arXiv subjects

A. Braun

Publications and source records attributed to A. Braun.

13 recordsLinked to original sources

arXiv2024

Frequency-dependent amplitude correction to free-precession scalar magnetometers

Pump and probe scalar atomic magnetometers show incredible potential for real-world, traditionally difficult measurement environments due to their high dynamic range and linearity. Previously, it has been assumed these scalar magnetometer have a flat response across their bandwidth, and flat noise floor. Here we show that standard fitting routines, used to extract the magnetic field, result in a non-linear frequency dependent response across the sensor bandwidth, due to the time-averaged nature of such free precession measurements. We present an analytic correction dependent on dead-time, and show how this equation can also correct the sensor spectral density. The maximum in-band amplitude loss approaches 29\% as the frequency of interest becomes the Nyquist frequency, making a significant correction for applications such as source localization in magnetoencephalography. These pump and probe atomic magnetometers also are known to have large aliasing of out-of-band signals, and we propose a scheme where the frequency of out-of-band signals can be identified by performing fits with varying dead-time on the raw free-precession sensor data.

physics.atom-ph↗

arXiv2020

Portable magnetometry for detection of biomagnetism in ambient environments

We present a method of optical magnetometry with parts-per-billion resolution that is able to detect biomagnetic signals generated from the human brain and heart in Earth's ambient environment. Our magnetically silent sensors measure the total magnetic field by detecting the free-precession frequency of highly spin-polarized alkali metal vapor. A first-order gradiometer is formed from two magnetometers that are separated by a 3 cm baseline. Our gradiometer operates from a laptop consuming 5 W over a USB port, enabled by state-of-the-art micro-fabricated alkali vapor cells, advanced thermal insulation, custom electronics, and laser packages within the sensor head. The gradiometer obtains a sensitivity of 16 fT/cm/Hz$^{1/2}$ outdoors, which we use to detect neuronal electrical currents and magnetic cardiography signals. Recording of neuronal magnetic fields is one of a few available methods for non-invasive functional brain imaging that usually requires extensive magnetic shielding and other infractructure. This work demonstrates the possibility of a dense array of portable biomagnetic sensors that are deployable in a variety of natural environments.

physics.med-ph↗

arXiv2016

Attosecond physics at the nanoscale

Recently two emerging areas of research, attosecond and nanoscale physics, have started to come together. Attosecond physics deals with phenomena occurring when ultrashort laser pulses, with duration on the femto- and sub-femtosecond time scales, interact with atoms, molecules or solids. The laser-induced electron dynamics occurs natively on a timescale down to a few hundred or even tens of attoseconds, which is comparable with the optical field. On the other hand, the second branch involves the manipulation and engineering of mesoscopic systems, such as solids, metals and dielectrics, with nanometric precision. Although nano-engineering is a vast and well-established research field on its own, the merger with intense laser physics is relatively recent. In this article we present a comprehensive experimental and theoretical overview of physics that takes place when short and intense laser pulses interact with nanosystems, such as metallic and dielectric nanostructures. In particular we elucidate how the spatially inhomogeneous laser induced fields at a nanometer scale modify the laser-driven electron dynamics. Consequently, this has important impact on pivotal processes such as ATI and HHG. The deep understanding of the coupled dynamics between these spatially inhomogeneous fields and matter configures a promising way to new avenues of research and applications. Thanks to the maturity that attosecond physics has reached, together with the tremendous advance in material engineering and manipulation techniques, the age of atto-nano physics has begun, but it is in the initial stage. We present thus some of the open questions, challenges and prospects for experimental confirmation of theoretical predictions, as well as experiments aimed at characterizing the induced fields and the unique electron dynamics initiated by them with high temporal and spatial resolution.

physics.atom-ph↗

arXiv2014

Concentrated sunlight for accelerated stability testing of organic photovoltaic materials: Towards decoupling light intensity and temperature

