173 research outputs found
Probing Wigner correlations in a suspended carbon nanotube
The influence of the electron-vibron coupling on the transport properties of
a strongly interacting quantum dot built in a suspended carbon nanotube is
analyzed. The latter is probed by a charged AFM tip scanned along the axis of
the CNT which induces oscillations of the chemical potential and of the linear
conductance. These oscillations are due to the competition between finite-size
effects and the formation of a Wigner molecule for strong interactions. Such
oscillations are shown to be suppressed by the electron-vibron coupling. The
suppression is more pronounced in the regime of weak Coulomb interactions,
which ensures that probing Wigner correlations in such a system is in principle
possible
Theory of the STM detection of Wigner molecules in spin incoherent CNTs
The linear conductance of a carbon nanotube quantum dot in the Wigner
molecule regime, coupled to two scanning tunnel microscope tips is inspected.
Considering the high temperature regime, the nanotube quantum dot is described
by means of the spin-incoherent Luttinger liquid picture. The linear
conductance exhibits spatial oscillations induced by the presence of the
correlated, molecular electron state. A power-law scaling of the electron
density and of the conductance as a function of the interaction parameter are
found. They confirm local transport as a sensitive tool to investigate the
Wigner molecule. The double-tip setup allows to explore different transport
regimes with different shapes of the spatial modulation, all bringing
information about the Wigner molecule
Fractional Wigner crystal in the helical Luttinger liquid
The properties of the strongly interacting edge states of two dimensional
topological insulators in the presence of two particle backscattering are
investigated. We find an anomalous behavior of the density-density correlation
functions, which show oscillations that are neither of Friedel nor of Wigner
type: they instead represent a Wigner crystal of fermions of fractional charge
e/2, with e the electron charge. By studying the Fermi operator, we show that
the state characterized by such fractional oscillations still bears the
signatures of spin momentum locking. Finally, we compare the spin-spin
correlation functions and the density-density correlation functions to argue
that the fractional Wigner crystal is characterized by a non trivial spin
texture.Comment: 5 pages, 2 figure
AFM probe for the signatures of Wigner correlations in the conductance of a one-dimensional quantum dot
The transport properties of an interacting one-dimensional quantum dot
capacitively coupled to an atomic force microscope probe are investigated. The
dot is described within a Luttinger liquid framework which captures both
Friedel and Wigner oscillations. In the linear regime, we demonstrate that both
the conductance peak position and height oscillate as the tip is scanned along
the dot. A pronounced beating pattern in the conductance maximum is observed,
connected to the oscillations of the electron density. Signatures of the
effects induced by a Wigner molecule are clearly identified and their stability
against the strength of Coulomb interactions are analyzed. While the
oscillations of the peak position due to Wigner get enhanced at strong
interactions, the peak height modulations are suppressed as interactions grow.
Oscillations due to Friedel, on the other hand, are robust against interaction.Comment: 9 figure
Temperature-induced emergence of Wigner correlations in a STM-probed one-dimensional quantum dot
The temperature-induced emergence of Wigner correlations over finite-size
effects in a strongly interacting one-dimensional quantum dot are studied in
the framework of the spin coherent Luttinger liquid. We demonstrate that, for
temperatures comparable with the zero mode spin excitations, Friedel
oscillations are suppressed by the thermal fluctuations of higher spin modes.
On the other hand, the Wigner oscillations, sensitive to the charge mode only,
are stable and become more visible. This behavior is proved to be robust both
in the thermal electron density and in the linear conductance in the presence
of an STM tip. This latter probe is not directly proportional to the electron
density and may confirm the above phenomena with complementary and additional
information
Correlation functions for the detection of Wigner molecules in a one-channel Luttinger liquid quantum dot
In one-channel, finite-size Luttinger one-dimensional quantum dots, both
Friedel oscillations and Wigner correlations induce oscillations in the
electron density with the same wavelength, pinned at the same position.
Therefore, observing such a property does not provide any hint about the
formation of a Wigner molecule when electrons interact strongly and other tools
must be employed to assess the formation of such correlated states. We compare
here the behavior of three different correlation functions and demonstrate that
the integrated two point correlation function, which represents the probability
density of finding two particles at a given distance, is the only faithful
estimator for the formation of a correlated Wigner molecule.Comment: 6 pages, 5 figure
Carbon nanotube sensor for vibrating molecules
The transport properties of a CNT capacitively coupled to a molecule
vibrating along one of its librational modes are studied and its transport
properties analyzed in the presence of an STM tip. We evaluate the linear
charge and thermal conductances of the system and its thermopower. They are
dominated by position-dependent Franck-Condon factors, governed by a
position-dependent effective coupling constant peaked at the molecule position.
Both conductance and thermopower allow to extract some information on the
position of the molecule along the CNT. Crucially, however, thermopower sheds
also light on the vibrational levelspacing, allowing to obtain a more complete
characterization of the molecule even in the linear regime
An electrical probe for mechanical vibrations in suspended carbon nanotubes
The transport properties of a suspended carbon nanotube probed by means of a
STM tip are investigated. A microscopic theory of the coupling between
electrons and mechanical vibrations is developed. It predicts a
position-dependent coupling constant, sizeable only in the region where the
vibron is located. This fact has profound consequences on the transport
properties, which allow to extract information on the location and size of the
vibrating portions of the nanotube.Comment: 11 Pages, 12 color figures, accepted for publication on Physical
Review
Oils and fats based biofuels : technological chalendges
Periodically, during petroleum shortage, fatty acids and their derivatives have been used as alternative fuels to those derived from petroleum. Different approaches have been proposed, including the use of neat fats and oils or their derivatives. Indeed, the utilization of biodiesel produced by alcoholysis of triacilglycerides or esterification of fatty acids, or hydrocarbons obtained from cracking of fatty materials were studied and used in several countries. Increasing concerns about energy security and climate changes have lead several countries, including Brazil, to start up biofuels programs. Different technologies are currently being developed in order to produce biofuels with economical feasibility. In this work are discussed alternative fatty raw-materials and processing technologies that are currently being studied in order to produce fuels suitable to sustainable substitute diesel fuel
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