Dominant Relaxation Mechanism Research Articles

SMM features of a large lanthanide family of butterfly CrIII2LnIII2 pivalate complexes (Ln = Gd, Tb, Dy, Ho, Er, Tm and Yb).

In this work we report the synthesis, structural characterization and magnetic properties of a family of butterfly complexes {Cr2Ln2} with Ln = Tb (4), Ho (5), Er (6), Tm (7) and Yb (8), extending the family of previously reported isostructural compounds with Gd (1), Dy (2) and Y (3). As in 1 and 2, an anti-ferromagnetic Cr(III)-Ln(III) exchange interaction is found. For oblate ions 4 and 5, SMM behavior with a purely Orbach relaxation mechanism is observed with thermal barriers of 38 cm-1 (4) and 32 cm-1 (5). Complex 4 displays hysteresis opening up to 2.4 K with loss of magnetization at zero field. The prolate complex ions 6 and 7 are field-induced SMM's with dominant Orbach, direct and QTM relaxation mechanisms. Compound 8 did not show SMM properties.

Influence of the phonon-bottleneck effect and low-energy vibrational modes on the slow spin-phonon relaxation in Kramers-ions-based Cu(II) and Co(II) complexes with 4-amino-3,5-bis-(pyridin-2-yl)-1,2,4-triazole and dicyanamide.

Two new complexes, bis-[4-amino-3,5-bis-(pyridin-2-yl)-1,2,4-triazole-κ2N2,N6]bis-(dicyanamide-κN8)copper(II), [Cu(abpt)2(dca)2] (1) and bis-[4-amino-3,5-bis-(pyridin-2-yl)-1,2,4-triazole-κ2N2,N6]bis-(dicyanamide-κN8)cobalt(II), [Co(abpt)2(dca)2] (2), have been prepared and magneto-structurally characterised. Single crystal studies of both complexes have shown that their crystal structures are molecular, in which the central atoms are six-coordinated in the form of a distorted octahedron by two bidentate abpt and two monodentate dca ligands. Even if both complexes have the same composition and crystallize in the same P1̄ space group, they are not isostructural. Both structures contain strong intermolecular N-H⋯N hydrogen bonds and π-π stacking interactions. IR spectra are consistent with the solved structures of both complexes and confirmed the terminal character of the dca ligands and the bidentate coordination of the abpt ligands. The analysis of the magnetic properties showed that both complexes exhibit field-induced slow spin-phonon relaxation. In both complexes, the slow spin-phonon relaxation is influenced by a severe phonon-bottleneck effect that affects the direct process, a dominant relaxation mechanism at low temperatures in both complexes. The phonon-bottleneck effect in 1 was suppressed by simply reducing the crystallite size, and further analysis of the field dependence of the relaxation time yielded the characteristic energy of vibrational modes of 11 cm-1 participating in the Raman process at low magnetic fields. The analysis of magnetic properties and ab initio calculations confirmed that 2 represents a system with a moderate uniaxial anisotropy yielding an average energy barrier of 82 cm-1 (from all four nonequivalent Co(II) sites in the structure of 2).

