J.-G. Cheng

ZnO-based film bulk acoustic resonator for high sensitivity biosensor applications

Zinc oxide (ZnO)-based film bulk acoustic resonator consisting of a piezoelectric element (Au∕ZnO∕Pt) and a Bragg reflector (ZnO∕Pt multilayer structure) has been fabricated by magnetron sputtering. The transmission electron microscopy and x-ray diffraction measurements revealed that all thin film layers in the device were well crystallized and highly textured. By electrical measurements, it was found that the device had a high resonant frequency (3.94GHz) and mass sensitivity (8970Hzcm2∕ng). The use of the device as a biosensor was demonstrated by comparing the resonant properties of the device with/without coatings of biospecies.

Superconductivity in the vicinity of antiferromagnetic order in CrAs

One of the common features of unconventional superconducting systems such as the heavy-fermion, high transition-temperature cuprate and iron-pnictide superconductors is that the superconductivity emerges in the vicinity of long-range antiferromagnetically ordered state. In addition to doping charge carriers, the application of external pressure is an effective and clean approach to induce unconventional superconductivity near a magnetic quantum critical point. Here we report on the discovery of superconductivity on the verge of antiferromagnetic order in CrAs via the application of external pressure. Bulk superconductivity with Tc≈2 K emerges at the critical pressure Pc≈8 kbar, where the first-order antiferromagnetic transition at T(N)≈265 K under ambient pressure is completely suppressed. The close proximity of superconductivity to an antiferromagnetic order suggests an unconventional pairing mechanism for CrAs. The present finding opens a new avenue for searching novel superconductors in the Cr and other transition metal-based systems.

PbZr0.5Ti0.5O3/La0.5Sr0.5CoO3 heterostructures prepared by chemical solution routes on silicon with no fatigue polarization

PbZr0.5Ti0.5O3/La0.5Sr0.5CoO3 (PZT/LSCO) heterostructures have been grown directly on (100)Si by chemical solution routes. Field-emission scanning electron microscopy and x-ray diffraction analysis show that PZT and LSCO thin films are polycrystalline and entirely perovskite phase. PZT thin films grown on LSCO thin films showed no preferential orientation and excellent ferroelectricity. The remnant polarization Pr is about 22.8 μC/cm2 and the coercive field Ec is about 95 kV/cm at an applied electric field of 400 kV/cm. The ferroelectric capacitor has been fabricated and showed no polarization fatigue after 3×109 fatigue cycles. The Pt/PZT/LSCO capacitor is suitable for nonvolatile random access memory application.

Dome-shaped magnetic order competing with high-temperature superconductivity at high pressures in FeSe

The coexistence and competition between superconductivity and electronic orders, such as spin or charge density waves, have been a central issue in high transition-temperature (Tc) superconductors. Unlike other iron-based superconductors, FeSe exhibits nematic ordering without magnetism whose relationship with its superconductivity remains unclear. Moreover, a pressure-induced fourfold increase of Tc has been reported, which poses a profound mystery. Here we report high-pressure magnetotransport measurements in FeSe up to ∼15 GPa, which uncover the dome shape of magnetic phase superseding the nematic order. Above ∼6 GPa the sudden enhancement of superconductivity (Tc≤38.3 K) accompanies a suppression of magnetic order, demonstrating their competing nature with very similar energy scales. Above the magnetic dome, we find anomalous transport properties suggesting a possible pseudogap formation, whereas linear-in-temperature resistivity is observed in the normal states of the high-Tc phase above 6 GPa. The obtained phase diagram highlights unique features of FeSe among iron-based superconductors, but bears some resemblance to that of high-Tc cuprates.

Properties of highly (100) oriented Ba0.9Sr0.1TiO3/LaNiO3 heterostructures prepared by chemical solution routes

Highly (100) oriented Ba0.9Sr0.1TiO3/LaNiO3 heterostructures have been grown on Si(100) using chemical solution routes. X-ray diffraction analysis shows that Ba0.9Sr0.1TiO3 thin films are high (100) orientation (α100=0.92). Atomic force microscopy investigation shows that they have large grains about 80–200 nm. A Pt/Ba0.9Sr0.1TiO3/LaNiO3 capacitor has been fabricated and showed excellent ferroelectricity, the remnant polarization and coercive field are 10.8 μC/cm2 and 96 kV/cm, respectively. The electric field dependence of capacitance measurement shows that the capacitor has large capacitance tuning ([Cmax−Cmin]/Cmax×100%) of 63%. The Ba0.9Sr0.1TiO3 thin films have high dielectric constant (e) of 200 at 1 MHz.

