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CNST Group Seminars: 2012

CNST Electron Physics Group Seminar


Tom Silva
NIST PML Boulder

Friday December 7, 2012, 3:30 PM, Rm H107, Bldg H107

I will describe two new techniques for measuring magnetization dynamics. The first is the heterodyne magneto-optic microwave microscope, which measures the ferromagnetic resonance properties of nanoscale magnetic features as small as 50 nm. Using this instrument, we have measured the Landau-Lifshitz damping of spin-wave modes present in lithographically patterned nanomagnets. We find that the damping has a significant dependence on the spatial profile of the mode, and we explain these results in terms of "intra-layer" itinerant spin-currents generated by gradients in the magnetization dynamics, originally proposed by Tserkovnyak, et al. (PRB, 79, 094415, 2009). This has important implications for spintronic memory devices currently under development by a number of semiconductor companies. The second technique utilizes extreme ultraviolet (EUV) light generated via high harmonic generation, an optical technique that employs strong nonlinear interactions between an intense ultrafast infrared beam and noble gas atoms to produce harmonic photons as high as 1 keV (Science, 336,1287, 2012). We utilize the M-edge absorption peaks (50-70 eV) of the ferromagnetic transition metals (Fe, Ni) to simultaneously yet distinctly detect the magnetization dynamics of each element for multilayers and alloys in a transverse magneto-optic Kerr effect (T-MOKE) geometry. We have shown that ultrafast demagnetization of thin magnetic films gives rise to giant, quasi-ballistic spin-currents that are sufficiently large to drive both demagnetization and enhanced magnetization in a second proximate magnetic layer (Nat.Commun. 3:1037 doi: 10.1038/ncomms2029 (2012)).

For further information please contact John Unguris, 301-975-3712, John.Unguris@nist.gov

CNST Electron Physics Group Seminar


Nancy A. Burnham
Worcester Polytechnic Institute

Friday November 30, 2012, 11:00 AM, Rm H107, Bldg H107

Whereas calibration of the flexural spring constant of cantilevers in atomic force microscopy (AFM) has become part of commercial AFM packages, calibration of lateral forces is not yet routine. Many approaches have been published, but there are as yet no clearly preferred methods. I describe our method [1], which is a variation of the "wedge" technique. Instead of varying the load, we vary the angle of the sample. The method is quick, simple, and has better than 10% precision. Possible future work includes determing its accuracy, which was done only on an order-of-magnitude basis. When the tip-sample adhesion in AFM changes, the researcher must make assumptions about the stability of the tip in order to interpret the results (or go through a long validation process with expensive equipment). The tip could have changed geometrically, chemically, or both. We are developing an adhesion standard -- possibly for wholesale to AFM suppliers -- which is chemically inert, smooth, flat, durable, insensitive to storage environment, and shows changes in AFM adhesion after the tip has touched adhesive samples that would be expected to leave residues. We are providing evaluation samples to interested groups. In this talk, I'll start with a short introduction to my other activities, then concentrate on lateral-force calibration and the adhesion standard. For both projects, I credit my former student Evan Anderson, MS in Nanophysics, '12. 1. Anderson et al, ACS Appl. Mater. Interfaces 2011, 3, 3256–3260.

For further information please contact Joseph Stroscio, 301-975-3716, joseph.stroscio@nist.gov

CNST Energy Research Group Seminar

Novel Methods and New Materials for Current-Induced Domain-Wall Motion

Rembert Duine
Institute for Theoretical Physics, Utrecht University

Monday November 5, 2012, 10:30 AM, Rm H107, Bldg H107

Over the last few years, a great deal of research in the field of spintronics has been devoted to manipulation of magnetization of conducting ferromagnets using electric currents rather than magnetic fields, with the long-term goal of advancing magnetic-memory technology. A prime example is current-driven motion of ferromagnet domain walls. The underlying physical mechanism of current-induced magnetization dynamics and domain-wall motion, dubbed spin transfer, is usually understood in terms of conservation of spin angular momentum. In this talk I will discuss new directions for current-driven domain-wall motion that go beyond this "state-of-the-art". These include current-induced torques in antiferromagnets and in systems with strong spin-orbit coupling. In antiferromagnets current-induced torques exist even though such materials have no net magnetic moment and a description based on conservation of spin angular momentum does not apply. I will discuss the phenomenology of such torques and show that it is somewhat similar to ferromagnets. In particular, a current can lead to motion of domain walls in antiferromagnetic metals. In systems with strong spin-orbit coupling new types of torques are found to exist due to the interplay of magnetization gradients and spin-orbit coupling. These torques are shown to qualitatively alter current-driven domain wall motion with respect to the situation without spin-orbit coupling. Effects of heat currents in strongly spin-orbit coupled systems are also discussed.

For further information please contact Paul Haney, 301-975-4025, paul.haney@nist.gov

CNST Electron Physics Group Seminar


Shinji Yuasa
National Institute of Advanced Industrial Science and Technology (AIST)

Thursday November 1, 2012, 10:30 AM, Rm H107, Bldg H107

A magnetic tunnel junction (MTJ) consisting of a thin insulating layer (a tunnel barrier) sandwiched between two ferromagnetic electrodes exhibits the tunnel magnetoresistance (TMR) effect due to spin-dependent electron tunneling. Since the discovery of room-temperature TMR in the mid-1990s, MTJs with an amorphous aluminum oxide (Al–O) tunnel barrier have been studied extensively. Such MTJs exhibit a magnetoresistance (MR) ratio of several tens of percent at room temperature (RT) and have been applied to magnetoresistive random access memory (MRAM) and the read heads of hard disk drives. MTJs with MR ratios substantially higher than 100%, however, are desired for next-generation spintronic devices. In 2001, first-principle theories predicted that the MR ratios of epitaxial Fe/MgO/Fe MTJs with a crystalline MgO(001) barrier would be over 1000% due to the coherent tunneling of specific Bloch states. In 2004, MR ratios of about 200% were obtained for MgO-based MTJs [1]. MTJs with a CoFeB/MgO/CoFeB structure were developed for practical application and found to have MR ratios of above 200% and other practical properties [1,2]. This lecture focuses on the physics of magnetoresistance and spin-transfer torque in MTJs and the application of MTJs to various spintronic devices such as magnetic sensors, spin-transfer-torque MRAM (STT-RAM or spin-RAM) with perpendicular magnetization, and novel spin-torque oscillators. In addition, new types of MTJs such as spin-filter junctions with a ferromagnetic tunnel barrier will be discussed.
[1] S. Yuasa and D. D. Djayaprawira, J. Phys. D: Appl. Phys. 40, R337 (2007). [2] D. D. Djayaprawira, K. Tsunekawa, M. Nagai, H. Maehara, S. Yamagata, N. Watanabe, S. Yuasa, Y. Suzuki and K. Ando, Appl. Phys. Lett. 86, 092502 (2005).

