Emergent Spin-Singlet Pairing in the Frustrated Kagome Metal Sc$_3$Mn$_3$Al$_7$Si$_5$

TL;DR

This study addresses whether a disorder-free metallic kagome magnet can host spin-singlet correlations without long-range magnetic order. It combines bulk thermodynamics, transport on FIB-fabricated devices, ESR, and site-specific $^{55}$Mn NMR to map low-energy spin dynamics in Sc$_3$Mn$_3$Al$_7$Si$_5$. The authors find strongly suppressed magnetic entropy, a low-$T$ resistivity upturn with negative magnetoresistance, and a subtle heat-capacity feature near $T^{*}\sim12$ K, accompanied by a Hebel-Slichter-like peak in $T_1^{-1}$ around 10 K, all indicative of emergent short-range spin-singlet correlations. NMR further reveals a pronounced $T_2^{-1}$ at low $T$ arising from indirect nuclear coupling via electronic spin fluctuations, consistent with partially gapped spin excitations and a spin-gap scale of $ ilde{T}\approx7.1$ K. Collectively, these results place Sc$_3$Mn$_3$Al$_7$Si$_5$ among metallic kagome systems near a quantum spin liquid and highlight a BR-type indirect nuclear coupling as a sensitive probe of hidden magnetic dynamics in correlated metals.

Abstract

The metallic kagome compound Sc$_3$Mn$_3$Al$_7$Si$_5$ has attracted attention as a candidate platform where geometric frustration and itinerant electrons may cooperate to stabilize a quantum-disordered magnetic ground state. Here, we combine bulk thermodynamic probes, low-noise FIB-device transport, and comprehensive $^{55}$Mn Nuclear Magnetic Resonance (NMR) measurements to elucidate the low-temperature spin dynamics of this system. The bulk data reveal strongly reduced magnetic entropy, a negative magnetoresistance arising from spin scattering, and field-dependent transport indicates the spin fluctuations, while showing no signatures of long-range magnetic order. NMR provides a direct local view of the correlated Mn moments: the nuclear spin-spin relaxation $T_2$ exhibits a pronounced low-temperature enhancement driven by an indirect internuclear coupling through electronic spin fluctuations, whose temperature and distance dependence point to partially gapped low-energy spin excitations. The spin-lattice relaxation rate $T_1^{-1}$ displays a Hebel-Slichter-like coherence peak near \SI{10}{K}, coincident with the resistivity crossover and a subtle heat-capacity anomaly, indicating the formation of short-range spin-singlet correlations. Together, our results demonstrate that Sc$_3$Mn$_3$Al$_7$Si$_5$ hosts an unconventional correlated state dominated by frustrated, gapped spin dynamics, placing it among the rare metallic kagome systems proximate to a quantum spin liquid.

Figures (13)

• Figure 1: (a) Crystal structure of Sc$_3$Mn$_3$Al$_7$Si$_5$, (b) Temperature dependent DC susceptibility $\chi(T)$ and modified CW with the field along $c$ and $ab$. (c) Temperature dependent DC susceptibility $\chi(T)$ at different fields for $B_0\parallel c$. (d) $C_p/T$ versus $T$ in the presence of various fields. (e) Details of the temperature dependent longitudinal resistivity $(\rho_{\mathrm{xx}}$) at various fields. See Fig. \ref{['FigS04']} in the SI for the full temperature range. (f) Field dependence of magnetoresistance $\textrm{MR}(\%)=100\times(\rho-\rho(0)) / \rho(0)$ at various temperatures for $I\parallel ab$ and $B\parallel c$.
• Figure 2: (a) The $^{55}$Mn central transition (CT) in frequency units (kHz) at 75K for 3 different magnetic fields applied parallel to the $c$-axis. The gray line shows the approximate line broadening as expected from direct dipole-dipole interactions ($\sim10kHz$) Vleck1948. (b) Temperature dependent linewidth (full width at half maximum) in frequency units of the $^{55}$Mn CT for 3 different fields.
• Figure 3: (a) Signal intensity ($T_2$ corrected) for various pulse lengths (excitation band widths) and two example resonances for short ($1\mu s$) and long ($6\mu s$) pulses. (b) The spin-spin relaxation rate ($T_2^{-1}$) and inverse CT lifetime ($T_{1,\mathrm{exp}}^{-1}$) as functions of temperature for the 3 external fields 3.86T, 7.72T, and 11.53T. The grey bar separates the two temperature regimes (see text for details). The very small relaxation rates at the lowest temperatures (pink circle) denote isolated spins that decay via direct dipolar couplings. (c) Spin-spin relaxation rate after subtraction of the corresponding CT lifetime. Grey parallel lines are guides to the eye, corresponding to an exponential function with $\tilde{T}=7.1K$. (d) $T_2$ measurement (signal intensity as function of pulse delay) for 4.2K, 7.73T, and various excitation conditions (100%, 73%, and 44% excited nuclei). Solid lines are fittings using a sum of a single exponential ($T_2$) and a Gaussian decay ($T_{\mathrm{2G}}$). (e) The spin-spin relaxation rates as a function of the mean distance between Mn nuclei for various temperatures.
• Figure 4: (a) The spin-lattice relaxation rate divided by the temperature $(T_1T)^{-1}$ as a function of temperature for 3 different magnetic fields, including the calculated behavior from the DOS (blue). (inset) The density of states (DOS) for Mn $d$-orbitals (red) near the Fermi level. To extract the temperature dependence of the spin-lattice relaxation, we calculated $T_1^{-1}\propto\int f(E-\mu_c)[1-f(E-\mu_c)]D(E)^2\mathrm{d}E$, using the Fermi function $f(E-\mu_c)$ for various temperatures. The blue line denotes the integrant for 300K representing the range of thermally excited states available for nuclear relaxation.
• Figure S1: Photographic image of as grown crystals of Sc$_3$Mn$_3$Al$_7$Si$_5$ single crystals.