12,683 research outputs found
Theoretical Study of Spin-dependent Electron Transport in Atomic Fe Nanocontacts
We present theoretical predictions of spintronic transport phenomena that
should be observable in ferromagnetic Fe nanocontacts bridged by chains of Fe
atoms. We develop appropriate model Hamiltonians based on semi-empirical
considerations and the known electronic structure of bulk Fe derived from ab
initio density functional calculations. Our model is shown to provide a
satisfactory description of the surface properties of Fe nano-clusters as well
as bulk properties. Lippmann-Schwinger and Green's function techniques are used
together with Landauer theory to predict the current, magneto-resistance, and
spin polarization of the current in Fe nanocontacts bridged by atomic chains
under applied bias. Unusual device characteristics are predicted including
negative magneto-resistance and spin polarization of the current, as well as
spin polarization of the current for anti-parallel magnetization of the Fe
nanocontacts under moderate applied bias. We explore the effects that
stretching the atomic chain has on the magneto-resistance and spin polarization
and predict a cross-over regime in which the spin polarization of the current
for parallel magnetization of the contacts switches from negative to positive.
We find resonant transmission due to dangling bond formation on tip atoms as
the chain is stretched through its breaking point to play an important role in
spin-dependent transport in this regime. The physical mechanisms underlying the
predicted phenomena are discussed.Comment: 13 pages, 6 figures, Accepted for publication in Physical Review
Energy and information flows in strongly coupled rotary machines
Living systems at the molecular scale are composed of many coupled components with interactions varying in nature and strength. Microscopic biological systems operate far from equilibrium and are subject to strong fluctuations. These conditions pose significant challenges to efficient, precise, and rapid free-energy transduction, yet nature has evolved numerous molecular machines that do just this. We present a model of strongly coupled stochastic rotary motors inspired by FoF1-ATP synthase and study its behavior. Rather than aiming for the most accurate model of ATP synthase, the model is meant to be a starting point to explore the effect of less-than-tight coupling between components. To this end, we aim to give the model a minimum level of complexity while keeping biological considerations in mind. Energy and information flows are studied numerically and through analytically tractable limiting cases. The limiting cases provide bounds on the system’s performance. We find that the output power of a work-to-work converter consisting of two coupled subsystems in the presence of energy barriers can be maximized at intermediate-strength coupling rather than at tight coupling. This phenomenon is backed up by a simple theory that predicts the power maximizing coupling strength, and agrees well with numerical results. We observe several characteristics that show up at the coupling strength that maximizes output power: a maximum in power transmitted from Fo to F1, a maximum in information flow, and equal subsystem entropy production rates. Finally, we derive a bound on the machine’s input and output power, which accounts for the energy and information passed between subsystems. We conclude that intermediate-strength coupling is a realistic option for biological systems passing on energy and information to downstream processes
The quantized Hall conductance of a single atomic wire: A proposal based on synthetic dimensions
We propose a method by which the quantization of the Hall conductance can be
directly measured in the transport of a one-dimensional atomic gas. Our
approach builds on two main ingredients: (1) a constriction optical potential,
which generates a mesoscopic channel connected to two reservoirs, and (2) a
time-periodic modulation of the channel, specifically designed to generate
motion along an additional synthetic dimension. This fictitious dimension is
spanned by the harmonic-oscillator modes associated with the tightly-confined
channel, and hence, the corresponding "lattice sites" are intimately related to
the energy of the system. We analyze the quantum transport properties of this
hybrid two-dimensional system, highlighting the appealing features offered by
the synthetic dimension. In particular, we demonstrate how the energetic nature
of the synthetic dimension, combined with the quasi-energy spectrum of the
periodically-driven channel, allows for the direct and unambiguous observation
of the quantized Hall effect in a two-reservoir geometry. Our work illustrates
how topological properties of matter can be accessed in a minimal
one-dimensional setup, with direct and practical experimental consequences.
