arXiv subjects

Jing-Yu Zhao

Publications and source records attributed to Jing-Yu Zhao.

15 recordsLinked to original sources

arXiv2026

Magnetic Breakdown and Anomalous Quantum Oscillation in Rhombohedral Tetralayer Graphene

We investigate magnetic breakdown near Van Hove singularities (VHSs) in the electron-doped rhombohedral tetralayer graphene, where chiral superconductivity has recently been reported. Using the noninteracting band structure and Kubo formula, we identify anomalous Shubnikov-de Haas effects: Ring-like structures in the Landau fan and anomalous high-frequency peaks in the frequency spectra. These anomalous quantum oscillations can be understood by the reconstruction from magnetic breakdown among three nearby Fermi pockets separated by VHSs. Remarkably, these qualitative anomalous features persist into a stronger-VHS regime, where the semiclassical picture breaks down. The temperature and (weak) disorder dependence of the oscillations are also investigated. Our results establish that the magnetic-breakdown-induced anomalous quantum oscillation provides a general distinctive probe for the underlying Fermi-surface geometry associated with VHSs and may explain the recent quantum oscillation experiment in rhombohedral tetralayer graphene [arXiv:2606.05356].

cond-mat.mes-hall↗

arXiv2026

Displacement-Field-Driven Semimetal-Superconductor Transition in Magic-Angle Twisted Trilayer Graphene

Magic-angle twisted trilayer graphene(MATTG) hosts versatile displacement-field-tuned correlated phenomena. MATTG consists of a dispersive Dirac cone which hybridizes with the flat band from a twisted bilayer graphene (TBG) sector. The hybridization strength increases with the displacement field $D$ and naively one may expect D-driven heavy fermion physics. However, the TBG Hubbard bands have a momentum-selective Mott gap, which is small at the $\Gamma$ point due to the band topology, and a rigid local moment description as in the familiar Kondo lattice model is invalid. Here we show that the dominant effect of the displacement field is to induce an energy shift of the Dirac cone and self-doping into the TBG sector. We illustrate this picture in a concrete calculation using a slave-particle theory at the filling $\nu=\pm 2$. We find that increasing $D$ drives a transition from a semimetal into a superconducting state. We also discuss the enhancement of the superconductivity by $D$ near $\nu=\pm2$ and the particle-hole asymmetry of the phase diagram. Our results provide a unified picture for electric-field-tunable superconductivity, Mottness, and heavy-fermion-like behavior in MATTG.

cond-mat.str-el↗

arXiv2026

Pseudogap and Non-Fermi-liquid criticality in double Kondo model for bilayer nickelates

Motivated by recent experimental progress on high-temperature superconductivity in bilayer nickelates, we investigate the phase diagram of the normal state in a bilayer Kondo lattice model using single-site dynamical mean-field theory (DMFT). When the interlayer tunneling $t_\perp$ is absent, we identify a non-Fermi-liquid (NFL) critical point tuned by the interlayer spin coupling $J_\perp$ or hole doping $x$, which separates a standard Fermi liquid in the overdoped region from a distinct pseudogap (PG) metal in the underdoped regime. This PG phase, which we term the `second Fermi liquid' (sFL), exhibits small hole pockets and violates the perturbative Luttinger theorem despite the absence of symmetry breaking or fractionalization. The PG metal behaves like a heavy Fermi liquid, with small quasi-particle residue and large effective mass. We also provide an intuitive analytical description of the pseudogap and the ground-state wave function based on an ancilla-fermion framework. Inside the PG phase, we interpret the ancilla fermion as a spin-polaron and demonstrate a Kondo-like resonance peak in the spectral function of this composite fermion directly in DMFT calculation. Extending the analysis to finite $t_\perp$, we apply this framework to the bilayer nickelate $\mathrm{La}_3\mathrm{Ni}_2\mathrm{O}_7$. We propose that current experimental samples ($x \approx 0.5$) reside in the overdoped FL regime, suggesting that the pseudogap phase and the NFL criticality may be accessed via electron doping.

