Energy Gap Structures Research Articles

Low-energy band structure in Bernal stacked six-layer graphene: Landau fan diagram and resistance ridge

The low-energy band structure of Bernal-stacked hexlayer graphene was investigated by the measuring transport properties of high-mobility graphene samples equipped with a top and a bottom gate electrode at low temperature and in a magnetic field. By analyzing the Landau fan diagram, it was found that the hexlayer graphene consisted of three different bilayer-like bands that overlap and form a semimetal. The electronic band structure was significantly influenced by the perpendicular electric field generated by the top and bottom gate voltages. The Not fan diagram shows splitting of zero-mode Landau levels, while also the top and bottom gate voltage dependence of the resistivity at zero magnetic field shows ridge-like structure, similar to the one recently found in tetralayer graphene. It is shown that the ridge structure originates from the band structure of the Bernalstacked hexlayer graphene. Most of the ridges originate from the bottoms of the bilayer-like bands, and a few ridges originate from the energy gap structures which locally close due to trigonal warping and expected to form mini-Dirac cones.

Andreev reflections and magnetotransport in 2D Josephson junctions

In this paper, the subharmonic energy gap structures (SGS) and induced superconductivity, due to multiple Andreev reflections, in indium gallium arsenide (In0.75Ga0.25As) two-dimensional electron gas (2DEG) are discussed at different temperatures and magnetic fields. Strong suppression of both SGS and induced gap are observed as a function of temperature and magnetic field. The differential conductance of the Josephson junctions (JJs) as a function of external in-plane magnetic fields shows an asymmetric response to positive and negative magnetic field sweeps with a conductance maximum at close to zero. Our approach to quantum transport studies of the ballistic 2D JJs may open a new road towards scalable and integrated quantum processing and help pave the way for the development of topological circuits for the realization of the next generation of quantum processors.

Proximity induced superconductivity in indium gallium arsenide quantum wells

We report on the experimental observation of the proximity induced superconductivity in an indium gallium arsenide (In0.75Ga0.25As) quantum well. The Josephson junction was fabricated by several photo-lithographic processes on an InGaAs heterojunction and Niobium (Nb) was used as superconducting electrodes. Owing to the Andreev reflections and Andreev bound states at the Nb-In0.75Ga0.25As quantum well-Nb interfaces, the subharmonic energy gap structures (SGS) are observed at the differential conductance (dI/dV) versus voltage (V) plots when the applied source-drain bias voltages satisfy the expression VSD = 2Δ/ne. The dI/dV as a function of applied magnetic field B shows a maximum at zero B which decreases by increasing B. When decreasing B to below ±0.4 T, a hysteresis and shift of the conductance maxima close to B = 0 T are observed. Our results help to pave the way to the development of integrated coherent quantum circuitry.

Improved Performance of CuInS2 Quantum Dot-Sensitized Solar Cells Based on a Multilayered Architecture

This article describes a CuInS2 quantum dot (QD)-sensitized solar cell (QDSSC) with a multilayered architecture and a cascaded energy-gap structure fabricated using a successive ionic-layer adsorption and reaction process. We initially used different metal chalcogenides as interfacial buffer layers to improve unmatched band alignments between the TiO2 and CuInS2 QD sensitizers. In this design, the photovoltaic performance, in terms of the short-circuit current density (JSC), open-circuit voltage (VOC), fill factor (FF), and power conversion efficiency (PCE), was significantly improved. Both JSC and VOC were improved in CuInS2-based QDSSCs in the presence of interfacial buffer layers because of proper band alignment across the heterointerface and the negative band edge movement of TiO2. The PCE of CuInS2-based QDSSCs containing In2Se3 interfacial buffer layers was 1.35%, with JSC=5.83 mA/cm2, VOC=595 mV, and FF=39.0%. We also examined the use of alternative CdS and CdSe hybrid-sensitized layers, which were sequentially deposited onto the In2Se3/CuInS2 configuration for creating favorable cascaded energy-gap structures. Both JSC (11.3 mA cm(-2)) and FF (47.3%) for the CuInS2/CdSe hybrid-sensitized cells were higher than those for CuInS2-based cells (JSC=5.83 mA cm(-2) and FF=39.0%). In addition, the hybrid-sensitized cells had PCEs that were 1.3 times those of cells containing identically pretreated In2Se3 interfacial buffer layers. Additionally, we determined that ZnSe served as a good passivation layer on the surface of CuInS2/CdSe hybrid-sensitized QDs, prevented current leakage from the QDs to electrolytes, and lowered interfacial charge recombination. Under simulated illumination (AM 1.5, 100 mW cm(-2)), multilayered QDSSCs with distinct architectures delivered a maximum external quantum efficiency of 80% at 500 nm and a maximum PCE of 4.55%, approximately 9 times that of QDSSCs fabricated with pristine CuInS2.

