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In this work, the transverse magnetic focusing of pseudospin-1 fermions in two-dimensional materials with a superconducting contact is studied. The transmission coefficient and Andreev reflection coefficient from injector lead to collector lead are calculated based on the Landauer–Büttiker formula combined with the non-equilibrium Green's function method. The local state density diagrams of the electron and hole transmission in the center region are given, which provide a complete image of the ballistic electron–hole cyclotron orbits. Using these images, the positive longitudinal shift in Andreev reflection of the pseudospin-1 fermion can be obviously seen. Furthermore, it is found that the total offset is proportional to the number of Andreev reflection occurrence. This setup provides the possibility of detecting this shift experimentally. [ABSTRACT FROM AUTHOR]

We investigate the transport properties in nodal-line Weyl semimetal resonant tunneling junction. Double reflections and double transmissions appear, which are specular reflection, retroreflection, normal transmission, and specular transmission. The incident angle dependence transmissions and reflections are studied with different incident energies and different quantum well widths. The retroreflection and the specular transmission appear only when the incident angle of the electron θ is less than the critical value θc. As the incident energy approximately equals the z component of the momentum, double transmissions disappear completely, perfect retroreflection (specular reflection) can be found when θ is smaller (bigger) than θc. Under certain conditions, the transmission and reflection types and their magnitudes can be controlled. For the different cases that the nodal line of the Weyl semimetal is parallel to and perpendicular to the interface, the conductance spectra exhibit strong anisotropy. These physical phenomena help us to design nodal-line Weyl semimetal-based nanodevices. [ABSTRACT FROM AUTHOR]

The transport properties in graphene-based asymmetric double velocity well (Fermi velocity inside the well less than that outside the well) and electrostatic well structures are investigated using the transfer matrix method. The results show that quantum beats occur in the oscillations of the conductance for asymmetric double velocity wells. The beating effect can also be found in asymmetric double electrostatic wells, but only if the widths of the two wells are different. The beat frequency for the asymmetric double well is exactly equal to the frequency difference between the oscillation rates in two isolated single wells with the same structures as the individual wells in the double well structure. A qualitative interpretation is proposed based on the fact that the resonant levels depend upon the sizes of the quantum wells. The beating behavior can provide a new way to identify the symmetry of double well structures. [ABSTRACT FROM AUTHOR]

By using the nonequilibrium Green's function combined with the tight-binding Hamiltonian, we have studied the effects of the edge states on the conductance and the thermopower for zigzag bilayer graphene nanoribbons (ZBGNs). It is shows that the band structure, conductance, and thermopower can be modulated by the boundary potentials and the bias voltages to the layers of the ZBGNs. When the boundary potentials are adjusted to the nearest neighbor hopping energy, two gapless edge modes with opposite velocities appear in the vicinity of the two Dirac points, and the quantized conductance has a transition from 2(n+1)G0 to 2(n+1/2)G0 with G0=2e2/h being the conductance unit and n an integer. Particularly, under the strong bias voltage, compare to the gapless perfect ZBGN, the thermopower can be enhanced more than twice for the gapless edge modes with opposite velocities. Combining the reduced thermal conductivity in few-layer graphene, our results show that the modulated ZBGNs are more reliable in thermoelectric application. [ABSTRACT FROM AUTHOR]

The Andreev reflection in topological nodal-line semimetal–superconductor (TNLSM–SC) junctions is studied based on the Landauer–Büttiker formula combined with the non-equilibrium Green's function method. It is found that the Andreev reflection coefficient can reach large values of several tens for a TNLSM–SC junction. In the energy gap, two new peaks for the Andreev reflection appear. Outside the energy gap, the Andreev reflection shows a quasi-periodic variation. With the incident energy increasing, the Andreev reflection decreases periodically. This phenomenon originates from the peculiar energy spectrum of the TNLSMs. The value and quasi-period of the Andreev reflection coefficient depend on the size of the system in different directions. These anomalous Andreev reflections are helpful to study the transport properties in normal metal–superconductor junctions. [ABSTRACT FROM AUTHOR]

