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Professor Jae Sung Lee continues to work on various emerging phenomena and important questions raised in physics via methodologies developed in statistical physics. One of his main interests is the stochastic thermodynamics of microscopic systems. Early studies in thermodynamics focused on macroscopic equilibrium constructs; however, new technologies and advanced experimental techniques have enabled a closer look at the thermodynamics of these systems at microscopic levels, such as that of cellsize biological systems. With a focus on fundamental thermodynamic principles such as the second law of thermodynamics and thermodynamic tradeoff relations, which have received much attention during the last decade, he applies these principles to resolving the mechanisms and thermodynamic properties of microscopic heat engines, activeparticle systems, and open quantum systems. In addition, his research interests also comprise a broad range of statistical physics phenomena, including the phase transitions of complex systems and the resistive switching behavior of resistive random access memory devices via dynamic percolation models.



Publication at kias
NUMBER  P20018 
AUTHOR  Park, Hyunggyu,Park, JongMin,Lee, Jae Sung,Lee, Jae Sung 
TITLE  Brownian heat engine with active reservoirs 
ARCHIVE  condmat/2003.13189 
FILE  
JOURNAL  PHYSICAL REVIEW E, 2020 
ABSTRACT  Microorganisms such as bacteria are active matter which consume chemical energy and generate their unique runandtumble motion. A swarm of such microorganisms provide a nonequilibrium active environment whose noise characteristics are different from those of thermal equilibrium reservoirs. One important difference is a finite persistence time, which is considerably large compared to that of the equilibrium noise, that is, the active noise is colored. Here we study a mesoscopic energyharvesting device (engine) with active reservoirs harnessing this noise nature. For an exactly solvable linear model, we show that the performance from the active environment can surpass that from the equilibrium environment. Furthermore, we propose a proper definition of the activereservoir temperature and show that the engine efficiency can overcome the conventional Carnot bound, thus the powerefficiency tradeoff constraint is released. We also show that the efficiency at the maximum power can surpass the CurzonAhlborn efficiency. This remarkable enhancement originates from the extra unconventional entropy production beyond the conventional Clausius entropy production, due to the nonMarkovian nature of the active reservoirs. Interestingly, the supremacy of the active engine critically depends on the timescale symmetry of two active reservoirs. 

Publication at kias
NUMBER  P20017 
AUTHOR  Park, Hyunggyu,Park, JongMin,Lee, Jae Sung,Lee, Jae Sung 
TITLE  Exactly solvable twoterminal heat engine with asymmetric Onsager coefficients: Origin of the powerefficiency bound 
ARCHIVE  condmat/2003.01869 
FILE  
JOURNAL  PHYSICAL REVIEW E, 2020 
ABSTRACT  An engine producing a finite power at the ideal (Carnot) efficiency is a dream engine which is not prohibited by the thermodynamic second law. Some years ago, a twoterminal heat engine with asymmetric Onsager coefficients in the linear response regime was suggested by Benenti et al. [Phys. Rev. Lett. 106, 230602 (2011)], as a prototypical system to make such a dream come true with nondivergent system parameter values. However, such a system has never been realized, in spite of many trials. Here, we introduce an exactly solvable twoterminal Brownian heat engine with the asymmetric Onsager coefficients in the presence of a Lorenz (magnetic) force. Nevertheless, we show that the dream engine regime cannot be accessible, even with the asymmetric Onsager coefficients, due to an instability keeping the engine from reaching its steady state. This is consistent with recent tradeoff relations between the engine power and efficiency, where the (cyclic) steadystate condition is implicitly presumed. We conclude that the inaccessibility to the dream engine originates from the steadystate constraint on the engine. 

