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To access superconductivity via the electric field effect in a clean, two-dimensional device is a central goal of nanoelectronics. Recently, superconductivity has been realized in graphene moiré heterostructures.sup.1-4; however, many of these structures are not mechanically stable, and experiments show signatures of strong disorder. Here we report the observation of superconductivity--manifesting as low or vanishing resistivity at sub-kelvin temperatures--in crystalline rhombohedral trilayer graphene.sup.5,6, a structurally metastable carbon allotrope. Superconductivity occurs in two distinct gate-tuned regions (SC1 and SC2), and is deep in the clean limit defined by the ratio of mean free path and superconducting coherence length. Mapping of the normal state Fermi surfaces by quantum oscillations reveals that both superconductors emerge from an annular Fermi sea, and are proximal to an isospin-symmetry-breaking transition where the Fermi surface degeneracy changes.sup.7. SC1 emerges from a paramagnetic normal state, whereas SC2 emerges from a spin-polarized, valley-unpolarized half-metal.sup.17 and violates the Pauli limit for in-plane magnetic fields by at least one order of magnitude.sup.8,9. We discuss our results in view of several mechanisms, including conventional phonon-mediated pairing.sup.10,11, pairing due to fluctuations of the proximal isospin order.sup.12, and intrinsic instabilities of the annular Fermi liquid.sup.13,14. Our observation of superconductivity in a clean and structurally simple two-dimensional metal provides a model system to test competing theoretical models of superconductivity without the complication of modelling disorder, while enabling new classes of field-effect controlled electronic devices based on correlated electron phenomena and ballistic electron transport. Superconductivity is observed in rhombohedral trilayer graphene in the absence of a moiré superlattice, with two distinct superconducting states both occurring at a symmetry-breaking transition where the Fermi surface degeneracy changes., Author(s): Haoxin Zhou [sup.1] , Tian Xie [sup.1] , Takashi Taniguchi [sup.2] , Kenji Watanabe [sup.3] , Andrea F. Young [sup.1] Author Affiliations: (1) Department of Physics, University of California, [...]

Strange metals possess highly unconventional electrical properties, such as a linear-in-temperature resistivity.sup.1-6, an inverse Hall angle that varies as temperature squared.sup.7-9 and a linear-in-field magnetoresistance.sup.10-13. Identifying the origin of these collective anomalies has proved fundamentally challenging, even in materials such as the hole-doped cuprates that possess a simple bandstructure. The prevailing consensus is that strange metallicity in the cuprates is tied to a quantum critical point at a doping p* inside the superconducting dome.sup.14,15. Here we study the high-field in-plane magnetoresistance of two superconducting cuprate families at doping levels beyond p*. At all dopings, the magnetoresistance exhibits quadrature scaling and becomes linear at high values of the ratio of the field and the temperature, indicating that the strange-metal regime extends well beyond p*. Moreover, the magnitude of the magnetoresistance is found to be much larger than predicted by conventional theory and is insensitive to both impurity scattering and magnetic field orientation. These observations, coupled with analysis of the zero-field and Hall resistivities, suggest that despite having a single band, the cuprate strange-metal region hosts two charge sectors, one containing coherent quasiparticles, the other scale-invariant 'Planckian' dissipators. Measurements of high-field magnetotransport in overdoped cuprates indicate that the strange-metal regime exists beyond the critical doping, and that it has both coherent and incoherent contributions., Author(s): J. Ayres [sup.1] [sup.2] [sup.3] , M. Berben [sup.2] [sup.3] , M. Culo [sup.2] [sup.3] [sup.10] , Y.-T. Hsu [sup.2] [sup.3] , E. van Heumen [sup.4] [sup.5] , Y. [...]

