1,891 research outputs found
Nodal-antinodal dichotomy from pairing disorder in d-wave superconductors
We study the basic features of the local density of states (LDOS) observed in
STM experiments on high-T d-wave superconductors in the context of a
minimal model of a d-wave superconductor which has {\it weakly} modulated
off-diagonal disorder. We show that the low and high energy features of the
LDOS are consistent with the observed experimental patterns and in particular,
the anisotropic local domain features at high energies. At low energies, we
obtain not only the scattering peaks predicted by the octet model, but also
weak features that should be experimentally accessible. Finally, we show that
the emerging features of the LDOS lose their correspondence with the features
of the imposed disorder, as its complexity increases spatially
A Striped Holographic Superconductor
We study inhomogeneous solutions of a 3+1-dimensional Einstein-Maxwell-scalar
theory. Our results provide a holographic model of superconductivity in the
presence of a charge density wave sourced by a modulated chemical potential. We
find that below a critical temperature superconducting stripes develop. We show
that they are thermodynamically favored over the normal state by computing the
grand canonical potential. We investigate the dependence of the critical
temperature on the modulation's wave vector, which characterizes the
inhomogeneity. We find that it is qualitatively similar to that expected for a
weakly coupled BCS theory, but we point out a quantitative difference. Finally,
we use our solutions to compute the conductivity along the direction of the
stripes.Comment: 30 pages, 11 figures, v2: references added, figure 4 replace
On the behaviour of structure-sensitive reactions on single atom and dilute alloy surfaces
Materials that are composed of atomically dispersed platinum group metal (PGM) atoms on coinage metal surfaces show remarkable catalytic performance in a number of chemical reactions. On these single atom alloy (SAA) surfaces, the isolated PGM atoms exhibit unique reactivity features owing to their distinctive, and often limited, interactions with the surrounding coinage metal atoms. In this work, we use density functional theory to investigate the reactivity of numerous SAA(100) and (111) surfaces, focusing on typically structure-sensitive reactions, which include the direct dissociations of NO, CO_{2} and N_{2}. Our results suggest that the structure-sensitivity of these three reactions is considerably reduced on SAA surfaces as compared to pure platinum group metal surfaces (Rh, Pt, Pd and Ni). Additionally, we examine the reactivity of small Rh and Ni ensembles doped on Cu(100) and (111) facets. We determine that NiāNi dimers and Ni trimers outperform the studied SAAs in the activation of N[double bond, length as m-dash]O, C[double bond, length as m-dash]O and N[triple bond, length as m-dash]N bonds, and are also capable of performing facile association reactions. This work can guide future theoretical and surface science studies on SAAs, as well as the development of highly dilute alloys, which can efficiently catalyse chemistries of industrial significance
The Catalytic Decomposition of Nitrous Oxide and the NO + CO Reaction over Ni/Cu Dilute and Single Atom Alloy Surfaces: First-principles Microkinetic Modelling
The development of platinum group metal-free (PGM-free) catalysts, which can efficiently reduce pollution-causing emissions, is an important task for overcoming major environmental challenges. In particular, nitrogen oxides (NOx) are major contributors to air pollution, being one of the culprits for smog and ozone depletion. In this work, we employ density functional theory (DFT) and microkinetic modelling to investigate the decomposition of N2O and the NO + CO reaction over two PGM-free Ni/Cu dilute alloys. On the first surface, Ni atoms are isolated on the host Cu(111), thereby forming a single atom alloy surface (i.e. Ni/Cu(111) SAA), while on the second, the same atoms are organised as Ni-Ni dimers (i.e. Ni2Cu(111)). The same reactions are also simulated on pure Cu(111) (i.e. the host surface), and on Rh(111), which is used for benchmarking as Rh is a well-established PGM in emissions control catalysis. Our results suggest that the addition of trace amounts of Ni on Cu(111) may bring about significant improvement to the catalytic performance with regard to the catalytic decomposition of N2O. Additionally, we determine that Ni2Cu(111) shows equivalent, or under some circumstances even better, performance as compared to Rh(111) for the NO + CO reaction. This work contributes to the longāstanding efforts toward the design of efficient PGM-free catalytic materials for the reduction of noxious gases
Magnetoresistance of atomic-sized contacts: an ab-initio study
The magnetoresistance (MR) effect in metallic atomic-sized contacts is
studied theoretically by means of first-principle electronic structure
calculations. We consider three-atom chains formed from Co, Cu, Si, and Al
atoms suspended between semi-infinite Co leads. We employ the screened
Korringa-Kohn-Rostoker Green's function method for the electronic structure
calculation and evaluate the conductance in the ballistic limit using the
Landauer approach. The conductance through the constrictions reflects the
spin-splitting of the Co bands and causes high MR ratios, up to 50%. The
influence of the structural changes on the conductance is studied by