Refined Universality for Critical KCM: Upper Bounds

We study a general class of interacting particle systems called kinetically constrained models (KCM) in two dimensions. They are tightly linked to the monotone cellular automata called bootstrap percolation. Among the three classes of such models (Bollobás et al. in Combin Probab Comput 24(4):687–722, 2015), the critical ones are the most studied. Together with the companion paper by Marêché and the author (Hartarsky and Marêché in Combin Probab Comput 31(5):879–906, 2022), our work determines the logarithm of the infection time up to a constant factor for all critical KCM. This was previously known only up to logarithmic corrections (Hartarsky et al. in Probab Theory Relat Fields 178(1):289–326, 2020, Ann Probab 49(5):2141–2174, 2021, Martinelli et al. in Commun Math Phys 369(2):761–809, 2019). We establish that on this level of precision critical KCM have to be classified into seven categories. This refines the two classes present in bootstrap percolation (Bollobás et al. in Proc Lond Math Soc (3) 126(2):620–703, 2023) and the two in previous rougher results (Hartarsky et al. in Probab Theory Relat Fields 178(1):289–326, 2020, Ann Probab 49(5):2141–2174, 2021, Martinelli et al. in Commun Math Phys 369(2):761–809, 2019). In the present work we establish the upper bounds for the novel five categories and thus complete the universality program for equilibrium critical KCM. Our main innovations are the identification of the dominant relaxation mechanisms and a more sophisticated and robust version of techniques recently developed for the study of the Fredrickson-Andersen 2-spin facilitated model (Hartarsky et al. in Probab Theory Relat Fields 185(3):993–1037, 2023).

Eight-coordinate mono- and dinuclear Dy(III) complexes containing a rigid equatorial plane and an anisobidentate carboxylate ligand in the axial position: synthesis, structure and magnetism.

A rigid pentadentate chelating ligand (H2L) has been utilized to synthesize a series of octacoordinate mononuclear complexes, [Dy(L)(Ph3PO)(OOCR)] (where R = C6H5 (1), C(CH3)3 (2), CF3 (3)) and a dinuclear complex, [Dy2(L)2(Ph3PO)2{(OOC)2C6H4}] (4) based on the highly anisotropic Dy(III) ion. All the complexes were structurally characterized by single-crystal X-ray diffraction studies. The complexes were formed by the coordination action of the dianionic pentadentate ligand [L]2-, one phosphine oxide, and carboxylate ligands. DC and AC magnetic measurements were performed on 1-4. Complexes 1-4 show SMM behaviour, under zero DC field for 1 and 4, and under 500 Oe and 1000 Oe DC fields for 2 and 3 respectively, with thermally activated, Raman, and Raman and quantum tunnelling dominant relaxation mechanisms for 1 and 2, 3 and 4, respectively.

Hilbert Space in Isotopologue Dy(III) SMM Dimers: Dipole Interaction Limit in [163/164Dy2(tmhd)6(tape)]0 Complexes.

Single-molecule magnets are molecular complexes proposed to be useful for information storage and quantum information processing applications. In the quest for multilevel systems that can act as Qudits, two dysprosium-based isotopologues were synthesized and characterized. The isotopologues are [164Dy2(tmhd)6(tape)] (1(I=0)) and [163Dy2(tmhd)6(tape)] (2(I=5/2)), where tmhd = 2,2,6,6-tetramethylheptandionate and tape = 1,6,7,12-tetraazaperylene. Both complexes showed slow relaxation at a zero applied magnetic field with dominant Orbach and Raman relaxation mechanisms. μSQUID studies at milli-Kelvin temperatures reveal quasi-single ion loops, in contrast with the expected S-shape (near zero field) butterfly loops, characteristic of antiferromagnetically coupled dimeric complexes. Through analysis of the low-temperature data, we find that the interaction operating between Dy(III) is small, leading to a small exchange biasing from the zero-field transition. The resulting indirectly coupled nuclear states are degenerate or possess a small energy difference between them. We, therefore, conclude that for the creation of Qudits with enlarged Hilbert spaces, shorter Dy(III)···Dy(III) distances are deemed essential.

Effect of Dipole Interactions on Blocking Temperature and Relaxation Dynamics of Superparamagnetic Iron-Oxide (Fe3O4) Nanoparticle Systems.

