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Physics Condensed Matter Mesoscale and Nanoscale Physics

cond-mat.mes-hall Watch Mesoscale and Nanoscale Physics

Tuesday 29 September

Mesoscale and Nanoscale Physics

Interlayer dark excitons in a van der Waals heterostructure

Rundong Ma, Konstantin Davydov, Liuxin Gu et al. (+9 more)

Interlayer excitons in transition metal dichalcogenide (TMD) heterostructures exhibit long lifetimes and long-range transport, making them promising for excitonic devices and quantum many-body phases, such as Bose-Einstein condensates. Achieving these goals requires a precise understanding of spin-allowed bright and nominally spin-forbidden dark excitons, because the lowest-energy exciton species governs population, transport, and condensation. Despite substantial progress, unambiguously distinguishing singlet and triplet interlayer excitons has been challenging, as moiré excitons in these heterostructures can mimic their optical signatures. Here, we report the direct spectroscopic identification of bright (singlet) and dark (triplet) interlayer excitons in high-quality, dual-gated WSe$_2$/hBN/WSe$_2$ homobilayers. Electric-field-dependent photoluminescence and reflectance reveal two momentum-direct interlayer transitions with distinct spin configurations. The interlayer dark excitons obey selection rules that differ from those of bright excitons. Strikingly, interlayer dark excitons retain strong valley polarization, even with their ultralong lifetime exceeding microseconds. Finally, we demonstrate twist-angle control, wherein twist-induced electron-hole momentum mismatch modulates interlayer exciton emission. These results provide critical insights into the electronic and excitonic structure of TMD heterostructures, opening new avenues for excitonic many-body physics and optoelectronic devices.
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All authors Rundong Ma, Konstantin Davydov, Liuxin Gu, Lifu Zhang, Hassan Alnatah, Beini Gao, Ruihao Ni, Suji Park, Houk Jang, Takashi Taniguchi, Kenji Watanabe, You Zhou

Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.supr-con

Reversible field-free superconducting diode effect controlled by an antiferromagnet

Filip Krizek, Kamil Olejník, Tobias Edwinson et al. (+19 more)

The semiconductor diode, which allows current to flow preferentially in one direction, is a fundamental building block of numerous modern electronic circuits. Its superconducting analogue—the superconducting diode effect—enables directional dissipationless current flow and may provide similar functionality in future superconducting quantum circuits. Realization of such a nonreciprocal supercurrent requires broken time-reversal symmetry. At zero applied field, this has been typically associated either with intrinsic unconventional superconductivity or extrinsic spin-split electronic states induced by magnetic proximity. Here we demonstrate a field-free superconducting diode effect in conventional superconducting Al proximitized by collinear antiferromagnetic CuMnAs, whose electronic structure breaks time-reversal symmetry without generating spin splitting. By tuning the proximity effect through an insulating AlAs interlayer and correlating the reversal of the diode polarity with the reversal of the remanent Néel state, we establish that the antiferromagnet controls the superconducting diode effect. Our results show that neither spin-split bands nor net magnetization is required for a magnetically controlled field-free superconducting diode effect, extending superconducting nonreciprocity to a broader class of collinear compensated magnets.
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All authors Filip Krizek, Kamil Olejník, Tobias Edwinson, August Jacobi, Athira Suresh, Andrej Farkaš, Jan Kraus, Vít Novák, Christoph Müller, Vojtěch Pařízek, Niclas Heinsdorf, Peter Wadley, Oliver Amin, Kevin Edmonds, Tomas Jungwirth, Libor Šmejkal, Anna Birk Hellenes, Sumit Ghosh, Michal Mazur, Dominik Kriegner, Lucas Casparis, Saulius Vaitiekėnas

Mesoscale and Nanoscale Physics

Multiscale modelling of nanoscaled FETs based on 2D ferroelectric materials

Alejandro Toral-Lopez, Michele Virgilio, Gianluca Fiori et al. (+1 more)

Ferroelectric materials are highly attractive for low-power, high-speed electronics and emerging neuromorphic computing architectures. However, the severe physical scaling limits of conventional bulk (3D) ferroelectrics at the nanoscale have shifted attention towards two-dimensional (2D) ferroelectric monolayers. Accurate device-level performance predictions are essential to accelerate the experimental testing and screening of these novel materials. In this work, we present a multiscale simulation framework that bridges first-principles density functional theory with Non-Equilibrium Green's Function (NEGF) transport calculations. Using a 2D Indium Phosphide monolayer as a case study, our approach leverages a continuously interpolated, polarization-dependent Hamiltonian embedded within a self-consistent Poisson-NEGF solver. The model captures the dynamic interplay between ion movement and electronic transport, naturally reproducing macroscopic hysteresis loops and memory windows without empirical parameters. This predictive pipeline provides a computationally efficient tool to evaluate and optimize next-generation 2D ferroelectric field-effect transistors.
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All authors Alejandro Toral-Lopez, Michele Virgilio, Gianluca Fiori, Damiano Marian

