Electron Tunneling into Intermediate-Valence Materials

Electron tunneling spectra of TmSe, SmS, SmB6, and CePd& have been measured with the GaAs Schottky-barrier probe tunneling method. Antiferromagnetic TmSe shows an energy gap 2D (ful] width at half maximum) =1. 2 meV, in situ pressure-transformed metallic SmS exhibits a gap of 1. 7 meV, and SmB6 shows a gap of 2. 7 meV, which is independent of magnetic field. For CePd3 an inelastic excitation is found near + 14 meV, which is absent in YPd&.

The interaction of localized 4f states with con- duction-band states at the Fermi level EF in in- termediate-valence (IV) materials raises funda- mental questions concerning the nature of the 'low-temperature coherent ground state and its low-energy excited states.A central issue in recent research concerns the possibility of an insulating singlet ground state.' Although elec- tron tunneling spectroscopy appears to be extremely well suited for probing the low-energy excitations in the immediate vicinity of EF, measurements have only been performed on SmB, .' We report on electron tunneling investigations of IV materials using the GaAs Schottky-barrier probe tunneling method.' We have found well-de- fined energy-gap structures in the tunneling spectra of antiferromagnetic TmSe, of SmB" and of in situ pressure-transformed metallic SmS.While the gap of SmB, is insensitive to magnetic fields, it disappears in TmSe for field- induced ferromagnetic spin alignment.No such pronounced gaplike structure has been found in CePd"which exhibits an inelastic excitation near ~14 meV, In certain IV ma, terials, such as SmB"SmS, or TmSe, the low-temperature resistivity in- crease, partly affected by sample purity and stoichiometry, has been subject to various con- jectures in terms of gaps due to hybridization or antiferromagnetism, impurity or Kondo scattering, Anderson localization, or Wigner lattice formation.'"' Deviations at the lowest tempera- tures from an activated resistivity behavior have been attributed to extrinsic effects, ' ' partly in view of theoretical predictions' of gaps in the low-energy excitation spectrum of IV states us-ing, e.g. , the Anderson lattice model.'"" On the other hand, the above materials can be con- trasted with the majority of IV materials which show a decrease of the resistivity at low temperatures, like for instance the intermetal.lic CePd ." We have applied the GaAs probe tunneling method, ' which overcomes inherent difficulties in fabricating oxide barrier tunnel junctions.We have measured the differential resistance R' =dV/dI versus the applied bias voltage V up to + 100 meV at temperatures between 1.5&T &4.2 K and in magnetic fields up to 20 kOe.Reproducible, symmetric tunneling spectra were ob- tained only after carefully removing surface con- taminants by an in situ (within liquid helium) sputter cleaning method.' The preparation of single crystals of SmS, YSe, and TmSe, " and of SmB, (Ref.14) has been described elsewhere.The annealed polycrystalline Cepd, (p/O, = 5) and YPd, samples have been prepared by E. Cat- taneo, Universita, t Koln.
The proper handling of the tunneling and sput- tering techniques has been tested with appropri- ate reference materials.
A cleaved (100) face of nominally stoichiomet- ric TmSe (a, = 5.705 A) shows at 4.2 K a. struc- tureless tunneling resistance (R' = 900 0) as shown in Fig. 1.Just below the Neel tempera- ture T~= 3.5 K (Ref. 13)we observe a tunneling spectrum similar to that of YSe but with R'(V re lacing 0.9 in the above definition by 0.5.i.e. , replacing . in t s the quantitative difference between To s ress e uote also a TmSe and superconducting YSe we quo e e 2A =2.0 meV.The BCS-defined  i VI .The shape of the spectrum in 1 for H ) 13 kOe is attributed to a zero- t a e h t the 10 meV excitation ' foun tt ' below 50 K results from exneutron scattering e ow citations acro a.cross the hybridization gap. of a, Fi ure 2 shows the tunneling spectrum o a Figure 2 s ows K.At H = 0 we deduce SmB, single crystal at 2.0 K. Vc 0 is con- still defined for II g 0 since R ., V 0 ' con- 0 meV.(2A = 4.9 meV at H = 0 is stant up to + 6 me uoted for comparison.) Changes inR' quo e ' ld h ever cannot be a.ccount- with magnetic fie, ow, ted for by the rather small negative magnetore-