We have demonstrated OPV accelerated degradation studies using concentrated sunlight, where the atmosphere, temperature and illumination intensity were independently controlled. Testing various schemes for controlling the sample temperature under concentrated sunlight showed that heating of P3HT:PCBM was caused by photons at the absorbed wavelength range and dissipation of excess photon energy, and not necessarily by IR photon absorption. Sunlight chopping was found to be an effective method for independent temperature control under illumination by concentrated sunlight. The first accelerated degradation tests using sunlight concentration applied to P3HT:PCBM blends were reported. P3HT:PCBM blends exposed to concentrated sunlight in the presence of traces of oxygen/ humidity showed degradation induced by photo-oxidation of the P3HT backbone within the P3HT:PCBM blend, which is significantly thermally accelerated, in agreement with previous observations. However, this could be demonstrated in a time scale of minutes and hours, i.e., significantly accelerated. Exposure of well encapsulated P3HT:PCBM films to concentrated sunlight demonstrated stability up to 3,600 sun*hours, corresponding to about 1.6 years of operating time. This result was obtained at 300 suns exposure after merely 12 hours, demonstrating the advantage of using concentrated sunlight for accelerated stability tests. These tests can therefore combine extremely high acceleration factors with profound understanding of the effect of various, independently controlled factors on the degradation mechanisms.

cond-mat.mtrl-sci↗

arXiv2011

Correlation of conductivity and angle integrated valence band photoemission characteristics in single crystal iron perovskites for 300 K < T < 800 K: Comparison of surface and bulk sensitive methods

A single crystal monolith of La0.9Sr0.1FeO3 and thin pulsed laser deposited film of La0.8Sr0.2Fe0.8Ni0.2O3 were subject to angle integrated valence band photoemission spectroscopy in ultra high vacuum and conductivity experiments in ambient air at temperatures from 300 K to 800 K. Except for several sputtering and annealing cycles, the specimen were not prepared in-situ.. Peculiar changes in the temperature dependent, bulk representative conductivity profile as a result of reversible phase transitions, and irreversible chemical changes are semi-quantitatively reflected by the intensity variation in the more surface representative valence band spectra near the Fermi energy. X-ray photoelectron diffraction images reflect the symmetry as expected from bulk iron perovskites. The correlation of spectral details in the valence band photoemission spectra (VB PES) and details of the conductivity during temperature variation suggest that valuable information on electronic structure and transport properties of complex materials may be obtained without in-situ preparation.

cond-mat.str-el↗

arXiv2011

Electron hole - phonon interaction and structural changes in La0.9Sr0.1FeO3 by temperature dependent conductivity and valence band photoemission spectros-copy

Electric conductivity and structural details of the hole-doped polaron conductor La0.9Sr0.1FeO3 (LSF10) are reported. The conductivity of a single crystal shows an exponential increase with temperature with a maxi-mum of 100 S/cm at 700 K and with activation energy of about 375 meV, and a decrease for higher temperatures that follows a power law. The exponential increase of the electric conductivity for 300 K {\leg} T {\leg} 700 is accompanied by a shift of spectral weight in the photoemission valence band towards the Fermi level, indicative of a strong electron-phonon interaction. The subsequent decrease of the conductivity for T > 700 K is accompanied by a reversible phase transformation from orthorhombic to rhombohedral symmetry. The decreasing conductivity for T > 700 K is likely due to the reduction of the iron due to oxygen loss causing a decreasing hole concentration, as evidenced by a substantial chemical shift in the Fe K-shell x-ray absorption spectra. Two additional fine structures in the conductivity data at 357 K, this is, a small temperature reversible jump in the conductivity, and at 573 K, a slight reversible increase of the polaron activation energy, are correlated with an exceptionally strong decrease in spectral valence band intensity near the Fermi level, and with the onset of a corresponding structural transition.

cond-mat.str-el↗

arXiv2011

Correlation of high temperature X-ray photoemission spectral features and conductivity of epitaxially strained (La0.8Sr0.2)0.95Ni0.2Fe0.8O3/SrTiO3(110)

