Pressure Tuning of the Interplay of Magnetism and Superconductivity in ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$

We carried out specific-heat and ac-susceptibility experiments under hydrostatic pressure to investigate the interplay of spin-density-wave antiferromagnetism ($A$) and superconductivity ($S$) in single-crystalline $AS$-type ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$. We find evidence for a line of magnetic-field- and pressure-tuned quantum critical points in the normal state in the zero-temperature magnetic field–pressure plane. Our analysis suggests an extension of this line into the superconducting state and corroborates the close connection of the underlying mechanisms leading to the formation of the antiferromagnetic and the superconducting states in $AS$-type ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$.

Figure 1

$C_{\mathrm{el}}(T)/T$ as function of $T$ of $AS$-type ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$ for different magnetic fields $B=0$, 0.5, 1, and 2 T ($B\parallel c$) and at four different pressures as indicated in (a)–(d). The solid arrows mark $T_N$ and $T_c$ at zero magnetic field. Dashed arrows denote the AFM transition at 2 T at 0.06 and 0.09 GPa. The transition temperatures were determined by an equal-entropy approximation. The insets of (c) and (d) show ${\chi }_{\mathrm{ac}}(T)$ for different magnetic fields as indicated.

Figure 2

Effect of a magnetic field ($B\parallel c$) on the low-pressure $T\mathrm{\text{−}}p$ phase diagram of $AS$-type ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$. Full symbols correspond to $T_N$, while open symbols mark $T_c$. The dashed lines extrapolate $T_N(p)$ to zero temperature (see the text for details). The arrows mark the critical pressures $p_c$ for the corresponding magnetic field.

Figure 3

Magnetic and superconducting phase diagram of $AS$-type ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$ as a function of $p$ and $B$ for $B\parallel c$. Stars and circles correspond to $T_N(p,B)$ and $T_c(p,B)$, respectively. Black lines represent fits to the $T_N(p,B=\mathrm{const})$ data and white lines to the $T_N(B,p=\mathrm{const})$ data. A detailed description can be found in the text. The critical points $B_c^0(p)$ obtained by the extrapolation of the fitting curves to zero temperature are marked with bullets in the $p\mathrm{\text{−}}B$ plane. “SC” and “AFM” mark the superconducting and the antiferromagnetic phase, respectively. $B_{c2}(T)$ data at ambient pressure are shown. The extrapolated upper-critical field $B_{c2}^0(p)$ is indicated by the solid line.

Figure 4

Specific heat of $AS$-type ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$ at $B=6\mathrm{T}$ ($B\parallel c$) for different pressures. $C_{\mathrm{el}}/T$ is plotted as a function of $\sqrt{T}$. The straight lines correspond to fits of $C_{\mathrm{el}}(T)/T={\gamma }_0-a\sqrt{T}$ to the data. For clarity, only two lines for $p=0$ and 1.07 GPa are indicated.