The electronic states of manganese in p-type GaN are investigated using photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopies. A series of sharp PL lines at 1.0 eV is observed in codoped GaN and attributed to the intra d-shell transition 4T2(F)-4T1(F) of Mn4+ ions. PLE spectrum of the Mn4+ [4T2(F)-4T1(F)] luminescence reveals intra center excitation processes via the excited states of Mn4+ ions. PLE peaks observed at 1.79 and 2.33 eV are attributed to the intra-d-shell 4T1(P)-4T1(F) and 4A2(F)-4T1(F) transitions of Mn4+, respectively. In addition to the intra shell excitation processes, a broad PLE band involving charge-transfer transition of the Mn4+/3+ deep level is observed, which is well described by the Lucovsky model. As determined from the onset of this PLE band, the position of the Mn4+/3+ deep level is 1.11 eV above the valence band maximum, which is consistent with prior theory using ab initio calculations. Our work indicates 4+ is the predominant oxidation state of Mn ions in p-type GaN:Mn when the Fermi energy is lower than 1.11 eV above the valence band maximum.
https://arxiv.org/abs/cond-mat/0412301
This paper proposes a software architecture for dynamical service adaptation. The services are constituted by reusable software components. The adaptation’s goal is to optimize the service function of their execution context. For a first step, the context will take into account just the user needs but other elements will be added. A particular feature in our proposition is the profiles that are used not only to describe the context’s elements but also the components itself. An Adapter analyzes the compatibility between all these profiles and detects the points where the profiles are not compatibles. The same Adapter search and apply the possible adaptation solutions: component customization, insertion, extraction or replacement.
https://arxiv.org/abs/cs/0411056
Using first-principles total-energy methods, we investigate Jahn-Teller distortions in III-V dilute magnetic semiconductors, GaAs:Mn and GaN:Mn in the cubic zinc blende structure. The results for an isolated Mn impurity on a Ga site show that there is no appreciable effect in GaAs, whereas, in GaN there is a Jahn-Teller effect in which the symmetry around the impurity changes from T${d}$ to D${2d}$ or to C$_{2v}$. The large effect in GaN occurs because of the localized d$^4$ character, which is further enhanced by the distortion. The lower symmetry should be detectable experimentally in cubic GaN with low Mn concentration, and should be affected by charge compensation (reductions of holes and conversion of Mn ions to d$^5$ with no Jahn-Teller effect). Jahn-Teller effect is greatly reduced because the symmetry at each Mn site is lowered due to the Mn-Mn interaction. The tendency toward ferromagnetism is found to be stronger in GaN:Mn than in GaAs:Mn and to be only slightly reduced by charge compensation.
https://arxiv.org/abs/cond-mat/0411394
Spontaneous formation of grains has been observed for the MnAs layer grown by means of MBE on the GaN(0001)-(1x1) surface. Electronic structure of the system was investigated in situ by resonant photoemission spectroscopy. Density of the valence band states of MnAs and its changes due to increase of the layer thickness were revealed.
https://arxiv.org/abs/cond-mat/0408111
Current approaches to the annotation process focus on annotation schemas, languages for annotation, or are very application driven. In this paper it is proposed that a more flexible architecture for annotation requires a knowledge component to allow for flexible search and navigation of the annotated material. In particular, it is claimed that a general approach must take into account the needs, competencies, and goals of the producers, annotators, and consumers of the annotated material. We propose that a user-model based approach is, therefore, necessary.
https://arxiv.org/abs/cs/0410059
We analyze the lineshape of x-ray diffraction profiles of GaN epitaxial layers with large densities of randomly distributed threading dislocations. The peaks are Gaussian only in the central, most intense part of the peak, while the tails obey a power law. The $q^{-3}$ decay typical for random dislocations is observed in double-crystal rocking curves. The entire profile is well fitted by a restricted random dislocation distribution. The densities of both edge and screw threading dislocations and the ranges of dislocation correlations are obtained.
https://arxiv.org/abs/cond-mat/0410510
First-principles investigations of the structural, electronic and magnetic properties of Cr-doped AlN/GaN (0001) heterostructures reveal that Cr segregates into the GaN region, that these interfaces retain their important half-metallic character and thus yield efficient (100 %) spin polarized injection from a ferromagnetic GaN:Cr electrode through an AlN tunnel barrier - whose height and width can be controlled by adjusting the Al concentration in the graded bandgap engineered Al(1-x)Ga(x)N (0001) layers.
