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Strain-induced multiferroicity in Cr1/3NbS2
Authors:
Y. Sun,
Y. Ahn,
D. Sapkota,
H. S. Arachchige,
R. Xue,
S. Mozaffari,
D. G. Mandrus,
L. Zhao,
J. Orenstein,
V. Sunko
Abstract:
Multiferroic materials, in which electric polarization and magnetic order coexist and couple, offer rich opportunities for both fundamental discovery and technology. However, multiferroicity remains rare due to conflicting electronic requirements for ferroelectricity and magnetism. One route to circumvent this challenge is to exploit the noncollinear ordering of spin cycloids, whose symmetry permi…
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Multiferroic materials, in which electric polarization and magnetic order coexist and couple, offer rich opportunities for both fundamental discovery and technology. However, multiferroicity remains rare due to conflicting electronic requirements for ferroelectricity and magnetism. One route to circumvent this challenge is to exploit the noncollinear ordering of spin cycloids, whose symmetry permits the emergence of polar order. In this work, we introduce another pathway to multiferroic order in which strain generates polarization in materials that host nonpolar spin spirals. To demonstrate this phenomenon, we chose the spin spiral in the well-studied helimagnet Cr1/3NbS2. To detect the induced polarization, we introduce the technique of magnetoelectric birefringence (MEB), an optical probe that enables spatially-resolved and unambiguous detection of polar order. By combining MEB imaging with strain engineering, we confirm the onset of a polar vector at the magnetic transition, establishing strained Cr1/3NbS2 as a type-II multiferroic.
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Submitted 13 October, 2025;
originally announced October 2025.
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Intertwined polar, chiral, and ferro-rotational orders in a rotation-only insulator
Authors:
Weizhe Zhang,
June Ho Yeo,
Xiaoyu Guo,
Tony Chiang,
Nishkarsh Agarwal,
John T. Heron,
Kai Sun,
Junjie Yang,
Sang-Wook Cheong,
Youngjun Ahn,
Liuyan Zhao
Abstract:
Intertwined orders refer to strongly coupled and mutually dependent orders that coexist in correlated electron systems, often underpinning key physical properties of the host materials. Among them, polar, chiral, and ferro-rotational orders have been theoretically known to form a closed set of intertwined orders. However, experimental investigation into their mutual coupling and physical consequen…
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Intertwined orders refer to strongly coupled and mutually dependent orders that coexist in correlated electron systems, often underpinning key physical properties of the host materials. Among them, polar, chiral, and ferro-rotational orders have been theoretically known to form a closed set of intertwined orders. However, experimental investigation into their mutual coupling and physical consequences has remained elusive. In this work, we employ the polar-chiral insulator Ni$_3$TeO$_6$ as a platform and utilize a multimodal optical approach to directly probe and reveal the intertwining among polarity, chirality, and ferro-rotational order. We demonstrate how their coupling governs the formation of domains and dictates the nature of domain walls. Within the domains, we identify spatial inversion symmetry as the operation connecting two domain states of opposite polarity and chirality, with a common ferro-rotational state serving as the prerequisite for these interlocked configurations. At the domain walls, we observe a pronounced enhancement of in-plane polarization accompanied by a suppression of chirality. By combining with Ginzburg-Landau theory within the framework of a pre-existing ferro-rotational background, we uncover the emergence of mixed Néel- and Bloch-type domain walls. Our findings highlight the critical role of intertwined orders in defining domain and domain wall characteristics and open pathways for domain switching and domain wall control via intertwined order parameters.
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Submitted 10 September, 2025;
originally announced September 2025.
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Rigid body rotation and chiral reorientation combine in filamentous E. coli swimming in low-Re flows
Authors:
Richard Z. DeCurtis,
Yongtae Ahn,
Jane Hill,
Sara M. Hashmi
Abstract:
Antibiotic doses below a minimum inhibitory concentration turn off bacteria cell division, but not cell growth. As a result, rod-like bacteria including E. coli can elongate many times their original length without increasing their width. The swimming behavior of these filamentous bacteria through small channels may provide insights into how antibiotic-resistant bacteria swim to channel walls. Suc…
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Antibiotic doses below a minimum inhibitory concentration turn off bacteria cell division, but not cell growth. As a result, rod-like bacteria including E. coli can elongate many times their original length without increasing their width. The swimming behavior of these filamentous bacteria through small channels may provide insights into how antibiotic-resistant bacteria swim to channel walls. Such swimming behaviors in settings like hospital tubing may facilitate adhesion, biofilm formation and infection. Despite the importance of understanding the behavior of bacteria not killed by antibiotics, the swimming of filamentous bacteria in external flows has not received much attention. We study the swimming of stressed, filamentous E. coli. In quiescence, highly elongated E. coli swim with a sinusoidal undulation, suggesting rigid body rotation of the long, slightly segmented cell bodies. In low Reynolds number pressure-driven flows through a microchannel, undulation becomes irregular and may stop and start within a particular bacteria trajectory. We refer to this behavior in flow as `wiggling.' Rigid body rotation persists in flow, appearing as a high frequency change in body orientation on top of a slower frequency of reorientation. Chiral reorientation can explain the slower reorientation. We quantify swimming in two flow rates and observe both rheotaxis and preferential orientation of bacteria bodies. We find that the faster flow constrains wiggling bacteria trajectories and orientations more than slower flow does. Interestingly, not all bacteria in flow wiggle. Populations of `non-wiggling' filamentous E. coli follow streamlines, without preferential orientation, flowing faster than wigglers. Non-wigglers do not behave like chiral rods propelled by flagellar bundles, but like rigid rods. Differentiating these populations may have important implications for understanding motility loss.
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Submitted 1 August, 2025;
originally announced August 2025.
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Photo-induced Dynamics and Momentum Distribution of Chiral Charge Density Waves in 1T-TiSe$_{2}$
Authors:
Qingzheng Qiu,
Sae Hwan Chun,
Jaeku Park,
Dogeun Jang,
Li Yue,
Yeongkwan Kim,
Yeojin Ahn,
Mingi Jho,
Kimoon Han,
Xinyi Jiang,
Qian Xiao,
Tao Dong,
Jia-Yi Ji,
Nanlin Wang,
Jeroen van den Brink,
Jasper van Wezel,
Yingying Peng
Abstract:
Exploring the photoinduced dynamics of chiral states offers promising avenues for advanced control of condensed matter systems. Photoinduced or photoenhanced chirality in 1T-TiSe$_{2}$ has been suggested as a fascinating platform for optical manipulation of chiral states. However, the mechanisms underlying chirality training and its interplay with the charge density wave (CDW) phase remain elusive…
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Exploring the photoinduced dynamics of chiral states offers promising avenues for advanced control of condensed matter systems. Photoinduced or photoenhanced chirality in 1T-TiSe$_{2}$ has been suggested as a fascinating platform for optical manipulation of chiral states. However, the mechanisms underlying chirality training and its interplay with the charge density wave (CDW) phase remain elusive. Here, we use time-resolved X-ray diffraction (tr-XRD) with circularly polarized pump lasers to probe the photoinduced dynamics of chirality in 1T-TiSe$_{2}$. We observe a notable ($\sim$20%) difference in CDW intensity suppression between left- and right-circularly polarized pumps. Additionally, we reveal momentum-resolved circular dichroism arising from domains of different chirality, providing a direct link between CDW and chirality. An immediate increase in CDW correlation length upon laser pumping is detected, suggesting the photoinduced expansion of chiral domains. These results both advance the potential of light-driven chirality by elucidating the mechanism driving chirality manipulation in TiSe$_2$, and they demonstrate that tr-XRD with circularly polarized pumps is an effective tool for chirality detection in condensed matter systems.
