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Abstract The magnetism of Kitaev materials has been widely studied, but their charge properties and the coupling to other degrees of freedom are less known. Here we investigate the charge states of α-RuCl3, a promising Kitaev quantum spin liquid candidate, in proximity to graphite. We discover that few-layered α-RuCl3 experiences a clear modulation of charge states, where a Mott-insulator to weak charge-transfer-insulator transition in the 2D limit occurs by means of heterointerfacial polarization. More notably, distinct signals of incommensurate charge and lattice super-modulations, regarded as an unconventional charge order, accompanied in the insulator. Our theoretical calculations have reproduced the incommensurate charge order by taking into account the antiferroelectricity of α-RuCl3 that is driven by dipole order in the internal electric fields. The findings imply that there is strong coupling between the charge, spin, and lattice degrees of freedom in layered α-RuCl3 in the heterostructure, which offers an opportunity to electrically access and tune its magnetic interactions inside the Kitaev compounds.

Abstract Thermoelectric generators held great promise through energy harvesting from waste heat. Their practical application, however, is greatly constrained by poor raw material utilization and tedious processing in fabricating desired shapes. Herein, a state‐of‐the‐art process is reported for 3D printing the half‐Heusler (Nb0.88Hf0.12FeSb) thermoelectric material using laser powder bed fusion (LPBF). The multi‐dimensional intra‐ and inter‐granular defects created by this process greatly suppress thermal conductivity by providing numerous phonon scattering centers. The resulting LPBF‐fabricated half‐Heusler exhibits a high figure of merit ≈1.2 at 923 K and a single‐leg maximum efficiency of ≈3.3% at a temperature difference (ΔT) of 371 K. Hafnium oxide nanoparticles generated during LPBF effectively prevent crack propagation, ensuring competent mechanical performance and reliable thermoelectric output. The findings highlight the significant potential of LPBF in driving the next industrial revolution of highly efficient and customizable thermoelectric materials.

Recently, there has been a growing interest in two-dimensional van der Waals (vdW) magnets owing to their unique two-dimensional magnetic phenomena and potential applications. Most vdW ferromagnets have the Curie temperature below room temperature, highlighting the need to explore how to enhance their magnetism. In our previous report, we successfully increased the Curie temperature of the prototypical vdW magnet Cr2Ge2Te6 using a NiO overlayer. In layered materials, the presence of wrinkles is often observed and evaluating them using optical microscopy proves to be useful; however, there have been limited investigations into the optical constants of vdW magnets, hampering progress in understanding their optical properties. In this study, we present the optical constants of Cr2Ge2Te6 obtained through ellipsometry measurements. To account for the presence of wrinkles, we model a vacuum region between the substrate and the vdW magnet, and we calculate the reflectivity as a function of wavelength and vacuum thickness to visualize the optical image. Furthermore, we discuss the relationship between the optical constants and the electronic structure of the material.

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.

Dirac cone surface states rectify an ultralow dissipative spin and charge current, but it is yet to be confirmed in devices. Here, Tuet al. observe p-type electrical transport on one half surface and n-type on the other in Bi2−xSbxTe3−ySeythin films, realizing a topological p-n junction.

Low-temperature specific heat (SH) is measured on the postannealed Ba(Fe1−xNix)2As2 single crystal with x = 0.058 under different magnetic fields. The sample locates on the overdoped sides and the critical transition temperature Tc 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 term emerges 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.

In order to clarify electronic states and energy dissipations due to a motion of a vortex core in pure FeSe, which is a candidate superconductor possessing a super-clean core, we measured the microwave surface impedance of pure FeSe single crystals under finite magnetic fields. From the magnetic-field dependence of the flux-flow resistivity, we found that a barometer of electronic states inside the vortex core ω0τcore is 1 ± 0.5, suggesting that the vortex core of pure FeSe is in the moderately clean regime contrary to the expectation of the super-clean core. We also found that the mean-free path inside the vortex core is suppressed at the distance of the order of the core radius. Based on observed results and previous reports, we discussed possible origins of rather small ω0τcore value in terms of the multiple-bands nature of FeSe and additional mechanisms producing extra energy dissipations specific to the vortex core in motion. [ABSTRACT FROM AUTHOR]

