<p> This work contributes to the study of magnetic interactions in the low-dimensional antiferromagnets <i>M</i>(Sb,Ta)₂O₆, where <i> M</i> is a transition metal. By virtue of the trirutile structure, <i> M-O-O-M</i> chains propagate along [110] at <i>z</i> = 0 and [1<span style="text-decoration:overline">1</span>0] at <i>z</i> = 1/2 of the unit cell. These chains are separated along [001] by sheets of weakly-interacting diamagnetic ions. The spin-exchange coupling perpendicular to the chains is weak, permitting the low-dimensional classification. Single crystals have been grown using chemical vapor deposition and the floating zone method. Magnetization, in-field heat capacity, and high-resolution thermal expansion measurements have been performed along various axes, revealing significant anisotropy due to the peculiar magnetic structures and low dimensionality.</p><p> The Neel temperature, <i>T_N,</i> at which long-range order occurs is found to be unstable against the application of magnetic field above 2 T. Large fields tend to lower <i>T_N</i> of the set of moments with projections along the applied field. Moments which are aligned perpendicular to the field are significantly less affected. This can lead to the formation of a secondary peak in heat capacity when magnetic field is along either [110] or [1<span style="text-decoration:overline">1</span>0]. The change in heat capacity at the location of the newly formed peak means there is a change in entropy, which depends upon the direction of applied field with respect to the magnetic moments. Consequently, an anisotropic magnetocaloric effect arises due to the unique magnetic structure. The anisotropic nature of this effect has potential applications in magnetic refrigeration.</p>

Transport measurements and fabrication of superconductor-exchange spring magnet-superconductor systems

<p> The NPDGamma Experiment is measuring the parity-violating correlation A_&gamma; between neutron spin and gamma momentum in the radiative capture of a polarized cold neutron beam on a cryogenic liquid parahydrogen target. This measurement is expected to give insight into theories that incorporate the weak interaction into what is primarily a strongly interacting system. This dissertation discusses the operation and characterization of the liquid hydrogen target, including the calibration of the instrumentation that monitors the state of the hydrogen. An important consideration is the fact that for safety reasons the instrumentation in direct contact with the hydrogen is limited, and so a detailed understanding of the target design and of the properties of hydrogen is required in order to interpret the state of the system. For this experiment, it is essential that the hydrogen be kept mostly in the para state in order to prevent the beam from being significantly depolarized before capture. Since the uncatalyzed conversion rate is slow, an ortho-para conversion loop is used to promote conversion from the room temperature orthohydrogen fraction to the fraction associated with the temperatures of the cryogenic vessel. In addition to the calibration and characterization studies, a method is introduced for placing an empirical limit on the deviation of the orthohydrogen fraction inside the vessel from the desired level associated with the temperature of the ortho-para conversion catalyst and vessel. This method, which does not require precise knowledge of the parahydrogen cross section, involves observing the transmission of the beam through the target while the rate of flow of hydrogen through the ortho-para conversion loop is changed. In addition to the studies of the hydrogen target, this dissertation discusses a calibration of some ³He ion chambers that monitor the flux of the neutron beam and that are used to perform beam transmission measurements. This calibration, which involves a study of the noise inherent in the signal due to neutron capture, does not involve comparison to a separate calibrated detector.</p>

We have used the narrow 2 S 1/2 [arrow right] 3 P 3/2 transition in the ultraviolet (UV) to laser cool and magneto-optically trap (MOT) 6 Li atoms. Laser cooling of lithium atoms is usually performed on the 2 S 1/2 [arrow right] 2 P 3/2 (D2) transition, where temperatures of twice the Doppler limit, or ∼300 μ K for lithium, are achieved. The linewidth of the UV transition is seven times narrower than the D2 line, resulting in a lower Doppler limit. We show that a MOT operating on the UV transition reaches temperatures as low as 59 μ K. We load 6 million atoms from this UV MOT into a 1070 nm optical dipole trap (ODT). We show that the light shift of the UV transition in the ODT is small and blue-shifted, facilitating efficient loading. Evaporative cooling of a two spin-state mixture of 6 Li in the ODT produces a quantum degenerate Fermi gas with 3 million atoms in only 11 seconds.

