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Vanderbilt University
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physics-astronomy

Condensed Matter and Optics Seminar, 2011-2012

Condensed Matter and Optics Seminar, 2011-2012

Condensed Matter and Optics Seminar, 2010-2011

Condensed Matter and Optics Seminar, 2009-2010

Seminars are held on Fridays at 3pm in Stevenson 6501, unless otherwise noted.
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Fall 2011

Friday, September 2

Justin Gregory, Interdisciplinary Graduate Program in Materials Science, Vanderbilt University

Characterization of Ion Implantation-Induced Vacancy Defects and Graphitization in Diamond Lattices by Coherent Acoustic Phonon Spectroscopy   (show abstract)

I will describe implantation damage experiments on single-crystal diamond samples and subsequent characterization using optical pump-probe techniques. Samples are irradiated using He+ ions with energies on the order of 1 MeV, generating lattice defects and buried layers of graphitic carbon under high pressure. Using the time-resolved optical technique known as coherent acoustic phonon spectroscopy, these samples are characterized in a depth-dependent manner. The resulting oscillation patterns reveal clues about the optical and electronic structure of the defects and the graphitic layers.

Host:N. Tolk

Friday, September 9

tba

Friday, September 16

tba

Friday, September 23

tba

Friday, September 30, 3:00 pm in SC 6501

Isabel Gonzalo-Juan, Department of Physics and Astronomy, Vanderbilt University

*Controlled Monolayer Growth of Ultra-thin Films of TiO2 Nanocrystals by Electrophoretic Deposition   (show abstract)

The work discussed in this presentation contributes to the production of continuous, thin films of densely-packed titanium oxide nanocrystals, synthesized by solvothermal synthesis and assembled by electrophoretic deposition using non polar solvents. TiO2 has attracted significant research interest due to its photochemical activity and optical properties. Colloidal nanocrystals (NCs) have increasingly been employed due to their attractive size-dependent optical properties. Precise control of the morphology and the size of these materials at nanometric scale is of a basic importance to fine-tuning of their physical properties, such as electrical conductivity or magnetic, optical and mechanical characteristics. Hence, to improve the preparation method of nanocrystalline TiO2 with a narrow size distribution is of great importance. Electrophoretic deposition (EPD) of particles from colloidal suspensions stands out as a powerful and versatile colloidal process to face inexpensive and mass production of films using low concentrated suspensions. Ceramic NCs can be easily deposited by EPD on any conductive substrate. Furthermore, EPD can facilitate much thinner films than can be produced by other techniques, such as tape casting. Interestingly self-ordering occurs during EPD in monolayers of certain colloidal nanocrystals, which may allow the production of highly ordered 3D NC structures The electrical parameters of the TiO2 nanocrystal suspensions were optimized to achieve the necessary NC deflocculated conditions and to produce homogeneous films of nanocrystals by SLED using non polar solvents. The associated physical properties, current density, potential, electric field and deposition time, were optimized to produce homogeneous self supporting films.

Host:J. Dickerson

Friday, September 30, 3:30 pm in SC 6501

Alex Krejci, Department of Physics and Astronomy, Vanderbilt University

*The Evolution of Ordering in Iron Oxide Nanoparticle Monolayers Using Electrophoretic Deposition   (show abstract)

Iron oxide nanoparticle monolayers and multilayers were assembled using dc electrophoretic deposition. The rate of deposition and the total particle deposition were controlled by varying the concentration of nanoparticles and the deposition time, respectively. Using scanning electron microscopy, we performed a time resolved study that demonstrated the growth of the monolayer from a single isolated nanoparticle to a nearly complete layer. We observed tight, hexagonal packing of the nanoparticles indicating strong particle particle interaction. Future research topics include generalizing the process for various types of nanoparticles and controlling the total particle deposition with current measurements.

