49 research outputs found
The thermal Hall effect of spin excitations in a Kagome magnet
At low temperatures, the thermal conductivity of spin excitations in a
magnetic insulator can exceed that of phonons. However, because they are charge
neutral, the spin waves are not expected to display a thermal Hall effect in a
magnetic field. Recently, this semiclassical notion has been upended in quantum
magnets in which the spin texture has a finite chirality. In the Kagome
lattice, the chiral term generates a Berry curvature. This results in a thermal
Hall conductivity that is topological in origin. Here we report
observation of a large in the Kagome magnet Cu(1-3, bdc) which
orders magnetically at 1.8 K. The observed undergoes a remarkable
sign-reversal with changes in temperature or magnetic field, associated with
sign alternation of the Chern flux between magnon bands. We show that thermal
Hall experiments probe incisively the effect of Berry curvature on heat
transport.Comment: 6 pages, 3 figure
Anomalous Nernst Effect in Dirac Semimetal Cd3As2
Dirac and Weyl semimetals display a host of novel properties. In
CdAs, the Dirac nodes lead to a protection mechanism that strongly
suppresses backscattering in zero magnetic field, resulting in ultrahigh
mobility ( 10 cm V s). In applied magnetic field,
an anomalous Nernst effect is predicted to arise from the Berry curvature
associated with the Weyl nodes. We report observation of a large anomalous
Nernst effect in CdAs. Both the anomalous Nernst signal and transport
relaxation time begin to increase rapidly at 50 K. This
suggests a close relation between the protection mechanism and the anomalous
Nernst effect. In a field, the quantum oscillations of bulk states display a
beating effect, suggesting that the Dirac nodes split into Weyl states,
allowing the Berry curvature to be observed as an anomalous Nernst effect.Comment: 13 pages, 7 figure
Evidence for massive bulk Dirac Fermions in PbSnSe from Nernst and thermopower experiments
The lead chalcogenides (Pb,Sn)Te and (Pb,Sn)Se are the first examples of
topological crystalline insulators (TCI) predicted \cite{Fu,Hsieh} (and
confirmed \cite{Hasan,Story,Takahashi}) to display topological surface Dirac
states (SDS) that are protected by mirror symmetry. A starting premise
\cite{Hsieh} is that the SDS arise from bulk states describable as massive
Dirac states \cite{Wallis,Svane}, but this assumption is untested. Here we show
that the thermoelectric response of the bulk states display features specific
to the Dirac spectrum. We show that, in the quantum limit, the lowest Landau
Level (LL) is singly spin-degenerate, whereas higher levels are doubly
degenerate. The abrupt change in spin degeneracy leads to a large step-decrease
in the thermopower . In the lowest LL, displays a striking
linear increase vs. magnetic field. In addition, the Nernst signal undergoes an
anomalous sign change when the bulk gap inverts at 180 K.Comment: 16 pages, 8 figure
Correlation of Crystal Quality and Extreme Magnetoresistance of WTe
High quality single crystals of WTe were grown using a Te flux followed
by a cleaning step involving self-vapor transport. The method is reproducible
and yields consistently higher quality single crystals than are typically
obtained via halide assisted vapor transport methods. Magnetoresistance
(MR)values at 9 Tesla and 2 Kelvin as high as 1.75 million \%, nearly an order
of magnitude higher than previously reported for this material, were obtained
on crystals with residual resistivity ratio (RRR) of approximately 1250. The MR
follows a near B law (B = 1.95(1)) and, assuming a semiclassical model, the
average carrier mobility for the highest quality crystal was found to be
~167,000 cm/Vs at 2 K. A correlation of RRR, MR ratio and average carrier
mobility () is found with the cooling rate during the flux growth.Comment: 7 pages, 3 figures, 1 tabl
Kagome lattice promotes chiral spin fluctuations
Magnetic materials with tilted electron spins often exhibit conducting
behavior that cannot be explained from semiclassical theories without invoking
fictitious (emergent) electromagnetic fields. Quantum-mechanical models
explaining such phenomena are rooted in the concept of a moving quasiparticle's
Berry phase, driven by a chiral (left- or right-handed) spin-habit. Dynamical
and nearly random spin fluctuations, with a slight bent towards left- or
right-handed chirality, represent a promising route to realizing Berry-phase
phenomena at elevated temperatures, but little is known about the effect of
crystal lattice geometry on the resulting macroscopic observables. Here, we
report thermoelectric and electric transport experiments on two metals with
large magnetic moments on a triangular and on a slightly distorted kagom\'e
lattice, respectively. We show that the impact of chiral spin fluctuations is
strongly enhanced for the kagom\'e lattice. Both these spiral magnets have
similar magnetic phase diagrams including a periodic array of magnetic
skyrmions. However, our modelling shows that the geometry of the kagom\'e
lattice, with corner-sharing spin-trimers, helps to avoid cancellation of
Berry-phase contributions; spin fluctuations are endowed with a net chiral
habit already in the thermally disordered (paramagnetic) state. Hence, our
observations for the kagom\,e material contrast with theoretical models
treating magnetization as a continuous field, and emphasize the role of lattice
geometry on emergent electrodynamic phenomena.Comment: 16 pages, 4 figure