Rm Sc Research Articles

Properties of scalar wave emission in a scalar-tensor theory with kinetic screening

We study numerically the scalar wave emission by a non-spherical oscillation of neutron stars in a scalar-tensor theory of gravity with kinetic screening, considering both the monopole and quadrupole mode emission. In agreement with previous results in the literature, we find that the monopole is always suppressed by the screening effect, regardless of the size of the screening radius, $r_{\rm sc}$. For the quadrupole mode, however, our analysis shows that the suppression only occurs for screening radius larger than the wavelength of scalar waves, $\lambda_{\rm wave}$, but not for $r_{\rm sc} < \lambda_{\rm wave}$. This demonstrates that to fully understand the nature of this theory, it is necessary to study other more complex systems, such as neutron star binaries, considering a wide range of $r_{\rm sc}$ values.

Filamentation of fast radio bursts in magnetar winds

ABSTRACT Magnetars are the most promising progenitors of fast radio bursts (FRBs). Strong radio waves propagating through the magnetar wind are subject to non-linear effects, including modulation/filamentation instabilities. We derive the dispersion relation for modulations of strong waves propagating in magnetically dominated pair plasmas focusing on dimensionless strength parameters a0 ≲ 1, and discuss implications for FRBs. As an effect of the instability, the FRB-radiation intensity develops sheets perpendicular to the direction of the wind magnetic field. When the FRB front expands outside the radius where the instability ends, the radiation sheets are scattered due to diffraction. The FRB-scattering time-scale depends on the properties of the magnetar wind. In a cold wind, the typical scattering time-scale is τsc ∼ $\mu$s–ms at the frequency $\nu \sim 1\, {\rm GHz}$. The scattering time-scale increases at low frequencies, with the scaling τsc ∝ ν−2. The frequency-dependent broadening of the brightest pulse of FRB 181112 is consistent with this scaling. From the scattering time-scale of the pulse, one can estimate that the wind Lorentz factor is larger than a few tens. In a warm wind, the scattering time-scale can approach $\tau _{\rm sc}\sim \, {\rm ns}$. Then scattering produces a frequency modulation of the observed intensity with a large bandwidth, $\Delta \nu \sim 1/\tau _{\rm sc}\gtrsim 100\, {\rm MHz}$. Broad-band frequency modulations observed in FRBs could be due to scattering in a warm magnetar wind.

Pairing interaction in superconducting UCoGe tunable by magnetic field

The mechanism of unconventional superconductivity, such as high-temperature-cuprate, Fe-based, and heavy-fermion superconductors, has been studied as a central issue in condensed-matter physics. Spin fluctuations, instead of phonons, are considered to be responsible for the formation of Cooper pairs, and many efforts have been made to confirm this mechanism experimentally. Although a qualitative consensus seems to have been obtained, experimental confirmation has not yet been achieved. This is owing to a lack of the quantitative comparison between theory and experiments. Here, we show a semi-quantitative comparison between the superconducting-transition temperature ($T_{\rm SC}$) and spin fluctuations derived from the nuclear magnetic resonance (NMR) experiment on the ferromagnetic (FM) superconductor UCoGe, in which the FM fluctuations and superconductivity are tunable by external fields. The enhancement and abrupt suppression of $T_{\rm SC}$ by applied fields, as well as the pressure variation of $T_{\rm SC}$ around the FM criticality are well understood by the change in the FM fluctuations on the basis of the single-band spin-triplet theoretical formalism. The present comparisons strongly support the theoretical formalism of spin-fluctuation-mediated superconductivity, particularly in UCoGe.

