9 research outputs found
Tuning phase transition between quantum spin Hall and ordinary insulating phases
An effective theory is constructed for analyzing a generic phase transition
between the quantum spin Hall and the insulator phases. Occurrence of
degeneracies due to closing of the gap at the transition are carefully
elucidated. For systems without inversion symmetry the gap-closing occurs at
\pm k_0(\neq G/2) while for systems with inversion symmetry, the gap can close
only at wave-numbers k=G/2, where G is a reciprocal lattice vector. In both
cases, following a unitary transformation which mixes spins, the system is
represented by two decoupled effective theories of massive two-component
fermions having masses of opposite signs. Existence of gapless helical modes at
a domain wall between the two phases directly follows from this formalism. This
theory provides an elementary and comprehensive phenomenology of the quantum
spin Hall system.Comment: 6 pages, 2 figures, to appear in Phys. Rev.
Josephson diode effect on the surface of topological insulators
We study the Josephson diode effect in a superconductor-normal
metal-superconductor junction on the surface of a three-dimensional topological
insulator when an in-plane magnetic field is applied. We find strong dependence
of the current-phase relation on the transverse momentum and sign changes of
the quality factor as the magnitude of magnetic field varies. These sign
changes require a long junction length and are absent in the one-dimensional
limit. Our work indicates that high-dimensional rectification systems have rich
properties due to multiple transport channels, which can be used for designing
realistic superconducting diode devices.Comment: 6 pages, 5 figure
RESPONSE CHARACTERISTICS OF A TRAILER TYPE MULTI-CONNECTED BARGE SYSTEM IN WAVES
ABSTRACT A trailer type multi-connected barge system is a new type of ocean transportation system composed of several barges connected to each other with a mechanical connecting device and a tug ship. To design the trailer type multi-connected barge system, it is necessary to grasp the exact response characteristics of the trailer type multi-connected barge system in waves. A model basin test was conducted for the trailer type multiconnected barge system in waves. In the model basin test, the tug ship and three barges were connected to each other by the mechanical connecting device, and the tug ship towed three barges in waves. As a result, the response characteristics of the trailer type multi-connected barge system in waves were obtained. A nonlinear phenomenon of a longitudinal connecting force caused by relative pitch motions of barges was observed. A way to avoid the non-linear phenomenon and reduce the longitudinal connecting force could be found, and the realization of the trailer type multi-connected barge system was experimentally confirmed
DEVELOPMENT OF MULTI-CONNECTED FLOATING BODY SYSTEM
ABSTRACT A multi-connected floating body system composed of many connected small-scale floating body units has a number of advantages with regard to construction and performance. To realize the multi-connected floating body system, a new type of mechanical connecting device was developed. It is composed of multi-degrees of freedom connecting mechanism and restricting mechanism to control a restricting condition between floating body units, and its effectiveness was confirmed by model basin test in waves. Next, friction and wear tests in the seawater environment led to the discovery of suitable materials for oscillatory sliding parts in multi-degrees of freedom connecting mechanism. Finally, a field test performed for two floating body units connected by use of the mechanical connecting device proved that the developed mechanical connecting device was of practical use
Observation of Fluctuation Spin Hall Effect in Antiferromagnet
The spin Hall effect (SHE) can generate a pure spin current by an electric
current, which is promisingly used to electrically control magnetization. To
reduce power consumption of this control, a giant spin Hall angle (SHA) in the
SHE is desired in low-resistivity systems for practical applications. Here,
critical spin fluctuation near the antiferromagnetic (AFM) phase-transition is
proved as an effective mechanism to create an additional part of SHE, named as
fluctuation spin Hall effect (FSHE). This FSHE enhances the SHA due to the AFM
spin fluctuation between conduction electrons and local spins. We detect the
FSHE with the inverse and direct spin Hall effect (ISHE and DSHE) set-up and
their temperature (T) dependences in the Cr/MgO/Fe magnetic tunnel junctions
(MTJs). The SHA is significantly enhanced when temperature is approached to the
N\'eel temperature (T_N) and has a peak value of -0.34 at 200 K near T_N. This
value is higher than the room-temperature value by 240% and comparable to that
of heavy metals Ta and W. Furthermore, the spin Hall resistivity of Cr well
fits the modeled T-dependence when T approaches T_N from low temperatures,
implying the AFM spin fluctuation nature of strong SHA enhancement. Thus, this
study demonstrates the critical spin fluctuation as a prospective way of
increasing SHA and enriches the AFM material candidates for spin-orbitronic
devices.Comment: 27 pages, 9 figure
Effects of disorder on two coupled Hubbard chains at half-filling
We investigate the effects of quenched disorder on two chain Hubbard models
at half-filling by using bosonization and renormalization group methods. It is
found that the sufficiently strong forward scattering due to impurities and the
random gauge field, which is generated by impurity backward scattering, destroy
the charge gaps as well as the spin gaps. Random backward scattering due to
impurities then drives the resulting massless phase to the Anderson
localization phase. For intermediate strength of random forward scattering,
however, the spin gaps still survive, and only one of the charge gaps is
collapsed. In this parameter region, one of the charge degrees of freedom is in
the Anderson localized state, while the other one is still in the massive
state.Comment: 10 pages, RevTex, 3 eps figure