Publication at kias
NUMBER  Q19010 
AUTHOR  Park, Hyunggyu,Lee, Jae Sung,Lee, Jae Sung 
TITLE  Carnot Efficiency and ZeroEntropy Production Rate Do Not Guarantee Reversibility of a Process 
ARCHIVE  condmat/1812.10651 
FILE  
JOURNAL  JOURNAL OF THE KOREAN PHYSICAL SOCIETY, 2019 
ABSTRACT  A thermodynamic process at a zeroentropy production (EP) rate has been regarded as a reversible process. A process achieving the Carnot efficiency is also considered a reversible process. Therefore, the condition, Carnot efficiency at zeroEP rate can be regarded as a strong condition for a reversible process. Here, however, we show that the detailed balance can be broken for a zeroEP rate process and even for a process achieving the Carnot efficiency at a zeroEP rate in an example of a quantumdot model. This clearly demonstrates that Carnot efficiency at zeroEP rate or just zeroEP rate is not a sufficient condition for a reversible process. 

Publication at kias
NUMBER  P19052 
AUTHOR  Park, Hyunggyu,Park, JongMin,Lee, Jae Sung,Lee, Jae Sung 
TITLE  Thermodynamic uncertainty relation for underdamped Langevin systems driven by a velocitydependent force 
ARCHIVE  condmat.statmech/1907.06221 
FILE  
JOURNAL  PHYSICAL REVIEW E, 2019 
ABSTRACT  Recently, it has been shown that there is a tradeoff relation between thermodynamic cost and current fluctuations, referred to as the thermodynamic uncertainty relation (TUR). The TUR has been derived for various processes, such as discretetime Markov jump processes and overdamped Langevin dynamics. For underdamped dynamics, it has recently been reported that some modification is necessary for application of the TUR. However, the previous TUR for underdamped dynamics is not applicable to a system driven by a velocitydependent force. In this study, we present a TUR, applicable to a system driven by a velocitydependent force in the context of underdamped Langevin dynamics, by extending the theory of Vu and Hasegawa [Phys. Rev. E 100, 032130 (2019)]. We show that our TUR accurately describes the tradeoff properties of a molecular refrigerator (cold damping), Brownian dynamics in a magnetic field, and an active particle system. 

Publication at kias
NUMBER  Q18009 
AUTHOR  Park, Hyunggyu,Lee, Jae Sung,Lee, Jae Sung 
TITLE  Stochastic thermodynamics and hierarchy of fluctuation theorems with multiple reservoirs 
ARCHIVE  
FILE  
JOURNAL  NEW JOURNAL OF PHYSICS, 2018 
ABSTRACT  We reformulate stochastic thermodynamics in terms of noise realizations for Langevin systems in contact with multiple reservoirs and investigate the structure of the second laws of thermodynamics. We derive a hierarchy of fluctuation theorems when one degree of freedom of the system is affected by multiple reservoirs simultaneously, that is, when noisemixing occurs. These theorems and the associated second laws of thermodynamics put stricter bounds on the thermodynamics of Langevin systems. We apply our results to a stochastic machine in noisemixing environments and demonstrate that our new bounds play a crucial role in determining the potential function and performance of the machine. 

Publication at kias
NUMBER  Q17062 
AUTHOR  Park, Hyunggyu,Lee, Jae Sung,Lee, Jae Sung 
TITLE  Additivity of multiple heat reservoirs in the Langevin equation 
ARCHIVE  arXiv:1712.00972 
FILE  
JOURNAL  PHYSICAL REVIEW E, 2018 
ABSTRACT  The Langevin equation greatly simplifies the mathematical expression of the effects of thermal noise by using only two terms, a dissipation term, and a randomnoise term. The Langevin description was originally applied to a system in contact with a single heat reservoir; however, many recent studies have also adopted a Langevin description for systems connected to multiple heat reservoirs. This is accomplished through the introduction of a simple summation for the dissipation and randomnoise terms associated with each reservoir. However, the validity of this simple addition has been the focus of only limited discussion and has raised several criticisms. Moreover, this additive description has never been either experimentally or numerically verified, rendering its validity is still an open question. Here we perform molecular dynamics simulations for a Brownian system in simultaneous contact with multiple heat reservoirs to check the validity of this additive approach. Our simulation results confirm that the effect of multiple heat reservoirs is additive in general. A very small deviation in the total amount of dissipation and associated noise is found but seems not significant within statistical errors. We find that the steadystate properties satisfy the additivity perfectly and are not affected by this deviation. 