Using a mode matched scattering matrix method, we study the effect of Rashba spin-orbit coupling and a tunnel barrier on the zero conductance and the tunnel magnetoresistance (TMR) in a one-dimensional conducting Aharonov–Bohm ring symmetrically coupled to two ferromagnetic leads. It is found that the conductances of spin-up and spin-down electrons show periodic variations as the ring’s length changes. The conductance amplitude in antiferromagnetic alignment is very different from that in ferromagnetic alignment. The lifting of the real zero conductance is changed by the spin-orbit interaction and the tunnel barrier. The numerical results also show that this structure can provide a large TMR. When the ring’s length is increased, the TMR varies its period. [ABSTRACT FROM AUTHOR]

The spin-dependent Andreev reflection (AR) tunneling through a ferromagnet double-quantum-dot superconductor (F-DQD-S) is studied using the nonequilibrium Green’s function method. The F-DQD-S is a T-shaped hybrid structure where one of the dots is coupled to the F and S electrodes and the other dot is coupled to the first dot only. On each dot the intradot spin-flip process is present. The behavior of the F-DQD-S tunneling conductance is determined by the competition among the interdot coupling, the intradot spin-flip process, and the coupling of the first dot to the electrodes. When the interdot coupling is present, the quantum interference effects between the two dots cause secondary peaks in the tunneling conductance besides the primary one, indicating a structure in the AR spectrum. On the other hand, the spin-flip scattering process suppresses the AR peaks. With the spin-flip scattering strength increasing, the resonance peaks are split. [ABSTRACT FROM AUTHOR]

Using the mode-matching method, spin-dependent electron transport properties in multichannel ferromagnet/semiconductor/ferromagnet nanowires are studied. The significant quantum confinement and Rashba spin-orbit interaction are considered simultaneously. As the width of the wire varies, corresponding channels for electrons are opened, and the conductance oscillates as a function of the length of the semiconductor. As the strength of the spin-orbit interaction increases, the phase of the conductance changes and the resonant peaks shift to the right. When two electron channels are open, the conductance oscillates anomalously and the polarization becomes smaller than that for only one open channel. [ABSTRACT FROM AUTHOR]

The Nernst and Seebeck effects in graphene with uniform Kekulé lattice distortion have been studied using the tight-binding model combined with the nonequilibrium Green's function method. Numerical results of this work showed that due to the electron–hole symmetry, the Nernst coefficient is an even function of the Fermi energy, while the Seebeck coefficient is an odd function regardless of the magnetic field. The Nernst and Seebeck coefficients show peaks when the Fermi energy crosses the Landau levels at high magnetic fields or crosses the transverse subbands at the zero magnetic fields. The peak height can be very large when the Fermi energy approaches the Dirac point, the Seebeck coefficient can reach about 0.78 mV/K, and the Nernst coefficient can reach about 0.95 mV/K at the corresponding hopping energy modification parameter δ = 0.03 and T = 0.009 t / k B ≈ 288 K. When δ = 0.08 and T = 0.024 t / k B ≈ 766 K, the Seebeck coefficient (or Nernst coefficient) is still up to about 0.78 mV/K (or 0.95 mV/K). This suggests that tunable Seebeck and Nernst coefficients can be achieved because the bandgap is a function of the corresponding hopping energy modification parameter δ. Experimentally, δ can be modulated by changing the type and amount of atoms adsorbed on graphene. In strong magnetic fields, the Nernst coefficient does not depend on the chirality of the nanoribbon. [ABSTRACT FROM AUTHOR]

