Showing papers on "Transition temperature published in 2013"

Phase diagram and electronic indication of high-temperature superconductivity at 65 K in single-layer FeSe films

Abstract: The recent discovery of possible high-temperature superconductivity in single-layer FeSe films has generated significant experimental and theoretical interest. In both the cuprate and the iron-based high-temperature superconductors, superconductivity is induced by doping charge carriers into the parent compound to suppress the antiferromagnetic state. It is therefore important to establish whether the superconductivity observed in the single-layer sheets of FeSe--the essential building blocks of the Fe-based superconductors--is realized by undergoing a similar transition. Here we report the phase diagram for an FeSe monolayer grown on a SrTiO3 substrate, by tuning the charge carrier concentration over a wide range through an extensive annealing procedure. We identify two distinct phases that compete during the annealing process: the electronic structure of the phase at low doping (N phase) bears a clear resemblance to the antiferromagnetic parent compound of the Fe-based superconductors, whereas the superconducting phase (S phase) emerges with the increase in doping and the suppression of the N phase. By optimizing the carrier concentration, we observe strong indications of superconductivity with a transition temperature of 65±5 K. The wide tunability of the system across different phases makes the FeSe monolayer ideal for investigating not only the physics of superconductivity, but also for studying novel quantum phenomena more generally.

Control of the metal-insulator transition in vanadium dioxide by modifying orbital occupancy

Abstract: Bulk vanadium dioxide undergoes a metal–insulator transition near room temperature. It is now shown that by putting a thin layer of vanadium dioxide on a buffer, and varying the buffer’s thickness, the orbital occupancy in the metallic state and the transition temperature can be tuned.

Ultrathin Nanosheets of Vanadium Diselenide: A Metallic Two-Dimensional Material with Ferromagnetic Charge-Density-Wave Behavior

Abstract: A new metallic 2D material with high electrical conductivity (1×10(3) S m(-1)) consists of VSe2 ultrathin nanosheets with 4-8 Se-V-Se atomic layers. This is the first 2D transition-metal dichalcogenide with intrinsic room-temperature ferromagnetism. The nanosheets increase the charge-density-wave transition temperature to 135 K by dimensional reduction.

Role of Thermal Heating on the Voltage Induced Insulator-Metal Transition in VO 2

Abstract: We show that the main mechanism for the dc voltage or dc current induced insulator-metal transition in vanadium dioxide VO(2) is due to local Joule heating and not a purely electronic effect. This "tour de force" experiment was accomplished by using the fluorescence spectra of rare-earth doped micron sized particles as local temperature sensors. As the insulator-metal transition is induced by a dc voltage or dc current, the local temperature reaches the transition temperature indicating that Joule heating plays a predominant role. This has critical implications for the understanding of the dc voltage or dc current induced insulator-metal transition and has a direct impact on applications which use dc voltage or dc current to externally drive the transition.

Mg-doped VO2 nanoparticles: hydrothermal synthesis, enhanced visible transmittance and decreased metal–insulator transition temperature

Abstract: This paper reports the successful preparation of Mg-doped VO2 nanoparticles via hydrothermal synthesis. The metal–insulator transition temperature (Tc) decreased by approximately 2 K per at% Mg. The Tc decreased to 54 °C with 7.0 at% dopant. The composite foils made from Mg-doped VO2 particles displayed excellent visible transmittance (up to 54.2%) and solar modulation ability (up to 10.6%). In addition, the absorption edge blue-shifted from 490 nm to 440 nm at a Mg content of 3.8 at%, representing a widened optical band gap from 2.0 eV for pure VO2 to 2.4 eV at 3.8 at% doping. As a result, the colour of the Mg-doped films was modified to increase their brightness and lighten the yellow colour over that of the undoped-VO2 film. A first principle calculation was conducted to understand how dopants affect the optical, Mott phase transition and structural properties of VO2.

The Effects of Environmental Water and Oxygen on the Temperature-Dependent Friction of Sputtered Molybdenum Disulfide