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arXiv2026

Minimal loop currents in doped Mott insulators

For the $t$-$J$ model, variational wave functions can generally be constructed based on an accurate description of antiferromagnetism (AFM) at half-filling and an exact phase-string sign structure under doping. The single-hole-doped and two-hole-doped states, as determined by variational Monte Carlo (VMC) simulations, display sharply contrasting behaviors. The single-hole state constitutes a ``cat state'' that resonates strongly between a quasiparticle component and a local loop-current component, with approximately equal weights. In the ground state, the quasiparticle spectral weight $Z_{\mathbf{k}}$ peaks at momenta $\mathbf{k}_0 \equiv (\pm\frac{\pi}{2},\pm\frac{\pi}{2})$. The total-energy dispersion versus $\mathbf{k}$ agrees remarkably well with the Green function Monte Carlo results. However, Landau's one-to-one correspondence hypothesis for quasiparticles breaks down here with the incoherent component exhibiting intrinsic magnetization originating from a minimal $2\times2$ loop current that forms a $4\times4$ pattern on the square lattice--a finding in excellent agreement with density matrix renormalization group (DMRG) calculations. In the two-hole ground state, a new pairing mechanism is revealed: the two holes are automatically fused into a tightly bound object consisting of an incoherent $d_{xy}$ pairing along the diagonal direction by compensating the local loop currents. This hole pair is again a ``cat state'' that resonates strongly between the incoherent $d_{xy}$ and a coherent $d_{x^2-y^2}$ Cooper channel to gain substantial hopping energy. Its size extends over an area of about $4\times 4$ lattice spacings, much smaller than the divergent AFM correlation length, implying that it should survive as a minimal superconducting building block even in the dilute doping regime. Experimental implications and the generalization to the finite-doping case are briefly addressed.

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arXiv2025

Resonating-valence-bond superconductor from small Fermi surface in twisted bilayer graphene

Mechanism of superconductivity in twisted bilayer graphene (TBG) remains one of the central problems in correlated moir\'e materials. The most intriguing question is about the nature of the normal state: is the Cooper pair formed from small Fermi surface or large Fermi surface? In this work we point out the possibility of a symmetric pseudogap metal with small hole pockets, dubbed as second Fermi liquid (sFL). In the sFL phase at $\nu=-2-x$, there is a two-component picture: two electrons mainly localize at each AA site and form a paired singlet due to anti-Hund's coupling mediated by the optical phonon, while additional holes doped into the AA sites form small Fermi surfaces. The sFL phase corresponds to an intrinsically strongly interacting fixed point and is topologically distinct to the conventional Fermi liquid. We develop a unified framework to describe both a renormalized FL phase and an sFL phase. We propose that the TBG superconductor emerges from the sFL phase, but is close to the transition toward the FL phase under increasing hole doping. In the superconductor, pairing of local moments is shared to the mobile carriers and a smaller superconducting gap with nodal $p_x$ symmetry is opened on the small hole pockets. This work provides, to our knowledge, the first unified theory that explains both the pseudogap metal above $T_c$ and the two-gap nematic superconductivity below it.

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arXiv2025

Yamaji effect in models of underdoped cuprates

Recent angle-dependent magnetoresistance measurements in underdoped cuprates have revealed compelling evidence for small hole pockets in the pseudogap regime, including observation of the Yamaji effect in HgBa$_2$CuO$_{4+\delta}$ (Chan et al., Nature Physics 10.1038/s41567-025-03032-2 (2025)). A key distinction between theories is their predicted Fermi volumes, measured as fractions of the square lattice Brillouin zone: $p/4$ per pocket for spin density wave (SDW) versus $p/8$ for fractionalized Fermi liquid (FL*), where $p$ is the hole doping. We calculate the $c$-axis magnetoresistance $\rho_{zz}(\theta, \phi)$ within the semiclassical Boltzmann formalism for both states, and using the ancilla layer model (ALM) for FL* in a single-band Hamiltonian. The results from the $\text{FL}^*$ phase show good consistency with current experimental data. Conversely, the results for the SDW phase are highly sensitive to the ordering momentum along the $z$-direction. An ordering vector of $Q = (\pi, \pi, \pi)$ yields predictions that starkly disagree with the experiment. The only possibility for agreement within the SDW scenario is to assume an ordering momentum of $Q = (\pi, \pi, 0)$. However, even in this specific case, the SDW scenario predicts a marginally smaller Yamaji angle at $\phi=0$ than the FL* theory, and a second Yamaji peak near in-plane angle $\phi = 45^\circ$, which was not observed in the experiment. In reality, the N\'eel ordering vector is likely uncorrelated between adjacent layers, so that there is no coherent interlayer transport of hole-pocket quasiparticles in the SDW scenario, and consequently no Yamaji effect. Our results support the FL* interpretation of Fermi arcs in the pseudogap phase, and establish Yamaji angle measurements as a discriminatory tool between theoretical models.

