We demonstrate theoretically that the interaction of electrons in gapped Dirac materials (gapped graphene and transition-metal dichalchogenide monolayers) with a strong off-resonant electromagnetic field (dressing field) substantially renormalizes the band gaps and the spin-orbit splitting. Moreover…

[Phys. Rev. B 95, 125401] Published Wed Mar 01, 2017

Turbostratic graphene disks feature multiple layers with rotational stacking order, which ensures electronic decoupling of adjacent layers. Despite there being up to 100 graphene sheets per disk, the authors find the signature of $two-dimensional$ charge transport. With its central region well protected from environmental influence, this system combines the merits of single-layer graphene with a remarkable, consistent robustness, making it eminently suitable for device integration.

[Phys. Rev. Applied 7, 024022] Published Thu Feb 23, 2017

The single-particle Green's function (GF) of mesoscopic structures plays a central role in mesoscopic quantum transport. The recursive GF technique is a standard tool to compute this quantity numerically, but it lacks physical transparency and is limited to relatively small systems. Here we present …

[Phys. Rev. B 95, 075421] Published Fri Feb 17, 2017

We investigate the local electronic structure of a Sinai-like, quadrilateral graphene quantum billiard with zigzag and armchair edges using scanning tunneling microscopy (STM) at room temperature. It is revealed that besides the $(\sqrt{3}\times \sqrt{3})R{30}^{\circ }$ superstructure, which is caused by the intervalley scattering…

[Phys. Rev. B 95, 075123] Published Mon Feb 13, 2017

When exposed to an ultrafast laser pulse, electrons within graphite are warmed to the same temperature as the surface of the Sun. This behavior could explain how laser light can turn graphite into diamond and allow for efficient chemistry on the surface of graphitic materials.

[Phys. Rev. X 7, 011004] Published Tue Jan 17, 2017

[Phys. Rev. B 95, 039903] Published Tue Jan 17, 2017

[Phys. Rev. B 95, 039901] Published Tue Jan 10, 2017

Normal-mode expansion is routinely used to model multimodal mechanical systems, but at the nanoscale its assumptions are not always valid. The authors find that in the presence of $spatiallyheterogeneous$ dissipation, the thermal noise spectrum of a hybrid nanomechanical system includes anomalous lineshapes quite different from model predictions. To address this, they measure the $local$ mechanical susceptibility, and verify the validity of the Fluctuation-Dissipation Theorem in the coupled device.

[Phys. Rev. Applied 6, 064021] Published Thu Dec 29, 2016

We investigated the stability and mechanical and electronic properties of 15 metastable mixed $s{p}^2\text{−}s{p}^3$ carbon allotropes in the family of interpenetrating graphene networks (IGNs) using density functional theory (DFT). IGN allotropes exhibit nonmonotonic bulk and linear compressibilities before their…

[Phys. Rev. B 94, 245422] Published Mon Dec 19, 2016

According to Lieb's theorem the ferromagnetic interaction in graphene-based materials with bipartite lattice is a result of disbalance between the number of sites available for $p_z$ electrons in different sublattices. Here we report on another mechanism of the ferromagnetism in functionalized graphene…

[Phys. Rev. B 94, 214411] Published Mon Dec 12, 2016

Graphene-based spintronics hinges on controlling spin currents through ferromagnet/graphene interfaces. By measuring devices with different contact resistances, the authors show that the spin-transport properties of the graphene—particularly its spin resistance—can be controlled by a gate voltage. Due to reduced flow of spins from graphene into the ferromagnetic contacts, spin accumulation increases with contact resistance, scaling with the ratio of contact resistance to graphene’s spin resistance. This is an important, quantitative demonstration that the effect of this ratio on the spin signal is consistent with the theory of contact-induced spin relaxation.

[Phys. Rev. Applied 6, 054015] Published Mon Nov 28, 2016

Recent reports of spin-orbit coupling enhancement in chemically modified graphene have opened doors to studies of the spin Hall effect with massless chiral fermions. Here, we theoretically investigate the interaction and impurity density dependence of the extrinsic spin Hall effect in spin-orbit cou…

[Phys. Rev. B 94, 201402(R)] Published Mon Nov 07, 2016

The Coulomb interaction is widely known to enhance the effective mass of interacting particles and therefore tends to favor a localized state at commensurate filling. Here, we will show that, in contrast to this consensus, in a van der Waals heterostructure consisting of graphene and hexagon boron n…

[Phys. Rev. B 94, 195103] Published Tue Nov 01, 2016

Metasurfaces (metamaterials of near-vanishing thickness) are of interest for engineering subwavelength structures with complex responses. Typically a graphene metasurface exhibits just one or two resonances and absorbs light in a narrow frequency band, but this study demonstrates tunable broadband absorption due to hierarchical, fractal structuring. Over a very wide band (about 190% of the central frequency), the average absorption exceeds 20%, without any metallic mirror. This approach to broadband absorbers, and plasmonic components more generally, offers substantially improved performance at terahertz and optical frequencies.

