arXiv subjects

Congcong Le

Publications and source records attributed to Congcong Le.

At least 19 recordsLinked to original sources

arXiv2026

Catalog of Altermagnetism in Magnetic Wallpaper/Space Groups and Nonsymmorphic Altermagnets

Conventional altermagnetism, characterized by compensated collinear spin alignment and spin splitting, exhibits identical spin states at opposite momenta. In this work, we employ a non-spatial global symmetry $S$, the spinless time-reversal symmetry, which effectively replaces inversion symmetry in preserving the spin-state equivalence; hence, we systematically extend the classification of altermagnetism to all possible non-centrosymmetric crystals. By analyzing the necessary symmetry conditions, we provide a complete catalog of altermagnetic orders for all 2D magnetic wallpaper groups and all 3D magnetic space groups, identifying 17 altermagnetic wallpaper groups (12 centrosymmetric and 5 non-centrosymmetric) and 422 altermagnetic space groups (160 centrosymmetric and 262 non-centrosymmetric). This catalog assigns each altermagnetic wallpaper and space group to one of the six altermagnetic wave types established in the literature and presents its distinct spin distribution in the Brillouin zone (BZ); notably, the low-energy wave-type description does not necessarily extend throughout the full BZ, since the spin-degenerate nodal lines and planes can be unpinned from the high-symmetry planes. Beyond the catalog, nonsymmorphic symmetries further bring new patterns of the altermagnetic BZs through the emergence of hourglass dispersions, which arise from the compatibility relations between two symmetry-protected degenerate manifolds: same-spin and opposite-spin degeneracies. In both the non-centrosymmetric altermagnetism and the emergence of the hourglass dispersion, the spinless time-reversal symmetry plays the key role. Our work extends the symmetry catalog of altermagnetism and reveals that nonsymmorphic symmetries are essential for realizing altermagnetic band structures beyond the six established wave types, such as an $i$-wave-like spin winding in a tetragonal BZ.

cond-mat.mtrl-sci↗

arXiv2026

Chiral-Structured Superconductors TrX4 (Tr = Rh, Ir; X = Ge, Si): A Platform for Mixed-Parity Pairing and Topological States

Chiral-structured superconductors, with simultaneous broken mirror and inversion symmetries, promote unconventional superconductivity through parity-mixing mechanisms. Yet a few bulk chiral-structured superconductors are known, partly due to the difficulty in directly determining their atomic-scale chirality. Here we report three chiral-structured superconductors, , RhGe4, IrGe4, and IrSi4, synthesized under high pressure, with Tc values of about 1.6 K, 1.1 K, and 2.5 K, respectively.Using atomic resolution Cs-corrected scanning transmission electron microscopy (STEM) combined with X-ray diffraction characterizations, we directly confirm their chiral structure (space group P3121). This real space imaging approach overcomes ambiguities in traditional diffraction based methods. These materials exhibit type-II superconductivity, and the enhancement of spin-orbit coupling (SOC) leads to the emergence of mixed parity pairing. Calculations also reveal symmetry protected Weyl points near the Fermi level, which is robust against the SOC. Our work not only expands the family of chiral-structured superconductors but also demonstrates the indispensable role of STEM in directly determining chiral crystal structures. These materials thus offer a clean platform to explore the interplay among structural chirality, SOC, mixed parity superconductivity, and topological quantum phenomena.

cond-mat.supr-con↗

arXiv2026

Termination-Dependent Surface States and Magnetic Fingerprints of Chiral Helimagnet Cr1/3TaS2

