D. Fobes et al ., Phys. Rev. B 84, 014406 (2011)

1. Magnetic phase transitions and bulk spin-valve effect tuned by in-plane field orientation in Ca3Ru2O7Zhiqiang Mao, Tulane University, DMR 0645305 Perovskite ruthenates exhibit exceptionally rich quantum phenomena such as spin-triplet superconductivity [1], antiferromagnetic (AFM) Mott insulating behavior [2], orbital ordering [3] and a field-tuned nematic phase [4]. These rich properties headline the complex interplay between the charge, spin, lattice, and orbital degrees of freedom in ruthenates, and provide fantastic opportunities to study the novel quantum phenomena tuned by external stimuli. Ca3Ru2O7 is of particular interest in this aspect due to its unique magnetic properties. It orders antiferromatically at 56 K [5] and exhibits bulk spin-valve behavior under the application of magnetic fields [6]. We have performed systematic in-plane angle-dependent c-axis transverse magnetotransport measurements on Ca3Ru2O7. Our results reveal that its magnetic state unusually evolves with in-plane rotation of magnetic field. When magnetic field is applied along the b-axis, we probe crossover magnetic states in close proximity to phase boundaries of long-range ordered antiferromagnetic (AFM) states. These crossover magnetic states are characterized by short-range AFM order and switch to polarized paramagnetic states at critical angles as the in-plane field is rotated from the b to the a-axis. Additionally, we observe bulk spin-valve behavior resulting from spin-flop transitions tuned by in-plane rotation of magnetic field. Our results post a challenging question for the theoretical community: why does the change of magnetic-field orientation result in magnetic phase transitions. Magnetic phase diagram of Ca3Ru2O7 for magnetic field parallel to the a- and b-axes. AFM-a, the AFM state with magnetic moments along the a axis; AFM-b, the AFM state with magnetic moments along the b axis; CAFM, the canted AFM state; EPM, enhanced paramagnetic state; COM1 and COM2, two various crossover magnetic states. The inset in (a) shows magnetic structures of AFM-a, AFM-b and CAFM phase.. D. Fobes et al., Phys. Rev. B 84, 014406 (2011)

2. Broader Impacts Zhiqiang Mao, Tulane University, DMR 0645305 1. Developing national and international collaboration Mao’s research group currently focuses on two research areas: novel physics of strongly correlated electron systems and unconventional superconductivity. His group’s expertise is high-quality single crystal growth of materials and low temperature measurements. He has established extensive collaboration with researchers in these fields. He has collaborated with Collin Broholm at Johns Hopkins University, Ruslan Prozorov at Ames Lab, Ying Liu and Peter Schiffer at the Pennsylvania State University, Zhixun Shen at Stanford University, Yimin Qiu at NIST, Leonard Spinu at University of New Orleans, Wei Bao at Renmin University in China and Dimitri Argyriou at Helmholtz-Zentrum Berlin für Materialen in studies of iron chalcogenide superconductors. He has also collaborated with Xianglin Ke and David J. Singh at Oak Ridge National Lab, Jeffrey W. Lynn at NIST, John Freeland at Argonne National Lab, Jiandi Zhang at Louisiana State University, Xiaoshan Wu at Nanjing University in China, etc., in studies of perovskite ruthenates. These collaborations have been very effective in solving complex problems. For example, Mao’s recent achievement in studies of iron chalcogenides is an excellent example. Several important results in this study, including the determination of antiferromagnetic structure in FeTe (PRL 102, 247001 (2009)), the observation of superconducting spin resonance (PRL 103, 067008 (2009)) and the establishment of phase diagram (Nature Materials 9, 716 (2010)) were all obtained through the collaboration with Bao, Qiu and Broholm et al. 2. Involving undergraduate students in the research Three undergraduate students have recently been involved in Mao’s research. One of them, who was supported by this project, completed his honor thesis under Mao’s guidance. The motivation of his research is to search for novel superconductors through chemical substitution in materials exhibiting a charge-density wave such as LaTe2 and CeTe3. The other two students, supported by the Louisiana Alliance for Simulation-Guided Materials REU program, were trained to synthesize novel materials using a chemical intercalation method. All of them have obtained interesting results.