Mesoscale and Nanoscale Physics

Multistate Torsion in Zitterbewegung

Wojciech J. Jankowski

We derive nonlinear Zitterbewegung velocity of electrons augmented by band torsion in crystals. The equation of motion with multistate quantum geometric tensors and torsion culminates in transient nonlinear photocurrent responses distinct from the recently measured resonant steady-state photovoltaic effects induced by circularly polarized light. Tomography protocols for measuring the multistate quantum geometric torsion tensors are proposed.
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Mesoscale and Nanoscale Physics

Rotational symmetry and common-mode phase drift in a counter-wound S-shaped Aharonov–Bohm interferometer

Halil Serdar Solak and Afif Siddiki

We study common flux offsets in a three-dimensional InAs Aharonov-Bohm waveguide formed by two quarter-circle bends of opposite curvature. Each bend contains a through-opening that separates two conducting arms and encloses an independently specified flux. For a spin-independent, two-terminal model with ideal confined fluxes, microreversibility and a rotation exchanging the complete modules imply $\mathcal{T}(\phi_1,\phi_2)=\mathcal{T}(-\phi_2,-\phi_1)$. The counter-wound configuration is therefore stationary against common offsets, while the same-winding configuration is stationary against differential offsets. This constraint does not require cancellation of every interference phase, and it extends to a spatially uniform ambient field perpendicular to the plane of the guide, even though that field also penetrates the conductor. Three-dimensional scattering calculations at $R=350$ nm and $E_F\simeq 7.283$ meV retain three open orbital lead modes and strong reflection at the openings. The finest sampled counter-wound sweep has a visibility of approximately 3.56%; referring the grids to a common threshold gives an estimated continuum visibility of 2.6-2.9%. The response coefficients vary strongly with energy, while a 100 mK electron temperature retains about 98% of the visibility. For weak, purely common Gaussian phase noise, the leading transmission variance is quartic in noise amplitude, whereas the mean shift remains quadratic. Flux imbalance and asymmetric scalar potentials restore a linear response; symmetric scalar disorder preserves stationarity without guaranteeing high transmission. The proposed experimental test requires few-mode coherent transport, calibrated flux controls, and a magnetic-source design whose leakage fields and spin-dependent terms are assessed explicitly.
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Mesoscale and Nanoscale Physics

Nonlinear optical responses of semiconducting graphene nanoribbons: Strong constraint from a hidden symmetry

Jiawei Ruan, Weichen Tang, Yang-Hao Chan et al. (+3 more)

Nonlinear optical (NLO) responses are powerful tools for probing material structures, as they are highly sensitive to spatial symmetries. For instance, it is well known that having inversion symmetry forbids even-order NLO effects. Here, we show that nonspatial symmetries can also significantly constrain NLO responses. In graphene nanoribbons (GNRs) with bipartite-lattice structures, we identify the approximate bipartite-lattice particle-hole (BLPH) symmetry strongly suppresses even-order NLO effects, even for systems with no inversion symmetry. Importantly, we demonstrate that this approximate symmetry enables the optical generation of nearly pure spin shift currents in ferromagnetic GNRs, with the charge shift currents strongly suppressed. The spin orientation of these photo-induced DC spin currents can be easily tuned by an external magnetic field, providing distinct advantages over previous schemes for generating pure spin shift currents in antiferromagnetic systems. We illustrate our findings using a recently synthesized ferromagnetic Janus GNR. Our results underscore the significance of an overlooked symmetry aspect of NLO responses and highlight magnetic GNRs as promising candidates for spintronic devices.
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All authors Jiawei Ruan, Weichen Tang, Yang-Hao Chan, Chen Hu, Xiaoxun Gong, Steven G. Louie

Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.mtrl-sci

Topological Hall Effect Induced by Chiral Spin Textures at the Ferroelectric/Ferromagnetic Interface

Jingkuan Xiao, Yaqing Han, Jianfeng Guo et al. (+25 more)

Chiral spin textures, largely driven by the Dzyaloshinskii-Moriya interaction, offer significant potential for next-generation computing technologies due to their chirality and topological stability. Ferroelectric/ferromagnetic van der Waals heterostructures are particularly appealing because they can combine interfacial inversion-symmetry breaking and spin-orbit coupling to promote interfacial Dzyaloshinskii-Moriya interaction, while switchable ferroelectric polarization provides a nonvolatile tuning knob. This study investigates interfacial chiral spin textures in few-layer Fe$_3$GeTe$_2$/$\alpha$-In$_2$Se$_3$ heterostructures. Two groups of topological Hall signals are identified just below and above the coercive field, and thickness-dependent transport reveals a notable reduction in critical temperature with increasing Fe$_3$GeTe$_2$ layer thickness. Low-temperature magnetic force microscopy images reveal two types of magnetic bubbles with opposite magnetic contrasts near the coercive field, each associated with distinct topological Hall signals. Together with atomistic spin-dynamics simulations and first-principles calculations, these results support the formation of interfacial DMI-stabilized chiral spin textures. Switching the ferroelectric polarization of the $\alpha$-In$_2$Se$_3$ layer further enables nonvolatile modulation of both anomalous and topological Hall effects. The resulting ferroelectric and magnetic bistabilities generate four distinguishable Hall resistance states programmable by electric and magnetic fields. These findings highlight the potential of van der Waals interfaces for advanced device applications.
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All authors Jingkuan Xiao, Yaqing Han, Jianfeng Guo, Renjun Du, Jiawei Jiang, Baoshan Cui, Runnong Zhou, Siqin Wang, Siqi Jiang, Fuzhuo Lian, Di Zhang, Guodong Ma, Jiabei Huang, Zhaochen Qu, Wanting Xu, Kenji Watanabe, Takashi Taniguchi, Alexander S. Mayorov, Jinsheng Wen, Haifeng Ding, Gong Chen, Ahmet Avsar, Hongxin Yang, Lihong Bao, Hong-Jun Gao, Shiyu Zhu, Lei Wang, Geliang Yu

