arXiv subjects

A. Jenkins

Publications and source records attributed to A. Jenkins.

At least 19 recordsLinked to original sources

arXiv2026

Remotely programming the weights of a spintronic neural network by a radiofrequency broadcast signal

Selectively programming large number of non-volatile synaptic weights without compromising scalability is a key challenge for in-memory computing. Here, we demonstrate remote programming of synaptic weights in series-connected chains of 11 vortex-based magnetic tunnel junctions using broadcast radiofrequency signals applied through a shared strip line. The programming relies on frequency-selective reversal of the vortex-core polarity and therefore does not require individual access lines or selector devices. By reconfiguring the binary states of these chains, we reshape the weighted sums they perform on frequency-multiplexed RF inputs. Using a 22-synapse network composed of two such chains, we remotely reconfigure the same hardware to perform two distinct tasks: handwritten-digit classification and drone RF-signature identification. The digit-optimized configuration reaches 94.91 +/- 0.26% accuracy on handwritten digits but only 13.17 +/- 0.47% on drone RF signatures, whereas the drone-optimized configuration reaches 97.33 +/- 0.62% on drones but only 47.59 +/- 1.5% on digits. Broadcast RF programming thus provides a compact and scalable route to rapidly reconfigurable spintronic neuromorphic hardware.

cs.ET↗

arXiv2026

Field-driven triggering of self-induced Floquet magnons in a magnetic vortex

We report the experimental control of Floquet magnons in a magnetic vortex. Using microwave spectroscopy of vortex state magnetic tunnel junctions (MTJs), we find that self-induced Floquet sidebands form frequency combs whose existence depend on the vortex core orbit. By shifting the vortex core with an applied magnetic field, we switch the system between regular and Floquet magnons at identical drive conditions, demonstrating hysteretic control of the Floquet spectrum. A nonlinear vortex-magnon model shows that this behavior originates from multiple stable vortex gyration radii created by Floquet-mediated feedback. These results establish magnetic state initialization as a means to switch between regular and Floquet magnons.

cond-mat.mtrl-sci↗

arXiv2025

Time-resolved observation of magnon splitting into vortex gyration and Floquet spin waves

Forced excitations at frequencies in the range of the first order azimuthal spin waves of a magnetic disk in the vortex state are known to scatter into the vortex gyration mode, thereby allowing the growth of Floquet spin waves forming a frequency comb. We study the temporal emergence of this dynamical state using time-resolved microwave electrical measurements. The most intense Floquet mode emerges synchronously with the gyration mode after a common incubation delay which diverges at the scattering threshold. This delay is minimal when the drive is resonant with one of the first order azimuthal spin waves. It can be as short as 3 ns for the maximum investigated power. We conclude that the first-to-occur scattering mechanism is the three-wave splitting of a regular azimuthal eigenmode into a coherent pair formed by a gyration magnon and a Floquet spin wave.

cond-mat.mtrl-sci↗

arXiv2020

Experimental demonstration of a rapid sweep-tuned spectrum analyzer with temporal resolution based on a spin-torque nano-oscillator

It is demonstrated experimentally that a spin-torque nano-oscillator (STNO) rapidly sweep-tuned by a bias voltage can be used for time-resolved spectrum analysis of frequency-manipulated microwave signals with complicated multi-tone spectra. The critical reduction in the time of spectrum analysis comes from the naturally small time constants of a nano-sized STNO (1-100 ns). The demonstration is performed on a vortex-state STNO generating in a frequency range around 300 MHz, with frequency down-conversion and matched filtering used for signal processing. It is shown that this STNO-based spectrum analyzer can perform analysis of multi-tone signals, and signals with rapidly changing frequency components with time resolution on a $\mu$s time scale and frequency resolution limited only by the 'bandwidth' theorem. The proposed concept of rapid time-resolved spectrum analysis can be implemented with any type of micro and nano-scale frequency-swept oscillators having low time constants and high oscillation frequency.