Superconductivity and charge density wave order in the two-dimensional Holstein model

The Holstein Hamiltonian describes fermions hopping on a lattice and interacting locally with dispersionless phonon degrees of freedom. In the low density limit, dressed quasiparticles, polarons and bipolarons, propagate with an effective mass. At higher densities, pairs can condense into a low temperature superconducting phase and, at or near commensurate filling on a bipartite lattice, to charge density wave (CDW) order. CDW formation breaks a discrete symmetry and hence occurs via a second order (Ising) transition, and therefore at a finite $T_{\rm cdw}$ in two dimensions. Quantum Monte Carlo calculations have determined $T_{\rm cdw}$ for a variety of geometries, including square, honeycomb, and Lieb lattices. The superconducting transition, on the other hand, in $d=2$ is in the Kosterlitz-Thouless (KT) universality class, and is much less well characterized. In this paper we determine $T_{\rm sc}$ for the square lattice, for several values of the density $\rho$ and phonon frequency $\omega_0$. We find that quasi-long range order sets in at $T_{\rm sc} \lesssim t/20$, where $t$ is the near neighbor hopping amplitude, consistent with previous rough estimates from simulations which only extrapolated to the temperatures we reach from considerably higher $T$. We also show evidence for a discontinuous evolution of the density as the CDW transition is approached at half-filling.

Numerical simulation of the electron traps effect created by neutron irradiation on $$p^+-n-n^+$$ GaAs solar cell performance

Numerical simulation is used in this work to model the effect of 1 MeV neutron irradiation on the performance degradation of a $$p^+-n-n^+$$ GaAs solar cell. The effect is predicted by the calculation of the current–voltage characteristics under AM0 illumination for a constant dose of neutron irradiation. The solar cell output parameters (the short-circuit current density $$J_{{\rm sc}}$$ , the open-circuit voltage $$V_{{\rm oc}}$$ , the fill factor FF and the conversion efficiency $$\eta$$ ) are extracted from these characteristics. The neutron irradiation induced five electron traps En1, En2, En3, En4 and En5 in the energy gap either as recombination centers or traps. The degradation by the induced traps is widely attributed to the first type of defects. Simulating the effect of each trap level separately helps to find out which of them is responsible for the degradation of a particular output parameter. The simulation results have shown that the $$p^+-n-n^+$$ GaAs solar cell degradation is very apparent at $$10^{14}$$ cm $$^{-2}$$ neutron irradiation fluence. The deepest electron trap En5, with largest capture cross section, is responsible for the degradation of $$J_{{\rm sc}}$$ and $$\eta$$ . The other electron traps En1, En2, En3 and En4 have a no significant effect on the solar cell output parameters, particularly on the open-circuit voltage $$V_{{\rm oc}}$$ . Finally, the solar cell resistivity to the neutron irradiation can be improved by decreasing the thickness of $$p^+ {\rm GaAs}$$ emitter layer from 0.44 to 0.1 $$\upmu$$ m with keeping a gradual $${\rm Al}_x$$ $${\rm Ga}_{1-x}{\rm As}$$ window thickness of 0.09 $$\upmu$$ m.

Universal relationship between the energy scales of the pseudogap phase, the superconducting state, and the charge-density-wave order in copper oxide superconductors

We report the hole doping dependencies of the pseudogap phase energy scale, $2\Delta_{\rm PG}$, the anti-nodal (nodal) superconducting energy scales $2\Delta^{AN}_{\rm SC}$ ($2\Delta^{N}_{\rm SC}$) and the charge density wave energy scale, $2\Delta_{\rm CDW}$. They have been extracted from the electronic Raman responses of distinct copper oxide families. For all the cuprates studied, we reveal universal doping dependencies which suggest that $2\Delta_{\rm PG}$, $2\Delta^{AN}_{\rm SC}$ and $2\Delta_{\rm CDW}$ are governed by common microscopic interactions and that these interactions become relevant well above the superconducting transition at $T_c$. In sharp contrast, $2\Delta^N_{\rm SC}$ tracks the doping dependence of $T_c$, appearing to be controlled by a different kind of interactions than the energy scales above.

