arXiv subjects

Canxun Zhang

Publications and source records attributed to Canxun Zhang.

15 recordsLinked to original sources

arXiv2026

Visualizing orbital magnetism in electron doped rhombohedral multilayer graphene

Electron doped rhombohedral multilayer graphene at high displacement field features an exceptionally flat band minimum with near-ideal quantum geometry. Experiments in this regime observe the formation of a 'quarter metal,' in which the electron liquid condenses into a single spin- and valley flavor. Remarkably, recent experiments have found a zero resistance state in the same region of the density- and displacement-field-tuned parameter space, attributed to the formation of a chiral superconductor from an orbitally ferromagnetic normal state. Here, we use nanoSQUID-on-tip magnetometry to map the orbital magnetization of electron-doped rhombohedral graphene devices ranging in thickness between 3 and 15 layers. Magnetization within the quarter metal phases peaks at finite density, consistent with concentration of the Berry curvature in a finite-momentum 'ring of fire'. Correlating transport and local magnetometry data in a superconducting tetralayer sample reveals a finite orbital ferromagnetic moment, providing direct evidence of valley polarization in the superconducting ground state. We further show that widely observed stochastic switching of the resistivity in both metallic and superconducting regimes arises from a density-tuned sign change in the valley-resolved total magnetic moment. This leads to the formation of metastable magnetic domains under typical gate control sequences and can also be harnessed for electric-field controlled switching of the magnetization across the entire device. Finally, high resolution measurements of the magnetization across a superconducting transition allow us to put an upper bound on the 'condensation magnetization' of 0.1 Bohr magneton per carrier, placing a strong quantitative restriction on theoretical models for ferromagnetic superconductivity.

cond-mat.mes-hall↗

arXiv2026

Imaging flat band electron hydrodynamics in biased bilayer graphene

Hydrodynamic electron transport arises when carrier kinetics are dominated by interelectron collisions rather than the relaxation of momentum out of the electron system. In recent years, signatures of electron hydrodynamics have been reported in graphene devices owing to the low disorder and weak electron-phonon coupling. However, these experiments have been performed in regimes where the carrier mass is light, and the electron-electron collision length--though smaller than corresponding lengths for phonon or impurity scattering--remains large in absolute terms, typically several hundred nanometers. This restricts hydrodynamic transport phenomena to large length scales, limiting miniaturization of devices based on hydrodynamic flow. The advent of dual-gated rhombohedral graphene multilayers introduces a new route toward enhanced hydrodynamic behavior via their large--and tunable--effective mass. Here, we employ a scanning superconducting magnetic sensor to image local current flow in dual-gated bilayer graphene. Exploiting a sample geometry sensitive to both laminar and vortical flow, we identify three distinct transport regimes--ballistic, hydrodynamic, and diffusive--across the full phase space spanned by carrier density and displacement field. The strongest hydrodynamic transport is observed in the flat band regime, where fitting our results to a unified Boltzmann transport model reveals the electron-electron scattering length to be comparable to the Fermi wavelength of ~50 nm. High-current measurements, meanwhile, reveal striking nonlinearities in the flow pattern. Our results pave the way for miniaturized electronic devices based on linear and nonlinear electron hydrodynamics.

cond-mat.mes-hall↗

arXiv2026

Van Hove Singularity-Driven Topological Magnetism in Twisted MoTe2

Van Hove singularities (vHSs) strongly amplify electron interactions and can stabilize correlated phases in topological bands. Here we report signatures of topological magnetism in large-angle twisted bilayer MoTe2 driven by the interplay of vHSs, strong correlations, and valley topology. In a 4.8 degree device, electrostatic tuning to a vHS produces a spontaneous anomalous Hall hot spot near nu = -1. Combined transport and reflective magnetic circular dichroism measurements indicate that this regime is not governed by magnetization alone, but instead emerges from a correlated intervalley-coherent antiferromagnetic state that evolves with doping into a canted phase. With increasing magnetic field, the Hall response develops an additional finite-field component consistent with a topological Hall effect from a noncoplanar spin texture, before transitioning into a C = -1 Chern insulator. Our results establish tunable vHSs in moire topological bands as a route to chiral magnetism and engineering topological phase transitions.

cond-mat.mes-hall↗

arXiv2026

Cryogenic shock exfoliation for ultrahigh mobility rhombohedral graphite nanoelectronics

