Justin C. W. Song

Hot carrier-assisted intrinsic photoresponse in graphene.

Photoexcited electrons in graphene remain thermally excited because they cannot transfer this energy to lattice vibrations. We report on the intrinsic optoelectronic response of high-quality dual-gated monolayer and bilayer graphene p-n junction devices. Local laser excitation (of wavelength 850 nanometers) at the p-n interface leads to striking six-fold photovoltage patterns as a function of bottom- and top-gate voltages. These patterns, together with the measured spatial and density dependence of the photoresponse, provide strong evidence that nonlocal hot carrier transport, rather than the photovoltaic effect, dominates the intrinsic photoresponse in graphene. This regime, which features a long-lived and spatially distributed hot carrier population, may offer a path to hot carrier–assisted thermoelectric technologies for efficient solar energy harvesting.

Hot Carrier Transport and Photocurrent Response in Graphene

Strong electron-electron interactions in graphene are expected to result in multiple-excitation generation by the absorption of a single photon. We show that the impact of carrier multiplication on photocurrent response is enhanced by very inefficient electron cooling, resulting in an abundance of hot carriers. The hot-carrier-mediated energy transport dominates the photoresponse and manifests itself in quantum efficiencies that can exceed unity, as well as in a characteristic dependence of the photocurrent on gate voltages. The pattern of multiple photocurrent sign changes as a function of gate voltage provides a fingerprint of hot-carrier-dominated transport and carrier multiplication.

Detecting topological currents in graphene superlattices.

Making use of graphenes valleys Graphene has two distinct valleys in its electronic structure, in which the electrons have the same energy. Theorists have predicted that creating an asymmetry between the two valleys will coax graphene into exhibiting the so-called valley Hall effect (VHE). In this effect, electrons from the two valleys move across the sample in opposite directions when the experimenters run current along the sample. Gorbachev et al. achieved this asymmetry by aligning graphene with an underlying layer of hexagonalboron nitride (hBN) (see the Perspective by Lundeberg and Folk). The authors measured the transport characteristics of the sample, which were consistent with the theoretical predictions for the VHE. The method may in the future lead to information processing using graphenes valleys. Science, this issue p. 448; see also p. 422 Graphene is aligned with a layer of hexagonal boron nitride to achieve the valley Hall effect. [Also see Perspective by Lundeberg and Folk] Topological materials may exhibit Hall-like currents flowing transversely to the applied electric field even in the absence of a magnetic field. In graphene superlattices, which have broken inversion symmetry, topological currents originating from graphene’s two valleys are predicted to flow in opposite directions and combine to produce long-range charge neutral flow. We observed this effect as a nonlocal voltage at zero magnetic field in a narrow energy range near Dirac points at distances as large as several micrometers away from the nominal current path. Locally, topological currents are comparable in strength with the applied current, indicating large valley-Hall angles. The long-range character of topological currents and their transistor-like control by means of gate voltage can be exploited for information processing based on valley degrees of freedom.

Disorder-assisted electron-phonon scattering and cooling pathways in graphene.

We predict that graphene is a unique system where disorder-assisted scattering (supercollisions) dominates electron-lattice cooling over a wide range of temperatures, up to room temperature. This is so because for momentum-conserving electron-phonon scattering the energy transfer per collision is severely constrained due to a small Fermi surface size. The characteristic T(3) temperature dependence and power-law cooling dynamics provide clear experimental signatures of this new cooling mechanism. The cooling rate can be changed by orders of magnitude by varying the amount of disorder providing means for a variety of new applications that rely on hot-carrier transport.

Electron interactions and gap opening in graphene superlattices.

We develop a theory of interaction effects in graphene superlattices, where tunable superlattice periodicity can be used as a knob to control the gap at the Dirac point. Applied to graphene on hexa-boron-nitride (G/h-BN), our theory predicts substantial many-body enhancement of this gap. Tunable by the moiré superlattice periodicity, a few orders of magnitude enhancement is reachable under optimal conditions. The Dirac point gap enhancement can be much larger than that of the minigaps opened by Bragg scattering at principal superlattice harmonics. This naturally explains the conundrum of large Dirac point gaps recently observed in G/h-BN heterostructures and their tunability by the G/h-BN twist angle.

