665 research outputs found
Anderson impurity in the one-dimensional Hubbard model on finite size systems
An Anderson impurity in a Hubbard model on chains with finite length is
studied using the density-matrix renormalization group (DMRG) technique. In the
first place, we analyzed how the reduction of electron density from
half-filling to quarter-filling affects the Kondo resonance in the limit of
Hubbard repulsion U=0. In general, a weak dependence with the electron density
was found for the local density of states (LDOS) at the impurity except when
the impurity, at half-filling, is close to a mixed valence regime. Next, in the
central part of this paper, we studied the effects of finite Hubbard
interaction on the chain at quarter-filling. Our main result is that this
interaction drives the impurity into a more defined Kondo regime although
accompanied in most cases by a reduction of the spectral weight of the impurity
LDOS. Again, for the impurity in the mixed valence regime, we observed an
interesting nonmonotonic behavior. We also concluded that the conductance,
computed for a small finite bias applied to the leads, follows the behavior of
the impurity LDOS, as in the case of non-interacting chains. Finally, we
analyzed how the Hubbard interaction and the finite chain length affect the
spin compensation cloud both at zero and at finite temperature, in this case
using quantum Monte Carlo techniques.Comment: 9 pages, 9 figures, final version to be published in Phys. Rev.
Caracterización de algunos nuevos complejos de Fe (II) en alto spin, con tioureas substituidas como ligantes
El estudio de nuevos complejos de Fe (Il) en alto spin con tioureas, utilizando X = ClO4 o BF4 como aniones, muestra que sólo hay complejos de fórmula [FeL6 X 2] cuando L = tiourea, N-metiltiourea, N-etiltiourea, N-N'-di-n-propiltiourea o N,N'-diciclohexiltiourea. Cuando el ligante es N,N'dimetiltiourea (DMTU) o N ,N'-dietiltiourea (DETU) se observa la formación de complejos de fórmula [Fe (DMTU)n (BF4)2] (n = 4, 5 ó 7), [Fe (DMTU)n (ClO4 } 2 ) (n = 4 ó 6) y [Fe (DETUJn X2 ](n = 4 ó 6). En la caracterización de estos complejos, mediante espectroscopia IR, electrónica y Mossbauer, se encontraron complejos tetraédricos ([FeL4 X2 ]), octaédricos ( [FeL 6 X2 ]) y [Fe (DMTU)6 • (BF4}2 • DMTU]) y pentacoordinados ([Fe (DMTU)5 (BF4)2 ))
Interpretación de los espectros Mössbauer y electrónicos de un complejo pentacoordinado de Fe (II) en alto spin
En este trabajo se ha calculado el desdoblamiento de los orbitales 3d y de los términos del Fe (II) libre, en un campo cristalino de una pirámide de base cuadrada distorsionada, para poder interpretar los espectros Mossbauer y electrónicos del complejo [Fe (N,N'-dimetiltiourea)5 ] ( BF4 )2. Dichos espectros son explicados satisfactoriamente considerando que el átomo metálico se encuentra sobre el plano basal, de manera que el orden de energía de los orbitales 3d es dxy < dxz, dyz < dz2 < dx2 y2
Numerical study of finite size effects in the one-dimensional two-impurity Anderson model
We study the two-impurity Anderson model on finite chains using numerical
techniques. We discuss the departure of magnetic correlations as a function of
the interimpurity distance from a pure 2k_F oscillation due to open boundary
conditions. We observe qualitatively different behaviors in the interimpurity
spin correlations and in transport properties at different values of the
impurity couplings. We relate these different behaviors to a change in the
relative dominance between the Kondo effect and the
Ruderman-Kittel-Kasuya-Yoshida (RKKY) interaction. We also observe that when
RKKY dominates there is a definite relation between interimpurity magnetic
correlations and transport properties. In this case, there is a recovery of
2k_F periodicity when the on-site Coulomb repulsion on the chain is increased
at quarter-filling. The present results could be relevant for electronic
nanodevices implementing a non-local control between two quantum dots that
could be located at variable distance along a wire.Comment: final version to appear in Phys. Rev.
Are older people putting themselves at risk when using their walking frames?
Background Walking aids are issued to older adults to prevent falls, however, paradoxically their use has been identified as a risk factor for falling. To prevent falls, walking aids must be used in a stable manner, but it remains unknown to what extent associated clinical guidance is adhered to at home, and whether following guidance facilitates a stable walking pattern. It was the aim of this study to investigate adherence to guidance on walking frame use, and to quantify user stability whilst using walking frames. Additionally, we explored the views of users and healthcare professionals on walking aid use, and regarding the instrumented walking frames (‘Smart Walkers’) utilized in this study.
Methods This observational study used Smart Walkers and pressure-sensing insoles to investigate usage patterns of 17 older people in their home environment; corresponding video captured contextual information. Additionally, stability when following, or not, clinical guidance was quantified for a subset of users during walking in an Activities of Daily Living Flat and in a gait laboratory. Two focus groups (users, healthcare professionals) shared their experiences with walking aids and provided feedback on the Smart Walkers.