considering different geometrical arrangements of atoms forming the chains. Our
results show that the conductance through s-like states is robust against
geometrical changes, whereas the transmission is strongly influenced by the
atomic arrangement if p or d states contribute to the current.Comment: Revised version, presentation of results is improved, figure 2 is
splitted to two figure
Engineering the Surface Architecture of Highly Dilute Alloys: An ab Initio Monte Carlo Approach
Highly dilute alloys of platinum group metals (PGMs) - (Pt, Rh, Ir, Pd, and Ni) with coinage metals (Cu, Au and Ag) serve as highly selective and coke-resistant catalysts in a number of applications. The catalytic behaviour of these materials is governed by the size and shape of the surface āensemblesā of PGM atoms. Therefore, establishing a means of control over the topological architecture of highly dilute alloy surfaces is crucial to optimising their catalytic performance. In the present work, we use on-lattice Monte Carlo (MC) simulations that are parameterised by density functional theory (DFT) derived energetics, in order to investigate the surface aggregation of PGM atoms under vacuum conditions and in the presence of CO. We study several highly dilute alloy surfaces at various PGM loadings, including Pd/Au(111), Pd/Ag(111), Pt/Cu(111), Rh/Cu(111), Ir/Ag(111) and Ni/Cu(111). Under vacuum conditions, we observe a thermodynamic preference for dispersion of PGM as single atoms in the surface of the coinage metal host, on all examined alloy surfaces except Ir/Ag(111), where Ir atom aggregation and island formation is preferred. By evaluating the alloy surface structure in the presence of CO, we determine that the size and shape of PGM ensembles can be manipulated by tuning the partial pressure of CO (PCO) on the Pd/Au(111), Pd/Ag(111), Ir/Ag(111) and Ni/Cu(111) surfaces. In contrast, we determine that Pt/Cu(111) and Rh/Cu(111) highly dilute alloys are unresponsive to changes in PCO with Rh and Pt dispersing as isolated single atoms within the host matrix, irrespective of gaseous composition. Our findings suggest that it may be possible to fine-tune the surface architecture of highly dilute binary alloys for optimised catalytic performance
Ballistic Spin Injection from Fe into ZnSe and GaAs with a (001), (111), and (110) orientation
We present first-principles calculations of ballistic spin injection in
Fe/GaAs and Fe/ZnSe junctions with orientation (001), (111), and (110). We find
that the symmetry mismatch of the Fe minority-spin states with the
semiconductor conduction states can lead to extremely high spin polarization of
the current through the (001) interface for hot and thermal injection
processes. Such a symmetry mismatch does not exist for the (111) and (110)
interfaces, where smaller spin injection efficiencies are found. The presence
of interface states is found to lower the current spin polarization, both with
and without a Schottky barrier. Finally, a higher bias can also affect the spin
injection efficiency.Comment: 12 pages, 18 figure
Energy efficiency parametric design tool in the framework of holistic ship design optimization
Recent International Maritime Organization (IMO) decisions with respect to measures to reduce the emissions from maritime greenhouse gases (GHGs) suggest that the collaboration of all major stakeholders of shipbuilding and ship operations is required to address this complex techno-economical and highly political problem efficiently. This calls eventually for the development of proper design, operational knowledge, and assessment tools for the energy-efficient design and operation of ships, as suggested by the Second IMO GHG Study (2009). This type of coordination of the efforts of many maritime stakeholders, with often conflicting professional interests but ultimately commonly aiming at optimal ship design and operation solutions, has been addressed within a methodology developed in the EU-funded Logistics-Based (LOGBASED) Design Project (2004ā2007). Based on the knowledge base developed within this project, a new parametric design software tool (PDT) has been developed by the National Technical University of Athens, Ship Design Laboratory (NTUA-SDL), for implementing an energy efficiency design and management procedure. The PDT is an integral part of an earlier developed holistic ship design optimization approach by NTUA-SDL that addresses the multi-objective ship design optimization problem. It provides Pareto-optimum solutions and a complete mapping of the design space in a comprehensive way for the final assessment and decision by all the involved stakeholders. The application of the tool to the design of a large oil tanker and alternatively to container ships is elaborated in the presented paper
Universality of liquid-gas Mott transitions at finite temperatures
We explain in a consistent manner the set of seemingly conflicting
experiments on the finite temperature Mott critical point, and demonstrate that
the Mott transition is in the Ising universality class. We show that, even
though the thermodynamic behavior of the system near such critical point is
described by an Ising order parameter, the global conductivity can depend on
other singular observables and, in particular, on the energy density. Finally,
we show that in the presence of weak disorder the dimensionality of the system
has crucial effects on the size of the critical region that is probed
experimentally.Comment: 4 pages, 3 figure
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