J. L. Zhu

Superconductivity in Topological Insulator Sb2Te3 Induced by Pressure

Topological superconductivity is one of most fascinating properties of topological quantum matters that was theoretically proposed and can support Majorana Fermions at the edge state. Superconductivity was previously realized in a Cu-intercalated Bi2Se3 topological compound or a Bi2Te3 topological compound at high pressure. Here we report the discovery of superconductivity in the topological compound Sb2Te3 when pressure was applied. The crystal structure analysis results reveal that superconductivity at a low-pressure range occurs at the ambient phase. The Hall coefficient measurements indicate the change of p-type carriers at a low-pressure range within the ambient phase, into n-type at higher pressures, showing intimate relation to superconducting transition temperature. The first principle calculations based on experimental measurements of the crystal lattice show that Sb2Te3 retains its Dirac surface states within the low-pressure ambient phase where superconductivity was observed, which indicates a strong relationship between superconductivity and topology nature.

Li(Zn,Mn)As as a new generation ferromagnet based on a I–II–V semiconductor

In a prototypical ferromagnet (Ga,Mn)As based on a III-V semiconductor, substitution of divalent Mn atoms into trivalent Ga sites leads to severely limited chemical solubility and metastable specimens available only as thin films. The doping of hole carriers via (Ga,Mn) substitution also prohibits electron doping. To overcome these difficulties, Masek et al. theoretically proposed systems based on a I-II-V semiconductor LiZnAs, where isovalent (Zn,Mn) substitution is decoupled from carrier doping with excess/deficient Li concentrations. Here we show successful synthesis of Li(1+y)(Zn(1-x)Mn(x))As in bulk materials. Ferromagnetism with a critical temperature of up to 50 K is observed in nominally Li-excess (y=0.05-0.2) compounds with Mn concentrations of x=0.02-0.15, which have p-type metallic carriers. This is presumably due to excess Li in substitutional Zn sites. Semiconducting LiZnAs, ferromagnetic Li(Zn,Mn)As, antiferromagnetic LiMnAs, and superconducting LiFeAs systems share square lattice As layers, which may enable development of novel junction devices in the future.

Mn-doped 0.71Pb(Mg1∕3Nb2∕3)O3–0.29PbTiO3 pyroelectric crystals for uncooled infrared focal plane arrays applications

3mol%Mn-doped 0.71Pb(Mg1∕3Nb2∕3)O3–0.29PbTiO3 single crystals were grown by a modified Bridgman technique. The pyroelectric properties and thermal stability of the crystals were investigated. Mn substitution resulted in an enhanced pyroelectric coefficient and a lower dielectric loss, which led to the improvement of the detectivity figure of merit of doped crystals by about a factor of 4 at 50Hz compared with that of pure crystals. Moreover, the thermal stability was enhanced by Mn substitution. The mechanism of doping effect is explained by the fact that the domain walls are pinned by the dopant dipolar defects, which optimizes the pyroelectric performance of 0.71Pb(Mg1∕3Nb2∕3)O3–0.29PbTiO3 for uncooled infrared focal plane arrays applications.

Large pyroelectric response in relaxor-based ferroelectric (1−x)Pb(Mg1∕3Nb2∕3)O3–xPbTiO3 single crystals

The pyroelectric properties of (1−x)Pb(Mg1∕3Nb2∕3)O3–xPbTiO3 (PMN–xPT) single crystals have been investigated over a broad composition range of 0.13⩽x⩽0.40. The best pyroelectric performances are achieved in ⟨111⟩-oriented PMN–0.26PT single crystal. At room temperature, the figures of merit for voltage responsivity and detectivity reach up to 0.11m2∕C and 15.3×10−5Pa−1∕2, respectively. These properties are superior to those of conventional pyroelectric ceramics that have been widely used in device applications. PMN–0.26PT single crystal also possesses a relative high Curie temperature (∼120°C) and a low thermal diffusivity (∼4.4×10−7m2∕s). Furthermore, the pyroelectric properties of PMN–xPT (x⩾0.26) single crystals are weak dependent on temperature and nearly independent of frequency in the experimental temperature range of 20–55°C and frequency range of 50–10000Hz. The superior pyroelectric performances of the single crystal make it a promising candidate for high-performance uncooled infrared detectors a...

Superconductivity in Topological Insulator Sb[subscript 2]Te[subscript 3] Induced by Pressure

Topological superconductivity is one of most fascinating properties of topological quantum matters that was theoretically proposed and can support Majorana Fermions at the edge state. Superconductivity was previously realized in a Cu-intercalated Bi2Se3 topological compound or a Bi2Te3 topological compound at high pressure. Here we report the discovery of superconductivity in the topological compound Sb2Te3 when pressure was applied. The crystal structure analysis results reveal that superconductivity at a low-pressure range occurs at the ambient phase. The Hall coefficient measurements indicate the change of p-type carriers at a low-pressure range within the ambient phase, into n-type at higher pressures, showing intimate relation to superconducting transition temperature. The first principle calculations based on experimental measurements of the crystal lattice show that Sb2Te3 retains its Dirac surface states within the low-pressure ambient phase where superconductivity was observed, which indicates a strong relationship between superconductivity and topology nature.

