The ground state of a mixed-valence compound is a quantum mechanical mixture of two or more electronic configurations as explained phenomenologically by the valence fluctuation model or theoretically by the one-impurity Anderson model and shown experimentally by e. The most elegant way to tune T K is the application of pressure P.
T K decreases increases with P for mixed valent Yb or Sm Ce compounds and drives the materials towards a trivalent tetravalent state.
The latter techniques e. Dallera et al. The lines T and T m which represent, respectively, the crossover temperatures to a low temperature semiconducting phase and a magnetically-ordered state need still to be determined. Nowadays it appears that the temperature-pressure phase diagram of SmS Figure 57 is much more intricate, particularly concerning the nature of the golden phase.
At low temperature below T and up to 20 kbar the high pressure SmS phase actually behaves like a semiconductor but with a small energy gap several meV often named hybridisation gap,. Trivalent Sm ions are expected to be stabilised above 20 kbar in the metallic phase and band structure calculations predict the occurrence of a magnetic ground state at T m although all attempts have failed to find magnetic order and thus a QCP.
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The valence transition at P c also manifests itself in lattice dynamics, investigated in the past by neutron scattering in a restricted pressure range up to 7 kbar and extended to 80 kbar by Raymond et al. These measurements enabled the characterisation of novel phonon anomalies in the longitudinal acoustic mode. T m is the ordering temperature and JN is the f-d exchange coupling times the f density of states at the Fermi energy.
When the valence of the f-ion is an integer or close to and the corresponding total angular momentum of the ion is non-zero i. The most interesting situation occurs in the heavy fermion regime where the compound is near a magnetic instability, i.
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In these systems the strong interaction between electrons leads to a very large effective mass of the quasiparticles hence the term heavy fermion and the large spin fluctuations which develop at the QCP can induce a breakdown of the conventional Fermi liquid FL theory e. The effect of pressure was investigated by Barla et al. They showed that U In 0. NFL behaviour n 2 is observed just at P c. The superconductivity observed in heavy fermions is unconventional in the sense that the pairing mechanism of the Cooper pairs cannot be explained by the standard Bardeen-Cooper-Schriffer BCS theory, which involves an attractive interaction between the electrons mediated by lattice vibrations.
Nowadays it is generally accepted that the superconductivity in heavy fermions arises from magnetically-mediated pairing. Moreover, when the magnetic correlations are ferromagnetic UGe 2 , URhGe  , pairing must result from electrons with parallel spin orientation triplet superconductivity. The 39 K superconductivity discovered in MgB 2 seems to require a phonon mediated pairing mechanism .
This new superconductor differs in several important ways from ordinary metallic superconductors e.
The determination of the phonon dispersion and lifetimes in MgB 2 by Shukla et al. References  H. Gschneidner Jr. Eyring, G. Lander and G. Chopin Eds , North Holland, Amsterdam, Mathur, F. Grosche, S. Cookies are used by this site. To decline or learn more, visit our Cookies page. Knovel offers following tools to help you find materials and properties data Material Property Search. Also known as Data Search, find materials and properties information from technical references.
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Physics of Dilute Magnetic Alloys Details Available for the first time in English, this classic text describes the Kondo effect thoroughly and intuitively. Its clear and concise treatment makes this book of interest to graduate students and researchers in condensed matter physics. The first half of the book describes the rudiments of the theory of metals at a level that is accessible for undergraduate students. The second half discusses key developments in the Kondo problem, covering topics including magnetic impurities in metals, the resistance minimum phenomenon, infrared divergence in metals and scaling theory, including Wilson's renormalization group treatment and the exact solution by the Bethe ansatz.
A new chapter has been added covering advances made since the Japanese edition was published, such as the quantum dot and heavy fermion systems. Show less. View More. Back to Table of Contents.
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