Publication at kias
NUMBER  Q17027 
AUTHOR  Park, Hyunggyu,Lee, Jae Sung,Lee, Jae Sung 
TITLE  Carnot efficiency is reachable in an irreversible process 
ARCHIVE  arXiv:1611.07665 
FILE  
JOURNAL  SCIENTIFIC REPORTS, 2017 
ABSTRACT  In thermodynamics, there exists a conventional belief that the Carnot efficiency is reachable only in the reversible ( zero entropy production) limit of nearly reversible processes. However, there is no theorem proving that the Carnot efficiency is unattainable in an irreversible process. Here, we show that the Carnot efficiency is reachable in an irreversible process through investigation of the FeynmanSmoluchowski ratchet (FSR). We also show that it is possible to enhance the efficiency by increasing the irreversibility. Our result opens a new possibility of designing an efficient heat engine in a highly irreversible process and also answers the longstanding question of whether the FSR can operate with the Carnot efficiency. 

Publication at kias
NUMBER  Q17028 
AUTHOR  Lee, Jae Sung,Lee, Jae Sung 
TITLE  Brownmillerite thin films as fast ion conductors for ultimateperformance resistance switching memory 
ARCHIVE  
FILE  
JOURNAL  NANOSCALE, 2017 
ABSTRACT  An oxidebased resistance memory is a leading candidate to replace Sibased flash memory as it meets the emerging specifications for future memory devices. The nonuniformity in the key switching parameters and low endurance in conventional resistance memory devices are preventing its practical application. Here, a novel strategy to overcome the aforementioned challenges has been unveiled by tuning the growth direction of epitaxial brownmillerite SrFeO2.5 thin films along the SrTiO3 [111] direction so that the oxygen vacancy channels can connect both the top and bottom electrodes rather directly. The controlled oxygen vacancy channels help reduce the randomness of the conducting filament (CF). The resulting device displayed high endurance over 10(6) cycles, and a short switching time of similar to 10 ns. In addition, the device showed very high uniformity in the key switching parameters for devicetodevice and within a device. This work demonstrates a feasible example for improving the nanoscale device performance by controlling the atomic structure of a functional oxide layer. 

Publication at kias
NUMBER  Q17035 
AUTHOR  Lee, Jae Sung,Lee, Jae Sung 
TITLE  Understanding the transition between memory and threshold switching due to the compliance current 
ARCHIVE  
FILE  
JOURNAL  JOURNAL OF THE KOREAN PHYSICAL SOCIETY, 2016 
ABSTRACT  The switchingtype transition induced by external parameters, e.g., the thermal conductance and the temperature, has been intensively investigated by using the thermal random circuit breaker (RCB) network model. Recently, some researchers argued that the thermal RCB network model was not able to predict the switchingtype transition induced by a new parameter, i.e., the compliance current. However, we demonstrate that the compliance currentinduced transition can be explained by using the thermal RCB network model. This work clearly demonstrates that the basic mechanism of unipolar resistance switching is closely related to the formation and the rupture of conducting filaments due to the bias voltage and thermal effects. 

Publication at kias
NUMBER  Q17029 
AUTHOR  Noh, Jae Dong,Lee, Jae Sung,Lee, Jae Sung 
TITLE  Optimal tuning of a confined Brownian information engine 
ARCHIVE  
FILE  
JOURNAL  PHYSICAL REVIEW E, 2016 
ABSTRACT  A Brownian information engine is a device extracting mechanical work from a single heat bath by exploiting the information on the state of a Brownian particle immersed in the bath. As for engines, it is important to find the optimal operating condition that yields the maximum extracted work or power. The optimal condition for a Brownian information engine with a finite cycle time tau has been rarely studied because of the difficulty in finding the nonequilibrium steady state. In this study, we introduce a model for the Brownian information engine and develop an analytic formalism for its steadystate distribution for any tau. We find that the extracted work per engine cycle is maximum when t approaches infinity, while the power is maximum when t approaches zero. 

