Local Phase Transition Research Articles

Inverted many-body mobility edge in a central qudit problem

Many interesting experimental systems, such as cavity QED or central spin models, involve global coupling to a single harmonic mode. Out of equilibrium, it remains unclear under what conditions localized phases survive such global coupling. We study energy-dependent localization in the disordered Ising model with transverse and longitudinal fields coupled globally to a $d$-level system (qudit). Strikingly, we discover evidence for an inverted mobility edge where high-energy states are localized whereas low-energy states are delocalized. This prediction is supported by shift-and-invert eigenstate targeting and Krylov time evolution up to $L=13$ and 18, respectively. We argue for a critical energy of the localization phase transition which scales as ${E}_{c}\ensuremath{\propto}{L}^{1/2}$, consistent with finite-size numerics. We also show evidence for a reentrant many-body localization phase at even lower energies despite the presence of strong effects of the central mode in this regime. Similar results should occur in the central spin-$S$ problem at large $S$ and in certain models of cavity QED.

Sharp, localized phase transitions in single neuronal cells

The origin of nonlinear responses in cells has been suggested to be crucial for various cell functions including the propagation of the nervous impulse. In physics, nonlinear behavior often originates from phase transitions. Evidence for such transitions on the single-cell level, however, has so far not been provided, leaving the field unattended by the biological community. Here, we demonstrate that single cells of a human neuronal cell line display all optical features of a sharp, highly nonlinear phase transition within their membrane. The transition is reversible and does not originate from protein denaturation. Triggered by temperature and modified by pH here, a thermodynamic approach strongly suggests that similar nonlinear state changes can be induced by other variables such as calcium or mechanical stress. At least in lipid membranes, such state changes are accompanied by significant changes in permeability, enzyme activity, elastic, and electrical properties.

Inhomogeneous spin excitations in weakly coupled spin- 12 chains

We present a systematic inelastic neutron scattering and neutron diffraction study on the magnetic structure of the quasi-one-dimensional spin-$\frac{1}{2}$ magnet ${\mathrm{SrCo}}_{2}{\mathrm{V}}_{2}{\mathrm{O}}_{8}$, where the interchain coupling in the N\'eel-type antiferromagnetic ground state breaks the static spin lattice into two independent domains. At zero magnetic field, we have observed two new spin excitations with small spectral weights inside the gapped region defined by the spinon bound states. In an external magnetic field along the chain axis, the N\'eel order gets partially destabilized at ${\ensuremath{\mu}}_{0}{H}^{★}=2.0\phantom{\rule{0.16em}{0ex}}\mathrm{T}$ and completely suppressed at ${\ensuremath{\mu}}_{0}{H}_{\mathrm{p}}=3.9\phantom{\rule{0.16em}{0ex}}\mathrm{T}$, above which a quantum disordered Tomonaga--Luttinger liquid (TLL) prevails. The low-energy spin excitations between ${\ensuremath{\mu}}_{0}{H}^{★}$ and ${\ensuremath{\mu}}_{0}{H}_{\mathrm{p}}$ are not homogeneous, containing the dispersionless (or weakly dispersive) spinon bound states excited in the N\'eel phase and the highly dispersive psinon-antipsinon mode characteristic of a TLL. We propose that the two new modes at zero field are spinon excitations inside the domain walls. Since they have a smaller gap than those excited in the N\'eel domains, the underlying spin chains enter the TLL state via a local quantum phase transition at ${\ensuremath{\mu}}_{0}{H}^{★}$, making the N\'eel/TLL coexistence a stable configuration until the excitation gap in the N\'eel domains closes at ${\ensuremath{\mu}}_{0}{H}_{\mathrm{p}}$.

Controlled Localized Phase Transition of Selenium for Color-Selective Photodetectors.

Color-selective photodetectors are widely applied in several fields; however, they suffer from complex fabrication and low resolution. Herein, we propose a simple and convenient design to achieve a logical color-selective heterojunction photodetector composed of CdS and Se with a crystalline/amorphous mixed state. The as-deposited amorphous Se top layer in the heterojunction is partly transformed to trigonal crystalline Se by localized in situ phase transformation during annealing. As these two heterojunctions have different photoresponses under the same wavelength, the integrated device can accurately identify red, green, and blue light via logical judgment. Finally, the device exhibits high recognition ability in actual tests. This work provides a potential development of high-resolution color-selective photodetectors for visible light communication and logical photoelectric devices.

