arXiv subjects

C. -Y. Lim

Publications and source records attributed to C. -Y. Lim.

7 recordsLinked to original sources

arXiv2026

Symmetry-Selective Stabilization of Charge-Density Wave in ScV$_6$Sn$_6$

Charge-density-wave (CDW) order in kagome metals is highly sensitive to external tuning parameters such as chemical substitution and hydrostatic pressure, which generally suppress long-range order. Here, using high-resolution X-ray diffraction under controlled uniaxial strain, we show that anisotropic lattice deformation instead stabilizes and enhances the CDW state in ScV$_6$Sn$_6$. Compression along the [H00] and [HH0] directions lowers the crystal symmetry from hexagonal to orthorhombic, lifts the degeneracy between symmetry-equivalent in-plane CDW domains, and promotes long-range order while preserving the underlying trimer instability. Phonon calculations indicate only a moderate stabilization of the imaginary flat phonon mode, demonstrating that the increase in T$_\mathrm{CDW}$ is primarily driven by the in-plane ordering of the Sn$^\mathrm{T}$--Sc--Sn$^\mathrm{T}$ \textit{rattling} chains within the frustrated kagome lattice. A phenomenological model incorporating strain-dependent Ising couplings within a three-state Potts framework successfully reproduces the evolution of T$_\mathrm{CDW}$ under compression and captures the continuous nature of the transition. Our results establish uniaxial strain as a powerful symmetry-selective tuning parameter for order-disorder transformations in frustrated lattices.

cond-mat.str-el↗

arXiv2026

Flat band driven competing charge and spin instabilities in the altermagnet CrSb

The confinement of electronic wavefunctions in momentum space can give rise to flat electronic bands, where the quenching of kinetic energy enhances the density of states and amplifies interaction effects. Such conditions are fertile ground for emergent quantum phases, as spin, charge and lattice degrees of freedom become strongly entangled. In these regimes, subtle competitions between intertwined order parameters often dictate the macroscopic ground state, producing complex and sometimes unexpected collective behavior. Here we show that the altermagnet CrSb provides a realization of this scenario, and uncover short-range charge-order fluctuations at the M point of the Brillouin zone, q*=(1/2 0), persisting above the Neel temperature (TN). Remarkably, these fluctuations collapse upon entering the magnetically ordered phase, revealing a direct and robust competition between charge and spin order. At TN, the phonon dispersion at q* develops a pronounced Kohn-like anomaly, signaling strong electron-phonon coupling in the vicinity of the magnetic transition. Below TN, exchange striction dramatically renormalizes the associated soft phonon mode by approximately ~6 meV, the largest spin-phonon coupling ever reported. First-principles calculations attribute this behavior to a strong coupling between nearly dispersionless electronic states and a phonon branch that appears unstable at the harmonic level only when no magnetic order is considered, revealing the large sensitivity of the lattice to magnetic symmetry breaking. The competition between charge and spin order parameters, amplified by flat-band physics, drives the observed phonon anomaly and its abrupt reconstruction at TN. With its chemically simple structure and symmetry-protected altermagnetic state, CrSb emerges as a model platform to explore how flat electronic bands mediate giant spin-phonon coupling and competing broken symmetries.

cond-mat.str-el↗

arXiv2025

Flat band driven itinerant magnetism in the Co-pnictides CaCo$_2$As$_2$ and LaCo$_2$P$_2$

Flat bands can induce strong electron correlation effects that help stabilize both magnetic and superconducting states. Here, we carry out angle-resolved photoemission spectroscopy and density functional theory calculations to study the electronic structure of the Co-pnictides CaCo$_2$As$_2$ and LaCo$_2$P$_2$. We find that, while the $k_z$ Fermi topology of ferromagnetic LaCo$_2$P$_2$ is markedly 2-dimensional, antiferromagnetic CaCo$_2$As$_2$ develops a 3D Fermi surface described by a $zig-zag$-like band dispersion perpendicular to the Co-As plane. Furthermore, the magnetism is driven by the electronic correlations of the flat bands with $d_{xy}$ and $d_{z^2}$ orbital character at the Fermi level. Our results link the electronic dimensionality and the magnetic order, and emphasize the critical role of the As-As and P-P bond strength along the $c$-direction to understand the electronic band structure and the rich phase diagram of transition metal pnictides.