We have studied tunneling transport properties through multi-quantum-dot coupled to Majorana bound states (MBSs), in which only one quantum dot is connected with two normal leads. When the MBSs are sandwiched between two quantum dots (dot-MBSs-dot), the signature of Majorana fermions; namely, that the conductance is e2/h at zero fermion energy, is robust against the coupling (εM) between the two Majorana bound states as well as the coupling between the MBSs and the quantum dots which, in the dot-MBSs structure, will change even with a very small value of εM. The model that the Majorana bound states are coupled with one of a series of connected quantum dots (three connected quantum dots coupled with MBSs, e.g., dot-dot-dot-MBSs) is also investigated. Unlike the resonant condition for quantum dots structure with zero MBSs, if the MBSs couple with an even number of quantum dots, the conductance at zero Fermion energy is 2e2/h. On the other hand, in case of an odd number of quantum dots, it is e2/h. [ABSTRACT FROM AUTHOR]

The Seebeck and Nernst effects in HgTe/CdTe quantum wells are studied using the tight-binding Hamiltonian and the nonequilibrium Green's function method. The Seebeck coefficient, Sc, and the Nernst coefficient, Nc, oscillate as a function of EF, where EF is the Fermi energy. The Seebeck coefficient shows peaks when the Fermi energy crosses the discrete transverse channels, and the height of the nth peak of the Sc is [ln2/ (1/2+∣n∣)] for EF>0. For the case EF<0, the values of the peaks are negative, but the absolute values of the first five peaks are the same as those for EF>0. The 6th peak of Sc reaches the value [ln2=1:35 due to a higher density of states. When a magnetic field is applied, the Nernst coefficient appears. However, the values of the peaks for Nc are all positive. For a weak magnetic field, the temperature suppresses the oscillation of the Seebeck and Nernst coefficients but increases their magnitude. For a large magnetic field, because of the highly degenerate Landau levels, the peaks of the Seebeck coefficient at position EF = -12, 10, 28 meV, and Nernst coefficient at EF = -7, 10 meV are robust against the temperature. [ABSTRACT FROM AUTHOR]

We have studied in detail the influence of same-orbit and different-orbit hopping disorders in HgTe/CdTe quantum wells. Intriguingly, similar to the behavior of the on-site Anderson disorder, a phase transition from a topologically trivial phase to a topological phase is induced at a proper strength of the same-orbit hopping disorder. For different-orbit hopping disorder, however, the phase transition does not occur. The results have been analytically verified by using effective medium theory. A consistent conclusion can be obtained by comparing phase diagrams, conductance, and conductance fluctuations. In addition, the influence of Rashba spin-orbit interaction (RSOI) on the system has been studied for different types of disorder, and the RSOI shows different influence on topological phase at different disorders. The topological phase induced by same-orbit hopping disorder is more robust against the RSOI than that induced by on-site Anderson disorder. For different-orbit hopping disorder, no matter whether the RSOI is included or not, the phase transition does not occur. The results indicate, whether or not the topological Anderson insulator can be observed depends on a competition between the different types of the disorder as well as the strength of the RSOI in a system. [ABSTRACT FROM AUTHOR]

We investigate spin-filtering effect in multilayered ferromagnetic (F)/semiconductor (S) heterostructures within the Landauer framework of ballistic transport. Spin-dependent transmission and polarization are calculated and analyzed for different magnetizations of three ferromagnetic layers in a F/S/F/F structure proposed in this work. The results indicate that in such a multilayered configuration and when the magnetizations of the middle and the right ferromagnetic layers are antiparallel, the transmission for spin-up and spin-down electrons can be separated, which is quite different from the transport properties in the F/S/F structure, where electrons of different spin orientations have exactly the same contributions to transmission if the magnetic moments of the two ferromagnetic layers are antiparallel. It is also shown that the F/S/F/F structure can have big values of the polarization than the F/S/F structure. The quantum size effect of the length of the middle ferromagnetic layer and that of the semiconductor layer are discussed. Moreover, the polarization can be reversed when we switch the magnetizations of the middle and the right ferromagnetic layers. [ABSTRACT FROM AUTHOR]

