Genda Gu

Quantum magnetic excitations from stripes in copper oxide superconductors

In the copper oxide parent compounds of the high-transition-temperature superconductors the valence electrons are localized—one per copper site—by strong intra-atomic Coulomb repulsion. A symptom of this localization is antiferromagnetism, where the spins of localized electrons alternate between up and down. Superconductivity appears when mobile ‘holes’ are doped into this insulating state, and it coexists with antiferromagnetic fluctuations. In one approach to describing the coexistence, the holes are believed to self-organize into ‘stripes’ that alternate with antiferromagnetic (insulating) regions within copper oxide planes, which would necessitate an unconventional mechanism of superconductivity. There is an apparent problem with this picture, however: measurements of magnetic excitations in superconducting YBa2Cu3O6+x near optimum doping are incompatible with the naive expectations for a material with stripes. Here we report neutron scattering measurements on stripe-ordered La1.875Ba0.125CuO4. We show that the measured excitations are, surprisingly, quite similar to those in YBa2Cu3O6+x (refs 9, 10) (that is, the predicted spectrum of magnetic excitations is wrong). We find instead that the observed spectrum can be understood within a stripe model by taking account of quantum excitations. Our results support the concept that stripe correlations are essential to high-transition-temperature superconductivity.

Ubiquitous Interplay Between Charge Ordering and High-Temperature Superconductivity in Cuprates

Besides superconductivity, copper-oxide high-temperature superconductors are susceptible to other types of ordering. We used scanning tunneling microscopy and resonant elastic x-ray scattering measurements to establish the formation of charge ordering in the high-temperature superconductor Bi2Sr2CaCu2O8+x. Depending on the hole concentration, the charge ordering in this system occurs with the same period as those found in Y-based or La-based cuprates and displays the analogous competition with superconductivity. These results indicate the similarity of charge organization competing with superconductivity across different families of cuprates. We observed this charge ordering to leave a distinct electron-hole asymmetric signature (and a broad resonance centered at +20 milli–electron volts) in spectroscopic measurements, indicating that it is likely related to the organization of holes in a doped Mott insulator. 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.

Electron–phonon coupling reflecting dynamic charge inhomogeneity in copper oxide superconductors

The attempt to understand copper oxide superconductors is complicated by the presence of multiple strong interactions in these systems. Many believe that antiferromagnetism is important for superconductivity, but there has been renewed interest in the possible role of electron–lattice coupling. The conventional superconductor MgB2 has a very strong electron–lattice coupling, involving a particular vibrational mode (phonon) that was predicted by standard theory and confirmed quantitatively by experiment. Here we present inelastic scattering measurements that show a similarly strong anomaly in the Cu–O bond-stretching phonon in the copper oxide superconductors La2-xSrxCuO4 (with x = 0.07, 0.15). Conventional theory does not predict such behaviour. The anomaly is strongest in La1.875Ba0.125CuO4 and La1.48Nd0.4Sr0.12CuO4, compounds that exhibit spatially modulated charge and magnetic order, often called stripe order; it occurs at a wave vector corresponding to the charge order. These results suggest that this giant electron–phonon anomaly, which is absent in undoped and over-doped non-superconductors, is associated with charge inhomogeneity. It follows that electron–phonon coupling may be important to our understanding of superconductivity, although its contribution is likely to be indirect.

Fluctuating stripes at the onset of the pseudogap in the high-Tc superconductor Bi2Sr2CaCu2O8+x

Doped Mott insulators have a strong propensity to form patterns of holes and spins often referred to as stripes. In copper oxides, doping also gives rise to the pseudogap state, which can be transformed into a high-temperature superconducting state with sufficient doping or by reducing the temperature. A long-standing issue has been the interplay between the pseudogap, which is generic to all hole-doped copper oxide superconductors, and stripes, whose static form occurs in only one family of copper oxides over a narrow range of the phase diagram. Here we report observations of the spatial reorganization of electronic states with the onset of the pseudogap state in the high-temperature superconductor Bi2Sr2CaCu2O8+x, using spectroscopic mapping with a scanning tunnelling microscope. We find that the onset of the pseudogap phase coincides with the appearance of electronic patterns that have the predicted characteristics of fluctuating stripes. As expected, the stripe patterns are strongest when the hole concentration in the CuO2 planes is close to 1/8 (per copper atom). Although they demonstrate that the fluctuating stripes emerge with the onset of the pseudogap state and occur over a large part of the phase diagram, our experiments indicate that the stripes are a consequence of pseudogap behaviour rather than its cause.

