636 research outputs found
The nature and strength of inter-layer binding in graphite
We computed the inter-layer bonding properties of graphite using an ab-initio
many body theory. We carried out variational and diffusion quantum Monte Carlo
calculations and found an equilibrium inter-layer binding energy in good
agreement with most recent experiments. We also analyzed the behavior of the
total energy as a function of interlayer separation at large distances
comparing the results with the predictions of the random phase approximation.Comment: 5 pages; to appear in Phys. Rev. Let
The Resonating-Valence-Bond Ground State of Li Nanoclusters
We have performed Diffusion Quantum Monte Carlo simulations of Li clusters
showing that Resonating-Valence-Bond (RVB) pairing correlations between
electrons provide a substantial contribution to the cohesive energy. The RVB
effects are identified in terms of electron transfers from s- to p-like
character, constituting a possible explanation for the breakdown of the Fermi
liquid picture observed in recent high resolution Compton scattering
experiments for bulk Li.Comment: 4 pages, 2 figures, 3 table
Electric-field-dependent empirical potentials for molecules and crystals: a first application to flexible water molecule adsorbed in zeolites
A general method to include electric-field-dependent terms in empirical potential functions representing interatomic interactions is proposed. It is applied to derive an intramolecular potential model for the water molecule able to reproduce the effects of an electric field on its geometry and dynamics: to enlarge the HOH angle, to increase slightly the OH bond lengths, to red-shift the stretching vibrational frequencies, and to blue-shift slightly the bending mode frequency. These effects have been detected experimentally for water adsorbed in zeolites and have been confirmed by quantum mechanical calculations. The electric-field-dependent intramolecular potential model for water has been combined with a newly refined intermolecular potential for bulk water and with new potentials representing cation–water and aluminosilicate–water interactions in order to simulate, by classical molecular dynamics (MD) technique, the behavior of water adsorbed in zeolites. The performances of the model have been checked by a MD simulation of liquid water at room temperature, by the structural and vibrational properties of the water dimer, and by test MD calculations on a hydrated natural zeolite (natrolite). The results are encouraging, and the simulations will be extended to study the behavior of water adsorbed in other zeolites, including diffusion and some aspects of ion exchange processes
An organic transistor-based system for reference-less electrophysiological monitoring of excitable cells
In the last four decades, substantial advances have been done in the understanding of the electrical behavior
of excitable cells. From the introduction in the early 70’s of the Ion Sensitive Field Effect Transistor (ISFET),
a lot of effort has been put in the development of more and more performing transistor-based devices to
reliably interface electrogenic cells such as, for example, cardiac myocytes and neurons. However, depending
on the type of application, the electronic devices used to this aim face several problems like the intrinsic
rigidity of the materials (associated with foreign body rejection reactions), lack of transparency and the
presence of a reference electrode. Here, an innovative system based on a novel kind of organic thin film
transistor (OTFT), called organic charge modulated FET (OCMFET), is proposed as a flexible, transparent,
reference-less transducer of the electrical activity of electrogenic cells. The exploitation of organic
electronics in interfacing the living matters will open up new perspectives in the electrophysiological field
allowing us to head toward a modern era of flexible, reference-less, and low cost probes with high-spatial and
high-temporal resolution for a new generation of in-vitro and in-vivo monitoring platforms
Submicrometer-Channel Organic Transistors with MHz Operation Range on Flexible Substrates by a Low-Resolution Fabrication Technique
In this paper, the development of a simple and reproducible approach for the fabrication of n-type organic field-effect transistors with a 350 nm-long channel on flexible substrates is reported. The critical feature of the device, the channel length, is obtained using a self-alignment process that exploits the vertical step of a plasma-etched thin Parylene C layer, according to the so-called step-edge architecture. The fabricated devices can operate in continuous mode and show an average and maximum transition frequency of 2.5 MHz and 5.5 MHz, respectively. The possibility of easily obtaining high-performing, short channel organic transistors on flexible substrates, without the use of expensive and high-resolution techniques, represents an interesting step toward the miniaturization of flexible circuits in the field of large-area organic electronics
Wearable System Based on Ultra-Thin Parylene C Tattoo Electrodes for EEG Recording
In an increasingly interconnected world, where electronic devices permeate every aspect of our lives, wearable systems aimed at monitoring physiological signals are rapidly taking over the sport and fitness domain, as well as biomedical fields such as rehabilitation and prosthetics. With the intent of providing a novel approach to the field, in this paper we discuss the development of a wearable system for the acquisition of EEG signals based on a portable, low-power custom PCB specifically designed to be used in combination with non-conventional ultra-conformable and imperceptible Parylene-C tattoo electrodes. The proposed system has been tested in a standard rest-state experiment, and its performance in terms of discrimination of two different states has been compared to that of a commercial wearable device for EEG signal acquisition (i.e., the Muse headset), showing comparable results. This first preliminary validation demonstrates the possibility of conveniently employing ultra-conformable tattoo-electrodes integrated portable systems for the unobtrusive acquisition of brain activity
Magnetism and superconductivity in the model
We present a systematic study of the phase diagram of the
model by using the Green's function Monte Carlo (GFMC) technique, implemented
within the fixed-node (FN) approximation and a wave function that contains both
antiferromagnetic and d-wave pairing. This enables us to study the interplay
between these two kinds of order and compare the GFMC results with the ones
obtained by the simple variational approach. By using a generalization of the
forward-walking technique, we are able to calculate true FN ground-state
expectation values of the pair-pair correlation functions. In the case of
, there is a large region with a coexistence of superconductivity
and antiferromagnetism, that survives up to for
and for . The presence of a finite
induces a strong suppression of both magnetic (with ,
for and ) and pairing correlations. In particular,
the latter ones are depressed both in the low-doping regime and around , where strong size effects are present.Comment: 10 pages, 9 figure
Weakly frustrated two-dimensional Heisenberg antiferromagnets: thermodynamic properties from a non-perturbative approach
We analyze the thermodynamic properties of the spin-S two-dimensional quantum
Heisenberg antiferromagnet on a square lattice with nearest and next-nearest
neighbor couplings in the Neel phase (J_2/J_1<0.4) employing the quantum
hierarchical reference theory (QHRT), a non-perturbative implementation of the
renormalization group method to quantum systems. We investigate the staggered
susceptibility, the structure factors and the correlation length at finite
temperature and for different values of the frustration ratio. From the finite
temperature results, we also extrapolate ground state properties, such as spin
stiffness and spontaneous staggered magnetization, providing an estimate of the
extent of quantum corrections. The behavior of these quantities as a function
of frustration may provide some hint on the breakdown of the Neel phase at zero
temperature for larger values of J_2
Recommended from our members
In planta proteomics and proteogenomics of the biotrophic barley fungal pathogen blumeria f.sp. hordei
Whilst there is increasing evidence tht the outcome of the interation between a pathogen and a host is dependent on protein-protein interactions, very little information is available on in planta proteomics of biotrophic plant pathogens. Here a proteogenomic approach has been employed to supplement the annotation of the recently sequenced genome and to cast light on the biology of the infection process of the economically important barley powdery mildew pathogen, Blumeria graminis f.sp horde
- …