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 $f_{0}$ in microwave magneto-electronic measurements. The large frequency tunability of the STVO with respect to $f_{0}$ allowed the device to be locked to multiple sub-harmonics of the microwave frequency $f_{RF}$, or to the same sub-harmonic over a wide range of $f_{RF}$ 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

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

Multi-color Optical and NIR Light Curves of 64 Stripped-Envelope Core-Collapse Supernovae

We present a densely-sampled, homogeneous set of light curves of 64 low redshift (z < 0.05) stripped-envelope supernovae (SN of type IIb, Ib, Ic and Ic-bl). These data were obtained between 2001 and 2009 at the Fred L. Whipple Observatory (FLWO) on Mt. Hopkins in Arizona, with the optical FLWO 1.2-m and the near-infrared PAIRITEL 1.3-m telescopes. Our dataset consists of 4543 optical photometric measurements on 61 SN, including a combination of UBVRI, UBVr'i', and u'BVr'i', and 2142 JHKs near-infrared measurements on 25 SN. This sample constitutes the most extensive multi-color data set of stripped-envelope SN to date. Our photometry is based on template-subtracted images to eliminate any potential host galaxy light contamination. This work presents these photometric data, compares them with data in the literature, and estimates basic statistical quantities: date of maximum, color, and photometric properties. We identify promising color trends that may permit the identification of stripped-envelope SN subtypes from their photometry alone. Many of these SN were observed spectroscopically by the CfA SN group, and the spectra are presented in a companion paper (Modjaz et al. 2014). A thorough exploration that combines the CfA photometry and spectroscopy of stripped-envelope core-collapse SN will be presented in a follow-up paper.Comment: 26 pages, 17 figures, 8 tables. Revised version resubmitted to ApJ Supplements after referee report. Additional online material is available through http://cosmo.nyu.edu/SNYU

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

Supernovae as seen by off-center observers in a local void

Inhomogeneous universe models have been proposed as an alternative explanation for the apparent acceleration of the cosmic expansion that does not require dark energy. In the simplest class of inhomogeneous models, we live within a large, spherically symmetric void. Several studies have shown that such a model can be made consistent with many observations, in particular the redshift--luminosity distance relation for type Ia supernovae, provided that the void is of Gpc size and that we live close to the center. Such a scenario challenges the Copernican principle that we do not occupy a special place in the universe. We use the first-year Sloan Digital Sky Survey-II supernova search data set as well as the Constitution supernova data set to put constraints on the observer position in void models, using the fact that off-center observers will observe an anisotropic universe. We first show that a spherically symmetric void can give good fits to the supernova data for an on-center observer, but that the two data sets prefer very different voids. We then continue to show that the observer can be displaced at least fifteen percent of the void scale radius from the center and still give an acceptable fit to the supernova data. When combined with the observed dipole anisotropy of the cosmic microwave background however, we find that the data compells the observer to be located within about one percent of the void scale radius. Based on these results, we conclude that considerable fine-tuning of our position within the void is needed to fit the supernova data, strongly disfavouring the model from a Copernican principle point of view.Comment: 20 pages, 6 figures, matches the published versio

Crystallization of Ge2Sb2Te5 films by amplified femtosecond optical pulses

Copyright © 2012 American Institute of PhysicsThe phase transition between the amorphous and crystalline states of Ge2Sb2Te5 has been studied by exposure of thin films to series of 60 femtosecond (fs) amplified laser pulses. The analysis of microscope images of marks of tens of microns in size provide an opportunity to examine the effect of a continuous range of optical fluence. For a fixed number of pulses, the dependence of the area of the crystalline mark upon the fluence is well described by simple algebraic results that provide strong evidence that thermal transport within the sample is one-dimensional (vertical). The crystalline mark area was thus defined by the incident fs laser beam profile rather than by lateral heat diffusion, with a sharp transition between the crystalline and amorphous materials as confirmed from line scans of the microscope images. A simplified, one-dimensional model that accounts for optical absorption, thermal transport and thermally activated crystallization provides values of the optical reflectivity and mark area that are in very good quantitative agreement with the experimental data, further justifying the one-dimensional heat flow assumption. Typically, for fluences below the damage threshold, the crystalline mark has annular shape, with the fluence at the centre of the irradiated mark being sufficient to induce melting. The fluence at the centre of the mark was correlated with the melt depth from the thermal model to correctly predict the observed melt fluence thresholds and to explain the closure and persistence of the annular crystalline marks as functions of laser fluence and pulse number. A solid elliptical mark may be obtained for smaller fluences. The analysis of marks made by amplified fs pulses present a new and effective means of observing the crystallization dynamics of phase-change material at elevated temperatures near the melting point, which provided estimates of the growth velocity in the range 7-9 m/s. Furthermore, finer control over the crystallization process in phase-change media can be obtained by controlling the number of pulses which, along with the laser fluence, can be tailored to any medium stack with relaxed restrictions on the thermal properties of the layers in the stack

Use of microscale coplanar striplines with indium tin oxide windows in optical ferromagnetic resonance measurements

Copyright © 2005 American Institute of PhysicsIt is shown that a coplanar stripline structure containing indium tin oxide windows can be used to perform optical ferromagnetic resonance measurements on a sample grown on an opaque substrate, using a pulsed magnetic field of any desired orientation. The technique is demonstrated by applying it to a thin film of permalloy grown on a Si substrate. The measured precession frequency was found to be in good agreement with macrospin simulations. The phase of the oscillatory Kerr response was observed to vary as the probe spot was scanned across the coplanar stripline structure, confirming that the orientation of the pulsed field varied from parallel to perpendicular relative to the plane of the sample

Band-filling-controlled magnetism from transition metal intercalation in N1/3NbS2 revealed with first-principles calculations

We present a first-principles study of the effect of 3d transition metal intercalation on the magnetic properties of the 2H-NbS2 system, using spin-resolved density functional theory calculations to investigate the electronic structure of N1/3NbS2 (N=Ti, V, Cr, Mn, Fe, Co, Ni). We are able to accurately determine the magnetic moments and crystal-field splitting, and find that the magnetic properties of the materials are determined by a mechanism based on filling rigid bands with electrons from the intercalant. We predict the dominant magnetic interaction of these materials by considering Fermi-surface topology, finding agreement with experiment where data are available

Third quantization of f(R)f(R)-type gravity

We examine the third quantization of $f(R)$-type gravity, based on its effective Lagrangian in the case of a flat Friedmann-Lemaitre-Robertson-Walker metric. Starting from the effective Lagrangian, we execute a suitable change of variable and the second quantization, and we obtain the Wheeler-DeWitt equation. The third quantization of this theory is considered. And the uncertainty relation of the universe is investigated in the example of $f(R)$-type gravity, where $f(R)=R^2$. It is shown, when the time is late namely the scale factor of the universe is large, the spacetime does not contradict to become classical, and, when the time is early namely the scale factor of the universe is small, the quantum effects are dominating.Comment: 9 pages, Arbitrary constants in (4.19) are changed to arbitrary functions of $\varphi$. Conclusions are not changed. References are added. Typos are correcte