Coslovich Giacomo, Kemper Alexander F, Behl Sascha, Huber Bernhard, Bechtel Hans A, Sasagawa Takao, Martin Michael C, Lanzara Alessandra, Kaindl Robert A
Materials Sciences Division, E.O. Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, USA.
SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, CA 94025, USA.
Sci Adv. 2017 Nov 24;3(11):e1600735. doi: 10.1126/sciadv.1600735. eCollection 2017 Nov.
The ability to probe symmetry-breaking transitions on their natural time scales is one of the key challenges in nonequilibrium physics. Stripe ordering represents an intriguing type of broken symmetry, where complex interactions result in atomic-scale lines of charge and spin density. Although phonon anomalies and periodic distortions attest the importance of electron-phonon coupling in the formation of stripe phases, a direct time-domain view of vibrational symmetry breaking is lacking. We report experiments that track the transient multi-terahertz response of the model stripe compound LaSrNiO, yielding novel insight into its electronic and structural dynamics following an ultrafast optical quench. We find that although electronic carriers are immediately delocalized, the crystal symmetry remains initially frozen-as witnessed by time-delayed suppression of zone-folded Ni-O bending modes acting as a fingerprint of lattice symmetry. Longitudinal and transverse vibrations react with different speeds, indicating a strong directionality and an important role of polar interactions. The hidden complexity of electronic and structural coupling during stripe melting and formation, captured here within a single terahertz spectrum, opens new paths to understanding symmetry-breaking dynamics in solids.
在自然时间尺度上探测对称性破缺转变的能力是非平衡物理学中的关键挑战之一。条纹有序化代表了一种有趣的对称性破缺类型，其中复杂的相互作用导致电荷和自旋密度的原子尺度线。尽管声子异常和周期性畸变证明了电子 - 声子耦合在条纹相形成中的重要性，但缺乏对振动对称性破缺的直接时域观点。我们报告了追踪模型条纹化合物LaSrNiO瞬态太赫兹响应的实验，对其在超快光学猝灭后的电子和结构动力学产生了新的见解。我们发现，尽管电子载流子立即离域，但晶体对称性最初保持冻结——这由作为晶格对称性指纹的折叠区Ni - O弯曲模式的延迟抑制所证明。纵向和横向振动以不同速度反应，表明强烈的方向性和极性相互作用的重要作用。在单个太赫兹光谱中捕获的条纹熔化和形成过程中电子与结构耦合的隐藏复杂性，为理解固体中的对称性破缺动力学开辟了新途径。
