Showing 1–24 of 24 results for author: Bang, J

Electrolyte Bonding Engineering for Highly Uniform GeTe-based CBRAM and Parallel Hebbian Learning in Selector-free Hopfield Networks

Authors: Jiin Bang, Jingyeong Hwang, Unhyeon Kang, Seungmin Oh, Kyungmin Lee, Jaehyun Park, Younghyun Lee, Hyun Jae Jang, Seongsik Park, YeonJoo Jeong, Inho Kim, Jong Keuk Park, Suyoun Lee

Abstract: Hopfield networks offer a hardware-friendly framework for energy-efficient associative memory, yet their practical realization in memristor crossbar arrays is critically hindered by device-to-device (D2D) variability, which prevents reliable parallel programming. Here, we address this bottleneck through systematic composition engineering of the Ge-Te solid electrolyte in conductive bridge random a… ▽ More Hopfield networks offer a hardware-friendly framework for energy-efficient associative memory, yet their practical realization in memristor crossbar arrays is critically hindered by device-to-device (D2D) variability, which prevents reliable parallel programming. Here, we address this bottleneck through systematic composition engineering of the Ge-Te solid electrolyte in conductive bridge random access memory (CBRAM) devices. By varying the Ge:Te ratio, we identify Ge3.5Te1 as an optimal electrolyte composition that suppresses stochastic resistance variation by approximately three orders of magnitude compared to GeSe-based devices. Raman spectroscopy reveals that this dramatic improvement originates from a bonding network dominated by asymmetric-stretching GeTe4 tetrahedral units, which form interconnected free-volume channels that confine and stabilize Cu+ ion migration pathways. Leveraging this enhanced uniformity, we fabricate a selector-less 16x16 Cu/Ge3.5Te1 CBRAM crossbar array and demonstrate a 4x4 Hopfield associative network capable of learning and recalling binary pattern pairs via fully parallel programming using a half-selection scheme. Successful pattern recall is achieved for up to two stored associations despite the absence of selector elements, establishing a proof-of-concept for selector-free hardware implementations of associative memory. These results highlight the critical role of electrolyte bonding structure in determining memristor uniformity and provide a materials-driven pathway toward scalable, parallel neuromorphic computing systems. △ Less

Submitted 4 June, 2026; originally announced June 2026.

Hot carrier diffusion-assisted ideal carrier multiplication in monolayer MoSe2

Authors: Joonsoo Kim, Hong-Guk Min, Sehwan Park, Jin Cheol Park, Junhyeok Bang, Youngkuk Kim, Ji-Hee Kim

Abstract: Carrier multiplication (CM), the process of generating multiple charge carriers from a single photon, offers an opportunity to exceed the Shockley-Queisser limit in photovoltaic applications. Despite extensive research, no material has yet achieved ideal CM efficiency, primarily owing to significant energy losses from carrier-lattice scattering. In this study, we demonstrate that monolayer MoSe2 c… ▽ More Carrier multiplication (CM), the process of generating multiple charge carriers from a single photon, offers an opportunity to exceed the Shockley-Queisser limit in photovoltaic applications. Despite extensive research, no material has yet achieved ideal CM efficiency, primarily owing to significant energy losses from carrier-lattice scattering. In this study, we demonstrate that monolayer MoSe2 can attain the theoretical maximum CM efficiency permitted by energy-momentum conservation principle, using ultrafast transient absorption spectroscopy. By resolving the scatter-free ballistic transport of hot carriers and validating our findings with first-principles calculations, we identify the cornerstone of optimal CM in monolayer MoSe2: superior hot-carrier dynamics characterized by suppressed energy dissipation via minimized carrier-lattice scattering, and the availability of abundant CM pathways facilitated by 2Eg band nesting. Comparative analysis with bulk MoSe2 further emphasizes the enhanced CM efficiency in the monolayer, attributed by superior hot-carrier diffusion and access to additional CM pathways. These results position monolayer MoSe2 as a promising candidate for high-performance optoelectronic applications, providing a robust platform for next-generation energy conversion technologies. △ Less

Submitted 31 May, 2026; originally announced June 2026.

Imaging asymmetric Coulomb blockade phenomena across metallic nanoislands

Authors: Junho Bang, Byeongin Lee, Hankyu Lee, Jian-Feng Ge, Doohee Cho

Abstract: Coulomb blockade (CB) arises in nanoscale systems with ultra-small capacitance, where discrete charging effects dictate electron transport, enabling wide-ranging applications based on single-electron transistors. Despite established electrostatic control of charge states in quantum dots and nanoislands, a rigorous quantitative link between junction parameters and the CB spectrum remains elusive. H… ▽ More Coulomb blockade (CB) arises in nanoscale systems with ultra-small capacitance, where discrete charging effects dictate electron transport, enabling wide-ranging applications based on single-electron transistors. Despite established electrostatic control of charge states in quantum dots and nanoislands, a rigorous quantitative link between junction parameters and the CB spectrum remains elusive. Here, using scanning tunneling spectroscopy, we investigate the spatial variation of CB in indium nanoislands on semiconducting black phosphorus. We observe spatially dispersive charging resonances whose trajectories exhibit a finite shift of the symmetry axis in bias as well as a pronounced asymmetric curvature. By comparing the experimental results with calculations based on orthodox theory, we show that these features originate from work function differences in the junctions, underscoring the importance of junction-specific electrostatics in nanoscale charge transport. △ Less

Submitted 4 March, 2026; originally announced March 2026.