Moiré quantum matter has emerged as a materials platform in which correlated and topological phases can be explored with unprecedented control. Among them, magic-angle systems constructed from two or three layers of graphene have shown robust superconducting phases with unconventional characteristics.sup.1-5. However, direct evidence of unconventional pairing remains to be experimentally demonstrated. Here we show that magic-angle twisted trilayer graphene exhibits superconductivity up to in-plane magnetic fields in excess of 10 T, which represents a large (2-3 times) violation of the Pauli limit for conventional spin-singlet superconductors.sup.6,7. This is an unexpected observation for a system that is not predicted to have strong spin-orbit coupling. The Pauli-limit violation is observed over the entire superconducting phase, which indicates that it is not related to a possible pseudogap phase with large superconducting amplitude pairing. Notably, we observe re-entrant superconductivity at large magnetic fields, which is present over a narrower range of carrier densities and displacement fields. These findings suggest that the superconductivity in magic-angle twisted trilayer graphene is likely to be driven by a mechanism that results in non-spin-singlet Cooper pairs, and that the external magnetic field can cause transitions between phases with potentially different order parameters. Our results demonstrate the richness of moiré superconductivity and could lead to the design of next-generation exotic quantum matter. A large violation of the Pauli limit and re-entrant superconductivity in a magnetic field is reported for magic-angle twisted trilayer graphene, suggesting that the spin configuration of the superconducting state of this material is unlikely to consist of spin singlets., Author(s): Yuan Cao [sup.1] , Jeong Min Park [sup.1] , Kenji Watanabe [sup.2] , Takashi Taniguchi [sup.3] , Pablo Jarillo-Herrero [sup.1] Author Affiliations: (1) Department of Physics, Massachusetts Institute of [...]

Superconductivity and magnetism have been observed in layered graphene in which the sheets are twisted with respect to each other. But a simpler, more stable graphene system also exhibits these phases. Trilayer graphene system without a twist shows different phases., Author(s): Thiti Taychatanapat Author Affiliations: Untwisted trilayer graphene hosts superconductivity and magnetism Electrons typically propagate without interacting in graphene, a single layer of carbon atoms arranged in a honeycomb lattice. [...]

The behaviour of strongly correlated materials, and in particular unconventional superconductors, has been studied extensively for decades, but is still not well understood. This lack of theoretical understanding has motivated the development of experimental techniques for studying such behaviour, such as using ultracold atom lattices to simulate quantum materials. Here we report the realization of intrinsic unconventional superconductivitywhich cannot be explained by weak electronphonon interactionsin a two-dimensional superlattice created by stacking two sheets of graphene that are twisted relative to each other by a small angle. For twist angles of about 1.1the first magic anglethe electronic band structure of this twisted bilayer graphene exhibits flat bands near zero Fermi energy, resulting in correlated insulating states at half-filling. Upon electrostatic doping of the material away from these correlated insulating states, we observe tunable zero-resistance states with a critical temperature of up to 1.7 kelvin. The temperaturecarrier-density phase diagram of twisted bilayer graphene is similar to that of copper oxides (or cuprates), and includes dome-shaped regions that correspond to superconductivity. Moreover, quantum oscillations in the longitudinal resistance of the material indicate the presence of small Fermi surfaces near the correlated insulating states, in analogy with underdoped cuprates. The relatively high superconducting critical temperature of twisted bilayer graphene, given such a small Fermi surface (which corresponds to a carrier density of about 10[sup.11] per square centimetre), puts it among the superconductors with the strongest pairing strength between electrons. Twisted bilayer graphene is a precisely tunable, purely carbon-based, two-dimensional superconductor. It is therefore an ideal material for investigations of strongly correlated phenomena, which could lead to insights into the physics of high-critical-temperature superconductors and quantum spin liquids., Author(s): Yuan Cao (corresponding author) [1]; Valla Fatemi [1]; Shiang Fang [2]; Kenji Watanabe [3]; Takashi Taniguchi [3]; Efthimios Kaxiras [2, 4]; Pablo Jarillo-Herrero (corresponding author) [1] Strong interactions among [...]

A material system known as magic-angle twisted trilayer graphene exhibits superconductivity. The observation that this superconductivity persists under a strong magnetic field could lead to advances in quantum computation. Evidence of spin-triplet superconductivity in a quasi-2D system., Author(s): Yi-Ting Hsu Author Affiliations: Superconductivity in a graphene system survives a strong magnetic field Quantum phases of matter known as superconductors transmit electrical current with zero resistance. Microscopically, this [...]

