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SPECIAL TOPIC — States and new effects in nonequilibrium
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Attosecond ionization time delays in strong-field physics
Yongzhe Ma(马永哲), Hongcheng Ni(倪宏程), and Jian Wu(吴健)
Chin. Phys. B, 2024, 33 (
1
): 013201. DOI:
10.1088/1674-1056/ad0e5d
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200
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Electronic processes within atoms and molecules reside on the timescale of attoseconds. Recent advances in the laser-based pump-probe interrogation techniques have made possible the temporal resolution of ultrafast electronic processes on the attosecond timescale, including photoionization and tunneling ionization. These interrogation techniques include the attosecond streak camera, the reconstruction of attosecond beating by interference of two-photon transitions, and the attoclock. While the former two are usually employed to study photoionization processes, the latter is typically used to investigate tunneling ionization. In this review, we briefly overview these timing techniques towards an attosecond temporal resolution of ionization processes in atoms and molecules under intense laser fields. In particular, we review the backpropagation method, which is a novel hybrid quantum-classical approach towards the full characterization of tunneling ionization dynamics. Continued advances in the interrogation techniques promise to pave the pathway towards the exploration of ever faster dynamical processes on an ever shorter timescale.
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Core-level spectroscopy of the photodissociation process of BrCN molecule
Kun Zhou(周坤) and Han Wang(王涵)
Chin. Phys. B, 2024, 33 (
1
): 018702. DOI:
10.1088/1674-1056/ad0cc9
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Fewest-switches surfacing hopping (FSSH) simulations have been performed with the high-level multi-reference electronic structure method to explore the coupled electronic and nuclear dynamics upon photoexcitation of cyanogen bromide (BrCN). The potential energy surfaces (PES) of BrCN are charted as functions of the Jacobi coordinates (
R
,
θ
). An in-depth examination of the FSSH trajectories reveals the temporal dynamics of the molecule and the population changes of the lowest twelve states during BrCN's photodissociation process, which presents a rich tapestry of dynamical information. Furthermore, the carbon
K
-edge x-ray absorption spectroscopy (XAS) is calculated with multi-reference inner-shell spectral simulations. The rotation of the CN fragment and the elongation of the C—Br bond are found to be the reason for the peak shifting in the XAS. Our findings offer a nuanced interpretation for inner-shell probe investigations of BrCN, setting the stage for a deeper understanding of the photodissociation process of cyanogen halides molecules.
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Photophysics of metal-organic frameworks: A brief overview
Qingshuo Liu(刘晴硕), Junhong Yu(余俊宏), and Jianbo Hu(胡建波)
Chin. Phys. B, 2024, 33 (
1
): 017204. DOI:
10.1088/1674-1056/acfe00
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Metal-organic frameworks (MOFs), which are self-assembled porous coordination materials, have garnered considerable attention in the fields of optoelectronics, photovoltaic, photochemistry, and photocatalysis due to their diverse structures and excellent tunability. However, the performance of MOF-based optoelectronic applications currently falls short of the industry benchmark. To enhance the performance of MOF materials, it is imperative to undertake comprehensive investigations aimed at gaining a deeper understanding of photophysics and sequentially optimizing properties related to photocarrier transport, recombination, interaction, and transfer. By utilizing femtosecond laser pulses to excite MOFs, time-resolved optical spectroscopy offers a means to observe and characterize these ultrafast microscopic processes. This approach adds the time coordinate as a novel dimension for comprehending the interaction between light and MOFs. Accordingly, this review provides a comprehensive overview of the recent advancements in the photophysics of MOFs and additionally outlines potential avenues for exploring the time domain in the investigation of MOFs.
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Optical manipulation of the topological phase in ZrTe
5
revealed by time- and angle-resolved photoemission
Chaozhi Huang(黄超之), Chengyang Xu(徐骋洋), Fengfeng Zhu(朱锋锋), Shaofeng Duan(段绍峰), Jianzhe Liu(刘见喆), Lingxiao Gu(顾凌霄), Shichong Wang(王石崇), Haoran Liu(刘浩然), Dong Qian(钱冬), Weidong Luo(罗卫东), and Wentao Zhang(张文涛)
Chin. Phys. B, 2024, 33 (
1
): 017901. DOI:
10.1088/1674-1056/ad0d9d
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149
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High-resolution time- and angle-resolved photoemission measurements were conducted on the topological insulator ZrTe
5
. With strong femtosecond photoexcitation, a possible ultrafast phase transition from a weak to a strong topological insulating phase was experimentally realized by recovering the energy gap inversion in a time scale that was shorter than 0.15 ps. This photoinduced transient strong topological phase can last longer than 2 ps at the highest excitation fluence studied, and it cannot be attributed to the photoinduced heating of electrons or modification of the conduction band filling. Additionally, the measured unoccupied electronic states are consistent with the first-principles calculation based on experimental crystal lattice constants, which favor a strong topological insulating phase. These findings provide new insights into the longstanding controversy about the strong and weak topological properties in ZrTe
5
, and they suggest that many-body effects including electron—electron interactions must be taken into account to understand the equilibrium weak topological insulating phase in ZrTe
5
.