Comments: 23 pages, 4 main figure, 7 supplementary figures

Journal ref: Phys. Rev. B 113, 165414 (2026)

Identifying field-tunable surface resonance states on black phosphorus

Authors: Dongming Zhao, Byeongin Lee, Junho Bang, Claudia Felser, Jian-Feng Ge, Doohee Cho

Abstract: Surface resonance states are electronic states localized near the surface while remaining hybridized with bulk bands. These states can strongly modify the electric-field response of semiconductors. Here, we demonstrate using scanning tunneling spectroscopy that on black phosphorus, surface resonance states near the valence-band edge dominate the screening of a strong external electric field. We ob… ▽ More Surface resonance states are electronic states localized near the surface while remaining hybridized with bulk bands. These states can strongly modify the electric-field response of semiconductors. Here, we demonstrate using scanning tunneling spectroscopy that on black phosphorus, surface resonance states near the valence-band edge dominate the screening of a strong external electric field. We observe in the tunneling conductance spectrum a pronounced dip with an energy continuously tunable by the local electric field in the tunneling junction. Meanwhile, we also notice that the bulk band edges remain effectively pinned, indicating efficient surface screening and suppression of bulk band bending. We interpret the conductance dip as the consequence of a field-dependent tunneling barrier: as the external electric field drives the surface resonance band into the band gap, the coupling between the surface resonance states and the bulk states is suppressed, leading to a reduced tunneling probability. Our simplified model based on this mechanism reproduces our main experimental findings. Our results highlight surface-localized states as a critical component in the electrostatic response of semiconductors, which must be taken into consideration in the design and operation of nanoscale semiconductor devices. △ Less

Submitted 2 March, 2026; originally announced March 2026.

Unfolding Bloch States in Disordered Systems

Authors: T. Thuy Hoang, Kunihiro Yananose, Sungjong Woo, Seongjin Ahn, Dong Han, Xian-Bin Li, Junhyeok Bang

Abstract: In crystalline solids, disorder breaks translational symmetry and obscures k-resolved Bloch states, limiting an accurate description of wavefunction-based observables. In this work, we present a method that unfolds not only the band structures but also the corresponding Bloch states in disordered systems, going beyond conventional band-unfolding techniques. As a prototype application, we study def… ▽ More In crystalline solids, disorder breaks translational symmetry and obscures k-resolved Bloch states, limiting an accurate description of wavefunction-based observables. In this work, we present a method that unfolds not only the band structures but also the corresponding Bloch states in disordered systems, going beyond conventional band-unfolding techniques. As a prototype application, we study defective graphene and demonstrate the capabilities by capturing key wavefunction-level responses, including disorder-driven redistribution of Berry curvature. △ Less

Submitted 1 March, 2026; originally announced March 2026.

Comments: 18 pages, 4 figures

Anomalous impurity-induced charge modulations in black phosphorus

Authors: Byeongin Lee, Junho Bang, Sayan Banerjee, João Augusto Sobral, Young Woo Choi, Claudia Felser, Mathias S. Scheurer, Jian-Feng Ge, Doohee Cho

Abstract: We observe anomalous charge modulations induced by ionized indium impurities on the surface of the semiconductor black phosphorus by scanning tunneling microscopy (STM). When the impurities are switched into a negatively charged state by the STM tip, periodic charge modulations emerge around the impurity center, but strictly confined by the nanoscale impurity potential. These modulations form a di… ▽ More We observe anomalous charge modulations induced by ionized indium impurities on the surface of the semiconductor black phosphorus by scanning tunneling microscopy (STM). When the impurities are switched into a negatively charged state by the STM tip, periodic charge modulations emerge around the impurity center, but strictly confined by the nanoscale impurity potential. These modulations form a distorted triangular pattern, whose periodicity remains unchanged in a wide range of positive bias. Furthermore, these local charge orders exhibit an anisotropy opposite to that expected based on the anisotropy of the Fermi surface, challenging a simple band-structure interpretation. Our experiment demonstrates the possibility of creating and manipulating macroscopic charge orders through impurity engineering. △ Less

Submitted 4 December, 2025; originally announced December 2025.

