171 research outputs found
Universal quantum computation with ordered spin-chain networks
It is shown that anisotropic spin chains with gapped bulk excitations and
magnetically ordered ground states offer a promising platform for quantum
computation, which bridges the conventional single-spin-based qubit concept
with recently developed topological Majorana-based proposals. We show how to
realize the single-qubit Hadamard, phase, and pi/8 gates as well as the
two-qubit CNOT gate, which together form a fault-tolerant universal set of
quantum gates. The gates are implemented by judiciously controlling Ising
exchange and magnetic fields along a network of spin chains, with each
individual qubit furnished by a spin-chain segment. A subset of single-qubit
operations is geometric in nature, relying on control of anisotropy of spin
interactions rather than their strength. We contrast topological aspects of the
anisotropic spin-chain networks to those of p-wave superconducting wires
discussed in the literature.Comment: 9 pages, 3 figure
The giant aortic root aneurysm related to bicuspid aortic valve treated with valve-sparing operation
Wave function engineering in quantum dot-ring nanostructures
Modern nanotechnology allows producing, depending on application, various
quantum nanostructures with the desired properties. These properties are
strongly influenced by the confinement potential which can be modified, e.g.,
by electrical gating. In this paper we analyze a nanostructure composed of a
quantum dot surrounded by a quantum ring. We show that depending on the details
of the confining potential the electron wave functions can be located in
different parts of the structure. Since the properties of such a nanostructure
strongly depend on the distribution of the wave functions, varying the applied
gate voltage one can easily control them. In particular, we illustrate the high
controllability of the nanostructure by demonstrating how its coherent,
optical, and conducting properties can be drastically changed by a small
modification of the confining potential.Comment: 8 pages, 10 figures, 2 tables, revte
Scanning Tunneling Microscope Operating as a Spin-diode
We theoretically investigate spin-polarized transport in a system composed of
a ferromagnetic Scanning Tunneling Microscope (STM) tip coupled to an adsorbed
atom (adatom) on a host surface. Electrons can tunnel directly from the tip to
the surface or via the adatom. Since the tip is ferromagnetic and the host
surface (metal or semiconductor) is non-magnetic we obtain a spin-diode effect
when the adatom is in the regime of single occupancy. This effect leads to an
unpolarized current for direct bias (V > 0) and polarized current for reverse
(V < 0) bias voltages, if the tip is nearby the adatom. Within the
nonequilibrium Keldysh technique we analyze the interplay between the lateral
displacement of the tip and the intra adatom Coulomb interaction on the
spindiode effect. As the tip moves away from the adatom the spin-diode effect
vanishes and the currents become polarized for both V > 0 and V < 0. We also
find an imbalance between the up and down spin populations in the adatom, which
can be tuned by the tip position and the bias. Finally, due to the presence of
the adsorbate on the surface, we observe spin-resolved Friedel oscillations in
the current, which reflects the oscillations in the calculated LDOS of the
subsystem surface+adatom.Comment: 11 pages, 4 figures. Submitte
Momentum dependence of the spin susceptibility in two dimensions: nonanalytic corrections in the Cooper channel
We consider the effect of rescattering of pairs of quasiparticles in the
Cooper channel resulting in the strong renormalization of second-order
corrections to the spin susceptibility in a two-dimensional electron system. We
use the Fourier expansion of the scattering potential in the vicinity of the
Fermi surface to find that each harmonic becomes renormalized independently.
Since some of those harmonics are negative, the first derivative of the spin
susceptibility is bound to be negative at small momenta, in contrast to the
lowest order perturbation theory result, which predicts a positive slope. We
present in detail an effective method to calculate diagrammatically corrections
to the spin susceptibility to infinite order
Group Irregularity Strength of Connected Graphs
We investigate the group irregularity strength () of graphs, i.e. the
smallest value of such that taking any Abelian group \gr of order ,
there exists a function f:E(G)\rightarrow \gr such that the sums of edge
labels at every vertex are distinct. We prove that for any connected graph
of order at least 3, if and otherwise,
except the case of some infinite family of stars
Modelling and analysis of nonlinear guided waves interaction at a breathing crack using time-domain spectral finite element method
Published 20 June 2017This study proposes a time-domain spectral finite element (SFE) model and investigates nonlinear guided wave interaction at a breathing crack. An extended time-domain SFE method based on the Mindlin–Hermann rod and Timoshenko beam theory is proposed to predict the nonlinear guided wave generation at the breathing crack. An SFE crack element is proposed to simulate the mode-conversion effect, in which a bilinear crack mechanism is implemented to take into account the contact nonlinearity at the breathing crack. There is good agreement between the results calculated using the proposed time-domain SFE method and threedimensional finite element simulation. This demonstrates the accuracy of the proposed SFE method in simulating contact nonlinearity at the breathing crack. Parametric studies using the fundamental symmetric (S₀) and anti-symmetric (A₀) modes of guided waves are also carried out to provide physical insights into the higher harmonics generated due to the contact nonlinearity at the breathing crack. The magnitude of the higher harmonics generated as a function of the crack depth is investigated in detail. The results show that the mode-converted higher harmonic guided waves provide valuable information for damage detection.Shuai He and Ching Tai N
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