46,993 research outputs found
Experimental verification of reciprocity relations in quantum thermoelectric transport
Symmetry relations are manifestations of fundamental principles and
constitute cornerstones of modern physics. An example are the Onsager relations
between coefficients connecting thermodynamic fluxes and forces, central to
transport theory and experiments. Initially formulated for classical systems,
these reciprocity relations are also fulfilled in quantum conductors.
Surprisingly, novel relations have been predicted specifically for
thermoelectric transport. However, whereas these thermoelectric reciprocity
relations have to date not been verified, they have been predicted to be
sensitive to inelastic scattering, always present at finite temperature. The
question whether the relations exist in practice is important for
thermoelectricity: whereas their existence may simplify the theory of complex
thermoelectric materials, their absence has been shown to enable, in principle,
higher thermoelectric energy conversion efficiency for a given material
quality. Here we experimentally verify the thermoelectric reciprocity relations
in a four-terminal mesoscopic device where each terminal can be electrically
and thermally biased, individually. The linear response thermoelectric
coefficients are found to be symmetric under simultaneous reversal of magnetic
field and exchange of injection and emission contacts. Intriguingly, we also
observe the breakdown of the reciprocity relations as a function of increasing
thermal bias. Our measurements thus clearly establish the existence of the
thermoelectric reciprocity relations, as well as the possibility to control
their breakdown with the potential to enhance thermoelectric performanceComment: 7 pages, 5 figure
Scaling forces to asteroid surfaces: The role of cohesion
The scaling of physical forces to the extremely low ambient gravitational
acceleration regimes found on the surfaces of small asteroids is performed.
Resulting from this, it is found that van der Waals cohesive forces between
regolith grains on asteroid surfaces should be a dominant force and compete
with particle weights and be greater, in general, than electrostatic and solar
radiation pressure forces. Based on this scaling, we interpret previous
experiments performed on cohesive powders in the terrestrial environment as
being relevant for the understanding of processes on asteroid surfaces. The
implications of these terrestrial experiments for interpreting observations of
asteroid surfaces and macro-porosity are considered, and yield interpretations
that differ from previously assumed processes for these environments. Based on
this understanding, we propose a new model for the end state of small, rapidly
rotating asteroids which allows them to be comprised of relatively fine
regolith grains held together by van der Waals cohesive forces.Comment: 54 pages, 7 figure
Density modulations in an elongated Bose-Einstein condensate released from a disordered potential
We observe large density modulations in time-of-flight images of elongated
Bose-Einstein condensates, initially confined in a harmonic trap and in the
presence of weak disorder. The development of these modulations during the
time-of-flight and their dependence with the disorder are investigated. We
render an account of this effect using numerical and analytical calculations.
We conclude that the observed large density modulations originate from the weak
initial density modulations induced by the disorder, and not from initial phase
fluctuations (thermal or quantum).Comment: Published version; 4+ pages; 4 figure
ESR study of the single-ion anisotropy in the pyrochlore antiferromagnet Gd2Sn2O7
Single-ion anisotropy is of importance for the magnetic ordering of the
frustrated pyrochlore antiferromagnets Gd2Ti2O7 and Gd2Sn2O7. The anisotropy
parameters for the Gd2Sn2O7 were measured using the electron spin resonance
(ESR) technique. The anisotropy was found to be of the easy plane type, with
the main constant D=140mK. This value is 35% smaller than the value of the
corresponding anisotropy constant in the related compound Gd2Ti2O7.Comment: 8 pages, 3 figure
Probe of Lorentz Invariance Violation effects and determination of the distance of PG 1553+113
The high frequency peaked BL Lac object PG 1553+113 underwent a flaring event
in 2012. The High Energy Stereoscopic System (H.E.S.S.) observed this source
for two consecutive nights at very high energies (VHE, 100~GeV). The data
show an increase of a factor of three of the flux with respect to archival
measurements with the same instrument and hints of intra-night variability. The
data set has been used to put constraints on possible Lorentz invariance
violation (LIV), manifesting itself as an energy dependence of the velocity of
light in vacuum, and to set limits on the energy scale at which Quantum Gravity
effects causing LIV may arise. With a new method to combine H.E.S.S. and Fermi
large area telescope data, the previously poorly known redshift of PG 1555+113
has been determined to be close to the value derived from optical measurements.Comment: 2014 Fermi Symposium proceedings - eConf C14102.
Assessment of the U and Co magnetic moments in UCoGe by X-ray magnetic circular dichroism
The ferromagnetic superconductor UCoGe has been investigated by high field
X-ray magnetic circular dichroism (XMCD) at the U-M and Co/Ge-K edges.
The analysis of the branching ratio and XMCD at the U-M edges reveals
that the U-5 electrons count is close to 3. The orbital ()
and spin () moments of U at 2.1K and 17T (H//c) have been
determined. Their ratio () suggests a significant delocalization of
the 5 electron states. The similar field dependences of the local U/Co and
the macroscopic magnetization indicate that the Co moment is induced by the U
moment. The XMCD at the Co/Ge-K edges reveal the presence of small Co-4 and
Ge-4 orbital moments parallel to the macroscopic magnetization. In addition,
the Co-3 moment is estimated to be at most of the order of 0.1 at
17T. Our results rule out the possibility of an unusual polarisability of the U
and Co moments as well as their antiparallel coupling. We conclude that the
magnetism which mediates the superconductivity in UCoGe is driven by U.Comment: 4 figures + supplementary materia
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