Showing 1–50 of 87 results for author: Lin, K

Interaction driven charge transfer transitions in closely spaced graphene double layers

Authors: Kenneth A. Lin, Unmesh Ghorai, Kenji Watanabe, Takashi Taniguchi, Emanuel Tutuc, Rafi Bistritzer

Abstract: Charge transfer between two conductors is conventionally viewed as a single-particle process governed by electrostatics and band alignment. Using tunneling spectroscopy, we show that charge transfer in closely spaced graphene double layer quantum Hall ferromagnets instead proceeds through a sequence of interaction driven phase transitions governed by the competition between capacitive charging and… ▽ More Charge transfer between two conductors is conventionally viewed as a single-particle process governed by electrostatics and band alignment. Using tunneling spectroscopy, we show that charge transfer in closely spaced graphene double layer quantum Hall ferromagnets instead proceeds through a sequence of interaction driven phase transitions governed by the competition between capacitive charging and Coulomb exchange interactions. A comparison of experimental data and theoretical calculations identifies spectroscopic signatures of the interaction driven charge transfer transitions, and reveals that this charge transfer reconstructs the quasiparticle spectrum. While intralayer exchange favors abrupt transfer of entire spin-valley subbands between the layers, interlayer exchange stabilizes coherent intermediate phases that enable gradual charge transfer. Our results establish interlayer tunneling as a powerful probe of interacting electronic systems whose quasiparticle spectrum is itself bias dependent. △ Less

Submitted 14 August, 2026; originally announced August 2026.

Comments: 30 pages, 7 figures

Magnetic Order in bilayer Ruddlesden-Popper Nickelates

Authors: Yiming Wang, Guijing Duan, Zhiguang Liao, Kuan-Sen Lin, Rong Yu, Qimiao Si

Abstract: The recent discovery of high-temperature superconductivity in the bilayer nickelate La$_3$Ni$_2$O$_7$ has led to extensive interest in the correlation physics of its normal state. Given that the superconducitivity develops near a density wave order in the phase diagram, it is important to elucidate the nature of this order. Based on the accumulated experimental evidence for a bad metal state in pr… ▽ More The recent discovery of high-temperature superconductivity in the bilayer nickelate La$_3$Ni$_2$O$_7$ has led to extensive interest in the correlation physics of its normal state. Given that the superconducitivity develops near a density wave order in the phase diagram, it is important to elucidate the nature of this order. Based on the accumulated experimental evidence for a bad metal state in proximity to an orbital-selective Mott phase, here we describe magnetic correlations of the system in a conceptually new way -- in terms of effective local moments experiencing a combination of RKKY and superexchange interactions. This gives rise to a magnetic order with a wavevector that is close to $\mathbf{Q}=(π/2,π/2)$ and, at the same time, yields a clear understanding of the associated spin dynamics. Our results are consistent with the rapidly emerging experiments about the magnetic correlations in the density wave order of the bilayer nickelate. Implications for unconventional superconductivity in this and related multiorbital systems are discussed. △ Less

Submitted 16 July, 2026; originally announced July 2026.

Comments: 7+3 pages, 5+1 figures

Destructive interference of second harmonic generation in AA stacked MoTe$_2$/WSe$_2$

Authors: Yiduo Wang, Yao Lu, Changshen Chen, Xiaotong Liao, Siyu Fan, Zhenyu Wang, Yaotian Liu, Subi Du, Yingze Jia, Ye Zhu, Yingwei Wang, Jun He, Song Liu, Jiawei Ruan, Zhen Chen, Kai-Qiang Lin, Yang Xu

Abstract: The stacking configuration of two-dimensional materials critically governs their optical and electronic responses. Monolayer transition-metal dichalcogenides (TMDC) lack inversion symmetry and exhibit exciton-enhanced second-harmonic generation (SHG). In TMDC bilayers, 60° (0°) stacking is conventionally expected to suppress (enhance) SHG owing to destructive (constructive) interference of the lay… ▽ More The stacking configuration of two-dimensional materials critically governs their optical and electronic responses. Monolayer transition-metal dichalcogenides (TMDC) lack inversion symmetry and exhibit exciton-enhanced second-harmonic generation (SHG). In TMDC bilayers, 60° (0°) stacking is conventionally expected to suppress (enhance) SHG owing to destructive (constructive) interference of the layer-resolved nonlinear polarizations. Here, we report an unconventional destructive SHG interference in nearly 0°-stacked (AA-stacked) MoTe2/WSe2 heterobilayers using two independent probes: atomic-resolution imaging and stacking-sensitive exciton hybridization measurements. Supported by ab initio GW and Bethe-Salpeter equation calculations, we show that distinct two-photon resonances associated with the WSe2 C exciton and the MoTe2 D exciton generate a nearly $π$ phase difference ($Δφ$) in their second-order nonlinear susceptibilities $χ^{(2)}$, leading to the anomalous destructive interference. We further demonstrate that in small-angle twisted MoTe2/WSe2, the SHG polarization state is governed by the interplay between twist angle $α$ and phase difference $Δφ$, and can be mapped onto trajectories on the Poincaré sphere. At excitation energies satisfying $Δφ$ + 3$α$ = 180°, the SHG output becomes nearly circularly polarized (ellipticity ~ 0.91) and undergoes an abrupt 90° azimuthal rotation, corresponding to a geometric polarization singularity in the parameter space. Our findings open new routes for exciton-resonance engineered nonlinear photonics and stacking-resolved optical functionality in moiré materials. △ Less

Submitted 20 May, 2026; originally announced May 2026.

Comments: 35 pages, 4 main figures and 10 supplemental figures

Interband Berry connection measurement in the optical honeycomb lattice

Authors: Shao-Wen Chang, Malte N. Schwarz, Erin G. Moloney, Ke Lin, Dan M. Stamper-Kurn

Abstract: The geometry of Bloch bands affects many physical properties of crystalline solids and other spatially periodic systems. Direct experimental determination of such geometry is an active area of research. In this work, we focus on the fundamental connection between optical excitations and the relative geometry of pairs of Bloch bands, as characterized by the interband Berry connection. We simulate t… ▽ More The geometry of Bloch bands affects many physical properties of crystalline solids and other spatially periodic systems. Direct experimental determination of such geometry is an active area of research. In this work, we focus on the fundamental connection between optical excitations and the relative geometry of pairs of Bloch bands, as characterized by the interband Berry connection. We simulate the response of electrons in solids to optical excitation by the response of ultracold fermionic atoms in optical lattices to periodic modulation of the lattice position. The strength of resonant excitation between bands, measured at each quasimomentum and for various lattice-shaking polarizations, directly maps out the interband Berry connection. We apply this method to the optical honeycomb lattice, driving excitations between the ground $n=1$ band and the excited $n'=\{2,3,4\}$ bands. We observe transparency lines of quasimomenta at which the response to excitation of specific polarization is zero. Further, the interband Berry connection between bands 1 and 3 shows irreducible Dirac strings connecting the $K$ and $K'$ points in the Brillouin zone, lines along which the interband Berry connection abruptly changes orientation. Our work establishes optical response as a powerful tool for characterizing geometrical and topological properties of band structure. △ Less

Submitted 19 May, 2026; v1 submitted 12 May, 2026; originally announced May 2026.

Comments: 10 pages, 10 figures; added references

Landau free energy and the absence of spontaneous magnetization of the one-dimensional Ising model

Authors: Z. F. Zheng, R. K. Lin, J. M. Zhang

Abstract: We revisit the problem of spontaneous magnetization of the one-dimensional Ising model from the Landau free energy perspective. To this end, we define and calculate the density of states of the one-dimensional Ising model following a technique introduced by Ising. The observed monotonicity property of the density of states suggests heuristically that the model does not exhibit spontaneous magnetiz… ▽ More We revisit the problem of spontaneous magnetization of the one-dimensional Ising model from the Landau free energy perspective. To this end, we define and calculate the density of states of the one-dimensional Ising model following a technique introduced by Ising. The observed monotonicity property of the density of states suggests heuristically that the model does not exhibit spontaneous magnetization at any finite temperature. Subsequently, we solve the model exactly in the thermodynamic limit by employing the maximum-term approximation, which is feasible due to the simple analytical expression of the density of states. We also show that the Landau free energy is an increasing function of $|m|$ and its second derivative at $m=0$ is positive and non-analytic in temperature, proving rigorously the absence of spontaneous magnetization of the model at any finite temperature. △ Less

Submitted 2 May, 2026; originally announced May 2026.

Comments: 13 pages, 3 figs. To appear in European Journal of Physics. Comments are welcome

Journal ref: European Journal of Physics 47, 035103 (2026)

Photoluminescence Line Shapes of Nanocrystals: Contributions from First- and Second-Order Vibronic Couplings

Authors: Kaiyue Peng, Bokang Hou, Kailai Lin, Caroline Chen, Hendrik Utzat, Eran Rabani

Abstract: We present a microscopic, parameter-free approach for computing the photoluminescence spectra of a single semiconductor nanocrystal. The method derives exciton-phonon coupling directly from the semi-empirical pseudopotential framework and systematically incorporates both diagonal and off-diagonal interactions, expanded to second-order in the phonon modes. The dipole-dipole correlation function was… ▽ More We present a microscopic, parameter-free approach for computing the photoluminescence spectra of a single semiconductor nanocrystal. The method derives exciton-phonon coupling directly from the semi-empirical pseudopotential framework and systematically incorporates both diagonal and off-diagonal interactions, expanded to second-order in the phonon modes. The dipole-dipole correlation function was calculated using a Dyson expansion within the Kubo-Toyozawa formalism, enabling a consistent description of the role of pure dephasing and population-transfer on the photoluminescence spectral features. Applied to CdSe/CdS core-shell nanocrystals, the approach quantitatively reproduces experimental photoluminescence spectra over a wide temperature range, revealing that quadratic phonon couplings account for nearly half of the homogeneous linewidth above 100-150 K, while off-diagonal couplings leading to exciton thermalization play only a minor role and only as T approaches 300K. △ Less

Submitted 26 February, 2026; originally announced February 2026.

Chiral Weyl-Kondo semimetals and hexagonal heavy fermion systems

Authors: Kuan-Sen Lin, Yuan Fang, Henrique Fabrelli, Runhan Li, Andrey Prokofiev, Fang Xie, Jennifer Cano, Maia G. Vergniory, Silke Paschen, Qimiao Si

Abstract: Strong correlation, in concert with symmetry and topology, engenders novel gapless phases of matter, though only a tip of the iceberg has been seen. An exemplary framework is provided by Weyl-Kondo semimetals, in which Weyl fermions develop through crystalline symmetry constraints on the emergent low-energy heavy-fermion excitations. This paradigm has opened up new opportunities to explore correla… ▽ More Strong correlation, in concert with symmetry and topology, engenders novel gapless phases of matter, though only a tip of the iceberg has been seen. An exemplary framework is provided by Weyl-Kondo semimetals, in which Weyl fermions develop through crystalline symmetry constraints on the emergent low-energy heavy-fermion excitations. This paradigm has opened up new opportunities to explore correlated topologies without a noninteracting counterpart, but fully realizing this potential requires a large base of candidate materials. Here we confront the challenge on both fronts by studying heavy fermion systems with hexagonal space groups. This family contains a large number of chiral nonsymmorphic crystal structures that promote Weyl degeneracies and, in addition, feature geometric frustration in the $f$-electron magnetism. Our calculations for the heavy fermion states identify Weyl-Kondo semimetals with chiral or achiral Weyl nodes in the respective structural classes. We also develop the first search strategy of any kind for the difficult case of strongly correlated materials, which is also suitable for automation, using a combination of materials database, symmetry classification and search for desired experimental properties, and propose as candidate topological heavy fermion systems the chiral CePt$_2$B and achiral Ce$_2$NiGe$_3$ and Ce$_6$Co$_{2-δ}$Si$_3$. Our findings raise the prospect for strongly correlated metallic topology in the unusual setting of exotic quantum magnetism and, moreover, point a way to go beyond serendipity in the search for novel strongly correlated quantum materials. △ Less

Submitted 9 March, 2026; v1 submitted 25 February, 2026; originally announced February 2026.

