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M. Betz

Publications and source records attributed to M. Betz.

16 recordsLinked to original sources
arXiv2025

Enhancing Spin Diffusion in GaAs Quantum Wells: The Role of Electron Density and Channel Width

This study explores the relationship between spin diffusion, spin lifetime, electron density and lateral spatial confinement in two-dimensional electron gases hosted in GaAs quantum wells. Using time-resolved magneto-optical Kerr effect microscopy, we analyze how Hall-bar channel width and back-gate voltage modulation influence spin dynamics. The results reveal that the spin diffusion coefficient increases with reduced channel widths, a trend further amplified at lower electron concentrations achieved via back-gate voltages, where it increases up to 150% for the narrowest channels. The developed theoretical model confirms the spatial inhomogeneities in the spin diffusion as arising from electron-density variations within the channels. The results underscore the importance of tuning electron density and spatial geometry to optimize spin transport and coherence, providing valuable design considerations for spintronic devices where efficient spin manipulation is crucial.

cond-mat.mes-hall↗
arXiv2022

Spin helices in GaAs quantum wells: Interplay of electron density, spin diffusion, and spin lifetime

To establish a correlation between the spin diffusion, the spin lifetime, and the electron density, we study, employing time-resolved magneto-optical Kerr effect microscopy, the spin polarization evolution in low-dimensional GaAs semiconductors hosting two-dimensional electron gases. It is shown that for the establishment of the longest spin-lifetime, the variation of scattering rate with the electron density is of higher importance than the fulfilling of the persistent spin helix condition when the Rashba $α$ and Dresselhaus $β$ parameters are balanced. More specifically, regardless of the $α$ and $β$ linear dependencies on the electron density, the spin relaxation rate is determined by the spin diffusion coefficient that depends on electron density nonmonotonously. The longest experimental spin-lifetime occurs at an electron density, corresponding to the transition from Boltzmann to Fermi-Dirac statistics, which is several times higher than that when the persistent spin helix is expected. These facts highlight the role the electron density may play when considering applications for spintronic devices.

cond-mat.mes-hall↗
arXiv2021

Non-Degenerate Two-Photon Absorption in ZnSe: Experiment and Theory

We experimentally and theoretically investigate the non-degenerate two-photon absorption coefficient $β(ω_1,ω_2)$ in the prototypical semiconductor ZnSe. In particular, we provide a comprehensive data set on the dependence of $β(ω_1,ω_2)$ on the non-degeneracy parameter $ω_1/ω_2$ with the total frequency sum $ω_1+ω_2$ kept constant. We find a substantial increase of the two-photon absorption strength with increasing $ω_1/ω_2$. In addition, different crystallographic orientations and polarization configurations are investigated. The nonlinear optical response is analyzed theoretically by evaluating the multiband semiconductor Bloch equations including inter- and intraband excitations in the length gauge. The band structure and the matrix elements are taken an eight-band k.p model. The simulation results are in very good agreement with the experiment.

cond-mat.mes-hall↗
arXiv2019

Low phase noise master oscillator generation and distribution for ALS and ALS-U

The coax based MO distribution system in the ALS is going to be replaced by a modernized, lower phase noise and more interference tolerant version, ready to support ALS-U operation. System aspects are shown and several commercial analog and digital optical transceiver modules are compared for their suitability in this application. Furthermore, recent phase noise optimizing efforts in the ALS RF system are discussed and several prototypes for a custom built, low phase noise, frequency adjustable master oscillator around 500 MHz are shown.

physics.acc-ph↗
arXiv2018

Persistent Spin Helix Manipulation by Optical Doping of a CdTe Quantum Well

Time-resolved Kerr-rotation microscopy explores the influence of optical doping on the persistent spin helix in a [001]-grown CdTe quantum well at cryogenic temperatures. Electron spin diffusion dynamics reveal a momentum-dependent effective magnetic field providing SU(2) spin-rotation symmetry, consistent with kinetic theory. The Dresselhaus and Rashba spin-orbit coupling parameters are extracted independently from rotating the spin helix with external magnetic fields applied parallel and perpendicular to the effective magnetic field. Most importantly, a non-uniform spatiotemporal precession pattern is observed. The kinetic theory framework of spin diffusion allows for modeling of this finding by incorporating the photocarrier density into the Rashba ($α$) and the Dresselhaus ($β_3$) parameters. Corresponding calculations are further validated by an excitation-density dependent measurement. This work shows universality of the persistent spin helix by its observation in a II-VI compound and the ability to fine-tune it by optical doping.

