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Physics Condensed Matter Mesoscale and Nanoscale Physics

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

Monday 21 September

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

Kitaev spin liquid in superconducting networks

Guilherme Delfino, Mehmet Dede, Dmitry Green et al. (+3 more)

We propose a realization of the Kitaev honeycomb Hamiltonian – an archetypal spin-liquid model – in a superconducting metamaterial. The architecture consists of Cooper-pair boxes coupled through depleted semiconductor–superconductor heterostructures that do not require spin–orbit coupling. The Cooper-pair boxes encode effective spin degrees of freedom, while normal and anomalous virtual propagation through the heterostructures mediate bond-directional interactions. Two key control parameters are an out-of-plane magnetic flux and the semiconductor Fermi energy. The former controls interference and distinguishes the bond directions, while tuning the latter close to the bottom of the band gives rise to an emergent Nambu-exchange symmetry that enforces the required bond directionality. Through numerical calculations, we identify an operating regime with controlled corrections, with associated energy and length scales within experimental reach. These results establish a route toward equilibrium quantum spin liquids in engineered superconducting networks.
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All authors Guilherme Delfino, Mehmet Dede, Dmitry Green, Michael J. Manfra, Charles M. Marcus, Claudio Chamon

Mesoscale and Nanoscale Physics

Collective Coordinate Dynamics of Antiferromagnet: Gravity by Quantum Metric

Kuangyin Deng and Ran Cheng

The collective coordinate dynamics of antiferromagnetic and ferrimagnetic textures emerges as a geodesic equation governed by the quantum geometry of the CP$^1$ state associated with the Néel vector, rendering an effective Riemannian gravity and a Lorentz force in the texture motion. This unveils a physical scenario fundamentally distinct from the Thiele equation widely used for ferromagnetic textures, which is inertialess without any gravity analogy. Applying our framework to an optically driven ultrafast antiferromagnetic domain wall, we can explain the helicity-dependent asymmetric motion observed experimentally and predict a vibrantly breathing wall width.
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Mesoscale and Nanoscale Physics

Model of the magnon Kerr effect in a highly anisotropic ferromagnet

Davit Petrosyan, Hiroki Matsumoto, Hanchen Wang et al. (+3 more)

The magnon Kerr effect is a nonlinear phenomenon that occurs universally in any ferromagnet with finite magnetic anisotropy. It manifests itself as a magnon population-dependent change in the magnon dispersion relation. Here, we derive the Hamiltonian for the magnon Kerr effect for a thin film with uniaxial anisotropy. We consider the external field, parallel, perpendicular, and at an intermediate angle to the anisotropy axis, and determine the Kerr coefficient in each case. We show how the nonlinearity scales for different magnetic materials as a function of magnon frequency and magnetic sample volume. Moreover, we derive the equations of motion for a hybrid cavity-magnon system with such thin-film magnon Kerr nonlinearity, to obtain populations of photons and magnons, and scattering matrix terms for the microwave transmission of the cavity, relevant for experimental observations.
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All authors Davit Petrosyan, Hiroki Matsumoto, Hanchen Wang, Richard Schlitz, Pietro Gambardella, William Legrand

Mesoscale and Nanoscale Physics

Modified Low-Temperature Scanning Tunneling Microscope for Ultrafast Pump-Probe Spectroscopy

Vibhuti N. Rai, Junyoung Sim, Nils Bogdanoff et al. (+7 more)

Combining scanning tunneling microscopy (STM) with terahertz pulses (THz-STM) enables ultrafast dynamics of phonons, spins, and charge carriers to be probed with atomic-scale spatial and sub-picosecond temporal resolution. However, coupling THz radiation into a low-temperature STM junction presents some technical challenges as it requires optical access for laser illumination while maintaining junction stability. Instead of designing a completely new STM head with customized optical access and optics within the ultra-high vacuum chamber, we modified a Besocke "Beetle" style STM with minimal changes of the cryogenic radiation shields. Despite the given geometrical conditions, we achieved efficient coupling of THz and optical pulses to the STM junction while maintaining temperatures below 6 K. We designed the optical setup on a separate laser table, starting with femtosecond laser pulses that are used to generate THz pulses in a LiNbO$_3$ crystal. We show the obtained THz pulse shapes and determine the effective time resolution using cross-correlation measurements. We further demonstrate the stable performance of the modified STM by measuring THz-induced tunneling currents from monolayer molybdenum disulfide (MoS$_2$) grown on Au(111). The resulting current maps resolve atomic-scale contrast revealing a point defect. The measurements confirm efficient THz coupling and stable low-temperature STM operation with atomic resolution.
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All authors Vibhuti N. Rai, Junyoung Sim, Nils Bogdanoff, Sergey Trishin, Florian Faaber, Paul Wiechers, Caroline Firschke, Tobias Kampfrath, Christian Lotze, Katharina J. Franke

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

Overcoming the Hong-Ou-Mandel interference limitation for the second photon from a two-photon cascade

Raphael Joos, Michal Vyvlecka, Tim Strobel et al. (+8 more)

