arXiv subjects

D. Fuchs

Publications and source records attributed to D. Fuchs.

At least 55 records · Page 3Linked to original sources

arXiv2022

Probing spin-dependent dark matter interactions with $^6$Li

CRESST is one of the most prominent direct detection experiments for dark matter particles with sub-GeV/c$^2$ mass. One of the advantages of the CRESST experiment is the possibility to include a large variety of nuclides in the target material used to probe dark matter interactions. In this work, we discuss in particular the interactions of dark matter particles with protons and neutrons of $^{6}$Li. This is now possible thanks to new calculations on nuclear matrix elements of this specific isotope of Li. To show the potential of using this particular nuclide for probing dark matter interactions, we used the data collected previously by a CRESST prototype based on LiAlO$_2$ and operated in an above ground test-facility at Max-Planck-Institut für Physik in Munich, Germany. In particular, the inclusion of $^{6}$Li in the limit calculation drastically improves the result obtained for spin-dependent interactions with neutrons in the whole mass range. The improvement is significant, greater than two order of magnitude for dark matter masses below 1 GeV/c$^2$, compared to the limit previously published with the same data.

physics.ins-det↗

arXiv2021

A family of integrable transformations of centroaffine polygons: geometrical aspects

Two polygons, $(P_1,\ldots,P_n)$ and $(Q_1,\ldots,Q_n)$ in ${\mathbb R}^2$ are $c$-related if $\det(P_i, P_{i+1})=\det(Q_i, Q_{i+1})$ and $\det(P_i, Q_i)=c$ for all $i$. This relation extends to twisted polygons (polygons with monodromy), and it descends to the moduli space of $SL(2,{\mathbb R}^2)$-equivalent polygons. This relation is an equiaffine analog of the discrete bicycle correspondence studied by a number of authors. We study the geometry of this relations, present its integrals, and show that, in an appropriate sense, these relations, considered for different values of the constants $c$, commute. We relate this topic with the dressing chain of Veselov and Shabat. The case of small-gons is investigated in detail.

math.DS↗

arXiv2021

Operation of an archaeological lead PbWO$_4$ crystal to search for neutrinos from astrophysical sources with a Transition Edge Sensor

The experimental detection of the CE$ν$NS allows the investigation of neutrinos and neutrino sources with all-flavor sensitivity. Given its large content in neutrons and stability, Pb is a very appealing choice as target element. The presence of the radioisotope $^{210}$Pb (T$_{1/2}\sim$22 yrs) makes natural Pb unsuitable for low-background, low-energy event searches. This limitation can be overcome employing Pb of archaeological origin, where several half-lives of $^{210}$Pb have gone by. We present results of a cryogenic measurement of a 15g PbWO$_4$ crystal, grown with archaeological Pb (older than $\sim$2000 yrs) that achieved a sub-keV nuclear recoil detection threshold. A ton-scale experiment employing such material, with a detection threshold for nuclear recoils of just 1 keV would probe the entire Milky Way for SuperNovae, with equal sensitivity for all neutrino flavors, allowing the study of the core of such exceptional events.

physics.ins-det↗

arXiv2020

Cryogenic characterization of a LiAlO$_{2}$ crystal and new results on spin-dependent dark matter interactions with ordinary matter

In this work, a first cryogenic characterization of a scintillating LiAlO$_{2}$ single crystal is presented. The results achieved show that this material holds great potential as a target for direct dark matter search experiments. Three different detector modules obtained from one crystal grown at the Leibniz-Institut für Kristallzüchtung (IKZ) have been tested to study different properties at cryogenic temperatures. Firstly, two 2.8 g twin crystals were used to build different detector modules which were operated in an above-ground laboratory at the Max Planck Institute for Physics (MPP) in Munich, Germany. The first detector module was used to study the scintillation properties of LiAlO$_{2}$ at cryogenic temperatures. The second achieved an energy threshold of (213.02$\pm$1.48) eV which allows setting a competitive limit on the spin-dependent dark matter particle-proton scattering cross section for dark matter particle masses between 350 MeV/c$^{2}$ and 1.50 GeV/c$^{2}$. Secondly, a detector module with a 373 g LiAlO$_{2}$ crystal as the main absorber was tested in an underground facility at the Laboratori Nazionali del Gran Sasso (LNGS): from this measurement it was possible to determine the radiopurity of the crystal and study the feasibility of using this material as a neutron flux monitor for low-background experiments.