Theoretical study of chemisorption of hydrogen atoms on the sidewalls of armchair single-walled carbon nanotubes with Stone–Wales defect

The geometrical structures and electronic properties as well as thermal stabilities of hydrogenated armchair (n,n) (n=4–8) SWCNTs with Stone–Wales (SW) defect were investigated using density functional theory (DFT-B3LYP) in combination with the 6-31G* basis set. It is found that the chemisorptions of two hydrogen atoms inside and outside the SW defective SWCNTs are exothermic processes. As the nanotube diameter increases, the reaction energy of exohedral addition decreases, whereas that of endohedral addition increases. Exohedral nanotube adsorption is energetically more favorable than endohedral adsorption for smaller diameter (4,4)-SW, (5,5)-SW SWCNTs, but not for larger diameter (7,7)-SW and (8,8)-SW SWCNTs. The result is different from hydrogen on perfect nanotubes. Significantly, compared with the adsorption on the exterior sidewalls of perfect SWCNTs, the reaction energies of the endo-hydrogenated (7,7)-SW and (8,8)-SW SWCNTs are more exothermic, meaning that the central CC bond of SW defect inside the armchair (7,7)-SW and (8,8)-SW SWCNT sidewalls is more reactive than that in perfect sites. This is different from the result reported on the sidewall reactivity of SW defects outside the armchair SWCNTs. The calculated energy gaps indicate that the hydrogen-chemisorbed SW defective armchair tubes are always wide energy gap structures. The energy gaps of endohedral hydrogen-chemisorbed tubes are higher than those of the corresponding perfect tubes.

Efficient “green” quantum dot-sensitized solar cells based on Cu2S–CuInS2–ZnSe architecture

A Cu(2)S-CuInS(2)-ZnSe quantum dot (QD)-sensitized solar cell with cascaded energy gap structures has been fabricated. Under simulated illumination (AM 1.5, 100 mW cm(-2)), the best device is obtained with a Cu(2)S-CuInS(2)-ZnSe QD-sensitized solar cell, yielding a power conversion efficiency of 2.52%.

Density functional theory study of the hydrogen chemisorption of single-walled carbon nanotubes with carbon ad-dimer defect

The structural and electronic properties of hydrogenated armchair and zigzag SWCNTs with carbon ad-dimer (CD) defect were investigated by means of the B3LYP hybrid density functional method using 6-31G∗ basis set. It is found that the chemisorptions of two hydrogen atoms inside and outside the CD defective SWCNTs are exothermic processes. Exohedral nanotube adsorption is energetically more favorable than endohedral adsorption. These results are in agreement with hydrogen on pristine nanotubes. The positional preference for the chemisorption of two hydrogen atoms is the same for the CD defective armchair and zigzag nanotubes. However, the reaction energy of two hydrogen atoms on the exterior sidewalls of CD defective SWCNTs is almost independent of the tube diameter. This is different from the results reported on pristine nanotubes. The calculated energy gaps indicate that the hydrogen-chemisorbed CD defective armchair tubes are always wide energy gap structures, while the hydrogen-chemisorbed CD defective zigzag tubes have significantly lower gaps. The HOMO–LUMO gap and reaction energy for the chemisorption of more hydrogen atoms on the exterior sidewalls of CD defective armchair SWCNTs were also explored.