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Ultrafast carrier dynamics in GeSn thin film based on time-resolved terahertz spectroscopy
Panpan Huang(黄盼盼), Youlu Zhang(张有禄), Kai Hu(胡凯), Jingbo Qi(齐静波), Dainan Zhang(张岱南), and Liang Cheng(程亮)
Chin. Phys. B, 2024, 33 (
1
): 017201. DOI:
10.1088/1674-1056/accf7f
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138
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We measure the time-resolved terahertz spectroscopy of GeSn thin film and studied the ultrafast dynamics of its photo-generated carriers. The experimental results show that there are photo-generated carriers in GeSn under femtosecond laser excitation at 2500 nm, and its pump-induced photoconductivity can be explained by the Drude—Smith model. The carrier recombination process is mainly dominated by defect-assisted Auger processes and defect capture. The first- and second-order recombination rates are obtained by the rate equation fitting, which are (2.6±1.1)×10
-2
ps
-1
and (6.6±1.8)×10
-19
cm
3
·ps
-1
, respectively. Meanwhile, we also obtain the diffusion length of photo-generated carriers in GeSn, which is about 0.4 μm, and it changes with the pump delay time. These results are important for the GeSn-based infrared optoelectronic devices, and demonstrate that GeSn materials can be applied to high-speed optoelectronic detectors and other applications.
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Ultrafast dynamics in photo-excited Mott insulator Sr
3
Ir
2
O
7
at high pressure
Xia Yin(尹霞), Jianbo Zhang(张建波), Wang Dong(王东), Takeshi Nakagawa, Chunsheng Xia(夏春生), Caoshun Zhang(张曹顺), Weicheng Guo(郭伟程), Jun Chang(昌峻), and Yang Ding(丁阳)
Chin. Phys. B, 2024, 33 (
1
): 016103. DOI:
10.1088/1674-1056/acdc8c
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130
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High-pressure ultrafast dynamics, as a new crossed research direction, are sensitive to subtle non-equilibrium state changes that might be unresolved by equilibrium states measurements, providing crucial information for studying delicate phase transitions caused by complex interactions in Mott insulators. With time-resolved transient reflectivity measurements, we identified the new phases in the spin—orbit Mott insulator Sr
3
Ir
2
O
7
at 300 K that was previously unidentified using conventional approaches such as x-ray diffraction. Significant pressure-dependent variation of the amplitude and lifetime obtained by fitting the reflectivity Δ
R
/
R
reveal the changes of electronic structure caused by lattice distortions, and reflect the critical phenomena of phase transitions. Our findings demonstrate the importance of ultrafast nonequilibrium dynamics under extreme conditions for understanding the phase transition of Mott insulators.
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Progress and realization platforms of dynamic topological photonics
Qiu-Chen Yan(闫秋辰), Rui Ma(马睿), Xiao-Yong Hu(胡小永), and Qi-Huang Gong(龚旗煌)
Chin. Phys. B, 2024, 33 (
1
): 010301. DOI:
10.1088/1674-1056/acf284
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149
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Dynamic topological photonics is a novel research field, combining the time-domain optics and topological physics. In this review, the recent progress and realization platforms of dynamic topological photonics have been well introduced. The definition, measurement methods and the evolution process of the dynamic topological photonics are demonstrated to better understand the physical diagram. This review is meant to bring the readers a different perspective on topological photonics, grasp the advanced progress of dynamic topology, and inspire ideas about future prospects.
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Capturing the non-equilibrium state in light—matter—free-electron interactions through ultrafast transmission electron microscopy
Wentao Wang(汪文韬), Shuaishuai Sun(孙帅帅), Jun Li(李俊), Dingguo Zheng(郑丁国), Siyuan Huang(黄思远), Huanfang Tian(田焕芳), Huaixin Yang(杨槐馨), and Jianqi Li(李建奇)
Chin. Phys. B, 2024, 33 (
1
): 010701. DOI:
10.1088/1674-1056/ad0141
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146
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Ultrafast transmission electron microscope (UTEM) with the multimodality of time-resolved diffraction, imaging, and spectroscopy provides a unique platform to reveal the fundamental features associated with the interaction between free electrons and matter. In this review, we summarize the principles, instrumentation, and recent developments of the UTEM and its applications in capturing dynamic processes and non-equilibrium transient states. The combination of the transmission electron microscope with a femtosecond laser via the pump—probe method guarantees the high spatiotemporal resolution, allowing the investigation of the transient process in real, reciprocal and energy spaces. Ultrafast structural dynamics can be studied by diffraction and imaging methods, revealing the coherent acoustic phonon generation and photo-induced phase transition process. In the energy dimension, time-resolved electron energy-loss spectroscopy enables the examination of the intrinsic electronic dynamics of materials, while the photon-induced near-field electron microscopy extends the application of the UTEM to the imaging of optical near fields with high real-space resolution. It is noted that light—free-electron interactions have the ability to shape electron wave packets in both longitudinal and transverse directions, showing the potential application in the generation of attosecond electron pulses and vortex electron beams.
