SOLID STATE SEMINARS
Schedule For Fall, 2008
All talks are in Room B-131, except when otherwise noted. Regular seminar time is Friday 1:30PM. Please send a Mail to Marivi Fernandez-Serra with the name of the speaker, date and title and abstract for the seminar. Follow the links to see the schedule in past semesters.
Nanowire Chemical Gas Sensors and Their Heterogeneous integration for Electronic Nose Applications
The development of a low-footprint integrated "electronic nose" (E-nose) will open vast markets spanning a wide variety of applications. However, despite several decades of intensive research, the goal of creating such an artificial E-nose that can compete with a biological olfactory system has yet to be achieved. This talk presents an approach to integrate a high sensitive nanowire sensor array with large chemical diversity and massive parallelism which are the key characteristics of a smart artificial E-nose. The talk starts with electrochemical synthesis of single nanowire sensors with a wide range of chemical diversity including conducting polymers (PEDOT/PSS) and metal oxides (TiO2, SnO2 and ZnO). Their electronic and gas-sensing properties including thermal effects were also investigated. As a second part of this talk, we demonstrate a new electrochemical approach to integrate these heterogeneous nanowire sensors on chip. This approach has a potential to meet all requirements of an sophisticated E-nose system. Metal oxide nanowire sensors operating at elevated temperatures are not compatible with CMOS circuits and sensors operating at low temperatures such as conducting polymers. As a thought for future work, on-chip microhoplates are proposed to locally heat up metal oxide sensors to solve this issue. Briefly, this talk has demonstrated to overcome all challenges of developing a sophisticated E-nose. It still may need years to develop such an E-nose. But successful development of such an E-nose will generate a huge impact on current electronic nose technology.
Using Synthetic Inorganic Chemistry to learn about the Active Site in Hydrogenase Enzymes
The hydrogenase enzymes can efficiently catalyze the interconversion of protons and electrons and hydrogen gas: 2 H+ + 2 e- = H2. The only other system, which catalyzes this technologically important reaction, is platinum metal. The active site of the hydrogenase enzymes contains the far less expensive metals, nickel and iron. We endeavor to understand how and why Nature has chosen to construct the unusual [(RS)4Ni-Fe(CO)(CN)2 ] active site center, which contain cyanide and carbon monoxide as native ligands.
Dynamical response of 1D Bose liquids. Generation of solitons.
We discuss the dynamical excitation of one-dimensional system of bosons in the context of Brag scattering experiments with cold atoms. The dynamical structure factor (DSF) is introduced to characterize the energy absorption by the system under the harmonic perturbation. The numerical and analytical results on DSF indicate the finite absorption rate in the wide range of frequencies not withstanding the classical picture of the absorption. The lower bound on the frequency of the probe of a given wave-number to result in finite absorption is set by the energy of the soliton solution of the semi-classical Gross-Pitaevskii equation. We argue that the harmonic probe inevitably leads to soliton generation when its frequency exceeds the kinematical threshold. We find the probability of soliton creation to have a power-law dependence on the frequency detuning from the threshold. This dependence is a signature of the quantum nature of the absorption process and the orthogonality catastrophe phenomenon associated with it.
The k-space inhomogeneity in heavy fermion materials
I will outline some recent developments in exploratory synthesis of two model materials: CeCoIn5 and FeSb2. Presentation will include characteristics of magnetic field induced quantum critical point and magnetic field/composition tuned magnetism in CeCoIn5. This will be followed by design, discovery and characterization of new FeSi - like narrow gap nearly magnetic (or "heavy fermion") semiconductor FeSb2. Selected results will be presented, progressing from historical aspects of materials discovery to perspective outlook on applications and design of new materials.
Shot noise in nano-electronic devices
Shot noise is widely observed in everyday life, such as counting the number of rain drops falling into a bucket or the number of vehicles arriving at a parking lot in a given time interval. In electrical systems, shot noise is the time-dependent fluctuations in electrical currents due to the discreteness of the electron charge. It exists in various systems, including vacuum tubes, transistors, and nano-scale electronic devices. A powerful attribute of shot noise is that it reveals information on temporal correlations in electronic transport that is not accessible by typical transport investigations, such as current versus voltage measurements. In this talk, I will explore the exciting possibilities of studying shot noise in mesoscopic tunnel barriers using different measurement techniques. We first investigate single tunnel barriers and find that localized states in such systems have a significant effect on their shot noise behavior. By fabricating extremely small tunnel barriers, we are able to demonstrate theoretically predicted shot noise behavior of an ideal tunnel barrier that does not contain any localized state. We then perform shot noise cross spectrum measurements in a beam splitter configuration using tunnel barriers as electron sources. In certain cases, we find a surprising positive cross spectrum that is not expected to exist in electronic systems. Shot noise suppression due to quantum interference is also observed. I will end with a discussion of some interesting future possibilities, such as using shot noise to probe quantum entanglement.
