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12 pages, 1199 KiB

Open AccessArticle

Bloch Oscillations Due to Quantum Domain Breathing in One-Dimensional Electronic Photoinduced Phase Transitions

by Kaoru Iwano

Appl. Sci. 2019, 9(12), 2461; https://doi.org/10.3390/app9122461 - 16 Jun 2019

Cited by 1 | Viewed by 2263

Abstract

We theoretically predict a novel oscillation that will be observed during the dynamical processes of one-dimensional electronic photoinduced phase transitions. This oscillation is considered to be a breathing mode of a quantum domain of a photoinduced phase in the background of the initial [...] Read more.

We theoretically predict a novel oscillation that will be observed during the dynamical processes of one-dimensional electronic photoinduced phase transitions. This oscillation is considered to be a breathing mode of a quantum domain of a photoinduced phase in the background of the initial phase. When the initial phase is sufficiently stable, being far apart from the phase boundary, the domain feels a constant attractive force depending on its size or the distance between the two domain walls. This fact allows an interpretation that this oscillation is essentially the same as a so-called Bloch oscillation seen for the Stark ladder. Full article

(This article belongs to the Special Issue Photoinduced Cooperative Phenomena)

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12 pages, 1927 KiB

Open AccessFeature PaperArticle

Optical Study of Electronic Structure and Photoinduced Dynamics in the Organic Alloy System [(EDO-TTF)_0.89(MeEDO-TTF)_0.11]₂PF₆

by Tadahiko Ishikawa, Yohei Urasawa, Taiki Shindo, Yoichi Okimoto, Shin-ya Koshihara, Seiichi Tanaka, Ken Onda, Takaaki Hiramatsu, Yoshiaki Nakano, Koichiro Tanaka and Hideki Yamochi

Appl. Sci. 2019, 9(6), 1174; https://doi.org/10.3390/app9061174 - 20 Mar 2019

Cited by 2 | Viewed by 2702

Abstract

Over the past two decades (EDO-TTF)₂PF₆ (EDO-TTF = 4,5-ethylenedioxytetrathiafulvalene), which exhibits a metal–insulator (M–I) phase transition with charge–ordering (CO), has been investigated energetically because of attractive characteristics that include ultrafast and massive photoinduced spectral and structural changes. In contrast, while [...] Read more.

Over the past two decades (EDO-TTF)₂PF₆ (EDO-TTF = 4,5-ethylenedioxytetrathiafulvalene), which exhibits a metal–insulator (M–I) phase transition with charge–ordering (CO), has been investigated energetically because of attractive characteristics that include ultrafast and massive photoinduced spectral and structural changes. In contrast, while its crystal structure has much in common with the (EDO-TTF)₂PF₆ crystal, the organic alloy system of [(EDO-TTF)_0.89(MeEDO-TTF)_0.11]₂PF₆ (MeEDO-TTF = 4,5-ethylenedioxy-4′-methyltetrathiafulvalene) exhibits a quite different type of M–I phase transition that is attributed to Peierls instability. Here, an optical study of the static absorption spectra and the time-resolved changes in the absorption spectra of [(EDO-TTF)_0.89(MeEDO-TTF)_0.11]₂PF₆ are reported. The observed absorption spectra related to the electronic structure are highly anisotropic. With a reduction in temperature (T), the opening of a small optical gap and a small shift in the center frequency of the C=C stretching mode are observed along with the M–I phase transition. Additionally, photoinduced transient states have been assigned based on their relaxation processes and transient intramolecular vibrational spectra. Reflecting small valence and structural changes and weak donor–anion interactions, a photoinduced transient state that is similar to the thermal-equilibrium high-T metallic phase appears more rapidly in the alloy system than that in (EDO-TTF)₂PF₆. Full article

(This article belongs to the Special Issue Photoinduced Cooperative Phenomena)

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11 pages, 2013 KiB

Open AccessArticle

The Importance of Topological Defects in Photoexcited Phase Transitions Including Memory Applications

by Dragan Mihailovic

Appl. Sci. 2019, 9(5), 890; https://doi.org/10.3390/app9050890 - 2 Mar 2019

Cited by 5 | Viewed by 4417

Abstract

Photoinduced phase transitions have become a very important field of study with the advent of diverse time-resolved experimental techniques whose time resolution matches the electron, lattice, and spin relaxation dynamics associated with elementary excitations in quantum materials. Most techniques currently available rely on [...] Read more.

