Search

The rare decay $K_L \!\to\! \pi^0 \nu \overline{\nu}$ was studied with the dataset taken at the J-PARC KOTO experiment in 2016, 2017, and 2018. With a single event sensitivity of $( 7.20 \pm 0.05_{\rm stat} \pm 0.66_{\rm syst} ) \times 10^{-10}$, three candidate events were observed in the signal region. After unveiling them, contaminations from $K^{\pm}$ and scattered $K_L$ decays were studied, and the total number of background events was estimated to be $1.22 \pm 0.26$. We conclude that the number of observed events is statistically consistent with the background expectation. For this dataset, we set an upper limit of $4.9 \times 10^{-9}$ on the branching fraction of $K_L \!\to\! \pi^0 \nu \overline{\nu}$ at the 90% confidence level.

Ohm's law is a fundamental paradigm in the electrical transport of metals. Any transport signatures violating Ohm's law would give an indisputable fingerprint for a novel metallic state. Here, we uncover the breakdown of Ohm's law owing to a topological structure of the chiral anomaly in the Weyl metal phase. We observe nonlinear I-V characteristics in Bi0.96Sb0.04 single crystals in the diffusive limit, which occurs only for a magnetic-field-aligned electric field (E∥B). The Boltzmann transport theory with the charge pumping effect reveals the topological-in-origin nonlinear conductivity, and it leads to a universal scaling function of the longitudinal magnetoconductivity, which completely describes our experimental results. As a hallmark of Weyl metals, the nonlinear conductivity provides a venue for nonlinear electronics, optical applications, and the development of a topological Fermi-liquid theory beyond the Landau Fermi-liquid theory.

A search for the rare decay K_{L}→π^{0}νν[over ¯] was performed. With the data collected in 2015, corresponding to 2.2×10^{19} protons on target, a single event sensitivity of (1.30±0.01_{stat}±0.14_{syst})×10^{-9} was achieved and no candidate events were observed. We set an upper limit of 3.0×10^{-9} for the branching fraction of K_{L}→π^{0}νν[over ¯] at the 90% confidence level (C.L.), which improved the previous limit by almost an order of magnitude. An upper limit for K_{L}→π^{0}X^{0} was also set as 2.4×10^{-9} at the 90% C.L., where X^{0} is an invisible boson with a mass of 135 MeV/c^{2}.

After the classification of topological states of matter has been clarified for non-interacting electron systems, the theoretical connection between gapless boundary modes and nontrivial bulk topological structures, and their evolutions as a function of dimensions are now well understood. However, such dimensional hierarchy has not been well established experimentally although some indirect evidences were reported, for example, such as the half-quantized Hall conductance via quantum Hall effect and extrapolation in the quantum-oscillation measurement. In this paper, we report the appearance of the possible chiral edge mode from the surface state of topological insulators under magnetic fields, confirming the dimensional hierarchy in three dimensional topological insulators. Applying laser pulses to the surface state of Bi1-xSbx, we find that the sign of voltage relaxation in one edge becomes opposite to that in the other edge only when magnetic fields are applied to the topological insulating phase. We show that this sign difference originates from the chirality of edge states, based on coupled time-dependent Poisson and Boltzmann equations., 17 pages, 6 figures

We have measured photoluminescence properties of cerium-doped Gd 3 Al 2 Ga 3 O 12 (Ce:GAGG) crystals at low temperatures with use of synchrotron radiation. Excitation spectra for the Ce 3+ 5d–4f emission exhibit prominent peaks at Gd 3+ intra-4f absorption bands. The Gd 3+ intra-4f emission band is observed at 3.91 eV, but is not in resonance with the lowest energy Gd 3+ intra-4f absorption band at 3.95 eV. The temperature dependence of the Gd 3+ emission intensity is not correlated with that of the Ce 3+ emission intensity. Decay curves of the Ce 3+ emission were also measured at 9 K under excitation at various photon energies. The decay curve is remarkably changed, depending on the excitation photon energies. The present results give us hints to understand the whole of energy transfer processes in Ce:GAGG crystals.

