arXiv subjects

H. Okabe

Publications and source records attributed to H. Okabe.

At least 37 records · Page 2Linked to original sources

arXiv2022

Local spin dynamics in geometrically frustrated Mo pyrochlore antiferromagnet Lu$_2$Mo$_2$O$_{5-y}$N$_2$

The magnetic ground state of oxynitride pyrochlore Lu$_2$Mo$_2$O$_{5-y}$N$_2$, a candidate compound for the quantum spin liquid ($S=1/2$, Mo$^{5+}$), was studied by muon spin rotation/relaxation experiment. In contrast to Lu$_2$Mo$_2$O$_7$ ($S=1$, Mo$^{4+}$) which exhibits the spin-glass behavior with a freezing temperature $T_g\simeq16$ K, no such spin freezing or long range magnetic order was observed down to 0.3 K. Moreover, two separate magnetic domains were detected below $\sim$13 K, which were characterized by differences in spin dynamics. The first is the "sporadic" spin fluctuation seen in frustrated antiferromagnets, where the amplitude of the hyperfine fields suggests that the excitation comprises a local cluster of unpaired spins. The other is rapid paramagnetic fluctuation, which is only weakly suppressed at low temperatures. In place of the paramagnetic fluctuation, the volume fraction for the sporadic fluctuation steadily increases with decreasing temperature, indicating the formation of an excitation gap with a broad distribution of the gap energy involving null gap.

cond-mat.str-el↗

arXiv2022

Organic Molecular Dynamics and Charge-carrier Lifetime in Lead Iodide Perovskite MAPbI$_3$

The long charge carrier lifetime of the hybrid organic-inorganic perovskites (HOIPs) is the key for their remarkable performance as a solar cell material. The microscopic mechanism for the long lifetime is still in debate. Here, by using muon spin relaxation technique that probes the fluctuation of local magnetic fields, we show that the muon depolarization rate ($Δ$) of a prototype HOIP methylammonium lead iodide (MAPbI$_3$) shows a sharp decrease with increasing temperature in two steps above 120 K and 190 K across the structural transition from orthorhombic to tetragonal structure at 162 K. Our analysis shows that the reduction of $Δ$ is quantitatively in agreement with the expected behavior due to the rapid development of MA jumping rotation around the $C_3$ and $C_4$ symmetry axes. Our results provide direct evidence for the intimate relation between the rotation of the electric dipoles of MA molecules and the charge carrier lifetime in HOIPs.

cond-mat.mtrl-sci↗

arXiv2022

Ambipolar Property of Isolated Hydrogen in Oxide Materials Revealed by Muon

The study on the electronic state of muon as pseudo-hydrogen (represented by the elemental symbol Mu) has long been appreciated as one of the few methods to experimentally access the electronic state of dilute hydrogen (H) in semiconductors and dielectrics. Meanwhile, theoretical predictions on the electronic state of H in these materials by first-principles calculations using density functional theory (DFT) do not always agree with the observed states of Mu, standing as an obstacle to integrating the findings of both Mu and H. In this paper, we provide resolution to this problem by a semi-quantitative model that enables systematic understanding of the electronic states of Mu in most oxides.

cond-mat.mtrl-sci↗

arXiv2021

Anomalous diamagnetism of electride electrons in transition metal silicides

Intermetallic silicide compounds, LaScSi and Y$_5$Si$_3$, known for being hydrogen (H) storage materials, are drawing attention as candidates for electrides in which anions are substituted by unbound electrons. It is inferred from a muon spin rotation experiment that the local field at the muon site (which is the same site as that for H) in these compounds exhibits a large negative shift under an external magnetic field, which is mostly independent of temperature. Such anomalous diamagnetism signals a unique property of electride electrons associated with transition metals. Moreover, the diamagnetic shift decreases with increasing H content, suggesting that the electride electrons existing coherently in the hollow interstitial positions are adsorbed by H to form hydride ions (H$^-$)

