Nanoscale control of an interfacial metal-insulator transition at room temperature

Experimental and theoretical investigations have demonstrated that a quasi-two-dimensional electron gas (q-2DEG) can form at the interface between two insulators: non-polar SrTiO3 and polar LaTiO3 (ref. 2), LaAlO3 (refs 3-5), KTaO3 (ref. 7) or LaVO3 (ref. 6). Electronically, the situation is analogous to the q-2DEGs formed in semiconductor heterostructures by modulation doping. LaAlO3/SrTiO3 heterostructures have recently been shown to exhibit a hysteretic electric-field-induced metal-insulator quantum phase transition for LaAlO3 thicknesses of 3 unit cells. Here, we report the creation and erasure of nanoscale conducting regions at the interface between two insulating oxides, LaAlO3 and SrTiO3. Using voltages applied by a conducting atomic force microscope (AFM) probe, the buried LaAlO3/SrTiO3 interface is locally and reversibly switched between insulating and conducting states. Persistent field effects are observed using the AFM probe as a gate. Patterning of conducting lines with widths of approximately 3 nm, as well as arrays of conducting islands with densities >10(14) inch(-2), is demonstrated. The patterned structures are stable for >24 h at room temperature.     

Electrical control of antiferromagnetic domains in multiferroic BiFeO3 films at room temperature

Multiferroic materials, which offer the possibility of manipulating the magnetic state by an electric field or vice versa, are of great current interest. In this work, we demonstrate the first observation of electrical control of antiferromagnetic domain structure in a single-phase multiferroic material at room temperature. High-resolution images of both antiferromagnetic and ferroelectric domain structures of (001)-oriented multiferroic BiFeO3 films revealed a clear domain correlation, indicating a strong coupling between the two types of order. The ferroelectric structure was measured using piezo force microscopy, whereas X-ray photoemission electron microscopy as well as its temperature dependence was used to detect the antiferromagnetic configuration. Antiferromagnetic domain switching induced by ferroelectric polarization switching was observed, in agreement with theoretical predictions.     

四方相纳米氧化锆低温稳定机制的研究现状

介绍了近几十年t-ZrO2纳米晶在室温下稳定机制的研究现状,并且重点分析表面能理论和氧空位理论等主要机制,在此基础上对镀膜、水蒸气、杂质离子、形核位置以及结构相似对其稳定性的影响进行进一步的讨论.     

Interdiffusion and phase formation at room temperature in evaporated gold-tin films

Interdiffusion at room temperature in evaporated Au-Sn films was studied by the in situ backscattering of 2.0 MeV ⁴He ions. The interdiffusion resulted in the formation and growth of an AuSn phase region. The growth of the phase followed a parabolic growth rate law, and the growth rate constant was found to be about 9 × 10^?15 cm²s^?1 at room temperature.     

Ar ion induced copper germanide phase formation at room temperature

The copper germanide phase Cu₃Ge which is emerging as an alternative material for making contacts and interconnects for semiconductor industry has been produced across the interface of Cu/Ge bilayers by ion beam mixing at room temperature using 1 MeV Ar ions. The dose dependence of the thickness of the mixed region shows a diffusion controlled mixing process. The experimental mixing rate and efficiency for this phase are 5·35 nm⁴ and 10·85 nm⁵/keV respectively. At doses above 8 × 10¹⁵ Ar/cm² the formation and growth of another copper rich phase Cu₅Ge has been observed. The present theoretical models are inadequate to explain the observed experimental mixing rate.     

Magnetic refrigeration in the temperature range from 10 K to room temperature: the ferromagnetic refrigerants

The criteria for selecting refrigerant materials for a magnetic refrigerator in the temperature range from 4.2 K to 300 K have been studied. First, by assuming the molecular field approximation, we calculated the magnetic entropy S_M (T, H) of a typical 3-dimensional ferromagnetic substance near the Curie temperature as a function of temperature and magnetic field strength. In order to prove the validity of the above theoretical results, the magnetocaloric effects of Cr₃Te₄ and Gd near the room temperature have been investigated experimentally. Comparison between the calculated and experimental results confirms that they are in good agreement. Second, by using the Debye approximation, the temperature variation of the lattice entropy S_L (T) as a function of the Debye temperature is clarified. Using the above S_M (T, H) and S_L (T), the temperature and magnetic field dependences of the calculated total entropy of several typical ferromagnets are obtained. In view of the above results we have discussed several criteria which should be satisfied by a magnetic refrigerant to be used near room temperature.     

