[Paper Review] Nearly Singular Magnetic Fluctuations in the Normal State of a High-T_c Cuprate Superconductor

G. Aeppli, T. E. Mason+3 more|arXiv (Cornell University)|Jan 16, 1998

Physics of Superconductivity and Magnetism4 citations

TL;DR

This study uses polarized and unpolarized neutron scattering to investigate magnetic fluctuations in the normal state of La_{1.86}Sr_{0.14}CuO_4, a high-T_c cuprate superconductor. It reveals nearly singular magnetic fluctuations that scale with temperature and energy transfer, indicating proximity to a quantum critical point, with fluctuation amplitudes decreasing as T^{-2} and widths increasing linearly with k_B T.

ABSTRACT

Polarized and unpolarized neutron scattering was used to measure the wave vector- and frequency-dependent magnetic fluctuations in the normal state (from the superconducting transition temperature, T_c=35, up to 350 K) of single crystals of La_{1.86}Sr_{0.14}CuO_4. The peaks which dominate the fluctuations have amplitudes that decrease as T^{-2} and widths that increase in proportion to the thermal energy, k_B T (where k_B is Boltzmann's constant), and energy transfer added in quadrature. The nearly singular fluctuations are consistent with a nearby quantum critical point.

Motivation & Objective

• To investigate the nature of magnetic fluctuations in the normal state of a high-T_c cuprate superconductor.
• To determine whether these fluctuations exhibit characteristics of quantum critical behavior.
• To measure the wave vector- and frequency-dependent magnetic response across a wide temperature range (35 K to 350 K).
• To assess the scaling behavior of fluctuation amplitudes and widths with temperature and energy transfer.
• To test the hypothesis that the normal state is influenced by critical fluctuations near a quantum critical point.

Proposed method

• Polarized and unpolarized neutron scattering was performed on single crystals of La_{1.86}Sr_{0.14}CuO_4.
• Measurements were conducted over a temperature range from T_c = 35 K up to 350 K in the normal state.
• The wave vector- and frequency-dependent magnetic structure factor was extracted from scattering data.
• Fluctuation amplitudes and widths were analyzed as functions of temperature and energy transfer.
• Scaling behavior was examined by comparing amplitude suppression (T^{-2}) and width broadening (proportional to k_B T) with theoretical expectations.
• The data were interpreted in the context of a quantum critical point scenario, with fluctuations becoming increasingly singular as temperature decreases.

Experimental results

Research questions

• RQ1 Do magnetic fluctuations in the normal state of La_{1.86}Sr_{0.14}CuO_4 exhibit scaling behavior consistent with a quantum critical point?
• RQ2 How do the amplitudes and widths of magnetic fluctuations vary with temperature and energy transfer?
• RQ3 Are the observed fluctuations consistent with a nearly singular response near a critical point?
• RQ4 What is the role of thermal energy in broadening the fluctuation width in the normal state?
• RQ5 Can the data be explained by a critical point scenario without long-range order?

Key findings

• Magnetic fluctuations in the normal state of La_{1.86}Sr_{0.14}CuO_4 exhibit nearly singular behavior, with peak amplitudes decreasing as T^{-2}.
• The width of the fluctuation peaks increases in proportion to the thermal energy, k_B T, and energy transfer, added in quadrature.
• The scaling of amplitude and width with temperature and energy transfer is consistent with a system near a quantum critical point.
• The fluctuations are dominant at wave vectors corresponding to antiferromagnetic order, suggesting a connection to spin dynamics near long-range order.
• The data show no evidence of long-range magnetic order, yet the fluctuations remain strong and sharply peaked, indicating critical-like behavior.
• The nearly singular nature of the fluctuations supports the idea that the superconducting state may be driven by critical spin fluctuations near a quantum critical point.