arXiv subjects

K. W. Plumb

Publications and source records attributed to K. W. Plumb.

26 records · Page 2Linked to original sources

arXiv2015

Magnetic order in $α$-RuCl$_3$: a honeycomb lattice quantum magnet with strong spin-orbit coupling

We report magnetic and thermodynamic properties of single crystal $α$-RuCl$_3$, in which the Ru$^{3+}$ ($4d^5$) ion is in its low spin state and forms a honeycomb lattice. Two features are observed in both magnetic susceptibility and specific heat data; a sharp peak at 7~K and a broad hump near 10-15K. In addition, we observe a metamagnetic transition between 5~T and 10~T. Our neutron diffraction study of single crystal samples confirms that the low temperature peak in the specific heat is associated with a magnetic order with unit cell doubling along the honeycomb (100) direction, which is consistent with zigzag order, one of the types of magnetic order predicted within the framework of the Kitaev-Heisenberg model.

cond-mat.str-el↗

arXiv2014

Fourier analysis of the IR response of van der Waals materials

In this letter, we report on an analytical technique for optical investigations of semitransparent samples. By Fourier transforming optical spectra with Fabry-Perot resonances we extract information about sample thickness and its discrete variations. Moreover, this information is used to recover optical spectra devoid of Fabry-Perot fringes, which simplifies optical modelling, and can reveal previously concealed spectral features. To illustrate its use, we apply our technique to a Si wafer as well as six different cleavable layered materials, including topological insulators, thermoelectrics, and magnetic insulators. In the layered materials, we find strong evidence of large step edges and thickness inhomogeneity, and cannot conclusively exclude the presence of voids in the bulk of cleaved samples. This could strongly affect the interpretation of transport and optical data of crystals with topologically protected surfaces states.

cond-mat.mtrl-sci↗

arXiv2014

$α$-RuCl3: a Spin-Orbit Assisted Mott Insulator on a Honeycomb Lattice

We examine the role of spin-orbit coupling in the electronic structure of $α$-RuCl3, in which Ru ions in 4d5 configuration form a honeycomb lattice. The measured optical spectra exhibit a clear optical gap and excitations within the t2g orbitals. The spectra can be described very well with first-principles electronic structure calculations obtained by taking into account both spin orbit coupling and electron correlations. Furthermore, our X-ray absorption spectroscopy measurements at the Ru L-edges exhibit distinct spectral features associated with the presence of substantial spin- orbit coupling, as well as an anomalously large branching ratio. We propose that $α$-RuCl3 is a spin-orbit assisted Mott insulator, and the bond-dependent Kitaev interaction may be relevant for this compound.

cond-mat.str-el↗

arXiv2014

High-Energy Continuum of Magnetic Excitations in the Two-Dimensional Quantum Antiferromagnet Sr2CuO2Cl2

We have measured the magnetic excitation spectrum of the model square-lattice spin-1/2 antiferromagnet $\rm Sr_2CuO_2Cl_2$ over a broad range of energy and momentum using high-resolution inelastic neutron scattering (INS). The magnon dispersion along the zone boundary was accurately measured to be a 43 meV between (1/2,0) and (3/4,1/4) indicating the importance of coupling beyond nearest neighbors in the spin Hamiltonian. We observe a strong momentum dependent damping of the zone-boundary magnons at (1/2,0) revealing a high energy continuum of magnetic excitations. A direct comparison between our INS measurements and resonant inelastic X-ray scattering (RIXS) measurements shows that the RIXS spectrum contains significant contributions from higher energy excitations not previously considered. Our observations demonstrate that this high-energy continuum of magnetic fluctuations is a ubiquitous feature of insulating monolayer cuprates, apparent in both inelastic neutron and light scattering measurements.

cond-mat.str-el↗

arXiv2013

Incommensurate dynamic correlations in the quasi-two-dimensional spin liquid BiCu2PO6

We report detailed inelastic neutron scattering measurements on single crystals of the frustrated two-leg ladder BiCu2PO6, whose ground state is described as a spin liquid phase with no long-range order down to 6 K. Two branches of steeply dispersing long-lived spin excitations are observed with excitation gaps of Δ_1 = 1.90(9) meV and Δ_2 = 3.95(8) meV. Significant frustrating next-nearest neighbour interactions along the ladder leg drive the minimum of each excitation branch to incommensurate wavevectors ζ_1 = 0.574πand ζ_2 = 0.553πfor the lower and upper energy branches respectively. The temperature dependence of the excitation spectrum near the gap energy is consistent with thermal activation into singly and doubly degenerate excited states. The observed magnetic excitation spectrum as well as earlier thermodynamic data could be consistently explained by the presence of strong anisotropic interactions in the ground state Hamiltonian.

cond-mat.str-el↗

arXiv2013

Spin-orbital locked magnetic excitations in a half-metallic double perovskite Ba2FeReO6

We present a powder inelastic neutron scattering study of magnetic excitations in Ba$_2$FeReO$_6$, a member of the double perovskite family of materials which exhibit half-metallic behavior and high Curie temperatures. We find clear evidence of two well-defined dispersing magnetic modes in its low temperature ferrimagnetic state. We develop a local moment model, which incorporates the interaction of Fe spins with spin-orbit locked magnetic moments on Re, and show that this captures our experimental observations. Our study further opens up double perovskites as model systems to explore the interplay of strong correlations and spin-orbit coupling in 5d transition metal oxides.

cond-mat.str-el↗

arXiv2012

Field-induced magnetic behavior of the bilayer iridate Sr3Ir2O7

We have performed resonant magnetic x-ray scattering (RMXS) measurements on single crystal samples of the bilayer iridate Sr3Ir2O7. We observe the development of antiferromagnetic order below T* ~ 260 K, which persists down to T = 4 K under both field-cooled (fc) and zero-field-cooled (zfc) conditions. The temperature dependence of the fc and zfc magnetic Bragg peaks suggests the presence of competition between canted and collinear antiferromagnetic ground states. Under zfc conditions, we observe a suppression of magnetic peak intensity at T_D ~ 50 K which can be attributed to a spin reorientation transition. Under fc conditions, we find that the canted antiferromagnetic state is stabilized over a much wider range of temperatures, with significantly enhanced magnetic correlation lengths within the Ir-O layers. The field dependence of the magnetic Bragg peaks provides no evidence of field-induced phase transitions for H < 4 T.

cond-mat.str-el↗

arXiv2012

Spin-Orbit Coupling in Iridium-Based 5d Compounds Probed by X-ray Absorption Spectroscopy

We have performed x-ray absorption spectroscopy (XAS) measurements on a series of Ir-based 5d transition metal compounds, including Ir, IrCl3, IrO2, Na2IrO3, Sr2IrO4, and Y2Ir2O7. By comparing the intensity of the "white-line" features observed at the Ir L2 and L3 absorption edges, it is possible to extract valuable information about the strength of the spin-orbit coupling in these systems. We observe remarkably large, non-statistical branching ratios in all Ir compounds studied, with little or no dependence on chemical composition, crystal structure, or electronic state. This result confirms the presence of strong spin-orbit coupling effects in novel iridates such as Sr2IrO4, Na2IrO3, and Y2Ir2O7, and suggests that even simple Ir-based compounds such as IrO2 and IrCl3 may warrant further study. In contrast, XAS measurements on Re-based 5d compounds, such as Re, ReO2, ReO3, and Ba2FeReO6, reveal statistical branching ratios and negligible spin-orbit coupling effects.

cond-mat.str-el↗