arXiv subjects

Xiaochen Fang

Publications and source records attributed to Xiaochen Fang.

4 recordsLinked to original sources

arXiv2026

Giant spontaneous Kerr effect and its self-doping dependence in altermagnetic MnTe

Altermagnetism, a third class of collinear magnetism with spin-split bands and vanishing net magnetization, has emerged in hexagonal {\alpha}-MnTe, a promising platform for ultrafast, stray-field-free spintronics. Whether MnTe's macroscopic symmetry-breaking signatures reflect ideal altermagnetic order or are activated by defects remains open. Here we report giant spontaneous Kerr rotations of up to 1500 rad in {\alpha}-MnTe single crystals at the telecommunication wavelength of 1550 nm, onsetting precisely at the N\'eel temperature TN = 307 K. The signal appears in disjoint macroscopic patches whose amplitude tracks the sample conductivity, and a hole-conducting {\alpha}-MnTe film reproduces this response together with an anomalous Hall effect. Within a single crystal, Kerr-active regions are optically distinct from their Kerr-silent surroundings in co-registered reflection maps. These observations indicate that carrier self-doping, rather than ideal altermagnetic order alone, governs the giant magneto-optical response, and establish telecom-wavelength Kerr imaging as a practical readout for altermagnetic spintronics.

cond-mat.str-el↗

arXiv2023

Kibble-Zurek mechanism of Ising domains

The formation of topological defects after a symmetry-breaking phase transition is an overarching phenomenon that encodes rich information about the underlying dynamics. Kibble-Zurek mechanism (KZM), which describes these nonequilibrium dynamics, predicts defect densities of these second-order phase transitions driven by thermal fluctuations. It has been verified as a successful model in a wide variety of physical systems, finding applications from structure formation in the early universe to condensed matter systems. However, whether topologically-trivial Ising domains, one of the most common and fundamental types of domains in condensed matter systems, also obey the KZM has never been investigated in the laboratory. We examined two different kinds of three-dimensional (3D) structural Ising domains: clockwise (CW)/counter-clockwise (CCW) ferro-rotation domains in NiTiO3 and up/down polar domains in BiTeI. While the KZM slope of ferro-rotation domains in NiTiO3 agrees well with the prediction of the 3D Ising model, the KZM slope of polar domains in BiTeI surprisingly far exceeds the theoretical limit, setting an exotic example where possible weak long-range dipolar interactions play a critical role in steepening the KZM slope of non-topological quantities. Our results demonstrate the validity of KZM for Ising domains and reveal an enhancement of the power-law exponent and a possible reduction of the dynamic critical exponent z for transitions with long-range interactions.

cond-mat.stat-mech↗

arXiv2022

Ferro-rotational domain walls revealed by electric quadrupole second harmonic generation microscopy

Domain walls are ubiquitous in materials that undergo phase transitions driven by spontaneous symmetry breaking. Domain walls in ferroics and multiferroics have received tremendous attention recently due to their emergent properties distinct from their domain counterparts, for example, their high mobility and controllability, as well as their potential applications in nanoelectronics. However, it is extremely challenging to detect, visualize and study the ferro-rotational (FR) domain walls because the FR order, in contrast to ferromagnetism (FM) and ferroelectricity (FE), is invariant under both the spatial-inversion and the time-reversal operations and thus hardly couple with conventional experimental probes. Here, an FR candidate $\mathrm{NiTiO_{3}}$ is investigated by ultrasensitive electric quadrupole (EQ) second harmonic generation rotational anisotropy (SHG RA) to probe the point symmetries of the two degenerate FR domain states, showing their relation by the vertical mirror operations that are broken below the FR critical temperature. We then visualize the real-space FR domains by scanning EQ SHG microscopy, and further resolve the FR domain walls by revealing a suppressed SHG intensity at domain walls. By taking local EQ SHG RA measurements, we show the restoration of the mirror symmetry at FR domain walls and prove their unconventional nonpolar nature. Our findings not only provide a comprehensive insight into FR domain walls, but also demonstrate a unique and powerful tool for future studies on domain walls of unconventional ferroics, both of which pave the way towards future manipulations and applications of FR domain walls.

cond-mat.mtrl-sci↗

arXiv2022

Vibrational properties of CuInP2S6 across the ferroelectric transition

In order to explore the properties of a two-sublattice ferroelectric, we measured the infrared and Raman scattering response of CuInP2S6 across the ferroelectric and glassy transitions and compared our findings to a symmetry analysis, calculations of phase stability, and lattice dynamics. In addition to uncovering displacive character and a large hysteresis region surrounding the ferroelectric transition temperature T_C, we identify the vibrational modes that stabilize the polar phase and confirm the presence of two ferroelectric variants with opposite polarizations. Below TC, a poorly understood relaxational or glassy transition at Tg is characterized by local structure changes in the form of subtle peak shifting and activation of low frequency out-of-plane Cu- and In-containing modes. The latter are due to changes in the Cu/In coordination environments and associated order-disorder processes. Moreover, Tg takes place in two steps with another large hysteresis region and significant underlying scattering. Combined with imaging of the room temperature phase separation, this effort lays the groundwork for studying CuInP2S6 under external stimuli and in the ultra-thin limit.

cond-mat.mtrl-sci↗