For further information please contact John Unguris, 301-975-3712, John.Unguris@nist.gov

CNST Electron Physics Group Seminar


Huanlong Liu
New York University

Monday October 15, 2012, 10:30 AM, Rm H107, Bldg H107

The interaction between the intrinsic spin of itinerant electrons and the magnetization of ferromagnetic materials is of great interest for both fundamental physics and applications. While a thick ferromagnetic layer can polarize the spin of the electrons passing through, a spin-polarized current can also transfer the angular momentum to the magnetization via spin-transfer torque (STT) and change the magnetization orientation correspondingly. We experimentally study the magnetization reversal, relaxation and precession in magnetic nanostructures such as spin-valves and magnetic tunnel junctions (MTJs) subject to spin-polarized current, as well as applied magnetic field and thermal fluctuation. We explore the magnetization dynamics in a wide range of timescales, from sub-nanosecond to second and we are able to resolve these dynamics in real-time under 50 ps. We examine the magnetization configurations by resistance measurements through Giant Magnetoresistance (GMR) and Tunnel Magnetoresistance (TMR) effects. We use a macro-spin model with analytical solutions for direct comparison with experimental data on spin-valves with perpendicular anisotropies. We also carry out a Fokker-Planck calculation in order to address the influence of thermal fluctuation on magnetization dynamics. This research was supported at NYU by NSF Grant No. DMR- 0706322, DMR- 1006575, USARO Grant No. W911NF0710643, the Partner University Fund (PUF) of the Embassy of France and Spin Transfer Technology.

For further information please contact Robert McMichael, 301-975-5121, robert.mcmichael@nist.gov

CNST Energy Research Group Seminar


Alexander Tselev
Oak Ridge National Laboratory

Friday October 12, 2012, 9:15 AM, Rm H107, Bldg H107

Electrochemical energy storage and conversion (EESC) substantially relies on phenomena at solid-solid, solid-gas, and solid liquid interphase. In EESC systems considered as complex dynamic entities with scores of coupled chemical and physical phenomena, interfacial processes at the nanoscale are critical for performance of a system as a whole. Mechanisms of many of such processes are currently far from being understood and pose significant characterization challenge. In this talk, we will discuss two scanning probe techniques—electrochemical strain microscopy (ESM) and scanning near-field microwave microscopy (SMM)—in applications directly related to studies of interfacial processes in EESC materials at mesoscopic and nanometer length scales. ESM addresses one of key challenges, namely capability for probing ionic transport on the nanometer length scale. ESM utilizes the dependence of material molar volume on ionic concentration and detects ionic transport through local strain. The lateral resolution of the technique is as high as 10 nm. ESM can be implemented, for example, to explore the critical bias required for the onset of electrochemical transformation and provide information on kinetic parameters of ionic transport in Li batteries (Li ions) and fuel cell materials (oxygen vacancies). In contrast, SMM is sensitive to electron transport and local dielectric properties and relies on electric sample-probe capacitive coupling, which becomes sufficiently strong at microwave frequencies of a few GHz. Due to the relatively high measurements frequency, the technique has a significant potential for application for in-situ imaging of processes at solid-liquid electrolyte interfaces in batteries. We will demonstrate application of SMM in studies of mesoscopic metal-insulator transitions at ferroelastic domain walls of a strongly-correlated-electron material VO2 and in application to imaging of conductance inhomogeneities in thin films and isolated islands of CVD graphene with a sub-100 nm lateral resolution.

For further information please contact Nikolai Zhitenev, 301-975-6039, nikolai.zhitenev@nist.gov

CNST Energy Research Group Seminar


Laxmikant Saraf
Pacific Northwest National Laboratory

Wednesday, Ocotober 10, 2012, 10:30AM, RM H107, Bldg. 217

In solid oxide fuel cell (SOFC), electricity is electrochemically generated using oxygen ion mobility from the oxidizing fuel at high temperatures. Lowering of SOFC temperatures have benefits like reductions in the cost, carbon contamination, thermal stresses and broader choice of components. The performance at low temperatures is limited due to poor oxygen ion conductivity. Electrode polarization, sulfur poisoning and modifications at triple phase boundary (TPB) length also affect the outcome. TPB in SOFC is electrochemically active region where porous, ionic and electronic pathways meet. Effects of poor conductivity could be compensated if we take advantage of modified properties of SOFC materials at nanoscale and address the issue of effective electrochemical energy conversion (EEC) at TPB. In this presentation, I will discuss the usefulness of a variety of nanoscale and bulk analysis methods to interpret oxygen mobility and its impact on EEC. The studies are done with popular SOFC materials like YSZ, Ni-YSZ and CeO2. Using oxygen isotope studies, I will present complex oxygen mobility effects in nanocrystalline CeO2 and correlate oxygen desorption, diffusivity with oxygen transport properties. Even though exact quantification of complex oxygen mobility effects like exchange interaction, diffusion, adsorption and desorption in the bulk remains a challenge, a shift in the 18O concentration maxima during nuclear reaction analysis indicated oxygen desorption from the surface region. Due to the lack of long range lattice ordering, high defect density in nano-CeO2 did not convert into high conductivity. MBE was used to grow epitaxial Gd-CeO2/YSZ interfaces. Improvements in the conductivity as a function of layers (constant total thickness) were observed at intermediate temperatures. Growth of high density CeO2 /YSZ vertical interfaces resulted in distinct contributions from CeO2 and YSZ. Reduction of Ni-YSZ show mobile nature of Ni on YSZ surface. Finally, based on my research experience, I will discuss future nanoscale measurement capability development needs to address effective EEC and performance challenges in SOFCs at the intermediate operating temperatures.