physics.app-ph↗

arXiv2020

Imaging the breakdown of ohmic transport in graphene

Ohm's law describes the proportionality of current density and electric field. In solid-state conductors, Ohm's law emerges due to electron scattering processes that relax the electrical current. Here, we use nitrogen-vacancy center magnetometry to directly image the local breakdown of Ohm's law in a narrow constriction fabricated in a high mobility graphene monolayer. Ohmic flow is visible at room temperature as current concentration on the constriction edges, with flow profiles entirely determined by sample geometry. However, as the temperature is lowered below 200 K, the current concentrates near the constriction center. The change in the flow pattern is consistent with a crossover from diffusive to viscous electron transport dominated by electron-electron scattering processes that do not relax current.

cond-mat.mes-hall↗

arXiv2019

Analog and digital phase modulation of spin torque nano-oscillators

Spin torque nano-oscillators (STNO) are nanoscale devices with wide band frequency tunability. Their multifunctional RF properties are well suited to define novel schemes for wireless communications that use basic protocols for data transmission such as amplitude, frequency and phase shift keying (ASK, FSK, PSK). In contrast to ASK and FSK, implementation of PSK is more challenging for STNOs because of their relatively high phase noise. Here we introduce a special PSK technique by combining their modulation and injection locking functionality. The concept is validated using magnetic tunnel junction based vortex STNOs for injection locking at 2f and f/2 showing phase shifts up to 2.1rad and data transmission rates up to 4Mbit/s. Quadrature phase shift keying and analog phase modulation are also implemented, where the latter is employed for voice transmission over a distance of 10 meters. This demonstrates that STNO phase noise and output power meet the requested performances for operation in existing communication schemes.

physics.app-ph↗

arXiv2015

Evolution of galaxy stellar masses and star formation rates in the EAGLE simulations

We investigate the evolution of galaxy masses and star formation rates in the Evolution and Assembly of Galaxies and their Environment (EAGLE) simulations. These comprise a suite of hydrodynamical simulations in a $Λ$CDM cosmogony with subgrid models for radiative cooling, star formation, stellar mass loss, and feedback from stars and accreting black holes. The subgrid feedback was calibrated to reproduce the observed present-day galaxy stellar mass function and galaxy sizes. Here we demonstrate that the simulations reproduce the observed growth of the stellar mass density to within 20 per cent. The simulation also tracks the observed evolution of the galaxy stellar mass function out to redshift z = 7, with differences comparable to the plausible uncertainties in the interpretation of the data. Just as with observed galaxies, the specific star formation rates of simulated galaxies are bimodal, with distinct star forming and passive sequences. The specific star formation rates of star forming galaxies are typically 0.2 to 0.4 dex lower than observed, but the evolution of the rates track the observations closely. The unprecedented level of agreement between simulation and data makes EAGLE a powerful resource to understand the physical processes that govern galaxy formation.

astro-ph.GA↗

arXiv2015

Understanding of phase noise squeezing under fractional synchronization of non-linear spin transfer vortex oscillator

We investigate experimentally the synchronization of a vortex based spin transfer oscillator to an external rf current whose frequency is at multiple integers, as well as half integer, of the oscillator frequency. Through a theoretical study of the locking process, we highlight both the crucial role of the symmetries of the spin torques acting on the magnetic vortex and the nonlinear properties of the oscillator on the phase locking process. Through the achievement of a perfect injection locking state, we report a record phase noise reduction down to -90dBc/Hz at 1 kHz offset frequency. The phase noise of these nanoscale oscillators is demonstrating as being low and controllable which is of significant importance for real applications using spin transfer devices.

cond-mat.mes-hall↗

arXiv2013

The journey of QSO haloes from z=6 to the present

We apply a recently developed scaling technique to the Millennium-XXL, one of the largest cosmological N-body simulations carried out to date 3x10^11 particles within a cube of volume ~70Gpc^3). This allows us to investigate the cosmological parameter dependence of the mass and evolution of haloes in the extreme high-mass tail of the z=6 distribution. We assume these objects to be likely hosts for the population of rare but ultraluminous high-redshift quasars discovered by the Sloan Digital Sky Survey. Haloes with a similar abundance to these quasars have a median mass of 9x10^12 Msun in the currently preferred cosmology, but do not evolve into equally extreme objects at z=0. Rather, their descendants span the full range conventionally assigned to present-day clusters, 6x10^13 to 2.5x10^15 Msun for this same cosmology. The masses both at z=6 and at z=0 shift up or down by factors exceeding two if cosmological parameters are pushed to the boundaries of the range discussed in published interpretations of data from the WMAP satellite. The main factor determining the future growth of a high-mass z=6 halo is the mean overdensity of its environment on scales of 7 to 14 Mpc, and descendant masses can be predicted 6 to 8 times more accurately if this density is known than if it is not. All these features are not unique to extreme high-z haloes, but are generic to hierarchical growth. Finally, we find that extreme haloes at z=6 typically acquired about half of their total mass in the preceding 100 Myr, implying very large recent accretion rates which may be related to the large black hole masses and high luminosities of the SDSS quasars.