Mesoscale and Nanoscale Physics

Atomic-scale control of spin transport in exchange-coupled molecules

A. Fétida, R. Robles, F. Scheurer et al. (+4 more)

Spin transport between coupled spins is governed by tunneling and exchange interactions, yet their microscopic relation in real space has remained experimentally elusive. To address this, we functionalize the tip of a scanning tunneling microscope with a single molecule and use it as a movable spin sensor positioned above a second molecule on a ferromagnetic surface, forming an exchange-coupled molecular junction. From the same spectroscopic dataset, we independently extract the junction conductance, exchange coupling, and spin polarization with atomic-scale resolution. Conductance and exchange display nearly identical spatial dependences, consistent with a minimal Hubbard description in which both are governed by the same hopping matrix between the molecular frontier orbitals. In contrast, the spin polarization depends on the orbital composition and the relative weight of the resulting transport channels. These results establish a real-space connection between tunneling, exchange, and spin transport in magnetically coupled molecules.
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All authors A. Fétida, R. Robles, F. Scheurer, M. Romeo, J. Kröger, N. Lorente, L. Limot

Mesoscale and Nanoscale Physics

Engineering Vacuum Fluctuations in Hyperbolic Heterostructures

Jie-Cheng Feng, Xinle Cheng, Itai Keren et al. (+5 more)

Vacuum fluctuations can be engineered using optical cavities, resonators, and surface-polaritonic modes, but extending such control deep inside a material remains challenging. Here we show that the vacuum field inside a hyperbolic material can be reshaped by a much thinner overlayer with opposite hyperbolicity. This effect requires overlap between the hyperbolic bands of the two materials and modifies the coupling between the vacuum field and matter excitations within the bottom material, without changing its intrinsic dielectric properties. As an application, we use Eliashberg theory to study a two-dimensional superconducting layer inside the bottom material, showing that the overlayer can remotely change its critical temperature. Our results may shed light on recent experiments reporting changes in superconductivity arising from a frequency-matched hyperbolic overlayer, and suggest an on-chip route to long-range control of matter excitations and collective material properties.
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All authors Jie-Cheng Feng, Xinle Cheng, Itai Keren, Dante M. Kennes, Abhay N. Pasupathy, Dmitri N. Basov, Emil Vinas Bostrom, Angel Rubio

Mesoscale and Nanoscale Physics · Cross-listed from physics.optics

Optically-Induced Modulation and Programming of Diamond Photonic Cavities

Yuchun Zhu, Amirali Arabmoheghi, Nicolas Le Thomas et al. (+3 more)

Single-crystal diamond combines exceptional optical, thermal, and mechanical properties while hosting optically addressable and spin-coherent colour centres, making it a promising platform for integrated quantum and nonlinear photonics. However, practical post-fabrication mechanisms for dynamically controlling and tuning monolithic diamond photonic circuits remain limited. Here, we demonstrate all-optical modulation and long-lived tuning of suspended diamond Fabry–Perot nanobeam cavities containing nitrogen-vacancy centres. Under 532~nm illumination, the infrared (1000 to 1100~nm) cavity response is governed by two competing contributions: a well-understood thermo-optic red-shift, and a novel charge-mediated blue-shift that we attribute to the free-carrier plasma dispersion effect. With 10~mW of 20~kHz-modulated green light, we achieve an infrared modulation depth of approximately 45% of the available reflection contrast. Green illumination also produces quasi-permanent photo-refractive resonance blue-shifts, with a maximum observed tuning of 3.15~nm (0.87~THz), without measurable degradation of the cavity linewidth or contrast. Achieving the same cavity shift in a lithium-niobate Pockels modulator would require 200 to 500~V across a \(5~\mu\mathrm{m}\) electrode gap. These results establish a monolithic diamond nanophotonic platform combining agile and reversible modulation with long-lived optical reconfiguration, two key ingredients in photonic technologies.
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All authors Yuchun Zhu, Amirali Arabmoheghi, Nicolas Le Thomas, Niels Quack, Valentin Goblot, Christophe Galland

Mesoscale and Nanoscale Physics

Symmetry-enforced topological parity: revisiting $2\mathbb{Z}$ classifications beyond cellwise symmetry actions

