Random Local Fields Research Articles

Magnetic domain dynamics in an insulating quantum ferromagnet

The statistics and form of avalanches in a driven system reveal the nature of the underlying energy landscape and dynamics. In conventional metallic ferromagnets, eddy-current back action can dominate the dynamics. Here, we study Barkhausen noise in Li(Ho,Y)F4, an insulating Ising ferromagnet that cannot sustain eddy currents. For large avalanches at temperatures approaching the Curie point, we find a symmetric response free of drag effects. In the low temperature limit, drag effects contribute to the dynamics, which we link to enhanced pinning from local random fields that are enabled by the microscopic dipole-coupled Hamiltonian (the Ising model in transverse field).

Electronic structure of interstitial hydrogen in In-Ga-Zn-O semiconductor simulated by muon

We report on the local electronic structure of an interstitial muon (Mu) as pseudohydrogen in In-Ga-Zn oxide (IGZO) semiconductors studied by muon spin rotation/relaxation (μSR) experiment. In polycrystalline (c-) IGZO, it is inferred that Mu is in a diamagnetic state, where the μSR time spectra under zero external field are perfectly described by the Gaussian Kubo-Toyabe relaxation function with the linewidth Δ serving as a sensitive measure for the random local fields from In/Ga nuclear magnetic moments. The magnitude of Δ combined with the density functional theory calculations for H (to mimic Mu) suggests that Mu occupies Zn-O bond-center site (MuBC) similar to the case in crystalline ZnO. This implies that the diamagnetic state in c-IGZO corresponds to MuBC+, thus serving as an electron donor. In amorphous (a-) IGZO, the local Mu structure in the as-deposited films is nearly identical to that in c-IGZO, suggesting MuBC+ for the electronic state. In contrast, the diamagnetic signal in heavily hydrogenated a-IGZO films exhibits the Lorentzian Kubo-Toyabe relaxation, implying that Mu accompanies more inhomogeneous distribution of the neighboring nuclear spins that may involve a Mu− H−-complex state in an oxygen vacancy.

Relaxor behavior and tunable property of lead-free Li+-doped Ba0.9Ca0.1Zr0.2Ti0.8O3 ceramics

Li+ doped lead-free Ba0.9Ca0.1Zr0.2Ti0.8O3 (abbreviated as BCZTL) ceramics were prepared through solid-state reaction method. Effects of Li+ on the formation of oxygen vacancies, change of lattice parameters and grain sizes were investigated. The disordered distribution of Li+ ions on B sites of BCZTL were found to give rise to the emergence of local random fields and facilitate the appearence of polar nanoregions (PNRs), which led to the relaxor behavior observed in dielectric measurements. The physical mechanisms for the influences of Li+ substitution and sintered temperature on the variation of PNRs, relaxor behavior and tunability were also proposed. Compared with other sintered temperatures, the 1300 °C sintered BCZTL sample exhibited relatively lower degree of relaxor behavior (γ = 1.89) and higher tunability (k = 56%) owning to the smaller PNRs size.

Phase structure dependence of acceptor doping effects in (Bi0.5Na0.5)TiO3–BaTiO3 lead-free ceramics

The introduction of acceptor dopants into perovskites is a common method for designing hard piezoelectric ceramics by forming defect dipoles. It is interesting to find that the doping effect of a few amount of MnO2 in (Bi0.5Na0.5)TiO3–BaTiO3 lead-free ceramics significantly depends on the local symmetry. A conventional hardening effect corresponding to an obviously enhanced mechanical quality factor and a decreased piezoelectric coefficient can be generated in the tetragonal phase rich zone, rather than in the rhombohedral-rich zone, where a typically amphoteric characteristic can be observed unexpectedly. The result was well interpreted by means of the formation of a local random field as a result of the local symmetry-breaking effect of point defects after doping MnO2 in addition to a traditional internal bias field as a result of the pinning effect of defect dipoles. Compared with tetragonal-rich ones, the obviously enhanced random field in rhombohedral-rich compositions tends to induce a weakening ferroelectricity, as evidenced by composition dependent normal ferroelectric-relaxor ferroelectric phase transition. This work provides a new understanding of how charged defects affect the domain stability in the matrix with different symmetries through the competition between random electric fields and internal bias fields.