Publication at kias
NUMBER  Q17030 
AUTHOR  Lee, J. S.,Lee, J. S. 
TITLE  Hybrid Percolation Transition in Cluster Merging Processes: Continuously Varying Exponents 
ARCHIVE  
FILE  
JOURNAL  PHYSICAL REVIEW LETTERS, 2016 
ABSTRACT  Consider growing a network, in which every new connection is made between two disconnected nodes. At least one node is chosen randomly from a subset consisting of g fraction of the entire population in the smallest clusters. Here we show that this simple strategy for improving connection exhibits a more unusual phase transition, namely a hybrid percolation transition exhibiting the properties of both firstorder and secondorder phase transitions. The cluster size distribution of finite clusters at a transition point exhibits powerlaw behavior with a continuously varying exponent tau in the range 2 < tau (g) <= 2.5. This pattern reveals a necessary condition for a hybrid transition in cluster aggregation processes, which is comparable to the powerlaw behavior of the avalanche size distribution arising in models with linkdeleting processes in interdependent networks. 

Publication at kias
NUMBER  Q17031 
AUTHOR  Lee, Jae Sung,Lee, Jae Sung 
TITLE  Resistive switching phenomena: A review of statistical physics approaches 
ARCHIVE  
FILE  
JOURNAL  APPLIED PHYSICS REVIEWS, 2015 
ABSTRACT  Resistive switching (RS) phenomena are reversible changes in the metastable resistance state induced by external electric fields. After discovery similar to 50 years ago, RS phenomena have attracted great attention due to their potential application in nextgeneration electrical devices. Considerable research has been performed to understand the physical mechanisms of RS and explore the feasibility and limits of such devices. There have also been several reviews on RS that attempt to explain the microscopic origins of how regions that were originally insulators can change into conductors. However, little attention has been paid to the most important factor in determining resistance: how conducting local regions are interconnected. Here, we provide an overview of the underlying physics behind connectivity changes in highly conductive regions under an electric field. We first classify RS phenomena according to their characteristic currentvoltage curves: unipolar, bipolar, and threshold switchings. Second, we outline the microscopic origins of RS in oxides, focusing on the roles of oxygen vacancies: the effect of concentration, the mechanisms of channel formation and rupture, and the driving forces of oxygen vacancies. Third, we review RS studies from the perspective of statistical physics to understand connectivity change in RS phenomena. We discuss percolation model approaches and the theory for the scaling behaviors of numerous transport properties observed in RS. Fourth, we review various switchingtype conversion phenomena in RS: bipolarunipolar, memorythreshold, figureofeight, and counterfigureofeight conversions. Finally, we review several related technological issues, such as improvement in high resistance fluctuations, sneakpath problems, and multilevel switching problems. (C) 2015 AIP Publishing LLC. 

