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Related Concept Videos

Atomic Nuclei: Nuclear Relaxation Processes01:23

Atomic Nuclei: Nuclear Relaxation Processes

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In the absence of an external magnetic field, nuclear spin states are degenerate and randomly oriented. When a magnetic field is applied, the spins begin to precess and orient themselves along (lower energy) or against (higher energy) the direction of the field. At equilibrium, a slight excess population of spins exists in the lower energy state. Because the direction of the magnetic field is fixed as the z-axis,  the precessing magnetic moments are randomly oriented around the z-axis.
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Ferromagnetism

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Materials like iron, nickel, and cobalt consist of magnetic domains, within which the magnetic dipoles are arranged parallel to each other. The magnetic dipoles are rigidly aligned in the same direction within a domain by quantum mechanical coupling among the atoms. This coupling is so strong that even thermal agitation at room temperature cannot break it. The result is that each domain has a net dipole moment. However, some materials have weaker coupling, and are ferromagnetic at lower...
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Coordination compounds and complexes exhibit different colors, geometries, and magnetic behavior, depending on the metal atom/ion and ligands from which they are composed. In an attempt to explain the bonding and structure of coordination complexes, Linus Pauling proposed the valence bond theory, or VBT, using the concepts of hybridization and the overlapping of the atomic orbitals. According to VBT, the central metal atom or ion (Lewis acid) hybridizes to provide empty orbitals of suitable...
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The number of nuclear spins aligned in the lower energy state is slightly greater than those in the higher energy state. In the presence of an external magnetic field, as the spins precess at the Larmor frequency, the excess population results in a net magnetization oriented along the z axis. When a pulse or a short burst of radio waves at the Larmor frequency is applied along the x axis, the coupling of frequencies causes resonance and flips the nuclear spins of the excess population from the...
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Current-Driven Magnetization Reversal in Orbital Chern Insulators.

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|February 19, 2021
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Graphene multilayers exhibiting the quantized anomalous Hall effect can reverse their Hall sign using magnetic fields or transport currents. A novel current-driven mechanism allows for controlled reversal along specific lines in the current-magnetic field parameter space.

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Area of Science:

  • Condensed matter physics
  • Materials science
  • Quantum Hall effect

Background:

  • Graphene multilayers with flat moiré minibands can host the quantized anomalous Hall effect (QAHE).
  • QAHE arises from spontaneous valley polarization and topologically nontrivial valley-projected bands.
  • The Hall effect sign in Chern insulators can be reversed by external magnetic fields or transport currents.

Purpose of the Study:

  • To propose and analyze a current-driven mechanism for reversing the Hall effect sign in graphene-based Chern insulators.
  • To identify the control parameters and conditions under which this reversal occurs.

Main Methods:

  • Theoretical analysis of transport properties in graphene multilayers.
  • Investigation of the interplay between spontaneous valley polarization, topological bands, and applied electric/magnetic fields.
  • Derivation of the relationship between current, magnetic field, and material parameters for Hall effect reversal.

Main Results:

  • A novel current-driven mechanism for reversing the Hall effect sign is proposed.
  • Reversal occurs along specific lines in the (current I, magnetic-field B) control parameter space.
  • The slope of these reversal lines is determined by magnetization (M), moiré unit cell area (A_M), and a ratio (γ) related to chemical potential differences and electrical bias.

Conclusions:

  • The study presents a new method for controlling the topological properties of graphene-based Chern insulators via electrical current.
  • This provides a pathway for tunable quantum devices based on the quantized anomalous Hall effect.
  • The findings offer a deeper understanding of the interplay between topology, magnetism, and transport in moiré materials.