Strongly angle-dependent magnetoresistance in Weyl semimetals with long-range disorder

The chiral anomaly in Weyl semimetals states that the left- and right-handed Weyl fermions, constituting the low energy description, are not individually conserved, resulting, for example, in a negative magnetoresistance in such materials. Recent experiments see strong indications of such an anomalous resistance response; however, with a response that at strong fields is more sharply peaked for parallel magnetic and electric fields than expected from simple theoretical considerations. Here, we uncover a mechanism, arising from the interplay between the angle-dependent Landau level structure and long-range scalar disorder, that has the same phenomenology. In particular, we ana- lytically show, and numerically confirm, that the internode scattering time decreases exponentially with the angle between the magnetic field and the Weyl node separation in the large field limit, while it is insensitive to this angle at weak magnetic fields. Since, in the simplest approximation, the internode scattering time is proportional to the anomaly-related conductivity, this feature may be related to the experimental observations of a sharply peaked magnetoresistance.Comment: 8 pages, 4 figure

Transversal magnetotransport in Weyl semimetals: Exact numerical approach

Magnetotransport experiments on Weyl semimetals are essential for investigating the intriguing topological and low-energy properties of Weyl nodes. If the transport direction is perpendicular to the applied magnetic field, experiments have shown a large positive magnetoresistance. In this work, we present a theoretical scattering matrix approach to transversal magnetotransport in a Weyl node. Our numerical method confirms and goes beyond the existing perturbative analytical approach by treating disorder exactly. It is formulated in real space and is applicable to mesoscopic samples as well as in the bulk limit. In particular, we study the case of clean and strongly disordered samples.Comment: 10 pages, 4 figure

Moskau und Chicago als Metropolen der Moderne: Sozialer Konflikt und gesellschaftliche Integration 1870-1914

Das WZB-Discussion Paper verwendet Georg Simmels Aufsatz „Die Großstädte und das Geistesleben“ von 1903 als Ausgangspunkt für eine vergleichende Diskussion der sozialen Frage in Moskau und Chicago in der klassischen Moderne. Der vergleichende Blick auf Russland und die USA verdeutlicht, dass die spezifische Perspektive Georg Simmels auf das Großstadtleben jenseits der europäischen Metropolen eher einer Zielvorstellung denn der Beschreibung sozialer Realitäten entsprach. Der durchschnittliche Bewohner von Moskau oder Chicago konnte sich die von Simmel gepriesene „Blasiertheit“ kaum zu eigen machen. „Hass und Kampf“ bestimmten häufig seinen Alltag. In einer segregierten Stadt musste er versuchen, Fuß zu fassen und sich zu behaupten. Das unabhängige Individuum im Sinne Georg Simmels war in diesen Metropolen nur in den Eliten anzutreffen. Eine Gesellschaft autonomer Stadtbürger war bestenfalls im Entstehen begriffen und die zahlreichen gewalttätigen Auseinandersetzungen und letztlich die russische Revolution zeigen, welch fragile Werte Frieden und Zivilität in der modernen Metropole waren. Das Papier gibt einen Überblick über die urbanen Lebenswelten der russischen und der amerikanischen Stadt und über die unterschiedlichen Versuche sozialer Reform.This WZB Discussion Paper takes Georg Simmel’s classic 1903 essay, “The Metropolis and Mental Life,” as the starting point for a comparative analysis of social issues and city reform in Moscow and Chicago in the period of classic modernity. Scrutinizing each of these cities, it becomes clear that many of Simmel’s observations about the European metropolis did not hold true in these countries. The average Muscovite or Chicagoan was hardly in a position to adopt the “blasé attitude” attributed to the average European, characterized by a kind of cool, remote, and intellectualized approach to things. To the contrary, Muscovites and Chicagoans during this period were struggling for survival in urban environments where authority was often absent or corrupt, and where physical violence shaped everyday life. The “urbane” city dweller, as described by Simmel, could only be found among elites. In densely populated, largely segregated cities with anonymous and estranged citizenries, peace and civility remained fragile, to which the upheavals of the Russian revolution and the Chicago riots dramatically testify. This paper provides an overview of everyday life in Moscow and Chicago, and considers different approaches to social reform in the United States and Russia

