Session M11: 2D Materials: Electronic Phenomena and Criticality

Correlated Phases and Quantum Anomalous Hall Effects in Multilayer Graphene

Recent experiments on multilayer graphene materials have unearthed a zoo of phases, including correlated insulators, ferromagnetism, and quantum anomalous Hall states. In this work, we investigate the microscopic origin of these phases employing Hartree-Fock methods to understand the renormalized electronic bandstructure and the ground states that can be supported in rhombohedral stacked pentalayer graphene. Our findings highlight the fertile ground of two-dimensional materials as a platform for realizing correlated and quantum anomalous Hall phases of matter.   

Tunable van Hove Singularities and Competing Orders in Bernal Bilayer Graphene

Recent experiments on hole-doped Bernal bilayer graphene under strong displacement field discovered that two seemingly separate knobs -- in-plane magnetic field or proximity spin-orbit coupling -- promote superconductivity. Without these knobs, a competing phase featuring high resistivity and non-linear charge transport reminiscent of the charge density wave depinning appears instead. While previous works pointed out various possible mechanisms for the superconductivity, the key question of how two separate knobs are promoting superconductivity as well as the nature of the competing phase have not been clear. Here, we study instabilities arising from repulsive interactions near van Hove singularities in Bernal bilayer graphene through parquet renormalization group. We note that both the in-plane field and the proximity spin-orbit coupling have the effect of lifting the spin degeneracy. This observation opens an angle to study the shared aspect of two knobs. When there is a spin degeneracy, we find that the intra-valley charge density wave wins over superconductivity. Upon lifting the spin degeneracy, superconductivity becomes the primary instability, followed by inter-valley charge density waves. Hence, we propose that the competing intra-valley charge density wave phase suppresses superconductivity in spin-degenerate Bernal bilayer graphene. Our analysis also suggests the spin-degenerate Bernal bilayer graphene as a possible platform for a PDW state and a charge-4e superconductivity.   

Towards quantum pumps: Charge pumping in boron-nitride-encapsulated low-dimensional conductors using surface acoustic waves

Interfacing surface acoustic waves with nanomaterials is a promising approach for sensor technology [1] and for probing previously inaccessible phenomena in low-dimensional electronic systems [2,3]. However, achieving the full potential of these systems requires pristine materials with low electronic disorder. 

We present results on acoustoelectric charge pumping in boron nitride-encapsulated graphene on lithium niobate. Our device uses a solid-state graphene gate, which avoids the drawbacks of ionic liquid gates, such as acoustic damping, while still allowing for strong modulation of charge carrier concentration. We demonstrate that our device produces higher-magnitude and lower-noise acoustoelectric current than previously reported devices, and we discuss plans for using this platform to achieve topologically protected charge pumping.

[1] S. Yang, C. Jiang, and S-huai Wei, “Gas Sensing in 2D Materials,” Appl. Phys. Rev. 4, 021304 (2017)

[2] Y. Fang, Y. Xu, K. Kang, B. Davaji, et al., “Quantum Oscillations in Graphene Using Surface Acoustic Wave Resonators,” Phys. Rev. Lett. 130, 246201 (2023).

[3] D. S. Novikov, “Devil’s Staircase of Incompressible Electron States in a Nanotube,” Phys. Rev. Lett. 95, 1 (2005).   

High frequency conductance of a quantum Hall insulator

Insulators do not carry DC current but can conduct electricity at high frequencies. In a quantum Hall insulator, this capacitive current is enabled by the time-dependent polarization of Landau orbits and is expected to carry information about the quantum geometry of the underlying Hilbert space. To measure this conductance, we embed a graphene quantum Hall insulator in a resonator where a dispersive shift of the resonator frequency sensitively tracks the capacitive response of these insulating states. Finite temperature and disorder broadening creates a conventional quantum capacitive response that competes with the quantum geometric contribution. We discuss ways to disentangle these contributions and show preliminary experiments.   

Stretch-Induced Tunability of Electrical Transport in 3D Graphene Foam

3D graphene foam with fast electron transport and mechanical flexibility finds wide applications in stretchable electronics. This study investigates the electrical transport properties of graphene foam by analyzing its temperature-dependent electrical resistance (R(T)) under various pre-stretching levels. Experimental results show that R(T) changes as pre-stretching level increases, some even exhibiting a notable transition from insulating to metallic behavior. This indicates a stretch-induced modulation of the electrical transport properties of graphene foam. Considering the interconnected polycrystalline graphene domains in graphene foam, we propose a conduction network model that effectively explains R(T) of graphene foam by incorporating thermally activated conduction and phonon-limited conduction within each graphene domain, along with the fluctuation-induced tunneling conduction between neighbouring domains. By fitting experimentally obtained R(T) to the model, we probed the stretch-induced modulation of the electrical transport of graphene foam by discerning the alterations of the conduction mechanisms and the conduction networks. The deduced conduction network changes aligns with the optically observed breakage or reconnection of graphene foam network branches under increasing pre-stretching levels. These findings provide valuable insights into the modulation of electrical transport properties in graphene foam-based stretchable electronics, offering opportunities for further refinement.   

