Session N20: Focus Session: Mesoscopics - Tunneling

Scanning gate spectroscopy of a quantum Hall island near a quantum point contact

We report on low temperature (100 mK) scanning gate experiments performed at high magnetic field (around 10 T) on a mesoscopic device patterned in an InGaAs/InAlAs heterostructure. Magnetotransport measurements yield signatures of ultra-small Quantum Hall Islands (QHI) formed by closed quantum Hall edge states and connected to propagating edge channels through tunnel barriers. Scanning gate microscopy and scanning gate spectroscopy are used to locate and probe a single QHI near a quantum point contact. The presence of Coulomb diamonds in the local spectroscopy confirms that Coulomb blockade governs transport across the QHI. Varying the microscope tip bias as well as current bias across the device, we uncover the QHI discrete energy spectrum arising from electronic confinement and we extract estimates of the gradient of the confining potential and of the edge state velocity.   

Noise Intensity-Intensity Correlations and the Fourth Cumulant of Current Fluctuations

We report measurements of the correlation between intensities of noise at different frequencies on a tunnel junction under ac excitation. We show that such correlations exist only for certain relations between the excitation frequency and the two detection frequencies, which are similar to three-wave and four-wave mixing in optics, depending on the dc bias of the sample. We demonstrate that the correlation we measure is proportional to the fourth cumulant of current fluctuations.   

Optical Spectra of Au Nanoparticle Arrays in Grating Templates on a Silver Mirror

Reflection spectra of close packed spherical gold nanoparticle assemblies in grating templates on a silver film covered with a thin dielectric spacer are studied in a wide range of incidence angles. Wide spectral minina corresponding to the plasmonic eigen-modes of the nanoparticle arrays are observed and compared with spectra of empty gratings. These minima correspond to extended optical bands of the arrays formed due to the strong interactions between localized plasmons modes of nanoparticles, silver film surface plasmons and grating resonances. From the angular variations of the spectra we obtain the dispersion of plasmonic excitations which yield a strong amplificatin of the light intensity in our system. Raman signal enhancement for Benzenethiol molecules in the gaps between nanoparticles is estimated as 3x10$^{10}$. The intense light amplification in a wide spectral range and the large number of regular hot spots makes our structures an advanced platform for optical sensing, solid state lighting, and solar harvesting technologies.   

Quantum Mesoscopic Physics of Electrons and Photons

We first review basic notions of coherent quantum transport at the mesoscopic scale for both electronic and photonic systems. We then show that successful descriptions developed for coherent electronic transport (e.g. weak localization and UCF) and thermodynamics (persistent currents), noise and full counting statistics can be extended and applied to the study of Quantum Electrodynamics of quantum conductors and of quantum optics based on photons emitted by such conductors. In this context, we discuss the two following specific problems : (1) Ramsey fringes and time domain interference for particle creation form a quantum vacuum with a specific application to dynamical Coulomb blockade. In that setup, the current noise of a coherent conductor is biased by two successive voltage pulses. An interference pattern between photon assisted processes is observed which is explained by the contribution of several processes to the probability to emit photons after each pulse. Recent experiments in this context will be discussed. (2) Quantum emitter coupled to a fractal environment. A new and unexpected type of oscillatory structures for the probability of spontaneous emission has been obtained which results from the fractal nature of the quantum vacuum. When applied to the case of a tunnel junction as a quantum emitter of photons, the same oscillatory structure arises for the conductance of the tunnel junction.   

Squeezing in Photo-assisted Electron Quantum Shot Noise

The current/voltage fluctuations generated by a conductor are another point of view of a randomly fluctuating electromagnetic field, i.e. ``white'' light. We demonstrate experimentally that this light is naturally squeezed, i.e. that the noise on one quadrature can go below the vacuum fluctuations, for a tunnel junction at very low temperature irradiated by a microwave. A classical current in a conductor generates a coherent state of light. We show that a quantum current can emit non-classical light.   

Electron tunneling in chaotic quantum ring

Single electron confinement states of two dimensional InAs/GaAs quantum ring (QR) are considered under the effective approach [1]. The symmetry of the QR shape is violated as it is in well-known Bohigas annular billiard [2]. For weak violation of the symmetry, the energy spectrum may be represented by a set of quasi-doublets. We study the correlation between electron localizations and quasi-doublet splitting for complete spectrum. The bands with different ``radial'' quantum numbers are well determined within our calculations. The inter-band tunneling is considered in relation to the chaotic properties of the QR. We propose an alternative interpretation of the experimental data [3] to that made in Ref. [3], where the ``first experimental evidence for chaos-assisted tunneling'' in a microwave annular billiard was reported. We show that this effect can be explained by inter-band tunneling that occurs due to the anti-crossing of the levels having different ``radial'' quantum numbers. [1] I. Filikhin, V. M. Suslov and B. Vlahovic, Phys. Rev. B 73, 205332 (2006). [2] O. Bohigas, D. Boose, R. Egydio de Carvallho, and V. Marvulle, Nucl. Phys. A 560, 197 (1993). [3] C. Dembowski et al., PRL 84 (2000) 867; R. Hofferbert et al., Phys. Rev. E 71, 046201 (2005).   