Nodal-line semimetals from Weyl superlattices

The existence and topological classification of lower-dimensional Fermi surfaces is often tied to the crystal symmetries of the underlying lattice systems. Artificially engineered lattices, such as heterostructures and other superlattices, provide promising avenues to realize desired crystal symmetries that protect lower-dimensional Fermi surface, such as nodal lines. In this work, we investigate a Weyl semimetal subjected to spatially periodic onsite potential, giving rise to several phases, including a nodal-line semimetal phase. In contrast to proposals that purely focus on lattice symmetries, the emergence of the nodal line in this setup does not require small spin-orbit coupling, but rather relies on its presence. We show that the stability of the nodal line is understood from reflection symmetry and a combination of a fractional lattice translation and charge-conjugation symmetry. Depending on the choice of parameters, this model exhibits drumhead surface states that are exponentially localized at the surface, or weakly localized surface states that decay into the bulk at all energies.Comment: 11 pages, 8 figures, Editors' Suggestio

Theoretical Description of Pulsed RYDMR: Refocusing Zero-Quantum and Single Quantum Coherences

A theoretical description of pulsed reaction yield detected magnetic resonance (RYDMR) is proposed. In RYDMR, magnetic resonance spectra of radical pairs (RPs) are indirectly detected by monitoring their recombination yield. Such a detection method is significantly more sensitive than conventional electron paramagnetic resonance (EPR), but design of appropriate pulse sequences for RYDMR requires additional effort because of a different observable. In this work various schemes for generating spin-echo like signals and detecting them by RYDMR are treated. Specifically, we consider refocusing of zero-quantum coherences (ZQCs) and single-quantum coherences (SQCs) by selective as well as by non-selective pulses and formulate a general analytical approach to pulsed RYDMR, which makes an efficient use of the product operator formalism. We anticipate that these results are of importance for RYDMR studies of elusive paramagnetic particles, notably, in organic semiconductors

Photoexcited triplet states of twisted acenes investigated by Electron Paramagnetic Resonance

Twisting of the acene backbone out of planarity in twisted acenes leads to a variation in their optical and electronic properties. The effect of increasing twist angles on the properties of the photoexcited triplet states of a series of anthracene-based helically tethered twisted acenes is investigated here by Electron Paramagnetic Resonance (EPR) spectroscopy. Increasing signal intensities with increasing twist angles indicate increased intersystem crossing efficiencies for the twisted molecules compared to the untethered reference compound. Variations in the electron spin polarisation observed in the transient EPR spectra, in particular for the compound with the shortest tether, imply changes in the sublevel population kinetics depending on molecular geometry. Changes in the zero-field splitting parameters and in the proton hyperfine couplings for compounds with short tethers and therefore higher twist angles point towards a slight redistribution of the spin density compared to the parent compound. The experimental results can be explained by considering both an increase in twist angle and a related decrease in the dihedral angle between the phenyl side groups and the acene core. The observation of a clear excitation-wavelength dependence suggests preferential excitation of different molecular conformations, with conformers characterised by higher twist angles selected at higher wavelengths

Transient electrically detected magnetic resonance spectroscopy applied to organic solar cells

The influence of light-induced paramagnetic states on the photocurrent generated by polymer:fullerene solar cells is studied using spin-sensitive techniques in combination with laser-flash excitation. For this purpose, we developed a setup that allows for simultaneous detection of transient electron paramagnetic resonance as well as transient electrically detected magnetic resonance (trEDMR) signals from fully processed and encapsulated solar cells. Combining both techniques provides a direct link between photoinduced triplet excitons, charge transfer states, and free charge carriers as well as their influence on the photocurrent generated by organic photovoltaic devices. Our results obtained from solar cells based on poly(3-hexylthiophene) as electron donor and a fullerene-based electron acceptor show that the resonant signals observed in low-temperature (T = 80 K) trEDMR spectra can be attributed to positive polarons in the polymer as well as negative polarons in the fullerene phase, indicating that both centers are involved in spin-dependent processes that directly influence the photocurrent

