Lattice Fluctuations Research Articles

Neutron noise observations in German KWU built PWRs and analyses with the reactor dynamics code DYN3D

Low-frequency neutronic noise with magnitudes of up to ten percent of the reactor power have been observed in Konvoi-type PWR in Germany in the last years. Several attempts were made to identify the reasons for increased magnitudes of the neutronic fluctuations in comparison with pre-Konvoi reactors, and various hypotheses have been created to explain this effect.In this paper, results of noise simulations performed with use of the reactor dynamics code DYN3D are presented. Fluctuations of the coolant inlet temperature and mass flow rate were considered. Besides of un-correlated fluctuations, correlated temperature fluctuations were simulated. In a preliminary analysis with harmonic temperature oscillations, the importance of existing correlations between temperature fluctuations in the individual fuel assemblies was pointed out. In our analyses, the correlations between fluctuations in the individual fuel assemblies were obtained based on an experimentally validated coolant mixing model. However, the features of the neutronic noise found in the simulations do not correspond to the measurements.Obviously, more complex mechanisms than only temperature and/or mass flow fluctuations have to be considered. Simulated generic fluctuations of the local moderator density, that were introduced independently from thermal hydraulics, indicate that potentially deformations or vibrations of the fuel rod lattice or turbulent fluctuations of the coolant temperature and density inside the fuel rod bundles leading to variations of the local moderator content might be responsible for the observed neutronic noise. Therefore, advanced models coupling neutronics, thermal hydraulics, turbulence and mechanical modelling have to be developed.

Charge and Lattice Fluctuations in Molecule-Based Spin Liquids

Spin liquid (SL) systems have been the subject of much attention recently, as they have been theoretically predicted to not freeze, even at 0 K. Despite extensive searches being made for such a system, only a few candidates have been found. All of these candidates share geometrical frustrations that are based on triangular lattices. We applied vibrational spectroscopy to one of the candidates of a molecule-based SL system, and we compared its results against three antiferromagnetic compounds and four charge-ordered compounds. All of their structural motifs belong to triangular lattices. The C=C stretching modes in the SL state indicated that there were charge and lattice fluctuations. These fluctuations were suppressed but non-negligible in the AF compounds. This finding is potentially significant, as it indicates that a hidden lattice and charge fluctuation are the driving force of a geometrical frustration, which eventually leads to a SL state.

The Deformations of Carbon Nanotubes under Cutting.

The determination of structural evolution at the atomic level is essential to understanding the intrinsic physics and chemistries of nanomaterials. Mechanochemistry represents a promising method to trace structural evolution, but conventional mechanical tension generates random breaking points, which makes it unavailable for effective analysis. It remains difficult to find an appropriate model to study shear deformations. Here, we synthesize high-modulus carbon nanotubes that can be cut precisely, and the structural evolution is efficiently investigated through a combination of geometry phase analysis and first-principles calculations. The lattice fluctuation depends on the anisotropy, chirality, curvature, and slicing rate. The strain distribution further reveals a plastic breaking mechanism for the conjugated carbon atoms under cutting. The resulting sliced carbon nanotubes with controllable sizes and open ends are promising for various applications, for example, as an anode material for lithium-ion batteries.

Effect of atomic flux reversal in a fluctuating moving optical lattice

The dynamics of an ensemble of noninteracting cold atoms is considered in the field of a moving optical lattice subjected to random amplitude and phase fluctuations. The effect of the reversal of an atomic ensemble under the action of lattice fluctuations is demonstrated, in which the atoms begin to move in the direction opposite to that of the lattice motion. A kinetic model is constructed that reproduces this effect and makes it possible to relate its emergence to the asymmetry of the amplitudes of the interlevel transitions in a momentum space.

The order-disorder evolution in quasicrystals through phason flips

Abstract The problem of incorporating phason flips in the structural investigation of aperiodic systems is still an open question and a challenge in crystallography. Phasons are understood both as atomic fluctuation and positional disorder of the quasicrystalline lattice. Popular correction to diffraction peaks' intensities takes the form of generalized Debye-Waller factor, assuming Gaussian distribution of fluctuations in the perpendicular space of higher-dimensional periodic lattice. Although proven to work in case of random tiling types of structures recent evidence indicates improper handling of peaks with high perpendicular space scattering vector whenever structure is far from random tiling regime. We introduce the concept of a series expansion of the characteristic function of the statistical distribution to properly correct the peaks' intensities with respect to phasonic fluctuations. Calculations are performed upon Penrose tiling. Such approximation of the structure factor works correctly even in cases for which the Debye-Waller correction fails. Even more we investigate transition to random tiling through phason flips by means of the statistical approach which results in interesting scaling properties (ordered → disordered → random → amorphous structure).

