Session T53: Invited Session: DMP Prize Session

James C. McGroddy Prize Lecutre: Iron-Based Superconductors: Discovery and Progress

The largest breakthrough in the history is the discovery of high Tc Cuprates by G.Bednorz and A.Muller in 1986 and the maximum Tc exceeded 77K, boiling temperature of liquid nitrogen in 1987. However, no new superconductors with high Tc had been reported since then except MgB2 (Tc$=$39K) discovered by J.Akimitsu in 2001.We found LaFePO superconductor with Tc$=$3K in 2006 and LaFeAsO1-xFx with Tc$=$26K (42K at under high pressure of 5GPa) in early 2008. The latter discovery rekindled the extensive superconductivity research globally, and more than 10,000 papers have been published to now. This excitement originates from disprovement of a widely accepted belief that iron with a large magnetic moment is harmful for emergence of superconductivity and relatively high Tc. Extensive research on iron-based superconductors pushed up the maximal Tc to 56K, which is next to high Tc cuprates and has led to the discovery of more than 50 new iron-based superconducting materials to date. Seen are so many advances in elucidation of superconducting properties and pairing mechanism. In this talk, I introduce a tale to the discovery and show the current status by reviewing progresses in materials, properties, mechanism and the application covering the recent hot topics. Emphases are placed on the unique characteristics arising from multi-orbital nature which totally differs from high Tc cuprates.   

David Adler Lectureship Award Talk: Friction and energy dissipation mechanisms in adsorbed molecules and molecularly thin films

Studies of the fundamental origins of friction have undergone rapid progress in recent years, with the development of new experimental and computational techniques for measuring and simulating friction at atomic length and time scales [1]. The increased interest has sparked a variety of discussions and debates concerning the nature of the atomic-scale and quantum mechanisms that dominate the dissipative process by which mechanical energy is transformed into heat. Measurements of the sliding friction of physisorbed monolayers and bilayers can provide information on the relative contributions of these various dissipative mechanisms. Adsorbed films, whether intentionally applied or present as trace levels of physisorbed contaminants, moreover are ubiquitous at virtually all surfaces. As such, they impact a wide range of applications whose progress depends on precise control and/or knowledge of surface diffusion processes. Examples include nanoscale assembly, directed transport of Brownian particles, material flow through restricted geometries such as graphene membranes and molecular sieves, passivation and edge effects in carbon-based lubricants, and the stability of granular materials associated with frictional and frictionless contacts. \\[4pt] [1] J. Krim, Advances in Physics, \textbf{61}, (2012) pp155-323   

Nanoscale Impedance Imaging of Novel Quantum Materials

The research of complex quantum materials, in which a dazzling number of emergent phenomena take place in the nanoscale, is a major theme in modern condensed matter physics. For real-space mapping of complex systems, electrical impedance microscopy fills an important void that is not well represented by the existing local probes. Using shielded cantilever probes and sensitive microwave electronics, we can now perform non-invasive electrical imaging with unprecedented resolution (10-100nm) and sensitivity (sub-aF). To date, this powerful technique has enabled us to visualize the electronic inhomogeneity in colossal magnetoresistance manganites, spatially resolve the topological edge channels, image the metal-insulator transition in novel field-effect transistors, and probe the anomalous conduction in multiferroic domain walls. The sub-surface imaging capability is also ideal for understanding the evolution of chemical reaction involving low-dimensional layered materials. Further development of the technique will allow us to perform local dielectric spectroscopy across a large frequency span, explore the localized microwave magnetic resonance, and study the nanoscale nonlinear electromagnetic response in complex materials.   

