Multiple Energy Gaps Research Articles

Unveiling the charge density wave mechanism in vanadium-based Bi-layered kagome metals

The charge density wave (CDW), as a hallmark of vanadium-based kagome superconductor AV3Sb5 (A = K, Rb, Cs), has attracted intensive attention. However, the fundamental controversy regarding the underlying mechanism of CDW therein persists. Recently, the vanadium-based bi-layered kagome metal ScV6Sn6, reported to exhibit a long-range charge order below 94 K, has emerged as a promising candidate to further clarify this core issue. Here, employing micro-focusing angle-resolved photoemission spectroscopy (μ-ARPES) and first-principles calculations, we systematically studied the unique CDW order in vanadium-based bi-layered kagome metals by comparing ScV6Sn6 with its isostructural counterpart YV6Sn6, which lacks a CDW ground state. Combining ARPES data and the corresponding joint density of states (DOS), we suggest that the VHS nesting mechanism might be invalid in these materials. Besides, in ScV6Sn6, we identified multiple hybridization energy gaps resulting from CDW-induced band folding, along with an anomalous band dispersion, implying a potential electron-phonon coupling-driven mechanism underlying the formation of the CDW order. Our finding not only comprehensively maps the electronic structure of V-based bi-layer kagome metals but also provides constructive experimental evidence for the unique origin of CDW in this system.

Crossing multiple solar energy gaps: A Dutch case study on intermediation for building-integrated photovoltaics

Building-integrated photovoltaics are a promising technology to enhance renewable energy production in the built environment while improving the aesthetics of buildings at the same time. Several challenges hinder this technology's uptake, such as information asymmetry and limited value chain coordination. Prior work demonstrates that the support of intermediaries can play a crucial role in coping with these challenges, but this aspect has not yet been previously investigated for this technology. A comprehensive overview is lacking of how various intermediaries can support a multi-stage decision process. Rather than focusing on specific intermediary actors, we explore the Dutch system for building-integrated photovoltaics, identifying which actors act or can act as an intermediary, and what intermediation activities can support the decision process. This article identifies the need for intermediation at the various stages of the decision process and the actors best suited to providing this. Drawing from our empirical findings from 26 in-depth interviews and the literature on innovation adoption and intermediaries, the results reveal that a dynamic ‘ecology of intermediaries’ is necessary to perform various intermediation activities at different system levels in the multi-stage decision process. As these activities and actors are highly interrelated and interdependent, we argue that it is vital to assess intermediation in a holistic way. These findings are significant for suppliers, potential intermediaries, and governments because they can support improving the decision process with the help of intermediation. The present paper contributes to innovation and intermediation studies by demonstrating that intermediation is an interrelated, multi-level, and variegated phenomenon.

Superconductivity of K‐Intercalated Epitaxial Bilayer Graphene

AbstractGraphene‐based materials are among the most promising candidates for studying superconductivity arising from reduced dimensionality. Apart from doping by twisted stacking, superconductivity can also be achieved by metal‐intercalation of stacked graphene sheets, where the properties depend on the choice of the metal atoms and the number of graphene layers. Many different and even unconventional pairing mechanisms and symmetries are predicted in the literature for graphene monolayers and few‐layers. However, those theoretical predictions have yet to be verified experimentally. Here, it is shown that potassium‐intercalated epitaxial bilayer graphene is a superconductor with a critical temperature of Tc = 3.6 ± 0.1 K. By scanning‐tunneling microscopy and angle‐resolved photoelectron spectroscopy, the physical mechanisms are analyzed in great detail, using laboratory equipment. The data demonstrate that electron–phonon coupling is the driving force enabling superconductivity. Although the consideration of an s‐wave pairing symmetry is sufficient to explain the experimental data, evidence is found for the existence of multiple energy gaps. Furthermore, it is shown that low‐dimensional effects are most likely the cause of a gap ratio of 6.1 ± 0.2 that strongly exceeds the Bardeen‐Cooper‐Schrieffer (BCS) value of 3.52 for conventional superconductors. These results highlight the importance of reduced dimensionality yielding unusual superconducting properties of K‐intercalated epitaxial bilayer graphene.

