arXiv subjects

K. Kudo

Publications and source records attributed to K. Kudo.

At least 19 recordsLinked to original sources

arXiv2026

Strain-tunable charge localization coupled to complex magnetic orders in EuAl$_4$

Charge localization is particularly interesting when coupled to antiferromagnetic spin structures. Coupled spin-charge orders are well established in elemental chromium and correlated oxide superconductors, yet the interplay between charge order and more complex magnetic textures -- such as skyrmion lattices and chiral spin structures -- remains largely unexplored. Here we report a comprehensive study of how charge localization couples to the unusually rich sequence of magnetic phases in EuAl$_4$. Using x-ray diffraction under applied magnetic field and uniaxial pressure, we demonstrate a direct coupling between the charge and spin order parameters. In the absence of external stimuli, charge localization is markedly enhanced upon entering the magnetically ordered phases. Strikingly, this effect is highly susceptible to strain: uniaxial pressure applied along the charge-order propagation direction further enhances localization, whereas pressure applied perpendicular to it weakens it. Application of magnetic field reveals both competitive and possible collaborative interactions between spin and charge ordering.This flexible coupling between spin and charge ordering opens a new route to designing symmetry-breaking states. Chiral charge order may for example be patterned from spin structures with that symmetry.

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arXiv2024

Spontaneous magnetic field and disorder effects in BaPtAs_1-x_Sb_x_ with honeycomb network

Chiral superconductivity exhibits the formation of novel electron pairs that breaks the time-reversal symmetry and has been actively studied in various quantum materials in recent years. However, despite its potential to provide definitive information, effects of disorder in the crystal structure on the chiral superconductivity has not yet been clarified, and therefore the investigation using a solid-solution system is desirable. We report muon-spin-relaxation (muSR) results of layered pnictide BaPtAs_1-x_Sb_x_ with a honeycomb network composed of Pt and (As, Sb). We observed an increase of the zero-field muon-spin relaxation rate in the superconducting (SC) state at the Sb end of x=1.0, suggesting the occurrence of spontaneous magnetic field due to the time-reversal symmetry breaking in the SC state. On the other hand, spontaneous magnetic field was almost and completely suppressed for the As-Sb mixed samples of x=0.9 and 0.2, respectively, suggesting that the time-reversal symmetry breaking SC state in x=1.0 is sensitive to disorder. The magnetic penetration depth estimated from transverse-field muSR measurements at x=1.0 and 0.2 behaved like weak-coupling s-wave superconductivity. These seemingly incompatible zero-field and transverse-field muSR results of BaPtAs_1-x_Sb_x_ with x=1.0 could be understood in terms of chiral d-wave superconductivity with point nodes on the three-dimensional Fermi surface.

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arXiv2024

Observation of two-level critical-state in a van-der-Waals superconductor Pt(Bi$_{1-x}$Se$_x$)$_2$

Trigonal PtBi$_2$ is one of the attractive van-der-Waals materials because of the enhancement of its superconducting transition temperature $T_{\rm{c}}$ by doping chalcogen elements such as Se and Te. Recently, it has been reported that $T_{\rm{c}}$ of Pt(Bi$_{1-x}$Se$_x$)$_2$ is enhanced by a factor of 4, compared to the pristine PtBi$_2$, together with the polar-nonpolar structural phase transition. Thus, it is desirable to study electrical transport properties for this new superconducting compound. Here, we have performed magnetotransport measurements for Pt(Bi$_{1-x}$Se$_x$)$_2$ ($x$ = 0.06 and 0.08) thin-film devices and have observed a peculiar magnetoresistance where a finite hysteresis appears when the superconducting state is broken. By measuring the magnetoresistance systematically, we have attributed this magnetoresistance to the two-level critical-state where fluxons pinned in Pt(Bi$_{1-x}$Se$_x$)$_2$ play an important role.

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arXiv2023

Phase separation and phase transitions in undoped and Rh3+ doped iron pnictide CaFe2As2: a Raman scattering study

Iron-pnictides Ca(Fe1-xRhx)2As2 (x = 0, 0.035, and 0.19) were studied across the tetragonal-orthorhombic and uncollapsed to collapsed tetragonal phase transitions using Raman spectroscopy. The effect of the phase separation was observed in the high-temperature phase for the first time. Two phases with the low-spin and high-spin states of the Fe2+ ions coexist in the undoped (x = 0) and doped (x = 0.035) samples at ambient conditions. These two phases are present on a larger length scale and they are not limited to the local distortions. Both phases in the undoped sample successfully undergo tetra-to-ortho phase transitions approximately at the same temperature T0 = 160 K. In doped samples, a cascade of phase transitions is observed at temperatures T* = 100 K and Tc = 72 K with cooling for the first time. A complex type of the re-entrant magnetic structure is realized in the x = 0.035 sample at temperatures below Tc, which is distinct from the usual orthorhombic twofold one. The overdoped sample (x = 0.19) also shows phase separation at temperatures 7 - 295 K in Raman spectra similar to these of the remnant phase.

