Observation of the Superstructure in Fe5-xGeTe2 by X-ray Fluorescence Holography.

Fe5-xGeTe2 is a two-dimensional van der Waals material that exhibits ferromagnetic order with a high Curie temperature (TC) of around room temperature. In addition to TC, two magnetic transitions occur with decreasing temperature, and a charge-ordered state is observed at low temperatures. We employed Ge Kα X-ray fluorescence holography (XFH) for Fe5-xGeTe2 to directly investigate the local structure in the charge-ordered state, i.e., the 3×3 superstructure. The Ge Kα XFH results revealed local atomic structures around the Ge atom, thus clarifying the simultaneous locations and arrangements of the Te, Fe, and Ge atoms. The atomic positions relative to the Ge atom are useful for understanding the coexistence of the ideal 1 × 1 structure and 3×3 superstructure found in the charge-ordered state.

Pressure Dependence of the Structural and Superconducting Properties of the Bi-Based Superconductor Bi2Pd3Se2.

The structural and superconducting properties of the Bi-based compound Bi2Pd3Se2 were investigated over a wide pressure range. The prepared Bi2Pd3Se2 sample was a superconductor with a superconducting transition temperature, Tc, of approximately 3.0 K, which differed from a previous report (Tc of less than 1.0 K). At ambient pressure, the powder X-ray diffraction (XRD) pattern of the Bi2Pd3Se2 sample was consistent with that previously reported for Bi2Pd3Se2. The Rietveld method was used to refine the crystal structure, which had a space group of C2/m (No. 12), as reported previously. This compound showed no clear anomaly due to the charge-density-wave (CDW) transition, as seen from the temperature dependence of magnetic susceptibility. However, the temperature dependence of electrical resistivity indicated a clear anomaly, presumably because of the CDW transition in the low-pressure range; the CDW transition temperature was approximately 230 K. The XRD patterns of the Bi2Pd3Se2 sample were measured at 0.160-22.7 GPa, and the patterns were well analyzed by both the Le Bail and Rietveld refinement methods, showing no structural phase transitions in the above pressure range. The pressure dependence of Tc of Bi2Pd3Se2 was recorded based on the temperature dependence of the electrical resistance, which showed an almost constant Tc at 0-13.7 GPa, and the Tc-pressure (p) behavior was fully discussed.

Pressure Dependence of Superconductivity in a Charge-Density-Wave Superconductor Bi2Rh3Se2.

The structural and superconducting properties of a Bi-based compound, Bi2Rh3Se2, are investigated over a wide pressure range. Bi2Rh3Se2 is a superconductor with a superconducting transition temperature, Tc, of 0.7 K. This compound is in a charge-density-wave (CDW) state below 240 K, which implies the coexistence of superconducting and CDW states at low temperatures. Here, the superconducting properties of Bi2Rh3Se2 are studied from the perspective of the temperature dependence of electrical resistance (R) at high pressures (p's). The pressure dependence of Tc of Bi2Rh3Se2 shows a slow increase in Tc at 0-15.5 GPa, and the Tc slowly decreases with pressure above 15.5 GPa, which is markedly different from that of normal superconductors because the value of Tc should simply decrease owing to the decrease in density of states (DOS) on the Fermi level, N(εF), driven by a simple shrinkage of the lattice under pressure. To ascertain the origin of such a dome-like Tc-p behavior, the crystal structure of Bi2Rh3Se2 was explored over a wide pressure range of 0-20 GPa on the basis of powder X-ray diffraction; no structural phase transitions or simple shrinkage of the lattice was observed. This result implies that the increase in Tc against pressure cannot simply be explained from a structural point of view. In other words, a direct relation between superconductivity and crystal structure was not found. On the other hand, the CDW transition became ambiguous at pressures higher than 3.8 GPa, suggesting that the Tc had been suppressed by the CDW transition in a low pressure range. Thus, the findings suggest that for Bi2Rh3Se2, Tc is enhanced through the suppression of CDW transition, which may be reasonable because the CDW-ordered state restrains the charge fluctuation to weaken the electron-phonon coupling and opens the gap to decrease the density of states on the Fermi level. The obtained dome-like Tc-p behavior indicates the possibility of Bi2Rh3Se2 being an exotic superconductor.

