Two-step Transition Research Articles

Crystal Field Effects in Polymorphic Compound TbIr2Si2

The tetragonal ternary compound TbIr2Si2 shows polymorphism; the ThCr2Si2-type structure as a low temperature phase (I-phase) and the CaBe2Ge2-type one as a high temperature phase (P-phase) exist. In order to compare with magnetism of the morphs, magnetic measurements were performed on the I- and P-phase single crystals. The magnetic behavior changes drastically depending on the structure. The TbIr2Si2 (I) shows an antiferromagnetic ordering below TN=80K, a strong uniaxial magnetic anisotropy with the easy [001] direction, and isotropic magnetic behavior in the basal plane. The [001] magnetization shows a clear two-step metamagnetic transition: an Ising-like behavior at low temperatures. On the other hand, the TbIr2Si2 (P) has the comparatively low ordering temperature of TN1=11.5K and an additional transition temperature of TN2=5K, and exhibits an easy-plane magnetic anisotropy with the easy [100] direction. A strong magnetic anisotropy appears for high magnetic fields between the directions in the basal plane. Multi-step metamagnetic processes appear. In the both phases, the χ-T behavior suggests the existence of component-separated magnetic transitions. The ab-component of magnetic moments orders at the higher transition temperature TN1 for the P-phase compound, which is contrast to the I-phase behavior; the c-component orders firstly at TN in the I-phase. The crystalline electric field analysis was made, and the CEF effects explain the different of magnetic anisotropy between the both phases.

Theoretical modeling of two-step spin-crossover transitions in FeII dinuclear systems

Calculations using the hybrid meta-GGA TPSSh exchange–correlation functional have been employed to model the spin-transition in dinuclear iron(ii) systems.

Out-of-equilibrium dynamics of photoexcited spin-state concentration waves.

The spin crossover compound [FeIIH2L2-Me][PF6]2 presents a two-step phase transition. In the intermediate phase, a spin state concentration wave (SSCW) appears resulting from a symmetry breaking (cell doubling) associated with a long-range order of alternating high and low spin molecular states. By combining time-resolved optical and X-ray diffraction measurements on a single crystal, we study how such a system responds to femtosecond laser excitation and we follow in real time the erasing and rewriting of the SSCW.

Cosmological phase transitions and their properties in the NMSSM

We study cosmological phase transitions in the Next-to-Minimal Supersymmetric Standard Model (NMSSM) in light of the Higgs discovery. We use an effective field theory approach to calculate the finite temperature effective potential, focusing on regions with significant tree-level contributions to the Higgs mass, a viable neutralino dark matter candidate, 1-2 TeV stops, and with the remaining particle spectrum compatible with current LHC searches and results. The phase transition structure in viable regions of parameter space exhibits a rich phenomenology, potentially giving rise to one- or two-step first-order phase transitions in the singlet and/or $SU(2)$ directions. We compute several parameters pertaining to the bubble wall profile, including the bubble wall width and $\Delta\beta$ (the variation of the ratio in Higgs vacuum expectation values across the wall). These quantities can vary significantly across small regions of parameter space and can be promising for successful electroweak baryogenesis. We estimate the wall velocity microphysically, taking into account the various sources of friction acting on the expanding bubble wall. Ultra-relativistic solutions to the bubble wall equations of motion typically exist when the electroweak phase transition features substantial supercooling. For somewhat weaker transitions, the bubble wall instead tends to be sub-luminal and, in fact, likely sub-sonic, suggesting that successful electroweak baryogenesis may indeed occur in regions of the NMSSM compatible with the Higgs discovery.