[Phys. Rev. Applied 6, 044019] Published Fri Oct 28, 2016

Spin-orbit interactions are responsible for intriguing phenomena such as topological insulating states. Now, scientists study the spin-orbit interactions of electrons directly at the interface between graphene and transition-metal dichalcogenides.

[Phys. Rev. X 6, 041020] Published Wed Oct 26, 2016

Chemically functionalized graphene quantum dots (GQDs) are promising for photonic and optoelectronic applications. The authors’ calculations show that shaping destructive interference of the quantum states of GQDs can yield dark states at the red end of the spectrum for efficient electron transfer, or highly coherent bright states for photonic applications, and that strain can induce closely spaced, brighter states. This study offers a guide for the rational design of GQDs to obtain desired optical properties.

[Phys. Rev. Applied 6, 044014] Published Mon Oct 24, 2016

The optical Kerr effect is the variation of a material’s refractive index in response to high-intensity light, and as such causes the spectrum of the light to change. The authors measure the spectral broadening of chirped laser pulses in graphene-covered silicon waveguides, and determine the sign of graphene’s Kerr index to be $negative$, contrary to common assumption. This finding suggests greatly extended applicability of graphene in nonlinear photonic devices.

[Phys. Rev. Applied 6, 044006] Published Thu Oct 13, 2016

We report unusual physics associated with wave scattering in pseudospin-1 systems whose band structure consists of a conventional Dirac cone and a topologically flat band. First, for small scatterer size, we find a surprising revival resonant scattering phenomenon and identify a peculiar type of bou…

[Phys. Rev. B 94, 165405] Published Fri Oct 07, 2016

Graphene could be a wonder material for electronics, due to not just its electrical but also its $thermal$ conductivity, to help keep nanodevices cool. Laying graphene on a substrate tends to spoil this terrific property, but the authors find that atop $h$-BN, graphene surprisingly retains 90% of its thermal conductivity, due to polarization of the graphene and thus strong interlayer adhesion. This study highlights the importance of understanding the interlayer interactions of graphene with dielectric materials in devices.

[Phys. Rev. Applied 6, 034015] Published Mon Sep 26, 2016

Our understanding of charge transport across interfaces needs to be reconsidered at the nanoscale, especially for advanced optoelectronic applications. Systems based on graphene, for example, present highly nonparabolic energy dispersion, for which the Schottky equation fails. The authors formulate a Kane-Schottky scaling relation for current with temperature, bridging the nonrelativistic $T^2$ and relativistic $T^3$ regimes. They show that using incorrect scaling relations to analyze experimental data can produce errors of up to two orders of magnitude.

[Phys. Rev. Applied 6, 034013] Published Wed Sep 21, 2016

The random quivering of graphene membranes could be exploited to generate electricity.

[Phys. Rev. Lett. 117, 126801] Published Tue Sep 13, 2016

[Phys. Rev. B 94, 119901] Published Tue Sep 06, 2016

Zhang et al. [Phys. Rev. B 77, 241402(R) (2008)] reported a theoretical study of the optical spectra of monolayer graphene employing the Kubo formula within a tight-binding model. Their calculations predicted that at high frequencies the optical conductivity of graphene becomes strongly anisotropic.…

[Phys. Rev. B 94, 117401] Published Thu Sep 01, 2016

[Phys. Rev. B 94, 079904] Published Fri Aug 12, 2016

In recent years the LaAlO${}_3$/SrTiO${}_3$ (LAO/STO) interface has been a playground of condensed matter physics, as it exhibits exotic electronic properties not seen in either constituent. Furthermore, combining this system with graphene has its own charm: The photoconductivity of graphene is a good probe of the natural polar field within the LAO layer, as evidenced by hole doping in a graphene/LAO/STO hybrid system under pulsed deep-ultraviolet illumination. This also renders graphene/LAO/STO a convenient deep-ultraviolet sensor, and suggests its use in broad-spectrum photodetectors.