Rhombohedral multilayer graphene (RMG) offers a highly tunable platform for correlated electron physics, featuring field-effect control of magnetic, superconducting, and topological phases[1-24]. The promise of these materials has been held back by the limited abundance of rhombohedral stacking in natural graphite, which constrains both sample yield and useful area. Here we introduce 'cryogenic shock exfoliation' to produce large area rhombohedral graphene flakes which, combined with a low-pressure van der Waals assembly technique that preserves stacking order, enable highly uniform devices exceeding 1300 $\mu m^2$ with fabrication yields of 90%. Using scanning nanoSQUID-on-tip imaging, we demonstrate uniform spin magnetism over the full central 10 times 10 $\mu m^2$ area of our devices. Transverse magnetic focusing reveals a disorder mean free path exceeding 200 $\mu m$ at low temperatures. Within the flat surface bands of RMG[20], we observe a size-driven crossover from Poiseuille to porous electron flow in the intermediate-temperature regime of strong electron-electron hydrodynamics[16, 25], providing a further signature of ultrahigh device quality. Our approach overcomes a key materials bottleneck in the fabrication of mesoscopic rhombohedral graphene devices, paving the way for incorporating strongly correlated phases into two-dimensional nanoelectronics.

cond-mat.mes-hall↗

arXiv2026

Imaging the Meissner effect and local superfluid stiffness in a graphene superconductor

We report the observation of the Meissner effect in a rhombohedral graphene superconductor, realized via direct imaging of the static fringe magnetic field. In our few-micron sample, the onset of superconductivity manifests as a diamagnetic response that screens only $\sim 100$ ppm of the applied magnetic field. Tracking the evolution of the resulting nanotesla-scale fields in real space allows us to observe the entry of superconducting vortices and map the local superfluid stiffness, $ρ_s$. Correlating fringe field signals from both Meissner screening and magnetically ordered states, we show that superconductivity onsets in the midst of a continuous quantum phase transition to a canted spin ferromagnet. Within the superconducting state, we find the temperature dependence of $ρ_s$ to be incompatible with isotropic Bardeen-Cooper-Schrieffer theory and the zero-temperature stiffness $ρ_s^0$ to be linearly proportional to $T_c$, constraining future theoretical models of superconductivity in this system.

cond-mat.supr-con↗

arXiv2025

Universal Magnetic Phases in Twisted Bilayer MoTe$_2$

Twisted bilayer MoTe$_2$ (tMoTe$_2$) has emerged as a robust platform for exploring correlated topological phases, notably supporting fractional Chern insulator (FCI) states at zero magnetic field across a wide range of twist angles. The evolution of magnetism and topology with twist angle remains an open question. Here, we systematically map the magnetic phase diagram of tMoTe$_2$ using local optical spectroscopy and scanning nanoSQUID-on-tip (nSOT) magnetometry. We identify spontaneous ferromagnetism at moiré filling factors $ν= -1$ and $-3$ over a twist angle range from 2.1$^\circ$ to 3.7$^\circ$, revealing a universal, twist-angle-insensitive ferromagnetic phase. At 2.1$^\circ$, we further observe robust ferromagnetism at $ν= -5$, absent in the devices with larger twist angle -- a signature of the flattening of higher bands in this twist angle range. Temperature-dependent measurements reveal a contrasting twist-angle dependence of the Curie temperatures between $ν= -1$ and $ν= -3$, indicating distinct interplay between exchange interaction and bandwidth for the two Chern bands. Despite spontaneous time-reversal symmetry breaking, we find no evidence of a topological gap at $ν= -3$; however, fragile correlated topological phases could be obscured by the device disorder evident in our spatially resolved measurements. Our results establish a global framework for understanding and controlling magnetic order in tMoTe$_2$ and highlight its potential for accessing correlated topological phases in higher energy Chern band.

cond-mat.mes-hall↗

arXiv2025

Evolution of charge correlations in the hole-doped kagome superconductor CsV$_{3-x}$Ti$_x$Sb$_5$

The interplay between superconductivity and charge correlations in the kagome metal CsV$_3$Sb$_5$ can be tuned by external perturbations such as doping or pressure. Here we present a study of charge correlations and superconductivity upon hole doping via Ti substitution on the V kagome sites in CsV$_{3-x}$Ti$_x$Sb$_5$ via synchrotron x-ray diffraction and scanning SQUID measurements. While the superconducting phase, as viewed via the vortex structure, remains conventional and unchanged across the phase diagram, the nature of charge correlations evolves as a function of hole-doping from the first superconducting dome into the second superconducting dome. For Ti doping in the first superconducting dome, competing $2\times 2 \times 2$ and $2\times 2 \times 4$ supercells form within the charge density wave state and are suppressed rapidly with carrier substitution. In the second superconducting dome, no charge correlations are detected. Comparing these results to those observed for CsV$_3$Sb$_{5-x}$Sn$_x$ suggests important differences between hole doping via chemical substitution on the V and Sb sites, particularly in the disorder potential associated with each dopant.