Ken Shiozaki

Momentum-dependent symmetry actions can turn the even-integer constraint of a $2\mathbb{Z}$ classification of topological insulators and superconductors into an odd-integer constraint without changing the symmetry algebra. For internal and order-two crystalline symmetries in up to four spatial dimensions, we derive parity relations between the Chern or winding number of a gapped Hamiltonian and topological invariants of its momentum-dependent antiunitary symmetry matrices. Whenever the symmetry action enforces odd parity, any symmetry-preserving gapped phase must be topologically nontrivial. Using cellwise actions, which are represented by momentum-independent matrices in a fixed unit-cell basis, as a reference, we determine how locality constrains the realization of odd topological numbers. Explicit model constructions and no-go theorems distinguish three cases: odd values can be realized by finite-range symmetry actions, can be realized by exponentially decaying quasilocal actions but not by finite-range actions, or are impossible in finite-dimensional Bloch systems.
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Mesoscale and Nanoscale Physics

Cavity-QED enhancement of quantum entanglement and battery performance in double quantum dots

Hamid Arian Zad, Usama Shoukat, Michal Jaščur et al. (+2 more)

We theoretically investigate quantum correlations and energy storage in a single-electron silicon double quantum dot (eDQD) coupled to a single-mode microwave cavity. The charge and spin degrees of freedom are hybridized by Rashba spin-orbit coupling (SOC), while the cavity interacts with the eDQD through spin-photon and charge-photon couplings. Using the reduced thermal density matrix, we characterize spin-charge entanglement by concurrence and quantum coherence by the $l_1$ norm. At low temperature, the cavity strongly modifies the spin-charge correlations, producing distinct regimes of enhanced and suppressed entanglement. We identify a nonlinear crossover between an eDQD-dominated regime and a photon-dressed regime. This crossover is revealed independently by two signatures: a rapid change in the concurrence and the onset of finite photon occupation in the cavity. Its boundary exhibits a dominant $G_{\rm c}\propto\sqrt{\Omega}$ dependence and is robust against cavity Hilbert-space truncation. Beyond the Rashba coupling studied in [Ferreira et al., Phys. Rev. A 107, 052408 (2023)], we show that the cavity provides an additional tunable means of controlling quantum correlations and energy storage. We further characterize the eDQD as a quantum battery under coherent charging and evaluate its stored energy and ergotropy. The ergotropy varies markedly across a cavity-dressing crossover resembling that identified from entanglement, connecting cavity-induced modification of the dressed states to extractable work. Optimization over the charge-photon coupling reveals parameter-dependent optimal charging regimes and enhanced maximum ergotropy with increasing Rashba coupling. Our results establish cavity coupling and SOC as complementary controls of quantum correlations and extractable energy in semiconductor eDQD-cavity systems.
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All authors Hamid Arian Zad, Usama Shoukat, Michal Jaščur, Hazrat Ali, Saeed Haddadi

Mesoscale and Nanoscale Physics

Spin separation of Dirac electrons by twisted and vector optical beams

A. A. Gunyaga, M. V. Durnev and S. A. Tarasenko

We demonstrate that twisted and vector optical beams can spatially separate two-dimensional electrons with opposite spin projections. Spin accumulation arises from diverging spin currents induced by structured light together with spin relaxation and diffusion. We derive analytical expressions for the resulting spin density in the quasi-local and diffusion-controlled regimes and numerically calculate the spin textures for the parameters relevant to transition metal dichalcogenide monolayers. Our results suggest a mechanism by which the twist and vector pattern of optical beams can be imprinted onto the electron spin degree of freedom.
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Mesoscale and Nanoscale Physics · Cross-listed from physics.comp-ph

From Bellissard's gap labeling to atomistic quasiperiodic bilayers

Thomas D. Kühne, Vladislav Efremkin and Emil Prodan

Bellissard's framework uses covariant observables to describe aperiodic solids. We examine how butterfly-like spectra in twisted bilayers can be related to bulk gap labels and topological response. A bulk gap permits a spectral projector and an integrated density of states. A topological response requires an additional pairing of that projector. In an illustrative finite honeycomb bilayer, an exact second-moment identity isolates the interlayer contribution to spectral variance. Angle-resolved spectra, finite state counts, an uncoupled reference, and size and broadening checks show how coupling redistributes spectral weight without assigning a bulk gap label. We then examine the requirements for atomistic electronic-structure calculations, including nonorthogonal orbitals, projected densities of states, and basis functions that move during relative sliding. The resulting framework specifies the steps needed to connect atomistic spectra with bulk state counting and topological response.
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Mesoscale and Nanoscale Physics

Deep Learning GW Quasiparticle Hamiltonians for Many-Body Excited-State Electronic Structure at Scale

Xiaoxun Gong, Zechen Tang, Woochang Kim et al. (+4 more)

Accurate quasiparticle electronic structures are the foundation for understanding excited-state properties of materials and explaining optoelectronic, quantum, and transport phenomena. First-principles GW calculations nevertheless remain computationally intensive for large or configurationally complex systems. Here we introduce DeepH-GW, a deep-learning framework that predicts an effective GW quasiparticle Hamiltonian directly from atomic structure. Building on the local, equivariant message-passing architecture of DeepH, DeepH-GW is trained on high-fidelity plane-wave GW calculations through a real-space Hamiltonian-reconstruction interface. This approach combines the systematic accuracy and broad chemical applicability of plane-wave methods with linear-scaling neural-network inference. Across the systems examined, DeepH-GW reproduces quasiparticle band structures with errors on the order of a few meV. Moreover, we show that, despite the intrinsic nonlocality of many-body interactions, DeepH-GW exhibits strong cross-scale transferability from relatively small training structures to substantially larger supercells. We demonstrate that the framework can accurately capture GW-level electron-phonon band-gap renormalization through predictions in supercells with thermal displacements, illustrating the power of the approach. DeepH-GW therefore provides a practical route toward large-scale many-body simulations and foundation models for excited-state electronic structure.
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All authors Xiaoxun Gong, Zechen Tang, Woochang Kim, Yang Li, Wenhui Duan, Yong Xu, Steven G. Louie