Photoinduced phase transitions have become a very important field of study with the advent of diverse time-resolved experimental techniques whose time resolution matches the electron, lattice, and spin relaxation dynamics associated with elementary excitations in quantum materials. Most techniques currently available rely on stroboscopic data-averaging over multiple transition outcomes. However, each time a transition takes place, fluctuations close to the time of the transition ensure that the phase transition outcome is different, with the emergence of different topological defect textures. In this paper, we briefly review the non-perturbative processes in selected charge-ordered quantum systems and the methods for their observation with different time-resolved techniques and scanning tunneling microscopy, which avoids the problem of averaging. The topological defect dynamics are seen to play an essential role in stabilizing emergent states in non-equilibrium transitions, appearing on different timescales as well as determining the emergent properties of the system. The phenomena are fundamentally important for understanding the fabric of matter in the Universe, as well as for possible applications in non-volatile memory devices. Full article

(This article belongs to the Special Issue Photoinduced Cooperative Phenomena)

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11 pages, 501 KiB

Open AccessArticle

Kinetics of a Phonon-Mediated Laser-Driven Structural Phase Transition in Sn₂P₂Se₆

by Martin Kubli, Matteo Savoini, Elsa Abreu, Bulat Burganov, Gabriel Lantz, Lucas Huber, Martin J. Neugebauer, Larissa Boie, Vincent Esposito, Elisabeth M. Bothschafter, Sergii Parchenko, Sebastian Grübel, Michael Porer, Jochen Rittmann, Paul Beaud, Urs Staub, Makina Yabashi, Yoshikazu Tanaka, Tetsuo Katayama, Tadashi Togashi, Anton A. Kohutych, Yulian M. Vysochanskii and Steven L. Johnson Show full author list Hide full author list

Appl. Sci. 2019, 9(3), 525; https://doi.org/10.3390/app9030525 - 4 Feb 2019

Cited by 3 | Viewed by 3591

Abstract

We investigate the structural dynamics of the incommensurately modulated phase of Sn

_{2}

P

_{2}

Se

_{6}

by means of time-resolved X-ray diffraction following excitation by an optical pump. Tracking the incommensurable distortion in the time domain enables us to identify the transport [...] Read more.

We investigate the structural dynamics of the incommensurately modulated phase of Sn

_{2}

P

_{2}

Se

_{6}

by means of time-resolved X-ray diffraction following excitation by an optical pump. Tracking the incommensurable distortion in the time domain enables us to identify the transport effects leading to a complete disappearance of the incommensurate phase over the course of 100 ns. These observations suggest that a thin surface layer of the high-temperature phase forms quickly after photo-excitation and then propagates into the material with a constant velocity of 3.7 m/s. Complementary static structural measurements reveal previously unreported higher-order satellite reflection in the incommensurate phase. These higher-order reflections are attributed to cubic vibrational terms in the Hamiltonian. Full article

(This article belongs to the Special Issue Photoinduced Cooperative Phenomena)

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13 pages, 1179 KiB

Open AccessArticle

Photoinduced Dynamics of Commensurate Charge Density Wave in 1T-TaS₂ Based on Three-Orbital Hubbard Model

by Tatsuhiko N. Ikeda, Hirokazu Tsunetsugu and Kenji Yonemitsu

Appl. Sci. 2019, 9(1), 70; https://doi.org/10.3390/app9010070 - 25 Dec 2018

Cited by 5 | Viewed by 5238

Abstract

We study the coupled charge-lattice dynamics in the commensurate charge density wave (CDW) phase of the layered compound 1T-TaS

_{2}

driven by an ultrashort laser pulse. For describing its electronic structure, we employ a tight-binding model of previous studies including the effects of [...] Read more.