Phonon-Bottleneck-Driven Relaxation and Tunneling in Single-Molecule Magnets
A single-molecule magnets is - true to the name - a magnet made out of a single molecule. The magnetic moment of such a system shows hysteresis like a classical magnet, yet it can tunnel between different orientation states. This talk will focus on recent experiments to study the spin relaxation in single-molecule magnets when subjected to pulsed microwave radiation. We find that, for the Fe8 single-molecule magnet, intense, short pulses of radiation induce a phonon bottleneck with decay time ~5 microseconds. This allows us to observe the thermally assisted resonant tunneling process in real time down to ~100 ns time scales. Detailed numerical modeling of the process quantitatively agrees with our data.
Growth of KNbO3 Nanowires and Carbon Nanotubes for Functional Devices
The first part of the talk is dedicated to the oriented growth of KNbO3 nanowires by hydrothermal synthesis. Alkali niobate materials are believed to be the best candidates for replacing Pb containing piezoelectric materials. Furthermore, they are foreseen for photocatalytic production of H2 from water splitting. Therefore, the preparation of large arrays of oriented KNbO3 nanowires would drive the development of novel generation of devices. The second part of the talk deals with the newly discovered chemical mechanism in the synthesis of of Carbon Nanotubes (CNTs). Mixing C2H2 and CO2 in an equimolar proportion (C2H2/CO2 =1) provides outstanding kinetics characteristics of the reaction, leading to a large-scale production of carbon nanotubes free of amorphous carbon. The equimolar C2H2-CO2 reaction allows CNTs growth at temperatures well below 500°C without any arduous pre-activation of the catalyst, on numerous functional materials like oxides, nitrides, carbides or metals. It is an attractive synthesis pathway for the direct integration of CNTs into devices which do not support the traditional high temperature synthesis of CNTs.
First principles simulations of transition metal compounds: Magnetism, crystal field excitations and spin transitions
Pure density functional theory (DFT) approaches often yield poor results for many computed properties in transition metal (TM) compounds. A number of underlying factors contribute to this deficit, not least of which are the incomplete cancellation of electron self-interactions, and an inferior description of the on-site interactions in the d-orbital manifolds of the TM sites. Hybrid DFT has the potential to improve upon the description of such materials. The seminar will outline the basic principles of hybrid DFT, discussing the strengths and weaknesses of the approach. A number of case studies will be presented, including investigations of the magnetism and spin transitions in Prussian blue and related compounds, and in novel organic-inorganic hybrid materials.
Energy-scale Phenomenology and Spin-mediated Pairing for FeAs, CuO, heavy-fermion and other exotic superconductors
Recent discovery of the FeAs based superconductors has brought renewed excitement to studies of superconductivity in correlated-electron systems. Important energy scales of these superconductors include: (a) “superfluid density” (ns/m*) that can be determined by muon spin relaxation measurements of the magnetic-field penetration depth; (b) magnetic resonance mode energy from inelastic neutron scattering measurements; and (c) spin fluctuation / exchange interaction energy J.
Accumulated results demonstrate: (a) nearly linear correlations between Tc and the superfluid density for FeAs, CuO, A3C60, BEDT, and some heavy-fermion systems; (b) resonance mode energy scaling as 4-5kBTc for FeAs, CuO, CeCu2Si2. CeCoIn5 as well as rotons in superfluid He; (c) nearly linear relationship between Tc and the ``spin fluctuation energy scale’’ at the zone boundary in CuO, FeAs, and heavy fermion superconductors; and (d) all of these correlated-electron superconductors have parent compounds with static magnetic order, and superconductivity appears when static magnetic order disappears, often in first-order phase transitions involving abrupt change and/or phase separation near the phase boundary.
We discuss implication of these results with respect to pictures [1] based on Bose-Einstein to BCS crossover, resonance mode as roton analogue soft-mode excitations, and an extremely strong spin-charge coupling and superconducting pairing caused by charge motion resonating with spin fluctuations, which we shall call “traffic-light resonance”.
[1] Y.J. Uemura, Physica B374-375 (2006) 1; arXiv:0811.1546 (2008)
Charge transfer and electronic structure of graphene and graphite intercalation compounds
In graphite intercalation compounds (GIC), layers of different chemical species (intercalants) are introduced between graphene sheets. Due to the charge transfer between the intercalant and graphene layers, intercalation allows a controlled doping of graphene sheets and a broad variation of many physical properties, including the emergence of relatively high transition temperature superconductivity in some GICs. We have studied the changes in the electronic structure of various GICs in angle-resolved photoemission spectroscopy and found that, with the doping of graphene sheets, the electronic correlations become stronger and more anisotropic.