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- ItemOrganic carbon burial is paced by a ∼173-ka obliquity cycle in the middle to high latitudes(Washington, DC [u.a.] : Assoc., 2021) Huang, He; Gao, Yuan; Ma, Chao; Jones, Matthew M.; Zeeden, Christian; Ibarra, Daniel E.; Wu, Huaichun; Wang, ChengshanEarth’s climate system is complex and inherently nonlinear, which can induce some extraneous cycles in paleoclimatic proxies at orbital time scales. The paleoenvironmental consequences of these extraneous cycles are debated owing to their complex origin. Here, we compile high-resolution datasets of total organic carbon (TOC) and stable carbon isotope (δ13Corg) datasets to investigate organic carbon burial processes in middle to high latitudes. Our results document a robust cyclicity of ~173 thousand years (ka) in both TOC and δ13Corg. The ~173-ka obliquity–related forcing signal was amplified by internal climate feedbacks of the carbon cycle under different geographic and climate conditions, which control a series of sensitive climatic processes. In addition, our new and compiled records from multiple proxies confirm the presence of the obliquity amplitude modulation (AM) cycle during the Mesozoic and Cenozoic and indicate the usefulness of the ~173-ka cycle as geochronometer and for paleoclimatic interpretation.
- ItemFingerprint of volcanic forcing on the ENSO-Indian monsoon coupling(Washington, DC [u.a.] : Assoc., 2020) Singh, M.; Krishnan, R.; Goswami, B.; Choudhury, A. D.; Swapna, P.; Vellore, R.; Prajeesh, A. G.; Sandeep, N.; Venkataraman, C.; Donner, R. V.; Marwan, N.; Kurths, J.Coupling of the El Niño-Southern Oscillation (ENSO) and Indian monsoon (IM) is central to seasonal summer monsoon rainfall predictions over the Indian subcontinent, although a nonstationary relationship between the two nonlinear phenomena can limit seasonal predictability. Radiative effects of volcanic aerosols injected into the stratosphere during large volcanic eruptions (LVEs) tend to alter ENSO evolution; however, their impact on ENSO-IM coupling remains unclear. Here, we investigate how LVEs influence the nonlinear behavior of the ENSO and IM dynamical systems using historical data, 25 paleoclimate reconstructions, last-millennium climate simulations, large-ensemble targeted climate sensitivity experiments, and advanced analysis techniques. Our findings show that LVEs promote a significantly enhanced phase-synchronization of the ENSO and IM oscillations, due to an increase in the angular frequency of ENSO. The results also shed innovative insights into the physical mechanism underlying the LVE-induced enhancement of ENSO-IM coupling and strengthen the prospects for improved seasonal monsoon predictions.
- ItemQuantitative hyperspectral coherent diffractive imaging spectroscopy of a solid-state phase transition in vanadium dioxide(Washington, DC [u.a.] : Assoc., 2021) Johnson, Allan S.; Conesa, Jordi Valls; Vidas, Luciana; Perez-Salinas, Daniel; Günther, Christian M.; Pfau, Bastian; Hallman, Kent A.; Haglund, Richard F.; Eisebitt, Stefan; Wall, SimonSolid-state systems can host a variety of thermodynamic phases that can be controlled with magnetic fields, strain, or laser excitation. Many phases that are believed to exhibit exotic properties only exist on the nanoscale, coexisting with other phases that make them challenging to study, as measurements require both nanometer spatial resolution and spectroscopic information, which are not easily accessible with traditional x-ray spectromicroscopy techniques. Here, we use coherent diffractive imaging spectroscopy (CDIS) to acquire quantitative hyperspectral images of the prototypical quantum material vanadium oxide across the vanadium L2,3 and oxygen K x-ray absorption edges with nanometer-scale resolution. We extract the full complex refractive indices of the monoclinic insulating and rutile conducting phases of VO2 from a single sample and find no evidence for correlation-driven phase transitions. CDIS will enable quantitative full-field x-ray spectromicroscopy for studying phase separation in time-resolved experiments and other extreme sample environments where other methods cannot operate.
- ItemStrongly enhanced and tunable photovoltaic effect in ferroelectric-paraelectric superlattices(Washington, DC [u.a.] : Assoc., 2021) Yun, Yeseul; Mühlenbein, Lutz; Knoche, David S.; Lotnyk, Andriy; Bhatnagar, AkashEver since the first observation of a photovoltaic effect in ferroelectric BaTiO3, studies have been devoted to analyze this effect, but only a few attempted to engineer an enhancement. In conjunction, the steep progress in thin-film fabrication has opened up a plethora of previously unexplored avenues to tune and enhance material properties via growth in the form of superlattices. In this work, we present a strategy wherein sandwiching a ferroelectric BaTiO3 in between paraelectric SrTiO3 and CaTiO3 in a superlattice form results in a strong and tunable enhancement in photocurrent. Comparison with BaTiO3 of similar thickness shows the photocurrent in the superlattice is 103 times higher, despite a nearly two-thirds reduction in the volume of BaTiO3. The enhancement can be tuned by the periodicity of the superlattice, and persists under 1.5 AM irradiation. Systematic investigations highlight the critical role of large dielectric permittivity and lowered bandgap.
- ItemUltrafast optically induced spin transfer in ferromagnetic alloys(Washington, DC [u.a.] : Assoc., 2020) Hofherr, M.; Häuser, S.; Dewhurst, J.K.; Tengdin, P.; Sakshath, S.; Nembach, H.T.; Weber, S.T.; Shaw, J.M.; Silva, T.J.; Kapteyn, H.C.; Cinchetti, M.; Rethfeld, B.; Murnane, M.M.; Steil, D.; Stadtmüller, B.; Sharma, S.; Aeschlimann, M.; Mathias, S.The vision of using light to manipulate electronic and spin excitations in materials on their fundamental time and length scales requires new approaches in experiment and theory to observe and understand these excitations. The ultimate speed limit for all-optical manipulation requires control schemes for which the electronic or magnetic subsystems of the materials are coherently manipulated on the time scale of the laser excitation pulse. In our work, we provide experimental evidence of such a direct, ultrafast, and coherent spin transfer between two magnetic subsystems of an alloy of Fe and Ni. Our experimental findings are fully supported by time-dependent density functional theory simulations and, hence, suggest the possibility of coherently controlling spin dynamics on subfemtosecond time scales, i.e., the birth of the research area of attomagnetism.