Coupled superconducting and magnetic order in CeCoIn5

Strong magnetic fluctuations can provide a coupling mechanism for electrons that leads to unconventional superconductivity. Magnetic order and superconductivity have been found to coexist in a number of magnetically mediated superconductors, but these order parameters generally compete. We report that close to the upper critical field, CeCoIn5 adopts a multicomponent ground state that simultaneously carries cooperating magnetic and superconducting orders. Suppressing superconductivity in a first-order transition at the upper critical field leads to the simultaneous collapse of the magnetic order, showing that superconductivity is necessary for the magnetic order. A symmetry analysis of the coupling between the magnetic order and the superconducting gap function suggests a form of superconductivity that is associated with a nonvanishing momentum.     

Strongly correlated superconductivity

High-temperature superconductivity in doped Mott insulators such as the cuprates contradicts the conventional wisdom that electron repulsion is detrimental to superconductivity. Because doped fullerene conductors are also strongly correlated, the recent discovery of high-critical-temperature, presumably s-wave, superconductivity in C60 field effect devices is even more puzzling. We examine a dynamical mean-field solution of a model for electron-doped fullerenes that shows how strong correlations can indeed enhance superconductivity close to the Mott transition. We argue that the mechanism responsible for this enhancement could be common to a wider class of strongly correlated models, including those for cuprate superconductors.     

Magnetic field-induced superconductivity in the ferromagnet URhGe

In several metals, including URhGe, superconductivity has recently been observed to appear and coexist with ferromagnetism at temperatures well below that at which the ferromagnetic state forms. However, the material characteristics leading to such a state of coexistence have not yet been fully elucidated. We report that in URhGe there is a magnetic transition where the direction of the spin axis changes when a magnetic field of 12 tesla is applied parallel to the crystal b axis. We also report that a second pocket of superconductivity occurs at low temperature for a range of fields enveloping this magnetic transition, well above the field of 2 tesla at which superconductivity is first destroyed. Our findings strongly suggest that excitations in which the spins rotate stimulate superconductivity in the neighborhood of a quantum phase transition under high magnetic field.     

Lattice effect of strong electron correlation: implication for ferroelectricity and superconductivity

Much theoretical work has been devoted to understanding the role of strong electron correlations in high-temperature superconductivity mainly through magnetic interactions, but the possible role of electron correlation in ferroelectricity of metal oxides has not received attention. Diagonalization of a simple many-body, tight-binding Hamiltonian shows that the electron-lattice interaction is dramatically enhanced in some cases by strong electron correlation because of deformation-induced charge transfer. This effect may be closely related to ferroelectricity and superconductivity in transition metal oxides.     

The challenge of unconventional superconductivity

During the past few decades, several new classes of superconductors have been discovered that do not appear to be related to traditional superconductors. The source of the superconductivity of these materials is likely different from the electron-ion interactions that are at the heart of conventional superconductivity. Developing a rigorous theory for any of these classes of materials has proven to be a difficult challenge and will remain one of the major problems in physics in the decades to come.     

Orbital-independent superconducting gaps in iron pnictides

The origin of superconductivity in the iron pnictides has been attributed to antiferromagnetic spin ordering that occurs in close combination with a structural transition, but there are also proposals that link superconductivity to orbital ordering. We used bulk-sensitive laser angle-resolved photoemission spectroscopy on BaFe(2)(As(0.65)P(0.35))(2) and Ba(0.6)K(0.4)Fe(2)As(2) to elucidate the role of orbital degrees of freedom on the electron-pairing mechanism. In strong contrast to previous studies, an orbital-independent superconducting gap magnitude was found for the hole Fermi surfaces. Our result is not expected from the superconductivity associated with spin fluctuations and nesting, but it could be better explained invoking magnetism-induced interorbital pairing, orbital fluctuations, or a combination of orbital and spin fluctuations. Regardless of the interpretation, our results impose severe constraints on theories of iron pnictides.     

