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Field-controlled breaking and restoration of parity-time symmetry in Josephson interference
Authors:
Yi-Chen Tsai,
Yung-Yeh Chang,
Tao-Yi Hsu,
Thomas Kuo,
Chia-Nung Kuo,
Chin-Shan Lue,
Kuei-Lin Chiu,
Chen-Hsuan Hsu,
Chung-Ting Ke
Abstract:
Symmetry plays a fundamental role in determining the phases and physical properties of quantum matter. Controlling symmetry in mesoscopic superconducting devices provides a route to reconfigure their phase-coherent transport. Here we demonstrate symmetry-selective Josephson interferometry in lateral NbTi/PtTe2/NbTi junctions by controlling the relative orientations of the current and magnetic fiel…
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Symmetry plays a fundamental role in determining the phases and physical properties of quantum matter. Controlling symmetry in mesoscopic superconducting devices provides a route to reconfigure their phase-coherent transport. Here we demonstrate symmetry-selective Josephson interferometry in lateral NbTi/PtTe2/NbTi junctions by controlling the relative orientations of the current and magnetic field. From the supercurrent interference patterns, we construct a field-current symmetry map that identifies configurations exhibiting or violating the device-level parity (\mathcal{P}), time-reversal (\mathcal{T}) and their combined \mathcal{P}\mathcal{T} symmetry. In the absence of an in-plane field, the junction exhibits a symmetric Fraunhofer pattern. An in-plane field parallel to the current produces a pronounced side-lobe asymmetry, whereas reversing both the current and the complete magnetic-field configuration restores a generalized \mathcal{T} relation. Remarkably, orienting the in-plane field perpendicular to the current restores the \mathcal{P}\mathcal{T}-symmetric Fraunhofer response even at substantial field strengths. A microscopic model attributes this behavior to the interplay between disorder-induced potential variations and flux dipoles generated by in-plane-field Meissner focusing near the superconducting electrodes. Our results establish a reconfigurable Josephson interferometer in which the field-current geometry selects the symmetry operation being probed and switches the device between symmetry-broken and symmetry-restored interference states.
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Submitted 17 August, 2026;
originally announced August 2026.
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Proton-electron coupled catalyst for ionomer-free electrochemical energy conversion
Authors:
Ao Zhang,
Ran Wang,
Mohammed O. Bazaid,
Shiyi Wang,
Ting-Jung Hsiao,
Yibo Wang,
Antonio Sorrentino,
Yang Liu,
Yu-Han Joseph Tsai,
Boxuan Zhou,
Bosi Peng,
Zeyan Liu,
Xiangfeng Duan,
Adam Z. Weber,
William A. Goddard III,
Seung Soon Jang,
Yu Huang
Abstract:
Efficient electrochemical energy devices are vital to renewable energy technology, yet coordinating the effective flow of electrons, ions, and chemical species continues to be a major challenge. In conventional proton-exchange membrane fuel cell (PEMFC) catalyst layers, proton and electron transport are supplied separately through percolating carbon networks and ionomer binders, rendering the cata…
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Efficient electrochemical energy devices are vital to renewable energy technology, yet coordinating the effective flow of electrons, ions, and chemical species continues to be a major challenge. In conventional proton-exchange membrane fuel cell (PEMFC) catalyst layers, proton and electron transport are supplied separately through percolating carbon networks and ionomer binders, rendering the catalyst largely passive and imposing fundamental trade-offs between reactant accessibility, ionic conductivity, and catalyst activity. Here, we introduce a one-dimensional proton-electron coupled catalyst (PECC) design, a transport-integrated electrocatalyst architecture in which the catalyst itself simultaneously supplies electronic and protonic transport to catalyst active sites. Using this PECC, PEMFCs can have an ionomer-free cathode catalyst layer (CCL), resulting in a dramatic 95% reduction in non-Fickian oxygen transport and boosting power density by 34% and 85% compared to traditional CCLs, with cathode Pt loadings of approximately 0.090 mg/cm^2 and 0.037 mg/cm^2, respectively. Meanwhile, PECC retains 65% of its mass activity and exhibits 32% higher power density than its ionomer-based CCL counterpart after 30k accelerated stressed test. Similar mass transport improvements have been observed in the electrochemical hydrogen pump (EHP) using PECC in the catalyst layers. Molecular dynamics simulations show the PECC's proton conductivity is 249% higher than Nafion. This PECC catalyst structure addresses core transport problems in PEMFCs, leading to almost 20% improvement in fuel efficiency and opens up new possibilities for designing high-performance, cost-effective electrochemical devices.
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Submitted 23 May, 2026;
originally announced May 2026.
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High-Throughput GW Calculations via Machine Learning
Authors:
Ragab. A. Abdelghany,
Chih-En Hsu,
Hung-Chung Hsueh,
Yuan-Hong Tsai,
Ming-Chiang Chung
Abstract:
We present a machine learning (ML) framework that predicts $G_0W_0$ quasiparticle energies across molecular dynamics (MD) trajectories with high accuracy and efficiency. Using only DFT-derived mean-field eigenvalues and exchange-correlation potentials, the model is trained on 25\% of MD snapshots and achieves RMSEs below 0.1 eV. It accurately reproduces k-resolved quasiparticle band structures and…
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We present a machine learning (ML) framework that predicts $G_0W_0$ quasiparticle energies across molecular dynamics (MD) trajectories with high accuracy and efficiency. Using only DFT-derived mean-field eigenvalues and exchange-correlation potentials, the model is trained on 25\% of MD snapshots and achieves RMSEs below 0.1 eV. It accurately reproduces k-resolved quasiparticle band structures and density of states, even for BN polymorphs excluded from the training data. This approach bypasses the computational bottlenecks of $G_0W_0$ simulations over dynamic configurations, offering a scalable route to excited-state electronic structure simulations with many-body accuracy.
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Submitted 5 May, 2025;
originally announced May 2025.