Emergence of the collective-response of granular solid - liquid mixtures to wave- pulse excitations

The phenomenon of emergence of new properties observed in the collection of solid particles in liquid, due to pulse-wave excitations, can be found in many macroscopic systems. In this paper the uniform mixtures of solid spherical grains in water were subjected to high intensity, 60-Volts amplitude, pulsed -Ultrasonic waves of 45 kHz peak frequency. The observed response of the mixture was imbedded in the modified transmitted pulse, and could be extracted and compared to that of a reference pulse. Analysis of the results, in the frequency and time domains, includes; Fast Fourier Transform, amplitude and phase changes, and frequency dependent attenuation. The experimental findings and numerical results show that, the response of the mixture can be made independent of the scale, up to relatively small scale. The findings also show that, several collective- response to elastic wave propagation in the crystalline solids at the atomic scale, such as; cut-off frequency, tunneling effect, and absorption and conduction bands, can also have analogous ones in intermediary, and equivalences in these relatively simple mixtures.   

Theory of Solvent-Mediated Environmental Effects on Transport in Molecular Junctions

Single-molecule junctions, formed with well-defined and robust metal-molecule contacts, can provide an ideal model system to study mechanisms of charge transport at the molecular scale. However, the presence of solvent is often unavoidable, and recent experiments have shown that the junction conductance can be altered by a factor of two depending on the solvent present. Here, we use a first-principles scattering-state approach, based on self-energy corrected density functional theory (DFT), to explore how solvent and coverage impacts the transmission and conductance of bipyridine-Au molecular junctions. We find the conductance can shift by more than a factor of 5 by varying the bipyridine coverage, which is an effect associated with work function shifts that can be understood with a 2D polarizable dipole model fit with DFT values. A generalization of this electrostatic model to include solvent molecules allows us to estimate the work function shift for a mixed molecular coverage based both on the experimental parameters and system thermodynamics. By combining the results of our transmission calculations and the electrostatic model, we can accurately describe the conductance shifts observed experimentally. We acknowledge DOE for support, and NERSC for computational resources.   

Correlating Molecular Energy Level Alignment with the Conductance of Single Molecular Junctions

There has been increased interest in understanding electronic and thermoelectric transport in single molecule junctions and metal/organic interfaces. While the ionization potential and electron affinity of molecules can be calculated for molecules in the gas-phase, additional physical effects, such as charge transfer and rearrangement, hybridization, and electrode polarization are expected to alter these electronic energies for molecular junctions. Therefore, it is hard to determine energy level alignments in molecular junctions. Here, we determine the relationship between electronic energy level alignment at a metal-molecule-metal interface and single-molecule junction conductance properties for 4,4'-bipyridine via direct and simultaneous measurement of electrical and thermoelectric currents using a scanning tunneling microscope-based break-junction technique. We measure directly, the position of the lowest unoccupied molecular orbital (LUMO) relative to the Au Fermi level assuming a Lorentzian resonance lineshape. Furthermore, we correlate the energy level alignment and coupling strength between two conductance states through repeated junction elongation and compression. We find that these values are in excellent agreement with our self-energy corrected density functional theory calculations. These results thus provide the first evidence for correlation between energy level alignment and single molecule transport.   

Transition from Coulomb Diamonds to Checherboard-like Spectroscopies in a Mesoscopic Quantum Hall Interferometer

We report low temperature ($\sim$ 100 mK) magnetotransport, scanning gate microscopy and scanning gate spectroscopy measurements in an $\rm In_{0.7}Ga_{0.3}As/In_{0.52}Al_{0.48}As$ quantum point contact (QPC). The magnetoresistance of the QPC shows oscillations in the vicinity of integer quantum Hall states. We attribute these magnetoresistance oscillations to the formation of an electron interferometer around a small, disorder-induced quantum Hall island located within the constriction. The magnetic field $B$ tunes the edge states configuration in the QPC, leading to different signatures in the transport measurements. Interestingly, near the Landau level filling factor $\nu=3$, the spectroscopy measurements performed on the quantum Hall interferometer, as a function of $B$ or scanning gate tip voltage, exhibit a smooth transition from Coulomb diamonds to a checkerboard pattern.   

Fluctuations of g factors of discrete levels in ferromagnetic nanoparticles

It has been known that the interplay between electron-electron interactions and spin-orbit scattering can cause a wide distribution of g factors in tunneling spectra of metallic nanoparticles, including g-factors much larger than 2 if electron-electron interactions are strong. Here, we present our studies of single Co nanoparticles in Al/Al$_2$O$_3$/(Co nanoparticles)/Al$_2$O$_3$/Al tunnel junctions using electron tunneling spectroscopy at mK-temperatures. The g factor of discrete energy levels exhibits significant difference between minority-spin and majority-spin levels. We have clearly observed large g factors ($\approx$ 6) in one sample at magnetic field greater than 4T, suggesting $\Delta S=3/2$ in the tunneling transition, $S$ is the magnitude of the spin. We will present the latest results on tunneling junctions containing Ni, Permalloy or Gd nanoparticles, which have weaker magnetic anisotropy fluctuations.   

Band edge noise spectroscopy

Although metal/insulator interfaces are expected to play a major role in charge, spin and phonon flow, little is known about the real underlying band structure. The reason is the difficulty in directly obtaining this information from interfaces by the use of a non-invasive physical tool. Here we introduce and demonstrate the feasibility of a conceptually new method that enables us to gather information on the interface electron bands. The low frequency and low temperature noise measurements as a function of applied bias voltage clearly reveal the appearance of the electron band edges at the Fermi level. By analyzing the bias dependence of the normalized 1/f noise (Hooge factor) in Fe1-xVx/MgO/Fe (with 0\textless\ x\textless\ 0.16) epitaxial magnetic tunnel junctions with diminished misfit dislocations, we observe strong anomalies in the 1/f noise at specific voltages where the band edges of the ferromagnetic electrodes which form the tunnel junction are expected to cross the Fermi level. These effects, understood within a simple model of 1/f noise due to localized states near the band edges, open up new perspectives for a reliable ``in situ'' characterization of electron bands in normal metal or spintronic devices.