92 research outputs found
Relevance of nonadiabatic effects in TiOCl
We analyze the effect of the phonon dynamics on a recently proposed model for
the uniform-incommensurate transition seen in TiOX compounds. The study is
based on a recently developed formalism for nonadiabatic spin-Peierls systems
based on bosonization and a mean field RPA approximation for the interchain
coupling. To reproduce the measured low temperature spin gap, a spin-phonon
coupling quite bigger than the one predicted from an adiabatic approach is
required. This high value is compatible with the renormalization of the phonons
in the high temperature phase seen in inelastic x-ray experiments. Our theory
accounts for the temperature of the incommensurate transition and the value of
the incommensurate wave vector at the transition point.Comment: 4 pages, 2 figure
RKKY interaction and intervalley processes in p-doped transition metal dichalcogenides
We study the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction in p-doped
transition metal dichalcogenides such as MoS and WS. We consider
magnetic impurities hybridized to the Mo d-orbitals characteristic of the
valence bands. Using the Matsubara Green's function formalism, we obtain the
two-impurity interaction vs their separation and chemical potential of the
system, accounting for the important angular dependence which reflects the
underlying triangular lattice symmetry. The inclusion of the valence band
valley at the point results in a strong enhancement of the
interaction. Electron scattering processes transferring momentum between
valleys at different symmetry points give rise to complex spatial oscillation
patterns. Variable doping would allow the exploration of rather interesting
behavior in the interaction of magnetic impurities on the surfaces of these
materials, including the control of the interaction symmetry, which can be
directly probed in STM experiments.Comment: Includes supplemental materia
Radar sounding using the Cassini altimeter waveform modeling and Monte Carlo approach for data inversion observations of Titan's seas
Recently, the Cassini RADAR has been used as a sounder to probe the depth and constrain the composition of hydrocarbon seas on Saturn's largest moon, Titan. Altimetry waveforms from observations over the seas are generally composed of two main reflections: the first from the surface of the liquid and the second from the seafloor. The time interval between these two peaks is a measure of sea depth, and the attenuation from the propagation through the liquid is a measure of the dielectric properties, which is a sensitive property of liquid composition. Radar measurements are affected by uncertainties that can include saturation effects, possible receiver distortion, and processing artifacts, in addition to thermal noise and speckle. To rigorously treat these problems, we simulate the Ku-band altimetry echo received from Titan's seas using a two-layer model, where the surface is represented by a specular reflection and the seafloor is modeled using a facet-based synthetic surface. The simulation accounts for the thermal noise, speckle, analog-to-digital conversion, and block adaptive quantization and allows for possible receiver saturation. We use a Monte Carlo method to compare simulated and observed waveforms and retrieve the probability distributions of depth, surface/subsurface intensity ratio, and subsurface roughness for the individual double-peaked waveform of Ligeia Mare acquired by the Cassini spacecraft in May 2013. This new analysis provides an update to the Ku-band attenuation and results in a new estimate for its loss tangent and composition. We also demonstrate the ability to retrieve bathymetric information from saturated altimetry echoes acquired over Ontario Lacus in December 2008
Liquid filled canyons on Titan
In May 2013 the Cassini RADAR altimeter observed channels in Vid Flumina, a drainage network
connected to Titan’s second largest hydrocarbon sea, Ligeia Mare. Analysis of these altimeter echoes shows
that the channels are located in deep (up to ~570 m), steep-sided, canyons and have strong specular surface
reflections that indicate they are currently liquid filled. Elevations of the liquid in these channels are at the
same level as Ligeia Mare to within a vertical precision of about 0.7 m, consistent with the interpretation of
drowned river valleys. Specular reflections are also observed in lower order tributaries elevated above the
level of Ligeia Mare, consistent with drainage feeding into the main channel system
Quantum phase transitions into Kondo states in bilayer graphene
We study a magnetic impurity intercalated in bilayer graphene. A
representative configuration generates a hybridization function with strong
dependence on the conduction-electron energy, including a full gap with one
hard and one soft edge. Shifts of the chemical potential via gating or doping
drive the system between non-Kondo (free-moment) and Kondo-screened phases,
with strong variation of the Kondo scale. Quantum phase transitions near the
soft edge are of Kosterlitz-Thouless type, while others are first order. Near
the hard edge, a bound-state singlet appears inside the gap; although of
single-particle character, its signatures in scanning tunneling spectroscopy
are very similar to those arising from a many-body Kondo resonance
Microscopic theory for the incommensurate transition in TiOCl
We propose a microscopic mechanism for the incommensurate phase in TiOX
compounds. The model includes the antiferromagnetic chains of Ti ions immersed
in the phonon bath of the bilayer structure. Making use of the Cross-Fisher
theory, we show that the geometrically frustrated character of the lattice is
responsible for the structural instability which leads the chains to an
incommensurate phase without an applied magnetic field. In the case of TiOCl,
we show that our model is consistent with the measured phonon frequencies at
and the value of the incommensuration vector at the transition
temperature. Moreover, we find that the dynamical structure factor shows a
progressive softening of an incommensurate phonon near the zone boundary as the
temperature decreases. This softening is accompanied by a broadening of the
peak which gets asymmetrical as well when going towards the transition
temperature. These features are in agreement with the experimental inelastic
X-ray measurements.Comment: 6 pages, 5 figures. Published versio
Geologically recent areas as one key target for identifying active volcanism on Venus
The recently selected NASA VERITAS and DAVINCI missions, the ESA EnVision, the Roscosmos Venera-D will open a new era in the exploration of Venus. One of the key targets of the future orbiting and in situ investigations of Venus is the identification of volcanically active areas on the planet. The study of the areas characterized by recent or ongoing volcano-tectonic activity can inform us on how volcanism and tectonism are currently evolving on Venus. Following this key target, Brossier et al. (2022, https://doi.org/10.1029/2022GL099765) extend the successful approach and methodology used by previous works to Ganis Chasma in Atla Regio. Here we comment on the main results published in Brossier et al. (2022, https://doi.org/10.1029/2022GL099765) and discuss the important implications of their work for the future orbiting and in situ investigation of Venus. Their results add further lines of evidence indicating possibly recent volcanism on Venus
Titan's cold case files - Outstanding questions after Cassini-Huygens
The entry of the Cassini-Huygens spacecraft into orbit around Saturn in July 2004 marked the start of a golden era in the exploration of Titan, Saturn's giant moon. During the Prime Mission (2004–2008), ground-breaking discoveries were made by the Cassini orbiter including the equatorial dune fields (flyby T3, 2005), northern lakes and seas (T16, 2006), and the large positive and negative ions (T16 & T18, 2006), to name a few. In 2005 the Huygens probe descended through Titan's atmosphere, taking the first close-up pictures of the surface, including large networks of dendritic channels leading to a dried-up seabed, and also obtaining detailed profiles of temperature and gas composition during the atmospheric descent. The discoveries continued through the Equinox Mission (2008–2010) and Solstice Mission (2010–2017) totaling 127 targeted flybys of Titan in all. Now at the end of the mission, we are able to look back on the high-level scientific questions from the start of the mission, and assess the progress that has been made towards answering these. At the same time, new scientific questions regarding Titan have emerged from the discoveries that have been made. In this paper we review a cross-section of important scientific questions that remain partially or completely unanswered, ranging from Titan's deep interior to the exosphere. Our intention is to help formulate the science goals for the next generation of planetary missions to Titan, and to stimulate new experimental, observational and theoretical investigations in the interim
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