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The mechanism of high-temperature superconductivity of copper oxides (cuprates) remains unsolved puzzle in condensed matter physics. The cuprates represent extremely complicated system, showing fascinating variety of quantum phenomena and rich phase diagram as a function of doping. In the suggested “superconducting glue” mechanisms, phonon and spin excitations are invoked most frequently, and it appears that only spin excitations cover the energy scale required to justify very high transition temperature Tc ∼ 165 K (as in mercury-based triple layer cuprates compressed to 30 GPa). It appears that pressure is quite important variable helping to boost the Tc record by almost 30°. Pressure may be also considered as a clean tuning parameter, helping to understand the underlying balance of various energy scales and ordered states in cuprates. In this paper, a review of mostly our work on cuprates under pressure will be given, with the emphasis on the interactions between phonon and spin excitations. It appears ...

We report on superconducting properties of high-quality single crystals of F-substituted NdOBiS2 using low-temperature magnetization and transport measurements. Using the mixture of CsCl and KCl as the flux, we have synthesized our single crystals. This compound exhibits bulk superconductivity with a transition temperature of about T c∼4.6 K. The critical current density J c as a function of temperature has been derived and decreases with the increasing temperature. We construct the phase diagram H c2(T). The zero-temperature value for $H_{\mathrm {c2}}^{B\parallel c}$ for value for $T_{c}^{90~\%}$ and $T_{c}^{0~\%}$ is estimated to be approximately 2.17 and 1.72 T respectively by using Werthamer-Helfand-Hohenberg model.

An effective method to synthesize gold nanoclusters that can specifically recognize fibroblast growth factor receptor2 (FGFR2) was reported. We designed a new peptide sequence, cysteine–cysteine–tyrosine–leucine–glutamine–leucine–glutamine–alanine–glutamic–glutamic–arginine–NH2, and then used it as the precursor to synthesize red luminescent gold nanoclusters (GNCs) in one step. In order to better understand the properties of the GNCs, ultraviolet-visible spectra, transmission electron microscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and matrix-assisted laser desorption/ionization time of flight mass spectrometry were used to characterize the GNCs. We also successfully applied the GNCs in fluorescence imaging of esophageal cancer KYSE510 cells that express in a high level of the FGFR2. The results demonstrated that the GNCs possess good luminescence, high stability, nontoxicity and good biocompatibility. We expect that the GNCs could find more applications in overexpression of the FGFR2 tumor cells imaging.

Vibrational properties and ultrafast photocarrier dynamics of 2H–TaS2 under pressures are studied by the experiments of Raman scattering and femtosecond time resolved spectroscopy. With increasing pressure, the linewidth of two-phonon Raman mode broadens, especially features with a sudden rapid increase above 10 GPa. Meanwhile, the ultrafast dynamics show that the fast decay through optical phonon scattering is significantly suppressed in amplitude and prolonged in lifetime; however, the weight of the slow decay component originating from acoustic phonon scattering is enhanced at high pressures. Furthermore, the electron-phonon (e-ph) coupling constant, λq, and the density of states (DOS) near the Fermi level, N(0), are evaluated qualitatively, which reveals a drastic decrease in e-ph coupling strength and a sudden increase in the DOS beyond 10 GPa. This turning pressure is consistent with the critical point where T c of superconductivity peaks while the charge density wave (CDW) vanishes, indicating the direct correlations between λq, N(0), superconductivity, and the CDW under pressure. The anomaly of the DOS may imply the occurrence of a topological transition of the electronic structure. Our work not only provides valuable information for the understanding of competition between the CDW and superconductivity in 2H–TaS2 but also may open up a routine to unveil the pressure-induced electronic topological transition.

Chalcopyrite compounds are promising high efficient thermoelectric materials. However, the relatively high lattice thermal conductivity at modest temperatures limits their performance. Here, we investigate the lattice dynamics in a polycrystalline CuInTe2 with a combined experimental and computational approach. The phonon dispersion and density of states are computed using the density functional theory. Raman scattering is performed to investigate the phonon dynamical properties. Together with the bulk modulus from X-ray diffraction, the mode-Gruneisen parameters are determined. The low energy B 1 1 and E2 modes characterize the weak anharmonicity, thus responsible for the high lattice thermal conductivity. Meanwhile, B 1 1 and E2 modes display energy redshift under pressure. The softening and enhanced anharmonicity of these modes naturally result in the reduction of the thermal conductivity. Our study suggests that pressure is a routine to reduce the phonon heat conduction at modest temperatures in chalcopyrites.

