Photovoltaic efficiency in solar cells is hindered by many unwanted effects. Radiative channels (emission of photons) sometimes mediated by non-radiative ones (emission of phonons) are principally responsible for the decrease in exciton population before charge separation can take place. Another unwanted effect is electron-hole recombination at surfaces where in-gap edge states serve as the non-radiative channels. Topological insulators (TIs), in particular the most common ones characterized by a Z2 invariant, are rarely explored from an optoelectronics standpoint, possibly because of their typically small gaps. Otherwise, they are not much different from small-gap semiconductors and excitons are also expected to be their simplest excitations. Here we show that one can take advantage of the non-radiative decay channel due to the surface states to generate a topologically protected photovoltaic current. Focusing on two-dimensional TIs, and specifically for illustration purposes on a Bi(111) monolayer, we show the potential of TI nanoribbons to generate edge charge accumulation and edge currents under illumination.
Silicene is a two-dimensional quantum spin-Hall insulator. We study the edge channels of silicene nanoribbons from the viewpoint of the topological protection and the interference between the two edges. It is found that the behaviors of the helical edge channels (HECs) are completely different between the armchair and zigzag edges. The penetration depths $ξ$ of the HEC is antiproportional to the spin-orbit gap for the armchair edge $ξ_{\rm arm} \sim \hbar v_\text{F}/ Δ$ ($v_\text{F}$: the Fermi velocity, $Δ$: the gap due to the spin-orbit interaction), while it remains as short as the lattice …
Excitonic emission from atomically precise graphene nanoribbons (GNRs) synthesised on a metal surface is probed with atomic-scale spatial resolution using a scanning tunneling microscopy (STM) approach. A STM-based strategy to transfer the GNRs to a partially insulating surface is used to prevent light emission quenching of the ribbons by the metal substrate. Sub-nanometer resolved STM-induced fluorescence spectra reveal emission from localized dark excitons build upon the topological end states of the GNRs. A low frequency vibronic emission comb whose characteristics change with the GNR lengt…
We numerically compute the density of states (DOS) of interacting disordered zigzag graphene nanoribbon (ZGNR) having midgap states showing $e/2$ fractional edge charges. The computed Hartree-Fock DOS is linear at the critical disorder strength where the gap vanishes. This implies an $I\mbox{-}V$ curve of $I\propto V^2$. Thus, $I\mbox{-}V$ curve measurement may yield evidence of fractional charges in interacting disordered ZGNR. We show that even a weak disorder potential acts as a singular perturbation on zigzag edge electronic states, producing drastic changes in the energy spectrum. Spin-ch…
An intense laser field in the high-frequency regime drives carriers in graphene nanoribbons (GNRs) out of equilibrium and creates topologically-protected edge states. Using Floquet theory on driven GNRs, we calculate the time evolution of local excitations of these edge states and show that they exhibit a robust dynamics also in the presence of very localized lattice defects (atomic vacancies), which is characteristic of topologically non-trivial behavior. We show how it is possible to control them by a modulated electrostatic potential: They can be fully transmitted on the same edge, reflecte…
Silicene (a monolayer of silicon atoms) is a quantum spin-Hall insulator, which undergoes a topological phase transition into other insulators by applying external field such as electric field, photo-irradiation and antiferromagnetic order. We investigate the electronic and transport properties of silicene nanoribbons based on the Landauer formalism. We propose to determine topological phase transitions by measuring the density of states and conductance. The conductance is quantized and changes its value when the system transforms into different phases. We show that a silicene nanoribbon near …
Previously, graphene nanoribbons set in lateral heterostructures with hexagonal boron nitride were predicted to support topologically protected states at low energy. We investigate how robust the transport properties of these states are against lattice disorder. We find that forms of disorder that do not couple the two valleys of the zigzag graphene nanoribbon do not impact the transport properties at low bias, indicating that these lateral heterostructures are very promising candidates for chip-scale conducting interconnects. Forms of disorder that do couple the two valleys, such as vacancies…
A simple surface band structure and a large bulk band gap have allowed Bi2Se3 to become a reference material for the newly discovered three-dimensional topological insulators, which exhibit topologically-protected conducting surface states that reside inside the bulk band gap. Studying topological insulators such as Bi2Se3 in nanostructures is advantageous because of the high surface-to-volume ratio, which enhances effects from the surface states; recently reported Aharonov-Bohm oscillation in topological insulator nanoribbons by some of us is a good example. Theoretically, introducing magneti…
