Extended Data Fig. 5: Comparison of NQR measured 1/T₁T at 0.08 GPa and 2.00 GPa.

a, Low-temperature 1/T₁T at 0.08 GPa by ¹²³Sb NQR measurement on Sb1 sites. The dashed line is the guide for the eyes; b, temperature-dependent 1/T₁T at 2.00 GPa by ¹²³Sb and ¹²¹Sb NQR measurements on Sb1 sites. The filled and open pentagons represent 1/T₁T measured on ¹²³Sb and ¹²¹Sb respectively. The longitudinal axis scale on the right-hand side belonging to ¹²¹Sb NQR is 3.41 times larger than that on the left-hand side belonging to ¹²³Sb NQR. Usually, the relaxation rate for the T₁ process in NQR has two kinds of relaxation channels. One is the magnetic relaxation channel and the other is the quadrupole relaxation channel. These two relaxation channels can be identified by checking the ratio between the relaxation rates at different isotopes. The magnetic relaxation channel requires \({T}_{1}^{-1}\propto {\gamma }_{n}^{2}\), where γ_n is the nuclear gyromagnetic ratio. The expected ratio between nuclear spin-lattice relaxation rate at ¹²¹Sb and ¹²³Sb through the magnetic relaxation channel is expressed as \(\frac{{T}_{1M}^{-1}(121)}{{T}_{1M}^{-1}(123)}=\frac{{(10.189)}^{2}}{{(5.51756)}^{2}}=3.41\). The quadrupole relaxation channel requires \({T}_{1}^{-1}\propto 3(2I+3){Q}^{2}/[10(2I-1){I}^{2}]\), where Q is the quadrupole moment. Thus, the expected ratio between the nuclear spin-lattice relaxation rate at ¹²¹Sb and ¹²³Sb through the quadrupole relaxation channel is \(\frac{{T}_{1Q}^{-1}(121)}{{T}_{1Q}^{-1}(123)}=1.5\) (ref. ⁶⁶). We checked this ratio at two temperatures above T_c and found that it is very close to 3.41. Therefore, the magnetic relaxation channel dominates T₁ in the NQR measurement.

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