2,005 research outputs found
Exploring Topological Phases With Quantum Walks
The quantum walk was originally proposed as a quantum mechanical analogue of
the classical random walk, and has since become a powerful tool in quantum
information science. In this paper, we show that discrete time quantum walks
provide a versatile platform for studying topological phases, which are
currently the subject of intense theoretical and experimental investigation. In
particular, we demonstrate that recent experimental realizations of quantum
walks simulate a non-trivial one dimensional topological phase. With simple
modifications, the quantum walk can be engineered to realize all of the
topological phases which have been classified in one and two dimensions. We
further discuss the existence of robust edge modes at phase boundaries, which
provide experimental signatures for the non-trivial topological character of
the system
Discovering new two-dimensional topological insulators from computational screening
We have performed a computational screening of topological two-dimensional
(2D) materials from the Computational 2D Materials Database (C2DB) employing
density functional theory. A full \textit{ab initio} scheme for calculating
hybrid Wannier functions directly from the Kohn-Sham orbitals has been
implemented and the method was used to extract indices, Chern
numbers and Mirror Chern numbers of 3331 2D systems including both
experimentally known and hypothetical 2D materials. We have found a total of 46
quantum spin Hall insulators, 7 quantum anomalous Hall insulators and 9
crystalline topological insulators that are all predicted to be dynamically
stable. Roughly one third of these were known prior to the screening. The most
interesting of the novel topological insulators are investigated in more
detail. We show that the calculated topological indices of the quantum
anomalous Hall insulators are highly sensitive to the approximation used for
the exchange-correlation functional and reliable predictions of the topological
properties of these materials thus require methods beyond density functional
theory. We also performed calculations, which yield a gap of 0.65 eV for
the quantum spin Hall insulator PdSe in the MoS crystal structure. This
is significantly higher than any known 2D topological insulator and three times
larger than the Kohn-Sham gap.Comment: 12 page
Phase sensitive measurements of order parameters for ultracold atoms through two particles interferometry
Nontrivial symmetry of order parameters is crucial in some of the most
interesting quantum many-body states of ultracold atoms and condensed matter
systems. Examples in cold atoms include p-wave Feshbach molecules and d-wave
paired states of fermions that could be realized in optical lattices in the
Hubbard regime. Identifying these states in experiments requires measurements
of the relative phase of different components of the entangled pair
wavefunction.
We propose and discuss two schemes for such phase sensitive measurements,
based on two-particle interference revealed in atom-atom or atomic density
correlations. Our schemes can also be used for relative phase measurements for
non-trivial particle-hole order parameters, such as d-density wave order.Comment: 4 pages, 4 figure
Parametrizations of triaxial deformation and E2 transitions of the wobbling band
By the very definition the triaxial deformation parameter is related
to the expectation values of the K=0 and K=2 components of the intrinsic
quadrupole tensor operator. On the other hand, using the same symbol
"", various different parametrizations of triaxial deformation have
been employed, which are suitable for various types of the mean-field
potentials. It is pointed out that the values of various "" are quite
different for the same actual triaxial deformation, especially for the large
deformation; for example, the difference can be almost a factor two for the
case of the triaxial superdeformed bands recently observed in the Hf and Lu
nuclei. In our previous work, we have studied the wobbling band in Lu nuclei by
using the microscopic framework of the cranked Nilsson mean-field and the
random phase approximation, where the most serious problem is that the
calculated B(E2) value is about factor two smaller. It is shown that the origin
of this underestimation can be mainly attributed to the small triaxial
deformation; if is used the same triaxial deformation as in the analysis of the
particle-rotor model, the calculated B(E2) increases and gives correct
magnitude compared with the experimental data.Comment: 10 pages, 9 figure
Electronic-Structure-Driven Magnetic Ordering in a Kondo Semiconductor CeOs2Al10
We report the anisotropic changes in the electronic structure of a Kondo
semiconductor CeOsAl across an anomalous antiferromagnetic ordering
temperature () of 29 K, using optical conductivity spectra. The spectra
along the - and -axes indicate that a - hybridization gap emerges
from a higher temperature continuously across . Along the b-axis, on the
other hand, a different energy gap with a peak at 20 meV appears below 39 K,
which is higher temperature than , because of structural distortion. The
onset of the energy gap becomes visible below . Our observation reveals
that the electronic structure as well as the energy gap opening along the
b-axis due to the structural distortion induces antiferromagnetic ordering
below .Comment: 4 pages, 4 figure
Combined theoretical and experimental approaches for development of squaraine dyes with small energy barrier for electron injection
A series of far-red sensitizing squaraine dyes has been systematically designed and synthesized in order to correlate the theoretically calculated values with their corresponding experimental parameters. Efforts have been directed towards determining the minimum thermodynamic energy barrier for the electron injection in the nanoporous TiO2 by logical molecular design. Theoretical calculations using Gaussian program package were performed for ground and excited states in both of the isolated gaseous state as well as in solution including the solvent effect using a self-consistent reaction field polarizable continuum model (PCM). Implementation of the PCM model or use of LSDA functional under TD-DFT calculations gives much better results for energetics as well as absorption maximum for all of the sensitizers used in this work. Newly designed symmetrical squaraine dye SQ-5 exhibits a minimum energy barrier of 0.16 eV for electron injection and shows photon harvesting behavior in far-red region with external photoconversion efficiency of 2.02% under simulated solar irradiation
Neurite imaging reveals microstructural variations in human cerebral cortical gray matter
We present distinct patterns of neurite distribution in the human cerebral cortex using diffusion magnetic resonance imaging (MRI). We analyzed both high-resolution structural (T1w and T2w images) and diffusion MRI data in 505 subjects from the Human Connectome Project. Neurite distributions were evaluated using the neurite orientation dispersion and density imaging (NODDI) model, optimized for gray matter, and mapped onto the cortical surface using a method weighted towards the cortical mid-thickness to reduce partial volume effects. The estimated neurite density was high in both somatosensory and motor areas, early visual and auditory areas, and middle temporal area (MT), showing a strikingly similar distribution to myelin maps estimated from the T1w/T2w ratio. The estimated neurite orientation dispersion was particularly high in early sensory areas, which are known for dense tangential fibers and are classified as granular cortex by classical anatomists. Spatial gradients of these cortical neurite properties revealed transitions that colocalize with some areal boundaries in a recent multi-modal parcellation of the human cerebral cortex, providing mutually supportive evidence. Our findings indicate that analyzing the cortical gray matter neurite morphology using diffusion MRI and NODDI provides valuable information regarding cortical microstructure that is related to but complementary to myeloarchitecture
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