9 research outputs found

    Electrostatically confined monolayer graphene quantum dots with orbital and valley splittings

    Full text link
    The electrostatic confinement of massless charge carriers is hampered by Klein tunneling. Circumventing this problem in graphene mainly relies on carving out nanostructures or applying electric displacement fields to open a band gap in bilayer graphene. So far, these approaches suffer from edge disorder or insufficiently controlled localization of electrons. Here we realize an alternative strategy in monolayer graphene, by combining a homogeneous magnetic field and electrostatic confinement. Using the tip of a scanning tunneling microscope, we induce a confining potential in the Landau gaps of bulk graphene without the need for physical edges. Gating the localized states towards the Fermi energy leads to regular charging sequences with more than 40 Coulomb peaks exhibiting typical addition energies of 7-20 meV. Orbital splittings of 4-10 meV and a valley splitting of about 3 meV for the first orbital state can be deduced. These experimental observations are quantitatively reproduced by tight binding calculations, which include the interactions of the graphene with the aligned hexagonal boron nitride substrate. The demonstrated confinement approach appears suitable to create quantum dots with well-defined wave function properties beyond the reach of traditional techniques

    Large tunable valley splitting in edge-free graphene quantum dots on boron nitride

    Full text link
    Coherent manipulation of binary degrees of freedom is at the heart of modern quantum technologies. Graphene offers two binary degrees: the electron spin and the valley. Efficient spin control has been demonstrated in many solid state systems, while exploitation of the valley has only recently been started, yet without control on the single electron level. Here, we show that van-der Waals stacking of graphene onto hexagonal boron nitride offers a natural platform for valley control. We use a graphene quantum dot induced by the tip of a scanning tunneling microscope and demonstrate valley splitting that is tunable from -5 to +10 meV (including valley inversion) by sub-10-nm displacements of the quantum dot position. This boosts the range of controlled valley splitting by about one order of magnitude. The tunable inversion of spin and valley states should enable coherent superposition of these degrees of freedom as a first step towards graphene-based qubits

    Electronic and optical properties of graphene and large-scale graphene nanodevices

    No full text
    Zusammenfassung in deutscher SpracheParallelt. [Übers. des Autors]: Elektronische und optische Eigenschaften von Graphen und Graphen-NanodevicesGraphen, eine Monolage von Kohlenstoffatomen mit der Gitterstruktur einer Bienenwabe, hat außergewöhnliche elektronische Eigenschaften, die es für zukünftiger Elektronikanwendungen sehr interessant machen. Allerdings hat Graphen keine Bandlücke, die für die Anwendung in logischen Schaltungen entscheidend ist. Elektronentransport durch Graphen ist sehr empfindlich gegenüber Defekten der Kristallstruktur am Rand und im Inneren der Monolage. Diese reduzieren die Mobilität der Elektronen. Neue Substate, wie hexagonales Bornitrid, reduzieren die Anzahl der Defekte im Inneren von Graphen und induzieren auch eine Bandlücke von etwa 40 meV. Verbesserte Probenqualität und optimierte Substrate verringern Defecte im Inneren. Daher dominieren nun Randdefekte den Elektronentransport. Diese Arbeit zielt darauf ab, realistische Graphen-Nanostructuren zu simulieren und theoretische Studien aktueller Experimente mit Graphen bereitzustellen. Wir untersuchen: (i) Eigenschaften des Elektronentransports durch Graphen-Nanostructuren; (ii) elektronische und optische Eigenschaften von Graphen auf hexagonalem Bornitrid; und (iii) nicht-lineare optische Eigenschaften von Graphen. Insbesondere finden wir, dass die Leitfähigkeit von Graphen-Nanostructuren die Physik an den Rändern der Strukturen widerspiegelt. Durch Messung der Leitfähigkeit kann die Zustandsdichte lokalisierter Randzustände gefunden werden. Wir zeigen auch, dass neue Substrate die Bandstruktur von Graphen ändern können. Die Substrate führen zu einer kleinen Bandlücke am Diracpunkt und zu Mini-Bandlücken oberhalb und unterhalb des Diracpunkts. Die Zustandsdichte von Graphen im Magnetfeld mit einem zusätzlichen Substratpotenzial kann durch magneto-optische Spectroskopie analysiert werden. Darüberhinaus zeigt Graphen starke nichtlineare Effekte im THz Bereich aufgrund seiner linearen Energiedispersion. Diese Eigenschaften haben eine große Bedeutung für den Bau von THz Lasern und Detektoren. Wir zeigen, dass Graphen unter Belichtung mit THz Laserpulsen high-harmonic (HHG) Strahlung bilden kann. Die HHG Spektren von Graphen ähneln deren in Gasen.Graphene, a one-atom thin honeycomb lattice of carbon, has exceptional electronic properties making it a prime candidate for future electronic applications. However, graphene has no band gap which is essential for building logical circuits, and electronic transport is highly sensitive to the edge or (substrate-induced) bulk disorder reducing carrier mobility. In an attempt to overcome these issues, new substrates such as hexagonal boron nitride have proven to reduce the bulk disorder in graphene and even to open a small band gap of 40 meV. Although new substrates help to reduce bulk disorder, electronic transport is still affected by edge roughness and the surrounding chemical environment. The thesis aims to simulate realistic graphene devices and to provide a theoretical study of several recent experiments performed with graphene. It addresses: (i) electron transport properties of graphene nanoconstrictions; (ii) electronic and optical properties of graphene on hexagonal boron nitride; and (iii) the nonlinear optical response of graphene. In particular, we predict that the conductance of small graphene nanodevices can probe the physics at the edges of the device by extracting the density of localized or trapped edge states from the conductance trace measurements. We also show that new substrates may modify the bandstructure of graphene by opening a small band gap and by creating mini-gaps above and below the Dirac cone. The density of states of graphene with an additional substrate potential in the magnetic field can be probed by optical magneto-spectroscopy. Furthermore, due to its linear energy dispersion, graphene demonstrates strong nonlinear response in the THz range highlighting its importance for building THz lasers and detectors. We also prove that graphene can form high-harmonic generation (HHG) spectra under the application of THz laser pulses similar to the HHG in gases.13