The discovery of high-temperature superconductivity in the copper oxides in 1986 triggered a huge amount of innovative scientific inquiry. In the almost three decades since, much has been learned about the novel forms of quantum matter that are exhibited in these strongly correlated electron systems. A qualitative understanding of the nature of the superconducting state itself has been achieved. However, unresolved issues include the astonishing complexity of the phase diagram, the unprecedented prominence of various forms of collective fluctuations, and the simplicity and insensitivity to material details of the 'normal' state at elevated temperatures., The discovery of high-temperature superconductivity in the copper oxide perovskite [La.sub.2-x][Ba.sub.x]Cu[O.sub.4] (ref. 1) ranks among the major scientific events of the twentieth century. The superconducting transition temperatures in the copper [...]

Terahertz-frequency optical pulses can resonantly drive selected vibrational modes in solids and deform their crystal structures (1-3). In complex oxides, this method has been used to melt electronic order (4-6), drive insulator-to-metal transitions (7) and induce superconductivity (8). Strikingly, coherent interlayer transport strongly reminiscent of superconductivity can be transiently induced up to room temperature (300 kelvin) in Y[Ba.sub.2][Cu.sub.3][O.sub.6+x] (refs 9,10). Here we report the crystal structure of this exotic non-equilibrium state, determined by femtosecond X-ray diffraction and ab initio density functional theory calculations. We find that nonlinear lattice excitation in normal-state Y[Ba.sub.2][Cu.sub.3][O.sub.6+x] at above the transition temperature of 52 kelvin causes a simultaneous increase and decrease in the Cu-[O.sub.2] intra-bilayer and, respectively, inter-bilayer distances, accompanied by anisotropic changes in the in-plane O-Cu-O bond buckling. Density functional theory calculations indicate that these motions cause drastic changes in the electronic structure. Among these, the enhancement in the [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] character of the in-plane electronic structure is likely to favour superconductivity., The response of a crystal lattice to strong, resonant excitation of an infrared-active phonon mode can be described by separating the crystal Hamiltonian into its linear and nonlinear terms: H [...]

An outstanding problem in the field of high-transition-temperature (high- [T.sub.c]) superconductivity is the identification of the normal state out of which superconductivity emerges in the mysterious underdoped regime (1). The [...]

We have employed the relativistic mean field (RMF)+BCS approach, to study ground state properties of drip line isotopes of Si. In this approach the single particle continuum corresponding to the RMF is replaced by a set of discrete positive energy states for the calculations of pairing energy. Detailed analysis of separation energy, single particle spectrum, pairing energies, and densities of the nuclei predict weakly bound structure in neutron-rich Si isotopes, which is seen to be due to the large spatial extension of the wave functions for the weakly bound single particle states with low orbital angular momentum having very small centrifugal barriers. PACS Nos.: 21.10.-k, 21.10.Ft, 21.10.Dr, 21.10.Gv, 21.10.-n, 21.60.Jz. Nous avons utilise une approche de champ moyen relativiste (RMF)+BCS pour etudier les proprietes des isotopes du Si pres de la drip line. Ici, le continuum a une particule correspondant au RMF est remplace par un ensemble d'etats discrets a energie positive pour le calcul de l'energie de pairage. L'analyse detaillee de l'energie de separation, du spectre a une particule, des energies de pairage et des densites nucleaires, decrit une structure faiblement liee pour les isotopes de Si riches en neutrons. Nous l'expliquons par la grande extension spatiale des fonctions d'onde pour les etats a une particule faiblement lies, avec faible moment angulaire et de tres basses barrieres centrifuges. [Traduit par la Redaction], 1. Introduction With advancements in nuclear experimental techniques and the development of radioactive nuclear beams facilities around the world, it has been possible to study for the first time the [...]

The physics of the superconducting state in two-dimensional (2D) electron systems is relevant to understanding the high-transition-temperature copper oxide superconductors and for the development of future superconductors based on interface electron systems (1). But it is not yet understood how fundamental superconducting parameters, such as the spectral density of states, change when these superconducting electron systems are depleted of charge carriers. Here we use tunnel spectroscopy with planar junctions to measure the behaviour of the electronic spectral density of states as a function of carrier density, clarifying this issue experimentally. We chose the conducting LaAl[O.sub.3]SrTi[O.sub.3]interface (2) as the 2D superconductor, because this electron system can be tuned continuously with an electric gate field (3). We observed an energy gap of the order of 40 microelectronvolts in the density of states, whose shape is well described by the Bardeen-Cooper-Schrieffer superconducting gap function. In contrast to the dome-shaped dependence of the critical temperature, the gap increases with charge carrier depletion in both the underdoped region and the overdoped region. These results are analogous to the pseudogap behaviour of the high-transition-temperature copper oxide superconductors and imply that the smooth continuation of the superconducting gap into pseudogap-like behaviour could be a general property of 2D superconductivity., One of the main challenges in understanding the superconductivity of copper oxide superconductors is to identify the origin and nature of the pseudogap phase in the underdoped regime (4,5). This [...]

