1,091 research outputs found
X-shaped and Y-shaped Andreev resonance profiles in a superconducting quantum dot
The quasi-bound states of a superconducting quantum dot that is weakly
coupled to a normal metal appear as resonances in the Andreev reflection
probability, measured via the differential conductance. We study the evolution
of these Andreev resonances when an external parameter (such as magnetic field
or gate voltage) is varied, using a random-matrix model for the
scattering matrix. We contrast the two ensembles with broken time-reversal
symmetry, in the presence or absence of spin-rotation symmetry (class C or D).
The poles of the scattering matrix in the complex plane, encoding the center
and width of the resonance, are repelled from the imaginary axis in class C. In
class D, in contrast, a number of the poles has zero real
part. The corresponding Andreev resonances are pinned to the middle of the gap
and produce a zero-bias conductance peak that does not split over a range of
parameter values (Y-shaped profile), unlike the usual conductance peaks that
merge and then immediately split (X-shaped profile).Comment: Contribution for the JETP special issue in honor of A.F. Andreev's
75th birthday. 9 pages, 8 figure
HFR Code: A Flexible Replication Scheme for Cloud Storage Systems
Fractional repetition (FR) codes are a family of repair-efficient storage
codes that provide exact and uncoded node repair at the minimum bandwidth
regenerating point. The advantageous repair properties are achieved by a
tailor-made two-layer encoding scheme which concatenates an outer
maximum-distance-separable (MDS) code and an inner repetition code. In this
paper, we generalize the application of FR codes and propose heterogeneous
fractional repetition (HFR) code, which is adaptable to the scenario where the
repetition degrees of coded packets are different. We provide explicit code
constructions by utilizing group divisible designs, which allow the design of
HFR codes over a large range of parameters. The constructed codes achieve the
system storage capacity under random access repair and have multiple repair
alternatives for node failures. Further, we take advantage of the systematic
feature of MDS codes and present a novel design framework of HFR codes, in
which storage nodes can be wisely partitioned into clusters such that data
reconstruction time can be reduced when contacting nodes in the same cluster.Comment: Accepted for publication in IET Communications, Jul. 201
Quantum theory of the charge stability diagram of semiconductor double quantum dot systems
We complete our recently introduced theoretical framework treating the double
quantum dot system with a generalized form of Hubbard model. The effects of all
quantum parameters involved in our model on the charge stability diagram are
discussed in detail. A general formulation of the microscopic theory is
presented, and truncating at one orbital per site, we study the implication of
different choices of the model confinement potential on the Hubbard parameters
as well as the charge stability diagram. We calculate the charge stability
diagram keeping three orbitals per site and find that the effect of additional
higher-lying orbitals on the subspace with lowest-energy orbitals only can be
regarded as a small renormalization of Hubbard parameters, thereby justifying
our practice of keeping only the lowest-orbital in all other calculations. The
role of the harmonic oscillator frequency in the implementation of the Gaussian
model potential is discussed, and the effect of an external magnetic field is
identified to be similar to choosing a more localized electron wave function in
microscopic calculations. The full matrix form of the Hamiltonian including all
possible exchange terms, and several peculiar charge stability diagrams due to
unphysical parameters are presented in the appendix, thus emphasizing the
critical importance of a reliable microscopic model in obtaining the system
parameters defining the Hamiltonian.Comment: 19 pages, 15 figure
Implementation of a color-capable optofluidic microscope on a RGB CMOS color sensor chip substrate
We report the implementation of a color-capable on-chip lensless microscope system, termed color optofluidic microscope (color OFM), and demonstrate imaging of double stained Caenorhabditis elegans with lacZ gene expression at a light intensity about 10 mW/cm^2
Classifying Pattern Formation in Materials via Machine Learning
Scanning probe experiments such as scanning tunneling microscopy (STM) and atomic force microscopy (AFM) on strongly correlated materials often reveal complex pattern formation that occurs on multiple length scales. We have shown in two disparate correlated materials that the pattern formation is driven by proximity to a disorder-driven critical point. We developed new analysis concepts and techniques that relate the observed pattern formation to critical exponents by analyzing the geometry and statistics of clusters observed in these experiments and converting that information into critical exponents. Machine learning algorithms can be helpful correlating data from scanning probe experiments to theoretical models of pattern formation. We analyze the use of machine learning algorithms for the identification of critical behavior and universality underlying scanning probe data sets taken from correlated materials. This method has complementary strengths to the cluster analysis methods. The cluster techniques have a clear physical interpretation while machine learning algorithms require less expertise from the user and are faster to implement. The complementary nature of the two techniques further facilitates our understanding of correlated materials. The training of machine learning algorithms has been done through artificial neural networks. The neural net was trained using data from theoretical simulations of percolation and Ising models. The trained net had a 3.00% average classification error during testing. This proves that machine learning algorithms can successfully distinguish whether the complex pattern formation of a specific novel material is governed by uncorrelated percolation or by an interaction model fixed point
Wigner-Poisson statistics of topological transitions in a Josephson junction
The phase-dependent bound states (Andreev levels) of a Josephson junction can
cross at the Fermi level, if the superconducting ground state switches between
even and odd fermion parity. The level crossing is topologically protected, in
the absence of time-reversal and spin-rotation symmetry, irrespective of
whether the superconductor itself is topologically trivial or not. We develop a
statistical theory of these topological transitions in an N-mode quantum-dot
Josephson junction, by associating the Andreev level crossings with the real
eigenvalues of a random non-Hermitian matrix. The number of topological
transitions in a 2pi phase interval scales as sqrt(N) and their spacing
distribution is a hybrid of the Wigner and Poisson distributions of
random-matrix theory.Comment: 12 pages, 15 figures; v2 to appear in PRL, with appendix in the
supplementary materia
- …