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Electron spin resonance driven photogalvanic effect in graphene-based structures
Authors:
C. Bray,
I. Yahniuk,
L. E. Golub,
M. Marocko,
C. Consejo,
B. Benhamou-Bui,
Ziyang Gan,
A. George,
A. Turchanin,
P. Sadovyi,
K. Watanabe,
T. Taniguchi,
J. Eroms,
J. Fabian,
F. Teppe,
S. D. Ganichev
Abstract:
We report an electron-spin-resonance-driven linear photogalvanic effect (LPGE) in unbiased monolayer graphene and WSe_2/graphene heterostructures. Under linearly polarized 45--75 GHz radiation, the photovoltage exhibits pronounced resonant features in both Faraday and Voigt geometries. Multiple resonances associated with the electron spin resonance in graphene are observed for both out-of-plane an…
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We report an electron-spin-resonance-driven linear photogalvanic effect (LPGE) in unbiased monolayer graphene and WSe_2/graphene heterostructures. Under linearly polarized 45--75 GHz radiation, the photovoltage exhibits pronounced resonant features in both Faraday and Voigt geometries. Multiple resonances associated with the electron spin resonance in graphene are observed for both out-of-plane and in-plane magnetic-field orientations. Their magnetic-field positions vary linearly with frequency, their amplitudes reverse sign across the charge-neutrality point, and the resonant contribution has the opposite sign to the nonresonant Drude photogalvanic background. We develop a microscopic theory in which radiation-induced momentum alignment followed by skew scattering generates both contributions. Their opposite signs originate from the orthogonal momentum alignments produced by indirect Drude absorption and direct spin-resonant transitions. The theory describes well the main features of the observed resonant photocurrent and provides a microscopic description of ESR-induced LPGE in two-dimensional systems. These results establish the photogalvanic response as a probe of ESR in unbiased micron-scale graphene-based devices.
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Submitted 26 August, 2026;
originally announced August 2026.
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Optimized near-field optical response via adaptive tip illumination
Authors:
Tao Chen,
Wei Wang,
Ziyang Gan,
Daniel Repp,
Jinxin Zhan,
Antony George,
Henrik Schneidewind,
Ulf Peschel,
Andrey Turchanin,
Volker Deckert
Abstract:
The performance of tip-enhanced optical microscopy is often limited by inefficient coupling of the excitation field to the plasmonic tip apex, as well as by thermal drift and optical aberrations. Here, we demonstrate that adaptive wavefront shaping based on Zernike mode provides a practical approach to achieving robust near-field optimisation at the tip apex. Using a sequential feedback algorithm,…
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The performance of tip-enhanced optical microscopy is often limited by inefficient coupling of the excitation field to the plasmonic tip apex, as well as by thermal drift and optical aberrations. Here, we demonstrate that adaptive wavefront shaping based on Zernike mode provides a practical approach to achieving robust near-field optimisation at the tip apex. Using a sequential feedback algorithm, initially using the near-field signal, we narrow the illumination point-spread function and suppress sidelobes. This demonstrates that Zernike-mode control can be used for both aberration correction and field engineering. In tip-enhanced Raman measurements of a Janus MoSSe monolayer, conventional near-field optimisation increases the signal intensity by around 1.4 fold. A second optimisation step based directly on the Raman-band intensity yields a further 5 to 15 fold enhancement, depending on the specific tips used. These results establish a systematic, optics-based strategy for optimising tip fields, providing a transferable framework for improving tip-enhanced and related near-field spectroscopies.
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Submitted 15 May, 2026;
originally announced May 2026.
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Designing Coulombic Contact Interactions between Polarizable Particles through Asymmetry
Authors:
Yanyu Duan,
Zecheng Gan
Abstract:
Polarizable particle systems, including charged colloids, polarizable ions, biomolecular assemblies, and soft nanomaterials, can exhibit contact electrostatic interactions that depart strongly from Coulomb behavior when dielectric mismatch and geometric singularities amplify polarization effects. Here we use charged dielectric spheres as a model system and show that these polarization contribution…
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Polarizable particle systems, including charged colloids, polarizable ions, biomolecular assemblies, and soft nanomaterials, can exhibit contact electrostatic interactions that depart strongly from Coulomb behavior when dielectric mismatch and geometric singularities amplify polarization effects. Here we use charged dielectric spheres as a model system and show that these polarization contributions can be canceled by jointly tuning size, charge, and dielectric asymmetries. By extending a recently developed image-charge formula to contacting dielectric spheres, we derive analytical conditions under which the contact interaction reduces to the bare Coulomb form. Accurate two-sphere calculations validate the resulting contact design rules with relative errors below $3\%$. Strikingly, many-body molecular dynamics simulations reveal that systems satisfying these two-body rules self-assemble into structures that closely match their pure Coulomb references. These results establish asymmetry as a route for turning electrostatic complexity into Coulombic simplicity at contact, with implications for controlled self-assembly and materials design.
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Submitted 19 August, 2026; v1 submitted 12 May, 2026;
originally announced May 2026.
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An O(N) quasi-Ewald splitting method for nanoconfined electrostatics
Authors:
Zecheng Gan,
Xuanzhao Gao,
Yuqing Li
Abstract:
Simulating the dynamics of charged particles in quasi-two-dimensional (quasi-2D) nanoconfined systems presents a significant computational challenge due to the long-range nature of electrostatic interactions and the geometric anisotropy. To address this, we introduce a novel quasi-Ewald splitting strategy tailored for particle-based simulations in such geometry. Our splitting strategy seamlessly i…
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Simulating the dynamics of charged particles in quasi-two-dimensional (quasi-2D) nanoconfined systems presents a significant computational challenge due to the long-range nature of electrostatic interactions and the geometric anisotropy. To address this, we introduce a novel quasi-Ewald splitting strategy tailored for particle-based simulations in such geometry. Our splitting strategy seamlessly integrates a collection of advanced numerical techniques, including optimal quadrature rules [L. N. Trefethen, SIAM Rev. 64(1)(2022), pp.132-150], fast pairwise kernel summation methods [S. Jiang and L. Greengard, Commun. Comput. Phys. 31(1)(2022), pp.1-26], and the random batch method with importance sampling in k-space [S. Jin, L. Li, Z. Xu et al., SIAM J. Sci. Comput. 43(4)(2021), pp.B937-B960]. The resulting algorithm achieves an O(N) overall computational complexity, where N denotes the total number of confined particles. Simulations of several prototype systems validate the accuracy and efficiency of our method. Furthermore, we present numerical observations specifically related to nanoconfined charged many-body systems, highlighting phenomena such as dielectric boundary effects, anisotropic diffusion, and the structure of the electrical double layer (EDL) under conditions of charge asymmetry.
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Submitted 6 January, 2026;
originally announced January 2026.
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Tuning proximity-induced spin-orbit coupling in graphene/WSe$_{2}$ heterostructures
Authors:
Tobias Rockinger,
Bálint Szentpéteri,
Szabolcs Csonka,
Marina Marocko,
Julia Amann,
Ziyang Gan,
Antony George,
Andrey Turchanin,
Kenji Watanabe,
Takashi Taniguchi,
Dieter Weiss,
Péter Makk,
Jonathan Eroms
Abstract:
Recently, proximity-induced spin-orbit coupling (SOC) has been observed in heterostructures consisting of monolayer graphene (ML-G) and transition metal dichalcogenides (TMDCs) such as WSe$_{2}$. Successful tuning of SOC in graphene/WSe$_{2}$ heterostructures by applying mechanical pressure and electric fields was also demonstrated in previous studies. In addition, theoretical calculations predict…
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Recently, proximity-induced spin-orbit coupling (SOC) has been observed in heterostructures consisting of monolayer graphene (ML-G) and transition metal dichalcogenides (TMDCs) such as WSe$_{2}$. Successful tuning of SOC in graphene/WSe$_{2}$ heterostructures by applying mechanical pressure and electric fields was also demonstrated in previous studies. In addition, theoretical calculations predicted a strong dependence of the proximity-induced SOC on the twist angle between graphene and TMDC. Here, we put these predictions to experimental test in ML-G/ML-WSe$_{2}$/hBN-heterostructures, where the twist angle is determined by aligning fractured edges, and by crystallographic etching of graphene. By performing weak anti-localization measurements, we determine the strength of the Rasbha-type SOC ($λ_\mathrm{R}$) and the valley-Zeeman-type SOC ($λ_\mathrm{VZ}$). Our experiments confirm a strong twist angle dependence of the proximity-induced SOC in agreement with theoretical predictions. Finally, we demonstrate the tunability of the SOC strength via mechanical pressure, which is in agreement with earlier findings.
