Phonon Bloch Oscillations.

One-dimensional models have been instrumental in enhancing our understanding of the vibrational and electronic properties of crystalline as well as aperiodic structures. Here, we show that the classical motion of a one-dimensional chain of atoms coupled through a specific force function that depends on position shows features very similar to the Wannier-Stark problem of a quantum particle under the combined effects of a periodic lattice potential and a constant electric field. Both problems exhibit localized modes and a ladder of equally spaced eigenfrequencies, leading to temporal dynamics characterized by periodic oscillations. These findings apply broadly to a variety of synthetic systems including acoustic metamaterials and functionally graded composites.

Easy Synthetic Access to High-Melting Sulfurated Copolymers and their Self-Assembling Diblock Copolymers from Phenylisothiocyanate and Oxetane.

Although sulfurated polymers promise unique properties, their controlled synthesis, particularly when it comes to complex and functional architectures, remains challenging. Here, we show that the copolymerization of oxetane and phenyl isothiocyanate selectively yields polythioimidocarbonates as a new class of sulfur containing polymers, with narrow molecular weight distributions (Mn=5-80 kg/mol with D≤1.2; Mn,max=124 kg/mol) and high melting points of up to 181 °C. The method tolerates different substituent patterns on both the oxetane and the isothiocyanate. Self-nucleation experiments reveal that π-stacking of phenyl substituents, the presence of unsubstituted polymer backbones, and the kinetically controlled linkage selectivity are key factors in maximising melting points. The increased tolerance to macro-chain transfer agents and the controlled propagation allows the synthesis of double crystalline and amphiphilic diblock copolymers, which can be assembled into micellar- and worm-like structures with amorphous cores in water. In contrast, crystallization driven self-assembly in ethanol gives cylindrical micelles or platelets.

Parametric amplification of optical phonons.

We use coherent midinfrared optical pulses to resonantly excite large-amplitude oscillations of the Si-C stretching mode in silicon carbide. When probing the sample with a second pulse, we observe parametric optical gain at all wavelengths throughout the reststrahlen band. This effect reflects the amplification of light by phonon-mediated four-wave mixing and, by extension, of optical-phonon fluctuations. Density functional theory calculations clarify aspects of the microscopic mechanism for this phenomenon. The high-frequency dielectric permittivity and the phonon oscillator strength depend quadratically on the lattice coordinate; they oscillate at twice the frequency of the optical field and provide a parametric drive for the lattice mode. Parametric gain in phononic four-wave mixing is a generic mechanism that can be extended to all polar modes of solids, as a means to control the kinetics of phase transitions, to amplify many-body interactions or to control phonon-polariton waves.

A crossover comparative study to assess efficacy of competency based medical education (CBME) and the traditional structured (TS) method in selected competencies of living anatomy of first year MBBS curriculum: A pilot study.

BACKGROUND: Competency based medical education (CBME) is outcome based teaching methodology where a student learns a set of measurable competencies for early clinical exposure. Inspite of ample resources on CBME, there are limited studies on its implementation. This study will try to demonstrate improvement in the performance of students using CBME as a teaching tool over the traditional structured method (TS). METHODS: Forty student volunteers were chosen and divided into two groups. The crossover design exposed the group of students to CBME and TS spread over two periods with a wash out period in between. The intervention group was exposed to selected list of competencies in living Anatomy with feedbacks and formative assessments. The summative assessments were held at the end of each period. RESULTS: The mean scores of CBME and TS in group 1 is 130.625 and 113.65 while in group 2 is 139.425 and 112.075 respectively. The treatment and period effect is significant. Estimate of treatment effect is 22.1625. The average improvement in treatment scores is by 11%. Two tailed paired sample T test reveals significant improvement in the scores post intervention. CONCLUSION: CBME method produces better performance of the students in the competencies of living anatomy.

Dynamical Stability Limit for the Charge Density Wave in K_{0.3}MoO_{3}.

