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Register a new userElectron localization and possible phase separation in the absence of a charge density wave in single-phase 1T-VS$_2$
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We report on a systematic study of the structural, magnetic and transport properties of high-purity 1T-VS$_2$ powder samples prepared under high pressure. The results differ notably from those previously obtained by de-intercalating Li from LiVS$_2$. First, no Charge Density Wave (CDW) is found by transmission electron microscopy down to 94 K. Though, textit{ab initio} phonon calculations unveil a latent CDW instability driven by an acoustic phonon softening at the wave vector ${bf q}_{CDW} approx$ (0.21,0.21,0) previously reported in de-intercalated samples. A further indication of latent lattice instability is given by an anomalous expansion of the V-S bond distance at low temperature. Second, infrared optical absorption and electrical resistivity measurements give evidence of non metallic properties, consistent with the observation of no CDW phase. On the other hand, magnetic susceptibility and NMR data suggest the coexistence of localized moments with metallic carriers, in agreement with textit{ab initio} band structure calculations. This discrepancy is reconciled by a picture of electron localization induced by disorder or electronic correlations leading to a phase separation of metallic and non-metallic domains in the nm scale. We conclude that 1T-VS$_2$ is at the verge of a CDW transition and suggest that residual electronic doping in Li de-intercalated samples stabilizes a uniform CDW phase with metallic properties.
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We report a rectangular charge density wave (CDW) phase in strained 1T-VSe$_2$ thin films synthesized by molecular beam epitaxy on c-sapphire substrates. The observed CDW structure exhibits an unconventional rectangular 4a{times}{sqrt{3a}} periodicity, as opposed to the previously reported hexagonal $4atimes4a$ structure in bulk crystals and exfoliated thin layered samples. Tunneling spectroscopy shows a strong modulation of the local density of states of the same $4atimessqrt{3}a$ CDW periodicity and an energy gap of $2Delta_{CDW}=(9.1pm0.1)$ meV. The CDW energy gap evolves into a full gap at temperatures below 500 mK, indicating a transition to an insulating phase at ultra-low temperatures. First-principles calculations confirm the stability of both $4atimes4a$ and $4atimessqrt{3}a$ structures arising from soft modes in the phonon dispersion. The unconventional structure becomes preferred in the presence of strain, in agreement with experimental findings.
We study the structural, electronic and vibrational properties of single-layer 1TNbSe$_2$ from first principles. Within the generalized gradient approximation, the 1T polytype is highly unstable with respect to the 2H. The DFT+U method improves the stability of the 1T phase, explaining its detection in experiments. A charge density wave occurs with a $sqrt{13}timessqrt{13}~R30^{circ}$ periodicity, in agreement with STM data. At $U=0$, the David-star reconstruction displays a flat band below the Fermi level with a marked d$_{z^2-r^2}$ orbital character of the central Nb. The Hubbard interaction induces a magnetic Mott insulating state. Magnetism distorts the lattice around the central Nb atom in the star, reduces the hybridization between the central Nb d$_{z^2-r^2}$ orbital and the neighbouring Se p-states and lifts in energy the flat band becoming non-bonding. This cooperative lattice and magnetic effect amplifies the Mott gap. Single-layer 1TNbSe$_2$ is then a phonon-assisted spin-$1/2$ Magnetic Mott insulator.
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How magnetism emerges in low-dimensional materials such as transition metal dichalcogenides at the monolayer limit is still an open question. Herein, we present a comprehensive study of the magnetic properties of single crystal and monolayer VSe$_{2}$, both experimentally and emph{ab initio}. Magnetometry, X-ray magnetic circular dichrosim (XMCD) and emph{ab initio} calculations demonstrate that the charge density wave in bulk stoichiometric VSe$_{2.0}$ causes a structural distortion with a strong reduction in the density of sates at the Fermi level, prompting the system towards a non-magnetic state but on the verge of a ferromagnetic instability. In the monolayer limit, the structural rearrangement induces a Peierls distortion with the opening of an energy gap at the Fermi level and the absence of magnetic order. Control experiments on defect-induced VSe$_{2-delta}$ single crystals show a breakdown of magnetism, discarding vacancies as a possible origin of magnetic order in VSe$_{2}$.
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