In situ nanoindentation: probing nanoscale multifunctionality

Nanoindentation is the leading technique for evaluating nanoscale mechanical properties of materials. Consistent developments in instrumentation and their capabilities are transforming nanoindentation into a powerful tool for characterization of multifunctionality at the nanoscale. This review outlines the integration of nanoindentation with real-time electron imaging, high temperature measurements, electrical characterization, and a combination of these. In situ nanoindentation measurements have enabled the real-time study of the interplay between mechanical, thermal, and electrical effects at the nanoscale. This review identifies previous reviews in this area, traces developments and pinpoints significant recent advances (post-2007), with emphasis on the applications of in situ nanoindentation techniques to materials systems, and highlighting the new insights gained from these in situ techniques. Based on this review, future directions and applications of in situ nanoindentation are identified, which highlight the potential of this suite of techniques for materials scientists from all disciplines

Distinct neurocomputational mechanisms support informational and socially normative conformity

A change of mind in response to social influence could be driven by informational conformity to increase accuracy, or by normative conformity to comply with social norms such as reciprocity. Disentangling the behavioural, cognitive, and neurobiological underpinnings of informational and normative conformity have proven elusive. Here, participants underwent fMRI while performing a perceptual task that involved both advice-taking and advice-giving to human and computer partners. The concurrent inclusion of 2 different social roles and 2 different social partners revealed distinct behavioural and neural markers for informational and normative conformity. Dorsal anterior cingulate cortex (dACC) BOLD response tracked informational conformity towards both human and computer but tracked normative conformity only when interacting with humans. A network of brain areas (dorsomedial prefrontal cortex (dmPFC) and temporoparietal junction (TPJ)) that tracked normative conformity increased their functional coupling with the dACC when interacting with humans. These findings enable differentiating the neural mechanisms by which different types of conformity shape social changes of mind

Entorhinal and ventromedial prefrontal cortices abstract and generalize the structure of reinforcement learning problems

Knowledge of the structure of a problem, such as relationships between stimuli, enables rapid learning and flexible inference. Humans and other animals can abstract this structural knowledge and generalize it to solve new problems. For example, in spatial reasoning, shortest-path inferences are immediate in new environments. Spatial structural transfer is mediated by cells in entorhinal and (in humans) medial prefrontal cortices, which maintain their co-activation structure across different environments and behavioral states. Here, using fMRI, we show that entorhinal and ventromedial prefrontal cortex (vmPFC) representations perform a much broader role in generalizing the structure of problems. We introduce a task-remapping paradigm, where subjects solve multiple reinforcement learning (RL) problems differing in structural or sensory properties. We show that, as with space, entorhinal representations are preserved across different RL problems only if task structure is preserved. In vmPFC and ventral striatum, representations of prediction error also depend on task structure

High performance CMOS-compatible perovskite oxide memristors: compositional control and nanoscale switching characteristics

Nanoscale memristive devices have been dubbed as one of the main contenders for the next generation nonvolatile memories (NVM) and alternative logic architectures. Passive two-terminal metal-insulator-metal (MIM) memristive crossbar configurations based on functional transition metal-oxides (e.g. TiO2, SrTiO3) offer great potential for ultimate integration in contemporary electronic industry. This thesis focuses on the realization and nanoscale characterization of high performance CMOS-compatible memristive devices utilizing functional perovskite oxides. A PVD based synthesis route for the realization of functional perovskite oxides with control over their composition and structure has been established. Utilizing the synthesis approach, first realization of memristive devices based on oxygen deficient amorphous SrTiO3 (a-STO) oxides has been demonstrated and their resistive switching performance has been studied in detail utilizing micro-scale crossbar MIM arrays and a sophisticated conductive nano-contact technique based on in situ electrical nanoindentation. RF magnetron sputtering has been used in this work to synthesis perovskite oxide thin films on conventional silicon substrates. Firstly, a lead-free ferro/piezoelectric perovskite oxide (KxNa1‑xNbO3) was chosen to study the effects of sputtering parameters and post-deposition treatments on the composition and the structure of sputtered thin films. This study demonstrates that the crystal orientation, thickness and the elemental composition of the thin films sputtered from the same ceramic target can be effectively and reliably controlled via tuning the sputtering parameters (process gas, substrate temperature, etc.) and the oxide structure and secondary phases can be engineered through post-annealing treatments. The same procedure was employed for the synthesis of SrTiO3 thin films as a reliable resistive switching perovskite oxide. A low temperature synthesis of amorphous SrTiO3 (a-STO) thin films with precise control over the thickness, oxygen deficiency and A‑site/B-site dopants has been demonstrated for the first time. The switching characteristics of a-STO cross-point devices suggest the possibility of fine tuning the memristive performance through tailoring the oxide composition and device structure. Outstanding switching performance (high switching ratios, excellent endurance and retention) is demonstrated in oxygen deficient a-STO devices. Also, it is shown that niobium doping through low temperature co-sputtering of Nb: a-STO result in significant improvements in device energy requirements. Furthermore, nanoscale conduction and resistive switching mechanisms of these devices have been studied in detail utilizing a sophisticated in situ electrical nanoindentation technique, capable of forming nano‑contacts with controlled size and mechanical force. To this end, a unique empirical model has been developed that allows for a complete characterization of the electrical properties of the load controlled nano‑contact and therefore yields quantified insights into the conduction and switching mechanisms of a‑STO based memristive device at nanoscale. The results exhibit ultimately scalable and isolatedly controllable switching characteristics in these devices and also suggest the possibility of mechanically modulated nanoscale resistive switching in a‑STO based devices. Overall, this thesis highlights a‑STO based devices as strong candidates for the ongoing development of the alternative memory technologies as well as applications in MEMS/NEMS devices