[Phys. Rev. Applied 6, 014005] Published Thu Jul 14, 2016

One of graphene’s interesting properties is that, when pumped by a laser at near-infrared and visible frequencies, it offers amplification at terahertz (THz) frequencies, which are useful for remote sensing in security applications. The authors explain how to take advantage of exotic parity-time ($PT$) symmetry in an active graphene metasurface that realizes reciprocal, unidirectional reflectionless propagation of THz waves. This suggests exciting prospects for detecting chemical and biological agents with ultrahigh sensitivity.

[Phys. Rev. Applied 5, 064018] Published Wed Jun 29, 2016

Angle-resolved photoemission spectroscopy (ARPES) provides direct access to the electronic band structure of materials. Very recently, the spatial resolution of ARPES setups has dramatically increased, with a spot size at the sample shrinking from 50$\times$50 $\mu$m${}^2$, typical of most synchrotron-based setups, down to 100$\times$100 nm${}^2$ or less. In this work, the authors demonstrate the impact of increased spatial resolution by evidencing faint spatial inhomogeneity in the ARPES signal of a graphene sample that would go undetected in ARPES setups possessing lower spatial resolution.

[Phys. Rev. B 93, 241101(R)] Published Thu Jun 09, 2016

The remarkable ability of phase change materials (PCM) to switch between amorphous and crystalline states on a nanosecond time scale could provide new opportunities for graphene engineering. We have used density functional calculations to investigate the structures and electronic properties of heter…

[Phys. Rev. B 93, 195443] Published Tue May 31, 2016

We present a molecular modeling study analyzing nanometer-scale strain variations in graphene as a function of externally applied tensile strain. We consider two different mechanisms that could underlie nanometer-scale strain variations: static perturbations from lattice imperfections of an underlyi…

[Phys. Rev. B 93, 195438] Published Thu May 26, 2016

We engineer topological insulating phases in a fermion-fermion mixture on the honeycomb lattice, without resorting to artificial gauge fields or spin-orbit couplings and considering only local interactions. Essentially, upon integrating out the fast component (characterized by a larger hopping ampli…

[Phys. Rev. B 93, 195153] Published Tue May 24, 2016

Hybrid quantum Hall (QH) junctions have been extensively studied by transport measurements due to their exciting physics and device applications. Here we report on spatially resolving electronic properties of such a junction on the nanoscale. We present a subnanometer-resolved scanning tunneling mic…

[Phys. Rev. B 93, 195408] Published Fri May 06, 2016

Graphene is the wonder material hoped to enable the energy-efficient spin-based information processing that could replace today’s electronics. Actually making working devices from this stuff, however, is not trivial. This study presents a milestone in harnessing spins in graphene for computation: The authors have produced one of the building blocks needed to make spintronic integrated circuits.

[Phys. Rev. Applied 5, 044003] Published Mon Apr 04, 2016

Motivated by recent free-standing graphene experiments, we show how thermal fluctuations affect the mechanical properties of microscopically thin solid ribbons, which can be many thousand times wider than their atomic thickness. A renormalization group analysis of flexural phonons reveals that elong…

[Phys. Rev. B 93, 125431] Published Thu Mar 24, 2016

We report on microscopic measurements of the low-energy electronic structures both at the zigzag and armchair edges of bilayer graphene using scanning tunneling microscopy and spectroscopy (STM and STS). We have found that, both in the absence and in the presence of a magnetic field, an almost zero-…

[Phys. Rev. B 93, 125422] Published Fri Mar 18, 2016

The possibility of observing a strongly interacting quantum critical fluid of electrons in a metal has intrigued physicists for a long time. The authors combine insight from string theory and condensed matter physics to develop a hydrodynamic theory of the thermal and electrical conductivity of electron fluids. They use this formalism to explain experimental data from a novel state of Dirac electron fluid in clean samples of graphene near the charge neutrality point, and observe substantially improved quantitative agreement over the existing hydrodynamic theories. This study marks the first quantitative connection between these exotic models of transport and experimentally realizable condensed matter systems.

[Phys. Rev. B 93, 075426] Published Tue Feb 16, 2016

Although phonon-mediated thermal conduction in pristine graphene and hexagonal boron nitride is well understood, less is known about phonon transport in single-sheet graphene-hexagonal boron nitride ($\mathrm{Gr}/h-\mathrm{BN}$) lateral heterostructures, where the thermal resistance of the interfaces plays an important…

[Phys. Rev. B 93, 075406] Published Tue Feb 02, 2016

The graphene twist bilayer represents the prototypical system for investigating the stacking degree of freedom in few-layer graphenes. The electronic structure of this system changes qualitatively as a function of angle, from a large-angle limit in which the two layers are essentially decoupled—with…

[Phys. Rev. B 93, 035452] Published Wed Jan 27, 2016

Casimir and van der Waals forces, induced by dipolar fluctuations, are commonplace, and now a new study shows that fluctuations induced by monopolar charges can create similar or stronger forces in solid-state devices.

