Content uploaded by Igor Khmelinskii
Author content
All content in this area was uploaded by Igor Khmelinskii on Jun 11, 2018
Content may be subject to copyright.
A preview of the PDF is not available
Anticrossing Spectroscopy in Multi-Nanolayer Structures
Abstract
Presently we explored nanosandwich structures with graphite (Gt) and graphene (Gn) nanolayers. We found that in Pt-SiO2-Gt, Pt-BN-Gt and Pt-SiO2-Ni-Gn structures the spectra may be decomposed into several components, each corresponding to a different value of the total spin angular momentum S. Only one component was required to describe the Pt-SiO 2-Ni-Gn spectra at 5.3 K, with additional components appearing at higher temperatures. On the other hand, a single component described the Pt-BN-Ni-Gn spectra at all temperatures. Temperature dependence of the spectra of the Pt-SiO2-Ni-Gn system was studied in the 5.3-75.3 K range. Presently we obtained experimental results for novel sandwich systems, with the Gn layer only two monoatomic layers thick. Thus, we compared experimental spectra of a three-nanolayer sandwich system containing a Gt nanolayer with those of a four-nanolayer system containing a diatomic Gn layer. The experimental results were discussed using a theoretical model of the respective physical mechanisms. We propose an exchange anticrossing mechanism, whereby the spin-state polarization of the given Zeeman×.
Background
It is increasingly recognized that existing diagnostic approaches do not capture the underlying heterogeneity and complexity of psychiatric disorders such as depression. This study uses a data-driven approach to define fluid depressive states and explore how patients transition between these states in response to cognitive behavioural therapy (CBT).
Methods
Item-level Patient Health Questionnaire (PHQ-9) data were collected from 9891 patients with a diagnosis of depression, at each CBT treatment session. Latent Markov modelling was used on these data to define depressive states and explore transition probabilities between states. Clinical outcomes and patient demographics were compared between patients starting at different depressive states.
Results
A model with seven depressive states emerged as the best compromise between optimal fit and interpretability. States loading preferentially on cognitive/affective v. somatic symptoms of depression were identified. Analysis of transition probabilities revealed that patients in cognitive/affective states do not typically transition towards somatic states and vice-versa. Post-hoc analyses also showed that patients who start in a somatic depressive state are less likely to engage with or improve with therapy. These patients are also more likely to be female, suffer from a comorbid long-term physical condition and be taking psychotropic medication.
Conclusions
This study presents a novel approach for depression sub-typing, defining fluid depressive states and exploring transitions between states in response to CBT. Understanding how different symptom profiles respond to therapy will inform the development and delivery of stratified treatment protocols, improving clinical outcomes and cost-effectiveness of psychological therapies for patients with depression.
We report absorption spectra of the 7.3- and 11.3-nm Co nanolayers and emission of a structured Co nanolayer. The structure contains a 7.3-nm Co nanolayer covering a 25×25mm2 fused silica substrate, with a thicker 11.3-nm Co track in the middle of the substrate. We report that the radiation energy absorbed by the entire Co nanolayer is transferred to the thicker nanotrack. The transferred energy is reemitted by the track, with the emission spectra containing well-defined emission bands, strongly dependent on the excitation wavelength. We report that the bands appearing in the emission spectra of the nanotrack correspond to the transitions from the higher electronic excited states of the nanotrack to its first excited state. We therefore identify the observed emission as the superemission of the Co nanotrack. The superemission quantum yield is dependent on the excitation wavelength, decreasing at higher excitation energies. We propose a theoretical model that explains the results obtained. The model analysis produced estimates of several model parameters.
Presently we explored quantum confinement (QC) in three-nanolayer sandwich systems, composed of Au-SnO2-Fe, Au-SnO2-Si and Au-SnO2-Ag layers. We recorded the absorption spectra of these sandwich systems, all with discrete structure. We recorded the action spectra of the photocurrent for the Au-SnO2-Fe sandwich system, with the photocurrent quantum yields increasing with the photon energy, achieving 3.1 at 4.7 × 10⁴ cm⁻¹. The photocurrent action spectra correlate with high accuracy with optical absorption spectra. We discuss the mechanisms determining the absorption bandwidth value, including surface imperfections, thermal distribution of the vibrational level populations in the electronic ground state, and the diabatic coupling of levels of the excited state to those of a "dark" state. Volt-Ampere (V/A) characteristics were recorded for all three of the sandwich systems, quite similar to those of a Schottky diode. We report the parameter values of the V/A characteristics, found by fitting the experimental data by a theoretical curve. We also report charge density changes in the SnO2 layer caused by low constant voltage applied to the sandwich structure, observed as changes in the absorption band intensity.