The electron-pairing mechanism of iron-based superconductors

The past three years have witnessed the discovery of a series of novel high-temperature superconductors. Trailing behind the cuprates, these iron-based compounds are the second-highest-temperature superconducting material family known to date. Despite the marked differences in the chemical composition, these materials share many properties with the cuprates and offer the hope of finally unveiling the secret of high-temperature superconductivity. The main theme of this review is the electron-pairing mechanism responsible for their superconductivity. We discuss the progress in this young field and point out the open issues.     

The relationship between high-temperature superconductivity and the fractional quantum Hall effect

The case is made that the spin-liquid state of a Mott insulator, hypothesized to exist by Anderson and identified by him as the correct context for discussing high-temperature superconductors, occurs in these materials and exhibits the principles of fractional quantization identified in the fractional quantum Hall effect. The most important of these is that particles carrying a fraction of an elementary quantum number, in this case spin, attract one another by a powerful gauge force, which can lead to a new kind of superconductivity. The temperature scale for the superconductivity is set by an energy gap in the spin-wave spectrum, which is also the fundamental measure of how "liquid" the spins are.     

Electronic States of KxC60: Insulating, Metallic, and Superconducting Character

The recent report of electrical conductivity in the alkali metal fullerides and the discovery of superconductivity at 18 K for KxC(60) has raised fundamental questions about the electronic states on either side of the Fermi level, their occupancy with K intercalation, and the mechanism of superconductivity. Direct photoemission evidence is presented of filling of bands derived from the lowest unoccupied molecular orbital as a function of K incorporation for the metallic and insulating phases. This filling is not rigid band-like, and it reflects disorder in the K sites. Theoretical analysis indicates that KxC(60) is a strong coupling superconductor, and we suggest that the enhanced electron-phonon interaction is related to the unique hybridization of the C sp-derived states.     

Metallic CsI at pressures of up to 220 gigapascals

Direct electrical transport measurements in a diamond anvil cell provide evidence for the metallization of cesium iodide (CsI) at a pressure of 115 gigapascals. A drop in the temperature dependence of the resistance was found at pressures above 180 gigapascals, indicating that the CsI was superconductive. The superconductivity changed under the influence of a magnetic field to a lower critical temperature and disappeared above 0.3 tesla. The highest critical temperature at which superconductivity was observed was 2 kelvin, and the critical temperature decreased with increasing pressure.     

Light-induced superconductivity in a stripe-ordered cuprate

One of the most intriguing features of some high-temperature cuprate superconductors is the interplay between one-dimensional "striped" spin order and charge order, and superconductivity. We used mid-infrared femtosecond pulses to transform one such stripe-ordered compound, nonsuperconducting La(1.675)Eu(0.2)Sr(0.125)CuO(4), into a transient three-dimensional superconductor. The emergence of coherent interlayer transport was evidenced by the prompt appearance of a Josephson plasma resonance in the c-axis optical properties. An upper limit for the time scale needed to form the superconducting phase is estimated to be 1 to 2 picoseconds, which is significantly faster than expected. This places stringent new constraints on our understanding of stripe order and its relation to superconductivity.     

A superconducting field-effect switch

We report here on a novel realization of a field-effect device that allows switching between insulating and superconducting states, which is the widest possible variation of electrical properties of a material. We chose C(60) as the active material because of its low surface state density and observed superconductivity in alkali metal-doped C(60). We induced three electrons per C(60) molecule in the topmost molecular layer of a crystal with the field-effect device, creating a superconducting switch operating up to 11 kelvin. An insulator was thereby transformed into a superconductor. This technique offers new opportunities for the study of superconductivity as a function of carrier concentration.     

