arXiv subjects

P. Diener

Publications and source records attributed to P. Diener.

At least 19 recordsLinked to original sources

arXiv2024

Fast dynamic ejecta in neutron star mergers

The ejection of neutron-rich matter is one of the most important consequences of a neutron star merger. While the bulk of the matter is ejected at fast, but non-relativistic velocities ($\sim0.2c$), a small amount of mildly relativistic dynamic ejecta have been seen in a number of numerical simulations. Such ejecta can have far reaching observational consequences ranging from the shock breakout burst of gamma-rays promptly after the merger, to an early ($\sim 1$ hour post-merger) blue kilonova precursor signal, to synchrotron emission years after the merger ("kilonova afterglow"). These all potentially carry the imprint of the binary system parameters and the equation of state. By analyzing Lagrangian simulations in full General Relativity, performed with the code SPHINCS_BSSN, we identify two ejection mechanisms for fast ejecta: i) about 30\% of the ejecta with {$v> 0.4c$} are "sprayed out" from the shear interface between the merging stars and escape along the orbital plane and ii) the remaining $\sim$ 70\% of the fast ejecta result from the central object "bouncing back" after strong, general-relativistic compression. This "bounce component" is ejected in a rather isotropic way and reaches larger velocities (by $\sim0.1c$) so that its faster parts can catch up with and shock slower parts of the spray ejecta. Even for a case that promptly collapses to a black hole, we find fast ejecta with similar properties to the non-collapsing case, while slower matter parts are swallowed by the forming black hole. We discuss observational implications of these fast ejecta, including shock breakout and kilonova afterglow.

astro-ph.HE↗

arXiv2024

SPHINCS_BSSN: Numerical Relativity with Particles

In this book chapter we describe the {\em Lagrangian} numerical relativity code \sphi. This code evolves spacetimes in full General Relativity by integrating the BSSN equations on structured meshes with a simple dynamical mesh refinement strategy. The fluid is evolved by means of freely moving Lagrangian particles, that are evolved using a modern Smooth Particle Hydrodynamics (SPH) formulation. To robustly and accurately capture shocks, our code uses artificial dissipation terms, but, similar to Finite Volume schemes, we apply a slope-limited reconstruction within the dissipative terms and we use in addition time-dependent dissipation parameters, so that dissipation is only applied where needed. The technically most complicated, but absolutely crucial part of the methodology, is the coupling between the particles and the mesh. For the mapping of the energy-momentum tensor $T_{\mu\nu}$ from the particles to the mesh, we use a sophisticated combination of "Local Regression Estimate" (LRE) method and a "multi-dimensional optimal order detection" (MOOD) approach which we describe in some detail. The mapping of the metric quantities from the grid to the particles is achieved by a quintic Hermite interpolation. Apart from giving an introduction to our numerical methods, we demonstrate the accurate working of our code by presenting a set of representative relativistic hydrodynamics tests. We begin with a relativistic shock tube test, then compare the frequencies of a fully relativistic neutron star with reference values from the literature and, finally, we present full-blown merger simulations of irrotational binary systems, one case where a central remnant survives and another where a black hole forms, and of a binary where only one of the stars is rapidly spinning.

gr-qc↗

arXiv2023

Mergers of double neutron stars with one high-spin component: brighter kilonovae and fallback accretion, weaker gravitational waves

Neutron star (NS) mergers where both stars have negligible spins are commonly considered as the most likely ``standard'' case. In globular clusters, however, the majority of NSs have been spun up to millisecond (ms) periods and, based on observed systems, we estimate that a non-negligible fraction of all double NS mergers ($\sim 4\pm2\;\%$) contains one component with a spin of a (few) ms. We use the Lagrangian numerical relativity code \SpB to simulate mergers where one star has no spin and the other has a dimensionless spin parameter of $\chi=0.5$. Such mergers exhibit several distinct signatures compared to irrotational cases. They form only one, very pronounced spiral arm and they dynamically eject an order of magnitude more mass of unshocked material at the original, very low electron fraction. One can therefore expect particularly bright, red kilonovae. Overall, the spinning case collisions are substantially less violent and they eject smaller amounts of shock-generated semi-relativistic material. Therefore, the ejecta produce a weaker blue/UV kilonova {\em precursor} signal, but -- since the total amount is larger -- brighter kilonova {\em afterglows} months after the merger. The spinning cases also have significantly more fallback accretion and thus could power late-time X-ray flares. Since the post-merger remnant loses energy and angular momentum significantly less efficiently to gravitational waves, such systems can delay a potential collapse to a black hole and are therefore candidates for merger-triggered gamma-ray bursts with longer emission time scales.

