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Topic Review
Chirality
A chiral phenomenon is one that is not identical to its mirror image (see the article on mathematical chirality). The spin of a particle may be used to define a handedness, or helicity, for that particle, which, in the case of a massless particle, is the same as chirality. A symmetry transformation between the two is called parity transformation. Invariance under parity transformation by a Dirac fermion is called chiral symmetry.
  • 514
  • 27 Oct 2022
Topic Review
Nature of the Bounce in LQC and PQM
We present a review concerning quantum cosmology in the presence of cut-off physics. We analyze how the Wheeler–DeWitt equation describes the quantum Universe dynamics when a pure metric approach is concerned, showing that the primordial singularity is not removed by the quantum effects. We then analyze the main implications of applying Loop Quantum Gravity prescriptions to the minisuperspace model, i.e., we discuss the basic features of Loop Quantum Cosmology. For the isotropic Universe dynamics, we compare the original m0 scheme and the most commonly accepted formulation, i.e. the so-called ¯m scheme. Furthermore, some fundamental results concerning the Bianchi Universes are discussed. Finally, we consider some relevant criticisms about the real link between the full theory of LQG and its minisuperspace implementation, especially with respect to the preservation of the internal SU(2) symmetry. In the second part of the review, we consider the dynamics of the isotropic Universe and of the Bianchi models in the framework of Polymer Quantum Mechanics. We first address the polymerization in termsof the Ashtekar–Barbero–Immirzi connection and show how the resulting dynamics is isomorphic to the m0 scheme of LQC with a critical energy density of the Universe that depends on the initial conditions for the dynamics. Then we analyze the polymerization of volume-like variables, both for the isotropic and Bianchi I models, and we see that if the Universe volume (the cubed scale factor) is one of the configurational variables, then the resulting dynamics is isomorphic to that for the ¯m scheme of LQC, with the critical energy density value being fixed only by fundamental constants and the Immirzi parameter. Finally, we consider the polymer quantum dynamics of the homogeneous and inhomogeneous Mixmaster model by means of a metric approach. In particular, we compare the results obtained by using the volume variable, which leads to the emergence of a singularity-free and chaos-free cosmology, to the use of the standard Misner variable. In the latter case we deal with the surprising result of a cosmology that is still singular, and its chaotic properties depend on the ratio between the lattice steps for the isotropic and anisotropic variables. We conclude the review with some considerations of the problem of changing variables in polymer cosmology. In particular, on a semiclassical level, we consider how the dynamics can be properly mapped in two different sets of variables (at the price of having to deal with a coordinate dependent lattice step), and we infer some possible implications on the equivalence of the m0 and ¯m scheme of LQC.
  • 514
  • 09 Oct 2021
Topic Review
Vibrational Spectroscopy of Linear Molecules
To determine the vibrational spectroscopy of linear molecules, the rotation and vibration of linear molecules are taken into account to predict which vibrational (normal) modes are active in the infrared spectrum and the Raman spectrum.
  • 513
  • 11 Nov 2022
Topic Review
Enchroma
EnChroma lenses are glasses designed to improve and modify some aspects of color vision deficiency for color blind people. The glasses were invented by Dr. Donald McPherson in 2002. Wearing the glasses results in subtle differences when color blind people look longer and more carefully.
  • 513
  • 17 Nov 2022
Topic Review
Udwadia–Kalaba Equation
In theoretical physics, the Udwadia–Kalaba equation is a method for deriving the equations of motion of a constrained mechanical system. The equation was first described by Firdaus E. Udwadia and Robert E. Kalaba in 1992. The approach is based on Gauss's principle of least constraint. The Udwadia–Kalaba equation applies to both holonomic constraints and nonholonomic constraints, as long as they are linear with respect to the accelerations. The equation generalizes to constraint forces that do not obey D'Alembert's principle.
  • 513
  • 14 Oct 2022
Topic Review
Mid-Infrared External Cavity Quantum Cascade Lasers
External cavity quantum cascade lasers (ECQCLs) in the mid-infrared band have a series of unique spectral properties, which can be widely used in spectroscopy, gas detection, protein detection, medical diagnosis, free space optical communication, and so on, especially wide tuning range, the tuning range up to hundreds of wavenumbers; therefore, ECQCLs show great applications potential in many fields. 
  • 512
  • 29 Nov 2022
Topic Review
Vascular Endothelial Dysfunction-Related Disease
Oxidative stress and chronic inflammation play an important role in the pathogenesis of atherosclerosis. Atherosclerosis develops as the first step of vascular endothelial dysfunction induced by complex molecular mechanisms. Vascular endothelial dysfunction leads to oxidative stress and inflammation of vessel walls, which in turn enhances vascular endothelial dysfunction. Vascular endothelial dysfunction and vascular wall oxidative stress and chronic inflammation make a vicious cycle that leads to the development of atherosclerosis.
  • 512
  • 03 Feb 2023
Topic Review
Types of Liquid Crystal Biosensors
Interactions between liquid crystal molecules and target analytes open up various biosensing applications for quick screening and point-of-care applications. There exist different kinds of biosensors, such as electrochemical, wearable, amperometric, potentiometric, optical, impedimetric and thermometric. Liquid crystal (LC) biosensors belong to the type of optical biosensors, which employ its anisotropy and sensitivity to external stimuli. As it is known, LCs are highly sensitive to environment changes, e.g., temperature, electric fields and surface interactions. The inherent property of LCs to respond to external stimuli gives LC biosensors a high potential to contribute to the new biosensing era. 
  • 511
  • 02 Jan 2024
Topic Review
Orion
Orion, one of the most recognizable constellations in the night sky, is steeped in mythology and astronomical significance. Named after the legendary hunter from Greek mythology, Orion is adorned with bright stars, including the famous Orion's Belt, and hosts a plethora of celestial wonders, such as the Orion Nebula, making it a favorite among stargazers.
  • 511
  • 15 Mar 2024
Topic Review
Surface-Extended X-Ray Absorption Fine Structure
Surface-extended X-ray absorption fine structure (SEXAFS) is the surface-sensitive equivalent of the EXAFS technique. This technique involves the illumination of the sample by high-intensity X-ray beams from a synchrotron and monitoring their photoabsorption by detecting in the intensity of Auger electrons as a function of the incident photon energy. Surface sensitivity is achieved by the interpretation of data depending on the intensity of the Auger electrons (which have an escape depth of ~1–2 nm) instead of looking at the relative absorption of the X-rays as in the parent method, EXAFS. The photon energies are tuned through the characteristic energy for the onset of core level excitation for surface atoms. The core holes thus created can then be filled by nonradiative decay of a higher-lying electron and communication of energy to yet another electron, which can then escape from the surface (Auger emission). The photoabsorption can therefore be monitored by direct detection of these Auger electrons to the total photoelectron yield. The absorption coefficient versus incident photon energy contains oscillations which are due to the interference of the backscattered Auger electrons with the outward propagating waves. The period of this oscillations depends on the type of the backscattering atom and its distance from the central atom. Thus, this technique enables the investigation of interatomic distances for adsorbates and their coordination chemistry. This technique benefits from long range order not being required, which sometimes becomes a limitation in the other conventional techniques like LEED (about 10 nm). This method also largely eliminates the background from the signal. It also benefits because it can probe different species in the sample by just tuning the X-ray photon energy to the absorption edge of that species. Joachim Stöhr played a major role in the initial development of this technique.
  • 510
  • 24 Oct 2022
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