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Submitted 4 February, 2025;
originally announced February 2025.
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Progress and Prospects in Two-Dimensional Magnetism of van der Waals Materials
Authors:
Youngjun Ahn,
Xiaoyu Guo,
Suhan Son,
Zeliang Sun,
Liuyan Zhao
Abstract:
Two-dimensional (2D) magnetism in van der Waals (vdW) atomic crystals and moiré superlattices has emerged as a topic of tremendous interest in the fields of condensed matter physics and materials science within the past half-decade since its first experimental discovery in 2016 - 2017. It has not only served as a powerful platform for investigating phase transitions in the 2D limit and exploring n…
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Two-dimensional (2D) magnetism in van der Waals (vdW) atomic crystals and moiré superlattices has emerged as a topic of tremendous interest in the fields of condensed matter physics and materials science within the past half-decade since its first experimental discovery in 2016 - 2017. It has not only served as a powerful platform for investigating phase transitions in the 2D limit and exploring new phases of matter, but also provided new opportunities for applications in microelectronics, spintronics, magnonics, optomagnetics, and so on. Despite the flourishing developments in 2D magnetism over this short period of time, further efforts are welcome in multiple forefronts of 2D magnetism research for achieving the ultimate goal of routinely implementing 2D magnets as quantum electronic components. In this review article, we will start with basic concepts and properties of 2D magnetism, followed by a brief overview of historical efforts in 2D magnetism research and then a comprehensive review of vdW material-based 2D magnetism. We will conclude with discussions on potential future research directions for this growing field of 2D vdW magnetism.
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Submitted 24 January, 2024;
originally announced January 2024.
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Chiral magnetic waves in strongly coupled Weyl semimetals
Authors:
Yongjun Ahn,
Matteo Baggioli,
Yan Liu,
Xin-Meng Wu
Abstract:
Propagating chiral magnetic waves (CMW) are expected to exist in chiral plasmas due to the interplay between the chiral magnetic and chiral separation effects induced by the presence of a chiral anomaly. Unfortunately, it was pointed out that, because of the effects of electric conductivity and dissipation, CMW are overdamped and therefore their signatures are unlikely to be seen in heavy-ion coll…
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Propagating chiral magnetic waves (CMW) are expected to exist in chiral plasmas due to the interplay between the chiral magnetic and chiral separation effects induced by the presence of a chiral anomaly. Unfortunately, it was pointed out that, because of the effects of electric conductivity and dissipation, CMW are overdamped and therefore their signatures are unlikely to be seen in heavy-ion collision experiments and in the quark gluon plasma. Nonetheless, the chiral anomaly plays a fundamental role in Weyl semimetals and their anomalous transport properties as well. Hence, CMW could be potentially observed in topological semimetals using table-top experiments. By using a holographic model for strongly coupled Weyl semimetals, we investigate in detail the nature of CMW in presence of Coulomb interactions and axial charge relaxation and estimate whether, and in which regimes, CMW could be observed as underdamped collective excitations in topological materials.
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Submitted 30 March, 2024; v1 submitted 15 January, 2024;
originally announced January 2024.
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Electric quadrupole second harmonic generation revealing dual magnetic orders in a magnetic Weyl semimetal
Authors:
Youngjun Ahn,
Xiaoyu Guo,
Rui Xue,
Kejian Qu,
Kai Sun,
David Mandrus,
Liuyan Zhao
Abstract:
Broken symmetries and electronic topology are nicely manifested together in the second order nonlinear optical responses from topologically nontrivial materials. While second order nonlinear optical effects from the electric dipole (ED) contribution have been extensively explored in polar Weyl semimetals (WSMs) with broken spatial inversion (SI) symmetry, they are rarely studied in centrosymmetric…
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Broken symmetries and electronic topology are nicely manifested together in the second order nonlinear optical responses from topologically nontrivial materials. While second order nonlinear optical effects from the electric dipole (ED) contribution have been extensively explored in polar Weyl semimetals (WSMs) with broken spatial inversion (SI) symmetry, they are rarely studied in centrosymmetric magnetic WSMs with broken time reversal (TR) symmetry due to complete suppression of the ED contribution. Here, we report experimental demonstration of optical second harmonic generation (SHG) in a magnetic WSM Co$_{3}$Sn$_{2}$S$_{2}$ from the electric quadrupole (EQ) contribution. By tracking the temperature dependence of the rotation anisotropy (RA) of SHG, we capture two magnetic phase transitions, with both the SHG intensity increasing and its RA pattern rotating at $T_{C,1}$=175K and $T_{C,2}$=120K subsequently. The fitted critical exponents for the SHG intensity and RA orientation near $T_{C,1}$ and $T_{C,2}$ suggest that the magnetic phase at $T_{C,1}$ is a 3D Ising-type out-of-plane ferromagnetism while the other at $T_{C,2}$ is a 3D XY-type all-in-all-out in-plane antiferromagnetism. Our results show the success of detection and exploration of EQ SHG in a centrosymmetric magnetic WSM, and hence open the pathway towards the future investigation of its tie to the band topology.
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Submitted 23 October, 2023;
originally announced October 2023.
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Inability of linear axion holographic Gubser-Rocha model to capture all the transport anomalies of strange metals
Authors:
Yongjun Ahn,
Matteo Baggioli,
Hyun-Sik Jeong,
Keun-Young Kim
Abstract:
In the last decade, motivated by the concept of Planckian relaxation and the possible existence of a quantum critical point in cuprate materials, holographic techniques have been extensively used to tackle the problem of strange metals and high-$T_c$ superconductors. Among the various setups, the linear axion Gubser-Rocha model has often been considered as a promising holographic model for strange…
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In the last decade, motivated by the concept of Planckian relaxation and the possible existence of a quantum critical point in cuprate materials, holographic techniques have been extensively used to tackle the problem of strange metals and high-$T_c$ superconductors. Among the various setups, the linear axion Gubser-Rocha model has often been considered as a promising holographic model for strange metals since endowed with the famous linear in $T$ resistivity property. As fiercely advocated by Phil Anderson, beyond $T$-linear resistivity, there are several additional anomalies unique to the strange metal phase, as for example a Fermi liquid like Hall angle -- the famous problem of the two relaxation scales. In this short note, we show that the linear axion holographic Gubser-Rocha model, which presents a single momentum relaxation time, fails in this respect and therefore is not able to capture the transport phenomenology of strange metals. We prove our statement by means of a direct numerical computation, a previously demonstrated scaling analysis and also a hydrodynamic argument. Finally, we conclude with an optimistic discussion on the possible improvements and generalizations which could lead to a holographic model for strange metals in all their glory.
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Submitted 2 December, 2023; v1 submitted 10 July, 2023;
originally announced July 2023.
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Impurity-Driven Metal-Insulator Transitions in Holography
Authors:
Yunseok Seo,
Youngjun Ahn,
Keun-Young Kim,
Sang-Jin Sin,
Kyung Kiu Kim
Abstract:
In this work, we study Metal-Insulator transition in a holographic model containing an interaction between the order parameter and charge-carrier density. It turns out that the impurity density of this model can drive the phase transition whose ordered phase corresponds to the insulating phase. The temperature behavior of DC conductivity distinguishes the insulating phase from the metal phase. We…
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In this work, we study Metal-Insulator transition in a holographic model containing an interaction between the order parameter and charge-carrier density. It turns out that the impurity density of this model can drive the phase transition whose ordered phase corresponds to the insulating phase. The temperature behavior of DC conductivity distinguishes the insulating phase from the metal phase. We confirm this behavior by a numerical method and an analytic calculation. As a byproduct, we show the existence of a `quantum phase transition' supported by the Breitenlohner-Freedman bound argument.