Chiral helimagnets based on intercalated transition-metal dichalcogenides, characterized by nano-scale spin ordering, provide a powerful route to engineer chiral spin textures (e.g. the topologically protected magnetic solitons) and emergent electronic functionality at reduced dimensions, where surface and interface states often dominate device operation. However, despite growing interest, direct experimental studies of termination-dependent surface electronic structures and their temperature-driven magnetic evolution remain largely unexplored, hindering a microscopic understanding of the electronic states that is crucial for the development of low-dimensional spintronic devices. Here, for the first time, taking Cr1/3TaS2 as a representative example, we systematically investigate the termination-dependent surface electronic states of the chiral helimagnets and uncover their distinct temperature evolution across the magnetic transition (TC~142K) by combining high-resolution ARPES with a micro-focused beam and surface-state-resolved first-principles calculations. The TaS2-terminated surface hosts folded monolayer-like TaS2 bands under the $\sqrt3\times\sqrt3$ superlattice potential and a shallow triangular electron pocket at the superlattice $\bar K$ point arising from Cr-Ta orbital hybridization. In contrast, the Cr-terminated surface exhibits reconstructed hole pockets with pronounced magnetic band splitting. This splitting disappears above TC and closely follows the chiral helimagnetic order parameter, providing a direct spectroscopic fingerprint of chiral helimagnetic order. In addition, multiple ultranarrow Cr-d-derived surface flat bands are resolved. These findings establish Cr1/3TaS2 as a model system in which surface electronic states are strongly coupled to chiral magnetism, opening new opportunities for chiral spintronic and valleytronic micro/nanodevices.

cond-mat.mtrl-sci↗

arXiv2026

Contrasting Momentum-Selective Spin-Density-Wave Gaps in Bilayer and Trilayer Nickelates

Resolving where the density-wave gap opens in momentum space is essential for identifying the microscopic origin of the instability in layered nickelates. Using polarization-resolved electronic Raman scattering, we map the momentum selectivity of the spin-density-wave (SDW) gap in trilayer La4Ni3O10. We observe a SDW-induced redistribution of spectral weight on both the $\alpha$ pocket at the Brillouin-zone centre and a portion of the $\beta$ pocket near the zone boundary, characterized by gap energies of approximately 55~meV. In contrast, no comparable spectral weight suppression is observed along the diagonal region of $\beta$ pockets, implying little or no gap opening. This gap topology contrasts sharply with that in La3Ni2O7, where anisotropic SDW gaps open solely on the $\beta$ pocket. Our results establish a distinct momentum-space gap topology between bilayer and trilayer nickelates, placing new constraints on the ordering wave vector and the mechanism of the density-wave instability relevant to superconductivity.

cond-mat.supr-con↗

arXiv2026

Common sublattice-pure van Hove singularities in the kagome superconductors $\textit{A}$V$_{3}$Sb$_{5}$ ($\textit{A}$ = K, Rb, Cs)

Kagome materials offer a versatile platform for exploring correlated and topological quantum states, where van Hove singularities (VHSs) play a pivotal role in driving electronic instabilities, exhibiting distinct behaviors depending on electron filling and interaction settings. In the recently discovered kagome superconductors $\textit{A}$V$_{3}$Sb$_{5}$ ($\textit{A}$ = K, Rb, Cs), unconventional charge density wave order, superconductivity, and electronic chirality emerge, yet the nature of VHSs near the Fermi level ($\textit{E}$$_{F}$) and their connection to these exotic orders remain elusive. Here, using high-resolution polarization-dependent angle-resolved photoemission spectroscopy, we uncover a universal electronic structure across $\textit{A}$V$_{3}$Sb$_{5}$ that is distinct from density-functional theory predictions that show noticeable discrepancies. We identify multiple common sublattice-pure VHSs near $\textit{E}$$_{F}$, arising from strong V-$\textit{d}$/Sb-$\textit{p}$ hybridization, which significantly promote bond-order fluctuations and likely drive the observed charge density wave order. These findings provide direct spectroscopic evidence for hybridization-driven VHS formation in kagome metals and establish a unified framework for understanding the intertwined electronic instabilities in $\textit{A}$V$_{3}$Sb$_{5}$.

cond-mat.supr-con↗

arXiv2025

Nonthermal melting and density wave instability coupled to the lattice in La$_4$Ni$_3$O$_{10}$

The recent discovery of high-temperature superconductivity in pressurized nickelates has renewed interest in the broken-symmetry states of their ambient-pressure parent phases, where a density-wave (DW) order emerges and competes with superconductivity, but its microscopic origin remains unresolved. Using ultrafast optical spectroscopy, we track quasiparticle relaxation dynamics across the DW transition at $T_{\rm DW} \approx$ 136 K in trilayer nickelate {\LNO} single crystals, revealing the opening of an energy gap of $\sim$52 meV. Multiple coherent phonons, including $A_g$ modes near 3.88, 5.28, and 2.09 THz, display pronounced mode-selective anomalies across the transition, indicating that the DW is strongly coupled to lattice degrees of freedom and suggesting an important role of electron-phonon coupling. At higher excitation densities, the DW is nonthermally suppressed, producing a temperature-fluence phase diagram that parallels pressure-tuned behavior. These results establish the DW in {\LNO} as a lattice-entangled instability involving multiple phonon modes, and highlight ultrafast optical excitation as a nonequilibrium tuning parameter for suppressing density-wave order in nickelates.