Publication at kias
NUMBER  Q17032 
AUTHOR  Lee, J. S.,Lee, J. S. 
TITLE  AshkinTeller model and diverse opinion phase transitions on multiplex networks 
ARCHIVE  
FILE  
JOURNAL  PHYSICAL REVIEW E, 2015 
ABSTRACT  Multiplex networks (MNs) have become a platform of recent research in network sciences because networks in many realworld systems interact and function together. One of the main scientific issues in MNs is how the interdependence changes the emerging patterns or phase transitions. Until now, studies of such an issue have concentrated on clusterbreakdown phenomena, aiming to understand the resilience of the system under random failures of edges. These studies have revealed that various phase transition (PT) types emerge in MNs. However, such studies are rather limited to percolationrelated problems, i.e., the limit q > 1 of the qstate Potts model. Thus, a systematic study of opinion formation in social networks with the effect of interdependence between different social communities, which may be seen as the study of the emerging pattern of the Ising model on MNs, is needed. Here we study a wellknown spin model called the AshkinTeller (AT) model in scalefree networks. The AT model can be regarded as a model for interacting systems between two species of Ising spins placed on respective layers in doublelayer networks. Our study shows that, depending on the interlayer coupling strength and a network topology, unconventional PT patterns can also emerge in interactionbased phenomena: continuous, discontinuous, successive, and mixedorder PTs and a continuous PT not satisfying the scaling relation. The origins of such rich PT patterns are elucidated in the framework of LandauGinzburg theory. 

Publication at kias
NUMBER  Q17034 
AUTHOR  Lee, J. S.,Lee, J. S. 
TITLE  Groundstate energy of the qstate Potts model: The minimum modularity 
ARCHIVE  
FILE  
JOURNAL  PHYSICAL REVIEW E, 2014 
ABSTRACT  A wide range of interacting systems can be described by complex networks. A common feature of such networks is that they consist of several communities or modules, the degree of which may quantified as the modularity. However, even a random uncorrelated network, which has no obvious modular structure, has a finite modularity due to the quenched disorder. For this reason, the modularity of a given network is meaningful only when it is compared with that of a randomized network with the same degree distribution. In this context, it is important to calculate the modularity of a random uncorrelated network with an arbitrary degree distribution. The modularity of a random network has been calculated [Reichardt and Bornholdt, Phys. Rev. E 76, 015102 (2007)]; however, this was limited to the case whereby the network was assumed to have only two communities, and it is evident that the modularity should be calculated in general with q(>= 2) communities. Here we calculate the modularity for q communities by evaluating the groundstate energy of the qstate Potts Hamiltonian, based on replica symmetric solutions assuming that the mean degree is large. We found that the modularity is proportional to /< k > regardless of q and that only the coefficient depends on q. In particular, when the degree distribution follows a power law, the modularity is proportional to < k >(1/2). Our analytical results are confirmed by comparison with numerical simulations. Therefore, our results can be used as reference values for realworld networks. 

Publication at kias
NUMBER  Q17033 
AUTHOR  Lee, Jae Sung 
TITLE  Novel ElectroformingFree Nanoscaffold Memristor with Very High Uniformity, Tunability, and Density 
ARCHIVE  
FILE  
JOURNAL  ADVANCED MATERIALS, 2014 
ABSTRACT  A novel device structure is developed, which uses easytogrow nano scaffold films to localize oxygen vacancies at vertical heterointerfaces. The strategy is to design vertical interfaces using two structurally incompatible oxides, which are likely to generate a highconcentration oxygen vacancy. Nonlinear electroresistance at room temperature is demonstrated using these nano scaffold devices. The resistance variations exceed two orders of magnitude with very high uniformity and tunability. 

Publication at kias
NUMBER  Q17036 
AUTHOR  Lee, Jae Sung,Lee, Jae Sung 
TITLE  Anomalous effect due to oxygen vacancy accumulation below the electrode in bipolar resistance switching Pt/Nb:SrTiO3 cells 
ARCHIVE  
FILE  
JOURNAL  APL MATERIALS, 2014 
ABSTRACT  In conventional semiconductor theory, greater doping decreases the electronic resistance of a semiconductor. For the bipolar resistance switching (BRS) phenomena in oxides, the same doping principle has been used commonly to explain the relationship between the density variation of oxygen vacancies (Vo(center dot center dot)) and the electronic resistance. We find that the Vo(center dot center dot) density can change at a depth of similar to 10 nm below the Pt electrodes in Pt/Nb:SrTiO3 cells, depending on the resistance state. Using electron energy loss spectroscopy and secondary ion mass spectrometry, we found that greater Vo(center dot center dot) density underneath the electrode resulted in higher resistance, contrary to the conventional doping principle of semiconductors. To explain this seemingly anomalous experimental behavior, we provide quantitative explanations on the anomalous BRS behavior by simulating the mobile Vo(center dot center dot) [J. S. Lee et al., Appl. Phys. Lett. 102, 253503 (2013)] near the Schottky barrier interface. (C) 2014 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License. 

