592 research outputs found
Super-harmonic injection locking of nano-contact spin-torque vortex oscillators
Super-harmonic injection locking of single nano-contact (NC) spin-torque
vortex oscillators (STVOs) subject to a small microwave current has been
explored. Frequency locking was observed up to the fourth harmonic of the STVO
fundamental frequency in microwave magneto-electronic measurements. The
large frequency tunability of the STVO with respect to allowed the
device to be locked to multiple sub-harmonics of the microwave frequency
, or to the same sub-harmonic over a wide range of by tuning
the DC current. In general, analysis of the locking range, linewidth, and
amplitude showed that the locking efficiency decreased as the harmonic number
increased, as expected for harmonic synchronization of a non-linear oscillator.
Time-resolved scanning Kerr microscopy (TRSKM) revealed significant differences
in the spatial character of the magnetization dynamics of states locked to the
fundamental and harmonic frequencies, suggesting significant differences in the
core trajectories within the same device. Super-harmonic injection locking of a
NC-STVO may open up possibilities for devices such as nanoscale frequency
dividers, while differences in the core trajectory may allow mutual
synchronisation to be achieved in multi-oscillator networks by tuning the
spatial character of the dynamics within shared magnetic layers.Comment: 21 pages, 8 figure
Spectroscopic study of optically induced ultrafast electron dynamics in gold
Copyright © 2007 The American Physical SocietyUsing a supercontinuum pulse as a probe, we have measured the transient reflectivity spectra of a thin film of gold for different values of the pump-probe time delay. The wavelength lambda(x) at which the measured transient reflectivity changes sign has been found to depend upon the time delay, leading to bipolar time resolved signals. The time dependence of lambda(x) has been shown to be consistent with calculations that take into account the full dependence of the reflectivity upon the electron occupation number, and to contradict qualitatively a model in which the signal is assumed to be directly proportional to the occupation number. The shift of lambda(x) has been found to persist at time delays that are much longer than the time required for the electrons to thermalize. Therefore the bipolar reflectivity signals do not necessarily contain a contribution from nonthermalized electrons, as has been previously assumed
Simple theory of hot electron dynamics observed by femtosecond ellipsometry
Copyright © 2006 American Institute of PhysicsThe dynamics of the linear and angular momenta of hot electrons in metals are of key importance for the design and operation of hot electron devices such as spin and tunnel valve transistors. The corresponding relaxation times are expected to lie in the subpicosecond range and must be studied with experimental techniques of adequate (femtosecond) temporal resolution. Here we report a simple theory of the ultrafast ellipsometric response of metals after excitation with femtosecond optical pulses. Although developed in the relaxation time approximation, the theory allows electron linear and angular momentum relaxation times to be extracted
Direct observation of magnetization dynamics generated by nano-contact spin-torque vortex oscillators
Time-resolved scanning Kerr microscopy has been used to directly image the
magnetization dynamics of nano-contact (NC) spin-torque vortex oscillators
(STVOs) when phase-locked to an injected microwave (RF) current. The Kerr
images reveal free layer magnetization dynamics that extend outside the NC
footprint, where they cannot be detected electrically, but which are crucial to
phase-lock STVOs that share common magnetic layers. For a single NC, dynamics
were observed not only when the STVO frequency was fully locked to that of the
RF current, but also for a partially locked state characterized by periodic
changes in the core trajectory at the RF frequency. For a pair of NCs, images
reveal the spatial character of dynamics that electrical measurements show to
have enhanced amplitude and reduced linewidth. Insight gained from these images
may improve understanding of the conditions required for mutual phase-locking
of multiple STVOs, and hence enhanced microwave power emission.Comment: 10 pages, 3 figure
SN 2006bt: A Perplexing, Troublesome, and Possibly Misleading Type Ia Supernova
SN 2006bt displays characteristics unlike those of any other known Type Ia
supernova (SN Ia). We present optical light curves and spectra of SN 2006bt
which demonstrate the peculiar nature of this object. SN 2006bt has broad,
slowly declining light curves indicative of a hot, high-luminosity SN, but
lacks a prominent second maximum in the i band as do low-luminosity SNe Ia. Its
spectra are similar to those of low-luminosity SNe Ia, containing features that
are only present in cool SN photospheres. Light-curve fitting methods suggest