xii, 106 p. : ill. (some col.) / Quantum dots are systems in which all three spatial sizes are comparable to the Fermi wavelength. The strong confinement leads to a discrete energy spectrum. A goal of thermoelectric research is to find a system with a high thermoelectric figure of merit, which is related to the efficiency of solid-state heat engines. The delta-like density of states of quantum dots has been predicted to boost this figure of merit. This dissertation addresses some thermoelectric properties relevant to the thermal-to-electric energy conversion using InAs/InP quantum dots embedded in nanowires. In thermoelectric experiments, a temperature difference must be established and its value needs to be determined. A novel technique for measuring electron temperature across the dot is presented. A strong nonlinearity of the thermocurrent as a function of temperature difference is observed at a small ratio of temperature gradient and cryostat temperature. At large heating currents, a sign reversal is observed. Numerical calculations explore the contribution of the energy dependence of the transmission function to this effect. Depending on the relative contributions from sequential tunneling and co-tunneling, thermovoltages of quantum dots generally have one of two different lineshapes: a sawtooth shape or a shape similar to the derivative of the conductance peak. Here a simple picture is presented that shows that thermovoltage lineshape is accurately predicted from the energy level spacing inside the dot and the width of the transmission function. An important figure of merit of all heat engines is the efficiency at maximum power. Here the thermoelectric efficiency at maximum power of quantum dots is numerically compared to that of two other low-dimensional systems: an ideal one-dimensional conductor (1D) and a thermionic power generator (TI). The numerical calculations show that either 1D or TI systems can produce the highest maximum power depending on the operating temperature, the effective mass of the electron, and the effective area of the TI system. In spite of this, 1D systems yield the highest efficiency at maximum power. / Committee in charge: Dr. Richard Taylor, Chair; Dr. Heiner Linke, Research Advisor; Dr. Dietrich Belitz; Dr. David Johnson; Dr. David Strom

<p> Quantum computers will potentially outperform classical computers for certain applications by employing quantum states to store and process information. However, algorithms using quantum states are prone to errors through continuous decay, posing unique challenges to engineering a quantum system with enough quantum bits and sufficient controls to solve interesting problems. A promising platform for implementing quantum computers is that of circuit quantum electrodynamics (cQED) using superconducting qubits. Here, two energy levels of a resonant circuit endowed with a Josephson junction serve as the qubit, which is coupled to a microwave-frequency electromagnetic resonator. Modern quantum circuits are reaching size and complexity that puts extreme demands on input/output connections as well as selective isolation among internal elements. Continued progress will require adapting sophisticated 3D integration and RF packaging techniques found in today's high-density classical devices to the cQED platform. This novel technology will take the form of multilayer microwave integrated quantum circuits (MMIQCs), combining the superb coherence of three-dimensional structures with the advantages of lithographic integrated circuit fabrication. Several design and fabrication techniques are essential to this new physical architecture, notably micromachining, superconducting wafer bonding, and out-of-plane qubit coupling. This thesis explores these techniques and culminates in the design, fabrication, and measurement of a two-cavity/one-qubit MMIQC featuring qubit coupling to a superconducting micromachined cavity resonator in silicon wafers. Current prototypes are extensible to larger scale MMIQCs for scalable quantum information processing.</p><p>

