Search

This chapter deals with semiconducting structures that are inhomogeneous, because of non-uniform doping or non-uniform band structure, or both. Particular attention is given to the bipolar junction transistor and to the metal-oxide-semiconductor field-effect transistor, because of their prominent fundamental and technological importance. We begin this chapter with a discussion of p-n homojunctions, and p-n-p (or n-p-n) bipolar structures, where the transistor action is achieved through the interaction of two back-to-back junctions. We then consider a variety of structures that can be envisaged when the flexibility in doping is accompanied also by the flexibility in the composing materials; this analysis includes semiconductor heterostructures, metal-semiconductor contacts, metal-oxide-semiconductor junctions. Finally we discuss the field-effect transistor action, achieved through modulation of the width of the conductivity channel in doped semiconductors.

Within the general framework of the adiabatic principle, we study the nuclear dynamics in periodic systems. We are interested in small nuclear displacements around the equilibrium configuration of the ground adiabatic energy surface. Within the harmonic approximation, we describe the dispersion curves for normal mode propagation in crystals, and introduce the concept of phonons, as traveling quanta of vibrational energy. In the study of the lattice vibrations of ionic or partially ionic crystals (polar crystals), essentially new features appear in the long wavelength limit; in polar crystals, the coupling of optical vibrational branches with the electromagnetic field leads to the concept of mixed phonon-photon quasiparticles, known as polaritons.

Low dimensionality offers a unique opportunity to introduce some relevant concepts of solid state physics, keeping the treatment at a simple level. This chapter begins with the presentation of the Bloch theorem for one-dimensional periodic lattices. A peculiar aspect of the energy spectrum of an electron in a periodic potential is the presence of allowed and forbidden energy regions. One-dimensional approaches are particularly suited to show from different points of view (weak binding, tight-binding, quantum tunneling, continued fractions) the mechanism of formation of energy bands in solids. The focus of this chapter is on one-dimensional periodic potentials, with traditional contributions going back to the early times of the foundation of quantum mechanics; brief digressions are given to evidence the conceptual link between this historical area of solid state and other much younger and very active areas of research such as artificial structures and superlattices, photonic crystals, atomic condensates.

The occurrence of magnetic ordering in crystals implies some coupling mechanism between localized or delocalized magnetic moments. Among the microscopic models of coupling between the localized moments, we focus on the Heisenberg spin Hamiltonian, because of its formal simplicity and the clear insight into the electrostatic origin of the effective spin-spin interaction. The phase transition to magnetic ordering is first studied within the Weiss mean field theory. We also consider aspects beyond mean field theories, with the spin-wave theory for the elementary excitations (magnons) of the Heisenberg ferromagnets. Universal aspects at or near the critical temperature region are examined with the renormalization group formalism, which establishes the common language for quantum critical phenomena. We close this chapter with a discussion of the Stoner-Hubbard model of itinerant magnetism, the spin-dependent transport in bulk and layered metallic ferromagnetic structures, and the extraordinary impact of giant magnetoresistance from both fundamental and technological points of view.

Aims and background This report retrospectively analyzes 106 cases of endometrioid carcinoma of the ovary treated at the National Cancer Institute of Milan from 1974 through December 1993. In 12 of the 106 cases (11.3%) a synchronous carcinoma of the uterine body was observed. Methods and study design Only patients who had previously untreated disease were included in the study. Patients with synchronous tumors were staged according to their ovarian cancer and treated according to the stage of that disease. Results Thirty-nine patients (36.8%) had stage I, 17 (16.0%) stage II, 43 (40.6%) stage III, and 7 (6.6%) stage IV disease. Moderately plus poorly differentiated tumors were present in 76 of the 106 cases (71.7%). Considering the 67 patients with advanced disease, residual tumor was absent in 27 cases (40.3%), ≤ 2 cm in 17 (25.4%), and > 2 cm in 23 (34.3%) cases. Systematic pelvic and para-aortic lymphadenectomy was performed in 60 patients (56.6%); selective sampling was carried out in 23 cases (21.7%). After surgery, 77 patients underwent various chemotherapy regimens. Conclusion Using univariate analysis, FIGO stage, tumor grade, residual disease after surgery, lymph node status, and platinum in the chemotherapy regimen significantly influenced 5-year survival. However, when all these variables were included in a multivariate analysis only FIGO stage still had a significant impact on survival. Survival analysis also showed a trend towards longer survival in patients with synchronous tumors.

