Showing papers by "K. S. Novoselov published in 2016"

Quantum oscillations of the critical current and high-field superconducting proximity in ballistic graphene

Abstract: Josephson junctions based on graphene exhibit tunable proximity effects. The appearance of superconducting states when changing magnetic field and carrier concentration has now been investigated—some proximity effect survives for fields above 1 T.

Exciton and trion dynamics in atomically thin MoSe2 and WSe2: Effect of localization

Abstract: We present a detailed investigation of the exciton and trion dynamics in naturally doped ${\mathrm{MoSe}}_{2}$ and ${\mathrm{WSe}}_{2}$ single atomic layers as a function of temperature in the range 10--300 K under above band-gap laser excitation. By combining time-integrated and time-resolved photoluminescence (PL) spectroscopy, we show the importance of exciton and trion localization in both materials at low temperatures. We also reveal the transition to delocalized exciton complexes at higher temperatures where the exciton and trion thermal energy exceeds the typical localization energy. This is accompanied by strong changes in PL including suppression of the trion PL and decrease of the trion PL lifetime, as well as significant changes for neutral excitons in the temperature dependence of the PL intensity and the appearance of a pronounced slow PL decay component. In ${\mathrm{MoSe}}_{2}$ and ${\mathrm{WSe}}_{2}$ studied here, the temperatures where such strong changes occur are observed at around 100 and 200 K, respectively, in agreement with their inhomogeneous PL linewidth of 8 and 20 meV at $T\ensuremath{\approx}10\phantom{\rule{4.pt}{0ex}}\text{K}$. The observed behavior is a result of a complex interplay between influences of the specific energy ordering of bright and dark excitons in ${\mathrm{MoSe}}_{2}$ and ${\mathrm{WSe}}_{2}$, sample doping, trion, and exciton localization and various temperature-dependent nonradiative processes.

Macroscopic self-reorientation of interacting two-dimensional crystals

Abstract: Microelectromechanical systems, which can be moved or rotated with nanometre precision, already find applications in such fields as radio-frequency electronics, micro-attenuators, sensors and many others. Especially interesting are those which allow fine control over the motion on the atomic scale because of self-alignment mechanisms and forces acting on the atomic level. Such machines can produce well-controlled movements as a reaction to small changes of the external parameters. Here we demonstrate that, for the system of graphene on hexagonal boron nitride, the interplay between the van der Waals and elastic energies results in graphene mechanically self-rotating towards the hexagonal boron nitride crystallographic directions. Such rotation is macroscopic (for graphene flakes of tens of micrometres the tangential movement can be on hundreds of nanometres) and can be used for reproducible manufacturing of aligned van der Waals heterostructures.

Phonon-Assisted Resonant Tunneling of Electrons in Graphene-Boron Nitride Transistors

Abstract: We observe a series of sharp resonant features in the differential conductance of graphene-hexagonal boron nitride-graphene tunnel transistors over a wide range of bias voltages between 10 and 200 mV. We attribute them to electron tunneling assisted by the emission of phonons of well-defined energy. The bias voltages at which they occur are insensitive to the applied gate voltage and hence independent of the carrier densities in the graphene electrodes, so plasmonic effects can be ruled out. The phonon energies corresponding to the resonances are compared with the lattice dispersion curves of graphene-boron nitride heterostructures and are close to peaks in the single phonon density of states.

Tuning the valley and chiral quantum state of Dirac electrons in van der Waals heterostructures

Abstract: Chirality is a fundamental property of electrons with the relativistic spectrum found in graphene and topological insulators. It plays a crucial role in relativistic phenomena, such as Klein tunneling, but it is difficult to visualize directly. Here, we report the direct observation and manipulation of chirality and pseudospin polarization in the tunneling of electrons between two almost perfectly aligned graphene crystals. We use a strong in-plane magnetic field as a tool to resolve the contributions of the chiral electronic states that have a phase difference between the two components of their vector wave function. Our experiments not only shed light on chirality, but also demonstrate a technique for preparing graphene’s Dirac electrons in a particular quantum chiral state in a selected valley.

Valley addressable exciton-polaritons in atomically thin semiconductors

Abstract: While conventional semiconductor technology relies on the manipulation of electrical charge for the implementation of computational logic, additional degrees of freedom such as spin and valley offer alternative avenues for the encoding of information. In transition metal dichalcogenide (TMD) monolayers, where spin-valley locking is present, strong retention of valley chirality has been reported for MoS$_2$, WSe$_2$ and WS$_2$ while MoSe$_2$ shows anomalously low valley polarisation retention. In this work, chiral selectivity of MoSe$_2$ cavity polaritons under helical excitation is reported with a polarisation degree that can be controlled by the exciton-cavity detuning. In contrast to the very low circular polarisation degrees seen in MoSe$_2$ exciton and trion resonances, we observe a significant enhancement of up to 7 times when in the polaritonic regime. Here, polaritons introduce a fast decay mechanism which inhibits full valley pseudospin relaxation and thus allows for increased retention of injected polarisation in the emitted light. A dynamical model applicable to cavity-polaritons in any TMD semiconductor, reproduces the detuning dependence through the incorporation of the cavity-modified exciton relaxation, allowing an estimate of the spin relaxation time in MoSe$_2$ which is an order of magnitude faster than those reported in other TMDs. The valley addressable exciton-polaritons reported here offer robust valley polarised states demonstrating the prospect of valleytronic devices based upon TMDs embedded in photonic structures, with significant potential for valley-dependent nonlinear polariton-polariton interactions.