SciPost Phys. 4, 001 (2018) ·
published 17 January 2018

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The Heisenberg model for S=1/2 describes the interacting spins of electrons
localized on lattice sites due to strong repulsion. It is the simplest
strongcoupling model in condensed matter physics with widespread
applications. Its relevance has been boosted further by the discovery of curate
hightemperature superconductors. In leading order, their undoped parent
compounds realize the Heisenberg model on squarelattices. Much is known about
the model, but mostly at small wave vectors, i.e., for longrange processes,
where the physics is governed by spin waves (magnons), the Goldstone bosons of
the longrange ordered antiferromagnetic phase. Much less, however, is known
for shortrange processes, i.e., at large wave vectors. Yet these processes are
decisive for understanding hightemperature superconductivity. Recent reports
suggest that one has to resort to qualitatively different fractional
excitations, spinons. By contrast, we present a comprehensive picture in terms
of dressed magnons with strong mutual attraction on short length scales. The
resulting spectral signatures agree strikingly with experimental data.
SciPost Phys. 3, 032 (2017) ·
published 29 October 2017

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Topological aspects represent currently a boosting area in condensed matter
physics. Yet there are very few suggestions for technical applications of
topological phenomena. Still, the most important is the calibration of
resistance standards by means of the integer quantum Hall effect. We propose
modifications of samples displaying the integer quantum Hall effect which
render the tunability of the Fermi velocity possible by external control
parameters such as gate voltages. In this way, so far unexplored possibilities
arise to realize devices such as tunable delay lines and interferometers.
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