We studied the spin-polarized state transport in Fe – SnO2 – Ag and Fe – BeO – Ag three-nanolayer sandwich structures. The exchange-resonance spectra of these sandwich structures are quite specific and different from those observed earlier in other three-nanolayer structures. The presently recorded spectra comprise a set of discrete lines, their width increasing with the sample temperature and also with the Ag layer thickness, for both samples. The linewidth dependences on temperature and Ag layer thickness were studied in detail. Effect of thickness of the intermediate nanolayers of SnO2 and BeO on the linewidth was observed as well. To explain the observed line broadening effects, we proposed and developed the spin-orbit (SO) coupling mechanism of the electron spin relaxation. In the frameworks of this mechanism, we assumed that the electron spin of a bonding electron in one of the layers of the sandwich system is coupled by SO interaction with the other layers. We found that the change in phonon densities affects the linewidths of the exchange resonance spectra. We estimated the values of the model parameters from the analysis of the experimental data. To that end, we continue further development of our earlier theoretical model, using it to interpret the current experimental results, including ab initio calculations of the electronic structure. The exchange resonance spectra were simulated using phenomenological model, where anisotropy of Δg-factor was introduced. We performed ab initio simulations of the exchange resonance spectra and their linewidths, using Gaussian-2000 and a homemade FORTRAN code.
The body of research on (III,Mn)V diluted magnetic semiconductors initiated during the 1990's has concentrated on three major fronts: i) the microscopic origins and fundamental physics of the ferromagnetism that occurs in these systems, ii) the materials science of growth and defects and iii) the development of spintronic devices with new functionalities. This article reviews the current status of the field, concentrating on the first two, more mature research directions. From the fundamental point of view, (Ga,Mn)As and several other (III,Mn)V DMSs are now regarded as textbook examples of a rare class of robust ferromagnets with dilute magnetic moments coupled by delocalized charge carriers. Both local moments and itinerant holes are provided by Mn, which makes the systems particularly favorable for realizing this unusual ordered state. Advances in growth and post-growth treatment techniques have played a central role in the field, often pushing the limits of dilute Mn moment densities and the uniformity and purity of materials far beyond those allowed by equilibrium thermodynamics. In (III,Mn)V compounds, material quality and magnetic properties are intimately connected. In the review we focus on the theoretical understanding of the origins of ferromagnetism and basic structural, magnetic, magneto-transport, and magneto-optical characteristics of simple (III,Mn)V epilayers, with the main emphasis on (Ga,Mn)As. The conclusions we arrive at are based on an extensive literature covering results of complementary ab initio and effective Hamiltonian computational techniques, and on comparisons between theory and experiment. Comment: 58 pages, 49 figures Version accepted for publication in Rev. Mod. Phys. Related webpage: http://unix12.fzu.cz/ms/
We present a simple and very convincing approach to visualizing that subsequent layers of graphene grow between the existing monolayer graphene and the copper catalyst in chemical vapor deposition (CVD). Graphene samples were grown by CVD and then transferred onto glass substrates by the bubbling method in two ways, either direct-transfer (DT) to yield poly (methyl methacrylate) (PMMA)/graphene/glass or (2) inverted transfer (IT) to yield graphene/PMMA/glass. Field emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM) were used to reveal surface features for both the DT and IT samples. The results from FE-SEM and AFM topographic analyses of the surfaces revealed the underlayer growth of subsequent layers. The subsequent layers in the IT samples are visualized as 3D structures, where the smaller graphene layers lie above the larger layers stacked in a concentric manner. The results support the formation of the so-called “inverted wedding cake” stacking in multilayer graphene growth.
We studied photodecomposition dynamics of (SO2⋯XH) Van der Waals' (VdW) complexes and clusters in gas phase, with X = C2H, C2H3, and C2H5. SO2 was excited by frequency-doubled radiation of a tunable dye laser and resonance-enhanced multiphoton ionization was used to detect the C2H (m/z 25), C2H3 (m/z 27), and C2H5 (m/z 29) ions by time-of-flight mass spectroscopy. Spectra obtained at higher nozzle pressures (P0 > 2.5 atm) indicate formation of clusters. Detailed studies of the VdW complex structure were carried out by analyzing the rotational structure of the respective action spectra. We also performed ab initio theoretical analysis of structures of the VdW complexes and transitional states leading to photodecomposition. We find that the structure of the transition state is significantly different as compared to the equilibrium ground-state structure of the respective complex. The photodecomposition mechanism depends on the hydrocarbon molecule bound to SO2.
