When bulk helium-4 is cooled below T = 2.18 K, it undergoes a phase transition to a superfluid, characterized by a complex wave function with a macroscopic phase and exhibits inviscid, quantized flow. The macroscopic phase coherence can be probed in a container filled with helium-4, by reducing one or more of its dimensions until they are smaller than the coherence length, the spatial distance over which order propagates. As this dimensional reduction occurs, enhanced thermal and quantum fluctuations push the transition to the superfluid state to lower temperatures. However, this trend can be countered via the proximity effect, where a bulk 3-dimensional (3d) superfluid is coupled to a low (2d) dimensional superfluid via a weak link producing superfluid correlations in the film at temperatures above the Kosterlitz-Thouless temperature. Recent experiments probing the coupling between 3d and 2d superfluid helium-4 have uncovered an anomalously large proximity effect, leading to an enhanced superfluid density that cannot be explained using the correlation length alone. In this work, we have determined the origin of this enhanced proximity effect via large scale quantum Monte Carlo simulations of helium-4 in a topologically non-trivial geometry that incorporates the important aspects of the experiments. We find that due to the bosonic symmetry of helium-4, identical particle permutations lead to correlations between contiguous spatial regions at a length scale greater than the coherence length. We show that quantum exchange plays a large role in explaining the anomalous experimental results while simultaneously showing how classical arguments fall short of this task.

The number of Web traffic flows dominates Internet traffic today and most Web interactions are short-lived HTTP connections handled by TCP. Most core Internet routers use Drop Tail queuing which produces bursts of packet drops that contribute to unfair service. This thesis introduces two new active queue management (AQM) algorithms, PISA (PI with Short-lived flows Adjustment) and PIMC (PI with Minimum Cwnd). These AQMs are built on top of the PI (Proportional Integrator). To evaluate the performance of PISA and PIMC, a new simple model of HTTP traffic was developed for the NS-2 simulation. TCP sources inform PISA and PIMC routers of their congestion window by embedding a source hint in the packet header. Using the congestion window, PISA drops packets from short-lived Web flows less than packets from long-lived flows. Using a congestion window, PIMC does not drop a packet when congestion window is below a fixed threshold. This study provides a series of NS-2 experiments to investigate the behavior of PISA and PIMC. The results show fewer drops for both PISA and PIMC that avoids timeouts and increases the rate at which Web objects are sent. PISA and PIMC improve the performance of HTTP flows significantly over PI. PISA performs slightly better than PIMC.

本文透過國衛院所發行的精神疾病住院病患歸人檔，串連用藥、醫師、承保人，以及醫院檔，對於台灣抗憂鬱劑的使用情形做描述。發現抗憂鬱劑的使用次數是逐年增加的，且新藥成長快速，同時舊藥的使用卻是逐年下降，而國內製藥的使用也是逐年下降。總費用的部份，即使經過消費者物價指數的平減，仍是逐年上升；另外，醫院的特性，特別是醫院層級與醫院權屬別，對於抗憂鬱劑使用的種類與國內製藥使用的情形，都扮演重要的角色。

Path integral Monte Carlo (PIMC) is a method suitable for quantum liquid simulations at finite temperature. We present in this thesis a study of PIMC dealing with the theory and algorithms related to it, and then two applications of PIMC to current research problems of quantum fluids in the Bolzmann regime. The first part encompasses a study of the different ingredients of a PIMC code: action, sampling and physical property estimators. Particular attention has been paid to Li-Broughton's higher order approximation to the action. Regarding sampling, several collective movement methods have been derived, including the bisection algorithm, that has been thoroughly tested. We also include a study of estimators for different physical properties, such as, the energy (through the thermodynamic and virial estimators), the pair distribution function, the structure factor, and the momentum distribution. In relation to the momentum distribution, we have developed a novel algorithm for its estimation, the trail method. It surmounts some of the problems exposed by previous approaches, such as the open chain method or McMillan's algorithm.The Richardson extrapolation used within PIMC simulations, is another contribution of this thesis. Up until now, this extrapolation has not been used in this context. We present studies of the energy dependence on the number of "beads", along with the betterment provide by the Richardson extrapolation. Inasmuch as our goal is to perform research of quantum liquids at finite temperature, we have produced a library of codes, written from scratch, that implement most of the features theoretically developed. The most elaborated parts of these codes are included in some of the appendixes.The second part shows two different applications of the algorithms coded. We present results of a PIMC calculation of the momentum distribution of Ne and normal 4He at low temperatures. In the range of temperatures analysed, exchanges can be disregarded and both systems are considered Boltzmann quantum liquids. Their quantum character is well reflected in their momentum distributions witch show clear departures from the classical limit. The PIMC momentum distributions which show clear departures from the classical limit. The PIMC momentum distributions are sampled using the trail method. Kinetic energies of both systems, as a function of temperature and at a fixed density, are also reported. Finally, the solid-liquid neon phase transition along the 35 K isotherm has been characterized.While thermodynamic properties of the solid phase are well known the behaviour of some properties, such as the energy or the dessity, during the trasition presen6 some uncertainties For example, experimental data for the place diagram, which determines solid and liquid boundaries, present sizeable differences. The temperature chosen is high enough so that Bose or Fermi statistics corrections are small, although the system is strongly quantum mechanical. The results obtained show a discontinuity in the kinetic energy during the transition.