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Submitted 10 June, 2026; v1 submitted 2 December, 2025;
originally announced December 2025.
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Enhancement of the WS$_2$ A$_{1\text{g}}$ Raman Mode in MoS$_2$/WS$_2$ Heterostructures
Authors:
Annika Bergmann-Iwe,
Tomasz Woźniak,
Mustafa Hemaid,
Oisín Garrity,
Patryk Kusch,
Rico Schwartz,
Ziyang Gan,
Antony George,
Ludger Wirtz,
Stephanie Reich,
Andrey Turchanin,
Tobias Korn
Abstract:
When combined into van der Waals heterostructures, transition metal dichalcogenide monolayers enable the exploration of novel physics beyond their unique individual properties. However, for interesting phenomena such as interlayer charge transfer and interlayer excitons to occur, precise control of the interface and ensuring high-quality interlayer contact is crucial. Here, we investigate bilayer…
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When combined into van der Waals heterostructures, transition metal dichalcogenide monolayers enable the exploration of novel physics beyond their unique individual properties. However, for interesting phenomena such as interlayer charge transfer and interlayer excitons to occur, precise control of the interface and ensuring high-quality interlayer contact is crucial. Here, we investigate bilayer heterostructures fabricated by combining chemical-vapor-deposition-grown MoS$_2$ and exfoliated WS$_2$ monolayers, allowing us to form several heterostructures with various twist angles within one preparation step. In case of sufficiently good interfacial contact, evaluated by photoluminescence quenching, we observe a twist-angle-dependent enhancement of the WS$_2$ A$_{1g}$ Raman mode. In contrast, other WS$_2$ and MoS$_2$ Raman modes (in particular, the MoS$_2$ A$_{1g}$ mode) do not show a clear enhancement under the same experimental conditions. We present a systematic study of this mode-selective effect using nonresonant Raman measurements that are complemented with ab-initio calculations of Raman spectra. We find that the selective enhancement of the WS$_2$ A$_{1g}$ mode exhibits a strong dependence on interlayer distance. We show that this selectivity is related to the A$_{1g}$ eigenvectors in the heterolayer: the eigenvectors are predominantly localized on one of the two layers; yet, the intensity of the MoS$_2$ mode is attenuated because the WS$_2$ layer is vibrating (albeit with much lower amplitude) out of phase, while the WS$_2$ mode is amplified because the atoms on the MoS$_2$ layer are vibrating in phase. To separate this eigenmode effect from resonant Raman enhancement, our study is extended with near-resonant Raman measurements.
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Submitted 6 February, 2026; v1 submitted 1 October, 2025;
originally announced October 2025.
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Fabrication of hydrogen-bonded metal inorganic-organic complex glasses by ligand-tuning approach
Authors:
Tianzhao Xu,
Zhencai Li,
Jia-Xin Wu,
Zihao Wang,
Hanmeng Zhang,
Huotian Zhang,
Lars R. Jensen,
Kenji Shinozaki,
Feng Gao,
Haomiao Zhu,
Ivan Hung,
Zhehong Gan,
Jinjun Ren,
Zheng Yin,
Ming-Hua Zeng,
Yuanzheng Yue
Abstract:
Metal inorganic-organic complex (MIOC) crystals are a new category of hybrid glass formers. However, the glass-forming compositions of MIOC crystals are limited due to lack of both a general design principle for such compositions and a deep understanding of the structure and formation mechanism for MIOC glasses. This work reports a general approach for synthesizing glass-forming MIOC crystals. In…
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Metal inorganic-organic complex (MIOC) crystals are a new category of hybrid glass formers. However, the glass-forming compositions of MIOC crystals are limited due to lack of both a general design principle for such compositions and a deep understanding of the structure and formation mechanism for MIOC glasses. This work reports a general approach for synthesizing glass-forming MIOC crystals. In detail, the principle of this approach is based on the creation of hydrogen-bonded structural network by substituting acid anions for imidazole or benzimidazole ligands in the tetrahedral units of zeolitic imidazolate framework crystals. By tuning the metal centers, anions, and organic ligands of MIOCs, supramolecular unit structures can be designed to construct supramolecular networks and thereby enable property modulation. Furthermore, mixed-ligand synthesis yielded a mixed-crystal system in which the glass-transition temperature (Tg) can be linearly tuned from 282 K to 360 K through gradual substitution of benzimidazole for imidazole. Interestingly, upon vitrification, MIOCs were observed to undergo reorganization of hydrogen-bonded networks, with retention of tetrahedral units, short-range disorder, and the freezing of multiple conformations. This work offers a new strategy to systematically expand the glass-forming compositional range of MIOCs and to develop functional MIOC glasses.
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Submitted 29 September, 2025;
originally announced September 2025.
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Quantitative Theory for Critical Conditions of Like-Charge Attraction Between Polarizable Spheres
Authors:
Yanyu Duan,
Zecheng Gan
Abstract:
Despite extensive experimental and theoretical efforts, a concise quantitative theory to predict the occurrence of like-charge attraction (LCA) between polarizable spheres remains elusive. In this work, we first derive a novel three-point image formula, based on a key observation that connects the classical Neumann's image principle with the incomplete beta function. This approach naturally yields…
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Despite extensive experimental and theoretical efforts, a concise quantitative theory to predict the occurrence of like-charge attraction (LCA) between polarizable spheres remains elusive. In this work, we first derive a novel three-point image formula, based on a key observation that connects the classical Neumann's image principle with the incomplete beta function. This approach naturally yields simple yet precise critical conditions for LCA, with a relative discrepancy of less than $1\%$ compared to numerical simulations, validated across diverse parameter settings. The obtained critical conditions may provide physical insights into various processes potentially involving LCA, such as self-assembly, crystallization, and phase separation across different length scales. Additionally, the new image formula is directly applicable to enhance the efficiency of polarizable force field calculations involving polarizable spheres.
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Submitted 20 March, 2025; v1 submitted 17 January, 2025;
originally announced January 2025.
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Selective excitation of collective modes in multiband superconductor MgB2
Authors:
Jiayu Yuan,
Liyu Shi,
Tiequan Xu,
Yue Wang,
Zizhao Gan,
Hao Wang,
Tianyi Wu,
Dong Wu,
Tao Dong,
Nanlin Wang
Abstract:
Recent developments in nonequilibrium and nonlinear terahertz (THz) spectroscopies have significantly advanced our understanding of collective excitations in superconductors. However, there is still debate surrounding the identification of Higgs or Leggett modes, as well as BCS charge fluctuations, in the well-known two-band superconductor MgB$_2$. Here, we utilized both multi-cycle and single-cyc…
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Recent developments in nonequilibrium and nonlinear terahertz (THz) spectroscopies have significantly advanced our understanding of collective excitations in superconductors. However, there is still debate surrounding the identification of Higgs or Leggett modes, as well as BCS charge fluctuations, in the well-known two-band superconductor MgB$_2$. Here, we utilized both multi-cycle and single-cycle THz pump-broadband THz probe techniques to investigate the THz nonlinear response of MgB$_2$. Through multicycle THz pump-THz probe experiments on MgB$_2$, we observed distinct nonlinear signals at both the fundamental frequency ($ω$) and the second harmonic frequency (2$ω$) of the pump pulses, which exhibited resonant enhancement at temperatures where their frequencies respectively match 2$Δ_π(T)$. They are mainly attributed to the $π$-band Higgs mode. By adjusting the THz pump pulse to a single-cycle waveform that satisfies non-adiabatic excitation criteria, we observed an over-damped oscillation corresponding to the Leggett mode. Our findings contribute to solving the ongoing debates and demonstrate the selective excitation of collective modes in multiband superconductors, offering new insights into the interaction between Higgs and Leggett modes.