For further information contact Nikolai Zhitenev, 301-975-6039, nikolai.zhitenev@nist.gov

CNST Energy Research Group Seminar


Howard Wang
State University of New York, Binghamton

Friday, Ocotober 5, 2012, 10:30AM, RM H107, Bldg. 217

The science and technology of nanoscale materials have been greatly advanced in the past two decades with new approaches for synthesizing novel nanomaterials and innovative measurements revealing characteristics of individual nanoparticulates from quantum dots to nanotubes to graphene to 3D nano-objects. Relatively limited attention has been paid to collective behaviors of nanomaterials, yet most real-world applications of nanotechnology require using large amounts of nanomaterials collectively. For example, to accommodate one terawatt power generation from alternative energy sources, one million tons of nanostructured energy storage materials are needed. In this talk, the speaker will share his measurements on the synthesis, dispersion, aggregation, transformation and applications of large quantities of nanomaterials. Our studies often reveal compelling stories that challenge the intuitive wisdom. For example, better dispersion of carbon nanotubes is controlled by the amount of surfactants instead of the ratio of surfactants to nanotubes, and metastable network formation stabilizes the suspension of carbon nanotubes. In the case of using metal nanoparticles for printing electronic devices, dispersion is often at odd with electric conductance, which in turn at odd with mechanical reliability. Throughout the process of nanoparticle synthesis, ink formulation, jet printing, particle sintering and device operation, the physical and chemical interactions among and the electric transport across nanoparticles play a dominant role. In batteries made of nanomaterials, ion transport through electrodes and electrolytes depends heavily on topological structures and interfacial properties of active materials. The talk will finish with a discussion on measurement needs for nanomaterial ensembles in general and their behaviors in energy storage systems in particular.

For further information contact Nikolai Zhitenev, 301-975-6039, nikolai.zhitenev@nist.gov

CNST Electron Physics Group Seminar


Ivana Petkovi
Service de Physique de l’Etat Condensé (SPEC), CEA Saclay, France

Monday, September 17, 2012, 10:30AM, RM H107, Bldg. 217

Edges in two dimensional electron systems (2DES) are central to the quantum Hall effect which depends for its existence on chiral edge currents with vanishing backscattering. But edges also host collective excitations in the form of chiral, globally neutral edge magnetoplasmons (EMP). Much investigated in conventional 2DES, little is known about them in the newly discovered graphene system where electrons obey massless relativistic dynamics. By timing the propagation of narrow wave-packets along the edge of an exfoliated graphene sample on picosecond time scales, we find that the propagation is chiral with low attenuation and the velocity is quantized on the Hall plateaus. We extract the electron drift velocity, which we show to be slightly less than the Fermi velocity, as expected for the abrupt edge of graphene. It is substantially higher than in conventional quantum 2DES, which makes it experimentally accessible. Spatial spread of dynamical charge imbalance is shown to be narrower than for conventional soft edged systems.

For further information contact Joseph Strocio, 301-975-3716, joseph.stroscio@nist.gov

CNST Electron Physics Group Seminar


Nitin Samarth
Penn State University

Wednesday, September 12, 2012, 10:30AM, RM H107, Bldg. 217

The influence of spin-orbit coupling on the electronic band structure of semiconductors has provided a long standing basis for phenomena in semiconductor spintronics [1], ranging from the spin Hall effect to the magnetic behavior of ferromagnetic semiconductors. Recent theoretical developments [2,3] have led to a resurgent interest in the implications of spin-orbit coupling for interfacial/surface states in narrow band gap semiconductors [4]. The modern perspective [2,3] recognizes deep and fundamental connections between topological invariants, time reversal symmetry, spin-orbit coupling and surface/edge states, resulting in "topological insulators" possessing gapped interior bulk states and metallic surface/edge states with a spin textured Dirac cone dispersion. Interfacing topological insulator surface states with order parameters such as superconductivity, ferromagnetism and antiferromagnetism is predicted to result in condensed matter analogs of exotic quantum particles such as Majorana fermions, dyons, axions and magnetic monopoles [2,3]. Motivated by these ideas. we have undertaken a concerted program to develop epitaxial topological insulator heterostructures [5] wherein "candidate" topological insulators are interfaced with a variety of magnetic systems. I will provide a general overview of this topic, followed by a focus on the synthesis and characterization of ferromagnetic topological insulator heterostructures [6] and the effects of breaking time-reversal symmetry on the spin texture of Dirac cone surface states [7]. 1. D.D. Awschalom and N. Samarth, Physics 2, 50 (2009). 2. M. Z. Hasan and C. L. Kane, Rev. Mod. Phys. 82, 3045 (2010). 3. X. -L. Qi and S. -C. Zhang, Rev. Mod. Phys. 83, 1057 (2011). 4. B. A. Volkov and O. A. Pankratov, JETP Lett. 42, 178 (1985). 5. A. Richardella et al., Appl. Phys. Lett. 97, 262104 (2010). 6. Duming Zhang et al., arxiv: 1206.2908. 7. Su-Yang Xu et al., Nature Physics 8, 616 (2012).

For further information contact Joseph Strocio, 301-975-3716, joseph.stroscio@nist.gov

CNST Energy Research Group Seminar


Clint Novotny
BAE Systems, Inc.

Wednesday, September 5, 2012, 10:30AM, RM H107, Bldg. 217

Semiconductor nanowires have recently gained much attention due to their promising electrical, optical, and thermal properties. As the desire to commercialize nanotechnology increases, so does the need for a better understanding of the basic principles behind these technologies. This research takes a bottom up approach to the study of nanowires by first investigating and controlling the growth of vertically aligned, self-catalyzed InP nanowires. Second, a novel fabrication method is developed that allows for the first measurement of the linear electro-optic coefficient of nanowires. Finally, the growth of nanowires directly on an ITO electrode is examined for use in nanowire/polymer hybrid solar cells.

