Bulk Transition Research Articles

Superconductivity at Pd/Bi2Se3 Interfaces Due to Self-Formed PdBiSe Interlayers

Understanding the physical and chemical processes at the interfaces of metals and topological insulators is crucial for the development of the next generation of topological quantum devices. Here, we report the discovery of robust superconductivity in Pd/Bi2Se3 bilayers fabricated by sputtering Pd on the surface of Bi2Se3. Through transmission electron microscopy measurements, we identify that the observed interfacial superconductivity originates from the diffusion of Pd into Bi2Se3. In the diffusion region, Pd chemically reacts with Bi2Se3 and forms a layer of PdBiSe, a known superconductor with a bulk transition temperature of 1.5 K. Our work provides a method for the introduction of superconductivity into Bi2Se3, laying the foundation for the development of sophisticated Bi2Se3-based topological devices.

Stabilizing 4.6 V LiCoO2 via Surface‐to‐Bulk Titanium Modification

AbstractElevating the charging cut‐off voltage is an effective strategy to increase the energy density of LiCoO2. However, unstable interfacial structures and unfavorable phase transitions in bulk are inevitably triggered during deep de‐lithiation at high voltage. Herein, an integrated surface‐to‐bulk Ti‐modification strategy is applied to LiCoO2, enabling uniform Li2TiO3 coating on the surface and gradient Ti‐doping toward the structural bulk. The resultant Ti‐modified LiCoO2 (T‐LCO) electrode can be stably cycled up to 4.6 V, providing a high‐rate capability of 137 mAh g−1 at 5C and a long‐life stability with 80.5% capacity retention after 400 cycles at 1C, far outperforming the unmodified LiCoO2 electrode with only 50.7% capacity retention. In situ X‐ray diffraction characterization and density functional theory calculation reveal that the synergistic modification of T‐LCO enhances Li+ diffusion, facilitates the construction of high‐quality cathode/electrolyte interphase, reduces the phase transition from O3 to H1‐3 and Co3d/O2p band overlap, and restrains layer‐to‐spinel phase distortion, thus improving structural stability at 4.6 V. This work presents a “two birds one step” strategy to enhance the cycling stability and achievable capacity of high‐voltage LiCoO2 for developing high energy density lithium‐ion batteries.

Microscopic Thermo‐Mechanical Properties and Phase Transition of Bulk Ice‐Ih

Microscopic Thermo‐Mechanical Properties and Phase Transition of Bulk Ice‐Ih

Unraveling the Role of the Stoichiometry of Atomic Layer Deposited Nickel Cobalt Oxides on the Oxygen Evolution Reaction.

Nickel cobalt oxides (NCOs) are promising, non-precious oxygen evolution reaction (OER) electrocatalysts. However, the stoichiometry-dependent electrochemical behavior makes it crucial to understand the structure-OER relationship. In this work, NCO thin film model systems are prepared using atomic layer deposition. In-depth film characterization shows the phase transition from Ni-rich rock-salt films to Co-rich spinel films. Electrochemical analysis in 1m KOH reveals a synergistic effect between Co and Ni with optimal performance for the 30 at.% Co film after 500 CV cycles. Electrochemical activation correlates with film composition, specifically increasing activation is observed for more Ni-rich films as its bulk transitions to the active (oxy)hydroxide phase. In parallel to this transition, the electrochemical surface area (ECSA) increases up to a factor 8. Using an original approach, the changes in ECSA are decoupled from intrinsic OER activity, leading to the conclusion that 70 at.% Co spinel phase NCO films are intrinsically the most active. The studies point to a chemical composition dependent OER mechanism: Co-rich spinel films show instantly high activities, while the more sustainable Ni-rich rock-salt films require extended activation to increase the ECSA and OER performance. The results highlight the added value of working with model systems to disclose structure-performance mechanisms.