https://arxiv.org/abs/cond-mat/0410455
We report the observation of ferromagnetism at over 900K in Cr-GaN and Cr-AlN thin films. The saturation magnetization moments in our best films of Cr-GaN and Cr-AlN at low temperatures are 0.42 and 0.6 u_B/Cr atom, respectively, indicating that 14% and 20%, of the Cr atoms, respectively, are magnetically active. While Cr-AlN is highly resistive, Cr-GaN exhibits thermally activated conduction that follows the exponential law expected for variable range hopping between localized states. Hall measurements on a Cr-GaN sample indicate a mobility of 0.06 cm^2/V.s, which falls in the range characteristic of hopping conduction, and a free carrier density (1.4E20/cm^3), which is similar in magnitude to the measured magnetically-active Cr concentration (4.9E19/cm^3). A large negative magnetoresistance is attributed to scattering from loose spins associated with non-ferromagnetic impurities. The results indicate that ferromagnetism in Cr-GaN and Cr-AlN can be attributed to the double exchange mechanism as a result of hopping between near-midgap substitutional Cr impurity bands.
https://arxiv.org/abs/cond-mat/0402103
Density-functional studies of the electron states in the dilute magnetic semiconductor GaN:Mn reveal major differences for the case of the Mn impurity at the substitutional site Mn_Ga versus the interstitial site Mn_I. The splitting of the two-fold and the three-fold degenerate Mn(d)states in the gap are reversed between the two cases, which is understood in terms of the symmetry-controlled hybridization with the neighboring atoms. In contrast to Mn_Ga, which acts as a deep acceptor, Mn_I acts as a donor, suggesting the formation of Coulomb-stabilized complexes such as (Mn_Ga Mn_I Mn_Ga), where the acceptor level of Mn_Ga is passivated by the Mn_I donor. Formation of such passivated clusters might be the reason for the observed low carrier-doping efficiency of Mn in GaN. Even though the Mn states are located well inside the gap,the wave functions are spread far away from the impurity center. This is caused by the hybridization with the nitrogen atoms, which acquire small magnetic moments aligned with the Mn moment. Implications of the differences in the electronic structure for the optical properties are discussed.
https://arxiv.org/abs/cond-mat/0409366
We have demonstrated surface-plasmon induced change in spontaneous emission rate in the ultraviolet regime at ~ 375-380 nm, using AlN/GaN quantum dots (QD). Using time-resolved and continuous-wave photoluminescence measurements, the recombination rate in AlN/GaN QD is shown to be enhanced when spontaneous emission is resonantly coupled to a metal-surface plasmon mode. The exciton recombination process via Ag-surface plasmon modes is observed to be as much as 3-7 times faster than in normal QD spontaneous emission and depends strongly on the emission wavelength and silver thickness.
https://arxiv.org/abs/cond-mat/0409089
Many data-based statistical algorithms require that one find \textit{near or nearest neighbors} to a given vector among a set of points in that vector space, usually with Euclidean topology. The k-d data structure and search algorithms are the generalization of classical binary search trees to higher dimensional spaces, so that one may locate near neighbors to an example vector in $O(\log N)$ time instead of the brute-force O(N) time, with $N$ being the size of the data base. KDTREE2 is a Fortran 95 module, and a parallel set of C++ classes which implement tree construction and search routines to find either a set of $m$ nearest neighbors to an example, or all the neighbors within some Euclidean distance $r.$ The two versions are independent and function fully on their own. Considerable care has been taken in the implementation of the search methods, resulting in substantially higher computational efficiency (up to an order of magnitude faster) than the author’s previous Internet-distributed version. Architectural improvements include rearrangement for memory cache-friendly performance, heap-based priority queues for large $m$searches, and more effective pruning of search paths by geometrical constraints to avoid wasted effort. The improvements are the most potent in the more difficult and slowest cases: larger data base sizes, higher dimensionality manifolds containing the data set, and larger numbers of neighbors to search for. The C++ implementation requires the Standard Template Library as well as the BOOST C++ library be installed.
https://arxiv.org/abs/physics/0408067
We present calculations of magnetic exchange interactions and critical temperature T_c in Mn:GaAs, Cr:GaAs and Cr:GaN. The local spin density approximation is combined with a linear-response technique to map the magnetic energy onto a Heisenberg hamiltonion, but no significant further approximations are made. Special quasi-random structures in large unit cells are used to accurately model the disorder. T_c is computed using both a spin-dynamics approach and the cluster variation method developed for the classical Heisenberg model. We show the following: (i) configurational disorder results in large dispersions in the pairwise exchange interactions; (ii) the disorder strongly reduces T_c; (iii) clustering in the magnetic atoms, whose tendency is predicted from total-energy considerations, further reduces T_c. Additionally the exchange interactions J(R) are found to decay exponentially with distance R^3 on average; and the mean-field approximation is found to be a very poor predictor of T_c, particularly when J(R) decays rapidly. Finally the effect of spin-orbit coupling on T_c is considered. With all these factors taken into account, T_c is reasonably predicted by the local spin-density approximation in MnGaAs without the need to invoke compensation by donor impurities.
