Quest for new states of matter near an ordered phase is a promising route for making modern physics forward. By probing thermal properties of a ferroelectric (FE) crystal Ba1-xSrxAl2O4, we have clarified that low-energy excitation of acoustic phonons is remarkably enhanced with critical behavior at the border of the FE phase. The phonon spectrum is significantly damped toward the FE phase boundary and transforms into glasslike phonon excitation which is reminiscent of a boson peak. This system thus links long-standing issues of amorphous solids and structural instability in crystals to pave the way to controlling lattice fluctuation as a new tuning parameter.
Origin of the large phonon band-gap in SrTiO3 and the vibrational signatures of ferroelectricity in ATiO3 perovskite: First principles lattice dynamics and inelastic neutron scattering of PbTiO3, BaTiO3 and SrTiO3
Many crystalline solids possess strongly anharmonic soft phonon modes characterized by diminishing frequency as temperature approaches a critical point associated with a symmetry breaking phase transition. While electron-soft phonon coupling can introduce unique scattering channels for charge carriers in ferroelectrics, recent studies on the non-ferroelectric lead halide perovskites have also suggested the central role of anharmonic phonons bearing resemblance to soft modes in charge carrier screening. Here we apply coherent phonon spectroscopy to directly study electron coupling to the soft t…
Ultrafast optical control of ferroelectricity using intense terahertz fields has attracted significant interest. Here we show that the nonlinear interactions between two optical phonons in SnTe, a two-dimensional in-plane ferroelectric material, enables a dynamical amplification of the electric polarization within subpicoseconds time domain. Our first principles time dependent simulations show that the infrared-active out-of-plane phonon mode, pumped to nonlinear regimes, spontaneously generates in-plane motions, leading to rectified oscillations in the in-plane electric polarization. We sugge…
Investigating the competition between ferroelectric ordering and quantum fluctuations is essential to tailor the desired functionalities of mixed ferroelectric and incipient ferroelectric systems, like, (Ba,Sr)TiO3 and (Eu,Ba)TiO3. Recently, it has been shown that suppression of quantum fluctuations increases ferroelectric ordering in (Eu,Ba)TiO3 and since these phenomena are coupled to crystallographic phase transitions it is essential to understand the role of phonons. Here, we observe that the unusual temperature dependence of phonons in BaTiO3 gets suppressed when Ba2+ is replaced by Eu2+.…
We use first-principles methods to study doped strong ferroelectrics (taking BaTiO$_3$ as a prototype). Here we find a strong coupling between itinerant electrons and soft polar phonons in doped BaTiO$_3$, contrary to Anderson/Blount's weakly coupled electron mechanism for "ferroelectric-like metals". As a consequence, across a polar-to-centrosymmetric phase transition in doped BaTiO$_3$, the total electron-phonon coupling is increased to about 0.6 around the critical concentration, which is sufficient to induce phonon-mediated superconductivity of about 2 K. Lowering the crystal symmetry of d…
Domain Walls Stabilized by Intrinsic Phonon Modes and Engineered Defects Enable Robust Ferroelectricity in HfO2
School of Integrated Circuits, Peking University（北京大学集成电路学院）；Advanced Instrumental Analysis Center, School of Chemical Engineering and Technology, Tianjin University（天津大学化学工程与技术学院先进仪器分析中心）
We investigate the effect of local electron-phonon coupling on nonlinear optical conductivity in an interacting ferroelectric system. Using real-time simulations, we show an enhancement in nonlinear conductivity under linearly-polarized light due to generation of the phonon-assisted ballistic-current in addition to the injection-current generated by electron-hole pairs. The optically excited phonon modes generate an asymmetric carrier distribution that causes a strong directional ballistic-current. The ballistic-current enhances the photocurrent several times at above band-gap excitation frequ…
Spontaneous decay of a soft optical phonon in the relaxor ferroelectric PbMg$_{1/3}$Nb$_{2/3}$O$_{3}$
我们报道了固体中软光学声子的自发衰变。利用中子能谱，我们发现在弛豫体PbMg $_{1/3}$ Nb $_{2/3}$ O $_{3}$中的比声子寿命在定义好的能量和动量范围内非常短。这种行为是独立的铁电秩序和发生时，具有特定的能量和动量的光学声子可以运动学衰减成两个声学声子具有较低的相速度。我们解释了著名的弛豫“瀑布”效应作为一种形式的准粒子衰变类似于以前报道的量子自旋液体和量子流体。
We present a detailed analysis of the temperature dependence of the thermal conductivity of a ferroelectric PbTiO3 thin film deposited in a composition-spread geometry enabling a continuous range of compositions from ~25% titanium-deficient to ~20% titanium-rich to be studied. By fitting the experimental results to the Debye model we deconvolve and quantify the two main phonon scattering sources in the system: ferroelectric domain walls (DWs) and point defects. Our results prove that ferroelectric DWs are the main agent limiting the thermal conductivity in this system, not only in the stoichio…