Electron beam irradiation-induced atomically thin domes of two-dimensional materials: Graphene and MoS[formula omitted]

The local strain-induced indirect-to-direct band gap transition in bulk transition metal dichalcogenides (TMDCs) holds great potential for optoelectronic applications. The formation of domes on the topmost layer of a multilayered flake through hydrogen ion H+ irradiation is an effective technique for tuning the band gap locally via molecular hydrogen (H2) confinement. In this work, we present an alternative robust, simpler, and faster technique for concurrently forming and visualizing dome-like structures of two-dimensional (2D) materials through low-energy electron beam irradiation. We investigated the growth mechanism and dynamics of the domes under exposure to electron beams at different energy levels. The simultaneous production and imaging of atomically thin domes of 2D materials has several practical and fundamental applications, such as mass storage and transport, single photon emission, nano/micro-electromechanical sensors, strain engineering, etc.

Plasmon-assisted boosting of strong self-hybrid exciton–anapole coupling in bulk transition metal dichalcogenides nanoresonators

Intrinsic strong coupling between excitons and anapole mode in self-hybridizing transition metal dichalcogenide (TMD) nanostructures has recently attracted much attention for their capability of improving exciton–anapole interactions as a consequence of the nonradiative feature of anapole mode and the excellent mode overlap in the self-hybridizing geometry. Here, we propose a strategy for further enhancing exciton–anapole coupling. By embedding Au dimers as plasmonic components in slotted bulk WS2 nanodisks, a plasmon-induced electric field strength enhancement over nearly 140 times is achieved compared to that of the bulk WS2 nanodisk, boosting the strong exciton–anapole hybridization with greatly enhanced Rabi splitting up to 532 meV. Importantly, we demonstrate not only the effect of the plasmon mode on the initial Rabi phase of the anapole mode but also the great reduction of the ultrafast energy transfer time by the constructed full-quantum model. The plasmon-assisted hybrid system proposed in this paper provides a promising approach for enhancing strong light–matter interactions and may pave the way for the development of high-performance TMD-based photonic devices.

Competition for resources in an exclusion model with biased lane-changing mechanism.

The motivation for the proposed work is drawn from the attachment-detachment observed in biological and physical transport processes that entail finite resources. We investigate the influence of limited particle availability on particle dynamics within two parallel totally asymmetric simple exclusion lanes, with one lane incorporating only particle detachment and the other considering particle attachment. We establish a theoretical framework by employing vertical mean-field theory in conjunction with singular perturbation technique. The analytical findings are supported by numerical and stochastic validation using a finite-difference scheme and the Gillespie algorithm. By utilizing these approaches, we scrutinize various stationary properties, including particle densities, phase boundaries, and particle currents for both lanes. Our analysis reveals that the complexity of the phase diagram exhibits a nonmonotonic trend in the number of stationary phases as the particle count increases. Each phase diagram is constructed with respect to the intrinsic boundary parameters, illustrating both bulk and surface transitions occurring within the lanes. The interplay between finite resources and coupling mechanisms gives rise to two phases involving upward shock in one of the lanes, while two phases exhibit synchronized downward shock in both lanes. Finally, we delve into shock dynamics to comprehend critical phase transitions occurring in the system.

New insights into lamellar and twinning transformations in TiAl/Nb composites with core-shell structure during high temperature annealing

Lamellar and twinning transformations play a crucial role in stabilizing the ordered phases in TiAl based composites for property improvement. In this work, we have systematically investigated the lamellar and twinning transformations in TiAl/Nb composites with core-shell structure and proposed novel insights on γ → α2 and γ → γT processes from an atomistic perspective. The results show that high temperature annealing facilitates the formation of lamellar microstructure including γ + α2 and γT twin lamellae in the vicinity of core/shell interface, due to rapid diffusion of principal atoms followed by atomic shuffling. Specifically, plenty of lenticular γT can nucleate in B2 regime by accidental misalignment of atoms on a 110B2 plane adhering to Kurdjumov-Sachs relationship, and then grow even into α2 lath driven by the release of internal stress via Shockley partial slips on the 111γ plane. The formation of γT mediated by 9R involves γ → 9R and γ ← 9R → γT transformations. Among them, a full period of atomic stacking …ABC|BCA|CAB… (LPSO) can be achieved through three Shockley partials being sequentially activated on the C, A and B atomic layers in the stacking of …ABC|ABC|ABC… (L10) with the total shift of 3δA. Then 9R will change synchronously to γ and γT once the migrating Shockley partials react with Frank partials to yield a full dislocation, i.e., δA + Dδ→DA and δA + δC + δD→BD, which can also be considered as detwinning and twinning behaviors, respectively. Additionally, the transition of bulk γ to α2 or γT lamellae is closely associated with the stacking faults, i.e. whether the activated atomic layer is located at the boundary or periphery of the updated stacking fault region.