https://arxiv.org/abs/cond-mat/0408185
The structure of disordered GaN:O films grown by ion-assisted deposition is investigated using x-ray absorption near-edge spectroscopy and Raman spectroscopy. It is found that between 4 and 21 % of the nitrogen in the films is in the form of molecular N_2 that interacts only weakly with the surrounding matrix. The anion to cation ratio in the GaN:O host remains close to unity, and there is a close correlation between the N_2 fraction, the level of oxygen impurities, and the absence of short-range order in the GaN:O matrix.
https://arxiv.org/abs/cond-mat/0408174
In the preprint cond-mat/0408050 M. V. Cheremisin noticed that some results (namely Fig. 4) of our recent work (cond-mat/0407364) have been taken from his paper cond-mat/0405176. As clear from the caption to Fig. 4 as well as from the text of our paper, we did not consider the curves shown in that Figure as our original results, but emphasized that they have been obtained in the well known paper Chiu and Quinn, Phys. Rev. B 9, 4724 (1974) 30 years ago. It is a surprise, that M. V. Cheremisin presents the 30-years-old results of Chiu and Quinn as his own results, obtained for the first time in cond-mat/0405176.
在预印本中，M.V.Cheremisin注意到我们最近的工作（cond-mat / 0407364）的一些结果（即图4）已经从他的论文cond-mat / 0405176中获得了。从图4的标题以及本文的文本中可以清楚看出，我们没有将该图中所示的曲线作为原始结果，而是强调已经在着名的论文Chiu和Quinn中得到，物理学。 Rev. B 9，4724（1974）30年前。令人惊讶的是，M. Cheremisin先生将Chiu和Quinn的30年历史结果作为自己的成果，首次在cond-mat / 0405176中获得。
https://arxiv.org/abs/cond-mat/0408094
Withdrawn by the author.
https://arxiv.org/abs/cond-mat/0107612
Ion assisted deposition (IAD) has been investigated for the growth of GaN, and the resulting films studied by x-ray diffraction and absorption spectroscopy and by transmission electron microscopy. IAD grown stoichiometric GaN consists of random-stacked quasicrystals of some 3 nm diameter. Amorphous material is formed only by incorporation of 15% or more oxygen, which we attribute to the presence of non-tetrahedral bonds centered on oxygen. The ionic favourability of heteropolar bonds and its strikingly simple constraint to even-membered rings is the likely cause of the instability of stoichiometric a-GaN.
https://arxiv.org/abs/cond-mat/0407659
This paper describes the architecture of MOSE (My Own Search Engine), a scalable parallel and distributed engine for searching the web. MOSE was specifically designed to efficiently exploit affordable parallel architectures, such as clusters of workstations. Its modular and scalable architecture can easily be tuned to fulfill the bandwidth requirements of the application at hand. Both task-parallel and data-parallel approaches are exploited within MOSE in order to increase the throughput and efficiently use communication, storing and computational resources. We used a collection of html documents as a benchmark, and conducted preliminary experiments on a cluster of three SMP Linux PCs.
https://arxiv.org/abs/cs/0407053
The electronic states of manganese in p-type GaN are investigated using photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopies. A series of sharp PL lines at 1.0 eV is observed in codoped GaN and attributed to the intra d-shell transition 4T2(F)-4T1(F) of Mn4+ ions. PLE spectrum of the Mn4+ [4T2(F)-4T1(F)] luminescence reveals intra-center excitation processes via the excited states of Mn4+ ions. PLE peaks observed at 1.79 and 2.33 eV are attributed to the intra d-shell 4T1(P)-4T1(F) and 4A2(F)-4T1(F) transitions of Mn4+, respectively. In addition to the intra-shell excitation processes, a broad PLE band involving charge-transfer transition of the Mn4+/3+ deep level is observed, which is well described by the Lucovsky model. As determined from the onset of this PLE band, the position of the Mn4+/3+ deep level is 1.11 eV above the valence band maximum, which is consistent with prior theory using ab initio calculations. Our work indicates 4+ is the predominant oxidation state of Mn ions in p-type GaN:Mn when the Fermi energy is lower than 1.11 eV above the valence band maximum.
https://arxiv.org/abs/cond-mat/0406228
Now no web search engine can cover more than 60 percent of all the pages on Internet. The update interval of most pages database is almost one month. This condition hasn’t changed for many years. Converge and recency problems have become the bottleneck problem of current web search engine. To solve these problems, a new system, search engine based on DNS is proposed in this paper. This system adopts the hierarchical distributed architecture like DNS, which is different from any current commercial search engine. In theory, this system can cover all the web pages on Internet. Its update interval could even be one day. The original idea, detailed content and implementation of this system all are introduced in this paper.
