arXiv subjects

Yun-Mei Li

Publications and source records attributed to Yun-Mei Li.

10 recordsLinked to original sources

arXiv2026

Symmetry-enforced third-order nonlinear thermal Hall effects in altermagnets

Thermal Hall effect (THE) is a powerful probe of material properties, even in insulators. Here we investigate the thermal response of altermagnets by developing a theory on Berry curvature driven nonlinear THE from both electrons and magnons. We identify symmetry conditions under which the third-order THE dominates while linear and second-order contributions vanish. Notably, d-wave and i-wave altermagnets with out-of-plane Neel order satisfy these conditions, whereas in-plane order yields a dominant linear THE. Using KV2Se2O (a d-wave altermagnetic metal) and MnF2 (an altermagnetic insulator) as concrete examples, we show nonzero third-order nonlinear THE from electrons and magnons, respectively, both exhibiting a pi-periodic dependence on the direction of temperature gradient. These symmetry-guaranteed Berry curvature phenomena provide a diagnostic for altermagnetism in candidate materials and enable determination of the Neel vector orientation.

cond-mat.mtrl-sci↗

arXiv2025

Quantized Quadrupole Superconductors

We introduce a class of superconductors termed "quantized quadrupole superconductors" that support Majorana corner modes according to the bulk-corner correspondence, distinct from previous works on the second-order topological superconductors. An intrinsic physical quantity for superconductors, i.e., the quadrupole moment serves as the topological invariant, which is always half-quantized due to the particle-hole symmetry. As examples, two types of mixed pairings, $d_{x^{2}-y^{2}}\pm id_{xy}$ and $d_{x^{2}-y^{2}}\pm is$, induced in the bilayer two-dimensional electron gases with Rashba spin-orbit coupling give the quadrupole phase. Extended discussions indicate that the nontrivial phase is robust against relative phase fluctuations in the mixed pairings and the disorders. Our schemes provide realistic platforms to implement Majorana zero modes, paving the way for studying the Majorana physics.

cond-mat.supr-con↗

arXiv2025

Finite-temperature topological magnons in honeycomb ferromagnets with sublattice asymmetries

The Comment [Y.-M. Li, B. Wei, and K. Chang, Phys. Rev. Lett. 132, 219601 (2024)] pointed out that it is incorrect to predict the temperature-driven topological phase transition of Dirac magnons in honeycomb ferromagnets with Dzyaloshinskii-Moriya interactions based on the theory in Lu et al. [Y.-S. Lu, J.-L. Li, and C.-T. Wu, Phys. Rev. Lett. 127, 217202 (2021)]. Here we propose that by breaking the sublattice symmetries in honeycomb ferromagnets, increasing temperature could induce topological transitions from the trivial phase at zero temperature based on the linear spin wave theory to the Chern insulating phase above a critical temperature without changing any spin-spin interactions. The key to the finite-temperature topological magnons is considering the magnon-magnon interactions (MMIs) at a mean-field level. A self-consistently renormalized spin wave theory is employed to include self-energy corrections from MMIs, guaranteeing that the critical temperatures for topological transitions are below the Curié temperatures. Across the critical temperatures, the magnon band gap closes and reopens at K or K? points in the Brillouin zone, accompanied by nontrivial Berry curvature transitions. However, in stark contrast to the work of Lu et al. [Phys. Rev. Lett. 127, 217202 (2021)], the topological transitions cannot be revealed by the thermal Hall effect of magnons. Our work provides a realistic scheme for achieving a finite-temperature topological phase in honeycomb ferromagnets.

cond-mat.other↗

arXiv2024

Stark Effects of Rydberg Excitons in a Monolayer WSe2 P-N Junction

The enhanced Coulomb interaction in two-dimensional (2D) semiconductors leads to the tightly bound electron-hole pairs known as excitons. The large binding energy of excitons enables the formation of Rydberg excitons with high principal quantum numbers (n), analogous to Rydberg atoms. Rydberg excitons possess strong interactions among themselves, as well as sensitive responses to external stimuli. Here, we probe Rydberg exciton resonances through photocurrent spectroscopy in a monolayer WSe2 p-n junction formed by a split-gate geometry. We show that an external in-plane electric field not only induces a large Stark shift of Rydberg excitons up to quantum principal number n=3 but also mixes different orbitals and brightens otherwise dark states such as 3p and 3d. Our study provides an exciting platform for engineering Rydberg excitons for new quantum states and quantum sensing.

cond-mat.mes-hall↗

arXiv2023

Anti-helical edge magnons in patterned antiferromagnetic thin film

Helical edge states in topological insulators give counterpropagating spin current on the two parallel edges. We here propose anti-helical edge states of magnons in patterned antiferromagnetic thin films, which host copropagating spin current on the two parallel edges, where the two magnon modes with opposite chirality act like the spin. The embedded heavy metal dot array in the thin film induces interfacial Dzyaloshinskii-Moriya interactions (iDMIs), drives the magnon bands into nontrivial topological phases, characterized by spin Chern number. The resulting helical edge modes lead to spin current with the direction dependent on the sign of iDMI parameter. In a strip geometry, we combine two subsystems with two embedded metal dot arrays, which give opposite iDMI parameters. Anti-helical edge states emerge, compensated by the counterpropagating bulk confined states. Helical and anti-helical edge states are verified by the micromagnetic simulations. Our work is quite helpful in the field of magnon spintronics based on antiferromagnets.

