 Award ID(s):
 1709255
 NSFPAR ID:
 10265868
 Date Published:
 Journal Name:
 Nature Communications
 Volume:
 12
 Issue:
 1
 ISSN:
 20411723
 Format(s):
 Medium: X
 Sponsoring Org:
 National Science Foundation
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BACKGROUND Landau’s Fermi liquid theory provides the bedrock on which our understanding of metals has developed over the past 65 years. Its basic premise is that the electrons transporting a current can be treated as “quasiparticles”—electronlike particles whose effective mass has been modified, typically through interactions with the atomic lattice and/or other electrons. For a long time, it seemed as though Landau’s theory could account for all the manybody interactions that exist inside a metal, even in the socalled heavy fermion systems whose quasiparticle mass can be up to three orders of magnitude heavier than the electron’s mass. Fermi liquid theory also lay the foundation for the first successful microscopic theory of superconductivity. In the past few decades, a number of new metallic systems have been discovered that violate this paradigm. The violation is most evident in the way that the electrical resistivity changes with temperature or magnetic field. In normal metals in which electrons are the charge carriers, the resistivity increases with increasing temperature but saturates, both at low temperatures (because the quantized lattice vibrations are frozen out) and at high temperatures (because the electron mean free path dips below the smallest scattering pathway defined by the lattice spacing). In “strange metals,” by contrast, no saturation occurs, implying that the quasiparticle description breaks down and electrons are no longer the primary charge carriers. When the particle picture breaks down, no local entity carries the current. ADVANCES A new classification of metallicity is not a purely academic exercise, however, as strange metals tend to be the hightemperature phase of some of the best superconductors available. Understanding hightemperature superconductivity stands as a grand challenge because its resolution is fundamentally rooted in the physics of strong interactions, a regime where electrons no longer move independently. Precisely what new emergent phenomena one obtains from the interactions that drive the electron dynamics above the temperature where they superconduct is one of the most urgent problems in physics, attracting the attention of condensed matter physicists as well as string theorists. One thing is clear in this regime: The particle picture breaks down. As particles and locality are typically related, the strange metal raises the distinct possibility that its resolution must abandon the basic building blocks of quantum theory. We review the experimental and theoretical studies that have shaped our current understanding of the emergent strongly interacting physics realized in a host of strange metals, with a special focus on their posterchild: the copper oxide hightemperature superconductors. Experiments are highlighted that attempt to link the phenomenon of nonsaturating resistivity to parameterfree universal physics. A key experimental observation in such materials is that removing a single electron affects the spectrum at all energy scales, not just the lowenergy sector as in a Fermi liquid. It is observations of this sort that reinforce the breakdown of the singleparticle concept. On the theoretical side, the modern accounts that borrow from the conjecture that strongly interacting physics is really about gravity are discussed extensively, as they have been the most successful thus far in describing the range of physics displayed by strange metals. The foray into gravity models is not just a pipe dream because in such constructions, no particle interpretation is given to the charge density. As the breakdown of the independentparticle picture is central to the strange metal, the gravity constructions are a natural tool to make progress on this problem. Possible experimental tests of this conjecture are also outlined. OUTLOOK As more strange metals emerge and their physical properties come under the scrutiny of the vast array of experimental probes now at our disposal, their mysteries will be revealed and their commonalities and differences cataloged. In so doing, we should be able to understand the universality of strange metal physics. At the same time, the anomalous nature of their superconducting state will become apparent, offering us hope that a new paradigm of pairing of nonquasiparticles will also be formalized. The correlation between the strength of the linearintemperature resistivity in cuprate strange metals and their corresponding superfluid density, as revealed here, certainly hints at a fundamental link between the nature of strange metallicity and superconductivity in the cuprates. And as the gravityinspired theories mature and overcome the challenge of projecting their powerful mathematical machinery onto the appropriate crystallographic lattice, so too will we hope to build with confidence a complete theory of strange metals as they emerge from the horizon of a black hole. Curved spacetime with a black hole in its interior and the strange metal arising on the boundary. This picture is based on the string theory gaugegravity duality conjecture by J. Maldacena, which states that some strongly interacting quantum mechanical systems can be studied by replacing them with classical gravity in a spacetime in one higher dimension. The conjecture was made possible by thinking about some of the fundamental components of string theory, namely Dbranes (the horseshoeshaped object terminating on a flat surface in the interior of the spacetime). A key surprise of this conjecture is that aspects of condensed matter systems in which the electrons interact strongly—such as strange metals—can be studied using gravity.more » « less

