Exotic Transition Research Articles

Unconventional excited-state dynamics in the concerted benzyl (C7H7) radical self-reaction to anthracene (C14H10)

Polycyclic aromatic hydrocarbons (PAHs) are prevalent in deep space and on Earth as products in combustion processes bearing direct relevance to energy efficiency and environmental remediation. Reactions between hydrocarbon radicals in particular have been invoked as critical molecular mass growth processes toward cyclization leading to these PAHs. However, the mechanism of the formation of PAHs through radical – radical reactions are largely elusive. Here, we report on a combined computational and experimental study of the benzyl (C7H7) radical self-reaction to phenanthrene and anthracene (C14H10) through unconventional, isomer-selective excited state dynamics. Whereas phenanthrene formation is initiated via a barrierless recombination of two benzyl radicals on the singlet ground state surface, formation of anthracene commences through an exotic transition state on the excited state triplet surface through cycloaddition. Our findings challenge conventional wisdom that PAH formation via radical-radical reactions solely operates on electronic ground state surfaces and open up a previously overlooked avenue for a more “rapid” synthesis of aromatic, multi-ringed structures via excited state dynamics in the gas phase.

Valence transition in topological Kondo insulator

We investigate the valence transition in three-dimensional topological Kondo insulator through slave-boson analysis of periodic Anderson model. By including the effect of intra-atomic Coulomb correlation between conduction and local electrons, we find a first-order valence transition from Kondo region to mixed valence state upon ascending of local f- level above a critical , and this valence transition usually occurs very close to or simultaneously with a topological transition. Near the parameter region of zero-temperature valence transition, rise of temperature can generate a thermal valence transition from mixed valence to Kondo region, accompanied by a first-order topological transition. Remarkably, above a critical which is considerably smaller than that generating paramagnetic valence transition, the original continuous antiferromagnetic transition is shifted to first order one, at which a discontinuous valence shift takes place. Upon increasing , the paramagnetic valence transition approaches then converges with the first-order antiferromagnetic transition, leaving a significant valence shift on the magnetic boundary. The continuous antiferromagnetic transition, first-order antiferromagnetic transition, paramagnetic valence transition and topological transitions are all summarized in a global phase diagram. Our proposed exotic transition processes can help to understand the thermal valence variation as well as the valence shift around the pressure-induced magnetic transition in topological Kondo insulator candidates and in other heavy-fermion systems.

Synthesis of 2,2’-Bipyridine Via a Phosphorus Extrusion Reaction

The formation of carbon-carbon bonds has long been a synthetic goal for chemists. This has lead to the development of many novel and highly synthetically useful reactions. One of the most notable reactions to form sp2 C - sp2 C bonds has been the cross-coupling reaction.1 Although these transformations have been very well studied and have recently produced a Nobel Prize, these transformations suffer from two drawbacks.1 First, they often require expensive and exotic transition metal complexes and second, the reaction conditions often require inert or anhydrous conditions.1 Therefore, any reactions which can substitute these reactions that do not require expensive metals or can be run under less sensitive conditions are in great demand. Our research group in conjunction with the Wicker group at Southeastern Louisiana University has developed a phosphonium species that we are exploring as a novel precursor for the formation of 2,2’-bipyridine and its derivatives. 2,2’-bipyridines and their derivatives are of increasing interest to the chemistry community due to their unique properties as ligands.2 However, the majority of syntheses of these species rely on the use of cross coupling reactions to form unsymmetrically substituted bipyridine species. Our protocol, once optimized, will allow us to form these species without the use of cross-coupling reactions.

Human behavioral regularity, fractional Brownian motion, and exotic phase transition

The mix of competition and cooperation (C&C) is ubiquitous in human society, which, however, remains poorly explored due to the lack of a fundamental method. Here, by developing a Janus game for treating C&C between two sides (suppliers and consumers), we show, for the first time, experimental and simulation evidences for human behavioral regularity. This property is proved to be characterized by fractional Brownian motion associated with an exotic transition between periodic and nonperiodic phases. Furthermore, the periodic phase echoes with business cycles, which are well-known in reality but still far from being well understood. Our results imply that the Janus game could be a fundamental method for studying C&C among humans in society, and it provides guidance for predicting human behavioral activity from the perspective of fractional Brownian motion.