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Sustained Robust Exciton Emission in Suspended Monolayer WSe_2 within the Low Carrier Density Regime for Quantum Emitter Applications
Authors:
Zheng-Zhe Chen,
Chiao-Yun Chang,
Ya-Ting Tsai,
Po-Cheng Tsai,
Shih-Yen Lin,
Min-Hsiung Shih
Abstract:
The development of semiconductor optoelectronic devices is moving toward low power consumption and miniaturization, especially for high-efficiency quantum emitters. However, most of these quantum sources work at low carrier density region, where the Shockley-Read-Hall recombination may dominant and seriously reduce the emission efficiency. In order to diminish the affection of carrier trapping and…
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The development of semiconductor optoelectronic devices is moving toward low power consumption and miniaturization, especially for high-efficiency quantum emitters. However, most of these quantum sources work at low carrier density region, where the Shockley-Read-Hall recombination may dominant and seriously reduce the emission efficiency. In order to diminish the affection of carrier trapping and sustain a strong photoluminescence emission under low power pumping condition, we investigated on the influence of Suspending to monolayered tungsten diselenide, novel two-dimensional quantum material. Not only the PL intensity, but also the fundamental photoluminescence quantum yield has exhibited a huge, order-scale enhancement through suspending, even surprisingly, we found the PLQY improvement revealed far significantly under small pumping power and came out an exponential increase tendency toward even lower carrier density region. With its strong excitonic effect, suspended WSe_2 offers a solution to reduce carrier trapping and participate in non-radiative processes. Moreover, in the low-power range where SRH recombination dominates, suspended WSe_2 exhibited remarkably higher percentage of excitonic radiation compared to contacted WSe_2. Herein, we quantitatively demonstrate the significance of suspended WSe_2 monolayer at low carrier density region, highlighting its potential for developing compact, low-power quantum emitters in the future.
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Submitted 27 February, 2024;
originally announced February 2024.
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Integration of graphene-based superconducting quantum circuits in 3D cavity
Authors:
Kuei-Lin Chiu,
Youyi Chang,
Avishma J. Lasrado,
Cheng-Han Lo,
Yung-Hsiang Chen,
Tao-Yi Hsu,
Yen-Chih Chen,
Yi-Chen Tsai,
Samina,
Yen-Hsiang Lin,
Chung-Ting Ke
Abstract:
Integrating 2D materials into circuit quantum electrodynamics (c-QED) devices is an emerging filed in recent years. This integration not only facilitates the exploration of potential applications in quantum information processing but also enables the study of material's fundamental properties using microwave techniques. While most studies employ 2D coplanar architectures with scalability potential…
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Integrating 2D materials into circuit quantum electrodynamics (c-QED) devices is an emerging filed in recent years. This integration not only facilitates the exploration of potential applications in quantum information processing but also enables the study of material's fundamental properties using microwave techniques. While most studies employ 2D coplanar architectures with scalability potential, 3D cavity based c-QED devices, due to their simpler design, offer the advantage of a quicker turnaround to probe the composite Josephson junctions (JJs). Here, we construct the first flux-tunable, 3D cavity-compatible superconducting quantum circuit based on 2D materials, featuring a graphene superconducting quantum interference device (SQUID) shunted by a capacitor that is accessible by both DC and microwave probes. We have shown how flux-modulated cavity frequency can be linked to the SQUID critical current under the influence of Fraunhofer pattern. In addition, we extracted the symmetry information of the SQUIDs based on DC analysis, and correlated this with the flux-modulated cavity frequency observed in microwave measurements. Our platform can extend to topological materials, holding the prospect of establishing valid topological JJs with DC probe while allowing fast microwave probe to avoid quasiparticle poisoning.
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Submitted 2 March, 2025; v1 submitted 6 December, 2023;
originally announced December 2023.
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Physical Insights of Low Thermal Expansion Coefficient Electrode Stress Effect on Hafnia-Based Switching Speed
Authors:
Y. -T. Tsai,
C. -R. Liu,
Y. -T. Chen,
S. -M. Wang,
Z. -K. Chen,
C. -S. Pai,
Z. -R. Haung,
F. -S. Chang,
Z. -X. Li,
K. -Y. Hsiang,
M. -H. Lee,
Y. -T. Tang
Abstract:
In this report, we investigate the effect of low coefficient of thermal expansion (CTE) metals on the operating speed of hafnium-based oxide capacitance. We found that the cooling process of low CTE metals during rapid thermal annealing (RTA) generates in-plane tensile stresses in the film, This facilitates an increase in the volume fraction of the o-phase and significantly improves the domain swi…
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In this report, we investigate the effect of low coefficient of thermal expansion (CTE) metals on the operating speed of hafnium-based oxide capacitance. We found that the cooling process of low CTE metals during rapid thermal annealing (RTA) generates in-plane tensile stresses in the film, This facilitates an increase in the volume fraction of the o-phase and significantly improves the domain switching speed. However, no significant benefit was observed at electric fields less than 1 MV/cm. This is because at low voltage operation, the defective resistance (dead layer) within the interface prevents electron migration and the increased RC delay. Minimizing interface defects will be an important key to extending endurance and retention.
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Submitted 10 July, 2023;
originally announced July 2023.
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Liquid Droplet as Adaptive Material while Levitating via Coupling between Plasma and Kelvin Force
Authors:
Ping-Rui Tsai,
Hong-Yue Huang,
Ying-Pin Tsai,
Chih-Jung Lin,
Bo-Kai Xu,
Jih-Kang Hsieh,
Yu-Ting Cheng,
Cheng-Wei Lai,
Yu Hsuan Kao,
Wen-Chi Chen,
Fu-Li Hsiao,
Yu-Jane Sheng,
Po-Heng Lin,
Tzay-Ming Hong
Abstract:
Fascinating in art and science, the ability to float is also captivating and relevant in practical applications, such as Penning and ion traps that are fundamental to quantum computing. In this work, we first reproduce the classic water bridge by glycerol and, as it breaks down due to thermal agitation, observe that a lump of glycerol with mass~2.5 g can float and exhibit near-periodic oscillation…
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Fascinating in art and science, the ability to float is also captivating and relevant in practical applications, such as Penning and ion traps that are fundamental to quantum computing. In this work, we first reproduce the classic water bridge by glycerol and, as it breaks down due to thermal agitation, observe that a lump of glycerol with mass~2.5 g can float and exhibit near-periodic oscillations. Through experiments, finite element analysis, and simulations, we discover that the stability of the floating droplet is made possible by the interaction between three mechanisms: Deformation, Plasma, and Kelvin force. Note that glycerol cluster (GC) falls in the class of adaptive materials that can change their properties or behavior in response to varying environmental conditions, i.e., stimuli-responsive. Furthermore, the stimuli, modified by the deformation of GC, collaborate with it to create this unique simple, yet stable, floating system. Backed up by simulations, this process, operated by only a single pair of electrodes, holds the potential to develop a simple yet powerful railgun.
