Showing papers on "Transition temperature published in 2017"

Thermal processing of diblock copolymer melts mimics metallurgy

Abstract: Small-angle x-ray scattering experiments conducted with compositionally asymmetric low molar mass poly(isoprene)-b-poly(lactide) diblock copolymers reveal an extraordinary thermal history dependence. The development of distinct periodic crystalline or aperiodic quasicrystalline states depends on how specimens are cooled from the disordered state to temperatures below the order-disorder transition temperature. Whereas direct cooling leads to the formation of documented morphologies, rapidly quenched samples that are then heated from low temperature form the hexagonal C14 and cubic C15 Laves phases commonly found in metal alloys. Self-consistent mean-field theory calculations show that these, and other associated Frank-Kasper phases, have nearly degenerate free energies, suggesting that processing history drives the material into long-lived metastable states defined by self-assembled particles with discrete populations of volumes and polyhedral shapes.

Intercalant-independent transition temperature in superconducting black phosphorus

Abstract: Research on black phosphorus has been experiencing a renaissance over the last years, after the demonstration that few-layer crystals exhibit high carrier mobility and a thickness-dependent bandgap. Black phosphorus is also known to be a superconductor under high pressure exceeding 10 GPa. The superconductivity is due to a structural transformation into another allotrope and accompanied by a semiconductor-metal transition. No superconductivity could be achieved for black phosphorus in its normal orthorhombic form, despite several reported attempts. Here we describe its intercalation by several alkali metals (Li, K, Rb and Cs) and alkali-earth Ca. All the intercalated compounds are found to be superconducting, exhibiting the same (within experimental accuracy) critical temperature of 3.8±0.1 K and practically identical characteristics in the superconducting state. Such universal superconductivity, independent of the chemical composition, is highly unusual. We attribute it to intrinsic superconductivity of heavily doped individual phosphorene layers, while the intercalated layers of metal atoms play mostly a role of charge reservoirs. No superconductivity could so far be achieved in black phosphorus in its normal orthorhombic form. Here, the authors demonstrate that intercalation with alkali metals makes black phosphorus superconducting with intercalant-independent transition temperature and near-identical superconducting characteristics.

Superconducting order from disorder in 2H-TaSe 2− x S x

Abstract: We report on the emergence of robust superconducting order in single crystal alloys of TaSe2− x S x (0 ≤ × ≤ 2). The critical temperature of the alloy is surprisingly higher than that of the two end compounds TaSe2 and TaS2. The evolution of superconducting critical temperature T c(x) correlates with the full width at half maximum of the Bragg peaks and with the linear term of the high-temperature resistivity. The conductivity of the crystals near the middle of the alloy series is higher or similar than that of either one of the end members 2H-TaSe2 and/or 2H-TaS2. It is known that in these materials superconductivity is in close competition with charge density wave order. We interpret our experimental findings in a picture where disorder tilts this balance in favor of superconductivity by destroying the charge density wave order. Substituting sulfur into TaSe2 induces disorder, which further helps to enhance superconductivity, with a higher transition temperature. It is higher than that of either TaSe2 or TaS2. An international team of researchers led by Cedomir Petrovic at Brookhaven national laboratory of USA synthesized single crystal alloys of TaSe2− x S x and measured the electrical conductivity and superconducting transition temperature as a function of x. They found that the transition temperature optimally increased when a maximal disorder is introduced by substituting sulfur into TaSe2. The role of such a disorder was understood as to suppress other competing orders while keeping superconductivity intact. By breaking other orders, conducting carriers were released so that they contributed further to superconductivity. These results highlight a benefit role of disorder and provide a possible way to enhance superconductivity.