Effect of Thermal Fluctuations on the Radiative Rate in Core/Shell Quantum Dots.

The effect of lattice fluctuations and electronic excitations on the radiative rate is demonstrated in CdSe/CdS core/shell spherical quantum dots (QDs). Using a combination of time-resolved photoluminescence spectroscopy and atomistic simulations, we show that lattice fluctuations can change the radiative rate over the temperature range from 78 to 300 K. We posit that the presence of the core/shell interface plays a significant role in dictating this behavior. We show that the other major factor that underpins the change in radiative rate with temperature is the presence of higher energy states corresponding to electron excitation into the shell. These effects should be present in other core/shell samples and should also affect other excited state rates, such as the rate of Auger recombination or the rate of charge transfer.

Quantum-classical model of retinal photoisomerization reaction in visual pigment rhodopsin.

A quantum-classical model of photoisomerization of the visual pigment rhodopsin chromophore is proposed. At certain (and more realistic) parameter value combinations, the model is shown to accurately reproduce a number of independent experimental data on the photoreaction dynamics: the quantum yield, the time to reach the point of conical intersection of potential energy surfaces, the termination time of the evolution of quantum subsystem, as well as the characteristic low frequencies of retinal molecular lattice fluctuations during photoisomerization. In addition, the model behavior is in good accordance with experimental data about coherence and local character of quantum transition.

Comment on "Modeling the electrical conduction in DNA nanowires: Charge transfer and lattice fluctuation theories".

In a recent paper [S. Behnia and S. Fathizadeh, Phys. Rev. E 91, 022719 (2015)10.1103/PhysRevE.91.022719] an analytical approach is proposed for the investigation of the conductivity properties of DNA. The authors use mean Lyapunov exponent methods as the backbone of their approach and try to interpret properties of the system based on its behavior. Their interpretation regarding the change in nature of the mean Lyapunov exponent at the denaturation temperatures and discussions of stability and instability based on the mean Lyapunov exponent method are questioned. Moreover there is misunderstanding between mean Lyapunov exponent and Lyapunov exponent.

Enhancement of Curie Temperature (T c) and Magnetization of Fe-Ni Invar alloy Through Cu Substitution and with He+2 Ion Irradiation

The magnetic properties of ternary Fe-Ni-Cu invar alloys are affected by ion irradiation, which goes on increasing with increasing ion fluence (Φ), and by increasing Cu content. In the present study, the ions used are He+2 with 2 MeV energy and with 1 × 1013 cm−2, 1 × 1014 cm−2, 5 × 1014 cm−2, 1 × 1015 cm−2 and 5 × 1015 cm−2 fluence (dose) for irradiation purpose. The face centered cubic structure of the alloy was investigated after ion irradiation using x-ray diffraction (XRD) and found unchanged. However, the peaks become broader with increasing ion dose. Additionally, the lattice fluctuations were observed in XRD study. Curie temperature (Tc) is also increased after irradiation. Many factors are considered here for the reason for increasing Tc, such as the stopping of incident ions, atomic mixing effect at micro scale level owing to ion irradiation, which might change local concentration and ordering already reported in diffuse scattering, and as a result the Fe-Fe interatomic distance and the Fe-Fe coupling are changed. A comparative study shows that the effect of irradiation on Tc and magnetization with increasing ion fluence is more distinctive than the addition of Cu.

Quantum Lattice Fluctuations in the Charge Density Wave State beyond the Adiabatic Approximation

We have developed a tractable numerical method in which large-amplitude quantum lattice fluctuations can be described beyond the adiabatic approximation using the coherent state representation of phonons. A many-body wave function is constructed by the superposition of direct products of non-orthogonal Slater determinants for electrons and coherent states of phonons. Both orbitals in all the Slater determinants and the amplitudes of all the coherent states are simultaneously optimized. We apply the method to the one-dimensional Su–Schrieffer–Heeger model with the on-site and nearest-neighbor-site Coulomb interactions. It is shown the lattice fluctuations in doped charge density wave (CDW) systems are described by the translational and vibrational motion of lattice solitons. Such lattice solitons induce bond alternation in the doped CDW system while the lattice becomes equidistant in the half-filled CDW system.

