The spontaneous symmetry breaking in Ta2_2NiSe5_5 is structural in nature

The excitonic insulator is an electronically-driven phase of matter that emerges upon the spontaneous formation and Bose condensation of excitons. Detecting this exotic order in candidate materials is a subject of paramount importance, as the size of the excitonic gap in the band structure establishes the potential of this collective state for superfluid energy transport. However, the identification of this phase in real solids is hindered by the coexistence of a structural order parameter with the same symmetry as the excitonic order. Only a few materials are currently believed to host a dominant excitonic phase, Ta$_2$NiSe$_5$ being the most promising. Here, we test this scenario by using an ultrashort laser pulse to quench the broken-symmetry phase of this transition metal chalcogenide. Tracking the dynamics of the material's electronic and crystal structure after light excitation reveals surprising spectroscopic fingerprints that are only compatible with a primary order parameter of phononic nature. We rationalize our findings through state-of-the-art calculations, confirming that the structural order accounts for most of the electronic gap opening. Not only do our results uncover the long-sought mechanism driving the phase transition of Ta$_2$NiSe$_5$, but they also conclusively rule out any substantial excitonic character in this instability

25th annual computational neuroscience meeting: CNS-2016

The same neuron may play different functional roles in the neural circuits to which it belongs. For example, neurons in the Tritonia pedal ganglia may participate in variable phases of the swim motor rhythms [1]. While such neuronal functional variability is likely to play a major role the delivery of the functionality of neural systems, it is difficult to study it in most nervous systems. We work on the pyloric rhythm network of the crustacean stomatogastric ganglion (STG) [2]. Typically network models of the STG treat neurons of the same functional type as a single model neuron (e.g. PD neurons), assuming the same conductance parameters for these neurons and implying their synchronous firing [3, 4]. However, simultaneous recording of PD neurons shows differences between the timings of spikes of these neurons. This may indicate functional variability of these neurons. Here we modelled separately the two PD neurons of the STG in a multi-neuron model of the pyloric network. Our neuron models comply with known correlations between conductance parameters of ionic currents. Our results reproduce the experimental finding of increasing spike time distance between spikes originating from the two model PD neurons during their synchronised burst phase. The PD neuron with the larger calcium conductance generates its spikes before the other PD neuron. Larger potassium conductance values in the follower neuron imply longer delays between spikes, see Fig. 17.Neuromodulators change the conductance parameters of neurons and maintain the ratios of these parameters [5]. Our results show that such changes may shift the individual contribution of two PD neurons to the PD-phase of the pyloric rhythm altering their functionality within this rhythm. Our work paves the way towards an accessible experimental and computational framework for the analysis of the mechanisms and impact of functional variability of neurons within the neural circuits to which they belong

Analysis of Mo sidewall ohmic contacts to InGaAs fins

Thesis: S.M. in Electrical Engineering, Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 2017.Cataloged from PDF version of thesis.Includes bibliographical references (pages 79-80).As transistor size is scaled down, the performance is degraded and many problems, so called shortchannel effects, arise. To address this problem, a vertical transistor structure such as vertical nanowire is suggested. In a vertical nanowire field-effect-transistor, the Ohmic contact at the top of the nanowire not only covers the top surface, but also wraps around the sidewall. Because the sidewall is considered to be different from the top surface, it is necessary to study the sidewall Ohmic contact properties such as the contact resistivity. In this thesis, to explore sidewall contact resistivity, a theoretical model for sidewall contacts is developed. For the suggested test structure, the fin sidewall contact (FSWC) structure, the sidewall contact is modeled with a transmission line model (TLM), and by using TLM, the sidewall contact resistance is derived. Also, an extraction method of the sidewall contact resistivity from the total resistance measured in FSWC structure is developed. Next, process steps to fabricate FSWC structure are developed. FSWC structure is made for Mo/n+-InGaAs contacts. The key step is that the fin etch mask on top of InGaAs is not removed and the metal (Mo) is sputtered so that InGaAs is contacted by the Mo only through the sidewall. Therefore, only a sidewall contact is made without a top contact. Also, to investigate the way to improve the sidewall contact resistivity, the effect of digital etch and annealing on the sidewall contact resistivity is explored. With the measured total resistance in FSWC structure and the extraction method for sidewall contact resistivity, sidewall contact resistivity for each split of digital etch and annealing are extracted. As a summary of the effect of digital etch and annealing, two cycles of digital etch or sequential annealing up to 400 °C improves the sidewall contact resistivity with little sacrifice in semiconductor resistivity. The best result of sidewall contact resistivity is 3.7±0.01[Omega] x [mu]m2 at 400 °C annealing, which is about 1.9 times improvement over the non-annealed value, 6.9±0.05 [Omega] x [mu]m2 but still about 5.4 times larger than the reported top contact resistivity of 0.69±0.3 [Omega] x [mu]m2.by Dongsung ChoiS.M. in Electrical Engineerin

Phase Retrieval of High-Power Gyrotron Orbital Angular Momentum Beams

We present an effective phase retrieval technique for determining the vortex charge of a high-power orbital angular momentum (OAM) beam based on phase coefficients optimization. The high-power OAM beam is generated by transmitting a 25 kW, 95 GHz Gaussian like gyrotron beam through a spiral phase plate (SPP). Determination of the vortex charge of the OAM beam is carried out using a modified phase retrieval algorithm. Retrieved intensity profile show 99.27% intensity regeneration at measurement planes for the high-power OAM beam