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Jiangfan Wang

Publications and source records attributed to Jiangfan Wang.

At least 19 recordsLinked to original sources
arXiv2026

Interlayer pairing mechanism for bilayer nickelate superconductors

The discovery of superconductivity in Ruddlesden-Popper bilayer nickelates under both high pressure and ambient conditions has opened a new paradigm for exploring unconventional superconductivity. This review provides a brief survey of theoretical progress on bilayer nickelate superconductors. Drawing from the key experimental observations, we summarize essential physical ingredients including the hybridized Ni-3$d_{x^2-y^2}$ and 3$d_{z^2}$ electronic structure, orbital-dependent electronic correlation, Hund's coupling, and strong interlayer magnetic coupling. The fundamental theoretical models including the bilayer two-orbital Hubbard model and its minimal $t$-$J$ variants are introduced. Starting from the atomic-limit interlayer valence bond picture of the half-filled $d_{z^2}$ orbital, we elaborate on strong correlation interlayer pairing mechanisms based on different limiting considerations. Specific emphasis is placed on the hybridization mechanism, where the $d_{z^2}$ local singlet pairs provide the pairing energy and their hybridization with itinerant $d_{x^2-y^2}$ promotes superconducting phase coherence. We further analyze the pairing symmetry, the dependence of $T_c$ on various internal and external parameters, the nontrivial normal state properties including the Fermi liquid, non-Fermi liquid, weakly insulating and pseudogap behaviors. Effects of pressure tuning, oxygen content, and Kondo scattering induced by oxygen vacancies are also discussed. Finally, weak correlation theories based on spin fluctuations associated with Fermi surface nesting are briefly covered.

cond-mat.supr-con↗
arXiv2026

Topological phase transition driven by Hatsugai-Kohmoto interaction on the Kagome lattice

The interplay between band topology and strong correlations is central to modern condensed matter physics, but exact solutions are rare. Here, we present an exactly solvable model on the Kagome lattice by combining a Kane-Mele-type spin-orbit coupling with the Hatsugai-Kohmoto interaction. At 1/3 filling, we uncover a continuous topological quantum phase transition driven by electron interaction. A weakly correlated $\mathbb{Z}_2$ topological insulator gives way to a strongly correlated insulator that, while $\mathbb{Z}_2$-trivial, hosts a nontrivial spin Chern number $C_s=2$. The transition exhibits critical scaling consistent with the universality class of two-dimensional Dirac fermions. At half-filling, the same model yields a non-Fermi-liquid to Mott-insulator transition, demonstrating that correlation-driven topological and Mott transitions can be unified within a single solvable framework. Our results establish the Kagome Hatsugai-Kohmoto model as a valuable benchmark for interacting topological systems.

cond-mat.str-el↗
arXiv2026

A unified theory of thin film and bulk bilayer nickelates

The discovery of bilayer nickelate superconductivity in both pressurized bulk and thin films has drawn enormous attention on their similarity and distinction. Here we provide a unified explanation based on the two-component scenario for a number of key experimental observations reported recently. Our theory predicts two superconducting domes upon electron or hole doping, separated by a valence bond state near $d_{z^2}$ half filling for strong interlayer superexchange coupling $J$, and a single dome across half filling with a lower $T_c$ for weak or moderate $J$. Increasing doping drives the normal state from a Fermi liquid to non-Fermi liquid or weak insulating behaviors, with quasi-linear-in-$T$ scattering rate near optimal $T_c$, while breaking the interlayer valence bonds by oxygen vacancies or chemical substitution simultaneously suppresses the superconductivity and causes local Kondo scattering of $d_{x^2-y^2}$ electrons. These explain the different superconducting transitions and normal states in bulk and thin films, the effect of $d_{z^2}$ hole or electron doping, and the Kondo effect in non-superconducting samples. We propose bulk superconductivity at ambient pressure by doping or reducing the interlayer magnetic coupling and predict even higher $T_c$ upon electron doping.

cond-mat.supr-con↗
arXiv2026

Auxiliary-Bath Numerical Renormalization Group Method and Successive Collective Screening in Multi-Impurity Kondo Systems