For further information contact Fred Sharifi, 301-975-4633, fred.sharifi@nist.gov

CNST Energy Research Group Seminar


Ekaterina A. Pomerantseva
University of Maryland

Thursday, August 2, 2012, 10:30AM, RM H107, Bldg. 217

Miniaturization of energy storage devices (batteries and supercapacitors) has not kept in pace with constantly evolving Microelectromechanical Systems (MEMS) technology. As a result, autonomy and functionality of microsystems such as microsensors, microactuators, and miniaturized medical implants are limited by the lack of suitable power sources. The sizes of these devices are often determined by the size of the power supply, and an urgent need exists for scaled-down power sources without compromising their performance. In this work we present a fabrication approach that combines MEMS methods with the use of biologically templated nanomaterials for lithium-ion battery and supercapacitor hierarchical electrodes. These methods enable fabrication of three-dimensional elements that increase surface area and electrochemically active material loading for the same electrode footprint. The use of nanostructured materials for energy storage device electrodes has advantages of better mechanical integrity, higher electrode/electrolyte contact area and shorter diffusion distances for electrons and ions. The key enabling technology is the use of the Tobacco mosaic virus (TMV) as a template for the synthesis of nanostructures. Previously we have demonstrated core/shell virus-templated Li-ion battery electrodes as well as photolithographic patterning and 3D assembly of the particles. This novel technology enables a significant increase in energy density without increasing electrode footprint or compromising power density, and it has demonstrated compatibility with a variety of energy storage materials (Si, TiO2, and V2O5). In this work, hierarchical electrodes are fabricated and their energy and power density are for the first time characterized. The hierarchical electrodes studied in this work consist of virus-templated nanostructures self-assembled on three-dimensional micropillars. Active battery material (V2O5) is conformally deposited using atomic layer deposition (ALD) on the hierarchical micro/nano network. Electrochemical characterization of these electrodes indicates a 3-fold increase in energy density compared to nanostructures alone, while maintaining the high power characteristics. Investigation of capacity scaling for varying active material thickness reveals underlying limitations and highlights the importance of our method in controlling both energy and power density with structural hierarchy. The presented approach demonstrates a new direction for the design of nanostructured electrodes for energy storage microdevices.

For further information contact Alec Talin, 301-975-4724, alec.talin@nist.gov

CNST Electron Physics Group Seminar


Kamaram Munira
University of Virginia

Tuesday, July 24, 2012, 10:30AM, RM H107, Bldg. 217

The ability to rotate the magnetization of a single domain nanomagnet using spin polarized current or uniaxial strain leads to exciting possibilities for low-power embedded memory and logic applications. Realizing those applications for real life usage requires addressing a complex and interlinked set of problems: material properties of the ferromagnet-oxide heterostructure, spin transport, micromagnetics and thermal stochasticity of the free layer. A particular challenge the STT-RAM industry faces is maintaining a high thermal stability while trying to switch within a given voltage pulse with an acceptably low error rate and energy cost. While operating at lower barrier increases the static error in STT-RAMs, it decreases the dynamic write error rate associated with the spins freezing around stagnation points along the potential energy landscape of the nanomagnets. We introduce a comprehensive and predictive STT-RAM modeling platform that operates at different levels of complexities, ranging from a quasi-analytical model for the energy-delay-reliability tradeoffs to a fully atomistic, chemistry based multi-orbital model for predictive material design and optimization. Using this platform, we identify suitable alloys for perpendicular, in-plane and partially perpendicular magnets and underscore the dual role of thermal fluctuations, both in hindering rotation and also in releasing spins from their stagnation points. A similar set of challenges confronts ‘straintronics’ based multiferroic logic, where once again thermal perturbations play a decisive role on the dynamic writing error rate. In presence of stagnation points, applied stress, demag field and dipole-dipole interactions, the error rate and switching delay can be controlled by material design and by engineering the stress profile on the nanomagnets.

For further information contact Mark Stiles, 301-975-3745, mark.stiles@nist.gov

CNST Nanofabrication Research Group Seminar


Shin Grace Chou
Radiation and Biomolecular Physics Division, Physical Measurement Laboratory, NIST

Wednesday, July 11, 2012, 10:30AM, Rm. H107, Bldg. 217

In this talk, I will discuss the applications of a number of spectroscopic techniques (resonance Raman, photoluminescence, and THz spectroscopy) in probing the different types of excitation and relaxation processes in condensed matter systems. Depending on the level of spectral resolution and the nature of the materials system, theories ranging from first principles calculations to the lowest-order, symmetry-adapted models were applied to interpret the spectroscopic finding and to obtain physical insights into the structural-property relationship of the materials and, sometimes, the roles of the interfacial water. Specifically, the talk will discuss illustrative examples of optical studies of carbon nanotubes and peptide-based biomaterials.

For further information contact Rachel Cannara, 301-975-4258, rachel.cannara@nist.gov

CNST Nanofabrication Research Group Seminar


Friday June 22, 2012, 10:30AM

Wei Wang
Penn State University

The nano/micro motor research in Mallouk group, and in the department of chemistry at Penn State in general, focuses on a number of aspects of motion at small scale. Firstly we design and fabricate autonomously moving nano/micro objects, or nano/micro motors, that demonstrate interesting individual and collective behaviors. Secondly, through our collaboration with Physicists and chemical engineers, we investigate the fundamental aspects of these motors such as the motion at viscosity-dominated low Reynolds number regime, and the energy transduction at small scale. We also actively collaborate with groups in bioengineering and mechanical engineering on applying these nano/micro motors in various applications. The first part of my research is about elucidating and improving the energy efficiency of the catalytic micromotor system, which has low energy efficiency (ca. 10-9 for Au-Pt micromotors). We estimate a 10-2 energy loss due to the non-electrochemical decomposition of H2O2 at the Pt end of the Au-Pt motor, and another 10-2 or 10-3 energy loss due to the fast diffusion of protons. Replacing Pt with the catalytically less active metal Ru increases the energy efficiency 10-fold, and confining the proton flux is also a promising way to improve the energy efficiency of catalytic motors. Additionally we show that the electrophoretic mechanism is intrinsically energy inefficient. In the second part of my research, we discovered a new propulsion mechanism for autonomously moving rod-shaped metallic micromotors using ultrasound. Ultrasonic standing waves operating in the MHz frequency range can be used to levitate, propel, rotate, align and assemble metal nanowires (2-3 µm long and ~300 nm diameter) in water. A self-acoustophoresis mechanism based on the shape asymmetry of the metal nanowires is proposed to explain the axial propulsion of the rods. The ultrasonically driven movement of the metal nanowires opens the possibility of driving and controlling metallic micromotors in biologically relevant media.

