[Paper Review] From Magnons to the Resonance Peak: Spin Dynamics in High-T_C Superconducting Cuprates by Inelastic Neutron Scattering

P. Bourges|ArXiv.org|Jan 28, 1999

Physics of Superconductivity and Magnetism4 references6 citations

TL;DR

This paper reviews inelastic neutron scattering measurements of spin dynamics in high-Tc cuprates, revealing a evolution from magnon-like excitations in the insulating state to a sharp magnetic resonance peak in the superconducting state. The resonance energy scales linearly with Tc, and underdoped samples exhibit strong antiferromagnetic fluctuations linked to a spin pseudogap, while these correlations vanish in overdoped regimes.

ABSTRACT

The spin dynamics of high temperature superconductors measured by inelastic neutron scattering is reviewed. The spin susceptibility evolves a lot with increasing doping from the undoped insulating state to the overdoped metallic state. In the superconducting state, a strong magnetic resonance peak occurs in the spin excitation spectrum whose energy is proportional to the superconducting temperature. In underdoped and optimally doped regimes, normal state antiferromagnetic fluctuations are observed, proving the existence of strong electronic correlations in cuprates. They are characterized by a spin pseudo-gap. In contrast, these dynamical spin correlations vanish in overdoped samples.

Motivation & Objective

• To understand the evolution of spin excitation spectra across the phase diagram of high-Tc superconducting cuprates.
• To investigate the role of electronic correlations and spin fluctuations in the normal and superconducting states.
• To determine how doping affects magnetic response, particularly the emergence of the magnetic resonance mode.
• To clarify the connection between the spin pseudogap and antiferromagnetic fluctuations in underdoped cuprates.
• To contrast the spin dynamics in underdoped/optimally doped versus overdoped regimes, especially the disappearance of spin correlations in the latter.

Proposed method

• Employing inelastic neutron scattering to probe the dynamic spin structure factor S(q,ω) in various cuprate compounds.
• Measuring spin excitation spectra across different doping levels, from the insulating Mott state to the overdoped metallic state.
• Analyzing the energy and momentum dependence of spin excitations to identify magnon-like modes and the resonance peak.
• Comparing the energy of the resonance peak with the superconducting transition temperature Tc to test scaling behavior.
• Using the absence of long-range antiferromagnetic order to infer the role of short-range spin correlations and the pseudogap.
• Focusing on the evolution of spin susceptibility with doping to distinguish between gapped and coherent spin excitations.

Experimental results

Research questions

• RQ1 How does the spin excitation spectrum evolve from the insulating Mott state to the overdoped metallic state in high-Tc cuprates?
• RQ2 What is the origin and nature of the sharp magnetic resonance peak observed in the superconducting state?
• RQ3 How do antiferromagnetic spin fluctuations in the normal state relate to the pseudogap and electronic correlations?
• RQ4 Why do spin correlations vanish in the overdoped regime despite the presence of superconductivity?
• RQ5 Is the energy of the resonance peak universally proportional to Tc across different cuprate families?

Key findings

• In the superconducting state, a sharp magnetic resonance peak emerges in the spin excitation spectrum, with its energy linearly proportional to Tc.
• In underdoped and optimally doped cuprates, strong antiferromagnetic spin fluctuations persist in the normal state, indicating strong electronic correlations.
• These spin fluctuations are associated with a spin pseudogap, which suppresses low-energy spin excitations.
• In contrast, the spin dynamics in overdoped samples show no evidence of long-range or short-range antiferromagnetic order, indicating the breakdown of spin correlations.
• The evolution of spin susceptibility from magnon-like modes in the insulating state to a resonance peak in the superconducting state reflects a crossover driven by doping.
• The absence of spin fluctuations in the overdoped regime suggests a decoupling of spin and charge degrees of freedom, consistent with a more conventional metallic state.