17 research outputs found
Ultrasonic Sensitivity of Strain-Insensitive Fiber Bragg Grating Sensors and Evaluation of Ultrasound-Induced Strain
In conventional ultrasound detection in structures, a fiber Bragg grating (FBG) is glued on or embedded in the structure. However, application of strain to the structure can influence the sensitivity of the FBG toward ultrasound and can prevent its effective detection. An FBG can work as a strain-insensitive ultrasound sensor when it is not directly glued to the monitored structure, but is instead applied to a small thin plate to form a mobile sensor. Another possible configuration is to affix an FBG-inscribed optical fiber without the grating section attached to the monitored structure. In the present study, sensitivity to ultrasound propagated through an aluminum plate was compared for a strain-insensitive FBG sensor and an FBG sensor installed in a conventional manner. Strains induced by ultrasound from a piezoelectric transducer and by quasi-acoustic emission of a pencil lead break were also quantitatively evaluated from the response amplitude of the FBG sensor. Experimental results showed that the reduction in the signal-to-noise ratio for ultrasound detection with strain-insensitive FBG sensors, relative to traditionally-installed FBG sensors, was only 6 dB, and the ultrasound-induced strain varied within a range of sub-micron strains
Superconductivity in quantum-dot superlattices composed of quantum wire networks
Based on calculations using the local density approximation, we propose
quantum wire networks with square and plaquette type lattice structures that
form quantum dot superlattices. These artificial structures are well described
by the Hubbard model. Numerical analysis reveals a superconducting ground state
with transition temperatures of up to 90 mK for the plaquette, which is
more than double the value of 40 mK for the square lattice type and is
sufficiently high to allow for the experimental observation of
superconductivity.Comment: 10 pages, 4 figure
Flat-band ferromagnetism in quantum dot superlattices
Possibility of flat-band ferromagnetism in quantum dot arrays is
theoretically discussed. By using a quantum dot as a building block, quantum
dot superlattices are possible. We consider dot arrays on Lieb and kagome
lattices known to exhibit flat band ferromagnetism. By performing an exact
diagonalization of the Hubbard Hamiltonian, we calculate the energy difference
between the ferromagnetic ground state and the paramagnetic excited state, and
discuss the stability of the ferromagnetism against the second nearest neighbor
transfer. We calculate the dot-size dependence of the energy difference in a
dot model and estimate the transition temperature of the
ferromagnetic-paramagnetic transition which is found to be accessible within
the present fabrication technology. We point out advantages of semiconductor
ferromagnets and suggest other interesting possibilities of electronic
properties in quantum dot superlattices.Comment: 15 pages, 7 figures (low resolution). High-resolution figures are
available at
http://www.brl.ntt.co.jp/people/tamura/Research/PublicationPapers.htm
In situ annealing of superconducting MgB2 films prepared by pulsed laser deposition
The in situ annealing conditions of pulsed laser deposited MgB2 films were
studied. The precursor films were deposited at 250 C from a stoichiometric MgB2
target in a 120mTorr Ar atmosphere. The films were then in situ annealed at a
temperature from 450 C to 800 C and an annealing time from 1 minute to 10
minutes. We found that the superconducting properties depend in a crucial way
on the annealing conditions: temperature, heating rate and time. The best film
with a thickness of ~600nm was obtained under the following annealing
conditions: Tanneal=680-690 C, tanneal=1 min, heating rate= 38 C/min. The Tc
onset of the film is 28K with a transition width of ~10K. The hysteresis loop
of magnetic moment of the film indicates weak field dependence in high fields.
Magneto-optical imaging of the film showed quite homogeneous magnetic flux
penetration, indicating structural homogeneity. The films without annealing
showed no superconductivity.Comment: 12 pages, 6 figure
Self- and Cross-Fusing of Furan-Based Polyurea Gels Dynamically Cross-Linked with Maleimides
Bio-based polyureas (PUs) with main-chain furan rings were synthesized by the polyaddition of 2,5-bis(aminomethyl)furan with various diisocyanates, such as methylene diphenyl diisocyanate. Several PU’s were soluble in polar organic solvents, and were cast to form thermomechanically stable films with softening temperatures of over 100 °C. The furan rings of the PU main chains underwent a dynamic Diels-Alder (DA) reaction with bismaleimide (BMI) cross-linkers. While the mixed solution of PU and BMI did not show any apparent signs of reaction at room temperature, the DA reaction proceeded to form gels upon heating to 60 °C, which became a solution again by further heating to 80 °C (retro-DA reaction). The solution phase was maintained by rapid quenching from 80 °C to room temperature, while the gel was reformed upon slow cooling. The recovered gels exhibited self-healing properties. A scratch made by a hot knife at temperatures above 80 °C disappeared spontaneously. When two different gels were cut using a knife at room temperature, placed in contact with each other, and heated to 60 °C, they fused. The ability to control the DA/retro-DA reaction allowed gels of varying composition to heal