Charge-spin correlation in van der Waals antiferromagenet NiPS3

Strong charge-spin coupling is found in a layered transition-metal trichalcogenide NiPS3, a van derWaals antiferromagnet, from our study of the electronic structure using several experimental and theoretical tools: spectroscopic ellipsometry, x-ray absorption and photoemission spectroscopy, and density-functional calculations. NiPS3 displays an anomalous shift in the optical spectral weight at the magnetic ordering temperature, reflecting a strong coupling between the electronic and magnetic structures. X-ray absorption, photoemission and optical spectra support a self-doped ground state in NiPS3. Our work demonstrates that layered transition-metal trichalcogenide magnets are a useful candidate for the study of correlated-electron physics in two-dimensional magnetic material.Comment: 6 pages, 3 figur

Doping-dependent charge dynamics in CuₓBi₂Se₃

Superconducting CuₓBi₂Se₃ has attracted significant attention as a candidate topological superconductor. Besides inducing superconductivity, the introduction of Cu atoms to this material has also been observed to produce a number of unusual features in DC transport and magnetic susceptibility measurements. To clarify the effect of Cu doping, we have performed a systematic optical spectroscopic study of the electronic structure of CuₓBi₂Se₃ as a function of Cu doping. Our measurements reveal an increase in the conduction band effective mass, while both the free carrier density and lifetime remain relatively constant for Cu content greater than x=0.15. The increased mass naturally explains trends in the superfluid density and residual resistivity as well as hints at the complex nature of Cu doping in Bi₂Se₃

Unconventional spin-phonon coupling via the Dzyaloshinskii???Moriya interaction

Spin-phonon coupling (SPC) plays a critical role in numerous intriguing phenomena of transition metal oxides (TMOs). In 3d and 4d TMOs, the coupling between spin and lattice degrees of freedom is known to originate from the exchange interaction. On the other hand, the origin of SPC in 5d TMOs remains to be elucidated. To address this issue, we measured the phonon spectra of the 5d pyrochlore iridate Y 2 Ir 2 O 7 using optical spectroscopy. Three infrared-active phonons soften below the N??el temperature of T N ??? 170 K, indicating the existence of strong SPC. Simulations using density functional theory showed that the coupling is closely related to the Ir???O???Ir bond angle. A tight-binding model analysis reveals that this SPC is mainly mediated by the Dzyaloshinskii???Moriya interaction rather than the usual exchange interaction. We suggest that such unconventional SPC may be realized in other 5d TMOs with non-collinear magnetic order

Strong Spin-Phonon Coupling Mediated by Single Ion Anisotropy in the All-In-All-Out Pyrochlore Magnet Cd2Os2 O7

Spin-phonon coupling mediated by single ion anisotropy was investigated using optical spectroscopy and first-principles calculations in the all-in-all-out pyrochlore magnet Cd2Os2O7. Clear anomalies were observed in both the phonon frequencies and linewidths at the magnetic ordering temperature. The renormalization of the phonon modes was exceptionally large, signifying the presence of an unconventional magnetoelastic term from large spin-orbit coupling. In addition, the relative phonon frequency shifts show a strong correlation with the modulation of noncubic crystal field by the corresponding lattice distortion. Our observation establishes a new type of spin-phonon coupling through single ion anisotropy, a second-order spin-orbit coupling term, in Cd2Os2O7. © 2017 American Physical Society4

Spin-Orbit Coupling and Interband Transitions in the Optical Conductivity of Sr2RhO4

The prototypical correlated metal Sr2RhO4 was studied using optical and photoemission spectroscopy. At low energies and temperatures, the optical data reveal a complex, multicomponent response that on the surface points to an unconventional metallic state in this material. Via a comparison with photoemission, the anomalous optical response may be attributed to an unexpectedly strong interband transition near 180 meV between spin-orbit coupled bands that are nearly parallel along ΓX. This spin-orbit coupling effect is shown to occur in a number of related metallic ruthenates and explains the previously puzzling optical properties reported for these materials. © 2017 American Physical Society101sciescopu

Optical probe of Heisenberg-Kitaev magnetism in alpha-RuCl3

We report a temperature-dependent optical spectroscopic study of the Heisenberg-Kitaev magnet alpha-RuCl3. Our measurements reveal anomalies in the optical response near the magnetic ordering temperature. At higher temperatures, we observe a redistribution of spectral weight over a broad energy range that is associated with nearest-neighbor spin-spin correlations. This finding is consistent with highly frustrated magnetic interactions and in agreement with theoretical expectations for this class of material. The optical data also reveal significant electron-hole interaction effects, including a bound excitonic state. These results demonstrate a clear coupling between charge and spin degrees of freedom and provide insight into the properties of thermally disordered Heisenberg-Kitaev magnets

Electronic structure of HxVO2 probed with in-situ spectroscopic ellipsometry

Vanadium dioxide (VO$_{\mathrm{2}})$ undergoes a metal-to-insulator transition (MIT) near 340K. Despite extensive studies on this material, the role of electronelectron correlation and electron-lattice interactions in driving this MIT is still under debate. Recently, it was demonstrated that hydrogen can be reversibly absorbed into VO$_{\mathrm{2}}$ thin film without destroying the lattice framework. This H-doping allows systematic control of the electron density and lattice structure which in turn leads to a insulator (VO$_{\mathrm{2}})$ - metal (H$_{x}$VO$_{\mathrm{2}})$ - insulator (HVO$_{\mathrm{2}})$ phase modulation [Yoon \textit{et al.}, Nat. Mat. \textbf{15}, 1113-1119 (2016)]. To better understand the phase modulation of H$_{x}$VO$_{\mathrm{2}}$, we used \textit{in-situ} spectroscopic ellipsometry to monitor the electronic structure during the hydrogenization process, i.e. we measured the optical conductivity of H$_{x}$VO$_ {\mathrm{2}}$ while varying $x$. Starting in the high temperature rutile metallic phase of VO$_{\mathrm{2}}$, we observed a large change in the electronic structure upon annealing in H gas at 370K: the low energy conductivity is continuously suppressed, consistent with reported DC resistivity data, while the conductivity peaks at high energy show strong changes in energy and spectral weight. The implications of our results for the MIT in H$_{x}$VO$_{\mathrm{2}}$ will be discussed.1