Host:J. Dickerson

Friday, October 7

AKM Newaz, Department of Physics and Astronomy, Vanderbilt University

*Probing charge scattering mechanisms in suspended graphene by varying its dielectric environment

Host:K. Bolotin

Friday, October 14, 3:00 pm in SC 6501

Yunhao Cao, Department of Electrical Engineering and Computer Science, Vanderbilt University

*Controlling growth morphology of carbon nanotubes: from suspended bridges to upright forests   (show abstract)

We have developed two strategies to produce carbon nanotubes (CNTs) from low-density surface growth to high-density forest growth. We have demonstrated that by introducing a C2H2 pulse at the beginning of the growth, where methane is still used as the main carbon feeding gas, the growth tendency of CNTs can be changed and the resulting growth morphology will vary from surface growth to forest growth. Similarly, the growth morphology can be changed when the growth temperature is risen. Furthermore, by appropriately adjusting the growth temperature, we have managed to produce both suspended CNT bridges and upright forests within a single growth procedure. The further characterization via Raman spectroscopy indicates that the increasing C2H2 pulse time will lead to the rise of D peak for as-grown CNTs, due to the formation of more multi-walled CNTs and the amorphous carbon contamination introduced by extra C2H2, while high growth temperature tends to produce high-quality CNTs and reduce the amorphous carbon contamination. The systematic analysis for presented strategies will be important to understand the growth mechanism, and also help with the process development of full CNT-based electronic architecture in future.

Host:Y. Xu

Friday, October 14, 3:30 pm in SC 6501

Tu Hong, Department of Electrical Engineering and Computer Science, Vanderbilt University

*Carbon nanotube-mediated siRNA delivery for gene silencing in cancer cells   (show abstract)

Small interfering RNA (siRNA) is potentially a promising tool in influencing gene expression with a high degree of target specificity. However, its poor intracellular uptake, instability in vivo, and non-specific immune stimulations impeded its effect in clinical applications. In this study, carbon nanotubes (CNTs) functionalized with phospholipid-polyethylene glycol (PEG) have shown capabilities to stabilize siRNA in cell culture medium during the transfection and efficiently deliver siRNA into neuroblastoma and breast cancer cells. Moreover, the intrinsic optical properties of CNTs have been investigated through absorption and fluorescence measurements. We have found that the directly-functionalized groups play an important role on the fluorescence imaging of functionalized CNTs. The unique fluorescence imaging and high delivery efficiency make CNTs a promising material to deliver drugs and evaluate the treatment effect simultaneously.

Host:Y. Xu

Friday, October 21, 3:00 pm in SC 6501

Terry Musho, Department of Mechanical Engineering, Vanderbilt University

NEGF Quantum Simulation of Nanotip Field and Thermionic Emitters for Direct Energy Conversion   (show abstract)

Wide band-gap diamond nanotip field emission devices have been experimentally shown to have superior performance and lifetime. However, theoretical studies of the electronic emission from these devices using standard Fowler-Nordheim (FN) theory does not fully capture the physics as a result of the fitting parameters inherent to the FN approximation. The following research computationally models wide band-gap nanotip field emission devices from a quantum point of view, using a novel non-equilibrium Green's function (NEGF) approach previously applied to modeling the transport in solid-state electronic devices. In this research the IV characteristics of a single square tip diamond emitter and diamond films are investigated under several bias conditions. Those bias conditions include both field emission and thermionic emission regimes with comparison to availible experimental values. The NEGF model calculates the ballistic transport using a self-consistent Schrödinger-Poisson solver that calculates the transmission at discrete energy levels which is used in conjuction with a Landauer formalism to determine the total current. Ultimately, this model allows the inherent quantum mechanical transport to be captured at sub-micron device sizes without need for fitting parameters. Findings from this research have confirmed non-linearities in the FN curve and have demonstrated the experimental transport trends. Additionally, thermionic emission trends suggest that select material parameters are target for enhanced emission.

