Pressure Induced Stripe-Order Antiferromagnetism and First-Order Phase Transition in FeSe

To elucidate the magnetic structure and the origin of the nematicity in FeSe, we perform a high-pressure ${}^{77}\mathrm{Se}$ NMR study on FeSe single crystals. We find a suppression of the structural transition temperature with pressure up to about 2 GPa from the anisotropy of the Knight shift. Above 2 GPa, a stripe-order antiferromagnetism that breaks the spatial fourfold rotational symmetry is determined by the NMR spectra under different field orientations and with temperatures down to 50 mK. The magnetic phase transition is revealed to be first-order type, implying the existence of a concomitant structural transition via a spin-lattice coupling. Stripe-type spin fluctuations are observed at high temperatures, and remain strong with pressure. These results provide clear evidence for strong coupling between nematicity and magnetism in FeSe, and therefore support a universal scenario of magnetic driven nematicity in iron-based superconductors.

Figure 1

The ($P$, $T$) phase diagram with $T_s$, $T_N$, $T_c$, and $T^{*}$ (see text for definition). The S-AFM phase is observed below $T_N$ above 2 GPa. The first-order magnetic transition is indicated by the bold solid line of $T_N$. The solid line connecting the $T_c$ points presents superconductivity observed in both the rf inductance and the $1/T_1$, and the dashed line presents superconductivity only seen in the rf inductance.

Figure 2

(a) The characteristic ${}^{77}\mathrm{Se}$ spectra showing the structure transition by the line splitting with $H\parallel a\&b$ at $P=0.56\mathrm{GPa}$. (b) The difference of Knight shifts, ${\mathrm{\Delta }}^{77}{K}_{a,b}={|}^{77}{K}_a{-}^{77}{K}_b|$, measured as functions of temperature at various pressures. The solid lines are fits to mean-field functions ${\mathrm{\Delta }}^{77}{K}_{a,b}\sim 1/(T_s-T{)}^{1/2}$.

Figure 3

(a) and (b) The NMR spectra with $H\parallel a\&b$ and $H\parallel c$ at $P=2.4\mathrm{GPa}$. (c) The normalized total spectral weight as a function of temperature at high pressures. $T_c$ are determined by the rf inductance under the same field. (d) The $1/{}^{77}{T}_1$ for $H\parallel a\&b$ (averaged for two frequency peaks below $T_s$) and $H\parallel c$, at $P=1.34$ and 2.4 GPa. Inset: the anisotropy factor $R=(1{/}^{77}{T}_1{)}_{H\parallel a\&b}/(1{/}^{77}{T}_1{)}_{H\parallel c}$ as a function of temperature. The dashed and dotted horizontal lines are the theoretical values for the stripe-type ($R=1.5$) and for the checkerboard-type ($R=0.5$) spin fluctuations.

Figure 4

(a),(b),(c) The spin-lattice relaxation rate divided by temperature ($1{/}^{77}{T}_{1}T$) with $H\parallel a\&b$ at typical pressures. Only the data measured on the high-frequency peaks are presented below $T_s$. The $T_c$ (determined by rf inductance under the same field), the $T_N$, and the $T^{*}$ are marked by the arrows. (d) The plot of $1{/}^{77}{T}_{1}T$ at 100 K as a function of pressure.