arXiv subjects

D. Meyer

Publications and source records attributed to D. Meyer.

At least 19 recordsLinked to original sources

arXiv2026

Programmable CMOS DAC Operating in Cryogenic Environments for Controlling Superconducting Qubits

This paper presents the design and test results of a CMOS current-based Digital to Analog Converter (DAC) that operates at cryogenic temperatures and that can be used to precisely control the amount of flux coupled to qubits or that can be used in the readout of superconducting circuits. The current pulse output can be controlled in terms of its amplitude, rise and fall slopes, via digital controls, and pulse width, via external triggers, while driving a superconducting circuit. The design has been implemented in a planar 90 nm CMOS process and test results closely match circuit predictions. The solution's wide degree of digital tunability affords potential application of the same device to many types of quantum circuits, beyond those discussed here. Due to the wide-ranging flexibility of digital CMOS control, we envision that this DAC design will enable the next generation of high-fidelity cryo-CMOS control architectures for superconducting qubits.

quant-ph↗

arXiv2026

Fiber-coupling of Fourier Transform Spectrographs

Fourier Transform Spectrographs (FTS) are versatile tools for measuring accurate, high resolution spectra. They are internally calibrated by a reference laser that runs in parallel to the science light. Therefore it is crucial to properly align these two beams with respect to each other. We show how this can be achieved by feeding a part of the reference light into the optical path of the science beam. For astronomical applications it's often useful to use optical fibers. We present a coupling setup for our Bruker Optics IFS 125 FTS, consisting of (1) two hexagonal input fibers, (2) dichroic beam-combining for measuring two light sources simultaneously and (3) optimized optics to match the original Bruker design. The hexagonal shape of the fiber cores secures sufficient mode scrambling inside the fibers, resulting in constant beam parameters and a more homogeneous illumination of the entrance aperture of the FTS.

astro-ph.IM↗

arXiv2021

Atomic scale spectral control of thermal transport in phononic crystal superlattices

We present experimental and theoretical investigations of phonon thermal transport in (LaMnO$_3$)$_m$/(SrMnO$_3$)$_n$ superlattices (LMO/SMO SLs) with the thickness of individual layers $m,n = 3 - 10\;$ u.c. and the thickness ratio $m/n = 1, 2$. Optical transient thermal reflectivity measurements reveal a pronounced difference in the thermal conductivity between SLs with $m/n = 1$, and SLs with $m/n = 2$. State-of-the art electron microscopy techniques and ab-initio density functional calculations enables us to assign the origin of this difference to the absence ($m/n = 1$) or presence ($m/n = 2$) of spatially periodic, static oxygen octahedral rotation (OOR) inside the LMO layers. The experimental data analysis shows that the effective thermal conductance of the LMO/SMO interfaces strongly changes from $0.3$ GW/m$^2$K for $m/n = 2$ SLs with OOR to a surprisingly large value of $1.8$ GW/m$^2$K for $m/n = 1$ SLs without OOR. An instructive lattice dynamical model rationalizes our experimental findings as a result of coherent phonon transmission for $m/n = 1$ versus coherent phonon blocking in SLs with $m/n = 2$. We briefly discuss the possibilities to exploit these results for atomic-scale engineering of a crystalline phonon insulator. The thermal resistivity of this proposal for a thermal metamaterial surpasses the amorphous limit, although phonons still propagate coherently.

cond-mat.mtrl-sci↗

arXiv2019

Open source QGIS toolkit for the Advanced Research WRF modelling system

The Advanced Research WRF (Weather Research and Forecasting) model is a popular atmospheric model used for research and Numerical Weather Prediction (NWP). However, despite its popularity, its set-up and configuration often demand several interdisciplinary skills that go beyond the understanding of physical processes. Pre-processing tasks, such as importing custom high-resolution datasets in the WRF Pre-processing System (WPS), still require a considerable effort from the user. We present GIS4WRF, a free, open-source, and cross-platform QGIS Python plug-in to help scientists and practitioners with their Advanced Research WRF modelling workflows. GIS4WRF incorporates new and existing tools for data-processing, configuration, simulation, and visualization into a single graphical environment, and offers WRF-CMake binary distributions for Windows, macOS, and Linux. We highlight its main features and provide useful insights into several key approaches and techniques used in its development. We end with two example applications highlighting the contributions of GIS4WRF in simplifying several WRF-related tasks.

physics.ao-ph↗

arXiv2012

Jets and Accretion Disks in Astrophysics - A Brief Review

The significance of jets and accretion disks in Astrophysics may be growing far beyond any single example of recent finds in the scientific journals. This brief review will summarize recent, significant manifestations of accretion disk powered jets in the universe. We then introduce supplemental contemporary finds in physics and astrophysics which might bear tangential or direct implications for astrophysics toward rethinking the universe with a major role of relativistic jets powered by accretion disks. We conclude with the direction our research will take in order to establish a new perspective on the universe.

