Session Y17: Nanostructures and Interfaces: Electrons, Phonons, and Plasmons

Transport and Kondo correlations in magnetic break junction devices

A single molecule transistor device fabricated with a break junction technique is utilized as a tunable model system for probing transport properties of the highly correlated Kondo state. The emergence of this collective phenomenon occurs due to an antiferromagnetic interaction between conduction electrons and a local magnet moment represented by an unpaired spin on the molecule. Low energy non-equilibrium conductance measured in the Kondo regime has been shown to obey a particular scaling relationship with respect to different perturbations. Devices are now fabricated with ferromagnetic and strongly paramagnetic electrodes, and the applicability of such scaling behavior is investigated in the presence of magnetic interactions and anomalous transport characteristics.   

Phonon transmission across Si/Ge interface from first-principles by the Green's function method

Modeling phonon transmission as a function of phonon frequency and incidence angle is vital for multiscale modeling of heat transport in nanostructured materials. In this study, we calculate the phonon transmission in three-dimensions via Green's function method. It will be applied to silicon/germanium interface for which the force constants will be calculated from either the Stillinger-Weber semi-empirical potential, or from first-principles density functional methods. Both the perfect interface and the rough interface will be investigated. The transmission as a function of interface roughness will give us more guidance for surface engineering. Results between first-principles and the SW potential will be compared to see how reliable the predictions from SW potential are. The contribution of optical modes is illustrated by comparing the results will the prediction of the acoustic mismatch model (AMM) which is also harmonic and the long-wavelength limit of the general theory. It will now be possible to integrate the information on momentum and frequency-dependent transmission and the bulk mean free paths, both calculated from first-principles DFT, to accurately model heat transport in complex nanostructured materials.   

Ultrafast Characterization of Nanostructures in GaAs

We combine ultrafast pump-probe and optoacoustic spectroscopy with magneto-optical Kerr rotation measurements to characterize embedded, self-assembled magnetically active nanostructures in a GaAs host matrix. We observe variations in the pump-probe and optoacoustic signals depending on the composition and growth characteristics of the embedded layers. Further, we observe (a) distinct behaviors in the femtosecond response of the composite structures when the probe photon energy is tuned near the GaAs band edge and (b) strong modulation of the optoacoustic signal inside the embedded layer. These results indicate an effective change in the transient femtosecond response of the composite structure, likely originating in strain effects due to the presence of nanoparticles within the host lattice. We additionally probe ultrafast magneto-optical interactions through time-resolved Kerr measurements. Finally, we present a potential method for high-resolution depth-dependent magnetic characterization by combining the Kerr rotation and optoacoustic experimental techniques.   

Position-dependent Diffusion Coefficient as Localization Criterion in Non-Conservative Random Media

Characterization of different regimes of electromagnetic wave transport in random media is an important area of research with ramifications in condensed matter physics. For passive systems, transport can be either ballistic, diffusive, or localized. Wave corrections to otherwise classical transport in a finite random media result in position-dependent diffusion coefficient $D(z)$. If media is active or dissipative (contains optical gain or absorption), then $D(z)$ can be used to distinguish a multitude of distinct wave transport regimes. Using a numerical model, we validate the existence of the position-dependent diffusion, including for media with absorption. We have previously developed a phase space enumerating all regimes exhibiting distinct transport behavior. Here we present recent results from our numerical simulations which demonstrate the ability to use $D(z)$ to distinguish between these regimes.   

Probing phonon surface scattering in nanostructures

In insulating materials, heat is transmitted by atomic vibrations (``phonons''). In nanostructured materials such as nanowires and nanosheets, the characteristic length scale of a material can be less than the mean free path of a phonon. The phonon transport is then drastically altered and becomes dominated by scattering from surfaces. We demonstrate a method to assess the scattering rate and transmission factor of phonons traversing a silicon nanosheet. Generation and detection of phonons is accomplished by a superconducting tunnel junction attached to the silicon nanostructure and operated at a temperature of 0.3K. Decay of excited states in the superconductor is employed as a tunable narrow-band source of phonons [1,2]. This tunable source enables investigation of the phonon mean free path as a function of phonon frequency and surface roughness, for frequencies from $\sim$100 GHz to $\sim$500 GHz in nanosheets 100 to 200 nm thick. This work is supported by DOE (DE-SC0001086). \\[4pt] [1] H. Kinder. Phys. Rev. Lett. 28, 1564 (1972)\\[0pt] [2] J. B. Hertzberg et al, Rev. Sci. Inst. 82, 104905 (2011).   

