# Highlights

Publication Highlights

### Emerging local Kondo screening and spatial coherence in the heavy-fermion metal YbRh$_2$Si$_2$

The entanglement of quantum states is both a central concept in fundamental physics and a potential tool for realizing advanced materials and applications. The quantum superpositions underlying entanglement are at the heart of the intricate interplay of localized spin states and itinerant electronic states that gives rise to the Kondo effect in certain dilute magnetic alloys. In systems where the density of localized spin states is sufficiently high, they can no longer be treated as non-interacting; if they form a dense periodic array, a Kondo lattice may be established1. Such a Kondo lattice gives rise to the emergence of charge carriers with enhanced effective masses, but the precise nature of the coherent Kondo state responsible for the generation of these heavy fermions remains highly debated. Here we use atomic-resolution tunnelling spectroscopy to investigate the low-energy excitations of a generic Kondo lattice system, YbRh$_2$Si$_2$. We find that the hybridization of the conduction electrons with the localized 4f electrons results in a decrease in the tunnelling conductance at the Fermi energy. In addition, we observe unambiguously the crystal-field excitations of the Yb$^{3+}$ ions. A strongly temperature-dependent peak in the tunnelling conductance is attributed to the Fano resonance resulting from tunnelling into the coherent heavy-fermion states that emerge at low temperature. Taken together, these features reveal how quantum coherence develops in heavy 4f-electron Kondo lattices. Our results demonstrate the efficiency of real-space electronic structure imaging for the investigation of strong electronic correlations, specifically with respect to coherence phenomena, phase coexistence and quantum criticality. S. Ernst, S. Kirchner, C. Krellner, C. Geibel, G. Zwicknagel, F. Steglich & S. Wirth Nature 474, 362 (2011)
Institute's News

### New research group 'Topology and Correlations in Condensed Matter'

The research group is headed by Dr. Frank Pollmann and focuses on the study of topological phases of matter, the dynamics in disordered systems, and the applications of quantum information concepts to quantum many-body systems.
Awards and Honors

### EMBO Young Investigator Award 2010

Dr. Stephan Grill EMBO chooses some of the best young group leaders in Europe, through a highly competitive annual selection to join the EMBO Young Investigator programme. During their three-year tenure in the programme, EMBO Young Investigators receive a range of financial and practical benefits. These support them during a critical stage of their career, and help them realize their potential as world-class researchers.
Awards and Honors

### Gustav-Hertz-Preis 2010 der DPG

Thomas Pohl vom Dresdner Max-Planck-Institut für Physik komplexer Systeme erhält den mit 7.500 Euro dotierten „Gustav-Hertz-Preis“ der Deutschen Physikalischen Gesellschaft. Der 31-jährige Wissenschaftler wird für seine theoretischen Arbeiten zur Erzeugung von Anti-Wasserstoff ausgezeichnet. Dieses künstlich hergestellte Element ist das Pendant zum natürlich vorkommenden Wasserstoff. Es besteht aus Antimaterie: nämlich aus einem elektrisch negativ geladenen Anti-Proton und einem positiv geladenen Positron. Das Interesse für Anti-Wasserstoff rührt daher, dass sich an derlei künstlich hergestellten Atomen fundamentale Theorien der Physik überprüfen lassen.
Institute's News

### New research group 'Dynamical Systems and Social Dynamics'

The Otto Hahn Group is headed by Dr. Eduardo Altmann and applies complexity theory to data of human activities.
Publication Highlights

### Coupling a sensory hair-cell bundle to cyber clones enhances nonlinear amplification

The vertebrate ear benefits from nonlinear mechanical amplification to operate over a vast range of sound intensities. The amplificatory process is thought to emerge from active force production by sensory hair cells. The mechano-sensory hair bundle that protrudes from the apical surface of each hair cell can oscillate spontaneously and function as a frequency-selective, nonlinear amplifier. Intrinsic fluctuations, however, jostle the response of a single hair bundle to weak stimuli and seriously limit amplification. Most hair bundles are mechanically coupled by overlying gelatinous structures. Here, we assayed the effects of mechanical coupling on the hair-bundle amplifier by combining dynamic force clamp of a hair bundle from the bullfrog’s saccule with real-time stochastic simulations of hair-bundle mechanics. This setup couples the hair bundle to two virtual hair bundles, called cyber clones, and mimics a situation in which the hair bundle is elastically linked to two neighbors with similar characteristics. We found that coupling increased the coherence of spontaneous hair-bundle oscillations. By effectively reducing noise, the synergic interplay between the hair bundle and its cyber clones also enhanced amplification of sinusoidal stimuli. All observed effects of coupling were in quantitative agreement with simulations. We argue that the auditory amplifier relies on hair-bundle cooperation to overcome intrinsic noise limitations and achieve high sensitivity and sharp frequency selectivity. Jérémie Barral, Kai Dierkes, Benjamin Lindner, Frank Jülicher, Pascal Martin PNAS 107 (18), 8079 (2010)
Awards and Honors

### Stephan Grill wird mit dem Arches Award 2009 der Minerva Stiftung ausgezeichnet

Der Forschungspreis 'Award for Research Cooperation and High Excellence in Science' zeichnet deutsch-israelische Kooperationen aus und wird vom Bundesministerium für Bildung und Forschung finanziert.
Awards and Honors

### Prof. Dr. Peter Fulde wird mit dem Tsungming Tu Award des National Science Councils, Taiwan geehrt

Der Tsungming-Tu-Preis wird verliehen vom National Science Councils Taiwan und ist die höchste akademische Ehre für Ausländer in Taiwan. Der erstmals 2007 verliehene Preis wird jährlich an höchstens drei Personen für außerordentliche Verdienste oder Entdeckungen in einem Forschungsgebiet vergeben und ist mit einem Preisgeld von zwischen 50.000 und 100.000 US-Dollar dotiert. Namensgeber ist der taiwanische Mediziner Tsungming Tu (1893–1986). Partnerorganisation des NSC für den Preis ist die Alexander von Humboldt-Stiftung.
### Magnetically driven superconductivity in CeCu$_2$Si$_2$
The origin of unconventional superconductivity, including high-temperature and heavy-fermion superconductivity, is still a matter of controversy. Spin excitations instead of phonons are thought to be responsible for the formation of Cooper pairs. Using inelastic neutron scattering, we present the first in-depth study of the magnetic excitation spectrum in momentum and energy space in the superconducting and the normal states of CeCu$_2$Si$_2$. A clear spin excitation gap is observed in the superconducting state. We determine a lowering of the magnetic exchange energy in the superconducting state, in an amount considerably larger than the superconducting condensation energy. Our findings identify the antiferromagnetic excitations as the main driving force for superconducting pairing in this prototypical heavy-fermion compound located near an antiferromagnetic quantum critical point. O. Stockert, J. Arndt, E. Faulhaber, C. Geibel, H.S. Jeevan, S. Kirchner, M. Loewenhaupt. K. Schmalz, W. Schmidt, Q. Si & F. Steglich Nature Physics 7, 119 (2010)