arXiv subjects

K. M. Kojima

Publications and source records attributed to K. M. Kojima.

At least 37 records · Page 2Linked to original sources

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↗

arXiv2016

Metal-Insulator Transition and Pseudogap in Bi$_{1.76}$Pb$_{0.35}$Sr$_{1.89}$CuO$_{6+δ}$ High-$T_c$ Cuprates

It is inferred from bulk-sensitive muon Knight shift measurement for a Bi$_{1.76}$Pb$_{0.35}$Sr$_{1.89}$CuO$_{6+δ}$ single-layer cuprate that metal-insulator (MI) transition (in the low temperature limit, $T\rightarrow0$) occurs at the critical hole concentration $p=p_{\rm MI}=0.09(1)$, where the electronic density of states (DOS) at the Fermi level is reduced to zero by the pseudogap irrespective of the Néel order or spin glass magnetism. Superconductivity also appears for $p>p_{\rm MI}$, suggesting that this feature is controlled by the MI transition. More interestingly, the magnitude of the DOS reduction induced by the pseudogap remains unchanged over a wide doping range ($0.1\le p\le0.2$), indicating that the pseudogap remains as a hallmark of the MI transition for $p>p_{\rm MI}$.

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↗

arXiv2015

Temperature evolution of correlation strength in the superconducting state of high-Tc cuprates

We have performed an angle-resolved photoemission study of the nodal quasi-particle spectra of the high-Tc cuprate tri-layer Bi2Sr2Ca2Cu3O10+d (Tc~ 110 K). The spectral weight Z of the nodal quasi-particle increases with decreasing temperature across the Tc. Such a temperature dependence is qualitatively similar to that of the coherence peak intensity in the anti nodal region of various high-Tc cuprates although the nodal spectral weight remains finite and large above Tc. We attribute this observation to the reduction of electron correlation strength in going from the normal metallic state to the superconducting state, a characteristic behavior of a superconductor with strong electron correlation.

cond-mat.str-el↗

arXiv2015

Evidence for ordered magnetic moments at oxygen sites in antiferromagnetic Sr$_2$IrO$_4$ and Sr$_3$Ir$_2$O$_7$

In this study, the magnetic ground state of iridium perovskites (Sr$_{n+1}$Ir$_{n}$O$_{3n+1}$, where $n=1$, 2, and $\infty$) was considered using muon spin spectroscopy ($μ$SR). When probed by muons in Sr$_2$IrO$_4$ and Sr$_3$Ir$_2$O$_7$ ($n=1,2$), the internal field ($B_{\rm loc}$) showed clear sign of a magnetic order in two stages at transition temperatures $T_{\rm N}\simeq 230$ K and $T_{\rm m}\simeq 90$ K in Sr$_2$IrO$_4$ and $T_{\rm N}\simeq 280$ K and $T_{\rm m}\simeq 70$ K in Sr$_3$Ir$_2$O$_7$, respectively. In contrast, no long-range magnetic order was observed in orthorhombic SrIrO$_3$ ($n=\infty$). Based on the known magnetic structure in Sr$_2$IrO$_4$ and Sr$_3$Ir$_2$O$_7$, we successfully identified muon sites in these compounds from the magnitude of $B_{\rm loc}$ in the first stage ($T_{\rm m}\le T\le T_{\rm N}$). Below $T_{\rm m}$, $B_{\rm loc}$ probed by a fraction of muons occupying sites near the apical oxygen of IrO$_6$ octahedra exhibited a further increase but remained mostly unchanged for sites close to the in-plane oxygen. While such behavior cannot be explained by the alteration of the Ir spin structure, it is consistent with the selective appearance of ordered magnetic moments on the apical oxygen. The oxygen polarization was also in line with the reported magnetization anomalies in these compounds below $\sim T_{\rm m}$. A possible link between the oxygen polarization and ferroelectric (multiferroic) behavior in Sr$_2$IrO$_4$ was considered.

cond-mat.str-el↗

arXiv2014

Bulk superconductivity in undoped T'-La$_{1.9}$Y$_{0.1}$CuO$_4$ probed by muon spin rotation

The Meissner effect has been directly demonstrated by depth-resolved muon spin rotation measurements in high-quality thin films of the T'-structured cuprate, T'-La$_{1.9}$Y$_{0.1}$CuO$_4$, to confirm bulk superconductivity ($T_c\simeq21$ K) in its {\sl undoped} state. The gradual expelling of an external magnetic field is observed over a depth range of $\sim$100 nm in films with a thickness of 275(15) nm, from which the penetration depth is deduced to be 466(22) nm. Based on this result, we argue that the true ground state of the "parent" compound of the $n$-type cuprates is not a Mott insulator but a strongly correlated metal with colossal sensitivity to apical oxygen impurities.

