2,775 research outputs found
Hydration properties of carbohydrates: A clue from molecular dynamics
A molecular dynamics (MD) simulation study is presented for the dynamics of the polarizability anisotropy of pure water and of water-trehalose solutions as a function of concentration. The calculated time correlation functions (TCF) of the water collective polarizability for the different mixtures are compared and analyzed in terms of two distinct relaxation dynamics in the picoseconds timescale. The two processes have been attributed to the dynamics of bulk and hydration water molecules, respectively, with a retardation factor ∼ 5. The whole picture has been confirmed by the further separation of the total water TCFs into the ones arising from bulk, hydration molecules and a cross term between the two, that allowed a
separate estimate of the two relaxation times
Frequency-dependent Thermal Response of the Charge System and Restricted Sum Rules in La(2-x)Sr(x)CuO(4)
By using new and previous measurements of the -plane conductivity
of LaSrCuO (LSCO) it is shown that
the spectral weight
obeys the same law which holds for a conventional
metal like gold, for 's below the plasma frequency. However
, which measures the "thermal response" of the charge system, in
LSCO exhibits a peculiar behavior which points towards correlation effects. In
terms of hopping models, is directly related to an energy scale
, smaller by one order of magnitude than the full bandwidth .Comment: 4 pages with 3 fig
Growing massive black holes through super-critical accretion of stellar-mass seeds
The rapid assembly of the massive black holes that power the luminous quasars
observed at remains a puzzle. Various direct collapse models have
been proposed to head-start black hole growth from initial seeds with masses
, which can then reach a billion solar mass while
accreting at the Eddington limit. Here we propose an alternative scenario based
on radiatively inefficient super-critical accretion of stellar-mass holes
embedded in the gaseous circum-nuclear discs (CNDs) expected to exist in the
cores of high redshift galaxies. Our sub-pc resolution hydrodynamical
simulations show that stellar-mass holes orbiting within the central 100 pc of
the CND bind to very high density gas clumps that arise from the fragmentation
of the surrounding gas. Owing to the large reservoir of dense cold gas
available, a stellar-mass black hole allowed to grow at super-Eddington rates
according to the "slim disc" solution can increase its mass by 3 orders of
magnitudes within a few million years. These findings are supported by
simulations run with two different hydro codes, RAMSES based on the Adaptive
Mesh Refinement technique and GIZMO based on a new Lagrangian Godunov-type
method, and with similar, but not identical, sub-grid recipes for star
formation, supernova feedback, black hole accretion and feedback. The low
radiative efficiency of super-critical accretion flows are instrumental to the
rapid mass growth of our black holes, as they imply modest radiative heating of
the surrounding nuclear environment.Comment: 12 pages, 8 figures, 2 tables. Accepted for publication in MNRA
Chiral spin currents and spectroscopically accessible single merons in quantum dots
We provide unambiguous theoretical evidence for the formation of
correlation-induced isolated merons in rotationally-symmetric quantum dots. Our
calculations rely on neither the lowest-Landau-level approximation, nor on the
maximum-density-droplet approximation, nor on the existence of a spin-polarized
state. For experimentally accessible system parameters, unbound merons condense
in the ground state at magnetic fields as low as T and for as few
as N = 3 confined fermions. The four-fold degenerate ground-state at
corresponds to four orthogonal merons characterized by their
topological chirality and charge . This degeneracy is lifted by the
Rashba and Dresselhaus spin-orbit interaction, which we include perturbatively,
yielding spectroscopic accessibility to individual merons. We further derive a
closed-form expression for the topological chirality in the form of a chiral
spin current and use it to both characterize our states and predict the
existence of other topological textures in other regions of phase space, for
example, at N=5. Finally, we compare the spin textures of our numerically exact
meron states to ansatz wave-functions of merons in quantum Hall droplets and
find that the ansatz qualitatively describes the meron states.Comment: 4 pages, 5 figures; minor title change, typos fixe
Optical Properties of (SrMnO3)n/(LaMnO3)2n superlattices: an insulator-to-metal transition observed in the absence of disorder
We measure the optical conductivity of (SrMnO3)n/(LaMnO3)2n superlattices
(SL) for n=1,3,5, and 8 and 10 < T < 400 K. Data show a T-dependent insulator
to metal transition (IMT) for n \leq 3, driven by the softening of a polaronic
mid-infrared band. At n = 5 that softening is incomplete, while at the
largest-period n=8 compound the MIR band is independent of T and the SL remains
insulating. One can thus first observe the IMT in a manganite system in the
absence of the disorder due to chemical doping. Unsuccessful reconstruction of
the SL optical properties from those of the original bulk materials suggests
that (SrMnO3)n/(LaMnO3)2n heterostructures give rise to a novel electronic
state.Comment: Published Online in Nano Letters, November 8, 2010;
http://pubs.acs.org/doi/abs/10.1021/nl1022628; 5 pages, 3 figure
Urban commoning in a civic social network: the case study of FirstLife