Comments: 24+58 pages, 3+30 figures

The Role of THz Phonons in the Ionic Conduction Mechanism of $Li_7La_3Zr_2O_{12}$ Polymorphs

Authors: Amy K. Lin, Natan A. Spear, Geoffrey A. Blake, Scott K. Cushing

Abstract: Superionic conduction in solid-state materials is governed not only by static factors, such as structure and composition, but also by dynamic interactions between the mobile ion and the crystal lattice. Specifically, the dynamics of lattice vibrations, or phonons, have attracted interest because of their hypothesized ability to facilitate superionic conduction. However, direct experimental measure… ▽ More Superionic conduction in solid-state materials is governed not only by static factors, such as structure and composition, but also by dynamic interactions between the mobile ion and the crystal lattice. Specifically, the dynamics of lattice vibrations, or phonons, have attracted interest because of their hypothesized ability to facilitate superionic conduction. However, direct experimental measurement of the role of phonons in ionic conduction is challenging due to the fast intrinsic timescales of ion hopping and the difficulty of driving relevant phonon modes, which often lie in the low-energy THz regime. To overcome these limitations, we use laser-driven ultrafast impedance spectroscopy (LUIS). LUIS resonantly excites phonons using a THz field and probes ion hopping with picosecond time resolution. We apply LUIS to understand the dynamical role of phonons in $Li_7La_3Zr_2O_{12}$ (LLZO). When in its cubic phase (c-LLZO), this garnet-type solid electrolyte has an ionic conductivity two orders of magnitude greater than its tetragonal phase (t-LLZO). T-LLZO is characterized by an ordered and filled $Li^+$ sublattice necessitating synchronous ion hopping. In contrast, c-LLZO is characterized by a disordered and vacancy-rich $Li^+$ sublattice, and has a conduction mechanism dominated by single hops. We find that, upon excitation of phonons in the 0.5-7.5 THz range, the impedance of t-LLZO experiences a longer ion hopping decay signal in comparison to c-LLZO. The results suggest that phonon-mediated ionic conduction by THz modes may lead to larger ion displacements in ordered and fully occupied mobile ion sublattices as opposed to those that are disordered and vacancy-rich. Overall, this work highlights the interplay between static and dynamic factors that enables improved ionic conductivity in otherwise poorly conducting inorganic solids. △ Less

Submitted 26 December, 2025; originally announced December 2025.

Comments: 12 pages, 4 figures

Classical-to-Quantum Crossover in 2D TMD Field-Effect Transistors: A First-Principles Study via Sub-10 nm Channel Scaling Beyond the Boltzmann Tyranny

Authors: Yu-Chang Chen, Chia-Yang Ling, Ken-Ming Lin

Abstract: Scaling field-effect transistors (FETs) into the sub-10-nm regime fundamentally alters the transport mechanism, challenging long-standing design rules. This study investigates monolayer TMD FETs with channel lengths from 12 nm to 3 nm, quantifying the competition between semiclassical thermionic current and quantum tunneling. We show that quantum transport, as described by the Landauer formula, as… ▽ More Scaling field-effect transistors (FETs) into the sub-10-nm regime fundamentally alters the transport mechanism, challenging long-standing design rules. This study investigates monolayer TMD FETs with channel lengths from 12 nm to 3 nm, quantifying the competition between semiclassical thermionic current and quantum tunneling. We show that quantum transport, as described by the Landauer formula, asymptotically approaches classical thermionic emission in the long-channel and high-temperature limit, in accordance with Richardson law. A competition parameter $ζ$ cleanly delineates the semiclassical-to-quantum transition, and two characteristic temperatures emerge: $T_{op}$ (minimizing $J_{OFF}$ and $T_{c}$ (thermionic onset). For $L_{ch}<9$ nm, $T_{op}<300$ K and $J_{OFF}$ is tunneling-dominated; the 3 nm device remains tunneling-dominated up to 500 K and achieves a subthreshold swing overcoming Boltzmann tyranny via the steep slope of $τ(E)$. However, the short-channel effect also generates leakage current and makes the transistor difficult to turn off. For $L_{ch} \geq 9$ nm, $T_{op}>300$ K and $J_{OFF}$ is thermionic-dominated, and the subthreshold swing approaches Boltzmann tyranny scaled by $α_{in}}$. Consequently, the ideal channel length for 2D FETs is $L_{ch} \approx 10$ nm. These results provide criteria for selecting the optimal operating temperature and gate-voltage windows in miniaturizing 2D FETs, and pinpoint the crossover at which quantum tunneling current becomes comparable to semiclassical thermionic emission. △ Less

Submitted 17 November, 2025; v1 submitted 4 August, 2025; originally announced August 2025.

Comments: 12 pages, 7 figures

Journal ref: Applied Physics Reviews, 13, 011409 (2026)

Quantum transport calculations: An effective medium theory based on the projector augmented wave method with the plane-wave basis

Authors: Yi-Cheng Lin, Ken-Ming Lin, Yu-Chang Chen

Abstract: We present an effective medium theory based on density functional theory that is implemented in VASP using the PAW method with a plane wave basis set. The transmission coefficient is derived through three complementary approaches: the current density relation J=nqv, the field operator method, and the nonquilibrium Green's function formalism. We compare transmission coefficients calculated using EM… ▽ More We present an effective medium theory based on density functional theory that is implemented in VASP using the PAW method with a plane wave basis set. The transmission coefficient is derived through three complementary approaches: the current density relation J=nqv, the field operator method, and the nonquilibrium Green's function formalism. We compare transmission coefficients calculated using EMT-PW with results from NEGF-DFT, based on the NanoDCAL package utilizing a linear combination of atomic orbitals (LCAO) basis set, for both periodic and nonperiodic boundary conditions. The minor discrepancies observed are attributed to differences in basis sets, pseudopotentials, and the treatment of lead regions. Notably, the EMT-PW framework avoids the common issue of overcompleteness encountered in non-equilibrium transport theories and allows for the decomposition of the total transmission coefficient into contributions from individual eigenstates. Furthermore, when combined with an effective gate model, EMT-PW is shown to be a powerful tool for analyzing current characteristics in nanodevices under applied gate voltages. By leveraging one-electron wavefunctions in eigenstates, this method provides a robust foundation for exploring the quantum statistics of electrons and current quantum correlations within the second quantization framework. △ Less

Submitted 31 August, 2025; v1 submitted 9 July, 2025; originally announced July 2025.

Comments: 14 pages, 6 Figures

Journal ref: Physical Review B 112, 085155 (2025)

Deep-learning atomistic semi-empirical pseudopotential model for nanomaterials

Authors: Kailai Lin, Matthew J. Coley-O'Rourke, Eran Rabani

Abstract: The semi-empirical pseudopotential method (SEPM) has been widely applied to provide computational insights into the electronic structure, photophysics, and charge carrier dynamics of nanoscale materials. We present "DeepPseudopot", a machine-learned atomistic pseudopotential model that extends the SEPM framework by combining a flexible neural network representation of the local pseudopotential wit… ▽ More The semi-empirical pseudopotential method (SEPM) has been widely applied to provide computational insights into the electronic structure, photophysics, and charge carrier dynamics of nanoscale materials. We present "DeepPseudopot", a machine-learned atomistic pseudopotential model that extends the SEPM framework by combining a flexible neural network representation of the local pseudopotential with parameterized non-local and spin-orbit coupling terms. Trained on bulk quasiparticle band structures and deformation potentials from GW calculations, the model captures many-body and relativistic effects with very high accuracy across diverse semiconducting materials, as illustrated for silicon and group III-V semiconductors. DeepPseudopot's accuracy, efficiency, and transferability make it well-suited for data-driven in silico design and discovery of novel optoelectronic nanomaterials. △ Less

Submitted 30 December, 2025; v1 submitted 14 May, 2025; originally announced May 2025.

Journal ref: npj Comput Mater 11, 381 (2025)

Textured growth and electrical characterization of Zinc Sulfide on back-end-of-the-line (BEOL) compatible substrates

Authors: Claire Wu, Mythili Surendran, Anika Tabassum Priyoti, Gokul Anilkumar, Cheng-Hsien Wu, Chun-Chen Wang, Cheng-Chen Kuo, Harish Kumarasubramanian, Kenta Lin, Amari Butler, Rehan Kapadia, Xinyu Bao, Jayakanth Ravichandran

Abstract: Scaling of transistors has enabled continuous improvements in logic device performance, especially through materials engineering. However, surpassing horizontal limitations in chip manufacturing requires a vertical, third dimension. Three-dimensional integration of high-performance logic demands solving the challenge of low-temperature (less than 450°C) synthesis of high-mobility n-type and p-type… ▽ More Scaling of transistors has enabled continuous improvements in logic device performance, especially through materials engineering. However, surpassing horizontal limitations in chip manufacturing requires a vertical, third dimension. Three-dimensional integration of high-performance logic demands solving the challenge of low-temperature (less than 450°C) synthesis of high-mobility n-type and p-type semiconductor thin films for back-end-of-line (BEOL) compatible transistors. Metal oxides, particularly indium oxides alloyed with gallium and tungsten, are promising n-type channel materials, but suitable p-type materials for BEOL remain scarce. Zinc sulfide (ZnS), a wide band-gap semiconductor, shows room-temperature p-type conductivity when doped with copper and crystallizes below 400°C. Here, we report growth of crystalline ZnS thin films by pulsed laser deposition on amorphous and polycrystalline surfaces including silicon nitride, thermal silicon dioxide, yttrium oxide, hafnium dioxide, sapphire, platinum, and titanium nitride. X-ray diffraction reveals out-of-plane texturing across all surfaces, while grazing incidence wide-angle X-ray scattering probes in-plane crystalline quality. Surface and interface properties are assessed using X-ray reflectivity and atomic force microscopy. Electrical characterization via J-V measurements (ZnS on Pt) and metal-oxide-semiconductor capacitor (ZnS on silicon dioxide) measurements show low leakage current ($10^{-5} A/cm^2$ at 0.40 MV/cm) and bilayer capacitor behavior, suggesting ZnS is highly intrinsic with minimal electrically active defects. Further work on doping ZnS with copper or other p-type elements is needed to realize ZnS as a dopable wide band-gap semiconductor for BEOL integration. This work demonstrates a novel thin-film growth method for sulfide semiconductors under BEOL-compatible conditions. △ Less

Submitted 5 May, 2025; v1 submitted 28 April, 2025; originally announced April 2025.