Mesoscale and Nanoscale Physics

Ohm's law for information revealed by a skyrmion Maxwell's demon

Soma Miki, Yoshishige Suzuki, Ryo Ishikawa et al. (+10 more)

Reducing the energy cost of computation-a pressing issue in the age of AI-requires a framework for circuit design based on information thermodynamics. By extending non-equilibrium statistical mechanics to information thermodynamics, it has been shown that a probability flow carrying information-hereinafter referred to as an 'information current'-is driven by its conjugate thermodynamic force, namely 'information affinity'. Since information affinity plays a role analogous to that of voltage in an electrical circuit, it is suggested that information transport also follows a linear response relationship analogous to Ohm's law. However, despite this theoretical prediction, experimentally establishing an Ohm-like constitutive relation for information transport remains an important unsolved problem. Here, we realise an information engine consisting of two magnetic skyrmions undergoing Brownian motion at room temperature. We demonstrate that the resulting information current is proportional to the information affinity generated by Maxwell's demon over a relatively wide range, establishing Ohm's law for information. Furthermore, we clarify that, in real information circuits, both Ohm's law for information and Kirchhoff's voltage law must be slightly modified to account for interactions between information carriers and the scattering of the information current.
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All authors Soma Miki, Yoshishige Suzuki, Ryo Ishikawa, Yuki Hibino, Shunya Morishita, Hiroto Imanishi, Keigo Adachi, Yoichi Shiota, Takayuki Nozaki, Minori Goto, Hikaru Nomura, Shigemi Mizukami, Eiiti Tamura

Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.mtrl-sci

Orientation engineering is a universal strategy for ferroelectric trans-switching

Cameron Scott and Jorge Íñiguez-González

About two decades ago, “strain engineering” emerged as a powerful strategy to tune epitaxial thin films. Here we contend that ”orientation engineering” - that is, controlling the film's growth direction - constitutes a complementary design route that opens unprecedented opportunities and deserves greater attention. Specifically, we show that orientation engineering can endow standard ferroelectric compounds with exotic and useful behaviors that may be difficult or impossible to obtain otherwise. Critically, we predict it readily enables ”trans-switching,” where polarization components switch perpendicularly to the applied electric field, paving the way for novel 3D device architectures. We also show that, by controlling the electric screening provided by electrodes in contact with the film, ferrielectric behavior can be obtained. Notably, one can even design an operational mode where the switching is, in effect, fully transversal to the applied field. Supported by phenomenological calculations for representative compounds LiNbO$_3$ and BaTiO$_3$, we thus propose orientation engineering as a powerful and general method to unlock unprecedented functionalities in common ferroelectrics.
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Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.mtrl-sci

Evidence for Distributed Fault Energetics and Their Impact on Deformation in a Chemically Complex Alloy

Kaijun Yin, Jun-Ping Du, Peijun Yu et al. (+7 more)

Chemically complex alloys feature intrinsically heterogeneous local chemical environments and, consequently, fluctuations in local fault energetics. However, experimentally quantifying their relationship remains challenging, leaving the role of this distributed energy landscape in deformation mechanisms incompletely resolved. Here, we develop a distribution based framework linking experimentally measured stacking fault widths to deformation relevant apparent fault energy, revealing a distributed local fault-energy landscape in CrCoNi. The framework reveals the stabilizing role of energy fluctuations and captures an upward shift in the apparent fault energetics, from negative values toward zero following heat treatment, which atomistic simulations associate with the emergence of L12 type chemical short-range order. Using one-dimensional kinetic Monte Carlo simulations, supported by electron microscopy observations, we further show that history-dependent changes in the local fault-energy landscape bias the competition among stacking faulting, nano-twinning and HCP transformation in CrCoNi. Our results provide an experimentally anchored, distribution-based framework for understanding deformation in chemically complex alloys as it evolves within a distributed fault-energy landscape shaped by local chemical order.
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All authors Kaijun Yin, Jun-Ping Du, Peijun Yu, Rui Feng, Hanyu Hou, Haw-Wen Hsiao, Ke An, Peter K. Liaw, Shigenobu Ogata, Jian-Min Zuo

Mesoscale and Nanoscale Physics · Cross-listed from quant-ph

Quantum States and Spectra of Small Cylindrical and Toroidal Lattices

Caelan Brooks and Kunal K. Das

We examine the spectrum and quantum states of small lattices with cylindrical and toroidal topology subject to a scalar gauge potential that introduces a position dependent phase in the inter-site coupling. Equivalency of gauges assumed in infinite lattices is generally lost due to the periodic boundary conditions, and conditions that restore it are identified. We trace the impact of various system parameters including gauge choice, boundary conditions and inter-site coupling strengths, and an additional axial field. We find gauge dependent appearance of avoided crossings and persistent degeneracies, and we show their impact on the associated eigenstates. Smaller lattices develop prominent gaps in spectral lines associated with edge states, which are suppressed in the thermodynamic limit. Toroidal lattices have counterparts of most of the features observed in cylindrical lattices, but notably they display a transition from localization to delocalization determined by the relation between the field parameter and the number of lattice sites.
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Mesoscale and Nanoscale Physics

