Proton Tomography in Momentum Space using an Electron Beam

Sucheta Jawalkar
Department of Physics, Duke University (Truman alum)

Measurements in the late 1980s at CERN revealed that quark spins account for a small fraction of the proton’s spin. This so-called spin crisis spurred a number of new experiments to understand the spin structure of the proton, starting with identifying the proton’s silent spin contributors, namely, the spin of the gluons, which hold the quarks together, and the orbital angular momentum of both quarks and gluons. One such experiment was “eg1-dvcs” at the Thomas Jefferson National Accelerator Facility in Newport News, Va., which ran in 2009 and collected approximately 19 (7) billion electron triggers for hydrogen (deuterium). I will discuss measurements of the single and double-spin asymmetries AUL and ALL for π⁺, π^- and π⁰ , measured as a function of Bjorken x, squared momentum transfer Q², hadron energy fraction z, and hadron transverse momentum p_T. These asymmetries correlate with the transverse momentum, and therefore with the orbital angular momentum, of the struck quark.

Level: Advanced

Novel Effects of High Pressure on Superconductivity

Narelle J. Hillier
Department of Physics, Washington University

Superconductivity has been studied extensively since it was first discovered by Kamerlingh Onnes over 100 years ago. High pressure studies are vital in furthering our understanding of this novel state, as pressure allows researchers to enhance the properties of existing superconductors, to find new superconductors, and to test the validity of theoretical models. Following an introduction to superconductivity I will give a brief overview of high pressure techniques used to study materials at extreme pressures, including He-gas and diamond anvil cell (DAC) systems. The focus will then turn to the novel properties of various materials under extreme pressure. At high pressure Li shows a marked deviation from free-electron behavior, leading to a superconducting transition temperature that is among the highest of all elemental superconductors. Mg, however, has not been found to be superconducting. By substituting divalent Mg for monovalent Li to create Li(Mg) alloys, it is possible to explore the effect of increasing electron concentration on the phase diagram, T_c(P). Finally, the cuprate and iron-based materials are considered high-T_c superconductors. These layered compounds exhibit anisotropic behavior under pressure. Precise hydrostatic measurements of dT_c/dP on HgBa₂CuO_4+d carried out in conjunction with uniaxial pressure experiments provide insight into the effect of each of the lattice parameters on T_c. Among the iron-based materials, the LnFePO (Ln = La, Pr, Nd) materials have not been studied extensively due to their relatively low transition temperatures. However, various high pressure measurements indicate a startling array of pressure dependences. For various pressure media, the superconducting transition temperature may either increase or decrease with increasing pressure! These findings, along with other high-pressure studies on iron-based compounds, show startling evidence that the superconducting state in the iron-based superconductors is highly sensitive to lattice strain. Hydrostatic and uniaxial measurements are therefore vital for understanding the nature of superconductivity in these materials and for determining how to enhance T_c.

Level: Advanced

Efficiency of a microwave photon detector based on a current-biased Josephson junction

Amrit Poudel
Department of Physics, University of Wisconsin—Madison

Superconducting qubits are promising candidates for quantum information processing. Recently, circuit quantum electrodynamics (cQED) has emerged as a novel paradigm for the study of radiation-matter interaction in mesoscopic systems. Moreover, cQED is an attractive candidate for scalable quantum computing and transmission of quantum information. Recently, there has been a growing interest in the development of a superconducting microwave photon detector that has possible applications to quantum information processing and communication. In this talk, I will discuss the efficiency of a microwave photon detector based on a current-biased Josephson junction. Our results indicate that junctions with modest coherence properties can provide efficient detection of single microwave photons. I will also discuss several possible set-ups one can use for microwave photon detection and present a systematic way to compute the power absorbed by the detector in these set-ups.

