27 research outputs found
The Non-traditional Methods of Robotic Wheelchair Multi-channel Control
The most common way to control a wheelchair is the joystick. However, for some people the joystick control is difficult or impossible for one reason or another. For such people, other (non-traditional) control methods are being developed using methods of robotics. In this article, we consider possible non-traditional control methods that can be used to allow patients control a robotic wheelchair by themselves.
Keywords: robotic wheelchair, multi-channel control, non-traditional control methods, robotic
NMR study of trialuminide intermetallics
We present a systematic study of the DO22-structure trialuminide intermetallic alloys using 27Al NMR spectroscopy. The quadrupole splittings, Knight shifts, and spin-lattice relaxation times on Al3Ti, Al3V, Al3Nb, and Al3Ta have been identified. Knight-shift tensors were isolated by observation of quadrupole satellite lines and fitting to the central-transition powder patterns. The results are associated with the local electronic density of states for each crystallographic site. Universally small isotropic Knight shifts and long T1’s are consistent with low Fermi-surface densities of states indicating the importance of Fermi-surface features for the phase stability of these alloys. Larger anisotropic Knight shifts occurring at aluminum site I indicate strong hybridization at this site, and the electric-field-gradient tensors confirm the strong ab plane bonding configuration. Local-moment magnetism is found in Al3V, yet electrically this material appears very similar to the other DO22 aluminides
Separation of quadrupolar and magnetic contributions to spin-lattice relaxation in the case of a single isotope
We present a NMR pulse double-irradiation method which allows one to separate
magnetic from quadrupolar contributions in the spin-lattice relaxation. The
pulse sequence fully saturates one transition while another is observed. In the
presence of a Delta m = 2 quadrupolar contribution, the intensity of the
observed line is altered compared to a standard spin-echo experiment. We
calculated analytically this intensity change for spins I=1, 3/2, 5/2, thus
providing a quantitative analysis of the experimental results. Since the pulse
sequence we used takes care of the absorbed radio-frequency power, no problems
due to heating arise. The method is especially suited when only one NMR
sensitive isotope is available. Different cross-checks were performed to prove
the reliability of the obtained results. The applicability of this method is
demonstrated by a study of the plane oxygen 17O (I = 5/2) in the
high-temperature superconductor YBa_2Cu_4O_8: the 17O spin-lattice relaxation
rate consists of magnetic as well as quadrupolar contributions.Comment: 7 pages, 6 figure
Magnetic-field effects in NbSe3
We have performed NMR studies on an aligned, multicrystalline NbSe3 sample at various temperatures. We find conclusive evidence of field-induced Fermi-surface changes at low temperatures, and associate these changes with charge-density-wave (CDW) enhancement mainly localized on the yellow crystallographic site, contrary to expectations, since the low-temperature CDW is mainly localized on the orange site
59Co Nuclear Quadrupole Resonance Studies of Superconducting and Non-superconducting Bilayer Water Intercalated Sodium Cobalt Oxides NaxCoO2.yH2O
We report 59Co nuclear quadrupole resonance (NQR) studies of bilayer water
intercalated sodium cobalt oxides NaxCoO2.yH2O (BLH) with the superconducting
transition temperatures, 2 K < T_c <= 4.6 K, as well as a magnetic BLH sample
without superconductivity. We obtained a magnetic phase diagram of T_c and the
magnetic ordering temperature T_M against the peak frequency nu_3 59Co NQR
transition I_z = +- 5/2 +-7/2 and found a dome shape superconducting phase.
The 59Co NQR spectrum of the non-superconducting BLH shows a broadening below
T_M without the critical divergence of 1/T_1 and 1/T_2, suggesting an
unconventional magnetic ordering. The degree of the enhancement of 1/T_1T at
low temperatures increases with the increase of nu_3 though the optimal
nu_3~12.30 MHz. In the NaxCoO2.yH2O system, the optimal-T_c superconductivity
emerges close to the magnetic instability. T_c is suppressed near the phase
boundary at nu_3~12.50 MHz, which is not a conventional magnetic quantum
critical point.Comment: 4 pages, 5 figure
Microscopic Coexistence of Ferromagnetism and Superconductivity in Single-Crystal UCoGe
Unambiguous evidence for the microscopic coexistence of ferromagnetism and
superconductivity in UCoGe ( K and
0.6 K) is reported from Co nuclear quadrupole resonance (NQR). The
Co-NQR signal below 1 K indicates ferromagnetism throughout the sample
volume, while nuclear spin-lattice relaxation rate in the ferromagnetic
(FM) phase decreases below due to the opening of the
superconducting(SC) gap. The SC state was found to be inhomogeneous, suggestive
of a self-induced vortex state, potentially realizable in a FM superconductor.