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Submitted 26 July, 2025; v1 submitted 3 July, 2023;
originally announced July 2023.
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Planar and Nematic Aerogels: DLCA and Superfluid 3He
Authors:
M. D. Nguyen,
J. S. Simon,
J. W. Scott,
Y. C. Cincia Tsai,
A. M. Zimmerman,
W. P. Halperin
Abstract:
We perform cluster aggregation simulations to model the structure of anisotropic aerogel. By biasing the diffusion process, we are able to obtain two distinct types of globally anisotropic aerogel structures which we call "nematic", with long strands along the anisotropy axis, and "planar", with long strands in planes perpendicular to the anisotropy axis. We calculate the auto-correlation function…
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We perform cluster aggregation simulations to model the structure of anisotropic aerogel. By biasing the diffusion process, we are able to obtain two distinct types of globally anisotropic aerogel structures which we call "nematic", with long strands along the anisotropy axis, and "planar", with long strands in planes perpendicular to the anisotropy axis. We calculate the auto-correlation function, the structure factor, and the angular dependence of the free-path distribution for these samples. The calculated structure factor from simulated aerogels can be compared with data from small-angle X-ray scattering (SAXS) of lab-grown aerogel allowing us to classify the spatial structure of the lab-grown samples. We find that the simulated "nematic" aerogel has a structure factor consistent with lab-grown, axially-compressed silica aerogel while the simulated "planar" aerogel has a structure factor consistent with lab-grown "stretched" silica aerogel. Unexpectedly, compressing previously isotropic silica aerogel leads to the formation of long strands along the compression axis while stretching silica aerogel leads to formation of planes perpendicular to the stretching axis. We discuss the implication of this determination on experiments of superfluid $^3$He in anisotropic aerogel, in particular the orbital analog of the spin-flop transition.
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Submitted 26 January, 2023;
originally announced January 2023.
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Deep Learning of Phase Transitions for Quantum Spin Chains from Correlation Aspects
Authors:
Ming-Chiang Chung,
Guang-Yu Huang,
Ian P. McCulloch,
Yuan-Hong Tsai
Abstract:
Using machine learning (ML) to recognize different phases of matter and to infer the entire phase diagram has proven to be an effective tool given a large dataset. In our previous proposals, we have successfully explored phase transitions for topological phases of matter at low dimensions either in a supervised or an unsupervised learning protocol with the assistance of quantum information related…
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Using machine learning (ML) to recognize different phases of matter and to infer the entire phase diagram has proven to be an effective tool given a large dataset. In our previous proposals, we have successfully explored phase transitions for topological phases of matter at low dimensions either in a supervised or an unsupervised learning protocol with the assistance of quantum information related quantities. In this work, we adopt our previous ML procedures to study quantum phase transitions of magnetism systems such as the XY and XXZ spin chains by using spin-spin correlation functions as the input data. We find that our proposed approach not only maps out the phase diagrams with accurate phase boundaries, but also indicates some new features that have not observed before. In particular, we define so-called relevant correlation functions to some corresponding phases that can always distinguish between those and their neighbors. Based on the unsupervised learning protocol we proposed [Phys. Rev. B 104, 165108 (2021)], the reduced latent representations of the inputs combined with the clustering algorithm show the connectedness or disconnectedness between neighboring clusters (phases), just corresponding to the continuous or disrupt quantum phase transition, respectively.
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Submitted 9 May, 2023; v1 submitted 16 January, 2023;
originally announced January 2023.
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Sparse Edge Encoder (SEE): I. Visual recognition in neuronal networks
Authors:
Chia-Ying Lin,
Mei Ian Sam,
Yi-Ching Tsai,
Hsiu-Hau Lin
Abstract:
In the past few decades, there have been intense debates whether the brain operates at a critical state. To verify the criticality hypothesis in the neuronal networks is challenging and the accumulating experimental and theoretical results remain controversial at this point. Here we simulate how visual information of a nature image is processed by the finite Kinouchi-Copelli neuronal network, extr…
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In the past few decades, there have been intense debates whether the brain operates at a critical state. To verify the criticality hypothesis in the neuronal networks is challenging and the accumulating experimental and theoretical results remain controversial at this point. Here we simulate how visual information of a nature image is processed by the finite Kinouchi-Copelli neuronal network, extracting the trends of the mutual information (how sensible the neuronal network is), the dynamical range (how sensitive the network responds to external stimuli) and the statistical fluctuations (how criticality is defined in conventional statistical physics). It is rather remarkable that the optimized state for visual recognition, although close to, does not coincide with the critical state where the statistical fluctuations reach the maximum. Different images and/or network sizes of course lead to differences in details but the trend of the information optimization remains the same. Our findings pave the first step to investigate how the information processing is optimized in different neuronal networks and suggest that the criticality hypothesis may not be necessary to explain why a neuronal network can process information smartly.
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Submitted 25 February, 2025; v1 submitted 28 November, 2022;
originally announced November 2022.
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Deep learning of topological phase transitions from entanglement aspects: An unsupervised way
Authors:
Yuan-Hong Tsai,
Kuo-Feng Chiu,
Yong-Cheng Lai,
Kuan-Jung Su,
Tzu-Pei Yang,
Tsung-Pao Cheng,
Guang-Yu Huang,
Ming-Chiang Chung
Abstract:
Machine learning techniques have been shown to be effective to recognize different phases of matter and produce phase diagrams in the parameter space interested, while they usually require prior labeled data to perform well. Here, we propose a machine learning procedure, mainly in an unsupervised manner, which can first identify topological/non-topological phases and then refine the locations of p…
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Machine learning techniques have been shown to be effective to recognize different phases of matter and produce phase diagrams in the parameter space interested, while they usually require prior labeled data to perform well. Here, we propose a machine learning procedure, mainly in an unsupervised manner, which can first identify topological/non-topological phases and then refine the locations of phase boundaries. By following this proposed procedure, we expand our previous work on the one-dimensional $p$-wave superconductor [Phys. Rev. B 102, 054512 (2020)] and further on the Su-Schrieffer-Heeger model, with an emphasis on using the quantum entanglement-based quantities as the input features. We find that our method not only reproduces similar results to the previous work with sharp phase boundaries but importantly it also does not rely on prior knowledge of the phase space, e.g., the number of phases present. We conclude with a few remarks about its potential, limitations, and explainabilities.
