254 research outputs found

### Phonon-assisted magnetic Mott-insulating state in the charge density wave phase of single-layer 1TNbSe2

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
$\sqrt{13}\times\sqrt{13}~R30^{\circ}$ 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.Comment: 6 pages, 9 picture

### Chemically-exfoliated single-layer MoS$_2$ : stability, lattice dynamics and catalytic adsorption from first principles

Chemically and mechanically exfoliated MoS$_2$ single-layer samples have
substantially different properties. While mechanically exfoliated single-layers
are mono-phase (1H polytype with Mo in trigonal prismatic coordination), the
chemically exfoliated samples show coexistence of three different phases, 1H,
1T (Mo in octahedral coordination) and 1T$^{'}$ (a distorted $2\times 1$
1T-superstructure). By using first-principles calculations, we investigate the
energetics and the dynamical stability of the three phases. We show that the 1H
phase is the most stable one, while the metallic 1T phase, strongly unstable,
undergoes a phase transition towards a metastable and insulating 1T$^{'}$
structure composed of separated zig-zag chains. We calculate electronic
structure, phonon dispersion, Raman frequencies and intensities for the
1T$^{'}$ structure. We provide a microscopical description of the J$_1$, J$_2$
and J$_3$ Raman features first detected more then $20$ years ago, but
unexplained up to now. Finally, we show that H adsorbates, that are naturally
present at the end of the chemical exfoliation process, stabilize the
1T$^{\prime}$ over the 1H one.Comment: 7 Pages, 8 Pictures, To appear on Phys. Rev.

### Projector augmented wave calculation of x-ray absorption spectra at the L2,3 edges

We develop a technique based on density functional theory and the projector
augmented wave method in order to obtain the x-ray absorption cross section at
a general edge, both in the electric dipole and quadrupole approximations. The
method is a generalization of Taillefumier et al., PRB 66, 195107 (2002). We
apply the method to the calculation of the Cu L2,3 edges in fcc copper and
cuprite (Cu2O), and to the S L2,3 edges in molybdenite (2H-MoS2). The role of
core-hole effects, modeled in a supercell approach, as well as the
decomposition of the spectrum into different angular momentum channels are
studied in detail. In copper we find that the best agreement with experimental
data is obtained when core-hole effects are neglected. On the contrary,
core-hole effects need to be included both in Cu2O and 2H-MoS2. Finally we show
that a non-negligible component of S L2,3 edges in 2H-MoS2 involves transition
to states with s character at all energy scales. The inclusion of this angular
momentum channel is mandatory to correctly describe the angular dependence of
the measured spectra. We believe that transitions to s character states are
quantitatively significant at the L2,3 edges of third row elements from Al to
Ar.Comment: 12 pages, 10 picture

### Possibility of superconductivity in graphite intercalated with alkaline earths investigated with density functional theory

Using density functional theory we investigate the occurrence of
superconductivity in AC$_6$ with A=Mg,Ca,Sr,Ba. We predict that at zero
pressure, Ba and Sr should be superconducting with critical temperatures
(T$_c$) 0.2 K and 3.0 K, respectively. We study the pressure dependence of
T$_c$ assuming the same symmetry for the crystal structures at zero and finite
pressures. We find that the SrC$_6$ and BaC$_6$ critical temperatures should be
substantially enhanced by pressure. On the contrary, for CaC$_6$ we find that
in the 0 to 5 GPa region, T$_c$ weakly increases with pressure. The increase is
much smaller than what shown in several recent experiments. Thus we suggest
that in CaC$_6$ a continous phase transformation, such as an increase in
staging, occurs at finite pressure. Finally we argue that, although MgC$_6$ is
unstable, the synthesis of intercalated systems of the kind
Mg$_x$Ca$_{1-x}$C$_y$ could lead to higher critical temperatures.Comment: 9 page

### Charge density wave and superconducting dome in TiSe2 from electron-phonon interaction

At low temperature TiSe2 undergoes a charge density wave instability.
Superconductivity is stabilized either by pressure or by Cu intercalation. We
show that the pressure phase diagram of TiSe2 is well described by
first-principles calculations. At pressures smaller than 4 GPa charge density
wave ordering occurs, in agreement with experiments. At larger pressures the
disappearing of the charge density wave is due to a stiffening of the
short-range force-constants and not to the variation of nesting with pressure.
Finally we show that the behavior of Tc as a function of pressure is entirely
determined by the electron-phonon interaction without need of invoking
excitonic mechanisms. Our work demonstrates that phase-diagrams with competing
orders and a superconducting dome are also obtained in the framework of the
electron-phonon interaction.Comment: 4 pages, 7 picture

### Electronic structure of heavily-doped graphene: the role of foreign atom states

Using density functional theory calculations we investigate the electronic
structure of graphene doped by deposition of foreign atoms. We demonstrate
that, as the charge transfer to the graphene layer increases, the band
structure of the pristine graphene sheet is substantially affected. This is
particularly relevant when Ca atoms are deposed on graphene at CaC$_{6}$
stoichiometry. Similarly to what happens in superconducting graphite
intercalated compounds, a Ca bands occurs at the Fermi level. Its hybridization
with the C states generates a strong non-linearity in one of the $\pi^{*}$
bands below the Fermi level, at energies comparable to the graphene E$_{2g}$
phonon frequency. This strong non-linearity, and not manybody effects as
previously proposed, explains the large and anisotropic values of the apparent
electron-phonon coupling measured in angular resolved photoemission.Comment: 4 pages, 2 figures, see also M. Calandra and F. Mauri,arXiv:0707.146