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Universal basis underlying temperature, pressure and size induced dynamical evolution in metallic glass-forming liquids
H P Zhang(张华平), B B Fan(范蓓蓓), J Q Wu(吴佳琦), and M Z Li(李茂枝)
Chin. Phys. B, 2024, 33 (
1
): 016101. DOI:
10.1088/1674-1056/acf994
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147
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The dramatic temperature-dependence of liquids dynamics has attracted considerable scientific interests and efforts in the past decades, but the physics of which remains elusive. In addition to temperature, some other parameters, such as pressure, loading and size, can also tune the liquid dynamics and induce glass transition, which makes the situation more complicated. Here, we performed molecular dynamics simulations for Ni
50
Zr
50
bulk liquid and nanodroplet to study the dynamics evolution in the complex multivariate phase space, especially along the isotherm with the change of pressure or droplet size. It is found that the short-time Debye—Waller factor universally determines the long-time relaxation dynamics no matter how the temperature, pressure or size changes. The basic correlation even holds at the local atomic scale. This finding provides general understanding of the microscopic mechanism of dynamic arrest and dynamic heterogeneity.
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Anelasticity to plasticity transition in a model two-dimensional amorphous solid
Baoshuang Shang(尚宝双)
Chin. Phys. B, 2024, 33 (
1
): 016102. DOI:
10.1088/1674-1056/acf82c
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Anelasticity, as an intrinsic property of amorphous solids, plays a significant role in understanding their relaxation and deformation mechanism. However, due to the lack of long-range order in amorphous solids, the structural origin of anelasticity and its distinction from plasticity remain elusive. In this work, using frozen matrix method, we study the transition from anelasticity to plasticity in a two-dimensional model glass. Three distinct mechanical behaviors, namely, elasticity, anelasticity, and plasticity, are identified with control parameters in the amorphous solid. Through the study of finite size effects on these mechanical behaviors, it is revealed that anelasticity can be distinguished from plasticity. Anelasticity serves as an intrinsic bridge connecting the elasticity and plasticity of amorphous solids. Additionally, it is observed that anelastic events are localized, while plastic events are subextensive. The transition from anelasticity to plasticity is found to resemble the entanglement of long-range interactions between element excitations. This study sheds light on the fundamental nature of anelasticity as a key property of element excitations in amorphous solids.
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Ab initio
nonadiabatic molecular dynamics study on spin—orbit coupling induced spin dynamics in ferromagnetic metals
Wansong Zhu(朱万松), Zhenfa Zheng(郑镇法), Qijing Zheng(郑奇靖), and Jin Zhao(赵瑾)
Chin. Phys. B, 2024, 33 (
1
): 016301. DOI:
10.1088/1674-1056/acf91d
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180
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Understanding the photoexcitation induced spin dynamics in ferromagnetic metals is important for the design of photo-controlled ultrafast spintronic device. In this work, by the
ab initio
nonadiabatic molecular dynamics simulation, we have studied the spin dynamics induced by spin—orbit coupling (SOC) in Co and Fe using both spin-diabatic and spin-adiabatic representations. In Co system, it is found that the Fermi surface (
E
F
) is predominantly contributed by the spin-minority states. The SOC induced spin flip will occur for the photo-excited spin-majority electrons as they relax to the
E
F
, and the spin-minority electrons tend to relax to the
E
F
with the same spin through the electron—phonon coupling (EPC). The reduction of spin-majority electrons and the increase of spin-minority electrons lead to demagnetization of Co within 100 fs. By contrast, in Fe system, the
E
F
is dominated by the spin-majority states. In this case, the SOC induced spin flip occurs for the photo-excited spin-minority electrons, which leads to a magnetization enhancement. If we move the
E
F
of Fe to higher energy by 0.6 eV, the
E
F
will be contributed by the spin-minority states and the demagnetization will be observed again. This work provides a new perspective for understanding the SOC induced spin dynamics mechanism in magnetic metal systems.
