arXiv subjects

Yun-Peng Huang

Publications and source records attributed to Yun-Peng Huang.

9 recordsLinked to original sources

arXiv2026

Visualizing the interplay of dual electronic nematicities in kagome superconductors

Kagome superconductor AV$_3$Sb$_5$ (A stands for K, Rb, and Cs) hosts a wealth of intertwined electronic orders driven by geometric frustration and electron correlations. Among them, the breaking of rotational and/or time-reversal symmetry, observed within the triple-$Q$ charge density wave (CDW) phase yet exhibiting a more complex temperature dependence, remains a central puzzle. Here, by using scanning tunneling microscopy to study the electronic structures of CsV$_3$Sb$_5$ as a function of temperature and Ti doping, we disentangle the interrelation between two distinct nematic order parameters, one associated with the CDW and the other manifested as $C_2$ distortion of the V-$d_{x^{2}-y^{2}}$ Fermi pockets without breaking transition symmetry. The latter persists to high doping levels and high temperatures where the long-range CDW is fully suppressed. Moreover, its nematic director is oriented in a lattice direction distinct from that of the CDW-induced nematicity at intermediate doping, and eventually aligns with the strong nematic CDW order in the pristine compound where the quasiparticles of vanadium orbitals become coherent below a lower characteristic temperature. These observations, combined with Ginzburg-Landau analysis, reveal a rich interplay between two nematic orders that can be assigned to distinct kagome-lattice orbitals. Our results shed new light on the enigmatic intertwined orders in this family and establish a rare material platform in which dual nematic orders coexist and couple to give rise to unusual correlated phenomena.

cond-mat.supr-con↗

arXiv2025

Topological Degeneracy Induced Flat Bands in two-Dimensional Holed Systems

Systems hosting flat bands offer a powerful platform for exploring strong correlation physics. Theoretically topological degeneracy rising in systems with non-trivial topological orders on periodic manifolds of non-zero genus can generate ideal flat bands. However, experimental realization of such geometrically engineered systems is very difficult. In this work, we demonstrate that flat planes with strategically patterned hole defects can engineer ideal flat bands. We constructing two families of models, singular flat band systems where degeneracy is stabilized by non-contractible loop excitations tied to hole defects and perfectly nested van Hove systems where degeneracy arises from line excitations in momentum space. These models circumvent the need for exotic manifolds while retaining the essential features of topological flat bands. By directly linking defect engineering to degeneracy mechanisms, our results establish a scalable framework for experimentally accessible flat band design.

cond-mat.supr-con↗

arXiv2024

Imprinting Ground State Chirality on Adatom Spins

We propose an alternative experimental protocol for the detection of doped Chern insulators and chiral superconductors. Our approach relies on coupling the target chiral system to adatom spins. Due to the substrate chirality, the adatom spins are expected to order in a noncoplanar configuration with a nonzero spin chirality. Here, we obtain concrete results for chiral substrates which are invariant under arbitrary spin rotations, and are coupled to three adatoms carrying classical moments. By exploring all the accessible magnetic ground states, we identify the regimes in which nonzero spin chirality is induced on the adatom complex. We apply our method to valley-polarized bilayer graphene and $d+id$ superconductors, and find qualitatively different ground state diagrams. Our analysis shows that the adatom spin chirality fully encodes the properties of the substrate chirality.

cond-mat.mes-hall↗

arXiv2024

van Hove Singularity-Driven Emergence of Multiple Flat Bands in Kagome Superconductors

The newly discovered Kagome superconductors AV$_3$Sb$_5$ (A=K, Rb and Cs) continue to bring surprises in generating unusual phenomena and physical properties, including anomalous Hall effect, unconventional charge density wave, electronic nematicity and time-reversal symmetry breaking. Here we report an unexpected emergence of multiple flat bands in the AV$_3$Sb$_5$ superconductors. By performing high-resolution angle-resolved photoemission (ARPES) measurements, we observed four branches of flat bands that span over the entire momentum space. The appearance of the flat bands is not anticipated from the band structure calculations and cannot be accounted for by the known mechanisms of flat band generation. It is intimately related to the evolution of van Hove singularities. It is for the first time to observe such emergence of multiple flat bands in solid materials. Our findings provide new insights in revealing the underlying mechanism that governs the unusual behaviors in the Kagome superconductors. They also provide a new pathway in producing flat bands and set a platform to study the flat bands related physics.

