Bulk Transition Research Articles

Surface-induced transition of nematic liquid crystals on graphene/SiC substrate

A nematic liquid-crystal director aligns along the armchair direction of graphene grown on a SiC substrate. The temperature-dependent textural change in the nematic phase is different from the usual texture on the alignment layer. The isotropic-nematic phase transition temperature decreases over time after cell fabrication. Tiny domains occur with the phase transition and appear to be merged from a critical temperature with decreasing temperature in the nematic phase. This is thought to be due to the transition occurring at the interface. On the other hand, the nematic liquid crystal in graphene grown on a Cu foil does not show textural changes, and the phase transition temperature does not decrease even after a long time has elapsed. X-ray fluorescence measurements indicate that silicon atoms exist in the liquid crystal possibly extracted from the SiC substrate. A model of first-order phase transition on the graphene surface has been proposed. This transition is accompanied by an inhomogeneous distribution of scalar order parameters and the transformation from a small multi-grained structure to a large domain distribution in the director field. This structural transformation takes place at a temperature different from that corresponding to the bulk transition. Such behavior may be explained by the adsorption process of silicon atoms in contact with the SiC-graphene interface.

Spin-Resolved Electronic Response to the Phase Transition in MoTe2

The semimetal MoTe_{2} is studied by spin- and angle-resolved photoemission spectroscopy across the centrosymmetry-breaking structural transition temperature of the bulk. A three-dimensional spin-texture is observed in the bulk Fermi surface in the low temperature, noncentrosymmetric phase that is consistent with first-principles calculations. The spin texture and two types of surface Fermi arc are not completely suppressed above the bulk transition temperature. The lifetimes of quasiparticles forming the Fermi arcs depend on thermal history and lengthen considerably upon cooling toward the bulk structural transition. The results indicate that a new form of polar instability exists near the surface when the bulk is largely in a centrosymmetric phase.

First principles phase diagram calculation for the 2D TMD system [formula omitted

First principles phase diagram calculations, that included van der Waals interactions, were performed for the bulk transition metal dichalcogenide system (1−X)·WS2−(X)·WTe2. To obtain a converged phase diagram, a series of cluster expansion calculations were performed with increasing numbers of structural energies, (Nstr) up to Nstr=435, used to fit the cluster expansion Hamiltonian. All calculated formation energies are positive and all ground-state analyses predict that formation energies for supercells with 16 or fewer anion sites are positive; but when 150⪅Nstr⪅376, false ordered ground-states are predicted. With Nstr≥399, only a miscibility gap is predicted, but one with dramatic asymmetry opposite to what one expects from size-effect considerations; i.e. the calculations predict more solubility on the small-ion S-rich side of the diagram and less on the large-ion Te-rich side. This occurs because S-rich low-energy metastable ordered configurations have lower energies than their Te-rich counterparts which suggests that elastic relaxation effects are not dominant for the shape of the miscibility gap.

Ferroelectric Problem beyond the Conventional Scaling Law

Ferroelectric (FE) size effects against the scaling law were reported recently in ultrathin group-IV monochalcogenides, and extrinsic effects (e.g., defects and lattice strains) were often resorted to. Via first-principles based finite-temperature (T) simulations, we reveal that these abnormalities are intrinsic to their unusual symmetry breaking from bulk to thin film. Changes of the electronic structures result in different order parameters characterizing the FE phase transition in bulk and in thin films, and invalidation of the scaling law. Beyond the scaling law T_{c} limit, this mechanism can help predict materials that are promising for room-T ultrathin FE devices of broad interest.

Percolative nature of the direct-current paraconductivity in cuprate superconductors

Despite extraordinary scientific efforts over the past three decades, the cuprate high-temperature superconductors continue to pose formidable challenges. A pivotal problem, essential for understanding both the normal and superconducting states, is to clarify the nature of the superconducting pre-pairing above the bulk transition temperature Tc. Different experimental probes have given conflicting results, in part due to difficulties in discerning the superconducting response from the complex normal-state behavior. Moreover, it has proven challenging to separate common properties of the cuprates from compound-specific idiosyncrasies. Here we investigate the paraconductivity—the superconducting contribution to the direct-current (dc) conductivity—of the simple-tetragonal model cuprate material HgBa2CuO4+δ. We are able to separate the superconducting and normal-state responses by taking advantage of the Fermi-liquid nature of the normal state in underdoped HgBa2CuO4+δ; the robust and simple quadratic temperature-dependence of the normal-state resistivity enables us to extract the paraconductivity above the macroscopic Tc with great accuracy. We find that the paraconductivity exhibits unusual exponential temperature dependence, and that it can be quantitatively explained by a simple superconducting percolation model. Consequently, the emergence of superconductivity in this model system is dominated by the underlying intrinsic gap inhomogeneity. Motivated by these insights, we reanalyze published results for two other cuprates and find exponential behavior as well, with nearly the same characteristic temperature scale. The universal intrinsic gap inhomogeneity is not only essential for understanding the supercoducting precursor, but will also have to be taken into account in the analysis of other bulk measurements of the cuprates.

