Paul Goddard

Abstract:

We report quantum oscillations in the underdoped high-temperature superconductor YBa2 Cu3 O6+x over a wide range in magnetic field 28≤ μ0 H≤85 T corresponding to ≈12 oscillations, enabling the Fermi surface topology to be mapped to high resolution. As earlier reported by Sebastian [Nature (London) 454, 200 (2008)10.1038/nature07095], we find a Fermi surface comprising multiple pockets, as revealed by the additional distinct quantum oscillation frequencies and harmonics reported in this work. We find the originally reported broad low-frequency Fourier peak at ≈535 T to be clearly resolved into three separate peaks at ≈460, ≈532, and ≈602 T, in reasonable agreement with the reported frequencies of Audouard [Phys. Rev. Lett. 103, 157003 (2009)10.1103/PhysRevLett.103.157003]. However, our increased resolution and angle-resolved measurements identify these frequencies to originate from two similarly sized pockets with greatly contrasting degrees of interlayer corrugation. The spectrally dominant frequency originates from a pocket (denoted α) that is almost ideally two-dimensional in form (exhibiting negligible interlayer corrugation). In contrast, the newly resolved weaker adjacent spectral features originate from a deeply corrugated pocket (denoted γ). On comparison with band structure, the d -wave symmetry of the interlayer dispersion locates the minimally corrugated α pocket at the "nodal" point knodal = (π/2,π/2), and the significantly corrugated γ pocket at the "antinodal" point kantinodal = (π,0) within the Brillouin zone. The differently corrugated pockets at different locations indicate creation by translational symmetry breaking-a spin-density wave has been suggested from the suppression of Zeeman splitting for the spectrally dominant pocket. In a broken-translational symmetry scenario, symmetry points to the nodal (α) pocket corresponding to holes, with the weaker antinodal (γ) pocket corresponding to electrons-likely responsible for the negative Hall coefficient reported by LeBoeuf [Nature (London) 450, 533 (2007)10.1038/nature06332]. Given the similarity in α and γ pocket volumes, their opposite carrier type and the previous report of a diverging effective mass in Sebastian [Proc. Nat. Am. Soc. 107, 6175 (2010)10.1073/pnas.0913711107], we discuss the possibility of a secondary Fermi surface instability at low dopings of the excitonic insulator type, associated with the metal-insulator quantum critical point. Its potential involvement in the enhancement of superconducting transition temperatures is also discussed. © 2010 The American Physical Society.

Abstract:

We describe how the presence of magnetic breakdown affects the appearance of angle-dependent magnetoresistance oscillations in an organic metal. The model takes account of all the contributions from quasiparticles undergoing both magnetic breakdown and Bragg reflection at each junction and allows extremely efficient simulation of data which can be compared with experimental results on κ-(BEDT-TTF)2Cu(NCS)2. © 2010 Elsevier B.V. All rights reserved.

Abstract:

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).

Abstract:

We propose a model which suggests that structural martensitic transitions are related to significant changes in the electronic structure, and are effected by high-magnetic fields. The magnetic field dependence is considered unusual as many influential investigations of martensitic transitions have emphasized that the structural transitions are primarily lattice dynamical and are driven by the entropy due to the phonons. We provide a theoretical framework which can be used to describe the effect of high magnetic field on the transition and lattice dynamics in which the field dependence originates from the dielectric constant. The model is compared with some recent experimental results. © 2010 IOP Publishing Ltd.

Abstract:

The objective of this work is to study the magnetic properties of arrays of Ni-Fe nanowires electrodeposited in different template materials such as porous silicon, polycarbonate and alumina. Magnetic properties were studied as a function of template material, applied magnetic field (parallel and perpendicular) during deposition, wire length, as well as magnetic field orientation during measurement. The results show that the application of magnetic field during deposition strongly influences the c-axis preferred orientation growth of the Ni-Fe nanowires. The samples with magnetic field perpendicular to the template plane during deposition exhibit strong perpendicular anisotropy with greatly enhanced coercivity and squareness ratio, particularly in the Ni-Fe nanowires deposited in polycarbonate templates. In the case of polycarbonate template, as magnetic field during deposition increases, both coercivity and squareness ratio also increase. The wire length dependence was also measured for polycarbonate templates. As wire length increases, coercivity and squareness ratio decrease, saturation field increases. Such magnetic behaviour (dependence on template material, magnetic field, wire length) can be qualitatively explained by preferential growth phenomena, dipolar interactions among nanowires and perpendicular shape anisotropy in individual nanowires. © 2009 IOP Publishing Ltd.