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Thermodynamic phase diagram and phase competition in BaFe2(As1-xPx)2 studied by thermal expansion

High-resolution thermal-expansion and specific-heat measurements were performed on single crystalline BaFe2(As1-xPx)2 (0 < x < 0.33, x = 1). The observation of clear anomalies allows to establish the thermodynamic phase diagram which features a small coexistence region of SDW and superconductivity with a steep rise of Tc on the underdoped side. Samples that undergo the tetragonal-orthorhombic structural transition are detwinned in situ, and the response of the sample length to the magneto-structural and superconducting transitions is studied for all three crystallographic directions. It is shown that a reduction of the magnetic order by superconductivity is reflected in all lattice parameters. On the overdoped side, superconductivity affects the lattice parameters in much the same way as the SDW on the underdoped side, suggesting an intimate relation between the two types of order. Moreover, the uniaxial pressure derivatives of Tc are calculated using the Ehrenfest relation and are found to be large and anisotropic. A correspondence between substitution and uniaxial pressure is established, i.e., uniaxial pressure along the b-axis (c-axis) corresponds to a decrease (increase) of the P content. By studying the electronic contribution to the thermal expansion we find evidence for a maximum of the electronic density of states at optimal doping.