Mitigating Jahn–Teller Effects by Fast Electrode Kinetics Inducing Charge Redistribution

AbstractManganese hexacyanomanganates have attracted significant attention as promising cathode materials for sodium‐ion batteries owing to the high theoretical capacity and low cost of Mn resources. Because of the strong Jahn–Teller effect, causing unstable local phase transition and distortion, the redox reactions of the high‐spin state of Mn(III) are considered to be a conundrum. Herein, it is reported that the charge‐redistribution mechanism of low‐spin MnIII + high‐spin MnIII → low‐spin MnII + high‐spin MnIV in manganese hexacyanomanganates can decrease unstable high‐spin MnIII, leading to the mitigation of Jahn–Teller distortion. X‐ray absorption near‐edge spectroscopy and X‐ray photoelectron spectroscopy suggest that the fast reaction rate activates charge redistribution. Moreover, different crystal structures, reported using post‐mortem synchrotron X‐ray diffraction analysis, confirm a large orthorhombic structure, thus verifying the presence of charge redistribution based on the superexchange rule. These results demonstrate that manganese hexacyanomanganate in an aqueous electrolyte can achieve long‐term cyclability, thus paving the way for high‐performance batteries.

Topological delocalization transitions and mobility edges in the nonreciprocal Maryland model

Non-Hermitian effects could trigger spectrum, localization and topological phase transitions in quasiperiodic lattices. We propose a non-Hermitian extension of the Maryland model, which forms a paradigm in the study of localization and quantum chaos by introducing asymmetry to its hopping amplitudes. The resulting nonreciprocal Maryland model is found to possess a real-to-complex spectrum transition at a finite amount of hopping asymmetry, through which it changes from a localized phase to a mobility edge phase. Explicit expressions of the complex energy dispersions, phase boundaries and mobility edges are found. A topological winding number is further introduced to characterize the transition between different phases. Our work introduces a unique type of non-Hermitian quasicrystal, which admits exactly obtainable phase diagrams, mobility edges, and holding no extended phases at finite nonreciprocity in the thermodynamic limit.

MoS2 nanosheet induced destructive alterations in the Escherichia coli bacterial membrane.

Two dimensional molybdenum disulfide (MoS2) nanosheets have recently gained wide recognition for their efficient broad-spectrum antibacterial activity complemented with great biocompatibility and minimal bacterial resistance inducing capabilities. However, despite the numerous investigations, the molecular level interactions at the nano-bio interface responsible for their bactericidal activity remain obscure. Herein, through an atomistic molecular dynamics study, we attempt to seek an in-depth understanding of the atomic level details of the underlying mechanism of their antibacterial action against the Escherichia coli (E. coli) bacterial membrane. Our study reveals a two-step MoS2 nanosheet interaction pathway with the bacterial membrane. The nanosheets spontaneously adhere to the membrane surface and prompt vigorous phospholipid extraction majorly via strong van der Waals interactions with lipid hydrophobic tails. The lipid extraction process originates a significant water intrusion in the bilayer hydrophobic region, signifying the onset of cytoplasmic leakage under realistic conditions. Further, a synergistic effect of lipid-lipid self-interactions and lipid-MoS2 dispersion interactions drags the nanosheet to completely immerse in the bilayer hydrophobic core. The embedded nanosheets induce a layerwise structural rearrangement of the membrane lipids in their vicinity, thus altering the structural and dynamic features of the membrane in a localized manner by (i) increasing the lipid fatty acyl tail ordering and (ii) alleviating the lipid lateral dynamics. The detrimental efficacy of the nanosheets can be magnified by enlarging the nanosheet size or by increasing the nanosheet concentration. Our study concludes that the MoS2 nanosheets can exhibit their antibacterial action through destructive phospholipid extraction as well as by altering the morphology of the membrane by embedding in the membrane core.

From symmetry breaking in the bulk to phase transitions at the surface: a quantum-mechanical exploration of Li6PS5Cl argyrodite superionic conductor.

Lithium superionic conductor electrolytes may enable the safe use of metallic lithium anodes in all-solid-state batteries. The key to a successful application is a high Li conductivity in the electrolyte material, to be achieved through the maintenance of intimate contact with the electrodes and the knowledge of the chemical nature of that contact. In this manuscript, we tackle this issue by a theoretical ab initio approach. Focusing on the Li6PS5Cl, a thiophosphate with high ionic conductivity, we carry on thorough modeling of the surfaces together with the prediction of the thermal and elastic behaviour. Our investigation leads to some new findings: the bulk structure, as reported in the literature, appears to be metastable, with spontaneous symmetry breaking. Moreover, the relevant stoichiometric surfaces identified for stable and metastable crystal structures are not up-down symmetry related and they expose from one side Li2S and LiCl. Surface reconstructions can be interpreted as local phase transitions. We also predict entirely ab initio the morphology of crystallites, charge, and electrostatic potential at surfaces, together with the effect of temperature on structural properties and the elastic behaviour of this material. Such findings may constitute the relevant groundwork for a better understanding of ionic transport in Li-ion conductors at the electrolyte/anode and electrolyte/cathode interfaces.

