Showing papers on "Transition temperature published in 2019"

High-temperature superconductivity in monolayer Bi 2 Sr 2 CaCu 2 O 8+δ

Abstract: Although copper oxide high-temperature superconductors constitute a complex and diverse material family, they all share a layered lattice structure. This curious fact prompts the question of whether high-temperature superconductivity can exist in an isolated monolayer of copper oxide, and if so, whether the two-dimensional superconductivity and various related phenomena differ from those of their three-dimensional counterparts. The answers may provide insights into the role of dimensionality in high-temperature superconductivity. Here we develop a fabrication process that obtains intrinsic monolayer crystals of the high-temperature superconductor Bi2Sr2CaCu2O8+δ (Bi-2212; here, a monolayer refers to a half unit cell that contains two CuO2 planes). The highest superconducting transition temperature of the monolayer is as high as that of optimally doped bulk. The lack of dimensionality effect on the transition temperature defies expectations from the Mermin–Wagner theorem, in contrast to the much-reduced transition temperature in conventional two-dimensional superconductors such as NbSe2. The properties of monolayer Bi-2212 become extremely tunable; our survey of superconductivity, the pseudogap, charge order and the Mott state at various doping concentrations reveals that the phases are indistinguishable from those in the bulk. Monolayer Bi-2212 therefore displays all the fundamental physics of high-temperature superconductivity. Our results establish monolayer copper oxides as a platform for studying high-temperature superconductivity and other strongly correlated phenomena in two dimensions. Transport and scanning tunnelling microscopy studies of freestanding monolayers of an unconventional layered copper oxide establish that the superconducting properties of copper oxides are not changed in the 2D limit.

Enhanced Spontaneous Polarization in Ultrathin SnTe Films with Layered Antipolar Structure.

Abstract: 2D SnTe films with a thickness of as little as 2 atomic layers (ALs) have recently been shown to be ferroelectric with in-plane polarization. Remarkably, they exhibit transition temperatures (Tc ) much higher than that of bulk SnTe. Here, combining molecular beam epitaxy, variable temperature scanning tunneling microscopy, and ab initio calculations, the underlying mechanism of the Tc enhancement is unveiled, which relies on the formation of γ-SnTe, a van der Waals orthorhombic phase with antipolar inter-layer coupling in few-AL thick SnTe films. In this phase, 4n - 2 AL (n = 1, 2, 3…) thick films are found to possess finite in-plane polarization (space group Pmn21 ), while 4n AL thick films have zero total polarization (space group Pnma). Above 8 AL, the γ-SnTe phase becomes metastable, and can convert irreversibly to the bulk rock salt phase as the temperature is increased. This finding unambiguously bridges experiments on ultrathin SnTe films with predictions of robust ferroelectricity in GeS-type monochalcogenide monolayers. The observed high transition temperature, together with the strong spin-orbit coupling and van der Waals structure, underlines the potential of atomically thin γ-SnTe films for the development of novel spontaneous polarization-based devices.

Photoinduced possible superconducting state with long-lived disproportionate band filling in FeSe

Abstract: Photoexcitation is a very powerful way to instantaneously drive a material into a novel quantum state without any fabrication, and variable ultrafast techniques have been developed to observe how electron, lattice, and spin degrees of freedom change. One of the most spectacular phenomena is photoinduced superconductivity, and it has been suggested in cuprates that the transition temperature Tc can be enhanced from the original Tc with significant lattice modulations. Here, we show a possibility for another photoinduced high-Tc superconducting state in the iron-based superconductor FeSe. The transient electronic state over the entire Brillouin zone is directly observed by time- and angle-resolved photoemission spectroscopy using extreme ultraviolet pulses obtained from high harmonic generation. Our results of dynamical behaviors from 50 fs to 800 ps consistently support the favourable superconducting state after photoexcitation well above Tc. This finding demonstrates that multiband iron-based superconductors emerge as an alternative candidate for photoinduced superconductors. Light-matter interactions can be used to induce a superconducting-like state in some cuprate superconductors at temperatures above the expected transition temperature. Here, the authors provide time and angle resolved spectroscopic evidence to suggest that photo induced superconductivity can also be achieved in Fe-based superconductors

Probing the Magnetic Ordering of Antiferromagnetic MnPS3 by Raman Spectroscopy.