cond-mat.mes-hall↗
arXiv2017

Gate-Control of Anisotropic Spin Transport and Spin Helix Dynamics in a Modulation-Doped GaAs Quantum Well

Electron spin transport and dynamics are investigated in a single, high-mobility, modulation-doped, GaAs quantum well using ultrafast two-color Kerr-rotation micro-spectroscopy, supported by qualitative kinetic theory simulations of spin diffusion and transport. Evolution of the spins is governed by the Dresselhaus bulk and Rashba structural inversion asymmetries, which manifest as an effective magnetic field that can be extracted directly from the experimental coherent spin precession. A spin precession length L-SOI is defined as one complete precession in the effective magnetic field. It is observed that application of (a) an out-of-plane electric field changes the spin decay time and L-SOI through the Rashba component of the spin-orbit coupling, (b) an in-plane magnetic field allows for extraction of the Dresselhaus and Rashba parameters, and (c) an in-plane electric field markedly modifies both the L-SOI and diffusion coefficient. While simulations reproduce the main features of the experiments, the latter results exceed the corresponding simulations and extend previous studies of drift-current-dependent spin-orbit interactions.

cond-mat.mes-hall↗
arXiv2014

Conceptual Design of the International Axion Observatory (IAXO)

The International Axion Observatory (IAXO) will be a forth generation axion helioscope. As its primary physics goal, IAXO will look for axions or axion-like particles (ALPs) originating in the Sun via the Primakoff conversion of the solar plasma photons. In terms of signal-to-noise ratio, IAXO will be about 4-5 orders of magnitude more sensitive than CAST, currently the most powerful axion helioscope, reaching sensitivity to axion-photon couplings down to a few $\times 10^{-12}$ GeV$^{-1}$ and thus probing a large fraction of the currently unexplored axion and ALP parameter space. IAXO will also be sensitive to solar axions produced by mechanisms mediated by the axion-electron coupling $g_{ae}$ with sensitivity $-$for the first time$-$ to values of $g_{ae}$ not previously excluded by astrophysics. With several other possible physics cases, IAXO has the potential to serve as a multi-purpose facility for generic axion and ALP research in the next decade. In this paper we present the conceptual design of IAXO, which follows the layout of an enhanced axion helioscope, based on a purpose-built 20m-long 8-coils toroidal superconducting magnet. All the eight 60cm-diameter magnet bores are equipped with focusing x-ray optics, able to focus the signal photons into $\sim 0.2$ cm$^2$ spots that are imaged by ultra-low-background Micromegas x-ray detectors. The magnet is built into a structure with elevation and azimuth drives that will allow for solar tracking for $\sim$12 h each day.

physics.ins-det↗
arXiv2013

First results of the CERN Resonant WISP Search (CROWS)

The CERN Resonant WISP Search (CROWS) probes the existence of Weakly Interacting Sub-eV Particles (WISPs) like axions or hidden sector photons. It is based on the principle of an optical light shining through the wall experiment, adapted to microwaves. Critical aspects of the experiment are electromagnetic shielding, design and operation of low loss cavity resonators and the detection of weak sinusoidal microwave signals. Lower bounds were set on the coupling constant $g = 4.5 \cdot 10^{-8} $GeV$^{-1}$ for axion like particles with a mass of $m_a = 7.2 μ$eV. For hidden sector photons, lower bounds were set for the coupling constant $χ= 4.1 \cdot 10^{-9}$ at a mass of $m_{γ'} = 10.8 μ$eV. For the latter we were probing a previously unexplored region in the parameter space.

physics.ins-det↗
arXiv2013

Status report and first results of the microwave LSW experiment at CERN

To detect or exclude the existence of hidden sector photons or axion like particles, a table-top "microwaves shining through the wall" experiment has been set up at CERN. An overview of the experimental layout is given, the technical challenges involved are reviewed and the measurement procedure including data-evaluation and its results to date are shown.