Structural heterogeneity has been proposed as a key intrinsic feature and mechanism underline unique properties and dynamic behavior of metallic glasses; however, it is still challenging in precisely describing and effectively characterizing the atomic-scale heterogeneity in metallic glasses. In this work, we employed the Meissner effect of superconductivity in magnetic susceptibility measurements as a sensitive bulk probe and successfully revealed the three-dimensional structural heterogeneity and its two-way evolution tuned by structural aging or rejuvenation in a La-based metallic glass. Compared with the resistivity measurements which only signal the most superconductive loop in inhomogeneous materials, the diamagnetic susceptibility signal of the Meissner effect maps the volumetric distribution of all superconductive regions and the corresponding structural heterogeneity in metallic glasses with high sensitivity. The experimental results reported in this article can be well interpreted based on a structural model of tunable “soft liquid-like” regions with soft vibration modes mixed with “hard solid-like” regions, validating the heterogeneity models of metallic glasses with new experimental data and approach.

Recent reports of the superconductivity in hydrides of two different families (covalent lattice, as in SH3 and clathrate-type H-cages containing La and Y atoms, as in LaH10 and YH6) have revealed new families of high-Tc materials with Tc’s near room temperature values. These findings confirm earlier expectations that hydrides may have very high Tc’s due to the fact that light H atoms have very high vibrational frequencies, leading to high Tc values within the conventional Bardeen–Cooper–Schrieffer phonon mechanism of superconductivity. However, as is pointed out by Ashcroft, it is important to have the metallic hydrogen “alloyed” with the elements added to it. This concept of a metallic alloy containing a high concentration of metal-like hydrogen atoms has been instrumental in finding new high-Tc superhydrides. These new superhydride “room-temperature” superconductors are stabilized only at very high pressures above 100 GPa, making the experimental search for their superconducting properties very difficult. We will review the current experimental and theoretical results for LaH10−x and YH6−x superhydrides.

Phenyl molecules are proposed as potential high-temperature superconductors due to exhibiting interesting properties. Here, we report the discovery of superconductivity with the critical temperature (Tc) of ∼7.2 Kelvin in potassium (K)-doped biphenyl (C12H10). The dc magnetic susceptibility measurements provide solid evidence for the presence of the Meissner effect in KxC12H10. The Raman spectra detected bipolaronic characteristics in this superconducting state, which are proposed to account for the electron pairing. Theoretical simulations provided the information of the crystal structure of KxC12H10. Combining XRD data with formation energy, we suggest that the superconducting phase corresponds to K2C12H10 or with a small charge fluctuation in a layered structure. The discovery of superconductivity in K-doped biphenyl vastly expands the potential applications in the superconducting field.

Intermetallic lithium compounds have found a wide range of applications owing to their light mass and desirable electronic and mechanical properties. Here, by compressing pure lithium and zinc mixtures in a diamond-anvil cell, we observe a direct reaction forming the stoichiometric compound LiZn, at pressures below 1 GPa. On further compression above 10 GPa, we observe the formation of Li 2 Zn, which is the highest lithium content compound to be discovered in the Li-Zn system. Our results constrain the structures of these compounds and their evolution with pressure, furthering our understanding of potentially useful light volume-efficient energy storage materials.

The electrical conductivity and Raman spectroscopy measurements have been performed on ${\mathrm{MoS}}_{2}$ at high pressures up to 80 GPa and variable temperatures down to 5 K. We find that the temperature dependence of the resistance in a mixed phase has an anomaly (a hump) which shifts with pressure to higher temperature. Concomitantly, a different Raman phonon mode appears in the mixed state suggesting that the electrical resistance anomaly may be related to a structural transformation. We suggest that this anomalous behavior is due to a charge-density wave state, the presence of which is indicative for an emergence of superconductivity at higher pressures.

