arXiv subjects

Yusuke Iguchi

Publications and source records attributed to Yusuke Iguchi.

18 recordsLinked to original sources

arXiv2026

Magnetic Quantum Criticality inside the Superconducting State Revealed by Penetration Depth Scaling with Local $T_{\mathrm c}$

We demonstrate a magnetic quantum critical point embedded within the superconducting state of Zn-doped CeCoIn$_5$, revealed by a pronounced peak in the magnetic penetration depth at zero temperature $\lambda(0)$. Using scanning SQUID microscopy, we determine the local superconducting transition temperature $T_{\mathrm c}$ and $\lambda(0)$. By parameterizing $\lambda(0)$ in terms of the local $T_{\mathrm c}$ rather than nominal Zn substitution, we circumvent the ambiguity caused by doping inhomogeneity and enable a more precise extraction of the critical exponent. The extracted exponent exceeds the clean spin-density-wave value, indicating a disorder-modified quantum critical regime. The enhancement of $\lambda(0)$ reflects the suppression of the superfluid stiffness and is consistent with critical scaling. Our approach provides a route to uncover intrinsic quantum critical behavior hidden by inhomogeneity in unconventional superconductors.

cond-mat.supr-con↗

arXiv2026

Visualizing Vortex Cluster Dynamics in the Weak Type-II Superconductor CaSb$_2$

Scanning SQUID imaging of CaSb$_2$ reveals dense vortex clusters with enhanced boundary susceptibility and suppressed internal vortex motion, which features inconsistent with both isolated vortex and flux tube behaviors. These measurements provide the first local visualization of magnetic dynamics within vortex clusters in a weakly pinned superconductor, offering a new route to probe non-monotonic vortex-vortex interactions that are typically expected in single-band type-II/1 or multiband type-1.5 superconductors. Although the superfluid density follows a single-gap BCS model and the Ginzburg-Landau parameter of CaSb$_2$ lies slightly outside the type-II/1 regime, vortex clustering and spatially inhomogeneous dynamics are clearly observed, indicating physics beyond existing microscopic theories for single-band superconductors.

cond-mat.supr-con↗

arXiv2025

Vortex Motion Induced Losses in Tantalum Resonators

Tantalum (Ta) based superconducting circuits have been demonstrated to enable record qubit coherence times and quality factors, motivating a careful study of the microscopic origin of the remaining losses that limit their performance. We have recently shown that the losses in Ta-based resonators are dominated by two-level systems (TLSs) at low microwave powers and millikelvin temperatures. We also observe that some devices exhibit loss that is exponentially activated at a lower temperature inconsistent with the superconducting critical temperature (Tc) of the constituent film. Specifically, dc resistivity measurements show a Tc of over 4 K, while microwave measurements of resonators fabricated from these films show losses that increase exponentially with temperature with an activation energy as low as 0.3 K. Here, we present a comparative study of the structural and thermodynamic properties of Ta-based resonators and identify vortex motion-induced loss as the source of thermally activated microwave loss. Through careful magnetoresistance and x-ray diffraction measurements, we observe that the increased loss occurs for films that are in the clean limit, where the superconducting coherence length is shorter than the mean free path. Vortex motion-induced losses are suppressed for films in the dirty limit, which show evidence of structural defects that can pin vortices. We verify this hypothesis by explicitly pinning vortices via patterning and find that we can suppress the loss by microfabrication.

cond-mat.supr-con↗

arXiv2024

Magnetic edge fields in UTe$_2$ near zero background fields

Chiral superconductors are theorized to exhibit spontaneous edge currents. Here, we found magnetic fields at the edges of UTe$_2$, a candidate odd-parity chiral superconductor, that seem to agree with predictions for a chiral order parameter. However, we did not detect the chiral domains that would be expected, and recent polar Kerr and muon spin relaxation data in nominally clean samples argue against chiral superconductivity. Our results show that hidden sources of magnetism must be carefully ruled out when using spontaneous edge currents to identify chiral superconductivity.

cond-mat.supr-con↗

arXiv2024

In-situ local imaging of ferromagnetism and superconductivity in RbEuFe$_4$As$_4$