A fundamental and previously unobserved aspect of the Josephson effect is revealed through spectroscopic measurements of the excited Andreev states in superconducting atomic contacts. A new twist on the Josephson effect Brian Josephson proposed in 1962 that a dissipationless 'supercurrent' should flow between two superconductors separated by a weak link such as a tunnel junction. He was right, and the Josephson effect launched a new field of research with applications to magnetometry, medicine and astronomy. This paper describes an aspect of the Josephson effect that has previously been overlooked. Existing Josephson junction applications are based on properties of just the ground state, in which the electron pairs that carry the supercurrent localize at the weak link and form so-called Andreev doublets of ground and excited pair states. Bretheau et al. establish the existence of excited Andreev pair states through spectroscopic measurements of superconducting atomic contacts. This previously overlooked degree of freedom for tunneling electron pairs is a new quantum resource that could be exploited in novel types of superconducting qubits. The Josephson effect describes the flow of supercurrent in a weak link--such as a tunnel junction, nanowire or molecule--between two superconductors.sup.1. It is the basis for a variety of circuits and devices, with applications ranging from medicine.sup.2 to quantum information.sup.3. Experiments using Josephson circuits that behave like artificial atoms.sup.4 are now revolutionizing the way we probe and exploit the laws of quantum physics.sup.5,6. Microscopically, the supercurrent is carried by Andreev pair states, which are localized at the weak link. These states come in doublets and have energies inside the superconducting gap.sup.7,8,9,10. Existing Josephson circuits are based on properties of just the ground state of each doublet, and so far the excited states have not been directly detected. Here we establish their existence through spectroscopic measurements of superconducting atomic contacts. The spectra, which depend on the atomic configuration and on the phase difference between the superconductors, are in complete agreement with theory. Andreev doublets could be exploited to encode information in novel types of superconducting qubits.sup.11,12,13., Author(s): L. Bretheau [sup.1] , Ç. Ã-. Girit [sup.1] , H. Pothier [sup.1] , D. Esteve [sup.1] , C. Urbina [sup.1] Author Affiliations: (1) Quantronics Group, Service de Physique de [...]

The superconducting pairing instability--as determined by a divergence of the two-particle susceptibility--is obtained in the mean field (BCS) approximation in the thermodynamic limit. The usual practice is to examine this property for a finite lattice. We illustrate that, while the conclusions remain unchanged, the technical features are very different in the thermodynamic limit and conform more closely with the usual treatment of phase transitions encountered in, for example, the mean-field paramagnetic-ferromagnetic transition. Furthermore, by going to the extreme dilute limit, one can distinguish three dimensions from one and two dimensions, in which a pairing instability occurs even for two particles. PACS Nos: 74.20.-z, 74.20.Fg L'instabilite de pairage en supraconductivite--determinee par une divergence de la susceptibilite des deux particules --est obtenue ici en approximation de champ moyen (BCS) dans la limite thermodynamique. Il est plus habituel d'examiner cette propriete dans un reseau fini. Nous illustrons que, alors que les conclusions demeurent les memes, les caracteristiques techniques sont tres differentes dans la limite thermodynamique et sont plus conformes au cadre habituel des transitions de phase rencontrees, par exemple, dans la transition en champ moyen paramagnetique-ferromagnetique. De plus, en allant a l'extreme limite de dilution, on peut y distinguer le cas a trois dimensions des cas a une et deux dimensions, dans lesquels une instabilite de pairage apparait, meme pour deux particules. [Traduit par la Redaction], 1. Introduction The Bardeen, Cooper, and Schrieffer (BCS) theory of superconductivity, following the original literature [1], is typically first presented in textbooks as a proposed variational ground state, whose energy [...]