physics.ins-det↗

arXiv2020

An extensive spectroscopic time series of three Wolf-Rayet stars -- II. A search for wind asymmetries in the dust-forming WC7 binary WR137

We present the results of a four-month, spectroscopic campaign of the Wolf-Rayet dust-making binary, WR137. We detect only small-amplitude, random variability in the CIII5696 emission line and its integrated quantities (radial velocity, equivalent width, skewness, kurtosis) that can be explained by stochastic clumps in the wind of the WC star. We find no evidence of large-scale, periodic variations often associated with Corotating Interaction Regions that could have explained the observed intrinsic continuum polarization of this star. Our moderately high-resolution and high signal-to-noise average Keck spectrum shows narrow double-peak emission profiles in the Halpha, Hbeta, Hgamma, HeII6678 and HeII5876 lines. These peaks have a stable blue-to-red intensity ratio with a mean of 0.997 and a root-mean-square of 0.004, commensurate with the noise level; no variability is found during the entire observing period. We suggest that these profiles arise in a decretion disk around the O9 companion, which is thus an O9e star. The characteristics of the profiles are compatible with those of other Be/Oe stars. The presence of this disk can explain the constant component of the continuum polarization of this system, for which the angle is perpendicular to the plane of the orbit, implying that the rotation axis of the O9e star is aligned with that of the orbit. It remains to be explained why the disk is so stable within the strong ultraviolet radiation field of the O star. We present a binary evolutionary scenario that is compatible with the current stellar and system parameters.

astro-ph.SR↗

arXiv2020

Anomalous pressure dependence of the electronic transport and anisotropy in SrIrO3 films

Iridate oxides display exotic physical properties that arise from the interplay between a large spin-orbit coupling and electron correlations. Here, we present a comprehensive study of the effects of hydrostatic pressure on the electronic transport properties of SrIrO3 (SIO), a system that has recently attracted a lot of attention as potential correlated Dirac semimetal. Our investigations on untwinned thin films of SIO reveal that the electrical resistivity of this material is intrinsically anisotropic and controlled by the orthorhombic distortion of the perovskite unit cell. These effects provide another evidence for the strong coupling between the electronic and lattice degrees of freedom in this class of compounds. Upon increasing pressure, a systematic increase of the transport anisotropies is observed. The anomalous pressure-induced changes of the resistivity cannot be accounted for by the pressure dependence of the density of the electron charge carriers, as inferred from Hall effect measurements. Moreover, pressure-induced rotations of the IrO6 octahedra likely occur within the distorted perovskite unit cell and affect electron mobility of this system.

cond-mat.str-el↗

arXiv2020

New limits on the resonant absorption of solar axions obtained with a $^{169}$Tm-containing cryogenic detector

A search for resonant absorption of solar axions by $^{169}$Tm nuclei was carried out. A newly developed approach involving low-background cryogenic bolometer based on Tm$_3$Al$_5$O$_{12}$ crystal was used that allowed for significant improvement of sensitivity in comparison with previous $^{169}$Tm based experiments. The measurements performed with $8.18$ g crystal during $6.6$ days exposure yielded the following limits on axion couplings: $|g_{Aγ} (g_{AN}^0 + g_{AN}^3) \leq 1.44 \times 10^{-14}$ GeV$^{-1}$ and $|g_{Ae} (g_{AN}^0 + g_{AN}^3) \leq 2.81 \times 10^{-16}$.

