Well-defined Phase Transition Research Articles

A broad-spectrum gas sensor based on correlated two-dimensional electron gas

Designing a broad-spectrum gas sensor capable of identifying gas components in complex environments, such as mixed atmospheres or extreme temperatures, is a significant concern for various technologies, including energy, geological science, and planetary exploration. The main challenge lies in finding materials that exhibit high chemical stability and wide working temperature range. Materials that amplify signals through non-chemical methods could open up new sensing avenues. Here, we present the discovery of a broad-spectrum gas sensor utilizing correlated two-dimensional electron gas at a delta-doped LaAlO3/SrTiO3 interface with LaFeO3. Our study reveals that a back-gating on this two-dimensional electron gas can induce a non-volatile metal to insulator transition, which consequently can activate the two-dimensional electron gas to sensitively and quantitatively probe very broad gas species, no matter whether they are polar, non-polar, or inert gases. Different gas species cause resistance change at their sublimation or boiling temperature and a well-defined phase transition angle can quantitatively determine their partial pressures. Such unique correlated two-dimensional electron gas sensor is not affected by gas mixtures and maintains a wide operating temperature range. Furthermore, its readout is a simple measurement of electric resistance change, thus providing a very low-cost and high-efficient broad-spectrum sensing technique.

Emergence of mesoscale quantum phase transitions in a ferromagnet.

Mesoscale patterns as observed in, for example, ferromagnets, ferroelectrics, superconductors, monomolecular films or block copolymers1,2 reflect spatial variations of a pertinent order parameter at length scales and timescales that may be described classically. This raises the question for the relevance of mesoscale patterns near zero-temperature phase transitions, also known as quantum phase transitions. Here we report the magnetic susceptibility of LiHoF4-a dipolar Ising ferromagnet-near a well-understood transverse-field quantum critical point (TF-QCP)3,4. When tilting the magnetic field away from the hard axis such that the Ising symmetry is always broken, a line of well-defined phase transitions emerges from the TF-QCP, characteristic of further symmetry breaking, in stark contrast to a crossover expected microscopically. We show that the scenario ofa continuous suppression of ferromagnetic domains, representing a breaking of translation symmetry on mesoscopic scales in an environment of broken magnetic Ising symmetry on microscopic scales, is in excellent qualitative and quantitative agreement with the field and temperature dependence of the susceptibility and the magnetic phase diagram of LiHoF4 under tilted field. This identifies a new type of phase transition that may be referred to as mesoscale quantum criticality, which emanates from the textbook example of a microscopic ferromagnetic TF-QCP. Our results establish the surroundings of quantum phase transitions as a regime of mesoscale pattern formation, in which non-analytical quantum dynamics and materials properties without classical analogue may be expected.

Proton strings and rings in atypical nucleation of ferroelectricity in ice

Ordinary ice has a proton-disordered phase which is kinetically metastable, unable to reach, spontaneously, the ferroelectric (FE) ground state at low temperature where a residual Pauling entropy persists. Upon light doping with KOH at low temperature, the transition to FE ice takes place, but its microscopic mechanism still needs clarification. We introduce a lattice model based on dipolar interactions plus a competing, frustrating term that enforces the ice rule (IR). In the absence of IR-breaking defects, standard Monte Carlo (MC) simulation leaves this ice model stuck in a state of disordered proton ring configurations with the correct Pauling entropy. A replica exchange accelerated MC sampling strategy succeeds, without open path moves, interfaces, or off-lattice configurations, in equilibrating this defect-free ice, reaching its low-temperature FE order through a well-defined first-order phase transition. When proton vacancies mimicking the KOH impurities are planted into the IR-conserving lattice, they enable standard MC simulation to work, revealing the kinetics of evolution of ice from proton disorder to partial FE order below the transition temperature. Replacing ordinary nucleation, each impurity opens up a proton ring generating a linear string, an actual FE hydrogen bond wire that expands with time. Reminiscent of those described for spin ice, these impurity-induced strings are proposed to exist in doped water ice too, where IRs are even stronger. The emerging mechanism yields a dependence of the long-time FE order fraction upon dopant concentration, and upon quenching temperature, that compares favorably with that known in real-life KOH doped ice.

