arXiv subjects

Migaku Oda

Publications and source records attributed to Migaku Oda.

11 recordsLinked to original sources

arXiv2026

Magnetism and Electrical Conduction in Lightly-Doped Single-Layer High-$T_c$ Cuprate $\mathrm{La}_2\mathrm{CuO}_{4+\delta}$

The temperature dependences of magnetization and electrical resistivity as well as their magnetic field dependences have been examined in lightly-doped single-layer cuprate $\mathrm{La}_2\mathrm{CuO}_{4+\delta}$ (LCO, hole-doping level $p \, (2\delta) \cong 0.03$) single crystals, in comparison with those in the extremely low doping region of $p \lesssim 0.015$ to uncover the intrinsic magnetism and electrical conduction of the $\mathrm{Cu\text{-}O}$ plane that exhibits both antiferromagnetic (AF) and superconducting (SC) orders simultaneously. In $p \cong 0.03$ SC LCO, the sub-lattice moments on $\mathrm{Cu}$ sites and their AF couplings are only $\sim 15\,\%$ smaller than those of the Mott-insulator parent material, suggesting that the localization of $\mathrm{Cu}$ $3d$ electrons remains very strong. Furthermore, we report that in the SC LCO, two-dimensional AF spin correlations develop rapidly from $T^* \cong 280\text{ K}$ towards N\'eel temperature $T_{\mathrm{N}} = 266\text{ K}$, where the out-of-plane resistivity starts to decrease largely. This might be responsible for the AF ordering at such a high temperature in the SC single-layer cuprate with $p \cong 0.03$.

cond-mat.supr-con↗

arXiv2026

Direct High-Magnetic-Field Coupling to Stripe Order in a Cuprate Superconductor

Superconductivity in cuprates emerges out of a complex normal state that hosts density waves, pseudogap physics, and strange metal properties. Here, we access this normal state by synchronizing free-electron laser x-rays with high-magnetic-field pulses up to 44 T. We observe a linear increase in charge order amplitude and correlation length that persists far above the vortex melting transition. This behavior is incompatible with standard phase competition between charge order and superconductivity. By means of conventional hard x-ray diffraction and magnetostriction, we show that applied fields also enhance monoclinic lattice distortions. However, this magnetoelastic response is weaker and an epiphenomenon of the stripe order enhancement. Combined with recent observations of field-linear spin freezing, our results point to a direct coupling between magnetic field and the spin component of stripe order in the high-field normal state -- a mechanism independent of superconductivity suppression that has so far remained hidden from scattering probes.

cond-mat.str-el↗

arXiv2026

Suppression of Spectral Gap and Flat Bands on a Cuprate Superconductor Side-Surface

Side surfaces of cuprate superconductors are expected to display a suppressed $d$-wave order parameter and zero-energy topological flat bands with a large density of states, making them susceptible to symmetry broken orders. Yet such surfaces have never been investigated with momentum-resolved, surface-sensitive probes, because high-temperature superconductors rarely cleave along them. Using focused-ion-beam milling to define a controlled breaking point, we expose pristine (110) side surfaces of overdoped La$_{2-x}$Sr$_x$CuO$_4$ ($x=0.22$) suitable for angle-resolved photoemission. We observe the suppression of the superconducting spectral gap within our energy resolution ($\sim 4~\mathrm{meV}$), and surprisingly, the expected zero-energy flat band peak is also suppressed, despite the high topographic quality of the surface. Self-consistent Bogoliubov--de~Gennes calculations show that the measured geometric roughness of the cleaved surface is too weak to eliminate these modes. The calculations further demonstrate that bulk inhomogeneities characteristic of high-temperature superconductors, modelled as moderate Anderson-type disorder, can broaden the flat-band states beyond detectability. Our results provide the first momentum-resolved view of the electronic structure on a cuprate side surface and reveal disorder as the key factor currently preventing appearance of flat bands and their associated correlated orders.

cond-mat.supr-con↗

arXiv2025

Superconductivity emerging from the N${\'e}$el state in ${\it infinite}$-${\it stage}$ single-layer cuprate La$_2$CuO$_{4+\delta}$

In copper oxides (cuprates) with single CuO$_2$ layer such as La$_{2-x}$Ba(Sr)$_x$CuO$_4$, antiferromagnetism coexists with superconductivity at small doping levels $x$, where chemical disorders are significant. Here, we report that superconductivity occurs in a uniform and fully ordered N${\'e}$el state in a single-layer cuprate La$_2$CuO$_{4+\delta}$ with a small amount of excess oxygen $(\delta = 0.015)$ as demonstrated by the $^{139}$La nuclear quadrupole resonance measurement. A uniform oxygen distribution in the crystal is crucial for achieving microscopic phase coexistence and overcoming the miscibility gap associated with the staging instability; self-organized periodic oxygen arrangement driven by mobile oxygen atoms. This finding prompts the reconsideration of superconductivity in cuprates, highlighting that it can emerge in a robust N${\'e}$el state that retains sizable magnetic moments and hosts only a small carrier density.

cond-mat.supr-con↗

arXiv2023

Weak-signal extraction enabled by deep-neural-network denoising of diffraction data

Removal or cancellation of noise has wide-spread applications for imaging and acoustics. In every-day-life applications, denoising may even include generative aspects, which are unfaithful to the ground truth. For scientific use, however, denoising must reproduce the ground truth accurately. Here, we show how data can be denoised via a deep convolutional neural network such that weak signals appear with quantitative accuracy. In particular, we study X-ray diffraction on crystalline materials. We demonstrate that weak signals stemming from charge ordering, insignificant in the noisy data, become visible and accurate in the denoised data. This success is enabled by supervised training of a deep neural network with pairs of measured low- and high-noise data. We demonstrate that using artificial noise does not yield such quantitatively accurate results. Our approach thus illustrates a practical strategy for noise filtering that can be applied to challenging acquisition problems.