Quantum emitters such as semiconductor quantum dots can provide entangled photon pairs emitting from a system of cascaded states. However, the resulting temporal correlation between the two photons from the three-level ladder system has been shown to set a limit on the achievable two-photon interference. In this work, we show that such limitation, for photons emitted by the second transition, is only due to reduced temporal overlap at the interference beam splitter. We investigate this theoretically and experimentally by determining time-resolved four-photon coincidences between two successively emitted photon pairs. Post-emission synchronization shows the recovery of maximum interference visibility; in prospective quantum networks this could be achieved via deterministic quantum memories.
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All authors Raphael Joos, Michal Vyvlecka, Tim Strobel, Benjamin Breiholz, Furkan Aglarci, Ponraj Vijayan, Hans-Georg Babin, Arne Ludwig, Michael Jetter, Simone L. Portalupi, Peter Michler

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

Electronic Reconstruction Towards Topological Superconductivity in FeTe

Hongtao Rong, Yang Ge, Zi-Jie Yan et al. (+13 more)

The recent discovery of intrinsic superconductivity in stoichiometric FeTe films has renewed interest in the Te-rich end member of the iron chalcogenides for studies of unconventional and topological superconductivity, yet its intrinsic electronic structure remains unresolved. In this work, we combine molecular beam epitaxy, angle-resolved photoemission spectroscopy (ARPES), electrical transport measurements, density functional theory, and embedded dynamical mean-field theory to track the electronic reconstruction of 20-unit-cell FeTe films as Te annealing progressively removes excess interstitial Fe and drives the system from an antiferromagnetic metal to a superconductor. We find that this evolution is accompanied by recovered quasiparticle coherence, reduced electronic correlations, a Lifshitz transition, and a topological phase transition, yielding dxy-dominated hole and electron pockets that favor inter-pocket scattering. In addition, a shallow dxz/dyz-derived hole band located about 2 meV below the Fermi level may provide an incipient-band pairing channel, while scattering between the two electron pockets at M may offer additional pairing channels. High-resolution polarization-dependent laser ARPES measurements further reveal a topological surface state whose circular dichroism is consistent with the expected orbital-angular-momentum texture of stoichiometric FeTe. These results establish the intrinsic low-energy electronic structure of superconducting FeTe and identify the electronic states most relevant to superconductivity. The coexistence of intrinsic superconductivity and a topological surface state establishes stoichiometric FeTe as a promising platform for exploring topological superconductivity and possible Majorana bound states.
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All authors Hongtao Rong, Yang Ge, Zi-Jie Yan, Haoran Lin, Bing Xia, Xiaoda Liu, Zihao Wang, Pu Xiao, Lok-Kan Lai, Stephen Paplini, Jiatao Song, Jiangang Yang, Peter J. Hirschfeld, Shuolong Yang, Jiabin Yu, Cui-Zu Chang

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

Broadband Single-Particle Absorption Circular Dichroism Reveals Chiroptical Heterogeneity in Gold Helicoids

Rohit B. Raj, Susanna Bertuletti, Jeong Hyun Han et al. (+7 more)

Quantitative measurements of absorption circular dichroism (CD) at the single-particle level are essential for understanding how individual chiral nanostructures dissipate optical energy, yet broadband absorptance measurements remain experimentally challenging. Here, we introduce a wavelength-tunable integrating-sphere microscope that determines the absorptance of individual nanoparticles under right- and left-circularly polarized illumination through direct optical energy balance, enabling broadband measurement of the absorption dissymmetry factor, gabs. Measurements of achiral gold nanospheres and strongly anisotropic gold nanorods establish the apparent absorption CD baseline and demonstrate minimal detectable linear-dichroism-to-circular-dichroism (LD-to-CD) leakage under the experimental conditions. Applying this approach to 87 chiral gold helicoids synthesized using L-glutathione (L-helicoids) and 96 synthesized using D-glutathione (D-helicoids) reveals mean particle-level gabs values of opposite sign, with a statistically significant difference between the two populations. Individual particles nevertheless exhibit pronounced heterogeneity in response sign, magnitude, spectral position, and line shape. More than one-third of the particles in each population display opposite-sign absorption CD responses relative to their population-average tendency. Correlative SEM analysis of the L-helicoid population further shows that opposite-sign responses persist among isolated particles exhibiting the characteristic projected helicoid morphology, indicating that aggregation and gross differences in projected morphology are insufficient to explain the observed heterogeneity. These results establish broadband single-particle absorption CD spectroscopy as a direct probe of absorptive chiroptical heterogeneity and reveal particle-specific responses obscured by ensemble averaging.
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All authors Rohit B. Raj, Susanna Bertuletti, Jeong Hyun Han, Sepin Cho, Debapriya Pal, Nick Feldman, Ki Tae Nam, Willem L. Noorduin, A. Femius Koenderink, Erik C. Garnett

Mesoscale and Nanoscale Physics

Observation of Kondo Effect in Rhombohedral Graphene Superlattices

Xin Liao, Qing Yin, Huiwen Wang et al. (+10 more)

