We report the structural, vibrational and electrical transport properties up to ~ 16 GPa of the 1T-TiTe2, a prominent layered 2D system, which is predicted to show a series of topologically trivial -nontrivial transitions under hydrostatic compression. We clearly show signatures of two iso-structural transition at ~ 2 GPa and ~ 4 GPa obtained from the minima in c/a ratio concomitant with the phonon linewidth anomalies of Eg and A1g modes at around the same pressures, providing strong indication of unusual electron-phonon coupling associated to these transitions. Resistivity presents nonlinear behavior over similar pressure ranges providing a strong indication of the electronic origin of these pressure driven isostructural transitions. Our data thus provide clear evidences of topological changes at A and L point of the Brillouin zone predicted to be present in the compressed 1T-TiTe2. Between 4 GPa and ~ 8 GPa, the c/a ratio shows a plateau suggesting a transformation from an anisotropic 2D layer to a quasi 3D crystal network. First principles calculations suggest that the 2D to quasi 3D evolution without any structural phase transitions is mainly due to the increased interlayer Te-Te interactions (bridging) via the charge density overlap. In addition to the pressure dependent isostructural phase transitions, our data also evidences the occurrence of a first order structural phase transition from the trigonal (P3 ̅ m1) phase at higher pressures. We estimate the start of this structural phase transition to be ~ 8 GPa and the symmetric of the new high-pressure phase to be monoclinic (C2/m).2

We report pressure evolution of charge density wave (CDW) order and emergence of superconductivity (SC) in 1T-VSe2 single crystal by studying resistance and magnetoresistance behavior under high pressure. With increasing quasi-hydrostatic pressure the CDW order enhances with increase of the ordering temperature up to 240K at 12 GPa. Upon further increase of pressure, the resistance anomaly due to CDW order gets suppressed drastically and superconductivity emerges at ∼15 GPa, with the onset critical temperature (Tc) ∼4K. The pressure dependence of Tc is found negligible, different from the significant increase or a dome-shape seen in iso-structural layered diselenide superconductors. The high pressure magnetoresistance and Hall measurements suggest successive electronic structural changes with Fermi surface modifications at 6 GPa and 12 GPa. From the observed negative magnetoresistance in this pressure range and absence of coexisting CDW and SC phases, we propose that intra-layer spin-fluctuation can play a role in the emergence of superconductivity in the high pressure phase. PACS numbers:Layered transition metal dichalcogenide (TMDC) compounds provide an ideal platform to explore exotic ground-state electronic orders by tuning the Fermi surface topology and many-body effects through various external stimulations. 1 Among these, 1T-structured correlated metals (e.g., 1T-TaS 2 , 1T-TaSe 2 , 1T-TiSe 2 and 1T-TiTe 2 ) have been extensively studied for understanding the mechanism of charge density wave (CDW) order at low temperature and its coexistence with superconductivity (SC) in some part of the phase diagram. 2-8 An unconventional (exciton or band-type Jahn-Teller) mechanism for the CDW ordering has been established in these compounds by several experimental and theoretical studies, rather than the conventional Fermi surface nesting mechanism. However, the underlying mechanism for superconductivity is not conclusive so far. In some systems, the dome-shaped superconducting window in the vicinity of the CDW suppressed quantum critical point (QCP) supports unconventional SC scenario where CDW amplitude fluctuation is believed to be responsible for the Cooper pair formation. [9][10][11] In some other systems phonon mediated (BCS type) SC appears in the phase diagram window (separated from the CDW region) that is believed to originate in phase-separated metallic domains. 12,13 1T-VSe 2 is one of the rare correlated metallic systems where three dimensional (3D) nesting of the Fermi surface gives rise to 3D-CDW ordering (having commensurate in-plane wave vector 0.25a* with an incommensurate out-of-plane component). [14][15][16][17] The CDW transition temperature (T CDW ) is ∼110K, as seen in resistivity and susceptibility measurements. 18,19 Due to weak nesting condition, the CDW distortion (amplitude) is small, making the superstructure bands not observable below T CDW in ARPES measurements . 15,16 Although the high resolution ARPES measurements on single crystal 1T-VSe 2 show the presence of only hole pocket (of V 3d...

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