Oral: Acoustoelectric Control of Charge Carriers in Graphene

Surface acoustic wave (SAW) potentials have previously been shown to modulate the distribution of charge carriers in two-dimensional electron systems via the acoustoelectric effect. Here, we observe a strong interaction between SAWs in LiNbO3 and carriers in hexagonal boron nitride encapsulated graphene devices. By constructively interfering counterpropagating SAWs, we demonstrate the ability to engineer dynamic artificial periodic potentials and charge distributions in van der Waals heterostructures without the use of static gates or moiré patterns. Our results suggest that SAW potentials are a prime candidate for use as a platform for quantum simulation in two-dimensional materials.    

Exploring Electron Tunneling in Kek-Y Patterned Graphene Using the WKB Theory

We have developed a comprehensive semi-classical WKB theory for graphene with a recently-discovered Y-shaped Kekulé (Kek-Y) distortion pattern and a unique folding of the K and K' valleys. This lattice distortion results in a highly specific linear energy dispersion characterized by two non-equivalent Dirac cones, each described by different Fermi velocities, v_F(1 ± Δ₀). We derive the semi-classical action, electron momentum, and wave functions to analyze electron tunneling dynamics and resonant scattering through non-square potential barriers. Additionally, we have formulated and solved a set of transport equations that connect successive pairs of our model wavefunctions using a perturbative approach, assuming a small, strain-induced coupling parameter, Δ₀. These equations enable us to determine electron transmission amplitudes into regions that are classically inaccessible to electrons. We have conducted a detailed exploration of the dependence of electron transmission amplitudes on the potential and the band parameters of Kek-Y-patterned graphene. Our findings have practical applications in the development and operation of the next generation of opto-electronic and valleytronics devices, as the derived electron transmission amplitudes directly impact quantum transport in this model material.   

Composite (multi)ferroic order

The formalism of composite and intertwined orders has been remarkably successful in discussing the complex phase diagrams of strongly correlated materials and high-T_c superconductors. A generalization of the formalism to other ordered states of matter has remained largely unexplored. Here, we show that conventional ferromagnetic and ferroelectric materials can exhibit composite orders, emerging above the critical temperature for the transition into the ferroic phase. The existence of this transition depends on the anisotropy of the ferroic phase. We show that the quadrupole magnetic and electric orders couple to the shear elastic constant, which explains experimental findings for elastic precursors of ferromagnetic and ferroelectric phase transitions, showing a softening of shear modes in various materials. Furthermore, we extend our formalism to strongly coupled multiferroic materials, which can form composites of magnetic and ferroelectric orders. This gives rise to novel kinds of "hidden orders", not interacting with electric, magnetic, or strain as well as insights into the formation of toroidal moments in multiferroics. As the multipolar and composite orders discussed here are emerging above the ferroic transition temperatures, they might be relevant for explaining precursor phenomena in incipient (multi)ferroic materials.

[1] R. M. Geilhufe, Hidden composite (multi)ferroic order, arXiv:2309.03312 (2023)

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

Double layer graphene, which consists of two graphene layers separated by a few-layer-thick insulating barrier, provides a highly tunable platform to investigate the rich phase diagram of strongly correlated bosons in the form of interlayer exciton. For instance, it has long been established that a superfluid phase of interlayer excitons emerges In the quantum Hall regime, when each graphene layer is tuned to half filling of the lowest landau level. In this work, we report an excitonic insulating phase stabilized in the dilute limit of the exciton phase diagram, achieved in the regime of large layer imbalance. Two observations point towards the exotic nature of this insulator. First, with increasing temperature, the insulator undergoes a melting transition into a high temperature phase that behaves like a superfluid. Secondly, we show that the phase boundary between the insulator and superfluid to be hysteretic, the location of which depends on the direction of temperature sweep. The temperature hysteresis offers a strong indication for a first order transition, which arises from a non-zero latent heat associated with solid melting. Together, our observations point towards a superfluid-to-solid phase transition in interlayer excitons.