cond-mat.str-el↗

arXiv2025

Comparative Raman study of Ruddlesden-Popper nickelates and the monolayer-trilayer polymorph

Ruddlesden-Popper (RP) nickelates have attracted intense interest following the discovery of superconductivity in several members of the series, including bilayer (BL) La$_3$Ni$_2$O$_7$, trilayer (TL) La$_4$Ni$_3$O$_{10}$, and structural polymorphs composed of monolayer-bilayer or monolayer-trilayer (ML-TL) units. However, an inherent propensity of the RP series to form intergrown phases during single-crystal synthesis, together with spatial variations in oxygen stoichiometry, has complicated the determination of their intrinsic material properties. As a consequence, conflicting reports have emerged on both their electronic phase transitions and lattice dynamics. In this work, we perform a comparative study of the phononic and electronic Raman responses of high-quality ML-TL single crystals and contrast them with those of other RP nickelates, using samples with optimized oxygen content. We establish several Raman spectral features that enable unambiguous phase identification across the series. Moreover, we uncover characteristics in the phononic and electronic Raman response of ML-TL that are not reflected in the pure ML and TL compounds. We attribute these differences to a distinctive electronic structure arising from self-doping and confinement effects induced by the ML unit within the ML-TL lattice architecture.

cond-mat.str-el↗

arXiv2025

Mirror-Selective Quasiparticle Interference in Bilayer Nickelate Superconductor

The recent discovery of high-temperature superconductivity in both bulk and thin-film bilayer nickelates has garnered significant attention. In this study, inspired by recent STM experiments on thin films, we investigate the quasiparticle interference (QPI) characteristics of bilayer nickelates in both normal and superconducting states to identify their Fermiology and pairing symmetry. We demonstrate that the mirror symmetry inherent in the bilayer structure induces mirror-selective quasiparticle scattering by establishing selection rules based on the mirror properties of impurities and the mirror eigenvalues of electronic wavefunctions. This mirror-selective scattering allows for the differentiation of distinct Fermiologies, as QPI patterns vary markedly between scenarios with and without the $d_{z^2}$-bonding Fermi surface (FS). Furthermore, it enables the separate detection of sign changes in superconducting gaps both within the same FS and between different FSs. Crucially, if the mirror-symmetry-enforced selection rules are ignored, the QPI response of an $s_\pm$-wave state can masquerade as that of a conventional $s$-wave state, leading to a misidentification of the pairing symmetry. When combined with field-dependent and reference QPI measurements, this approach facilitates the precise determination of pairing symmetry, even in the presence of FS-dependent gaps and gap anisotropy. Additionally, we discuss practical considerations for STM measurements to effectively identify the pairing symmetry. Our findings demonstrate that mirror-selective QPI is a powerful tool for distinguishing between different Fermiologies and pairing states, which is helpful in pinning down pairing symmetry and revealing the pairing mechanism in bilayer nickelates.

cond-mat.supr-con↗

arXiv2025

Multiorbital character of the density wave in trilayer nickelate superconductors

Ruddlesden-Popper nickelates exhibit high-temperature superconductivity closely intertwined with charge and spin density waves. However, fundamental questions persist regarding the interplay between the associated density wave (DW) fluctuations and superconductivity, as well as the orbital character and symmetry underlying the DW instabilities. Here we utilize polarized Raman scattering to investigate the phononic and electronic Raman responses of the trilayer nickelate La$_4$Ni$_3$O$_{10}$ across its concomitant charge and spin density wave transitions. In addition to distinct phonon anomalies occurring below the transition temperature, we observe a depletion of continuum spectral weight up to 114 meV and a pronounced peak centered at this energy. By combining momentum-selective information from polarized electronic Raman scattering with Raman-response model calculations based on a multiorbital Raman vertex in a reconstructed two-orbital DW state involving both Ni-3$d_{x^2 - y^2}$ and Ni-3$d_{z^2}$ orbitals, we identify 114 meV as the energy scale $2\Delta_\mathrm{DW}$ of the DW gap, characterized by incoherent opening and non-mean-field behavior. Furthermore, the model calculations reveal that the corresponding $2\Delta_\mathrm{DW}$ peak has a multiorbital origin, requiring both orbital contributions and their mixing beyond single-orbital projections, thus shedding light on the nature of the DW instabilities in La$_4$Ni$_3$O$_{10}$.

