Dislocation

The study was done on effects of temperature, Langevin friction and external shear stress on the rates for dislocation to overcome the energy barrier interactions in FCC copper.

They used Kramers theory equations.(Kramers Rate Equation helps predict how dislocations (line defects) move through a crystal, which directly affects the strength, ductility, and failure of materials)


Langevin friction (gamma Y) refers to the damping force, or resistance to motion, that a particle experiences when interacting with a surrounding medium. Higher Langevin friction makes it slower for dislocations to move. It Does Not Change the Energy Barrier Itself.


At higher temperatures, the dislocation motion doesn't follow the usual straight-line pattern with temp which happens in Arrhenius behavior.


The non-Arrhenius behavior is stronger when the shear stress gets to near  τ cross which forces the enthalpy barrier to decrease. 


Source:

https://arxiv.org/html/2504.21246v1


Defects in Quartz

When radiation hits quartz, it excites electrons. electrons jump from the valence band (low energy) to the conduction band (high energy). This leaves behind holes in the valence band. The free electrons and holes can then move through the crystal.

If a defect (like an oxygen vacancy or an iron impurity) is present, it creates localized energy levels inside the wide bandgap. These levels can trap electrons or holes. The electron is trapped at an energy level lower than the conduction band.


Quartz is used in luminescence dating and ESR (Electron Spin Resonance) dosimetry, methods that measure radiation doses or date archaeological/geological samples. These applications depend on defects in the quartz lattice, which act as traps for charge carriers (electrons and holes) after exposure to radiation.


It is found that peroxy defects( two oxygen atoms bond directly to each other) can be formed in the presence of either excess Oxygen or due to the absence of Silicon (Si4+).


Oxygen vacancies and oxygen interstitials (excess oxygen) create trap states -they are responsible for electron and hole trapping, important for luminescence and ESR.


Passivated oxygen vacancies (with H or OH) do not create trap states- they neutralize defects.


https://arxiv.org/pdf/2504.18077


Wavefront Sensor

  

By using a convolutional neural network (CNN) researchers develop a method to calculate the Fried parameter with high accuracy using just one frame from a wavefront sensor (WFS), either Shack-Hartmann or Pyramid type. 

Shack-Hartmann Wavefront Sensor (SH-WFS) is used in Adaptive optics systems, to capture the incoming wavefront and convert it into an intensity image. SH-WFS consists of an array of micro-lenses, each focusing a small portion of the wavefront onto a spot on an image sensor. 

The displacement of these spots from the centre of each sub-aperture is proportional to the local wavefront gradient.

In Pyramid wavefront sensor (Py-WFS), a pyramid-shaped optical prism placed at the focal plane to split the incoming wavefront into four separate images, each sampling a different quadrant of the focal plane. 

Fried parameter, r0, measures the strength of atmospheric turbulence. Cn^2⁢(h) is atmospheric refractive index structure constant at h height.

Source :https://arxiv.org/html/2504.17029v1


Speckle pattern

As the excitation light propagates through biological tissue it undergoes multiple scattering events. These scattering processes distort the original wavefront of the light and  its propagation paths is randomized, so interference pattern speckle pattern generates.

When the excitation beam is tilted by a small angle, the speckle pattern at the object plane remains unchanged, this is called memory effect.


In deep tissue imaging laser scanning microscopy is ineffective. Here the point spread function (PSF) is used having non linear behavior with fluorescence intensity.


Source:

https://arxiv.org/html/2504.10423v1#Sx1.F2



Comets’ brightness


Brightening is due to a combination of observing geometry, as the comet approaches both the Sun and the Earth, and increasing back-scattering cross-section.


Total Magnitude(brightness) of comet:

 M1 absolute magnitude of the comet (a baseline brightness value).

Δ: geocentric distance (distance between the comet and Earth, inAU).

r: The heliocentric distance (distance between the comet and the Sun, in AU).

K1: brightness slop


Dynamically new comets have a lower heliocentric brightening slope (kr), meaning they brighten at a slow rate.

Higher kr means the comet brightens more rapidly as it goes near the sun. Older comets tend to have more scattered kr values due to surface evolution and patchy mantles of refractory material.


New dynamically young comets often experience strong post-perihelion fading because they have more pristine, volatile-rich surfaces.


New comets appear intrinsically brighter than old comets because they -contain pristine, highly volatile ices that sublimate easily, have higher surface activity with a larger fraction of the nucleus actively.


The dynamically new comets produce more CO2 than CO, while dynamically old comets appear to be more CO-dominant.


source:

https://arxiv.org/html/2504.00565v1


Meteoroid in earth atmosphere:

Fragmentation, rather than ablation, is the dominant mechanism of mass loss. Here Rayleigh–Taylor (Hydrodynamic )instability occurs at the interface between two fluids of different densities, lighter fluid is pushing the heavier fluid.

Meteoroid Trajectory equation:

Cd is the drag coefficient of meteoroid, ρ is atmospheric density, A is the cross-sectional area, m is the mass of the impactor, g(z) gravitational acceleration


Mass ablation: It  is the process by which an object moving through an atmosphere loses material due to intense heat and pressure.

.

ξ is the energy required to heat the surface, Ch heat transfer coefficient.


Fragmentation may occur either when the stagnation pressure of the atmospheric flow exceeds the material’s compressive strength.


source:

https://arxiv.org/html/2503.22632v1




Babinet's Principle for non linear optics


Babinet’s principle states that if you have an opaque object and its complementary aperture, they should produce identical diffraction patterns when light passes through them. Ex. A thin metal rod blocking light should create the same diffraction pattern as a slit of the same shape cut into a metal sheet.


