New Class Of High-temperature Superconductors Research Articles

The Superconducting Dome in Artificial High-Tc Superlattices Tuned at the Fano–Feshbach Resonance by Quantum Design

While the search for new high-temperature superconductors had been driven by the empirical “trials and errors” method for decades, we now report the synthesis of Artificial High-Tc Superlattices (AHTS) designed by quantum mechanics theory at the nanoscale. This discovery paves the way for engineering a new class of high-temperature superconductors, following the predictions of the Bianconi Perali Valletta (BPV) theory recently implemented in 2022 by Mazziotti et al. including Rashba spin-orbit coupling to create nanoscale AHTS composed of quantum wells. The high-Tc superconducting properties within these superlattices are controlled by a conformational parameter of the superlattice geometry, specifically, the ratio L/d which represents the thickness of La2CuO4 layers (L) relative to the superlattice period (d). Using molecular beam epitaxy, we have successfully grown numerous AHTS samples. These samples consist of initial layers of stoichiometric La2CuO4 units with a thickness L, doped by interface space charge, and intercalated with second layers of non-superconducting metallic material, La1.55Sr0.45CuO4 with thickness denoted as W = d − L. This configuration forms a quantum superlattice with periodicity d. The agreement observed between the experimental dependence Tc (the superconducting transition temperature) versus L/d ratio and the predictions of the BPV theory for AHTS in the form of the superconducting dome validates the hypothesis that the superconducting dome arises from the Fano–Feshbach or shape resonance in multigap superconductivity driven by quantum nanoscale confinement.

Special Issue on the Fundamental Problems of High-Temperature Superconductivity

The conference “Fundamental Problems of High Temperature Superconductivity-2015,” FPS’15, is the fifth conference in a row, organized by the P.N. Lebedev Physical Institute in the suburbs of Moscow (http://fps15.lebedev. ru/en/). The previous conference, FPS’11, was held in the year celebrating the 100th anniversary of the discovery of superconductivity. Its scientific content was mainly devoted to the “iron-based superconductors,” a new class of hightemperature superconductors discovered in 2008 whose physical research reached maturity by 2011. The years preceding FPS’15 have been also exciting for superconductivity due to the advent of hydrogen sulfide (2014–2015) with the record critical Tc value 203 K [1], with impressive achievements in interfacial superconductivity, including LaAlO3/SrTiO3 interface [2] and FeSe monolayers on the SrTiO3 [3, 4], and with the emergence of very interesting physics of topologically non-trivial materials, including topological superconductivity [5–7]. The discovery of high-temperature superconductivity in hydrogen sulfide is important not only due to its record Tc value; more importantly, it has injected a new vitality into the wineskins of the conventional electron-phonon pairing, already revitalizing with the discovery of MgB2. There

Spectroscopy of metal “superatom” nanoclusters and high-Tcsuperconducting pairing

A unique property of metal nanoclusters is the "superatom" shell structure of their delocalized electrons. The electronic shell levels are highly degenerate and therefore represent sharp peaks in the density of states. This can enable exceptionally strong electron pairing in certain clusters composed of tens to hundreds of atoms. In a finite system, such as a free nanocluster or a nucleus, pairing is observed most clearly via its effect on the energy spectrum of the constituent fermions. Accordingly, we performed a photoionization spectroscopy study of size-resolved aluminum nanoclusters and observed a rapid rise of the near-threshold density of states of several clusters ($Al_{37,44,66,68}$) with decreasing temperature. The characteristics of this behavior are consistent with compression of the density of states by a pairing transition into a high-temperature superconducting state with $T_c$>~100 K. This value exceeds that of bulk aluminum by two orders of magnitude. These results highlight the potential of novel pairing effects in size-quantized systems and the possibility to attain even higher critical temperatures by optimizing the particles' size and composition. As a new class of high-temperature superconductors, such metal nanocluster particles are promising building blocks for high-$T_c$ materials, devices, and networks.

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

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 high-temperature superconductors. Finally, we show how point-contact 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(Fe1−x Co x )2As2.

