Excitation of a bosonic mode by electron tunneling into a cuprate superconductor ${\text{NdBa}}_2{\text{Cu}}_3{\text{O}}_{7-\delta }$

We performed scanning tunneling spectroscopic experiments on hole-doped ${\text{NdBa}}_2{\text{Cu}}_3{\text{O}}_{7-\delta }$. The $\text{d}I/\text{d}V$ curves obtained at 4.2 K are asymmetric with clear peak, dip, and hump structures. Energy derivatives of these curves show peaks at energies beyond the dip features. Highly precise full potential band-structure calculations confirm a featureless electronic density of states in that energy region. Our results indicate that tunneling electrons couple to a collective mode in the ${\text{CuO}}_2$ plane.

Figure 1

(Color online) $\text{d}I/\text{d}V$ curves (shifted along $y$ axis for clarity) obtained at 4.2 K along a 20 nm line. The peaks and humps are indicated by dotted vertical lines.

Figure 2

(Color online) Typical $\text{d}I/\text{d}V$ curve (triangles) containing peak-dip-hump structures together with the energy derivative (circles). The smooth dark line is a guide to the eyes. The arrows show the inelastic features at both empty and filled states. The boson mode energy $\left(\Omega \right)$ at empty state is shown.

Figure 3

(Color online) Calculated total electronic density of states (shaded gray) in the vicinity of the Fermi level (zero energy) for NdBCO. The sharp features at about 85 and 110 meV binding energy can be assigned to Van Hove singularities originating from the Cu-O chains [(red) dot-dashed line]. The contribution from the plane [(green) dashed line] is rather flat in the shown energy window.

Figure 4

(Color online) Mode energy $\Omega \left(r\right)$ vs energy gap $2\Delta \left(r\right)$ obtained from the $\text{d}I/\text{d}V$ curves like in Fig. 2. The dashed line emphasizes the tentative correlation. The uncertainties in the evaluation of $\Delta$ and $\Omega$ are $\pm \left(0.5–1\right)\text{meV}$ and $\pm \left(1–1.5\right)\text{meV}$, respectively.