Electronic Fluctuations Research Articles

First-principles modeling of solute effects on thermal properties of nickel alloys

The effects of Cr and W on high-temperature properties of Ni-based alloys are derived from first-principles calculations combined with a quasi-harmonic Debye model. The modeling procedure integrates all contributions from electronic excitations, magnetic fluctuations and lattice vibrations. The predicted lattice parameter and thermal expansion coefficient agree well with available reference values. The temperature dependence of elastic moduli for Ni-Cr alloys is described satisfactorily. Debye temperature has been calculated to show a strong volume dependence but a weak temperature dependence. A close relationship between the concentration dependence of elastic moduli and the volume dependence of Debye temperature, and thus the vibrational free energy, has been established. Besides, the softening of elastic moduli with increasing temperature is shown to be a result of thermal expansion which mainly comes from lattice vibrations. Therefore, lattice vibrations affect both temperature and concentration dependencies of elastic properties at elevated temperatures. With the help of Labusch–Nabarro model, solid solution hardening caused by Cr and W solutes in Ni has been analyzed. Moreover, the presence of solute atoms increases elastic anisotropy. The effects of Cr or W additions on the thermodynamic properties are found to be small compared to the effect of temperature.

Impact of partially bosonized collective fluctuations on electronic degrees of freedom

In this work we present a comprehensive analysis of collective electronic fluctuations and their effect on single-particle properties of the Hubbard model. Our approach is based on a standard dual fermion/boson scheme with the interaction truncated at the two-particle level. Within this framework we compare various approximations that differ in the set of diagrams (ladder vs exact diagrammatic Monte Carlo), and/or in the form of the four-point interaction vertex (exact vs partially bosonized). This allows to evaluate the effect of all components of the four-point vertex function on the electronic self-energy. In particular, we observe that contributions that are not accounted for by the partially bosonized approximation for the vertex have only a minor effect on electronic degrees of freedom in a broad range of model parameters. In addition, we find that in the regime, where the ladder dual fermion approximation provides an accurate solution of the problem, the leading contribution to the self-energy is given by the longitudional bosonic modes. This can be explained by the fact that contributions of transverse particle-hole and particle-particle modes partially cancel each other. Our results justify the applicability of the recently introduced dual triply irreducible local expansion (D-TRILEX) method that represents one of the simplest consistent diagrammatic extensions of the dynamical mean-field theory. We find that the self-consistent D-TRILEX approach is reasonably accurate also in challenging regimes of the Hubbard model, even where the dynamical mean-field theory does not provide the optimal local reference point (impurity problem) for the diagrammatic expansion.

Distributed charge models of liquid methane and ethane for dielectric effects and solvation

Liquid hydrocarbons are often modelled with fixed, symmetric, atom-centred charge distributions and Lennard-Jones interaction potentials that reproduce many properties of the bulk liquid. While useful for a wide variety of applications, such models cannot capture dielectric effects important in solvation, self-assembly, and reactivity. The dielectric constants of hydrocarbons, such as methane and ethane, physically arise from electronic polarisation fluctuations induced by the fluctuating liquid environment. In this work, we present non-polarisable, fixed-charge models of methane and ethane that break the charge symmetry of the molecule to create fixed molecular dipoles, the fluctuations of which reproduce the experimental dielectric constant. These models can be considered a mean-field-like approximation that can be used to include dielectric effects in large-scale molecular simulations of polar and charged molecules in liquid methane and ethane. We further demonstrate that solvation of model ionic solutes and a water molecule in these fixed-dipole models improve upon dipole-free models.

Dynamic Core Polarization in High Harmonic Generation from Solids: The Example of MgO Crystals.

Previously, the strong field processes in solids have always been explained by the single-active-electron (SAE) model with a frozen core excluding the fluctuation of background electrons. In this work, we demonstrate the strong field induced dynamic core polarization effect and propose a model for revealing its role in high harmonic generation (HHG) from solids. We show that the polarized core induces an additional polarization current beyond the SAE model based on the frozen cores. It gives a new mechanism for HHG and leads to new anisotropic structures, which are experimentally observed with MgO. Our experiments indicate that the influences of dynamic core polarization on HHG are obvious for both linearly and elliptically polarized laser fields. Our work establishes the bridge between the HHG and the dynamic changes of the effective many-electron interaction in solids, which paves the way to probe the ultrafast electron dynamics.

System-on-chip upgrade of millimeter-wave imaging diagnostics for fusion plasma.

