A central theme in modern condensed matter physics is the exploration of emergent phenomena, novel collective behaviors that arise from interactions among an enormous number of quantum particles, typically on the order of Avogadro’s number ( \(\sim \) 10 \(^{23}\) ). These phenomena, often not predictable from the properties of individual particles, manifest in quantum materials, where charge, spin, orbital, and lattice degrees of freedom interact in complex and often competing ways. Traditional experimental probes such as electrical transport, thermodynamic measurements, and linear spectroscopies have provided invaluable insights, but frequently fall short in disentangling the intricate couplings among these degrees of freedom. A promising strategy to overcome these limitations involves selective perturbation: exciting one degree of freedom and tracking the resulting response in others. This approach has been made possible by the advent of ultrafast laser techniques, which offer femtosecond ( \(10^{-15}\) s) time resolution and the ability to drive materials far from equilibrium. Such ultrafast pump-probe spectroscopy has proven powerful not only for resolving the microscopic dynamics of complex materials, but also for enabling transient control over their ordered phases. These techniques effectively open a new laboratory for studying quantum materials in the nonequilibrium regime, where novel phases and enhanced functionalities can emerge. This dissertation contributes to these efforts by developing and applying new ultrafast spectroscopic tools to probe and control lattice, magnetic, and electronic properties in low-dimensional and layered quantum materials. In particular, it focuses on inducing a transient magnetic state with net magnetization in the van der Waals antiferromagnet FePS \(_3\) , using intense, low-energy terahertz (THz) pulses. The results reveal that critical fluctuations near the antiferromagnetic phase transition enhance both the amplitude and lifetime of this induced magnetic state. Additionally, a broadband two-dimensional (2D) THz spectroscopy technique is developed and employed to investigate low-energy collective excitations, allowing for the direct identification of phonon modes responsible for magnetic phase switching. In a related study, time-resolved pump-probe spectroscopy in the visible and near-infrared ranges is used to detect a bound state between phonons and electronic excitations in the sister compound NiPS \(_3\) , and to observe magnetostriction, a coupling between magnetism and lattice distortion, in FePS \(_3\) via coherent phonon generation, which eluded detection by conventional diffraction techniques. Finally, second harmonic generation (SHG) microscopy with microscale spatial resolution is applied to the layered multiferroic material NiI \(_2\) , revealing the persistence of its nonlinear optical response down to the monolayer limit, a demonstration of long-range multiferroic order in a two-dimensional system, and a first of its kind.

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发表日期：

2025年

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Batyr Ilyas

摘要

A central theme in modern condensed matter physics is the exploration of emergent phenomena, novel collective behaviors that arise from interactions among an enormous number of quantum particles, typically on the order of Avogadro’s number ( \(\sim \) 10 \(^{23}\) ). These phenomena, often not predictable from the properties of individual particles, manifest in quantum materials, where charge, spin, orbital, and lattice degrees of freedom interact in complex and often competing ways. Traditional experimental probes such as electrical transport, thermodynamic measurements, and linear spectroscopies have provided invaluable insights, but frequently fall short in disentangling the intricate couplings among these degrees of freedom. A promising strategy to overcome these limitations involves selective perturbation: exciting one degree of freedom and tracking the resulting response in others. This approach has been made possible by the advent of ultrafast laser techniques, which offer femtosecond ( \(10^{-15}\) s) time resolution and the ability to drive materials far from equilibrium. Such ultrafast pump-probe spectroscopy has proven powerful not only for resolving the microscopic dynamics of complex materials, but also for enabling transient control over their ordered phases. These techniques effectively open a new laboratory for studying quantum materials in the nonequilibrium regime, where novel phases and enhanced functionalities can emerge. This dissertation contributes to these efforts by developing and applying new ultrafast spectroscopic tools to probe and control lattice, magnetic, and electronic properties in low-dimensional and layered quantum materials. In particular, it focuses on inducing a transient magnetic state with net magnetization in the van der Waals antiferromagnet FePS \(_3\) , using intense, low-energy terahertz (THz) pulses. The results reveal that critical fluctuations near the antiferromagnetic phase transition enhance both the amplitude and lifetime of this induced magnetic state. Additionally, a broadband two-dimensional (2D) THz spectroscopy technique is developed and employed to investigate low-energy collective excitations, allowing for the direct identification of phonon modes responsible for magnetic phase switching. In a related study, time-resolved pump-probe spectroscopy in the visible and near-infrared ranges is used to detect a bound state between phonons and electronic excitations in the sister compound NiPS \(_3\) , and to observe magnetostriction, a coupling between magnetism and lattice distortion, in FePS \(_3\) via coherent phonon generation, which eluded detection by conventional diffraction techniques. Finally, second harmonic generation (SHG) microscopy with microscale spatial resolution is applied to the layered multiferroic material NiI \(_2\) , revealing the persistence of its nonlinear optical response down to the monolayer limit, a demonstration of long-range multiferroic order in a two-dimensional system, and a first of its kind.