450 research outputs found
Low-Dimensional Transport and Large Thermoelectric Power Factors in Bulk Semiconductors by Band Engineering of Highly Directional Electronic States
Thermoelectrics are promising to address energy issues but their exploitation
is still hampered by low efficiencies. So far, much improvement has been
achieved by reducing the thermal conductivity but less by maximizing the power
factor. The latter imposes apparently conflicting requirements on the band
structure: a narrow energy distribution and a low effective mass. Quantum
confinement in nanostructures or the introduction of resonant states were
suggested as possible solutions to this paradox but with limited success. Here,
we propose an original approach to fulfill both requirements in bulk
semiconductors. It exploits the highly-directional character of some orbitals
to engineer the band-structure and produce a type of low-dimensional transport
similar to that targeted in nanostructures, while retaining isotropic
properties. Using first-principles calculations, the theoretical concept is
demonstrated in FeYZ Heusler compounds, yielding power factors 4-5 times
larger than in classical thermoelectrics at room temperature. Our findings are
totally generic and rationalize the search of alternative compounds with a
similar behavior. Beyond thermoelectricity, these might be relevant also in the
context of electronic, superconducting or photovoltaic applications.Comment: 6 pages, 2 figure
On the use of U-Net for dominant melody estimation in polyphonic music
International audienceEstimation of dominant melody in polyphonic music remains a difficult task, even though promising breakthroughs have been done recently with the introduction of the Harmonic CQT and the use of fully convolutional networks. In this paper, we build upon this idea and describe how U-Net-a neural network originally designed for medical image segmentation-can be used to estimate the dominant melody in polyphonic audio. We propose in particular the use of an original layer-by-layer sequential training method, and show that this method used along with careful training data conditioning improve the results compared to plain convolutional networks
Characterization of wake- and tip-vortex-induced unsteady blade response in multistage compressor environment
Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 2002.Includes bibliographical references (p. 90-91).by Geoffroy Lenglin.S.M
Topological effects of a vorticity filament on the coherent backscattering cone
In this Letter, we report on the effects of a vorticity filament on the coherent backscattering cone. Using ultrasonic waves in a strongly reverberating cavity, we experimentally show that the discrete number of loops of acoustic paths around a pointlike vortex located at the center of the cavity drives the cancellation and the potential rebirth of the coherent backscattering enhancement. The vorticity filament behaves, then, as a topological anomaly for wave propagation that provides some new insight between reciprocity and weak localization
Logarithmic scaling for height variables in the Abelian sandpile model
We report on the exact computation of the scaling form of the 1-point
function, on the upper-half plane, of the height 2 variable in the
two-dimensional Abelian sandpile model. By comparing the open versus the closed
boundary condition, we find that the scaling field associated to the height 2
is a logarithmic scalar field of scaling dimension 2, belonging to a c=-2
logarithmic conformal field theory. This identification is confirmed by
numerical simulations and extended to the height 3 and 4 variables, which
exhibit the same scaling form. Using the conformal setting, we make precise
proposals for the bulk 2-point functions of all height variables.Comment: 7 pages, 2 figure
Pre-logarithmic and logarithmic fields in a sandpile model
We consider the unoriented two-dimensional Abelian sandpile model on the
half-plane with open and closed boundary conditions, and relate it to the
boundary logarithmic conformal field theory with central charge c=-2. Building
on previous results, we first perform a complementary lattice analysis of the
operator effecting the change of boundary condition between open and closed,
which confirms that this operator is a weight -1/8 boundary primary field,
whose fusion agrees with lattice calculations. We then consider the operators
corresponding to the unit height variable and to a mass insertion at an
isolated site of the upper half plane and compute their one-point functions in
presence of a boundary containing the two kinds of boundary conditions. We show
that the scaling limit of the mass insertion operator is a weight zero
logarithmic field.Comment: 18 pages, 9 figures. v2: minor corrections + added appendi
Height variables in the Abelian sandpile model: scaling fields and correlations
We compute the lattice 1-site probabilities, on the upper half-plane, of the
four height variables in the two-dimensional Abelian sandpile model. We find
their exact scaling form when the insertion point is far from the boundary, and
when the boundary is either open or closed. Comparing with the predictions of a
logarithmic conformal theory with central charge c=-2, we find a full
compatibility with the following field assignments: the heights 2, 3 and 4
behave like (an unusual realization of) the logarithmic partner of a primary
field with scaling dimension 2, the primary field itself being associated with
the height 1 variable. Finite size corrections are also computed and
successfully compared with numerical simulations. Relying on these field
assignments, we formulate a conjecture for the scaling form of the lattice
2-point correlations of the height variables on the plane, which remain as yet
unknown. The way conformal invariance is realized in this system points to a
local field theory with c=-2 which is different from the triplet theory.Comment: 68 pages, 17 figures; v2: published version (minor corrections, one
comment added
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