Abstract: Ferromagnetic/antiferromagnetic materials are of crucial importance in information storage and spintronics devices. Herein we present a comprehensive study of 2D Heisenberg-like antiferromagnetic material MnPS3 by optical contrast and Raman spectroscopy. We propose a criterion of 0.1 × ( N - 1) < (Δ R/ R)max < 0.1 × N ( N ≤ 7) to quickly identify the layer number N by using maximum optical contrast (Δ R/ R)max of few-layer MnPS3 on a SiO2/Si substrate (90 nm thick SiO2). The Raman modes are also identified by polarization Raman spectroscopy. Furthermore, by temperature-dependent Raman measurements, we obtain three phase transition temperatures of MnPS3. The transition temperature at around 80 K corresponds to the transition from the antiferromagnetic to paramagnetic phase; the one at around 120 K is related to its second magnetic phase transition temperature due to two-dimensional spin critical fluctuations; the one at around 55 K is associated with unbinding of spin vortices. Our studies provide more evidence to advance knowledge of the magnetic critical dynamics of 2D ferromagnetic/antiferromagnetic systems.

Generalized way to make temperature tunable conductor–insulator transition liquid metal composites in a diverse range

Abstract: Liquid materials with the ability to transit between conductor and insulator are of great scientific and practical significance. However, achieving the conduction of a liquid metal droplet network is still a challenge. To solve these problems, a generalized method is proposed to fabricate temperature tunable liquid conductor–insulator transition composites, which is achieved firstly via freezing and thawing liquid metal droplets dispersed in dimethicone. Such composites also impart conductivity to the dispersed liquid metal droplet network. To illustrate the typical application of the thus realized materials, a visualized circuit is constructed based on the relationship between the color and the conduction. In addition, reconfigurable and repairable circuits are fabricated depending on the inherent liquid properties of these materials. Furthermore, this universal mechanism has been revealed via the abnormal volume expansion ratio of the liquid metal droplets during the phase change. By calculating the volume change ratio of all metals, we speculate and confirm that gallium-based alloys and bismuth-based alloys can be used to prepare such conductive transition materials. Accordingly, we identify more eligible materials with suitable phase transition points, which significantly extends the transition temperature from insulator to conductor. The liquid material preparation strategy proposed here provides a novel paradigm for achieving the conductor–insulator transition at a wide temperature range and offers promising potential for future applications.

Understanding ferroelectric phase formation in doped HfO2 thin films based on classical nucleation theory

Abstract: Ferroelectricity in doped HfO2 thin films has attracted increasing attention since 2011. The origin of the unexpected ferroelectric property is now accepted as the formation of the noncentrosymmetric Pca21 orthorhombic phase. However, the mechanism for the ferroelectric phase formation is still under debate. In this paper, the classical nucleation theory is revisited to understand the ferroelectric phase formation in doped HfO2 thin films. From nucleation theory, it can be identified that there is an appropriate doping concentration range for the ferroelectric phase formation. The doping concentration should be sufficiently high to suppress the monoclinic phase formation during the crystallization annealing process. Once the stable monoclinic phase is formed, the transformation into the other metastable phases is improbable. For appropriate doping concentrations, a transition to the second most stable tetragonal phase is kinetically enhanced, whereas that to the most stable monoclinic phase is kinetically suppressed at the annealing temperature. During cooling, the transition of the tetragonal phase to the second most stable orthorhombic phase is kinetically enhanced, whereas that to the most stable monoclinic phase is suppressed near room temperature. However, the doping concentration should not be too high. Otherwise, the tetragonal phase formed during the crystallization annealing process cannot be transformed into the ferroelectric orthorhombic phase during cooling. This is because high doping concentration lowers the transition temperature and makes the transition reaction difficult. The appropriate doping concentration range is dependent on the types of dopants, but the general governing process was the kinetic nucleation of the tetragonal phase during crystallization and its transformation into the ferroelectric orthorhombic phase during cooling.