hep-ex↗

arXiv2020

Magnetotransport of SrIrO3 films on (110) DyScO3

Epitaxial perovskite (110) oriented SrIrO3 (SIO) thin films were grown by pulsed laser deposition on (110) oriented DyScO3 (DSO) substrates with various film thickness t (2 nm < t < 50 nm). All the films were produced with stoichiometric composition, orthorhombic phase, and with high crystallinity. The nearly perfect in-plane lattice matching of DSO with respect to SIO and same symmetry result in a full epitaxial inplane alignment, i.e., the c-axis of DSO and SIO are parallel to each other with only slightly enlarged d110 out-of-plane lattice spacing (+0.38%) due to the small in-plane compressive strain caused by the DSO substrate. Measurements of the magnetoresistance MR were carried out for current flow along the [001] and [1-10] direction of SIO and magnetic field perpendicular to the film plane. MR appears to be distinctly different for both directions. The anisotropy MR001/MR1-10 > 1 increases with decreasing T and is especially pronounced for the thinnest films, which likewise display a hysteretic field dependence below T* ~ 3 K. The coercive field Hc amounts to 2-5 T. Both, T* and Hc are very similar to the magnetic ordering temperature and coercivity of DSO which strongly suggests substrate-induced mechanism as a reason for the anisotropic magnetotransport in the SIO films.

cond-mat.str-el↗

arXiv2019

An improved upper limit on the neutrino mass from a direct kinematic method by KATRIN

We report on the neutrino mass measurement result from the first four-week science run of the Karlsruhe Tritium Neutrino experiment KATRIN in spring 2019. Beta-decay electrons from a high-purity gaseous molecular tritium source are energy analyzed by a high-resolution MAC-E filter. A fit of the integrated electron spectrum over a narrow interval around the kinematic endpoint at 18.57 keV gives an effective neutrino mass square value of $(-1.0^{+0.9}_{-1.1})$ eV$^2$. From this we derive an upper limit of 1.1 eV (90$\%$ confidence level) on the absolute mass scale of neutrinos. This value coincides with the KATRIN sensitivity. It improves upon previous mass limits from kinematic measurements by almost a factor of two and provides model-independent input to cosmological studies of structure formation.

hep-ex↗

arXiv2019

First operation of the KATRIN experiment with tritium

The determination of the neutrino mass is one of the major challenges in astroparticle physics today. Direct neutrino mass experiments, based solely on the kinematics of beta-decay, provide a largely model-independent probe to the neutrino mass scale. The Karlsruhe Tritium Neutrino (KATRIN) experiment is designed to directly measure the effective electron antineutrino mass with a sensitivity of 0.2 eV 90% CL. In this work we report on the first operation of KATRIN with tritium which took place in 2018. During this commissioning phase of the tritium circulation system, excellent agreement of the theoretical prediction with the recorded spectra was found and stable conditions over a time period of 13 days could be established. These results are an essential prerequisite for the subsequent neutrino mass measurements with KATRIN in 2019.

physics.ins-det↗

arXiv2019

Suppression of twinning and enhanced electronic anisotropy of SrIrO3 films

The spin-orbit coupling and electron correlation in perovskite SrIrO3 (SIO) strongly favor new quantum states and make SIO very attractive for next generation quantum information technology. In addition, the small electronic band-width offers the possibility to manipulate anisotropic electronic transport by strain. However, twinned film growth of SIO often masks electronic anisotropy which could be very useful for device applications. We demonstrate that the twinning of SIO films on (001) oriented SrTiO3 (STO) substrates can be strongly reduced for thin films with thickness t less than 30 nm by using substrates displaying a TiO2-terminated surface with step-edge alignment parallel to the a- or b-axis direction of the substrate. This allows us to study electronic anisotropy of strained SIO films which hitherto has been reported only for bulk-like SIO. For films with t < 30 nm electronic anisotropy increases with increasing t and becomes even twice as large compared to nearly strain-free films grown on (110) DyScO3. The experiments demonstrate the high sensitivity of electronic transport towards structural distortion and the possibility to manipulate transport by substrate engineering.

