130 research outputs found
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
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.
Chandra Observations of the Gamma-ray Binary LSI+61303: Extended X-ray Structure?
We present a 50 ks observation of the gamma-ray binary LSI+61303 carried out
with the ACIS-I array aboard the Chandra X-ray Observatory. This is the highest
resolution X-ray observation of the source conducted so far. Possible evidence
of an extended structure at a distance between 5 and 12 arcsec towards the
North of LSI+61303 have been found at a significance level of 3.2 sigma. The
asymmetry of the extended emission excludes an interpretation in the context of
a dust-scattered halo, suggesting an intrinsic nature. On the other hand, while
the obtained source flux, of F_{0.3-10 keV}=7.1^{+1.8}_{-1.4} x 10^{-12}
ergs/cm^2/s, and hydrogen column density, N_{H}=0.70+/-0.06 x 10^{22} cm^{-2},
are compatible with previous results, the photon index Gamma=1.25+/-0.09 is the
hardest ever found. In light of these new results, we briefly discuss the
physics behind the X-ray emission, the location of the emitter, and the
possible origin of the extended emission ~0.1 pc away from LSI+61303.Comment: 4 pages, 3 figures. Accepted for publication in ApJ Letter
Evaluation of 224Ra as a tracer for submarine groundwater discharge in Long Island Sound (NY)
Altres ajuts: the government of Spain and the Fulbright Commission for a post-doctoral fellowship to J.G-O. (ref 2007-0516)The approach to quantify submarine groundwater discharge using Ra isotopes generally involves developing a Ra mass balance in an estuary, bay or lagoon. In this work we present a 224Ra mass balance used to evaluate the importance of the submarine groundwater discharge (SGD) in Long Island Sound (NY, US), the third most important estuary in US, located between Long Island and Connecticut that is usually affected by summertime hypoxia in the western basin. Three surveys were conducted between April 2009 and August 2010 where 25 water stations were sampled for Ra isotopes, oxygen and Mn. Stations were oriented along 4 transects: one axial extending from the western to the eastern Sound and three longitudinal transects in the western, central and eastern Sound. The inventory of 224Ra in the water column in summer was circa 2 times greater than in winter, suggesting an increased 224Ra flux to the Sound in summer. A mass balance for 224Ra was constructed considering tidal exchange, inputs from rivers, desorption from resuspended particles, diffusive fluxes (including bioirrigation) from bottom sediments and radioactive decay in the water column. Fluxes of 224Ra from bottom sediments were measured by incubating cores under oxic conditions in a continuous flow mode such that the overlying water was circulated through a Mn-oxide fiber to maintain a constant activity of 224Ra. Fluxes from muddy sediments (comprising ~67% of the Sound bottom) ranged from 127 to 312dpmm-2d-1 and were ~60dpmm-2d-1 in sandy sediments (33% of the Sound). Incubations under hypoxic conditions showed variable fluxes depending on reduction and mobilization of Mn. The 224Ra mass balance shows a net input of Ra to the Sound of 106±50×1012dpmy-1 in spring and 244±112×1012dpmy-1 in the summer that is attributed to SGD. Elevated 224Ra values were observed near shore and in the pore fluids of the coarse beach sands along the Long Island and Connecticut coasts, suggesting that SGD driven by tidal recirculation through the beach face is a major source of 224Ra to the Sound. Seasonal variation in this source seems unlikely, and the calculated 224Ra SGD fluxes for spring and summer overlap within the uncertainties. Nevertheless we conclude that variations in the 224Ra water column inventories could be produced by seasonal changes in bioirrigation and/or redox cycling of Mn as well as sediment resuspension and desorption of 224Ra from resuspended particles, and that our mass balance underestimates these terms, particularly in the summer. 224Ra fluxes from sediments in estuaries, especially those with significant areas of muddy sediments and seasonal hypoxia, are important and should be well constrained in future uses of this isotope as a tracer for SGD
Electronic structure studies of BaFe2As2 by angle-resolved photoemission spectroscopy
We report high resolution angle-resolved photoemission spectroscopy (ARPES)
studies of the electronic structure of BaFeAs, which is one of the
parent compounds of the Fe-pnictide superconductors. ARPES measurements have
been performed at 20 K and 300 K, corresponding to the orthorhombic
antiferromagnetic phase and the tetragonal paramagnetic phase, respectively.
Photon energies between 30 and 175 eV and polarizations parallel and
perpendicular to the scattering plane have been used. Measurements of the Fermi
surface yield two hole pockets at the -point and an electron pocket at
each of the X-points. The topology of the pockets has been concluded from the
dispersion of the spectral weight as a function of binding energy. Changes in
the spectral weight at the Fermi level upon variation of the polarization of
the incident photons yield important information on the orbital character of
the states near the Fermi level. No differences in the electronic structure
between 20 and 300 K could be resolved. The results are compared with density
functional theory band structure calculations for the tetragonal paramagnetic
phase.Comment: 11 pages, 5 figure
Spin gap formation in the quantum spin systems TiOX, X=Cl and Br
In the layered quantum spin systems TiOCl and TiOBr the magnetic
susceptibility shows a very weak temperature dependence at high temperatures
and transition-induced phenomena at low temperatures. There is a clear
connection of the observed transition temperatures to the distortion of the
octahedra and the layer separation. Band structure calculations point to a
relation of the local coordinations and the dimensionality of the magnetic
properties. While from magnetic Raman scattering only a small decrease of the
magnetic exchange by -5-10% is derived comparing TiOCl with TiOBr, the
temperature dependence of the magnetic susceptibility favors a much bigger
change.Comment: 5 figures, 15 pages, further information see
http://www.peter-lemmens.d
The Mott-Hubbard Transition on the D=infinity Bethe Lattice
In view of a recent controversy we investigated the Mott-Hubbard transition
in D=infinity with a novel cluster approach. i) We show that any truncated
Bethe lattice of order n can be mapped exactly to a finite Hubbard-like
cluster. ii) We evaluate the self-energy numerically for n=0,1,2 and compare
with a series of self-consistent equation-of-motion solutions. iii) We find the
gap to open continously at the critical U_c~2.5t* (t = t* / sqrt{4d}). iv) A
low-energy theory for the Mott-Hubbard transition is developed and relations
between critical exponents are presented.Comment: Replaced with the published versio
Hubbard band or oxygen vacancy states in the correlated electron metal SrVO?
We study the effect of oxygen vacancies on the electronic structure of the
model strongly correlated metal SrVO. By means of angle-resolved
photoemission (ARPES) synchrotron experiments, we investigate the systematic
effect of the UV dose on the measured spectra. We observe the onset of a
spurious dose-dependent prominent peak at an energy range were the lower
Hubbard band has been previously reported in this compound, raising questions
on its previous interpretation. By a careful analysis of the dose dependent
effects we succeed in disentangling the contributions coming from the oxygen
vacancy states and from the lower Hubbard band. We obtain the intrinsic ARPES
spectrum for the zero-vacancy limit, where a clear signal of a lower Hubbard
band remains. We support our study by means of state-of-the-art ab initio
calculations that include correlation effects and the presence of oxygen
vacancies. Our results underscore the relevance of potential spurious states
affecting ARPES experiments in correlated metals, which are associated to the
ubiquitous oxygen vacancies as extensively reported in the context of a
two-dimensional electron gas (2DEG) at the surface of insulating
transition metal oxides.Comment: Manuscript + Supplemental Material, 12 pages, 9 figure
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