<p> Ice Ih, formed by freezing liquid water below 273&sim;K at atmospheric pressure, is well-known and highly-studied, but some of its fundamental physical properties have mystified scientists since the early twentieth century. The thermal expansion is one of those properties; the low relative-resolution of past measurements has left questions regarding the structural isotropy and negative thermal expansion (NTE). Furthermore, the existence of relaxation phenomena near 100&sim;K, related to the residual entropy at 0&sim;K, may reveal itself through subtle features in the thermal expansion and, thus, warrants further investigation. Here we measure the thermal expansion of ultra-pure single crystal ice from 5&ndash;265&sim;K with 10⁶ times higher relative resolution than has previously been made. The data reveal a distinct crossover to NTE below 62&sim;K, and a third-order transition along the crystallographic \(c\)-axis near 100&sim;K, as evident by an unambiguous relaxational decrease in the thermal expansion coefficient on cooling. To further understand the nature of the transition, isotopic substitution and dielectric measurements were performed. </p><p> Three properties of the dielectric relaxation in ice were probed at temperatures between 80--250&sim;K; the thermally stimulated depolarization (TSD) current, static electrical conductivity, and dielectric relaxation time. The dielectric data agree with relaxation-based models and provide for the determination of activation energies which identify the dielectric relaxation in ice as being dominated by Bjerrum defects below 140&sim;K. An anisotropy was also found in the data which revealed that molecular reorientations, in the form of propagating Bjerrum point defects, are energetically favored along the \(c\)-axis between 80--140&sim;K. Furthermore, a similar relaxational effect to that observed in the thermal expansion was observed in the TSD along \(c\), providing a strong correlation between dielectric relaxation and inherent thermodynamic relaxation in ice. Finally, isotopic substitution in both measurement sets indicates the transition is related the movements of hydrogen nuclei, not those of the whole molecule, and provides details about the low-temperature phonon modes. These findings paint a picture of ice as a proton-disordered crystal which undergoes a partial ordering on cooling near 100&sim;K but, before an ordered equilibrium state is realized, the exponentially increasing relaxation time rapidly slows the ordering and ultimately freezes-in the residual entropy, causing a continuous decrease in the thermal expansion coefficient. </p><p>

This dissertation concentrates on the existence, stability and dynamical properties of nonlinear waves in Bose-Einstein condensates (BECs) trapped in doublewell potentials (DWPs). The fundamental model of interest will be the nonlinear Schrödinger equation, the so-called Gross-Pitaevskii (GP) equation, contributed to the well-established mean-field description of BECs. In this context of the GP equation with DWP, a Galerkin-type few-mode approach provides us a powerful handle towards studying the stationary states and predicting the bifurcation diagram including the occurrence of spontaneous symmetry breaking (SSB). Such method and the corresponding phenomena are discussed based on a prototypical quasi-1D model in Chapter 2. The systematic analysis progresses by considering various modified models, starting with the ones involving different interatomic interactions, e.g., collisionally inhomogeneous interactions, long-range interactions, and competing of short- and long-range interactions. We observe how the basic SSB bifurcation structure persists or is appropriately modified in the presence of these interactions in Chapter 3. We also extend the study to multi-component systems, including nonlinearly coupled two-component settings and F = 1 spinor BECs (genuinely three-component settings) confined in DWPs, where besides the one-component stationary states, combined states involving two or three components appear as well, and novel SSB phenomena emerge within them. Finally the trapped stationary modes of a twodimensional (2D) GP equation with a symmetric four-well potential are explored, providing the picture of SSB in the fundamental 2D setting. These various systems are studied in Chapter 4 - 6. In all models, our analytical predictions based on the few-mode approximation are in excellent agreement with the numerical results of the full GP equations.

No description available.

We cool and study trans-Stilbene, Nile Red and Benzonitrile in a cryogenic (7K) cell filled with low density helium buffer gas. No molecule-helium cluster formation is observed, indicating limited atom-molecule sticking in this system. We place an upper limit of 5% on the population of clustered He-trans-Stilbene, consistent with a measured He-molecule collisional residence time of less than \(1 \mu s\). With several low energy torsional modes, trans-Stilbene is less rigid than any molecule previously buffer gas cooled into the Kelvin regime. We report cooling and gas phase visible spectroscopy of Nile Red, a much larger molecule. Our data suggest that buffer gas cooling will be feasible for a variety of small biological molecules. The same cell is also ideal for studying collisional relaxation cross sections. Measurements of Benzonitrile vibrational state decay results in determination of the vibrational relaxation cross sections of \(\sigma_{22} = 8x10^{-15} cm^2\) and \(\sigma_{21} = 6x10^{-15} cm^2\) for the 22 (v=1) and 21 (v=1) states. For the first time, we directly observe formation of cold molecular dimers in a cryogenic buffer gas cell and determine the dimer formation cross section to be \(\sim10^{-13} cm^2\). / Physics