Magnetic phase diagram and multiferroicity ofBa3MnNb2O9: A spin-52triangular lattice antiferromagnet with weak easy-axis anisotropy

We have performed magnetic, electric, thermal, and neutron powder diffraction (NPD) experiments as well as density functional theory (DFT) calculations on ${\mathrm{Ba}}_{3}{\mathrm{MnNb}}_{2}{\mathrm{O}}_{9}$. All results suggest that ${\mathrm{Ba}}_{3}{\mathrm{MnNb}}_{2}{\mathrm{O}}_{9}$ is a spin-5/2 triangular lattice antiferromagnet (TLAF) with weak easy-axis anisotropy. At zero field, we observed a narrow two-step transition at ${T}_{\mathrm{N}1}=3.4$ K and ${T}_{\mathrm{N}2}=3.0$ K. The neutron diffraction measurement and the DFT calculation indicate a ${120}^{\ensuremath{\circ}}$ spin structure in the $ab$ plane with out-of-plane canting at low temperatures. With increasing magnetic field, the ${120}^{\ensuremath{\circ}}$ spin structure evolves into up-up-down ($uud$) and oblique phases showing successive magnetic phase transitions, which fits well to the theoretical prediction for the 2D Heisenberg TLAF with classical spins. Multiferroicity is observed when the spins are not collinear but suppressed in the $uud$ and oblique phases.

Diverse photoinduced dynamics in an organic charge-transfer complex having strong electron-phonon interactions.

CONSPECTUS: Phenomena that occur in nonequilibrium states created by photoexcitation differ qualitatively from those that occur at thermal equilibrium, and various physical theories developed for thermal equilibrium states can hardly be applied to such phenomena. Recently it has been realized that understanding phenomena in nonequilibrium states in solids is important for photoenergy usage and ultrafast computing. Consequently, much effort has been devoted to revealing such phenomena by developing various ultrafast observation techniques and theories applicable to nonequilibrium states. This Account describes our recent studies of diverse photoinduced dynamics in a strongly correlated organic solid using various ultrafast techniques. Solids in which the electronic behavior is affected by Coulomb interactions between electrons are designated as strongly correlated materials and are known to exhibit unique physical properties even at thermal equilibrium. Among them, many organic charge-transfer (CT) complexes have low dimensionality and flexibility in addition to strong correlations; thus, their physical properties change sensitively in response to changes in pressure or electric field. Photoexcitation is also expected to drastically change their physical properties and would be useful for ultrafast photoswitching devices. However, in nonequilibrium states, the complicated dynamics due to these characteristics prevents us from understanding and using these materials for photonic devices. The CT complex (EDO-TTF)2PF6 (EDO-TTF = 4,5-ethylenedioxytetrathiafulvalene) exhibits unique photoinduced dynamics due to strong electron-electron and electron-phonon interactions. We have performed detailed studies of the dynamics of this complex using transient electronic spectroscopy at the 10 and 100 fs time scales. These studies include transient vibrational spectroscopy, which is sensitive to the charges and structures of constituent molecules, and transient electron diffraction, which provides direct information on the crystal structure. Photoexcitation of the charge-ordered low-temperature phase of (EDO-TTF)2PF6 creates a new photoinduced phase over 40 fs via the Franck-Condon state, in which electrons and vibrations are coherently and strongly coupled. This new photoinduced phase is assigned to an insulator-like state in which the charge order differs from that of the initial state. In the photoinduced phase, translations of component molecules proceed before the rearrangements of intramolecular conformations. Subsequently, the charge order and structure gradually approach those of the high-temperature phase over 100 ps. This unusual two-step photoinduced phase transition presumably originates from steric effects due to the bent EDO-TTF as well as strong electron-lattice interactions.