[Phys. Rev. X 6, 011004] Published Thu Jan 21, 2016

Massless Dirac fermions with a pseudospin of $\frac{1}{2}$ are tied to many intriguing properties of graphene. Previous studies have discussed the possibility of constructing a higher pseudospin system in artificial lattices of ultracold atoms, which can also support equally interesting physical properties. The authors of this paper demonstrate that photon transport in certain photonic crystals corresponds to a pseudospin-1 system. This is a significant development because, contrary to ultracold systems, photonic crystals offer a better opportunity to study novel pseudospin-1 physics in experimentally realizable materials and at room temperature.

[Phys. Rev. B 93, 035422] Published Thu Jan 14, 2016

Inducing magnetism in graphene holds great promises, such as controlling the exchange interaction with a gate electrode, and generating exotic magnetic phases. Coating graphene with magnetic molecules or atoms has so far mostly led to decreased graphene mobility. In the present work, we show that Pt…

[Phys. Rev. B 93, 045403] Published Tue Jan 05, 2016

Snake states are open trajectories for charged particles propagating in two dimensions under the influence of a spatially varying perpendicular magnetic field. In the quantum limit they are protected edge modes that separate topologically inequivalent ground states and can also occur when the partic…

[Phys. Rev. B 92, 235438] Published Mon Dec 28, 2015

Majorana particles, which are their own antiparticles and whose recent detection in solid-state systems remains controversial, are expected to play an important role in future quantum computing. Now, scientists predict that graphene may host Majorana particles.

[Phys. Rev. X 5, 041042] Published Tue Dec 15, 2015

Experiments have confirmed that double monolayer graphene does not generate finite-temperature electron-hole superfluidity, because of very strong screening of the pairing attraction. The linear dispersing energy bands in monolayer graphene block any attempt to reduce the strength of the screening. …

[Phys. Rev. B 92, 220502(R)] Published Mon Dec 14, 2015

We have investigated ABA-stacked trilayer graphene under a perpendicular electric field by using the density functional theory (DFT) calculations, which may contribute to the resolution of the discrepancies between experimental and theoretical results on the electric-field-induced band gap and topol…

[Phys. Rev. B 92, 245416] Published Thu Dec 10, 2015

The use of graphene in spintronic devices depends, among other things, on its ability to convert a spin excitation into an electric charge signal, a phenomenon that requires a spin-orbit coupling (SOC). Here we report the observation of two effects that show the existence of SOC in large-area CVD gr…

[Phys. Rev. Lett. 115, 226601] Published Wed Nov 25, 2015

We show theoretically that in an external magnetic field, like charges on top of graphene monolayer may be mutually attracted to form macromolecules. For this to happen, graphene needs to be in a quantum Hall plateau state with the local chemical potential being between the Landau levels. One or sev…

[Phys. Rev. B 92, 195426] Published Mon Nov 23, 2015

Heterostructures of two-dimensional materials have shown unusual properties and rich physical phenomena. This paper reports on micrometer-scale angle-resolved photoemission spectroscopy of van der Waals heterostructures of graphene and MoS${}_2$ monolayers. The authors directly measured the electronic structure of monolayer stacking and its tunability due to the twist-angle between the layers. They show that the electronic states of graphene and MoS${}_2$ are not hybridized, and the band gap of MoS${}_2$ can be engineered by changing the orientation of the two layers.

[Phys. Rev. B 92, 201409(R)] Published Mon Nov 16, 2015

Modulating the phase of electromagnetic waves has many applications in photonic research. A new mechanism allows a thin graphene metasurface to reliably achieve an extremely large phase modulation in THz radiation.