cond-mat.supr-con↗

arXiv2014

Magnetic frustration in iridium spinel compound CuIr$_2$S$_4$

We demonstrate via a muon spin rotation experiment that the electronic ground state of the iridium spinel compound, CuIr$_2$S$_4$, is not the presumed spin-singlet state but a novel paramagnetic state, showing a quasistatic spin glass-like magnetism below ~100 K. Considering the earlier indication that IrS$_6$ octahedra exhibit dimerization associated with the metal-to-insulator transition below 230 K, the present result suggests that a strong spin-orbit interaction may be playing an important role in determining the ground state that accompanies magnetic frustration.

cond-mat.str-el↗

arXiv2014

Cooperative order in the weakly magnetic domain of LaFeAsO$_{1-x}$F$_{x}$ near the doping phase boundary

Spatial phase separation into mesoscopic domains of non-magnetic [superconducting (SC) below Tc = 25.5 K] and two kinds of magnetic phases, one showing disordered spin density wave (d-SDW) order and another associated with glassy weak magnetism (WM), are observed below ~100 K by muon spin rotation (muSR) in LaFeAsO1-xFx for x = 0.057(3) which is near the boundary of these phases on the doping phase diagram. In contrast to the competing order observed in the regular SDW phase of Ba(Fe1-xCox)2As2, the WM domain exhibits cooperative coupling of superconducting and magnetic order parameters as inferred from strong diamagnetism and associated negative shift of muSR frequency just below Tc.

cond-mat.str-el↗

arXiv2012

Effect of electron-phonon coupling in the ARPES spectra of the tri-layer cuprate Bi$_2$Sr$_2$Ca$_2$Cu$_3$O$_{10+δ}$

Angle-resolved photoemission spectroscopy using tunable low energy photons allows us to study the quasi-particle (QP) dispersions of the inner and outer CuO2 planes (IP and OP) separately in the tri-layer cuprate Bi$_2$Sr$_2$Ca$_2$Cu$_3$O$_{10+δ}$ (Bi2223). The kink energy of the OP band is $\sim$ 70 meV, as observed in various high-$T_c$ cuprates, while that of the IP band is as large as 100 meV in the superconducting (SC) state. This large kink energy is attributed to the $\sim$ 35 meV buckling mode plus the large ($\sim$ 60 meV) SC gap of IP. The IP band also shows a weak kink feature at 70 meV in the SC state. The latter feature can be explained either by the 70 meV half-breathing mode or by the $\sim$ 35 meV buckling-phonon mode plus the $\sim$ 40 meV SC gap of OP if interlayer scattering of QP is involved.

cond-mat.supr-con↗

arXiv2011

Quasi-One-Dimensional Spin Dynamics in $d$-Electron Heavy-Fermion Metal Y$_{1-x}$Sc$_x$Mn$_2$

Slow spin fluctuations ($ν< 10^{12}$ s$^-1$) observed by the muon spin relaxation technique in Y$_{1-x}$Sc$_x$Mn$_2$ exhibits a power law dependence on temperature ($ν\propto T^α$), where the power converges asymptotically to unity ($α\rightarrow 1$) as the system moves away from spin-glass instability with increasing Sc content $x$. This linear $T$ dependence, which is common to that observed in LiV$_2$O$_4$, is in line with the prediction of the "intersecting Hubbard chains" model for a metallic pyrochlore lattice, suggesting that the geometrical constraints to t2g bands specific to the pyrochlore structure serve as a basis of the $d$-electron heavy-fermion state.

cond-mat.str-el↗

arXiv2010

Strong carrier-scattering in iron-pnictide superconductors with highest Tc obtained from charge transport experiments

Characteristic normal-state charge transport is found in the oxygen-deficient iron-arsenides LnFeAsO1-y (Ln: La and Nd) with the highest Tc's among known Fe-based superconductors. The effect of "doping" in this system is mainly on the carrier scattering, quite distinct from that in high-Tc cuprates. In the superconducting regime of the La system with maximum Tc = 28 K, the low-temperature resistivity is dominated by a T^2 term. On the other hand, in the Nd system with Tc higher than 40 K, the carriers are subject to stronger scattering showing T-linear resistivity and small magnetoresistance. Such strong scattering appears crucial for high-Tc superconductivity in the iron-based system.