For further informaiton contact James Alex Liddle, 301-975-6050, james.liddle@nist.gov

CNST Electron Physics Group Seminar


Tuesday June 19, 2012, 10:30AM, Rm. H107, Bldg. B217

Wang-Kong Tse
University of Texas at Austin

Topological insulators are a novel class of materials theoretically predicted several years ago whose existence has subsequently been confirmed by experiments. These materials are insulating in the bulk but carry gapless surface states that are protected by time-reversal symmetry. Under broken time-reversal symmetry condition, the resulting half-quantized surface quantum Hall effect is a distinctive signature of the topological insulating state and can be probed via the associated magneto-optical response. In this talk, I will present a microscopic theory for the magneto-electric and magneto-optical effects of topological insulator films. When time-reversal symmetry is broken, our theory predicts that the low-frequency Faraday effect is quantized in integer multiples of the fine structure constant 1/137, whereas the Kerr effect exhibits a giant full-quarter rotation from the incident polarization plane. Possible experimental detection schemes for these effects will be discussed.

For further information contact Mark Stiles, 301-975-3745, mark.stiles@nist.gov

CNST Electron Physics Group Seminar


Thursday June 14, 2012, 10:30AM, Rm. H107, Bldg. 217

Professor Hyun-Woo Lee
Pohang University

Rashba spin-orbit coupling arises generically when structural inversion symmetry is broken. Recently it was suggested [1-3] that ultrathin ferromagnetic layers with broken structural inversion symmetry may be subject to the Rashba coupling. If the coupling is strong, it can affect magnetization and conduction electron spin dynamics of the ultrathin layers considerably. In this talk, we first explore the magnetization dynamics of such ultrathin layers or Rashba ferromagnets. We demonstrate that the coupling modifies properties of the spin torque and the current-driven motion of magnetic domain walls. Next we discuss effects of the coupling on spin-dependent electromagnetic fields, of which existence has been known for many years [4-7] but commonly ignored due to their weakness. We demonstrate that the coupling can enhance the spin-dependent fields considerably, elevating the fields from pure scientific curiosity to relevant factors for the magnetization dynamics of Rashba ferromagnets.

For further information contact, Mark Stiles, 301-975-3745, mark.stiles@nist.gov

CNST Electron Physics Group Seminar


Friday May 25, 2012, 1:30PM, Rm. H107, Bldg. 217

Liang Fu
Assistant Professor, MIT

Topological superconductors are unconventional superconductors which have protected gapless surface Andreev bound states josting itinerant Majorana fermions. There is currently intensive search for topological superconductors. In this talk, I will describe our theoretical proposal of odd-parity pairing in a recently discovered superconductor Cu-doped Bi2Se3, which leads to topological superconductivity. Properties of this superconducting state will be discussed in connection with recent experiments.

For further information contact Joseph Stroscio, 301-975-3716, joseph.stroscio@nist.gov

CNST Energy Research Group Seminar


Tuesday May 15, 2012, 2:00PM, Rm. H107, Bldg. 217

Daniel Thompson
Clemson University

Thermoelectric materials provide a means for the direct conversion between thermal and electrical energy. The historic thermoelectric material for power generation, Silicon Germanium (SiGe), was investigated to determine means of controlling the lattice thermal conductivity. Various avenues for reducing lattice thermal conductivity were investigated and these methods along with their performance will be discussed.

For further information contact Fred Sharifi, 301-975-4633, fred.sharifi@nist.gov

CNST Electron Physics Group Seminar


Tuesday May 15, 2012, 10:30AM, Rm. H107, Bldg. 217

Benjamin Pigeau
CEA Saclay, France

The main purpose of this seminar is to introduce an experimental tool to perform magnetic resonance spectroscopy and imaging: the Magnetic Resonance Force Microscope or MRFM. I will present the general principles of this measurement, invented in the early 90’s by John Sidles to perform nuclear magnetic resonance imaging. Then I will show how we have adapted this technique to detect the Ferro-Magnetic Resonance (FMR).
In our FMRFM apparatus, a tiny magnetic iron sphere of 600 nm in diameter is glued at the apex of a very soft AFM cantilever (k ? 5 mN/m). This probe is placed just above the sample where it experiences dipolar interactions with the sample. When resonances are excited in the sample by the micro-wave magnetic field, the sample’s stray field is varied, modifying the dipolar coupling with the sphere. The changing force between the probe and the sample results in a cantilever deflection which is then optically detected with an interferometer (see figure 1 a)). The MRFM is well adapted to study individual magnetic nanostrucures, like disks. I will present two interesting results:
-A ferromagnetic disk (made of NiMnSb)in the magnetic vortex state. The magnetization is curling in the plane and present a singularity, the so called vortex core, in the center. It is possible to play with the dynamics and relative orientation of the core with microwave fields.
-A system of two adjacent disks (made of FeV), dipolarly coupled, that can exhibit a coupled magnetization dynamics. This is an interesting "two level" system where the MRFM can tune the strength of the coupling.

