14 research outputs found
Enhancing superconductivity in MXenes through hydrogenation
Two-dimensional transition metal carbides and nitrides (MXenes) are an
emerging class of atomically-thin superconductors, whose characteristics are
highly prone to tailoring by surface functionalization. Here we explore the use
of hydrogen adatoms to enhance phonon-mediated superconductivity in MXenes,
based on first-principles calculations combined with Eliashberg theory. We
first demonstrate the stability of three different structural models of
hydrogenated Mo- and W-based MXenes. Particularly high critical temperatures of
over 30 K are obtained for hydrogenated MoN and WN. Several mechanisms
responsible for the enhanced electron-phonon coupling are uncovered, namely (i)
hydrogen-induced changes in the phonon spectrum of the host MXene, (ii)
emerging hydrogen-based phonon modes, and (iii) charge transfer from hydrogen
to the MXene layer, boosting the density of states at the Fermi level. Finally,
we demonstrate that hydrogen adatoms are moreover able to induce
superconductivity in MXenes that are not superconducting in pristine form, such
as NbC
Superconductivity in functionalized niobium-carbide MXenes
We show the effect of Cl and S functionalization on the superconducting
properties of layered (bulk) and monolayer niobium carbide (NbC) MXene
crystals, based on first-principles calculations combined with Eliashberg
theory. For the bulk layered NbCCl, the calculated superconducting
transition temperature () is in very good agreement with the recently
measured value of 6 K. We show that is enhanced to 10 K for monolayer
NbCCl, due to an increase in the density of states at the Fermi level,
and the corresponding electron-phonon coupling. We further demonstrate a
feasible gate-induced enhancement of up to 40 K for both bulk-layered and
monolayer NbCCl crystals. For the S-functionalized cases our
calculations reveal the importance of phonon softening in understanding their
superconducting properties. Finally, we predict that NbCS in
bulk-layered and monolayer form is potentially superconducting, with a
around 30 K. Considering that NbC is not superconducting in pristine form,
our findings promote functionalization as a pathway towards robust
superconductivity in MXenes
Intrinsic control of interlayer exciton generation rate in van der Waals materials via Janus layers
We demonstrate the possibility of engineering the optical properties of
transition metal dichalcogenide heterobilayers when one of the constitutive
layers has a Janus structure. This has important consequences for the charge
separation efficiency. We investigate different MoS@Janus layer
combinations using first-principles methods including electron-hole
interactions (excitons) and exciton-phonon coupling. The direction of the
intrinsic electric field from the Janus layer modifies the electronic band
alignments and, consequently, the energy separation between interlayer exciton
states -- which usually have a very low oscillator strength and hence are
almost dark in absorption -- and bright in-plane excitons. We find that
in-plane lattice vibrations strongly couple the two states, so that
exciton-phonon scattering may be a viable generation mechanism for interlayer
excitons upon light absorption. In particular, in the case of MoS@WSSe, the
energy separation of the low-lying interlayer exciton from the in-plane exciton
is resonant with the transverse optical phonon modes (40 meV). We thus identify
this heterobilayer as a prime candidate for efficient electron-hole pair
generation with efficient charge carrier separation
Fast micromagnetic simulations on GPU : recent advances made with mumax³
In the last twenty years, numerical modeling has become an indispensable part of magnetism research. It has become a standard tool for both the exploration of new systems and for the interpretation of experimental data. In the last five years, the capabilities of micromagnetic modeling have dramatically increased due to the deployment of graphical processing units (GPU), which have sped up calculations to a factor of 200. This has enabled many studies which were previously unfeasible. In this topical review, we give an overview of this modeling approach and show how it has contributed to the forefront of current magnetism research
Chester supersolid of spatially indirect excitons in double-layer semiconductor heterostructures
A supersolid, a counter-intuitive quantum state in which a rigid lattice of
particles flows without resistance, has to date not been unambiguously
realised. Here we reveal a supersolid ground state of excitons in a
double-layer semiconductor heterostructure over a wide range of layer
separations outside the focus of recent experiments. This supersolid conforms
to the original Chester supersolid with one exciton per supersolid site, as
distinct from the alternative version reported in cold-atom systems of a
periodic modulation of the superfluid density. We provide the phase diagram
augmented by the supersolid. This new phase appears at layer separations much
smaller than the predicted exciton normal solid, and it persists up to a
solid--solid transition where the quantum phase coherence collapses. The ranges
of layer separations and exciton densities in our phase diagram are well within
reach of the current experimental capabilities
Metastable states and hidden phase slips in nanobridge SQUIDs
We fabricated an asymmetric nanoscale SQUID consisting of one nanobridge weak
link and one Dayem bridge weak link. The current phase relation of these
particular weak links is characterized by multivaluedness and linearity. While
the latter is responsible for a particular magnetic field dependence of the
critical current (so-called vorticity diamonds), the former enables the
possibility of different vorticity states (phase winding numbers) existing at
one magnetic field value. In experiments the observed critical current value is
stochastic in nature, does not necessarily coincide with the current associated
with the lowest energy state and critically depends on the measurement
conditions. In this work, we unravel the origin of the observed metastability