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Submitted 3 June, 2021; v1 submitted 9 May, 2021;
originally announced May 2021.
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Deep learning of topological phase transitions from entanglement aspects for two-dimensional chiral p-wave superconductors
Authors:
Ming-Chiang Chung,
Tsung-Pao Cheng,
Guang-Yu Huang,
Yuan-Hong Tsai
Abstract:
Applying deep learning to investigate topological phase transitions (TPTs) becomes a useful method due to not only its ability to recognize patterns but also its statistical excellency to examine the amount of information carried by different types of data inputs. Among possible data types, entanglement-related quantities, such as Majorana correlation matrices (MCMs), one-particle entanglement spe…
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Applying deep learning to investigate topological phase transitions (TPTs) becomes a useful method due to not only its ability to recognize patterns but also its statistical excellency to examine the amount of information carried by different types of data inputs. Among possible data types, entanglement-related quantities, such as Majorana correlation matrices (MCMs), one-particle entanglement spectra (OPES), and entanglement eigenvectors (OPEEs), have been proved effective, however, are to date mostly restricted to one dimension. Here, we propose practical input data forms based on those quantities to study TPTs and to compare the efficiency of each form on classic two-dimensional chiral $p$-wave superconductors via the deep learning approach. First, we find that different input forms, either matrices or tensors both originated from real MCMs, can affect the precise locations of the predicted transition points. Next, due to the complex nature of OPEEs, we extract three spatially dependent quantities from OPEEs, one related to the "intensity", and the other two related to "phases" of particle and hole components. We show that similar to taking OPES directly as inputs, solely using "intensity" quantity can only distinguish topological phases from trivial ones, whereas using either whole MCMs or complete OPEE-extracted quantities can provide sufficient information for deep learning to distinguish between phases of matter with different $U(1)$ gauges or Chern numbers. Finally, we discuss certain characteristic features in the deep learning approach and, in particular, they reveal that our trained models indeed learn physically meaningful features, which confirms the potential use even at high dimensions.
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Submitted 23 July, 2021; v1 submitted 8 April, 2021;
originally announced April 2021.
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Deep learning of topological phase transitions from entanglement aspects
Authors:
Yuan-Hong Tsai,
Meng-Zhe Yu,
Yu-Hao Hsu,
Ming-Chiang Chung
Abstract:
The one-dimensional $p$-wave superconductor proposed by Kitaev has long been a classic example for understanding topological phase transitions through various methods, such as examining Berry phase, edge states of open chains and, in particular, aspects from quantum entanglement of ground states. In order to understand the amount of information carried in the entanglement-related quantities, here…
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The one-dimensional $p$-wave superconductor proposed by Kitaev has long been a classic example for understanding topological phase transitions through various methods, such as examining Berry phase, edge states of open chains and, in particular, aspects from quantum entanglement of ground states. In order to understand the amount of information carried in the entanglement-related quantities, here we study topological phase transitions of the model with emphasis of using the deep learning approach. We feed different quantities, including Majorana correlation matrices (MCMs), entanglement spectra (ES) or entanglement eigenvectors (EE) originated from Block correlation matrices (BCMs), into the deep neural networks for training, and investigate which one could be the most useful input format in this approach. We find that ES is indeed too compressed information compared to MCM or EE. MCM and EE can provide us abundant information to recognize not only the topological phase transitions in the model but also phases of matter with different $U$(1) gauges, which is not reachable by using ES only.
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Submitted 26 December, 2019; v1 submitted 10 September, 2019;
originally announced September 2019.
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Contact Engineering High Performance n-Type MoTe2 Transistors
Authors:
Michal J. Mleczko,
Andrew C. Yu,
Christopher M. Smyth,
Victoria Chen,
Yong Cheol Shin,
Sukti Chatterjee,
Yi-Chia Tsai,
Yoshio Nishi,
Robert M. Wallace,
Eric Pop
Abstract:
Semiconducting MoTe2 is one of the few two-dimensional (2D) materials with a moderate band gap, similar to silicon. However, this material remains under-explored for 2D electronics due to ambient instability and predominantly p-type Fermi level pinning at contacts. Here, we demonstrate unipolar n-type MoTe2 transistors with the highest performance to date, including high saturation current (>400…
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Semiconducting MoTe2 is one of the few two-dimensional (2D) materials with a moderate band gap, similar to silicon. However, this material remains under-explored for 2D electronics due to ambient instability and predominantly p-type Fermi level pinning at contacts. Here, we demonstrate unipolar n-type MoTe2 transistors with the highest performance to date, including high saturation current (>400 $μA/μm$ at 80 K and >200 $μA/μm$ at 300 K) and relatively low contact resistance (1.2 to 2 $kΩ\cdotμm$ from 80 to 300 K), achieved with Ag contacts and AlOx encapsulation. We also investigate other contact metals, extracting their Schottky barrier heights using an analytic subthreshold model. High-resolution X-ray photoelectron spectroscopy reveals that interfacial metal-Te compounds dominate the contact resistance. Among the metals studied, Sc has the lowest work function but is the most reactive, which we counter by inserting monolayer h-BN between MoTe2 and Sc. These metal-insulator-semiconductor (MIS) contacts partly de-pin the metal Fermi level and lead to the smallest Schottky barrier for electron injection. Overall, this work improves our understanding of n-type contacts to 2D materials, an important advance for low-power electronics.
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Submitted 4 July, 2019;
originally announced July 2019.
