908 research outputs found
Magnetotransport in Sr3PbO antiperovskite with three-dimensional massive Dirac electrons
Novel topological phenomena are anticipated for three-dimensional (3D) Dirac
electrons. The magnetotransport properties of cubic
antiperovskite, theoretically proposed to be a 3D massive Dirac electron
system, are studied. The measurements of Shubnikov-de Haas oscillations and
Hall resistivity indicate the presence of a low density ( ) of holes with an extremely small cyclotron mass of
0.01-0.06. The magnetoresistance is linear in
magnetic field with the magnitude independent of temperature. These results
are fully consistent with the presence of 3D massive Dirac electrons in . The chemical flexibility of the antiperovskites and our findings
in the family member, , point to their potential as a model
system in which to explore exotic topological phases
Generation of femtosecond light pulses in the near infrared around λ = 850 nm
Femtosecond light pulses tunable between 840 nm and 880 nm are generated in a synchronously pumped ring dye laser. The laser emits nearly bandwidth-limited pulses (Δv tp = 0.45) with pulse durations down to 65 fs. At a pumping power of 450 mW of a mode-locked Ar-ion laser (λ = 514 nm) the infrared femtosecond dye laser has an output of up to 15 mW
Excited-state reaction dynamics of bacteriorhodopsin studied by femtosecond spectroscopy
The photodynamics of bacteriorhodopsin were studied by transient absorption and gain measurements after excitation with femtosecond pulses at 620 nm. With probing pulses at longer wavelengths (λ > 770 nm) the previously reported formation of the J intermediate (with a time constant of 500±100 fs) was confirmed. With probing pulses around 700 nm, a faster process with a relaxation time of 200±70 fs was observed. The data analysis strongly suggests that this kinetic constant describes the reactive motion of the polyatomic molecule on its excited-state potential energy surface, i.e. one observes directly the incipient isomerization of the retinal molecule. The minimum of the S1 potential energy surface reached in 200 fs lies approximately 13300 cm−1 above the ground state of bacteriorhodopsin and from this minimum the intermediate J is formed with a time constant of 500 fs
The influence of phase-modulation on femtosecond time-resolved coherent Raman spectroscopy
The influence of phase-modulation on femtosecond time-resolved coherent Raman scattering is investigated theoretically and experimentally. The coherent Raman signal taken as a function of the spectral position shows unexpected temporal oscillations close to time zero. A theoretical analysis of the coherent Raman scattering process indicates that the femtosecond light pulses are amplitude and phase modulated. The pulses are asymmetric in time with more slowly decaying trailing wings. The phase of the pulse amplitude contains quadratic and higher-order contributions
Kelvin Waves and Internal Bores in the Marine Boundary Layer Inversion and Their Relationship to Coastally Trapped Wind Reversals
Detailed observations of a coastally trapped disturbance, or wind reversal, on 10–11 June 1994 along the
California coast provide comprehensive documentation of its structure, based on aircraft, wind profiler, radio
acoustic sounding system, and buoy measurements. Unlike the expectations from earlier studies based on limited
data, which concluded that the deepening of the marine boundary layer (MBL) was a key factor, the 1994 data
show that the perturbation was better characterized as an upward thickening of the inversion capping the MBL.
As the event propagated over a site, the reversal in the alongshore wind direction occurred first within the
inversion and then 3–4 h later at the surface. A node in the vertical structure (defined here as the altitude of
zero vertical displacement) is found just above the inversion base, with up to 200-m upward displacements of
isentropic surfaces above the node, and 70-m downward displacements below.
Although this is a single event, it is shown that the vertical structure observed is representative of most other
coastally trapped wind reversals. This is determined by comparing a composite of the 10–11 June 1994 event,
based on measurements at seven buoys, with surface pressure perturbations calculated from aircraft data. These
results are compared to the composite of many events. In each case a weak pressure trough occurred between
2.4 and 4.0 h ahead of the surface wind reversal, and the pressure rose by 0.32–0.48 mb between the trough
and the wind reversal. The pressure rise results from the cooling caused by the inversion’s upward expansion.
