When electrons pass through a cylindrical electrical conductor aligned in a magnetic ®eld, their wave-like nature manifests itself as a periodic oscillation in the electrical resistance as a function of the enclosed magnetic¯ux 1 . This phenomenon re¯ects the dependence of the phase of the electron wave on the magnetic ®eld, known as the Aharonov±Bohm effect 2 , which causes a phase difference, and hence interference, between partial waves encircling the conductor in opposite directions. Such oscillations have been observed in micrometre-sized thin-walled metallic cylinders 3±5 and lithographically fabricated rings 6±8 . Carbon nanotubes 9,10 are composed of individual graphene sheets rolled into seamless hollow cylinders with diameters ranging from 1 nm to about 20 nm. They are able to act as conducting molecular wires 11±18 , making them ideally suited for the investigation of quantum interference at the single-molecule level caused by the Aharonov±Bohm effect. Here we report magnetoresistance measurements on individual multi-walled nanotubes, which display pronounced resistance oscillations as a function of magnetic ux. We ®nd that the oscillations are in good agreement with theoretical predictions for the Aharonov±Bohm effect in a hollow conductor with a diameter equal to that of the outermost shell of the nanotubes. In some nanotubes we also observe shorter-period oscillations, which might result from anisotropic electron currents caused by defects in the nanotube lattice.In a diffusive and thin-walled metallic cylinder, a prominent periodic quantum correction to the resistance arises from the interference of closed electron trajectories that encircle the cylinder once. The phase difference Df between each such trajectory ¡ and the time-reversed counter-propagating trajectory ¡9 (Fig. 1a) is solely determined by the magnetic¯ux © enclosed: Df 2p2e=h©, where e and h are the electron charge and Planck's constant, respectively. Consequently, the electrical resistance has an oscillating contribution with period h/2e, known as the Altshuler± Aronov±Spivak (AAS) effect 1 . For zero magnetic¯ux, these interference terms add up constructively, increasing electron backscattering and thereby the electrical resistance, an effect known as weak localization 19 . For a thin metallic ®lm in a perpendicular magnetic ®eld, the weak-localization resistance correction monotonically disappears in higher ®elds (negative magnetoresistance, MR). In contrast, for a cylinder in a parallel magnetic ®eld, weak localization is periodically modulated with a magnetic-®eld period given by DB h=2e=r 2 p, where r is the radius of the cylinder.We have carried out electric-transport measurements on multiwalled carbon nanotubes (MWNTs) composed of multiple coaxial graphene cylinders, in a magnetic ®eld parallel to the axis of the nanotubes. An example is shown in Fig. 1b. Coulomb blockade can strongly affect electrical transport in small structures, and we therefore use samples containing only a single contacted nanotube with low contact resis...

Single-layer WS$_2$ is a direct-gap semiconductor showing strong excitonic photoluminescence features in the visible spectral range. Here, we present temperature-dependent photoluminescence measurements on mechanically exfoliated single-layer WS$_2$, revealing the existence of neutral and charged excitons at low temperatures as well as at room temperature. By applying a gate voltage, we can electrically control the ratio of excitons and trions and assert a residual n-type doping of our samples. At high excitation densities and low temperatures, an additional peak at energies below the trion dominates the photoluminescence, which we identify as biexciton emission.Comment: 6 pages, 5 figure

(24). Single-crystal films are essential for devices based on superconductor, giant magnetoresistance, thermionic, piezoelectric, and ferroelectric metal oxides because the intrinsic properties of the material, rather than its grain boundaries, can be exploited. The most active crystallographic orientation can also be selected. Our results show that epitaxy can be achieved even for systems with very high lattice mismatch, and they provide a method for producing other nonequilibrium phases that cannot be accessed by traditional thermal processing. Golden, ibid. 258, 1918Golden, ibid. 258, (1992. 3. J. A. Switzer et al., ibid. 264, 1573Switzer et al., ibid. 264, (1994 A Hanbury Brown and Twiss experiment for a beam of electrons has been realized in a two-dimensional electron gas in the quantum Hall regime. A metallic split gate serves as a tunable beam splitter to partition the incident beam into transmitted and reflected partial beams. In the nonequilibrium case the fluctuations in the partial beams are shown to be fully anticorrelated, demonstrating that fermions exclude each other. In equilibrium, the crosscorrelation of current fluctuations at two different contacts is also found to be negative and nonzero, provided that a direct transmission exists between the contacts.