Self-assembled, vertically aligned, epitaxial nanoscale $p-n$ heterojunctions for thin film based photovoltaic applications

Using \textit{rf-} sputtering technique we have exploited phase-separated self-assembly and developed epitaxial, nanostructured composite films composed of phase separated, and vertically oriented $p-n$ interfacial nanocolumns of Cu$_{2}$O (p type; 2eV bandgap) and TiO$_{2 }$(n type; 3.2 eV bandgap). The characteristic band gaps of these phases allow extension of the solar capture from ultraviolet to a visible wavelenght. The composite films were grown on perovskite substrates and exhibit single crystalline epitaxy in both phases. We have investigated crystalline structure, interfacial quality and optical properties of the nanopillar arrays using XRD, TEM, SEM, AFM, and optical absorption techniques. Here, we show nearly complete atomic order at Cu$_{2}$O-TiO$_{2}$ interface (i.e., $p-n $junction) and an absorption profile that captures a wide range of solar spectrum extending from ultraviolet to visible wavelengths. Compared to layered thin film architectures, the use of such vertically aligned nanostructures in solar cells can promote cost-effective fabrication of high efficiency PV devices by providing low defect concentrations, improved absorption and light trapping capabilities, and increased minority carrier diffusion lengths.   

Ab initio calculation of the electron-phonon coupling for transport

We have developed an approach which enables us to compute matrix elements of the electron-phonon coupling within the density functional perturbation theory for the electronic interaction with short-wavelength phonons.\footnote{J. Sjakste, N. Vast, V. Tyuterev, Phys. Rev. Lett. 99, 236405 (2007).} Combining this {\it ab initio} approach to the Boltzmann transport equation, we have obtained the thermoelectric coefficients of silicon.\footnote{Z. Wang, S. Wang, S. Obukhov, N. Vast, J. Sjakste, V. Tyuterev, and N. Mingo, Phys. Rev. B 83, 205208 (2011).} The lifetime of the 2p$_0$ shallow impurity state in doped-silicon turns out to be shorter than expected.\footnote{V. Tyuterev, J. Sjakste, N. Vast, Phys. Rev. B 81, 245212 (2010)} The lifetime of the exciton in germanium under pressure\footnote{V.G. Tyuterev and S.V. Obukhov N. Vast and J. Sjakste,Phys. Rev. B 84, 035201} is found to be well described. Effect of the material nanostructuring on the electron-phonon coupling constants will be shown for small semiconducting superlattices. Finally, the calculation of deformation potentials for intravalley scattering will be discussed, and results shown for silicon and for bismuth, which is the prototype material for thermoelectricity.   

Effect of Interlayer Interaction on the Structural, Electronic, and Thermal Properties of Layered MS$_2$ (M=W, Mo) Structures

Using density functional theory (DFT) supplemented with van der Waals interaction, we investigate the effect of interlayer interaction on the structural, electroinic, and thermal properties of transition-metal disulfides MS$_2$, such as MoS$_2$ and WS$_2$. We calculate the relative stability of various layer-layer stacking configurations determined by considering relative positions and orientations between neighboring layers. We find that MS$_2$ layers may slide over each other with a small sliding barrier. We explore the effect of layer stacking on the electronic structure, and find an intriguing coupling effect. We also calculate their thermal properties including thermal expansion behavior especially along the direction normal to the plane. Such thermal expansion behavior is considered for our study of Li-intercalation into layered MS$_2$, which may become a fundamental understanding for future development of Li-ion battery. We evaluate thoroughly the diffusion paths and barriers in between layers and compare them with those on the surface. Interestingly we find that the diffusion barrier between layers is ${\sim}100$~meV smaller than that on a single layer, implying layered MS$_2$ may be a good candidate for Li-ion battery electrodes.   

