Electrical generation and absorption of phonons in carbon nanotubes

Two-Dimensional Spectroscopy of Open Quantum Systems: Nonequilibrium Green's Function Formulation

Two-dimensional spectroscopy is examined for open quantum systems featuring multiple simultaneously measurable fluxes. Specifically, we explore a junction where optical measurements of photon flux are paired with concurrent transport measurements of electron currents. The theory of two-dimensional spectroscopy for both fluxes is developed using a nonself-consistent nonequilibrium Green's function formulation. Theoretical derivations are demonstrated through numerical simulations within a generic junction model.

Charge-transfer dipole low-frequency vibronic excitation at single-molecular scale

Scanning tunneling microscopy (STM) vibronic spectroscopy, which has provided submolecular insights into electron-vibration (vibronic) coupling, faces challenges when probing the pivotal low-frequency vibronic excitations. Because of eigenstate broadening on solid substrates, resolving low-frequency vibronic states demands strong decoupling. This work designs a type II band alignment in STM junction to achieve effective charge-transfer state decoupling. This strategy enables the successful identification of the lowest-frequency Hg(ω1) (Raman-active Hg mode) vibronic excitation within single C60 molecules, which, despite being notably pronounced in electron transport of C60 single-molecule transistors, has remained hidden at submolecular level. Our results show that the observed Hg(ω1) excitation is "anchored" to all molecules, irrespective of local geometry, challenging common understanding of structural definition of vibronic excitation governed by Franck-Condon principle. Density functional theory calculations reveal existence of molecule-substrate interfacial charge-transfer dipole, which, although overlooked previously, drives the dominant Hg(ω1) excitation. This charge-transfer dipole is not specific but must be general at interfaces, influencing vibronic coupling in charge transport.

Voltage controlled shutter regulates channel size and motion direction of protein aperture as durable nano-electric rectifier-----An opinion in biomimetic nanoaperture

In optical devices such as camera or microscope, an aperture is used to regulate light intensity for imaging. Here we report the discovery and construction of a durable bio-aperture at nanometerscale that can regulate current at the pico-ampere scale. The nano-aperture is made of 12 identical protein subunits that form a 3.6-nm channel with a shutter and "one-way traffic" property. This shutter responds to electrical potential differences across the aperture and can be turned off for double stranded DNA translocation. This voltage enables directional control, and three-step regulation for opening and closing. The nano-aperture was constructed in vitro and purified into homogeneity. The aperture was stable at pH2-12, and a temperature of -85C-60C. When an electrical potential was held, three reproducible discrete steps of current flowing through the channel were recorded. Each step reduced 32% of the channel dimension evident by the reduction of the measured current flowing through the aperture. The current change is due to the change of the resistance of aperture size. The transition between these three distinct steps and the direction of the current was controlled via the polarity of the voltage applied across the aperture. When the C-terminal of the aperture was fused to an antigen, the antibody and antigen interaction resulted in a 32% reduction of the channel size. This phenomenon was used for disease diagnosis since the incubation of the antigen-nano-aperture with a specific cancer antibody resulted in a change of 32% of current. The purified truncated cone-shape aperture automatically self-assembled efficiently into a sheet of the tetragonal array via head-to-tail self-interaction. The nano-aperture discovery with a controllable shutter, discrete-step current regulation, formation of tetragonal sheet, and one-way current traffic provides a nanoscale electrical circuit rectifier for nanodevices and disease diagnosis.

Charge-State Dependent Vibrational Relaxation in a Single-Molecule Junction

The outcome of an electron-transfer process is determined by the quantum-mechanical interplay between electronic and vibrational degrees of freedom. Nonequilibrium vibrational dynamics are known to direct electron-transfer mechanisms in molecular systems; however, the structural features of a molecule that lead to certain modes being pushed out of equilibrium are not well understood. Herein, we report on electron transport through a porphyrin dimer molecule, weakly coupled to graphene electrodes, that displays sequential tunneling within the Coulomb-blockade regime. The sequential transport is initiated by current-induced phonon absorption and proceeds by rapid sequential transport via a nonequilibrium vibrational distribution of low-energy modes, likely related to torsional molecular motions. We demonstrate that this is an experimental signature of slow vibrational dissipation, and obtain a lower bound for the vibrational relaxation time of 8 ns, a value dependent on the molecular charge state.