Anisotropy of the Upper Critical Field in the Heavy-Fermion Superconductor UTe2 under Pressure

We studied the anisotropy of the superconducting upper critical field $H_{\rm c2}$ in the heavy-fermion superconductor UTe$_2$ under hydrostatic pressure by magnetoresistivity measurements. In agreement with previous experiments we confirm that superconductivity disappears near a critical pressure $p_{\rm c} \approx 1.5$~GPa, and a magnetically ordered state appears. The unusual $H_{\rm c2}(T)$ at low temperatures for $H \parallel a$ suggests that the multiple superconducting phases which appear under pressure have quite different $H_{\rm c2}$. For a field applied along the hard magnetization $b$ axis $H_{\rm c2} (0)$ is glued to the metamagnetic transition $H_{\rm m}$ which is suppressed near $p_{\rm c}$. The suppression of $H_{\rm m}$ with pressure follows the decrease of temperature $T_{\chi}^{\rm max}$, at the maximum in the susceptibility along $b$. The strong reinforcement of $H_{\rm c2}$ at ambient pressure for $H \parallel b$ above 16~T is rapidly suppressed under pressure due to the increase of $T_{\rm sc}$ and the decrease of $H_{\rm m}$. The change in the hierarchy of the anisotropy of $H_{\rm c2}(0)$ on approaching $p_{\rm c}$ points out that the $c$ axis becomes the hard magnetization axis.

A randomized trial of nurse-administered behavioral interventions to manage anticipatory nausea and vomiting in chemotherapy.

PurposeChemotherapy side effects diminish quality of life and can lead to treatment delay. Nausea and vomiting can occur prior to chemotherapy because of classical conditioning. We studied the effects of 20‐minute behavioral interventions, administered by oncology nurses, of higher intensity (mindfulness relaxation—MR) or lower intensity (relaxing music—RM), on anticipatory nausea and vomiting (ANV).Patients and methodsPatients undergoing chemotherapy for solid tumors were randomized to MR (N = 160), RM (N = 159), or standard care SC (N = 155). Subjects were mostly female (91.8%) and white (86.1%) with breast cancer (85%). Most patients had early stage disease (Stage I: 26%; II: 52.9%; III: 19%; IV: 0.1%). Anticipatory nausea and vomiting were assessed at the midpoint and end of the chemotherapy course using the Morrow Assessment of Nausea and Emesis (MANE).ResultsCompared to SC, there was reduced anticipatory nausea at the midpoint of chemotherapy in those receiving MR (OR 0.44, 95% CI 0.20‐0.93) and RM (OR 0.40, 95% CI 0.20‐0.93), controlling for age, sex, cancer stage, and emetogenic level of chemotherapy. There was no difference between treatment groups in anticipatory nausea at the end of chemotherapy or in anticipatory vomiting and postchemotherapy nausea and vomiting at either time point.ConclusionA brief nurse‐delivered behavioral intervention can reduce midpoint ANV associated with chemotherapy.

VLA Observations of Single Pulses from the Galactic Center Magnetar

Abstract We present results of a 7–12 GHz phased-array study of the Galactic center magnetar J1745−2900 with the Karl G. Jansky Very Large Array (VLA). Using data from two 6.5 hr observations from 2014 September, we find that the average profile is comprised of several distinct components at these epochs and is stable over ∼day timescales and ∼GHz frequencies. Comparison with additional phased VLA data at 8.7 GHz shows significant profile changes on longer timescales. The average profile at 7–12 GHz is dominated by the jitter of relatively narrow pulses. The pulses in each of the four main profile components seen in 2014 September are uncorrelated in phase and amplitude, though there is a small but significant correlation in the occurrence of pulses in two of the profile components. Using the brightest pulses, we measure the dispersion and scattering parameters of J1745−2900. A joint fit of 38 pulses gives a 10 GHz pulse-broadening time of and a dispersion measure of . Both of these results are consistent with previous measurements, which suggests that the scattering and dispersion measure of J1745−2900 may be stable on timescales of several years.

Anisotropic Grüneisen Parameter and Diverse Order Parameter Fluctuations in Iron-Based Superconductor Ba(Fe1−xCox)2As2