[Phys. Rev. X 5, 041027] Published Mon Nov 16, 2015

The prospect of a Dirac half metal, a material which is characterized by a band structure with a gap in one spin channel but a Dirac cone in the other, is of both fundamental interest and a natural candidate for use in spin-polarized current applications. However, while the possibility of such a mat…

[Phys. Rev. B 92, 201403(R)] Published Mon Nov 09, 2015

A nonlocal Hall bar geometry is used to detect neutral-current Hall effects in graphene on silicon dioxide. Disorder is tuned by the addition of $\mathrm{Au}$ or $\mathrm{Ir}$ adatoms in ultrahigh vacuum. A reproducible neutral-current Hall effect is found in both as-fabricated and adatom-decorated graphene. The Hall ang…

[Phys. Rev. B 92, 161411(R)] Published Mon Oct 26, 2015

[Phys. Rev. B 92, 159902] Published Mon Oct 19, 2015

We study tunneling of charge carriers in single- and bilayer graphene. We propose an explanation for nonzero “magic angles” with 100% transmission for the case of symmetric potential barrier, as well as for their almost-survival for slightly asymmetric barrier in the bilayer graphene known previousl…

[Phys. Rev. B 92, 165407] Published Fri Oct 09, 2015

We use an on-chip superconducting resonator as a sensitive meter to probe the properties of graphene double quantum dots at microwave frequencies. Specifically, we investigate the charge dephasing rates in a circuit quantum electrodynamics architecture. The dephasing rates strongly depend on the num…

[Phys. Rev. Lett. 115, 126804] Published Thu Sep 17, 2015

The electron-hole symmetry in graphene monolayer, which is analogous to the inherent symmetric structure between electrons and positrons of the Universe, plays a crucial role in the chirality and chiral tunneling of massless Dirac fermions. Here we demonstrate that both strain and charged-defect sca…

[Phys. Rev. B 92, 121405(R)] Published Mon Sep 14, 2015

A moderate in-plane magnetic field can reduce spin-related decoherence in epitaxial graphene.

[Phys. Rev. Lett. 115, 106602] Published Thu Sep 03, 2015

Using x-ray photoelectron spectroscopy, thermal desorption spectroscopy, and scanning tunneling microscopy, we show that upon keV ${\mathrm{Xe}}^{+}$ irradiation of graphene on Ir(111), Xe atoms are trapped under the graphene. Upon annealing, aggregation of Xe leads to graphene bulges and blisters. The efficient tr…

[Phys. Rev. B 92, 085429] Published Wed Aug 26, 2015

We consider a waveguide formed in a clean graphene monolayer by a spatially inhomogeneous magnetic field. The single-particle dispersion relation for this waveguide exhibits a zero-energy Landau-like flat band, while finite-energy bands have dispersion and correspond, in particular, to snake orbits.…

[Phys. Rev. B 92, 085422] Published Thu Aug 20, 2015

Nanoscale materials can be extremely strong compared to their bulk forms, but this strength can be accompanied by significant sample-to-sample fluctuations. The authors combine simulations and analytical theory to describe the size-dependent distribution of fracture strength in defective graphene flakes, highlighting their differences from systems of macroscopic extent. This understanding may be important for controlling thermally activated failure in nano- and micromechanical systems.

[Phys. Rev. Applied 4, 024011] Published Wed Aug 19, 2015

Non-Abelian gauge potentials are quite relevant in subatomic physics, but they are relatively rare in a condensed matter context. Here we report the experimental evidence for non-Abelian gauge potentials in twisted graphene bilayers by scanning tunneling microscopy and spectroscopy. At a magic twist…

[Phys. Rev. B 92, 081406(R)] Published Wed Aug 19, 2015

A wish list for a chemical sensor would include sensitivity, selectivity, and speed. The authors present a sensor design in which sample molecules adsorbed on graphene modify its conductivity, which in turn modifies the reflectance of a coupled terahertz resonator. By reading the resulting change in reflectance optically, the authors show that such a sensor can hit all three marks on the list, for example producing a sensitivity around 1 ppm for various molecules while detecting within a subwavelength layer.

[Phys. Rev. Applied 4, 024007] Published Tue Aug 11, 2015

The prototypical exciton model of two interacting Dirac particles in graphene was analyzed in J. Sabio et al., Phys. Rev. B 81, 045428 (2010) and it was found that in one of the electron-hole scattering channels the total kinetic energy vanishes, resulting in a singular behavior. We show that this …

[Phys. Rev. B 92, 085409] Published Fri Aug 07, 2015

We present a systematic study of the electronic, transport, and optical properties of disordered graphene, including the next-nearest-neighbor hopping. We show that this hopping has a nonnegligible effect on resonant scattering but is of minor importance for long-range disorder such as charged impur…

[Phys. Rev. B 92, 045437] Published Fri Jul 31, 2015

Retardation effects (REs) are known to cause a crossover from linear to sublinear behaviors in the dispersion relation of two-dimensional (2D) plasma waves at long wavelengths. In the present work, we systematically analyze REs on plasma waves in both 2D and 1D electron gases, and we clarify the exp…

