We report on measurements of quantum many-body modes in ballistic wires and their dependence on Coulomb interactions, obtained by tunneling between two parallel wires in an GaAs/AlGaAs heterostructure while varying electron density. We observed two spin modes and one charge mode of the coupled wires and mapped the dispersion velocities of the modes down to a critical density, at which spontaneous localization was observed. Theoretical calculations of the charge velocity agree well with the data, although they also predict an additional charge mode that was not observed. The measured spin velocity was smaller than theoretically predicted.

The collective excitation spectrum of interacting electrons in one dimension has been measured by controlling the energy and momentum of electrons tunneling between two closely spaced, parallel quantum wires in a GaAs/AlGaAs heterostructure while measuring the resulting conductance. The excitation spectrum deviates from the noninteracting spectrum, attesting to the importance of Coulomb interactions. An observed 30% enhancement of the excitation velocity relative to noninteracting electrons with the same density, a parameter determined experimentally, is consistent with theories on interacting electrons in one dimension. In short wires, 6 and 2 micrometers long, finite size effects, resulting from the breaking of translational invariance, are observed.

We have measured the low temperature conductance of a one-dimensional island embedded in a single mode quantum wire. The quantum wire is fabricated using the cleaved edge overgrowth technique and the tunneling is through a single state of the island. Our results show that while the resonance line shape fits the derivative of the Fermi function the intrinsic line width decreases in a power law fashion as the temperature is reduced. This behavior agrees quantitatively with Furusaki's model for resonant tunneling in a Luttinger-liquid.PACS numbers: 73.20. Dx, 73.23.Ad, 73.23.Ps, 73.50.Jt One-dimensional (1D) electronic systems are expected to show unique transport behavior as a consequence of the Coulomb interaction between carriers [1]. Unlike in two and three dimensions [2], where the Coulomb interaction affects the transport properties only perturbatively, in 1D it completely modifies the ground state from its well-known Fermi-liquid form and the Fermi surface is qualitatively altered even for weak interactions. Today, it is well established theoretically that the low temperature transport properties of interacting 1D-electron systems are described in terms of a Luttinger-liquid rather than a Fermi-liquid [3,4]. The difference between a Luttingerliquid and Fermi-liquid becomes dramatic already in the presence of a single impurity. According to Landauer's theory the conductance of a single channel wire with a barrier is given by G = |t| 2 · e 2 /h, where |t| 2 is the transmission probability through the barrier. This result holds even at finite temperatures, assuming the transmission probability is independent of energy, as is often the case for barriers that are sufficiently above or below the Fermi energy. In 1D, interactions play a crucial role in that they form charge density correlations. These correlations, similar in nature to charge density waves [5], are easily pinned by even the smallest barrier, resulting in zero transmission and, hence, a vanishing conductance at zero temperature. At finite temperatures the correlation length is finite and the conductance decreases as a power-law of temperature,Herewhere U is the Coulomb energy between particles and E F is the Fermi energy in the wire. Despite the vast theoretical understanding of Luttingerliquids only a handful of experiments have been interpreted using such models. For example, in clean semiconductor wires prepared by the cleaved edge overgrowth (CEO) method [6], contrary to theory, the conductance is suppressed from its universal value [7]. Although not fully understood this suppression is believed to be a result of Coulomb interactions that suppress the coupling between the reservoirs and the wire region. Other measurements done on weakly disordered wires [8] show a weak temperature dependence of the conductance that is attributed to the Coulomb forces between electrons in the wire. Finally, The strongest manifestation of interaction in the clean limit comes from tunneling experiments such as the one recently reported on single walled carbon na...

. Although vortex matter has been studied extensively 1,6,7 , the static and dynamic properties of an individual vortex have not. Here, we use magnetic force microscopy (MFM) to image and manipulate individual vortices in a detwinned YBa 2 Cu 3 O 6.991 single crystal, directly measuring the interaction of a moving vortex with the local disorder potential. We find an unexpected and marked enhancement of the response of a vortex to pulling when we wiggle it transversely. In addition, we find enhanced vortex pinning anisotropy that suggests clustering of oxygen vacancies in our sample and demonstrates the power of MFM to probe vortex structure and microscopic defects that cause pinning.