We propose an auxiliary-bath algorithm for the numerical renormalization group (NRG) method to solve multi-impurity models with shared electron baths. The method allows us to disentangle the electron baths into independent Wilson chains to perform standard NRG procedures beyond the widely adopted independent bath approximation. Its application to the 2-impurity model immediately reproduces the well-known even- and odd-parity channels. For 3-impurity Kondo models, we find collective screening of cluster degrees of freedom depending on impurity configurations and clarify the false prediction of a non-Fermi liquid ground state for the $C_3$ symmetric case in previous literature due to improper treatment of disentanglement. Our work highlights the importance of nonlocal spatial correlations due to shared baths and reveals a generic picture of successive collective screening for entropy depletion that is crucial in real correlated systems. Our method greatly expands the applicability of the NRG and opens an avenue for its further development.

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arXiv2025

Fermi liquid and isotropic superconductivity of Hund scenario for bilayer nickelates

Recent experiments on bulk and thin film bilayer nickelate high-$T_c$ superconductors urge for clarification of their pairing mechanism. Debates exist on whether the hybridization or the Hund's coupling between the nickel $d_{x^2-y^2}$ and $d_{z^2}$ orbitals plays a primary role in driving the superconductivity. Here, we study the Hund scenario and make comparisons with the hybridization scenario using the same dynamic Schwinger boson approach. Our calculations reveal several key features of the Hund-driven superconductivity, including an isotropic $s$-wave gap, a lower maximum $T_c$, and Fermi liquid normal states, that differ from the hybridization-driven mechanism. We attribute these differences to their distinct low-energy dynamics. Comparison with recent experiments suggests that the Hund scenario alone is not enough to explain the bilayer nickelate superconductivity in both bulk and thin films.

cond-mat.supr-con↗
arXiv2024

Frustrated superconductivity and intrinsic reduction of $T_c$ in trilayer nickelate

Identifying the key factors controlling the magnitude of $T_c$ is of critical importance in the pursuit of high-temperature superconductivity. In cuprates, $T_c$ reaches its maximal value in trilayer structure, leading to the belief that interlayer coupling may help promote the pairing. In contrast, for the recently discovered nickelate superconductors under high pressure, the maximum $T_c$ is reduced from about 80 K in the bilayer La$_3$Ni$_2$O$_{7}$ to 30 K in the trilayer La$_4$Ni$_3$O$_{10}$. Motivated by this opposite trend, we propose an interlayer pairing scenario for the superconductivity of La$_4$Ni$_3$O$_{10}$. Our theory reveals intrinsic frustration in the spin-singlet pairing that the inner layer tends to form with both of the two outer layers respectively, leading to strong superconducting fluctuations between layers. This explains the reduction of its maximum $T_c$ compared to that of the bilayer La$_3$Ni$_2$O$_{7}$. Our findings support a fundamental distinction between multilayer nickelate and cuprate superconductors, and ascribe it to their different (interlayer versus intralayer) pairing mechanisms. Furthermore, our theory predicts extended $s^\pm$-wave gap structures in La$_4$Ni$_3$O$_{10}$, with varying signs and possible nodes on different Fermi pockets. We also find an intrinsic Josephson coupling with potentially interesting consequences that may be examined in future experiments. Our work reveals the possibility of rich novel physics in multilayer superconductors with interlayer pairing.

cond-mat.supr-con↗
arXiv2024

Highly asymmetric superconducting dome and strange metallicity in La$_3$Ni$_2$O$_7$

The recent discovery of high-temperature superconductivity in La$_3$Ni$_2$O$_7$ under high pressure has stimulated intensive investigations concerning its paring mechanism and a correct description of its effective low-energy physics. Notable experimental observations include the right-triangle-shaped superconducting dome with the maximum $T_c$ of about 80 K and the strange metal behavior in the normal state above $T_c$. Here we apply the Schwinger boson approach to the bilayer $t$-$V$-$J$ model, which allows us to treat well both the magnetic correlations and the superconducting instability. We obtain a global phase diagram with both metallic and superconducting ground states separated by a quantum phase transition, and predict a minimal interlayer superexchange $J$ and a minimal $d_{z^2}$-hole concentration necessary for the superconductivity, with a highly asymmetric right-triangle-like shape for the $T_c$ curve as well as a maximum $T_c$ comparable with experiments. The normal state is featured with a pseudogap in the $d_{x^2-y^2}$ spectra associated with preformed Cooper pairs and non-Fermi liquid strange metal behavior due to hybridization-induced spinon-holon-electron scattering. Our work clarifies the key difference of the two-component scenario of the superconductivity from other mechanisms, and provides a consistent understanding of the high-temperature superconductivity and strange metallic properties of La$_{3}$Ni$_2$O$_7$ under high pressure.