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Submitted 15 February, 2023;
originally announced February 2023.
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Dynamical criticality of spin-shear coupling in van der Waals antiferromagnets
Authors:
Faran Zhou,
Kyle Hwangbo,
Qi Zhang,
Chong Wang,
Lingnan Shen,
Jiawei Zhang,
Qianni Jiang,
Alfred Zong,
Yifan Su,
Marc Zajac,
Youngjun Ahn,
Donald Walko,
Richard Schaller,
Jiun-Haw Chu,
Nuh Gedik,
Xiaodong Xu,
Di Xiao,
Haidan Wen
Abstract:
The interplay between a multitude of electronic, spin, and lattice degrees of freedom underlies the complex phase diagrams of quantum materials. Layer stacking in van der Waals (vdW) heterostructures is responsible for exotic electronic and magnetic properties, which inspires stacking control of two-dimensional magnetism. Beyond the interplay between stacking order and interlayer magnetism, we dis…
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The interplay between a multitude of electronic, spin, and lattice degrees of freedom underlies the complex phase diagrams of quantum materials. Layer stacking in van der Waals (vdW) heterostructures is responsible for exotic electronic and magnetic properties, which inspires stacking control of two-dimensional magnetism. Beyond the interplay between stacking order and interlayer magnetism, we discover a spin-shear coupling mechanism in which a subtle shear of the atomic layers can have a profound effect on the intralayer magnetic order in a family of vdW antiferromagnets. Using time-resolved x-ray diffraction and optical linear dichroism measurements, interlayer shear is identified as the primary structural degree of freedom that couples with magnetic order. The recovery times of both shear and magnetic order upon optical excitation diverge at the magnetic ordering temperature with the same critical exponent. The time-dependent Ginzburg-Landau theory shows that this concurrent critical slowing down arises from a linear coupling of the interlayer shear to the magnetic order, which is dictated by the broken mirror symmetry intrinsic to the monoclinic stacking. Our results highlight the importance of interlayer shear in ultrafast control of magnetic order via spin-mechanical coupling.
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Submitted 15 November, 2022;
originally announced November 2022.
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Optically Induced Picosecond Lattice Compression in the Dielectric Component of a Strongly Coupled Ferroelectric/Dielectric Superlattice
Authors:
Deepankar Sri Gyan,
Hyeon Jun Lee,
Youngjun Ahn,
Jerome Carnis,
Tae Yeon Kim,
Sanjith Unithrattil,
Jun Young Lee,
Sae Hwan Chun,
Sunam Kim,
Intae Eom,
Minseok Kim,
Sang-Youn Park,
Kyung Sook Kim,
Ho Nyung Lee,
Ji Young Jo,
Paul G. Evans
Abstract:
Above-bandgap femtosecond optical excitation of a ferroelectric/dielectric BaTiO3/CaTiO3 superlattice leads to structural responses that are a consequence of the screening of the strong electrostatic coupling between the component layers. Time-resolved x-ray free-electron laser diffraction shows that the structural response to optical excitation includes a net lattice expansion of the superlattice…
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Above-bandgap femtosecond optical excitation of a ferroelectric/dielectric BaTiO3/CaTiO3 superlattice leads to structural responses that are a consequence of the screening of the strong electrostatic coupling between the component layers. Time-resolved x-ray free-electron laser diffraction shows that the structural response to optical excitation includes a net lattice expansion of the superlattice consistent with depolarization-field screening driven by the photoexcited charge carriers. The depolarization-field-screening-driven expansion is separate from a photoacoustic pulse launched from the bottom electrode on which the superlattice was epitaxially grown. The distribution of diffracted intensity of superlattice x-ray reflections indicates that the depolarization-field-screening-induced strain includes a photoinduced expansion in the ferroelectric BaTiO3 and a contraction in CaTiO3. The magnitude of expansion in BaTiO3 layers is larger than the contraction in CaTiO3. The difference in the magnitude of depolarization-field-screening-driven strain in the BaTiO3 and CaTiO3 components can arise from the contribution of the oxygen octahedral rotation patterns at the BaTiO3/CaTiO3 interfaces to the polarization of CaTiO3. The depolarization-field-screening-driven polarization reduction in the CaTiO3 layers points to a new direction for the manipulation of polarization in the component layers of a strongly coupled ferroelectric/dielectric superlattice.
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Submitted 3 November, 2022;
originally announced November 2022.
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Ferro-rotational domain walls revealed by electric quadrupole second harmonic generation microscopy
Authors:
Xiaoyu Guo,
Rachel Owen,
Austin Kaczmarek,
Xiaochen Fang,
Chandan De,
Youngjun Ahn,
Wei Hu,
Nishkarsh Agarwal,
Suk Hyun Sung,
Robert Hovden,
Sang-Wook Cheong,
Liuyan Zhao
Abstract:
Domain walls are ubiquitous in materials that undergo phase transitions driven by spontaneous symmetry breaking. Domain walls in ferroics and multiferroics have received tremendous attention recently due to their emergent properties distinct from their domain counterparts, for example, their high mobility and controllability, as well as their potential applications in nanoelectronics. However, it…
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Domain walls are ubiquitous in materials that undergo phase transitions driven by spontaneous symmetry breaking. Domain walls in ferroics and multiferroics have received tremendous attention recently due to their emergent properties distinct from their domain counterparts, for example, their high mobility and controllability, as well as their potential applications in nanoelectronics. However, it is extremely challenging to detect, visualize and study the ferro-rotational (FR) domain walls because the FR order, in contrast to ferromagnetism (FM) and ferroelectricity (FE), is invariant under both the spatial-inversion and the time-reversal operations and thus hardly couple with conventional experimental probes. Here, an FR candidate $\mathrm{NiTiO_{3}}$ is investigated by ultrasensitive electric quadrupole (EQ) second harmonic generation rotational anisotropy (SHG RA) to probe the point symmetries of the two degenerate FR domain states, showing their relation by the vertical mirror operations that are broken below the FR critical temperature. We then visualize the real-space FR domains by scanning EQ SHG microscopy, and further resolve the FR domain walls by revealing a suppressed SHG intensity at domain walls. By taking local EQ SHG RA measurements, we show the restoration of the mirror symmetry at FR domain walls and prove their unconventional nonpolar nature. Our findings not only provide a comprehensive insight into FR domain walls, but also demonstrate a unique and powerful tool for future studies on domain walls of unconventional ferroics, both of which pave the way towards future manipulations and applications of FR domain walls.
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Submitted 7 September, 2022;
originally announced September 2022.