No abstract

We investigate low-temperature transport properties of thin TiN superconducting films in the vicinity of the disorder-driven superconductor-insulator transition. In a zero magnetic field, we find an extremely sharp separation between superconducting and insulating phases, evidencing a direct superconductor-insulator transition without an intermediate metallic phase. At moderate temperatures, in the insulating films we reveal thermally activated conductivity with the magnetic field-dependent activation energy. At very low temperatures, we observe a zero-conductivity state, which is destroyed at some depinning threshold voltage VT . These findings indicate formation of a distinct collective state of the localized Cooper pairs in the critical region at both sides of the transition.An early suggestion that tuning disorder strength can cause a direct superconductor-insulator transition (SIT) in two-dimensional systems [1] triggered explosive activity in experimental studies of superconductor films [2]. Experimentally, the SIT can be induced by decreasing the film thickness [3] and/or, close to the critical thickness, also by the magnetic field [4]. These scenarios are commonly referred to as disorder-driven SIT (D-SIT) and magnetic-field driven SIT. Recent studies on the Binduced insulator revealed several striking features: a magnetic-field-dependent thermally activated behavior of the conductivity [5] and a threshold response to the dc voltage [6], indicating the possible formation of a distinct collective insulating state. Importantly, these findings refer to the films belonging to the superconducting side of the D-SIT. This rises the question of whether the above findings are specific only to the superconducting side of the D-SIT or a characteristic feature of the whole critical region including both the insulating and superconducting sides of the D-SIT.In this Letter we focus on the insulating side of the disorder-driven superconductor-insulator transition in TiN films. The transition itself turns out to be exceptionally sharp. At zero and low magnetic fields we find thermally activated behavior of the conductivity. A positive magnetoresistance and a distinct threshold behavior in the low-temperature I-V characteristics persist on the insulating side of the D-SIT. Our results clearly indicate that, in the vicinity of the D-SIT, the response to applied magnetic and/or electric fields, is the same irrespective of whether the underlying ground state is superconducting or insulating.The 5-nm thick TiN films were grown by atomic layer chemical vapor deposition onto a Si/SiO 2 substrate. The samples for transport measurements were patterned into Hall bridges using conventional UV lithography and subsequent plasma etching. To increase sheet resistances (R ) without introducing structural changes, the films were thinned by an additional soft plasma etching. Electron transmission micrographs and diffraction patterns revealed a polycrystalline structure in both initial and etched films, the interfaces separating densely-p...

Synchronized oscillators are ubiquitous in nature, and synchronization plays a key part in various classical and quantum phenomena. Several experiments have shown that in thin superconducting films, disorder enforces the droplet-like electronic texture--superconducting islands immersed into a normal matrix--and that tuning disorder drives the system from superconducting to insulating behaviour. In the vicinity of the transition, a distinct state forms: a Cooper-pair insulator, with thermally activated conductivity. It results from synchronization of the phase of the superconducting order parameter at the islands across the whole system. Here we show that at a certain finite temperature, a Cooper--air insulator undergoes a transition to a superinsulating state with infinite resistance. We present experimental evidence of this transition in titanium nitride films and show that the superinsulating state is dual to the superconducting state: it is destroyed by a sufficiently strong critical magnetic field, and breaks down at some critical voltage that is analogous to the critical current in superconductors.

We report equilibrium electric resistance R and tunneling spectroscopy ( d I/ dV ) measurements obtained on single multi-wall nanotubes contacted by four metallic Au fingers from above. At low temperature quantum interference phenomena dominate the magnetoresistance. The phasecoherence (l φ ) and elastic-scattering lengths (l e ) are deduced.Because l e is of order of the circumference of the nanotubes, transport is quasi-ballistic. This result is supported by a dI/ dV spectrum which is in good agreement with the density of states (DOS) due to the one-dimensional subbands expected for a perfect single-wall tube. As a function of temperature T the resistance increases on decreasing T and saturates at ≈ 1-10 K for all measured nanotubes. R(T ) cannot be related to the energy-dependent DOS of graphene but is mainly caused by interaction and interference effects. On a relatively small voltage scale of the order ≈ 10 meV, a pseudogap is observed in d I/ dV which agrees with Luttingerliquid theories for nanotubes. Because we have used quantum diffusion based on Fermi-liquid as well as Luttinger-liquid theory in trying to understand our results, a large fraction of this paper is devoted to a careful discussion of all our results.

No abstract