Showing papers on "Transition temperature published in 2015"
TL;DR: S sulfur hydride is investigated, and it is argued that the phase responsible for high-Tc superconductivity in this system is likely to be H3S, formed from H2S by decomposition under pressure, which raises hope for the prospects for achieving room-temperature super conductivity in other hydrogen-based materials.
Abstract: A superconductor is a material that can conduct electricity without resistance below a superconducting transition temperature, Tc. The highest Tc that has been achieved to date is in the copper oxide system: 133 kelvin at ambient pressure and 164 kelvin at high pressures. As the nature of superconductivity in these materials is still not fully understood (they are not conventional superconductors), the prospects for achieving still higher transition temperatures by this route are not clear. In contrast, the Bardeen-Cooper-Schrieffer theory of conventional superconductivity gives a guide for achieving high Tc with no theoretical upper bound--all that is needed is a favourable combination of high-frequency phonons, strong electron-phonon coupling, and a high density of states. These conditions can in principle be fulfilled for metallic hydrogen and covalent compounds dominated by hydrogen, as hydrogen atoms provide the necessary high-frequency phonon modes as well as the strong electron-phonon coupling. Numerous calculations support this idea and have predicted transition temperatures in the range 50-235 kelvin for many hydrides, but only a moderate Tc of 17 kelvin has been observed experimentally. Here we investigate sulfur hydride, where a Tc of 80 kelvin has been predicted. We find that this system transforms to a metal at a pressure of approximately 90 gigapascals. On cooling, we see signatures of superconductivity: a sharp drop of the resistivity to zero and a decrease of the transition temperature with magnetic field, with magnetic susceptibility measurements confirming a Tc of 203 kelvin. Moreover, a pronounced isotope shift of Tc in sulfur deuteride is suggestive of an electron-phonon mechanism of superconductivity that is consistent with the Bardeen-Cooper-Schrieffer scenario. We argue that the phase responsible for high-Tc superconductivity in this system is likely to be H3S, formed from H2S by decomposition under pressure. These findings raise hope for the prospects for achieving room-temperature superconductivity in other hydrogen-based materials.
1,756 citations
TL;DR: FeSe/STO is confirmed as an ideal material for studying high-Tc superconductivity by means of in situ four-point probe electrical transport measurements, and rekindle the long-standing idea that electron pairing at interfaces between two different materials can be tailored to achieve high-temperaturesuperconductivity.
Abstract: Monolayer iron selenide grown on SrTiO3 has recently gained attention due to suggestive evidence it superconducts at high temperature. In situ electrical transport measurements now reveal a transition temperature above 100 K.
968 citations
TL;DR: In this paper, the structural and optical properties of CH3NH3PbI3 from room temperature to 5 K were investigated, and X-ray diffraction reveals an extremely sharp transition at 163 K from a twinned tetragonal I4/mcm phase to a low temperature phase characterized by complex twinning and possible frozen disorder.
Abstract: Hybrid organometal halide perovskites have been demonstrated to have outstanding performance as semiconductors for solar energy conversion. Further improvement of the efficiency and stability of these devices requires a deeper understanding of their intrinsic photophysical properties. Here, the structural and optical properties of high-quality single crystals of CH3NH3PbI3 from room temperature to 5 K are investigated. X-ray diffraction reveals an extremely sharp transition at 163 K from a twinned tetragonal I4/mcm phase to a low-temperature phase characterized by complex twinning and possible frozen disorder. Above the transition temperature, the photoluminescence is in agreement with a band-edge transition, explaining the outstanding performances of the solar cells. Whereas below the transition temperature, three different excitonic features arise, one of which is attributed to a free-exciton and the other two to bound excitons (BEs). The BEs are characterized by a decay dynamics of about 5 μs and by a saturation phenomenon at high power excitation. The long lifetime and the saturation effect make us attribute these low temperature features to bound triplet excitons. This results in a description of the room temperature recombination as being due to spontaneous band-to-band radiative transitions, whereas a diffusion-limited behavior is expected for the low-temperature range.