Host:G. Walker

Friday, October 28, 3:00 pm in SC 6501

Hadley Lawler, Department of Physics and Astronomy, Vanderbilt University

*First-principles methods for evaluation of the light-matter interaction in insulators and semiconductors   (show abstract)

Polarization currents and their description within density-functional theory and many-body theory are introduced. Calculations of dielectric spectra for several simple systems demonstrate the essential physics in these approaches. For far-infrared spectra, relevant quantities discussed are Born-effective charges, optical phonon self-energies, and the electrical anharmonicity. For optical and UV spectra, various levels of interaction will be introduced, from single-particle transitions, to the random-phase approximation, to treatment of electron-hole interactions with the Bethe-Salpeter equation.

Host:N. Tolk

Friday, October 28, 3:30 pm in SC 6501

Hiram Conley, Department of Physics and Astronomy, Vanderbilt University

*Strained Graphene: Suppressing Flexural Phonons and Exploring the Psuedo Quantum Hall Effect   (show abstract)

The interplay between the mechanical and electrical properties of materials is interesting and technologically relevant. Exploring this interplay between the mechanical and electrical properties in graphene may enable tuning graphene's electrical properties as well as opening up new exotic electronic states. We have found several promising methods to engineer strain in graphene and will discuss future experiments to explore the connection between strain and graphene's electrical properties.

Host:K. Bolotin

Friday, November 4, 3:00 pm in SC 6501

Shuren Hu, Department of Physics and Astronomy, Vanderbilt University

*Creating Defects in Suspended and Supported Graphene FET Devices   (show abstract)

Creating Defects in Suspended and Supported Graphene FET Devices Defects (such as dislocations or vacancies) in graphene can cause strong electron scattering. Therefore, to achieve high quality graphene-based electronic devices it is important to elucidate the mechanisms of defect formation. In this talk, we present our recent results on controllable creation of vacancy defects in graphene and measuring the impact of these defect on electron transport. Specifically, we focus on understanding the effect of the substrate under graphene on formation of defects. The vacancy defects were created by irradiating both suspended and supported-on-SiO2 graphene devices with high energy Ga ion beams. Electron transport was measured in these devices in situ in high vacuum as a function of irradiation dose. We find that the number of defect created in supported devices is dramatically higher in comparison with suspended devices. We identify formation of defects by secondary ions in the case of supported devices as a likely explanation of these results. These results may be important in designing devices for high radiation working environment, such as space electronics.

Host:K. Bolotin

Friday, November 4, 3:30 pm in SC 6501

Dhiraj Prasai, Department of Physics and Astronomy, Vanderbilt University

*Interaction between photoexcited quantum dots and graphene   (show abstract)

A quantum dot in its excited state can decay via two different pathways. First, it can fluoresce and relieve its energy by emitting a photon. Second, if a metal is placed near the dot, the excitation can be non-radiatevely transferred into that metal via the so-called Foster Resonance Transfer (FRET). In that second case the fluorescence is quenched. Here, we study fluorescence of quantum dots placed in the vicinity of graphene. By electrostatic gating we expect to tune the Fermi energy of graphene and hence control the efficiency of FRET. We thus expect to be able to electrically switch the fluorescence of the quantum dot between "on" and "off" states. In addition, by studying the detailed magnitude of the fluorescence quenching we expect to extract the spectrum of plasmons in graphene.

Host:K. Bolotin

Friday, November 18, 3:00 pm in SC 6501

Sergiy Bubin, Department of Physics and Astronomy, Vanderbilt University

*Modeling Coulomb explosions of small hydrocarbons driven by intense femtosecond laser pulses   (show abstract)

In the framework of real-time real-space time-dependent density functional theory (TDDFT) complemented with classical molecular dynamics for ions, we have studied strong field ionization and subsequent molecular fragmentation (Coulomb explosion) in small hydrocarbon molecules - methane and 1,3-butadiene. The mechanism of the process has been investigated and the spectra of the ejected protons have been computed. The results of the TDDFT simulations are compared with some recent experimental data.

Host:K. Varga

Friday, November 18, 3:30 pm in SC 6501

Travis Wade, Interdisciplinary Graduate Program in Materials Science, Vanderbilt University

*Nanostructure analysis of diamond cold cathode field emitters   (show abstract)

Diamond field emitter arrays have demonstrated extraordinary brightness and durability, but face fabrication yield issues before being adopted commercially. Transmission electron microscopy performed in this study provides new insight into tip structure and composition with implications for field emission and diamond growth. I will discuss the development of a process for TEM analysis of surface features without graphitization and how we have examined sub-surface cross-sections to offer feedback for growth parameter conditions.

