- Email: [email protected]
An enhancement of superconductivity in Al1−xSix solid solutions
Physica B 284}288 (2000) 1115}1116
An enhancement of superconductivity in Al Si \V V solid solutions N.E. Sluchanko *, V.V. Glushkov , S.V. Demishev , M.V. Kondrin , A.A. Pronin , N.A. Samarin , Y. Bruynseraede, V.V. Moshchalkov, V.V. Brazhkin Low Temperatures Laboratory, General Physics Institute RAS, 38 Vavilov str., 117942 Moscow, Russia Laboratorium voor Vaste-Stowysica en Magnetisme, K.U. Leuven, Celestijnenlaan 200, B-3001 Leuven, Belgium High Pressure Physics Institute RAS, 142092, Troitsk, Moscow region, Russia
Abstract Electronic, structural and thermodynamical properties of high-pressure produced non-equilibrium superconductors Al Si have been studied. The experimental results indicate that the enhancement of the electron}phonon interaction \V V with soft `cluster modesa is the origin of the tenfold increase of ¹ in these compounds. 2000 Elsevier Science B.V. All rights reserved. Keywords: Lattice instability; Non-equilibrium superconductivity
The signi"cant elevation of the Si and Ge solubility in Al under high pressure (up to 10 GPa) makes it possible to synthesize non-equilibrium FCC solid solutions Al Si and Al Ge in the concentration range \V V \V V x(0.2 [1]. These non-equilibrium superconductors Al Si are characterized by ¹ changes within an \V V order of magnitude (between 1.18 and 11 K) in homogeneous single-phase FCC metallic compounds. Moreover, a simple Fermi surface constructed from s- and p-electronic states and a simple ¹ (x) dependence allow one to consider these alloys as a very promising model system to study the nature of the superconductivity enhancement in the vicinity of the lattice instability in metals. In a traditional approach based on McMillan's equation for strong coupling superconductors (constant of electron}phonon interaction j(Al)"0.38), the ¹ en hancement in these non-equilibrium solid solutions cannot be attributed to the transformation of phonon density of states F(u) that changes only slightly with the substitution of Al by Si [2]. Magnetotransport [3], Al ¸ X-ray emission spectra together with the low''\'''
* Corresponding author. Fax: #7-095-135-8129. E-mail address: [email protected] (N.E. Sluchanko)
temperature heat capacity and Al NMR Knight shift [4] show neither a reconstruction of the conduction band structure nor the density of states N(E ) variation. Tak$ ing into account the noticeable increase (by a factor of about 1.5) of the linear electronic speci"c heat coe$cient c"(p/3)k N(E )(1#j) in the Si concentration range $ 0.01}0.08 [4] the only parameter responsible for the rapid changes of ¹ is j. Moreover, low-temperature anomalies of the Seebeck coe$cient observed in Ref. [5] have been interpreted in terms of an enhancement of the electron}phonon coupling in the energy range below 20 meV. Additional and direct evidence of the electron}phonon interaction enhancement can be obtained from pointcontact (PC) spectroscopy measurements. The second harmonic of PC voltage}current characteristics dI/d; is shown in Fig. 1. Data for pure aluminium (curve 1) are presented as a reference and reproduce well-known results [6]: two peaks (marked by dashed lines in Fig. 1) at 21 and 36 meV correspond to electron}phonon interaction with LA and TA modes, respectively. However, in the case of the non-equilibrium substitutional solid solutions Al Si with high Si content it is \V V possible to expect a transition from ballistic to thermal regime of the point contact. As a result, the phonon features of the point-contact spectra decrease by a factor
0921-4526/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 2 4 6 1 - 8
1116
N.E. Sluchanko et al. / Physica B 284}288 (2000) 1115}1116