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arXiv2021

Domain dependent Fermi arcs observed in a striped phase dichalcogenide

Angle-resolved photoemission spectromicroscopy on IrTe2 reveals evolution of mesoscopic striped domains across its first order phase transition at about 280 K. The striped texture of the domains is characterized by a herringbone arrangement of the electronic anisotropy axes. Under further cooling down to 47 K, the striped domains evolve into trijunction domains with the electronic anisotropy in three directions. Each domain harbors quasi one-dimensional surface bands forming Fermi arcs with peculiar spin polarization. The Fermi arc corresponds to an edge state of the two-dimensional bulk electronic states truncated at the surface, indicating an interesting interplay between the symmetry breaking and the surface electronic states.

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arXiv2019

Thermal Conductivity and Magnetic Phase Diagram of CuB2O4

We have measured temperature and magnetic field dependences of the thermal conductivity along the c-axis, kc, and that along the [110] direction, k110, of CuB2O4 single crystals in zero field and magnetic fields along the c-axis and along the [110] direction. It has been found that the thermal conductivity is nearly isotropic and very large in zero field and that the thermal conductivity due to phonons is dominant in CuB2O4. The temperature and field dependences of kc and k110 have markedly changed at phase boundaries in the magnetic phase diagram, which has been understood to be due to the change of the mean free path of phonons caused by the change of the phonon-spin scattering rate at the phase boundaries. It has been concluded that thermal conductivity measurements are very effective for detecting magnetic phase boundaries.

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arXiv2018

Temperature-dependent local structure of superconducting BaPd$_2$As$_2$ and SrPd$_2$As$_2$

The local structures of 122-type paradium arsenides, namely BaPd$_2$As$_2$ and SrPd$_2$As$_2$, are examined by As K-edge extended x-ray absorption fine structure measurements to find a possible correlation between the variation of their superconducting transition temperature and the local structure. The local atomic distances are found to be consistent with average distances measured by diffraction techniques. The temperature dependence of mean square relative displacements reveal that, while BaPd$_2$As$_2$ is characterized by a local As-Pd soft mode, albeit with larger atomic disorder, SrPd$_2$As$_2$ shows anomalous As-Pd correlations with a kink at $\sim$160 K due to hardening by raising temperature. We have discussed implications of these results and possible mechanism of differing superconducting transition temperature in relation with the structural instability.

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arXiv2018

Commensurate vs incommensurate charge ordering near the superconducting dome in Ir$_{1-x}$Pt$_x$Te$_2$ revealed by resonant x-ray scattering

The electronic-structural modulations of Ir$_{1-x}$Pt$_x$Te$_2$ (0 $\leqq x\leqq$ 0.12) have been examined by resonant elastic x-ray scattering (REXS) and resonant inelastic x-ray scattering (RIXS) techniques at both the Ir and Te edges. Charge-density-wave-like superstructure with wave vectors of $\mathbf{Q}$=(1/5 0 $-$1/5), (1/8 0 $-$1/8), and (1/6 0 $-$1/6) are observed on the same sample of IrTe$_2$ at the lowest temperature, the patterns of which are controlled by the cooling speeds. In contrast, superstructures around $\mathbf{Q}$=(1/5 0 $-$1/5) are observed for doped samples (0.02 $\leqq x\leqq$ 0.05). The superstructure reflections persist to higher Pt substitution than previously assumed, demonstrating that a charge density wave (CDW) can coexists with superconductivity. The analysis of the energy-dependent REXS and RIXS lineshape reveals the importance of the Te 5$p$ state rather than the Ir 5$d$ states in the formation of the spatial modulation of these systems. The phase diagram re-examined in this work suggests that the CDW incommensurability may correlate the emergence of superconducting states as-like Cu$_x$TiSe$_2$ and Li$_x$TaS$_2$.

