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.

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.

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.

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.

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, T_c, 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 T_c of Bi₂Rh₃Se₂ shows a slow increase in T_c at 0-15.5 GPa, and the T_c slowly decreases with pressure above 15.5 GPa, which is markedly different from that of normal superconductors because the value of T_c should simply decrease owing to the decrease in density of states (DOS) on the Fermi level, N(ε_F), driven by a simple shrinkage of the lattice under pressure. To ascertain the origin of such a dome-like T_c-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 T_c 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 T_c had been suppressed by the CDW transition in a low pressure range. Thus, the findings suggest that for Bi₂Rh₃Se₂, T_c 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 T_c-p behavior indicates the possibility of Bi₂Rh₃Se₂ being an exotic superconductor.

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, T_c, of 3.50 and 4.21 K at ambient pressure, respectively. The onset superconducting transition temperature, T_c^onset, 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 T_c^onset was observed for KBi₂ at 9 GPa, which reflects the decomposition of KBi₂ into KBi and Bi. By contrast, the value of T_c^onset 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 T_c 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 T_c and verified the chemical stability of KBi₂ under pressure.

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, μ_eff, 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 μ_eff 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 μ_eff for each FET. The FET performance based on the values of μ_eff obtained in this study is discussed in detail. In particular, the μ_eff values of single-crystal FETs are almost consistent with the μ values that were reported previously, but the μ_eff 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 μ_eff values. The FET performance is discussed based on the newly evaluated μ_eff values, and the future prospects of using phenacene molecules in FET devices are demonstrated.

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, T_c, showed a slight decrease (or almost constant T_c) against pressure up to 17.2 GPa. The values of the upper critical field, H_c2, 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 β 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 T_cversus p and V versus p of BaBi₃ are depicted over a wide pressure range for the first time.

Superconductivity of topological insulator Sb2Te3-ySeyunder pressure

The crystal structures of Sb₂Te_3-ySe_y(y= 0.6 andy= 1.2) at 0-24 GPa were investigated by synchrotron x-ray diffraction. The stoichiometry of Sb₂Te_3-ySe_yused in this study was determined to be Sb₂Te_2.19(9)Se_0.7(2)fory= 0.6 and Sb₂Te_1.7(1)Se_1.3(3)fory= 1.2, on the basis of energy-dispersive x-ray spectroscopy. The sample of Sb₂Te_2.19(9)Se_0.7(2)showed a structural phase transition from a rhombohedral structure (space group No. 166,R3¯m) (phase I) to a monoclinic structure (space group No. 12,C2/m) (phase II), with increasing pressure up to ∼9 GPa. A new structural phase (phase II') emerged at 17.7 GPa, a monoclinic structure with the space groupC2/c(No. 15). Finally, a 9/10-fold monoclinic structure (space group No. 12,C2/m) (phase III) was observed at 21.8 GPa. In contrast, the sample of Sb₂Te_1.7(1)Se_1.3(3)provided only phase I (space group No. 166,R3¯m) and phase II (space group No. 12,C2/m), showing one structural phase transition from 0-19.5 GPa. These samples were not superconductors at ambient pressure, but superconductivity suddenly appeared with increasing pressure. Superconductivity with superconducting transition temperatures (T_c's) of 2 and 4 K was observed above 6 GPa in phase I of Sb₂Te_2.19(9)Se_0.7(2). In this sample, theT_cvalues of 6 and 9 K were observed in phase II and phase II' or III of Sb₂Te_2.19(9)Se_0.7(2), respectively. Superconductivity withT_c's of 4 and 5 K suddenly emerged in Sb₂Te_1.7(1)Se_1.3(3)at 13.6 GPa, which corresponds to phase II, and it evolved to 6.0 K under further increased pressure. AT_cvalue of 9 K was finally found above 15 GPa. The magnetic field dependence ofT_cin phase II of Sb₂Te_2.19(9)Se_0.7(2)and Sb₂Te_1.7(1)Se_1.3(3)followed ap-wave polar model, suggesting topologically nontrivial superconductivity.

