Ming-Hui Lu

Substitution-induced phase transition and enhanced multiferroic properties of Bi1−xLaxFeO3 ceramics

Single-phase, insulating Bi1−xLaxFeO3 (BLFOx, x=0.05, 0.10, 0.15, 0.20, 0.30, and 0.40) ceramics were prepared. An obvious phase transition from rhombohedral to orthorhombic phase was observed near x=0.30. It is found that the phase transition destructs the spin cycloid of BiFeO3 (BFO), and therefore, releases the locked magnetization and enhances magnetoelectric interaction. As a result, improved multiferroic properties of the BLFO0.30 ceramics with remnant polarization and magnetization (2Pr and 2Mr) of 22.4μC∕cm2 and 0.041emu∕g, respectively, were established.

Acoustic topological insulator and robust one-way sound transport

The acoustic analogue of a topological insulator is shown: a metamaterial exhibiting one-way sound transport along its edge. The system — a graphene-like array of stainless-steel rods — is a promising new platform for exploring topological phenomena.

Preparation, structures, and multiferroic properties of single phase Bi1−xLaxFeO3(x=0–0.40) ceramics

A simple and effective method that solid state reaction followed by quenching process is developed to prepare multiferroic La-substituted BiFeO3 (Bi1−xLaxFeO3 (BLFOx) with x=0–0.40) ceramics. X-ray diffraction, x-ray photoelectron spectroscopy, and inductively coupled plasma studies show that the ceramics prepared under the optimized conditions are single phase. A phase transition from rhombohedral to orthorhombic phase is observed near x=0.30. This transition has great effects on the multiferroic properties. Magnetic and electric measurements reveal that the BLFO0.30 has enhanced multiferroic properties with two times remnant magnetization and polarization of 0.041emu∕g and 22.4μC∕cm2, respectively. The enhanced multiferroic properties are attributed to the enhanced magnetoelectric interaction, which results from the La substitution-induced destruction of the spin cycloid. These results show that BiFeO3-based perovskite solid solution with no other ferroelectric end member can have improved multiferroic ...

Semiconductor/relaxor 0–3 type composites without thermal depolarization in Bi0.5Na0.5TiO3-based lead-free piezoceramics

Commercial lead-based piezoelectric materials raised worldwide environmental concerns in the past decade. Bi₀.₅Na₀.₅TiO₃-based solid solution is among the most promising lead-free piezoelectric candidates; however, depolarization of these solid solutions is a longstanding obstacle for their practical applications. Here we use a strategy to defer the thermal depolarization, even render depolarization-free Bi₀.₅Na₀.₅TiO₃-based 0-3-type composites. This is achieved by introducing semiconducting ZnO particles into the relaxor ferroelectric 0.94Bi₀.₅Na₀.₅TiO₃-0.06BaTiO₃ matrix. The depolarization temperature increases with increasing ZnO concentration until depolarization disappears at 30 mol% ZnO. The semiconducting nature of ZnO provides charges to partially compensate the ferroelectric depolarization field. These results not only pave the way for applications of Bi₀.₅Na₀.₅TiO₃-based piezoceramics, but also have great impact on the understanding of the mechanism of depolarization so as to provide a new design to optimize the performance of lead-free piezoelectrics.

Observation of Temperature-Induced Crossover to an Orbital-Selective Mott Phase in AxFe2-ySe2 (A=K, Rb) Superconductors

Using angle-resolved photoemission spectroscopy, we observe the low-temperature state of the A(x)Fe(2-y)Se(2) (A=K, Rb) superconductors to exhibit an orbital-dependent renormalization of the bands near the Fermi level-the d(xy) bands heavily renormalized compared to the d(xz)/d(yz) bands. Upon raising the temperature to above 150 K, the system evolves into a state in which the d(xy) bands have depleted spectral weight while the d(xz)/d(yz) bands remain metallic. Combined with theoretical calculations, our observations can be consistently understood as a temperature-induced crossover from a metallic state at low temperatures to an orbital-selective Mott phase at high temperatures. Moreover, the fact that the superconducting state of A(x)Fe(2-y)Se(2) is near the boundary of such an orbital-selective Mott phase constrains the system to have sufficiently strong on-site Coulomb interactions and Hunds coupling, highlighting the nontrivial role of electron correlation in this family of iron-based superconductors.

Broadband Photovoltaic Detectors Based on an Atomically Thin Heterostructure

van der Waals junctions of two-dimensional materials with an atomically sharp interface open up unprecedented opportunities to design and study functional heterostructures. Semiconducting transition metal dichalcogenides have shown tremendous potential for future applications due to their unique electronic properties and strong light-matter interaction. However, many important optoelectronic applications, such as broadband photodetection, are severely hindered by their limited spectral range and reduced light absorption. Here, we present a p-g-n heterostructure formed by sandwiching graphene with a gapless band structure and wide absorption spectrum in an atomically thin p-n junction to overcome these major limitations. We have successfully demonstrated a MoS2-graphene-WSe2 heterostructure for broadband photodetection in the visible to short-wavelength infrared range at room temperature that exhibits competitive device performance, including a specific detectivity of up to 10(11) Jones in the near-infrared region. Our results pave the way toward the implementation of atomically thin heterostructures for broadband and sensitive optoelectronic applications.

