Partial absorption and “virtual” traps

The spatial probability distribution associated with diffusion and attenuation in partially absorbing media is studied. An equivalence is established between a system with free diffusion for x>0 and partial absorption for x<0, and a semi-infinite system (x>0) with a radiation boundary condition at x=0. By exploiting this equivalence, it is shown that the effect of a partially absorbing medium in the long-time limit is equivalent to that of a perfect, virtual trap whose size is smaller than the original absorbing medium. For short times, however, there is substantial penetration of diffusing particles into the absorber. The virtual trap approach is readily generalized to higher dimensions. This allows one to obtain the density profile of diffusing particles around a partially absorbing spherical trap. An unusual crossover between short-time penetration and long-time trapping occurs in two dimensions; the size of the virtual trap is exponentially small in the case of weak absorption, corresponding to an absorption time which is exponentially large.     

Experiments with single atoms in cavities and traps

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Are there traps in quantum control landscapes?

There has been great interest in recent years in quantum control landscapes. Given an objective J that depends on a control field ε the dynamical landscape is defined by the properties of the Hessian δ2J/δε2 at the critical points δJ/δε=0. We show that contrary to recent claims in the literature the dynamical control landscape can exhibit trapping behavior due to the existence of special critical points and illustrate this finding with an example of a 3-level Λ system. This observation can have profound implications for both theoretical and experimental quantum control studies.     

Are the native states of proteins kinetic traps?

Four proteins were selected to represent each of the four different CATH classes and, for each protein, three decoys were constructed with structures totally alien to the native state. The decoys were scored against the native state with the help of the AMBER force field, using three measures: the average energy, the average fluctuation and the resistance to a heat pulse. Two sets of simulations were performed, one with explicit solvent and the other with implicit solvent. The overall conclusion is that, of these three measures, the most successful in picking out the native states was the last one, since the native structures take a consistently longer time to be destabilized in this manner. However, the general conclusion is also that none of the measures is completely effective in discriminating all the decoys, a result that supports other studies, according to which the native state is reached by a kinetic step.     

Two-dimensional novel optical lattices with multi-well traps for cold atoms or molecules

We propose some new schemes to constitute two-dimensional (2D) array of multi-well optical dipole traps for cold atoms (or molecules) by using an optical system consisting of a binaryπ-phase grating and a 2D array of rectangle microlens. We calculate the intensity distribution of each optical well in 2D array of multi-well traps and its geometric parameters and so on. The proposed 2D array of multi-well traps can be used to form novel 2D optical lattices with cold atoms (or molecules), and form various novel optical crystals with cold atoms (or molecules), or to perform quantum computing and quantum information processing on an atom chip, even to realize an array of all-optical multi-well atomic (or molecular) Bose-Einstein condensates (BECs) on an all-optical integrated atom (or molecule) chip.     

Photo- and electroluminescence of nitrogen isoelectronic traps in GaAs1–xPx

The variation with temperature of the charge on the surface of NaCl single crystals has been measured directly by means of vibrating capacitor probe. The charge is negative at room temperature, in agreement with theory based on values of the free energy of vacancy formation, decreases with increase of temperature in two stages to a zero value at the isoelectric temperature, and then becomes positive. Values of isoelectric temperature obtained on surfaces are consistent with those obtained by experiments on charged dislocations, which suggests that surfaces could be used to obtain accurate values of the free energy of formation of cation and anion vacancies.     

Electron transport in corona charged 12 μm teflon FEP with saturable deep traps

Surface-potential measurements carried out in negatively corona charged 12 m samples of fluorethylenepropylene (Teflon FEP) showed the following characteristics: 1) with a constant charging current, the potential initially rises linearly, and then sublinearly; 2) the potential saturates irrespectively of the charging process and 3) practically no potential decay is observed after switching off the corona. These results have been interpreted in terms of an usual model (field-independent trapping time) for charge transport in insulators, with saturable deep traps in both surface and bulk of the sample and a relatively high electron mobility in order to prevent free-space charge accumulation. The partial differential equations derived from the model are numerically solved and it was found that only the product of the mobility with the trapping time is relevant to the fitting of experimental results, provided that >10^–8 cm²/Vs. A field-dependent trapping time model leads to poorer fittings.     

Effects of spacing on dynamics of two coupled Bose-Einstein condensates in two finite traps

Interaction between two coupled Bose-Einstein condensates (BECs) is investigated by the variational approach in two finite traps, and the effects of the spacing between the two traps on dynamics of the two BECs are analyzed. The spacing determines the stable condition of stationary states, affects the existence condition of each BEC, and changes the switching and self-trapping effects on the two BECs. The dynamic mechanism is demonstrated by performing a coordinate of classical particle moving in an effective potential field, and confirmed by the evolution of the atom population transferring ratio.     