Two Nonlinear Optical Thiophosphates Cu5Hg0.5P2S8 and AgHg3PS6 Activated by Their Tetrahedra-Stacking Architecture

Infrared (IR) lasers are very critical in military and civil affairs, but it is also challenging and difficult to develop new infrared nonlinear optical (NLO) crystals. Herein, two new mixed-metal thiophosphates, Cu₅Hg_0.5P₂S₈ and AgHg₃PS₆ were discovered with the noncentrosymmetric (NCS) space group Pmn2₁ (No. 31) and Cc (No. 9). Cu₅Hg_0.5P₂S₈ displays a three-dimensional (3D) defective diamond-like structure stacked by _∞²[Cu_2.5Hg_0.25PS₈]^8- layers. AgHg₃PS₆ is characterized by a 3D framework consisting of distorted tetrahedrons. Moreover, the optical spectra show the band gaps of Cu₅Hg_0.5P₂S₈ and AgHg₃PS₆ are 2.12 and 1.85 eV, respectively, with a broad transparent range of 0.7-16.0 μm. In these two compounds, the dipole moments of nonlinear active units are strengthened due to the high-valence element P and the Hg-S bonds with large susceptibility. Therefore, AgHg₃PS₆ exhibits a moderate second harmonic generation (SHG) response that is half that of AgGaS₂ (AGS) at 30-45 μm, while Cu₅Hg_0.5P₂S₈ performs a phase-matching (PM) behavior with a good SHG signal of 0.8 × AGS at 150-200 μm. The origin of NLO performance and electronic structures were revealed by the calculated dipole moments of distorted tetrahedra and theoretical calculations on the basis of density functional theory.

Interrelation of Elasticity and Thermal Bath in Nanotube Cantilevers

We report the first study on the thermal behavior of the stiffness of individual carbon nanotubes, which is achieved by measuring the resonance frequency of their fundamental mechanical bending modes. We observe a reduction of the Young's modulus over a large temperature range with a slope -(173±65)  ppm/K in its relative shift. These findings are reproduced by two different theoretical models based on the thermal dynamics of the lattice. These results reveal how the measured fundamental bending modes depend on the phonons in the nanotube via the Young's modulus. An alternative description based on the coupling between the measured mechanical modes and the phonon thermal bath in the Akhiezer limit is discussed.

Vibron-assisted spin excitation in a magnetically anisotropic molecule

The electrical control and readout of molecular spin states are key for high-density storage. Expectations are that electrically-driven spin and vibrational excitations in a molecule should give rise to new conductance features in the presence of magnetic anisotropy, offering alternative routes to study and, ultimately, manipulate molecular magnetism. Here, we use inelastic electron tunneling spectroscopy to promote and detect the excited spin states of a prototypical molecule with magnetic anisotropy. We demonstrate the existence of a vibron-assisted spin excitation that can exceed in energy and in amplitude a simple excitation among spin states. This excitation, which can be quenched by structural changes in the magnetic molecule, is explained using first-principles calculations that include dynamical electronic correlations.

Manipulation of non-linear heat currents in the dissipative Anderson-Holstein model

The precise control of phonon heat currents will be of primary importance in emerging phononic devices. In this paper, a detailed analysis of electronically controled phonon transport is carried out using an Anderson-Holstein based dissipative quantum dot setup. We consider two relevant electronic bias situations: (a) a voltage bias in the absence of an electronic temperature gradient and (b) an electronic temperature gradient at zero voltage. It is shown that the direction of phonon transport in the non-linear regime is different in the two cases since the first case facilitates the accumulation of phonons in the dot and the second case leads to the absorption of phonons in the dot. In the linear regime, both the phonon and electronic transport get decoupled and Onsager's symmetry is verified. We explain the observed cumulative effects of voltage and electronic temperature gradients on the non-linear phonon currents by introducing a new transport coefficient that we term as the electron induced phonon thermal conductivity. It is demonstrated that under suitable operating conditions in Case (a) the dot can pump in phonons into the hotter phonon reservoirs and in Case (b) the dot can extract phonons out of the colder phonon reservoirs. Finally, as a corollary, we elaborate on how the non-linear electronic heat current can be stimulated and controlled by manipulating the temperature of the phonon reservoirs even under vanishing effective electronic charge flow.