The electronic structure of n - i - p - i Si superlattices is investigated by the tight-binding renormalization method. Strong anisotropy of the hole masses in the in-plane direction is found and strong localization of the electron and hole wave-functions in the growth direction is reported. For the structure studied, a type-II configuration is found with nearly zero overlap between electron and hole wave-functions. The latter property is discussed in view of its possible device applications.

Background: Colorectal cancer (CRC) is a leading cause of cancer morbidity and mortality. People at higher risk are those individuals with a family history of CRC and familial adenomatous polyposis. Prevention and screening are two milestones for this disease. The aim of this study is to evaluate the chemopreventive role of non-steroidal anti-inflammatory drugs (NSAIDs), including aspirin and cyclooxygenase 2 inhibitors, some micronutrients (folic acid, calcium, selenium, antioxidants) and probiotics. Discussion: The studies on aspiring reported promising results, but it is debatable whether aspirin should be used as chemoprevention, because of its side effects and because of poor efficacy evident in subjects at high risk. Similar results were reported for other non-aspirin NSAIDs, such as sulindac and celecoxib, which the potential adverse effects limit their use. Selenium role in prevention of various types of cancer as well as in colon adenomas are often inconclusive or controversial. Several studies suggested that calcium may have a possible chemopreventive effect on colon adenomas and CRC, although contrasting results are reported for the latter. A recent meta-analysis including 13 randomized trial suggested that folic acid supplementation had not a chemiopreventive action on CRC. Several studies investigated the association between antioxidants, administered alone or in combination, and CRC risk, both among general and at risk population, but only few of them supported statistically significant results. Conclusion: The results of this literature review showed an unclear role in CRC prevention of both pharmacological and dietary intervention. Despite several options are available to prevent colon cancer, it is challenging to identify a correct strategy to prevent CRC through pharmacological and dietary intervention due to the long latency of cancer promotion and development. Since some of the drugs investigated may have uncertain individual effects, it can be suggested to potentiate such effects by adding them together.

Background In the late '80s the successes of the laparoscopic surgery for gallbladder disease laid the foundations on the modern use of this surgical technique in a variety of diseases. In the last 20 years, laparoscopic colorectal surgery had become a popular treatment option for colorectal cancer patients. Discussion Many studies emphasized on the benefits stating the significant advantages of the laparoscopic approach compared with the open surgery of reduced blood loss, early return of intestinal motility, lower overall morbidity, and shorter duration of hospital stay, leading to a general agreement on laparoscopic surgery as an alternative to conventional open surgery for colon cancer. The reduced hospital stay may also decrease the cost of the laparoscopic surgery for colorectal cancer, despite th higher operative spending compared with open surgery. The average reduction in total direct costs is difficult to define due to the increasing cost over time, making challenging the comparisons between studies conducted during a time range of more than 10 years. However, despite the theoretical advantages of laparoscopic surgery, it is still not considered the standard treatment for colorectal cancer patients due to technical limitations or the characteristics of the patients that may affect short and long term outcomes. Conclusions The laparoscopic approach to colectomy is slowly gaining acceptance for the management of colorectal pathology. Laparoscopic surgery for colon cancer demonstrates better short-term outcome, oncologic safety, and equivalent long-term outcome of open surgery. For rectal cancer, laparoscopic technique can be more complex depending on the tumor location. The advantages of minimally invasive surgery may translate better care quality for oncological patients and lead to increased cost saving through the introduction of active enhanced recovery programs which are likely cost-effective from the perspective of the hospital health-care providers.

Cytoreductive surgery and HIPEC after neoadjuvant chemotherapy for advanced epithelial ovarian cancer

Lymphatic spread pattern in 17 cases of adenocarcinoma of fallopian tube is reported. Median age of the patients was 48 years. All patients underwent surgical staging including total abdominal hysterectomy, bilateral salpingo-oopherectomy omentectomy, and appendectomy. Systematic pelvic and paraaortic lymphadenectomy was feasible in 15 cases. Majority of the patients (11 of 17 cases, 64%) had advanced disease and showed serous adenocarcinoma (83%). Lymph nodes were involved in 10 of 17 cases (59%). Node metastases rate increased significantly (P < 0.01) with intraperitoneal stage of disease and with grading. Interestingly, positive nodes were also found in 2 cases (33%) of 6 patients with disease still limited to fallopian tube. Overall, patients with negative nodes had a median survival of 76 months, compared with only 33 months if node metastases were found. In conclusion, combined pelvic and para-aortic lymphadenectomy seems to be necessary for staging and perhaps for prognosis of this disease.

