arXiv subjects

Y. Aiura

Publications and source records attributed to Y. Aiura.

At least 19 recordsLinked to original sources

arXiv2015

Coordination of the energy and temperature scales of pairing across the doping phase diagram of Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$

Using a new variant of photoelectron spectroscopy, we measure the homogeneous near-nodal pairing ($Δ$) and pair-breaking self-energy ($Γ_S$) processes for a wide range of doping levels of Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$. For all samples we find that the pairing extends above the superconducting transition T$_c$ to a scale T$_{Pair}$ that is distinct from the antinodal pseudogap scale T$^*$ and near but slightly above T$_c$. We find that $Δ$ and T$_{Pair}$ are related with a strong coupling ratio 2$Δ$ /k$_B$T$_{Pair}\approx6$ across the entire doping phase diagram, i.e. independent of the effects of antinodal pseudogaps or charge-density waves.

cond-mat.supr-con↗

arXiv2014

Pre-pairing and the "Filling" Gap in the Cuprates From the Tomographic Density of States

We use the tomographic density of states (TDoS), which is a measure of the density of states for a single slice through the band structure of a solid, to study the temperature evolution of the superconducting gap in the cuprates. The TDoS provides unprecedented accuracy in determining both the superconducting pair-forming strength, $Δ$, and the pair-breaking rate, $Γ$. In both optimally- and under-doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$, we find the near-nodal $Δ$ smoothly evolves through the superconducting transition temperature - clear evidence for the existence of pre-formed pairs. Additionally, we find the long observed `filling' of the superconducting gap in the cuprates is due to the strongly temperature dependent $Γ$.

cond-mat.supr-con↗

arXiv2014

The Origin and Non-quasiparticle Nature of Fermi Arcs in Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$

A Fermi arc is a disconnected segment of a Fermi surface observed in the pseudogap phase of cuprate superconductors. This simple description belies the fundamental inconsistency in the physics of Fermi arcs, specifically that such segments violate the topological integrity of the band. Efforts to resolve this contradiction of experiment and theory have focused on connecting the ends of the Fermi arc back on itself to form a pocket, with limited and controversial success. Here we show the Fermi arc, while composed of real spectral weight, lacks the quasiparticles to be a true Fermi surface. To reach this conclusion we developed a new photoemission-based technique that directly probes the interplay of pair-forming and pair-breaking processes with unprecedented precision. We find the spectral weight composing the Fermi arc is shifted from the gap edge to the Fermi energy by pair-breaking processes. While real, this weight does not form a true Fermi surface, because the quasiparticles, though significantly broadened, remain at the gap edge. This non-quasiparticle weight may account for much of the unexplained behavior of the pseudogap phase of the cuprates.

cond-mat.supr-con↗

arXiv2014

Absence of the 1/8 -anomaly in oxygen isotope effect on YBa$_{2}$Cu$_{3}$O$_{y}$

We have prepared oxygen isotope exchanged crystals of impurity-free YBCO with various oxygen concentents, and examined pure doping ($p$) dependance of isotope effect on superconducting transition temperature. With decreasing oxygen contents, the isotope exponent $α$ monotonously increases without any anomaly around $p = 1/8$. The monotonous increase in $α$ indicates that phonons are involved in the mechanism which causes the monotonous $T_c$ suppression with underdoping.

cond-mat.supr-con↗

arXiv2013

Large momentum-dependence of the main dispersion "kink" in the high-Tc superconductor Bi2Sr2CaCu2O8+δ

Ultrahigh resolution angle-resolved photoemission spectroscopy with low-energy photons is used to study the detailed momentum dependence of the well-known nodal "kink" dispersion anomaly of Bi2Sr2CaCu2O8+δ. We find that the kink's location transitions smoothly from a maximum binding energy of about 65 meV at the node of the d-wave superconducting gap to 55 meV roughly one-third of the way to the antinode. Meanwhile, the self-energy spectrum corresponding to the kink dramatically sharpens and intensifies beyond a critical point in momentum space. We discuss the possible bosonic spectrum in energy and momentum space that can couple to the k-space dispersion of the electronic kinks.

