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Single crystals of Mg1-xAlxB2 have been grown at a pressure of 30 kbar usingthe cubic anvil technique. Precipitation free crystals with x < 0.1 wereobtained as a result of optimization of already developed MgB2 crystal growthprocedure. Systematic decrease of the c-axis lattice constant with increasingAl content, when the a-axis lattice constant is practically unchanged, wasobserved. Variation of the critical temperature on Al content in Mg1-xAlxB2crystals was found to be slightly different than that one observed forpolycrystalline samples since, even a very small substitution of 1-2% of Alleads to the decrease of Tc by about 2-3 K. X-ray and high resolutiontransmission electron microscopy investigations indicate on the appearance ofsecond precipitation phase in the crystals with x > 0.1. This is in a form ofnon-superconducting MgAlB4 domains in the structure of superconductingMg1-xAlxB2 matrix. Resistivity and magnetic investigations show the slightincrease of the upper critical field, Hc2, for H//c for the samples with smallx, significant reduction of the Hc2 anisotropy at lower temperatures, anddecrease of the residual resistance ratio value for Al substituted samples ascompared to those of unsubstituted crystals. Superconducting gaps variation asa function of Al content, investigated with point contact spectroscopy for theseries of the crystals with Tc in the range from 20 to 37 K, does not indicateon the merging of the gaps with decreasing Tc down to 20 K. It may be relatedto an appearance of the precipitation phase in the Mg1-xAlxB2 structure.Comment: PDF only, 33 pages, 4 tables, 18 figure

physical review. b, condensed matter and materials physics

Al substitution in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mi>MgB</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:mrow></mml:math>crystals: Influence on superconducting and structural properties

Al substitution inMgB2crystals: Influence on superconducting and structural properties