Pulsed versus DC I-V characteristics of resistive manganites

We report on pulsed and DC I-V characteristics of polycrystalline samples of three charge-ordered manganites, Pr_{2/3}Ca_{1/3}MnO_3, Pr_{1/2}Ca_{1/2}MnO_3, Bi_{1/2}Sr_{1/2}MnO_3 and of a double-perovskite Sr_2MnReO_6, in a temperature range where their ohmic resistivity obeys the Efros-Shklovskii variable range hopping relation. For all samples, the DC I(V) exhibits at high currents negative differential resistance and hysteresis, which mask a perfectly ohmic or a moderately nonohmic conductivity obtained by pulsed measurements. This demonstrates that the widely used DC I-V measurements are usually misleading.Comment: 6 pages, 4 figures. Accepted for publication to AP

Inter-grain tunneling in the half-metallic double-perovskites Sr2_2BB'O6_6 (BB'-- FeMo, FeRe, CrMo, CrW, CrRe

The zero-field conductivities ($\sigma$) of the polycrystaline title materials, are governed by inter-grain transport. In the majority of cases their $\sigma$(T) can be described by the "fluctuation induced tunneling" model. Analysis of the results in terms of this model reveals two remarkable features: 1. For \emph{all} Sr$_2$FeMoO$_6$ samples of various microstructures, the tunneling constant (barrier width $\times$ inverse decay-length of the wave-function) is $\sim$ 2, indicating the existence of an intrinsic insulating boundary layer with a well defined electronic (and magnetic) structure. 2. The tunneling constant for \emph{all} cold-pressed samples decreases linearly with increasing magnetic-moment/formula-unit.Comment: 10 pages, 2 tables, 3 figure

Cognitive-postural multitasking training in older adults: Effects of input-output modality mappings on cognitive performance and postural control

Older adults exhibit impaired cognitive and balance performance, particularly under multi-task conditions, which can be improved through training. Compatibility of modality mappings in cognitive tasks (i.e., match between stimulus modality and anticipated sensory effects of motor responses), modulates physical and cognitive dual-task costs. However, the effects of modality specific training programs have not been evaluated yet. Here, we tested the effects of cognitive-postural multi-tasking training on the ability to coordinate task mappings under high postural demands in healthy older adults. Twenty-one adults aged 65-85 years were assigned to one of two groups. While group 1 performed cognitive-postural triple-task training with compatible modality mappings (i.e., visual-manual and auditory-vocal dual n-back tasks), group 2 performed the same tasks with incompatible modality mappings (i.e., visual-vocal and auditory-manual n-back tasks). Throughout the 6-weeks balance training intervention, working-memory load was gradually increased while base-of-support was reduced. Before training (T0), after a 6-week passive control period (T1), and immediately after the intervention (T2), participants performed spatial dual one-back tasks in semi-tandem stance position. Our results indicate improved working-memory performance and reduced dual-task costs for both groups after the passive control period, but no training-specific performance gains. Furthermore, balance performance did not improve in response to training. Notably, the cohort demonstrated meaningful interindividual variability in training responses. Our findings raise questions about practice effects and age-related heterogeneity of training responses following cognitive-motor training. Following multi-modal balance training, neither compatible nor incompatible modality mappings had an impact on the observed outcomes

Dark Photocatalysis: Storage of Solar Energy in Carbon Nitride for Time-Delayed Hydrogen Generation

While natural photosynthesis serves as the model system for efficient charge separation and decoupling of redox reactions, bio-inspired artificial systems typically lack applicability owing to synthetic challenges and structural complexity. We present herein a simple and inexpensive system that, under solar irradiation, forms highly reductive radicals in the presence of an electron donor, with lifetimes exceeding the diurnal cycle. This radical species is formed within a cyanamide-functionalized polymeric network of heptazine units and can give off its trapped electrons in the dark to yield H$_{2}$ , triggered by a co-catalyst, thus enabling the temporal decoupling of the light and dark reactions of photocatalytic hydrogen production through the radical's longevity. The system introduced here thus demonstrates a new approach for storing sunlight as long-lived radicals, and provides the structural basis for designing photocatalysts with long-lived photo-induced states.This work was supported by the Deutsche Forschungsgemeinschaft (project LO1801/1-1) and an ERC Starting Grant (B.V.L., grant number 639233), the Max Planck Society, the cluster of excellence Nanosystems Initiative Munich (NIM), and the Center for Nanoscience (CeNS). We acknowledge support by the Christian Doppler Research Association (Austrian Federal Ministry of Science, Research and Economy, National Foundation for Research, Technology and Development) and the OMV Group (H.K., E.R.). V.W.-h.L. gratefully acknowledges a postdoctoral scholarship from the Max Planck Society

The pre-WDVV ring of physics and its topology

We show how a simplicial complex arising from the WDVV (Witten-Dijkgraaf-Verlinde-Verlinde) equations of string theory is the Whitehouse complex. Using discrete Morse theory, we give an elementary proof that the Whitehouse complex $\Delta_n$ is homotopy equivalent to a wedge of $(n-2)!$ spheres of dimension $n-4$. We also verify the Cohen-Macaulay property. Additionally, recurrences are given for the face enumeration of the complex and the Hilbert series of the associated pre-WDVV ring.Comment: 13 pages, 4 figures, 2 table

Thermocapillary actuation of liquid flow on chemically patterned surfaces

We have investigated the thermocapillary flow of a Newtonian liquid on hydrophilic microstripes which are lithographically defined on a hydrophobic surface. The speed of the microstreams is studied as a function of the stripe width w, the applied thermal gradient |dT/dx| and the liquid volume V deposited on a connecting reservoir pad. Numerical solutions of the flow speed as a function of downstream position show excellent agreement with experiment. The only adjustable parameter is the inlet film height, which is controlled by the ratio of the reservoir pressure to the shear stress applied to the liquid stream. In the limiting cases where this ratio is either much smaller or much larger than unity, the rivulet speed shows a power law dependency on w, |dT/dx| and V. In this study we demonstrate that thermocapillary driven flow on chemically patterned surfaces can provide an elegant and tunable method for the transport of ultrasmall liquid volumes in emerging microfluidic technologies