Condensed Matter Theory

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Absolutely Stable Spatiotemporal Order in Noisy Quantum Systems

Physical Review Letters American Physical Society

Authors:

Max McGinley, Sthitadhi Roy, Sa Parameswaran
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Active Extensile Stress Promotes 3D Director Orientations and Flows

Physical Review Letters 128 (4), 048001

Authors:

Mehrana R. Nejad, Julia M. Yeomans

Abstract:

We use numerical simulations and linear stability analysis to study an active nematic layer where the director is allowed to point out of the plane. Our results highlight the difference between extensile and contractile systems. Contractile stress suppresses the flows perpendicular to the layer and favors in-plane orientations of the director. By contrast extensile stress promotes instabilities that can turn the director out of the plane, leaving behind a population of distinct, in-plane regions that continually elongate and divide. This supports extensile forces as a mechanism for the initial stages of layer formation in living systems, and we show that a planar drop with extensile (contractile) activity grows into three dimensions (remains in two dimensions). The results also explain the propensity of disclination lines in three dimensional active nematics to be of twist type in extensile or wedge type in contractile materials.
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Active forces in confluent cell monolayers

Authors:

Guanming Zhang, Julia M Yeomans
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Activity pulses induce spontaneous flow reversals in viscoelastic environments

Authors:

Emmanuel LC VI M Plan, Julia M Yeomans, Amin Doostmohammadi
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An ideal Weyl semimetal induced by magnetic exchange

Authors:

J-R Soh, FD Juan, Vergniory, NBM Schröter, MC Rahn, DY Yan, M Bristow, PA Reiss, JN Blandy, YF Guo, YG Shi, TK Kim, A McCollam, SH Simon, Y Chen, AMALIA Coldea, AT Boothroyd

Abstract:

Weyl semimetals exhibit exceptional quantum electronic transport due to the presence of topologically-protected band crossings called Weyl nodes. The nodes come in pairs with opposite chirality, but their number and location in momentum space is otherwise material specific. Following the initial discoveries there is now a need for better material realizations, ideally comprising a single pair of Weyl nodes located at or very close to the Fermi level and in an energy window free from other overlapping bands. Here we propose the layered intermetallic EuCd$_2$As$_2$ to be such a system. We show that Weyl nodes in EuCd$_2$As$_2$ are magnetically-induced via exchange coupling, emerging when the Eu spins are aligned by a small external magnetic field. The identification of EuCd$_2$As$_2$ as a model magnetic Weyl semimetal, evidenced here by ab initio calculations, photoemission spectroscopy, quantum oscillations and anomalous Hall transport measurements, opens the door to fundamental tests of Weyl physics.
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