cond-mat.supr-con↗
arXiv2024

Pair density wave, unconventional superconductivity, and non-Fermi liquid quantum critical phase in frustrated Kondo lattice

Motivated by the recent discovery of an intermediate quantum critical phase between the antiferromagnetic order and the Fermi liquid in the frustrated Kondo lattice CePdAl, we study here a Kondo-Heisenberg chain with frustrated $J_1$-$J_2$ XXZ interactions among local spins using the density matrix renormalization group method. Our simulations reveal a global phase diagram with rich ground states including the antiferromagnetic order, the valence-bond-solid and bond-order-wave orders, the pair density wave state, the uniform superconducting state, and the Luttinger liquid state. We show that both the pair density wave and uniform superconductivity belong to the family of Luther-Emery liquids and may arise from pair instability of an intermediate quantum critical phase with medium Fermi volume in the presence of strong quantum fluctuations, while the Luttinger liquid has a large Fermi volume. This suggests a deep connection between the pair density wave, the unconventional superconductivity, and the non-Fermi liquid quantum critical phase.

cond-mat.str-el↗
arXiv2023

Pseudogap Behavior in the Local Spinon Spectrum of Power-Law Diverging Multichannel Kondo Model

Motivated by the emergence of higher-order van Hove singularities (VHS) with power-law divergent density of states (DOS) ($ρ_c(ω)=ρ_0/|ω|^{r}$, $0<r<1$) in materials, we investigate a multichannel Kondo model involving conduction electrons near the higher-order van Hove filling. This model considers $M$ channel and $N$ spin degrees of freedom. Employing a renormalization group analysis and dynamical large-$N$ approach, our results reveal a crossover from a non-Fermi liquid to pseudogap behavior in the spectral properties of the local impurity at the overscreened fixed point. We precisely determine the conditions under which the crossover occurs, either by tuning the exponent $r$ or the ratio $κ=M/N$ to a critical value. This pseudogap phase of spinon exhibits distinct physical properties that could have an impact on the properties of real systems. The results of this study provide novel insights into the non-Fermi liquid and pseudogap behaviors observed in strongly correlated systems and offer a playground to study the interplay between higher-order van Hove singularities and multichannel Kondo physics.

cond-mat.str-el↗
arXiv2023

Slave fermion interpretation of the pseudogap in doped Mott insulators

We apply the recently developed slave fermion approach to study the doped Mott insulator in the one-band Hubbard and Hubbard-Heisenberg models. Our results produce several subtle features in the electron spectra and confirm the key role of antiferromagnetic (AFM) correlations in the appearance of the pseudogap. Upon hole doping, the electron spectra exhibit a single peak near the Fermi energy in the local approximation of the Hubbard model where AFM correlations are not included. When AFM correlations are included through an explicit mean-field Heisenberg interaction, a second peak emerges at slightly lower energy and pushes the other peak to higher energy, so that a pseudogap emerges between the two peaks at small doping. Both peaks grow rapidly with increasing doping and eventually merge together, where the pseudogap no longer exists. Detailed analyses of the spectral evolution with doping and the strength of the Heisenberg interaction confirm that the lower-energy peak comes from a polaronic mechanism due to the holon-spinon interaction in the AFM-correlated background and the higher-energy peak arises from the holon hybridization to form the electron quasiparticles. Thus, the pseudogap arises from the interplay of the polaronic and hybridization mechanisms. Our results are in good agreement with previous numerical calculations using the dynamical mean-field theory and its cluster extensions, but give a clearer picture of the underlying physics. Our work provides a promising perspective for clarifying the nature of doped Mott insulators and may serve as a starting point for more elaborate investigations in the future.

