Enhancement of the Critical Temperature of ${\mathrm{HgBa}}_2{\mathrm{CuO}}_{4+\delta }$ by Applying Uniaxial and Hydrostatic Pressure: Implications for a Universal Trend in Cuprate Superconductors

It is well known that the superconducting transition temperature ($T_c$) of cuprate superconductors can be enhanced by varying certain structural and electronic parameters, such as the flatness of the ${\mathrm{CuO}}_2$ planes or their doping level. We determine the uniaxial and hydrostatic pressure derivatives of $T_c$ in the structurally simple tetragonal compound ${\mathrm{HgBa}}_2{\mathrm{CuO}}_{4+\delta }$ near optimal doping. Our results provide experimental evidence for two further methods to enhance $T_c$: (i) reducing the area of the ${\mathrm{CuO}}_2$ planes, and (ii) increasing the separation of the ${\mathrm{CuO}}_2$ planar groups. $T_c$ is found to couple much more strongly to the ratio $c/a$ of the lattice constants than to the unit cell volume. A comparison with prior results for structurally more complicated cuprates reveals a general trend of uniaxial pressure derivatives with $T_c$.

Figure 1

$T_c$ versus pressure from susceptibility data in inset. Numbers give order of measurement. See text for details. (inset) Real part of ac susceptibility at 0.1 Oe and 1023 Hz versus temperature for single-crystalline Hg-1201 at different hydrostatic (He-gas) pressures.

Figure 2

(a) Temperature-dependent linear thermal expansion coefficients $\alpha$ along the $a$ and $c$ axes for tetragonal Hg-1201 crystal at 0 T magnetic field. Dashed vertical line marks superconducting transition. (b) Change in temperature-dependent expansion coefficients ${\alpha }_i$ and scaled specific heat $C_p$ data (solid black curves) from Ref. 14 in 0 and 10 T magnetic field.

Figure 3

Partial pressure derivatives $d{T}_c/d{P}_i$ for $i=a$, $b$, and $c$ for Hg-1201 compared to those for La-214 [7], Bi-2212 [8], and Hg-1223 [17]. The combined partial derivatives in the $a$ and $b$ directions are positive and seen to nearly cancel the negative derivatives in the $c$ direction. Dashed vertical lines are guides to the eye.