The coexistence of superconductivity and ferromagnetism is an intrinsically interesting research focus in condensed matter physics but the study is limited by low superconducting ($T_c$) and magnetic ($T_m$) transition temperatures in related materials. Here, we used a scanning superconducting quantum interference device to image the in-situ diamagnetic and ferromagnetic responses of RbEuFe$_4$As$_4$ with high $T_c$ and $T_m$. We observed significant suppression of superfluid density in vicinity of the magnetic phase transition, signifying fluctuation-enhanced magnetic scatterings between Eu spins and Fe 3$d$ conduction electrons. Intriguingly, we observed multiple ferromagnetic domains which should be absent in an ideal magnetic helical phase. The formation of these domains demonstrates a weak $c$-axis ferromagnetic component probably arising from Eu spin-canting effect, indicative of possible superconductivity-driven domain Meissner and domain vortex-antivortex phases as revealed in EuFe$_2$(As$_{0.79}$P$_{0.21}$)$_2$. Our observations highlight RbEuFe$_4$As$_4$ is a unique system which includes multiple interplay channels between superconductivity and ferromagnetism.

cond-mat.supr-con↗

arXiv2024

Scanning SQUID study of ferromagnetism and superconductivity in infinite-layer nickelates

Infinite-layer nickelates $R_{1-x}$Sr$_{x}$NiO$_{2}$ ($R$ = La, Pr, Nd) are a class of superconductors with structural similarities to cuprates. Although long-range antiferromagnetic order has not been observed for these materials, magnetic effects such as antiferromagnetic spin fluctuations and spin-glass behavior have been reported. Different experiments have drawn different conclusions about whether the pairing symmetry is $s$- or $d$ wave. In this paper, we applied a scanning superconducting quantum interference device (SQUID) to probe the magnetic behavior of film samples of three infinite-layer nickelates (La$_{0.85}$Sr$_{0.15}$NiO$_2$, Pr$_{0.8}$Sr$_{0.2}$NiO$_2$, and Nd$_{0.775}$Sr$_{0.225}$NiO$_2$) grown on SrTiO$_3$ (STO), each with a nominal thickness of 20 unit cells. In all three films, we observed a ferromagnetic background. We also measured the magnetic susceptibility above the superconducting critical temperature in Pr$_{0.8}$Sr$_{0.2}$NiO$_2$ and La$_{0.85}$Sr$_{0.15}$NiO$_2$ and identified a non-Curie-Weiss dynamic susceptibility. Both magnetic features are likely due to NiO$_x$ nanoparticles. Additionally, we investigated superconductivity in Pr$_{0.8}$Sr$_{0.2}$NiO$_2$ and Nd$_{0.775}$Sr$_{0.225}$NiO$_2$, which exhibited inhomogeneous diamagnetic screening. The superfluid density inferred from the diamagnetic susceptibility in relatively homogeneous regions shows $T$-linear behavior in both samples. Finally, we observed superconducting vortices in Nd$_{0.775}$Sr$_{0.225}$NiO$_2$. We determined a Pearl length of 330 $\upmu$m for Nd$_{0.775}$Sr$_{0.225}$NiO$_2$ at 300 mK both from the strength of the diamagnetism and from the size and shape of the vortices. These results highlight the importance of considering NiO$_x$ particles when interpreting experimental results for these films.

cond-mat.supr-con↗

arXiv2023

Anomalous superfluid density in a disordered charge density wave material: Pd-intercalated ErTe$_3$

We image local superfluid density in single crystals of Pd-intercalated ErTe$_3$ below the superconducting critical temperature, $T_c$, well below the onset temperature, $T_{CDW}$, of (disordered) charge-density-wave order. We find no detectable inhomogeneities. We observe a rapid increase of the superfluid density below $T_c$, deviating from the behavior expected in conventional Bardeen-Cooper-Schrieffer, and show that the temperature dependence is qualitatively consistent with a combination of quantum and thermal phase fluctuations.

cond-mat.supr-con↗

arXiv2023

Superconducting Penetration Depth Through a Van Hove Singularity: Sr$_2$RuO$_4$ Under Uniaxial Stress

In the unconventional superconductor Sr$_2$RuO$_4$, uniaxial stress along the $[100]$ direction tunes the Fermi level through a Van Hove singularity (VHS) in the density of states, causing a strong enhancement of the superconducting critical temperature $T_\textrm{c}$. Here, we report measurements of the London penetration depth $\lambda$ as this tuning is performed. We find that the zero-temperature superfluid density, here defined as $\lambda(0)^{-2}$, increases by $\sim$15%, with a peak that coincides with the peak in $T_\textrm{c}$. We also find that the low temperature form of $\lambda(T)$ is quadratic over the entire strain range. Using scanning tunneling microscopy, we find that the gap increases from $\Delta_0 \approx 350~\mu$eV in unstressed Sr$_2$RuO$_4$ to $\Delta_0 \approx 600~\mu$eV in a sample strained to near the peak in $T_c$. With a nodal order parameter, an increase in the superconducting gap could bring about an increase in the superfluid density through reduced sensitivity to defects and through reduced non-local effects in the Meissner screening. Our data indicate that tuning to the VHS increases the gap throughout the Brillouin zone, and that non-local effects are likely more important than reduced scattering.

