arXiv subjects

S. Calder

Publications and source records attributed to S. Calder.

60 records · Page 4Linked to original sources

arXiv2013

Doping dependence of the spin excitations in Fe-based superconductors Fe1+yTe1-xSex

The Fe1+yTe1-xSex series of materials is one of the prototype families of Fe-based superconductors. To provide further insight into these materials we present systematic inelastic neutron scattering measurements of the low energy spin excitations for x=0.27, 0.36, 0.40, 0.49. These measurements show an evolution of incommensurate spin excitations towards the (1/2 1/2 0) wave vector with doping. Concentrations (x=0.40 and 0.49) which exhibit the most robust superconducting properties have spin excitations closest to (1/2 1/2 0) and also exhibit a strong spin resonance in the spin excitation spectrum below Tc. The resonance signal appears to be closer to (1/2 1/2 0) than the underlying spin excitations. We discuss the possible relationship between superconductivity and spin excitations at the (1/2 1/2 0) wave vector and the role that interstitial Fe may play.

cond-mat.supr-con↗

arXiv2013

Magnetism and electronic structure of La2ZnIrO6 and La2MgIrO6: candidate Jeff =1/2 Mott insulators

We study experimentally and theoretically the electronic and magnetic properties of two insulating double perovskites that show similar atomic and electronic structure, but different magnetic properties. In magnetization measurements, La2ZnIrO6 displays weak ferromagnetic behavior below 7.5 K whereas La2MgIrO6 shows antiferromagnetic behavior (AFM) below TN = 12 K. Electronic structure calculations find that the weak ferromagnetic behavior observed in La2ZnIrO6 is in fact due to canted antiferromagnetism. The calculations also predict canted antiferromagnetic behavior in La2MgIrO6, but intriguingly this was not observed. Neutron diffraction measurements confirm the essentially antiferromagnetic behavior of both systems, but lack the sensitivity to resolve the small (0.22 μB/Ir) ferromagnetic component in La2ZnIrO6. Overall, the results presented here indicate the crucial role of spin-orbit coupling (SOC) and the on-site Coulomb repulsion on the magnetic, transport, and thermodynamic properties of both compounds. The electronic structure calculations show that both compounds, like Sr2IrO4, are Jeff = 1/2 Mott insulators. Our present findings suggest that La2ZnIrO6 and La2MgIrO6 provide a new playground to study the interplay between SOC and on-site Coulomb repulsion in a 5d transition metal oxide.

cond-mat.str-el↗

arXiv2012

Magnetic structural change of Sr2IrO4 upon Mn doping

The layered 5d transition metal oxide Sr2IrO4 has been shown to host a novel Jeff=1/2 Mott spin orbit insulating state with antiferromagnetic ordering, leading to comparisons with the layered cuprates. Here we study the effect of substituting Mn for Ir in single crystals of Sr2Ir0.9Mn0.1O4 through an investigation involving bulk measurements and resonant x-ray and neutron scattering. We observe a new long range magnetic structure emerge upon doping through a reordering of the spins from the basal plane to the c-axis with a reduced ordering temperature compared to Sr2IrO4. The strong enhancement of the magnetic x-ray scattering intensity at the L3 edge relative to the L2 edge indicates that the Jeff=1/2 state is robust and capable of hosting a variety of ground states.

cond-mat.str-el↗

arXiv2012

Magnetic structure determination of Ca$_3$LiOsO$_6$ using neutron and x-ray scattering

We present a neutron and x-ray scattering investigation of Ca$_3$LiOsO$_6$, a material predicted to host magnetic ordering solely through an extended superexchange pathway involving two anions, an interaction mechanism that has undergone relatively little investigation. This contrasts with the ubiquitous superexchange interaction mechanism involving a single anion that has well defined and long standing rules. Despite the apparent 1D nature and triangular units of magnetic osmium ions the onset of magnetic correlations has been observed at a high temperature of 117 K in bulk measurements. We experimentally determine the magnetically ordered structure and show it to be long range and three dimensional. Our results support the model of extended superexchange interaction.

cond-mat.str-el↗

arXiv2012

Magnetically driven metal-insulator transition in NaOsO3

The metal-insulator transition (MIT) is one of the most dramatic manifestations of electron correlations in materials. Various mechanisms producing MITs have been extensively considered, including the Mott (electron localization via Coulomb repulsion), Anderson (localization via disorder) and Peierls (localization via distortion of a periodic 1D lattice). One additional route to a MIT proposed by Slater, in which long-range magnetic order in a three dimensional system drives the MIT, has received relatively little attention. Using neutron and X-ray scattering we show that the MIT in NaOsO3 is coincident with the onset of long-range commensurate three dimensional magnetic order. Whilst candidate materials have been suggested, our experimental methodology allows the first definitive demonstration of the long predicted Slater MIT. We discuss our results in the light of recent reports of a Mott spin-orbit insulating state in other 5d oxides.

cond-mat.str-el↗

arXiv2009

Magnetic Charge Transport

It has recently been predicted that certain magnetic materials contain mobile magnetic charges or `monopoles'. Here we address the question of whether these magnetic charges and their associated currents (`magnetricity') can be directly measured in experiment, without recourse to any material-specific theory. By mapping the problem onto Onsager's theory of weak electrolytes, we show that this is possible, and devise an appropriate method. Then, using muon spin rotation as a convenient local probe, we apply the method to a real material: the spin ice Dy2Ti2O7. Our experimental measurements prove that magnetic charges exist in this material, interact via a Coulomb interaction, and have measurable currents. We further characterise deviations from Ohm's Law, and determine the elementary unit of magnetic charge to be 5 muB per Angstrom, which is equal to that predicted by Castelnovo, Moessner and Sondhi using the microscopic theory of spin ice. Our demonstration of magnetic charge transport has both conceptual and technological implications.

cond-mat.other↗