335 citations
TL;DR: Experimental evidences of a liquid–liquid transition in glass-forming La50Al35Ni15 melt above its liquidus temperature by 27Al nuclear magnetic resonance including the temperature dependence of cage volume fluctuations and atomic diffusion and the need of two-order parameters in describing phase transitions of liquids are reported.
Abstract: Non-density driven liquid-liquid transition has been predicted in theories, but direct experimental verification is challenging because liquid often remains metastable at transition temperature. Here, Xu et al. provide evidence in a lanthanum-based metallic glass above its liquidus temperature.
107 citations
TL;DR: It is found that thermally excited intrinsic carriers are responsible for this behavior by introducing enough loss in the context of the radically modified electromagnetic environment in the vicinity of the nanogaps to extend the versatility of insulator to metal transition devices to encompass their semiconductor properties.
Abstract: We report that vanadium dioxide films patterned with λ/100000 nanogaps exhibit an anomalous transition behavior at millimeter wavelengths. Most of the hybrid structure's switching actions occur well below the insulator to metal transition temperature, starting from 25 °C, so that the hysteresis curves completely separate themselves from their bare film counterparts. It is found that thermally excited intrinsic carriers are responsible for this behavior by introducing enough loss in the context of the radically modified electromagnetic environment in the vicinity of the nanogaps. This phenomenon newly extends the versatility of insulator to metal transition devices to encompass their semiconductor properties.
101 citations
TL;DR: The structure, critical exponents and magnetocaloric effect (MCE) of the second-order magnetic phase transition were investigated in detail in this article, showing that the sample adopts an orthorhombic structure with Pnma space group.
87 citations
TL;DR: In this paper, it was shown that the behavior exhibited by several quantities that characterise these caloric effects (isothermal entropy change, adiabatic temperature change and refrigerant capacity) can be rationalised in terms of the relative distance between the Curie point of the austenite and the martensitic transition temperature.
80 citations
TL;DR: In this article, the authors investigated the aggregation of P3HT in THF solution within a temperature range from 300 to 5 K. By detailed steadystate, site-selective, and time-resolved fluorescence spectroscopy combined with Franck-Condon analyses, they showed that below a certain transition temperature (265 K) aggregates are formed that prevail in different polymorphs.
Abstract: In an endeavor to correlate the optoelectronic properties of π-conjugated polymers with their structural properties, we investigated the aggregation of P3HT in THF solution within a temperature range from 300 to 5 K. By detailed steady-state, site-selective, and time-resolved fluorescence spectroscopy combined with Franck–Condon analyses, we show that below a certain transition temperature (265 K) aggregates are formed that prevail in different polymorphs. At 5 K, we can spectroscopically identify two H-type aggregates with planar polymer backbones yet different degree of order regarding their side chains. Upon heating, the H-character of the aggregates becomes gradually eroded, until just below the transition temperature the prevailing “aggregate” structure is that of still phase-separated, yet disordered main and side chains. These conclusions are derived by analyzing the vibrational structure of the spectra and from comparing the solution spectra with those obtained from thin films that were cooled slo...
79 citations
TL;DR: In this paper, a reactively sputtered, undoped film was shown to achieve a transition temperature of 45°C by restricting grain size to approximately 30'nm, which is the smallest grain size possible.
Abstract: Vanadium(IV) oxide (VO2) is a unique material that undergoes a reversible phase transformation around 68 °C. The material could potentially be used as an energy-efficient coating for windows since its reflectance in the infrared (IR) increases significantly more than in the visible region. Currently, VO2 is limited by a transition temperature (τc) that is too high, luminous transmittance that is too low or both. In this study, a transition temperature of 45 °C is achieved for a reactively sputtered, undoped film by restricting grain size to approximately 30 nm. It is concluded that a higher density of grain boundaries (smaller grain size) provides a greater number of nucleating defects which in turn reduces τc. Similarly, a higher density of grain boundaries may reduce the hysteresis width (difference between transition temperatures in heating and cooling). Also in this study, a new set of optical performance metrics is proposed in which the solar spectrum is divided into the ultraviolet (UV), visible and...