Photochemical synthesis and device application of acene-phenacene hybrid molecules, dibenzo[n]phenacenes (n = 5-7)

Dibenzo[n]phenecenes (DBnPs, n = 5-7) were conveniently synthesised through Mallory photocyclization as the key step. Effective mobilities of single-crystal field-effect transistors of DBnPs were evaluated to demonstrate that C2h-symmetrical DB6P shows higher performance than C2v-symmetrical DB5P and DB7P.

Balanced Ambipolar Charge Transport in Phenacene/Perylene Heterojunction-Based Organic Field-Effect Transistors

Electronic devices relying on the combination of different conjugated organic materials are considerably appealing for their potential use in many applications such as photovoltaics, light emission, and digital/analog circuitry. In this study, the electrical response of field-effect transistors achieved through the evaporation of picene and PDIF-CN₂ molecules, two well-known organic semiconductors with remarkable charge transport properties, was investigated. With the main goal to get a balanced ambipolar response, various device configurations bearing double-layer, triple-layer, and codeposited active channels were analyzed. The most suitable choices for the layer deposition processes, the related characteristic parameters, and the electrode position were identified to this purpose. In this way, ambipolar organic field-effect transistors exhibiting balanced mobility values exceeding 0.1 cm² V^-1 s^-1 for both electrons and holes were obtained. These experimental results highlight also how the combination between picene and PDIF-CN₂ layers allows tuning the threshold voltages of the p-type response. Scanning Kelvin probe microscopy (SKPM) images acquired on picene/PDIF-CN₂ heterojunctions suggest the presence of an interface dipole between the two organic layers. This feature is related to the partial accumulation of space charge at the interface being enhanced when the electrons are depleted in the underlayer.

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

We investigated the pressure dependence of electric transport in a superconducting sample, Ba_0.77Na_0.23Ti₂Sb₂O, to complete the phase diagram of superconducting transition temperature (T_c) against pressure (p). This superconducting sample exhibits a T_c value of 5.8 K at ambient pressure. Here, the superconductivity of the recently reported sample was investigated over a wide pressure range. The T_c value monotonously decreased with pressure below 8 GPa. Interestingly, the T_c 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 Ba_0.77Na_0.23Ti₂Sb₂O 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 T_c value rapidly increased against pressure. This study provides detailed information on the superconductivity of Ba_0.77Na_0.23Ti₂Sb₂O under pressure, which will lead to a comprehensive understanding of pressure-driven superconductivity.

Superconductivity in topological insulator-PdBi2under pressure

The topological insulator PdBi2exhibits two different crystal phases at ambient pressure, i.e., 'α-PdBi2' and ' -PdBi2'. The pressure dependence of crystal structure and superconductivity of α-PdBi2has been fully elucidated thus far. However, the physical properties of β-PdBi2crystals under pressure have not been sufficiently investigated. In this study, we fully investigate the crystal structure and superconductivity of β-PdBi2under pressure based on synchrotron x-ray diffraction (XRD) patterns. The temperature dependence of β-PdBi2indicates its superconductivity with a superconducting transition temperature (Tc) as high as 4.10 K, and its crystal structure is tetragonal [space group ofI4/mmm(no. 139)]. The XRD patterns at 0-22.0 GPa indicate no structural phase transitions, and the unit cell volume shrinks monotonically with pressure, unlike the behavior of α-PdBi2. Furthermore, α-PdBi2transformed to β-PdBi2under pressure. This suggests that β-PdBi2is stable under pressure. The superconductivity is clearly observed at 0-11.8 GPa, and the value ofTcis almost constant at ∼4.4 K. The temperature dependence of the upper critical field at ambient pressure and 10.7 GPa indicates that the superconductivity is not attributed to a simples-wave dirty limit but ans-wave clean orp-wave polar model. This is the first systematic study of superconductivity of topological insulator β-PdBi2under pressure.