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Optical control of carrier wavefunction in magnetic quantum dots
Authors:
P. Zhang,
T. Norden,
J. M. Pientka,
R. Oszwałdowski,
A. Najafi,
B. Barman,
Y. Tsai,
W-C. Fan,
W-C. Chou,
J. E. Han,
I. Žutić,
B. D. McCombe,
A. Petrou
Abstract:
Spatially indirect Type-II band alignment in magnetically-doped quantum dot (QD) structures provides unexplored opportunities to control the magnetic interaction between carrier wavefunction in the QD and magnetic impurities. Unlike the extensively studied, spatially direct, QDs with Type-I band alignment where both electrons and holes are confined in the QD, in ZnTe QDs embedded in a (Zn,Mn)Se ma…
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Spatially indirect Type-II band alignment in magnetically-doped quantum dot (QD) structures provides unexplored opportunities to control the magnetic interaction between carrier wavefunction in the QD and magnetic impurities. Unlike the extensively studied, spatially direct, QDs with Type-I band alignment where both electrons and holes are confined in the QD, in ZnTe QDs embedded in a (Zn,Mn)Se matrix only the holes are confined in the QDs. Photoexcitation with photon energy 3.06 eV (2.54 eV) generates electron-hole pairs predominantly in the (Zn,Mn)Se matrix (ZnTe QDs). The photoluminescence (PL) at 7 K in the presence of an external magnetic field exhibits an up to three-fold increase in the saturation red shift with the 2.54 eV excitation compared to the shift observed with 3.06 eV excitation. This unexpected result is attributed to multiple hole occupancy of the QD and the resulting increased penetration of the hole wavefunction tail further into the (Zn,Mn)Se matrix. The proposed model is supported by microscopic calculations which accurately include the role of hole-hole Coulomb interactions as well as the hole-Mn spin exchange interactions.
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Submitted 15 January, 2018;
originally announced January 2018.
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Tailoring Semiconductor Lateral Multi-junctions for Giant Photoconductivity Enhancement
Authors:
Yutsung Tsai,
Zhaodong Chu,
Yimo Han,
Chih-Piao Chuu,
Di Wu,
Alex Johnson,
Fei Cheng,
Mei-Yin Chou,
David A. Muller,
Xiaoqin Li,
Keji Lai,
Chih-Kang Shih
Abstract:
Semiconductor heterostructures have played a critical role as the enabler for new science and technology. The emergence of transition metal dichalcogenides (TMDs) as atomically thin semiconductors has opened new frontiers in semiconductor heterostructures either by stacking different TMDs to form vertical heterojunctions or by stitching them laterally to form lateral heterojunctions via direct gro…
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Semiconductor heterostructures have played a critical role as the enabler for new science and technology. The emergence of transition metal dichalcogenides (TMDs) as atomically thin semiconductors has opened new frontiers in semiconductor heterostructures either by stacking different TMDs to form vertical heterojunctions or by stitching them laterally to form lateral heterojunctions via direct growth. In conventional semiconductor heterostructures, the design of multi-junctions is critical to achieve carrier confinement. Analogously, we report successful synthesis of monolayer WS2/WS2(1-x)Se2x/WS2 multi-junction lateral heterostructure via direct growth by chemical vapor deposition. The grown structures are characterized by Raman, photoluminescence, and annular dark-field scanning transmission electron microscopy to determine its lateral compositional profile. More importantly, using microwave impedance microscopy, we demonstrate that the local photoconductivity in the alloy region can be tailored and enhanced by 2 orders of magnitude over pure WS2. Finite element analysis confirms that this effect is due to the carrier diffusion and confinement into the alloy region. Our work exemplifies the technological potential of atomically thin lateral heterostructures in optoelectronic applications.
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Submitted 15 September, 2017;
originally announced September 2017.
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Turbulence scaling laws across the superfluid to supersolid transition
Authors:
C. -H. Hsueh,
Y. -C. Tsai,
T. -L. Horng,
M. Tsubota,
W. C. Wu
Abstract:
We investigate quantum turbulence in a two-dimensional trapped supersolid and demonstrate that both the wave and vortex turbulence involve triple rather than dual cascades, as in a superfluid. Because of the presence of a second gapless mode associated with translation symmetry breaking, a new $k^{-13/3}$ scaling law is predicted to occur in the wave turbulence. Simultaneous fast vortex-antivortex…
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We investigate quantum turbulence in a two-dimensional trapped supersolid and demonstrate that both the wave and vortex turbulence involve triple rather than dual cascades, as in a superfluid. Because of the presence of a second gapless mode associated with translation symmetry breaking, a new $k^{-13/3}$ scaling law is predicted to occur in the wave turbulence. Simultaneous fast vortex-antivortex creation and annihilation in the interior of the oscillating supersolid results in a $k^{-1}$ scaling law in the vortex turbulence. Numerical simulations based on the Gross-Pitaevskii equation confirmed the predictions.
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Submitted 4 May, 2017;
originally announced May 2017.
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Growth rate of crystal surfaces with several dislocation centers
Authors:
Takeshi Ohtsuka,
Yen-Hsi Richard Tsai,
Yoshikazu Giga
Abstract:
We study analytically and numerical the growth rate of a crystal surface growing by several screw dislocations. To describe several spiral steps we use the revised level set method for spirals by the authors (Journal of Scientific Computing 62, 831-874, 2015). We carefully compare our simulation results on the growth rates with predictions in a classical paper by Burton et al. (Philos Trans R Soc…
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We study analytically and numerical the growth rate of a crystal surface growing by several screw dislocations. To describe several spiral steps we use the revised level set method for spirals by the authors (Journal of Scientific Computing 62, 831-874, 2015). We carefully compare our simulation results on the growth rates with predictions in a classical paper by Burton et al. (Philos Trans R Soc Lond Ser A Math Phys Sci 243,299-358, 1951). Then, we propose improved estimates on the growth rate with several different configurations, which are in agreement with our numerical simulations. The influence of distribution of screw dislocations in a group on a line to the growth rate, and the growth rate by a group including different rotational orientations of spirals are also studied in this paper.
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Submitted 15 June, 2017; v1 submitted 28 December, 2016;
originally announced December 2016.
