103 research outputs found
Alloyed surfaces: new substrates for graphene growth
We report a systematic ab-initio density functional theory investigation of Ni(111) surface alloyed with elements of group
IV (Si, Ge and Sn), demonstrating the possibility to use it to grow high quality graphene. Ni(111) surface represents
an ideal substrate for graphene, due to its catalytic properties and perfect matching with the graphene lattice constant.
However, Dirac bands of graphene growth on Ni(111) are completely destroyed due to the strong hybridization between
carbon pz and Ni d orbitals. Group IV atoms, namely Si, Ge and Sn, once deposited on Ni(111) surface, form an ordered
alloyed surface with √
3 ×
√
3-R30◦
reconstruction. We demonstrate that, at variance with the pure Ni(111) surface,
alloyed surfaces effectively decouple graphene from the substrate, resulting unstrained due to the nearly perfect lattice
matching and preserves linear Dirac bands without the strong hybridization with Ni d states. The proposed surfaces can
be prepared before graphene growth without resorting on post-growth processes which necessarily alter the electronic
and structural properties of graphene
Clarifying the apparent flattening of the graphene band near the van Hove singularity
Graphene band renormalization near the van Hove singularity (VHS) has been investigated by angle-resolved photoemission spectroscopy (ARPES) on Li-doped quasifreestanding graphene on a 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 an investigation. A clear ARPES signal is detected along the KMK direction 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 around the M point originates from the "tail"of the spectral function of the unoccupied band above the Fermi level. Such an interpretation puts forward the absence of any additional strong correlation effects near the VHS, reconciling the mean-field description of the graphene band structure even in a highly doped scenario
On the importance of measuring accurately LDOS maps using scanning tunneling spectroscopy in materials presenting atom-dependent charge order: the case of the correlated Pb/Si(111) single atomic layer
We show how to properly extract the local charge order in two-dimensional
materials from scanning tunneling microscopy/spectroscopy (STM/STS)
measurements. When the charge order presents spatial variations at the atomic
scale inside the unit cell and is energy dependent, particular care should be
taken. In such cases the use of the lock-in technique, while acquiring an STM
topography in closed feedback loop, leads to systematically incorrect dI/dV
measurements giving a false local charge order. A correct method is either to
perform a constant height measurement or to perform a full grid of dI/dV(V)
spectroscopies, using a bias voltage setpoint outside the material bandwidth
where the local density-of-states (LDOS) is spatially homogeneous. We take as a
paradigmatic example of two-dimensional material the 1/3 single-layer
Pb/Si(111). As large areas of this phase cannot be grown, charge ordering in
this system is not accessible to angular resolved photoemission or grazing
x-ray diffraction. Previous investigations by STM/STS supplemented by {\it ab
initio} Density Functional Theory (DFT) calculations concluded that this
material undergoes a phase transition to a low-temperature
reconstruction where one Pb atom moves up, the two remaining Pb atoms shifting
down. A third STM/STS study by Adler {\it et al.} [PRL 123, 086401 (2019)] came
to the opposite conclusion, i.e. that two Pb atoms move up, while one Pb atom
shifts down. This latter erroneous conclusion comes from a misuse of the
lock-in technique. In contrast, using a full grid of dI/dV(V) spectroscopy
measurements, we show that the energy-dependent LDOS maps agree very well with
state-of-the-art DFT calculations confirming the one-up two-down charge
ordering. This structural and charge re-ordering in the unit cell
is equally driven by electron-electron interactions and the coupling to the
substrate.Comment: 11 pages, 3 figure
Nonlinear rheology of colloidal dispersions
Colloidal dispersions are commonly encountered in everyday life and represent
an important class of complex fluid. Of particular significance for many
commercial products and industrial processes is the ability to control and
manipulate the macroscopic flow response of a dispersion by tuning the
microscopic interactions between the constituents. An important step towards
attaining this goal is the development of robust theoretical methods for
predicting from first-principles the rheology and nonequilibrium microstructure
of well defined model systems subject to external flow. In this review we give
an overview of some promising theoretical approaches and the phenomena they
seek to describe, focusing, for simplicity, on systems for which the colloidal
particles interact via strongly repulsive, spherically symmetric interactions.