A novel method for measurement of g-factor and spin-lattice relaxation time of spin-polarized states in nano-layers of different chemical nature was developed. This method is based on usage of spin-polarized state quantum filter, which was created and tested earlier [V. I. Makarov et al., J Appl. Phys. 110, 063717 (2011) and V. I. Makarov et al., J Appl. Phys. 112, 084310 (2012). The spin state parameters were measured in nanolayers of different materials (Fe, Au, and Si) in function of such experimental parameters as the layer thickness and temperature. The phenomenological model developed earlier for steady-state conditions was presently extended to include time dependence and successfully used in the data analysis. Qualitative models were proposed that explain the observed dependences, forming the basis for future theoretical developments.
We continue the work on the quantum filter of spin polarized states induced by magnetic field in an iron nanolayer. Properties of a three-layer ferromagnetic (Fe)–dielectric (SiO2)–conductor (Au) device performing selective transport of spin polarized states were investigated. Reduced diameter of the input magnetic core and thinner conductive layer improved the filter resolution. Output signal amplitude decayed exponentially with the thickness of the dielectric layer. The filter properties were analyzed and explained using the previously developed theoretical approach, based on exchange interaction of the electronic energy levels located in the ferromagnetic and conductive layers. We also studied a five-layer Fe–SiO2–Au–SiO2–Fe sandwich system. Here the transmitted signal structure was more complex than that in a three-layer device. Theoretical model for the five-layer spin state filter device was proposed, based on an extension of that for the three-level device.
The level-crossing technique of atomic spectroscopy utilizes the spatial interference in the scattering of resonance radiation which can occur when two Zeeman levels of an atom are brought into coincidence ("crossed") by the application of an external magnetic field. "Anticrossing" refers to the case where the two levels involved are coupled by a small static interaction. In this paper the general expression for an anticrossing signal is given and its predictions compared with signals observed in the 2 2P term of Li. It is shown that anticrossings produce signals in many experimental situations for which there would be no signal from a normal level crossing.
An interaction Hamiltonian analogous to that in a coupled Lee model is used to calculate the transition matrix appropriate for the description of the anticrossing effect in optical resonance fluorescence. The transition matrix is found to contain two terms (in addition to those for resonance scattering through each of the coupled states) which exist in the amplitude only because the damping matrix in the coupled representation is not diagonal. This matrix is diagonal when the uncoupled states have equal radiative widths, and the resonance scattering intensity is then given by the Breit formula. This result justifies the application by Eck, Foldy, and Wieder of the Breit equation to their initial discovery of the anticrossing effect even though this formula would not be expected to apply to the general anticrossing situation. The crossing effect can also be discussed as a special case of the approach presented.
In the present study, a quantum filter of spin-polarized states induced by magnetic fields in an iron nanolayer was assembled and experimentally studied. We found the device to pass certain spin-polarized states from the iron nanolayer to a conductive gold nanolayer through a dielectric silicon dioxide nanolayer. A theoretical model developed earlier was successfully applied to qualitatively interpret the experimental data.
The geometries, stabilities, and electronic and magnetic properties of MAln (M=Cr, Mn, Fe, Co, Ni; n=1–7, 12) clusters have been investigated systematically within the framework of the gradient-corrected density-functional theory. MAln clusters have similar geometries as that of Aln+1 clusters. For MAl12 clusters, only cobalt and the nickel atoms, whose atom radius are smaller, prefer a position at the cluster center of icosahedron. MAl3 clusters possess relatively higher stability. All the HOMO and LUMO states are non-degenerate and delocalized. The computed total magnetic moments of the lowest-energy structures oscillate with the cluster size, and NPA shows that the 3d electrons play a dominant role for the magnetism of the system.
Nanocrystalline TiN (or nc-TiN) has been imbedded in amorphous silicon nitride (a-SiN x) matrix to form a nanocomposite thin film (nc-TiN/a-SiN x) via magnetron sputtering deposition on silicon wafer. Two important effects of the Si 3 N 4 sputtering target power on the formation of nc-TiN/a-SiN x have been studied: (1) Aside from forming a-SiN x in the matrix, Si atoms also imbed into TiN to form (Ti,Si)N solid solution crystallites. At low target power, the solid solution is substitutional. With increase of power, the amount of silicon "dissolved" in the TiN crystallite increases, and in the meantime, the interstitial components increase which is manifested in the increase in the TiN lattice parameter. (2) The crystallites have a preferred orientation varying with the deposition target power. As conveniently described by the coefficient of texture, the degree of preferred orientation along [111] direction decreases and finally tails off with increase of power. At the same time, the crystallites orient along [200] and [220] direction and eventually [220] direction dominants.