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The search of novel topological phases, such as the quantum anomalous Hall insulator (QAHI) or the axion insulator, has motivated different schemes to introduce magnetism into topological insulators. One scheme is to introduce ferromagnetic dopants in topological insulators. However, it is generally challenging and requires carefully engineered growth/heterostructures or relatively low temperatures to observe the QAHI due to issues such as the added disorder with ferromagnetic dopants. Another promising scheme is using the magnetic proximity effect with a magnetic insulator to magnetize the topological insulator. Most of these heterostructures are synthesized so far by growth techniques such as molecular beam epitaxy and metallic organic chemical vapor deposition. These are not readily applicable to allow mixing and matching many of the available ferromagnetic and topological insulators due to difference in growth conditions and lattice mismatch. Here, we demonstrate that the magnetic proximity effect can still be obtained in stacked heterostructures assembled via the dry transfer of exfoliated micrometer-sized thin flakes of van der Waals topological insulator and magnetic insulator materials (BiSbTeSe2/Cr2Ge2Te6), as evidenced in the observation of an anomalous Hall effect (AHE). Furthermore, devices made from these heterostructures can allow modulation of the AHE when controlling the carrier density via electrostatic gating. These results show that simple mechanical transfer of magnetic van der Waals materials provides another possible avenue to magnetize topological insulators by magnetic proximity effect, a key step towards further realization of novel topological phases such as QAHI and axion insulators., Comment: 22 pages, 10 figures

Different fluxes, Ga, Bi, In, Sb, and Sn, have been used to grow clathrate Ba8Cu5.3Ge40.7 (BCG) single crystals using a flux method, and BCG single crystals have been successfully synthesized by Bi-, In- and Sn-flux. The grown crystals were characterized by single crystal and power X-ray diffraction as well as energy dispersive X-ray spectroscopy measurements. The solubility of flux inside the crystals was determined and the actual chemical composition was decided to be: Ba8Cu5.1Ge40.5 (BCG-Bi), Ba8Cu4.6In1.4Ge40.0 (BCG-In) and Ba8Cu5.1Ge40.2Sn0.7 (BCG-Sn) for the compounds grown from Bi-, In- and Sn-flux, respectively. A high electron mobility (μ), 44.8 cm2 V−1 s−1 at 300 K, is observed for BCG-In, which is almost 5 times higher than the reported data at a similar carrier concentration. Therefore, enhanced thermoelectric power factors and ZT values are obtained. The relationship between the structure and electrical/thermoelectric properties is discussed. The high μ is most probably due to the relatively ordered cage framework structure in BCG-In, because the 6c site is fully occupied by Cu and In, and the other two sites are uniformly occupied by Ge.

To develop high-performance organic light-emitting organic field-effect transistors (LE-OFETs), a fundamental problem in organic semiconductors is to compromise light luminescent efficiency for high carrier mobility and vice versa. Therefore, LE-OFETs can avoid this problem by separating the light-emission and carrier-transport functions. Here, a bilayer LE-OFET composed of a tetracene crystal as a carrier transporter (bottom crystal) and a 4-(dicyanomethylene)-2-methyl-6-( p-dimethylaminostyryl)-4 H-pyran (DCM1)-doped tetracene crystal as a light emitter (top crystal) was fabricated. Red light-emission color, which is distinct from the green emission color of tetracene, was detected in the top crystal. Light emission from the top layer was prohibited when an insulating thin film was inserted between the two crystals. These observations indicate that excitons are formed in the bottom crystal and transferred to the top crystal, emitting reddish light. Bilayer LE-OFETs have the advantage of providing both high current density and a bright emission for high-performance light-emitting FETs.

A very large negative magnetoresistance (LNMR) is observed in the insulating regime of the antiferromagnet ${\mathrm{BaMn}}_{2}{\mathrm{Bi}}_{2}$ when a magnetic field is applied perpendicular to the direction of the sublattice magnetization. A high perpendicular magnetic field eventually suppresses the insulating behavior and allows ${\mathrm{BaMn}}_{2}{\mathrm{Bi}}_{2}$ to reenter a metallic state. This effect is seemingly unrelated to any field-induced magnetic phase transition, as measurements of magnetic susceptibility and specific heat did not find any anomaly as a function of magnetic fields at temperatures above $2\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. The LNMR appears in both current-in-plane and current-out-of-plane settings, and Hall effects suggest that its origin lies in an extreme sensitivity of conduction processes of holelike carriers to the infinitesimal field-induced canting of the sublattice magnetization. The LNMR-induced metallic state may thus be associated with the breaking of the antiferromagnetic parity-time symmetry by perpendicular magnetic fields and/or the intricate multiorbital electronic structure of ${\mathrm{BaMn}}_{2}{\mathrm{Bi}}_{2}$.

