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Register a new userDegeneracy doubling and sublattice polarization in strain-induced pseudo-Landau levels
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The degeneracy and spatial support of pseudo-Landau levels (pLLs) in strained honeycomb lattices systematically depends on the geometry -- for instance, in hexagonal and rectangular flakes the 0th pLL displays a twofold increased degeneracy, while the characteristic sublattice polarization of the 0th pLL is only fully realized in a zigzag-terminated triangle. These features are dictated by algebraic constraints in the atomistic theory, and signify a departure from the standard picture in which all qualitative differences between pLLs and Landau levels induced by a magnetic field trace back to the valley-antisymmetry of the pseudomagnetic field.
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Using an array of coupled microwave resonators arranged in a deformed honeycomb lattice, we experimentally observe the formation of pseudo-Landau levels in the whole crossover from vanishing to large pseudomagnetic field strength. This is achieved by utilizing an adaptable set-up in a geometry that is compatible with the pseudo-Landau levels at all field strengths. The adopted approach enables to observe fully formed flat-band pseudo-Landau levels spectrally as sharp peaks in the photonic density of states, and image the associated wavefunctions spatially, where we provide clear evidence for a characteristic nodal structure reflecting the previously elusive supersymmetry in the underlying low-energy theory. In particular, we resolve the full sublattice polarization of the anomalous 0th pseudo-Landau level, which reveals a deep connection to zigzag edge states in the unstrained case.
As a canonical response to the applied magnetic field, the electronic states of a metal are fundamentally reorganized into Landau levels. In Dirac metals, Landau levels can be expected without magnetic fields, provided that an inhomogeneous strain is applied to spatially modulate electron hoppings in a similar way as the Aharonov-Bohm phase. We here predict that a twisted zigzag nanoribbon of graphene exhibits strain-induced pseudo Landau levels of unexplored but analytically solvable dispersions at low energies. The presence of such dispersive pseudo Landau levels results in a negative strain resistivity characterizing the $(1+1)$-dimensional chiral anomaly if partially filled and can greatly enhance the thermopower when fully filled.
Combining the tight-binding approximation and linear elasticity theory for a planar membrane, we investigate stretching of a graphene flake assuming that two opposite edges of the sample are clamped by the contacts. We show that, depending on the aspect ratio of the flake and its orientation, gapped states may form in the membrane in the vicinity of the contacts. This gap in the pre-contact region should be biggest for the armchair orientation of the flake and width to length ratio of around 1.
Energy spectroscopy of strongly interacting phases requires probes which minimize screening while retaining spectral resolution and local sensitivity. Here we demonstrate that such probes can be realized using atomic sized quantum dots bound to defects in hexagonal Boron Nitride tunnel barriers, placed at nanometric distance from graphene. With dot energies capacitively tuned by a planar graphite electrode, dot-assisted tunneling becomes highly sensitive to the graphene excitation spectrum. The spectra track the onset of degeneracy lifting with magnetic field at the ground state, and at unoccupied exited states, revealing symmetry-broken gaps which develop steeply with magnetic field - corresponding to Lande $g$ factors as high as 160. Measured up to $B = 33$ T, spectra exhibit a primary energy split between spin-polarized excited states, and a secondary spin-dependent valley-split. Our results show that defect dots probe the spectra while minimizing local screening, and are thus exceptionally sensitive to interacting states.
Effects of disorder and valley polarization in graphene are investigated in the quantum Hall regime. We find anomalous localization properties for the lowest Landau level (LL), where disorder can induce wavefunction delocalization (instead of localization), both for white-noise and gaussian-correlated disorder. We quantitatively identify the contribution of each sublattice to wavefunction amplitudes. Following the valley (sublattice) polarization of states within LLs for increasing disorder we show: (i) valley mixing in the lowest LL is the main effect behind the observed anomalous localization properties, (ii) the polarization suppression with increasing disorder depends on the localization for the white-noise model, while, (iii) the disorder induces a partial polarization in the higher Landau levels for both disorder models.
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