Angle-resolved photoemission study of the strongly correlated semiconductor FeSi

The temperature-dependent electronic states of FeSi have been studied by using high-resolution angle-resolved photoemission spectroscopy (ARPES) and using low-energy tunable photons. At low temperatures, a peak indicating the valence-band maximum (VBM) exists at a binding energy of $\ensuremath{\sim}20\text{ }\text{meV}$ along the $\ensuremath{\Gamma}R$ direction. The observed dispersional width of the energy bands is narrower than that given by the band-structure calculation, and the width of the ARPES peak near the VBM rapidly broadens as the binding energy increases. Analysis of a model self-energy reveals the importance of electron correlation, especially near the VBM. We observed an unusual temperature dependence of the ARPES spectral features near the Fermi level $({E}_{F})$: Below $\ensuremath{\sim}100\text{ }\text{K}$, the peak at the VBM and the energy gap structures are almost unchanged, while at $\ensuremath{\sim}100--350\text{ }\text{K}$, the peak gradually moves toward ${E}_{F}$ and the gap is filled. The present results indicate that FeSi is a strongly correlated semiconductor, with a renormalized band near ${E}_{F}$ being responsible for the rapid collapse of the peak and the coherent energy gap upon heating.

Expanding the World of Endohedral Fullerenes—The Tm3N@C2n (39≤n≤43) Clusterfullerene Family

A new family of endohedral fullerenes, based on an encaged trithulium nitride (Tm(3)N) cluster, was synthesised, isolated and characterised by HPLC, mass spectrometry, and visible-NIR and FTIR spectroscopy. Tm(3)N clusterfullerenes with cages as small as C(76) and as large as C(88) were prepared and six of them were isolated. Tm(3)N@C(78) is a small clusterfullerene. The two isomers of Tm(3)N@C(80) (I and II) were the most abundant structures in the fullerene soot. Tm(3)N@C(82), Tm(3)N@C(84), and Tm(3)N@C(86) represent a new series of higher clusterfullerenes. All six isolated Tm(3)N clusterfullerenes were classified as large energy-gap structures with optical energy gaps between approximately 1.2 and approximately 1.75 eV. Tm(3)N@C(80) (I) and Tm(3)N@C(80) (II) were assigned to the C(80) cages C(80):7 (I(h)) and C(80):6 (D(5h)). For Tm(3)N@C(78), the analysis pointed to an elliptical carbon cage with C(78):1 (D(3)) or C(78):4 (D(3h)) being the probable structures.

Spin-injection properties and conductance spectra of Co/Au/YBaCuO and Co/Au/BiSrCaCuO tunnel junctions

Tunnelling spectra of Co/Au/YBa2Cu3Oy (YBaCuO) and Co/Au/Bi2Sr2CaCu2Ox (BiSrCaCuO) tunnel junctions have been studied in order to understand the electrical properties of ferromagnet/high-Tc superconductor junctions, and to study the difference between YBaCuO and BiSrCaCuO. The energy gap structures and a zero-bias conductance peak (ZBCP) are clearly observed in conductance spectra for both Co/Au/YBaCuO and Co/Au/BiSrCaCuO junctions. This result can be compared with that of Au/YBaCO and Au/BiSrCaCuO junctions, a normal metal/superconductor (S/N) junction. In the case of Co/Au/YBaCuO and Co/Au/BiSrCaCuO junctions, the amplitude of the ZBCP is smaller than that of the S/N junctions. This result indicates that the tunnelling properties are strongly affected by a spin polarization of electrons flowing through the junctions. Furthermore, the amplitude of the ZBCP observed in the Co/Au/YBaCuO junction is smaller than that of the Co/Au/BiSrCaCuO junction. The difference is attributed to the difference in anisotropy of the two superconductors.