The magnetic proximity effect is one of the powerful approaches to realize quantum anomalous Hall effect in topological insulators (TIs) targeting at high temperatures. Various TI/ferromagnetic-insulator (FMI) heterostructures have extensively been investigated by using a van der Waals epitaxial growth technique of TI films grown on FMI substrates. However, FMI materials which can be used as a substrate are strictly limited due to the lattice mismatching between TI and FMI, not succeeding to boost the quantization temperature to be higher. Here, we show that a large anomalous Hall effect, comparable to the best value so far reported for the heterostructure interfaces fabricated by epitaxial growth techniques, is realized for ${\mathrm{Bi}}_{1.5}{\mathrm{Sb}}_{0.5}{\mathrm{Te}}_{1.7}{\mathrm{Se}}_{1.3}$ (BSTS)/${\mathrm{Cr}}_{2}{\mathrm{Ge}}_{2}{\mathrm{Te}}_{6}$ (CGT) heterostructure by transferring an epitaxially grown BSTS thin film floating on a ultrapure water directly on a CGT substrate (wet transfer method). The fundamental discussions about the nature of magnetic proximity effect were given in the aspect of the quality of the CGT substrate and the atomic orientation of the TI/CGT interface. The large magnetic proximity effect shown in our present studies can be applicable for a variety of FMI substrates and therefore can pave a route for promoting magnetic TIs both in basic science and applications.

A family of layered manganese-pnictide antiferromagnets ${\mathrm{BaMn}}_{2}{Pn}_{2}$ ($\mathit{Pn}$ stands for P, As, Sb, and Bi) has been recently shown to host anomalously large magnetoresistance (MR) with both positive and negative MR components. In search for the microscopic picture of MR in this family, we here report a local-probe $^{55}\mathrm{Mn}$ nuclear magnetic resonance (NMR) study. The zero-field NMR spectra and the temperature dependence of the spin-lattice relaxation rates are fully consistent with the proposed G-type antiferromagnetic order. However, a close inspection of the $^{55}\mathrm{Mn}$ NMR spectra reveals a fine structure, which is due to the weakly localized charge carriers. As these carriers localize and segregate in the presence of electron correlations, they also undergo collective spin fluctuations which freeze-out at low temperatures and thus contribute to the $^{55}\mathrm{Mn}$ spin-lattice relaxation. The characteristic temperatures of the appearance of these additional features in the magnetic response probed by $^{55}\mathrm{Mn}$ NMR correlate well with the anomalies in the resistivity and anomalously large MR, which hints that the two phenomena are connected.

Topological surface Dirac states (TSDSs) generated in three-dimensional topological insulators (3D-TIs) are currently of significant interest for new science and advanced technologies. In contrast to many other thermoelectric materials, 3D-TIs exhibit a significant potential to achieve a large enhancement in thermoelectric power factor ($PF=\ensuremath{\sigma}{S}^{2}$) due to their special topological symmetry. However, only limited experiments and discussions have been made so far for elucidating the thermoelectric properties of TSDS. Herein, we report a large $S$ and $\mathit{PF}$ observed for high-quality single-crystal flakes of $\mathrm{Sn}\text{\ensuremath{-}}\mathrm{B}{\mathrm{i}}_{1.1}{\mathrm{Sb}}_{0.9}\mathrm{Te}{\mathrm{S}}_{2}$ (Sn-BSTS). Accurate interpretations that the energy-dependent relaxation times $\ensuremath{\tau}(E)$ play an important role in thermoelectrical transport of 3D-TIs are provided. Among 3D-TIs, Sn-BSTS has the highest bulk insulation and shows intrinsic TSDS transport without bulk contributions, along with its hallmark of quantum integer Hall effect at high temperatures. Based on the longitudinal/transverse electrical transport and the thermoelectric coefficient, $\ensuremath{\tau}(E)\ensuremath{\propto}{E}^{0.21}$ is accurately deduced. As a consequence of the energy-dependent $\ensuremath{\tau}(E)$, a large enhancement in both $S$ and $\mathit{PF}$ is obtained ($S=58\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{V}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ and $PF=5.0\phantom{\rule{0.16em}{0ex}}\mathrm{mW}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.28em}{0ex}}{\mathrm{K}}^{\ensuremath{-}2}$ at 77 K), leading to a large increase of 160% for $S$ and 280% for $\mathit{PF}$ when compared to those of graphene at 77 K. The potential thermoelectric performance of the pure TSDS is discussed based on the Boltzmann transport equations.