Fabrication of ring oscillators using organic molecules of phenacene and perylenedicarboximide

Organic field-effect transistors (FETs) can be applied to radio-frequency identification tags (RFIDs) and active-matrix flat-panel displays. For RFID application, a cardinal functional block is a ring oscillator using an odd number of inverters to convert DC voltage to AC. Herein, we report the properties of two ring oscillators, one formed with [6]phenacene for a p-channel FET and N,N′-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8) for an n-channel FET, and one formed with 3,10-ditetradecylpicene ((C₁₄H₂₉)₂-picene) for a p-channel FET and PTCDI-C8 for an n-channel FET. The former ring oscillator provided a maximum oscillation frequency, f_osc of 26 Hz, and the latter a maximum f_osc of 21 Hz. The drain–drain voltage, V_DD, applied to these ring oscillators was 100 V. This may be the first step towards a future practical ring oscillator using phenacene molecules. The values of field-effect mobility, μ in the p-channel [6]phenacene FET and n-channel PTCDI-C8 FET, which form the building blocks in the ring oscillator with an f_osc value of 26 Hz, are 1.19 and 1.50 × 10⁻¹ cm² V⁻¹ s⁻¹, respectively, while the values in the p-channel (C₁₄H₂₉)₂-picene FET and n-channel PTCDI-C8 FET, which form the ring oscillator with an f_osc of 21 Hz, are 1.85 and 1.54 × 10⁻¹ cm² V⁻¹ s⁻¹, respectively. The μ values in the p-channel FETs are higher by one order of magnitude than those of the n-channel FET, which must be addressed to increase the value of f_osc. Finally, we fabricated a ring oscillator with ZrO₂ instead of parylene for the gate dielectric, which provided the low-voltage operation of the ring oscillator, in which [6]phenacene and PTCDI-C8 thin-film FETs were employed. The value of f_osc obtained in the ring oscillator was 24 Hz. In this ring oscillator, the V_DD value applied was limited to 20 V. The durability of the ring oscillators was also investigated, and the bias stress effect on the f_osc and the amplitude of the output voltage, V_out are discussed. This successful operation of ring oscillators represents an important step towards the realization of future practical integrated logic gate circuits using phenacene molecules.

A ring oscillator consisting of p-channel and n-channel organic FETs.

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.

Superconductivity in Bi2-xSbxTe3-ySey(x= 1.0 andy= 2.0) under pressure

The crystal structure of BiSbTeSe2(Bi2-xSbxTe3-ySey(x= 1.0 andy= 2.0)) at 0-29 GPa is investigated through synchrotron x-ray diffraction (XRD) and two structural phase transitions are discovered. The stoichiometry of BiSbTeSe2employed in this study is Bi1.19(4)Sb0.81(4)Te0.83(4)Se2.17(4), as determined from energy-dispersive x-ray spectroscopy. The sample demonstrated structural transitions, from a rhombohedral structure (space group no 166,R3¯m) (phase I) to a monoclinic structure (space group no 12,C2/m) (phase II), and from phase II to a 9/10-fold monoclinic structure (space group no 12,C2/m) (phase III). The temperature dependence of resistance (R-Tplot) exhibited a semiconducting behavior in a low pressure range and changed from semiconducting to metallic behavior with increasing pressure. Pressure-driven superconductivity is observed above 9.1 GPa in Bi1.19(4)Sb0.81(4)Te0.83(4)Se2.17(4). The pressure phase corresponds to phase II. The superconducting transition temperature,Tc, increased with pressure. The maximumTcvalue is 8.3 K at 19.1 GPa. The magnetic field dependence ofTcin phase II of Bi1.19(4)Sb0.81(4)Te0.83(4)Se2.17(4)is proceeded by ap-wave polar model, indicating topologically nontrivial superconductivity. In addition, the emergence of superconductivity and the change in superconducting behavior are closely associated with the structural transitions.

Structure and superconducting properties of multiple phases of (NH3)yAExFeSe (AE: Ca, Sr and Ba)