cond-mat.supr-con↗

arXiv2013

Fermi-surface reconstruction involving two Van Hove singularities across the antiferromagnetic transition in BaFe2As2

We report an angle-resolved photoemission study of BaFe2As2, a parent compound of iron-based superconductors. Low-energy tunable excitation photons have allowed the first observation of a saddle-point singularity at the Z point, as well as the Gamma point. With antiferromagnetic ordering, both of these two van Hove singularities come down below the Fermi energy, leading to a topological change in the innermost Fermi surface around the kz axis from cylindrical to tear-shaped, as expected from first-principles calculation. These singularities may provide an additional instability for the Fermi surface of the superconductors derived from BaFe2As2.

cond-mat.supr-con↗

arXiv2009

Low-energy (< 10 meV) feature in the nodal electron self-energy and strong temperature dependence of the Fermi velocity in Bi(2)Sr(2)CaCu(2)O(8+delta)

Using low-photon energy angle-resolved photoemission (ARPES), we study the low-energy dispersion along the nodal (pi, pi) direction in Bi(2)Sr(2)CaCu(2)O(8+delta) (Bi2212) as a function of temperature. Less than 10 meV below the Fermi energy, the high-resolution data reveals a novel "kink"-like feature in the real part of the electron self-energy that is distinct from the larger well-known kink roughly 70 meV below E_F. This new kink is strongest below the superconducting critical temperature and weakens substantially as the temperature is raised. A corollary of this finding is that the Fermi velocity, as measured over this energy range, varies rapidly with temperature - increasing by almost 30% from 70 to 110 K.

cond-mat.supr-con↗

arXiv2008

An Isotopic Fingerprint of Electron-Phonon Coupling in High-Tc Cuprates

Angle-resolved photoemission spectroscopy with low-energy tunable photons along the nodal direction of oxygen isotope substituted Bi2Sr2CaCu2O8+delta reveals a distinct oxygen isotope shift near the electron-boson coupling "kink" in the electronic dispersion. The magnitude (a few meV) and direction of the kink shift are as expected due to the measured isotopic shift of phonon frequency, which are also in agreement with theoretical expectations. This demonstrates the participation of the phonons as dominant players, as well as pinpointing the most relevant of the phonon branches.

cond-mat.supr-con↗

arXiv2006

Interplay of electron-lattice interactions and superconductivity in Bi2Sr2CaCu2O8+d

Formation of electron pairs is essential to superconductivity. For conventional superconductors, tunnelling spectroscopy has established that pairing is mediated by bosonic modes (phonons); a peak in the second derivative of tunnel current d2I/dV2 corresponds to each phonon mode . For high-transition-temperature (high-Tc) superconductivity, however, no boson mediating electron pairing has been identified. One explanation could be that electron pair formation and related electron-boson interactions are heterogeneous at the atomic scale and therefore challenging to characterize. However, with the latest advances in d2I/dV2 spectroscopy using scanning tunnelling microscopy, it has become possible to study bosonic modes directly at the atomic scale . Here we report d2I/dV2 imaging studies of the high-Tc superconductor Bi2Sr2CaCu2O8+d. We find intense disorder of electron-boson interaction energies at the nanometre scale, along with the expected modulations in d2I/dV2 (refs 9,10). Changing the density of holes has minimal effects on both the average mode energies and the modulations, indicating that the bosonic modes are unrelated to electronic or magnetic structure. Instead, the modes appear to be local lattice vibrations, as substitution of 18O for 16O throughout the material reduces the average mode energy by approximately 6 per cent - the expected effect of this isotope substitution on lattice vibration frequencies. Significantly, the mode energies are always spatially anticorrelated with the superconducting pairing-gap energies, suggesting an interplay between these lattice vibration modes and the superconductivity.