Early stage ageing effects and shallow positron traps in Al–Mg–Si alloys

Room temperature ageing, so-called natural ageing, of Al–Mg–Si alloys has a subtle but striking influence on the mechanical properties achievable by subsequent ageing at more elevated temperatures. Though strongly debated, different clustering processes are generally accepted to give rise to this effect. Using temperature-dependent positron lifetime measurements of naturally aged Al–Mg–Si alloys, it is shown that in the early stages of ageing, small clusters of alloying atoms without embedded vacancies take part in the decomposition process. These clusters serve as shallow positron traps with a binding energy of about 55(10) meV, grow in the course of natural ageing and transform to deep positron traps with binding energies well above thermal energies. Thus, results of positron annihilation spectroscopy techniques need to be interpreted carefully with respect to the microstructure of age-hardenable Al alloys. Moreover, it is shown that a simple approach to bind positron states using a three-dimensional potential well and (bulk) positron affinities cannot explain the present findings.     

Bose–Einstein condensates in concentrically coupled annular traps with spin–orbit coupling and rotation

We investigate Bose–Einstein condensates in concentrically coupled annular traps with spin–orbit coupling and rotation. The ground state wave functions are computed by minimizing the Gross–Pitaevskii energy functional, and the combined effects of system?s parameters, especially the spin–orbit coupling and rotating, are investigated. The results show that for a finite fixed spin–orbit coupling, with increasing the angular frequency of rotation, the system is always in phase coexistence. Moreover, phase transitions between different ground state phases can be induced not only by spin–orbit coupling, but also rotation, which resembles very much the one where the s-wave interactions are varied.     

Near-interface oxide traps in 4H–SiC MOS structures fabricated with and without annealing in NO

Near-interface oxide traps(NIOTs)in 4H–Si C metal–oxide–semiconductor(MOS)structures fabricated with and without annealing in NO are systematically investigated in this paper.The properties of NIOTs in Si C MOS structures prepared with and without annealing in NO are studied and compared in detail.Two main categories of the NIOTs,the"slow"and"fast"NIOTs,are revealed and extracted.The densities of the"fast"NIOTs are determined to be 0.76×10~(11)cm~(-2)and0.47×10~(11)cm~(-2)for the N_2 post oxidation annealing(POA)sample and NO POA sample,respectively.The densities of"slow"NIOTs are 0.79×10~(11)cm~(-2)and 9.44×10~(11)cm~(-2)for the NO POA sample and N_2POA sample,respectively.It is found that the NO POA process only can significantly reduce"slow"NIOTs.However,it has a little effect on"fast"NIOTs.The negative and positive constant voltage stresses(CVS)reveal that electrons captured by those"slow"NIOTs and bulk oxide traps(BOTs)are hardly emitted by the constant voltage stress.     

On luminescence stimulated from deep traps using thermally-assisted time-resolved optical stimulation in α-Al2O3:C

We report a study of charge transfer mechanisms of electrons stimulated optically from very deep traps, also known as donor traps, in α-Al₂O₃:C. The investigations were carried out using thermally-assisted time-resolved optical stimulation, thermoluminescence and by way of residual thermoluminescence from the main electron trap. When the charges are optically stimulated from the deep traps, they are redistributed via the conduction band to the main electron trap and the shallow trap from where they are optically or thermally released for recombination at luminescence centres. The luminescence is strongly quenched at high measurement temperature as evident by very short luminescence lifetimes at these temperatures. The main peak due to residual thermoluminescence is located at a higher temperature than the conventional main peak.     

Influence of shallow traps on holographic recording in Bi12SiO20 in the temperature range 200–350 K

The photoconductivity response time in Bi₁₂SiO₂₀ grows 3 orders of magnitude when the temperature changes from 350 to 200 K. We attribute this to the influence of shallow traps with activation energy 0.4 eV. For low temperatures, the time of holographic grating formation is determined by the time of shallow-level filling. Received: 29 October 1998 / Revised version: 18 December 1998 / Published online: 12 April 1999     

Brouwer?s problem on a heavy particle in a rotating vessel: Wave propagation, ion traps, and rotor dynamics

In 1918 Brouwer considered stability of a heavy particle in a rotating vessel. This was the first demonstration of a rotating saddle trap which is a mechanical analogue for quadrupole particle traps of Penning and Paul. We revisit this pioneering work in order to uncover its intriguing connections with classical rotor dynamics and fluid dynamics, stability theory of Hamiltonian and non-conservative systems as well as with the modern works on crystal optics and atomic physics. In particular, we find that the boundary of the stability domain of the undamped Brouwer?s problem possesses the Swallowtail singularity corresponding to the quadruple zero eigenvalue. In the presence of dissipative and non-conservative positional forces there is a couple of Whitney umbrellas on the boundary of the asymptotic stability domain. The handles of the umbrellas form a set where all eigenvalues of the system are pure imaginary despite the presence of dissipative and non-conservative positional forces.     