Glassy magnetic freezing and exchange bias effect in NiCo2O4/(Co-Ni)O nanoparticles composite

An anomalous magnetic behavior and exchange bias effect are systematically studied for NiCo2O4/(Co-Ni)O nanoparticles synthesized by a microwave combustion method. Based on a complementary characterization by powder X-ray diffraction and high-resolution transmission electron microscopy, a composite of spinel Ni-cobaltite, NiCo2O4, and rock-salt-structured Co-Ni monoxide, (Co0.65Ni0.35)O, nanoparticles was synthesized in almost uniform elemental distribution. Optical absorbance measurements indicated multiple optical energy gaps with values of 2.02, 3.63 eV and 1.54, 4.015 eV attributed to the cobaltite and monoxide phases, respectively. A magnetic cluster-glass-like behavior with a blocking temperature of ∼ 75 K and irreversibility between the ZFC and FC magnetizations below ∼ 254 K were observed at low fields. An inherent exchange bias effect with significant shifts in both coercive field and remanent magnetization at temperatures up to room temperature is explored.

Influence of pairing symmetry and multiple energy gaps on the behaviour of critical current through the Josephson junction

Influence of pairing symmetry and multiple energy gaps on the behaviour of critical current through the Josephson junction

Effect of magnetic field on the preparation of Cu doped zinc oxide nanostructures in different temperatures

We tried to overcome some industrial defects of crystals (Cu doped ZnO) while improving some of its features through the use of three important factors: Doping (5% Cu), temperature, and magnetic field effect. We have identified some of its properties and dimensions through UV, XRD and FE-SEM measurements. We have succeeded in preparing Cu doped ZnO Nanostructure with different lengths and diameters, more uniform crystals, as well as multiple energy gap values. This allows it to be used in a wide range of technology applications. Energy gap values are clearly affected by the increase in temperature and the presence of the magnetic field. The size of the crystals increases with the increase in temperature and the presence of the magnetic field. The crystals become more uniform and the artificial defects decrease. The change in the dimensions of the crystals with increasing temperature depends on the nature of doping and the nature of semiconductor metallurgy. Lengths and diameters of the Nanostructure increase with increasing temperature and with the presence of the magnetic field, which means more attractive and useful features. Enhanced catalytic efficiency of Cu-doped ZnO is attributed to intrinsic oxygen vacancies mainly due to high surface to volume ratio in nanorods and as well as extrinsic defect due to Cu doping. While the method (Cu doped ZnO) demonstrates the half-metallic ground state as well and the high ferromagnetic stability for all the measured Cu concentrations mostly within the generalized gradient approximation.

Topological Insulator-to-Weyl Semimetal Transition in Strongly Correlated Actinide System UNiSn

Although strong electronic correlations are known to be responsible for some highly unusual behaviors of solids such as, metal--insulator transitions, magnetism and even high--temperature superconductivity, their interplay with recently discovered topological states of matter awaits full exploration. Here we use a modern electronic structure method combining density functional theory of band electrons with dynamical self-energies of strongly correlated states to predict that two well-known phases of actinide compound UNiSn, a paramagnetic semiconducting and antiferromagnetic metallic, correspond to Topological Insulator (TI) and Weyl semimetal (WSM) phases of topological quantum matter. Thus, the famous unconventional insulator-metal transition observed in UNiSn is also a TI-to-WSM transition. Driven by a strong hybridization between U f-electron multiplet transitions and band electrons, multiple energy gaps open up in the single-particle spectrum whose topological physics is revealed using the calculation of Z2 invariants in the strongly correlated regime. A simplified physical picture of these phenomena is provided based on a periodic Anderson model of strong correlations and multiple band inversions that occur in this fascinating compound. Studying the topology of interacting electrons reveals interesting opportunities for finding new exotic phase transitions in strongly correlated systems.

Anomalous enhancement of the lower critical field deep in the superconducting state of LaRu4As12

LaRu4As12 with the critical temperature Tc = 10.4K displays several features which point at a non-singlet superconducting order parameter, although the bcc crystal structure of the filled skutterudites does not favour the emergence of multiple energy gaps. LaRu4As12 displays an unexpected enhancement of the lower critical field deep in superconducting state which can be attributed to the existence of two superconducting gaps. At T = 0.4K, the local magnetization measurements were performed utilizing miniaturized Hall sensors.

Single-crystal growth and physical property characterization of the intermediate-valence compound YbFe2Al10

We report single crystal growth and physical properties characterization of YbFe$_2$Al$_{10}$ compounds. The measurements of resistivity, magnetic susceptibility, and specific heat show different behaviors from previous studies on polycrystal samples. A mixed valent characteristic with moderate mass enhancement is indicated. In particular, the optical spectroscopy measurement reveals formation of multiple hybridization energy gaps which become progressively pronounced at low temperature. The multiple hybridization energy gaps are likely caused by the hybridizations between the flat band from Yb 4$f$ electrons and different bands of conduction electrons.

