Showing 1–50 of 144 results for author: Lee, J S

Nonequilibrium Fluctuation-Response Theory in the Frequency Domain

Authors: Euijoon Kwon, Hyun-Myung Chun, Hyunggyu Park, Jae Sung Lee

Abstract: We establish a unified fluctuation-response theory in the frequency domain for nonequilibrium steady states governed by overdamped Langevin dynamics and Markov jump processes. The central identity is an exact fluctuation-response relation that reconstructs the power spectrum of general observables from local responses measured at the same frequency. This relation applies to state-dependent observa… ▽ More We establish a unified fluctuation-response theory in the frequency domain for nonequilibrium steady states governed by overdamped Langevin dynamics and Markov jump processes. The central identity is an exact fluctuation-response relation that reconstructs the power spectrum of general observables from local responses measured at the same frequency. This relation applies to state-dependent observables, current-like observables, and their combinations, and reveals how local dynamics shape global fluctuations. From this identity, we derive a hierarchy of relations in the frequency domain, including response uncertainty relations, kinetic and thermodynamic uncertainty relations, the equilibrium fluctuation-dissipation theorem, and Harada-Sasa relations. This establishes an overarching framework for fluctuation-response theory in the frequency domain. Applications to stochastic networks and driven diffusive systems demonstrate how the theory decomposes fluctuation spectra into local contributions and reveals frequency-dependent tradeoffs between fluctuations, response, and dissipation. △ Less

Submitted 13 August, 2026; v1 submitted 6 May, 2026; originally announced May 2026.

Comments: 16 pages and 3 figures for main text, 14 pages for appendix

Fluctuation-Response Theory for Nonequilibrium Langevin Dynamics

Authors: Hyun-Myung Chun, Euijoon Kwon, Hyunggyu Park, Jae Sung Lee

Abstract: We establish a unified fluctuation-response relation for Langevin dynamics. By exploiting the common mathematical structures underlying fluctuations and responses of empirical density and current, we derive a unified identity that generalizes the fluctuation-dissipation theorem from equilibrium to nonequilibrium settings. This relation connects global fluctuations of observables with their local r… ▽ More We establish a unified fluctuation-response relation for Langevin dynamics. By exploiting the common mathematical structures underlying fluctuations and responses of empirical density and current, we derive a unified identity that generalizes the fluctuation-dissipation theorem from equilibrium to nonequilibrium settings. This relation connects global fluctuations of observables with their local responses to perturbations in force, mobility, and temperature. We further derive finite-time fluctuation-response inequalities, leading to response uncertainty relations that complement the identity by providing more practical bounds. These derivations establish a unified theoretical framework linking the fluctuation-dissipation theorem and thermodynamic uncertainty relations. Using the $F_1$-ATPase molecular motor model, we illustrate how these response-based bounds constrain the long-time diffusion coefficient. △ Less

Submitted 22 January, 2026; originally announced January 2026.

Comments: 8 pages and 2 figures for main text, 14 pages for supplemental material

Coherence enhanced by detrained oscillators: Breaking $π$-reflection symmetry

Authors: Hyunsuk Hong, Jae Sung Lee, Hyunggyu Park

Abstract: We study a generalized Kuramoto model in which each oscillator carries two coupled phase variables, representing a minimal swarmalator system. Assuming perfect correlation between the intrinsic frequencies associated with each phase variable, we identify a novel dynamic mode characterized by bounded oscillatory motion that breaks the $π$-reflection symmetry. This symmetry breaking enhances global… ▽ More We study a generalized Kuramoto model in which each oscillator carries two coupled phase variables, representing a minimal swarmalator system. Assuming perfect correlation between the intrinsic frequencies associated with each phase variable, we identify a novel dynamic mode characterized by bounded oscillatory motion that breaks the $π$-reflection symmetry. This symmetry breaking enhances global coherence and gives rise to a non-trivial mixed state, marked by distinct degrees of ordering in each variable. Numerical simulations confirm our analytic predictions for the full phase diagram, including the nature of transition. Our results reveal a fundamental mechanism through which detrained (dynamic) oscillators can promote global synchronization, offering broad insights into coupled dynamical systems beyond the classical Kuramoto paradigm. △ Less

Submitted 11 November, 2025; originally announced November 2025.

Comments: 15 pages, 5 figures. To appear in Chaos

Strain Engineering of van Hove Singularity and Coupled Itinerant Ferromagnetism in Quasi-2D Oxide Superlattices

Authors: Seung Gyo Jeong, Minjae Kim, Jin Young Oh, Youngeun Ham, In Hyeok Choi, Seong Won Cho, Jihyun Kim, Huimin Jeong, Byungmin Sohn, Tuson Park, Suyoun Lee, Jong Seok Lee, Deok-Yong Cho, Bongjae Kim, Woo Seok Choi

Abstract: Engineering van Hove singularities (vHss) near the Fermi level, if feasible, offers a powerful route to control exotic quantum phases in electronic and magnetic behaviors. However, conventional approaches, which rely primarily on chemical and electrical doping, focus mainly on local electrical or optical measurements, limiting their applicability to coupled functionalities. In this study, a vHs-in… ▽ More Engineering van Hove singularities (vHss) near the Fermi level, if feasible, offers a powerful route to control exotic quantum phases in electronic and magnetic behaviors. However, conventional approaches, which rely primarily on chemical and electrical doping, focus mainly on local electrical or optical measurements, limiting their applicability to coupled functionalities. In this study, a vHs-induced insulator-metal transition coupled with a ferromagnetic phase transition was empirically achieved in atomically designed quasi-2D SrRuO3 (SRO) superlattices via epitaxial strain engineering, which has not been observed in conventional 3D SRO systems. Theoretical calculations revealed that epitaxial strain effectively modulates the strength and energy positions of vHs of specific Ru orbitals, driving correlated phase transitions in the electronic and magnetic ground states. X-ray absorption spectroscopy confirmed the anisotropic electronic structure of quasi-2D SRO modulated by epitaxial strain. Magneto-optic Kerr effect and electrical transport measurements demonstrated modulated magnetic and electronic phases. Furthermore, magneto-electrical measurements detected significant anomalous Hall effect signals and ferromagnetic magnetoresistance, indicating the presence of magnetically coupled charge carriers in the 2D metallic regime. This study establishes strain engineering as a promising platform for tuning vHss and resultant itinerant ferromagnetism of low-dimensional correlated quantum systems. △ Less

Submitted 28 October, 2025; originally announced October 2025.

Comments: 29 pages, 3 figures

Journal ref: published 2025

Demagnetization-Driven Nanoscale Chirality-Selective Thermal Switch

Authors: In Hyeok Choi, Daeheon Kim, Yeon Jong Jin, Seungmo Yang, Tae-Seong Ju, Changsoo Kim, Chanyong Hwang, Dongbin Shin, Jong Seok Lee

Abstract: Chiral-lattice degrees of freedom can offer novel chirality-selective functionalities for thermotronic applications. Chiral phonons, carrying both heat and angular momentum, can emerge through a breaking of chiral degeneracy in the phonon bands, either via an intrinsic chiral crystal structure or by angular momentum transfer from photons or spins. This chiral controllability of the lattice dynamic… ▽ More Chiral-lattice degrees of freedom can offer novel chirality-selective functionalities for thermotronic applications. Chiral phonons, carrying both heat and angular momentum, can emerge through a breaking of chiral degeneracy in the phonon bands, either via an intrinsic chiral crystal structure or by angular momentum transfer from photons or spins. This chiral controllability of the lattice dynamics enables a design of chiral thermo-devices by integrating ferromagnets with chiral materials. Here, we present a nanoscale chirality-selective thermal switch realized using a simple heterostructure composed of ferromagnetic [Co/Pt] multilayers and insulating chiral $α$-SiO2, where an external magnetic field can control thermal transport properties. Our experimental results based on the magneto-optic thermometry reveal that the thermal conductivity of $α$-SiO2 exhibits a clear dependence on both the magnetization direction of [Co/Pt] multilayers and the structural chirality of $α$-SiO2, which is supported well by the first-principles-based molecular dynamic simulations. The magnetization-dependent thermal on/off ratio amounts to 1.07 at room temperature and increases to about 1.2 as temperature decreases to 50 K, due to a reduction of Umklapp phonon-phonon scattering rate in $α$-SiO2. These findings provide the first experimental demonstration of the nanoscale chirality-selective thermal switch based on the ferromagnetic/chiral material heterostructure, highlighting its potential as a key technology for addressing heat dissipation challenges in nanoscale electronic devices. △ Less

Submitted 28 September, 2025; originally announced September 2025.

Strain-induced Dynamic Spin-Phonon Coupling in Epitaxial RuO2 Films

Authors: In Hyeok Choi, Seung Gyo Jeong2, Jae Hyuck Lee, San Kang, Sreejith Nair, Changyoung Kim, Dirk Wulferding, Bharat Jalan, Jong Seok Lee

Abstract: Magnetic order parameters in altermagnets can couple to quantized lattice vibration via both piezomagnetic and magnetoelastic effects, leading to the renormalization of phonon dispersion. Here, we demonstrate photo-induced dynamic frequency modulation of THz phonons excited in anisotropically-strained epitaxial RuO2 thin films using ultrafast coherent phonon spectroscopy and time-resolved magneto-… ▽ More Magnetic order parameters in altermagnets can couple to quantized lattice vibration via both piezomagnetic and magnetoelastic effects, leading to the renormalization of phonon dispersion. Here, we demonstrate photo-induced dynamic frequency modulation of THz phonons excited in anisotropically-strained epitaxial RuO2 thin films using ultrafast coherent phonon spectroscopy and time-resolved magneto-optic Kerr effect measurement. A coherent oscillation of a transverse acoustic phonon appears in the sub-THz range with increasing film thickness above 4 nm due to local dislocation arising from the anisotropic strain relaxation, which hosts large non-zero shear strain. Interestingly, this phonon mode exhibits a time-varying mode hardening below ~ 500 K. Furthermore, an optical phonon oscillation emerges in magnetization dynamics of the photo-induced non-equilibrium state, and it becomes significantly softened near the critical temperature, while there is no observable magneto-optic signal in fully-strain-relaxed films. Such notable dynamic frequency modulations in acoustic and optical phonons offer an opportunity to manipulate phonons in the THz range through the spin-phonon coupling controlled by epitaxial design, which can inspire the new class of altermagnetic applications in the ultrafast quantum opto-spintronics. △ Less

Submitted 28 September, 2025; originally announced September 2025.

