Chia-Jung Yang

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Yang

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Chia-Jung

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0000-0001-8858-1548

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Publications 1 - 10 of 11

Magnetoelectric Phase Control at Domain-Wall-Like Epitaxial Oxide Multilayers
Gradauskaite, Elzbieta; Yang, Chia-Jung; Efe, Ipek; et al. (2025)
Advanced Functional Materials
Ferroelectric domain walls are nanoscale objects that can be created, positioned, and erased on demand. They often embody functional properties that are distinct from the surrounding bulk material. Enhanced conductivity, for instance, is observed at charged ferroelectric domain walls. Regrettably, domain walls of this type are scarce because of the energetically unfavorable electrostatics. This hinders the current technological development of domain-wall nanoelectronics. Here this constraint is overcome by creating robust domain-wall-like objects in epitaxial oxide heterostructures. Charged head-to-head (HH) and tail-to-tail (TT) junctions are designed with two ferroelectric layers (BaTiO3 and BiFeO3) that have opposing out-of-plane polarization. To test domain-wall-like functionalities, an ultrathin ferromagnetic La0.7Sr0.3MnO3 layer is inserted into the junctions. The interfacial electron or hole accumulation at the interfaces, set by the HH and TT polarization configurations, respectively, controls the LSMO conductivity and magnetization. Thus it is proposed that trilayers reminiscent of artificial domain walls provide magnetoelectric functionality and may constitute an important building block in the design of oxide-based electronic devices.
Origin of Terahertz Soft-Mode Nonlinearities in Ferroelectric Perovskites
Pal, Shovon; Strkalj, Nives; Yang, Chia-Jung; et al. (2021)
Physical Review X
Soft modes are intimately linked to structural instabilities and are key for the understanding of phase transitions. The soft modes in ferroelectrics, for example, map directly the polar order parameter of a crystal lattice. Driving these modes into the nonlinear, frequency-changing regime with intense terahertz (THz) light fields is an efficient way to alter the lattice and, with it, the physical properties. However, recent studies show that the THz electric-field amplitudes triggering a nonlinear soft-mode response are surprisingly low, which raises the question on the microscopic picture behind the origin of this nonlinear response. Here, we use linear and two-dimensional terahertz (2D THz) spectroscopy to unravel the origin of the soft-mode nonlinearities in a strain-engineered epitaxial ferroelectric SrTiO3 thin film. We find that the linear dielectric function of this mode is quantitatively incompatible with pure ionic or pure electronic motions. Instead, 2D THz spectroscopy reveals a pronounced coupling of electronic and ionic-displacement dipoles. Hence, the soft mode is a hybrid mode of lattice (ionic) motions and electronic interband transitions. We confirm this conclusion with model calculations based on a simplified pseudopotential concept of the electronic band structure. It reveals that the entire THz nonlinearity is caused by the off-resonant nonlinear response of the electronic interband transitions of the lattice-electronic hybrid mode. With this work, we provide fundamental insights into the microscopic processes that govern the softness that any material assumes near a ferroic phase transition. This knowledge will allow us to gain an efficient all-optical control over the associated large nonlinear effects.
Kondo coherence versus superradiance in terahertz radiation-driven heavy-fermion systems
Yang, Chia-Jung; Woerner, Michael; Stockert, Oliver; et al. (2024)
Physical Review B
In strongly correlated systems such as heavy-fermion materials, the coherent superposition of localized and mobile spin states leads to the formation of Kondo resonant states, which on a dense, periodic array of Kondo ions develop lattice coherence. Characteristically, these quantum-coherent superposition states respond to a terahertz (THz) excitation by a delayed THz pulse on the scale of the material's Kondo energy scale and hence independent of the pump-light intensity. However, a delayed response is also typical for superradiance in an ensemble of excited atoms. In this case, quantum coherence is established by the coupling to an external, electromagnetic mode and hence dependent on the pump-light intensity. In the present paper, we investigate the physical origin of the delayed pulse, i.e., inherent, correlation-induced versus light-induced coherence, in the prototypical heavy-fermion compound CeCu5.9Au0.1. We study the delay, duration, and amplitude of the THz pulse at various temperatures in dependence on the electric-field strength of the incident THz excitation. We observe a robust delayed response at approximately 6 ps with an amplitude proportional to the amplitude of the incident THz wave. This is consistent with theoretical expectation for the Kondo-like coherence and thus provides compelling evidence for the dominance of condensed-matter versus optical coherence in the heavy-fermion compound.
Exploring ultrafast dynamics of electrons in heavy fermions by THz time-domain spectroscopy
Yang, Chia-Jung (2023)
Terahertz crystal electric field transitions in a Kondo-lattice antiferromagnet
Shee, Payel; Yang, Chia-Jung; Kumar Pandey, Shishir; et al. (2024)
Physical Review B
Hybridization between the localized f electrons and the delocalized conduction electrons together with the crystal electric field (CEF) play a determinant role in governing the many-body ground state of a correlated-electron system. Here, we investigate the low-energy CEF states in CeAg₂Ge₂, a prototype Kondo-lattice antiferromagnet where Kondo correlation is found to exist within the antiferromagnetic phase. Using time-domain THz reflection spectroscopy, we show direct evidence of two low-energy CEF transitions at 0.6 THz (2.5 meV) and 2.1 THz (8.7 meV). The presence of low-frequency infrared-active phonon modes further manifests as a Fano-modified line shape of the 2.1 THz CEF conductivity peak. The temporal spectral weights obtained directly from the THz time traces, in addition, corroborate the corresponding CEF temperature scales of the compound.