cond-mat.str-el↗

arXiv2025

Mixed valence Mott insulator and composite excitation in twisted bilayer graphene

Interplay of strong correlation and flat topological band has been a central problem in moir\'e systems such as the magic angle twisted bilayer graphene (TBG). Recent studies show that Mott-like states may still be possible in TBG despite the Wannier obstruction. However, the nature of such unconventional states is still not well understood. In this work we construct the ground state wavefunction and exotic excitations of a symmetric correlated semimetal or insulator at even integer filling using a parton mean field theory of the topological heavy fermion model. We label the valence of the $f$ orbital based on its occupation $n_f$. At $\nu=-2$, we show that the $f$ orbital is not in the simple $f^{2+}$ valence expected from a trivial Mott localization. Instead, around $1/3$ of AA sites are self doped, with holes entering the $c$ orbitals away from AA sites. As a result, the $f$ orbital is in a superposition of $f^{2+}$ and $f^{3+}$ valences and should not be viewed as local moment. We dub the phase as \textit{mixed valence Mott insulator}. This unconventional insulator has a large hybridization $\langle c^\dagger f \rangle\neq 0$ and is sharply distinct from the usual `kondo breakdown' picture. In most of the momentum space away from the $\Gamma$ point, there is a Mott gap equal to the Hubbard $U$. At the $\Gamma$ point, we have a `charge transfer gap' much smaller than $U$. In particular, the top of the lower band is dominated by a composite excitation, which is a linear combination of $|f^{1+}\rangle\langle f^{2+}|$ and $|f^{2+}\rangle\langle f^{3+}|$ with a sign structure such that it is orthogonal to the microscopic $f$ operator. At $\nu=0$, similar approach leads to a Mott semimetal. We hope this work will inspire more explorations of the Anderson models with a large hybridization, a regime which may host new physics beyond the familiar Kondo or heavy fermion systems.

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arXiv2025

Ancilla theory of twisted bilayer graphene I: topological Mott localization and pseudogap metal in twisted bilayer graphene

The recent experimental studies of twisted bilayer graphene (TBG) raise a fundamental question: how do we understand Mott localization in a topological band? In this work, we offer a new perspective of Mott physics, which can be generalized to TBG directly in momentum space. In our theory, the Mott gap is understood as from an exciton-like hybridization $\Phi(\mathbf k) c^\dagger(\mathbf k)\psi(\mathbf k)$ between the physical electron $c$ and an ancilla fermion $\psi$. In the conventional Mott insulator of trivial band, the hybridization is $s$-wave with $\Phi(\mathbf k)=\frac{U}{2}$, where $U$ is the on-site Hubbard interaction. On the other hand, the band topology in TBG enforces a topological Mott hybridization with $\Phi(\mathbf k)\sim k_x \pm i k_y$ in a small region around $\mathbf{k}=0$. We dub this new Mott state as topological Mott localization because of the $p\pm ip$ order parameter analogous to the topological superconductor. At $\nu=0$, we find a topological Mott semimetal with a low energy effective theory resembling that of the untwisted bilayer graphene. For $\nu=\pm 1, \pm 2, \pm 3$, we show transitions from correlated insulators to Mott semimetals at smaller $U$. In the most intriguing density region $\nu=-2-x$, we propose a symmetric pseudogap metal at small $x$, which hosts a small Fermi surface and violates the perturbative Luttinger theorem. Interestingly, the quasiparticle is primarily formed by ancilla fermion, which we interpret as a composite fermion formed by a hole bound to a particle-hole pair. Our theory offers a unified language to describe the Mott localization in both trivial and topological bands in momentum space, and we anticipate applications in other moir\'e systems with topological Wannier obstruction, such as the twisted transition-metal dichalcogenide (TMD) homobilayer.