that SN 2006bt is reddened by a significant amount of dust; however, it
occurred in the outskirts of its early-type host galaxy and has no strong Na D
absorption in any of its spectra, suggesting a negligible amount of host-galaxy
dust absorption. C II is possibly detected in our pre-maximum spectra, but at a
much lower velocity than other elements. The progenitor was likely very old,
being a member of the halo population of a galaxy that shows no signs of recent
star formation. SNe Ia have been very successfully modeled as a one-parameter
family, and this is fundamental to their use as cosmological distance
indicators. SN 2006bt is a challenge to that picture, yet its relatively normal
light curves allowed SN 2006bt to be included in cosmological analyses. We
generate mock SN Ia datasets which indicate that contamination by similar
objects will both increase the scatter of a SN Ia Hubble diagram and
systematically bias measurements of cosmological parameters. However, spectra
and rest-frame i-band light curves should provide a definitive way to identify
and eliminate such objects.Comment: ApJ, accepted. 13 pages, 13 figure
Time-resolved investigation of magnetization dynamics of arrays of non-ellipsoidal nanomagnets with a non-uniform ground state
We have performed time-resolved scanning Kerr microscopy (TRSKM) measurements
upon arrays of square ferromagnetic nano-elements of different size and for a
range of bias fields. The experimental results were compared to micromagnetic
simulations of model arrays in order to understand the non-uniform precessional
dynamics within the elements. In the experimental spectra two branches of
excited modes were observed to co-exist above a particular bias field. Below
the so-called crossover field, the higher frequency branch was observed to
vanish. Micromagnetic simulations and Fourier imaging revealed that modes from
the higher frequency branch had large amplitude at the center of the element
where the effective field was parallel to the bias field and the static
magnetization. Modes from the lower frequency branch had large amplitude near
the edges of the element perpendicular to the bias field. The simulations
revealed significant canting of the static magnetization and the effective
field away from the direction of the bias field in the edge regions. For the
smallest element sizes and/or at low bias field values the effective field was
found to become anti-parallel to the static magnetization. The simulations
revealed that the majority of the modes were de-localized with finite amplitude
throughout the element, while the spatial character of a mode was found to be
correlated with the spatial variation of the total effective field and the
static magnetization state. The simulations also revealed that the frequencies
of the edge modes are strongly affected by the spatial distribution of the
static magnetization state both within an element and within its nearest
neighbors
Heavily loaded ferrite-polymer composites to produce high refractive index materials at centimetre wavelengths
A cold-pressing technique has been developed for fabricating composites composed of a polytetrafluoroethylene-polymer matrix and a wide range of volume-fractions of MnZn-ferrite filler (0%–80%). The electromagnetic properties at centimetre wavelengths of all prepared composites exhibited good reproducibility, with the most heavily loaded composites possessing simultaneously high permittivity (180 ± 10) and permeability (23±2). The natural logarithm of both the relative complex permittivity and permeability shows an approximately linear dependence with the volume fraction of ferrite. Thus, this simple method allows for the manufacture of bespoke materials required in the design and construction of devices based on the principles of transformation optics
Hubble Residuals of Nearby Type Ia Supernovae Are Correlated with Host Galaxy Masses
From Sloan Digital Sky Survey u'g'r'i'z' imaging, we estimate the stellar
masses of the host galaxies of 70 low redshift SN Ia (0.015 < z < 0.08) from
the hosts' absolute luminosities and mass-to-light ratios. These nearby SN were
discovered largely by searches targeting luminous galaxies, and we find that
their host galaxies are substantially more massive than the hosts of SN
discovered by the flux-limited Supernova Legacy Survey. Testing four separate
light curve fitters, we detect ~2.5{\sigma} correlations of Hubble residuals
with both host galaxy size and stellar mass, such that SN Ia occurring in
physically larger, more massive hosts are ~10% brighter after light curve
correction. The Hubble residual is the deviation of the inferred distance
modulus to the SN, calculated from its apparent luminosity and light curve
properties, away from the expected value at the SN redshift. Marginalizing over
linear trends in Hubble residuals with light curve parameters shows that the
correlations cannot be attributed to a light curve-dependent calibration error.
