Tunable orbital susceptibility in α\alpha-T3{\cal T}_3 tight-binding models

We study the importance of interband effects on the orbital susceptibility of three bands $\alpha$-${\cal T}_3$ tight-binding models. The particularity of these models is that the coupling between the three energy bands (which is encoded in the wavefunctions properties) can be tuned (by a parameter $\alpha$) without any modification of the energy spectrum. Using the gauge-invariant perturbative formalism that we have recently developped, we obtain a generic formula of the orbital susceptibility of $\alpha$-${\cal T}_3$ tight-binding models. Considering then three characteristic examples that exhibit either Dirac, semi-Dirac or quadratic band touching, we show that by varying the parameter $\alpha$ and thus the wavefunctions interband couplings, it is possible to drive a transition from a diamagnetic to a paramagnetic peak of the orbital susceptibility at the band touching. In the presence of a gap separating the dispersive bands, we show that the susceptibility inside the gap exhibits a similar dia to paramagnetic transition.Comment: 15 pages,5 figs. Proceedings of the International Workshop on Dirac Electrons in Solids 2015Proceedings of the International Workshop on Dirac Electrons in Solids 201

Positive and negative well-being and objectively measured sedentary behaviour in older adults: evidence from three cohorts

Background: Sedentary behaviour is related to poorer health independently of time spent in moderate to vigorous physical activity. The aim of this study was to investigate whether wellbeing or symptoms of anxiety or depression predict sedentary behaviour in older adults. Method: Participants were drawn from the Lothian Birth Cohort 1936 (LBC1936) (n = 271), and the West of Scotland Twenty-07 1950s (n = 309) and 1930s (n = 118) cohorts. Sedentary outcomes, sedentary time, and number of sit-to-stand transitions, were measured with a three-dimensional accelerometer (activPAL activity monitor) worn for 7 days. In the Twenty-07 cohorts, symptoms of anxiety and depression were assessed in 2008 and sedentary outcomes were assessed ~ 8 years later in 2015 and 2016. In the LBC1936 cohort, wellbeing and symptoms of anxiety and depression were assessed concurrently with sedentary behaviour in 2015 and 2016. We tested for an association between wellbeing, anxiety or depression and the sedentary outcomes using multivariate regression analysis. Results: We observed no association between wellbeing or symptoms of anxiety and the sedentary outcomes. Symptoms of depression were positively associated with sedentary time in the LBC1936 and Twenty-07 1950s cohort, and negatively associated with number of sit-to-stand transitions in the LBC1936. Meta-analytic estimates of the association between depressive symptoms and sedentary time or number of sit-to-stand transitions, adjusted for age, sex, BMI, long-standing illness, and education, were β = 0.11 (95% CI = 0.03, 0.18) and β = − 0.11 (95% CI = − 0.19, −0.03) respectively. Conclusion: Our findings indicate that depressive symptoms are positively associated with sedentary behavior. Future studies should investigate the causal direction of this association

Enhancement of Ti3C2 MXene Pseudocapacitance after Urea Intercalation Studied by Soft X ray Absorption Spectroscopy

MXenes have shown outstanding properties due to their highly active hydrophilic surfaces coupled with high metallic conductivity. Many applications rely on the intercalation between Ti3C2Tx Tx describes the OH, F and O surface terminations flakes by ions or molecules, which in turn might alter the Ti3C2Tx surface chemistry and electrochemical properties. In this work, we show that the capacitance, rate capability, and charge carrier kinetics in Ti3C2Tx MXene electrodes are remarkably enhanced after urea intercalation u Ti3C2Tx . In particular, the areal capacitance increased to 1100 mF cm2, which is 56 higher than that of pristine Ti3C2Tx electrodes. We attribute this dramatic improvement to changes in the Ti3C2Tx surface chemistry upon urea intercalation. The oxidation state and the oxygen bonding of individual Ti3C2Tx flakes before and after urea intercalation are probed by soft X ray absorption spectroscopy XAS at the Ti L and O K edges with 30 nm spatial resolution in vacuum. After urea intercalation, a higher Ti oxidation state is observed across the entire flake compared to pristine Ti3C2Tx. Additionally, in situ XAS of u Ti3C2Tx aqueous dispersions reveal a higher Ti oxidation similar to dry samples, while for pristine Ti3C2Tx the Ti atoms are significantly reduced in water compared to dry sample

An optoelectronic framework enabled by low-dimensional phase-change films.

