Search

The growth of two dimensional sp2-bonded boron nitride (2D-BN) was studied in a plasma-assisted molecular beam epitaxy set-up, using independent boron and nitrogen sources. We studied the growth conditions on polycrystalline Ni foils: B and N respective fluxes, growth temperature and time, which are influencing the surface morphology, stoichiometry and the 2D-BN domain size. Using a B/N precursor flux ratio ≫1 yields films with incorporated boron largely in excess and intermixed with 2D-BN. On the contrary, precursor flux ratios from moderately B-rich to moderately N-rich leads to stoichiometric 2D-BN. The optimum growth temperature is found to be 900 °C, a temperature for which the crystallographic quality is improved compared to lower temperatures thanks to the increased adatom surface mobility although a partial sublimation of BN occurs. Increasing the growth time under the optimized settings shows that the growth does not occur in a layer-by-layer mode, but rather by stacking BN domains on top of each other with a rather slow lateral extension of the domains. [ABSTRACT FROM AUTHOR]

2D boron nitride (2D-BN) was synthesized by gas-source molecular beam epitaxy on polycrystalline and monocrystalline Ni substrates using gaseous borazine and active nitrogen generated by a remote plasma source. The excess of nitrogen atoms allows to overcome the thickness self-limitation active on Ni when using borazine alone. The nucleation density and the shape of the 2D-BN domains are clearly related to the Ni substrate preparation and to the growth parameters. Based on spatially-resolved photoemission spectroscopy and on the detection of the π plasmon peak, we discuss the origin of the N1s and B1s components and their relationship with an electronic coupling at the interface. After optimization of the growth parameters, a full 2D-BN coverage is obtained, although the material thickness is not evenly distributed. The 2D-BN presents a granular structure on (111) oriented Ni grains, showing a rather poor cristallographic quality. On the contrary, high quality 2D-BN is found on (101) and (001) Ni grains, where triangular islands are observed whose lateral size is limited to ∼20 μ m. [ABSTRACT FROM AUTHOR]

Extremely large magnetoresistance (XMR) has been explored in many nonmagnetic topologically nontrivial/trivial semimetals, while it is experimentally ambiguous which mechanism should be responsible in a specific material due to the complex electronic structures. In this paper, the magnetoresistance origin of single crystal CaAl4 with C2/m structure at low temperature is investigated, exhibiting unsaturated magnetoresistance of ∼ 3000% at 2.5 K and 14 T as the fingerprints of XMR materials. By the combination of ARPES and the first-principles calculations, we elaborate multiband features and anisotropic Fermi surfaces, which can explain the mismatch of isotropic two-band model. Although the structural phase transition from I4/mmm to C2/m has been recognized, the subtle impact on electronic structure is revealed by our ARPES measurements. Considering that both charge compensation and potential topologically nontrivial band structure exist in CaAl4, our findings report CaAl4 as a new reference material for exploring the XMR phenomena. [ABSTRACT FROM AUTHOR]

Co3Sn2S2 has attracted a lot of attention for its multiple novel physical properties, including topological nontrivial surface states, anomalous Hall effect, and anomalous Nernst effect. Vacancies, which play important roles in functional materials, have attracted increasing research attention. In this paper, by using density functional theory calculations, we first obtain band structures and magnetic moments of Co3Sn2S2 with exchange-correlation functionals at different levels. It is found that the generalized gradient approximation gives the positions of Weyl points consistent with experiments in bulk Co3Sn2S2. We then investigate the electronic structures of defects on surfaces with S and Sn terminations which have been observed in experiments. The results show that the single sulfur vacancy on the S-terminated surface introduces localized bond states inside the bandgap near the Fermi level. For di- and tri-sulfur vacancies, the localized defect states hybridize with neighboring ones, forming bonding states as well as anti-bonding states. The Sn vacancy on the Sn-terminated surface also introduces localized bond states, which are merged with the valence bands. These results provide a reference for future experimental investigations of vacancies in Co3Sn2S2. [ABSTRACT FROM AUTHOR]

