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Y. Toda

Publications and source records attributed to Y. Toda.

11 recordsLinked to original sources
arXiv2025

Possible Spatial Correlation of Superconducting and Pseudogap Dynamics in a Bi-based Cuprate

Understanding the interplay between superconductivity and the pseudogap phase is essential for elucidating the mechanism of high-temperature superconductivity in cuprates. Here we provide direct spatial evidence that these two states are locally and intrinsically correlated. Using spatially and temporally resolved measurements of photoinduced quasiparticle dynamics in optimally doped Bi$_2$Sr$_{1.7}$La$_{0.3}$CuO$_{6+\delta}$ (La-Bi2201), we reveal micrometer-scale spatial contrasts in the transient reflectivity that arise from local variations in the threshold fluence required to disrupt either the superconducting or pseudogap state. The superconducting response remains spatially uniform, whereas the pseudogap exhibits intrinsic inhomogeneity, yet the spatial variations of their threshold fluences closely track each other, establishing a robust local correlation between the two. These results introduce a bulk-sensitive ultrafast optical methodology for visualizing hidden spatial correlations in correlated materials and provide new benchmarks for understanding the intertwined phases in cuprates.

cond-mat.supr-con↗
arXiv2025

Critical photoinduced reflectivity relaxation dynamics in single-layer Bi-based cuprates near the pseudogap end point

A comprehensive study of photoinduced transient reflectivity dynamics in heavily overdoped single-layer cuprate (Bi,Pb)$_{2}$Sr$_{2}$CuO$_{6+\delta}$ (Pb-Bi-2201) across the end points of the pseudogap and superconducting phases was conducted using optical ultrafast time-resolved pump-probe spectroscopy. In the Pb-Bi-2201 near the proposed pseudogap end-point doping, the transient reflectivity dynamics above $T_{{\rm c}}$ resemble the pseudogap response observed in the optimally doped La-Bi-2201. With decreasing temperature, however, the relaxation time exhibits a power-law divergence, $\tau\sim10\hbar/k_{\mathrm{B}}T$, consistent with quantum critical behavior near the proposed pseudogap end-point doping. A similar power-law increase in relaxation time is also observed at a slightly more overdoped composition, though it is less pronounced.

cond-mat.supr-con↗
arXiv2018

Incommensurate Chiral CDW in $1T$-VSe$_2$

We have investigated the chiral charge-density wave (CDW) in $1T$-VSe$_2$ using scanning tunneling microscopy (STM) measurements and optical polarimetry measurements. With the STM mesurements, we revealed that the CDW intensities along each triple-$q$ directions are different. Thus the rotational symmetry of $1T$-VSe$_2$ is lower than that in typical two-dimentional triple-$q$ CDWs. We found that the CDW peaks form a kagome lattice rather than a triangular lattice. The Friedel oscillations have the chirality and the periodicity reflected properties of the background CDW. With the optical measurements in $1T$-VSe$_2$, we also observed a lower rotational symmetry with the polarization dependence of the transient reflectivity variation, which is consistent with the STM result on a microscopic scale. Both $1T$-TiSe$_2$ and $1T$-VSe$_2$ show chiral CDWs, which implies that such waves are usual for CDWs with the condition $H_\mathrm{CDW} \equiv q_{1}\cdot(q_{2} \times q_{3}) \neq0$.

cond-mat.str-el↗
arXiv2017

Dynamics of the superconducting order parameter through ultrafast normal-to-superconducting phase transition in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+δ}$ from multi-pulse polarization-resolved transient optical reflectivity

A systematic temperature dependent study of the femtosecond optical superconducting (SC) state destruction and recovery in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+δ}$ cuprate superconductor by means of the all-optical polarization-sensitive multi-pulse spectroscopy is presented. At low temperatures and a partial SC state suppression an anisotropic SC-gap recovery-timescale is suggested by the data. The SC state destruction and recovery dynamics are compared to the recent TR-ARPES-inferred SC-gap dynamics and a qualitative agreement is found. Using a phenomenological response function the experimental data are also compared to time dependent Ginzburg-Landau model simulations.

cond-mat.supr-con↗
arXiv2014

Strain-induced enhancement of the electron energy relaxation in strongly correlated superconductors

We use femtosecond optical spectroscopy to systematically measure the primary energy relaxation rate k1 of photoexcited carriers in cuprate and pnictide superconductors. We find that k1 increases monotonically with increased negative strain in the crystallographic a-axis. Generally, the Bardeen-Shockley deformation potential theorem and, specifically, pressure-induced Raman shifts reported in the literature suggest that increased negative strain enhances electron-phonon coupling, which implies that the observed direct correspondence between a and k1 is consistent with the canonical assignment of k1 to the electron-phonon interaction. The well-known non-monotonic dependence of the superconducting critical temperature Tc on the a-axis strain is also reflected in a systematic dependence Tc on k1, with a distinct maximum at intermediate values (~16 ps-1 at room temperature). The empirical non-monotonic systematic variation of Tc with the strength of the electron-phonon interaction provides us with unique insight into the role of electron-phonon interaction in relation to the mechanism of high-Tc superconductivity as a crossover phenomenon.