as a result of the phase dynamics happening during the freezing process and
while sweeping the current. Moreover, we employ special measurement protocols
to prepare the desired vorticity state and identify the (hidden) phase slip
dynamics ruling the detected state of these nanodevices. In order to gain
insights into the dynamics of the condensate and, more specifically the hidden
phase slips, we performed time-dependent Ginzburg-Landau simulations.Comment: 10 pages, 4 figures, 1 supplementary vide
Observation of a gel of quantum vortices in a superconductor at very low magnetic fields
A gel consists of a network of particles or molecules formed for example using the sol-gel process, by which a solution transforms into a porous solid. Particles or molecules in a gel are mainly organized on a scaffold that makes up a porous system. Quantized vortices in type-II superconductors mostly form spatially homogeneous ordered or amorphous solids. Here we present high-resolution imaging of the vortex lattice displaying dense vortex clusters separated by sparse or entirely vortex-free regions in β-Bi2Pd superconductor. We find that the intervortex distance diverges upon decreasing the magnetic field and that vortex lattice images follow a multifractal behavior. These properties, characteristic of gels, establish the presence of a novel vortex distribution, distinctly different from the well-studied disordered and glassy phases observed in high-temperature and conventional superconductors. The observed behavior is caused by a scaffold of one-dimensional structural defects with enhanced stress close to the defects. The vortex gel might often occur in type-II superconductors at low magnetic fields. Such vortex distributions should allow to considerably simplify control over vortex positions and manipulation of quantum vortex states.Fil: Llorens, José Benito. Universidad Autónoma de Madrid; EspañaFil: Embon, Lior. Weizmann Institute Of Science.; IsraelFil: Correa, Alexandre. Consejo Superior de Investigaciones Científicas; España. Instituto de Ciencia de Materiales de Madrid; EspañaFil: González, Jesús David. Universidad del Magdalena; Colombia. Universiteit Antwerp; BélgicaFil: Herrera, Edwin. Universidad Autónoma de Madrid; España. Universidad Central; ColombiaFil: Guillamón, Isabel. Universidad Autónoma de Madrid; EspañaFil: Luccas, Roberto F.. Consejo Superior de Investigaciones Científicas; EspañaFil: Azpeitia, Jon. Consejo Superior de Investigaciones Científicas; EspañaFil: Mompeán, Federico J.. Consejo Superior de Investigaciones Científicas; EspañaFil: García Hernández, Mar. Consejo Superior de Investigaciones Científicas; EspañaFil: Munuera, Carmen. Consejo Superior de Investigaciones Científicas; EspañaFil: Aragón Sánchez, Jazmín. Comisión Nacional de Energía Atómica. Gerencia del Area de Investigación y Aplicaciones No Nucleares. Gerencia de Física (Centro Atómico Bariloche). División Bajas Temperaturas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología - Nodo Bariloche | Comisión Nacional de Energía Atómica. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología - Nodo Bariloche; ArgentinaFil: Fasano, Yanina. Comisión Nacional de Energía Atómica. Gerencia del Area de Investigación y Aplicaciones No Nucleares. Gerencia de Física (Centro Atómico Bariloche). División Bajas Temperaturas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología - Nodo Bariloche | Comisión Nacional de Energía Atómica. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología - Nodo Bariloche; ArgentinaFil: Milosevic, Milorad V.. Universiteit Antwerp; BélgicaFil: Suderow, Hermann. Universidad Autónoma de Madrid; EspañaFil: Anahory, Yonathan. The Hebrew University of Jerusalem; Israe
Shape-resonant superconductivity in nanofilms: from weak to strong coupling
Ultrathin superconductors of different materials are becoming a powerful
platform to find mechanisms for enhancement of superconductivity, exploiting
shape resonances in different superconducting properties. Here we evaluate the
superconducting gap and its spatial profile, the multiple gap components, and
the chemical potential, of generic superconducting nanofilms, considering the
pairing attraction and its energy scale as tunable parameters, from weak to
strong coupling, at fixed electron density. Superconducting properties are
evaluated at mean field level as a function of the thickness of the nanofilm,
in order to characterize the shape resonances in the superconducting gap. We
find that the most pronounced shape resonances are generated for weakly coupled
superconductors, while approaching the strong coupling regime the shape
resonances are rounded by a mixing of the subbands due to the large energy gaps
extending over large energy scales. Finally, we find that the spatial profile,
transverse to the nanofilm, of the superconducting gap acquires a flat behavior
in the shape resonance region, indicating that a robust and uniform multigap
superconducting state can arise at resonance.Comment: 7 pages, 4 figures. Submitted to the Proceedings of the Superstripes
2016 conferenc
Intrinsic Control of Interlayer Exciton Generation in Van der Waals Materials via Janus Layers
We demonstrate the possibility of engineering the optical properties of transition metal dichalcogenide heterobilayers when one of the constitutive layers has a Janus structure. We investigate different MoS2@Janus layer combinations using first-principles methods including excitons and exciton–phonon coupling. The direction of the intrinsic electric field from the Janus layer modifies the electronic band alignments and, consequently, the energy separation between dark interlayer exciton states and bright in-plane excitons. We find that in-plane lattice vibrations strongly couple the two states, so that exciton–phonon scattering may be a viable generation mechanism for interlayer excitons upon light absorption. In particular, in the case of MoS2@WSSe, the energy separation of the low-lying interlayer exciton from the in-plane exciton is resonant with the transverse optical phonon modes (40 meV). We thus identify this heterobilayer as a prime candidate for efficient generation of charge-separated electron–hole pairs