cond-mat.mtrl-sci↗

arXiv2021

Precise Neutron Lifetime Measurement Using Pulsed Neutron Beams at J-PARC

A neutron decays into a proton, an electron, and an anti-neutrino through the beta-decay process. The decay lifetime ($\sim$880 s) is an important parameter in the weak interaction. For example, the neutron lifetime is a parameter used to determine the |$V_{\rm ud}$| parameter of the CKM quark mixing matrix. The lifetime is also one of the input parameters for the Big Bang Nucleosynthesis, which predicts light element synthesis in the early universe. However, experimental measurements of the neutron lifetime today are significantly different (8.4 s or 4.0$σ$) depending on the methods. One is a bottle method measuring surviving neutron in the neutron storage bottle. The other is a beam method measuring neutron beam flux and neutron decay rate in the detector. There is a discussion that the discrepancy comes from unconsidered systematic error or undetectable decay mode, such as dark decay. A new type of beam experiment is performed at the BL05 MLF J-PARC. This experiment measured neutron flux and decay rate simultaneously with a time projection chamber using a pulsed neutron beam. We will present the world situation of neutron lifetime and the latest results at J-PARC.

hep-ex↗

arXiv2020

Magnetism behavior of $T^{\prime}$-type Eu$_2$CuO$_4$ revealed by muon spin rotation/relaxation measurements

We performed muon spin rotation/relaxation measurements to investigate the magnetic behavior of $T^{\prime}$-type Eu$_2$CuO$_4$ (ECO), which is the parent compound of electron-doped cuprate superconductors, and the effects of oxygen-reduction annealing on its magnetism. In as-sintered (AS) ECO, we clarified the development of magnetic correlations upon cooling below $T_\mathrm{N1}$ (= 265 K) as well as the coexistence of a dominant fluctuating spin state and partially ordered spin state in the temperature range between $\sim$150 K and $T_\mathrm{N1}$. Upon further cooling, uniform long-range magnetic order was observed below $T_\mathrm{N2} = 110$ K, which is close to the ordering temperature of 115 K in $T^{\prime}$-type La$_2$CuO$_4$ (LCO) [Phys. Rev. B {\bf 82}, 180508(R) (2010)]. For oxygen-reduction-annealed ECO, a similar ordering sequence with the same $T_\mathrm{N2}$ was observed but without the partially ordered spin state. Therefore, the fluctuating spin state over a wide temperature range and a $T_\mathrm{N2}$ less than the N{é}el temperature ($T_\mathrm{N2} \approx T_\mathrm{N1}$) in $T$-type LCO are common features of the $T^{\prime}$-type parent $R_2$CuO$_4$ ($R$CO, $R$: rare-earth ion). The origin of the partially ordered spin state in AS ECO is discussed from the viewpoint of chemical defect. Furthermore, we discuss the roles of electron doping and repairing defect in the observed effect of annealing on the magnetism of $T^{\prime}$-type $R$CO.

cond-mat.str-el↗

arXiv2020

Neutron lifetime measurement with pulsed cold neutrons

The neutron lifetime has been measured by comparing the decay rate with the reaction rate of $^3$He nuclei of a pulsed neutron beam from the spallation neutron source at the Japan Proton Accelerator Research Complex (J-PARC). The decay rate and the reaction rate were determined by simultaneously detecting electrons from the neutron decay and protons from the $^3$He(n,p)$^3$H reaction using a gas chamber of which working gas contains diluted $^3$He. The measured neutron lifetime was $898\,\pm\,10\,_{\rm stat}\,^{+15}_{-18}\,_{\rm sys}\,$s.

hep-ex↗

arXiv2020

Electronic charge transfer driven by spin cycloidal structure

Muon spin rotation and resonant soft X-ray scattering experiments on prototype multiferroics RMn2O5 (R = Y, Sm) are used to demonstrate that the local electric displacements are driven by the spin-current (SC) mechanism. Small local electric displacements were evaluated by observing spin polarization at ligand O ions, for which implanted muons served as an extremely sensitive probe. Our results for YMn2O5 provide evidence that the spin polarization of O ions forming a spin cycloid chain with Mn spins increases in proportion to the vector spin chirality (Si x Sj ) of the Mn ions. This relationship strongly indicates that the charge transfer between O and Mn ions is driven by the SC mechanism, which leads to the ferroelectricity accompanying O spin polarization.

cond-mat.str-el↗

arXiv2020

Magnetism driven by strong electronic correlation in the heavily carrier-doped iron oxypnictide LaFeAsO$_{0.49}$H$_{0.51}$