Harnessing Machine Learning for Quantum-Accurate Predictions of Non-Equilibrium Behavior in 2D Materials

Authors: Yue Zhang, Robert J. Appleton, Kui Lin, Megan J. McCarthy, Jeffrey T. Paci, Subramanian K. R. S. Sankaranarayanan, Alejandro Strachan, Horacio D. Espinosa

Abstract: Accurately predicting the non-equilibrium mechanical properties of two-dimensional (2D) materials is essential for understanding their deformation, thermo-mechanical properties, and failure mechanisms. In this study, we parameterize and evaluate two machine learning (ML) interatomic potentials, SNAP and Allegro, for modeling the non-equilibrium behavior of monolayer MoSe2. Using a density function… ▽ More Accurately predicting the non-equilibrium mechanical properties of two-dimensional (2D) materials is essential for understanding their deformation, thermo-mechanical properties, and failure mechanisms. In this study, we parameterize and evaluate two machine learning (ML) interatomic potentials, SNAP and Allegro, for modeling the non-equilibrium behavior of monolayer MoSe2. Using a density functional theory (DFT) derived dataset, we systematically compare their accuracy and transferability against the physics-based Tersoff force field. Our results show that SNAP and Allegro significantly outperform Tersoff, achieving near-DFT accuracy while maintaining computational efficiency. Allegro surpasses SNAP in both accuracy and efficiency due to its advanced neural network architecture. Both ML potentials demonstrate strong transferability, accurately predicting out-of-sample properties such as surface stability, inversion domain formation, and fracture toughness. Unlike Tersoff, SNAP and Allegro reliably model temperature-dependent edge stabilities and phase transformation pathways, aligning closely with DFT benchmarks. Notably, their fracture toughness predictions closely match experimental measurements, reinforcing their suitability for large-scale simulations of mechanical failure in 2D materials. This study establishes ML-based force fields as a powerful alternative to traditional potentials for modeling non-equilibrium mechanical properties in 2D materials. △ Less

Submitted 13 April, 2025; originally announced April 2025.

Comments: 23 pages, 5 figures, Supplementary Information (5 figures and two tables)

The dynamical role of optical phonons and sub-lattice screening in a solid-state ion conductor

Authors: Kim H. Pham, Vijaya Begum-Hudde, Amy K. Lin, Natan A. Spear, Jackson McClellan, Michael W. Zuerch, Andre Schleife, Kimberly A. See, Scott Cushing

Abstract: Solid-state electrolytes (SSEs) require ionic conductivities that are competitive with liquid electrolytes to realize applications in all-solid state batteries. Although numerous materials have been discovered, the underlying mechanisms enabling superionic conduction remain elusive. In particular, the role of ultrafast lattice dynamics in mediating ion migration, which involves couplings between i… ▽ More Solid-state electrolytes (SSEs) require ionic conductivities that are competitive with liquid electrolytes to realize applications in all-solid state batteries. Although numerous materials have been discovered, the underlying mechanisms enabling superionic conduction remain elusive. In particular, the role of ultrafast lattice dynamics in mediating ion migration, which involves couplings between ions, phonons, and electrons, is rarely explored experimentally at their corresponding timescales. To investigate the contributions of coupled lattice dynamics on ion migration, we modulate the charge density occupations within the crystal, and then measure the time-resolved change in impedance on picosecond timescales for a candidate SSE, Li0.5La0.5TiO3. Upon perturbation, we observe enhanced ion migration at ultrafast timescales that are shorter than laser-induced heating. The respective transients match the timescales of optical and acoustic phonon vibrations, suggesting their involvement in ion migration. We further computationally evaluate the effect of a charge transfer from the O 2p to Ti 3d band on the electronic and physical structure of LLTO. We hypothesize that the charge transfer excitation distorts the TiO6 polyhedra by altering the local charge density occupancy of the hopping site at the migration pathway saddle point, thereby causing a reduction in the migration barrier for the Li+ hop, shown using computation. We rule out the contribution of photogenerated electrons and laser heating. Overall, our investigation introduces a new spectroscopic tool to probe fundamental ion hopping mechanisms transiently at ultrafast timescales, which has previously only been achieved in a time-averaged manner or solely via computational methods. Our proposed technique expands our capability to answer dynamical questions previously limited by incumbent spectroscopic strategies. △ Less

Submitted 11 April, 2025; v1 submitted 9 April, 2025; originally announced April 2025.

Comments: 7 figures, 1 table

Chaos-Assisted Dynamical Tunneling in Flat Band Superwires

Authors: Anton Marius Graf, Ke Lin, MyeongSeo Kim, Joonas Keski-Rahkonen, Alvar Daza, Eric Heller

Abstract: Recent theoretical investigations have revealed unconventional transport mechanisms within high Brilliouin zones of two-dimensional superlattices. Electrons can navigate along channels we call superwires, gently guided without brute force confinement. Such dynamical confinement is caused by weak superlattice deflections, markedly different from the static or energetic confinement observed in tradi… ▽ More Recent theoretical investigations have revealed unconventional transport mechanisms within high Brilliouin zones of two-dimensional superlattices. Electrons can navigate along channels we call superwires, gently guided without brute force confinement. Such dynamical confinement is caused by weak superlattice deflections, markedly different from the static or energetic confinement observed in traditional wave guides or one-dimensional electron wires. The quantum properties of superwires give rise to elastic dynamical tunneling, linking disjoint regions of the corresponding classical phase space, and enabling the emergence of several parallel channels. This paper provides the underlying theory and mechanisms that facilitate dynamical tunneling assisted by chaos in periodic lattices. Moreover, we show that the mechanism of dynamical tunneling can be effectively conceptualized through the lens of a paraxial approximation. Our results further reveal that superwires predominantly exist within flat bands, emerging from eigenstates that represent linear combinations of conventional degenerate Bloch states. Finally, we quantify tunneling rates across various lattice configurations, and demonstrate the tunneling can be suppressed in a controlled fashion, illustrating potential implications in future nanodevices. △ Less

Submitted 29 April, 2024; originally announced April 2024.

Comments: 12 pages, 6 Figures

Journal ref: Entropy 26, 492 (2024)

Electrically tunable layer-hybridized trions in doped WSe$_2$ bilayers

Authors: Raul Perea-Causin, Samuel Brem, Fabian Buchner, Yao Lu, Kenji Watanabe, Takashi Taniguchi, John M. Lupton, Kai-Qiang Lin, Ermin Malic

Abstract: Doped van der Waals heterostructures host layer-hybridized trions, i.e. charged excitons with layer-delocalized constituents holding promise for highly controllable optoelectronics. Combining a microscopic theory with photoluminescence (PL) experiments, we demonstrate the electrical tunability of the trion energy landscape in naturally stacked WSe$_2$ bilayers. We show that an out-of-plane electri… ▽ More Doped van der Waals heterostructures host layer-hybridized trions, i.e. charged excitons with layer-delocalized constituents holding promise for highly controllable optoelectronics. Combining a microscopic theory with photoluminescence (PL) experiments, we demonstrate the electrical tunability of the trion energy landscape in naturally stacked WSe$_2$ bilayers. We show that an out-of-plane electric field modifies the energetic ordering of the lowest lying trion states, which consist of layer-hybridized $Λ$-point electrons and layer-localized K-point holes. At small fields, intralayer-like trions yield distinct PL signatures in opposite doping regimes characterized by weak Stark shifts in both cases. Above a doping-asymmetric critical field, interlayer-like species are energetically favored and produce PL peaks with a pronounced Stark red-shift and a counter-intuitively large intensity arising from efficient phonon-assisted recombination. Our work presents an important step forward in the microscopic understanding of layer-hybridized trions in van der Waals heterostructures and paves the way towards optoelectronic applications based on electrically controllable atomically-thin semiconductors. △ Less

Submitted 7 August, 2024; v1 submitted 29 April, 2024; originally announced April 2024.

Journal ref: Nat Commun 15, 6713 (2024)

Probing the band splitting near the $Γ$ point in the van der Waals magnetic semiconductor CrSBr

Authors: Kaiman Lin, Yi Li, Mahdi Ghorbani-Asl, Zdenek Sofer, Stephan Winnerl, Artur Erbe, Arkady V. Krasheninnikov, Manfred Helm, Shengqiang Zhou, Yaping Dan, Slawomir Prucnal

Abstract: This study investigates the electronic band structure of Chromium Sulfur Bromide (CrSBr) through comprehensive photoluminescence (PL) characterization. We clearly identify low-temperature optical transitions between two closely adjacent conduction-band states and two different valence-band states. The analysis of the PL data robustly unveils energy splittings, bandgaps and excitonic transitions ac… ▽ More This study investigates the electronic band structure of Chromium Sulfur Bromide (CrSBr) through comprehensive photoluminescence (PL) characterization. We clearly identify low-temperature optical transitions between two closely adjacent conduction-band states and two different valence-band states. The analysis of the PL data robustly unveils energy splittings, bandgaps and excitonic transitions across different thicknesses of CrSBr, ranging from monolayer to bulk. Temperature-dependent PL measurements elucidate the stability of the band splitting below the Néel temperature, suggesting that magnons coupled with excitons are responsible for the symmetry breaking and brightening of the transitions from the secondary conduction band minimum (CBM2) to the global valence band maximum (VBM1). Collectively, these results not only reveal band splitting in both the conduction and valence bands, but also point to an intricate interplay between the optical, electronic and magnetic properties of antiferromagnetic two-dimensional van der Waals crystals. △ Less

Submitted 2 April, 2024; originally announced April 2024.

Ultralarge polarization in ferroelectric hafnia-based thin films

Authors: Han Wu, Kun Lin, Qinghua Zhang, Qian Yu, Xiaoqian Fu, Qiang Li, Meera Cheviri, Oswaldo Dieguez, Shuai Xu, Lin Gu, Yili Cao, Jiaou Wang, Zhen Wang, Yu Chen, Huanhua Wang, Jinxia Deng, Jun Miao, Xianran Xing

Abstract: Hafnia-based ferroelectrics have become a valuable class of electronic functional materials at the nanoscale, showing great potential for next-generation memory and logic devices. However, more robust ferroelectric properties and better understanding of the polarization mechanisms are currently needed both in technology and science. Herein, we report the properties of oxygen-deficient Hf0.5Zr0.5O2… ▽ More Hafnia-based ferroelectrics have become a valuable class of electronic functional materials at the nanoscale, showing great potential for next-generation memory and logic devices. However, more robust ferroelectric properties and better understanding of the polarization mechanisms are currently needed both in technology and science. Herein, we report the properties of oxygen-deficient Hf0.5Zr0.5O2 films with ultralarge remanent polarization (Pr) of 387 uC cm-2 at room temperature (1 kHz). Structure characterizations identify a new ferroelectric monoclinic Pc phase in these Hf0.5Zr0.5O2 films. The in-situ STEM measurements evidence polar displacements of the oxygen atoms, which move up and down in the Pc structure under applied DC bias fields, showing a huge displacement (1.6 A). DFT calculations optimized the Pc structure and also predicted a large polarization. The coexistence of the ferroelectric monoclinic (Pc) phases and orthorhombic (Pca21) is responsible for this superior ferroelectric properties. These findings are promising for hafnia-based ferroelectric applications in integrated ferroelectric devices, energy harvesting and actuators, etc. △ Less

Submitted 18 March, 2024; originally announced March 2024.

