Recent progress of probing correlated electron states by point contact spectroscopy

Abstract

We review recent progress in point contact spectroscopy (PCS) to extract spectroscopic information out of correlated electron materials, with the emphasis on non-superconducting states. PCS has been used to detect bosonic excitations in normal metals, where signatures (e.g. phonons) are usually less than 1% of the measured conductance. In the superconducting state, point contact Andreev reflection (PCAR) has been widely used to study properties of the superconducting gap in various superconductors. It has been well-recognized that the corresponding conductance can be accurately fitted by the Blonder-Tinkham-Klapwijk (BTK) theory in which the AR occurring near the point contact junction is modeled by three parameters; the superconducting gap, the quasiparticle scattering rate, and a dimensionless parameter, Z, describing the strength of the potential barrier at the junction. AR can be as large as 100% of the background conductance, and only arises in the case of superconductors. In the last decade, there have been more and more experimental results suggesting that the point contact conductance could reveal new features associated with the unusual single electron dynamics in non-superconducting states, shedding a new light on exploring the nature of the competing phases in correlated materials. To correctly interpret these new features, it is crucial to re-examine the modeling of the point contact junctions, the formalism used to describe the single electron dynamics particularly in point contact spectroscopy, and the physical quantity that should be computed to understand the conductance. We will summarize the theories for point contact spectroscopy developed from different approaches and highlight these conceptual differences distinguishing point contact spectroscopy from tunneling-based probes. Moreover, we will show how the Schwinger-Kadanoff-Baym-Keldysh (SKBK) formalism together with the appropriate modeling of the nano-scale point contacts randomly distributed across the junction leads to the conclusion that the point contact conductance is proportional to the effective density of states, a physical quantity that can be computed if the electron self energy is known. The experimental data on iron based superconductors and heavy fermion compounds will be analyzed in this framework. These recent developments have extended the applicability of point contact spectroscopy to correlated materials, which will help us achieve a deeper understanding of the single electron dynamics in strongly correlated systems.

Heavy fermion thin films: progress and prospects

“…Superconducting topological surface sates in such proximitized topological insulator heterostructures was utilized to experimentally demonstrate the phenomenon of perfect transmission through a potential barrier, known as Klein tunneling [155][156][157]. One way to establish this phenomenon is by demonstrating doubling of conductance within the superconducting gap by point-contact Andreev reflection (PCAR) measurements [157,158]. In the Andreev process at the interface between a superconductor and a normal state material, an electron in the normal state material propagates as a Cooper pair within the gap of the superconductor creating a super current.…”

Analysis of Anomalous Dips in the Differential Conductivity of Heterostructures Based on Superconductors

“…Точково-контактні експерименти [1] стали одним із найбільш поширених методів спектроскопії металічних матеріалів. Одна з першочергових областей застосування цієї методики стали дослідження приповерхневих властивостей надпровідників [2]. Коли гострий металевий наконечник нормального (N) металу знаходиться у безпосередньому контакті із надпровідником (S), струм I нормальних електронів, обумовлений різницею електричних потенціалів V між двома електродами, перетворюється у надструм за допомогою квантового процесу, відомого як андріївське відбиття.…”

Electrical and Thermal Transport in Antiferromagnet-Superconductor Junctions

“…The simplest superconducting heterostructure is a normal metal (N)-superconductor (S) junction. The low bias transport at low temperature is dominated by Andreev reflection (AR) [8,23,24]. In conventional AR, an incident electron is retroreflected as a hole of the opposite spin, and a Cooper pair is transmitted into the S layer.…”