Perturbative finiteness of three-dimensional supersymmetric QED to all orders

Within the superfield formalism, we study the ultraviolet properties of the three-dimensional super-symmetric quantum electrodynamics. The theory is shown to be finite at all loop orders in a particular gauge.

Finiteness of the noncommutative supersymmetric Maxwell-Chern-Simons theory

Within the superfield approach, we prove the absence of UV/IR mixing in the three-dimensional noncommutative supersymmetric Maxwell-Chern-Simons theory at any loop order and demonstrate its finiteness in one, three, and higher loop orders.

Spin-Orbit Coupling in n-Type PbTe/PbEuTe Quantum Wells

Magnetoresistance measurements were performed on an n-type PbTe/PbEuTe quantum well and weak antilocalization effects were observed. This indicates the presence of spin orbit coupling phenomena and we showed that the Rashba effect is the main mechanism responsible for this spin orbit coupling. Using the model developed by Iordanskii et al., we fitted the experimental curves and obtained the inelastic and spin orbit scattering times. Thus we could compare the zero field energy spin-splitting predicted by the Rashba theory with the energy spin-splitting obtained from the analysis of the experimental curves. The final result confirms the theoretical prediction of strong Rashba effect on IV-VI based quantum wells.

Thermally activated intersubband scattering and oscillating magnetoresistance in quantum wells

Experimental studies of magnetoresistance in high-mobility wide quantum wells reveal oscillations which appear with an increase in temperature to 10 K and whose period is close to that of Shubnikov-de Haas oscillations. The observed phenomenon is identified as magnetointersubband oscillations caused by the scattering of electrons between two occupied subbands and the third subband which becomes occupied as a result of thermal activation. These small-period oscillations are less sensitive to thermal suppression than the large-period magnetointersubband oscillations caused by the scattering between the first and the second subbands. Theoretical study, based on consideration of electron scattering near the edge of the third subband, gives a reasonable explanation of our experimental findings.

Microwave Zero-Resistance States in a Bilayer Electron System

Magnetotransport measurements on a high-mobility electron bilayer system formed in a wide GaAs quantum well reveal vanishing dissipative resistance under continuous microwave irradiation. Profound zero-resistance states (ZRS) appear even in the presence of additional intersubband scattering of electrons. We study the dependence of photoresistance on frequency, microwave power, and temperature. Experimental results are compared with a theory demonstrating that the conditions for absolute negative resistivity correlate with the appearance of ZRS.

Crossover between distinct mechanisms of microwave photoresistance in bilayer systems

We report on temperature-dependent magnetoresistance measurements in balanced double quantum wells exposed to microwave irradiation for various frequencies. We have found that the resistance oscillations are described by the microwave-induced modification of electron distribution function limited by inelastic scattering (inelastic mechanism), up to a temperature of T*similar or equal to 4 K. With increasing temperature, a strong deviation of the oscillation amplitudes from the behavior predicted by this mechanism is observed, presumably indicating a crossover to another mechanism of microwave photoresistance, with similar frequency dependence. Our analysis shows that this deviation cannot be fully understood in terms of contribution from the mechanisms discussed in theory.

Theory of near-gap second harmonic generation in centrosymmetric magnetic semiconductors: Europium chalcogenides

Second harmonic generation is strictly forbidden in centrosymmetric materials, within the electric dipole approximation. Recently, it was found that the centrosymmetric magnetic semiconductors EuTe and EuSe can generate near-gap second harmonics, if the system is submitted to an external magnetic field. Here, a theoretical model is presented, which well describes the observed phenomena. The model shows that second harmonic generation becomes efficient when the magnetic dipole oscillations between the band-edge excited states of the system, induced by the excitation light, enter the in-phase regime, which can be achieved by applying a magnetic field to the material.

Magnetoresistance oscillations in multilayer systems: Triple quantum wells

Magnetoresistance of two-dimensional electron systems with several occupied subbands oscillates owing to periodic modulation of the probability of intersubband transitions by the quantizing magnetic field. In addition to previous investigations of these magnetointersubband (MIS) oscillations in two-subband systems, we report on both experimental and theoretical studies of such a phenomenon in three-subband systems realized in triple quantum wells. We show that the presence of more than two subbands leads to a qualitatively different MIS oscillation picture, described as a superposition of several oscillating contributions. Under a continuous microwave irradiation, the magnetoresistance of triple-well systems exhibits an interference of MIS oscillations and microwave-induced resistance oscillations. The theory explaining these phenomena is presented in the general form, valid for an arbitrary number of subbands. A comparison of theory and experiment allows us to extract temperature dependence of quantum lifetime of electrons and to confirm the applicability of the inelastic mechanism of microwave photoresistance for the description of magnetotransport in multilayer systems.

Nonlinear transport and oscillating magnetoresistance in double quantum wells

The nonlinear regime of low-temperature magnetoresistance of double quantum wells in the region of magnetic fields below 1 T is studied both experimentally and theoretically. The observed inversion of the magnetointersubband oscillation peaks with increasing electric current and splitting of these peaks are described by the theory based on the kinetic equation for the isotropic nonequilibrium part of electron distribution function. The inelastic-scattering time of electrons is determined from the current dependence of the inversion field.