Rabi oscillations as a tool to detect strong coupling of paramagnetic center with the nuclear spin bath

We show that strong coupling with the nuclear spin ensemble leaves an imprint on the nutation dynamics of the electron spin in the form of forced oscillations. The frequency of these oscillations equals Larmor precession frequency of the nuclear spins. This effect is evidenced by our experimental data on Rabi oscillations of paramagnetic nitroxyl radical TEMPO ((2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl) dispersed in glassy and crystalline ethanol. The data are interpreted in terms of a simple model that represents an impact of the proton spin bath as random local field with certain probability distribution. The electron spin coherence times of TEMPO in ethanol obtained by means of Hahn spin echo and Carr-Purcell-Meiboom-Gill sequences are compared with the calculations based on a spin diffusion model.

Motional narrowing under Markovian and non-Markovian hopping transitions in inhomogeneous broadened absorption line shape.

Inspired by recent experiments showing a minimum of electron spin or paramagnetic resonance (ESR and EPR) line width as a function of inverse temperature, we studied the motional narrowing effect by considering a combined model of carrier transitions and static dispersion of the angular frequency giving rise to an inhomogeneous broadening in the spectrum. The dispersion of the angular frequency results from the distribution of the local field. The transition between the sites under inhomogeneous static local field induces adiabatic relaxation of the spin. We also considered the on-site inherent (nonadiabatic) relaxation of the spin. We obtained the exact solution of the spin correlation function by explicitly considering transitions between two sites for both Markovian and non-Markovian transition processes. The absorption line shape is expressed in terms of the Voigt function, which is a convolution of a Gaussian function and a Lorentzian function. Using the known properties of the Voigt function, we discuss the correlation between the change in the full-width at half-maximum and the change in line shape, both of which are induced by motional narrowing. By assuming thermal activation processes for both the hopping transition and the on-site inherent relaxation, we show that the minimum of the width appears as a function of inverse temperature as observed experimentally in organic materials. Contrary to the general belief, we also show that the narrowing of the Gaussian line shape under a local random field did not necessarily lead to a Lorentzian line shape in particular under the presence of heavy tail property in the waiting time distribution of hopping transitions.

Soft mode driven local ferroelectric transition in lead-based relaxors

Recently, we have analyzed the complex infrared (IR) and terahertz (THz) response of the relaxor ferroelectric Pb(Mg1/3Nb2/3)O3 (PMN) and suggested a new interpretation of the so-called intermediate temperature T* ≈ 400 K, where the split soft phonon mode undergoes incomplete softening. IR-THz reflectivity was fitted using effective medium approximation, which describes the uniaxially anisotropic response of polar nanodomains (PNDs). It indicated that the response perpendicular to the local polarization undergoes a classical Cochran softening toward T* with the extrapolated low-frequency permittivity obeying the Curie-Weiss (CW) law with TC ≈ T*. In this letter, we report analysis performed in a similar way on the published IR-THz response in closely related relaxor Pb(Mg1/3Ta2/3)O3 and have revealed a similar behavior: The lowest-frequency phonon polarized perpendicularly to the local polarization of PNDs softens below ∼900 K according to the Cochran law and the corresponding low-frequency permittivity obeys the CW law with TC = 340 K ≈ T* and Curie constant C = 1.16 × 105 K. This is comparable to similar parameters for PMN, TC = 380 K ≈ T* and C = 1.7 × 105 K. This explains the high-temperature CW anomaly in the low-frequency dielectric response of both materials and strongly suggests that both relaxors undergo a local ferroelectric phase transition near T* which induces a new polarization component perpendicular to the local random electric field.