Observation of mirror-odd and mirror-even spin texture in ultrathin epitaxially strained RuO2 films

Authors: Yichen Zhang, Seung Gyo Jeong, Luca Buiarelli, Seungjun Lee, Yucheng Guo, Jiaqin Wen, Hang Li, Sreejith Nair, In Hyeok Choi, Zheng Ren, Ziqin Yue, Jounghoon Hyun, Tieqiong Zhang, Alexei Fedorov, Sung-Kwan Mo, Hojoon Lim, Adrian Hunt, Iradwikanari Waluyo, Junichiro Kono, Jan Minar, Jong Seok Lee, Tony Low, Turan Birol, Rafael M. Fernandes, Milan Radovic , et al. (2 additional authors not shown)

Abstract: Recently, rutile ruthenium dioxide (RuO$_2$) has attracted renewed interest due to expectations of prominent altermagnetic spin splitting. However, accumulating experimental evidence suggests that, in its bulk and thick-film forms, RuO$_2$ does not display any form of magnetic ordering. Despite this, the spin structure of RuO$_2$ remains largely unexplored in the ultrathin limit, where substrate-i… ▽ More Recently, rutile ruthenium dioxide (RuO$_2$) has attracted renewed interest due to expectations of prominent altermagnetic spin splitting. However, accumulating experimental evidence suggests that, in its bulk and thick-film forms, RuO$_2$ does not display any form of magnetic ordering. Despite this, the spin structure of RuO$_2$ remains largely unexplored in the ultrathin limit, where substrate-imposed epitaxial strain can be substantial. Here, we use spin-resolved angle-resolved photoemission spectroscopy, supported by ab initio calculations, to reveal the electronic structure of 2-nanometer-thick epitaxial RuO$_2$ heterostructures. We observe an unconventional spin texture characterized by the coexistence of mirror-even and mirror-odd momentum-dependent components. A comprehensive symmetry analysis rules out nonmagnetic origins of this spin texture. These findings suggest an emergent nonrelativistic spin structure enabled by epitaxial strain in the ultrathin limit, marking a distinct departure from the behavior of relaxed or bulk RuO$_2$. Our work opens previously unexplored perspectives for exploring symmetry-breaking mechanisms and spin textures in oxide heterostructures. △ Less

Submitted 1 August, 2026; v1 submitted 19 September, 2025; originally announced September 2025.

Comments: 71 pages. 5 main figures, 2 main tables, 19 supplementary figures, 1 supplementary table

Journal ref: Sci. Adv. 12, eaec2917 (2026)

Violation of kinetic uncertainty relation in maser heat engines: Role of spontaneous emission

Authors: Varinder Singh, Euijoon Kwon, Jae Sung Lee

Abstract: We investigate the kinetic uncertainty relation (KUR)-a fundamental trade-off between dynamical activity and current fluctuations-in two configurations of a maser heat engine. We find that KUR violations arise only in one model. This asymmetry originates from spontaneous emission, which breaks the structural symmetry between the configurations and modifies their coherence dynamics. While we analyz… ▽ More We investigate the kinetic uncertainty relation (KUR)-a fundamental trade-off between dynamical activity and current fluctuations-in two configurations of a maser heat engine. We find that KUR violations arise only in one model. This asymmetry originates from spontaneous emission, which breaks the structural symmetry between the configurations and modifies their coherence dynamics. While we analyze several contributing factors-including statistical signatures such as the Fano factor and the ratio of dynamical activity to current-our results show that the decisive mechanism is the slower decoherence in one configuration, which enables quantum violations of the classical steady-state KUR bound. By contrast, the faster coherence decay in the other configuration suppresses such violations, driving it closer to classical behavior. These findings highlight the critical role of decoherence mechanisms in determining fundamental thermodynamic bounds and provide insights for the design of quantum heat engines in which the control of decoherence is central to suppressing fluctuations and enhancing reliable performance. △ Less

Submitted 10 September, 2025; v1 submitted 25 August, 2025; originally announced August 2025.

Memory-aware feedback enhances power in active information engines

Authors: Sehoon Bahng, Jae Sung Lee, Cheol-Min Ghim

Abstract: We study an information engine operating in an active bath, where a Brownian particle confined in a harmonic trap undergoes feedback-driven displacement cycles. Unlike thermal environments, active baths exhibit temporally correlated fluctuations, introducing memory effects that challenge conventional feedback strategies. Extending the framework of stochastic thermodynamics to account for such memo… ▽ More We study an information engine operating in an active bath, where a Brownian particle confined in a harmonic trap undergoes feedback-driven displacement cycles. Unlike thermal environments, active baths exhibit temporally correlated fluctuations, introducing memory effects that challenge conventional feedback strategies. Extending the framework of stochastic thermodynamics to account for such memory, we analyze a feedback protocol that periodically shifts the potential minimum based on noisy measurements of the particle's position. We show that conventional feedback schemes, optimized for memoryless thermal baths, can degrade performance in active media due to the disruption of bath-particle memory by abrupt resetting. To overcome this degradation, we introduce a class of memory-preserving feedback protocols that partially retain the covariance between the particle's displacement and active noise, thereby exploiting the temporal persistence of active fluctuations. Through asymptotic analysis, we show how the feedback gain -- which quantifies the strength of positional shifts -- nontrivially shapes the engine's work and power profiles. In particular, we demonstrate that in active media, intermediate gains outperform full-shift resetting. Our results reveal the critical interplay between bath memory, measurement noise, and feedback gain, offering guiding principles for designing high-performance information engines in nonequilibrium environments. △ Less

Submitted 26 November, 2025; v1 submitted 22 August, 2025; originally announced August 2025.

Journal ref: Phys. Rev. E 112, 054134 (2025)

Momentum-Resolved Relaxation-Time Approach for Size-Dependent Conductivity in Anisotropic Metallic Films

Authors: YoungJun Lee, Jin Soo Lee, Seungjun Lee, Seoung-Hun Kang, Young-Kyun Kwon

Abstract: Shrinking CMOS interconnect dimensions to the nanometer scale intensifies electron scattering at surfaces, interfaces, and grain boundaries, causing severe conductivity loss and challenging copper-based designs. Here we present a momentum-resolved relaxation time framework that integrates density functional theory with the semiclassical Boltzmann transport equation to predict size-dependent resist… ▽ More Shrinking CMOS interconnect dimensions to the nanometer scale intensifies electron scattering at surfaces, interfaces, and grain boundaries, causing severe conductivity loss and challenging copper-based designs. Here we present a momentum-resolved relaxation time framework that integrates density functional theory with the semiclassical Boltzmann transport equation to predict size-dependent resistivity in metallic thin films. Electron phonon interactions are computed from first principles, and anisotropic surface and grain boundary scattering is captured through a momentum dependent mean free path, allowing relaxation times to vary spatially and directionally without empirical fitting. Applied to isotropic (Cu, Ag, Au) and anisotropic (W, Ti$_2$GeC) metals, the model achieves excellent agreement with experiments and uncovers the critical role of crystallographic anisotropy in transport. We further identify layered MAX phase compounds as promising ultrathin interconnects. This work provides a predictive, physically rigorous, and computationally efficient route to designing high-performance conductors for next generation nanoelectronics. △ Less

Submitted 12 August, 2025; originally announced August 2025.

Quantum confinement effect in Sb thin films

Authors: Anuradha Wijesinghe, Yongxi Ou, Anjali Rathore, Chandima Edirisinghe, Pradip Adhikari, An-Hsi Chen, Dustin Gilbert, Anthony Richardella, Nitin Samarth, Joon Sue Lee

Abstract: Antimony (Sb), an element with strong spin-orbit coupling, is predicted to undergo a topological phase transition from a topological semimetal to a topological insulator as its dimensionality approaches the two-dimensional limit, driven by the quantum confinement effect. In this study, we investigate this transition in Sb thin films grown by molecular beam epitaxy, employing electrical transport m… ▽ More Antimony (Sb), an element with strong spin-orbit coupling, is predicted to undergo a topological phase transition from a topological semimetal to a topological insulator as its dimensionality approaches the two-dimensional limit, driven by the quantum confinement effect. In this study, we investigate this transition in Sb thin films grown by molecular beam epitaxy, employing electrical transport measurements and angle-resolved photoemission spectroscopy (ARPES). Electrical transport measurements revealed signatures of a modified electronic band structure, including a Hall response with multiple carrier types, a decreasing carrier concentration, and a transition in the curvature of the longitudinal resistance from quadratic to linear with decreasing film thickness. Temperature-dependent magnetoresistance further showed weak antilocalization below 16 K, indicating strong spin-orbit coupling and suggesting the presence of non-trivial topological states. Analysis of the WAL characteristics revealed a single coherent conducting channel and a thickness-dependent change in the phase decoherence mechanism. Complementary ARPES measurements confirmed that reducing the film thickness lifts the conduction band at the M-point, consistent with the emergence of a band gap. These findings support theoretical predictions of a thickness-dependent band structure evolution driven by the quantum confinement effect, providing a foundation for further exploration of topological phase transitions in Sb as well as Bi1-xSbx. The realization of an elemental topological material with simplified stoichiometry and semiconductor compatibility presents a promising avenue for next-generation hybrid systems and applications in spintronics and quantum technologies. △ Less

Submitted 30 July, 2025; originally announced July 2025.

Comments: 19 pages, 10 figures

Strain-Stabilized Interfacial Polarization Tunes Work Function Over 1 eV in RuO2/TiO2 Heterostructures

Authors: Seung Gyo Jeong, Bonnie Y. X. Lin, Mengru Jin, In Hyeok Choi, Seungjun Lee, Zhifei Yang, Sreejith Nair, Rashmi Choudhary, Juhi Parikh, Anand Santhosh, Matthew Neurock, Kelsey A. Stoerzinger, Jong Seok Lee, Tony Low, Qing Tu, James M. LeBeau, Bharat Jalan

Abstract: Interfacial polarization-charge accumulation at the heterointerface-is a well-established tool in semiconductors, but its influence in metals remains unexplored. Here, we demonstrate that interfacial polarization can robustly modulate surface work function in metallic rutile RuO2 layers in epitaxial RuO2/TiO2 heterostructures grown by hybrid molecular beam epitaxy. Using multislice electron ptycho… ▽ More Interfacial polarization-charge accumulation at the heterointerface-is a well-established tool in semiconductors, but its influence in metals remains unexplored. Here, we demonstrate that interfacial polarization can robustly modulate surface work function in metallic rutile RuO2 layers in epitaxial RuO2/TiO2 heterostructures grown by hybrid molecular beam epitaxy. Using multislice electron ptychography, we directly visualize polar displacements of transition metal ions relative to oxygen octahedra near the interface, despite the conductive nature of RuO2. This interfacial polarization enables over 1 eV modulation of the RuO2 work function, controlled by small thickness variation (2-4 nm) as measured by Kelvin probe probe microscopy, with a critical thickness of 4 nm - corresponding to the transition from fully strained to relaxed film. These results establish interfacial polarization as a powerful route to control electronic properties in metals and have implications for designing tunable electronic, catalytic, and quantum devices through interfacial control in polar metallic systems. △ Less

Submitted 10 July, 2025; originally announced July 2025.