It is at the heart of modern condensed matter physics to investigate the role of a topological structure in anomalous transport phenomena. In particular, chiral anomaly turns out to be the underlying mechanism for the negative longitudinal magnetoresistivity in a Weyl metal phase. The existence of a dissipationless current channel causes enhancement of electric currents along the direction of a pair of Weyl points or applied magnetic fields $(B)$. However, temperature $(T)$ dependence of the negative longitudinal magnetoresistivity has not been understood yet in the presence of disorder scattering since it is not clear at all how to introduce effects of disorder scattering into the topological-in-origin transport coefficient at finite temperatures. The calculation based on the Kubo formula of the current-current correlation function is simply not known for this anomalous transport coefficient. Combining the renormalization group analysis with the Boltzmann transport theory to encode the chiral anomaly, we reveal how disorder scattering renormalizes the distance between a pair of Weyl points and such a renormalization effect modifies the topological-in-origin transport coefficient at finite temperatures. As a result, we find breakdown of $B/T$ scaling, given by $B/{T}^{1+\ensuremath{\eta}}$ with $0l\ensuremath{\eta}l1$. This breakdown may be regarded to be a fingerprint of the interplay between disorder scattering and topological structure in a Weyl metal phase.

The magnetic properties of β -FeSi 2 single crystals were investigated using magnetization and anomalous Hall effect measurements. These measurements clearly show the appearance of superparamagnetism, the origin of which is attributed to defect-induced magnetic aggregates. Unlike previous reports, based on our combined analysis using magnetization and the anomalous Hall effect, we found that two kinds of aggregates, which are magnetically distinct, exist in our samples. Among the two types found, one gives an anomalous Hall term, which follows a side-jump mechanism while the other is a completely new kind, suggesting that magnetic properties of β -FeSi 2 are very unconventional. The sizes of the aggregates were estimated from our experimental data.

Valence electronic structure of β - FeSi 2 single crystal has been investigated by photoelectron spectroscopy using synchrotron radiation. The plate-type β - FeSi 2 single crystal has been grown by chemical vapor transport method. The surface contamination and oxide layer were removed by Ne+ ion sputtering. The dependence of the valence band spectra on exciting photon energy shows that the strong peak at the binging energy ( E Bin ) of 0.5 eV and the shoulder peak at E Bin = 8 eV are assigned to be Fe 3 d and Si 3 p orbitals, respectively. Upon the excitation of Fe 2 p 3 / 2 , resonant enhancement of the valence band was observed at E Bin = 3.7 eV, which is very similar to Fe metal. This fact suggests the existence of the two-hole satellite states from Fe 3 d orbitals.

The transport and magnetic properties of single-crystalline Fe-intercalation-compound Fe x TiSe2 have been compared with the transport properties of single-crystalline Ti-self-intercalation-compound Ti x TiSe2. At low concentration of intercalated guest atoms, Fe x TiSe2 exhibits carrier localization behavior without magnetic ordering, while Ti x TiSe2 shows an itinerant behavior. The superstructure and the carrier localization observed in Fe x TiSe2 single crystals simultaneously disappear at the critical concentration x c∼0.07, which agrees well with theoretical value based on a percolation theory. Fe intercalation compounds show paramagnetism and nonexistence of magnetism is attributed to the large distance between guest Fe atoms. We discuss the effect of guest intercalation on the stability of superstructure formed in the host.

Time-of-Flight (TOF) neutron diffraction measurements with the 6Li/7Li isotopic substitution technique were carried out for aqueous 18 mol% lithium alaninate solutions in D2O in order to obtain structural information on the interaction between Li+ and the amino acid molecule in the concentrated aqueous solution. The first-order difference function, ΔLi(Q), was obtained from the difference between scattering cross sections observed for solutions involving dl -CH3CH(ND2)COO6Li and dl -CH3CH(ND2)COO7Li. The distribution function around Li+, GLi(r), exhibits well resolved first and second peaks that are tentatively attributable to the nearest neighbor Li+ ⋯ O and Li+ ⋯ D interactions. However, the coordination number, nLiD, estimated from the area under the second peak is found to be much larger than the value, 2nLiO, evaluated from the first peak of the present GLi(r). This implies that the nearest neighbor Li+ ⋯ alaninate ion interaction may contribute to the second peak. Structural parameters concerning the first coordination shell of the Li+ has been determined through the least squares fitting analysis of the observed ΔLi(Q). It has been revealed that the Li+ is surrounded by 2.4(1) D2O molecules and 2.3(2) alaninate ions with interatomic distances of r(Li+ ⋯ OD2O) = 1.97(1) A, r(Li+ ⋯ DD2O) = 2.62(1) A and r(Li+ ⋯ Oalaninate) = 2.38(1) A, respectively.