76 citations
TL;DR: In this paper, the results of an experimental investigation on the magnetocaloric properties of hydrogenated La(Fe-Mn-Si)13-H with Mn substituting Fe to finely tune the transition temperature were presented.
Abstract: In this paper, we present the results of an experimental investigation on the magnetocaloric properties of hydrogenated La(Fe-Mn-Si)13-H with Mn substituting Fe to finely tune the transition temperature. We measured the specific heat under magnetic field cp(H, T) and the magnetic field induced isothermal entropy change Δs(H, T) of a series of compounds by direct Peltier calorimetry. Results show that increasing Mn from 0.06 to 0.46 reduces the transition temperature from 339 K to 270 K whilst the total entropy change due to a 1.5 T field is depressed from 18.7 J kg−1 K−1 to 10.2 J kg−1 K−1 and the thermal hysteresis similarly is reduced from 1.5 K to zero. In the paper, we interpret the results in terms of a magnetic phase transition changing from the first to the second order with increasing Mn content, and we discuss the value of the results for magnetic cooling applications.
70 citations
TL;DR: In this article, the coexistence of bulk superconductivity and magnetic ordering was observed in the case of the CeO1−xFxBiS2 (x = 0-1.0) prepared by annealing under high pressure.
Abstract: We show the observation of the coexistence of bulk superconductivity and magnetic ordering in CeO1−xFxBiS2 (x = 0–1.0) prepared by annealing under high pressure. In the CeO1−xFxBiS2 system, both superconductivity and two types of ferromagnetic-like ordering with magnetic transition temperatures of 4.5 and 7.5 K are induced upon systematic F substitution. This fact suggests that carriers generated by the substitution of O by F are supplied to not only the BiS2 superconducting layers but also the CeO blocking layers. Furthermore, the highest superconducting transition temperature is observed when the ferromagnetic-like transition temperature increases, which implies that superconductivity and this magnetic ordering are linked to each other in the CeO1−xFxBiS2 system.
TL;DR: In this paper, the effect of Ba substitution on the structural, mechanical, electrical transport and thermal properties of La0.65Ca0.35� xBaxMnO3 (0.00 6 x 6 0.25) manganites have been investigated.
TL;DR: Results of extensive molecular simulations that mimic a number of features of the experimental vapor deposition process are presented, including a mechanism based on distinct orientations observed at equilibrium near the surface of the film, which get trapped within the film during the non-equilibrium process of vapor deposition.
Abstract: Enhanced kinetic stability of vapor-deposited glasses has been established for a variety of glass organic formers. Several recent reports indicate that vapor-deposited glasses can be orientationally anisotropic. In this work, we present results of extensive molecular simulations that mimic a number of features of the experimental vapor deposition process. The simulations are performed on a generic coarse-grained model and an all-atom representation of N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine (TPD), a small organic molecule whose vapor-deposited glasses exhibit considerable orientational anisotropy. The coarse-grained model adopted here is found to reproduce several key aspects reported in experiments. In particular, the molecular orientation of vapor-deposited glasses is observed to depend on substrate temperature during deposition. For a fixed deposition rate, the molecular orientation in the glasses changes from isotropic, at the glass transition temperature, Tg, to slightly normal to the substrate at temperatures just below Tg. Well below Tg, molecular orientation becomes predominantly parallel to the substrate. The all-atom model is used to confirm some of the equilibrium structural features of TPD interfaces that arise above the glass transition temperature. We discuss a mechanism based on distinct orientations observed at equilibrium near the surface of the film, which get trapped within the film during the non-equilibrium process of vapor deposition.
