arXiv subjects

Justin C. W. Song

Publications and source records attributed to Justin C. W. Song.

68 records · Page 4Linked to original sources

arXiv2015

Topological Bloch Bands in Graphene Superlattices

We outline an approach to endow a plain vanilla material with topological properties by creating topological bands in stacks of manifestly nontopological atomically thin materials. The approach is illustrated with a model system comprised of graphene stacked atop hexagonal-boron-nitride. In this case, the Berry curvature of the electron Bloch bands is highly sensitive to the stacking configuration. As a result, electron topology can be controlled by crystal axes alignment, granting a practical route to designer topological materials. Berry curvature manifests itself in transport via the valley Hall effect and long-range chargeless valley currents. The non-local electrical response mediated by such currents provides diagnostics for band topology.

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arXiv2015

Accessing phonon polaritons in hyperbolic crystals by ARPES

Recently studied hyperbolic materials host unique phonon-polariton (PP) modes. The ultra-short wavelengths of these modes, which can be much smaller than those of conventional exciton-polaritons, are of high interest for extreme sub-diffraction nanophotonics schemes. Polar hyperbolic materials such as hexagonal boron nitride can be used to realize strong long-range coupling between PP modes and extraneous charge degrees of freedom. The latter, in turn, can be used to control and probe PP modes. Of special interest is coupling between PP modes and plasmons in an adjacent graphene sheet, which opens the door to accessing PP modes by angle-resolved photoemission spectroscopy (ARPES). A rich structure in the graphene ARPES spectrum due to PP modes is predicted, providing a new probe of PP modes and their coupling to graphene plasmons.

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arXiv2015

Shockley-Ramo theorem and long-range photocurrent response in gapless materials

Scanning photocurrent maps of gapless materials, such as graphene, often exhibit complex patterns of hot spots positioned far from current-collecting contacts. We develop a general framework that helps to explain the unusual features of the observed patterns, such as the directional effect and the global character of photoresponse. We show that such a response is captured by a simple Shockley-Ramo-type approach. We examine specific examples and show that the photoresponse patterns can serve as a powerful tool to extract information about symmetry breaking, inhomogeneity, chirality, and other local characteristics of the system.

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arXiv2014

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. The system ground state hosts dissipationless persistent valley currents existing even when topologically protected edge modes are absent or when they are localized due to edge roughness. Topological valley currents in such materials are dominated by bulk currents produced by electronic states just beneath the gap rather than by edge modes. Dissipationless currents induced by an external bias are characterized by a quantized half-integer valley Hall conductivity. The under-gap currents dominate magnetization and the charge Hall effect in a light-induced valley-polarized state.

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arXiv2014

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 graphene's 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.

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arXiv2013

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 moire 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.

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arXiv2013

Hall Drag and Magnetodrag in Graphene

Massless Dirac fermions in graphene at charge neutrality form a strongly interacting system in which both charged and neutral (energy) modes play an important role. These modes are essentially decoupled in the absence of a magnetic field, but become strongly coupled when a field is applied. We show that these ideas explain the recently observed giant magnetodrag, arising in classically weak fields when electron density is tuned near charge neutrality. We predict strong Hall drag in this regime, which is in stark departure from the weak coupling regime, where theory predicts the absence of Hall drag. Energy-driven magnetodrag and Hall drag arise in a wide temperature range and at weak magnetic fields, and feature an unusually strong dependence on field and carrier density.

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arXiv2013

Ballistic Heat Transfer and Energy Waves in an Electron System

Materials in which heat and entropy can be transmitted by directed ballistic pulses can trigger new approaches to energy transduction in solids. We predict that a ballistic energy transfer mode, with heat propagation governed by a wave equation rather than a diffusion equation, can be realized for a thermal electron-hole plasma in graphene. The new behavior originates from rapid exchange of energy and momentum in particle collisions leading to energy propagation as a collective weakly-damped oscillation. Due to the electronic nature of this mode, the estimated propagation velocity can be ~10^3 times larger than that for previously studied phonon mechanisms. The energy mode is uncharged at charge neutrality, but becomes coupled to charge dynamics upon doping. This coupling can be used for all-electric excitation and detection of energy transport.

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arXiv2013

Coulomb Drag Mechanisms in Graphene

Recent measurements revealed an anomalous Coulomb drag in graphene, hinting at new physics at charge neutrality. The anomalous drag is explained by a new mechanism based on energy transport, which involves interlayer energy transfer, coupled to charge flow via lateral heat currents and thermopower. The old and new drag mechanisms are governed by distinct physical effects, resulting in starkly different behavior, in particular for drag magnitude and sign near charge neutrality. The new mechanism explains the giant enhancement of drag near charge neutrality, as well as its sign and anomalous sensitivity to magnetic field. Under realistic conditions, energy transport dominates in a wide temperature range, giving rise to a universal value of drag which is essentially independent of the electron-electron interaction strength.

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arXiv2012

Photo-excited Carrier Dynamics and Impact Excitation Cascade in Graphene

Photo-excitation in solids can trigger a cascade in which multiple particle-hole excitations are generated. We analyze the carrier multiplication cascade of impact excitation processes in graphene and show that the number of pair excitations has a strong dependence on doping, which makes carrier multiplication gate-tunable. We also predict that the number of excited pairs as well as the characteristic time of the cascade scale linearly with photo-excitation energy. These dependences, as well as sharply peaked angular distribution of pair excitations, provide clear experimental signatures of carrier multiplication.

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arXiv2012

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 which offers means for a variety of new applications that rely on hot-carrier transport.

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arXiv2012

Energy-driven Drag at Charge Neutrality in Graphene

Coulomb coupling between proximal layers in graphene heterostructures results in efficient energy transfer between the layers. We predict that, in the presence of correlated density inhomogeneities in the layers, vertical energy transfer has a strong impact on lateral charge transport. In particular, for Coulomb drag it dominates over the 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 experiment. We predict distinct features such as a peak at zero doping and a multiple sign reversal, which provide diagnostics for this new drag mechanism.

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arXiv2011

Hot Carrier-Assisted Intrinsic Photoresponse in Graphene

Graphene is a new material showing high promise in optoelectronics, photonics, and energy-harvesting applications. However, the underlying physical mechanism of optoelectronic response has not been established. Here, we report on the intrinsic optoelectronic response of high-quality dual-gated monolayer and bilayer graphene p-n junction devices. Local laser excitation 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 non-local hot-carrier transport, rather than the photovoltaic effect, dominates the intrinsic photoresponse in graphene. This novel regime, which features a long-lived and spatially distributed hot carrier population, may open the doorway for optoelectronic technologies exploiting efficient energy transport at the nanoscale.

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arXiv2011

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.

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