Time-resolved collapse and revival of the Kondo state near a quantum phase transition

One of the most successful paradigms of many-body physics is the concept of quasiparticles: excitations in strongly interacting matter behaving like weakly interacting particles in free space. Quasiparticles in metals are very robust objects. Yet, when a system's ground state undergoes a qualitative change at a quantum critical point (QCP), the quasiparticles may disintegrate and give way to an exotic quantum-fluid state of matter. The nature of this breakdown is intensely debated, because the emergent quantum fluid dominates the material properties up to high temperature and might even be related to the occurence of superconductivity in some compounds. Here we trace the dynamics of heavy-fermion quasiparticles in CeCu$_{6-x}$Au$_{x}$ and monitor their evolution towards the QCP in time-resolved experiments, supported by many-body calculations. A terahertz pulse disrupts the many-body heavy-fermion state. Under emission of a delayed, phase-coherent terahertz reflex the heavy-fermion state recovers, with a coherence time 100 times longer than typically associated with correlated metals. The quasiparticle weight collapses towards the QCP, yet its formation temperature remains constant -- phenomena believed to be mutually exclusive. Coexistence in the same experiment calls for revisions in our view on quantum criticality.Comment: Published version, including data on CeCu6, CeCu5.9Au0.1, and CeCu5Au1 and extended Supplementary Information. 7 pages, 4 figures, Supplementary Information: 5 pages, 3 figure

Tricritical behaviour of Ising spin glasses with charge fluctuations

We show that tricritical points displaying unusal behaviour exist in phase diagrams of fermionic Ising spin glasses as the chemical potential or the filling assumes characteristic values. Exact results for infinite range interaction and a one loop renormalization group analysis of thermal tricritical fluctuations for finite range models are presented. Surprising similarities with zero temperature transitions and a new $T=0$ tricritical point of metallic quantum spin glasses are derived.Comment: 4 pages, 1 Postscript figure, minor change

Fermionic random transverse-field Ising spin chain

The interplay of spin and charge fluctuations in the random transverse-field Ising spin chain on the fermionic space is investigated. The finite chemical potential, which controls the charge fluctuations, leads to the appearance of the quantum critical region in the phase diagram where the magnetic correlations are quenched by nonmagnetic sites. Regions of nonmonotonous temperature dependence of spin-spin correlation length appear at nonzero $\mu$. The results on the one-fermion density of states of the model are discussed.Comment: 10 pages, 2 figures included, citation correcte

conInvestigation of Sr2RuO4-Pt tacts in an applied magnetic field

We investigated the spin-triplet superconductor Sr 2 RuO 4 by means of conductance measurements of normal conductor/ superconductor point contacts. We present results that show a linear dependence of the excess current as a function of temperature and applied magnetic fields over a wide range of the phase diagram. Applying a magnetic field larger than the upper critical field B c2 suppresses completely the features related to superconductivity and a conductance minimum at V = 0 occurs which becomes more pronounced with increasing field and persists even above T c up to several K. \ua9 2003 Elsevier B.V. All rights reserved

conInvestigation of Sr2RuO4-Pt tacts in an applied magnetic field

We investigated the spin-triplet superconductor Sr 2 RuO 4 by means of conductance measurements of normal conductor/ superconductor point contacts. We present results that show a linear dependence of the excess current as a function of temperature and applied magnetic fields over a wide range of the phase diagram. Applying a magnetic field larger than the upper critical field B c2 suppresses completely the features related to superconductivity and a conductance minimum at V = 0 occurs which becomes more pronounced with increasing field and persists even above T c up to several K. \ua9 2003 Elsevier B.V. All rights reserved

Physikalisch-technische Eigenschaften oxidischer Supraleiter und ihre potentiellen Anwendungen in der Mikroelektronik Schlussbericht

With 121 refs., 6 tabs., 85 figs.SIGLEAvailable from TIB Hannover: F93B455+a / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman