Anticrossing Region Research Articles

Symmetries and couplings in double quantum dot molecule

AbstractElectronic transport through a quantum dot molecule attached to leads has been very successfully studied finding very interesting effects like Fano and Kondo. In particular, the symmetry in quantum dot molecules going from the parallel to series coupling seems to play a very interesting role showing very different density of states spectra, these results have been obtained by fixing the interdot coupling equal to the coupling to the leads. In this work we go a step further by changing the value of the interdot coupling and the addition of on‐site Coulomb interaction with the help of Slave Boson Mean Field theory (SBMFT), Keldysh non equilibrium Green function technique and the equation of motion approach the density of states and the conductance of this system are calculated and analyzed in the Kondo regime. We also studied the asymmetric molecule set up of dots of different size in order to analyze the density of states and the Fano interference in an anticrossing region. Comparison of the results taking into account the Coulomb interaction is presented, which gives rise to the appearance of the Kondo effect in these systems. We also studied the interplay between the Kondo and Fano effects. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electric-field-induced redistribution of radiation transition probabilities in atomic multiplet lines

Analytical dependences on a static electric field F0 are derived for the wavefunctions, matrix elements and probabilities of radiation transitions between multiplet substates interacting in field in pairs, ranging from ordinary doublet states with spin S = 1/2, pairwise interacting sublevels of triplet and quintet states with the magnetic quantum number M = 0, to the most general case of states with arbitrary angular L and spin S momenta and maximal magnitude of the magnetic quantum number |M| = L + S − 1. Equalization of doublet line intensities in the anticrossing field region and vanishing of one of the two doublet lines in the high-field regime are demonstrated. A general relation is determined between the anticrossing field FA, multiplet splitting and tensor polarizability. The data of numerical computations with the Fues' model potential for polarizabilities of triplet states in helium and doublet states in alkali atoms are extrapolated with a three-term polynomial including 5th, 6th and 7th powers of effective principal quantum number and providing accuracy better than 0.5% in a wide range of the principal quantum number n, up to n = 1000.

Spectral modulation of exciton Fano resonance due to Zener breakdown in strongly biased superlattices

The effects of the Wannier-Stark ladder (WSL) resonance on the optical absorption spectra of strongly biased superlattices are theoretically investigated. Exciton Fano resonance (FR) states in this system are calculated based on the multichannel scattering theory. When the bias of a static electric field $(F)$ is applied such that a WSL subband state is energetically aligned with its adjacent ones, resulting in an anticrossing with strong repulsion due to Zener resonance, we obtain the following findings. (i) The onset of exciton absorption noticeably shifts toward the lower energy side, accompanying the oscillation of the magnitude of the absorption tail with the change in $F$. However, for the anticrossing, this effect is limited to a small localized region of $F$. In fact, a slight change in $F$ away from the anticrossing leads to a peculiar suppression of the redshift of the absorption tail edge. (ii) The absorption intensities and the positions of the FR states vary in an irregular manner due to the Zener breakdown as $F$ traverses the anticrossing region. For instances in a certain WSL state, the oscillator strength is reduced by a large extent only in the anticrossing region, followed by recovery of the intensity out of this region. Moreover, the changes in Fano's $q$ values with respect to $F$ are also discussed. As described in (i) and (ii), these two effects on the exciton spectra are due to the delocalization of WSL wave functions across several periods that accompany the strong anticrossing.

Optically detected heavy- and light-hole anti-crossing in GaAs quantum wells under pulsed magnetic fields

We report magneto-photoluminescence studies of two undoped GaAs–Al0.3Ga0.7As single quantum well (SQW) samples (120 and 60 Å) in pulsed magnetic fields up to ∼65 T. Both samples exhibit exciton transitions due to the ground-state (1s) electron-heavy-hole recombination which undergoes diamagnetic energy shifts at low fields and has a liner dependence at high fields. The 120 Å SQW shows the electron-light-hole exciton transition at zero magnetic field. However at about 30 T, the electron-heavy-hole and the electron-light-hole transitions intersect and show an anti-crossing behavior. The 60 Å SQW shows a similar behavior but the splitting between the heavy- and light-hole excitons can only be observed in the anti-crossing region at about 35 T. The results indicate that the valence band mixing plays a significant role at high magnetic fields.