Graphs, local zeta functions, log-Coulomb gases, and phase transitions at finite temperature

We study a log-gas on a network (a finite, simple graph) confined in a bounded subset of a local field (i.e., R, C, and Qp being the field of p-adic numbers). In this gas, a log-Coulomb interaction between two charged particles occurs only when the sites of the particles are connected by an edge of the network. The partition functions of such gases turn out to be a particular class of multivariate local zeta functions attached to the network and a positive test function, which is determined by the confining potential. The methods and results of the theory of local zeta functions allow us to establish that the partition functions admit meromorphic continuations in the parameter β (the inverse of the absolute temperature). We give conditions on the charge distributions and the confining potential such that the meromorphic continuations of the partition functions have a pole at a positive value βUV, which implies the existence of phase transitions at finite temperature. In the case of p-adic fields, the meromorphic continuations of the partition functions are rational functions in the variable p−β. We give an algorithm for computing such rational functions. For this reason, we can consider the p-adic log-Coulomb gases as exact solvable models. We expect that all these models for different local fields share common properties and that they can be described by a uniform theory.

Photoinduced phase-transition on CuO electrospun nanofibers over the TiO2 photosensitizer for enhancing non-enzymatic glucose-sensing performance

Constructing metal-oxide-based composites with the multiple valence-states and redox-couples has been proved as an effective strategy to enhance the performance of non-enzymatic glucose sensing. Herein, we report a mild and efficient method to prepare multiple-valent copper-based oxide composite nanofibers (CuxO CNFs) through introducing the TiO2 photosensitizer into the CuO NFs for actuating the localized phase transition from tenorite CuO (Cu2+) to cuprite Cu2O (Cu+) upon UV–visible light irradiation. The sensitivity of the obtained TiO2/Cu2O/CuO CNFs for glucose sensing (0–2 mM) could be enhanced by 1.12–1.31 times as compared to that of the TiO2/CuO CNFs. The enhanced sensitivity is dependent on the TiO2-content in the primal TiO2/CuO CNFs. When introducing 5.0 at.% of TiO2 into the CuO nanofibers, the photoinduced phase-transition process could result in the formation of optimal TiO2/Cu2O/CuO CNFs with the component ratio of Cu2O to CuO at 0.46:1. This TiO2/Cu2O/CuO CNFs (2074.7 µA·mM−1·cm−2; 0.25 µM) exhibited nearly 1.32-fold improvement on the sensitivity and 3.0-fold enhancement on the detection limit as compared to the corresponding values of the pure CuO electrospun nanofibers (1581 µA·mM−1·cm−2; 0.75 µM). After the continuous measurements for 14 days, the sensitivity of the optimal sample could maintain 89.4% of its initial value. Notably, its sensitivity could be further recovered to 97.4% of the initial value after UV–visible light irradiation for 1 h.

Non-Hermiticity-induced reentrant localization in a quasiperiodic lattice

In this paper, we demonstrate that the non-Hermiticity can induce reentrant localization in a generalized quasiperiodic lattice. Specifically, by considering a nonreciprocal dimerized lattice with staggered quasiperiodic disorder, we find that the localization transition can appear twice by increasing the disorder strength. We also unravel a multi-complex-real eigenenergy transition, whose transition points coincide with those in the localization phase transitions. Moreover, the impacts of boundary conditions on the localization properties have been clarified. Finally, we study the wavepacket dynamics in different parameter regimes, which offers an experimentally feasible route to detect the reentrant localization.

Phenomenology of Spectral Functions in Disordered Spin Chains at Infinite Temperature.

Studies of disordered spin chains have recently experienced a renewed interest, inspired by the question to which extent the exact numerical calculations comply with the existence of a many-body localization phase transition. For the paradigmatic random field Heisenberg spin chains, many intriguing features were observed when the disorder is considerable compared to the spin interaction strength. Here, we introduce a phenomenological theory that may explain some of those features. The theory is based on the proximity to the noninteracting limit, in which the system is an Anderson insulator. Taking the spin imbalance as an exemplary observable, we demonstrate that the proximity to the local integrals of motion of the Anderson insulator determines the dynamics of the observable at infinite temperature. In finite interacting systems our theory quantitatively describes its integrated spectral function for a wide range of disorders.

