Long-range incommensurate charge fluctuations in (Y,Nd)Ba2Cu3O(6+x) (1207.0915v1)

Abstract: There are increasing indications that superconductivity competes with other orders in cuprate superconductors, but obtaining direct evidence with bulk-sensitive probes is challenging. We have used resonant soft x-ray scattering to identify two-dimensional charge fluctuations with an incommensurate periodicity of $\bf \sim 3.2$ lattice units in the copper-oxide planes of the superconductors (Y,Nd)Ba$2$Cu$_3$O${6+x}$ with hole concentrations $0.09 \leq p \leq 0.13$ per planar Cu ion. The intensity and correlation length of the fluctuation signal increase strongly upon cooling down to the superconducting transition temperature, $T_c$; further cooling below $T_c$ abruptly reverses the divergence of the charge correlations. In combination with prior observations of a large gap in the spin excitation spectrum, these data indicate an incipient charge-density-wave instability that competes with superconductivity.

• The paper demonstrates that charge-density-wave correlations exhibit a 3.2 lattice unit periodicity, highlighting a competing order with superconductivity.
• It shows that charge fluctuations intensify near Tc and then sharply decline below the transition, emphasizing a narrow doping dependence.
• Resonant soft x-ray scattering distinguishes these charge signals from spin fluctuations, providing new insights into the mechanisms of high-Tc superconductivity.

Long-Range Incommensurate Charge Fluctuations in (Y,Nd)Ba$_2$Cu$_3$O$_{6+x}$

The paper presents a detailed investigation into the charge order and fluctuations within the copper oxide planes of the high-temperature cuprate superconductor family, specifically (Y,Nd)Ba$_2$Cu$_3$O$_{6+x}$, using resonant soft x-ray scattering (RSXS). The paper aims to bridge the understanding of how charge fluctuations may interplay with superconductivity and how they may compete with other ordered states in underdoped superconductors.

The research identifies two-dimensional charge fluctuations in the copper-oxide planes, with an incommensurate periodicity of approximately 3.2 lattice units, within the range of $0.09 \leq p \leq 0.13$ holes per Cu ion in the plane. These fluctuations increase in intensity and correlation length as the system approaches the superconducting transition temperature, $T_c$; however, this trend reverses sharply below $T_c$, indicating a suppression of charge correlations by the onset of superconductivity. This observation suggests a competing order with an incipient charge-density-wave (CDW) instability that may be pivotal to understanding the superconducting state in cuprates.

The paper discusses previously identified incommensurate spin fluctuations in this and similar systems, conflicting with the current observations of charge order without a clear spin correlation. The paper instruments include resonant x-ray scattering tuned at the Cu $L_3$ absorption edge, enhancing sensitivity to the valence electronic system, and allowing experimental discrimination between elastic and quasielastic signals in energy-resolved scattering experiments.

Several pertinent outcomes include:

1. Polarization and Energy-Dependence: The paper elucidates the characteristics of the CDW signal, establishing its charge nature over spin by analyzing the behavior in different polarization states and energy transfers.
2. Doping Dependence: The CDW signal is consistently observed within a narrow doping range ($0.09 \leq p \leq 0.13$), corresponding to an anomaly in the superconducting transition temperature ($T_c$), suggesting that charge-density correlations within this range are competitive with the superconducting state.
3. Theoretical Implications: The findings advocate for CDW correlations as a possible generic feature of copper oxide superconductors, challenging the previous generalization of protection against or necessity for fluctuating spin or stripe orders for superconductivity in these materials.
4. Temperature Dependence: Correlation lengths sharply increase approaching $T_c$, indicating growing CDW correlations, but these are suppressed below $T_c$, aligning with the notion of the CDW being suppressed by the onset of superconductivity, thereby supporting the competition hypothesis.

These results reinforce the notion that density wave orders—such as the CDW observed here—are integral to the electronic phase diagram of cuprates and must be seriously considered in any comprehensive theory of high-temperature superconductivity in these systems. Future research directions could involve further exploration into the mechanisms underpinning this competition and how they might be tuned or controlled to optimize superconducting properties. Understanding the precise nature of these orders and how they might be decoupled or coupled with superconductivity could open new pathways for developing materials with higher superconducting transition temperatures.