The effects of dipole interactions on magnetic nanoparticle magnetization and relaxation dynamics were investigated using five nanoparticle (NP) systems with different surfactants, carrier liquids, size distributions, inter-particle spacing, and NP confinement. Dipole interactions were found to play a crucial role in modifying the blocking temperature behavior of the superparamagnetic nanoparticles, where stronger interactions were found to increase the blocking temperatures. Consequently, the blocking temperature of a densely packed nanoparticle system with stronger dipolar interactions was found to be substantially higher than those of the discrete nanoparticle systems. The frequencies of the dominant relaxation mechanisms were determined by magnetic susceptibility measurements in the frequency range of 100 Hz-7 GHz. The loss mechanisms were identified in terms of Brownian relaxation (1 kHz-10 kHz) and gyromagnetic resonance of Fe3O4 (~1.12 GHz). It was observed that the microwave absorption of the Fe3O4 nanoparticles depend on the local environment surrounding the NPs, as well as the long-range dipole-dipole interactions. These significant findings will be profoundly important in magnetic hyperthermia medical therapeutics and energy applications.

Emergent Pauli Blocking in a Weakly Interacting Bose Gas

The relationship between many-body interactions and dimensionality is integral to numerous emergent quantum phenomena. A striking example is the Bose gas, which upon confinement to one dimension (1D) obeys an infinite set of conservation laws, prohibiting thermalization and constraining dynamics. In our experiment, we demonstrate that such a 1D behavior can extend much further into the dimensional crossover toward 3D than expected. Starting from a weakly interacting Bose gas trapped in a highly elongated potential, we perform a quench to instigate the dynamics of a single density mode. Employing the theory of generalized hydrodynamics, we identify the dominant relaxation mechanism as the 1D dephasing of the relevant collective excitations of the system, the rapidities. Surprisingly, the dephasing remains dominant even for temperatures far exceeding conventional limits of one dimensionality where thermalization should occur. We attribute our observations to an emergent Pauli blocking of transverse excitations caused by the rapidities assuming fermionic statistics, despite the gas being purely bosonic. Thus, our study suggests that 1D physics is less fragile than previously thought, as it can persist even in the presence of significant perturbations. More broadly, by employing the exact Bethe ansatz solutions of the many-body system, we facilitate an interpretation of how the emergent macroscopic behavior arises from the microscopic interactions.5 MoreReceived 24 June 2022Accepted 15 November 2022DOI:https://doi.org/10.1103/PhysRevX.12.041032Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasNonequilibrium statistical mechanicsQuantum quenchPhysical Systems1-dimensional systemsUltracold gasesCondensed Matter, Materials & Applied PhysicsStatistical Physics

Quantitative explanation of the difference in AC magnetization curves between suspended and immobilized magnetic nanoparticles for biomedical application

We study the difference in AC magnetization (M−H) curves between suspended and immobilized magnetic nanoparticles (MNPs). We use three commercial MNP samples that are often used for biomedical application. First, we measure the hysteresis area of the AC M−H curve, A, when the amplitude of the excitation field, Hac, is changed. The A vs Hac curve is compared with previously obtained analytical results by taking account of the MNP size distribution in the sample. We show that they quantitatively agree for both suspended and immobilized samples. From the comparison, we clarify the mechanism that determines the AC M−H curve of a suspended sample. For high Hac, the Néel relaxation becomes dominant in the suspended sample, and alignment of easy axes caused by the AC field increases the hysteresis loss compared to the immobilized case. The effect of Brownian relaxation in the suspended sample increases with decreasing Hac and gives additional loss at low field. The portion of Néel-relaxation- and Brownian-relaxation-dominant MNPs in the sample is quantitatively evaluated. We also clarify the difference in harmonic signals between suspended and immobilized samples. Finally, we discuss the condition for the MNP parameters and excitation field that determines the dominant relaxation mechanism in a suspended sample.