Results Incorrect use was observed for 16% of single support periods and for 29% of dual support periods, and was associated with environmental constraints and a specific frame design feature. Incorrect use was associated with reduced stability. Participants and healthcare professionals perceived the Smart Walker technology positively.
Conclusions Clinical guidance cannot easily be adhered to and self-selected strategies reduce stability, hence are placing the user at risk. Current guidance needs to be improved to address environmental constraints whilst facilitating stable walking. The research is highly relevant considering the rising number of walking aid users, their increased falls-risk, and the costs of falls.
Trial Registration Not applicable
Environmental and economic assessment of hydrogen compression with the metal hydride technology
Research and development of hydrogen carrier based solutions for hydrogen compression and storage
Melting of Partially Fluorinated Graphene: From Detachment of Fluorine Atoms to Large Defects and Random Coils
The melting of fluorographene is very unusual and depends strongly on the
degree of fluorination. For temperatures below 1000 K, fully fluorinated
graphene (FFG) is thermo-mechanically more stable than graphene but at
T2800 K FFG transits to random coils which is almost twice lower
than the melting temperature of graphene, i.e. 5300 K. For fluorinated graphene
(PFG) up to 30 % ripples causes detachment of individual F-atoms around 2000 K
while for 40-60 % fluorination, large defects are formed beyond 1500 K and
beyond 60% of fluorination F-atoms remain bonded to graphene until melting. The
results agree with recent experiments on the dependence of the reversibility of
the fluorination process on the percentage of fluorination.Comment: 16 pages, 6 figure
Objective measures of rollator user stability and device loading during different walking scenarios
Walking aids are widely used by older adults, however, alarmingly, their use has been linked to increased falls-risk, yet clinicians have no objective way of assessing user stability. This work aims to demonstrate the application of a novel methodology to investigate how the type of walking task, the amount of body weight supported by the device (i.e., device loading), and task performance strategy affect stability of rollator users. In this context, ten users performed six walking tasks with an instrumented rollator. The combined stability margin “SM” was calculated, which considers user and rollator as a combined system. A Friedman Test was used to investigate the effects of task on SM and a least-squares regression model was applied to investigate the relationship between device loading and SM. In addition, the effects of task performance strategy on SM were explored. As a result, it was found that: the minimum SM for straight line walking was higher than for more complex tasks (p<0.05); an increase in device loading was associated with an increase in SM (p<0.05); stepping up a kerb with at least 1 rollator wheel in ground contact at all times resulted in higher SM than lifting all four wheels simultaneously. Hence, we conclude that training should not be limited to straight line walking but should include various everyday tasks. Within person, SM informs on which tasks need practicing, and which strategy facilitates stability, thereby enabling person-specific guidance/training. The relevance of this work lies in an increase in walking aid users, and the costs arising from fall-related injuries.
Supplementary data is available in Figshare
Photocatalytic reactions of a nickel(II) annulene complex incorporated in polymeric structures
The photochemical reactions of the Ni(II) annulene complex, [NiII([5,7,12,14]-tetra methyl dibenzo[2,3- b:2,3-b,i][1,4,8,11]tetraaza[14]annulenate)], grafted into a poly(isobutylene-alt-maleate) backbone were investigated in aqueous media. The grafted Ni(II) complex becomes soluble in aqueous and organic solvents where the strands form aggregates with medium-dependent shapes. Irradiation of the polymer at 532 or 351 nm produce charge-separated macrocyclic pendants, CS, with a lifetime s 30 ns. CS reacts with electron donors and acceptors before it decays with a lifetime s 1 ms. In parallel to the decay of CS, an excited state-excited state annihilation process gives rise to luminescence whose spectrum spans wavelengths shorter than the wavelength of the irradiation, lex > 500 nm. Theoretical calculations were carried out with the aim of understanding the morphology and structures of strand aggregates, to confirm the nature of reaction products and to account for the spectroscopic and photochemical properties of the Ni(II) pendants. The endothermic reduction of CO2 to CO by S(IV) species was used as a test of the Ni(II) complex´s ability to photocatalyze the reaction. In the photoprocess, the Ni(II) complex fulfills the double role of antenna and catalyst.Fil: Estiu, G.. University Of Notre Dame-indiana; Estados UnidosFil: Ferraudi, G.. University Of Notre Dame-indiana; Estados UnidosFil: Lappin, A. G.. University Of Notre Dame-indiana; Estados UnidosFil: Ruiz, Gustavo Teodosio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico la Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; Argentina. Universidad Nacional de La Plata; ArgentinaFil: Vericat, Carolina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico la Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; Argentina. Universidad Nacional de La Plata; ArgentinaFil: Costamagna, J.. Universidad de Santiago de Chile; ChileFil: Villagrán, M.. Universidad de Santiago de Chile; Chil
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