A. R. Champagne

Mechanical Control of Spin States in Spin-1 Molecules and the Underscreened Kondo Effect

Spin Control Through Molecular Stretching Molecules with high symmetry, such as metal complexes with several equivalent ligands, can, in principle, have this symmetry broken by stresses that lengthen bonds in one direction. Parks et al. (p. 1370; see the Perspective by Jarillo-Herrero) placed cobalt complexes in a break-junction contact and then applied a mechanical force to slowly open the contact. Low-temperature measurement of differential conductance revealed a splitting of the Kondo peak at zero-applied voltage into two features, which occurred by breaking the degeneracy of S = 1 triplet states. This assignment of the spin state was confirmed by the evolution of splitting with magnetic field and by comparison to theory for a case where the conduction electrons only partially screen the spin states. Controlled stretching of individual transition-metal complexes enables direct manipulation of the molecule’s spin states. The ability to make electrical contact to single molecules creates opportunities to examine fundamental processes governing electron flow on the smallest possible length scales. We report experiments in which we controllably stretched individual cobalt complexes having spin S = 1, while simultaneously measuring current flow through the molecule. The molecule’s spin states and magnetic anisotropy were manipulated in the absence of a magnetic field by modification of the molecular symmetry. This control enabled quantitative studies of the underscreened Kondo effect, in which conduction electrons only partially compensate the molecular spin. Our findings demonstrate a mechanism of spin control in single-molecule devices and establish that they can serve as model systems for making precision tests of correlated-electron theories.

Tuning the Kondo Effect with a Mechanically Controllable Break Junction

We study electron transport through C(60) molecules in the Kondo regime using a mechanically controllable break junction. By varying the electrode spacing, we are able to change both the width and the height of the Kondo resonance, indicating modification of the Kondo temperature and the relative strength of coupling to the two electrodes. The linear conductance as a function of T/T(K) agrees with the scaling function expected for the spin-1/2 Kondo problem. We are also able to tune finite-bias Kondo features which appear at the energy of the first C(60) intracage vibrational mode.

Nanometer-scale scanning sensors fabricated using stencil lithography

We describe a flexible technique for fabricating 10-nm-scale devices for use as high-resolution scanning sensors and functional probes. Metallic structures are deposited directly onto atomic force microscope tips by evaporation through nanoscale holes fabricated in a stencil mask. We report on the lithographic capabilities of the technique and discuss progress in one initial application, to make high-spatial-resolution magnetic force sensors.

Electronic thermal conductivity measurements in intrinsic graphene

The electronic thermal conductivity of graphene and two-dimensional Dirac materials is of fundamental interest and can play an important role in the performance of nanoscale devices. We report the electronic thermal conductivity Ke in suspended graphene in the nearly intrinsic regime over a temperature range of 20‐300 K. We present a method to extract Ke using two-point dc electron transport at low bias voltages, where the electron and lattice temperatures are decoupled. We find Ke ranging from 0.5 to 11 W/m K over the studied temperature range. The data are consistent with a model in which heat is carried by quasiparticles with the same mean free path and velocity as graphene’s charge carriers.

Wiedemann-Franz relation and thermal-transistor effect in suspended graphene.

We extract experimentally the electronic thermal conductivity, Ke, in suspended graphene that we dope using a back-gate electrode. We make use of two-point dc electron transport at low bias voltages and intermediate temperatures (50-160 K), where the electron and lattice temperatures are decoupled. The thermal conductivity is proportional to the charge conductivity times the temperature, confirming that the Wiedemann-Franz relation is obeyed in suspended graphene. We extract an estimate of the Lorenz coefficient as 1.1-1.7 × 10(-8) W ΩK(-2). Ke shows a transistor effect and can be tuned with the back-gate by more than a factor of 2 as the charge carrier density ranges from ∼0.5 to 1.8 × 10(11) cm(-2).