Babinet’s principle does not hold for nonlinear optics.


super-resolution effect:

In linear optics, only Fourier components inside the fundamental resolution limit (2π/λ) contribute to the far-field pattern. This means that small details in the near-field do not appear in the far-field image.


In nonlinear optics,  The third-harmonic wave has a much shorter wavelength (λ/3), so the far-field image contains Fourier components up to 3 times and allows finer details visible.


Eddy current in slits:

In the rod, the polarization is mostly cosine-shaped, aligned with the electric field.

The Third Harmonic Generation process is highly localized and depends on the third power of the electric field.


The eddy currents in slits create extra localized hot spots, which do not appear in rods.

These extra hot spots cause the THG pattern of slits and rods to be completely different, violating Babinet’s principle.


Source: 

https://arxiv.org/html/2503.14773v1


Study of birefringence in shear thinning fluid


Birefringence is an optical property of materials where light splits into two different rays, each traveling at a different speed and direction when passing through the material.

Phase retardation (∆) refers to the difference in the phase of light waves as they pass through a birefringent material (like a flowing fluid with aligned molecules). 

∆ is Phase retardation, C₁: First-order stress-optic coefficient, σxx, σyy: Stress components in x and y directions


The second-order stress-optic law is needed to accurately predict birefringence in thin fluid flows. Shear-thinning affects birefringence, mainly due to changes in second order Stress optics coefficient c2 rather than just viscosity.

As shear rate increases, C₂ decreases and phase retardation increases. The phase retardation decreases in the radial direction and increases with increasing flow rate.


source:

https://arxiv.org/html/2503.10261v1


Symmetry change in Magnetite

Below the Verwey transition temperature (~110K in this experiment), Magnetite Fe3O4 structure distorts into a monoclinic shape, where one of the angles slightly deviates from 90° (β ≈ 90.23°). This distortion introduces a shear strain. 

The change is accompanied by charge ordering of the Fe2+ and Fe3+ ions to form “trimerons”, valence-ordered Fe3+-Fe2+-Fe3+ linear structures, which confine the electrons and lead to an insulating behavior at low temperatures.


The transition involves both electronic localization and a significant lattice distortion, evidenced by a change in symmetry and unit cell dimensions.


Below the Verwey transition, the strain distribution becomes increasingly fragmented, which indicates localized stress concentrations due to internal layered phase variations. Here higher temperatures allow for stress relaxation, while lower temperatures promote strain localization.


Bragg Coherent X-ray Diffraction Imaging method was used here.


Source:

https://arxiv.org/html/2503.10417v1


two superconducting gaps observed for CsV₃Sb₅

The two superconducting gaps have been observed for CsV₃Sb₅ material. 

A charge density wave (CDW) is a state where the electron density in a material forms a periodic pattern instead of being uniform. This happens because of electron-phonon interactions, where electrons couple with vibrations in the crystal lattice.


As per CDW in CsV₃Sb₅, electron density and lattice distortions repeat every two unit cells in different directions. 


Ginzburg-Landau Theory, Tinkham Model were used here.


In a conventional superconductor, the energy gap is uniform, meaning all Cooper pairs have the same energy. In a nodal superconductor, the superconducting gap has zero-energy points (nodes) where quasiparticles can exist even in the superconducting state.


The charge order changes the natural symmetry of the kagome lattice from having six-fold rotational symmetry to only two-fold symmetry. When a magnetic field is rotated within the plane of the material, the upper critical field also follows a two-fold pattern. The superconducting state itself has become nematic, meaning different along different directions-not uniform.


https://arxiv.org/abs/2411.15333


Chromium based tantalum disulfide properties

Study was done on Cr₁/₃TaS₂ (Cr atoms are inserted into a layered compound TaS₂ (tantalum disulfide)). The Raman Scattering method was used here.

Pressure changes bond lengths and angles and, at the same time, systematically reduces the van der Waals distance and modifies the structural c/a ratio.


Cr₁/₃TaS₂ has a smaller van der Waals gap, making it less change to structural distortions than Fe based material.


When pressure increases Cr₁/₃TaS₂ magnetic properties remain stable but Fe₁/₃TaS₂ magnetic property decreases drastically.


source:

https://link.springer.com/article/10.1038/s41535-025-00734-x


Study of Cesium lead bromide- local polarization

Material CsPbBr₃ (cesium lead bromide) was studied here. It is widely used in solar cells, LEDs.

At high temperatures (>100K), transport shifts to a band-like motion, where charge carriers move smoothly.


At low temperatures (<100K), charge carriers move by hopping transport (jumping between localized spots). spontaneous grain boundaries form inside the material. 


These grain boundaries create regions of local polarization meaning certain parts of the material develop an electric dipole moment. Here Displacement of Pb Ions occurs. This shift causes an imbalance in charge distribution, leading to localized electric fields.


GB⊥ have been observed here. It means the crystal tilts in a direction that is perpendicular to the boundary. Twinning boundaries have lower energy( more stable) when the in-phase octahedral tilting axis is rotated perpendicularly rather than twisted.


Low temperature (<100K) transport can be explained by the Mott’s variable range hopping (VRH) equation. Its conductivity is given by,

P is Local polarization, V is volume, d is displacement of ion


Source: https://arxiv.org/html/2502.20261v1


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