Hierarchical Coherent Phonons in a Superatomic Semiconductor

The coupling of phonons to electrons and other phonons plays a defining role in material properties, such as charge and energy transport, light emission, and superconductivity. In atomic solids, phonons are delocalized over the 3D lattice, in contrast to molecular solids where localized vibrations dominate. Here, a hierarchical semiconductor that expands the phonon space by combining localized 0D modes with delocalized 2D and 3D modes is described. This material consists of superatomic building blocks (Re₆ Se₈ ) covalently linked into 2D sheets that are stacked into a layered van der Waals lattice. Using transient reflectance spectroscopy, three types of coherent phonons are identified: localized 0D breathing modes of isolated superatom, 2D synchronized twisting of superatoms in layers, and 3D acoustic interlayer deformation. These phonons are coupled to the electronic degrees of freedom to varying extents. The presence of local phonon modes in an extended crystal opens the door to controlling material properties from hierarchical phonon engineering.

Negative Differential Conductance Assisted by Optical Fields in a Single Quantum Dot with Ferromagnetic Electrodes

In a single quantum dot (QD) system connected with ferromagnetic electrodes, the electron transport properties, assisted by the thermal and Fock state optical fields, are theoretically studied by the Keldysh nonequilibrium Green’s function approach. The results show that the evolution properties of the density of state and tunneling current assisted by the Fock state optical field, are quite different from those of the thermal state. The photon sideband shift decreases monotonously with the increase in the electron–photon coupling strength for the case of the thermal state, while the shift is oscillatory for the case of the Fock state. Negative differential conductance (NDC) appears obviously in a QD system contacted with parallel (P) and antiparallel (AP) magnetization alignment of the ferromagnetic electrode leads, assisted by the Fock state optical field in a wide range of electron–photon interaction parameters. Evident NDC usually only arises in an AP configuration QD system assisted by the thermal state optical field. The results have the potential to introduce a new way to actively manipulate and control the single-electron tunneling transport on a QD system by the quantum states of the optical field.

Biological Self-Assembly and Recognition Used to Synthesize and Surface Guide Next Generation of Hybrid Materials

Free-standing, high aspect ratio sulfur-doped carbon nanodot-based hybrid nanowires with a microtubular aspect were synthesized using self-recognition and self-assembly processes of tubulin, a biological molecule precursor of the cytoskeletal microtubule. Physicochemical characterizations (e.g., morphology, diameter, spectral characteristics, etc.) of such user-synthesized hybrid bionanowires were performed using classical atomic and spectroscopic techniques, whereas bioactivity and functionality testing was demonstrated by mimicking cellular transport based on kinesin, a motor protein capable to recognize, and move on the microtubules. Our results indicate that user-synthesized hybrid nanowires could be manipulated in vitro under constant chemical energy of adenosine triphosphate and have the potential to be implemented in the next generation of synthetic applications from drug delivery to diagnosis systems, and photocatalytic to optical devices.

Non-linear phonon Peltier effect in dissipative quantum dot systems

Solid state thermoelectric cooling is based on the electronic Peltier effect, which cools via an electronic heat current in the absence of an applied temperature gradient. In this work, we demonstrate that equivalently, a phonon Peltier effect may arise in the non-linear thermoelectric transport regime of a dissipative quantum dot thermoelectric setup described via Anderson-Holstein model. This effect leads to an electron induced phonon heat current in the absence of a thermal gradient. Utilizing the modification of quasi-equilibrium phonon distribution via charge induced phonon accumulation, we show that in a special case the polarity of the phonon heat current can be reversed so that setup can dump heat into the hotter reservoirs. In further exploring possibilities that can arise from this effect, we propose a novel charge-induced phonon switching mechanism that may be incited via electrostatic gating.