The interplay of orbital and spin degrees of freedom is the fundamental characteristic in numerous condensed matter phenomena, including high temperature superconductivity, quantum spin liquids, and topological semimetals. In iron-based superconductors (FeSCs), this causes superconductivity to emerge in the vicinity of two other instabilities: nematic and magnetic. Unveiling the mutual relationship among nematic order, spin fluctuations, and superconductivity has been a major challenge for research in FeSCs, but it is still controversial. Here, by carrying out 77Se nuclear magnetic resonance (NMR) measurements on FeSe single crystals, doped by cobalt and sulfur that serve as control parameters, we demonstrate that the superconducting transition temperature Tc increases in proportion to the strength of spin fluctuations, while it is independent of the nematic transition temperature Tnem. Our observation therefore directly implies that superconductivity in FeSe is essentially driven by spin fluctuations in the intermediate coupling regime, while nematic fluctuations have a marginal impact on Tc., Comment: 20 pages, 3 figures; includes supplementary materials (1 note and 3 figures)

Realization of intrinsic surface dominant transport in a wide temperature region for topological insulators (TIs) is an important frontier research to promote the progress of TIs toward future electronics. We report here systematic measurements of longitudinal electrical transport, Shubnikov--de Haas (SdH) quantum oscillations, the Hall coefficient (${R}_{\mathrm{H}}^{2\mathrm{D}}$), and the Seebeck coefficient as a function of film thickness ($d$) and temperature using high-quality $\mathrm{B}{\mathrm{i}}_{2\ensuremath{-}x}\mathrm{S}{\mathrm{b}}_{x}\mathrm{T}{\mathrm{e}}_{3\ensuremath{-}y}\mathrm{S}{\mathrm{e}}_{y}$ single-crystal thin films grown by physical vapor-phase deposition. The thickness dependence of sheet conductance and the Seebeck coefficient clearly shows the suppression of semiconducting hole carriers of bulk states by reducing film thickness, reaching to the surface dominant transport at below ${d}_{\mathrm{c}}=14\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$. Quantitative arguments are made as to how the contribution of itinerant carrier number ($n$) can be suppressed, using both ${R}_{\mathrm{H}}^{2\mathrm{D}}$ (${n}_{\mathrm{Hall}}^{2\mathrm{D}}$) and SdH (${n}_{\mathrm{SdH}}$). Intriguingly, the value of ${n}_{\mathrm{Hall}}^{2\mathrm{D}}$ approaches being twice that of ${n}_{\mathrm{SdH}}$ below ${d}_{\mathrm{c}}$. While ${R}_{\mathrm{H}}^{2\mathrm{D}}$ shows a negative sign in the whole temperature region, a change from negative to positive polarity is clearly observed for $S$ at high temperatures when $d$ is thick. We point out that this inconsistency observed between ${R}_{\mathrm{H}}^{2\mathrm{D}}$ and $S$ is intrinsic in three-dimensional (3D) TIs and its origin is the large difference in carrier mobility between the bulk and the topological surface. We propose that the Seebeck coefficient can become a convenient and effective tool to evaluate the intrinsic topological surface transport of 3D TIs in the absence of magnetic field.