Higher triplet state of fullerene C70 revealed by electron spin relaxation

Spin-lattice relaxation timesT1 of photoexcited triplets 3C70 in glassy decalin were obtained from electron spin echo inversion recovery dependences. In the range 30–100 K, the temperature dependence of T1 was fitted by the Arrhenius law with an activation energy of 172 cm−1. This indicates that the dominant relaxation process of 3C70 is described by an Orbach-Aminov mechanism involving the higher triplet state t2 which lies 172 cm−1 above the lowest triplet state t1. Chemical modification of C70fullerene not only decreases the intrinsic triplet lifetime by about ten times but also increases T1 by several orders of magnitude. The reason for this is the presence of a low-lying excited triplet state in 3C70 and its absence in triplet C70 derivatives. The presence of the higher triplet state in C70 is in good agreement with the previous results from phosphorescence spectroscopy

The Post-Merger Magnetized Evolution of White Dwarf Binaries: The Double-Degenerate Channel of Sub-Chandrasekhar Type Ia Supernovae and the Formation of Magnetized White Dwarfs

Type Ia supernovae (SNe Ia) play a crucial role as standardizable cosmological candles, though the nature of their progenitors is a subject of active investigation. Recent observational and theoretical work has pointed to merging white dwarf binaries, referred to as the double-degenerate channel, as the possible progenitor systems for some SNe Ia. Additionally, recent theoretical work suggests that mergers which fail to detonate may produce magnetized, rapidly-rotating white dwarfs. In this paper, we present the first multidimensional simulations of the post-merger evolution of white dwarf binaries to include the effect of the magnetic field. In these systems, the two white dwarfs complete a final merger on a dynamical timescale, and are tidally disrupted, producing a rapidly-rotating white dwarf merger surrounded by a hot corona and a thick, differentially-rotating disk. The disk is strongly susceptible to the magnetorotational instability (MRI), and we demonstrate that this leads to the rapid growth of an initially dynamically weak magnetic field in the disk, the spin-down of the white dwarf merger, and to the subsequent central ignition of the white dwarf merger. Additionally, these magnetized models exhibit new features not present in prior hydrodynamic studies of white dwarf mergers, including the development of MRI turbulence in the hot disk, magnetized outflows carrying a significant fraction of the disk mass, and the magnetization of the white dwarf merger to field strengths $\sim 2 \times 10^8$ G. We discuss the impact of our findings on the origins, circumstellar media, and observed properties of SNe Ia and magnetized white dwarfs.Comment: Accepted ApJ version published on 8/20/13, with significant additional text added discussing the nature of the magnetized outflows, and possible CSM observational features relevant to NaID detection

Compact electrically detected magnetic resonance setup

Electrically detected magnetic resonance (EDMR) is a commonly used technique for the study of spin-dependent transport processes in semiconductor materials and electro-optical devices. Here, we present the design and implementation of a compact setup to measure EDMR, which is based on a commercially available benchtop electron paramagnetic resonance (EPR) spectrometer. The electrical detection part uses mostly off-the-shelf electrical components and is thus highly customizable. We present a characterization and calibration procedure for the instrument that allowed us to quantitatively reproduce results obtained on a silicon-based reference sample with a “large-scale” state-of- the-art instrument. This shows that EDMR can be used in novel contexts relevant for semiconductor device fabrication like clean room environments and even glove boxes. As an application example, we present data on a class of environment-sensitive objects new to EDMR, semiconducting organic microcrystals, and discuss similarities and differences to data obtained for thin-film devices of the same molecule