Unusual Transition Research Articles

Suppressing Electron-Phonon Coupling through Laser-Induced Phase Transition.

Using first-principle calculations, we introduced a strategy of laser-induced phase transition that suppress electron-phonon couplings in crystal lattice. We explained unusual irreversible phase transitions in previous experiments on MoTe2 and NaYF4 crystals. Laser irradiations produced local heats in 2H-MoTe2 and Hex NaYF4, driving atom reorganizations toward new lattices. The reorganization with effective electron-phonon couplings continues with spontaneously generated heats, whereas a 1T'-MoTe2 and a metastable cubic NaYF4 phases were kept because of suppressed vibrational relaxations. Long time laser treatments create phases with weak electron-phonon couplings. Such irreversible transitions guarantee complete conversions, opening a new door to selective material modifications.

Extinction phase transitions in a model of ecological and evolutionary dynamics

We study the non-equilibrium phase transition between survival and extinction of spatially extended biological populations using an agent-based model. We especially focus on the effects of global temporal fluctuations of the environmental conditions, i.e., temporal disorder. Using large-scale Monte-Carlo simulations of up to $3\times 10^7$ organisms and $10^5$ generations, we find the extinction transition in time-independent environments to be in the well-known directed percolation universality class. In contrast, temporal disorder leads to a highly unusual extinction transition characterized by logarithmically slow population decay and enormous fluctuations even for large populations. The simulations provide strong evidence for this transition to be of exotic infinite-noise type, as recently predicted by a renormalization group theory. The transition is accompanied by temporal Griffiths phases featuring a power-law dependence of the life time on the population size.

Unconventional phases of attractive Fermi gases in synthetic Hall ribbons

A novel way to produce quantum Hall ribbons in a cold atomic system is to use M hyperfine states of atoms in a 1D optical lattice to mimic an additional "synthetic dimension". A notable aspect here is that the SU(M) symmetric interaction between atoms manifests as "infinite ranged" along the synthetic dimension. We study the many body physics of fermions with attractive interactions in this system. We use a combination of analytical field theoretic and numerical density matrix renormalization group (DMRG) methods to reveal the rich ground state phase diagram of the system, including novel phases such as squished baryon fluids. Remarkably, changing the parameters entails unusual crossovers and transitions, e. g., we show that increasing the magnetic field (that produces the Hall effect) may convert a "ferrometallic" state at low fields to a "squished baryon superfluid" (with algebraic pairing correlations) at high fields. We also show that this system provides a unique opportunity to study quantum phase separation in a multiflavor ultracold fermionic system.

The Effect of Water upon Deep Eutectic Solvent Nanostructure: An Unusual Transition from Ionic Mixture to Aqueous Solution.

The nanostructure of a series of choline chloride/urea/water deep eutectic solvent mixtures was characterized across a wide hydration range by neutron total scattering and empirical potential structure refinement (EPSR). As the structure is significantly altered, even at low hydration levels, reporting the DES water content is important. However, the DES nanostructure is retained to a remarkably high level of water (ca. 42 wt % H2O) because of solvophobic sequestration of water into nanostructured domains around cholinium cations. At 51 wt %/83 mol % H2O, this segregation becomes unfavorable, and the DES structure is disrupted; instead, water–water and DES–water interactions dominate. At and above this hydration level, the DES–water mixture is best described as an aqueous solution of DES components.

The Effect of Water upon Deep Eutectic Solvent Nanostructure: An Unusual Transition from Ionic Mixture to Aqueous Solution

AbstractThe nanostructure of a series of choline chloride/urea/water deep eutectic solvent mixtures was characterized across a wide hydration range by neutron total scattering and empirical potential structure refinement (EPSR). As the structure is significantly altered, even at low hydration levels, reporting the DES water content is important. However, the DES nanostructure is retained to a remarkably high level of water (ca. 42 wt % H2O) because of solvophobic sequestration of water into nanostructured domains around cholinium cations. At 51 wt %/83 mol % H2O, this segregation becomes unfavorable, and the DES structure is disrupted; instead, water–water and DES–water interactions dominate. At and above this hydration level, the DES–water mixture is best described as an aqueous solution of DES components.