Comments: 25 pages, 4 figures

Topological phase transition to a hidden charge density wave liquid

Authors: Joshua S. H. Lee, Thomas M. Sutter, Goran Karapetrov, Pietro Musumeci, Anshul Kogar

Abstract: Charge density waves (CDWs), electronic crystals that form within a host solid, have long been speculated to melt into a spatially textured electronic liquid. Though they have not been previously detected, liquid CDWs may nonetheless be fundamental to the phase diagrams of many correlated electron systems, including high temperature superconductors and quantum Hall states. In one of the most promi… ▽ More Charge density waves (CDWs), electronic crystals that form within a host solid, have long been speculated to melt into a spatially textured electronic liquid. Though they have not been previously detected, liquid CDWs may nonetheless be fundamental to the phase diagrams of many correlated electron systems, including high temperature superconductors and quantum Hall states. In one of the most promising candidate materials capable of hosting a liquid CDW, 1T-TaS2, a structural phase transition impedes its observation. Here, by irradiating the material with a femtosecond light pulse, we circumvent the structural phase transition to reveal how topological defect dynamics govern the otherwise invisible CDW correlations. Upon photoexcitation, the CDW diffraction peaks broaden azimuthally, initially revealing a hexatic state. At higher temperatures, photoexcitation completely destroys translational and orientational order and only a ring of diffuse scattering is observed, a key signature of a liquid CDW. Our work provides compelling evidence for a defect-unbinding transition to a CDW liquid and presents a protocol for uncovering states that are hidden by other transitions in thermal equilibrium. △ Less

Submitted 7 May, 2025; originally announced May 2025.

Thermodynamic anomalies in overdamped systems with time-dependent temperature

Authors: Shakul Awasthi, Hyunggyu Park, Jae Sung Lee

Abstract: One of the key objectives in investigating small stochastic systems is the development of micrometer-sized engines and the understanding of their thermodynamics. However, the primary mathematical tool used for this purpose, the overdamped approximation, has a critical limitation: it fails to fully capture the thermodynamics when the temperature varies over time, as the velocity is not considered i… ▽ More One of the key objectives in investigating small stochastic systems is the development of micrometer-sized engines and the understanding of their thermodynamics. However, the primary mathematical tool used for this purpose, the overdamped approximation, has a critical limitation: it fails to fully capture the thermodynamics when the temperature varies over time, as the velocity is not considered in the approximation. Specifically, we show that heat dissipation and entropy production calculated under the overdamped approximation deviate from their true values. These discrepancies are termed thermodynamic anomalies. To overcome this limitation, we analytically derive expressions for these anomalies in the presence of a general time-varying temperature. One notable feature of the result is that high viscosity and small mass, though both leading to the same overdamped dynamic equations, result in different thermodynamic anomaly relations. Our results have significant implications, particularly for accurately calculating the efficiency of heat engines operating in overdamped environments with time-varying temperatures, without requiring velocity measurements. Additionally, our findings offer a simple method for estimating the kinetic energy of an overdamped system. △ Less

Submitted 28 March, 2025; originally announced March 2025.

Comments: 21 pages, 5 figures

Evolution of surface morphology from Stranski Krastanov growth mode to step flow growth mode in InSbBi thin films

Authors: Chandima Kasun Edirisinghe, Anuradha Wijesinghe, Anjali Rathore, Pradip Adhikari, Christopher Rouleau, Joon Sue Lee

Abstract: The incorporation of dilute concentrations of bismuth (Bi) into traditional III V alloys leads to significant reduction in bandgap energy, making InSbBi is a promising candidate for long wavelength infrared photodetection sensors due to its small bandgap (<0.17 eV). Furthermore, InSbBi could serve as a valuable platform for spin dynamics and quantum phenomena due to its strong spin-orbit coupling.… ▽ More The incorporation of dilute concentrations of bismuth (Bi) into traditional III V alloys leads to significant reduction in bandgap energy, making InSbBi is a promising candidate for long wavelength infrared photodetection sensors due to its small bandgap (<0.17 eV). Furthermore, InSbBi could serve as a valuable platform for spin dynamics and quantum phenomena due to its strong spin-orbit coupling. Despite its potential, the material quality of InSbBi alloys lags behind that of conventional III V semiconductors, primarily due to the substantial challenges associated with incorporating Bi into InSb and producing high-quality InSbBi with varying Bi compositions. In this study, we address these issues by developing a method for growing smooth InSbBi thin films with tunable Bi incorporation up to 1.81% by the dynamic adjustment of Sb flux and careful control of the interplay between growth temperature and Bi flux using molecular beam epitaxy. This work paves the path for high-quality InSbBi thin films for applications in photodetection, spintronics, and quantum technology. △ Less

Submitted 9 May, 2025; v1 submitted 14 March, 2025; originally announced March 2025.

Comments: 21 pages, 6 figures

Journal ref: J. Vac. Sci. Technol. A 43, 052701 (2025)

Enhancing Electrical Properties of Selectively Grown In-Plane InAs Nanowires using InGaAs Buffer and Capping Layers

Authors: Pradip Adhikari, Anjali Rathore, Dayrl P Briggs, Srijanto R Bernadeta, Joon Sue Lee

Abstract: In-plane semiconductor nanowires with complex branched geometries, prepared via selective area growth (SAG), offer a versatile platform for advanced electronics, optoelectronics, and quantum devices. However, defects and disorder at the interfaces and top surfaces of the nanowires can significantly degrade their electrical properties. One effective method to mitigate these issues is the incorporat… ▽ More In-plane semiconductor nanowires with complex branched geometries, prepared via selective area growth (SAG), offer a versatile platform for advanced electronics, optoelectronics, and quantum devices. However, defects and disorder at the interfaces and top surfaces of the nanowires can significantly degrade their electrical properties. One effective method to mitigate these issues is the incorporation of buffer and capping layers. In this work, we achieved a wider growth selectivity window of InGaAs in the presence of atomic hydrogen (H) and employed it as buffer and capping layers for SAG InAs nanowires to enhance their electrical properties. Hall measurements on InAs nanowires, with and without InGaAs buffer and/or capping layers, revealed that incorporating closely lattice-matched InGaAs buffer and capping layers to InAs nanowires nearly tripled the electron mobility and doubled the phase coherence length compared to nanowires without these layers. These findings demonstrate that the use of InGaAs buffer and capping layers is a crucial strategy for significantly enhancing the quality of InAs nanowires, unlocking their full potential for high performance electronics and quantum devices. △ Less

Submitted 9 May, 2025; v1 submitted 18 February, 2025; originally announced February 2025.

Anisotropic Strain Relaxation-Induced Directional Ultrafast Carrier Dynamics in RuO2 Films

Authors: S. G. Jeong, I. H. Choi, S. Lee, J. Y. Oh, S. Nair, J. H. Lee, C. Kim, A. Seo, W. S. Choi, T. Low, J. S. Lee, B. Jalan

Abstract: Ultrafast light-matter interactions inspire potential functionalities in picosecond optoelectronic applications. However, achieving directional carrier dynamics in metals remains challenging due to strong carrier scattering within a multiband environment, typically expected to isotropic carrier relaxation. In this study, we demonstrate epitaxial RuO2/TiO2 (110) heterostructures grown by hybrid mol… ▽ More Ultrafast light-matter interactions inspire potential functionalities in picosecond optoelectronic applications. However, achieving directional carrier dynamics in metals remains challenging due to strong carrier scattering within a multiband environment, typically expected to isotropic carrier relaxation. In this study, we demonstrate epitaxial RuO2/TiO2 (110) heterostructures grown by hybrid molecular beam epitaxy to engineer polarization-selectivity of ultrafast light-matter interactions via anisotropic strain engineering. Combining spectroscopic ellipsometry, X-ray absorption spectroscopy, and optical pump-probe spectroscopy, we revealed the strong anisotropic transient optoelectronic response of strain-engineered RuO2/TiO2 (110) heterostructures along both in-plane [001] and [1-10] crystallographic directions. Theoretical analysis identifies strain-induced modifications in band nesting as the underlying mechanism for enhanced anisotropic carrier relaxation. These findings establish epitaxial strain engineering as a powerful tool for tuning anisotropic optoelectronic responses in metallic systems, paving the way for next-generation polarization-sensitive ultrafast optoelectronic devices. △ Less

Submitted 12 February, 2025; originally announced February 2025.

Comments: 18 pages

Metallicity and Anomalous Hall Effect in Epitaxially-Strained, Atomically-thin RuO2 Films

Authors: Seung Gyo Jeong, Seungjun Lee, Bonnie Lin, Zhifei Yang, In Hyeok Choi, Jin Young Oh, Sehwan Song, Seung wook Lee, Sreejith Nair, Rashmi Choudhary, Juhi Parikh, Sungkyun Park, Woo Seok Choi, Jong Seok Lee, James M. LeBeau, Tony Low, Bharat Jalan

Abstract: The anomalous Hall effect (AHE), a hallmark of time-reversal symmetry breaking, has been reported in rutile RuO2, a debated metallic altermagnetic candidate. Previously, AHE in RuO2 was observed only in strain-relaxed thick films under extremely high magnetic fields (~50 T). Yet, in ultrathin strained films with distinctive anisotropic electronic structures, there are no reports, likely due to dis… ▽ More The anomalous Hall effect (AHE), a hallmark of time-reversal symmetry breaking, has been reported in rutile RuO2, a debated metallic altermagnetic candidate. Previously, AHE in RuO2 was observed only in strain-relaxed thick films under extremely high magnetic fields (~50 T). Yet, in ultrathin strained films with distinctive anisotropic electronic structures, there are no reports, likely due to disorder and defects suppressing metallicity thus hindering its detection. Here, we demonstrate that ultrathin, fully-strained 2 nm TiO2/t nm RuO2/TiO2 (110) heterostructures, grown by hybrid molecular beam epitaxy, retain metallicity and exhibit a sizeable AHE at a significantly lower magnetic field (< 9 T). Density functional theory calculations reveal that epitaxial strain stabilizes a non-compensated magnetic ground state and reconfigures magnetic ordering in RuO2 (110) thin films. These findings establish ultrathin RuO2 as a platform for strain-engineered magnetism and underscore the transformative potential of epitaxial design in advancing spintronic technologies. △ Less

Submitted 19 January, 2025; originally announced January 2025.

Comments: 23 pages

A unified framework for classical and quantum uncertainty relations using stochastic representations

Authors: Euijoon Kwon, Jae Sung Lee

Abstract: Thermodynamic uncertainty relations (TURs) and kinetic uncertainty relations (KURs) provide tradeoff relations between measurement precision and thermodynamic cost such as entropy production and activity. Conventionally, these relations are derived using the Cramér-Rao inequality, which involves an auxiliary perturbation in deterministic differential equations governing the time evolution of the s… ▽ More Thermodynamic uncertainty relations (TURs) and kinetic uncertainty relations (KURs) provide tradeoff relations between measurement precision and thermodynamic cost such as entropy production and activity. Conventionally, these relations are derived using the Cramér-Rao inequality, which involves an auxiliary perturbation in deterministic differential equations governing the time evolution of the system's probability distribution. In this study, without relying on the previous formulation based on deterministic evolving equation, we demonstrate that all previously discovered uncertainty relations can be derived solely through the stochastic representation of the same dynamics. For this purpose, we propose a unified method based on stochastic representations for general Markovian dynamics. Extending beyond classical systems, we apply this method to Markovian open quantum systems by unraveling their dynamics, deriving quantum uncertainty relations that are physically more accessible and tighter in regimes where quantum effects play a significant role. This fully establishes uncertainty relations for both classical and quantum systems as intrinsic properties of their stochastic nature. △ Less

Submitted 13 December, 2024; v1 submitted 6 December, 2024; originally announced December 2024.