In addition, the Co-NQR spectrum around show that the FM
transition in UCoGe possesses a first-order character, which is consistent with
the theoretical prediction that the low-temperature FM transition in itinerant
magnets is generically of first-order.Comment: 5 pages, 5 figure
Na content dependence of superconductivity and the spin correlations in Na_{x}CoO_{2}\cdot 1.3H_{2}O
We report systematic measurements using the ^{59}Co nuclear quadrupole
resonance(NQR) technique on the cobalt oxide superconductors Na_{x}CoO_{2}\cdot
1.3H_{2}O over a wide Na content range x=0.25\sim 0.34. We find that T_c
increases with decreasing x but reaches to a plateau for x \leq0.28. In the
sample with x \sim 0.26, the spin-lattice relaxation rate 1/T_1 shows a T^3
variation below T_c and down to T\sim T_c/6, which unambiguously indicates the
presence of line nodes in the superconducting (SC) gap function. However, for
larger or smaller x, 1/T_1 deviates from the T^3 variation below T\sim 2 K even
though the T_c (\sim 4.7 K) is similar, which suggests an unusual evolution of
the SC state. In the normal state, the spin correlations at a finite wave
vector become stronger upon decreasing x, and the density of states at the
Fermi level increases with decreasing x, which can be understood in terms of a
single-orbital picture suggested on the basis of LDA calculation.Comment: version published in J. Phys. Condens. Matter (references updated and
more added
Evidence for Uniform Coexistence of Ferromagnetism and Unconventional Superconductivity in UGe_2: A ^73Ge-NQR Study under Pressure
We report on the itinerant ferromagnetic superconductor UGe_2 through
^73Ge-NQR measurements under pressure (P). The P dependence of the NQR spectrum
signals a first-order transition from the low-temperature (T) and low-P
ferromagnetic phase (FM2) to high-T and high-P one (FM1) around a critical
pressure of P_x ~ 1.2 GPa. The superconductivity exhibiting a maximum value of
T_sc=0.7 K at P_x ~ 1.2 GPa, was found to take place in connection with the
P-induced first-order transition. The nuclear spin-lattice relaxation rate
1/T_1 has probed the ferromagnetic transition, exhibiting a peak at the Curie
temperature as well as a decrease without the coherence peak below T_sc. These
results reveal the uniformly coexistent phase of ferromagnetism and
unconventional superconductivity with a line-node gap. We remark on an intimate
interplay between the onset of superconductivity and the underlying electronic
state for the ferromagnetic phases.Comment: 8 pages, 9 figures. to appear in J. Phys. Soc. JPN, 74 No.2 (2005
Spin dynamics and ordering of a cuprate stripe-antiferromagnet
In La1.48Nd0.4Sr0.12CuO4 the 139La and 63Cu NQR relaxation rates and signal
wipe-out upon lowering temperature are shown to be due to purely magnetic
fluctuations. They follow the same renormalized classical behavior as seen in
neutron data, when the electronic spins order in stripes, with a small spread
in spin stiffness (15% spread in activation energy). The La signal, which
reappears at low temperatures, is magnetically broadened and experiences
additional wipe-out due to slowing down of the Nd fluctuations.Comment: 4 pages including 3 figures - ref. 16 adde
Electrodeionization Using Microseparated Bipolar Membranes
An electrochemical technique for deionizing water, now under development, is intended to overcome a major limitation of prior electrically-based water-purification techniques. The limitation in question is caused by the desired decrease in the concentration of ions during purification: As the concentration of ions decreases, the electrical resistivity of the water increases, posing an electrical barrier to the removal of the remaining ions. In the present technique, this limitation is overcome by use of electrodes, a flowfield structure, and solid electrolytes configured to provide conductive paths for the removal of ions from the water to be deionized, even when the water has already been purified to a high degree. The technique involves the use of a bipolar membrane unit (BMU), which includes a cation-exchange membrane and an anion-exchange membrane separated by a nonconductive mesh that has been coated by an ionically conductive material (see figure). The mesh ensures the desired microseparation between the ion-exchange membranes: The interstices bounded by the inner surfaces of the membranes and the outer surfaces of the coated mesh constitute a flow-field structure that allows the water that one seeks to deionize (hereafter called "process water" for short) to flow through the BMU with a low pressure drop. The flow-field structure is such that the distance between any point in the flow field and an ionically conductive material is small; thus, the flow-field structure facilitates the diffusion of molecules and ions to and from the ion-exchange membranes. The BMU is placed between an anode and a cathode, but not in direct contact with these electrodes. Instead, the space between the anion-exchange membrane and the anode is denoted the anode compartment and is filled with an ionic solution. Similarly, the space between the cation-exchange membrane and the cathode is denoted the cathode compartment and is filled with a different ionic solution. The electrodes are made of titanium coated with platinum