Electronic and optical properties of BaTiO3 across tetragonal to cubic phase transition: An experimental and theoretical investigation

Abstract: Temperature dependent diffuse reflectance spectroscopy measurements were carried out on polycrystalline samples of BaTiO3 across the tetragonal to cubic structural phase transition temperature (TP). The values of various optical parameters such as band gap (Eg), Urbach energy (Eu), and Urbach focus (E0) were estimated in the temperature range of 300 K to 480 K. It was observed that with increasing temperature, Eg decreases and shows a sharp anomaly at TP. First principle studies were employed in order to understand the observed change in Eg due to the structural phase transition. Near TP, there exist two values of E0, suggesting the presence of electronic heterogeneity. Further, near TP, Eu shows metastability, i.e., the value of Eu at temperature T is not constant but is a function of time (t). Interestingly, it is observed that the ratio of Eu (t=0)/Eu (t = tm), almost remains constant at 300 K (pure tetragonal phase) and at 450 K (pure cubic phase), whereas this ratio decreases close to the transition temperature, which confirms the presence of electronic metastability in the pure BaTiO3. The time dependence of Eu, which also shows an influence of the observed metastability can be fitted with the stretched exponential function, suggesting the presence of a dynamic heterogeneous electronic disorder in the sample across TP. First principle studies suggest that the observed phase coexistence may be due to a very small difference between the total cohesive energy of the tetragonal and the cubic structure of BaTiO3. The present work implies that the optical studies may be a sensitive probe of disorder/heterogeneity in the sample.

Giant electric field-induced strain at room temperature in LiNbO3-doped 0.94(Bi0.5Na0.5)TiO3-0.06BaTiO3

Abstract: We report experimental investigation on the ferroelectricity and electric field-induced strain response in LiNbO 3 -doped 0.94(Bi 0.5 Na 0.5 )TiO 3 -0.06BaTiO 3 (BNT-BT) piezoelectric ceramics. At room temperature, a large strain of 0.6% (at 70 kV/cm) is achieved in the 2.5%-LiNbO 3 -doped BNT-BT, higher than that of commercially-utilized Pb(Zr,Ti)O 3 . The corresponding piezoelectric coefficient d * 33 reaches 857 pm/V, which is high among these of BNT-based ceramics at room temperature. Further study indicates that the superior piezoelectric properties are realized at the ferroelectric-relaxor transition temperature T F-R , which is pushed to room temperature with 2.5% LiNbO 3 doping. This indicates that large electromechanical response can be induced via delicate mixing of the ferroelectric rhombohedral phase and the polar nanoregions (PNRs) relaxor-ferroelectric tetragonal phase.

Disorder and superfluid density in overdoped cuprate superconductors

Abstract: One of the hallmarks of clean, $d$-wave superconductivity is a linear temperature dependence of superfluid density in the low-temperature limit. Here, the authors show that weak scattering, combined with a realistic model of the cuprate Fermi surface, allows the linear behavior of superfluid density to persist in the presence of disorder even in the regime where the superfluid density and the transition temperature are strongly suppressed from their clean-limit values. This leads to a superfluid density that is both correlated with the transition temperature and linear in temperature, consistent with recent experiments on overdoped La${}_{2-x}$Sr${}_{x}$CuO${}_{4}$.

Dimensional crossover of the charge density wave transition in thin exfoliated VSe 2

Abstract: Isolating single unit-cell thin layers from the bulk matrix of layered compounds offers tremendous opportunities to design novel functional electronic materials. However, a comprehensive thickness dependence study is paramount to harness the electronic properties of such atomic foils and their stacking into synthetic heterostructures. Here we show that a dimensional crossover and quantum confinement with reducing thickness result in a striking non-monotonic evolution of the charge density wave transition temperature in VSe2. Our conclusion is drawn from a direct derivation of the local order parameter and transition temperature from the real space charge modulation amplitude imaged by scanning tunnelling microscopy. This study lifts the disagreement of previous independent transport measurements. We find that thickness can be a non-trivial tuning parameter and demonstrate the importance of considering a finite thickness range to accurately characterize its influence.