cond-mat.supr-con↗

arXiv2005

Laser ARPES, the sudden approximation, and quasiparticle-like peaks in Bi2Sr2CaCu2O8+delta

A new low photon energy regime of angle resolved photoemission spectroscopy is accessed with lasers and used to study the superconductor Bi2Sr2CaCu2O8+delta. The low energy increases bulk sensitivity, reduces background, and improves resolution. With this we observe spectral peaks which are sharp on the scale of their binding energy - the clearest evidence yet for quasiparticles in the normal state. Crucial aspects of the data such as the dispersion, superconducting gaps, and the bosonic coupling kink and associated weight transfer are robust to a possible breakdown of the sudden approximation.

cond-mat.supr-con↗

arXiv2005

Orbital selectivity of the kink in the dispersion of Sr2RuO4

We present detailed energy dispersions near the Fermi level on the monolayer perovskite ruthenate Sr2RuO4, determined by high-resolution angle-resolved photoemission spectroscopy. An orbital selectivity of the kink in the dispersion of Sr2RuO4 has been found: A kink for the Ru 4d_xy orbital is clearly observed, but not for the Ru 4d_yz and 4d_zx ones. The result provides insight into the origin of the kink.

cond-mat.supr-con↗

arXiv2005

Unusual electron-doping effects in Sr$_{2-x}$La$_x$FeMoO$_6$ observed by photoemission spectroscopy

We have investigated the electronic structure of electron-doped Sr$_{2-x}$La$_x$FeMoO$_6$ ($x$=0.0 and 0.2) by photoemission spectroscopy and band-structure calculations within the local-density approximation+$U$ (LDA+$U$) scheme. A characteristic double-peak feature near the Fermi level ($E_{\rm F}$) has been observed in the valence-band photoemission spectra of both $x$=0.0 and 0.2 samples. A photon-energy dependence of the spectra in the Mo 4$d$ Cooper minimum region compared with the band-structure calculations has shown that the first peak crossing $E_{\rm F}$ consists of the (Fe+Mo) $t_{2g\downarrow}$ states (feature A) and the second peak well below $E_{\rm F}$ is dominated by the Fe $e_{g\uparrow}$ states (feature B). Upon La substitution, the feature A moves away from $E_{\rm F}$ by $\sim$50 meV which is smaller than the prediction of our band theory, 112 meV. In addition, an intensity enhancement of $both$ A and B has been observed, although B is not crossing $E_{\rm F}$. Those two facts are apparently incompatible with the simple rigid-band shift due to electron doping. We point out that such phenomena can be understood in terms of the strong Hund's rule energy stabilization in the 3$d^5$ configuration at the Fe sites in this compound. From an observed band-narrowing, we have also deduced a mass enhancement of $\sim$2.5 with respect to the band theory, in good agreement with a specific heat measurement.

cond-mat.str-el↗

arXiv2003

Effects of next-nearest-neighbor hopping $t^{\prime}$ on the electronic structure of cuprates

Photoemission spectra of underdoped and lightly-doped Bi$_{2-z}$Pb$_z$Sr$_2$Ca$_{1-x}${\it R}$_{x}$Cu$_2$O$_{8+y}$ ($R=$ Pr, Er) (BSCCO) have been measured and compared with those of La$_{2-x}$Sr$_x$CuO$_4$ (LSCO). The lower-Hubbard band of the insulating BSCCO, like Ca$_2$CuO$_2$Cl$_2$, shows a stronger dispersion than La$_2$CuO$_4$ from ${\bf k}\sim$($π/2,π/2$) to $\sim$($π,0$). The flat band at ${\bf k}\sim$($π,0$) is found generally deeper in BSCCO. These observations together with the Fermi-surface shapes and the chemical potential shifts indicate that the next-nearest-neighbor hopping $|t^{\prime}|$ of the single-band model is larger in BSCCO than in LSCO and that $|t^{\prime}|$ rather than the super-exchange $J$ influences the pseudogap energy scale.

