Single crystals of Mg1−xAlxB2 have been grown at a pressure of 30 kbar using the cubic anvil technique. Precipitation free crystals with x<0.1 were obtained as a result of the optimization of an already developed MgB2 crystal growth procedure. A systematic decrease of the c-axis lattice constant with increasing Al content, when the a-axis lattice constant is practically unchanged, was observed. Variation of the critical temperature on Al content in Mg1−xAlxB2 crystals was found to be slightly different than that one observed for polycrystalline samples, since even a very small substitution of 1%–2% of Al leads to the decrease of Tc by about 2–3 K. X-ray and high resolution transmission electron microscopy investigations indicate the appearance of the second precipitation phase in the crystals with x>0.1. This is in a form of nonsuperconducting MgAlB4 domains in the structure of a superconducting Mg1−xAlxB2 matrix. Resistivity and magnetic investigations show the slight increase of the upper critical field, Hc2, for H|| c for the samples with small x, a significant reduction of the Hc2 anisotropy at lower temperatures, and a decrease of the residual resistance ratio value for Al substituted samples as compared to those of unsubstituted crystals. Superconducting gaps variation as a function of Al content, investigated with point contact spectroscopy for the series of the crystals with Tc in the range from 20 to 37 K, does not indicate on the merging of the gaps with decreasing Tc down to 20 K. It may be related to an appearance of the precipitation phase in the Mg1−xAlxB2 structure.

Al substitution in MgB2 crystals: Influence on superconducting and structural properties / Karpinski, J.; Zhigadlo, N. D.; Schuck, G.; Kazakov, S. M.; Batlogg, B.; Rogacki, K.; Puzniak, R.; Jun, J.; Muller, E.; Wagli, P.; Gonnelli, Renato; Daghero, Dario; Ummarino, Giovanni; Stepanov, V. A.. - In: PHYSICAL REVIEW. B, CONDENSED MATTER AND MATERIALS PHYSICS. - ISSN 1098-0121. - 71:(2005), pp. 174506-1-174506-15. [10.1103/PhysRevB.71.174506]

Al substitution in MgB2 crystals: Influence on superconducting and structural properties

GONNELLI, Renato;DAGHERO, Dario;UMMARINO, Giovanni;
2005

Abstract

Single crystals of Mg1−xAlxB2 have been grown at a pressure of 30 kbar using the cubic anvil technique. Precipitation free crystals with x<0.1 were obtained as a result of the optimization of an already developed MgB2 crystal growth procedure. A systematic decrease of the c-axis lattice constant with increasing Al content, when the a-axis lattice constant is practically unchanged, was observed. Variation of the critical temperature on Al content in Mg1−xAlxB2 crystals was found to be slightly different than that one observed for polycrystalline samples, since even a very small substitution of 1%–2% of Al leads to the decrease of Tc by about 2–3 K. X-ray and high resolution transmission electron microscopy investigations indicate the appearance of the second precipitation phase in the crystals with x>0.1. This is in a form of nonsuperconducting MgAlB4 domains in the structure of a superconducting Mg1−xAlxB2 matrix. Resistivity and magnetic investigations show the slight increase of the upper critical field, Hc2, for H|| c for the samples with small x, a significant reduction of the Hc2 anisotropy at lower temperatures, and a decrease of the residual resistance ratio value for Al substituted samples as compared to those of unsubstituted crystals. Superconducting gaps variation as a function of Al content, investigated with point contact spectroscopy for the series of the crystals with Tc in the range from 20 to 37 K, does not indicate on the merging of the gaps with decreasing Tc down to 20 K. It may be related to an appearance of the precipitation phase in the Mg1−xAlxB2 structure.
File in questo prodotto:
Non ci sono file associati a questo prodotto.
Pubblicazioni consigliate

I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.

Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11583/1400927
 Attenzione

Attenzione! I dati visualizzati non sono stati sottoposti a validazione da parte dell'ateneo