Electronic excitation spectrum of metallic carbon nanotubes

We have studied the discrete electronic spectrum of closed metallic nanotube quantum dots. At low temperatures, the stability diagrams show a very regular four-fold pattern that allows for the determination of the electron addition and excitation energies. The measured nanotube spectra are in excellent agreement with theoretical predictions based on the nanotube band structure. Our results permit the complete identification of the electron quantum states in nanotube quantum dots.Comment: 4 pages, 3 figure

Electronic Transport Spectroscopy of Carbon Nanotubes in a Magnetic Field

We report magnetic field spectroscopy measurements in carbon nanotube quantum dots exhibiting four-fold shell structure in the energy level spectrum. The magnetic field induces a large splitting between the two orbital states of each shell, demonstrating their opposite magnetic moment and determining transitions in the spin and orbital configuration of the quantum dot ground state. We use inelastic cotunneling spectroscopy to accurately resolve the spin and orbital contributions to the magnetic moment. A small coupling is found between orbitals with opposite magnetic moment leading to anticrossing behavior at zero field.Comment: 7 pages, 4 figure

Josephson current through a single Anderson impurity coupled to BCS leads

We investigate the Josephson current J(\phi) through a quantum dot embedded between two superconductors showing a phase difference \phi. The system is modeled as a single Anderson impurity coupled to BCS leads, and the functional and the numerical renormalization group frameworks are employed to treat the local Coulomb interaction U. We reestablish the picture of a quantum phase transition occurring if the ratio between the Kondo temperature T_K and the superconducting energy gap \Delta or, at appropriate T_K/\Delta, the phase difference \phi or the impurity energy is varied. We present accurate zero- as well as finite-temperature T data for the current itself, thereby settling a dispute raised about its magnitude. For small to intermediate U and at T=0 the truncated functional renormalization group is demonstrated to produce reliable results without the need to implement demanding numerics. It thus provides a tool to extract characteristics from experimental current-voltage measurements.Comment: version accepted for publication in PR

Las microexposiciones del parque de las ciencias de Granada. otra forma de enseñar y aprender ciencias

Los museos de Ciencias tienen un papel importante en la tarea de proporcionar a estudiantes y personas adultas una formación científica básica para desenvolverse en el mundo actual. El Parque de las Ciencias de Granada, en colaboración con la Consejería de Educación de la Junta de Andalucía, elabora desde hace dos años una serie de microexposiciones que presta a los centros educativos para que sea un grupo de estudiantes, con su profesor, quien se responsabilice de organizarlas y presentarlas en el centro. El alumnado tiene así ocasión de aprender ciencias de otra forma, investigando y enseñando a los demás. En este trabajo se describen las características de las microexposiciones, especialmente de la dedicada a Faraday, las actividades paralelas organizadas por los centros al presentarla y la valoración que hace el profesorado de la experiencia

Las microexposiciones del parque de las ciencias de Granada. otra forma de enseñar y aprender ciencias

Los museos de Ciencias tienen un papel importante en la tarea de proporcionar a estudiantes y personas adultas una formación científica básica para desenvolverse en el mundo actual. El Parque de las Ciencias de Granada, en colaboración con la Consejería de Educación de la Junta de Andalucía, elabora desde hace dos años una serie de microexposiciones que presta a los centros educativos para que sea un grupo de estudiantes, con su profesor, quien se responsabilice de organizarlas y presentarlas en el centro. El alumnado tiene así ocasión de aprender ciencias de otra forma, investigando y enseñando a los demás. En este trabajo se describen las características de las microexposiciones, especialmente de la dedicada a Faraday, las actividades paralelas organizadas por los centros al presentarla y la valoración que hace el profesorado de la experiencia

Observation of suppressed terahertz absorption in photoexcited graphene

When light is absorbed by a semiconductor, photoexcited charge carriers enhance the absorption of far-infrared radiation due to intraband transitions. We observe the opposite behavior in monolayer graphene, a zero-gap semiconductor with linear dispersion. By using time domain terahertz (THz) spectroscopy in conjunction with optical pump excitation, we observe a reduced absorption of THz radiation in photoexcited graphene. The measured spectral shape of the differential optical conductivity exhibits non-Drude behavior. We discuss several possible mechanisms that contribute to the observed low-frequency non-equilibrium optical response of graphene.United States. Dept. of Energy. Office of Basic Energy Sciences (Grant DE-SC0006423)National Science Foundation (U.S.). Graduate Research Fellowship ProgramUnited States. Air Force Office of Scientific ResearchUnited States. Office of Naval Research. Multidisciplinary University Research Initiative. Graphene Approaches to Terahertz ElectronicsNational Science Foundation (U.S.) (Award DMR-0819762)National Science Foundation (U.S.) (Grant ECS-0335765

Coupling carbon nanotube mechanics to a superconducting circuit

The quantum behaviour of mechanical resonators is a new and emerging field driven by recent experiments reaching the quantum ground state. The high frequency, small mass, and large quality-factor of carbon nanotube resonators make them attractive for quantum nanomechanical applications. A common element in experiments achieving the resonator ground state is a second quantum system, such as coherent photons or superconducting device, coupled to the resonators motion. For nanotubes, however, this is a challenge due to their small size. Here, we couple a carbon nanoelectromechanical (NEMS) device to a superconducting circuit. Suspended carbon nanotubes act as both superconducting junctions and moving elements in a Superconducting Quantum Interference Device (SQUID). We observe a strong modulation of the flux through the SQUID from displacements of the nanotube. Incorporating this SQUID into superconducting resonators and qubits should enable the detection and manipulation of nanotube mechanical quantum states at the single-phonon level

Defining and controlling double quantum dots in single-walled carbon nanotubes

We report the experimental realization of double quantum dots in single-walled carbon nanotubes. The device consists of a nanotube with source and drain contact, and three additional top-gate electrodes in between. We show that, by energizing these top-gates, it is possible to locally gate a nanotube, to create a barrier, or to tune the chemical potential of a part of the nanotube. At low temperatures we find (for three different devices) that in certain ranges of top-gate voltages our device acts as a double quantum dot, evidenced by the typical honeycomb charge stability pattern.Comment: 9 pages, 3 figure

Noisy Kondo impurities

The anti-ferromagnetic coupling of a magnetic impurity carrying a spin with the conduction electrons spins of a host metal is the basic mechanism responsible for the increase of the resistance of an alloy such as Cu${}_{0.998}$Fe${}_{0.002}$ at low temperature, as originally suggested by Kondo . This coupling has emerged as a very generic property of localized electronic states coupled to a continuum . The possibility to design artificial controllable magnetic impurities in nanoscopic conductors has opened a path to study this many body phenomenon in unusual situations as compared to the initial one and, in particular, in out of equilibrium situations. So far, measurements have focused on the average current. Here, we report on \textit{current fluctuations} (noise) measurements in artificial Kondo impurities made in carbon nanotube devices. We find a striking enhancement of the current noise within the Kondo resonance, in contradiction with simple non-interacting theories. Our findings provide a test bench for one of the most important many-body theories of condensed matter in out of equilibrium situations and shed light on the noise properties of highly conductive molecular devices.Comment: minor differences with published versio