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Submitted 18 December, 2024;
originally announced December 2024.
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Mechanisms of electrostatic interactions between two charged dielectric spheres inside a polarizable medium: An effective-dipole analysis
Authors:
Yanyu Duan,
Zecheng Gan,
Ho-Kei Chan
Abstract:
The mechanisms of electrostatic interactions between two charged dielectric spheres inside a polarizable medium have been investigated, in terms of hypothetical effective dipoles that depict how the positive and negative charge in each particle are separated. Our findings, which revealed that it is possible for polarization-induced opposite-charge repulsion to occur at short interparticle separati…
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The mechanisms of electrostatic interactions between two charged dielectric spheres inside a polarizable medium have been investigated, in terms of hypothetical effective dipoles that depict how the positive and negative charge in each particle are separated. Our findings, which revealed that it is possible for polarization-induced opposite-charge repulsion to occur at short interparticle separations if the dielectric constant of the medium is greater than the dielectric constants of both spheres, provide insights into the physics of charge separation in each sphere and of polarization in the medium behind such counterintuitive behaviour.
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Submitted 4 February, 2025; v1 submitted 7 October, 2024;
originally announced October 2024.
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arXiv:2409.05693
[pdf]
physics.app-ph
cond-mat.mtrl-sci
physics.chem-ph
physics.comp-ph
physics.optics
Photoluminescence-Based Gas Sensing with MoS2 Monolayers
Authors:
Gia Quyet Ngo,
Chanaprom Cholsuk,
Sebastian Thiele,
Ziyang Gan,
Antony George,
Joerg Pezoldt,
Andrey Turchanin,
Tobias Vogl,
Falk Eilenberger
Abstract:
Two-dimensional transition metal dichalcogenides (TMDs) are highly appealing for gas sensors, lab-on-a-chip devices and bio-sensing applications because of their strong light-matter interaction and high surface-to-volume ratio. The ability to grow these van der Waals materials on different substrates and waveguide geometries opens a horizon toward scalable on-chip photonic nanodevices. Here, we re…
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Two-dimensional transition metal dichalcogenides (TMDs) are highly appealing for gas sensors, lab-on-a-chip devices and bio-sensing applications because of their strong light-matter interaction and high surface-to-volume ratio. The ability to grow these van der Waals materials on different substrates and waveguide geometries opens a horizon toward scalable on-chip photonic nanodevices. Here, we report on a versatile technique for real time remote optical gas sensing using two-dimensional TMDs. The adsorption of the gas molecules on the monolayer surface provides a gateway for gas sensing based on charge-transfer-induced photoluminescence variation. For gases that are weakly adsorbed on the surface of monolayer TMDs, purging the monolayers' surface by an inert gas like N2 can desorb gases from the monolayers at room temperature. We demonstrate CO, NO and NO2 detection by monitoring photoluminescence from semiconducting MoS2 monolayers grown on SiO2/Si chips at a level of 10 ppm with fast response time. Observations are supported by our density functional theory calculations, which predict a significant interaction between these gases and MoS2 monolayers. These findings may lead to advances in remote sensing, surface-sensitive bioanalytics and lab-on-a-chip sensors.
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Submitted 2 June, 2025; v1 submitted 9 September, 2024;
originally announced September 2024.
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Learning local equivariant representations for quantum operators
Authors:
Zhanghao Zhouyin,
Zixi Gan,
MingKang Liu,
Shishir Kumar Pandey,
Linfeng Zhang,
Qiangqiang Gu
Abstract:
Predicting quantum operator matrices such as Hamiltonian, overlap, and density matrices in the density functional theory (DFT) framework is crucial for material science. Current methods often focus on individual operators and struggle with efficiency and scalability for large systems. Here we introduce a novel deep learning model, SLEM (strictly localized equivariant message-passing) for predictin…
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Predicting quantum operator matrices such as Hamiltonian, overlap, and density matrices in the density functional theory (DFT) framework is crucial for material science. Current methods often focus on individual operators and struggle with efficiency and scalability for large systems. Here we introduce a novel deep learning model, SLEM (strictly localized equivariant message-passing) for predicting multiple quantum operators, that achieves state-of-the-art accuracy while dramatically improving computational efficiency. SLEM's key innovation is its strict locality-based design for equivariant representations of quantum tensors while preserving physical symmetries. This enables complex many-body dependency without expanding the effective receptive field, leading to superior data efficiency and transferability. Using an innovative SO(2) convolution and invariant overlap parameterization, SLEM reduces the computational complexity of high-order tensor products and is therefore capable of handling systems requiring the $f$ and $g$ orbitals in their basis sets. We demonstrate SLEM's capabilities across diverse 2D and 3D materials, achieving high accuracy even with limited training data. SLEM's design facilitates efficient parallelization, potentially extending DFT simulations to systems with device-level sizes, opening new possibilities for large-scale quantum simulations and high-throughput materials discovery.
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Submitted 24 January, 2025; v1 submitted 8 July, 2024;
originally announced July 2024.
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Unveiling the origin of unconventional moire ferroelectricity
Authors:
Ruirui Niu,
Zhuoxian Li,
Xiangyan Han,
Qianling Liu,
Zhuangzhuang Qu,
Zhiyu Wang,
Chunrui Han,
Kenji Watanabe,
Takashi Taniguchi,
Kaihui Liu,
Jinhai Mao,
Wu Shi,
Bo Peng,
Zheng Vitto Han,
Zizhao Gan,
Jianming Lu
Abstract:
Interfacial ferroelectricity emerges in heterostructures consisting of nonpolar van der Waals (vdW) layers, greatly expanding the scope of two dimensional ferroelectrics. In particular, the unconventional moire ferroelectricity observed in bilayer graphene/boron nitride (BN) heterostructures, exhibits promising functionalities with topological current, superconductivity and synaptic responses. How…
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Interfacial ferroelectricity emerges in heterostructures consisting of nonpolar van der Waals (vdW) layers, greatly expanding the scope of two dimensional ferroelectrics. In particular, the unconventional moire ferroelectricity observed in bilayer graphene/boron nitride (BN) heterostructures, exhibits promising functionalities with topological current, superconductivity and synaptic responses. However, the debate about its mechanism - correlation driven charge transfer between two graphene layers - limits device reproducibility and hence large-scale production. Here by designing a single-layer graphene encapsulated by lattice-mismatched WSe2, we identify the ferroelectricity as stemming from - instead of graphene moire bands - the particular BN, where interfacial sliding ferroelectricity must play a role. With similar structures, multilayer twisted MoS2 is found to reproduce the ferroelectricity. The key is a conductive moire ferroelectric, where the screened gate and the pinned domain wall together result in unchanged electronic states, i.e. anomalous screening. The intimate connection to interfacial sliding ferroelectricity thus provides advantages of diverse choices of constituent materials and robust polarization switching while preserving the unique anomalous screening, paving the way to reproducible and reliable memory-based devices in artificial intelligence.
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Submitted 25 March, 2024;
originally announced March 2024.