Weak topological insulators (TIs) host non-trivial topological surface state bands (SSBs) at the side surface of the crystal while the top (001) surface can be topologically either dark or light. In our detailed angle-resolved photoelectron spectroscopy (ARPES) study we found Dirac-like linearly dispersive SSBs on the (001) surface of BiSe and Sb-doped (8%) BiSe. The lower part of the SSBs buries deep in the bulk valence band (BVB) and the overlap region between the SSBs and BVB increases significantly with Sb doping. These results highlight the role of the interlayer coupling between the Bi bilayer and the Bi2Se3 quintuple layer (QL). Furthermore, in the overlap region the SSBs deviate from the Dirac-like linear dispersion. Interestingly, we observed a large intensity imbalance) in the SSBs located at the positive and negative directions. This asymmetry pattern) gradually reverses to) as the excitation energy scans from low (14 eV) to high (34 eV) values. The observed photon energy-dependent intensity variation could be a signature of the mixing between the spin and the orbit texture of the SSBs. [ABSTRACT FROM AUTHOR]

Graphane is formed by bonding hydrogen (and deuterium) atoms to carbon atoms in the graphene mesh, with modification from the pure planar sp2 bonding towards an sp3 configuration. Atomic hydrogen (H) and deuterium (D) bonding with C atoms in fully free-standing nano porous graphene (NPG) is achieved, by exploiting low-energy proton (or deuteron) non-destructive irradiation, with unprecedented minimal introduction of defects, as determined by Raman spectroscopy and by the C 1s core level lineshape analysis. Evidence of the H- (or D-) NPG bond formation is obtained by bringing to light the emergence of a H- (or D-) related sp3-distorted component in the C 1s core level, clear fingerprint of H–C (or D–C) covalent bonding. The H (or D) bonding with the C atoms of free-standing graphene reaches more than 1/4 (or 1/3) at% coverage. This non-destructive H-NPG (or D-NPG) chemisorption is very stable at high temperatures up to about 800 K, as monitored by Raman and x-ray photoelectron spectroscopy, with complete healing and restoring of clean graphene above 920 K. The excellent chemical and temperature stability of H- (and D-) NPG opens the way not only towards the formation of semiconducting graphane on large-scale samples, but also to stable graphene functionalisation enabling futuristic applications in advanced detectors for the β-spectrum analysis. [ABSTRACT FROM AUTHOR]

In this perspective we discuss three emerging fields of condensed matter physics which in recent years have attracted considerable attention. In particular, we consider the recent challenging topics on time-dependent phase transitions, topological phenomena, and unconventional superconductivity, with the aim to foster the community towards new applications and technological advancements. As for the time-dependent phase transitions, in recent years the experiments have shown light-induced phase transitions and new fields of application are emerging, including material design. Regarding topological materials, new challenges have arisen to detect the Majorana origin of quantized conductance in superconducting hybrid structures, as well as the effect of interaction on edge channels. Finally, concerning superconductivity, non-conventional pairing and correlation effects dominate the physics of a vast class of two-dimensional materials and novel devices were recently conceived. This work offers a comprehensive overview on these topics for promoting new ideas in these fertile fields of research. [ABSTRACT FROM AUTHOR]

Photoelectron spectroscopy resolved in three-dimensional momentum space with a microscopic field of view is realized by combining a so-called Momentum Microscope with a soft X-ray synchrotron radiation source. A new experimental station for momentum-resolved photoelectron micro-spectroscopy and spectro-microscopy has been built at BL6U, an undulator-based soft X-ray beamline of the UVSOR synchrotron facility. This experimental station specializes in characterizing the electronic structure of surface atomic sites, thin films, molecular adsorbates, and bulk crystals. The instrument details are described along with possible measurement techniques. [ABSTRACT FROM AUTHOR]