The propagation and structure of the event are shown to be best characterized as a mixed Kelvin wave–bore
propagating within the inversion above the MBL, with the MBL acting as a quasi-rigid lower boundary. If the
MBL is instead assumed to respond in unison with the inversion, then the theoretically predicted intrinsic phase
speeds significantly exceed the observed intrinsic phase speed. The hybrid nature of the event is indicated by
two primary characteristics: 1) the disturbance had a much shallower slope than expected for an internal bore,
while at the same time the upward perturbation within the inversion was quasi-permanent rather than sinusoidal,
which more closely resembles a bore; and 2) the predicted phase speeds for the ‘‘solitary’’ form of nonlinear
Kelvin wave and for an internal bore are both close to the observed intrinsic phase speed
Optical and Thermal-Transport Properties of an Inhomogeneous d-Wave Superconductor
We calculate transport properties of disordered 2D d-wave superconductors
from solutions of the Bogoliubov-de Gennes equations, and show that weak
localization effects give rise to a finite frequency peak in the optical
conductivity similar to that observed in experiments on disordered cuprates. At
low energies, order parameter inhomogeneities induce linear and quadratic
temperature dependencies in microwave and thermal conductivities respectively,
and appear to drive the system towards a quasiparticle insulating phase.Comment: 5 pages,3 figure
Inverse-perovskites A3BO (A = Sr, Ca, Eu/B = Pb, Sn) : a platform for control of Dirac and Weyl fermions
This work was partly supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI (Grant Nos. 24224010, 15K13523, JP15H05852, JP15K21717, and 17H01140), EPSRC (Grant No. EP/P024564/1), and the Alexander von Humboldt FoundationBulk Dirac electron systems have attracted strong interest for their unique magnetoelectric properties as well as their close relation to topological (crystalline) insulators. Recently, the focus has been shifting toward the role of magnetism in stabilizing Weyl fermions as well as chiral surface states in such materials. While a number of nonmagnetic systems are well known, experimental realizations of magnetic analogs are a key focus of current studies. Here, we report on the physical properties of a large family of inverse perovskites A3BO (A = Sr, Ca, Eu/B = Pb, Sn) in which we are able to not only stabilize 3D Dirac electrons at the Fermi energy but also chemically control their properties. In particular, it is possible to introduce a controllable Dirac gap, change the Fermi velocity, tune the anisotropy of the Dirac dispersion, and—crucially—introduce complex magnetism into the system. This family of compounds therefore opens up unique possibilities for the chemical control and systematic investigation of the fascinating properties of such topological semimetals.Publisher PDFPeer reviewe
Quasiparticle-quasiparticle Scattering in High Tc Superconductors
The quasiparticle lifetime and the related transport relaxation times are the
fundamental quantities which must be known in order to obtain a description of
the transport properties of the high T_c superconductors. Studies of these
quantities have been undertaken previously for the d-wave, high T_c
superconductors for the case of temperature-independent elastic impurity
scattering. However, much less is known about the temperature-dependent
inelastic scattering. Here we give a detailed description of the
characteristics of the temperature-dependent quasiparticle-quasiparticle
scattering in d-wave superconductors, and find that this process gives a
natural explanation of the rapid variation with temperature of the electrical
transport relaxation rate.Comment: 4 page
Theory of Thermal Conductivity in YBa_2Cu_3O_{7-\delta}
We calculate the electronic thermal conductivity in a d-wave superconductor,
including both the effect of impurity scattering and inelastic scattering by
antiferromagnetic spin fluctuations. We analyze existing experiments,
particularly with regard to the question of the relative importance of
electronic and phononic contributions to the heat current, and to the influence
of disorder on low-temperature properties. We find that phonons dominate heat
transport near T_c, but that electrons are responsible for most of the peak
observed in clean samples, in agreement with a recent analysis of Krishana et
al. In agreement with recent data on YBa_2(Cu_1-xZn_x)_3O_7-\delta the peak
position is found to vary nonmonotonically with disorder.Comment: 4 pages, 4 figures, to be published in Phys. Rev. Let
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