Specific heat and nonlinear susceptibility in spin glasses with random fields

We study magnetic properties of spin glass (SG) systems under a random field (RF), based on the suggestion that RFs can be induced by a weak transverse field in the compound ${\mathrm{LiHo}}_{x}{\mathrm{Y}}_{1\ensuremath{-}x}{\mathrm{F}}_{4}$. We consider a cluster spin model that allows long-range disordered interactions among clusters and short-range interactions inside the clusters, besides a local RF for each spin following a Gaussian distribution with standard deviation $\mathrm{\ensuremath{\Delta}}$. We adopt the one-step replica symmetry breaking approach to get an exactly solvable single-cluster problem. We discuss the behavior of order parameters, specific heat ${C}_{m}$, nonlinear susceptibility ${\ensuremath{\chi}}_{3}$, and phase diagrams for different disorder configurations. In the absence of RF, the ${\ensuremath{\chi}}_{3}$ exhibits a divergence at ${T}_{f}$, while the ${C}_{m}$ shows a broad maximum at a temperature ${T}^{**}$ around $30%$ above ${T}_{f}$, as expected for conventional SG systems. The presence of RF changes this scenario. The ${C}_{m}$ still shows the maximum at ${T}^{**}$ that is weakly dependent on $\mathrm{\ensuremath{\Delta}}$. However, the ${T}_{f}$ is displaced to lower temperatures, enhancing considerably the ratio ${T}^{**}/{T}_{f}$. Furthermore, the divergence in ${\ensuremath{\chi}}_{3}$ is replaced by a rounded maximum at a temperature ${T}^{*}$, which becomes increasingly higher than ${T}_{f}$ as $\mathrm{\ensuremath{\Delta}}$ is enhanced. As a consequence, the paramagnetic phase is unfolded in three regions: (i) a conventional paramagnetism ($T>{T}^{**}$); (ii) a region with formation of short-range order with frozen spins (${T}^{*}<T<{T}^{**}$); (iii) a region with slow growth of free-energy barriers slowing down the spin dynamics before the SG transition (${T}_{f}<T<{T}^{*}$) suggesting an intermediate Griffiths phase before the SG state. Our results reproduce qualitatively some findings of ${\mathrm{LiHo}}_{x}{\mathrm{Y}}_{1\ensuremath{-}x}{\mathrm{F}}_{4}$ as the rounded maximum of ${\ensuremath{\chi}}_{3}$ behavior triggered by RF and the deviation of the conventional relationship between the ${T}_{f}$ and ${T}^{**}$.

Quantum-confined stark effect in the ensemble of phase-pure CdSe/CdS quantum dots.

Colloidal semiconductor quantum dots (QDs) have recently attracted great attention in electric field sensing via the quantum-confined Stark effect (QCSE), but they suffer from the random local electric field around the charged QDs through the Auger process or defect traps. Here, QCSE in the ensemble of phase-pure wurtzite CdSe/CdS QDs was studied by applying a uniform external electric field. We observed clear field-dependent photoluminescence (PL) and absorption characteristics in thick-shell CdSe/CdS QDs with 11 CdS monolayers (11 MLs) including a pronounced spectral redshift in PL of ∼2.3 nm and absorption of ∼2.1 nm. The time-dependent PL intensity traces implied that the thick-shell QDs were conducive to realize the Stark shift in QD ensembles due to the effective suppression of the main sources of the local field. These findings were in stark contrast to those of moderate-shell (5 MLs) and ultrathick-shell (15 MLs) CdSe/CdS QDs. The measurement value of exciton polarizability was smaller than the theoretical value, which may be influenced by very few exciton traps. Moreover, the amplified stimulated emission also exhibited obvious optical modulations under the electric field with decreased emission intensity and an increased ultrafast lifetime. Finally, the temporal evolution of the multiexciton process in thick-shell CdSe/CdS QDs indicated that the multiexciton state induced a higher energy state near the band edge, which may weaken the QCSE of a single exciton. Therefore, it was demonstrated that efficient field control over the optical properties of these nanomaterials is feasible and this can open up potential applications in field-controlled electro-optic modulators.

