Neutral and charged dark excitons in monolayer WS2_2

Low temperature and polarization resolved magneto-photoluminescence experiments are used to investigate the properties of dark excitons and dark trions in a monolayer of WS$_2$ encapsulated in hexagonal BN (hBN). We find that this system is an $n$-type doped semiconductor and that dark trions dominate the emission spectrum. In line with previous studies on WSe$_2$, we identify the Coulomb exchange interaction coupled neutral dark and grey excitons through their polarization properties, while an analogous effect is not observed for dark trions. Applying the magnetic field in both perpendicular and parallel configurations with respect to the monolayer plane, we determine the g-factor of dark trions to be $g\sim$-8.6. Their decay rate is close to 0.5 ns, more than 2 orders of magnitude longer than that of bright excitons.Comment: 6 pages, 6 figures, supplemental materia

Raman scattering from the bulk inactive out-of-plane B2g1^{1}_{2\text{g}} mode in few-layer MoTe2_{2}

Raman scattering from the out-of-plane vibrational modes (A$_{1\text{g}}$/A'$_{1}$), which originate from the bulk-inactive out-of-plane B$^{1}_{2\text{g}}$ mode, are studied in few-layer MoTe$_{2}$. Temperature-dependent measurements reveal a doublet structure of the corresponding peaks in the Raman scattering spectra of tetralayer and pentalayer samples. A strong enhancement of their lower energy components is recorded at low temperature for 1.91 eV and 1.96 eV laser excitation. We discuss the attribution of the peaks to the inner modes of the respective Raman-active vibrations. The temperature evolution of their intensity strongly suggests a resonant character of the employed excitation, which leads to the mode enhancement at low temperature. The resonance of the laser light with the singularity of the electronic density of states at the $M$ point of the Brillouin zone in MoTe$_{2}$ is proposed to be responsible for the observed effects.Comment: 10 pages, 5 figure

Optimization of broadband semiconductor chirped mirrors with genetic algorithm

Genetic algorithm was applied for optimization of dispersion properties in semiconductor Bragg reflectors for applications in femtosecond lasers. Broadband, large negative group-delay dispersion was achieved in the optimized design: The group-delay dispersion (GDD) as large as −3500 fs2 was theoretically obtained over a 10-nm bandwidth. The designed structure was manufactured and tested, providing GDD −3320 fs2 over a 7-nm bandwidth. The mirror performance was verified in semiconductor structures grown with molecular beam epitaxy. The mirror was tested in a passively mode-locked Yb:KYW laser

Assessment of prognostic significance of cytoplasmic survivin expression in advanced oesophageal cancer.

Survivin is a member of the family of proteins, which inhibit apoptosis (inhibitor of apoptosis proteins - IAP). Expression of survivin was found in colorectal cancer, neuroblastoma, bladder cancer, non-small cell lung cancer, and breast cancer. There is some recent data indicating the correlation of poor prognosis and worse response to chemotherapy in patients with oesophageal squamous cell carcinoma (OSCC) expressing survivin. The aim of the present study was to assess survivin expression in cancerous tissue of patients with advanced OSCC and to test the potential correlation between survivin expression and clinicopathological data. Forty two patients (mean age 58.36+/-8.97 yrs), who were oesophagectomised due to squamous cell carcinoma of the thoracic oesophagus between 1998 and 2000, were retrospectively analysed. Cytoplasmic survivin expression, examined immunohistochemically, was found in 35 (83.33%) cases. No statistically significant correlation between survivin expression in the tumour and patients' gender, TNM stage, or vascular involvement was noted. The mean survival of patients with cytoplasmic survivin expression (17.81+/-5.51 months) was not statistically different to those with negative survivin staining (16+/-6.28 months) as assessed by Mantel-Cox test (p=0.49). Univariate regression analysis revealed UICC staging as the only predictor of survival in the analysed group (

Anatomical classification of the shape and topography of the operated stomach

The aim of the study was to present the classification of anatomical variances of the operated stomach, based on the radiological and historical data. Different anatomical variations of the operated organ were revealed in 431 out of 2034 patients examined in years 2006-2010. Four primary groups were established: abnormal position along longitudinal (I) and horizontal axis (II), as well as abnormal shape (III) and stomach connections (IV). An additional group (V) encloses mixed forms that connect features of two or more primary groups. The first group contains the partial and total translocation of the stomach into the thoracic cavity after the partial or total esophagectomy. Depends on the applied surgical techniques used during the total esophagectomy, the stomach could be located in the front or back to the pericardial sac. An elongated and gestrectatical form often with signs of pylorostenosis is visible in patients treated by the vagotomy. The consequences of fundoplication included: lack or narrow cardiac angle, and often mild form of the stomach cascade. The most common abnormal shape of the stomach was secondary to the gastrectomy and gastric bending. The final organ shape depends on the type of applied surgical procedure that maintains physiological connection with the duodenum or un-anatomical one, mostly with the jejunal loop. In banding, the body of the stomach forms hourglass on the level of the artificial adjustable band, typically fitted for the surgical slim purpose

