Yb4LiGe4 - A Yb Mixed Valent Zintl Phase with Strong Electronic Correlations

Single-phase samples of Yb4LiGe4 and Yb5Ge4 were synthesized using high frequency (HF) heat treatment. Yb4LiGe4 crystallizes in orthorhombic space group Pnma with the Gd5Si4 type of crystal structure and lattice parameters a = 7.0571(1) Angs, b = 14.6239(1) Angs, and c = 7.6155(1) Angs. One Yb position in Yb5Ge4 is substituted by the lithium atom and causes a distortion of the germanium tetragons in Yb4LiGe4. Investigation of the electronic state of Yb via magnetic susceptibility and X-ray absorption near-edge spectroscopy (XANES) revealed a presence of two electronic states of ytterbium, 4f13 and 4f14 (mixed valence), in Yb5Ge4 and Yb4LiGe4. Studies of the temperature dependence of the electrical resistivity, magnetization, 7Li spin-lattice relaxation rate and the specific heat indicate that strong electronic correlations are present in Yb4LiGe4, and below approximately 50 K there is a competition between ferromagnetic and antiferromagnetic correlations. Magnetic ordering in Yb4LiGe4, if present, occurs below the reported antiferromagnetic transition temperature of 1.7 K for Yb5Ge4.Comment: 27 Pages, 9 figures, Uncder revie

Exploring More-Coherent Quantum Annealing

In the quest to reboot computing, quantum annealing (QA) is an interesting candidate for a new capability. While it has not demonstrated an advantage over classical computing on a real-world application, many important regions of the QA design space have yet to be explored. In IARPA's Quantum Enhanced Optimization (QEO) program, we have opened some new lines of inquiry to get to the heart of QA, and are designing testbed superconducting circuits and conducting key experiments. In this paper, we discuss recent experimental progress related to one of the key design dimensions: qubit coherence. Using MIT Lincoln Laboratory's qubit fabrication process and extending recent progress in flux qubits, we are implementing and measuring QA-capable flux qubits. Achieving high coherence in a QA context presents significant new engineering challenges. We report on techniques and preliminary measurement results addressing two of the challenges: crosstalk calibration and qubit readout. This groundwork enables exploration of other promising features and provides a path to understanding the physics and the viability of quantum annealing as a computing resource.Comment: 7 pages, 3 figures. Accepted by the 2018 IEEE International Conference on Rebooting Computing (ICRC

Anneal-path correction in flux qubits

Quantum annealers require accurate control and optimized operation schemes to reduce noise levels, in order to eventually demonstrate a computational advantage over classical algorithms. We study a high coherence four-junction capacitively shunted flux qubit (CSFQ), using dispersive measurements to extract system parameters and model the device. Josephson junction asymmetry inherent to the device causes a deleterious nonlinear cross-talk when annealing the qubit. We implement a nonlinear annealing path to correct the asymmetry in-situ, resulting in a substantial increase in the probability of the qubit being in the correct state given an applied flux bias. We also confirm the multi-level structure of our CSFQ circuit model by annealing it through small spectral gaps and observing quantum signatures of energy level crossings. Our results demonstrate an anneal-path correction scheme designed and implemented to improve control accuracy for high-coherence and high-control quantum annealers, which leads to an enhancement of success probability in annealing protocols.Comment: v2 published versio

Multiferroicity in doped hexagonal LuFeO3

The hexagonal phase of LuFeO3 is a rare example of a multiferroic material possessing a weak ferromagnetic moment, which is predicted to be switchable by an electric field. We stabilize this structure in bulk form though Mn and Sc doping, and determine the complete magnetic and crystallographic structures using neutron-scattering and magnetometry techniques. The ferroelectric P6(3)cm space group is found to be stable over a wide concentration range, ordering antiferromagnetically with Neel temperatures that smoothly increase following the ratio of c to a (c/a) lattice parameters up to 172 K, the highest found in this class of materials to date. The magnetic structure for a range of temperatures and dopings is consistent with recent studies of high quality epitaxial films of pure hexagonal LuFeO3 including a ferromagnetic moment parallel to the ferroelectric axis. We propose a mechanism by which room-temperature multiferroicity could be achieved in this class of materialsopen