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Structural evidence for ultrafast polarization rotation in ferroelectric/dielectric superlattice nanodomains
Authors:
Hyeon Jun Lee,
Youngjun Ahn,
Samuel D. Marks,
Eric C. Landahl,
Shihao Zhuang,
M. Humed Yusuf,
Matthew Dawber,
Jun Young Lee,
Tae Yeon Kim,
Sanjith Unithrattil,
Sae Hwan Chun,
Sunam Kim,
Intae Eom,
Sang-Yeon Park,
Kyung Sook Kim,
Sooheyong Lee,
Ji Young Jo,
Jiamian Hu,
Paul G. Evans
Abstract:
Weakly coupled ferroelectric/dielectric superlattice thin film heterostructures exhibit complex nanoscale polarization configurations that arise from a balance of competing electrostatic, elastic, and domain-wall contributions to the free energy. A key feature of these configurations is that the polarization can locally have a significant component that is not along the thin-film surface normal di…
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Weakly coupled ferroelectric/dielectric superlattice thin film heterostructures exhibit complex nanoscale polarization configurations that arise from a balance of competing electrostatic, elastic, and domain-wall contributions to the free energy. A key feature of these configurations is that the polarization can locally have a significant component that is not along the thin-film surface normal direction, while maintaining zero net in-plane polarization. PbTiO3/SrTiO3 thin-film superlattice heterostructures on a conducting SrRuO3 bottom electrode on SrTiO3 have a room-temperature stripe nanodomain pattern with nanometer-scale lateral period. Ultrafast time-resolved x-ray free electron laser diffraction and scattering experiments reveal that above-bandgap optical pulses induce rapidly propagating acoustic pulses and a perturbation of the domain diffuse scattering intensity arising from the nanoscale stripe domain configuration. With 400 nm optical excitation, two separate acoustic pulses are observed: a high-amplitude pulse resulting from strong optical absorption in the bottom electrode and a weaker pulse arising from the depolarization field screening effect due to absorption directly within the superlattice. The picosecond scale variation of the nanodomain diffuse scattering intensity is consistent with a larger polarization change than would be expected due to the polarization-tetragonality coupling of uniformly polarized ferroelectrics. The polarization change is consistent instead with polarization rotation facilitated by the reorientation of the in-plane component of the polarization at the domain boundaries of the striped polarization structure. The complex steady-state configuration within these ferroelectric heterostructures leads to polarization rotation phenomena that have been previously available only through the selection of bulk crystal composition.
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Submitted 8 July, 2021; v1 submitted 6 July, 2021;
originally announced July 2021.
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Atomic scale mechanisms controlling the oxidation of polyethylene: a first principles study
Authors:
Yunho Ahn,
Xavier Colin,
Guido Roma
Abstract:
Understanding the degradation mechanisms of aliphatic polymers by thermal oxidation and radio-oxidation is very important in order to assess their lifetime in a variety of industrial applications. We focus here on polyethylene as a prototypical aliphatic polymer. Kinetic models describing the time evolution of the concentration of chain defects and radicals species in the material identify a relev…
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Understanding the degradation mechanisms of aliphatic polymers by thermal oxidation and radio-oxidation is very important in order to assess their lifetime in a variety of industrial applications. We focus here on polyethylene as a prototypical aliphatic polymer. Kinetic models describing the time evolution of the concentration of chain defects and radicals species in the material identify a relevant step in the formation and subsequent decomposition of transient hydroperoxides species, finally leading to carbonyl defects, in particular ketones. In this paper we first summarize the most relevant mechanistic paths proposed in the literature for hydroperoxide formation and decomposition and, second, revisit them using first principles calculations based on Density Functional Theory (DFT). We investigate the reaction paths for several chemical reactions, for both isolated alkane molecules and a crystalline model of polyethylene, and confirm, in some cases, the accepted activation energies; in some other cases, we challenge the accepted view finding alternative, more favourable, reaction paths for which we estimate the activation energy. We highlight the infuence of the environment -- crystalline or not -- on the outcome of some of the studied chemical reactions. A remarkable results of our calculations is that hydroxyl radicals play an important role in the decomposition of hydroperoxides. Based on our findings, it should be possible to improve the set of equations and parameters used in current kinetic simulations of polyethylene radio-oxidation.
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Submitted 7 June, 2021; v1 submitted 12 February, 2021;
originally announced February 2021.
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Subterahertz collective dynamics of polar vortices
Authors:
Qian Li,
Vladimir A. Stoica,
Marek Paściak,
Yi Zhu,
Yakun Yuan,
Tiannan Yang,
Margaret R. McCarter,
Sujit Das,
Ajay K. Yadav,
Suji Park,
Cheng Dai,
Hyeon Jun Lee,
Youngjun Ahn,
Samuel D. Marks,
Shukai Yu,
Christelle Kadlec,
Takahiro Sato,
Matthias C. Hoffmann,
Matthieu Chollet,
Michael E. Kozina,
Silke Nelson,
Diling Zhu,
Donald A. Walko,
Aaron M. Lindenberg,
Paul G. Evans
, et al. (7 additional authors not shown)
Abstract:
The collective dynamics of topological structures have been of great interest from both fundamental and applied perspectives. For example, the studies of dynamical properties of magnetic vortices and skyrmions not only deepened the understanding of many-body physics but also led to potential applications in data processing and storage. Topological structures constructed from electrical polarizatio…
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The collective dynamics of topological structures have been of great interest from both fundamental and applied perspectives. For example, the studies of dynamical properties of magnetic vortices and skyrmions not only deepened the understanding of many-body physics but also led to potential applications in data processing and storage. Topological structures constructed from electrical polarization rather than spin have recently been realized in ferroelectric superlattices, promising for ultrafast electric-field control of topological orders. However, little is known about the dynamics of such complex extended nanostructures which in turn underlies their functionalities. Using terahertz-field excitation and femtosecond x-ray diffraction measurements, we observe ultrafast collective polarization dynamics that are unique to polar vortices, with orders of magnitude higher frequencies and smaller lateral size than those of experimentally realized magnetic vortices. A previously unseen soft mode, hereafter referred to as a vortexon, emerges as transient arrays of nanoscale circular patterns of atomic displacements, which reverse their vorticity on picosecond time scales. Its frequency is significantly reduced at a critical strain, indicating a condensation of structural dynamics. First-principles-based atomistic calculations and phase-field modeling reveal the microscopic atomic arrangements and frequencies of the vortex modes. The discovery of subterahertz collective dynamics in polar vortices opens up opportunities for applications of electric-field driven data processing in topological structures with ultrahigh speed and density.
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Submitted 10 February, 2021;
originally announced February 2021.
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Dynamic Tilting of Ferroelectric Domain Walls via Optically Induced Electronic Screening
Authors:
Youngjun Ahn,
Arnoud S. Everhardt,
Hyeon Jun Lee,
Joonkyu Park,
Anastasios Pateras,
Silvia Damerio,
Tao Zhou,
Anthony D. DiChiara,
Haidan Wen,
Beatriz Noheda,
Paul G. Evans
Abstract:
Optical excitation perturbs the balance of phenomena selecting the tilt orientation of domain walls within ferroelectric thin films. The high carrier density induced in a low-strain BaTiO3 thin film by an above-bandgap ultrafast optical pulse changes the tilt angle that 90° a/c domain walls form with respect to the substrate-film interface. The dynamics of the changes are apparent in time-resolved…
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Optical excitation perturbs the balance of phenomena selecting the tilt orientation of domain walls within ferroelectric thin films. The high carrier density induced in a low-strain BaTiO3 thin film by an above-bandgap ultrafast optical pulse changes the tilt angle that 90° a/c domain walls form with respect to the substrate-film interface. The dynamics of the changes are apparent in time-resolved synchrotron x-ray scattering studies of the domain diffuse scattering. Tilting occurs at 298 K, a temperature at which the a/b and a/c domain phases coexist but is absent at 343 K in the better ordered single-phase a/c regime. Phase coexistence at 298 K leads to increased domain-wall charge density, and thus a larger screening effect than in the single-phase regime. The screening mechanism points to new directions for the manipulation of nanoscale ferroelectricity.
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Submitted 14 October, 2022; v1 submitted 15 August, 2020;
originally announced August 2020.