Kondo effect in strongly correlated systems arises from the antiferromagnetic coupling between itinerant conduction electrons and localized magnetic moments, giving rise to a variety of exotic quantum phenomena. Two-dimensional moiré superlattice systems provide a highly tunable platform featuring topological flat bands, where Wannier orbitals are spatially confined by the periodic moiré potential and serve as localized magnetic moments, enabling the observable Kondo effect. Here we experimentally demonstrate Kondo interactions in hexalayer rhombohedral graphene moiré superlattices through magneto-transport and temperature-dependent measurements. With increasing magnetic field, the magnetoresistance exhibits an increase-decrease transition across a critical field B_c, while the Hall resistance R_xy undergoes a sign reversal near B_c. Moreover, as temperature decreases, the longitudinal resistance R_xx first increases logarithmically and then decreases following a T^2 behavior, indicating a transition to heavy fermion liquid. These behaviors can be consistently explained by the breakdown of Kondo singlets induced by either magnetic field or temperature, which liberates carriers previously bound to localized moments, thereby enhancing conductivity and altering the dominant carrier type. Furthermore, our results demonstrate that the Kondo interaction can be continuously tuned by both carrier density n and displacement electric field D, and suggest the emergence of a Kondo insulating state. Our findings provide deep insight into the Kondo effect in moiré engineered flat-band systems, paving the path for exploring exotic correlated quantum phases.
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All authors Xin Liao, Qing Yin, Huiwen Wang, Jing-Wei Dong, Jun-Xi Chen, Si-Li Wu, Cai-Zhen Li, Guowei Lyu, Kenji Watanabe, Takashi Taniguchi, Wei Jiang, Yu-Gui Yao, Zhi-Min Liao

Mesoscale and Nanoscale Physics

Corbino-Enhanced Supersonic Acoustic-Emission Threshold in a GaAs Two-Dimensional Electron System

A. D. Levin, A. S. Jaroshevich, Z. D. Kvon et al. (+5 more)

We report nonlinear differential-resistance measurements in a high-mobility GaAs/AlGaAs two-dimensional electron system patterned in a Corbino geometry. At zero magnetic field, $R_{\mathrm{diff}}(I_{dc})$ exhibits a pronounced polarity-selective threshold peak on the negative-current branch, gradually suppressed by a perpendicular magnetic field. We interpret the Corbino anomaly as a local Cherenkov-like threshold for acoustic phonon emission, enabled by the radial current concentration $j_r(r)=I/(2\pi r)$, which drives the local electron velocity above the sound velocity in a region adjacent to the inner contact. The polarity selectivity is attributed to Peltier heating and cooling, which modify the local thermal and boundary conditions near the contact, rather than to the kinematic threshold itself. Our results identify the Corbino geometry as a sensitive platform for probing local nonequilibrium electron–phonon processes in a high-mobility electron system within an independently established hydrodynamic-crossover regime and suggest that the inner Corbino contact can act as a geometry-defined source for supersonic acoustic emission.
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All authors A. D. Levin, A. S. Jaroshevich, Z. D. Kvon, V. A. Chitta, M. S. Aksenov, D. V. Dmitriev, A. K. Bakarov, G. M. Gusev

Mesoscale and Nanoscale Physics

The Superconducting Talbot Effect in Phased-Array Josephson Junctions

Hechen Ren and Ziying Li

We introduce the superconducting Talbot effect—a macroscopic quantum self-imaging phenomenon occurring when proximitized Cooper pairs propagate through a ballistic two-dimensional electron gas. By configuring periodic superconducting leads into a phased-array Josephson junction with programmable phase differences, we demonstrate active steering of the resulting superconducting Talbot carpet. To overcome transport resolution limits, we design a Vernier-scale collector array that performs sub-wavelength sampling of the fractional Talbot pattern. This approach maps real-space quantum interference with high robustness to disorder, enabling direct extraction of Fermi wavelengths across helical, spin-degenerate, and spin-orbit-split Fermi surfaces. Tight-binding numerical calculations on a square lattice validate the real-space interference patterns. Our results establish a versatile framework for coherent wavefront engineering and quantum materials diagnostics in superconducting optics.
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Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.str-el

Carrier-mediated spin helices in low-dimensional systems: Josephson-interference features and correlation renormalization

Yung-Yeh Chang and Chen-Hsuan Hsu

Carrier-induced helical magnetism can arise in interacting one-dimensional systems coupled to localized moments. Here, we investigate the coupled-wire sliding-Luttinger-liquid counterparts with and without valley degrees of freedom, using isolated Tomonaga-Luttinger liquids as reference limits. The carrier spin susceptibility mediates a Ruderman-Kittel-Kasuya-Yosida interaction that selects the helical ordering wave vector, while interchannel correlations lock the relative helix phases and stabilize a collective quasi-two-dimensional texture. Using a variational treatment, we estimate the helix-induced partial gap and analyze the reconstruction of the remaining gapless carrier modes. To probe the spatial texture through transport, we incorporate the helix into an anisotropic superconductor-normal-metal-superconductor Josephson junction motivated by the coupled-channel structure of twisted bilayer WTe$_2$. We exploit flux focusing near the junction interfaces to make an applied in-plane field a controlled probe of the spin helix. Relative to junctions without the helix, the helical exchange field modifies the central-lobe width and relative lobe weights. In the transverse-channel geometry, it also produces field-dependent side-lobe asymmetry and central-peak displacement, which remain negligible in the matched references for the parameters considered. The reconstructed carrier sector also exhibits modified local-density-of-states exponents, spin-relaxation behavior, and competing correlations, with distinct signatures in isolated and coupled-wire systems. Our results connect collective helical order in coupled-wire systems to carrier-sector reconstruction and identify field-tunable Josephson interferometry as a complementary transport probe of the spatial spin texture.
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Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.stat-mech

Derivation of the Thiele equation for magnetic skyrmions using the Mori-Zwanzig formalism

