8,271 research outputs found
Geometry and the anomalous Hall effet in ferromagnets
The geometric ideas underlying the Berry phase and the modern viewpoint of
Karplus and Luttinger's theory of the anomalous Hall effect are discussed in an
elementary way. We briefly review recent Hall and Nernst experiments which
support the dominant role of the KL velocity term in ferromagnets.Comment: 6 pages, 4 figures, conference proceedings, tutorial revie
Phase fluctuations versus Gaussian fluctuations in optimally-doped YBaCuO
We analyze recent torque measurements of the magnetization vs. magnetic
field in optimally doped YBaCuO (OPT YBCO) to argue against
a recent proposal by Rey et al that the magnetization results above are
consistent with Gaussian fluctuations. We find that, despite its strong
interlayer coupling, OPT YBCO displays an anomalous non-monotonic dependence of
on which represents direct evidence for the locking of the pair
wavefunction phase at and the subsequent unlocking by a
relatively weak . These unusual features characterize the unusual nature of
the transition to the Meissner state in cuprate superconductors. They are
absent in low- superconductors to our knowledge. We also stress the
importance of the vortex liquid state, as well as the profiles of the melting
field and the upper critical field curve in the -
plane. Contrary to the claims of Rey et al, we show that the curves of the
magnetization and the Nernst signal illustrate the inaccessibility of the
line near . The prediction of the line by Rey et al is
shown to be invalid in OPT YBCO.Comment: 6 pages, 6 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
Phase coherence and the Nernst effect at magic angles in organic conductors
A giant Nernst signal was recently observed for fields near crystallographic
directions in (TMTSF)PF. Such large Nernst signals are most naturally
associated with the motion of pancake vortices. We propose a model in which
phase coherence is destroyed throughout the sample except in planes closely
aligned with the applied field . A small tilt above or below the plane
changes the direction and density of the penetrating vortices and leads to a
Nernst signal that varies with the tilt angle of as observed. The
resistance notches at magic angles are understood in terms of flux-flow
dissipation from field-induced vortices.Comment: 4 pages, 4 figure
The Nernst effect in high- superconductors
The observation of a large Nernst signal in an extended region above
the critical temperature in hole-doped cuprates provides evidence that
vortex excitations survive above . The results support the scenario that
superfluidity vanishes because long-range phase coherence is destroyed by
thermally-created vortices (in zero field), and that the pair condensate
extends high into the pseudogap state in the underdoped (UD) regime. We present
a series of measurements to high fields which provide strong evidence for
this phase-disordering scenario.Comment: 21 pages, 28 figure
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
Unusual Nernst effect suggestive of time-reversal violation in the striped cuprate LaBaCuO
The striped cuprate LaBaCuO ( undergoes several
transitions below the charge-ordering temperature = 54 K. From Nernst
experiments, we find that, below , there exists a large, anomalous
Nernst signal that is symmetric in field , and remains
finite as . The time-reversal violating signal suggests that, below
, vortices of one sign are spontaneously created to relieve interlayer
phase frustration.Comment: 5 pages, 4 figure
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