457 research outputs found
Optical properties of periodic systems within the current-current response framework: pitfalls and remedies
We compare the optical absorption of extended systems using the
density-density and current-current linear response functions calculated within
many-body perturbation theory. The two approaches are formally equivalent for a
finite momentum of the external perturbation. At
, however, the equivalence is maintained only if a small
expansion of the density-density response function is used. Moreover, in
practical calculations this equivalence can be lost if one naively extends the
strategies usually employed in the density-based approach to the current-based
approach. Specifically we discuss the use of a smearing parameter or of the
quasiparticle lifetimes to describe the finite width of the spectral peaks and
the inclusion of electron-hole interaction. In those instances we show that the
incorrect definition of the velocity operator and the violation of the
conductivity sum rule introduce unphysical features in the optical absorption
spectra of three paradigmatic systems: silicon (semiconductor), copper (metal)
and lithium fluoride (insulator). We then demonstrate how to correctly
introduce lifetime effects and electron-hole interactions within the
current-based approach.Comment: 17 pages, 6 figure
Exciton-Exciton transitions involving strongly bound Frenkel excitons: an ab initio approach
In pump-probe spectroscopy, two laser pulses are employed to garner dynamical
information from the sample of interest. The pump initiates the optical process
by exciting a portion of the sample from the electronic ground state to an
accessible electronic excited state, an exciton. Thereafter, the probe
interacts with the already excited sample. The change in the absorbance after
pump provides information on transitions between the excited states and their
dynamics. In this work we study these exciton-exciton transitions by means of
an ab initio real time propagation scheme based on dynamical Berry phase
formulation. The results are then analyzed taking advantage of a Fermi-golden
rule approach formulated in the excitonic basis-set and in terms of the
symmetries of the excitonic states. Using bulk LiF and 2D hBN as two prototype
materials, we discuss the selection rules for transitions involving strongly
bound Frenkel excitons, for which the hydrogen model cannot be used
A parallel multigrid solver for multi-patch Isogeometric Analysis
Isogeometric Analysis (IgA) is a framework for setting up spline-based
discretizations of partial differential equations, which has been introduced
around a decade ago and has gained much attention since then. If large spline
degrees are considered, one obtains the approximation power of a high-order
method, but the number of degrees of freedom behaves like for a low-order
method. One important ingredient to use a discretization with large spline
degree, is a robust and preferably parallelizable solver. While numerical
evidence shows that multigrid solvers with standard smoothers (like Gauss
Seidel) does not perform well if the spline degree is increased, the multigrid
solvers proposed by the authors and their co-workers proved to behave optimal
both in the grid size and the spline degree. In the present paper, the authors
want to show that those solvers are parallelizable and that they scale well in
a parallel environment.Comment: The first author would like to thank the Austrian Science Fund (FWF)
for the financial support through the DK W1214-04, while the second author
was supported by the FWF grant NFN S117-0
Effects of remote digital monitoring on oral hygiene of orthodontic patients: a prospective study
Background: Remote digital monitoring during orthodontic treatment can help patients in improving their oral hygiene performance and reducing the number of appointments due to emergency reasons, especially in time of COVID-19 pandemic where non-urgent appointments might be discouraged. Methods: Thirty patients scheduled to start an orthodontic treatment were divided into two groups of fifteen. Compared to controls, study group patients were provided with scan box and cheek retractor (Dental Monitoring®) and were instructed to take monthly intra-oral scans. Plaque Index (PI), Gingival Index (GI), and White Spot Lesions (WSL) were recorded for both groups at baseline (t0), every month for the first 3 months (t1, t2, t3), and at 6 months (t4). Carious Lesions Onset (CLO) and Emergency Appointments (EA) were also recorded during the observation period. Inter-group differences were assessed with Student's t test and Chi-square test, intra-group differences were assessed with Cochran’s Q-test (significance α = 0.05). Results: Study group patients showed a significant improvement in plaque control at t3 (p = 0.010) and t4 (p = 0.039), compared to control group. No significant difference was observed in the number of WSL between the two groups. No cavities were detected in the study group, while five CLO were diagnosed in the control group (p = 0.049). A decreased number of EA was observed in the study group, but the difference was not significant. Conclusions: Integration of a remote monitoring system during orthodontic treatment was effective in improving plaque control and reducing carious lesions onset. The present findings encourage orthodontists to consider this technology to help maintaining optimal oral health of patients, especially in times of health emergency crisis
Anomalous Aharonov--Bohm gap oscillations in carbon nanotubes
The gap oscillations caused by a magnetic flux penetrating a carbon nanotube
represent one of the most spectacular observation of the Aharonov-Bohm effect
at the nano--scale. Our understanding of this effect is, however, based on the
assumption that the electrons are strictly confined on the tube surface, on
trajectories that are not modified by curvature effects. Using an ab-initio
approach based on Density Functional Theory we show that this assumption fails
at the nano-scale inducing important corrections to the physics of the
Aharonov-Bohm effect. Curvature effects and electronic density spilled out of
the nanotube surface are shown to break the periodicity of the gap
oscillations. We predict the key phenomenological features of this anomalous
Aharonov-Bohm effect in semi-conductive and metallic tubes and the existence of
a large metallic phase in the low flux regime of Multi-walled nanotubes, also
