224 research outputs found
Surface effects on the Mott-Hubbard transition in archetypal VO
We present an experimental and theoretical study exploring surface effects on
the evolution of the metal-insulator transition in the model Mott-Hubbard
compound Cr-doped VO. We find a microscopic domain formation that is
clearly affected by the surface crystallographic orientation. Using scanning
photoelectron microscopy and X-ray diffraction, we find that surface defects
act as nucleation centers for the formation of domains at the
temperature-induced isostructural transition and favor the formation of
microscopic metallic regions. A density functional theory plus dynamical mean
field theory study of different surface terminations shows that the surface
reconstruction with excess vanadyl cations leads to doped, and hence more
metallic surface states, explaining our experimental observations.Comment: 5 pages, 4 figure
Investigation on the mechanically-induced nanocrystallization in metallic glasses
Shear-induced nanocrystallization in bent ribbons of Pd40Cu30Ni10P20 metallic glass has been quantitatively investigated via synchrotron radiation. The formed nanocrystals volume fraction during deformation has been directly estimated from X-ray diffraction spectra using peaks area integration. The nanocrystallization process during deformation was found to be strongly linked with the microstructure configuration of shear bands in amorphous alloys. A constitutive model based on free volume approach has been introduced to describe the kinetic of mechanically induced nanocrystallization. The solution of the coupled constitutive equations of the model, fitted to experimental data, permits to determine the physical and mechanical parameters governing the phenomena of shear-induced crystallization in metallic glasses
Selective scattering between Floquet-Bloch and Volkov states in a topological insulator
The coherent optical manipulation of solids is emerging as a promising way to
engineer novel quantum states of matter. The strong time periodic potential of
intense laser light can be used to generate hybrid photon-electron states.
Interaction of light with Bloch states leads to Floquet-Bloch states which are
essential in realizing new photo-induced quantum phases. Similarly, dressing of
free electron states near the surface of a solid generates Volkov states which
are used to study non-linear optics in atoms and semiconductors. The
interaction of these two dynamic states with each other remains an open
experimental problem. Here we use Time and Angle Resolved Photoemission
Spectroscopy (Tr-ARPES) to selectively study the transition between these two
states on the surface of the topological insulator Bi2Se3. We find that the
coupling between the two strongly depends on the electron momentum, providing a
route to enhance or inhibit it. Moreover, by controlling the light polarization
we can negate Volkov states in order to generate pure Floquet-Bloch states.
This work establishes a systematic path for the coherent manipulation of solids
via light-matter interaction.Comment: 21 pages, 6 figures, final version to appear in Nature Physic
Observation of Weyl and Dirac fermions at smooth topological Volkov-Pankratov heterojunctions
Weyl and Dirac relativistic fermions are ubiquitous in topological matter.
Their relativistic character enables high energy physics phenomena like the
chiral anomaly to occur in solid state, which allows to experimentally probe
and explore fundamental relativistic theories. Here we show that on smooth
interfaces between a trivial and a topological material, massless Weyl and
massive Dirac fermions intrinsically coexist. The emergence of the latter,
known as Volkov-Pankratov states, is directly revealed by magneto-optical
spectroscopy, evidencing that their energy spectrum is perfectly controlled by
the smoothness of topological interface. Simultaneously, we reveal the optical
absorption of the zero-energy chiral Weyl state, whose wavefunction is
drastically transformed when the topological interface is smooth. Artificial
engineering of the topology profile thus provides a novel textbook system to
explore the rich relativistic energy spectra in condensed matter
heterostructures.Comment: 21 pages 10 figure
A new critical curve for the Lane-Emden system
We study stable positive radially symmetric solutions for the Lane-Emden
system in , in , where .
We obtain a new critical curve that optimally describes the existence of such
solutions.Comment: 13 pages, 1 figur
Tuning a Schottky barrier in a photoexcited topological insulator with transient Dirac cone electron-hole asymmetry
The advent of Dirac materials has made it possible to realize two dimensional
gases of relativistic fermions with unprecedented transport properties in
condensed matter. Their photoconductive control with ultrafast light pulses is
opening new perspectives for the transmission of current and information. Here
we show that the interplay of surface and bulk transient carrier dynamics in a
photoexcited topological insulator can control an essential parameter for
photoconductivity - the balance between excess electrons and holes in the Dirac
cone. This can result in a strongly out of equilibrium gas of hot relativistic
fermions, characterized by a surprisingly long lifetime of more than 50 ps, and
a simultaneous transient shift of chemical potential by as much as 100 meV. The
unique properties of this transient Dirac cone make it possible to tune with
ultrafast light pulses a relativistic nanoscale Schottky barrier, in a way that
is impossible with conventional optoelectronic materials.Comment: Nature Communications, in press (12 pages, 6 figures
Ultrafast surface carrier dynamics in the topological insulator Bi2Te3
We discuss the ultrafast evolution of the surface electronic structure of the
topological insulator BiTe following a femtosecond laser excitation.
Using time and angle resolved photoelectron spectroscopy, we provide a direct
real-time visualisation of the transient carrier population of both the surface
states and the bulk conduction band. We find that the thermalization of the
surface states is initially determined by interband scattering from the bulk
conduction band, lasting for about 0.5 ps; subsequently, few ps are necessary
for the Dirac cone non-equilibrium electrons to recover a Fermi-Dirac
distribution, while their relaxation extends over more than 10 ps. The surface
sensitivity of our measurements makes it possible to estimate the range of the
bulk-surface interband scattering channel, indicating that the process is
effective over a distance of 5 nm or less. This establishes a correlation
between the nanoscale thickness of the bulk charge reservoir and the evolution
of the ultrafast carrier dynamics in the surface Dirac cone
Chemical composition and antibacterial activity of essential oils from the medicinal plant Mentha cervina L. grown in Portugal
Mentha cervina is a medicinal plant traditionally
used in Portugal in folk medicine, in different gastric
disorders and inflammations of the respiratory tract. In
order to validate those traditional uses, M. cervina essential
oils (EOs) were characterized by GC and GC–MS and their
antimicrobial activity was tested against 23 bacterial strains
(including multiresistant strains). The EOs were dominated
by the monoterpenes pulegone (52–75%), isomenthone
(8–24%), limonene (4–6%), and menthone (1–2%). The
antibacterial activity of these EOs was compared to that of
the main components standards. The most effective antibacterial
activity was expressed by the EOs against the
Gram-negative bacteria, Escherichia coli and Acinetobacter
baumanni, with MIC values of 1 mg/ml. The EOs complex mixtures were more active than the individual
aromatic components supporting the hypothesis that the
EOs antibacterial activity is a function of the synergistic
effect of their different aromatic components. These results
show the potential role of M. cervina EOs as antibacterial
agents and validate the traditional use of this plant
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