arXiv subjects

S. Souma

Publications and source records attributed to S. Souma.

49 records · Page 3Linked to original sources

arXiv2009

Superconducting gap symmetry of Ba0.6K0.4Fe2As2 studied by angle-resolved photoemission spectroscopy

We have performed high-resolution angle-resolved photoemission spectroscopy on the optimally-doped Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ compound and determined the accurate momentum dependence of the superconducting (SC) gap in four Fermi-surface sheets including a newly discovered outer electron pocket at the M point. The SC gap on this pocket is nearly isotropic and its magnitude is comparable ($Δ$ $\sim$ 11 meV) to that of the inner electron and hole pockets ($\sim$12 meV), although it is substantially larger than that of the outer hole pocket ($\sim$6 meV). The Fermi-surface dependence of the SC gap value is basically consistent with $Δ$($k$) = $Δ$$_0$cos$k_x$cos$k_y$ formula expected for the extended s-wave symmetry. The observed finite deviation from the simple formula suggests the importance of multi-orbital effects.

cond-mat.supr-con↗

arXiv2008

Electronic structure of optimally doped pnictide Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$: a comprehensive ARPES investigation

We have conducted a comprehensive angle-resolved photoemission study on the normal state electronic structure of the Fe-based superconductor Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$. We have identified four dispersive bands which cross the Fermi level and form two hole-like Fermi surfaces around $Γ$ and two electron-like Fermi surfaces around M. There are two nearly nested Fermi surface pockets connected by an antiferromagnetic ($π$, $π$) wavevector. The observed Fermi surfaces show small $k_z$ dispersion and a total volume consistent with Luttinger theorem. Compared to band structure calculations, the overall bandwidth is reduced by a factor of 2. However, many low energy dispersions display stronger mass renormalization by a factor of $\sim$ 4, indicating possible orbital (energy) dependent correlation effects. Using an effective tight banding model, we fitted the band structure and the Fermi surfaces to obtain band parameters reliable for theoretical modeling and calculations of the important physical quantities, such as the specific heat coefficient.

cond-mat.supr-con↗

arXiv2008

Band Structure and Fermi Surface of an Extremely Overdoped Iron-Based Superconductor KFe2As2

We have performed high-resolution angle-resolved photoemission spectroscopy on heavily overdoped KFe_2As_2 (transition temperature (Tc = 3 K). We observed several renormalized bands near the Fermi level with a renormalization factor of 2-4. While the Fermi surface (FS) around the Brillouin-zone center is qualitatively similar to that of optimally-doped Ba_{1-x}K_xFe_2As_2 (x = 0.4; Tc = 37 K), the FS topology around the zone corner (M point) is markedly different: the two electron FS pockets are completely absent due to excess of hole doping. This result indicates that the electronic states around the M point play an important role in the high-Tc superconductivity of Ba$_{1-x}$K$_x$Fe$_2$As$_2$ and suggests that the interband scattering via the antiferromagnetic wave vector essentially controls the Tc value in the overdoped region.

cond-mat.supr-con↗

arXiv2008

Observation of an Orbital Selective Electron-Mode Coupling in Fe-Based High-$T_c$ Superconductors

We have performed an angle-resolved photoemission spectroscopy study of the new superconductor Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ in the low energy range. We report the observation of an anomaly around 25 meV in the dispersion of superconducting Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ samples that nearly vanishes above $T_c$. The energy scale of the related mode (13$\pm$2 meV) and its strong dependence on orbital and temperature indicates that it is unlikely related to phonons. Moreover, the momentum locations of the kink can be connected by the antiferromagnetic wavevector. Our results point towards an unconventional electronic origin of the mode and the superconducting pairing in the Fe-based superconductors, and strongly support the anti-phase s-wave pairing symmetry.

cond-mat.supr-con↗

arXiv2008

Observation of Fermi-surface-dependent nodeless superconducting gaps in Ba0.6K0.4Fe2As2

The recent discovery of superconductivity in iron-arsenic compounds below a transition temperature (Tc) as high as 55K ended the monopoly of copper oxides (cuprates) in the family of high-Tc superconductors. A critical issue in understanding this new superconductor, as in the case of cuprates, is the nature, in particular the symmetry and orbital dependence, of the superconducting gap. There are conflicting experimental results, mostly from indirect measurements of the low energy excitation gap, ranging from one gap to two gaps, from line nodes to nodeless gap function in momentum space. Here we report a direct observation of the superconducting gap, including its momentum, temperature, and Fermi surface (FS) dependence in Ba0.6K0.4Fe2As2 (Tc = 37 K) using angle-resolved photoelectron spectroscopy. We find two superconducting gaps with different values: a large gap (~ 12 meV) on the two small hole-like and electron-like FS sheets, and a small gap (~ 6 meV) on the large hole-like FS. Both gaps, closing simultaneously at the bulk Tc, are nodeless and nearly isotropic around their respective FS sheets. The isotropic pairing interactions are strongly orbital dependent, as the ratio 2Delta/kBTc switches from weak to strong coupling on different bands. The same and surprisingly large superconducting gap due to strong pairing on the two small FS, which are connected by the (pi, 0) spin-density-wave vector in the parent compound, strongly suggests that the pairing mechanism originates from the inter-band interactions between these two nested FS sheets.

cond-mat.supr-con↗

arXiv2008

Superconducting Gap and Pseudogap in Iron-Based Layered Superconductor La(O$_{1-x}$F$_x$)FeAs

