Search

Magnetotransport studies have established the existence of exotic electronic properties in materials of technological and fundamental interest. However, measurements of the Shubnikov-de Haas oscillations, intended to reveal information about Fermi surfaces (FSs), have mostly been carried out in magnetic fields perpendicular to the applied currents. Here, using magnetic fields not only perpendicular but also parallel to the applied currents in a given contact configuration, we investigated the anisotropic magnetotransport and the anisotropic FS properties of Bi

Ce-doped (Gd, La)2Si2O7 (Ce:La-GPS) scintillator was found to have a good light output of around 40 000 photons/MeV, while some other parameters have not been determined so far. In order to clarify the cause for such a high light yield, some physical parameters have been estimated in this paper. The bandgap energy of Ce:La-GPS was evaluated to be over 7.13 ± 0.03 eV for (Gd0.75, La0.25)2Si2O7. Although Ce:La-GPS had wider bandgap compared with the other oxide scintillators, its light output was larger than values for some other scintillators. Since quantum yield of Ce:La-GPS was as high as around 0.8, the product of transfer efficiency and electron–hole pair generation efficiency was estimated to be around 0.9, which was higher than for the other scintillators. Thus, Ce:La-GPS was suggested to have high the product of transfer efficiency and/or electron–hole pair generation efficiency.

Local structures of Gd atoms in Gd3(Al, Ga)5O12 garnet solid solutions, grown using the Czochralski method, have been studied at low temperatures through X-ray and UV absorption spectroscopy experiments. From the Fourier transforms (FTs) of Gd K-edge extended X-ray absorption fine structure (EXAFS) oscillations, the preferential sites for the occupation of Ga and Al atoms were assigned to the 24d and 16a sites, respectively. The interatomic distances between Gd and neighboring atoms, and the Debye–Waller factors were determined through the fit to the inverse FTs. Positional fluctuations of neighboring O atoms buffered local distortions induced by the occupation of Ga and Al atoms at the same crystallographic sites. The change in the high-resolution UV absorption spectra due to Gd intra-4f transitions on the Ga content is explained by considering the crystal field and Coulomb repulsion on the Gd atom.

Magnetotransport studies have established the existence of exotic electronic properties in materials of technological and fundamental interest. However, measurements of the Shubnikov–de Haas oscillations, intended to reveal information about Fermi surfaces (FSs), have mostly been carried out in magnetic fields perpendicular to the applied currents. Here, using magnetic fields not only perpendicular but also parallel to the applied currents in a given contact configuration, we investigated the anisotropic magnetotransport and the anisotropic FS properties of Bi2−x Sn x Te3 (0 ⩽ x ⩽ 0.0075) and Bi2Se3. While the magnetotransport properties of Bi2Te3 and Bi2Se3 were nearly isotropic, Bi1.995Sn0.005Te3 exhibited quite anisotropic features. These observations are attributed to the nonparabolicity of the associated bands, which evolved to more anisotropic band structures with Sn concentration. This sensitivity of the band anisotropy was rather unexpected because only a small number of dopants are known to increase disorder levels in the degenerate region. Our approach, using two different magnetic field directions in the measurements of the Shubnikov–de Haas oscillations, is a simple and easily adoptable method for shedding more light on the FSs of functional materials.

Excitation spectra for 5d–4f photoluminescence (PL) bands of Ce3+ and Eu2+ ions doped in SrGa2S4 crystals have been measured at room temperature in the energy region up to 25 eV. These spectra allow us to investigate the excitation process of luminescent ions under host excitation with vacuum ultraviolet (VUV) photons. A gentle increase in intensity is commonly observed at around 16.5 eV in the excitation spectra. Optical constant spectra were obtained from measurements of reflectivity and absorption edge spectra of undoped SrGa2S4 crystals. These data were compared with the results of a relativistic DV-Xα calculation, to clarify the electronic structure of SrGa2S4. Ultraviolet photoelectron spectroscopy (UPS) has been carried out under the excitation with photons in the 12–20 eV range. The detailed analysis of UPS spectra reveals that the multiplication of electronic excitations (MEEs) occurs above 16.5 eV. The excitation process of Ce3+ and Eu2+ ions is discussed together with the processes of MEEs.