We study the response of the one-dimensional charge density wave in K_{0.3}MoO_{3} to different types of excitation with femtosecond optical pulses. We compare direct excitation of the lattice at midinfrared frequencies with injection of quasiparticles across the low energy charge density wave gap and with charge transfer excitation in the near infrared. For all three cases, we observe a fluence threshold above which the amplitude-mode oscillation frequency is softened and the mode becomes increasingly damped. We show that all the data can be collapsed onto a universal curve in which the melting of the charge density wave occurs abruptly at a critical lattice excursion. These data highlight the existence of a universal stability limit for a charge density wave, reminiscent of the Lindemann criterion for the melting of a crystal lattice.

Real-Time Observation of Phonon-Mediated σ-π Interband Scattering in MgB_{2}.

In systems having an anisotropic electronic structure, such as the layered materials graphite, graphene, and cuprates, impulsive light excitation can coherently stimulate specific bosonic modes, with exotic consequences for the emergent electronic properties. Here we show that the population of E_{2g} phonons in the multiband superconductor MgB_{2} can be selectively enhanced by femtosecond laser pulses, leading to a transient control of the number of carriers in the σ-electronic subsystem. The nonequilibrium evolution of the material optical constants is followed in the spectral region sensitive to both the a- and c-axis plasma frequencies and modeled theoretically, revealing the details of the σ-π interband scattering mechanism in MgB_{2}.

Physiological adaptations to chronic stress in healthy humans - why might the sexes have evolved different energy utilisation strategies?

KEY POINTS: The human stress response activates the autonomic nervous system and endocrine systems to increase performance during environmental challenges. This response is usually beneficial, improving the chance of overcoming environmental challenges, but costs resources such as energy. Humans and other animals are known to adapt their responses to acute stress when they are stimulated chronically, presumably to optimise resource utilisation. Characterisation of these adaptations has been limited. Using advanced imaging techniques, we show that cardiovascular and endocrine physiology, reflective of energy utilisation during acute stress, and energy storage (fat) differ between the sexes when they are exposed to chronic stress. We examine possible evolutionary explanations for these differences, related to energy use, and point out how these physiological differences could underpin known disparities between the sexes in their risk of important cardiometabolic disorders such as obesity and cardiovascular disease. ABSTRACT: Obesity and associated diseases, such as cardiovascular disease, are the dominant human health problems in the modern era. Humans develop these conditions partly because they consume excess energy and exercise too little. Stress might be one of the factors contributing to these disease-promoting behaviours. We postulate that sex-specific primordial energy optimisation strategies exist, which developed to help cope with chronic stress but have become maladaptive in modern societies, worsening health. To demonstrate the existence of these energy optimisation strategies, we recruited 88 healthy adults with varying adiposity and chronic stress exposure. Cardiovascular physiology at rest and during acute stress (Montreal Imaging Stress Task), and body fat distribution were measured using advanced magnetic resonance imaging methods, together with endocrine function, cardiovascular energy use and cognitive performance. Potential confounders such as lifestyle, social class and employment were accounted for. We found that women exposed to chronic stress had lower adiposity, greater acute stress cardiovascular responses and better cognitive performance. Conversely, chronic stress-exposed men had greater adiposity and lower cardiovascular responses to acute stress. These results provide initial support for our hypothesis that differing sex-specific energy conservation strategies exist. We propose that these strategies have initially evolved to benefit humans but are now maladaptive and increase the risk of disorders such as obesity, especially in men exposed to chronic stress.

Photonic crystals as topological high-Q resonators.

It is well known that defects, such as holes, inside an infinite photonic crystal can sustain localized resonant modes whose frequencies fall within a forbidden band. Here we prove that finite, defect-free photonic crystals behave as mirrorless resonant cavities for frequencies within but near the edges of an allowed band, regardless of the shape of their outer boundary. The resonant modes are extended, surface-avoiding (nearly-Dirichlet) states that may lie inside or outside the light cone. Independent of the dimensionality, quality factors and finesses are on the order of, respectively, (L/λ)3 and L/λ, where λ is the vacuum wavelength and L >> λ is a typical size of the crystal. Similar topological modes exist in conventional Fabry-Pérot resonators, and in plasmonic media at frequencies just above those at which the refractive index vanishes.

Observation of standing waves of electron-hole sound in a photoexcited semiconductor.