astro-ph.HE↗

arXiv2022

Thinking outside the box: Numerical Relativity with particles

To date, essentially all simulation codes that solve the full set of Einstein's equations are performed in the framework of Eulerian hydrodynamics. The exception is our recently developed Numerical Relativity code SPHINCS_BSSN which solves the commonly used BSSN formulation of the Einstein equations on a structured mesh and the matter equations via Lagrangian particles. We show here, for the first time, SPHINCS_BSSN neutron star merger simulations with piecewise polytropic approximations to four nuclear matter equations of state. In this set of neutron star merger simulations we focus on perfectly symmetric binary systems that are irrotational and have 1.3 $M_\odot$ masses. We introduce some further methodological refinements (a new way of steering dissipation, an improved particle-mesh mapping) and we explore the impact of the exponent that enters in the calculation of the thermal pressure contribution. We find that it leaves a noticeable imprint on the gravitational wave amplitude (calculated via both quadrupole approximation and the $Ψ_4$-formalism) and has a noticeable impact on the amount of dynamic ejecta. Consistent with earlier findings, we only find a few times $10^{-3}$ \Msun as dynamic ejecta in the studied equal mass binary systems, with softer equations of state (which are more prone to shock formation) ejecting larger amounts of matter. In all of the cases, we see a credible high-velocity ($\sim0.5 .. 0.7c$) ejecta component of $\sim 10^{-4}$ \Msun that is launched at contact from the interface between the two neutron stars. Such a high-velocity component has been suggested to produce an early, blue precursor to the main kilonova emission and it could also potentially cause a kilonova afterglow.

gr-qc↗

arXiv2022

Simulating neutron star mergers with the Lagrangian Numerical Relativity code SPHINCS_BSSN

We present the first neutron star merger simulations performed with the newly developed Numerical Relativity code SPHINCS_BSSN. This code evolves the spacetime on a mesh using the BSSN formulation, but matter is evolved via Lagrangian particles according to a high-accuracy version of general-relativistic Smooth Particle Hydrodynamics (SPH). Our code contains a number of new methodological elements compared to other Numerical Relativity codes. The main focus here is on the new elements that were introduced to model neutron star mergers. These include a) a refinement (fixed in time) of the spacetime-mesh, b) corresponding changes in the particle--mesh mapping algorithm and c) a novel way to construct SPH initial data for binary systems via the recently developed "Artificial Pressure Method." This latter method makes use of the spectral initial data produced by the library LORENE, and is implemented in a new code called SPHINCS_ID. While our main focus is on introducing these new methodological elements and documenting the current status of SPHINCS_BSSN, we also show as a first application a set of neutron star merger simulations employing "soft" ($Γ=2.00$) and "stiff" ($Γ=2.75$) polytropic equations of state.

astro-ph.HE↗

arXiv2021

Resolving the structure of the striped Ge layer on Ag(111): Ag$_2$Ge surface alloy with alternate fcc and hcp domains

Two-dimensional (2D) honeycomb lattices beyond graphene, such as germanene, promise new physical properties such as quantum spin Hall effect. While there have been many claims of growth of germanene, the lack of precise structural characterization of the epitaxial layers synthesized hinders further research. The striped layer formed by Ge deposition on Ag(111) has been recently ascribed as a stretched germanene layer. Using surface X-ray diffraction and density functional theory calculations, we demonstrate that it corresponds in fact to a Ag$_2$Ge surface alloy with an atomic density 6.45% higher than the Ag(111) atomic density. The overall structure is formed by stripes associated with a face-centered cubic top-layer alignment, alternating with stripes associated with an hexagonal-close-packed top-layer alignment, in great analogy with the ($22\times\sqrt3$) Au(111) reconstruction.

cond-mat.mtrl-sci↗

arXiv2020

SPHINCS_BSSN: A general relativistic Smooth Particle Hydrodynamics code for dynamical spacetimes