cond-mat.str-el↗

arXiv2025

Impact of Nonlocal Coulomb Repulsion on Superconductivity and Density-Wave Orders in Bilayer Nickelates

The recent discovery of high-temperature superconductivity in pressurized bilayer nickelate La$_3$Ni$_2$O$_7$ and its thin films has generated significant interest in uncovering the underlying pairing mechanisms and correlated electronic states. While earlier theoretical studies have mainly focused on onsite Coulomb interactions, the role of nonlocal Coulomb repulsion has remained largely unexplored. In this work, we systematically investigate the effects of nonlocal Coulomb interactions, in the presence of onsite interactions, on both superconducting and density-wave instabilities using the functional renormalization group (FRG) approach. We find that the interlayer intraorbital repulsion suppresses the interlayer intraorbital $s_{\pm}$-wave pairing and spin-density-wave (SDW) order, while promoting a transition to an interlayer interorbital $d_{x^2-y^2}$-wave pairing state and a mirror-symmetry-breaking charge order. Remarkably, the critical scale of the interorbital $d_{x^2-y^2}$-wave superconductivity is significantly lower than that of the intraorbital $s_{\pm}$-wave superconductivity, indicating that the former is unlikely to account for the observed high-$T_c$ superconductivity. Moreover, the interlayer interorbital repulsion suppresses this $d_{x^2-y^2}$-wave pairing but enhances the $s_{\pm}$-wave pairing through strengthened interlayer charge fluctuations. In addition, the intralayer nearest-neighbor repulsion favors an in-plane charge-density-wave (CDW) order with wave vector $(\pi,\pi)$. Our findings reveal the profound impact of nonlocal Coulomb repulsion and underscore the robustness of interlayer pairing rooted in the bilayer structure and multi-orbital nature, thereby advancing the understanding of the intricate correlation effects in bilayer nickelates.

cond-mat.supr-con↗

arXiv2025

Opposite-Mirror-Parity Scattering as the Origin of Superconductivity in Strained Bilayer Nickelates

We study the electronic structure and doping-dependent instabilities of strained La$_3$Ni$_2$O$_7$ thin films using first-principles and functional renormalization group methods. We demonstrate that ordering tendencies are governed by Fermi surface scattering between electrons of opposite mirror parity. Under moderate hole doping, when the $d_{z^2}$ bonding band becomes incipient or crosses the Fermi level, robust $s_{\pm}$-wave superconductivity emerges from cooperative interlayer pairing reinforced by two competing spin-density-wave fluctuations. Compressive strain favors superconductivity in NiO$_2$ bilayers slightly away from the interface, whereas tensile strain induces pair-breaking nesting that suppresses pairing. Our results establish a unified microscopic scenario for superconductivity in pressurized bulk and strained thin-film nickelates, providing new insights into high-T$_c$ pairing in correlated quantum materials.

cond-mat.supr-con↗

arXiv2024

Topological electronic structure and electronic nematicity in candidate kagome superconductors, ATi$_{3}$Bi$_{5}$ (A = Rb, Cs)

The newly discovered family of titanium-based kagome metals, ATi$_{3}$Bi$_{5}$ (where A can be Rb or Cs), has been found to exhibit non-trivial band topology and fascinating electronic instabilities, including electronic nematicity and potential bulk superconductivity. Distinct from their vanadium-based counterparts (AV$_{3}$Sb$_{5}$), which display a charge density wave (CDW) phase that already breaks rotational symmetry, ATi$_{3}$Bi$_{5}$ shows no evidence of CDW, providing a unique platform to study nematicity in its pure form and its interplay with other correlated quantum phenomena, such as superconductivity. In this review, we highlight recent progress in both experimental and theoretical research on ATi$_{3}$Bi$_{5}$ and discuss the unresolved questions and challenges in this burgeoning field.