https://arxiv.org/abs/cs/0405099
We present a protocol for transfer of an unknown quantum state. The protocol is based on a two-mode cavity interacting dispersively in a sequential manner with three-level atoms in $\Lambda$ configuration. We propose a scheme for quantum networking using an atomic channel. We investigate the effect of cavity decoherence in the entire process. Further, we demonstrate the possibility of an efficient quantum memory for arbitrary superposition of two modes of a cavity contaning one photon.
https://arxiv.org/abs/quant-ph/0305101
We discuss the performance of the Search and Fourier Transform algorithms on a hybrid computer constituted of classical and quantum processors working together. We show that this semi-quantum computer would be an improvement over a pure classical architecture, no matter how few qubits are available and, therefore, it suggests an easier implementable technology than a pure quantum computer with arbitrary number of qubits.
https://arxiv.org/abs/quant-ph/0304085
Evolutionary artificial neural networks (EANNs) refer to a special class of artificial neural networks (ANNs) in which evolution is another fundamental form of adaptation in addition to learning. Evolutionary algorithms are used to adapt the connection weights, network architecture and learning algorithms according to the problem environment. Even though evolutionary algorithms are well known as efficient global search algorithms, very often they miss the best local solutions in the complex solution space. In this paper, we propose a hybrid meta-heuristic learning approach combining evolutionary learning and local search methods (using 1st and 2nd order error information) to improve the learning and faster convergence obtained using a direct evolutionary approach. The proposed technique is tested on three different chaotic time series and the test results are compared with some popular neuro-fuzzy systems and a recently developed cutting angle method of global optimization. Empirical results reveal that the proposed technique is efficient in spite of the computational complexity.
https://arxiv.org/abs/cs/0405033
The optical properties of Mn-Mg codoped epitaxial GaN were studied. Addition of Mg acceptors quenches the weak manganese-related photoluminescence (PL) band at 1.3 eV in GaN:Mn and a series of sharp PL peaks are observed at 1 eV in codoped epilayers. The change in PL spectra indicates that Mg addition stabilizes the Mn4+ charge state by decreasing the Fermi level. The 1 eV PL peaks are tentatively attributed to intra center transitions involving Mn4+ ions. Spin allowed 3d-shell 4T2-4T1 transitions and their phonon replicas are involved. The relative intensities of the sharp peaks are strongly dependent on the excitation wavelength, indicating the optically active Mn4+ centers involved in the separate peaks are different. The temperature dependence of the PL spectrum suggests the presence of at least three distinct Mn4+ complex centers.
https://arxiv.org/abs/cond-mat/0405097
Systematic experimental data on resonant inelastic X-ray scattering (RIXS) in GaN near the N K-edge are presented for the first time. Excitation energy dependence of the spectral structures manifests the band structure effects originating from momentum selectivity of the RIXS process. This finding allows obtaining k-resolved band structure information for GaN crystals and nanostructures.
https://arxiv.org/abs/cond-mat/0404421
We present the analysis of four 3.6 cm radio continuum archival observations of Orion obtained using the Very Large Array in its A-configuration, with $0\rlap.{‘‘}3$ angular resolution. The observations were made during the period 1994-1997. In a region of $4’ \times 4’$, we detect a total of 77 compact radio sources. Of the total of detected sources, 54 are detected in one or more of the individual observations and 36 of these show time variability (by more than 30%) between the observed epochs. A deep image made from averaging all data shows an additional 23 faint sources, in the range of 0.1 to 0.3 mJy. Of the total of 77 sources, 39 are new centimeter detections. However, only 9 of the 77 sources do not have a previously reported counterpart at near-infrared, optical, or X-ray wavelengths. In particular, we detect three faint sources in the OMC-1S region that may be related to the sources that power the multiple outflows that emanate from this part of the Orion nebula. %We discuss the nature of these sources and its relation with the %near-infrared, optical, and X-ray objects in the region.
https://arxiv.org/abs/astro-ph/0403350
We study the ability of linear recurrent networks obeying discrete time dynamics to store long temporal sequences that are retrievable from the instantaneous state of the network. We calculate this temporal memory capacity for both distributed shift register and random orthogonal connectivity matrices. We show that the memory capacity of these networks scales with system size.
https://arxiv.org/abs/cond-mat/0402452
Negative Dark Electric Matter Objects, daemons, have been detected by means of ZnS(Ag) scintillator screens. These objects are apparently relic elementary Planckian black holes. The scintillations in ZnS(Ag) are excited by electrons and nucleons ejected as the daemon captures a nucleus of Zn (or S). By catalyzing the decay of protons in the remainder of the nucleus, the daemon becomes capable of capturing a new nucleus, and so on. The time elapsed between successive captures (scintillations) is used to estimate the daemon velocity and the proton decay time (~1 microsec). The flux of daemons with velocities from ~5 to ~30 km/s is ~10-9 cm-2s-1 and varies with a period of 0.5 yr. This could indicate a preferred direction of the flux, which is a result of the Sun’s moving relative to daemons in the Galactic disk and of their capture into helio- and geocentric orbits. Daemons crowd at the centres of the Earth, the Sun, and the Galaxy, where they catalyze proton decay, a process capable of accounting from a common standpoint a variety of observations and phenomena unexplained heretofore.
