Low High

1. Imaging the impact of single oxygen atoms on superconducting Bi2+ySr2-yCaCu2O8+xJennifer E. Hoffman, Harvard University, DMR 0847433 The superconducting transition temperature of the cuprate Bi2+ySr2-yCaCu2O8+x (BSCCO) can be controlled by tuning the concentration of extra oxygen atoms (dopants) in the crystal. We used scanning tunneling microscopy (STM) to image two types of extra oxygen atoms (type-A and type-B) as well as missing oxygen atoms (vacancies) at the apical site of the copper oxide octahedron. We correlate the distribution of these dopants to the spatial distribution of the inhomogeneous spectral gap measured via STM (“pseudogap”) to understand how the chemical disorder in the crystal is responsible for the observed inhomogeneity of the pseudogap state. We conclude that it is not the extra oxygen atoms, but the apical oxygen vacancies that are most strongly correlated with the areas of the largest pseudogap(see figure on right). C B A Low High O, type-B O, type-A apical O vacancy F D E 40 mV 100mV Figure 1: Panels A-C show type-B oxygen dopants, type-A oxygen dopants and apical oxygen vacancies respectively as bright atomic-scale features in STM dI/dV (differential conductance) images at bias voltages -1 V, -1.5 V and +1 V respectively over a 35 nm field-of-view. The same set of dopants is shown in panels D-F, superimposed on the spatial distribution map of the inhomogeneous spectral gap (“pseudogap”). Zeljkovic et al., Science337, 320 (2012)

2. Imaging the impact of single oxygen atoms on superconducting Bi2+ySr2-yCaCu2O8+xJennifer E. Hoffman, Harvard University, DMR 0847433 We have imaged oxygen dopants and apical oxygen vacancies in four samples with different superconducting transition temperatures (Tc). We find that the number of apical oxygen vacancies increases with decreasing doping in BSCCO crystals (figure 2A on left). It has been theoretically proposed that controlling the chemical disorder in these materials would result in increasing the critical temperature by ~15% (Figure 2B). Thus, exploring different crystal growth recipes to minimize the number of apical oxygen vacancies could potentially increase Tc in this material as theoretically predicted. Furthermore, in order to image the dopants and vacancies (Figure 1 A-C), we developed a single-layer data acquisition technique tailored to high voltage studies of a fragile material like BSCCO. We believe that this technique could be utilized to study several other sensitive quasi-2D crystals. A B Figure 2: Panel A shows the number of type-A, type-B, and apical oxygen vacancies as a function of transition temperature Tcin BSCCO. Panel B shows a theoretical prediction on how Tccould be increased in these materials through homogeneous doping. Adapted from Goren, PRB 84, 094508 (2011)