P. E. Lindelof

Kondo physics in carbon nanotubes

The connection of electrical leads to wire-like molecules is a logical step in the development of molecular electronics, but also allows studies of fundamental physics. For example, metallic carbon nanotubes are quantum wires that have been found to act as one-dimensional quantum dots, Luttinger liquids, proximity-induced superconductors and ballistic and diffusive one-dimensional metals. Here we report that electrically contacted single-walled carbon nanotubes can serve as powerful probes of Kondo physics, demonstrating the universality of the Kondo effect. Arising in the prototypical case from the interaction between a localized impurity magnetic moment and delocalized electrons in a metallic host, the Kondo effect has been used to explain enhanced low-temperature scattering from magnetic impurities in metals, and also occurs in transport through semiconductor quantum dots. The far greater tunability of dots (in our case, nanotubes) compared with atomic impurities renders new classes of Kondo-like effects accessible. Our nanotube devices differ from previous systems in which Kondo effects have been observed, in that they are one-dimensional quantum dots with three-dimensional metal (gold) reservoirs. This allows us to observe Kondo resonances for very large electron numbers (N) in the dot, and approaching the unitary limit (where the transmission reaches its maximum possible value). Moreover, we detect a previously unobserved Kondo effect, occurring for even values of N in a magnetic field.

Ballistic hall micromagnetometry

We report a magnetization measurement technique which allows quantitative studies of thermodynamic properties of individual submicron superconducting and ferromagnetic particles.

Bias and temperature dependence of the 0.7 conductance anomaly in quantum point contacts

The 0.7 (2e^2/h) conductance anomaly is studied in strongly confined, etched GaAs/GaAlAs quantum point contacts, by measuring the differential conductance as a function of source-drain and gate bias as well as a function of temperature. We investigate in detail how, for a given gate voltage, the differential conductance depends on the finite bias voltage and find a so-called self-gating effect, which we correct for. The 0.7 anomaly at zero bias is found to evolve smoothly into a conductance plateau at 0.85 (2e^2/h) at finite bias. Varying the gate voltage the transition between the 1.0 and the 0.85 (2e^2/h) plateaus occurs for definite bias voltages, which defines a gate voltage dependent energy difference

Ultrafast local field dynamics in photoconductive THz antennas

\Delta

Mesoscopic decoherence in Aharonov-Bohm rings

. This energy difference is compared with the activation temperature T_a extracted from the experimentally observed activated behavior of the 0.7 anomaly at low bias. We find \Delta = k_B T_a which lends support to the idea that the conductance anomaly is due to transmission through two conduction channels, of which the one with its subband edge \Delta below the chemical potential becomes thermally depopulated as the temperature is increased.

Electron transport in single-wall carbon nanotube weak links in the Fabry-Perot regime.

We demonstrate a new ultrafast pump‐probe technique using terahertz pulses to investigate carrier transport and screening in semiconductors. As an example we have studied the temporal evolution of the local electric field in a dipole antenna, used for generation of ultrafast terahertz pulses. Ultrafast screening effects are shown to be important for both carrier transport and the emission of THz radiation. At high carrier densities the external bias field is screened on a time scale comparable to the duration of the THz pulse, giving rise to changes in the shape and bandwidth of the radiated pulses.

OBSERVATION OF QUANTUM ASYMMETRY IN AN AHARONOV-BOHM RING

We study electron decoherence by measuring the temperature dependence of Aharonov-Bohm (AB) oscillations in quasi-one-dimensional rings, etched in a high-mobility

Conductance quantization above 30 K in GaAlAs shallow-etched quantum point contacts smoothly joined to the background 2DEG

\mathrm{GaAs}/{\mathrm{Ga}}_{x}{\mathrm{Al}}_{1\ensuremath{-}x}\mathrm{As}

A Triple Quantum Dot in a Single-Wall Carbon Nanotube

heterostructure. The oscillation amplitude is influenced both by phase breaking and by thermal averaging. Thermal averaging is important when the temperature approaches the energy scale on which the AB oscillations shift their phase. For the phase breaking, it is demonstrated that the damping of the oscillation amplitude is proportional to the length of the interfering paths. For temperatures T from 0.3 to 4 K we find the phase-coherence length

Critical current 0-π transition in designed josephson quantum dot junctions

{L}_{\ensuremath{\varphi}}