Observation of two-dimensional Anderson localisation of ultracold atoms

Anderson localisation —the inhibition of wave propagation in disordered media— is a surprising interference phenomenon which is particularly intriguing in two-dimensional (2D) systems. While an ideal, non-interacting 2D system of infinite size is always localised, the localisation length-scale may be too large to be unambiguously observed in an experiment. In this sense, 2D is a marginal dimension between one-dimension, where all states are strongly localised, and three-dimensions, where a well-defined phase transition between localisation and delocalisation exists as the energy is increased. Here, we report the results of an experiment measuring the 2D transport of ultracold atoms between two reservoirs, which are connected by a channel containing pointlike disorder. The design overcomes many of the technical challenges that have hampered observation of localisation in previous works. We experimentally observe exponential localisation in a 2D ultracold atom system.

Electrowetting on Langmuir-Blodgett films from epicuticular wax

Langmuir and Langmuir-Blodgett (LB) films were fabricated from epicuticular wax, that had been extracted from Vitis vinifera. For Langmuir films, the influence of subphase temperature on their structure was examined, whereas for LB films, the surface morphology at various temperatures and the effect of the layer number on electrowetting were investigated. Isotherms of surface pressure revealed no well-defined phase transition or collapse on Langmuir films at 22 °C, whereas Brewster angle microscopy showed that even before starting the compression procedure, there are platelets of epicuticular wax floating on the subphase. Those platelets give rise to the Langmuir films of epicuticular wax during compression. In contrast, for higher temperatures (40 °C and 50 °C), a well-defined phase transition was found – without epicuticular wax platelets, with a behavior close to those found in typical fatty acids. For LB films, atomic force microscopy images indicated a decreasing of roughness as a function of the subphase temperature as a result of greater organization of the epicuticular wax film. A significant modulation of about 20 ° (under applied voltage range of 0 to 100 V) was demonstrated of the electrowetting on LB films, which did not depend on the LB layer number. Our results suggest that epicuticular wax LB films have the potential for being applied in electrowetting-based devices. Furthermore, the Langmuir-Blodgett technique is feasible to fabricate films with various compositions focusing on the electrowetting.

Correlation between the tolerance factor and phase transition in A4–xBxNi3O10 ( A and B=La,Pr,and Nd;x=0,1,2,and 3 )

We have synthesized a series of the Ruddlesden-Popper nickelate solid solution Ln4-xLn'xNi3O10 (Ln and Ln' = La, Pr and Nd; x = 0, 1, 2 and 3) via the citrate precursor method at different reacting atmospheres. Both the electronic-transport and magnetization measurements on these samples show well-defined phase transitions at temperatures between 135 K and 165 K. These transition temperatures, the room-temperature resistivities, as well as the changes of the Pauli-paramagnetic susceptibilities at the respective phase transitions, strongly correlate with the Goldschmidt tolerance factor t, irrespective of the combination of the magnetic rare-earth ions with unmagnetic La3+. We conclude that these changes of the electronic properties are mostly related to the distortion of the NiO6 octahedra at the phase transition which is strongly correlated with the tolerance factor t, but are rather insensitive to the magnetism of the rare-earth ions Ln3+ and Ln'3+.

Experimental observation of ferroelectricity in ferrimagnet MnCr2S4

Ferrimagnetic spinel compounds AB2X4 (A and B are the magnetic transition elements) are considered to be promising candidates for multiferroics with large magnetization and polarization. In this work, we synthesize polycrystalline spinel MnCr2S4 and characterize the magnetic and ferroelectric properties. Two well-defined ferroelectric phase transitions are demonstrated. The first one occurs at the Cr3+ ferromagnetic phase transition temperature of TC = 65 K, and the other takes place at the Yafet–Kittel (YK) magnetic phase transition temperature of TYK ≈ 5 K. It is suggested that ferroelectricity in the YK phase is driven by the noncollinear triangular YK spin orders and can be greatly tuned by an external magnetic field. Between TYK and TC ranges, another opposite electric polarization sublattice appears, which is enhanced by the external magnetic field just near TC, revealing that this opposite electric polarization is likely related to magnetostriction and the magnetic field can enhance the lattice distortions near TC. Thus, this work paves the way for exploiting ferrimagnetic multiferroicity although more studies are needed to clarify the ferroelectricity mechanism in the Cr3+ ferromagnetic phase.

