arXiv subjects

Youichi Murakami

Publications and source records attributed to Youichi Murakami.

At least 19 recordsLinked to original sources

arXiv2023

Distinctive Doping Dependence of Upper Critical Field in Iron-Based Superconductor LaFeAsO$_{1-x}$H$_{x}$

High magnetic fields up to 105 T have been utilized in deriving the upper critical field $B_{\rm c2}$ of LaFeAsO$_{1-x}$H$_x$ throughout whole temperatures below $T_{\rm c}$. Resistivity measurements demonstrate that $B_{\rm c2}$ behaves differently in samples with $x = 0.12$ (SC1) from those with 0.32 (SC2). In SC1, the two-band model assuming the $s$-wave pairing gives a good fitting with repulsive intraband interaction and dominant interband coupling. In SC2, we have to assume attractive intraband interaction with weak interband coupling, which in fact suggests a non-$s$-wave pairing in view of the strong Coulomb repulsion. These results support the possibility that SC1 and SC2 have different pairing symmetries.

cond-mat.supr-con↗

arXiv2021

Photoinduced Transient States of Antiferromagnetic Orderings in La${}_{1/3}$Sr${}_{2/3}$FeO${}_{3}$ and SrFeO${}_{3}$ Thin Films Observed through Time-resolved Resonant Soft X-ray Scattering

The relationship between the magnetic interaction and photoinduced dynamics in antiferromagnetic perovskites is investigated in this study. In La${}_{1/3}$Sr${}_{2/3}$FeO${}_{3}$ thin films, commensurate spin ordering is accompanied by charge disproportionation, whereas SrFeO${}_{3}$ thin films show incommensurate helical antiferromagnetic spin ordering due to increased ferromagnetic coupling compared to La${}_{1/3}$Sr${}_{2/3}$FeO${}_{3}$. To understand the photoinduced spin dynamics in these materials, we investigate the spin ordering through time-resolved resonant soft X-ray scattering. In La${}_{1/3}$Sr${}_{2/3}$FeO${}_{3}$, ultrafast quenching of the magnetic ordering within 130 fs through a nonthermal process is observed, triggered by charge transfer between the Fe atoms. We compare this to the photoinduced dynamics of the helical magnetic ordering of SrFeO${}_{3}$. We find that the change in the magnetic coupling through optically induced charge transfer can offer an even more efficient channel for spin-order manipulation.

cond-mat.str-el↗

arXiv2019

Metamagnetic transitions and magnetoelectric responses in a chiral polar helimagnet Ni$_2$InSbO$_6$

Magnetic-field effect on the magnetic and electric properties in a chiral polar ordered corundum Ni$_2$InSbO$_6$ has been investigated. Single-crystal soft x-ray and neutron diffraction measurements confirm long-wavelength magnetic modulation. The modulation direction tends to align along the magnetic field applied perpendicular to the polar axis, suggesting that the nearly proper-screw type helicoid should be formed below 77\,K. The application of a high magnetic field causes a metamagnetic transition. In a magnetic field applied perpendicular to the polar axis, a helix-to-canted antiferromagnetic transition takes place through the intermediate soliton lattice type state. On the other hand, a magnetic field applied along the polar axis induces a first-order metamagnetic transition. These metamagnetic transitions accompany a change in the electric polarization along the polar axis.

cond-mat.mtrl-sci↗

arXiv2019

Quantum dynamics of hydrogen in iron-based superconductor LaFeAsO0.9D0.1 measured with inelastic neutron spectroscopy

Inelastic neutron scattering was performed for an iron-based superconductor LaFeAsO0.9D0.1, where most of D (deuterium) replaces oxygen, while a tiny amount goes into interstitial sites. By first-principle calculation, we characterize the interstitial sites for D (and for H slightly mixed) with four equivalent potential minima. Below the superconducting transition temperature Tc = 26 K,new excitations emerge in the range 5-15 meV, while they are absent in the reference system LaFeAsO0.9F0.1. The strong excitations at 14.5 meV and 11.1 meV broaden rapidly around 15 K and 20 K, respectively, where each energy becomes comparable to twice of the superconducting gap. The strong excitations are ascribed to a quantum rattling, or a band motion of hydrogen, which arises only if the number of potential minima is larger than two.

cond-mat.supr-con↗

arXiv2017

Growth of Antiperovskite Oxide Ca3SnO Films by Pulsed Laser Deposition

We report the epitaxial growth of Ca3SnO antiperovskite oxide films on (001)-oriented cubic yttria-stabilized zirconia (YSZ) substrates by using a conventional pulsed laser deposition (PLD) technique. In this work, a sintered Ca3SnO pellet is used as the ablation target. X-ray diffraction measurements demonstrate the (001) growth of Ca3SnO films with the antiperovskite structure and a cube-on-cube orientation relationship to the YSZ substrate. The successful synthesis of the antiperovskite phase is further confirmed by x-ray photoemission spectroscopy. These results strongly suggest that antiperovskite-oxide films can be directly grown on substrates from the target material using a PLD technique.

cond-mat.mtrl-sci↗

arXiv2017

Thickness dependence and dimensionality effects of charge and magnetic orderings in La1/3Sr2/3FeO3 thin films

We investigate the thickness effects on charge and magnetic orderings in Fe perovskite oxide La1/3Sr2/3FeO3/SrTiO3 thin films by hard x-ray and resonant soft x-ray scattering (RSXS) with changing thin film thickness systematically. We found that the correlation lengths of the magnetic ordering along the in-plane and out-of-plane directions are comparable and proportional to the thickness, and shows stronger thickness dependence than those of charge orderg. %the thickness dependence of correlation length of charge ordering is smaller than that of magnetic orderings. The magnetic ordered states disappear when the correlation length of magnetic ordering decreases to that of charge ordering through the intrinsic thickness effects. Surface sensitive grazing-incident RSXS revealed that the orderings exist even in the surface region, which indicates that the observed orderings is not affected by surface effect like oxygen vacancies. Critical thickness is in 5-15 nm, which corresponds to 4-11 antiferromagnetic ordering period. This critical value seems to be common to other ferromagnetic oxide thin films.

cond-mat.str-el↗

arXiv2017

Absence of magnetic long range order in Ba$_3$ZnRu$_2$O$_9$: A spin-liquid candidate in the $S=3/2$ dimer lattice

We have discovered a novel candidate for a spin liquid state in a ruthenium oxide composed of dimers of $S = $ 3/2 spins of Ru$^{5+}$,Ba$_3$ZnRu$_2$O$_9$. This compound lacks a long range order down to 37 mK, which is a temperature 5000-times lower than the magnetic interaction scale of around 200 K. Partial substitution for Zn can continuously vary the magnetic ground state from an antiferromagnetic order to a spin-gapped state through the liquid state. This indicates that the spin-liquid state emerges from a delicate balance of inter- and intra-dimer interactions, and the spin state of the dimer plays a vital role. This unique feature should realize a new type of quantum magnetism.

cond-mat.str-el↗

arXiv2016

Microscopic examinations of Co valences and spin states in electron-doped LaCoO$_{3}$

We studied the Co valences and spin states in electron-doped LaCo$_{1-y}$Te$_{y}$O$_3$ by measuring x-ray absorption spectra and electron spin resonance. The low-temperature insulating state involves the low-spin Co$^{3+}$ ($S=0$) and the high-spin Co$^{2+}$ state, which is described by $g=3.8$ and $j_{\rm eff}=1/2$. The results, in concurrence with the electron-hole asymmetry confirmed in electrical resistivity, coincide with a spin-blockade phenomenon in this system. Further, we discuss the $g$ factor in terms of the strong covalent-bonding nature and consider multiple origins of this phenomenon.

cond-mat.str-el↗

arXiv2016

Semimetallic "Electride Bands" Derived from Interlayer Electrons in Quasi-Two-Dimensional Electride Y$_2$C

Two-dimensional (2D) electrides are a new concept material in which anionic electrons are confined in the interlayer space between positively charged layers. We have performed angle-resolved photoemission spectroscopy measurements on Y$_2$C, which is a possible 2D electride, in order to verify the formation of 2D electride states in Y$_2$C. We clearly observe the existence of semimetallic "electride bands" near the Fermi level, as predicted by ${ab}$ ${initio}$ calculations, conclusively demonstrating that Y$_2$C is a quasi-2D electride with electride bands derived from interlayer anionic electrons.

cond-mat.mtrl-sci↗

arXiv2016

Effects of $c/a$ Anisotropy and Local Crystal Structure on Superconductivity in $A\mathrm{Fe_{2}}(\mathrm{As}_{1-x}\mathrm{P}_{x}\mathrm{)_{2}}$ ($A$=Ba$_{1-y}$Sr$_y$, Sr$_{1-y}$Ca$_y$ and Eu)

We investigated the effects of $c/a$ anisotropy and local crystal structure on superconductivity (SC) in As/P solid solution systems, $A\mathrm{Fe_{2}}(\mathrm{As}_{1-x}\mathrm{P}_{x}\mathrm{)_{2}}$ ($A$122P) with various $A$ ions. With decreasing $A$ site atomic size from $A$=Ba to Eu, the structural anisotropy decreases, and the rate of decreasing with $x$ also increases. The rapid narrowing of the region of antiferromagnetic composition ($x$) can be considered to be a result of this anisotropy change due mainly to the change in the Fermi surface (FS) nesting condition. By contrast, although the structural anisotropy systematically changes, the maximum $T_{\mathrm{c}}$ values are almost the same in all $A$122P systems except for Eu122P. These results indicate that the modification of the FS topology via the structural anisotropy does not affect SC. However local structural parameters, such as pnictogen height, are crucial for $T_{\mathrm{c}}$.

cond-mat.supr-con↗

arXiv2016

Novel Charge Ordering in the Trimer Iridium Oxide BaIrO3

We have prepared polycrystalline samples of the trimer Ir oxide BaIrO3 with face-shared Ir3O12 trimers, and have investigated the origin of the phase transition at 182 K by measuring resistivity, thermopower, magnetization and synchrotron x-ray diffraction. We propose a possible electronic model and transition mechanism, starting from a localized electron picture on the basis of the Rietveld refinement. Within this model, BaIrO3 can be basically regarded as a Mott insulator, when the Ir3O12 trimer is identified to one pseudo-atom or one lattice site. The transition can be viewed as a transition from the Mott insulator phase to a kind of charge ordered insulator phase.

cond-mat.str-el↗

arXiv2015

Strong charge density wave fluctuation and sliding state in PdTeI with quasi-1D PdTe chains

In quasi-one-dimensional (quasi-1D) system, the charge density wave (CDW) transition temperature TCDW is usually lower than the mean-field-theory predicted TMF and a CDW fluctuation region exists between them. Here, we investigate the physical properties of PdTeI single crystal containing quasi-1D PdTe chains. Surprisingly, we find that the carrier concentration decreases gradually before the long-range CDW ordering state occurring at T1 ~ 110 K, reflecting the existence of strong CDW fluctuation with possible pseudogap state at T >> T1 because of dynamic charge separation of Pd ions (Pd3+ -> Pd2+ + Pd4+). Moreover, the sliding CDW state appears below T2 ~ 6 K. Combined such low T2 with the feature of multiple quasi-1D bands, PdTeI exhibits exotic crossover behavior from negative to huge positive magnetoresistance under magnetic field and field-induced localization. Thus, PdTeI provides a novel platform for studying the CDW fluctuation and the interplay between magnetic field and CDW state.

cond-mat.str-el↗

arXiv2015

Structural anomalies and short-range magnetic correlations in the orbitally degenerated system Sr$_2$VO$_4$

We report on the electronic ground state of a layered perovskite vanadium oxide Sr$_2$VO$_4$ studied by the combined use of synchrotron radiation x-ray diffraction (SR-XRD) and muon spin rotation/relaxation ($μ$SR) techniques, where $μ$SR measurements were extended down to 30 mK. We found an intermediate orthorhombic phase between $T_{\rm c2} \sim$~130 K and $T_{\rm c1} \sim$~100 K, whereas a tetragonal phase appears for $T > T_{\rm c2}$ and $T < T_{\rm c1}$. The absence of long-range magnetic order was confirmed by $μ$SR at the reentrant tetragonal phase below $T_{\rm c1}$, where the relative enhancement in the $c$-axis length versus that of the $a$-axis length was observed. However, no clear indication of the lowering of the tetragonal lattice symmetry with superlattice modulation, which is expected in the orbital order state with superstructure of $d_{yz}$ and $d_{zx}$ orbitals, was observed by SR-XRD below $T_{\rm c1}$. Instead, it was inferred from $μ$SR that a magnetic state developed below $T_{\rm c0} \sim$~10 K, which was characterized by the highly inhomogeneous and fluctuating local magnetic fields down to 30 mK. We argue that the anomalous magnetic ground state below $T_{\rm c0}$ originates from the coexistence of ferromagnetic and antiferromagnetic correlations.

cond-mat.str-el↗

arXiv2014

Unconventional magnetism in the layered oxide LaSrRhO$_4$

We have prepared polycrystalline samples of LaSrRh$_{1-x}$Ga$_x$O$_4$ and LaSr$_{1-x}$Ca$_x$RhO$_4$,and have measured the x-ray diffraction, resistivity, Seebeck coefficient, magnetization and electron spin resonance in order to evaluate their electronic states. The energy gap evaluated from the resistivity and the Seebeck coefficient systematically changes with the Ga concentration, and suggests that the system changes from a small polaron insulator to a band insulator. We find that all the samples show Curie-Weiss-like susceptibility with a small Weiss temperature of the order of 1 K, which is seriously incompatible with the collective wisdom that a trivalent rhodium ion is nonmagnetic. We have determined the $g$ factor to be $g$=2.3 from the electron spin resonance, and the spin number to be $S$=1 from the magnetization-field curves by fitting with a modified Brillouin function. The fraction of the $S$=1 spins is 2--5%, which depends on the degree of disorder in the La/Sr/Ca-site, which implies that disorder near the apical oxygen is related to the magnetism of this system. A possible origin for the magnetic Rh$^{3+}$ ions is discussed.

cond-mat.str-el↗

arXiv2014

Hydrogen Ordering and New Polymorph of Layered Perovskite Oxyhydrides: Sr2VO4-xHx

Compositionally tunable vanadium oxyhydrides Sr2VO4-xHx (x = 0 - 1) without considerable anion vacancy were synthesized by high-pressure solid state reaction. The crystal structures and their properties were characterized by powder neutron diffraction, synchrotron X-ray diffraction, thermal desorption spectroscopy, and first-principles density functional theory (DFT) calculations. The hydrogen anions selectively replaced equatorial oxygen sites in the VO6 layers via statistical substitution of hydrogen in the low x region (x < 0.2). A new orthorhombic phase (Immm) with an almost entirely hydrogen-ordered structure formed from the K2NiF4-type tetragonal phase with x > 0.7. Based on the DFT calculations, the degree of oxygen/hydrogen anion ordering is strongly correlated with the bonding interaction between vanadium and the ligands.

cond-mat.mtrl-sci↗

arXiv2014

Bipartite magnetic parent phases in the iron-oxypnictide superconductor

High-temperature (high-$T_{\rm c}$) superconductivity appears as a consequence of the carrier-doping of an undoped parent compound exhibiting antiferromagnetic order; thereby, ground-state properties of the parent compound are closely relevant to the superconducting state. On the basis of the concept, a spin-fluctuation has been addressed as an origin of pairing of the superconducting electrons in cuprates. Whereas, there is growing interest in the pairing mechanism such as an unconventional spin-fluctuation or an advanced orbital-fluctuation due to the characteristic multi-orbital system in iron-pnictides. Here, we report the discovery of an antiferromagnetic order as well as a unique structural transition in electron-overdoped LaFeAsO$_{1-x}$H$_x$ ($x$ ~ 0.5), whereby another parent phase was uncovered, albeit heavily doped. The unprecedented two-dome superconducting phases observed in this material can be interpreted as a consequence of the carrier-doping starting from the original at $x\sim0$ and advanced at $x\sim0.5$ parent phases toward the intermediate region. The bipartite parent phases with distinct physical properties in the second magnetic phase provide us with an interesting example to illustrate the intimate interplay among the magnetic interaction, structural change and orbital degree of freedom in iron-pnictides.

cond-mat.str-el↗

arXiv2013

Spin state of Co$^{3+}$ in LaCo$_{1-x}$Rh$_{x}$O$_{3}$ investigated by structural phenomena

Neutron diffraction for a polycrystalline sample of LaCo$_{0.8}$Rh$_{0.2}$O$_{3}$ and synchrotron x-ray diffraction for polycrystalline samples of LaCo$_{0.9}$Rh$_{0.1}$O$_{3}$ and LaCo$_{0.8}$Rh$_{0.2}$O$_{3}$ have been carried out in order to investigate the structural properties related with the spin state of Co$^{3+}$ ions. We have found that the values of the Co(Rh)-O bond lengths in the Co(Rh)O$_{6}$ octahedron of LaCo$_{0.8}$Rh$_{0.2}$O$_{3}$ are nearly identical at 10 K. The lattice volume for the Rh$^{3+}$ substituted samples decreases with the thermal expansion coefficient similar to that of LaCoO$_{3}$ from room temperature, and ceases to decrease around 70 K. These experimental results favor a mixed state consisting of the high-spin-state and low-spin-state Co$^{3+}$ ions, and suggest that the high-spin-state Co$^{3+}$ ions are thermally excited in addition to those pinned by the substituted Rh$^{3+}$ ions.

cond-mat.str-el↗

arXiv2012

Spin-state responses to light impurity doping in low-spin perovskite LaCoO$_{3}$

We studied the spin-state responses to light impurity doping in low-spin perovskite LaCoO$_{3}$ (Co^3+: d^6) through magnetization and X-ray fluorescence measurements of single-crystal LaCo$_{0.99}$$M_{0.01}$O$_{3}$ ($M$ = Cr, Mn, Fe, Ni). In the magnetization curves measured at 1.8 K, a change in the spin-state was not observed for Cr, Mn, or Fe doping but was observed for Ni doping. Strong magnetic anisotropy along the [100] easy axis was also found in the Ni-doped sample. The fluorescence measurements revealed that the valences were roughly estimated to be Cr^3+, Mn^4+, Fe^(3+delta)+, and Ni^3+. From the observed chemical trends, we propose that the chemical potential is a key factor in inducing the change of the low-spin state. By expanding a model of the ferromagnetic spin-state heptamer generated by hole doping, we discuss the emergence of highly anisotropic ferromagnetic spin-state clusters induced by low-spin Ni^3+ with Jahn-Teller activity. We also discuss applicability of the present results to mantle materials and impurity-doped pyrites with Fe (d^6).

cond-mat.str-el↗