Structural and Magnetic Phase Transitions in Manganese Arsenide Thin-Films Grown by Molecular Beam Epitaxy

Phase transitions play an important role in many fields of physics and engineering, and their study in bulk materials has a long tradition. Many of the experimental techniques involve measurements of thermodynamically extensive parameters. With the increasing technological importance of thin-film technology there is a pressing need to find new ways to study phase transitions at smaller length-scales, where the traditional methods are insufficient. In this regard, the phase transitions observed in thin-films of MnAs present interesting challenges. As a ferromagnetic material that can be grown epitaxially on a variety of technologically important substrates, MnAs is an interesting material for spintronics applications. In the bulk, the first order transition from the low temperature ferromagnetic $\alpha$-phase to the $\beta$-phase occurs at 313~K. The magnetic state of the $\beta$-phase has remained controversial. A second order transition to the paramagnetic $\gamma$-phase takes place at 398~K. In thin-films, the anisotropic strain imposed by the substrate leads to the interesting phenomenon of coexistence of $\alpha$- and $\beta$-phases in a regular array of stripes over an extended temperature range. In this dissertation these phase transitions are studied in films grown by molecular beam epitaxy on GaAs (001). The films are confirmed to be of high structural quality and almost purely in the $A_0$ orientation. A diverse set of experimental techniques, germane to thin-film technology, is used to probe the properties of the film: Temperature-dependent X-ray diffraction and atomic-force microscopy (AFM), as well as magnetotransport give insights into the structural properties, while the anomalous Hall effect is used as a probe of magnetization during the phase transition. In addition, reflectance difference spectroscopy (RDS) is used as a sensitive probe of electronic structure. Inductively coupled plasma etching with BCl$_3$ is demonstrated to be effective for patterning MnAs. We show that the evolution of electrical resistivity in the coexistence regime of $\alpha$- and $\beta$-phase can be understood in terms of a simple model. These measurements allow accurate extraction of the order-parameter phase fraction and thus permit us to study the hysteresis of the phase transition in detail. Major features in the hysteresis can be correlated to the ordering observed in the array of $\alpha$- and $\beta$-stripes. As the continuous ferromagnetic film breaks up into isolated stripes of $\alpha$-phase, a hysteresis in the out-of-plane magnetization is detected from measurements of the anomalous Hall effect. The appearance of out-of-plane domains can be understood from simple shape-anisotropy arguments. Remarkably, an anomaly of the Hall effect at low fields persists far into the $\beta$-phase. Signatures of the more elusive $\beta$- to $\gamma$-transition are found in the temperature-dependence of resistivity, the out-of-plane lattice constant, and reflectance difference spectra. The transition temperature is significantly lowered compared to the bulk, consistent with the strained state of the material. The negative temperature coefficient of resistivity, as well as its anisotropic changes, lend support to the idea of an antiferromagnetic order within the $\beta$-phase

Riding the dark matter wave: Novel limits on general dark photons from LISA Pathfinder

We note the possibility to perform a parametrically improved search for gauged baryon (B) and baryon minus lepton ($B-L$) Dark Photon Dark Matter (DPDM) using auxiliary channel data from LISA Pathfinder. In particular we use the measurement of the differential movement between the test masses (TMs) and the space craft (SC) which is nearly as sensitive as the tracking between the two TMs. TMs and SC are made from different materials and therefore have different charge-to-mass ratios for both $B-L$ and B. Thus, the surrounding DPDM field induces a relative acceleration of nearly constant frequency. For the case of $B-L$, we find that LISA Pathfinder can constrain previously unexplored parameter space, providing the world leading limits in the mass range $4\cdot10^{-19}\space{eV}<m<3\cdot10^{-17}\space{eV}$. This limit can easily be recast also for dark photons that arise from gauging other global symmetries of the SM

A High Spectral Resolution Study of the Soft X-ray Background with the X-ray Quantum Calorimeter

We present here a combined analysis of four high spectral resolution observations of the Diffuse X-ray Background (DXRB), made using the University of Wisconsin-Madison/Goddard Space Flight Center X-ray Quantum Calorimeter (XQC) sounding rocket payload. The observed spectra support the existence of a $\sim0.1~$keV Local Hot Bubble and a $\sim0.2~$keV Hot Halo, with discrepancies between repeated observations compatible with expected contributions of time-variable emission from Solar Wind Charge Exchange (SWCX). An additional component of $\sim0.9~$keV emission observed only at low galactic latitudes can be consistently explained by unresolved dM stars.Comment: 21 pages, 6 figures, accepted for publication in Ap

Minicharges and Magnetic Monopoles

Minicharged particles arise naturally in extensions of the Standard Model with a kinetic mixing term between the ordinary electromagnetic U(1) and an extra "hidden sector" U(1). In this note we study the compatibility of these particles with the existence of magnetic monopoles. We find that angular momentum quantization allows only certain combinations of ordinary and hidden monopole charge. Using the example where one of the U(1)s originates from a spontaneously broken SU(2), we demonstrate that exactly the allowed types of monopoles arise as 't Hooft-Polyakov monopoles.Comment: 9 pages, 1 figur