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Thickness-dependent Dielectric Constant of Few-layer In2Se3 Nano-flakes
Authors:
Di Wu,
Alexander J. Pak,
Yingnan Liu,
Yu Zhou,
Xiaoyu Wu,
Yihan Zhu,
Min Lin,
Yu Han,
Yuan Ren,
Hailin Peng,
Yu-Hao Tsai,
Gyeong S. Hwang,
Keji Lai
Abstract:
The dielectric constant or relative permittivity of a dielectric material, which describes how the net electric field in the medium is reduced with respect to the external field, is a parameter of critical importance for charging and screening in electronic devices. Such a fundamental material property is intimately related to not only the polarizability of individual atoms, but also the specific…
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The dielectric constant or relative permittivity of a dielectric material, which describes how the net electric field in the medium is reduced with respect to the external field, is a parameter of critical importance for charging and screening in electronic devices. Such a fundamental material property is intimately related to not only the polarizability of individual atoms, but also the specific atomic arrangement in the crystal lattice. In this letter, we present both experimental and theoretical investigations on the dielectric constant of few-layer In2Se3 nano-flakes grown on mica substrates by van der Waals epitaxy. A nondestructive microwave impedance microscope is employed to simultaneously quantify the number of layers and local electrical properties. The measured dielectric constant increases monotonically as a function of the thickness and saturates to the bulk value at around 6 ~ 8 quintuple layers. The same trend of layer-dependent dielectric constant is also revealed by first-principle calculations. Our results of the dielectric response, being ubiquitously applicable to layered 2D semiconductors, are expected to be significant for this vibrant research field.
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Submitted 23 November, 2015;
originally announced November 2015.
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Time-resolved magnetophotoluminescence studies of magnetic polaron dynamics in type-II quantum dots
Authors:
B. Barman,
R. Oszwałdowski,
L. Schweidenback,
A. H. Russ,
J. M. Pientka,
Y. Tsai,
W-C. Chou,
W. C. Fan,
J. R. Murphy,
A. N. Cartwright,
I. R. Sellers,
A. G. Petukhov,
I. Žutić,
B. D. McCombe,
A. Petrou
Abstract:
We used continuous wave photoluminescence (cw-PL) and time resolved photoluminescence (TR-PL) spectroscopy to compare the properties of magnetic polarons (MP) in two related spatially indirect II-VI epitaxially grown quantum dot systems. In the ZnTe/(Zn,Mn)Se system the holes are confined in the non-magnetic ZnTe quantum dots (QDs), and the electrons reside in the magnetic (Zn,Mn)Se matrix. On the…
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We used continuous wave photoluminescence (cw-PL) and time resolved photoluminescence (TR-PL) spectroscopy to compare the properties of magnetic polarons (MP) in two related spatially indirect II-VI epitaxially grown quantum dot systems. In the ZnTe/(Zn,Mn)Se system the holes are confined in the non-magnetic ZnTe quantum dots (QDs), and the electrons reside in the magnetic (Zn,Mn)Se matrix. On the other hand, in the (Zn,Mn)Te/ZnSe system, the holes are confined in the magnetic (Zn,Mn)Te QDs, while the electrons remain in the surrounding non-magnetic ZnSe matrix. The magnetic polaron formation energies in both systems were measured from the temporal red-shift of the band-edge emission. The magnetic polaron exhibits distinct characteristics depending on the location of the Mn ions. In the ZnTe/(Zn,Mn)Se system the magnetic polaron shows conventional behavior with decreasing with increasing temperature T and increasing magnetic field B. In contrast, in the (Zn,Mn)Te/ZnSe system has unconventional dependence on temperature T and magnetic field B; is weakly dependent on T as well as on B. We discuss a possible origin for such a striking difference in the MP properties in two closely related QD systems.
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Submitted 14 October, 2015;
originally announced October 2015.
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Spin orders in the supersolid phases in binary Rydberg-dressed Bose-Einstein condensates
Authors:
C. -H. Hsueh,
Y. -C. Tsai,
K. -S. Wu,
M. -S. Chang,
W. C. Wu
Abstract:
We show that the five possible ordered states in a quantum spin-1/2 system with long-range exchange interactions: Neel, ladder, Peierls, coincidence, and domain states, can be realized in a binary Rydberg-dressed BEC system in the supersolid phase. In such a system, blockade phenomenon is shown to also occur for pairs of different excited-state atoms, which results in similar intra- and inter-spec…
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We show that the five possible ordered states in a quantum spin-1/2 system with long-range exchange interactions: Neel, ladder, Peierls, coincidence, and domain states, can be realized in a binary Rydberg-dressed BEC system in the supersolid phase. In such a system, blockade phenomenon is shown to also occur for pairs of different excited-state atoms, which results in similar intra- and inter-species long-range interactions between ground-state atoms. It suggests that a pseudo spin-1/2 system can be possibly formed in the ground state of ultracold rudibium.
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Submitted 23 May, 2013;
originally announced May 2013.
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Imaginary polarization as a way to surmount the sign problem in ab initio calculations of spin-imbalanced Fermi gases
Authors:
Jens Braun,
Jiunn-Wei Chen,
Jian Deng,
Joaquín E. Drut,
Bengt Friman,
Chen-Te Ma,
Yu-Dai Tsai
Abstract:
From ultracold atoms to quantum chromodynamics, reliable ab initio studies of strongly interacting fermions require numerical methods, typically in some form of quantum Monte Carlo calculation. Unfortunately, (non)relativistic systems at finite density (spin polarization) generally have a sign problem, such that those ab initio calculations are impractical. It is well-known, however, that in the r…
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From ultracold atoms to quantum chromodynamics, reliable ab initio studies of strongly interacting fermions require numerical methods, typically in some form of quantum Monte Carlo calculation. Unfortunately, (non)relativistic systems at finite density (spin polarization) generally have a sign problem, such that those ab initio calculations are impractical. It is well-known, however, that in the relativistic case imaginary chemical potentials solve this problem, assuming the data can be analytically continued to the real axis. Is this feasible for nonrelativistic systems? Are the interesting features of the phase diagram accessible in this manner? By introducing complex chemical potentials, for real total particle number and imaginary polarization, the sign problem is avoided in the nonrelativistic case. To give a first answer to the above questions, we perform a mean-field study of the finite-temperature phase diagram of spin-1/2 fermions with imaginary polarization.
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Submitted 1 April, 2013; v1 submitted 14 September, 2012;
originally announced September 2012.
