36 research outputs found
Persistent Orbital Degeneracy in Carbon Nanotubes
The quantum-mechanical orbitals in carbon nanotubes are doubly degenerate
over a large number of states in the Coulomb blockade regime. We argue that
this experimental observation indicates that electrons are reflected without
mode mixing at the nanotube-metal contacts. Two electrons occupying a pair of
degenerate orbitals (a ``shell'') are found to form a triplet state starting
from zero magnetic field. Finally, we observe unexpected low-energy excitations
at complete filling of a four-electron shell.Comment: 6 pages, 4 figure
Four-Probe Measurements of Carbon Nanotubes with Narrow Metal Contacts
We find that electrons in single-wall carbon nanotubes may propagate
substantial distances (tens of nanometers) under the metal contacts. We perform
four-probe transport measurements of the nanotube conductance and observe
significant deviations from the standard Kirchhoff's circuit rules. Most
noticeably, injecting current between two neighboring contacts on one end of
the nanotube, induces a non-zero voltage difference between two contacts on the
other end.Comment: 4 pages, 5 figures; submitte
Kondo effect in a carbon nanotube with spin-orbit interaction and valley mixing: A DM-NRG study
We investigate the effects of spin-orbit interaction (SOI) and valley mixing
on the transport and dynamical properties of a carbon nanotube (CNT) quantum
dot in the Kondo regime. As these perturbations break the pseudo-spin symmetry
in the CNT spectrum but preserve time-reversal symmetry, they induce a finite
splitting between formerly degenerate Kramers pairs. Correspondingly,
a crossover from the SU(4) to the SU(2)-Kondo effect occurs as the strength of
these symmetry breaking parameters is varied. Clear signatures of the crossover
are discussed both at the level of the spectral function as well as of the
conductance. In particular, we demonstrate numerically and support with scaling
arguments, that the Kondo temperature scales inversely with the splitting
in the crossover regime. In presence of a finite magnetic field, time
reversal symmetry is also broken. We investigate the effects of both parallel
and perpendicular fields (with respect to the tube's axis), and discuss the
conditions under which Kondo revivals may be achieved.Comment: 13 pages, 17 figure
Evolution of SU(4) Transport Regimes in Carbon Nanotube Quantum Dots
We study the evolution of conductance regimes in carbon nanotubes with doubly
degenerate orbitals (``shells'') by controlling the contact transparency within
the same sample. For sufficiently open contacts, Kondo behavior is observed for
1, 2, and 3 electrons in the topmost shell. As the contacts are opened more,
the sample enters the ``mixed valence'' regime, where different charge states
are strongly hybridized by electron tunneling. Here, the conductance as a
function of gate voltage shows pronounced modulations with a period of four
electrons, and all single-electron features are washed away at low temperature.