A retrospective study of 488 patients with untreated advanced ovarian cancer is presented. Systematic pelvic and paraaortic lymphadenectomy was performed in 248 cases (50.8%). Selective sampling and node biopsy was performed in 33 (6.7%) and 47 (9.6%) patients, respectively. Node metastases were found in 194 of 328 patients (59.1%). The incidence of metastatic nodes significantly increased with more advanced stages, with serous histology, and with a greater amount of residual tumor. Node status appeared to be related to pathology findings at second-look. A complete pathologic response was documented in 26 of 31 (83.8%) patients with negative nodes and in 38 of 59 (64.6%) with positive nodes at first surgery. Patients with negative nodes survived significantly longer (5-year survival, 46%; median, 60 months) than those who had node metastases (5-year survival, 25%; median, 36 months). Using multivariate analysis, lymph node status, together with the stage of disease and residual tumor, still had a significant impact on 5-year survival. Moreover, among patients with optimal cytoreduction, 5-year survival was 46% (median, 56 months) and 30% (median, 41 months) for patients who did and did not undergo lymphadenectomy, respectively ( P = 0.05). Likewise, when suboptimal cytoreduction was considered, a median 5-year survival of 24 months was obtained in patients who underwent lymphadenectomy compared with 14 months in patients who did not ( P

We provide a very efficient procedure for obtaining the excited states of a quantum operator {ital H}, in any arbitrary chosen energy range, independently from the knowledge of the states at lower energies. Our procedure consists in determining, within the Lanczos algorithm, the ground state of the auxiliary operator {ital A}=({ital H}{minus}{ital E}{sub {ital t}}){sup 2}, and hence the eigenvalue of {ital H} nearest in energy to {ital E}{sub {ital t}}, where {ital E}{sub {ital t}} is any chosen trial energy in the energy range of interest. We show that a variational method exploiting diagonalization of 2{times}2 Lanczos matrices, combined with a two-pass Lanczos procedure of relatively small number of iterations, produces eigenvalues of {ital H} within any desired accuracy, as well as the corresponding eigenfunctions. We discuss in particular the {ital T}{direct_product}{tau} Jahn-Teller model, as a realistic prototype of a quantum system with a very large number of degrees of freedom.

The augmented space formalism has been used so far for the study of disordered crystals. We stress here that the topological structure of the augmented space of a binary alloy is formally equivalent to the set of configurations of an Ising system. In particular in both cases a very useful binary representation for labelling the vectors of the space can be done. Starting from this observation we have developed a very efficient description of the Ising hamiltonian, and we have shown that it can be succesfully applied both in the direct diagonalization approach and in conjunction with the renormalization scheme. Within this last case we have proposed a generalization of the techniques commonly used in the literature, obtaining very good results.

Nous presentons un traitement general pour les etats electroniques d'alliages desordonnes dans lequel on considere des elements de matrice hors de diagonale dependants de la composition. Nous exploitons la methode des recurrence a l'interieur d'un espace augmente d'un reticule desordonne. Nouveaute principale de notre travail c'est la simple description des etats de cet espace a travers une logique binaire qui va nous permettre un emploi maniable des relations de recurrence. On a envisage des differents exemples d'alliages desordonnes dans une dimension apres quoi nous avons etabli une comparaison entre la structure electronique obtenue par notre et la simulation numerique de petits clusters

The use of recurrence relations in Lanczos procedure is often limited in practice by loss of numerical stability. In many applications this fact severely limits the numerical reliability of the results and hence the interpretation of the physical models proposed. In this paper we focus on the effects of finite precision arithmetic in the Lanczos algorithm and examine some methods for assessing and testing the reliability of the tridiagonal matrix generated. In addition to the residual vector test we propose an initial vector test, and an orthogonalization test to assess the quality of the eigenvalues. We also implement the use of multiple precision to obtain a significant improvement in the tridiagonal chain transformation. As a final resort, we consider the modified Lanczos procedure to obtain one by one, energies and wavefunctions of a system, within any desired energy range and with any desired accuracy.

