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Authors
Ivar Martin
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Recently, superconductivity was discovered at very low densities in slightly misaligned graphene multilayers. Surprisingly, despite extremely low electronic density (about $10^{-4}$ electrons per unit cell), these systems realize strong-coupling superconductivity, with the transition temperature being a large fraction of the Fermi energy ($T_csim 0.1 epsilon_F$). Here we propose a qualitative explanation for this remarkable phenomenon, highlighting similarities and qualitative differences with the conventional uniform high-density superconductivity. Most importantly, we find that periodic superimposed potential generically enhances local interactions relative to nonlocal (for instance, Coulomb) interactions. In addition, the density of states is enhanced as well, exponentially in modulation strength for low lying bands in some cases. Combination of these two effects makes moire systems natural intermediate or strong-coupled superconductors, with potential for very high transition temperatures.
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The search for artificial topological superconductivity has been limited by the stringent conditions required for its emergence. As exemplified by the recent discoveries of various correlated electronic states in twisted van der Waals materials, moire patterns can act as a powerful knob to create artificial electronic structures. Here we demonstrate that a moire pattern between a van der Waals superconductor and a monolayer ferromagnet creates a periodic potential modulation that enables the realization of a topological superconducting state that would not be accessible in the absence of the moire. We show that the existence of a magnetic moire pattern gives rise to Yu-Shiba-Rusinov minibands and periodic modulation of the Majorana edge modes that we detect using low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS). Our results put forward moire patterns as a powerful tool to overcome conventional constrains for topological superconductivity in van der Waals heterostructures. In a broader picture, periodic potential modulation provides a general way of controlling topological superconductivity towards the realisation of topological qubits in the future.
We study the quantum many-body instabilities of interacting electrons with SU(2)$times$SU(2) symmetry in spin and orbital degrees of freedom on the triangular lattice near van-Hove filling. Our work is motivated by effective models for the flat bands in hexagonal moire heterostructures like twisted bilayer boron nitride and trilayer graphene-boron nitride systems. We consider an extended Hubbard model including onsite Hubbard and Hunds couplings, as well as nearest-neighbor exchange interactions and analyze the different ordering tendencies with the help of an unbiased functional renormalization group approach. We find three classes of instabilities controlled by the filling and bare interactions. For a nested Fermi surface at van-Hove filling, Hund-like couplings induce a weak instability towards spin or orbital density wave phases. An SU(4) exchange interaction moves the system towards a Chern insulator, which is robust with respect to perturbations from Hund-like interactions or deviations from perfect nesting. Further, in an extended range of fillings and interactions, we find topological $dpm id$ and (spin-singlet)-(orbital-singlet) $f$-wave superconductivity.
It was recently suggested that the topology of magic-angle twisted bilayer graphenes (MATBG) flat bands could provide a novel mechanism for superconductivity distinct from both weakly-coupled BCS theory and the $d$-wave phenomenology of the high-$T_c$ cuprates. In this work, we examine this possibility using a density matrix renormalization group (DMRG) study of a model which captures the essential features of MATBGs symmetry and topology. Using large scale cylinder-DMRG calculations to obtain the ground state and its excitations as a function of the electron doping, we find clear evidence for superconductivity driven by the binding of electrons into charge-$2e$ skyrmions. Remarkably, this binding is observed even in the regime where the unscreened Coulomb repulsion is by-far the largest energy scale, demonstrating the robustness of this topological, all-electronic pairing mechanism.
The search for a material platform for topological quantum computation has recently focused on unconventional superconductors. Such material systems, where the superconducting order parameter breaks a symmetry of the crystal point group, are capable of hosting novel phenomena, including emergent Majorana quasiparticles. Unique among unconventional superconductors is the recently discovered UTe2, where spin-triplet superconductivity emerges from a paramagnetic normal state. Although UTe2 could be considered a relative of a family of known ferromagnetic superconductors, the unique crystal structure of this material and experimentally suggested zero Curie temperature pose a great challenge to determining the symmetries, magnetism, and topology underlying the superconducting state. These emergent properties will determine the utility of UTe2 for future spintronics and quantum information applications. Here, we report observations of a non-zero polar Kerr effect and of two transitions in the specific heat upon entering the superconducting state, which together show that the superconductivity in UTe2 is characterized by an order parameter with two components that breaks time reversal symmetry. These data allow us to place firm constraints on the symmetries of the order parameter, which strongly suggest that UTe2 is a Weyl superconductor that hosts chiral Fermi arc surface states.
Actinide materials play a special role in condensed matter physics, spanning behaviours of itinerant d-electron and localized 4f-electron materials. An intermediate state, found notably in Pu-based materials whose 5f electrons are neither fully localized nor itinerant, is particularly challenging to understand. Superconductivity appearing in some actinide materials provides clues to the nature of the 5f electrons. PuCoGa5, the first Pu-based superconductor, is superconducting at Tc=18.5 K. This relatively high Tc is unprecedented in any other actinide system but is typical of itinerant electron compounds in which superconductivity is mediated by phonons. Recent studies of PuCoGa5 show that its superconductivity is not phonon-mediated; rather, these experiments are consistent with superconductivity produced by antiferromagnetic fluctuations of nearly localized 5f electrons. Similarities of PuCoGa5 with the superconducting and normal states of isostructural 4f analogues CeMIn5 (M=Co, Rh, Ir) and high-Tc cuprates enable new perspectives on the 5f electrons of Pu.
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