Pressure-mediated structural transitions in bulk EuTiO3

The high pressure structural phase diagram of ${\mathrm{EuTiO}}_{3}$ is revisited by means of single crystal x-ray diffraction at pressures below $30\phantom{\rule{4.pt}{0ex}}\text{GPa}$. We report a transition at about $3\phantom{\rule{4.pt}{0ex}}\text{GPa}$ with a surprisingly small lattice distortion. At $23\phantom{\rule{4.pt}{0ex}}\text{GPa}$ we have detected a yet unknown transition into a new orthorhombic phase (space group $Ibam$). Combining our observations on structures obtained on single crystals and polycrystalline samples with complementary methods, such as electrical resistivity and nuclear forward scattering of synchrotron radiation measured on polycrystalline samples, we address issues relevant to chemistry and disorder.

ZnTe semiconductor nanoparticles: A chemical approach of the mechanochemical synthesis

To have a better understanding on how ZnTe semiconductor nanoparticles were obtained by high-energy milling under composition, pressure and temperature vial conditions reached during milling, a molar Zn–Te–O ternary representation was disclosed. Through this approach, all the bulk and surface chemical transitions were revealed as: i) stable, ii) stoichiometric and iii) metastable or non-stoichiometric reaction products in the Zn–Te–O molar system. Surface studies (via X-ray photoelectron spectroscopy and high-resolution transmission electron microscopy) and bulk analyses (X-ray diffraction and secondary ion mass spectrometry) were carried out to find out the mechanism of formation of the ZnTe semiconductor nanoparticles. The results showed that the as-milled ZnTe nanoparticles were polycrystalline – with a slight presence of amorphous phases – and chemically homogeneous (according to elemental chemical mapping of Zn K-line and Te L-line). Thus, the way in which the oxygen chemical potential was decreased from Zn2Te3O8 to ZnTe nanoparticles was proposed. Consequently, predicted transitions in the ternary molar representation were consistent with experimental results. A blue shift of the ZnTe nanoparticles (at 10 h milling time) was observed by optical measurements. It implied an enhancement in its direct band gap, attributed to the presence of nanoparticles and coarse particles with both, embedded quantum dots and nanoparticles.

Vibrational and dielectric properties of the bulk transition metal dichalcogenides

Interest in the bulk transition metal dichalcogenides for their electronic, photovoltaic, and optical properties has grown and led to their use in many technological applications. We present a systematic investigation of their interlinked vibrational and dielectric properties using density functional theory and density functional perturbation theory, studying the effects of the spin-orbit interaction and of the long-range ${e}^{\ensuremath{-}}\phantom{\rule{0.28em}{0ex}}\ensuremath{-}\phantom{\rule{0.28em}{0ex}}{e}^{\ensuremath{-}}$ correlation as part of our investigation. This study confirms that the spin-orbit interaction plays a small role in these physical properties, while the direct contribution of dispersion corrections is of crucial importance in the description of the interatomic force constants. Here, our analysis of the structural and vibrational properties, including the Raman spectra, compare well to experimental measurement. Three materials with different point groups are showcased, and data trends on the full set of 15 existing hexagonal, trigonal, and triclinic materials are demonstrated. This overall picture will enable the modeling of devices composed of these materials for novel applications.

Consequences of Dispersity on the Self-Assembly of ABA-Type Amphiphilic Block Co-Oligomers.