TL;DR: In this paper, the importance of open volume defects onto the magnetic phase transition was investigated by means of magneto-optical and spectroscopy techniques, i.e., Kerr effect, positron annihilation, and Mossbauer spectrograms.
Abstract: Magnetic phase transition in the Fe60Al40 transition metal aluminide from the ferromagnetic disordered A2-phase to the paramagnetic ordered B2-phase as a function of annealing up to 1000 °C has been investigated by means of magneto-optical and spectroscopy techniques, i.e., Kerr effect, positron annihilation, and Mossbauer spectroscopy. The positron annihilation spectroscopy has been performed in-situ sequentially after each annealing step at the Apparatus for In-situ Defect Analysis that is a unique tool combining positron annihilation spectroscopy with temperature treatment, material evaporation, ion irradiation, and sheet resistance measurement techniques. The overall goal was to investigate the importance of the open volume defects onto the magnetic phase transition. No evidence of variation in the vacancy concentration in matching the magnetic phase transition temperature range (400–600 °C) has been found, whereas higher temperatures showed an increase in the vacancy concentration.
TL;DR: In this article, X-ray diffraction (XRD) analysis was confirmed by XRD analysis which is in coherence with the results obtained by Rietveld refinement.
TL;DR: In this paper, the authors show that the magnetostructural transition temperature is tunable in a wide temperature range of about 280 K. They also suggest that MnNiSi-Fe2Ge material system is a promising platform for tunable magnetic transition and the associated magnetocaloric effect.
Abstract: Magnetostructural transition from ferromagnetic orthorhombic phase to paramagnetic hexagonal phase can be obtained by chemically alloying appropriate amount of Fe2Ge into MnNiSi. The magnetostructural transition temperature is tunable in a wide temperature range of about 280 K. Saturation moment of the ferromagnetic orthorhombic phase increases from 2.62 μB/f.u. to 3.17 μB/f.u. with Fe2Ge-doping. The magnetostructural transition is accompanied by a large change of magnetization over 80 Am2/kg under magnetic field of 5 T. Relatively large magnetic entropy changes and working temperature ranges were observed in the vicinity of room temperature. Our findings suggest that MnNiSi-Fe2Ge material system is a promising platform for tunable magnetostructural transition and the associated magnetocaloric effect.
TL;DR: In this article, the effect of Gd-Sn substitution on the structural, electrical and magnetic properties of Sr 1− x Gd x Fe 12− y Sn y O 19 was investigated.
TL;DR: In this article, the high pressure electronic properties of BaReH9 were investigated using diamond-anvil-cell electrical conductivity techniques to megabar (100 GPa) pressures.
Abstract: BaReH9 is an exceedingly high-hydrogen-content metal hydride that is predicted to exhibit interesting behavior under pressure. The high-pressure electronic properties of this material were investigated using diamond-anvil-cell electrical conductivity techniques to megabar (100 GPa) pressures. The measurements show that BeReH9 transforms into a metal and then a superconductor above 100 GPa with a maximum transition temperature (Tc) near 7 K. The occurrence of superconductivity was confirmed by the suppression of the resistance drop upon application of magnetic fields. The transition to the metallic phase is sluggish, but it is accelerated by laser irradiation. Raman scattering and X-ray diffraction measurements, used to supplement the electrical measurements, indicate that the Ba–Re sublattice is largely preserved upon compression under the conditions explored, but there is a possibility that hydrogen atoms are gradually disordered under pressure. This is suggested by the sharpening of the peaks in Raman s...
TL;DR: Thin films of the spin-crossover (SCO) molecule Fe{[Me2Pyrz]3BH}2 (Fe-pyrz) were sublimed on Si/SiO2 and quartz substrates, and their properties investigated by X-ray absorption and photoemission spectroscopies, optical absorption, atomic force microscopy, and superconducting quantum interference device.