Optical control of exciton spin dynamics in layered metal halide perovskites via polaronic state formation

One of the open challenges of spintronics is to control the spin relaxation mechanisms. Layered metal-halide perovskites are an emerging class of semiconductors which possess a soft crystal lattice that strongly couples electronic and vibrational states and show promise for spintronic applications. Here, we investigate the impact of such strong coupling on the spin relaxation of excitons in the layered perovskite BA2FAPbI7 using a combination of cryogenic Faraday rotation and transient absorption spectroscopy. We report an unexpected increase of the spin lifetime by two orders of magnitude at 77 K under photoexcitation with photon energy in excess of the exciton absorption peak, and thus demonstrate optical control over the dominant spin relaxation mechanism. We attribute this control to strong coupling between excitons and optically excited phonons, which form polaronic states with reduced electron-hole wave function overlap that protect the exciton spin memory. Our insights highlight the special role of exciton-lattice interactions on the spin physics in the layered perovskites and provide a novel opportunity for optical spin control.

Q-dependent collective relaxation dynamics of glass-forming liquid Ca0.4K0.6(NO3)1.4 investigated by wide-angle neutron spin-echo

The relaxation behavior of glass formers exhibits spatial heterogeneity and dramatically changes upon cooling towards the glass transition. However, the underlying mechanisms of the dynamics at different microscopic length scales are not fully understood. Employing the recently developed wide-angle neutron spin-echo spectroscopy technique, we measured the Q-dependent coherent intermediate scattering function of a prototypical ionic glass former Ca0.4K0.6(NO3)1.4, in the highly viscous liquid state. In contrast to the structure modulated dynamics for Q < 2.4 Å−1, i.e., at and below the structure factor main peak, for Q > 2.4 Å−1, beyond the first minimum above the structure factor main peak, the stretching exponent exhibits no temperature dependence and concomitantly the relaxation time shows smaller deviations from Arrhenius behavior. This finding indicates a change in the dominant relaxation mechanisms around a characteristic length of 2π/(2.4 Å−1) ≈ 2.6 Å, below which the relaxation process exhibits a temperature independent distribution and more Arrhenius-like behavior.

Modeling orthogonal superposition rheometry to probe nonequilibrium dynamics of entangled polymers

Understanding changes to microstructural dynamics under nonlinear deformations is critical for designing flow processes of entangled polymeric fluids, motivating the development of experimental methods to probe strain- and rate- dependent modifications to relaxation mechanisms. Although orthogonal superposition rheometry (OSR) holds promise as such a probe, the ability to interpret the superposition moduli accessible by OSR in the context of entangled polymer dynamics remains an open question. To fill this gap, we report model OSR predictions using detailed microstructural models for both monodisperse and polydisperse entangled polymers, i.e., the Rolie-Poly and the Rolie-Double-Poly models, respectively, which account for reptation, chain retraction, and convective constraint release. By combining numerical calculations with a perturbation analysis, we demonstrate that for polymers that can be described by a single-mode model, the OSR superposition moduli at different shear rates and frequencies can generally be collapsed onto a single master curve, with rate-dependent shift factors that depend on the nonlinear rate-dependent modification of polymer conformation and relaxation rates without changing the dominant relaxation mechanisms. We systematically study how the OSR moduli are sensitive to the shape and dispersity of the molecular weight distribution. We discuss the generality of our results for a broad class of constitutive models and suggest an analogy to Laun’s rule to relate OSR moduli to the first normal stress difference. Our results provide a foundation to guide the design and interpretation of future experiments and demonstrate that orthogonal superposition rheometry often probes features in nonlinear dynamics more directly than conventional rheometry techniques.

Transient interfacial rheology and polar component dynamics at oil-brine interfaces

