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Related Concept Videos

Colors and Magnetism03:02

Colors and Magnetism

Color in Coordination Complexes
When atoms or molecules absorb light at the proper frequency, their electrons are excited to higher-energy orbitals. For many main group atoms and molecules, the absorbed photons are in the ultraviolet range of the electromagnetic spectrum, which cannot be detected by the human eye. For coordination compounds, the energy difference between the d orbitals often allows photons in the visible range to be absorbed and emitted, which is seen as colors by the human eye.
¹H NMR: Complex Splitting01:13

¹H NMR: Complex Splitting

A proton M that is coupled to a proton X results in doublet signals for M. However, NMR-active nuclei can be simultaneously coupled to more than one nonequivalent nucleus. When M is coupled to a second proton A, such as in styrene oxide, each peak in the doublet is split into another doublet.
Splitting diagrams or splitting tree diagrams are routinely used to depict such complex couplings. While drawing splitting diagrams, the splitting with the larger coupling constant is usually applied first.
¹H NMR: Interpreting Distorted and Overlapping Signals01:02

¹H NMR: Interpreting Distorted and Overlapping Signals

Spin systems where the difference in chemical shifts of the coupled nuclei is greater than ten times J are called first-order spin systems. These nuclei are weakly coupled, and their chemical shifts and coupling constant can generally be estimated from the well-separated signals in the spectrum.
As Δν decreases and the signals move closer, the doublets appear increasingly distorted. The intensities of the inner lines increase at the cost of those of the outer lines as the signals are slanted or...
Spin–Spin Coupling Constant: Overview01:08

Spin–Spin Coupling Constant: Overview

In bromoethane, the three methyl protons are coupled to the two methylene protons that are three bonds away. In accordance with the n+1 rule, the signal from the methyl protons is split into three peaks with 1:2:1 relative intensities. The methylene protons appear as a quartet, with the relative intensities of 1:3:3:1.
Qualitatively, any spin plus-half nucleus polarizes the spins of its electrons to the minus-half state. Consequently, the paired electron in the hydrogen–carbon bond must have a...
IR Spectroscopy: Hooke's Law Approximation of Molecular Vibration01:16

IR Spectroscopy: Hooke's Law Approximation of Molecular Vibration

A covalently bonded heteronuclear diatomic molecule can be modeled as two vibrating masses connected by a spring. The vibrational frequency of the bond can be expressed using an equation derived from Hooke's law, which describes how the force applied to stretch or compress a spring is proportional to the displacement of the spring. In this case, the atoms behave like masses, and the bond acts like a spring.
According to Hooke's law, the vibrational frequency is directly proportional to the...
Atomic Nuclei: Types of Nuclear Relaxation01:28

Atomic Nuclei: Types of Nuclear Relaxation

Nuclear relaxation restores the equilibrium population imbalance and can occur via spin–lattice or spin–spin mechanisms, which are first-order exponential decay processes.
In spin–lattice or longitudinal relaxation, the excited spins exchange energy with the surrounding lattice as they return to the lower energy level. Among several mechanisms that contribute to spin–lattice relaxation, magnetic dipolar interactions are significant. Here, the excited nucleus transfers energy to a nearby...

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Measurement of Coherence Decay in GaMnAs Using Femtosecond Four-wave Mixing
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Published on: December 3, 2013

Phonon dispersion and lifetimes in MgB2.

Abhay Shukla1, Matteo Calandra, Matteo D'Astuto

  • 1Laboratoire de Minéralogie-Cristallographie, case 115, 4 Place Jussieu, 75252, Paris cedex 05, France.

Physical Review Letters
|April 12, 2003
PubMed
Summary

We measured phonon behavior in magnesium diboride (MgB2) using inelastic X-ray scattering. Electron-phonon coupling significantly influences phonon linewidth, matching theoretical predictions.

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Area of Science:

  • Condensed Matter Physics
  • Materials Science
  • Solid-State Physics

Background:

  • Magnesium diboride (MgB2) is a superconductor with unique electronic and vibrational properties.
  • Understanding phonon behavior is crucial for comprehending its superconducting mechanisms and material characteristics.

Purpose of the Study:

  • To experimentally measure phonon dispersion and linewidth in MgB2.
  • To theoretically investigate the contributions of electron-phonon coupling and anharmonicity to phonon linewidth.
  • To elucidate the dominant factors influencing phonon broadening in MgB2.

Main Methods:

  • Inelastic X-ray scattering (IXS) was employed to measure phonon dispersion and linewidth along specific crystallographic directions (Gamma-A, Gamma-M, A-L).
  • Density functional theory (DFT) calculations were performed to model the effects of electron-phonon coupling and anharmonicity on phonon linewidth.
  • Experimental results were compared with theoretical predictions to validate the models.

Main Results:

  • Phonon dispersion and linewidth were successfully measured in a single crystal of MgB2.
  • Anomalous broadening of the E(2g) phonon mode was observed along the Gamma-A direction.
  • Theoretical calculations showed excellent agreement with experimental data, indicating the accuracy of the employed models.
  • Electron-phonon coupling was identified as the dominant contributor to the phonon linewidth across all measured directions.

Conclusions:

  • The study provides a comprehensive understanding of phonon dynamics in MgB2.
  • Electron-phonon coupling plays a critical role in determining phonon linewidth and broadening.
  • The findings offer valuable insights into the electron-phonon interaction in MgB2, relevant for superconductivity research.