Scanning tunneling spectroscopy of high-temperature superconductors

Abstract

Tunneling spectroscopy has played a central role in the experimental verification of the microscopic theory of superconductivity in classical superconductors. Initial attempts to apply the same approach to high-temperature superconductors were hampered by various problems related to the complexity of these materials. The use of scanning tunneling microscopy and spectroscopy ͑STM and STS͒ on these compounds allowed the main difficulties to be overcome. This success motivated a rapidly growing scientific community to apply this technique to high-temperature superconductors. This paper reviews the experimental highlights obtained over the last decade. The crucial efforts to gain control over the technique and to obtain reproducible results are first recalled. Then a discussion on how the STM and STS techniques have contributed to the study of some of the most unusual and remarkable properties of high-temperature superconductors is presented: the unusually large gap values and the absence of scaling with the critical temperature, the pseudogap and its relation to superconductivity, the unprecedented small size of the vortex cores and its influence on vortex matter, the unexpected electronic properties of the vortex cores, and the combination of atomic resolution and spectroscopy leading to the observation of periodic local density of states modulations in the superconducting and pseudogap states and in the vortex cores.

Unconventional superconductivity in the strong-coupling limit for the heavy fermion system CeCoIn5

“…[5] that this phase is reminiscent to the pseudogap of underdoped cuprates. In several families of cuprates, the pseudogap temperature follows a phenomenological scaling 2∆/kT * = 4.3, [18] suggesting that in all these materials the temperature-scale that governs the opening of ∆ at a local scale is T * . In our sample, the value we detect of T * is of 2.85 K at best, which gives a ratio 2∆/kT * = 6.…”

“…Since the value we detect of T * is of at best 2.85 K, this pseudogap phase does not seem to follow the phenomenological scaling 2∆/kT * found in numerous cuprate families. [18] Therefore the nature of this pseudogap phase in CeCoIn 5 deserves further investigation. The value of ∆(0) provides a BCS ratio that suggests that the material is in the strong coupling limit.…”

“Checkerboard Stripe” Electronic State on Cleaved Surface of NdO0.7F0.3BiS2 Probed by Scanning Tunneling Microscopy

“…One of the powerful techniques for addressing directly these issues is a scanning tunneling spectroscopy (STS) using a scanning tunneling microscopy (STM), which has been used to determine the pairing symmetries in a large variety of superconductors [38][39][40][41][42] . Additionally, this technique has made a significant contribution not only to visualize the CDW 3,6 and other electronic ordered states 13,14,16,17 but also to disentangle the relationship between the superconductivity and the electronic ordered states.…”

Interplay of chemical disorder and electronic inhomogeneity in unconventional superconductors

“…In contrast, as-grown YBCO surfaces show a gap plausibly identified as the superconducting gap, but neither atomic resolution nor type I inhomogeneity have been observed. 122 The cleaved CuO chain surface exhibits a complicated onedimensional modulation (Fig. 13a) initially attributed to a CDW state.…”