Neutron diffraction measurements were carried out on 14N/15N isotopically substituted aqueous 5 mol % sodium L-hydrogenglutamate solutions in D2O in order to obtain direct information concerning th...

Time-of-flight neutron diffraction measurements were carried out for 6Li/7Li isotopically substituted 10 mol % LiPF6-propylene carbonate-d6 (PC-d6) solutions, in order to obtain structural information on the first solvation shell of Li+. Structural parameters concerning the nearest neighbor Li+...PC and Li+...PF6- interactions were determined through least-squares fitting analysis of the observed difference function, DeltaLi(Q). It has been revealed that the first solvation shell of Li+ consists in average of 4.5(1) PC molecules with an intermolecular Li+...O(PC) distance of 2.04(1) A. The angle Li+...O=C bond angle has been determined to be 138(2) degrees.

We show experimental and theoretical evidence that BiTeI hosts a novel disordered metallic state named diffusive helical Fermi liquid (DHFL), characterized by a pair of concentric spin-chiral Fermi surfaces with negligible inter-valley scattering. Key experimental observations are extreme disparity of the mobility between inner and outer helical Fermi surfaces near the Weyl point and existence of the so called universal scaling behavior for the Hall resistivity. Although the extreme enhancement of the inner-Fermi-surface mobility near the Weyl point is quantitatively explained within the self-consistent Born approximation, the existence of universal scaling in the Hall resistivity shows its breakdown, implying necessity of mass renormalization in the inner Fermi-surface beyond the independent electron picture.

Zero-field-cooled (ZFC) and field-cooled (FC) magnetizations have been measured for β-FeSi2 single crystals from 4 to 300 K up to 60 kOe using a SQUID magnetometer. The ZFC and FC magnetizations at low field below 500 Oe show the characteristic temperature dependence due to superparamagnetism (SP). The blocking temperature Tbwas well explained by the theory for the SP. We have found an anomalous peak (AP) at around 55 K, which is caused by some kind of magnetic transition. The AP is enhanced appreciably with increasing magnetic field but whose peak temperature is field-independent. Moreover, its magnetization develops temporally with the delay time 16.5 h.

Magnetotransport and magnetic properties have been measured for β-FeSi2 single crystals from 4.2 to 300 K using a pulsed magnet, a superconducting magnet, and a SQUID magnetometer. Nonlinear Hall effect followed by a normal Hall effect was observed, involving a hysteresis effect at low temperature. The signs of normal Hall coefficients changed from positive to negative ones at around 180 K. The magnetization curves above 180 K are well explained by considering the superparamagnetism. Below 150 K we found that the aggregates of Fe atoms exhibit an inter-aggregate ferromagnetic interaction. At low temperature we also found that the cohesive forces obtained from the ρH-B curves are larger than those from the M-Bcurves. This discrepancy was discussed qualitatively by a proposed model.

Neutron diffraction measurements were carried out at 25°C on aqueous 15 mol % urea heavy water solutions, [(ND 2 ) 2 *C=O] 0.15 (D 2 O) 0.85 , in which the 12 C/ 13 C isotopic composition of the carbonyl-carbon atom was changed. The hydration structure around the carbonyl-carbon atom was derived from the least-squares fitting analysis of the observed first-order difference function, Δ C (Q). From the analysis, 4.3(3) water molecules are hydrogen-bonded to the carbonyl-oxygen atom with the intermolecular C···D W1 (D W1 : water deuterium atom) distance of 2.46(3) A. The average angle, LC...D W1 -O W (O W : water oxygen atom), was determined to be 124(5)°.

Neutron diffraction measurements were carried out for anion and cation exchange resins (AER and CER, respectively) absorbing H/D isotopically substituted water molecules in order to obtain structural information on the hydrogen-bonded network among water molecules within the resins. Partial structure factors, a HH (Q), a XH (Q), and a XX (Q) (X: atoms involved in the sample except for water-hydrogen atom), for AER and CER samples were respectively deduced from combined analyses of intermolecular interference terms observed for resins containing D 2 O (99.8% D), 0 H 2 O (35.9% D), and 0-2 H 2 O (67.9% D). The nearest neighbor distances, r(O-H) = 1.95(3) A and r(H···H) = 2.40(3) A, determined for the AER sample agree well with those reported for pure liquid water. On the other hand, considerably shorter intermolecular distances, r(O···H) = 1.78(3) A and r(H···H) = 2.28(3) A, were found for the CER sample, implying that the hydrogen-bonded network among water molecules in CER would be strongly affected by a highly acidic environment.

