Florian Bärtl | Searcharxiv

arXiv2025

Transmission of radio-frequency waves and nuclear magnetic resonance in lanthanum superhydrides

The discovery of near-room temperature superconductivity in the lanthanum hydride LaH$_{10}$ has revolutionized this field of research. However, the need to use diamond anvils for the synthesis of such superconductors severely limits the number of experimental techniques to study these materials. Nuclear magnetic resonance (NMR) is one of the key methods for probing spin systems of superconductors. In this work, we show how $^1$H NMR measurements can be realized in diamond anvil cells to study high-temperature superconductivity in lanthanum polyhydrides at pressures up to 165 GPa. In the newly discovered superhydride LaH$_{12}$, we observed a pronounced suppression of the $^1$H NMR signal intensity below $\textit{T$_{c}$(onset)}$ = 260 K in a magnetic field of 7 T, corresponding to the screening of the radio-frequency pulses. Below the critical temperature of superconductivity, all $^1$H NMR characteristics, including the spin-lattice relaxation rate $\textit{1/T$_{1}$T}$, demonstrate pronounced features, evidencing the bulk nature of the superconducting transition. In zero field, the radio-frequency signal transmission through the LaH$_{12}$ sample shows a pronounced drop below $\textit{T$_{c}$(onset)}$ = 267 K, confirming the superconducting nature of the transition. A description of the $\textit{1/T$_{1}$T}$ data with an exponential form allows the estimation of the superconducting gap $\textit{$Δ$(0)}$ lying between 427 and 671 K (corresponding to 36.8 to 57.8 meV), and the ratio $\textit{R$_Δ$ = 2$Δ$(0)/k$_B$T$_c$}$ between 3.76 and 5.16 in the synthesized hydride sample.

cond-mat.supr-con↗

arXiv2025

Evidence of pseudogap and absence of spin magnetism in the time-reversal-symmetry-breaking state of Ba$_{1-x}$K$_x$Fe$_2$As$_2$

Muon-spin-rotation ($μ$SR) experiments and the observation of a spontaneous Nernst effect indicate time-reversal symmetry breaking (BTRS) at $T_{\rm c}^{\rm Z2}$ above the superconducting transition temperature $T_{\rm c}$ in Ba$_{1-x}$K$_x$Fe$_2$As$_2$, with $x\approx0.8$. Further studies have pointed out that BTRS is caused by the formation of a new state of matter associated with the condensation of pairs of electron pairs. Despite exhibiting multiple unconventional effects that warrant further investigation, the electronic spectral properties of this electron quadrupling state remain largely unexplored. Here, we present detailed $^{75}$As nuclear magnetic resonance (NMR) measurements of Ba$_{1-x}$K$_x$Fe$_2$As$_2$, with $x = 0.77$, which has $T_{\rm c}^{\rm Z2}$ > $T_{\rm c}$ according to measurements of the spontaneous Nernst effect. The NMR data obtained in this work provide the first direct electronic spectral characteristics of the electron quadrupling state by indicating that it evolves from a pseudogap that sets in at $T^*$ well above $T_{\rm c}^{\rm Z2}$. This pseudogap behavior is consistent with $μ$SR Knight-shift, specific-heat, and transport data indicating the formation of a bound state of electrons. According to a theory of electron quadrupling condensates, such bound-state formations should precede the onset of BTRS correlations between pairs of electron pairs. The second important insight from NMR data is the absence of spin-related magnetism. The temperature dependence of the spin-lattice relaxation rate $1/T_1T$ and the evolution of the NMR linewidth prove the absence of a magnetic transition at $T_{\rm c}^{\rm Z2}$ and rule out even a proximity to some magnetic instability. This indicates that the spontaneous magnetic fields detected in this compound are not caused by spin magnetism but are associated with persistent real-space currents.

cond-mat.supr-con↗