Detection of metastable electronic states by Penning trap mass spectrometry.

State-of-the-art optical clocks1 achieve precisions of 10-18 or better using ensembles of atoms in optical lattices2,3 or individual ions in radio-frequency traps4,5. Promising candidates for use in atomic clocks are highly charged ions6 (HCIs) and nuclear transitions7, which are largely insensitive to external perturbations and reach wavelengths beyond the optical range8 that are accessible to frequency combs9. However, insufficiently accurate atomic structure calculations hinder the identification of suitable transitions in HCIs. Here we report the observation of a long-lived metastable electronic state in an HCI by measuring the mass difference between the ground and excited states in rhenium, providing a non-destructive, direct determination of an electronic excitation energy. The result is in agreement with advanced calculations. We use the high-precision Penning trap mass spectrometer PENTATRAP to measure the cyclotron frequency ratio of the ground state to the metastable state of the ion with a precision of 10-11-an improvement by a factor of ten compared with previous measurements10,11. With a lifetime of about 130 days, the potential soft-X-ray frequency reference at 4.96 × 1016 hertz (corresponding to a transition energy of 202 electronvolts) has a linewidth of only 5 × 10-8 hertz and one of the highest electronic quality factors (1024) measured experimentally so far. The low uncertainty of our method will enable searches for further soft-X-ray clock transitions8,12 in HCIs, which are required for precision studies of fundamental physics6.

Photon-Modulated Bond Covalency of [Sm(II)(η9-C9H9)2].

Lanthanides are widely assumed not to form covalent bonds due to the localized nature of their 4f valence electrons. This work demonstrates that the ionic bond of Sm(II) with cyclononatetraenyl (η9-C9H9-) in [Sm(η9-C9H9)2] can be modulated and becomes more covalent by photon-induced transfer of Sm 4f electrons to Sm 5d orbitals. This photon-induced change in bonding properties facilitates a subsequent reconfiguration of [Sm(η9-C9H9)2]. As a result, Sm-C bond length contraction is detected and the local Sm coordination environment exhibits more extensive disorder. Both Sm 4f and 5d electrons have increased participation in covalent Sm-ligand interactions. The Sm L3-edge valence band resonant inelastic X-ray scattering (VB-RIXS), high-resolution X-ray absorption near-edge structure (HR-XANES), and quantum chemical computations showcase a spectroscopic methodology for in-depth studies of bond covalency of lanthanide atoms.

Maximal Rashba-like spin splitting via kinetic-energy-coupled inversion-symmetry breaking.

Engineering and enhancing the breaking of inversion symmetry in solids-that is, allowing electrons to differentiate between 'up' and 'down'-is a key goal in condensed-matter physics and materials science because it can be used to stabilize states that are of fundamental interest and also have potential practical applications. Examples include improved ferroelectrics for memory devices and materials that host Majorana zero modes for quantum computing. Although inversion symmetry is naturally broken in several crystalline environments, such as at surfaces and interfaces, maximizing the influence of this effect on the electronic states of interest remains a challenge. Here we present a mechanism for realizing a much larger coupling of inversion-symmetry breaking to itinerant surface electrons than is typically achieved. The key element is a pronounced asymmetry of surface hopping energies-that is, a kinetic-energy-coupled inversion-symmetry breaking, the energy scale of which is a substantial fraction of the bandwidth. Using spin- and angle-resolved photoemission spectroscopy, we demonstrate that such a strong inversion-symmetry breaking, when combined with spin-orbit interactions, can mediate Rashba-like spin splittings that are much larger than would typically be expected. The energy scale of the inversion-symmetry breaking that we achieve is so large that the spin splitting in the CoO2- and RhO2-derived surface states of delafossite oxides becomes controlled by the full atomic spin-orbit coupling of the 3d and 4d transition metals, resulting in some of the largest known Rashba-like spin splittings. The core structural building blocks that facilitate the bandwidth-scaled inversion-symmetry breaking are common to numerous materials. Our findings therefore provide opportunities for creating spin-textured states and suggest routes to interfacial control of inversion-symmetry breaking in designer heterostructures of oxides and other material classes.

Direct Q-Value Determination of the ß

The cyclotron frequency ratio of ^{187}Os^{29+} to ^{187}Re^{29+} ions was measured with the Penning-trap mass spectrometer PENTATRAP. The achieved result of R=1.000 000 013 882(5) is to date the most precise such measurement performed on ions. Furthermore, the total binding-energy difference of the 29 missing electrons in Re and Os was calculated by relativistic multiconfiguration methods, yielding the value of ΔE=53.5(10) eV. Finally, using the achieved results, the mass difference between neutral ^{187}Re and ^{187}Os, i.e., the Q value of the ß^{-} decay of ^{187}Re, is determined to be 2470.9(13) eV.