Photoexcited carrier dynamics and impact-excitation cascade in graphene

In materials with strong electron-electron interactions, photoexcitation can trigger a cascade in which multiple particle-hole excitations are generated. Here we analyze the cascade of impact-excitation processes in graphene in which many hot carriers are generated by a single absorbed photon. We show that the number of generated carriers has a strong dependence on doping (gate tunability). Linear scaling with photon energy is predicted for the number of pairs and for the duration of the cascade. These dependencies, along with a sharply peaked angular distribution of excited carriers, provide clear experimental signatures of hot carrier multiplication.

Topological Valley Currents in Gapped Dirac Materials

Gapped 2D Dirac materials, in which inversion symmetry is broken by a gap-opening perturbation, feature a unique valley transport regime. Topological valley currents in such materials are dominated by bulk currents produced by electronic states just beneath the gap rather than by edge modes. The system ground state hosts dissipationless persistent valley currents existing even when topologically protected edge modes are absent. Valley currents induced by an external bias are characterized by a quantized half-integer valley Hall conductivity. The undergap currents dominate magnetization and the charge Hall effect in a light-induced valley-polarized state.

Chiral plasmons without magnetic field

Significance A class of collective excitations is introduced, arising from the combined action of electron interactions and Berry curvature. These excitations manifest as chiral plasmonic modes at zero magnetic field. These collective modes are predicted to arise in a range of anomalous Hall metals, yielding a fundamental characteristic of interacting topological metals with non-zero Berry flux. Featuring a rich set of properties, these modes will enable versatile new tools for molding the flow of light. Plasmons, the collective oscillations of interacting electrons, possess emergent properties that dramatically alter the optical response of metals. We predict the existence of a new class of plasmons—chiral Berry plasmons (CBPs)—for a wide range of 2D metallic systems including gapped Dirac materials. As we show, in these materials the interplay between Berry curvature and electron–electron interactions yields chiral plasmonic modes at zero magnetic field. The CBP modes are confined to system boundaries, even in the absence of topological edge states, with chirality manifested in split energy dispersions for oppositely directed plasmon waves. We unveil a rich CBP phenomenology and propose setups for realizing them, including in anomalous Hall metals and optically pumped 2D Dirac materials. Realization of CBPs will offer a powerful paradigm for magnetic field-free, subwavelength optical nonreciprocity, in the mid-IR to terahertz range, with tunable splittings as large as tens of THz, as well as sensitive all-optical diagnostics of topological bands.

Energy flows in graphene: hot carrier dynamics and cooling

Long lifetimes of hot carriers can lead to qualitatively new types of responses in materials. The magnitude and time scales for these responses reflect the mechanisms governing energy flows. We examine the microscopics of two processes which are key for energy transport, focusing on the unusual behavior arising due to graphenes unique combination of material properties. One is hot carrier generation in its photoexcitation dynamics, where hot carriers multiply through an Auger type carrier-carrier scattering cascade. The hot-carrier generation manifests itself through elevated electronic temperatures which can be accessed in a variety of ways, in particular optical conductivity measurements. Another process of high interest is electron-lattice cooling. We survey different cooling pathways and discuss the cooling bottleneck arising for the momentum-conserving electron-phonon scattering pathway. We show how this bottleneck can be relieved by higher-order collisions—supercollisions—and examine the variety of supercollision processes that can occur in graphene.

Energy-Driven Drag at Charge Neutrality in Graphene

Coulomb coupling in graphene heterostructures results in vertical energy transfer between electrons in proximal layers. We show that, in the presence of correlated density inhomogeneity in the layers, vertical energy transfer has a strong impact on lateral charge transport. In particular, for Coulomb drag, its contribution dominates over conventional momentum drag near zero doping. The dependence on doping and temperature, which is different for the two drag mechanisms, can be used to separate these mechanisms in experiments. We predict distinct features such as a peak at zero doping and a multiple sign reversal, which provide diagnostics for this new drag mechanism.