Higher orders of perturbation theory for the Stark effect on an atomic multiplet

The contribution of higher order corrections to the Stark energy is calculated in the anticrossing region of atomic multiplet sublevels. Perturbation theory for close-lying levels is presented that is based on the Schro dinger integral equation with a completely reduced Green’s function. Analytic formulas are obtained for the splitting of two interacting fine-structure sublevels as a function of the field strength. These formulas take into account fourth-order resonance and nonresonance corrections to both the diagonal and the off-diagonal matrix elements of the dipole moment operator. By the method of the Fues model potential, a numerical analysis of radial matrix elements of the second, third, and fourth orders is carried out that determine a variation in the transition energy between n 3 P 0 and n 3 P 2 sublevels of a helium atom for n=2, 3, 4, 5 in a uniform electric field. It is shown that the contribution of the fourth-order corrections in the vicinity of anticrossing of levels for n=2, 3, 4, 5 amounts to 0.1, 5, 10, and 15% of the total variation of energy, respectively. A comparative anal-ysis is carried out with the results of calculations obtained by the method of diagonalization of the energy matrix, which, together with resonance terms, takes into account other states of the discrete spectrum with n≤6.

Characterization of Bernstein modes in quantum dots

The dipole modes of non-parabolic quantum dots are studied by means of their current and density patterns as well as with their local absorption distribution. The anticrossing of the so-called Bernstein modes originates from the coupling with electron-hole excitations of the two Landau bands which are occupied at the corresponding magnetic fields. Non-quadratic terms in the potential cause an energy separation between bulk and edge current modes in the anticrossing region. On a local scale the fragmented peaks absorb energy in complementary spatial regions which evolve with the magnetic field.

Magnetic effects on the dynamics of organic triplets in the level anti-crossing region

A fast magnetic field sweep (2.0 × 105 T s−1) is applied to probe the spin dynamics in the level anti-crossing (LAC) region of the photo-excited triplet states of pentacene-h 14 in p-terphenyl (PHPT) and pentacene-d 14 in p-terphenyl (PDPT) systems. From the pulsed zero-field FID (ZF FID) EPR measurements of the triplet signal at room temperature, the crossing is found to be a non-adiabatic passage at such a high jumping rate. Detailed examination of the initial population of the photo-excited triplet states of these two systems indicates that the non-adiabatic state mixing is the dominant mechanism for the polarization transfer (electron-nuclear spins) in the LAC region. The time dependent Schrödinger equation is applied to calculate the non-adiabatic transition probabilities, and to simulate the observed triplet population profiles with fast field sweeping.

Polariton acceleration in a microcavity wedge

We report an experimental evidence of the acceleration of exciton-polaritons propagating in-plane of a semiconductor microcavity. The acceleration is achieved due to the gradient of the thickness of the cavity and results from the reciprocal space filtering effect of the cavity. The wave packet propagating in the cavity plane is constantly filtered by the cavity eigenmode that shifts towards larger wave vectors as the cavity width increases. The decay time of the emission of light by the propagating polaritons increases as one approaches the anticrossing region. We show that it is a consequence of the elastic scattering of the polaritons. The spatial size of propagating polariton packet decreases with time due to the same effect.

Measurements of energy gap at the level anti-crossing region of the photo-excited triplet state of organic molecules

By the application of zero-field magnetic resonance (ZFMR)/FID technique and fast-field sweeping around the level anti-crossing region with fixed rf frequency, we obtained the (minimum) energy gaps between the two crossing levels as 16.7±2.0 and 14.3±3.7 MHz for pentacene-h 14 in p-terphenyl and pentacene-d 14 in p-terphenyl, respectively, when B 0∥ x. The minimum energy gaps occur at 14 mT for both systems. The hyperfine interaction contributes only partially to the observed energy gap.

Effects of an external magnetic field on shallow donor levels in semiconductors

The recent progress on investigation of the effects of external magnetic field on shallow impurity centers in semiconductors is discussed. As an example, the shallow donor levels of phosphorus in ultra pure silicon are investigated under magnetic fields and by use of photothermal ionization spectroscopy(PTIS). By selecting the configuration of B∥∥ kββ 〈100〉, the linear and quadratic Zeeman splittings of phosphorus donor states are observed. By selecting the configuration of B∥∥k∥∥ 〈111〉, the anticrossing between hybridized Zeeman states and the intensity evolution of Zeeman transitions near the anticrossing region are observed and measured quantitatively, which, in turn, gives the measurement of wavefunction composition and their evolutions with magnetic fields fox the hybridized Zeeman states of bounded electrons of impurities. An extension of Faulkner's method is made and a variational calculation in the framework of the effective mass approach(EMA) is performed to estimate theoretically the effects of magnetic fields on shallow donor levels and impurity transitions, especially to estimate the wave function compositions of hybridized states. A comparison has been made between the theoretical and experimental results.

Wave-function mixture and composition for hybridized Zeeman states of P in Si.