Neutron diffraction measurements have been carried out at 25 °C for aqueous 18 mol % lithium alaninate heavy water solutions, [C*H3C*H(*ND2)COO*Li]0.18(D2O)0.82, in which the isotopic compositions ...

Time-of-flight neutron diffraction and infrared (IR) double difference spectral measurements have been carried out on aqueous 4 mol% lithium benzoate solutions. The neutron diffraction with isotopic substitution technique has been applied to both lithium and benzyl-hydrogen atoms within the benzoate ion in order to obtain information on the hydration structure around both the lithium ion and the benzyl-hydrogen atom within the benzoate ion. The observed first-order difference functions, ΔLi(Q) and ΔHinter(Q), were analyzed by the least squares fitting procedure. The number of water molecules within the first hydration shell of the Li+ was determined to be 5.3(1) with the Li+···O and Li+···D distances of 1.95(1) and 2.58(1) A, respectively. A well-defined second hydration shell involving 6.0(1) water molecules, has been found and characterized by the Li+···O and Li+···D distances of 4.12(1) and 4.73(1) A, respectively. The first hydration shell of the benzyl-hydrogen atoms (Hb) within the benzoate ion was found to consist of 1.0(1) water molecule with the average Hb···D2O intermolecular distance of 2.78(1) A. Results of the IR double difference spectra exhibited that the number of water molecules within the first hydration shell of Li+ is 5.7(1), in reasonable agreement with that obtained from the neutron diffraction data. The hydration number of the carboxyl group within the benzoate ion was determined to be 4.9(1) from the IR difference spectra.

Time-of-Flight (TOF) neutron diffraction measurements were carried out for 12 mol% *HCOO7Li solutions in D2O. The H/D isotopic substitution technique was applied to the formyl-hydrogen atom of the ...

Time-of-Flight (TOF) and steady state reactor neutron diffraction measurements have been carried out for aqueous 2.5 mol% DL- and L-alanine solutions in order to invest gate the differences in intermolecular hydrogen-bonded structure between solutions involving amino acid molecules with different optical activities. The observed difference functions, Δi(Q) and Δi i n t e r (Q), between observed scattering cross sections for DL- and L-alanine solutions with a D content of 96.1% exhibit the first peak located at Q = 2A - 1 followed by oscillatory features extending up to the higher-Q region. The difference distribution function, Δg(r), obtained from the Fourier transform of Δi(Q), clearly indicates negative peaks at r = 2 and 2.5 A and a positive one at r = 3.5 A. Partial structure factors, a H H (Q), a X H (Q), and a X X (Q) (X: O, N, C, H M , and H M ' where H M and H M ' denote methyl and methine hydrogen atoms within the alanine molecule, respectively) for 2.5 mol% DL-alanine solutions are successfully determined from combined analyses of intermolecular interference terms observed for solutions with 96.1, 66.0, and 35.9% exchangeable deuterium content. The nearest neighbor O...H e x and H e x ...H e x (H e x : exchangeable hydrogen atom) distances are determined from the least squares fit of the observed partial structure factors, 1.90(1) and 2.48(1) A, respectively. These values correspond to the positions of negative peaks observed in the present Δg(r) function. The least squares fitting analysis of the observed Ai(Q) revealed that the difference in the coordination number of the nearest neighbor O...H e x and H e x ...H e x interactions between the DL- and L-alanine solutions are -0.031(5) and --0.072(5), respectively. It is concluded that the intermolecular hydrogen bonds among solvent water molecules in the DL-alanine solution are ca. 2% weaker than those in the L-alanine solution.

