15 research outputs found
Charge density wave and spin insulating state in single layer 1T-NbS
In bulk samples and few layer flakes, the transition metal dichalcogenides
NbS and NbSe assume the H polytype structure with trigonal prismatic
coordination of the Nb atom. Recently, however, single and few layers of
1T-NbSe with octahedral coordination around the transition metal ion were
synthesized. Motivated by these experiments and by using first-principles
calculations, we investigate the structural, electronic and dynamical
properties of single layer 1T-NbS. We find that single-layer 1T-NbS
undergoes a star-of-David charge density wave.
Within the generalized gradient approximation, the weak interaction between the
stars leads to an ultraflat band at the Fermi level isolated from all other
bands. The spin-polarized generalized gradient approximation stabilizes a total
spin magnetic state with opening of a eV band gap and a
magnetic moment localized on the central Nb in the star. Within
GGA+U, the magnetic moment on the central Nb is enhanced to and a
larger gap occurs. Most important, this approximation gives a small energy
difference between the 1T and 1H polytypes (only mRy/Nb), suggesting
that the 1T-polytype can be synthesized in a similar way as done for single
layer 1T-NbSe. Finally we compute first and second nearest neighbors
magnetic inter-star exchange interactions finding =9.5~K and =0.4~K
ferromagnetic coupling constants
Compressed tetragonal phase in XFe2As2 (X = Na, K, Rb, Cs) and in the alloy Na0.5K0.5Fe2As2
Motivated by the recent discovery of a high-temperature superconducting
phase in KFe2As2 at 16 GPa accompanied by an isostructural phase
transition from the tetragonal to the compressed tetragonal phase, we
extend the study of pressure effects to the whole XFe2As2 family (X =
Na, K, Rb, Cs). We demonstrate that the ionic radius of the X atom
determines the transition to the compressed phase which induces relevant
changes in electronic properties and the Fermi surface which can enhance
the superconducting pairing. Based on these results and in analogy with
KFe2As2, we theoretically propose Na0.5K0.5Fe2As2 as a possible new
superconductor material in its compressed tetragonal phase which we
predict to happen above 6 GPa
Strain effects in monolayer iron-chalcogenide superconductors
Successful fabrication of one monolayer FeSe on SrTiO3 represented a real breakthrough in searching for high-Tc Fe-based superconductors ([1]). Motivated by this important discovery, we studied the effects of tensile strain on one monolayer and bulk iron-chalcogenide superconductors (FeSe and FeTe), showing that it produces important magnetic and electronic changes in the systems. We found that the magnetic ground state of bulk and monolayer FeSe is the block-checkerboard phase, which turns into the collinear stripe phase under in-plane tensile strain. FeTe, in both bulk and monolayer phases, shows two magnetic transitions upon increasing the tensile strain: from bicollinear in the ground state to block-checkerboard ending up to the collinear antiferromagnetic phase which could bring it in the superconducting state. Finally, the study of the mechanical properties of both FeSe and FeTe monolayers reveals their enormous tensile strain limits and opens the possibility to grow them on different substrates
Ab initio study of the (2 x 2) phase of barium on graphene
We present a first-principles density functional theory study on the structural, electronic and dynamical properties of a novel barium doped graphene phase. Low energy electron diffraction of barium doped graphene presents clear evidence of (2 x 2) spots induced by barium adatoms with BaC8 stoichiometry. First principles calculations reveals that the phase is thermodynamically stable but unstable to segregation towards the competitive BaC6 monolayer phase. The calculation of phonon spectrum confirms the dynamical stability of the BaC8 phase indicating its metastability, probably stabilized by doping and strain conditions due to the substrate. Barium induces a relevant doping of the graphene pi states and new barium-derived hole Fermi surface at the M-point of the (2 x 2) Brillouin zone. In view of possible superconducting phase induced by foreign dopants in graphene, we studied the electron-phonon coupling of this novel (2 x 2) obtaining lambda = 0.26, which excludes the stabilization of a superconducting phase
Why mercury is a superconductor
Despite being the oldest known superconductor, solid mercury is mysteriously absent from all current computational databases of superconductors. In this Research Letter, we present a critical study of its superconducting properties based on state-of-the-art superconducting density functional theory. Our calculations reveal numerous anomalies in electronic and lattice properties, which can mostly be handled, with due care, by modern ab initio techniques. In particular, we highlight an anomalous role of spin-orbit coupling in the dynamical stability and of semicore d levels in the effective Coulomb interaction and, ultimately, the critical temperature
Clarifying the apparent flattening of the graphene band near the van Hove singularity
Graphene band renormalization at the proximity of the van Hove singularity
(VHS) has been investigated by angle-resolved photoemission spectroscopy