Interfacial deformation processes have diverse impacts on numerous industrial applications, including the petroleum industry. This study aimed to investigate the interfacial rheology and polar component dynamics in crude oil and brine systems using a wide range of salinity. The oscillating pendant-drop technique was utilized to evaluate the transient and equilibrium oil-brine interfacial rheological behavior and the associated diffusion, adsorption, and rearrangement processes of the bulk-dispersed and interfacially adsorbed polar components. A new logistic equation is proposed to model the transitory rheology of oil-brine interfaces, enabling the quantitative comparison of their behaviors. The conducted modeling approach indicated that the diffusion/adsorption processes are the dominant relaxation mechanisms over reorientation/rearrangement processes (negligible) when the interfaces are perturbed at different aging times. Furthermore, the results show that (a) the rate at which apparent interfacial elasticity increases is frequency-independent, (b) there are two local maximums for apparent interfacial viscoelasticity with brine salinity variation, (c) polar component dynamics in the studied crude oil are mainly controlled by diffusion, and (d) absence of significant lateral interactions of adsorbed species on the interface. Transient interfacial rheology is demonstrated to be of great importance when defining final apparent interfacial properties, and the potential consequences of these new findings on enhanced oil recovery applications are discussed.

Magnetic Characterization and Moderate Cytotoxicity of Magnetic Mesoporous Silica Nanocomposite for Drug Delivery of Naproxen.

In this study, we describe the magnetic and structural properties and cytotoxicity of drug delivery composite (DDC) consisting of hexagonally ordered mesoporous silica, iron oxide magnetic nanoparticles (Fe2O3), and the drug naproxen (Napro). The nonsteroidal anti-inflammatory drug (NSAID) naproxen was adsorbed into the pores of MCM-41 silica after the ultra-small superparamagnetic iron oxide nanoparticles (USPIONs) encapsulation. Our results confirm the suppression of the Brownian relaxation process caused by a “gripping effect” since the rotation of the whole particle encapsulated in the porous system of mesoporous silica was disabled. This behavior was observed for the first time, to the best of our knowledge. Therefore, the dominant relaxation mechanism in powder and liquid form is the Néel process when the rotation of the nanoparticle’s magnetic moment is responsible for the relaxation. The in vitro cytotoxicity tests were performed using human glioma U87 MG cells, and the moderate manifestation of cell death, although at high concentrations of studied systems, was observed with fluorescent labeling by AnnexinV/FITC. All our results indicate that the as-prepared MCM-41/Napro/Fe2O3 composite has a potential application as a drug nanocarrier for magnetic-targeted drug delivery.

Improved Analysis of Nuclear-Magnetic-Resonance Measurements in Organic-Rich Mudrocks Through Experimental Quantification of the Hydrocarbon/Kerogen Intermolecular-Interfacial-Relaxation Mechanism

Summary Nuclear-magnetic-resonance (NMR) measurements have become a popular choice for estimating hydrocarbon saturations in organic-rich mudrock reservoirs. Previous publications have shown that the dominant mechanism for surface relaxation during NMR measurements in organic pores is intramolecular dipolar coupling among hydrocarbon protons. However, the influence of kerogen/hydrocarbon intermolecular interactions and kerogen thermal maturity on the surface relaxivity has not been reliably quantified. The objectives of this paper are to experimentally quantify the influence of intermolecular coupling on kerogen surface relaxivities; compare the experimentally determined surface relaxivities with those obtained from our previously published analytical model; and quantify the effect of intermolecular coupling on estimates of the adsorbed-hydrocarbon phase volume in simple geometries. First, we selected two organic-rich mudrock formations with different kerogen thermal maturities and extracted pure kerogen from them. The extracted-kerogen samples were synthetically matured by increasing the temperature at 4°C/min from 25 to 450°C under a controlled environment. The petrophysical properties of kerogen samples at different thermal maturities were quantified using pyrolysis and Brunauer-Emmett-Teller (BET) measurements. The untreated and thermally mature kerogen samples were then saturated with protonated and partially deuterated chloroform mixtures. Consequently, we performed longitudinal (T1) and transverse (T2) measurements on the kerogen/chloroform mixtures. Then, we compared the surface relaxivities estimated from T1/T2 and BET surface-area measurements with those predicted by a previously published theoretical model derived from generalized adsorption theory. Finally, we performed a sensitivity study demonstrating the effect of intermolecular dipolar coupling on estimates of adsorbed-hydrocarbon volume by modeling kerogen pores as synthetic spherical objects. Results indicate that synthetic maturation of kerogen samples relatively increased their specific surface areas by up to 97.1%. When chloroform deuteration is kept constant and kerogen samples were heat treated from temperatures of 25 to 450°C, the T1 and T2 surface relaxivities relatively decreased by up to 70.1 and 80.3%, respectively. Our recently introduced analytical model was able to reliably quantify the kerogen surface relaxivities estimated from experimental measurements with a relative error of 30.5%. The results of the sensitivity analysis showed that improved assessment of kerogen surface relaxivity by including intermolecular coupling enhanced the NMR-based adsorbed-hydrocarbon-volume estimates relatively by up to 41.9% when kerogen pores were modeled as synthetic spherical objects. The results of the experimental measurements support the observations of the analytically developed surface-relaxivity model derived from the generalized adsorption theory. Accurately quantifying the mechanism contributing to surface relaxation helps in providing accurate temperature and frequency corrections for T2 and T1/T2 cutoff values. Such cutoff values can then be extended to in-situ conditions improving downhole estimates of NMR-based hydrocarbon saturations in organic-rich mudrocks.