cond-mat.str-el↗

arXiv2024

Fusion of Low-Entanglement Excitations in 2D Toric Code

On top of a $D$-dimensional gapped bulk state, Low Entanglement Excitations (LEE) on $d$($<D$)-dimensional sub-manifolds can have extensive energy but preserves the entanglement area law of the ground state. Due to their multi-dimensional nature, the LEEs embody a higher-category structure in quantum systems. They are the ground state of a modified Hamiltonian and hence capture the notions of `defects' of generalized symmetries. In previous works, we studied the low-entanglement excitations in a trivial phase as well as those in invertible phases. We find that LEEs in these phases have the same structure as lower-dimensional gapped phases and their defects within. In this paper, we study the LEEs inside non-invertible topological phases. We focus on the simple example of $\mathbb{Z}_2$ toric code and discuss how the fusion result of 1d LEEs with 0d morphisms can depend on both the choice of fusion circuit and the ordering of the fused defects.

cond-mat.str-el↗

arXiv2024

Variational Monte Carlo Study of the Doped $t$-$J$ Model on Honeycomb Lattice

The ground state of the bipartite $t$-$J$ model must satisfy a specific sign structure, based on which the single-hole and two-hole ground state $Ans\ddot{a}tze$ on honeycomb lattice are constructed and studied by a variational Monte Carlo (VMC) method. The VMC results are in good agreement with the exact diagonalization (ED) calculation. For the single-hole case, the degenerate ground states are characterized by quantum numbers of a spin-1/2 and an orbital angular momentum $L_z=\pm 2$. The latter is associated with the emergent chiral spin/hole currents mutually surrounding the hole/spin-1/2 as a composite object or ``twisted hole''. A vanishing quasiparticle spectral weight is shown in the large-sample limit. In the two-hole ground state, the holes form a spin-singlet pairing with $d$+$id$ symmetry in the Cooper channel, but are of $s$-wave symmetry as a tightly bound pair of the ``twisted holes''. Such a pairing mechanism of dichotomy can be attributed to eliminating the local spin currents which has nothing to do with the long-range antiferromagnetic correlation. Superconducting ground state at finite doping is briefly discussed in terms of the tightly bound hole pairs as the building blocks.

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arXiv2023

Composite Structure of Single-Particle Spectral Function in Lightly-Doped Mott Insulators

The internal structure of doped holes in the Mott insulator may provide important insight into the physics of doped cuprates. Its observability via a single-particle probe by scanning tunneling spectroscopy (STS) and angle-resolved photo-emission spectroscopy (ARPES) is explored in this paper. Specifically we study the single-particle spectral function based on a two-hole variational ground state wavefunction [Phys. Rev. X 12, 011062 (2022)] in the $t$-$J$ model. The latter as a strongly correlated state possesses a dichotomy of $d$-wave Cooper pairing and $s$-wave ``twisted'' hole pairing. This pairing structure will give rise to two branches of local spectral function at finite energies. The low-lying one corresponds to a nodal-like quasiparticle excitation and the higher branch is associated with the pair breaking of ``twisted'' quasiparticles, with the threshold energy resembling a pseudogap, which is consistent with the recent STS observation. It can be further extended into energy spectra in momentum space measurable by ARPES, where the low-energy dispersion is also shown to agree well with the Quantum Monte Carlo numerical result for a single hole. It implies that the dominant pairing force arises from the ``twisted'' holes showing up in the high-energy branch. The effect of the next nearest neighbor hopping integral $t'$ is also examined, which shows interesting distinction between $t'/t > 0$ and $t'/t \leq 0$ with a dramatic shift of the low-lying excitation from the nodal region to the antinodal region, but with the high-energy branch remaining insensitive to $t'$. Finally, a possible ``orthogonality catastrophe'' effect, namely, a ``dark matter'' component in the strongly correlated wavefunction that cannot be directly detected by the single-electron spectroscopy, is briefly discussed.