Abstract: Molybdenum disulfide (MoS2) is well known for exceptional friction and wear properties in inert and high vacuum environments. However, these tribological properties degrade in humid and high temperature environments for reasons that are not fully understood. A prevailing hypothesis suggests that moisture and thermal energy facilitate oxidation, which increases the shear strength of the sliding interface. The purpose of this study is to elucidate the contributions of water, oxygen, and temperature to the tribological degradation of MoS2. Generally speaking, we found a minimum friction coefficient that occurred at a temperature we defined as the transition temperature. This transition temperature ranged from 100 to 250 °C and was a strong function of the MoS2 preparation and thermal sliding history. Below the transition temperature, friction increased with increased water, but was insensitive to oxygen. Above the transition, friction increased with increased oxygen, but decreased to a limited extent with increased water. These results are generally consistent with prior results, but clarify some inconsistencies in the literature discussions. Contrary to the prevailing hypothesis, the results suggest that water does not promote oxidation near room temperature, but directly interferes with lamellar shear through physical bonding. Increased temperatures drive off water and thereby reduce friction up to the transition temperature. The results suggest that oxidation causes increased friction with increased temperature above the transition temperature. The data also suggest that water helps mitigate high temperature oxidation by displacing the environmental oxygen or by preferentially adsorbing to the surface.

Enhanced cooperative interactions at the nanoscale in spin-crossover materials with a first-order phase transition.

Abstract: We analyzed the size effect on a first-order spin transition governed by elastic interactions. This study was performed in the framework of a nonextensive thermodynamic core-shell model. When decreasing the particle size, differences in surface energies between the two phases lead to the shrinking of the thermal hysteresis width, the lowering of the transition temperature, and the increase of residual fractions at low temperature, in good agreement with recent experimental observations on spin transition nanomaterials. On the other hand, a modification of the particle-matrix interface may allow for the existence of the hysteresis loop even at very low sizes. In addition, an unexpected reopening of the hysteresis, when the size decreases, is also possible due to the hardening of the nanoparticles at very small sizes, which we deduced from the size dependence of the Debye temperature of a series of coordination nanoparticles.

Phase transitions, relaxor behavior, and large strain response in LiNbO3-modified Bi0.5(Na0.80K0.20)0.5TiO3 lead-free piezoceramics

Abstract: The effect of LiNbO3 (LN) addition on the ferroelectric behavior and piezoelectric properties of Bi0.5(Na0.80K0.20)0.5TiO3 (BNKT20) lead-free piezoceramics were systematically investigated. Results showed that the LN substitution into BNKT20 induced a transition from coexistence of ferroelectric tetragonal and rhombohedral phases to relaxor pseudocubic phases, which is accompanied by the significant disruption of ferroelectric order and with the shift of the ferroelectric-relaxor transition temperature TF-R down to room temperature. Accordingly, a large accompanying normalized strain of ∼0.38% (corresponding to a large signal d33* of ∼475 pm/V) were obtained in BNKT20 with 2.5 mol. %LN addition near the phase boundary. Temperature-dependent measurements of both polarization and strain from room temperature to 120 °C suggested that the origin of the large strain is due to a reversible field-induced ergodic relaxor-to-ferroelectric phase transformation. Moreover, an attractive property for application as hi...

Ethylenediamine-Mediated Wurtzite Phase Formation in ZnS

Abstract: The usual high temperature wurtzite phase of ZnS was successfully obtained at low temperature (170 °C) in the presence of ethylenediamine (EN) as the soft template. X-ray diffraction and Raman spectroscopy analysis confirmed the EN-mediated phase transformation (zinc blende to wurtzite) of ZnS. X-ray photoelectron spectroscopy (XPS) showed that all the samples were sulfur deficient. A high temperature X-ray diffraction (XRD) study showed that ZnS samples, both EN-mediated and without EN, retained their phases except small changes in the unit cell dimension. Besides the EN-mediated phase transition, morphology transformations from nearly spherical shape to nanorods are also observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The coupling between EN molecules with ZnS is confirmed by Fourier transform infrared spectroscopy. A significant reduction in the phase transition temperature of ZnS has been achieved as compared to the bulk transition temperature (1020 °C). Mech...

High-temperature surface superconductivity in rhombohedral graphite

Abstract: Surface superconductivity in rhombohedral graphite is a robust phenomenon which can exist even when higher order hoppings between the layers lift the topological protection of the surface flat band and introduce a quadratic dispersion of electrons with a heavy effective mass. We show that for weak pairing interaction, the flat-band character of the surface superconductivity transforms into a BCS-like relation with high critical temperature characterized by a higher coupling constant due to a much larger density of states than in the bulk. Our results offer an explanation for the recent findings of graphite superconductivity with an unusually high transition temperature.

Magnetism driven ferroelectricity above liquid nitrogen temperature in Y2CoMnO6

Abstract: We report multiferroic behaviour in double perovskite Y2CoMnO6 with ferroelectric transition temperature Tc = 80 K. Both X-ray diffraction and neutron scattering data confirm a centro-symmetric crystal structure of space group P21/n at room temperature. The saturation polarization and magnetization are estimated to be 65 μC/m2 and 6.2 μB/f.u. respectively. The magneto-electric coupling parameter, on the other hand, is small as a 5 T field suppresses the electric polarization by only ∼8%. The origin of ferroelectricity is associated with magnetic ordering of Co2+ and Mn4+ moments in ↑↑-↓↓ arrangement. A model based on exchange-striction is proposed to explain the observed high temperature ferroelectricity.