Pressure-induced structural phase transitions in the multiferroic four-layer Aurivillius ceramic Bi5FeTi3O15

The structural stability of the multiferroic four-layer Aurivillius ceramic Bi5FeTi3O15 (BFTO) under hydrostatic pressure was investigated in situ by synchrotron X-ray powder diffraction (SXRPD) and Raman spectroscopy. Measurements performed up to 15 GPa allowed the identification of two well-defined structural phase transitions. The orthorhombic phase (at ambient conditions) transforms into a tetragonal one around 3.2 GPa and a new orthorhombic phase rises around 7.5 GPa. Both phase transitions are evidenced by the pressure dependence of the lattice parameters and vibrational mode frequencies. The analysis of the induced strain indicates that the transitions are of second-order and first-order, respectively. Subtle changes in the Raman spectra and powder patterns suggest the onset of a new phase around 10.5 GPa.

Enhanced localization and protection of topological edge states due to geometric frustration

Topologically nontrivial phases are linked to the appearance of localized modes in the boundaries of an open insulator. On the other hand, the existence of geometric frustration gives rise to degenerate localized bulk states. The interplay of these two phenomena may, in principle, result in an enhanced protection/localization of edge states. In this paper, we study a two-dimensional Lieb-based topological insulator with staggered hopping parameters and diagonal open boundary conditions. This system belongs to the ${C}_{2v}$ class and sustains one-dimensional (1D) boundary modes except at the topological transition point, where the ${C}_{4v}$ symmetry allows for the existence of localized (0D) corner states. Our analysis reveals that, while a large set of boundary states have a common well-defined topological phase transition, other edge states reflect a topological nontrivial phase for any finite value of the hopping parameters, are completely localized (compact) due to destructive interference, and evolve into corner states when reaching the higher symmetry point. We consider the robustness of these compact edge states with respect to time-dependent perturbations and indicate ways that these states could be prepared and measured in experiments with ultracold atoms.

Percolation phase transition by removal of k^{2}-mers from fully occupied lattices.

Numerical simulations and finite-size scaling analysis have been carried out to study the problem of inverse site percolation by the removal of k×k square tiles (k^{2}-mers) from square lattices. The process starts with an initial configuration, where all lattice sites are occupied and, obviously, the opposite sides of the lattice are connected by occupied sites. Then the system is diluted by removing k^{2}-mers of occupied sites from the lattice following a random sequential adsorption mechanism. The process finishes when the jamming state is reached and no more objects can be removed due to the absence of occupied sites clusters of appropriate size and shape. The central idea of this paper is based on finding the maximum concentration of occupied sites, p_{c,k}, for which the connectivity disappears. This particular value of the concentration is called the inverse percolation threshold and determines a well-defined geometrical phase transition in the system. The results obtained for p_{c,k} show that the inverse percolation threshold is a decreasing function of k in the range 1≤k≤4. For k≥5, all jammed configurations are percolating states, and consequently, there is no nonpercolating phase. In other words, the lattice remains connected even when the highest allowed concentration of removed sites is reached. The jamming exponent ν_{j} was measured, being ν_{j}=1 regardless of the size k considered. In addition, the accurate determination of the critical exponents ν, β, and γ reveals that the percolation phase transition involved in the system, which occurs for k varying between one and four, has the same universality class as the standard percolation problem.

