arXiv subjects

Chi Ian Jess Ip

Publications and source records attributed to Chi Ian Jess Ip.

4 recordsLinked to original sources

arXiv2026

A Fermi Surface Driven Spiral Spin Liquid

EuAg$_4$Sb$_2$ is a model material to study the interplay of electronic and spin texture degrees of freedom, exhibiting numerous multi-$q$ magnetic textures coupled with the electronic properties. It is generally understood that some combination of conduction-electron mediated interactions, frustration, and higher order interactions are responsible for complex incommensurate spin textures in centrosymmetric lanthanide materials. Here, we refine an effective model of the magnetic interactions in EuAg$_4$Sb$_2$ through measurements of diffuse magnetic neutron scattering above the ordering temperature. These diffuse measurements reveal a ring of fluctuating spin modulations that reflects a manifold of nearly degenerate propagation vectors known as a spiral spin liquid (SSL). We further identify that this approximate $U$(1) symmetric SSL emerges from magnetic interactions mediated by a quasi-2D hole pocket and exhibits critical scaling of the spatial correlations. Further, Monte Carlo simulations reveal excellent agreement with experiment and provide a comprehensive understanding of the phase diagram. This study emphasizes the connection between the rich spin textures in this material, the electronic structure, and spin liquidity$\unicode{x2014}$uncovering new insights into design principles for nano-scale spin texture materials with advantageous intertwined electronic, magnetic, and topological properties, and new mechanisms for generating the physics of spiral spin liquids.

cond-mat.str-el↗

arXiv2025

Cascade of Spin Moiré Superlattices with In-Plane Field in Triangle Lattice Semimetal EuAg$_4$Sb$_2$

EuAg$_4$Sb$_2$ is a rhombohedral europium triangle lattice material that exhibits a rich phase diagram of spin moiré superlattices (SMS) and single-$q$ magnetic phases. In this paper, we characterize the incommensurate phases accessible with field applied in the plane with small angle neutron scattering (SANS). A variety of phases with unusual SANS patterns are accessible with magnetic field applied along the $a$ and $a^*$ directions. Many of these phases can be understood to be multi-$q$ phases. One phase in particular, ICM2b (ICM=incommensurate magnetic phase), is rather unconventional in that it is an anisotropic multi-$q$ phase that can rotate freely within the $ab$-plane, dependent on magnetic field direction and history. The stabilization of tunable multi-$q$ incommensurate spin textures \textit{via} in-plane field sets this class of materials apart from conventional skyrmion materials. We further identify that the propagation vectors of the in-plane phases have a significant commensuration with the diameter of the smallest pocket of the Fermi surface ($2k_{\text{F}}$). The multi/single-$q$ nature is also correlated with the enhancement of resistivity, suggesting that a gap opens in the electron bands at $q=2k_{\text{F}}$. We also compare with a phenomenological model of the phase diagram. The richness of phases revealed in this study hint at the frustrated nature of the incommensurate magnetism present in EuAg$_4$Sb$_2$ and motivate further probes of these phases and the origin of the stability of spin moiré superlattices. Finally, the coupling of the multi-$q$ nature and $q=2k_{\text{F}}$ commensuration condition reveals the key requirements for a strong SMS transport response.

cond-mat.mes-hall↗

arXiv2024

Distinguishing Surface and Bulk Electromagnetism via Their Dynamics in an Intrinsic Magnetic Topological Insulator

The indirect exchange interaction between local magnetic moments via surface electrons has been long predicted to bolster the surface ferromagnetism in magnetic topological insulators (MTIs), which facilitates the quantum anomalous Hall effect. This unconventional effect is critical to determining the operating temperatures of future topotronic devices. However, the experimental confirmation of this mechanism remains elusive, especially in intrinsic MTIs. Here we combine time-resolved photoemission spectroscopy with time-resolved magneto-optical Kerr effect measurements to elucidate the unique electromagnetism at the surface of an intrinsic MTI MnBi2Te4. Theoretical modeling based on 2D Ruderman-Kittel-Kasuya-Yosida interactions captures the initial quenching of a surface-rooted exchange gap within a factor of two but over-estimates the bulk demagnetization by one order of magnitude. This mechanism directly explains the sizable gap in the quasi-2D electronic state and the nonzero residual magnetization in even-layer MnBi2Te4. Furthermore, it leads to efficient light-induced demagnetization comparable to state-of-the-art magnetophotonic crystals, promising an effective manipulation of magnetism and topological orders for future topotronics.

cond-mat.str-el↗

arXiv2024

Preservation of Topological Surface States in Millimeter-Scale Transferred Membranes

Ultrathin topological insulator membranes are building blocks of exotic quantum matter. However, traditional epitaxy of these materials does not facilitate stacking in arbitrary orders, while mechanical exfoliation from bulk crystals is also challenging due to the non-negligible interlayer coupling therein. Here we liberate millimeter-scale films of topological insulator Bi$_2$Se$_3$, grown by molecular beam epitaxy, down to 3 quintuple layers. We characterize the preservation of the topological surface states and quantum well states in transferred Bi$_{2}$Se$_{3}$ films using angle-resolved photoemission spectroscopy. Leveraging the photon-energy-dependent surface sensitivity, the photoemission spectra taken with $6$ eV and $21.2$ eV photons reveal a transfer-induced migration of the topological surface states from the top to the inner layers. By establishing clear electronic structures of the transferred films and unveiling the wavefunction relocation of the topological surface states, our work paves the physics foundation crucial for the future fabrication of artificially stacked topological materials with single-layer precision.

cond-mat.mtrl-sci↗