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Energy gap in Lu-substituted YbB12 probed by break junction
Physica B 281&282 (2000) 278}279
Energy gap in Lu-substituted YbB probed by break junction 12 T. Ekino!,",*, H. Umeda!,1, J. Klijn",#, F. Iga", T. Takabatake", H. Fujii! !Faculty of Integrated Arts and Sciences, Hiroshima University, Higashi-Hiroshima 739-8521, Japan "Department of Quantum Matter, ADSM, Hiroshima University, Higashi-Hiroshima 739-8526, Japan #Van der Waals}Zeeman Institute, Universiteit van Amsterdam, 1018XE Amsterdam, The Netherlands
Abstract Tunneling measurements of Yb Lu B have been carried out with break junction. The gap magnitude rapidly 1~x x 12 reduces with increasing x up to +0.5, but the decreasing rate becomes weak for x'0.5. By contrast, the zero-bias conductance increases rather smoothly with increasing x up to +0.8. These results clarify the nontrivial relationship between the Lu substitution and the gap suppression in Yb Lu B . ( 2000 Elsevier Science B.V. All rights reserved. 1~x x 12 Keywords: Kondo semiconductor; Break junction; Yb Lu B 1~x x 12
The cubic compound YbB has been known as the 12 representative Yb-based Kondo semiconductor [1]. Our previous tunneling measurements using single crystals clari"ed the conductance features having very sharp gap-edge structure and quite low leakage of the zerobias conductance [2,3]. The gap magnitude of 2*+ 200}300 meV determined unambiguously from these data is extremely larger than the transport gap of +10 meV [2,3]. For the further study, the e!ects of Lu substitution on the gap have been examined through the transport, magnetic, and speci"c-heat measurements [4]. These experiments suggest that the energy gap closes at x+0.5 in Yb Lu B . However, the direct evidence 1~x x 12 for the change in the gap upon Lu substitution has not been given to date. In this work, tunneling measurements of Yb Lu B 1~x x 12 single crystals are carried out to examine the Lu-substitution e!ects on the energy gap in a straightforward way. The tunneling technique directly measures the energy gap through the current (I) } voltage (<) characteristics of the junction. The break-junction setup was used for the measurements with the cleanest interface as a result of
* Corresponding author. Tel.: #81-824-24-6552; fax: #81824-24-0757. E-mail address: [email protected] (T. Ekino) 1 Present address: TDK Materials Research Center, Chiba 286-0124, Japan.
in situ cracking of the sample at 4.2 K. This method has been demonstrated to be e!ective for measuring the Cebased compounds with reactive surface characteristics [3,5,6]. The di!erential conductance, dI/d<, was obtained by an AC-modulation technique with four-probe geometry. Fig. 1 shows the tunneling conductance for Yb Lu B break junctions with x"0, 0.5, and 0.75. 1~x x 12 The junction resistance R (0.3 V) ranges as J +200}500 ), which is lower than h/2e2+13 k). The conductance curves are symmetric with respect to zero bias, and well-de"ned gap edges are seen for all the data. Notice that the spread of the gap was observed in every x. This indicates microscopic variations of stoichiometry, while the data presented here are the representative ones. The peak-to-peak magnitudes of the conductance, i.e., 0.46, 0.19, and 0.17 V for x"0, 0.5, and 0.75, respectively, are attributed to 4*/e of the regular semiconductor}insulator}semiconductor break junction. We have also observed the half-magnitudes owing to the grain boundary cracking which forms a semiconductor}insulator}metal break junction [2,3]. With increasing x the gap magnitude decreases concomitant with the "lling up of the midgap states. This systematic change indicates that the extremely large gap in YbB observed by tunneling is 12 the intrinsic property. The sharp and intensive gap structure is observed even for x"0.5, while it is suppressed for x"0.75. These features are a general tendency in the present measurements. As shown in the spectrum of
0921-4526/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 1 1 9 3 - X
T. Ekino et al. / Physica B 281&282 (2000) 278}279
279
Fig. 2. Energy gap and normalized zero-bias conductance (NZBC) for several x in Yb Lu B . 1~x x 12
Fig. 1. Tunneling conductance for Yb Lu B break junc1~x x 12 tions.