Extensive soft x-ray photoelectron spectroscopy studies are performed on Ba8 Ga16 Ge30 (BGG) and Sr8Ga16Ge30 (SGG) single crystals ranging from Fermi to core levels, at a high-energy facility. Valence band x-ray photoelectron spectroscopy (XPS) experiments with theoretical calculations revealed that the valence band is mainly constructed by the Ge/Ga 4s and 4p wave functions with little contribution of the Ba/Sr atomic orbitals. Surprisingly, unexpected features evidencing the different shift for the 2a- and 6d- sites between Ba 4d and Sr 3d are observed. The detailed analyses including theoretical support by first-principles band-structure calculations lead to the conclusion that the component distributions of the larger tetrakaidecahedral cage are different depending on the endohedral atoms, which contrasts with the past consensus that BGG and SGG have the same framework structure. This may give thorough reconsiderations on earlier interpretations of experimental data.

Charge transfer (CT) at metal-graphene contacts induces a potential variation from the contact edges that extend to ~1 µm. Potential variations with a similar length should be observed around charge-transferring surface adsorbates. Thus, it is expected that a competition exists between these two CT sources when one source is within ~1 µm from the other. In this letter, weakly coupled Ni contacts and 7,7,8,8-tetracyanoquinodimethan molecules are employed as the CT sources to investigate their possible competition. The CT from the molecules adsorbed only in the channel region change the charge density of the graphene in the under-contact regions. The extent of the CT effect in the under-contact region is as long as ~4 µm. The considerably long CT is ascribed to the high effective dielectric constant of the graphene under the contacts, resulting from a thin interfacial NiOx layer containing carbon impurities acquired from the graphene. [ABSTRACT FROM AUTHOR]

We report the growth of high quality Bi2-xSbxTe3-ySey ultrathin nanoplates (BSTS-NPs) on an electrically insulating fluorophlogopite mica substrate using a catalyst-free vapor solid method. Under an optimized pressure and suitable Ar gas flow rate, we control the thickness, the size, and the composition of the BSTS-NPs. Raman spectra showing systematic change indicate that the thicknesses and compositions of the BSTS-NPs are indeed accurately controlled. Electrical transport demonstrates a robust Dirac cone carrier transport in the BSTS-NPs. Since the BSTS-NPs provide superior dominant surface transport of the tunable Dirac cone surface states with negligible contribution of the conduction of the bulk states, the BSTS-NPs provide an ideal platform to explore intrinsic physical phenomena as well as technological applications of 3-dimensional topological insulators in the future. [ABSTRACT FROM AUTHOR]

Graphene field-effect transistors with source/drain contacts made of metals that can be easily oxidized such as ferromagnetic metals often display a double dip structure in the transfer characteristics because of charge density depinning at the contacts. Generally, transfer characteristics of field-effect transistors show no dependence on the length of the source/drain contacts because charge carrier injection occurs mainly at the edges of the contact. However, the shape of the transfer characteristics of devices fabricated using Ni contacts is found to be dependent on the length of the contact. This peculiar behavior was attributed to charge carrier injection from near the center of the contacts. This is because of oxygen diffusion and the resultant formation of an interfacial oxide layer of non-uniform thickness. The observed contact length dependent transfer characteristics were reproduced using a model calculation that includes charge carrier injection from the center of the electrode and subsequent charge transport underneath the metal contact. [ABSTRACT FROM AUTHOR]

High quality bulk samples of anthracene (AN) doped with potassium (K) in 1 : 1 and 2 : 1 stoichiometries were successfully prepared by a method involving a room temperature solid-state mechanical diffusion process prior to intercalation reactions during heat treatment, and their physical properties were studied using both magnetic and optical measurements. The transfer of almost one electron from K to AN in K1(AN) was confirmed by SQUID and ESR measurements. A pronounced magnetic hump centered at 150 K associated with antiferromagnetic interactions was observed, which can most likely be interpreted in terms of on-site Coulomb repulsions of the Mott insulating states. Optical spectra of K1(AN) clearly showed the insulating states, as well as the electron occupation of the LUMO-derived band of AN. Our results demonstrated tuning of the ground state of a typical bulk hydrocarbon by alkali metal intercalation. [ABSTRACT FROM AUTHOR]

