arXiv subjects

Naiyuan J. Zhang

Publications and source records attributed to Naiyuan J. Zhang.

10 recordsLinked to original sources

arXiv2026

Coexisting Charge Density Wave and Superconducting Order in Quantizing Magnetic Fields

Charge density wave (CDW) and superconductivity are both common in strongly interacting electron systems. While CDW order is ubiquitous in both quantum Hall systems and unconventional superconductors, superconductivity is generally suppressed by the strong magnetic fields required for Landau quantization. Here we investigate the intertwined CDW and superconducting phases of rhombohedral hexalayer graphene (R6G) in a large displacement field, which generates tunable flat band edges, and a strong magnetic field, which generates a manifold of nearly degenerate Landau levels. We find a series of integer quantum Hall effects with Hall conductance quantum numbers that deviate from nearby integer filling factors, an observation that can be explained only by CDW order that mixes many Landau levels. We also find a nearby superconducting phase stabilized by perpendicular magnetic fields and persists deep within the quantum Hall regime. This intertwinement provides new insight into superconductivity in R6G at zero magnetic field.

cond-mat.mes-hall↗

arXiv2026

Stripe Order in the Metallic and Superconducting Phases of Rhombohedral Hexalayer Graphene

In strongly correlated electronic systems, Coulomb interactions frequently give rise to emergent electronic orders that spontaneously break rotational symmetry. Understanding how such symmetry breaking intertwines with other collective phenomena-such as unconventional superconductivity-and how it shapes experimental observables, particularly transport responses, remains a central challenge in modern condensed-matter physics. Here we report experimental signatures of charge stripe order, with a transport anisotropy rivaling that of quantum Hall stripe phases, coexisting with superconductivity and magnetism in rhombohedral hexalayer graphene. Strikingly, the low-temperature superconducting state not only inherits strong anisotropy but also exhibits a wide range of hysteretic transitions arising from the tunability of the underlying stripe order. Together, these findings reveal a previously unrecognized coexistence between superconductivity and charge stripe, shedding new light on the role of rotational symmetry breaking in shaping unconventional superconductivity in rhombohedral graphene.

cond-mat.mes-hall↗

arXiv2026

Continuously tunable dipolar exciton geometry for controlling bosonic quantum phase transitions

The geometry and binding energy of excitons, set by electron-hole wavefunction distributions, are fundamental factors that underpin their many-body interactions and determine optoelectronic properties of semiconductors. However, in typical solid-state systems, these quantities are fixed by material composition and structure. Here we introduce a polarizable interlayer exciton hosted in a two-dimensional tetralayer heterostructure whose dipole length, in-plane radius, and binding energy can be continuously programmed in situ over a wide range, enabling direct control over the nature of excitonic many-body phase transitions. An out-of-plane electric field redistributes layer-hybridized electron-hole wavefunctions, realizing in situ control of exciton geometry through a strong quadratic Stark response. This tunability further regulates the nature of interaction-driven Mott transition, transforming it from gradual to abrupt. Our results establish exciton geometry as a continuously tunable materials parameter, opening routes to exciton-based quantum phase-transition simulators and guiding the design of emergent optoelectronic functionalities from programmable excitonic materials.

cond-mat.mtrl-sci↗

arXiv2025

Bilayer Excitons in the Laughlin Fractional Quantum Hall State

The Laughlin state embodies a universal class of fractional quantum Hall effects arising in two-dimensional electron systems subjected to strong perpendicular magnetic fields. Conventionally described by a single-component wavefunction, the Laughlin state features fractionally charged quasiparticles arising from correlations within one electron species. Here, we explore a novel physical situation by introducing inter-species Coulomb coupling between two intra-species Laughlin states in a quantum Hall graphene bilayer structure. Although quasiparticle excitations typically exhibit charge gaps of tens of Kelvin, we observe that this energy scale is significantly lowered through interlayer excitonic pairing between quasiparticles and quasiholes. Identified via transport measurements, these excitons belong to an unprecedented category of charge-neutral anyons, opening a new avenue for investigating exotic quantum statistics and phases of matter.

cond-mat.mes-hall↗

arXiv2025

A Hierarchy of Superconductivity and Topological Charge Density Wave States in Rhombohedral Graphene

Superconductivity and the quantum Hall effect are conventionally regarded as mutually exclusive: superconductivity is suppressed by magnetic fields, whereas the quantum Hall effect relies on them. Here we report a striking exception, where an unconventional superconducting phase is stabilized by an out-of-plane magnetic field and coexists with a re-entrant integer quantum Hall (RIQH) effect in moiré-less rhombohedral hexalayer graphene. The re-entrant quantum Hall state, arising from a bubble-like charge density wave (CDW), provides a natural backdrop for the emergence of superconductivity. Angle-resolved transport reveals that the field-stabilized superconducting phase occupies the same density--displacement-field regime as a stripe-ordered phase at zero field, yet only develops once the stripe is replaced by a bubble-like CDW at finite field. These findings demonstrate a decisive role of CDW order in stabilizing superconductivity in rhombohedral graphene, establishing a new paradigm for the interplay between superconductivity and quantum Hall physics.