For further information contact Robert McMichael, 301-975-5121, robert.mcmichael@nist.gov

CNST Electron Physics Group Seminar


Kohei Ueda
Institute for Chemical Research, Kyoto University

Monday, May 14, 2012, 10:30AM, Rm. H107, Bldg. 217

The spin transfer effect has attracted much attention from the viewpoints of not only the fundamental physics but also of potential application to spintronics devices. One of the factors determining the performance of spintronics devices is carrier spin polarization (P). Not only the search for materials with high P (or structures exhibiting high tunnel magnetoresistance ratio) but also the development of methods to determine the current polarization are crucial. Current-induced magnetic domain wall motion (CIDWM) has been widely investigated, and theory [6] predicts that DW velocity induced by currents (v) is proportional to P, suggesting that P can be determined from v experimentally. In this talk, I will present an overview of the DW motion in perpendicularly magnetized Co/Ni nanowires that our group has reported and then talk about new measurements of the temperature dependence of the CIDWM. As main research result, temperature dependence of P in Co/Ni determined from CIDWM will be reported over a wide temperature range from the low temperature to near up to Currie temperature. In addition, it was found that P strongly depends on temperature and the result is in good agreement with an earlier study using spin wave Doppler technique reported by M. Zhu et al.

For further information contact Robert McMichael, 301-975-5121, robert.mcmichael@nist.gov

CNST Energy Research Group Seminar


Duming Zhang
Penn State University

Tuesday, May 8, 2012, 10:00AM, Rm. H107, Bldg. 217

In this talk, I will discuss our recent experiments on topological insulators. First, I will demonstrate proximity-induced superconductivity in topological insulator (Bi2Se3) nanoribbons as a possible route towards the search for Majorana fermions in condensed matter. We observe distinct signatures of the superconducting proximity effect which couples preferentially to a ballistic surface channel. In addition, we also provide evidence for possible formation of vortices in the proximity-induced region. These two key results provide an important step towards realizing a condensed matter analog of Majorana fermions. Second, I will discuss our research on a magnetically-doped topological insulator (Mn-doped Bi2Se3) to induce a surface state gap, which might lead to exotic phenomena such as the topological magnetoelectric effect and the induction of magnetic monopoles. Our systematic measurements reveal a close correlation between the onset of ferromagnetism and quantum corrections to diffusive transport, which crosses over from the symplectic (weak anti-localization) to the unitary (weak localization) class. These observations are consistent with the prediction of a time-reversal symmetry breaking gap, which is further supported by angle-resolved photoemission spectroscopy measurements.

For further information contact Fred Sharifi, 301-975-4633, fred.sharifi@nist.gov

CNST Electron Physics Group Seminar


Morgan Trassin
Post Doc, UC Berkeley

Wednesday, May 2, 2012, 10:30AM, Rm. H107, Bldg. 217

Controlling magnetism using solely electric fields is interesting not only from a fundamental standpoint, but presents great potential for ultimately low energy consumption logic and memory. The evidence of the electrically controllable antiferromagnetic ordering in the multiferroic magnetoelectric bismuth ferrite (BiFeO3) drew an increasing interest in the pursuit for new emerging devices. To use such functionality for device applications, deterministic control not only of antiferromagnetism, but also ferromagnetism is essential. To achieve this goal, a ferromagnet/multiferroic heterostructure has been proposed based on the combination of magnetoelectric coupling in BiFeO3 and exchange coupling between magnetic materials and offers a new pathway for the electrical control of magnetism. By combination of a piezoresponse force microscopy, photoemission electron microscopy and anisotropic magnetoresistance measurements, we demonstrated the non-volatile reversal of a CoFe layer magnetization induced solely by the application of an electric field at room temperature. This 180° rotation of the magnetization of the ferromagnetic layer is mediated by a strong interfacial coupling. The correlation between the ferroelectric state in the multiferroic layer and the CoFe ferromagnetic domain architecture is evidenced. The projection of this strong magnetoelectric coupling in an out-of-plane configuration, allowing the reduction by an order of magnitude of voltage required, will be discussed. Our results show the high potential of magnetoelectric-based heterostructures for future low energy consumption data storage devices.

For further information contact John Unguris, 301-975-3712,john.unguris@nist.gov

CNST Energy Research Group Seminar


Duming Zhang
Penn State University

Tuesday, May 8, 2012, 10:00AM, Rm. H107, Bldg. 217

In this talk, I will discuss our recent experiments on topological insulators. First, I will demonstrate proximity-induced superconductivity in topological insulator (Bi2Se3) nanoribbons as a possible route towards the search for Majorana fermions in condensed matter. We observe distinct signatures of the superconducting proximity effect which couples preferentially to a ballistic surface channel. In addition, we also provide evidence for possible formation of vortices in the proximity-induced region. These two key results provide an important step towards realizing a condensed matter analog of Majorana fermions. Second, I will discuss our research on a magnetically-doped topological insulator (Mn-doped Bi2Se3) to induce a surface state gap, which might lead to exotic phenomena such as the topological magnetoelectric effect and the induction of magnetic monopoles. Our systematic measurements reveal a close correlation between the onset of ferromagnetism and quantum corrections to diffusive transport, which crosses over from the symplectic (weak anti-localization) to the unitary (weak localization) class. These observations are consistent with the prediction of a time-reversal symmetry breaking gap, which is further supported by angle-resolved photoemission spectroscopy measurements.

For further information contact Fred Sharifi, 301-975-4633, fred.sharifi@nist.gov

CNST Electron Physics Group Seminar


Jennifer Hoffman
Professor of Physics, Harvard University

Friday, April 27, 2012, 1:30PM, RM H107, Bldg. 217

Although superconductors recently celebrated their 100th 'birthday', these fascinating materials have yet to be fully understood or tamed for widespread application. High-Tc cuprate superconductors display startling nanoscale disorder in essential properties such as Fermi surface and superconducting critical temperature. However, the underlying cause of this disorder remains mysterious: does it arise from spontaneous electronic phase separation which is unavoidable, or from atomic scale chemistry which may be subject to our control? The highest Tc superconductors to date are all non-stoichiometric materials, but atomic scale chemical mapping has remained elusive. We extend the energy range of scanning tunneling spectroscopy, allowing the first-ever direct mapping of all three types of oxygen dopants in Bi2+ySr2-yCaCu2O8+x with maximum superconducting Tc ~ 90K. We show that a subset of these dopants are indeed the direct cause of the nanoscale disorder. We explain how the spatial variations in competing electronic orders, such as the notorious 'pseudogap' and the charge density wave, are governed by the disorder in the dopant concentrations, which suggests a possible avenue to raise Tc in this material.

