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We experimentally investigate capacitance response of a thick ferroelectric GeTe single-crystal flake on the Si/SiO2 substrate, where p-doped Si layer serves as a gate electrode. We confirm by resistance measurements, that for three-dimensional flakes, electron concentration is not sensitive to the gate electric field due to the screening by bulk carriers. Unexpectedly, we observe that sample capacitance C is strongly diminishing for both gate field polarities, so C(Vg) is a maximum near the zero gate voltage. Also, we observe well-developed hysteresis with the gate voltage sweep direction for the experimental C(Vg) curves. From our analysis, the capacitance behavior is explained by the known dependence of the Rashba parameter on the electric field for giant Rashba splitting in GeTe. In this case, the hysteresis in capacitance should be ascribed to polarization evolution in GeTe surface layers, which also allows to realize the regime of dynamic negative capacitance. The latter can be directly observed in time-dependent resistive measurements, as non-monotonic evolution of voltage response to the step-like current pulse. Thus, the negative capacitance regime can indeed improve performance and, therefore, the energy efficiency of electronic devices.

ZnO/ZnMgO multiple quantum wells are investigated in terms of intersubband transitions within the conduction band. The Schrodinger's and Poisson's equations are solved self-consistently. The intersubband transition energy is represented as a function of their physical dimension and Mg concentration. The effects of the depolarization and excitonic shifts on the transition energy are considered. A high level of doping, such as 1 × 10 25 m −3 , is required to screen the built-in electric field, which is around 0.355 MV/cm at most. In case of screening, it is possible to have quantum wells aligned in energy and a homogeneous electron distribution over the quantum wells. The transition energy is between 51 and 403 meV if the width of the constituting quantum wells is between 20 and 100 A. The contribution of the many-body effects is 29 meV at most. However, this contribution strongly depends on the well width: it can be as low as −1 meV.

This paper reports a research of the magneto-transport property of an AlInN/AlN/GaN heterostructure. A large positive magnetoresistance (MR) up to 79% under 9 T is observed, which is indicative of potential sensor application. The observed positive MR is attributed to the presence of the parallel conduction. Meanwhile, the Shubnikov-de Hass (SdH) oscillations of the longitudinal MR appear up at high magnetic field. From the SdH oscillations, beating patterns are observed in the AlInN/AlN/GaN heterostructure. Neither the magneto-intersubband oscillation nor the zero-field spin splitting can explain the observed beating patterns, which can be attributed to the sample inhomogeneity.

A study of the electron states of a semiconductor core/shell quantum well wire and the electron Raman scattering are presented. Thus, the system has been modeled considering a G a A s / A l x G a 1 − x A s / A l A s structure. To carry out this study, the exact mathematical expressions that describe the electron states and the Raman differential cross-section in the aforementioned system were obtained. Our model considers T = 0 K and that, as a consequence of the presence of the confinement, the electron bands unfold in a subband system. In addition, the expressions that describe the differential cross-section and the selection rules for the emission spectra are shown. On the other hand, the net gain of Raman is studied through the transition rate, since they are directly proportional. It should be point out that our results are compared with the results obtained in previous works, especially with the step-quantum well wire. Finally, it is shown that although the core/shell quantum well wire and the step-quantum well wire are very similar systems, the Raman net gain for the step-quantum well wire is much greater than the one for the core/shell quantum well wire.

We obtain exact expressions for the local current and shot noise and the nonlocal shot noise in the first propagating channel of an NSN (normal-superconducting-normal) nanowire. Using high-$T_c$ cuprate superconductors (HTS) as our model systems we utilize a scattering theory approach to derive the scattering matrix for the systems in the high transparency limit and without the Andreev approximation. The local current and shot noise spectrum is calculated and plotted. In view of recent work in the area of cooper-pair splitting (CPS) devices, we investigate the nonlocal shot noise and relate the behavior of the noise as we change parameters such as the system size, the orientation of the order parameter and the biasing of the junction in the HTS NSN system., Comment: 25 pages, 9 figures

We propose a scheme to manipulate the shot noise spectrum via the gate voltage Vg(t) of the quantum point contact. It has been known that shot noise can be used as a hallmark of electron temperature in the reservoirs. We show that, by choosing the pulse shape of Vg(t) properly, the zero-frequency shot noise can be tuned so that it is the same with the DC shot noise of two electron reservoirs at different temperatures. The voltage pulse width is rather fixed and cannot be changed efficiently. We provide a way to construct an amplitude-modulated pulse train to increase the signal strength of shot noise, which is useful in the study of shot noise experimentally.