Comments: 16 pages, 2 figures

Fluctuation-response inequalities for kinetic and entropic perturbations

Authors: Euijoon Kwon, Hyun-Myung Chun, Hyunggyu Park, Jae Sung Lee

Abstract: We derive fluctuation-response inequalities for Markov jump processes that link the fluctuations of general observables to the response to perturbations in the transition rates within a unified framework. These inequalities are derived using the Cramér-Rao bound, enabling broader applicability compared to existing fluctuation-response relations formulated for static responses of current-like obser… ▽ More We derive fluctuation-response inequalities for Markov jump processes that link the fluctuations of general observables to the response to perturbations in the transition rates within a unified framework. These inequalities are derived using the Cramér-Rao bound, enabling broader applicability compared to existing fluctuation-response relations formulated for static responses of current-like observables. The fluctuation-response inequalities are valid for a wider class of observables and are applicable to finite observation times through dynamic responses. Furthermore, we extend these inequalities to open quantum systems governed by the Lindblad quantum master equation and find the quantum fluctuation-response inequality, where dynamical activity plays a central role. △ Less

Submitted 27 November, 2024; originally announced November 2024.

Comments: 8 pages, 3 figures

Journal ref: Phys. Rev. Lett. 135, 097101 (2025)

Thermodynamic uncertainty relation for systems with active Ornstein-Uhlenbeck particles

Authors: Hyeong-Tark Han, Jae Sung Lee, Jae-Hyung Jeon

Abstract: Thermodynamic uncertainty relations (TURs) delineate tradeoff relations between the thermodynamic cost and the magnitude of an observable's fluctuation. While TURs have been established for various nonequilibrium systems, their applicability to systems influenced by active noise remains largely unexplored. Here, we present an explicit expression of TUR for systems with active Ornstein-Uhlenbeck pa… ▽ More Thermodynamic uncertainty relations (TURs) delineate tradeoff relations between the thermodynamic cost and the magnitude of an observable's fluctuation. While TURs have been established for various nonequilibrium systems, their applicability to systems influenced by active noise remains largely unexplored. Here, we present an explicit expression of TUR for systems with active Ornstein-Uhlenbeck particles (AOUPs). Our findings reveal that active noise introduces modifications to the terms associated with the thermodynamic cost in the TUR expression. The altered thermodynamic cost encompasses not only the conventional entropy production but also the energy consumption induced by the active noise. We examine the capability of this TUR as an accurate estimator of the extent of anomalous diffusion in systems with active noise driven by a constant force in free space. By introducing the concept of a contracted probability density function, we derive a steady-state TUR tailored to this system. Moreover, through the adoption of a new scaling parameter, we enhance and optimize the TUR bound further. Our results demonstrate that active noise tends to hinder accurate estimation of the anomalous diffusion extent. Our study offers a systematic approach for exploring the fluctuation nature of biological systems operating in active environments. △ Less

Submitted 18 June, 2025; v1 submitted 29 October, 2024; originally announced October 2024.

Electric Control of Polarity in Spin-Orbit Josephson Diode

Authors: Junghyun Shin, Jae-Ho Han, Anjali Rathore, Joon Sue Lee, Seung-Bo Shim, Jinwoong Cha, Sunghun Park, Junho Suh

Abstract: The effect of spin-orbit coupling in a Josephson diode has not been elucidated due to its interplay with the complexity of Josephson devices. Here, we systematically control local electric fields in epitaxial Al-InAs Josephson junctions under in-plane magnetic fields and observe a polarity reversal of the Josephson diode. We interpret this polarity reversal as an effect of field-tunable spin-orbit… ▽ More The effect of spin-orbit coupling in a Josephson diode has not been elucidated due to its interplay with the complexity of Josephson devices. Here, we systematically control local electric fields in epitaxial Al-InAs Josephson junctions under in-plane magnetic fields and observe a polarity reversal of the Josephson diode. We interpret this polarity reversal as an effect of field-tunable spin-orbit coupling on nonreciprocal Josephson currents. A theoretical model, accounting for Rashba and Dresselhaus spin-orbit couplings in a planar Josephson junction containing many transverse subbands, aligns with the observed polarity reversal and its dependence on magnetic field. Our finding addresses spin-orbit control in a Josephson diode, enabling manipulation of Josephson harmonics. △ Less

Submitted 3 May, 2025; v1 submitted 26 September, 2024; originally announced September 2024.

Comments: 39 pages, 18 figures, 1 table

Journal ref: Phys. Rev. Applied 25, 034070 (2026)

Nonlocal phase-change metaoptics for reconfigurable nonvolatile image processing

Authors: Guoce Yang, Mengyun Wang, June Sang Lee, Nikolaos Farmakidis, Joe Shields, Carlota Ruiz de Galarreta, Stuart Kendall, Jacopo Bertolotti, Andriy Moskalenko, Kairan Huang, Andrea Alù, C. David Wright, Harish Bhaskaran

Abstract: The next generation of smart imaging and vision systems will require compact and tunable optical computing hardware to perform high-speed and low-power image processing. These requirements are driving the development of computing metasurfaces to realize efficient front-end analog optical pre-processors, especially for edge-detection capability. Yet, there is still a lack of reconfigurable or progr… ▽ More The next generation of smart imaging and vision systems will require compact and tunable optical computing hardware to perform high-speed and low-power image processing. These requirements are driving the development of computing metasurfaces to realize efficient front-end analog optical pre-processors, especially for edge-detection capability. Yet, there is still a lack of reconfigurable or programmable schemes, which may drastically enhance the impact of these devices at the system level. Here, we propose and experimentally demonstrate a reconfigurable flat optical image processor using low-loss phase-change nonlocal metasurfaces. The metasurface is configured to realize different transfer functions in spatial frequency space, when transitioning the phase-change material between its amorphous and crystalline phases. This enables edge detection and bright-field imaging modes on the same device. The metasurface is compatible with a large numerical aperture of ~0.5, making it suitable for high resolution coherent optical imaging microscopy. The concept of phase-change reconfigurable nonlocal metasurfaces may enable emerging applications of artificial intelligence-assisted imaging and vision devices with switchable multitasking. △ Less

Submitted 17 September, 2024; originally announced September 2024.

Comments: 20 pages, 5 figures

Journal ref: Light Sci Appl 14, 182 (2025)

Controlling structural phases of Sn through lattice engineering

Authors: Chandima Kasun Edirisinghe, Anjali Rathore, Taegeon Lee, Daekwon Lee, An-Hsi Chen, Garrett Baucom, Eitan Hershkovitz, Anuradha Wijesinghe, Pradip Adhikari, Sinchul Yeom, Hong Seok Lee, Hyung-Kook Choi, Hyunsoo Kim, Mina Yoon, Honggyu Kim, Matthew Brahlek, Heesuk Rho, Joon Sue Lee

Abstract: Topology and superconductivity, two distinct phenomena offer unique insight into quantum properties and their applications in quantum technologies, spintronics, and sustainable energy technologies if system can be found where they coexist. Tin (Sn) plays a pivotal role here as an element due to its two structural phases, $α$-Sn and $β$-Sn, exhibiting topological characteristics ($α$-Sn) and superc… ▽ More Topology and superconductivity, two distinct phenomena offer unique insight into quantum properties and their applications in quantum technologies, spintronics, and sustainable energy technologies if system can be found where they coexist. Tin (Sn) plays a pivotal role here as an element due to its two structural phases, $α$-Sn and $β$-Sn, exhibiting topological characteristics ($α$-Sn) and superconductivity ($β$-Sn). In this study we show how precise control of $α$ and $β$ phases of Sn thin films can be achieved by using molecular beam epitaxy grown buffer layers with systematic control over the lattice parameter. The resulting Sn films showed either $β$-Sn or $α$-Sn phases as the lattice constant of the buffer layer was varied from 6.10 A to 6.48 A, covering the range between GaSb (closely matched to InAs) and InSb. The crystal structures of the $α$- and $β$-Sn films were characterized by x-ray diffraction and confirmed by Raman spectroscopy and scanning transmission electron microscopy. The smooth and continuous surface morphology of the Sn films was validated using atomic force microscopy. The characteristics of $α$- and $β$-Sn phases were further verified using electrical transport measurements by observing resistance drop near 3.7 K for superconductivity of the $β$-Sn phase and Shubnikov-de Haas oscillations for the $α$-Sn phase. Density functional theory calculations showed that the stability of the Sn phases is highly dependent on lattice strain, with $α$-Sn being more stable under tensile strain and $β$-Sn becoming favorable under compressive strain, which is in good agreement with experimental observations. Hence, this study sheds light on controlling Sn phases through lattice engineering, enabling innovative applications in quantum technologies and beyond. △ Less

Submitted 21 September, 2024; v1 submitted 24 July, 2024; originally announced July 2024.

Is active motion beneficial for target search with resetting in a thermal environment?

Authors: Priyo Shankar Pal, Jong-Min Park, Arnab Pal, Hyunggyu Park, Jae Sung Lee

Abstract: Stochastic resetting has recently emerged as an efficient target-searching strategy in various physical and biological systems. The efficiency of this strategy depends on the type of environmental noise, whether it is thermal or telegraphic (active). While the impact of each noise type on a search process has been investigated separately, their combined effects have not been explored. In this work… ▽ More Stochastic resetting has recently emerged as an efficient target-searching strategy in various physical and biological systems. The efficiency of this strategy depends on the type of environmental noise, whether it is thermal or telegraphic (active). While the impact of each noise type on a search process has been investigated separately, their combined effects have not been explored. In this work, we explore the effects of stochastic resetting on an active system, namely a self-propelled run-and-tumble particle immersed in a thermal bath. In particular, we assume that the position of the particle is reset at a fixed rate with or without reversing the direction of self-propelled velocity. Using standard renewal techniques, we compute the mean search time of this active particle to a fixed target and investigate the interplay between active and thermal fluctuations. We find that the active search can outperform the Brownian search when the magnitude and flipping rate of self-propelled velocity are large and the strength of environmental noise is small. Notably, we find that the presence of thermal noise in the environment helps reduce the mean first passage time of the run-and-tumble particle compared to the absence of thermal noise. Finally, we observe that reversing the direction of self-propelled velocity while resetting can also reduce the overall search time. △ Less

Submitted 4 July, 2024; originally announced July 2024.