Superconductivity and magnetism generally do not coexist. Changing the relative number of up and down spin electrons disrupts the basic mechanism of superconductivity, where atoms of opposite momentum and spin form Cooper pairs. Nearly forty years ago Fulde and Ferrell (1) and Larkin and Ovchinnikov (2) (FFLO) proposed an exotic pairing mechanism in which magnetism is accommodated by the formation of pairs with finite momentum. Despite intense theoretical and experimental efforts, however, polarized superconductivity remains largely elusive (3). Unlike the three-dimensional (3D) case, theories predict that in one dimension (ID) a state with FFLO correlations occupies a major part of the phase diagram (4-12). Here we report experimental measurements of density profiles of a two-spin mixture of ultracold [sup.6]Li atoms trapped in an array of 1D tubes (a system analogous to electrons in 1D wires). At finite spin imbalance, the system phase separates with an inverted phase profile, as compared to the 3D case. In 1D, we find a partially polarized core surrounded by wings which, depending on the degree of polarization, are composed of either a completely paired or a fully polarized Fermi gas. Our work paves the way to direct observation and characterization of FFLO pairing., The FFLO states are perhaps the most interesting of a number of exotic polarized superconducting phases proposed in the past 40 years. In the original concept of Fulde and Ferrell, [...]

Motivated by the potential high-temperature superconductivity in hydrogen-rich materials, the high-pressure structures of Si[H.sub.4][([H.sub.2]).sub.2] in the pressure range 50-300 GPa were extensively explored by using a genetic algorithm. An intriguing layered orthorhombic (Ccca) structure was revealed to be energetically stable above 248 GPa with the inclusion of zero-point energy. The Ccca structure is metallic and composed of hydrogen shared Si[H.sub.8] dodecahedra layers intercalated by orientationally ordered molecular [H.sub.2]. Application of the Allen-Dynes modified McMillan equation yields remarkably high superconducting temperatures of 98-107 K at 250 GPa, among the highest values reported so far for phonon-mediated superconductors. Analysis reveals a unique superconducting mechanism that the direct interactions between [H.sub.2] and Si[H.sub.4] molecules at high pressure play the major role in the high superconductivity, while the contribution from [H.sub.2] vibrons is minor. H2-containing compounds | high-pressure | high-temperature superconductors | metallization doi/ 10.1073/pnas.1007354107

Finding ways to achieve higher values of the transition temperature, [T.sub.c], in superconductors remains a great challenge. The super-conducting phase is often one of several competing types of electronic order, including antiferromagnetism and charge density waves (1-5). An emerging trend documented in heavy-fermion (1) and organic (2) conductors is that the maximum [T.sub.c] for superconductivity occurs under external conditions that cause the critical temperature for a competing order to go to zero. Recently, such competition has been found in multilayer copper oxide high-temperature superconductors (HTSCs (3-5)) that possess two crystallographically inequivalent Cu[O.sub.2] planes in the unit cell. However, whether the competing electronic state can be suppressed to enhance [T.sub.c] in HTSCs remains an open question. Here we show that pressure-driven phase competition leads to an unusual two-step enhancement of [T.sub.c] in optimally doped trilayer [Bi.sub.2][Sr.sub.2][Ca.sub.2][Cu.sub.3][O.sub.10+δ] (Bi2223). We find that [T.sub.c] first increases with pressure and then decreases after passing through a maximum. Unexpectedly, [T.sub.c] increases again when the pressure is further raised above a critical value of around 24 GPa, surpassing the first maximum. The presence of this critical pressure is a manifestation of the crossover from the competing order to superconductivity in the inner of the three Cu[O.sub.2] planes. We suggest that the increase at higher pressures occurs as a result of competition between pairing and phase ordering in different Cu[O.sub.2] planes., We used Bi2223 single crystals in the present investigation. The structure of Bi2223 consists of two insulating Bi-O and Sr-O blocking layers intercalating in the c-axis direction with two structurally [...]