Suppression of structural phase transition by Sr substitution in the improper ferroelectric BaAl2O4

To clarify lattice fluctuations and precursor phenomena accompanied by structural phase transition in stuffed tridymite compounds, changes in diffuse scattering as a function of temperature in Ba0.6Sr0.4Al2O4 have been carefully investigated by powder X-ray diffraction using synchrotron radiation, electron diffraction and transmission electron microscopy (TEM) experiments. In situ electron diffraction experiments revealed that Ba0.6Sr0.4Al2O4 exhibits lattice fluctuation manifested as a unique honeycomb-shaped diffuse scattering in the wide temperature range between 298 and 100 K. Unlike in the case of BaAl2O4, Ba0.6Sr0.4Al2O4 shows no structural phase transition to the ferroelectric structure with the hexagonal P63 space group in the temperature range. In contrast, it is revealed that the electron beam irradiation to the Ba0.6Sr0.4Al2O4 sample inside the transmission electron microscope induced structural change from the hexagonal P6322 structure to the modulated structure with double periodicity in the three equivalent 〈110〉 directions in the low-temperature region. This implies that the total energy difference between these two structures is small. The hexagonal P6322 structure transforms into the modulated one with short correlation length owing to some small external perturbations.

Probing the electron-phonon interaction in correlated systems with coherent lattice fluctuation spectroscopy

Tailoring the properties of correlated oxides is accomplished by chemical doping, pressure, temperature or magnetic field. Photoexcitation is a valid alternative to reach out-of-equilibrium states otherwise inaccessible. Here, we quantitatively estimate the coupling between a lattice distortion and the charge-transfer excitation in (La$_2$CuO$_{4+\delta}$). We photoinduce a coherent La ion vibration and monitor the response of the optical constants in a broad energy range, providing quantitative information on the electron-phonon matrix element that can be compared to theoretical models. We propose the same methodology to probe electron-electron interactions in other materials.

Calculating linear-response functions for finite temperatures on the basis of the alloy analogy model

A scheme is presented that is based on the alloy analogy model and allows one to account for thermal lattice vibrations as well as spin fluctuations when calculating response quantities in solids. Various models to deal with spin fluctuations are discussed concerning their impact on the resulting temperature-dependent magnetic moment, longitudinal conductivity, and Gilbert damping parameter. It is demonstrated that, by using the Monte Carlo (MC) spin configuration as input, the alloy analogy model is capable of reproducing the results of MC simulations on the average magnetic moment within all spin fluctuation models under discussion. On the other hand, the response quantities are much more sensitive to the spin fluctuation model. Separate calculations accounting for the thermal effect due to either lattice vibrations or spin fluctuations show that they give comparable contributions to the electrical conductivity and Gilbert damping. However, comparison to results accounting for both thermal effects demonstrates violation of Matthiessen's rule, showing the nonadditive effect of lattice vibrations and spin fluctuations. The results obtained for bcc Fe and fcc Ni are compared with the experimental data, showing rather good agreement for the temperature-dependent electrical conductivity and the Gilbert damping parameter.

Modeling the electrical conduction in DNA nanowires: charge transfer and lattice fluctuation theories.

An analytical approach is proposed for the investigation of the conductivity properties of DNA. The charge mobility of DNA is studied based on an extended Peyrard-Bishop-Holstein model when the charge carrier is also subjected to an external electrical field. We have obtained the values of some of the system parameters, such as the electron-lattice coupling constant, by using the mean Lyapunov exponent method. On the other hand, the electrical current operator is calculated directly from the lattice operators. Also, we have studied Landauer resistance behavior with respect to the external field, which could serve as the interface between chaos theory tools and electronic concepts. We have examined the effect of two types of electrical fields (dc and ac) and variation of the field frequency on the current flowing through DNA. A study of the current-voltage (I-V) characteristic diagram reveals regions with a (quasi-)Ohmic property and other regions with negative differential resistance (NDR). NDR is a phenomenon that has been observed experimentally in DNA at room temperature. We have tried to study the affected agents in charge transfer phenomena in DNA to better design nanostructures.

Lifshitz transitions and zero point lattice fluctuations in sulfur hydride showing near room temperature superconductivity

AbstractEmerets’s experiments on pressurized sulfur hydride have shown that H3S metal has the highest known superconducting critical temperature Tc = 203 K. The Emerets data show pressure induced changes of the isotope coefficient between 0.25 and 0.5, in disagreement with Eliashberg theory which predicts a nearly constant isotope coefficient.We assign the pressure dependent isotope coefficient to Lifshitz transitions induced by pressure and zero point lattice fluctuations. It is known that pressure could induce changes of the topology of the Fermi surface, called Lifshitz transitions, but were neglected in previous papers on the H3S superconductivity issue. Here we propose thatH3S is a multi-gap superconductor with a first condensate in the BCS regime (located in the large Fermi surface with high Fermi energy) which coexists with second condensates in the BCS-BEC crossover regime (located on the Fermi surface spots with small Fermi energy) near the and Mpoints.We discuss the Bianconi-Perali-Valletta (BPV) superconductivity theory to understand superconductivity in H3S since the BPV theory includes the corrections of the chemical potential due to pairing and the configuration interaction between different condensates, neglected by the Eliashberg theory. These two terms in the BPV theory give the shape resonance in superconducting gaps, similar to Feshbach resonance in ultracold fermionic gases, which is known to amplify the critical temperature. Therefore this work provides some key tools useful in the search for new room temperature superconductors.

