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When a magnetic phase transition in a metal is driven to absolute zero
temperature by a non-thermal control parameter, such as
pressure or composition, the quantum fluctuations
of the order parameter induce new universality classes
and, possibly, new ground states of matter.
Experiments suggest that near such a quantum phase transition
(QPT) in heavy-fermion (HF) compounds, notably CeCu6-xAux,
the HF quasiparticles, formed by the Kondo effect below the Kondo
temperature TK, disintegrate.
The conditions for this breakdown have, however, remained obscure.
We propose a new criterion for distinguishing the Hertz-Millis (HM)
and the local quantum critical (LQC) mechanism in heavy-fermion systems
with a magnetic quantum phase transition (QPT) [1,2]. The criterion is
based on
our finding that the complete spin screening of Kondo ions can be
suppressed by the RKKY coupling to the surrounding magnetic ions even
without
magnetic ordering and that, consequently, the signature of this
suppression
can be observed in spectroscopic measurements above the magnetic
ordering temperature. Applying the criterion to high-resolution
photoemission measurements on CeCu6-xAux [1,3] suggests that
the QPT in this system is dominated by the LQC scenario, in agreement
with previous experiments.
[1] M. Klein, A. Nuber, F. Reinert, J. Kroha, O. Stockert and H. v. Löhneysen, Phys. Rev Lett. 101, 266404 (2008). [2] J. Kroha, M. Klein, A. Nuber, F. Reinert, O. Stockert and H. v. Löhneysen, J. Phys. Cond. Mat. 22, 164203 (2010). [3] M. Klein, J. Kroha, H. v. Löhneysen, O. Stockert, and F. Reinert, Phys. Rev. B 79, 075111 (2009). |
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