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Physics Condensed Matter Mesoscale and Nanoscale Physics

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

Wednesday 19 August

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

Self-calibrating thermal interferometry of vortex parity in a two-dimensional chiral superconductor

Kumar Ghosh

A chiral superconductor carries chiral Majorana modes along its boundary, and the integer that counts them fixes everything that follows, yet that integer has never been measured together with a local parity observable on one object. Proximitized one-dimensional wires read fermion parity rapidly but diagnose bulk topology through a separate protocol. Here we show that a reconfigurable domain wall between regions of opposite Chern number in an intrinsic two-dimensional chiral superconductor performs both functions. Opened to its contacts the wall is a ballistic channel whose quantized thermal conductance counts its Majorana modes; closed, the same wall is a Fabry–Pérot resonator whose spectrum shifts by half a level spacing when the parity of the enclosed vortices changes, giving a two-level heat conductance. We derive the exact transmission, the elastic heat full counting statistics, and a theorem showing that linear-response heat scattering of a fixed quadratic problem resolves vortex parity but not the fusion channel of well-separated cores. An outside vortex hybridized with the wall is an intrinsic false positive; temperature, geometry and a finite-bias mean–noise test separates it. Rhombohedral-graphene parameters place submicron loops in the resolved regime at millikelvin temperatures, where chiral-domain reconfiguration and noise thermometry are both established.
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Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.str-el

Interferometric Signatures of Zero Modes in Fractional Quantum Hall-Superconductor Heterostructures

Junyi Cao, Ramanjit Sohal, Angela Kou et al. (+1 more)

Fractional quantum Hall-superconductor (FQH-SC) heterostructures are predicted to host defect-bound parafermion zero modes (PZMs). We propose two related configurations to probe their fusion structure. In a Josephson junction coupled to a single quantum point contact (QPC), quasiparticle tunneling switches the defect fusion channel, producing stochastic transitions between branches of the fractional Josephson spectrum. Embedding the junction in a two-QPC Fabry-Pérot interferometer provides a complementary probe. Weak zero mode tunneling produces fusion-channel-dependent interference while strong tunneling makes the interferometer probe a superposition of fusion channels and strongly suppresses the signal: in the topological limit it vanishes exactly, revealing the defects' non-Abelian nature even when the parent FQH state is Abelian.
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All authors Junyi Cao, Ramanjit Sohal, Angela Kou, Eduardo Fradkin

Mesoscale and Nanoscale Physics

Extension of the Shockley-Queisser Limit for Nanostructured Solar Cells

Rivo Herivola Manjakamanana Ravelonjato, Jean Patrice Rakotoniaina, Ravo Tokiniaina Ranaivoson et al. (+1 more)

This article extends the Shockley-Queisser limit to nanostructured solar cells using the quantum phase space formalism. The parameter B_ll represents the momentum variance in each confinement direction and acts as a variance-covariance matrix linking the nanostructure geometry to thermodynamic properties. Electron-electron interactions are included via an exchange-correlation energy with an adjustable coefficient theta. The authors derive an analytical expression for the maximum efficiency as a function of size, shape, temperature, and doping. For the cylindrical geometry, the exact confinement energy uses the first zero of the Bessel function j_0,1. Numerical simulations are performed with Python 3.8.1, NumPy, and Matplotlib for PbS quantum dots in four geometries: cube, square parallelepiped, cylinder, and sphere. The integral is evaluated using an exact convergent series expansion. Results show that the maximum efficiency reaches 48.7 percent for a 5 nanometre cube, 49.0 percent for flattened parallelepiped and cylinder shapes, and 49.1 percent for a 3 nanometre sphere. These values greatly exceed the bulk PbS efficiency of 15.8 percent and surpass classical Shockley-Queisser limits. For constant-volume shapes, two efficiency peaks appear corresponding to different aspect ratios. The model correctly returns to classical values for large sizes. This approach provides a theoretical framework for optimising nanostructured solar cells and demonstrates that quantum confinement offers a promising route to surpass traditional photovoltaic limits.
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All authors Rivo Herivola Manjakamanana Ravelonjato, Jean Patrice Rakotoniaina, Ravo Tokiniaina Ranaivoson, Wilfrid Chrysante Solofoarisina