Inverse percolation by removing straight rigid rods from square lattices in the presence of impurities

Numerical simulations and finite-size scaling analysis have been carried out to study the problem of inverse percolation by removing straight rigid rods from square lattices contaminated with non-conducting impurities. The presence of impurities provides a more realistic approach to the deposited monolayer, which usually presents inhomogeneities due to the irregular arrangement of surface and bulk atoms, the presence of various chemical species, etc. The process starts with an initial configuration, where all lattice sites are occupied by an impurity (with a concentration ) or a conducting particle (with a concentration ). Then, the system is diluted by randomly removing linear k-mers (linear clusters of k consecutive conducting particles) from the lattice. The impurities remain fixed in its position and cannot be removed. The central idea of this paper is based on finding the maximum concentration of conducting sites (minimum concentration of empty sites) for which the connectivity disappears. This particular value of the concentration is called inverse percolation threshold, and determines a well-defined geometrical phase transition in the system. On the other hand, the inverse jamming coverage is the coverage of the limit state, in which no more objects can be removed from the lattice due to the absence of linear clusters of nearest-neighbour sites of appropriate size. The dependence of percolation and jamming thresholds on the concentration of defects was investigated for different values of k, ranging from 1 to 120. The obtained results show that the behaviour of the system is significantly affected by the presence of impurities. In addition, the nature of the jamming and percolation transitions was studied. In the first case, the corresponding spatial correlation length critical exponent was measured, being . This value coincides with previous calculations of this exponent for the standard random sequential adsorption of linear k-mers on square lattices. Critical exponents were also calculated for the percolation phase transition, showing that the universality class corresponding to ordinary percolation is preserved regardless the values of k and considered.

The learning curve of robot-assisted laparoscopic pyeloplasty in children: a multi-outcome approach

Few studies have evaluated the learning curve (LC) for robot-assisted laparoscopic pyeloplasty (RALP) for ureteropelvic junction obstruction in children. It was attempted to assess the LC of this procedure using a multi-outcome approach, accounting for patient complexity. Data on the first series of children undergoing RALP between November 2007 and December 2017at the study institution were prospectively collected. Patient complexity factors and peri-operative data including operative time (OT) were retrospectively analyzed. The LC was analyzed using cumulative sum (CUSUM) methodology for OT and a composite parameter (combination of 3 parameters: OT adjusted for patient complexity factors (AOT), complications, and surgical success). Two surgeons without any experience in robotic surgeryperformed 42 consecutive RALP in 41 patients. Median age at surgery was 5 years (6 months-15 years), and mean OT was 200±72.8min. Cumulative sum chart demonstrated biphasic LC for OT and multiphasic LC for composite factor. Based on the CUSUM analysis for composite outcome, the LC for RALP could be divided into three different phases: phase 1, the learning period (1-12 cases); phase 2, the consolidation period (13-22 cases); and phase 3, representing the period of increased competence (23-39cases). Interphase comparison showed a significant reduction in OT, length of stay, and postoperative pain (P=0.0001; P=0.0076; P=0.039, respectively) CONCLUSION: Numerous distinctly shaped LCs depending on the outcome measures and well-defined learning phase transition points were demonstrated. Patient complexity factors were accounted for, which can influence surgical outcomes. Because there is no perfect indicator of proficiency, a multi-outcome approach was adopted to provide a comprehensive view of the learning process for RALP. More than 41 cases are needed to achieve mastery.

Magnetic Bistability in Naphtho-1,3,2-dithiazolyl: Solid State Interconversion of a Thiazyl π-Radical and Its N–N σ-Bonded Dimer

Crystals of the heterocyclic radical naphtho-1,3,2-dithiazolyl NDTA display magnetic bistability with a well-defined hysteretic phase transition at Tc↓ = 128(2) K and Tc↑ = 188(2) K. The magnetic signature arises from a radical/dimer interconversion involving one of the two independent π-radicals in the P1̅ unit cell. Variable temperature X-ray crystallography has established that while all the radicals in HT-NDTA serve as paramagnetic ( S = 1/2) centers, half of the radicals in LT-NDTA form closed-shell N-N σ-bonded dimers ( S = 0) and half retain their S = 1/2 spin state. The wide window of bistability (60 K) may be attributed to the large structural changes that accompany the phase transition.

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.