We report high-resolution photoemission spectroscopy of newly-discovered iron-based layered superconductor La(O$_{0.93}$F$_{0.07}$)FeAs (Tc = 24 K). We found that the superconducting gap shows a marked deviation from the isotropic s-wave symmetry. The estimated gap size at 5 K is 3.6 meV in the s- or axial p-wave case, while it is 4.1 meV in the polar p- or d-wave case. We also found a pseudogap of 15-20 meV above Tc, which is gradually filled-in with increasing temperature and closes at temperature far above Tc similarly to copper-oxide high-temperature superconductors.

cond-mat.supr-con↗

arXiv2007

Protected nodes and the collapse of the Fermi arcs in high Tc cuprates

Angle resolved photoemission on underdoped Bi2Sr2CaCu2O8 reveals that the magnitude and d-wave anisotropy of the superconducting state energy gap are independent of temperature all the way up to Tc. This lack of T variation of the entire k-dependent gap is in marked contrast to mean field theory. At Tc the point nodes of the d-wave gap abruptly expand into finite length ``Fermi arcs''. This change occurs within the width of the resistive transition, and thus the Fermi arcs are not simply thermally broadened nodes but rather a unique signature of the pseudogap phase.

cond-mat.supr-con↗

arXiv2006

Shadow bands in single-layered Bi_2Sr_2CuO_6 studied by angle-resolved photoemission spectroscopy

We have performed systematic angle-resolved photoemission spectroscopy (ARPES) on single-layered cuprate superconductor Bi2Sr2CuO6 to elucidate the origin of shadow band. We found that the shadow band is exactly the c(2x2) replica of the main band irrespective of the carrier concentration and its intensity is invariable with respect to temperature, doping, and substitution constituents of block layers. This result rules out the possibility of antiferromagnetic correlation and supports the structural origin of shadow band. ARPES experiments on optimally doped La1.85Sr0.15CuO4 also clarified the existence of the c(2x2) shadow band, demonstrating that the shadow band is not a unique feature of Bi-based cuprates. We conclude that the shadow band is related to the orthorhombic distortion at the crystal surface.

cond-mat.supr-con↗

arXiv2006

Fermi Surface and Anisotropic Spin-Orbit Coupling of Sb(111) studied by Angle-Resolved Photoemission Spectroscopy

High-resolution angle-resolved photoemission spectroscopy has been performed on Sb(111) to elucidate the origin of anomalous electronic properties in group-V semimetal surfaces. The surface was found to be metallic despite the semimetallic character of bulk. We clearly observed two surface-derived Fermi surfaces which are likely spin split, demonstrating that the spin-orbit interaction plays a dominant role in characterising the surface electronic states of group-V semimetals. Universality/disimilarity of the electronic structure in Bi and Sb is discussed in relation to the granular superconductivity, electron-phonon coupling, and surface charge/spin density wave.

cond-mat.other↗

arXiv2005

Fermi surface evolution and Luttinger theorem in Na$_x$CoO$_2$: a systematic photoemission study

We report a systematic angle-resolved photoemission study on Na$_x$CoO$_2$ for a wide range of Na concentrations ($0.3 \leq x \leq 0.72$). In all the metallic samples at different $x$, we observed (i) only a single hole-like Fermi surface centered around $Γ$ and (ii) its area changes with $x$ according to the Luttinger theorem. We also observed a surface state that exhibits a larger Fermi surface area. The $e_g^\prime$ band and the associated small Fermi surface pockets near the $K$ points predicted by band calculations are found to ``sink'' below the Fermi energy in a manner almost independent of the doping and temperature.

cond-mat.str-el↗

arXiv2004

Fermi surface topology of Ca_{1.5}Sr_{0.5}RuO_{4} determined by ARPES

We report ARPES results of the Fermi surface of Ca_{1.5}Sr_{0.5}RuO$_{4}, which is at the boundary of magnetic/orbital instability in the phase diagram of the Ca-substituted Sr ruthenates. Three t_{2g} energy bands and the corresponding Fermi surface sheets are observed, which are also present in the Ca-free Sr_{2}RuO$_{4}. We find that while the Fermi surface topology of the alpha, beta (d_{yz, zx}) sheets remains almost the same in these two materials, the gamma (d_{xy}) sheet exhibits a hole-like Fermi surface in Ca_{1.5}Sr_{0.5}RuO_{4} in contrast to being electron-like in Sr_{2}RuO_{4}. Our observation of all three volume conserving Fermi surface sheets clearly demonstrates the absence of orbital-selective Mott transition, which was proposed theoretically to explain the unusual transport and magnetic properties in Ca_{1.5}Sr_{0.5}RuO_{4}.

cond-mat.str-el↗

arXiv2003

ARPES on Na$_{0.6}$CoO$_{2}$: Fermi surface, extended flat dispersion, and unusual band splitting

The electronic structure of single crystals Na$_{0.6}$CoO$_2$, which are closely related to the superconducting Na$_{0.3}$CoO$_2$.$y$H$_2$O ($T_c \sim 5K$), is studied by angle-resolved photoelectron spectroscopy. While the measured Fermi surface is found to be consistent with the prediction of a local density band theory, the energy dispersion is highly renormalized, with an anisotropy along the two principle axes ($Γ$-$K$, $Γ$-$M$). Our ARPES result also indicates that an extended flat band is formed slightly above $E_F$ along $Γ$-$K$. In addition, an unusual band splitting is observed in the vicinity of the Fermi surface along the $Γ$-$M$ direction, which differs from the predicted bilayer splitting.

cond-mat.supr-con↗

arXiv2001

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 of 4.5 meV at 15 K. The temperature dependence (15 - 40 K) of gap value follows well the BCS form, suggesting that 2Delta/kBTc 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.

cond-mat.str-el↗