Recent theory predicted that the Quantum Spin Hall Effect, a fundamentally novel quantum state of matter that exists at zero external magnetic field, may be realized in HgTe/(Hg,Cd)Te quantum wells. We have fabricated such sample structures with low density and high mobility in which we can tune, through an external gate voltage, the carrier conduction from n-type to the p-type, passing through an insulating regime. For thin quantum wells with well width d < 6.3 nm, the insulating regime shows the conventional behavior of vanishingly small conductance at low temperature. However, for thicker quantum wells (d > 6.3 nm), the nominally insulating regime shows a plateau of residual conductance close to 2e 2 /h. The residual conductance is independent of the sample width, indicating that it is caused by edge states. Furthermore, the residual conductance is destroyed by a small external magnetic field.

Capacitance measurements provide a powerful means of probing the density of states. The technique has proved particularly successful in studying 2D electron systems, revealing a number of interesting many-body effects. Here, we use large-area high-quality graphene capacitors to study behavior of the density of states in this material in zero and high magnetic fields. Clear renormalization of the linear spectrum due to electron-electron interactions is observed in zero field. Quantizing fields lead to splitting of the spin-and valley-degenerate Landau levels into quartets separated by interaction-enhanced energy gaps. These many-body states exhibit negative compressibility but the compressibility returns to positive in ultrahigh B. The reentrant behavior is attributed to a competition between field-enhanced interactions and nascent fractional states.Fermi velocity-renormalization | boron nitride | 2D based heterostructures T he Dirac-like spectrum of charge carriers in graphene (1) gives rise to a constant ratio between their kinetic and Coulomb energies (2). The ratio is given by the coupling constant α = e 2 /eZv F , where e is the electron charge, Z is the reduced Planck constant, and e is the effective dielectric constant (2). Because the Fermi velocity v F is 300× smaller than the speed of light c, α is close to unity, that is, much larger than the finestructure constant e 2 /Zc. This regime of strong relativistic-like coupling presents considerable interest from the point of view of many-body physics (2). For example, the large α leads to a noticeable renormalization of v F in the vicinity of the Dirac point (3), which has clear analogies with quantum-field theory (4). In the presence of a magnetic field B, many-body effects become more pronounced, generating large interaction-enhanced gaps at filling factors ν = 0, ±1, ±3, ±4, ±5, ±7,. . .,. (5). Still, despite the intensive research in recent years (2), many-body physics in graphene is far from complete.Unlike in the conventional 2D systems, the density of states (DoS) in graphene depends on n; in capacitance measurements, this makes even small DoS contributions readily noticeable on top of a constant geometrical capacitance C G . This allowed several recent observations of graphene's quantum compressibility (6-12). However, considerable charge inhomogeneity typical for graphene deposited on silicon oxide leads to strong spatial averaging. For example, graphene-on-SiO 2 devices normally exhibit pronounced quantum Hall effect (QHE) features, but Landau quantization in their total capacitance C is seen only as weak oscillations (9). This inhomogeneity obscures finer details in the DoS which can indicate new phenomena.In this article, we use graphene deposited on hexagonal boron nitride (hBN), which has dramatically reduced charge inhomogeneity. Our devices have relatively large area, S ∼ 10 3 μm 2 , to increase their capacitance C but, despite this, quantum oscillations are pronounced already in B below 1 T and correspond to changes in C by a factor of >10....

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The topological properties of fermions arise from their low-energy Dirac-like band dispersion and associated chiralities. Initially confined to points, extensions of the Dirac dispersion to lines and even loops have now been uncovered and semimetals hosting such features have been identified. However, experimental evidence for the enhanced correlation effects predicted to occur in these topological semimetals has been lacking. Here, we report a quantum oscillation study of the nodal loop semimetal ZrSiS in high magnetic fields that reveals significant enhancement in the effective mass of the quasiparticles residing near the nodal loop. Above a threshold field, magnetic breakdown occurs across gaps in the loop structure with orbits that enclose different windings around its vertices, each winding accompanied by an additional π Berry phase. The amplitudes of these breakdown orbits exhibit an anomalous temperature dependence. These findings demonstrate the emergence of novel, correlation-driven physics in ZrSiS associated with the Dirac-like quasiparticles.