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Exchange Narrowing Effects in the EPR Linewidth of Gd Diluted in Ce Compounds
Anomalous thermal behavior on the EPR linewidths of Gd impurities diluted in
Ce compounds has been observed. In metals, the local magnetic moment EPR
linewidth, \Delta H, is expected to increase linearly with the temperature. In
contrast, in Ce_{x}La_{1-x}Os_{2} the Gd EPR spectra show a nonlinear increase.
In this work, the mechanisms that are responsible for the thermal behavior of
the EPR lines in Ce_{x}La_{1-x}Os_{2} are examined. We show that the exchange
interaction between the local magnetic moments and the conduction electrons are
responsible for the narrowing of the spectra at low temperatures. At high
temperatures, the contribution to the linewidth of the exchange interaction
between the local magnetic moments and the ions has an exponential
dependence on the excitation energy of the intermediate valent ions. A complete
fitting of the EPR spectra for powdered samples is obtained.Comment: 11 pages, 1 figur
Gauge invariance of the background average effective action
Using the background field method for the functional renormalization group
approach in the case of a generic gauge theory, we study the background field
symmetry and gauge dependence of the background average effective action, when
the regulator action depends on external fields. The final result is that the
symmetry of the average effective action can be maintained for a wide class of
regulator functions, but in all cases the dependence of the gauge fixing
remains on-shell. The Yang-Mills theory is considered as the main particular
example.Comment: Fits the version accepted in EPJ
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