Intriguingly, little is known about the impact of dispersity on the crystallization driven self-assembly (CDSA) of amphiphilic block copolymers in aqueous media. Here, we investigate the influence of dispersity on the CDSA of ABA-type amphiphilic block co-oligomers (ABCOs). Two pairs of ABCOs are synthesized comprising discrete (Đ = 1.00) or disperse (Đ = 1.20) isotactic l-lactic acid 16-mers as the semicrystalline hydrophobic block and either oligo(ethylene glycol) methyl ether (MeOoEG) or oligo(tetraethylene glycol succinate) (oTEGSuc) as the discrete hydrophilic block. Self-assembly studies in water with 10% THF reveal uniform nanofibers/2D sheets for the discrete oligomers, but such structural regularity is largely compromised in the disperse oligomers. The results are corroborated by sharp melting transitions in both solution and bulk for the discrete ABCOs, unlike their disperse analogues that show a lack of crystallization. Interestingly, the discrete MeOoEG-LLA oligomer reveals crystallization driven gelation, illustrating the contrasting differences between the discrete oligomers and their disperse counterparts.

Electronic Band Structure of Rhenium Dichalcogenides

The band structures of bulk transition metal dichalcogenides ReS2 and ReSe2 are presented, showing the complicated nature of interband transitions in these materials, with several close-lying band gaps. Three-dimensional plots of constant energy surfaces in the Brillouin zone at energies near the band extrema are used to show that the valence band maximum and conduction band minimum may not be located at special high symmetry points. We find that both materials are indirect gap materials and that one must be careful to consider the whole Brillouin zone volume in addressing this question.

Phonon Scattering and Electron Doping by 2D Structural Defects in In/ZnO.

In/ZnO bulk compounds have been synthesized using a simple solid-state process. In this study, both the structural features and thermoelectric properties of the Zn1-xInxO series with ultralow indium content (0 ≤ x ≤ 0.02) have been studied. High-angle annular dark-field scanning transmission electron microscopy analyses highlight that indium has the ability to create multiple basal plane and pyramidal defects that produce ZnO domains with inverted polarity starting from dopant concentrations as low as 0.25 atom %. Interestingly, the formation of parallel inversion boundaries consisting of InO6 octahedra in the ZnO4 tetrahedra matrix is responsible for phonon scattering while increasing electrical conductivity, thereby enhancing the thermoelectric properties. This effect of multiple extended two-dimensional defects on the thermoelectric properties of ZnO is reported for the first time with such low indium doping. On the chemistry side, the present results point toward a lack of In solubility in the ZnO structure. Moreover, this study is a step forward to the synthesis of other thermoelectric compounds where dopant-induced planar defects in bulk transition metal compounds have the potential to enhance both phonon scattering and electronic conductivity.

Transition metal dichalcogenides MX2 (M=Mo, W; X=S, Se, Te) and MX2-CIP composites: Promising materials with high microwave absorption performance

The microwave absorption (MA) performance of bulk transition metal dichalcogenides (TMDCs) MX2 (M = Mo, W; X = S, Se, Te) and their composites with carbonyl iron powder (CIP) were investigated for the first time. The results show that among the investigated MX2, MoSe2 exhibited the best comprehensive MA performance. For MoSe2/paraffin with 80 wt% loading, the minimum reflection loss (RL) value was −60.23 dB with the matching thickness of 2.56 mm, and the corresponding bandwidth with effective attenuation (<−10 dB) was up to 5.68 GHz (6.32–12.0 GHz). Furthermore, compositing MX2 with CIP by vibrating ball milling significantly improved their MA performance. For 80 wt% filler loading of WSe2-CIP-1-3, the matching thickness decreased to 1.86 mm, while a minimum RL of −68.14 dB and an effective bandwidth of 6.32 GHz were achieved. Our results demonstrated that TMDCs-based materials are promising for the potential applications as novel, efficient microwave absorbents.

Superconductivity, Kondo effect, and observation of self-organized pattern formation in intermetallic NiBi3 thin films

The superconducting properties of the intermetallic compound NiBi3 have been studied mostly in bulk crystals. In this report we have studied several aspects of NiBi3 thin films, spontaneously formed at the interface of Ni and Bi multilayers. We have observed superconducting transition around 3.9 K, close to the bulk transition temperature of 4.1 K. Due to the diffusive mechanism of formation of NiBi3, additional phenomena such as magnetic (Nickel) impurity induced Kondo effect and self organized Turing pattern formation also come into picture. From X-ray diffraction and Magneto-resistance measurements we have established the presence of atomic Ni impurities in the films. Kondo minima were found to scale with varied concentration of Ni impurities in a series of Ni-Bi multilayer films (with fixed Ni thickness and varied Bi thickness). Similarly, saturation magnetization and superconducting transition temperature were also found to follow the concentration of Ni impurities. In addition, we have also observed signatures of Turing patterns, from magnetic force microscopy, naturally formed with time without any external heat treatment.

