184 research outputs found
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
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
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
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
Persistence of structural distortion and bulk band Rashba splitting in SnTe above its ferroelectric critical temperature
The ferroelectric semiconductor -SnTe has been regarded as a
topological crystalline insulator and the dispersion of its surface states has
been intensively measured with angle-resolved photoemission spectroscopy
(ARPES) over the last decade. However, much less attention has been given to
the impact of the ferroelectric transition on its electronic structure, and in
particular on its bulk states. Here, we investigate the low-energy electronic
structure of -SnTe with ARPES and follow the evolution of the
bulk-state Rashba splitting as a function of temperature, across its
ferroelectric critical temperature of about K. Unexpectedly, we
observe a persistent band splitting up to room temperature, which is consistent
with an order-disorder contribution to the phase transition that requires the
presence of fluctuating local dipoles above . We conclude that no
topological surface state can occur at the (111) surface of SnTe, at odds with
recent literature.Comment: 26 pages, 8 figure
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
Atomically Sharp Interface in an h-BN-epitaxial graphene van der Waals Heterostructure
International audienceStacking various two-dimensional atomic crystals is a feasible approach to creating unique multilayered van der Waals heterostructures with tailored properties. Herein for the first time, we present a controlled preparation of large-area h-BN/graphene heterostructures via a simple chemical deposition of h-BN layers on epitaxial graphene/SiC(0001). Van der Waals forces, which are responsible for the cohesion of the multilayer system, give rise to an abrupt interface without interdiffusion between graphene and h-BN, as shown by X-ray Photoemission Spectroscopy (XPS) and direct observation using scanning and High-Resolution Transmission Electron Microscopy (STEM/HRTEM). The electronic properties of graphene, such as the Dirac cone, remain intact and no significant charge transfer i.e. doping, is observed. These results are supported by Density Functional Theory (DFT) calculations. We demonstrate that the h-BN capped graphene allows the fabrication of vdW heterostructures without altering the electronic properties of graphene
Altermagnetic lifting of Kramers spin degeneracy
Lifted Kramers spin-degeneracy has been among the central topics of
condensed-matter physics since the dawn of the band theory of solids. It
underpins established practical applications as well as current frontier
research, ranging from magnetic-memory technology to topological quantum
matter. Traditionally, lifted Kramers spin-degeneracy has been considered to
originate from two possible internal symmetry-breaking mechanisms. The first
one refers to time-reversal symmetry breaking by magnetization of ferromagnets,
and tends to be strong due to the non-relativistic exchange-coupling origin.
The second mechanism applies to crystals with broken inversion symmetry, and
tends to be comparatively weaker as it originates from the relativistic
spin-orbit coupling. A recent theory work based on spin-symmetry classification
has identified an unconventional magnetic phase, dubbed altermagnetic, that
allows for lifting the Kramers spin degeneracy without net magnetization and
inversion-symmetry breaking. Here we provide the confirmation using
photoemission spectroscopy and ab initio calculations. We identify two distinct
unconventional mechanisms of lifted Kramers spin degeneracy generated by the
altermagnetic phase of centrosymmetric MnTe with vanishing net magnetization.
Our observation of the altermagnetic lifting of the Kramers spin degeneracy can
have broad consequences in magnetism. It motivates exploration and exploitation
of the unconventional nature of this magnetic phase in an extended family of
materials, ranging from insulators and semiconductors to metals and
superconductors, that have been either identified recently or perceived for
many decades as conventional antiferromagnets
Nonlinear Dynamics Analysis of FGM Shell Structures with a Higher Order Shear Strain Enhanced Solid-Shell Element
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