picture of the moon's surface. It is equivalent to a crater 20 m in diameter and 6 m deep which, according to the Ranger photographs, should occur on the average with a frequency of one crater per 10 000 m 2 . 4 Since the energy partition ratio y as well as r depends on kz, a detailed analysis of the seismograph response with time as a function of frequency should give information about the distribution in depth of the principal obstacles and tell whether one is dealing with surface craters or heavy meteorites embedded at some depth. That roughness could have a bigger effect on surface waves on the moon than on the earth is of course not unreasonable: The absence of atmospheric weathering and the larger number of embedded meteorites would make the moon's surface much rougher on the scale of tens of meters.An alternative explanation of the duration of seismic signals is that the original disturbances set off secondary events due to instabilities of the lunar surface. The trapping of surface-wave energy proposed by us would also tend to enhance the period during which secondary disturbances could occur. In that case, however, the spectral decay should depart significantly from Eq. (4).A new electronic configuration involving localized 6s electrons is hypothesized for the Sm ions in SmB 6 . The model is successful in explaining earlier results of susceptibility measurements and new Mossbauer spectroscopy data reported here.Recently, a number of reports 1 " 3 discussing the unusual electric and magnetic properties of SmB 6 have appeared. This paper presents new data, obtained by Mossbauer-effect measurements on Sm 149 , which help establish the electronic structure of the Sm ions. The results of the Mossbauer experiments are combined with the previously published information to develop a new model which seems to explain the observed properties.Briefly outlining the results reported in Ref. 1, the electrical characteristics of the material appear to be semiconducting, with resistance sharply increasing down to 3°K, and thereafter increasing only very slowly. The system does not order magnetically down to below 0.35°K, and the temperature dependence of the susceptibility is complex and not consistent with what would be expected from either Sm 2+ or Sm 3+ ions. The earlier work attributed the unusual behavior to a thermally excited electronic transition; the Sm ions were considered to be divalent at low temperature and trivalent at high temperature.This electronic transition has been used as the basis for recent theoretical articles 2 ' 3 discussing the temperature dependence of the conductivity. We present here a different analysis of the SmB 6 electronic structure.The Mossbauer isomer shift measures the density of s electrons at the Sm nucleus. In rareearth ions, a change in the number of 4/electrons results in a changed shielding of the s electrons (especially the two 5s electrons), so that an increase in the number of 4/'s increases the shielding and decreases the net s density at the nucleus. This shielding effect ...
PHYSICAL REVIEW LETTERS 21 FEBRUARY 1972 7 D. W. Ross, unpublished calculation (cf. Ref. 3, Appendix C). The important point is that the resonant contribution to the growth or damping rate contains the average along a field line of $ cos(moJ b Idl/v H) which is small because of the oscillating cosine factor.The nature of the electronic state of transition metals and alloys remains a subject of discussion. 1 There are mainly two quite distinct approaches to this problem: The first is the rigidband model which makes no distinction between the different constituents of the alloy and attributes a common band to all electrons. The second is the virtual-bound-state model (preferred for dilute impurities) which assumes screening of the host electrons at the site of the impurity. Neither of these two limiting models can fully describe the actual situation. Though many experimental facts favor the virtual-bound-state model, its application is difficult in the case of concentrated alloys, especially when the magnetic electrons have much itinerant character as, for example, in CuNi alloys. 2 Soven 3 and Velicky, Kirkpatrick, and Ehrenreich 4 have therefore used the coherent-potential approximation (CPA) to describe the electronic properties of binary alloys. The results of these calculations are very promising, and invite comparison with experimental results.The CuNi system may be regarded as one of the test cases for theoretical descriptions of magnetic alloys. 5 This system had long been considered the prototype alloy whose magnetic properties are explained by the rigid-band model. Yet simple theoretical considerations 6 * 7 and much recent experimental evidence 5 ' 8 " 12 show the inadequacy of that model for the CuNi system. Ehrenreich and co-workers 4 ' 6t 7 * 13 find that an approach based on the CPA (which they call the minimum polarity model) does indeed account much more satisfactorily for the experimental observations. Recently Stocks, Williams, and Faulkner 14 have re-examined the CuNi problem using the CPA, Here, m is the closest integer to lq(r). The mode is not assumed to be of the form e im ® but instead slowly varying along the field line at each radius. 9 R. Z. Sagdeev and A. A. Galeev, Dokl. Akad. Nauk SSSR 180, 839 (1968) [Sov. Phys. Dokl. 13, 562 (1968)].following the work of Kirkpatrick, Velicky, and Ehrenreich. 13 We report here x-ray photoemission spectroscopy (XPS) data for the density of states of NiCu alloys and compare them with the theoretical predictions and with other measurements. These include (1) the specific-heat measurements of Gupta, Cheng, and Beck, 8 which strongly favor a CPA description; (2) the very extensive uv-photoemission data of Seib and Spicer 9 ' 10 which were originally interpreted in terms of the virtual-bound-state model, but can indeed be very well interpreted 13 ' 14 in the CPA; and (3) the extensive x-ray-emission investigation of Clift, Curry, and Thompson, u who found that the sharing of electrons between Cu and Ni is small but were unable to determine the form ...
The unusual magnetic and electrical properties of the semiconductor SmB6 have been closely investigated recently. Primarily from magnetic susceptibility measurements, it has been hypothesized that the Sm valence changes from 3 + (4f5, 6H5/2) at high temperatures to 2 + (4f6, 7F0) at low temperatures. We have used the Mössbauer effect in 149Sm to make a direct determination of the valence state by means of the ``isomer shift,'' which measures the s-electron density at the Sm nucleus. This density changes with the 4f configuration because of coulombic shielding effects. Unfortunately, the change between 4f5 and 4f6 is too small to resolve resonance lines from the two charge states. The observed isomer shift of −0.4 mm/sec lies approximately halfway between the values (∼−0.9 and ∼0. mm/sec−1 respectively) obtained for well characterized ionic di- and trivalent Sm compounds, and is not observed to vary between room temperature and 1.1°K. A new model is presented to explain both the magnetic susceptibility data and the Mössbauer results.
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