Presence of Pseudo-Path in the Interplane Penetration Depth of Layered YBa2Cu3Oy

The concept of magnetic field effect (MFE) on cuprates is not new. However, little has been discussed on the magnetic field effects on the interplane penetration depth of yttrium barium copper oxide (YBCO). The orbital magnetoresistance of the YBCO were derived and analyzed to reflect its effect on the interplane penetration of layered YBCO. Though most of the anomalies in the YBCO can be associated with the lattice fluctuations, the interplane penetration depth is dependent on the orbital anomaly which dictates the interactions of particulates. The presence of pseudo-path was discovered to exist in the interplane penetration depth of the YBCO. The pseudo-path is observed to control other vital parameters of superconductivity, e.g., vortex dynamics, particulate interactions, etc.

Thermal phase transitions of strongly correlated bosons with spin-orbit coupling.

Experiments on ultracold atoms have started to explore lattice effects and thermal fluctuations for two-component bosons with spin-orbit coupling (SOC). Motivated by this, we derive and study a tJ model for lattice bosons with equal Rashba-Dresselhaus SOC and strong Hubbard repulsion in a uniform Zeeman magnetic field. Using the Gutzwiller ansatz, we find strongly correlated ground states with stripe superfluid (SF) order. We formulate a finite temperature generalization of the Gutzwiller method, and show that thermal fluctuations in the doped Mott insulator drive a two-step melting of the stripe SF, revealing a wide regime of a stripe normal fluid.

Lattice macro and microstrain fluctuations in the calcified byssus of Anomia simplex

In biomineralization the crystallization of minerals takes place in intimate contact with bio-organic molecules. These influence the lattice constants of the forming mineral. Herein we study this effect in the mineralized attachment organ (byssus) of the marine bivalve Anomia simplex . We compare two approaches to studying the biomolecule-induced lattice deformation: X-ray diffraction of annealed samples and in situ heating X-ray diffraction. In the latter case, the lattice deformation is obtained by extrapolation of the thermal expansion from high to low temperature. A positive lattice deformation was found along all three axes of the orthorhombic aragonite lattice. The microstrain fluctuations were found to be anisotropic and largest in the direction of smallest macrostrain. The all-positive macrostrain is in contrast to previous data from shells, showing that much remains to be learnt about the impact of biomolecules on minerals.

Atomistic modeling of peptide adsorption on rutile (100) in the presence of water and of contamination by low molecular weight alcohols.

Previous models for the interface between titanium implants and biosystems take into account the oxide passivation layer and the hydroxylation, but omit the hydrocarbon contamination on air-exposed samples. The authors develop a consistent model for the contamination of the rutile (100) surface by small alcohols, which are known to be present in ambient atmosphere, and use this approach in molecular dynamics calculations. Contact angle evaluation reveals that hydrophobic surfaces can be generated. During molecular dynamics simulations with three peptides (RPRGFGMSRERQ, WFCLLGCDAGCW, and RKLPDA), polar side chains penetrate the hydrocarbons and become immobilized on the titanium dioxide. In the carbon layer, the peptide recognizes a hydrophobic environment, which was not present on the clean surface, and the authors attribute changes in the secondary structure in one case to this interaction. The authors further include the popular Matsui-Akaogi approach [M. Matsui and M. Akaogi, Mol. Simul. 6, 239 (1991)] into the frame of the AMBER force field and quote van der Waals parameters for fitting the original Buckingham part. With the new potential, the authors evaluated lattice parameters, thermal fluctuation, and bulk modulus. Translational diffusion coefficients and dipole autocorrelation functions of water on the surface are discussed in relation to surface properties, and it is shown that the water layers are more rigid than on earlier titanium dioxide models, and that contacts between peptide and surface are less direct.

Quantum fluctuations in anisotropic triangular lattices with ferromagnetic and antiferromagnetic exchange

The Heisenberg model on a triangular lattice is a prime example for a geometrically frustrated spin system. However most experimentally accessible compounds have spatially anisotropic exchange interactions. As a function of this anisotropy, ground states with different magnetic properties can be realized. Motivated by recent experimental findings on Cs$_{2}$CuCl$_{4-x}$Br$_{x}$, we discuss the full phase diagram of the anisotropic model with two exchange constants $J_{1}$ and $J_{2}$, including possible ferromagnetic exchange. Furthermore a comparison with the related square lattice model is carried out. We discuss the zero-temperature phase diagram, ordering vector, ground-state energy, and ordered moment on a classical level and investigate the effect of quantum fluctuations within the framework of spin-wave theory. The field dependence of the ordered moment is shown to be nonmonotonic with field and control parameter.