Monolithic, millimeter wave "system-on-chip" technology has been employed in chip heterodyne radiometers in a newly developed Electron Cyclotron Emission Imaging (ECEI) system on the DIII-D tokamak for 2D electron temperature and fluctuation diagnostics. The system employs 20 horn-waveguide receiver modules each with customized W-band (75-110GHz) monolithic microwave integrated circuit chips comprising a W-band low noise amplifier, a balanced mixer, a ×2 local oscillator (LO) frequency doubler, and two intermediate frequency amplifier stages in each module. Compared to previous quasi-optical ECEI arrays with Schottky mixer diodes mounted on planar antennas, the upgraded W-band array exhibits >30 dB additional gain and 20× improvement in noise temperature; an internal eight times multiplier chain is used to provide LO coupling, thereby eliminating the need for quasi-optical coupling. The horn-waveguide shielding housing avoids out-of-band noise interference on each module. The upgraded ECEI system plays an important role for absolute electron temperature and fluctuation measurements for edge and core region transport physics studies. An F-band receiver chip (up to 140GHz) is under development for additional fusion facilities with a higher toroidal magnetic field. Visualization diagnostics provide multi-scale and multi-dimensional data in plasma profile evolution. A significant aspect of imaging measurement is focusing on artificial intelligence for science applications.

Electronic noise of warm electrons in semiconductors from first principles

The ab-initio theory of low-field electronic transport properties such as carrier mobility in semiconductors is well-established. However, an equivalent treatment of electronic fluctuations about a non-equilibrium steady state, which are readily probed experimentally, remains less explored. Here, we report a first-principles theory of electronic noise for warm electrons in semiconductors. In contrast with typical numerical methods used for electronic noise, no adjustable parameters are required in the present formalism, with the electronic band structure and scattering rates calculated from first-principles. We demonstrate the utility of our approach by applying it to GaAs and show that spectral features in AC transport properties and noise originate from the disparate time scales of momentum and energy relaxation, despite the dominance of optical phonon scattering. Our formalism enables a parameter-free approach to probe the microscopic transport processes that give rise to electronic noise in semiconductors.

Increased day-to-day fluctuations in exhaled breath profiles after a rhinovirus challenge in asthma.

BackgroundEarly detection/prediction of flare‐ups in asthma, commonly triggered by viruses, would enable timely treatment. Previous studies on exhaled breath analysis by electronic nose (eNose) technology could discriminate between stable and unstable episodes of asthma, using single/few time‐points. To investigate its monitoring properties during these episodes, we examined day‐to‐day fluctuations in exhaled breath profiles, before and after a rhinovirus‐16 (RV16) challenge, in healthy and asthmatic adults.MethodsIn this proof‐of‐concept study, 12 atopic asthmatic and 12 non‐atopic healthy adults were prospectively followed thrice weekly, 60 days before, and 30 days after a RV16 challenge. Exhaled breath profiles were detected using an eNose, consisting of 7 different sensors. Per sensor, individual means were calculated using pre‐challenge visits. Absolute deviations (|%|) from this baseline were derived for all visits. Within‐group comparisons were tested with Mann‐Whitney U tests and receiver operating characteristic (ROC) analysis. Finally, Spearman's correlations between the total change in eNose deviations and fractional exhaled nitric oxide (FeNO), cold‐like symptoms, and pro‐inflammatory cytokines were examined.ResultsBoth groups had significantly increased eNose fluctuations post‐challenge, which in asthma started 1 day post‐challenge, before the onset of symptoms. Discrimination between pre‐ and post‐challenge reached an area under the ROC curve of 0.82 (95% CI = 0.65–0.99) in healthy and 0.97 (95% CI = 0.91–1.00) in asthmatic adults. The total change in eNose deviations moderately correlated with IL‐8 and TNFα (ρ ≈ .50–0.60) in asthmatics.ConclusionElectronic nose fluctuations rapidly increase after a RV16 challenge, with distinct differences between healthy and asthmatic adults, suggesting that this technology could be useful in monitoring virus‐driven unstable episodes in asthma.

