[Show abstract][Hide abstract] ABSTRACT: In an armchair carbon nanotube pn junction the p and n regions are separated by a region of a Mott insulator, which can backscatter electrons only in pairs. We predict a quantum-critical behavior in such a pn junction. Depending on the junction's built-in electric field E, its conductance G scales either to zero or to 4e(2)/h as the temperature T is lowered. The two types of the G(T) dependence indicate the existence, at some special value of E, of an intermediate quantum-critical point with a finite conductance G<4e(2)/h. This makes the pn junction drastically different from a simple potential barrier in a Luttinger liquid.
[Show abstract][Hide abstract] ABSTRACT: We evaluate the rate of energy loss of a plasmon in a disorder-free carbon
nanotube. The plasmon decays into neutral bosonic excitations of the electron
liquid. The process is mediated either by phonon-assisted backscattering of a
single electron, or Umklapp backscattering of two electrons. To lowest order in
the backscattering interactions the partial decay rates are additive. At zero
doping the corresponding decay rates scale as power-laws of the temperature
with positive and negative exponents for the two mechanisms, respectively. The
precise values of the exponents depend on the Luttinger liquid parameter. At
finite doping the decay rates are described by universal crossover functions of
frequency and chemical potential measured in units of temperature. In the
evaluation of the plasmon decay, we concentrate on a finite-length geometry
allowing excitation of plasma resonances.
[Show abstract][Hide abstract] ABSTRACT: Many strongly correlated electronic materials have a domain structure that greatly influences the bulk properties and obscures the fundamental properties of the homogeneous material. Nanoscale samples, on the other hand, can be smaller than the characteristic domain size, thus making it possible to explore these fundamental properties in detail. Here, we report new aspects of the metal-insulator transition, studied in single-domain vanadium dioxide nanobeams. We have observed supercooling of the metallic phase by 50 degrees C, an activation energy in the insulating phase that is consistent with the optical gap, and a connection between the metal-insulator transition and the equilibrium carrier density in the insulating phase. Our devices also provide a nanomechanical method for determining the transition temperature, enable measurements on individual metal-insulator interphase walls to be made, and allow general investigations of phase transitions in quasi-one-dimensional geometries.
[Show abstract][Hide abstract] ABSTRACT: In many correlated materials, domain structure causes the bulk
properties to differ from those on the sub-domain level. In addition,
near first-order phase transitions it leads to transition broadening,
hysteresis, and sample degradation. Studies of nanoscale crystals enable
investigations of the domain-free homogeneous material. We demonstrate
this by working with nanobeams of vanadium dioxide, thereby discovering
or clarifying multiple aspects of its famous metal-insulator transition
at 67^oC. Amongst them are that the transition to the metal
occurs at a constant value of the resistivity of the insulating phase;
large supercooling of the homogeneous metallic phase is possible; and
the activation energy in the insulating phase is consistent with the
optical gap, in contrast with earlier reports on bulk samples. The
nanobeams also enable new classes of experiments, including
investigating a single metal-insulator interphase wall, employing
nanomechanical effects to determine the equilibrium transition
temperature, and investigating the dynamics of a phase transition in
[Show abstract][Hide abstract] ABSTRACT: We show that strong Luttinger correlations of the electron liquid in armchair carbon nanotubes significantly enhance the onset temperature of the putative twist Peierls instability and lead to its 1/R3 dependence on the tube radius. Depending on the values of the coupling constants the umklapp processes can either assist or compete with the twist instability. In the case of a competition the umklapp processes win in wide tubes. In narrow tubes the outcome of the competition depends on the relative strength of the e-e and e-ph backscattering. Our estimates show that the twist instability may be realized in freestanding (5,5) tubes.
[Show abstract][Hide abstract] ABSTRACT: We study the conductance of an interconnect between two graphene leads formed
by a single-atom carbon chain. Its dependence on the chemical potential and the
number of atoms in the chain is qualitatively different from that in the case
of normal metal leads. Electron transport proceeds via narrow resonant states
in the wire. The latter arise due to strong reflection at the junctions between
the chain and the leads, which is caused by the small density of states in the
leads at low energy. The energy dependence of the transmission coefficient near
resonance is asymmetric and acquires a universal form at small energies. We
find that in the case of leads with the zigzag edges the dispersion of the edge
states has a significant effect on the device conductance.