Comments: 6 pages, 4 figures

Journal ref: Phys. Rev. B 113, L041105 (2026)

Imaging 3D polarization dynamics via deep learning 4D-STEM

Authors: Jinho Byun, Keeyong Lee, Myoungho Jeong, Eunha Lee, Jeongil Bang, Haeryong Kim, Geun Ho Gu, Sang Ho Oh

Abstract: Recent advances in ferroelectrics highlight the role of three-dimensional (3D) polar entities in forming topological polar textures and generating giant electromechanical responses, during polarization rotation. However, current electron microscopy methods lack the depth resolution to resolve the polarization component along the electron beam direction, which restricts full characterization. Here,… ▽ More Recent advances in ferroelectrics highlight the role of three-dimensional (3D) polar entities in forming topological polar textures and generating giant electromechanical responses, during polarization rotation. However, current electron microscopy methods lack the depth resolution to resolve the polarization component along the electron beam direction, which restricts full characterization. Here, we present a deep learning framework combined with four-dimensional scanning transmission electron microscopy to reconstruct 3D polarization maps in Ba0.5Sr0.5TiO3 thin-film capacitors with picometer-level accuracy under applied electric fields. Our approach enables observation of polar nanodomains consistent with the polar slush model and shows that switching occurs through coordinated vector rotation toward <111> energy minima, rather than magnitude changes. Furthermore, regions with higher topological density exhibit smaller polarization variation when the electric field changes, indicating topological protection. Our work reveals the value of 3D polarization mapping in elucidating complex nanoscale polar phenomena, with broad implications for emergent ferroelectrics. △ Less

Submitted 6 June, 2025; originally announced June 2025.

Comments: 30 pages, 14 figures

Symmetry breaking in Prussian Blue Analogues via growth--guided local ordering of hexacyanometallate vacancies

Authors: Yevheniia Kholina, Thomas Weber, Joohee Bang, Arthur Baroni, Marianne Liebi, Semen Gorfman, Ido Biran, Mark Warren, Dmitriy Chernyshov, Arkadiy Simonov

Abstract: We report Growth--Guided Local Ordering, a novel mechanism of symmetry reduction in disordered crystals. This mechanism operates through the directional ordering of point defects during crystal growth, where defect correlations develop preferentially along the growth direction, resulting in reduced symmetry that persists in the final structure through the spatial distribution of defects. We demons… ▽ More We report Growth--Guided Local Ordering, a novel mechanism of symmetry reduction in disordered crystals. This mechanism operates through the directional ordering of point defects during crystal growth, where defect correlations develop preferentially along the growth direction, resulting in reduced symmetry that persists in the final structure through the spatial distribution of defects. We demonstrate this phenomenon in Mn[Co]-Prussian Blue Analogues, disordered cyanide crystals containing numerous Co(CN)$_6$ vacancies. Single crystal diffuse scattering reveals pronounced anisotropy in vacancy distribution: strong correlations along [001] growth direction contrast with weak correlations perpendicular to it. This local ordering reduces the Laue symmetry to tetragonal $4/mmm$, evident in properties such as birefringence, while the average structure retains cubic $m\bar 3m$ symmetry. When growth proceeds along [111] direction, the same mechanism produces domains with trigonal symmetry. Because this mechanism relies on fundamental aspects of crystal growth rather than specific material properties, it offers a general strategy for symmetry control in disordered crystals. Crucially, it transforms the complex task of altering crystal symmetry into the more manageable challenge of controlling growth direction, achievable through various established techniques such as the use of surfactants during crystallization. △ Less

Submitted 9 February, 2025; originally announced February 2025.

Comments: 19 pages, 11 figures

Critical Current, Lengthwise Fluctuations, and Flux Jumps in REBCO CC: A Torque Magnetometry Study up to 45 T

Authors: J. Jaroszynski, A-M Constantinescu, D. Kolb-Bond, A. Francis, A. Xu, R. Ries, G. Bradford, J. Bang, J. Lee, D. Larbalestier

Abstract: Rare Earth Barium Copper Oxide (REBCO) coated conductors (CCs) have emerged for future high field magnets in fields and temperatures inaccessible for Nb based superconductors. However, their exceptionally high current densities pose challenges for low temperature characterization. This paper presents the design and implementation of a simple torque magnetometer that is particularly suitable for ch… ▽ More Rare Earth Barium Copper Oxide (REBCO) coated conductors (CCs) have emerged for future high field magnets in fields and temperatures inaccessible for Nb based superconductors. However, their exceptionally high current densities pose challenges for low temperature characterization. This paper presents the design and implementation of a simple torque magnetometer that is particularly suitable for characterizing REBCO CC. It details the construction and underlying physics, with a particular emphasis on its capability to assess the angular critical currents Ic in high magnetic fields and low temperatures. The study includes characterizations of multiple REBCO samples from different manufacturers, performed under magnetic fields up to 45 T, demonstrating the exceptional capabilities of REBCO CCs in extreme fields. The results revealed significant lengthwise Ic variations, especially in tapes cut from the edges of 12 mm-wide production tapes compared to those cut from the center. These variations are most pronounced when the field is near the ab plane. Importantly, flux jumps are observed in samples with thick REBCO layers and thin stabilizers, underscoring the potential thermal instabilities. These findings provide valuable insights into the performance of REBCO tapes under extreme magnetic fields, highlighting their relevance for high-field magnet and nuclear fusion applications. △ Less

Submitted 2 June, 2026; v1 submitted 4 February, 2025; originally announced February 2025.