General nonequilibrium quantum transport equations are derived for a coupled system of charge carriers, Dirac spin, isospin (or valley spin), and pseudospin, such as either one of the band, layer, impurity, and boundary pseudospins. Limiting cases are obtained for one, two or three different kinds of spin occurring in a system. We show that a characteristic integer number Ns determines the formal form of spin quantum transport equations, irrespective of the type of spins or pseudospins, as well as the maximal entanglement entropy. The results may shed a new perspective on the mechanism leading to zero modes and chiral/helical edge states in topological insulators, integer quantum Hall effect topological insulator (QHE-TI), quantum spin Hall effect topological insulator (QSHE-TI) and Kondo topological insulator (Kondo-TI). It also shed new light in the observed competing weak localization and antilocalization in spin-dependent quantum transport measurements. In particular, a novel mechanism of localization and delocalization, as well as the new mechanism leading to the onset of superconductivity in bilayer systems seems to emerge naturally from torque entanglements in nonequilibrium quantum transport equations of spin and pseudospins. Moreover, the general results may serve as a foundation for engineering approximations of the quantum transport simulations of spintronic devices based on graphene and other 2-D materials such as the transition metal dichalcogenides (TMDs), silicene, as well as based on topological materials exhibiting quantum spin Hall effects. The extension of the formalism to spincaloritronics and pseudo-spincaloritronics is straightforward.

Thermal energy transfer across two-dimensional silicon and diamond thin films are considered and the influence of the thermal boundary resistance, due to mismatching of properties, on the acoustic phonon transport characteristics is studied. The transient form of the Boltzmann equation with frequency dependency of phonons is incorporated simulating the phonon transport. Thermal boundary resistance is considered incorporating mismatches of interface conditions of films, namely cut-off and diffusive models. The ratios of longitudinal and transverse acoustic phonon wave intensities because mismatch models (cut-off and diffusive mismatches) are predicted for temperature oscillating boundary condition at the silicon edge. It is found that the ratio of the longitudinal acoustic phonon intensities ratio because of mismatch models (cut-off over diffusive) does not behave exactly the oscillation of temperature at the edge of the silicon film due to scattering of phonons in the films. The same arguments are applicable for the ratio of transverse acoustic phonon intensities because of the interfacial mismatch models (cut-off over diffusive). The consideration of the phonon frequencies cut-off results in the ballistic phonon transfer in between films, which lowers the interfacial thermal resistance.

We present results for the nonlinear optical rectification of an asymmetric GaAs/AlGaAs quantum dot structure coupled to a gold nanosphere under the influence of a strong electromagnetic field. We show that the nonlinear optical rectification coefficient depends strongly on the distance between the quantum dot and the metallic nanosphere and on the direction of the applied electromagnetic field.

Formalism is proposed for studying of the thermodynamic properties of ferromagnetic nanowires, on the basis of which it is established that at temperatures below the Debye temperature the contribution of a domain wall to the heat capacity of ferrite-garnet nanowires can exceed the phonon and magnon contributions. The conditions for realization of this phenomenon are determined. It is shown that this effect has a nanoscale nature. It is found that by varying the magnitude of the nanowire magnetization it is possible to influence its temperature and dimensional characteristics.

In the present work we theoretically study thermoelectric transport and heat transfer in a junction including a $T$-shaped double quantum dot coupled to nonmagnetic electrodes and supplemented with a third dot in a parallel configuration. We focus on the combined effect of Coulomb interactions and quantum interference occurring in the $T$-shaped portion of the considered triple quantum dot on the thermoelectric electron transport. The transport through the system is studied within the Coulomb blockade regime in the limit of strong intra-dot interactions between electrons beyond the linear response regime. It is shown that under these conditions both charge and heat transfer through the considered system may be significantly affected by the quantum interference and inter-dot Coulomb interactions., Comment: 8 pages, 6 figures

Based on transfer matrix method, the giant magnetoresistance (GMR) effect of 2-dimensional electron gas (2DEG) under magnetic and electrical potential is investigated. Such system can be obtained easily by depositing two ferromagnetic (FM) stripes and one schottky metal (SM) on the surface of semiconductor heterostructure. Our study shows a great difference in electrons tunneling through parallel (P) and antiparallel (AP) magnetization of FM stripes. The corresponding magnetoresistance ratio (MRR) depends strongly on the magnetic intensity of magnetic barrier (MB) and the height (U(x)) of electric barrier (EB). Furthermore, we also study the effect of SM stripe width (ds) on MRR for a given value of U(x). It indicates that the MRR of the device is sensitive to the ds. These results may offer an alternative to get a tunable GMR device by adjusting U(x) and width of the SM stripe.

WOS: 000457720900013, We have investigated the binding energies of the s-symmetric ground and first excited shallow donor impurity states and the total 1s -> 2s absorption coefficient, including the first and third order corrections, in "12-6" tuned GaAs/GaAlAs double quantum well as a function of the impurity position, size of the structure, and the electric and magnetic field intensities. The obtained results show that the electronic and optical properties of tuned GaAs/GaAlAs double quantum well can be adjustable by an appropriate choice of the sample geometry, material parameters and applied external fields which will lead to new potential applications in optoelectronics., Colombian Agency: CODI-Universidad de Antioquia (Estrategia de Sostenibilidad de la Universidad de Antioquia); Colombian Agency: Facultad de Ciencias Exactas y Naturales-Universidad de Antioquia (CAD-exclusive dedication project 2017-2018); Cumhuriyet University (CUBAP) [F-557], This research was partially supported by Colombian Agencies: CODI-Universidad de Antioquia (Estrategia de Sostenibilidad de la Universidad de Antioquia) and Facultad de Ciencias Exactas y Naturales-Universidad de Antioquia (CAD-exclusive dedication project 2017-2018). This work used resources of the Centro Nacional de Processamento de Alto Desempenho em Sao Paulo (CENAPAD-SP). The authors are grateful to The Scientific Research Project Fund of Cumhuriyet University (CUBAP) under the project number F-557.