Giant electron-hole transport asymmetry in ultra-short quantum transistors

Making use of bipolar transport in single-wall carbon nanotube quantum transistors would permit a single device to operate as both a quantum dot and a ballistic conductor or as two quantum dots with different charging energies. Here we report ultra-clean 10 to 100 nm scale suspended nanotube transistors with a large electron-hole transport asymmetry. The devices consist of naked nanotube channels contacted with sections of tube under annealed gold. The annealed gold acts as an n-doping top gate, allowing coherent quantum transport, and can create nanometre-sharp barriers. These tunnel barriers define a single quantum dot whose charging energies to add an electron or a hole are vastly different (e−h charging energy asymmetry). We parameterize the e−h transport asymmetry by the ratio of the hole and electron charging energies η_e−h. This asymmetry is maximized for short channels and small band gap tubes. In a small band gap device, we demonstrate the fabrication of a dual functionality quantum device acting as a quantum dot for holes and a much longer quantum bus for electrons. In a 14 nm-long channel, η_e−h reaches up to 2.6 for a device with a band gap of 270 meV. The charging energies in this device exceed 100 meV.

By utilizing electron-hole asymmetry in ultra-short single-walled carbon nanotube (SWCNT) transistors, McRae et al., develop ‘two-in-one' SWCNT quantum devices that can switch from behaving as quantum-dot transistors for holes to quantum buses for electrons by changing the transistor's gate voltage

Nanoelectromechanical Heat Engine Based on Electron-Electron Interaction

We theoretically show that a nanoelectromechanical system can be mechanically actuated by a heat flow through it via an electron-electron interaction. In contrast to most known actuation mechanisms in similar systems, this new mechanism does not involve an electronic current nor external ac fields. Instead, the mechanism relies on deflection-dependent tunneling rates and a heat flow which is mediated by an electron-electron interaction while an electronic current through the device is prohibited by, for instance, a spin-valve effect. Therefore, the system resembles a nanoelectromechanical heat engine. We derive a criterion for the mechanical instability and estimate the amplitude of the resulting self-sustained oscillations. Estimations show that the suggested phenomenon can be studied using available experimental techniques.

Ground-State Cooling of a Mechanical Oscillator by Interference in Andreev Reflection

We study the ground-state cooling of a mechanical oscillator linearly coupled to the charge of a quantum dot inserted between a normal metal and a superconducting contact. Such a system can be realized, e.g., by a suspended carbon nanotube quantum dot with a capacitive coupling to a gate contact. Focusing on the subgap transport regime, we analyze the inelastic Andreev reflections which drive the resonator to a nonequilibrium state. For small coupling, we obtain that vibration-assisted reflections can occur through two distinct interference paths. The interference determines the ratio between the rates of absorption and emission of vibrational energy quanta. We show that ground-state cooling of the mechanical oscillator can be achieved for many of the oscillator's modes simultaneously or for single modes selectively, depending on the experimentally tunable coupling to the superconductor.

Nonlinear thermoelectric transport in single-molecule junctions: the effect of electron-phonon interactions

In this paper, we theoretically analyze steady-state thermoelectric transport through a single-molecule junction with a vibrating bridge. The thermally induced charge current in the system is explored using a nonequilibrium Green function formalism. We study the combined effects of Coulomb interactions between charge carriers on the bridge and electron-phonon interactions on the thermocurrent beyond the linear response regime. It is shown that electron-vibron interactions may significantly affect both the magnitude and the direction of the thermocurrent, and vibrational signatures may appear.

Structures and electrical properties of single nanoparticle junctions assembled using LaC2-encapsulating carbon nanocapsules

As the miniaturization of integrated circuits advances, electronics using single molecules and nanosize particles are being studied increasingly. Single nanoparticle junctions (SNPJs) consist of two electrodes sandwiching a single nanoparticle. Nanocarbons with nanospaces in their center, such as fullerenes, carbon nanotubes, and carbon nanocapsules (CNCs), are expected to be elements of advanced SNPJs. In this study, SNPJs were assembled using lanthanum dicarbide (LaC2)-encapsulating CNCs and two gold (Au) electrodes by a nanotip operation inside a high-resolution transmission electron microscope. The atomic configuration and electrical resistance of the SNPJs were investigated in situ. The results implied that the electrical resistance of the SNPJ depended on the interface structures of the contacts between the CNC and Au electrodes, i.e., the contact electrical resistance, and the greatest portion of the current through the SNPJ flowed along the outermost carbon layer of the CNC. Thus, the resistance of the SNPJs using the CNCs was demonstrated and the electrical conduction mechanism of one of the CNC was discussed in this study.