Functional renormalization group approach to correlated fermion systems
Numerous correlated electron systems exhibit a strongly scale-dependent
behavior. Upon lowering the energy scale, collective phenomena, bound states,
and new effective degrees of freedom emerge. Typical examples include (i)
competing magnetic, charge, and pairing instabilities in two-dimensional
electron systems, (ii) the interplay of electronic excitations and order
parameter fluctuations near thermal and quantum phase transitions in metals,
(iii) correlation effects such as Luttinger liquid behavior and the Kondo
effect showing up in linear and non-equilibrium transport through quantum wires
and quantum dots. The functional renormalization group is a flexible and
unbiased tool for dealing with such scale-dependent behavior. Its starting
point is an exact functional flow equation, which yields the gradual evolution
from a microscopic model action to the final effective action as a function of
a continuously decreasing energy scale. Expanding in powers of the fields one
obtains an exact hierarchy of flow equations for vertex functions. Truncations
of this hierarchy have led to powerful new approximation schemes. This review
is a comprehensive introduction to the functional renormalization group method
for interacting Fermi systems. We present a self-contained derivation of the
exact flow equations and describe frequently used truncation schemes. Reviewing
selected applications we then show how approximations based on the functional
renormalization group can be fruitfully used to improve our understanding of
correlated fermion systems.Comment: Review article, final version, 59 pages, 28 figure
What are communities of practice? A comparative review of four seminal works
This paper is a comparative review of four seminal works on communities of practice. It is argued that the ambiguities of the terms community and practice are a source of the concept's reusability allowing it to be reappropriated for different purposes, academic and practical. However, it is potentially confusing that the works differ so markedly in their conceptualizations of community, learning, power and change, diversity and informality. The three earlier works are underpinned by a common epistemological view, but Lave and Wenger's 1991 short monograph is often read as primarily about the socialization of newcomers into knowledge by a form of apprenticeship, while the focus in Brown and Duguid's article of the same year is, in contrast, on improvising new knowledge in an interstitial group that forms in resistance to management. Wenger's 1998 book treats communities of practice as the informal relations and understandings that develop in mutual engagement on an appropriated joint enterprise, but his focus is the impact on individual identity. The applicability of the concept to the heavily individualized and tightly managed work of the twenty-first century is questionable. The most recent work by Wenger – this time with McDermott and Snyder as coauthors – marks a distinct shift towards a managerialist stance. The proposition that managers should foster informal horizontal groups across organizational boundaries is in fact a fundamental redefinition of the concept. However it does identify a plausible, if limited, knowledge management (KM) tool. This paper discusses different interpretations of the idea of 'co-ordinating' communities of practice as a management ideology of empowerment
Cooperative Behavior of Kinetically Constrained Lattice Gas Models of Glassy Dynamics
Kinetically constrained lattice models of glasses introduced by Kob and
Andersen (KA) are analyzed. It is proved that only two behaviors are possible
on hypercubic lattices: either ergodicity at all densities or trivial
non-ergodicity, depending on the constraint parameter and the dimensionality.
But in the ergodic cases, the dynamics is shown to be intrinsically cooperative
at high densities giving rise to glassy dynamics as observed in simulations.
The cooperativity is characterized by two length scales whose behavior controls
finite-size effects: these are essential for interpreting simulations. In
contrast to hypercubic lattices, on Bethe lattices KA models undergo a
dynamical (jamming) phase transition at a critical density: this is
characterized by diverging time and length scales and a discontinuous jump in
the long-time limit of the density autocorrelation function. By analyzing
generalized Bethe lattices (with loops) that interpolate between hypercubic
lattices and standard Bethe lattices, the crossover between the dynamical
transition that exists on these lattices and its absence in the hypercubic
lattice limit is explored. Contact with earlier results are made via analysis
of the related Fredrickson-Andersen models, followed by brief discussions of
universality, of other approaches to glass transitions, and of some issues
relevant for experiments.Comment: 59 page
Putting the Lab in the Lab Book: Supporting Coordination in Large, Multi-site Research
Large and distributed science projects present researchers with a challenging environment for interaction and collaboration. While digital technologies offer promises in supporting these difficulties, researchers appear reluctant to discontinue their use of analogue resources. We present a study of communication practices in very large-scale collaborative scientific research programmes that involve multidisciplinary and multinational research consortia. Qualitative data collection with researchers, principal investigators and project coordinators were carried out to examine the conduct and coordination of biological, biomedical and chemistry experiments that were distributed over multiple geographical locations. Results show that many problems in collaboration appear to result from the collective documentation of experimental operating procedures, tracking of experimental samples, and the sharing and cross-association of physical and digital experimental materials. Our analysis highlights the crucial but problematic role of the laboratory notebook as a driver for collaboration, most notably in supporting traceability of the distributed experimental process. We identify opportunities for improving experimental coordination, scientific communication and project synchronisation, drawing implications for digital interaction design that offers opportunities to enhance research coordination
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