cond-mat.str-el↗
arXiv2023

A Simple Solvable Model for Heavy Fermion Superconductivity from the Two-Fluid Normal State

We propose an exactly solvable momentum-space Kondo-BCS model to study heavy fermion superconductivity. The Kondo interaction is local in momentum space, which can be derived from an Anderson lattice with a Hatsugai-Kohmoto interaction between $f$-electrons. By increasing the Kondo interaction, the model exhibits a crossover from a weak-coupling BCS superconductor to a strong-coupling heavy fermion superconductor featured with a large gap ratio and a large specific heat jump anomaly. Accordingly, the normal state evolves from a Fermi liquid above the BCS superconductor to a non-Fermi liquid two-fluid state above the heavy fermion superconductor. The two-fluid normal state also leads to two types of Cooper pairs, one between conduction electrons, the other between composite fermions formed by conduction electrons and $f$-spins, which is responsible for the strong coupling behaviors of heavy fermion superconductivity.

cond-mat.str-el↗
arXiv2023

Nonlocal Kondo effect and two-fluid picture revealed in an exactly solvable model

Understanding the nature of local-itinerant transition of strongly correlated electrons is one of the central problems in condensed matter physics. Heavy fermion systems describe the f-electron delocalization through Kondo interactions with conduction electrons. Tremendous efforts have been devoted to the so-called Kondo-destruction scenario, which predicts a dramatic local-to-itinerant quantum phase transition of f-electrons at zero temperature. On the other hand, two-fluid behaviors have been observed in many materials, suggesting coexistence of local and itinerant f-electrons over a broad temperature range but lacking a microscopic theoretical description. To elucidate this fundamental issue, here we propose an exactly solvable Kondo-Heisenberg model in which the spins are defined in the momentum space and the k-space Kondo interaction corresponds to a highly nonlocal spin scattering in the coordinate space. Its solution reveals a continuous evolution of the Fermi surfaces with Kondo interaction and two-fluid behaviors similar to those observed in real materials. The electron density violates the usual Luttinger's theorem, but follows a generalized one allowing for partially enlarged Fermi surfaces due to partial Kondo screening in the momentum space. Our results highlight the consequence of nonlocal Kondo interaction relevant for strong quantum fluctuation regions, and provide important insight into the microscopic description of two-fluid phenomenology in heavy fermion systems.

cond-mat.str-el↗
arXiv2022

Dynamic charge Kondo effect and a slave fermion approach to the Mott transition

Mott transition plays a key role in strongly correlated physics but its nature is not yet fully understood. Motivated by recent development of Schwinger boson approach for the Kondo lattice, we propose in this work a novel slave fermion algorithm to study the Mott transition. Upon local approximation, our method yields a phase diagram with a zero-temperature continuous (Mott) metal-insulator transition at finite Coulomb interaction $U$ for the half-filled one-band Hubbard model on a square lattice, and the resistivity exhibits a critical scaling around the quantum Widom line. We argue that the Mott transition may be associated with a dynamic charge Kondo effect of local degenerate doublon and holon states, causing sharp resonances on the doublon/holon and electron spectra. The transition is pushed to $U=0$ once intersite antiferromagnetic correlations are included, in agreement with exact numerical calculations. Our approach captures some essential features of the Mott transition and offers an alternative angle to view this important problem. It can be extended to study other correlated electron models with more complicated local interactions.

cond-mat.str-el↗
arXiv2022

Continuous ferromagnetic quantum phase transition on an anisotropic Kondo lattice

Motivated by the recent discovery of ferromagnetic quantum criticality in the heavy fermion compound CeRh$_6$Ge$_4$, we develop a numerical algorithm of infinite projected entangled pair states for the anisotropic ferromagnetic Kondo-Heisenberg model in two dimensions and study the ferromagnetic quantum phase transitions with varying magnetic and hopping anisotropy. Our calculations reveal a continuous ferromagnetic quantum phase transition in the large anisotropic region and first-order quantum phase transitions for smaller anisotropy. Our results highlight the importance of magnetic anisotropy on ferromagnetic quantum criticality in Kondo lattice systems and provide a possible explanation for the experimental observation in CeRh$_6$Ge$_4$ with a quasi-one-dimensional magnetic structure. Our work opens the avenue for future studies of the rich Kondo lattice physics using state-of-the-art tensor network approaches.