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Second-Harmonic Young Interference in Atom-Thin Heterocrystals
Authors:
Wontaek Kim,
Je Yhoung Ahn,
Juseung Oh,
Ji Hoon Shim,
Sunmin Ryu
Abstract:
Optical second-harmonic generation (SHG) is a nonlinear parametric process that doubles the frequency of incoming light. Only allowed in non-centrosymmetric materials, it has been widely used in frequency modulation of lasers, surface scientific investigation, and label-free imaging in biological and medical sciences. Two-dimensional crystals are ideal SHG-materials not only for their strong light…
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Optical second-harmonic generation (SHG) is a nonlinear parametric process that doubles the frequency of incoming light. Only allowed in non-centrosymmetric materials, it has been widely used in frequency modulation of lasers, surface scientific investigation, and label-free imaging in biological and medical sciences. Two-dimensional crystals are ideal SHG-materials not only for their strong light-matter interaction and atomic thickness defying the phase-matching requirement but also for their stackability into customized hetero-crystals with high angular precision and material diversity. Here we directly show that SHG in hetero-bilayers of transition metal dichalcogenides (TMDs) is governed by optical interference between two coherent SH fields with material-dependent phase delays using spectral phase interferometry. We also quantify the frequency-dependent phase difference between MoS2 and WS2, which also agrees with polarization-resolved data and first-principles calculations on complex susceptibility. The second-harmonic analogue of Young double-slit interference shown in this work demonstrates the potential of custom-designed parametric generation by atom-thick nonlinear optical materials.
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Submitted 4 January, 2021; v1 submitted 5 August, 2020;
originally announced August 2020.
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Dynamical Scattering in Coherent Hard X-Ray Nanobeam Bragg Diffraction
Authors:
A. Pateras,
J. Park,
Y. Ahn,
J. A. Tilka,
M. V. Holt,
H. Kim,
L. J. Mawst,
P. G. Evans
Abstract:
Unique intensity features arising from dynamical diffraction arise in coherent x-ray nanobeam diffraction patterns of crystals having thicknesses larger than the x-ray extinction depth or exhibiting combinations of nanoscale and mesoscale features. We demonstrate that dynamical scattering effects can be accurately predicted using an optical model combined with the Darwin theory of dynamical x-ray…
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Unique intensity features arising from dynamical diffraction arise in coherent x-ray nanobeam diffraction patterns of crystals having thicknesses larger than the x-ray extinction depth or exhibiting combinations of nanoscale and mesoscale features. We demonstrate that dynamical scattering effects can be accurately predicted using an optical model combined with the Darwin theory of dynamical x-ray diffraction. The model includes the highly divergent coherent x-ray nanobeams produced by Fresnel zone plate focusing optics and accounts for primary extinction, multiple scattering, and absorption. The simulation accurately reproduces the dynamical scattering features of experimental diffraction patterns acquired from a GaAs/AlGaAs epitaxial heterostructure on a GaAs (001) substrate.
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Submitted 25 February, 2020;
originally announced February 2020.
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Stressor-Layer-Induced Elastic Strain Sharing in SrTiO3 Complex Oxide Sheets
Authors:
J. A. Tilka,
J. Park,
Y. Ahn,
A. Pateras,
Z. Cai,
P. G. Evans
Abstract:
A precisely selected elastic strain can be introduced in submicron-thick single-crystal SrTiO3 sheets using a silicon nitride stressor layer. A conformal stressor layer deposited using plasma-enhanced chemical vapor deposition produces an elastic strain in the sheet consistent with the magnitude of the nitride residual stress. Synchrotron x-ray nanodiffraction reveals that the strain introduced in…
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A precisely selected elastic strain can be introduced in submicron-thick single-crystal SrTiO3 sheets using a silicon nitride stressor layer. A conformal stressor layer deposited using plasma-enhanced chemical vapor deposition produces an elastic strain in the sheet consistent with the magnitude of the nitride residual stress. Synchrotron x-ray nanodiffraction reveals that the strain introduced in the SrTiO3 sheets is on the order of 10 4, matching the predictions of an elastic model. This approach to elastic strain sharing in complex oxides allows the strain to be selected within a wide and continuous range of values, an effect not achievable in heteroepitaxy on rigid substrates.
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Submitted 25 February, 2020;
originally announced February 2020.
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Mesoscopic Elastic Distortions in GaAs Quantum Dot Heterostructures
Authors:
Anastasios Pateras,
Joonkyu Park,
Youngjun Ahn,
Jack A. Tilka,
Martin V. Holt,
Christian Reichl,
Werner Wegscheider,
Timothy A. Baart,
Juan Pablo Dehollain,
Uditendu Mukhopadhyay,
Lieven M. K. Vandersypen,
Paul G. Evans
Abstract:
Quantum devices formed in high-electron-mobility semiconductor heterostructures provide a route through which quantum mechanical effects can be exploited on length scales accessible to lithography and integrated electronics. The electrostatic definition of quantum dots in semiconductor heterostructure devices intrinsically involves the lithographic fabrication of intricate patterns of metallic ele…
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Quantum devices formed in high-electron-mobility semiconductor heterostructures provide a route through which quantum mechanical effects can be exploited on length scales accessible to lithography and integrated electronics. The electrostatic definition of quantum dots in semiconductor heterostructure devices intrinsically involves the lithographic fabrication of intricate patterns of metallic electrodes. The formation of metal/semiconductor interfaces, growth processes associated with polycrystalline metallic layers, and differential thermal expansion produce elastic distortion in the active areas of quantum devices. Understanding and controlling these distortions presents a significant challenge in quantum device development. We report synchrotron x-ray nanodiffraction measurements combined with dynamical x-ray diffraction modeling that reveal lattice tilts with a depth-averaged value up to 0.04 deg. and strain on the order of 10^-4 in the two-dimensional electron gas (2DEG) in a GaAs/AlGaAs heterostructure. Elastic distortions in GaAs/AlGaAs heterostructures modify the potential energy landscape in the 2DEG due to the generation of a deformation potential and an electric field through the piezoelectric effect. The stress induced by metal electrodes directly impacts the ability to control the positions of the potential minima where quantum dots form and the coupling between neighboring quantum dots.
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Submitted 25 February, 2020;
originally announced February 2020.
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Role of temperature-dependent electron trapping dynamics in the optically driven nanodomain transformation in a PbTiO${_3}$/SrTiO${_3}$ superlattice
Authors:
Joonkyu Park,
Youngjun Ahn,
Jack A. Tilka,
Hyeon Jun Lee,
Anastasios Pateras,
Mohammed H. Yusuf,
Matthew Dawber,
Haidan Wen,
Paul G. Evans
Abstract:
The spontaneously formed striped polarization nanodomain configuration of a PbTiO${_3}$/SrTiO${_3}$ superlattice transforms to a uniform polarization state under above-bandgap illumination with a time dependence varying with the intensity of optical illumination and a well-defined threshold intensity. Recovery after the end of illumination occurs over a temperature-dependent period of tens of seco…
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The spontaneously formed striped polarization nanodomain configuration of a PbTiO${_3}$/SrTiO${_3}$ superlattice transforms to a uniform polarization state under above-bandgap illumination with a time dependence varying with the intensity of optical illumination and a well-defined threshold intensity. Recovery after the end of illumination occurs over a temperature-dependent period of tens of seconds at room temperature and shorter times at elevated temperatures. A model in which the screening of the depolarization field depends on the population of trapped electrons correctly predicts the observed temperature and optical intensity dependence.
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Submitted 10 December, 2019;
originally announced December 2019.