cond-mat.supr-con↗

arXiv2023

Magnetic-coupled electronic landscape in bilayer-distorted titanium-based kagome metals

Quantum materials whose atoms are arranged on a lattice of corner-sharing triangles, $\textit{i.e.}$, the kagome lattice, have recently emerged as a captivating platform for investigating exotic correlated and topological electronic phenomena. Here, we combine ultra-low temperature angle-resolved photoemission spectroscopy (ARPES) with scanning tunneling microscopy and density functional theory calculations to reveal the fascinating electronic structure of the bilayer-distorted kagome material $\textit{Ln}$Ti${_3}$Bi${_4}$, where $\textit{Ln}$ stands for Nd and Yb. Distinct from other kagome materials, $\textit{Ln}$Ti${_3}$Bi${_4}$ exhibits two-fold, rather than six-fold, symmetries, stemming from the distorted kagome lattice, which leads to a unique electronic structure. Combining experiment and theory we map out the electronic structure and discover double flat bands as well as multiple van Hove singularities (VHSs), with one VHS exhibiting higher-order characteristics near the Fermi level. Notably, in the magnetic version NdTi${_3}$Bi${_4}$, the ultra-low base temperature ARPES measurements unveil an unconventional band splitting in the band dispersions which is induced by the ferromagnetic ordering. These findings reveal the potential of bilayer-distorted kagome metals $\textit{Ln}$Ti${_3}$Bi${_4}$ as a promising platform for exploring novel emergent phases of matter at the intersection of strong correlation and magnetism.

cond-mat.mtrl-sci↗

arXiv2023

Classification of High-Ordered Topological Nodes towards Moir\'e Flat Bands in Twisted Bilayers

At magic twisted angles, Dirac cones in twisted bilayer graphene (TBG) can evolve into flat bands, serving as a critical playground for the study of strongly correlated physics. When chiral symmetry is introduced, rigorous mathematical proof confirms that the flat bands are locked at zero energy in the entire Moir\'e Brillouin zone (BZ). Yet, TBG is not the sole platform that exhibits this absolute band flatness. Central to this flatness phenomenon are topological nodes and their specific locations in the BZ. In this study, considering twisted bilayer systems that preserve chiral symmetry, we classify various ordered topological nodes in base layers and all possible node locations across different BZs. Specifically, we constrain the node locations to rotational centers, such as {\Gamma} and M points, to ensure the interlayer coupling retains equal strength in all directions. Using this classification as a foundation, we systematically identify the conditions under which Moir\'e flat bands emerge. Additionally, through the extension of holomorphic functions, we provide proof that flat bands are locked at zero energy, shedding light on the origin of the band flatness. Remarkably, beyond Dirac cones, numerous twisted bilayer nodal platforms can host flat bands with a degeneracy number of more than two, such as four-fold, six-fold, and eight-fold. This multiplicity of degeneracy in flat bands might unveil more complex and enriched correlation physics.

cond-mat.str-el↗

arXiv2023

Effective model and pairing tendency in bilayer Ni-based superconductor La$_3$Ni$_2$O$_7$

Since the discovery of cuprate, the origin of high-T$_c$ superconductivity has been an outstanding puzzle. Recently, high-T$_c$ superconductivity was observed in a bilayer nickelate La$_3$Ni$_2$O$_7$ under pressure, whose structure hosts the apical oxygen between two layers, distinct from multi-layer cuprates. Motivated by this discovery, we investigate its electronic structure using first-principle calculations and superconducting instabilities from both weak-coupling and strong-coupling perspective. Based on the first-principle band structures, we construct a bilayer two-orbital model on a square lattice, consisting of $d_{x^2-y^2}$ and $d_{z^2}$ orbitals, which accurately captures the low-energy electronic properties. Within this model, we study pairing instability using both functional renormalization group approach and multi-orbital t-J model. An $s_{\pm}$-wave pairing with sign-reversal gaps on different Fermi surfaces is revealed, reminiscent of iron based superconductors. The Ni-$d_{z^2}$ orbital and its associated interlayer and intralayer exchange couplings are found to be crucial for the high-T$_c$ superconductivity. Our study provides valuable insights into unique nature of electronic structure and superconductivity in La$_3$Ni$_2$O$_7$ and contributes to the understanding of unconventional superconductors.