We study the formation of local moments in a metallic host due to a localized spin-orbit (Rashba) interaction. Using the Anderson model we describe the occurrence of magnetic moments through calculation of the magnetic-non-magnetic phase transition within the mean-field scenario. We compare our results with those in the absence of the Rashba interaction. The symmetry and regime of the magnetic phase demonstrates a dependence on the above interaction. The conductance values through the impurities are also exhibited for the above phases.

We analyze the nonlinear optical properties in Weyl semimetals which result from the intraband and the interband contributions around the Weyl points respectively. Based on the Boltzmann equation and the Floquet method to the Schrodinger equation under the tight-binding model we can calculate any order of nonlinear optical conductivity in the Terahertz regime with the effective chiral Hamiltonian used. We find that the influences of the interband transition and intraband term on the optical properties of the system are opposite each other when increasing frequency and interband transitions dominates the optical responses. The part of the linear conductivity of the system which is contributed from the intraband electric motion increases when the relaxation time reduces and the part of the linear conductivity from the interband transitions increases with the decrease of the temperature. The part of the third harmonic generation which is contributed to the interband transitions is proportional to ω−3 and will be considerable when the frequency become small enough. The nonlinear terms enhance the optical reponses of Weyl semimetals and provide more information about the critical properties.

Stability of the fully exo-hydrogenated zigzag (n,0) and armchair (n,n) single wall carbon nanotubes (CnHn) is studied using first-principles calculations. Zigzag and armchair carbon nanotubes (CNTs) were found to be different in electronic and atomic structures with respect to chemical hydrogenation. It was noted that the binding energy of hydrogenated carbon nanotubes revealed an approximately inverse proportionality behaviour with their diameter. Moreover, each single hydrogen atom was attached to nanotube by an exothermic process. The similar diameter exo-hydrogenated zigzag nanotubes were found to be energetically more favourable than armchair nanotubes. The calculated values of band gaps in small hydrogenated zigzag CNTs were found to be higher in both types of hydrogenated CNTs. These findings were important for particular functionalization of hydrogenated carbon nanotubes and improve the understanding to separate similar diameter zigzag and armchair nanotubes from each other through chemical functionalization.

The role of hydrostatic pressure and temperature on the optical properties of a two dimensional electron gas confined in a quantum wire having Rashba spin orbit interaction is studied in this work. The energy band gap and thus the effective mass of the charge carriers are found to have strong dependence of the hydrostatic pressure on the quantum wire and its absolute temperature. This in turn affects the linear as well as the nonlinear optical properties of the quantum wire. The external magnetic field, temperature, hydrostatic pressure and the Rashba spin orbit interaction are found to interplay in determining the linear and nonlinear absorption coefficient and refractive change in the quantum wire as a response to incident electromagnetic radiation.

A theoretical study of the spectrum of quasiparticles in a superlattice of spherical quantum dots (QDs) with a trapezoidal potential profile with a weak coupling of quasiparticles between QD layers is carried out. The calculations were performed using the example of spherical QDs I n G a A s embedded in G a A s in the form of a superlattice. The model of the trapezoidal QD profile with three variable parameters is especially suitable for considering such zero-dimensional structures when, due to the different lattice constants of the QD material and the matrix medium, the boundary between them is fuzzy and is modeled by a certain transition region. The dependences of the energies of the underlying bands on the geometric parameters of the superlattice are investigated, which can be used to determine the metric of zero-dimensional structures by comparing it with the experimental intraband absorption spectra. The sensitivity of the position of the bands to changes in the parameters of the potential profile is revealed, which makes it possible to efficiently analyze the tunneling passage of carriers between QDs, to study the processes of controlled relaxation of carriers in QDs, which is important for the operation of lasers based on zero-dimensional structures with mismatched lattice constants of the QD material and the matrix medium.

In this paper, we have discussed theoretically the effect of an applied bias voltage and the temperature on the spin-tunneling time, spin-dependent polarization and current densities of holes in GaAs/GaMnAs double barriers using the transfer matrix method (TMM). The behavior of the transmission coefficients for different spin orientations (up and down), various applied voltages and different temperatures were calculated. Our findings indicate that the maxima of the spin-up transmission coefficient shift towards the lower energies by increasing the applied bias voltage. Furthermore, our structure is almost transparent for spin-down holes and presents some resonances for spin-up ones. In addition, we have found that the tunneling time decreases progressively by enhancing the applied voltage. It is also shown that the current densities of holes with both spin-up and down orientations present a negative differential resistance (NDR). This (NDR) resistance is more intense for spin-up holes than spin-down ones. Consequently, the obtained results pointed out that we can design and fabricate a high-speed spin-filters and diodes by fixing the appropriate values of applied voltages and temperatures.

