Evidence for two superconducting gaps in MgB2

We have measured the Raman spectra of polycrystalline MgB2 from 25 to 1200 cm(-1). A superconductivity-induced redistribution in the electronic Raman continuum was observed. Two pair-breaking peaks appear in the spectra, suggesting the presence of two superconducting gaps. The measured spectra were analyzed using a quasi-two-dimensional model in which two s-wave superconducting gaps open on two sheets of Fermi surface. For the gap values we have obtained Delta(1) = 22 cm(-1) ( 2.7 meV) and Delta(2) = 50 cm(-1) ( 6.2 meV). Our results suggest that a conventional phonon-mediated pairing mechanism occurs in the planar boron sigma bands and is responsible for the superconductivity of MgB2.     

Strongly correlated s-wave superconductivity in the N-type infinite-layer cuprate

Quasiparticle tunneling spectra of the electron-doped ( n-type) infinite-layer cuprate Sr0.9La0.1CuO2 reveal characteristics that counter a number of common phenomena in the hole-doped ( p-type) cuprates. The optimally doped Sr0.9La0.1CuO2 with T(c) = 43 K exhibits a momentum-independent superconducting gap Delta = 13.0+/-1.0 meV that substantially exceeds the BCS value, and the spectral characteristics indicate insignificant quasiparticle damping by spin fluctuations and the absence of pseudogap. The response to quantum impurities in the Cu sites also differs fundamentally from that of the p-type cuprates with d(x(2)-y(2))-wave pairing symmetry.     

Spin resonance in the d-wave superconductor CeCoIn5

Neutron scattering is used to probe antiferromagnetic spin fluctuations in the d-wave heavy fermion superconductor CeCoIn5 (T_(c)=2.3 K). Superconductivity develops from a state with slow (variant Planck's over 2piGamma=0.3+/-0.15 meV) commensurate [Q_(0)=(1/2,1/2,1/2)] antiferromagnetic spin fluctuations and nearly isotropic spin correlations. The characteristic wave vector in CeCoIn5 is the same as CeIn3 but differs from the incommensurate wave vector measured in antiferromagnetically ordered CeRhIn5. A sharp spin resonance (variant Planck's over 2piGamma<0.07 meV) at variant Planck's over 2piomega=0.60+/-0.03 meV develops in the superconducting state removing spectral weight from low-energy transfers. The presence of a resonance peak is indicative of strong coupling between f-electron magnetism and superconductivity and consistent with a d-wave gap order parameter satisfying Delta(q+Q0)=-Delta(q).     

Experimental evidence of s-wave superconductivity in bulk CaC6

The temperature dependence of the in-plane magnetic penetration depth, lambda(ab)(T), has been measured in a c-axis oriented polycrystalline CaC(6) bulk sample using a high-resolution mutual inductance technique. A clear exponential behavior of lambda(ab)(T) has been observed at low temperatures, strongly suggesting isotropic s-wave pairing. Data fit using the standard BCS theory yields lambda(ab)(0) = (720 +/- 80) A and delta(0) = (1.79 +/- 0.08) meV. The ratio 2delta(0)/k(B)T(c) = (3.6 +/- 0.2) gives indication for a weakly coupled superconductor.     

High-resolution photoemission study of MgB2

We have performed high-resolution photoemission spectroscopy on MgB2 and observed opening of a superconducting gap with a narrow coherent peak. We found that the superconducting gap is s like with the gap value ( Delta) of 4.5+/-0.3 meV at 15 K. The temperature dependence (15-40 K) of the gap value follows well the BCS form, suggesting that 2Delta/k(B)T(c) at T = 0 is about 3. No pseudogap behavior is observed in the normal state. The present results strongly suggest that MgB2 is categorized into a phonon-mediated BCS superconductor in the weak-coupling regime.     

Indication of charge-density-wave formation in Bi(111)

Photoemission spectroscopy of Bi(111) reveals a small hexagonal two-dimensional Fermi surface (FS) associated with an electron band centered in the surface Brillouin zone. Along the hexagon the Fermi momentum k(F) ranges from 0.053 to 0.061 A(-1). Temperature dependent valence band spectra show an anisotropic energy gap Delta near the Fermi level. We find a transition temperature of about 75 K. At 11 K, the gap is Delta=4 meV at the corner and Delta=7.5 meV at the side of the hexagon. Arguments based on susceptibility chi(--> q) calculations of a hexagonal FS are used to discuss an incommensurate charge-density-wave (CDW) formation associated with a q(CDW)=0.106 A(-1).     