cond-mat.supr-con↗
arXiv2013

Dynamics of broken symmetry nodal and anti-nodal excitations in Bi_{2} Sr_{2} CaCu_{2} O_{8+δ} probed by polarized femtosecond spectroscopy

The dynamics of excitations with different symmetry is investigated in the superconducting (SC) and normal state of the high-temperature superconductor Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+δ}$ (Bi2212) using optical pump-probe (Pp) experiments with different light polarizations at different doping levels. The observation of distinct selection rules for SC excitations, present in A$_{\rm 1g}$ and B$_{\rm 1g}$ symmetries, and for the PG excitations, present in A$_{\rm 1g}$ and B$_{\rm 2g}$ symmetries, by the probe and absence of any dependence on the pump beam polarization leads to the unequivocal conclusion of the existence of a spontaneous spatial symmetry breaking in the pseudogap (PG) state.

cond-mat.supr-con↗
arXiv2011

Mechanisms of nonthermal destruction of the superconducting state and melting of the charge-density-wave state by femtosecond laser pulses

The processes leading to nonthermal condensate vaporization and charge-density wave (CDW) melting with femtosecond laser pulses is systematically investigated in different materials. We find that vaporization is relatively slow (tau_v ~ 1 ps) and inefficient in superconductors, exhibiting a strong systematic dependence of the vaporization energy Uv on Tc. In contrast, melting of CDW order proceeds rapidly (tau_m = 50 ~ 200 fs) and more efficiently. A quantitative model describing the observed systematic behavior in superconductors is proposed based on a phonon-mediated quasi-particle (QP) bottleneck mechanism. In contrast, Fermi surface disruption by hot QPs is proposed to be responsible for CDW state melting.

cond-mat.supr-con↗
arXiv2010

Chiral charge-density-waves

We discovered the chirality of charge density waves (CDW) in 1T-TiSe$_2$ by using scanning tunnelling microscopy (STM) and optical ellipsometry. We found that the CDW intensity becomes $I{a_1}:I{a_2}:I{a_3} = 1:0.7 \pm 0.1:0.5 \pm 0.1$, where $Ia_i$ (i =1, 2, 3) is the amplitude of the tunnelling current contributed by the CDWs. There were two states, in which the three intensity peaks of the CDW decrease \textit{clockwise} and \textit{anticlockwise} when we index each nesting vector in order of intensity in the Fourier transformation of the STM images. The chirality in CDW results in the three-fold symmetry breaking. Macroscopically, two-fold symmetry was indeed observed in optical measurement. We propose the new generalized CDW chirality $H_{CDW} \equiv {\boldmath $q_1$} \cdot ({\boldmath $q_2$}\times {\boldmath $q_3$})$, where ${\boldmath $q_i$}$ are the nesting vectors, which is independent of the symmetry of components. The nonzero $H_{CDW}$ - the triple-${\boldmath $q$}$ vectors do not exist in an identical plane in the reciprocal space - should induce a real-space chirality in CDW system.

cond-mat.other↗
arXiv2008

Direct observation of the coexistence of pseudogap and superconducting quasi-particles in Bi2212 by time-resolved optical spectroscopy

We report the ultra-fast optical response of quasi-particles (QPs) in both the pseudogap (PG) and superconducting (SC) states of underdoped (UD) Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+δ}$ (Bi2212) single crystal measured with the time-resolved pump-probe technique. At a probe energy $\hbarω_{pr}$=1.55 eV, it is found that the reflectivity change $Δ$R/R changes its sign at exactly $T_{c}$, which allows the direct separation of the charge dynamics of PG and SC QPs. Further systematic investigations indicate that the transient signals associated with PG and SC QPs depend on the probe beam energy and polarization. By tuning them below $T_{c}$ two distinct components can be detected simultaneously, providing evidence for the coexistence of PG and SC QPs.

cond-mat.supr-con↗
arXiv2004

Optical diffraction spectroscopy of excitons in uniaxially-strained GaN films

The degenerate four-wave mixing spectroscopy of uniaxially strained GaN layers is demonstrated using colinearly polarized laser pulses. The nonlinear response of FWM signal on exciton oscillator strength enhances the sensitivity for polarized exciton, allowing for mapping out the in-plane anisotropy of the strain field. The observed high-contrast spectral polarization clearly shows fine structure splittings of excitons, which are also confirmed in the change of quantum beating periods of time.

cond-mat.other↗