We report the discovery of a large-magnetoresistance (LMR) phenomenon in a family of ${\mathrm{BaMn}}_{2}{Pn}_{2}$ antiferromagnets ($Pn$ = P, As, Sb, and Bi) with parity-time symmetry. The resistivities of these materials are reduced by 60 times in magnetic fields $\mathbf{H}$, thus yielding a LMR of about $\ensuremath{-}98%$. The LMR changes systematically along with the $Pn$ elements, hinting that its origin is the spin-orbit coupling and/or $d\ensuremath{-}p$ orbital hybridization. A positive MR component emerging on top of the negative LMR at low temperatures suggests an orbital-sensitive magnetotransport as $\mathbf{H}$ suppresses the conduction of the electronlike carriers in the $d$-like band but enhances that of holelike ones in the $d\ensuremath{-}p$ hybridized band. The anisotropy of the LMR reveals that the electrical conductivity is extremely sensitive to the minute changes in the direction of the antiferromagnetic moments induced by the parity-time symmetry-breaking $\mathbf{H}$, which is perpendicular to the antiferromagnetic spin-ordered direction. We attribute the observed LMR to the nontrivial low-energy bands of ${\mathrm{BaMn}}_{2}{Pn}_{2}$ antiferromagnets, which are governed by the parity-time symmetry.

The conditions for determining the oscillation wavelength of organic distributed-feedback (DFB) lasers were investigated by employing 5,5″-bis(biphenyl-4-yl)-2,2′:5′,2″-terthiophene (BP3T) single crystals with various thicknesses as an active layer. Analysis using experimentally obtained refractive indexes revealed that the oscillation wavelengths of some BP3T DFB lasers are at the edge of a stopband, whereas the others are at an edge of a mini stopband, which satisfies the phase-matching condition among different transverse modes and has not been considered in DFB lasers so far. The coupling of different transverse modes was verified by finite-element optical-waveguide simulations of the BP3T DFB laser.

Single crystals of type I clathrate \(\hbox {K}_{8}\hbox {Zn}_4\hbox {Sn}_{42}\) and \(\hbox {K}_8\hbox {In}_8\hbox {Sn}_{38}\) were grown by the flux method. The structure properties of both clathrates were investigated by single-crystal x-ray diffraction. Both clathrates crystallize in space group \(Pm\bar{3}n\), and no phase transition was observed in the investigated temperature range (100 K–450 K). The Debye temperature, the Einstein temperature, and the static disorder parameters of each clathrate were derived. The spread of the Einstein temperatures for \(\hbox {K}_8\hbox {In}_8\hbox {Sn}_{38}\), \(\hbox {K}_{8}\hbox {Zn}_4\hbox {Sn}_{42}\), and \(\hbox {K}_8\hbox {In}_8\hbox {Sn}_{38}\) suggest that the charge distribution on the cage framework is crucial to the energy of rattling mode in the tetrakaidecahedron of the type-I clathrate.

Magnetism in two-dimensional van der Waals materials has received significant attention recently. The Curie temperature reported for those materials, however, has been so far remained relatively low. Here, we measure magneto-optical Kerr effects (MOKE) under perpendicular magnetic field for van der Waals ferromagnet Cr2Ge2Te6 as well as its heterostructure with antiferromagnetic insulator NiO. We observe a notable increase in both Curie temperature and magnetic perpendicular anisotropy in Cr2Ge2Te6/NiO heterostructures compared to those in Cr2Ge2Te6. Measurements on the same exfoliated Cr2Ge2Te6 flake (on a SiO2/Si substrate) before and after depositing NiO show that the hysteresis loop can change into a square shape with larger coercive field for Cr2Ge2Te6/NiO. The maximum Curie temperature (TC) observed for Cr2Ge2Te6/NiO reaches ~120 K, is nearly twice the maximum TC ~ 60 K reported for Cr2Ge2Te6 alone. Both enhanced perpendicular anisotropy and increased Curie temperature are observed for Cr2Ge2Te6 flakes with a variety of thicknesses ranging from ~5 nm to ~200 nm. The results indicate that magnetic properties of two-dimensional van der Waals magnets can be engineered and controlled by using the heterostructure interface with other materials.