Different spectral features near the energy bandgaps of normal and inverse heterostructures of In<sub>0.52</sub>Al<sub>0.48</sub>As/InP

Photoconductivity (PC) spectroscopy and photoluminescence (PL) spectroscopy were used to characterize two heterostructure configurations of InAlAs/InP grown by molecular beam epitaxy (MBE) on the InP (100) substrate. The sample A is the type called normal heterostructure, which has an In<sub>0.52</sub>Al<sub>0.48</sub>As layer grown on InP, while sample B is called the inverse type formed by an InP cap layer on In<sub>0.52</sub>Al<sub>0.48</sub>As. The front excitation was employed in both PC experiment and PL experiment and the measurements were conducted at 77 K. The PC spectrum of sample A shows an abnormal step-like drop when the photon energy is larger than the energy band gap of In<sub>0.52</sub>Al<sub>0.48</sub>As. The phenomenon implies that the conductance of sample is a multilayer effect including the contribution of interfacial two-dimensional electron gas (2DEG). Moreover, a conductance peak is observed at 916 nm below the bandgap of InP. Accordingly, an intense luminescent peak at the wavelength manifests in the PL spectrum. The origin of the 916 nm peak is attributed to the recombination of 2DEG electrons with the valence band holes excited near the interface. However, the spectral feature of the above energy does not exist in both PC and PL spectra of sample B. This difference may be explained by the different interface electronic structures of the inverse interface. For the latter case, considering that a graded variation in In-As-P composition is related to the inverse interface of InP/InAlAs, the band bending effect should be weak. In such a case, the bound energy of 2DEG in the interface potential well is raised closer to the conductance band of the bulk. Consequently, the recombination energy of 2DEG at the inverse interface with the holes in the valence band is close to the band-to-band transition of InP bulk and the luminescence is difficult to be distinguished from that of bulk InP. The work also demonstrates that the comparative study with both PC technique and PL technique is helpful to provide a full insight into the interface electronic property.

A comparative study between Monte Carlo entropic sampling method and local mean field investigations of thermal properties of spin-crossover nanoparticles based on Ising-like model

The thermally induced transitions between low-spin (LS) and high-spin (HS) configurations of spin-crossover (SCO) nanoparticles are simulated, focusing on the effects of localized surface and bulk interactions on the average magnetization of 2D square lattices. The thermal behaviors and hysteresis cycles are investigated within the framework of the Ising model Hamiltonian and are conducted following two approaches: local mean field approximation (LMFA) and Monte Carlo entropic sampling (MCES) techniques. The results obtained by these two methods are compared for the two square lattice sizes, 6 $$ \times $$ 6 and 7 $$ \times $$ 7. Thus, when the bulk-surface interaction term is set to zero, the two approaches lead to identical values of the surface and bulk transition temperatures separated by a long intermediate plateau in both cases. Although hysteresis curves exhibit a similar shape, LMFA shows slightly larger widths $$ \Delta T $$ than MCES. On increasing bulk-surface interaction term, the two methods lead to different shifts in equilibrium temperature values for both bulk and surface components, respectively, to lower and higher values by MCES. In general, it is found that LMFA shifts surface equilibrium temperature differently to lower values and enhances the hysteresis effect, particularly for surface molecules. On the other hand, for the 7 $$ \times $$ 7 square lattice, the equilibrium temperatures are slightly higher by 1.5% and 3.2% for bulk and surface molecules, respectively, with a narrower hysteresis width in the surface. Moreover, with the MCES method, an abrupt transition instead of a hysteresis transition is calculated for surface molecules ($$ \Delta T_{surf}^{MCES}=0 $$ K).