Time-of-Flight (TOF) neutron diffraction measurements have been carried out on aqueous 8 mol% sodium acetate solutions in D2O. Scattering cross sections that were observed for sample solutions involving 12C/13C and H/D isotopically substituted acetate ions were used to derive the first-order difference functions, ΔH(Q) and ΔC(Q), and corresponding distribution functions, G H(r;r) and G C(r;r), which describe the environmental structure around the methyl and the carboxyl groups within the acetate ion, respectively. Structural parameters concerning the first hydration shell of the carboxyl group within the acetate ion were obtained through the least squares fit to the observed intermolecular difference function, ΔC inter(Q). The nearest neighbor C O...D W1 (CO: carboxyl carbon atom, DW1: water deuterium atom) distance, r(C O...D W1 ), and the angle, ∠ C O ...D W1 -O W (O W : water oxygen atom), were determined to be 2.63(1) A and 120(1)°, respectively. The coordination number, n(C O ...D W1 ), was obtained to be 4.0(1). These results are consistent with the hydration structure in which water molecules in the first hydration shell of the carboxyl group are hydrogen-bonded with oxygen atoms of the carboxyl group.

Neutron diffraction measurements have been carried out on aqueous 4 mol% glycine solutions. The H/D isotopic substitution technique was applied to both methylene-hydrogen (HM) and water-hydrogen (HW) atoms in order to obtain information concerning the hydration structure around the methylene group within the glycine molecule. Structural parameters for the first hydration shell of the methylene-hydrogen atoms were determined as r(HM···OW) = 2.64(1) A and r(HM···HW) = 2.87(1) A from the least squares fitting analysis of the observed first-order difference function, 0HΔH(Q), and the partial structure factor, aHMHex(Q), respectively. The first hydration shell of the methylene group involves ca. 2 water molecules per one glycine molecule.

Time-of-flight (TOF) neutron scattering measurements have been carried out for liquid null-H2O, in which the average coherent scattering length of hydrogen atoms is zero. In order to determine the inelasticity effect depending on both the scattering angle and the neutron flight path ratio, γ [ = l_{s}/(l_{0} + l_{s}), l 0 and l s denote the moderator-sample and sample-detector distances, respectively], neutron scattering measurements have been performed using three neutron spectrometers, HIT-II, RAT, and SWAN, installed at KENS, Tsukuba, Japan. The self-scattering intensity for the null-H2O was derived by subtracting the known O–O partial structure factor from the observed scattering cross-section. It has been revealed that the magnitude of the inelasticity distortion involved in the self-scattering term is still significant even at a smaller scattering angle than that expected from the first-order inelasticity correction formulas proposed in the literature. The inelasticity distortion in the self-scatter...

We have performed photo-induced transient thermoelectric effect (TTE) experiments for charge-density-wave (CDW) material 1T-TaS 2 single crystals in hexagonal and triclinic nearly commensurate phases up to 5 T. The TTE voltages decay with the characteristic relaxation times τ 1 , τ 2 , and τ 3 . τ 1 and τ 2 in the hexagonal phase oscillate with magnetic field B, while those in the triclinic phase are independent of B. The oscillatory phenomenon is interpreted by the Altshuler-Aronov-Spivak effect around the hexagonal arrays of commensurate CDW domains.

We have measured electrical resistivity, Hall coefficient, thermoelectric power, and magnetization for charge-density wave (CDW) material 1T-TaS2 single crystals grown by varying the excess sulfur content xes. We have revealed that a small mid-gap state is formed inside the Mott gap and that anomalous low temperature transport is not governed by the Mott gap state itself but by the mid-gap state. The electric properties of the mid-gap state are modified by increasing xes (or hole doping), and we have found the insulator-metal transition occurs by hole doping below 60 K.

Weyl metal is regarded as a platform toward interacting topological states of matter, where its topological structure gives rise to anomalous transport phenomena, referred to as chiral magnetic effect and ``negative'' magnetoresistivity, the origin of which is chiral anomaly. Recently, the negative magnetoresistivity has been observed with the signature of weak antilocalization at $x=3--4%$ in ${\mathrm{Bi}}_{1\ensuremath{-}x}$${\mathrm{Sb}}_{x}$, where a magnetic field is applied in parallel with an electric field ($\mathbit{E}\phantom{\rule{0.16em}{0ex}}\ensuremath{\parallel}\phantom{\rule{0.16em}{0ex}}\mathbit{B}$). Based on the Boltzmann equation approach, we find the negative magnetoresistivity in the presence of weak antilocalization. An essential ingredient is to introduce the topological structure of chiral anomaly into the Boltzmann equation approach, resorting to semiclassical equations of motion with Berry curvature.