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Non-thermal fluence threshold for femtosecond pulsed x-ray radiation damage in perovskite complex oxide epitaxial heterostructures
Authors:
Hyeon Jun Lee,
Youngjun Ahn,
Samuel D. Marks,
Eric C. Landahl,
Jun Young Lee,
Tae Yeon Kim,
Sanjith Unithrattil,
Ji Young Jo,
Sae Hwan Chun,
Sunam Kim,
Sang-Yeon Park,
Intae Eom,
Carolina Adamo,
Darrell G. Schlom,
Haidan Wen,
Paul G. Evans
Abstract:
Intense hard x-ray pulses from a free-electron laser induce irreversible structural damage in a perovskite oxide epitaxial heterostructure when pulse fluences exceed a threshold value. The intensity of x-ray diffraction from a 25-nm thick epitaxial BiFeO$_{3}$ layer on a SrTiO$_{3}$ substrate measured using a series of pulses decreases abruptly with a per-pulse fluence of 2.7 x 10$^{6}$ photons…
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Intense hard x-ray pulses from a free-electron laser induce irreversible structural damage in a perovskite oxide epitaxial heterostructure when pulse fluences exceed a threshold value. The intensity of x-ray diffraction from a 25-nm thick epitaxial BiFeO$_{3}$ layer on a SrTiO$_{3}$ substrate measured using a series of pulses decreases abruptly with a per-pulse fluence of 2.7 x 10$^{6}$ photons $μ$m$^{-2}$ at 9.7 keV photon energy, but remains constant for 1.3 x 10$^{6}$ photons $μ$m$^{-2}$ or less. The damage resulted in the destruction of the BiFeO$_{3}$ thin film within the focal spot area and the formation of a deep cavity penetrating into the STO substrate via the removal of tens of nanometers of material per pulse. The damage threshold occurs at a fluence that is insufficient to heat the absorption volume to the melting point. The morphology of the ablated sample is consistent with fracture rather than melting. Together these results indicate that the damage occurs via a non-thermal process consistent with ultrafast ionization of the absorption volume.
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Submitted 10 December, 2019; v1 submitted 6 December, 2019;
originally announced December 2019.
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Linear-$T$ resistivity from low to high temperature: axion-dilaton theories
Authors:
Yongjun Ahn,
Hyun-Sik Jeong,
Dujin Ahn,
Keun-Young Kim
Abstract:
The linear-$T$ resistivity is one of the hallmarks of various strange metals regardless of their microscopic details. Towards understanding this universal property, the holographic method or gauge/gravity duality has made much progress. Most holographic models have focused on the low temperature limit, where the linear-$T$ resistivity has been explained by the infrared geometry. We extend this ana…
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The linear-$T$ resistivity is one of the hallmarks of various strange metals regardless of their microscopic details. Towards understanding this universal property, the holographic method or gauge/gravity duality has made much progress. Most holographic models have focused on the low temperature limit, where the linear-$T$ resistivity has been explained by the infrared geometry. We extend this analysis to high temperature and identify the conditions for a robust linear-$T$ resistivity up to high temperature. This extension is important because, in experiment, the linear-$T$ resistivity is observed in a large range of temperatures, up to room temperature. In the axion-dilaton theories we find that, to have a robust linear-$T$ resistivity, the strong momentum relaxation is a necessary condition, which agrees with the previous result for the Guber-Rocha model. However, it is not sufficient in the sense that, among large range of parameters giving a linear-$T$ resistivity in low temperature limit, only very limited parameters can support the linear-$T$ resistivity up to high temperature even in strong momentum relaxation. We also show that the incoherent term in the general holographic conductivity formula or the coupling between the dilaton and Maxwell term is responsible for a robust linear-$T$ resistivity up to high temperature.
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Submitted 21 April, 2020; v1 submitted 28 July, 2019;
originally announced July 2019.
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Strain-engineering of Berry curvature dipole and valley magnetization in monolayer MoS$_2$
Authors:
Joolee Son,
Kyung-Han Kim,
Y. H. Ahn,
Hyun-Woo Lee,
Jieun Lee
Abstract:
The Berry curvature dipole is a physical quantity that is expected to allow various quantum geometrical phenomena in a range of solid-state systems. Monolayer transition metal dichalcogenides provide an exceptional platform to modulate and investigate the Berry curvature dipole through strain. Here we theoretically demonstrate and experimentally verify for monolayer MoS$_\rm{2}$ the generation of…
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The Berry curvature dipole is a physical quantity that is expected to allow various quantum geometrical phenomena in a range of solid-state systems. Monolayer transition metal dichalcogenides provide an exceptional platform to modulate and investigate the Berry curvature dipole through strain. Here we theoretically demonstrate and experimentally verify for monolayer MoS$_\rm{2}$ the generation of valley orbital magnetization as a response to an in-plane electric field due to the Berry curvature dipole. The measured valley orbital magnetization shows excellent agreement with the calculated Berry curvature dipole which can be controlled by the magnitude and direction of strain. Our results show that the Berry curvature dipole acts as an effective magnetic field in current-carrying systems, providing a novel route to generate magnetization.
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Submitted 2 July, 2019; v1 submitted 28 June, 2019;
originally announced July 2019.
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Nanosecond Optically Induced Phase Transformation in Compressively Strained BiFeO3 on LaAlO3
Authors:
Youngjun Ahn,
Anastasios Pateras,
Samuel D. Marks,
Han Xu,
Tao Zhou,
Zhenlin Luo,
Zuhuang Chen,
Lang Chen,
Xiaoyi Zhang,
Anthony D. DiChiara,
Haidan Wen,
Paul G. Evans
Abstract:
Above-bandgap optical illumination induces a transformation from tilted tetragonal-like (T-like) and rhombohedral-like (R-like) phases to an untilted T-like phase in compressively strained BiFeO3. Optical excitation leads to an out-of-plane lattice expansion in the T-like phase. The transformation proceeds in regions with boundaries between the T-like and tilted R-like and tilted T-like phases, co…
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Above-bandgap optical illumination induces a transformation from tilted tetragonal-like (T-like) and rhombohedral-like (R-like) phases to an untilted T-like phase in compressively strained BiFeO3. Optical excitation leads to an out-of-plane lattice expansion in the T-like phase. The transformation proceeds in regions with boundaries between the T-like and tilted R-like and tilted T-like phases, consistent with the motion of a boundary. The optically induced transformation indicates that there are new optically driven routes towards nanosecond-scale control of phase transformations in ferroelectrics and multiferroics.
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Submitted 8 October, 2019; v1 submitted 24 January, 2019;
originally announced January 2019.
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Robust operation of a GaAs tunable barrier electron pump
Authors:
S. P. Giblin,
M. -H. Bae,
N. Kim,
Ye-Hwan Ahn,
M. Kataoka
Abstract:
We demonstrate the robust operation of a gallium arsenide tunable-barrier single-electron pump operating with 1 part-per-million accuracy at a temperature of $1.3$~K and a pumping frequency of $500$~MHz. The accuracy of current quantisation is investigated as a function of multiple control parameters, and robust plateaus are seen as a function of three control gate voltages and RF drive power. The…
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We demonstrate the robust operation of a gallium arsenide tunable-barrier single-electron pump operating with 1 part-per-million accuracy at a temperature of $1.3$~K and a pumping frequency of $500$~MHz. The accuracy of current quantisation is investigated as a function of multiple control parameters, and robust plateaus are seen as a function of three control gate voltages and RF drive power. The electron capture is found to be in the decay-cascade, rather than the thermally-broadened regime. The observation of robust plateaus at an elevated temperature which does not require expensive refrigeration is an important step towards validating tunable-barrier pumps as practical current standards.
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Submitted 28 September, 2018;
originally announced October 2018.