The ground state of the pseudogap in cuprate superconductors

We present studies of the electronic structure of La2–xBaxCuO4, a system where the superconductivity is strongly suppressed as static spin and charge orders or “stripes” develop near the doping level of x = ⅛. 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 = ⅛. Thus, the nonsuperconducting, striped state at x = ⅛ is consistent with a phase-incoherent d-wave superconductor whose Cooper pairs form spin-charge–ordered structures instead of becoming superconducting.

Microstructure and structural defects in MgB2 superconductor

Abstract We report a detailed study of the microstructure and defects in sintered polycrystalline magnesium diboride (MgB2). Both transmission electron microscopy and X-ray data reveal that MgO is the major second-phase in our bulk samples. Although MgB2 and MgO have different crystal symmetries, being P6/mmm and Fm-3m, respectively, their stacking sequence of Mg and B (or O) and lattice spacings in certain crystallographic orientations are very similar. The size of MgO varies from 10–500 nm, and its mismatch with the MgB2 matrix can be a source for dislocations. Dislocations in MgB2 often have a Burgers vector of 〈1 0 0〉 . 1/3 〈1 −1 0〉 and 1/3 〈2 1 0〉 partial dislocations and their associated stacking faults were also observed. Since both dislocations and stacking faults are located in the ( 0 0 1 ) basal plane, flux pinning anisotropy is expected. Diffuse scattering analysis suggests that the correlation length along the c-axis for defect-free basal planes is about 50 nm. ( 0 0 1 ) twist grain boundaries (GBs), formed by rotations along the c-axis, are major grain boundaries in MgB2 as a result of the out-of-plane weak bonding between Mg and B atoms. An excess of Mg was observed in some grain boundaries. High-resolution nano-probe electron-energy loss spectroscopy reveals that there is a difference in near edge structure of the boron K-edge acquired from GBs and grain interiors. The change at the edge threshold may be suggestive of variation of the hole concentration that would significantly alter boundary superconductivity.

Enhanced Superconducting Transition Temperature in FeSe0.5Te0.5 Thin Films

We report magnetoresistive and structural measurements of superconducting FeSe0.5Te0.5 epitaxial thin films grown by pulsed laser deposition. Enhanced onset superconducting transition temperature (∼17 K) is observed in some of these films. Structural analysis by x-ray diffraction and high resolution transmission electron microscopy reveal that these films generally have significantly shorter out-of-plane lattice constant c than the bulk value, suggesting that the out-of-plane changes have a dominating impact on the superconducting transition in iron-based superconductors. Our data also indicate that the upper critical field Hc2(0) of those films may reach as high as 50 T.

Electronic Origin of the Inhomogeneous Pairing Interaction in the High-Tc Superconductor Bi2Sr2CaCu2O8+δ

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+δ 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.

Proximity-induced high-temperature superconductivity in the topological insulators Bi 2 Se 3 and Bi 2 Te 3

Interest in the superconducting proximity effect has been reinvigorated recently by novel optoelectronic applications as well as by the possible emergence of the elusive Majorana fermion at the interface between topological insulators and superconductors. Here we produce high-temperature superconductivity in Bi(2)Se(3) and Bi(2)Te(3) via proximity to Bi(2)Sr(2)CaCu(2)O(8+δ), to access higher temperature and energy scales for this phenomenon. This was achieved by a new mechanical bonding technique that we developed, enabling the fabrication of high-quality junctions between materials, unobtainable by conventional approaches. We observe proximity-induced superconductivity in Bi(2)Se(3) and Bi(2)Te(3) persisting up to at least 80 K-a temperature an order of magnitude higher than any previous observations. Moreover, the induced superconducting gap in our devices reaches values of 10 mV, significantly enhancing the relevant energy scales. Our results open new directions for fundamental studies in condensed matter physics and enable a wide range of applications in spintronics and quantum computing.

Fully gapped topological surface states in Bi2Se3 films induced by ad-wave high-temperature superconductor

By growing a topological insulator on top of a high-temperature superconducting substrate it is possible to induce superconductivity in the surface states of the topological insulator. Moreover, the pairing symmetry of the induced superconductivity is s-wave, unlike the d-wave symmetry of the substrate.