Comments: 22 pages, 10 figures

Origin of Distinct Insulating Domains in the Layered Charge Density Wave Material 1T-TaS2

Authors: Hyungryul Yang, Byeongin Lee, Junho Bang, Sunghun Kim, Dirk Wulferding, Sung-Hoon Lee, Doohee Cho

Abstract: Vertical charge order shapes the electronic properties in layered charge density wave (CDW) materials. Various stacking orders inevitably create nanoscale domains with distinct electronic structures inaccessible to bulk probes. Here, the stacking characteristics of bulk 1$T$-TaS$2$ are analyzed using scanning tunneling spectroscopy (STS) and density functional theory (DFT) calculations. It is obse… ▽ More Vertical charge order shapes the electronic properties in layered charge density wave (CDW) materials. Various stacking orders inevitably create nanoscale domains with distinct electronic structures inaccessible to bulk probes. Here, the stacking characteristics of bulk 1$T$-TaS$2$ are analyzed using scanning tunneling spectroscopy (STS) and density functional theory (DFT) calculations. It is observed that Mott-insulating domains undergo a transition to band-insulating domains restoring vertical dimerization of the CDWs. Furthermore, STS measurements covering a wide terrace reveal two distinct band insulating domains differentiated by band edge broadening. These DFT calculations reveal that the Mott insulating layers preferably reside on the subsurface, forming broader band edges in the neighboring band insulating layers. Ultimately, buried Mott insulating layers believed to harbor the quantum spin liquid phase are identified. These results resolve persistent issues regarding vertical charge order in 1$T$-TaS$2$, providing a new perspective for investigating emergent quantum phenomena in layered CDW materials. △ Less

Submitted 12 June, 2024; originally announced June 2024.

Comments: 26 pages and 13 figures

Charge ordered phases in the hole-doped triangular Mott insulator 4Hb-TaS2

Authors: Junho Bang, Byeongin Lee, Hyungryul Yang, Sunghun Kim, Dirk Wulferding, Doohee Cho

Abstract: 4Hb-TaS2 has been proposed to possess unconventional superconductivity with broken time reveral symmetry due to distinctive layered structure, featuring a heterojunction between a 2D triangular Mott insulator and a charge density wave metal. However, since a frustrated spin state in the correlated insulating layer is susceptible to charge ordering with carrier doping, it is required to investigate… ▽ More 4Hb-TaS2 has been proposed to possess unconventional superconductivity with broken time reveral symmetry due to distinctive layered structure, featuring a heterojunction between a 2D triangular Mott insulator and a charge density wave metal. However, since a frustrated spin state in the correlated insulating layer is susceptible to charge ordering with carrier doping, it is required to investigate the charge distribution driven by inter-layer charge transfer to understand its superconductivity. Here, we use scanning tunneling microscopy and spectroscopy (STM/S) to investigate the charge ordered phases of 1T-TaS2 layers within 4Hb-TaS2, explicitly focusing on the non-half-filled regime. Our STS results show an energy gap which exhibits an out-of-phase relation with the charge density. We ascribe the competition between on-site and nonlocal Coulomb repulsion as the driving force for the charge-ordered insulating phase of a doped triangular Mott insulator. In addition, we discuss the role of the insulating layer in the enhanced superconductivity of 4Hb-TaS2. △ Less

Submitted 17 June, 2024; v1 submitted 12 June, 2024; originally announced June 2024.

Comments: 18 pages, 6 figures

Journal ref: Phys. Rev. B 109, 195170 (2024)

Melting of unidirectional charge density waves across twin domain boundaries in GdTe$_{3}$

Authors: Sanghun Lee, Eunseo Kim, Junho Bang, Jongho Park, Changyoung Kim, Dirk Wulferding, Doohee Cho

Abstract: Solids undergoing a transition from order to disorder experience the proliferation of topological defects. The melting process generates transient quantum states. However, their dynamical nature with femtosecond lifetime hinders exploration with atomic precision. Here, we suggest an alternative approach to the dynamical melting process by focusing on the interface created by competing degenerate q… ▽ More Solids undergoing a transition from order to disorder experience the proliferation of topological defects. The melting process generates transient quantum states. However, their dynamical nature with femtosecond lifetime hinders exploration with atomic precision. Here, we suggest an alternative approach to the dynamical melting process by focusing on the interface created by competing degenerate quantum states. We use a scanning tunneling microscope (STM) to visualize the unidirectional charge density wave (CDW) and its spatial progression ("static melting") across a twin domain boundary (TDB) in the layered material GdTe$_{3}$. Combining STM with a spatial lock-in technique, we reveal that the order parameter amplitude attenuates with the formation of dislocations and thus two different unidirectional CDWs coexist near the TDB, reducing the CDW anisotropy. Notably, we discover a correlation between this anisotropy and the CDW gap. Our study provides valuable insight into the behavior of topological defects and transient quantum states. △ Less

Submitted 14 December, 2023; originally announced December 2023.