Mesoscale and Nanoscale Physics

Ultra-low energy (20 - 300 eV) electron imaging and diffraction

Tatiana Latychevskaia

The lecture discusses the properties of low-energy electrons compared to the electrons of conventional energies employed in commercial transmission electron microscopes. Low-energy electron diffraction and low-energy electron holography principles and experimental schemes are presented and discussed.
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Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.supr-con

Large Field-Free Superconducting Diode Effect with Nonmonotonic Polarity Reversals in NbSe$_2$/CrBr$_3$ Heterostructures

Shengbiao Sun, Lang Xiao, Chenghao Shen et al. (+10 more)

The superconducting diode effect (SDE), characterized by nonreciprocal dissipationless supercurrent, offers a promising route toward ultralow-power superconducting electronics. Yet most realizations require an external magnetic field, and achieving a large, controllable SDE at zero field remains challenging. Here we report a field-free SDE with an efficiency reaching 35.7% in a van der Waals NbSe$_2$/CrBr$_3$ heterostructure, whose polarity is programmable by magnetic-field history. Unlike conventional mechanisms based on finite-momentum pairing, the SDE originates from the leading symmetry-allowed cubic term odd in Cooper-pair momentum, which yields unequal critical currents while leaving the equilibrium condensate at zero momentum. Remarkably, upon sweeping a perpendicular magnetic field, the diode polarity undergoes nonmonotonic and hysteretic reversals that cannot be explained by Meissner screening or conventional ferromagnetic proximity. Combining transport measurements, micromagnetic simulations, and a generalized Ginzbur-Landau theory, we attribute these unconventional behaviors to layered ferrimagnetism in CrBr$_3$, where coexisting ferromagnetic and antiferromagnetic interlayer couplings produce a history-dependent interfacial exchange field acting on NbSe$_2$. Our results reveal a distinct mechanism for nonreciprocal superconductivity and establish layered van der Waals magnetism as a versatile platform for high-efficiency, programmable, field-free superconducting diodes.
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All authors Shengbiao Sun, Lang Xiao, Chenghao Shen, Bowen Hao, Jia-Peng Peng, Ziye Zhu, Feiyue Wang, Qilin Han, Ya-Qing Bie, Shuo Wang, Tong Zhou, Dapeng Yu, Ben-Chuan Lin

Mesoscale and Nanoscale Physics

Flat and Topological Floquet Minibands from Patterned Light in Untwisted Bilayer Graphene

Muhammad Faisal and Michael Vogl

Two-dimensional superlattices in van der Waals materials host flat bands and nontrivial topology, most famously at the so-called magic angles of twisted bilayer graphene, where flat bands give rise to correlated and topological phases. Yet these superlattices are usually created by twisting the layers or applying strain, and once a sample is fabricated, their period is fixed and extremely difficult to tune. Here we propose an alternative route: imprint the superlattice optically, using patterned electromagnetic fields rather than a physical twist or strain. We show that patterned in-plane cir- cularly polarized light and a combined drive consisting of a patterned out-of-plane longitudinal field and a uniform circularly polarized field produce isolated bands in both AA- and AB-stacked bilayer graphene. In AB stacking, the central bands additionally become nearly flat, capturing key features of a driven moire superlattice. In this approach, the superlattice period is set by the illumination and is straightforward to tune, and circularly polarized light breaks time-reversal symmetry. Computing the valley Chern numbers of the central bands, we find a rich topological structure with several phase transitions in both stackings. Our results establish light-induced superlattices as a flexible and tunable platform for engineering flat bands and topological phases in bilayer graphene.
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Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.mtrl-sci

Ultrafast order-selective electron imaging and spectroscopy

Jan Philipp Bange, Till Domröse, Claus Ropers et al. (+2 more)