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Numerical Simulation of Quantized Current Generated by a Quantum Dot Pump
Authors:
Ye-Hwan Ahn,
Yunchul Chung
Abstract:
The quantized current generated by a quantum dot pump is calculated numerically. The numerical simulation is done by dividing the time varying potential into many static potentials with a short time interval and calculating the electron capture and pumping rate with the time independent Schrodinger equation. The simulation results show good agreement with reported experimental results qualitativel…
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The quantized current generated by a quantum dot pump is calculated numerically. The numerical simulation is done by dividing the time varying potential into many static potentials with a short time interval and calculating the electron capture and pumping rate with the time independent Schrodinger equation. The simulation results show good agreement with reported experimental results qualitatively. The calculated 2D pump current map and the plateau width dependence on the modulation gate voltage show good agreement with the experimental results. From the simulation results, it is explained how the back-tunneling process affects the accuracy of the current plateaus quantitatively. Also, the energy distribution of the pumped electron is calculated, which can be measured experimentally. Finally, it is found that the pump current accuracy can be enhanced by increasing the entrance gate width, which is important to realize the quantum current standard.
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Submitted 15 January, 2018;
originally announced January 2018.
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Transport spectroscopy for Paschen-Back splitting of Landau levels in InAs nanowires
Authors:
Bum-Kyu Kim,
Sang-Jun Choi,
Jae Cheol Shin,
Minsoo Kim,
Ye-Hwan Ahn,
H. -S. Sim,
Ju-Jin Kim,
Myung-Ho Bae
Abstract:
The coupling of electron orbital motion and spin leads to nontrivial changes in energy-level structures, leading to various spectroscopies and applications. In atoms, such spin-orbit coupling (SOC) causes anomalous Zeeman splitting, known as the Paschen-Back (PB) effect, in the pres-ence of a strong magnetic field. In solids, SOC generates energy-band inversion or splitting, a prerequisite for top…
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The coupling of electron orbital motion and spin leads to nontrivial changes in energy-level structures, leading to various spectroscopies and applications. In atoms, such spin-orbit coupling (SOC) causes anomalous Zeeman splitting, known as the Paschen-Back (PB) effect, in the pres-ence of a strong magnetic field. In solids, SOC generates energy-band inversion or splitting, a prerequisite for topological phases or Majorana fermions, at zero or weak magnetic fields. Here, we present the first observation of PB splitting of Landau levels (LLs) in indium arsenide nan-owires in a strong-field regime. Our energy-resolved transport spectroscopy results indicated the presence of LL-dependent anomalous Zeeman splitting in these nanowires, analogous to the atomic PB effect. This result was found to be in good agreement with a theoretical analysis based on Rashba SOC. Our findings also suggested a way of generating spin-resolved electron transport in nanowires.
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Submitted 18 November, 2017; v1 submitted 15 November, 2017;
originally announced November 2017.
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Thermal diffusivity and butterfly velocity in anisotropic Q-Lattice models
Authors:
Dujin Ahn,
Yongjun Ahn,
Hyun-Sik Jeong,
Keun-Young Kim,
Wei-Jia Li,
Chao Niu
Abstract:
By using a holographic method we study a relation between the thermal diffusivity ($D_T$) and two quantum chaotic properties, Lyapunov time ($τ_L$) and butterfly velocity ($v_B$) in strongly correlated systems. It has been shown that $D_T/(v_B^2 τ_L)$ is universal in some holographic models as well as condensed matter systems including the Sachdev-Ye-Kitaev (SYK) models. We investigate to what ext…
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By using a holographic method we study a relation between the thermal diffusivity ($D_T$) and two quantum chaotic properties, Lyapunov time ($τ_L$) and butterfly velocity ($v_B$) in strongly correlated systems. It has been shown that $D_T/(v_B^2 τ_L)$ is universal in some holographic models as well as condensed matter systems including the Sachdev-Ye-Kitaev (SYK) models. We investigate to what extent this relation is universal in the Q-lattice models with infrared (IR) scaling geometry, focusing on the effect of spatial anisotropy. Indeed it was shown that $\mathcal{E}_i := D_{T,i}/(v_{B,i}^2 τ_L)$ ($i=x,y$) is determined only by some scaling exponents of the IR metric in the low temperature limit regardless of the matter fields and ultraviolet data. Inspired by this observation, in this work, we find the concrete expressions for $\mathcal{E}_i$ in terms of the critical dynamical exponents $z_i$ in each direction. By analyzing the IR scaling geometry we identify the allowed scaling parameter regimes, which enable us to compute the allowed range of $\mathcal{E}_i$. We find the lower bound of $\mathcal{E}_i$ is always $1/2$, which is not affected by anisotropy, contrary to the $η/s$ case. However, there may be an upper bound determined by anisotropy.
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Submitted 10 September, 2017; v1 submitted 29 August, 2017;
originally announced August 2017.
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Photoinduced Domain Pattern Transformation in Ferroelectric/Dielectric Superlattices
Authors:
Youngjun Ahn,
Joonkyu Park,
Anastasios Pateras,
Matthew B. Rich,
Qingteng Zhang,
Pice Chen,
Mohammed H. Yusuf,
Haidan Wen,
Matthew Dawber,
Paul G. Evans
Abstract:
The nanodomain pattern in ferroelectric/dielectric superlattices transforms to a uniform polarization state under above-bandgap optical excitation. X-ray scattering reveals a disappearance of domain diffuse scattering and an expansion of the lattice. The reappearance of the domain pattern occurs over a period of seconds at room temperature, suggesting a transformation mechanism in which charge car…
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The nanodomain pattern in ferroelectric/dielectric superlattices transforms to a uniform polarization state under above-bandgap optical excitation. X-ray scattering reveals a disappearance of domain diffuse scattering and an expansion of the lattice. The reappearance of the domain pattern occurs over a period of seconds at room temperature, suggesting a transformation mechanism in which charge carriers in long-lived trap states screen the depolarization field. A Landau-Ginzburg-Devonshire model predicts changes in lattice parameter and a critical carrier concentration for the transformation.
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Submitted 4 August, 2017; v1 submitted 4 May, 2017;
originally announced May 2017.
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Imaging ultrafast carrier transport in nanoscale devices using femtosecond photocurrent microscopy
Authors:
B. H. Son,
Jae-Ku Park,
J. T. Hong,
Ji-Yong. Park,
Soonil. Lee,
Y. H. Ahn
Abstract:
One-dimensional nanoscale devices, such as semiconductor nanowires (NWs) and single- walled carbon nanotubes (SWNTs), have been intensively investigated because of their potential application of future high-speed electronic, optoelectronic, and sensing devices. To overcome current limitations on the speed of contemporary devices, investigation of charge carrier dynamics with an ultrashort time sca…
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One-dimensional nanoscale devices, such as semiconductor nanowires (NWs) and single- walled carbon nanotubes (SWNTs), have been intensively investigated because of their potential application of future high-speed electronic, optoelectronic, and sensing devices. To overcome current limitations on the speed of contemporary devices, investigation of charge carrier dynamics with an ultrashort time scale is one of the primary steps necessary for developing high-speed devices. In the present study, we visualize ultrafast carrier dynamics in nanoscale devices using a combination of scanning photocurrent microscopy and time- resolved pump-probe techniques. We investigate transit times of carriers that are generated near one metallic electrode and subsequently transported toward the opposite electrode based on drift and diffusion motions. Carrier dynamics have been measured for various working conditions. In particular, the carrier velocities extracted from transit times increase for a larger negative gate bias, because of the increased field strength at the Schottky barrier.
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Submitted 1 June, 2014;
originally announced June 2014.