cond-mat.supr-con↗

arXiv2010

Evolution of the optical spectrum with doping in iron pnictides Ba(Fe1-xCox)2As2

We investigated the optical spectrum of Ba(Fe1-xCox)2As2 single crystals with various doping levels. It is found that the low-energy optical conductivity spectrum of this system can be decomposed into two components: a sharp Drude term and a broad "incoherent" term. For the compounds showing magnetic order, a gap appears predominantly in the "incoherent" component, while an s-wave like superconducting gap opens in both components for highly doped compounds. The Drude weight steadily increases as doping proceeds, consistent with electron doping in this system. On the other hand, the "incoherent" spectral weight is almost doping independent, but its spectral feature is intimately connected with the magnetism. We demonstrate that the presence of two distinct components in the optical spectrum well explains the doping and temperature dependences of the dc resistivity.

cond-mat.supr-con↗

arXiv2010

Characteristic electronic structure and its doping evolution in lightly-doped to underdoped YBa2Cu3Oy

We have performed an angle resolved photoemission spectroscopy (ARPES) study of lightly-doped to underdoped YBa2Cu3Oy (YBCO) untwinned single crystals and a core-level x-ray photoemission spectroscopy (XPS) study of YBCO single and polycrystals. In the zone diagonal (nodal) direction, dispersive quasi-particle (QP) features crossing the Fermi level were observed down to the hole concentration of ~ 4%, which explains the metallic transport of the lightly-doped YBCO. The chemical potential shift estimated from XPS was more rapid than in the Bi2212 cuprates. Upon hole doping, very rapid spectral weight transfer from high binding energies to the QP feature, even faster than La2-xSrxCuO4, was observed.

cond-mat.supr-con↗

arXiv2009

Field-induced magnetism in the high-$T_c$ superconductor Ca$_{2-x}$Na$_x$CuO$_2$Cl$_2$ with flat CuO$_2$ planes

The internal magnetic field distribution in a mixed state of a cuprate superconductor, Ca$_{2-x}$Na$_x$CuO$_2$Cl$_2$ ($T_{\rm c}\simeq28.5$ K, near the optimal doping), was measured by muon spin rotation ($μ$SR) technique up to 60 kOe. The $μ$SR linewidth $Λ(B)$ which exhibits excess broadening at higher fields ($B>5$ kOe) due to field-induced magnetism (FIM), is described by a relation, $Λ(B)\propto\sqrt{B}$. This suggests that the orbital current and associated quasiparticle excitation plays predominant roles in stabilizing the quasistatic correlation. Moreover, a slowing down of the vortex fluctuation sets in well above $T_{\rm c}$, as inferred from the trace of FIM observed up to $\sim80$ K, and develops continuously without a singularity at $T_{\rm c}$ as the temperature decreases.

cond-mat.supr-con↗

arXiv2009

Enhanced superconducting gaps in the tri-layer high-$T_c$ cuprate Bi$_2$Sr$_2$Ca$_2$Cu$_3$O$_{10+δ}$

We have investigated the optimally doped tri-layer cuprate Bi$_2$Sr$_2$Ca$_2$Cu$_3$O$_{10+δ}$ (Bi2223) by angle-resolved photoemission spectroscopy, and observed energy bands and Fermi surfaces originated from the outer and inner CuO$_2$ planes (OP and IP) separately. The OP band is overdoped (hole density $\sim$ 0.26/Cu) with a large d-wave gap of $Δ_0$ $\sim$ 43 meV while the IP is underdoped ($\sim$ 0.06/Cu) with an even large gap of $Δ_0$ $\sim$ 60 meV. We propose that the enhancement of the $Δ_0$ of IP is due to the minimal influence of out-of-plane disorder and that the OP gap is then enhanced through interlayer coupling, most likely a proximity effect from the IP.

cond-mat.supr-con↗

arXiv2009

Enhancement of the superconducting critical temperature in Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ by controlling disorder outside CuO$_2$ planes

We investigate the effect of disorder at various lattice sites outside the CuO$_2$ plane on the superconducting critical temperature $T_c$ of the cuprate superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ (Bi2212). The most effective disorder turns out to be that at the Sr site in the neighboring blocks, which contain apical-oxygen atoms. The Sr-site disorder reduces $T_c$ and also produces a residual component ($ρ_0$) in the in-plane resistivity, the magnitude of which is found to be proportional to the magnitude of $T_c$ reduction. We demonstrate that both $T_c$-degradation rate and $ρ_0$ decrease as the number of the CuO$_2$ planes increases in the unit cell, that is, as the number of the neighboring SrO blocks decreases. In real crystals of Bi-based cuprates, main source of disorder is Bi atoms randomly occupying the Sr sites. We have succeeded in reducing the Bi content at Sr-sites as much as possible and achieved $T_c$=98K, a high-$T_c$ record in Bi2212.

cond-mat.supr-con↗