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arXiv2017

Charge-Stripe Order and Superconductivity in $\mathrm{Ir_{1-x}Pt_xTe_2}$

A combined resistivity and hard x-ray diffraction study of superconductivity and charge ordering in $\mathrm{Ir_{1-x}Pt_xTe_2}$, as a function of Pt substitution and externally applied hydrostatic pressure, is presented. Experiments are focused on samples near the critical composition $x_c\sim 0.045$ where competition and switching between charge order and superconductivity is established. We show that charge order as a function of pressure in $\mathrm{Ir_{0.95}Pt_{0.05}Te_{2}}$ is preempted - and hence triggered - by a structural transition. Charge ordering appears uniaxially along the short crystallographic (1,0,1) domain axis with a $\mathrm{(\frac{1}{5},0,\frac{1}{5})}$ modulation. Based on these results we draw a charge-order phase diagram and discuss the relation between stripe ordering and superconductivity.

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arXiv2016

High-resolution magnetic penetration depth and inhomogeneities in locally noncentrosymmetric SrPtAs

We present a magnetic-penetration-depth study on polycrystalline and granular samples of SrPtAs, a pnictide superconductor with a hexagonal structure containing PtAs layers that individually break inversion symmetry (local noncentrosymmetry). Compact samples show a clear-cut s-wave-type BCS behavior, which we consider to be the intrinsic penetration depth of SrPtAs. Granular samples display a sample-dependent second diamagnetic drop, attributed to the intergrain coupling. Our experimental results point to a nodeless isotropic superconducting energy gap in SrPtAs, which puts strong constraints on the driven mechanism for superconductivity and the order parameter symmetry of this compound.

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arXiv2015

Real Space Imaging of Spin Polarons in Zn Doped SrCu2(BO3)2

We report on the real space profile of spin polarons in the quasi two-dimensional frustrated dimer spin system SrCu2(BO3)2 doped with 0.16% of Zn. The 11B nuclear magnetic resonance spectrum exhibits 15 additional boron sites near non-magnetic Zn impurities. With the help of exact diagonalizations of finite clusters, we have deduced from the boron spectrum the distribution of local magnetizations at the Cu sites with fine spatial resolution, providing direct evidence for an extended spin polaron. The results are confronted with those of other experiments performed on doped and undoped samples of SrCu2(BO3)2.

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arXiv2014

Effect of Pt substitution on the electronic structure of AuTe2

We report a photoemission and x-ray absorption study on Au1-xPtxTe2 (x = 0 and 0.35) triangular lattice in which superconductivity is induced by Pt substitution for Au. Au 4f and Te 3d core-level spectra of AuTe2 suggests a valence state of Au2+(Te2)2-, which is consistent with its distorted crystal structure with Te-Te dimers and compressed AuTe6 otahedra. On the other hand, valence-band photoemission spectra and pre-edge peaks of Te 3d absorption edge indicate that Au 5d bands are almost fully occupied and that Te 5p holes govern the transport properties and the lattice distortion. The two apparently conflicting pictures can be reconciled by strong Au 5d/Au 6s-Te 5p hybridization. Absence of a core-level energy shift with Pt substitution is inconsistent with the simple rigid band picture for hole doping. The Au 4f core-level spectrum gets slightly narrow with Pt substitution, indicating that the small Au 5d charge modulation in distorted AuTe2 is partially suppressed.

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arXiv2014

Superconductivity in Noncentrosymmetric Iridium Silicide Li2IrSi3

The effects of lithium absorption on the crystal structure and electronic properties of IrSi3, a binary silicide with a noncentrosymmetric crystal structure, were studied. X-ray and neutron diffraction experiments revealed that hexagonal IrSi3 (space group P6_3mc) transforms into trigonal Li2IrSi3 (space group P31c) upon lithium absorption. The structure of Li2IrSi3 is found to consist of a planar kagome network of silicon atoms with Li and Ir spaced at unequal distances between the kagome layers, resulting in a polar structure along the c-axis. Li2IrSi3 exhibited type-II superconductivity with a transition temperature Tc of 3.8 K, displaying a structure type that no previous superconductors have been reported to have.