We investigated theoretically the effect of introducing Si impurities in a single layer graphene (1LG) that had been deposited on a polar substrate on the transport properties of the graphene layer. We consider in our analysis the scattering effects due to the surface optical (SO) phonons located at the interface of the 1LG with various polar substrates such as S i C , hexagonal B N , S i O 2 a n d H f O 2 . Our results demonstrate a reduction of SO phonon-limited (SOPL) mobility, and SOPL conductivity as well as an increase of the SOPL resistivity and of the scattering rate in the presence of Si impurities in the 1LG. Further, we studied the effect of Si impurities on the electron-surface phonon interaction . For our analysis we used the eigenenergies aquired from the tight-binding Hamiltonian in 1LG. Indeed the presence of the Si impurities induces a decrement in the resonant coupling between the electronic sub-levels and the surface vibration modes in monolayer graphene deposited on polar substrates. Finally, we investigated the effect of Si impurities on the Auger scattering process which affects the carriers relaxation. Our results show an enhancement of the Auger scattering rate in the case of the Si-doped 1LG compared to the undoped 1LG.

Nonlinear optical absorption spectra are analyzed and thereby the electromagnetically induced transparency is investigated in a GaAs/InAs/GaAs quantum well. The taken system is InAs sandwiched between GaAs semiconducting material. The confinement potential and well size of the quantum well are fixed. The three atomic sub-levels are considered and the system is analyzed using a density matrix approach. The intersubband electromagnetically induced transparency in the three level single quantum well is investigated theoretically. The optical susceptibilities, the Rabi frequency and the detuning parameters are investigated in the present work. The variation of real and imaginary parts of optical susceptibility as a function of normalized detuning is brought out. The refractive index and the group velocity of the probe light pulse are described. The variation of group index as a function of probe field energy is investigated. The dependence of the optical susceptibilities on the normalized detuning, refractive index and the group index is brought out.

We have studied the collective excitation of fractional quantum Hall effect (FQHE) in the rotating Bose-Einstein condensate (BEC) using CF theory at the filling fraction $\nu=1/2$. The roton type of excitation in the FQHE of electron system is established over the years for all the filling fraction, whereas the collective excitation at $\nu=1/2$ filling fraction in the rotating BEC shows no roton minimum. We have investigated this using composite fermion theory with the P$\ddot{o}$schl-Teller interaction potential between the particles. We have seen that the long range interaction will give the roton, whereas short range interaction gives no roton minimum.

In this paper linear and nonlinear optical dielectric function of a slab of CdSe/ZnS quantum dot matrix associated with intersubband transitions is investigated theoretically. To this end, by using effective mass approximation and density matrix, the electronic structure and dielectric function of quantum dots are calculated. Afterwards, the Clausius-Mossotti relation and TKF model is used to calculate the effective dielectric function and susceptibility of slabs of ffc and randomly ordered quantum dots. The dependence of dielectric function, susceptibility, refractive index (RI) and absorption coefficient (AC) of the slab on the size and density of quantum dots have been studied. The results show that for all radii the AC and RI of fcc ordered quantum dots have higher magnitude than randomly ordered ones. It is also found that magnitude of the dielectric function of the slab increases by increasing radius and intensity. At fixed radii the dielectric function, RI and AC increases by increasing number density of quantum dots in the slab.

The gettering effect by rare earths (RE) in semiconductor CdTe crystals was studied using the photoluminescence (PL) measurements. It was shown that the low-temperature PL spectra of Dy-doped CdTe crystals exhibit two acceptor-bound exciton lines, namely, the most intense sharp A1oX-line and a low-intensity A2oX-line associated with the residual Na(Li) impurity atoms and the cadmium vacancies, respectively. In addition, the donor-bound exciton lines are also observed. It was shown that these lines are caused by the presence of the Cl residual impurity atoms, interstitial Сdi or Nai(Lii) atoms. The PL of donor-acceptor pairs and the emission caused by optical transitions of (e-A) type were revealed. The nature and photoionization energy of different donor and acceptor centers were determined. It was found that the emission associated with the presence of complex acceptor centers is absent in PL spectra. This indicates that the concentration of dislocations is very small for the investigated crystals. The presence of bound and free excitons in the PL spectra as well as free excitons in the reflection spectrum shows high crystalline and optical quality of CdTe:Dy crystals that is due to so-called “cleaning” process of the semiconductor materials by their doping with RE elements.

We analyze the spin-dependent thermoelectric transport through the correlated quantum dot. In our model we assume that the coupling between the dot and external metallic leads depends on the dot's filling. The assisted hopping processes break the particle-hole symmetry of thermoelectric transport characteristics, such as the thermopower or electric and thermal conductance measured as a function of the gate voltage. We show that assisted hopping can be the mechanism which is responsible for the plateau generation in the dependence of the linear conductance on the gate voltage. Moreover, in the case of dot's nonzero spin splitting energy, which is generated by an external magnetic field, we analyze the dependence of the spin-Seebeck coefficient and the spin polarization of linear conductance on the value of assisted hopping parameter. We demonstrate that the assisted hopping processes can enhance the spin dependence of the relevant transport characteristics.