cond-mat.mtrl-sci↗

arXiv2019

Tuning the superconducting transition of SrTiO$_3$-based 2DEGs with light

The resistivity of the two dimensional electron gas that forms at the interface of strontium titanate with various oxides is sensitive to irradiation with visible light. In this letter we present data on the interface between the band gap insulators LaAlO$_3$ (LAO) and SrTiO$_3$ (STO). We operate a light emitting diode at temperatures below 1 K and utilize it to irradiate the LAO/STO interface at ultra low temperatures. On irradiation the resistance of this system is lowered continuously by a factor of five and the resistance change is persistent at low temperatures as long as the sample is kept in the dark. This makes a characterization of transport properties in different resistive states over extended time periods possible. Our pristine sample gets superconducting below 265 mK. The transition temperature $T_c$ shifts downwards on the persistent photo-induced lowering of the resistance. The persistent photoconductance can be completely reverted by heating the structure above 10 K in which case $T_c$ as well takes on its original value. Thus very similar to field effect control of electron densities irradiation at low temperatures offers a versatile tuning knob for the superconducting state of STO-based interfaces which in addition has the advantage to be nonvolatile.

cond-mat.supr-con↗

arXiv2018

Structural properties and anisotropic electronic transport in SrIrO3 films

Perovskite SrIrO3 (SIO) films epitaxially deposited with a thickness of about 60 nm on various substrate materials display nearly strain-relieved state. Films grown on orthorhombic (110) DyScO3 (DSO) are found to display untwinned bulk-like orthorhombic structure. However, film deposition on cubic (001) SrTiO3 induces a twinned growth of SIO. Resistance measurements on the SIO films reveal only weak temperature dependence, where the resistance R increases with decreasing temperature T. Hall measurements show dominant electron-like transport throughout the temperature range from 2 K to 300 K. At 2 K, the electron concentration and resistivity for SIO on STO amount to ne = 1.4*10^20 cm-3 and 1 mohmcm. Interestingly, the film resistance of untwinned SIO on DSO along the [1-10] and the [001] direction differs by up to 25% indicating pronounced anisotropic electronic transport. The anisotropy of the resistance increases with decreasing T and displays a distinct maximum around 86 K. The specific T-dependence is similar to that of the structural anisotropy sqrt(a2+b2)/c of bulk SIO. Therefore, anisotropic electronic transport in SIO is very likely induced by the orthorhombic distortion. Consequently, for twinned SIO films on STO anisotropy vanishes nearly completely. The experimental results show that structural changes are very likely responsible for the observed anisotropic electronic transport. The strong sensitivity of the electronic transport in SIO films may be explained in terms of the narrow electron-like bands in SIO caused by spin-orbit-coupling and orthorhombic distortion.

cond-mat.str-el↗

arXiv2018

Anisotropic electronic transport and Rashba effect of the two-dimensional electron system in (110) SrTiO$_3$-based heterostructures

The two-dimensional electron system in (110)Al$_2$O$_{3-δ}$/SrTiO$_3$ heterostructures displays anisotropic electronic transport. Largest and lowest conductivity and electron mobility $μ$ are observed along the $[001]$ and $[1\bar{1}0]$ direction, respectively. The anisotropy of the sheet resistance and $μ$ likewise leads to a distinct anisotropic normal magnetotransport (MR) for T < 30K. However, at temperatures T<5K and magnetic field B<2T MR is dominated by weak antilocalization.Despite the rather strong anisotropy of the Fermi surfaces, the in-plane anisotropic magnetoresistance (AMR) displays two-fold non-crystalline anisotropy. However, the AMR-amplitude is found to be anisotropic with respect to the current direction, leading to a 60% larger AMR amplitude for current I along the $[001]$ direction compared to I parallel to $[1\bar{1}0]$. Tight binding calulations evidence an anisotropic Rashba-induced band splitting with dominant linear k-dependence. In combination with semiclassical Boltzmann theory the non-crystalline AMR is well described, despite the anisotropic Fermi surface.

cond-mat.str-el↗

arXiv2017

Anisotropic electronic transport of the two-dimensional electron system in Al2O3/SrTiO3 heterostructures