Differences in the resistive and thermodynamic properties of the single crystalline chiral superconductor candidate SrPtAs

The locally noncentrosymmetric superconductor SrPtAs is proposed to host a topological chiral $d$-wave state, but experimental reports have been limited to polycrystalline samples. Here we report the synthesis of single crystalline SrPtAs grown from Pb flux. SrPtAs crystallizes in the hexagonal space group $P{6}_{3}/mmc$ with lattice parameters $a=4.2445(4)$ \AA{} and $c=8.9513(18)$ \AA{}. Magnetic susceptibility and electrical resistivity measurements reveal a superconducting transition at ${T}_{c}\phantom{\rule{4pt}{0ex}}\ensuremath{\sim}2.2$ K, in agreement with previous reports on polycrystalline samples. Surprisingly, heat capacity data show only a small bulk transition at 0.7 K. We discuss the possible origins of the discrepancy between the various measurements.

The origin of the band-splitting and the spin polarization in bulk 2H-WSe2

Recently, band-splitting associated with spin polarization at K- and K′-points of the Brillouin zone has been found in centrosymmetric transition metal dichalcogenide materials. This discovery offers a possibility on centrosymmetric crystals for potential valleytronic applications. However, the origin of the band-splitting and the spin polarization in multilayer and bulk transition metal dichalcogenides remains unclear as the interlayer coupling should play a role when compared with that in monolayers. Here, by performing spin- and angle-resolved photoemission spectroscopy in bulk 2H-WSe2 at variable temperatures, we have quantitatively established contributions of the intralayer spin–orbit coupling and interlayer coupling. While the strength of the intralayer spin–orbit coupling is determined to be 450 meV, independent of the temperature, the strength of the interlayer coupling is found to increase from 68 to 172 meV as the temperature decreases from 300 to 80 K. This is also accompanied by an increase in the total band-splitting and a decrease in the spin polarization. This work reveals the micro-mechanism of spin and interlayer interaction in centrosymmetric materials, which provides a basis for the development of next-generation energy-efficient valleytronic devices.

Strong light-matter interactions of exciton in bulk WS2 and a toroidal dipole resonance.

Exciton-polaritonic states are generated by strong interactions between photons and excitons in nanocavities. Bulk transition metal dichalcogenides (TMDCs) host excitons with a large binding energy at room temperature, and thus are regarded as an ideal platform for realizing exciton-polaritons. In this work, we investigate the strong coupling properties between high-Q toroidal dipole (TD) resonance and bulk WS2 excitons in a hybrid metasurface, consisting of Si3N4 nanodisk arrays with embedded WS2. Multipole decomposition and near-field distribution confirm that Si3N4 nanodisk arrays support strong TD resonance. The TD resonance wavelength is easily tuned to overlap with the bulk WS2 exciton wavelength, and strong coupling is observed when the bulk WS2 is integrated with the hollow nanodisk and the oscillator strength of the WS2 material is adjusted to be greater than 0.6. The Rabi splitting of the hybrid device is up to 65 meV. In addition, strong coupling is confirmed by the anticrossing of fluorescence enhancement in the hybrid Si3N4-WS2 metastructure. Our findings are expected to be of importance for both fundamental research in TMDC-based light-matter interactions and practical applications in the design of high-performance exciton-polariton devices.

Starches of Varied Digestibilities Differentially Modify Intestinal Function in Rats

Starches of different digestibilities may enter the colon to different extents and alter colonic function. Male Fischer 344 rats were fed diets containing 25% cooked potato starch, arrowroot starch, high amylose cornstarch or raw potato starch for 6 wk. Fecal weight, transit time, colonic thymidine kinase activity (a marker for cell proliferation), and weight, starch content and pH of the cecum and proximal and distal colon were measured. Raw potato starch was much less completely digested than high amylose cornstarch, resulting in a 32-fold greater amount of undigested starch entering the cecum in the raw potato starch group. Both the high amylose cornstarch and raw potato starch diets significantly enhanced fecal weight and produced large intestinal hypertrophy, effects that were greatest in the raw potato starch group. Raw potato starch feeding was associated with the highest level of thymidine kinase activity, although the differences in thymidine kinase activity among the four groups were not significant. This diet also produced a 50% longer transit time. Entry of a large amount of raw potato starch into the colon resulted in greater luminal acidity, greater luminal bulk and slower transit. A much smaller amount of starch entered the colon in the high amylose cornstarch group and resulted in fecal bulking but no alteration in transit.

Catalytic Activity of BaTiO3 Nanoparticles for Wastewater Treatment: Piezo- or Sono-Driven?