Collective energy gap of preformed Cooper pairs in disordered superconductors

Abstract: In most superconductors, the transition to the superconducting state is driven by the binding of electrons into Cooper pairs1. The condensation of these pairs into a single, phase-coherent, quantum state takes place at the same time as their formation at the transition temperature, Tc. A different scenario occurs in some disordered, amorphous, superconductors: instead of a pairing-driven transition, incoherent Cooper pairs first preform above Tc, causing the opening of a pseudogap, and then, at Tc, condense into the phase-coherent superconducting state2–11. Such a two-step scenario implies the existence of a new energy scale, Δc, driving the collective superconducting transition of the preformed pairs2–6. Here we unveil this energy scale by means of Andreev spectroscopy5,12 in superconducting thin films of amorphous indium oxide. We observe two Andreev conductance peaks at ±Δc that develop only below Tc and for highly disordered films on the verge of the transition to insulator. Our findings demonstrate that amorphous superconducting films provide prototypical disordered quantum systems to explore the collective superfluid transition of preformed Cooper pairs. Experiments show two different energy scales associated with the onset of superconductivity in an amorphous superconductor. This validates the theory of Cooper pairs that condense to a superfluid at lower temperature than they form.

Decomposition-Induced Room-Temperature Magnetism of the Na-Intercalated Layered Ferromagnet Fe3-xGeTe2.

Abstract: The creation of 2D van der Waals materials with ferromagnetism above room temperature is an essential goal toward their practical utilization in spin-based applications. Recent studies suggest that intercalating lithium in exfoliated flakes of the ferromagnet Fe3-xGeTe2 induces a nonzero magnetization at T ∼ 300 K. However, the nanoscale nature of such experiments precludes precise observations of structural and chemical changes upon intercalation. Here, we report the preparation of sodium-intercalated NaFe2.78GeTe2 as well as the investigation into its structure and magnetic properties. Sodium readily intercalates into the van der Waals gap, as revealed by synchrotron X-ray diffraction. Concurrently, the Fe2.78GeTe2 layer becomes heavily charge doped and strained via chemical pressure, yet retains its structure and ferromagnetic transition temperature of ∼140 K. However, we observe the presence of a ferromagnetic amorphous iron germanide impurity over a wide range of synthetic conditions, leading to room-temperature magnetization. This work highlights the importance of strain and electronic control for manipulating the Curie temperature in 2D ferromagnets, while emphasizing the need for careful chemical analysis when exploring phenomena in exfoliated layers.

Superconductivity in High-Entropy-Alloy Telluride AgInSnPbBiTe5

Abstract: A polycrystalline sample of the high-entropy-alloy-type telluride AgInSnPbBiTe5 was synthesized using high-pressure synthesis. Superconductivity with a transition temperature of 2.6 K was observed ...

High-temperature mechanical properties and thermal shock behavior of ternary-layered MAB phases Fe2AlB2

Abstract: The high-temperature flexural and compressive strengths, and thermal shock behavior in water of Fe2AlB2 were investigated from room-temperature (RT) to 1000 °C. The flexural strength varies in a narrow range of 200–250 MPa from room temperature to 1000 °C, without evident plastic deformation. The compressive strength of Fe2AlB2 decreases gradually from 1992 ± 176 MPa at RT to 1482 ± 127 MPa at 600 °C. However, the further increasing temperature results in quicker decrease of the compressive strength to 245 ± 7 MPa at 1000 °C although no plastic deformation is present in the temperature range of 600–800 °C. The brittle-ductile transition temperature (BDTT) is higher under flexure (>1000 °C) than compression (800–900 °C), which is attributed to the higher shear stress under compression. The water-quenched flexural strength exhibits features consistent with the quasi-static propagation of “long initial cracks”, with a critical temperature difference of 200–300 °C. The deduced cracks contribute to the decreasing retained strength. The uniaxial compress during hot pressing results in a weak anisotropy of mechanical properties.