Longitudinal magnetoresistivity ${\ensuremath{\rho}}_{xx}(B)$ and Hall resistivity ${\ensuremath{\rho}}_{xy}(B)$ of KMo${}_{6}$O${}_{17}$ single crystals have been measured up to 60 T in a pulse magnet. Remarkable and unusual features at characteristic magnetic fields were found in both ${\ensuremath{\rho}}_{xx}(B)$ and ${\ensuremath{\rho}}_{xy}(B)$, which were attributed to Fermi surface reconstructions caused by field-induced density-wave transitions. To understand these transitions in detail, we analyzed ${\ensuremath{\rho}}_{xx}(B)$ and ${\ensuremath{\rho}}_{xy}(B)$ based on a multiband model. This model successfully described the experimental results, revealing that charge carriers are increased significantly at these transitions with increasing magnetic field as a result of the Fermi surface reconstruction. In particular, the charge carriers are increased by two orders of magnitude at ${B}_{\mathrm{osc}}=\ensuremath{\sim}40$ T, above which a quantum oscillation emerges in both ${\ensuremath{\rho}}_{xx}(B)$ and ${\ensuremath{\rho}}_{xy}(B)$. This strongly suggests that the field-induced quantum oscillation is closely related with the induced charge carriers.

Landau's Fermi-liquid theory is the standard model for metals, characterized by the existence of electron quasiparticles near a Fermi surface as long as Landau's interaction parameters lie below critical values for instabilities. Recently, this fundamental paradigm has been challenged by physics of strong spin-orbit coupling although the concept of electron quasiparticles remains valid near the Fermi surface, where the Landau's Fermi-liquid theory fails to describe electromagnetic properties of this novel metallic state, referred to as Weyl metal. A novel ingredient is that such a Fermi surface encloses a Weyl point with definite chirality, referred to as a chiral Fermi surface, which can arise from breaking of either time reversal or inversion symmetry in systems with strong spin-orbit coupling, responsible for both Berry curvature and chiral anomaly. As a result, electromagnetic properties of the Weyl metallic state are described not by conventional Maxwell equations but by axion electrodynamics, where Maxwell equations are modified with a topological-in-origin spatially modulated $��(\bm{r}) \bm{E} \cdot \bm{B}$ term. This novel metallic state has been realized recently in Bi$_{1-x}$Sb$_{x}$ around $x \sim 3%$ under magnetic fields, where the Dirac spectrum appears around the critical point between the normal semiconducting ($x < 3%$) and topological semiconducting phases ($x > 3%$) and the time reversal symmetry breaking perturbation causes the Dirac point to split into a pair of Weyl points along the direction of the applied magnetic field for such a strong spin-orbit coupled system. In this review article, we discuss how the topological structure of both the Berry curvature and chiral anomaly (axion electrodynamics) gives rise to anomalous transport phenomena in Bi$_{1-x}$Sb$_{x}$ around $x \sim 3%$ under magnetic fields, modifying the Drude model of Landau's Fermi liquids., An invited review article for the STAM (Science and Technology of Advanced Materials (ISSN 1468-6996)) Focus issue "Materials Science of Topological Insulators and Superconductors"

Dirac metals (gapless semiconductors) are believed to turn into Weyl metals when perturbations, which break either time reversal symmetry or inversion symmetry, are employed. However, no experimental evidence has been reported for the existence of Weyl fermions in three dimensions. Applying magnetic fields near the topological phase transition from a topological insulator to a band insulator in ${\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x}$ we observe not only the weak antilocalization phenomenon in magnetoconductivity near zero magnetic fields ($Bl0.4\text{ }\text{ }\mathrm{T}$), but also its upturn above 0.4 T only for $E//B$. This ``incompatible'' coexistence between weak antilocalization and ``negative'' magnetoresistivity is attributed to the Adler-Bell-Jackiw anomaly (``topological'' $E\ifmmode\cdot\else\textperiodcentered\fi{}B$ term) in the presence of weak antilocalization corrections.

Time-of-flight (TOF) neutron diffraction measurements were carried out for 12C/13C isotopically substituted aqueous 10 mol% K2CO3 solutions in D2O in order to obtain structural information on the hydration structure of CO32−. Structural parameters concerning water molecules in the first hydration shell of CO32− were determined through the least-squares fitting analysis of the observed intermolecular difference function, ΔCinter(Q). It has been revealed that the first hydration shell of CO32− consists of 9.1(5) water molecules with the intermolecular C(CO32−)⋯DW1(D2O) distance of 2.68(1) A.