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Unveiling the early stage evolution of local atomic structures in the crystallization process of a metallic glass
Lin Ma(马琳), Xiao-Dong Yang(杨晓东), Feng Yang(杨锋), Xin-Jia Zhou(周鑫嘉), and Zhen-Wei Wu(武振伟)
Chin. Phys. B, 2024, 33 (
3
): 036402. DOI:
10.1088/1674-1056/ad24d6
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The early stage evolution of local atomic structures in a multicomponent metallic glass during its crystallization process has been investigated via molecular dynamics simulation. It is found that the initial thermal stability and earliest stage evolution of the local atomic clusters show no strong correlation with their initial short-range orders, and this leads to an observation of a novel symmetry convergence phenomenon, which can be understood as an atomic structure manifestation of the ergodicity. Furthermore, in our system we have quantitatively proved that the crucial factor for the thermal stability against crystallization exhibited by the metallic glass is not the total amount of icosahedral clusters, but the degree of global connectivity among them.
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Ultrafast magneto-optical dynamics in nickel (111) single crystal studied by the integration of ultrafast reflectivity and polarimetry probes
Hao Kuang(匡皓), Junxiao Yu(余军潇), Jie Chen(陈洁), H. E. Elsayed-Ali, Runze Li(李润泽), and Peter M. Rentzepis
Chin. Phys. B, 2024, 33 (
3
): 037802. DOI:
10.1088/1674-1056/ad1983
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110
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With the integration of ultrafast reflectivity and polarimetry probes, we observed carrier relaxation and spin dynamics induced by ultrafast laser excitation of Ni (111) single crystals. The carrier relaxation time within the linear excitation range reveals that electron-phonon coupling and dissipation of photon energy into the bulk of the crystal take tens of picoseconds. On the other hand, the observed spin dynamics indicate a longer time of about 120 ps. To further understand how the lattice degree of freedom is coupled with these dynamics may require the integration of an ultrafast diffraction probe.
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Ultrafast photoemission electron microscopy: A multidimensional probe of nonequilibrium physics
Yanan Dai(戴亚南)
Chin. Phys. B, 2024, 33 (
3
): 038703. DOI:
10.1088/1674-1056/ad174a
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137
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Exploring the realms of physics that extend beyond thermal equilibrium has emerged as a crucial branch of condensed matter physics research. It aims to unravel the intricate processes involving the excitations, interactions, and annihilations of quasi- and many-body particles, and ultimately to achieve the manipulation and engineering of exotic non-equilibrium quantum phases on the ultrasmall and ultrafast spatiotemporal scales. Given the inherent complexities arising from many-body dynamics, it therefore seeks a technique that has efficient and diverse detection degrees of freedom to study the underlying physics. By combining high-power femtosecond lasers with real- or momentum-space photoemission electron microscopy (PEEM), imaging excited state phenomena from multiple perspectives, including time, real space, energy, momentum, and spin, can be conveniently achieved, making it a unique technique in studying physics out of equilibrium. In this context, we overview the working principle and technical advances of the PEEM apparatus and the related laser systems, and survey key excited-state phenomena probed through this surface-sensitive methodology, including the ultrafast dynamics of electrons, excitons, plasmons, spins, etc., in materials ranging from bulk and nano-structured metals and semiconductors to low-dimensional quantum materials. Through this review, one can further envision that time-resolved PEEM will open new avenues for investigating a variety of classical and quantum phenomena in a multidimensional parameter space, offering unprecedented and comprehensive insights into important questions in the field of condensed matter physics.
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K
-core attack, equilibrium
K
-core, and kinetically constrained spin system
Hai-Jun Zhou(周海军)
Chin. Phys. B, 2024, 33 (
6
): 066402. DOI:
10.1088/1674-1056/ad4329
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Kinetically constrained spin systems are toy models of supercooled liquids and amorphous solids. In this perspective, we revisit the prototypical Fredrickson-Andersen (FA) kinetically constrained model from the viewpoint of $K$-core combinatorial optimization. Each kinetic cluster of the FA system, containing all the mutually visitable microscopic occupation configurations, is exactly the solution space of a specific instance of the $K$-core attack problem. The whole set of different jammed occupation patterns of the FA system is the configuration space of an equilibrium $K$-core problem. Based on recent theoretical results achieved on the $K$-core attack and equilibrium $K$-core problems, we discuss the thermodynamic spin glass phase transitions and the maximum occupation density of the fully unfrozen FA kinetic cluster, and the minimum occupation density and extreme vulnerability of the partially frozen (jammed) kinetic clusters. The equivalence between $K$-core attack and the fully unfrozen FA kinetic cluster also implies a new way of sampling $K$-core attack solutions.