Light-Induced Charge Density Wave in LaTe$_3$ (1904.07472v1)

Abstract: When electrons in a solid are excited with light, they can alter the free energy landscape and access phases of matter that are beyond reach in thermal equilibrium. This accessibility becomes of vast importance in the presence of phase competition, when one state of matter is preferred over another by only a small energy scale that, in principle, is surmountable by light. Here, we study a layered compound, LaTe$_3$, where a small in-plane (a-c plane) lattice anisotropy results in a unidirectional charge density wave (CDW) along the c-axis. Using ultrafast electron diffraction, we find that after photoexcitation, the CDW along the c-axis is weakened and subsequently, a different competing CDW along the a-axis emerges. The timescales characterizing the relaxation of this new CDW and the reestablishment of the original CDW are nearly identical, which points towards a strong competition between the two orders. The new density wave represents a transient non-equilibrium phase of matter with no equilibrium counterpart, and this study thus provides a framework for unleashing similar states of matter that are "trapped" under equilibrium conditions.

• The paper demonstrates that photoexcitation induces a transient a-axis CDW in LaTe3 while suppressing the equilibrium c-axis CDW.
• It employs ultrafast electron diffraction to capture non-equilibrium dynamics with both CDWs relaxing on a similar ~1.7 picosecond timescale.
• The work provides key insights into manipulating competitive phases and accessing hidden states in complex condensed matter systems.

Light-Induced Charge Density Wave in LaTe$_3$

The paper "Light-Induced Charge Density Wave in LaTe$_3$" presents an exploration of photoinduced phase transitions in the layered compound LaTe$_3$, emphasizing the dynamics and interactions of competing charge density waves (CDWs). Utilizing ultrafast electron diffraction (UED) techniques, the authors investigate the transient appearance of a non-equilibrium CDW upon photoexcitation, a phenomenon significant for understanding phase competition in condensed matter systems.

Overview of Experimental Findings

The authors begin by detailing the structural and electronic properties of LaTe$_3$, a quasi-two-dimensional material characterized by an in-plane lattice anisotropy that supports a unidirectional CDW along the c-axis under equilibrium conditions. This inherent CDW coexists within a framework of competing phases, made accessible through photoexcitation.

The experimental methodology involves the application of ultrafast electron diffraction (UED) to capture the real-time dynamics of the CDW transitions. Upon photoexcitation with femtosecond laser pulses, a suppression of the original c-axis CDW is observed, accompanied by the emergence of a transient a-axis CDW. This second CDW is noted to possess a wavevector distinct from that of any equilibrium states in the rare-earth tritelluride series, indicating a non-trivial, non-equilibrium phase. Remarkably, the dynamics governing the relaxation of this transient CDW and the re-establishment of the original CDW exhibit nearly identical timescales, suggesting a robust competitive interaction between these phases.

Strong Numerical Results and Key Observations

• Wavevector Determination: The transient a-axis CDW is characterized by a wavevector $\widetilde{q}_a=0.291(13)$ reciprocal lattice units, a value smaller than that of the stable c-axis CDW and any equilibrium a-axis CDWs in heavier rare-earth tritellides.
• Relaxation Timescales: Both CDWs display relaxation dynamics on the order of 1.7 picoseconds, elucidating the intense competition inherent in these systems.
• Energy Landscape Modulation: Through photoexcitation, a shift in the energy landscape is achieved, allowing the observation of a CDW phase with no equilibrium counterpart.

Theoretical Implications

The introduction of a transient CDW, activated by light, opens intriguing theoretical implications. The configuration of this CDW lacks a corresponding equilibrium state—a feature that underlines its unique origin as being driven by phase competition. A Ginzburg-Landau formalism provides a theoretical underpinning, supporting the hypothesis that transient CDWs emerge in regions where the equilibrium order is locally suppressed, potentially by topological defects like dislocations.

This mechanistic framework implies potential routes to manipulate material properties and access hidden phases using optical control, particularly given how subtle alterations in symmetry and electronic interactions can reveal entirely new states of matter. The paper enriches the theoretical understanding of far-from-equilibrium behavior in strongly correlated electron systems.

Future Developments and Applications

The implications of this research extend to the broader field of photoinduced phase transitions and the use of light as a control mechanism in quantum materials. As techniques such as UED develop and computational modeling becomes more sophisticated, researchers could aim to explore these dynamics across different material systems, including those relevant for high-temperature superconductivity and other emergent phenomena. Furthermore, the insights gained here could inform the design of materials and devices that leverage controlled non-equilibrium states for novel functionalities, potentially influencing future developments in quantum computing and advanced optoelectronic applications.

In summary, this paper advances our understanding of nonequilibrium phases by demonstrating the power of ultrafast techniques in modulating and uncovering hidden competitive interactions within complex materials.