Fabrication of MoSe2 nanoribbons via an unusual morphological phase transition

Transition metal dichalcogenides (TMDs) are a family of van der Waals layered materials exhibiting unique electronic, optical, magnetic and transport properties. Their technological potentials hinge critically on the ability to achieve controlled fabrication of desirable nanostructures, such as nanoribbons and nanodots. To date, nanodots/nanoislands have been regularly observed, while controlled fabrication of TMD nanoribbons remains challenging. Here we report a bottom-up fabrication of MoSe2 nanoribbons using molecular beam epitaxy, via an unexpected temperature-induced morphological phase transition from the nanodot to nanoribbon regime. Such nanoribbons are of zigzag nature, characterized by distinct chemical and electronic properties along the edges. The phase space for nanoribbon growth is narrowly defined by proper Se:Mo ratios, as corroborated experimentally using different Se fluxes, and supported theoretically using first-principles calculations that establish the crucial role of the morphological reconstruction of the bare Mo-terminated edge. The growth mechanism revealed should be applicable to other TMD systems.

One-dimensional irreversible aggregation with dynamics of a totally asymmetric simple exclusion process.

We define and study one-dimensional model of irreversible aggregation of particles obeying a discrete-time kinetics, which is a special limit of the generalized Totally Asymmetric Simple Exclusion Process (gTASEP) on open chains. The model allows for clusters of particles to translate as a whole entity one site to the right with the same probability as single particles do. A particle and a cluster, as well as two clusters, irreversibly aggregate whenever they become nearest neighbors. Nonequilibrium stationary phases appear under the balance of injection and ejection of particles. By extensive Monte Carlo simulations it is established that the phase diagram in the plane of the injection-ejection probabilities consists of three stationary phases: a multiparticle (MP) one, a completely filled (CF) phase, and a "mixed" (MP+CF) one. The transitions between these phases are: an unusual transition between MP and CF with jump discontinuity in both the bulk density and the current, a conventional first-order transition with a jump in the bulk density between MP and MP+CF, and a continuous clustering-type transition from MP to CF, which takes place throughout the MP+CF phase between them. By the data collapse method a finite-size scaling function for the current and bulk density is obtained near the unusual phase transition line. A diverging correlation length, associated with that transition, is identified and interpreted as the size of the largest cluster. The model allows for a future extension to account for possible cluster fragmentation.

Unusual magnetic transition near metal-insulator transition and paramagnetic anomaly in VO2

Experiments for vanadium dioxide show a magnetic transition characterized by a sharp variation in susceptibility associated with the metal-insulator transition at the temperature Tc. The sample is shown to be of Pauli paramagnetism above Tc but of anomalous paramagnetism below Tc. Considering co-contributions of Pauli paramagnetism of residual V4+ ions and Curie-like paramagnetism of V-V dimers, we propose a phenomenological expression for the temperature-dependent susceptibility below Tc, which yields an excellent agreement with the experimental data. The unusual temperature dependence of Curie-like susceptibility below Tc is explained to be due to the fact that every dimer is formed by spin paring at an angle θ close to 180° and hence has an effective spin expressed as Seff=cos(θ/2) which decreases with lowering temperature. The observed metal-insulator and magnetic transitions are argued to be due to a transition from the high-temperature Pauli paramagnetic state of V4+ ions to the low-temperature dimerized state.

Nanoscale Stabilization of Nonequilibrium Rock Salt BiAgSeS: Colloidal Synthesis and Temperature Driven Unusual Phase Transition

Stabilization of nonequilibrium phases of inorganic solids is an art in synthetic chemistry. Herein, we demonstrate the entrapping of nonequilibrium rock salt phase of BiAgSeS in the nanocrystalline form at room temperature via kinetic colloidal synthesis. High symmetry rock salt nanocrystals undergo an unusual irreversible phase transition to a lower symmetry thermodynamic trigonal structure upon thermal treatment. To get fundamental insights into such unusual finding, we have performed temperature dependent synchrotron powder X-ray diffraction, positron annihilation spectroscopy, and density functional theory based structural energy and phonon modes investigations. Kinetically trapped strained rock salt BiAgSeS nanocrystals irreversibly transform to thermodynamically stable trigonal structure upon heating via Ag–Bi exchange, which is alleviated by a significant increase in the Ag vacancy.