Many-Body Physics of Single and Double Spin-Flip Excitations in NiO.

Understanding many-body physics of elementary excitations has advanced our control over material properties. Here, we study spin-flip excitations in NiO using Ni L_{3}-edge resonant inelastic x-ray scattering (RIXS) and present a strikingly different resonant energy behavior between single and double spin-flip excitations. Comparing our results with single-site full-multiplet ligand field theory calculations we find that the spectral weight of the double-magnon excitations originates primarily from the double spin-flip transition of the quadrupolar RIXS process within a single magnetic site. Quadrupolar spin-flip processes are among the least studied excitations, despite being important for multiferroic or spin-nematic materials due to their difficult detection. We identify intermediate state multiplets and intra-atomic core-valence exchange interactions as the key many-body factors determining the fate of such excitations. RIXS resonant energy dependence can act as a convincing proof of existence of nondipolar higher-ranked magnetic orders in systems for which, only theoretical predictions are available.

Noncollinear Ordering of the Orbital Magnetic Moments in Magnetite.

The magnitude of the orbital magnetic moment and its role as a trigger of the Verwey transition in the prototypical Mott insulator, magnetite, remain contentious. Using 1s2p resonant inelastic x-ray scattering angle distribution (RIXS-AD), we prove the existence of noncollinear orbital magnetic ordering and infer the presence of dynamical distortion creating a polaronic precursor for the metal to insulator transition. These conclusions are based on a subtle angular shift of the RIXS-AD spectral intensity as a function of the magnetic field orientation. Theoretical simulations show that these results are only consistent with noncollinear magnetic orbital ordering. To further support these claims we perform Fe K-edge x-ray magnetic circular dichroism in order to quantify the Fe average orbital magnetic moment.

4f Crystal Field Ground State of the Strongly Correlated Topological Insulator SmB_{6}.

We investigated the crystal-electric field ground state of the 4f manifold in the strongly correlated topological insulator SmB_{6} using core-level nonresonant inelastic x-ray scattering. The directional dependence of the scattering function that arises from higher multipole transitions establishes unambiguously that the Γ_{8} quartet state of the Sm f^{5} J=5/2 configuration governs the ground-state symmetry and, hence, the topological properties of SmB_{6}. Our findings contradict the results of density functional calculations reported so far.

Spin-orbital separation in the quasi-one-dimensional Mott insulator Sr2CuO3.

When viewed as an elementary particle, the electron has spin and charge. When binding to the atomic nucleus, it also acquires an angular momentum quantum number corresponding to the quantized atomic orbital it occupies. Even if electrons in solids form bands and delocalize from the nuclei, in Mott insulators they retain their three fundamental quantum numbers: spin, charge and orbital. The hallmark of one-dimensional physics is a breaking up of the elementary electron into its separate degrees of freedom. The separation of the electron into independent quasi-particles that carry either spin (spinons) or charge (holons) was first observed fifteen years ago. Here we report observation of the separation of the orbital degree of freedom (orbiton) using resonant inelastic X-ray scattering on the one-dimensional Mott insulator Sr2CuO3. We resolve an orbiton separating itself from spinons and propagating through the lattice as a distinct quasi-particle with a substantial dispersion in energy over momentum, of about 0.2 electronvolts, over nearly one Brillouin zone.

Crossover from Collective to Incoherent Spin Excitations in Superconducting Cuprates Probed by Detuned Resonant Inelastic X-Ray Scattering.

Spin excitations in the overdoped high temperature superconductors Tl_{2}Ba_{2}CuO_{6+δ} and (Bi,Pb)_{2}(Sr,La)_{2}CuO_{6+δ} were investigated by resonant inelastic x-ray scattering (RIXS) as functions of doping and detuning of the incoming photon energy above the Cu-L_{3} absorption peak. The RIXS spectra at optimal doping are dominated by a paramagnon feature with peak energy independent of photon energy, similar to prior results on underdoped cuprates. Beyond optimal doping, the RIXS data indicate a sharp crossover to a regime with a strong contribution from incoherent particle-hole excitations whose maximum shows a fluorescencelike shift upon detuning. The spectra of both compound families are closely similar, and their salient features are reproduced by exact-diagonalization calculations of the single-band Hubbard model on a finite cluster. The results are discussed in the light of recent transport experiments indicating a quantum phase transition near optimal doping.