The fine structure and temperature evolution of infrared spectra have been intensively used to probe the nature of Jahn-Teller dynamics in correlated materials. At the same time, theoretical framework to adequately extract the information on the complicated vibronic dynamics from infrared spectra is still lacking. In this work, the first-principles theory of the infrared spectra of dynamical Jahn-Teller system is developed and applied to the Mott-insulating Cs$_3$C$_{60}$. With the calculated coupling parameters for Jahn-Teller and infrared active vibrational modes, the manifestation of the dynamical Jahn-Teller effect in infrared spectra is elucidated. In particular, the temperature evolution of the infrared line shape is explained. The transformation of the latter into Fano resonance type in metallic fulleride is discussed on the basis of obtained results., 10 pages

High-quality bulk samples of the first four polyacenes, which are naphthalene, anthracene, tetracene, and pentacene, doped with alkali metal in 1 : 1 and 1 : 2 stoichiometries were prepared and their fundamental properties were systematically studied. A new systematic understanding on the electronic states of electron-doped polyacenes sensitive to the energetic balance among on-site Coulomb repulsion, bandwidth and the Peierls instability was provided. The carrier-doped typical aromatic hydrocarbons showed a large variety of properties as well as charge transfer complexes and metal-doped fullerides. We open a new avenue for organometallic and inorganic chemistry.

Three new 2-positional pyrene end-capped oligothiophene co-oligomers, BPynT (n = 1, 2, 3), have been synthesized for application in organic field effect transistors (OFETs). BPy2T showed the highest hole mobility of 3.3 cm(2) V(-1) s(-1) in a single crystal OFET and a good photoluminescence efficiency of 32% in the crystalline state. A green light emission was observed for the OFET based on a BPy2T single crystal.

We study type I clathrate Ba8Cu5.1Ge40.2Sn0.7 single crystals (space group Pmn, no. 223, a = 10.7151(3)) grown using a Sn flux method. Microprobe analysis and single-crystal X-ray diffraction reveal a small amount of Sn embedded in the Cu/Ge framework, which increases disorder at the guest Ba sites. Ba8Cu5.1Ge40.2Sn0.7 is diamagnetic with a susceptibility of ∼2.8 × 10−7 emu g−1 and shows metal-like behavior (dρ/dT > 0) with a low charge carrier concentration of 0.5 e− per unit cell at 300 K. The single crystals show a relatively high carrier mobility (μ (300 K) = 11.9 cm2 V−1) and very low lattice thermal conductivity (∼0.6 W m−1 K−1). The thermoelectric figure of merit ZT of Ba8Cu5.1Ge40.2Sn0.7 single crystals reaches a maximum value of 0.6 at 773 K, which can be further improved by adjusting the chemical composition.

Low-temperature specific heat (SH) is measured on the postannealed Ba(Fe_{1-x}Ni_x)_2As_2 single crystal with x = 0.058 under different magnetic fields. The sample locates on the overdoped sides and the critical transition temperature is determined to be 14.8 K by both the magnetization and SH measurements. A simple and reliable analysis shows that, besides the phonon and normal electronic contributions, a clear T2 termemerges in the low temperature SH data.Our observation is similar to that observed in the Co-doped system in our previous work and is consistent with the theoretical prediction for a superconductor with line nodes in the energy gap., 5 pages, 4 figures

Graphene with excellent 2D characters has been required to expand their potentials for meeting applicational demands in recent years. As one avenue, several approaches for fablicating 3D porous graphene network structures have been developed to realize multi-functional graphene materials with 2D graphene. Here we overview characteristics of 3D porous graphene for future electronic device applications along with physical insights into "2D to 3D graphene", where the characters of 2D graphene such as massless Dirac fermions are well preserved. The present review thus summarizes recent 3D porous graphene studies with a perspective for providing new and board graphene electronic device applications., 27 page, 12 figures

Organic semiconductors have attracted much attention for low-cost, flexible and human-friendly optoelectronics. However, achieving high electron-injection efficiency is difficult from air-stable electrodes and cannot be equivalent to that of holes. Here, we present a novel concept of electrode composed of a bilayer of tetratetracontane (TTC) and polycrystalline organic semiconductors (pc-OSC) covered by a metal layer. Field-effect transistors of single-crystal organic semiconductors with the new electrodes of M/pc-OSC/TTC (M: Ca or Au) show both highly efficient electron and hole injection. Contact resistance for electron injection from Au/pc-OSC/TTC and hole injection from Ca/pc-OSC/TTC are comparable to those for electron injection from Ca and hole injection from Au, respectively. Furthermore, the highest field-effect mobilities of holes (22 cm2 V–1 s–1) and electrons (5.0 cm2 V–1 s–1) are observed in rubrene among field-effect transistors with electrodes so far proposed by employing Ca/pc-OSC/TTC and Au/pc-OSC/TTC electrodes for electron and hole injection, respectively., One of technological challenges building organic electronics is efficient injection of electrons at metal-semiconductor interfaces compared to that of holes. The authors show an air-stable electrode design with induced gap states, which support Fermi level pinning and thus ambipolar carrier injection.