Michael te Vrugt and Mathias Kläui

We use the Mori-Zwanzig formalism to derive an exact generalized Langevin equation governing the dynamics of magnetic skyrmions. Using standard approximations, this generalized Langevin equation is found to reduce to the usual Thiele equation. Our approach provides a general route for obtaining extensions of the Thiele equation applicable to magnetic textures and degrees of freedom not covered by the standard derivation route. It also provides some insights into the microscopic justification of the standard Thiele equation, specifically concerning contributions of fast modes to the form of the drift and diffusion terms in the Fokker-Planck equation for skyrmions.
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Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.mtrl-sci

Enhanced thermal conductivity of (010) (AlxGa1-x)2O3 epitaxial films utilizing indium-catalyzed molecular beam epitaxy

Shivashree Gowda, Stephen Schaefer, Ethan A. Scott et al. (+3 more)

(AlxGa1-x)2O3/beta-Ga2O3 transistors are an emerging candidate for high-power and high-frequency electronic devices. beta-Ga2O3 in particular is notably limited for anisotropic low thermal conductivity, which is further reduced in (AlxGa1-x)2O3 due to alloy and other defect-driven phonon scattering mechanisms. In this work we show that the thermal conductivity of (010) oriented (AlxGa1-x)2O3 thin films for 0.01 < x < 0.20, measured using time-domain thermoreflectance (TDTR), is limited by alloy scattering without observable adverse scattering by additional defects. This is enabled through the synthesis of the (AlxGa1-x)2O3 films using molecular beam epitaxy (MBE) on e̱ta}-Ga2O3 substrates, leveraging indium-catalyzed growth to suppress dislocation formation and phase separation to achieve single-phase pseudomorphic films up to x = 0.2. This growth process improved the thermal conductivity of (AlxGa1-x)2O3 by 2X as compared to previously reported values. For increasing Al composition (x), we observe a steady decline in (AlxGa1-x)2O3 thermal conductivity due to alloy scattering which is validated using virtual crystal approximation (VCA) model. We also show that the thermal boundary conductance across the Al/(AlxGa1-x)2O3 interface is reduced with increasing x, which we posit is due to the stiffening of the (AlxGa1-x)2O3 acoustic modes with increasing x by comparing experimental results with a diffuse mismatch model (DMM). Overall, these thermal characteristics provide valuable insights for designing heterostructures with optimized interfaces and composition.
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All authors Shivashree Gowda, Stephen Schaefer, Ethan A. Scott, Samreen Khan, Patrick E. Hopkins, M. Brooks Tellekamp

Mesoscale and Nanoscale Physics

Scalable Multiple Electron Transport Architectures for Feedback Traceable Current Sources

Agustin Javier Lapi, Guillermo Fernandez Moroni, Fernando Chierchie et al. (+8 more)

We present a scalable electron-counting current cell based on a floating gate architecture operated as a Multiple Electron Transport device (MET). The system enables controlled injection, transport, and precise quantification of discrete charge packets using the sensor non-destructive multiple readout capability. We repurposed the charge-injection technique, jointly with the electron-resolution capability, for a controlled generation of quantized charge packets for an electron-traceable current source. Scalable architectures based on parallel and series multi-amplifier configurations are explored. Experimental results confirm noise reduction following the square root of the number of independent measurements and demonstrate stable, programmable output current. This approach provides a compact and scalable platform for electron-counting current sources and precision quantum metrology applications.
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All authors Agustin Javier Lapi, Guillermo Fernandez Moroni, Fernando Chierchie, Fabricio Alcalde Bessia, Miqueas Ezequiel Gamero, Brenda Aurea Cervantes Vergara, Blas Junior Irigoyen Gimenez, Eduardo Paolini, Claudio Rodrigo Chavez Blanco, Juan Estrada, Javier Tiffenberg

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

Transitions between bulk and interfacial fracture in diamond/$c$BN heterostructures

Wei Qiu, Feiyu Zhou, Xiaonan Wang et al. (+5 more)

Whether an initially crack-free heterostructure fails at its interface or within an adjoining phase is controlled by the relative cohesion of competing atomic planes, but how interfacial chemistry, crystallographic orientation, and intermixing reshape this competition remains unclear. Here, we combine density functional theory (DFT) with a fine-tuned atomistic foundation model to resolve tensile fracture in coherent diamond/cubic boron nitride (cBN) heterostructures. The resulting potential reproduces independent DFT tensile responses, including an unseen (001) interface orientation. Interfacial termination, orientation, and diffusion-induced intermixing jointly determine fracture resistance and fracture-plane selection. C-N-bonded (111) and C-B-bonded (001) remain interface-controlled throughout the investigated intermixing range, whereas pristine C-B-bonded (111) fractures at a neighboring B-N plane inside cBN because the interface is more strongly bound. Increasing the diffusion fraction from 0 to 50.0% causes a nonlinear decrease in fracture strength from 50.3 to 15.8 GPa and drives a bulk-to-interface transition through three regimes: cBN fracture up to 4.86%, configuration-dependent competition between 7.6 and 10.1%, and interfacial fracture at 12.5% and above. Atom-resolved stress fields and DFT separation energetics show that this transition is associated with stress relocation and a reversal in the relative cohesion of competing planes, while electron localization analysis connects the cohesion hierarchy to termination- and orientation-dependent bonding. These results establish an atomistic framework for controlling fracture resistance and fracture pathways in strongly bonded heterostructures.
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All authors Wei Qiu, Feiyu Zhou, Xiaonan Wang, Feng Xie, Yan Chen, Shengying Yue, Yilun Liu, Penghua Ying