Host:J. Davidson

Friday, November 25

Thanksgiving Holidays, no seminar

Friday, December 2, 3:00 pm in SC 6501

Davon Ferrara, Department of Physics and Astronomy, Vanderbilt University

*Correlated electron—Plasmon Coupling in Gold::Vanadium Dioxide Nanocomposites   (show abstract)

The localized surface plasmon resonance (LSPR) frequency of gold nanoparticle (NP) arrays embedded in vanadium dioxide (VO­2) is modulated by the semiconductor-to-metal phase transition (SMT) of the film. Since the 180-nm diameter particles are on the order of the VO­2 grain size, the NPs serve as sensitive nanoantennas for measuring the changing VO­2 dielectric function. As the split vanadium d­-bands come together to form the metallic conduction band, the interaction between the plasmon and correlated electrons of the VO­2 film decreases the Au LSPR dephasing time by 30% during the transition.

Host:R. Haglund

Friday, December 2, 3:30 in SC 6501

Krishen Appavoo, Department of Physics and Astronomy, Vanderbilt University

*Role of defects in phase transition of VO2 nanoparticles probed by plasmon resonance spectroscopy   (show abstract)

Defects are known to affect nanoscale phase transitions, but their specific role in the metal-to-insulator transition in VO2 has remained elusive. By combining plasmon resonance nanospectroscopy with density-functional calculations, we correlate decrease in phase-transition energy with oxygen vacancies created by strain at grain boundaries. By measuring the degree of metallization in the lithographically-defined VO2 nanoparticles we find that hysteresis width narrows with increasing size, thus illustrating the potential for domain boundary engineering in phase-changing nanostructures.

Host:R. Haglund

Friday, December 9

Charles Adams, Department of Physics and Astronomy, Vanderbilt University

*Measuring the Nonlinear Refractive Index of VOx Nanotubes   (show abstract)

I will discuss using the Z-scan method to measure the nonlinear index of VOx Nanotubes as well as preliminary results and comparison with results from measurements on carbon nanotubes.

Host:R. Haglund

Friday, December 16, 3:00 pm in SC 6501

Robert Marvel, Department of Physics and Astronomy, Vanderbilt University

*Deposition of vanadium dioxide films by electron beam evaporation   (show abstract)

Thin films of vanadium dioxide with good uniformity and low surface roughness are desired for many optical applications. In addition, the ability to deposit VO2 on silicon substrates is of critical importance for integration with current silicon based electronics. Films deposited on silicon by Pulsed Laser Deposition (PLD) have been limited to 100nm minimum thickness, in contrast, good quality 30nm thick films have been deposited by electron beam evaporation. In addition, electron beam evaporation is significantly cheaper than PLD and is capable of much higher deposition rates. Switching contrast and hysteresis width of the VO2 films deposited by electron beam evaporation has also been observed to be much less affected by conditions during post-deposition annealing when compared with PLD films.

Host:R. Haglund

Friday, December 16, 3:30 in SC 6501

Daniel Mayo, Department of Physics and Astronomy, Vanderbilt University

*Metal Oxide Nanoparticle Film Deposition Using Resonant Infrared Matrix-Assisted Pulsed Laser Evaporation (RIR-MAPLE)   (show abstract)

The successful development of flexible, high performance solar cells that are competitive with silicon-based technology will likely depend on the ability to fabricate hybrid materials that incorporate nanomaterials, glasses, ceramics, polymers, and thin films. Resonant Infrared Matrix-Assisted Pulsed Laser Evaporation (RIR-MAPLE) is an ideal method for depositing organic materials and nanoparticles with minimal photochemical or photothermal damage to the deposited material. Furthermore, there are many non-hazardous solvents containing chemical functional groups with infrared absorptions accessible using IR lasers. We report here the results from recent work using RIR-MAPLE to deposit films of TiO2 nanoparticles on Si substrates. Using an Er:YAG laser (λ = 2.94 μm), we investigated a variety of MAPLE solvents containing an –OH moiety, including all four isomers of butyl alcohol. We compare the effects of varying concentration and laser fluence. Results of our analysis of films of TiO2 nanoparticles using scanning electron microscopy, surface metrology, and energy dispersive X-ray spectroscopy will be presented.