Superconductivity of the topological insulator Bi2Se3 at high pressure

The pressure-induced superconductivity and structural evolution of Bi2Se3 single crystals are studied. The emergence of superconductivity at an onset transition temperature (Tc) of about 4.4 K is observed at around 12 GPa. Tc increases rapidly to a maximum of 8.2 K at 17.2 GPa, decreases to around 6.5 K at 23 GPa, and then remains almost constant with further increases in pressure. Variations in Tc with respect to pressure are closely related to the carrier density, which increases by over two orders of magnitude from 2 to 23 GPa. High-pressure synchrotron radiation measurements reveal structural transitions at around 12, 20, and above 29 GPa. A phase diagram of superconductivity versus pressure is also constructed.

Superconductivity in Strong Spin Orbital Coupling Compound Sb2Se3

Recently, A2B3 type strong spin orbital coupling compounds such as Bi2Te3, Bi2Se3 and Sb2Te3 were theoretically predicated to be topological insulators and demonstrated through experimental efforts. The counterpart compound Sb2Se3 on the other hand was found to be topological trivial, but further theoretical studies indicated that the pressure might induce Sb2Se3 into a topological nontrivial state. Here, we report on the discovery of superconductivity in Sb2Se3 single crystal induced via pressure. Our experiments indicated that Sb2Se3 became superconductive at high pressures above 10 GPa proceeded by a pressure induced insulator to metal like transition at ~3 GPa which should be related to the topological quantum transition. The superconducting transition temperature (TC) increased to around 8.0 K with pressure up to 40 GPa while it keeps ambient structure. High pressure Raman revealed that new modes appeared around 10 GPa and 20 GPa, respectively, which correspond to occurrence of superconductivity and to the change of TC slop as the function of high pressure in conjunction with the evolutions of structural parameters at high pressures.

The comprehensive phase evolution for Bi2Te3 topological compound as function of pressure

The recently discovered three-dimensional topological insulator Bi2Te3 is studied as function of pressure in terms of crystal structures, resistance, and Hall coefficient. The superconductivity is found in phase I (ambient phase) Bi2Te3 with Tc ∼ 3 K, which is related to the topological features. The evolution of crystal structure with pressure is investigated by high pressure synchrotron radiation experiments that reveal structural transitions occurring at about 8 GPa, 13 GPa, and 16 GPa, respectively. Furthermore, the high pressure phases of Bi2Te3 are also superconducting but with much higher Tc ∼ 8 K. The superconducting transitions are compared with those for Bi, Te elements. A global phase diagram of Bi2Te3 as function of pressure up to 30 GPa is obtained.

Determination of the intrinsic ferroelectric polarization in orthorhombic HoMnO3

Whether or not a large ferroelectric polarization P exists in the orthorhombic HoMnO3 with E-type antiferromagnetic spin ordering remains one of the unresolved, challenging issues in the physics of multiferroics. The issue is closely linked to an intriguing experimental difficulty in determining the P of polycrystalline specimens, namely that conventional pyroelectric current measurements performed after a poling procedure under high dc electric fields are subject to large errors due to the problems caused by leakage currents or space charges. To overcome the difficulty, we employed the positive-up negative-down (PUND) method, which uses successively the two positive and two negative electrical pulses, to directly measure electrical hysteresis loops in several polycrystalline HoMnO3 specimens below their Neel temperatures. We found that all the HoMnO3 samples had similar remnant polarization Pr values at each temperature, regardless of their variation in resistivity, dielectric constant and pyroelectric current levels. Moreover, the Pr value of 0.07µCcm 2 at 6K is consistent with the P value obtained from the pyroelectric current measurement performed after a short pulse poling. Our findings suggest that the intrinsic P of polycrystalline HoMnO3 can be determined through the PUND method and P at 0K may reach 0.24µCcm 2 in a single crystalline specimen. This P value is still much smaller than the theoretically predicted one but is one of the largest observed in magnetism induced ferroelectrics.

Diluted ferromagnetic semiconductor Li(Zn,Mn)P with decoupled charge and spin doping

We report the discovery of a diluted magnetic semiconductor, Li(Zn,Mn)P, in which charge and spin are introduced independently via lithium off-stoichiometry and the isovalent substitution of Mn2+ for Zn2+, respectively. Isostructural to (Ga,Mn)As, Li(Zn, Mn) P was found to be a p-type ferromagnetic semiconductor with excess lithium providing charge doping. First-principles calculations indicate that excess Li is favored to partially occupy the Zn site, leading to hole doping. Ferromagnetism with Curie temperature up to 34 K is achieved while the system still shows semiconducting transport behavior.