Excellent comprehensive energy storage properties of novel lead-free NaNbO3-based ceramics for dielectric capacitor applications

Excellent energy storage properties were achieved in NaNbO3-based ceramics by enhancing antiferroelectricity and constructing local random field simultaneously.

A change detection framework by fusing threshold and clustering methods for optical medium resolution remote sensing images

ABSTRACT In change detection (CD) of medium-resolution remote sensing images, the threshold and clustering methods are two kinds of the most popular ones. It is found that the threshold method of the expectation maximization (EM) algorithm usually generates a CD map including many false alarms but almost detecting all changes, and the fuzzy local information c-means algorithm (FLICM) obtains a homogenous CD map but with some missed detections. Therefore, a framework is designed to improve CD results by fusing the advantages of the threshold and clustering methods. The CD map generated by the clustering method of FLICM is used to remove false alarms in the CD map obtained by EM threshold method by an overlap fusion. Then, the local Markov random field model is implemented to verify the potentially missed detections. Finally, a fused CD map with less false alarms and missed detections is achieved. Two experiments were carried out on two Landsat ETM+ data sets. The proposed method obtained the least errors (1.11% and 3.51%) and the highest kappa coefficient (0.9366 and 0.8834), respectively, when compared with five popular CD methods.

Influence of structural evolution on electrocaloric effect in Bi0.5Na0.5TiO3-SrTiO3 ferroelectric ceramics

The electrocaloric effect (ECE) in lead-free relaxor ferroelectrics has gained significant interest over the past decades. However, certain aspects of the ECE in relaxor ferroelectrics, such as Bi0.5Na0.5TiO3, remain poorly understood. In this work, we investigate the ECE by considering Bi0.5Na0.5TiO3-SrTiO3 (BNT-ST) as an example. The results show that, for BNT-0.25ST ceramics, the directly-measured ECE is optimal when the freezing temperature is tailored to be about room temperature. For this material, ΔT = 0.51 K under an electric field of 6 kV/mm and the ECE has excellent thermal stability (the instability η ≤ 20% in the range 30–120 °C). The addition of strontium increases the cubic-phase fraction, enhances the local random field, and changes the local structure, as clarified by in situ Raman spectroscopy and piezoelectric force morphology. In addition, we discuss in detail the correlation between the ECE and local structure. This work thus improves our understanding of the ECE in BNT-based materials for EC cooling technologies.

Compressive Binary Patterns: Designing a Robust Binary Face Descriptor with Random-Field Eigenfilters.

A binary descriptor typically consists of three stages: image filtering, binarization, and spatial histogram. This paper first demonstrates that the binary code of the maximum-variance filtering responses leads to the lowest bit error rate under Gaussian noise. Then, an optimal eigenfilter bank is derived from a universal assumption on the local stationary random field. Finally, compressive binary patterns (CBP) is designed by replacing the local derivative filters of local binary patterns (LBP) with these novel random-field eigenfilters, which leads to a compact and robust binary descriptor that characterizes the most stable local structures that are resistant to image noise and degradation. A scattering-like operator is subsequently applied to enhance the distinctiveness of the descriptor. Surprisingly, the results obtained from experiments on the FERET, LFW, and PaSC databases show that the scattering CBP (SCBP) descriptor, which is handcrafted by only 6 optimal eigenfilters under restrictive assumptions, outperforms the state-of-the-art learning-based face descriptors in terms of both matching accuracy and robustness. In particular, on probe images degraded with noise, blur, JPEG compression, and reduced resolution, SCBP outperforms other descriptors by a greater than 10 percent accuracy margin.

Obtaining highly excited eigenstates of the localized XX chain via DMRG-X.