We synthesized the alkaline-earth metal-doped FeSe compounds (NH3)yAExFeSe (AE: Ca, Sr and Ba), using the liquid NH3technique, to determine their superconducting properties and crystal structures. Multiple superconducting phases were obtained in each sample of (NH3)yCaxFeSe and (NH3)yBaxFeSe, which showed two superconducting transition temperatures (Tc's) as high as 37-39 K and 47-48 K at ambient pressure, hereinafter referred to as the 'low-Tcphase' and 'high-Tcphase', respectively. The high-Tcphases in (NH3)yCaxFeSe and (NH3)yBaxFeSe were metastable, and rapidly converted to their low-Tcphases. However,Tcvalues of 38.4 K and 35.6 K were recorded for (NH3)ySrxFeSe, which displayed different behavior than (NH3)yCaxFeSe and (NH3)yBaxFeSe. The Le Bail fitting of x-ray diffraction (XRD) patterns provided lattice constants ofc= 16.899(1) Å andc= 16.8630(8) Å for the low-Tcphases of (NH3)yCaxFeSe and (NH3)yBaxFeSe, respectively. The lattice constants of their high-Tcphases could not be determined due to the disappearance of the highTcphase within a few days. The XRD pattern for (NH3)ySrxFeSe indicated the coexistence of two phases withc= 16.899(3) Å andc= 15.895(4) Å. The former value ofcin (NH3)ySrxFeSe is almost the same as those of the low-Tcphases in (NH3)yCaxFeSe and (NH3)yBaxFeSe. Therefore, the phase withc= 16.899(3) Å in (NH3)ySrxFeSe must correspond to the superconducting phase with theTcof 38.4 K, while the superconducting phase withTc= 35.6 K is assigned to the crystal phase withc= 15.895(4) Å. For (NH3)ySrxFeSe, a high-Tcphase withTc= 47-48 K has not yet been obtained, but a new phase showing theTcvalue of 35.6 K was clearly obtained. This is the first systematic study of the preparation, crystal structure, and superconductivity of alkaline-earth metal-doped FeSe, (NH3)yAExFeSe.

Superconductivity in 5d transition metal Laves phase SrIr2

We report here the superconducting properties of a Laves phase superconductor SrIr₂, which has a cubic MgCu₂ structure. SrIr₂ is a type-II superconductor, with a T _c of 5.9 K. The estimated superconducting parameters of lower critical field µ ₀ H _c1 and upper critical field µ ₀ H _c2, coherence length ξ(0), penetration depth λ(0) and Ginzburg-Landau (GL) parameter κ(0) are approximately µ ₀ H _c1  =  101 Oe, µ ₀ H _c2(0)  =  5.9 T, ξ(0)  =  7.47 nm, λ(0)  =  237 nm, and κ(0)  =  31.7, respectively. The specific-heat data indicate that SrIr₂ is a strong-coupling superconductor because the value of ΔC/γT _c is approximately 1.71, which is larger than the value of 1.43 that is expected from the BCS theory. The physical properties obtained in this study are explained well by theoretical calculations including spin-orbit coupling (SOC). This result indicates that the physical properties of SrIr₂ are strongly affected by the presence of SOC.

Superconducting behavior of a new metal iridate compound, SrIr2, under pressure

Herein, we investigated the pressure dependence of electric transport in a new type of superconducting metal iridate compound, SrIr₂, that exhibits a superconducting transition temperature, T _c, as high as 6.6 K at ambient pressure, in order to complete the T _c-pressure (p ) phase diagram. Very recently, this sample's superconductivity was discovered by our group, but the superconducting behavior has not yet been clarified under pressure. In this study, we fully investigated this sample's superconductivity in a wide pressure range. The T _c value decreased with an increase in pressure, but the onset superconducting transition temperature, [Formula: see text], increased above a pressure of 8 GPa, indicating an unconventional superconductivity different from a BCS-type superconductor. The magnetic field dependence of electric resistance (R) against temperature (R  -  T plot) recorded at 7.94 and 11.3 GPa suggested an unconventional superconductivity, followed by a p -wave polar model, supporting the deviation from a simple s-wave pairing. Moreover, we fully investigated the pressure dependence of crystal structure in SrIr₂ and discussed the correlation between superconductivity and crystal structure. This is the first systematic study on superconducting behavior of a new type of metal iridate compound, MIr₂ (M: alkali-earth metal atom), under pressure.

Facile synthesis of picenes incorporating imide moieties at both edges of the molecule and their application to n-channel field-effect transistors