https://arxiv.org/abs/astro-ph/0402367
We study the multi-exciton optical spectrum of vertically coupled GaN/AlN quantum dots with a realistic three-dimensional direct-diagonalization approach for the description of few-particle Coulomb-correlated states. We present a detailed analysis of the fundamental properties of few-particle/exciton interactions peculiar of nitride materials. The giant intrinsic electric fields and the high electron/hole effective masses give rise to different effects compared to GaAs-based quantum dots: intrinsic exciton-exciton coupling, non-molecular character of coupled dot exciton wavefunction, strong dependence of the oscillator strength on the dot height, large ground state energy shift for dots separated by different barriers. Some of these effects make GaN/AlN quantum dots interesting candidates in quantum information processing.
https://arxiv.org/abs/cond-mat/0402260
A paramagnetic muonium (Mu) state with an extremely small hyperfine parameter was observed for the first time in single-crystalline GaN below 25 K. It has a highly anisotropic hyperfine structure with axial symmetry along the [0001] direction, suggesting that it is located either at a nitrogen-antibonding or a bond-centered site oriented parallel to the c-axis. Its small ionization energy (=< 14 meV) and small hyperfine parameter (–10^{-4} times the vacuum value) indicate that muonium in one of its possible sites produces a shallow state, raising the possibility that the analogous hydrogen center could be a source of n-type conductivity in as-grown GaN.
https://arxiv.org/abs/cond-mat/0402155
Hexagonal to cubic phase transformation is studied in focused ion beam assisted Ga+-implanted GaN nanowires. Optical photoluminescence and cathodoluminescence studies along with high-resolution transmission electron microscopic structural studies are performed to confirm the phase transformation. In one possibility, sufficient accumulation of Ga from the implanted source might have reduced the surface energy and simultaneously stabilized the cubic phase. Other potential reason may be that the fluctuations in the short-range order induced by enhanced dynamic annealing (defect annihilation) with the irradiation process stabilize the cubic phase and cause the phase transformation.
https://arxiv.org/abs/cond-mat/0402062
Mobile access to information is a considerable problem for many users, especially to information found on the Web. In this paper, we explore how a voice-controlled service, accessible by telephone, could support mobile users’ needs for refinding specific information previously found on the Web. We outline challenges in creating such a service and describe architectural and user interfaces issues discovered in an exploratory prototype we built called WebContext. We also present the results of a study, motivated by our experience with WebContext, to explore what people remember about information that they are trying to refind and how they express information refinding requests in a collaborative conversation. As part of the study, we examine how end-usercreated Web page annotations can be used to help support mobile information re-finding. We observed the use of URLs, page titles, and descriptions of page contents to help identify waypoints in the search process. Furthermore, we observed that the annotations were utilized extensively, indicating that explicitly added context by the user can play an important role in re-finding.
https://arxiv.org/abs/cs/0402001
We present a statistical study of the detectability of lightcurves of Kuiper Belt objects (KBOs). Some Kuiper Belt objects display lightcurves that appear “flat”, i.e., there are no significant brightness variations within the photometric uncertainties. Under the assumption that KBO lightcurves are mainly due to shape, the lack of brightness variations may be due to (1) the objects have very nearly spherical shapes, or (2) their rotation axes coincide with the line of sight. We investigate the relative importance of these two effects and relate it to the observed fraction of “flat” lightcurves. This study suggests that the fraction of KBOs with detectable brightness variations may provide clues about the shape distribution of these objects. Although the current database of rotational properties of KBOs is still insufficient to draw any statistically meaningful conclusions, we expect that, with a larger dataset, this method will provide a useful test for candidate KBO shape distributions.
https://arxiv.org/abs/astro-ph/0401300
We present data on mobility lifetime, $\tau_t$, quantum lifetime, $\tau_q$, and cyclotron resonance lifetime, $\tau_{CR}$, of a sequence of high-mobility two-dimensional electron gases in the AlGaN/GaN system, covering a density range of $\sim1-4.5\times10^{12}$cm$^{-2}$. We observe a large discrepancy between $\tau_q$ and $\tau_{CR}$ ($\tau_q\sim\tau_{CR}$/6) and explain it as the result of density fluctuations of only a few percent. Therefore, only $\tau_{CR}$ –and not $\tau_q$ – is a reliable measure of the time between electron scattering events in these specimens. The ratio $\tau_t / \tau_{CR}$ increases with increasing density in this series of samples, but scattering over this density range remains predominantly in the large-angle scattering regime.