Light baryons below and above the deconfinement transition: medium effects and parity doubling

Abstract: We study what happens to the N , Δ and Ω baryons in the hadronic gas and the quark-gluon plasma, with particular interest in parity doubling and its emergence as the plasma is heated. This is done using simulations of lattice QCD, employing the FASTSUM anisotropic N f = 2 + 1 ensembles, with four temperatures below and four above the deconfinement transition temperature. Below T c we find that the positive-parity groundstate masses are largely temperature independent, whereas the negative-parity ones are reduced considerably as the temperature increases. This may be of interest for heavy-ion phenomenology. Close to the transition, the masses are nearly degenerate, in line with the expectation from chiral symmetry restoration. Above T c we find a clear signal of parity doubling in all three channels, with the effect of the heavier s quark visible.

Elevated transition temperature in Ge doped VO2 thin films

Abstract: Thermochromic GexV1−xO2+y thin films have been deposited on Si (100) substrates by means of reactive magnetron sputtering. The films were then characterized by Rutherford backscattering spectrometry (RBS), four-point probe electrical resistivity measurements, X-ray diffraction, and atomic force microscopy. From the temperature dependent resistivity measurements, the effect of Ge doping on the semiconductor-to-metal phase transition in vanadium oxide thin films was investigated. The transition temperature was shown to increase significantly upon Ge doping (∼95 °C), while the hysteresis width and resistivity contrast gradually decreased. The precise Ge concentration and the film thickness have been determined by RBS. The crystallinity of phase-pure VO2 monoclinic films was confirmed by XRD. These findings make the use of vanadium dioxide thin films in solar and electronic device applications—where higher critical temperatures than 68 °C of pristine VO2 are needed—a viable and promising solution.

Tuning the ferroelectric-relaxor transition temperature in NBT-based lead-free ceramics by Bi nonstoichiometry

Abstract: In this study, the Bi-nonstoichiometric 099Bix(Na08K02)05TiO3-001SrTiO3 (BNKST) ceramics with x = 05–0535 mol (Bi50-Bi535) were prepared by a conventional solid-state reaction method The effects of Bi excess on structural transition and ferroelectric stability of BNKST ceramics were systematically investigated by the Raman spectra, dielectric analyses and electromechanical measurements The introduction of excess Bi3+ could significantly break the long-range ferroelectric order and favor the presence of relaxor phase, then the ferroelectric-relaxor transition temperature (TFR) can be effectively tuned to around room temperature by Bi nonstoichiometry, giving rise to an enhanced room-temperature strain property The positive strain Spos and dynamic piezoelectric constant d33* of Bi525 critical composition reach 033% and 440 pm/V, respectively at 6 kV/mm The high recoverable strain of Bi525 sample can be attributed to the electric-field-induced reversible relaxor-ferroelectric phase transition The present work may be helpful for further understanding and designing high-performance NBT-based lead-free ceramics for piezoelectric actuator applications

On the origin of critical temperature enhancement in atomically thin superconductors

Abstract: Recent experiments showed that thinning gallium, iron selenide and 2H tantalum disulfide to single/several monoatomic layer(s) enhances their superconducting critical temperatures. Here, we characterize these superconductors by extracting the absolute values of the London penetration depth, the superconducting energy gap, and the relative jump in specific heat at the transition temperature from their self-field critical currents. Our central finding is that the enhancement in transition temperature for these materials arises from the opening of an additional superconducting gap, while retaining a largely unchanged 'bulk' superconducting gap. Literature data reveals that ultrathin niobium films similarly develop a second superconducting gap. Based on the available data, it seems that, for type-II superconductors, a new superconducting band appears when the film thickness becomes smaller than the out-of-plane coherence length. The same mechanism may also be the cause of enhanced interface superconductivity.

Room-temperature helimagnetism in FeGe thin films.

