Linear and nonlinear optical response of crystals using length and velocity gauges: Effect of basis truncation

We study the effects of a truncated band structure on the linear and nonlinear optical response of crystals using four methods. These are constructed by (i) choosing either length or velocity gauge for the perturbation and (ii) computing the current density either directly or via the time-derivative of the polarization density. In the infinite band limit, the results of all four methods are identical, but basis truncation breaks their equivalence. In particular, certain response functions vanish identically and unphysical low-frequency divergences are observed for few-band models in the velocity gauge. Using hexagonal boron nitride (hBN) monolayer as a case study, we analyze the problems associated with all methods and identify the optimal one. Our results show that the length gauge calculations provide the fastest convergence rates as well as the most accurate spectra for any basis size and, moreover, that low-frequency divergences are eliminated.Comment: 11 pages, 7 figure

A library of ab initio Raman spectra for automated identification of 2D materials

Raman spectroscopy is frequently used to identify composition, structure and layer thickness of 2D materials. Here, we describe an efficient first-principles workflow for calculating resonant first-order Raman spectra of solids within third-order perturbation theory employing a localized atomic orbital basis set. The method is used to obtain the Raman spectra of 733 different monolayers selected from the computational 2D materials database (C2DB). We benchmark the computational scheme against available experimental data for 15 known monolayers. Furthermore, we propose an automatic procedure for identifying a material based on an input experimental Raman spectrum and illustrate it for the cases of MoS$_2$ (H-phase) and WTe$_2$ (T$^\prime$-phase). The Raman spectra of all materials at different excitation frequencies and polarization configurations are freely available from the C2DB. Our comprehensive and easily accessible library of \textit{ab initio} Raman spectra should be valuable for both theoreticians and experimentalists in the field of 2D materialsComment: 17 pages, 7 figure

Gauge invariance of excitonic linear and nonlinear optical response

We study the equivalence of four different approaches to calculate the excitonic linear and nonlinear optical response of multiband semiconductors. These four methods derive from two choices of gauge, i.e. length and velocity gauges, and two ways of computing the current density, i.e. direct evaluation and evaluation via the time-derivative of the polarization density. The linear and quadratic response functions are obtained for all methods by employing a perturbative density matrix approach within the mean-field approximation. The equivalence of all four methods is shown rigorously, when a correct interaction Hamiltonian is employed for the velocity gauge approaches. The correct interaction is written as a series of commutators containing the unperturbed Hamiltonian and position operators, which becomes equivalent to the conventional velocity gauge interaction in the limit of infinite Coulomb screening and infinitely many bands. As a case study, the theory is applied to hexagonal boron nitride monolayers, and the linear and nonlinear optical response found in different approaches are compared.Comment: 11 pages, 3 figure

Nonlinear optical response of doped mono- and bilayer graphene: length gauge tight-binding model

We compute the nonlinear optical response of doped mono- and bilayer graphene using the full dispersion based on tight-binding models. The response is derived with the density matrix formalism using the length gauge and is valid for any periodic system, with arbitrary doping. By collecting terms that define effective nonlinear response tensors, we identify all nonlinear Drude-like terms (up to third-order) and show that all additional spurious divergences present in the induced current vanish. The nonlinear response of graphene comprises a large Drude-like divergence and three resonances that are tightly connected with transitions occurring in the vicinity of the Fermi level. The analytic solution derived using the Dirac approximation captures accurately the first- and third-order responses in graphene, even at very high doping levels. The quadratic response of gapped graphene is also strongly enhanced by doping, even for systems with small gaps such as commensurate structures of graphene on SiC. The nonlinear response of bilayer graphene is significantly richer, combining the resonances that stem from doping with its intrinsic strong low-energy resonances.Comment: Revised manuscript 18 pages, 6 figure

Effects of different heat processing methods of rapeseed on ruminal and post-ruminal nutrient disappearance