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Influence of resonant plasmonic nanoparticles on optically accessing the valley degree of freedom in 2D semiconductors
Authors:
Tobias Bucher,
Zlata Fedorova,
Mostafa Abasifard,
Rajeshkumar Mupparapu,
Matthias J. Wurdack,
Emad Najafidehaghani,
Ziyang Gan,
Heiko Knopf,
Antony George,
Falk Eilenberger,
Thomas Pertsch,
Andrey Turchanin,
Isabelle Staude
Abstract:
The valley degree of freedom is one of the most intriguing properties of atomically thin transition metal dichalcogenides. Together with the possibility to address this degree of freedom by valley-contrasting optical selection rules, it has the potential to enable a completely new class of future electronic and optoelectronic devices. Resonant optical nanostructures emerge as promising tools for i…
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The valley degree of freedom is one of the most intriguing properties of atomically thin transition metal dichalcogenides. Together with the possibility to address this degree of freedom by valley-contrasting optical selection rules, it has the potential to enable a completely new class of future electronic and optoelectronic devices. Resonant optical nanostructures emerge as promising tools for interacting with and controlling the valley degree of freedom at the nanoscale. However, a critical understanding gap remains in how nanostructures and their nearfields affect the circular polarization properties of valley-selective emission hindering further developments in this field. In order to address this issue, our study delves into the experimental investigation of a hybrid model system where valley-specific emission from a monolayer of molybdenum disulfide is interacting with a resonant plasmonic nanosphere. Contrary to the simple intuition suggesting that a centrosymmetric nanoresonator preserves the degree of circular polarization in the forward scattered farfield by angular momentum conservation, our cryogenic photoluminescence microscopy reveals that the light emitted from the nanoparticle position is largely unpolarized, i.e. we observe depolarization. We rigorously study the nature of this phenomenon numerically considering the monolayer-nanoparticle interaction at different levels including excitation and emission. In doing so, we find that the farfield degree of polarization strongly reduces in the hybrid system when including excitons emitting from outside of the system's symmetry point, which in combination with depolarisation at the excitation level causes the observed effect. Our results highlight the importance of considering spatially distributed emitters for precise predictions of polarization responses in these hybrid systems.
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Submitted 20 June, 2024; v1 submitted 24 January, 2024;
originally announced January 2024.
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Possible Eliashberg-type superconductivity enhancement effects in a two-band superconductor MgB2 driven by narrow-band THz pulses
Authors:
Sergei Sobolev,
Amon Lanz,
Tao Dong,
Amrit Pokharel,
Viktor Kabanov,
Tie-Quan Xu,
Yue Wang,
Zi-Zhao Gan,
L. Y. Shi,
Nan-Lin Wang,
Alexej Pashkin,
Ece Uykur,
Stephan Winnerl,
Manfred Helm,
Jure Demsar
Abstract:
We study THz-driven condensate dynamics in epitaxial thin films of MgB$_{2}$, a prototype two-band superconductor (SC) with weak interband coupling. The temperature and excitation density dependent dynamics follow the behavior predicted by the phenomenological bottleneck model for the single-gap SC, implying adiabatic coupling between the two condensates on the ps timescale. The amplitude of the T…
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We study THz-driven condensate dynamics in epitaxial thin films of MgB$_{2}$, a prototype two-band superconductor (SC) with weak interband coupling. The temperature and excitation density dependent dynamics follow the behavior predicted by the phenomenological bottleneck model for the single-gap SC, implying adiabatic coupling between the two condensates on the ps timescale. The amplitude of the THz-driven suppression of condensate density reveals an unexpected decrease in pair-breaking efficiency with increasing temperature - unlike in the case of optical excitation. The reduced pair-breaking efficiency of narrow-band THz pulses, displaying minimum near $\approx0.7$ T$_{c}$, is attributed to THz-driven, long-lived, non-thermal quasiparticle distribution, resulting in Eliashberg-type enhancement of superconductivity, competing with pair-breaking.
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Submitted 1 August, 2023;
originally announced August 2023.
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Kapitza-resistance-like exciton dynamics in atomically flat MoSe$_{2}$-WSe$_{2}$ lateral heterojunction
Authors:
Hassan Lamsaadi,
Dorian Beret,
Ioannis Paradisanos,
Pierre Renucci,
Delphine Lagarde,
Xavier Marie,
Bernhard Urbaszek,
Ziyang Gan,
Antony George,
Kenji Watanabe,
Takashi Taniguchi,
Andrey Turchanin,
Laurent Lombez,
Nicolas Combe,
Vincent Paillard,
Jean-Marie Poumirol
Abstract:
Being able to control the neutral excitonic flux is a mandatory step for the development of future room-temperature two-dimensional excitonic devices. Semiconducting Monolayer Transition Metal Dichalcogenides (TMD-ML) with extremely robust and mobile excitons are highly attractive in this regard. However, generating an efficient and controlled exciton transport over long distances is a very challe…
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Being able to control the neutral excitonic flux is a mandatory step for the development of future room-temperature two-dimensional excitonic devices. Semiconducting Monolayer Transition Metal Dichalcogenides (TMD-ML) with extremely robust and mobile excitons are highly attractive in this regard. However, generating an efficient and controlled exciton transport over long distances is a very challenging task. Here we demonstrate that an atomically sharp TMD-ML lateral heterostructure (MoSe$_{2}$-WSe$_{2}$) transforms the isotropic exciton diffusion into a unidirectional excitonic flow through the junction. Using tip-enhanced photoluminescence spectroscopy (TEPL) and a modified exciton transfer model, we show a discontinuity of the exciton density distribution on each side of the interface. We introduce the concept of exciton Kapitza resistance, by analogy with the interfacial thermal resistance referred to as Kapitza resistance. By comparing different heterostructures with or without top hexagonal boron nitride (hBN) layer, we deduce that the transport properties can be controlled, over distances far greater than the junction width, by the exciton density through near-field engineering and/or laser power density. This work provides a new approach for controlling the neutral exciton flow, which is key toward the conception of excitonic devices.
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Submitted 23 June, 2023;
originally announced June 2023.
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Interface engineering of charge-transfer excitons in 2D lateral heterostructures
Authors:
Roberto Rosati,
Ioannis Paradisanos,
Libai Huang,
Ziyang Gan,
Antony George,
Kenji Watanabe,
Takashi Taniguchi,
Laurent Lombez,
Pierre Renucci,
Andrey Turchanin,
Bernhard Urbaszek,
Ermin Malic
Abstract:
The existence of bound charge transfer (CT) excitons at the interface of monolayer lateral heterojunctions has been debated in literature, but contrary to the case of interlayer excitons in vertical heterostructure their observation still has to be confirmed. Here, we present a microscopic study investigating signatures of bound CT excitons in photoluminescence spectra at the interface of hBN-enca…
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The existence of bound charge transfer (CT) excitons at the interface of monolayer lateral heterojunctions has been debated in literature, but contrary to the case of interlayer excitons in vertical heterostructure their observation still has to be confirmed. Here, we present a microscopic study investigating signatures of bound CT excitons in photoluminescence spectra at the interface of hBN-encapsulated lateral MoSe$_2$-WSe$_2$ heterostructures. Based on a fully microscopic and material-specific theory, we reveal the many-particle processes behind the formation of CT excitons and how they can be tuned via interface- and dielectric engineering. For junction widths smaller than the Coulomb-induced Bohr radius we predict the appearance of a low-energy CT exciton. The theoretical prediction is compared with experimental low-temperature photoluminescence measurements showing emission in the bound CT excitons energy range. Our joint theory-experiment study presents a significant step towards a microscopic understanding of optical properties of technologically promising 2D lateral heterostructures.
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Submitted 6 February, 2023;
originally announced February 2023.