One of central issues in iron-based superconductors is the role of structural change to the superconducting transition temperature (T_c). It was found in FeSe that the lattice strain leads to a drastic increase in T_c, accompanied by suppression of nematic order. By angle-resolved photoemission spectroscopy on tensile- or compressive-strained and strain-free FeSe, we experimentally show that the in-plane strain causes a marked change in the energy overlap (DeltaE_{h-e}) between the hole and electron pockets in the normal state. The change in DeltaE_{h-e} modifies the Fermi-surface volume, leading to a change in T_c. Furthermore, the strength of nematicity is also found to be characterized by DeltaE_{h-e}. These results suggest that the key to understanding the phase diagram is the fermiology and interactions linked to the semimetallic band overlap., 6 pages, 3 figures

Uniform and large-area synthesis of bulk insulating ultrathin films is an important subject toward applications of a surface of three-dimensional topological insulators (3D-TIs) in various electronic devices. Here we report epitaxial growth of bulk insulating three-dimensional topological insulator (3D-TI) Bi

High-quality polycrystalline polyacenes (PLAs) of anthracene (AN), tetracene (TN), and pentacene (PN) doped with potassium in the 1:1 and 1:2 stoichiometries are prepared and their physical properties are systematically studied by magnetic measurements. The magnetic susceptibilities as a function of temperature for Kx(PLAs) are interpreted in the framework of a band-filling model. Clear evidence for the occurrence of charge transfer from K to the organic semiconductors is provided. K1(AN) prepared by a solid-state diffusion process exhibits a pronounced magnetic hump at 140 K, which is associated with antiferromagnetic interactions ascribed to the on-site Coulomb repulsion in a Mott insulating state, whereas both K1(TN) and K1(PN) show a conventional Curie–Weiss behavior at low temperatures. We successfully demonstrate the tuning of the electronic states of PLA-type semiconductors in bulk. Comments are given about the possibility of metallic states in potassium-doped PLAs, which have recently been reported for other polycyclic aromatic hydrocarbons.

Bicontinuous nanoporous N-doped graphene with tunable pore size is synthesized by nanoporous Ni-based chemical vapor deposition. The novel 3D graphene material shows an outstanding catalytic activity towards the oxygen reduction reaction with a low onset potential of -0.08 V and a high kinetic current density of 8.2 mA cm(-2) at -0.4 V.

We report three-dimensional (3D) nanoporous graphene with preserved 2D electronic properties, tunable pore sizes, and high electron mobility for electronic applications. The complex 3D network comprised of interconnected graphene retains a 2D coherent electron system of massless Dirac fermions. The transport properties of the nanoporous graphene show a semiconducting behavior and strong pore-size dependence, together with unique angular independence. The free-standing, large-scale nanoporous graphene with 2D electronic properties and high electron mobility holds great promise for practical applications in 3D electronic devices.

The quantum spin Hall effect (QSHE) in three-dimensional topological insulators (3D-TIs) is an important hallmark of realization of ultralow dissipative electrical transport originating from nontrivial topological surface Dirac states (TSDSs). The observation of QSHE, however, had been limited at low temperatures for thin film crystals prepared by epitaxial growth or ultrathin (nanometer-level) flakes, and the finite Fermi energy tuning was required using field effect transistor (FET) construction. Here, we show that a clear observation of QSHE with a quantized level of e 2 / h can be possible for high quality single crystal flakes of Sn-(Bi,Sb)2(Te,S)3 (Sn-BSTS) prepared by an exfoliation method without any gate control under FET construction. The QSHE is observed in an extremely large sample area of millimeter square at the largest thickness of micrometer and up to the highest temperature of 20 K, which is greatly superior to those reported for other 3D-TIs so far. The accurate band picture of both top and bottom topological surface states of Sn-BSTS is determined from both the QSHE and the Shubnikov-de Hass (SdH) oscillations. High quality Sn-BSTS of 3D-TIs will effectively be used for experiments targeting on science as well as many technological applications expected in the future.

