Intrinsic Versus Extrinsic Pseudogaps in Photoemission Spectra of Poorly Conducting Solids

Abstract

In his report Robert Joynt (1) argued that the pseudogap observed in photoemission experiments in the manganites, and possibly in many other compounds, may be due to extrinsic loss effects. Joynt considered that the electric field created by the outgoing photoelectron would produce ohmic losses, lowering the kinetic energy of the ejected photoelectron. This would show up in the spectra as a pseudogap depression of weight near E F , even if there was not one in the true density of states, and should be most important for highly resistive samples. Although Joynt's general arguments are plausible, he was not able to determine the magnitude of many of the important parameters of his model, including at what resistivity these effects should become realizable in a high energy resolution experiment. Joynt stated that the extrinsic loss effects are not expected to be momentum-or angle-dependent, so that we can test such extrinsic effects in an angle-resolved photoemission (ARPES) experiment. Figure 1 shows data obtained from a cleaved single crystal of the colossal magnetoresistive (CMR) manganite La 1.2 Sr 1.8 Mn 2 O 7. The temperatures studied were well below T c (blue) and well above T c (red), between which there is about a two order of magnitude change in resistivity (see inset). The curves at k (, 0.28) show a significant temperature-dependent shift away from E F with temperature, while the curves at k (, 0.09) and k (0.46, 0) show an intermediate effect and a minimal effect, respectively. The minimal temperature-dependent changes observed at (0.46, 0) put an upper limit on the magnitude of the effects that could be due to Joynt's extrinsic losses; hence, the dramatic changes observed at k (, 0.28) should be of intrinsic origin. Because there is at some angles no effect from a two order of magnitude change of resistivity, it should also be clear that the weaker pseu-dogap observed at (, 0.28) at 50 K in the spectra is not due to ohmic losses [the pseu-dogap should be measured at the k where the peak is closest to E F , which occurs at (, 0.28)]. Because these samples have high resistivi-ties of about 0.1 ohm-cm (three orders of magnitude larger than the value of 10 4 ohm-cm at which Joynt expected these effects to become relevant), this should also alleviate concerns about these extrinsic effects for other samples with a conductivity of this order or better.