Session D15: Focus Session: Atomic Tunneling, Films, Nanostructures

Dephasing of Atomic Tunneling by Nuclear Quadrupoles

A few years ago strong magnetic field effects have been observed in low-temperature dielectric susceptibility measurements and in coherent polarization echo experiments on certain glasses and disordered crystals. The low-temperature dielectric properties of such materials are dominated by atomic tunneling systems. It has been suggested that the surprising magnetic field dependence is caused by tunneling particles with nuclear quadrupole moment. For such systems there exists a coupling of the tunneling motion with the nuclear quadrupole moments experiencing the electric field gradients in the localized states. This coupling gives rise to a quadrupole splitting of the energy levels of the tunneling systems. The applied magnetic field then leads to an additional Zeeman splitting of the nuclear levels. We review the results of polarization echo experiments in magnetic fields and discuss the role of the nuclear spins in such experiments.   

Electronic Structures of Quasiperiodically Modulated Thin Ag films

When electrons move in crystalline solids, the coherent scattering with the periodic potential enables them to sneak by all ions uninhibited, forming Bloch electrons with their own E vs. k dispersion relationship. This ``Bloch description'' underlies the electronic structures of all crystalline solids. In quasicrystals, however, such a description encounters conceptual difficulties since the Bloch theorem no longer applies. While theoretical investigations showed exotic characteristics such as the Cantor-set energy spectrum early experiments did not reveal such exotic properties. Recent attempt to measure E vs. k relation in a quasicrytal using angle-resolved photoemission revealed free-electron like states. Nevertheless, critical questions remain as how electronic states in a quasiperiodic solid differ from those in a periodic solid. By using low-tempeature scanning tunneling spectroscopy (STS) to probe a quasiperiodically modulated Ag metallic thin film, and by using Fourier analysis, we unravel the influences of individual Fourier components of the scattering potential (periodic vs. quasiperiodic) on the electronic structures.   

Theoretical study of the morphology of stretched gold nanowires

Gold nanowires (NW) have been proposed for a variety of applications, including as intrinsic force standards in the nanoregime. To determine if they are effectively suitable for such an usage, we analyzed the dependence of the evolving NW morphology on different strain rates and directions, using semi-static, density functional (DFT) calculations. The NW under examination are very thin, initially fcc and have the wire axis along the (110) crystallographic orientation. The uniqueness of the necking pathway and the corresponding pre- failure structure are analyzed. The structure evolution and the ultimate length of the NWs are shown to be extremely sensitive to the tensile direction, and possibly on the strain rate as well, indicating that the corresponding energy surface is complex and that any experimental quantitative evaluation of the forces necessary to break the NW should keep parameters such as stretching direction and strain rate under careful control.   

Optical Properties of Silver Nanowires

We report on the propagation of light through silver nanowires mediated by plasmons. Light is observed to couple through crossed nanowires and around soft bends. Farfield Emission is observed from multiple bends in the nanowire. Propagation lengths of several microns are observed through structures less than 100nm in diameter. Light is coupled into silver nanowire fibers with a focused laser beam at both 830nm and 532nm.   

Computer Simulation of Copper Nanowires

Any device to operate, no matter how large or small, needs metallic contacts. The trend towards nano-miniaturization, with the development of nanodevices, will require the understanding of the behavior of metals at the nanoscale, especially gold and copper. Gold has received a lot of attention, while copper, also an important metal has not been studied as much. Recently the dynamical evolution of copper nanowires was observed with high resolution transmission electron microscopy (HRTEM) experiments [1]. We have performed tight binding molecular dynamics (TB-MD) simulations of copper nanowires under stress along some directions until their rupture. The TB-MD calculations have been show to be an excellent tool to study gold nanowires [2]. We present the time evolution of the structures, the formation of defects and the forces sustained by the nanowires. We discuss the possibility of the formation of short linear atomic chains before the breaking. Our results are in good agreement with experimental results. [1] J. C. Gonzalez, et al., Phys. Rev. Lett. \textbf{93}, 126103-1 (2004). [2] E. Z. da Silva, A. J. R. da Silva and A. Fazzio, Phys. Rev. Lett. \textbf{87}, 256102 (2001).   

