Suppression of electron correlations in the collapsed tetragonal phase of $\mathrm{Ca}{\mathrm{Fe}}_2{\mathrm{As}}_2$ under ambient pressure demonstrated by ${}^{75}\mathrm{As}$ NMR/NQR measurements

The static and the dynamic spin correlations in the low-temperature collapsed tetragonal and the high-temperature tetragonal phase in $\mathrm{Ca}{\mathrm{Fe}}_2{\mathrm{As}}_2$ have been investigated by ${}^{75}\mathrm{As}$ nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements. Through the temperature ($T$) dependence of the nuclear spin lattice relaxation rates (1$/T_1$) and the Knight shifts, although stripe-type antiferromagnetic (AFM) spin correlations are realized in the high-temperature tetragonal phase, no trace of the AFM spin correlations can be found in the nonsuperconducting, low-temperature, collapsed tetragonal (c$\mathcal{T}$) phase. Given that there is no magnetic broadening in ${}^{75}\mathrm{As}$ NMR spectra, together with the $T$-independent behavior of magnetic susceptibility $\chi$ and the $T$ dependence of 1$/T_{1}T\chi$, we conclude that Fe spin correlations are completely quenched statically and dynamically in the nonsuperconducting c$\mathcal{T}$ phase in $\mathrm{Ca}{\mathrm{Fe}}_2{\mathrm{As}}_2$.

Figure 1

(a) Field-swept ${}^{75}\mathrm{As}$-NMR spectra for the as-grown $\mathrm{Ca}{\mathrm{Fe}}_2{\mathrm{As}}_2$ crystal (quenched from 960 ${}^{\circ }$C) at $f$ = 51 MHz in the high-temperature tetragonal phase (measured at $T=11$0 K) for magnetic field parallel to the $c$ axis (bottom) and perpendicular to the $c$ axis (middle), together with ${}^{75}\mathrm{As}$-NMR spectrum at $T=200$ K for the annealed $\mathrm{Ca}{\mathrm{Fe}}_2{\mathrm{As}}_2$ crystal (quenched from 400 ${}^{\circ }$C after annealed, see text) with $H$ perpendicular to the $c$ axis (top). The black and red lines are observed and simulated spectra, respectively. Expected lines above 8.5 T are not measured due to the limited maximum magnetic field for our SC magnet. (b) Same as described in (a) but in the low-temperature collapsed tetragonal phase (measured at $T=4.2$ K). (Inset) ${}^{75}\mathrm{As}$ NQR spectrum at $T=4.2$ K and $H=0$ T.

Figure 2

(a) Temperature $T$ dependence of ${}^{75}\mathrm{As}$ NMR shifts $K_{\mathrm{ab}}$ and $K_{\mathrm{c}}$ for the as-grown $\mathrm{Ca}{\mathrm{Fe}}_2{\mathrm{As}}_2$. (b) Anisotropic magnetic susceptibility $\chi \equiv M/H$ vs $T$ (where $M$ is magnetization and $H$ is applied magnetic field) for the as-grown $\mathrm{Ca}{\mathrm{Fe}}_2{\mathrm{As}}_2$ crystal measured at $H=1$ T. From the NMR Knight shift measurements shown in (a), the upturns in $\chi \left(T\right)$ below $\sim$50 K are not intrinsic, originating from impurities. The solid lines are corrected $\chi \left(T\right)$ by subtracting the impurity contributions.

Figure 3

Temperature dependence of 1$/T_{1}T$ for both magnetic field directions, $H\parallel c$ axis and $H\parallel ab$ plane and at zero field (NQR) for the as-grown $\mathrm{Ca}{\mathrm{Fe}}_2{\mathrm{As}}_2$, together with the data measured in the annealed $\mathrm{Ca}{\mathrm{Fe}}_2{\mathrm{As}}_2$.

Figure 4

(a) $1/T_{1}T\chi$ vs $T$ for both magnetic field directions, $H\parallel c$ axis and $H\parallel ab$ plane in the as-grown sample. $T$ dependencies of $1/T_{1}T\chi$ for both $H$ directions in the annealed $\mathrm{Ca}{\mathrm{Fe}}_2{\mathrm{As}}_2$ are also plotted for comparison. The increase of $1/T_{1}T\chi$ observed above $T_{\mathrm{s}}$ indicates the growth of the strip-type AFM spin correlations. (b) $T$ dependence of the ratio $r\equiv T_{1,\mathrm{c}}/T_{1,\mathrm{ab}}$. The solid line is an eye guide.