arXiv subjects

Mengqiao Geng

Publications and source records attributed to Mengqiao Geng.

3 recordsLinked to original sources

arXiv2025

Robust triple-q magnetic order with trainable spin vorticity in Na$_2$Co$_2$TeO$_6$

Recent studies suggest that the candidate Kitaev magnet Na$_2$Co$_2$TeO$_6$ possesses novel triple-$\mathbf{q}$ magnetic order instead of conventional single-$\mathbf{q}$ zigzag order. Here we present dedicated experiments in search for distinct properties expected of the triple-$\mathbf{q}$ order, namely, insensitivity of the magnetic domains to weak $C_3$ symmetry-breaking fields and fictitious magnetic fields generated by the spin vorticity. In structurally pristine single crystals, we show that $C_3$ symmetry-breaking in-plane uniaxial strains do not affect the order's magnetic neutron diffraction signals. We further show that $\mathbf{c}$-axis propagating light exhibits large Faraday rotations in the ordered state due to the spin vorticity, the sign of which can be trained via the system's ferrimagnetic moment. These results are in favor of the triple-$\mathbf{q}$ order in Na$_2$Co$_2$TeO$_6$ and reveal its unique emerging behavior.

cond-mat.str-el↗

arXiv2024

A High Resolution Dilatometer Using Optical Fiber Interferometer

We introduce a high performance differential dilatometer based on an all-fiber Michelson interferometer at cryogenic temperature with $10^{-10}$ resolution in $δL/L$. It resolve the linear thermal expansion coefficient by measuring the oscillating changes of sample thickness and sample temperature with the interferometer and in-situ thermometer, respectively. By measuring the linear thermal expansion coefficient $α$ near the antiferromagnetic transition region of BaFe$_2$As$_2$ as a demonstration, we show our dilatometer is able to measure thin samples with sub-pm-level length change resolution and mK-level temperature resolution. Despite there is residual background thermal expansion of a few nm/K in measurement result, our new dilatometer is sitll a powerful tool for study of phase transition in condensed matter physics, especially significant advantages in fragile materials with sub-100$μ$m thickness and being integrated with multiple synchronous measurements and tuning thanks to the extremely high resolution and contactless nature. The prototype design of this setup can be further improved in many aspects for specific applications.

physics.ins-det↗

arXiv2024

Electronic Phase Propagation Speed in BaFe$_2$As$_2$ Revealed by Dilatometry

Thermal expansion offers deep insights into phase transitions in condensed matter physics. Utilizing an advanced AC-temperature dilatometer with picometer resolution, this study clearly resolves the antiferromagnetic and structural transition in BaFe$_2$As$_2$. The implementation of temperature oscillation reveals a hysteresis near the transition temperature $T_\mathrm{N}$ with unprecedented resolution. Unexpectedly, we find that the hysteretic width exhibits a universal dependence on the parameters of temperature oscillation and the sample's longidutinal dimension, which in turn reveals a finite transition speed. Our quantitative analysis shows that this phase boundary propagates at a mere 188 $μ$m/s - a speed seven orders of magnitude slower than acoustic waves. It suggests a hidden thermodynamic constraint imposed by the electronic degrees of freedom. Our research not only sheds light on the dynamics of phase transitions between different correlated phases, but also establishes high precision dilatometry as a powerful tool for material studies. This measurement technique, when properly modified, can be extended to studies of other material properties such as piezoelectric, magneto-restriction, elastic modulus, etc.

cond-mat.supr-con↗