We theoretically analyze the optoelectronic properties of single crystals of 2,5-bis(4-biphenylyl) bithiophene (BP2T) and 2-(4-biphenyl)-5-[5- (4-biphenyl)-2-thienyl] furan (BPFT) molecules, aiming to provide a guiding principle for the material design of organic light-emitting transistors. The X-ray structure analysis and the density functional theory (DFT) calculations indicate that half of the BPFT molecules bend the π-conjugation plane in the crystal. The Marcus theory parametrized by the DFT calculations indicates anisotropic charge mobilities. The emission spectra of the BP2T and BPFT crystals are analyzed by the time-dependent DFT calculations in conjunction with the Frenkel exciton model and the vibronic coupling analysis. We revealed that the high photoluminescence efficiency of the BPFT crystal originates from the symmetry breaking of the H-aggregate, where the transition dipole of the dark state does not cancel out.

We report thermoelectric (TE) properties of topological surface Dirac states (TSDS) in three-dimensional topological insulators (3D-TIs) purely isolated from the bulk by employing single crystal Bi$_{2-x}$Sb$_x$Te$_{3-y}$Se$_y$ films epitaxially grown in the ultrathin limit. Two intrinsic nontrivial topological surface states, a metallic TSDS (m-TSDS) and a gap-opened semiconducting topological state (g-TSDS), are successfully observed by electrical transport, and important TE parameters (electrical conductivity (${\sigma}$), thermal conductivity (${\kappa}$), and thermopower ($S$)) are accurately determined. Pure m-TSDS gives $S$=-44 {\mu}VK$^{-1}$, which is an order of magnitude higher than those of the conventional metals and the value is enhanced to -212 {\mu}VK$^{-1}$ for g-TSDS. It is clearly shown that the semi-classical Boltzmann transport equation (SBTE) in the framework of constant relaxation time (${\tau}$) most frequently used for conventional analysis cannot be valid in 3D-TIs and strong energy dependent relaxation time ${\tau}(E)$ beyond the Born approximation is essential for making intrinsic interpretations. Although ${\sigma}$ is protected on the m-TSDS, ${\kappa}$ is greatly influenced by the disorder on the topological surface, giving a dissimilar effect between topologically protected electronic conduction and phonon transport., Comment: 15 pages, 4 figures

Three organic semiconductors consisting of thiophene, furan, and phenylene groups showed either one or two gain-narrowed emission peaks by excitation with a laser pulse. The two gain-narrowed emission peaks are optically studied and assigned to 0 → 1 and 0 → 2 vibronic transitions. The number of gain-narrowed emission peaks is determined by a degree of overlap of the vibronic emissions with ground-state and excited-state absorption of the material. Dependence of the gained emission intensity both on the total input energy and the density of the excited states shows that the two gain-narrowed peaks are attributed to two different decay processes: amplified spontaneous emission (ASE) and superfluorescence (SF). The input energy to be conserved in total is distributed between ASE and SF processes strongly dependent on the transition probability and the self-absorption of organic semiconductors as a gain media.

Oxygen molecules are found to exhibit non-negligible reactivity with graphene under strong light irradiation in the presence of water. The reaction is triggered by the laser Raman spectroscopy measurement itself, and the D band (ca. 1340 cm-1) becomes larger as the laser irradiation is prolonged. The electronic transport properties of the graphene derivative are also investigated and both the electron and hole mobility are found to be reduced. These results are attributed to oxidation of graphene. This primitive modification method can be exploited to manipulate the structural and electronic properties of graphene., 22 pages including supporting information