### Charge density wave and spin $1/2$ insulating state in single layer 1T-NbS$_2$

In bulk samples and few layer flakes, the transition metal dichalcogenides
NbS$_2$ and NbSe$_2$ assume the H polytype structure with trigonal prismatic
coordination of the Nb atom. Recently, however, single and few layers of
1T-NbSe$_2$ 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$_2$. We find that single-layer 1T-NbS$_2$
undergoes a $\sqrt{13}\times\sqrt{13}$ 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 $1/2$ magnetic state with opening of a $0.15$ eV band gap and a
$0.21\mu_B$ magnetic moment localized on the central Nb in the star. Within
GGA+U, the magnetic moment on the central Nb is enhanced to $0.41\mu_{B}$ and a
larger gap occurs. Most important, this approximation gives a small energy
difference between the 1T and 1H polytypes (only $+0.5$ mRy/Nb), suggesting
that the 1T-polytype can be synthesized in a similar way as done for single
layer 1T-NbSe$_2$. Finally we compute first and second nearest neighbors
magnetic inter-star exchange interactions finding $J_1$=9.5~K and $J_2$=0.4~K
ferromagnetic coupling constants

### Superconductivity in C6Ca

Using density functional theory we demonstrate that superconductivity in C6Ca
is due to a phonon-mediated mechanism with electron-phonon coupling 0.83 and
phonon-frequency logarithmic-average 24.7 meV. The calculated isotope exponents
are 0.24 for Ca and 0.26 for C. Superconductivity is mostly due C vibrations
perpendicular and Ca vibrations parallel to the graphite layers. Since the
electron-phonon couplings of these modes are activated by the presence of an
intercalant Fermi surface, the occurrence of superconductivity in graphite
intercalated compounds requires a non complete ionization of the intercalant.Comment: 4 pages, 3 figure

### Electronic structure and magnetic properties of few-layer Cr$_2$Ge$_2$Te$_6$: the key role of nonlocal electron-electron interaction effects

Atomically-thin magnetic crystals have been recently isolated experimentally,
greatly expanding the family of two-dimensional materials. In this Article we
present an extensive comparative analysis of the electronic and magnetic
properties of ${\rm Cr}_2{\rm Ge}_2{\rm Te}_6$, based on density functional
theory (DFT). We first show that the often-used ${\rm DFT}+U$ approaches fail
in predicting the ground-state properties of this material in both its
monolayer and bilayer forms, and even more spectacularly in its bulk form. In
the latter case, the fundamental gap {\it decreases} by increasing the
Hubbard-$U$ parameter, eventually leading to a metallic ground state for
physically relevant values of $U$, in stark contrast with experimental data. On
the contrary, the use of hybrid functionals, which naturally take into account
nonlocal exchange interactions between all orbitals, yields good account of the
available ARPES experimental data. We then calculate all the relevant exchange
couplings (and the magneto-crystalline anisotropy energy) for monolayer,
bilayer, and bulk ${\rm Cr}_2{\rm Ge}_2{\rm Te}_6$ with a hybrid functional,
with super-cells containing up to $270$ atoms, commenting on existing
calculations with much smaller super-cell sizes. In the case of bilayer ${\rm
Cr}_2{\rm Ge}_2{\rm Te}_6$, we show that two distinct intra-layer
second-neighbor exchange couplings emerge, a result which, to the best of our
knowledge, has not been noticed in the literature.Comment: 13 pages, 6 figures, 3 table

### High-$T_c$ superconductivity in weakly electron-doped HfNCl

We investigate the magnetic and superconducting properties in electron-doped
Li$_x$HfNCl. HfNCl is a band insulator that undergoes an insulator to
superconductor transition upon doping at $x\approx0.13$. The persistence of the
insulating state for $x<0.13$ is due to an Anderson transition probably related
to Li disorder. In the metallic and superconducting phase, Li$_x$HfNCl is a
prototype two-dimensional two-valley electron gas with parabolic bands. By
performing a model random phase approximation approach as well as
first-principles range-separated Heyd-Scuseria-Ernzerhof (HSE06) calculations,
we find that the spin susceptibility $\chi_s$ is strongly enhanced in the low
doping regime by the electron-electron interaction. Furthermore, in the low
doping limit, the exchange interaction renormalizes the intervalley
electron-phonon coupling and results in a strong increase of the
superconducting critical temperature for $x<0.15$. On the contrary, for
$x>0.15$, $T_c$ is approximately constant, in agreement with experiments. At
$x=0.055$ we found that $T_c$ can be as large as 40 K, suggesting that the
synthesis of cleaner samples of Li$_x$HfNCl could remove the Anderson
insulating state competing with superconductivity and generate a high-$T_c$
superconductor.Comment: 8 pages, 6 figure

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