accounts for changes in temperature derivative of resistivity, which has been found earlier [5] in the same concentration range. Finally, the point-contact spectra measurements corroborate the conclusion that the electron}phonon interaction enhancement in the vicinity of the FCC lattice instability is likely to be the main reason for the dramatic increase in ¹ in the solid solutions Al Si . The substi \V V tution of Al by Si in these non-equilibrium metals causes a strong phonon mode softening. This behaviour is possibly connected with the local `soft microregionsa of about 1 nm in size in the FCC lattice that are destroyed under pressure. The FCC atomic clusters and the corresponding `cluster phonon modesa may give rise to a noticeable enhancement of the electron}phonon interaction and lead to an increase of ¹ by an order of magnitude. Fig. 1. Point-contact spectra of pure Al (curve 1) and solid solutions Al Si : x"0.015 (2) and 0.025 (3, 4) at ¹"1.8 K. \V V
l /d (where d is the point-contact diameter and l the G G mean free path), and hence only a range of small Si concentrations is suitable for the qualitative analysis of the point-contact Eliashberg function behaviour. The main result of the present study can be formulated in terms of a strong increase of the electron}phonon coupling in the low-frequency range ( u)15 meV, see Fig. 1). This increase causes a dramatic renormalization of point-contact Eliashberg function even for non-equilibrium solid solutions with Si content x*0.02. It should be noted that processing of the point-contact spectra measured in thermal regime allows one to estimate a temperature dependence of the phonon part of resistivity given by the Ziman formula. The result of the calculation done for solid solutions Al Si x"0.015, 0.025 \V V
Acknowledgements This work was supported by INTAS program 96-451, the RFBR Grants 16067, 17163 and 96929, Programs `Fundamental Spectroscopya and `Microwavesa of the Russian Ministry of Science and Technology and Copernicus Network ERB IC15 CT98 0812.
References [1] V.F. Degtyareva et al., Phys. Stat. Sol. A 89 (1985) K127. [2] J. Chevrier et al., Phys. Rev. B 49 (1994) 961. [3] V.V. Brazhkin et al., J. Phys.: Condens. Matter 5 (1993) 5933. [4] N.E. Sluchanko et al., Phys. Rev. B 51 (1995) 1112. [5] N.E. Sluchanko et al., JETF 86 (1998) 190. [6] M. Pepper, J. Phys. C 13 (1980) L717.
An enhancement of superconductivity in Al Si \V V solid solutions N.E. Sluchanko *, V.V. Glushkov , S.V. Demishev , M.V. Kondrin , A.A. Pronin , N.A. Samarin , Y. Bruynseraede, V.V. Moshchalkov, V.V. Brazhkin Low Temperatures Laboratory, General Physics Institute RAS, 38 Vavilov str., 117942 Moscow, Russia Laboratorium voor Vaste-Stowysica en Magnetisme, K.U. Leuven, Celestijnenlaan 200, B-3001 Leuven, Belgium High Pressure Physics Institute RAS, 142092, Troitsk, Moscow region, Russia
Abstract Electronic, structural and thermodynamical properties of high-pressure produced non-equilibrium superconductors Al Si have been studied. The experimental results indicate that the enhancement of the electron}phonon interaction \V V with soft `cluster modesa is the origin of the tenfold increase of ¹ in these compounds. 2000 Elsevier Science B.V. All rights reserved. Keywords: Lattice instability; Non-equilibrium superconductivity
The signi"cant elevation of the Si and Ge solubility in Al under high pressure (up to 10 GPa) makes it possible to synthesize non-equilibrium FCC solid solutions Al Si and Al Ge in the concentration range \V V \V V x(0.2 [1]. These non-equilibrium superconductors Al Si are characterized by ¹ changes within an \V V order of magnitude (between 1.18 and 11 K) in homogeneous single-phase FCC metallic compounds. Moreover, a simple Fermi surface constructed from s- and p-electronic states and a simple ¹ (x) dependence allow one to consider these alloys as a very promising model system to study the nature of the superconductivity enhancement in the vicinity of the lattice instability in metals. In a traditional approach based on McMillan's equation for strong coupling superconductors (constant of electron}phonon interaction j(Al)"0.38), the ¹ en hancement in these non-equilibrium solid solutions cannot be attributed to the transformation of phonon density of states F(u) that changes only slightly with the substitution of Al by Si [2]. Magnetotransport [3], Al ¸ X-ray emission spectra together with the low''\'''