-BiPd: a clean noncentrosymmetric superconductor

We present a comprehensive review of the normal and superconducting state properties of a high-quality single crystal of monoclinic BiPd (-BiPd, space group ). The superconductivity of this crystal below 3.8 K is established by measuring its properties using bulk as well as spectroscopic techniques. BiPd is one of the cleanest noncentrosymmetric superconductors that display superconductivity with multiple energy gaps. Evidence of multiple energy gaps was found in heat capacity, point contact (PC) spectroscopy, penetration depth, muon spin rotation, small angle neutron scattering and NMR/NQR measurements. Moreover, Muon spin rotation measurements also suggest strong field dependence of the penetration depth of this superconductor. Unusual superconducting properties due to possible s and p wave mixing are shown by the observation of Andreev bound state in PC measurements as well as the suppressed coherence peak in the temperature dependence of the spin–lattice relaxation in the NQR measurements. This surmise is at variance with the recent STM measurements (different crystal). The observed unusual properties and multiband superconductivity are extremely sensitive to disorder in BiPd. Finally, there is a possibility of tuning the electron correlations by selective substitution in BiPd, thus making it an important system for further investigations.

Optical properties and photocatalytic activity of CdS-TiO2/graphite composite

AbstractTiO2film was applied to graphite substrate through chemical bath deposition (CBD) method with TiCl4as precursor. CdS was deposited on TiO2/graphite (TiO2/G) by CBD with precursors of CdSO4. A UV-Vis diffuse reflectance (UV-Vis DR) analysis was used to analyze the optical properties and band gap energy. Meanwhile, photocatalytic activity was determined from the quantum yield (QY) value of isopropanol degradation. The X-ray diffraction pattern shows that the TiO2film contains rutile and anatase phases. UV-Vis DR spectrum shows the photoactivity in the visible light area and provides lower band gap value due to CdS deposition. TiO2/G provide energy gap at 3.0 eV, which indicates that rutile phase is dominant. Meanwhile, CdS-TiO2/G shows multiple energy gaps representing CdS at 2.4 eV, rutile at 3.0 eV, and anatase at 3.2 eV. The QY values of CdS-TiO2/G are 7.98×10-3and 8.62×10-4at 450 and 380 nm of radiation light, respectively. These values are higher than the QY of TiO2/G and TiO2-P25/graphite (TiO2-P25/G), which are 1.19×10-4and 5.0×10-4, respectively. The photocatalytic reaction follows the first-order reaction. CdS deposition allows the rate constant to increase from 2×10-3to 13×10-3under UV radiation.

Single-gap superconductivity inβ−Bi2Pd

beta-Bi2Pd compound has been proposed as another example of a multi-gap superconductor [Y. Imai et al., J. Phys. Soc. Jap. 81, 113708 (2012)]. Here, we report on measurements of several important physical quantities capable to show a presence of multiple energy gaps on our superconducting single crystals of beta-Bi2Pd with the critical temperature Tc close to 5 K. The calorimetric study via a sensitive ac technique shows a sharp anomaly at the superconducting transition, however only a single energy gap is detected. Also other characteristics inferred from calorimetric measurements as the field dependence of the Sommerfeld coefficient and the temperature and angular dependence of the upper critical magnetic field point unequivocally to standard single s-wave gap superconductivity. The Hall-probe magnetometry provides the same result from the analysis of the temperature dependence of the lower critical field. A single-gapped BCS density of states is detected by the scanning tunneling spectroscopy measurements. Then, the bulk as well as the surface sensitive probes evidence a standard conventional superconductivity in this system where the topologically protected surface states have been recently detected by ARPES [M. Sakano et al., Nature Comm. 6, 8595 (2015)] .

Noncentrosymmetric superconductivity in a clean crystal of type II superconductor Bi-Pd

In this work, we present the bulk superconductivity of a high-quality single crystal of monoclinic BiPd (α-BiPd, space group P21) below 3.8 K by studying its electrical resistivity, magnetic susceptibility, and heat capacity. This is the cleanest noncentrosymmetric superconductor (NCS) that display anisotropy due to spin-orbit scattering and also exhibits unusual superconducting properties due to s and p wave mixing as evidenced by the observation of Andreev bound state and multiple energy gaps via point contact measurements. In addition, Fermi surface studies suggest multiband superconductivity in this compound. Penetration depth studies and NQR investigations support mixing of s and p wave Copper paring in this crystal. Moroever, Muon spin rotation measurements indicate strong field dependence of the Ginzburg- Landau coefficient of this superconductor. Unusual pairing and multiband superconductivity are extremely sensitive to disorder and they can be observed only in cleanest (RRR > 170) single crystals.