We study the coupled charge-lattice dynamics in the commensurate charge density wave (CDW) phase of the layered compound 1T-TaS

_{2}

driven by an ultrashort laser pulse. For describing its electronic structure, we employ a tight-binding model of previous studies including the effects of lattice distortion associated with the CDW order. We further add on-site Coulomb interactions and reproduce an energy gap at the Fermi level within a mean-field analysis. On the basis of coupled equations of motion for electrons and the lattice distortion, we numerically study their dynamics driven by an ultrashort laser pulse. We find that the CDW order decreases and even disappears during the laser irradiation while the lattice distortion is almost frozen. We also find that the lattice motion sets in on a longer time scale and causes a further decrease in the CDW order even after the laser irradiation. Full article

(This article belongs to the Special Issue Photoinduced Cooperative Phenomena)

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10 pages, 899 KiB

Open AccessArticle

Excitation and Relaxation Dynamics of the Photo-Perturbed Correlated Electron System 1T-TaS₂

by Isabella Avigo, Ping Zhou, Matthias Kalläne, Kai Rossnagel, Uwe Bovensiepen and Manuel Ligges

Appl. Sci. 2019, 9(1), 44; https://doi.org/10.3390/app9010044 - 24 Dec 2018

Cited by 7 | Viewed by 6188

Abstract

We investigate the perturbation and subsequent recovery of the correlated electronic ground state of the Mott insulator 1T-TaS

_{2}

by means of femtosecond time-resolved photoemission spectroscopy in normal emission geometry. Upon an increase of near-infrared excitation strength, a considerable collapse of [...] Read more.

We investigate the perturbation and subsequent recovery of the correlated electronic ground state of the Mott insulator 1T-TaS

_{2}

by means of femtosecond time-resolved photoemission spectroscopy in normal emission geometry. Upon an increase of near-infrared excitation strength, a considerable collapse of the occupied Hubbard band is observed, which reflects a quench of short-range correlations. It is furthermore found that these excitations are directly linked to the lifting of the periodic lattice distortion which provides the localization centers for the formation of the insulating Mott state. We discuss the observed dynamics in a localized real-space picture. Full article

(This article belongs to the Special Issue Photoinduced Cooperative Phenomena)

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15 pages, 33910 KiB

Open AccessArticle

Photocontrollable Mixed-Valent States in Platinum-Halide Ladder Compounds

by Jun Ohara and Shoji Yamamoto

Appl. Sci. 2018, 8(11), 2126; https://doi.org/10.3390/app8112126 - 2 Nov 2018

Viewed by 2246

Abstract

Employing a two-orbital extended Peierls–Hubbard model, we demonstrate the photomanipulation of mixed-valent states in platinum-halide ladder compounds. There are two types of interchain valence arrangements, namely in-phase and out-of-phase types. The conversion of the in-phase structure to the out-of-phase structure is induced by [...] Read more.

Employing a two-orbital extended Peierls–Hubbard model, we demonstrate the photomanipulation of mixed-valent states in platinum-halide ladder compounds. There are two types of interchain valence arrangements, namely in-phase and out-of-phase types. The conversion of the in-phase structure to the out-of-phase structure is induced by photoirradiation, which is accelerated with increasing light intensity, while the reverse process hardly occurs. The out-of-phase arrangement is highly stabilized in the photoexcited states by the interchain electron transfer. Full article

(This article belongs to the Special Issue Photoinduced Cooperative Phenomena)

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8 pages, 1528 KiB

Open AccessArticle

Theoretical Analysis of a Molecular Optical Modulator for a Continuous-Wave Laser Based on a Hollow-Core Photonic Crystal Fiber

by Shin-ichi Zaitsu, Takumi Tanabe, Kota Oshima and Hiroyuki Hirata

Appl. Sci. 2018, 8(10), 1895; https://doi.org/10.3390/app8101895 - 12 Oct 2018

Viewed by 2338

Abstract

A THz optical modulator for a continuous-wave laser using a hollow-core photonic crystal fiber (HC-PCF) was proposed and theoretically analyzed. Lightwaves traveling through the HC-PCF is modulated by interactions with coherently driven Raman-active molecules in the core. The coherent molecular motion is excited [...] Read more.