Subharmonic energy-gap structures and 1/ f noise properties in Nb/Al+AlO x/Nb tunnel junctions

We have investigated subharmonic energy-gap structures (SGSs) and 1/ f noise spectra in Nb/Al+AlO x /Nb tunnel junctions as a function of bias voltage. The clear SGSs were observed in d V/d I vs. V curves at 4.2 K. The quasiparticle characteristic was explained by the Octavio, Tinkham, Blonder and Klapwijk theory based on multiple-Andreev reflection model. To estimate the 1/ f noise parameter α/ N c in the Nb/Al+AlO x /Nb junctions, we used Hooge's empirical theory. The α/ N c of these junctions biased above superconducting energy-gap did not depend on bias voltage, while below the gap voltage it was larger than that above the gap voltage. Moreover, some peak structures related to the SGSs were observed in α/ N c vs. V characteristics biased below the gap voltage. This enhanced 1/ f noise might be caused by MAR current.

The tunneling conductance inside the unit cell of Bi 2Sr 2CaCu 2O 8

We model the unit cell of Bi 2Sr 2CaCu 2O 8 as six equally spacing layers based on the layered nature of the material and solve the Hamiltonian matrix of the system in Nambu representation. The tunneling current through the junction between Cu O planes in the unit cell is calculated and then differentiated to obtain the tunneling conductance. Our calculation results indicate that the conductance has peak values near double energy gap and characteristic fine structures. The results may be applied for accurate calculation of behaviors of single-electron devices made by unit cells of Bi 2Sr 2CaCu 2O 8.

Photon-assisted Andreev transport and sub-gap structures

We report new measurements of microwave-induced perturbations of the sub-harmonic energy gap structures in the current–voltage characteristics of superconductor–semiconductor-superconductor junctions. Around the sub-gap bias voltages associated with the enhanced quasi-particle transfer mediated by multiple Andreev reflection processes we observe microwave induced satellites, shifted in voltage by multiples of hf/ en, where hf is the photon energy and n is the number of quasi-particle traversals as determined by the Andreev processes. The observed behavior is the analogue of the so-called photon-assisted tunneling but here associated with the multiple Andreev reflections.

D-wave superconductivity of the hole-doped(SrCa)10Cu17O29ladder compound

We performed tunneling experiments on single crystals of hole-doped two-leg ladders $(\mathrm{SrCa}{)}_{10}{\mathrm{Cu}}_{17}{\mathrm{O}}_{29}$ with ${T}_{c}\ensuremath{\cong}75$ K and, for reference purposes, on textured bulk samples of ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8}$ (Bi2212). Point-contact and break-junction (Bi2212 only) junction configurations were used. The point-contact junctions, both superconductor--insulator--normal-metal and superconductor--normal-metal, were using Ag counterelectrodes. In all experiments the tunneling direction was along ${\mathrm{Cu}}_{2}{\mathrm{O}}_{3}$ planes (for the ladders) and ${\mathrm{CuO}}_{2}$ planes (for Bi2212). The recorded tunneling conductance spectra were reproducible with sharp, well-defined energy-gap structures. All junctions exhibited a pronounced zero-bias conductance peak disappearing at the critical temperature. Experimental results are consistent with the formation of zero-energy Andreev bound states at the surface of the superconductors and ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$-wave symmetry of the order parameter $\ensuremath{\Delta}$ in both compounds.

Evidence ofdx2−y2symmetry in the tunneling conductance density of states ofTl2Ba2CuO6

Tunnel junctions have been made on single crystals of ${\mathrm{Tl}}_{2}{\mathrm{Ba}}_{2}{\mathrm{CuO}}_{6}$ with a ${T}_{c}$\ensuremath{\sim}91 K using a mechanical point-contact method. Both superconductor--insulator--normal-metal (SIN) and superconductor-insulator-superconductor (SIS) junctions were obtained using Au and Nb tips, respectively. The tunneling conductances obtained are reproducible, with sharp, well-defined energy gap structures and are characterized by a cusplike feature at zero bias. These conductances can be fit to a simple, momentum-averaged density of states (DOS) obtained from a superconducting order parameter with ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ symmetry. The SIS data display a weak Nb gap feature which is consistent with the subgap DOS observed in the SIN data.