High-pressure structures of disilane ([Si.sub.2][H.sub.6]) are investigated extensively by means of first-principles density functional theory and a random structure-searching method. Three metallic structures with P-1, Pm-3m, and C2/c symmetries are found, which are more stable than those of [XY.sub.3]-type candidates under high pressure. Enthalpy calculations suggest a remarkably wide decomposition (Si and [H.sub.2]) pressure range below 135 GPa, above which three metallic structures are stable. Perturbative linear-response calculations for Pm-3m disilane at 275 GPa show a large electron-phonon coupling parameter [lambda] of 1.397 and the resulting superconducting critical temperature beyond the order of [10.sup.2] K. metallization | new phase | solid disilane doi/ 10.1073/pnas.1005242107

Ca-III, the first superconducting calcium phase under pressure, was identified as simple-cubic (sc) by previous X-ray diffraction (XRD) experiments. In contrast, all previous theoretical calculations showed that sc had a higher enthalpy than many proposed structures and had an imaginary (unstable) phonon branch. By using our newly developed submicrometer high-pressure single-crystal XRD, cryogenic high-pressure XRD, and theoretical calculations, we demonstrate that Ca-III is neither exactly sc nor any of the lower-enthalpy phases, but sustains the sc-like, primitive unit by a rhombohedral distortion at 300 K and a monoclinic distortion below 30 K. This surprising discovery reveals a scenario that the high-pressure structure of calcium does not go to the zero-temperature global enthalpy minimum but is dictated by high-temperature anharmonicity and low-temperature metastability fine-tuned with phonon stability at the local minimum. high pressure | phase transition | superconductivity doi/ 10.1073/pnas.1005279107

The apparently inimical relationship between magnetism and superconductivity has come under increasing scrutiny in a wide range of material classes, where the free energy landscape conspires to bring them in close proximity to each other. Particularly enigmatic is the case when these phases microscopically interpenetrate, though the manner in which this can be accomplished remains to be fully comprehended. Here, we present combined measurements of elastic neutron scattering, magnetotransport, and heat capacity on a prototypical heavy fermion system, in which antiferromagnetism and superconductivity are observed. Monitoring the response of these states to the presence of the other, as well as to external thermal and magnetic perturbations, points to the possibility that they emerge from different parts of the Fermi surface. Therefore, a single 4f state could be both localized and itinerant, thus accounting for the coexistence of magnetism and superconductivity. strongly correlated electron systems | antiferromagnetism www.pnas.org/cgi/doi/10.1073/pnas.1004958107

The long-standing prediction that hydrogen can assume a metallic state under high pressure, combined with arguments put forward more recently that this state might even be superconducting up to high temperatures, continues to spur tremendous research activities toward the experimental realization of metallic hydrogen. These efforts have however so far been impeded by the enormous challenges associated with the exceedingly large required pressure. Hydrogen-dense materials, of the M[H.sub.4] form (where M can be, e.g., Si, Ge, or Sn) or of the M[H.sub.3] form (with M being, e.g., Al, Sc, Y, or La), allow for the rather exciting opportunity to carry out a proxy study of metallic hydrogen and associated high-temperature superconductivity at pressures within the reach of current techniques. At least one experimental report indicates that a superconducting state might have been observed already in Si[H.sub.4], and several theoretical studies have predicted superconductivity in pressurized hydrogen-rich materials; however, no systematic dependence on the applied pressure has yet been identified so far. In the present work, we have used first-principles methods in an attempt to predict the superconducting critical temperature ([T.sub.c]) as a function of pressure (P) for three metal-hydride systems of the M[H.sub.3] form, namely Sc[H.sub.3], Y[H.sub.3], and La[H.sub.3]. By comparing the obtained results, we are able to point out a general trend in the [T.sub.c]-dependence on P. These gained insights presented here are likely to stimulate further theoretical studies of metallic phases of hydrogen-dense materials and should lead to new experimental investigations of their superconducting properties. ab initio calculations | high pressure | lattice dynamics | metal-hydride | superconductivity www.pnas.org/cgi/doi/10.1073/pnas.0914462107

In quantum information processing, qudits (d-Level systems) are an extension of qubits that could speed up certain computing tasks. We demonstrate the operation of a superconducting phase qudit with a number of levels d up to d = 5 and show how to manipulate and measure the qudit state, including simultaneous control of multiple transitions. We used the qudit to emulate the dynamics of single spins with principal quantum number s = 1/2, 1, and 3/2, allowing a measurement of Berry's phase and the even parity of integer spins (and odd parity of half-integer spins) under 2[pi]-rotation. This extension of the two-Level qubit to a multilevel qudit holds promise for more-complex quantum computational architectures and for richer simulations of quantum mechanical systems.

