54 research outputs found
Electronic Transport on the Shastry-Sutherland Lattice in Ising-type Rare Earth Tetraborides
In the presence of a magnetic field frustrated spin systems may exhibit
plateaus at fractional values of saturation magnetization. Such plateau states
are stabilized by classical and quantum mechanisms including order-by-disorder,
triplon crystallization, and various competing order effects. In the case of
electrically conducting systems, free electrons represent an incisive probe for
the plateau states. Here we study the electrical transport of Ising-type rare
earth tetraborides B (Er, Tm), a metallic Shastry-Sutherland lattice
showing magnetization plateaus. We find that the longitudinal and transverse
resistivities reflect scattering with both the static and dynamic plateau
structure. We model these results consistently with the expected strong
uniaxial anisotropy in a quantitative level, providing a framework for the
study of plateau states in metallic frustrated systems.Comment: 18 pages, 5 figure
The zero-energy state in graphene in a high magnetic field
The fate of the charge-neutral Dirac point in graphene in a high magnetic
field has been investigated at low temperatures ( 0.3 K). In samples
with small (the gate voltage needed to access the Dirac point), the
resistance at the Dirac point diverges steeply with , signalling a
crossover to an insulating state in intense field. The approach to the
insulating state is highly unusual. Despite the steep divergence in , the
profile of vs. in fixed saturates to a -independent value
below 2 K, consistent with charge carrying gapless excitations.Comment: 4 pages, 4 figures. Four new sub-figures have been added. Text
expanded to discuss data from more sample
Transport Signatures of Fermi Surface Topology Change in BiTeI
We report a quantum magnetotransport signature of a change in Fermi surface
topology in the Rashba semiconductor BiTeI with systematic tuning of the Fermi
level . Beyond the quantum limit, we observe a marked increase/decrease in
electrical resistivity when is above/below the Dirac node that we show
originates from the Fermi surface topology. This effect represents a
measurement of the electron distribution on the low-index () Landau
levels and is uniquely enabled by the finite bulk dispersion along the
-axis and strong Rashba spin-orbit coupling strength of the system. The
Dirac node is independently identified by Shubnikov-de Haas oscillations as a
vanishing Fermi surface cross section at . Additionally we find that the
violation of Kohler's rule allows a distinct insight into the temperature
evolution of the observed quantum magnetoresistance effects.Comment: 12 pages, 4 figure
Extreme Magnetoresistance in Magnetic Rare Earth Monopnictides
The acute sensitivity of the electrical resistance of certain systems to
magnetic fields known as extreme magnetoresistance (XMR) has recently been
explored in a new materials context with topological semimetals. Exemplified by
WTe and rare earth monopnictide La(Sb,Bi), these systems tend to be
non-magnetic, nearly compensated semimetals and represent a platform for large
magnetoresistance driven by intrinsic electronic structure. Here we explore
electronic transport in magnetic members of the latter family of semimetals and
find that XMR is strongly modulated by magnetic order. In particular, CeSb
exhibits XMR in excess of % at fields of 9 T while the
magnetoresistance itself is non-monotonic across the various magnetic phases
and shows a transition from negative magnetoresistance to XMR with field above
magnetic ordering temperature . The magnitude of the XMR is larger than
in other rare earth monopnictides including the non-magnetic members and
follows an non-saturating power law to fields above 30 T. We show that the
overall response can be understood as the modulation of conductivity by the Ce
orbital state and for intermediate temperatures can be characterized by an
effective medium model. Comparison to the orbitally quenched compound GdBi
supports the correlation of XMR with the onset of magnetic ordering and
compensation and highlights the unique combination of orbital inversion and
type-I magnetic ordering in CeSb in determining its large response. These
findings suggest a paradigm for magneto-orbital control of XMR and are relevant
to the understanding of rare earth-based correlated topological materials.Comment: 21 pages, 6 figure
The thermopower and Nernst Effect in graphene in a magnetic field
We report measurements of the thermopower and Nernst signal in
graphene in a magnetic field . Both quantities show strong quantum
oscillations vs. the gate voltage . Our measurements for Landau Levels of
index are in quantitative agreement with the edge-current model of
Girvin and Jonson (GJ). The inferred off-diagonal thermoelectric conductivity
comes close to the quantum of Amps per Kelvin. At the Dirac point
(), however, the width of the peak in is very narrow. We
discuss features of the thermoelectric response at the Dirac point including
the enhanced Nernst signal.Comment: 4 pages, 4 figures. In new version, the sign of the Nernst signal is
corrected. Text revised and expanded. 2 Figures amended. One new panel adde
Band engineering of a magnetic thin film rare earth monopnictide
Realizing quantum materials in few atomic layer morphologies is a key to both
observing and controlling a wide variety of exotic quantum phenomena. This
includes topological electronic materials, where the tunability and
dimensionality of few layer materials have enabled the detection of ,
Chern, and Majorana phases. Here, we report the development of a platform for
thin film correlated, topological states in the magnetic rare-earth
monopnictide () system GdBi synthesized by molecular beam epitaxy. This
material is known from bulk single crystal studies to be semimetallic
antiferromagnets with Neel temperature 28 K and is the magnetic analog
of the non--electron containing system LaBi proposed to have topological
surface states. Our transport and magnetization studies of thin films grown
epitaxially on BaF reveal that semimetallicity is lifted below
approximately 8 crystallographic unit cells while magnetic order is maintained
down to our minimum thickness of 5 crystallographic unit cells.
First-principles calculations show that the non-trivial topology is preserved
down to the monolayer limit, where quantum confinement and the lattice symmetry
give rise to a Chern insulator phase. We further demonstrate the
stabilization of these films against atmospheric degradation using a
combination of air-free buffer and capping procedures. These results together
identify thin film materials as potential platforms for engineering
topological electronic bands in correlated magnetic materials
Creating Weyl nodes and controlling their energy by magnetization rotation
As they do not rely on the presence of any crystal symmetry, Weyl nodes are
robust topological features of an electronic structure that can occur at any
momentum and energy. Acting as sinks and sources of Berry curvature, Weyl nodes
have been predicted to strongly affect the transverse electronic response, like
in the anomalous Hall or Nernst effects. However, to observe large anomalous
effects the Weyl nodes need to be close to or at the Fermi-level, which implies
the band structure must be tuned by an external parameter, e.g. chemical doping
or pressure. Here we show that in a ferromagnetic metal tuning of the Weyl node
energy and momentum can be achieved by rotation of the magnetization. Taking
CoSnS as an example, we use electronic structure calculations based
on density-functional theory to show that not only new Weyl fermions can be
created by canting the magnetization away from the easy axis, but also that the
Weyl nodes can be driven exactly to the Fermi surface. We also show that the
dynamics in energy and momentum of the Weyl nodes strongly affect the
calculated anomalous Hall and Nernst conductivities.Comment: Supp. Material adde
- …