29,154 research outputs found
Diffuse MeV Gamma-rays and Galactic 511 keV Line from Decaying WIMP Dark Matter
The origin of both the diffuse high-latitude MeV gamma-ray emission and the
511 keV line flux from the Galactic bulge are uncertain. Previous studies have
invoked dark matter physics to independently explain these observations, though
as yet none has been able to explain both of these emissions within the
well-motivated framework of Weakly-Interacting Massive Particles (WIMPs). Here
we use an unstable WIMP dark matter model to show that it is in fact possible
to simultaneously reconcile both of these observations, and in the process show
a remarkable coincidence: decaying dark matter with MeV mass splittings can
explain both observations if positrons and photons are produced with similar
branching fractions. We illustrate this idea with an unstable branon, which is
a standard WIMP dark matter candidate appearing in brane world models with
large extra dimensions. We show that because branons decay via three-body final
states, they are additionally unconstrained by searches for Galactic MeV
gamma-ray lines. As a result, such unstable long-lifetime dark matter particles
provide novel and distinct signatures that can be tested by future observations
of MeV gamma-rays.Comment: 19 pages, 4 figure
Finite-Size Scaling Analysis of the Eigenstate Thermalization Hypothesis in a One-Dimensional Interacting Bose gas
By calculating correlation functions for the Lieb-Liniger model based on the
algebraic Bethe ansatz method, we conduct a finite-size scaling analysis of the
eigenstate thermalization hypothesis (ETH) which is considered to be a possible
mechanism of thermalization in isolated quantum systems. We find that the ETH
in the weak sense holds in the thermodynamic limit even for an integrable
system although it does not hold in the strong sense. Based on the result of
the finite-size scaling analysis, we compare the contribution of the weak ETH
to thermalization with that of yet another thermalization mechanism, the
typicality, and show that the former gives only a logarithmic correction to the
latter.Comment: 5 pages, 3 figure
Carbon nanotube quantum dots on hexagonal boron nitride
We report the fabrication details and low-temperature characteristics of the
first carbon nanotube (CNT) quantum dots on flakes of hexagonal boron nitride
(hBN) as substrate. We demonstrate that CNTs can be grown on hBN by standard
chemical vapor deposition and that standard scanning electron microscopy
imaging and lithography can be employed to fabricate nanoelectronic structures
when using optimized parameters. This proof of concept paves the way to more
complex devices on hBN, with more predictable and reproducible characteristics
and electronic stability.Comment: 4 pages, 4 figure
Mechanical cleaning of graphene
Contamination of graphene due to residues from nanofabrication often
introduces background doping and reduces charge carrier mobility. For samples
of high electronic quality, post-lithography cleaning treatments are therefore
needed. We report that mechanical cleaning based on contact mode AFM removes
residues and significantly improves the electronic properties. A mechanically
cleaned dual-gated bilayer graphene transistor with hBN dielectrics exhibited a
mobility of ~36,000 cm2/Vs at low temperature.Comment: 4 pages, 4 figure
Ferromagnetism in a Hubbard model for an atomic quantum wire: a realization of flat-band magnetism from even-membered rings
We have examined a Hubbard model on a chain of squares, which was proposed by
Yajima et al as a model of an atomic quantum wire As/Si(100), to show that the
flat-band ferromagnetism according to a kind of Mielke-Tasaki mechanism should
be realized for an appropriate band filling in such a non-frustrated lattice.
Reflecting the fact that the flat band is not a bottom one, the ferromagnetism
vanishes, rather than intensified, as the Hubbard U is increased. The exact
diagonalization method is used to show that the critical value of U is in a
realistic range. We also discussed the robustness of the magnetism against the
degradation of the flatness of the band.Comment: misleading terms and expressions are corrected, 4 pages, RevTex, 5
figures in Postscript, to be published in Phys. Rev. B (rapid communication
Discreteness-induced resonances and AC voltage amplitudes in long one-dimensional Josephson junction arrays
New resonance steps are found in the experimental current-voltage
characteristics of long, discrete, one-dimensional Josephson junction arrays
with open boundaries and in an external magnetic field. The junctions are
underdamped, connected in parallel, and DC biased. Numerical simulations based
on the discrete sine-Gordon model are carried out, and show that the solutions
on the steps are periodic trains of fluxons, phase-locked by a finite amplitude
radiation. Power spectra of the voltages consist of a small number of harmonic
peaks, which may be exploited for possible oscillator applications. The steps
form a family that can be numbered by the harmonic content of the radiation,
the first member corresponding to the Eck step. Discreteness of the arrays is
shown to be essential for appearance of the higher order steps. We use a
multi-mode extension of the harmonic balance analysis, and estimate the
resonance frequencies, the AC voltage amplitudes, and the theoretical limit on
the output power on the first two steps.Comment: REVTeX, 17 pages, 7 figures, psfig; to appear in J. Applied Physic
Entanglement Purification of Any Stabilizer State
We present a method for multipartite entanglement purification of any
stabilizer state shared by several parties. In our protocol each party measures
the stabilizer operators of a quantum error-correcting code on his or her
qubits. The parties exchange their measurement results, detect or correct
errors, and decode the desired purified state. We give sufficient conditions on
the stabilizer codes that may be used in this procedure and find that Steane's
seven-qubit code is the smallest error-correcting code sufficient to purify any
stabilizer state. An error-detecting code that encodes two qubits in six can
also be used to purify any stabilizer state. We further specify which classes
of stabilizer codes can purify which classes of stabilizer states.Comment: 11 pages, 0 figures, comments welcome, submitting to Physical Review
Uncertainty Relation Revisited from Quantum Estimation Theory
By invoking quantum estimation theory we formulate bounds of errors in
quantum measurement for arbitrary quantum states and observables in a
finite-dimensional Hilbert space. We prove that the measurement errors of two
observables satisfy Heisenberg's uncertainty relation, find the attainable
bound, and provide a strategy to achieve it.Comment: manuscript including 4 pages and 2 figure
Quantum Effects in Small-Capacitance Single Josephson Junctions
We have measured the current-voltage (I-V) characteristics of
small-capacitance single Josephson junctions at low temperatures (T=0.02-0.6
K), where the strength of the coupling between the single junction and the
electromagnetic environment was controlled with one-dimensional arrays of dc
SQUIDs. The single-junction I-V curve is sensitive to the impedance of the
environment, which can be tuned IN SITU. We have observed Coulomb blockade of
Cooper-pair tunneling and even a region of negative differential resistance,
when the zero-bias resistance R_0' of the SQUID arrays is much higher than the
quantum resistance R_K = h/e^2 = 26 kohm. The negative differential resistance
is evidence of coherent single-Cooper-pair tunneling within the theory of
current-biased single Josephson junctions. Based on the theory, we have
calculated the I-V curves numerically in order to compare with the experimental
ones at R_0' >> R_K. The numerical calculation agrees with the experiments
qualitatively. We also discuss the R_0' dependence of the
single-Josephson-junction I-V curve in terms of the superconductor-insulator
transition driven by changing the coupling to the environment.Comment: 11 pages with 14 embedded figures, RevTeX4, final versio
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