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Transport Measurement of Andreev Bound States in a Kondo-Correlated Quantum Dot
Authors:
Bum-Kyu Kim,
Ye-Hwan Ahn,
Ju-Jin Kim,
Mahn-Soo Choi,
Myung-Ho Bae,
Kicheon Kang,
Jong Soo Lim,
Rosa Lopez,
Nam Kim
Abstract:
We report transport measurements of gate-tunable Andreev bound states in a carbon nanotube quantum dot coupled to two superconducting leads. In particular, we observe clear features of two types of Kondo ridges, which can be understood in terms of the interplay between the Kondo effect and superconductivity. In the first type (type I), the coupling is strong and the Kondo effect is dominant. Level…
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We report transport measurements of gate-tunable Andreev bound states in a carbon nanotube quantum dot coupled to two superconducting leads. In particular, we observe clear features of two types of Kondo ridges, which can be understood in terms of the interplay between the Kondo effect and superconductivity. In the first type (type I), the coupling is strong and the Kondo effect is dominant. Levels of the Andreev bound states display anti-crossing in the middle of the ridge. On the other hand, crossing of the two Andreev bound states is shown in the second type (type II) together with the 0-$π$ transition of the Josephson junction. Our scenario is well understood in terms of only a single dimensionless parameter, $k_BT_K^{min}/Δ$, where $T_K^{min}$ and $Δ$ are the minimum Kondo temperature of a ridge and the superconducting order parameter, respectively. Our observation is consistent with measurements of the critical current, and is supported by numerical renormalization group calculations.
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Submitted 5 December, 2012; v1 submitted 21 September, 2012;
originally announced September 2012.
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Robustness and modular structure in networks
Authors:
James P. Bagrow,
Sune Lehmann,
Yong-Yeol Ahn
Abstract:
Complex networks have recently attracted much interest due to their prevalence in nature and our daily lives [1, 2]. A critical property of a network is its resilience to random breakdown and failure [3-6], typically studied as a percolation problem [7-9] or by modeling cascading failures [10-12]. Many complex systems, from power grids and the Internet to the brain and society [13-15], can be mode…
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Complex networks have recently attracted much interest due to their prevalence in nature and our daily lives [1, 2]. A critical property of a network is its resilience to random breakdown and failure [3-6], typically studied as a percolation problem [7-9] or by modeling cascading failures [10-12]. Many complex systems, from power grids and the Internet to the brain and society [13-15], can be modeled using modular networks comprised of small, densely connected groups of nodes [16, 17]. These modules often overlap, with network elements belonging to multiple modules [18, 19]. Yet existing work on robustness has not considered the role of overlapping, modular structure. Here we study the robustness of these systems to the failure of elements. We show analytically and empirically that it is possible for the modules themselves to become uncoupled or non-overlapping well before the network disintegrates. If overlapping modular organization plays a role in overall functionality, networks may be far more vulnerable than predicted by conventional percolation theory.
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Submitted 6 January, 2016; v1 submitted 24 February, 2011;
originally announced February 2011.
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Googling Social Interactions: Web Search Engine Based Social Network Construction
Authors:
Sang Hoon Lee,
Pan-Jun Kim,
Yong-Yeol Ahn,
Hawoong Jeong
Abstract:
Social network analysis has long been an untiring topic of sociology. However, until the era of information technology, the availability of data, mainly collected by the traditional method of personal survey, was highly limited and prevented large-scale analysis. Recently, the exploding amount of automatically generated data has completely changed the pattern of research. For instance, the enormou…
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Social network analysis has long been an untiring topic of sociology. However, until the era of information technology, the availability of data, mainly collected by the traditional method of personal survey, was highly limited and prevented large-scale analysis. Recently, the exploding amount of automatically generated data has completely changed the pattern of research. For instance, the enormous amount of data from so-called high-throughput biological experiments has introduced a systematic or network viewpoint to traditional biology. Then, is "high-throughput" sociological data generation possible? Google, which has become one of the most influential symbols of the new Internet paradigm within the last ten years, might provide torrents of data sources for such study in this (now and forthcoming) digital era. We investigate social networks between people by extracting information on the Web and introduce new tools of analysis of such networks in the context of statistical physics of complex systems or socio-physics. As a concrete and illustrative example, the members of the 109th United States Senate are analyzed and it is demonstrated that the methods of construction and analysis are applicable to various other weighted networks.
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Submitted 21 July, 2010; v1 submitted 17 October, 2007;
originally announced October 2007.
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Photocurrent Imaging of p-n Junctions and Local Defects in Ambipolar Carbon Nanotube Transistors
Authors:
Y. H. Ahn,
Wei Tsen,
Bio Kim,
Yung Woo Park,
Jiwoong Park
Abstract:
We use scanning photocurrent microscopy (SPCM) to investigate the properties of internal p-n junctions as well as local defects in ambipolar carbon nanotube (CNT) transistors. Our SPCM images show strong signals near metal contacts whose polarity and positions change depending on the gate bias. SPCM images analyzed in conjunction with the overall conductance also indicate the existence and gate-…
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We use scanning photocurrent microscopy (SPCM) to investigate the properties of internal p-n junctions as well as local defects in ambipolar carbon nanotube (CNT) transistors. Our SPCM images show strong signals near metal contacts whose polarity and positions change depending on the gate bias. SPCM images analyzed in conjunction with the overall conductance also indicate the existence and gate-dependent evolution of internal p-n junctions near contacts in the n-type operation regime. To determine the p-n junction position and the depletion width with a nanometer scale resolution, a Gaussian fit was used. We also measure the electric potential profile of CNT devices at different gate biases, which shows that both local defects and induced electric fields can be imaged using the SPCM technique. Our experiment clearly demonstrates that SPCM is a valuable tool for imaging and optimizing electrical and optoelectronic properties of CNT based devices.
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Submitted 20 July, 2007;
originally announced July 2007.
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Epidemic Dynamics of Interacting Two Particle Species on Scale-free Networks
Authors:
Yong-Yeol Ahn,
Naoki Masuda,
Hawoong Jeong,
Jae Dong Noh
Abstract:
We study the non-equilibrium phase transition in a model for epidemic spreading on scale-free networks. The model consists of two particle species $A$ and $B$, and the coupling between them is taken to be asymmetric; $A$ induces $B$ while $B$ suppresses $A$. This model describes the spreading of an epidemic on networks equipped with a reactive immune system. We present analytic results on the ph…
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We study the non-equilibrium phase transition in a model for epidemic spreading on scale-free networks. The model consists of two particle species $A$ and $B$, and the coupling between them is taken to be asymmetric; $A$ induces $B$ while $B$ suppresses $A$. This model describes the spreading of an epidemic on networks equipped with a reactive immune system. We present analytic results on the phase diagram and the critical behavior, which depends on the degree exponent $γ$ of the underlying scale-free networks. Numerical simulation results that support the analytic results are also presented.
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Submitted 21 August, 2006;
originally announced August 2006.
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Growing network model for community with group structure
Authors:
Jae Dong Noh,
Hyeong-Chai Jeong,
Yong-Yeol Ahn,
Hawoong Jeong
Abstract:
We propose a growing network model for a community with a group structure. The community consists of individual members and groups, gatherings of members. The community grows as a new member is introduced by an existing member at each time step. The new member then creates a new group or joins one of the groups of the introducer. We investigate the emerging community structure analytically and n…
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We propose a growing network model for a community with a group structure. The community consists of individual members and groups, gatherings of members. The community grows as a new member is introduced by an existing member at each time step. The new member then creates a new group or joins one of the groups of the introducer. We investigate the emerging community structure analytically and numerically. The group size distribution shows a power law distribution for a variety of growth rules, while the activity distribution follows an exponential or a power law depending on the details of the growth rule. We also present an analysis of empirical data from on the online communities, the ``Groups'' in \url{http://www.yahoo.com} and the ``Cafe'' in \url{http://www.daum.net}, which shows a power law distribution for a wide range of group sizes.
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Submitted 24 March, 2005; v1 submitted 7 December, 2004;
originally announced December 2004.