Metal/semiconductor interfaces govern the operation of semiconductor devices through the formation of charge injection barriers that can be controlled by tuning the metal work function. However, the controlling ability is typically limited to being static. We show that a dynamic nature can be imparted to the interfaces using electrode surface modification with a structurally disordered molecular monolayer. The barrier height at the interfaces is altered significantly in a reversible way by an external electric field. As a result, a dramatic change in the carrier transport properties through the interfaces is observed, such as a reversible polarity reversion of metal/organic-semiconductor/metal diodes. [ABSTRACT FROM AUTHOR]

A small amount of defects (less than 0.01%) were introduced into graphene by irradiating it with ultraviolet (UV) light in water. The chemisorbed oxygen species caused a limited amount of degradation in the charge carrier mobility, while the physisorbed water molecules caused both a reduction in the mobility and hole doping. The oxidation was nonuniform, owing to variations in the potential caused by the metal contacts. Raman spectroscopy measurements revealed that UV irradiation in water promoted mild oxidation of graphene’s basal plane, which enhanced the electrical sensing response of the adsorption of water molecules. The enhanced electrical response was achieved by the high binding energy of the water molecules at the oxidized sites and the near-zero Dirac point voltage, easily obtained by desorbing the physisorbed water molecules. [ABSTRACT FROM AUTHOR]

Because of the large reactivity of single layer graphene to electron-transfer chemistries, 4-nitrobenzene diazonium tetrafluoroborate is employed to modify the electrical properties of graphene field-effect transistors. After modification, the transfer characteristics of chemically modified graphene show a reduction in the minimum conductivity, electron-hole mobility asymmetry, a decrease in the electron/hole mobility, and a positive shift of the charge neutrality point with broadening of the minimum conductivity region. These phenomena are attributed to a dediazoniation reaction and the adsorbates on the graphene surface. [ABSTRACT FROM AUTHOR]

Superconductivity in aromatic hydrocarbons doped with potassium (K) such as K3 [picene (PCN)] and K3 [phenanthrene (PHN)] is found for only armchair-type polycyclic aromatic hydrocarbon. In this paper the thermodynamics of the reaction processes of PHN or anthracene (AN, zigzag type) with K was studied using differential scanning calorimetry and x-ray diffraction. We show that PHN decomposes during the reaction, triggered by hydrogen abstraction, to give metal hydride KH and unknown amorphous. No stable doped phases exist in Kx(PHN) with stoichiometries of x=1-3. However, in the case of AN, a stable doped phase forms. We claim that PHN, which has been reported to be energetically more stable in the ground state than AN by first principle calculations, is unstable upon doping. We also suggest that the superconductivity in K3(PCN) is due to the misinterpretation of experimental data, which actually arises from ferromagnetic impurities. We have never detected the superconductivity above 2 K in these compounds. The superconductivity in both Kx(PHN) and Kx(PCN) is concluded to be highly questionable. [ABSTRACT FROM AUTHOR]

Among many two-dimensional (2D) high Tc superconductors, graphite intercalation compounds (GICs) are the most famous intercalation family, which are classified as typical electron-phonon mediated superconductors. We show unambiguous experimental facts that BaC6, the superconductivity of which has been missing for many years sofar among various alkaline earth metal (Ca, Sr, and Ba) intercalted GICs, exhibits superconductivity at Tc = 65 mK. By adding this finding as the additional experimental point, a complete figure displaying the relationship between Tc and interlayer distance (d) for GICs is now provided, and their possible superconducting mechanisms raised sofar are revisited. The present study settles a long-running debate between theories and experiments on the superconductivity in the first stage GICs. [ABSTRACT FROM AUTHOR]

The thermodynaqmics for intercalation of phenanthrene (PHN) with Ba, for which superconductivity has been reported, is studied in comparison with its isomer of a linear aromatic hydrocarbon of anthracene (AN). Contrary to previous reports by other authors, the important observation that Ba is intercalated into neither PHN nor AN without affecting their molecular structures is unambiguously made by differential scanning calorimetry measurements and annealing time dependences observed by powder x-ray diffraction (XRD) measurements. The reactions of Ba and PHN at elevated temperatures lead this system to molecular decomposition instead of intercalation, resulting in the BaC2 carbide or amorphous carbon formation, which is clearly supported by XRD and Raman spectroscopy. The phenomena of metallicity and superconductivity in PHN intercalated with alkaline-earth metals (Ba or Sr) should be reconsidered. [ABSTRACT FROM AUTHOR]

