top of page

Dynamical decoding of the relationship between proimate phase of Matter

HN_icons-01.png
HN_icons-02.png
HN_icons-03.png
HN_icons-04.png

​​​↘ Research Directions

* Denotes equal contribution  ✉️ Denotes corresponding author

Joint commensurate charge density wave hosts shear-type topological defects

Commensuration, the condition where the ratio between two physical quantities is a rational number, can lead to various exotic phenomena affecting both electrons and the underlying lattice in quantum materials. This concept is particularly crucial in charge density waves (CDWs), periodic modulations of electron density resulting from the interplay between electronic and lattice degrees of freedom. A CDW is termed commensurate when its wavelength is a simple rational multiple of the lattice constant. Beyond conventional incommensurate and commensurate CDWs, a new type called jointly commensurate CDW has emerged, where multiple coexisting incommensurate CDWs collectively exhibit commensurability. However, the absence of standard experimental methods for identifying these orders has limited their reports, and the collective response of the individual intercorrelated incommensurate CDWs to external perturbations remains unknown.

 

Here, we address these challenges by employing a combination of time-resolved techniques to directly probe the dynamics of CDWs in the jointly commensurate CDW candidate EuTe₄ upon photoexcitation. Time-resolved X-ray diffraction confirms that two incommensurate CDWs form a jointly commensurate CDW, preserving coherence and wavevectors even out of equilibrium. Complementary ultrafast electron diffraction and time- and angle-resolved photoemission spectroscopy reveal the creation of uncharted shear-type topological defects, which disrupt coherence only along the direction perpendicular to the wavevectors. These findings establish the persistence and unique dynamics of jointly commensurate CDWs out of equilibrium and identify a new platform for optical manipulation of topological defects.

Controllable enhancement and reduction of charge density waves mediated by phase competition

Coexisting and competing phases not only exhibit sensitivity to static tuning parameters such as pressure, electric field, magnetic field, and chemical doping, but also demonstrate remarkable susceptibility to ultrafast manipulation via light pulses. The intricate competition between coexisting CDWs can lead to rich phenomena, offering unique opportunities for phase manipulation through electromagnetic stimuli.

Fig2-1.png
Fig2-2.png

Leveraging time-resolved X-ray diffraction, we demonstrate nonmonotonic control of a CDW in EuTe₄ upon optical excitation. At low excitation intensities, the amplitude of this CDW order increases at the expense of competing orders, whereas at high intensities, it exhibits a nonmonotonic response characterized by both enhancement and reduction. This bidirectional controllability, tunable by adjusting the total deposited pump energy, arises fromthe interplay between optical quenchingand phase-competition-induced enhancement.Our findings, supported by phenomenological time-dependent Landau theory simulations, not only clarify the relationships between various CDWs in EuTe₄, but also underscore the versatility of optical control over order parameters enabled by phase competition.

See more details in the article: H. Ning*, K. H. Oh*, Y. Su*, A. Zong, D. Wu, B. Q. Lv., D. Z. Shi, Z. Shen, G. Kang, H. Choi, H. W. Kim, S. Ha, J. Kim, S. Sarker, J. P. C. Ruff, B. J. Kim, N. L. Wang, H. Jang, N. Gedik✉️, Optically controllable enhancement and reduction of charge density waves mediated by phase competition, in prep (2024). 

Dynamical decoding of competition between charge density waves in a kagome superconductor

The kagome superconductor CsV₃Sb₅ possesses a plethora of proximal phases stemming from the interplay between the non-trivial band topology, lattice geometric frustration, and electronic correlations. Ensuing or accompanying the emergence of CDWs, a cascade of symmetry breaking phases including orbital flux state, electronic nematicity, and superconductivity accompanied by a pair density wave arise. Therefore, deciphering the nature of the CDW phases in this series of compounds is of paramount significance.  However, identifying the precise structure of these CDW phases and their intricate relationships remain the subject of intense debate, due to the lack of static probes that can distinguish the CDW phases with identical spatial periodicity.

Fig2-3.png

Here, we unveil the out-of-equilibrium competition between two coexisting 2x2x2 CDWs in CsV₃Sb₅ harnessing time-resolvedX-ray diffraction. By analyzing the light-inducedchanges in the intensity of CDW superlattice peaks, we demonstrate the presence of both phases, each displaying a significantly different amount of melting upon excitation. The anomalous light-induced sharpening of peak width further shows that the phase that is more resistant to photo-excitation exhibits an increase in domain size at the expense of the other, thereby showcasing a hallmark of phase competition. Our results not only shed light on the interplay between the multiple CDW phases in CsV₃Sb₅, but also establish a non-equilibrium framework for comprehending complex phase relationships that are challenging to disentangle using static techniques.

See more details in the article: H. Ning*,  K. H. Oh* , Y. Su*, A. von Hoegen, Z. Porter, A. Capa Salinas, Q. L. Nguyen, M. Chollet, T. Sato, V. Esposito, M. C. Hoffmann, A. White, C. Melendrez, D. Zhu, S. D. Wilson, and N. Gedik✉️, Dynamical decoding of the competition between charge density waves in a kagome superconductor, Nat. Commun. 15, 7286 (2024).

See more details in the article: K. H. Oh*, Y. Su*, H. Ning*, A. Zong, D. Wu, B. Q. Lv., D. Z. Shi, Z. Shen, G. Kang, H. Choi, H. W. Kim, S. Ha, J. Kim, X. Shen, D. Luo, S. Weathersby, P. Kramer, X.Cheng, S. Sarker, J. P. C. Ruff, B. J. Kim, N. L. Wang, H. Jang, N. Gedik, Shear-type topological defects in jointly commensurate charge density wave, in prep (2024). 

bottom of page