In this study, the temperature dependence of elastic constants $C_{11}$, $C_{33}$, $C_{\rm E} = (C_{11}-C_{12})/2$, $C_{66}$ and $C_{44}$ of the iron-based superconductor Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ (0 $\leqq x \leqq$ 0.245) have been measured. This system shows a large elastic softening in $C_{66}$ towards low temperatures. In addition to $C_{66}$, which originates from orthorhombic structural fluctuation, the samples near the optimal concentration show remarkable structural fluctuation in $C_{11}$ and $C_{33}$ elastic modes, which correspond to $\Gamma_{1}$ (C4) symmetry. It suggests the existence of diverse fluctuations in this system. Gr{\" u}neisen parameters were analyzed under some assumptions for structural and magnetic transition temperatures. Results showed that the Gr{\" u}neisen parameters for the inter-plane strain are remarkably enhanced toward the QCP, while those for the in-plane stress tend to turn down near the QCP. Gr{\" u}neisen parameters for the superconducting transition are anisotropic and shows remarkable Co-concentration dependence, suggesting that the in-plane isotropic compression and inter-layer elongation enhance the superconductivity. The correlation of Gr{\" u}neisen parameters between $T_{\rm S}$, $T_{\rm N}$ and $T_{\rm sc}$ shows $c$-axis elongation and its relevant role in the emergence of superconductivity in this system.

Theoretical investigation of two-particle two-hole effects on spin-isospin excitations through charge-exchange reactions

Coherent one-particle one-hole (1p1h) excitations have given us effective insights into general nuclear excitations. However, the two-particle two-hole (2p2h) excitation beyond 1p1h is now recognized as critical for the proper description of experimental data of various nuclear responses. The spin-flip charge-exchange reactions $^{48}{\rm Ca}(p,n)^{48}{\rm Sc}$ are investigated to clarify the role of the 2p2h effect on their cross sections. The Fermi transition of $^{48}{\rm Ca}$ via the $(p,n)$ reaction is also investigated in order to demonstrate our framework. The transition density is calculated microscopically with the second Tamm-Dancoff approximation, and the distorted-wave Born approximation is employed to describe the reaction process. A phenomenological one-range Gaussian interaction is used to prepare the form factor. For the Fermi transition, our approach describes the experimental behavior of the cross section better than the Lane model, which is the conventional method. For spin-flip excitations including the GT transition, the 2p2h effect decreases the magnitude of the cross section and does not change the shape of the angular distribution. The $\Delta l=2$ transition of the present reaction is found to play a negligible role. The 2p2h effect will not change the angular-distributed cross section of spin-flip responses. This is because the transition density of the Gamow-Teller response, the leading contribution to the cross section, is not significantly varied by the 2p2h effect.

A Single-Stage Stand-Alone Photovoltaic Energy System With High Tracking Efficiency

A single-stage flyback PV inverter with maximum power point tracking (MPPT) high-speed capability is presented in this paper. The proposed stand-alone photovoltaic energy system (SAPES) incorporates the use of hybrid MPPT to ensure peak energy harvesting under all weather conditions. The proposed hybrid MPPT method combines the conventional short current pulse (SCP) MPPT method and the perturb and observe (P&O) methods. Measurement of the offline parameter [short-circuit current of PV module ( ${I}_{\rm sc}$ )] for SCP is made on the basis of the difference between the offline parameter and the instantaneous current of PV module ( ${I}_{\rm pv}$ ). Use of a single power conversion stage is achieved by employing a modified flyback inverter operating in a discontinuous-conduction mode. The control structure of the conventional flyback inverter is also modified to incorporate the hybrid MPPT algorithm. The proposed SAPES method is tested using computer-aided simulations and real-time hardware in the loop experimentation using the dSPACE DS1104 board. The results obtained using the proposed systems are better than those for the conventional algorithms under the environmental conditions tested.

Interplay of Pomeranchuk instability and superconductivity in the two-dimensional repulsive Hubbard model

Interplay of Pomeranchuk instability (spontaneous symmetry breaking of the Fermi surface) and d-wave superconductivity is studied for the repulsive Hubbard model on the square lattice with the dynamical mean field theory combined with the fluctuation exchange approximation (FLEX+DMFT). We show that the four-fold symmetric Fermi surface becomes unstable against a spontaneous distortion into two-fold near the van Hove filling, where the symmetry of superconductivity coexisting with the Pomeranchuk distorted Fermi surface is modified from the d-wave pairing to (d+s)-wave. By systematically shifting the position of van Hove filling with varied second- and third-neighbor hoppings, we find that the transition temperature $T_{\rm c}^{\rm PI}$ of Pomeranchuk instability is more sensitively affected by the position of van Hove filling than the superconducting $T_{\rm c}^{\rm SC}$. This implies that the filling region for strong Pomeranchuk instability and that for strong superconducting fluctuations can be separated, and Pomeranchuk instability can appear even if the peak of $T_c^{\rm PI}$ is lower than the peak of $T_c^{\rm SC}$. An interesting observation is that the Fermi surface distortion can enhance the superconducting $T_{\rm c}^{\rm SC}$ in the overdoped regime, which is explained with a perturbation picture for small distortions.