Non-Resonant n = 1 Helical Core Induced by m/n = 2/1 Neoclassical Tearing Mode in JT-60U

In JT-60U, simultaneous excitation of n = 1 helical cores (HCs) and m/n = 2/1 Tearing Modes (TMs) was observed [T. Bando et al., Plasma Phys. Control. Fusion 61 115014 (2019)]. In this paper, we have investigated the excitation mechanism of n = 1 HCs with m/n = 2/1 TMs based on the experimental observations and a simple quasi-linear MHD model. In the previous study, it was reported that a "coupling" on the phase of the MHD mode is observed between n = 1 HCs and m/n = 2/1 TMs. In this study, it is found that the coupling is observed with the mode frequency from several Hz to 6 kHz. This indicates that the resistive wall and the plasma control system do not induce the coupling because the both time scales are different from the mode frequency. In addition, n = 1 HCs appear to be the non-resonant mode from the two observations: n = 1 HCs do not rotate with the plasma around the q = 1 surface in the core and the coupling is also observed even when qmin > 1. It is also observed that the electron fluctuation due to an n = 1 HC in the core region disappears with the stabilization of an m/n = 2/1 neoclassical tearing mode by electron cyclotron current drive, implying that n = 1 HCs are driven by m/n = 2/1 TMs. This perspective, n = 1 HCs are driven by m/n = 2/1 TMs, is supported by the observation that the saturated amplitude of the m/n = 1/1 component of the radial displacement in the core is smaller than that of the m/n = 2/1 component. Finally, we revisit a quasi-linear MHD model where the m/n = 1/1 HC is induced directly by the sideband of the current for the m/n = 2/1 TM, which allows to excite the non-resonant m/n = 1/1 mode. The model also describes the characteristic of the coupling, fm/n=1/1(HC) = 2fm/n=2/1(TM).

Dipolar dephasing for structure determination in a paramagnetic environment

We demonstrate the efficacy of the REDOR-type sequences in determining dipolar coupling strength in a paramagnetic environment. Utilizing paramagnetic effects of enhanced relaxation rates and rapid electronic fluctuations in Cu(II)-(DL-Ala)2.H2O, the dipolar coupling for the methyl C–H that is 4.20 ​Å (methyl carbon) away from the Cu2+ ion, was estimated to be 8.8 ​± ​0.6 ​kHz. This coupling is scaled by a factor of ~0.3 in comparison to the rigid limit value of ~32 ​kHz, in line with partial averaging of the dipolar interaction by rotational motion of the methyl group. Limited variation in the scaling factor of the dipolar coupling strength at different temperatures is observed. The C–H internuclear distance derived from the size of the dipolar coupling is similar to that observed in the crystal structure. The errors in the dipolar coupling strength observed in the REDOR-type experiments are similar to those reported for diamagnetic systems. Increase in resolution due to the Fermi contact shifts, coupled with MAS frequencies of 30–35 ​kHz allowed to estimate the hyperfine coupling strengths for protons and carbons from the temperature dependence of the chemical shift and obtain a high resolution 1H–1H spin diffusion spectrum. This study shows the utility of REDOR-type sequences in obtaining reliable structural and dynamical information from a paramagnetic complex. We believe that this can help in studying the active site of paramagnetic metalloproteins at high resolution.

Electronic Fluctuation of Graphene Nanoribbon MOSFETs Under a Full Quantum Dynamics Framework

The electronic fluctuation issue of the armchair graphene nanoribbons (AGNRs) is theoretically investigated under a full quantum dynamics framework. Besides the electrons, the behaviors of the C and H nuclei are also quantized, more rigorous than the common classical particle treatment. Simulation results show that the nuclear quantization will boost the atomic vibration and lead to the band structure deformation, resulting in the energy gap ( E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><b>g</b></sub> ) variation range from -49% to 32% at room temperature. The decrease of E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><b>g</b></sub> is more favorable compared with the increase. Therefore, quantum transport simulations show that the E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><b>g</b></sub> fluctuations will increase the leakage current by more than three orders of magnitude in AGNR MOSFETs, degrading the off-state behavior significantly. Moreover, the out-of-order channel structure can also boost the scattering effect, degrading the on-state behavior. The electronic fluctuation caused by atomic vibrations should be highlighted in low-dimensional semiconductor device applications.

Electronic heat current fluctuations in a quantum dot

The fluctuations of the heat current in a quantum dot coupled to electron reservoirs are calculated at finite frequency, voltage, and temperature using the nonequilibrium Green function technique. The nonsymmetrized heat noise is expressed as an integral on energy containing four contributions, each of which includes transmission amplitudes, electron-hole pair distribution functions, and energy difference factors. The effect of the asymmetry of the couplings between the quantum dot and the reservoirs is studied. Features of the heat noise are highlighted and discussed for an equilibrium and an out-of-equilibrium quantum dot. In the latter case and within the high transmission limit, the heat noise is closely related to the radiative power spectrum, leading to an out-of-equilibrium Planck's law. Proposals for the measurement of the heat noise are discussed.