Solution of the explosive percolation quest: scaling functions and critical exponents.

Percolation refers to the emergence of a giant connected cluster in a disordered system when the number of connections between nodes exceeds a critical value. The percolation phase transitions were believed to be continuous until recently when, in a new so-called "explosive percolation" problem for a competition-driven process, a discontinuous phase transition was reported. The analysis of evolution equations for this process showed, however, that this transition is actually continuous, though with surprisingly tiny critical exponents. For a wide class of representative models, we develop a strict scaling theory of this exotic transition which provides the full set of scaling functions and critical exponents. This theory indicates the relevant order parameter and susceptibility for the problem and explains the continuous nature of this transition and its unusual properties.

Exotic continuous quantum phase transition betweenZ2topological spin liquid and Néel order

Recent numerical simulations with different techniques have all suggested the existence of a continuous quantum phase transition between the ${\mathbb{Z}}_{2}$ topological spin-liquid phase and a conventional N\'eel order. Motivated by this numerical progress, we propose a candidate theory for such ${\mathbb{Z}}_{2}$-N\'eel transition. We first argue on general grounds that, for a SU(2)-invariant system, this transition can not be interpreted as the condensation of spinons in the ${\mathbb{Z}}_{2}$ spin-liquid phase. Then, we propose that such ${\mathbb{Z}}_{2}$-N\'eel transition is driven by proliferating the bound state of the bosonic spinon and vison excitation of the ${\mathbb{Z}}_{2}$ spin liquid, i.e., the so-called $(e,m)$-type excitation. Universal critical exponents associated with this exotic transition are computed using $1/N$ expansion. This theory predicts that at the ${\mathbb{Z}}_{2}$-N\'eel transition, there is an emergent quasi-long-range power-law correlation of columnar valence bond solid order parameter.

Anomalies in nonstoichiometric uranium dioxide induced by a pseudo phase transition of point defects

A uniform distribution of point defects in an otherwise perfect crystallographic structure usually describes a unique pseudo phase of that state of a non-stoichiometric material. With off-stoichiometric uranium dioxide as a prototype, we show that analogous to a conventional phase transition, these pseudo phases also will transform from one state into another via changing the predominant defect species when external conditions of pressure, temperature, or chemical composition are varied. This exotic transition is numerically observed along shock Hugoniots and isothermal compression curves in UO2 with first-principles calculations. At low temperatures, it leads to anomalies (or quasi-discontinuities) in thermodynamic properties and electronic structures. In particular, the anomaly is pronounced in both shock temperature and the specific heat at constant pressure. With increasing of the temperature, however, it transforms gradually to a smooth cross-over, and becomes less discernible. The underlying physical mechanism and characteristics of this type of transition are encoded in the Gibbs free energy, and are elucidated clearly by analyzing the correlation with the variation of defect populations as a function of pressure and temperature. The opportunities and challenges for a possible experimental observation of this phase change are also discussed.

Exotic transition in the three-dimensional spin-liquid candidate Tb2Ti2O7

The insulator pyrochlore Tb${}_{2}$Ti${}_{2}$O${}_{7}$ is a three-dimensional spin liquid candidate: the strength of the magnetic interaction would normally induce order at $\ensuremath{\approx}$20 K, but according to published muon-spin relaxation and neutron diffraction measurements, it is still in a correlated paramagnetic state down to a temperature of 0.05 K. Our detailed experimental investigation shows that an exotic transition takes place at ${T}_{\mathrm{t}}=0.15\phantom{\rule{0.16em}{0ex}}(2)$ K. It is characterized by an anomaly in the muon precession frequency shift but not in the specific heat, which nevertheless reveals the signature of degrees of freedom at very low energy. It might be associated with a cooperative Jahn-Teller transition. The robustness of the experimental results is discussed.

Evidence of spin density wave of CeOs 4Sb 12

Elastic neutron diffraction measurements were performed on single crystals to study the ground state below the mysterious exotic transition temperature 0.86 K. An antiferromagnetic order with a tiny moment of 0.027 μB per formula is formed as the ground state for CeOs 4Sb 12 below the transition point. Our neutron data gives the evidence of spin density wave state for CeOs 4Sb 12 in this work.