Forming Long-range Order of Semiconducting Polymers through Liquid-phase Directional Molecular Assemblies

Authors: Minh Nhat Pham, Chun-Jen Su, Yu-Ching Huang, Kun-Ta Lin, Ting-Yu Huang, Yu-Ying Lai, Chen-An Wang, Yong-Kang Liaw, Ting-Han Lin, U-Ser Jeng, Jrjeng Ruan, Chan Luo, Ye Huang, Guillermo C. Bazan, Ben B. Y. Hsu

Abstract: Intermolecular interactions are crucial in determining the morphology of solution-processed semiconducting polymer thin films. However, these random interactions often lead to disordered or short-range ordered structures. Achieving long-range order in these films has been a challenge due to limited control over microscopic interactions in current techniques. Here, we present a molecular-level meth… ▽ More Intermolecular interactions are crucial in determining the morphology of solution-processed semiconducting polymer thin films. However, these random interactions often lead to disordered or short-range ordered structures. Achieving long-range order in these films has been a challenge due to limited control over microscopic interactions in current techniques. Here, we present a molecular-level methodology that leverages spatial matching of intermolecular dynamics among solutes, solvents, and substrates to induce directional molecular assembly in weakly bonded polymers. Within the optimized dynamic scale of 2.5 Å between polymer side chains and self-assembled monolayers (SAMs) on nanogrooved substrates, our approach transforms random aggregates into unidirectional fibers with a remarkable increase in the anisotropic stacking ratio from 1 to 11. The Flory-Huggins-based molecular stacking model accurately predicts the transitioning order on various SAMs, validated by morphologic and spectroscopic observations. The enhanced structural ordering spans over 3 orders of magnitude in length, raising from the smallest 7.3 nm random crystallites to >14 um unidirectional fibers on sub-millimeter areas. Overall, this study provides insights into the control of complex intermolecular interactions and offers enhanced molecular-level controllability in solution-based processes. △ Less

Submitted 19 February, 2024; originally announced February 2024.

Comments: 40 pages, 6 figures, 2 tables

Origin of magic angles in twisted bilayer graphene: The magic ring

Authors: Wei-Chen Wang, Feng-Wu Chen, Kuan-Sen Lin, Justin T. Hou, Ho-Chun Lin, Mei-Yin Chou

Abstract: The unexpected discovery of superconductivity and strong electron correlation in twisted bilayer graphene (TBG), a system containing only sp electrons, is considered as one of the most intriguing developments in two-dimensional materials in recent years. The key feature is the emergent flat energy bands near the Fermi level, a favorable condition for novel many-body phases, at the so-called "magic… ▽ More The unexpected discovery of superconductivity and strong electron correlation in twisted bilayer graphene (TBG), a system containing only sp electrons, is considered as one of the most intriguing developments in two-dimensional materials in recent years. The key feature is the emergent flat energy bands near the Fermi level, a favorable condition for novel many-body phases, at the so-called "magic angles". The physical origin of these interesting flat bands has been elusive to date, hindering the construction of an effective theory for the unconventional electron correlation. In this work, we have identified the importance of charge accumulation in the AA region of the moire supercell and the most critical role of the Fermi ring in AA-stacked bilayer graphene. We show that the magic angles can be predicted by the moire periodicity determined by the size of this Fermi ring. The resonant criterion in momentum space makes it possible to coherently combine states on the Fermi ring through scattering by the moire potential, leading to flat bands near the Fermi level. We thus establish the physical origin of the magic angles in TBG and identify the characteristics of one-particle states associated with the flat bands for further many-body investigations. △ Less

Submitted 21 April, 2025; v1 submitted 18 September, 2023; originally announced September 2023.

Comments: 12 pages, 4 figures

Journal ref: Phys. Rev. B 110, 115154 (2024)

Phantom energy in the nonlinear response of a quantum many-body scar state

Authors: Kangning Yang, Yicheng Zhang, Kuan-Yu Li, Kuan-Yu Lin, Sarang Gopalakrishnan, Marcos Rigol, Benjamin L. Lev

Abstract: Quantum many-body scars are notable as nonthermal states that exist at high energies. Here, we use attractively interacting dysprosium gases to create scar states that are stable enough be driven into a strongly nonlinear regime while retaining their character. We uncover an emergent nonlinear many-body phenomenon, the effective transmutation of attractive interactions into repulsive interactions.… ▽ More Quantum many-body scars are notable as nonthermal states that exist at high energies. Here, we use attractively interacting dysprosium gases to create scar states that are stable enough be driven into a strongly nonlinear regime while retaining their character. We uncover an emergent nonlinear many-body phenomenon, the effective transmutation of attractive interactions into repulsive interactions. We measure how the kinetic and total energies evolve after quenching the confining potential. Although the bare interactions are attractive, the low-energy degrees of freedom evolve as if they repel each other: Thus, their kinetic energy paradoxically decreases as the gas is compressed. The missing ``phantom'' energy is quantified by benchmarking our experimental results against generalized hydrodynamics calculations. We present evidence that the missing kinetic energy is stored in very high-momentum modes. △ Less

Submitted 9 September, 2023; v1 submitted 22 August, 2023; originally announced August 2023.

Comments: 6 pages, 4 figures with 15-page supplement including 9 figures

Journal ref: Science 385, 1063 (2024)

Strong Exciton-Phonon Coupling as a Fingerprint of Magnetic Ordering in van der Waals Layered CrSBr

Authors: Kaiman Lin, Xiaoxiao Sun, Florian Dirnberger, Yi Li, Jiang Qu, Peiting Wen, Zdenek Sofer, Aljoscha Söll, Stephan Winnerl, Manfred Helm, Shengqiang Zhou, Yaping Dan, Slawomir Prucnal

Abstract: The layered, air-stable van der Waals antiferromagnetic compound CrSBr exhibits pronounced coupling between its optical, electronic, and magnetic properties. As an example, exciton dynamics can be significantly influenced by lattice vibrations through exciton-phonon coupling. Using low-temperature photoluminescence spectroscopy, we demonstrate the effective coupling between excitons and phonons in… ▽ More The layered, air-stable van der Waals antiferromagnetic compound CrSBr exhibits pronounced coupling between its optical, electronic, and magnetic properties. As an example, exciton dynamics can be significantly influenced by lattice vibrations through exciton-phonon coupling. Using low-temperature photoluminescence spectroscopy, we demonstrate the effective coupling between excitons and phonons in nanometer-thick CrSBr. By careful analysis, we identify that the satellite peaks predominantly arise from the interaction between the exciton and an optical phonon with a frequency of 118 cm-1 (~14.6 meV) due to the out-of-plane vibration of Br atoms. Power-dependent and temperature-dependent photoluminescence measurements support exciton-phonon coupling and indicate a coupling between magnetic and optical properties, suggesting the possibility of carrier localization in the material. The presence of strong coupling between the exciton and the lattice may have important implications for the design of light-matter interactions in magnetic semiconductors and provides new insights into the exciton dynamics in CrSBr. This highlights the potential for exploiting exciton-phonon coupling to control the optical properties of layered antiferromagnetic materials. △ Less

Submitted 31 January, 2024; v1 submitted 9 August, 2023; originally announced August 2023.

Comments: 21 pages, together with suppl

Journal ref: ACS Nano 18, 2898-2905 (2024)

Perpendicular separations of a binary mixture under van der Waals confinement

Authors: Kui Lin

Abstract: We investigated the dynamics of a binary mixture confined within van der Waals (vdW) walls using molecular dynamics simulations. We discovered a novel phenomenon named perpendicular separations of two phases (PSTP). In the initial stage, central water molecules diffused, subsequently condensing symmetrically within the confinement's mid-plane. In the later stage, as water droplets nucleate and gro… ▽ More We investigated the dynamics of a binary mixture confined within van der Waals (vdW) walls using molecular dynamics simulations. We discovered a novel phenomenon named perpendicular separations of two phases (PSTP). In the initial stage, central water molecules diffused, subsequently condensing symmetrically within the confinement's mid-plane. In the later stage, as water droplets nucleate and grow, the resin separates perpendicularly into two films due to the action of bubblers and vdW walls, resulting in a hollow nanochannel. The mechanisms and conditions underlying PSTS are discussed. The results indicate that the concentration (C) of resin in the middle region is linearly decreased with temporal power. We propose a new mechanism for stabilizing nanochannels and films: dynamic "soft pillars" that prevent Rayleigh-like instability. Our findings could shed light on the manufacture of nanofilms and organic nanochannels, which could help advance bio-detection and energy fields. △ Less

Submitted 11 July, 2024; v1 submitted 20 July, 2023; originally announced July 2023.

Comments: 6 pages, 6 figures

Triplet State and Auger-Type Excitation Originating from Two-Electron Tunneling in Field Emission Resonance on Ag(100)

Authors: Shin-Ming Lu, Ho-Hsiang Chang, Wei-Bin Su, Wen-Yuan Chan, Kung-Hsuan Lin, Chia-Seng Chang

Abstract: In this study, we discovered that the energy gap above the vacuum level in the projected bulk band structure of Ag(100) prevents electrons in the first-order field emission resonance (FER) from inducing the surface plasmons. This mechanism allows light emission from FER to reveal characteristics of triplet states and Auger-type excitation resulting from two-electron tunneling in FER. According to… ▽ More In this study, we discovered that the energy gap above the vacuum level in the projected bulk band structure of Ag(100) prevents electrons in the first-order field emission resonance (FER) from inducing the surface plasmons. This mechanism allows light emission from FER to reveal characteristics of triplet states and Auger-type excitation resulting from two-electron tunneling in FER. According to optical spectra, surface plasmons can be induced by electrons in the zeroth-order FER. However, corresponding radiative decay can also trigger Auger-type excitation, whose energy state is influenced by the sharpness-dependent image potential acting on the scanning tunneling microscope tip. △ Less

Submitted 7 June, 2023; originally announced June 2023.

Comments: 19 pages, 8 figures

MSC Class: 81-02

Ferromagnetic interlayer coupling in CrSBr crystals irradiated by ions

Authors: Fangchao Long, Mahdi Ghorbani-Asl, Kseniia Mosina, Yi Li, Kaiman Lin, Fabian Ganss, René Hübner, Zdenek Sofer, Florian Dirnberger, Akashdeep Kamra, Arkady V. Krasheninnikov, Slawomir Prucnal, Manfred Helm, Shengqiang Zhou

Abstract: Layered magnetic materials are becoming a major platform for future spin-based applications. Particularly the air-stable van der Waals compound CrSBr is attracting considerable interest due to its prominent magneto-transport and magneto-optical properties. In this work, we observe a transition from antiferromagnetic to ferromagnetic behavior in CrSBr crystals exposed to high-energy, non-magnetic i… ▽ More Layered magnetic materials are becoming a major platform for future spin-based applications. Particularly the air-stable van der Waals compound CrSBr is attracting considerable interest due to its prominent magneto-transport and magneto-optical properties. In this work, we observe a transition from antiferromagnetic to ferromagnetic behavior in CrSBr crystals exposed to high-energy, non-magnetic ions. Already at moderate fluences, ion irradiation induces a remanent magnetization with hysteresis adapting to the easy-axis anisotropy of the pristine magnetic order up to a critical temperature of 110 K. Structure analysis of the irradiated crystals in conjunction with density functional theory calculations suggest that the displacement of constituent atoms due to collisions with ions and the formation of interstitials favors ferromagnetic order between the layers. △ Less

Submitted 27 September, 2023; v1 submitted 30 May, 2023; originally announced May 2023.