The magnetism of the second antiferromagnetic phase (AF2) arising in the iron-based LaFeAsO$_{1-x}$H$_{x}$ superconductor for $x\gtrsim0.4$ was investigated by muon spin rotation measurements under hydrostatic pressure up to 2.6 GPa. The Néel temperature ($T_{\rm N}$) obtained for a sample with $x=0.51$ exhibits considerably greater sensitivity to pressure than that in the pristine antiferromagnetic phase (AF1, $x\lesssim0.06$). Moreover, while the AF1 phase is always accompanied by the structural transition (from tetragonal to orthorhombic) at a temperature ($T_{\rm s}$) which is slightly higher than $T_{\rm N}$, the AF2 phase prevails at higher pressures above $\sim$1.5 GPa where the structural transition is suppressed ($T_{\rm s}=0$). These features indicate that the microscopic origin of the AF2 phase is distinct from that of AF1, suggesting that electronic correlation plays important role in the former phase. We argue that the orbital-selective Mott transition is a plausible scenario to account for the observed pressure dependence of $T_{\rm N}$ and $T_{\rm s}$ in the AF2 phase.

cond-mat.str-el↗

arXiv2019

Coupled Spin-Charge-Phonon Fluctuation in the All-In/All-Out Antiferromagnet Cd$_2$Os$_2$O$_7$

We report on a novel spin-charge fluctuation in the all-in-all-out pyrochlore magnet Cd$_2$Os$_2$O$_7$, where the spin fluctuation is driven by the conduction of thermally excited electrons/holes and associated fluctuation of Os valence. The fluctuation exhibits an activation energy significantly greater than the spin-charge excitation gap and a peculiar frequency range of $10^{6}$--$10^{10}$ s$^{-1}$. These features are attributed to the hopping motion of carriers as small polarons in the insulating phase, where the polaron state is presumably induced by the magnetoelastic coupling via the strong spin-orbit interaction. Such a coupled spin-charge-phonon fluctuation manifests as a part of the metal-insulator transition that is extended over a wide temperature range due to the modest electron correlation comparable with other interactions characteristic for 5$d$-subshell systems.

cond-mat.str-el↗

arXiv2019

Electronic structure of interstitial hydrogen in In-Ga-Zn-O semiconductor simulated by muon

We report on the local electronic structure of interstitial muon (Mu) as pseudo-hydrogen in In-Ga-Zn oxide (IGZO) semiconductor studied by muon spin rotation/relaxation ($μ$SR) experiment. In polycrystalline (c-) IGZO, it is inferred that Mu is in a diamagnetic state, where the $μ$SR time spectra under zero external field is perfectly described by the Gaussian Kubo-Toyabe relaxation function with the linewidth $Δ$ serving as a sensitive measure for the random local fields from In/Ga nuclear magnetic moments. The magnitude of $Δ$ combined with the density functional theory calculations for H (to mimic Mu) suggests that Mu occupies Zn-O bond-center site (Mu$_{\rm BC}$) similar to the case in crystalline ZnO. This implies that the diamagnetic state in c-IGZO corresponds to Mu$_{\rm BC}^+$, thus serving as an electron donor. In amorphous (a-) IGZO, the local Mu structure in as-deposited films is nearly identical with that in c-IGZO, suggesting Mu$_{\rm BC}^+$ for the electronic state. In contrast, the diamagnetic signal in heavily hydrogenated a-IGZO films exhibits the Lorentzian Kubo-Toyabe relaxation, implying that Mu accompanies more inhomogeneous distribution of the neighboring nuclear spins that may involve Mu$^-$H$^-$-complex state in an oxygen vacancy.

cond-mat.mtrl-sci↗

arXiv2019

Quantum magnetisms in uniform triangular lattices Li2AMo3O8 (A = In, Sc)