We report on the Andreev spectroscopy and specific heat of high-quality single crystals of ${\mathrm{BaFe}}_{1.9}{\mathrm{Ni}}_{0.1}{\mathrm{As}}_{2}$. The intrinsic multiple Andreev reflection spectroscopy reveals two anisotropic superconducting gaps ${\mathrm{\ensuremath{\Delta}}}_{L}\ensuremath{\approx}3.2\ensuremath{-}4.5\phantom{\rule{4pt}{0ex}}\mathrm{meV}$, ${\mathrm{\ensuremath{\Delta}}}_{S}\ensuremath{\approx}1.2\ensuremath{-}1.6\phantom{\rule{4pt}{0ex}}\mathrm{meV}$ (the ranges correspond to the minimum and maximum value of the coupling energy in the ${k}_{x}{k}_{y}$ plane). The $25%\ensuremath{-}30%$ anisotropy shows the absence of nodes in the superconducting gaps. Using a two-band model with $s$-wave-like gaps ${\mathrm{\ensuremath{\Delta}}}_{L}\ensuremath{\approx}3.2\phantom{\rule{4pt}{0ex}}\mathrm{meV}$ and ${\mathrm{\ensuremath{\Delta}}}_{S}\ensuremath{\approx}1.6\phantom{\rule{4pt}{0ex}}\mathrm{meV}$, the temperature dependence of the electronic specific heat can be well described. A linear magnetic field dependence of the low-temperature specific heat offers further support of $s$-wave type of the order parameter. We find that a $d$-wave or single-gap BCS theory under the weak-coupling approach cannot describe our experiments.

Raman scattering measurements are performed on crystalline biphenyl at low temperatures. The properties of the vibrational modes focused on the intra- and intermolecular terms are analyzed in detail. Nearly all of the vibration modes exhibit hardening and simultaneously sharpen with decreasing temperature, whereas the modes at around 250 cm−1 and 1280 cm−1 soften their energies as temperature is decreased. Moreover, all the internal modes have anomalous reversals at around 45 K on the frequencies, widths, and intensities, and below 45 K, several new internal modes appear. Results of the analyses indicate that the reemergence of the interring tilt angle of the molecules at 45 K has a significant impact on the vibrational properties of biphenyl. Our work thus paves interesting and essential groundwork for the further study of biphenyl.

The effect of adding high pressure as a control parameter in solids, liquids, and gases expands opportunities to observe unexpected novel phenomena and understand matter in extreme environments. This review on high pressure science highlights subjects ranging from quantum criticality to Earth science. State-of-the-art experimental methods at megabar pressures are also discussed. The proliferation of pressure-induced phases illustrate promising new directions for this field of research.

Measurement of the electrical, thermal, and structural properties of palladium sulfide (PdS) has been conducted in order to investigate its thermoelectric performance. A tetragonal structure with the space group P42/m for PdS was determined from X-ray diffraction measurement. The obtained power factor of 27 μW cm−1 K−2 at 800 K is the largest value obtained for the transition metal sulfides studied so far. The maximum value of the dimensionless figure of merit is 0.33 at 800 K. These results indicate that binary bulk PdS has promising potential for good thermoelectric performance.

Raman scattering measurements are performed on benzene and a number of $p$-oligophenyls including biphenyl, $p$-terphenyl, $p$-quaterphenyl, $p$-quinquephenyl, and $p$-sexiphenyl at ambient conditions. The vibrational modes of the intra- and intermolecular terms in these materials are analyzed and compared. Chain length effects on the vibrational properties are examined for the C-H in-plane bending mode and the inter-ring C-C stretching mode at around 1200 cm$^{-1}$ and 1280 cm$^{-1}$, respectively, and the C-C stretching modes at around 1600 cm$^{-1}$. The complex and fluctuating properties of these modes result in an imprecise estimation of the chain length of these molecules. Meanwhile, the obtained ratio of the intensities of the 1200 cm$^{-1}$ mode and 1280 cm$^{-1}$ mode is sensitive to the applied lasers. A librational motion mode with the lowest energy is found to have a monotonous change with the increase in the chain length. This mode is simply relevant to the $c$ axis of the unit cell. Such an obvious trend makes it a better indicator for determining the chain length effects on the physical and chemical properties in these molecules., 7 pages and 5 figures

A half-Heusler material FeNb$_{0.8}$Ti$_{0.2}$Sb has been identified as a promising thermoelectric material due to its excellent thermoelectric performance at high temperatures. The origins of the efficient thermoelectric performance are investigated through a series of low-temperature (2 - 400 K) measurements. The high data coherence of the low and high temperatures is observed. An optimal and nearly temperature-independent carrier concentration is identified, which is ideal for the power factor. The obtained single type of hole carrier is also beneficial to the large Seebeck coefficient. The electronic thermal conductivity is found to be comparable to the lattice thermal conductivity and becomes the dominant component above 200 K. These findings again indicate that electron scattering plays a key role in the electrical and thermal transport properties. The dimensionless figure of merit is thus mainly governed by the electronic properties. These effects obtained at low temperatures with the avoidance of possible thermal fluctuations together offer the physical origin for the excellent thermoelectric performance in this material., 8 pages, 7 figures