Journal ref: Nano Lett. 23, 11219 (2023)

Quantum electron liquid and its possible phase transition

Authors: Sunghun Kim, Joonho Bang, Chan-young Lim, Seung Yong Lee, Jounghoon Hyun, Gyubin Lee, Yeonghoon Lee, Jonathan D. Denlinger, Soonsang Huh, Changyoung Kim, Sang Yong Song, Junpil Seo, Dinesh Thapa, Seong-Gon Kim, Young Hee Lee, Yeongkwan Kim, Sung Wng Kim

Abstract: Purely quantum electron systems exhibit intriguing correlated electronic phases by virtue of quantum fluctuations in addition to electron-electron interactions. To realize such quantum electron systems, a key ingredient is dense electrons decoupled from other degrees of freedom. Here, we report the discovery of a pure quantum electron liquid, which spreads up to ~ 3 Å in the vacuum on the surface… ▽ More Purely quantum electron systems exhibit intriguing correlated electronic phases by virtue of quantum fluctuations in addition to electron-electron interactions. To realize such quantum electron systems, a key ingredient is dense electrons decoupled from other degrees of freedom. Here, we report the discovery of a pure quantum electron liquid, which spreads up to ~ 3 Å in the vacuum on the surface of electride crystal. An extremely high electron density and its weak hybridisation with buried atomic orbitals evidence the quantum and pure nature of electrons, that exhibit a polarized liquid phase as demonstrated by our spin-dependent measurement. Further, upon enhancing the electron correlation strength, the dynamics of quantum electrons changes to that of non-Fermi liquid along with an anomalous band deformation, suggestive of a transition to a hexatic liquid crystal phase. Our findings cultivate the frontier of quantum electron systems, and serve as a platform for exploring correlated electronic phases in a pure fashion. △ Less

Submitted 30 September, 2022; originally announced September 2022.

Comments: 29 pages, 4 figures, 10 extended data figures

Journal ref: Nature Material (2022)

Non-reciprocal energy transfer through the Casimir effect

Authors: Zhujing Xu, Xingyu Gao, Jaehoon Bang, Zubin Jacob, Tongcang Li

Abstract: A fundamental prediction of quantum mechanics is that there are random fluctuations everywhere in a vacuum because of the zero-point energy. Remarkably, quantum electromagnetic fluctuations can induce a measurable force between neutral objects, known as the Casimir effect, which has attracted broad interests. The Casimir effect can dominate the interaction between microstructures at small separati… ▽ More A fundamental prediction of quantum mechanics is that there are random fluctuations everywhere in a vacuum because of the zero-point energy. Remarkably, quantum electromagnetic fluctuations can induce a measurable force between neutral objects, known as the Casimir effect, which has attracted broad interests. The Casimir effect can dominate the interaction between microstructures at small separations and has been utilized to realize nonlinear oscillation, quantum trapping, phonon transfer, and dissipation dilution. However, a non-reciprocal device based on quantum vacuum fluctuations remains an unexplored frontier. Here we report quantum vacuum mediated non-reciprocal energy transfer between two micromechanical oscillators. We modulate the Casimir interaction parametrically to realize strong coupling between two oscillators with different resonant frequencies. We engineer the system's spectrum to have an exceptional point in the parameter space and observe the asymmetric topological structure near it. By dynamically changing the parameters near the exceptional point and utilizing the non-adiabaticity of the process, we achieve non-reciprocal energy transfer with high contrast. Our work represents an important development in utilizing quantum vacuum fluctuations to regulate energy transfer at the nanoscale and build functional Casimir devices. △ Less

Submitted 29 May, 2021; v1 submitted 25 February, 2021; originally announced February 2021.

Journal ref: Nature Nanotechnology 17, 148 (2022)

Phase Transition in a Memristive Suspended MoS2 Monolayer Probed by Opto- and Electro-Mechanics

Authors: Julien Chaste, Imen Hnid, Lama Khalil, Chen Si, Alan Durnez, Xavier Lafosse, Meng-Qiang Zhao, A. T. Charlie Johnson, Shengbai Zhang, Junhyeok Bang, Abdelkarim Ouerghi

Abstract: Semiconducting monolayer of 2D material are able to concatenate multiple interesting properties into a single component. Here, by combining opto-mechanical and electronic measurements, we demonstrate the presence of a partial 2H-1T phase transition in a suspended 2D monolayer membrane of MoS2. Electronic transport shows unexpected memristive properties in the MoS2 membrane, in the absence of any e… ▽ More Semiconducting monolayer of 2D material are able to concatenate multiple interesting properties into a single component. Here, by combining opto-mechanical and electronic measurements, we demonstrate the presence of a partial 2H-1T phase transition in a suspended 2D monolayer membrane of MoS2. Electronic transport shows unexpected memristive properties in the MoS2 membrane, in the absence of any external dopants. A strong mechanical softening of the membrane is measured concurrently and may only be related to the phase 2H-1T phase transition which imposes a 3percent directional elongation of the topological 1T phase with respect to the semiconducting 2H. We note that only a few percent 2H- 1T phase switching is sufficient to observe measurable memristive effects. Our experimental results combined with First-principles total energy calculations indicate that sulfur vacancy diffusion plays a key role in the initial nucleation of the phase transition. Our study clearly shows that nanomechanics represents an ultrasensitive technique to probe the crystal phase transition in 2D materials or thin membranes. Finally, a better control of the microscopic mechanisms responsible for the observed memristive effect in MoS2 is important for the implementation of future devices. △ Less

Submitted 23 November, 2020; originally announced November 2020.