x"0.5, the ingap structures of +$0.04}0.05 V occur at half the biases of the main gap edges at $2*/e. Since their separation of 0.08}0.10 V is actually observed as a single gap feature, they can be attributed to $*/e. The zero-bias cusp with the width +0.01}0.02 eV found at x"0.5 suggests the formation of impurity states inside the gap, which is observed for every x depending on the junction condition. This would explain why the transport gap is much smaller than the tunneling gap. For all the spectra, the lost spectral weight inside the gap is not fully transferred just above the gap, although the gap-edge peaks are sharp. This feature may involve the unsolved issue of whether the lost spectral weight is recovered to energies of the renormalized band or much higher energies of the initial states [7]. In Fig. 2, we plot the x dependence of the energy gap and the normalized zero-bias conductance (NBZC) which is proportional to the thermally smeared density of states at the Fermi energy. Here, only the clear spectra are taken into account for the plot. Wide spreads of the gap +$0.05 V are seen for every x. The preliminary data of x"0.25 show that the gap takes the value less than +0.2 eV, which "ts the expected position in the "gure. With increasing x the average gap magnitude rapidly decreases up to x+0.5, where it reaches the value +0.1 eV or less. This is +1 of the gap magnitude 3 at x"0. This supression indicates that the interaction for the gap creation is signi"cantly weakened upon Lu substitution, which is most likely due to the "lling up of 4f holes to disturb the formation of the coherent hydridized band consisting of 4f and conduction electrons. This seems to be consistent with the speci"c-heat measurements which suggest that the gap exsits for x(0.5 [4]. On the other hand, the gap of the present
tunneling measurements is noted to survive beyond x+0.5, where the average gap magnitude does not so much di!erent to that of x+0.5. The observed lower boundary of the gap magnitude at +0.01}0.02 eV for x"0.5 and 0.75 in Fig. 2 suggests the existence of the impurity phase produced by the Lu substitution. Actually, this magnitude is consistent with that of the cusp-like ingap feature observed in Fig. 1. In contrast to the behavior of the gap magnitude in which the reduced average magnitude no longer depends on x for x'0.5, the NZBC increases smoothly with increasing x even for x'0.5. The decreasing rate of the gap is +3}5]10~3 eV/x (%) for x(0.5, and the increasing rate of NZBC +8]10~3/x (%) for x(0.75. Such a nontrivial relation between the energy gap and NZBC upon Lu substitution can be involved with the introduction of f electrons and randomness in the system at the same time. Further, it would be valuable to note that the feature above x+0.5 somewhat resembles that of the gapless superconductivity, where the pairing coherence remains with no energy gap [8]. In summary, we present tunneling spectroscopy of Yb Lu B using break junction. Upon Lu substitu1~x x 12 tion, the gap magnitude rapidly decreases for x(0.5, while the zero-bias conductance increases rather smoothly in the whole x region. This would give a key to understand what occurs upon "lling up of 4f holes in this compound.
References [1] [2] [3] [4] [5] [6] [7] [8]
F. Iga et al., J. Magn. Magn. Mater. 52 (1985) 279. T. Ekino et al., Physica B 259}261 (1999) 315. T. Ekino et al., Jpn. J. Appl. Phys. Ser. 11 (1999) 97. F. Iga et al., Physica B 259}261 (1999) 312. T. Ekino et al., Phys. Rev. Lett. 75 (1995) 4262. T. Ekino et al., J. Magn. Magn. Mater. 177}181 (1998) 379. H. Okamura et al., Phys. Rev. B 58 (1998) R7496. P.G. de Gennes, Superconductivity of Metals and Alloys, Addison-Wesley, Reading, MA, 1989.