Abstract: Thin films of the spin-crossover (SCO) molecule Fe{[Me2Pyrz]3BH}2 (Fe-pyrz) were sublimed on Si/SiO2 and quartz substrates, and their properties investigated by X-ray absorption and photoemission spectroscopies, optical absorption, atomic force microscopy, and superconducting quantum interference device. Contrary to the previously studied Fe(phen)2(NCS)2, the films are not smooth but granular. The thin films qualitatively retain the typical SCO properties of the powder sample (SCO, thermal hysteresis, soft X-ray induced excited spin-state trapping, and light induced excited spin-state trapping) but present intriguing variations even in micrometer-thick films: the transition temperature decreases when the thickness is decreased, and the hysteresis is affected. We explain this behavior in the light of recent studies focusing on the role of surface energy in the thermodynamics of the spin transition in nano-structures. In the high-spin state at room temperature, the films have a large optical gap (∼5 eV), decreasing at thickness below 50 nm, possibly due to film morphology.
TL;DR: It is concluded that the increase in the gauche-gauche gg conformers, which shows the conformation change under a low stress level, reduces the critical onset stress for shear yielding.
TL;DR: A key role of magnetic-field-induced structural instability in enhancing the magnetocaloric effect for MnCo1−xZnxGe alloys is shown, pointing to the importance of the magnetostructural coupling for the design of high-performance magnetocalorics.
Abstract: Magnetocaloric effect is the phenomenon that temperature change of a magnetic material is induced by application of a magnetic field. This effect can be applied to environmentally-benign magnetic refrigeration technology. Here we show a key role of magnetic-field-induced structural instability in enhancing the magnetocaloric effect for MnCo1−xZnxGe alloys (x = 0–0.05). The increase in x rapidly reduces the martensitic transition temperature while keeping the ferromagnetic transition around room temperature. Fine tuning of x around x = 0.03 leads to the concomitant structural and ferromagnetic transition in a cooling process, giving rise to enhanced magnetocaloric effect as well as magnetic-field-induced structural transition. Analyses of the structural phase diagrams in the T-H plane in terms of Landau free-energy phenomenology accounts for the characteristic x-dependence of the observed magnetocaloric effect, pointing to the importance of the magnetostructural coupling for the design of high-performance magnetocalorics.
TL;DR: In this article, the structural, electrical, magnetic and thermo-electric properties of La 0.8 Ba x Ca 0.2 − x MnO 3 (0,⩽ ǫ xǫ ⩽ Á 2 ) manganites are analyzed using the phenomenological percolation model.
TL;DR: In this paper, structural, magnetic and transport studies on monovalent doped La1−xNaxMnO3 mixed valent manganites synthesized by conventional ceramic route are reported.
TL;DR: In-situ synchrotron XRD measurements of the magnetocaloric material LaFe118Si12 are used to understand the virgin effects and asymmetry of the underlying first order magnetovolume transition as mentioned in this paper.
Abstract: In-situ synchrotron XRD measurements of the magnetocaloric material LaFe118Si12 are used to understand virgin effects and asymmetry of the underlying first order magnetovolume transition A remarkable change of the transition kinetics occurs after the first cycle, which we attribute to the formation of cracks originating from the volume change Tomographic imaging revealed that the bulk material disintegrates via an interlocked state where fragments are loosely connected Though cracks have opened between the fragments, the transition is sharp, which we attribute to magnetostatic interactions In the cycled sample we find a strong asymmetry between the transition interval upon heating and cooling, which we explain by isostatic pressure acting on parts of the sample during the cooling transition (© 2015 WILEY-VCH Verlag GmbH &Co KGaA, Weinheim)
TL;DR: The dynamics of the imidazolium based room temperature ionic liquid Bmim Tf2N was investigated by means of nuclear magnetic resonance relaxation dispersion (NMRD) and NMR-PFG diffusion experiments on the bulk liquid in a wide range of temperatures, giving evidence of heterogeneities in the dynamics.