Dichalcogenides are known to exhibit layered solid phases, at ambient and high pressures, where 2D layers of chemically bonded formula units are held together by van der Waals forces. These materials are of great interest for solid-state sciences and technology, along with other 2D systems such as graphene and phosphorene. SiS

We performed high pressure x-ray diffraction (XRD), Raman, and transport measurements combined with first-principles calculations to investigate the behavior of tin diselenide (SnSe$_2$) under compression. The obtained single-crystal XRD data indicate the formation of a $(1/3,1/3,1)$-type superlattice above 17 GPa. According to our density functional theory results, the pressure-induced transition to the commensurate periodic lattice distortion (PLD) phase is due to the combined effect of strong Fermi surface nesting and electron-phonon coupling at a momentum wave vector $\mathbf{q}=(1/3,1/3,1)$. In contrast, similar PLD transitions associated with charge density wave (CDW) orderings in transition metal dichalcogenides (TMDs) do not involve significant Fermi surface nesting. The discovered pressure-induced PLD is quite remarkable, as pressure usually suppresses CDW phases in related materials. Our findings, therefore, provide new playgrounds to study the intricate mechanisms governing the emergence of PLD in TMD-related materials., Comment: 6 pages, 4 figures + Supplemental (12 pages, 14 figures)

An extended study on PdS is carried out with the measurements of the resistivity, Hall coefficient, Raman scattering, and X-ray diffraction at high pressures up to 42.3 GPa. With increasing pressure, superconductivity is observed accompanying with a structural phase transition at around 19.5 GPa. The coexistence of semiconducting and metallic phases observed at normal state is examined by the Raman scattering and X-ray diffraction between 19.5 and 29.5 GPa. After that, only the metallic normal state maintains with an almost constant superconducting transition temperature. The similar evolution between the superconducting transition temperature and carrier concentration with pressure supports the phonon-mediated superconductivity in this material. These results highlight the important role of pressure played in inducing superconductivity from these narrow band-gap semiconductors., Comment: 8 pages, 5 figures

The electrical conductivity and Raman spectroscopy measurements have been performed on MoS$_2$ at high pressures up to 90 GPa and variable temperatures down to 5 K. We find that the temperature dependence of the resistance in a metallic 2H$_a$ phase has an anomaly (a hump) which shifts with pressure to higher temperature. Concomitantly, a new Raman phonon mode appears in the metallic state suggesting that the electrical resistance anomaly may be related to a structural transformation. We suggest that this anomalous behavior is due to a charge density wave state, the presence of which is indicative for a possibility for an emergence of superconductivity at higher pressures., Comment: 6 pages, 4 figures

The original version of this Article omitted references to previous experimental reports on solid hydrogen that are relevant for a full understanding of the context of the previous work. The added references are: 47. Akahama, Y. et al. Evidence from x-ray diffraction of orientational ordering in phase III of solid hydrogen at pressures up to 183 GPa. Phys. Rev. B82, 060101 (2010). 48. Zha, C.-S., Liu, Z. & Hemley, R. J. Synchrotron infrared measurements of dense hydrogen to 360 GPa. Phys. Rev. Lett. 108, 146402 (2012). 49. Dias, R. & Silvera, I. Observation of the Wigner-Huntington transition to metallic hydrogen. Science355, 715–718 (2017). 50. Eremets, M. I. & Drozdov, A. P. Comments on the claimed observation of the Wigner-Huntington transition to metallic hydrogen. Preprint at http://arxiv.org/abs/1702.05125 (2017). 51. Loubeyre, P., Occelli, F. & Dumas, P. Comment on: “Observation of the Wigner-Huntington transition to metallic hydrogen”. Preprint at http://arxiv.org/abs/1702.07192 (2017). 52. Goncharov, A. F. & Struzhkin, V. V. Comment on “Observation of the Wigner-Huntington transition to metallic hydrogen”. Science357, eaam9736 (2017). 53. Liu, X.-D., Dalladay-Simpson, P., Howie, R. T., Li, B. & Gregoryanz, E. Comment on “Observation of the Wigner-Huntington transition to metallic hydrogen”. Science357, eaan2671 (2017). Citations to these reference, plus reference 21, have been added to the fourth sentence of the Introduction: ‘The experimental realisation of atomic metallic hydrogen has remained elusive despite intense research efforts lasting over 30 years4–7,21,47–53.’ This has been corrected in the PDF and HTML versions of the Article.