astro-ph.CO↗

arXiv2013

Scaling relations for galaxy clusters in the Millennium-XXL simulation

We present a very large high-resolution cosmological N-body simulation, the Millennium-XXL or MXXL, which uses 303 billion particles to represent the formation of dark matter structures throughout a 4.1Gpc box in a LambdaCDM cosmology. We create sky maps and identify large samples of galaxy clusters using surrogates for four different observables: richness estimated from galaxy surveys, X-ray luminosity, integrated Sunyaev-Zeldovich signal, and lensing mass. The unprecedented combination of volume and resolution allows us to explore in detail how these observables scale with each other and with cluster mass. The scatter correlates between different mass-observable relations because of common sensitivities to the internal structure, orientation and environment of clusters, as well as to line-of-sight superposition of uncorrelated structure. We show that this can account for the apparent discrepancies uncovered recently between the mean thermal SZ signals measured for optically and X-ray selected clusters by stacking data from the Planck satellite. Related systematics can also affect inferences from extreme clusters detected at high redshift. Our results illustrate that cosmological conclusions from galaxy cluster surveys depend critically on proper modelling, not only of the relevant physics, but also of the full distribution of the observables and of the selection biases induced by cluster identification procedures.

astro-ph.CO↗

arXiv2012

The Phoenix Project: the Dark Side of Rich Galaxy Clusters

[abridged] We introduce the Phoenix Project, a set of $Λ$CDM simulations of the dark matter component of nine rich galaxy clusters. Each cluster is simulated at least at two different numerical resolutions. For eight of them, the highest resolution corresponds to $\sim 130$ million particles within the virial radius, while for one this number is over one billion. We study the structure and substructure of these systems and contrast them with six galaxy-sized dark matter haloes from the Aquarius Project, simulated at comparable resolution. This comparison highlights the approximate mass invariance of CDM halo structure and substructure. We find little difference in the spherically-averaged mass, pseudo-phase-space density, and velocity anisotropy profiles of Aquarius and Phoenix haloes. When scaled to the virial properties of the host halo, the abundance and radial distribution of subhaloes are also very similar, despite the fact that Aquarius and Phoenix haloes differ by roughly three decades in virial mass. The most notable difference is that cluster haloes have been assembled more recently and are thus significantly less relaxed than galaxy haloes, which leads to decreased regularity, increased halo-to-halo scatter and sizable deviations from the mean trends. This accentuates the effects of the strong asphericity of individual clusters on surface density profiles, which may vary by up to a factor of three at a given radius, depending on projection. The high apparent concentration reported for some strong-lensing clusters might very well reflect these effects. A more recent assembly also explains why substructure in some Phoenix haloes is slightly more abundant than in Aquarius, especially in the inner regions. Resolved subhaloes nevertheless contribute only $11 \pm 3%$ of the virial mass in Phoenix clusters. .

astro-ph.CO↗

arXiv2012

The Aquila comparison Project: The Effects of Feedback and Numerical Methods on Simulations of Galaxy Formation

We compare the results of thirteen cosmological gasdynamical codes used to simulate the formation of a galaxy in the LCDM structure formation paradigm. The various runs differ in their hydrodynamical treatment (SPH, moving-mesh and AMR) but share the same initial conditions and adopt their latest published model of cooling, star formation and feedback. Despite the common halo assembly history, we find large code-to-code variations in the stellar mass, size, morphology and gas content of the galaxy at z=0, due mainly to the different implementations of feedback. Compared with observation, most codes tend to produce an overly massive galaxy, smaller and less gas-rich than typical spirals, with a massive bulge and a declining rotation curve. A stellar disk is discernible in most simulations, though its prominence varies widely from code to code. There is a well-defined trend between the effects of feedback and the severity of the disagreement with observation. Models that are more effective at limiting the baryonic mass of the galaxy come closer to matching observed galaxy scaling laws, but often to the detriment of the disk component. Our conclusions hold at two different numerical resolutions. Some differences can also be traced to the numerical techniques: more gas seems able to cool and become available for star formation in grid-based codes than in SPH. However, this effect is small compared to the variations induced by different feedback prescriptions. We conclude that state-of-the-art simulations cannot yet uniquely predict the properties of the baryonic component of a galaxy, even when the assembly history of its host halo is fully specified. Developing feedback algorithms that can effectively regulate the mass of a galaxy without hindering the formation of high-angular momentum stellar disks remains a challenge.