Piezocatalytic reactions are generally excited by ultrasonication and, as such, should occur simultaneously with other catalytic reactions caused by different mechanisms such as sonocatalysis or tribocatalysis. One of the main challenges is how to discriminate between these effects in a catalysis experiment and quantify each contribution. In order to distinguish these effects, we use nano- and micro-particles of piezoelectric barium titanate (BaTiO3) and then compare their catalytic properties below and close to the piezoelectric to non-piezoelectric phase transition temperature at which the crystalline structure changes from the low-temperature tetragonal, piezoelectric phase to the higher-temperature cubic, non-piezoelectric phase. All other parameters, such as particle size, surface termination, etc., remain unaltered. The phase transition in bulk occurs at about 120 °C, but the transition temperature decreases as a function of particle size and reaches room temperature for particle sizes of about 20 nm. Transmission electron microscopy and X-ray diffraction were used to characterize the morphology and crystalline phase of the nanoparticles, respectively. Since the piezoelectric properties and the lattice dynamics are closely related, temperature-dependent Raman spectroscopy provides an even better insight into the nano- and micro-structural properties, allowing the ferroelectric phase transition temperature to be estimated. The catalytic activities of the BaTiO3 nanoparticles were determined by monitoring the optical absorption of a solution containing the model pollutant methyl orange, after ultrasonication of the solution to which were added the dispersed piezoelectric BaTiO3 particles as catalysts. An exponential-like correlation has been found between the catalytic reaction rates and the tetragonal distortion of the crystal structure of the BaTiO3 particles. Our study has also established that while 10% of the catalytic reaction of BaTiO3 is related to either sonocatalysis or tribocatalysis, the remaining 90% of the overall catalytic activity is ascribed to the piezocatalytic contribution.

Strong coupling between excitons and quasibound states in the continuum in bulk transition metal dichalcogenides

We investigate the strong coupling between the excitons and quasi-bound states in the continuum (BIC) resonance in a bulk WS$_2$ metasurface. Here we employ the bulk WS$_2$ to construct an ultrathin nanodisk metasurface, supporting the symmetry-protected magnetic dipole (MD) quasi-BIC resonance, which can self-hybridize with the excitons and lead to a strong light-matter interaction enhancement within the structure without the necessity for an external cavity. This strong coupling can be charactered by the considerable Rabi splitting of 159 meV and the clearly anti-crossing behavior appeared in the absorption spectrum. Furthermore, we analyze such light-matter coupling by constructing a Hamiltonian model including the surplus excitons, and tune the interaction from weak coupling to strong coupling regimes via the tunability radiation loss of the quasi-BIC resonance. Our results have great potential for manipulating the exciton-polaritons at room temperature, and provide a promising prospect for photonic devices that exploit strong coupling in applications.

Effects of Exchange Correlation Functional (Vwdf3) on the Structural, Elastic, and Electronic Properties of Transition Metal Dichalogenides

In this research, the effects of Van der Waals forces on the structural, elastic, electronic, and optical properties of bulk transition metals dichalcogenides (TMDs) were studied using a novel exchange-correlation functional, vdW-DF3. This new functional tries to correct the hidden Van der Waals problems which are not reported by the previous exchange functionals. Molybdenum dichalcogenide, MoX 2 (X = S, Se, Te) was chosen as a representative transition metal dichalcogenide to compare the performance of the newly designed functional with the other two popular exchange-correlation functional; PBE and rVV10. From the results so far obtained, the analysis of the structural properties generally revealed better performance by vdW-DF3 via the provision of information on lattice parameters very closer to the experimental value. For example, the lattice constant obtained by vdW-DF3 was 3.161 Å which is very close to 3.163 Å and 3.160 Å experimental and theoretical values respectively. Calculations of the electronic properties revealed good performance by vdW-DF3 functional. Furthermore, new electronic features were revealed for MoX2 (X = S, Se, Te). In terms of optical properties, PBE functional demonstrates lower absorption than vdW-DF3, as such it can be reported that vdW-DF3 improves photon absorption by TMDs. However, our results also revealed that vdW-DF3 performed well for MoS2 than for MoSe2 and MoTe2 because of the lower density observed for the S atom in MoS2.

