario, in which the transition is due to a combination of Jahn-Teller effects and exciton formation. The hybrid exciton-phonons which cause the CDW formation interpolate between a purely structural and a purely electronic type of transition. Above the transition temperature, the electron-phonon coupling becomes ineffective but a finite mean-field density of excitons remains and gives rise to the observed diffuse ARPES signals.
Dimensionality is crucial in determining or controlling the magnetic, electronic and structural properties of condensed matter systems. The case of 2D and of graphene is of course a very topical example. The pairing mechanisms of unconventional superconductors such as high-temperature superconductors and FeSe are often seen to be strengthened in 2D. Additionally, new magnetic phases and structures can be found in thin films or layers of otherwise simplistic materials. Crystals with layered structures of atomic planes separated by van-der-Waals gaps form an ideal case for studying a wide
The strong coupling between lattice modes and charges which leads to the formation of charge density waves in materials such as the transition-metal dichalcogenides may also give rise to superconductivity in the same materials, mediated by the same exciton or phonon modes that dominate the charge ordered state. Such a superconducting phase has recently been observed for example in TiSe2, both upon intercalation with Copper and in the pristine material under pressure. Here we investigate the interplay of exciton formation and electron-phonon coupling within a simplified model description. We find that the combined exciton-phonon modes previously suggested to drive the charge density wave instability in TiSe2 are also responsible for the pairing of electrons in its superconducting regions. Based on these results, it is suggested that both of the observed domes form part of a single superconducting phase. We also study the effect of the quantum critical fluctuations emerging from the suppressed charge order on the transport properties directly above the superconducting region, and compare our finding with reported experimental results.
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