https://arxiv.org/abs/cond-mat/0312638
In this paper we report first results of our study on network characteristics of a reference-based, bibliographically coupled (BC) publication network structure. We find that this network of clustered publications shows different topologies depending on the age of the references used for building the network. A remarkable finding is that only the network structure based on all references within publications is characterized by a degree distribution with a power-law dependence. This topology, which is typical for scale-free networks, disappears when selecting references of a specific age for the clustering process. Structuring the publication network as a function of reference age, allows ‘tuning through the episodic memory’ of the nodes of the network. We find that the older the references, the more the network tends to change its topology towards a Gaussian degree distribution.
https://arxiv.org/abs/cond-mat/0311318
We demonstrate that the 3.42 eV photoluminescence (PL) band in GaN is of the same intrinsic origin as the near-edge $\sim$3.47 eV band, but arises from regions of inversed polarity characterized by different strain and growth rate. Two absorption edges are thermally detected at 0.35 K in nanocolumn structures, exhibiting both bands. Micro-PL studies have shown similar temperature/power behavior of these bands, with a competition in intensity in closely spaced spots accompanied by alterations of exciton level ordering. Strain-induced one-dimensional carrier confinement in small inversion domains likely explains the discrete narrow lines observed between the bands.
https://arxiv.org/abs/cond-mat/0311191
We show that the large band offsets between GaN and InN and the heavy carrier effective masses preclude the use of the Virtual Crystal Approximation to describe the electronic structure of Ga_(1-x)In_(x)N/GaN heterostructures while this approximation works very well for the Ga_(1-x)In_(x)As/GaAs heterostructures.
https://arxiv.org/abs/cond-mat/0311133
We study the nucleation of GaN islands grown by plasma-assisted molecular-beam epitaxy on AlN(0001) in a Stranski-Krastanov mode. In particular, we assess the variation of their height and density as a function of GaN coverage. We show that the GaN growth passes four stages: initially, the growth is layer-by-layer; subsequently, two-dimensional precursor islands form, which transform into genuine three-dimensional islands. During the latter stage, island height and density increase with GaN coverage until the density saturates. During further GaN growth, the density remains constant and a bimodal height distribution appears. The variation of island height and density as a function of substrate temperature is discussed in the framework of an equilibrium model for Stranski-Krastanov growth.
https://arxiv.org/abs/cond-mat/0311050
We show that large amplitude, coherent acoustic phonon wavepackets can be generated and detected in In$x$Ga${1-x}$N/GaN epilayers and heterostructures in femtosecond pump-probe differential reflectivity experiments. The amplitude of the coherent phonon increases with increasing Indium fraction $x$ and unlike other coherent phonon oscillations, both \textit{amplitude} and \textit{period} are strong functions of the laser probe energy. The amplitude of the oscillation is substantially and almost instantaneously reduced when the wavepacket reaches a GaN-sapphire interface below the surface indicating that the phonon wavepackets are useful for imaging below the surface. A theoretical model is proposed which fits the experiments well and helps to deduce the strength of the phonon wavepackets. Our model shows that localized coherent phonon wavepackets are generated by the femtosecond pump laser in the epilayer near the surface. The wavepackets then propagate through a GaN layer changing the local index of refraction, primarily through the Franz-Keldysh effect, and as a result, modulate the reflectivity of the probe beam. Our model correctly predicts the experimental dependence on probe-wavelength as well as epilayer thickness.
https://arxiv.org/abs/cond-mat/0310654
We study a model of associative memory based on a neural network with small-world structure. The efficacy of the network to retrieve one of the stored patterns exhibits a phase transition at a finite value of the disorder. The more ordered networks are unable to recover the patterns, and are always attracted to mixture states. Besides, for a range of the number of stored patterns, the efficacy has a maximum at an intermediate value of the disorder. We also give a statistical characterization of the attractors for all values of the disorder of the network.
https://arxiv.org/abs/nlin/0310033
We investigate exciton states theoretically in strained GaN/AlN quantum dots with wurtzite (WZ) and zinc-blende (ZB) crystal structures, as well as strained WZ GaN/AlGaN quantum dots. We show that the strain field significantly modifies the conduction and valence band edges of GaN quantum dots. The piezoelectric field is found to govern excitonic properties of WZ GaN/AlN quantum dots, while it has a smaller effect on WZ GaN/AlGaN, and very little effect on ZB GaN/AlN quantum dots. As a result, the exciton ground state energy in WZ GaN/AlN quantum dots, with heights larger than 3 nm, exhibits a red shift with respect to the bulk WZ GaN energy gap. The radiative decay time of the red-shifted transitions is large and increases almost exponentially from 6.6 ns for quantum dots with height 3 nm to 1100 ns for the quantum dots with height 4.5 nm. In WZ GaN/AlGaN quantum dots, both the radiative decay time and its increase with quantum dot height are smaller than those in WZ GaN/AlN quantum dots. On the other hand, the radiative decay time in ZB GaN/AlN quantum dots is of the order of 0.3 ns, and is almost independent of the quantum dot height. Our results are in good agreement with available experimental data and can be used to optimize GaN quantum dot parameters for proposed optoelectronic applications.
