Contrasting impurity-induced magnetism and dynamics in 2H-MoTe2

We investigate the behavior of interstitial $^8$Li$^+$ implanted near the surface of 2H-MoTe$2$ using $β$-detected NMR. We find that, unlike the muon, $^8$Li$^+$ does not show any signature of induced magnetism. This result is consistent with density functional theory, which identifies the Li stopping site at the 2a Wyckoff position in the van der Waals gap and confirms the absence of detectable Li-induced electronic spin polarization. Both the spin-lattice relaxation and the resonance lines show evidence of strong spin dynamics above $\sim 200$ K, reminiscent of local stochastic $^8$Li$^+$ motion within a cage. The resonance line shape consists of quadrupolar satellites on top of a broad central peak. To better understand the interaction of $^8$Li$^+$ with the host material, we employ a frequency-comb measurement, by simultaneously exciting four frequencies corresponding to the first-order quadrupolar satellite transitions, $ν_0 \pm 3ν{\mathrm{comb}}$ and $ν_0 \pmν_{\mathrm{comb}}$ around the Larmor frequency $ν_0$ as a function of $ν_{\mathrm{comb}}$. This offers an enhanced sensitivity to the quadrupolar split portion of the line. Using this method, we find a small decrease of the quadrupolar frequency with increasing temperature, showing the typical behavior associated with thermally excited phonons and the absence of any magnetic response which was observed with other defects in 2H-MoTe$_2$.

💡 Research Summary

In this work the authors employ β‑detected nuclear magnetic resonance (β‑NMR) to probe the local magnetic and electric environment of spin‑polarized ⁸Li⁺ ions implanted near the surface of a 2H‑MoTe₂ single crystal. The motivation stems from earlier μ⁺ studies that reported magnetic signatures when muons occupy the van‑der‑Waals (vdW) gap of this transition‑metal dichalcogenide (TMD). By contrast, ⁸Li⁺ carries a nuclear spin I = 2, a sizable quadrupole moment, and can be used as a charged impurity probe. The ions were implanted at 22.5 keV, yielding an average depth of ~110 nm, as confirmed by TRIM.SP simulations. The β‑NMR experiments were carried out on the ISA‑C beamline at TRIUMF, with spin‑lattice relaxation (SLR) measured in a 10 mT in‑plane field and resonance spectra recorded in a 6.55 T out‑of‑plane field.

Density‑functional theory (DFT) calculations (PBE functional, VASP/OPENMX) were performed on a 3 × 3 × 1 supercell to locate the energetically favorable Li⁺ site and to evaluate diffusion pathways. The calculations identify a single stable site at the 2a Wyckoff position within the vdW gap, possessing D₃d point‑group symmetry. This site generates a non‑zero electric‑field gradient (EFG), predicting a quadrupolar splitting of the ⁸Li resonance into four satellite lines. Importantly, the spin‑resolved density of states shows no induced magnetic moment at the Li site for either neutral or positively charged supercells, indicating that Li⁺ does not polarize the surrounding electrons.

Experimentally, the β‑NMR spectra consist of a broad central line present at all temperatures, accompanied by weak quadrupolar satellites that become resolvable below ~260 K. Above this temperature the satellites merge into the central line, suggesting motional averaging of the EFG. To enhance the detection of the satellites, the authors introduce a frequency‑comb technique: four RF frequencies (ν₀ ± ν_comb and ν₀ ± 3ν_comb) are applied simultaneously, exciting all first‑order quadrupolar transitions at once. This method dramatically improves the signal‑to‑noise ratio of the satellite peaks, allowing precise extraction of the quadrupolar frequency ν_q as a function of temperature.

The temperature dependence of ν_q follows a power‑law ν_q = ν_q0 (1 − c T^{3/2}) with ν_q0 ≈ 3.603 kHz and c ≈ 2.33 × 10⁻⁵ kHz K⁻³⁄², a behavior typical of phonon‑induced softening of the EFG. The central line frequency is essentially temperature‑independent, while its linewidth broadens on cooling, reflecting static disorder or slow fluctuations of the local environment.

Spin‑lattice relaxation data reveal a bi‑exponential decay, indicating two distinct Li⁺ populations or dynamical regimes. The fast component (1/T₁,f) and the slow component (1/T₁,s) each display temperature‑dependent peaks: a prominent peak near 240 K and a smaller one around 15 K. An Arrhenius fit to the low‑temperature flank of the 240 K peak yields an activation energy E_A ≈ 0.2 eV. The authors interpret this as the onset of local stochastic motion of Li⁺ within a “cage” formed by the surrounding lattice, which modulates the EFG and enhances quadrupolar relaxation. However, this activation energy is considerably lower than the ~1 eV diffusion barrier predicted by DFT, suggesting that the observed dynamics may involve localized hopping or anharmonic vibrations rather than long‑range diffusion.

Crucially, throughout all measurements no signature of magnetic ordering or impurity‑induced magnetism is observed. This contrasts sharply with μ⁺ results and confirms the DFT prediction of negligible spin polarization around Li⁺. The combination of β‑NMR and the frequency‑comb approach thus proves highly sensitive to electric‑field gradients and ionic dynamics, while remaining blind to magnetic effects that are absent for this impurity.

In summary, the study demonstrates that interstitial ⁸Li⁺ in 2H‑MoTe₂ acts as a non‑magnetic charge probe that reveals temperature‑dependent lattice dynamics and phonon‑driven EFG softening, but does not induce local magnetic moments. The work validates the use of β‑NMR with advanced excitation schemes for probing subtle quadrupolar interactions in layered vdW materials and provides valuable insight into ion mobility and defect behavior in TMDs, which is relevant for applications in ion‑based energy storage, catalysis, and the engineering of heterostructures.