arXiv subjects

Yi-feng Yang

Publications and source records attributed to Yi-feng Yang.

At least 37 records · Page 2Linked to original sources

arXiv2025

Possible Fano effect and suppression of Andreev reflection in La$_3$Ni$_2$O$_7$

The recently-discovered high-temperature superconductor La$_3$Ni$_2$O$_7$ under high pressure has stimulated intensive debates. Key controversies concern interlayer versus intralayer pairing scenarios and if the hybridization plays a key role in establishing the superconductivity. But experimental clarification is difficult due to the limitation of employing state-of-the-art techniques under high pressure. Here we propose that quasiparticle tunneling and Andreev reflection may provide a feasible way to distinguish different pairing scenarios. We predict that an asymmetric Fano line shape may be induced by the hybridization between the $d_{x^2-y^2}$ metallic bands and the strongly renormalized flat $d_{z^2}$ quasiparticle bands. In the superconducting state, we show that the Andreev reflection should be greatly suppressed for interlayer pairing superconductivity with a small interlayer hopping. We propose future experiments to examine these predictions and help clarify the fundamental physics of superconducting La$_3$Ni$_2$O$_7$ and other multilayer nickelate superconductors.

cond-mat.supr-con↗

arXiv2025

Intrinsic constraint on $T_c$ for unconventional superconductivity

Can room temperature superconductivity be achieved in correlated materials under ambient pressure? Our answer to this billion-dollar question is probably no, at least for realistic models within the current theoretical framework. This is shown by our systematic simulations on the pairing instability of some effective models for two-dimensional superconductivity. For a square lattice model with nearest-neighbour pairing, we find a plaquette state formed of weakly-connected $2\times2$ blocks for sufficiently large pairing interaction. The superconductivity is suppressed on both sides away from its melting quantum critical point. Thus, the magnitude of $T_c$ is constrained by the plaquette state for the $d$-wave superconductivity, in resemblance of other competing orders. We then extend our simulations to a variety of effective models covering nearest-neighbour or onsite pairings, single layer or two-layer structures, intralayer or interlayer pairings, and find an intrinsic maximum of the ratio $T_c/J\approx 0.04-0.07$, where $J$ is the pairing interaction. Comparison with existing experiments supports this constraint in cuprate, iron-based, nickelate, and heavy fermion superconductors, despite that these compounds are so complicated well beyond our simplified models. As a result, the known families of unconventional superconductivity, possibly except the infinite-layer nickelates, seem to almost exhaust their potentials in reaching the maximal $T_c$ allowed by their respective $J$, while achieving room temperature superconductor would require a much larger $J$ beyond 400-700 meV, which seems unrealistic in existing correlated materials and hence demands novel pairing mechanisms. The agreement also implies some deep underlying principles of the constraint that urge for a more rigorous theoretical understanding.

cond-mat.supr-con↗

arXiv2025

Weak electronic correlations in the cobalt oxychalcogenide superconductor Na2CoSe2O

Motivated by the newly discovered Co-based superconductor Na2CoSe2O, we performed systematic calculations of its electronic band structures using the density functional theory (DFT) plus the dynamical mean-field theory (DMFT) approaches. Our comparative studies reveal weakly correlated and itinerant nature of the Co-3d electrons and show no sign of fluctuating local moments as expected in many other unconventional superconductors, although the Co eg orbitals are close to half filling. These suggest that Na2CoSe2O is a normal paramagnetic metal and its superconductivity might not be of strongly correlated nature, contrary to the initial speculation. We suggest future investigations of electron-phonon interactions to clarify its pairing mechanism.

cond-mat.str-el↗

arXiv2024

Spectroscopic origin of giant anomalous Hall effect in an interwoven magnetic kagome metal

The discovery of a giant anomalous Hall effect (AHE) and its novel mechanism holds significant promise for advancing both fundamental research and practical applications. Magnetic kagome lattice materials are uniquely suited for studying the AHE due to their interplay between electronic structure, topology, and magnetism. However, the geometric frustration inherent in kagome lattices often limits the configuration and tunability of magnetic order. Here, we present a new design strategy for kagome-lattice materials with emergent magnetism, exemplified by the magnetic kagome metal TbTi$_3$Bi$_4$, which features interwoven magnetic Tb zigzag chains and non-magnetic Ti kagome bilayers. This material exhibits a record-high anomalous Hall conductivity (AHC) of 10$^5$ $Ω^{-1}$ cm$^{-1}$. Spectroscopy measurements reveal a large band folding gap observed via angle-resolved photoemission spectroscopy, coexisting spin-density-wave (SDW) order detected through spin-polarized scanning tunneling spectroscopy, and a spiral magnetic order with large magnetic moments identified by neutron diffraction. These findings highlight a strong electron-magnetic coupling between itinerant charges and ordered magnetic moments, offering a spectroscopic explanation for the giant AHC in TbTi$_3$Bi$_4$. This work establishes a pathway for innovative material design strategies, unlocking new possibilities for future exploration and applications in quantum and spintronic technologies.