The high pressure vibrational properties of anthracene are investigated by using Raman scattering techniques in diamond anvil cells up to 7.1 GPa at room temperature. We present a detailed analysis of the normal vibrational modes of anthracene along with assignments of both symmetry and molecular motion. Our results demonstrate that almost all the modes shift toward higher frequencies and some peaks are broadened with increasing pressure. The smooth evolution of peaks corresponding to first and second nearest neighbor shell molecules with pressure rules out the possible existence of phase transformation. The mode Gruneisen parameters are obtained over the wide frequency range and relatively high pressure range.

Herein, we report the intriguing electrical transport and structural properties of compressed Alq3, which is an extensively used electron transport material in OLEDs. The bulk resistance (Rb) of Alq3 increases with uploading pressure, but drops markedly when the uploading pressure is above 8.6 GPa. In contrast, the grain boundary resistance (Rgb) varies smoothly below 16.4 GPa. With further compression, both Rb and Rgb increase with the amorphization of Alq3. The pressure-induced amorphization is found to be reversible at a low density amorphous state, while it is irreversible at a higher density state. Interestingly, XRD measurements indicate no structural transition at ∼8.0 GPa. The variation of Rb is found to be synchronous with the blue-shift of the Al–oxine deformation mode, which rationalizes the anomalous changes of Rb. The Al–oxine interaction is believed to be also significant in the electrical transport properties of dense Alq3, which provides insight into the correlation between its structural changes and electrical transport properties.

In situ high-pressure low-temperature high-quality Raman data for hydrogen and deuterium demonstrate the presence of a novel phase, phase ${\mathrm{II}}^{\ensuremath{'}}$, unique to deuterium and distinct from the known phase II. Phase ${\mathrm{II}}^{\ensuremath{'}}$ of ${\mathrm{D}}_{2}$ is not observed in hydrogen, making it the only phase that does not exist in both isotopes and occupies a significant part of $P\text{\ensuremath{-}}T$ space from $\ensuremath{\sim}25$ to 110 GPa and below 125 K. For ${\mathrm{H}}_{2}$, the data show that below 30 K the transition to phase II happens at as low as 73 GPa. The transformation from phase II to III commences at around $\ensuremath{\sim}155\text{ }\text{ }\mathrm{GPa}$ and is completed by 170 GPa with the average pressure of $\ensuremath{\sim}160\text{ }\text{ }\mathrm{GPa}$ being slightly higher than previously thought. The updated phase diagrams of ${\mathrm{H}}_{2}$ and ${\mathrm{D}}_{2}$ demonstrate the difference between the isotopes at low temperatures and moderate pressures, providing new information on the phase diagrams of both elements.

An unexpected superconductivity enhancement is reported in decompressed In2Se3. The onset of superconductivity in In2Se3 occurs at 41.3 GPa with a critical temperature (Tc) of 3.7 K, peaking at 47.1 GPa. The striking observation shows that this layered chalcogenide remains superconducting in decompression down to 10.7 GPa. More surprisingly, the highest Tc that occurs at lower decompression pressures is 8.2 K, a twofold increase in the same crystal structure as in compression. It is found that the evolution of Tc is driven by the pressure-induced R-3m to I-43d structural transition and significant softening of phonons and gentle variation of carrier concentration combined in the pressure quench. The novel decompression-induced superconductivity enhancement implies that it is possible to maintain pressure-induced superconductivity at lower or even ambient pressures with better superconducting performance.

Recently, there is a series of reports by Wang et al. on the superconductivity in K-doped p-terphenyl (KxC18H14) with the transition temperatures range from 7 to 123 Kelvin. Identifying the structural and bonding character is the key to understand the superconducting phases and the related properties. Therefore we carried out an extensive study on the crystal structures with different doping levels and investigate the thermodynamic stability, structural, electronic, and magnetic properties by the first-principles calculations. Our calculated structures capture most features of the experimentally observed X-ray diffraction patterns. The K doping concentration is constrained to within the range of 2 and 3. The obtained formation energy indicates that the system at x = 2.5 is more stable. The strong ionic bonding interaction is found in between K atoms and organic molecules. The charge transfer accounts for the metallic feature of the doped materials. For a small amount of charge transferred, the tilting force between the two successive benzenes drives the system to stabilize at the antiferromagnetic ground state, while the system exhibits non-magnetic behavior with increasing charge transfer. The multiformity of band structures near the Fermi level indicates that the driving force for superconductivity is complicated., 8 pages, 7 figures