Quantum phase transition of light in the dissipative Rabi-Hubbard lattice: A dressed-master-equation perspective

In this work, we investigate the quantum phase transition of light in the dissipative Rabi-Hubbard lattice under the framework of the mean-field theory and quantum dressed master equation. The order parameter of photons in strong qubit-photon coupling regime is derived analytically both at zero and low temperatures. Interestingly, we can locate the localization and delocalization phase transition very well in a wide parameter region. {In particular for the zero-temperature limit, the critical tunneling strength approaches zero generally in the deep-strong qubit-photon coupling regime, regardless of the quantum dissipation. This is contrary to the previous results with the finite minimal critical tunneling strength based on the standard Lindblad master equation. Moreover, a significant improvement of the critical tunneling is also observed at finite temperature, compared with the counterpart under the Lindblad description. We hope these results may deepen the understanding of the phase transition of photons in the Rabi-Hubbard model.

Competition between TCP and crystalline clusters during phase transition of rapidly super-cooled aluminum

Undercooling is usually a necessary condition for the starting of crystallization of molten metal under cooling. What structure hampered crystallization is however still unclear yet. The rapid cooling of pure aluminum has been conducted by a large-scale molecular dynamics simulation at different cooling rates. The evolution of local structures and phase transition mechanism are investigated in terms of the atom energy and the largest standard clusters. It is found that with high local density and low average potential energy (APE), the topologically close-packed (TCP) clusters in liquids increase exponentially with decreasing temperature, and have a number saturation stage whose survival time is related to cooling rate. When the crystalline clusters’ APE is lower than that of the system, crystal nucleation does not begin until the TCP clusters’ APE become higher than the system. Thus the competition between TCP and crystalline clusters results in the saturation of TCP clusters. These findings will provide new understandings on solidification and nucleation theory.

Exploring unconventional features of light dynamics in Aubrey–André–Harper model based quasi-periodic optical lattices

We report an Aubrey–André–Harper (AAH) model based quasi-periodic lossless evanescently coupled waveguide lattice to study the unconventional physics of light localization. We find an exclusively new way to establish localization phase transition in a 1D-AAH lattice by observing the slope variation of the band and accordingly study the modal characteristics to reveal the fact that a higher value of quasi-periodic modulation strength is imperative for observing a signature of fully localized light states having higher eigenenergy. This analytical concept has numerically been implemented in the proposed quasi-periodic lattice to achieve light localization, where we have shown that the supported states not only depend on localization phase transition parameter, but also on the specific location of excitation which is due to quasi-periodic nature of lattice. Furthermore, we have investigated a unique effect of the presence of disorder on light localization phenomenon, where it has been reported that the presence of off-diagonal disorder, which is otherwise detrimental, favours light localization in the proposed structure due to its quasi-periodic nature. The findings indeed have the potential to open up a fertile platform to manipulate light in passive photonic devices.

Antibacterial Mechanism of Multifunctional MXene Nanosheets: Domain Formation and Phase Transition in Lipid Bilayer.

MXenes, two-dimensional metal carbides or nitrides with multifunctional surfaces, are one of the most promising antibacterial nanoscale materials. However, their putative bactericidal mechanism is elusive. To study their bactericidal mechanism, we investigated the interaction between a MXene nanosheet and a model bacterial membrane by molecular dynamics simulations and found that an adsorbed MXene on a membrane surface induced a local phase transition in a domain where the fluidity of the phospholipid in this domain at room temperature was comparable with that of the gel phase. The domain also showed a denser and thinner phospholipid membrane structure than the peripheral phospholipids. By comparing it with our previous experiments of the bactericidal activity of MXenes, we proposed the leakage of intercellular molecules at the phase boundary defects as a possible bactericidal mechanism of MXenes that leads to cell lysis. This study provides a useful model for tailoring new bactericidal nanomaterials.

Real-space origin of topological band gaps, localization, and reentrant phase transitions in gyroscopic metamaterials.