Abstract: Chiral magnets are promising materials for the realisation of high-density and low-power spintronic memory devices. For these future applications, a key requirement is the synthesis of appropriate materials in the form of thin films ordering well above room temperature. Driven by the Dzyaloshinskii-Moriya interaction, the cubic compound FeGe exhibits helimagnetism with a relatively high transition temperature of 278 K in bulk crystals. We demonstrate that this temperature can be enhanced significantly in thin films. Using x-ray scattering and ferromagnetic resonance techniques, we provide unambiguous experimental evidence for long-wavelength helimagnetic order at room temperature and magnetic properties similar to the bulk material. We obtain α intr = 0.0036 ± 0.0003 at 310 K for the intrinsic damping parameter. We probe the dynamics of the system by means of muon-spin rotation, indicating that the ground state is reached via a freezing out of slow dynamics. Our work paves the way towards the fabrication of thin films of chiral magnets that host certain spin whirls, so-called skyrmions, at room temperature and potentially offer integrability into modern electronics.

Tuning Phase Transitions in 1T-TaS2 via the Substrate

Abstract: Phase transitions in 2D materials can lead to massive changes in electronic properties that enable novel electronic devices. Tantalum disulfide (TaS2), specifically the “1T” phase (1T-TaS2), exhibits a phase transition based on the formation of commensurate charge density waves (CCDW) at 180 K. In this work, we investigate the impact of substrate choice on the phase transitions in ultrathin 1T-TaS2. Doping and charge transfer from the substrate has little impact on CDW phase transitions. On the contrary, we demonstrated that substrate surface roughness is a primary extrinsic factor in CCDW transition temperature and hysteresis, where higher roughness leads to smaller transition hysteresis. Such roughness can be simulated via surface texturing of SiO2/Si substrates, which controllably and reproducibly induces periodic strain in the 1T-TaS2 and thereby enables the potential for engineering CDW phase transitions.

An approach for correlating the structural and electrical properties of Zr4+-modified SrBi4Ti4O15/SBT ceramic

Abstract: In the present work, B-site modified layer-structured strontium bismuth titanate (SBT) ceramics with the nominal formula SrBi4Ti4−xZrxO15 were prepared by solid state reaction route and the tailoring effects of zirconium (Zr) were investigated thoroughly. The X-ray diffraction analysis shows that the substitution leads to the formation of a single phase layered perovskite up to x ≤ 0.15 and a ZrO2-based secondary phase was detected for higher Zr-doped compositions. The higher grain-growth rate induced by the larger ionic radius of Zr4+ than Ti4+ was supported by the field emission scanning electron microscopy (FESEM) results. The transition temperature increases slightly for the Zr-modified compositions, which can be described in terms of structural distortion due to the internal stresses developed within the ceramics. The Cole–Cole plot analysis of impedance spectra enabled to distinguish two relaxation behaviours that were assigned to originate from grains and grain boundaries. It was also observed that the remnant polarization and piezoelectric coefficient increases up to the solubility limit of Zr and then decreases with higher doping content. The temperature dependent piezoelectric coefficient was also studied, and was found to be stable up to the transition temperature in all compositions. All these results were explained on the basis of occupancy of the Zr4+ ion at the B-site and for the higher composition ZrO2 secondary phase. Even the composition x = 0.15 exhibited low conductivity, a moderate dielectric constant and a highly stable d33 value, demonstrating that the ceramic is an excellent material for high-temperature piezoelectric application. The possible reason for the enhancement of the electrical properties in the Zr-modified ceramic was discussed based on the structural analysis, which may be used for designing and/or modifying properties of SBT-related ceramics.

Microstructures define melting of molybdenum at high pressures.

Abstract: High-pressure melting anchors the phase diagram of a material, revealing the effect of pressure on the breakdown of the ordering of atoms in the solid. An important case is molybdenum, which has long been speculated to undergo an exceptionally steep increase in melting temperature when compressed. On the other hand, previous experiments showed nearly constant melting temperature as a function of pressure, in large discrepancy with theoretical expectations. Here we report a high-slope melting curve in molybdenum by synchrotron X-ray diffraction analysis of crystalline microstructures, generated by heating and subsequently rapidly quenching samples in a laser-heated diamond anvil cell. Distinct microstructural changes, observed at pressures up to 130 gigapascals, appear exclusively after melting, thus offering a reliable melting criterion. In addition, our study reveals a previously unsuspected transition in molybdenum at high pressure and high temperature, which yields highly textured body-centred cubic nanograins above a transition temperature.