https://arxiv.org/abs/cond-mat/0310363
A currently active radio galaxy sits at the center of almost every strong cooling core. What effect does it have on the cooling core? Could its effect be strong enough to offset the radiative cooling which should be occuring in these cores? In order to answer these questions we need to know how much energy the radio jet carries to the cooling core; but we have no way to measure the jet power directly. We therefore need to understand how the radio source evolves with time, and how it radiates, in order to use the data to determine the jet power. When some simple models are compared to the data, we learn that cluster-center radio galaxies probably are energetically important – but not necessarily dominant – in cooling cores.
https://arxiv.org/abs/astro-ph/0310011
Using cyclotron resonance, we measure the effective mass, $m$, of electrons in AlGaN/GaN heterostructures with densities, $n_{2D}\sim 1-6\times10^{12}$cm$^{-2}$. From our extensive data, we extrapolate a band edge mass of $(0.208\pm0.002) m_e$. By comparing our $m$ data with the results of a multi-band \textbf{k.p} calculation we infer that the effect of remote bands is essential in explaining the observed conduction band non-parabolicity (NP). Our calculation of polaron mass corrections – including finite width and screening - suggests those to be negligible. It implies that the behavior of $m*(n_{2D})$ can be understood solely in terms of NP. Finally, using our NP and polaron corrections, we are able to reduce the large scatter in the published band edge mass values.
https://arxiv.org/abs/cond-mat/0309630
A ternary/binary data coding algorithm and conditions under which Hopfield networks implement optimal convolutional or Hamming decoding algorithms has been described. Using the coding/decoding approach (an optimal Binary Signal Detection Theory, BSDT) introduced a Neural Network Assembly Memory Model (NNAMM) is built. The model provides optimal (the best) basic memory performance and demands the use of a new memory unit architecture with two-layer Hopfield network, N-channel time gate, auxiliary reference memory, and two nested feedback loops. NNAMM explicitly describes the dependence on time of a memory trace retrieval, gives a possibility of metamemory simulation, generalized knowledge representation, and distinct description of conscious and unconscious mental processes. A model of smallest inseparable part or an “atom” of consciousness is also defined. The NNAMM’s neurobiological backgrounds and its applications to solving some interdisciplinary problems are shortly discussed. BSDT could implement the “best neural code” used in nervous tissues of animals and humans.
https://arxiv.org/abs/cs/0309036
On the basis of convolutional (Hamming) version of recent Neural Network Assembly Memory Model (NNAMM) for intact two-layer autoassociative Hopfield network optimal receiver operating characteristics (ROCs) have been derived analytically. A method of taking into account explicitly a priori probabilities of alternative hypotheses on the structure of information initiating memory trace retrieval and modified ROCs (mROCs, a posteriori probabilities of correct recall vs. false alarm probability) are introduced. The comparison of empirical and calculated ROCs (or mROCs) demonstrates that they coincide quantitatively and in this way intensities of cues used in appropriate experiments may be estimated. It has been found that basic ROC properties which are one of experimental findings underpinning dual-process models of recognition memory can be explained within our one-factor NNAMM.
https://arxiv.org/abs/cs/0309007
Mn doped semiconductors are extremely interesting systems due to their novel magnetic properties suitable for the spintronics applications. It has been shown recently by both theory and experiment that Mn doped GaN systems have a very high Curie temperature compared to that of Mn doped GaAs systems. To understand the electronic and magnetic properties, we have studied Mn doped GaN system in detail by a first principles plane wave method. We show here the effect of varying Mn concentration on the electronic and magnetic properties. For dilute Mn concentration, $d$ states of Mn form an impurity band completely separated from the valence band states of the host GaN. This is in contrast to the Mn doped GaAs system where Mn $d$ states in the gap lie very close to the valence band edge and hybridizes strongly with the delocalized valence band states. To study the effects of electron correlation, LSDA+U calculations have been performed. Calculated exchange interaction in (Mn,Ga)N is short ranged in contrary to that in (Mn,Ga)As where the strength of the ferromagnetic coupling between Mn spins is not decreased substantially for large Mn-Mn separation. Also, the exchange interactions are anisotropic in different crystallographic directions due to the presence or absence of connectivity between Mn atoms through As bonds.
https://arxiv.org/abs/cond-mat/0308527
In this work the properties of multi choice minority games are studied by means of extensive computational simulations. We have considered several ways of rewarding the strategies of the players and compared the resulting behaviours of the ensemble. It is shown that introducing a reward for the second place can improve the overall performance significantly. Our new but still very simple architecture of stategies shows interesting properties. In the version of the multi choice minority game which also rewards second best choice the unordered phase is strongly suppressed and the global performance of the ensemble is significantly improved. The efficiency of this model can come close to the one of standard minority games that use adaptive genetic algorithms for searching good strategies.