cond-mat.str-el↗

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

Decomposition of multilayer superconductivity with interlayer pairing

We prove that multilayer superconductivity with interlayer pairing may naturally decompose into a series of weakly-coupled bilayer and trilayer superconducting blocks in order to minimize its total free energy. Our work is motivated by the recent proposal of interlayer pairing induced by the interlayer superexchange interaction of nearly half-filled $d_{z^2}$ orbitals in the bilayer and trilayer nickelate superconductors. We explore general properties of interlayer pairing superconductivity and perform systematic Ginzburg-Landau analyses of an effective multilayer model. For real materials, our results imply strong superconducting order parameter modulation and short coherence length along the $z$-axis (perpendicular to the layers). This reveals a unique feature of multilayer superconductivity with interlayer pairing and provides a basic framework for future experimental and theoretical investigations.

cond-mat.supr-con↗

arXiv2024

Anomalous impurity effect in the heavy-fermion superconductor CeCu2Si2

Recent observations of contradictory impurity effect in the heavy-fermion superconductor CeCu2Si2 at ambient pressure have hindered the identification of its pairing symmetry. Here we perform theoretical analyses with both intraband and interband impurity scatterings for the nodeless $s^\pm$-wave pairing, and report an anomalous nonmonotonic variation of its $T_c$ suppression with the scattering strength. Our results reproduce the prominent reduction of $T_c$ in good agreement with earlier experiments by atomic substitution and explains as well its robustness against electron irradiation. We ascribe the latter to the screening of the interband impurity potential in the strong scattering or unitary region. This resolves the seeming contradiction in different experimental probes and provides an important support to the nodeless $s^\pm$-wave scenario in CeCu2Si2 at ambient pressure. Our theory may be extended to other narrow band systems such as twisted bilayer graphene and the recently discovered bilayer or trilayer nickelate superconductors, and provide a useful way to distinguish different paring candidates thereof.

cond-mat.supr-con↗

arXiv2024

Generalized Spectral Decomposition for Quantum Impurity Problems

Solving quantum impurity problems may advance our understanding of strongly correlated electron physics, but its development in multi-impurity systems has been greatly hindered due to the presence of shared bath. Here, we propose a general operation strategy to disentangle the shared bath into multiple auxiliary baths and relate the problem to a spectral decomposition problem of function matrix for applying the numerical renormalization group (NRG). We prove exactly that such decomposition is possible for models satisfying (block) circulant symmetry, and show how to construct the auxiliary baths for arbitrary impurity configuration by mapping its graph structure to the subgraph of a regular impurity configuration. We further propose an approximate decomposition algorithm to reduce the number of auxiliary baths and save the computational workload. Our work reveals a deep connection between quantum impurity problems and the graph theory, and provides a general scheme to extend the NRG applications for realistic multi-impurity systems.

cond-mat.str-el↗

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

Inter-layer valence bonds and two-component theory for high-$T_c$ superconductivity of La$_{3}$Ni$_{2}$O$_{7}$ under pressure

The recent discovery of high-$T_{c}$ superconductivity in bilayer nickelate La$_{3}$Ni$_{2}$O$_{7}$ under high pressure has stimulated great interest concerning its pairing mechanism. We argue that the weak coupling model from the almost fully-filled $d_{z^{2}}$ bonding band cannot give rise to its high $T_{c}$, and thus propose a strong coupling model based on local inter-layer spin singlets of Ni-$d_{z^{2}}$ electrons due to their strong on-site Coulomb repulsion. This leads to a minimal effective model that contains local pairing of $d_{z^{2}}$ electrons and a considerable hybridization with near quarter-filled itinerant $d_{x^{2}-y^{2}}$ electrons on nearest-neighbor sites. Their strong coupling provides a unique two-component scenario to achieve high-$T_{c}$ superconductivity. Our theory highlights the importance of the bilayer structure of superconducting La$_{3}$Ni$_{2}$O$_{7}$ and points out a potential route for the exploration of more high-$T_{c}$ superconductors.

cond-mat.supr-con↗

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

High-$T_c$ superconductivity by mobilizing local spin singlets and possible route to higher $T_c$ in pressurized La$_3$Ni$_2$O$_7$

We clarify the pairing mechanism of high-$T_c$ superconductivity in bilayer La$_3$Ni$_2$O$_7$ under high pressure by employing the static auxiliary field Monte Carlo approach to simulate a minimal effective model that contains local $d_{z^2}$ interlayer spin singlets and metallic $d_{x^2-y^2}$ bands. Superconductivity is induced when the local spin singlet pairs are mobilized and attain long-distance phase coherence by hybridization with the metallic bands. When projected onto realistic Fermi surfaces, it yields a nodeless $s$-wave gap on the $γ$ Fermi surface, and extended $s$-wave gaps of the same (opposite) sign on the $α$ ($β$) Fermi surface due to its bonding (antibonding) character, with nodes or gap minima along the diagonal direction of the two-dimensional Brillouin zone. We find a dual role of the hybridization that not only induces global phase coherence but also competes with the spin singlet formation. This lead to a tentative phase diagram where $T_c$ varies nonmonotonically with the hybridization, in good correspondence with experimental observations. A roughly linear relation is obtained for realistic hopping and hybridization parameters: $T_c\approx 0.04-0.05 J$, where $J$ is the interlayer superexchange interaction. We emphasize the peculiar tunability of the bilayer structure and propose that $T_c$ may be further enhanced by hole doping or applying uniaxial pressure along the $c$ axis on superconducting La$_3$Ni$_2$O$_7$. Our work provides reliable numerical evidence for the pairing mechanism of high-$T_c$ superconductivity in La$_3$Ni$_2$O$_7$ and points out a potential route to achieve even higher $T_c$.

