Uniaxial stress study of La$_{1.875}$Ba$_{0.125}$CuO$_{4}$ by $^{139}$La NMR

Abstract

We report 139 La nuclear magnetic resonance measurements on a single-crystal sample of La 1.875 Ba 0.125 CuO 4 under uniaxial stress. The spin order is shown to be more robust than at x = 0.115 doping, however, for magnetic field H in the c direction and the stress applied along the [110] direction (σ [110] ) the spin order transition temperature T SO is rapidly suppressed. This is in stark contrast to the behavior with stress in [100] direction (σ [100] ), which has virtually no effect on T SO . For H [110], σ [110] stress has a weakened effect, and the rate dT SO /dσ [110] is drastically reduced. Thus, H [110] acts as a stabilizing factor for spin-stripe order. Also, the onset temperature of the low-temperature tetragonal crystal structure T LTT is essentially unaffected by [110] stress, while it decreases slowly under compression along [100].We develop a Landau free energy model and interpret our findings as an interplay of symmetrybreaking terms driven by the orientation of spins. These findings put constraints on the applicability of theoretical models for the development of spin-stripe order.High-temperature superconductors present complex electronic behavior that has been the focus of intense research for almost four decades. One of the leading open questions is the relationship between competing electronic orders. Even though it has become clear that stripe charge order (CO) is ubiquitous in cuprates, the relationship between static charge and spin order (SO) remains incompletely understood. This is partly due to the limited number of systems in which both can be studied. The other reason is that the structural, electronic, and magnetic degrees of freedom are intertwined in these orders. Consequently, it remains a challenge to determine how they couple. In La 2−x Ba x CuO 4 (LBCO) close to x = 1/8 doping, CO becomes pinned as the symmetry of the lattice changes from low-temperature orthogonal (LTO) to low-temperature tetragonal (LTT) at T LTT = 57 K. At this doping, the SO transition temperature T SO reaches its maximum value [1-4] of ≈40 K, while the bulk superconducting transition temperature (T c ) is strongly suppressed. T c rapidly increases for doping away from 1/8, even though the structural transition and CO/SO persist. It was initially hypothesized that the LTT phase, in which the structural symmetry is locally lower than in the LTO phase, was necessary for CO/SO to condense. However, Hücker et al. [5] has shown in La 1.875 Ba 0.125 CuO 4 that hydrostatic pressure above ≈1.85 GPa suppresses the LTO/LTT transition while CO/SO survive, which indicated that the long-range LTT