Few-Hundred GHz Carbon Nanotube Nanoelectromechanical Systems (NEMS)

We study 23 to 30 nm long suspended single-wall carbon nanotube quantum dots and observe both their stretching and bending vibrational modes. We use low-temperature DC electron transport to excite and measure the tubes’ bending mode by making use of a positive feedback mechanism between their vibrations and the tunneling electrons. In these nano-electro-mechanical-systems (NEMS), we measure fundamental bending frequencies fbend ≈ 75 280 GHz, and extract quality factors Q ∼ 10. The NEMS’ frequencies can be tuned by a factor of two with tension induced by mechanical breakjunctions actuated by an electrostatic force, or tension from bent suspended electrodes.

Evidence for a Finite-Temperature Phase Transition in a Bilayer Quantum Hall System

We study the Josephson-like interlayer tunneling signature of the strongly correlated nuT=1 quantum Hall phase in bilayer two-dimensional electron systems as a function of the layer separation, temperature, and interlayer charge imbalance. Our results offer strong evidence that a finite temperature phase transition separates the interlayer coherent phase from incoherent phases which lack strong interlayer correlations. The transition temperature is dependent on both the layer spacing and charge imbalance between the layers.

Ultra-short suspended single-wall carbon nanotube transistors

We describe a method to fabricate clean suspended single-wall carbon nanotube (SWCNT) transistors hosting a single quantum dot ranging in length from a few 10 s of nm down to ≈3 nm. We first align narrow gold bow-tie junctions on top of individual SWCNTs and suspend the devices. We then use a feedback-controlled electromigration to break the gold junctions and expose nm-sized sections of SWCNTs. We measure electron transport in these devices at low temperature and show that they form clean and tunable single-electron transistors. These ultra-short suspended transistors offer the prospect of studying THz oscillators with strong electron-vibron coupling.

Charge imbalance and bilayer two-dimensional electron systems at nuT=1

We use interlayer tunneling to study bilayer two-dimensional electron systems at nuT=1 over a wide range of charge-density imbalance Deltanu=nu1−nu2 between the two layers. We find that the strongly enhanced tunneling associated with the coherent excitonic nuT=1 phase at small layer separation can survive at least up to an imbalance of Deltanu=0.5, i.e., (nu1,nu2)=(3/4,1/4). Phase transitions between the excitonic nuT=1 state and bilayer states which lack significant interlayer correlations can be induced in three different ways: by increasing the effective interlayer spacing d/[script-l], the temperature T, or the charge imbalance Deltanu. We observe that close to the phase boundary the coherent nuT=1 phase can be absent at Deltanu=0, present at intermediate Deltanu, and then absent again at large Deltanu, thus indicating an intricate phase competition between it and incoherent quasi-independent layer states. At zero imbalance, the critical d/[script-l] shifts linearly with temperature, while at Deltanu=1/3 the critical d/[script-l] is only weakly dependent on T. At Deltanu=1/3 we report on an observation of a direct phase transition between the coherent excitonic nuT=1 bilayer integer quantum Hall phase and the pair of single-layer fractional quantized Hall states at nu1=2/3 and nu2=1/3.

Tailoring 10 nm Scale Suspended Graphene Junctions and Quantum Dots

The possibility to make 10 nm scale, and low-disorder, suspended graphene devices would open up many possibilities to study and make use of strongly coupled quantum electronics, quantum mechanics, and optics. We present a versatile method, based on the electromigration of gold-on-graphene bow-tie bridges, to fabricate low-disorder suspended graphene junctions and quantum dots with lengths ranging from 6 nm up to 55 nm. We control the length of the junctions, and shape of their gold contacts by adjusting the power at which the electromigration process is allowed to avalanche. Using carefully engineered gold contacts and a nonuniform downward electrostatic force, we can controllably tear the width of suspended graphene channels from over 100 nm down to 27 nm. We demonstrate that this lateral confinement creates high-quality suspended quantum dots. This fabrication method could be extended to other two-dimensional materials.