Comments: 10 pages, 4 figures

Dimensionality Engineering of Magnetic Anisotropy from Anomalous Hall Effect in Synthetic SrRuO3 Crystals

Authors: Seung Gyo Jeong, Seong Won Cho, Sehwan Song, Jin Young Oh, Do Gyeom Jeong, Gyeongtak Han, Hu Young Jeong, Ahmed Yousef Mohamed, Woo-suk Noh, Sungkyun Park, Jong Seok Lee, Suyoun Lee, Young-Min Kim, Deok-Yong Cho, Woo Seok Choi

Abstract: Magnetic anisotropy in atomically thin correlated heterostructures is essential for exploring quantum magnetic phases for next-generation spintronics. Whereas previous studies have mostly focused on van der Waals systems, here, we investigate the impact of dimensionality of epitaxially-grown correlated oxides down to the monolayer limit on structural, magnetic, and orbital anisotropies. By designi… ▽ More Magnetic anisotropy in atomically thin correlated heterostructures is essential for exploring quantum magnetic phases for next-generation spintronics. Whereas previous studies have mostly focused on van der Waals systems, here, we investigate the impact of dimensionality of epitaxially-grown correlated oxides down to the monolayer limit on structural, magnetic, and orbital anisotropies. By designing oxide superlattices with a correlated ferromagnetic SrRuO3 and nonmagnetic SrTiO3 layers, we observed modulated ferromagnetic behavior with the change of the SrRuO3 thickness. Especially, for three-unit-cell-thick layers, we observe a significant 1,500% improvement of coercive field in the anomalous Hall effect, which cannot be solely attributed to the dimensional crossover in ferromagnetism. The atomic-scale heterostructures further reveal the systematic modulation of anisotropy for the lattice structure and orbital hybridization, explaining the enhanced magnetic anisotropy. Our findings provide valuable insights into engineering the anisotropic hybridization of synthetic magnetic crystals, offering a tunable spin order for various applications. △ Less

Submitted 3 July, 2024; originally announced July 2024.

Comments: 23 pages

Journal ref: published 2024

Discrete-time thermodynamic speed limit

Authors: Sangyun Lee, Jae Sung Lee, Jong-Min Park

Abstract: As a fundamental thermodynamic principle, speed limits reveal the lower bound of entropy production (EP) required for a system to transition from a given initial state to a final state. While various speed limits have been developed for continuous-time Markov processes, their application to discrete-time Markov chains remains unexplored. In this study, we investigate the speed limits in discrete-t… ▽ More As a fundamental thermodynamic principle, speed limits reveal the lower bound of entropy production (EP) required for a system to transition from a given initial state to a final state. While various speed limits have been developed for continuous-time Markov processes, their application to discrete-time Markov chains remains unexplored. In this study, we investigate the speed limits in discrete-time Markov chains, focusing on two types of EP commonly used to measure the irreversibility of a discrete-time process: time-reversed EP and time-backward EP. We find that time-reversed EP satisfies the speed limit for the continuous-time Markov processes, whereas time-backward EP does not. Additionally, for time-reversed EP, we derive practical speed limits applicable to systems driven by cyclic protocols or with unidirectional transitions, where conventional speed limits become meaningless or invalid. We show that these relations also hold for continuous-time Markov processes by taking the time-continuum limit of our results. Finally, we validate our findings through several examples. △ Less

Submitted 7 July, 2025; v1 submitted 25 June, 2024; originally announced June 2024.

Comments: 13 pages, 5 figures

Interfacially enhanced superconductivity in Fe(Te,Se)/Bi4Te3 heterostructures

Authors: An-Hsi Chen, Qiangsheng Lu, Eitan Hershkovitz, Miguel L. Crespillo, Alessandro R. Mazza, Tyler Smith, T. Zac Ward, Gyula Eres, Shornam Gandhi, Meer Muhtasim Mahfuz, Vitalii Starchenko, Khalid Hattar, Joon Sue Lee, Honggyu Kim, Robert G. Moore, Matthew Brahlek

Abstract: Realizing topological superconductivity by integrating high-transition-temperature ($T_C$) superconductors with topological insulators can open new paths for quantum computing applications. Here, we report a new approach for increasing the superconducting transition temperature ($T_{C}^{onset}$) by interfacing the unconventional superconductor Fe(Te,Se) with the topological insulator Bi-Te system… ▽ More Realizing topological superconductivity by integrating high-transition-temperature ($T_C$) superconductors with topological insulators can open new paths for quantum computing applications. Here, we report a new approach for increasing the superconducting transition temperature ($T_{C}^{onset}$) by interfacing the unconventional superconductor Fe(Te,Se) with the topological insulator Bi-Te system in the low-Se doping regime, near where superconductivity vanishes in the bulk. The critical finding is that the $T_{C}^{onset}$ of Fe(Te,Se) increases from nominally non-superconducting to as high as 12.5 K when $Bi_2Te_3$ is replaced with the topological phase $Bi_4Te_3$. Interfacing Fe(Te,Se) with $Bi_4Te_3$ is also found to be critical for stabilizing superconductivity in monolayer films where $T_{C}^{onset}$ can be as high as 6 K. Measurements of the electronic and crystalline structure of the $Bi_4Te_3$ layer reveal that a large electron transfer, epitaxial strain, and novel chemical reduction processes are critical factors for the enhancement of superconductivity. This novel route for enhancing $T_C$ in an important epitaxial system provides new insight on the nature of interfacial superconductivity and a platform to identify and utilize new electronic phases. △ Less

Submitted 24 May, 2024; originally announced May 2024.

Comments: 18 pages, 5 figures, accepted by Advanced Materials

Altermagnetic Polar Metallic phase in Ultra-Thin Epitaxially-Strained RuO2 Films

Authors: Seung Gyo Jeong, In Hyeok Choi, Sreejith Nair, Luca Buiarelli, Bita Pourbahari, Jin Young Oh, Nabil Bassim, Daigorou Hirai, Ambrose Seo, Woo Seok Choi, Rafael M. Fernandes, Turan Birol, Liuyan Zhao, Jong Seok Lee, Bharat Jalan

Abstract: Altermagnetism refers to a wide class of magnetic orders featuring magnetic sublattices with opposite spins related by rotational symmetries, resulting in non-trivial spin splitting and magnetic multipoles. However, the direct observation of the altermagnetic order parameter remains elusive. Here, by combining theoretical analysis, electrical transport, X-ray and optical spectroscopies, we establi… ▽ More Altermagnetism refers to a wide class of magnetic orders featuring magnetic sublattices with opposite spins related by rotational symmetries, resulting in non-trivial spin splitting and magnetic multipoles. However, the direct observation of the altermagnetic order parameter remains elusive. Here, by combining theoretical analysis, electrical transport, X-ray and optical spectroscopies, we establish a phase diagram in hybrid molecular beam epitaxy-grown RuO2/TiO2 (110) films, mapping symmetries along with altermagnetic/electronic/structural phase transitions as functions of film thickness and temperature. This features a novel altermagnetic metallic polar phase in epitaxially-strained 2 nm films, extending the concept of multiferroicity to altermagnets. Such a clear signature of a magnetic phase transition at ~500 K is observed exclusively in ultrathin strained films, unlike in bulk RuO2 single crystals. These results demonstrate the potential of epitaxial heterostructure design to induce altermagnetism, paving the way for emergent novel phases with multifunctional properties. △ Less

Submitted 13 February, 2025; v1 submitted 9 May, 2024; originally announced May 2024.

Comments: 23 pages

Broken inversion symmetry in van der Waals topological ferromagnetic metal iron germanium telluride

Authors: Kai-Xuan Zhang, Hwiin Ju, Hyuncheol Kim, Jingyuan Cui, Jihoon Keum, Je-Geun Park, Jong Seok Lee

Abstract: Inversion symmetry breaking is critical for many quantum effects and fundamental for spin-orbit torque, which is crucial for next-generation spintronics. Recently, a novel type of gigantic intrinsic spin-orbit torque has been established in the topological van-der-Waals (vdW) magnet iron germanium telluride. However, it remains a puzzle because no clear evidence exists for interlayer inversion sym… ▽ More Inversion symmetry breaking is critical for many quantum effects and fundamental for spin-orbit torque, which is crucial for next-generation spintronics. Recently, a novel type of gigantic intrinsic spin-orbit torque has been established in the topological van-der-Waals (vdW) magnet iron germanium telluride. However, it remains a puzzle because no clear evidence exists for interlayer inversion symmetry breaking. Here, we report the definitive evidence of broken inversion symmetry in iron germanium telluride directly measured by the second harmonic generation (SHG) technique. Our data show that the crystal symmetry reduces from centrosymmetric P63/mmc to noncentrosymmetric polar P3m1 space group, giving the three-fold SHG pattern with dominant out-of-plane polarization. Additionally, the SHG response evolves from an isotropic pattern to a sharp three-fold symmetry upon increasing Fe deficiency, mainly due to the transition from random defects to ordered Fe vacancies. Such SHG response is robust against temperature, ensuring unaltered crystalline symmetries above and below the ferromagnetic transition temperature. These findings add crucial new information to our understanding of this interesting vdW metal, iron germanium telluride: band topology, intrinsic spin-orbit torque and topological vdW polar metal states. △ Less

Submitted 7 January, 2025; v1 submitted 21 December, 2023; originally announced December 2023.

Comments: Accepted by Advanced Materials; 32 pages, 4 main figures, 5 supporting figures

Journal ref: Advanced Materials 36, 2312824 (2024)

Unified Hierarchical Relationship Between Thermodynamic Tradeoff Relations

Authors: Euijoon Kwon, Jong-Min Park, Jae Sung Lee, Yongjoo Baek

Abstract: Recent years have witnessed a surge of discoveries in the studies of thermodynamic inequalities: the thermodynamic uncertainty relation (TUR) and the entropic bound (EB) provide a lower bound on the entropy production (EP) in terms of nonequilibrium currents; the classical speed limit (CSL) expresses the lower bound on the EP using the geometry of probability distributions; the power-efficiency (P… ▽ More Recent years have witnessed a surge of discoveries in the studies of thermodynamic inequalities: the thermodynamic uncertainty relation (TUR) and the entropic bound (EB) provide a lower bound on the entropy production (EP) in terms of nonequilibrium currents; the classical speed limit (CSL) expresses the lower bound on the EP using the geometry of probability distributions; the power-efficiency (PE) tradeoff dictates the maximum power achievable for a heat engine given the level of its thermal efficiency. In this study, we show that there exists a unified hierarchical structure encompassing all of these bounds, with the fundamental inequality given by a novel extension of the TUR (XTUR) that incorporates the most general range of current-like and state-dependent observables. By selecting more specific observables, the TUR and the EB follow from the XTUR, and the CSL and the PE tradeoff follow from the EB. Our derivations cover both Langevin and Markov jump systems, with the first proof of the EB for the Markov jump systems and a more generalized form of the CSL. We also present concrete examples of the EB for the Markov jump systems and the generalized CSL. △ Less

Submitted 5 August, 2024; v1 submitted 2 November, 2023; originally announced November 2023.