cond-mat.mes-hall↗

arXiv2023

Temperature-Induced Magnonic Chern Insulator in Collinear Antiferromagnets

Thermal fluctuation in magnets will bring temperature-dependent self-energy corrections to the magnons, however, their effects on the topological orders of magnons is not well explored. Here we demonstrate that such corrections can induce a Chern insulating phase in two-dimensional collinear antiferromagnets with sublattice asymmetries by increasing temperature. We present the phase diagram of the system and show that the trivial magnon bands at zero temperature exhibit Chern insulating phase above a critical temperature before the paramagnetic phase transition. The self-energy corrections close and reopen the bandgap at Γ or K points, accompanied by a magnon chirality switch and nontrivial Berry curvature transition. The thermal Hall effect of magnons or detecting the magnon polarization can give experimentally prominent signatures of topological transitions. We include the numerical results based on van der Waals magnet MnPS3, calling for experimental implementation. Our work presents a new paradigm for constructing topological phase that is beyond the linear spin wave theory.

cond-mat.mes-hall↗

arXiv2022

Magnon Nernst Effect in Magnon Spin Hall Systems

Magnon spin Hall systems could hardly show experimentally observable particle and thermal transport phenomena intrinsically due to the spin cancellation. Here we demonstrated that the magnon spin Hall systems can exhibit magnon Nernst effect and thermal Hall effect under external magnetic field by considering two typical systems, i.e. the antiferromagnetically (AFM) coupled bilayer honeycomb ferromagnets and monolayer collinear honeycomb antiferromagnet. The both systems experience magnetic phase transitions from AFM phase to a field-polarized phase via a spin-flop (SF) phase or directly. In both systems, there exist magnon Nerst effect and also thermal Hall effect under a longitudinal temperature gradient, which can be regarded as the indicator of the magnetic phase transitions, with the Hall conductivity dependence on magnetic field consistent with the order of magnetic phase transition.

cond-mat.mes-hall↗

arXiv2022

Higher-order Topological Phases of Magnons in van der Waals Honeycomb Ferromagnets

We theoretically propose a second-order topological magnon insulator by stacking the van der Waals honeycomb ferromagnets with antiferromagnetic interlayer coupling. The system exhibits Z$_{2}$ topological phase, protected by pseudo-time-reversal symmetry (PTRS). An easy-plane anisotropy term breaks PTRS and destroys the topological phase. Nevertheless, it respects a magnetic two-fold rotational symmetry which protects a second-order topological phase with corner modes in bilayer and hinge modes along stacking direction. Moreover, an introduced staggered interlayer coupling establishes a Z$_{2}$$\times$Z topology, giving rise to gapped topological surface modes carrying non-zero Chern numbers. Consequently, chiral hinge modes propagate along the horizontal hinges in a cuboid geometry and are robust against disorders. Our work bridges the higher-order topology and magnons in van der Waals platforms, and could be used for constructing topological magnonic devices.

cond-mat.mes-hall↗

arXiv2021

$\mathcal{PT}$-Symmetry Enhanced Berezinskii-Kosterlitz-Thouless Superfluidity

Berezinskii-Kosterlitz-Thouless (BKT) transition, the topological phase transition to a quasi-long range order in a two-dimensional (2D) system, is a hallmark of low-dimensional topological physics. The recent emergence of non-Hermitian physics, particularly parity-time ($\mathcal{PT}$) symmetry, raises a natural question about the fate of low-dimensional orders (e.g., BKT transition) in the presence of complex energy spectrum. Here we investigate the BKT phase transition in a 2D degenerate Fermi gas with an imaginary Zeeman field obeying $\mathcal{PT}$-symmetry. Despite complex energy spectrum, $\mathcal{PT}$-symmetry guarantees that the superfluid density and many other quantities are real. Surprisingly, the imaginary Zeeman field enhances the superfluid density, yielding higher BKT transition temperature than that in Hermitian systems. In the weak interaction region, the transition temperature can be much larger than that in the strong interaction limit. Our work showcases a surprising interplay between low-dimensional topological defects and non-Hermitian effects, paving the way for studying non-Hermitian low-dimensional phase transitions.

cond-mat.quant-gas↗

arXiv2020

In-plane Zeeman field induced Majorana corner and hinge modes in an $s$-wave superconductor heterostructure

Second-order topological superconductors host Majorana corner and hingemodes in contrast to conventional edge and surface modes in two and three dimensions. However, the realization of such second-order corner modes usually demands unconventional superconducting pairing or complicated junctions or layered structures. Here we show that Majorana corner modes could be realized using a 2D quantum spin Hall insulator in proximity contact with an $s$-wave superconductor and subject to an in-plane Zeeman field. Beyond a critical value, the in-plane Zeeman field induces opposite effective Dirac masses between adjacent boundaries, leading to one Majorana mode at each corner. A similar paradigm also applies to 3D topological insulators with the emergence of Majorana hinge states. Avoiding complex superconductor pairing and material structure, our scheme provides an experimentally realistic platform for implementing Majorana corner and hinge states.

cond-mat.mes-hall↗