Superconductivity of underdoped PrFeAs(O,F) investigated via point-contact spectroscopy and nuclear magnetic resonance

Underdoped PrFeAs(O,F), one of the less known members of the 1111 family of iron-based superconductors, was investigated in detail by means of transport, SQUID magnetometry, nuclear magnetic resonance (NMR) measurements and point-contact Andreev-reflection spectroscopy (PCARS). PCARS measurements on single crystals evidence the multigap nature of PrFeAs(O,F) superconductivity, shown to host at least two isotropic gaps, clearly discernible in the spectra, irrespective of the direction of current injection (i.e., along the ab planes or along the c axis). Additional features at higher energy can be interpreted as signatures of a strong electron-boson coupling, as demonstrated by a model which combines Andreev reflection with the Eliashberg theory. Magnetic resonance measurements in the normal phase indicate the lack of a magnetic order in underdoped PrFeAs(O,F), while $^{75}$As NMR spin-lattice relaxation results suggest the presence of significant electronic spin fluctuations, peaking above $T_{c}$ and expected to mediate the superconducting pairing.

Probing nodeless superconductivity in LaMSi ( M=Ni , Pt) using muon-spin rotation and relaxation

Systems with strong spin-orbit coupling have been a topic of fundamental interest in condensed matter physics due to the exotic topological phases and the unconventional phenomenon they exhibit. In this particular study, we have investigated the superconductivity in the transition-metal ternary noncentrosymmetric compounds $\mathrm{La}M\mathrm{Si}$ ($M=\mathrm{Ni}$, Pt) with different spin-orbit coupling strength, using muon-spin rotation and relaxation measurements. Transverse-field measurements made in the vortex state indicate that the superconductivity in both materials is fully gapped, with a conventional $s$-wave pairing symmetry and BCS-like magnitudes for the zero-temperature gap energies. Zero-field measurements suggest a time-reversal symmetry preserved superconductivity in both the systems, though a small increase in muon depolarization is observed upon decreasing temperature. However, this has been attributed to quasistatic electronic fluctuations.

Fluctuation electron microscopy study of crystal nucleation in TiO2–SiO2 glass with heat treatment

We have used fluctuation electron microscopy (FEM) to investigate the nucleation stage of TiO2 crystal formation in binary TiO2–SiO2 glasses with heat treatment. It was found that spatial fluctuations of electron scattering in the glass with 13 wt% TiO2 increases with heat treatment above 800 °C but before any crystals precipitate, i.e. before crystals are detectable by electron diffraction. We have attributed this to TiO2 clustering and increasing medium-range order due to gradual ordering of TiO2 phase. Moreover, we have found that FEM is sensitive to structural changes at temperatures as low as 400 °C but the nature of the changes yet to be determined. This demonstrates that FEM can be sensitive to structural changes in oxide glasses occurring during thermal treatment but preceding detection of the first crystals.

Effect of topology on quasiparticle interactions in the Weyl semimetal WP2

We compare two crystallographic phases of the low-dimensional WP$_2$ to better understand features of electron-electron and electron-phonon interactions in topological systems. The topological $\beta$-phase, a Weyl semimetal with a giant magneto-resistance, shows a larger intensity of electronic Raman scattering compared to the topologically trivial $\alpha$-phase. This intensity sharply drops for $T < T^* = 20$ K which evidences a crossover in the topological phase from marginal quasiparticles to a coherent low temperature regime. In contrast, the non-topological $\alpha$-phase shows more pronounced signatures of electron-phonon interaction. Here there exist generally enlarged phonon linewidths and deviations from conventional anharmonicity in an intermediate temperature regime. These effects provide evidence for an interesting interplay of electronic correlations and electron-phonon coupling. Both interband and intraband electronic fluctuations are involved in these effects. Their dependence on symmetry as well as momentum conservation are critical ingredients to understand this interplay.

Investigation on elastic and thermodynamic properties of Fe25Cr20NiMnNb austenitic stainless steel at high temperatures from first principles

Temperature-dependent properties are very useful in modeling the behavior of materials servicing at high temperature such as austenitic stainless steels. Besides, for predicting the mechanical properties of such alloys, experimental data at high temperature are very important, but scarce. In this work, we use first-principles calculations combined with a quasi-harmonic Debye model to evaluate the Helmholtz free energy of paramagnetic Fe25Cr20NiMnNb austenitic stainless steel as a function of temperature and volume. The contributions due to electronic excitations, magnetic fluctuations, and lattice vibrations are taken into account. Elastic and other thermodynamic properties of the material, such as the equilibrium lattice parameter, elastic moduli, entropy, are derived from the free energy in the temperature range from 800 to 1100 K and compared with available experimental data. The influence of Nb concentration, as well as the effects of electronic excitations and lattice vibrations, on the elastic and thermodynamic properties are discussed in detail. The elastic moduli are found to decrease with increasing temperature. This elastic softening phenomenon is mainly due to thermal expansion; the contribution of thermal electronic excitations is found to be relatively small.

