Clarina de la Cruz

Structural and magnetic phase diagram of CeFeAsO 1− x F x and its relation to high-temperature superconductivity

Recently, high-transition-temperature (high-Tc) superconductivity was discovered in the iron pnictide RFeAsO(1-x)F(x) (R, rare-earth metal) family of materials. We use neutron scattering to study the structural and magnetic phase transitions in CeFeAsO(1-x)F(x) as the system is tuned from a semimetal to a high-Tc superconductor through fluorine (F) doping, x. In the undoped state, CeFeAsO develops a structural lattice distortion followed by a collinear antiferromagnetic order with decreasing temperature. With increasing fluorine doping, the structural phase transition decreases gradually and vanishes within the superconductivity dome near x=0.10, whereas the antiferromagnetic order is suppressed before the appearance of superconductivity for x>0.06, resulting in an electronic phase diagram remarkably similar to that of the high-Tc copper oxides. Comparison of the structural evolution of CeFeAsO(1-x)F(x) with other Fe-based superconductors suggests that the structural perfection of the Fe-As tetrahedron is important for the high-Tc superconductivity in these Fe pnictides.

Lattice and magnetic structures of PrFeAsO, PrFeAsO(0.85)F(0.15), and PrFeAsO(0.85)

We use powder neutron diffraction to study the spin and lattice structures of polycrystalline samples of nonsuperconducting PrFeAsO and superconducting PrFeAsO{sub 0.85}F{sub 0.15} and PrFeAsO{sub 0.85}. We find that PrFeAsO exhibits abrupt structural phase transitions at 153 K followed by static long-range antiferromagnetic order at 127 K. Both the structural distortion and magnetic order are similar to other rare-earth oxypnictides. Electron doping the system with either fluorine or oxygen deficiency suppresses the structural distortion and static long-range antiferromagnetic order, therefore placing these materials into the same class of FeAs-based superconductors.

Unusual relationship between magnetism and superconductivity in FeTe(0.5)Se(0.5).

We use neutron scattering to study magnetic excitations in crystals near the ideal superconducting composition of FeTe(0.5)Se(0.5). Two types of excitations are found, a resonance at (0.5,0.5,0) and incommensurate fluctuations on either side of this position. We show that the two sets of magnetic excitations behave differently with doping, with the resonance being fixed in position while the incommensurate excitations move as the doping is changed. These unusual results show that a common behavior of the low energy magnetic excitations is not necessary for pairing in these materials.

Magnetic Quantum Oscillations in YBa2Cu3O6.61 and YBa2Cu3O6.69 in Fields of Up to 85 T: Patching the Hole in the Roof of the Superconducting Dome

We measure magnetic quantum oscillations in the underdoped cuprates YBa2Cu3O6+x with x=0.61, 0.69, using fields of up to 85 T. The quantum-oscillation frequencies and effective masses obtained suggest that the Fermi energy in the cuprates has a maximum at hole doping p approximately 0.11-0.12. On either side, the effective mass may diverge, possibly due to phase transitions associated with the T=0 limit of the metal-insulator crossover (low-p side), and the postulated topological transition from small to large Fermi surface close to optimal doping (high p side).

Lattice distortion and magnetic quantum phase transition in CeFeAs(1-x)P(x)O.

We use neutron diffraction to study the structural and magnetic phase diagram of CeFeAs(1-x)P(x)O. We find that replacing the larger arsenic with smaller phosphorus in CeFeAs(1-x)P(x)O simultaneously suppresses the AFM order and orthorhombic distortion near x=0.4, thus suggesting the presence of a magnetic quantum critical point. Our detailed structural analysis reveals that the pnictogen height is an important controlling parameter for their electronic and magnetic properties, and may play an important role in electron pairing and superconductivity of these materials.

Spin ordering and electronic texture in the bilayer iridate Sr3Ir2O7

Through a neutron scattering, charge transport, and magnetization study, the correlated ground state in the bilayer iridium oxide Sr

Spin-state crossover and low-temperature magnetic state in yttrium-doped Pr0.7Ca0.3CoO3

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Neutron scattering study of magnetic phase separation in nanocrystalline La5/8Ca3/8MnO3

Ir

Structural and magnetic phase transitions in CeCu6-xTx (T = Ag,Pd)

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Structural and magnetic phase transitions in Na 1 − δ FeAs

O