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arXiv2014

Coexistence of Bloch electrons and glassy electrons in Ca10(Ir4As8)(Fe2_xIrxAs2)5 revealed by angle-resolved photoemission spectroscopy

Angle-resolved photoemission spectroscopy of Ca10(Ir4As8)(Fe2_xIrxAs2)5 shows that the Fe 3d electrons in the FeAs layer form the hole-like Fermi pocket at the zone center and the electron-like Fermi pockets at the zone corners as commonly seen in various Fe-based superconductors. The FeAs layer is heavily electron doped and has relatively good two dimensionality. On the other hand, the Ir 5d electrons are metallic and glassy probably due to atomic disorder related to the Ir 5d orbital instability. Ca10(Ir4As8)(Fe2_xIrxAs2)5 exhibits a unique electronic state where the Bloch electrons in the FeAs layer coexist with the glassy electrons in the Ir4As8 layer.

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arXiv2014

Bond-Order and the Role of Ligand States in Stripe-Modulated IrTe2

The coupled electronic-structural modulations of the ligand states in IrTe$_2$ have been studied by x-ray absorption spectroscopy (XAS) and resonant elastic x-ray scattering (REXS). Distinctive pre-edge structures are observed at the Te-$M_{4,5}$ (3$d$ $\rightarrow$ 5$p$) absorption edge, indicating the presence of a Te 5$p$-Ir 5$d$ covalent state near the Fermi level. An enhancement of the REXS signal near the Te 3$d$ $\rightarrow$ 5$p$ resonance at the $Q\!=\!(1/5,0,-1/5)$ superlattice reflection is observed below the structural transition temperature $T_s\sim$ 280 K. The analysis of the energy-dependent REXS lineshape reveals the key role played by the spatial modulation of the covalent Te 5$p$-Ir 5$d$ bond-density in driving the stripe-like order in IrTe$_2$, and uncovers its coupling with the charge and/or orbital order at the Ir sites. The similarity between these findings and the charge-ordering phenomenology observed in the high-T$_c$ superconducting cuprates suggests that the iridates may harbor similar exotic phases.

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arXiv2014

Spin-singlet superconductivity with a full gap in locally non-centrosymmetric SrPtAs

We report $^{195}$Pt-NMR and $^{75}$As-NQR measurements for the locally non-centrosymmetric superconductor SrPtAs where the As-Pt layer breaks inversion symmetry while globally the compound is centrosymmetric. The nuclear spin lattice relaxation rate $1/T_1$ shows a well-defined coherence peak below $T_c$ and decreases exponentially at low temperatures. The spin susceptibility measured by the Knight shift also decreases below $T_c$ down to $T<T_c/6$. These data together with the penetration depth obtained from the NMR spectra can be consistently explained by assuming a spin-singlet superconducting state with a full gap. Our results suggest that the spin-orbit coupling due to the local inversion-breaking is not large enough to bring about an exotic superconducting state, or the inter-layer hopping interaction is larger than the spin-orbit coupling.

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arXiv2014

Important Roles of Te 5p and Ir 5d Spin-orbit Interactions on the Multi-band Electronic Structure of Triangular Lattice Superconductor Ir1-xPtxTe2

We report an angle-resolved photoemission spectroscopy (ARPES) study on a triangular lattice superconductor Ir$_{1-x}$Pt$_{x}$Te$_2$ in which the Ir-Ir or Te-Te bond formation, the band Jahn-Teller effect, and the spin-orbit interaction are cooperating and competing with one another. The Fermi surfaces of the substituted system are qualitatively similar to the band structure calculations for the undistorted IrTe$_2$ with an upward chemical potential shift due to electron doping. A combination of the ARPES and the band structure calculations indicates that the Te $5p$ spin-orbit interaction removes the $p_x/p_y$ orbital degeneracy and induces $p_x \pm ip_y$ type spin-orbit coupling near the A point. The inner and outer Fermi surfaces are entangled by the Te $5p$ and Ir $5d$ spin-orbit interactions which may provide exotic superconductivity with singlet-triplet mixing.

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arXiv2013

Charge-orbital-lattice coupling effects in the dd-excitation profile of one dimensional cuprates

We identify dd-excitations in the quasi-one dimensional compound Ca$_2$Y$_2$Cu$_5$O$_{10}$ using resonant inelastic x-ray scattering. By tuning across the Cu L$_3$-edge, we observe abrupt shifts in the dd-peak positions as a function of incident photon energy. This observation demonstrates orbital-specific coupling of the high-energy excited states of the system to the low-energy degrees of freedom. A Franck-Condon treatment of electron-lattice coupling, consistent with other measurements in this compound, reproduces these shifts, explains the Gaussian lineshapes, and highlights charge-orbital-lattice renormalization in the high energy d-manifold.

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