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

Observation of precursor pair formation of recombining charge carriers

Journal ArticleAn experiment is presented which allows the observation of charge-carrier pair formation that precedes electronic transitions such as spin-dependent recombination or spin-dependent transport. It is based on an electrically detected magnetic-resonance-induced rotary echo sequence. The experimental demonstration is performed on precursor (spin) pairs of electrons in the emitter layer of crystalline silicon/amorphous silicon heterostructures. Precursor pair-generation-rate coefficients extracted from these measurements are studied as a function of light intensity and are found to show only a minor dependence on the illumination level indicating that the pair generation is not determined by charge-carrier densities

Supersymmetry in the Standard Sachdev-Ye-Kitaev Model.

Supersymmetry is a powerful concept in quantum many-body physics. It helps to illuminate ground-state properties of complex quantum systems and gives relations between correlation functions. In this Letter, we show that the Sachdev-Ye-Kitaev model, in its simplest form of Majorana fermions with random four-body interactions, is supersymmetric. In contrast to existing explicitly supersymmetric extensions of the model, the supersymmetry we find requires no relations between couplings. The type of supersymmetry and the structure of the supercharges are entirely set by the number of interacting Majorana modes and are thus fundamentally linked to the model's Altland-Zirnbauer classification. The supersymmetry we uncover has a natural interpretation in terms of a one-dimensional topological phase supporting Sachdev-Ye-Kitaev boundary physics and has consequences away from the ground state, including in q-body dynamical correlation functions

Framing fusion and fission

Engineering inter-triplet exchange coupling allows spin mixing between singlet and quintet manifolds in triplet–triplet pair states in metal–organic frameworks, demonstrating increased room-temperature triplet-fusion rates under relatively small applied magnetic fields

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

Coherent error threshold for surface codes from Majorana delocalization

Statistical mechanics mappings provide key insights on quantum error correction. However, existing mappings assume incoherent noise, thus ignoring coherent errors due to, e.g., spurious gate rotations. We map the surface code with coherent errors, taken as $X$- or $Z$-rotations (replacing bit or phase flips), to a two-dimensional (2D) Ising model with complex couplings, and further to a 2D Majorana scattering network. Our mappings reveal both commonalities and qualitative differences in correcting coherent and incoherent errors. For both, the error-correcting phase maps, as we explicitly show by linking 2D networks to 1D fermions, to a $\mathbb{Z}_2$-nontrivial 2D insulator. However, beyond a rotation angle $\phi_\text{th}$, instead of a $\mathbb{Z}_2$-trivial insulator as for incoherent errors, coherent errors map to a Majorana metal. This $\phi_\text{th}$ is the theoretically achievable storage threshold. We numerically find $\phi_\text{th}\approx0.14\pi$. The corresponding bit-flip rate $\sin^2(\phi_\text{th})\approx 0.18$ exceeds the known incoherent threshold $p_\text{th}\approx0.11$.Comment: 9 pages, 7 figure