We present a theory of superconducting pairing originating from soft critical fluctuations near isospin-polarized states in rhombohedral trilayer graphene. Using a symmetry-based approach, we determine possible isospin order types and derive the effe
ctive electron-electron interactions mediated by isospin fluctuations. Superconductitivty arising due to these interactions has symmetry and order parameter structure that depend in a unique way on the mother isospin order. This model naturally leads to a superconducting phase adjacent to isospin-ordering phase transition, which mimics the behavior observed in experiment. The symmetry of the paired state predicted for the isospin order type inferred in experiments matches the observations. These findings support a scenario of superconductivity originating from electron-electron interactions.
We discuss superconducting pairing in a narrow conduction band sandwiched between unoccupied and occupied bands, an arrangement that enables an unconventional pairing mechanism governed by Coulomb repulsion. Pairing interaction originates from repuls
ion-assisted scattering between far-out pair states in the higher-energy bands and those at the Fermi level. Optimizing the bandstructure design and carrier density in order to bring plasma frequency below the bandgap renders the repulsion unscreened for the processes with a large frequency transfer. This allows the pairing to fully benefit from the pristine Coulomb repulsion strength. The repulsion-induced attraction is particularly strong in two dimensions and is assisted by a low density of carriers and the resulting low plasma frequency values. We assess the possible connection of this mechanism to superconductivity in magic-angle twisted bilayer graphene where the bandstructure features wide dispersive upper and lower minibands. We use a simple model to illustrate the importance of the far-out pairs in these bands and predict testable signatures of this superconductivity mechanism.
One of humanitys earliest mathematical inquiries might have involved the geometric patterns in plants. The arrangement of leaves on a branch, seeds in a sunflower, and spines on a cactus exhibit repeated spirals, which appear with an intriguing regul
arity providing a simple demonstration of mathematically complex patterns. Surprisingly, the numbers of these spirals are pairs of Fibonacci numbers consecutive in the series 1, 2, 3, 5, 8, 13, 21, 34, 55... obeying a simple rule 1+2=3, 2+3=5, 5+8=13 and so on. This article describes how physics helps to clarify the origin of this fascinating behavior by linking it to the properties of deformable lattices growing and undergoing structural rearrangements under stress.
Collective plasma excitations in moire flat bands display unique properties reflecting strong electron-electron interactions and unusual carrier dynamics in these systems. Unlike the conventional two-dimensional plasmon modes, dispersing as $sqrt{k}$
at low frequencies and plunging into particle-hole continuum at higher frequencies, the moire plasmons pierce through the flat-band continuum and acquire a strong over-the-band character. Due to the complex structure of the moire superlattice unit cell, the over-the-band plasmons feature several distinct branches connected through zone folding in the superlattice Brillouin zone. Using a toy Hubbard model for the correlated insulating order in a flat band, we predict that these high-frequency modes become strongly dipole-active upon the system undergoing charge ordering, with the low-frequency modes gapped out within the correlated insulator gap. Strong dipole moments and sensitivity to charge order make these modes readily accessible by optical measurements, providing a convenient diagnostic of the correlated states.
Collective modes in two-dimensional electron fluids show an interesting response to a background carrier flow. Surface plasmons propagating on top of a flowing Fermi liquid acquire a non-reciprocal character manifest in a $pm k$ asymmetry of mode dis
persion. The nonreciprocity arises due to Fermi surface polarization by the flow. The flow-induced interactions between quasiparticles make collective modes of the system uniquely sensitive to subtle motional Fermi-liquid effects. The flow-induced Doppler-type frequency shift of plasmon resonances, arising due to electron interactions, can strongly deviate from the classical value. This opens a possibility to directly probe motional Fermi-liquid effects in plasmonic near-field imaging experiments.
Cold atoms embedded in a degenerate Fermi system interact via a fermionic analog of the Casimir force, which is an attraction of a -1/r form at distances shorter than the Fermi wavelength. Interestingly, the hydrogenic two-body bound states do not fo
rm in this regime because the interaction strength is too weak under realistic conditions, and yet the three-body bound states can have a considerably higher degree of stability. As a result, the trimer bound states can form even when the dimer states are unstable. A quasiclassical analysis of quantum states supported by periodic orbits singles out the figure-eight orbits, predicting bound states that are more stable than the ones originating from circular orbits. The discrete energies of these states form families of resonances with a distinct structure, enabling a direct observation of signatures of figure-eightbraiding dynamics.
Surface plasmons in 2-dimensional electron systems with narrow Bloch bands feature an interesting regime in which Landau damping (dissipation via electron-hole pair excitation) is completely quenched. This surprising behavior is made possible by stro
ng coupling in narrow-band systems characterized by large values of the fine structure constant $alpha=e^2/hbar kappa v_{rm F}$. Dissipation quenching occurs when dispersing plasmon modes rise above the particle-hole continuum, extending into the forbidden energy gap that is free from particle-hole excitations. The effect is predicted to be prominent in moire graphene, where at magic twist-angle values, flat bands feature $alphagg1$. The extinction of Landau damping enhances spatial optical coherence. Speckle-like interference, arising in the presence of disorder scattering, can serve as a telltale signature of undamped plasmons directly accessible in near-field imaging experiments.
Momentum-conserving quasiparticle collisions in two-dimensional Fermi gases give rise to a large family of exceptionally long-lived excitation modes. The lifetimes of these modes exceed by a factor $(T_F/T)^2gg 1$ the conventional Landau Fermi-liquid
lifetimes $tausim T_F/T^2$. The long-lived modes have a distinct angular structure, taking the form of $cos mtheta$ and $sin mtheta$ with odd $m$ values for a circular Fermi surface, with relaxation rate dependence on $m$ of the form $m^4log m$, valid at not-too-large $m$. In contrast, the even-$m$ harmonics feature conventional lifetimes with a weak $m$ dependence. The long-time dynamics, governed by the long-lived modes, takes the form of angular (super)diffusion over the Fermi surface. Altogether, this leads to unusual long-time memory effects, defining an intriguing transport regime that lies between the conventional ballistic and hydrodynamic regimes.
Controlling energy flows in solids through switchable electron-lattice cooling can grant access to a range of interesting and potentially useful energy transport phenomena. Here we discuss a unique switchable electron-lattice cooling mechanism arisin
g in graphene due to phonon emission mediated by resonant scattering on defects in crystal lattice, which displays interesting analogy to the Purcell effect in optics. This mechanism strongly enhances the electron-phonon cooling rate, since non-equilibrium carriers in the presence of momentum recoil due to disorder can access a larger phonon phase space and emit phonons more effciently. Resonant energy dependence of phonon emission translates into gate-tunable cooling rates, exhibiting giant enhancement of cooling occurring when the carrier energy is aligned with the electron resonance of the defect.
