Interplay between magnetism, structure, and strong electron-phonon coupling in binary FeAs under pressure

Unlike the ferropnictide superconductors, which crystallize in a tetragonal crystal structure, binary FeAs forms in an orthorhombic crystal structure, where the local atomic environment resembles a highly distorted variant of the FeAs${}_4$ tetrahdedral building block of the ferropnictide superconductors. However, like the parent compounds of the ferropnictide superconductors, FeAs undergoes magnetic ordering at low temperatures, with no evidence favoring a superconducting ground state at ambient pressure. We employ pressure-dependent electrical transport and x-ray diffraction measurements using diamond anvil cells to characterize the magnetic state and the structure as a function of pressure. While the MnP-type structure of FeAs persists up to 25 GPa, compressing continuously with no evidence of structural transformations under pressure, features in the magnetotransport measurements associated with magnetism are not observed for pressures in excess of 11 GPa. Where observable, the features associated with magnetic order at ambient pressure show remarkably little pressure dependence, and transport measurements suggest that a dynamical structural instability coupled to the Fermi surface via a strong electron-phonon interaction may play an important role in enabling magnetism in FeAs.

Figure 1

Crystal structures of some superconducting ferropnictide compounds (bottom) and binary FeAs (upper left). The superconducting compounds are composed of corrugated FeAs-based cages (upper right), which form regular, separated layers within the superconducting structures. A distorted version of these FeAs-based cages can be seen within the FeAs (Pnma) structure. Unit cells are outlined with light blue dashed lines.

Figure 2

Example x-ray diffraction pattern (solid line) obtained at 6.9 GPa. The pattern has been indexed to the FeAs Pnma structure, for which many of the prominent Bragg reflections have been labeled. The Bragg peaks from the Cu pressure marker are also labeled. Contributions from the gasket and the Ne pressure-transmitting medium are denoted by $g$ and n, respectively. The inset shows a low-angle portion of selected diffraction patterns, highlighting the shift of the (011), (102), and (111) Bragg peaks with compression of the lattice under pressure. Dashed lines are guides to the eye.

Figure 3

The evolution of the lattice parameters (a)–(c) and the unit cell volume (d) of FeAs under pressure as determined from this work (blue squares) and previously reported results of Lyman and Prewitt (red circles) (Ref. 24). The solid lines in (a)–(c) are guides to the eye, while the solid line in (d) is a fit to the third-order Birch-Murnaghan equation of state. Up to 25 GPa, the crystal structure remains well indexed by the orthorhombic (Pnma) MnP-type structure.

Figure 4

Normalized temperature-dependent electrical resistivity $(\rho -{\rho }_0)/{\rho }_0$ for selected pressures. The ambient-pressure curve corresponds to a measurement on a single crystal, and has been normalized to fit within the trend observed for the RRR in the powder sample under pressure.

Figure 5

(a) Temperature derivative of the resistance, $\mathit{dR}/\mathit{dT}$, versus temperature at pressures below 15 GPa. The dashed line is a guide to the eye, emphasizing the pressure dependence of the temperature where $\mathit{dR}/\mathit{dT}$ is a maximum ($T_1$). The downward arrow indicates a broad feature in the derivative associated with a subtle inflection point in $R(T)$ data ($T_2$). (b) The Hall coefficient $R_H$ as a function of temperature at various pressures, including ambient-pressure data from Segawa and Ando (Ref. 12). The onset of magnetism at ambient pressure corresponds to a cusp in the $R_H(T)$ curve. The downward arrows mark the positions of the cusp and kink in the ambient-pressure $R_H$ data.

Figure 6

Temperature-pressure phase diagram of FeAs characteristic temperatures corresponding to features in the electrical resistivity and Hall coefficient: red, closed circles, $T_N$; black, open circles, $T_1$; blue squares, $T_2$; green diamonds, $T_{H1}$; and orange triangles, $T_{H2}$. Pressure error bars include pressure changes with thermal contraction of the DAC, while temperature error bars estimate a reasonable uncertainty due to feature broadening. Regions A, B, and C are partitioned with vertical, dashed lines, and the red, shaded region corresponds to the proposed region of the phase diagram where SDW magnetism exists (Sec. 4). Solid and dashed lines connecting data points are guides to the eye. The left inset details the pressure dependence of $\Delta \rho /{\rho }_L$ for $T=290$ and at 15 K. The right inset displays the evolution of the RRR with applied pressure. Vertical gray bars indicate the pressure region where the pressure dependence of $\Delta \rho /{\rho }_L$ and the RRR change.

Figure 7

(a) High-pressure normalized electrical resistivity in region C of Fig. 6 with fits (solid lines) to Eq. (1). (b) Extracted characteristic temperature scale $T_0$ (red circles) from Eq. (1) likely corresponding to a zone boundary phonon that enables interband scattering and the effective electron-phonon coupling scaled to the residual resistivity ${\rho }_2/{\rho }_0$ (blue squares); the solid lines are guides to the eye.