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Revealing strong coupling of collective modes between superconductivity and pseudogap in cuprate superconductor by terahertz third harmonic generation
Authors:
J. Y. Yuan,
L. Y. Shi,
L. Yue,
B. H. Li,
Z. X. Wang,
S. X. Xu,
T. Q. Xu,
Y. Wang,
Z. Z. Gan,
F. C. Chen,
Z. F. Lin,
X. Wang,
K. Jin,
X. B. Wang,
J. L. Luo,
S. J. Zhang,
Q. Wu,
Q. M. Liu,
T. C. Hu,
R. S. Li,
X. Y. Zhou,
D. Wu,
T. Dong,
N. L. Wang
Abstract:
The study of interaction between different degrees of freedom in solids is of fundamental importance to understand the functionalities of materials. One striking example of such interaction is the intertwined coupling or competition between superconductivity (SC), charge density wave (CDW), pseudogap state (PG), and other exotic phases in cuprate superconductors. Recent emergence of nonlinear Tera…
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The study of interaction between different degrees of freedom in solids is of fundamental importance to understand the functionalities of materials. One striking example of such interaction is the intertwined coupling or competition between superconductivity (SC), charge density wave (CDW), pseudogap state (PG), and other exotic phases in cuprate superconductors. Recent emergence of nonlinear Terahertz (THz) third harmonic generation (THG) spectroscopy provides a powerful tool for exploring the collective (Higgs) modes of superconductivity order parameters, and its interaction with intertwined/competing phases. In this study, we report on nonlinear THz THG spectroscopy of the YBa$_2$Cu$_3$O$_{6+x}$ (YBCO) thin films with different doping. We identify a characteristic temperature $T_{THG}$, below which third order suscepetility $χ^{(3)}$ emerges. Notably, the $T_{THG}$ is coincident with the crossover temperature $T^*$ of pseudogap in a wide range doping of phase diagram. Upon entering the superconducting state, THG increases sharply but exhibits an abnormal dip feature near $T_c$ which is more clearly seen in optimally doped sample. Strikingly, we observe a beating structure directly in the measured real time waveform of THG signal. Fourier transformation of the time domain waveform gives two separate modes below and above original THG frequency. The observation strongly indicates that an additional mode, presumably Higgs mode, appears at $T_c$ and couples to the mode already developed below $T^*$. The strong coupling effect offers new insight into the interplay between superconductivity and pseudogap. The result unambiguously suggests that the pseudogap phase is not a precursor of superconductivity but represents a distinct order.
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Submitted 13 November, 2022;
originally announced November 2022.
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High-order nonlinear terahertz probing of the two-band superconductor MgB$_2$: Third- and fifth-order harmonic generation
Authors:
C. Reinhoffer,
P. Pilch,
A. Reinold,
P. Derendorf,
S. Kovalev,
J. -C. Deinert,
I. Ilyakov,
A. Ponomaryov,
Min Chen,
Tie-Quan Xu,
Yue Wang,
Zi-Zhao Gan,
De-Sheng Wu,
Jian-Lin Luo,
S. Germanskiy,
E. A. Mashkovich,
P. H. M. van Loosdrecht,
I. M. Eremin,
Zhe Wang
Abstract:
We report on high-order harmonic generation in the two-band superconductor MgB$_2$ driven by intense terahertz electromagnetic pulses. Third- and fifth-order harmonics are resolved in time domain and investigated as a function of temperature and in applied magnetic fields crossing the superconducting phase boundary. The high-order harmonics in the superconducting phase reflects nonequilibrium dyna…
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We report on high-order harmonic generation in the two-band superconductor MgB$_2$ driven by intense terahertz electromagnetic pulses. Third- and fifth-order harmonics are resolved in time domain and investigated as a function of temperature and in applied magnetic fields crossing the superconducting phase boundary. The high-order harmonics in the superconducting phase reflects nonequilibrium dynamics of the superconducting order parameter in MgB$_2$, which is probed via nonlinear coupling to the terahertz field. The observed temperature and field dependence of the nonlinear response allows to establish the superconducting phase diagram.
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Submitted 16 December, 2022; v1 submitted 25 July, 2022;
originally announced July 2022.
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Broken Symmetries in Quasi-2D Charged Systems via Negative Dielectric Confinement
Authors:
Xuanzhao Gao,
Zecheng Gan
Abstract:
We report spontaneous symmetry breaking (SSB) phenomena in symmetrically charged binary particle systems under planar nanoconfinement with negative dielectric constants.The SSB is triggered $solely$ via the dielectric confinement effect, without any external fields. The mechanism of SSB is found to be caused by the strong polarization field enhanced by nanoconfinement, giving rise to charge/field…
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We report spontaneous symmetry breaking (SSB) phenomena in symmetrically charged binary particle systems under planar nanoconfinement with negative dielectric constants.The SSB is triggered $solely$ via the dielectric confinement effect, without any external fields. The mechanism of SSB is found to be caused by the strong polarization field enhanced by nanoconfinement, giving rise to charge/field oscillations in the transverse directions. Interestingly, dielectric contrast can even determine the degree of SSB in transverse and longitudinal dimensions, forming charge-separated interfacial liquids and clusters on square lattices. Furthermore, we analytically show that the formed lattice constant is determined by the dielectric mismatch and the length scale of confinement, which is validated via molecular dynamics simulations. The novel broken symmetry mechanism may provide new insights in the study of quasi-2D systems and the design of future nanodevices.
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Submitted 4 August, 2024; v1 submitted 11 July, 2022;
originally announced July 2022.
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Direct Growth of Monolayer MoS$_2$ on Nanostructured Silicon Waveguides
Authors:
Athira Kuppadakkath,
Emad Najafidehaghani,
Ziyang Gan,
Alessandro Tuniz,
Gia Quyet Ngo,
Heiko Knopf,
Franz J. F. Löchner,
Fatemeh Abtahi,
Tobias Bucher,
Sai Shradha,
Thomas Käsebier,
Stefano Palomba,
Nadja Felde,
Pallabi Paul,
Tobias Ullsperger,
Sven Schröder,
Adriana Szeghalmi,
Thomas Pertsch,
Isabelle Staude,
Uwe Zeitner,
Antony George,
Andrey Turchanin,
Falk Eilenberger
Abstract:
We report for the first time the direct growth of Molybdenum disulfide (MoS$_2$) monolayers on nanostructured silicon-on-insulator waveguides. Our results indicate the possibility of utilizing the Chemical Vapour Deposition (CVD) on nanostructured photonic devices in a scalable process. Direct growth of 2D material on nanostructures rectifies many drawbacks of the transfer-based approaches. We sho…
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We report for the first time the direct growth of Molybdenum disulfide (MoS$_2$) monolayers on nanostructured silicon-on-insulator waveguides. Our results indicate the possibility of utilizing the Chemical Vapour Deposition (CVD) on nanostructured photonic devices in a scalable process. Direct growth of 2D material on nanostructures rectifies many drawbacks of the transfer-based approaches. We show that the van der Waals materials grow conformally across the curves, edges, and the silicon-SiO$_2$ interface of the waveguide structure. Here, the waveguide structure used as a growth substrate is complex not just in terms of its geometry but also due to the two materials (Si and SiO$_2$) involved. A transfer-free method like this yields a novel approach for functionalizing nanostructured, integrated optical architectures with an optically active direct semiconductor.
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Submitted 27 May, 2022;
originally announced June 2022.
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Tuning nanowire lasers via hybridization with two-dimensional materials
Authors:
Edwin Eobaldt,
Francesco Vitale,
Maximilian Zapf,
Margarita Lapteva,
Tarlan Hamzayev,
Ziyang Gan,
Emad Najafidehaghani,
Christof Neumann,
Antony George,
Andrey Turchanin,
Giancarlo Soavi,
Carsten Ronning
Abstract:
Mixed dimensional hybrid structures have recently gained increasing attention as promising building blocks for novel electronic and optoelectronic devices. In this context, hybridization of semiconductor nanowires with two-dimensional materials could offer new ways to control and modulate lasing at the nanoscale. In this work, we deterministically fabricate hybrid mixed-dimensional heterostructure…
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Mixed dimensional hybrid structures have recently gained increasing attention as promising building blocks for novel electronic and optoelectronic devices. In this context, hybridization of semiconductor nanowires with two-dimensional materials could offer new ways to control and modulate lasing at the nanoscale. In this work, we deterministically fabricate hybrid mixed-dimensional heterostructures composed of ZnO nanowires and MoS2 monolayers with micrometer control over their relative position. First, we show that our deterministic fabrication method does not degrade the optical properties of the ZnO nanowires. Second, we demonstrate that the lasing wavelength of ZnO nanowires can be tuned by several nanometers by hybridization with CVD-grown MoS2 monolayers. We assign this spectral shift of the lasing modes to an efficient carrier transfer at the heterointerface and the subsequent increase of the optical band gap in ZnO (Moss-Burstein effect).