Host:R. Haglund

Spring 2012

Friday, January 6

Mark T. Lusk, Department of Physics,Colorado School of Mines (mlusk@mines.edu)

Exciton Engineering for Improved Quantum Dot Photovoltaics    (show abstract)

Recent advances in the synthesis of inorganic quantum dots (QDs) opens up an intriguing opportunity for exploiting quantum confinement as a means of improving photovoltaic energy conversion efficiency. The design space is rich since size, shape and surface termination of these robust nanostructures can be used to change the nature of quantum confinement and thus modify the character of photon absorption/emission and excitonic relaxation rates. As an alternative to immediate charge separation, quantum confinement can also be exploited to design assemblies that promote exciton transport between quantum dots. Within this setting of nanostructured exciton dynamics, two facets of exciton engineering will be discussed. The first is multi-exciton generation (MEG), the production of more than one exciton from a single photon, and the MEG rate is quantified as a function of dot size. The second topic, exciton transport, elucidates the way in which dot size and surface termination modify inter-dot transport dynamics. In both cases, we show that the relevant process efficiency increases as dot size decreases. Excitonically speaking, photo-energy conversion favors assemblies composed of smaller quantum dots.

Hosts:R. Mu and N. Tolk

Friday, January 27

AKM Newaz, Department of Physics and Astronomy, Vanderbilt University

Probing streaming potential using a graphene transistor   (show abstract)

Recent advances in micro- and nano- fluidics spawned a great interest in miniaturized nanoscale probes that can detect fluidic dynamic with nanometer precision. Graphene, being one-atom-thin material, strongly interacts with its environment and is a perfect material to be used in application where precise sensing is required. We present the development of a graphene-based field effect transistor sensor capable of probing the velocity of a fluid, potentially with nanometer precision. Our sensor detects flow rate as small as ~4mm/sec, can measure the flow direction, and is also sensitive to the ionic strength of the solution flowing past it. We are currently working towards experiments where motion of a single cell past graphene transistor are analyzed using the graphene transistor.

Host: K. Bolotin

Friday, February 3

Zenghu Chang, Department of Physics, University of Central Florida

Approaches to producing attosecond pulses   (show abstract)

The field of ultrafast optics has experienced a revolution during the last decade. Coherent XUV and soft x-ray pulses as short as 80 attoseconds can now be generated, which is quickly approaching one atomic unit of time-the natural time scale of electron motion in atoms and molecules. Here, I will describe the theoretical and experimental challenges in producing such pulses.

Host: N. Tolk

Friday, February 17

Juan Carlos Idrobo, Materials Science and Technology Division, Oak Ridge National Laboratory

Atomic-scale plasmonics in graphene   (show abstract)

In this talk, I will present recent results of how defects and impurities change the physical properties of graphene. In particular, using a synergistic combination of aberration-corrected scanning transmission electron microscopy techniques and first-principles calculations, we find that the graphene optical and plasmon properties can be manipulated all the way down to the atomic level. We find that single substitutional atoms enhance the graphene response at the atomic scale and that they can act as atomic antennas in the petahertz (1015 Hz) frequency range. In the case of interfaces, we observe that the graphene optical response exhibits properties equivalent to that of one-dimensional plasmons. We also find that the assemblage of impurities forming different two-dimensional patterns on the graphene surface result in plasmonic quantum corrals. A theoretical explanation of the, so far unique, optical and plasmon properties of graphene will be presented.

