  We have investigated the elastic constant C33 of Ba(Fe1-xCox)2As2 with eight
different Co concentrations by ultrasonic measurement. We found remarkable
elastic anomalies near the quantum critical point. We have studied them by
measuring the electrical resistivity, heat capacity, and ultrasonic attenuation
in addition to the elastic constant. These results have revealed that the
inter-layer three-dimensional properties appearing in C33 to be possibly
originated from the magnetic character of these materials. Our data about the
elastic constant C33 highlight the importance of controlling the c-axis length
in the emergence of superconductivity in iron-based superconductors.
