19 research outputs found
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-wall 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 higher 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 number (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.Comment: 7 pages, pdf onl
Electron-hole symmetry in a semiconducting carbon nanotube quantum dot
Optical and electronic phenomena in solids arise from the behaviour of
electrons and holes (unoccupied states in a filled electron sea). Electron-hole
symmetry can often be invoked as a simplifying description, which states that
electrons with energy above the Fermi sea behave the same as holes below the
Fermi energy. In semiconductors, however, electron-hole symmetry is generally
absent since the energy band structure of the conduction band differs from the
valence band. Here we report on measurements of the discrete, quantized-energy
spectrum of electrons and holes in a semiconducting carbon nanotube. Through a
gate, an individual nanotube is filled controllably with a precise number of
either electrons or holes, starting from one. The discrete excitation spectrum
for a nanotube with N holes is strikingly similar to the corresponding spectrum
for N electrons. This observation of near perfect electron-hole symmetry
demonstrates for the first time that a semiconducting nanotube can be free of
charged impurities, even in the limit of few-electrons or holes. We furthermore
find an anomalously small Zeeman spin splitting and an excitation spectrum
indicating strong electron-electron interactions.Comment: 12 pages, 4 figure
Spectral weight transfer in a disorder-broadened Landau level
In the absence of disorder, the degeneracy of a Landau level (LL) is
, where is the magnetic field, is the area of the sample
and is the magnetic flux quantum. With disorder, localized states
appear at the top and bottom of the broadened LL, while states in the center of
the LL (the critical region) remain delocalized. This well-known phenomenology
is sufficient to explain most aspects of the Integer Quantum Hall Effect (IQHE)
[1]. One unnoticed issue is where the new states appear as the magnetic field
is increased. Here we demonstrate that they appear predominantly inside the
critical region. This leads to a certain ``spectral ordering'' of the localized
states that explains the stripes observed in measurements of the local inverse
compressibility [2-3], of two-terminal conductance [4], and of Hall and
longitudinal resistances [5] without invoking interactions as done in previous
work [6-8].Comment: 5 pages 3 figure
Observation of the Fractional Quantum Hall Effect in Graphene
When electrons are confined in two dimensions and subjected to strong
magnetic fields, the Coulomb interactions between them become dominant and can
lead to novel states of matter such as fractional quantum Hall liquids. In
these liquids electrons linked to magnetic flux quanta form complex composite
quasipartices, which are manifested in the quantization of the Hall
conductivity as rational fractions of the conductance quantum. The recent
experimental discovery of an anomalous integer quantum Hall effect in graphene
has opened up a new avenue in the study of correlated 2D electronic systems, in
which the interacting electron wavefunctions are those of massless chiral
fermions. However, due to the prevailing disorder, graphene has thus far
exhibited only weak signatures of correlated electron phenomena, despite
concerted experimental efforts and intense theoretical interest. Here, we
report the observation of the fractional quantum Hall effect in ultraclean
suspended graphene, supporting the existence of strongly correlated electron
states in the presence of a magnetic field. In addition, at low carrier density
graphene becomes an insulator with an energy gap tunable by magnetic field.
These newly discovered quantum states offer the opportunity to study a new
state of matter of strongly correlated Dirac fermions in the presence of large
magnetic fields
Electric Field Control of Spin Transport
Spintronics is an approach to electronics in which the spin of the electrons
is exploited to control the electric resistance R of devices. One basic
building block is the spin-valve, which is formed if two ferromagnetic
electrodes are separated by a thin tunneling barrier. In such devices, R
depends on the orientation of the magnetisation of the electrodes. It is
usually larger in the antiparallel than in the parallel configuration. The
relative difference of R, the so-called magneto-resistance (MR), is then
positive. Common devices, such as the giant magneto-resistance sensor used in
reading heads of hard disks, are based on this phenomenon. The MR may become
anomalous (negative), if the transmission probability of electrons through the
device is spin or energy dependent. This offers a route to the realisation of
gate-tunable MR devices, because transmission probabilities can readily be
tuned in many devices with an electrical gate signal. Such devices have,
however, been elusive so far. We report here on a pronounced gate-field
controlled MR in devices made from carbon nanotubes with ferromagnetic
contacts. Both the amplitude and the sign of the MR are tunable with the gate
voltage in a predictable manner. We emphasise that this spin-field effect is
not restricted to carbon nanotubes but constitutes a generic effect which can
in principle be exploited in all resonant tunneling devices.Comment: 22 pages, 5 figure
Electrical generation and absorption of phonons in carbon nanotubes
The interplay between discrete vibrational and electronic degrees of freedom
directly influences the chemical and physical properties of molecular systems.
This coupling is typically studied through optical methods such as
fluorescence, absorption, and Raman spectroscopy. Molecular electronic devices
provide new opportunities for exploring vibration-electronic interactions at
the single molecule level. For example, electrons injected from a scanning
tunneling microscope tip into a metal can excite vibrational excitations of a
molecule in the gap between tip and metal. Here we show how current directly
injected into a freely suspended individual single-wall carbon nanotube can be
used to excite, detect, and control a specific vibrational mode of the
molecule. Electrons inelastically tunneling into the nanotube cause a
non-equilibrium occupation of the radial breathing mode, leading to both
stimulated emission and absorption of phonons by successive electron tunneling
events. We exploit this effect to measure a phonon lifetime on the order of 10
nanoseconds, corresponding to a quality factor well over 10000 for this
nanomechanical oscillator.Comment: 17 pages, 4 figure
Spin-resolved Quantum Interference in Graphene
The unusual electronic properties of single-layer graphene make it a
promising material system for fundamental advances in physics, and an
attractive platform for new device technologies. Graphene's spin transport
properties are expected to be particularly interesting, with predictions for
extremely long coherence times and intrinsic spin-polarized states at zero
field. In order to test such predictions, it is necessary to measure the spin
polarization of electrical currents in graphene. Here, we resolve spin
transport directly from conductance features that are caused by quantum
interference. These features split visibly in an in-plane magnetic field,
similar to Zeeman splitting in atomic and quantum dot systems. The
spin-polarized conductance features that are the subject of this work may, in
the future, lead to the development of graphene devices incorporating
interference-based spin filters.Comment: 12 pages, 4 figures, plus supplementary (11 pages, 9 figures
Magnetic Behavior of Surface Nanostructured 50-nm Nickel Thin Films
Thermally evaporated 50-nm nickel thin films coated on borosilicate glass substrates were nanostructured by excimer laser (0.5 J/cm2, single shot), DC electric field (up to 2 kV/cm) and trench-template assisted technique. Nanoparticle arrays (anisotropic growth features) have been observed to form in the direction of electric field for DC electric field treatment case and ruptured thin film (isotropic growth features) growth for excimer laser treatment case. For trench-template assisted technique; nanowires (70–150 nm diameters) have grown along the length of trench template. Coercive field and saturation magnetization are observed to be strongly dependent on nanostructuring techniques