Structural Characterization of Graphite Analogue BCx Synthesized Under Various Conditions and Its Application to Ti Intercalation.

A graphite-like material boron carbide (BCx) was synthesized under various heat treatment conditions and extensively characterized. First, we synthesized the BCx precursor phase by a single-step reaction using a mixed solution of BBr3 and C6H6. We confirmed that the precursor phase had a graphite-like structure with B-C chemical bonds, but its crystallinity was poor. To improve their crystallinity, we annealed the precursor sample at high temperature using a high-frequency furnace and determined the annealing condition. We also investigated the magnetic properties of BCx. The high-temperature annealing for the precursor phase yields the highest Pauli paramagnetic susceptibility χPauli, indicating the highest density of states at the Fermi level. Accordingly, the high-temperature treatment for the precursor phase is significant to improve its crystallinity and physical properties. In addition, we synthesized a Ti-intercalated material TiBC by using the same procedure as that for making the BCx precursor phase. The crystal structure can be indexed by the AlB2 structure, indicating that Ti atoms are intercalated between the BC layers. The χPauli value of TiBC is obtained to be 1 order of magnitude smaller than that of BCx, suggesting the compensation of hole carriers by electron doping through Ti intercalation into the BCx system.

Superconducting Behavior of BaTi2(Sb1-yBiy)2O under Pressure.

The crystal structure and superconducting properties of a new type of titanium-pnictide superconductor, BaTi2(Sb1-yBiy)2O (y = 0.2, 0.5, and 0.8), are comprehensively investigated over a wide pressure range to elucidate the effect of substituting Bi for Sb on the superconducting behavior. The behavior of superconducting properties under pressure changes drastically with y, as expected from the double-dome Tc-y phase diagram obtained at ambient pressure. In this study, three BaTi2(Sb1-yBiy)2O samples (y = 0.2, 0.5, and 0.8) are considered, which correspond to the first superconducting dome, nonsuperconducting part, and second superconducting dome, respectively, in the Tc-y phase diagram. The crystal of BaTi2(Sb1-yBiy)2O with y = 0.2 shows a clear collapse transition, i.e., a drastic shrinkage of the lattice constant c at ca. 5 GPa. Strictly speaking, the collapsed crystal phase coexists with the noncollapsed phase above 5 GPa. On the other hand, BaTi2(Sb1-yBiy)2O with y = 0.8 shows a continuous change in the crystal lattice with pressure, i.e., no collapse transitions. The pressure dependence of Tc for BaTi2(Sb1-yBiy)2O with y = 0.2 shows a drastic increase in Tc at approximately 5 GPa, where the collapse transition occurs, indicating a clear pressure-induced superconducting phase transition related to the collapse transition. The value of Tc for BaTi2(Sb1-yBiy)2O with y = 0.8 increases slightly up to ∼2 GPa and is almost constant at 2-13 GPa. It is found that the superconducting behavior under pressure can be unambiguously classified by y based on the double-dome Tc-y phase diagram, indicative of distinguishable superconducting features at different y values. In this study, we comprehensively discuss the superconducting properties of the exotic material, BaTi2(Sb1-yBiy)2O, with a double-dome Tc-y phase diagram.

Pressure dependence of superconductivity in alkali-Bi compounds KBi2 and RbBi2.