Ultrafast pump-probe spectroscopies in energy and momentum have become indispensable for probing non-equilibrium dynamics in quantum materials, revealing pathways in ultrafast energy conversion and light-induced phase transitions. Different modalities uniquely access the coupled lattice, charge, and spin degrees of freedom that underpin the functionality of these materials. However, investigating technologically relevant nanoscale devices also requires high spatial resolution. Although electron microscopy provides nanometer-scale imaging, it frequently lacks the simultaneous ultrafast temporal resolution and spectroscopic specificity needed for such studies. In this perspective, we review recent advances in dark-field electron and photoelectron microscopy, focusing on two complementary techniques: femtosecond photoelectron momentum microscopy and ultrafast transmission electron microscopy. Their application enables comprehensive insights into the ultrafast dynamics of the electron, lattice, and spin subsystems. These order-selective electron imaging and spectroscopy techniques open broad scientific opportunities for a microscopic understanding of non-equilibrium phenomena in quantum materials, nanostructures and devices.
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All authors Jan Philipp Bange, Till Domröse, Claus Ropers, Stefan Mathias, Marcel Reutzel.

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

Molecular Insights into Gas Nanofilms Confined Between Bulk Liquid Phases

Yafan Yang, Zufeng Zuo, Xingyu Zhao et al. (+2 more)

Nanometer-thick fluid films play a critical role in confined multiphase processes, yet the thermodynamics and stability of free gas nanofilms remain poorly understood compared with their liquid counterparts. Here, molecular dynamics (MD) simulations are employed to systematically investigate gas nanofilms confined between bulk liquid phases using Lennard-Jones argon as a model system. The results show that the surface tension decreases exponentially with decreasing film thickness, accompanied by an increasing magnitude of the negative disjoining pressure. Upon thinning, the planar gas film undergoes a distinct morphological transition from a stable planar state to a transient or persistent spherical bubble through the formation and growth of a liquid bridge. The film surface area strongly affects its thermodynamic properties, with larger areas producing stronger thickness dependence and larger deviations from classical density functional theory (cDFT) predictions. The closer agreement between MD and cDFT at smaller surface areas suggests that the discrepancy primarily arises from thermal capillary-wave fluctuations, which are included in MD but omitted in mean-field cDFT. Moreover, at small film thicknesses, the magnitude of the disjoining pressure increases with decreasing temperature, consistent with the enhanced sensitivity of the confined gas phase to thickness variations and contrasting with the trend generally reported for liquid nanofilms. These findings provide molecular insights into the thermodynamics and stability of gas nanofilms, with implications for confined multiphase transport and droplet coalescence.
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All authors Yafan Yang, Zufeng Zuo, Xingyu Zhao, Shuyu Sun, Denvid Lau

Mesoscale and Nanoscale Physics

Spectral Fingerprints of Resonant Defect Scattering by Substitutional Mn in Graphene

Ahmed Samir Lotfy, Zviadi Zarkua, Renan Villarreal et al. (+10 more)

Using substitutional Mn in graphene/Cu(111) as a model point defect, we combine scanning tunneling microscopy (STM) and angle-resolved photoemission spectroscopy (ARPES) to test the predicted fingerprints of resonant scattering. As the Mn concentration increases to 0.44%, the Dirac point stretching reaches 0.52 eV, while momentum broadening remains energy independent. These spectral fingerprints classify substitutional Mn as a strong resonant scatterer and establish the combination of STM and ARPES as a powerful approach to identify and characterize resonant disorder in graphene.
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All authors Ahmed Samir Lotfy, Zviadi Zarkua, Renan Villarreal, Rikkie Joris, Muhammad Saad, Koen van Stiphout, Karina Landivar, Steven Brems, Giovanni Di Santo, Aleksandr Seliverstov, Simona Achilli, Luca Petaccia, Lino M. C. Pereira

Mesoscale and Nanoscale Physics

Electromagnetic and Acoustic Fano Interference in Surface Acoustic Wave Resonators

P.K. Rath, J.M. Kitzman, M. Mesbah et al. (+2 more)

Surface acoustic wave-based resonators are sensitive probes of condensed matter systems, as well as surface-selective sensors for chemistry and biology. Surface acoustic wave devices have also been integrated into hybrid quantum systems with qubit platforms for applications in quantum information processing and sensing. The sensitivity of piezoelectric surface wave-based resonators to investigate these various systems can be enhanced by optimizing the device architecture and electrical measurement techniques. Alternatively, tailoring the spectral symmetry, arising from interference effects, offers a promising route to further improve sensitivity. In this work, we demonstrate the simultaneous introduction of both electromagnetic and acoustic Fano interference to shape the spectral response of GHz-frequency surface acoustic wave resonators. By systematically modifying the acoustic reflectivity of the resonators, we are able to isolate and analyze each interference mechanism independently. The broad range of temperature operation, from ambient to cryogenic temperatures highlights the potential for both classical and quantum sensing applications.
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All authors P.K. Rath, J.M. Kitzman, M. Mesbah, C. Undershute, J. Pollanen