By numerically solving the Bogoliubov-de Gennes equations, Friedel oscillations of single quasiparticles in nanoscale superconducting square disks are systematically studied. We find that Anderson's approximation, which assumes the electron- and hole-like wave functions of a quasiparticle are proportional to a certain single-electron wave function, breaks down under strong quantum confinement, i.e. the electron- and hole-like components of single quasiparticles may show Friedel oscillations with different amplitude and normalized profile, especially when the corresponding single-electron energy moves away from the Fermi level. Such oscillation behavior is referred as dual Friedel oscillations of single quasiparticles, and is expected to be observed the local density of states spectrum. The resulting density of states (DOS) becomes asymmetric and shows significant oscillations with respect to the applied bias.

We investigate the triple quantum dots with triangular geometry, in which one of the dots (embedded dot) is coupled to pseudo-gap Fermi system and electrons in the dot interact with a local phonon mode. The model, which we treat using the analytical argument and the numerical renormalization group, exhibits the quantum phase transition between ferromagnetically-coupled local moment phase and antiferromagnetically-coupled strong coupling phase depending upon electron-phonon coupling. We restrict our discussion to the case that inter-dot exchange couplings are much smaller than the Kondo temperature for the embedded dot. Since Fermi system has the density of states dependent on energy in power-law fashion, the embedded dot retains an unscreened spin degree of freedom even in Kondo regime. For the coupling to such Fermi system, the simultaneous increase of electron-phonon coupling and pseudo-gap exponent has the contrary effects on the quantum phase transition depending upon inter-dot couplings.

The thermoelectric transport properties of a parallel-coupled double quantum dot (PCDQD) system with side-coupled quantum dots (QDs) is investigated by using the Keldysh non-equilibrium Green's function technique. The thermoelectric quantities, including the thermal conductance, thermopower, and thermoelectric figure of merit denoted by ZT, are sensitive to the inter-dot coupling strength. With the help of side-coupled QD, unusual double Fano resonances are created in the conductance spectra to largely enhance the thermoelectric effect at low-temperature. Benefited from the coexistence of local bipolar effect and Fano resonance, the ZT can be improved by one-fold higher than that of original PCDQD system. Moreover, when the asymmetry parameter α, which indicates the geometric arrangement of coupled QDs with a given lead, takes appropriate value, the optimization of ZT can be achieved at high temperature. Our work suggests that the side-coupled QDs scheme holds promise for the designing of high-efficiency thermoelectric conversion devices.

We study the impact of non-square potential well structure on the electron mobility μ of double quantum well (DQW) based field effect transistors carved out of the AlxGa1-xAs alloy. The barriers lying towards the substrate and surface sides of the DQW are delta doped with Si. We consider DQWs having V-shaped (VDQW), parabolic (PDQW), cubic (CDQW) and square (SDQW) potential wells to obtain the low temperature double subband electron mobility μ. We consider ionized impurity (Imp-) and alloy disorder (Al-) scatterings to calculate μ as a function of well width w and surface electron density NS. We show that the changes in the structure potentials influence the interplay of intersubband effects on the scattering mechanisms differently causing μ(VDQW)

We report Shubnikov-de Haas (SdH) and Hall oscillations in Cu-doped high quality bismuth selenide single crystals. To increase the accuracy of Berry phase determination by means of the of the SdH oscillations phase analysis we present a study of n-type samples with bulk carrier density n ∼ 10 19 − 10 20 cm − 3 at high magnetic field up to 60 Tesla. In particular, Landau level fan diagram starting from the value of the Landau index N = 4 was plotted. Thus, from our data we found π -Berry phase that directly indicates the Dirac nature of the carriers in three-dimensional topological insulator (3D TI) based on Cu-doped bismuth selenide. We argued that in our samples the magnetotransport is determined by a general group of carriers that exhibit quasi-two-dimensional (2D) behaviour and are characterized by topological π -Berry phase. Along with the main contribution to the conductivity the presence of a small group of bulk carriers was registered. For 3D-pocket Berry phase was identified as zero, which is a characteristic of trivial metallic states.

This paper reports about excitons in strongly correlated oxide nanocrystals NicMg1-cO (c = 0.008 and c = 1). At 8 K two weak peaks were firstly observed in the optical density spectrum of NiO nanocrystals at the energies of 3.510 eV and 3.543 eV. The intensity of the peaks subsides with an increase of temperature and a decrease of nanoparticle sizes from 25 nm to 10 nm. The peaks were attributed to the formation of p-d charge transfer excitons {d9h}. A tunneling annihilation for {d9h} excitons diminishes their lifetime drastically, even at low temperatures. This fact is considered as an inherent feature in the p-d charge transfer excitons {d9h}, which makes them significantly different from the Wannier-Mott excitons for semiconductors with direct allowed transitions. We believe that energy shift between two peaks originates due to the spin-orbit splitting of the top of the valence band, equals 33 meV. In p-d charge transfer photoluminescence excitation spectrum of NicMg1-cO (c = 0.008), we have revealed two [d9h] exciton lines near the charge transfer band edge. Energy shift of these lines (equals 25 meV) is due to the spin-orbit splitting of MgO valence band top.

