Ultrahigh-resolution Photoemission Spectroscopy Research Articles

Bulk superconducting gap of V3Si studied by low-energy ultrahigh-resolution photoemission spectroscopy

We have performed low-energy ultrahigh-resolution photoemission spectroscopy (PES) of A15 compound V3Si with a xenon-plasma discharge lamp to elucidate the bulk superconducting gap. Below the superconducting transition temperature (Tc=15.9K), we found a sharp quasiparticle peak at the Fermi level in the PES spectrum. The gap spectrum is well fitted by a single s-wave superconducting-gap function together with a dip structure at ∼30meV suggestive of a strong electron-phonon coupling. The anomalous in-gap state previously observed in the PES measurement with high-energy photons is absent or negligibly small in the present bulk-sensitive measurement. The present PES result shows that V3Si is a single-gap s-wave superconductor.

Study of the superconducting gap in a Fe0.5TiSe2 single crystal by ultrahigh-resolution photoemission spectroscopy

The 1T-Fe0.5TiSe2 compound has been investigated by ultrahigh-resolution photoemission spectroscopy in order to reveal changes in the density of states at a possible superconducting transition. The experimental results showed the presence of a superconducting gap with a width of about 2 meV in the spectrum measured at 4.5 K.

Superconducting Gap of Fe-Intercalated TiSe<sub>2</sub> Observed Using Ultrahigh-Resolution Photoemission Spectroscopy

Ultrahigh-resolution photoemission spectroscopy of 1T-Fe0.5TiSe2 was performed in order to study the changes in the density of states across the superconducting transition. We observed the gap about 2 meV in spectrum measured at 4.5 K.

Doping Dependence of Pseudogap in LaFeAsO1-xFx

We have performed systematic ultrahigh-resolution photoemission spectroscopy on the iron-based superconductor LaFeAsO 1- x F x ( x =0-14%) to elucidate the evolution of electronic states with doping. We found that the density of states (DOS) around the Fermi level of the superconducting sample is markedly smaller than that of the undoped sample in contrast to conventional phonon-mediated superconductors. We observed a pseudogap in the DOS above the superconducting transition temperature ( T c ) which opens/closes at a temperature ( T * ) well above T c . The energy scale of pseudogap and the T * value are gradually reduced upon fluorine substitution. The origin of pseudogap is discussed in terms of the partial nesting of Fermi surfaces due to the spin-density-wave formation or the short-range spin correlations.

Revelation of the Role of Impurities and Conduction Electron Density in the High Resolution Photoemission Study of Ferromagnetic Hexaborides

We investigate the temperature evolution of the electronic structure of ferromagnetic CaB6 using ultrahigh resolution photoemission spectroscopy; the electronic structure of paramagnetic LaB6 is used as a reference. High resolution spectra of CaB6 reveal a finite density of states at the Fermi level E(F) at all the temperatures and evidence of impurity induced localized features in the vicinity of E(F), which are absent in the spectra of LaB6. Analysis of the high resolution spectra suggests that disorder in the B sublattice inducing partial localization in the mobile electrons and low electron density at E(F) is important to achieve ferromagnetism in these systems.

Anomalous superconducting-gap symmetry of noncentrosymmetricLa2C3observed by ultrahigh-resolution photoemission spectroscopy

We have performed ultrahigh-resolution photoemission spectroscopy on noncentrosymmetric superconductor ${\mathrm{La}}_{2}{\mathrm{C}}_{3}$ to study the electronic structure near the Fermi level $({E}_{F})$ and the superconducting-gap symmetry. We clearly observed the opening of the superconducting gap, as evidenced by the emergence of a sharp quasiparticle peak just below ${E}_{F}$ together with the leading-edge shift toward higher binding energy. Quantitative analysis of the gap function has revealed a marked deviation from the single $s$- or $d$-wave symmetry, suggesting the existence of multiple superconducting gaps.

Xenon-plasma-light low-energy ultrahigh-resolution photoemission study ofCo(S1−xSex)2(x=0.075)

We have performed low-energy ultrahigh-resolution photoemission spectroscopy on $\mathrm{Co}{({\mathrm{S}}_{1\ensuremath{-}x}{\mathrm{Se}}_{x})}_{2}$ $(x=0.075)$ to elucidate the bulk electronic states responsible for the ferromagnetic transition. By using a newly developed plasma-driven low-energy xenon (Xe) discharge lamp $(h\ensuremath{\nu}=8.436\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$, we clearly observed a sharp quasiparticle peak at the Fermi level together with the remarkable temperature dependence of the electron density of states across the transition temperature. Comparison with the experimental result by the $\mathrm{He}I\ensuremath{\alpha}$ resonance line $(h\ensuremath{\nu}=21.218\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$ indicates that the sharp quasiparticle is of bulk origin and is produced by the Fermi-level crossing of the Co $3d$ ${e}_{g\ensuremath{\downarrow}}$ subband.

