98,503 research outputs found
From the Mendeleev periodic table to particle physics and back to the periodic table
We briefly describe in this paper the passage from Mendeleev's chemistry
(1869) to atomic physics (in the 1900's), nuclear physics (in the 1932's) and
particle physics (from 1953 to 2006). We show how the consideration of
symmetries, largely used in physics since the end of the 1920's, gave rise to a
new format of the periodic table in the 1970's. More specifically, this paper
is concerned with the application of the group SO(4,2)xSU(2) to the periodic
table of chemical elements. It is shown how the Madelung rule of the atomic
shell model can be used for setting up a periodic table that can be further
rationalized via the group SO(4,2)xSU(2) and some of its subgroups. Qualitative
results are obtained from this nonstandard table.Comment: 15 pages; accepted for publication in Foundations of Chemistry
(special issue to commemorate the one hundredth anniversary of the death of
Mendeleev who died in 1907); version 2: 16 pages; some sentences added;
acknowledgment and references added; misprints correcte
Unsupervised Learning of Long-Term Motion Dynamics for Videos
We present an unsupervised representation learning approach that compactly
encodes the motion dependencies in videos. Given a pair of images from a video
clip, our framework learns to predict the long-term 3D motions. To reduce the
complexity of the learning framework, we propose to describe the motion as a
sequence of atomic 3D flows computed with RGB-D modality. We use a Recurrent
Neural Network based Encoder-Decoder framework to predict these sequences of
flows. We argue that in order for the decoder to reconstruct these sequences,
the encoder must learn a robust video representation that captures long-term
motion dependencies and spatial-temporal relations. We demonstrate the
effectiveness of our learned temporal representations on activity
classification across multiple modalities and datasets such as NTU RGB+D and
MSR Daily Activity 3D. Our framework is generic to any input modality, i.e.,
RGB, Depth, and RGB-D videos.Comment: CVPR 201
Shift insulators: rotation-protected two-dimensional topological crystalline insulators
We study a two-dimensional (2D) tight-binding model of a topological
crystalline insulator (TCI) protected by rotation symmetry. The model is built
by stacking two Chern insulators with opposite Chern numbers which transform
under conjugate representations of the rotation group, e.g. orbitals.
Despite its apparent similarity to the Kane-Mele model, it does not host stable
gapless surface states. Nevertheless the model exhibits topological responses
including the appearance of quantized fractional charge bound to rotational
defects (disclinations) and the pumping of angular momentum in response to
threading an elementary magnetic flux, which are described by a mutual
Chern-Simons coupling between the electromagnetic gauge field and an effective
gauge field corresponding to the rotation symmetry. In addition, we show that
although the filled bands of the model do not admit a symmetric Wannier
representation, this obstruction is removed upon the addition of appropriate
atomic orbitals, which implies `fragile' topology. As a result, the response of
the model can be derived by representing it as a superposition of atomic
orbitals with positive and negative integer coefficients. Following the
analysis of the model, which serves as a prototypical example of 2D TCIs
protected by rotation, we show that all TCIs protected by point group
symmetries which do not have protected surface states are either atomic
insulators or fragile phases. Remarkably, this implies that gapless surface
states exist in free electron systems if and only if there is a stable Wannier
obstruction. We then use dimensional reduction to map the problem of
classifying 2D TCIs protected by rotation to a zero-dimensional (0D) problem
which is then used to obtain the complete non-interacting classification of
such TCIs as well as the reduction of this classification in the presence of
interactions.Comment: 33 pages, 16 figure
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