Performing the non-equilibrium Green's function method, we calculate the Seebeck coefficient (S) and the thermoelectrical figure of merit (Z T) in the quantum anomalous Hall insulator-topological superconductor-quantum anomalous Hall insulator hybrid structure with temperature gradient. It is found that a high thermoelectrical figure of merit can be achieved with the helical topological superconductor phase. The S and the Z T are mainly affected by the temperature and the device size. At a certain temperature and the right device size, the Seebeck coefficient and the figure of merit can reach very high values. [ABSTRACT FROM AUTHOR]

We discuss the transport properties of thermal electrons in double quantum dots that are coupled with Majorana bound states (MBSs) corresponding to two model systems with T-type structure and series connection structure. It has been found that the thermoelectric figure of merit ZT in these model systems is suppressed when we consider the effects of the Majorana bound states. Here, ZT = GS²T/κ, where G is the electric conductance, S is the thermopower, T is the temperature and κ is the thermal conductance. The sign of the thermopower S changes from negative to positive when the energy levels of the quantum dots are less than μ while the sign of the thermopower S changes from positive to negative when the energy levels of the quantum dots are above μ in the model system of T-type structure, where μ is the chemical potential. As a result, the figure of merit ZT first decreases and then increases as the temperature kBT increases. This behavior is different from what is seen in the general quantum dot structure without MBSs. It is interesting to showthat in the series connection structure, the thermopower S and ZT are robustness and do not vary with changes in εM when |εd| < λ, even if κ changes with εM. [ABSTRACT FROM AUTHOR]

Nanomechanics and real-time buckling deformation of an individual multi-walled carbon nanotube (MWCNT) were investigated through in situ nanoindentation within a transmission electron microscope (TEM). These in situ observations reveal a significant shell-to-Euler phase transformation in the buckling response of the nanotube. Objective evidences that the MWCNT possesses time-dependent characteristic were first suggested by combining in situ TEM nanoindentation performed strain rate influences on an individual MWCNT with classical molecular dynamics simulations. Structural evolutions and buckling instabilities for thin-wall and thick-wall CNTs are theoretically studied, indicating the role of the tube thickness and interwall van der Waals interactions in governing buckling behavior. [ABSTRACT FROM AUTHOR]

The conductance and the shot noise were investigated in graphene superlattice. A new Dirac point appears in the graphene superlattice and the position of the new Dirac point depends on the ratio of the width of the barriers to that of the wells. At the new Dirac point the conductance is minimum and the Fano factor reaches its max value 1/3. This Dirac point is robust against the magnetic field. In the energy window below the Dirac point, the magnetic field weakens the conductance but enhances the Fano factor. [ABSTRACT FROM AUTHOR]

Ba0.4Sr0.6TiO3 ceramics by adding mental Fe powders have been fabricated via the solid-state reaction method. The microstructures and optical properties of samples are systematically studied in order to establish the effects of Fe powder additives on microwave dielectric properties of Ba0.4Sr0.6TiO3 ceramics by x-ray diffraction, x-ray photoelectron spectroscopy, and optical reflective spectrum. The results show the coexistence of Fe2+ and Fe3+ in Ba0.4Sr0.6TiO3 ceramics, the decrease of O vacancy concentrations, and their incorporation into the B-site (Ti) of the Ba0.4Sr0.6TiO3 host lattice give rise to excellent microwave dielectric properties. All samples have a higher Q value above 290 while maintaining relatively high tunability above 16.6%. In particular, the sample with the composition of x = 0.035 mol has the dielectric constant of 889, Q × f value of 826 (at 1.370 GHz), and tunability of 24%, which are very promising for high power tunable devices. In comparison, Fe2O3 oxide doped Ba0.4Sr0.6TiO3 ceramics with the same molar ratios of Fe exhibit inferior microwave properties. It indicates that additives of the metal Fe powders can more effectively improve dielectric properties of BaxSr1-xTiO3 system than Fe2O3 oxide. [ABSTRACT FROM AUTHOR]