1,721,008 research outputs found
Gauge approach to the specific heat in the normal state of cuprates
No full textMany experimental features of the electronic specific heat and entropy of high-Tc cuprates in the normal state, including the nontrivial temperature dependence of the specific-heat coefficient γ and the negative intercept of the extrapolated entropy to T=0 for underdoped cuprates, are reproduced using the spin-charge gauge approach to the t-J model. The entropy turns out to be basically due to fermionic excitations but with a temperature dependence of the specific-heat coefficient controlled by fluctuations of a gauge field coupling them to gapful bosonic excitations. In particular the negative intercept of the extrapolated entropy at T=0 in the pseudogap “phase” is attributed to the scalar component of the gauge field, which implements the local no-double occupancy constraint
Trends in the Change in Graphene Conductivity upon Gas Adsorption: The Relevance of Orbital Distortion
No full textThe experimental ability to alter graphene (G) conductivity by adsorption of a single gas molecule is promoting the development of ultra-high-sensitivity gas detectors and could ultimately provide a novel playground for future nanoelectronics devices. At present, the underpinning effect is broadly attributed to a variation of G carrier concentration, caused by an adsorption-induced Fermi-level shift. By means of first-principle Kubo-Greenwood calculations, here we demonstrate that adsorbate-induced orbital distortion could also lead to small but finite G conductivity changes, even in the absence of Fermi-level shifts. This mechanism enables a sound physical interpretation of the observed variable sensitivity of G devices to different chemical moieties, and it can be strongly enhanced by using a suitable Ni substrate, thereby opening new pathways for the optimal design of operational nanoscale detectors
Liquid-glass transition in monoatomic vanadium: A first-principles study
No full textMonoatomic metallic glasses, successfully obtained in recent experiments, represent ideal systems to investigate the local atomic structures in glasses and glass-forming processes. By ab initio molecular dynamics we simulate the formation of a metal glass of vanadium and compute different structural, energetic, and electronic properties, including the electrical and thermal conductivities, which are compared to those of vanadium in the standard, solid-state, bcc crystal phase (obtained by adopting a slower quenching rate) and in the liquid phase too. As found for other monoatomic metallic glasses, we show that the fundamental structural process of V glass formation is represented by the tendency of V atoms to form icosahedral structures. This conclusion, together with the analysis of the electronic-charge distribution and the estimate of the electrical conductivity, suggests that the glass state of vanadium can be interpreted as a "frozen" (inherent) liquid configuration
Inclusion of screening effects in the van der Waals corrected DFT simulation of adsorption processes on metal surfaces
No full textThe DFT/vdW-WF2 method, recently developed to include the van der Waals (vdW) interactions in density functional theory (DFT) using the maximally localized Wannier functions, is improved by taking into account screening effects and applied to the study of adsorption of rare gases and small molecules, H-2, CH4, and H2O on the Cu(111) metal surface, and of H-2 on Al(111), and Xe on Pb(111), which are all cases where screening effects are expected to be important. Screening is included in DFT/vdW-WF2 by following different recipes, also considering the single-layer approximation adopted to mimic a screened metal substrate. Comparison of the computed equilibrium binding energies and distances, and the C-3 coefficients characterizing the adparticle-surface van der Waals interactions, with available experimental and theoretical reference data show that the improvement with respect to the original unscreened approach is remarkable. The results are also compared with those obtained by other vdW-corrected DFT schemes
Cohesive properties of noble metals by van der Waals-corrected density functional theory: Au, Ag, and Cu as case studies
No full textThe cohesive energy, equilibrium lattice constant, and bulk modulus of Au, Ag, and Cu noble metals are computed by different van der Waals (vdW)-corrected density functional theory (DFT) methods, including vdW-DF, vdW-DF2, vdW-DF-cx, rVV10, and PBE-D. Two specifically designed methods are also developed in order to effectively include dynamical screening effects: the DFT/vdW-WF2p method, based on the generation of maximally localized Wannier functions, and the RPAp scheme (in two variants), based on a single-oscillator model of the localized electron response. Comparison with results obtained without explicit inclusion of van der Waals effects, such as with the local density approximation (LDA), PBE, PBEsol, or the hybrid PBE0 functional, elucidates the importance of a suitable description of screened van der Waals interactions even in the case of strong metal bonding. Many-body effects are also quantitatively evaluated within the RPAp approach
Hidden by graphene – Towards effective screening of interface van der Waals interactions via monolayer coating