Mesoscale and Nanoscale Physics

A miniature evaporator for in-operando deposition of isolated atoms in a low-temperature scanning tunneling microscope

Jeongmin Oh, Hermann Osterhage, Vasily Cherepanov et al. (+5 more)

Depositing dilute atomic ensembles onto cold samples is challenging in low-temperature scanning tunneling microscopes (STM) because radiation shields and restricted internal geometries often preclude a direct deposition path, particularly in instruments designed for millikelvin operation. We present a compact, milliwatt-range evaporation source fabricated from a commercial miniature incandescent lamp and integrated directly into a millikelvin STM head. The exposed tungsten filament is coated with a micrometre-thick Fe film and positioned about 1 cm from the sample. We deposit isolated Fe atoms onto MgO/Ag(100) while operating the microscope near 5 K. Evaporation increases the STM-body temperature by only about 2 K, and the same nanoscopic surface region can be readily scanned after deposition with a lateral displacement of less than 5 nm. Differential-conductance spectra displaying symmetric inelastic steps near $\pm$14 mV identify the deposited atoms on MgO as Fe. From STM images, we estimate a local deposition flux of $1.5\times10^{-5}~\mathrm{nm^{-2}\,s^{-1}}$, corresponding to a nominal evaporator lifetime of ~150 h. The fixed evaporator enables repeated low-flux deposition without a room-temperature line of sight, the need for movable radiation shields, or mechanical evaporator access after cooldown, while preserving access to the same atomic-scale surface region before and after deposition.
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All authors Jeongmin Oh, Hermann Osterhage, Vasily Cherepanov, Sven Just, Denis Krylov, F. Stefan Tautz, Taner Esat, Ruslan Temirov

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

An ultracompact dilution refrigerator for fast quantum device characterization

Clḿent Geffroy, Dorian Nicolas, Eric Eyraud et al. (+10 more)

Rapid thermal cycling is a central bottleneck in the development of superconducting quantum devices: conventional dilution refrigerators require cooldowns of a day or more and substantial cryogenic infrastructure, which throttles the fabricate-measure-redesign loop. We present an ultracompact dilution refrigerator (3 kg in mass and 100 mm in diameter) that completes a full cooldown-warm-up cycle to a base temperature of 70 mK in 1.2 hours when unloaded, and in 2.1 hours when fully equipped with the microwave wiring required for qubit measurements, while delivering 20 microW of cooling power at 100 mK. We validate the platform through a complete characterization of a two-fluxonium device: we extract the full circuit Hamiltonian by two-tone spectroscopy, measure energy-relaxation and coherence times, and benchmark single-qubit control. Although the relaxation time is limited by the base temperature of the system, we reach a single-qubit gate fidelity of up to 99%, at the coherence limit set by our operating temperature. These results demonstrate that compact, fast-cycling dilution refrigeration can support state-of-the-art quantum-device characterization without sacrificing measurement quality, offering a practical route to high-throughput quantum-hardware development.
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All authors Clḿent Geffroy, Dorian Nicolas, Eric Eyraud, Shelender Kumar, Supriya Mandal, Julien Jarreau, Laura Kowalski, Laurent Del-Rey, Didier Dufeu, Nicolas Roch, Wolfgang Wernsdorfer, Quentin Ficheux, Matias Urdampilleta

Mesoscale and Nanoscale Physics

Floquet engineering of topological bands in semiconductor van der Waals heterobilayers

Eréndira Santana-Suárez, Brayan E. Walteros-Mendivelso, A. Jazmín Tapia-de-la-Rosa et al. (+2 more)