Publication at kias
NUMBER  Q17037 
AUTHOR  Park, Hyunggyu,Lee, Jae Sung,Lee, Jae Sung 
TITLE  Modified saddlepoint integral near a singularity for the large deviation function 
ARCHIVE  
FILE  
JOURNAL  JOURNAL OF STATISTICAL MECHANICSTHEORY AND EXPERIMENT, 2013 
ABSTRACT  Longtimeintegrated quantities in stochastic processes, in or out of equilibrium, usually exhibit rare but huge fluctuations. Work or heat production is such a quantity, for which the probability distribution function displays an exponential decay characterized by the large deviation function (LDF). The LDF is often deduced from the cumulant generating function through the inverse Fourier transformation. The saddlepoint integration method is a powerful technique to obtain the asymptotic results in the Fourier integral, but special care should be taken when the saddle point is located near a singularity of the integrand. In this paper, we present a modified saddlepoint method to handle such a difficulty efficiently. We investigate the dissipated and injected heat production in equilibration processes with various initial conditions, for example, where the generating functions contain branchcut singularities as well as powerlaw ones. Exploiting the new modified saddlepoint integrations, we obtain the leading finitetime corrections for the LDFs, which are confirmed by numerical results. 

Publication at kias
NUMBER  Q17041 
AUTHOR  Lee, Jae Sung,Lee, Jae Sung 
TITLE  Two opposite hysteresis curves in semiconductors with mobile dopants 
ARCHIVE  
FILE  
JOURNAL  APPLIED PHYSICS LETTERS, 2013 
ABSTRACT  Semiconductors with mobile dopants (SMDs), which are distinct from conventional semiconductors, exhibit hysteretic currentvoltage curves. The fundamental feature of this hysteresis curve is that it exhibits two oppositely rotating directions, whose origin is not clarified yet. Here, we investigate microscopic origin of the two types of curves and show that they result from the spatial inhomogeneity of the mobile dopant distribution in the SMD. In particular, we observed an abnormal modulation of the electronic energy band due to mobile dopants; lower (higher) density of dopants near a metalsemiconductor interface lead to higher (lower) conductance, whereas the conventional ionic models predict the reverse behaviors. (C) 2013 AIP Publishing LLC. 

Publication at kias
NUMBER  Q17040 
AUTHOR  Lee, Jae Sung,Lee, Jae Sung 
TITLE  Statistical properties of the electrically induced contact resistance between two stainless steel balls 
ARCHIVE  
FILE  
JOURNAL  APPLIED PHYSICS LETTERS, 2013 
ABSTRACT  The electrical contact resistance between two stainless steel balls was measured to study the electrical conduction of a metallic contact separated by a thin oxide layer. Through a statistical approach, the contact resistance was found to have a bimodal lognormal distribution. This result reflects conduction by tunneling and electrically induced metal bridge, which was explained by the simulation of a random circuit breaker model, inside the insulating layer. Based on the results of this study, we suggest an effective conduction model to explain macroscopic electrical contact systems with a nanoor microscopic filamentary conduction mechanism. (C) 2013 AIP Publishing LLC. 

