Binding energy of localized biexcitons in quantum wells

A variational calculation of the ground state energy of a biexciton in a GaAs/AlGaAs quantum well is presented. The well width fluctuations leading to trapping of the biexcitons are modeled by a parabolic potential. The results obtained for different well widths are compared with recent experimental data. Good agreement is obtained both for the biexciton binding energy and for the Haynes factor. We find that the structure of a biexciton is similar to the one of the H_2 molecule.Comment: 9 pages, 4 figure

Ground state and optical conductivity of interacting polarons in a quantum dot

The ground-state energy, the addition energies and the optical absorption spectra are derived for interacting polarons in parabolic quantum dots in three and two dimensions. A path integral formalism for identical particles is used in order to take into account the fermion statistics. The approach is applied to both closed-shell and open-shell systems of interacting polarons. Using a generalization of the Jensen-Feynman variational principle, the ground-state energy of a confined N-polaron system is analyzed as a function of N and of the electron-phonon coupling constant. As distinct from the few-electron systems without the electron-phonon interaction, three types of spin polarization are possible for the ground state of the few-polaron systems: (i) a spin-polarized state, (ii) a state where the spin is determined by Hund's rule, (iii) a state with the minimal possible spin. A transition from a state fulfilling Hund's rule, to a spin-polarized state occurs when decreasing the electron density. In the strong-coupling limit, the system of interacting polarons turns into a state with the minimal possible spin. These transitions should be experimentally observable in the optical absorption spectra of quantum dots.Comment: 33 pages, 9 figures, E-mail addresses: [email protected], [email protected], [email protected], [email protected], accepted for Phys. Rev.

Spin-orbit coupling and crystal-field splitting in the electronic and optical properties of nitride quantum dots with a wurtzite crystal structure

We present an $sp^3$ tight-binding model for the calculation of the electronic and optical properties of wurtzite semiconductor quantum dots (QDs). The tight-binding model takes into account strain, piezoelectricity, spin-orbit coupling and crystal-field splitting. Excitonic absorption spectra are calculated using the configuration interaction scheme. We study the electronic and optical properties of InN/GaN QDs and their dependence on structural properties, crystal-field splitting, and spin-orbit coupling.Comment: 9 pages, 6 figure

Bipolaron Binding in Quantum Wires

A theory of bipolaron states in quantum wires with a parabolic potential well is developed applying the Feynman variational principle. The basic parameters of the bipolaron ground state (the binding energy, the number of phonons in the bipolaron cloud, the effective mass, and the bipolaron radius) are studied as a function of sizes of the potential well. Two cases are considered in detail: a cylindrical quantum wire and a planar quantum wire. Analytical expressions for the bipolaron parameters are obtained at large and small sizes of the quantum well. It is shown that at $R\gg 1$ [where $R$ means the radius (halfwidth) of a cylindrical (planar) quantum wire, expressed in Feynman units], the influence of confinement on the bipolaron binding energy is described by the function $\sim 1/R^{2}$ for both cases, while at small sizes this influence is different in each case. In quantum wires, the bipolaron binding energy $W(R)$ increases logarithmically with decreasing radius. The shapes and the sizes of a nanostructure, which are favorable for observation of stable bipolaron states, are determined.Comment: 17 pages, 6 figures, E-mail addresses: [email protected]; [email protected]

Wave-packet dynamics in slowly perturbed crystals: Gradient corrections and Berry-phase effects

We present a unified theory for wave-packet dynamics of electrons in crystals subject to perturbations varying slowly in space and time. We derive the wave-packet energy up to the first order gradient correction and obtain all kinds of Berry-phase terms for the semiclassical dynamics and the quantization rule. For electromagnetic perturbations, we recover the orbital magnetization energy and the anomalous velocity purely within a single-band picture without invoking inter-band couplings. For deformations in crystals, besides a deformation potential, we obtain a Berry-phase term in the Lagrangian due to lattice tracking, which gives rise to new terms in the expressions for the wave-packet velocity and the semiclassical force. For multiple-valued displacement fields surrounding dislocations, this term manifests as a Berry phase, which we show to be proportional to the Burgers vector around each dislocation.Comment: 12 pages, RevTe

Optical Absorption Spectra of Bipolarons

The absorption of large bipolarons is investigated using the path-integral method. The response of a bipolaron to an external electromagnetic field is derived in the framework of the memory-function approach. The bipolaron optical absorption spectrum consists of a series of relatively narrow peaks. The peculiarities of the bipolaron optical absorption as a function of the frequency of the electromagnetic field may be attributed to the transitions involving relaxed excited states and scattering states of a bipolaron.Comment: 14 pages, 3 figures, E-mail addresses: [email protected], [email protected]; to be published in Phys. Rev.