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Submitted 26 May, 2022;
originally announced May 2022.
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One pot chemical vapor deposition of high optical quality large area monolayer Janus transition metal dichalcogenides
Authors:
Ziyang Gan,
Ioannis Paradisanos,
Ana Estrada-Real,
Julian Picker,
Emad Najafidehaghani,
Francis Davies,
Christof Neumann,
Cedric Robert,
Peter Wiecha,
Kenji Watanabe,
Takashi Taniguchi,
Xavier Marie,
Arkady V. Krasheninnikov,
Bernhard Urbaszek,
Antony George,
Andrey Turchanin
Abstract:
We report one-pot chemical vapor deposition (CVD) growth of large-area Janus SeMoS monolayers, with the asymmetric top (Se) and bottom (S) chalcogen atomic planes with respect to the central transition metal (Mo) atoms. The formation of these two-dimensional semiconductor monolayers takes place upon the thermodynamic equilibrium-driven exchange of the bottom Se atoms of the initially grown MoSe2 s…
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We report one-pot chemical vapor deposition (CVD) growth of large-area Janus SeMoS monolayers, with the asymmetric top (Se) and bottom (S) chalcogen atomic planes with respect to the central transition metal (Mo) atoms. The formation of these two-dimensional semiconductor monolayers takes place upon the thermodynamic equilibrium-driven exchange of the bottom Se atoms of the initially grown MoSe2 single crystals on gold foils with S atoms. The growth process is characterized by complementary experimental techniques including Raman and X-ray photoelectron spectroscopy and the growth mechanisms are rationalized by first principle calculations. The remarkably high optical quality of the synthesized Janus monolayers is demonstrated by optical and magneto-optical measurements which reveal the strong exciton-phonon coupling and enable to obtain the exciton g-factor of -3.3.
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Submitted 10 May, 2022;
originally announced May 2022.
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Exciton spectroscopy and diffusion in MoSe2-WSe2 lateral heterostructures encapsulated in hexagonal boron nitride
Authors:
Dorian Beret,
Ioannis Paradisanos,
Ziyang Gan,
Emad Naja dehaghani,
Antony George,
Tibor Lehnert,
Johannes Biskupek,
Shivangi Shree,
Ana Estrada-Real,
Delphine Lagarde,
Jean-Marie Poumirol,
Vincent Paillard,
Kenji Watanabe,
Takashi Taniguchi,
Xavier Marie,
Ute Kaiser,
Pierre Renucci,
Laurent Lombez,
Andrey Turchanin,
Bernhard Urbaszek
Abstract:
Chemical vapor deposition (CVD) allows lateral edge epitaxy of transition metal dichalcogenide heterostructures with potential applications in optoelectronics. Critical for carrier and exciton transport is the quality of the two materials that constitute the monolayer and the nature of the lateral heterojunction. Important details of the optical properties were inaccessible in as-grown heterostruc…
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Chemical vapor deposition (CVD) allows lateral edge epitaxy of transition metal dichalcogenide heterostructures with potential applications in optoelectronics. Critical for carrier and exciton transport is the quality of the two materials that constitute the monolayer and the nature of the lateral heterojunction. Important details of the optical properties were inaccessible in as-grown heterostructure samples due to large inhomogeneous broadening of the optical transitions. Here we perform optical spectroscopy at T = 4 K and also at 300 K to access the optical transitions in CVD grown MoSe2-WSe2 lateral heterostructures that are transferred from the growth-substrate and are encapsulated in hBN. Photoluminescence (PL), reflectance contrast and Raman spectroscopy reveal considerably narrowed optical transition linewidth similar to high quality exfoliated monolayers. In high-resolution transmission electron microscopy (HRTEM) we find near-atomically sharp junctions with a typical extent of 3nm for the covalently bonded MoSe2-WSe2. In PL imaging experiments we find effective excitonic diffusion length that are longer for WSe2 than for MoSe2 at low T=4 K, whereas at 300 K this trend is reversed.
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Submitted 15 April, 2022;
originally announced April 2022.
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Signatures of multi-band effects in high-harmonic generation in monolayer MoS$_2$
Authors:
Lun Yue,
Richard Hollinger,
Can B. Uzundal,
Bailey Nebgen,
Ziyang Gan,
Emad Najafidehaghani,
Antony George,
Christian Spielmann,
Daniil Kartashov,
Andrey Turchanin,
Diana Y. Qiu,
Mette B. Gaarde,
Michael Zuerch
Abstract:
High-harmonic generation (HHG) in solids has been touted as a way to probe ultrafast dynamics and crystal symmetries in condensed matter systems. Here, we investigate the polarization properties of high-order harmonics generated in monolayer MoS$_2$, as a function of crystal orientation relative to the mid-infrared laser field polarization. At several different laser wavelengths we experimentally…
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High-harmonic generation (HHG) in solids has been touted as a way to probe ultrafast dynamics and crystal symmetries in condensed matter systems. Here, we investigate the polarization properties of high-order harmonics generated in monolayer MoS$_2$, as a function of crystal orientation relative to the mid-infrared laser field polarization. At several different laser wavelengths we experimentally observe a prominent angular shift of the parallel-polarized odd harmonics for energies above approximately 3.5 eV, and our calculations indicate that this shift originates in subtle differences in the recombination dipole strengths involving multiple conduction bands. This observation is material specific and is in addition to the angular dependence imposed by the dynamical symmetry properties of the crystal interacting with the laser field, and may pave the way for probing the vectorial character of multi-band recombination dipoles.
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Submitted 15 February, 2022; v1 submitted 24 December, 2021;
originally announced December 2021.
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All-optical polarization and amplitude modulation of second-harmonic generation in atomically thin semiconductors
Authors:
Sebastian Klimmer,
Omid Ghaebi,
Ziyang Gan,
Antony George,
Andrey Turchanin,
Giulio Cerullo,
Giancarlo Soavi
Abstract:
Second-harmonic generation is of paramount importance in several fields of science and technology, including frequency conversion, self-referencing of frequency combs, nonlinear spectroscopy and pulse characterization. Advanced functionalities are enabled by modulation of the harmonic generation efficiency, which can be achieved with electrical or all-optical triggers. Electrical control of the ha…
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Second-harmonic generation is of paramount importance in several fields of science and technology, including frequency conversion, self-referencing of frequency combs, nonlinear spectroscopy and pulse characterization. Advanced functionalities are enabled by modulation of the harmonic generation efficiency, which can be achieved with electrical or all-optical triggers. Electrical control of the harmonic generation efficiency offers large modulation depth at the cost of low switching speed, by contrast to all-optical nonlinear devices, which provide high speed and low modulation depth. Here we demonstrate all-optical modulation of second-harmonic generation in MoS2 with a modulation depth of close to 100% and speed limited only by the fundamental pulse duration. This result arises from a combination of D3h crystal symmetry and the deep subwavelength thickness of the sample, it can therefore be extended to the whole family of transition metal dichalcogenides to provide great flexibility in the design of advanced nonlinear optical devices such as high-speed integrated frequency converters, broadband autocorrelators for ultrashort pulse characterization, and tunable nanoscale holograms.