The structure stability and electronic properties of edge carboxylated hexagonal and triangular graphene quantum dots are investigated by using density functional theory. The calculated binding energies show that the hexagonal clusters with armchair edges have the highest stability among all other flakes. The binding energy of carboxylated graphene quantum dots increases by increasing the number of attached carboxyl groups. Our study shows that the total dipole moment significantly increases by adding COOH with the highest values observed in triangular clusters. The edge states in triangular graphene with zigzag edges produce completely different energy spectrum from other shapes as (a) the energy gap in triangular zigzag cluster is very small compared to other clusters and (b) the highest occupied molecular orbital is localized at the edges which is in contrast to other clusters where it is distributed over the cluster surface. The enhanced reactivity and the controllable energy gap by shape and edge termination make graphene quantum dots ideal nanodevices for various applications such as sensors. The infrared spectra for different flakes are presented for confirmation and detection of the obtained results.

In a quantum dot hybrid superconducting junction, the behavior of the supercurrent is dominated by Coulomb blockade physics, which determines the magnetic state of the dot. In particular, in a single level quantum dot singly occupied, the sign of the supercurrent can be reversed, giving rise to a π -junction. This 0 − π transition, corresponding to a singlet-doublet transition, is then driven by the gate voltage or by the superconducting phase in the case of strong competition between the superconducting proximity effect and Kondo correlations. In a two-level quantum dot, such as a clean carbon nanotube, 0– π transitions exist as well but, because more cotunneling processes are allowed, are not necessarily associated to a magnetic state transition of the dot. In this proceeding, after a review of 0– π transitions in Josephson junctions, we present measurements of current-phase relation in a clean carbon nanotube quantum dot, in the single and two-level regimes. In the single level regime, close to orbital degeneracy and in a regime of strong competition between local electronic correlations and superconducting proximity effect, we find that the phase diagram of the phase-dependent transition is a universal characteristic of a discontinuous level-crossing quantum transition at zero temperature. In the case where the two levels are involved, the nanotube Josephson current exhibits a continuous 0 − π transition, independent of the superconducting phase, revealing a different physical mechanism of the transition.

A 1D anharmonic chain with a single impurity particle is used to study the center deviation and stability of the localized modes. The displacement patterns of the localized modes for a variable impurity mass and anharmonic parameter are studied. The pattern center is shifted away from the impurity with decreasing anharmonic parameter for both symmetric and asymmetric anharmonic impurity modes. In the limit of a heavy-mass impurity, the energy localization is constrained to the three particles nearest to the impurity.

The magnetic field dependence of the optical properties of CdSe nanocrystal quantum dots (NQDs) is investigated theoretically using a perturbation method within the effective-mass approximation. The results show that the magnetic field lifts the degeneracy of the electron (hole) states. A blue-shift in the absorption spectra of m ≥ 0 exciton states is observed while the absorption peak of m

In this paper, we explore THz range optical intersubband transition energies in a donor doped quantum well of a GaAs/AlGaAs system as a function of the insertion position of an AlAs monolayer in the GaAs quantum well. In simulated models, the optical transition energies between electron subband levels 1 and 2 were higher in the doped structure than in the undoped structure. This may be because the envelope wave function of the second electron subband strongly overlapped the envelope wave function of the first electron subband and influenced the optical intersubband transition between the two levels in the THz range. At different levels of bias voltage at the Schottky barrier on the donor doped structure, the electric field in the growth direction of the structure linearly increased the further away the AlAs monolayer was placed from the reference position. We also simulated the optical transition energies between acceptor energy levels of the acceptor doped structure as a function of the insertion position of the AlAs monolayer. The acceptor doped structure induced THz range emission whereas the undoped structure induced mid-IR emission.

The behavior of InN/InxGa1-xN spherical quantum dots solar cell is investigated, considering the internal electric field induced by the polarization of the junction. In order to determine the position of the intermediate band (IB), we present an efficient numerical technique based on difference finite method to solve the 3D time-independent Schrodinger's equation in spherical coordinates. The resultant n × n Hamiltonian matrix when considering n discrete points in spatial direction is diagonalized in order to calculate energy levels. Thus, the interband and intersubband transitions are determined, taking into consideration the effect of the internal electric field, size dots, interdot distances, and indium content on the energy levels, optical transition, photo-generated current density, open-circuit voltage and power conversion efficiency of the QD-IBSCs.

The effect of confined optical phonons and laser radiation are taken into account to study the Hall effect in a compositional semiconductor superlattice (CSSL). The new theoretical expressions of the Hall conductivity, the Hall coefficient and the magnetoresistance are obtained as functions of the external fields (the magnetic field B , the laser frequency Ω and amplitude E ), the CSSL parameters (the well length d I , the barrier length d II and height U ) and the quantum number m characterizing the phonon confinement effect. The magneto-confined phonon resonance condition in a CSSL is also carried out to explain the numerical evaluation results for the GaAs / Al x Ga 1 − x As CSSL. When optical phonons are confined, the Hall conductivity has more resonance peaks in comparison with that for bulk phonons. In addition, these sub-peaks are more separate from the main peaks as the CSSL layers thickness decreases. The tighter CSSL period is, the more forcefully the phonon confinement makes. When the CSSL period d is smaller than 30 nm , the contribution of confined optical phonons is significant and could not be neglected. The magnetophonon resonance for bulk semiconductor could be achieved in the limit the CSSL period goes to infinity. Furthermore, the Hall conductivity increases and the Hall coefficient, the magnetoresistance decrease due to the laser radiation and the effect of confined optical phonons. Especially, the near-linear dependence of the magnetoresistance on temperature is in good agreement with the experimental result.