Nuclear Dynamics at Molecule-Metal Interfaces: A Pseudoparticle Perspective

We discuss nuclear dynamics at molecule-metal interfaces including nonequilibrium molecular junctions. Starting from the many-body states (pseudoparticle) formulation of the molecule-metal system in the molecular vibronic basis, we introduce gradient expansion to reduce the adiabatic nuclear dynamics (that is, nuclear dynamics on a single molecular potential surface) into its semiclassical form while maintaining the effect of the nonadiabatic electronic transitions between different molecular charge states. This yields a set of equations for the nuclear dynamics in the presence of these nonadiabatic transitions, which reproduce the surface-hopping formulation in the limit of small metal-molecule coupling (where broadening of the molecular energy levels can be disregarded) and Ehrenfest dynamics (motion on the potential of mean force) when information on the different charging states is traced out.

Below-gap excitation of semiconducting single-wall carbon nanotubes

We investigate the optoelectronic properties of the semiconducting (6,5) species of single-walled carbon nanotubes by measuring ultrafast transient transmission changes with 20 fs time resolution. We demonstrate that photons with energy below the lowest exciton resonance efficiently lead to linear excitation of electronic states. This finding challenges the established picture of a vanishing optical absorption below the fundamental excitonic resonance. Our result points towards below-gap electronic states as an intrinsic property of semiconducting nanotubes.

Vibrational Excitation in Electron Transport through Carbon Nanotube Quantum Dots

Electron transport in single-walled carbon nanotubes (SWCNTs) is extremely sensitive to environmental effects. SWCNTs experiencing an inhomogeneous environment are effectively subjected to a disorder potential, which can lead to localized electronic states. An important element of the physical picture of such states localized on the nanometer-scale is the existence of a local vibronic mainfold resulting from the localization-enhanced electron-vibrational coupling. In this Letter, scanning tunneling spectroscopy (STS) is used to study the quantum-confined electronic states in SWCNTs deposited on the Au(111) surface. STS spectra show the vibrational overtones identified as D-band Kekulé vibrational modes and K-point transverse out-of plane phonons. The presence of these vibrational modes in the STS spectra suggests rippling distortion and dimerization of carbon atoms on the SWCNT surface. The present study thus, for the first time, experimentally connects the properties of well-defined localized electronic states to the properties of their associated vibronic states.

Fast synthesis of multilayer carbon nanotubes from camphor oil as an energy storage material

Among the wide range of renewable energy sources, the ever-increasing demand for electricity storage represents an emerging challenge. Utilizing carbon nanotubes (CNTs) for energy storage is closely being scrutinized due to the promising performance on top of their extraordinary features. In this work, well-aligned multilayer carbon nanotubes were successfully synthesized on a porous silicon (PSi) substrate in a fast process using renewable natural essential oil via chemical vapor deposition (CVD). Considering the influx of vaporized multilayer vertical carbon nanotubes (MVCNTs) to the PSi, the diameter distribution increased as the flow rate decreased in the reactor. Raman spectroscopy results indicated that the crystalline quality of the carbon nanotubes structure exhibits no major variation despite changes in the flow rate. Fourier transform infrared (FT-IR) spectra confirmed the hexagonal structure of the carbon nanotubes because of the presence of a peak corresponding to the carbon double bond. Field emission scanning electron microscopy (FESEM) images showed multilayer nanotubes, each with different diameters with long and straight multiwall tubes. Moreover, the temperature programmed desorption (TPD) method has been used to analyze the hydrogen storage properties of MVCNTs, which indicates that hydrogen adsorption sites exist on the synthesized multilayer CNTs.

Ground-state cooling of a carbon nanomechanical resonator by spin-polarized current

We study the nonequilibrium steady state of a mechanical resonator in the quantum regime realized by a suspended carbon nanotube quantum dot in contact with two ferromagnets. Because of the spin-orbit interaction and/or an external magnetic field gradient, the spin on the dot couples directly to the flexural eigenmodes. Accordingly, the nanomechanical motion induces inelastic spin flips of the tunneling electrons. A spin-polarized current at finite bias voltage causes either heating or active cooling of the mechanical modes. We show that maximal cooling is achieved at resonant transport when the energy splitting between two dot levels of opposite spin equals the vibrational frequency. Even for weak electron-resonator coupling and moderate polarizations we can achieve ground-state cooling with a temperature of the leads, for instance, of T = 10 ω.