Comments: 25 pages including the supporting information, published as Nano Lett. 23, 8468-8473 (2023)

Journal ref: Nano Lett. 23, 8468-8473 (2023)

Correlated Terahertz phonon-ion interactions control ion conduction in a solid electrolyte

Authors: Kim H. Pham, Kiarash Gordiz, Natan A. Spear, Amy K. Lin, Jonathan M. Michelsen, Hanzhe Liu, Daniele Vivona, Geoffrey A. Blake, Yang Shao-Horn, Asegun Henry, Kimberly A. See, Scott K. Cushing

Abstract: Ionic conduction in solids that exceeds 1 mS/cm is predicted to involve coupled phonon-ion interactions in the crystal lattice. Here, we use theory and experiment to measure the possible contribution of coupled phonon-ion hopping modes which enhance Li+ migration in Li0.5La0.5TiO3 (LLTO). The ab initio calculations predict that the targeted excitation of individual TiO6 rocking modes greatly incre… ▽ More Ionic conduction in solids that exceeds 1 mS/cm is predicted to involve coupled phonon-ion interactions in the crystal lattice. Here, we use theory and experiment to measure the possible contribution of coupled phonon-ion hopping modes which enhance Li+ migration in Li0.5La0.5TiO3 (LLTO). The ab initio calculations predict that the targeted excitation of individual TiO6 rocking modes greatly increases the Li+ jump rate as compared to the excitation of vibrational modes associated with heating. Experimentally, coherently driving TiO6 rocking modes via terahertz (THz) illumination leads to a ten-fold decrease in the differential impedance compared to the excitation of acoustic and optical phonons. Additionally, we differentiate the ultrafast responses of LLTO due to ultrafast heating and THz-range vibrations using laser-driven spectroscopy (LUIS), finding a unique long-lived response for the THz-range excitation. These findings provide new insights into coupled ion migration mechanisms, indicating the important role of THz-range coupled phonon-ion hopping modes in enabling fast ion conduction at room temperature. △ Less

Submitted 25 December, 2025; v1 submitted 2 May, 2023; originally announced May 2023.

Comments: 13 pages, 6 figures

Crossover from non-thermal to thermal photoluminescence from metals excited by ultrashort light pulses

Authors: Y. Sivan, I. W. Un, I. Kalyan, K. -Q. Lin, J. M. Lupton, S. Bange

Abstract: Photoluminescence from metal nanostructures following intense ultrashort illumination is a fundamental aspect of light-matter interactions. Surprisingly, many of its basic characteristics are under ongoing debate. Here, we resolve many of these debates by providing a comprehensive theoretical framework that describes this phenomenon, and support it by experimental confirmation. Specifically, we id… ▽ More Photoluminescence from metal nanostructures following intense ultrashort illumination is a fundamental aspect of light-matter interactions. Surprisingly, many of its basic characteristics are under ongoing debate. Here, we resolve many of these debates by providing a comprehensive theoretical framework that describes this phenomenon, and support it by experimental confirmation. Specifically, we identify aspects of the emission that are characteristic to either non-thermal or thermal emission, in particular, differences in the spectral and electric field-dependence of these two contributions to the emission. Overall, non-thermal emission is characteristic of the early stages of light emission, while the later stages show thermal characteristics. The former dominate only for moderately high illumination intensities for which the electron temperature reached after thermalization remains close to room temperature. The theory is complemented by experimental evidence that demonstrates the novel aspects of our considerations. △ Less

Submitted 19 April, 2023; v1 submitted 15 February, 2023; originally announced February 2023.

A high-cooperativity confocal cavity QED microscope

Authors: Ronen M. Kroeze, Brendan P. Marsh, Kuan-Yu Lin, Jonathan Keeling, Benjamin L. Lev

Abstract: Cavity QED with cooperativity far greater than unity enables high-fidelity quantum sensing and information processing. The high-cooperativity regime is often reached through the use of short, single-mode resonators. More complicated multimode resonators, such as the near-confocal optical Fabry-Pérot cavity, can provide intracavity atomic imaging in addition to high cooperativity. This capability h… ▽ More Cavity QED with cooperativity far greater than unity enables high-fidelity quantum sensing and information processing. The high-cooperativity regime is often reached through the use of short, single-mode resonators. More complicated multimode resonators, such as the near-confocal optical Fabry-Pérot cavity, can provide intracavity atomic imaging in addition to high cooperativity. This capability has recently proved important for exploring quantum many-body physics in the driven-dissipative setting. In this work, we show that a confocal cavity QED microscope can realize cooperativity in excess of 110. This cooperativity is on par with the very best single-mode cavities (which are far shorter) and 21$\times$ greater than single-mode resonators of similar length and mirror radii. The 1.7-$μ$m imaging resolution is naturally identical to the photon-mediated interaction range. We measure these quantities by determining the threshold of cavity superradiance when small, optically tweezed Bose-Einstein condensates are pumped at various intracavity locations. Transmission measurements of an ex situ cavity corroborate these results. We provide a theoretical description that shows how cooperativity enhancement arises from the dispersive coupling to the atoms of many near-degenerate modes. △ Less

Submitted 13 December, 2022; originally announced December 2022.

Comments: 20 pages including 4 figures, four appendices, and a supplement

Journal ref: PRX Quantum 4, 020326 (2023)

Theory of photoluminescence spectral line shapes of semiconductor nanocrystals

Authors: Kailai Lin, Dipti Jasrasaria, Jason J. Yoo, Moungi Bawendi, Hendrik Utzat, Eran Rabani

Abstract: Single-molecule photoluminescence (PL) spectroscopy of semiconductor nanocrystals (NCs) reveals the nature of exciton-phonon interactions in NCs. Understanding the narrow line shapes at low temperatures and the significant broadening as temperature increases remains an open problem. Here, we develop an atomistic model to describe the PL spectrum of NCs, accounting for excitonic effects, phonon dis… ▽ More Single-molecule photoluminescence (PL) spectroscopy of semiconductor nanocrystals (NCs) reveals the nature of exciton-phonon interactions in NCs. Understanding the narrow line shapes at low temperatures and the significant broadening as temperature increases remains an open problem. Here, we develop an atomistic model to describe the PL spectrum of NCs, accounting for excitonic effects, phonon dispersion relations, and exciton-phonon couplings. We use single-molecule PL measurements on CdSe/CdS core-shell NCs from T=4 to T=290K to validate our model and find that the slightly-asymmetric main peak at low temperatures is comprised of a narrow zero-phonon line (ZPL) and several acoustic phonon sidebands. Furthermore, we identify the distinct CdSe optical modes that give rise to the optical phonon sidebands. As the temperature increases, the spectral width shows a stronger dependence on temperature, which we demonstrated to be correlated with frequency shifts and mode-mixing, reflected as higher-order exciton-phonon couplings (Duschinsky rotations). We also model the PL dependence on core size and shell thickness and provide strategies for the design of NCs with narrow linewidths at elevated temperatures. △ Less

Submitted 12 December, 2022; originally announced December 2022.

Journal ref: J. Phys. Chem. Lett. 2023 14 (32), 7241-7248

Strain Engineering on the Excitonic Properties of Monolayer GaSe

Authors: Vo Khuong Dien, Nguyen Thi Han, Wei-Bang Li, Kuang-I Lin, Ming-Fa Lin

Abstract: This paper investigates strain effects on the electronic and optical properties of monolayer GaSe using first-principles calculations. The deformation significantly alters energy dispersion, band gap, and the band edge states of GaSe. The band gap evolution exhibits both linearly and nonlinearly with the strains, and strongly depending on the types of deformation and the direction of the modificat… ▽ More This paper investigates strain effects on the electronic and optical properties of monolayer GaSe using first-principles calculations. The deformation significantly alters energy dispersion, band gap, and the band edge states of GaSe. The band gap evolution exhibits both linearly and nonlinearly with the strains, and strongly depending on the types of deformation and the direction of the modifications. The external mechanical strains also significantly tailor the optical properties of GaSe, the exciton binding energy is strongly reduced when the tensile strain is applied, while the opposite way is true for compressive stress. Moreover, the inhomogeneous strain also induces strong polarization in the absorption spectra. Our calculations demonstrate that the electronic and optical properties of GaSe monolayer can be significantly tuned by using strain engineering which appears as a promising way to design novel optoelectronic devices. △ Less

Submitted 2 January, 2023; v1 submitted 7 December, 2022; originally announced December 2022.

Achieving Bright Organic Light Emitting Field Effect Transistors with Sustained Efficiency through Hybrid Contact Design

Authors: Shih-Wei Chiu, An Hsu, Lei Ying, Yong-Kang Liaw, Kun-Ta Lin, Jrjeng Ruan, Ifor D. W. Samuel, Ben Bang-Yu Hsu

Abstract: Organic light emitting field effect transistors (OLEFETs) with bilayer structures have been widely studied due to their potential to integrate high-mobility organic transistors and efficient organic light emitting diodes. However, these devices face a major challenge of imbalance charge transport leading to severe efficiency roll-off at high brightness. Here, we propose a solution to this challeng… ▽ More Organic light emitting field effect transistors (OLEFETs) with bilayer structures have been widely studied due to their potential to integrate high-mobility organic transistors and efficient organic light emitting diodes. However, these devices face a major challenge of imbalance charge transport leading to severe efficiency roll-off at high brightness. Here, we propose a solution to this challenge by introducing a transparent organic/inorganic hybrid contact with specially designed electronic structures. Our design aims to steadily accumulate the electrons injected to the emissive polymer, allowing the light emitting interface to effectively capture more holes even when hole current increases. Our numerical simulations show that the capture efficiency of these steady electrons will dominate charge recombination and lead to a sustained external quantum efficiency of 0.23% over 3 orders of magnitude of brightness (4 to 7700 cd/m2) and current density (1.2 to 2700 mA/cm2) from -4 to -100 V. The same enhancement is retained even after increasing EQE to ~0.51%. The high and tunable brightness with stable efficiency offered by hybrid-contact OLEFETs make them ideal light emitting devices for various applications. These devices have the potential to revolutionize the field of organic electronics by overcoming the fundamental challenge of imbalance charge transport. △ Less

Submitted 13 June, 2023; v1 submitted 6 December, 2022; originally announced December 2022.

Comments: 21 pages and 5 figures 2 Tables

Rapidity and momentum distributions of 1D dipolar quantum gases

Authors: Kuan-Yu Li, Yicheng Zhang, Kangning Yang, Kuan-Yu Lin, Sarang Gopalakrishnan, Marcos Rigol, Benjamin L. Lev

Abstract: We explore the effect of tunable integrability breaking dipole-dipole interactions in the equilibrium states of highly magnetic 1D Bose gases of dysprosium at low temperatures. We experimentally observe that in the strongly correlated Tonks-Girardeau regime, rapidity and momentum distributions are nearly unaffected by the dipolar interactions. By contrast, we also observe that significant changes… ▽ More We explore the effect of tunable integrability breaking dipole-dipole interactions in the equilibrium states of highly magnetic 1D Bose gases of dysprosium at low temperatures. We experimentally observe that in the strongly correlated Tonks-Girardeau regime, rapidity and momentum distributions are nearly unaffected by the dipolar interactions. By contrast, we also observe that significant changes of these distributions occur when decreasing the strength of the contact interactions. We show that the main experimental observations are captured by modeling the system as an array of 1D gases with only contact interactions, dressed by the contribution of the short-range part of the dipolar interactions. Improvements to theory-experiment correspondence will require new tools tailored to near-integrable models possessing both short and long-range interactions. △ Less

Submitted 29 June, 2023; v1 submitted 16 November, 2022; originally announced November 2022.