Picene derivatives incorporating imide moieties along the long-axis direction of the picene core (C_n-PicDIs) were conveniently synthesized through a four-step synthesis. Photochemical cyclization of dinaphthylethenes was used as the key step for constructing the picene skeleton. Field-effect transistor (FET) devices of C_n-PicDIs were fabricated by using ZrO₂ as a gate substrate and their FET characteristics were investigated. The FET devices showed normally-off n-channel operation; the averaged electron mobility (μ) was evaluated to be 2(1) × 10⁻⁴, 1.0(6) × 10⁻¹ and 1.4(3) × 10⁻² cm² V⁻¹ s⁻¹ for C₄-PicDI, C₈-PicDI and C₁₂-PicDI, respectively. The maximum μ value as high as 2.0 × 10⁻¹ cm² V⁻¹ s⁻¹ was observed for C₈-PicDI. The electronic spectra of C_n-PicDIs in solution showed the same profiles irrespective of the alkyl chain lengths. In contrast, in thin films, the UV absorption and photoelectron yield spectroscopy (PYS) indicated that the lowest unoccupied molecular orbital (LUMO) level of C_n-PicDIs gradually lowered upon the elongation of the alkyl chains, suggesting that the alkyl chains modify intermolecular interactions between the C_n-PicDI molecules in thin films. The present results provide a new strategy for constructing a high performance n-channel organic semiconductor material by utilizing the electronic features of phenacenes.

Picenediimide derivatives serve as the active layer of n-channel thin-film field-effect transistors displaying a maximum charge carrier mobility as high as 2.0 × 10⁻¹ cm² V⁻¹ s⁻¹.

A new protocol for the preparation of superconducting KBi2

A superconducting KBi₂ sample was successfully prepared using a liquid ammonia (NH₃) technique. The temperature dependence of the magnetic susceptibility (M/H) showed a superconducting transition temperature (T_c) as high as 3.6 K. In addition, the shielding fraction at 2.0 K was evaluated to be 87%, i.e., a bulk superconductor was realized using the above method. The T_c value was the same as that reported for the KBi₂ sample prepared using a high-temperature annealing method. An X-ray diffraction pattern measured based on the synchrotron X-ray radiation was analyzed using the Rietveld method, with a lattice constant, a, of 9.5010(1) Å under the space group of Fd3̄m (face-centered cubic, no. 227). The lattice constant and space group found for the KBi₂ sample using a liquid NH₃ technique were the same as those reported for KBi₂ through a high-temperature annealing method. Thus, the superconducting behavior and crystal structure of the KBi₂ sample obtained in this study are almost the same as those for the KBi₂ sample reported previously. Strictly speaking, the magnetic behavior of the superconductivity was different from that of a KBi₂ sample reported previously, i.e., the KBi₂ sample prepared using a liquid NH₃ technique was a type-II like superconductor, contrary to that prepared using a high-temperature annealing method, the reason for which is fully discussed. These results indicate that the liquid NH₃ technique is effective and simple for the preparation of a superconducting KBi₂. In addition, the topological nature of the superconductivity for KBi₂ was not confirmed.

A superconducting KBi₂ sample was successfully prepared using a liquid ammonia (NH₃) technique.

Synthesis of the extended phenacene molecules, [10]phenacene and [11]phenacene, and their performance in a field-effect transistor

The [10]phenacene and [11]phenacene molecules have been synthesized using a simple repetition of Wittig reactions followed by photocyclization. Sufficient amounts of [10]phenacene and [11]phenacene were obtained, and thin-film FETs using these molecules have been fabricated with SiO₂ and ionic liquid gate dielectrics. These FETs operated in p-channel. The averaged measurements of field-effect mobility, <μ>, were 3.1(7) × 10⁻² and 1.11(4) × 10⁻¹ cm² V⁻¹ s⁻¹, respectively, for [10]phenacene and [11]phenacene thin-film FETs with SiO₂ gate dielectrics. Furthermore, [10]phenacene and [11]phenacene thin-film electric-double-layer (EDL) FETs with ionic liquid showed low-voltage p-channel FET properties, with <μ> values of 3(1) and 1(1) cm² V⁻¹ s⁻¹, respectively. This study also discusses the future utility of the extremely extended π-network molecules [10]phenacene and [11]phenacene as the active layer of FET devices, based on the experimental results obtained.

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.