In presenting the various theories, we will consider first low volume fraction
systems, for which a number of exact results may be derived, before moving on
to consider the intermediate and high volume fraction states which present both
the most interesting physics and the most demanding technical challenges. In
the high volume fraction regime particular emphasis will be given to the
rheology of dynamically arrested states.Comment: Review articl
Laser pulse propagation and enhanced energy coupling to fast electrons in dense plasma gradients
Laser energy absorption to fast electrons during the interaction of an ultra-intense (1020 W/cm2), picosecond laser pulse with a solid is investigated, experimentally and numerically, as a function of the plasma density scale length at the irradiated surface. It is shown that there is an optimum density gradient for efficient energy coupling to electrons and that this arises due to strong self-focusing and channeling driving energy absorption over an extended length in the preformed plasma. At longer density gradients the laser laments, resulting in significantly lower overall energy coupling. As the scale length is further increased, a transition to a second laser energy absorption process is observed experimentally via multiple diagnostics. The results demonstrate that it is possible to significantly enhance laser energy absorption and coupling to fast electrons by dynamically controlling the plasma density gradient
Injection and transport properties of fast electrons in ultraintense laser-solid interactions
Fast electron injection and transport in solid foils irradiated by sub-picosecond-duration laser pulses with peak intensity equal to 4 x 10(20)W/cm(2) is investigated experimentally and via 3D simulations. The simulations are performed using a hybrid-particle-in-cell (PIC) code for a range of fast electron beam injection conditions, with and without inclusion of self-generated resistive magnetic fields. The resulting fast electron beam transport properties are used in rear-surface plasma expansion calculations to compare with measurements of proton acceleration, as a function of target thickness. An injection half-angle of similar to 50 degrees - 70 degrees is inferred, which is significantly larger than that derived from previous experiments under similar conditions
Deep-underground dark matter search with a COSINUS detector prototype
Sodium iodide (NaI) based cryogenic scintillating calorimeters using quantum
sensors for signal read out have shown promising first results towards a
model-independent test of the annually modulating signal detected by the
DAMA/LIBRA dark matter experiment. The COSINUS collaboration has previously
reported on the first above-ground measurements using a dual channel readout of
phonons and light based on transition edge sensors (TESs) that allows for
particle discrimination on an event-by-event basis. In this letter, we outline
the first underground measurement of a NaI cryogenic calorimeter read out via
the novel remoTES scheme. A 3.67 g NaI absorber with an improved silicon light
detector design was operated at the Laboratori Nazionali del Gran Sasso, Italy.
A significant improvement in the discrimination power of /-events
to nuclear recoils was observed with a five-fold improvement in the nuclear
recoil baseline resolution, achieving = 441 eV. Furthermore, we
present a limit on the spin-independent dark-matter nucleon elastic scattering
cross-section achieving a sensitivity of (pb) with an exposure of
only 11.6 g d.Comment: 11 pages, 14 figure
Particle discrimination in a NaI crystal using the COSINUS remote TES design
The COSINUS direct dark matter experiment situated at Laboratori Nazionali
del Gran Sasso in Italy is set to investigate the nature of the annually
modulating signal detected by the DAMA/LIBRA experiment. COSINUS has already
demonstrated that sodium iodide crystals can be operated at mK temperature as
cryogenic scintillating calorimeters using transition edge sensors, despite the
complication of handling a hygroscopic and low melting point material. With
results from a new COSINUS prototype, we show that particle discrimination on
an event-by-event basis in NaI is feasible using the dual-channel readout of
both phonons and scintillation light. The detector was mounted in the novel
remoTES design and operated in an above-ground facility for 9.06 gd of
exposure. With a 3.7 g NaI crystal, e/ events could be clearly
distinguished from nuclear recoils down to the nuclear recoil energy threshold
of 15 keV.Comment: 7 pages, 9 figure
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