The interplay of intralead and dot-lead Coulomb interactions on shot noise of a quantum dot coupled to Luttinger-liquid leads is systematically investigated in the Kondo regime. By equation of motion and canonical transformation method, a general shot noise formula with transmission coefficient is derived, the factors of the above two types of interactions and on-dot Coulomb interaction being taken into account. Then we study in detail the shot noise and Fano factor as a function of bias voltage. Numerical results show for weak and moderately strong intralead Coulomb interactions, the dot-lead interaction not only helps to improve the shot noise, but also narrows the zero-bias Kondo dip. A series of polaron peaks appear in the differential shot noise when the renormalized parameter Y 1 , a combination effect of the intralead and dot-lead Coulomb interactions. For Y > 1 , a novel type of excitonic dips appear only in the case of moderately strong intralead interaction. In the very strong intralead interaction limit, all features of excitonic peaks or dips disappear, and shot noise shows power-law dependence on the voltage with the renormalized exponent Y . The Fano factor is always sub-Poisson noise no matter whether Y is greater or less than 1.

We propose a model with a theory to simulate the ferromagnetism increasing with the temperature observed from some ferromagnetic n-type ZnOs. This phenomenon reveals the possible mechanism of ferromagnetism for us to propose that ferromagnetism originates from the electron's spin split in the dopant-induced impurity states by Coulomb excitations. Our theoretical results indicate that the electron-phonon interaction effectively influences ferromagnetism, which can localize the electrons in the impurity states leading to the enhancement of ferromagnetism. At the same time, the electron-phonon interaction suppresses the hybridization coupling between the impurity state and the conduction band, and this hybridization coupling can increase the electron density and ferromagnetism of the impurity state.

We investigate linear and nonlinear optical absorption coefficients (OACs) and refractive index changes(RICs) of a Morse quantum well when a quantum-sized metal nanoparticle is placed nearby. The exciton–plasmon coupling induces the modification of the energy interval and the relaxation rate of the Morse quantum well. The spontaneous decay rate of the Morse quantum well is also modified due to Purcell effect. These modification above reduce the magnitudes of the linear and nonlinear OACs and RICs of the Morse quantum well. More colorful results are demonstrated by controlling the size of the metal nanoparticle and the distance between the Morse quantum well and the metal nanoparticle. Our findings provide a way for manipulating the linear and nonlinear optical properties of the Morse quantum well.

I theoretically determine the electronic and optical signatures of CdSe/CdS core/shell dot-in-hexagonal platelet nanocrystals with experimentally synthesized dimensions exploiting the atomistic tight-binding theory. With the increasing lateral sizes and thicknesses of the CdS hexagonal platelets, the reduction of excitonic band gaps is ascribed to the quantum confinement consequence. The excitonic band gaps emitting the visible spectra are carried out by CdS shell sizes. The optical characteristic of CdSe/CdS dot-in-hexagonal platelet nanocrystals is advanced when comparing with bare CdSe nanocrystal. The reduction of oscillator strengths, excitonic binding energies and stokes shift with the increasing sizes of CdSe shell is ascribed to the electron-hole wave function overlaps. The key findings from the tight-binding analysis agree well with the experimental observation. Finally, this theoretical study delivers the essential direction to predesign of core/shell dot-in-hexagonal platelet nanocrystals with specific characteristics for various optoelectronic technologies.

A new category of tubular nanostructures solely composed of selenium was explored by using the density functional theory. Similar to those in carbon nanotubes, the armchair and zigzag tube-like structures formed as a roll-up α-Se nanoribbon. The zigzag (n ≤ 16, 0) nanotubes exhibit decreasingly narrow direct band gap, and the armchair (n, n) possess decreasingly moderate indirect gap. The strain energy of nanotubes is inversely proportional to the second power of the nanotube diameter. With increasing tube diameter, an initial rise and consequent decrease occurred in the effective masses to the limit value of α-Se monolayer. The carrier mobility of nanotubes is comparable with that of its monolayer with orders of 103 and 102 cm2V−1s−1 for electron and hole, respectively. The optical absorption spectra of α-Se nanotubes exhibit high absorbance in energy window cover the entire visible light spectrum. Therefore, α-Se nanotubes hold great potential for future applications in high-performance optoelectronics.

The integer quantum Hall Effect (QHE) and magnetoresistance measurements are carried out at temperature range 1.8 K and 40 K under a magnetic field up to 11 T to investigate the influence of the thermal annealing process and thermal annealing time on the transport parameters of as-grown and annealed n-type Ga0.68In0.32N0.017As0.983/GaAs quantum well (QW) structures. The electron effective mass, two-dimensional (2D) carrier density, quantum lifetime, and Fermi level are obtained by analyzing both oscillatory parts of magnetoresistance measurements (Shubnikov de Haas oscillations) and QHE oscillations. The compatibility of two different methods with each other is discussed, and the dependence of physical parameters obtained from both methods on annealing time was discussed. Our results reveal that the thermal annealing process and annealing duration time affect 2D carrier density, electron effective mass, quantum lifetime, transport lifetime, and Fermi level. The increase in annealing time showed that μ t / μ q ratios caused the shift from long-range scattering to short-range scattering in the low-temperature region.