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Carrier-Dopant Exchange Interactions in Mn-doped PbS Colloidal Quantum Dots
Authors:
Gen Long,
Biplob Barman,
Savas Delikanli,
Yu Tsung Tsai,
Peihong Zhang,
Athos Petrou,
Hao Zeng
Abstract:
Carrier-dopant exchange interactions in Mn-doped PbS colloidal quantum dots were studied by circularly polarized magneto-photoluminescence. Mn substitutional doping leads to paramagnetic behavior down to 5 K. While undoped quantum dots show negative circular polarization, Mn doping changes its sign to positive. A circular polarization value of 40% was achieved at T=7 K and B=7 tesla. The results a…
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Carrier-dopant exchange interactions in Mn-doped PbS colloidal quantum dots were studied by circularly polarized magneto-photoluminescence. Mn substitutional doping leads to paramagnetic behavior down to 5 K. While undoped quantum dots show negative circular polarization, Mn doping changes its sign to positive. A circular polarization value of 40% was achieved at T=7 K and B=7 tesla. The results are interpreted in terms of Zeeman splitting of the band edge states in the presence of carrier-dopant exchange interactions that are qualitatively different from the s,p-d exchange interactions in II-VI systems.
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Submitted 3 August, 2012;
originally announced August 2012.
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Crystallized merons and inverted merons in the condensation of spin-1 Bose gases with spin-orbit coupling
Authors:
S. -W. Su,
I. -K. Liu,
Y. -C. Tsai,
W. M. Liu,
S. -C. Gou
Abstract:
The non-equilibrium dynamics of a rapidly quenched spin-1 Bose gas with spin-orbit coupling is studied. By solving the stochastic projected Gross-Pitaevskii equation, we show that crystallization of merons can occur in a spinor condensate of ^{87}Rb. Analytic form and stability of the crystal structure are given. Likewise, inverted merons can be created in a spin-polarized spinor condensate of ^{2…
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The non-equilibrium dynamics of a rapidly quenched spin-1 Bose gas with spin-orbit coupling is studied. By solving the stochastic projected Gross-Pitaevskii equation, we show that crystallization of merons can occur in a spinor condensate of ^{87}Rb. Analytic form and stability of the crystal structure are given. Likewise, inverted merons can be created in a spin-polarized spinor condensate of ^{23}Na. Our studies provide a chance to explore the fundamental properties of meron-like matter.
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Submitted 18 December, 2011; v1 submitted 27 November, 2011;
originally announced November 2011.
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Spontaneous Crystallization of Skyrmions and Fractional Vortices in the Fast-rotating and Rapidly-quenched Spin-1 Bose-Einstein Condensates
Authors:
S. -W. Su,
C. -H. Hsueh,
I. -K. Liu,
T. -L. Horng,
Y. -C. Tsai,
S. -C. Gou,
W. M. Liu
Abstract:
We investigate the spontaneous generation of crystallized topological defects via the combining effects of fast rotation and rapid thermal quench on the spin-1 Bose-Einstein condensates. By solving the stochastic projected Gross-Pitaevskii equation, we show that, when the system reaches equilibrium, a hexagonal lattice of skyrmions, and a square lattice of half-quantized vortices can be formed in…
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We investigate the spontaneous generation of crystallized topological defects via the combining effects of fast rotation and rapid thermal quench on the spin-1 Bose-Einstein condensates. By solving the stochastic projected Gross-Pitaevskii equation, we show that, when the system reaches equilibrium, a hexagonal lattice of skyrmions, and a square lattice of half-quantized vortices can be formed in a ferromagnetic and antiferromagnetic spinor BEC, respetively, which can be imaged by using the polarization-dependent phase-contrast method.
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Submitted 31 May, 2011;
originally announced May 2011.
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Electronic structure and transport properties of La_0.7Ce_0.3MnO_3
Authors:
W. J. Chang,
J. Y. Tsai,
H. -T. Jeng,
J. -Y. Lin,
Kenneth Y. -J. Zhang,
H. L. Liu,
J. M. Lee,
J. M. Chen,
K. H. Wu,
T. M. Uen,
Y. S. Gou,
J. Y. Juang
Abstract:
X-ray absorption spectroscopy (XAS), optical reflectance spectroscopy, and the Hall effect measurements were used to investigate the electronic structure in La_0.7Ce_0.3MnO_3 thin films (LCeMO). The XAS results are consistent with those obtained from LDA+U calculations. In that the doping of Ce has shifted up the Fermi level and resulted in marked shrinkage of hole pockets originally existing in…
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X-ray absorption spectroscopy (XAS), optical reflectance spectroscopy, and the Hall effect measurements were used to investigate the electronic structure in La_0.7Ce_0.3MnO_3 thin films (LCeMO). The XAS results are consistent with those obtained from LDA+U calculations. In that the doping of Ce has shifted up the Fermi level and resulted in marked shrinkage of hole pockets originally existing in La_0.7Ca_0.3MnO_3 (LCaMO). The Hall measurements indicate that in LCeMO the carriers are still displaying the characteristics of holes as LDA+U calculations predict. Analyses of the optical reflectance spectra evidently disapprove the scenario that the present LCeMO might have been dominated by the La-deficient phases.
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Submitted 13 September, 2005;
originally announced September 2005.
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Near-Equilibrium Dynamics of Crystalline Interfaces with Long-Range Interactions in 1+1 Dimensional Systems
Authors:
Yan-Chr Tsai
Abstract:
The dynamics of a one-dimensional crystalline interface model with long-range interactions is investigated. In the absence of randomness, the linear response mobility decreases to zero when the temperature approaches the roughening transition from above, in contrast to a finite jump at the critical point in the Kosterlitz-Thouless (KT) transition. In the presence of substrate disorder, there exist…
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The dynamics of a one-dimensional crystalline interface model with long-range interactions is investigated. In the absence of randomness, the linear response mobility decreases to zero when the temperature approaches the roughening transition from above, in contrast to a finite jump at the critical point in the Kosterlitz-Thouless (KT) transition. In the presence of substrate disorder, there exists a phase transition into a low-temperature pinning phase with a continuously varying dynamic exponent $z>1$. The expressions for the non-linear response mobility of a crystalline interface in both cases are also derived.
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Submitted 1 March, 1995;
originally announced March 1995.