Dynamical Effects in Resonant X-Ray Diffraction.

Using resonant magnetic diffraction at the Ni L_{2,3} edge in a LaNiO_{3} superlattice, we show that dynamical effects beyond the standard kinematic approximation can drastically modify the resonant scattering cross section. In particular, the combination of extinction and refraction convert maxima to minima in the azimuthal-angle dependence of the diffracted intensity, which is commonly used to determine orbital and magnetic structures by resonant x-ray diffraction. We provide a comprehensive theoretical description of these effects by numerically solving Maxwell's equations in three dimensions. The understanding and description of dynamical diffraction enhances the capabilities of resonant x-ray scattering as a probe of electronic ordering phenomena in solids.

Photoelectron spin-polarization control in the topological insulator Bi2Se3.

We study the manipulation of the spin polarization of photoemitted electrons in Bi2Se3 by spin- and angle-resolved photoemission spectroscopy. General rules are established that enable controlling the photoelectron spin-polarization. We demonstrate the ± 100% reversal of a single component of the measured spin-polarization vector upon the rotation of light polarization, as well as full three-dimensional manipulation by varying experimental configuration and photon energy. While a material-specific density-functional theory analysis is needed for the quantitative description, a minimal yet fully generalized two-atomic-layer model qualitatively accounts for the spin response based on the interplay of optical selection rules, photoelectron interference, and topological surface-state complex structure. It follows that photoelectron spin-polarization control is generically achievable in systems with a layer-dependent, entangled spin-orbital texture.

Spin-orbital entanglement and the breakdown of singlets and triplets in Sr2RuO4 revealed by spin- and angle-resolved photoemission spectroscopy.

Spin-orbit coupling has been conjectured to play a key role in the low-energy electronic structure of Sr2RuO4. By using circularly polarized light combined with spin- and angle-resolved photoemission spectroscopy, we directly measure the value of the effective spin-orbit coupling to be 130±30 meV. This is even larger than theoretically predicted and comparable to the energy splitting of the dxy and dxz,yz orbitals around the Fermi surface, resulting in a strongly momentum-dependent entanglement of spin and orbital character in the electronic wavefunction. As demonstrated by the spin expectation value ⟨sk⃗·s-k⃗⟩ calculated for a pair of electrons with zero total momentum, the classification of the Cooper pairs in terms of pure singlets or triplets fundamentally breaks down, necessitating a description of the unconventional superconducting state of Sr2RuO4 in terms of these newly found spin-orbital entangled eigenstates.

Orbital control of noncollinear magnetic order in nickel oxide heterostructures.

We have used resonant x-ray diffraction to develop a detailed description of antiferromagnetic ordering in epitaxial superlattices based on two-unit-cell thick layers of the strongly correlated metal LaNiO3. We also report reference experiments on thin films of PrNiO3 and NdNiO3. The resulting data indicate a spiral state whose polarization plane can be controlled by adjusting the Ni d-orbital occupation via two independent mechanisms: epitaxial strain and spatial confinement of the valence electrons. The data are discussed in light of recent theoretical predictions.

Layer-by-layer entangled spin-orbital texture of the topological surface state in Bi2Se3.

We study Bi(2)Se(3) by polarization-dependent angle-resolved photoemission spectroscopy and density-functional theory slab calculations. We find that the surface state Dirac fermions are characterized by a layer-dependent entangled spin-orbital texture, which becomes apparent through quantum interference effects. This explains the discrepancy between the spin polarization obtained in spin and angle-resolved photoemission spectroscopy-ranging from 20% to 85%-and the 100% value assumed in phenomenological models. It also suggests a way to probe the intrinsic spin texture of topological insulators, and to continuously manipulate the spin polarization of photoelectrons and photocurrents all the way from 0 to ±100% by an appropriate choice of photon energy, linear polarization, and angle of incidence.

Determining the in-plane orientation of the ground-state orbital of CeCu2Si2.

We have successfully determined the hitherto unknown sign of the B(4)(4) Stevens crystal-field parameter of the tetragonal heavy-fermion compound CeCu(2)Si(2) using vector q-dependent nonresonant inelastic x-ray scattering experiments at the cerium N(4,5) edge. The observed difference between the two different directions, q∥[100] and q∥[110], is due to the anisotropy of the crystal-field ground state in the (001) plane and is observable only because of the utilization of higher than dipole transitions possible in nonresonant inelastic x-ray scattering. This approach allows us to go beyond the specific limitations of dc magnetic susceptibility, inelastic neutron scattering, and soft x-ray spectroscopy, and provides us with a reliable information about the orbital state of the 4f electrons relevant for the quantitative modeling of the quasiparticles and their interactions in heavy-fermion systems.