Direct S0→T Excitation of a Conjugated Polymer Repeat Unit: Unusual Spin-Forbidden Transitions Probed by Time-Resolved Electron Paramagnetic Resonance Spectroscopy.

A detailed understanding of the electronic structure of semiconducting polymers and their building blocks is essential to develop efficient materials for organic electronics. (Time-resolved) electron paramagnetic resonance (EPR) is particularly suited to address these questions, allowing one to directly detect paramagnetic states and to reveal their spin-multiplicity, besides its clearly superior resolution compared to optical methods. We present here evidence for a direct S0→T optical excitation of distinct triplet states in the repeat unit of a conjugated polymer used in organic photovoltaics. These states differ in their electronic structure from those populated via intersystem crossing from excited singlet states. This is an additional and so far unconsidered route to triplet states with potentially high impact on efficiency of organic electronic devices.

Grand-canonical solution of semiflexible self-avoiding trails on the Bethe lattice.

We consider a model of semiflexible interacting self-avoiding trails (sISATs) on a lattice, where the walks are constrained to visit each lattice edge at most once. Such models have been studied as an alternative to the self-attracting self-avoiding walks (SASAWs) to investigate the collapse transition of polymers, with the attractive interactions being on site as opposed to nearest-neighbor interactions in SASAWs. The grand-canonical version of the sISAT model is solved on a four-coordinated Bethe lattice, and four phases appear: non-polymerized (NP), regular polymerized (P), dense polymerized (DP), and anisotropic nematic (AN), the last one present in the phase diagram only for sufficiently stiff chains. The last two phases are dense, in the sense that all lattice sites are visited once in the AN phase and twice in the DP phase. In general, critical NP-P and DP-P transition surfaces meet with a NP-DP coexistence surface at a line of bicritical points. The region in which the AN phase is stable is limited by a discontinuous critical transition to the P phase, and we study this somewhat unusual transition in some detail. In the limit of rods, where the chains are totally rigid, the P phase is absent and the three coexistence lines (NP-AN, AN-DP, and NP-DP) meet at a triple point, which is the endpoint of the bicritical line.

Uniaxial pressure effects on the unusual antiferromagnetic transition in the Kondo semiconductor CeOs2Al10

We have studied the relation between the unusual antiferromagnetic (AFM) ordering and anisotropic c-f hybridization in CeOs2Al10 by measuring the magnetic susceptibility χ under uniaxial pressures P up to 0.45 GPa. For P // B // a and P // B // c, the AFM ordering temperature TN = 28.5 K hardly changes with increasing P, while the value of χ decreases and the maximal temperature of χ(T) shifts to higher temperatures. These findings suggest that the intensity of c-f hybridization is enhanced along the pressurized a and c axes. On the other hand, for P // b, TN increases up to 30 K at 0.15 GPa, while the value of χ remains unchanged. This observation for P // b suggests that the net intensity of c-f hybridization decreases, while the c-f hybridization along the b-axis hardly changes. We discuss these results in terms of the atomic displacements of Os, Al(3) and Al(4) under uniaxial pressures.

Supramolecular "Big Bang" in a Single-Ionic Surfactant/Water System Driven by Electrostatic Repulsion: From Vesicles to Micelles.

In aqueous solution, dimethyldi-n-octylammonium chloride, [DiC8][Cl], spontaneously forms dimers at low concentrations (1-10 mM) to decrease the strength of the hydrophobic-water contact. Dimers represent ideal building blocks for the abrupt edification of vesicles at 10 mM. These vesicles are fully characterized by dynamic and static light scattering, self-diffusion nuclear magnetic resonance, and freeze-fracture transmission electron microscopy. An increase in concentration leads to electrostatic repulsion between vesicles that explode into small micelles at 30 mM. These transitions are detected by means of surface tension, conductivity, and solubility of hydrophobic solutes as well as by isothermal titration microcalorimetry. These unusual supramolecular transitions emerge from the surfactant chemical structure that combines two contradictory features: (i) the double-chain structure tending to form low planar aggregates with low water solubility and (ii) the relatively short chains giving high hydrophilicity. The well-balanced hydrophilic-hydrophobic character of [DiC8][Cl] is then believed to be at the origin of the unusual supramolecular sequence offering new opportunities for drug delivery systems.