Characterization of the N-acetylneuraminic acid synthase (NeuB) from the psychrophilic fish pathogen Moritella viscosa

Moritella viscosa is a Gram-negative psychrophilic bacterium that causes winter ulcer disease in Atlantic salmon and cod. Its genome reveals that it possesses the ability to synthesize sialic acids. Indeed, sialic acid can be isolated from the bacterium and when analyzed using HPLC-MS/MS, the presence of N-acetylneuraminic acid was confirmed. Thus, the N-acetylneuraminic acid synthase NeuB from M. viscosa (MvNeuB) was recombinantly produced and characterized. The optimum pH and temperature for MvNeuB activity are 7.5 and 30°C, respectively. The KM for N-acetylmannosamine and phosphoenolpyruvate is 18±5 and 0.8±0.2mM, respectively. The kcat value (∼225min−1) for both N-acetylmannosamine and phosphoenolpyruvate is the highest turnover number found for an enzyme in this class until the date. A calorimetric study of MvNeuB shows that the enzyme has a two-step transition peak probably reflecting the two domains these proteins consist of. MvNeuB is less stable at higher temperature and has a high catalytic activity at lower temperature compared to mesophilic counterparts. Enzymes from psychrophilic organisms are generally cold adapted meaning they can maintain adequate function near the freezing point of water. Cold adapted enzymes are catalytically more efficient at lower temperature and are more thermo-labile compared to their mesophilic counterparts. MvNeuB is a typical cold adapted enzyme and could be further explored for production of sialic acids and derivates at low temperatures.

Temperature-responsive membrane for hydrophobic interaction based chromatographic separation of proteins in bind-and-elute mode

Butyl-acrylate (BA) containing branched poly-(N-isopropyl acrylamide) or PNIPAM copolymer was grafted on filter paper to prepare composite membrane for carrying out temperature-responsive hydrophobic interaction membrane chromatography (HIMC) of proteins. The copolymer demonstrated two-step phase transition behavior due to the presence of BA and PNIPAM, making the changeover from hydrophilic to hydrophobic state more tunable. The feasibility of using the temperature-responsive membrane was demonstrated by controlling human immunoglobulin G (hIgG) binding and release using temperature change. Higher temperature promoted protein binding and lower temperature favored protein release in this process, which was carried out using lyotropic salt-free mobile phase. The purification of monoclonal antibody (mAb) hIgG1-CD4 from simulated CHO cell culture supernatant was carried out to demonstrate practical application for the novel temperature-responsive membrane.

Doppler-corrected Balmer spectroscopy of Rydberg positronium

The production of long-lived Rydberg positronium (Ps) and correction for Doppler shifts in the excitation laser frequencies are crucial elements of proposed measurements of the gravitational freefall of antimatter and for precision measurements of the optical spectrum of Ps. Using a two-step optical transition via $2P$ levels, we have prepared Ps atoms in Rydberg states up to the term limit. The spectra are corrected for the first-order Doppler shift using measured velocities, and the Balmer transitions are resolved for $15\ensuremath{\le}n\ensuremath{\le}31$. The excitation signal amplitude begins to decrease for $ng50$, consistent with the onset of motional electric field ionization in the 3.5-mT magnetic field at the Ps formation target.

Magnetic phase diagram of theS=12triangular-lattice Heisenberg antiferromagnetBa3CoNb2O9

We report the results of low-temperature thermal and magnetic measurements on ${\mathrm{Ba}}_{3}\mathrm{Co}{\mathrm{Nb}}_{2}{\mathrm{O}}_{9}$ powder, described as a uniform triangular-lattice antiferromagnet (TLAF) with a fictitious spin-1/2. ${\mathrm{Ba}}_{3}\mathrm{Co}{\mathrm{Nb}}_{2}{\mathrm{O}}_{9}$ is found to undergo two-step antiferromagnetic transitions at ${T}_{\mathrm{N}1}=1.39\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ and ${T}_{\mathrm{N}2}=1.13\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. As the magnetic field is increased, both ${T}_{\mathrm{N}1}$ and ${T}_{\mathrm{N}2}$ monotonically decrease. The magnetic field vs temperature phase diagram indicates that the exchange interactions are nearly of the Heisenberg type with weak easy-axis anisotropy and that the exchange interaction between triangular lattices is crucial, in contrast to the case of the quasi-two-dimensional TLAF ${\mathrm{Ba}}_{3}\mathrm{Co}{\mathrm{Sb}}_{2}{\mathrm{O}}_{9}$ [Susuki et al., Phys. Rev. Lett. 110, 267201 (2013)].