Two-spinon and orbital excitations of the spin-Peierls system TiOCl.

We combine high-resolution resonant inelastic x-ray scattering with cluster calculations utilizing a recently derived effective magnetic scattering operator to analyze the polarization, excitation energy, and momentum-dependent excitation spectrum of the low-dimensional quantum magnet TiOCl in the range expected for orbital and magnetic excitations (0-2.5 eV). Ti 3d orbital excitations yield complete information on the temperature-dependent crystal-field splitting. In the spin-Peierls phase we observe a dispersive two-spinon excitation and estimate the inter- and intradimer magnetic exchange coupling from a comparison to cluster calculations.

Theory of resonant inelastic x-ray scattering by collective magnetic excitations.

I present a tractable theory for the resonant inelastic x-ray scattering (RIXS) of magnons. The low-energy transition operator is written as a product of local spin operators and fundamental x-ray absorption spectral functions. This leads to simple selection rules. The scattering cross section linear (quadratic) in spin operators is proportional to the fundamental magnetic circular (linear) dichroic spectral function. RIXS is a novel tool to measure magnetic quasiparticles (magnons) and the incoherent spectral weight, as well as multiple magnons up to very high energy losses, in small samples, thin films, and multilayers, complementary to neutron scattering.

Inequivalent routes across the Mott transition in V2O3 explored by X-ray absorption.

The changes in the electronic structure of V2O3 across the metal-insulator transition induced by temperature, doping, and pressure are identified using high resolution x-ray absorption spectroscopy at the V pre-K edge. Contrary to what has been taken for granted so far, the metallic phase reached under pressure is shown to differ from the one obtained by changing doping or temperature. Using a novel computational scheme, we relate this effect to the role and occupancy of the a{1g} orbitals. This finding unveils the inequivalence of different routes across the Mott transition in V2O3.

Detection of the t(14;18) translocation in frozen and formalin-fixed tissue.

As part of a retrospective study into the prevalence of the t(14;18) translocation in B-cell lymphomas, we assessed the suitability of the polymerase chain reaction (PCR) to amplify the t(14;18) major breakpoint region (MBR) in frozen and formalin-fixed tissue. Considering Southern blotting as a standard, the sensitivity of PCR was 81%. Of the various procedures used to extract DNA from paraffin-embedded tissue (PET), proteinase K digestion in the presence of nonionic detergents gave the highest yield and quality of DNA and the most efficient amplification rate. Using this method, excellent amplification rates (100%) were obtained for both the beta-globin control sequence and the MBR t(14;18) for fixed follicular lymphoma specimens collected in the previous 2 to 6 years (n = 27). Of nine older PETs, PCR on six gave inconsistent results, probably because of the poorer-quality substrate used for amplification. Specimens exposed to formol sublimate or formalin-acetic acid-alcohol were as suitable for amplification as tissues fixed in neutral-buffered formalin. The overall incidence of the MBR t(14;18) in all follicular lymphoma specimens as detected by both Southern blotting and PCR was 59% (23 of 39).

Use of human keratinocyte and fibroblast cultures for toxicity studies of topically applied compounds.

Penetration enhancers are often used as additives in pharmaceutical and dermatological preparations. It should be expected that in many cases penetration enhancers not only enter the stratum corneum but also reach the viable cells of the epidermis and exert a toxic effect. This study focused on a series of well-known compounds that are often used as skin penetration enhancers; namely, ethanol, propylene glycol, dimethylsulfoxide, dimethylformamide, and Brij 96. In order to obtain more insight in the potential skin toxicity of these agents, they were administrated to cultured human keratinocytes and fibroblasts and the following cytotoxicity assays were performed: inhibition of the proliferation of fibroblasts and keratinocytes; inhibition of collagen contraction by fibroblasts; and cell morphology changes in confluent cultures of fibroblasts and keratinocytes. In all assays performed, the same trend was observed: ethanol was the least toxic, propylene glycol, dimethylsulfoxide, and dimethylformamide were moderately potent, and Brij 96 was the most toxic agent. An obvious advantage of the in vitro model presented here is its immediate availability and reproducibility, which allows for the comparison of a large series of topical agents (e.g., penetration enhancers) with respect to their cell toxicity under standardized conditions. However, this single-cell model lacks some of the properties found in intact skin, such as the stratum corneum barrier, and interactions between keratinocytes and other cells, such as Langerhans cells. Hence, extrapolation of these data to in vivo should be done with caution.