MULTIPLE SUPERCONDUCTING GAPS, ANISOTROPIC SPIN FLUCTUATIONS AND SPIN-ORBIT COUPLING IN IRON-PNICTIDES

This article reviews the NMR and NQR studies on iron-based high-temperature superconductors by the IOP/Okayama group. It was found that the electron pairs in the superconducting state are in the spin-singlet state with multiple fully-opened energy gaps. The antiferromagnetic spin fluctuations in the normal state are found to be closely correlated with the superconductivity. Also, the antiferromagnetic spin fluctuations are anisotropic in the spin space, which is different from the case in copper oxide superconductors. This anisotropy originates from the spin-orbit coupling and is an important reflection of the multiple-bands nature of this new class of superconductors.

Interface-induced complex electronic interference structures in Ag films on Ge(111)

We have mapped the electronic structure of atomically uniform films of Ag grown on Ge(111) by angle-resolved photoemission. Cuts in the momentum space at constant energies near the Fermi surface reveal intricate patterns resembling interfering waves emanating from multiple centers. The measured dispersion relations exhibit zigzag patterns with multiple energy gaps. These features are attributed to the mixing of electronic standing waves by the Ag-Ge interface potential as confirmed by the observed pattern symmetry and by an experimentally deduced interaction strength that scales as the inverse film thickness.

Surface-plasmon hopping along coupled coplanar cavities

We report on surface-plasmon propagation along coupled coplanar cavities periodically distributed in an otherwise unperturbed plasmonic crystal. We show that the dispersion of particular Bloch modes can exhibit multiple energy gaps that can be adjusted by choosing the cavity size. For resonant sizes, the composite crystals composed of juxtaposed cavities can support plasmon modes at frequencies within the gap of the unperturbed grating. In this case, we demonstrate that the surface-plasmon propagation relies on a hopping mechanism.

Transition spectra for a BCS superconductor with multiple gaps: Model calculations forMgB2

We analyze the qualitative features in the transition spectra of a model superconductor with multiple energy gaps, using a simple extension of the Mattis-Bardeen expression for probes with case I and case II coherence factors. At temperature T = 0, the far infrared absorption edge is, as expected, determined by the smallest gap. However, the large thermal background may mask this edge at finite temperatures and instead the secondary absorption edges found at Delta_i+Delta_j may become most prominent. At finite T, if certain interband matrix elements are large, there may also be absorption peaks at the gap difference frequencies | Delta_i-Delta_j | . We discuss the effect of sample quality on the measured spectra and the possible relation of these predictions to the recent infrared absorption measurement on MgB_2.

Quantum interference in carbon nanotubes

We present magnetotransport properties of a multiwalled carbon nanotube. We have developed a technique using electron beam lithography to connect a carbon nanotube with a small contact resistance. The temperature dependence of the nanotube is thermally activated down to 30 mK but with multiple energy gap. At low temperature, the magnetoconductance can be analysed in terms of quantum interferences. Both weak localisation and universal conductance fluctuations have been observed. The phase coherence length extracted from the weak localisation is consistent with the amplitude of the conductance fluctuations.

Ground-state properties of field-induced spin-density waves in Bechgaard salts.

Introducing the effect of multiple energy gaps within the pair approximation, we reexamine the ground-state properties of the field-induced spin-density wave in Bechgaard salts. We show that the condensation energy and the magnetization are much smaller than those obtained with the single-gap approximation, except in the high-field limit (Hg15 T) where the single-gap result becomes exact.

Limiting efficiencies of ideal single and multiple energy gap terrestrial solar cells

The maximum efficiencies of ideal solar cells are calculated for both single and multiple energy gap cells using a standard air mass 1.5 terrestrial solar spectrum. The calculations of efficiency are made by a simple graphical method, which clearly exhibits the contributions of the various intrinsic losses. The maximum efficiency, at a concentration of 1 sun, is 31%. At a concentration of 1000 suns with the cell at 300 K, the maximum efficiencies are 37, 50, 56, and 72% for cells with 1, 2, 3, and 36 energy gaps, respectively. The value of 72% is less than the limit of 93% imposed by thermodynamics for the conversion of direct solar radiation into work. Ideal multiple energy gap solar cells fall below the thermodynamic limit because of emission of light from the forward-biased p-n junctions. The light is radiated at all angles and causes an entropy increase as well as an energy loss.