Magnetic and transport properties of (Mn, Co)-codoped ZnO films prepared by radio-frequency magnetron cosputtering

(Mn, Co)-codoped ZnO films have been synthesized on c-sapphire (0001) by a radio-frequency magnetron sputtering system in which two targets were sputtered together. X-ray-diffraction measurements indicate that the films are highly c-axis oriented. X-ray photon spectra show that the doped Mn and Co ions in (Mn, Co) ZnO films are both in the divalent states. The films show ferromagnetic behavior with a coercivity of about 90 Oe and a saturation moment of 0.11μB∕(0.3Mn2++0.7Co2+) at 300 K. In the lower temperatures between 5 and 20 K, a relatively large positive magnetoresistance over 10% was observed for (Mn0.03,Co0.07)Zn0.90O film. The number of carrier concentration is experimentally established to be 1.5613×1018cm−3 and the mobility to be 2.815cm2V−1s−1 for (Mn0.03,Co0.07)Zn0.90O film by Hall measurements at 300 K. The origins of the room-temperature magnetism and the large positive magnetoresistance are also discussed.(Mn, Co)-codoped ZnO films have been synthesized on c-sapphire (0001) by a radio-frequency magnetron sputtering system in which two targets were sputtered together. X-ray-diffraction measurements indicate that the films are highly c-axis oriented. X-ray photon spectra show that the doped Mn and Co ions in (Mn, Co) ZnO films are both in the divalent states. The films show ferromagnetic behavior with a coercivity of about 90 Oe and a saturation moment of 0.11μB∕(0.3Mn2++0.7Co2+) at 300 K. In the lower temperatures between 5 and 20 K, a relatively large positive magnetoresistance over 10% was observed for (Mn0.03,Co0.07)Zn0.90O film. The number of carrier concentration is experimentally established to be 1.5613×1018cm−3 and the mobility to be 2.815cm2V−1s−1 for (Mn0.03,Co0.07)Zn0.90O film by Hall measurements at 300 K. The origins of the room-temperature magnetism and the large positive magnetoresistance are also discussed.

Photonic topological insulator with broken time-reversal symmetry.

Significance Topological insulators are first discovered in electronic systems. A key factor is the Kramers doublet for the spin-1/2 electrons under fermionic time-reversal symmetry Tf2=−1. Unlike electrons, photons are massless bosons with spin-1. Therefore, the Kramers degeneracy theorem cannot readily apply to photons under the bosonic time-reversal symmetry. So far, there has been no coherent physical explanation for the symmetry protection mechanism behind the photonic topological insulator. Here, we design a photonic topological insulator that violates the bosonic time-reversal symmetry but complies with a fermionic-like pseudo time-reversal symmetry. The analyses and results, through comprehensive investigations on the properties of edge states, validate that the topological edge states are, in fact, protected by the fermionic-like pseudo time-reversal symmetry Tp (Tp2=−1). A topological insulator is a material with an insulating interior but time-reversal symmetry-protected conducting edge states. Since its prediction and discovery almost a decade ago, such a symmetry-protected topological phase has been explored beyond electronic systems in the realm of photonics. Electrons are spin-1/2 particles, whereas photons are spin-1 particles. The distinct spin difference between these two kinds of particles means that their corresponding symmetry is fundamentally different. It is well understood that an electronic topological insulator is protected by the electron’s spin-1/2 (fermionic) time-reversal symmetry Tf2=−1. However, the same protection does not exist under normal circumstances for a photonic topological insulator, due to photon’s spin-1 (bosonic) time-reversal symmetry Tb2=1. In this work, we report a design of photonic topological insulator using the Tellegen magnetoelectric coupling as the photonic pseudospin orbit interaction for left and right circularly polarized helical spin states. The Tellegen magnetoelectric coupling breaks bosonic time-reversal symmetry but instead gives rise to a conserved artificial fermionic-like-pseudo time-reversal symmetry, Tp (Tp2=−1), due to the electromagnetic duality. Surprisingly, we find that, in this system, the helical edge states are, in fact, protected by this fermionic-like pseudo time-reversal symmetry Tp rather than by the bosonic time-reversal symmetry Tb. This remarkable finding is expected to pave a new path to understanding the symmetry protection mechanism for topological phases of other fundamental particles and to searching for novel implementations for topological insulators.

Topologically protected one-way edge mode in networks of acoustic resonators with circulating air flow

Recent explorations of topology in physical systems have led to a new paradigm of condensed matters characterized by topologically protected states and phase transition, for example, topologically protected photonic crystals enabled by magneto-optical effects. However, in other wave systems such as acoustics, topological states cannot be simply reproduced due to the absence of similar magnetics-related sound-matter interactions in naturally available materials. Here, we propose an acoustic topological structure by creating an effective gauge magnetic field for sound using circularly flowing air in the designed acoustic ring resonators. The created gauge magnetic field breaks the time-reversal symmetry, and therefore topological properties can be designed to be nontrivial with non-zero Chern numbers verified by a tight-binding model and thus to enable a topological sonic crystal, in which the topologically protected acoustic edge-state transport is observed, featuring robust one-way propagation characteristics against a variety of topological defects and impurities. Interestingly, the one-way propagation direction is relevant to the azimuthal order of the resonant mode in the ring resonator which influences the corresponding topological Chern number. Our results open a new venue to non-magnetic topological structures and promise a unique approach to effective manipulation of acoustic interfacial transport at will.

Acoustic cloaking by a near-zero-index phononic crystal

Zero-refractive-index materials may lead to promising applications in various fields. Here, we design and fabricate a near Zero-Refractive-Index (ZRI) material using a phononic crystal (PC) composed of a square array of densely packed square iron rods in air. The dispersion relation exhibits a nearly flat band across the Brillouin zone at the reduced frequency f = 0.5443c/a, which is due to Fabry-Perot (FP) resonance. By using a retrieval method, we find that both the effective mass density and the reciprocal of the effective bulk modulus are close to zero at frequencies near the flat band. We also propose an equivalent tube network model to explain the mechanisms of the near ZRI effect. This FP-resonance-induced near ZRI material offers intriguing wave manipulation properties. We demonstrate both numerically and experimentally its ability to shield a scattering obstacle and guide acoustic waves through a bent structure.