Missing spectral weight in a heavy-fermion system far above the Néel temperature
Li, Jingwen; Priyadarshi, Debankit; Yang, Chia-Jung; et al. (2025)
Physical Review B
The competition between the Kondo spin-screening effect and the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction in heavy-fermion systems drives the quantum phase transition between the magnetically ordered and the heavy-Fermi-liquid ground states. Despite intensive investigations of heavy quasiparticles on the Kondo-screened side of the quantum phase transition and of their breakdown at the quantum critical point, studies on the magnetically ordering side are scarce. Using terahertz time-domain spectroscopy, we report a suppression of the Kondo quasiparticle weight in CeCu6-xAux samples on the antiferromagnetic side of the quantum phase transition at temperatures as much as two orders of magnitude above the Neel temperature TN. The suppression results from a quantum frustration effect induced by the temperature-independent RKKY interaction. Hence, our results emphasize that besides critical fluctuations, the RKKY interaction may play an important role in the quantum-critical scenario.
Terahertz control of many-body dynamics in quantum materials
Yang, Chia-Jung; Li, Jingwen; Fiebig, Manfred; et al. (2023)
Nature Reviews Materials
Quantum-mechanical phenomena underpin the behaviour of quantum materials at the microscopic level. The description of several essential properties of these materials surpasses the generic treatment of electrons as classical non-interacting entities. Owing to the many-body nature of quantum materials, a microscopic understanding of the interactions dictating their ground state is indispensable for acquiring control over their dynamics. Non-equilibrium measurements can characterize such interactions both in time and in space, and the temporal evolution of the relaxation processes after excitation sheds light on the underlying correlations among charge, spin, orbital and lattice degrees of freedom. The energy scales of these interactions fall within the terahertz (THz) range, making THz radiation not only an effective probe but also an ideal tool for non-equilibrium perturbation, and perhaps a future tool for manipulation. In this Review, we survey how THz light has been used to drive quantum materials out of equilibrium and to retrieve information on the associated correlation processes and many-body dynamics. In particular, we show how THz light can induce superconducting-like features in layered superconductors and drive quasiparticles in heavy-fermion systems out of equilibrium. We also provide several examples of phase transitions driven dynamically by pumping correlated systems using THz light.
Birefringence of orthorhombic DyScO3: Toward a terahertz quarter-wave plate
Yang, Chia-Jung; Li, Jingwen; Lehmann, Jannis; et al. (2021)
Applied Physics Letters
A perspective on nonlinearities in coherent magnetization dynamics
Li, Jingwen; Yang, Chia-Jung; Mondal, Ritwik; et al. (2022)
Applied Physics Letters
The recent thrust in ultrafast magnetization dynamics aims at extending spintronic functionalities to terahertz frequencies. Deterministic manipulation of magnetization at the corresponding ultrashort timescales requires minute control not only over the magnetization itself but also the reservoirs it is interacting with. Although the various intricate couplings between spins, phonons, and electrons - all of which are susceptible to ultrashort laser pulses - lead to many (often nonlinear) coupling routes, magnetization-dynamical nonlinearities have remained largely underexplored. In this Perspective, we highlight recent advances and foresee future developments in the rapidly evolving field of nonlinear magnetization dynamics. Given the elementary character of coherent excitations, we put particular emphasis on their nonlinearities. We briefly review theoretical aspects and assess excitation mechanisms to reach the nonlinear regime of magnetic excitations in a broad class of magnetic materials, such as ferromagnets, antiferromagnets, and ferrimagnets. We present an overview of the groundbreaking experiments that showcase the unique insights provided by magnetic nonlinearities. We conclude by discussing open challenges and opportunities that underpin the potential of nonlinear magnetization dynamics for the advancement of spintronics and cavity quantum electrodynamics with spin waves at terahertz frequencies.
Terahertz conductivity of heavy-fermion systems from time-resolved spectroscopy
Yang, Chia-Jung; Pal, Shovon; Zamani, Farzaneh; et al. (2020)
Physical Review Research
The Drude model describes the free-electron conduction in simple metals, governed by the freedom that the mobile electrons have within the material. In strongly correlated systems, however, a significant deviation of the optical conductivity from the simple metallic Drude behavior is observed. Here, we investigate the optical conductivity of the heavy-fermion system CeCu6−xAux , using time-resolved, phase-sensitive terahertz spectroscopy. The terahertz electric field creates two types of excitations in heavy-fermion materials: First, the intraband excitations that leave the heavy quasiparticles intact. Second, the resonant interband transitions between the heavy and light parts of the hybridized conduction band that break the Kondo singlet. We find that the Kondo-singlet-breaking interband transitions do not create a Drude peak, while the Kondo-retaining intraband excitations yield the expected Drude response. This makes it possible to separate these two fundamentally different correlated contributions to the optical conductivity.

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Chia-Jung Yang

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