Qiuju Li

Revisiting the low-temperature dielectric properties of ZnO

Electric modulus spectroscopy and impedance analysis were used to investigate the low-temperature (∼100–333 K) dielectric properties of ZnO crystals. Two relaxations were observed. The low-temperature relaxation (R1) features a thermally activated behavior resulting from the bulk response. Our results convincingly demonstrate that this relaxation is a polaronic relaxation. The relaxing species for R1 are localized holes created by zinc vacancies instead of the commonly agreed oxygen vacancies. The high-temperature relaxation (R2) is a Maxwell-Wagner relaxation due to skin-layer effect as it can be eliminated by grinding off the sample surface. The inhomogeneous distribution of zinc interstitials leads to the formation of the skin layer. Interestingly, an abnormal dielectric behavior contrary to the thermally activated behavior was found for the R2 relaxation. This abnormal behavior was confirmed to be related to the positive temperature coefficient of resistance due to the metal-insulator transition occur...

Vortex-Glass Phase Transition and Superconductivity in Fe1.01Te0.62Se0.38 Single Crystal

Transport properties and current–voltage (I–V) characteristics for the Fe1.01Te0.62Se0.38 single crystal were measured under dc magnetic field up to 14 T. The temperature dependence of resistivity (ρ–T) displayed major differences when magnetic fields were applied along and perpendicular to the c-axis, respectively, which distinguishes Fe1.01Te0.62Se0.38 from the isotropic β-FeSe. Strong evidence of the existence of vortex-glass transition was provided by both the tail behavior in the ρ–T curves and successful vortex-glass scaling of the I–V isotherms, implying excellent sustainability under high magnetic field and potential application of the superconducting system. In contrast to the typical three-dimensional vortex behavior of the AxFe2−ySe2 superconductors, the Fe1.01Te0.62Se0.38 single crystals displayed quasi two-dimensional vortex behavior. A vortex phase diagram is presented, based on the evolution of the upper critical field and the vortex-glass transition temperature.

Point-defect-induced colossal dielectric behavior in GaAs single crystals

We herein reported colossal dielectric constant (CDC) behavior in GaAs single crystals. This behavior appears in the temperature range above room temperature and results from the bulk effect due to polaron relaxation caused by hopping motion of EL2 defects. When temperature rises higher than 420 K, the interfacial contribution due to Maxwell–Wagner relaxation caused by sample/electrode contacts appears. When temperature is higher than 560 K, the CDC behavior is mainly contributed by the interfacial effect. These features are quite different from the CDC behavior found in oxides, and therefore, the CDC behavior in GaAs single crystals is considered as a new type of the CDC family. Our results underscore the role of point-defects in CDC behavior and suggest that defect engineering can be a promising strategy to achieve superior CDC behavior in both oxide and non-oxide materials.

Normal and abnormal dielectric relaxation behavior in KTaO3 ceramics

KTaO3 ceramic samples were prepared via a conventional solid state reaction route. The dielectric properties of KTaO3 were investigated in temperatures from room temperature to 1000 K and the frequency range of 102–106 Hz. The sample exhibits an abnormal dielectric behavior contrary to the traditional thermally activated behavior in the temperature range below 450 K. Our results revealed that the sample was very sensitive to humidity, leading to a metal-insulator transition (MIT) at 473 K. It is the positive temperature coefficient of resistance of the MIT that results in the abnormal dielectric behavior. When the temperature is higher than 500 K, the sample shows two normal dielectric relaxations following thermally activated behavior. The low- and high-temperature relaxations were argued, respectively, to be related to dipolar relaxation and Maxwell–Wagner relaxations due to oxygen vacancies hopping inside grains and then being blocked by grain boundaries.

High-temperature colossal dielectric behavior of BaZrO3 ceramics

BaZrO3 (BZO) powders were synthesized by solution combustion based on the glycine nitrate process. The sintering conditions of the BZO sample were optimized by varying the sintering temperature and dwell time. The dielectric properties of the sintered samples were investigated in the temperature range of 300–1080 K and frequency range of 102 to 106 Hz. The sample sintered at 1973 K for 12 h was found to show the best dielectric properties. Detailed investigations on this sample reveal an incipient ferroelectric behavior in the temperature range below 420 K and a colossal dielectric behavior above 420 K. The colossal dielectric behavior is composed of two thermally activated relaxations. The low-temperature relaxation was argued to be a dipolar relaxation caused by an oxygen vacancy hopping motion inside grains and the high-temperature one was ascribed to a Maxwell–Wagner relaxation due to the oxygen vacancies being blocked by the sample/electrode contacts.