Comments: ACS nano (2020)

Quantum dynamics of hydrogen in iron-based superconductor LaFeAsO0.9D0.1 measured with inelastic neutron spectroscopy

Authors: Jun-ichi Yamaura, Haruhiro Hiraka, Soshi Iimura, Yoshinori Muraba, Joonho Bang, Kazuhiko Ikeuchi, Mitsutaka Nakamura, Yasuhiro Inamura, Takashi Honda, Masatoshi Hiraishi, Kenji M. Kojima, Ryosuke Kadono, Yoshio Kuramoto, Youichi Murakami, Satoru Matsuishi, Hideo Hosono

Abstract: Inelastic neutron scattering was performed for an iron-based superconductor LaFeAsO0.9D0.1, where most of D (deuterium) replaces oxygen, while a tiny amount goes into interstitial sites. By first-principle calculation, we characterize the interstitial sites for D (and for H slightly mixed) with four equivalent potential minima. Below the superconducting transition temperature Tc = 26 K,new excitat… ▽ More Inelastic neutron scattering was performed for an iron-based superconductor LaFeAsO0.9D0.1, where most of D (deuterium) replaces oxygen, while a tiny amount goes into interstitial sites. By first-principle calculation, we characterize the interstitial sites for D (and for H slightly mixed) with four equivalent potential minima. Below the superconducting transition temperature Tc = 26 K,new excitations emerge in the range 5-15 meV, while they are absent in the reference system LaFeAsO0.9F0.1. The strong excitations at 14.5 meV and 11.1 meV broaden rapidly around 15 K and 20 K, respectively, where each energy becomes comparable to twice of the superconducting gap. The strong excitations are ascribed to a quantum rattling, or a band motion of hydrogen, which arises only if the number of potential minima is larger than two. △ Less

Submitted 25 June, 2019; v1 submitted 29 May, 2019; originally announced May 2019.

Comments: 6 pages, 4 figures

Journal ref: Phys. Rev. B 99, 220505(R) (2019)

Stable emission and fast optical modulation of quantum emitters in boron nitride nanotubes

Authors: Jonghoon Ahn, Zhujing Xu, Jaehoon Bang, Andres E. Llacsahuanga Allcca, Yong P. Chen, Tongcang Li

Abstract: Atom-like defects in two-dimensional (2D) hexagonal boron nitride (hBN) have recently emerged as a promising platform for quantum information science. Here we investigate single-photon emissions from atomic defects in boron nitride nanotubes (BNNTs). We demonstrate the first optical modulation of the quantum emission from BNNTs with a near-infrared laser. This one-dimensional system displays brigh… ▽ More Atom-like defects in two-dimensional (2D) hexagonal boron nitride (hBN) have recently emerged as a promising platform for quantum information science. Here we investigate single-photon emissions from atomic defects in boron nitride nanotubes (BNNTs). We demonstrate the first optical modulation of the quantum emission from BNNTs with a near-infrared laser. This one-dimensional system displays bright single-photon emission as well as high stability at room temperature and is an excellent candidate for optomechanics. The fast optical modulation of single-photon emission from BNNTs shows multiple electronic levels of the system and has potential applications in optical signal processing. △ Less

Submitted 15 June, 2018; originally announced June 2018.

Journal ref: Optics Letters. Vol. 43, Issue 15, pp. 3778-3781 (2018)

Experimental realization of Feynman's ratchet

Authors: Jaehoon Bang, Rui Pan, Thai M. Hoang, Jonghoon Ahn, Christopher Jarzynski, H. T. Quan, Tongcang Li