physics.ins-det↗
arXiv2013

Status report of the CERN microwave axion experiment

"Light Shining Through the Wall" experiments can probe the existence of "axion like particles" through their weak coupling to photons. We have adapted such an experiment to the microwave regime and constructed the table top apparatus. This work presents an overview of the experimental setup and then focuses on our latest measurement run and its results. By operating the apparatus within a superconducting MRI magnet, competitive exclusion limits for axion like particles to the first generation optical light shining through the wall experiments have been achieved.

physics.ins-det↗
arXiv2012

A microwave paraphoton and axion detection experiment with 300 dB electromagnetic shielding at 3 GHz

For the microwave equivalent of "light shining through the wall" (LSW) experiments, a sensitive microwave detector and very high electromagnetic shielding is required. The screening attenuation between the axion generating cavity and the nearby detection cavity should be greater than 300 dB, in order to improve over presently existing exclusion limits. To achieve these goals in practice, a "box in a box" concept was utilized for shielding the detection cavity, while a vector signal analyzer was used as a microwave receiver with a very narrow resolution bandwidth in the order of a few micro-Hz. This contribution will present the experimental layout and the results to date.

physics.ins-det↗
arXiv2012

Status report of the CERN light shining through the wall experiment with microwave axions and related aspects

One way to proof or exclude the existence of axion like particles is a microwave light shining through the wall experiment. In this publication we will emphasize on the engineering aspects of such a setup, currently under development at CERN. One critical point, to achieve meaningful results, is the electromagnetic shielding between axion-emitter and -receiver cavity, which needs to be in the order of 300 dB to improve over existing experimental bounds. The RF leakage or electromagnetic crosstalk between both cavities must be well controlled and quantified during the complete duration of the experiment. A very narrow band (in the 10^-6 Hz range) homodyne detection method is used to reveal the axion signal from background thermal noise. The current status of the experiment is presented.

physics.ins-det↗
arXiv2012

High fidelity optical preparation and coherent Larmor precession of a single hole in an InGaAs quantum dot molecule

We employ ultrafast pump-probe spectroscopy with photocurrent readout to directly probe the dynamics of a single hole spin in a single, electrically tunable self-assembled quantum dot molecule formed by vertically stacking InGaAs quantum dots. Excitons with defined spin configurations are initialized in one of the two dots using circularly polarized picosecond pulses. The time-dependent spin configuration is probed by the spin selective optical absorption of the resulting few Fermion complex. Taking advantage of sub-5 ps electron tunneling to an orbitally excited state of the other dot, we initialize a single hole spin with a purity of >96%, i.e., much higher than demonstrated in previous single dot experiments. Measurements in a lateral magnetic field monitor the coherent Larmor precession of the single hole spin with no observable loss of spin coherence within the ~300 ps hole lifetime. Thereby, the purity of the hole spin initialization remains unchanged for all investigated magnetic fields.

cond-mat.mes-hall↗
arXiv2011

Excited State Quantum Couplings and Optical Switching of an Artificial Molecule

We optically probe the spectrum of ground and excited state transitions of an individual, electrically tunable self-assembled quantum dot molecule. Photocurrent absorption measurements show that the spatially direct neutral exciton transitions in the upper and lower dots are energetically separated by only ~2 meV. Excited state transitions ~8-16 meV to higher energy exhibit pronounced anticrossings as the electric field is tuned due to the formation of hybridized electron states. We show that the observed excited state transitions occur between these hybridized electronic states and different hole states in the upper dot. By simultaneously pumping two different excited states with two laser fields we demonstrate a strong (88% on-off contrast) laser induced switching of the optical response. The results represent an electrically tunable, discrete coupled quantum system with a conditional optical response.

cond-mat.mes-hall↗
arXiv2002

Role of Delta exchange for proton-antiproton annihilation into two-pion and three-pion channels

Proton-antiproton annihilation into two pions and three pions is studied in a baryon-exchange model. Annihilation diagrams involving nucleon as well as Delta-resonance exchanges are included consistently in the two- and three-pion channels. Effects from the initial-state interaction are fully taken into account. A comparison of the influence of the Delta exchange on the considered annihilation channels is made and reveals that its importance for three-pion annihilation is strongly reduced as compared to two-pion annihilation. It is found that annihilation into three uncorrelated pions can yield up to 10 % of the total experimentally observed three-pion annihilation cross section.

nucl-th↗