Edge effects in bilayer graphene nanoribbons: Ab initio total-energy density functional theory calculations

We show that the ground state of zigzag bilayer graphene nanoribbons is nonmagnetic. It also possesses a finite gap, which has a nonmonotonic dependence with the width as a consequence of the competition between bulk and strongly attractive edge interactions. All results were obtained using ab initio total-energy density functional theory calculations with the inclusion of parametrized van der Waals interactions.

Magnetic resonant x-ray diffraction study of europium telluride

Here we use magnetic resonant x-ray diffraction to study the magnetic order in a 1.5 mu m EuTe film grown on (111) BaF(2) by molecular-beam epitaxy. At Eu L(II) and L(III) absorption edges, a resonant enhancement of more than two orders was observed for the sigma ->pi(') diffracted intensity at half-order reciprocal-lattice points, consistent with the magnetic character of the scattering. We studied the evolution of the (1/21/21/2) magnetic reflection with temperature. When heating toward the Neel temperature (T(N)), the integrated intensity decreased monotonously and showed no hysteresis upon cooling again, indicating a second-order phase transition. A power-law fit to the magnetization versus temperature curve yielded T(N)=9.99(1) K and a critical exponent beta=0.36(1), which agrees with the renormalization theory results for three-dimensional Heisenberg magnets. The fits to the sublattice magnetization dependence with temperature, disregarding and considering fourth-order exchange interactions, evidenced the importance of the latter for a correct description of magnetism in EuTe. A value of 0.009 was found for the (2j(1)+j(2))/J(2) ratio between the Heisenberg J(2) and fourth-order j(1,2) exchange constants. The magnetization curve exhibited a round-shaped region just near T(N) accompanied by an increase in the magnetic peak width, which was attributed to critical scattering above T(N). The comparison of the intensity ratio between the (1/21/21/2) and the (1/21/21/2) magnetic reflections proved that the Eu(2+) spins align within the (111) planes, and the azimuthal dependence of the (1/21/21/2) magnetic peak is consistent with the model of equally populated S domains.

Approximation to density functional theory for the calculation of band gaps of semiconductors

The local-density approximation (LDA) together with the half occupation (transitionstate) is notoriously successful in the calculation of atomic ionization potentials. When it comes to extended systems, such as a semiconductor infinite system, it has been very difficult to find a way to half ionize because the hole tends to be infinitely extended (a Bloch wave). The answer to this problem lies in the LDA formalism itself. One proves that the half occupation is equivalent to introducing the hole self-energy (electrostatic and exchange correlation) into the Schrodinger equation. The argument then becomes simple: The eigenvalue minus the self-energy has to be minimized because the atom has a minimal energy. Then one simply proves that the hole is localized, not infinitely extended, because it must have maximal self-energy. Then one also arrives at an equation similar to the self- interaction correction equation, but corrected for the removal of just 1/2 electron. Applied to the calculation of band gaps and effective masses, we use the self- energy calculated in atoms and attain a precision similar to that of GW, but with the great advantage that it requires no more computational effort than standard LDA.

Optical absorption and electronic band structure first-principles calculations of alpha-glycine crystals

Light absorption of alpha-glycine crystals grown by slow evaporation at room temperature was measured, indicating a 5.11 +/- 0.02 eV energy band gap. Structural, electronic, and optical absorption properties of alpha-glycine crystals were obtained by first-principles quantum mechanical calculations using density functional theory within the generalized gradient approximation in order to understand this result. To take into account the contribution of core electrons, ultrasoft and norm-conserving pseudopotentials, as well as an all electron approach were considered to compute the electronic density of states and band structure of alpha-glycine crystals. They exhibit three indirect energy band gaps and one direct Gamma-Gamma energy gap around 4.95 eV. The optical absorption related to transitions between the top of the valence band and the bottom of the conduction band involves O 2p valence states and C, O 2p conduction states, with the carboxyl group contributing significantly to the origin of the energy band gap. The calculated optical absorption is highly dependent on the polarization of the incident radiation due to the spatial arrangement of the dipolar glycine molecules; in the case of a polycrystalline sample, the first-principles calculated optical absorption is in good agreement with the measurement when a rigid energy shift is applied.

(7,9,10)Be elastic scattering and total reaction cross sections on a (12)C target

Elastic scattering angular distributions for (7)Be, (9)Be, and (10)Be isotopes on (12)C target were measured at laboratory energies of 18.8, 26.0, and 23.2 MeV, respectively. The analysis was performed in terms of optical model potentials using Woods-Saxon and double-folding form factors. Also, continuum discretized coupled-channels calculations were performed for (7)Be and (9)Be + (12)C systems to infer the role of breakup in the elastic scattering. For the (10)Be + (12)C system, bound states coupled-channels calculations were considered. Moreover, total reaction cross sections were deduced from the elastic scattering analysis and compared with published data on other weakly and tightly bound projectiles elastically scattered on the (12)C target, as a function of energy.

alpha plus alpha scattering reexamined in the context of the Sao Paulo potential

We have analyzed a large set of alpha + alpha elastic scattering data for bombarding energies ranging from 0.6 to 29.5 MeV. Because of the complete lack of open reaction channels, the optical interaction at these energies must have a vanishing imaginary part. Thus, this system is particularly important because the corresponding elastic scattering cross sections are very sensitive to the real part of the interaction. The data were analyzed in the context of the velocity-dependent Sao Paulo potential, which is a successful theoretical model for the description of heavy-ion reactions from sub-barrier to intermediate energies. We have verified that, even in this low-energy region, the velocity dependence of the model is quite important for describing the data of the alpha + alpha system.