Microwave measurements have recently been successfully applied to measure ferroelectric materials on the nanoscale, including detection of polarization switching and ferroelectric domain walls. Here we discuss the question whether scanning probe microscopy (SPM) operating at microwave frequency can identify the changes associated with the soft phonon dynamics in a ferroic. The analytical expressions for the electric potential, complex impedance and dielectric losses are derived and analyzed, since these physical quantities are linked to experimentally-measurable properties of the ferroic. As a…
First principles investigation of ferroelectricity in epitaxially strained Pb$_2$TiO$_4$
本文用密度泛函理论研究了假设的Ruddlesden-Popper化合物Pb $_2$ TiO $_4$的结构和极化。计算了空间群为$I4/mmm$的高对称性K-$_2$-NiF-$_4$-型参考结构的带内声子.在理论基态晶格常数下，存在一个不稳定的红外激活声子。该声子冻结以产生$I2mm$铁电态。作为外延应变的函数，两个额外的铁电相被发现，与空间群$I4mm$和$F2mm$在压缩和拉伸应变，分别。
Anomalous transverse acoustic phonon broadening in the relaxor ferroelectric Pb(Mg_1/3Nb_2/3)O_3
在弛豫铁电化合物Pb（Mg）$_{1/3}$ Nb $_{2/3})_{0.8}$ Ti $_{0.2}$ O $_3$（PMN-20%PT）中观察到的横向声学（TA）声子的本征线宽$Γ_{TA}$在650 K附近开始随温度的降低而变宽，比铁电转变温度$T_c$（$\sim 360$ K）高近300 K.我们推测，这种异常的行为是直接相关的极化，纳米尺寸，在伯恩斯温度$T_d$区域的冷凝。我们还观察到以前在纯PMN中看到的“瀑布”异常，其中横向光学（TO）分支似乎在有限的动量转移$q_{wf} \sim 0.15$ $^{-1}$处急剧下降到TA分支。即使在高于$T_d$的温度下也能看到瀑布特征。后一个结果表明PNR作为动态实体存在于$T_d$之上。
Historically, the soft mode theory of ferroelectric phase transitions has been developed for the high-temperature (paraelectric) phase, where the phonon mode softens upon decreasing the temperature. In the low-temperature ferroelectric phase, a similar phonon softening occurs, also leading to a bosonic condensation of the frozen-in mode at the transition, but in this case the phonon softening occurs upon increasing the temperature. Here we present a soft mode theory of ferroelectric and displacive phase transitions by describing what happens in the low-temperature phase in terms of phonon soft…
We present a minimal theory of superconductivity enhancement in ferroelectric-type materials. Simple expressions for the optical mode responsible for the soft mode transition are assumed. A key role is played by the anharmonic phonon damping which is modulated by an external control parameter (electron doping or mechanical strain) causing the appearance of the soft mode. It is shown that the enhancement in the superconducting critical temperature $T_{c}$ upon approaching the ferroelectric transition from either side is due to the Stokes electron-phonon scattering processes promoted by strong p…
Multiferroics possess two or more switchable states such as polarization, magnetization, etc. Phonon excitations in multiferroic phase are strongly modified by magnetoelectric coupling, spin-phonon coupling, and anharmonic phonon-phonon interactions. Here, we have investigated the correlation between phonons and multiferroic order parameters in hexagonal BaMO3: M = Ti1-x(Mn/Fe)x systems using powder x-ray diffraction (PXRD), Raman spectroscopic, and magnetic measurements. The structural transformation from a polar tetragonal to a non-polar 6H-type hexagonal phase is observed as a function of d…
IR Phonon signatures of multiferroicity in TbMn$_{2}$O$_{5}$
研究了多铁性TbMn $_2$ O $_5$的红外（IR）激活声子随温度的变化.观察到了大部分沿\textit{a}和\textit{b}晶轴极化的对称性允许的红外模式，并且几个\textit{b}极化声子的行为与磁和铁电跃迁相关。顺电相只允许拉曼的高频\textit{b}极化声子在铁电相变时变为IR活性。该模式的IR强度与铁电体序参量的平方成比例，并且给出了降低铁电体相中晶格畸变的对称性的灵敏量度。
The symmetry breaking induced by the ferroelectric transition often triggers the emergence of topological electronic states such as Weyl fermions in ferroelectric-like metals/semimetals. Such strong coupling between the lattice deformation and electronic states is therefore essentially important for the control of novel topological phases. Here, we study the terahertz lattice and charge dynamics in ferroelectric semiconductor SnxPb1-xTe thin films hosting versatile topological phases by means of the terahertz time-domain spectroscopy. With lowering the temperature, the resonant frequency of tr…
The proximity to structural phase transitions in IV-VI thermoelectric materials is one of the main reasons for their large phonon anharmonicity and intrinsically low lattice thermal conductivity $κ$. However, the $κ$ of GeTe increases at the ferroelectric phase transition near $700$ K. Using first-principles calculations with the temperature dependent effective potential method, we show that this rise in $κ$ is the consequence of negative thermal expansion in the rhombohedral phase and increase in the phonon lifetimes in the high-symmetry phase. Negative thermal expansion increases phonon grou…
Coherent seeding and control of dynamical ferroelectricity by phonon anharmonicity
Peking University（北京大学）；Beihang University（北京航空航天大学）；Institute of Physics, Chinese Academy of Sciences（中国科学院物理研究所）；Beijing National Laboratory for Condensed Matter Physics（北京凝聚态物理国家研究中心）；Beijing Academy of Quantum Information Sciences（北京量子信息科学研究院）
