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Quantum critical systems derive their finite-temperature properties from the influence of a zero-temperature quantum phase transition. The paradigm is essential for understanding unconventional high-Tc superconductors and the non-Fermi liquid properties of heavy fermion compounds. However, the microscopic origins of quantum phase transitions in complex materials are often debated. Here we demonstrate experimentally, with support from numerical renormalization group calculations, a universal crossover from quantum critical non-Fermi liquid behaviour to distinct Fermi liquid ground states in a highly controllable quantum dot device. Our device realizes the non-Fermi liquid two-channel Kondo state, based on a spin-1/2 impurity exchange-coupled equally to two independent electronic reservoirs. On detuning the exchange couplings we observe the Fermi liquid scale T*, at energies below which the spin is screened conventionally by the more strongly coupled channel. We extract a quadratic dependence of T* on gate voltage close to criticality, and validate an asymptotically exact description of the universal crossover between strongly correlated non-Fermi liquid and Fermi liquid states. [ABSTRACT FROM AUTHOR]

Quantum impurity models, which describe how a local degree of freedom interacts with a continuum, are central to condensed-matter physics. Such models may be naturally implemented with quantum dots coupled to each other and to metallic leads. Here we detail a many-body Kondo state occurring when two quantum dots are coupled electrostatically. We use orbital state-resolved bias spectroscopy to demonstrate the entanglement of spin and charge between spatially separated orbitals of the Kondo state. Detailed agreement between transport measurements and numerical renormalization group calculations suggests an emergent SU(4) symmetry. [ABSTRACT FROM AUTHOR]

Understanding the flow of spins in magnetic layered structures has resulted in an increase in data storage density in hard drives over the past decade of more than two orders of magnitude. Following this remarkable success, the field of ‘spintronics’ or spin-based electronics is moving beyond effects based on local spin polarization and is turning towards spin–orbit interaction (SOI) effects, which hold promise for the production, detection and manipulation of spin currents, allowing coherent transmission of information within a device. Although SOI-induced spin transport effects have been observed in two- and three-dimensional samples, these have been subtle and elusive, often detected only indirectly in electrical transport or else with more sophisticated techniques. Here we present the first observation of a predicted ‘spin–orbit gap’ in a one-dimensional sample, where counter-propagating spins, constituting a spin current, are accompanied by a clear signal in the easily measured linear conductance of the system. We first introduce the class of phenomena we dub ‘the one-dimensional spin–orbit gap’ using a simple example adapted from ref. 10, then describe our experiment in detail and finally present a more elaborate model that captures most of the features seen in our data. [ABSTRACT FROM AUTHOR]

Recent advances in the fabrication of microelectromechanical systems and their evolution into nanoelectromechanical systems have enabled researchers to measure extremely small forces, masses and displacements. In particular, researchers have developed position transducers with resolution approaching the uncertainty limit set by quantum mechanics. The achievement of such resolution has implications not only for the detection of quantum behaviour in mechanical systems, but also for a variety of other precision experiments including the bounding of deviations from newtonian gravity at short distances and the measurement of single spins. Here, we demonstrate the use of a quantum point contact as a sensitive displacement detector capable of sensing the low-temperature thermal motion of a nearby micromechanical cantilever. Advantages of this approach include versatility due to its off-board design, compatibility with nanoscale oscillators and, with further development, the potential to achieve quantum-limited displacement detection. [ABSTRACT FROM AUTHOR]

GaAs-based two-dimensional electron gases (2DEGs) show a wealth of remarkable electronic states, and serve as the basis for fast transistors, research on electrons in nanostructures and prototypes of quantum-computing schemes. All of these uses depend on the extremely low levels of disorder in GaAs 2DEGs, with low-temperature mean free paths ranging from micrometres to hundreds of micrometres. Here we study how disorder affects the spatial structure of electron transport by imaging electron flow in three different GaAs/AlGaAs 2DEGs, whose mobilities range over an order of magnitude. As expected, electrons flow along narrow branches that we find remain straight over a distance roughly proportional to the mean free path. We also observe two unanticipated phenomena in high-mobility samples. In our highest-mobility sample we observe an almost complete absence of sharp impurity or defect scattering, indicated by the complete suppression of quantum coherent interference fringes. Also, branched flow through the chaotic potential of a high-mobility sample remains stable to significant changes to the initial conditions of injected electrons. [ABSTRACT FROM AUTHOR]

Some of the most intriguing problems in solid-state physics arise when the motion of one electron dramatically affects the motion of surrounding electrons. Traditionally, such highly correlated electron systems have been studied mainly in materials with complex transition metal chemistry. Over the past decade, researchers have learned to confine one or a few electrons within a nanometre-scale semiconductor ‘artificial atom’, and to understand and control this simple system in detail3. Here we combine artificial atoms to create a highly correlated electron system within a nano-engineered semiconductor structure. We tune the system in situ through a quantum phase transition between two distinct states, each a version of the Kondo state, in which a bound electron interacts with surrounding mobile electrons. The boundary between these competing Kondo states is a quantum critical point—namely, the exotic and previously elusive two-channel Kondo state, in which electrons in two reservoirs are entangled through their interaction with a single localized spin. [ABSTRACT FROM AUTHOR]

Presents research which reported measurements on single-electron transistors (SETs) smaller than those made previously, exhibiting all of the predicted aspects of the Kondo effect. Interaction between localized and delocalized electrons in solid-state physics; What the Kondo effect is; Formation of a spin singlet state; Theories about Kondo singlets; Previous research.

Carbon nanotubes are interesting for studying the remarkable electronic properties of one-dimensional (1D) quantum systems. Electron flow in such systems is not described by Fermi liquid theory-restricted dimensionality leads to the appearance of collective excitations-or Luttinger liquid behavior. Previous studies have probed Luttinger liquid behavior by tunneling into or between one-dimensional systems. We propose to extend these studies by using narrow top gates to introduce tunable tunnel barriers within nanotubes. We report on the scalable fabrication of carbon nanotube-based transistors with nanowire top gates. We have used electron-beam lithography (EBL) to create single-walled carbon nanotube (SWNT) transistors with source-drain spacings down to 200 nm and with sub-30 nm metal top gates for creating tunable tunnel barriers. The top metal gate is isolated from the nanotube by a thin aluminum oxide layer deposited by atomic layer deposition. We fabricated chips with 100 devices using multiple electron-beam lithography alignment steps and achieved overall placement better than 30 nm. The details of top-gated SWNT transistor fabrication are presented, and initial transport measurements on fabricated devices are discussed. [ABSTRACT FROM AUTHOR]