Interlayer coupling between twisted graphenes through atomically-precise barriers

Yueyang Wang, Ben Fuller, Hongxia Xue et al. (+5 more)

Understanding and controlling interlayer coupling in van der Waals (vdW) materials is crucial for engineering novel electronic phenomena, including correlated states, topological phases, and superconductivity. Twisted bilayer graphene (TBG) offers a highly tunable platform to explore how interlayer orientation and separation influence quantum transport and moiré physics. Here, we introduce a method for achieving precise, atomic-scale control of interlayer coupling in TBG using dual-gated devices separated by ultrathin, thickness-tunable hBN spacers. This approach enables systematic control of interlayer interactions, demonstrating that the critical displacement field required for electron-hole bilayer behavior decreases as the layer separation increases at a fixed twist angle. Remarkably, at small twist angles, moiré-related side peaks persist even when layers are separated by tetralayers of hBN (approximately 1.3 nm), indicating robust interlayer band hybridization. By combining atomic-layer-precise control of interlayer coupling with independent twist angle tunability, our platform opens new avenues for discovering novel moiré physics governed by engineered interlayer coupling and moiré patterns.
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All authors Yueyang Wang, Ben Fuller, Hongxia Xue, Tianyu Zhang, Kenji Watanabe, Takashi Taniguchi, Vladimir Fal'ko, Dong-Keun Ki

Mesoscale and Nanoscale Physics

Linear and nonlinear transport responses of topological nodal-line semimetals

L. E. Sosa-Arias and A. Martín-Ruiz

Topological nodal-line semimetals are three-dimensional quantum materials characterized by band crossings that form closed loops in momentum space. In $\mathcal{PT}$-symmetric realizations, these nodal rings are stabilized in the absence of spin-orbit coupling, giving rise to drumhead surface states and unconventional transport responses. In this work, we study charge transport across a nodal-line semimetal containing a finite electrostatic barrier, with both leads described by the same equilibrium material. By solving the corresponding scattering problem, we show that the transmission across the barrier exhibits {Klein-tunneling behavior protected at normal incidence by the nodal topology}, despite the extended nodal-line dispersion, which can be traced back to Berry-curvature-induced momentum locking. Using the Landauer-Büttiker formalism, we derive general expressions for the linear and nonlinear conductances, including both longitudinal and Hall components, and evaluate them at zero and finite temperature. Our analytical and numerical results elucidate the dependence of the conductance on barrier height and width, as well as on a $\mathcal{PT}$-breaking mass term. We identify distinct transport regimes in which nonlinear contributions are strongly enhanced and transverse Hall currents emerge, providing clear transport signatures of nodal-line topology and suggesting potential routes toward device applications.
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Mesoscale and Nanoscale Physics

Magnetic-field-activated transport from band geometry in gapped nodal-line semimetals

L. Medel Onofre, L. E. Sosa-Arias and A. Martín-Ruiz

We develop a nonperturbative semiclassical theory of magnetotransport in gapped nodal-line semimetals, retaining the full magnetic-field dependence of Berry-curvature and orbital-magnetic-moment corrections. Starting from a minimal two-band model, we derive exact expressions for both the intrinsic Hall response and the dissipative Fermi-surface conductivity, valid to all orders in the magnetic field within the semiclassical regime. We show that the orbital magnetic moment reconstructs the geometrically active Fermi surface, breaking the azimuthal cancellation imposed by the nodal-line geometry and thereby activating an intrinsic Hall current that is absent at zero magnetic field. The Hall response exhibits a pronounced nonmonotonic dependence on the chemical potential and a strongly nonlinear magnetic-field evolution, reflecting the redistribution of geometrically active states around the nodal ring. We further demonstrate that the Fermi-surface conductivities recover the conventional Drude behavior in the weak-field limit, while acquiring quadratic and ultimately nonperturbative magnetic-field corrections together with a field-induced transport anisotropy that gives rise to a measurable planar Hall effect at intermediate fields. Both the intrinsic and dissipative responses are strongly enhanced when the Fermi level lies close to the nodal ring, where Berry curvature and orbital magnetic moment are largest. Our results identify magnetic-field-activated geometric transport as a characteristic signature of nodal-line semimetals and provide a unified framework for describing magnetotransport beyond perturbative magnetic-field expansions, together with experimentally accessible signatures of orbital-magnetic-moment physics.
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Mesoscale and Nanoscale Physics

Effects of Longitudinal Spin Current Density Gradient on Spin-Orbit Torque Switching of Perpendicular Magnetization