For further information contact Joseph Stroscio, 301-975-3716, joseph.stroscio@nist.gov

CNST Nanofabrication Research Group Seminar


Professor Judith Yang
Dept. of Chemical and Petroleum Engineering, Dept. of Physics, University of Pittsburgh

Thursday, March 29, 2012, 10:30AM, Rm. H107, Bldg. 217

The transient stages of oxidation - from the nucleation of the metal oxide to the formation of the thermodynamically stable oxide - represent a scientifically challenging and technologically important terra incognito. These issues can only be understood through detailed study of the relevant microscopic processes at the nanoscale in situ. We are studying the dynamics of the initial and transient oxidation stages of a metal and alloys with in situ methods, including ultra-high vacuum (UHV) transmission electron microscopy (TEM). We have previously demonstrated that the formation of epitaxial cuprous oxide islands during the transient oxidation of Cu(100), (110) and (111) films bear a striking resemblance to heteroepitaxy, where the initial stages of growth are dominated by oxygen surface diffusion and strain impacts the evolution of the oxide morphologies. We are presently investigating the early stages of oxidation of Cu-Au and Cu-Ni as a function of oxygen partial pressures and temperatures. For Cu-Au oxidation, the oxidation mechanisms change where the cuprous oxide reveals a dendritic growth. For Cu-Ni oxidation, the addition of Ni causes the formation cuprous oxide and/or NiO where the oxide type(s) and the relative orientation with the film depend on the Ni concentration, oxygen partial pressure and temperature.

For further information contact Renu Sharma, 301-975-2418, renu.sharma@nist.gov

CNST Electron Physics Group Seminar


Gene Mele
Department of Physics and Astronomy University of Pennsylvania

Friday, March 23, 2012, 10:30AM, RM H107, Bldg. 217

Topological insulators are a recently discovered quantum electronic phase of matter. This talk will give a brief overview of the known electronic phases of matter, focusing on the unique properties of topological insulators and their discovery from a careful consideration of the low energy electronic physics of single-layer graphene. Closely related topological ideas are then used to analyze the mysterious electronic behavior of a family of multilayer graphenes known as "twisted" graphenes in which a rotation of neighboring layers leads to unexpectedly rich low energy physics.

For further information contact Shaffique Adam, 301-975-6187, shaffique.adam@nist.gov

CNST Energy Research Group Seminar


Francesco Stellacci
Institute of Materials, Ecole Polytechnique Fédérale, Lausanne (EPFL), Switzerland

Friday, March 16, 2012, 1:30PM, Rm. C103-106, Bldg. 215

A bird eye view of any folded protein shows a complex surface composed of hydrophobic and hydrophilic patches closely packed. To date little is known on the fundamental properties that such packing determines. In this talk I will present my group's endeavor into the synthesis, characterization, and understanding of a family of nanomaterials (mixed monolayer protected nanoparticles) that posses a surface coexistence of patches of opposite hydrophilicity resembling that present on folded protein. I will show that these materials are ideal model compound to uncover the basic properties that such coexistence determines at the solid liquid interface, and will conclude with example of application of these nanoparticles when used as mimic of biological entities (e.g. as cell penetrating peptides, as nano-enzymes, etc.).

For further information contact Andrea Centrone, 301-975-8225, andrea.centrone@nist.gov

CNST Nanofabrication Research Group Seminar


Mirjam Leunissen
Foundation for Fundamental Research on Matter, AMOLF Institute. Utrecht, The Netherlands

Friday, February 24, 2012, 1:30PM, Rm. H107, Bldg. 217

In this lecture, I will focus on the exciting new possibilities that synthetic DNA offers for the creation of self-organizing and self-replicating materials of nano- and micro-particles. DNA 'sticky ends' with complementary nucleotide sequences, for instance, form highly specific and reversible links between the particles. Besides their application in directed self-assembly, I will discuss the intriguing physics and broader implications of such (collections of) weak ligand-receptor-like bonds.

For further information contact J. Alex Liddle, 301-975-6050, james.liddle@nist.gov

CNST Electron Physics Group Seminar


Hyun-Woo Lee
Associate Professor, Pohang University

Friday, February 24, 2012, 10:30AM, Rm H107, Bldg. 217

Rashba spin-orbit coupling (RSOC) arises generically when structural inversion symmetry is broken. Surface electronic structure of heavy metallic elements such as Au or Bi/Ag alloy is known to exhibit large RSOC with characteristic RSOC parameter of the order of 1 eV?A. Topological insulator is another class of material with large RSOC. In this talk, we explore magnetization dynamics of ultrathin magnetic layers in contact with such strong RSOC sources. Since RSOC induces the deviation of conduction electron spin direction from local magnetization direction, it modifies the spin torque considerably. It will be demonstrated that this modification can be quite significant and current-driven magnetization dynamics properties can change considerably. For instance, the current-driven magnetic domain wall moves against electron flow instead of the conventional direction, which is along it. We also discuss the phenomenon of the spin-dependent electric field induction by magnetization dynamics. It will be illustrated that RSOC can strengthen the spin-dependent electric field more than one order of magnitude, so that the field can induce spin current sufficiently large enough to modify the magnetization dynamics itself. Thus ultrathin magnetic layers with strong RSOC are good systems to test various aspects of RSOC effects.

For further information contact Mark Stiles, 301-975-3745, mark.stiles@nist.gov

CNST Nanofabrication Research Group Seminar


Tiffany Cheng
Ph.D Candidate, Cornell University

Wednesday, February 22, 2012, 10:30AM, Rm. H107, Bldg. 217

In this talk, I will address the improvement of quality factor, electromechanical coupling, and impedance of MEMS resonators, which are all important issues in improving the performance of MEMS resonators in integrated CMOS systems. I will show how we lowered the impedance of electrostatically actuated resonators by using extremely thin air gaps. In the second part of my talk, I will present some of my work with piezoelectric transducers and resonators. In particular, we are developing a high frequency piezoelectrically driven device with potential uses in high resolution measurement tools, tip-based nanofabrication, and applications that require a nanosecond ultrasonic source.