Difference in gating and doping effects on the band gap in bilayer graphene

A band gap is opened in bilayer graphene (BLG) by applying an electric field perpendicular to the layer, which offers versatility and controllability in graphene-based electronics. The presence of the band gap has been confirmed using double-gated BLG devices in which positive and negative gate voltages are applied to each side of BLG. An alternative method to induce the electric field is electron and hole doping of each side of BLG using electron-transfer adsorbates. However, the generation of the band gap by carrier doping is still under investigation. Here, we determined whether the electron/hole doping can produce the electric field required to open the band gap by measuring the temperature dependence of conductivity for BLG placed between electron-donor self-assembled monolayers (SAMs) and electron-acceptor molecules. We found that some devices exhibited a band gap and others did not. The potentially irregular and variable structure of SAMs may affect the configuration of the electric field, yielding variable electronic properties. This study demonstrates the essential differences between gating and doping.

Preparation and characterization of a new graphite superconductor: Ca0.5Sr0.5C6

We have produced a superconducting binary-elements intercalated graphite, CaxSr1-xCy, with the intercalation of Sr and Ca in highly-oriented pyrolytic graphite; the superconducting transition temperature, T c, was ~3 K. The superconducting CaxSr1-xCy sample was fabricated with the nominal x value of 0.8, i.e., Ca0.8Sr0.2Cy. Energy dispersive X-ray (EDX) spectroscopy provided the stoichiometry of Ca0.5(2)Sr0.5(2)Cy for this sample, and the X-ray powder diffraction (XRD) pattern showed that Ca0.5(2)Sr0.5(2)Cy took the SrC6-type hexagonal-structure rather than CaC6-type rhombohedral-structure. Consequently, the chemical formula of CaxSr1-xCy sample could be expressed as 'Ca0.5(2)Sr0.5(2)C6'. The XRD pattern of Ca0.5(2)Sr0.5(2)C6 was measured at 0-31 GPa, showing that the lattice shrank monotonically with increasing pressure up to 8.6 GPa, with the structural phase transition occurring above 8.6 GPa. The pressure dependence of T c was determined from the DC magnetic susceptibility and resistance up to 15 GPa, which exhibited a positive pressure dependence of T c up to 8.3 GPa, as in YbC6, SrC6, KC8, CaC6 and Ca0.6K0.4C8. The further application of pressure caused the rapid decrease of T c. In this study, the fabrication and superconducting properties of new binary-elements intercalated graphite, CaxSr1-xCy, are fully investigated, and suitable combinations of elements are suggested for binary-elements intercalated graphite.

Chemical analysis of superconducting phase in K-doped picene

Potassium-doped picene (K3.0picene) with a superconducting transition temperature (T C) as high as 14 K at ambient pressure has been prepared using an annealing technique. The shielding fraction of this sample was 5.4% at 0 GPa. The T C showed a positive pressure-dependence and reached 19 K at 1.13 GPa. The shielding fraction also reached 18.5%. To investigate the chemical composition and the state of the picene skeleton in the superconducting sample, we used energy-dispersive x-ray (EDX) spectroscopy, MALDI-time-of-flight (MALDI-TOF) mass spectroscopy and x-ray diffraction (XRD). Both EDX and MALDI-TOF indicated no contamination with materials other than K-doped picene or K-doped picene fragments, and supported the preservation of the picene skeleton. However, it was also found that a magnetic K-doped picene sample consisted mainly of picene fragments or K-doped picene fragments. Thus, removal of the component contributing the magnetic quality to a superconducting sample should enhance the volume fraction.

Recent progress on carbon-based superconductors

This article reviews new superconducting phases of carbon-based materials. During the past decade, new carbon-based superconductors have been extensively developed through the use of intercalation chemistry, electrostatic carrier doping, and surface-proving techniques. The superconducting transition temperature T c of these materials has been rapidly elevated, and the variety of superconductors has been increased. This review fully introduces graphite, graphene, and hydrocarbon superconductors and future perspectives of high-T c superconductors based on these materials, including present problems. Carbon-based superconductors show various types of interesting behavior, such as a positive pressure dependence of T c. At present, experimental information on superconductors is still insufficient, and theoretical treatment is also incomplete. In particular, experimental results are still lacking for graphene and hydrocarbon superconductors. Therefore, it is very important to review experimental results in detail and introduce theoretical approaches, for the sake of advances in condensed matter physics. Furthermore, the recent experimental results on hydrocarbon superconductors obtained by our group are also included in this article. Consequently, this review article may provide a hint to designing new carbon-based superconductors exhibiting higher T c and interesting physical features.