Epitaxial growth and tunneling properties of YBa 2Cu 3O 7−x/PrBa 2Cu 3O 7−x/YBa 2Cu 3O 7−x trilayer structures

Using a multitarget high oxygen pressure sputtering system, we have produced in situ high quality epitaxial YBa 2Cu 3O 7−x/ PrBa 2Cu 3O 7−x/YBa 2Cu 3O 7−x trilayer structures on SrTiO 3 (001) substrates with PrBa 2Cu 3O 7−x barrier thicknesses ranging from 10 to 30 nm. The structural properties were determined by x-ray diffraction. Rutherford Back Scattering (RBS) and Transmission Electron Microscopy (TEM) analysis, which showed an epitaxial growth of the trilayers with sharp and clean interfaces. I–V and dI/dV vs V measurements showed a clear SIS quasiparticle tunneling behavior with well developed peaks at ± 45m V indicating the presence of energy gap structures at ±2Δ and flat background conductances at high bias. Ratios of the conductance at zero bias, G(0), to the normal conductance at 150 mV, G(150), of about 10 % are observed. These characteristics can be related to the very good interfaces and superconducting properties observed in our heterostructures.

MONTE CARLO MODEL FOR ELECTRON TRANSPORT IN THE MATERIAL 3C-SiC

Static electron transport in the material 3C-SiC is analyzed by using single particle Monte Carlo method at high field and high temperature. The physical model used in the simulation is developed by considering the energy gap structures and the main scattering mechanisms in details. The results show the excellent high field and high temperature properties of the material. The scattering mechanisms at high temperature and high field are discussed by analysing the results.

Tunneling characteristics of Y1−xPrxBa2Cu3O7−y/Pb planar junctions

We have fabricated electron tunneling junctions on thin films of Y1−xPrxBa2Cu3O7−y, with x=0−0.45, which were grown by laser ablation on (100) MgO substrates. The tunneling characteristics of the YBCO/Pb and YPBCO/Pb planar junctions were measured. They all exhibited Pb energy gap and phonon structures as well as the gap-like structure of YBCO at 4.2 K. However, no gap-like structure of YPBCO was observed with high Pr concentration. The differential conductance of YPBCO films changes discontinuously both in shape and in value at Tc. A tunneling model is proposed.

Andreev-reflection in highly doped Pb/n?InSb/Pb junctions

We studied the carrier transport through n-InSb-films sandwiched between two superconducting lead electrodes. These n-InSb-films were about 30 nm thick, polycrystalline and very highly doped (1021 cm−3). TheV(I)- and dV/dI (V)-characteristics indicated all the properties of Andreev-reflection (subharmonic energygap structures (SGS) and excess current) according to the theory of Octavio et al. (OTBK-model) [1, 2]. Our measurements show the temperature dependence of these structures and also the influence of magnetic fields up to 80 mT and give a further reference to the importance of Andreev-reflection in semiconductor-coupled superconducting junctions (SSmS junctions).

Transport properties in an InAs-inserted-channel In0.52Al0.48As/In0.53Ga0.47As heterostructure coupled superconducting junction.

Supercurrent is observed in a superconductor-semiconductor-superconductor (S-Sm-S) junction using a two-dimensional-electron-gas system in an InAs-inserted-channel ${\mathrm{In}}_{0.52}$${\mathrm{Al}}_{0.48}$As/${\mathrm{In}}_{0.53}$${\mathrm{Ga}}_{0.47}$As heterostructure. The temperature dependence of the supercurrent cannot be explained by a simple exponential expression, but is here explained in terms of the clean-limit theory, taking account of the higher-order effect of the coherence length. The differential resistance of the junction shows clear subharmonic energy-gap structures due to multiple Andreev reflection since the phase coherence is maintained during the Andreev-reflection process. The Cooper pair transport explained by the clean-limit theory is attributed to the fact that the mean free path is longer than the coherence length and the length between the superconducting electrodes.