Molecular based spin-1/2 triangular lattice systems such as LiZn$_2$Mo$_3$O$_8$ have attracted research interest. Distortions, defects, and intersite disorder are suppressed in such molecular-based magnets, and intrinsic geometrical frustration gives rise to unconventional and unexpected ground states. Li$_2$$A$Mo$_3$O$_8$ ($A$ = In or Sc) is such a compound where spin-1/2 Mo$_3$O$_{13}$ clusters in place of Mo ions form the uniform triangular lattice. Their ground states are different according to the $A$ site. Li$_2$InMo$_3$O$_8$ undergoes conventional $120^\circ$ long-range magnetic order below $T_\text{N}=12$~K whereas isomorphic Li$_2$ScMo$_3$O$_8$ exhibits no long-range magnetic order down to 0.5~K. Here, we report exotic magnetisms in Li$_2$InMo$_3$O$_8$ and Li$_2$ScMo$_3$O$_8$ investigated by muon spin rotation ($μ$SR) and inelastic neutron scattering (INS) spectroscopies using polycrystalline samples. Li$_2$InMo$_3$O$_8$ and Li$_2$ScMo$_3$O$_8$ show completely different behaviors observed in both $μ$SR and INS measurements, representing their different ground states. Li$_2$InMo$_3$O$_8$ exhibits spin wave excitation which is quantitatively described by the nearest neighbor anisotropic Heisenberg model based on the $120^\circ$ spin structure. In contrast, Li$_2$ScMo$_3$O$_8$ undergoes short-range magnetic order below 4~K with quantum-spin-liquid-like magnetic fluctuations down to the base temperature. Origin of the different ground states is discussed in terms of anisotropies of crystal structures and magnetic interactions.

cond-mat.str-el↗

arXiv2019

Successive magnetic ordering sequence observed from muon spin rotation in T'-structured R2CuO4 (R = Eu, Nd)

The magnetic behavior and effects of oxygen-reduction annealing on the magnetism of T'-structured R2CuO4 (R = Eu and Nd), which is the parent compound of electron-doped cuprate superconductors, have been investigated from muon spin rotation (muSR) measurements. In the muon time spectra of as-sintered R2CuO4, an oscillation component with a small amplitude and a dominant exponential-type decay component were observed for the temperature range between 150 K and TN1 (=250-270 K). This observation suggests an inhomogeneous magnetic phase of a fluctuating spin state and a locally ordered spin state at these temperatures. With decreasing temperature,the small oscillation component disappears, and the exponential one develops below 150 K. Upon further cooling, well-defined oscillation spectra corresponding to uniform long-ranged magnetic order emerge below TN2 = 110 K. Except for the absence of the small oscillation component, similar magnetic behavior is confirmed for the annealed compounds. Therefore, the successive development of magnetism characterized by the fluctuating spin state over a wide temperature range is a common feature for T'-R2CuO4. We discuss a possible picture for the ordering sequence in T'-R2CuO4 and the origin of the small oscillation component in the as-sintered compounds from the viewpoint of local defects.

cond-mat.str-el↗

arXiv2018

Metallic Spin Liquid-like Behavior of LiV$_2$O$_4$

LiV$_2$O$_4$ spinel is known to exhibit heavy fermion-like behavior below a characteristic temperature $T_K\simeq 20$ K, while it preserves a paramagnetic state down to $T\sim10^{-2}$ K due to geometrical frustration. Here, it is shown that the dynamical spin susceptibility $χ({\bf q},ω)$ in LiV$_2$O$_4$ exhibits anomalous duality which is modeled as a sum of itinerant ($χ_{\rm F}$) and local ($χ_{\rm L}$) components,and that the local spin dynamics inferred from $χ_{\rm L}({\bf q},ω)$ is qualitatively different from that expected from time-averaged bulk properties. The anomaly coexists with the marginal Fermi liquid behavior inferred from the $-\ln T$ dependence of the electronic specific heat over a wide temperature range below $T_K$. We argue that such unusual properties of LiV$_2$O$_4$ can be attributed to the putative metallic spin liquid state emerging near the quantum critical point between spin glass and Fermi liquid states.