Study of the critical behavior of antiferromagnetic thin films by computer modeling

In article the study of the surface phase changes in the thin films described by the antiferromagnetic Ising model, by computer modeling is executed. The Metropolis algorithm is used. Modeling is executed at various values of exchange integrals relation on a surface and in bulk of films RS. The difference of exchange integral between the surface spins and the first subsurface layer RSB from volume value is considered. The limiting cases of the value RSB are considered. Two order parameters are used. The first order parameter defines an antiferromagnetic order in the bulk of system. It is calculated as chess magnetization of the spins located not on a surface. Its value is equal to the difference of magnet moments for two sublattices. For study of the surface phase transition, the second order parameter is entered. It is calculated as chess magnetization of the spins located on the free surface. For definition of transition temperature, bulk and surface Binder’s cummulants are used. The computer experiment for various values of film thickness from 4 to 12 layers is made. The relation of exchange integrals changed from 0.5 to 2.0. Temperatures of bulk and surface phase transition are identical at all relations of exchange integrals. Transition temperature grows at increase in the exchange integrals relation RS. Growth rate of transition temperature depends on thickness of a film and velue RSB. The difference of exchange integral between the surface layer and the first subsurface layer leads to more rapid growth of the transition temperature. For all values of exchange integrals there is a cross point for temperature curves at any thickness of a film.

Mesoscopic simulations of temperature-dependent anchoring and wetting behavior at aqueous-liquid crystal interfaces in the presence of a rod-coil amphiphilic monolayer.

Dissipative particle dynamics simulations have been applied to study the temperature dependent anchoring and wetting behavior of thermotropic liquid crystals (LCs) in the presence of a rod–coil amphiphilic monolayer at the aqueous–LC interface. Upon cooling in the nematic phase, a thermally-induced anchoring transition from homeotropic through tilted to planar has been observed. The growth and propagation of smectic order from the interfaces to the bulk nematic LCs are demonstrated to be mainly responsible for this novel transition sequence. In particular, when a complete smectic layer in the amphiphile monolayer is induced around the bulk transition of nematic–smectic-A, the propagation of homeotropic alignment fails instantly and a unique planar anchoring configuration is formed instead. While heating towards the isotropic phase, simulation results show that the nematic–isotropic transition of confined LCs is slightly shifted to a higher temperature, and a nematic wetting layer with homeotropic alignment appears in the rod block monolayer when the bulk LCs is isotropic. Our systematic simulations throughout the whole phase regimes of LCs provide important molecular-level insight into how the coupling between the ordering of LCs and adsorbents and their temperature dependencies affect the anchoring behavior in this complex system, which should be instrumental in the rational design and application of advanced LC-based biosensors with optimal operating temperature range.

Valley-contrasting optics of interlayer excitons in Mo- and W-based bulk transition metal dichalcogenides.

Recently, spatially indirect ("interlayer") excitons have been discovered in bulk 2H-MoTe2. They are theoretically predicted to exist in other Mo-based transition metal dichalcogenides (TMDCs) and are expected to be present in W-based TMDCs as well. We investigate interlayer excitons (XIL) in bulk 2H-MoSe2 and 2H-WSe2 using valley-resolved magneto-reflectance spectroscopy under high magnetic fields of up to 29 T combined with ab initio GW-BSE calculations. In the experiments, we observe interlayer excitons in MoSe2, while their signature is surprisingly absent in WSe2. In the calculations, we find that interlayer excitons exist in both Mo- and W-based TMDCs. However, their energetic positions and their oscillator strengths are remarkably different. In Mo-based compounds, the interlayer exciton resonance XIL is clearly separated from the intralayer exciton X1sA and has a high amplitude. In contrast, in W-based compounds, XIL is close in energy to the intralayer A exciton X1sA and possesses a small oscillator strength, which explains its absence in the experimental data of WSe2. Our combined experimental and theoretical observations demonstrate that interlayer excitons can gain substantial oscillator strength by mixing with intralayer states and hence pave the way for exploring interlayer exciton physics in Mo-based bulk transition metal dichalcogenides.