Comments: 23 pages, 5 figures

3D Heisenberg universality in the Van der Waals antiferromagnet NiPS$_3$

Authors: Rajan Plumley, Sougata Mardanya, Cheng Peng, Johannes Nokelainen, Tadesse Assefa, Lingjia Shen, Nicholas Burdet, Zach Porter, Alexander Petsch, Aidan Israelski, Hongwei Chen, Jun Sik Lee, Sophie Morley, Sujoy Roy, Gilberto Fabbris, Elizabeth Blackburn, Adrian Feiguin, Arun Bansil, Wei-Sheng Lee, Aaron Lindenberg, Sugata Chowdhury, Mike Dunne, Joshua J. Turner

Abstract: Van der Waals (vdW) magnetic materials are comprised of layers of atomically thin sheets, making them ideal platforms for studying magnetism at the two-dimensional (2D) limit. These materials are at the center of a host of novel types of experiments, however, there are notably few pathways to directly probe their magnetic structure. We report the magnetic order within a single crystal of NiPS$_3$… ▽ More Van der Waals (vdW) magnetic materials are comprised of layers of atomically thin sheets, making them ideal platforms for studying magnetism at the two-dimensional (2D) limit. These materials are at the center of a host of novel types of experiments, however, there are notably few pathways to directly probe their magnetic structure. We report the magnetic order within a single crystal of NiPS$_3$ and show it can be accessed with resonant elastic X-ray diffraction along the edge of the vdW planes in a carefully grown crystal by detecting structurally forbidden resonant magnetic X-ray scattering. We find the magnetic order parameter has a critical exponent of $β\sim0.36$, indicating that the magnetism of these vdW crystals is more adequately characterized by the three-dimensional (3D) Heisenberg universality class. We verify these findings with first-principle density functional theory, Monte-Carlo simulations, and density matrix renormalization group calculations. △ Less

Submitted 18 October, 2024; v1 submitted 11 October, 2023; originally announced October 2023.

Stochastic Differential Equation for a System Coupled to a Thermostatic Bath via an Arbitrary Interaction Hamiltonian

Authors: Jong-Min Park, Hyunggyu Park, Jae Sung Lee

Abstract: The conventional Langevin equation offers a mathematically convenient framework for investigating open stochastic systems interacting with their environment or a bath. However, it is not suitable for a wide variety of systems whose dynamics rely on the nature of the environmental interaction, as the equation does not incorporate any specific information regarding that interaction. Here, we present… ▽ More The conventional Langevin equation offers a mathematically convenient framework for investigating open stochastic systems interacting with their environment or a bath. However, it is not suitable for a wide variety of systems whose dynamics rely on the nature of the environmental interaction, as the equation does not incorporate any specific information regarding that interaction. Here, we present a universal formulation of the stochastic differential equation (SDE) for an open system coupled to a thermostatic bath via an arbitrary interaction Hamiltonian. This SDE encodes the interaction information to a fictitious potential (mean force) and a position-dependent damping coefficient. Surprisingly, we find that the conventional Langevin equation can be recovered in the presence of arbitrary strong interactions given two conditions: translational invariance of the potential and disjoint separability of the bath particles. Our results provide a universal framework for studying open stochastic systems with an arbitrary interaction Hamiltonian and yield deeper insight into why various experiments fit the conventional Langevin description regardless of the strength or type of interaction. △ Less

Submitted 26 September, 2023; originally announced September 2023.

Comments: 5 figures, 10 pages + supplemental material, Comments are welcome

Journal ref: Phys. Rev. E 110, 014143 (2024)

Estimating Entanglement Entropy via Variational Quantum Circuits with Classical Neural Networks

Authors: Sangyun Lee, Hyukjoon Kwon, Jae Sung Lee

Abstract: Entropy plays a crucial role in both physics and information science, encompassing classical and quantum domains. In this work, we present the Quantum Neural Entropy Estimator (QNEE), a novel approach that combines classical neural network (NN) with variational quantum circuits to estimate the von Neumann and Renyi entropies of a quantum state. QNEE provides accurate estimates of entropy while als… ▽ More Entropy plays a crucial role in both physics and information science, encompassing classical and quantum domains. In this work, we present the Quantum Neural Entropy Estimator (QNEE), a novel approach that combines classical neural network (NN) with variational quantum circuits to estimate the von Neumann and Renyi entropies of a quantum state. QNEE provides accurate estimates of entropy while also yielding the eigenvalues and eigenstates of the input density matrix. Leveraging the capabilities of classical NN, QNEE can classify different phases of quantum systems that accompany the changes of entanglement entropy. Our numerical simulation demonstrates the effectiveness of QNEE by applying it to the 1D XXZ Heisenberg model. In particular, QNEE exhibits high sensitivity in estimating entanglement entropy near the phase transition point. We expect that QNEE will serve as a valuable tool for quantum entropy estimation and phase classification. △ Less

Submitted 16 December, 2023; v1 submitted 25 July, 2023; originally announced July 2023.

Comments: 14 pages, 5 figures; see also independent researches of Shin, Lee, and Jeong at arXiv:2306.14566v1 and Goldfeld, Patel, Sreekumar, Wilde at arXiv:2307.01171

Supercurrent through a single transverse mode in nanowire Josephson junctions

Authors: B. Zhang, Z. Li, H. Wu, M. Pendharkar, C. Dempsey, J. S. Lee, S. D. Harrington, C. J. Palmstrom, S. M. Frolov

Abstract: Hybrid superconductor-semiconductor materials are fueling research in mesoscopic physics and quantum technology. Recently demonstrated smooth $β$-Sn superconductor shells, due to the increased induced gap, are expanding the available parameter space to new regimes. Fabricated on quasiballistic InSb nanowires, with careful control over the hybrid interface, Sn shells yield measurable switching curr… ▽ More Hybrid superconductor-semiconductor materials are fueling research in mesoscopic physics and quantum technology. Recently demonstrated smooth $β$-Sn superconductor shells, due to the increased induced gap, are expanding the available parameter space to new regimes. Fabricated on quasiballistic InSb nanowires, with careful control over the hybrid interface, Sn shells yield measurable switching currents even when nanowire resistance is of order 10kohm. In this regime Cooper pairs travel through a purely 1D quantum wire for at least part of their trajectory. Here, we focus on the evolution of proximity-induced supercurrent in magnetic field parallel to the nanowire. Long decay up to fields of 1T is observed. At the same time, the decay for higher occupied subbands is notably faster in some devices but not in others. We analyze this using a tight-binding numerical model that includes the Zeeman, orbital and spin-orbit effects. When the first subband is spin polarized, we observe a dramatic suppression of supercurrent, which is also confirmed by the model and suggests an absence of significant triplet supercurrent generation. △ Less

Submitted 2 February, 2025; v1 submitted 31 May, 2023; originally announced June 2023.

Zero-bias conductance peaks at zero applied magnetic field due to stray fields from integrated micromagnets in hybrid nanowire quantum dots

Authors: Y. Jiang, M. Gupta, C. Riggert, M. Pendharkar, C. Dempsey, J. S. Lee, S. D. Harrington, C. J. Palmstrøm, V. S. Pribiag, S. M. Frolov

Abstract: Many recipes for realizing topological superconductivity rely on broken time-reversal symmetry, which is often attained by applying a substantial external magnetic field. Alternatively, using magnetic materials can offer advantages through low-field operation and design flexibility on the nanoscale. Mechanisms for lifting spin degeneracy include exchange coupling, spin-dependent scattering, spin i… ▽ More Many recipes for realizing topological superconductivity rely on broken time-reversal symmetry, which is often attained by applying a substantial external magnetic field. Alternatively, using magnetic materials can offer advantages through low-field operation and design flexibility on the nanoscale. Mechanisms for lifting spin degeneracy include exchange coupling, spin-dependent scattering, spin injection-all requiring direct contact between the bulk or induced superconductor and a magnetic material. Here, we implement locally broken time-reversal symmetry through dipolar coupling from nearby micromagnets to superconductor-semiconductor hybrid nanowire devices. Josephson supercurrent is hysteretic due to micromangets switching. At or around zero external magnetic field, we observe an extended presence of Andreev bound states near zero voltage bias. We also show a zero-bias peak plateau of a non-quantized value. Our findings largely reproduce earlier results where similar effects were presented in the context of topological superconductivity in a homogeneous wire, and attributed to more exotic time-reversal breaking mechanisms [1]. In contrast, our stray field profiles are not designed to create Majorana modes, and our data are compatible with a straightforward interpretation in terms of trivial states in quantum dots. At the same time, the use of micromagnets in hybrid superconductor-semiconductor devices shows promise for future experiments on topological superconductivity. △ Less

Submitted 21 May, 2025; v1 submitted 31 May, 2023; originally announced May 2023.

Journal ref: SciPost Phys. 19, 030 (2025)

Structural Anisotropy in Sb Thin Films

Authors: Pradip Adhikari, Anuradha Wijesinghe, Anjali Rathore, Timothy Jinsoo Yoo, Gyehyeon Kim, Hyoungtaek Lee, Sinchul Yeom, Alessandro R. Mazza, Changhee Sohn, Hyeong-Ryeol Park, Mina Yoon, Matthew Brahlek, Honggyu Kim, Joon Sue Lee

Abstract: Sb thin films have attracted wide interests due to their tunable band structure, topological phases, and remarkable electronic properties. We successfully grow epitaxial Sb thin films on a closely lattice-matched GaSb(001) surface by molecular beam epitaxy. We find a novel anisotropic directional dependence of their structural, morphological, and electronic properties. The origin of the anisotropi… ▽ More Sb thin films have attracted wide interests due to their tunable band structure, topological phases, and remarkable electronic properties. We successfully grow epitaxial Sb thin films on a closely lattice-matched GaSb(001) surface by molecular beam epitaxy. We find a novel anisotropic directional dependence of their structural, morphological, and electronic properties. The origin of the anisotropic features is elucidated using first-principles density functional theory (DFT) calculations. The growth regime of crystalline and amorphous Sb thin films was determined by mapping the surface reconstruction phase diagram of the GaSb(001) surface under Sb$_2$ flux, with confirmation of structural characterizations. Crystalline Sb thin films show a rhombohedral crystal structure along the rhombohedral (104) surface orientation parallel to the cubic (001) surface orientation of the GaSb substrate. At this coherent interface, Sb atoms are aligned with the GaSb lattice along the [1-10] crystallographic direction but are not aligned well along the [110] crystallographic direction, which results in anisotropic features in reflection high-energy electron diffraction patterns, surface morphology, and transport properties. Our DFT calculations show that the anisotropic features originate from the GaSb surface, where Sb atoms align with the Ga and Sb atoms on the reconstructed surface. The formation energy calculations confirm that the stability of the experimentally observed structures. Our results provide optimal film growth conditions for further studies of novel properties of Bi$_{1-x}$Sb$_x$ thin films with similar lattice parameters and an identical crystal structure as well as functional heterostructures of them with III-V semiconductor layers along the (001) surface orientation, supported by a theoretical understanding of the anisotropic film orientation. △ Less

Submitted 11 May, 2023; originally announced May 2023.