[Phys. Rev. B 92, 045434] Published Tue Jul 28, 2015

We study the multicritical behavior for the semimetal-insulator transitions on graphene's honeycomb lattice using the Gross-Neveu-Yukawa effective theory with two order parameters: the SO(3) (Heisenberg) order parameter describes the antiferromagnetic transition, and the ${\mathbb{Z}}_2$ (Ising) order parameter d…

[Phys. Rev. B 92, 035429] Published Fri Jul 24, 2015

Single- and few-layer graphene have been used as electrodes in organic optoelectronic devices, however limited attention has been devoted to understanding and optimizing the transfer of excited electrons at the organic-graphene interface. The authors find that the charge-transfer rate depends on the number of graphene layers and their stacking, and can be explained by considering the electronic coupling between graphene layers. This work suggests clear ways to control the key process in this class of devices, which may see use as photodetectors or solar cells.

[Phys. Rev. Applied 4, 014016] Published Fri Jul 24, 2015

[Phys. Rev. Lett. 115, 049901] Published Wed Jul 22, 2015

Graphene’s remarkable properties include its outstanding thermal conductivity $\kappa$, and understanding heat flow at its interfaces is key to thermal management of graphene-based electronics. The authors study the ties between interfacial morphology and thermal conductivity and find that, rather than the magnitude of van der Waals interactions, it is uniformity (or lack thereof) in the contact-force distribution that primarily sets $\kappa$ in devices.

[Phys. Rev. Applied 4, 014006] Published Thu Jul 16, 2015

We calculate the spin-dependent zero-bias conductance $G_{\sigma {\sigma }^{\prime }}$ in armchair graphene nanoribbons with hydrogen adsorbates employing a DFT-based ab initio transport formalism including spin-orbit interaction. We find that the spin-flip conductance $G_{\sigma \overline{\sigma }}$ can reach the same order of magnitude as the spin-con…

[Phys. Rev. B 92, 014405] Published Mon Jul 06, 2015

We show that graphene possesses a strong nonlinear optical response in the form of multiplasmon absorption, with exciting implications in classical and quantum nonlinear optics. Specifically, we predict that graphene nanoribbons can be used as saturable absorbers with low saturation intensity in the…

[Phys. Rev. Lett. 114, 236801] Published Wed Jun 10, 2015

Interlayer coupling in graphene multilayers with certain commensurate twists is predicted to form a 2D topological insulator.

[Phys. Rev. Lett. 114, 226802] Published Tue Jun 02, 2015

This Rapid Communication reports on the modulation of the position and linewidth of the $G$ and $2D$ Raman bands for monolayer graphene through coupling to surface plasmon polaritons (SPPs). It is shown that gold nanoresonators tuned to the graphene Stokes emission frequency, not the laser pump, broaden…

[Phys. Rev. B 91, 201408(R)] Published Thu May 21, 2015

We present a detailed theoretical study of bilayer-graphene's electronic properties in the presence of electric and magnetic fields. Using group-theoretical methods, we derive an invariant expansion of the Hamiltonian for electron states near the $\mathrm{K}$ point of the Brillouin zone. In contrast to known m…

[Phys. Rev. B 91, 205312] Published Wed May 20, 2015

The discovery of light emission from metal-insulator-metal tunnel junctions in the 1970s suggested a low-energy, broadband source of visible light. Presently this technology is also of interest for generating subwavelength surface plasmons electrically. The authors show that inelastic electron tunneling excitation is potentially 10${}^5$ times as efficient for producing mid-infrared plasmons in graphene than in metal, offering great promise for on-chip integrated nanophotonics.

[Phys. Rev. Applied 3, 054001] Published Fri May 08, 2015

In ultrafast laser systems, graphene has become a popular saturable absorber to provide passive mode-locking with tunable wavelength. The authors elucidate the pulse width saturated mode-locking mechanism, focusing on the soliton compression that can be achieved by strengthening self-phase modulation in the regime of weakly negative group-delay dispersion. This mechanistic understanding allows for an optimized soliton pulse width of 500 femtoseconds or less from an erbium-doped fiber laser, for photonics and optoelectronics applications.

[Phys. Rev. Applied 3, 044016] Published Fri Apr 24, 2015

We propose a detailed experimental procedure for preparing relativistic vortices, governed by the nonlinear Dirac equation, in a two-dimensional Bose-Einstein condensate (BEC) in a honeycomb optical lattice. Our setup contains Dirac points, in direct analogy to graphene. We determine a range of prac…

[Phys. Rev. A 91, 043609] Published Thu Apr 09, 2015

A theoretical study of electronic interactions in graphene shows that a transverse component of valley conductivity emerges, while the longitudinal component cancels.