    Classical and Quantum-mechanical Description of the Arnold Diffusion in a System with 2.5 Degrees of Freedom

    Get PDF
    We study a universal phenomenon of nonlinear dynamics the Arnol'd Diffusion in a model system with 2.5 degrees of freedom. In the model an influence of three main resonances which take place in a phase space of the system is considered. The results obtained during classical and quantum-mechanical observation are compared. It was shown that a dependence of a rate of the quantum Arnol'd diffusion on parameters of the model behave alike classical one, however a value of the diffusion rate using methods of quantum mechanics lesser then that in classical case approximately at one of the order. It was found that presence of a threshold by the perturbation parameters is not necessarily feature of the Arnol'd diffusion. Also it was shown that there can occur a hybrid process in the quantum system in weak chaotic regime what doesn't have classical analogue diffusion along resonance plus oscillations across overlapped resonances.Работа посвящена изучению универсального явления нелинейной динамики - диффузии Арнольда в модельной системе с 2.5 степенями свободы. При учете влияния лишь трех основных резонансов, действующих в фазовом пространстве, проводится сопоставление результатов, полученных при классическом и квантово-механическом подходах. Показано, что зависимость скорости квантовой диффузии Арнольда от параметров задачи имеет характер, подобный классическому, при этом по абсолютной величине коэффициент квантовой диффузии отстает примерно на порядок. Обнаружено, что наличие порога по параметрам возмущения, на который указывалось ранее, не является обязательной чертой квантовой диффузии Арнольда. Также в работе показано, что в квантовой системе в режиме слабого перекрытия резонансов возможно и протекание гибридного процесса, не имеющего классического аналога, диффузии вдоль резонанса, на которую накладывается процесс колебаний поперек перекрывшихся резонансов

    Classical and Quantum-mechanical Description of the Arnold Diffusion in a System with 2.5 Degrees of Freedom

    No full text
    We study a universal phenomenon of nonlinear dynamics the Arnol'd Diffusion in a model system with 2.5 degrees of freedom. In the model an influence of three main resonances which take place in a phase space of the system is considered. The results obtained during classical and quantum-mechanical observation are compared. It was shown that a dependence of a rate of the quantum Arnol'd diffusion on parameters of the model behave alike classical one, however a value of the diffusion rate using methods of quantum mechanics lesser then that in classical case approximately at one of the order. It was found that presence of a threshold by the perturbation parameters is not necessarily feature of the Arnol'd diffusion. Also it was shown that there can occur a hybrid process in the quantum system in weak chaotic regime what doesn't have classical analogue diffusion along resonance plus oscillations across overlapped resonances.Работа посвящена изучению универсального явления нелинейной динамики - диффузии Арнольда в модельной системе с 2.5 степенями свободы. При учете влияния лишь трех основных резонансов, действующих в фазовом пространстве, проводится сопоставление результатов, полученных при классическом и квантово-механическом подходах. Показано, что зависимость скорости квантовой диффузии Арнольда от параметров задачи имеет характер, подобный классическому, при этом по абсолютной величине коэффициент квантовой диффузии отстает примерно на порядок. Обнаружено, что наличие порога по параметрам возмущения, на который указывалось ранее, не является обязательной чертой квантовой диффузии Арнольда. Также в работе показано, что в квантовой системе в режиме слабого перекрытия резонансов возможно и протекание гибридного процесса, не имеющего классического аналога, диффузии вдоль резонанса, на которую накладывается процесс колебаний поперек перекрывшихся резонансов
    corecore