Guowen Gong, Qianbiao Liu and Lijun Zhu

It has remained a critical open question as to whether a longitudinal spin current gradient (e.g., due to a gradient in the thickness, composition, or width) can replace the longitudinal magnetic field required for deterministic spin-orbit torque switching of normal metal/ferromagnet heterostructures with perpendicular magnetic anisotropy. Here, we report robust micromagnetic and experimental evidence that any realistic longitudinal spin-current density gradient cannot be an effective replacement for a longitudinal magnetic field to enable deterministic switching of a perpendicular magnetic anisotropy device. Instead, the longitudinal spin current density can only modify the nucleation and pulse-timing-sensitive oscillations of magnetic domains and thus the picosecond-scale strip-like indeterministic switching windows. The same conclusions hold robustly when the transverse effective field associated with the spin-current pulse (the sum of the Oersted field and field-like torque) is taken into account. The indeterministic switching prevents applications in the presence of finite device-to-device variations and drifts in write current pulse and thermal fluctuations of device parameters. We also experimentally show that, instead of a longitudinal spin current density gradient, perpendicular spins are much more effective in deterministic switching of perpendicular spin torque devices.
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Monday 28 September

Mesoscale and Nanoscale Physics · Cross-listed from physics.comp-ph

Second-Order Homogenization of Particulate Elastomeric Composites for Structural-Scale Damage Analysis

Krishna Murthy Pabbu, Tarkes Dora Pallicity and Nelson Muthu

Damage prediction in particulate elastomeric composites is challenging due to the strong material heterogeneity, nearly incompressible behavior of the matrix, and the presence of localized stress concentrations arising from matrix-inclusion interactions. In the present work, a computational framework is developed for damage propagation analysis of particulate elastomeric composite structures. The effective constitutive response of the heterogeneous material is first determined using the second-order homogenization method. The resulting homogenized response is subsequently used to identify equivalent hyperelastic material parameters for structural simulations. A three-parameter isotropic damage model is incorporated to capture damage-induced softening. A mixed displacement-pressure finite element formulation, is used to alleviate volumetric locking associated with nearly incompressible property of elastomers. The proposed framework is validated through full-scale simulations and homogenized response involving composites with varying inclusion volume fractions. Numerical investigations demonstrate that the homogenized models is able to reproduce the global force-displacement response within 2% error and damage evolution characteristics of the heterogeneous composites while significantly reducing the computational cost.
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Mesoscale and Nanoscale Physics

Magnon chiral damping beyond the Dzyaloshinskii-Moriya interaction

Darpa Narayan Basu, Subhadip Ghosh, Peter M. Oppeneer et al. (+2 more)

Magnon chiral damping denotes damping that is asymmetric with respect to the magnon wave vector. Such nonreciprocity has been attributed to antisymmetric Dzyaloshinskii-Moriya spin interactions. Here, we provide an alternative theoretical explanation of magnon chiral damping using linear spin-wave theory and antisymmetric Gilbert damping introduced within the Landau-Lifshitz-Gilbert equation of motion. First, employing a minimal model of a single sublattice ferromagnet on a rectangular lattice, we show that the effective magnon damping is independent of the system's energy, including contributions from Dzyaloshinskii-Moriya interactions. Considering scalar nonlocal Gilbert damping, the effective damping parameter $\alpha_{\rm eff}$ becomes wave-vector dependent. In the long-wavelength limit, $\alpha_{\rm eff}$ acquires a quadratic dependence on $\bf{k}$. Furthermore, the antisymmetric component of the Gilbert damping tensor gives rise to asymmetric (nonreciprocal) magnon damping. We find the asymmetry to be linear in $\bf{k}$ in the long-wavelength limit, consistent with recent experiments. Extending the analysis to a two-sublattice system, we consider cross-sublattice Gilbert damping. In the presence of only scalar Gilbert damping, broken reciprocity in magnon relaxation arises from Dzyaloshinskii-Moriya interactions. In contrast, the cross-sublattice antisymmetric component of the Gilbert damping tensor can generate nonreciprocal magnon damping even if the Dzyaloshinskii-Moriya interaction is parametrically set to zero, providing an alternative mechanism for asymmetric spin-wave relaxation. Notably, the nonreciprocity induced by antisymmetric Gilbert damping is roughly an order of magnitude larger than that arising from Dzyaloshinskii-Moriya interactions.
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All authors Darpa Narayan Basu, Subhadip Ghosh, Peter M. Oppeneer, Alexander Mook, Ritwik Mondal

Mesoscale and Nanoscale Physics

Unveiling Three-Dimensional Skyrmion Transitions Through Vortices and Monopoles

Melissa E. Henderson, Daichi Kurebayashi, Benjamin Heacock et al. (+9 more)

Magnetic skyrmions represent vortex-like spin configurations that provide a robust platform for next-generation spintronic technologies. Although they are often treated as two-dimensional objects with integer topological charge, their extension into three-dimensional strings realizes other composite structures with unique device functionalities that transcend planar frameworks. Unfortunately, a lack of bulk probes has failed to realize such higher-dimensional topological structures and their implementations. Here, we report the first experimental visualization of three-dimensional topological $Q = 0$ skyrmion structures using neutron scattering tomography techniques across the equilibrium phase of a Co$_8$Zn$_8$Mn$_4$ sample. Disordered skyrmion states reveal metastable skyrmioniums and composite topological objects novel to bulk systems. Vortex-antivortex lattices mediate changes in topology, with unprecedented transition pathways via a coupling of merons and monopoles. The present realization of bulk Q = 0 quasiparticles and meron-mediated dynamics paves the way for higher-dimensional spintronic frameworks through multi-bit encoding architectures, unidirectional transport schemes, and monopole-mediated controls.
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All authors Melissa E. Henderson, Daichi Kurebayashi, Benjamin Heacock, Wangchun Chen, Charles W. Clark, David G. Cory, Dusan Sarenac, Shannon Watson, Jonathan S. White, Lisa DeBeer-Schmitt, Oleg A. Tretiakov, Dmitry A. Pushin