Emergence of double-dome superconductivity in ammoniated metal-doped FeSe

The pressure dependence of the superconducting transition temperature (T_c) and unit cell metrics of tetragonal (NH₃)_yCs_0.4FeSe were investigated in high pressures up to 41 GPa. The T_c decreases with increasing pressure up to 13 GPa, which can be clearly correlated with the pressure dependence of c (or FeSe layer spacing). The T_cvs. c plot is compared with those of various (NH₃)_yM_xFeSe (M: metal atoms) materials exhibiting different T_c and c, showing that the T_c is universally related to c. This behaviour means that a decrease in two-dimensionality lowers the T_c. No superconductivity was observed down to 4.3 K in (NH₃)_yCs_0.4FeSe at 11 and 13 GPa. Surprisingly, superconductivity re-appeared rapidly above 13 GPa, with the T_c reaching 49 K at 21 GPa. The appearance of a new superconducting phase is not accompanied by a structural transition, as evidenced by pressure-dependent XRD. Furthermore, T_c slowly decreased with increasing pressure above 21 GPa, and at 41 GPa superconductivity disappeared entirely at temperatures above 4.9 K. The observation of a double-dome superconducting phase may provide a hint for pursuing the superconducting coupling-mechanism of ammoniated/non-ammoniated metal-doped FeSe.

An EXAFS Investigation of Local Structure around Rb+ in Aqueous Solution

EXAFS spectra have been measured in order to elucidate the local structure around Rb+ in aqueous solution. It has been found that the Rb+ is surrounded by ca. six O atoms of H2O molecules. The Rb -O distance was determined to be 2.90 (3) Å. The coordination number and Rb -O distance compare well with those of the other alkaline metal ions estimated by X-ray diffraction.

EXAFS Study on the Phase Transition (Phase α’-δ) in CH3NH3I

The local structure of CH3NH3I around the I- ion in Phase α' and δ was studied by iodine K-edge EXAFS. The crystal structure in Phase α' was redetermined by single crystal X-ray diffraction. The Debye-Waller factor σ(2) derived from EXAFS spectra shows an anomalous behavior around the order-disorder transition temperature from Phase δ to α'. The phase transition has been interpreted as the disordering process which involves not only the H atoms but also the C atoms in the cation.

Transistor application of alkyl-substituted picene

Field-effect transistors (FETs) were fabricated with a thin film of 3,10-ditetradecylpicene, picene-(C₁₄H₂₉)₂, formed using either a thermal deposition or a deposition from solution (solution process). All FETs showed p-channel normally-off characteristics. The field-effect mobility, μ, in a picene-(C₁₄H₂₉)₂ thin-film FET with PbZr_0.52Ti_0.48O₃ (PZT) gate dielectric reached ~21 cm² V⁻¹ s⁻¹, which is the highest μ value recorded for organic thin-film FETs; the average μ value (<μ>) evaluated from twelve FET devices was 14(4) cm² V⁻¹ s⁻¹. The <μ> values for picene-(C₁₄H₂₉)₂ thin-film FETs with other gate dielectrics such as SiO₂, Ta₂O₅, ZrO₂ and HfO₂ were greater than 5 cm² V⁻¹ s⁻¹, and the lowest absolute threshold voltage, |V_th|, (5.2 V) was recorded with a PZT gate dielectric; the average |V_th| for PZT gate dielectric is 7(1) V. The solution-processed picene-(C₁₄H₂₉)₂ FET was also fabricated with an SiO₂ gate dielectric, yielding μ = 3.4 × 10⁻² cm² V⁻¹ s⁻¹. These results verify the effectiveness of picene-(C₁₄H₂₉)₂ for electronics applications.

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.

Antiferromagnetic resonance in the Mott insulator fcc-Cs3C60

The magnetic ground state of the fcc phase of the Mott insulator Cs3C60 was studied using a low-temperature electron spin resonance technique, and antiferromagnetic resonance (AFMR) below 1.57 K was directly observed at ambient pressure. The AFMR modes for the fcc phase of Cs3C60 were investigated using a conventional two-sublattice model with uniaxial anisotropy, and the spin-flop field was determined to be 4.7 kOe at 1.57 K. The static magnetic exchange interactions and anisotropy field for fcc-Cs3C60 were also estimated.

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.

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.

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.