Design of New Superconducting Materials, and Point-Contact Spectroscopy as a Probe of Strong Electron Correlations

Abstract

At this centenary of the discovery of superconductivity, the design of new and more useful superconductors remains as enigmatic as ever. These materials play crucial roles both for fundamental science and applications, and they hold great promise in addressing our global energy challenge. The recent discovery of a new class of high-temperature superconductors has made the community more enthusiastic than ever about finding new superconductors. Historically, these discoveries were almost completely guided by serendipity, and now, researchers in the field have grown into an enthusiastic global network to find a way, together, to predictively design new superconductors. After a short history of discoveries of superconducting materials, we share our own guidelines for searching for hightemperature superconductors. Finally, we show how pointcontact spectroscopy (PCS) is used to detect strong correlations in the normal state, with a focus on the normal state region of the Fe-based superconductors, defining a new region in the phase diagram of Ba(Fe 1−x Co x ) 2 As 2 .Keywords History of superconducting materials · Design of new superconductors · Point-contact spectroscopy · Fe-based superconductors · High-temperature superconductors At this ten carat diamond anniversary (100 years) of the discovery of superconductivity, the silver anniversary (25 years) of the discovery of high-temperature superconductivity, and the blue topaz anniversary (4 years) of the discovery of a new class of high-temperature Fe-based superconductors, it is worth looking back at the history, and modes of discovery of the many classes of known superconductors. We do this not only for historical breadth, but also to see if we can learn to turn our serendipitous discoveries into predictive design of new superconductors. We start with a short history of the field, mention the ubiquitous connection between the tunable superconductors we know of today, their phase diagrams, and demonstrate how we can measure strong correlations in these materials with pointcontact spectroscopy (PCS).The Physics Today article by van Delft and Kes [1] beautifully relates the discovery of superconductivity by Heike Kamerlingh Onnes: The quest for achieving lower temperatures was followed by the curiosity-driven measurements of the resistance of metals at these newly achieved liquid helium temperatures. Mercury was chosen since it was a liquid at room temperature, and therefore easily purified. As described by Stephen Blundell [2], there existed three popular predictions on how the resistance would behave upon approaching absolute zero: Lord Kelvin predicted an insulating up-turn at low temperatures due to a freezing out of charge carriers; Matthiessen thought the resistance would decrease to a finite value; and Dewar predicted a smooth decrease to zero resistance at zero temperature. The abrupt drop in resistance at ∼4 K was a complete surprise! Over the next few decades, the critical temperature slowly increased through systematic tests of elements, alloys and compounds...

Correlated materials design: prospects and challenges

“…The eigenvalues kj of the solution of the self-consistent set of Eqs. (1) and (2) are not to be interpreted as excitation energies. Konh Sham excitations are not Landau quasiparticle excitations.…”

“…Through this paper we use a matrix notation loosely and view operators as matrices. For example, in the Dyson equations W, v Coul , Π are operators (matrices) with matrix elements r| W (ω) |r = W (ω, r, r ), r| Π(ω) |r = Π(ω, r, r ), r| v Coul |r = 1 |r−r | . This G 0 W 0 method, introduced by Hybertsen and Louie 16 , systematically improves the gaps of all semiconducting materials.…”

“…Materials discovery has historically proceeded via trial and error, with a mixture of serendipity and intuition being the most fruitful path. For example, all major classes of superconductors-from elemental mercury in 1911, to the heavy fermions, cuprates and most recently, the iron-based superconductors-have been discovered largely by chance [1].…”

Nature of magnetic excitations in the iron pnictides and its pertinence to superconductivity as studied by inelastic neutron scattering

“…are not entirely self-consistent by design in order to create materials with competing phases, from which superconductivity might emerge [17]. These rules point to normal state behavior with which previous theories are inconsistent, and significant improvements are required for the previous theories to become consistent with this novel behavior [5].…”

Single 20 meV boson mode inKFe2As2detected by point-contact spectroscopy

“…In particular, several authors have recently underlined the potential importance of PCS in iron pnictides and chalchogenides, as a tool to identify new featuress due to the interplay of strong electronic correlations with spin and orbital fluctuations, close to an orbital-selective Mott transition. [11][12][13] .…”