We successfully describe this behavior by a simple formula with no fitting
parameters. Finally, we find a surprisingly small energy scale that controls
the temperature evolution of conductance and the tunneling density of states in
the mixed valence regime.Comment: 4 pages + supplementary info. The second part of the original
submission is now split off as a separate paper (0709.1288
Two-Stage Kondo Effect and Kondo Box Level Spectroscopy in a Carbon Nanotube
The concept of the "Kondo box" describes a single spin, antiferromagnetically
coupled to a quantum dot with a finite level spacing. Here, a Kondo box is
formed in a carbon nanotube interacting with a localized electron. We
investigate the spins of its first few eigenstates and compare them to a recent
theory. In an 'open' Kondo-box, strongly coupled to the leads, we observe a
non-monotonic temperature dependence of the nanotube conductance, which results
from a competition between the Kondo-box singlet and the 'conventional' Kondo
state that couples the nanotube to the leads.Comment: 5 pages, 3 figure
SU(4) and SU(2) Kondo Effects in Carbon Nanotube Quantum Dots
We study the SU(4) Kondo effect in carbon nanotube quantum dots, where doubly
degenerate orbitals form 4-electron ``shells''. The SU(4) Kondo behavior is
investigated for one, two and three electrons in the topmost shell. While the
Kondo state of two electrons is quenched by magnetic field, in case of an odd
number of electrons two types of SU(2) Kondo effect may survive. Namely, the
spin SU(2) state is realized in the magnetic field parallel to the nanotube
(inducing primarily orbital splitting). Application of the perpendicular field
(inducing Zeeman splitting) results in the orbital SU(2) Kondo effect.Comment: 5 pages. Some material was previously posted in cond-mat/0608573, v
Resonant Tunneling in a Dissipative Environment
We measure tunneling through a single quantum level in a carbon nanotube
quantum dot connected to resistive metal leads. For the electrons tunneling
to/from the nanotube, the leads serve as a dissipative environment, which
suppresses the tunneling rate. In the regime of sequential tunneling, the
height of the single-electron conductance peaks increases as the temperature is
lowered, although it scales more weekly than the conventional 1/T. In the
resonant tunneling regime (temperature smaller than the level width), the peak
width approaches saturation, while the peak height starts to decrease. Overall,
the peak height shows a non-monotonic temperature dependence. We associate this
unusual behavior with the transition from the sequential to the resonant
tunneling through a single quantum level in a dissipative environment.Comment: 5 pages, 5 figure
Transition between SU(4) and SU(2) Kondo effect
Motivated by experiments in nanoscopic systems, we study a generalized
Anderson, which consists of two spin degenerate doublets hybridized to a
singlet by promotion of an electron to two conduction bands, as a function of
the energy separation between both doublets. For =0 or very
large, the model is equivalent to a one-level SU(N) Anderson model, with N=4
and 2 respectively. We study the evolution of the spectral density for both
doublets ( and ) and their
width in the Kondo limit as is varied, using the non-crossing
approximation (NCA). As increases, the peak at the Fermi energy in the
spectral density (Kondo peak) splits and the density of the doublet of higher
energy shifts above the Ferrmi energy. The Kondo
temperature (determined by the half width at half maximum of the Kondo
peak in density of the doublet of lower energy )
decreases dramatically. The variation of with is reproduced by a
simple variational calculation.Comment: 8 pages, 4 figures. Presented at the conference "Frontiers of
Condensed Matter Physics", December 201
Universal conductance enhancement and reduction of the two-orbital Kondo effect
We investigate theoretically the linear and nonlinear conductance through a
nanostructure with two-fold degenerate single levels, corresponding to the
transport through nanostructures such as a carbon nanotube, or double dot
systems with capacitive interaction. It is shown that the presence of the
interaction asymmetry between orbits/dots affects significantly the profile of
the linear conductance at finite temperature, and, of the nonlinear
conductance, particularly around half-filling, where the two-particle Kondo
effect occurs. Within the range of experimentally feasible parameters, the
SU(4) universal behavior is suggested, and comparison with relevant experiments
is made.Comment: 10 pages, 16 figure
Noisy Kondo impurities
The anti-ferromagnetic coupling of a magnetic impurity carrying a spin with
the conduction electrons spins of a host metal is the basic mechanism
responsible for the increase of the resistance of an alloy such as
CuFe at low temperature, as originally suggested by
Kondo . This coupling has emerged as a very generic property of localized
electronic states coupled to a continuum . The possibility to design artificial
controllable magnetic impurities in nanoscopic conductors has opened a path to
study this many body phenomenon in unusual situations as compared to the
initial one and, in particular, in out of equilibrium situations. So far,
measurements have focused on the average current. Here, we report on
\textit{current fluctuations} (noise) measurements in artificial Kondo
impurities made in carbon nanotube devices. We find a striking enhancement of
the current noise within the Kondo resonance, in contradiction with simple
non-interacting theories. Our findings provide a test bench for one of the most
important many-body theories of condensed matter in out of equilibrium
situations and shed light on the noise properties of highly conductive
molecular devices.Comment: minor differences with published versio