Zirconia nanoparticles: a martensitic phase transition at low temperature

The low temperature (450–600°C) amorphous to tetragonal and tetragonal to monoclinic phase transformations in zirconia nanoparticles, produced by an aqueous sol–gel route, are analysed in terms of their changes of lattice parameters and by using the phenomenological theory of martensitic transformation (Bowles–Mackenzie theory) to explore whether or not those crystallographic changes may be considered a martensitic transformation within the zirconia nanoparticles.     

Electric-field control of spin waves at room temperature in multiferroic BiFeO3

To face the challenges lying beyond present technologies based on complementary metal-oxide-semiconductors, new paradigms for information processing are required. Magnonics proposes to use spin waves to carry and process information, in analogy with photonics that relies on light waves, with several advantageous features such as potential operation in the terahertz range and excellent coupling to spintronics. Several magnonic analog and digital logic devices have been proposed, and some demonstrated. Just as for spintronics, a key issue for magnonics is the large power required to control/write information (conventionally achieved through magnetic fields applied by strip lines, or by spin transfer from large spin-polarized currents). Here we show that in BiFeO(3), a room-temperature magnetoelectric material, the spin-wave frequency (>600 GHz) can be tuned electrically by over 30%, in a non-volatile way and with virtually no power dissipation. Theoretical calculations indicate that this effect originates from a linear magnetoelectric effect related to spin-orbit coupling induced by the applied electric field. We argue that these properties make BiFeO(3) a promising medium for spin-wave generation, conversion and control in future magnonics architectures.     

Electrical control of the superconducting-to-insulating transition in graphene-metal hybrids

Graphene is a sturdy and chemically inert material exhibiting an exposed two-dimensional electron gas of high mobility. These combined properties enable the design of graphene composites, based either on covalent or non-covalent coupling of adsorbates, or on stacked and multilayered heterostructures. These systems have shown tunable electronic properties such as bandgap engineering, reversible metal-insulating transition or supramolecular spintronics. Tunable superconductivity is expected as well, but experimental realization is lacking. Here, we show experiments based on metal-graphene hybrid composites, enabling the tunable proximity coupling of an array of superconducting nanoparticles of tin onto a macroscopic graphene sheet. This material allows full electrical control of the superconductivity down to a strongly insulating state at low temperature. The observed gate control of superconductivity results from the combination of a proximity-induced superconductivity generated by the metallic nanoparticle array with the two-dimensional and tunable metallicity of graphene. The resulting hybrid material behaves, as a whole, like a granular superconductor showing universal transition threshold and localization of Cooper pairs in the insulating phase. This experiment sheds light on the emergence of superconductivity in inhomogeneous superconductors, and more generally, it demonstrates the potential of graphene as a versatile building block for the realization of superconducting materials.     

Divacancies at room temperature in germanium

We have studied the annealing of vacancy defects in neutron and proton irradiated germanium. After neutron irradiation the Sb-doped samples were annealed at 473, 673 and 773 K for 30 min. The positron lifetime was measured as a function of temperature (30 - 295 K). A lifetime component of 330 ps with no temperature dependence is observed in as irradiated samples, identified as the positron lifetime in a neutral divacancy. The average positron lifetime in the samples annealed at 473 K has a definite temperature dependence, suggesting that the divacancies become negative as the crystal recovers and the Fermi level moves upward in the band gap. Proton irradiation of germanium at 37 K with subsequent room temperature annealing also resulted in a similar lifetime component 315 ps, in good agreement with the neutron irradiation experiment.     

High ferromagnetic transition temperature in PbS and PbS:Mn nanowires

Spontaneous magnetization measured in the temperature range 5-300 K with high ferromagnetic transition temperature (T(c)) has been observed in both undoped and Mn doped (2-8 mol %) PbS nanowires (diameter 30 nm) in polymer. For undoped sample, we find T(c) ~ 290 K while for doped samples T(c) varies between 310-340 K depending on Mn concentrations. Both T(c) and coercive fields are critically dependent on Mn concentrations. Coercive fields show a T(0.5) dependence with temperature for a moderate concentration of Mn (4 mol %) in PbS while it deviates from T(0.5) behavior for higher Mn concentrations. Anionic defects arising out of nonstoichiometric growth is solely responsible for the observed magnetism in undoped PbS nanowires. The role of intrinsic strain along with reduced dimensionality in determining such high T(c) and overall magnetizations has been discussed.     