cond-mat.mtrl-sci↗

arXiv2024

Microscopic model realization of $\boldsymbol{d}$-wave pseudospin current order in Sr$_{\boldsymbol{2}}$IrO$_{\boldsymbol{4}}$

The $d$-wave pseudospin current order ($d$PSCO) with staggered circulating pseudospin current has been proposed as the hidden electronic order to describe the unexpected breaking of spatial symmetries in stoichiometric Sr$_{2}$IrO$_{4}$ and the unconventional pseudogap phenomena in electron doped Sr$_{2}$IrO$_{4}$. However, a microscopic model for the emergence of $d$PSCO is still lacking. The nearest neighbor Coulomb repulsion $V$, which is expected to be significant in Sr$_{2}$IrO$_{4}$ due to the large spatial extension of the Ir $5d$ orbitals, is capable of driving $d$PSCO on the mean-field level, albeit the latter is energetically degenerate to the staggered flux phase with circulating charge current. We find the in-plane anisotropy $Γ_2$ in the effective superexchange interaction between $J_\text{eff}={1\over 2}$ pseudospins, originating from the cooperative interplay between Hund's rule coupling and spin-orbit coupling of Ir $5d$ electrons, is able to lift the degeneracy and stabilize the pseudospin currents. The effective single-orbital model of $J_\text{eff}={1\over 2}$ electrons, including onsite Coulomb repulsion $U$, nearest neighbor Coulomb repulsion $V$, and the in-plane anisotropy $Γ_2$, is then studied. We obtain the mean-field ground states, analyze their properties, and determine the phase diagram of stoichiometric Sr$_{2}$IrO$_{4}$ in the plane spanned by $U$ and $V$ at a fixed $Γ_2$. We demonstrate the realization of $d$PSCO, and its competition and coexistence with antiferromagnetism. Remarkably, we find the coexistence of $d$PSCO and antiferromagnetism naturally leads to spin bond nematicity, with the spin directions of these three orders forming nontrivial chirality. Furthermore, we show that the emergence of the coexistent state and its chirality can be tuned by carrier doping.

cond-mat.str-el↗

arXiv2023

New mechanisms to engineer magnetic skyrmions and topological superconductors

We propose an alternative route to stabilize magnetic skyrmions which does not require Dzyaloshinkii-Moriya interactions, magnetic anisotropy, or an external Zeeman field. Our so-called magnetic skyrmion catalysis (MSC) solely relies on the emergence of flux in the system's ground state. We review scenarios that allow for a nonzero flux and summarize the magnetic skyrmion phases that it induces. Among these, we focus on the so-called skyrmionic spin-whirl crystal (Sk-SWC$_4$) phase. We discuss aspects of MSC using a concrete model for topological superconductivity, which describes the surface states of a topological crystalline insulator in the presence of proximity induced pairing. By assuming that the surface states can exhibit the Sk-SWC$_4$ phase, we detail how the addition of a pairing gap generates a chiral superconductor. For this purpose, we construct a low-energy model which renders the mechanism for topological superconductivity transparent. Moreover, by employing this model, we perform a self-consistent investigation of the appearance of the Sk-SWC$_4$ phase for different values of the pairing gap and the ground state's flux. Our analysis verifies the catalytic nature of our mechanism in stabilizing the Sk-SWC$_4$ phase, since the magnetization modulus becomes enhanced upon ramping up the flux. The involvement of MSC further shields magnetism against the suppression induced by the pairing gap. Remarkably, even if the pairing gap fully suppresses the Sk-SWC$_4$ phase for a given value of flux, this skyrmion phase can be restored by further increasing the flux. Our findings demonstrate that MSC enables topological superconductivity in a minimal and robust fashion.