Hardness, elastic, and electronic properties of chromium monoboride

We report high-pressure synthesis of chromium monoboride (CrB) at 6 GPa and 1400 K. The elastic and plastic behaviors have been investigated by hydrostatic compression experiment and micro-indentation measurement. CrB is elastically incompressible with a high bulk modulus of 269.0 (5.9) GPa and exhibits a high Vickers hardness of 19.6 (0.7) GPa under the load of 1 kg force. Based on first principles calculations, the observed mechanical properties are attributed to the polar covalent Cr-B bonds interconnected with strong zigzag B-B covalent bonding network. The presence of metallic Cr bilayers is presumably responsible for the weakest paths in shear deformation.

Abnormal elastic and vibrational behaviors of magnetite at high pressures.

Magnetite exhibits unique electronic, magnetic, and structural properties in extreme conditions that are of great research interest. Previous studies have suggested a number of transitional models, although the nature of magnetite at high pressure remains elusive. We have studied a highly stoichiometric magnetite using inelastic X-ray scattering, X-ray diffraction and emission, and Raman spectroscopies in diamond anvil cells up to ~20 GPa, while complementary electrical conductivity measurements were conducted in a cubic anvil cell up to 8.5 GPa. We have observed an elastic softening in the diagonal elastic constants (C11 and C44) and a hardening in the off-diagonal constant (C12) at ~8 GPa where significant elastic anisotropies in longitudinal and transverse acoustic waves occur, especially along the [110] direction. An additional vibrational Raman band between the A1g and T2g modes was also detected at the transition pressure. These abnormal elastic and vibrational behaviors of magnetite are attributed to the occurrence of the octahedrally-coordinated Fe2+-Fe3+-Fe2+ ions charge-ordering along the [110] direction in the inverse spinel structure. We propose a new phase diagram of magnetite in which the temperature for the metal-insulator and distorted structural transitions decreases with increasing pressure while the charge-ordering transition occurs at ~8 GPa and room temperature.

Ultrastrong Boron Frameworks in ZrB12: A Highway for Electron Conducting

ZrB12 , with a high symmetrical cubic structure, possesses both high hardness ≈27.0 GPa and ultralow electrical resistivity ≈18 µΩ cm at room temperature. Both the superior conductivity and hardness of ZrB12 are associated with the extended BB 3D covalent bonding network as it is not only favorable for achieving high hardness, but also provides conducting channels for transporting electrons.

Investigation of interface and bulk fatigue scenarios in sol-gel derived Pb(Zr0.5Ti0.5)O3 films by asymmetric field driving

A method to distinguish the bulk and interface scenarios of fatigue in sol-gel derived Pb(Zr0.5Ti0.5)O3 (PZT) thin films on Pt and LaNiO3 (LNO) substrates is proposed based on the asymmetric electric field driving of capacitors. The “hard” and “soft” failures of polarization, which are typical for sputtering and sol-gel deposited PZT, respectively, are realized in sol-gel derived Pt/PZT/Pt capacitors by changing the asymmetricity of driving pulses, indicating that the interface plays an important role in the polarization breakdown. For Pt/PZT/LNO capacitors, it is shown that the bulk pinning of domains appears to be more evident than the interface pinning. A model based on the electromigration and entrapment of charged defects subjected to the asymmetric field is also given, and the plausible approximation of space charge field versus charge density is discussed.

Maximizing T c by tuning nematicity and magnetism in FeSe 1−x S x superconductors

A fundamental issue concerning iron-based superconductivity is the roles of electronic nematicity and magnetism in realising high transition temperature (Tc). To address this issue, FeSe is a key material, as it exhibits a unique pressure phase diagram involving non-magnetic nematic and pressure-induced antiferromagnetic ordered phases. However, as these two phases in FeSe have considerable overlap, how each order affects superconductivity remains perplexing. Here we construct the three-dimensional electronic phase diagram, temperature (T) against pressure (P) and isovalent S-substitution (x), for FeSe1−xSx. By simultaneously tuning chemical and physical pressures, against which the chalcogen height shows a contrasting variation, we achieve a complete separation of nematic and antiferromagnetic phases. In between, an extended non-magnetic tetragonal phase emerges, where Tc shows a striking enhancement. The completed phase diagram uncovers that high-Tc superconductivity lies near both ends of the dome-shaped antiferromagnetic phase, whereas Tc remains low near the nematic critical point.The overlap between different phases has hindered the understanding of how each phase affects superconductivity in FeSe. Here, Matsuura et al. achieve a complete separation of non-magnetic nematic and antiferromagnetic phases for FeSe1-xSx, observing a tetragonal phase in between with a strikingly enhanced Tc.