Mean parameter model for the Pekar-Fr\"{o}hlich polaron in a multilayered heterostructure

The polaron energy and the effective mass are calculated for an electron confined in a finite quantum well constructed of $GaAs/Al_x Ga_{1-x} As$ layers. To simplify the study we suggest a model in which parameters of a medium are averaged over the ground-state wave function. The rectangular and the Rosen-Morse potential are used as examples. To describe the confined electron properties explicitly to the second order of perturbations in powers of the electron-phonon coupling constant we use the exact energy-dependent Green function for the Rosen-Morse confining potential. In the case of the rectangular potential, the sum over all intermediate virtual states is calculated. The comparison is made with the often used leading term approximation when only the ground-state is taken into account as a virtual state. It is shown that the results are quite different, so the incorporation of all virtual states and especially those of the continuous spectrum is essential. Our model reproduces the correct three-dimensional asymptotics at both small and large widths. We obtained a rather monotonous behavior of the polaron energy as a function of the confining potential width and found a peak of the effective mass. The comparison is made with theoretical results by other authors. We found that our model gives practically the same (or very close) results as the explicit calculations for potential widths $L \geq 10 \AA$.Comment: 12 pages, LaTeX, including 5 PS-figures, subm. to Phys. Rev. B, new data are discusse

Contemporary snapshot of tumor regression grade (TRG) distribution in locally advanced rectal cancer: a cross sectional multicentric experience.

Pre-operative chemoradiotherapy (CRT) followed by surgical resection is still the standard treatment for locally advanced low rectal cancer. Nowadays new strategies are emerging to treat patients with a complete response to pre-operative treatment, rendering the optimal management still controversial and under debate. The primary aim of this study was to obtain a snapshot of tumor regression grade (TRG) distribution after standard CRT. Second, we aimed to identify a correlation between clinical tumor stage (cT) and TRG, and to define the accuracy of magnetic resonance imaging (MRI) in the restaging setting. Between January 2017 and June 2019, a cross sectional multicentric study was performed in 22 referral centers of colon-rectal surgery including all patients with cT3-4Nx/cTxN1-2 rectal cancer who underwent pre-operative CRT. Shapiro-Wilk test was used for continuous data. Categorical variables were compared with Chi-squared test or Fisher's exact test, where appropriate. Accuracy of restaging MRI in the identification of pathologic complete response (pCR) was determined evaluating the correspondence with the histopathological examination of surgical specimens.In the present study, 689 patients were enrolled. Complete tumor regression rate was 16.9%. The "watch and wait" strategy was applied in 4.3% of TRG4 patients. A clinical correlation between more advanced tumors and moderate to absent tumor regression was found (p = 0.03). Post-neoadjuvant MRI had low sensibility (55%) and high specificity (83%) with accuracy of 82.8% in identifying TRG4 and pCR.Our data provided a contemporary description of the effects of pre-operative CRT on a large pool of locally advanced low rectal cancer patients treated in different colon-rectal surgical centers