The integration of ICTs in the urban management is increasing at all levels of public administrations in order to improve efficiency and effectiveness of public services, but their role is still instrumental rather than drive a change toward a more collaborative local governance. On the other hand, there is a raising expectation of the civil society to participate in decision making processes and contribute in defining local policies about sensitive topics. These purposes are often addressed by using or creating community digital tools designed for a specific contextual scope, resulting in a deep fragmentation of information about civic initiatives and social innovation projects and a lack of continuous communication among urban stakeholders even working in the same area. The challenge is to design an ICT solution to refactor the current practices of cooperation between private and public sector and support a real change in the city management processes from the local to the territorial level. In this contribution, we present the development of FirstLife, a map-based civic social network, designed to represent the complex environment of the city through geo-referenced time framed crowdsourced data about urban entities as events, places, groups, initiatives, projects, stories, news, etc. The main goals of the platform are to support the action of multiple stakeholders in alternative processes of co-management of common or shared resources, as for instance public spaces, green areas and buildings hosting collective institutions, to enable the co-production of services based on a reform of local administrative protocols toward the We-government model, and to empower mixed local networks. The development of FirstLife followed a participatory action design research methodology involving several stakeholders among associations, local authorities and institutions, businesses and the University in the city of Turin in the last two years. The participatory process started from the requirement elicitation, and continued with the collection of applicative scenarios based on the context analysis of internal/external relations of groups of stakeholders and the balancing of their goals in a common platform. Then, the co-design of features has been undertaken in the stakeholders’ working environments to model the platform functionalities on the real processes and practices defining social acceptable technological solutions, ready to be adopted by institutional and civic organizations. The platform has been tested-in-use in multiple living labs and pilot projects, experimenting a number of use patterns representing the common actions in the city carried out by public or private actors. These activities have been integrated in an iterative development cycle that brought so far at four progressively improved versions of FirstLife, from a map-based tool to share georeferenced information to a common workbench for multiple stakeholders acting in the same area (from the neighbourhood to the city level) where open groups can self-organize initiatives and cooperate with others. The result of this process is a digital space for urban commoning practices,reflecting the organization of society in individuals and structured public and private entities, the spatial framework of their actions and the temporal development of city transformations and initiatives
Far-infrared absorption and the metal-to-insulator transition in hole-doped cuprates
By studying the optical conductivity of BSLCO and YCBCO, we show that the
metal-to-insulator transition (MIT) in these hole-doped cuprates is driven by
the opening of a small gap at low T in the far infrared. Its width is
consistent with the observations of Angle-Resolved Photoemission Spectroscopy
in other cuprates, along the nodal line of the k-space. The gap forms as the
Drude term turns into a far-infrared absorption, whose peak frequency can be
approximately predicted on the basis of a Mott-like transition. Another band in
the mid infrared softens with doping but is less sensitive to the MIT.Comment: To be published on Physical Review Letter
Optical properties of V2O3 in its whole phase diagram
Vanadium sesquioxide V2O3 is considered a textbook example of Mott-Hubbard
physics. In this paper we present an extended optical study of its whole
temperature/doping phase diagram as obtained by doping the pure material with
M=Cr or Ti atoms (V1-xMx)2O3. We reveal that its thermodynamically stable
metallic and insulating phases, although macroscopically equivalent, show very
different low-energy electrodynamics. The Cr and Ti doping drastically change
both the antiferromagnetic gap and the paramagnetic metallic properties. A
slight chromium content induces a mesoscopic electronic phase separation, while
the pure compound is characterized by short-lived quasiparticles at high
temperature. This study thus provides a new comprehensive scenario of the
Mott-Hubbard physics in the prototype compound V2O3
Evidence of a pressure-induced metallization process in monoclinic VO
Raman and combined trasmission and reflectivity mid infrared measurements
have been carried out on monoclinic VO at room temperature over the 0-19
GPa and 0-14 GPa pressure ranges, respectively. The pressure dependence
obtained for both lattice dynamics and optical gap shows a remarkable stability
of the system up to P*10 GPa. Evidence of subtle modifications of V ion
arrangements within the monoclinic lattice together with the onset of a
metallization process via band gap filling are observed for PP*. Differently
from ambient pressure, where the VO metal phase is found only in
conjunction with the rutile structure above 340 K, a new room temperature
metallic phase coupled to a monoclinic structure appears accessible in the high
pressure regime, thus opening to new important queries on the physics of
VO.Comment: 5 pages, 3 figure
A combined experimental and computational study of the pressure dependence of the vibrational spectrum of solid picene C_22H_14
We present high-quality optical data and density functional perturbation
theory calculations for the vibrational spectrum of solid picene
(CH) under pressure up to 8 GPa. First-principles calculations
reproduce with a remarkable accuracy the pressure effects on both frequency and
intensities of the phonon peaks experimentally observed . Through a detailed
analysis of the phonon eigenvectors, We use the projection on molecular
eigenmodes to unambiguously fit the experimental spectra, resolving complicated
spectral structures, in a system with hundreds of phonon modes. With these
projections, we can also quantify the loss of molecular character under
pressure. Our results indicate that picene, despite a \sim 20 % compression of
the unit cell, remains substantially a molecular solid up to 8 GPa, with phonon
modes displaying a smooth and uniform hardening with pressure. The Grueneisen
parameter of the 1380 cm^{-1} a_1 Raman peak () is much lower
than the effective value () due to K doping. This is an
indication that the phonon softening in K doped samples is mainly due to charge
transfer and electron-phonon coupling.Comment: Replaced with final version (PRB
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