Franck-Condon blockade in a single-molecule transistor

We investigate vibron-assisted electron transport in single-molecule transistors containing an individual Fe4 Single-Molecule Magnet. We observe a strong suppression of the tunneling current at low bias in combination with vibron-assisted excitations. The observed features are explained by a strong electron-vibron coupling in the framework of the Franck-Condon model supported by density-functional theory.

Suppression of electron-vibron coupling in graphene nanoribbons contacted via a single atom

Graphene nanostructures, where quantum confinement opens an energy gap in the band structure, hold promise for future electronic devices. To realize the full potential of these materials, atomic-scale control over the contacts to graphene and the graphene nanostructure forming the active part of the device is required. The contacts should have a high transmission and yet not modify the electronic properties of the active region significantly to maintain the potentially exciting physics offered by the nanoscale honeycomb lattice. Here we show how contacting an atomically well-defined graphene nanoribbon to a metallic lead by a chemical bond via only one atom significantly influences the charge transport through the graphene nanoribbon but does not affect its electronic structure. Specifically, we find that creating well-defined contacts can suppress inelastic transport channels.

Ultraclean single, double, and triple carbon nanotube quantum dots with recessed Re bottom gates

We demonstrate that ultraclean single, double, and triple quantum dots (QDs) can be formed reliably in a carbon nanotube (CNT) by a straightforward fabrication technique. The QDs are electrostatically defined in the CNT by closely spaced metallic bottom gates deposited in trenches in SiO2 by sputter deposition of Re. The carbon nanotubes are then grown by chemical vapor deposition (CVD) across the trenches and contacted using conventional resist-based electron beam lithography. Unlike in previous work, the devices exhibit reproducibly the characteristics of ultraclean QDs behavior even after the subsequent electron beam lithography and chemical processing steps. We specifically demonstrate the high quality using CNT devices with two narrow bottom gates and one global back gate. Tunable by the gate voltages, the device can be operated in four different regimes: (i) fully p-type with ballistic transport between the outermost contacts (over a length of 700 nm), (ii) clean n-type single QD behavior where a QD can be induced by either the left or the right bottom gate, (iii) n-type double QD, and (iv) triple bipolar QD where the middle QD has opposite doping (p-type). Our simple fabrication scheme opens up a route to more complex devices based on ultraclean CNTs, since it allows for postgrowth processing.

Franck–Condon factors—Computational approaches and recent developments

Algorithms and methodologies for the calculation of Franck–Condon integrals are reviewed. Starting from the standard approach based on the Cartesian representation of the normal modes and the use of Duschinsky's transformation and recursion formulas, methods for treating large displacements of the equilibrium positions and anharmonic and non-Condon effects are considered. Finally, focusing attention on problems arising from the use of recurrence relations, some of the proposed solutions are critically reviewed along with new alternative approaches.

Transient heat generation in a quantum dot under a step-like pulse bias

We study the transient heat generation in a quantum dot system driven by a step-like or a square-shaped pulse bias. We find that a periodically oscillating heat generation arises after adding the sudden bias. One particularly surprising result is that there exists a heat absorption from the zero-temperature phonon subsystem. Thus the phonon population in non-equilibrium can be less than that of the equilibrium electron-phonon system. In addition, we also ascertain the optimal conditions for the operation of a quantum dot with the minimum heat generation.

Few-hundred GHz carbon nanotube nanoelectromechanical systems (NEMS)

We study 23-30 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 nanoelectromechanical systems (NEMS), we measure fundamental bending frequencies f(bend) ≈ 75-280 GHz and extract quality factors Q ∼ 10(6). The NEMS's frequencies can be tuned by a factor of 2 with tension induced by mechanical breakjunctions actuated by an electrostatic force or tension from bent suspended electrodes.

Sensing with superconducting point contacts

Superconducting point contacts have been used for measuring magnetic polarizations, identifying magnetic impurities, electronic structures, and even the vibrational modes of small molecules. Due to intrinsically small energy scale in the subgap structures of the supercurrent determined by the size of the superconducting energy gap, superconductors provide ultrahigh sensitivities for high resolution spectroscopies. The so-called Andreev reflection process between normal metal and superconductor carries complex and rich information which can be utilized as powerful sensor when fully exploited. In this review, we would discuss recent experimental and theoretical developments in the supercurrent transport through superconducting point contacts and their relevance to sensing applications, and we would highlight their current issues and potentials. A true utilization of the method based on Andreev reflection analysis opens up possibilities for a new class of ultrasensitive sensors.