The local environment of W and Mo atoms in CaW1−x Mo x O4 (x = 0.12) solid solution (mixed crystal) scintillator crystal was analyzed by X-ray diffraction (XD), extended X-ray absorption fine structure (EXAFS), and X-ray fluorescence holography (XFH). The XD analysis revealed that W and Mo atoms occupy the same crystallographic site. The analyzes of XFH and EXAFS exhibited that the local environment of Mo atoms is different from that of W atoms. Atomic images of neighbor atoms around the Mo and W atoms were reconstructed using their positional fluctuations. The positional fluctuation of the W atoms was apparently larger than that of Mo atoms. This feature explained the absence of the peak due to next nearest neighbor O atoms around W atoms in the Fourier transfer magnitudes for the W-L 3 edge EXAFS spectrum. It is, therefore, most likely that the different positional fluctuations of the Mo and W atoms are responsible for different atomic images of neighbor atoms between them.

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.

In this paper, we report a new type Auger-free luminescence (AFL) of Rb2ZnCl4 crystals. Until now AFL has been reported in alkali and alkali-earth halides. This luminescence originates from electon transitions from the valence band of halogen p orbitals to the top of outermost core levels of alkali or alkali-earth p orbitals (p-p transitions). On the other hand, the AFL of Rb2ZnCl4 is expected to be of p-d type, in which the electronic transitions from Cl 3p orbitals are terminated by outermost Zn 3d core levels. The electronic states concerning are apparently different from those of alkali and alkali-earth halides. The finding of such a peculiar AFL is achieved by comparison between results of spectroscopy experiment and cluster calculation. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

It is reported that Auger-free (AF) luminescence appears with two bands at 4.5 and 6.3 eV in Rb 2 ZnCl 4 . This luminescence originates from a radiative transition of the Cl 3p valence electrons into the Zn 3d outermost-core holes. The present work is the first observation of AF luminescence due to interatomic p–d transitions in halide crystals. The appearance of two AF luminescence bands suggests the existence of two types of AF transitions following core hole creation. A largely Stokes-shifted luminescence band is also found to appear at 1.9 eV. This band has an excitation threshold at the fundamental absorption edge, and is ascribed to the radiative decay of a self-trapped exciton.

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.

We have studied the electronic structure within the topmost two quintuple layers (QL) on vacuum-cleaved n- and p-type Bi2Te3(111) at 10 K by angle-resolved photoelectron spectroscopy. Tunable synchrotron radiation maximized the surface sensitivity of photoelectrons. The gapless surface-state band (SSB) in the angle-resolved photoelectron spectra clearly demonstrates the topological nature of the samples. Below the SSB, the bulk valence bands (BVB) is observed. Against the three-fold symmetry of the crystal structure, the k|| dispersion of the BVB is highly symmetrical around the Γ, which indicates the six-fold symmetry. Moreover, most of the BVB show almost flat k⊥ dispersion perpendicular to the surface. These facts are the experimental evidence of a strong modification of the valence bands within the topmost two QLs into highly two-dimensional states by the advent of the surface. [DOI: 10.1380/ejssnt.2012.117]

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.

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.

To clarify the origin of shallow electron traps in cerium-doped Gd3Al2Ga3O12 (Ce:GAGG), the optical properties of cerium-doped Lu3−xGdxAl2Ga3O12 crystals were investigated. Absorption spectra for x = 3 exhibited a prominent band at 12 000 cm−1 when excited by 3.31-eV ultraviolet light. This band has been previously attributed to shallow electron traps at defect complexes associated with oxygen vacancies. When Gd3+ ions were replaced with Lu3+ ions, the 12 000 cm−1 band weakened and disappeared completely for Ce:Lu3Al2Ga3O12. In addition, thermally stimulated luminescence glow curves were observed. Optically stimulated luminescence indicated that the energy of the conduction band minimum did not change by the presence of Lu3+ ions. Thus, the Gd3+ ions were important for the formation of shallow electron traps in Ce:GAGG. First-principles calculations implied that Gd3+ ions responsible for shallow electron traps formed antisite defects at GAGG octahedral sites. Hence, defect complexes of antisite Gd2+ ions adjacent to oxygen vacancies were the most plausible candidates for shallow electron traps in Ce:GAGG.

The energy levels of electron traps (namely, defect complexes associated with oxygen vacancies) in Gd3Al2Ga3O12:Ce (GAGG:Ce) were studied at 12 K using mid-infrared (MIR) light pulses from a free-electron laser (FEL) as the probe light. Ce3+ 5d–4f luminescence was stimulated by the MIR light pulse following an ultraviolet light pulse. Stimulation of Ce3+ 5d–4f luminescence by MIR light pulses was pronounced above 0.31 eV. This result is consistent with that of previous work based on a trap-mediated luminescence model. It is concluded that the electron trap levels are located 0.31 eV below the bottom of the conduction band. This study demonstrates that MIR-FEL is applicable for the determination of hidden electron trap levels.