Enhancement of superconductivity near the pressure-induced semiconductor–metal transition in the BiS2-based superconductors LnO0.5F0.5BiS2 (Ln = La, Ce, Pr, Nd)

Abstract: Measurements of electrical resistivity were performed between 3 and 300 K at various pressures up to 2.8 GPa on the BiS2-based superconductors LnO0.5F0.5BiS2 (Ln=Pr, Nd). At lower pressures, PrO0.5F0.5BiS2 and NdO0.5F0.5BiS2 exhibit superconductivity with critical temperatures Tc of 3.5 and 3.9 K, respectively. As pressure is increased, both compounds undergo a transition at a pressure Pt from a low Tc superconducting phase to a high Tc superconducting phase in which Tc reaches maximum values of 7.6 and 6.4 K for PrO0.5F0.5BiS2 and NdO0.5F0.5BiS2, respectively. The pressure-induced transition is characterized by a rapid increase in Tc within a small range in pressure of ∼0.3 GPa for both compounds. In the normal state of PrO0.5F0.5BiS2, the transition pressure Pt correlates with the pressure where the suppression of semiconducting behaviour saturates. In the normal state of NdO0.5F0.5BiS2, Pt is coincident with a semiconductor–metal transition. This behaviour is similar to the results recently reported for the LnO0.5F0.5BiS2 (Ln=La, Ce) compounds. We observe that Pt and the size of the jump in Tc between the two superconducting phases both scale with the lanthanide element in LnO0.5F0.5BiS2 (Ln=La, Ce, Pr, Nd).

Superconductivity in two-dimensional NbSe2 field effect transistors

Abstract: We describe investigations of superconductivity in few molecular layer NbSe2 field effect transistors. While devices fabricated from NbSe2 flakes less than eight molecular layers thick did not conduct, thicker flakes were superconducting with an onset Tc that was only slightly depressed from the bulk value for 2H-NbSe2 (7.2 K). The resistance typically showed a small, sharp high temperature transition followed by one or more broader transitions which usually ended in a wide tail to zero resistance at low temperatures. We speculate that these multiple resistive transitions are related to disorder in the layer stacking. The behavior of several flakes has been characterized as a function of temperature, applied field and back-gate voltage. We find that the conductance in the normal state and transition temperature depend weakly on the gate voltage, with both conductivity and Tc decreasing as the electron concentration is increased. The application of a perpendicular magnetic field allows the evolution of different resistive transitions to be tracked and values of the zero temperature upper critical field, Hc2.0/, and coherence length,. 0/, to be independently estimated. Our results are analyzed in terms of available theories for these phenomena. (Some figures may appear in colour only in the online journal)

Theoretical study on the tungsten-induced reduction of transition temperature and the degradation of optical properties for VO2

Abstract: We have studied the effect of the doped tungsten on the phase transition temperature and the optical properties between the monoclinic phase and the rutile phase of VO2 by performing first-principles calculations It is found that the introduction of tungsten (W) significantly decreases the transition temperature between the rutile VO2 and the monoclinic VO2, with reducing the energy difference between the two phases Meanwhile, our calculations indicate that the doped W lowers the infrared light transmittance of the monoclinic VO2, being consistent with the experimental observations The nature about the reduction of the transition temperature and the change of the optical properties arising from the W dopants is revealed Our results are valuable for application of VO2 as an optical energy material in future

Superconductivity with extremely large upper critical fields in Nb2Pd0.81S5

Abstract: Here, we report the discovery of superconductivity in a new transition metal-chalcogenide compound, i.e. Nb2Pd0.81S5, with a transition temperature Tc ≅ 6.6 K. Despite its relatively low Tc, it displays remarkably high and anisotropic superconducting upper critical fields, e.g. μ0Hc2 (T → 0 K) > 37 T for fields applied along the crystallographic b-axis. For a field applied perpendicularly to the b-axis, μ0Hc2 shows a linear dependence in temperature which coupled to a temperature-dependent anisotropy of the upper critical fields, suggests that Nb2Pd0.81S5 is a multi-band superconductor. This is consistent with band structure calculations which reveal nearly cylindrical and quasi-one-dimensional Fermi surface sheets having hole and electron character, respectively. The static spin susceptibility as calculated through the random phase approximation, reveals strong peaks suggesting proximity to a magnetic state and therefore the possibility of unconventional superconductivity.