Abstract: Feynman's ratchet is a microscopic machine in contact with two heat reservoirs, at temperatures $T_A$ and $T_B$, that was proposed by Richard Feynman to illustrate the second law of thermodynamics. In equilibrium ($T_A=T_B$), thermal fluctuations prevent the ratchet from generating directed motion. When the ratchet is maintained away from equilibrium by a temperature difference ($T_A \ne T_B$), it… ▽ More Feynman's ratchet is a microscopic machine in contact with two heat reservoirs, at temperatures $T_A$ and $T_B$, that was proposed by Richard Feynman to illustrate the second law of thermodynamics. In equilibrium ($T_A=T_B$), thermal fluctuations prevent the ratchet from generating directed motion. When the ratchet is maintained away from equilibrium by a temperature difference ($T_A \ne T_B$), it can operate as a heat engine, rectifying thermal fluctuations to perform work. While it has attracted much interest, the operation of Feynman's ratchet as a heat engine has not been realized experimentally, due to technical challenges. In this work, we realize Feynman's ratchet with a colloidal particle in a one dimensional optical trap in contact with two heat reservoirs: one is the surrounding water, while the effect of the other reservoir is generated by a novel feedback mechanism, using the Metropolis algorithm to impose detailed balance. We verify that the system does not produce work when $T_A=T_B$, and that it becomes a microscopic heat engine when $T_A \ne T_B$. We analyze work, heat and entropy production as functions of the temperature difference and external load. Our experimental realization of Feynman's ratchet and the Metropolis algorithm can also be used to study the thermodynamics of feedback control and information processing, the working mechanism of molecular motors, and controllable particle transportation. △ Less

Submitted 14 November, 2017; originally announced November 2017.

Journal ref: New J. Phys. 20, 103032 (2018)

Experimental test of the differential fluctuation theorem and a generalized Jarzynski equality for arbitrary initial states

Authors: Thai M. Hoang, Rui Pan, Jonghoon Ahn, Jaehoon Bang, H. T. Quan, Tongcang Li

Abstract: Nonequilibrium processes of small systems such as molecular machines are ubiquitous in biology, chemistry and physics, but are often challenging to comprehend. In the past two decades, several exact thermodynamic relations of nonequilibrium processes, collectively known as fluctuation theorems, have been discovered and provided critical insights. These fluctuation theorems are generalizations of t… ▽ More Nonequilibrium processes of small systems such as molecular machines are ubiquitous in biology, chemistry and physics, but are often challenging to comprehend. In the past two decades, several exact thermodynamic relations of nonequilibrium processes, collectively known as fluctuation theorems, have been discovered and provided critical insights. These fluctuation theorems are generalizations of the second law, and can be unified by a differential fluctuation theorem. Here we perform the first experimental test of the differential fluctuation theorem, using an optically levitated nanosphere in both underdamped and overdamped regimes, and in both spatial and velocity spaces. We also test several theorems that can be obtained from it directly, including a generalized Jarzynski equality that is valid for arbitrary initial states, and the Hummer-Szabo relation. Our study experimentally verifies these fundamental theorems, and initiates the experimental study of stochastic energetics with the instantaneous velocity measurement. △ Less

Submitted 3 January, 2018; v1 submitted 29 June, 2017; originally announced June 2017.

Comments: accepted by Phys. Rev. Lett

Journal ref: Phys. Rev. Lett. 120, 080602 (2018)

Torsional optomechanics of a levitated nonspherical nanoparticle

Authors: Thai M. Hoang, Yue Ma, Jonghoon Ahn, Jaehoon Bang, F. Robicheaux, Zhang-Qi Yin, Tongcang Li

Abstract: An optically levitated nanoparticle in vacuum is a paradigm optomechanical system for sensing and studying macroscopic quantum mechanics. While its center-of-mass motion has been investigated intensively, its torsional vibration has only been studied theoretically in limited cases. Here we report the first experimental observation of the torsional vibration of an optically levitated nonspherical n… ▽ More An optically levitated nanoparticle in vacuum is a paradigm optomechanical system for sensing and studying macroscopic quantum mechanics. While its center-of-mass motion has been investigated intensively, its torsional vibration has only been studied theoretically in limited cases. Here we report the first experimental observation of the torsional vibration of an optically levitated nonspherical nanoparticle in vacuum. We achieve this by utilizing the coupling between the spin angular momentum of photons and the torsional vibration of a nonspherical nanoparticle whose polarizability is a tensor. The torsional vibration frequency can be one order of magnitude higher than its center-of-mass motion frequency, which is promising for ground state cooling. We propose a simple yet novel scheme to achieve ground state cooling of its torsional vibration with a linearly-polarized Gaussian cavity mode. A levitated nonspherical nanoparticle in vacuum will also be an ultrasensitive nanoscale torsion balance with a torque detection sensitivity on the order of $10^{-29} ~\mathrm{N}\cdot \mathrm{m}/\sqrt{\mathrm{ Hz}}$ under realistic conditions. △ Less

Submitted 13 August, 2016; v1 submitted 12 May, 2016; originally announced May 2016.

Journal ref: Phys. Rev. Lett. 117, 123604 (2016)

Carrier Multiplication-Induced Structural Change during Ultrafast Carrier Relaxation and Non-Thermal Phase Transition in Semiconductors

Authors: Junhyeok Bang, Y. Y. Sun, X. -Q. Liu, F. Gao, S. B. Zhang

Abstract: While being extensively studied as an important physical process to alter exciton population in nanostructures at fs time scale, carrier multiplication has not been considered seriously as a major mechanism for phase transition. Real-time time-dependent density functional theory study of Ge2Sb2Te5 reveals that carrier multiplication can induce ultrafast phase transition in solid state despite that… ▽ More While being extensively studied as an important physical process to alter exciton population in nanostructures at fs time scale, carrier multiplication has not been considered seriously as a major mechanism for phase transition. Real-time time-dependent density functional theory study of Ge2Sb2Te5 reveals that carrier multiplication can induce ultrafast phase transition in solid state despite that the lattice remains cold. The results also unify the experimental findings in other semiconductors for which the explanation remains to be the 30-year old phenomenological plasma annealing model. △ Less

Submitted 17 September, 2016; v1 submitted 17 January, 2016; originally announced January 2016.