Ligand-binding pocket bridges DNA-binding and dimerization domains of the urate-responsive MarR homologue MftR from Burkholderia thailandensis.

Members of the multiple antibiotic resistance regulator (MarR) family often regulate gene activity by responding to a specific ligand. In the absence of ligand, most MarR proteins function as repressors, while ligand binding causes attenuated DNA binding and therefore increased gene expression. Previously, we have shown that urate is a ligand for MftR (major facilitator transport regulator), which is encoded by the soil bacterium Burkholderia thailandensis. We show here that both mftR and the divergently oriented gene mftP encoding a major facilitator transport protein are upregulated in the presence of urate. MftR binds two cognate sites in the mftR-mftP intergenic region with equivalent affinity and sensitivity to urate. Mutagenesis of four conserved residues previously reported to be involved in urate binding to Deinococcus radiodurans HucR and Rhizobium radiobacter PecS significantly reduced protein stability and DNA binding affinity but not ligand binding. These data suggest that residues equivalent to those implicated in ligand binding to HucR and PecS serve structural roles and that MftR relies on distinct residues for ligand binding. MftR exhibits a two-step melting transition suggesting independent unfolding of the dimerization and DNA-binding regions; urate binding or mutations in the predicted ligand-binding sites result in one-step unfolding transitions. We suggest that MftR binds the ligand in a cleft between the DNA-binding lobes and the dimer interface but that the mechanism of ligand-mediated attenuation of DNA binding differs from that proposed for other urate-responsive MarR homologues. Since DNA binding by MftR is attenuated at 37 °C, our data also suggest that MftR responds to both ligand and a thermal upshift by attenuated DNA binding and upregulation of the genes under its control.

Composition-dependent melting behaviour of NaxK55–x core–shell nanoalloys

Molecular dynamics simulations at constant temperature are performed to investigate melting-like transition in Na13K42, Na19K36 and Na26K29 nanoalloys using a second-moment-approximation tight-binding analytic potential to calculate the forces on the constituent atoms. A weighted histogram analysis method is employed to remove non-ergodicity issues due to the complex potential energy surface of these nanoalloys. The heat capacity shows three distinctive steps in melting for Na13K42, while Na26K29 and Na19K36 have two-step and one-step melting transition, respectively. The steepest descent method is used to quench the configurations in a given interval during the simulation and also study the isomerisation processes occurring at different temperatures. Analysing the configuration energies of quenched structures for the entire nanoalloy and the core atoms separately gives more details about the melting mechanism. The Lindemann parameter is also calculated at several temperatures during the simulation which shows a gradual increase for Na13K42 and Na26K29 while a sharp change is observed for Na19K36. These findings are in agreement with the multi-step nature of the phase transition in Na13K42 and Na26K29 and one-step melting of the Na19K36 magic composition.

Phase diagram of two-dimensional hard ellipses.

We report the phase diagram of two-dimensional hard ellipses as obtained from replica exchange Monte Carlo simulations. The replica exchange is implemented by expanding the isobaric ensemble in pressure. The phase diagram shows four regions: isotropic, nematic, plastic, and solid (letting aside the hexatic phase at the isotropic-plastic two-step transition [E. P. Bernard and W. Krauth, Phys. Rev. Lett. 107, 155704 (2011)]). At low anisotropies, the isotropic fluid turns into a plastic phase which in turn yields a solid for increasing pressure (area fraction). Intermediate anisotropies lead to a single first order transition (isotropic-solid). Finally, large anisotropies yield an isotropic-nematic transition at low pressures and a high-pressure nematic-solid transition. We obtain continuous isotropic-nematic transitions. For the transitions involving quasi-long-range positional ordering, i.e., isotropic-plastic, isotropic-solid, and nematic-solid, we observe bimodal probability density functions. This supports first order transition scenarios.