Sunday 20 September

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

On the Fast Electrons-Optical Modes Interaction Problem Error

Svetlana Churochkina and Dmitry Churochkin

A recently developed relativistic quantum optics approach for a fast electron beam-optical modes interaction problem (Optica 6, 1524–1534 (2019)) contains suggestion about a smallness of the error induced by replacement $\Psi_{\vec k}\approx\Psi_{\vec k_0}$ for noninteracting free electron Dirac spinors at such wave vector $\vec k$ that $\left|\vec k-\vec k_0 \right| \ll\left|\vec k_0\right|$ with $\vec k_0$ as a peaked wave vector of a focused electron beam. Here, based on quantum information approach we strictly investigated a degree of coincidence of spinors $\Psi_{\vec k}$ and $\Psi_{\vec k_0}$ by calculating fidelity for them. The analytical expression is obtained. It is shown that the fidelity inevitably depends on not only wave vectors $\vec k$ and $\vec k_0$ but also on mutual orientation of $[\vec k_0,\vec k]$ and $\hat{\vec{s}}^{\dagger}\vec{\sigma}\hat{\vec{s}}$ with $\vec{s}$ is a two-component spin unit vector and $\vec{\sigma}$ is a vector of Pauli matrices. This result can be conceived as a chirality-like effect in the fidelity and constraints spin arbitrariness when applying the developed relativistic quantum optics approach to the problems of interest. A geometrical configuration of $\vec k_0$, $\vec k-\vec k_0$, and $\hat{\vec{s}}^{\dag}\vec\sigma\hat{\vec{s}}$ that is optimal in terms of fidelity is established.
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Mesoscale and Nanoscale Physics

Interplay of spin-orbit coupling and magnetic chirality: Multidirectional spin polarization in a helical antiferromagnet

Kallol Mondal, Sudin Ganguly and Santanu K. Maiti

We investigate spin-dependent transport in a non-collinear helical antiferromagnet in the presence of spin-orbit coupling within a tight-binding framework. Using the Landauer-Büttiker formalism, we analyze the generation of spin polarization arising from the combined effects of the helical magnetic texture and spin-orbit interaction. We find that finite spin polarization can be generated simultaneously along the $\hat{x}$, $\hat{y}$, and $\hat{z}$-directions, in contrast to the predominantly single-axis spin polarization commonly observed in conventional spin-filtering systems. The multidirectional spin polarization originates from the symmetry breaking introduced by the non-collinear magnetic order. We further show that the spin polarization is modified significantly in the presence of spin-orbit coupling and it has strong dependence on the parameters associated with the helical system. In particular, long-range hopping significantly enhances the spin-filtering effect, leading to large spin polarization components along all three spatial directions.
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Mesoscale and Nanoscale Physics

Multiscale Modeling of Ion Transport in Nanopores: Fitting Implicit-Water Radial Diffusion Profiles to Explicit-Water Molecular Dynamics

Mónika Valiskó, Salman Shabbir, Eszter Molnárné Lakics et al. (+2 more)

We develop a multiscale approach for incorporating molecular-scale transport information into computationally efficient models of ion transport through nanopores. A radially varying effective diffusion coefficient profile is fitted to radial conductivity profiles obtained from explicit-water molecular dynamics (MD) simulations. The fitting is performed within the NP+LEMC framework, which combines the Nernst–Planck equation with Local Equilibrium Monte Carlo to account for ion correlations beyond mean-field approximation. We apply the approach to NaCl, CaCl$_2$, and their mixtures in a negatively charged silica nanopore. The resulting diffusion coefficient profiles reproduce the radial current distributions of the MD simulations, including the strong suppression of ionic mobility near the pore wall that cannot be captured by a spatially constant diffusion coefficient inside the pore. NaCl and CaCl$_2$ exhibit qualitatively different transport behavior: the former is cation selective due to enhanced near-wall Na$^+$ conduction, whereas the latter shows weak anion selectivity because strongly bound Ca$^{2+}$ ions have strongly suppressed mobility near the surface. For NaCl–CaCl$_2$ mixtures, preferential Ca$^{2+}$ binding leads to nonlinear changes in ionic conductance. The approach establishes a bridge between experimentally relevant device behavior and computationally efficient reduced models, with explicit-water MD providing the molecular-scale information that can be incorporated into the effective transport coefficients.
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All authors Mónika Valiskó, Salman Shabbir, Eszter Molnárné Lakics, Zoltán Ható, Dezső Boda

Mesoscale and Nanoscale Physics

Universal Generalized Brillouin Zone Theory I: Review of the Spectral Approach

Zeqi xu, Jiangping Hu and Zhesen Yang

This series of papers aims to establish a universal generalized Brillouin zone (GBZ) theory for higher-dimensional non-Hermitian systems. As the starting point of this series, we emphasize a fundamental question: while the conventional one-dimensional (1D) GBZ condition, $|\beta_p| = |\beta_{p+1}|$, is well known to fail in two dimensions, how does this breakdown actually occur as a system gradually crosses over from 1D to 2D? Investigating this question reveals that the existing 1D GBZ theory itself remains incomplete. In this first paper, we therefore systematically review the 1D spectral approach and clarify where the underlying difficulties lie. Our work identifies the key challenges that motivate the wavefunction approach developed in Paper~II.
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Mesoscale and Nanoscale Physics