Inhomogeneous spatial distribution of the magnetic transition in an iron-rhodium thin film.

Abstract: Monitoring a magnetic state using thermal or electrical activation is mandatory for the development of new magnetic devices, for instance in heat or electrically assisted magnetic recording or room-temperature memory resistor. Compounds such as FeRh, which undergoes a magnetic transition from an antiferromagnetic state to a ferromagnetic state around 100 °C, are thus highly desirable. However, the mechanisms involved in the transition are still under debate. Here we use in situ heating and cooling electron holography to quantitatively map at the nanometre scale the magnetization of a cross-sectional FeRh thin film through the antiferromagnetic-ferromagnetic transition. Our results provide a direct observation of an inhomogeneous spatial distribution of the transition temperature along the growth direction. Most interestingly, a regular spacing of the ferromagnetic domains nucleated upon monitoring of the transition is also observed. Beyond these findings on the fundamental transition mechanisms, our work also brings insights for in operando analysis of magnetic devices.

Locking and Unlocking the Molecular Spin Crossover Transition

Abstract: The Fe(II) spin crossover complex [Fe{H2 B(pz)2 }2 (bipy)] (pz = pyrazol-1-yl, bipy = 2,2'-bipyridine) can be locked in a largely low-spin-state configuration over a temperature range that includes temperatures well above the thermal spin crossover temperature of 160 K. This locking of the spin state is achieved for nanometer thin films of this complex in two distinct ways: through substrate interactions with dielectric substrates such as SiO2 and Al2 O3 , or in powder samples by mixing with the strongly dipolar zwitterionic p-benzoquinonemonoimine C6 H2 (-⋯ NH2 )2 (-⋯ O)2 . Remarkably, it is found in both cases that incident X-ray fluences then restore the [Fe{H2 B(pz)2 }2 (bipy)] moiety to an electronic state characteristic of the high spin state at temperatures of 200 K to above room temperature; that is, well above the spin crossover transition temperature for the pristine powder, and well above the temperatures characteristic of light- or X-ray-induced excited-spin-state trapping. Heating slightly above room temperature allows the initial locked state to be restored. These findings, supported by theory, show how the spin crossover transition can be manipulated reversibly around room temperature by appropriate design of the electrostatic and chemical environment.

Cation–Eutectic Transition via Sublattice Melting in CuInP2S6/In4/3P2S6 van der Waals Layered Crystals

Abstract: Single crystals of the van der Waals layered ferrielectric material CuInP2S6 spontaneously phase separate when synthesized with Cu deficiency. Here we identify a route to form and tune intralayer heterostructures between the corresponding ferrielectric (CuInP2S6) and paraelectric (In4/3P2S6) phases through control of chemical phase separation. We conclusively demonstrate that Cu-deficient Cu1–xIn1+x/3P2S6 forms a single phase at high temperature. We also identify the mechanism by which the phase separation proceeds upon cooling. Above 500 K both Cu+ and In3+ become mobile, while P2S64– anions maintain their structure. We therefore propose that this transition can be understood as eutectic melting on the cation sublattice. Such a model suggests that the transition temperature for the melting process is relatively low because it requires only a partial reorganization of the crystal lattice. As a result, varying the cooling rate through the phase transition controls the lateral extent of chemical domains over several decades in size. At the fastest cooling rate, the dimensional confinement of the ferrielectric CuInP2S6 phase to nanoscale dimensions suppresses ferrielectric ordering due to the intrinsic ferroelectric size effect. Intralayer heterostructures can be formed, destroyed, and re-formed by thermal cycling, thus enabling the possibility of finely tuned ferroic structures that can potentially be optimized for specific device architectures.