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Submitted 28 October, 2021;
originally announced October 2021.
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In-fiber second-harmonic generation with embedded two-dimensional materials
Authors:
Gia Quyet Ngo,
Emad Najafidehaghani,
Ziyang Gan,
Sara Khazaee,
Antony George,
Erik P. Schartner,
Heike Ebendorff-Heidepriem,
Thomas Pertsch,
Alessandro Tuniz,
Markus A. Schmidt,
Ulf Peschel,
Andrey Turchanin,
Falk Eilenberger
Abstract:
Silica-based optical fibers are a workhorse of nonlinear optics. They have been used to demonstrate nonlinear phenomena such as solitons and self-phase modulation. Since the introduction of the photonic crystal fiber, they have found many exciting applications, such as supercontinuum white light sources and third-harmonic generation, among others. They stand out by their low loss, large interactio…
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Silica-based optical fibers are a workhorse of nonlinear optics. They have been used to demonstrate nonlinear phenomena such as solitons and self-phase modulation. Since the introduction of the photonic crystal fiber, they have found many exciting applications, such as supercontinuum white light sources and third-harmonic generation, among others. They stand out by their low loss, large interaction length, and the ability to engineer its dispersive properties, which compensate for the small chi(3) nonlinear coefficient. However, they have one fundamental limitation: due to the amorphous nature of silica, they do not exhibit second-order nonlinearity, except for minor contributions from surfaces. Here, we demonstrate significant second-harmonic generation in functionalized optical fibers with a monolayer of highly nonlinear MoS2 deposited on the fiber guiding core. The demonstration is carried out in a 3.5 mm short piece of exposed core fiber, which was functionalized in a scalable process CVD-based process, without a manual transfer step. This approach is scalable and can be generalized to other transition metal dichalcogenides and other waveguide systems. We achieve an enhancement of more than 1000x over a reference sample of equal length. Our simple proof-of-principle demonstration does not rely on either phase matching to fundamental modes, or ordered growth of monolayer crystals, suggesting that pathways for further improvement are within reach. Our results do not just demonstrate a new path towards efficient in-fiber SHG-sources, instead, they establish a platform with a new route to chi(2)-based nonlinear fiber optics, optoelectronics, and photonics platforms, integrated optical architectures, and active fiber networks.
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Submitted 11 August, 2021;
originally announced August 2021.
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Band-selective third-harmonic generation in superconducting MgB$_2$: Possible evidence for Higgs amplitude mode in the dirty limit
Authors:
Sergey Kovalev,
Tao Dong,
Li-Yu Shi,
Chris Reinhoffer,
Tie-Quan Xu,
Hong-Zhang Wang,
Yue Wang,
Zi-Zhao Gan,
Semyon Germanskiy,
Jan-Christoph Deinert,
Igor Ilyakov,
Paul H. M. van Loosdrecht,
Dong Wu,
Nan-Lin Wang,
Jure Demsar,
Zhe Wang
Abstract:
We report on time-resolved linear and nonlinear terahertz spectroscopy of the two-band superconductor MgB$_2$ with the superconducting transition temperature $T_c \approx 36$ K. Third-harmonic generation (THG) is observed below $T_c$ by driving the system with intense narrowband THz pulses. For the pump-pulse frequencies $f=$ 0.3, 0.4, and 0.5 THz, temperature-dependent evolution of the THG signal…
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We report on time-resolved linear and nonlinear terahertz spectroscopy of the two-band superconductor MgB$_2$ with the superconducting transition temperature $T_c \approx 36$ K. Third-harmonic generation (THG) is observed below $T_c$ by driving the system with intense narrowband THz pulses. For the pump-pulse frequencies $f=$ 0.3, 0.4, and 0.5 THz, temperature-dependent evolution of the THG signals exhibits a resonance maximum at the temperatures with the resonance conditions $2f=2Δ_π(T)$ fulfilled, for the dirty-limit superconducting gap $2Δ_π$. In contrast, for $f=$ 0.6 and 0.7 THz with $2f>2Δ_π(T\rightarrow0)=1.03$ THz, the THG intensity increases monotonically with decreasing temperature. Moreover, for $2f<2Δ_π(T\rightarrow0)$ the THG is found nearly isotropic with respect to the pump-pulse polarization. These results suggest a predominant contribution of the driven Higgs amplitude mode of the dirty-limit $π$-band superconducting gap, pointing to the importance of scattering for observation of the Higgs mode in superconductors.
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Submitted 24 October, 2021; v1 submitted 10 October, 2020;
originally announced October 2020.
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Scalable functionalization of optical fibers using atomically thin semiconductors
Authors:
Gia Quyet Ngo,
Antony George,
Robin Tristan Klaus Schock,
Alessandro Tuniz,
Emad Najafidehaghani,
Ziyang Gan,
Nils C. Geib,
Tobias Bucher,
Heiko Knopf,
Christof Neumann,
Tilman Lühder,
Stephen Warren-Smith,
Heike Ebendorff-Heidepriem,
Thomas Pertsch,
Markus A. Schmidt,
Andrey Turchanin,
Falk Eilenberger
Abstract:
Atomically thin transition metal dichalcogenides are highly promising for integrated optoelectronic and photonic systems due to their exciton-driven linear and nonlinear interaction with light. Integrating them into optical fibers yields novel opportunities in optical communication, remote sensing, and all-fiber optoelectronics. However, scalable and reproducible deposition of high quality monolay…
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Atomically thin transition metal dichalcogenides are highly promising for integrated optoelectronic and photonic systems due to their exciton-driven linear and nonlinear interaction with light. Integrating them into optical fibers yields novel opportunities in optical communication, remote sensing, and all-fiber optoelectronics. However, scalable and reproducible deposition of high quality monolayers on optical fibers is a challenge. Here, we report the chemical vapor deposition of monolayer MoS2 and WS2 crystals on the core of microstructured exposed core optical fibers and their interaction with the fibers' guided modes. We demonstrate two distinct application possibilities of 2D-functionalized waveguides to exemplify their potential. First, we simultaneously excite and collect excitonic 2D material photoluminescence with the fiber modes, opening a novel route to remote sensing. Then we show that third harmonic generation is modified by the highly localized nonlinear polarization of the monolayers, yielding a new avenue to tailor nonlinear optical processes in fibers. We anticipate that our results may lead to significant advances in optical fiber based technologies.
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Submitted 2 September, 2020; v1 submitted 8 May, 2020;
originally announced May 2020.
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Disentangling the magnetoelectric and thermoelectric transport in topological insulator thin films
Authors:
Jinsong Zhang,
Xiao Feng,
Yong Xu,
Minghua Guo,
Zuocheng Zhang,
Yunbo Ou,
Yang Feng,
Kang Li,
Haijun Zhang,
Lili Wang,
Xi Chen,
Zhongxue Gan,
Shou-Cheng Zhang,
Ke He,
Xucun Ma,
Qi-Kun Xue,
Yayu Wang
Abstract:
We report transport studies on (Bi,Sb)2Te3 topological insulator thin films with tunable electronic band structure. We find a doping and temperature regime in which the Hall coefficient is negative indicative of electron-type carriers, whereas the Seebeck coefficient is positive indicative of hole-type carriers. This sign anomaly is due to the distinct transport behaviors of the bulk and surface s…
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We report transport studies on (Bi,Sb)2Te3 topological insulator thin films with tunable electronic band structure. We find a doping and temperature regime in which the Hall coefficient is negative indicative of electron-type carriers, whereas the Seebeck coefficient is positive indicative of hole-type carriers. This sign anomaly is due to the distinct transport behaviors of the bulk and surface states: the surface Dirac fermions dominate magnetoelectric transport while the thermoelectric effect is mainly determined by the bulk states. These findings may inspire new ideas for designing topological insulator-based high efficiency thermoelectric devices.
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Submitted 26 February, 2015;
originally announced February 2015.