We present a new methodology for simulating self-gravitating general-relativistic fluids. In our approach the fluid is modelled by means of Lagrangian particles in the framework of a general-relativistic (GR) Smooth Particle Hydrodynamics (SPH) formulation, while the spacetime is evolved on a mesh according to the BSSN formulation that is also frequently used in Eulerian GR-hydrodynamics. To the best of our knowledge this is the first Lagrangian fully general relativistic hydrodynamics code (all previous SPH approaches used approximations to GR-gravity). A core ingredient of our particle-mesh approach is the coupling between the gas (represented by particles) and the spacetime (represented by a mesh) for which we have developed a set of sophisticated interpolation tools that are inspired by other particle-mesh approaches, in particular by vortex-particle methods. One advantage of splitting the methodology between matter and spacetime is that it gives us more freedom in choosing the resolution, so that -- if the spacetime is smooth enough -- we obtain good results already with a moderate number of grid cells and can focus the computational effort on the simulation of the matter. Further advantages of our approach are the ease with which ejecta can be tracked and the fact that the neutron star surface remains well-behaved and does not need any particular treatment. In the hydrodynamics part of the code we use a number of techniques that are new to SPH, such as reconstruction, slope limiting and steering dissipation by monitoring entropy conservation. We describe here in detail the employed numerical methods and demonstrate the code performance in a number of benchmark problems ranging from shock tube tests, over Cowling approximations to the fully dynamical evolution of neutron stars in self-consistently evolved spacetimes.

gr-qc↗

arXiv2014

Evidence of a nonequilibrium distribution of quasiparticles in the microwave response of a superconducting aluminium resonator

In a superconductor absorption of photons with an energy below the superconducting gap leads to redistribution of quasiparticles over energy and thus induces a strong non-equilibrium quasiparticle energy distribution. We have measured the electrodynamic response, quality factor and resonant frequency, of a superconducting aluminium microwave resonator as a function of microwave power and temperature. Below 200 mK, both the quality factor and resonant frequency decrease with increasing microwave power, consistent with the creation of excess quasiparticles due to microwave absorption. Counterintuitively, above 200 mK, the quality factor and resonant frequency increase with increasing power. We demonstrate that the effect can only be understood by a non-thermal quasiparticle distribution.

cond-mat.supr-con↗

arXiv2013

Design and Testing of Kinetic Inductance Detectors Made of Titanium Nitride

To use highly resistive material for Kinetic Inductance Detectors (KID), new designs have to be done, in part due to the impedance match needed between the KID chip and the whole 50 ohms readout circuit. Chips from two new hybrid designs, with an aluminum throughline coupled to titanium nitride microresonators, have been measured and compared to a TiN only chip. In the hybrid chips, parasitic temperature dependent box resonances are absent. The dark KID properties have been measured in a large set of resonators. A surprisingly long lifetime, up to 5.6 ms is observed in a few KIDs. For the other more reproducible devices, the mean electrical Noise Equivalent Power is 5.4 10-19 W.Hz1/2.

astro-ph.IM↗

arXiv2013

Cryogenic Calibration Setup for Broadband Complex Impedance Measurements

Reflection measurements give access to the complex impedance of a material on a wide frequency range. This is of interest to study the dynamical properties of various materials, for instance disordered superconductors. However reflection measurements made at cryogenic temperature suffer from the difficulty to reliably subtract the circuit contribution. Here we report on the design and first tests of a setup able to precisely calibrate in situ the sample reflection, at 4.2 K and up to 2 GHz, by switching and measuring, during the same cool down, the sample and three calibration standards.

physics.ins-det↗

arXiv2013

S-wave superconductivity probed by measuring magnetic penetration depth and lower critical field of MgCNi$_{3}$ single crystals

The magnetic penetration depth $λ$ has been measured in MgCNi$_{3}$ single crystals using both a high precision Tunnel Diode Oscillator technique (TDO) and Hall probe magnetization (HPM). In striking contrast to previous measurements in powders, $δλ$(T) deduced from TDO measurements increases exponentially at low temperature, clearly showing that the superconducting gap is fully open over the whole Fermi surface. An absolute value at zero temperature $λ(0)=230 $nm is found from the lower critical field measured by HPM. We also discuss the observed difference of the superfluid density deduced from both techniques. A possible explanation could be due to a systematic decrease of the critical temperature at the sample surface.

cond-mat.supr-con↗

arXiv2012

Homogeneous superconducting phase in TiN film : a complex impedance study

The low frequency complex impedance of a high resistivity 92 μΩ cm and 100 nm thick TiN superconducting film has been measured via the transmission of several high sensitivity GHz microresonators, down to Tc/50. The temperature dependence of the kinetic inductance follows closely BCS local electrodynamics, with one well defined superconducting gap. This evidences the recovery of an homogeneous superconducting phase in TiN far from the disorder and composition driven transitions. Additionally, we observe a linearity between resonator quality factor and frequency temperature changes, which can be described by a two fluid model.