Mesoscale and Nanoscale Physics

Topology Obstructs Nodeless Excitons

Lumen Eek

Lowest-energy excitons are usually expected to have nodeless $1s$-like envelope wavefunctions. We show that band topology can obstruct this expectation. The exciton envelope wavefunction is a section of the valence-conduction transition bundle, whose topology can enforce momentum-space zeros, with their parity or signed total index fixed by the corresponding characteristic class. We exemplify this behavior in a minimal $PT$-symmetric three-band model with a rank-two transition bundle characterized by Euler number $\nu^e=2$. By directly solving the Bethe-Salpeter equation with both contact and Rytova-Keldysh interactions, we find that the lowest exciton contains two zeros, each with index $+1$, and is less strongly bound than its topologically trivial counterpart. We further classify topology-enforced exciton nodal structures for real and complex transition bundles in dimensions $d \leq 3$. Our results establish the topology of the transition bundle as a direct constraint on exciton wavefunctions and spectra.
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Mesoscale and Nanoscale Physics

Magnetoplasmon molecule

A. V. Larionov, A. V. Gorbunov, A. B. Van'kov et al. (+3 more)

We investigate a two-dimensional electron system in an external magnetic field. An intrinsic four-particle excitation of the electron system, which we term a magnetoplasmon molecule, is observed. The molecule comprises two identical electrons and two Fermi holes with opposite spins. Both electrons and one Fermi hole participate in the collective plasma oscillations of the electron system, whereas the second Fermi hole neutralizes the excess electronic charge. We examine the interaction between the magnetoplasmon molecule and a magnetoexciton condensate.
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All authors A. V. Larionov, A. V. Gorbunov, A. B. Van'kov, D. A. Shchigarev, L. V. Kulik, V. Umansky

Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.str-el

Spectroscopy of phonon-coupled integer and fractional Chern insulators: emergence of polarons and chirality deficit of graviton mode

Min Long, Yuzhu Wang and Zi Yang Meng

Strong electron-phonon coupling can destabilize both Chern insulators (CIs) and fractional Chern insulators (FCIs) in favor of charge order, but whether the instabilities in CI and FCI share a common microscopic mechanism remains unclear. In this paper, we address this question by studying a flat-band Haldane model coupled to dynamical Holstein phonons at integer and fractional fillings using density-matrix renormalization group with local basis optimization. We find that the low-energy effects of phonons are dominated by their dressing of charge-neutral collective modes. In the CI and FCI, phonons dress excitons and magnetorotons into composite modes that we identify as exciton polarons and magnetoroton polarons, respectively. These modes soften strongly upon approaching the transition, while the momenta of their energy minima anticipate the ordering wave vectors of the charge-ordered phases. The phonon spectrum, in turn, acquires dispersive features inherited from the exciton and magnetoroton modes, providing direct lattice signatures of these neutral excitations. Within the FCI phase, electron-phonon coupling also enhances the opposite-chirality spectral weight of the graviton response, thereby resulting in a chirality deficit. Our results provide a unified excitation-based picture of phonon-driven instabilities in CIs and FCIs and establish lattice dynamics as a probe of their neutral collective modes.
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Mesoscale and Nanoscale Physics · Cross-listed from quant-ph

Characterizing quantum precision enhancement for multiple currents in open quantum systems

Khalak Mahadeviya, Sheikh Parvez Mandal, Mahasweta Pandit et al. (+3 more)

The kinetic uncertainty relation (KUR) constrains fluctuations of individual currents in classical nonequilibrium systems in terms of the dynamical activity, which quantifies the average number of transitions per unit of time. Here, we derive a multi-current KUR (MKUR) establishing the precision limit for multiple currents generated in classical stochastic processes in the nonequilibrium steady state. We then apply our theory to identify quantum devices whose correlated current fluctuations surpass this classical limit by comparing the quantum system to a classical emulator with the same average currents in the steady state. In this way, violations of the emulator's MKUR have a clear physical meaning, i.e., that the quantum-coherent evolution reduces joint current fluctuations below what is possible for a classical Markovian system with the same energy-level structure and incoherent resources. Since the MKUR incorporates correlations between currents, its violation pinpoints nonclassical fluctuations in parameter regimes where the single-current KUR is satisfied. We illustrate the essential and complementary role of these bounds for characterizing joint current precision with two examples: a coherently driven two-level system and an experimentally motivated model of a three-level heat engine subject to parasitic environmental couplings.
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All authors Khalak Mahadeviya, Sheikh Parvez Mandal, Mahasweta Pandit, Javier Prior, Mark T. Mitchison, Saulo V. Moreira