Magnetostructural coupling in spinel oxides

Spinels oxides are of great interest functionally as multiferroic, battery, and magnetic materials as well as fundamentally because they exhibit novel spin, structural, and orbital ground states. Competing interactions are at the heart of novel functional behavior in spinels. Here, we explore the intricate landscape of spin, lattice, and orbital interactions in magnetic spinels by employing variable-temperature high-resolution synchrotron x-ray powder diffraction, total neutron scattering, magnetic susceptibility, dielectric, and heat capacity measurements. We show that the onset of long-range magnetic interactions often gives rise to lattice distortions. Our work illustrates that the spinels NiCr$_2$O$_4$, CuCr$_2$O$_4$, and Mn$_3$O$_4$, which are tetragonal at room temperature due to Jahn-Teller ordering, undergo further spin-driven structural distortions at the onset of long-range ferrimagnetic order. We have also studied the complete structural description of the ground states of several spinels including the geometrically frustrated spinels ZnCr$_2$O$_4$ and MgCr$_2$O$_4$. The detailed spin-lattice studies of spinel oxides presented here illustrate the prevalence of structural phase coexistence when magnetostructural changes occur below 50\,K. The new understanding of structural ground states in spinel oxides will guide the design of structure-property relationships in these materials. Broadly, this work highlights the importance of variable-temperature high-resolution synchrotron x-ray diffraction in understanding phase transitions in functional materials. \\[4pt] [1] M. C. Kemei, J. K. Harada, M. R. Suchomel, and R. Seshadri, Structural changes and phase coexistence in the magnetodielectric spinel Mn$_3$O$_4$, \textit{Phys. Rev. B} \textbf{90} (2014) 064418. \\[0pt] [2] M. C. Kemei, S. L. Moffitt, L. E. Dagaro, R. Seshadri, M. R. Suchomel, D. P. Shoemaker, K. Page, and J. Siewenie, Structural ground states of ($A,A'$)Cr$_2$O$_4$($A$=Mg, Zn; $A^{\prime}$ = Co, Cu) spinel solid solutions: Spin-Jahn-Teller and Jahn-Teller effects, \textit{Phys. Rev. B} \textbf{89} (2014) 174410. \\[0pt] [3] M. C. Kemei, P. T. Barton, S. L. Moffitt, M. W. Gaultois, J. A. Kurzman, R. Seshadri, M. R. Suchomel, and Y-Il. Kim, Crystal structures of spin-Jahn-Teller-ordered MgCr$_2$O$_4$ and ZnCr$_2$O$_4$, \textit{J. Phys.: Condens. Matter} \textbf{25} (2013) 326001. \\[0pt] [4] M. R. Suchomel, D. P. Shoemaker, L. Ribaud, M. C. Kemei and R. Seshadri, Spin-induced symmetry breaking in orbitally ordered NiCr$_2$O$_4$ and CuCr$_2$O$_4$, \textit{Phys. Rev. B} \textbf{86} (2012) 0544061.   

Entanglement and Quantum Error Correction with Superconducting Qubits

Quantum information science seeks to take advantage of the properties of quantum mechanics to manipulate information in ways that are not otherwise possible. Quantum computation, for example, promises to solve certain problems in days that would take a conventional supercomputer the age of the universe to decipher. This power does not come without a cost however, as quantum bits are inherently more susceptible to errors than their classical counterparts. Fortunately, it is possible to redundantly encode information in several entangled qubits, making it robust to decoherence and control imprecision with quantum error correction. I studied one possible physical implementation for quantum computing, employing the ground and first excited quantum states of a superconducting electrical circuit as a quantum bit. These “transmon" qubits are dispersively coupled to a superconducting resonator used for readout, control, and qubit-qubit coupling in the cavity quantum electrodynamics (cQED) architecture. In this talk I will give an general introduction to quantum computation and the superconducting technology that seeks to achieve it before explaining some of the specific results reported in my thesis. One major component is that of the first realization of three-qubit quantum error correction in a solid state device, where we encode one logical quantum bit in three entangled physical qubits and detect and correct phase- or bit-flip errors using a three-qubit Toffoli gate. My thesis is available at arXiv:1311.6759.