Magnetism driven ferroelectricity above liquid nitrogen temperature in Y2CoMnO6

Abstract: We report multiferroic behavior in double perovskite Y2CoMnO6 with ferroelectric transition temperature Tc = 80K. The origin of ferroelectricity is associated with magnetic ordering of Co2+ and Mn4+ moments in a up-up-down-down arrangement. The saturation polarization and magnetization are estimated to be 65 uC/m2 and 6.2 Bohr magneton/f.u. respectively. The magnetoelectric coupling parameter, on the other hand, is small as a 5 Tesla field suppresses the electric polarization by only ~8%. This is corroborated with observed hysteretic behaviour at 5K that remains unsaturated even upto 7 Tesla. A model based on exchange-striction is proposed to explain the observed high temperature ferroelectricity.

Anionic depolymerization transition in IrTe2.

Abstract: Selenium substitution drastically increases the transition temperature of iridium ditelluride (IrTe(2)) to a diamagnetic superstructure from 278 to 560 K. Transmission electron microscopy experiments revealed that this enhancement is accompanied by the evolution of nonsinusoidal structure modulations from q = 1/5(101) to q = 1/6(101) types. These comprehensive results are consistent with the concept of the destabilization of polymeric Te-Te bonds at the transition, the temperature of which is increased by chemical and hydrostatic pressure and by the substitution of Te with the more electronegative Se. This temperature-induced depolymerization transition in IrTe(2) is unique in crystalline inorganic solids.

High-temperature surface superconductivity in rhombohedral graphite

Abstract: Surface superconductivity in rhombohedral graphite is a robust phenomenon which can exist even when higher order hoppings between the layers lift the topological protection of the surface flat band and introduce a quadratic dispersion of electrons with a heavy effective mass. We show that for weak pairing interaction, the flat band character of the surface superconductivity transforms into a BCS-like relation with high critical temperature characterized by a higher coupling constant due to a much larger density of states than in the bulk. Our results offer an explanation for the recent findings of graphite superconductivity with an unusually high transition temperature.

Dielectric and magnetoelectric properties of BaTiO3–Co0.6Zn0.4Fe1.7Mn0.3O4 composite

Abstract: In this paper we studied the structural, dielectric, magnetic and magnetoelectric properties of (x)BaTiO3–(1 − x)Co0.6Zn0.4Fe1.7Mn0.3O4 particulate composite series where x = 0.50, 0.60 and 0.70. BaTiO3–Co0.6Zn0.4Fe1.7Mn0.3O4 composite has the advantage of being non-toxic and environmental friendly from the point of view of device fabrication. High ME voltage coefficients were obtained in the whole series with the highest value of αE ∼ 73 mV/cm Oe achieved in sample x = 0.50 containing equal mole fractions of both the component phases. This value of αE is an order of magnitude higher than that of particulate sintered BaTiO3–CoFe2O4 composites (∼2–4 mV/cm Oe). Dielectric characteristics for these samples indicated two anomalies: (i) one at low temperature close to ferroelectric to paraelectric transition temperature of pure BaTiO3 and (ii) another at higher temperature related to the magnetic transition in ferrite, a characteristic dielectric feature of composite sample.

Effect of a built-in electric field in asymmetric ferroelectric tunnel junctions

Abstract: The contribution of a built-in electric field to ferroelectric phase transition in asymmetric ferroelectric tunnel junctions is studied using a multiscale thermodynamic model. It is demonstrated in detail that there exists a critical thickness at which an unusual ferroelectric-``polar nonferroelectric'' phase transition occurs in asymmetric ferroelectric tunnel junctions. In the ``polar nonferroelectric'' phase, there is only one nonswitchable polarization which is caused by the competition between the depolarizing field and the built-in field, and closurelike domains are proposed to form so as to minimize the system energy. The transition temperature is found to decrease monotonically as the ferroelectric barrier thickness is decreased and the reduction becomes more significant for the thinner ferroelectric layers. As a matter of fact, the built-in electric field not only results in smearing of the phase transition, but also forces the transition to take place at a reduced temperature. Such findings may impose a fundamental limit on the work temperature and thus should be further taken into account in the future ferroelectric-tunnel-junction--type or ferroelectric-capacitor--type devices.