Three-dimensional multicomponent plasmas composed of species with very different masses support a new branch of charge-density fluctuations known as acoustic plasmons. Here, we report on an ultrafast optical method to generate and probe coherent states of acoustic plasmons in a slab of GaAs, which relies on strong photoexcitation to create a large population of light electrons and heavy holes. Consistent with the random-phase-approximation theory, the data reveal standing plasma waves confined to these slabs, similar to those of conventional sound but with associated velocities that are significantly larger.

Improved access to polythioesters by heterobimetallic aluminium catalysis.

Bimetallic Al(III) catalysis mediates thioanhydride/epoxide copolymerisation at greatly improved rates and monomer tolerance than analogous Cr(III) catalysis. Moving to sulfurated monomers furthermore generally improves rates and selectivites.

Fluoride recovery in degradable fluorinated polyesters.

We report a new class of degradable fluorinated polymers through the copolymerization of tetrafluorophthalic anhydride and propylene oxide or trifluoropropylene oxide which show up to 20 times quicker degradation than the non-fluorinated equivalents and allow for fluoride recovery.

Optical probing of ultrafast electronic decay in Bi and Sb with slow phonons.

Illumination with laser sources leads to the creation of excited electronic states of particular symmetries, which can drive isosymmetric vibrations. Here, we use a combination of ultrafast stimulated and cw spontaneous Raman scattering to determine the lifetime of A(1g) and E(g) electronic coherences in Bi and Sb. Our results both shed new light on the mechanisms of coherent phonon generation and represent a novel way to probe extremely fast electron decoherence rates. The E(g) state, resulting from an unequal distribution of carriers in three equivalent band regions, is extremely short lived. Consistent with theory, the lifetime of its associated driving force reaches values as small as 2 (6) fs for Bi (Sb) at 300 K.

Habitual alcohol consumption is associated with lower cardiovascular stress responses--a novel explanation for the known cardiovascular benefits of alcohol?

In contrast to heavy alcohol consumption, which is harmful, light to moderate drinking has been linked to reduced cardiovascular morbidity and mortality. Effects on lipid status or clotting do not fully explain these benefits. Exaggerated cardiovascular responses to mental stress are detrimental to cardiovascular health. We hypothesized that habitual alcohol consumption might reduce these responses, with potential benefits. Advanced magnetic resonance techniques were used to accurately measure cardiovascular responses to an acute mental stressor (Montreal Imaging Stress Task) in 88 healthy adults (∼1:1 male:female). Salivary cortisol and task performance measures were used to assess endocrine and cognitive responses. Habitual alcohol consumption and confounding factors were assessed by questionnaire. Alcohol consumption was inversely related to responses of heart rate (HR) (r = -0.31, p = 0.01), cardiac output (CO) (r = -0.32, p = 0.01), vascular resistance (r = 0.25, p = 0.04) and mean blood pressure (r = -0.31, p = 0.01) provoked by stress, but not to stroke volume (SV), or arterial compliance changes. However, high alcohol consumers had greater cortisol stress responses, compared to moderate consumers (3.5 versus 0.7 nmol/L, p = 0.04). Cognitive measures did not differ. Findings were not explained by variations in age, sex, social class, ethnicity, physical activity, adrenocortical activity, adiposity, smoking, menstrual phase and chronic stress. Habitual alcohol consumption is associated with reduced cardiac responsiveness during mental stress, which has been linked to lower risk of hypertension and vascular disease. Consistent with established evidence, our findings suggest a mechanism by which moderate alcohol consumption might reduce cardiovascular disease, but not high consumption, where effects such as greater cortisol stress responses may negate any benefits.

Exactly solvable toy model of autocatalysis: Irreversible relaxation after a quantum quench.

A resolvable quantum many-body Hamiltonian is introduced that mimics the behavior of the autocatalytic chemical reaction A+B⇄2B involving two different molecular species, A and B. The model also describes two nonlinearly coupled modes of an optical cavity. Consistent with the current understanding of the relaxation dynamics of integrable systems in isolation, the wave function following a quantum quench exhibits irreversibility with retention of the memory about its initial conditions. Salient features of the model include a marked similarity with conventional quantum decay and a total B-to-A conversion, with associated classical-like behavior of the wave function, when the initial state does not contain A-type molecules.

Precise construction of weather-sensitive poly(ester-alt-thioesters) from phthalic thioanhydride and oxetane.