Emergent excitation in the paramagnetic phase of geometrically frustrated GdMn2O5

We have measured dielectric constant (ε) as well as magnetic susceptibility (χ) of GdMn2O5 in order to elucidate magnetoelectric phase transitions slightly above and below Néel temperature (TN). Our measurements clearly show that above TN there are short range magnetic correlations arising from geometrically frustrated Mn moments, which fluctuate with respect to field and frequency. These well-defined magnetoelectric phase transitions, together with other transitions below TN, have been observed and discussed in the light of the χ. Magnetoelectric phase diagram is drawn which corresponds well to polarization flip phenomenon as induced by 90° rotation of Gd moments in low temperature phase.

Triggering One-Dimensional Phase Transition with Defects at the Graphene Zigzag Edge.

One well-known argument about a one-dimensional (1D) system is that 1D phase transition at finite temperature cannot exist even though this concept depends on conditions such as range of interaction, external fields, and periodicity. Therefore, 1D systems usually have random fluctuations with intrinsic domain walls arising that naturally bring disorder during transition. Herein, we introduce a real 1D system in which artificially created defects can induce a well-defined 1D phase transition. The dynamics of structural reconstructions at graphene zigzag edges are examined by in situ aberration-corrected transmission electron microscopy. Combined with an in-depth analysis by ab initio simulations and quantum chemical molecular dynamics, the complete defect induced 1D phase transition dynamics at graphene zigzag edge is clearly demonstrated and understood on the atomic scale. Further, following this phase transition scheme, graphene nanoribbons (GNR) with different edge symmetries can be fabricated and, according to our electronic structure and quantum transport calculations, a metal-insulator-semiconductor transition for ultrathin GNRs is proposed.

Superlubric-pinned transition in sliding incommensurate colloidal monolayers

Two-dimensional (2D) crystalline colloidal monolayers sliding over a laser-induced optical lattice providing the periodic ``corrugation'' potential recently emerged as a new tool for the study of friction between ideal crystal surfaces. Here, we focus in particular on static friction, the minimal sliding force necessary to depin one lattice from the other. If the colloid and the optical lattices are mutually commensurate, the colloid sliding is always pinned by static friction; however, when they are incommensurate, the presence or absence of pinning can be expected to depend upon the system parameters, like in one-dimensional (1D) systems. If a 2D analogy to the mathematically established Aubry transition of one-dimensional systems were to hold, an increasing periodic corrugation strength ${U}_{0}$ should turn an initially free-sliding, superlubric colloid into a pinned state, where the static friction force goes from zero to finite through a well-defined dynamical phase transition. We address this problem by the simulated sliding of a realistic model 2D colloidal lattice, confirming the existence of a clear and sharp superlubric-pinned transition for increasing corrugation strength. Unlike the 1D Aubry transition, which is continuous, the 2D transition exhibits a definite first-order character, with a jump of static friction. With no change of symmetry, the transition entails a structural character, with a sudden increase of the colloid-colloid interaction energy, accompanied by a compensating downward jump of the colloid-corrugation energy. The transition value for the corrugation amplitude ${U}_{0}$ depends upon the misalignment angle $\ensuremath{\theta}$ between the optical and the colloidal lattices, superlubricity surviving until larger corrugations for angles away from the energetically favored orientation, which is itself generally slightly misaligned, as shown in recent work. The observability of the superlubric-pinned colloid transition is proposed and discussed.