astro-ph.GA↗

arXiv2011

Where will supersymmetric dark matter first be seen?

If the dark matter consists of supersymmetric particles, Gamma-ray observatories such as the Large Area Telescope aboard the Fermi satellite may detect annihilation radiation from the haloes of galaxies and galaxy clusters. Much recent effort has been devoted to searching for this signal around the Milky Way's dwarf satellites. Using a new suite of high-resolution simulations of galaxy cluster haloes (the Phoenix Project), together with the Aquarius simulations of Milky-Way-like galaxy haloes, we show that higher signal-to-noise and equally clean signals are, in fact, predicted to come from nearby rich galaxy clusters. Most of the cluster emission is produced by small subhaloes with masses less than that of the Sun. The large range of mass scales covered by our two sets of simulations allows us to deduce a physically motivated extrapolation to these small (and unresolved) masses. Since tidal effects destroy subhaloes in the dense inner regions of haloes, most cluster emission is then predicted to come from large radii, implying that the nearest and brightest systems should be much more extended than Fermi's angular resolution limit. The most promising targets for detection are clusters such as Coma and Fornax, but detection algorithms must be tuned to the predicted profile of the emission if they are to maximize the chance of finding this weak signal.

astro-ph.CO↗

arXiv2010

The evolution of galaxy cluster X-ray scaling relations

We use numerical simulations to investigate, for the first time, the joint effect of feedback from supernovae (SNe) and active galactic nuclei (AGN) on the evolution of galaxy cluster X-ray scaling relations. Our simulations are drawn from the Millennium Gas Project and are some of the largest hydrodynamical N-body simulations ever carried out. Feedback is implemented using a hybrid scheme, where the energy input into intracluster gas by SNe and AGN is taken from a semi-analytic model of galaxy formation. This ensures that the source of feedback is a population of galaxies that closely resembles that found in the real universe. We show that our feedback model is capable of reproducing observed local X-ray scaling laws, at least for non-cool core clusters, but that almost identical results can be obtained with a simplistic preheating model. However, we demonstrate that the two models predict opposing evolutionary behaviour. We have examined whether the evolution predicted by our feedback model is compatible with observations of high-redshift clusters. Broadly speaking, we find that the data seems to favour the feedback model for z<0.5, and the preheating model at higher redshift. However, a statistically meaningful comparison with observations is impossible, because the large samples of high-redshift clusters currently available are prone to strong selection biases. As the observational picture becomes clearer in the near future, it should be possible to place tight constraints on the evolution of the scaling laws, providing us with an invaluable probe of the physical processes operating in galaxy clusters.

astro-ph.CO↗

arXiv2010

Assembly History and Structure of Galactic Cold Dark Matter Halos

We use the Aquarius simulation series to study the imprint of assembly history on the structure of Galaxy-mass cold dark matter halos. Our results confirm earlier work regarding the influence of mergers on the mass density profile and the inside-out growth of halos. The inner regions that contain the visible galaxies are stable since early times and are significantly affected only by major mergers. Particles accreted diffusely or in minor mergers are found predominantly in the outskirts of halos. Our analysis reveals trends that run counter to current perceptions of hierarchical halo assembly. For example, major mergers (i.e. those with progenitor mass ratios greater than 1:10) contribute little to the total mass growth of a halo, on average less than 20 per cent for our six Aquarius halos. The bulk is contributed roughly equally by minor mergers and by "diffuse" material which is not resolved into individual objects. This is consistent with modeling based on excursion-set theory which suggests that about half of this diffuse material should not be part of a halo of any scale. Interestingly, the simulations themselves suggest that a significantly fraction is not truly diffuse, since it was ejected from earlier halos by mergers prior to their joining the main system. The Aquarius simulations resolve halos to much lower mass scales than are expected to retain gas or form stars. These results thus confirm that most of the baryons from which visible galaxies form are accreted diffusely, rather than through mergers, and they suggest that only relatively rare major mergers will affect galaxy structure at later times.

