775 research outputs found
Exploring local quantum many-body relaxation by atoms in optical superlattices
We establish a setting - atoms in optical superlattices with period 2 - in
which one can experimentally probe signatures of the process of local
relaxation and apparent thermalization in non-equilibrium dynamics without the
need of addressing single sites. This opens up a way to explore the convergence
of subsystems to maximum entropy states in quenched quantum many-body systems
with present technology. Remarkably, the emergence of thermal states does not
follow from a coupling to an environment, but is a result of the complex
non-equilibrium dynamics in closed systems. We explore ways of measuring the
relevant signatures of thermalization in this analogue quantum simulation of a
relaxation process, exploiting the possibilities offered by optical
superlattices.Comment: 4 pages, 3 figures, version to published in Physical Review Letter
The Digital Flynn Effect: Complexity of Posts on Social Media Increases over Time
Parents and teachers often express concern about the extensive use of social
media by youngsters. Some of them see emoticons, undecipherable initialisms and
loose grammar typical for social media as evidence of language degradation. In
this paper, we use a simple measure of text complexity to investigate how the
complexity of public posts on a popular social networking site changes over
time. We analyze a unique dataset that contains texts posted by 942, 336 users
from a large European city across nine years. We show that the chosen
complexity measure is correlated with the academic performance of users: users
from high-performing schools produce more complex texts than users from
low-performing schools. We also find that complexity of posts increases with
age. Finally, we demonstrate that overall language complexity of posts on the
social networking site is constantly increasing. We call this phenomenon the
digital Flynn effect. Our results may suggest that the worries about language
degradation are not warranted
Particle number conservation in quantum many-body simulations with matrix product operators
Incorporating conservation laws explicitly into matrix product states (MPS)
has proven to make numerical simulations of quantum many-body systems much less
resources consuming. We will discuss here, to what extent this concept can be
used in simulation where the dynamically evolving entities are matrix product
operators (MPO). Quite counter-intuitively the expectation of gaining in speed
by sacrificing information about all but a single symmetry sector is not in all
cases fulfilled. It turns out that in this case often the entanglement imposed
by the global constraint of fixed particle number is the limiting factor.Comment: minor changes, 18 pages, 5 figure
Orbital-order melting in rare-earth manganites: the role of super-exchange
We study the mechanism of orbital-order melting observed at temperature T_OO
in the series of rare-earth manganites. We find that many-body super-exchange
yields a transition-temperature T_KK that decreases with decreasing rare-earth
radius, and increases with pressure, opposite to the experimental T_OO. We show
that the tetragonal crystal-field splitting reduces T_KK further increasing the
discrepancies with experiments. This proves that super-exchange effects,
although very efficient, in the light of the experimentally observed trends,
play a minor role for the melting of orbital ordering in rare-earth manganites.Comment: 4 pages, 5 figure
Rigorous mean-field dynamics of lattice bosons: Quenches from the Mott insulator
We provide a rigorous derivation of Gutzwiller mean-field dynamics for
lattice bosons, showing that it is exact on fully connected lattices. We apply
this formalism to quenches in the interaction parameter from the Mott insulator
to the superfluid state. Although within mean-field the Mott insulator is a
steady state, we show that a dynamical critical interaction exists, such
that for final interaction parameter the Mott insulator is
exponentially unstable towards emerging long-range superfluid order, whereas
for the Mott insulating state is stable. We discuss the implications
of this prediction for finite-dimensional systems.Comment: 6 pages, 3 figures, published versio
Probing local relaxation of cold atoms in optical superlattices
In the study of relaxation processes in coherent non-equilibrium dynamics of
quenched quantum systems, ultracold atoms in optical superlattices with
periodicity two provide a very fruitful test ground. In this work, we consider
the dynamics of a particular, experimentally accessible initial state prepared
in a superlattice structure evolving under a Bose-Hubbard Hamiltonian in the
entire range of interaction strengths, further investigating the issues raised
in Ref. [Phys. Rev. Lett. 101, 063001 (2008)]. We investigate the relaxation
dynamics analytically in the non interacting and hard core bosonic limits,
deriving explicit expressions for the dynamics of certain correlation
functions, and numerically for finite interaction strengths using the
time-dependent density-matrix renormalization (t-DMRG) approach. We can