We benchmark a variant of the recently introduced density matrix renormalization group (DMRG)-X algorithm against exact results for the localized random field XX chain. We find that the eigenstates obtained via DMRG-X exhibit a highly accurate l-bit description for system sizes much bigger than the direct, many-body, exact diagonalization in the spin variables is able to access. We take advantage of the underlying free fermion description of the XX model to accurately test the strengths and limitations of this algorithm for large system sizes. We discuss the theoretical constraints on the performance of the algorithm from the entanglement properties of the eigenstates, and its actual performance at different values of disorder. A small but significant improvement to the algorithm is also presented, which helps significantly with convergence. We find that, at high entanglement, DMRG-X shows a bias towards eigenstates with low entanglement, but can be improved with increased bond dimension. This result suggests that one must be careful when applying the algorithm for interacting many-body localized spin models near a transition.This article is part of the themed issue 'Breakdown of ergodicity in quantum systems: from solids to synthetic matter'.

Local strain heterogeneity and elastic relaxation dynamics associated with relaxor behavior in the single-crystal perovskite Pb(In1/2Nb1/2)O3−PbZrO3−Pb(Mg1/3Nb2/3)O3−PbTiO3

Recently, $\mathrm{Pb}(\mathrm{I}{\mathrm{n}}_{1/2}\mathrm{N}{\mathrm{b}}_{1/2}){\mathrm{O}}_{3}\text{\ensuremath{-}}\mathrm{PbZr}{\mathrm{O}}_{3}\text{\ensuremath{-}}\mathrm{Pb}(\mathrm{M}{\mathrm{g}}_{1/3}\mathrm{N}{\mathrm{b}}_{2/3}){\mathrm{O}}_{3}\text{\ensuremath{-}}\mathrm{PbTi}{\mathrm{O}}_{3}$ (PIN-PZ-PMN-PT) relaxor single crystals were demonstrated to possess improved temperature-insensitive properties, which would be desirable for high-power device applications. The relaxor character associated with the development of local random fields (RFs) and a high rhombohedral-tetragonal (R-T) ferroelectric transition temperature $({T}_{\mathrm{R}\ensuremath{-}\mathrm{T}}g120{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C})$ would be critical for the excellent properties. A significant effect of the chemical substitution of $\mathrm{I}{\mathrm{n}}^{3+}$ and $\mathrm{Z}{\mathrm{r}}^{4+}$ in PMN-PT to give PIN-PZ-PMN-PT is the development of local strain heterogeneity, which acts to suppress the development of macroscopic shear strains without suppressing the development of local ferroelectric moments and contribute substantially to the RFs in PIN-PZ-PMN-PT. Measurements of elastic and anelastic properties by resonant ultrasound spectroscopy show that PIN-PZ-PMN-PT crystal has a quite different form of elastic anomaly due to Vogel-Fulcher freezing, rather than the a discrete cubic-$T$ transition seen in a single crystal of PMN-28PT. It also has high acoustic loss of the relaxor phase down to ${T}_{\mathrm{R}\ensuremath{-}\mathrm{T}}$. Analysis of piezoresponse force microscopy phase images at different temperatures provides a quantitative insight into the extent to which the RFs influence the microdomain structure and the short-range order correlation length $\ensuremath{\langle}\ensuremath{\xi}\ensuremath{\rangle}$.

Phase diagram for the O(n) model with defects of “random local field” type and verity of the Imry–Ma theorem

It is shown that the Imry-Ma theorem stating that in space dimensions d<4 the introduction of an arbitrarily small concentration of defects of the "random local field" type in a system with continuous symmetry of the n-component vector order parameter (O(n)model) leads to the long-range order collapse and to the occurrence of a disordered state, is not true if the anisotropic distribution of the defect-induced random local field directions in the n-dimensional space of the order parameter leads to the defect-induced effective anisotropy of the "easy axis" type. For a weakly anisotropic field distribution, in space dimensions 2<d<4 there exists some critical defect concentration, above which the inhomogeneous Imry-Ma state can exist as an equilibrium one. At lower defect concentration the long-range order takes place in the system. For a strongly anisotropic field distribution, the Imry-Ma state is suppressed completely and the long-range order state takes place at any defect concentration.

Robust Dehaze Algorithm for Degraded Image of CMOS Image Sensors.