Unusual physical behaviour and polymorphic phase transitions in crystalline bicyclic anhydrides

Bicyclic anhydrides function as versatile building blocks for polymeric materials, but molecular crystals of these monomeric units demonstrate interesting physicochemical properties. Here, we investigate two pairs of crystalline bicyclic anhydrides, where each pair contains a saturated and unsaturated component. One pair (RTM3/RTM4) is built around bicyclo[2.2.2]octane-2-endo,3-endo-dicarboxylic acid anhydride (BA) and another pair (BCA1/BCA2) consists of a BA-core with alkyl substituents. Differential scanning calorimetry (DSC), powder X-ray diffraction (PXRD) and reflective melting point determination shows that both components in the RTM3/RTM4 pair undergo reversible, polymorphic temperature-induced phase transitions to the same space group while the BCA1/BCA2 pair does not. The melting point (MP) behavior of the BCA1/BCA2 pair is interesting because it contradicts typical alkane/alkene MP trends, as the un-saturated component (BCA1) has the higher MP. Lattice enthalpies calculated via dispersion inclusive density functional theory (DFT+vdW) help to elucidate this unusual MP behavior. In addition, Hirshfeld surface analysis of both pairs demonstrates that variations in the intermolecular close contacts may have an effect as to whether the saturated or unsaturated component will exhibit the higher MP. Finally, BCA1 is found to possess unique liquid–solid phase transition properties for a single component organic molecular crystal (OMC), as it is supercoolable over a ∼60 K range.

Novel penta-graphene nanotubes: strain-induced structural and semiconductor-metal transitions.

Research into novel one-dimensional (1D) materials and associated structural transitions is of significant scientific interest. It is widely accepted that a 1D system with a short-range interaction cannot have 1D phase transition at finite temperature. Herein, we propose a series of new stable carbon nanotubes by rolling up penta-graphene sheets, which exhibit fascinating well-defined 1D phase transitions triggered by axial strain. Our first-principles calculations show that such penta-graphene nanotubes (PGNTs) are dynamically stable by phonon calculations, but transform from a tri-layer structure to a highly defective single-walled nanotube at low temperature in molecular dynamics simulations. We show that moderate compressive strains can drive structural transitions of (4,4), (5,5), and (6,6) PGNTs, during which the distances of neighboring carbon dimers in the inner shell have a sudden drop, corresponding to dimer-dimer nonbonding to bonding transitions. After such transition, the tubes become much more thermally stable and undergo semiconductor-metal transitions under increasing strain. The band gaps of PGNTs are not sensitive to chirality whereas they can be tuned effectively from visible to short-wavelength infrared by appropriate strain, making them appealing materials for flexible nano-optoelectronics. These findings provide useful insight into unusual phase transitions in low-dimensional systems.

Enhancement of multiferroic properties and unusual magnetic phase transition in Eu doped bismuth ferrite nanoparticles

Improvement of the multiferroic properties of bismuth ferrite is essential for its practical application and commercialization in the growing field of spintronics.

A frustrated phase driven by competition among layer structures.

Using our designed ternary system consisting of a semiperfluorinated smectic liquid crystal and two nematic liquid crystals, we investigated their phase transition behaviour using polarizing optical microscopy, differential scanning calorimetry and X-ray diffraction. A mixture containing 20 mol% of the semiperfluorinated compound was found to exhibit the following unusual phase sequence: isotropic liquid - isotropic liquid + smectic A phase - nematic phase - unidentified frustrated phase (X) - modulated phase (Y) - modulated phase (Z). The frustrated phase showed a fan-shaped texture characteristic of a smectic phase, but no peak corresponding to the layer spacing was detected. No difference was found in the shearing stress between the high-temperature N phase and the frustrated phase, but electro-optical switching observed in the N phase was not detected in the frustrated phase. We discuss how the molecules organize the unusual phase transition behaviour.