Journal ref: APL Mater. 12, 011116 (2024)

Thermodynamic Trade-off Relation for First Passage Time in Resetting Process

Authors: Priyo Shankar Pal, Arnab Pal, Hyunggyu Park, Jae Sung Lee

Abstract: Resetting is a strategy for boosting the speed of a target-searching process. Since its introduction over a decade ago, most studies have been carried out under the assumption that resetting takes place instantaneously. However, due to its irreversible nature, resetting processes incur a thermodynamic cost, which becomes infinite in case of instantaneous resetting. Here, we take into consideration… ▽ More Resetting is a strategy for boosting the speed of a target-searching process. Since its introduction over a decade ago, most studies have been carried out under the assumption that resetting takes place instantaneously. However, due to its irreversible nature, resetting processes incur a thermodynamic cost, which becomes infinite in case of instantaneous resetting. Here, we take into consideration both the cost and first passage time (FPT) required for a resetting process, in which the reset or return to the initial location is implemented using a trapping potential over a finite but random time period. An iterative generating function and counting functional method à la Feynman and Kac are employed to calculate the FPT and average work for this process. From these results, we obtain an explicit form of the time-cost trade-off relation, which provides the lower bound of the mean FPT for a given work input when the trapping potential is linear. This trade-off relation clearly shows that instantaneous resetting is achievable only when an infinite amount of work is provided. More surprisingly, the trade-off relation derived from the linear potential seems to be valid for a wide range of trapping potentials. In addition, we have also shown that the fixed-time or sharp resetting can further enhance the trade-off relation compared to that of the stochastic resetting. △ Less

Submitted 8 May, 2023; originally announced May 2023.

Comments: 14 pages, 8 figures

Swarmalators with thermal noise

Authors: Hyunsuk Hong, Kevin P. O'Keeffe, Jae Sung Lee, Hyunggyu Park

Abstract: We investigate a population of swarmalators, a mobile version of phase oscillators that both sync in time and swarm through space. We focus on a XY-type model of identical swarmalators running on a one-dimensional ring and subject to thermal noise. We uncover four distinct collective states, some of which capture the behavior of real-world swarmalators such as vinegar eels and sperm. Among these,… ▽ More We investigate a population of swarmalators, a mobile version of phase oscillators that both sync in time and swarm through space. We focus on a XY-type model of identical swarmalators running on a one-dimensional ring and subject to thermal noise. We uncover four distinct collective states, some of which capture the behavior of real-world swarmalators such as vinegar eels and sperm. Among these, the most intriguing is the `mixed state', which blends two of the other states. We present a comprehensive phase diagram from the Fourier mode analysis with a high accuracy, which is in excellent agreement with numerical simulation results. Our model serves as a tractable toy model for thermal systems that both self-synchronize and self-assemble interdependently. △ Less

Submitted 23 February, 2023; originally announced February 2023.

Comments: 10 pages, 4 figures

Journal ref: Phys. Rev. Research 5, 023105 (2023)

Evidence of $φ$0-Josephson junction from skewed diffraction patterns in Sn-InSb nanowires

Authors: B. Zhang, Z. Li, V. Aguilar, P. Zhang, M. Pendharkar, C. Dempsey, J. S. Lee, S. D. Harrington, S. Tan, J. S. Meyer, M. Houzet, C. J. Palmstrom, S. M. Frolov

Abstract: We study Josephson junctions based on InSb nanowires with Sn shells. We observe skewed critical current diffraction patterns: the maxima in forward and reverse current bias are at different magnetic flux, with a displacement of 20-40 mT. The skew is greatest when the external field is nearly perpendicular to the nanowire, in the substrate plane. This orientation suggests that spin-orbit interactio… ▽ More We study Josephson junctions based on InSb nanowires with Sn shells. We observe skewed critical current diffraction patterns: the maxima in forward and reverse current bias are at different magnetic flux, with a displacement of 20-40 mT. The skew is greatest when the external field is nearly perpendicular to the nanowire, in the substrate plane. This orientation suggests that spin-orbit interaction plays a role. We develop a phenomenological model and perform tight-binding calculations, both methods reproducing the essential features of the experiment. The effect modeled is the $φ$0-Josephson junction with higher-order Josephson harmonics. The system is of interest for Majorana studies: the effects are either precursor to or concomitant with topological superconductivity. Current-phase relations that lack inversion symmetry can also be used to design quantum circuits with engineered nonlinearity. △ Less

Submitted 24 September, 2024; v1 submitted 30 November, 2022; originally announced December 2022.

Journal ref: SciPost Phys. 18, 013 (2025)

Flat bands in Network Superstructures of Atomic Chains

Authors: Donghyeok Heo, Jun Seop Lee, Anwei Zhang, Jun-Won Rhim

Abstract: We investigate the origin of the ubiquitous existence of flat bands in the network superstructures of atomic chains, where one-dimensional(1D) atomic chains array periodically. While there can be many ways to connect those chains, we consider two representative ways of linking them, the dot-type and triangle-type links. Then, we construct a variety of superstructures, such as the square, rectangul… ▽ More We investigate the origin of the ubiquitous existence of flat bands in the network superstructures of atomic chains, where one-dimensional(1D) atomic chains array periodically. While there can be many ways to connect those chains, we consider two representative ways of linking them, the dot-type and triangle-type links. Then, we construct a variety of superstructures, such as the square, rectangular, and honeycomb network superstructures with dot-type links and the honeycomb superstructure with triangle-type links. These links provide the wavefunctions with an opportunity to have destructive interference, which stabilizes the compact localized state(CLS). The CLS is a localized eigenstate whose amplitudes are finite only inside a finite region and guarantees the existence of a flat band. In the network superstructures, there exist multiple flat bands proportional to the number of atoms of each chain, and the corresponding eigenenergies can be found from the stability condition of the compact localized state. Finally, we demonstrate that the finite bandwidth of the nearly flat bands of the network superstructures arising from the next-nearest-neighbor hopping processes can be suppressed by increasing the length of the chains consisting of the superstructures. △ Less

Submitted 28 November, 2022; originally announced November 2022.

Comments: 8pages, 4figures

Missing odd-order Shapiro steps do not uniquely indicate fractional Josephson effect

Authors: P. Zhang, S. Mudi, M. Pendharkar, J. S. Lee, C. P. Dempsey, A. P. McFadden, S. D. Harrington, J. T. Dong, H. Wu, A. -H. Chen, M. Hocevar, C. J. Palmstrøm, S. M. Frolov

Abstract: Topological superconductivity is expected to spur Majorana zero modes -- exotic states that are also considered a quantum technology asset. Fractional Josephson effect is their manifestation in electronic transport measurements, often under microwave irradiation. A fraction of induced resonances, known as Shapiro steps, should vanish, in a pattern that signifies the presence of Majorana modes. Her… ▽ More Topological superconductivity is expected to spur Majorana zero modes -- exotic states that are also considered a quantum technology asset. Fractional Josephson effect is their manifestation in electronic transport measurements, often under microwave irradiation. A fraction of induced resonances, known as Shapiro steps, should vanish, in a pattern that signifies the presence of Majorana modes. Here we report patterns of Shapiro steps expected in topological Josephson junctions, such as the missing first Shapiro step, or several missing odd-order steps. But our junctions, which are InAs quantum wells with Al contacts, are studied near zero magnetic field, meaning that they are not in the topological regime. We also observe other patterns such as missing even steps and several missing steps in a row, not relevant to topological superconductivity. Potentially responsible for our observations is rounding of not fully developed steps superimposed on non-monotonic resistance versus voltage curves, but several origins may be at play. Our results demonstrate that any single pattern, even striking, cannot uniquely identify topological superconductivity, and a multifactor approach is necessary to unambiguously establish this important phenomenon. △ Less

Submitted 24 May, 2025; v1 submitted 16 November, 2022; originally announced November 2022.

Comments: Written using The Block Method. Data on Zenodo DOI: https://zenodo.org/record/6416083. v2: Added Figs. S3, S15, discussion

Journal ref: SciPost Phys. 18, 203 (2025)

Large Second-Order Josephson Effect in Planar Superconductor-Semiconductor Junctions

Authors: P. Zhang, A. Zarassi, L. Jarjat, V. Van de Sande, M. Pendharkar, J. S. Lee, C. P. Dempsey, A. P. McFadden, S. D. Harrington, J. T. Dong, H. Wu, A. -H. Chen, M. Hocevar, C. J. Palmstrøm, S. M. Frolov

Abstract: We investigate the current-phase relations of Al/InAs-quantum well planar Josephson junctions fabricated using nanowire shadowing technique. Based on several experiments, we conclude that the junctions exhibit an unusually large second-order Josephson harmonic, the $\sin(2\varphi)$ term. First, superconducting quantum interference devices (dc-SQUIDs) show half-periodic oscillations, tunable by gat… ▽ More We investigate the current-phase relations of Al/InAs-quantum well planar Josephson junctions fabricated using nanowire shadowing technique. Based on several experiments, we conclude that the junctions exhibit an unusually large second-order Josephson harmonic, the $\sin(2\varphi)$ term. First, superconducting quantum interference devices (dc-SQUIDs) show half-periodic oscillations, tunable by gate voltages as well as magnetic flux. Second, Josephson junction devices exhibit kinks near half-flux quantum in supercurrent diffraction patterns. Third, half-integer Shapiro steps are present in the junctions. Similar phenomena are observed in Sn/InAs quantum well devices. We perform data fitting to a numerical model with a two-component current phase relation. Analysis including a loop inductance suggests that the sign of the second harmonic term is negative. The microscopic origins of the observed effect remain to be understood. We consider alternative explanations which can account for some but not all of the evidence. △ Less

Submitted 19 November, 2023; v1 submitted 14 November, 2022; originally announced November 2022.