Polariton condensate trapping by parametric pair scattering

Spatially confined, trapped polariton condensates have been shown to exhibit strong stochastic on-site spin polarization and in longer polariton condensate chains, distance controlled ferromagnetic and antiferromagnetic spin couplings. Until now, little is known, on how such polariton condensates spatially separated from their exciton reservoirs are trapped and formed. Here, we investigate the properties and formation dynamics of two main families of polariton condensates, those overlapping with the pump reservoir and those in confined geometries, under pulsed nonresonant excitation. The observed reduction in polariton condensation threshold and energy blueshift in trapped case is attributed to exciton reservoir-condensate spatial separation, whereas time-resolved photoluminescence measurements, reveal distinct relaxation and condensate formation dynamics with pair parametric scattering process being the dominant relaxation mechanism in trapped geometry.

Unconventional spin excitations in the S=32 triangular antiferromagnet RbAg2Cr[VO4]2

We present muon spin relaxation ($\ensuremath{\mu}\mathrm{SR}$) measurements of the $S=3/2$ undistorted triangular lattice established in ${\mathrm{RbAg}}_{2}\mathrm{Cr}{[{\mathrm{VO}}_{4}]}_{2}$. The zero- (ZF) and longitudinal-field $\ensuremath{\mu}\mathrm{SR}$ spectra evidence the absence of spin freezing and long-range magnetic ordering down to $T=25$ mK, supporting the formation of a dynamic ground state. Noticeably, we observe an anomalous temperature dependence of the ZF muon spin relaxation rate ${\ensuremath{\lambda}}_{Z}F(T)$, featuring a decrease below $T=20$ K. This suggests the alteration of the dominant relaxation mechanism by the development of short-range magnetic correlations. A subsequent leveling off of ${\ensuremath{\lambda}}_{Z}F(T)$ below $T=2$ K indicates persistent spin dynamics and reveals the presence of exotic magnetic excitations. The field dependence of the muon spin relaxation rate at $T=25$ mK is well described by a diffusive spin transport model with algebraic spin-spin correlations. The suppressed long-range order and the peculiar temperature-dependent behavior of ${\ensuremath{\lambda}}_{Z}F(T)$ will be discussed in terms of the exchange interaction between ${\mathrm{Cr}}^{3+}$ moments via nonmagnetic ${[{\mathrm{VO}}_{4}]}^{3\ensuremath{-}}$ entities. In the title compound, the degeneracy of the ${t}_{2g}$-orbital set is not lifted by a space group symmetry reduction or subject to significant anisotropy resulting from spin-orbit coupling.