The discovery in 2008 of the iron-based superconducting pnictide and chalcogenide compounds has provided an entirely new family of materials for studying the crucial interplay between superconductivity and magnetism in unconventional superconductors. The alkali-metal-intercalated iron selenide (AxFe2−ySe2, A = alkali metal) superconductors are of particular interest owing to their relatively high transition temperatures over 30 K and the co-existence of the superconducting state with antiferromagnetic ordering. Intrinsic phase separation on the mesoscopic scale is known to occur in ‘single crystals’ of these compounds, adding to the complexity of interpretation of bulk property measurements. In this study, we investigate the local electronic structure and chemistry of RbxFe2−ySe2 crystals using scanning microscopy techniques. Nano-focussed angle-resolved photoemission spectroscopy has enabled the band structure of the minority superconducting phase and the non-superconducting matrix to be measured independently and linked to the surface chemistry from the same regions using core level spectroscopy. Valence band mapping reveals the characteristic microstructure of these crystals, but does not have sufficient spatial resolution to enable the precise morphology of the superconducting phase to be elucidated. Cryogenic magnetic force microscopy has shown that the superconducting phase has a fine-scale stripey morphology that was not resolved in the scanning photoemission spectroscopy experiment. The correlation of these findings with previous microstructural studies, bulk measurements and first-principles density functional theory calculations paves the way for understanding the intriguing electronic and magnetic properties of these compounds. [ABSTRACT FROM AUTHOR]

Topological Dirac semimetals (DSMs) present a kind of topologically nontrivial quantum state of matter, which has massless Dirac fermions in the bulk and topologically protected states on certain surfaces. In superconducting DSMs, the effects of their nontrivial topology on superconducting pairing could realize topological superconductivity in the bulk or on the surface. As superconducting pairing takes place at the Fermi level EF, to make the effects possible, the Dirac points should lie in the vicinity of EF so that the topological electronic states can participate in the superconducting paring. Here, we show using angle-resolved photoelectron spectroscopy that in a series of (Ir1−xPtx)Te2 compounds, the type-II Dirac points reside around EF in the superconducting region, in which the bulk superconductivity has a maximum Tc of ∼ 3 K. The realization of the coexistence of bulk superconductivity and low-energy Dirac fermions in (Ir1−xPtx)Te2 paves the way for studying the effects of the nontrivial topology in DSMs on the superconducting state. [ABSTRACT FROM AUTHOR]

Wheat seeds (Triticumaestivum L.) were exposed to two different types of lasers, a He–Ne laser (633 nm) and an argon laser (514.5 nm), to study their effects on growth, germination, and physiological and biochemical characteristics such as the amounts of chlorophyll a, chlorophyll b, and carotenoids. A significant increase in the growth characteristics was monitored and studied keeping in view the length as well as fresh and dry weight of shoots and roots. Similarly, plant pigment contents such as chlorophyll a, chlorophyll b, and carotenoids were significantly changed. These significant alterations led to an increase in growth and germination compared with unexposed (control) wheat seeds. The experimental results of this in vitro study clearly demonstrate that different lasers like He–Ne laser and argon laser at low power significantly enhance the growth and germination of wheat seeds (Triticumaestivum L.) by positively influencing its physiological and biochemical characteristics. [ABSTRACT FROM AUTHOR]

Recently, the Dirac and Weyl semimetals have attracted extensive attention in condensed matter physics due to both the fundamental interest and the potential application of a new generation of electronic devices. Here we review the exotic electrical transport phenomena in Dirac and Weyl semimetals. Section 1 is a brief introduction to the topological semimetals (TSMs). In Section 2 and Section 3, the intriguing transport phenomena in Dirac semimetals (DSMs) and Weyl semimetals (WSMs) are reviewed, respectively. The most widely studied Cd3As2 and the TaAs family are selected as representatives to show the typical properties of DSMs and WSMs, respectively. Beyond these systems, the advances in other TSM materials, such as ZrTe5 and the MoTe2 family, are also introduced. In Section 4, we provide perspectives on the study of TSMs especially on the magnetotransport investigations. [ABSTRACT FROM AUTHOR]