Inclusive charmed-meson production from bottom hadron decays at the LHC

We present predictions for the inclusive productions of D mesons originating from bottom hadrons at the CERN large hadron collider in the general-mass variable-flavour-number scheme at next-to-leading order. We present results using two methods to describe the transition for b → D: a two-step transition b → B → D, based on the b → B fragmentation functions and the spectra for B → D as measured by CLEO and a one-step transition based on the fragmentation functions for b → D. The results of both approaches are compared.

Series of phase transitions and multiferroicity in the quasi-two-dimensional spin-12triangular-lattice antiferromagnetBa3CoNb2O9

We have investigated the magnetic and electric ground states of a quasi-two-dimensional triangular lattice antiferromagnet (TLAF), ${\mathrm{Ba}}_{3}{\mathrm{CoNb}}_{2}{\mathrm{O}}_{9}$, in which the effective spin of Co${}^{2+}$ is 1/2. At zero field, the system undergoes a two-step transition upon cooling at ${T}_{N2}=1.36$ K and ${T}_{N1}=1.10$ K and enters a 120${}^{\ensuremath{\circ}}$ ordered state. By applying magnetic fields, a series of spin states with fractions of the saturation magnetization ${M}_{s}$ are observed. They are spin states with 1/3, 1/2, 2/3 (or $\sqrt{3}/3$) ${M}_{s}$. The ferroelectricity emerges in all spin states, either with collinear or noncollinear spin structure, which makes ${\mathrm{Ba}}_{3}{\mathrm{CoNb}}_{2}{\mathrm{O}}_{9}$ another unique TLAF exhibiting both a series of magnetic phase transitions and multiferroicity. We discuss the role of quantum fluctuations and magnetic anisotropy in contributing more complex phase diagram compared to its sister multiferroic TLAF compound ${\mathrm{Ba}}_{3}$NiNb${}_{2}{\mathrm{O}}_{9}$ [J. Hwang et al., Phys. Rev. Lett. 109, 257205 (2012)].

Formation of an intermediate band in the energy gap of TiO2 by Cu–N-codoping: First principles study and experimental evidence

First principles calculations are performed to study the electronic and optical properties of Cu-doped, N-doped and (Cu+2N)-co-doped anatase TiO2. Strong hybridization between Cu 3d and N 2p orbitals above the valence band leads to the formation of an isolated intermediate band (IB) deep in the band gap of pure TiO2. The new energetic features, predicted by DFT, have been experimentally confirmed using UV–vis–NIR spectroscopy. In particular, the diffuse reflectance spectrum of the co-doped TiO2 sample shows the presence of two edges which confirm the existence of IB in the band gap. This IB in the band gap of TiO2 is responsible for high visible light absorption through a two-step optical transition between valence and conduction bands via the IB. In mono-doped samples, only a reduction of the band gap is observed which is consistent with the first-principles calculations. X-ray photoelectron spectroscopy of mono- and co-doped TiO2 samples establishes the chemical states of several atomic elements and especially clarifies the key presence of O-vacancies leading to a new position of conduction band minima. The presence of broad IB and the absence of dopant energy levels close to the conduction band minimum in (Cu+2N)-co-doped TiO2 qualify it to be an efficient material for photovoltaic conversion, photocatalytic water splitting, and photocatalytic degradation of pollutants.

An analytical model for solute diffusion in multicomponent alloy solidification

An analytical model describing steady-state plane front solidification of multicomponent alloys has been developed, taking into account both the diffusive interaction between the solutes and the local nonequilibrium diffusion effects in the bulk liquid. These effects strongly affect the solute distributions in the liquid phase ahead of the interface. At relatively low interface velocity the strong diffusional interaction between the species leads to a minimum or a maximum of solute concentration ahead of the interface. In ternary systems the model predicts two-step transition to diffusionless solidification.