Understanding the mechanism by which d wave superconductivity in the cuprates emerges and is optimized by doping the Mott insulator is one of the major outstanding problems in condensedmatter physics. Our high-resolution scanning tunneling microscopy measurements of the high--transition temperature ([T.sub.c]) superconductor [Bi.sub.2][Sr.sub.2]Ca[Cu.sub.2][0.sub.8+[delta]] show that samples with different [T.sub.c] values in the low doping regime follow a remarkably universal d wave low-energy excitation spectrum, indicating a doping-independent nodal gap. We demonstrate that [T.sub.c] instead correlates with the fraction of the Fermi surface over which the samples exhibit the universal spectrum. Optimal [T.sub.c] is achieved when all parts of the Fermi surface follow this universal, behavior. Increasing the temperature above [T.sub.c] turns the universal spectrum into an arc of gapless excitations, whereas overdoping breaks down the universal nodal behavior.

Superconductivity in the extreme two-dimensional limit is studied on ultrathin lead films down to two atomic layers, where only a single channel of quantum well states exists. Scanning tunneling spectroscopy reveals that local superconducting order remains robust until two atomic layers, where the transition temperature abruptly plunges to a lower value, depending sensitively on the exact atomic structure of the film. Our result shows that Cooper pairs can still form in the last two-dimensional channel of electron states, although their binding is strongly affected by the substrate.

Numerous phenomenological parallels have been drawn between f- and d- electron systems in an attempt to understand their display of unconventional superconductivity. The microscopics of how electrons evolve from participation in large moment antiferromagnetism to superconductivity in these systems, however, remains a mystery. Knowing the origin of Cooper paired electrons in momentum space is a crucial prerequisite for understanding the pairing mechanism. Of special interest are pressure-induced superconductors [Celn.sub.3] and [CeRhln.sub.5] in which disparate magnetic and superconducting orders apparently coexist--arising from within the same f-electron degrees of freedom. Here, we present ambient pressure quantum oscillation measurements on [Celn.sub.3] that crucially identify the electronic structure--potentially similar to high-temperature superconductors. Heavy hole pockets of f-character are revealed in [Celn.sub.3], undergoing an unexpected effective mass divergence well before the antiferromagnetic critical field. We thus uncover the softening of a branch of quasiparticle excitations located away from the traditional spin fluctuation-dominated antiferromagnetic quantum critical point. The observed Fermi surface of dispersive f-electrons in [Celn.sub.3] could potentially explain the emergence of Cooper pairs from within a strong moment antiferromagnet. Fermi surface | heavy fermion | quantum critical point | Lifshitz transition

We have fabricated nanocomposites consisting of 4-[Angstrom] carbon nanotubes embedded in the 0.7-nm pores of aluminophosphatefive (AFI) zeolite that display a superconducting specific heat transition at 15 K. MicroRaman spectra of the samples show strong and spatially uniform radial breathing mode (RBM) signals at 510 [cm.sup.-1] and 550 [cm.sup.-1], characteristic of the (4, 2) and (5, 0) nanotubes, respectively. The specific heat transition is suppressed at > 2 T, with a temperature dependence characteristic of finite-size effects. Comparison with theory shows the behavior to be consistent with that of a type II BCS superconductor, characterized by a coherence length of 14 [+ or -] 2 nm and a magnetic penetration length of 1.5 [+ or -] 0.7 [micro]m. Four probe and differential resistance measurements have also indicated a superconducting transition initiating at 15 K, but the magnetoresistance data indicate the superconducting network to be inhomogeneous, with a component being susceptible to magnetic fields below 3 T and other parts capable of withstanding a magnetic field of 5 T or beyond. resistance transition | specific heat | superconductivity

Superconductivity was recently observed (1-7) in iron-arsenic-based compounds with a superconducting transition temperature ([T.sub.c]) as high as 56 K, naturally raising comparisons with the high-[T.sub.c] copper oxides. The copper oxides [...]

This article reviews some of the experimental work on the Josephson effects. These effects occur in both superconductivity and superfluidity. In superconductivity they are found when two superconductors are separated by a thin insulating barrier (~[10.sup.-9] m thick), or connected by a weak link of metal. A supercurrent can flow between the superconductors, and electromagnetic and magnetic fields can interact with this supercurrent. The interactions demonstrate quantum effects in a particularly striking way, have led to a better understanding of the super-conducting state, and have also found some interesting applications.