On the microstructure and mechanical properties of an Fe-10Ni-7Mn martensitic steel processed by high-pressure torsion.

High-pressure torsion (HPT) processing was applied to an Fe-10Ni-7Mn (wt.%) martensitic steel at room temperature and the grain size was reduced from an initial value of ~5.5 μm to an ultrafine value of ~185 nm for the ferritic phase and around 30 nm for the austenitic phase after 20 HPT turns. The microstructure and mechanical properties of the as-processed material were evaluated using X-ray diffraction (XRD), electron backscatter diffraction (EBSD), field emission scanning electron microscopy (FESEM), microhardness measurements and tensile testing. In addition, annealing of an as-processed specimen was analyzed by differential scanning calorimetry (DSC). The results show that HPT processing increases the hardness and ultimate tensile strength to ~690 Hv and ~2230 MPa, respectively, but the ductility is decreased from ~16.5% initially to ~6.4% and ~3.1% after 10 and 20 turns, respectively. The hardness distributions and EBSD images show that a reasonably homogeneous microstructure is formed when applying a sufficient level of pressure and torsional strain. The DSC results demonstrate that processing by HPT reduces the start and finish temperatures of the reverse transformation of martensite to austenite and there is continuous re-crystallization after the recovery process

Causal role of a neural system for separating and selecting multidimensional social cognitive information

People are multi-faceted, typically good at some things but bad at others, and a critical aspect of social judgement is the ability to focus on those traits relevant for the task at hand. However, it remains unknown how the brain supports such context-dependent social judgement. Here, we examine how people represent multidimensional individuals, and how the brain extracts relevant information and filters out irrelevant information when comparing individuals within a specific dimension. Using human fMRI, we identify distinct neural representations in dorsomedial prefrontal cortex (dmPFC) and anterior insula (AI) supporting separation and selection of information for context-dependent social judgement. Causal evaluation using non-invasive brain stimulation shows that AI disruption alters the impact of relevant information on social comparison, whereas dmPFC disruption only affects the impact of irrelevant information. This neural circuit is distinct from the one supporting integration across, as opposed to separation of, different features of a multidimensional cognitive space

Correlations and the Cross Section of Exclusive (e,e′pe,e'p) Reactions for 16^{16}O

The reduced cross section for exclusive ($e,e'p$) reactions has been studied in DWIA for the example of the nucleus $^{16}$O using a spectral function containing effects of correlations. The spectral function is evaluated directly for the finite nucleus starting from a realistic nucleon-nucleon interaction within the framework of the Green's function approach. The emphasis is focused on the correlations induced by excitation modes at low energies described within a model-space of shell-model configurations including states up to the $sdg$ shell. Cross sections for the $p$-wave quasi-hole transitions at low missing energies are presented and compared with the most recent experimental data. In the case of the so-called perpendicular kinematics the reduced cross section derived in DWIA shows an enhancement at high missing momenta as compared to the PWIA result. Furthermore the cross sections for the $s$- and $d$-wave quasi-hole transitions are presented and compared to available data at low missing momenta. Also in these cases, which cannot be described in a model without correlations, a good agreement with the experiment is obtained.Comment: 12 pages, LaTeX, 4 figures include