Accepted author version. The definitive version was published in: Nature 511, 206–211 (10 July 2014) doi:10.1038/nature13487The development of materials whose refractive index can be optically transformed as desired, such as chalcogenide-based phase-change materials, has revolutionized the media and data storage industries by providing inexpensive, high-speed, portable and reliable platforms able to store vast quantities of data. Phase-change materials switch between two solid states--amorphous and crystalline--in response to a stimulus, such as heat, with an associated change in the physical properties of the material, including optical absorption, electrical conductance and Young's modulus. The initial applications of these materials (particularly the germanium antimony tellurium alloy Ge2Sb2Te5) exploited the reversible change in their optical properties in rewritable optical data storage technologies. More recently, the change in their electrical conductivity has also been extensively studied in the development of non-volatile phase-change memories. Here we show that by combining the optical and electronic property modulation of such materials, display and data visualization applications that go beyond data storage can be created. Using extremely thin phase-change materials and transparent conductors, we demonstrate electrically induced stable colour changes in both reflective and semi-transparent modes. Further, we show how a pixelated approach can be used in displays on both rigid and flexible films. This optoelectronic framework using low-dimensional phase-change materials has many likely applications, such as ultrafast, entirely solid-state displays with nanometre-scale pixels, semi-transparent 'smart' glasses, 'smart' contact lenses and artificial retina devices.Engineering and Physical Sciences Research Council (EPSRC)OUP John Fell Fun

Enabling Universal Memory by Overcoming the Contradictory Speed and Stability Nature of Phase-Change Materials

The quest for universal memory is driving the rapid development of memories with superior all-round capabilities in non-volatility, high speed, high endurance and low power. Phase-change materials are highly promising in this respect. However, their contradictory speed and stability properties present a key challenge towards this ambition. We reveal that as the device size decreases, the phase-change mechanism changes from the material inherent crystallization mechanism (either nucleation- or growth-dominated), to the hetero-crystallization mechanism, which resulted in a significant increase in PCRAM speeds. Reducing the grain size can further increase the speed of phase-change. Such grain size effect on speed becomes increasingly significant at smaller device sizes. Together with the nano-thermal and electrical effects, fast phase-change, good stability and high endurance can be achieved. These findings lead to a feasible solution to achieve a universal memory