Postsynthetic Route for Modifying the Metal—Insulator Transition of VO2 by Interstitial Dopant Incorporation

Abstract: The thermally driven orders-of-magnitude modulation of resistance and optical transmittance observed in VO2 makes it an archetypal first-order phase transition material and underpins functional applications in logic and memory circuitry, electromagnetic cloaking, ballistic modulation, and thermochromic glazing to provide just a few representative examples. VO2 can be reversibly switched from an insulating to a metallic state at an equilibrium transition temperature of 67 °C. Tuning the phase diagram of VO2 to bring the transition temperature closer to room temperature has been a longstanding objective and one that has tremendous practical relevance. Substitutional incorporation of dopants has been the most common strategy for modulating the metal—insulator transition temperature but requires that the dopants be incorporated during synthesis. Here we demonstrate a novel postsynthetic diffusive annealing approach for incorporating interstitial B dopants within VO2. The postsynthetic method allows for the tr...

Phonon-mediated superconductivity in Mg intercalated bilayer borophenes

Abstract: Using first-principles calculations, we investigate the structural, electronic and superconducting properties of Mg intercalated bilayer borophenes BxMgBx (x = 2–5). Remarkably, B2MgB2 and B4MgB4 are predicted to exhibit good phonon-mediated superconductivity with a high transition temperature (Tc) of 23.2 K and 13.3 K, respectively, while B4MgB4 is confirmed to be more practical based on the analyses of its stability. The densities of states of in-plane orbitals at the Fermi level are found to be dominant at the superconducting transition temperature in Mg intercalated bilayer borophenes, providing an effective avenue to explore Mg–B systems with high Tcs.

Skyrmion formation in a bulk chiral magnet at zero magnetic field and above room temperature

Abstract: We report that in a $\beta$-Mn-type chiral magnet Co$_9$Zn$_9$Mn$_2$, skyrmions are realized as a metastable state over a wide temperature range, including room temperature, via field-cooling through the thermodynamic equilibrium skyrmion phase that exists below a transition temperature $T_\mathrm{c}$ $\sim$ 400 K. The once-created metastable skyrmions survive at zero magnetic field both at and above room temperature. Such robust skyrmions in a wide temperature and magnetic field region demonstrate the key role of topology, and provide a significant step toward technological applications of skyrmions in bulk chiral magnets.

Magnetic properties of bilayer graphene: a Monte Carlo study

Abstract: The lattice structure of bilayer graphene atoms within the same layer is studied by Monte Carlo simulations. The ground-state phase diagrams of mixed spin-2 and spin-3/2 with Ising model on a bilayer graphene are investigated using the Monte Carlo simulations. The reduced transition temperatures for the plane and interplane exchange interactions and for different crystal fields have been obtained. The total magnetization and magnetic susceptibility with the crystal field have been established for different plane exchange interactions. We have also given the magnetic hysteresis cycle for different plane exchange interactions, different temperatures, and with different crystal fields in bilayer graphene. Finally, the system exhibits the superparamagnetic phase at the reduced transition temperature and for a fixed crystal field.

Layer- and substrate-dependent charge density wave criticality in 1T–TiSe 2

Abstract: TiSe2 exhibits an unconventional charge density wave (CDW) that has been associated with an excitonic insulator transition. Here we investigate how the CDW transition is changed for single to few layers compared to bulk TiSe2. TiSe2 grown by molecular beam epitaxy on HOPG- or MoS2-substrates is characterized by variable temperature scanning tunneling microscopy and spectroscopy. We show that the CDW state persists for the monolayer but the transition temperature T CDW is significantly increased compared to the bulk. Furthermore, T CDW is strongly dependent on the substrate material. Within the model of an excitonic insulator phase for TiSe2, the substrate dependence may be associated with variations of the excitonic binding energies by the dielectric properties of the substrate. Interestingly, for single layer TiSe2 on HOPG we also observe peaks in the tunneling spectra below 50 K, which are tentatively assigned to coherence peaks of an excitonic condensate. The peaks are observed below T CDW of ~230 K, suggesting that an excitonic insulator induced CDW can exist without a phase coherent state.