No full textRecent atomic force microscopy (AFM) experiments [ACS Nano 2014, 8, 12410-12417] conducted on graphene-coated SiO2 demonstrated that monolayer graphene (G) can effectively screen dispersion van der Waals (vdW) interactions deriving from the underlying substrate: despite the single-atom thickness of G, the AFM tip was almost insensitive to SiO2, and the tip-substrate attraction was essentially determined only by G. This G vdW opacity has far reaching implications, encompassing stabilization of multilayer heterostructures, micromechanical phenomena or even heterogeneous catalysis. Yet, detailed experimental control and high-end applications of this phenomenon await sound physical understanding of the underlying physical mechanism. By quantum many-body analysis and ab-initio Density Functional Theory, here we address this challenge providing theoretical rationalization of the observed G vdW opacity for weakly interacting substrates. The non-local density response and ultra slow decay of the G vdW interaction ensure compensation between standard attractive terms and many-body repulsive contributions, enabling vdW opacity over a broad range of adsorption distances. vdW opacity appears most efficient in the low frequency limit and extends beyond London dispersion including electrostatic Debye forces. By virtue of combined theoretical/experimental validation, G hence emerges as a promising ultrathin shield for modulation and switching of vdW interactions at interfaces and complex nanoscale devices
Erratum: van der Waals interactions in density functional theory using Wannier functions: Improved C_{6} and C_{3} coefficients by a different approach [Phys. Rev. B 85, 073101 (2012)]
No full textErratu
Gas Separation in Nanoporous Graphene from First Principle Calculations
No full textThanks to its single atom thickness and its mechanical strength,
nanoporous graphene is currently being regarded as a
promising candidate for efficient and reliable gas separation applications.
Clearly the accurate energetic characterization of the penetration
processes involving relevant gas-phase molecules is a fundamental
prerequisite for any possible application.
Here we evaluate permeation barriers and adsorption energies of the HO,
CH, CO, CO, O, H molecules, and of the Ar atom on two types of
hydrogen saturated pores by means of ab initio simulations, based on the
density functional theory (DFT), able to include dispersion corrections too.
We find that, although the qualitative trend followed by the
values of the permeation barriers of the considered molecules is
independent on the adopted DFT functional, at a quantitative level
the results are noticeably affected by the dispersion
corrections and the chosen exchange contribution characterizing the different
functionals, as well as by the allowed graphene sheet distortions.
Interestingly, we observe that, due to the occurrence of non-trivial
H-bond interactions with the pore-saturating H atoms, the permeation
barrier of water remains low even considering a small-size pore.
The barrier is further diminished when considering the interaction with a second
water molecule on the opposite side of the pore.
These observations, combined with the relatively strong binding of the water
molecule with the defected surface, suggests that porous graphene could also
represent a promising membrane for water filtration
van der Waals interactions in density functional theory using Wannier functions: Improved C_{6} and C_{3} coefficients by a different approach
No full textA new implementation is proposed for including van der Waals interactions
in Density Functional Theory using the Maximally-Localized Wannier
functions. With respect to the previous DFT/vdW-WF method, the
present DFT/vdW-WF2 approach, which is based on the simpler
London expression and takes into account the intrafragment overlap
of the localized Wannier functions, leads to a considerable improvement
in the evaluation of the van der Waals coefficients, as
shown by the application to a set of selected dimers.
Preliminary results on Ar on graphite and Ne on the Cu(111) metal
surface suggest that also the coefficients, characterizing
molecule-surfaces van der Waals interactions are better estimated
with the new scheme
Van Der Waals-Corrected Density Functional Theory Simulation of Adsorption Processes on Noble-Metal Surfaces: Xe on Ag(111), Au(111), and Cu(111)
No full textThe DFT/vdW-WF2s1 method based on the generation of localized Wannier functions, recently developed to include the van der Waals interactions in the density functional theory and describe adsorption processes on metal surfaces by taking metal-screening effects into account, is applied to the case of the interaction of Xe with noble-metal surfaces, namely Ag(111), Au(111), and Cu(111). The study is also repeated by adopting the DFT/vdW-QHO-WF variant relying on the quantum harmonic oscillator model which describes well many body effects. Comparison of the computed equilibrium binding energies and distances, and the coefficients characterizing the adatom-surface van der Waals interactions, with available experimental and theoretical reference data shows that the methods perform well and elucidates the importance of properly including screening effects. The results are also compared with those obtained by other vdW-corrected DFT schemes, including PBE-D, vdW-DF, vdW-DF2, rVV10, and by the simpler local density approximation and semi-local (PBE) generalized gradient approximation approaches
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