For further information contact Vladimir Aksuyuk, 301-975-2867, vladimir.aksyuk@nist.gov

CNST Nanofabrication Research Group Seminar


Mohsen Ahmadian
AEC- Project Manager for the Contrast Agent and Nanomaterial Sensors

Tuesday, February 14, 2012, 10:30AM, Rm. H107, Bldg. 217

The Advanced Energy Consortium (AEC) members (BG, BP, ConocoPhillips, Halliburton, Petrobras, Schlumberger, Shell, and Total) are fund and direct a multi-million $ collaborative effort in order to coordinate research projects which will accelerate the development of novel micro- and nanomaterial sensing technologies for oil exploration and recovery. This so-called "nanorealm" is so small and unexplored that it has never been seriously considered by most petroleum engineers, geophysicists, geologists, or geochemists. Given the heterogeneous nature of oil field rocks and fluids, and the harshness of the environment (high temperatures, high pressures, small pore spaces [30nm to 10µm], high salinity, and acidic conditions), this is a challenging and exciting proposition. In this talk I will give an overview of the three major technology research focus areas: 1) Contrast Agents 2) Nanomaterial Sensors 3) Microfabricated Sensors In addition, there is a collection of projects that address the critical and common need to enable particle/sensor transport deep into the reservoir. Issues include stability (lifetime) and transport (minimizing retention) enabled through coatings and/or custom synthesis.

For further information contact J. Alex Liddle, 301-975-6050, james.liddle@nist.gov

CNST Nanofabrication Research Group Seminar


Fahmida Ferdous
Research Assistant, Purdue University

Monday, February 6, 2012, 10:30AM, Rm. H107, Bldg. 217

Recently, on-chip comb generation methods based on nonlinear optical modulation in ultrahigh quality factor monolithic micro-resonators have been demonstrated, where two pump photons are transformed into sideband photons in a four wave mixing process mediated by the Kerr nonlinearity. We investigate line-by-line pulse shaping of such combs generated in silicon nitride ring resonators. We observe two distinct paths to comb formation which exhibit strikingly different time domain behaviors. For combs formed as a cascade of sidebands spaced by a single free spectral range (FSR) that spread from the pump termed as type I, we are able to stably compress to nearly bandwidth-limited pulses. This indicates high coherence across the spectra and provides new data on the high passive stability of the spectral phase. For combs where the initial sidebands are spaced by multiple FSRs which then fill in to give combs with single FSR spacing termed as type II, the time domain data reveal partially coherent behavior. We also investigate the behaviors of a few subfamilies of type II combs selected by a pulse shaper. We observe different coherent properties for different groups of comb lines.

For further information contact Vladimir Aksyuk, 301-975-2867, vladimir.aksyuk@nist.gov

CNST Nanofabrication Research Group Seminar


Jie Zou
Research Assistant, University of Florida. Gainesville, FL

Friday, January 20, 2012, 10:30AM, Rm. H107, Bldg. 217

The quantum vacuum might be devoid of matter, but its vacuum energy is always non-zero owing to ubiquitous quantum fluctuations. The presence of boundaries changes the total vacuum energy of electromagnetic fields and gives rise to the Casimir force that dominates the interaction between electrically neutral objects at nanoscale separations. The non-trivial dependence of the Casimir force on the geometry of the interacting objects provides opportunities to tailor this quantum electrodynamical force. In this talk, I will discuss our measurement of the Casimir force between a gold sphere and a silicon substrate with nanoscale trenches. The measured force deviates from the proximity force approximation, providing first evidence for the non-trivial boundary dependence of the Casimir force. Furthermore, I will describe our recent demonstration of the Casimir effect between two lithographically-defined micromechanical components on the same silicon substrate. Eliminating bulky off-chip positioners, this novel experimental setup not only represents the first step towards on-chip exploitation of the Casimir force, but also opens possibilities for engineering the Casimir force using unconventional geometries.

For further information contact Vladimir Aksyuk, 301-975-2867, vladimir.aksyuk@nist.gov

CNST Nanofabrication Research Group Seminar


Brian Piccione
Department of Materials Science and Engineering, University of Pennsylvania. Philadelphia, PA

Tuesday, January 17, 2012, 11:00AM, Rm. H107, Bldg. 217

As the bandwidth limitations of metals become more burdensome with each increase in electronic interconnect density, the hesitation towards investment in optical solutions has eroded. Nanophotonics is maturing at a rate where on-board and inter-chip optical interconnects appear likely if not inevitable, and solutions incorporating compound semiconductors remain important to this end. The exciting prospect of optical processing notwithstanding, even in a data transmission role, successful integration of optics requires a large toolbox of high-performance components: emitters, detectors, modulators, waveguides and switches, among others. Surface-passivated semiconductor nanowires, which exhibit vastly improved surface conditions when compared with comparable components produced via top-down methods, can serve as both candidates for future nanowire-based optical networks themselves, as well as model systems for furthering the understanding of optical processes in highly confined structures. Here we will show, for the first time, all-optical switching in individual CdS nanowire optical cavities with sub-wavelength dimensions, as well as a functional all-optical NAND gate built from multiple switches. The unique device designs utilize very strong light-matter coupling in the II-VI nanowire cavities not found in silicon and therefore result in total footprints a fraction that of comparable silicon-based dielectric contrast and photonic crystal devices, underscoring the continued relevance of compound semiconductor materials.

For further information contact Vladimir Aksyuk, 301-975-2867, vladimir.aksyuk@nist.gov

CNST NanoFab Operations Group Seminar


M. D. Stewart, Jr., PhD
Joint Quantum Institute & NIST

Thursday, January 12, 2012, 10:30AM, Rm. H107, Bldg. 217

Silicon based single electron devices for use as potential quantum bits have attracted much attention recently. The advantages derived from working in the most studied material system in the world as well as the ability to leverage decades of industrial fabrication techniques have helped fuel this work. Borrowing techniques from industry, we have fabricated and measured CMOS clean Si single electron devices of a unique architecture in the CNST for potential quantum information applications. As a first step toward these applications, we have succeeded in inducing and measuring the device response to a single electron. This talk will discuss why we believe CMOS processing is important in these devices, our current processing protocols and challenges, and our latest data on these devices.

For further information contact Vince Luciani, 301-975-2886, Vincent.luciani@nist.gov

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