Current-induced forces in mesoscopic systems: A scattering-matrix approach

Nanoelectromechanical systems are characterized by an intimate connection between electronic and mechanical degrees of freedom. Due to the nanoscopic scale, current flowing through the system noticeably impacts upons the vibrational dynamics of the device, complementing the effect of the vibrational modes on the electronic dynamics. We employ the scattering-matrix approach to quantum transport in order to develop a unified theory of nanoelectromechanical systems out of equilibrium. For a slow mechanical mode the current can be obtained from the Landauer-Büttiker formula in the strictly adiabatic limit. The leading correction to the adiabatic limit reduces to Brouwer's formula for the current of a quantum pump in the absence of a bias voltage. The principal results of the present paper are the scattering-matrix expressions for the current-induced forces acting on the mechanical degrees of freedom. These forces control the Langevin dynamics of the mechanical modes. Specifically, we derive expressions for the (typically nonconservative) mean force, for the (possibly negative) damping force, an effective "Lorentz" force that exists even for time-reversal-invariant systems, and the fluctuating Langevin force originating from Nyquist and shot noise of the current flow. We apply our general formalism to several simple models that illustrate the peculiar nature of the current-induced forces. Specifically, we find that in out-of-equilibrium situations the current-induced forces can destabilize the mechanical vibrations and cause limit-cycle dynamics.

Cooling of nanomechanical resonators by thermally activated single-electron transport

We show that the vibrations of a nanomechanical resonator can be cooled to near its quantum ground state by tunneling injection of electrons from a scanning tunneling microscope tip. The interplay between two mechanisms for coupling the electronic and mechanical degrees of freedom results in a bias-voltage-dependent difference between the probability amplitudes for vibron emission and absorption during tunneling. For a bias voltage just below the Coulomb blockade threshold, we find that absorption dominates, which leads to cooling corresponding to an average vibron population of the fundamental bending mode of 0.2.

A novel ionic polymer metal ZnO composite (IPMZC)

The presented research introduces a new Ionic Polymer-Metal-ZnO Composite (IPMZC) demonstrating photoluminescence (PL)-quenching on mechanical bending or application of an electric field. The newly fabricated IPMZC integrates the optical properties of ZnO and the electroactive nature of Ionic Polymer Metal Composites (IPMC) to enable a non-contact read-out of IPMC response. The electro-mechano-optical response of the IPMZC was measured by observing the PL spectra under mechanical bending and electrical regimes. The working range was measured to be 375–475 nm. It was noted that the PL-quenching increased proportionally with the increase in curvature and applied field at 384 and 468 nm. The maximum quenching of 53.4% was achieved with the membrane curvature of 78.74/m and 3.01% when electric field (12.5 × 10³ V/m) is applied. Coating IPMC with crystalline ZnO was observed to improve IPMC transduction.

Exploring the possibilities to control the molecular switching properties and dynamics: A field-switchable rotor-stator molecular system

A bistable, dipolar stator-rotor molecular system-candidate for molecular electronics is investigated. We demonstrate that it is possible to control the intramolecular torsional states and dynamics in this system by applying an appropriate additional electric field (instead of biasing one), achieving fine tuning and modulation of the relevant properties. The electric field effects on the quantities responsible for torsional dynamics (potential energy surface, potential barrier height, quantum and classical transition probabilities, correlation time, HOMO-LUMO gap) are studied from first principles. Our results indicate that it is possible to artificially stabilize the metastable conformational state of the studied molecule. The importance of this is evident, as the current-voltage characteristics of the metastable state are clearly distinguishable from the current-voltage characteristics of the two stable states. We report for the first time exact calculations related to the possibilities to control the thermally induced stochastic switching, and reduce the noise in a practical application. Thus, we believe that the molecule studied in this paper could operate as a field-switchable molecular device under real conditions.