astro-ph.CO↗

arXiv2010

The statistics of the subhalo abundance of dark matter haloes

We study the population statistics of the surviving subhaloes of LCDM dark matter haloes using a set of very high resolution N-body simulations. These include both simulations of representative regions of the Universe and ultra-high resolution resimulations of individual dark matter haloes. We find that more massive haloes tend to have a larger mass fraction in subhaloes. For example, cluster size haloes typically have 7.5 percent of their mass within R200 in substructures of fractional mass larger than 1e-5, which is 25 percent higher than galactic haloes. There is, however, a large variance in the subhalo mass fraction from halo to halo, whereas the subhalo abundance shows much higher regularity. For dark matter haloes of fixed mass, the subhalo abundance decreases by 30 percent between redshift 2 and 0. The subhalo abundance function correlates with the host halo concentration parameter and formation redshift. However, the intrinsic scatter is not significantly reduced for narrow ranges of concentration parameter or formation redshift, showing that they are not the dominant parameters that determine the subhalo abundance in a halo.

astro-ph.CO↗

arXiv2010

Galactic stellar haloes in the CDM model

We present six simulations of galactic stellar haloes formed by the tidal disruption of accreted dwarf galaxies in a fully cosmological setting. Our model is based on the Aquarius project, a suite of high resolution N-body simulations of individual dark matter haloes. We tag subsets of particles in these simulations with stellar populations predicted by the Galform semi-analytic model. Our method self-consistently tracks the dynamical evolution and disruption of satellites from high redshift. The luminosity function and structural properties of surviving satellites, which agree well with observations, suggest that this technique is appropriate. We find that accreted stellar haloes are assembled between 1<z<7 from less than 5 significant progenitors. These progenitors are old, metal-rich satellites with stellar masses similar to the brightest Milky Way dwarf spheroidals. In contrast to previous stellar halo simulations, we find that several of these major contributors survive as self-bound systems to the present day. Both the number of these significant progenitors and their infall times are inherently stochastic. This results in great diversity among our stellar haloes, which amplifies small differences between the formation histories of their dark halo hosts. The masses and density/surface-brightness profiles of the stellar haloes are consistent with expectations from the Milky Way and M31. Each halo has a complex structure, consisting of well-mixed components, tidal streams, shells and other subcomponents. This structure is not adequately described by smooth models. We find one example of an accreted thick disk. Contrasts in age and metallicity between halo stars and those in surviving satellites are in broad agreement with recent observations. [Abridged]

astro-ph.GA↗

arXiv2010

The earliest stars and their relics in the Milky Way

We have implemented a simple model to identify the likely sites of the first stars and galaxies in the high-resolution simulations of the formation of galactic dark matter halos of the Aquarius Project. The first star in a galaxy like the Milky Way formed around redshift z=35; by z=10, the young galaxy contained up to 30000 dark matter haloes capable of forming stars by molecular hydrogen cooling. These minihaloes were strongly clustered and feedback may have severely limited the actual number of Population III stars that formed. By the present day, the remnants of the first stars would be strongly concentrated to the centre of the main halo. If a second generation of long-lived stars formed near the first (the first star relics), we would expect to find half of them within 30kpc/h of the galactic centre and a significant fraction in satellites where they may be singled out by their anomalous metallicity patterns. The first halo in which gas could cool by atomic hydrogen line radiation formed at z=25; by z=10, the number of such first galaxies had increased to ~300. Feedback might have decreased the number of first galaxies at the time they undergo significant star formation, but not the number that survive to the present because near neighbours merge. Half of all the "first galaxies" that form before z=10 merge with the main halo before z~3 and most lose a significant fraction of their mass. However, today there should still be more than 20 remnants orbiting within the central ~30kpc/h of the Milky Way. These satellites have circular velocities of a few kilometers per second or more, comparable to those of known Milky Way dwarfs. They are a promising hunting ground for the remnants of the earliest epoch of star formation.

astro-ph.CO↗