Utilizing the operator projection technique, we theoretically study the contribution of different types of optical phonon modes to electron–phonon scattering in a quasi-two-dimensional system through calculating the expression for the magnetophonon resonance absorption power (MPR-AP) caused by scattering between the electrons with interface-optical phonons in parabolic quantum wells (PQWs). Electron interactions with different modes of interface-optical-phonons include the symmetric and antisymmetric modes in both the barrier and well layers together are considered in detail. The full width at half maximum (FWHM) of the resonant peaks due to different interface-optical-phonon modes was measured by using the profile method. Numerical results show that the strength of electron–interface-phonon interaction increases with confining frequency of parabolic confinement potential, temperature, and magnetic field under all antisymmetric- and symmetric-modes in both well and barrier layers. The electron-interaction with symmetric-interface-phonon has stronger strength compared to antisymmetric-interface-phonon in all well and barrier layers. The strongest-electron-scattering for symmetric phonon in well layer, meanwhile weakest that for antisymmetric phonon in barrier layer. At higher value of the photon frequency, the contributions of interface-optical-phonons is not significant. Our present calculations well accord with previous experimental and theoretical studies.

The second-harmonic generation (SHG) of parabolic quantum dots (QDs) with different confining potentials energies in the ρ and z directions under external electric and magnetic fields are studied theoretically. We have applied the compact-density-matrix approach and iterative method to obtain the SHG coefficients. The results indicate that the electric field and the size of QDs affect the peaks of SHG coefficient. In addition, the magnetic field and the radius of QDs not only strongly affect the value of the SHG peaks, but also cause the energy positions of the SHG coefficients to shift.

A theoretical method is applied to calculate the optical rectification (OR) in a cavity-quantum dot system based on open quantum systems theory. This method allows studying the linear and non-linear optical response of a nanostructure found within an ideal photonic crystal cavity by solving a Born–Markov master equation in the Lindblad form, including spontaneous emission processes and dephasing due to interaction with the environment. For a quantum dot (QD) interacting with non-classical light in a very high-quality factor cavity, an analytical expression for the OR was found when the average number of photons n ¯ is small, which explicitly depends on the quantum dot-cavity system’s initial state. The expression found was applied to a nanostructure constituted by a cylindrical quantum dot (CQD) of GaAs/Ga0.6Al0.4As with a donor impurity inside. It was found that the OR in nanostructures that interact with a quantum field of light exhibits an absorption peak at lower frequencies than in the case of interaction with classical light. The physical reason for this behavior is attributed to the number of quantum states involved in the optical transitions and, therefore, to the number of photons involved in the transitions.

We report the optical response of a periodic square array of GaAs nanowires embedded in epoxy as a candidate for photovoltaic solar cells. The simulated system is a multilayer array constituted by alternating layers of epoxy and an effective medium constituted by GaAs nanowires arrays embedded in epoxy. To discuss the optical response, we investigate the reflectance dependence on the number of bilayers considered in the array and the angle of incident light. The GaAs nanowire dielectric function is described in terms of Webb formalism to take into account the confinement energy of the excitons. The effective dielectric function of GaAs nanowires embedded in epoxy is evaluated within the Maxwell–Garnett theory. We evaluate the reflectance for s- and p-polarized light through the transfer matrix formalism for n bilayers. For both s- and p-polarization, we observe an oscillating behavior of the reflectance, similar to that reported in the literature. We have also obtained a feature peaked around 850 nm. While the oscillations can be ascribed to multiple interference from periodic bilayers, the peak at 850 nm can be understood in term of the gap energy in the nanowire dielectric function. Attending to the reflectance dependence on the light incidence angle, we have found that for s-polarized light, the reflectance is higher with increasing angles, in comparison to p-polarized light cases.

It is shown that in a germanium/silicon nanosystem with germanium quantum dots, the hole leaving the germanium quantum dot causes the appearance of the hole energy level in the bandgap energy in a silicon matrix. The dependences of the energies of the ground state of a hole and an electron, as well as spatially indirect excitons on the radius of the germanium QD and on the depth of the potential well for holes in the germanium QD are obtained. It is found that as a result of a direct electron transition in real space between the electron level, located in the conduction band of the silicon matrix, and the hole level located in the bandgap of the silicon matrix, the radiative recombination intensity in the germanium/silicon nanosystem with germanium quantum dots increases significantly.

An off-centre hydrogenic D 0 centre situated in a 2D Gaussian quantum dot is studied taking into accounts the effects of Rashba Dresselhaus spin-orbit interactions and an external magnetic field. The ground state binding energy of the D 0 complex is calculated using a variational method with an improved wave function. The magnetic susceptibility of the system is also calculated. The results show interesting behaviour with respect to different quantum dot parameters and spin-orbit couplings.