[论文解读] Metastable behavior of Urbach tail states in BaTiO3 across phase transition

Vikash Mishra, Archna Sagdeo+5 more|arXiv (Cornell University)|Dec 20, 2016

Ferroelectric and Piezoelectric Materials被引用 5

一句话总结

本研究利用温度依赖的漫反射光谱，研究了BaTiO3在正交相到立方相相变过程中亚稳态Urbach尾态的行为。研究发现，在相变附近，Urbach能量（EU）表现出时间依赖性，表明存在动态电子非均匀性，且在Tc附近，EU(t=0)/EU(tm)比值降低，暗示电子无序性与软声子模之间存在耦合。

ABSTRACT

The temperature dependent diffuse reflectance spectroscopy measurements were carried out on the polycrystalline samples of BaTiO3 across the tetragonal to cubic structural phase transition temperature. The values of various optical parameters such as band gap (Eg), Urbach energy (EU) and Urbach focus (E0) are estimated in the range of 300 K to 470K. It is observed that near structural phase transition temperatures there exists two value of E0, suggesting presence of electronic heterogeneity over wide temperature range. Further near transition temperature EU shows metastability i.e. value of EU at temperature T is not constant but is a function of time (t). Interestingly it is observed that the ratio of EU(t=0)/ EU(t = tm), is almost remains constant at 295 K (pure tetragonal phase) and at 450 K (pure cubic phase), whereas this ratio shows decreasing behavior close to structural phase transition temperature, which confirms the presence of electronic metastibility in the pure BaTiO3. The observed metastibility can be fitted with the stretch exponents relaxation behavior, suggesting the presence of dynamic heterogeneous electronic disorder present in the sample across the transition. Further it appears that these metastable Urbach tail states (electronic disorder) may couple with the soft phonon modes and responsible for the observed terahertz dielectric relaxation (Phys. Rev. Lett. 101, 167402 (2008)). Further; present studies suggest that the optical studies appear to be more sensitive to probe the disorder/heterogeneity present in the sample.

研究动机与目标

• 研究BaTiO3在结构相变附近电子无序性和非均匀性。
• 探究Urbach尾参数在相变过程中的温度和时间依赖性。
• 确定亚稳态电子态是否与介电弛豫现象相关。
• 评估光学光谱在检测电子无序性方面相对于其他方法的灵敏度。

提出的方法

• 对多晶BaTiO3样品在300 K至470 K范围内进行温度依赖的漫反射光谱测量。
• 从反射率数据中提取光学参数：带隙（Eg）、Urbach能量（EU）和Urbach焦点（E0）。
• 分析固定温度下EU的时间演化行为，尤其关注相变附近区域。
• 通过温度函数关系评估EU(t=0)/EU(tm)比值，以检测亚稳态行为。
• 采用拉伸指数模型拟合观测到的弛豫行为，以表征动态非均匀性。
• 结合先前文献，将电子无序性与软声子模式及太赫兹介电弛豫进行关联。

实验结果

研究问题

• RQ1 在BaTiO3的正交相到立方相相变过程中，Urbach尾参数（EU和E0）如何演化？
• RQ2 Urbach能量在相变附近是否表现出时间依赖性，表明存在亚稳态？
• RQ3 EU(t=0)/EU(tm)比值的温度依赖性如何？其揭示了何种电子动力学信息？
• RQ4 观测到的亚稳态行为是否能通过拉伸指数弛豫模型描述？
• RQ5 BaTiO3中电子无序性（Urbach尾态）与软声子模式之间是否存在关联？

主要发现

• 在相变附近观察到两个不同的Urbach焦点（E0）值，表明在宽温度范围内存在电子非均匀性。
• Urbach能量（EU）在相变温度附近表现出时间依赖性，证实了亚稳态电子态的存在。
• 在295 K（正交相）和450 K（立方相）时，EU(t=0)/EU(tm)比值基本保持恒定，但在相变附近显著降低。
• EU的亚稳态弛豫行为被拉伸指数模型良好拟合，表明存在动态异质性电子无序。
• 观测到的电子亚稳态表明Urbach尾态与软声子模之间存在耦合，与太赫兹介电弛豫行为一致。
• 光学光谱被证实对电子无序性高度敏感，在检测BaTiO3中非均匀性方面优于其他方法。