15 research outputs found
A Large Effective Phonon Magnetic Moment in a Dirac Semimetal
We investigated the magnetoterahertz response of the Dirac semimetal
CdAs and observed a particularly low frequency optical phonon, as well
as a very prominent and field sensitive cyclotron resonance. As the cyclotron
frequency is tuned with field to pass through the phonon, the phonon become
circularly polarized as shown by a notable splitting in their response to
right- and left-hand polarized light. This splitting can be expressed as an
effective phonon magnetic moment that is approximately 2.7 times the Bohr
magneton, which is almost four orders of magnitude larger than ab initio
calculations predict for phonon magnetic moments in nonmagnetic insulators.
This exceedingly large value is due to the coupling of the phonons to the
cyclotron motion and is controlled directly by the electron-phonon coupling
constant. This field tunable circular-polarization selective coupling provides
new functionality for nonlinear optics to create light-induced topological
phases in Dirac semimetals.Comment: 15 pages for main text and SI; To appear in Nano Letters (2020
Revealing novel aspects of light-matter coupling in terahertz two-dimensional coherent spectroscopy: the case of the amplitude mode in superconductors
Recently developed terahertz (THz) two-dimensional coherent spectroscopy
(2DCS) is a powerful technique to obtain materials information in a fashion
qualitatively different from other spectroscopies. Here, we utilized THz 2DCS
to investigate the THz nonlinear response of conventional superconductor NbN.
Using broad-band THz pulses as light sources, we observed a third-order
nonlinear signal whose spectral components are peaked at twice the
superconducting gap energy . With narrow-band THz pulses, a THz
nonlinear signal was identified at the driving frequency and exhibited
a resonant enhancement at temperature when . General
theoretical considerations show that such a resonance can only arise from a
disorder-activated paramagnetic coupling between the light and the electronic
current. This proves that the nonlinear THz response can access processes
distinct from the diamagnetic Raman-like density fluctuations, which are
believed to dominate the nonlinear response at optical frequencies in metals.
Our numerical simulations reveal that even for a small amount of disorder, the
resonance is dominated by the superconducting amplitude mode
over the entire investigated disorder range. This is in contrast to other
resonances, whose amplitude-mode contribution depends on disorder. Our findings
demonstrate the unique ability of THz 2DCS to explore collective excitations
inaccessible in other spectroscopies
Disorder-enhanced effective masses and deviations from Matthiessen's rule in PdCoO thin films
The observation of hydrodynamic transport in the metallic delafossite
PdCoO has increased interest in this family of highly conductive oxides,
but experimental studies so far have mostly been confined to bulk crystals. In
this work, the development of high-quality thin films of PdCoO has enabled
a thorough study of the conductivity as a function of film thickness using both
dc transport and time-domain THz spectroscopy. With increasing film thickness
from 12 nm to 102 nm, the residual resistivity decreases and we observe a large
deviation from Matthiessen's rule (DMR) in the temperature dependence of the
resistivity. We find that the complex THz conductivity is well fit by a single
Drude term. We fit the data to extract the spectral weight and scattering rate
simultaneously. The temperature dependence of the Drude scattering rate is
found to be nearly independent of the residual resistivity and cannot be the
primary mechanism for the observed DMR. Rather, we observe large changes in the
spectral weight as a function of disorder, changing by a factor of 1.5 from the
most disordered to least disordered films. We believe this corresponds to a
mass enhancement of times the value of the bulk effective mass which
increases with residual disorder. This suggests that the mechanism behind the
DMR observed in dc resistivity is primarily driven by changes in the electron
mass. We discuss the possible origins of this behavior including the
possibility of disorder-enhanced electron-phonon scattering, which can be
systematically tuned by film thickness.Comment: 10 pages, 8 figure
THz electrodynamics of mixed-valent YbAl
We present our results from time-domain THz spectroscopy measurements of thin films of mixed-valent YbAl and its structural analogue LuAl. Combined with Fourier transform infrared (FTIR) spectroscopy, the extended Drude formalism is utilized to study the quasiparticle scattering rate and effective masses in YbAl. We find that LuAl demonstrates conventional Drude transport whereas at low temperatures YbAl demonstrates a renormalized Drude peak and a mid-infrared (MIR) peak in the conductivity, indicative of the formation of a mass-enhanced Fermi liquid (FL). In YbAl the extended Drude analysis demonstrates consistency with FL behavior below the FL coherence temperature K with the scattering rate following proportionality and a moderate mass enhancement. Despite not observing a clear Fermi liquid-like frequency dependence the evidence is consistent with a moderate mass Fermi liquid, albeit one with a smaller mass than observed in single crystals. The extended Drude analysis also demonstrates a slow crossover between the FL state and the normal state above the in YbAl, indicative of incoherent hybridization effects persisting to high temperatures