TcSUH
Special Seminar
Zintl Phases as High Efficiency Thermoelectric Materials
by: Dr. Jeff Snyder
Date: Monday April 10, 2006
Time: 11:00 am – 12:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
An efficient thermoelectric material requires the combination of high electronic density of states with high electrical conductivity in combination with low thermal conductivity. Simple semiconductors can have electronic properties suitable for a thermoelectric material but often have high thermal conductivity. Amorphous conductors can have complex crystal structures for low thermal conductivity but also have broad electronic bands unsuitable for thermoelectrics. The ideal thermoelectric material has a complex, even disordered, structure at multiple Angstrom and nanometer length scales to scatter phonons, but also a covalently bonded network that provides high mobility, and heavy mass charge carriers. In addition, like high temperature superconductors, thermoelectrics require precise doping of electronic concentration without disrupting charge carrier pathways. Zintl phases are ideally suited for thermoelectrics because they can provide complex structures within a semiconducting framework that include ionic regions for doping. Examples of this principle in action are evident in the high thermoelectric figure of merit materials, Zn4Sb3, Yb14MnSb11 and the Skutterudites.
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Special Seminar
Single-Photon Optical Detectors Based on Superconducting Nanostructures
by: Prof. Roman Sobolewski
Date: Friday April 07, 2006
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
We review the current state-of-the-art in the development of superconducting single-photon detectors and demonstrate their advantages over conventional semiconductor avalanche photodiodes in terms of very efficient and ultrafast counting capabilities of both visible-light and infrared photons. Superconducting single-photon detectors (SSPDs) are quantum photon counters and their detection mechanism is based on photon-induced hotspot formation and, subsequently, generation of a voltage transient across a nanostructured NbN meander (4-nm-thick and ~100-nm-wide stripe). They operate at 4.2-2 K temperature range. Our best, 10X10-μm2-area devices exhibit quantum efficiency of up to ~30% in the visible to 1550 nm wavelength range, dark counts <0.01 per second, and the noise-equivalent power (NEP) of 5x10-21 W/Hz.1/2 The 4x4-μm2-area detectors are characterized by >2-GHz photon counting rate and timing jitter of <18 ps. The SSPD structures can be directly coupled to a single-mode optical fiber, packaged in a standard transport helium dewar, and regarded as a room-temperature-like apparatus. The SSPDs have already been successfully applied in commercial testers for debugging of VLSI CMOS circuits and are currently being implemented for free-space satellite optical communication links and in fiber-based quantum key distribution (quantum cryptography) systems.
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Special Seminar
The approach for nanolayered semiconductor-on-insulator
by: Dr. Lin Shao
Date: Thursday April 06, 2006
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
The creation of ever higher density chips with faster speeds and lower power consumption is one of the major goals being persistently pursed by the microelectronics industry. Device integration on silicon-on-insulator (SOI) wafers offer a sustainable, long-term pathway for scaling various devices such as sensors, photodiodes, and most importantly, complementary metal oxide semiconductor (CMOS). Fabrication of SOI wafers is typically performed using an ion-implantation-based technique in which implanted hydrogen atoms react with broken silicon bonds and create hydrogen-filled microcracks which can propagate in a direction parallel to the wafer surface. The top Si layer is then split apart and bonded to an oxide layer, to form SO1 wafers. However, there are technical challenges for the fabrication of nanolayered SO1 wafers, with the top Si layer less than 100 nm thick. We have recently developed novel approaches to transfer an ultrathin Si layer by using a buried strained layer or highly doped layer to provide H trapping centers during hydrogenation, with following advantages: 1) The crack location can be controlled by adjusting the position of the H-trapping layer; and 2) the crystalline quality of the transferred layer is greatly improved due to reduced irradiation damage. We have realized the lift-off of a Si layer with a thickness of 15 nm, which is not achievable by previous techniques. This talk will give an overview of the status and perspective of ion cutting techniques.
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Special Seminar
Colloids in External Fields: Crystallization, Melting, and Dynamics
by: Dr. Charles Reichhardt
Date: Monday April 03, 2006
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
Colloidal assemblies are ideal model systems in which to study basic equilibrium and non-equilibrium phenomena relevant to a wide range of condensed matter systems. Additionally, there are a variety of technological applications for self-organizing colloid structures, including photonic band gap materials and patterned nanostructures. Here we study the statics and dynamics of colloids interacting with external fields. When the fields are used to create a periodic substrate, we find a variety of novel crystalline states that we term "colloidal molecular crystals." These have interesting multi-step melting transitions and can be used to realize a variety of canonical statistical mechanics models physically. When the substrate is dynamic, we show that novel dynamical phases arise and lead to ratchet effects, which can be used to create new types of logic gates and new fractionation techniques. Many of these results have recently been realized experimentally for colloids interacting with periodic optical arrays.
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Special Seminar
Pseudogap, Superconducting Energy Scale, and Fermi Arcs in Cuprate Superconductors
by: Hai-Hu Wen
Date: Thursday March 02, 2006
Time: 4:00 pm – 5:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
Through low temperature specific heat and point contact tunneling measurement, we investigated the pairing symmetry and low energy quasiparticle excitation behavior in cuprate superconductors. The following conclusions are drawn:
1. For hole doped samples, the Volovik's relation predicted for the d-wave pairing symmetry has been well demonstrated by low temperature specific heat in wide doping regime in La2-xSrxCuO4. This is supported by the tunneling spectrum with a zero-bias-conductance peak along nodal direction. The nodal slope of the superconducting gap is thus derived and is found to follow the same doping dependence of the pseudogap Δp. Both indicate a close relationship between the pseudogap and superconductivity.
2. Still in the hole-doped side, it is also found that the critical temperature Tc is determined by the multiplication of the nodal gap slope and the residual Fermi arc length.
3. For electron doped samples, both specific heat and tunneling measurements reveal an unavoidable s-wave component which may be explained by the two-band and thus two-gap pictures. The evidence for pseudogap is presented from the tunneling data under a high magnetic field for electron-doped samples. It is found that the pseudogap does not change with temperature instead of being filled up by thermal excitation.
These observations put strong constraints on the theoretical models of high temperature superconductors.
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