Possible field-tuned SIT in high-Tc superconductors: implications for pairing at high magnetic fields

The behavior of some high temperature superconductors (HTSC) such as $\rm La_{2-x}Sr_{x}CuO_{4}$ and $\rm Bi_{2}Sr_{2-x}La_xCuO_{6+\delta}$, at very high magnetic field, is similar to that of thin films of amorphous InOx near the magnetic field-tuned superconductor-insulator transition. Analyzing the InOx data at high fields in terms of persisting local pairing amplitude, we argue by analogy that local pairing amplitude also persists well into the dissipative state of the HTSCs, the regime commonly denoted as the "normal state" in very high magnetic field experiments.Comment: Revised figures and reference

Fiber-coupled Antennas for Ultrafast Coherent Terahertz Spectroscopy in Low Temperatures and High Magnetic Fields

For the purposes of measuring the high-frequency complex conductivity of correlated-electron materials at low temperatures and high magnetic fields, a method is introduced for performing coherent time-domain terahertz spectroscopy directly in the cryogenic bore of existing dc and pulsed magnets. Miniature fiber-coupled THz emitters and receivers are constructed and are demonstrated to work down to 1.5 Kelvin and up to 17 Tesla, for eventual use in higher-field magnets. Maintaining the sub-micron alignment between fiber and antenna during thermal cycling, obtaining ultrafast (${<200}$~fs) optical gating pulses at the end of long optical fibers, and designing highly efficient devices that work well with low-power optical gating pulses constitute the major technical challenges of this project. Data on a YBCO superconducting thin film and a high mobility 2D electron gas is shown.Comment: 8 pages, 9 figure

In Situ Examination of Nanoscale Reaction Pathways in Battery Materials

In order to engineer less expensive and more energy-dense batteries, new materials that can reliably store and transport active ions must be first developed. However, these materials are known for their poor reversibility due to large morphological changes during cycling. To maximize reversibility during charge and discharge, we must be able to understand and control the electrochemical reaction mechanisms of these new electrode materials. This dissertation uses in situ experiments, primarily in situ transmission electron microscopy (TEM), to understand the nanoscale reaction pathways in various high-capacity electrode materials during reactions with Li+, Na+, and K+ ions. Upon reacting with alkali-metal ions, these electrode materials often exhibit much higher specific storage capacities than conventional Li-ion battery electrode materials. In addition, these types of materials can also be used in lower-cost sodium- and potassium-based systems. Hence, they could replace electrode materials in Li-ion batteries, which would make possible engineering batteries with higher specific energy. However, the more substantial volumetric changes that these electrode materials undergo during reaction cause a significant decrease in the capacity retention. This decrease in the capacity retention is caused by the mechanical fracture of the active material and continuous growth of the solid-electrolyte interphase (SEI) on the surface of the anode particles, which both lead to very low cyclability of these systems. If these battery systems are to be improved, it is critical to understand both how the larger Na+ and K+ ions affect the nanoscale phase transformations during these reactions and how to engineer high capacity battery materials with high coulombic efficiency and longer cycle life. As part of the research described in this dissertation, studies on the Cu2S and FeS2 active materials were conducted to examine the effect that larger alkali metal ions have on the reaction mechanisms of large-volume-change materials. Evidence obtained from extensive in situ and ex situ experiments suggests that the larger volume changes associated with the sodium/potassium reactions indicate that the different reaction pathways affect the materials behavior. This altered reaction behavior results in a more stable morphology for the overall cycling of the electrode material. In an effort to aid the engineering of a high capacity battery material with longer cycle life, a study was conducted on Sb nanocrystal electrode materials that exhibited stable electrochemical behavior. This study demonstrated that small spherical particles naturally formed uniform internal voids that were easily filled and vacated during cycling. This was found to be due to the resilient lithiated oxide layer that formed after the first lithiation and subsequently prevented shrinkage during delithiation. A chemomechanical model describing the void formation was developed; this model can serve as a tool to guide the creation of oxide or other shells that enable alloying materials to undergo voiding transformations in situ. When reacting with alkali ions of different sizes, all of these materials (Cu2S, FeS2, and Sb) exhibited counter-intuitive phase evolution and mechanical degradation behavior. The findings indicate that, thanks to their high energy density, large-volume-change materials could make possible the development of next-generation batteries, whether they be Li-ion batteries or batteries with other chemistries that undergo complex morphological changes.Ph.D

Strongly Enhanced Hole-Phonon Coupling in the Metallic State of the Dilute Two-Dimensional Hole Gas

We have studied the temperature dependent phonon emission rate $P$($T$) of a strongly interacting ($r_s\geq$22) dilute 2D GaAs hole system using a standard carrier heating technique. In the still poorly understood metallic state, we observe that $P$($T$) changes from $P$($T$)$\sim T^5$ to $P$($T$)$\sim T^7$ above 100mK, indicating a crossover from screened piezoelectric(PZ) coupling to screened deformation potential(DP) coupling for hole-phonon scattering. Quantitative comparison with theory shows that the long range PZ coupling between holes and phonons has the expected magnitude; however, in the metallic state, the short range DP coupling between holes and phonons is {\it almost twenty times stronger} than expected from theory. The density dependence of $P$($T$) shows that it is {\it easier} to cool low density 2D holes in GaAs than higher density 2D hole systems.Comment: To appear in Phys. Rev. Let

Metal-to-Insulator Crossover in the Low-Temperature Normal State of Bi_{2}Sr_{2-x}La_{x}CuO_{6+\delta}

We measure the normal-state in-plane resistivity of La-doped Bi-2201 single crystals at low temperatures by suppressing superconductivity with 60-T pulsed magnetic fields. With decreasing hole doping, we observe a crossover from a metallic to insulating behavior in the low-temperature normal state. This crossover is estimated to occur near 1/8 doping, well inside the underdoped regime, and not at optimum doping as reported for other cuprates. The insulating regime is marked by a logarithmic temperature dependence of the resistivity over two decades of temperature, suggesting that a peculiar charge localization is common to the cuprates.Comment: 4 pages, 5 figures, accepted for publication in PR