(ARPES) on the Li-doped quasi-freestanding graphene on the cobalt (0001)
surface. The absence of graphene band hybridization with the substrate, the
doping contribution well represented by a rigid energy shift and the excellent
electron-electron interaction screening ensured by the metallic substrate offer
a privileged point of view for such investigation. A clear ARPES signal is
detected along the M point of the graphene Brillouin zone, giving rise to an
apparent flattened band. By simulating the graphene spectral function from the
density functional theory calculated bands, we demonstrate that the
photoemission signal along the M point originates from the "shadow" of the
spectral function of the unoccupied band above the Fermi level. Such
interpretation put forward the absence of any additional strong correlation
effects at the VHS proximity, reconciling the mean field description of the
graphene band structure even in the highly doped scenario
Superconductivity induced by gate-driven hydrogen intercalation in the charge-density-wave compound 1T-TiSe2
Hydrogen (H) plays a key role in the near-to-room temperature superconductivity of hydrides at megabar pressures. This suggests that H doping could have similar effects on the electronic and phononic spectra of materials at ambient pressure as well. Here, we demonstrate the non-volatile control of the electronic ground state of titanium diselenide (1T-TiSe2) via ionic liquid gating-driven H intercalation. This protonation induces a superconducting phase, observed together with a charge-density wave through most of the phase diagram, with nearly doping-independent transition temperatures. The H-induced superconducting phase is possibly gapless-like and multi-band in nature, in contrast with those induced in TiSe2 via copper, lithium, and electrostatic doping. This unique behavior is supported by ab initio calculations showing that high concentrations of H dopants induce a full reconstruction of the bandstructure, although with little coupling between electrons and high-frequency H phonons. Our findings provide a promising approach for engineering the ground state of transition metal dichalcogenides and other layered materials via gate-controlled protonation.ISSN:2399-365
Coexisting superconductivity and charge-density wave in hydrogen-doped titanium diselenide via ionic liquid gating-induced protonation
The doping of correlated materials with various atomic and molecular species
is a staple in the tuning and understanding of their electronic ground states
and in the engineering of exotic quantum phenomena. In particular, the recent
discovery of near-to-room temperature superconductivity in hydrides under
pressure has highlighted the potential of hydrogen as a dopant and to tune the
electronic and the phononic spectra of a material. Here, we demonstrate the
non-volatile control of the electronic ground state of octahedral titanium
diselenide (1T-TiSe) by means of electric field-driven hydrogen
intercalation via the ionic liquid gating method. We show that in HTiSe
charge-density waves and superconductivity coexist through most of the phase
diagram, with nearly doping-independent transition temperatures. The
superconducting phase of HTiSe is gapless-like and possibly multi-band
in nature, setting it apart from what observed in TiSe via electrostatic
gating and copper- or lithium- intercalation. The uniqueness of hydrogen doping
is supported by ab initio calculations showing that its impact is not limited
to a rigid electron doping of pristine TiSe, but it can attain a full
reconstruction of the band structure. These findings open a new route towards
high-temperature superconductivity in hydrogen-rich layered compounds at
ambient pressure.Comment: Main text: 11 pages, 5 figures; Supplementary: 10 pages, 8 figure
Atomic-scale distortions and temperature-dependent large pseudogap in thin films of the parent iron-chalcogenide superconductor Fe1+yTe
We investigate with scanning tunneling microscopy/spectroscopy (STM/STS) and density functional theory (DFT) calculations the surface structures and the electronic properties of Fe1+yTe thin films grown by pulsed laser deposition. Contrary to the regular arrangement of antiferromagnetic nanostripes previously reported on cleaved single-crystal samples, the surface of Fe1+yTe thin films displays a peculiar distribution of spatially inhomogeneous nanostripes. Both STM and DFT calculations show the bias-dependent nature of such features and support the interpretation of spin-polarized tunneling between the FeTe surface and an unintentionally magnetized tip. In addition, the spatial inhomogeneity is interpreted as a purely electronic effect related to changes in hybridization and Fe-Fe bond length driven by local variations in the concentration of excess interstitial Fe cations. Unexpectedly, the surface density of states measured by STS strongly evolves with temperature in close proximity to the antiferromagnetic-paramagnetic first-order transition, and reveals a large pseudogap of 180-250 meV at about 50-65 K. We believe that in this temperature range a phase transition takes place, and the system orders and locks into particular combinations of orbitals and spins because of the interplay between excess interstitial magnetic Fe and strongly correlated d-electrons