Combining 180 higher-redshift ESSENCE, SNLS, and HigherZ SN with 30 nearby SN
whose host masses are less than 10^10.8 solar masses in a cosmology fit yields
1+w=0.22 +0.152/-0.143, while a combination where the 30 nearby SN instead have
host masses greater than 10^10.8 solar masses yields 1+w=-0.03 +0.217/-0.108.
Progenitor metallicity, stellar population age, and dust extinction correlate
with galaxy mass and may be responsible for these systematic effects. Host
galaxy measurements will yield improved distances to SN Ia.Comment: 16 pages, 6 figures, published in ApJ, minor change
Type II Supernova Light Curves and Spectra From the CfA
We present multiband photometry of 60 spectroscopically-confirmed supernovae
(SN): 39 SN II/IIP, 19 IIn, one IIb and one that was originally classified as a
IIn but later as a Ibn. Forty-six have only optical photometry, six have only
near infrared (NIR) photometry and eight have both optical and NIR. The median
redshift of the sample is 0.016. We also present 192 optical spectra for 47 of
the 60 SN. All data are publicly available. There are 26 optical and two NIR
light curves of SN II/IIP with redshifts z > 0.01, some of which may give rise
to useful distances for cosmological applications. All photometry was obtained
between 2000 and 2011 at the Fred Lawrence Whipple Observatory (FLWO), via the
1.2m and 1.3m PAIRITEL telescopes for the optical and NIR, respectively. Each
SN was observed in a subset of the bands. There are a total
of 2932 optical and 816 NIR light curve points. Optical spectra were obtained
using the FLWO 1.5m Tillinghast telescope with the FAST spectrograph and the
MMT Telescope with the Blue Channel Spectrograph. Our photometry is in
reasonable agreement with other samples from the literature. Comparison with
Pan-STARRS shows that two-thirds of our individual star sequences have
weighted-mean V offsets within 0.02 mag. In comparing our standard-system
SN light curves with common Carnegie Supernova Project objects using their
color terms, we found that roughly three-quarters have average differences
within 0.04 mag. The data from this work and the literature will provide
insight into SN II explosions, help with developing methods for photometric SN
classification, and contribute to their use as cosmological distance
indicators.Comment: Accepted to ApJS. TAR of light curves and star sequences here:
https://www.cfa.harvard.edu/supernova/fmalcolm2017/cfa_snII_lightcurvesndstars.june2017.tar
... Spectra can be found here:
https://www.cfa.harvard.edu/supernova/fmalcolm2017/cfaspec_snII.tar.gz ...
Passbands and plot of spectra can be found here:
https://www.cfa.harvard.edu/supernova/SNarchive.htm
The Luminous and Carbon-Rich Supernova 2006gz: A Double Degenerate Merger?
Spectra and light curves of SN 2006gz show the strongest signature of
unburned carbon and one of the slowest fading light curves ever seen in a type
Ia event (Delta m_15 = 0.69 +/- 0.04). The early-time Si II velocity is low,
implying it was slowed by an envelope of unburned material. Our best estimate
of the luminosity implies M_V = -19.74 and the production of ~ 1.2 M_sun of
56Ni. This suggests a super-Chandrasekhar mass progenitor. A double degenerate
merger is consistent with these observations.Comment: Accepted for publication in ApJL (5 pages, 4 figures). UBVr'i' light
curves, UVOIR light curves, and spectra available at
http://www.cfa.harvard.edu/supernova/SN2006g
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