Familial adenomatous polyposis

Familial adenomatous polyposis (FAP) is characterized by the development of many tens to thousands of adenomas in the rectum and colon during the second decade of life. FAP has an incidence at birth of about 1/8,300, it manifests equally in both sexes, and accounts for less than 1% of colorectal cancer (CRC) cases. In the European Union, prevalence has been estimated at 1/11,300-37,600. Most patients are asymptomatic for years until the adenomas are large and numerous, and cause rectal bleeding or even anemia, or cancer develops. Generally, cancers start to develop a decade after the appearance of the polyps. Nonspecific symptoms may include constipation or diarrhea, abdominal pain, palpable abdominal masses and weight loss. FAP may present with some extraintestinal manifestations such as osteomas, dental abnormalities (unerupted teeth, congenital absence of one or more teeth, supernumerary teeth, dentigerous cysts and odontomas), congenital hypertrophy of the retinal pigment epithelium (CHRPE), desmoid tumors, and extracolonic cancers (thyroid, liver, bile ducts and central nervous system). A less aggressive variant of FAP, attenuated FAP (AFAP), is characterized by fewer colorectal adenomatous polyps (usually 10 to 100), later age of adenoma appearance and a lower cancer risk. Some lesions (skull and mandible osteomas, dental abnormalities, and fibromas on the scalp, shoulders, arms and back) are indicative of the Gardner variant of FAP. Classic FAP is inherited in an autosomal dominant manner and results from a germline mutation in the adenomatous polyposis (APC) gene. Most patients (~70%) have a family history of colorectal polyps and cancer. In a subset of individuals, a MUTYH mutation causes a recessively inherited polyposis condition, MUTYH-associated polyposis (MAP), which is characterized by a slightly increased risk of developing CRC and polyps/adenomas in both the upper and lower gastrointestinal tract. Diagnosis is based on a suggestive family history, clinical findings, and large bowel endoscopy or full colonoscopy. Whenever possible, the clinical diagnosis should be confirmed by genetic testing. When the APC mutation in the family has been identified, genetic testing of all first-degree relatives should be performed. Presymptomatic and prenatal (amniocentesis and chorionic villous sampling), and even preimplantation genetic testing is possible. Referral to a geneticist or genetic counselor is mandatory. Differential diagnoses include other disorders causing multiple polyps (such as Peutz-Jeghers syndrome, familial juvenile polyps or hyperplastic polyposis, hereditary mixed polyposis syndromes, and Lynch syndrome). Cancer prevention and maintaining a good quality of life are the main goals of management and regular and systematic follow-up and supportive care should be offered to all patients. By the late teens or early twenties, colorectal cancer prophylactic surgery is advocated. The recommended alternatives are total proctocolectomy and ileoanal pouch or ileorectal anastomosis for AFAP. Duodenal cancer and desmoids are the two main causes of mortality after total colectomy, they need to be identified early and treated. Upper endoscopy is necessary for surveillance to reduce the risk of ampullary and duodenal cancer. Patients with progressive tumors and unresectable disease may respond or stabilize with a combination of cytotoxic chemotherapy and surgery (when possible to perform). Adjunctive therapy with celecoxib has been approved by the US Food and Drug Administration and the European Medicines Agency in patients with FAP. Individuals with FAP carry a 100% risk of CRC; however, this risk is reduced significantly when patients enter a screening-treatment program

Abdominal obesity and metabolic syndrome: exercise as medicine?

Background: Metabolic syndrome is defined as a cluster of at least three out of five clinical risk factors: abdominal (visceral) obesity, hypertension, elevated serum triglycerides, low serum high-density lipoprotein (HDL) and insulin resistance. It is estimated to affect over 20% of the global adult population. Abdominal (visceral) obesity is thought to be the predominant risk factor for metabolic syndrome and as predictions estimate that 50% of adults will be classified as obese by 2030 it is likely that metabolic syndrome will be a significant problem for health services and a drain on health economies.Evidence shows that regular and consistent exercise reduces abdominal obesity and results in favourable changes in body composition. It has therefore been suggested that exercise is a medicine in its own right and should be prescribed as such. Purpose of this review: This review provides a summary of the current evidence on the pathophysiology of dysfunctional adipose tissue (adiposopathy). It describes the relationship of adiposopathy to metabolic syndrome and how exercise may mediate these processes, and evaluates current evidence on the clinical efficacy of exercise in the management of abdominal obesity. The review also discusses the type and dose of exercise needed for optimal improvements in health status in relation to the available evidence and considers the difficulty in achieving adherence to exercise programmes. Conclusion: There is moderate evidence supporting the use of programmes of exercise to reverse metabolic syndrome although at present the optimal dose and type of exercise is unknown. The main challenge for health care professionals is how to motivate individuals to participate and adherence to programmes of exercise used prophylactically and as a treatment for metabolic syndrome