Coherence–incoherence crossover in the normal state of iron oxypnictides and importance of Hund's rule coupling

A new class of high-temperature superconductors based on iron and arsenic was recently discovered (Kamihara et al 2008 J. Am. Chem. Soc. 130 3296), with the superconducting transition temperature as high as 55 K (Ren et al 2008 Chin. Phys. Lett. 25 2215). Here we show, using microscopic theory, that the normal state of the iron pnictides at high temperatures is highly anomalous, displaying a very enhanced magnetic susceptibility and a linear temperature dependence of the resistivity. Below a coherence scale T*, the resistivity sharply drops and susceptibility crosses over to Pauli-like temperature dependence. Remarkably, the coherence–incoherence crossover temperature is a very strong function of the strength of Hund's rule coupling JHund. On the basis of the normal state properties, we estimate JHund to be ∼0.35 eV. In the atomic limit, this value of JHund leads to the critical ratio of the exchange constants J1/J2∼2. While normal state incoherence is common to all strongly correlated superconductors, the mechanism for emergence of the incoherent state in iron oxypnictides is unique due to its multiorbital electronic structure.

Unconventional superconductivity originating from disconnected Fermi surfaces in the iron-based compound

Abstract The iron-based LaFeAsO 1 - x F x recently discovered by Hosono's group is a fresh theoretical challenge as a new class of high-temperature superconductors. Here we describe the electronic structure of the material and the mechanism of superconductivity. We start with constructing a tight-binding model in terms of the maximally localised Wannier orbitals from a first-principles electronic structure calculation, which has turned out to involve all the five Fe 3d bands. This is used to calculate the spin and charge susceptibilities with the random phase approximation. The spin susceptibility has peaks around k = ( π , 0 ) , ( 0 , π ) arising from a nesting across disconnected Fermi surface pockets. We have then plugged the susceptibilities into the linearised Eliashberg equation. For the doping concentration x = 0.1 , we obtain an unconventional s-wave pairing, which is roughly an extended s in that the gap changes sign between the Fermi pockets, but the gap function is actually a 5 × 5 matrix. Its experimental implications are also discussed.

The new iron age

The discovery of a new class of high-temperature superconductors based on iron tests the limits of current theoretical and computational tools for the understanding of strongly correlated systems.

High-temperature superconductivity in the iron pnictides

A new class of high-temperature superconductors has been discovered in layered iron arsenic compounds. Results in this rapidly moving field may shed light on the still unsolved problem of high-temperature cuprate superconductivity.

Superconductors Beyond 1-2-3

In 1986 physicists K. Alex Mueller and J. Georg Bednorz of the IBM Zuerich Research Laboratory realized that La-Ba-Cu ceramics were the key to developing a new class of high-temperature superconductors that would revitalize superconductivity research and technology. Their discovery began one of the most startling revolutions in solid-state physics. In February, 1987, Maw-Kuen Wu of the University of Alabama, Ching-Wu (Paul) Chu of the University of Houston and their co-workers replaced lanthanum in the Bednorz-Mueller compound with yttrium--a smaller rare-earth element--to form a superconductor whose transition temperature was 90 kelvins. Soon after, R.Bruce van Dover, Bertram Batlogg and the author at AT T Bell Laboratories were the first to determine that the superconductor was the chemical compound now known as 1-2-3 because of the ratio of yttrium to barium to copper atoms. The 1-2-3 material was the first superconductor with a transition temperature above 77 kelvins and therefore could be cooled easily and inexpensively in a bath of liquid nitrogen. During the past four years the efforts of thousands of investigators around the world have produced about a dozen superconductors that have transition temperatures above 40 kelvins as well as a handful above 77 kelvins. So far the highestmore » transition temperature--which was achieved in a compound composed of thallium, barium, calcium, copper and oxygen--is an astounding 125 kelvins.« less

New family of layered copper oxide compounds with ordered cations: Prospective high-temperature superconductors.

A new family of layered copper oxide compounds with ordered rare-earth cations has been designed and synthesized. The compounds are partially Sr-substituted ${L}_{1}$${\mathrm{La}}_{1}$${\mathrm{CuO}}_{4}$, where L(=Sm, Eu, and Gd) and La ions are sequentially ordered thus: [-L-L-La-La-]. As a result, the Cu-O network of the new compound forms single-layer sheets of ${\mathrm{CuO}}_{5}$ pyramids, which is essentially the same as in the 30-K superconductor (Nd,Ce,Sr${)}_{2}$${\mathrm{CuO}}_{4}$ discovered recently by Akimitsu et al. Extensive oxygen annealing makes the new compounds metallic, indicating their possibility as a new class of high-temperature superconductors with hole-type charge carriers.