cond-mat.supr-con↗

arXiv2023

Constraints on a split superconducting transition under uniaxial strain in Sr$_2$RuO$_4$ from scanning SQUID microscopy

More than two decades after the discovery of superconductivity in Sr$_2$RuO$_4$, it is still unclear whether the order parameter has a single component or two degenerate components. For any two-component scenario, application of uniaxial strain is expected to lift the degeneracy, generating two distinct phase transitions. The presence of a second (lower-temperature) transition may be observable by probes that are sensitive to changes in the London penetration depth, $\lambda$, as a function of temperature, $T$. Here, we use scanning SQUID microscopy combined with a uniaxial strain device to test for a second transition under strain. We only observe a single transition. Within the temperature range where a second transition has been suggested by $\mu$SR measurements, we further place a tight upper bound of less than 1% on the change in the zero temperature superfluid density $n_s\propto\lambda^{-2}(0)$ due to a second transition, suggesting that such a transition does not occur. These results constrain theories of the order parameter in Sr$_2$RuO$_4$.

cond-mat.supr-con↗

arXiv2023

Observation of superconducting vortices carrying a temperature-dependent fraction of the flux quantum

The magnetic response is a state-defining property of superconductors. The magnetic flux penetrates type-II bulk superconductors by forming quantum vortices when the enclosed magnetic flux is equal to the magnetic flux quantum. The flux quantum is the universal quantity that depends only on the ratio of fundamental constants: the electron charge and the Planck constant. This work investigates the vortex state in the hole-overdoped Ba$_{1-x}$K$_x$Fe$_2$As$_2$ by using scanning superconducting quantum interference device (SQUID) magnetometry. We observed quantum vortices that carry only a fraction of the flux quantum, which vary continuously with temperature. This finding establishes the phenomenon that superconductors support quantum vortices with non-universally quantized magnetic flux. Furthermore, the demonstrations of the mobility of the fractional vortices and the manipulability of their positions open up a route for future fluxonics applications.

cond-mat.supr-con↗

arXiv2022

Microscopic imaging homogeneous and single phase superfluid density in UTe$_2$

The spin-triplet superconductor UTe$_2$ shows spontaneous time-reversal symmetry breaking and multiple superconducting phases in some crystals, implying chiral superconductivity. Here we microscopically image the local magnetic fields and magnetic susceptibility near the surface of UTe$_2$, observing a homogeneous superfluid density $n_s$ and homogeneous pinned vortices. The temperature dependence of $n_s$ is consistent with an anisotropic gap and shows no evidence for an additional kink that would be expected at any second phase transition. Our findings are consistent with a dominant $B_{3u}$ superconducting order parameter in the case of a quasi-2D Fermi surface and provide no evidence for multiple phase transitions in $n_s(T)$ in UTe$_2$.

cond-mat.supr-con↗

arXiv2021

Local observation of linear-$T$ superfluid density and anomalous vortex dynamics in URu$_2$Si$_2$

The heavy fermion superconductor URu$_2$Si$_2$ is a candidate for chiral, time-reversal symmetry-breaking superconductivity with a nodal gap structure. Here, we microscopically visualized superconductivity and spatially inhomogeneous ferromagnetism in URu$_2$Si$_2$. We observed linear-$T$ superfluid density, consistent with d-wave pairing symmetries including chiral d-wave, but did not observe the spontaneous magnetization expected for chiral d-wave. Local vortex pinning potentials had either four- or two-fold rotational symmetries with various orientations at different locations. Taken together, these data support a nodal gap structure in URu$_2$Si$_2$ and suggest that chirality either is not present or does not lead to detectable spontaneous magnetization.

