Unusual antiferromagnetic order and fluctuations in RbMn$_{6}$Bi$_{5}$

TL;DR

This work investigates the unusual antiferromagnetic order in the quasi-one-dimensional Mn-based superconductor candidate RbMn$_{6}$Bi$_{5}$. Using $^{55}$Mn and $^{87}$Rb NMR, the authors map a commensurate AFM structure with five non-equivalent Mn sites and a nonmagnetic central Mn, and detect strong AFM fluctuations in the paramagnetic state that intensify toward $T_{\rm N}$. They observe a first-order transition with phase coexistence and a charge rearrangement that contrasts with neutron-diffraction SDW models, supporting a localized-moment picture. The proximity of robust AFM fluctuations to the high-pressure superconducting phase suggests magnetic excitations could mediate unconventional pairing, providing insights into how geometric frustration and charge ordering interact with superconductivity in Mn-based materials.

Abstract

Quasi-one-dimensional RbMn$_{6}$Bi$_{5}$, the first pressure-induced ternary Mn-based superconductor, exhibits a phase diagram analogous to those of cuprate and iron-based superconductors, with superconductivity neighboring antiferromagnetic order. Here, we use $^{55}$Mn and $^{87}$Rb nuclear magnetic resonance (NMR) to unravel its magnetic structure and fluctuations. Above the Néel temperature ($T_{\rm N}$), strong antiferromagnetic fluctuations dominate, characteristic of a paramagnetic state with pronounced spin-lattice relaxation rate enhancement. Below $T_{\rm N}$, a first-order phase transition establishes a commensurate antiferromagnetic order, where Mn atoms at the pentagon corners exhibit distinct magnetic moments with different orientations, while the central Mn atom carries no magnetic moment. The complex magnetic architecture, revealed by zero-field and high-magnetic-field NMR spectra, contrasts with earlier neutron diffraction models proposing uniform spin density waves, instead supporting localized moments ordering with charge rearrangement. The proximity of robust antiferromagnetic fluctuations to the high-pressure superconducting phase suggests a potential role for magnetic excitations in mediating unconventional Cooper pairing, akin to paradigmatic high-$T_c$ systems. These findings provide critical insights into the interplay between geometric frustration, magnetic order, and superconductivity in manganese-based materials.

Figures (4)

• Figure 1: Zero field $^{55}$Mn NMR spectra. (a) The spectra of $A$Mn$_{6}$Bi$_{5}$ with $A=$ Cs, Rb, K, and Na at $4.2$ K. The inset illustrates the crystal structure of RbMn$_{6}$Bi$_{5}$. (b) The temperature-dependent spectra of NaMn$_{6}$Bi$_{5}$. The inset is the enlarged view of the coexistence zone. (c) The temperature-dependent spectra of RbMn$_{6}$Bi$_{5}$. The baselines of all curves are shifted to the value of their temperature on the right-hand ordinates. The spectra are scaled by intensity multiplied by temperature. The internal field, proportional to the resonance frequency, is labeled at the top.
• Figure 2: Magnetic structure is deduced from spectra and quadrupole splitting. (a) The spectrum of $^{55}$Mn at $7$ K. (b) The quadrupole splitting. The line widths are the full width at half maximum of each peak. The inset shows the magnetic moment directions on the pentagon. The dashed lines indicate the calculated EFG principal axes. The labels indicate the peaks from low to high frequency, and the colors represent the relationships between the peaks and the magnetic moments. (c) The enlarged view of the peaks from site 1. There are five peaks due to quadrupole splitting. (d) Comparison of spectra at zero field and an applied $0.215$ T field. (e,f,g,h) The $^{55}$Mn echo amplitude as a function of inter-pulse delay $\tau$ is measured at the frequencies marked by the corresponding colored dash-line in (a). The inverse of oscillation periods give the splitting frequencies which are shown in (b).
• Figure 3: Spectra of the central $^{55}$Mn (a)$B_{0} \parallel b$ and (b)$B_{0} \perp b$. Spectra of $^{87}$Rb (c)$B_{0} \parallel b$ and (d)$B_{0} \perp b$. The baselines of all curves are shifted to the value of their temperature on the right-hand ordinates. The spectral intensity of $^{55}$Mn at $4.2$ K is multiplied by $0.42$ to avoid overlap. The vertical dash-lines mark the frequency of $^{55}\gamma B_{0}$ and $^{87}\gamma B_{0}$, corresponding to $^{55}$Mn and $^{87}$Rb respectively. The spectra of $^{87}$Rb in both directions show the coexistence of the paramagnetic phase and ordered phase in the temperature range from $75$ K to $80$ K, which is shown on an enlarged scale. Below $T_{\rm N}$, the spectra of $^{55}$Mn split for both directions. For $^{87}$Rb, the spectra split only for $B_{0} \parallel b$, with two sets of splitting peaks are marked by red and blue arrows, respectively.
• Figure 4: NMR shift and spin-lattice relaxation rate of (a)(b) $^{55}$Mn and (c)(d) $^{87}$Rb are presented, respectively. The regions corresponding to magnetic order states are marked in green. (e) $K$ is plotted against the bulk susceptibility for both $^{55}$Mn and $^{87}$Rb. The lines represent the fits to a linear relation, and the slopes are the diagonal hyperfine coupling constants. The temperature dependence of $\nu_{\rm Q}$ for (f) Rb and (g) Mn is presented, respectively.