Comments: 5 pages, 4 figures plus supplement, as published

Journal ref: Phys. Rev. A 107, L061302 (2023)

Interpretation of spin-wave modes in Co/Ag nanodot arrays probed by broadband ferromagnetic resonance

Authors: Daniel Markó, Rajgowrav Cheenikundil, Julien Bauer, Kilian Lenz, Wan-Chen Chuang, Ko-Wei Lin, Jong-Ching Wu, Massimiliano d'Aquino, Riccardo Hertel, David S. Schmool

Abstract: We present a detailed investigation of the magnetization dynamics in Co/Ag nanodots, which due to their size can support standing spin-wave (SSW) modes with complex spectral responses. To interpret the experimentally measured broadband vector network analyzer ferromagnetic resonance data, we compare the spectra of the nanoarray structure with those of the unpatterned Co/Ag film of identical thickn… ▽ More We present a detailed investigation of the magnetization dynamics in Co/Ag nanodots, which due to their size can support standing spin-wave (SSW) modes with complex spectral responses. To interpret the experimentally measured broadband vector network analyzer ferromagnetic resonance data, we compare the spectra of the nanoarray structure with those of the unpatterned Co/Ag film of identical thickness, which serves as a baseline for obtaining the general magnetic parameters of the system. Using a novel frequency domain, matrix-free simulation method of the dynamic response, we identify the nature of the excitation modes, which allows us to assess the boundary conditions for the nanodots. We find an excellent agreement between the calculated and experimental values for the frequencies of the fundamental (uniform-like) (011) mode. The existence of an edge-localized mode in the experiment has been confirmed and fits very well with theory and micromagnetic simulations, having the form of a flapping mode at the extrema of the nanodot in one of the in-plane directions. Its frequency is below the fundamental mode's frequency and has been shown to be a consequence of the imaginary wave vector for such localized SSW modes. Higher order SSW modes can be generated from the theory, which allows us to find a probable mode number for the second bulk SSW (201 or 221 or 131), which lies at frequencies above the fundamental mode. △ Less

Submitted 20 July, 2023; v1 submitted 6 November, 2022; originally announced November 2022.

Comments: 20 pages, 14 figures

Decoration of graphene nanoribbons by $5d$ transition-metal elements

Authors: Wei-Bang Li, Kuang-I Lin, Yu-Ming Wang, Hsien-Ching Chung, Ming-Fa Lin

Abstract: Graphene is a famous truly two-dimensional (2D) material, possessing a cone-like energy structure near the Fermi level and treated as a gapless semiconductor. Its unique properties trigger researchers to find applications of it. The gapless feature shrinks the development of graphene nanoelectronics. Making one-dimensional (1D) strips of graphene nanoribbons (GNRs) could be one of the promising ro… ▽ More Graphene is a famous truly two-dimensional (2D) material, possessing a cone-like energy structure near the Fermi level and treated as a gapless semiconductor. Its unique properties trigger researchers to find applications of it. The gapless feature shrinks the development of graphene nanoelectronics. Making one-dimensional (1D) strips of graphene nanoribbons (GNRs) could be one of the promising routes to modulating the electronic and optical properties of graphene. The electronic and optical properties of GNRs are highly sensitive to the edge and width. The tunability in electronic and optical properties further implies the possibilities of GNR application. However, the dangling bonds at ribbon edges remain an open question in GNR systems. Various passivation at the ribbon edge might change the essential physical properties. In this work, $5d$ transition-metal elements are considered as the guest atoms at the edges. The geometric structure, energy bands, density of states, charge distribution, and optical transitions are discussed. △ Less

Submitted 18 September, 2022; v1 submitted 16 September, 2022; originally announced September 2022.

Journal ref: Chemical Modifications of Graphene-Like Materials, pp. 307-333 (2024)

Spin-Resolved Topology and Partial Axion Angles in Three-Dimensional Insulators

Authors: Kuan-Sen Lin, Giandomenico Palumbo, Zhaopeng Guo, Yoonseok Hwang, Jeremy Blackburn, Daniel P. Shoemaker, Fahad Mahmood, Zhijun Wang, Gregory A. Fiete, Benjamin J. Wieder, Barry Bradlyn

Abstract: Symmetry-protected topological crystalline insulators (TCIs) have primarily been characterized by their gapless boundary states. However, in time-reversal- ($\mathcal{T}$-) invariant (helical) 3D TCI$\unicode{x2014}$termed higher-order TCIs (HOTIs)$\unicode{x2014}$the boundary signatures can manifest as a sample-dependent network of 1D hinge states. We here introduce nested spin-resolved Wilson lo… ▽ More Symmetry-protected topological crystalline insulators (TCIs) have primarily been characterized by their gapless boundary states. However, in time-reversal- ($\mathcal{T}$-) invariant (helical) 3D TCI$\unicode{x2014}$termed higher-order TCIs (HOTIs)$\unicode{x2014}$the boundary signatures can manifest as a sample-dependent network of 1D hinge states. We here introduce nested spin-resolved Wilson loops and layer constructions as tools to characterize the intrinsic bulk topological properties of spinful 3D insulators. We discover that helical HOTIs realize one of three spin-resolved phases with distinct responses that are quantitatively robust to large deformations of the bulk spin-orbital texture: 3D quantum spin Hall insulators (QSHIs), "spin-Weyl" semimetals, and $\mathcal{T}$-doubled axion insulator (T-DAXI) states with nontrivial partial axion angles indicative of a 3D spin-magnetoelectric bulk response and half-quantized 2D TI surface states originating from a partial parity anomaly. Using ab-initio calculations, we demonstrate that $β$-MoTe$_2$ realizes a spin-Weyl state and that $α$-BiBr hosts both 3D QSHI and T-DAXI regimes. △ Less

Submitted 17 January, 2024; v1 submitted 20 July, 2022; originally announced July 2022.

Comments: Final version: 22+146 pages, 8+44 figures. Nested and spin-resolved Wilson loop code with example scripts and documentation freely available at https://github.com/kuansenlin/nested_and_spin_resolved_Wilson_loop

Journal ref: Nature Communications 15, 550 (2024)

Emergence of Interlayer Coherence in Twist-Controlled Graphene Double Layers

Authors: Kenneth Lin, Nitin Prasad, G. William Burg, Bo Zou, Keiji Ueno, Kenji Watanabe, Takashi Taniguchi, Allan H. MacDonald, Emanuel Tutuc

Abstract: We report enhanced interlayer tunneling with reduced linewidth at zero interlayer bias in a twist-controlled double monolayer graphene heterostructure in the quantum Hall regime, when the top ($ν_{\mathrm{T}}$) and bottom ($ν_{\mathrm{B}}$) layer filling factors are near $ν_{\mathrm{T}}=\pm1/2, \pm3/2$ and $ν_{\mathrm{B}}=\pm1/2, \pm3/2$, and the total filling factor $ν= \pm1$ or $\pm3$. The zero-… ▽ More We report enhanced interlayer tunneling with reduced linewidth at zero interlayer bias in a twist-controlled double monolayer graphene heterostructure in the quantum Hall regime, when the top ($ν_{\mathrm{T}}$) and bottom ($ν_{\mathrm{B}}$) layer filling factors are near $ν_{\mathrm{T}}=\pm1/2, \pm3/2$ and $ν_{\mathrm{B}}=\pm1/2, \pm3/2$, and the total filling factor $ν= \pm1$ or $\pm3$. The zero-bias interlayer conductance peaks are stable against variations of layer filling factor, and signal the emergence of interlayer phase coherence. Our results highlight twist control as a key attribute in revealing interlayer coherence using tunneling. △ Less

Submitted 21 November, 2022; v1 submitted 23 June, 2022; originally announced June 2022.

Comments: 5 pages, 4 figures, includes supplementary material

Photothermal responsivity of van der Waals material-based nanomechanical resonators

Authors: Myrron Albert C. Aguila, Joshoua C. Esmenda, Jyh-Yang Wang, Teik-Hui Lee, Yen-Chun Chen, Chi-Yuan Yang, Kung-Hsuan Lin, Kuei-Shu Chang-Liao, Sergey Kafanov, Yuri A. Pashkin, Chii-Dong Chen

Abstract: Nanomechanical resonators made from van der Waals materials (vdW NMRs) provide a new tool for sensing absorbed laser power. The photothermal response of vdW NMRs, quantified from the resonant frequency shifts induced by optical absorption, is enhanced when incorporated in a Fabry-Perot (FP) interferometer. Along with the enhancement comes the dependence of the photothermal response on NMR displace… ▽ More Nanomechanical resonators made from van der Waals materials (vdW NMRs) provide a new tool for sensing absorbed laser power. The photothermal response of vdW NMRs, quantified from the resonant frequency shifts induced by optical absorption, is enhanced when incorporated in a Fabry-Perot (FP) interferometer. Along with the enhancement comes the dependence of the photothermal response on NMR displacement, which lacks investigation. Here, we address the knowledge gap by studying electromotively driven niobium diselenide drumheads fabricated on highly reflective substrates. We use a FP-mediated absorptive heating model to explain the measured variations of the photothermal response. The model predicts a higher magnitude and tuning range of photothermal responses on few-layer and monolayer NbSe$_{2}$ drumheads, which outperform other clamped vdW drum-type NMRs at a laser wavelength of $532\,$nm. Further analysis of the model shows that both the magnitude and tuning range of NbSe$_{2}$ drumheads scale with thickness, establishing a displacement-based framework for building bolometers using FP-mediated vdW NMRs. △ Less

Submitted 18 April, 2022; originally announced April 2022.

Comments: 7 pages, 4 figures

Journal ref: Nanomaterials 12 (2022), 2675

Chemical design rules for non-fullerene acceptors in organic solar cells

Authors: A. Markina, K. -H. Lin, W. Liu, C. Poelking, Y. Firdaus, D. R. Villalva, J. I. Khan, S. H. K. Paleti, G. T. Harrison, J. Gorenflot, W. Zhang, S. De Wolf, I. McCulloch, T. D. Anthopoulos, D. Baran, F. Laquai, D. Andrienko

Abstract: Efficiencies of organic solar cells have practically doubled since the development of non-fullerene acceptors (NFAs). However, generic chemical design rules for donor-NFA combinations are still needed. Such rules are proposed by analyzing inhomogeneous electrostatic fields at the donor-acceptor interface. It is shown that an acceptor-donor-acceptor molecular architecture, and molecular alignment p… ▽ More Efficiencies of organic solar cells have practically doubled since the development of non-fullerene acceptors (NFAs). However, generic chemical design rules for donor-NFA combinations are still needed. Such rules are proposed by analyzing inhomogeneous electrostatic fields at the donor-acceptor interface. It is shown that an acceptor-donor-acceptor molecular architecture, and molecular alignment parallel to the interface, results in energy level bending that destabilizes the charge transfer state, thus promoting its dissociation into free charges. By analyzing a series of PCE10:NFA solar cells, with NFAs including Y6, IEICO, and ITIC, as well as their halogenated derivatives, it is suggested that the molecular quadrupole moment of ca 75 Debye A balances the losses in the open circuit voltage and gains in charge generation efficiency. △ Less

Submitted 27 January, 2022; originally announced January 2022.