physics.gen-ph↗

arXiv2012

Long-lived electron spins in a modulation doped (100) GaAs quantum well

We have measured T1 spin lifetimes of a 14 nm modulation-doped (100) GaAs quantum well using a time-resolved pump-probe Kerr rotation technique. The quantum well was selected by tuning the wavelength of the probe laser. T1 lifetimes in excess of 1 microsecond were measured at 1.5 K and 5.5 T, exceeding the typical T2* lifetimes that have been measured in GaAs and II-VI quantum wells by orders of magnitude. We observed effects from nuclear polarization, which were largely removable by simultaneous nuclear magnetic resonance, along with two distinct lifetimes under some conditions that likely result from probing two differently-localized subsets of electrons.

cond-mat.mes-hall↗

arXiv2012

Optimal uncertainty quantification for legacy data observations of Lipschitz functions

We consider the problem of providing optimal uncertainty quantification (UQ) --- and hence rigorous certification --- for partially-observed functions. We present a UQ framework within which the observations may be small or large in number, and need not carry information about the probability distribution of the system in operation. The UQ objectives are posed as optimization problems, the solutions of which are optimal bounds on the quantities of interest; we consider two typical settings, namely parameter sensitivities (McDiarmid diameters) and output deviation (or failure) probabilities. The solutions of these optimization problems depend non-trivially (even non-monotonically and discontinuously) upon the specified legacy data. Furthermore, the extreme values are often determined by only a few members of the data set; in our principal physically-motivated example, the bounds are determined by just 2 out of 32 data points, and the remainder carry no information and could be neglected without changing the final answer. We propose an analogue of the simplex algorithm from linear programming that uses these observations to offer efficient and rigorous UQ for high-dimensional systems with high-cardinality legacy data. These findings suggest natural methods for selecting optimal (maximally informative) next experiments.

math.PR↗

arXiv2005

Performance of an Operating High Energy Physics Data Grid: D0SAR-Grid

The D0 experiment at Fermilab's Tevatron will record several petabytes of data over the next five years in pursuing the goals of understanding nature and searching for the origin of mass. Computing resources required to analyze these data far exceed capabilities of any one institution. Moreover, the widely scattered geographical distribution of D0 collaborators poses further serious difficulties for optimal use of human and computing resources. These difficulties will exacerbate in future high energy physics experiments, like the LHC. The computing grid has long been recognized as a solution to these problems. This technology is being made a more immediate reality to end users in D0 by developing a grid in the D0 Southern Analysis Region (D0SAR), D0SAR-Grid, using all available resources within it and a home-grown local task manager, McFarm. We will present the architecture in which the D0SAR-Grid is implemented, the use of technology and the functionality of the grid, and the experience from operating the grid in simulation, reprocessing and data analyses for a currently running HEP experiment.

physics.data-an↗

arXiv2004

Phase diagram and dynamic response functions of the Holstein-Hubbard model

We present the phase diagram and dynamical correlation functions for the Holstein-Hubbard model at half filling and at zero temperature. The calculations are based on the Dynamical Mean Field Theory. The effective impurity model is solved using Exact Diagonalization and the Numerical Renormalization Group. Excluding long-range order, we find three different paramagnetic phases, metallic, bipolaronic and Mott insulating, depending on the Hubbard interaction U and the electron-phonon coupling g. We present the behaviour of the one-electron spectral functions and phonon spectra close to the metal insulator transitions.

cond-mat.str-el↗

arXiv2004

Renormalized Parameters for Impurity Models

We show that the low energy behaviour of quite diverse impurity systems can be described by a single renormalized Anderson model, with three parameters, an effective level $\tildeε_d$, an effective hybridization $\tilde V$, and a quasiparticle interaction $\tilde U$. The renormalized parameters are calculated as a function of the bare parameters for a number of impurity models, including those with coupling to phonons and a Falikov-Kimball interaction term. In the model with a coupling to phonons we determine where the interaction of the quasiparticles changes sign as a function of the electron-phonon coupling. In the model with a Falikov-Kimball interaction we show that to a good approximation the low energy behaviour corresponds to that of a bare Anderson model with a shifted impurity level.

cond-mat.str-el↗

arXiv2004

Dynamic response functions for the Holstein-Hubbard model

We present results on the dynamical correlation functions of the particle-hole symmetric Holstein-Hubbard model at zero temperature, calculated using the dynamical mean field theory which is solved by the numerical renormalization group method. We clarify the competing influences of the electron-electron and electron-phonon interactions particularity at the different metal to insulator transitions. The Coulomb repulsion is found to dominate the behaviour in large parts of the metallic regime. By suppressing charge fluctuations, it effectively decouples electrons from phonons. The phonon propagator shows a characteristic softening near the metal to bipolaronic transition but there is very little softening on the approach to the Mott transition.