cond-mat.supr-con↗

arXiv2012

Microwave-induced excess quasiparticles in superconducting resonators measured through correlated conductivity fluctuations

We have measured the number of quasiparticles and their lifetime in aluminium superconducting microwave resonators. The number of excess quasiparticles below 160 mK decreases from 72 to 17 $μ$m$^{-3}$ with a 6 dB decrease of the microwave power. The quasiparticle lifetime increases accordingly from 1.4 to 3.5 ms. These properties of the superconductor were measured through the spectrum of correlated fluctuations in the quasiparticle system and condensate of the superconductor, which show up in the resonator amplitude and phase respectively. Because uncorrelated noise sources vanish, fluctuations in the superconductor can be studied with a sensitivity close to the vacuum noise.

cond-mat.supr-con↗

arXiv2011

Photon noise limited radiation detection with lens-antenna coupled Microwave Kinetic Inductance Detectors

Microwave Kinetic Inductance Detectors (MKIDs) have shown great potential for sub-mm instrumentation because of the high scalability of the technology. Here we demonstrate for the first time in the sub-mm band (0.1...2 mm) a photon noise limited performance of a small antenna coupled MKID detector array and we describe the relation between photon noise and MKID intrinsic generation-recombination noise. Additionally we use the observed photon noise to measure the optical efficiency of detectors to be 0.8+-0.2.

astro-ph.IM↗

arXiv2011

Number fluctuations of sparse quasiparticles in a superconductor

We have directly measured quasiparticle number fluctuations in a thin film superconducting Al resonator in thermal equilibrium. The spectrum of these fluctuations provides a measure of both the density and the lifetime of the quasiparticles. We observe that the quasiparticle density decreases exponentially with decreasing temperature, as theoretically predicted, but saturates below 160 mK to 25-55 per cubic micron. We show that this saturation is consistent with the measured saturation in the quasiparticle lifetime, which also explains similar observations in qubit decoherence times.

cond-mat.supr-con↗

arXiv2010

Dynamics and Gravitational Wave Signature of Collapsar Formation

We perform 3+1 general relativistic simulations of rotating core collapse in the context of the collapsar model for long gamma-ray bursts. We employ a realistic progenitor, rotation based on results of stellar evolution calculations, and a simplified equation of state. Our simulations track self-consistently collapse, bounce, the postbounce phase, black hole formation, and the subsequent early hyperaccretion phase. We extract gravitational waves from the spacetime curvature and identify a unique gravitational wave signature associated with the early phase of collapsar formation.

astro-ph.HE↗

arXiv2010

Minimal resonator loss for circuit quantum electrodynamics

Single photon level quality factors of 500x10^3 are shown in NbTiN superconducting resonators at millikelvin temperatures. This result originates from the intrinsic low dielectric loss of NbTiN, as demonstrated by comparison with Ta, and by removing unnecessary parts of the dielectric substrate.

cond-mat.mes-hall↗

arXiv2009

A template bank for gravitational waveforms from coalescing binary black holes: non-spinning binaries

Gravitational waveforms from the inspiral and ring-down stages of the binary black hole coalescences can be modelled accurately by approximation/perturbation techniques in general relativity. Recent progress in numerical relativity has enabled us to model also the non-perturbative merger phase of the binary black-hole coalescence problem. This enables us to \emph{coherently} search for all three stages of the coalescence of non-spinning binary black holes using a single template bank. Taking our motivation from these results, we propose a family of template waveforms which can model the inspiral, merger, and ring-down stages of the coalescence of non-spinning binary black holes that follow quasi-circular inspiral. This two-dimensional template family is explicitly parametrized by the physical parameters of the binary. We show that the template family is not only \emph{effectual} in detecting the signals from black hole coalescences, but also \emph{faithful} in estimating the parameters of the binary. We compare the sensitivity of a search (in the context of different ground-based interferometers) using all three stages of the black hole coalescence with other template-based searches which look for individual stages separately. We find that the proposed search is significantly more sensitive than other template-based searches for a substantial mass-range, potentially bringing about remarkable improvement in the event-rate of ground-based interferometers. As part of this work, we also prescribe a general procedure to construct interpolated template banks using non-spinning black hole waveforms produced by numerical relativity.

gr-qc↗