A THz optical modulator for a continuous-wave laser using a hollow-core photonic crystal fiber (HC-PCF) was proposed and theoretically analyzed. Lightwaves traveling through the HC-PCF is modulated by interactions with coherently driven Raman-active molecules in the core. The coherent molecular motion is excited by a pulse train having an interval between successive pulses shorter than the molecular dephasing time. In principle, a rotational transition of molecular hydrogen (S

_{0}

(1)) at a pressure of 1 atm has a long enough dephasing time to maintain molecular coherence during a 1 GHz commercially available mode-locked pulse train. Optimization of the waveguide dispersion would enable phase-matching between the probe beam and generated sidebands during optical modulation. The proposed scheme would be achievable with a reasonable pump beam power and HC-PCF length, and with a feasible pressure of molecules in the core. Full article

(This article belongs to the Special Issue Photoinduced Cooperative Phenomena)

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Review

Jump to: Research

46 pages, 17220 KiB

Open AccessFeature PaperReview

Recent Advances in Ultrafast Structural Techniques

by Germán Sciaini

Appl. Sci. 2019, 9(7), 1427; https://doi.org/10.3390/app9071427 - 4 Apr 2019

Cited by 11 | Viewed by 6467

Abstract

A review that summarizes the most recent technological developments in the field of ultrafast structural dynamics with focus on the use of ultrashort X-ray and electron pulses follows. Atomistic views of chemical processes and phase transformations have long been the exclusive domain [...] Read more.

A review that summarizes the most recent technological developments in the field of ultrafast structural dynamics with focus on the use of ultrashort X-ray and electron pulses follows. Atomistic views of chemical processes and phase transformations have long been the exclusive domain of computer simulators. The advent of femtosecond (fs) hard X-ray and fs-electron diffraction techniques made it possible to bring such a level of scrutiny to the experimental area. The following review article provides a summary of the main ultrafast techniques that enabled the generation of atomically resolved movies utilizing ultrashort X-ray and electron pulses. Recent advances are discussed with emphasis on synchrotron-based methods, tabletop fs-X-ray plasma sources, ultrabright fs-electron diffractometers, and timing techniques developed to further improve the temporal resolution and fully exploit the use of intense and ultrashort X-ray free electron laser (XFEL) pulses. Full article

(This article belongs to the Special Issue Photoinduced Cooperative Phenomena)

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13 pages, 2463 KiB

Open AccessReview

Ultrafast Optical Control of Magnetic Interactions in Carrier-Density-Controlled Ferromagnetic Semiconductors

by Masakazu Matsubara

Appl. Sci. 2019, 9(5), 948; https://doi.org/10.3390/app9050948 - 6 Mar 2019

Cited by 3 | Viewed by 4726

Abstract

Investigation of the interaction of ultrashort laser pulses with magnetically ordered materials has become a fascinating research topic in modern magnetism. Especially, the control of magnetic order by sub-ps laser pulses has become a fundamentally important topic with a high potential for future [...] Read more.

Investigation of the interaction of ultrashort laser pulses with magnetically ordered materials has become a fascinating research topic in modern magnetism. Especially, the control of magnetic order by sub-ps laser pulses has become a fundamentally important topic with a high potential for future spintronics applications. This paper will review the recent success in optically controlling the magnetic interactions in carrier-density-controlled ferromagnetic semiconductor EuO doped with Gd ions. When the Gd concentration is low, the magnitude of the magnetic interaction is enhanced by the irradiation of ultrashort laser pulses, whereas it is attenuated when the Gd concentration is high. In ferromagnetic Eu_1−xGd_xO, we thereby demonstrate the strengthening as well as the weakening of the magnetic interaction by 10% and within 3 ps by optically controlling the magnetic exchange interaction. This principle—ultrafast optical control of magnetic interaction—can be applied to future ultrafast opto-spintronics. Full article

(This article belongs to the Special Issue Photoinduced Cooperative Phenomena)

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15 pages, 3076 KiB

Open AccessReview

Characteristics of Coherent Optical Phonons in a Hexagonal YMnO₃ Thin Film

by Takayuki Hasegawa

Appl. Sci. 2019, 9(4), 704; https://doi.org/10.3390/app9040704 - 18 Feb 2019

Cited by 4 | Viewed by 4517

Abstract

This paper reviews our recent study on a coherent optical phonon in a hexagonal YMnO₃ thin film together with related optical studies in hexagonal RMnO₃ (R = Y, Lu, Ho) compounds. Coherent phonons have been observed in RMnO [...] Read more.