We show that periodic driving with near-infrared to visible light drives topological phase transitions in the photon-dressed band structure of transition-metal dichalcogenide heterobilayers. We apply the Floquet formalism to a light-coupled lowest-order $\mathbf{k}\cdot\mathbf{p}$ Hamiltonian, and obtain an effective four-band model that correctly captures the essential photon-dressed bands of type-II TMD heterobilayers in the vicinity of the first photon resonance. Crossings between the zero- and one-photon sectors effectively invert the bands, yielding topological phases with Chern numbers up to $\pm 2$ and gaps of order 10 meV. Our results establish TMDs as prime candidates for engineering bands with higher Chern numbers, and exploring driven topological states in solid state media.
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All authors Eréndira Santana-Suárez, Brayan E. Walteros-Mendivelso, A. Jazmín Tapia-de-la-Rosa, Mahmoud M. Asmar, David A. Ruiz-Tijerina

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

Extracting Chern numbers from the equilibrium transport noise of multiband bosons

Zhi-Wei Wang and Samuel L. Braunstein

We show that the equilibrium transport noise of a multiband bosonic system carries a measurable imprint of its Chern numbers, and we give the protocol that extracts them. For electrons the topological part of the Berry curvature is absent from the transport noise: over a filled band it yields the quantized anomalous Hall conductivity, yet at a Fermi surface the Chern number is invisible to the fluctuations of the Hall current. That cancellation is an accident of the Fermi surface: magnons and phonons populate every band thermally, each with its own Chern number, so the harmonic sector of the Hodge decomposition is not a single constant, and a variance does not annihilate it. What survives is the occupation-weighted dispersion of Chern numbers. That current noise sees band geometry is known, and at zero temperature the antisymmetric part of the noise sum rule already returns a Chern number; the new element is an object with no zero-temperature analogue, existing only when several bands are populated and weighted differently. We derive it from a fluctuating Boltzmann equation and resolve the energy-magnetization subtraction at the level of fluctuations: the subtracted term is a curl, contributing exactly zero to the current a transport measurement records, and no magnetization auto-correlation enters the antisymmetric cross-spectrum at any wavevector. Measurement uses an equilibrium sum rule on that cross-spectrum, with no drive and hence no suppression: its frequency-integrated form cannot in practice separate topological from geometric content, for a rank reason, whereas the frequency-resolved form does, at accessible resolution. The protocol is specified for Cu(1,3-bdc), with calibration and robustness characterized. A driven alternative lies twelve orders below the equilibrium floor. Two independent implementations agree on every convention-free quantity.
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Mesoscale and Nanoscale Physics

Superfast hole spin qubits enabled by uniaxial strain-boosted spin-orbit coupling

Yi-Xu Wang, Yang Liu, Shan Guan et al. (+2 more)

Two-dimensional (2D) electron/hole gases confined in semiconductor heterostructures suffer from weak Rashba spin-orbit coupling (SOC) for manipulating spin degreee of freedom via an electric rather than a magnetic field. Here, we show that complementary metal-oxide-semiconductor technology-accessible strain could substantially enhance the linear Rashba SOC of the top hole subband in Ge/SiGe quantum wells (QWs) to a level comparable to that of 2D Rashba materials through enhancing the mixture of the light-hole and heavy-hole bands. We further show that strongly enhanced Rashba SOC boosts the Rabi frequency of hole spin qubits confined in Ge/SiGe QWs by two orders of magnitude to an unprecedented 40 GHz, more than one order of magnitude faster than other qubit platforms. We also demonstrate that the hole spin rotation with Rabi frequency > 25 GHz enters a new regime being immune to gate control-induced electric noise, opening a new avenue to simultaneously improve the gate speed and gate fidelity. Our findings provide a new routine to substantially enhance the Rashba SOC in 2D semiconductor hole gases to a level that is great for spintronic applications.
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All authors Yi-Xu Wang, Yang Liu, Shan Guan, Jun-Wei Luo, Shu-Shen Li

Tuesday 18 August

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

Collective photon echoes in the Tavis-Cummings model: distribution independence, two detuning regimes, and the Dicke ladder

