Session D37: Discovery of unexpected magnetism and superconductivity in natural bilayer graphene

Observation of magnetism and superconductivity in crystalline graphene multilayers

Bernal bilayer graphene and rhombohedral trilayer graphene are two crystalline allotropes of layered carbon materials. They inherited the simple energy band dispersion and gate tunability of monolayer graphene. In addition, the existence of Van Hove singularities and flat band edges opens up the opportunity for them to host correlation-driven electronic phases. In this talk, I will report our observation of magnetism and superconductivity in these two materials. We found that both of them host gate-tuned spin- and valley-ordered magnetic phases. The interplay of Coulomb interaction, inter-valley scattering, and subtle features of the single-particle energy bands makes the phase diagram rich and complex. More surprisingly, several superconducting phases were observed near some of the phase boundaries. They show distinct responses to external perturbations such as magnetic field and change of temperature, indicating different configurations of the Cooper pairs. Apart from gate-tuned phase transitions, we found that the proximity-induced spin-orbit coupling can alter the phase diagram and stabilize superconducting phases with significantly higher critical temperature. Our observation of the novel phases together with the homogeneity of the crystalline materials make the Bernal bilayer graphene and rhombohedral trilayer graphene ideal prototype materials to study the manybody physics and mechanisms of superconductivity.   

Nontrivial quantum phases in natural bilayer graphene accessed by control of bandstructure and screening

The exchange interaction can lead to correlated states in low dimensional systems such as the graphene family. Regions of large density of states are especially prone to correltaion effects, an example that will be discussed is the recently identified exchange driven quantum anomalous Hall (QAH) nu=2 state that exhibits quantized charge Hall conductance close to zero magnetic field as well as spin, valley and spin-valley anomalous quantum Hall effects and out-of-plane ferroelectricity in suspended bilayer graphene [1]. In the case that bilayers are encapsulated in h-BN, a large displacement field can be applied allowing the opening of a gap in the density of states with a concomitant van-Hove-singularity close to the band edges. We will discuss our recent measurements [2] in such device structures that indicate that close to the band edges novel states appear that are distinct from Stoner [3,4] and other single particle physics. For example, one identified state is consistent with a Chern insulating state at finite density in the valence band.

 

[1] F. R. Geisenhof, F. Winterer, A. M. Seiler, J. Lenz, T. Xu, F. Zhang and R. T. Weitz, "Quantum anomalous Hall octet driven by orbital magnetism in bilayer graphene", Nature 598, 53 (2021)

[2] A. M. Seiler, F. R. Geisenhof, F. Winterer, K. Watanabe, T. Taniguchi, T. Xu, F. Zhang and R. T. Weitz, "Quantum cascade of new correlated phases in trigonally warped bilayer graphene", Nature 608, (2022) 298

[3] H. Zhou, Y. Saito, L. Cohen, W. Huynh, C. L. Patterson, F. Yang, T. Taniguchi, K. Watanabe, A. F. Young, “Isospin magnetism and spin-triplet superconductivity in Bernal bilayer graphene”, Science 375 (2022), 774

[4] S. C. de la Barrera, S. Aronson, Z. Zheng, K. Watanabe, T. Taniguchi, Q. Ma, P. Jarillo-Herrero, R. Ashoori, “Cascade of isospin phase transitions in Bernal bilayer graphene at zero magnetic field” Nature Physics 18, (2022) 771   

Electric-field driven isospin order in Bernal bilayer graphene

The trigonally warped bands of Bernal-stacked bilayer graphene contain saddle points that give rise to divergences in the density of states. The magnitude of these divergences is tunable by an external electric field, providing a direct tuning knob of the density of states in a narrow window of energies. Here, we show that ultraclean samples of bilayer graphene display a cascade of electric-field driven broken-symmetry states with spontaneous spin and valley isospin ordering at zero magnetic field. We tune the carrier density and electric displacement field independently to explore the phase space of isospin order. Itinerant ferromagnetic states emerge near the conduction and valence band edges with complete spin and valley polarization. At larger hole densities, two-fold degenerate quantum oscillations manifest in an additional broken symmetry state that is enhanced by the application of an in-plane magnetic field. Both types of symmetry-broken states display enhanced layer polarization, suggesting a coupling to the layer degree of freedom. These states occur in the absence of a moiré superlattice and are intrinsic to natural graphene bilayers. These results demonstrate that bilayer graphene presents a related but distinct approach to produce interacting behavior from flat electronic dispersion, complementary to engineered moiré structures.   

Spin and valley T_1 times in graphene quantum dots

We measure spin T_1 times in graphene quantum dots containing one carrier of 10-50 ms using single shot read-out. Two and three electron states in graphene single- and double dots are investigated in magnetic fields parallel and perpendicular to the graphene plane. This way different configurations are found to display spin blockade, valley blockade or both. In the two carrier configuration we find valley T_1 times exceeding 500 ms.

This work was done in collaboration with C. Tong, R. Garreis, L. Gächter, W. Huang,  J. Gerber, A. Kurzmann, and T. Ihn.   

Imaging de Haas–van Alphen quantum oscillations in moiré graphene

Quantum oscillations originating from the quantization of the electron cyclotron orbits in presence of magnetic fields provide fundamental information about the band structure and interactions in novel materials. While the Shubnikov-de Haas resistivity oscillations are widely used for characterizing 2D electron systems including graphene, the thermodynamic magnetization oscillations due to the de Haas-van Alphen effect have evaded so far direct experimental observation in graphene structures. Here we report on scanning SQUID-on-tip imaging of the de Haas-van Alphen quantum oscillations in narrow electronic bands formed in AB-bilayer graphene/hBN superlattices. The local nanoscale measurements reveal very large magnetization oscillations vs. carrier density with amplitudes in excess of 500 Bohr magneton per electron in weak magnetic fields. The oscillations are position dependent and are highly sensitive to the superlattice filling fraction, appearing at elevated carrier densities where moiré bands overlap. We ascribe these observations to the formation of multiple Fermi surfaces, with the large, low-frequency oscillations originating from carriers occupying small Fermi pockets. The findings offer a unique tool for nanoscale mapping of the local band structure in a wide range of van der Waals structures with complex Fermi surfaces and strong electron interactions.