3,148 research outputs found
Patterns and driving forces of dimensionality-dependent charge density waves in 2H-type transition metal dichalcogenides
Two-dimensional (2D) materials have become a fertile playground for the
exploration and manipulation of novel collective electronic states. Recent
experiments have unveiled a variety of robust 2D orders in highly-crystalline
materials ranging from magnetism to ferroelectricity and from superconductivity
to charge density wave (CDW) instability. The latter, in particular, appears in
diverse patterns even within the same family of materials with isoelectronic
species. Furthermore, how they evolve with dimensionality has so far remained
elusive. Here we propose a general framework that provides a unfied picture of
CDW ordering in the 2H polytype of four isoelectronic transition metal
dichalcogenides 2H-MX (M=Nb, Ta and X=S, Se). We first show experimentally
that whilst NbSe exhibits a strongly enhanced CDW order in the 2D limit,
the opposite trend exists for TaSe and TaS, with CDW being entirely
absent in NbS from its bulk to the monolayer. Such distinct behaviours are
then demonstrated to be the result of a subtle, yet profound, competition
between three factors: ionic charge transfer, electron-phonon coupling, and the
spreading extension of the electronic wave functions. Despite its simplicity,
our approach can, in essence, be applied to other quasi-2D materials to account
for their CDW response at different thicknesses, thereby shedding new light on
this intriguing quantum phenomenon and its underlying mechanisms
Dissipative quantum dynamics, phase transitions and non-Hermitian random matrices
We explore the connections between dissipative quantum phase transitions and
non-Hermitian random matrix theory. For this, we work in the framework of the
dissipative Dicke model which is archetypal of symmetry-breaking phase
transitions in open quantum systems. We establish that the Liouvillian
describing the quantum dynamics exhibits distinct spectral features of
integrable and chaotic character on the two sides of the critical point. We
follow the distribution of the spacings of the complex Liouvillian eigenvalues
across the critical point. In the normal and superradiant phases, the
distributions are Poisson and that of the Ginibre Unitary random matrix
ensemble, respectively. Our results are corroborated by computing a recently
introduced complex-plane generalization of the consecutive level-spacing ratio
distribution. Our approach can be readily adapted for classifying the nature of
quantum dynamics across dissipative critical points in other open quantum
systems.Comment: 5 pages, 4 figures, and supplementary materia
Fractals in the Nervous System: conceptual Implications for Theoretical Neuroscience
This essay is presented with two principal objectives in mind: first, to
document the prevalence of fractals at all levels of the nervous system, giving
credence to the notion of their functional relevance; and second, to draw
attention to the as yet still unresolved issues of the detailed relationships
among power law scaling, self-similarity, and self-organized criticality. As
regards criticality, I will document that it has become a pivotal reference
point in Neurodynamics. Furthermore, I will emphasize the not yet fully
appreciated significance of allometric control processes. For dynamic fractals,
I will assemble reasons for attributing to them the capacity to adapt task
execution to contextual changes across a range of scales. The final Section
consists of general reflections on the implications of the reviewed data, and
identifies what appear to be issues of fundamental importance for future
research in the rapidly evolving topic of this review
Densification and Structural Transitions in Networks that Grow by Node Copying
We introduce a growing network model---the copying model---in which a new
node attaches to a randomly selected target node and, in addition,
independently to each of the neighbors of the target with copying probability
. When , this algorithm generates sparse networks, in which
the average node degree is finite. A power-law degree distribution also arises,
with a non-universal exponent whose value is determined by a transcendental
equation in . In the sparse regime, the network is "normal", e.g., the
relative fluctuations in the number of links are asymptotically negligible. For
, the emergent networks are dense (the average degree
increases with the number of nodes ) and they exhibit intriguing structural
behaviors. In particular, the -dependence of the number of -cliques
(complete subgraphs of nodes) undergoes transitions from normal to
progressively more anomalous behavior at a -dependent critical values of
. Different realizations of the network, which start from the same initial
state, exhibit macroscopic fluctuations in the thermodynamic limit---absence of
self averaging. When linking to second neighbors of the target node can occur,
the number of links asymptotically grows as as , so that the
network is effectively complete as .Comment: 15 pages, 12 figure
Introduction to Complex Systems, Sustainability and Innovation
The technological innovations have always proved the impossible possible. Humans have all the time obliterated barriers and set records with astounding regularity. However, there are issues springing up in terms of complexity and sustainability in this context, which we were ignoring for long. Today, in every walk of life, we encounter complex systems, whether it is the Internet, communication systems, electrical power grids, or the financial markets. Due to its unpredictable behavior, any creative change in a complex system poses a threat of systemic risks. This is because an innovation is always introducing something new, introducing a change, possibly to solve an existing problem, the effect of which is nonlinear. Failure to predict the future states of the system due to the nonlinear nature makes any system unsustainable. This necessitates the need for any development to be sustainable by meeting the needs of people today without destroying the potential of future generations to meet their needs. This chapter, which studies systems that are complex due to intricateness in their connectivity, gives insights into their ways of emergence and the nonlinear cause and effects pattern the complex systems use to follow, effectively paving way for sustainable innovation
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