Experimental evidence for two gaps in the high-temperature La1.83Sr0.17CuO4 superconductor

The in-plane magnetic field penetration depth (lambda(ab)) in single-crystal La1.83Sr0.17CuO4 was investigated by muon-spin rotation (muSR). The temperature dependence of lambda(ab)(-2) has an inflection point around 10-15 K, suggesting the presence of two superconducting gaps: a large gap (Delta(1)(d)) with d-wave and a small gap (Delta(2)(s)) with s-wave symmetry. The zero-temperature values of the gaps at mu(0)H=0.02 T were found to be Delta(1)(d)(0)=8.2(1) meV and Delta(2)(s)(0)=1.57(8) meV.     

Adsorbate-induced pinning of a charge-density wave in a quasi-1D metallic chains: Na on the In/Si(111)-(4x1) surface

We find that foreign adsorbates acting as local impurities can induce a metal-insulator transition by pinning a charge-density wave (CDW) on the quasi-1D metallic In/Si(111)-(4x1) chain system. Our scanning tunneling microscopy image clearly reveals the presence of a new local 4x2 structure nucleated by Na adatoms at room temperature, which turns out to be insulating with a doubled periodicity along the chains. We directly determine a CDW gap energy Delta = 105+/-8 meV by identifying a characteristic loss peak in our high-resolution electron-energy-loss spectra. We thus report the first observation of a local impurity-derived Peierls-like reconstruction of a quasi-1D system.     

Scanning tunneling spectroscopy on the novel superconductor CaC6

We present scanning tunneling microscopy and spectroscopy of the newly discovered superconductor CaC6. The tunneling conductance spectra, measured between 3 and 15 K, show a clear superconducting gap in the quasiparticle density of states. The gap function extracted from the spectra is in good agreement with the conventional BCS theory with Delta0=1.6+/-0.2 meV. The possibility of gap anisotropy and two-gap superconductivity is also discussed. In a magnetic field, direct imaging of the vortices allows us to deduce a coherence length in the ab plane xiab approximately 33 nm.     

Long-range electron binding to quadrupolar molecules

An excess electron can be bound to a molecule in a very diffuse orbital as a result of the long-range contributions of the molecular electrostatic field. Following a systematic search, we report experimental evidence that quadrupole binding occurs for the trans-succinonitrile molecule (EA=20+/-2 meV), while the gauche-succinonitrile conformer supports a dipole-bound anion state (EA=108+/-10 meV). Theoretical calculations at the DFT/B3LYP level support these interpretations and give electron affinities of 20 and 138 meV, respectively.     

Evidence for two superconducting energy gaps in MgB(2) by point-contact spectroscopy

Experimental support is found for the multiband model of the superconductivity in the recently discovered system MgB(2) with the transition temperature T(c) = 39 K. By means of Andreev reflection, evidence is obtained for two distinct superconducting energy gaps. The sizes of the two gaps ( Delta(S) = 2.8 meV and Delta(L) = 7 meV) are, respectively, smaller and larger than the expected weak coupling value. Because of the temperature smearing of the spectra the two gaps are hardly distinguishable at elevated temperatures, but when a magnetic field is applied the presence of two gaps can be demonstrated close to the bulk T(c) in the raw data.     

High-energy spin excitations in the electron-doped superconductor Pr(0.88)LaCe(0.12)CuO(4-delta) with T(c) = 21 K

We use high-resolution inelastic neutron scattering to study the low-temperature magnetic excitations of the electron-doping superconductor Pr(0.88)LaCe(0.12)CuO(4-delta) (T(c) = 21 +/- 1 K) over a wide energy range (4 meV < or = homega < or = 330 meV). The effect of electron doping is to cause a wave vector (Q) broadening in the low-energy (homega < or = 80 meV) commensurate spin fluctuations at (0.5, 0.5) and to suppress the intensity of spin-wave-like excitations at high energies (homega > or = 100 meV). This leads to a substantial redistribution in the spectrum of the local dynamical spin susceptibility chi'(omega), and reveals a new energy scale similar to that of the lightly hole-doped YB2Cu3O(6.353) (T(c) = 18 K).     

Zero-point vibration of hydrogen adsorbed on Si and Pt surfaces

Hydrogen atoms adsorbed on Si(111) and Pt(111) were investigated by nuclear reaction analysis (NRA) using 1H(15N,alphagamma)12C. From measurements of the NRA spectrum at normal and tilted ion incidences the zero-point vibrational energies of H on Si(111) in the perpendicular and parallel directions were found to be 123.4+/-4.6 and 44.6+/-6.2 meV, respectively, which are consistent with harmonic potentials. The zero-point energies obtained for Pt(111)-H were 80.8+/-3.9 and 62.1+/-6.0 meV for perpendicular and parallel directions, respectively. These results indicate that the stretching mode is harmonic, while the bending mode is strongly anharmonic.     