We have examined core-level photoemission spectra of 1T-TiSe 2 and 1T-TaS 2 to study chemical shifts associated with unusual superlattice formation. While large temperature-dependent core-level splitting was observed for TaS 2 , temperature-dependent chemical shift was not observed for TiSe 2 . On the other hand, angle-resolved valence-band photoemission spectra of TiSe 2 showed appearance of a new branch (band-folding) at the L points in the ordered phase at 50 K. These results suggest smaller lattice distortion in TiSe 2 than that in TaS 2 .

We propose a phase diagram for ${\mathrm{Fe}}_{x}{\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ ($0\ensuremath{\le}x\ensuremath{\le}0.1$) single crystals, which belong to a class of magnetically bulk-doped topological insulators. The evolution of magnetic correlations from ferromagnetic to antiferromagnetic gives rise to topological phase transitions, where the paramagnetic topological insulator of ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ turns into a band insulator with ferromagnetic-cluster glassy behavior around $x\ensuremath{\sim}0.025$, and it further evolves to a topological insulator with valence-bond glassy behavior, which spans over the region from $x\ensuremath{\sim}0.03$ up to $x\ensuremath{\sim}0.1$. This phase diagram is verified by measuring magnetization, magnetotransport, and angle-resolved photoemission spectra with theoretical discussions.

The electronic band structures of 1T-TiTe2 and Fe0.25TiTe2 have been clarified by angle-resolved resonant-photoemission spectroscopy. Upon Fe intercalation, while most of the band structure is unchanged, two new flat bands appear at the Fermi level and at E∼−0.5 eV. Constant-initial-state spectra indicate that these states are derived from both Ti 3d and Fe 3d states. While the Fermi wave number of the electron pocket around M point is reduced by Fe intercalation, those of hole pockets around Γ point are almost unchanged, suggesting that the additional electrons are accommodated by the new flat band at −0.5 eV.

Topological states of matter challenge the paradigm of symmetry breaking, characterized by gapless boundary modes and protected by the topological property of the ground state. Here, we present compelling evidence for the existence of gapless surface Dirac fermions from transport in Bi${}_{2}$Te${}_{3}$. We observe Sondheimer oscillation in magnetoresistance (MR). This oscillation originates from the quantization of motion due to the confinement of electrons within the surface layer. Based on Sondheimer's transport theory, we determine the thickness of the surface state from the oscillation data. In addition, we uncover the topological nature of the surface state, fitting consistently both the nonoscillatory part of MR and the Hall resistance. The side-jump contribution turns out to dominate around 1 T in Hall resistance while the Berry-curvature effect dominates in 3--4 T.

We performed a systematic, high-resolution, angle-resolved photoemission spectroscopy of Bi${}_{1\ensuremath{-}x}$Sb${}_{x}$ alloys to study the evolution of electronic states as a function of Sb concentration $x$. We revealed that the number of Fermi-level crossings is different between the topologically trivial ($x=$ 0.023) and nontrivial ($x=$ 0.17) phases: four and five times for the former and latter cases, respectively. We also found a systematic expansion of the bandwidth of spin-split surface bands at the $\mathrm{\ensuremath{\Gamma}\ifmmode \bar{}\else \={}\fi{}}$ point upon substituting Bi with Sb, which is indicative of the monotonous reduction of the spin-orbit coupling strength.

We investigate the interplay between the Kondo effect and randomness in ${M}_{x}{\text{TiSe}}_{2}$ ($M=\text{Co}$, Ni, and Fe) single crystals. Although the typical low-$T$ upturn of resistivity implies the Kondo effect around the single-ion Kondo temperature ${T}_{K}$, positive magnetoresistance linearly proportional to the magnetic field and the power-law scaling of magnetization suggest the forbidden coexistence between Kondo effect and time-reversal symmetry breaking. This puzzling result is resolved by the Griffiths scenario---disorder-induced distribution of the Kondo temperature produces an effective Kondo temperature $({\overline{T}}_{K})$ much lower than ${T}_{K}$, allowing unscreened local moments above ${\overline{T}}_{K}$ and resulting in non-Fermi-liquid properties in ${M}_{x}{\text{TiSe}}_{2}$ below the percolation threshold $(xl{x}_{\text{c}})$.

