345 research outputs found
Correlation effects on the electronic structure of TiOCl: a NMTO+DMFT study
Using the recently developed N-th order muffin-tin orbital-based downfolding
technique in combination with the Dynamical Mean Field theory, we investigate
the electronic properties of the much discussed Mott insulator TiOCl in the
undimerized phase. Inclusion of correlation effects through this approach
provides a description of the spectral function into an upper and a lower
Hubbard band with broad valence states formed out of the orbitally polarized,
lower Hubbard band. We find that these results are in good agreement with
recent photo-emission spectra.Comment: 4 pages, 3 figure
Internal screening and dielectric engineering in magic-angle twisted bilayer graphene
Magic-angle twisted bilayer graphene (MA-tBLG) has appeared as a tunable
testing ground to investigate the conspiracy of electronic interactions, band
structure, and lattice degrees of freedom to yield exotic quantum many-body
ground states in a two-dimensional Dirac material framework. While the impact
of external parameters such as doping or magnetic field can be conveniently
modified and analyzed, the all-surface nature of the quasi-2D electron gas
combined with its intricate internal properties pose a challenging task to
characterize the quintessential nature of the different insulating and
superconducting states found in experiments. We analyze the interplay of
internal screening and dielectric environment on the intrinsic electronic
interaction profile of MA-tBLG. We find that interlayer coupling generically
enhances the internal screening. The influence of the dielectric environment on
the effective interaction strength depends decisively on the electronic state
of MA-tBLG. Thus, we propose the experimental tailoring of the dielectric
environment, e.g. by varying the capping layer composition and thickness, as a
promising pursuit to provide further evidence for resolving the hidden nature
of the quantum many-body states in MA-tBLG.Comment: 9 pages, 3 figures, supplemental material included (8 figures
Spin-charge separation at small lengthscales in the 2D t-J model
We consider projected wavefunctions for the 2D model. For various
wavefunctions, including correlated Fermi-liquid and Luttinger-type
wavefunctions we present the static charge-charge and spin-spin structure
factors. Comparison with recent results from a high-temperature expansion by
Putikka {\it et al.} indicates spin-charge separation at small lengthscales.Comment: REVTEX, 5 pages, 5 figures hardcopies availabl
Ab initio investigation of VOSeO3, a spin gap system with coupled spin dimers
Motivated by an early experimental study of VOSeO3, which suggested that it
is a quasi-2D system of weakly coupled spin dimers with a small spin gap, we
have investigated the electronic structure of this material via
density-functional calculations. These ab initio results indicate that the
system is better thought of as an alternating spin-1/2 chain with moderate
interchain interactions, an analog of (VO)2P2O7. The potential interest of this
system for studies in high magnetic field given the presumably small value of
the spin gap is emphasized.Comment: 4 pages, 5 figure
tomo3d: a new 3-D joint refraction and reflection travel-time tomography code for active-source seismic data
European Geosciences Union General Assembly 22-27 April 2012, Vienna, Austria.-- 1 pageWe present the development state of tomo3d, a code for three-dimensional refraction and reflection travel-time tomography of wide-angle seismic data based on the previous two-dimensional version of the code, tomo2d. The core of both forward and inverse problems is inherited from the 2-D version. The ray tracing is performed by a hybrid method combining the graph and bending methods. The graph method finds an ordered array of discrete model nodes, which satisfies Fermat’s principle, that is, whose corresponding travel time is a global minimum within the space of discrete nodal connections. The bending method is then applied to produce a more accurate ray path by using the nodes as support points for an interpolation with beta-splines. Travel time tomography is formulated as an iterative linearized inversion, and each step is solved using an LSQR algorithm. In order to avoid the singularity of the sensitivity kernel and to reduce the instability of inversion, regularization parameters are introduced in the inversion in the form of smoothing and damping constraints. Velocity models are built as 3-D meshes, and velocity values at intermediate locations are obtained by trilinear interpolation within the corresponding pseudo-cubic cell. Meshes are sheared to account for topographic relief. A floating reflector is represented by a 2-D grid, and depths at intermediate locations are calculated by bilinear interpolation within the corresponding square cell. The trade-off between the resolution of the final model and the associated computational cost is controlled by the relation between the selected forward star for the graph method (i.e. the number of nodes that each node considers as its neighbors) and the refinement of the velocity mesh. Including reflected phases is advantageous because it provides a better coverage and allows us to define the geometry of those geological interfaces with velocity contrasts sharp enough to be observed on record sections. The code also offers the possibility of including water-layer multiples in the modeling, whenever this phase can be followed to greater offsets than the primary phases. This increases the quantity of useful information in the data and yields more extensive and better constrained velocity and geometry models. We will present results from benchmark tests for forward and inverse problems, as well as synthetic tests comparing an inversion with refractions only and another one with both refractions and reflectionPeer Reviewe
Fine-scale thermohaline ocean structure retrieved with 2-D prestack full-waveform inversion of multichannel seismic data: Application to the Gulf of Cadiz (SW Iberia)