Energy gap in Lu-substituted YbB probed by break junction 12 T. Ekino!,",*, H. Umeda!,1, J. Klijn",#, F. Iga", T. Takabatake", H. Fujii! !Faculty of Integrated Arts and Sciences, Hiroshima University, Higashi-Hiroshima 739-8521, Japan "Department of Quantum Matter, ADSM, Hiroshima University, Higashi-Hiroshima 739-8526, Japan #Van der Waals}Zeeman Institute, Universiteit van Amsterdam, 1018XE Amsterdam, The Netherlands
Abstract Tunneling measurements of Yb Lu B have been carried out with break junction. The gap magnitude rapidly 1~x x 12 reduces with increasing x up to +0.5, but the decreasing rate becomes weak for x'0.5. By contrast, the zero-bias conductance increases rather smoothly with increasing x up to +0.8. These results clarify the nontrivial relationship between the Lu substitution and the gap suppression in Yb Lu B . ( 2000 Elsevier Science B.V. All rights reserved. 1~x x 12 Keywords: Kondo semiconductor; Break junction; Yb Lu B 1~x x 12
The cubic compound YbB has been known as the 12 representative Yb-based Kondo semiconductor [1]. Our previous tunneling measurements using single crystals clari"ed the conductance features having very sharp gap-edge structure and quite low leakage of the zerobias conductance [2,3]. The gap magnitude of 2*+ 200}300 meV determined unambiguously from these data is extremely larger than the transport gap of +10 meV [2,3]. For the further study, the e!ects of Lu substitution on the gap have been examined through the transport, magnetic, and speci"c-heat measurements [4]. These experiments suggest that the energy gap closes at x+0.5 in Yb Lu B . However, the direct evidence 1~x x 12 for the change in the gap upon Lu substitution has not been given to date. In this work, tunneling measurements of Yb Lu B 1~x x 12 single crystals are carried out to examine the Lu-substitution e!ects on the energy gap in a straightforward way. The tunneling technique directly measures the energy gap through the current (I) } voltage (<) characteristics of the junction. The break-junction setup was used for the measurements with the cleanest interface as a result of
* Corresponding author. Tel.: #81-824-24-6552; fax: #81824-24-0757. E-mail address: [email protected] (T. Ekino) 1 Present address: TDK Materials Research Center, Chiba 286-0124, Japan.
in situ cracking of the sample at 4.2 K. This method has been demonstrated to be e!ective for measuring the Cebased compounds with reactive surface characteristics [3,5,6]. The di!erential conductance, dI/d<, was obtained by an AC-modulation technique with four-probe geometry. Fig. 1 shows the tunneling conductance for Yb Lu B break junctions with x"0, 0.5, and 0.75. 1~x x 12 The junction resistance R (0.3 V) ranges as J +200}500 ), which is lower than h/2e2+13 k). The conductance curves are symmetric with respect to zero bias, and well-de"ned gap edges are seen for all the data. Notice that the spread of the gap was observed in every x. This indicates microscopic variations of stoichiometry, while the data presented here are the representative ones. The peak-to-peak magnitudes of the conductance, i.e., 0.46, 0.19, and 0.17 V for x"0, 0.5, and 0.75, respectively, are attributed to 4*/e of the regular semiconductor}insulator}semiconductor break junction. We have also observed the half-magnitudes owing to the grain boundary cracking which forms a semiconductor}insulator}metal break junction [2,3]. With increasing x the gap magnitude decreases concomitant with the "lling up of the midgap states. This systematic change indicates that the extremely large gap in YbB observed by tunneling is 12 the intrinsic property. The sharp and intensive gap structure is observed even for x"0.5, while it is suppressed for x"0.75. These features are a general tendency in the present measurements. As shown in the spectrum of
0921-4526/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 1 1 9 3 - X
T. Ekino et al. / Physica B 281&282 (2000) 278}279
279
Fig. 2. Energy gap and normalized zero-bias conductance (NZBC) for several x in Yb Lu B . 1~x x 12
Fig. 1. Tunneling conductance for Yb Lu B break junc1~x x 12 tions.