Magnetic anisotropy and electronic transport properties in single-crystalline compounds of EuCu2Ge2 and EuCu2Si2

We present the temperature dependent magnetization, M(T), and the resistivity, ρ(T), of EuCu2Ge2 and EuCu2Si2 in single crystalline form. The EuCu2Ge2, which is synthesized in single-crystalline form, shows two different magnetic transitions in M(T) at TMc=8.5 K and TMab=4 K depending upon the magnetic field orientation. On the other hand, the ρ(T) reveals only one transition at T=4 K as a sudden drop of the ρ(T) without any noticeable change at T=8.5 K. For the EuCu2Si2, M(T) shows a two step antiferromagnetic transition at TN1≈10 K and TN2≈4 K, which is smeared out under H⩾1 T, probably due to field induced spin orientation. The transition at TN2 is also manifested as a decrease in ρ(T) due to loss of magnetic scattering. Interestingly, another exotic transition is found at T=2 K in ρ(T) data. The ρ(T) values rapidly increase by two orders of magnitude from ρ=20 μΩ cm at T=2 K and reach ρ=5 mΩ cm at T=0.5 K, which is similar to a metal–insulator transition.

Erratum: Coexisting static magnetic order and superconductivity in CeCu2Si2 found by nuclear quadrupole resonance

$^{63}\mathrm{Cu}$ nuclear quadrupole resonance (NQR) has been observed in a heavy-fermion system ${\mathrm{CeCu}}_{2}$${\mathrm{Si}}_{2}$. In the superconducting ${\mathrm{CeCu}}_{2}$${\mathrm{Si}}_{2}$ specimens (${\mathit{T}}_{\mathit{c}}$\ensuremath{\simeq}0.6 K), NQR experiments have revealed an onset of a phase transition below \ensuremath{\sim}1 K. Zero- and longitudinal-field muon-spin-relaxation measurements suggest a static magnetic ordering below \ensuremath{\sim}0.8 K, either in the spin-glass or in incommensurate spin-density-wave states. The exotic transition observed in NQR measurements should be closely related to the static magnetic ordering in muon-spin-relaxation measurements. NQR data from a nonsuperconducting ${\mathrm{CeCu}}_{2}$${\mathrm{Si}}_{2}$ sample are consistent with extensive disorder in Cu site occupation. Therefore, the spin-density wave may be depressed in the nonsuperconducting ${\mathrm{CeCu}}_{2}$${\mathrm{Si}}_{2}$. The superconducting transition temperature ${\mathit{T}}_{\mathit{c}}$ can be enhanced by the presence of spin-density waves such that, if the spin-density wave is depressed, ${\mathrm{CeCu}}_{2}$${\mathrm{Si}}_{2}$ will lose its superconducting properties.

Coexisting static magnetic order and superconductivity in CeCu2Si2 found by nuclear quadrupole resonance.

$^{63}\mathrm{Cu}$ nuclear quadrupole resonance (NQR) has been observed in a heavy-fermion system ${\mathrm{CeCu}}_{2}$${\mathrm{Si}}_{2}$. In the superconducting ${\mathrm{CeCu}}_{2}$${\mathrm{Si}}_{2}$ specimens (${\mathit{T}}_{\mathit{c}}$\ensuremath{\simeq}0.6 K), NQR experiments have revealed an onset of a phase transition below \ensuremath{\sim}1 K. Zero- and longitudinal-field muon-spin-relaxation measurements suggest a static magnetic ordering below \ensuremath{\sim}0.8 K, either in the spin-glass or in incommensurate spin-density-wave states. The exotic transition observed in NQR measurements should be closely related to the static magnetic ordering in muon-spin-relaxation measurements. NQR data from a nonsuperconducting ${\mathrm{CeCu}}_{2}$${\mathrm{Si}}_{2}$ sample are consistent with extensive disorder in Cu site occupation. Therefore, the spin-density wave may be depressed in the nonsuperconducting ${\mathrm{CeCu}}_{2}$${\mathrm{Si}}_{2}$. The superconducting transition temperature ${\mathit{T}}_{\mathit{c}}$ can be enhanced by the presence of spin-density waves such that, if the spin-density wave is depressed, ${\mathrm{CeCu}}_{2}$${\mathrm{Si}}_{2}$ will lose its superconducting properties.