An unexpected superconductivity enhancement is reported in decompressed In

Pressure-induced valence change and semiconductormetal transition were revealed in perovskite compound PbCrO3 by X-ray absorption spectroscopy (XAS) and resistance measurements, respectively. The Cr L-2,L-3 edge XAS spectra indicate a charge disproportionation of Cr (3Cr(4+) -> 2Cr(3+) + Cr6+) at ambient pressure, suggesting that the ambient-pressure phase of PbCrO3 has complex local structure and cannot be explained using the simple cubic perovskite structure (Pm-3m). Upon compression up to 4.2 GPa, Cr3+ and C-r6+ ions were converted to Cr4+ via charge transfer, associated with semiconductormetal transition. The high-pressure metal phase is determined to be cubic perovskite structure (Pm-3m). Compared with the previous X-ray diffraction experiment, the large volume collapse (similar to 9.8% at 1.6 GPa) may be related to valence change and semiconductormetal transition.

This paper introduces an electromagnetic induction heating method which is demonstrated to be able to bond carbon nanotubes and metal electrodes. Dielectrophoresis method is taken to realize the parallel directional assembly between carbon nanotubes and metal electrode, and then localized induction heating is adopted to bond them. This technique offers a convenient and reliable way in the practical applications of carbon nanotube interconnection. The bonding principle and process are introduced, and the experiment results are also discussed. The results show that the bonding interface exhibits good contact and electrical property.

Light-element compounds hold great promise of high critical temperature superconductivity judging from the theoretical perspective. Hydrogen-rich material, benzene, is such a kind of candidate but also an organic compound. A series of first-principles calculations are performed on the electronic structures, dynamics properties, and electron-phonon interactions of solid benzene at high pressures. Benzene is found to be dynamically stable in the pressure range of 180 - 200 GPa and to exhibit superconductivity with a maximum transition temperature of 20 K at 195 GPa. The phonon modes of carbon atoms are identified to mainly contribute to the electron-phonon interactions driving this superconductivity. The predicted superconductivity in this simplest pristine hydrocarbon shows a common feature in aromatic hydrocarbons and also makes it a bridge to organic and hydrogen-rich superconductors., 6 pages, 4 figures

Measurements of electrical transport and Raman spectroscopy are performed on 2H-TaS2 to study the formation and the character of the charge density wave. The entry of the charge-density-wave state below the lock-in temperature is detected by the sudden change of the resistivity slope with temperature, the sharp rise of the magneto-resistivity, and the significant change in the Hall coefficient. Meanwhile, the frequency of the two-phonon mode decreases with decreasing temperature. When approaching the lock-in temperature, the formation of the charge density wave is also evidenced by the disappearance of the two-phonon mode. In the charge density wave state, the newly born charge-density-wave modes in the E2g- and A1g-symmetries exhibit a blue-shift with decreasing temperature. These results suggest that the combined techniques of electrical transport measurement and Raman spectroscopy are powerful in studying the charge-density-wave order in materials with strong electron-phonon coupling.

Compared with graphene, 2H-MoS2 possesses a non-zero bandgap and thus has an unlimited potential for electronic, spintronic, and optoelectronic applications. Understanding of the phonon scattering mechanisms is crucial to its device applications because the heat flow and transport are the basic processes functioning at various temperatures. So far, the knowledge of the phonon anharmonicity of 2H-MoS2 is limited due to the availability from a narrow temperature range and the absence of the low frequency phonon information. Here, we report an experimental study on the temperature dependence of the frequency and linewidth of 2H-MoS2 by Raman scattering over a wide temperature range from 2.2 to 1000 K and down to the wavenumber of 10 cm−1. The cubic anharmonicity is found to be dominant at low temperatures, and quartic anharmonicity predominates at high temperatures. The obtained shear mode seems insensitive to the anharmonic effects. The damping effects are discussed based on the available experimental data. These phonon scattering behaviors of 2H-MoS2 are of great help to the future nanodevice developments and applications.

Journal of applied physics 124(19), 195107 (2018). doi:10.1063/1.5050697, Half-Heusler compounds have recently been identified as promising thermoelectric materials, but their relatively high thermal conductivities impede the further improvement of thermoelectric performance. The knowledge of phonon vibrational properties provides a fundamental understanding of the thermal transport behavior of solids and thus could serve as a guidance on further suppressing the thermal conductivity. Herein, a highly efficient p-type half-Heusler thermoelectric alloy FeNb$_{0.8}$Ti$_{0.2}$Sb is taken as an example to explore its phonon vibrational properties. Phonon spectrum with the frequencies down to 10 cm$^{−1}$ and its evolution with pressure for the studied material are provided by Raman scattering. It is found that two vibrational modes with the frequency > 200 cm$^{−1}$display a common mode frequency increase with increasing pressure. Based on the bulk modulus from synchrotron X-ray powder diffraction and phonon frequency shifts, the mode Grüneisen parameters are obtained. Our results establish characteristic phonon vibrational properties of this high-performance half-Heusler thermoelectric alloy., Published by American Inst. of Physics, Melville, NY