Level: Advanced

Physics at high energy density: bringing astrophysics to the lab

Carlos Di Stefano
Department of Atmospheric, Oceanic, and Space Sciences, University of Michigan (Truman alum)

High-energy-density (HED) physics is an emergent field exploring the properties of matter in an exotic regime of extreme temperatures and pressures. These conditions, occurring naturally only in astrophysical processes in the universe, such as supernovae, nebular evolution, and even the behavior of planetary interiors, are also of practical interest on earth (at the surface!) primarily due to the alluring potential of nuclear fusion as a clean energy source. In this talk, I will put the relationship of the HED regime to more familiar conditions into context, as well as describe the ensuing challenges of creating them in the laboratory. I will then discuss some of the exciting fundamental HED physics being done at the Center for Laser Experimental Astrophysical Research (CLEAR) at the University of Michigan.

Level: Intermediate

We inducted three more members into Sigma Pi Sigma on March 4: Lauren Liegey, Katherine Maxwell, and Casey Wetzel.

Photos of department chair Ian Lindevald welcoming them into Sigma Pi Sigma:

Study of the Electronic Pairing Symmetry in Iron-Based Superconductors

Ryan T. Gordon
Department of Physics, Western Illinois University

The study of the symmetry of the wavefunction (order parameter) for electrons in newly discovered iron-based superconductors is one of crucial importance for understanding how this fascinating state of matter is possible in them. There are two experimental probes that are very useful for investigating the symmetry of the order parameter in a superconductor: the London penetration depth and the thermal conductivity. The London penetration depth characterizes how externally applied magnetic fields are screened from a superconductor and it can be measured with remarkable sensitivity. The thermal conductivity, which is a measure of how efficiently materials can transport heat, can tell us how much entropy is available from normal state electrons in the limit of absolute zero temperature. In my talk, I will show data from both London penetration depth and thermal conductivity experiments on iron-based superconductors and outline in detail what we have concluded about the symmetry of the superconducting order parameter in the iron-based superconductors.

Level: Intermediate

Young Star Clusters and the Initial Mass Function

Bruce Wilking
Department of Physics and Astronomy, University of Missouri—St Louis

The distributions of stellar masses, or Initial Mass Functions (IMFs), in the disk of our Galaxy and in visible star clusters are remarkably similar. But subtle differences do exist and must trace back to the star formation process. By investigating the distribution of stellar masses in young star clusters still associated with molecular clouds, one can hope to connect differences in the IMFs with the physical conditions of the gas from which they formed. I will describe a spectroscopic survey of the young Rho Ophiuchi cluster and a comparison of its IMF with that of other young clusters.

Level: Intermediate

Magnetic field effects in organic semiconductors: from touch screen displays to bird navigation

Markus Wohlgenannt
Department of Physics and Astronomy, University of Iowa

Organic semiconductors are electrically conducting plastic materials, whose main applications are as display pixels as well as plastic solar cells. We show that these materials are however also promising as magnetic field sensors, similar to read-heads used in computer hard disk drives. We propose a novel application, combining the display pixel property with the magntoresistive property to construct a pen-input touch screen display. We will briefly discuss the physics that underlies the magnetic field effects. Interestingly, it has been proposed that a very similar mechanism is used by birds to navigate during migration season.

Level: Intermediate

Neural Networks vs. Self-Consistent Field Equations In Ab Initio Simulations

Paul Rulis
Department of Physics and Astronomy, University of Missouri – Kansas City

This presentation presents an exploration of an alternate method for calculating accurate total energies of complex defect containing solids that is based on machine learning. Within the ab initio orthogonalized linear combination of atomic orbitals (OLCAO) method for electronic structure calculation the solid state potential function is expanded as a summation of atom centered Gaussian functions. The coefficients of these functions are normally determined through self-consistent field iterations applied to the whole system. However, for large and complex systems where many atoms have substantially different local geometries this approach becomes excessively burdensome. A new approach to obtaining the potential function coefficients and subsequently computing the total energy is under development. Progress of the method development as applied to a passive defect model in silicon, a self-interstitial model in silicon, and a model of amorphous silicon will be presented.

Level: Advanced

Our own Women in Physics organization was prominently featured on the front page of November 28′s Kirksville Daily Express, due to a demonstration event they performed at the Adair County Public Library. (Story / Photos)