Stochastic Fermi Energization of Coronal Plasma during Explosive Magnetic Energy Release

Abstract The aim of this study is to analyze the interaction of charged particles (ions and electrons) with randomly formed particle scatterers (e.g., large-scale local “magnetic fluctuations” or “coherent magnetic irregularities”) using the setup proposed initially by Fermi. These scatterers are formed by the explosive magnetic energy release and propagate with the Alfvén speed along the irregular magnetic fields. They are large-scale local fluctuations (δB/B ≈ 1) randomly distributed inside the unstable magnetic topology and will here be called Alfvénic Scatterers (AS). We constructed a 3D grid on which a small fraction of randomly chosen grid points are acting as AS. In particular, we study how a large number of test particles evolves inside a collection of AS, analyzing the evolution of their energy distribution and their escape-time distribution. We use a well-established method to estimate the transport coefficients directly from the trajectories of the particles. Using the estimated transport coefficients and solving the Fokker–Planck equation numerically, we can recover the energy distribution of the particles. We have shown that the stochastic Fermi energization of mildly relativistic and relativistic plasma can heat and accelerate the tail of the ambient particle distribution as predicted by Parker &amp; Tidman and Ramaty. The temperature of the hot plasma and the tail of the energetic particles depend on the mean free path (λ sc) of the particles between the scatterers inside the energization volume.

On the Comprehensive Parametrization of the Photovoltaic (PV) Cells and Modules

In the classical parametrization of the single-diode model of photovoltaic (PV) cells and modules based on the datasheet values, first, the values of the five unknown parameters of the PV model are extracted via the values of the open-circuit voltage $V_{{\rm oc}}$ , the short-circuit current $I_{{\rm sc}}$ , and the voltage and current at the maximum power point $V_{m}$ , $I_m$ at standard test conditions (STC). Next, using some translational formulas, the STC values of the unknown parameters are projected to the new climatic conditions other than STC. A major problem of this approach is to determine the translational formulas of the five unknown parameters of the single-diode model as a function of both temperature and irradiation levels with a high degree of accuracy beforehand. This paper presents a new method to extract the parameters of the PV model that operates with a reverse identification process, as compared with the classical methods, in the sense that it starts with the translational formulas of the key parameters of $V_{{\rm oc}}$ , $I_{\rm{sc}}$ , $V_m$ , and $I_m$ and yields the variation of the all PV model unknown parameters as a function of both temperature and irradiation levels. The satisfactory operation of the proposed parametrization technique is evaluated by simulations, experiments, and comparative studies with the classical methods.

Visualization of the magnetic flux structure in phosphorus-doped EuFe2As2 single crystals

Magnetic flux structure on the surface of EuFe$_2$(As$\rm_{1-x}$P$\rm_x$)$_2$ single crystals with nearly optimal phosphorus doping levels $x=0.20$, and $x=0.21$ is studied by low-temperature magnetic force microscopy and decoration with ferromagnetic nanoparticles. The studies are performed in a broad temperature range. It is shown that the single crystal with $x=0.21$ in the temperature range between the critical temperatures $T_{\rm SC}=22$ K and $T_{\rm C}=17.7$ K of the superconducting and ferromagnetic phase transitions, respectively, has the vortex structure of a frozen magnetic flux, typical for type-II superconductors. The magnetic domain structure is observed in the superconducting state below $T_{\rm C}$. The nature of this structure is discussed.