A WSe2 monolayer of 1T' phase is a large band gap quantum spin Hall insulator, supporting dissipationless charge and spin transports through the topologically protected edge states. In this work, we explore the nanoribbon forms of 1T' phase WSe2 by first-principles density functional calculations and the many-body perturbation GW and Bethe-Salpeter equation method. We found that the 1T' WSe2 nanoribbon can show topological edge states with a ribbon width of ~4-6 nm. Those edge bands show crossing through the Fermi level an odd number of times, with one kind of spin-polarization connecting the …
We study the charge and spin transport in a two terminal graphene nanoribbon (GNR) decorated with random Gold (Au) adatoms using a Kane-Mele model. Two commonly used GNRs, that is, the armchair graphene nanoribbon (AGNR) and the zigzag graphene nanoribbon (ZGNR) are compared and contrasted, which shows that in presence of Au adatoms, a somewhat robust $2e^2/h$ conductance plateau occurs in the case of AGNR around the zero of the Fermi energy, while in ZGNR this plateau is fragile. We show that this flat plateau, having a conductance value $2e^2/h$, is not a hallmark signature of a topologicall…
In topological systems, a modulation in the gap onset near interfaces can lead to the appearance of massive edge states, as were first described by Volkov and Pankratov. In this work, we study graphene nanoribbons in the presence of intrinsic spin-orbit coupling smoothly modulated near the system edges. We show that this space modulation leads to the appearance of Volkov-Pankratov states, in addition to the topologically protected ones. We obtain this result by means of two complementary methods, one based on the effective low-energy Dirac equation description and the other on a fully numerica…
Mean-field theories have since long predicted edge magnetism in graphene nanoribbons, where the order parameter is given by the local magnetization. However, signatures of edge magnetism appears elusive in the experiments, suggesting another class of solutions. We employ a self-consistent mean field approximation within a multi-orbital tight-binding model and obtain particle-hole symmetry broken solutions, where the local filling plays the role of the order parameter. Unlike the magnetic edge solutions, these are topologically non-trivial and show zero local magnetization. A small and a large …
The topological properties of electronic edge states in time-periodically driven spatially-periodic corrugated zigzag graphene nanoribbons are studied. An effective one-dimensional Hamiltonian is used to describe the electronic properties of graphene and the time-dependence is studied within the Floquet formalism. Then the quasienergy spectrum of the evolution operator is obtained using analytical and numeric calculations, both in excellent agreement. Depending on the external parameters of the time-driving, two different kinds (type I and type II) of touching band points are found, which have…
We report the emergence of electronic edge states in time-periodically driven strained armchair terminated graphene nanoribbons. This is done by considering a short-pulse spatial-periodic strain field. Then, the tight-binding Hamiltonian of the system is mapped into a one dimensional ladder. The time periodicity is considered within the Floquet formalism. Thus the quasienergy spectrum is found numerically by diagonalizing the evolution operator. For some particular cases, the quasienergy spectrum is found analytically. We find that the system is able to support gapless and gapped phases. Very …
For most chiralities, semiconducting nanotubes display topologically protected end states of multiple degeneracies. We demonstrate using density matrix renormalization group based quantum chemistry tools that the presence of Coulomb interactions induces the formation of robust end spins. These are the close analogues of ferromagnetic edge states emerging in graphene nanoribbons. The interaction between the two ends is sensitive to the length of the nanotube, its dielectric constant, as well as the size of the end spins: for $S=1/2$ end spins their interaction is antiferromagnetic, while for $S…
Precise control over the size and shape of graphene nanostructures allows engineering spin-polarized edge and topological states, representing a novel source of non-conventional $π$-magnetism with promising applications in quantum spintronics. A prerequisite for their emergence is the existence of robust gapped phases, which are difficult to find in extended graphene systems: only armchair graphene nanoribbons (GNRs) show a band gap that, however, closes for any other GNR orientation. Here we show that semi-metallic chiral GNRs (chGNRs) narrowed down to nanometer widths undergoes a topological…
We study the emergence of electronic non-trivial topological flat bands in time-periodically driven strained graphene within a tight binding approach based on the Floquet formalism. In particular, we focus on uniaxial spatially periodic strain since it can be mapped onto an effective one-dimensional system. Also, two kinds of time-periodic driving are considered: a short pulse (delta kicking) and a sinusoidal variation (harmonic driving). We prove that for special strain wavelengths, the system is described by a two level Dirac Hamiltonian. Even though the study case is gapless, we find that t…
