Pressure-Driven Quantum Criticality in Iron-Selenide Superconductors

We report a finding of a pressure-induced quantum critical transition in ${\mathrm{K}}_{0.8}{\mathrm{Fe}}_x{\mathrm{Se}}_2$ ($x=1.7$ and 1.78) superconductors through in situ high-pressure electrical transport and x-ray diffraction measurements in diamond anvil cells. Transitions from metallic Fermi liquid behavior to non-Fermi liquid behavior and from antiferromagnetism to paramagnetism are found in the pressure range of 9.2–10.3 GPa, in which superconductivity tends to disappear. The change around the quantum critical point from the coexisting antiferromagnetism state and the Fermi liquid behavior to the paramagnetism state and the non-Fermi liquid behavior in the iron-selenide superconductors demonstrates a unique mechanism for their quantum critical transition.

Figure 1

Temperature dependence of the electrical resistance of a ${\mathrm{K}}_{0.8}{\mathrm{Fe}}_{1.7}{\mathrm{Se}}_2$ single crystal measured at different pressures and in the temperature range of (a) 4.2–290 K and (b) 4.2–50 K. The arrow in (a) shows a transition temperature $T_H$ of the resistance in a hump shape from its high-temperature semiconducting to low-temperature metallic behavior. The arrow in (b) denotes the superconducting transition temperature $T_c$.

Figure 2

(a) Pressure dependence of the superconducting transition temperature $T_c$ and power $\alpha$ obtained from fits with $\rho ={\rho }_0+A{T}^{\alpha }$ for ${\mathrm{K}}_{0.8}{\mathrm{Fe}}_x{\mathrm{Se}}_2$ ($x=1.7$ and 1.78) single crystals. The acronym SC stands for superconducting region. (b) The x-ray diffraction patterns of ${\mathrm{K}}_{0.8}{\mathrm{Fe}}_{1.78}{\mathrm{Se}}_2$, performed with a wavelength of 0.6888 Å. (c) Intensity of the superstructure peak (110) of Fe vacancies as a function of pressure. The insets of Fig. 2c display the schematics of the AFM state, in which the spin order is ferromagnetic groups oriented along the $c$ axis and couples antiferromagnetically, and the PM state, which is in a higher-pressure region.

Figure 3

(a) X-ray diffraction patterns of the ${\mathrm{K}}_{0.8}{\mathrm{Fe}}_{1.7}{\mathrm{Se}}_2$ sample collected at different temperatures and ambient pressure. (b),(c) The refined lattice parameters $a$ and $c$ as a function of temperature.

Figure 4

Temperature dependence of the resistance of a ${\mathrm{K}}_{0.8}{\mathrm{Fe}}_{1.7}{\mathrm{Se}}_2$ single crystal with magnetic fields applied along the $c$ axis of 0, 3, and 7 T, respectively. The inset is an extended view, with normalized resistance at 35 K around the superconducting transition.

Figure 5

Pressure dependence of (a) $T_H$ in the hump shape and (b) the activation energy of the electric transport of the high-temperature resistivity of a ${\mathrm{K}}_{0.8}{\mathrm{Fe}}_{1.7}{\mathrm{Se}}_2$ single crystal. The line in (b) is the linear fitting to the data points. The vertical dashed line denotes the phase boundary.