The CMOS (Complementary Metal-Oxide-Semiconductor) is a new type of solid image sensor device widely used in object tracking, object recognition, intelligent navigation fields, and so on. However, images captured by outdoor CMOS sensor devices are usually affected by suspended atmospheric particles (such as haze), causing a reduction in image contrast, color distortion problems, and so on. In view of this, we propose a novel dehazing approach based on a local consistent Markov random field (MRF) framework. The neighboring clique in traditional MRF is extended to the non-neighboring clique, which is defined on local consistent blocks based on two clues, where both the atmospheric light and transmission map satisfy the character of local consistency. In this framework, our model can strengthen the restriction of the whole image while incorporating more sophisticated statistical priors, resulting in more expressive power of modeling, thus, solving inadequate detail recovery effectively and alleviating color distortion. Moreover, the local consistent MRF framework can obtain details while maintaining better results for dehazing, which effectively improves the image quality captured by the CMOS image sensor. Experimental results verified that the method proposed has the combined advantages of detail recovery and color preservation.

Generic features of the primary relaxation in glass-forming materials (Review Article)

We discuss structural relaxation in molecular and polymeric supercooled liquids, metallic alloys and orientational glass crystals. The study stresses especially the relationships between observables raised from underlying constraints imposed on degrees of freedom of vitrification systems. A self-consistent parametrization of the α-timescale on macroscopic level results in the material-and-model independent universal equation, relating three fundamental temperatures, characteristic of the primary relaxation, that is numerically proven in all studied glass formers. During the primary relaxation, the corresponding small and large mesoscopic clusters modify their size and structure in a self-similar way, regardless of underlying microscopic realizations. We show that cluster-shape similarity, instead of cluster-size fictive divergence, gives rise to universal features observed in primary relaxation. In all glass formers with structural disorder, including orientational-glass materials (with the exception of plastic crystals), structural relaxation is shown to be driven by local random fields. Within the dynamic stochastic approach, the universal subdiffusive dynamics corresponds to random walks on small and large fractals.

Composition‐induced structural transitions and enhanced strain response in nonstoichiometric NBT‐based ceramics

AbstractIn this work, the nonstoichiometric 0.99Bi0.505(Na0.8K0.2)0.5‐xTiO3‐0.01SrTiO3 (BNKST(0.5‐x)) ceramics with x=0‐0.03 were synthesized by conventional solid‐state reaction method. The composition‐induced structural transitions were investigated by Raman spectra, dielectric analyses, and electrical measurements. It is found that the relaxor phase can be induced through the modulation of the (Na, K) content. The (Na, K) deficiency in BNKST(0.5‐x) ceramics favors a more disordered local structure and can result in the loss of long‐range ferroelectricity. The x=0.015 critical composition possesses relatively high positive strain Spos of 0.42% and large signal piezoelectric constant d33* of 479 pm V−1 at 6 kV mm−1, along with the good temperature (25‐120°C) and frequency (1‐20 Hz) stability. The recoverable large strain responses in nonstoichiometric ceramics can be attributed to the reversible relaxor‐ferroelectric phase transition, which is closely related to the complex defects (, , and ) and the local random fields. This work may be helpful for the exploration of high‐performance NBT‐based lead‐free materials by means of A‐site compositional modification.

Effective realization of random magnetic fields in compounds with large single-ion anisotropy

We show that spin $S=1$ system with large and random single--ion anisotropy can be at low energies mapped to a $S=1/2$ system with random magnetic fields. This is for example realized in \mbox{Ni(Cl$_{1-x}$Br$_{x}$)$_2$-4SC(NH$_2$)$_2$} compound (DTNX) and therefore it represents a long sought realization of random local (on-site) magnetic fields in antiferromagnetic systems. We support the mapping by numerical study of $S=1$ and effective $S=1/2$ anisotropic Heisenberg chains and find excellent agreement for static quantities and also for the spin conductivity. Such systems can therefore be used to study the effects of local random magnetic fields on transport properties.