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Dynamics of Particles Deposition on a Disordered Substrate: II. Far-from Equilibrium Behavior. -
Authors:
Yan-Chr Tsai,
Yonathan Shapir
Abstract:
The deposition dynamics of particles (or the growth of a rigid crystal) on a disordered substrate at a finite deposition rate is explored. We begin with an equation of motion which includes, in addition to the disorder, the periodic potential due to the discrete size of the particles (or to the lattice structure of the crystal) as well as the term introduced by Kardar, Parisi, and Zhang (KPZ) to…
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The deposition dynamics of particles (or the growth of a rigid crystal) on a disordered substrate at a finite deposition rate is explored. We begin with an equation of motion which includes, in addition to the disorder, the periodic potential due to the discrete size of the particles (or to the lattice structure of the crystal) as well as the term introduced by Kardar, Parisi, and Zhang (KPZ) to account for the lateral growth at a finite growth rate. A generating functional for the correlation and response functions of this process is derived using the approach of Martin, Sigga, and Rose. A consistent renormalized perturbation expansion to first order in the non-Gaussian couplings requires the calculation of diagrams up to three loops. To this order we show, for the first time for this class of models which violates the the fluctuation-dissipation theorem, that the theory is renormalizable. We find that the effects of the periodic potential and the disorder decay on very large scales and asymptotically the KPZ term dominates the behavior. However, strong non-trivial crossover effects are found for large intermediate scales.
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Submitted 21 September, 1994;
originally announced September 1994.
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Dynamics of Particle Deposition on a Disordered Substrate: I. Near- Equilibrium Behavior
Authors:
Yan-Chr Tsai,
Yonathan Shapir
Abstract:
A growth model which describes the deposition of particles (or the growth of a rigid crystal) on a disordered substrate is investigated. The dynamic renormalization group is applied to the stochastic growth equation using the Martin, Sigga, and Rose formalism. The periodic potential and the quenched disorder, upon averaging, are combined into a single term in the generating functional. Changing…
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A growth model which describes the deposition of particles (or the growth of a rigid crystal) on a disordered substrate is investigated. The dynamic renormalization group is applied to the stochastic growth equation using the Martin, Sigga, and Rose formalism. The periodic potential and the quenched disorder, upon averaging, are combined into a single term in the generating functional. Changing the temperature (or the inherent noise of the deposition process) two different regimes with a transition between them at $T_{sr}$, are found: for $T>T_{sr}$ this term is irrelevant and the surface has the scaling properties of a surface growing on a flat substrate in the rough phase. The height-height correlations behave as $C(L,τ)\sim \ln [L f(τ/L^2)]$. While the linear response mobility is finite in this phase it does vanish as $(T-T_{sr})^{1.78}$ when $T\rightarrow T_{sr}^+$. For $T<T_{sr}$ there is a line of fixed-point for the coupling constant. The surface is super-rough: the equilibrium correlation functions behave as $(\ln L)^2$ while their short time dependence is $(\ln τ) ^2$ with a temperature dependent dynamic exponent $z=2[1+1.78(1-T/T_{sr})]$. While the linear response mobility vanishes on large length scales, its scale-dependence leads to a non-linear response. For a small applied force $F$ the average velocity of the surface $v$ behaves as $v \sim F^{1+ζ}$. To first order $ζ=1.78(1-T/T_{sr})$. At the transition, $v\sim F/(1+C|\ln (F)|)^{1.78}$ and the crossover to the behavior to $T<T_{sr}$ is analyzed. These results also apply to two-dimensional vortex glasses with a parallel magnetic field.
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Submitted 6 September, 1994;
originally announced September 1994.
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On the Scale-Invariant Distribution of the Diffusion Coefficient for Classical Particles Diffusing in Disordered Media.-
Authors:
Yan-Chr Tsai,
Yonathan Shapir
Abstract:
The scaling form of the whole distribution P(D) of the random diffusion coefficient D(x) in a model of classically diffusing particles is investigated. The renormalization group approach above the lower critical dimension d=0 is applied to the distribution P(D) using the n-replica approach. In the annealed approximation (n=1), the inverse gaussian distribution is found to be the stable one under…
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The scaling form of the whole distribution P(D) of the random diffusion coefficient D(x) in a model of classically diffusing particles is investigated. The renormalization group approach above the lower critical dimension d=0 is applied to the distribution P(D) using the n-replica approach. In the annealed approximation (n=1), the inverse gaussian distribution is found to be the stable one under rescaling. This identification is made based on symmetry arguments and subtle relations between this model and that of fluc- tuating interfaces studied by Wallace and Zia. The renormalization-group flow for the ratios between consecutive cumulants shows a regime of pure diffusion for small disorder, in which P(D) goes to delta(D-<D>), and a regime of strong disorder where the cumulants grow infinitely large and the diffusion process is ill defined. The boundary between these two regimes is associated with an unstable fixed-point and a subdiffusive behavior: <x**2>=Ct**(1-d/2). For the quenched case (n goes to 0) we find that unphysical operators are generated raisng doubts on the renormalizability of this model. Implications to other random systems near their lower critical dimension are discussed.
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Submitted 16 October, 1992;
originally announced October 1992.
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Kinetic Roughening in Surfaces of Crystals Growing on Disordered Substrates
Authors:
Yan-Chr Tsai,
Yonathan Shapir
Abstract:
Substrate disorder effects on the scaling properties of growing crystalline surfaces in solidification or epitaxial deposition processes are investigated. Within the harmonic approach there is a phase transition into a low-temperature (low-noise) superrough phase with a continuously varying dynamic exponent z>2 and a non-linear response. In the presence of the KPZ nonlinearity the disorder cause…
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Substrate disorder effects on the scaling properties of growing crystalline surfaces in solidification or epitaxial deposition processes are investigated. Within the harmonic approach there is a phase transition into a low-temperature (low-noise) superrough phase with a continuously varying dynamic exponent z>2 and a non-linear response. In the presence of the KPZ nonlinearity the disorder causes the lattice efects to decay on large scales with an intermediate crossover behavior. The mobility of the rough surface hes a complex dependence on the temperature and the other physical parameters.
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Submitted 16 September, 1992;
originally announced September 1992.