Journal ref: Adv. Energy Mater., 2102363, 2-11, 2021

Robust excitons across the phase transition of two-dimensional hybrid perovskites

Authors: Jonas D. Ziegler, Kai-Qiang Lin, Barbara Meisinger, Xiangzhou Zhu, Manuel Kober-Czerny, Pabitra K. Nayak, Cecilia Vona, Takashi Taniguchi, Kenji Watanabe, Claudia Draxl, Henry J. Snaith, John M. Lupton, David A. Egger, Alexey Chernikov

Abstract: Two-dimensional halide perovskites are among intensely studied materials platforms profiting from solution based growth and chemical flexibility. They feature exceptionally strong interactions among electronic, optical as well as vibrational excitations and hold a great potential for future optoelectronic applications. A key feature for these materials is the occurrence of structural phase transit… ▽ More Two-dimensional halide perovskites are among intensely studied materials platforms profiting from solution based growth and chemical flexibility. They feature exceptionally strong interactions among electronic, optical as well as vibrational excitations and hold a great potential for future optoelectronic applications. A key feature for these materials is the occurrence of structural phase transitions that can impact their functional properties, including the electronic band gap and optical response dominated by excitons. However, to what extent the phase-transitions in two-dimensional perovskites alter the fundamental exciton properties remains barely explored so far. Here, we study the influence of the phase transition on both exciton binding energy and exciton diffusion, demonstrating their robust nature across the phase transition. These findings are unexpected in view of the associated substantial changes of the free carrier masses, strongly contrast broadly considered effective mass and drift-diffusion transport mechanisms, highlighting the unusual nature of excitons in two-dimensional perovskites. △ Less

Submitted 27 January, 2022; originally announced January 2022.

Supercurrent diode effect and magnetochiral anisotropy in few-layer NbSe$_2$

Authors: Lorenz Bauriedl, Christian Bäuml, Lorenz Fuchs, Christian Baumgartner, Nicolas Paulik, Jonas M. Bauer, Kai-Qiang Lin, John M. Lupton, Takashi Taniguchi, Kenji Watanabe, Christoph Strunk, Nicola Paradiso

Abstract: Nonreciprocal transport refers to charge transfer processes that are sensitive to the bias polarity. Until recently, nonreciprocal transport was studied only in dissipative systems, where the nonreciprocal quantity is the resistance. Recent experiments have, however, demonstrated nonreciprocal supercurrent leading to the observation of a supercurrent diode effect in Rashba superconductors, opening… ▽ More Nonreciprocal transport refers to charge transfer processes that are sensitive to the bias polarity. Until recently, nonreciprocal transport was studied only in dissipative systems, where the nonreciprocal quantity is the resistance. Recent experiments have, however, demonstrated nonreciprocal supercurrent leading to the observation of a supercurrent diode effect in Rashba superconductors, opening the vision of dissipationless electronics. Here we report on a supercurrent diode effect in NbSe$_2$ constrictions obtained by patterning NbSe$_2$ flakes with both even and odd layer number. The observed rectification is driven by valley-Zeeman spin-orbit interaction. We demonstrate a rectification efficiency as large as 60%, considerably larger than the efficiency of devices based on Rashba superconductors. In agreement with recent theory for superconducting transition metal dichalcogenides, we show that the effect is driven by an out-of-plane magnetic field component. Remarkably, we find that the effect becomes field-asymmetric in the presence of an additional in-plane field component transverse to the current direction. Supercurrent diodes offer a further degree of freedom in designing superconducting quantum electronics with the high degree of integrability offered by van der Waals materials. △ Less

Submitted 27 April, 2022; v1 submitted 29 October, 2021; originally announced October 2021.

Comments: 18 pages, 12 figures

Journal ref: Nature Communications volume 13, Article number: 4266 (2022)

Optoelectrical nanomechanical resonators made from multilayered 2D materials

Authors: Joshoua Condicion Esmenda, Myrron Albert Callera Aguila, Jyh-Yang Wang, Teik-Hui Lee, Yen-Chun Chen, Chi-Yuan Yang, Kung-Hsuan Lin, Kuei-Shu Chang-Liao, Sergey Kafanov, Yuri Pashkin, Chii-Dong Chen

Abstract: Studies involving nanomechanical motion have evolved from its detection and understanding of its fundamental aspects to its promising practical utility as an integral component of hybrid systems. Nanomechanical resonators' indispensable role as transducers between optical and microwave fields in hybrid systems, such as quantum communications interface, have elevated their importance in recent year… ▽ More Studies involving nanomechanical motion have evolved from its detection and understanding of its fundamental aspects to its promising practical utility as an integral component of hybrid systems. Nanomechanical resonators' indispensable role as transducers between optical and microwave fields in hybrid systems, such as quantum communications interface, have elevated their importance in recent years. It is therefore crucial to determine which among the family of nanomechanical resonators is more suitable for this role. Most of the studies revolve around nanomechanical resonators of ultrathin structures because of their inherently large mechanical amplitude due to their very low mass. Here, we argue that the underutilized nanomechanical resonators made from multilayered two-dimensional (2D) materials are the better fit for this role because of their comparable electrostatic tunability and larger optomechanical responsivity. To show this, we first demonstrate the electrostatic tunability of mechanical modes of a multilayered nanomechanical resonator made from graphite. We also show that the optomechanical responsivity of multilayered devices will always be superior as compared to the few-layer devices. Finally, by using the multilayered model and comparing this device with the reported ones, we find that the electrostatic tunability of devices of intermediate thickness is not significantly lower than that of ultrathin ones. Together with the practicality in terms of fabrication ease and design predictability, we contend that multilayered 2D nanomechanical resonators are the optimal choice for the electromagnetic interface in integrated quantum systems. △ Less

Submitted 11 February, 2022; v1 submitted 19 August, 2021; originally announced August 2021.

Essential properties of AlCl4-related graphite intercalation compounds of aluminum-ion-based battery cathodes

Authors: Wei-Bang Li, Ming-Hsiu Tsai, Shih-Yang Lin, Kuang-I Lin, Ming-Fa Lin

Abstract: Up to now, many guest atoms/molecules/ions have been successfully synthesized into graphite to form the various compounds. For example, alkali-atom graphite intercalation compounds are verified to reveal the stage-n structures, including LiC6n and LiM8n [M=K. Rb and Cs; n=1, 2, 3; 4]. On the other side, AlCl4-ion/molecule ones are examined to show stage-4 and stage-3 cases at room and lower temper… ▽ More Up to now, many guest atoms/molecules/ions have been successfully synthesized into graphite to form the various compounds. For example, alkali-atom graphite intercalation compounds are verified to reveal the stage-n structures, including LiC6n and LiM8n [M=K. Rb and Cs; n=1, 2, 3; 4]. On the other side, AlCl4-ion/molecule ones are examined to show stage-4 and stage-3 cases at room and lower temperatures, respectively. Stage-1 and stage-2 configurations, with the higher intercalant concentrations, are unable to synthesize in experimental laboratories. This might arise from the fact that it is quite difficult to build the periodical arrangements along the longitudinal z and transverse directions simultaneously for the large ions or molecules. Our works are mainly focused on stage-1 and stage-2 systems in terms of geometric and electronic properties. The critical features, being associated with the atom-dominated energy spectra and wave function within the specific energy ranges, the active multi-orbital hybridization in distinct chemical bonds, and atom- & orbital-decomposed van Hove singularities, will be thoroughly clarified by the delicate simulations and analyses. △ Less

Submitted 3 May, 2021; originally announced May 2021.

Enormous Berry-Curvature-Driven Anomalous Hall Effect in Topological Insulator (Bi,Sb)2Te3 on Ferrimagnetic Europium Iron Garnet beyond 400 K

Authors: Wei-Jhih Zou, Meng-Xin Guo, Jyun-Fong Wong, Zih-Ping Huang, Jui-Min Chia, Wei-Nien Chen, Sheng-Xin Wang, Keng-Yung Lin, Lawrence Boyu Young, Yen-Hsun Glen Lin, Mohammad Yahyavi, Chien-Ting Wu, Horng-Tay Jeng, Shang-Fan Lee, Tay-Rong Chang, Minghwei Hong, Jueinai Kwo

Abstract: To realize the quantum anomalous Hall effect (QAHE) at elevated temperatures, the approach of magnetic proximity effect (MPE) was adopted to break the time-reversal symmetry in the topological insulator (Bi0.3Sb0.7)2Te3 (BST) based heterostructures with a ferrimagnetic insulator europium iron garnet (EuIG) of perpendicular magnetic anisotropy. Here we demonstrate phenomenally large anomalous Hall… ▽ More To realize the quantum anomalous Hall effect (QAHE) at elevated temperatures, the approach of magnetic proximity effect (MPE) was adopted to break the time-reversal symmetry in the topological insulator (Bi0.3Sb0.7)2Te3 (BST) based heterostructures with a ferrimagnetic insulator europium iron garnet (EuIG) of perpendicular magnetic anisotropy. Here we demonstrate phenomenally large anomalous Hall resistance (RAHE) exceeding 8 Ω (\r{ho}AHE of 3.2 μΩ*cm) at 300 K and sustaining to 400 K in 35 BST/EuIG samples, surpassing the past record of 0.28 Ω (\r{ho}AHE of 0.14 μΩ*cm) at 300 K. The remarkably large RAHE as attributed to an atomically abrupt, Fe-rich interface between BST and EuIG. Importantly, the gate dependence of the AHE loops shows no sign change with varying chemical potential. This observation is supported by our first-principles calculations via applying a gradient Zeeman field plus a contact potential on BST. Our calculations further demonstrate that the AHE in this heterostructure is attributed to the intrinsic Berry curvature. Furthermore, for gate-biased 4 nm BST on EuIG, a pronounced topological Hall effect (THE) coexisting with AHE is observed at the negative top-gate voltage up to 15 K. Interface tuning with theoretical calculations has opened up new opportunities to realize topologically distinct phenomena in tailored magnetic TI-based heterostructures. △ Less

Submitted 30 September, 2021; v1 submitted 30 March, 2021; originally announced March 2021.