We have measured magnetoresistance (MR) for semimetals Bi and graphite, nearly gapless materials Bi-Sb and Bi-Te alloys, and narrow gap semiconductor Ag 2 Se single crystals at 4.2 K and up to 4 T. We have observed a giant MR for Bi, a giant linear MR (LMR) for Bi-Sb alloy and Ag 2 Se single crystals. Anomalous completely linear weak-field LMR has been found for nearly gapless materials: this is transferred into the ordinal MR or other LMR in relatively strong field range. It suggests the linear band dispersion in nearly gapless materials as proposed by Abrikosov.

Emission and excitation spectra of forsterite have been measured using synchrotron radiation. The 2.55 eV and 2.85 eV bands dominates the emission spectra under excitation above and below the fundamental absorption edge at 7.6 eV, respectively. From the feature of excitation spectra, the 2.55 eV band is assigned to an intrinsic origin in forsterite. That is, this band most likely originates from a self-trapped exciton (STE) state. The 2.85 eV band is ascribed to an extrinsic origin such as imperfections. The excitation spectrum for the 2.55 eV band exhibits the threshold for the electron-hole (e -h) multiplication at around 21 eV, which is caused by inelastic scattering of hot photoelectrons with valence electrons. The structure of STEs and e -h relaxation processes induced by vacuum ultraviolet (VUV) photons are discussed on the basis of a comparison of experiment and theoretical calculation. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Luminescence from BaFI and BaFBr1–xIx crystals has been investigated using vacuum ultraviolet photons from synchrotron radiation in the temperature range of 10–300 K. In BaFI, two kinds of luminescence bands are observed under band-to-band excitation: one is the 4.52 eV band appearing at low temperatures, and the other is the 3.44 eV band appearing as the 4.52 eV band weakens with increasing temperature. From the excitation spectra, the 4.52 eV band is attributed to the radiative decay of a self-trapped exciton (STE), and the 3.44 eV band to the extrinsic luminescence coming from an STE trapped by an I– vacancy. The change of luminescence spectra studied in BaFBr1–xIx suggests that the STE configuration of on-center type is responsible for the 4.52 eV intrinsic band, and the off-center instability hardly occurs in BaFI. The temperature dependence of the 4.52 eV and 3.44 eV band intensities is well explained by assuming an STE relaxation mechanism whereby the STE relaxes through phonon-assisted tunneling and hopping diffusion processes at high temperatures. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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.

Optical properties due to the intra-3d transitions of Mn2+ ions in YPO4: Mn, Zr phosphors have been studied by experiment and calculation. Following the general description on energy levels of Mn2+ impurities, calculation methods based on relativistic discrete variational Xα (DV-Xα) and discrete variational multielectron (DVME) methods are briefly explained. The effect of ligand field on Mn-3d orbitals is clarified from the results of calculation. On the basis of these data, the suitability of cluster size is shortly discussed. The theoretical spectrum of absorption is compared to the experimental spectrum. The origins of absorption peaks are interpreted by the configuration analysis of many-electron wave functions constructed under D2d symmetry. The energy position and lifetime of the Mn2+ emission is also evaluated using the relativistic DVME method. The validity of this method is verified by the comparison of experiment and calculation.

Reflectivity spectra of forsterite (α-Mg 2 SiO 4 ) single crystals for the polarization parallel to the a -axis ( E ∥ a ) and c -axis ( E ∥ c ) have been measured in the energy region up to 25 eV u...

Gd3Al2Ga3O12:Ce (GAGG:Ce) was studied by surface photovoltage spectroscopy using the ultraviolet photoelectron spectroscopy technique with synchrotron radiation and a laser source. The lowest Ce3+ 4f level is located below the conduction band minimum by 3.02 eV. This result is supported by the excitation spectrum for photo-stimulated luminescence and is compatible with the value predicted by the vacuum-referred binding energy scheme for GAGG:Ce. It is also found that GAGG:Ce is of the p-type. The information on the energy location of the Ce3+ 4f level and majority carrier type provides us with hints on how to improve the optical properties of GAGG:Ce for photonic device applications.