The structural and superconducting properties of alkali-Bi-based compounds, KBi2 and RbBi2, were investigated over a wide pressure range for the first time. The samples of KBi2 and RbBi2 were prepared using a liquid ammonia (NH3) technique, and demonstrated superconductivity with superconducting transition temperatures, Tc, of 3.50 and 4.21 K at ambient pressure, respectively. The onset superconducting transition temperature, Tconset, of KBi2 decreased slightly; however, it suddenly jumped at 2 GPa and increased gradually with pressure, indicating the presence of two superconducting phases in the low-pressure range. The pressure-dependent X-ray diffraction patterns indicate that the KBi2 sample decomposed into KBi and Bi at pressures higher than 2.5 GPa. Moreover, a discontinuous change in Tconset was observed for KBi2 at 9 GPa, which reflects the decomposition of KBi2 into KBi and Bi. By contrast, the value of Tconset of RbBi2 was almost constant over a pressure range of 0-8 GPa. Thus, the superconducting properties and stability of alkali-Bi-based compounds against pressure were comprehensively explored in this study. In addition, the superconducting Cooper pair symmetry was investigated from the magnetic field dependence of Tc of KBi2 at 0.790 and 2.32 GPa, and of RbBi2 at 1.17 GPa, indicating the exact deviation from the simple s-wave paring model, which may be due to the complex electronic structure of Bi. The results elucidated the exotic superconducting properties of KBi2 and RbBi2 based on the pressure and magnetic field dependence of Tc and verified the chemical stability of KBi2 under pressure.

Emergence of a Pressure-Driven Superconducting Phase in Ba0.77Na0.23Ti2Sb2O.

We investigated the pressure dependence of electric transport in a superconducting sample, Ba0.77Na0.23Ti2Sb2O, to complete the phase diagram of superconducting transition temperature (Tc) against pressure (p). This superconducting sample exhibits a Tc value of 5.8 K at ambient pressure. Here, the superconductivity of the recently reported sample was investigated over a wide pressure range. The Tc value monotonously decreased with pressure below 8 GPa. Interestingly, the Tc value rapidly increased above 8 GPa and slowly declined with pressure above 11 GPa. Thus, a new superconducting phase was discovered above ∼9 GPa. The crystal structure of Ba0.77Na0.23Ti2Sb2O was also elucidated at 0-22.0 GPa with synchrotron X-ray powder diffraction. Consequently, an evident relation between the crystal structure and the superconductivity was revealed, namely, a clear structural phase transition was observed at 8-11 GPa, where the Tc value rapidly increased against pressure. This study provides detailed information on the superconductivity of Ba0.77Na0.23Ti2Sb2O under pressure, which will lead to a comprehensive understanding of pressure-driven superconductivity.

Superconducting properties of BaBi3 at ambient and high pressures.

Herein, we report the preparation and characterization of BaBi3 clarified by DC magnetic susceptibility, powder X-ray diffraction (XRD), and electrical transport. The superconducting properties of BaBi3 were elucidated through the magnetic and electrical transport properties in a wide pressure range. The superconducting transition temperature, Tc, showed a slight decrease (or almost constant Tc) against pressure up to 17.2 GPa. The values of the upper critical field, Hc2, at 0 K, were determined to be 1.27 T at 0 GPa and 3.11 T at 2.30 GPa, using the formula, because p-wave pairing appeared to occur for this material at both pressures, indicating the unconventionality of superconductivity. This result appears to be consistent with the topological non-trivial nature of superconductivity predicted theoretically. The pressure-dependent XRD patterns measured at 0-20.1 GPa indicated no structural phase transitions up to 20.1 GPa, i.e., the structural phase transitions from the α phase to the ß or γ phase which are induced by an application of pressure were not observed, contrary to the previous report, demonstrating that the α phase is maintained over the entire pressure range. Admittedly, the lattice constants and the volume of the unit cell, V, steadily decrease with increasing pressure up to 20.1 GPa. In this study, the plots of Tcversus p and V versus p of BaBi3 are depicted over a wide pressure range for the first time.

Superconducting behavior of BaTi2Bi2O and its pressure dependence.