Publication at kias
NUMBER  Q17038 
AUTHOR  Park, Hyunggyu,Lee, J. S.,Lee, J. S. 
TITLE  Everlasting initial memory threshold for rare events in equilibration processes 
ARCHIVE  
FILE  
JOURNAL  PHYSICAL REVIEW E, 2013 
ABSTRACT  Conventional wisdom indicates that initial memory should decay away exponentially in time for general (noncritial) equilibration processes. In particular, timeintegrated quantities such as heat are presumed to lose initial memory in a sufficiently longtime limit. However, we show that the large deviation function of timeintegrated quantities may exhibit initial memory effect even in the infinitetime limit, if the system is initially prepared sufficiently far away from equilibrium. For a Brownian particle dynamics, as an example, we found a sharp finite threshold rigorously, beyond which the corresponding large deviation function contains everlasting initial memory. The physical origin for this phenomenon is explored with an intuitive argument and also from a toy model analysis. Our results can be applied to general nonequilibrium relaxation processes reaching (non)equilibrium steady states. DOI: 10.1103/PhysRevE.87.020104 

Publication at kias
NUMBER  Q17039 
AUTHOR  Lee, J. S.,Lee, J. S. 
TITLE  Origin of variation in switching voltages in thresholdswitching phenomena of VO2 thin films 
ARCHIVE  
FILE  
JOURNAL  APPLIED PHYSICS LETTERS, 2013 
ABSTRACT  We investigated the origin of the variation in switching voltages in thresholdswitching of VO2 thin films. When a triangularwaveform voltage signal was applied, the current changed abruptly at two switching voltages, i.e., VON (insulatortometal) and VOFF (metaltoinsulator). VON and VOFF were measured by changing the period of the voltage signal, the temperature of the environment, and the load resistance. We observed that either VON or VOFF varied significantly and had different dependences with respect to the external parameters. Based on the mechanism of the metalinsulator transition induced by Joule heating, numerical simulations were performed, which quantitatively reproduced all of the experimental results. From the simulation analysis, the variation in the switching voltages for thresholdswitching was determined to be thermal in origin. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4790842] 

Publication at kias
NUMBER  P12041 
AUTHOR  Lee, J. S. 
TITLE  Forming mechanism of the bipolar resistance switching in doublelayer memristive nanodevices 
ARCHIVE  
FILE  
JOURNAL  NANOTECHNOLOGY, 2012 
ABSTRACT  To initiate resistance switching phenomena, it is usually necessary to apply a strong electric field to a sample. This forming process poses very serious obstacles in real nanodevice applications. In unipolar resistance switching (URS), it is well known that the forming originates from soft dielectric breakdown. However, the forming in bipolar resistance switching (BRS) is poorly understood. In this study, we investigated the forming processes in Pt/Ta2O5/TaOx/Pt and Pt/TaOx/Pt nanodevices, which showed BRS and URS, respectively. By comparing the doubleand singlelayer systems, we were able to observe differences in the BRS and URS forming processes. Using computer simulations based on an interfacemodified random circuit breaker network model, we could explain most of our experimental observations. This success suggests that the BRS forming in our Pt/Ta2O5/TaOx/Pt doublelayer system can occur via two processes, i.e., polaritydependent resistance switching in the Ta2O5 layer and soft dielectric breakdown in the TaOx layer. This forming mechanism can be used to improve the performance of BRS devices. For example, we could improve the endurance properties of Pt/Ta2O5/TaOx/Pt cells by using a small forming voltage. 
 1998  2003 : Department of Physics, Seoul National University (BS)
 2003  2010 : Department of Physics, Seoul National University (PhD)
 2010  2011 : Research Fellow, Department of Physics, Seoul National University
 2011  2006 : Presidential PostDoc Fellow, School of Physics, KIAS
 2016  2021 : Research Professor, Quantum Universe Center, KIAS
 2021  present : Professor, School of Physics, KIAS
 2014 Haksul award, KIAS
 2019 Yongbong award, Korean Physics Society
 Office:1309 / TEL) 8229583802 / FAX) 8229582590
 Center for Advanced Computation, Korea Institute for Advanced Study
85 Hoegiro, Dongdaemungu, Seoul 02455, Korea