Measurement of Temperature-Dependent Conductivity and Seebeck Coefficient of Self-assembled Gold Nanoparticle Wires

We are exploring the electrical and thermoelectrical properties of wires composed of 12nm diameter gold nanoparticles. The wires are deposited on glass substrates via evaporation-driven vertical colloidal deposition (VCD). We then measure the temperature-dependence of the resistance and thermopower (Seebeck coefficient) between 30 and 120K. We will discuss these measurements in the context of the constituent nanoparticle size and the preparation of the deposited wire; we also make comparisons to similar measurements of bulk gold and gold-iron alloys.   

Electrical Detection of Surface Plasmon Resonance in Individual Gold Nanowires

Plasmon absorption in individual gold nanowires has been detected electrically.~ Gold nanowires, electrochemically synthesized in polycarbonate templates with diameters of 40 nm and lengths of 5 $\mu $m, are connected under an optical microscope using multiphoton absorption polymerization (MAP).~ MAP uses a prepolymer acrylic based resin and a focused ultrafast laser to stereolithographically pattern with sub-micron resolution.~ For this experiment, nonconducting acrylate leads are made by MAP to connect to a single gold nanowire.~ These polymer leads can then be made conductive by surface modification followed by electroless copper enhancement.~ In this way the resistance of a single nanowire can be measured in a four-probe scheme.~ The gold rod can then be optically excited near its plasmon absorption peak.~ The excitation is quickly converted to heat, which is observed as an increase in resistance.~ This electrically detected plasmon absorption has been found to be linearly dependent on light intensity and is also sensitive to the direction of the polarization with respect to the rod.~ This non-optical plasmon detection is of basic interest but may be applicable for intrabeam polarization detection or electrical detection of biomolecules.   

The Metal-Insulator Transition in Metal Transition Granular Films

We study with infrared reflectivity the concentration and temperature dependence of the regime change from metallic to insulating in granular films made of transition metals embedded in SO$_{2}$. The TM$_{x}$(SO$_{2}$)$_{1-x}$ (TM=Fe, Ni, Co), (0.25$ \leq $x$ \leq $0.85) systems yield spectra typical of conducting oxides where hopping carriers undergo electron-phonon interactions with localization enhanced by nanoparticles and substrate roughness. The distinct Drude component, extending beyond 1.3 eV in the metallic state, undergoes a dramatic change in intensity due to the progressive reduction of carriers critical paths as the transition temperature is reached in the glassy matrix. At the intermediate conducting state for x$\sim$ 0.55, about the percolation threshold, a well defined reflectivity edge and band, considered fingerprint for small polarons, emerges in addition to the vibrational bands. A very good agreement is found between the measured optical conductivity and current small polaron models. This, in addition to underlying the importance of polarization effects, provides grounds toward a quantitative microscopic description of transport properties. It also adds toward an understanding of a non-magnetic factor in the magnetoresistance and extraordinary Hall coefficient enhancements.   

Jahn-Teller distortion mediated phase separation in spinel thin films

Self-assembled nano-structures have been acquired in various forms, giving significant impacts both on industrial and academic realms. Recently, nano-structures in spinel oxides have been observed in bulk materials by harnessing the Jahn- Teller distortions. Realization of magnetic nano-stuructures could give a breakthrough in the magnetic recording industry. We have grown chemically phase separated spinel films utilizing pulsed laser deposition method, and the inter- relationship between structural and magnetic properties will be discussed.   