Sub-Thermionic Switching in a Negative-Capacitance Bandgap-Change Field-Effect Transistor

Rahul B Awale, Mike Klymenko, Yuefeng Yin et al. (+3 more)

Overcoming the approximately 60~mV/dec Boltzmann limit of the subthreshold swing remains a central challenge for low-power nanoelectronics. Negative-capacitance field-effect transistors address it by amplifying the channel potential, but their effectiveness is constrained by capacitance matching and by the quantum capacitance of conventional channels. Here we model a negative-capacitance bandgap-change field-effect transistor (NC-BCFET), in which the ferroelectric gate stack instead amplifies the electric field that opens the bandgap of the channel, using experimentally proven materials: bilayer graphene as a channel with an electrically tunable bandgap and Al$_{0.55}$Sc$_{0.45}$N as the negative-capacitance ferroelectric. Our self-consistent framework couples a four-band tight-binding Hamiltonian with GW-corrected screening, a quasistatic Landau–Devonshire ferroelectric response, and ballistic Landauer–Büttiker transport. Negative capacitance amplifies the interlayer potential difference, compressing the switching window 37-fold relative to a dielectric-gated control. At 300~K, the optimized NC-BCFET achieves a subthreshold swing of 15~mV/dec, four times below the Boltzmann limit, improving to 2.5~mV/dec at 100~K. The room-temperature on/off ratio is limited to $\sim 10^{2}$ by the bandgap of bilayer graphene. The concept extends directly to materials with larger field-tunable bandgaps, where deep subthermionic switching and high on/off ratios can be achieved together at room temperature.
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All authors Rahul B Awale, Mike Klymenko, Yuefeng Yin, Nikhil V Medhekar, Bhaskaran Muralidharan, Michael S Fuhrer

Mesoscale and Nanoscale Physics

Non-Hermitian Quantum Mechanics I: Instantaneous Self-Energy

Lingfeng Liu, Wei-Wei Yang, Jiangping Hu et al. (+1 more)

Starting from the unitary evolution of a closed quantum system, we rigorously demonstrate that the projection of the global wavefunction onto an arbitrary local subsystem is governed by an exact, time-dependent non-Hermitian Schrödinger equation. Crucially, the derivation does not rely on conventional approximations such as the Born approximation, the Markov approximation, or the wide-band limit. The central quantity is the instantaneous self-energy, a time-dependent and generally non-Hermitian operator that encodes environmental backaction and, together with the subsystem Hamiltonian, forms the exact time-local generator of the projected dynamics. By benchmarking the conventional non-Hermitian approximation against this exact framework, we systematically expose its limitations. These results provide a rigorous microscopic foundation for the emergence of effective non-Hermitian dynamics in quantum systems.
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All authors Lingfeng Liu, Wei-Wei Yang, Jiangping Hu, Zhesen Yang

Mesoscale and Nanoscale Physics

Non-Markovian Quantum Dynamics of Exciton-Polaritons

Rajanya Sarkar, Pritha Ghosh, Arshath Manjalingal et al. (+2 more)

Exciton-polaritons, hybrid light-matter quasiparticles formed when a material interacts with a confined electric field, have experimentally been shown to exhibit mesoscale coherent quantum propagation that remains robust at room temperature. However, an accurate and direct quantum dynamical simulation of this phenomenon that does not resort to semi-classical approximations is prohibitively expensive computationally, limiting the microscopic understanding of the rich dynamical interplay among phonons, photons, and electrons under collective light-matter coupling. To address this fundamental challenge, we develop a non-Markovian master equation approach which enables the fully quantum mechanical simulation of non-equilibrium exciton-polariton dynamics and captures phonon-induced decoherence and dissipation beyond the conventional Markovian limit. To carry out this task, a procedure is developed in which the wave vector space is coarse-grained and each diagonal element of the density matrix is evolved in parallel. To demonstrate the utility of this approach, we simulate exciton-polariton transport in TIPS-pentacene. We find that our approach reasonably captures the experimentally observed renormalization of the polariton group velocity, which originates from the phonon-induced non-Markovian Lamb shift. We further show that this renormalization cannot be reproduced within conventional Markovian theories.
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All authors Rajanya Sarkar, Pritha Ghosh, Arshath Manjalingal, David R. Reichman, Arkajit Mandal

Saturday 19 September

Mesoscale and Nanoscale Physics

Collective depinning of dipolar skyrmion chains: thermal and athermal rounding of an elastic polymer in a quenched landscape

R. L. Silva, R. C. Silva and R. L. Stamps

The dynamics of interacting dipole skyrmion chains in a weak pinning landscape is studied through numerical solutions of the stochastic Thiele equation. Chains of different lengths with periodic boundary conditions are driven across quenched attractive pinning sites under different magnetic fields. A clear threshold spin current is identified as the critical depinning force. Three main results emerge: (i) for chains of 50 skyrmions or more, the depinning threshold is independent of chain length and remains close to one third of that of an isolated skyrmion. (ii) The transition rounding is only partly thermal: the width changes from w = 0.334 +/- 0.004 decades at T = 0 to 0.633 +/- 0.002 at 300 K, while the intrinsic Larkin correlation length remains about 2.6 bonds below kBT ~ V0/25. (iii) The width is nearly unchanged up to kBT ~ 0.14 V0, but increases almost fourfold by kBT ~ 0.23 V0, mainly through broadening of the creep regime. It is independent of chain length, ruling out finite-size rounding. Increasing the field from 30 to 40 mT changes the transition shape, widening its core by 26% while shortening the creep tails. For chains of about 200 skyrmions, depinning proceeds through a sequence of internal deformations - growth of the Larkin length and roughness, transient bond stretching, and subsequent recovery of mobility and Hall angle. These collective effects are relevant for skyrmion-based devices in which chains, rather than isolated textures, act as information carriers.
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Mesoscale and Nanoscale Physics