We report the selective ring opening copolymerisation (ROCOP) of oxetane and phthalic thioanhydride by a heterobimetallic Cr(III)K catalyst precisely yielding semi-crystalline alternating poly(ester-alt-thioesters) which show improved degradability due to the thioester links in the polymer backbone.

Precise cooperative sulfur placement leads to semi-crystallinity and selective depolymerisability in CS2/oxetane copolymers.

CS2 promises easy access to degradable sulfur-rich polymers and insights into how main-group derivatisation affects polymer formation and properties, though its ring-opening copolymerisation is plagued by low linkage selectivity and small-molecule by-products. We demonstrate that a cooperative Cr(III)/K catalyst selectively delivers poly(dithiocarbonates) from CS2 and oxetanes while state-of-the-art strategies produce linkage scrambled polymers and heterocyclic by-products. The formal introduction of sulfur centres into the parent polycarbonates results in a net shift of the polymerisation equilibrium towards, and therefore facilitating, depolymerisation. During copolymerisation however, the catalyst enables near quantitative generation of the metastable polymers in high sequence selectivity by limiting the lifetime of alkoxide intermediates. Furthermore, linkage selectivity is key to obtain semi-crystalline materials that can be moulded into self-standing objects as well as to enable chemoselective depolymerisation into cyclic dithiocarbonates which can themselves serve as monomers in ring-opening polymerisation. Our report demonstrates the potential of cooperative catalysis to produce previously inaccessible main-group rich materials with beneficial chemical and physical properties.

Molecular docking analysis of syringic acid with proteins in inflammatory cascade.

Syringic Acid (SA) is a dimethoxybenzene derived from plants. Dietary SA possesses anti-obesity, anti-inflammatory and anti-steatotic effects and is of interest as a potential therapeutic medication in the treatment of obesity, diabetes, diabetic cataracts and asthma. It has anti-tumorigenic effect against hepatocellular carcinoma, lung carcinoma and oral mucosal carcinoma. It is also believed to have a protective effect on Acetaminophen induced damage in Wistar rats. Therefore, it is of interest to document the molecular docking analysis of syringic acid with proteins in inflammatory cascade such as TNF α, NFκB, P50, P65 and IKB for further consideration in drug discovery.

Observation of photo-induced plasmon-phonon coupling in PbTe via ultrafast x-ray scattering.

We report the observation of photo-induced plasmon-phonon coupled modes in the group IV-VI semiconductor PbTe using ultrafast x-ray diffuse scattering at the Linac Coherent Light Source. We measure the near-zone-center excited-state dispersion of the heavily screened longitudinal optical (LO) phonon branch as extracted from differential changes in x-ray diffuse scattering intensity following above bandgap photoexcitation. We suggest that upon photoexcitation, the LO phonon-plasmon coupled (LOPC) modes themselves become coupled to longitudinal acoustic modes that drive electron band shifts via acoustic deformation potentials and possibly to low-energy single-particle excitations within the plasma and that these couplings give rise to displacement-correlations that oscillate in time with a period given effectively by the heavily screened LOPC frequency.

Cherenkov radiation at speeds below the light threshold: phonon-assisted phase matching.

Charged particles traveling through matter at speeds larger than the phase velocity of light in the medium emit Cherenkov radiation. Calculations reveal that a given angle of the radiation conical wavefront is associated with two velocities, one above and one below a certain speed threshold. Emission at subluminal but not superluminal speeds is predicted and verified experimentally for relativistic dipoles generated with an optical method based on subpicosecond pulses moving in a nonlinear medium. The dipolar Cherenkov field, in the range of infrared-active phonons, is identical to that of phonon polaritons produced by impulsive laser excitation.

Synchrotron radiation from an accelerating light pulse.

Synchrotron radiation-namely, electromagnetic radiation produced by charges moving in a curved path-is regularly generated at large-scale facilities where giga-electron volt electrons move along kilometer-long circular paths. We use a metasurface to bend light and demonstrate synchrotron radiation produced by a subpicosecond pulse, which moves along a circular arc of radius 100 micrometers inside a nonlinear crystal. The emitted radiation, in the terahertz frequency range, results from the nonlinear polarization induced by the pulse. The generation of synchrotron radiation from a pulse revolving about a circular trajectory holds promise for the development of on-chip terahertz sources.