Abstract: The dynamics of the imidazolium based room temperature ionic liquid Bmim Tf2N was investigated by means of nuclear magnetic resonance relaxation dispersion (NMRD) and nuclear magnetic resonance pulsed field gradient (NMR-PFG) diffusion experiments on the bulk liquid in a wide range of temperatures. Relaxation and diffusion properties were determined for anions and cations individually, giving evidence of heterogeneities in the dynamics of the ionic liquid. The relevant NMR relaxation mechanisms are the inter- and intramolecular dipolar interactions between the molecular ions reflecting the molecular translational and rotational diffusion. Rotational and translational correlation times could be obtained and showed different dependences on temperature. The experimental diffusion values follow the Vogel–Fulcher–Tammann (VFT) relation above a transition temperature Tc ∼ 1.26 Tg, below which a deviation was observed. Differential scanning calorimetry experiments show a transition at the same temperature.
TL;DR: A negative magnetoresistance (MR) has been observed at all temperatures in contrast to positive behavior generally observed in strongly spin-orbit coupled materials, which implies the relevance of a quantum interference effect.
Abstract: We have investigated the temperature evolution of magnetism and its interrelation with structural parameters in the perovskite-based layered compound Sr2IrO4, which is believed to be a Jeff = 1/2 Mott insulator. The structural distortion plays an important role in this material and induces a weak ferromagnetism in an otherwise antiferromagnetically ordered magnetic state with a transition temperature around 240 K. Interestingly, at low temperatures, below around 100 K, a change in the magnetic moment has been observed. Temperature dependent x-ray diffraction measurements show that sudden changes in structural parameters around 100 K are responsible for this. Resistivity measurements show insulating behavior throughout the temperature range across the magnetic phase transition. The electronic transport can be described with Mott's two-dimensional variable range hopping (VRH) mechanism, however, three different temperature ranges are found for VRH, which is a result of varying the localization length with temperature. A negative magnetoresistance (MR) has been observed at all temperatures in contrast to positive behavior generally observed in strongly spin-orbit coupled materials. The quadratic field dependence of MR implies the relevance of a quantum interference effect.
TL;DR: In this paper, the structural and micro-structural properties of sol-gel grown nanostructured La0.6Nd0.1Sr0.3MnO3 (LNSMO) manganites sintered and annealed at different temperatures (viz., 700, 800 and 900 °C).
TL;DR: A way to achieve abrupt high-spin to low-spin transition with controllable transition temperature and hysteresis width is reported, relying not on solid-state cooperative interactions, but utilizing coherency between phase and spin transitions in neutral Fe(II) meltable complexes.
Abstract: Herein, we report a way to achieve abrupt high-spin to low-spin transition with controllable transition temperature and hysteresis width, relying not on solid-state cooperative interactions, but utilizing coherency between phase and spin transitions in neutral Fe(II) meltable complexes.
TL;DR: In this paper, the authors investigated the temperature dependent dielectric relaxation and conduction mechanism of multiferroic Y1−xGdxFe0.6Mn0.4O3 systems.
TL;DR: In this article, the phase transition temperature of VO2 films can be easily tuned from an intrinsic temperature to 44.7 °C and 40.2 °C on glass and sapphire by annealing oxygen pressure.
Abstract: A simple new way to tune the optical phase transition temperature of VO2 films was proposed by only controlling the pressure of oxygen during the annealing process. Vanadium films were deposited on glass by a large-scale magnetron sputtering coating system and then annealed in appropriate oxygen atmosphere to form the VO2 films. The infrared transmission change (at 2400 nm) is as high as 58% for the VO2 thin film on the glass substrate, which is very good for tuning infrared radiation and energy saving as smart windows. The phase transition temperature of the films can be easily tuned from an intrinsic temperature to 44.7 °C and 40.2 °C on glass and sapphire by annealing oxygen pressure, respectively. The mechanism is: V3+ ions form in the film when under anaerobic conditions, which can interrupt the V4+ chain and reduce the phase transition temperature. The existence of V3+ ions has been observed by x-ray photoelectron spectroscopy (XPS) experiments as proof.