We have investigated the conduction-band electronic structure of 1$T$-TaS${}_{2}$ in the incommensurate (IC) and commensurate (C) charge-density-wave (CDW) phases by means of angle-resolved inverse-photoemission spectroscopy (ARIPES) along the $\ensuremath{\Gamma}\phantom{\rule{0.28em}{0ex}}(A)--M\phantom{\rule{0.28em}{0ex}}(L)$ direction. In the ARIPES spectra for the ICCDW phase, a continuous and dispersive Ta 5$d$ band is clearly observed near the Fermi level (${E}_{\mathrm{F}}$). For the CCDW phase, on the other hand, the Ta 5$d$ band splits up and down into two manifolds around the $M\phantom{\rule{0.28em}{0ex}}(L)$ point, reflecting formation of the $\sqrt{13}\ifmmode\times\else\texttimes\fi{}\sqrt{13}$ superstructure of the Ta plane. In addition, the CCDW-ARIPES spectrum around the $M\phantom{\rule{0.28em}{0ex}}(L)$ points exhibits a shoulder structure at 0.7 eV above ${E}_{\mathrm{F}}$, ascribed to the upper Hubbard band folded from the $\ensuremath{\Gamma}\phantom{\rule{0.28em}{0ex}}(A)$ point. We have also observed an energy gap of $\ensuremath{\sim}$100 meV in the unoccupied density of states in the CCDW phase.

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.

Reflection spectra of (C 2 H 5 NH 3 ) 2 CdCl 4 single crystals have been investigated in the range of 3–30 eV using synchrotron radiation. Two kinds of exciton bands with the splitting of the order of 0.1 eV, which is almost equal to the spin-orbit splitting of a Cl atom, are observed at 6.19 and 8.20 eV at 7 K. These bands are located at the energy positions close to the lowest exciton bands of CdCl 2 and C 2 H 5 NH 3 Cl crystals, respectively. On rising the sample temperatures, the 8.20 eV band shows a discontinuous peak-shift of 0.1 eV around the temperature where a structural phase transition occurs, while such a result is not observed for the 6.19 eV band. The 6.19 eV band is assigned to the electronic transition from the Cl - 3 p valence band to the lower Cd 2+ 5 s conduction band, and the 8.20 eV band to that from the Cl - 3 p valence band to the upper NH 3 - s -like conduction band.

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.

The most preferable state of the two neutral anthracene molecules in a cylindrical molecule of γ -cyclodextrin is assigned as a face-to-face (coplanar, S-shape) configuration that is distinguished from the gas phase, where anthracenes form a face-to-edge (T-shape) configuration. Their faces are slightly tilted and twisted, thus the nearest sites are 9 and 9’ and the second nearest sites are 10 and 10’ between them. Excitation (300–400 nm) forms intermolecular bonds between two anthracenes in a γ -cyclodextrin. Absorption spectra were obtained from the reflection spectra of the powder sample using the Kubelka-Munk formula. Differential spectra of samples before and after irradiation (300– 400 nm) show a broad absorption band. This band corresponds to the transition from the ground state to the excited state of the supramolecular complex. The ab initio and molecular dynamics simulation reveal that the most probable bonds between them are the intermolecular bonds formed at the middle of them, namely, one between 9 and 9’ and the other between 10 and 10’ sites.

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 .

Polar crystals were formed using a chiral dinuclear nickel(II) complex [Ni2(bomp)(R/S-OCOCH(CH3)Ph)2]BPh4 as a building unit. [H(bomp): 2,6-bis[bis(2-methoxyethyl)aminomethyl]-4-methylphenol; H(R/S-OCOCH(CH3)Ph): R/S-2-phenylpropionic acid]. Single-crystal X-ray analysis revealed that the symmetry of the crystal was C2 and the complex cations were orientated along the polar axis. The crystals showed pyroelectric and second-order electro-optic effects, which are characteristic of polar materials.

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.

Recombination luminescence from (CnH2n+1NH3)2CdCl4 : n=1, 2, 3 has been studied using synchrotron radiation pulses. A largely stokes-shifted emission band was found to appear at 2.50eV for the n=2 crystal when excited into the excitonic absorption range, while at 2.25eV when excited into the band-to-band transition range. Decay curves of both bands are composed of fast and slow components. Non-exponential decay of slow components suggests that the 2.25eV band is induced by tunneling recombination between self-trapped electrons and self-trapped holes. As for the 2.50eV band, recombination of correlated electron-hole pairs with strong lattice relaxation is supposed.