Electronic origin of the inhomogeneous pairing interaction in the high-Tc superconductor Bi2Sr2CaCu2O8+delta

Identifying the mechanism of superconductivity in the high-temperature cuprate superconductors is one of the major outstanding problems in physics. We report local measurements of the onset of superconducting pairing in the high-transition temperature (Tc) superconductor Bi2Sr2CaCu2O8+delta using a lattice-tracking spectroscopy technique with a scanning tunneling microscope. We can determine the temperature dependence of the pairing energy gaps, the electronic excitations in the absence of pairing, and the effect of the local coupling of electrons to bosonic excitations. Our measurements reveal that the strength of pairing is determined by the unusual electronic excitations of the normal state, suggesting that strong electron-electron interactions rather than low-energy (<0.1 volts) electron-boson interactions are responsible for superconductivity in the cuprates.     

c-axis electrodynamics as evidence for the interlayer theory of high-temperature superconductivity

In the interlayer theory of high-temperature superconductivity, the interlayer pair tunneling energy (similar to the Josephson or Lawrence-Doniach energy) is the motivation for superconductivity. This connection requires two experimentally verifiable identities: the coherent normal-state conductance must be smaller than the "Josephson" coupling energy, and the Josephson coupling energy must be equal to the condensation energy of the superconductor. The first condition is well satisfied in the only case that is relevant, (La, Sr)2CuO4, but the second condition has been questioned. It is satisfied for all dopings in (La,Sr)2CuO4 and also in optimally doped Hg(Ba)2CuO5, which was measured recently, but seems to be strongly violated in measurements on single crystals of Tl2Ba2CuO6.     

Photoemission spectroscopy of the high-temperature superconductivity gap

Superconductivity is related to the presence of a narrow forbidden gap in the spectrum of the possible energies for the electrons in the material. These "superconductivity gaps" have traditionally been studied with tunneling and infrared absorption experiments. A third, powerful technique has been made possible by the discovery of hightransition temperature materials: the direct observation of the gap in photoemission spectra. The data analysis requires a careful reconsideration of the standard Einstein-Fermi model of the photoelectric effect. The conclusions are surprisingly simple and offer an alternate way to measure superconductivity gaps. This approach can also be used to study the directional properties of the gap, phenomena related to the coherence length, and possible departures from Fermi-liquid behavior.     

Superconductivity at 40 K in the Oxygen-Defect Perovskites La2-xSrxCuO4-y

Structural, magnetic, and electronic properties of compounds in the series La2-xSrx CuO4-y for 0.05 相似文献   

Superconductivity-induced transfer of in-plane spectral weight in Bi2Sr2CaCu2O8+delta

Optical data are reported on a spectral weight transfer over a broad frequency range of Bi2Sr2CaCu2O8+delta, when this material became superconducting. Using spectroscopic ellipsometry, we observed the removal of a small amount of spectral weight in a broad frequency band from 10(4) cm(-1) to at least 2 x 10(4) cm(-1), due to the onset of superconductivity. We observed a blue shift of the ab-plane plasma frequency when the material became superconducting, indicating that the spectral weight was transferred to the infrared range. Our observations are in agreement with models in which superconductivity is accompanied by an increased charge carrier spectral weight. The measured spectral weight transfer is large enough to account for the condensation energy in these compounds.     

Turning down the heat on thin films

Superconducting thin films will be essential to any practical application of superconductivity to microelectronics. Scientists have now succeeded in putting these thin films onto silicon, which is the base element in most integrated circuits.     

The ground state of the pseudogap in cuprate superconductors

We present studies of the electronic structure of La(2-x)BaxCuO4, a system where the superconductivity is strongly suppressed as static spin and charge orders or "stripes" develop near the doping level of x = (1/8). Using angle-resolved photoemission and scanning tunneling microscopy, we detect an energy gap at the Fermi surface with magnitude consistent with d-wave symmetry and with linear density of states, vanishing only at four nodal points, even when superconductivity disappears at x = (1/8). Thus, the nonsuperconducting, striped state at x = (1/8) is consistent with a phase-incoherent d-wave superconductor whose Cooper pairs form spin-charge-ordered structures instead of becoming superconducting.     

Superconductivity and antiferromagnetism in boron-rich lattices

Ferromagnetism, antiferromagnetism, or superconductivity has been discovered in most hexa- and dodecaborides.