Inverse percolation by removing straight rigid rods from square lattices

.Numerical simulations and finite-size scaling analysis have been carried out to study the problem of inverse percolation by removing straight rigid rods from square lattices. The process starts with an initial configuration, where all lattice sites are occupied and, obviously, the opposite sides of the lattice are connected by nearest-neighbor occupied sites. Then, the system is diluted by randomly removing straight rigid rods of length k (k-mers) from the surface. The central idea of this paper is based on finding the maximum concentration of occupied sites (minimum concentration of holes) for which connectivity disappears. This particular value of concentration is called the inverse percolation threshold, and determines a well-defined geometrical phase transition in the system. The results, obtained for k ranging from 2 to 256, showed a nonmonotonic size k dependence for the critical concentration, which rapidly decreases for small particle sizes (). Then, it grows for k = 4, 5 and 6, goes through a maximum at k = 7, and finally decreases again and asymptotically converges towards a definite value for large values of k. Percolating and non-percolating phases extend to infinity in the space of the parameter k and, consequently, the model presents percolation transition in all ranges of said value. This finding contrasts with the results obtained in literature for a complementary problem, where straight rigid k-mers are randomly and irreversibly deposited on a square lattice, and the percolation transition only exists for values of k ranging between 1 and approximately . The breaking of particle-hole symmetry, a distinctive characteristic of the k-mers statistics, is the source of this asymmetric behavior. Finally, the accurate determination of critical exponents reveals that the model belongs to the same universality class as random percolation regardless of the value of k considered.

Enhanced room temperature ferromagnetism in antiferromagnetic NiO nanoparticles

We report systematic investigations of structural, vibrational, resonance and magnetic properties of nanoscale NiO powders prepared by ball milling process under different milling speeds for 30 hours of milling. Structural properties revealed that both pure NiO and as-milled NiO powders exhibit face centered cubic structure, but average crystallite size decreases to around 11 nm along with significant increase in strain with increasing milling speed. Vibrational properties show the enhancement in the intensity of one-phonon longitudinal optical (LO) band and disappearance of two-magnon band due to size reduction. In addition, two-phonon LO band exhibits red shift due to size-induced phonon confinement effect and surface relaxation. Pure NiO powder exhibit antiferromagnetic nature, which transforms into induced ferromagnetic after size reduction. The average magnetization at room temperature increases with decreasing the crystallite size and a maximum moment of 0.016 μB/f.u. at 12 kOe applied field and coercivity of 170 Oe were obtained for 30 hours milled NiO powders at 600 rotation per minute milling speed. The change in the magnetic properties is also supported by the vibrational properties. Thermomagnetization measurements at high temperature reveal a well-defined magnetic phase transition at high temperature (TC) around 780 K due to induced ferromagnetic phase. Electron paramagnetic resonance (EPR) studies reveal a good agreement between the EPR results and magnetic properties. The observed results are described on the basis of crystallite size variation, defect density, large strain, oxidation/reduction of Ni and interaction between uncompensated surfaces and particle core with lattice expansion. The obtained results suggest that nanoscale NiO powders with high TC and moderate magnetic moment at room temperature with cubic structure would be useful to expedite for spintronic devices.

Live cell plasma membranes do not exhibit a miscibility phase transition over a wide range of temperatures.

Lipid/cholesterol mixtures derived from cell membranes as well as their synthetic reconstitutions exhibit well-defined miscibility phase transitions and critical phenomena near physiological temperatures. This suggests that lipid/cholesterol-mediated phase separation plays a role in the organization of live cell membranes. However, macroscopic lipid-phase separation is not generally observed in cell membranes, and the degree to which properties of isolated lipid mixtures are preserved in the cell membrane remain unknown. A fundamental property of phase transitions is that the variation of tagged particle diffusion with temperature exhibits an abrupt change as the system passes through the transition, even when the two phases are distributed in a nanometer-scale emulsion. We support this using a variety of Monte Carlo and atomistic simulations on model lipid membrane systems. However, temperature-dependent fluorescence correlation spectroscopy of labeled lipids and membrane-anchored proteins in live cell membranes shows a consistently smooth increase in the diffusion coefficient as a function of temperature. We find no evidence of a discrete miscibility phase transition throughout a wide range of temperatures: 14-37 °C. This contrasts the behavior of giant plasma membrane vesicles (GPMVs) blebbed from the same cells, which do exhibit phase transitions and macroscopic phase separation. Fluorescence lifetime analysis of a DiI probe in both cases reveals a significant environmental difference between the live cell and the GPMV. Taken together, these data suggest the live cell membrane may avoid the miscibility phase transition inherent to its lipid constituents by actively regulating physical parameters, such as tension, in the membrane.