Axial-Bonding-Driven Dimensionality Effect on the Charge-Density Wave in NbSe2.

2H-NbSe2 is a prototypical charge-density-wave (CDW) system, exhibiting such a symmetry-breaking quantum ground state in its bulk and down to a single-atomic-layer limit. However, how this state depends on dimensionality and what governs the dimensionality effect remain controversial. Here, we experimentally demonstrate a robust 3 × 3 CDW phase in both freestanding and substrate-supported bilayer NbSe2, far above the bulk transition temperature. We exclude environmental effects and reveal a strong temperature and thickness dependence of Raman intensity from an axially vibrating A1g phonon mode, involving Se ions. Using first-principles calculations, we show that these result from a delicate but profound competition between the intra- and interlayer bonding formed between Se-pz orbitals. Our results suggest the crucial role of Se out-of-plane displacement in driving the CDW distortion, revealing the Se-dominated dimensionality effect and establishing a new perspective on the chemical bonding and mechanical stability in layered CDW materials.

Ferromagnetism in Pr-rich binary Pr7Ru3 intermetallic compound

We present the synthesis and experimental characterization of the binary intermetallic compound Pr7Ru3. The polycrystalline sample was prepared by arc melting pure Pr and Ru, followed by homogenization at 500 °C and 600 °C for 48 and 89 h, respectively. Powder x-ray diffraction confirms that Pr7Ru3 crystallizes in an orthorhombic crystal structure (Pnma, space group no. 62) with the lattice parameters: a = 7.3606(7) Å, b = 23.120(1) Å and c = 6.5959(5) Å. Magnetization, resistivity, and heat capacity measurements reveal a ferromagnetic transition in Pr7Ru3 with the Curie temperature TC ∼ 24.5 K. The bulk transition is confirmed by a large λ-shape anomaly observed in the specific heat measurement. The magnetic susceptibility above the transition obeys the modified Curie-Weiss law with a positive Curie-Weiss temperature ϴCW = 30(1) K and an effective magnetic moment of 3.39(1) μB/Pr. Resistivity data for Pr7Ru3 reveals metallic-like behavior with a clear anomaly at the transition temperature which is smeared by an applied magnetic field. We also synthesized high-quality nonmagnetic analog La7Ru3 for which superconducting transition is observed with Tc = 1.95 K, in agreement with the literature.

STUDY OF THE FEATURES OF GEL FORMATION AND RHEOLOGICAL PROP-ERTIES OF HYDROGELS BASED ON AGAR-AGAR

The article shows studies on the study of the kinetics of gelation and the features of the rheological properties of hydrogels based on agar-agar. It has been found that hydrogels have attracted a lot of attention in the last 50 years due to their exceptional wide range of applications. They have a certain degree of elasticity, very similar to natural fabrics due to the high water content. Within the framework of the literature review, it has been shown that polymer hydrogels are usually obtained by polymerization of water-soluble unsaturated compounds in the presence of a bifunctional cross-linking agent or by structuring pre-prepared reactive prepolymers. The ability of polymer hydrogels to undergo bulk transitions between swollen and collapsed phases under the action of an external medium is one of the most important and universal properties of these materials. It has been shown that polymer hydrogels are known that are used as inert matrices for the controlled release of pre-absorbed biologically active substances. In this case, the release rate of the biologically active substance is determined by the nature and structure of the hydrogel polymer framework. The article studies the kinetics of gelation and the features of the rheological properties of hydrogels based on agar-agar in order to obtain modern biocompatible materials for the medical, cosmetological and pharmaceutical industries. It is shown that in compositions that consist exclusively of agar-agar and water, the shortest time for loss of stickiness and film formation is observed. At the same time, with an increase in the content of agar-agar from 25 to 50 %, one can see the most rapid processes of loss of stickiness and film formation. Compositions with the longest time for loss of stickiness and film formation were determined, which contain 25 % agar-agar and 50 % milk and honey, because they will be characterized by a long diffusion time of active substances to the human body. The scientific novelty of the study is related to the study of the effectiveness of gelation in conditions of highly concentrated agar-agar solutions with different contents of the polymer part and active substances, on the basis of which compositions for the preparation of face masks and medical tapes are proposed.