Lattices of interacting gyroscopes naturally support band gaps and topologically protected wave transport along material boundaries. Recently the authors and their collaborators found that amorphous arrangements of such coupled gyroscopes also support nontrivial topological phases. In contrast to periodic systems, for which there is a comprehensive understanding and predictive framework for band gaps and band topology, the theory of spectral gaps and topology for amorphous materials remains less developed. Here we use experiments, numerics, and analytic tools to address the relationship between local interactions and nontrivial topology. We begin with a derivation of the equations of motion within the framework of symplectic mechanics. We then present a general method for predicting whether a gap exists and for approximating the Chern number using only local features of a network, bypassing the costly diagonalization of the system's dynamical matrix. Finally we study how strong disorder interacts with band topology in gyroscopic metamaterials and find that amorphous gyroscopic Chern insulators exhibit similar critical behavior to periodic lattices. Our experiments and simulations additionally reveal a topological Anderson insulation transition, wherein disorder drives a trivial phase into a topological one.

PIV investigation of cavitating flows around circular cylinders with hydrophobic coatings

The treatment of the hydrophobic properties of solid surfaces is considered as a passive method to reduce the drag in water flows (Rothstein, 2010) and to potentially affect the flow separation and vortex shedding (Sooraj et al., 2020). The manufacturing of surfaces with micro- and nano-scale roughness allows to extend the hydrophobicity towards superhydrophobicity with the contact angle close to 180°. In such conditions the solid surface is not wetted completely and the air-water interphase partially remains on the surface texture. This results in so-called flow slip effect. Therefore, a local phase transition during the flow cavitation or gas effervescence in near-wall low-pressure regions may additionally affect the slip effect for hydrophobic surfaces. The present work is focused on the comparison between cavitating and noncavitating flows around circular cylinders with lateral sectors with hydrophobic and non-hydrophobic coatings. The experiments are performed in a water tunnel, which consists of a water outgassing and cooling/heating section, honeycomb, contraction section, test section and diffuser. The water flow is driven by an electric pump, providing a bulk velocity up to 10 m/s in the transparent test section with 1 m length and 80×150 mm2 rectangular cross-section. The facility is equipped with an ultrasonic flowmeter, temperature and pressure sensors. Besides, the static pressure inside the water tunnel can be varied by using a special shaft section. The measurements are performed by using high-repetition and low-repetition PIV systems. The former is used for the analysis of large-scale flow dynamics in the wake region, whereas the latter one is used for high-resolution measurements in near-wall regions by using a long-distance microscope. The Reynolds number based on the bulk velocity of the flow, diameter of the cylinders (D = 26 mm) and kinematic viscosity of the water is varied up to 2×105..

Atomic-resolution Probing of Anion Migration in Perovskites with In-situ (S)TEM

ABSTRACTPerovskites are promising functional materials for their optoelectronic properties and anion migration plays a key role in their functional performance [1-3]. By using in-situ (S)TEM mechanical and electrical testing in conjunction with 4D-STEM [4,5], we directly observed/probed anion migration in perovskites at atomic resolution (see Figure 1). Here, we studied the mechanism for the anion migration in perovskites such as (PbZr)TiO3 and BaTiO3, which is induced under the mechnaicl/electrical loading. To avoid the influence of the electron beam, we carried out the in-situ (S)TEM study at 60kv with low dose. And to avoid the possible strong size effect and the substrate (interface) influence, we prepared free-standing sub-micrometer single-crystalline structures to perform the experiments. Corresponding EDS and EELS examinations were performed to measure the local chemical change with applied stress and electrical currents. Our observations revealed the coexistence of multiple phase structures and hierarchical domain structures, as well as the greatly enhanced anion drifting and diffusion at the charged domain walls (Figure 2) and phase boundaries. The complex interaction between the local domain evolution and phase transition has been discussed. Based on above investigations, a model for anion migration in perovskire under mechanical/electrical loading has been presented.

Localized phase transition of TiO2 thin films induced by sub-bandgap laser irradiation

The ability to define the crystal phase of oxide semiconductors could benefit transparent electronics and catalysis. In this paper, laser-induced localized phase transitions of titanium dioxide (TiO2) thin films are reported. Irradiation was performed with a 532 nm continuous wave laser. Raman spectroscopy and micro-Raman mapping were used to identify the phase transformations. A Raman map of the anatase Eg mode (144 cm−1) and rutile Ag mode (608 cm−1) revealed the formation of crystalline microstructures due to the laser treatment. Laser irradiation under vacuum results in an anatase-to-rutile phase transition. Irradiating the rutile region in air changes the crystal structure back to anatase, despite the thermodynamic stability of rutile. The results suggest that irradiated photons are absorbed by defects, resulting in localized electronic excitation that leads to a mixture of amorphous and crystalline regions. The phase of the crystalline regions depends strongly on the ambient conditions (vacuum versus air).