Michael Tavis

An ensemble of $N$ two-level molecules prepared in its ground state and sharing a lossless cavity with a weak field does not simply absorb: the intensity collapses and then recurs, in a train of collective photon echoes. Working from the exact solution of the Tavis-Cummings model in the few-photon regime $\bar n<N$, we confirm the echo time $\tau_E=4\pi\sqrt{N+\Delta}/g$ numerically at resonance from $N=5$ to $400$, the small-$N$ end discriminating this form from the alternative $4\pi\sqrt{N-\bar n+\Delta}/g$ in its favor. The initial state requires no preparation, being the ground state. The echo time is independent of the initial photon distribution: coherent, thermal, squeezed and oscillatory-squeezed distributions spanning variances from 2 to 34 all recur together, as does a controlled pair with identical mean and variance differing only in the shape of $\rho_{nn}$. The echo amplitude is not: it varies at the tens-of-percent level at fixed mean, including a factor of 1.8 with the squeezing phase at fixed squeezing strength. Detuning organizes the dynamics into two clean regimes separated by a fragmented crossover, the dispersive-branch echo time approaching one-half the resonant one, and sufficient detuning removes the dependence on the initial Dicke state. For arbitrary initial Dicke state, emission replaces absorption at $m\simeq-N/2+\bar n$, and the echo envelope acquires one component per step up the ladder: a single-component echo occurs only at the ground state. A feasibility analysis against a five-qubit superconducting device, with Lindblad simulations of cavity decay and dephasing and full-Hilbert-space disorder simulations, shows the first echoes observable at $N\sim5$-$20$ on existing hardware: the echo survives the dominant loss channel with contrast $\sim e^{-\kappa\tau_E/2}$, photons being shielded from cavity decay while resident in the emitters.
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Mesoscale and Nanoscale Physics

Isolating the natural edges of bilayer graphene in gate-defined mesoscopic devices

Francesco Blanda, Grazia Raciti, Thilo Glatzel et al. (+8 more)

We introduce a graphite-gated architecture for bilayer graphene devices in which the active device is completely isolated from the natural graphene edges. Using a single patterned graphite-gate layer, we realize a fully electrostatically defined Hall-bar. Longitudinal and Hall measurements reveal mesoscopic transport features, including Hall-effect quenching and magnetoresistance peaks associated with boundary scattering. The dependence of the mesoscopic features on the carrierdensity shows that the effective channel width increases with the Fermi level and the electrostatic confinement at the gate-defined boundaries, and indicates that the carriers scatter at the electrostatic boundary. Raman spectroscopy and Kelvin probe force microscopy suggest that this boundary is disordered due to the used fabrication methods. Comparably, the quantum mobility in a fieldeffect transistor fabricated with the same architecture is not limited by boundary scattering and the visibility of quantum oscillations down to 4 mT suggests a record value of 2.5 x 10^6 cm2/Vs.
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All authors Francesco Blanda, Grazia Raciti, Thilo Glatzel, Aurin Strathmann, Fabrizio Volante, Kenji Watanabe, Takashi Taniguchi, Ilaria Zardo, Thomas Ihn, Klaus Ensslin, Andrea Hofmann

Mesoscale and Nanoscale Physics

Optical Voltage Profiling of 2D Semiconductors via Proximal Exciton Sensing

Ha-Leem Kim, Hyungbin Lim, Yuanyi Yang et al. (+6 more)

High contact resistances in atomically thin semiconductors often mask intrinsic electrical transport properties, particularly at low carrier densities where exotic correlated states emerge. We introduce optical voltage profiling, a noninvasive wide-field technique that replaces local voltage probes with a proximal monolayer MoSe$_2$ exciton sensor. Isolated by thin hexagonal boron nitride, this sensor converts the target's local electrostatic potential into spatially resolved modulations of exciton reflectance. Through pixel-wise in situ calibration, these signals yield quantitative two-dimensional voltage maps of an actively biased semiconductor device. Using this method, we demonstrate the carrier-density-driven metal-insulator transition in bilayer MoSe$_2$ and obtain channel resistances below 1 k$\Omega$ despite M$\Omega$-scale two-terminal resistances in the metallic region. The optically derived resistance exhibits a metal-insulator crossover near the resistance quantum $h/e^2$, and the voltage maps and reconstructed local conductivity reveal pronounced spatial heterogeneity in both insulating and metallic regimes. Beyond resolving channel resistance under high contact-resistance conditions, the technique provides spatially resolved access to microscopic transport heterogeneity in functional van der Waals devices.
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All authors Ha-Leem Kim, Hyungbin Lim, Yuanyi Yang, Ruishi Qi, Ruichen Xia, Can Uzundal, Takashi Taniguchi, Kenji Watanabe, Feng Wang