We report the experimental determination of the wave-function mixture and composition for hybridized Zeeman states of P in Si and give a related theoretical discussion. The intensity evolution of the Zeeman transitions near the anticrossing region for the isolated P donors in highly pure Si has been measured quantitatively by means of high-sensitivity and high-resolution photothermal ionization spectroscopy. The wave-function compositions and their evolutions with magnetic field for the hybridized Zeeman states of bounded electrons of impurities have been deduced from the measurement. A variational calculation in the framework of the effective-mass approach has been performed to estimate theoretically the wave-function compositions, and a comparison has been made between the theoretical results and experimental ones.

WAVEFUNCTION MIXTURE AND COMPOSITION FOR HYBRIDIZED ZEEMAN STATES OF P IN Si

We report direct experimental determination of the wavefunction mixture and composition for hybridized Zeeman states of P in Si. The intensity evolution of the Zeeman transitions nearby anti-crossing region for the isolated P donors in high pure Si bas been measured quantitatively by use of high sensitive and high resolution photothermal ionization spectroscopy (PTIS); The wavefunction compositions and their evolutions upon magnetic field for the hybridized Zeeman states of bounded electrons of impurities have been deduced from the measurement. A variational calculation under the frame of effective mass approach has been performed to estimate theoretically the wavefunction composition. The results are campare with experimental ones.

Anomaly in the in-plane polarization properties of (110)-oriented quantum wells under

The effects of uniaxial stress on the polarization properties of interband transitions are studied for (110) quantum wells under stress along the [110] axis. Within a multiband effective-mass approximation, an analytical method for calculating the hole energy levels as well as the optical transition matrix elements is presented. The calculated result for a (110) GaAs/${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As quantum well shows anticrossing behavior between the first two hole subbands in the compression dependence of the energy levels. It is shown that the dipole transition between the first electron state and the first hole state is forbidden for [001]-polarized light at a critical stress just beyond the anticrossing region, while that between the first electron and the second hole state becomes a forbidden transition for [1\ifmmode\bar\else\textasciimacron\fi{}10] polarization at a different critical stress just before the anticrossing.

Pressure dependence of zero-field splittings in organic triplets. II. Carbonyls

We have conducted optically detected magnetic resonance (ODMR) experiments at pressure up to 40 kbar for neat biactyl (BA), neat benzil (BZ), and acetophenone (AP) doped in dibromobenzene (DBB). The pressure dependences of their zero-field splitting (ZFS) parameters D and E are reported. For BA and BZ systems, the ‖D‖ value decreases greatly with increasing pressure. This behavior is in contrast with that of benzophenone (BP), whose ‖D‖ value increases sigmoidally 13% over the same pressure range. These results may be rationalized in a qualitative theory based on pressure modulation of the spin-orbit coupling (SOC) contribution to the ZFS. ln aromatic ketones, lattice compression modifies the twist angle of the phenyl ring(s) relative to the carbonyl frame, thus changing the energy of the 3ππ* state relative to that of the 3nπ* state. This variation of the energy denominator in a second order perturbation enhances the SOC contribution to the ZFS. In comparison, the increase of spin–spin (SS) dipolar interaction by isotropic compression is relatively unimportant. Consistent with this picture, the very small 3ππ*–3nπ* energy gap produces an enormous pressure sensitivity of D and E in AP/DBB. The behavior of the ZFS in this case may be interpreted as a consequence of pressure tuning of the 3ππ* state through an anticrossing region. In addition, a new set of high frequency ODMR signals appears under pressure. This is attributed to a new site of AP having the 3nπ* as the phosphorescent triplet state. The pressure dependence of ZFS for benzil shows complicated fine structure. This is a testimony to the flexible nature of benzil in both the dihedral angle of the dicarbonyl fragment and the phenyl twist angle.

The lineshape of quadrupolar satellites in level anticrossing (LAC) optical nuclear polarization (ONP) spectra

Satellite lines appear in the optical nuclear polarization (ONP) spectra of doped molecular single crystals when the guest or host contains bromine. The satellites, which are separated by the Br quadrupolar splitting, are observed in the level anticrossing (LAC) region of systems in the lowest excited triplet state. In the present paper we derive the satellite LAC ONP lineshape for a model system comprised of a triplet S = 1 coupled to a Br and a proton nuclear spin. We find that the satellite lineshape has the unusual form, L(B) ▪, where L is a Lorentzian function that depends only on the magnetic field strength, B, and bromine hyperfine (hf) coupling parameters. Surprisingly the proton hf coupling affects only the overall amplitude, not the form, of the lineshape function.