High temperature physical and mechanical properties of large-scale Ti2AlC bulk synthesized by self-propagating high temperature combustion synthesis with pseudo hot isostatic pressing

Abstract: The electrical, thermal, and mechanical properties as well as the effect of the temperature of large-scale Ti2AlC bulk synthesized by self-propagating high temperature combustion synthesis with pseudo hot isostatic pressing were investigated in detail. With increasing temperature, the lattice defects contribute to the decreasing phonon thermal conductivity, and the electrical resistivity increases linearly from room temperature (RT) to 900 °C. The RT flexural strength, compressive strength, fracture toughness, work of fracture, and Vickers hardness were measured to be 606 ± 20 MPa, 1057 ± 84 MPa, 6.9 ± 0.2 MPa m1/2, 158 ± 12 J/m2, and 4.7 ± 0.2 GPa, respectively. With increasing temperature, the flexural and compressive strengths both keep almost unchanged in the zone of brittle failure, but decrease sharply as the plastic deformation occurs. The brittle-plastic transition temperature under flexure (900–950 °C) is higher than compression (700–800 °C). Interestingly, a non-catastrophic failure is observed in the SENB test, with the high work of fracture (158 ± 12 J/m2).

Sharp semiconductor-to-metal transition of VO2 thin films on glass substrates

Abstract: Outstanding phase transition properties of vanadium dioxide (VO2) thin films on amorphous glass were achieved and compared with the ones grown on c-cut sapphire and Si (111) substrates, all by pulsed laser deposition. The films on glass substrate exhibit a sharp semiconductor-to-metal transition (∼4.3 °C) at a near bulk transition temperature of ∼68.4 °C with an electrical resistance change as high as 3.2 × 103 times. The excellent phase transition properties of the films on glass substrate are correlated with the large grain size and low defects density achieved. The phase transition properties of VO2 films on c-cut sapphire and Si (111) substrates were found to be limited by the high defect density.

Tailoring the Piezoelectric and Relaxor Properties of (Bi1/2Na1/2)TiO3–BaTiO3 via Zirconium Doping

Abstract: This article details the influence of zirconium doping on the piezoelectric properties and relaxor characteristics of 94(Bi1/2Na1/2)TiO3–6Ba(ZrxTi1−x)O3 (BNT–6BZT) bulk ceramics. Neutron diffraction measurements of BNT–6BZT doped with 0%–15% Zr revealed an electric-field-induced transition of the average crystal structure from pseudo-cubic to rhombohedral/tetragonal symmetries across the entire compositional range. The addition of Zr up to 10% stabilizes this transition, resulting in saturated polarization hysteresis loops with a maximum polarization of 40 μC/cm2 at 5.5 kV/mm, while corresponding strain hysteresis measurements yield a maximum strain of 0.3%. With further Zr addition, the ferroelectric order is progressively destabilized and typical relaxor characteristics such as double peaks in the current density loops are observed. In the strain hysteresis, this destabilization leads to an increase of the maximum strain by 0.05%. These changes to the physical behavior caused by Zr addition are consistent with a reduction of the transition temperature TF-R, above which the field-induced transformation from the relaxor to ferroelectric state becomes reversible.

CuNNi3: a new nitride superconductor with antiperovskite structure

Abstract: We have successfully synthesized a new Ni-based antiperovskite nitride, CuNNi3, which exhibits superconductivity with a transition temperature T-c of 3.2 K. This compound is the second nitride superconductor in the Ni-based antiperovskites. We report the synthesis and physical properties of CuNNi3, characterized via x-ray diffraction, magnetization, resistivity, and heat capacity measurements. The temperature dependence of the specific heat is consistent with an isotropic s-wave gap (Delta(0) = 0.42 meV). The estimated electron-phonon coupling strength (lambda(ep) = 0.53) suggests that the superconductivity in CuNNi3 is induced by electron-phonon coupling, and the gap ratio (2 Delta(0)/k(B)T(c)) of 3.05 indicates that it is a weak coupling superconductor. Furthermore, the relatively high Ginzburg-Landau parameter (kappa =24.6) signifies that CuNNi3 is a type II superconductor.

Superconductivity of topological insulator Bi2Se3 at high pressures

Abstract: The pressure induced superconductivity and structural evolution for Bi2Se3 single crystal have been studied. The emergence of superconductivity with onset transition temperature (Tc) about 4.4K is observed around 12GPa. Tc increases rapidly to the highest 8.5K at 16GPa, decreases to 6.5K at 21GPa, then keep almost constant. It is found that Tc versus pressure is closely related to the carrier density which increases by more than two orders of magnitude from 2GPa to 23GPa. High pressure synchrotron radiation measurements reveal structure transitions occur around 12GPa, 20GPa, and above 29GPa, respectively. A phase diagram of superconductivity versus pressure is obtained.