New peak shear strength model for cement filled rock joints

Grouting can significantly improve the mechanical properties of a fractured rock mass. A set of indoor grouting test systems is designed, and 3D scanning and direct shear tests are carried out for 10 groups of grouted joint specimens. In this study, grouted joints are simplified to unfilled joints and cement stone (including the bulk paste and transition zone). The impact of cement stone on unfilled joints is defined as the filling effect and bonding effect. First, 3D morphological characteristics of unfilled joints are analyzed. Then, a weak-filling joint shear model is derived based on the existing shear strength model for unfilled joints. Finally, the shear model for grouted joints is derived from the shear model for filled joints. Reinforcement mechanisms, failure modes and causes of error in grouted joints are discussed. The effects of roughness, filling degree, rock to filling material strength ratio, normal stress and cohesion on peak shear strength are considered comprehensively in the new model for grouted joints. The new model is consistent with the basic form of the Mohr-Coulomb criterion. The good agreement between test data and calculated results from the new model indicates the new model is suitable for predicting the shear strength of grouted joints.

Anomalous Corrosion of Bulk Transition Metal Diselenides Leading to Stable Monolayers.

In this paper we provide insight into an anomalous corrosion process, referred to as electroablation (EA), which converts multilayer flakes of transition metal diselenides like MoSe2 into their corresponding monolayers when micromechanically exfoliated on a conductive electrode and subsequently subjected to a high anodic potential inside a conventional electrochemical cell. Photoluminescence intensity maps and scanning transmission electron microscopy (STEM) images confirmed the single crystalline nature and 2H-hexagonal lattice structure of the remnant monolayer MoSe2 flakes, indicating the superior corrosion stability of the monolayers compared to that of the bulk counterpart. It is noted that the EA technique is a low-cost alternative for high-yield synthesis of single crystalline monolayer MoSe2 at room temperature. We also found that the dynamics of such an electro-oxidation-mediated and self-limiting corrosion process differs significantly for MoSe2 and WSe2. While we were able to engineer the corrosion conditions for the EA process to obtain monolayers of MoSe2, our attempts to obtain monolayers of WSe2 were largely unsuccessful. Finally, we constructed a phenomenological physical chemistry framework to explain such anomalous corrosion processes in transition metal diselenides.

Electron–phonon interaction and transport properties of metallic bulk and monolayer transition metal dichalcogenide TaS2

Transition metal dichalcogenides have recently emerged as promising two-dimensional materials with intriguing electronic properties. Existing calculations of intrinsic phonon-limited electronic transport so far have concentrated on the semicondcucting members of this family. In this paper we extend these studies by investigating the influence of electron–phonon coupling on the electronic transport properties and band renormalization of prototype inherent metallic bulk and monolayer TaS2. Based on density functional perturbation theory and semi-classical Boltzmann transport calculations, promising room temperature mobilities and sheet conductances are found, which can compete with other established 2D materials, leaving TaS2 as promising material candidate for transparent conductors or as atomically thin interconnects. Throughout the paper, the electronic and transport properties of TaS2 are compared to those of its isoelectronic counterpart TaSe2 and additional informations to the latter are given. We furthermore comment on the conventional superconductivity in TaS2, where no phonon-mediated enhancement of TC in the monolayer compared to the bulk state was found.

Compression-induced phase transition of GaN bulk from wurtzite phase to five-fold coordination hexagonal phase

The phase transformation of GaN bulk from the Wurtzite phase (WZ) to the hexagonal phase (HX) is studied by molecular dynamics simulation. The mechanical response and atomic structural evolution of transition are analyzed in detail. In addition, the loading rate effect on the phase transition is determined, that is, the phase transition ratio declines with a decrease of the strain rate. The WZ GaN bulk completely transforms into the HX phase in the case of compression at an ultrahigh strain rate. However, at a relatively slower strain rate, the HX phase of GaN partly nucleates and the untransformed regions are proved to be elastic deformed regions. Combined with an energy analysis, two atomic movement modes are recognized as the inducements for the phase transition and formation of elastic deformed regions. The first mode, which is responsible for the formation of elastic deformed regions, is an atomic sliding motion along the c {0001} planes. The second mode is a radial stretching atomic motion. The radial stretching motion, which requires a lot of energy, induces the WZ-HX phase transition. Moreover, the phase transition is affected drastically by the rise of temperature.