Electrical properties and relaxor phase evolution of Nb-Modified Bi0.5Na0.5TiO3-Bi0.5K0.5TiO3-SrTiO3 lead-free ceramics

Abstract: In this work, the crystalline phase, domain structure, and electrical properties of [Bi0.5(Na0.84K0.16)0.5]0.96Sr0.04Ti1-xNbxO3 (x = 0.010–0.030) ceramics are investigated. Increasing the Nb content induces the phase transition from coexistent rhombohedral and tetragonal phases to a single pseudo-cubic phase, and the lamellar ferroelectric domains evolve into polar nanoregions. Decreased ferroelectric-to-relaxor transition temperature and enhanced frequency dispersion are found in the temperature-dependent dielectric constant and loss, implying a transition from the non-ergodic to ergodic relaxor state. The Nb substitution significantly degrades the long-range ferroelectric order with sharply decreased piezoelectric coefficients from ˜ 140 to ˜ 1 pC/N. However, a large strain of 0.32% at 5 kV/mm (normalized strain of 640 pm/V) is obtained around the critical composition of x = 0.0225. The composition of x = 0.030 shows good temperature insensitivity of the strain response, characterized by 308 pm/V with less than 15% reduction from 25 °C to 125 °C.

Pressure-induced phase transitions and superconductivity in a quasi-1-dimensional topological crystalline insulator α-Bi4Br4.

Abstract: Great progress has been achieved in the research field of topological states of matter during the past decade. Recently, a quasi–1-dimensional bismuth bromide, Bi4Br4, has been predicted to be a rotational symmetry-protected topological crystalline insulator; it would also exhibit more exotic topological properties under pressure. Here, we report a thorough study of phase transitions and superconductivity in a quasihydrostatically pressurized α-Bi4Br4 crystal by performing detailed measurements of electrical resistance, alternating current magnetic susceptibility, and in situ high-pressure single-crystal X-ray diffraction together with first principles calculations. We find a pressure-induced insulator–metal transition between ∼3.0 and 3.8 GPa where valence and conduction bands cross the Fermi level to form a set of small pockets of holes and electrons. With further increase of pressure, 2 superconductive transitions emerge. One shows a sharp resistance drop to 0 near 6.8 K at 3.8 GPa; the transition temperature gradually lowers with increasing pressure and completely vanishes above 12.0 GPa. Another transition sets in around 9.0 K at 5.5 GPa and persists up to the highest pressure of 45.0 GPa studied in this work. Intriguingly, we find that the first superconducting phase might coexist with a nontrivial rotational symmetry-protected topology in the pressure range of ∼3.8 to 4.3 GPa; the second one is associated with a structural phase transition from monoclinic C2/m to triclinic P-1 symmetry.

Strain-engineering Mott-insulating La2CuO4.

Abstract: The transition temperature Tc of unconventional superconductivity is often tunable. For a monolayer of FeSe, for example, the sweet spot is uniquely bound to titanium-oxide substrates. By contrast for La2-xSrxCuO4 thin films, such substrates are sub-optimal and the highest Tc is instead obtained using LaSrAlO4. An outstanding challenge is thus to understand the optimal conditions for superconductivity in thin films: which microscopic parameters drive the change in Tc and how can we tune them? Here we demonstrate, by a combination of x-ray absorption and resonant inelastic x-ray scattering spectroscopy, how the Coulomb and magnetic-exchange interaction of La2CuO4 thin films can be enhanced by compressive strain. Our experiments and theoretical calculations establish that the substrate producing the largest Tc under doping also generates the largest nearest neighbour hopping integral, Coulomb and magnetic-exchange interaction. We hence suggest optimising the parent Mott state as a strategy for enhancing the superconducting transition temperature in cuprates.