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Enhanced Thermoelectric Performance and Anomalous Seebeck Effects in Topological Insulators
Authors:
Yong Xu,
Zhongxue Gan,
Shou-Cheng Zhang
Abstract:
Improving the thermoelectric figure of merit zT is one of the greatest challenges in material science. Recent discovery of topological insulators (TIs) offers new promise in this prospect. In this work, we demonstrate theoretically that zT is strongly size dependent in TI, and the size parameter can be tuned to enhance zT to be significantly greater than 1. Furthermore, we show that the life time…
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Improving the thermoelectric figure of merit zT is one of the greatest challenges in material science. Recent discovery of topological insulators (TIs) offers new promise in this prospect. In this work, we demonstrate theoretically that zT is strongly size dependent in TI, and the size parameter can be tuned to enhance zT to be significantly greater than 1. Furthermore, we show that the life time of the edge states in TI is strongly energy dependent, leading to large and anomalous Seebeck effects with an opposite sign to the Hall effect. These striking properties make TIs the promising material for thermoelectrics science and technology.
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Submitted 21 May, 2014; v1 submitted 12 March, 2014;
originally announced March 2014.
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Effects of image charges, interfacial charge discreteness, and surface roughness on the zeta potential of spherical electric double layers
Authors:
Zecheng Gan,
Xiangjun Xing,
Zhenli Xu
Abstract:
We investigate the effects of image charges, interfacial charge discreteness, and surface roughness on spherical electric double layers in electrolyte solutions with divalent counter-ions in the setting of the primitive model. By using Monte Carlo simulations and the image charge method, the zeta potential profile and the integrated charge distribution function are computed for varying surface cha…
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We investigate the effects of image charges, interfacial charge discreteness, and surface roughness on spherical electric double layers in electrolyte solutions with divalent counter-ions in the setting of the primitive model. By using Monte Carlo simulations and the image charge method, the zeta potential profile and the integrated charge distribution function are computed for varying surface charge strengths and salt concentrations. Systematic comparisons were carried out between three distinct models for interfacial charges: 1) SURF1 with uniform surface charges, 2) SURF2 with discrete point charges on the interface, and 3) SURF3 with discrete interfacial charges and finite excluded volume. By comparing the integrated charge distribution function (ICDF) and potential profile, we argue that the potential at the distance of one ion diameter from the macroion surface is a suitable location to define the zeta potential. In SURF2 model, we find that image charge effects strongly enhance charge inversion for monovalent interfacial charges, and strongly suppress charge inversion for multivalent interfacial charges. For SURF3, the image charge effect becomes much smaller. Finally, with image charges in action, we find that excluded volumes (in SURF3) suppress charge inversion for monovalent interfacial charges and enhance charge inversion for multivalent interfacial charges. Overall, our results demonstrate that all these aspects, i.e., image charges, interfacial charge discreteness, their excluding volumes have significant impacts on the zeta potential, and thus the structure of electric double layers.
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Submitted 17 October, 2012; v1 submitted 11 February, 2012;
originally announced February 2012.
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A sol-gel method for growing superconducting MgB2 films
Authors:
Liping Chen,
Chen Zhang,
Yinbo Wang,
Yue Wang,
Qingrong Feng,
Zizhao Gan,
Junzhi Yang,
Xingguo Li
Abstract:
In this paper we report a new sol-gel method for the fabrication of MgB2 films. Polycrystalline MgB2 films were prepared by spin-coating a precursor solution of Mg(BH_4)_2 diethyl ether on (001)Al2O3 substrates followed with annealing in Mg vapor. In comparison with the MgB2 films grown by other techniques, our films show medium qualities including a superconducting transition temperature of Tc ~…
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In this paper we report a new sol-gel method for the fabrication of MgB2 films. Polycrystalline MgB2 films were prepared by spin-coating a precursor solution of Mg(BH_4)_2 diethyl ether on (001)Al2O3 substrates followed with annealing in Mg vapor. In comparison with the MgB2 films grown by other techniques, our films show medium qualities including a superconducting transition temperature of Tc ~ 37 K, a critical current density of Jc(5 K, 0 T) ~ 5 {\times} 10^6 A cm^{-2}, and a critical field of H_{c2}(0) ~ 19 T. Such a sol-gel technique shows potential in the commercial fabrication of practically used MgB2 films as well as MgB2 wires and tapes.
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Submitted 28 February, 2011;
originally announced February 2011.
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Spectrally Narrowed Edge Emission from Organic Light-Emitting Diodes: Evidence for Amplified Spontaneous Emission and Mirrorless Lasing
Authors:
Yun Tian,
Zhengqing Gan,
Zhaoqun Zhou,
Ji-hun Kang,
Q-Han Park,
David W. Lynch,
Joseph Shinar
Abstract:
p-Conjugated materials, including small molecules and polymers, are attracting substantial attention as novel gain media in semiconductor lasers; they offer many potential advantages not achievable with conventional inorganic semiconductors: simple processing, low cost, easy tuneability of the spectrum, and large-area integration on flexible substrates. Optically pumped lasing action in various…
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p-Conjugated materials, including small molecules and polymers, are attracting substantial attention as novel gain media in semiconductor lasers; they offer many potential advantages not achievable with conventional inorganic semiconductors: simple processing, low cost, easy tuneability of the spectrum, and large-area integration on flexible substrates. Optically pumped lasing action in various small molecular and polymeric p-conjugated materials has been demonstrated using several resonator configurations. However, electrically pumped organic semiconductor lasers, i.e., organic injection or diode lasers, remain elusive, presumably due to various loss mechanisms, e.g., charge (polaron)-induced absorption and metal electrode absorption. Here we report on evidence for amplified spontaneous emission (ASE), also known as mirrorless lasing (i.e., wherein some of the spontaneously emitted photons are amplified by stimulated emission during their propagation) in DC-driven small molecular organic light-emitting diodes (SMOLEDs). The evidence includes a dramatic spectral line narrowing, with a full width at half maximum (FWHM) of only 5 - 10 nm, and optical gain, of the edge-emission from SMOLEDs at room temperature. However, there is no clear indication of threshold behavior associated with this spectral narrowing. Nevertheless, this discovery should pave the way towards the realization of an organic diode laser.
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Submitted 14 January, 2007;
originally announced January 2007.
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Preparation and transport properties of non-hydrated Na$_{0.5}$CoO$_2$ single crystals
Authors:
X. Z. Chen,
Z. A. Xu,
G. H. Cao,
J. Q. Shen,
L. M. Qiu,
Z. H. Gan
Abstract:
Single crystals of Na$_{0.5}$CoO$_2$ were obtained through a flux method followed by de-intercalation of sodium. The Na$_{0.5}$CoO$_2$ samples were found to be vulnerable to water in the air and a hydration process in which H$_2$O molecules fill oxygen vacancies in CoO$_2$ layers is suggested to be responsible for the unusual vulnerability to water. The transport properties, including resistivit…
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Single crystals of Na$_{0.5}$CoO$_2$ were obtained through a flux method followed by de-intercalation of sodium. The Na$_{0.5}$CoO$_2$ samples were found to be vulnerable to water in the air and a hydration process in which H$_2$O molecules fill oxygen vacancies in CoO$_2$ layers is suggested to be responsible for the unusual vulnerability to water. The transport properties, including resistivity (\emph{$ρ$}), thermopower (\emph{S}) and Hall coefficient (\emph{R$_H$}), were studied in a temperature range of 5-300 K. The compound shows a weak localization of carriers just below 200 K and Co$^{3+}$-Co$^{4+}$ charge ordering at about 30 K, a relatively lower temperature than previously reported. The results seem to be quite different from those previously reported for this system [Foo et al, Phys. Rev. Lett. 92 (2004) 247001]. Possible mechanism underlying this kind of inconsistency is discussed.
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Submitted 11 December, 2004;
originally announced December 2004.