Vertically aligned carbon nanotube films are good thermal interface materials because of their high thermal conductance and mechanical compliance. In this study, the authors bond carbon nanotube films and metallized silicon chips together using thermocompression method in a magnetic field to decrease the thermal contact resistance between them. Nickel is considered as the deposition metal because of its strong cohesion and good affinity on the carbon nanotube surface and it is ferromagnetic in nature. Bonding is demonstrated in vacuum chamber under the influence of temperature (300°C) and pressure (180 KPa). The resistance of the carbon nanotube film is measured with a thermal transient tester that can provide the structure function of the sample, so that people can know detailed thermal resistance. In general, the thermal contact resistance of the sample enhanced with magnetic field that the author measured is 34·24 mm2K/W that is 10% lower than the sample without enhancement. This study reveals that magnetic field can enhance the bonding strength to decrease the thermal contact resistance.

The structural, vibrational, and magnetic properties of well prepared Bi 1− x Tb x FeO 3 ( x =0–0.2) powders are investigated by combining the X-ray diffraction, Raman scattering, and magnetometry measurements. It is found that partial substitution of Tb (10–12.5%) in Bi site results in a structural transformation from the rhombohedral R 3 c symmetry to the orthorhombic Pnma phase, accompanied by a ferroelectric–paraelectric phase transition. The magnetization of Bi 1− x Tb x FeO 3 first gradually increases before reaching maximum at the polar–nonpolar phase boundary and then decreases for x ≥0.125 until x =0.20. The enhancement of the remnant magnetization is attributed to the suppression of the space-modulated spin structure in the rhombohedral R 3 c phase. Meanwhile, the reduction of the remnant magnetization in the orthorhombic Pnma phase is suggested to result from the establishment of well parallel antiferromagnetic ordering.

With finite bandgaps, g-BNC, a boron nitride monolayer (g-BN) phase within a graphene layer, is a promising semiconductor for next generation electronics. We report its mechanics dependence of the g-BN concentration, including the high order elastic constants and mechanical failure, through a first-principles study based on density functional theory. The in-plane stiffness as well as third order elastic constants of graphene can be linearly tuned with g-BN concentration. The longitudinal mode elastic constants are sensitive to the BN modification, in contrast to the shear mode elastic constants. This study may provide guidance in optimizing the mechanics of graphene-based nanodevices.

Structural, vibrational, and magnetic properties of Bi 1− x Sm x FeO 3 (0≤ x ≤0.20) are investigated by using x-ray powder diffraction, Raman scattering, and magnetometry techniques. We show that the rare-earth ion Sm substitution at Bi site, serving chemical pressure, causes the structural transformation from rhombohedral R3c phase to orthorhombic Pnma phase at x ∼0.15, accompanying with the ferroelectric–paraelectric transition. The magnetization is found to be enhanced initially with Sm substitution increasing before approaching ferroelectric–paraelectric boundary but it slightly decreases after passing a maximum. The enhanced magnetization is suggested to stem from the combined effects of the suppressed spin cycloids and increased degree of canting and magnetic interactions between magnetic ions.

In low-dimensional electron systems, charge density waves (CDW) and superconductivity are two of the most fundamental collective quantum phenomena. For all known quasi-two-dimensional superconductors, the origin and exact boundary of the electronic orderings and superconductivity are still attractive problems. Through transport and thermodynamic measurements, we report on the field-temperature phase diagram in 2H-TaS2 single crystals. We show that the superconducting transition temperature (Tc) increases by one order of magnitude from temperatures at 0.98 K up to 9.15 K at 8.7 GPa when the Tc becomes very sharp. Additionally, the effects of 8.7 GPa illustrate a suppression of the CDW ground state, with critically small Fermi surfaces. Below the Tc the lattice of magnetic flux lines melts from a solid-like state to a broad vortex liquid phase region. Our measurements indicate an unconventional s-wave-like picture with two energy gaps evidencing its multi-band nature.

