arXiv subjects

Kazuyasu Tokiwa

Publications and source records attributed to Kazuyasu Tokiwa.

8 recordsLinked to original sources

arXiv2026

BCS-BEC crossover driven by small Fermi pockets of a high-Tc cuprate superconductor

Fermi arcs observed in underdoped cuprates have sparked debate over whether they represent segments of a large Fermi surface or small Fermi pockets. This ambiguity has long hindered their classification as either the conventional Bardeen-Cooper-Schrieffer (BCS) regime or the strongly coupled Bose-Einstein condensation (BEC) crossover limit. Here, using angle-resolved photoemission spectroscopy and quantum oscillations, we demonstrate the coexistence of a small Fermi pocket and a large superconducting gap in the clean inner CuO2 layers of the four-layer cuprate Ba2Ca3Cu4O8(F,O)2. This coexistence constitutes a hallmark of the BCS-BEC crossover and has remained elusive for decades. Despite the presence of antiferromagnetic (AF) order, the superconducting gap in the small pocket is remarkably large, yielding a gap-to-Fermi-energy ratio (Delta_pocket/e_F ~ 0.6) and a critical-to-Fermi-temperature ratio (Tc/TF ~ 1.3) that reach the theoretical upper bound for two-dimensional superconductivity. Unexpectedly, this BCS-BEC crossover emerges not as the carrier density decreases but as it increases, abruptly within a narrow doping range of less than 1%. These results provide a long-sought microscopic foundation for the d-wave pairing mechanism in doped AF-Mott insulators.

cond-mat.supr-con↗

arXiv2025

Superconducting coherence boosted by outer-layer metallic screening in multilayered cuprates

In multilayered high-Tc cuprates with three or more CuO2 layers per unit cell, the inner CuO2 planes (IPs) are spatially separated from the dopant layers and thus remain cleaner than the outer planes (OPs). While both interlayer coupling and the presence of clean IPs have been proposed as key factors enhancing superconductivity, their individual roles have been difficult to disentangle, as IPs and OPs typically become superconducting simultaneously. Here we investigate five-layer (Cu,C)Ba2Ca4Cu5Oy (Cu1245) with Tc = 78 K and three-layer Ba2Ca2Cu3O6(F,O)2 (F0223) with Tc = 100 K using ARPES, and uncover an unprecedented situation, in which only the IPs become superconducting while the OPs remain metallic at low temperatures. Model calculations indicate that more than 95% of the OP wavefunction remains confined to OP itself, with minimal hybridization from the superconducting IPs. In particular, we experimentally realize an ideal configuration: a single superconducting CuO2 layer sandwiched between heavily overdoped metallic outer layers, which screen disorder originating from the dopant layers. Strikingly, this clean CuO2 layer exhibits the largest superconducting gap among all known cuprates and coherent Bogoliubov peaks extending beyond the antiferromagnetic zone boundary -- long regarded as the boundary beyond which coherence vanishes in heavily underdoped cuprates. Furthermore, a widely extended coherent flat band emerges at the Brillouin zone edge, overcoming the pseudogap damping effect. Our results introduce a new physical parameter, the degree of screening, to investigate the competition between superconductivity and the pseudogap, potentially shedding new light on its origin. The nearly disorder-free superconducting CuO2 layers offer a model platform for bridging the gap between disordered real materials and idealized theoretical models, which generally neglect disorder effects.

cond-mat.supr-con↗

arXiv2023

Unveiling phase diagram of the lightly doped high-Tc cuprate superconductors with disorder removed