Charge storage characteristics in Al/AlN/Si metal–insulator–semiconductor structure based on deep traps in AlN layer

Charge storage characteristics in an Al/AlN/p-Si metal–insulator–semiconductor (MIS) structure have been investigated by capacitance–voltage and long-term capacitance measurements. Good program/erase behavior is observed in the AlN/Si structure, which is attributed to the trapping and detrapping of charges in deep traps of the AlN layer. In the long-term retention mode, a clear memory window is found 2000 s after removing a program/erase voltage of ±3 V, indicating good charge retention capability of the MIS structure. Further investigation shows that for a program pulse width of 500 ms, the charge storage does not occur when the pulse amplitude is smaller than a threshold value of ∼1 V. The trapped charge density increases linearly with increase of the pulse amplitude (>1 V) and tends to saturate at 2.5 V. With increasing program pulse width, the trapped charged density increases a little more than logarithmically. PACS 73.40.Kp; 72.20.Jv; 71.55.Eq     

Effect of NO annealing on charge traps in oxide insulator and transition layer for 4H-SiC metal–oxide–semiconductor devices

The effect of nitric oxide(NO) annealing on charge traps in the oxide insulator and transition layer in n-type4H–Si C metal–oxide–semiconductor(MOS) devices has been investigated using the time-dependent bias stress(TDBS),capacitance–voltage(C–V),and secondary ion mass spectroscopy(SIMS).It is revealed that two main categories of charge traps,near interface oxide traps(Nniot) and oxide traps(Not),have different responses to the TDBS and C–V characteristics in NO-annealed and Ar-annealed samples.The Nniotare mainly responsible for the hysteresis occurring in the bidirectional C–V characteristics,which are very close to the semiconductor interface and can readily exchange charges with the inner semiconductor.However,Not is mainly responsible for the TDBS induced C–V shifts.Electrons tunneling into the Not are hardly released quickly when suffering TDBS,resulting in the problem of the threshold voltage stability.Compared with the Ar-annealed sample,Nniotcan be significantly suppressed by the NO annealing,but there is little improvement of Not.SIMS results demonstrate that the Nniotare distributed within the transition layer,which correlated with the existence of the excess silicon.During the NO annealing process,the excess Si atoms incorporate into nitrogen in the transition layer,allowing better relaxation of the interface strain and effectively reducing the width of the transition layer and the density of Nniot.     

Limits on a nucleon–nucleon monopole–dipole coupling from spin relaxation of polarized ultra-cold neutrons in traps

A new limit is presented on the axion-like monopole–dipole P, T-non-invariant interaction in a range (10⁻⁴–1) cm. The spin-dependent nucleon–nucleon potential between neutrons and nucleons of the walls of the cavity containing ultra-cold neutrons should affect the neutron depolarization probability at their reflection from the walls. The limit is obtained from existing data on the ultra-cold neutron depolarization probability per one collision with the walls.     

The contribution of hole traps to thermoluminescence of the dosimetric peak in anion-defect α-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> single crystals

The thermoluminescent properties of anion-defect alumina single crystals with different FWHMs of the main (dosimetric) peak at 400–500 K are studied. New experimental evidence in favor of the hole nature of traps associated with the high-temperature part of this peak are presented. The introduction of hole trap centers into analysis provided theoretical justification for the experimentally observed dependences of the thermoluminescence (TL) intensity, the temperature position of the main peak, and its FWHM on the occupancy of deep traps. The hole nature of traps of the high-temperature part of the main TL peak is confirmed by the results of examination of specific TL features of shallow trap centers, which govern TL at 350 K, and the temperature variation of the main TL peak spectrum.     

Ionic interactions and collision dynamics in cold traps: rotational quenching of OH<Superscript>−</Superscript>(<Superscript>1</Superscript>Σ<Superscript>+</Superscript>) by Rb(<Superscript>2</Superscript>S)

A newly computed potential energy surface, which describes the forces at play between the OH⁻ (X ¹Σ⁺) anionic molecule (treated as a rigid rotor at its equilibrium geometry) and the Rb(²S) atomic gas was obtained from fully ab initio methods, yielding highly correlated electronic wavefunctions for the interacting partners. It is in turn employed to calculate their rotationally quenching collision cross sections at ultralow energies and for different initial rotational states of the molecular anion. The results suggest that these strongly interacting partners are among the most efficient systems in providing very large internal de-excitation rates whenever the collision regime of an ultracold trap can occur for the partners. The collision encounters at such vanishing values of translational energies are seen to be strongly controlled by the behaviour of the real part of the scattering length as a function of the initial rotational state: the latter indicates, in fact, the presence of several virtual states close to the dissociation threshold of the complex.