67 research outputs found
Optical properties of V2O3 in its whole phase diagram
Vanadium sesquioxide V2O3 is considered a textbook example of Mott-Hubbard
physics. In this paper we present an extended optical study of its whole
temperature/doping phase diagram as obtained by doping the pure material with
M=Cr or Ti atoms (V1-xMx)2O3. We reveal that its thermodynamically stable
metallic and insulating phases, although macroscopically equivalent, show very
different low-energy electrodynamics. The Cr and Ti doping drastically change
both the antiferromagnetic gap and the paramagnetic metallic properties. A
slight chromium content induces a mesoscopic electronic phase separation, while
the pure compound is characterized by short-lived quasiparticles at high
temperature. This study thus provides a new comprehensive scenario of the
Mott-Hubbard physics in the prototype compound V2O3
Inequivalent routes across the Mott transition in V2O3 explored by X-ray absorption
The changes in the electronic structure of V2O3 across the metal-insulator
transition induced by temperature, doping and pressure are identified using
high resolution x-ray absorption spectroscopy at the V pre K-edge. Contrary to
what has been taken for granted so far, the metallic phase reached under
pressure is shown to differ from the one obtained by changing doping or
temperature. Using a novel computational scheme, we relate this effect to the
role and occupancy of the a1g orbitals. This finding unveils the inequivalence
of different routes across the Mott transition in V2O
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
Electronic structure of the α-(BEDT-TTF)2I3 surface by photoelectron spectroscopy
We report anomalies observed in photoelectron spectroscopy measurements performed on α-(BEDT-TTF)2I3 crystals. In particular, above its metal-insulator transition temperature (T ' 135 K), we observe the lack of a sharp Fermi edge in contradiction with the metallic transport properties exhib- ited by this quasi-bidimensional organic material. We interpret these unusual results as a signature of a one-dimensional electronic behavior confirmed by DFT band structure calculations. Using photoelectron spectroscopy we probe a Luttinger liquid with a large correlation parameter (α ą 1) that we interpret to be caused by the chain-like electronic structure of α-(BEDT-TTF)2I3 surface doped by iodine defects. These new surface effects are inaccessible by bulk sensitive measurements of electronic transport techniques
Valence band electronic structure of V2O3: identification of V and O bands
We present a comprehensive study of the photon energy dependence of the
valence band photoemission yield in the prototype Mott-Hubbard oxide V2O3. The
analysis of our experimental results, covering an extended photon energy range
(20-6000 eV) and combined with GW calculations, allow us to identify the nature
of the orbitals contributing to the total spectral weight at different binding
energies, and in particular to locate the V 4s at about 8 eV binding energy.
From this comparative analysis we can conclude that the intensity of the
quasiparticle photoemission peak, observed close to the Fermi level in the
paramagnetic metallic phase upon increasing photon energy, does not have a
significant correlation with the intensity variation of the O 2p and V 3d
yield, thus confirming that bulk sensitivity is an essential requirement for
the detection of this coherent low energy excitation
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