cond-mat.supr-con↗

arXiv2023

Mechanisms for Magnetic Skyrmion Catalysis and Topological Superconductivity

We propose an alternative route to stabilize magnetic skyrmion textures which does not require Dzyaloshinkii-Moriya interaction, magnetic anisotropy, or an external Zeeman field. Instead, it solely relies on the emergence of flux in the system's ground state. We discuss scenarios that lead to a nonzero flux, and identify the magnetic skyrmion ground states which become accessible in its presence. Moreover, we explore the chiral superconductors obtained for the surface states of a topological crystalline insulator when two types of magnetic skyrmion crystals coexist with a pairing gap. Our work opens perspectives for engineering topological superconductivity in a minimal fashion, and promises to unearth functional topological materials and devices which may be more compatible with electrostatic control than the currently explored skyrmion-Majorana platforms.

cond-mat.mes-hall↗

arXiv2021

Three-dimensional Stacking of Canted Antiferromagnetism and Pseudospin Current in Undoped Sr$_2$IrO$_4$: Symmetry Analysis and Microscopic Model Realization

Recent optical second-harmonic generation experiments observed unexpected broken spatial symmetries in the undoped spin-orbit Mott insulator Sr$_2$IrO$_4$, leading to intensive debates on the nature of its ground state. We propose that it is a canted antiferromagnetism with a hidden order of circulating staggered pseudospin current. Symmetry analysis shows that a proper $c$-axis stacking of the canted antiferromagnetism and the pseudospin current lead to a magnetoelectric coexistence state that breaks the two-fold rotation, inversion, and time-reversal symmetries, consistent with experimental observations. We construct a three-dimensional Hubbard model with spin-orbit coupling for the five localized 5$d$ Wannier orbitals centered at Ir sites, and demonstrate the microscopic realization of the desired coexistence state in a wide range of band parameters via a combination of self-consistent Hartree-Fock and variational calculations.

cond-mat.str-el↗

arXiv2020

Antiferromagnetic chiral spin density wave and strain-induced Chern insulator in the square lattice Hubbard model with frustration

We employ the Hartree-Fock approximation to identify the magnetic ground state of the Hubbard model on a frustrated square lattice. We investigate the phase diagram as a function of the Coulomb repulsion's strength $U$, and the ratio $t'/t$ between the nearest and next nearest neighbor hoppings $t$ and $t'$. At half-filling and for a sufficiently large $U$, an antiferromagnetic chiral spin density wave order with nonzero spin chirality emerges as the ground state in a wide regime of the phase diagram near $t'/t=1/\sqrt{2}$, where the Fermi surface is well-nested for both $(π,π)$ and $(π,0)/(0,π)$ wave vectors. This triple-${\bf Q}$ chiral phase is sandwiched by a single-${\bf Q}$ Néel phase and a double-${\bf Q}$ coplanar spin-vortex crystal phase, at smaller and larger $t'/t$, respectively. The energy spectrum in the chiral spin density wave phase consists of four pairs of degenerate bands. These give rise to two pairs of Dirac cones with the same chirality at the point $({π\over 2},{π\over 2})$ of the Brillouin zone. We demonstrate that the application of a diagonal strain induces a $d_{xy}$-wave next nearest neighbor hopping which, in turn, opens gaps in the two Dirac cones with opposite masses. As a result, four pairs of well-separated topologically-nontrivial bands emerge, and each pair of those contributes with a Chern number $\pm1$. At half-filling, this leads to a zero total Chern number and renders the topologically-notrivial properties observable only in the ac response regime. Instead, we show that at $3/4$ filling, the triple-${\bf Q}$ chiral phase yields a Chern insulator exhibiting the quantum anomalous Hall effect.

cond-mat.str-el↗