Fundamental Mechanisms Driving the Amorphous to Crystalline Phase Transformation

Phase transformations are ubiquitous, fundamental phenomena that lie at the heart of many structural, optical and electronic properties in condensed matter physics and materials science. Many transformations, especially those occurring under extreme conditions such as rapid changes in the thermodynamic state, are controlled by poorly understood processes involving the nucleation and quenching of metastable phases. Typically these processes occur on time and length scales invisible to most experimental techniques ({micro}s and faster, nm and smaller), so our understanding of the dynamics tends to be very limited and indirect, often relying on simulations combined with experimental study of the ''time infinity'' end state. Experimental techniques that can directly probe phase transformations on their proper time and length scales are therefore key to providing fundamental insights into the whole area of transformation physics and materials science. LLNL possesses a unique dynamic transmission electron microscope (DTEM) capable of taking images and diffraction patterns of laser-driven material processes with resolution measured in nanometers and nanoseconds. The DTEM has previously used time-resolved diffraction patterns to quantitatively study phase transformations that are orders of magnitude too fast for conventional in situ TEM. More recently the microscope has demonstrated the ability to directly image a reaction front moving at {approx}13 nm/ns and the nucleation of a new phase behind that front. Certain compound semiconductor phase change materials, such as Ge{sub 2}Sb{sub 2}Te{sub 5} (GST), Sb{sub 2}Te and GeSb, exhibit a technologically important series of transformations on scales that fall neatly into the performance specifications of the DTEM. If a small portion of such material is heated above its melting point and then rapidly cooled, it quenches into an amorphous state. Heating again with a less intense pulse leads to recrystallization into a vacancy-stabilized metastable rock salt structure. Each transformation takes {approx}10-100 ns, and the cycle can be driven repeatedly a very large number of times with a nanosecond laser such as the DTEM's sample drive laser. These materials are widely used in optical storage devices such as rewritable CDs and DVDs, and they are also applied in a novel solid state memory technology - phase change memory (PCM). PCM has the potential to produce nonvolatile memory systems with high speed, extreme density, and very low power requirements. For PCM applications several materials properties are of great importance: the resistivities of both phases, the crystallization temperature, the melting point, the crystallization speed, reversibility (number of phase-transformation cycles without degradation) and stability against crystallization at elevated temperature. For a viable technology, all these properties need to have good scaling behavior, as dimensions of the memory cells will shrink with every generation. In this LDRD project, we used the unique single-shot nanosecond in situ experimentation capabilities of the DTEM to watch these transformations in GST on the time and length scales most relevant for device applications. Interpretation of the results was performed in conjunction with atomistic and finite-element computations. Samples were provided by collaborators at IBM and Stanford University. We observed, and measured the kinetics of, the amorphous-crystalline and melting-solidification transitions in uniform thin-film samples. Above a certain threshold, the crystal nucleation rate was found to be enormously high (with many nuclei appearing per cubic {micro}m even after nanosecond-scale incubation times), in agreement with atomistic simulation and consistent with an extremely low nucleation barrier. We developed data reduction techniques based on principal component analysis (PCA), revealing the complex, multi-dimensional evolution of the material while suppressing noise and irrelevant information. Using a novel specimen geometry, we also achieved repeated switching between the amorphous and crystalline phases enabling in situ study of structural change after phase cycling, which is relevant to device performance. We also observed the coupling between the phase transformations and the evolution of morphology on the nanometer scale, revealing the gradual development of striations in uniform films and preferential melting at sharp edges in laser-heated nanopatterned GST. This nonuniform melting, interpreted through simulation as being a direct result of geometrical laser absorption effects, appears to be responsible for a marked loss in morphological stability even at moderate laser intensities and may be an important factor in the longevity of nanostructured phase change materials in memory applications

Fermi velocity engineering in graphene by substrate modification

The Fermi velocity is one of the key concepts in the study of a material, as it bears information on a variety of fundamental properties. Upon increasing demand on the device applications, graphene is viewed as a prototypical system for engineering Fermi velocity. Indeed, several efforts have succeeded in modifying Fermi velocity by varying charge carrier concentration. Here we present a powerful but simple new way to engineer Fermi velocity while holding the charge carrier concentration constant. We find that when the environment embedding graphene is modified, the Fermi velocity of graphene is (i) inversely proportional to its dielectric constant, reaching ~2.5$\times10^6$ m/s, the highest value for graphene on any substrate studied so far and (ii) clearly distinguished from an ordinary Fermi liquid. The method demonstrated here provides a new route toward Fermi velocity engineering in a variety of two-dimensional electron systems including topological insulators.Comment: accepted in Scientific Report

Structural study of an amorphous NiZr2 alloy by anomalous wide angle X-ray scattering and Reverse Monte Carlo simulations

The local atomic structure of an amorphous NiZr2 alloy was investigated using the anomalous wide-angle x-ray scattering (AWAXS), differential anomalous scattering (DAS) and reverse Monte Carlo (RMC) simulations techniques. The AWAXS measurements were performed at eight different incident photon energies, including some close to the Ni and Zr K edges. From the measurements eight total structure factor S(K,E) were derived. Using the AWAXS data four differential structure factors DSFi(K,Em,En) were derived, two about the Ni and Zr edges. The partial structure factors SNi-Ni(K), SNi-Zr(K) and SZr-Zr(K) were estimated by using two different methods. First, the S(K,E) and DSFi(K,Em,En) factors were combined and used in a matrix inversion process. Second, three S(K,E) factors were used as input data in the RMC technique. The coordination numbers and interatomic distances for the first neighbors extracted from the partial structure factors obtained by these two methods show a good agreement. By using the three-dimensional structure derived from the RMC simulations, the bond-angle distributions were calculated and they suggest the presence of distorted triangular-faced polyhedral units in the amorphous NiZr2 structure. We have used the Warren chemical short-range order parameter to evaluate the chemical short-range order for the amorphous NiZr2 alloy and for the NiZr2 compound. The calculated values show that the chemical short-range order found in these two materials is similar to that found in a solid solution.Comment: Submitted to Phys. Rev. B, 8 figure