cond-mat.supr-con↗

arXiv2019

Imaging anisotropic vortex dynamics in FeSe

Strong vortex pinning in FeSe could be useful for technological applications and could provide clues about the coexistence of superconductivity and nematicity. To characterize the pinning of individual, isolated vortices, we simultaneously apply a local magnetic field and image the vortex motion with scanning SQUID susceptibility. We find that the pinning is highly anisotropic: the vortices move easily along directions that are parallel to the orientations of twin domain walls and pin strongly in a perpendicular direction. These results are consistent with a scenario in which the anisotropy arises from vortex pinning on domain walls and quantify the dynamics of individual vortex pinning in FeSe.

cond-mat.supr-con↗

arXiv2017

Nanolayered diffusion couple technique for the reconsidering of the miscibility gap

Mysteries in binary phase diagram are still in existence, e.g., the case in Cu-Ni which is mutual miscible system, the question addressed that "Indirect experimental evidence indicates the presence of a miscibility gap in the fcc phase at a temperature somewhere between 450 and 923 K". Recently, author's group suggests new miscibility gap on Cupper-Nickel system by means of Secondary Neutral Mass Spectroscopy (SNMS) with nanolayered diffusion couple technique. Here we considered about problems in this technique, e.g., strain and size modified miscibility gap, equilibrium and its morphorogy, how to differentiate it, etc. Finally we discussed for justification of our procedure and phase diagrams on several binary alloys.

cond-mat.mtrl-sci↗

arXiv2016

On the miscibility gap of Cu-Ni system

The existence of the miscibility gap in the Cu-Ni system has been an issue in both computational and experimental discussions for half a century [Chakrabarti et al., Phase diagrams of binary nickel alloys, ASM, 1991]. Here we propose a new miscibility gap in the Cu-Ni system measured in nano-layered thin films by Secondary Neutral Mass Spectrometry. The maximum of the symmetrical miscibility curve is around 800~K at Cu50%Ni50%. To our best knowledge, this is the first experiment determining the miscibility from the measurement of the atomic fraction of Copper and Nickel in the whole composition range. Needless to say that Ni, Cu and its alloys are important in various fields, accordingly this result affects different areas to understand materials sciences.

cond-mat.mtrl-sci↗

arXiv2007

Two-Stage Formation Model and Helicity of Gold Nanowires

A model for formation of helical multishell gold nanowires is proposed and is confirmed with the quantum mechanical molecular dynamics simulations. The model can explain the magic number of the helical gold nanowires in the multishell structure. The reconstruction from ideal non-helical to realistic helical nanowires consists of two stages: dissociations of atoms on the outermost shell from atoms on the inner shell and slip deformations of atom rows generating (111)-like structure on the outermost shell. The elementary processes are governed by competition between energy loss and gain by s- and d-electrons together with the width of the d-band. The possibility for the helical nanowires of platinum, silver and copper is discussed.

cond-mat.mtrl-sci↗

arXiv2005

Nanoscale structures formed in silicon cleavage studied with large-scale electronic structure alculations; surface reconstruction, step and bending

The 10-nm-scale structure in silicon cleavage is studied by the quantum mechanical calculations for large-scale electronic structure. The cleavage process on the order of 10 ps shows surface reconstruction and step formation. These processes are studied by analyzing electronic freedom and compared with STM experiments. The discussion presents the stability mechanism of the experimentally observed mode, the $(111)$-$(2 x 1)$ mode, beyond the traditional approach with surface energy. Moreover, in several results, the cleavage path is bent into the experimentally observed planes, owing to the relative stability among different cleavage modes. Finally, several common aspects between cleavage and other phenomena are discussed from the viewpoints of the nonequilibrium process and the 10-nm-scale structure.

cond-mat.mtrl-sci↗

arXiv2005

The electronic structure around As antisite near (110) surface of GaAs

The electronic structure around a single As antisite in GaAs is investigated in bulk and near the surface both in the stable and the metastable atomic configurations. The most characteristic electronic structures of As antisite is the existence of the localized p-orbitals extending from the As antisite. The major component of the highest occupied state on As antisite in the stable configuration is s-orbital connecting with neighboring As atoms with nodes whereas that in the metastable configuration is p-orbital connecting without nodes. Localized p-orbitals on the surrounding As atoms around the As antisite exist in every configuration of As antisite. Such features are retained except the case of the As antisite located just in the surface layer in which the midgap level is smeared into the conduction band and no localized states exist near the top of the valence band. Scanning tunneling microscopic images of defects observed in low-temperature grown GaAs, possibly assigned as As antisite, the origin of the metastability, and the peculiarity of the defects in the surface layer are discussed.

cond-mat.mtrl-sci↗