Search

Spin glasses (SG) exhibit many of the common properties of glassy behavior: a phase transition at T [subscript g], gradual freezing below T [subscript g], and an out-of-equilibrium aging sensitive to initial conditions. The work in this dissertation focuses on four effects: glassy dynamics in SG thin films, magnetic field effects on SG dynamics, classification of aging rates of the SG correlation length, and SG temperature chaos. The experiments were performed on the canonical metallic SG, CuMn, studied in multilayer thin films, and on a CuMn single crystal. The beauty of working with a thin film geometry lies with the growing correlation length ξ⊥(t) saturating at the thin film thickness L on laboratory time scales, thereby creating a quasi-equilibrium state. From studying the crossover dynamics on 4.5, 9 and 20 nm thin films, a thermal activation picture was developed to describe the dynamics after dimensional crossover: from ξ⊥(t) < L (three dimensional) to ξ⊥(t) = L (two dimensional). The maximum free energy barrier height governing the dynamics scales as ℓnL, compatible with power law dynamics of the Parisi mean field solution, and not the activated dynamics of the droplet model of Fisher and Huse. The freezing temperature T [subscript f] < T [subscript g] in thin films was studied systematically and related to the observation time scale. The free energy barrier heights are reduced by an external magnetic field, enabling us to obtain the number of correlated spins in the 20 nm CuMn multilayer sample. The geometry of the correlated spins is found to be “pancake-like” or cylindrical in the thin films. In order to remove the finite size constraint on the growth of SG correlation length ξ⊥(t), aging rates were extracted from long time aging experiments on a single crystal CuMn sample. The extracted correlation length is roughly 200 lattice spacing, the largest ever achieved. The results were compared with large scale numerical simulation from the Janus Collaboration, and an almost perfect match was found. The aging rates slows down as the SG correlation length increases, indicated by T = 0 fixed point dynamics. Benefiting from quasi-equilibrium states in thin films after dimensional crossover, temperature chaos, the sensitivity of spin glass equilibrium state to external temperature changes, was explored in the 9 nm and 20 nm multilayer CuMn thin films. The results have been rather counterintuitive. The observable effective waiting time differs with respect to different cooling protocols. A deeper understanding of temperature chaos requires further investigation.

Spin glasses (SG) exhibit many of the common properties of glassy behavior: a phase transition at T [subscript g], gradual freezing below T [subscript g], and an out-of-equilibrium aging sensitive to initial conditions. The work in this dissertation focuses on four effects: glassy dynamics in SG thin films, magnetic field effects on SG dynamics, classification of aging rates of the SG correlation length, and SG temperature chaos. The experiments were performed on the canonical metallic SG, CuMn, studied in multilayer thin films, and on a CuMn single crystal. The beauty of working with a thin film geometry lies with the growing correlation length ξ⊥(t) saturating at the thin film thickness L on laboratory time scales, thereby creating a quasi-equilibrium state. From studying the crossover dynamics on 4.5, 9 and 20 nm thin films, a thermal activation picture was developed to describe the dynamics after dimensional crossover: from ξ⊥(t) < L (three dimensional) to ξ⊥(t) = L (two dimensional). The maximum free energy barrier height governing the dynamics scales as ℓnL, compatible with power law dynamics of the Parisi mean field solution, and not the activated dynamics of the droplet model of Fisher and Huse. The freezing temperature T [subscript f] < T [subscript g] in thin films was studied systematically and related to the observation time scale. The free energy barrier heights are reduced by an external magnetic field, enabling us to obtain the number of correlated spins in the 20 nm CuMn multilayer sample. The geometry of the correlated spins is found to be “pancake-like” or cylindrical in the thin films. In order to remove the finite size constraint on the growth of SG correlation length ξ⊥(t), aging rates were extracted from long time aging experiments on a single crystal CuMn sample. The extracted correlation length is roughly 200 lattice spacing, the largest ever achieved. The results were compared with large scale numerical simulation from the Janus Collaboration, and an almost perfect match was found. The aging rates slows down as the SG correlation length increases, indicated by T = 0 fixed point dynamics. Benefiting from quasi-equilibrium states in thin films after dimensional crossover, temperature chaos, the sensitivity of spin glass equilibrium state to external temperature changes, was explored in the 9 nm and 20 nm multilayer CuMn thin films. The results have been rather counterintuitive. The observable effective waiting time differs with respect to different cooling protocols. A deeper understanding of temperature chaos requires further investigation.