A low-noise cryogenic amplifier for the bandwidth from 100 kHz to 2 MHz with commercially available components is presented. The amplifier is mounted on the cold finger of our home-made liquid helium dipstick. The input impedance of the amplifier is 2 kΩ. The input-referred voltage noise of the amplifier at approximately 2 MHz is around 1 . We demonstrate the performance of the amplifier by measuring shot noise on the Al/AlOx/Al tunneling junction with resistance about 17 kΩ at liquid helium temperature. [ABSTRACT FROM AUTHOR]

Holes with an average size of 2–5 nm have been created in graphene layers by heating of graphite oxide (GO) in concentrated sulfuric acid followed by annealing in an argon flow. The hot mineral acid acts simultaneously as a defunctionalizing and etching agent, removing a part of oxygen-containing groups and lattice carbon atoms from the layers. Annealing of the holey reduced GO at 800 °C–1000 °C causes a decrease of the content of residual oxygen and the interlayer spacing thus producing thin compact stacks from holey graphene layers. Electrochemical tests of the obtained materials in half-cells showed that the removal of oxygen and creation of basal holes lowers the capacity loss in the first cycle and facilitates intercalation-deintercalation of lithium ions. This was attributed to minimization of electrolyte decomposition reactions, easier desolvation of lithium ions near the hole boundaries and appearance of multiple entrances for the naked ions into graphene stacks. [ABSTRACT FROM AUTHOR]

Dirac semimetals are materials in which the conduction and the valence bands have robust crossing points protected by topology or symmetry. Recently, a new type of Dirac semimetals, so called the Dirac line-node semimetals (DLNSs), have attracted a lot of attention, as they host robust Dirac points along the one-dimensional (1D) lines in the Brillouin zone (BZ). In this work, using angle-resolved photoemission spectroscopy (ARPES) and first-principles calculations, we systematically investigated the electronic structures of non-symmorphic ZrSiS crystal where we clearly distinguished the surface states from the bulk states. The photon-energy-dependent measurements further prove the existence of Dirac line node along the X–R direction. Remarkably, by in situ surface potassium doping, we clearly observed the different evolutions of the bulk and surface electronic states while proving the robustness of the Dirac line node. Our studies not only reveal the complete electronic structures of ZrSiS, but also demonstrate the method manipulating the electronic structure of the compound. [ABSTRACT FROM AUTHOR]

We report on the high-pressure Raman study of , a three-dimensional Dirac semimetal, up to 19 GPa at room temperature. Our study shows that light scattering by intervalley and intravalley density fluctuations give rise to electronic Raman scattering (ERS), with Lorentzian-like lineshape at low frequency. The strength and linewidth of the ERS are pressure dependent and exhibit a significant drop at , signifying a breakdown of the Dirac semimetal to a semiconducting phase. The first phase transition at Pc1 is also clearly identified by the significant changes in the pressure derivatives of phonon frequencies and linewidths. Pressure dependence of phonon parameters also reveal a second phase transition at , being reported for the first time. This semiconductor-to-semiconductor transition coincides with the abrupt changes seen in the activation energy (band gap) of the semiconductor phase. [ABSTRACT FROM AUTHOR]

We generalize the concept of triply-degenerate nodal points to non-collinear antiferromagnets. Here, we introduce this concept to insulating quantum antiferromagnets on the decorated honeycomb lattice, with spin-1 bosonic quasiparticle excitations known as magnons. We demonstrate the existence of magnonic surface states with constant energy contours that form pairs of magnonic arcs connecting the surface projection of the magnonic triple nodal points. The quasiparticle excitations near the triple nodal points represent three-component bosons beyond that of magnonic Dirac, Weyl, and nodal-line cases. They can be regarded as a direct reflection of the intrinsic spin carried by magnons. Furthermore, we show that the magnonic triple nodal points can split into magnonic Weyl points, as the system transits from a non-collinear spin structure to a non-coplanar one with a non-zero scalar spin chirality. Our results not only apply to insulating antiferromagnets, but also provide a platform to seek for triple nodal points in metallic antiferromagnets. [ABSTRACT FROM AUTHOR]