The currently established electronic phase diagram of cuprates is based on a study of single- and double-layered compounds. These CuO$_2$ planes, however, are directly contacted with dopant layers, thus inevitably disordered with an inhomogeneous electronic state. Here, we solve this issue by investigating a 6-layered Ba$_2$Ca$_5$Cu$_6$O$_{12}$(F,O)$_2$ with inner CuO$_2$ layers, which are clean with the extremely low disorder, by angle-resolved photoemission spectroscopy (ARPES) and quantum oscillation measurements. We find a tiny Fermi pocket with a doping level less than 1% to exhibit well-defined quasiparticle peaks which surprisingly lack the polaronic feature. This provides the first evidence that the slightest amount of carriers is enough to turn a Mott insulating state into a metallic state with long-lived quasiparticles. By tuning hole carriers, we also find an unexpected phase transition from the superconducting to metallic states at 4%. Our results are distinct from the nodal liquid state with polaronic features proposed as an anomaly of the heavily underdoped cuprates.

cond-mat.supr-con↗

arXiv2020

Observation of small Fermi pockets protected by clean CuO2 sheets of a high-Tc superconductor

The superconductivity of high transition temperature (Tc) occurs in copper oxides with carrier-doping to an antiferromagnetic (AF) Mott insulator. This discovery more than thirty years ago immediately led to a prediction about the formation of a small Fermi pocket. This structure, however, has not yet been detected, while it could be a key element in relating high-Tc superconductivity to Mott physics. To address this long-standing issue, we investigate the electronic structure of a five-layered Ba2Ca4Cu5O10(F,O)2 with inner CuO2 planes demonstrated to be cleanest ever in cuprates. Most surprisingly, we find small Fermi surface (FS) pockets closed around (pi/2,pi/2) consistently by angle-resolved photoemission spectroscopy (ARPES) and quantum oscillation measurements. The d-wave superconducting gap opens along the pocket, revealing the coexistence between the superconductivity and AF order in the same CuO2 sheet. Our data further indicate that the superconductivity can occur without contribution from the states near the antinodal region, which are shared by other competing excitations such as the charge density wave (CDW) and pseudogap states. This will have significant implications for understanding the superconductivity and puzzling Fermi arc phenomena in cuprates.

cond-mat.supr-con↗

arXiv2017

Multilayered cuprate superconductor Ba$_2$Ca$_5$Cu$_6$O$_{12}$(O$_{1-x}$,F$_x$)$_2$ studied by temperature-dependent scanning tunneling microscopy and spectroscopy

Scanning tunneling microscopy/spectroscopy (STM/STS) measurements were carried out on a multi-layered cuprate superconductor Ba$_2$Ca$_5$Cu$_6$O$_{12}$(O$_{1-x}$,F$_x$)$_2$. STM topography revealed random spot structures with the characteristic length $\le 0.5$ nm. The conductance spectra dI/dV(V) show the coexistence of smaller gaps $Δ_S$ and large gaps (pseudogaps) $Δ_L$. The pseudogap-related features in the superconducting state were traced with the spatial resolution of $\sim$ 0.07 nm. Here, $I$ and $V$ are the tunnel current and bias voltage, respectively. The temperature, $T$, dependence of $Δ_S$ follows the reduced Bardeen-Cooper-Schrieffer (BCS) dependence. The hallmark ratio 2$Δ_{S}(T=0)/k_B T_c$ equals to 4.9, which is smaller than those of other cuprate superconductors. Here, $T_c$ is the superconducting critical temperature and $k_B$ is the Boltzmann constant. The larger gap $Δ_L$ survives in the normal state and even increases with $T$ above $T_c$. The $T$ dependences of the spatial distributions for both relevant gaps ($Δ$ map), as well as for each gap separately ($Δ_S$ and $Δ_L$) were obtained. From the histogram of $Δ$ map, the averaged gap values were found to be $\bar Δ_S = \sim 24$ meV and $\bar Δ_L = \sim 79$ meV. The smaller gap $Δ_S$ shows a spatially homogeneous distribution while the larger gap $Δ_L$ is quite inhomogeneous, indicating that rather homogeneous superconductivity coexists with the patchy distributed pseudogap. The spatial variation length $ξ_{Δ_L}$ of $Δ_L$ correlates with the scale of the topography spot structures, being approximately 0.4 nm. This value is considerably smaller than the coherence length of this class of superconductors, suggesting that $Δ_L$ is strongly affected by the disorder of the apical O/F.