The pressure effect on the geometrical and electronic structures of crystalline naphthalene is calculated up to 30 GPa by performing density functional calculations. The lattice parameters a, b, and c, decrease by 1.77 A (–20.4%), 0.85 A (–14.1%), and 0.91 A (–8.2%), respectively, while the monoclinic angle β increases by 3.95° in this pressure region. At the highest pressure of 30 GPa the unit cell volume decreases by 62.7%. The detailed analysis of the molecular arrangement within crystal structure reveals that the molecular motion becomes more and more localized, and hints towards the evolution of intermolecular interaction with pressure. Moreover, the electronic structure of naphthalene under high pressure is also discussed. A pressure induced decrease of the band gap is observed.

We have performed measurements of Raman scattering, synchrotron x-ray diffraction and visible transmission spectroscopy combined with density functional theory calculations to study the pressure effect on solid triphenylene. The spectroscopic results demonstrate substantial change of the molecular configuration at 1.4 GPa from the abrupt change of splitting, disappearance and appearance of some modes. The structure of triphenylene is found be to stable at high pressures without any evidence of structural transition from the x-ray diffraction patterns. The obtained lattice parameters show a good agreement between experiments and calculations. The obtained band gap systematically decreases with increasing pressure. With the application of pressure, the molecular planes become more and more parallel relative to each other. The theoretical calculations indicate that this organic compound becomes metallic at 180 GPa, fueling the hope for the possible realization of superconductivity at high pressure.

Measurements of the resistivity, Hall coefficient, and Raman spectroscopy are performed on a Rashba semiconductor BiTeCl single crystal at high pressures up to 50 GPa. We find that applying pressure first induces a theoretically predicted insulating state, followed by a superconducting phase with an insulating normal state. Upon heavy compression, another different superconducting phase is entered into with a metallic normal state. A domelike evolution of the superconducting transition temperature with pressure is obtained with a crossover from the electron to hole carriers across the boundary of the two superconducting phases. These findings imply the possible realization of a topological state of the insulating and superconducting phases in this material.

A comprehensive study of the doping dependence of the phase diagram of FeSe-based superconductors is still required due to the lack of a clean and systematic means of doping control. Here, we report on the magneto-optical imaging, thermodynamic and transport properties, as well as in situ angle-resolved photoemission spectroscopy (ARPES) studies of impurity scattering in stoichiometric FeSe single crystals. Co doping at the Fe site is found to decrease the superconducting transition temperature $({T}_{c})$. The upper critical field and specific heat all indicate a possible multiband superconductivity with strong coupling in the Co-doped system. A remarkable feature in FeSe is that its temperature dependent resistivity exhibits a wide hump at high temperatures, a signature of a crossover from a semiconductinglike behavior to metallic behavior. A structural tetragonal-to-orthorhombic phase transition $({T}_{s})$ (a consequence of the electronic nematicity) is suppressed by either physical or chemical pressures. Due to the reconstruction of the Fermi surface at ${T}_{s}$, specific heat anomalies at ${T}_{s}$ present $\mathrm{\ensuremath{\Delta}}{C}_{p}/{T}_{s}\ensuremath{\approx}{\ensuremath{\gamma}}_{n}$, being ${\ensuremath{\gamma}}_{n}$ the Sommerfield coefficient at low temperature. This reflect an additional electronic instability in the ${\mathrm{FeSe}}_{1\ensuremath{-}x}{\mathrm{S}}_{x}$ system. ARPES data between 180 and 282 K indicates the existence of a chemical potential shift with increasing thermal excitations, resulting in a change of the Fermi-surface topology and exhibiting a semimetal behavior. We found that the temperature-induced Lifshitz transition is much higher than the temperature for the nematic order.

Elucidating the nature of the magnetism of a high-temperature superconductor is crucial for establishing its pairing mechanism. The parent compounds of the cuprate and iron-pnictide superconductors exhibit N\'eel and stripe magnetic order, respectively. However, FeSe, the structurally simplest iron-based superconductor, shows nematic order (Ts = 90 K), but not magnetic order in the parent phase, and its magnetic ground state is intensely debated. Here, we report inelastic neutron-scattering experiments that reveal both stripe and N\'eel spin fluctuations over a wide energy range at 110 K. On entering the nematic phase, a substantial amount of spectral weight is transferred from the N\'eel to the stripe spin fluctuations. Moreover, the total fluctuating magnetic moment of FeSe is ~ 60% larger than that in the iron pnictide BaFe2As2. Our results suggest that FeSe is a novel S = 1 nematic quantum-disordered paramagnet interpolating between the N\'eel and stripe magnetic instabilities., Comment: Supplementary information included; accepted by Nature Communications