Comments: Written using The Block Method. Data on Zenodo DOI: https://doi.org/10.5281/zenodo.6416083 v2: Added block "Non-identical negative and positive switching currents" and Figs.S4, S5. v3: Added Figs. 6, S6-S9, simulations with both inductive and second harmonic effects

Journal ref: SciPost Phys. 16, 030 (2024)

Planar Josephson Junctions Templated by Nanowire Shadowing

Authors: P. Zhang, A. Zarassi, M. Pendharkar, J. S. Lee, L. Jarjat, V. Van de Sande, B. Zhang, S. Mudi, H. Wu, S. Tan, C. P. Dempsey, A. P. McFadden, S. D. Harrington, B. Shojaei, J. T. Dong, A. -H. Chen, M. Hocevar, C. J. Palmstrøm, S. M. Frolov

Abstract: More and more materials, with a growing variety of properties, are built into electronic devices. This is motivated both by increased device performance and by the studies of materials themselves. An important type of device is a Josephson junction based on the proximity effect between a quantum material and a superconductor, useful for fundamental research as well as for quantum and other technol… ▽ More More and more materials, with a growing variety of properties, are built into electronic devices. This is motivated both by increased device performance and by the studies of materials themselves. An important type of device is a Josephson junction based on the proximity effect between a quantum material and a superconductor, useful for fundamental research as well as for quantum and other technologies. When both junction contacts are placed on the same surface, such as a two-dimensional material, the junction is called ``planar". One outstanding challenge is that not all materials are amenable to the standard planar junction fabrication. The device quality, rather than the intrinsic characteristics, may be defining the results. Here, we introduce a technique in which nanowires are placed on the surface and act as a shadow mask for the superconductor. The advantages are that the smallest dimension is determined by the nanowire diameter and does not require lithography, and that the junction is not exposed to chemicals such as etchants. We demonstrate this method with an InAs quantum well, using two superconductors - Al and Sn, and two semiconductor nanowires - InAs and InSb. The junctions exhibit critical current levels consistent with transparent interfaces and uniform width. We show that the template nanowire can be operated as a self-aligned electrostatic gate. Beyond single junctions, we create SQUIDs with two gate-tunable junctions. We suggest that our method can be used for a large variety of quantum materials including van der Waals layers, topological insulators, Weyl semimetals and future materials for which proximity effect devices is a promising research avenue. △ Less

Submitted 8 November, 2022; originally announced November 2022.

Comments: Written using The Block Method. Data on Zenodo DOI: https://doi.org/10.5281/zenodo.6416083

Multidimensional entropic bound: Estimator of entropy production for general Langevin dynamics with an arbitrary time-dependent protocol

Authors: Sangyun Lee, Dong-Kyum Kim, Jong-Min Park, Won Kyu Kim, Hyunggyu Park, Jae Sung Lee

Abstract: Entropy production (EP) is a key quantity in thermodynamics, and yet measuring EP has remained a challenging task. Here we introduce an EP estimator, called multidimensional entropic bound (MEB), utilizing an ensemble of trajectories without knowing the details of a given system. MEB is a unified method in the sense that it is applicable to both overdamped and underdamped Langevin dynamics, irresp… ▽ More Entropy production (EP) is a key quantity in thermodynamics, and yet measuring EP has remained a challenging task. Here we introduce an EP estimator, called multidimensional entropic bound (MEB), utilizing an ensemble of trajectories without knowing the details of a given system. MEB is a unified method in the sense that it is applicable to both overdamped and underdamped Langevin dynamics, irrespective of the time dependence of the driving protocol. In addition, MEB is computationally efficient because optimization is unnecessary. We apply our developed estimator to three physical systems driven by time-dependent protocols pertaining to experiments using optical tweezers: a dragged Brownian particle, a pulling process of a harmonic chain, and an unfolding process of an RNA hairpin. Numerical simulations confirm the validity and efficiency of our method. △ Less

Submitted 22 February, 2023; v1 submitted 13 July, 2022; originally announced July 2022.

Comments: 12 pages, 3 figures

Journal ref: Phys. Rev. Research 5, 013194 (2023)

Perturbative Steady States of Completely Positive Quantum Master Equations

Authors: Jae Sung Lee, Joonhyun Yeo

Abstract: The Lindblad form guarantees complete positivity of a Markovian quantum master equation (QME). However, its microscopic derivation for a quantum system weakly interacting with a thermal bath requires several approximations, which may result in inaccuracies in the QME. Recently, various Lindbladian QMEs were derived without resorting to the secular approximation from the Redfield equation which doe… ▽ More The Lindblad form guarantees complete positivity of a Markovian quantum master equation (QME). However, its microscopic derivation for a quantum system weakly interacting with a thermal bath requires several approximations, which may result in inaccuracies in the QME. Recently, various Lindbladian QMEs were derived without resorting to the secular approximation from the Redfield equation which does not guarantee the complete positivity. Here we explicitly calculate, in a perturbative manner, the equilibrium steady states of these Lindbladian QMEs. We compare the results with the steady state of the Redfield equation obtained from an analytic continuation method, which coincides with the so-called mean force Gibbs (MFG) state. The MFG state is obtained by integrating out the bath degrees of freedom for the Gibbs state of the total Hamiltonian. We explicitly show that the steady states of the Lindbladian QMEs are different from the MFG state. Our results indicate that manipulations of the Redfield equation needed to enforce complete positivity of a QME drives its steady state away from the MFG state. We also find that, in the high-temperature regime, both the steady states of the Lindbladian QMEs and MFG state reduce to the same Gibbs state of a system Hamiltonian under certain conditions. △ Less

Submitted 21 November, 2022; v1 submitted 7 July, 2022; originally announced July 2022.

Comments: 14 pages, 3 figures, 1 table

Journal ref: Phys. Rev. E 106, 054145 (2022)

Speed limit for a highly irreversible process and tight finite-time Landauer's bound

Authors: Jae Sung Lee, Sangyun Lee, Hyukjoon Kwon, Hyunggyu Park

Abstract: Landauer's bound is the minimum thermodynamic cost for erasing one bit of information. As this bound is achievable only for quasistatic processes, finite-time operation incurs additional energetic costs. We find a tight finite-time Landauer's bound by establishing a general form of the classical speed limit. This tight bound well captures the divergent behavior associated with the additional cost… ▽ More Landauer's bound is the minimum thermodynamic cost for erasing one bit of information. As this bound is achievable only for quasistatic processes, finite-time operation incurs additional energetic costs. We find a tight finite-time Landauer's bound by establishing a general form of the classical speed limit. This tight bound well captures the divergent behavior associated with the additional cost of a highly irreversible process, which scales differently from a nearly irreversible process. We also find an optimal dynamics which saturates the equality of the bound. We demonstrate the validity of this bound via discrete one-bit and coarse-grained bit systems. Our work implies that more heat dissipation than expected occurs during high-speed irreversible computation. △ Less

Submitted 16 June, 2022; v1 submitted 15 April, 2022; originally announced April 2022.

Journal ref: Phys. Rev. Lett. 129, 120603 (2022)

Flexoelectric control of a ferromagnetic metal

Authors: Wei Peng, Se Young Park, Chang Jae Roh, Junsik Mun, Hwiin Ju, Jinkwon Kim, Eun Kyo Ko, Zhengguo Liang, Sungsoo Hahn, Jinfeng Zhang, Liang Si, Yong Jin Jo, Tae Heon Kim, Changyoung Kim, Lingfei Wang, Miyoung Kim, Jong Seok Lee, Tae Won Noh, Daesu Lee

Abstract: Electric fields have played a key role in discovering and controlling exotic electronic states of condensed matter. However, electric fields usually do not work in metals as free carriers tend to screen electrostatic fields. While a pseudo-electric field generated by inhomogeneous lattice strain, namely a flexoelectric field, can in principle work in all classes of materials, it remains experiment… ▽ More Electric fields have played a key role in discovering and controlling exotic electronic states of condensed matter. However, electric fields usually do not work in metals as free carriers tend to screen electrostatic fields. While a pseudo-electric field generated by inhomogeneous lattice strain, namely a flexoelectric field, can in principle work in all classes of materials, it remains experimentally unexplored in metals. Here, using heteroepitaxy and atomic-scale imaging, we show that flexoelectric fields can polarize a metallic oxide SrRuO3 with unexpectedly large Ru off-center displacements. We also observe that the flexoelectrically induced polar state of SrRuO3 leads to sizable lattice expansion, similar to the electrostrictive expansion caused by ionic displacements in dielectrics under an external electric field. We further suggest that flexoelectrically driven Ru off-centering promotes strong coupling between lattice and electronic degrees of freedom, possibly enhancing the ferromagnetism of SrRuO3. Beyond conventional electric fields, flexoelectric fields may universally engender novel electronic states and their control via pure atomic displacements in a nondestructive and fast manner. △ Less

Submitted 7 March, 2022; originally announced March 2022.

Comments: 33 pages, 13 figures

Atomistic simulations of the deformation behavior of an Nb nanowire embedded in a NiTi shape memory alloy

Authors: Jung Soo Lee, Won-Seok Ko, Blazej Grabowski

Abstract: The influence of pre-strain and temperature on the superior properties exhibited by an Nb nanowire embedded in a NiTi shape memory alloy (SMA) are investigated via molecular dynamics simulations. To this end, a new Nb-Ni-Ti ternary interatomic potential based on the second nearest-neighbor modified embedded-atom method (2NN MEAM) is developed and employed. The origin of the unique phenomena of qua… ▽ More The influence of pre-strain and temperature on the superior properties exhibited by an Nb nanowire embedded in a NiTi shape memory alloy (SMA) are investigated via molecular dynamics simulations. To this end, a new Nb-Ni-Ti ternary interatomic potential based on the second nearest-neighbor modified embedded-atom method (2NN MEAM) is developed and employed. The origin of the unique phenomena of quasi-linear elasticity, slim hysteresis, and reduction in Young's modulus observed for pre-strained nanowire-SMA composites is uncovered. The results demonstrate the importance of plastic deformation in the embedded Nb nanowires and reveal how the deformation facilitates the just-mentioned, unprecedented phenomena. A simple and straightforwardly obtainable descriptor to correlate and monitor Young's modulus evolution during pre-straining is proposed. Furthermore, our simulations suggest that the desired Young's modulus can be obtained for a wide range of application temperatures through appropriate pre-straining. △ Less

Submitted 19 February, 2022; originally announced February 2022.

Comments: 59 pages, 25 figures, 10 tables

Journal ref: Acta Materialia 228 (2022) 117764

Observation of nonlocal Josephson effect on double InAs nanowires

Authors: Sadashige Matsuo, Joon Sue Lee, Chien-Yuan Chang, Yosuke Sato, Kento Ueda, Christopher J. Palmstrøm, Seigo Tarucha

Abstract: Short-range coherent coupling of two Josephson junctions (JJs) are predicted to generate a supercurrent in one JJ nonlocally modulated by the phase difference in the other. We report on observation of the nonlocal Josephson effect on double InAs nanowires as experimental evidence of the coherent coupling. We measured one JJ sharing one superconducting electrode with the other JJ and observed switc… ▽ More Short-range coherent coupling of two Josephson junctions (JJs) are predicted to generate a supercurrent in one JJ nonlocally modulated by the phase difference in the other. We report on observation of the nonlocal Josephson effect on double InAs nanowires as experimental evidence of the coherent coupling. We measured one JJ sharing one superconducting electrode with the other JJ and observed switching current oscillation as a control of the nonlocal phase difference. Our result is an important step toward engineering of novel superconducting phenomena with the short-range coherent coupling. △ Less

Submitted 24 December, 2021; originally announced December 2021.