Physics Theses and Dissertations
http://hdl.handle.net/1903/2800
2016-06-28T06:29:24ZExploiting Collective Effects to Direct Light Absorption in Natural and Artificial Light-harvesters
http://hdl.handle.net/1903/18389
Exploiting Collective Effects to Direct Light Absorption in Natural and Artificial Light-harvesters
Schroeder, Christopher
Photosynthesis –the conversion of sunlight to chemical energy –is fundamental for supporting life on our planet. Despite its importance, the physical principles that underpin the primary steps of photosynthesis, from photon absorption to electronic charge separation, remain to be understood in full. Electronic coherence within tightly-packed light-harvesting (LH) units or within individual reaction centers (RCs) has been recognized as an important ingredient for a complete understanding of the excitation energy transfer (EET) dynamics. However, the electronic coherence across units –RC and LH or LH and LH –has been consistently neglected as it does not play a significant role during these relatively slow transfer processes. Here, we turn our attention to the absorption process, which, as we will show, has a much shorter built-in timescale. We demonstrate that the- often overlooked- spatially extended but short-lived excitonic delocalization plays a relevant role in general photosynthetic systems. Most strikingly, we find that absorption intensity is, quite generally, redistributed from LH units to the RC, increasing the number of excitations which can effect charge separation without further transfer steps. A biomemetic nano-system is proposed which is predicted to funnel excitation to the RC-analogue, and hence is the first step towards exploiting these new design principles for efficient artificial light-harvesting.
2016-01-01T00:00:00ZSpectral graph theory with applications to quantum adiabatic optimization
http://hdl.handle.net/1903/18388
Spectral graph theory with applications to quantum adiabatic optimization
Baume, Michael Jarret
In this dissertation I draw a connection between quantum adiabatic optimization, spectral graph theory, heat-diffusion, and sub-stochastic processes through the operators that govern these processes and their associated spectra. In particular, we study Hamiltonians which have recently become known as ``stoquastic'' or, equivalently, the generators of sub-stochastic processes. The operators corresponding to these Hamiltonians are of interest in all of the settings mentioned above.
I predominantly explore the connection between the spectral gap of an operator, or the difference between the two lowest energies of that operator, and certain equilibrium behavior. In the context of adiabatic optimization, this corresponds to the likelihood of solving the optimization problem of interest. I will provide an instance of an optimization problem that is easy to solve classically, but leaves open the possibility to being difficult adiabatically.
Aside from this concrete example, the work in this dissertation is predominantly mathematical and we focus on bounding the spectral gap. Our primary tool for doing this is spectral graph theory, which provides the most natural approach to this task by simply considering Dirichlet eigenvalues of subgraphs of host graphs. I will derive tight bounds for the gap of one-dimensional, hypercube, and general convex subgraphs. The techniques used will also adapt methods recently used by Andrews and Clutterbuck to prove the long-standing ``Fundamental Gap Conjecture''.
2016-01-01T00:00:00ZMeasurement of Wγγ Cross Section and Limits on Anomalous Quartic Gauge Couplings in Proton-Proton Collisions at 8 TeV with the CMS detector
http://hdl.handle.net/1903/18384
Measurement of Wγγ Cross Section and Limits on Anomalous Quartic Gauge Couplings in Proton-Proton Collisions at 8 TeV with the CMS detector
Anelli, Christopher Ryan
Present the measurement of a rare Standard Model processes, pp →W±γγ
for the leptonic decays of the W±. The measurement is made with 19.4 fb−1 of
8 TeV data collected in 2012 by the CMS experiment. The measured cross section
is consistent with the Standard Model prediction and has a significance of 2.9σ.
Limits are placed on dimension-8 Effective Field Theories of anomalous Quartic
Gauge Couplings. The analysis has particularly sensitivity to the fT,0 coupling and
a 95% confidence limit is placed at −35.9 < fT,0/Λ4< 36.7 TeV−4. Studies of the
pp →Zγγ process are also presented. The Zγγ signal is in strict agreement with the Standard Model and has a significance of 5.9σ.
2016-01-01T00:00:00ZTHE PHYSICS OF IDEAS: INFERRING THE MECHANICS OF OPINION FORMATION FROM MACROSCOPIC STATISTICAL PATTERNS
http://hdl.handle.net/1903/18290
THE PHYSICS OF IDEAS: INFERRING THE MECHANICS OF OPINION FORMATION FROM MACROSCOPIC STATISTICAL PATTERNS
Burghardt, Keith A.
In a microscopic setting, humans behave in rich and unexpected ways. In a macroscopic setting, however, distinctive patterns of group behavior emerge, leading statistical physicists to search for an underlying mechanism. The aim of this dissertation is to analyze the macroscopic patterns of competing ideas in order to discern the mechanics of how group opinions form at the microscopic level.
First, we explore the competition of answers in online Q&A (question and answer) boards. We find that a simple individual-level model can capture important features of user behavior, especially as the number of answers to a question grows. Our model further suggests that the wisdom of crowds may be constrained by information overload, in which users are unable to thoroughly evaluate each answer and therefore tend to use heuristics to pick what they believe is the best answer.
Next, we explore models of opinion spread among voters to explain observed universal statistical patterns such as rescaled vote distributions and logarithmic vote correlations. We introduce a simple model that can explain both properties, as well as why it takes so long for large groups to reach consensus. An important feature of the model that facilitates agreement with data is that individuals become more stubborn (unwilling to change their opinion) over time.
Finally, we explore potential underlying mechanisms for opinion formation in juries, by comparing data to various types of models. We find that different null hypotheses in which jurors do not interact when reaching a decision are in strong disagreement with data compared to a simple interaction model. These findings provide conceptual and mechanistic support for previous work that has found mutual influence can play a large role in group decisions. In addition, by matching our models to data, we are able to infer the time scales over which individuals change their opinions for different jury contexts. We find that these values increase as a function of the trial time, suggesting that jurors and judicial panels exhibit a kind of stubbornness similar to what we include in our model of voting behavior.
2016-01-01T00:00:00Z