cond-mat.supr-con↗

arXiv2024

Direct magnetic imaging of fractional Chern insulators in twisted MoTe$_2$ with a superconducting sensor

In the absence of time reversal symmetry, orbital magnetization provides a sensitive probe of topology and interactions, with particularly rich phenomenology in Chern insulators where topological edge states carry large equilibrium currents. Here, we use a nanoscale superconducting sensor to map the magnetic fringe fields in twisted bilayers of MoTe$_2$, where transport and optical sensing experiments have revealed the formation of fractional Chern insulator (FCI) states at zero magnetic field. At a temperature of 1.6K, we observe oscillations in the local magnetic field associated with fillings $ν=-1,-2/3,-3/5,-4/7$ and $-5/9$ of the first moiré hole band, consistent with the formation of FCIs at these fillings. By quantitatively reconstructing the magnetization, we determine the local thermodynamic gaps of the most robust FCI state at $ν=-2/3$, finding $^{-2/3}Δ$ as large as 7 meV. Spatial mapping of the charge density- and displacement field-tuned magnetic phase diagram further allows us to characterize sample disorder, which we find to be dominated by both inhomogeneity in the effective unit cell area as well as inhomogeneity in the band edge offset and bound dipole moment. Our results highlight both the challenges posed by structural disorder in the study of twisted homobilayer moiré systems and the opportunities afforded by the remarkably robust nature of the underlying correlated topological states.

cond-mat.mes-hall↗

arXiv2023

Visualizing and manipulating chiral interface states in a moiré quantum anomalous Hall insulator

Moiré systems made from stacked two-dimensional materials host novel correlated and topological states that can be electrically controlled via applied gate voltages. We have used this technique to manipulate Chern domains in an interaction-driven quantum anomalous Hall insulator made from twisted monolayer-bilayer graphene (tMBLG). This has allowed the wavefunction of chiral interface states to be directly imaged using a scanning tunneling microscope (STM). To accomplish this tMBLG carrier concentration was tuned to stabilize neighboring domains of opposite Chern number, thus providing topological interfaces completely devoid of any structural boundaries. STM tip pulse-induced quantum dots were utilized to induce new Chern domains and thereby create new chiral interface states with tunable chirality at predetermined locations. Theoretical analysis confirms the chiral nature of observed interface states and enables the determination of the characteristic length scale of valley polarization reversal across neighboring tMBLG Chern domains. tMBLG is shown to be a useful platform for imaging the exotic topological properties of correlated moiré systems.

cond-mat.mes-hall↗

arXiv2023

Local spectroscopy of a gate-switchable moiré quantum anomalous Hall insulator

In recent years, correlated insulating states, unconventional superconductivity, and topologically non-trivial phases have all been observed in several moiré heterostructures. However, understanding of the physical mechanisms behind these phenomena is hampered by the lack of local electronic structure data. Here, we use scanning tunnelling microscopy and spectroscopy to demonstrate how the interplay between correlation, topology, and local atomic structure determines the behaviour of electron-doped twisted monolayer-bilayer graphene. Through gate- and magnetic field-dependent measurements, we observe local spectroscopic signatures indicating a quantum anomalous Hall insulating state with a total Chern number of $\pm 2$ at a doping level of three electrons per moiré unit cell. We show that the sign of the Chern number and associated magnetism can be electrostatically switched only over a limited range of twist angle and sample hetero-strain values. This results from a competition between the orbital magnetization of filled bulk bands and chiral edge states, which is sensitive to strain-induced distortions in the moiré superlattice.

cond-mat.mes-hall↗

arXiv2022

A novel $\sqrt{19}\times\sqrt{19}$ superstructure in epitaxially grown 1T-TaTe$_2$

The spontaneous formation of electronic orders is a crucial element for understanding complex quantum states and engineering heterostructures in two-dimensional materials. We report a novel $\sqrt{19}\times\sqrt{19}$ charge order in few-layer thick 1T-TaTe$_2$ transition metal dichalcogenide films grown by molecular beam epitaxy, which has not been realized. Our photoemission and scanning probe measurements demonstrate that monolayer 1T-TaTe$_2$ exhibits a variety of metastable charge density wave orders, including the $\sqrt{19}\times\sqrt{19}$ superstructure, which can be selectively stabilized by controlling the post-growth annealing temperature. Moreover, we find that only the $\sqrt{19}\times\sqrt{19}$ order persists in 1T-TaTe$_2$ films thicker than a monolayer, up to 8 layers. Our findings identify the previously unrealized novel electronic order in a much-studied transition metal dichalcogenide and provide a viable route to control it within the epitaxial growth process.