Magnetic Quadrupole Lens and Current-Induced Quadrupolar Dynamics of Antiskyrmion

Linfeng Du, Yang Liu, Peng Yan et al. (+1 more)

We demonstrate that antiskyrmions can serve as magnetic quadrupole lenses that focus or defocus electric currents traversing them. We showcase the antiskyrmion core as a focal point where electron beams converge for specific injection directions, leading to anisotropic and nonuniform current distributions. Thus the current-induced antiskyrmion dynamics is profoundly modified by the magnetic quadrupole lensing via the spin-transfer torque. By incorporating quantum transport and micromagnetic simulations iteratively, we self-consistently capture the coupled dynamics of conduction electrons and antiskyrmions. Our results unveil a quadrupolar antiskyrmion Hall effect that is highly anisotropic with respect to the current direction and can be effectively tuned by the spin-orbit coupling. In particular, the current focusing (defocusing) dramatically enhances (suppresses) the antiskyrmion speed while simultaneously reducing its Hall angle, thereby overcoming a key challenge for antiskyrmion racetrack memories. Our work establishes the intrinsic current lensing as a powerful mechanism for manipulating topological magnetic textures and opens new avenues for antiskyrmion-based spintronic devices.
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All authors Linfeng Du, Yang Liu, Peng Yan, Ying Su

Mesoscale and Nanoscale Physics

Non-Hermitian Topology from Edge Transport in Hermitian Quantum Anomalous Hall Systems

Humian Zhou, Ming Lu, Chui-Zhen Chen et al. (+1 more)

Non-Hermitian physics, known for exotic phenomena like exceptional points and the skin effect, has been most prominently realized in engineered systems relying on controlled gain and loss. Here we show that it can also arise naturally as an intrinsic transport response of a globally Hermitian quantum anomalous Hall system, without the need for external non-Hermitian engineering. We show that the interplay between unidirectional chiral edge modes and diffusive normal edge modes induces intrinsic non-reciprocal transport described by a continuum Hatano-Nelson model. Consequently, the non-Hermitian skin effect is encoded directly in experimentally accessible Hall-bar observables: the electrochemical potential and local heat dissipation acquire chirality-dependent exponential spatial profiles, while the longitudinal conductance decays exponentially with system size and the Hall conductance remains quantized. Using Landauer–B{"u}ttiker simulations, we confirm these transport signatures and identify magnetic topological insulators as a realistic platform for an intrinsic non-Hermitian transport response. Our results bridge non-Hermitian topology with mesoscopic transport, opening a pathway toward non-Hermitian topological devices in solid-state systems.
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All authors Humian Zhou, Ming Lu, Chui-Zhen Chen, X. C. Xie

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

Comment on "Topography of Fermi arcs in t-PtBi$_2$ using high-resolution angle-resolved photoemission spectroscopy" arXiv:2503.08841 (cond-mat)

Sergey Borisenko

Angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling spectroscopy have established surface superconductivity in the Weyl semimetal t-PtBi$_2$, whereas O'Leary et al. concluded from an independent ARPES experiment that superconducting signatures are absent above 3 K. Here we reanalyze the complete deposited data of O'Leary et al. Using only experimentally determined momentum-distribution-curve (MDC) maxima, energy-distribution-curve (EDC) peak positions, and leading edges, we find a strongly anisotropic low-energy gap and dispersion back-bending that closely reproduce the magnitude and angular dependence reported independently by Changdar et al. We further show that the temperature-dependent data of Ref.~\cite{Oleary} originate from the other surface termination and were presented using temperature-dependent energy translations, momentum translations, and momentum rescalings. Registering the original temperature-dependent datasets using the metallic bulk Fermi cutoff reveals finite gaps of approximately 2, 2, 1, and 0.8 meV at the four Fermi crossings. Thus the independently acquired data of O'Leary et al., obtained on different samples and with a different photon energy, confirm rather than contradict the previously reported anisotropic superconducting gap and additionally show that an anisotropic low-energy suppression persists to 19 K.
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Mesoscale and Nanoscale Physics

Electrical Probing of Dark Excitons through Microwave Permittivity

Alex Stram, Zhida Liu, Ziheng Zhang et al. (+17 more)