Microscopic pairing mechanism, order parameter, and disorder sensitivity in moiré superlattices: Applications to twisted double-bilayer graphene

Starting from a continuum-model description, we develop a microscopic weak-coupling theory for superconductivity in twisted double-bilayer graphene. We study both electron-phonon and entirely electronic pairing mechanisms. In each case, the leading superconducting instability transforms under the trivial representation, $A$, of the point group $C_3$ of the system, while the subleading pairing phases belong to the $E$ channel. We explicitly compute the momentum dependence of the associated order parameters and find that the leading state has no nodal points for electron-phonon pairing but exhibits six sign changes on the Fermi surface if the Coulomb interaction dominates. On top of these system-specific considerations, we also present general results relevant to other correlated graphene-based moir\'e superlattice systems. We show that, irrespective of microscopic details, triplet pairing will be stabilized if the collective electronic fluctuations breaking the enhanced $\text{SU}(2)_+ \times \text{SU}(2)_-$ spin symmetry of these systems are odd under time reversal, even when the main $\text{SU}(2)_+ \times \text{SU}(2)_-$-symmetric part of the pairing glue is provided by phonons. Furthermore, we discuss the disorder sensitivity of the candidate pairing states and demonstrate that the triplet phase is protected against disorder on the moir\'e scale.

Anderson transition in the cation-substituted compounds ReXMn1-XS

The electrical properties of cation-substituted ReXMn1-XS (Re = Gd, Sm, Ho) compounds are investigated in the temperature range of 77-1200 K. A change in the type of conductivity from semiconductor to “metal” in ReXMn1-XS compounds at a critical concentration of Xc with an increase in the degree of cationic substitution is detected. The metal-dielectric concentration transition in the GdXMn1-XS system is accompanied by a decrease in the value of the specific electrical resistance by 12 orders of magnitude. For Sm0.2Mn0.8S, a sharp maximum of resistance is detected at T = 100 K, which can be caused by scattering of conduction electrons on spin fluctuations of localized electrons. The metal type of conductivity was established for Sm0.25Mn0.75S. In the HoXMn1-XS system, the Anderson transition was detected for XC = 0.3 with a decrease in the value of the specific electrical resistance by 10 orders of magnitude.

Facile one-step sonochemical synthesis of AgCl quantum dots with enhanced photocatalytic activity and its size effects

In this work, a facile ultrasonic method for the fabrication of AgCl quantum dots (AgCl QDs) with an average diameter of about 2.5[Formula: see text]nm was reported for the first time. The material was analyzed by various techniques. In addition, effects of material’s size on its photocatalytic activities were studied. Results suggested that the AgCl QDs exhibited excellent photocatalytic activity to degrade Rhodamine B (RhB) and tetracycline (TC) under visible light illumination, and the degradation rate of RhB (TC) had reached up to 96.6% (72.2%) in 20 min, which was higher than that of AgCl nanoparticles (23[Formula: see text]nm) and AgCl nanospheres (114[Formula: see text]nm), respectively. Besides, the band gap of the material was increased when the size of material decreased from 23[Formula: see text]nm to 2.5[Formula: see text]nm. The significantly improved photocatalytic performance and increased band gap of AgCl QDs were mainly related to the quantum size effects of AgCl, which results in the more electron fluctuation in quantized energy levels and the lower recombination of electrons and holes.

Unconventional 17O and 63Cu NMR shift components in cuprate superconductors

Nuclear magnetic resonance (NMR) is a fundamental bulk probe that provides key information about the electronic properties of materials. Very recently, the analysis of all available planar copper shift as well as relaxation data proved that while the shifts cannot be understood in terms of a single temperature-dependent spin component, relaxation can be explained with one dominating Fermi liquid-like component, without enhanced electronic spin fluctuations. For the shifts, a doping-dependent isotropic term as well as doping-independent anisotropic term became obvious. Here, we focus on planar [Formula: see text]O NMR shifts and quadrupole splittings. Surprisingly, we find that they demand, independently, a similar two-component scenario and confirm most of the previous conclusions concerning the properties of the spin components, in particular that a negative spin polarization survives in the superconducting state. This should have consequences for the pairing scenario.