Crystal plasticity model for BCC iron atomistically informed by kinetics of correlated kinkpair nucleation on screw dislocation

The mobility of dislocation in body-centered cubic (BCC) metals is controlled by the thermally activated nucleation of kinks along the dislocation core. By employing a recent interatomic potential and the Nudged Elastic Band method, we predict the atomistic saddle-point state of 1/2〈111〉 screw dislocation motion in BCC iron that involves the nucleation of correlated kinkpairs and the resulting double superkinks. This unique process leads to a single-humped minimum energy path that governs the one-step activation of a screw dislocation to move into the adjacent {110} Peierls valley, which contrasts with the double-humped energy path and the two-step transition predicted by other interatomic potentials. Based on transition state theory, we use the atomistically computed, stress-dependent kinkpair activation parameters to inform a coarse-grained crystal plasticity flow rule. Our atomistically-informed crystal plasticity model quantitatively predicts the orientation dependent stress–strain behavior of BCC iron single crystals in a manner that is consistent with experimental results. The predicted temperature and strain-rate dependencies of the yield stress agree with experimental results in the 200–350K temperature regime, and are rationalized by the small activation volumes associated with the kinkpair-mediated motion of screw dislocations.

Isomeric chain structures of {[Mn(H₂O)₄]₂Ru₂(CO₃)₄Br₂}n(n-): syntheses, structural diversity and magnetic properties.

The self-assembly of Ru2(CO3)4(3-) paddle-wheel precursors and Mn(2+) ions in aqueous solution yields various carbonate complexes. With appropriate selection of the synthetic conditions, we are able to intentionally tune the composition and structure of Mn-Ru2-carbonate assemblies to form infinite chain structural complexes, e.g., K[{Mn(H2O)4}2Ru2(CO3)4Br2]·H2O (1) and H[{Mn(H2O)4}2Ru2(CO3)4Br2]·6H2O (2). Complexes 1 and 2 are obtained at different temperatures (25 °C for 1 and 5 °C for 2, respectively), and their crystal structures consist of brick-wall stacked chains, in which neighboring Ru2(CO3)4Br2(5-) units are linked by two disubstituted octahedral Mn(H2O)4(2+) in a cis manner, resulting in two isomeric (twisted and zigzag) negative double-chain α- and β-{[Mn(H2O)4]2Ru2(CO3)4Br2}n(n-). The magnetic properties of complexes 1 and 2 were highly characterized. The alternating current (AC) susceptibility analysis of complex 1 reveals a two-step magnetism transition at T1 = 5.0 K and T2 = 2.6 K, respectively. Complex 2 exhibits metamagnetism behavior, with a transition field H(C) = ~1.2 kOe at 2.0 K.

On the two-step phase transition behavior of the Poly(N-isopropylacrylamide) (PNIPAM) brush: different zones with different orders.

Poly(N-isopropylacrylamide) (PNIPAM) brushes integrating poly[oligo(ethylene glycol) methacrylate] (POEGMA) as the core were prepared via successive atom transfer radical polymerization (ATRP). The dynamic thermal phase transition behavior of PNIPAM brushes was studied by means of IR spectroscopy in combination with the perturbation correlation moving window (PCMW) technique and two-dimensional correlation spectroscopy (2Dcos) analysis. Compared with aqueous dispersions of PNIPAM brushes covalently bound to the surface of gold nanoparticles and hydrophobic hyperbranched polyester core, an increase in the double phase transition temperature was observed due to the existence of the POEGMA core, which was favourable to the hydrophilic condition. With PCMW analysis, the phase transition temperature (ca. 36 °C) as well as the transition temperature range (33-41 °C) during the heating process were determined. 2Dcos was employed to discern the sequence order of group motions during heating. It is concluded that the PNIPAM in the inner zone responds earlier than that located in the outer zone.