cond-mat.supr-con↗

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

An electronic origin of charge order in infinite-layer nickelates

A charge order (CO) with a wavevector $\mathbf{q}\simeq\left(\frac{1}{3},0,0\right)$ is observed in infinite-layer nickelates. Here we use first-principles calculations to demonstrate a charge-transfer-driven CO mechanism in infinite-layer nickelates, which leads to a characteristic Ni$^{1+}$-Ni$^{2+}$-Ni$^{1+}$ stripe state. For every three Ni atoms, due to the presence of near-Fermi-level conduction bands, Hubbard interaction on Ni-$d$ orbitals transfers electrons on one Ni atom to conduction bands and leaves electrons on the other two Ni atoms to become more localized. We further derive a low-energy effective model to elucidate that the CO state arises from a delicate competition between Hubbard interaction on Ni-$d$ orbitals and charge transfer energy between Ni-$d$ orbitals and conduction bands. With physically reasonable parameters, $\mathbf{q}=\left(\frac{1}{3},0,0\right)$ CO state is more stable than uniform paramagnetic state and usual checkerboard antiferromagnetic state. Our work highlights the multi-band nature of infinite-layer nickelates, which leads to some distinctive correlated properties that are not found in cuprates.

cond-mat.str-el↗

arXiv2023

Superconducting fluctuations and charge-4$e$ plaquette state at strong coupling

We apply the static auxiliary field Monte Carlo approach to study phase correlations of the pairing fields in a microscopic model with spin-singlet pairing interaction. We find that the short- and long-range phase correlations are well captured by the phase mutual information, which allows us to construct a theoretical phase diagram containing the uniform $d$-wave superconducting region, the phase fluctuating region, the local pairing region, and the disordered region. We show that the gradual development of phase coherence has a number of consequences on spectroscopic measurements, such as the development of the Fermi arc and the anisotropy in the angle-resolved spectra, scattering rate, entropy, specific heat, and quasiparticle dispersion, in good agreement with experimental observations. For strong coupling, our Monte Carlo simulation reveals an unexpected charge-4$e$ plaquette state with $d$-wave bonds, which competes with the uniform $d$-wave superconductivity and exhibits a U-shaped density of states.

cond-mat.supr-con↗

arXiv2023

Flat bands promoted by Hund's rule coupling in the candidate double-layer high-temperature superconductor La$_3$Ni$_2$O$_7$

We report strongly correlated electronic band structure calculations of the recently discovered double-layer high-temperature superconductor La$_3$Ni$_2$O$_7$ under pressure. Our calculations reveal dual nature of Ni-$d$ electrons with almost localized $d_{z^2}$ orbitals due to onsite Coulomb repulsion and very flat hybridization bands of Ni-$d_{x^2-y^2}$ and Ni-$d_{z^2}$ quasiparticles near the Fermi energy. We find that the quasiparticle effective mass are greatly enhanced by the Hund's rule coupling and their lifetimes are inversely proportional to the temperature, which explains the experimentally observed strange metal behavior in the normal state. We also find strong antiferromagnetic spin correlations of Ni-$d$ electrons, which may provide the pairing force of quasiparticles for the high-temperature superconductivity. These give a potential explanation of two key observations in experiment and connect the superconducting La$_3$Ni$_2$O$_7$ with cuprate high-temperature superconductors. The presence of flat bands and the interplay of orbital-selective Mott, Hund, and Kondo physics make La$_3$Ni$_2$O$_7$ a unique platform for exploring rich emergent quantum many-body phenomena in the future.

cond-mat.supr-con↗

arXiv2023

Universal Scaling Law of Quasiparticle Nernst Effect in Cuprates: A Unified Schematic Analysis for Transverse Transport

We discover a universal scaling law for the quasiparticle Nernst coefficient in underdoped cuprates, whose magnitude decreases exponentially with increasing temperature as confirmed in YBa$_2$Cu$_3$O$_y$. We attribute it to the basic mathematical structure of the conductivity formula with a narrow effective bandwidth of nonzero Berry curvatures associated with the pseudogap. A unified scheme is then developed to analyze transverse transport in hole-doped cuprates and clarify the puzzling disparity in determining the pseudogap temperature from different measurements. Our proposal opens the avenue for exploring potential scaling laws in the intermediate temperature region and may have broad applications in strongly correlated or narrow band systems.

cond-mat.str-el↗