Interplay of Superconductivity and Rattling Phenomena inβ-PyrochloreKOs2O6Studied by Photoemission Spectroscopy

The electronic structure near the Fermi level (EF) of the beta-pyrochlore superconductor KOs2O6 is studied using laser-excited ultrahigh-resolution photoemission spectroscopy. The superconducting gap clearly opens across the superconducting transition (Tc=9.6 K), with the strong electron-phonon coupling value of 2Delta(0)/k B Tc>or=4.56. A fitting analysis identifies clear anomalies at Tp=7.5 K in the temperature dependencies of the superconducting gap size and the quasiparticle relaxation lifetime. These anomalies and the fine spectral structures arising from phonons suggest that the existence of the rattling behavior of K ions significantly affects the superconductivity in KOs2O6.

Laser-excited ultrahigh-resolution photoemission spectroscopy of borocarbide superconductor RNi2B2C (R = Y and Er)

We present laser-excited ultrahigh-resolution photoemission spectroscopy of YNi2B2C and ErNi2B2C. The extremely high energy resolution enables us to discuss the superconducting (SC) gap function of YNi2B2C and to measure the density of states which is coexisted between superconductivity and antiferromagnetism in ErNi2B2C. For YNi2B2C, s+g-wave SC gap symmetry cannot reproduce the experimental data, implying importance of taking the complicated electronic structures into consideration. For ErNi2B2C, the observed coherent peak is very broad and finite density of state exists at Fermi level.

Laser-excited ultrahigh-resolution photoemission study of anisotropic s-wave superconductor YNi 2B 2C

We have performed laser-excited ultrahigh-resolution photoemission spectroscopy of Y(Ni 1− x Pt x ) 2B 2C ( x=0.0, 0.2) and numerical simulation using s, anisotropic s and even s+g symmetry to study the superconducting gap anisotropy in borocarbide supercodnuctors. We observed the reduction of gap anisotropy in the x=0.2 sample in comparison to the x=0.0 sample, confirming the anisotropic s-wave gap. We also found that the spectrum of x=0.0 at 3.5 K cannot be fitted well with the s+g-wave symmetry, implying that the s+g symmetry is too simple to describe over all supercodncuting gap electronic structure of borocarbides.

Laser-excited ultrahigh-resolution photoemission spectroscopy of superconducting Na0.35CoO2·1.3H2O

Abstract We have studied the electronic structure near the Fermi level of the layered cobaltate superconductor Na 0.35 CoO 2 ·1.3H 2 O using laser-excited ultrahigh-resolution photoemission spectroscopy (PES). In the normal state, we observed the formation of a pseudogap with an energy scale of ∼20 meV. Across the superconducting transition, we observed the leading-edge shift at 3.5 K compared to that at 10 K, indicating superconducting gap (SC gap) opening having a gap value Δ of 0.5–0.7 meV. The energy scale of the SC gap is ∼20 times smaller than that of the pseudogap, suggesting the existence of competing order parameters at low temperatures.

Ultrahigh-Resolution Photoemission Study ofh-ZrRuP

We have performed ultrahigh-resolution photoemission spectroscopy on the intermetallic superconductor hexagonal h -ZrRuP ( T c = 13 K). We observed the opening of a superconducting gap below T c , and found that the most probable order parameter is s -wave with a gap size (Δ) of 1.8 meV at 6 K. This gap size gives 2 Δ 0 / k B T c ∼3.5, suggesting the weak-coupling nature of the superconductivity in h -ZrRuP. We also observed a pseudogap-like suppression of spectral weight up to ∼45 meV from the Fermi level, indicative of the strong coupling of electrons with phonons and/or the charge density wave fluctuation.

Direct observation of superconducting gap in YB 6 by ultrahigh-resolution photoemission spectroscopy

We have performed ultrahigh-resolution photoemission spectroscopy (PES) on superconducting hexaboride YB 6 ( T c = 7.1 K) to investigate the electronic structure in the vicinity of the Fermi level ( E F ). We have succeeded in directly observing the superconducting gap together with a sharp coherent peak in the superconducting state (5 K). From the numerical simulation of the PES spectra by using isotropic Dynes function, we have estimated the size of the superconducting gap to be 0.9 meV, which corresponds to 2 Δ (0)/ k B T c = 3.8 . We found a dip structure around 10 meV with respect to E F in the PES spectrum in the superconducting state. This structure is attributed to the strong coupling feature of electrons with the 10 meV acoustic phonon. These results indicate that YB 6 is categorized into phonon-mediated BCS superconductor in the strong or intermediate coupling regime.

