Revisiting the EFG at Cd sites in metals: TDPAC experiments, APW+lo calculations and the nuclear quadrupole moment of 245 keV excited state of 111Cd

L. A: ERRICO; S. COTTENIER; M. RENTERÍA

Ginebra, Suiza

Conferencia; 3rd Joint International Conference on Hyperfine Interactions & International Symposium on Nuclear Quadrupole Interaction; 2010

CERN

Trabajo presentado en forma de poster durante la conferencia Several nuclear experimental techniques including nuclear magnetic resonance, nuclear, quadrupolar resonance (NQR), time-differential perturbed-angular correlations (TDPAC), Mossbauer spectroscopy, among others, are widely use for the study of the properties of solids through the determination of the electric-field-gradient (EFG) tensor at the position of a probe nucleus. All these nuclear experimental techniques do not determine directly the EFG but rather one or more characteristic nuclear quadrupole resonance frequencies νQ, which are proportional to the EFG and to the nuclear quadrupole moment Q. Provided an exact value of Q for the involved nuclear state involved is known, experimental EFG information can be deduced. However, in spite of the wide use of the experimental techniques, for many important probe nuclei the exact Q value is not known with sufficient accuracy and/or precision. Usually ratios between the quadrupole moments of different isotopes of the same element are easier to experimentally access than the quadrupole moment itself. This is a serious problem because all the information that νQ can provide about the system under study can be obtained only by confrontation with calculations of the EFG: EFG calculations assuming different structural and electronic scenarios are usually performed and compared with experimental results in order to characterize the system under study. Of course, accurate values of Q are essential for a correct comparison between the experimentally “determined” EFG and its theoretical prediction. For this reason, the knowledge of reliable Q values is of great importance in atomic, molecular, and condensed-matter physics, besides the direct interest in nuclear physics, where the determination of Q values can be used to check nuclear models.                 Ideally, theories of the nuclear state should provide values of the nuclear quadrupole moments but often the uncertainties involved are larger than what can be accepted for the interpretation of nuclear spectroscopy experiments. Another approach involves the calculation of EFG’s by ab initio electronic structure methods and comparison with experimental values of νQ frequencies.  Following this approach we present a determination of the nuclear-quadrupole moment of the I=5/2+ 245 keV excited state of 111Cd. This isotope and this particular state is the most frequently used tracer in TDPAC experiments and has been largely applied to study semiconductor physics, and a large amount of experimental work has been focused on the EFG characterization at 111Cd impurity sites in metals, semiconducting, and insulating compounds. Although the importance of this isotope, the Q value of the sensitive state is known with limited precision (i.e, with a relative error of 17%).        In this work we present an ab initio determination of the nuclear-quadrupole moment of the I=5/2+ 245 keV excited state of 111Cd. This determination was obtained calculating the EFG at Cd sites in metallic systems and using the common procedure of correlating ab initio calculated EFGs and experimentally determined quadrupole coupling constants measured with high precision. Calculations were performed with the Full-Potential Augmented Plane Waves plus Local orbitals (APW+lo) method, considering different approximations for the exchange and correlation potential.  In all cases, for a better comparison with the calculations, experimental results at very low temperatures (in the order of 4 K) were considered. Our Q value [Q(5/2+,  111Cd) = 0.70(5) b] is in excellent agreement with those reported in the literature [Q=0.83(13) b] but is obtained with largely better precision. The results obtained for the case of semiconducting and insulating systems is also discussed.