The crystalline phase stability of titania particles prepared at room temperature by the sol-gel method

Titania particles having anatase, brookite and rutile phase were prepared at various H⁺/TTIP (Titaniumtetraisopropoxide) mole ratios and room temperature by the sol-gel method. The crystalline phases according to the variation of the post heat treatment temperature were observed. The crystalline phase and the phase transformation, morphology, and crystallite size were identified by using XRD, TG/DTA, Raman spectroscopy and TEM. The brookite phase of titania particles prepared at the H⁺/TTIP mole ratio of 0.02 and room temperature was not transformed into anatase or rutile even with the heat treatment at 750°C, and also the anatase phase was stable at the temperature as high as 850°C. However, the titania particles prepared at the H⁺/TTIP mole ratio of 0.67, which contained the mixed phases of anatase, brookite, and rutile at room temperature, showed only rutile phase at temperature of 750°C. It was thus shown that the initial crystalline phase of the primary particles prepared at room temperature had an important effect on the phase transformation behavior upon post heating. Phase transformation from brookite to anatase and subsequently to rutile occurred with heating.     

Ilmenite phase nickel titanate nanowhiskers as highly sensitive LPG sensor at room temperature

The NiTiO₃ nanoparticles with ilmenite phase were synthesized by the sol gel method and investigated for sensing of various volatile organic compounds. The resistive type sensing by NiTiO₃ towards LPG was not reported earlier where the sensor showed fair response towards hydrogen sulfide gas, but the sensitivity towards LPG was very high. The response % for 150 ppm of LPG was approximately 3200% at room temperature. The high response towards LPG was due to the presence of some amount of rutile TiO₂ in the composites. The response and recovery times of these sensors were very less (about 2 s) which could be attributed to the whiskers like structure of NiTiO₃.     

室温铁磁性GaN:Cr薄膜的性质研究

采用高能离子注入的方式,在GaN衬底中掺杂引入Cr离子制备了磁性半导体.借助于X射线衍射仪(XRD)进行了注入前后结构的对比分析,没有发现新的衍射峰,并运用高分辨率X射线衍射仪(HR-XRD)分析了Mn离子注入后衬底(0002)峰的微小变化.根据原子力显微镜的结果,发现注入后的样品表面起伏比较大,发生明显的变化.通过使用超导量子干涉仪SQUID进行变温分析,在所分析的10～300K范围内,磁化强度变化幅度较小,样品在室温条件下仍然保持铁磁性.     

Visco-elasticity in alumina ceramics at room temperature

The decrease in the fracture strength of alumina ceramics at room temperature has usually been assumed to be caused by the effect of stress corrosion. However, at high temperatures, a viscous deformation will occur due to a glass-like phase which exists in the grain boundary. A study of the relation of the fracture strength of an alumina ceramics smooth specimen at room temperature to the visco-elastic property was performed. Mechanical tests and visco-elastic analysis, has shown that the fracture strength of this kind is controlled by the visco-elasticity.     

Computer simulation of ordering in bipolar nematogen H6CBP at phase transition temperature

The molecular ordering of 4-(6-hydroxyhexyloxy)-4′-cyanobiphenyl (H6CBP), a bipolar nematogen, has been carried out at nematic-isotropic transition temperature with respect to translatory and orientational motions. The complete neglect differential overlap (CNDO/2) method has been employed to compute the net atomic charge and atomic dipole moment at each atomic centre. The modified Rayleigh–Schrodinger perturbation theory with multicentred-multipole expansion method has been employed to evaluate long-range intermolecular interactions, while a 6-exp potential function has been assumed for short-range interactions. The interaction energy values obtained through these computations were used to calculate the probability of each configuration at nematic-isotropic transition temperature using the Maxwell–Boltzmann formula. On the basis of stacking, in-plane, and terminal interactions energy calculations, all possible geometrical arrangements of the molecular pair have been considered. The results are correlated with molecular parameters introduced in this article, and an attempt has been made to explain the nematic behaviour of the compound in terms of their relative order.     

Size control of magnetite nanoparticles by organic solvent-free chemical coprecipitation at room temperature

This study provides a facile single-step coprecipitation method for preparing size-controlled high crystalline magnetite nanoparticles in water system without using any organic solvents. In this method, an iron ions solution and an alkaline solution are simply mixed at room temperature without using any additional heating treatment. The size of obtained magnetite nanoparticles greatly depended on the coexisting anionic species in the starting solution because the coexisting anions greatly influenced both the formation of crystal nuclei and the dispersion stabilisation of formed precipitates. The size control of magnetite nanoparticles having high crystallinity and ferromagnetic property could be successfully achieved by using the effects of coexisting anions. For synthesising finer magnetite nanoparticles, the presence of lactate ion in the starting solution was effective, and coarser ones could be synthesised under higher ferrous/ferric ions molar ratios.