James M. Valles

Stable magnetic field gradient levitation of Xenopus laevis: toward low-gravity simulation

We have levitated, for the first time, living biological specimens, embryos of the frog Xenopus laevis, using a large inhomogeneous magnetic field. The magnetic field/field gradient product required for levitation was 1430 kG2/cm, consistent with the embryos susceptibility being dominated by the diamagnetism of water and protein. We show that unlike any other earth-based technique, magnetic field gradient levitation of embryos reduces the body forces and gravity-induced stresses on them. We discuss the use of large inhomogeneous magnetic fields as a probe for gravitationally sensitive phenomena in biological specimens.

Conductor--insulator quantum phase transitions

PART I: METAL-INSULATOR TRANSITIONS 1. Introduction to Metal-Insulator Transitions 2. Anderson Localisation 3. Visualizing Critical Correlations near the Metal-Insulator Transition in Ga1??xMnxAs 4. Local approaches to strongly correlated disordered systems 5. Penultimate fate of a dirty-Fermi-liquid 6. Glassy dynamics of electrons near the metal-insulator transition 7. Phase Competition and Inhomogeneous States as a New Paradigm for Complex Materials 8. Numerical Studies of Metal-Insulator Transitions in Disordered Hubbard Models PART II: SUPERCONDUCTOR-INSULATOR TRANSITIONS 9. Superconductor-Insulator Transitions: Present Status and Open Questions 10. Scaling Analysis of Direct Superconductor-Insulator Transitions in Disordered Ultrathin Films of Metals 11. Spin Effects Near the Superconductor-Insulator Transition 12. Magnetic field-induced novel insulating phase in 2D superconductors 13. SITs in ultrathin a-Pb films: comparisons of disorder, magnetic field and magnetic impurity tuned 14. Evidence of Cooper Pairs on the Insulating Side of the SIT 15. Spectroscopic Imaging STM Studies of Electronic Structure in both the Superconducting and Pseudogap Phases of Underdoped Cuprates 16. Suppression of Tunneling of Superconducting Vortices Caused by a Remote Gate: Andersons orthogonality catastrophe and localization 17. Theoretical Studies of Superconductor-Insulator TransitionsIn this article we study superconductor-insulator transitions within the general framework of an attractive Hubbard model. This is a well-defined model of s-wave superconductivity which permits different tuning parameters (disorder and field). Furthermore, it allows a comparison of various analytical and computational approaches in order to gain a complete understanding of the various effects of amplitude and phase fluctuations. We present a systematic pedagogical approach, aiming to equip the lay reader with enough apparatus to be able to understand the numerical calculations, reproduce some of the simpler results, and be able to tackle future problems related to inhomogeneous phases. We go into considerable detail on mean-field theory (MFT) and the Bogoliubov-de Gennes (BdG) approach, as these are a first line of attack which can capture much of the physics, but we also outline cases where this fails to capture phase fluctuations and more sophisticated Quantum Monte Carlo (QMC) calculations are necessary. We discuss the behavior of many observables, including densities of states, superfluid stiffness, and dynamical conductivity, for the disorder-tuned superconductor-insulator transition. We also discuss SITs tuned by parallel magnetic field, which are quite different due to pairbreaking.

Swimming Paramecium in magnetically simulated enhanced, reduced, and inverted gravity environments

Earths gravity exerts relatively weak forces in the range of 10–100 pN directly on cells in biological systems. Nevertheless, it biases the orientation of swimming unicellular organisms, alters bone cell differentiation, and modifies gene expression in renal cells. A number of methods of simulating different strength gravity environments, such as centrifugation, have been applied for researching the underlying mechanisms. Here, we demonstrate a magnetic force-based technique that is unique in its capability to enhance, reduce, and even invert the effective buoyancy of cells and thus simulate hypergravity, hypogravity, and inverted gravity environments. We apply it to Paramecium caudatum, a single-cell protozoan that varies its swimming propulsion depending on its orientation with respect to gravity, g. In these simulated gravities, denoted by fgm, Paramecium exhibits a linear response up to fgm = 5 g, modifying its swimming as it would in the hypergravity of a centrifuge. Moreover, experiments from fgm = 0 to −5 g show that the response is symmetric, implying that the regulation of the swimming speed is primarily related to the buoyancy of the cell. The response becomes nonlinear for fgm >5 g. At fgm = 10 g, many paramecia “stall” (i.e., swim in place against the force), exerting a maximum propulsion force estimated to be 0.7 nN. These findings establish a general technique for applying continuously variable forces to cells or cell populations suitable for exploring their force transduction mechanisms.

Superconducting Pair Correlations in an Amorphous Insulating Nanohoneycomb Film

The Cooper pairing mechanism that binds single electrons to form pairs in metals allows electrons to circumvent the exclusion principle and condense into a single superconducting or zero-resistance state. We present results from an amorphous bismuth film system patterned with a nanohoneycomb array of holes, which undergoes a thickness-tuned insulator-superconductor transition. The insulating films exhibit activated resistances and magnetoresistance oscillations dictated by the superconducting flux quantum h/2e. This 2e period is direct evidence indicating that Cooper pairing is also responsible for electrically insulating behavior.

Model of Magnetic Field-Induced Mitotic Apparatus Reorientation in Frog Eggs

Recent experiments have shown that intense static magnetic fields can alter the geometry of the early cell cleavages of Xenopus laevis eggs. The changes depend on field orientation, strength, and timing. We present a model that qualitatively accounts for these effects and which presumes that the structures involved in cell division are cylindrically symmetric and diamagnetically anisotropic and that the geometry of the centrosome replication and spreading processes dictates the nominal cleavage geometry. Within this model, the altered cleavage geometry results from the magnetic field-induced realignment of mitotic structures, which causes a realignment of the centrosome replication and spreading processes.

Absence of a zero-temperature vortex solid phase in strongly disordered superconducting Bi films

We present low-temperature measurements of the resistance in magnetic field of superconducting ultrathin amorphous Bi films with normal-state sheet resistances,

Observation of giant positive magnetoresistance in a Cooper pair insulator.

{R}_{N},

FORMATION OF POLYCRYSTALLINE STRUCTURE IN METALLIC FILMS IN THE EARLY STAGES OF ZONE I GROWTH

near the resistance quantum,

Cooper-pair insulator phase in superconducting amorphous Bi films induced by nanometer-scale thickness variations

{R}_{Q}=\ensuremath{\Elzxh}{/e}^{2}.

Varying the effective buoyancy of cells using magnetic force

For