cond-mat.supr-con↗

arXiv2016

Novel Interplay between High-Tc Superconductivity and Antiferromagnetism in Tl-based Six-CuO2-Layered Cuprates : 205 Tl and 63 Cu-NMR Probes

We report $^{63}$Cu- and $^{205}$Tl-NMR studies on six-layered ($n$=6) high-$T_c$ superconducting (SC) cuprate TlBa$_2$Ca$_5$Cu$_6$O$_{14+δ}$ (Tl1256) with $T_c\sim$100 K, which reveal that antiferromagnetic (AFM) order takes place below $T_{\rm N}\sim$170 K. In this compound, four underdoped inner CuO$_2$ planes ($n$(IP)=4) sandwiched by two outer planes (OPs) are responsible for the onset of AFM order, whereas the nearly optimally-doped OPs responsible for the onset of bulk SC. It is pointed out that an increase in the out-of-plane magnetic interaction within an intra-unit-cell causes $T_{\rm N}\sim$ 45 K for Tl1245 with $n$(IP)=3 to increase to $\sim$170 K for Tl1256 with $n$(IP)=4. It is remarkable that the marked increase in $T_{\rm N}$ and the AFM moments for the IPs does not bring about any reduction in $T_c$, since $T_c\sim 100$ K is maintained for both compounds with nearly optimally doped OP. We highlight the fact that the SC order for $n\ge5$ is mostly dominated by the long-range in-plane SC correlation even in the multilayered structure, which is insensitive to the magnitude of $T_{\rm N}$ and the AFM moments at the IPs or the AFM interaction among the IPs. These results demonstrate a novel interplay between the SC and AFM orders when the charge imbalance between the IPs and OP is significantly large.

cond-mat.supr-con↗

arXiv2010

Synthesis of ErFeAsO-based superconductors by hydrogen doping method

Here we demonstrate the technique to stabilize the ErFeAsO-based superconductor with the smallest lattice constants in LnFeAsO1-y (Ln = lanthanide) series using hydrogen doping method. Polycrystalline samples were synthesized by heating pellets with nominal compositions of ErFeAsO1-y (1-y = 0.75 ~ 0.95) sandwiched between pellets of LaFeAsO0.8H0.8 compositions at 1100 °C under a pressure of 5.0 - 5.5 GP. The sample with lattice constants of a = 3.8219 Å and c = 8.2807 Å shows the highest superconducting critical temperatures (Tc) of 44.5 K and 41.0 K determined by onset transitions of resistivity and susceptibility, respectively. We discuss phase diagram of Ln dependence of Tc in LnFeAsO-based superconductors.

cond-mat.supr-con↗

arXiv2006

Disorder-driven quantum phase transition from antiferromagnetic metal to insulating state in multilayered high-Tc cuprate (Cu,C)Ba2Ca4Cu5Oy

We report on superconducting(SC) characteristics for oxygen-reduced Cu-based five-layered high-temperature superconductor (Cu,C)Ba2Ca4Cu5Oy(Cu-1245(OPT)), which includes five-fold outer planes (OP) and four-fold inner planes (IP).As a result of the reduction of the carrier density, the bulk SC for Cu-1245 (OPT) takes place at the nearly optimally-doped OP with Tc= 98 K that is different from previously-reported Cu-1245(OVD) where IP plays a primary role for the onset of SC. It gives an evidence that the carrier density of the optimally-doped layer determines its bulk Tc.Static antiferromagnetic(AFM) order is evidenced at IP's by zero-field Cu-NMR at low temperature, irrespective of the SC transition at OP's below 98K. This AFM state at IP's is characterized by a carrier localization at low temperatures due to disorder effect, whereas the carrier densities in each layer are similar to Hg-1245(OPT) where the AFM metallic state are realized in IP's. This finding reinforces the phase diagram in which the AFM metallic phase exists between AFM insulator and SC states for the case of ideally-flat CuO2 plane without disorder.

cond-mat.supr-con↗