[Phys. Rev. X 5, 011040] Published Tue Mar 31, 2015

We modulate the atomic structure of bilayer graphene by driving its lattice at resonance with the in-plane $E_{1u}$ lattice vibration at $6.3\mu \mathrm{m}$. Using time- and angle-resolved photoemission spectroscopy (tr-ARPES) with extreme-ultraviolet (XUV) pulses, we measure the response of the Dirac electrons near…

[Phys. Rev. Lett. 114, 125503] Published Wed Mar 25, 2015

Previous studies of electron-phonon interaction in impure graphene have found that disorder can give rise to an enhancement of electronic cooling at high temperatures. We investigate the effect of lattice vacancy in both mono- and bilayer graphene and observe an order of magnitude suppression of ele…

[Phys. Rev. B 91, 121404(R)] Published Fri Mar 06, 2015

A monolayer of graphene irradiated with circularly polarized light suggests a unique platform for surface electromagnetic wave (plasmon-polariton) manipulation. In fact, the time periodicity of the Hamiltonian leads to a geometric Aharonov-Anandan phase and results in a photovoltaic Hall effect in g…

[Phys. Rev. B 91, 075419] Published Wed Feb 18, 2015

Graphene’s unique material properties enable the production of ultralightweight mechanical resonators that could yield significant progress in nanomechanical mass sensing and quantum optomechanics. The key to unlocking this potential is the precise measurement of a resonator’s oscillations. The authors use high-Q optical cavities to read out this motion with state-of-the-art precision and bandwidth.

[Phys. Rev. Applied 3, 024004] Published Tue Feb 17, 2015

Enhancement of the spin-orbit coupling in graphene may lead to various topological phenomena and also find applications in spintronics. Adatom adsorption has been proposed as an effective way to achieve the goal. In particular, great hope has been held for indium in strengthening the spin-orbit coup…

[Phys. Rev. B 91, 085411] Published Thu Feb 12, 2015

Graphene is a promising material for next-generation flexible electronic devices, but its behavior under real-world stresses is poorly understood. What if the graphene were to tear? Using in situ scanning probe microscopy and electrical transport measurements, the authors show that the deterioration of graphene’s mechanical and electrical properties due to strain-induced rips is in fact partially reversible. This limited self-healing presents exciting implications for device applications.

[Phys. Rev. Applied 3, 014010] Published Fri Jan 30, 2015

A simple means to convert waste heat into electrical energy is thermionic emission, in which the thermal energy of a hot cathode exceeds the work function for electrons to evaporate from its surface, for subsequent collection at a cold anode. Here thermionic emission from a monolayer of suspended graphene has been reconsidered, to account for the electrons in this material behaving like massless fermionic quasiparticles. The authors predict a scaling of current density as $T^3$, as opposed to the $T^2$ scaling predicted for metals by the traditional Richardson-Dushman equation. The efficiency of the authors’ proposed thermionic energy converter is about 45% at relatively low temperatures, with high current capability.

[Phys. Rev. Applied 3, 014002] Published Mon Jan 12, 2015

The cylindrical multishell structure is one of the prevalent atomic arrangements in nanowires. Being multishell, the well-defined atomic periodicity is hardly realized in it because the periodic units of individual shells therein generally do not match except for very few cases, posing a challenge t…

[Phys. Rev. B 91, 035405] Published Thu Jan 08, 2015

Electrons in graphene have four flavors associated with low-energy spin and valley degrees of freedom. The fractional quantum Hall effect in graphene is dominated by long-range Coulomb interactions, which are invariant under rotations in spin-valley space. This SU(4) symmetry is spontaneously broken…

[Phys. Rev. B 90, 235432] Published Mon Dec 22, 2014

An outstanding and fundamental problem in contemporary physics is to include and probe the many-body effect in the study of relativistic quantum manifestations of classical chaos. We address this problem using graphene systems described by the Hubbard Hamiltonian in the setting of resonant tunneling…

[Phys. Rev. B 90, 224301] Published Tue Dec 16, 2014

Mesoscopic graphene devices often exhibit complex transport properties, stemming both from the peculiar electronic band structure of graphene and from the high sensitivity of transport to local disorder in this two-dimensional crystal. To disentangle contributions of disorder in the different transp…

[Phys. Rev. B 90, 205433] Published Tue Nov 25, 2014

We show that the Landau levels in epitaxial graphene in the presence of localized defects are significantly modified compared to those of an ideal system. We report on magnetospectroscopy experiments performed on high-quality samples. Besides typical interband magneto-optical transitions, we clearly…