cond-mat.str-el↗
arXiv2022

$\mathbb{Z}_2$ metallic spin liquid on a frustrated Kondo lattice

Metallic spin liquid has been reported in several correlated metals, but a satisfactory theoretical description is not yet available. Here we propose a potential route to realize the metallic spin liquid and construct an effective $\mathbb{Z}_2$ gauge theory with charged fractionalized excitations on the triangular Kondo lattice. This leads to a $\mathbb{Z}_2$ metallic spin liquid featured with long-lived, heavy holon excitations of spin 0 and charge $+e$ and a partially enlarged electron Fermi surface. It differs from the weak-coupling FL$^*$ state proposed earlier and may be viewed as a fractionalized heavy fermion liquid. Our theory provides a general framework to describe the metallic spin liquid in frustrated Kondo lattice systems.

cond-mat.str-el↗
arXiv2021

A unified theory of ferromagnetic quantum phase transitions in heavy fermion metals

Motivated by the recent discovery of a continuous ferromagnetic quantum phase transition in CeRh$_6$Ge$_4$ and its distinction from other U-based heavy fermion metals such as UGe$_2$, we develop a unified explanation of their different ground state properties based on an anisotropic ferromagnetic Kondo-Heisenberg model. We employ an improved large-$N$ Schwinger boson approach and predict a full phase diagram containing both a continuous ferromagnetic quantum phase transition for large magnetic anisotropy and first-order transitions for relatively small anisotropy. Our calculations reveal three different ferromagnetic phases including a half-metallic spin selective Kondo insulator with a constant magnetization. The Fermi surface topologies are found to change abruptly between different phases, consistent with that observed in UGe$_2$. At finite temperatures, we predict the development of Kondo hybridization well above the ferromagnetic long-range order and its relocalization near the phase transition, in good agreement with band measurements in CeRh$_6$Ge$_4$. Our results highlight the importance of magnetic anisotropy and provide a unified theory for understanding the ferromagnetic quantum phase transitions in heavy fermion metals.

cond-mat.str-el↗
arXiv2021

Schwinger boson approach for the dynamical mean-field theory of the Kondo lattice

We apply the dynamical large-$N$ Schwinger boson technique as an impurity solver for the dynamical mean-field theory (DMFT) calculations of the Kondo lattice model. Our approach captures the hybridization physics through the DMFT self-consistency that is missing in the pure Schwinger boson calculations with independent electron baths. The resulting thermodynamic and transport properties are in qualitative agreement with more rigorous calculations and give the correct crossover behavior over a wide temperature range from the local moment regime to the Fermi liquid. Our method may be further extended to combine with the density functional theory for efficient material calculations.

cond-mat.str-el↗
arXiv2021

Nonlocal Kondo effect and quantum critical phase in heavy fermion metals

Heavy fermion metals typically exhibit unconventional quantum critical point or quantum critical phase at zero temperature due to competition of Kondo effect and magnetism. Previous theories were often based on certain local type of assumptions and a fully consistent explanation of experiments has not been achieved. Here we develop an efficient algorithm for the Schwinger boson approach to explore the effect of spatial correlations on the Kondo lattice and introduce the concept of nonlocal Kondo effect in the quantum critical region with deconfined spinons. We predict a global phase diagram containing a non-Fermi liquid quantum critical phase with a hidden holon Fermi surface and a partially enlarged electron Fermi surface for strong quantum fluctuations while a single quantum critical point for weak quantum fluctuations. This explains the unusual metallic spin liquid recently reported in the frustrated Kondo lattice CePdAl and resolves the Fermi volume puzzle in YbRh$_2$Si$_2$. Our theory highlights the importance of nonlocal physics and provides a unified understanding of heavy fermion quantum criticality.

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