arXiv subjects

Ross D. McDonald

Publications and source records attributed to Ross D. McDonald.

41 records · Page 3Linked to original sources

arXiv2011

Experimental realization of field-induced XY and Ising ground states in a quasi-2D S=1/2 Heisenberg antiferromagnet

High field specific heat, Cp, and magnetic susceptibility, \c{hi}, measurements were performed on the quasi-two dimensional Heisenberg antiferromagnet [Cu(pyz)2(pyO)2](PF6)2. While no Cp anomaly is observed down to 0.5 K in zero magnetic field, the application of field parallel to the crystallographic ab-plane induces a lambda-like anomaly in Cp, consistent with Ising-type magnetic order. On the other hand, when the field is parallel to the c-axis, Cp and \c{hi} show evidence of XY-type antiferromagnetism. We argue that it is a small but finite easy-plane anisotropy in quasi-two dimensional [Cu(pyz)2(pyO)2](PF6)2 that allows the unusual observation of field induced XY and Ising-type magnetic states.

cond-mat.mtrl-sci↗

arXiv2010

Bulk Fermi surface coexistence with Dirac surface state in Bi$_2$Se$_3$: a comparison of photoemission and Shubnikov-de Haas measurements

Shubnikov de Haas (SdH) oscillations and Angle Resolved PhotoEmission Spectroscopy (ARPES) are used to probe the Fermi surface of single crystals of Bi2Se3. We find that SdH and ARPES probes quantitatively agree on measurements of the effective mass and bulk band dispersion. In high carrier density samples, the two probes also agree in the exact position of the Fermi level EF, but for lower carrier density samples discrepancies emerge in the position of EF. In particular, SdH reveals a bulk three-dimensional Fermi surface for samples with carrier densities as low as 10^17cm-3. We suggest a simple mechanism to explain these differences and discuss consequences for existing and future transport studies of topological insulators.

cond-mat.mes-hall↗

arXiv2009

Quantum oscillations in the parent pnictide BaFe$_2$As$_2$ : itinerant electrons in the reconstructed state

We report quantum oscillation measurements that enable the direct observation of the Fermi surface of the low temperature ground state of \ba122. From these measurements we characterize the low energy excitations, revealing that the Fermi surface is reconstructed in the antiferromagnetic state, but leaving itinerant electrons in its wake. The present measurements are consistent with a conventional band folding picture of the antiferromagnetic ground state, placing important limits on the topology and size of the Fermi surface.

cond-mat.supr-con↗

arXiv2008

A mechanism for Fermi-surface-topology tuned superconductivity in the cuprates

Based on recent magnetic-quantum-oscillation, ARPES, neutron-scattering and other data, we propose that superconductivity in the cuprates occurs via a convenient matching of the spatial distribution of incommensurate spin fluctuations to the amplitude and phase of the $d_{x^2-y^2}$ Cooper-pair wavefunction; this establishes a robust causal relationship between the lengthscale of the fluctuations and the superconducting coherence length. It is suggested that the spin fluctuations are driven by the Fermi surface, which is prone to nesting; they couple to the itinerant holes via the on-site Coulomb correlation energy, which inhibits double occupancy of spins or holes. The maximum energy of the fluctuations gives an appropriate energy scale for the superconducting $T_{\rm c}$. Based on this model, one can specify the design of solids that will exhibit ``high $T_{\rm c}$'' superconductivity.

cond-mat.supr-con↗

arXiv2007

Cuprate Fermi orbits and Fermi arcs: the effect of short-range antiferromagnetic order

We consider the effect of a short antiferromagnetic correlation length $ξ$ on the electronic bandstructure of the underdoped cuprates. Starting with a Fermi-surface topology similar to that detected in magnetic quantum-oscillation experiments, we show that a reduced $ξ$ gives an assymmetric broadening of the quasiparticle dispersion, resulting in simulated ARPES data very similar to those observed in experiment. Predicted features include the presence of `Fermi arcs' close to $a{\bf k}=(π/2,π/2)$, without the need to invoke a d-wave pseudogap order parameter. The statistical variation in the ${\bf k}$-space areas of the reconstructed Fermi surface pockets causes the quantum oscillations to be strongly damped, even in very strong magnetic fields, in agreement with experiment.

cond-mat.supr-con↗