One-dimensional germanium nanostructures--formation and their electron field emission properties

Ge nanostructures were synthesized by reduction of GeO(2) in H(2) atmosphere at various temperatures. Entangled and straight Ge nanowires with oxide shells were grown at high temperatures. Ge nanowires with various numbers of nodules were obtained at low temperatures. Ge nanowires without nodules exhibited remarkable field emission properties with a turn-on field of 4.6 V µm(-1) and field enhancement factor of 1242.

Doped graphene as tunable electron-phonon coupling material

We present a new way to tune the electron-phonon coupling (EPC) in graphene by changing the deformation potential with electron/hole doping. We show the EPC for highest optical branch at the high symmetry point K acquires a strong dependency on the doping level due to electron-electron correlation not accounted in mean-field approaches. Such a dependency influences the dispersion (with respect to the laser energy) of the Raman D and 2D lines and the splitting of the 2D peak in multilayer graphene. Finally this doping dependence opens the possibility to construct tunable electronic devices through external control of the EPC.

Internal resonance of vibrational modes in single-walled carbon nanotubes

We show, by molecular dynamics simulations, that 2:1 internal resonance may occur between a radial breathing mode (RBM) and a circumferential flexural mode (CFM) in single-walled carbon nanotubes (SWCNTs). When the RBM vibration amplitude is greater than a critical value, automatic transformations between the RBM and CFM with approximately half RBM-frequency are observed. This discovery in the discrete SWCNT atom assembly is similar to the 2:1 internal resonance mechanism observed in continuum shells. A non-local continuum shell model is employed to determine the critical conditions for the occurrence of observed 2:1 internal resonance between the RBM and CFMs based on two non-dimensional parameters and the Mathieu stability diagram.

Interference effects on vibration-mediated tunneling through interacting degenerate molecular states

We study the combined effects of quantum electronic interference and Coulomb interaction on electron transport through near-degenerate molecular states with strong electron-vibration interaction. It is found that quantum electronic interference strongly affects the current and its noise properties. In particular, destructive interference induces pronounced negative differential conductances (NDCs) accompanying the vibrational excited states, and such NDC characters are not related to asymmetric tunnel coupling and are robust to the damping of a thermal bath. In a certain transport regime, the non-equilibrium vibration distribution even shows a peculiar sub-Poissonian behavior, which is enhanced by quantum electronic interference.

Pumping of vibrational excitations in the coulomb-blockade regime in a suspended carbon nanotube

Low-temperature transport spectroscopy measurements on a suspended few-hole carbon nanotube quantum dot are presented, showing a gate-dependent harmonic excitation spectrum which, strikingly, occurs in the Coulomb-blockade regime. The quantized excitation energy corresponds to the scale expected for longitudinal vibrations of the nanotube. The electronic transport processes are identified as cotunnel-assisted sequential tunneling, resulting from nonequilibrium occupation of the mechanical mode. They appear only above a high-bias threshold at the scale of electronic nanotube excitations. We discuss models for the pumping process that explain the enhancement of the nonequilibrium occupation and show that it is connected to a subtle interplay between electronic and vibrational degrees of freedom.

Orbital and charge-resolved polaron states in CdSe dots and rods probed by scanning tunneling spectroscopy

Conduction electrons interact with lattice vibrations, leading to coupled electron-phonon states. This effect is of fundamental importance in understanding electron transport and energy relaxation in nanoscale systems. We report quantitative results on the electron-phonon coupling strength in CdSe quantum dots (QDs) and rods, obtained by low-temperature scanning tunneling microscopy. We resolve the polaron eigenstates arising from coupling of the electrons to longitudinal optical phonons. The electron-phonon coupling strength is dependent on the electron orbital symmetry, the number of added electrons, and the size and dielectric environment of the QD.

Dynamical chiral symmetry breaking in sliding nanotubes

We discover in simulations of sliding coaxial nanotubes an unanticipated example of dynamical symmetry breaking taking place at the nanoscale. While both nanotubes are perfectly left-right symmetric and nonchiral, a nonzero angular momentum of phonon origin appears spontaneously at a series of critical sliding velocities, in correspondence with large peaks of the sliding friction. The nonlinear equations governing this phenomenon resemble the rotational instability of a forced string. However, several new elements, exquisitely "nano" appear here, with the crucial involvement of umklapp and of sliding nanofriction.