* Corresponding author. Fax: #7-095-135-8129. E-mail address: [email protected] (N.E. Sluchanko)
temperature heat capacity and Al NMR Knight shift [4] show neither a reconstruction of the conduction band structure nor the density of states N(E ) variation. Tak$ ing into account the noticeable increase (by a factor of about 1.5) of the linear electronic speci"c heat coe$cient c"(p/3)k N(E )(1#j) in the Si concentration range $ 0.01}0.08 [4] the only parameter responsible for the rapid changes of ¹ is j. Moreover, low-temperature anomalies of the Seebeck coe$cient observed in Ref. [5] have been interpreted in terms of an enhancement of the electron}phonon coupling in the energy range below 20 meV. Additional and direct evidence of the electron}phonon interaction enhancement can be obtained from pointcontact (PC) spectroscopy measurements. The second harmonic of PC voltage}current characteristics dI/d; is shown in Fig. 1. Data for pure aluminium (curve 1) are presented as a reference and reproduce well-known results [6]: two peaks (marked by dashed lines in Fig. 1) at 21 and 36 meV correspond to electron}phonon interaction with LA and TA modes, respectively. However, in the case of the non-equilibrium substitutional solid solutions Al Si with high Si content it is \V V possible to expect a transition from ballistic to thermal regime of the point contact. As a result, the phonon features of the point-contact spectra decrease by a factor
0921-4526/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 2 4 6 1 - 8
1116
N.E. Sluchanko et al. / Physica B 284}288 (2000) 1115}1116
accounts for changes in temperature derivative of resistivity, which has been found earlier [5] in the same concentration range. Finally, the point-contact spectra measurements corroborate the conclusion that the electron}phonon interaction enhancement in the vicinity of the FCC lattice instability is likely to be the main reason for the dramatic increase in ¹ in the solid solutions Al Si . The substi \V V tution of Al by Si in these non-equilibrium metals causes a strong phonon mode softening. This behaviour is possibly connected with the local `soft microregionsa of about 1 nm in size in the FCC lattice that are destroyed under pressure. The FCC atomic clusters and the corresponding `cluster phonon modesa may give rise to a noticeable enhancement of the electron}phonon interaction and lead to an increase of ¹ by an order of magnitude. Fig. 1. Point-contact spectra of pure Al (curve 1) and solid solutions Al Si : x"0.015 (2) and 0.025 (3, 4) at ¹"1.8 K. \V V
l /d (where d is the point-contact diameter and l the G G mean free path), and hence only a range of small Si concentrations is suitable for the qualitative analysis of the point-contact Eliashberg function behaviour. The main result of the present study can be formulated in terms of a strong increase of the electron}phonon coupling in the low-frequency range ( u)15 meV, see Fig. 1). This increase causes a dramatic renormalization of point-contact Eliashberg function even for non-equilibrium solid solutions with Si content x*0.02. It should be noted that processing of the point-contact spectra measured in thermal regime allows one to estimate a temperature dependence of the phonon part of resistivity given by the Ziman formula. The result of the calculation done for solid solutions Al Si x"0.015, 0.025 \V V
Acknowledgements This work was supported by INTAS program 96-451, the RFBR Grants 16067, 17163 and 96929, Programs `Fundamental Spectroscopya and `Microwavesa of the Russian Ministry of Science and Technology and Copernicus Network ERB IC15 CT98 0812.
References [1] V.F. Degtyareva et al., Phys. Stat. Sol. A 89 (1985) K127. [2] J. Chevrier et al., Phys. Rev. B 49 (1994) 961. [3] V.V. Brazhkin et al., J. Phys.: Condens. Matter 5 (1993) 5933. [4] N.E. Sluchanko et al., Phys. Rev. B 51 (1995) 1112. [5] N.E. Sluchanko et al., JETF 86 (1998) 190. [6] M. Pepper, J. Phys. C 13 (1980) L717.