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Spin–Spin Coupling: One-Bond Coupling01:17

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Coupling interactions are strongest between NMR-active nuclei bonded to each other, where spin information can be transmitted directly through the pair of bonding electrons. While nuclei polarize their electrons to the opposite spins, the bonding electron pair has opposite spins. Configurations with antiparallel nuclear spins are expected to be lower in energy. When coupling makes antiparallel states more favorable, J is considered to have a positive value. The one-bond coupling constant, 1J,...
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The spin state of an NMR-active nucleus can have a slight effect on its immediate electronic environment. This effect propagates through the intervening bonds and affects the electronic environments of NMR-active nuclei up to three bonds away; occasionally, even farther. This phenomenon is called spin–spin coupling or J-coupling. Coupling interactions are mutual and result in small changes in the absorption frequencies of both nuclei involved. While nuclei of the same element are involved...
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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.
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¹H NMR: Interpreting Distorted and Overlapping Signals01:02

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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.
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Spin–Spin Coupling: Three-Bond Coupling (Vicinal Coupling)01:22

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Vicinal or three-bond coupling is commonly observed between protons attached to adjacent carbons. Here, nuclear spin information is primarily transferred via electron spin interactions between adjacent C‑H bond orbitals. This generally favors the antiparallel arrangement of spins, so 3J values are usually positive.
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Spin–Spin Coupling: Two-Bond Coupling (Geminal Coupling)01:20

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Two NMR-active nuclei bonded to a central atom can be involved in geminal or two-bond coupling. Geminal coupling is commonly seen between diastereotopic protons in chiral molecules and unsymmetrical alkenes, among others.
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Updated: Sep 11, 2025

High Resolution Phonon-assisted Quasi-resonance Fluorescence Spectroscopy
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Quantum spin liquid from electron-phonon coupling.

Xun Cai1, Zhaoyu Han2, Zi-Xiang Li1

  • 1Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China.

Proceedings of the National Academy of Sciences of the United States of America
|August 11, 2025
PubMed
Summary
This summary is machine-generated.

Researchers found a quantum spin liquid (QSL) in a nonengineered model, a significant step for understanding these exotic states. This discovery opens new avenues for exploring QSLs in real materials and high-temperature superconductivity.

Keywords:
electron–phonon couplingquantum Monte Carloquantum spin liquids

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Area of Science:

  • Condensed Matter Physics
  • Quantum Materials Science

Background:

  • Quantum spin liquids (QSLs) are exotic phases of matter characterized by emergent gauge fields and fractionalized excitations.
  • Unambiguously demonstrating QSLs in nonengineered models or real materials remains a significant challenge in condensed matter physics.

Purpose of the Study:

  • To investigate the ground-state phase diagram of the bond Su-Schrieffer-Heeger model on a 2D triangular lattice.
  • To determine if a quantum spin liquid phase exists in this nonengineered electron-phonon model.

Main Methods:

  • Numerically exact, sign-problem-free quantum Monte Carlo simulations were employed.
  • The study focused on the bond Su-Schrieffer-Heeger model with one electron per site on a 2D triangular lattice.

Main Results:

  • A quantum spin liquid (QSL) phase was identified in the studied model.
  • The QSL phase is fully gapped, shows no symmetry-breaking order, and hosts deconfined fractionalized holon excitations.

Conclusions:

  • The findings demonstrate the existence of a QSL in a nonengineered electron-phonon model, addressing a key challenge in the field.
  • This work suggests promising strategies for discovering QSLs in realistic materials and for advancing the understanding of high-temperature superconductivity.