Host: S. Pantelides

Friday, February 24--March APS Meeting Preview

Stephanie Gilbert, Interdisciplinary Graduate Program in Materials Science, Vanderbilt University

**Opto-electronic and structural characterization of ErAs nanoparticles embedded in GaAsk   (show abstract)

We combine ultrafast pump-probe and optoacoustic spectroscopy with absorption measurements to characterize embedded, self-assembled 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 effects due to the presence of nanoparticles within the host lattice.

Host: N. Tolk

Alex Krejci, Department of Physics and Astronomy, Vanderbilt University

**Fabrication of Iron Oxide Nanoparticle Monolayers by Electrophoretic Deposition   (show abstract)

Magnetic nanoparticle (NP) films are potentially useful in a variety of applications, such as magnetic storage media and ultra-strong permanent magnets. Monolayers of magnetic NPs are specifically interesting as the monolayer geometry maximizes film interactions with dissimilar materials below and above the monolayer. However, many potential commercial and industrial applications of NP films rely on fabrication techniques that are facile, rapid, and site-selective which create homogenous, densely packed, defect-free thin films. Electrophoretic deposition (EPD) is a technique for forming thin films that meets all of these criteria. This work shows, for the first time, EPD's utility in forming monolayers of magnetic NPs. Iron oxide NPs ($\sim $14nm) have been synthesized using a solution phase synthesis technique. Repeated centrifugation of the particles prepares the NPs for EPD. The particles are then deposited onto silicon substrates with EPD using dc electric fields. Analysis of the films using scanning electron microscopy and atomic force microscopy shows the particles deposit as NP monolayers. The monolayer density and deposition rate are controlled by varying the suspension concentration and the deposition time. Future research will focus on creating long-range order within the monolayers.

Host: J. Dickerson

Dhiraj Prasai, Department of Physics and Astronomy, Vanderbilt University

**Fluorescence quenching of a quantum dot deposited onto a sheet of graphene   (show abstract)

We investigate fluorescence quenching of a quantum dot deposited onto a sheet of graphene as a function of graphene's Fermi energy. We fabricate devices where PbS quantum dots (QD) with a fluorescence peak at 1500 nm (0.83eV) are deposited at a controlled distance (10-50nm) from a single layer graphene sheet. The Fermi energy of graphene is controlled in the range between 0 to 0.6eV using a polymer electrolyte ionic gate. We observe strong quenching of the QD fluorescence unless the graphene is doped by approximately 0.5 eV. We interpret this as due to a resonant energy transfer into an interband excitation of graphene. For larger Fermi energies the interband transitions are blocked and we observe weaker quenching of fluorescence. We further investigate the potential of this gate-controlled fluorescence quenching as a sensitive probe of graphene's plasmon spectrum.

Host: K. Bolotin

AKM Newaz, Department of Physics and Astronomy, Vanderbilt University

**Scattering mechanisms in graphene suspended in liquids   (show abstract)

Enhanced dielectric screening of charged impurities by high-κ environment of graphene is predicted to improve the electronic quality of graphene devices by suppressing Coulomb scattering. However, experiments reported so far demonstrate that electronic transport in graphene is only modestly modified by a high-κ environment. Here we fabricate large area multi-terminal graphene devices suspended in liquids and study electronic transport in graphene as a function of liquid's dielectric constant. We observe a rapid increase of mobility µ with κ due to dielectric screening in non-polar solvents . We also find that charged ions present in polar solvents cause a drastic drop in mobility counteracting the gains by dielectric screening in polar high-κ liquids. We expect that our findings may provide avenues to control and reduce carrier scattering in future graphene-based electronic devices.

Host: K. Bolotin

Friday, March 2

tba   (show abstract)

Tuesday, March 6, 4pm in SC 6501---SPECIAL SEMINAR

Norbert Seifert, Logic Technology Development, Intel Corporation

Technology Development and Manufacturing Opportunities   (show abstract)

After a brief introduction of Intel Corporation, the presenter will describe how technology development is done and what job opportunities exist in the manufacturing branch of Intel. The second part of the talk focuses on the presenter's area of expertise "Single event effects" and on the technical challenges that exist at the 22nm technology node and beyond.