Shear localization as a mesoscopic stress-relaxation mechanism in fused silica glass at high strain rates

Molecular dynamics (MD) simulations of fused silica glass deforming in pressure-shear, while revealing useful insights into processes unfolding at the atomic level, fail spectacularly in that they grossly overestimate the magnitude of the stresses relative to those observed, e. g., in plate-impact experiments. We interpret this gap as evidence of relaxation mechanisms that operate at mesoscopic lengthscales and which, therefore, are not taken into account in atomic-level calculations. We specifically hypothesize that the dominant mesoscopic relaxation mechanism is shear banding. We evaluate this hypothesis by first generating MD data over the relevant range of temperature and strain rate and then carrying out continuum shear-banding calculations in a plate-impact configuration using a critical-state plasticity model fitted to the MD data. The main outcome of the analysis is a knock-down factor due to shear banding that effectively brings the predicted level of stress into alignment with experimental observation, thus resolving the predictive gap of MD calculations.

Relaxation of electrons in quantum-confined states in Pb/Si(111) thin films from master equation with first-principles-derived rates

Atomically thin films of Pb on Si(111) provide an experimentally tunable system comprising a highly structured electronic density of states. The lifetime of excited electrons in these states is limited by both electron–electron (e–e) and electron–phonon (e–ph) scattering. We employ the description by a master equation for the electronic occupation numbers to analyze the relative importance of both scattering mechanisms. The electronic and phononic band structures, as well as the matrix elements for electron–phonon coupling within deformation potential theory were obtained from density functional calculations, thus taking into account quantum confinement effects. For the relaxation dynamics, the contribution of impact ionization processes to the lifetime is estimated from the imaginary part of the electronic self-energy calculated in the GW approximation. By numerically solving rate equations for the occupations of the Pb-derived electronic states coupled to a phononic heat bath, we are able to follow the distribution of the electronic excitation energy to the various modes of Pb lattice vibrations. While e–e scattering is the dominant relaxation mechanism, we demonstrate that the e–ph scattering is highly phonon-mode-specific, with a large contribution from surface phonons. At electron energies of about 0.3 eV above the Fermi surface, a ‘phonon bottleneck’ characteristic of relaxation in nanostructures with well-separated electronic states is observed. The time scales extracted from the simulations are compared to data from pump-probe experiments using time-resolved two-photon photoemission.

Anomalous conductance scaling in strained Weyl semimetals

Magnetotransport provides key experimental signatures in Weyl semimetals. The longitudinal magnetoresistance is linked to the chiral anomaly and the transversal magnetoresistance to the dominant charge relaxation mechanism. Axial magnetic fields that act with opposite sign on opposite chiralities facilitate new transport experiments that probe the low-energy Weyl nodes. As recently realized, these axial fields can be achieved by straining samples or adding inhomogeneities to them. Here, we identify a robust signature of axial magnetic fields: an anomalous scaling of the conductance in the diffusive ultraquantum regime. In particular, we demonstrate that the longitudinal conductivity in the ultraquantum regime of a disordered Weyl semimetal subjected to an axial magnetic field increases with both the field strength and sample width due to a spatial separation of charge carriers. We contrast axial magnetic with real magnetic fields to clearly distinguish the different behavior of the conductance. Our results rely on numerical tight-binding simulations and are supported by analytical arguments. We argue that the spatial separation of charge carriers can be used for directed currents in microstructured electronic devices.

Mechanistic model for stresses in the oxide layer formed on zirconium alloys

Zirconium alloys are widely used as the cladding material in pressurized-water-reactors. The oxide formed is subjected to compressive stresses, which relax over time. This may affect the protectiveness of the oxide layer by allowing crack formation. We present a mechanistic model to predict these stresses as a function of temperature and oxidation kinetics. Material parameters for elastic deformation, creep, and thermal expansion are taken from appropriate experimental studies and the resulting predictions for the evolution of the stress distributions are compared with other experimental data. Dislocation glide in the oxide is found to be the dominant mechanism of stress relaxation for temperatures below 900 K.