x"0.5, the ingap structures of +$0.04}0.05 V occur at half the biases of the main gap edges at $2*/e. Since their separation of 0.08}0.10 V is actually observed as a single gap feature, they can be attributed to $*/e. The zero-bias cusp with the width +0.01}0.02 eV found at x"0.5 suggests the formation of impurity states inside the gap, which is observed for every x depending on the junction condition. This would explain why the transport gap is much smaller than the tunneling gap. For all the spectra, the lost spectral weight inside the gap is not fully transferred just above the gap, although the gap-edge peaks are sharp. This feature may involve the unsolved issue of whether the lost spectral weight is recovered to energies of the renormalized band or much higher energies of the initial states [7]. In Fig. 2, we plot the x dependence of the energy gap and the normalized zero-bias conductance (NBZC) which is proportional to the thermally smeared density of states at the Fermi energy. Here, only the clear spectra are taken into account for the plot. Wide spreads of the gap +$0.05 V are seen for every x. The preliminary data of x"0.25 show that the gap takes the value less than +0.2 eV, which "ts the expected position in the "gure. With increasing x the average gap magnitude rapidly decreases up to x+0.5, where it reaches the value +0.1 eV or less. This is +1 of the gap magnitude 3 at x"0. This supression indicates that the interaction for the gap creation is signi"cantly weakened upon Lu substitution, which is most likely due to the "lling up of 4f holes to disturb the formation of the coherent hydridized band consisting of 4f and conduction electrons. This seems to be consistent with the speci"c-heat measurements which suggest that the gap exsits for x(0.5 [4]. On the other hand, the gap of the present
tunneling measurements is noted to survive beyond x+0.5, where the average gap magnitude does not so much di!erent to that of x+0.5. The observed lower boundary of the gap magnitude at +0.01}0.02 eV for x"0.5 and 0.75 in Fig. 2 suggests the existence of the impurity phase produced by the Lu substitution. Actually, this magnitude is consistent with that of the cusp-like ingap feature observed in Fig. 1. In contrast to the behavior of the gap magnitude in which the reduced average magnitude no longer depends on x for x'0.5, the NZBC increases smoothly with increasing x even for x'0.5. The decreasing rate of the gap is +3}5]10~3 eV/x (%) for x(0.5, and the increasing rate of NZBC +8]10~3/x (%) for x(0.75. Such a nontrivial relation between the energy gap and NZBC upon Lu substitution can be involved with the introduction of f electrons and randomness in the system at the same time. Further, it would be valuable to note that the feature above x+0.5 somewhat resembles that of the gapless superconductivity, where the pairing coherence remains with no energy gap [8]. In summary, we present tunneling spectroscopy of Yb Lu B using break junction. Upon Lu substitu1~x x 12 tion, the gap magnitude rapidly decreases for x(0.5, while the zero-bias conductance increases rather smoothly in the whole x region. This would give a key to understand what occurs upon "lling up of 4f holes in this compound.
References [1] [2] [3] [4] [5] [6] [7] [8]
F. Iga et al., J. Magn. Magn. Mater. 52 (1985) 279. T. Ekino et al., Physica B 259}261 (1999) 315. T. Ekino et al., Jpn. J. Appl. Phys. Ser. 11 (1999) 97. F. Iga et al., Physica B 259}261 (1999) 312. T. Ekino et al., Phys. Rev. Lett. 75 (1995) 4262. T. Ekino et al., J. Magn. Magn. Mater. 177}181 (1998) 379. H. Okamura et al., Phys. Rev. B 58 (1998) R7496. P.G. de Gennes, Superconductivity of Metals and Alloys, Addison-Wesley, Reading, MA, 1989.