Modeling the Back Contact of Cu2ZnSnSe4Solar Cells

In this paper, we analyze the theoretical impact of the Molybdenum (Mo) back contact and the MoSe2 interfacial layer on the performances of a Cu2ZnSnSe4 (CZTSe)-based solar cell. MoSe2 layers are formed spontaneously in the Mo/CZTSe interface during the annealing of the absorber, but disagreeing interpretations about their actual role in affecting the device figures of merit ( $V_{{\rm OC}}, J_{{\rm SC}}$ , FF, and η) have been proposed in the literature. In our approach, we have simulated three structures presenting different conditions at the back contact: ideal-contact/CZTSe (flat-band), Mo/CZTSe, and Mo/MoSe2/CZTSe. For these three layers, an accurate explanation of the selection of critical material parameters is given. The numerical simulations, performed with SCAPS 3.2.01, show that the low values of Mo work function ( $\leq 4.95$ eV) would have a strong detrimental effect on the $V_{{\rm OC}}$ and FF of the cell if no interfacial layers were present at the Mo/CZTSe interface. On the other hand, a beneficial effect of the MoSe2 layer on the $V_{{\rm OC}}$ of the device is demonstrated when this layer is included in the structure. This trend is confirmed by experimental measurements. The expected band diagram of the full ZnO/CdS/CZTSe/MoSe2 /Mo structure is provided.

A Simple Cost-Effective Approach to Enhance Performance of Bifacial Dye-Sensitized Solar Cells

A simple cost-effective approach was demonstrated to increase the rear efficiency and ratio of rear-to-front efficiencies for bifacial dye-sensitized solar cells (DSCs). This was achieved by enhancing the transparency of the counter electrode of the DSC using a composite of platinum- and aluminum-doped zinc oxide (Pt/AZO). The thermally deposited composite Pt/AZO counter electrodes exhibited transmittance T550 nm = 0.70–0.74 versus Pt with 0.63. The comparable front power conversion efficiency (PCEfront) of fabricated DSCs using the composite Pt/AZO counter electrodes suggested that the presence of AZO in the composite had minimal effect on the catalytic activity of the platinum. DSCs with composite Pt/AZO counter electrode presented superior rear PCEs compared with platinum counterparts and was attributed to higher short-circuit current density $J_{{\rm sc}}$ resulting from the larger transmittance of the composite counter electrode. The higher PCErear / PCEfront of the composite-based DSCs compared with Pt-based DSC suggested that such transparent composite counter electrodes could be employed in bifacial DSCs.

Pattern Effects of Soil on Photovoltaic Surfaces

The texture or patterning of soil on PV surfaces may influence light capture at various angles of incidence (AOI). Accumulated soil can be considered a microshading element, which changes with respect to AOI. Laboratory deposition of simulated soil was used to prepare test coupons for simultaneous AOI and soiling loss experiments. A mixed solvent deposition technique was used to consistently deposit patterned test soils onto glass slides. Transmission decreased as soil loading and AOI increased. Dense aggregates significantly decreased transmission. However, highly dispersed particles are less prone to secondary scattering, improving overall light collection. In order to test AOI losses on relevant systems, uniform simulated soil coatings were applied to split reference cells to further examine this effect. The measured optical transmission and area coverage correlated closely to the observed $I_{{\rm SC}}$ . Angular losses were significant at angles as low as 25°.

Comparative Study of the Influence of Photoluminescence Quantum Yield and Molecular Orientation of Wide-Energy-Gap Small Molecules on the Performance of Organic Schottky Junctions

In this paper, we comparatively investigate the influence of absolute photoluminescence quantum yield (PLQY) and molecular orientation of a series of wide-energy-gap donors, which are usually used in organic light-emitting diodes, on the performance of organic Schottky junctions (OSJs) with low donor concentration. This work shows that PLQY of donors plays a crucial role on the photocurrent and fill factor of these types of OSJs. The PLQY of donors demonstrates a negative effect on the short-circuit current ( $J_{{\rm SC}}$ ) of OSJs. The molecular orientation of donors also influences the OSJ performance for both linear- or planar-shaped molecules, and horizontal orientation helps to achieve a higher $J_{{\rm SC}}$ . The influencing priority of molecule characteristics of donors is PLQY>orientation.