https://arxiv.org/abs/cond-mat/0308181
This article is concerned with the existence, status and description of the so-called emergent phenomena believed to occur in certain principally planar electronic systems. In fact, two distinctly different if inseparable tasks are accomplished. First, a rigorous mathematical model is proposed of emergent character, which is conceptually bonded with Quantum Mechanics while apparently non-derivable from the many-body Schrödinger equation. I call the resulting conceptual framework the Mesoscopic Mechanics (MeM). Its formulation is space-independent and comprises a nonlinear and holistic extension of the free electron model. Secondly, the question of relevancy of the proposed ``emergent mechanics” to the actually observed phenomena is discussed. In particular, I postulate a probabilistic interpretation, and indicate how the theory could be applied and verified by experiment. The Mesoscopic Mechanics proposed here has been deduced from the Nonlinear Maxwell Theory (NMT)–a classical in character nonlinear field theory. This latter theory has already been shown to provide a consistent phenomenological model of such phenomena as superconductivity, charge stripes, magnetic vortex lattice, and magnetic oscillations. The NMT, which arose from geometric considerations, has long been awaiting an explanation as to its ties with the fundamental principles. I believe the MeM provides at least a partial explanation to this effect.
https://arxiv.org/abs/cond-mat/0307716
High energy physics detectors can be described hierarchically from the different subsystems to their divisions in r, phi, theta and to the individual readout channels. An identification schema that follows the logical decomposition of the ATLAS detector has been introduced allowing identification of individual readout channels as well as other parts of the detector, in particular detector elements. These identifiers provide a sort of ?glue? allowing, for example, the connection of raw event data to their detector description for position calculation or alignment corrections, as well as fast access to subsets of the event data for event trigger selection. There are two important requirements on the software to support such an identification scheme. First is the possibility to formally specify these identifiers in terms of their structure and allowed values. And second is to generate different forms of the identifiers optimised in terms of access efficiency to information content, compactness or search key efficiency. We present here the generic toolkit developed in the context of the ATLAS experiment to primarily provide the identification of the readout channels and detector elements. The architecture of the toolkit is decomposed into three parts: an XML-based dictionary containing the formal specification of a particular range of identifiers, a set of various identifier classes (offering various level of compaction), and finally a set of ?helper? classes, specific for each detector system, which serve as intermediaries between the dictionary and the identifier classes to create, manipulate and interpret the identifiers. This architecture will be described as well as the various applications of this identification scheme.
https://arxiv.org/abs/physics/0306141
Distributed Services Architecture with support for mobile agents between services, offer significantly improved communication and computational flexibility. The uses of agents allow execution of complex operations that involve large amounts of data to be processed effectively using distributed resources. The prototype system Distributed Agents for Mobile and Dynamic Services (DIAMOnDS), allows a service to send agents on its behalf, to other services, to perform data manipulation and processing. Agents have been implemented as mobile services that are discovered using the Jini Lookup mechanism and used by other services for task management and communication. Agents provide proxies for interaction with other services as well as specific GUI to monitor and control the agent activity. Thus agents acting on behalf of one service cooperate with other services to carry out a job, providing inter-operation of loosely coupled services in a semi-autonomous way. Remote file system access functionality has been incorporated by the agent framework and allows services to dynamically share and browse the file system resources of hosts, running the services. Generic database access functionality has been implemented in the mobile agent framework that allows performing complex data mining and processing operations efficiently in distributed system. A basic data searching agent is also implemented that performs a query based search in a file system. The testing of the framework was carried out on WAN by moving Connectivity Test agents between AgentStations in CERN, Switzerland and NUST, Pakistan.
https://arxiv.org/abs/cs/0305062
The strain and electric fields present in free-standing AlGaN/GaN slabs are examined theoretically within the framework of fully-coupled continuum elastic and dielectric models. Simultaneous solutions for the electric field and strain components are obtained by minimizing the electric enthalpy. We apply constraints appropriate to pseudomorphic semiconductor epitaxial layers and obtain closed-form analytic expressions that take into account the wurtzite crystal anisotropy. It is shown that in the absence of free charges, the calculated strain and electric fields are substantially differently from those obtained using the standard model without electromechanical coupling. It is also shown, however, that when a two-dimensional electron gas is present at the AlGaN/GaN interface, a condition that is the basis for heterojunction field-effect transistors, the electromechanical coupling is screened and the decoupled model is once again a good approximation. Specific cases of these calculations corresponding to transistor and superlattice structures are discussed.
https://arxiv.org/abs/cond-mat/0306323