Xilin Zhou

A massive binary black-hole system in OJ 287 and a test of general relativity

Tests of Einstein’s general theory of relativity have mostly been carried out in weak gravitational fields where the space-time curvature effects are first-order deviations from Newton’s theory. Binary pulsars provide a means of probing the strong gravitational field around a neutron star, but strong-field effects may be best tested in systems containing black holes. Here we report such a test in a close binary system of two candidate black holes in the quasar OJ 287. This quasar shows quasi-periodic optical outbursts at 12-year intervals, with two outburst peaks per interval. The latest outburst occurred in September 2007, within a day of the time predicted by the binary black-hole model and general relativity. The observations confirm the binary nature of the system and also provide evidence for the loss of orbital energy in agreement (within 10 per cent) with the emission of gravitational waves from the system. In the absence of gravitational wave emission the outburst would have happened 20 days later.

Si-Sb-Te materials for phase change memory applications

Si-Sb-Te materials including Te-rich Si₂Sb₂Te₆ and Si(x)Sb₂Te₃ with different Si contents have been systemically studied with the aim of finding the most suitable Si-Sb-Te composition for phase change random access memory (PCRAM) use. Si(x)Sb₂Te₃ shows better thermal stability than Ge₂Sb₂Te₅ or Si₂Sb₂Te₆ in that Si(x)Sb₂Te₃ does not have serious Te separation under high annealing temperature. As Si content increases, the data retention ability of Si(x)Sb₂Te₃ improves. The 10 years retention temperature for Si₃Sb₂Te₃ film is ~393 K, which meets the long-term data storage requirements of automotive electronics. In addition, Si richer Si(x)Sb₂Te₃ films also show improvement on thickness change upon annealing and adhesion on SiO₂ substrate compared to those of Ge₂Sb₂Te₅ or Si₂Sb₂Te₆ films. However, the electrical performance of PCRAM cells based on Si(x)Sb₂Te₃ films with x > 3.5 becomes worse in terms of stable and long-term operations. Si(x)Sb₂Te₃ materials with 3 < x < 3.5 are proved to be suitable for PCRAM use to ensure good overall performance.

Carbon-doped Ge2Sb2Te5 phase change material: A candidate for high-density phase change memory application

Carbon-doped Ge2Sb2Te5 material is proposed for high-density phase-change memories. The carbon doping effects on electrical and structural properties of Ge2Sb2Te5 are studied by in situ resistance and x-ray diffraction measurements as well as optical spectroscopy. C atoms are found to significantly enhance the thermal stability of amorphous Ge2Sb2Te5 by increasing the degree of disorder of the amorphous phase. The reversible electrical switching capability of the phase-change memory cells is improved in terms of power consumption with carbon addition. The endurance of ∼2.1 × 104 cycles suggests that C-doped Ge2Sb2Te5 film will be a potential phase-change material for high-density storage application.

Phase-Change Memory Materials by Design: A Strain Engineering Approach

Van der Waals heterostructure superlattices of Sb2 Te1 and GeTe are strain-engineered to promote switchable atomic disordering, which is confined to the GeTe layer. Careful control of the strain in the structures presents a new degree of freedom to design the properties of functional superlattice structures for data storage and photonics applications.

Ti10Sb60Te30 for phase change memory with high-temperature data retention and rapid crystallization speed

With a high crystallization temperature of 211 °C, Ti10Sb60Te30 phase change material exhibits a data retention of 10-yr at 137 °C, which is much better than that of usual Ge2Sb2Te5. No other phase is formed in Ti10Sb60Te30 film except hexagonal Sb2Te phase. For Ti10Sb60Te30-based phase change memory cell, as short as 6 ns electric pulse can fulfill the Set operation, demonstrating an extremely rapid crystallization speed of Ti10Sb60Te30. The programming cycles can reach 2.2 × 104 with very short Set/Reset pulses of 100 ns/50 ns.

Al1.3Sb3Te material for phase change memory application

Comparing with Ge2Sb2Te5, Al1.3Sb3Te is proved to be a promising candidate for phase-change memory use because of its higher crystallization temperature (∼210 °C), larger crystallization activation energy (3.32 eV), and better data retention ability (124 °C for 10 yr). Furthermore, Al1.3Sb3Te shows fast phase change speed and crystallizes into a uniformly embedded crystal structure. As short as 10 ns width, voltage pulse can realize reversible operations for Al1.3Sb3Te based phase-change memory cell. Moreover, phase-change memory cell based on Al1.3Sb3Te material also has good endurance (∼2.5 × 104 cycles) and an enough resistance ratio of ∼102.

Understanding Phase-Change Behaviors of Carbon-Doped Ge2Sb2Te5 for Phase-Change Memory Application

Phase-change materials are highly promising for next-generation nonvolatile data storage technology. The pronounced effects of C doping on structural and electrical phase-change behaviors of Ge2Sb2Te5 material are investigated at the atomic level by combining experiments and ab initio molecular dynamics. C dopants are found to fundamentally affect the amorphous structure of Ge2Sb2Te5 by altering the local environments of Ge-Te tetrahedral units with stable C-C chains. The incorporated C increases the amorphous stability due to the enhanced covalent nature of the material with larger tetrahedral Ge sites. The four-membered rings with alternating atoms are reduced greatly with carbon addition, leading to sluggish phase transition and confined crystal grains. The lower RESET power is presented in the PCM cells with carbon-doped material, benefiting from its high resistivity and low thermal conductivity.

W-Sb-Te phase-change material: A candidate for the trade-off between programming speed and data retention

W-Sb-Te phase-change material has been proposed to improve the performance of phase-change memory (PCM). Crystallization temperature, crystalline resistance, and 10-year data retention of Sb2Te increase markedly by W doping. The Wx(Sb2Te)1−x films crystallize quickly into a stable hexagonal phase with W uniformly distributing in the crystal lattice, which ensures faster SET speed and better operation stability for the application in practical device. PCM device based on W0.07(Sb2Te)0.93 shows ultrafast SET operation (6 ns) and good endurance (1.8 × 105 cycles). W-Sb-Te material is a promising candidate for the trade-off between programming speed and data retention.

Strain-engineered diffusive atomic switching in two-dimensional crystals.

Strain engineering is an emerging route for tuning the bandgap, carrier mobility, chemical reactivity and diffusivity of materials. Here we show how strain can be used to control atomic diffusion in van der Waals heterostructures of two-dimensional (2D) crystals. We use strain to increase the diffusivity of Ge and Te atoms that are confined to 5 Å thick 2D planes within an Sb2Te3–GeTe van der Waals superlattice. The number of quintuple Sb2Te3 2D crystal layers dictates the strain in the GeTe layers and consequently its diffusive atomic disordering. By identifying four critical rules for the superlattice configuration we lay the foundation for a generalizable approach to the design of switchable van der Waals heterostructures. As Sb2Te3–GeTe is a topological insulator, we envision these rules enabling methods to control spin and topological properties of materials in reversible and energy efficient ways.

Phase transition characteristics of Al-Sb phase change materials for phase change memory application

The crystallization behavior of Al-Sb thin films is investigated for phase change memory application. The crystallization temperature and optical band gap of the amorphous material increase with Al content. The thermal stability and randomness in atomic configuration of the films are enhanced considerably. The shift of Raman modes associated mainly with Sb upon phase transformation is observed, and the co-existence of Sb-rich crystalline regions and Al-rich amorphous matrix is confirmed, revealing the amorphous nature of most Al components. Three distinct resistance levels are achieved in the devices using Al50Sb50, suggesting the potentiality for multilevel data storage application of the materials.