eess.IV↗

arXiv2022

Crystal Symmetry of Stripe Ordered La1.88Sr0.12CuO4

We present a combined x-ray and neutron diffraction study of the stripe ordered superconductor \lscox{0.12}. The average crystal structure is consistent with the orthorhombic $Bmab$ space group as commonly reported in the literature. This structure however is not symmetry compatible with a second order phase transition into the stripe order phase, and, as we report here numerous Bragg peaks forbidden in the $Bmab$ space group are observed. We have studied and analysed these $Bmab$-forbidden Bragg reflections. Fitting of the diffraction intensities yields monoclinic lattice distortions that are symmetry consistent with charge stripe order.

cond-mat.supr-con↗

arXiv2019

Hidden magnetism at the pseudogap critical point of a high temperature superconductor

The mysterious pseudogap phase of cuprate superconductors ends at a critical hole doping level p* but the nature of the ground state below p* is still debated. Here, we show that the genuine nature of the magnetic ground state in La2-xSrxCuO4 is hidden by competing effects from superconductivity: applying intense magnetic fields to quench superconductivity, we uncover the presence of glassy antiferromagnetic order up to the pseudogap boundary p* ~ 0.19, and not above. There is thus a quantum phase transition at p*, which is likely to underlie highfield observations of a fundamental change in electronic properties across p*. Furthermore, the continuous presence of quasi-static moments from the insulator up to p* suggests that the physics of the doped Mott insulator is relevant through the entire pseudogap regime and might be more fundamentally driving the transition at p* than just spin or charge ordering.

cond-mat.supr-con↗

arXiv2018

Spin Thermal Hall Conductivity of a Kagomé Antiferromagnet

A clear thermal Hall signal ($κ_{xy}$) was observed in the spin liquid phase of the $S=1/2$ kagomé antiferromagnet Ca kapellasite (CaCu$_3$(OH)$_6$Cl$_2\cdot 0.6$H$_2$O). We found that $κ_{xy}$ is well reproduced, both qualitatively and quantitatively, using the Schwinger-boson mean-field theory with the Dzyaloshinskii--Moriya interaction of $D/J \sim 0.1$. In particular, $κ_{xy}$ values of Ca kapellasite and those of another kagomé antiferromagnet, volborthite, converge to one single curve in simulations modeled using Schwinger bosons, indicating a common temperature dependence of $κ_{xy}$ for the spins of a kagomé antiferromagnet.

cond-mat.str-el↗

arXiv2014

Ultrafast carrier localisation in the pseudogap state of cuprate superconductors from coherent quench experiments

A pseudogap (PG) was introduced by Mott to describe a state of matter which has a minimum in the density of states at the Fermi level, deep enough for states to become localized. It can arise either from Coulomb repulsion between electrons, or due to an incipient charge or spin order, or a combination of the two. These states are rapidly fluctuating in time with random phase, so they are hard to observe experimentally. Here we present the first coherent quench measurements of the dynamical transition to the pseudogap state in the prototype high temperature superconductor Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+δ}$, revealing a marked absence of incipient collective ordering beyond a few coherence lengths on short timescales at any level of doping. Instead we find evidence for sub-picosecond carrier localization favouring a picture of pairing resulting from the competing Coulomb interaction and strain, enhanced by a Fermi surface instability.

cond-mat.str-el↗

arXiv2014

Separating pairing from quantum phase coherence dynamics above the superconducting transition by femtosecond spectroscopy

In classical superconductors an energy gap and phase coherence appear simultaneously with pairing at the transition to the superconducting state. In high-temperature superconductors, the possibility that pairing and phase coherence are distinct and independent processes has led to intense experimental search of their separate manifestations, but so far without success. Using femtosecond spectroscopy methods we now show that it is possible to clearly separate fluctuation dynamics of the superconducting pairing amplitude from the phase relaxation above the critical transition temperature. Empirically establishing a close correspondence between the superfluid density measured by THz spectroscopy and superconducting optical pump-probe response over a wide region of temperature, we find that in differently doped Bi_{2}Sr_{2}CaCu_{2}O_{8+δ} crystals the pairing gap amplitude monotonically extends well beyond T_{c}, while the phase coherence shows a pronounced power-law divergence as T\rightarrow T_{c}, thus showing for the first time that phase coherence and gap formation are distinct processes which occur on different timescales.

cond-mat.supr-con↗

arXiv2001

Observation of Andreev reflection in the c-axis transport of Bi_2Sr_2CaCu_2O_{8+x} single crystals near T_c and search for the preformed-pair state

We observed an enhancement of the $c$-axis differential conductance around the zero-bias in Au$/$Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$ (Bi2212) junctions near the superconducting transition temperature $T_c$. We attribute the conductance enhancement to the Andreev reflection between the surface Cu-O bilayer with suppressed superconductivity and the neighboring superconducting inner bilayer. The continuous evolution from depression to an enhancement of the zero-bias differential conductance, as the temperature approaches $T_c$ from below, points to weakening of the barrier strength of the non-superconducting layer between adjacent Cu-O bilayers. We observed that the conductance enhancement persisted up to a few degrees above $T_c$ in junctions prepared on slightly overdoped Bi2212 crystals. However, no conductance enhancement was observed above $T_c$ in underdoped crystals, although recently proposed theoretical consideration suggests an even wider temperature range of enhanced zero-bias conductance. This seems to provide negative perspective to the existence of the phase-incoherent preformed pairs in the pseudogap state.

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