cond-mat.str-el↗

arXiv2004

First- and Second Order Phase Transitions in the Holstein-Hubbard Model

We investigate metal-insulator transitions in the Holstein-Hubbard model as a function of the on-site electron-electron interaction U and the electron-phonon coupling g. We use several different numerical methods to calculate the phase diagram, the results of which are in excellent agreement. When the electron-electron interaction U is dominant the transition is to a Mott-insulator; when the electron-phonon interaction dominates, the transition is to a localised bipolaronic state. In the former case, the transition is always found to be second order. This is in contrast to the transition to the bipolaronic state, which is clearly first order for larger values of U. We also present results for the quasiparticle weight and the double-occupancy as function of U and g.

cond-mat.str-el↗

arXiv2002

Self-energy approach to the correlated Kondo-lattice model

We develop an interpolating self-energy approach to the correlated Kondo-lattice model. The correlation of the band electrons is taken into account by a Hubbard interaction. The method is based on a self-energy ansatz, the structure of which allows to fulfill a maximum number of exactly solvable limiting cases. The parameters of the ansatz are fitted to spectral moments via high-energy expansion of the self-energy. The band electron correlations are taken into account by an effective medium approach being correct in the strong coupling (U) regime. The theory is considered reliable for all temperatures, band occupations, and exchange couplings. Results are presented for the respective dependencies of spectral densities, quasiparticle densities of states, and characteristic correlation functions, and interpreted in terms of elementary spin exchange processes between itinerant conduction electrons and localized magnetic moments. The appearance of magnetic polarons, the typical quasiparticle of Kondo-lattices, in the energy spectrum is worked out. Spin exchange processes prevent a total spin polarization of the band electrons even for arbitrarily strong exchange couplings as long as the local moments are represented by quantum mechanical spins.

cond-mat.str-el↗

arXiv2002

Local Density of States in the Antiferromagnetic and Ferromagnetic Kondo Models

Based on a simple approximation scheme we have computed the local density of states (LDOS) of the antiferromagnetic and ferromagnetic Kondo models for the full range of band occupations and coupling strengths. For both models the LDOS with its full energy dependence has not been calculated before. Arguments are given for the results to be qualitatively trustworthy despite the simplicity of the approximation scheme.

cond-mat.str-el↗

arXiv2002

Gap formation and soft phonon mode in the Holstein model

We investigate electron-phonon coupling in many-electron systems using dynamical mean-field theory in combination with the numerical renormalization group. This non-perturbative method reveals significant precursor effects to the gap formation at intermediate coupling strengths. The emergence of a soft phonon mode and very strong lattice fluctuations can be understood in terms of Kondo-like physics due to the development of a double-well structure in the effective potential for the ions.

cond-mat.str-el↗

arXiv2001

Numerical Renormalization Group Study of the Anderson-Holstein Impurity Model

We present numerical renormalization group (NRG) calculations for a single-impurity Anderson model with a linear coupling to a local phonon mode. We calculate dynamical response functions, spectral densities, dynamic charge and spin susceptibilities. Being non-perturbative, the NRG is applicable for all parameter regimes. Our calculations cover both weak and strong electron-phonon coupling for zero and finite electron-electron interaction. We interpret the high- and low-energy features and compare our results to atomic limit calculations and perturbation theory. In certain restricted parameter regimes for strong electron-phonon coupling, a soft phonon mode develops inducing a very narrow resonance at the Fermi level.

cond-mat.str-el↗

arXiv2001

Ferromagnetism and disorder: A dynamical mean-field study

We investigate ferromagnetism in the periodic Anderson model with diagonal disorder. Using dynamical mean-field theory in combination with the modified perturbation theory, the disorder can be included in the calculation consistently, which turns out to be equivalent to the CPA method. Disorder generally reduces the Curie temperature and can for certain configurations completely suppress ferromagnetic order. This can be ascribed to the enhanced quasiparticle damping and the special structure of the density of states.

cond-mat.str-el↗

arXiv2001

Low density approach to the Kondo-lattice model

We propose a new approach to the (ferromagnetic) Kondo-lattice model in the low density region, where the model is thought to give a reasonable frame work for manganites with perovskite structure exhibiting the "colossal magnetoresistance" -effect. Results for the temperature- dependent quasiparticle density of states are presented. Typical features can be interpreted in terms of elementary spin-exchange processes between itinerant conduction electrons and localized moments. The approach is exact in the zero bandwidth limit for all temperatures and at T=0 for arbitrary bandwidths, fulfills exact high-energy expansions and reproduces correctly second order perturbation theory in the exchange coupling.

cond-mat.str-el↗