identify signatures of local relaxation that can be accessed experimentally
with present technology. While the global system preserves the information
about the initial condition, locally the system relaxes to the state having
maximum entropy respecting the constraints of the initial condition. For finite
interaction strengths and finite times, the relaxation dynamics contains
signatures of the relaxation dynamics of both the non-interacting and hard core
bosonic limits.Comment: 18 pages RevTex, 20 figures, final version (problem with figures
resolved
Probing the relaxation towards equilibrium in an isolated strongly correlated 1D Bose gas
The problem of how complex quantum systems eventually come to rest lies at
the heart of statistical mechanics. The maximum entropy principle put forward
in 1957 by E. T. Jaynes suggests what quantum states one should expect in
equilibrium but does not hint as to how closed quantum many-body systems
dynamically equilibrate. A number of theoretical and numerical studies
accumulate evidence that under specific conditions quantum many-body models can
relax to a situation that locally or with respect to certain observables
appears as if the entire system had relaxed to a maximum entropy state. In this
work, we report the experimental observation of the non-equilibrium dynamics of
a density wave of ultracold bosonic atoms in an optical lattice in the regime
of strong correlations. Using an optical superlattice, we are able to prepare
the system in a well-known initial state with high fidelity. We then follow the
dynamical evolution of the system in terms of quasi-local densities, currents,
and coherences. Numerical studies based on the time-dependent density-matrix
renormalization group method are in an excellent quantitative agreement with
the experimental data. For very long times, all three local observables show a
fast relaxation to equilibrium values compatible with those expected for a
global maximum entropy state. We find this relaxation of the quasi-local
densities and currents to initially follow a power-law with an exponent being
significantly larger than for free or hardcore bosons. For intermediate times
the system fulfills the promise of being a dynamical quantum simulator, in that
the controlled dynamics runs for longer times than present classical algorithms
based on matrix product states can efficiently keep track of.Comment: 8 pages, 6 figure
An investigation of the impact of data breach severity on the readability of mandatory data breach notification letters: evidence from U.S. firms
The aim of this article is to investigate the impact of data breach severity on the readability of mandatory data breach notification letters. Using a content analysis approach to determine data breach severity attributes (measured by the total number of breached records, type of data accessed, the source of the data breach, and how the data were used), in conjunction with readability measures (reading complexity, numerical intensity, length of letter, word size, and unique words), 512 data breach incidents from 281 U.S. firms across the 2012–2015 period were examined.
The results indicate that data breach severity has a positive impact on reading complexity, length of letter, word size, and unique words, and a negative impact on numerical terms. Interpreting the results collectively through the lens of impression management, it can be inferred that business managers may be attempting to obfuscate bad news associated with high data breach severity incidents by manipulating syntactical features of the data breach notification letters in a way that makes the message difficult for individuals to comprehend.
The study contributes to the information studies and impression management behavior literatures by analyzing linguistic cues in notifications following a data breach incident
What's hot in conservation biogeography in a changing climate? Going beyond species range dynamics
International audienceIn recent decades Earth's rapidly changing climate, driven by anthropogenic greenhouse gas emissions, has affected species distributions and phenology, ecological communities and ecosystem processes, effects that are increasingly being observed globally (Allen et al., 2010; Doney et al., 2012; Franklin, Serra‐Diaz, Syphard, & Regan, 2016; Parmesan, 2006; Walther et al., 2002). Pleistocene shifts in species ranges during glacial–interglacial transitions reveal large‐scale biome shifts and no‐analog species assemblages (MacDonald et al., 2008; Nolan et al., 2018; Williams & Jackson, 2007); the pace of current anthropogenic warming outstrips past changes in the Earth system and climate, however, leading to new climate novelties and ecological communities (Ordonez, Williams, & Svenning, 2016). Global scientific consensus now emphasizes that global warming should be kept to 1.5°C to avoid catastrophic changes in ecosystems and the services they provide to people (IPCC, 2018), and climate change threats to biodiversity are being prioritized in international policy response (Ferrier et al., 2016)
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