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arXiv2023

Sequential Quantum Circuits as Maps between Gapped Phases

Finite-depth quantum circuits preserve the long-range entanglement structure in quantum states and map between states within a gapped phase. To map between states of different gapped phases, we can use Sequential Quantum Circuits which apply unitary transformations to local patches, strips, or other sub-regions of a system in a sequential way. The sequential structure of the circuit on the one hand preserves entanglement area law and hence the gapped-ness of the quantum states. On the other hand, the circuit has generically a linear depth, hence it is capable of changing the long-range correlation and entanglement of quantum states and the phase they belong to. In this paper, we discuss systematically the definition, basic properties, and prototypical examples of sequential quantum circuits that map product states to GHZ states, symmetry-protected topological states, intrinsic topological states, and fracton states. We discuss the physical interpretation of the power of the circuits through connection to condensation, Kramers-Wannier duality, and the notion of foliation for fracton phases.

cond-mat.str-el↗

arXiv2023

Continuous transition from a Landau quasiparticle to a neutral spinon

We examine a wavefunction ansatz in which a doped hole can experience a quantum transition from a charge $+e$ Landau quasiparticle to a neutral spinon as a function of the underlying spin-spin correlation. As shown variationally, such a wavefunction accurately captures all the essential features revealed by exact diagonalization and density matrix renormalization group simulations in a two-leg $t$-$J$ ladder. Hence its analytic form can provide an explicit understanding of the mechanism for the unconventional ground state. The transition in the phase diagram is accompanied by a change of the hole composite from a tight charge-spin binding to a loosely-bound hole-spin pair. In the latter, the hole carries a \emph{finite} spin current but with vanishing charge current in the degenerate ground states. We show that the charge of the hole composite here is dynamically diminished due to an internal relative hole-spin motion, which is fundamentally distinct from a simple charge-spin separation in a one-dimensional case. We further show that the same effect is also responsible for a strong pairing between two doped holes in such a non-Landau quasiparticle regime.

cond-mat.str-el↗

arXiv2022

Mottness, Phase String, and High-$T_c$ Superconductivity

It is a great discovery in physics of the twentieth century that the elementary particles in Nature are dictated by gauge forces, characterized by a nonintegrable phase factor that an elementary particle of charge $q$ acquires from A to B points: $$P \exp\left(i\frac{q}{\hbar c} \int_A^B A_μdx^μ\right)$$ where $A_μ$ is the gauge potential and $P$ stands for path ordering. In a many-body system of strongly correlated electrons, if the so-called Mott gap is opened up by interaction, the corresponding Hilbert space will be fundamentally changed. A novel nonintegrable phase factor known as phase-string will appear and replace the conventional Fermi statistics to dictate the low-lying physics. Protected by the Mott gap, which is clearly identified in the high-$T_c$ cuprate with a magnitude > 1.5 eV, such a singular phase factor can enforce a fractionalization of the electrons, leading to a dual world of exotic elementary particles with a topological gauge structure. A non-Fermi-liquid "parent" state will emerge, in which the gapless Landau quasiparticle is only partially robust around the so-called Fermi arc regions, while the main dynamics are dominated by two types of gapped spinons. Antiferromagnetism, superconductivity, and a Fermi liquid with full Fermi surface can be regarded as the low-temperature instabilities of this new parent state. Both numerics and experiments provide direct evidence for such an emergent physics of the Mottness, which lies in the core of a high-$T_c$ superconducting mechanism.

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arXiv2022

Two-Hole Ground State: Dichotomy in Pairing Symmetry

A single-hole ground state Ansatz for the two-dimensional t-J model has been recently studied by the variational Monte Carlo (VMC) method. Such a doped hole behaves like a "twisted" non-Landau quasiparticle characterized by an emergent quantum number in agreement with exact numerics. In this work, we further investigate the ground state of two holes by VMC. It is found that the two holes strongly attract each other to form a pairing state with a new quantum number the same as obtained by the numerical exact diagonalization and density matrix renormalization group (DMRG) calculations. A unique feature of this pairing state is a dichotomy in the pairing symmetry, i.e., a d-wave in terms of the electron c operators and an s-wave in terms of the new quasiparticles, as explicitly illustrated in the ground state wave function. A similar VMC study of a two-hole wave function for the t-J two-leg ladder also yields a good agreement with the DMRG result. We demonstrate that the pairing mechanism responsible for the strong binding here is not due to the long-range antiferromagnetic order nor the resonating-valence-bound pairing in the spin background but is the consequence of the quantum phase-strings created by the hopping of holes. The resulting spin-current pattern mediating the pairing force is explicitly illustrated in the VMC calculation. Physical implications to superconductivity at finite doping are also discussed.

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