Stacking layers of atomically thin transitionmetal carbides and twodimensional (2D) semiconducting transitionmetal dichalcogenides, could lead to nontrivial superconductivity and other unprecedented phenomena yet to be studied. In this work, superconducting αphase thin molybdenum carbide flakes were first synthesized, and a subsequent sulfurization treatment induced the formation of vertical heterolayer systems consisting of different phases of molybdenum carbide—ranging from α to γ′ and γ phases—in conjunction with molybdenum sulfide layers. These transitionmetal carbide/disulfide heterostructures exhibited critical superconducting temperatures as high as 6 K, higher than that of the starting singlephased αMo 2 C (4 K). We analyzed possible interface configurations to explain the observed moiré patterns resulting from the vertical heterostacks. Our densityfunctional theory (DFT) calculations indicate that epitaxial strain and moiré patterns lead to a higher interfacial density of states, which favors superconductivity. Such engineered heterostructures might allow the coupling of superconductivity to the topologically nontrivial surface states featured by transitionmetal carbide phases composing these heterostructures potentially leading to unconventional superconductivity. Moreover, we envisage that our approach could also be generalized to other metal carbide and nitride systems that could exhibit hightemperature superconductivity.more » « less

Abstract Interest in topological states of matter burgeoned over a decade ago with the theoretical prediction and experimental detection of topological insulators, especially in bulk threedimensional insulators that can be tuned out of it by doping. Their superconducting counterpart, the fullygapped threedimensional timereversalinvariant topological superconductors, have evaded discovery in bulk
intrinsic superconductors so far. The recently discovered topological metalβ PdBi_{2}is a unique candidate for tunable bulk topological superconductivity because of its intrinsic superconductivity and spinorbitcoupling. In this work, we provide experimental transport signatures consistent with fullygapped 3D timereversalinvariant topological superconductivity in Kdopedβ PdBi_{2}. In particular, we find signatures of oddparity bulk superconductivity via uppercritical field and magnetization measurements— oddparity pairing can be argued, given the band structure ofβ PdBi_{2}, to result in 3D topological superconductivity. In addition, Andreev spectroscopy reveals surface states protected by timereversal symmetry which might be possible evidence of Majorana surface states (Majorana cone). Moreover, we find that the undoped bulk system is a trivial superconductor. Thus, we discoverβ PdBi_{2}as a unique bulk material that, on doping, can potentially undergo an unprecedented topological quantum phase transition in the superconducting state. 
Abstract The interplay between charge transfer and electronic disorder in transitionmetal dichalcogenide multilayers gives rise to superconductive coupling driven by proximity enhancement, tunneling and superconducting fluctuations, of a yet unwieldy variety. Artificial spacer layers introduced with atomic precision change the density of states by charge transfer. Here, we tune the superconductive coupling between
monolayers from proximityenhanced to tunnelingdominated. We correlate normal and superconducting properties in $\text{NbS}{\text{e}}_{\text{2}}$ tailored multilayers with varying SnSe layer thickness ( ${\left[{\left(\text{SnSe}\right)}_{1+\delta}\right]}_{m}{\left[\text{NbS}{\text{e}}_{\text{2}}\right]}_{1}$ ). From highfield magnetotransport the critical fields yield Ginzburg–Landau coherence lengths with an increase of $m=115$ crossplane ( $140\mathrm{\%}$ ), trending towards twodimensional superconductivity for $m=19$ . We show crossovers between three regimes: metallic with proximityenhanced coupling ( $m>9$ ), disorderedmetallic with intermediate coupling ( $m=14$ ) and insulating with Josephson tunneling ( $m=59$ ). Our results demonstrate that stacking metal mono and dichalcogenides allows to convert a metal/superconductor into an insulator/superconductor system, prospecting the control of twodimensional superconductivity in embedded layers. $m>9$ 
Abstract The mechanism of unconventional superconductivity in correlated materials remains a great challenge in condensed matter physics. The recent discovery of superconductivity in infinitelayer nickelates, as an analog to high
T _{c}cuprates, has opened a new route to tackle this challenge. By growing 8 nm Pr_{0.8}Sr_{0.2}NiO_{2}films on the (LaAlO_{3})_{0.3}(Sr_{2}AlTaO_{6})_{0.7}substrate, we successfully raise the superconducting onset transition temperatureT _{c}in the widely studied SrTiO_{3}substrated nickelates from 9 K into 15 K, which indicates compressive strain is an efficient protocol to further enhance superconductivity in infinitelayer nickelates. Additionally, the xray absorption spectroscopy, combined with the firstprinciples and manybody simulations, suggest a crucial role of the hybridization between Ni and O orbitals in the unconventional pairing. These results also suggest the increase ofT _{c}be driven by the change of chargetransfer nature that would narrow the origin of general unconventional superconductivity in correlated materials to the covalence of transition metals and ligands.