A topological p-n junction (TPNJ) is an important concept to control spin and charge transport on a surface of three dimensional topological insulators (3D-TIs). Here we report successful fabrication of such TPNJ on a surface of 3D-TI Bi$_{2-x}$Sb$_x$Te$_{3-y}$Se$_y$ thin films and experimental observation of the electrical transport. By tuning the chemical potential of n-type topological Dirac surface of BSTS on its top half by employing tetrafluoro-7,7,8,8-tetracyanoquinodimethane as an organic acceptor molecule, a half surface can be converted to p-type with leaving the other half side as the opposite n-type, and consequently TPNJ can be created. By sweeping the back-gate voltage in the field effect transistor structure, the TPNJ was controlled both on the bottom and the top surfaces. A dramatic change in electrical transport observed at the TPNJ on 3D-TI thin films promises novel spin and charge transport of 3D-TIs for future spintronics., 31pages, 5 figures

Two new regiospecific biphenyl end-capped bithiazole co-oligomers, BP2Tz(in) and BP2Tz(out), have been synthesized for application in thin film field effect transistors (TFTs). BP2Tz(in) with a 2,2'-bithiazole central unit exhibits a field effect hole mobility as high as 3.5 cm(2) V(-1) s(-1). Green light emission is demonstrated for highly balanced ambipoar TFTs based on both BP2Tz(in) and BP2Tz(out).

We report that anomalous low-energy excitation (ALE) peaks in the heat capacity emerging from single-crystal cage materials can be successfully rationalized in terms of a single unified exponential line for a variety of type-I clathrates by employing a parameter associated with the freedom of space and the modified radii of guest atoms estimated by band calculations. The origin of these low-energy excitations is interpreted in the framework of quasiharmonic van der Waals type guest-host interactions based on a unified picture with the help of first-principles calculations. It is shown that the influence of guest-host ionic and covalent bonding interactions on the phonon anharmonicity, which have so far been considered to play an important role, are not significant as long as high symmetry of the cage structure is preserved. The dominant van der Waals interactions explain the soft vibrational modes of the rattling, which suppress phonon transport and lead to the concept of ``phonon-glass electron-crystal'' (PGEC) for thermoelectric applications. A few exceptions existing in type-I clathrates, as indicated by deviations from the unified line, suggest that a quasiharmonic potential can become more asymmetric via lower symmetry of the cage structure, towards glasslike disordered states at even lower temperatures. Although the origin of the boson peaks appearing in disordered materials is still under debate due to incomplete information on the real structure, the understanding provided by the present paper for crystalline cage materials may provide information partly applicable to other disordered systems.

Polycrystalline samples of type I clathrate Ba8NixGe46−x (x = 3, 4, 6) have been synthesized using a radio-frequency (RF) induction furnace. Ba8NixGe46−x samples show metallic-like behavior (dρ/dT > 0) with high resistivity at room temperature, and diamagnetic susceptibility at 2 K and 300 K. The charge carriers vary from n type to p type depending on the Ni stoichiometry. The carrier concentrations at 300 K are calculated to be 5.84(3) e−/cell for x = 3, 2.29(1) e−/cell for x = 4, and 3.29(1) h+/cell for x = 6. The deviation of the carrier concentrations from the values expected based on 4-bonded Ni suggests that vacancies may play a very important role in the electronic states. The effective carrier masses are estimated from the low-temperature heat capacity data.

A history of the past researches on materials having nano spaces inside is described in connection to the present status and the future prospect. Specific features of such a family of nano materials are categorized and some of the important issues are discussed in more details by showing experimental examples. Atoms and molecules confined inside the nano spaces are focused from the viewpoint of anharmonic oscillations or energy non-dissipative isolation from outside. Glass-like low thermal conductivity and superconductivity appearing in this system are discussed on a basis of phonons.

Graphene fabricated on a SiO2/Si substrate by mechanical exfoliation has two features: one is charge (electron and hole) puddles which inhomogeneously distribute on graphene flakes, and the other i...

The heat capacity of single-crystalline samples of Sr8Ga16Ge30 (SGG) and Ba8Ga16Ge30 (BGG) clathrates was measured to investigate the anharmonicity of the encapsulated atoms. At low temperatures, BGG can be well described by a standard Debye model, and the Cp/T3 versus T plot can be fitted with two Einstein temperatures: θE1 = 42 K and θE2 = 74 K. On the other hand, SGG shows deviation from the Debye model. Moreover, neither the Einstein model nor the soft potential model (SPM) alone can fit the peak in the Cp/T3 versus T plot, and the peak should be fitted by employing both models. Our results indicate that the effective electron mass is enhanced by the anharmonic phonons.