Reversible and irreversible discontinuities at around 573 K and 823 K in the electric conductivity of a strained 175 nm thin film of (La0.8Sr0.2)0.95Ni0.2Fe0.8O3-δ grown by pulsed laser deposition on SrTiO3 (110) are reflected by valence band changes as monitored in photoemission and oxygen K-edge x-ray absorption spectra. The irreversible jump at 823 K is attributed to depletion of doped electron holes and reduction of Fe4+ to Fe3+, as evidenced by oxygen and iron core level soft x-ray spectroscopy, and possibly of a chemical origin, whereas the reversible jump at 573 K possibly originates from structural changes.

cond-mat.str-el↗

arXiv2010

Search for Stimulated Photon-Photon Scattering in Vacuum

We have searched for stimulated photon scattering in vacuum at a center of mass photon energy of 0.8 eV. The QED contribution to this process is equivalent to four wave mixing in vacuum. No evidence for gamma-gamma scattering was observed. The corresponding upper limit of the cross section is sigma_Lim=1.5 10^{-48}cm2.

physics.optics↗

arXiv2010

Resonance enhanced isotope-selective photoionization of YbI for ion trap loading

Neutral Ytterbium (YbI) and singly ionized Ytterbium (YbII) is widely used in experiments in quantum optics, metrology and quantum information science. We report on the investigation of isotope selective two-photoionisation of YbI that allows for efficient loading of ion traps with YbII. Results are presented on two-colour (399 nm and 369 nm) and single-colour (399 nm) photoionisation and their efficiency is compared to electron impact ionisation. Nearly deterministic loading of a desired number of YbII ions into a linear Paul trap is demonstrated.

physics.atom-ph↗

arXiv2010

Sample-to-sample fluctuations and bond chaos in the $m$-component spin glass

We calculate the finite size scaling of the sample-to-sample fluctuations of the free energy $ΔF$ of the $m$ component vector spin glass in the large-$m$ limit. This is accomplished using a variant of the interpolating Hamiltonian technique which is used to establish a connection between the free energy fluctuations and bond chaos. The calculation of bond chaos then shows that the scaling of the free energy fluctuaions with system size $N$ is $ΔF \sim N^μ$ with ${1/5}\leqμ<{3/10}$, and very likely $μ={1}{5}$ exactly.

cond-mat.dis-nn↗

arXiv2009

Individual addressing of trapped ions and coupling of motional and spin states using rf radiation

Individual electrodynamically trapped and laser cooled ions are addressed in frequency space using radio-frequency radiation in the presence of a static magnetic field gradient. In addition, an interaction between motional and spin states induced by an rf field is demonstrated employing rf-optical double resonance spectroscopy. These are two essential experimental steps towards realizing a novel concept for implementing quantum simulations and quantum computing with trapped ions.

quant-ph↗

arXiv2006

The m-component spin glass on a Bethe lattice

We study the m-component vector spin glass in the limit m to infinity on a Bethe lattice. The cavity method allows for a solution of the model in a self-consistent field approximation and for a perturbative solution of the full problem near the phase transition. The low temperature phase of the model is analyzed numerically and a generalized Bose-Einstein condensation is found, as in the fully connected model. Scaling relations between four distinct zero-temperature exponents are found.

cond-mat.dis-nn↗

arXiv2006

Electrodynamically trapped Yb+ ions for quantum information processing

Highly efficient, nearly deterministic, and isotope selective generation of Yb$^+$ ions by 1- and 2-color photoionization is demonstrated. State preparation and state selective detection of hyperfine states in \ybodd is investigated in order to optimize the purity of the prepared state and to time-optimize the detection process. Linear laser cooled Yb$^+$ ion crystals ions confined in a Paul trap are demonstrated. Advantageous features of different previous ion trap experiments are combined while at the same time the number of possible error sources is reduced by using a comparatively simple experimental apparatus. This opens a new path towards quantum state manipulation of individual trapped ions, and in particular, to scalable quantum computing.

quant-ph↗