cond-mat.mes-hall↗

arXiv2025

Evidence for a Superfluid-to-solid Transition of Bilayer Excitons

One of the most spectacular properties associated with Bose-Einstein condensation (BEC) is superfluidity in which the system exhibits zero viscosity and flows without dissipation. The superfluid phase has been observed in wide ranging Bosonic systems spanning naturally occurring quantum fluids, such as liquid helium, to engineered platforms such as bilayer excitons and cold atom systems. Theoretical works have proposed that interactions could drive the BEC ground state into another exotic phase that simultaneously exhibits properties of both a crystalline solid and a superfluid - termed a supersolid. Identifying a material system, however, that hosts the predicted BEC solid phase, driven purely by interactions and without imposing an external lattice potential, has remained elusive. Here we report observation of a superfluid to insulator transition in the layer-imbalanced regime of bilayer magneto-excitons. Mapping the transport behavior of the bilayer condensate as a function of density and temperature, suggests that the insulating phase is an ordered state of dilute excitons, stabilized by dipole interactions. The insulator melts into a recovered superfluid upon increasing the temperature, which could indicate that the low temperature solid is also a quantum coherent phase.

cond-mat.mes-hall↗

arXiv2025

Angular Interplay of Nematicity, Superconductivity, and Strange Metallicity in a Moiré Flat Band

Superconductivity in strongly correlated electron systems frequently exhibits broken rotational symmetry, raising fundamental questions about the underlying order parameter symmetry. In this work, we demonstrate that electronic nematicity--driven by Coulomb-mediated rotational symmetry breaking--serves as a crucial link to understanding the nature of superconductivity. Utilizing a novel framework of angle-resolved measurement, we reveal an interring angular interplay among nematicity, superconductivity, and strange metallicity in magic-angle twisted trilayer graphene. By establishing a direct correlation between the preferred superconducting transport direction and the principal axis of the metallic phase, our findings place strong constrains on the symmetry of the superconducting order parameter. This work introduces a new paradigm for probing the microscopic mechanisms governing superconductivity in strongly interacting two-dimensional systems.

cond-mat.mes-hall↗

arXiv2024

Excitons in the Fractional Quantum Hall Effect

Excitons, Coulomb-driven bound states of electrons and holes, are typically composed of integer charges. However, in bilayer systems influenced by charge fractionalization, a more exotic form of interlayer exciton can emerge, where pairing occurs between constituents that carry fractional charges. Despite numerous theoretical predictions for such fractional excitons, their experimental observation has remained elusive. Here, we report transport signatures of excitonic pairing within fractional quantum Hall effect states. By probing the composition of these excitons and their impact on the underlying wavefunction, we uncover two novel quantum phases of matter. One of these orders can be viewed as the fractional counterpart of the exciton condensate at a total filling of one, while the other involves a more unusual type of exciton that obeys fermionic and anyonic quantum statistics, challenging the standard paradigm of bosonic excitons.

cond-mat.mes-hall↗

arXiv2023

Spontaneous momentum polarization and diodicity in Bernal bilayer graphene

The low-temperature phase diagram of multilayer graphene heterostructures is largely defined by the exchange-driven instability that lifts the four-fold isospin degeneracy. Such instability gives rise to the quarter- and half-metal phases, which are key to our understanding of other emergent phenomena. Recent theoretical works shed light on a new type of Coulomb-driven instability. It is proposed that the exchange interaction between trigonal-warping-induced Fermi pockets could induce charge carriers to condense into one of the Fermi pockets, giving rise to a net polarization in the momentum space. Here, we report the observation of spontaneous momentum polarization in Bernal bilayer graphene using angle-resolved nonlinear transport measurement at the second-harmonic frequency. With excellent angular precision, we show that the polar axis of the momentum polarization is tunable with varying carrier density, electric field, and magnetic field. The dominating influence of the momentum-space instability reveals a natural connection between broken symmetries, and the isospin degeneracy lifting in the half- and quarter-metal phases.

cond-mat.mes-hall↗

arXiv2022

Electronic anisotropy in magic-angle twisted trilayer graphene

Due to its potential connection with nematicity, electronic anisotropy has been the subject of intense research effort on a wide variety of material platforms. The emergence of spatial anisotropy not only offers a characterization of material properties of metallic phases, which cannot be accessed via conventional transport techniques, but it also provides a unique window into the interplay between Coulomb interaction and broken symmetry underlying the electronic order. In this work, we utilize a new scheme of angle-resolved transport measurement (ARTM) to characterize electron anisotropy in magic-angle twisted trilayer graphene. By analyzing the dependence of spatial anisotropy on moiré band filling, temperature and twist angle, we establish the first experimental link between electron anisotropy and the cascade phenomenon, where Coulomb interaction drives a number of isospin transitions near commensurate band fillings. Furthermore, we report the coexistence between electron anisotropy and a novel electronic order that breaks both parity and time reversal symmetry. Combined, the link between electron anisotropy, cascade phenomenon and PT-symmetry breaking sheds new light onto the nature of electronic order in magic-angle graphene moiré systems.

cond-mat.mes-hall↗