Measurement of the hysteretic thermal properties of W-doped and undoped nanocrystalline powders of VO 2

Abstract: Hysteresis loops exhibited by the thermal properties of undoped and 0.8 at.% W-doped nanocrystalline powders of VO2 synthesized by means of the solution combustion method and compacted in pellets, are experimentally measured by photothermal radiometry. It is shown that: (i) the W doping reduces both the hysteresis loops of VO2 and its transition temperature up to 15 °C. (ii) The thermal diffusivity decreases (increases) until (after) the metallic domains become dominant in the VO2 insulating matrix, such that its variation across the metal-insulation transition is enhanced by 23.5% with W-0.8 at.% doping. By contrast, thermal conductivity (thermal effusivity) increases up to 45% (40%) as the metallic phase emerges in the VO2 structure due to the insulator-to-metal transition, and it enhances up to 11% (25%) in the insulator state when the local rutile phase is induced by the tungsten doping. (iii) The characteristic peak of the VO2 specific heat capacity is observed in both heating and cooling processes, such that the phase transition of the 0.8 at.% W-doped sample requires about 24% less thermal energy than the undoped one. (iv) The impact of the W doping on the four above-mentioned thermal properties of VO2 mainly shows up in its insulator phase, as a result of the distortion of the local lattice induced by the electrons of tungsten. W doping at 0.8 at.% thus enhances the VO2 capability to transport heat but diminishes its thermal switching efficiency.

Evolution of Anisotropy, First Order Hyperpolarizability and Electronic Parameters in p-Alkyl-p’-Cynobiphenyl Series of Liquid Crystals: Odd-Even Effect Revisited

Abstract: The homologues of p-alkyl-p’-cynobiphenyl (nCB) liquid crystal series are widely known for their diverse applications in display device industries, due to the low transition temperature. Th...

Phase transition in the extreme: a cubic-to-triclinic symmetry change in dielectrically switchable cyanide perovskites

Abstract: Two novel three-dimensional metal-organic compounds of formula FA2KM(CN)6, where M = Co, Fe and FA = formamidinium (CH(NH2)2+), have been found to crystallize in a perovskite-like architecture. They have been investigated by X-ray diffraction, dielectric and spectroscopic methods as a function of temperature in order to determine the interactions in the crystals and the mechanism of phase transitions occurring at ca. 321 K upon heating. The phase transitions in both crystals are of first order and originate from the ordering of the formamidinium cations below TC. Symmetry reduction (cubic-to-triclinic) seems to be the strongest upon temperature decrease among known metal-organic frameworks. These materials have been proposed as possible switchable dielectrics with a convenient near-room-temperature transition temperature.

Temperature vs. impedance dependencies of neutron-irradiated nanocrystalline silicon carbide (3C-SiC)

Abstract: At the present work, impedance spectroscopy of nanocrystalline silicon carbide (3C-SiC) has been investigated as a function of temperature. Nanocrystalline 3C-SiC particles irradiated by neutrons (2 × 1013 n cm− 2s− 1) up to 20 h. Impedance of neutron-irradiated nanocrystalline 3C-SiC has been studied at the temperature range of 100–400 K. Impedance spectra of the nanocrystal have been comparatively studied before and after neutron irradiation. The natures of conductivity and the metal—semiconductor transition temperature (TMS = 250 K, 325 K, and 370 K at the various frequencies) have been defined from the complex impedance spectroscopy. Polarization of nanocrystalline 3C-SiC increased corresponding to neutron irradiation duration. The mechanism of all effects observed in the experiments has been given in the work.

Stoichiometric evolutions of PH3 under high pressure: implication for high-Tc superconducting hydrides.