Raman scattering excitation in monolayers of semiconducting transition metal dichalcogenides

Raman scattering excitation (RSE) is an experimental technique in which the spectrum is made up by sweeping the excitation energy when the detection energy is fixed. We study the low-temperature ($T$=5~K) RSE spectra measured on four high quality monolayers (ML) of semiconducting transition metal dichalcogenides (S-TMDs), $i.e.$ MoS$_2$, MoSe$_2$, WS$_2$, and WSe$_2$, encapsulated in hexagonal BN. The outgoing resonant conditions of Raman scattering reveal an extraordinary intensity enhancement of the phonon modes, which results in extremely rich RSE spectra. The obtained spectra are composed not only of Raman-active peaks, $i.e.$ in-plane E$'$ and out-of-plane A$'_1$, but the appearance of 1$^{st}$, 2$^{nd}$, and higher-order phonon modes is recognised. The intensity profiles of the A$'_1$ modes in the investigated MLs resemble the emissions due to neutral excitons measured in the corresponding PL spectra for the outgoing type of resonant Raman scattering conditions. Furthermore, for the WSe$_2$ ML, the A$'_1$ mode was observed when the incoming light was in resonance with the neutral exciton line. The strength of the exciton-phonon coupling (EPC) in S-TMD MLs strongly depends on the type of their ground excitonic state, $i.e.$ bright or dark, resulting in different shapes of the RSE spectra. Our results demonstrate that RSE spectroscopy is a powerful technique for studying EPC in S-TMD MLs.Comment: 9 pages, 6 figures, ES

Resonance and antiresonance in Raman scattering in GaSe and InSe crystals

The temperature effect on the Raman scattering efficiency is investigated in ε-GaSe and γ-InSe crystals. We found that varying the temperature over a broad range from 5 to 350 K permits to achieve both the resonant conditions and the antiresonance behaviour in Raman scattering of the studied materials. The resonant conditions of Raman scattering are observed at about 270 K under the 1.96 eV excitation for GaSe due to the energy proximity of the optical band gap. In the case of InSe, the resonant Raman spectra are apparent at about 50 and 270 K under correspondingly the 2.41 eV and 2.54 eV excitations as a result of the energy proximity of the so-called B transition. Interestingly, the observed resonances for both materials are followed by an antiresonance behaviour noticeable at higher temperatures than the detected resonances. The significant variations of phonon-modes intensities can be explained in terms of electron-phonon coupling and quantum interference of contributions from different points of the Brillouin zone. Two-dimensional (2D) van der Waals crystals have recently attracted considerable attention due to their unique electronic band structure and functionalities 1,2. The main focus of researchers has been on semiconducting transition metal dichalcogenides (S-TMDs), e.g. MoS 2 , WSe 2 , and MoTe 2 3,4. Currently, another much larger group of layered materials, i.e. semiconducting post-transition metal chalcogenides (S-PTMCs), e.g. SnS, GaS, InSe, and GaTe, has drawn the attention of the 2D community. Among these crystals, Se-based compounds of S-PTMCs, i.e. InSe and GaSe, demonstrate a tunability of their optical response from the near infrared to the visible spectrum with decreasing layer thickness down to monolayers 5-7. Raman scattering (RS) spectroscopy is a powerful and nondestructive tool to get useful information about material properties 8. The RS measurements provide an insight into their vibrational and electronic structures and are of particular importance in studies of layered materials 9. The flake thickness, strain, stability, charge transfer, stoichiometry, and stacking orders of the layers can be accessed by monitoring parameters of the observed pho-non modes 10-17. RS experiments can be performed under non-resonant and resonant excitation conditions: 18. The resonant excitation may lead to a significant enhancement of the RS intensity in S-TMD as well as the activation of otherwise inactive modes. This offers supplementary information on the coupling of particular phonons to electronic transitions of a specific symmetry 19-21. The crossover between the non-resonant and resonant conditions can be achieved not only by the variation of the excitation energy but also by the modulation of temperature as it was recently reported 22-24. In such an approach, it is the band structure that changes with temperature allowing for resonance with particular excitation energy. In this work, we present a comprehensive investigation of the effect of temperature on the Raman scattering in ε-GaSe and γ-InSe crystals. It has been found that the intensity of some phonon modes exhibits a strong variation as a function of temperature under excitation with specific energy due to the resonant conditions of RS. Moreover, a significant antiresonance behaviour accompanies the resonances at higher temperatures, which leads to the vanishing of the modes intensities. The observed effects are discussed in terms of electron-phonon coupling and quantum interference of contributions from different points of the Brillouin zone (BZ)