Charge-density-wave phase, mottness and ferromagnetism in monolayer 1T1T-NbSe2_2

The recently investigated $1T$-polymorph of monolayer NbSe$_2$ revealed an insulating behaviour suggesting a star-of-David phase with $\sqrt{13}\,\times\sqrt{13}$ periodicity associated with a Mott insulator, reminiscent of $1T$-TaS$_2$. In this work, we examine this novel two-dimensional material from first principles. We find an instability towards the formation of an incommensurate charge-density-wave (CDW) and establish the star-of-David phase as the most stable commensurate CDW. The mottness in the star-of-David phase is confirmed and studied at various levels of theory: the spin-polarized generalized gradient approximation (GGA) and its extension involving the on-site Coulomb repulsion (GGA+$U$), as well as the dynamical mean-field theory (DMFT). Finally, we estimate Heisenberg exchange couplings in this material and find a weak nearest-neighbour ferromagnetic coupling, at odds with most Mott insulators. We point out the close resemblance between this star-of-David phase and flat-band ferromagnetism models

Excitonic effects in two-dimensional TiSe2_2 from hybrid density functional theory

Transition metal dichalcogenides (TMDs), whether in bulk or in monolayer form, exhibit a rich variety of charge-density-wave (CDW) phases and stronger periodic lattice distortions. While the actual role of nesting has been under debate, it is well understood that the microscopic interaction responsible for the CDWs is the electron-phonon coupling. The case of TiSe$_2$ is however unique in this family in that the normal state above the critical temperature $T_\mathrm{CDW}$ is characterized by a small quasiparticle bandgap as measured by ARPES, so that no nesting-derived enhancement of the susceptibility is present. It has therefore been argued that the mechanism responsible for this CDW should be different and that this material realizes the excitonic insulator phase proposed by Walter Kohn. On the other hand, it has also been suggested that the whole phase diagram can be explained by a sufficiently strong electron-phonon coupling. In this work, in order to estimate how close this material is to the pure excitonic insulator instability, we quantify the strength of electron-hole interactions by computing the exciton band structure at the level of hybrid density functional theory, focusing on the monolayer. We find that in a certain range of parameters the indirect gap at $q_{\mathrm{CDW}}$ is significantly reduced by excitonic effects. We discuss the consequences of those results regarding the debate on the physical mechanism responsible for this CDW. Based on the dependence of the calculated exciton binding energies as a function of the mixing parameter of hybrid DFT, we conjecture that a necessary condition for a pure excitonic insulator is that its noninteracting electronic structure is metallic.Comment: 6 pages, 3 figure

Crystal field, ligand field, and interorbital effects in two-dimensional transition metal dichalcogenides across the periodic table

Two-dimensional transition metal dichalcogenides (TMDs) exist in two polymorphs, referred to as $1T$ and $1H$, depending on the coordination sphere of the transition metal atom. The broken octahedral and trigonal prismatic symmetries lead to different crystal and ligand field splittings of the $d$ electron states, resulting in distinct electronic properties. In this work, we quantify the crystal and ligand field parameters of two-dimensional TMDs using a Wannier-function approach. We adopt the methodology proposed by Scaramucci et al. [A. Scaramucci et al., J. Phys.: Condens. Matter 27, 175503 (2015)]. that allows to separate various contributions to the ligand field by choosing different manifolds in the construction of the Wannier functions. We discuss the relevance of the crystal and ligand fields in determining the relative stability of the two polymorphs as a function of the filling of the $d$-shell. Based on the calculated parameters, we conclude that the ligand field, while leading to a small stabilizing factor for the $1H$ polymorph in the $d^1$ and $d^2$ TMDs, plays mostly an indirect role and that hybridization between different $d$ orbitals is the dominant feature. We investigate trends across the periodic table and interpret the variations of the calculated crystal and ligand fields in terms of the change of charge-transfer energy, which allows developing simple chemical intuition.Comment: 16 pages, 14 figure

Properties of galaxies in SDSS Quasar environments at z < 0.2

We analyse the environment of low redshift, z < 0.2, SDSS quasars using the spectral and photometric information of galaxies from the Sloan Digital Sky Survey Third Data Release (SDSS-DR3). We compare quasar neighbourhoods with field and high density environments through an analysis on samples of typical galaxies and groups. We compute the surrounding surface number density of galaxies finding that quasar environments systematically avoid high density regions. Their mean environments correspond to galaxy density enhancements similar to those of typical galaxies. We have also explored several galaxy properties in these environments, such as spectral types, specific star formation rates, concentration indexes, colours and active nuclei activity. We conclude that low redshift quasar neighbourhoods (r_p < 1 Mpc h^-1, Delta V < 500 km/s) are populated by bluer and more intense star forming galaxies of disk-type morphology than galaxies in groups and in the field. Although star formation activity is thought to be significantly triggered by interactions, we find that quasar fueling may not require the presence of a close companion galaxy (r_p < 100 kpc h^-1, Delta V< 350 km/s). As a test of the unified AGN model, we have performed a similar analysis to the neighbours of a sample of active galaxies. The results indicate that these neighbourhoods are comparable to those of quasars giving further support to this unified scenario.Comment: 7 pages, 8 figures, submitted to MNRA