cond-mat.mtrl-sci↗

arXiv2022

Large-gap insulating dimer ground state in monolayer IrTe2

Monolayers of two-dimensional van der Waals materials exhibit novel electronic phases distinct from their bulk due to the symmetry breaking and reduced screening in the absence of the interlayer coupling. In this work, we combine angle-resolved photoemission spectroscopy and scanning tunneling microscopy/spectroscopy to demonstrate the emergence of a unique insulating 2 x 1 dimer ground state in monolayer 1T-IrTe2 that has a large band gap in contrast to the metallic bilayer-to-bulk forms of this material. First-principles calculations reveal that phonon and charge instabilities as well as local bond formation collectively enhance and stabilize a charge-ordered ground state. Our findings provide important insights into the subtle balance of interactions having similar energy scales that occurs in the absence of strong interlayer coupling, which offers new opportunities to engineer the properties of 2D monolayers.

cond-mat.mtrl-sci↗

arXiv2022

Evidence for a spinon Kondo effect in cobalt atoms on single-layer 1T-TaSe$_2$

Quantum spin liquids (QSLs) are highly entangled, disordered magnetic states that arise in frustrated Mott insulators and host exotic fractional excitations such as spinons and chargons. Despite being charge insulators some QSLs are predicted to exhibit gapless itinerant spinons that yield metallic behavior in the spin channel. We have deposited isolated magnetic atoms onto single-layer (SL) 1T-TaSe$_2$, a gapless QSL candidate, to experimentally probe how itinerant spinons couple to impurity spin centers. Using scanning tunneling spectroscopy we observe the emergence of new, impurity-induced resonance peaks at the 1T-TaSe$_2$ Hubbard band edges when cobalt adatoms are positioned to have maximal spatial overlap with the Hubbard band charge distribution. These resonance peaks disappear when the spatial overlap is reduced or when the magnetic impurities are replaced with non-magnetic impurities. Theoretical simulations using a modified Anderson impurity model integrated with a gapless quantum spin liquid show that these resonance peaks are consistent with a Kondo resonance induced by spinons combined with spinon-chargon binding effects that arise due to QSL gauge-field fluctuations.

cond-mat.str-el↗

arXiv2021

Imaging gate-tunable Tomonaga-Luttinger liquids in 1H-MoSe$_2$ mirror twin boundaries

One-dimensional electron systems (1DESs) exhibit properties that are fundamentally different from higher-dimensional systems. For example, electron-electron interactions in 1DESs have been predicted to induce Tomonaga-Luttinger liquid behavior. Naturally-occurring grain boundaries in single-layer semiconducting transition metal dichalcogenides provide 1D conducting channels that have been proposed to host Tomonaga-Luttinger liquids, but charge density wave physics has also been suggested to explain their behavior. Clear identification of the electronic ground state of this system has been hampered by an inability to electrostatically gate such boundaries and thereby tune their charge carrier concentration. Here we present a scanning tunneling microscopy/spectroscopy study of gate-tunable mirror twin boundaries (MTBs) in single-layer 1H-MoSe$_2$ devices. Gating here enables STM spectroscopy to be performed for different MTB electron densities, thus allowing precise characterization of electron-electron interaction effects. Visualization of MTB electronic structure under these conditions allows unambiguous identification of collective density wave excitations having two distinct velocities, in quantitative agreement with the spin-charge separation predicted by finite-length Tomonaga-Luttinger-liquid theory.

cond-mat.str-el↗

arXiv2021

Visualizing delocalized correlated electronic states in twisted double bilayer graphene

The discovery of interaction-driven insulating and superconducting phases in moiré van der Waals heterostructures has sparked considerable interest in understanding the novel correlated physics of these systems. While a significant number of studies have focused on twisted bilayer graphene, correlated insulating states and a superconductivity-like transition up to 12 K have been reported in recent transport measurements of twisted double bilayer graphene. Here we present a scanning tunneling microscopy and spectroscopy study of gate-tunable twisted double bilayer graphene devices. We observe splitting of the van Hove singularity peak by ~20 meV at half-filling of the conduction flat band, with a corresponding reduction of the local density of states at the Fermi level. By mapping the tunneling differential conductance we show that this correlated system exhibits energetically split states that are spatially delocalized throughout the different regions in the moiré unit cell, inconsistent with order originating solely from onsite Coulomb repulsion within strongly-localized orbitals. We have performed self-consistent Hartree-Fock calculations that suggest exchange-driven spontaneous symmetry breaking in the degenerate conduction flat band is the origin of the observed correlated state. Our results provide new insight into the nature of electron-electron interactions in twisted double bilayer graphene and related moiré systems.

cond-mat.str-el↗