High-temperature (1000 °C) thermolytic decomposition of ethane 1,2-diamineborane (BH3NH2CH2CH2NH2BH3) deposited onto a Cu foil has been performed in an ultra-high-vacuum environment. A combined thermolytic, structural (x-ray diffraction), microscopic (scanning electron microscopy) and spectroscopic (Raman, x-ray photoemission spectroscopy) analysis, has identified a ternary borocarbonitride (BCN) compound as a result of the process. The obtained BCN compound is nanocrystalline, surrounded by crystallites of ammonium hydroxide borate hydrate. The ternary compound presents a 0.2:0.6:0.2 B:C:N composition in the bulk and 0.11:0.76:0.13 stoichiometry at the very surface, richer in C–C networks with respect to the bulk. Furthermore, the resulting BCN compound does not show oxidation at the surface due to the in-vacuum thermolysis of the single precursor. [ABSTRACT FROM AUTHOR]

The three-dimensional (3D) Dirac semimetals have linearly dispersive 3D Dirac nodes where the conduction band and valence band are connected. They have isolated 3D Dirac nodes in the whole Brillouin zone and can be viewed as a 3D counterpart of graphene. Recent theoretical calculations and experimental results indicate that the 3D Dirac semimetal state can be realized in a simple stoichiometric compound A3Bi (A = Na, K, Rb). Here we report comprehensive high-resolution angle-resolved photoemission (ARPES) measurements on the two cleaved surfaces, (001) and (100), of Na3Bi. On the (001) surface, by comparison with theoretical calculations, we provide a proper assignment of the observed bands, and in particular, pinpoint the band that is responsible for the formation of the three-dimensional Dirac cones. We observe clear evidence of 3D Dirac cones in the three-dimensional momentum space by directly measuring on the kx–ky plane and by varying the photon energy to get access to different out-of-plane kzs. In addition, we reveal new features around the Brillouin zone corners that may be related with surface reconstruction. On the (100) surface, our ARPES measurements over a large momentum space raise an issue on the selection of the basic Brillouin zone in the (100) plane. We directly observe two isolated 3D Dirac nodes on the (100) surface. We observe the signature of the Fermi-arc surface states connecting the two 3D Dirac nodes that extend to a binding energy of ∼150 meV before merging into the bulk band. Our observations constitute strong evidence on the existence of the Dirac semimetal state in Na3Bi that are consistent with previous theoretical and experimental work. In addition, our results provide new information to clarify on the nature of the band that forms the 3D Dirac cones, on the possible formation of surface reconstruction of the (001) surface, and on the issue of basic Brillouin zone selection for the (100) surface. [ABSTRACT FROM AUTHOR]

Na3Bi is a Dirac semimetal with protected nodes that may be sensitive to the breaking of time-reversal invariance in a magnetic field B. We report experiments which reveal that both the conductivity and resistivity tensors exhibit robust anomalies in B. The resistivity is B-linear up to 35 T, while the Hall angle exhibits an unusual profile approaching a step function. The conductivities and share identical power-law dependences at large B. We propose that these significant deviations from conventional transport result from an unusual sensitivity of the transport lifetime to B. The transport features are compared with those in Cd3As2. [ABSTRACT FROM AUTHOR]

Thermoelectrics has long been considered as a promising way of power generation for the next decades. So far, extensive efforts have been devoted to the search of ideal thermoelectric materials, which require both high electrical conductivity and low thermal conductivity. Recently, the emerging Dirac semimetal Cd3As2, a three-dimensional analogue of graphene, has been reported to host ultra-high mobility and good electrical conductivity as metals. Here, we report the observation of unexpected low thermal conductivity in Cd3As2, one order of magnitude lower than the conventional metals or semimetals with a similar electrical conductivity, despite the semimetal band structure and high electron mobility. The power factor also reaches a large value of 1.58 mW·m−1·K−2 at room temperature and remains non-saturated up to 400 K. Corroborating with the first-principles calculations, we find that the thermoelectric performance can be well-modulated by the carrier concentration in a wide range. This work demonstrates the Dirac semimetal Cd3As2 as a potential candidate of thermoelectric materials. [ABSTRACT FROM AUTHOR]