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We present the results of an optical spectroscopic study of two-dimensional atomic crystals of the high-temperature superconductor Bi2Sr2CaCu2O8+δ. Our measurements reveal a pronounced suppression of the optical conductivity σ1(ω) with thickness. Using an effective medium approximation, we interpret this in terms of an insulating surface layer. The surface layer explains the insulating behavior previously observed in exfoliated Bi2Sr2CaCu2O8+δ. and has implications for future studies and potential applications of these materials. [ABSTRACT FROM AUTHOR]

We demonstrate that structural and spectroscopic information can be obtained on exfoliated nanocrystals as thin as 6 nm. This can be achieved by using a combination of micro X-ray fluorescence (μXRF), micro X-ray absorption near-edge spectroscopy (μXANES), and X-ray microdiffraction (μXRD) techniques. Highly focused, tunable X-ray beams available at synchrotron sources enable one to use these non-invasive characterization tools to study exfoliated samples on a variety of substrates. As an example, we focused on exfoliated nanocrystals of the high temperature superconductor Bi2Sr2CaCu2O8+δ. μXRF is used to locate the sample of desired thickness; μXANES and μXRD are used to obtain electronic and structural information, respectively. We find that the '4.7b' structural modulation, characteristic of the bulk crystals, is drastically suppressed for exfoliated crystals thinner than 60 nm. [ABSTRACT FROM AUTHOR]

In the recently discovered class of materials known as topological insulators, the presence of strong spin-orbit coupling causes certain topological invariants in the bulk to differ from their values in vacuum. The sudden change in invariants at the interface results in metallic, time reversal invariant surface states whose properties are useful for applications in spintronics and quantum computation. However, a key challenge is to fabricate these materials on the nanoscale appropriate for devices and probing the surface. To this end we have produced 2 nm thick nanocrystals of the topological insulator Bi2Se3 via mechanical exfoliation. For crystals thinner than 10 nm we observe the emergence of an additional mode in the Raman spectrum. The emergent mode intensity together with the other results presented here provide a recipe for production and thickness characterization of Bi2Se3 nanocrystals. [ABSTRACT FROM AUTHOR]

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. 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. Furthermore the measured optical spectra can be described very well with first-principles electronic structure calculations obtained by taking into account both spin-orbit coupling and electron correlations. We propose that α-RuCl3 is a spin-orbit assisted Mott insulator, and that the bond-dependent Kitaev interaction may be important for understanding magnetism of this compound. [ABSTRACT FROM AUTHOR]

A key challenge in condensed matter research is the optimization of topological insulator (TI) compounds for the study and future application of their unique surface states. Truly insulating bulk states would allow the exploitation of predicted surface state properties, such as protection from backscattering, dissipationless spin-polarized currents, and the emergence of novel particles. Towards this end, major progress was recently made with the introduction of highly resistive Bi2Te2Se, in which surface state conductance and quantum oscillations are observed at low temperatures. Nevertheless, an unresolved and pivotal question remains: while room-temperature ARPES studies reveal clear evidence of TI surface states, their observation in transport experiments is limited to low temperatures. A better understanding of this surface state suppression at elevated temperatures is of fundamental interest, and crucial for pushing the boundary of device applications towards room-temperature operation. In this work, we simultaneously measure TI bulk and surface states via temperature-dependent optical spectroscopy, in conjunction with transport and ARPES measurements. We find evidence of coherent surface state transport at low temperatures, and propose that phonon-mediated coupling between bulk and surface states suppresses surface conductance as temperature rises. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We investigated the electronic structure of [(SrIrO3)m, (SrTiO3)] (m = 1, 2, and 8) superlattice (SL) thin films with optical spectroscopy and first principles calculations. Our optical results confirmed the existence of the Jeff = 1/2 states in SL samples, similar to the bulk Ruddlesden-Popper series Srn + 1 IrnO3n+1 iridates. Apart from this similarity, in the SL samples, we observed red shifts of the characteristic optical excitations in the J eff = 1 / 2 state and an enhancement of the low-energy spectral weight, which implies a reduction in the effective electron correlation for bands near the Fermi energy. The density functional theory plus Coulomb interactions (DFT + U) calculations suggested that the SrTiO3 layer intervened between SrIrO3 layers in the SLs activated additional hopping channels between the Ir ions, thus increasing the bandwidth and reducing the effective strength of the correlations. This paper demonstrates that fabrication of iridium-based heterostructures can be used to finely tune electronic structures via alteration of their local orbital environments. [ABSTRACT FROM AUTHOR]

We investigated the metal-insulator transition (MIT) driven by all-in-all-out (AIAO) antiferromagnetic ordering in the 5d pyrochlore Cd20 s20 7 using optical spectroscopy and first-principles calculations. We showed that the temperature evolution in the band-gap edge and free carrier density were consistent with rigid upward (downward) shifts of electron (hole) bands, similar to the case of Lifshitz transitions. The delicate relationship between the band gap and free carrier density provides experimental evidence for the presence of an AIAO metallic phase, a natural consequence of such MITs. The associated spectral weight change at high energy and first-principles calculations further support the origin of the MIT from the band shift near the Fermi level. Our data consistently support that the MIT induced by AIAO ordering in Cd20 s20 7 is not close to a Slater type but instead to a Lifshitz type. [ABSTRACT FROM AUTHOR]