Host: N. Tolk

Friday, March 16

Richard Haglund, Department of Physics and Astronomy, Vanderbilt University

Probing Ultrafast Changes in the Structural Symmetry with Light   (show abstract)

The symmetry of a crystallographic phase can be observed by the periodic positions of the constituent ions. These positions are determined by the symmetry of the underlying lattice potential. In equilibrium, phase transitions changes to the ionic positions and the lattice potential symmetry occur concomitantly. However, when a system is driven through a phase transition out of equilibrium, this may not be the case. The control of solid state properties with light requires development of equilibrium concepts in order to understand when and how different properties change. To date, most studies of ultrafast structural phase transitions have focused on measuring changes in ionic positions. Here, we show that changes in the lattice potential can be probed in the time domain via coherent phonon generation, enabling an all-optical probe of structural phase transitions. We demonstrate this principle with the monoclinic-to-rutile photoinduced structural transition of VO2 and show that strong electronic excitation changes the lattice potential on a sub phonon-period timescale.

Host: N. Tolk

Friday, March 23

Ke Chen, Department of Physics, Temple University

MgB2 Superconducting Energy Gaps and Josephson Junctions   (show abstract)

MgB2, a simple binary compound obtainable from chemical stores since 19th century, was discovered in 2001 to exhibit superconductivity up to 40 K, the highest among all discovered superconductors mediated by electron-phonon interaction. MgB2 is the first superconductor exhibiting prominent multigap feature. It has two superconducting energy gaps at about 2mV and 7mV. First-principles calculation has predicted that the energy gaps are momentum dependent with a distribution in energy. Electron tunneling spectroscopy by MgB2/native oxide/Pb juncitons shows that the distribution of energy gaps is in good agreement with theoretical calculation. This confirms the validity of the theoretical calculation. At zero temperature, the energy gaps MgB2 are larger than 2 mV, greater than that of Nb (1.5 mV), the mainstream superconductor, suggesting its potential in superconducting digital circuits that may work at 20 K, for high efficient cryocoolers, or work at a higher speed than Nb at 4 K. We have made sandwich-type MgB2/MgO/MgB2 Josephson junctions that can work at 0~37K, exhibiting Josephson effects in good agreement with Josephson junction theory. This ongoing research has proven that MgB2 Josephson junctions can work at a much higher temperature than Nb Josephson junctions and may lead to higher speed digital circuit at 4 K.

Host: Y. Xu

Friday, March 30

Halina Krzyzanowska, Department of Electrical and Computer Engineering, University of Rochester

Er doped SiO2/nanocrystalline Si multilayers: fabrication, optical and electrical properties   (show abstract)

Silicon photonics was pioneered in the 80's. Despite much research, highly efficient light sources based on silicon structures have not yet been realized. One of the most promising ways of making efficient electrically pumped Si light sources at the standard telecommunication wavelength (1535 nm) is to use the unique properties of Si nanostructures doped with rare earth ions. During this talk, optical and electrical studies of Er doped SiO2/nc-Si multilayers fabricated by rf magnetron sputtering will be presented. The mechanisms of effective energy transfer from the Si nanocrystals to Er and of strong optical confinement in the low refractive index medium (Er - doped SiO2) for TM polarization will be discussed. Understanding and optimization of energy transfer was achieved via visible and infrared time resolved photoluminescence. Ultrafast pump-probe spectroscopy, using a prism coupler that couples light into the multilayer waveguide, allowed us to study free carrier absorption and carrier dynamics in Si nanostructures. Carrier transport in lateral p-i-n junctions made in multilayers was studied as a function of various parameters. The strong electroluminescence generated in these structures has been characterized and optimized.

Host: N. Tolk

Friday, April 6

Ribhu Kaul, Department of Physics and Astronomy, University of Kentucky

Phase transitions in strongly correlated systems   (show abstract)

Host: K. Bolotin

Friday, April 13

tba   (show abstract)

Friday, April 20

tba   (show abstract)

Friday, April 27

tba   (show abstract)

Friday, May 4

tba   (show abstract)

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