A. Damascelli

Two gaps make a high-temperature superconductor?

One of the keys to the high-temperature superconductivity puzzle is the identification of the energy scales associated with the emergence of a coherent condensate of superconducting electron pairs. These might provide a measure of the pairing strength and of the coherence of the superfluid, and ultimately reveal the nature of the elusive pairing mechanism in the superconducting cuprates. To this end, a great deal of effort has been devoted to investigating the connection between the superconducting transition temperature Tc and the normal-state pseudogap crossover temperature T*. Here we present a review of a large body of experimental data that suggests a coexisting two-gap scenario, i.e. superconducting gap and pseudogap, over the whole superconducting dome.

Charge Order Driven by Fermi-Arc Instability in Bi2Sr2−xLaxCuO6+δ

The understanding of the origin of superconductivity in cuprates has been hindered by the apparent diversity of intertwining electronic orders in these materials. We combined resonant x-ray scattering (REXS), scanning-tunneling microscopy (STM), and angle-resolved photoemission spectroscopy (ARPES) to observe a charge order that appears consistently in surface and bulk, and in momentum and real space within one cuprate family, Bi 2Sr 2−xLa xCuO 6+δ. The observed wave vectors rule out simple antinodal nesting in the single-particle limit but match well with a phenomenological model of a many-body instability of the Fermi arcs. Combined with earlier observations of electronic order in other cuprate families, these findings suggest the existence of a generic charge-ordered state in underdoped cuprates and uncover its intimate connection to the pseudogap regime. Surface and bulk measurements in bismuth-based cuprates agree and indicate a short-range charge order. [Also see Perspective by Morr] Copper-Oxide Superconductors Copper-oxide superconductors have a complex electronic structure. A charge density order has been observed in two cuprate families; however, it has been unclear whether such an order exists in Bi-based compounds (see the Perspective by Morr). Comin et al. (p. 390, published online 19 December) and da Silva Neto et al. (p. 393, published online 19 December) address this question in single-layer and double-layer Bibased cuprates, respectively. For both families of materials, surface measurements by scanning tunneling spectroscopy agree with bulk measurements obtained through resonant elastic x-ray scattering, which suggests the formation of short-range correlations that modulate the charge density of the carriers over a range of dopings. Thus, charge ordering may represent a common characteristic of the major cuprate families.

Quantum critical behaviour in a high-Tc superconductor

Quantum criticality is associated with a system composed of a nearly infinite number of interacting quantum degrees of freedom at zero temperature, and it implies that the system looks on average the same regardless of the time- and length scale on which it is observed. Electrons on the atomic scale do not exhibit such symmetry, which can only be generated as a collective phenomenon through the interactions between a large number of electrons. In materials with strong electron correlations a quantum phase transition at zero temperature can occur, and a quantum critical state has been predicted, which manifests itself through universal power-law behaviours of the response functions. Candidates have been found both in heavy-fermion systems and in the high-transition temperature (high-Tc) copper oxide superconductors, but the reality and the physical nature of such a phase transition are still debated. Here we report a universal behaviour that is characteristic of the quantum critical region. We demonstrate that the experimentally measured phase angle agrees precisely with the exponent of the optical conductivity. This points towards a quantum phase transition of an unconventional kind in the high-Tc superconductors.In certain materials with strong electron correlations a quantum phase transition (QPT) at zero temperature can occur, in the proximity of which a quantum critical state of matter has been anticipated. This possibility has recently attracted much attention because the response of such a state of matter is expected to follow universal patterns defined by the quantum mechanical nature of the fluctuations. Forementioned universality manifests itself through power-law behaviours of the response functions. Candidates are found both in heavy fermion systems and in the cuprate high Tc superconductors. Although there are indications for quantum criticality in the cuprate superconductors, the reality and the physical nature of such a QPT are still under debate. Here we identify a universal behaviour of the phase angle of the frequency dependent conductivity that is characteristic of the quantum critical region. We demonstrate that the experimentally measured phase angle agrees precisely with the exponent of the optical conductivity. This points towards a QPT in the cuprates close to optimal doping, although of an unconventional kind.

Bilayer Splitting in the Electronic Structure of Heavily Overdoped Bi2Sr2CaCu2O8 + delta

The electronic structure of heavily overdoped Bi(2)Sr(2)CaCu(2)O(8+delta) is investigated by angle-resolved photoemission spectroscopy. The long-sought bilayer band splitting in this two-plane system is observed in both normal and superconducting states, which qualitatively agrees with the bilayer Hubbard model calculations. The maximum bilayer energy splitting is about 88 meV for the normal state feature, while it is only about 20 meV for the superconducting peak.

Probing the Electronic Structure of Complex Systems by ARPES

Angle-resolved photoemission spectroscopy (ARPES) is one of the most direct methods of studying the electronic structure of solids. By measuring the kinetic energy and angular distribution of the electrons photoemitted from a sample illuminated with sufficiently high-energy radiation, one can gain information on both the energy and momentum of the electrons propagating inside a material. This is of vital importance in elucidating the connection between electronic, magnetic, and chemical structure of solids, in particular for those complex systems which cannot be appropriately described within the independent-particle picture. The last decade witnessed significant progress in this technique and its applications, thus ushering in a new era in photoelectron spectroscopy; today, ARPES experiments with 2 meV energy resolution and 0.28 angular resolution are a reality even for photoemission on solids. In this paper we will review the fundamentals of the technique and present some illustrative experimental results; we will show how ARPES can probe the momentum-dependent electronic structure of solids providing detailed information on band dispersion and Fermi surface as well as on the strength and nature of many-body correlations, which may profoundly affect the oneelectron excitation spectrum and in turn the macroscopic physical properties.

Rashba spin-splitting control at the surface of the topological insulator Bi2Se3.