cond-mat.str-el↗

arXiv2018

Local Electronic Structure of Interstitial Hydrogen in Iron Disulfide

The electronic structure of interstitial hydrogen in a compound semiconductor FeS$_2$ (naturally $n$-type) is inferred from a muon study. An implanted muon (Mu, a pseudo-hydrogen) forms electronically different defect centers discerned by the hyperfine parameter ($ω_{\rm hf}$). A body of evidence indicates that one muon is situated at the center of an iron-cornered tetrahedron with nearly isotropic $ω_{\rm hf}$ (Mu$_{\rm p}$), and that the other exists as a diamagnetic state (Mu$_{\rm d}$, $ω_{\rm hf}\simeq 0$). Their response to thermal agitation indicates that the Mu$_{\rm d}$ center accompanies a shallow level (donor or acceptor) understood by effective mass model while the electronic structure of Mu$_{\rm p}$ center is more isolated from host than Mu$_{\rm d}$ to form a deeper donor level. These observations suggest that interstitial hydrogen also serves as an electronically active impurity in FeS$_2$. Based on earlier reports on the hydrogen diffusion in FeS$_2$, possibility of fast diffusion for Mu$_{\rm p}$ leading to formation of a complex defect state (Mu$^*_{\rm d}$, $T\le 100$ K) or to motional narrowing state (Mu$^*_{\rm p}$, $T\ge 150$ K) is also discussed.

cond-mat.mtrl-sci↗

arXiv2018

Electronic correlation in the quasi-two-dimensional electride Y$_2$C

Magnetic properties of the electride compound Y$_2$C were investigated by muon spin rotation and magnetic susceptibility on two samples with different form (poly- and single-crystalline), to examine the theoretically-predicted Stoner ferromagnetism for the electride bands. There was no evidence of static magnetic order in both samples even at temperatures down to 0.024 K. For the poly-crystalline sample, the presence of a paramagnetic moment at Y sites was inferred from the Curie-Weiss behavior of the muon Knight shift and susceptibility, whereas no such tendency was observed in the single-crystalline sample. These observations suggest that the electronic ground state of Y$_2$C is at the limit between weak-to-strong electronic correlation, where onsite Coulomb repulsion is sensitive to a local modulation of the electronic state or a shift in the Fermi level due to the presence of defects/impurities.

cond-mat.mtrl-sci↗

arXiv2017

Tomonaga-Luttinger spin liquid in the spin-1/2 inequilateral diamond-chain compound K$_3$Cu$_3$AlO$_2$(SO$_4$)$_4$

K$_3$Cu$_3$AlO$_2$(SO$_4$)$_4$ is a highly one-dimensional spin-1/2 inequilateral diamond-chain antiferromagnet. Spinon continuum and spin-singlet dimer excitations are observed in the inelastic neutron scattering spectra, which is in excellent agreement with a theoretical prediction: a dimer-monomer composite structure, where the dimer is caused by strong antiferromagnetic (AFM) coupling and the monomer forms an almost isolated quantum AFM chain controlling low-energy excitations. Moreover, muon spin rotation/relaxation spectroscopy shows no long-range ordering down to 90~mK, which is roughly three orders of magnitude lower than the exchange interaction of the quantum AFM chain. K$_3$Cu$_3$AlO$_2$(SO$_4$)$_4$ is, thus, regarded as a compound that exhibits a Tomonaga-Luttinger spin liquid behavior at low temperatures close to the ground state.

cond-mat.str-el↗

arXiv2016

Cage Electron-Hydroxyl Complex State as Electron Donor in Mayenite

It is inferred from the chemical shift of muon spin rotation ($μ$SR) spectra that muons implanted in pristine (fully oxidized) mayenite, [Ca$_{12}$Al$_{14}$O$_{32}]^{2+}$[$\square_5$O$^{2-}$] (C12A7, with $\square$ referring to the vacant cage), are bound to O$^{2-}$ at the cage center to form OMu$^-$ (where Mu represents muonium, a muonic analog of the H atom). However, an isolated negatively charged state (Mu$^-$, an analog of H$^-$) becomes dominant when the compound approaches the state of electride [Ca$_{12}$Al$_{14}$O$_{32}]^{2+}$[$\square_4$2e$^{-}$] as a result of the reduction process. Moreover, the OMu$^-$ state in the pristine specimen exhibits depolarization of paramagnetic origin at low temperatures (below $\sim$30~K), indicating that OMu$^-$ accompanies a loosely bound electron in the cage that can be thermally activated. This suggests that interstitial muons (and hence H) forming a "cage electron-hydroxyl" complex can serve as electron donors in C12A7.

cond-mat.mtrl-sci↗