De Novo Development Of A Cortisol-Producing Adrenocortical Carcinoma In A Patient With Primary Adrenal Insufficiency

ABSTRACT Objective: This case report describes the unusual transition from primary adrenal insufficiency to adrenocorticotropic hormone (ACTH)-independent Cushing syndrome (CS) associated with adrenocortical carcinoma (ACC). Methods: We report the clinical, laboratory, imaging, and pathological findings from a 57-year-old man with a history of primary adrenal insufficiency (Addison disease) who subsequently developed endogenous ACTH-independent CS associated with ACC. Results: Eight years after the patient was diagnosed with Addison disease, he developed features of CS, which led to cessation of the steroid replacement therapy he had been taking for his adrenal insufficiency. Hormonal tests, performed while the patient was not receiving corticosteroids, revealed elevated 24-hour urinary free cortisol (458 μg/24 h) and suppressed plasma ACTH. Computed tomography of the abdomen revealed a 10-cm heterogeneous right adrenal mass that did not exist on imaging studies performed 8 years earlier. The patient unde...

Room Temperature Magnetic Switchability Assisted by Hysteretic Valence Tautomerism in a Layered Two-Dimensional Manganese-Radical Coordination Framework

The manganese-nitronyl-nitroxide two-dimensional coordination polymer {[Mn2(NITIm)3]ClO4}n (1) (NITImH = 2-(2-imidazolyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-3-oxide-1-oxyl) undergoes an unusual hysteretic thermo-induced valence tautomeric transition near room temperature, during which the manganese(II) ions are oxidized to manganese(III) and two of the three deprotonated radicals (NITIm-) are reduced to their diamagnetic aminoxyl form (denoted NITRed2-). Upon cooling, the high-temperature species {[MnII2(NITIm)3]ClO4}n (1HT) turns into the low-temperature species {[MnIII2(NITRed)2(NITIm)]ClO4}n (1LT) around 274 K, while on heating the process is reversed at about 287 K. This valence tautomeric phenomenon is supported by temperature-dependent magnetic susceptibility measurements, differential scanning calorimetry (DSC), crystal structure determination, UV-vis absorption, X-ray absorption (XAS), and emission (XES) and electron paramagnetic resonance (EPR) spectroscopies in the solid state.

Tunable magnetism and structural transformations in mixed light- and heavy-lanthanide dialuminides

Rare-earth intermetallics play a critical yet often obscure role in numerous technological applications, including sensors, actuators, permanent magnets, and rechargeable batteries; therefore, understanding their basic science is of utmost importance. Here we report structural behaviors, specific heat, and magnetism of $\mathrm{P}{\mathrm{r}}_{1\text{--}x}\mathrm{E}{\mathrm{r}}_{x}\mathrm{A}{\mathrm{l}}_{2}$ studied by means of temperature-dependent x-ray powder diffraction, heat capacity, and magnetization measurements, in addition to first-principles calculations. Although the cubic lattice of $\mathrm{PrA}{\mathrm{l}}_{2}$ distorts tetragonally at the Curie temperature ${T}_{C}$, the distortion is rhombohedral in $\mathrm{ErA}{\mathrm{l}}_{2}$, creating a potential for instability in the pseudobinary $\mathrm{PrA}{\mathrm{l}}_{2}\text{\ensuremath{-}}\mathrm{ErA}{\mathrm{l}}_{2}$ system. When $0.05\ensuremath{\le}x\ensuremath{\le}0.5$, materials show complex magnetization behaviors, including metamagnetic transitions and Griffith-like phase. Unique among other mixed-lanthanide dialuminides, the substitution of Er for Pr in $\mathrm{P}{\mathrm{r}}_{1\text{--}x}\mathrm{E}{\mathrm{r}}_{x}\mathrm{A}{\mathrm{l}}_{2}$ results in unexpected ferrimagnetic behavior, and the ferrimagnetic interactions become strongest around $x=0.25$, where the compound shows unusual metamagnetic like transitions observed only in the odd-numbered quadrants of the full magnetic field cycles. The electronic structure calculations, including exchange interactions and crystal field splitting, magnetic moments, anisotropic $4f$ energy density, and magnetic surface potentials rationalize the interesting physics observed experimentally.