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arXiv2025

First-order phase transition driven by competing charge-order fluctuations in 1T'-TaTe$_{2}$

First-order phase transitions, characterized by a discontinuous change in the order parameter, are intriguing phenomena in condensed matter physics. However, the underlying, material-specific, microscopic mechanisms often remain unclear. Here, we unveil a high-temperature incommensurate charge-order precursor with the wave vector $\mathbf{q}^* = (0, \frac{1}{4}+δ, \frac{1}{2})$ in the 1T' phase of TaTe$_2$, which competes with fluctuating high-temperature Ta trimer bonding states at $\mathbf{q}_\mathrm{CO} =(0, \frac{1}{3}, 0)$. The precursor state follows the temperature dependence of the hidden incommensurability of the $\textit{quasi}$-1D nested Fermi surface. In contrast, the low-temperature commensurate charge order at $\mathbf{q}_\mathrm{CO}$, characterized by a charge disproportionation of the inequivalent Ta sites, appears to be driven by local chemical bonding. Dynamical lattice calculations identify an imaginary optical mode at $\mathbf{q}^*$, involving an in-plane vibration of the Ta atoms forming a chain-like structure that renormalizes below $T_\mathrm{CO}$. Our experimental and theoretical observations suggest that the controversial first-order phase transition, as captured by phenomenological Ginzburg-Landau theory, results from the competition between two order parameters: one involving Fermi surface nesting and the other involving local chemical bonding.

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arXiv2025

Unravelling disorder in kagome Yb$_{0.5}$Co$_3$Ge$_3$

The presence of phonon instabilities that leads to the formation of charge modulated states in kagome metals has allowed their classification based on the atomic displacements. Here, we use diffuse and inelastic x-ray scattering, backed by Monte Carlo simulations to describe a type-I instability in the kagome metal Yb$_{0.5}$Co$_3$Ge$_3$. We find that the in-plane distortion of Co in the kagome plane drives a structural transition with the appearance of new Bragg peaks at odd $L$, which are surrounded by a hexagonal diffuse signal. The anisotropic diffuse scattering, characteristic of a highly frustrated triangular lattice was simulated following a combination of Ising hamiltonian and Lennard-Jones potential, and demonstrate that the structural phase transition in Yb$_{0.5}$Co$_3$Ge$_3$ is of an order-disorder transformation type. The inelastic spectra reveals no softening but an anomalous broadening of the $Γ-A$ low energy acoustic mode. Our results highlight the critical role of the geometric frustration in promoting ordering from disorder in kagome lattices and power of diffuse scattering to disentangle the internal atomic displacements and correlated disorder.

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arXiv2025

Magnetoelastic coupling in intercalated transition metal dichalcogenides

The large van der Waals gap in transition metal dichalcogenides (TMDs) offers an avenue to host external metal atoms that modify the ground state of these 2D materials. Here, we experimentally and theoretically address the charge correlations in a family of intercalated TMDs. While short-range charge fluctuations develop in Co$_{1/3}$TaS$_{2}$ and Fe$_{1/3}$TaS$_{2}$, long-range charge order switches-on in Fe$_{1/3}$NbS$_{2}$ driven by the interplay of magnetic order and lattice degrees of freedom. The magnetoelastic coupling is demonstrated in Fe$_{1/3}$NbS$_{2}$ by the enhancement of the charge modulations upon magnetic field below T$_\mathrm{N}$, although Density Functional Perturbation Theory (DFPT) calculations predict negligible electron(spin)-phonon coupling. Furthermore, we show that Co-intercalated TaS$_2$ displays a kagome-like Fermi surface, hence opening the path to engineer electronic band structures and study the entanglement of spin, charge, and spin-phonon mechanisms in the large family of intercalated TMDs.

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arXiv2025

Cascade of pressure induced competing CDWs in the kagome metal FeGe

Electronic ordering is prevalent in correlated systems, which commonly exhibit competing interactions. Here, we use x-ray diffraction to demonstrate a cascade of pressure induced order-disorder transformations, with propagation vectors $\mathbf{q}_\mathrm{CDW}$= $\left(\frac{1}{2}\ 0\ \frac{1}{2}\right)$, $\mathbf{q}^*$=$\left(\frac{1}{3}\ \frac{1}{3}\ \frac{1}{2}\right)$ and $\mathbf{q}^\dagger$=$\left(\frac{1}{3}\ \frac{1}{3}\ \frac{1}{3}\right)$, in the kagome metal FeGe. In the pressure interval between 4$<p<$10 GPa, $\mathbf{q}_\mathrm{CDW}$ and $\mathbf{q}^*$ coexist and the spatial extent of the $\sqrt{3}\times\sqrt{3}$ order is nearly long-range at $\sim$15 GPa, $\sim$30 unit cells. Above $\sim$25 GPa, the periodic lattice distortion has a propagation wavevector of $\mathbf{q}^\dagger$=$\left(\frac{1}{3}\ \frac{1}{3}\ \frac{1}{3}\right)$ at room temperature. The cascade of phase transitions are captured by the Ising model of frustrated triangular lattices and modeled by Monte Carlo simulations based on the dimerization of trigonal Ge$_1$. The pressure dependence of the integrated intensities and correlation lengths of $\mathbf{q}_\mathrm{CDW}$, $\mathbf{q}^*$ and $\mathbf{q}^\dagger$ demonstrate a competition between the 2$\times$2 and $\sqrt{3}\times\sqrt{3}$ phases prove the tunability of order-disorder phase transitions under pressure and the rich landscape of metastable/fragile phases of FeGe.

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