This paper reviews our recent study on a coherent optical phonon in a hexagonal YMnO₃ thin film together with related optical studies in hexagonal RMnO₃ (R = Y, Lu, Ho) compounds. Coherent phonons have been observed in RMnO₃ compounds by pump-probe spectroscopy with subpicosecond laser pulses, whereas the observation of coherent optical phonons was reported only in LuMnO₃. Recently, we succeeded in the observation of the coherent optical phonon in a YMnO₃ thin film. The generation process of the coherent optical phonon is assigned to a displacive mechanism, which is identical to that in LuMnO₃. The coherent optical phonon is observed in the temperature range from 10 K to room temperature, while the oscillation intensity strongly decreases as the temperature increases to the Néel temperature of ~70 K from a lower temperature range. It is interesting that the temperature dependence is largely different from that in LuMnO₃. We describe that the result can be qualitatively explained by the property of an isostructural transition around the Néel temperature in RMnO₃ compounds. In addition, we briefly discuss ultrafast incoherent responses of excited electronic states from the viewpoint of the excitation photon energy of laser pulses. Full article

(This article belongs to the Special Issue Photoinduced Cooperative Phenomena)

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18 pages, 3447 KiB

Open AccessReview

Two Topics of Optical Excitation Dynamics, Newly Unveiled by the Time- and Momentum-Resolved Photo-Electron Emission from the Conduction Band of GaAs: A Theoretical Review

by Hiromasa Ohnishi and Norikazu Tomita

Appl. Sci. 2018, 8(10), 1788; https://doi.org/10.3390/app8101788 - 1 Oct 2018

Cited by 1 | Viewed by 3796

Abstract

We review the recent two topics of optical excitation and relaxation dynamics, newly unveiled by the time- and momentum-resolved photo-electron emission from the conduction band of GaAs. One is the real-time collective relaxation dynamics, resulting in the Fermi degeneracy formation in the [...] Read more.

We review the recent two topics of optical excitation and relaxation dynamics, newly unveiled by the time- and momentum-resolved photo-electron emission from the conduction band of GaAs. One is the real-time collective relaxation dynamics, resulting in the Fermi degeneracy formation in the

Γ

valley. We show that it takes almost infinite time to realize the exact Fermi degeneracy, due to a restricted selection rule for the intravalley transition of the photo-excited electrons. The other is the spontaneous and instantaneous intervalley transition from the

Γ

valley to the L one. By considering the electron-phonon coupling before the photo-excitation, such spontaneous intervalley transition is realized within the framework of the Franck–Condon principle of the photo-excitation. Full article

(This article belongs to the Special Issue Photoinduced Cooperative Phenomena)

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8 pages, 1265 KiB

Open AccessFeature PaperReview

Real Time Quantum Dynamics of Spontaneous Translational Symmetry Breakage in the Early Stage of Photo-Induced Structural Phase Transitions

by Keiichiro Nasu

Appl. Sci. 2018, 8(3), 332; https://doi.org/10.3390/app8030332 - 27 Feb 2018

Cited by 1 | Viewed by 2559

Abstract

Real time quantum dynamics of the spontaneous translational symmetry breakage in the early stage of photo-induced structural phase transitions is reviewed and supplementally explained, under the guide of the Toyozawa theory, which is exactly in compliance with the conservation laws of the total [...] Read more.

Real time quantum dynamics of the spontaneous translational symmetry breakage in the early stage of photo-induced structural phase transitions is reviewed and supplementally explained, under the guide of the Toyozawa theory, which is exactly in compliance with the conservation laws of the total momentum and energy. At the Franck-Condon state, an electronic excitation just created by a visible light, is in a plane wave state, which is extended all over the crystal. While, after the lattice relaxation having been completed, it is localized around a certain lattice site of the crystal, as a new excitation. Is there a sudden shrinkage of the excitation wave function, in between? No! The wave function never shrinks, but only the spatial (or inter lattice-site) quantum coherence (interference) of the excitation disappears, as the lattice relaxation proceeds. This is nothing but the spontaneous breakage of translational symmetry. Full article

(This article belongs to the Special Issue Photoinduced Cooperative Phenomena)

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