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We find that R'(V =0) is strongly resistance.
We in in the GaAsduced by excessively pressurizing e SmB, contact.
d state SmB a Wigner-crystal localize s a, wi a. 0 K 14 meV) has with a large gap of about 160 K me been invoked, w ic, o h h however, is not supported by our tunneling results.
The effective pressure in a Ga -p GaAs-robe pointtunneling experiment can actua y ll reach the yield pressure of the sample surface.' This unique op- t has been exploited for tunneling in o pressureran -transformed metallic IV-p ase o i j shows the tunneling spectrum of a R'(V) = 1 kO.Gradually pressurizing the SmS surface by the GaAs tip exhibits abruptly a tun- neling spectrum with a well pronounced gap 26(FWHM) =1.7 meV as shown in Fig. 3(b), with R'(V =0, H =0) =150 Q.The size of this gap remains unchanged upon further pressurizing SmS, with a change in R' up to only 5%%uo.The magnetic field dependence of the tunneling spectrum saturates beyond a field of 8.8 kOe, for which the gap can no more be defined (see above).A very similar tunneling spectrum is obtained for mechani- cally polished ("gold" ) SmS [2b (FWHM) = 2. 1 meV] showing the same magnetic field depen- dence.The unexpected' magnetic field depen- dence of the gap of metallic SmS may be due to magnetoresistance effects in the remaining non- transformed divalent SmS" outside of the point- contact region.
Asymmetric tunneling spectra have been ob- tained with SmB, -oxide-Pb junctions.A rough estimate of 2A(FWHM) = 10 meV (based on our definition) is four times larger than our value.
In our experiments when making contact to the SmB, surface without sputter cleaning we observe very similar spectra.The asymmetry in our case is attributed to leakage resistance, which is tunneling assisted by intermediate states due to surface contamination.It is possible that the ox- ide barrier of the SmB, -oxide-Pb junctions may not have been continuous throughout the junction.
The tunneling spectrum of CePd, at 4.2 K in Fig. 4 exhibits an inelastic excitation near 6' =+ 14 meV which is independent of magnetic fields up to 15 kOe and is absent in YPd, .This excita- tion can be correlated with a resonant scattering process of conduction electrons at empty 4f ' states 14 meV above EF as identified in an analy- sis of far infrared data, ' including those of Pink- erton, " Interestingly enough, we also observe the 14-meV excitation in asymmetric R'(V) characteristics of Ga-CePd, (metal-metal) point con- tacts, which can also be formed in situ by our method.This excitation is found for only one polarity of V for electrons injected into CePd, .
The characteristic features of our tunneling spectra seem to correlate qualitatively with the low-temperature resistivity behavior.The gap of TmSe appears to be primarily spin-superstructure induced.The tunneling spectrum of CePd, may qualitatively be considered as a broad- ened version of the pronounced spectra of SmB, and SmS.Our results raise fundamental questions about the nature of the gaps in IV materials and about what is observed by electron tunneling.Since the electronic density of states of IV ma- terials is believed to be asymmetric around E F, as for instance inferred from asymmetries in metal-metal point contact spectra lt ls surprising that we observe symmetric tunneling spectra in all materials.Instead of involving the one-electron density-of-states picture we propose a description of our spectra in terms of a more adequate particle representation.We consider 4' and 6, [corresponding more closely to d ' than 26(FWHM)/2] as the quasiparticle excitation energies of bound electron-hole pairs, with 2A' or 2A the electron-hole binding energy.For these electron-hole pairs a symmetric tunneling characteristic is obtained by either condensing (e +4f" '-4f") or breaking (4f"-4f" '+e )   pairs, in analogy to the Cooper-pair tunneling mechanism in superconductors.
1/2A' or 1/2h is assumed to be a measure of the electron-hole lifetime ~,&, which on the other hand is deter- mined by the charge relaxation rate v, &-1/I",.Indeed, the roughly five times larger 2~' of Ce Pd, compared to 24 of IV SmS or SmB, is con- sistent with the approximately five times larger I, of CePd, in comparison to IV SmS or SmB,." Consequently the "broadened" tunneling spectrum of CePd, compared to the pronounced one of SmB, or SmS is ascribed to the shorter 7. ,& in CePd, .
In the case of SmB, our interpretation is supported by recent far-infrared transmission measurements of a gap of similar magnitude." In metal-metal point-contact spectroscopy between Mo and SmB, (or TmSe) the resistance peak at zero voltage has been interpreted by anal- ogy to tunneling experiments of SmB, (Ref.2) as signature of a narrow gap." FIG. 1. Tunneling spectrum o a c1.8 K, the latter as a function of of TmSe at 4.2 and 1. um of supernetic field H. Ioset: Tunneling spectrum o s conducting YSe (T, =4.0 K) at fined gap 2D.

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Figure 3 a s ows gap for TmSe 2~= .me resistance minimum of TmSe nea r + 1.4 meV has