Surface plasmon modes management by Thompson plasmonics

We have studied the dispersion and propagation of the surface plasmons in a structure consisting of a metal slab and a dielectric slab, the latter of which contains randomly distributed small metal particles. In our model, the metal material is characterized by the Drude model and the pudding structure is studied with Maxwell-Garnett effective medium theory. This construction of material can bring a new hybridized band in the dispersion relation where light has a relatively small group velocity. The geometric profile of volume fraction of metal balls in pudding structure can effectively change the behaviour of the plasmon propagation. For example, by adding a parabolic confinement, it is shown by the Hamiltonian optics that the light propagation is trapped, i.e., the light experiences an oscillation in a small space. Experimentally, the confinement condition can be achieved with various means, thus it may be useful in development of new mechanism of solar cell.   

Tunable Propagation and Localization of Hybrid Surface Plasmon Polaritons in Chirped Metal-Dielectric Waveguides

We have studied the propagation of coupled surface plasmon polariton (SPP) waves in a metal-dielectric-metal (e.g. Au/MgF2/Au) waveguide by the transfer matrix method. Due to the evanescent coupling of the SPP waves at the two interfaces in the dielectric layer, three hybridized surface plasmon polariton (HSPP) branches are achieved with a nearly flat branch at intermediate frequencies. The flat HSPP branch is tunable by varying the thickness and/or the permittivity of the dielectric layer. Moreover, by imposing a gradual variation of the permittivity (or thickness) of the dielectric layer along the propagation direction, it is possible to alter the local dispersion relation so that a localization of SPP waves can be realized. Under a parabolic confinement, Hamiltonian optics is used for simulating the propagation and localization of HSSP waves. We demonstrate that HSSP are localized at different locations at different frequencies, which is useful for achieving trapped SPP rainbow. The results of this research can have fruitful applications in optical computing, etc.   

Energy concentration in plasmonic nanostructures: Green function formalism

We have developed the Green function formalism (GFF), which can be used to study the field distribution and electrostatic resonance of different nanostructures. In the GFF, a surface integral equation is formulated for the scalar potential for an arbitrary number of nanostructures of various shapes. This formalism has the advantage of avoiding matching the complicated boundary conditions on the surfaces of the nanostructure. In particular, we have studied the cases of two approaching metal cylinders and non-touching metal crescent under a uniform applied electric field. It is shown that there is an energy concentration within the air narrow gap and the metal narrow gap in the cases of approaching cylinders and non-touching crescent respectively. The numerical GFF results are compared with the analytic results by conformal transformation. The results are useful in designing plasmonic light-havesting devices.   

Circular Dichroism and Spin Polarization of Rashba Split Surface States on a Bi/Ag Surface Alloy

The Bi/Ag surface alloy possesses a huge Rashba splitting in its surface bands due to the prominent corrugation in the surface reconstruction and the large atomic spin-orbit coupling of the Bi atom. This system is an intriguing candidate to realize the 2D p$_{x}$+ip$_{y}$ superconductor and further, Majorana states. In this work, we study the electronic structure of the Bi/Ag surface alloy prepared by depositing Bi onto ultrathin Ag films followed by annealing. The electronic structure of the system is measured using circular angle resolved photoemission spectroscopy (CARPES). The results reveal two interesting phenomena: the hybridization of spin polarized surface states with Ag bulk quantum well states and the umklapp scattering by the perturbed surface potential. In addition, our CARPES spectra show clearly a unique dichroism pattern which is closely related to the spin texture of this 2D strongly spin-orbit coupled electron system.   

Atomic structure, energetics, and dynamicsof topological solitons inindium chains on Si(111) surfaces

Besides the presence of exotic ground states, potentially more intriguing are the elementary excitations of the One-dimensional charge density waves (1D-CDWs), including the nonlinear topological excitation or soliton. Solitons may possess spin-charge inversion properties, and act as the effective carriers that account for the high conductivity in conducting polymers. Howevercomprehensive quantitative study of topological solitary excitations at the atomic level remains a challenge. In this talk, I will present our recent work on the quantitative haracterization of solitons in In chains grown on Si(111) surfaces at atomic scale. The precise atomic structure of the topological soliton in In/Si(111) is determined based on scanning tunneling microscopy and first-principles calculations.Variable emperature measurements of the soliton population allow us to determine the soliton formation energy to be $\sim $60 meV, smaller than one-half of the band gap of $\sim $200 meV. Once created, these solitons have very low mobility; the sluggish nature is attributed to the exceptionally low attempt frequency for soliton migration. We furtherdemonstrate local electric ?eld enhanced soliton dynamics, and the feasibility of aggregating solitonsinto soliton polymers.