cond-mat.supr-con↗

arXiv2003

Spectroscopic Signatures on Increase in Charge-Density-Wave Potential of 1{\em T}-TaS$_{1.2}$Se$_{0.8}$

We present an angle-resolved photoemission (ARPES) study on the layered transition-metal dichalcogenide 1{\em T}-TaS$_{1.2}$Se$_{0.8}$ in the metallic commensurate charge-density-wave (CDW) phase. A model calculation of the spectral function captures the main features of the ARPES spectra well qualitatively, that is, the gross splits of unreconstructed band structure in the absence of the CDW superlattice. The observed enhancement of the size of the gap between the lower and middle fragments of the Ta {\em 5d} band along the $Γ$M line by cooling is interpreted in terms of the increase in the CDW-related potential.

cond-mat.str-el↗

arXiv2002

An intrinsic peak-dip-hump and strong coupling effects in $Bi_{2}Sr_{2}CaCu_{2}O_{8+δ}$ ARPES data near $(π,0)$

A well-known peak-dip-hump structure exists near $(π, 0)$ in superconducting state ARPES spectra of $Bi_{2}Sr_{2}CaCu_{2}O_{8+δ}$ (Bi2212). Here we report results on optimal and overdoped Bi2212 samples indicating the traditional peak-dip-hump structure observed near $(π,0)$ is largely due to bilayer splitting. However a separate, much weaker peak-dip hump (PDH) structure distinct from bilayer splitting can be detected near $(π, 0)$. This new PDH structure is consistent with electronic coupling to the magnetic resonance mode in Bi2212. Both the dispersion and line shape signatures indicate strong coupling to this mode.

cond-mat.supr-con↗

arXiv2002

Mass-renormalized electronic excitations at ($π$, 0) in the superconducting state of $Bi_{2}Sr_{2}CaCu_{2}O_{8+δ}$

Using high-resolution angle-resolved photoemission spectroscopy on $Bi_{2}Sr_{2}CaCu_{2}O_{8+δ}$, we have made the first observation of a mass renormalization or "kink" in the E vs. $\vec k$ dispersion relation localized near $(π, 0)$. Compared to the kink observed along the nodal direction, this new effect is clearly stronger, appears at a lower energy near 40 meV, and is only present in the superconducting state. The kink energy scale defines a cutoff below which well-defined quasiparticle excitations occur. This effect is likely due to coupling to a bosonic excitation, with the most plausible candidate being the magnetic resonance mode observed in inelastic neutron scattering.

cond-mat.supr-con↗

arXiv2001

Constancy of the bilayer splitting as a function of doping in $Bi_{2}Sr_{2}CaCu_{2}O_{8+δ}$

Using high energy resolution angle resolved photoemission spectroscopy, we have resolved the bilayer splitting effect in a wide range of dopings of the bilayer cuprate $Bi_{2}Sr_{2}CaCu_{2}O_{8+δ}$. This bilayer splitting is due to a nonvanishing intracell coupling $t_{\perp}$, and contrary to expectations, it is not reduced in the underdoped materials. This has implications for understanding the increased c-axis confinement in underdoped materials.

cond-mat.supr-con↗

arXiv2001

Doubling of the bands in overdoped Bi2Sr2CaCu2O8-probable evidence for c-axis bilayer coupling

We present high resolution ARPES data of the bilayer superconductor Bi2Sr2CaCu2O8 (Bi2212) showing a clear doubling of the near EF bands. This splitting approaches zero along the (0,0)-(pi,pi) nodal line and is not observed in single layer Bi2Sr2CuO6 (Bi2201), suggesting that the splitting is due to the long sought after bilayer splitting effect. The splitting has a magnitude of approximately 75 meV near the middle of the zone, extrapolating to about 100 meV near the (pi,0) point

cond-mat.supr-con↗