Comments: 66 pages, 16 figures

Twist-angle engineering of excitonic quantum interference and optical nonlinearities in stacked 2D semiconductors

Authors: Kai-Qiang Lin, Paulo E. Faria Junior, Jonas M. Bauer, Bo Peng, Bartomeu Monserrat, Martin Gmitra, Jaroslav Fabian, Sebastian Bange, John M. Lupton

Abstract: Twist-engineering of the electronic structure of van-der-Waals layered materials relies predominantly on band hybridization between layers. Band-edge states in transition-metal-dichalcogenide semiconductors are localized around the metal atoms at the center of the three-atom layer and are therefore not particularly susceptible to twisting. Here, we report that high-lying excitons in bilayer WSe2 c… ▽ More Twist-engineering of the electronic structure of van-der-Waals layered materials relies predominantly on band hybridization between layers. Band-edge states in transition-metal-dichalcogenide semiconductors are localized around the metal atoms at the center of the three-atom layer and are therefore not particularly susceptible to twisting. Here, we report that high-lying excitons in bilayer WSe2 can be tuned over 235 meV by twisting, with a twist-angle susceptibility of 8.1 meV/°, an order of magnitude larger than that of the band-edge A-exciton. This tunability arises because the electronic states associated with upper conduction bands delocalize into the chalcogenide atoms. The effect gives control over excitonic quantum interference, revealed in selective activation and deactivation of electromagnetically induced transparency (EIT) in second-harmonic generation. Such a degree of freedom does not exist in conventional dilute atomic-gas systems, where EIT was originally established, and allows us to shape the frequency dependence, i.e. the dispersion, of the optical nonlinearity. △ Less

Submitted 22 February, 2021; originally announced February 2021.

Journal ref: Nature Communications 12, 1553 (2021)

THz Acoustic Attenuations of Glycerol by Pump Probe Technique

Authors: Fan Jun Wei, Kung-Hsuan Lin

Abstract: The coherent acoustic waves can be created by InGaN double quantum wells. We measured the signals information of the coherent acoustic waves at the interface of GaN/glycerol to obtain the attenuations of glycerol at 295 K and 120 K by the femto-sec laser pump probe technique in THz regime. The attenuations of glycerol are calculated by using the acoustic mismatch model (AMM) through the complex ac… ▽ More The coherent acoustic waves can be created by InGaN double quantum wells. We measured the signals information of the coherent acoustic waves at the interface of GaN/glycerol to obtain the attenuations of glycerol at 295 K and 120 K by the femto-sec laser pump probe technique in THz regime. The attenuations of glycerol are calculated by using the acoustic mismatch model (AMM) through the complex acoustic impedance. We compared pump probe results with inelastic neutron scattering (INS) and inelastic X ray scattering (IXS), found they are matched within 0.5 THz at room temperature. The glycerol attenuations are found increasing monotonically to 1.75x10^8 m^(-1) and 1.3x10^8 m^(-1) till 0.5 THz by the pump probe technique at 295 K and 120 K, respectively. △ Less

Submitted 17 February, 2021; originally announced February 2021.

Comments: 5 pages, 8 figures

Impact of Band Structure on Wave Function Dissipation in Field Emission Resonance

Authors: Wei-Bin Su, Shin-Ming Lu, Ho-Hsiang Chang, Horng-Tay Jeng, Wen-Yuan Chan, Pei-Cheng Jiang, Kung-Hsuan Lin, Chia-Seng Chang

Abstract: We demonstrated on Ag(111) and Ag(100) surfaces that the reciprocal of the field emission resonance (FER) linewidth, which is proportional to the mean lifetime of resonant electrons in FER, may vary with the electric field. The variation on Ag(111) was nearly smooth, whereas that on Ag(100) was sporadic and fluctuated remarkably. This drastic difference can be explained through their dissimilar pr… ▽ More We demonstrated on Ag(111) and Ag(100) surfaces that the reciprocal of the field emission resonance (FER) linewidth, which is proportional to the mean lifetime of resonant electrons in FER, may vary with the electric field. The variation on Ag(111) was nearly smooth, whereas that on Ag(100) was sporadic and fluctuated remarkably. This drastic difference can be explained through their dissimilar projected bulk band structures and the ensemble interpretation of quantum mechanics, according to which all resonant electrons are governed by a single wave function. Ag(100) has an energy gap above its vacuum level, whereas Ag(111) does not. Consequently, the dissipation rate of the wave function, which is relevant to the FER linewidth, on Ag(111) was almost stable, whereas that on Ag(100) fluctuated. The fluctuation revealed that the quantum trapping effect and surface dipole layer on Ag(100) surface can be investigated through FER. △ Less

Submitted 29 December, 2020; originally announced December 2020.

Comments: 16 pages, 4 figures

Large-scale mapping of moiré superlattices by Raman imaging of interlayer breathing mode and moiré phonons

Authors: Kai-Qiang Lin, Johannes Holler, Jonas M. Bauer, Marten Scheuck, Bo Peng, Tobias Korn, Sebastian Bange, John M. Lupton, Christian Schüller

Abstract: Moiré superlattices can induce correlated-electronic phases in twisted van-der-Waals materials. Strongly correlated quantum phenomena emerge, such as superconductivity and the Mott-insulating state. However, moiré superlattices produced through artificial stacking can be quite inhomogeneous, which hampers the development of a clear correlation between the moiré period and the emerging electrical a… ▽ More Moiré superlattices can induce correlated-electronic phases in twisted van-der-Waals materials. Strongly correlated quantum phenomena emerge, such as superconductivity and the Mott-insulating state. However, moiré superlattices produced through artificial stacking can be quite inhomogeneous, which hampers the development of a clear correlation between the moiré period and the emerging electrical and optical properties. Here we demonstrate in twisted-bilayer transition-metal dichalcogenides that low-frequency Raman scattering can be utilized not only to detect atomic reconstruction, but also to map out the inhomogeneity of the moiré lattice over large areas. The method is established based on the finding that both the interlayer-breathing mode and moiré phonons are highly susceptible to the moiré period and provide characteristic fingerprints. We visualize microscopic domains with an effective twist-angle resolution of ~0.1°. This ambient non-invasive methodology can be conveniently implemented to characterize and preselect high-quality areas of samples for subsequent device fabrication, and for transport and optical experiments. △ Less

Submitted 26 December, 2020; originally announced December 2020.

Momentum-resolved observation of exciton formation dynamics in monolayer WS$_2$

Authors: Robert Wallauer, Raul Perea-Causin, Lasse Münster, Sarah Zajusch, Samuel Brem, Jens Güdde, Katsumi Tanimura, Kaiqiang Lin, Rupert Huber, Ermin Malic, Ulrich Höfer

Abstract: The dynamics of exciton formation in transition metal dichalcogenides is difficult to measure experimentally, since many momentum-indirect exciton states are not accessible to optical interband spectroscopy. Here, we combine a tuneable pump, high-harmonic probe laser source with a 3D momentum imaging technique to map photoemitted electrons from monolayer WS$_2$. This provides momentum-, energy- an… ▽ More The dynamics of exciton formation in transition metal dichalcogenides is difficult to measure experimentally, since many momentum-indirect exciton states are not accessible to optical interband spectroscopy. Here, we combine a tuneable pump, high-harmonic probe laser source with a 3D momentum imaging technique to map photoemitted electrons from monolayer WS$_2$. This provides momentum-, energy- and time-resolved access to excited states on an ultrafast timescale. The high temporal resolution of the setup allows us to trace the early-stage exciton dynamics on its intrinsic timescale and observe the formation of a momentum-forbidden dark K$Σ$ exciton a few tens of femtoseconds after optical excitation. By tuning the excitation energy we manipulate the temporal evolution of the coherent excitonic polarization and observe its influence on the dark exciton formation. The experimental results are in excellent agreement with a fully microscopic theory, resolving the temporal and spectral dynamics of bright and dark excitons in WS$_2$. △ Less

Submitted 28 May, 2021; v1 submitted 21 December, 2020; originally announced December 2020.

Comments: 18 pages, 5 figures

Simulating Higher-Order Topological Insulators in Density Wave Insulators

Authors: Kuan-Sen Lin, Barry Bradlyn

Abstract: Since the discovery of the Harper-Hofstadter model, it has been known that condensed matter systems with periodic modulations can be promoted to non-trivial topological states with emergent gauge fields in higher dimensions. In this work, we develop a general procedure to compute the gauge fields in higher dimensions associated to low-dimensional systems with periodic (charge- and spin-) density w… ▽ More Since the discovery of the Harper-Hofstadter model, it has been known that condensed matter systems with periodic modulations can be promoted to non-trivial topological states with emergent gauge fields in higher dimensions. In this work, we develop a general procedure to compute the gauge fields in higher dimensions associated to low-dimensional systems with periodic (charge- and spin-) density wave modulations. We construct two-dimensional (2D) models with modulations that can be promoted to higher-order topological phases with $U(1)$ and $SU(2)$ gauge fields in 3D. Corner modes in our 2D models can be pumped by adiabatic sliding of the phase of the modulation, yielding hinge modes in the promoted models. We also examine a 3D Weyl semimetal (WSM) gapped by charge-density wave (CDW) order, possessing quantum anomalous Hall (QAH) surface states. We show that this 3D system is equivalent to a 4D nodal line system gapped by a $U(1)$ gauge field with a nonzero second Chern number. We explain the recently identified interpolation between inversion-symmetry protected phases of the 3D WSM gapped by CDWs using the corresponding 4D theory. Our results can extend the search for (higher-order) topological states in higher dimensions to density wave systems. △ Less

Submitted 3 June, 2021; v1 submitted 24 November, 2020; originally announced November 2020.

Comments: v2: approximately published version, fixed typos and references, and added extensions to arbitrary orbital positions. 21+37 pages, 5+10 figures. v1: 20+33 pages, 5+10 figures

Journal ref: Phys. Rev. B 103, 245107 (2021)

Cost-Effective Methods to Nanopattern Thermally Stable Platforms on Kapton HN Flexible Films Using Inkjet Printing Technology to Produce Printable Nitrate Sensors, Mercury Aptasensors, Protein Sensors, and Organic Thin Film Transistors

Authors: Li Kai Lin, Jung Ting Tsai, Susana Diaz Amaya, Muhammed R Oduncu, Yifan Zhang, Peng Yuan Huang, Carlos Ostos, Jacob P Schmelzel, Raheleh Mohammadrahimi, Pengyu Xu, Nithin Raghunathan, Xinghang Zhang, Alexander Wei, David Bahr, Dimitrios Peroulis, Lia A Stanciu

Abstract: Kapton HN films, adopted worldwide due to their superior thermal durability (up to 400 °C), allow the high temperature sintering of nanoparticle based metal inks. By carefully selecting inks and Kapton substrates, outstanding thermal stability and anti-delaminating features are obtained in both aqueous and organic solutions and were applied to four novel devices: a solid state ion selective nitrat… ▽ More Kapton HN films, adopted worldwide due to their superior thermal durability (up to 400 °C), allow the high temperature sintering of nanoparticle based metal inks. By carefully selecting inks and Kapton substrates, outstanding thermal stability and anti-delaminating features are obtained in both aqueous and organic solutions and were applied to four novel devices: a solid state ion selective nitrate sensor, an ssDNA based mercury aptasensor, a low cost protein sensor, and a long lasting organic thin film transistor (OTFT). Many experimental studies on parameter combinations were conducted during the development of the above devices. The results showed that the ion selective nitrate sensor displayed a linear sensitivity range with a limit of detection of 2 ppm. The mercury sensor exhibited a linear correlation between the RCT values and the increasing concentrations of mercury. The protein printed circuit board (PCB) sensor provided a much simpler method of protein detection. Finally, the OTFT demonstrated a stable performance with mobility values for the linear and saturation regimes, and the threshold voltage. These devices have shown their value and reveal possibilities that could be pursued. △ Less

Submitted 13 August, 2020; originally announced August 2020.