Transport measurements on the two dimensional electron system in Al2O3 SrTiO3 heterostructures indicate significant noncrystalline anisotropic behavior below T = 30 K. Lattice dislocations in SrTiO3 and interfacial steps are suggested to be the main sources for electronic anisotropy. Anisotropic defect scattering likewise alters magnetoresistance at low temperature remarkably and influences spin-orbit coupling significantly by the Elliot Yafet mechanism of spin relaxation resulting in anisotropic weak localization. Applying a magnetic field parallel to the interface results in an additional field induced anisotropy of the conductance, which can be attributed to Rashba spin orbit interaction. Compared to LaAlO3 SrTiO3, Rashba coupling seems to be reduced indicating a weaker polarity in Al2O3 SrTiO3 heterostructures.

cond-mat.str-el↗

arXiv2016

Patterning of two-dimensional electron systems in SrTiO3 based heterostructures using a CeO2 template

Two-dimensional electron systems found at the interface of SrTiO3-based oxide heterostructures often display anisotropic electric transport whose origin is currently under debate. To characterize transport along specific crystallographic directions, we developed a hard-mask patterning routine based on an amorphous CeO2 template layer. The technique allows preparing well-defined microbridges by conventional ultraviolet photolithography which, in comparison to standard techniques such as ion- or wet-chemical etching, does not induce any degradation of interfacial conductance. The patterning scheme is described in details and the successful production of microbridges based on amorphous Al2O3-SrTiO3 heterostructures is demonstrated. Significant anisotropic transport is observed for T < 30 K which is mainly related to impurity/defect scattering of charge carriers in these heterostructures.

cond-mat.str-el↗

arXiv2016

Iterating evolutes and involutes

We study iterations of two classical constructions, the evolutes and involutes of plane curves, and we describe the limiting behavior of both constructions on a class of smooth curves with singularities given by their support functions. Next we study two kinds of discretizations of these constructions: the curves are replaced by polygons, and the evolutes are formed by the circumcenters of the triples of consecutive vertices, or by the incenters of the triples of consecutive sides. The space of polygons is a vector bundle over the space of the side directions, and both kinds of evolutes define vector bundle morphisms. In both cases, we describe the linear maps of the fibers. In the first case, the induced map of the base is periodic, whereas, in the second case, it is an averaging transformation. We also study the dynamics of the related inverse constructions, the involutes of polygons. In addition to the theoretical study, we performed numerous computer experiments; some of the observations remain unexplained.

math.DS↗

arXiv2016

An extensive spectroscopic time-series of three Wolf-Rayet stars. I. The lifetime of large-scale structures in the wind of WR 134

During the summer of 2013, a 4-month spectroscopic campaign took place to observe the variabilities in three Wolf-Rayet stars. The spectroscopic data have been analyzed for WR 134 (WN6b), to better understand its behaviour and long-term periodicity, which we interpret as arising from corotating interaction regions (CIRs) in the wind. By analyzing the variability of the He II $λ$5411 emission line, the previously identified period was refined to P = 2.255 $\pm$ 0.008 (s.d.) days. The coherency time of the variability, which we associate with the lifetime of the CIRs in the wind, was deduced to be 40 $\pm$ 6 days, or $\sim$ 18 cycles, by cross-correlating the variability patterns as a function of time. When comparing the phased observational grayscale difference images with theoretical grayscales previously calculated from models including CIRs in an optically thin stellar wind, we find that two CIRs were likely present. A separation in longitude of $Δϕ\simeq$ 90$^{\circ}$ was determined between the two CIRs and we suggest that the different maximum velocities that they reach indicate that they emerge from different latitudes. We have also been able to detect observational signatures of the CIRs in other spectral lines (C IV $λλ$5802,5812 and He I $λ$5876). Furthermore, a DAC was found to be present simultaneously with the CIR signatures detected in the He I $λ$5876 emission line which is consistent with the proposed geometry of the large-scale structures in the wind. Small-scale structures also show a presence in the wind, simultaneously with the larger scale structures, showing that they do in fact co-exist.

astro-ph.SR↗