From measuring absorption spectra of cerium-doped Gd3Al2Ga3O12 (Ce:GAGG) and undoped GAGG crystals at low temperatures under UV-light irradiation, we find that they exhibit a broad band at around 12000 cm−1. This band is enhanced by high-temperature annealing under a hydrogen atmosphere. On the basis of present experimental results, the UV-induced band is assigned to shallow electron traps of defect complexes associated with oxygen vacancies. The UV-induced band completely disappears with Mg2+ codoping. We conclude that the Mg2+ codoping has the effect of inhibiting the formation of shallow electron traps, which realizes a faster scintillation response of Ce:GAGG.

We have investigated the surface electronic structure on vacuum-cleaved $n$- and $p$-type Bi${}_{2}$Te${}_{3}$(111) at 10 K by high-resolution photoelectron spectroscopy. The surface sensitivity was maximized by tuning photon energy using synchrotron radiation as an excitation source, probing the topmost two quintuple layers (QLs). The Bi 5$d$ and Te 4$d$ core levels are also observed in high energy resolution. The absence of surface core-level shift of Bi 5$d$ clearly demonstrates that the topmost layer of the cleaved Bi${}_{2}$Te${}_{3}$(111) is composed of a Te layer. In the angle-resolved photoelectron spectra, the gapless surface-state bands (SSBs) as well as the bulk conduction and valence bands (BVBs) are clearly observed, ensuring the topological nature. The ${k}_{\ensuremath{\parallel}}$ dispersion of most of the BVBs along the $\overline{\ensuremath{\Gamma}}$-$\overline{\mathrm{M}}$ line in the surface Brillouin zone is highly symmetrical around the $\overline{\ensuremath{\Gamma}}$ against the threefold symmetry of the crystal structure, where ${k}_{\ensuremath{\parallel}}$ is the wave vector parallel to the surface. The topmost Te layer of sixfold structure must dominantly contribute to the symmetrical ${k}_{\ensuremath{\parallel}}$ dispersion. Moreover, the BVBs show almost flat ${k}_{\ensuremath{\perp}}$ dispersion, where ${k}_{\ensuremath{\perp}}$ is the wave vector perpendicular to the surface. These observations are the direct experimental evidence of a strong modification of the valence bands within the topmost two QLs into strongly two-dimensional states by the advent of the surface. In conclusion, the surface electronic structure within the two QLs is composed of the SSBs and modified two-dimensional valence bands.

Luminescence from self-trapped excitons in high-purity CdI 2 has been studied at liquid nitrogen temperature under intense excitation by a pulsed nitrogen laser. Luminescence intensity was carefully measured by changing excitation density and length of the excited region on a crystal surface. Obtained results are well explained by introducing a stimulated emission process into radiative decay channels of the self-trapped excitons. The net optical gain depends strongly on the excitation density and is estimated as 6 cm -1 in its maximum value which is extremely smaller than the previously reported one. Discussions will be made on consistency among the excitation-density dependence, the excitation-length dependence and the luminescence decay profiles.

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 $(xl{x}_{\text{c}})$.

The exciton-phonon interaction in a two-dimensional (C 2 H 5 NH 3 ) 2 CdCl 4 single crystal was studied by investigating the low energy tail of the first exciton absorption band in the temperature range of 100–440 K. The tail spectrum for different temperatures obeys the Urbach rule with small steepness parameters. From the analysis of those tails, it is determined that the steepness coefficient σ 0 is 0.60 and the exciton-phonon coupling constant g is 2.1. These indicate that the exciton-phonon interaction in (C 2 H 5 NH 3 ) 2 CdCl 4 is strong and the free exciton relaxes to the self-trapped exciton state. The electronic nature of the excitonic transition is discussed on the basis of the results in the present work.

We present electrical resistivity measurements for the intercalation compound FexTiSe2 0x0.16 over the temperature range from 4.2 to 300 K, and angle-resolved photoemission spectra for x=0, 0.05, and 0.14 at 50 and 250 K or 280 K. At 250 K, TiSe2 is a semimetal having hole pockets centered at the point and electron pockets around the L points of the Brillouin zone. Upon intercalation, Fe-derived flat bands appear just below the Fermi energy, and the Se 4p derived bands forming hole pockets at the point are lowered. At 50 K, band folding due to 2a2a2c superlattice is observed clearly near the L point for the host and x =0.05, while it vanishes for x=0.14, consistent with the -T data. The critical concentration for the suppression of the superstructure 0.05xc0.075 can be explained reasonably well by the percolation threshold of a two-dimensional-trianglar lattice consisting of seven Ti atoms, which is estimated to 1/14 =0.0714.