A new superconducting sample, BaTi2Bi2O, was synthesized and characterized over a wide pressure range. The superconducting transition temperature, Tc, of BaTi2Bi2O was 4.33 K at ambient pressure. The crystal structure was tetragonal (space group of P4/mmm (No. 123)), according to the X-ray diffraction (XRD) pattern at ambient pressure. The XRD pattern was analyzed using the Le Bail method. The magnetic-field dependence of the magnetization at different temperatures was precisely investigated to elucidate the characteristics of the superconductivity. The pressure-dependent XRD patterns showed absence of structural phase transitions up to 19.8 GPa. The superconducting properties of BaTi2Bi2O were investigated under pressure. Tc monotonously increased with the pressure (p) up to 4.0 GPa and saturated above 4.0 GPa. The variations in the Tc-p plot were thoroughly analyzed. The Cooper pair symmetry (or superconducting pairing mechanism) was analyzed based on the magnetic field dependence of the superconductivity at ambient and high pressures, which indicated a sign of p-wave pairing for the superconductivity of BaTi2Bi2O, i.e., topologically nontrivial sign was suggested for BaTi2Bi2O.

Surface Structure of Organic Semiconductor [ n]Phenacene Single Crystals.

Surface structure relaxation of organic semiconductors affects the properties of organic devices, although such relaxation has not been well explored. Only two examples have been experimentally reported; tetracene shows a large surface relaxation, while rubrene exhibits no relaxation. Therefore, a systematic investigation of the surface relaxation is conducted on [ n]phenacenes ( n = 5, 7, and 9). Electron density analyses are performed based on the synchrotron surface X-ray scattering with the aid of first-principles calculations. The results show little surface relaxation in [ n]phenacenes.

Superconductivity in alkali-metal-doped picene.

Efforts to identify and develop new superconducting materials continue apace, motivated by both fundamental science and the prospects for application. For example, several new superconducting material systems have been developed in the recent past, including calcium-intercalated graphite compounds, boron-doped diamond and-most prominently-iron arsenides such as LaO(1-x)F(x)FeAs (ref. 3). In the case of organic superconductors, however, no new material system with a high superconducting transition temperature (T(c)) has been discovered in the past decade. Here we report that intercalating an alkali metal into picene, a wide-bandgap semiconducting solid hydrocarbon, produces metallic behaviour and superconductivity. Solid potassium-intercalated picene (K(x)picene) shows T(c) values of 7 K and 18 K, depending on the metal content. The drop of magnetization in K(x)picene solids at the transition temperature is sharp (<2 K), similar to the behaviour of Ca-intercalated graphite. The T(c) of 18 K is comparable to that of K-intercalated C(60) (ref. 4). This discovery of superconductivity in K(x)picene shows that organic hydrocarbons are promising candidates for improved T(c) values.

Synthesis of methoxy-substituted picenes: substitution position effect on their electronic and single-crystal structures.

A series of picenes having methoxy groups was synthesized through Pd-catalyzed Suzuki-Miyaura couplings or Wittig reaction/intramolecular cyclization sequences, and their physicochemical properties and single-crystal structures were evaluated. The substitution position effects between the outer 1,12-, 2,11-, and 4,9-position and the inner 3,10-position are quite different; the former showed the same electronic structure as that of picene, but the latter results in a HOMO geometry different from those of picene and other methoxy picenes. In addition, crystal structures of four types of methoxy-substituted picenes 4a-c,e strongly depend on their substitution position and number of methoxy groups, which dramatically changes the structures from the fully anisotropic 1D π-stacked structure to a unique 3D herringbone structure due to steric hindrance of methoxy groups. The calculations of transfer integrals based on their single-crystal structures reveal that the methoxy picenes have intermolecular overlaps less effective than that of the parent nonsubstituted picene. These results are attributed not only to the packing structure but also to electronic structures such as the HOMO distribution. The preliminary OFET of the representative 4c,e showed hole mobilities significantly lower than that of picene due to their less effective intermolecular overlaps, as predicted by the calculated transfer integrals.