Predniso(lo)ne Dosage and Chance of Remission in Patients With Autoimmune Hepatitis

Background & Aims Patients with autoimmune hepatitis (AIH) commonly receive induction therapy with predniso(lo)ne followed by maintenance therapy with azathioprine. European Association for Study of the Liver clinical practice guidelines advise a predniso(lo)ne dose range of 0.50–1 mg/kg/day, which leaves room for practice variation. We performed a multicenter study to determine the efficacy of different dose ranges of predniso(lo)ne induction therapy in a large European cohort of patients with AIH. Methods We performed a retrospective cohort study using a comparative effectiveness design. We collected data from 451 adults with AIH who began treatment from 1978 through 2017 at 9 centers in 5 European countries. We assigned patients to a high-dose group (initial predniso(lo)ne dose ≥0.50 mg/kg/day; n=281) or a low-dose group (<0.50 mg/kg/day; n=170). Logistic regression was performed to determine difference in outcomes between the groups. The primary outcome was normal serum levels of transaminases at 6 months after initiation of therapy. Results There was no significant difference in rates of normalization of transaminases between the high-dose predniso(lo)ne group and the low-dose group (70.5% vs 64.7%; P =.20). After multivariable logistic regression with correction for confounders, there was no difference in the likelihood of normalization of transaminases between the groups (odds ratio, 1.21; 95% CI, 0.78 – 1.87; P =.38). Patients given an initial high dose of predniso(lo)ne received more predniso(lo)ne over time than patients started on a lower dose (median doses over 6 months: 3780 mg vs 2573 mg) ( P <.01). Conclusions In a retrospective study of patients with AIH in Europe, we found that the dose of predniso(lo)ne to induce remission in patients with AIH is less relevant than assumed. An initial predniso(lo)ne dose below 0.50 mg/kg/day substantially decreases unnecessary exposure to predniso(lo)ne in patients with AIH

High discontinuation rate of azathioprine in autoimmune hepatitis, independent of time of treatment initiation

Contains fulltext : 225262.pdf (Publisher’s version ) (Open Access)BACKGROUND: Guidelines regarding treatment for autoimmune hepatitis (AIH) favour two strategies for azathioprine (AZA) introduction: concurrent with steroids at induction or delayed by 2-4 weeks. The safety and efficacy of both strategies have been unexplored. METHODS: We established a cohort of 900 AIH patients from 12 centres in 7 European countries. There were 631 patients who used AZA as part of the therapeutic regimen. We distinguished two groups: patients with early AZA (<2 weeks) or delayed AZA initiation (≥2 weeks). Primary outcome was discontinuation of AZA in the first year of treatment. Cox regression and propensity score matching was performed to determine difference in outcomes between groups. RESULTS: Patients with early AZA initiation had significantly lower transaminases and bilirubin at baseline. Discontinuation rates of AZA did not differ between early and delayed starters (16.6% vs 14.2%), which did not reach statistical significance (hazard ratio 0.97, 95% confidence interval 0.61-1.55, P = .90). Stratification according to baseline disease activity or propensity score matching did not alter the results. Main reason for AZA discontinuation was intolerance to treatment (14.0% vs 13.2%, P = .78) with nausea and vomiting as main side effects. AIH remission rates were comparable among groups. CONCLUSION: The discontinuation rate of AZA in AIH treatment is ~15% in the first year of treatment. Early or delayed AZA initiation does not differ in remission and discontinuation rates in AIH induction therapy. Our data suggest that either strategy may be used as part of AIH treatment

Feebly-Interacting Particles:FIPs 2020 Workshop Report

With the establishment and maturation of the experimental programs searching for new physics with sizeable couplings at the LHC, there is an increasing interest in the broader particle and astrophysics community for exploring the physics of light and feebly-interacting particles as a paradigm complementary to a New Physics sector at the TeV scale and beyond. FIPs 2020 has been the first workshop fully dedicated to the physics of feebly-interacting particles and was held virtually from 31 August to 4 September 2020. The workshop has gathered together experts from collider, beam dump, fixed target experiments, as well as from astrophysics, axions/ALPs searches, current/future neutrino experiments, and dark matter direct detection communities to discuss progress in experimental searches and underlying theory models for FIPs physics, and to enhance the cross-fertilisation across different fields. FIPs 2020 has been complemented by the topical workshop "Physics Beyond Colliders meets theory", held at CERN from 7 June to 9 June 2020. This document presents the summary of the talks presented at the workshops and the outcome of the subsequent discussions held immediately after. It aims to provide a clear picture of this blooming field and proposes a few recommendations for the next round of experimental results.Comment: 240 pages, 71 figure