Comments: 19 pages, 4 figures

Journal ref: Phys. Rev. Lett. 117, 126402 (2016)

Electron spin control of optically levitated nanodiamonds in vacuum

Authors: Thai M. Hoang, Jonghoon Ahn, Jaehoon Bang, Tongcang Li

Abstract: Electron spins of diamond nitrogen-vacancy (NV) centers are important quantum resources for nanoscale sensing and quantum information. Combining NV spins with levitated optomechanical resonators will provide a hybrid quantum system for novel applications. Here we optically levitate a nanodiamond and demonstrate electron spin control of its built-in NV centers in low vacuum. We observe that the str… ▽ More Electron spins of diamond nitrogen-vacancy (NV) centers are important quantum resources for nanoscale sensing and quantum information. Combining NV spins with levitated optomechanical resonators will provide a hybrid quantum system for novel applications. Here we optically levitate a nanodiamond and demonstrate electron spin control of its built-in NV centers in low vacuum. We observe that the strength of electron spin resonance (ESR) is enhanced when the air pressure is reduced. To better understand this system, we investigate the effects of trap power and measure the absolute internal temperature of levitated nanodiamonds with ESR after calibration of the strain effect. We also observe that oxygen and helium gases have different effects on both the photoluminescence and the ESR contrast of nanodiamond NV centers, indicating potential applications of NV centers in oxygen gas sensing. Our results pave the way towards a levitated spin-optomechanical system for studying macroscopic quantum mechanics. △ Less

Submitted 19 May, 2016; v1 submitted 22 October, 2015; originally announced October 2015.

Journal ref: Nature Communications 7, 12250 (2016)

Hydrogen Ordering and New Polymorph of Layered Perovskite Oxyhydrides: Sr2VO4-xHx

Authors: Joonho Bang, Satoru Matsuishi, Haruhiro Hiraka, Fumika Fujisaki, Toshiya Otomo, Sachiko Maki, Jun-ichi Yamaura, Reiji Kumai, Youichi Murakami, Hideo Hosono

Abstract: Compositionally tunable vanadium oxyhydrides Sr2VO4-xHx (x = 0 - 1) without considerable anion vacancy were synthesized by high-pressure solid state reaction. The crystal structures and their properties were characterized by powder neutron diffraction, synchrotron X-ray diffraction, thermal desorption spectroscopy, and first-principles density functional theory (DFT) calculations. The hydrogen ani… ▽ More Compositionally tunable vanadium oxyhydrides Sr2VO4-xHx (x = 0 - 1) without considerable anion vacancy were synthesized by high-pressure solid state reaction. The crystal structures and their properties were characterized by powder neutron diffraction, synchrotron X-ray diffraction, thermal desorption spectroscopy, and first-principles density functional theory (DFT) calculations. The hydrogen anions selectively replaced equatorial oxygen sites in the VO6 layers via statistical substitution of hydrogen in the low x region (x < 0.2). A new orthorhombic phase (Immm) with an almost entirely hydrogen-ordered structure formed from the K2NiF4-type tetragonal phase with x > 0.7. Based on the DFT calculations, the degree of oxygen/hydrogen anion ordering is strongly correlated with the bonding interaction between vanadium and the ligands. △ Less

Submitted 12 May, 2014; originally announced May 2014.

Comments: 19 pages, 5 figures

Localization and One-Parameter Scaling in Hydrogenated Graphene

Authors: Junhyeok Bang, K. J. Chang

Abstract: We report a metal-insulator transition in disordered graphene with low coverages of hydrogen atoms. Hydrogen interacting with graphene creates short-range disorder and localizes states near the neutrality point. The energy range of localization grows with increasing of H concentration. Calculations show that the conductances through low-energy propagating channels decay exponentially with sample s… ▽ More We report a metal-insulator transition in disordered graphene with low coverages of hydrogen atoms. Hydrogen interacting with graphene creates short-range disorder and localizes states near the neutrality point. The energy range of localization grows with increasing of H concentration. Calculations show that the conductances through low-energy propagating channels decay exponentially with sample size and are well fitted by one-parameter scaling function, similar to a disorder-driven metal-insulator transition in 2-dimensional disordered systems. △ Less

Submitted 25 June, 2010; v1 submitted 10 March, 2010; originally announced March 2010.

Comments: 5 pages, 4 figures

Journal ref: Physical Review B 81, 193412 (2010) (http://link.aps.org/doi/10.1103/PhysRevB.81.193412)