To search the atomic sites of Mn impurities in Bi2 Te3 Mn0.1 topologica linsulator, a Mn Kα fuorescence x-ray holography experiment was performed. The reconstructed atomic image around the central Mn atom reveals a hexagonal confguration, where the local lattice constant is slightly larger than the original Bi2 Te3 crystal. Thus, the most plausible atomic position of Mn impurities is the substitution with Bi or Te atom. Thefurther-distant atomic images are hardly observed unlike usual crystals, indicating a formation of flat clusters around Mn in this functional crystal.

Photoluminescence (PL) and thermally stimulated luminescence (TSL) properties of Ce-doped Gd3Al2Ga3O12 (Ce:GAGG) crystals have been studied in the temperature range of 10–300 K under electron excitation from the 4f level to the 5d levels of Ce3+ ions. PL and TSL spectra commonly exhibit a prominent band around 2.31 eV, due to electron transitions from the lowest 5d (5d1) level to the 4f levels. The intensity of the Ce3+ PL band is significantly changed with temperature under excitation from the lowest 4f level to a higher-lying 5d (5d2) level, whereas it is scarcely changed under excitation from the 4f level to the 5d1 level. Such change correlates well with the appearance of TSL glow peaks at 84 and 175 K. The luminescence properties are apparently influenced by the thermal behavior of electrons trapped at some lattice site, but not by nonradiative processes of electrons starting from the 5d1 level. The phosphorescence process will be discussed using a phenomenological model based on the results obtained.

We investigate the effects of disorder in FexTi(Se1−ySy)2 single crystals. In MxTiSe2 (M = Fe, Co, and Ni), positive magnetoresistance proportional to magnetic field and power-law scaling of magnetization were observed. This observation has been attributed to disorder-induced distribution of Kondo temperature, which causes effective Kondo temperature much lower than single-ion Kondo temperature TK and which also results in non-Fermi liquid properties in MxTiSe2 below the percolation threshold (x < xc). FexTi(Se1−ySy)2, which connects magnetic phases in FexTiS2 with the disordered Kondo region in FexTiSe2 is an interesting system which provide a clue about how disorder-dominated Kondo region evolves into various magnetic phases. We discuss this interesting problem on the basis of our recent transport measurements on FexTi(Se1−ySy)2 single crystals.

Static transport properties and transient thermoelectric effect (TTE) of β-FeSi2 single crystals, which has received considerable attention as optical and thermoelectric devices, have been measured. The Hall and TTE data clearly showed the existence of multiple conduction carriers. From these results, the tentative band scheme is discussed.

BaIrO3 causes simultaneously charge-density wave and ferromagnetic phase transitions. The transient thermoelectric effect (TTE) has been investigated for this material. We have found that TTE signals are anisotropic on the 〈1 1 0〉, 〈1 0 1〉 and 〈0 0 1〉 axes, and that the magnitude of the TTE signals shows anomalies at the characteristic temperatures T c 1 =26 K , T c 2 =80 K and T c 3 =175 K (T c 3 : Curie temperature). A brief discussion is given on the nesting of the Fermi surface.

We report the dynamics of moving vortices in La1.86Sr0.14CuO4 single crystals studied via a transient voltage response (TVR) near the vortex melting transition. In the TVR signal, we could identify two types of relaxation caused by rearrangement of the vortex configuration. Based on careful temperature- and field-dependent measurements of these two components, we suggest that the fast component results from moving of the stable ordered vortex phase, while the slow one is generated during the annealing process of the metastable disordered vortex phase, which is injected through the surface. As the temperature is lowered across the melting line, the metastable disordered phase in the heterogeneous mixture gets a longer life time due to the reduced thermal fluctuations, occupying a larger portion of the sample. Consistent with this picture, the boundary of the slow relaxation is coincident with the irreversibility line, which is known to be a property of the surface barriers.

Excitation spectra for the 5d → 4f emission of Ce3+ ions in CaGa2S4 crystals have been measured at 300 K by using vacuum ultraviolet (VUV) photons, together with reflectivity and absorption-edge spectra. X-ray photoelectron spectroscopy (XPS) experiment has also been performed in order to clarify the contribution of shallow core states to the excitation and reflectivity spectra. Furthermore, cluster calculation by a relativistic discrete variational Xα (DV-Xα) method has been carried out to elucidate the optical transitions resulting in the creation of multiple e-h pairs in CaGa2S4. The excitation processes of Ce3+ ions in CaGa2S4 by hot photocarriers are discussed on the basis of the electronic structure determined from the experiments and calculation.