A systematic study on the anharmonicity of phonons is made for single-crystal type-I clathrates: n-typeBa8Ga16Ge30 (n-BGG), p-type Ba8Ga16Ge30 (p-BGG), n-type Sr8Ga16Ge30 (n-SGG), n-type K8Ga16Sn30 (n-KGSn), and n-type Ba8Ga16Sn30 (n-BGSn), based on their heat capacity Cρ at low temperatures (I) down to360 rnK. The low-T linear terms obsγT of Cρ, including the tunneling term of the atoms accommodated in thehost cages (γphT = aT) and the Sommerfeld itinerant-electron term (γeT), are successfully separated throughcareful measurements of single crystals with various carrier concentrations. The values of the minimum densityof anharmonic potentials are deduced from a to be 0.12 ± 0.24 for n-KGSn, 0.47 ± 0.24 for n-BGG, 1.9 ± 0.8for p-BGG, 6.0 ± 0.9 for n-SGG, and 10.9 ± 0 .7 for n-BGSn in units of ×1015 cm-3. The effective mass(m*/m0) is determined from γe to be 1.01 ± 0.25 for n-BGG, 1.20 ± 0.19 for p-BGG, 1.68 ± 0.24 for n-SGG,1.86 ± 0.54 for n-KGSn, and 2.05±0.48 for n-BGSn in units of free electron mass mo, and the electron-phononinteraction strength can be evaluated from these values. It is shown that both the thermal conductivity (κ) and theelectron-phonon interaction strength (λ) agree well with the a parameters deduced from Cρ. The differences in κ known between n- and p-BGGs are ascribed to the influence of defects at the crystallographic 6c sites, which areclearly indicated by magnetic susceptibility measurements. This is very different from the situation in the other clathrates. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The electronic states of potassium- (K-) intercalated zigzag-type polycyclic aromatic (PLA) hydrocarbon [polyacene PLAs] Kx(PLAs) are studied for a series of the four smallest molecules: naphthalene (NN), anthracene (AN), tetracene (TN), and pentacene (PN), focusing on their 1:1 stoichiometric phases. Clear experimental differences are identified between the first group [K1(NN) and K1(AN)] and the second group [K1(TN) and K1(PN)] by magnetic, vibrational, and optical measurements. The first group is categorized as a Mott insulator with an antiferromagnetic ground state with energy of ~10 meV, whereas the second group is classified as a band insulator via dimer formation due to the spin Peierls instability. In the latter system, the first thermally accessible triplet states are located far apart from the singlet ground states and are not detected by electron spin-resonance spectroscopy until 300 K being very different from what is observed for the hole-doped PN reported earlier. The results give a new systematic understanding on the electronic states of electron-doped PLAs sensitive to the energetic balance among on-site Coulomb repulsion, bandwidth, and the Peierls instability. [ABSTRACT FROM AUTHOR]

The superconducting critical temperature, Tc, of FeSe can be dramatically enhanced by intercalation of a molecular spacer layer. Here we report on a 77Se,7Li, and ¹H nuclear magnetic resonance (NMR) study of the powdered hyper-interlayer-expanded Lix(C2H8N2)yFe2-zSe2 with a nearly optimal Tc=45 K. The absence of any shift in the 7Li and ¹H NMR spectra indicates a complete decoupling of interlayer units from the conduction electrons in FeSe layers, whereas nearly temperature-independent 7Li and ¹H spin-lattice relaxation rates are consistent with the non-negligible concentration of Fe impurities present in the insulating interlayer space. On the other hand, the strong temperature dependence of 77Se NMR shift and spin-lattice relaxation rate, 1/77T1, is attributed to the holelike bands close to the Fermi energy. 1/77T1 shows no additional anisotropy that would account for the onset of electronic nematic order down to Tc. Similarly, no enhancement in 1/77T1 due to the spin fluctuations could be found in the normal state. Yet, a characteristic power-law dependence 1/77T1∝T4.5 still complies with the Cooper pairing mediated by spin fluctuations. [ABSTRACT FROM AUTHOR]