In the near future, we envision many mobile robots working for or interacting with humans in various scenarios such as guided shopping, policing, and senior care. In these scenarios, tracking humans and mobile robots is a crucial enabling technology as it provides their continuous locations and identities. We study each kind of tracking individually as humans and mobile robots differ in appearance, mobility, capability of computation and sensing, and feasibility of cooperation with tracking systems. In addition to these distinctions, the scale of a tracking system's environment is another key factor that determines the appropriate tracking algorithm. In small scale environments such as halls and rooms, tracking systems need to be lightweight with real-time performance. In large scale environments such as campuses and entire buildings, tracking systems face complex and noisy backgrounds. Tracking systems need to be robust and scalable. This dissertation examines problems for human and mobile robot tracking in both small and large scale environments in order to enable future human-robot applications. First, we study human tracking. In small scale environments like halls, multiple cameras are typically deployed to monitor most areas. We need to track multiple humans across multiple cameras accurately in real-time. We present VM-Tracking, a tracking system that achieves these goals. VM-Tracking aggregates motion sensor information from humans' mobile devices, which accompany them everywhere, and integrates it with visual data from physical locations. In large scale human tracking, determining humans' identities among different visual surveillance scenes is a major challenge, especially with noisy and incorrect data. Since people's mobile devices connect to widespread cellular networks, we propose EV-Matching to address these practical challenges. EV-Matching is a large scale human tracking system that matches humans recorded in both cellular network and visual surveillance datasets. It achieves efficient and robust tracking. Next, we study mobile robot tracking. Similar to human tracking, visual cameras are widely available in small scale environments. Mobile robots are capable of computation and cooperation but their on-board sensing capacities have room for improvement. Thus, we present S-Mirror, a novel approach using infrastructural cameras to “reflect” sensing signals towards mobile robots, which greatly extends their sensing abilities. S-Mirror is a lightweight infrastructure that is easily deployed as it leverages existing visual camera networks, leaving most computation to robots. In large scale mobile robot tracking, scalability is a major challenge as infrastructure does not cover all areas and augmenting it is expensive. Since robots have built-in sensing capabilities, we propose BridgeLoc, a novel vision-based robot tracking system that integrates both robots' and infrastructural camera views. We achieve accurate view bridging via visual tags, images with special patterns. We develop a key technology, rotation symmetric visual tag design, that greatly extends BridgeLoc's scalability.In this dissertation, we study human and mobile robot tracking in small and large scale environments. We design and implement all of the above systems. Our real-world experimental evaluations show the advantages of our work and demonstrate its potential for human-robot applications.

The primary objective of this research is to study the performance of a heat exchanger system with a bypass valve. At first a simplified double-pipe heat exchanger is considered and a computational fluid dynamic (CFD) code FluentTM 15.0 is used to analyze the heat exchanger performance. The geometry of the double-pipe heat exchanger is scaled down based on the prototypic heat exchanger design provided by Tenneco. Numerical computations are carried out for different inlet conditions provided by Tenneco. The results, including the pressure drop, thermal entrance length, and heat transfer coefficient are benchmarked against correlations available in the literature. Furthermore the performance of the heat exchanger system with a bypass valve is studied. In order to simplify the model, the heat exchanger is modeled using the porous media approach. In addition, the mal-distribution of the flow within the heat exchanger is investigated. The simulations are performed for a range of mass flow rate with both closed and open bypass valve positions. The results are compared with the values obtained using the in-house heat exchanger light tool. The ultimate goal of the thesis is to improve the performance of the overall heat exchanger system to minimize pressure losses and maximize the heat transfer efficiency. This goal could be achieved by optimizing the geometries of the bypass leg to improve the flow uniformity into the heat exchanger and optimizing the flow response dynamics for the open/closed valve positions.