High-temperature charge density wave correlations in La _1.875 Ba _0.125 CuO ₄ without spin–charge locking

Abstract

Although all superconducting cuprates display charge-ordering tendencies, their low-temperature properties are distinct, impeding efforts to understand the phenomena within a single conceptual framework. While some systems exhibit stripes of charge and spin, with a locked periodicity, others host charge density waves (CDWs) without any obviously related spin order. Here we use resonant inelastic x-ray scattering (RIXS) to follow the evolution of charge correlations in the canonical stripe ordered cuprate La 1.875 Ba 0.125 CuO 4 (LBCO 1/8) across its ordering transition. We find that hightemperature charge correlations are unlocked from the wavevector of the spin correlations, signaling analogies to CDW phases in various other cuprates. This indicates that stripe order at low temperatures is stabilized by the coupling of otherwise independent charge and spin density waves, with important implications for the relation between charge and spin correlations in the cuprates.Charge density waves | Stripes | Superconductivity | Cuprates W hen holes are doped into the Mott insulating parent compounds of the cuprates, multiple competing interactions conspire to form a rich phase diagram. In the underdoped regime, holes can save energy by clustering together on neighboring sites in order to minimize the number of broken magnetic bonds, but by doing so they pay an extra energy cost of the increased inter-site Coulomb repulsion and reduced kinetic energy. Several early theoretical works suggested that frustration between these different ordering tendencies generates an instability towards spin density wave (SDW) order (1-5) and low-energy incommensurate SDW correlations were indeed observed around the same time (6-8). Such considerations were key to the discovery of "stripes" in the La2−x−y(Nd/Eu)y(Sr/Ba)xCuO4 or 214 family of cuprates. These correlations were found to be strongest at a doping level of 1/8 for which static spin and charge order forms at wavevectors related by a factor of two (9, 10). This phase was often conceptualized in terms of a dominant spin degree of freedom, as the underdoped cuprates have a large magnetic energy scale and a relatively small electronic density of states at the Fermi level (1-5). Furthermore, although high-temperature spin correlations were easily seen (7,8,10), directly detecting high-temperature charge correlations proved beyond the sensitivity of standard x-ray and neutron scattering measurements. Most compellingly, charge and spin ordering appeared, until recently, to be absent in cuprates in which there was a low-energy spin gap such as YBa2Cu3O6+x (YBCO), Bi1.5Pb0.5Sr1.54CaCu2O 8+δ (BSCCO2212), and HgBa2CuO 4+δ (HBCO1201), so the discovery of CDW correlations in these systems generated great interest (11)(12)(13)(14)(15)(16)(17)(18)(19). While the similarity of CDW phase diagrams in these materials may indicate a unified CDW mechanism (20, 21), many of the CDW properties reported in these materials were, however, notably different than that in LBCO 1/8. The CDW incommensurability...

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