Abstract: The superconductivity of hydrides under high pressure has attracted a great deal of attention since the recent observation of the superconducting transition at 203 K in strongly compressed H2S. It has been realized that the stoichiometry of hydrides might change under high pressure, which is crucial in understanding the superconducting mechanism. In this study, PH3 was studied to understand its superconducting transition and stoichiometry under high pressure using Raman, IR and X-ray diffraction measurements, as well as theoretical calculations. PH3 is stable below 11.7 GPa and then it starts to dehydrogenate through two dimerization processes at room temperature and pressures up to 25 GPa. Two resulting phosphorus hydrides, P2H4 and P4H6, were verified experimentally and can be recovered to ambient pressure. Under further compression above 35 GPa, the P4H6 directly decomposed into elemental phosphorus. Low temperature can greatly hinder polymerization/decomposition under high pressure and retains P4H6 up to at least 205 GPa. The superconductivity transition temperature of P4H6 is predicted to be 67 K at 200 GPa, which agrees with the reported result, suggesting that it might be responsible for superconductivity at higher pressures. Our results clearly show that P2H4 and P4H6 are the only stable P-H compounds between PH3 and elemental phosphorus, which is helpful for shedding light on the superconducting mechanism.

Superconductivity and Fermi Surface Anisotropy in Transition Metal Dichalcogenide NbTe 2

Abstract: Transition metal dichalcogenides, featuring layered structures, have aroused enormous interest as a platform for novel physical phenomena and a wide range of potential applications. Among them, special interest has been placed upon WTe2 and MoTe2, which exhibit non-trivial topology both in single layer and bulk as well as pressure induced or enhanced superconductivity. We study another distorted 1T material NbTe2 through systematic electrical transport measurements. Intrinsic superconductivity with onset transition temperature ( ) up to 0.72 K is detected where the upper critical field (Hc) shows unconventional quasi-linear behavior, indicating spin-orbit coupling induced p-wave paring. Furthermore, a general model is proposed to fit the angle-dependent magnetoresistance, which reveals the Fermi surface anisotropy of NbTe2. Finally, non-saturating linear magnetoresistance up to 50 T is observed and attributed to the quantum limit transport.

Theory of Magnetic Domain Phases in Ferromagnetic Superconductors.

Abstract: Recently discovered superconducting P-doped EuFe_{2}As_{2} compounds reveal the situation when the superconducting critical temperature substantially exceeds the ferromagnetic transition temperature. The main mechanism of the interplay between magnetism and superconductivity occurs to be an electromagnetic one, and a short-period magnetic domain structure was observed just below Curie temperature [V. S. Stolyarov et al., Sci. Adv. 4, eaat1061 (2018)SACDAF2375-254810.1126/sciadv.aat1061]. We elaborate a theory of such a transition and demonstrate how the initial sinusoidal magnetic structure gradually transforms into a solitonlike domain one. Further cooling may trigger a first-order transition from the short-period domain Meissner phase to the self-induced ferromagnetic vortex state, and we calculate the parameters of this transition. The size of the domains in the vortex state is basically the same as in the normal ferromagnet, but with the domain walls which should generate the set of vortices perpendicular to the vortices in the domains.

Room-Temperature Ferromagnetism in Sulfur-Doped Graphdiyne Semiconductors

Abstract: The realization of magnetic ordering in a two-dimensional graphitic semiconductor, graphdiyne, has attracted great interest because of its promising potential application in semiconductor devices involving spin. Here, we propose a simple and feasible sulfuration strategy to induce robust ferromagnetic ordering into graphdiyne and realize the coexistence of room-temperature ferromagnetism and semiconductivity in graphdiyne without extrinsic magnetic impurity. The robust residual magnetization of more than 0.047 emu g–1 at room temperature and transition temperature of up to 460 K indicate great potential for application in magnetic storage. The subsequent spin-polarized density functional theory calculation reveals that the intrinsic ferromagnetic ordering originates from the enhanced local magnetic moment and nonlocal electron transfer between carbon atoms and sulfur atoms, which is well confirmed in our electrical measurements. This synthetic strategy could spur studies of two-dimensional magnetic semico...