The electronic structure of Bi(2)Se(3) is studied by angle-resolved photoemission and density functional theory. We show that the instability of the surface electronic properties, observed even in ultrahigh-vacuum conditions, can be overcome via in situ potassium deposition. In addition to accurately setting the carrier concentration, new Rashba-like spin-polarized states are induced, with a tunable, reversible, and highly stable spin splitting. Ab initio slab calculations reveal that these Rashba states are derived from 5-quintuple-layer quantum-well states. While the K-induced potential gradient enhances the spin splitting, this may be present on pristine surfaces due to the symmetry breaking of the vacuum-solid interface.

Symmetry of charge order in cuprates

Charge-ordered ground states permeate the phenomenology of 3d-based transition metal oxides, and more generally represent a distinctive hallmark of strongly correlated states of matter. The recent discovery of charge order in various cuprate families has fuelled new interest into the role played by this incipient broken symmetry within the complex phase diagram of high-T(c) superconductors. Here, we use resonant X-ray scattering to resolve the main characteristics of the charge-modulated state in two cuprate families: Bi2Sr(2-x)La(x)CuO(6+δ) (Bi2201) and YBa2Cu3O(6+y) (YBCO). We detect no signatures of spatial modulations along the nodal direction in Bi2201, thus clarifying the inter-unit-cell momentum structure of charge order. We also resolve the intra-unit-cell symmetry of the charge-ordered state, which is revealed to be best represented by a bond order with modulated charges on the O-2p orbitals and a prominent d-wave character. These results provide insights into the origin and microscopic description of charge order in cuprates, and its interplay with superconductivity.

Disentangling the Electronic and Phononic Glue in a High-Tc Superconductor

Electrons Beat Phonons The phenomenon of superconductivity, in which a material suddenly (below a certain transition temperature Tc) becomes a perfect conductor with zero electrical resistance, can be roughly explained in terms of Bose-Einstein condensation of pairs of electrons. In conventional superconductors, the formation of these so-called Cooper pairs is mediated by lattice deformations (phonons), but this mechanism is insufficient to explain the high Tc of cuprate superconductors. Other mechanisms, such as magnetic fluctuations, have been proposed which originate with the electrons themselves rather than the lattice. Dal Conte et al. (p. 1600) used time-resolved optical spectroscopy of an optimally doped cuprate to show that the temporal evolution of the reflectivity is consistent with the electronic contribution being dominant and is able to account for the high Tc by itself. A time-resolved optical technique resolves the influence of lattice dynamics on electron pairing in a cuprate. Unveiling the nature of the bosonic excitations that mediate the formation of Cooper pairs is a key issue for understanding unconventional superconductivity. A fundamental step toward this goal would be to identify the relative weight of the electronic and phononic contributions to the overall frequency (Ω)–dependent bosonic function, Π(Ω). We performed optical spectroscopy on Bi2Sr2Ca0.92Y0.08Cu2O8+δ crystals with simultaneous time and frequency resolution; this technique allowed us to disentangle the electronic and phononic contributions by their different temporal evolution. The spectral distribution of the electronic excitations and the strength of their interaction with fermionic quasiparticles fully account for the high critical temperature of the superconducting phase transition.

Evidence for superconductivity in Li-decorated monolayer graphene

Significance Although superconductivity is well-known in intercalated bulk graphite, the ultimate goal of inducing superconductivity in single-layer graphene has not yet been achieved. We have here developed an experiment that combines ultralow-temperature (5 K) and ultrahigh-vacuum (10−11 torr) sample preparation with high-resolution angle-resolved photoemission spectroscopy (ARPES). We show that decorating monolayer graphene with a layer of lithium atoms enhances the electron–phonon coupling to the point where a superconducting state can be stabilized at low temperature. Measurements of the size of the superconducting gap by ARPES suggest a Tc of about 5.9 K. This result constitutes the first observation, to our knowledge, of superconductivity in monolayer graphene. Given the massive scientific and technological interest in graphene, our findings will have significant cross-disciplinary impact. Monolayer graphene exhibits many spectacular electronic properties, with superconductivity being arguably the most notable exception. It was theoretically proposed that superconductivity might be induced by enhancing the electron–phonon coupling through the decoration of graphene with an alkali adatom superlattice [Profeta G, Calandra M, Mauri F (2012) Nat Phys 8(2):131–134]. Although experiments have shown an adatom-induced enhancement of the electron–phonon coupling, superconductivity has never been observed. Using angle-resolved photoemission spectroscopy (ARPES), we show that lithium deposited on graphene at low temperature strongly modifies the phonon density of states, leading to an enhancement of the electron–phonon coupling of up to λ≃0.58. On part of the graphene-derived π∗-band Fermi surface, we then observe the opening of a Δ≃0.9-meV temperature-dependent pairing gap. This result suggests for the first time, to our knowledge, that Li-decorated monolayer graphene is indeed superconducting, with Tc≃5.9 K.

Resonant X-Ray Scattering Studies of Charge Order in Cuprates

X-ray techniques have been used for more than a century to study the atomic and electronic structure in practically any type of material. The advent of correlated electron systems, in particular complex oxides, brought about new scientific challenges and opportunities for the advancement of conventional X-ray methods. In this context, the need for new approaches capable of selectively sensing new forms of orders involving all degrees of freedom—charge, orbital, spin, and lattice—paved the way for the emergence and success of resonant X-ray scattering, which has become an increasingly popular and powerful tool for the study of electronic ordering phenomena in solids. We review the recent resonant X-ray scattering breakthroughs in the copper oxide high-temperature superconductors, in particular regarding the phenomenon of charge order, a broken-symmetry state occurring when valence electrons self-organize into periodic structures. After a brief historical perspective on charge order, we outline the mileston...