363 research outputs found
High-field Studies on Layered Magnetic and Polar Dirac Metals
Recently, the interplay between the Dirac/Weyl fermion and various bulk
properties, such as magnetism, has attracted considerable attention, since
unconventional transport and optical phenomena were discovered. However, the
design principles for such materials have not been established well. Here, we
propose that the layered material Mn (: alkaline and rare-earth
ions, : Sb, Bi) is a promising platform for systematically exploring
strongly correlated Dirac metals, which consists of the alternative stack of
the square net layer hosting a 2D Dirac fermion and the
-Mn- magnetic block layer. In this article, we shall
review recent high-field studies on this series of materials to demonstrate
that various types of Dirac fermions are realized by designing the block layer.
First, we give an overview of the Dirac fermion coupled with the magnetic order
in EuMnBi (=Eu). This material exhibits large magnetoresistance by the
field-induced change in the magnetic order of Eu layers, which is associated
with the strong exchange interaction between the Dirac fermion and the local Eu
moment. Second, we review the Dirac fermion coupled with the lattice
polarization in BaMn (=Ba). There, spin-valley coupling manifests
itself owing to the Zeeman-type spin-orbit interaction, which is experimentally
evidenced by the bulk quantum Hall effect observed at high fields.Comment: 28 pages, 10 figures, published in JPSJ (Special Topics
Rich structural phase diagram and thermoelectric properties of layered tellurides Mo1-xNbxTe2
MoTe2 is a rare transition-metal ditelluride having two kinds of layered
polytypes, hexagonal structure with trigonal prismatic Mo coordination and
monoclinic structure with octahedral Mo coordination. The monoclinic distortion
in the latter is caused by anisotropic metal-metal bonding. In this work, we
have examined the Nb doping effect on both polytypes of MoTe2 and clarified a
structural phase diagram for Mo1-xNbxTe2 containing four kinds of polytypes. A
rhombohedral polytype crystallizing in polar space group has been newly
identified as a high-temperature metastable phase at slightly Nb-rich
composition. Considering the results of thermoelectric measurements and the
first principles calculations, the Nb ion seemingly acts as a hole dopant in
the rigid band scheme. On the other hand, the significant interlayer
contraction upon the Nb doping, associated with the Te p-p hybridization, is
confirmed especially for the monoclinic phase, which implies a shift of the
p-band energy level. The origin of the metal-metal bonding in the monoclinic
structure is discussed in terms of the d electron counting and the Te p-p
hybridization.Comment: 16 pages, 6 figures, 1 table, to be published in APL Material
Modified memoryless spectral-scaling Broyden family on Riemannian manifolds
This paper presents modified memoryless quasi-Newton methods based on the
spectral-scaling Broyden family on Riemannian manifolds. The method involves
adding one parameter to the search direction of the memoryless self-scaling
Broyden family on the manifold. Moreover, it uses a general map instead of
vector transport. This idea has already been proposed within a general
framework of Riemannian conjugate gradient methods where one can use vector
transport, scaled vector transport, or an inverse retraction. We show that the
search direction satisfies the sufficient descent condition under some
assumptions on the parameters. In addition, we show global convergence of the
proposed method under the Wolfe conditions. We numerically compare it with
existing methods, including Riemannian conjugate gradient methods and the
memoryless spectral-scaling Broyden family. The numerical results indicate that
the proposed method with the BFGS formula is suitable for solving an
off-diagonal cost function minimization problem on an oblique manifold.Comment: 20 pages, 8 figure
Effect of Applied Orthorhombic Lattice Distortion on the Antiferromagnetic Phase of CeAuSb
We study the response of the antiferromagnetism of CeAuSb to orthorhombic
lattice distortion applied through in-plane uniaxial pressure. The response to
pressure applied along a lattice direction shows a
first-order transition at zero pressure, which shows that the magnetic order
lifts the symmetry of the unstressed lattice. Sufficient
pressure appears to rotate the principal axes of the
order from to . At low pressure, the transition at is weakly first-order, however it
becomes continuous above a threshold pressure. We discuss
the possibility that this behavior is driven by order parameter fluctuations,
with the restoration of a continuous transition a result of reducing the
point-group symmetry of the lattice.Comment: 6 pages, 7 figure
Effect of uniaxial stress on the magnetic phases of CeAuSb
We present results of measurements of resistivity of \CAS{} under the
combination of -axis magnetic field and in-plane uniaxial stress. In
unstressed \CAS{} there are two magnetic phases. The low-field A phase is a
single-component spin-density wave (SDW), with , and the high-field B phase consists of microscopically coexisting
and spin-density waves. Pressure along
a lattice direction is a transverse field to both of
these phases, and so initially has little effect, however eventually induces
new low- and high-field phases in which the principal axes of the SDW
components appear to have rotated to the directions.
Under this strong compression, the field evolution of the
resistivity is much smoother than at zero strain: In zero strain, there is a
strong first-order transition, while under strong it
becomes much broader. We hypothesize that this is a consequence of the uniaxial
stress lifting the degeneracy between the (100) and (010) directions.Comment: 8 pages, 7 figure
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