18 pages, 9 figures, supporting information https://dx.doi.org/10.1002/2016JC011844This work demonstrates the feasibility of 2-D time-domain, adjoint-state acoustic full-waveform inversion (FWI) to retrieve high-resolution models of ocean physical parameters such as sound speed, temperature and salinity. The proposed method is first described and then applied to prestack multichannel seismic (MCS) data acquired in the Gulf of Cadiz (SW Iberia) in 2007 in the framework of the Geophysical Oceanography project. The inversion strategy flow includes specifically designed data preconditioning for acoustic noise reduction, followed by the inversion of sound speed in the shotgather domain. We show that the final sound speed model has a horizontal resolution of ∼ 70 m, which is two orders of magnitude better than that of the initial model constructed with coincident eXpendable Bathy Thermograph (XBT) data, and close to the theoretical resolution of O(λ). Temperature (T) and salinity (S) are retrieved with the same lateral resolution as sound speed by combining the inverted sound speed model with the thermodynamic equation of seawater and a local, depth-dependent T-S relation derived from regional conductivity-temperature-depth (CTD) measurements of the National Oceanic and Atmospheric Administration (NOAA) database. The comparison of the inverted T and S models with XBT and CTD casts deployed simultaneously to the MCS acquisition shows that the thermohaline contrasts are resolved with an accuracy of 0.18oC for temperature and 0.08 PSU for salinity. The combination of oceanographic and MCS data into a common, pseudo-automatic inversion scheme allows to quantitatively resolve submeso-scale features that ought to be incorporated into larger-scale ocean models of oceans structure and circulationThe work has been partially supported by the projects KALEIDOSCOPE and CO-Dos financed by REPSOL and MINECO project POSEIDON (CTM2010-21569) and HADES (CTM2011-30400-C02). B. Biescas work has been funded by the European Commission through the Marie Curie Action FP7-PEOPLE-2012-COFUND-600407Peer Reviewe
Cluster Dynamical Mean-field calculations for TiOCl
Based on a combination of cluster dynamical mean field theory (DMFT) and
density functional calculations, we calculated the angle-integrated spectral
density in the layered quantum magnet TiOCl. The agreement with recent
photoemission and oxygen K-edge X-ray absorption spectroscopy experiments is
found to be good. Th e improvement achieved with this calculation with respect
to previous single-site DMFT calculations is an indication of the correlated
nature and low-dimensionality of TiOCl.Comment: 9 pages, 3 figures, improved version as publishe
Novel Nonreciprocal Acoustic Effects in Antiferromagnets
The possible occurrence of nonreciprocal acoustic effects in antiferromagnets
in the absence of an external magnetic field is investigated using both (i) a
microscopic formulation of the magnetoelastic interaction between spins and
phonons and (ii) symmetry arguments. We predict for certain antiferromagnets
the existence of two new nonreciprocal (non-time invariant) effects:
A boundary-condition induced nonreciprocal effect and the occurrence of
transversal phonon modes propagating in opposite directions having different
velocities. Estimates are given and possible materials for these effects to be
observed are suggested.Comment: Euro. Phys. Lett. (in press
Multiphase tectonic interaction of Tyrrhenian - Tunisia Margin - Ionian systems: Implications for regional seismogenesis
European Geosciences Union (EGU) General Assembly 2020, 4-8 May 2020The region at the transition from the west to the east Mediterranean is a complex puzzle of terrains spanning in age from the Mesozoic Ionian lithosphere to the Pleistocene arc and back arc domains of the Tyrrhenian system. Although the region has had a complicated evolutionary history, the current configuration of terrains fundamentally denotes Miocene to recent kinematics.
In this contribution we present new data from Tunisia Margin showing the evolution from its formation in early Miocene to recent, the tectonic interaction with the opening of the Tyrrhenian system and its current inversion, and discuss the implications for the regional kinematics evolution.
The Tyrrhenian is no longer extending, but all basin borders indicate currently active large-scale thrusting to strike slip tectonics. Tunisia margins formed by a well-know contractional tectonic phase in early Miocene expressed in large-scale tectonics with a clearly imaged thrust and fold belt, cut by Messinian to Pliocene extensional faulting. However, high resolution multibeam bathymetry and images of the shallowest layers indicates ongoing inversion tectonics.
We compare the tectonic evolution of north Tunisia and Tyrrhenian with the patterns of deformation of the Ionian tectonic wedge observed in new and reprocessed seismic images. We interpret the current deformation of the Ionian tectonic wedge based on the integration of evolution of the kinematics from the data sets of observations from the three systems.
We conclude that the entire region is currently under collision of the Africa Plate with the Adria Plate and the Neogene terrains of the Tyrrhenian Domain. The corollary is the subduction of the Ionian lithosphere is fundamentally stalled so that the megathrust fault is possibly not any longer accumulating significant shortening and most deformation is currently occurring in steeper faults re-activation or cutting the previous structural framewor
Magnetic fluctuations and superconducting properties of CaKFe4As4 studied by 75As NMR
We report As nuclear magnetic resonance (NMR) studies on a new
iron-based superconductor CaKFeAs with = 35 K. As
NMR spectra show two distinct lines corresponding to the As(1) and As(2) sites
close to the K and Ca layers, respectively, revealing that K and Ca layers are
well ordered without site inversions. We found that nuclear quadrupole
frequencies of the As(1) and As(2) sites show an opposite
temperature () dependence. Nearly independent behavior of the Knight
shifts are observed in the normal state, and a sudden decrease in in
the superconducting (SC) state clearly evidences spin-singlet Cooper pairs.
As spin-lattice relaxation rates 1/ show a power law dependence
with different exponents for the two As sites. The isotropic antiferromagnetic
spin fluctuations characterized by the wavevector = (, 0) or (0,
) in the single-iron Brillouin zone notation are revealed by 1/ and
measurements. Such magnetic fluctuations are necessary to explain the
observed temperature dependence of the As quadrupole frequencies, as
evidenced by our first-principles calculations. In the SC state, 1/ shows
a rapid decrease below without a Hebel-Slichter peak and decreases
exponentially at low , consistent with an nodeless two-gap
superconductor.Comment: 9 pages, 6 figures, accepted for publication in Phys.Rev.
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