Due to improving the nutritive value of oilseeds and changing their digestion site in ruminants, processing, including heat treatment, seems necessary. The present research was conducted to evaluate the effect of applying heat processing such as roasting, microwaving and autoclaving on nutritive values; the amount and rate of degradability in rumen, and the disappearance of rapeseed nutrients in rumen, post-rumen and total tract. This evaluation was performed using mobile nylon bags techniques; three-step method of digestion and protein CNCPS fraction. A completely randomized design was used to investigate the effect of applying heat processing, and SAS software was used to analyze the data. The field Emission Scanning Electron Microscope was used to monitored the effect of heat treatment on surface of rapeseed. The application of heat processing in this research (roasting, microwaving and autoclaving) had no significant effect on the chemical composition of rapeseed. The results obtained from mobile nylon bags method and three-step digestion method showed that raw rapeseed has the highest disappearance of DM and CP in rumen and therefore has a significant difference with processed seeds (P<0.05). Also, the disappearance of DM and CP of processed rapeseed in intestines was significantly higher than raw seed (P<0.05), and this was higher than other processing for autoclaved rapeseed. According to the results obtained from CNCPS protein fractionation, applying heat processing altering protein fractionation (P<0.05). Applying microwave processing has created cracks in the surface of the rapeseed wall, and this condition was not observed in the wall surface of other heated seeds. In general, it can be said that in addition to increasing the digestibility of rapeseed in the entire gastrointestinal tract, applying heat processing reduces its degradability in the rumen and has increased the disappearance of nutrients in the intestine, that it can be stated the digestion site is altered from rumen to intestine, which can prevent the loss of protein sources in ruminant feed

Test-retest reliability of the Purdue Pegboard test in drug on-phase for patients with Parkinson's disease

Introduction: Parkinson's disease (PD) is a common progressive neurodegenerative disorder, which its onset is later in adults and it's incidence increases in people aged between 55 and 60 years. Dexterity, because of it's role at upper extremity function and person independency in function, is assessed in rehabilitation. Purdue- Pegboard test often is used in rehabilitation clinical settings. The purpose of this study was to determine the test-retest reliability and practice effects of the Purdue Pegboard test in persons with PD. Materials and Methods: 24 PD patients aging 50-89 of both sexes were non-randomly selected from rehabilitation clinics and hospitals of Tehran. A demographic questionnaire was used for documenting patient's general data and the Purdue pegboard test and a timer were used for determining of reliability. Each subtest was administered three times in a row. The time distance between test and retest was one week. Results: The relative reliability in repeated measures was above 90 in different items of Purdue pegboard test in single and mean of testes in off phase, thus indicates an excellent reliability. The absolute reliability (SEM) was lower than 10 of maximum obtained scores at each subtest in one and three trial of test that indicates acceptable errors of measurement. Conclusion: The results of the present study indicate that Purdue Pegboard test is enough reliable in evaluating a manual dexterity of Parkinson patients in drug on- phase

Predictive value of somatosensation for manual dexterity and upper Limb Motor function in stroke survivors

Objectives: One of the most disabling impairments following stroke is upper limb impairment. Despite the important role of somatosensory function in motor control and high prevalence of somatosensory deficits in stroke survivors, little attention has been paid to its effect on UE motor function in chronic stroke survivors. Thus, the aim of this study is to explore the correlation between different somatosensations and manual dexterity as well as UE motor function in these patients. Methods: In this correlational study, 225 chronic stroke survivors (112 female and 113 male) participated, selected by simple non-probability method among the stroke survivors admitted to the rehabilitation centers in Tehran. The lower-order somatosensations, including light touch and wrist Proprioception, were evaluated by Weinstein Enhanced Sensory Test (WEST) and Wrist Position Sense Test (WPST), respectively. The higher-order somatosensations were measured by static and moving 2-Point Discrimination (s2PD and m2PD, respectively), Hand Active Sensation Test (HAST) and Haptic Object Recognition Test (HORT). Gross and fine manual dexterity and UE motor function were assessed using Box-Block Test (BBT), Purdue Pegboard Test (PPT) and Wolf Motor Function Test (WMFT), respectively. Results: The multiple regression models showed that WPST alone accounted for the 38.8- 56.6 of the variance in manual dexterity and UE motor function. The WEST was the second most significant predictor in all regression models. The HORT, m2PD and HAST explained a small percent of the variance in regression models. Discussion: These results suggest that treatments that target somatosensory impairments, especially wrist Proprioception and light touch, may be particularly important for improving manual dexterity and UE motor function in chronic stroke survivors. © 2018, University of Social Welfare and Rehabilitation Sciences