[Show abstract][Hide abstract] ABSTRACT: Vanadium dioxide nanobeams show the same dramatic metal-insulator transition as does bulk VO2, occuring at about 67 degrees C under ambient conditions. The transition is first-order, accompanied by an abrupt and rapid changes in the electronic and optical properties, a latent heat, and a lattice distortion. In the bulk the transition is frustrated, leading to sample degradation, but in nanobeams this is not the case. As a result, in end-clamped nanobeams under tension we are able to investigate a regime of coexistence of the metallic and insulating phases. We find that the resistivity of the insulating phase along the phase boundary is independent of temperature. Furthermore the MIT occurs from the intermediate M2 insulating phase, which we detect near the transition by its higher resistivity, but not directly from the low-temperature M1 phase. These results imply that the MIT is triggered by carrier density and therefore involves electron correlations, and suggest that it takes place in the undimerized vanadium chains present in M2 but not in M1. More generally, these studies illustrate the scientific and technological potential of strongly correlated materials in nanoscale form.
[Show abstract][Hide abstract] ABSTRACT: We study the first-order metal-insulator transition (MIT) in vanadium dioxide nanobeams. The MIT occurs sharply at a temperature of Tc = 67^oC at ambient pressure. However, in nanobeams clamped at both ends, and hence subjected to a constant length condition, there is a wide coexistence regime between the two phases, which can be visualized in an optical microscope. Above Tc the beam is under axial tension and on warming up follows the phase boundary in the tension/temperature plane. Below Tc the beam buckles under compressive strain. The metallic phase can be supercooled by up to 50 °C. Usually there is a single metal-insulator domain wall in each beam, but a mobile bubble-like insulating domain can be induced by applying a nonuniform temperature profile..
[Show abstract][Hide abstract] ABSTRACT: We study the electrical properties of vanadium dioxide nanobeams undergoing the metal-insulator transition (MIT), which occurs at a temperature of 67^oC at ambient pressure. The nature of the MIT in bulk VO2, although known to involve electron-electron correlations, has remained elusive since its discovery fifty years ago. In nanobeams clamped at both ends there is a coexistence regime which allows electrical measurements along the phase boundary. Remarkably, the resistivity of the insulating phase turns out to be constant along the phase boundary implying that the transition is driven by electron density, consistent with a Mott-type mechanism. The measurements show that the resistance of a domain wall is negligible, and the resistance of a nanowire gives a direct measure of the length of insulating phase present in the wire, allowing one to study the motion of the domain wall electrically with high precision.
[Show abstract][Hide abstract] ABSTRACT: Adsorbates on a suspended single-walled carbon nanotube at a coverage of one monolayer or less offer the opportunity to study the various phases and phase transitions of a system where the dimensionality is below two. This is because such a monolayer resembles a well studied 2D monolayer on planar graphite, but with a tight cylindrical boundary condition imposed. The adsorbed density for any gas can be measured by using the nanotube itself as a vibrating microbalance, whose frequency varies with the adsorbed density and whose amplitude is detected by the way it modulates the conductance. We are focusing on two systems both thoroughly studied before on 2D graphite: the noble gases Xe and Kr; and oxygen. The noble gases are attractive for their simplicity, and because in 2D they exhibit discontinous phase transitions, which are not allowed in 1D according to an argument of Landau. They thus allow the possibility to confirm and explore this basic prediction of statistical mechanics for the first time. The magnetic and steric properties of phases of oxygen on 2D graphite, together with the question of its apparently unexplained large doping effect on nanotubes, make it particularly interesting and important. We have made suitable nanotube devices and will report on our progress in detecting monolayers on them.
[Show abstract][Hide abstract] ABSTRACT: VO2 undergoes a metal-insulator transition (MIT) around 67^oC. We investigate the transition in suspended crystalline nanobeams of VO2. The nanobeams are grown by vapor phase deposition on SiO2 substrates and contacted by electron beam lithography with chromium-gold metallisation. After suspending them by selectively etching away the substrate, the resulting nanobeams are firmly clamped at the contacts. Under some conditions the MIT occurs suddenly throughout the entire beam, associated with a single hysteretic conductance jump. This is in contrast with the behavior of nanobeams attached to the substrate in which alternating metallic and insulating domains form during the transition. Under other conditions, a single metallic domain forms and grows gradually as temperature is increased. At room temperature the longer beams are buckled, and on warming they unbuckle when the MIT occurs. When a force is applied to bend a suspended nanobeam, alternating domain patterns form in the bent region reflecting the strain field.
[Show abstract][Hide abstract] ABSTRACT: Both VO2 and V2O3 show dramatic metal-insulator transitions, whose manifestations on the nanoscale are not known. We investigate techniques to differentiate and pattern the metallic and insulating domains in small VO2 crystals and nanowires grown by vapor phase deposition. For instance, it has been reported that insulating VO2 can be metallized by electron beam exposure and by hydrogenation. We attempt to distinguish the domains by scanning probe techniques, including topography and electric force microscopy, and observe a pinning effect of the domains by oscillating strain variations when the nanowire is attached to a substrate. When the strain is released by etching, the pinning is removed. The VO2 crystals can be converted to V2O3 crystals by reducing in hydrogen and annealing. By patterning the V2O3 on the nanoscale we aim to realize strongly correlated quantum dots.