suggesting possible experiments to validate our results.Comment: 7 figure
PTSD symptoms as a consequence of breast cancer diagnosis : clinical implications
It is a well-established multidisciplinary practice at the European Institute of Oncology, that nurses and physicians often report their difficulties to clinical psychologists regarding adherence to hospital scheduling and procedures, when faced with women who, having been diagnosed with cancer, may be too overwhelmed to understand medical advice. We thus undertook an observational-prospective-cohort study, to investigate the prevalence and variation of PTSD symptomatology in women awaiting a mastectomy at a mean of 30 days after diagnosis and up to 2 years after discharge from hospital. The presence of any correlations between PTSD symptoms and medical and psycho-social variables was also investigated. Between March 2011 and June 2012, 150 women entered the study and were evaluated at four points in time: pre-hospital admission, admission for surgery, hospital discharge and two years later. The prevalence of distress at pre-hospital admission was 20% for intrusion symptoms, 19.1% for avoidance symptoms and 70.7% for state anxiety. Intrusion was negatively correlated with time from diagnosis independently of tumor dimensions, i.e. independently of the perceived seriousness of the illness. Even though at two-year follow up the prevalence of intrusion and avoidance is similar to that in the general population, patients with high levels of intrusion and avoidance at pre-hospital admission will maintain these levels, showing difficulties in adjusting to illness even two years later. As for psycho-social factors, the presence of a positive cancer family and relational history is associated with high levels of distress, in particular with intrusive thinking. Proper interventions aimed at the management of these issues and at their implications in clinical practice is clearly warranted
Radiation-induced sarcoma of the head and neck: A review of the literature
In the last decades, radiotherapy (RT) has become one of the cornerstones in the treatment of head and neck (HN) malignancies and has paralleled an increase in long-term patient survival. This lead to a concomitant increase in the incidence of radiation-induced sarcomas (RIS) of the irradiated field, with an annual rate up to 0.17%. The new techniques of irradiation do not seem to influence the risk of RIS of the HN (RISHN), which mainly develop within the middle-dose field. The median latency of RISHN after RT is 10-12 years and osteosarcoma is the most represented histotype, even though there is a high variability in time of occurrence and histological features observed. There is no clear evidence of predisposing factors for RISHN, and genetic findings so far have not revealed any common mutation. Early clinical diagnosis of RISHN is challenging, since it usually occurs within fibrotic and hardened tissues, while radiological findings are not pathognomonic and able to differentiate them from other neoplastic entities. Given the highly aggressive behavior of RISHN and its poor sensitivity to chemotherapy, radical surgery is the most important prognostic factor and the only curative option at present. Nevertheless, the anatomy of the HN district and the infiltrative nature of RIS do not always allow radical intervention. Therefore, a wise integration with systemic therapy and, when feasible, re-irradiation should be performed. Future findings in the genomic features of RISHN will be crucial to identify a possible sensitivity to specific drugs in order to optimize a multimodal treatment that will be ideally complementary to surgery and reirradiation
Radiological Features of Male Breast Neoplasms: How to Improve the Management of a Rare Disease
The primary aim of our study was to assess the main mammographic and ultrasonographic features of invasive male breast malignancies. The secondary aim was to evaluate whether a specific radiological presentation would be associated with a worse receptor profile. Radiological images (mammography and/or ultrasound) of all patients who underwent surgery for male invasive breast cancer in our institution between 2008 and 2023 were retrospectively analyzed by two breast radiologists in consensus. All significant features of radiological presentation known in the literature were re-evaluated. Fifty-six patients were selected. The mean age at surgery of patients was 69 years (range: 35-81); in 82% of cases (46 patients), the histologic outcome was invasive ductal carcinoma. A total of 28 out of 56 (50%) patients had preoperative mammography; in 9/28 cases (32%), we found a mass with microcalcifications on mammography. The mass presented high density in 25 out of 28 patients (89%); the mass showed irregular margins in 15/28 (54%) cases. A total of 46 out of 56 patients had preoperative ultrasounds. The lesion showed a solid mass in 41/46 (89%) cases. In 5/46 patients (11%), the lesion was a mass with a mixed (partly liquid-partly solid) structure. We did not find any statistically significant correlation between major types of radiological presentation and tumor receptor arrangement. Knowledge of the main radiologic presentation patterns of malignant male breast neoplasm can help better manage this type of disease, which is rare but whose incidence is increasing
Beyond the GW approximation: combining correlation channels
In many-body perturbation theory (MBPT) the self-energy \Sigma=iGW\Gamma
plays the key role since it contains all the many body effects of the system.
The exact self-energy is not known; as first approximation one can set the
vertex function \Gamma to unity which leads to the GW approximation. The latter
properly describes the high-density regime, where screening is important; in
the low-density regime, instead, other approximations are proposed, such as the
T matrix, which describes multiple scattering between two particles.
Here we combine the two approaches. Starting from the fundamental equations
of MBPT we show how one can derive the T-matrix approximation to the
self-energy in a common framework with GW. This allows us to elucidate several
aspects of this formulation, including the origin of, and link between, the
electron-hole and the particle-particle T matrix, the derivation of a screened
T matrix, and the conversion of the T matrix into a vertex correction. The
exactly solvable Hubbard molecule is used for illustration.Comment: 15 pages, 7 figure
- …