Parity effects in few-layer graphene.

We study the electronic properties in few-layer graphenes (FLGs) classified by even/odd layer number n. FLGs with even n have only parabolic energy dispersions, whereas FLGs with odd n have a linear dispersion besides parabolic ones. This difference leads to a distinct density of states in FLGs, experimentally confirmed by the gate-voltage dependence of the electric double-layer capacitance. Thus, FLGs with odd n are unique materials that have relativistic carriers originating in linear energy dispersion.

Edge-dependent transport properties in graphene.

Graphene has two kinds of edges which have different electronic properties. A singular electronic state emerges at zigzag edges, while it disappears at armchair edges. We study the edge-dependent transport properties in few-layer graphene by applying a side gate voltage to the edge with an ionic liquid. The devices indicating a conductance peak at the charge neutrality point have zigzag edges, confirmed by micro-Raman spectroscopy mapping. The hopping transport between zigzag edges increases the conductance.

Superstructure of Fe5-xGeTe2 Determined by Te K-Edge Extended X-ray Absorption Fine Structure and Te Kα X-ray Fluorescence Holography.

The local structure of the two-dimensional van der Waals material, Fe5-xGeTe2, which exhibits unique structural/magnetic phase transitions, was investigated by Te K-edge extended X-ray absorption fine structure (EXAFS) and Te Kα X-ray fluorescence holography (XFH) over a wide temperature range. The formation of a trimer of Te atoms at low temperatures has been fully explored using these methods. An increase in the Te-Fe distance at approximately 150 K was suggested by EXAFS and presumably indicates the formation of a Te trimer. Moreover, XFH displayed clear atomic images of Te atoms. Additionally, the distance between the Te atoms shortened, as confirmed from the atomic images reconstructed from XFH, indicating the formation of a trimer of Te atoms, i.e., a charge-ordered superstructure. Furthermore, Te Kα XFH provided unambiguous atomic images of Fe atoms occupying the Fe1 site; the images were not clearly observed in the Ge Kα XFH that was previously reported because of the low occupancy of Fe and Ge atoms. In this study, EXAFS and XFH clearly showed the local structure around the Te atom; in particular, the formation of Te trimers caused by charge-ordered phase transitions was clearly confirmed. The charge-ordered phase transition is fully discussed based on the structural variation at low temperatures, as established from EXAFS and XFH.

Fabrication of high performance/highly functional field-effect transistor devices based on [6]phenacene thin films.

Field-effect transistors (FETs) based on [6]phenacene thin films were fabricated with SiO2 and parylene gate dielectrics. These FET devices exhibit field-effect mobility in the saturation regime as high as 7.4 cm(2) V(-1) s(-1), which is one of the highest reported values for organic thin-film FETs. The two- and four-probe mobilities in the linear regime display nearly similar values, suggesting negligible contact resistance at 300 K. FET characteristics were investigated using two-probe and four-probe measurement modes at 50-300 K. The two-probe mobility of the saturation regime can be explained by the multiple shallow trap and release model, while the intrinsic mobility obtained by the four-probe measurement in the linear regime is better explained by the phenomenon of transport with charge carrier scattering at low temperatures. The FET device fabricated with a parylene gate dielectric on polyethylene terephthalate possesses both transparency and flexibility, implying feasibility of practical application of [6]phenacene FETs in flexible/transparent electronics. N-channel FET characteristics were also achieved in the [6]phenacene thin-film FETs using metals that possess a small work function for use as source/drain electrodes.

Semiconductor-metal transition in Bi2Se3 caused by impurity doping.

