Crystal Symmetry Lowering in Chiral Multiferroic Ba3_3TaFe3_3Si2_2O14_{14} observed by X-Ray Magnetic Scattering

Chiral multiferroic langasites have attracted attention due to their doubly-chiral magnetic ground state within an enantiomorphic crystal. We report on a detailed resonant soft X-ray diffraction study of the multiferroic Ba$_3$TaFe$_3$Si$_2$O$_{14}$ at the Fe $L_{2,3}$ and oxygen $K$ edges. Below $T_N$ ($\approx27K$) we observe the satellite reflections $(0,0,\tau)$, $(0,0,2\tau)$, $(0,0,3\tau)$ and $(0,0,1-3\tau)$ where $\tau \approx 0.140 \pm 0.001$. The dependence of the scattering intensity on X-ray polarization and azimuthal angle indicate that the odd harmonics are dominated by the out-of-plane ($\mathbf{\hat{c}}$-axis) magnetic dipole while the $(0,0,2\tau)$ originates from the electron density distortions accompanying magnetic order. We observe dissimilar energy dependences of the diffraction intensity of the purely magnetic odd-harmonic satellites at the Fe $L_3$ edge. Utilizing first-principles calculations, we show that this is a consequence of the loss of threefold crystal symmetry in the multiferroic phase

Energy dissipation in the time domain governed by bosons in a correlated material

In complex materials various interactions play important roles in determining the material properties. Angle Resolved Photoelectron Spectroscopy (ARPES) has been used to study these processes by resolving the complex single particle self energy $\Sigma(E)$ and quantifying how quantum interactions modify bare electronic states. However, ambiguities in the measurement of the real part of the self energy and an intrinsic inability to disentangle various contributions to the imaginary part of the self energy often leave the implications of such measurements open to debate. Here we employ a combined theoretical and experimental treatment of femtosecond time-resolved ARPES (tr-ARPES) and show how measuring the population dynamics using tr-ARPES can be used to separate electron-boson interactions from electron-electron interactions. We demonstrate the analysis of a well-defined electron-boson interaction in the unoccupied spectrum of the cuprate Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$ characterized by an excited population decay time constant $\tau_{QP}$ that maps directly to a discrete component of the equilibrium self energy not readily isolated by static ARPES experiments.Comment: 19 pages with 6 figure

Study of the phase transition dynamics of the L to H transition

A highly radiating zone (MARFE) just above the divertor X-point has been used to access the marginal transition regime P{sub sep} {approx} P{sub thres} to study the existence of a critical point for the L to H transition. Phase transition models predict that at the critical point, the transition duration increases and the plasma parameters vary continuously between L-mode and H-mode. In these experiments, the L to H transition duration increased 50--100 times over fast transitions. However, the evolution of E{sub r} shear, edge density gradient, H-mode pedestal, and fluctuations is essentially unchanged from that in fast transitions. The only difference is in the speed with which and the degree to which the fluctuation amplitudes are transiently reduced. This difference is understandable in terms of the time scales for fluctuation amplitude reduction ({le} 100 {micro}s) and edge pressure gradient increase (several ms), provided the edge fluctuations are pressure-gradient driven

Ultrafast Momentum-resolved Hot Electron Dynamics in the Two-dimensional Topological Insulator Bismuthene

Two-dimensional quantum spin Hall (QSH) insulators are a promising material class for spintronic applications based on topologically-protected spin currents in their edges. Yet, they have not lived up to their technological potential, as experimental realizations are scarce and limited to cryogenic temperatures. These constraints have also severely restricted characterization of their dynamical properties. Here, we report on the electron dynamics of the novel room-temperature QSH candidate bismuthene after photoexcitation using time- and angle-resolved photoemission spectroscopy. We map the transiently occupied conduction band and track the full relaxation pathway of hot photocarriers. Intriguingly, we observe photocarrier lifetimes much shorter than in \red{conventional} semiconductors. This is ascribed to the presence of topological in-gap states already established by local probes. Indeed, we find spectral signatures consistent with these earlier findings. Demonstration of the large band gap and the view into photoelectron dynamics mark a critical step toward optical control of QSH functionalities.Comment: 13 pages, 11 figure

Temporal and Geospatial Trends of Pediatric Cancer Incidence in Nebraska Over a 24-Year Period

BACKGROUND: Data from the Surveillance, Epidemiology, and End Results (SEER) revealed that the incidence of pediatric cancer in Nebraska exceeded the national average during 2009-2013. Further investigation could help understand these patterns. METHODS: This retrospective cohort study investigated pediatric cancer (0-19 years old) age adjusted incidence rates (AAR) in Nebraska using the Nebraska Cancer Registry. SEER AARs were also calculated as a proxy for pediatric cancer incidence in the United States (1990-2013) and compared to the Nebraska data. Geographic Information System (GIS) mapping was also used to display the spatial distribution of cancer in Nebraska at the county level. Finally, location-allocation analysis (LAA) was performed to identify a site for the placement of a medical center to best accommodate rural pediatric cancer cases. RESULTS: The AAR of pediatric cancers was 173.3 per 1,000,000 in Nebraska compared to 167.1 per 1,000,000 in SEER. The AAR for lymphoma was significantly higher in Nebraska (28.1 vs. 24.6 per 1,000,000; p = 0.009). For the 15-19 age group, the AAR for the 3 most common pediatric cancers were higher in Nebraska (p \u3c 0.05). Twenty-three counties located \u3e2 h driving distance to care facilities showed at least a 10% higher incidence than the overall state AAR. GIS mapping identified a second potential treatment site that would alleviate this geographic burden. CONCLUSIONS: Regional differences within Nebraska present a challenge for rural populations. Novel use of GIS mapping to highlight regional differences and identify solutions for access to care issues could be used by similar states

Ultrafast modulation of the chemical potential in BaFe2As2 by coherent phonons

Time- and angle-resolved extreme ultraviolet photoemission spectroscopy is used to study the electronic structure dynamics in BaFe2As2 around the high-symmetry points Γ and M. A global oscillation of the Fermi level at the frequency of the A1g(As) phonon mode is observed. It is argued that this behavior reflects a modulation of the effective chemical potential in the photoexcited surface region that arises from the high sensitivity of the band structure near the Fermi level to the A1g(As) phonon mode combined with a low electron diffusivity perpendicular to the layers. The results establish a novel way to tune the electronic properties of iron pnictides: coherent control of the effective chemical potential. The results further suggest that the equilibration time for the effective chemical potential needs to be considered in the ultrafast electronic structure dynamics of materials with weak interlayer coupling. © 2014 American Physical Society

Ultrafast Light-Induced Lifshitz Transition

Fermi surface is at the heart of our understanding of metals and strongly correlated many-body systems. An abrupt change in the Fermi surface topology, also called Lifshitz transition, can lead to the emergence of fascinating phenomena like colossal magnetoresistance and superconductivity. While Lifshitz transitions have been demonstrated for a broad range of materials and using different types of static external perturbations such as strain, doping, pressure and temperature, a non-equilibrium route toward ultrafast and transient modification of the Fermi surface topology has not been experimentally demonstrated. Combining time-resolved multidimensional photoemission spectroscopy with state-of-the-art TDDFT+U simulations, we introduce a scheme for driving an ultrafast Lifshitz transition in the correlated Weyl semimetal Td-MoTe2. We demonstrate that this non-equilibrium topological electronic transition finds its microscopic origin in the dynamical modification of the effective electronic correlations. These results shed light on a novel ultrafast and all-optical scheme for controlling the Fermi surface topology in correlated quantum materials