Physical properties of the noncentrosymmetric superconductor Mg10Ir19B16

Specific heat, electrical resistivity, and magnetic susceptibility measurements on a high quality sample of Mg10Ir19B16 provide a self-consistent determination of its superconducting properties. They indicate that Mg10Ir19B16 is a type-II superconductor [T{C}=4.45 K, kappa(0) approximately 20], with an electron-phonon coupling constant lambda{ep}=0.66. An analysis of the T-dependent specific heat shows that superconducting properties are dominated by an s-wave gap (Delta=0.7 meV). Point contact tunneling data provide evidence for multiple superconducting gaps, as expected from strong asymmetric spin-orbit coupling.     

Electron-phonon coupling in high-temperature cuprate superconductors determined from electron relaxation rates

We determined electronic relaxation times via pump-probe optical spectroscopy using sub-15 fs pulses for the normal state of two different cuprate superconductors. We show that the primary relaxation process is the electron-phonon interaction and extract a measure of its strength, the second moment of the Eliashberg function λ[ω2] = 800 ± 200 meV2 for La(1.85)Sr(0.15)CuO4 and λ[ω2] = 400 ± 100 meV2 for YBa(2)Cu(3)O(6.5). These values suggest a possible fundamental role of the electron-phonon interaction in the superconducting pairing mechanism.     

Direct measurement of the spin-orbit interaction in a two-electron InAs nanowire quantum dot

We demonstrate control of the electron number down to the last electron in tunable few-electron quantum dots defined in catalytically grown InAs nanowires. Using low temperature transport spectroscopy in the Coulomb blockade regime, we propose a method to directly determine the magnitude of the spin-orbit interaction in a two-electron artificial atom with strong spin-orbit coupling. Because of a large effective g factor |g(*)|=8+/-1, the transition from a singlet S to a triplet T+ ground state with increasing magnetic field is dominated by the Zeeman energy rather than by orbital effects. We find that the spin-orbit coupling mixes the T+ and S states and thus induces an avoided crossing with magnitude Delta(SO)=0.25+/-0.05 meV. This allows us to calculate the spin-orbit length lambda(SO) approximately 127 nm in such systems using a simple model.     

Search for T, CP, and CPT violation in B0-B0 mixing with inclusive dilepton events

We report the results of a search for T, CP, CPT, and violation in B0-B0 mixing using an inclusive dilepton sample collected by the BABAR experiment at the PEP-II factory. Using a sample of 232 x 10(6) BB pairs, we measure the T and CP violation parameter |q/p| - 1 = (-0.8 +/- 2.7(stat) +/- 1.9(syst) x 10(-3), and the CPT and CP parameters Imz = (13.9 +/- 7.3(stat) +/- 3.2(syst)) x 10(-3) and Delta Gamma x Rez = (7.1 +/- 3.9(stat) +/- 2.0(stat)) x 10(-3) ps(-1). The statistical correlation between the measurements of Imz and Delta Gamma x Rez is 76%.     

Bragg diffraction of microcavity polaritons by a surface acoustic wave

Bragg scattering of polaritons by a coherent acoustic wave is mediated and strongly enhanced by the exciton states resonant with the acoustic and optic fields in the intraband and interband transitions, respectively. In this case, in contrast with conventional acousto-optics, the resonantly enhanced Bragg spectra reveal the multiple orders of diffracted light. For polaritons in GaAs microcavities driven by a surface acoustic wave of nu(SAW)=1 GHz and I(ac)< or approximately 100 W/cm(2) the main acoustically induced band gap can be as large as Delta(MC)(ac) approximately equal to 0.6 meV and the Bragg replicas up to n=3 can be observed.     

In-chain tunneling through charge-density-wave nanoconstrictions and break junctions

We have fabricated longitudinal nanoconstrictions in the charge-density wave conductor (CDW) NbSe3 using a focused ion beam and using a mechanically controlled break-junction technique. Conductance peaks are observed below the TP1=145 K and TP2=59 K CDW transitions, which correspond closely with previous values of the full CDW gaps 2Delta1 and 2Delta2 obtained from photoemission. These results can be explained by assuming CDW-CDW tunneling in the presence of an energy gap corrugation epsilon2 comparable to Delta2, which eliminates expected peaks at +/-|Delta1+Delta2|. The nanometer length scales our experiments imply indicate that an alternative explanation based on tunneling through back-to-back CDW-normal-conductor junctions is unlikely.     

One- and two-triplon spectra of a cuprate ladder

We have performed inelastic neutron scattering on the near ideal spin-ladder compound La4Sr10Cu24O41 as a starting point for investigating doped ladders and their tendency toward superconductivity. A key feature was the separation of one-triplon and two-triplon scattering. Two-triplon scattering is observed quantitatively for the first time and so access is realized to the important strong magnetic quantum fluctuations. The spin gap is found to be 26.4+/-0.3 meV. The data are successfully modeled using the continuous unitary transformation method, and the exchange constants are determined by fitting to be Jleg=186 meV and Jrung=124 meV along the leg and rung, respectively; a substantial cyclic exchange of Jcyc=31 meV is confirmed.