cond-mat.supr-con↗

arXiv2022

Non-trivial band topology and orbital-selective electronic nematicity in a new titanium-based kagome superconductor

Electronic nematicity that spontaneously breaks rotational symmetry has been shown as a generic phenomenon in correlated quantum systems including high-temperature superconductors and the AV3Sb5 (A = K, Rb, Cs) family with a kagome network. Identifying the driving force has been a central challenge for understanding nematicity. In iron-based superconductors, the problem is complicated because the spin, orbital and lattice degrees of freedom are intimately coupled. In vanadium-based kagome superconductors AV3Sb5, the electronic nematicity exhibits an intriguing entanglement with the charge density wave order (CDW), making understanding its origin difficult. Recently, a new family of titanium-based kagome superconductors ATi3Bi5 has been synthesized. In sharp contrast to its vanadium-based counterpart, the electronic nematicity occurs in the absence of CDW. ATi3Bi5 provides a new window to explore the mechanism of electronic nematicity and its interplay with the orbital degree of freedom. Here, we combine polarization-dependent angle-resolved photoemission spectroscopy with density functional theory to directly reveal the band topology and orbital characters of the multi-orbital RbTi3Bi5. The promising coexistence of flat bands, type-II Dirac nodal line and nontrivial Z2 topological states is identified in RbTi3Bi5. Remarkably, our study clearly unveils the orbital character change along the G-M and G-K directions, implying a strong intrinsic inter-orbital coupling in the Ti-based kagome metals, reminiscent of iron-based superconductors. Furthermore, doping-dependent measurements directly uncover the orbital-selective features in the kagome bands, which can be well explained by the d-p hybridization. The suggested d-p hybridization, in collaboration with the inter-orbital coupling, could account for the electronic nematicity in ATi3Bi5.

cond-mat.supr-con↗

arXiv2022

Double and Quadruple Flat Bands tuned by Alternative magnetic Fluxes in Twisted Bilayer Graphene

Twisted bilayer graphene (TBG) can host the moir\'{e} energy flat bands with two-fold degeneracy serving as a fruitful playground for strong correlations and topological phases. However, the number of degeneracy is not limited to two. Introducing a spatially alternative magnetic field, we report that the induced magnetic phase becomes an additional controllable parameter and leads to an undiscovered generation of four-fold degenerate flat bands. This emergence stems from the band inversion at $\Gamma$ point near the Fermi level with a variation of both twisted angle and magnetic phase. We present the conditions for the emergence of multi-fold degenerate flat bands, which are associated with the eigenvalue degeneracy of a Birman-Schwinger operator. Using holomorphic functions, which explain the origin of the double flat bands in the conventional TBG, we can generate analytical wave functions in the magnetic TBG to show absolute flatness with four-fold degeneracy. Moreover, we identify an orbital-related intervalley coherent state as the many-body ground state at charge neutrality. In contrast, the conventional TBG has only two moir\'{e} energy flat bands, and the highly degenerate flat bands with additional orbital channels in this magnetic platform might bring richer correlation physics.

cond-mat.str-el↗

arXiv2022

Tunable Dirac Semimetals with Higher-order Fermi Arcs in Kagome Lattices Pd$_3$Pb$_2$X$_2$ (X = S, Se)

Bulk-boundary correspondence has achieved a great success in the identification of topological states. However, this elegant strategy doesn't apply to the Dirac semimetals (DSMs). Here, we propose that kagome lattices Pd$_3$Pb$_2$X$_2$ (X = S, Se) are unique type-I DSMs without surface Fermi arc states, which are different from the previous well-known DSMs, such as Na$_3$Bi and Cd$_3$As$_2$. Pd$_3$Pb$_2$X$_2$ are characterized by nontrivial topological invariant Z$_3$, guaranteeing a higher-order bulk-hinge correspondence and the existence of higher-order Fermi arcs, as well as fractional corner charges on the hinges. The type-I DSMs are located at the phase boundaries of several topological phases, including type-II DSMs and three-dimensional weak topological insulators. The phase transitions can be easily manipulated by external strain. Our results provide feasible platforms for the study of these unique DSMs and the related phase transitions.

cond-mat.mtrl-sci↗