Thickness effects on the epitaxial strain states and phase transformations in (001)-VO2/TiO2 thin films

Abstract: The thickness-dependent epitaxial strains and phase transformations of (001)-VO2/TiO2 thin films are investigated systematically in a wide thickness range (from 9 to 150 nm). Under a thickness of 18 nm, the tensile in-plane strain is maintained, owing to the good lattice and the symmetry matching between the VO2 thin film and the TiO2 substrate, but the compressive out-of-plane epitaxial strain is gradually relaxed. The epitaxial strains co-stabilize the rutile phase (R phase) in this thickness range. Beyond a thickness of 18 nm, the out-of-plane lattice c exhibits a sudden elongation and reaches the bulk level of 2.8528 A at a thickness of 20 nm, which indicates a structural phase transition (SPT). A further increase of the film thickness results in another new phase (tetragonal-like or T-like) with lattice distortion, which maintains the tetragonal symmetry in the thickness range of 20 to 55 nm. From a thickness of 60 nm, the monoclinic phase (M1 phase) appears, which indicates another SPT from T-like to the monoclinic M1 phase. This SPT is more favorable energetically, owing to the assistance of the strain relaxation in the thicker films. Additionally, the metal-insulator transition temperature positively increases as a function of the out-of-plane strain. This result is consistent with the fact that the tensile strain along the cR axis (V-V atom chain) is conducive for the stabilized insulating phase. This work highlights strain engineering as a crucial avenue for manipulating the phase transformations and properties in the correlated electron system.

A metastable nematic precursor accelerates polyethylene oligomer crystallization as determined by atomistic simulations and self-consistent field theory.

Abstract: Using PYS, TraPPE, OPLS-L, and Flexible-Williams (FW) force field models, atomistic simulations at temperatures ranging from 450 K to 600 K are performed to predict the melt density ρ, the persistence length Np, the nematic coupling constant α, and crystallization dynamics for pentacontane (C50). The coupling constant α arises from packing entropy of rodlike Kuhn segments and increases with increasing ρ and Np. Together with a self-consistent field theory, Np and α are then used to predict the isotropic-to-nematic (IN) transition temperature for polyethylene (PE) oligomers as a function of chain length. The nematic phase is found to be metastable since the IN transition temperature lies below the crystal melting temperatures for C50 in simulations using different force fields. Finally, isothermal simulations of crystallization for PE C50 oligomers and C1000 polymers show that crystal nucleation may be much accelerated by quenching below the IN transition temperature, where chains in the isotropic state first rapidly form nematic ordered domains, within which crystalline order then grows. We also find that the PYS, TraPPE, and FW models overpredict the melting temperature for C50 by around 50 K, while the most flexible OPLS-L model gives a melting temperature within around 10 K of the experimental value. Although giving a more accurate melting temperature, the slow crystallization kinetics of the OPLS-L model may limit its application in direct simulations of PE crystallization.

The Biaxial Strain Dependence of Magnetic Order in Spin Frustrated Mn3NiN Thin Films

Abstract: Multicomponent magnetic phase diagrams are a key property of functional materials for a variety of uses, such as manipulation of magnetization for energy efficient memory, data storage, and cooling applications. Strong spin‐lattice coupling extends this functionality further by allowing electric‐field‐control of magnetization via strain coupling with a piezoelectric. Here this work explores the magnetic phase diagram of piezomagnetic Mn3NiN thin films, with a frustrated noncollinear antiferromagnetic (AFM) structure, as a function of the growth induced biaxial strain. Under compressive strain, the films support a canted AFM state with large coercivity of the transverse anomalous Hall resistivity, ρxy, at low temperature, that transforms at a well‐defined Neel transition temperature (TN) into a soft ferrimagnetic‐like (FIM) state at high temperatures. In stark contrast, under tensile strain, the low temperature canted AFM phase transitions to a state where ρxy is an order of magnitude smaller and therefore consistent with a low magnetization phase. Neutron scattering confirms that the high temperature FIM‐like phase of compressively strained films is magnetically ordered and the transition at TN is first‐order. The results open the field toward future exploration of electric‐field‐driven piezospintronic and thin film caloric cooling applications in both Mn3NiN itself and the broader Mn3AN family.