[Phys. Rev. B 90, 195433] Published Thu Nov 20, 2014

In the Comment, Cooper et al. pointed out a disagreement between two recent density functional theory (DFT) studies ([Li, Phys. Rev. B 85, 235407 (2012)] and [Cooper et al., Phys. Rev. B 87, 035423 (2013)]) on the uniaxial tensile stress as a function of the applied strain along the zigzag $\left(x\right)$ and …

[Phys. Rev. B 90, 167402] Published Thu Oct 30, 2014

Futuristic electronic devices will rely on high electron speeds in graphene. A new investigation shows that random strain in the carbon honeycomb lattice limits the speed of electrons.

[Phys. Rev. X 4, 041019] Published Thu Oct 30, 2014

Li's paper [Phys. Rev. B 85, 235407 (2012)] presents density functional theory (DFT) results of stress as a function of different strain states. The work of Cooper et al. [Phys. Rev. B 87, 035423 (2013)] performs the same DFT calculations as part of an investigation into the nonlinear elastic proper…

[Phys. Rev. B 90, 167401] Published Thu Oct 30, 2014

Spintronic devices based on graphene are keenly anticipated for a new generation of technologies operating at higher speeds and smaller scales. Unfortunately, the low efficiency of spin injection from ferromagnetic electrodes into the carbon sheet poses a formidable challenge. Inspired by tunneling transport in everyday semiconductors, the authors calculate that a barrier layer of hexagonal boron nitride, unlike other materials, suppresses graphene’s minority spin channel. This reduces the spin-conductance mismatch between electrodes and graphene, enabling high spin polarization for efficient injection.

[Phys. Rev. Applied 2, 044008] Published Thu Oct 16, 2014

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 zeroth pLL displays a twofold increased degeneracy, while the characteristic sublattice polarization of the z…

[Phys. Rev. B 90, 155418] Published Fri Oct 10, 2014

Exotic properties of topological insulators (TIs) and graphene have captivated many condensed matter physicists, but are practical outcomes actually within reach? The authors propose an efficient, beyond-CMOS spin logic platform exploiting the strong exchange coupling between a ferromagnet and the Dirac fermion states of a TI and of graphene. A detailed theoretical analysis illustrates the desired ultralow-power performance under realistic conditions, leading to predictions for practical devices to be controlled by signals as small as atto- (10${}^{-18}$) Joules!

[Phys. Rev. Applied 2, 044003] Published Thu Oct 09, 2014

An investigation of the light scattering properties of monolayer graphene in the near-field regime finds, surprisingly, that inelastic scattering on the nanoscale is a partially coherent process.

[Phys. Rev. X 4, 031054] Published Fri Sep 26, 2014

We present ab initio calculations of the local current density $\mathbf{j}(\mathbf{r})$ as it arises in dc-transport measurements. We discover pronounced patterns in the local current density, ring currents (“eddies”), that go along with orbital magnetism. Importantly, the magnitude of the ring currents can exceed the …

[Phys. Rev. Lett. 113, 136602] Published Fri Sep 26, 2014

We report on the observation of strong backscattering of charge carriers in the quantum Hall regime of polycrystalline graphene, grown by chemical vapor deposition, which alters the accuracy of the Hall resistance quantization. The temperature and magnetic field dependence of the longitudinal conduc…

[Phys. Rev. B 90, 115422] Published Thu Sep 18, 2014

How the long-range ordering and local defect configurations modify the electronic structure of graphene remains an outstanding problem in nanoscience, which precludes the practical method of patterning graphene from being widely adopted for making graphene-based electronic and optoelectronic devices…

[Phys. Rev. B 90, 115415] Published Fri Sep 12, 2014

Thermophotovoltaic devices convert heat to electricity, and are valuable for both solar applications and waste-heat recovery. In this work, the authors show that a graphene-on-silicon Schottky photodiode both dramatically increases radiative heat transfer due to the materials’ plasmonic properties, and bypasses the need for $p-n$ junctions as are used in traditional semiconductors, making the device cheap and simple.

[Phys. Rev. Applied 2, 034006] Published Thu Sep 11, 2014

The nature of localized edge states in edge-disordered graphene nanoconstrictions connected to external leads is studied. Contrary to the general belief, the results show that the localization length is significantly longer than the size of the nanoconstriction, which results into charge spill over into the bulk as well.

[Phys. Rev. B 90, 115405] Published Thu Sep 04, 2014