Synthesis and properties of multiferroic Bi(Fe,Cr)O$_{3}$ thin films

Bi based magnetic oxide systems, including BiFeO$_{3}$ and BiCrO$_{3}$, have spontaneous magnetization and display charge polarization. Bi(Fe,Cr)O$_{3}$ system has been predicted to be a new candidate of multiferroic materials. In this study, we will present systematic investigation of multiferroic properties of Bi(Fe,Cr)O$_{3}$ thin films. The films are fabricated by pulsed laser deposition under various deposition conditions. We have succeeded in synthesizing single phase epitaxial BiCrO$_{3}$ films on LaAlO$_{3}$ (001), SrTiO$_{3}$ (001) and NdGaO$_{3}$ (110) substrates. It shows weak ferromagnetic behavior with the Curie temperature at 120 K. Pseudo-binary epitaxial thin film composition spreads of BiFe$_{1-x}$Cr$_{x}$O$_{3}$ have been fabricated using the combinatorial pulsed laser deposition technique in order to investigate their changing multiferroic properties as a function of composition. Their magnetic and dielectric properties mapped using low temperature scanning SQUID microscopy and microwave microscopy will be discussed. This work was supported by ONR N000140110761, ONR N000140410085, NSF DMR 0094265 (CAREER), NSF DMR 0231291, MRSEC DMR-00-0520471, and W. M. Keck Foundation.   

Domains in Multiferroic Films

The theory and modeling of equilibrium self-assembling nanostructures based on concept of elastic domains in multiphase epitaxial films successfully explained the morpholgies of CoFe$_{2}$O$_{4}$-PbTiO$_{3}$ films grown on differently oriented substrates. It is shown that it is possible to design the different nanostructure architectures: cobalt ferrite rods in lead titanate matrix and vise versa as well as differently oriented laminar structures. The elastic interactions between the phases determine the mutual change of order parameters: magnetization and polarization, that allow one to estimate the magneto-electric coupling through the study of constrained polarization and magnetization.   

Schottky-Barrier height at Si(001)/PtSi(010) and Si(111)/ErSi$_{2}$(001) interface

According to the Si roadmap (ITRS) one of the most pressing concerns of CMOS technology beyond the 65 nm node is the contact resistances in source/drain regions between the doped silicon and silicide. For the 45 nm node it is expected to amount to one-fourth of the total parasitic resistance. This contribution will clearly only rise as the scaling continues. Thus the roadmap calls for a new contact technology by the 45 nm node. A monosilicide would have an additional advantage in lower Si consumption. The transition from CoSi$_{2}$ to NiSi is a step in that direction. Physically, the contact resistance comes from a relatively high Schottky-barrier between Si and silicide. For both CoSi$_{2}$ and NiSi the barrier is about 0.5 eV. Thus, it is desirable to identify new metals or alloys that will have lower Schottky barrier to n- and p- type Si for use in NMOS and PMOS, respectively. In deep submicron regime, PtSi, ErSi$_{2}$ and NiSi have been shown to be prospective candidates for replacing conventional silicides. We have studied the electronic structure of bulk PtSi and ErSi$_{2}$ within the framework of density functional theory. We then calculate work functions and surface energies for different surface orientations and come up with plausible interface structures with Si. Our calculated valance band Schottky-barrier height of 0.26 eV at Si(001)/PtSi(010) is in excellent agreement with the experiments. We have , also, calculated the valance band Schottky-barrier height of 0.29 eV at Si(111)/ErSi$_{2}$(001) interface.   

Magnetic Resonance Force Microscopy investigations of a continuous ferromagnetic thin film

The ferromagnetic resonance of a 50nm thick Permalloy film has been investigated using a magnetic resonance force microscope operated at UHV and 4K. A low boron doped silicon cantilever with a hardmagnetic SmCo tip attached to its end was used to detect the locally excited ferromagnetic resonance in the film. The high coercivity of the tip magnet enables the measurement of FMRFM spectra with the tip magnetization aligned either parallel or antiparallel to the external magnetic field, while saturating the Permalloy film perpendicular to the film plane. The variation of the spectra as a function of the tip-film separation reveals a bulk resonance and the excitation of spin waves originating where the magnetic resonance condition is locally met. This local resonance may be used to characterize the dynamic properties of ferromagnetic thin film samples with high spatial resolution.