Doping a typical topological insulator, Bi2Se3, with Ag impurity causes a semiconductor-metal (S-M) transition at 35 K. To deepen the understanding of this phenomenon, structural and transport properties of Ag-doped Bi2Se3 were studied. Single-crystal X-ray diffraction (SC-XRD) showed no structural transitions but slight shrinkage of the lattice, indicating no structural origin of the transition. To better understand electronic properties of Ag-doped Bi2Se3, extended analyses of Hall effect and electric-field effect were carried out. Hall effect measurements revealed that the reduction of resistance was accompanied by increases in not only carrier density but carrier mobility. The field-effect mobility is different for positive and negative gate voltages, indicating that the EF is located at around the bottom of the bulk conduction band (BCB) and that the carrier mobility in the bulk is larger than that at the bottom surface at all temperatures. The pinning of the EF at the BCB is found to be a key issue to induce the S-M transition, because the transition can be caused by depinning of the EF or the crossover between the bulk and the top surface transport.

Accessing surface Brillouin zone and band structure of picene single crystals.

We have experimentally revealed the band structure and the surface Brillouin zone of insulating picene single crystals (SCs), the mother organic system for a recently discovered aromatic superconductor, with ultraviolet photoelectron spectroscopy (UPS) and low-energy electron diffraction with a laser for photoconduction. A hole effective mass of 2.24m(0) and the hole mobility µ(h)≥9.0 cm(2)/V s (298 K) were deduced in the Γ-Y direction. We have further shown that some picene SCs did not show charging during UPS even without the laser, which indicates that pristine UPS works for high-quality organic SCs.

Evaluation of Effective Field-Effect Mobility in Thin-Film and Single-Crystal Transistors for Revisiting Various Phenacene-Type Molecules.

The magnitude of the field-effect mobility µ of organic thin-film and single-crystal field-effect transistors (FETs) has been overestimated in certain recent studies. These reports set alarm bells ringing in the research field of organic electronics. Herein, we report a precise evaluation of the µ values using the effective field-effect mobility, µeff, a new indicator that is recently designed to prevent the FET performance of thin-film and single-crystal FETs based on various phenacene molecules from being overestimated. The transfer curves of a range of FETs based on phenacene are carefully categorized on the basis of a previous report. The exact evaluation of the value of µeff depends on the exact classification of each transfer curve. The transfer curves of all our phenacene FETs could be successfully classified based on the method indicated in the aforementioned report, which made it possible to evaluate the exact value of µeff for each FET. The FET performance based on the values of µeff obtained in this study is discussed in detail. In particular, the µeff values of single-crystal FETs are almost consistent with the µ values that were reported previously, but the µeff values of thin-film FETs were much lower than those previously reported for µ, owing to a high absolute threshold voltage, |V th|. The increase in the field-effect mobility as a function of the number of benzene rings, which was previously demonstrated based on the µ values of single-crystal FETs with phenacene molecules, is well reproduced from the µeff values. The FET performance is discussed based on the newly evaluated µeff values, and the future prospects of using phenacene molecules in FET devices are demonstrated.

Strong intramolecular electron-phonon coupling in the negatively charged aromatic superconductor picene.

Superconductivity was recently discovered in solid potassium-intercalated picene (K(3)22ph), in which the picene molecule becomes trianionic (22ph(3-)). In this Letter, we conduct a theory-based study of the superconductivity of 22ph(3-) within the framework of BCS theory. We estimate the density of states N(ε(F)) on the Fermi level to be 2.2 states per (eV molecule spin) by using the theoretical intramolecular electron-phonon coupling l(x) and the experimental superconducting transition temperature T(c) of 18 K. The theoretical value is consistent with the 1.2 states per (eV molecule spin) determined experimentally for K(3)22ph with T(c)=18 K, indicating the validity of our theoretical treatment and the electron-phonon mechanism for superconductivity. The predicted l(x), 0.206 eV, for 22ph(3-) is larger than any value reported for organic superconductors, so picene may have the largest l(x) among the superconductors reported so far.