Excitons in atomically thin semiconductors are almost always probed through their optical signatures, because the short lifetimes of these transient quasiparticles are generally assumed to preclude electrical detection. Here we show that photoexcited excitons in monolayer tungsten disulfide produce a large, optically tunable permittivity at gigahertz frequencies, and that the effect provides a contact-free electrical route to imaging dark excitons at the nanoscale. Using laser-illuminated microwave impedance microscopy, we find that high-purity encapsulated flakes exhibit a purely dielectric response resonant with the exciton spectrum, whereas defect-rich samples are governed by conventional photoconductivity. Spatial mapping of diffusion and sublinear power dependence identify long-lived dark excitons as the dominant contributors, and first-principles modelling of exciton polarizability reproduces the measured susceptibility. Our results establish excitons as optically tunable dielectric elements and introduce microwave microscopy as a direct electrical probe of dark-exciton transport with sub-100 nm resolution.
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All authors Alex Stram, Zhida Liu, Ziheng Zhang, Yangchen He, Xuejian Ma, Kyoung Pyo Lee, Lisa Frammolino, Fuxiang Chen, Robert J. Boyd, Sirapas Tangton, Anand Swain, Kan Yao, Kenji Watanabe, Takashi Taniguchi, Yuebing Zheng, Chih-Kang Shih, Daniel Rhodes, Li Yang, Xiaoqin Li, Keji Lai

Friday 18 September

Mesoscale and Nanoscale Physics

Vanadium doping induced valley asymmetries in WS$_2$ monolayers

Frederico B. Sousa, Boyang Zheng, Elizabeth Grace Houser et al. (+10 more)

Transition metal dichalcogenide (TMD) monolayers offer an innovative platform for encoding and manipulating information through the valley degree of freedom. While unique valley-related physical phenomena have been reported so far, practical applications still require advanced control over the valley polarization efficiency and the valley Zeeman effect. Recently, the introduction of spin-polarized metal atoms as substitutional defects was reported to break the time-reversal symmetry in TMD monolayers, consequently inducing a room-temperature ferromagnetic ordering and enhancing the valley-dependent optical responses. Here, we report valley asymmetries for vanadium-doped WS$_2$ monolayers. With a given magnetic polarization, one valley exhibits larger Zeeman slope and degree of circular polarization than the other valley. Additionally, the overall degree of circular polarization in the doped samples is approximately twice that of the pristine WS$_2$ monolayer. Density functional theory calculations in the doped structure show that different energy shifts in conduction band edges due to spin-dependent hybridization lead to different exciton energies between valleys, which is consistent with the experimental observations. Our results pave the way for valleytronic technologies based on defect-engineered two-dimensional materials.
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All authors Frederico B. Sousa, Boyang Zheng, Elizabeth Grace Houser, Paulo E. Faria Junior, Zhuohang Yu, Alessandra Ames, Gabriel A. D. Souza, Mingzu Liu, Gilmar Eugenio Marques, Leandro M. Malard, Mauricio Terrones, Vincent H. Crespi, Marcio D. Teodoro

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

Boundary-condition identification from a scale-invariant frequency ratio in shear-deformable and nonlocal beams and plates

Ak{\i}n Oktav

The ratio of a structure's first two natural frequencies depends on its boundary conditions but not on its size, material density, or absolute stiffness, which multiply every frequency equally and cancel in the quotient. We use this to identify the boundary conditions of a small-scale beam or plate from two ratio measurements, without knowing its exact dimensions or material. The paper first establishes how the ratio $r=\omega_2/\omega_1$, and the fractional shift $\Delta r$ it undergoes when a boundary condition changes, respond to the two effects that matter at small scale: transverse shear deformation, through Timoshenko beam and Mindlin plate theory, and small-scale elasticity, through Eringen's nonlocal Timoshenko model. Using a locking-free Rayleigh-Ritz beam solver and a finite-element Mindlin plate solver, each checked against benchmarks, we find an asymmetry. The absolute ratio moves appreciably with thickness, by 12 to 14 per cent over the shear-deformable range, and with orthotropy, by 6 to 9 per cent, but only weakly with nonlocality; the shift $\Delta r$, by contrast, varies by less than 0.2 per cent over the full nonlocal range and by a few per cent under thickness change. Because $\Delta r$ is largely independent of these details, a measured shift identifies which boundary-condition transition a deliberate change of support has produced, with a resolution we quantify and verify against molecular-dynamics data. Two spring continua then extend the method to estimating support compliance, with the non-uniqueness of the tip-support case stated explicitly. The method needs no value of the small-scale parameter or the material constants.
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Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.mtrl-sci

Laser-Ablated Au Electrodes with Preferred Orientation and Flat Interface on Oxides

Ambrose Seo, Richard Lai, Sujan Shrestha et al. (+5 more)

Gold (Au) thin-film electrodes deposited on oxides by pulsed laser deposition (PLD) exhibit distinctive structural properties compared with thermally evaporated Au films. While thermally evaporated Au forms polycrystalline films with randomly oriented grains, PLD-grown Au films show a strong preferred [111] orientation perpendicular to the substrate surface, likely due to the energetic nature of the laser-ablated Au plume. High-resolution transmission electron microscopy reveals atomically flat interfaces between the PLD-grown Au electrodes and epitaxial oxide thin films, separated by nanometer-scale gaps comparable to interlayer spacings in van der Waals materials. The highly ordered microstructure and flat interfacial morphology result in enhanced adhesion of the Au films to oxide surfaces. These results demonstrate that PLD can produce both high-quality Au electrodes and oxide thin films within a single deposition platform, offering improved interfacial control and potential benefits for a wide range of oxide electronic and functional devices.
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All authors Ambrose Seo, Richard Lai, Sujan Shrestha, Tina Tong, Joseph W. Brill, Menglin Zhu, Chris Chae, Jinwoo Hwang