Photoemission Spectroscopic Evidence of Gap Anisotropy in anf-Electron Superconductor

We used low-temperature ultrahigh-resolution (360 microeV) photoemission spectroscopy with a laser as a photon source (Laser-PES) to study the superconducting (SC) gap of an f-electron superconductor CeRu2. The unique combination of the large escape depth expected from the known universal behavior and extremely high-energy resolution has enabled us to directly measure the bulk SC gap of an f-electron superconductor for the first time. The present study provides direct evidence for an anisotropic SC gap in CeRu2, and also demonstrates the potential of Laser-PES in investigating unconventional superconductivity realized in correlated d- and f-electron superconductors.

Laser-excited ultrahigh-resolution photoemission spectroscopy ofNaxCoO2·yH2O: Evidence for pseudogap formation

We have studied the temperature-dependent electronic structure near the Fermi level (EF) of the layered cobaltate superconductor, Na0.35CoO2.1.3H2O, and related materials, using laser-excited ultrahigh-resolution photoemission spectroscopy. We observe the formation of a pseudogap with an energy scale of ~ 20 meV in Na0.35CoO2.1.3H2O and Na0.35CoO2.0.7H2O, which is clearly absent in Na0.7CoO2. The energy scale of the pseudogap is larger than the expected value for the superconducting gap, suggesting an additional competing order parameter at low temperatures. We discuss implications of the pseudogap in relation to available transport and magnetic susceptibility results.

Erratum: Electronic structure and superconducting gap of silicon clathrateBa8Si46studied with ultrahigh-resolution photoemission spectroscopy [Phys. Rev. B64, 172504 (2001)

Erratum: Electronic structure and superconducting gap of silicon clathrate<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mi mathvariant="normal">Ba</mml:mi><mml:mn>8</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">Si</mml:mi><mml:mn>46</mml:mn></mml:msub></mml:mrow></mml:math>studied with ultrahigh-resolution photoemission spectroscopy [Phys. Rev. B<b>64</b>, 172504 (2001)

Comparative study of the binary icosahedral quasicrystalCd5.7Yband its crystalline approximantCd6Ybby low-temperature ultrahigh-resolution photoemission spectroscopy

Low-temperature ultrahigh-resolution photoemission (PE) spectroscopy of the recently discovered binary icosahedral (i) ${\mathrm{Cd}}_{5.7}\mathrm{Yb}$ and its 1/1-cubic approximant Cd${}_{6}$Yb has commonly revealed a sharp Fermi edge with a marked spectral intensity decrease towards the Fermi level ${(E}_{F}),$ indicating the presence of a dip near ${E}_{F}$ which is consistent with the recent result of electronic structure calculations [Y. Ishii and T. Fujiwara, Phys. Rev. Lett. 87, 206408 (2001)], and the dip at ${E}_{F}$ is found to be much deeper in $i\ensuremath{-}{\mathrm{Cd}}_{5.7}\mathrm{Yb}$ than in $1/1\ensuremath{-}{\mathrm{Cd}}_{6}\mathrm{Yb}.$ Moreover, surface core-level shifts (SCS's) are clearly observed in the quasicrystal and the approximant, and the SCS's are found to be substantially different between these two compounds of different long-range orders.

Ultrahigh resolution photoemission study of CeRu 4Sb 12

Ultrahigh-resolution photoemission spectroscopy reveals the crossover behavior of CeRu 4Sb 12 into a low carrier state. The density of states (DOS) around the Fermi level ( E F) decreases on lowering the temperature. The temperature dependence of the DOS at E F is consistent with the results of the transport measurements and explained in terms of the development of the coherence.

Direct observation of pseudogap of SmB6 using ultrahigh-resolution photoemission spectroscopy

We have performed a temperature-dependent ultrahigh-resolution photoemission spectroscopy on SmB6 to study the “Kondo-insulator” nature. We found a sharp Sm 4f-derived peak about 18meV away from EF and a pseudogap at EF at low temperature. The pseudogap is gradually filled-in by the transfer of spectral weight from the sharp peak at high temperatures. This indicates that the pseudogap is a Kondo-insulator gap originating in the hybridization between the Sm 4f states and the conduction electrons.

Superconducting gap of MgB2 observed using ultrahigh-resolution photoemission spectroscopy

We study the superconducting gap of MgB2 using high-resolution photoemission. In the spectrum measured at 5.4K, a coherent peak with a shoulder structure is observed. We find that a simulation using two Dynes functions with the gap sizes of 1.7 and 5.6meV reproduces the superconducting-state spectrum better than that using a single Dynes function and anisotropic Dynes functions. We also find that both of the smaller and larger gap close at the bulk transition temperature. These results indicate a multiple superconducting gap of MgB2.