The self-duality property of one-dimensional tight-binding Hamiltonians, usually considered in the literature in the special case of the almost-Mathieu potential, is extended to more complex models, where this property can be achieved in the thermodynamic limit. We examine the consequences as they apply to the Lyapunov exponents, and we confirm our analytic deductions by means of a recently proposed numerical procedure based on the renormalization formalism

A new method for computing excited states of a given operatorH is here presented. Our procedure is of particular value when its representation requires an orthonormal basis set of large dimension. In order to obtain the excited state ofH nearest in energy to any chosen trial energyE t, we consider the auxiliary operatorA=(H−E t)2. We show that a reasonable number of relaxations on appropriately generated low-order Krylov subspaces forA is sufficient to produce better and better approximations of its ground state; a high-accuracy final refinement of the ground state ofA is then possible through the standard Lanczos procedure. An important feature of our method is that storage memory limitations, encountered in the conventional determination of all eigenvalues of large systems, are here overcome. As an illustration of the method two significant examples are discussed.

We study the localization-delocalization transition in one-dimensional incommensurate crystals both numerically and analytically. From the numerical point of view we provide an implementation of the renormalization method, which allows to process with high accuracy millions of sites (whenever necessary). From the analytic point of view we extend the envelope function concepts to incommensurate potentials, smoothly varying on lattice constant scale. The control of the transition is made by numerical calculation of the Lyapunov exponent: it presents surprising aspects of universality and simplicity, with plateaux, linear regions and, at times, much more complicated behaviours. The envelope function method, together with semi-analytic considerations, allows to understand, in a number of situations, the presence of mobility edges, pseudo-mobility edges, and gaps in the energy spectrum.

We present a numerically accurate procedure for the study of the electronic structure of one-dimensional incommensurate systems. Our method is used to infer the localized or extended nature of the electronic states by considering, at the same time, both diagonal and off-diagonal matrix elements of an appropriate effective Hamiltonian; we work out a convenient expression of the Lyapunov coefficient in terms of the off-diagonal effective matrix elements. With the further implementation of separately processing (though interdependently) the appropriate segments of the infinite chain, we provide a simple method to reach any desired numerical precision, so that physical aspects can be clearly worked out. Our procedure is tested on an incommensurate potential that exhibits mobility edges.

We treat the classical problem of a number of discrete states interacting with continua (Fano effect), by means of the recursion method. We first map a continuum of states on an equivalent semi-infinite linear chain; then we take advantage of the concepts and properties of the interaction and dipole carrying states to calculate the absorption spectrum. Our approach leads to a significant simplification of calculations without introducing approximations to the original problem

We present a theoretical study of the near gap electronic states of Si∕Ge based multiple quantum well systems composed of Si and Si1−xGex alloys coherently grown on (001)-Si or SiGe substrates. We interpret the experimental photoluminescence spectra recently reported [S. R. Sheng et al., Appl. Phys. Lett. 83, 857 (2003); 83, 2790 (2003)] in terms of direct or indirect k-space transitions. The effect of the spatial localization of the valence and conduction states is analyzed. We investigate the structures in the experiments within the tight binding renormalization method. Strain conditions, spin orbit effects, and quantum confinement are fully considered. Our calculations give an accurate description of the near gap experimental photoluminescence peaks.

The dynamic equations at the basis of the recursion method and of the moment method are discussed using the projection techniques borrowed from statistical mechanics. A local orbital is regarded as the system of interest and the rest of the crystal takes the place of a highly non-Markovian reservoir. A rewarding aspect of the formalism is an improved relation between the chain parameters of the recursion method and the moments. It is speculated that the concepts of random forces and their dynamics could be of help in determining the asymptotic behaviour of the chain variables.

The electronic band structure of the InAs-GaSb superlattice is studied within the localized-orbital framework by the renormalization method. The tight-binding Hamiltonian includes spin-orbit coupling and an accurate description of the composing crystals near relevant band edges. The tight-binding renormalization-group method is described in detail, and is shown to be conceptually simple and operatively efficient. The superlattice band structure is calculated for several unit-cell widths, and the semiconductor-semimetal transition is analyzed. A negative indirect band gap appears for long-period superlattices; its origin and the effects of band anisotropy and spin-orbit splitting are discussed.

The excited electron states in isolated rare-gas dimers are described by a model Hamiltonian with appropriate simplifications for the short-range terms. Then the energies of self-trapped excitons are obtained by including solid state effects. The results are compared with the available transient absorption spectroscopy measurements.

Valence excitons from the $3p$ shell and core excitons from the $3s$ and $2p$ shells in gaseous and solid argon are studied within the envelope-function formalism. First, semiempirical model potentials for atomic excitons are considered; then solid-state effects, which include band structure and electron-hole screening, are taken into account. The problem of a sufficiently accurate account of the "band kinetic operator" is solved with first-order perturbation theory. A new interpretation of the excitonic spectrum originating from the $2p$ shell is given in terms of first-class exciton ($1s$) and third-class excitons ($\mathrm{nd}$, $n\ensuremath{\ge}3$) associated with a unique band edge.

