Field-enhanced quantum fluctuation in an $S=\frac{1}{2}$ frustrated square lattice

We present a new model compound with the $S=1/2$ frustrated square lattice composed of the charge-transfer salt ($o$-MePy-V)${\mathrm{PF}}_6$. $Abinitio$ calculations indicate the formation of an $S=1/2$ square lattice, in which six types of nearest-neighbor ferromagnetic and antiferromagnetic interactions cause frustration. By applying a magnetic field, we observe an unusually gradual increase of magnetization and a subsequent 1/2-plateau-like behavior. A numerical analysis using the tensor network method qualitatively demonstrates such behaviors and suggests a collinear ordered state and a field-enhanced quantum fluctuation. Furthermore, the local magnetization and $T_1^{-1}$ probed by nuclear magnetic resonance measurements support these findings.

Figure 1

(a) Molecular structure of ($o$-MePy-V)${\mathrm{PF}}_6$. (b) Crystal structure of ($o$-MePy-V)${\mathrm{PF}}_6$ viewed parallel to the $a$ axis. Hydrogen atoms are omitted for clarity. The broken lines indicate the crystallographically independent sites, site 1 and site 2, along the $b$ axis. (c) $S=1/2$ square lattice composed of $J_i$ ($i=1-6$). The left table lists the values of the exchange interactions evaluated from the $abinitio$ MO calculation.

Figure 2

Specific heat of ($o$-MePy-V)${\mathrm{PF}}_6$ at (a) zero-field and (b) various magnetic fields. The solid line shows the fit of $C/T\propto T$ below approximately 1 K. The inset shows the evaluated entropy at zero-field. The arrows indicate the phase transition temperatures.

Figure 3

Magnetization curve of ($o$-MePy-V)${\mathrm{PF}}_6$ at 1.3 K. The lower inset shows the temperature dependence of magnetic susceptibility ($\chi =M/H$) at 0.5 T.

Figure 4

The ground states under magnetic fields obtained from $D=6$ iTPS calculation. (a) Normalized magnetization curve, (b) average local magnetization, (c) average local moment at $T=0$ calculated using the tensor network method assuming the ratios of the evaluated exchange constants. The illustrations describe the predicted collinear spin structure at $H=0$ and field-enhanced reduction of the local moment near the 1/2-plateau-like phase.

Figure 5

(a) ${}^{31}\mathrm{P}$-NMR spectra of ($o$-MePy-V)${\mathrm{PF}}_6$ at 1.4 K with various magnetic fields for $H//b$. (b) Local magnetic field $H_{\mathrm{loc}}$ of P(1) and P(2) evaluated from the spectra. (c) Temperature dependence of $T_1^{-1}$ for P(1) and P(2) at 2.5 and 5.0 T.