Ad Quantenphasenübergänge

Bei konventionellen Phasenübergängen wie z.B. dem Curiepunkt von Eisen wird die makroskopische Ordnung durch thermische Fluktuationen zerstört. Makroskopische Ordnung kann aber auch exakt am Temperaturnullpunkt zerstört werden, und zwar durch Quantenfluktuationen, die eine Folge der Heisenbergschen Unschärferelation sind.

Die Tieftemperatureigenschaften (Spezifische Wärme, Widerstand, usw.) von Systemen mit Anzeichen für quantenkritisches Verhalten ziehen derzeit breites Interesse an, insbesondere durch unerwartete Phänomene wie unkonventionelle Supraleitung. Auch die Hochtemperatursupraleitung wird in diesem Zusammenhang diskutiert.

Quantenphasenübergänge




















The family of RT9X4 compounds
The family of tetragonal (variant of the cubic NaZn13-type) compounds, RT9X4, has been very fertile in revealing a wide spectrum of important features including heavy fermion non-Fermi liquid behavior, model type Kondo lattice behavior, itinerant electron metamagnetism with indications for metamagnetic quantum criticality and others which are the subject of on-going research.
  • CeNi9Ge4 shows unusual non-Fermi liquid behaviour of the specific heat over at least two decades of temperature with a record value of the electronic specific heat coeficient γ at low temperature:γ ~ 6 J/molK2 at 80mK in this system without any sign of magnetic order. The ground state properties are currently studied by neutron and muon spectroscopy and single crystal investigations.
  • CeNi9Si4 is an exceptional system which exhibits a model type Kondo lattice behavior almost perfectly well described in terms of the degenerate (J = 5/2) Coqblin-Schrieffer model. It is hitherto the first Kondo lattice system where the thermoelectric power S(T) shows indeed close agreement with earlier theoretical results of the degenerate Anderson lattice without intersite interactions.
  • The observation of weak itinerant ferromagnetism in LaCo13-xSix with Tc approaching zero temperature approximately at the stoichiometric composition 1-9-4 suggests that LaCo9Si4 may be at or nearby a ferromagnetic QCP. A detailed investigation of this compound comprising magnetic, specific heat, and NMR measurements as well as ab initio electronic structure calculations, in fact revealed strongly exchange enhanced Pauli paramagnetism at magnetic fields below 3T and an itinerant metamagnetic phase transition with indications for metamagnetic quantum criticality at a critical field which is about 3.5T for H parallel and 6T for H perpendicular to the tetragonal c-axis. These critical fields for the appearance of metamagnetism are the lowest values ever found for rare earth intermetallic compounds.

Fig.2: The tetragonal crystal structure (I4/mcm)

  • Unconventional Superconductors


    • Recent Papers related to Strongly Correlated Electron Systems

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