Core excitons from 3 d levels of Kr and 4 d levels of Xe are examined within the integral equation formalism both in the isolated atoms and in the crystals. Earlier interpretation that resonances at the interband edge Kr-3 d and Xe-4 d are due to 1 s , 2 p , 3 p excitons is well supported by the present calculation.

The energy bands of solid neon are computed with a modified orthogonalized plane-wave method, which includes rigorously the Hartree-Fock exchange potential in its nonlocal form. Starting from the computed energy bands, the binding energies for valence and core excitons are obtained. The results are compared with earlier calculations and with experiments.

The recursion and renormalization methods have emerged as precious tools for treating a variety of electronic systems, when their complexity is so high to prevent direct diagonalization of the corresponding Hamiltonian. The two formalisms, introduced in the literature in independent contexts, are here shown to be algebraically equivalent. Moreover the discussion of their foundation is completed by a brief historical overview of other closely related methods: the Lanczos method, the moment method, the Mori method, and the equation of motion method. Finally we consider a number of detailed applications of the recursion and renormalization methods, and their perspectives in the electronic structure of lattices, superlattices, surfaces and coupled electron-phonon systems.

The consequences of layer compound polytypism on optical properties and electronic band structure are investigated for the case of 2H and 4H PbI2. Die Auswirkungen der Schichtstruktur-Polytypie anf optische Eigenschaften und elektronische Bandstruktur werden am Beispiel des 2H- und 4H-PbI2 untersucht.

We discuss the renormalization-group formalism for calculating the electronic structure of surfaces. To illustrate operatively the features of the method, we consider the ideal and reconstructed Si (100)-(2 × 1) surface, because of the interest of this prototype cleavage face from an experimental and theoretical point of view. The renormalization-group method is shown to be computationally efficient and conceptually simple; it provides a convenient description of the electronic structure of surfaces, as well as other multilayer systems.

The recursion and the renormalization methods have greatly enriched the traditional field of electronic state calculations of periodic and aperiodic materials. After discussing their formal relationship, we present problems where the mentioned iterative procedures provide significant breakthrough. We consider first the application of the recursion method in the reciprocal space for periodic structures, where the unprecedented number of recursions reached allows us to analyze directly the asymptotic region. The recursion method in real space is then discussed and extended to the treatment of large clusters, either regular or with built in foreign atoms or vacancies. Impurity states of arbitrary range can be studied and non-perturbative results concerning ionized substitutional atoms in silicon are here provided for the first time. Localized impurity atoms in the presence of a dynamical Jahn-Teller effect also benefit of the recursion method: several vibronic systems as well as infrared absorption spectrum of transition metal impurities in cubic semiconductors are discussed. Using a mixed real-space reciprocal-space representation, it is shown that layered structures (surfaces, heterostructures, quantum wells and multiple quantum wells) can be studied in a very elegant and economical way with the renormalization method. In particular, we interpret the intrinsic origin of the semiconductor-semimetal transition of InAs — GaSb superlattices.

A new, very efficient procedure is provided for obtaining a large number of continued-fraction parameters for the Green's function in translationally invariant systems. This is achieved by successive applications of the recursion method to different regions in k space. The well-known problems connected with computer-memory limitations are thus definitively overcome.

The problem of including rigorously the Hartree-Fock exchange potential in the orthogonalized-plane-wave method is considered. Exchange bielectronic integrals are conveniently evaluated analytically by means of the Dawson auxiliary function, upon expansion of Slater-type orbitals into Gaussian-type orbitals. As an example of our procedure, we compute the Hartree-Fock energy bands of solid lithium hydride. The results are compared with earlier calculations and with the energy bands of the other isoelectronic compounds.

We examine in this paper how van Hove critical points, in a given density-of-states distribution, modify the asymptotic behavior of the corresponding continued-fraction coefficients. Singularities within and at the boundary of a single connected band are discussed in detail, for one-, two-, and three-dimensional structures, in a simple and comprehensive way.

We consider many-body effects on the band structure of solid argon within the Coulomb-hole---plus---screened-exchange approximation, and with the use of a basis set of orthogonalized plane waves. The difficulties inherent to the nonlocal character of the self-energy operator are over-come by expressing the crystal density matrix in terms of localized Gaussian functions. A major advantage of this technique is that all the matrix elements necessary to perform the correlated band-structure calculation can be computed analytically. Our results for the case of argon are compared with experiments and with previous theoretical works.