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

Interface-Controlled Spin-Orbit Torques in Rare-Earth Synthetic Ferrimagnets Probed by Sagnac Magneto-Optics and Harmonic Hall Measurements

Akilan K, Koral Aykin, Jose-Luis Ampuero et al. (+6 more)

Spin-orbit torque (SOT) provides an efficient route for manipulating magnetization in spintronic devices, and its accurate quantification is essential. Here, we investigate Co/Gd-based synthetic ferrimagnets using complementary magnetotransport and magneto-optical techniques. We use a Sagnac magneto-optical interferometry method to directly quantify current-induced magnetization tilting and extract the damping-like SOT effective field. The Sagnac measurements show good agreement with harmonic Hall analysis across different Co/Gd and Gd/Pt/Co heterostructures, providing a quantitative determination of the damping-like SOT effective field that does not rely on electrical transport signatures. By varying the Gd thickness and multilayer stacking order, we demonstrate that the damping-like torque is strongly influenced by spin transport and angular momentum conversion at rare-earth and heavy-metal interfaces. The observation of a finite torque in Gd/Pt/Co/Pt structures, where conventional Pt spin Hall contributions are expected to compensate, highlights the active role of the Gd layer in SOT generation. Furthermore, interface engineering enables perpendicular magnetic anisotropy in Gd/Pt/Co/Al heterostructures with a 2-nm-thick Co layer, allowing current-induced SOT switching at current densities of 8-10 MA/cm$^2$. These results establish rare-earth-based synthetic ferrimagnets as a versatile platform for engineering spin-orbit phenomena and optimizing low-power spintronic devices.
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All authors Akilan K, Koral Aykin, Jose-Luis Ampuero, Laurent Badie, Stephane Mangin, Sébastien Petit-Watelot, Michel Hehn, Andrew D. Kent, J.-Carlos Rojas-Sánchez

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

Quantum-geometric bounds on Casimir repulsion

Adolfo G. Grushin

The quantum geometric tensor has been shown to bound the gap, optical absorption, and dielectric susceptibilities of materials. Here we derive new quantum-geometric bounds on the magnitude and sign of the Casimir force between two-dimensional plates in the long-distance limit. These bounds limit the previously attributed benefit of increasing the plate's Chern number to maximize repulsion, and give a quantum geometric origin to the stronger attractive force of metallic plates, regardless of their Chern number. These bounds allow us to infer that flat Chern bands that saturate geometric bounds, including Landau levels and moiré flat bands, enlarge the window where Casimir repulsion exists and bring the repulsive crossover to smaller, more experimentally relevant distances. We derive estimates for material platforms such as twisted MoTe$_2$. Our work shows that quantum-geometric bounds constrain repulsive Casimir forces beyond previously known theorems, and suggests new optimization strategies to observe repulsion.
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Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.supr-con

Engineering kekule superconductivity from layer-selective interactions in rhombohedral graphene

Hung Dinh Nguyen and Yafis Barlas

At weak coupling, finite-momentum superconductivity is typically associated with broken time-reversal or inversion symmetry of the Fermi surface. Here, we show that lattice-scale pair-density-wave order in rhombohedral multilayer graphene can arise from layer/orbital-dependent pairing interactions, band chirality, and Dirac-point-centered Fermi surface topology while preserving both symmetries. Using mean-field theory and comparing finite momentum sectors $Q = \pm 2 K_{D}$ with the $ Q = 0$ superconducting state, we find that layer-dependent interactions of opposite signs ($V_{1A}=-V_{JB}=-|V|$) favor an intra-valley Kekulè state with center-of-mass momentum ($ Q=\pm 2 K_D$). In the presence of a time-reversal and inversion symmetry-preserving Kane-Mele mass ($\lambda$), this state appears only above a critical carrier density ($n^{crit}_{K}(\lambda,J)$). The two superconducting condensates exhibit opposite chirality, $J(-J)$ for $K_D(-K_D) $ valleys, thereby preserving time-reversal and inversion symmetry. We map the phase diagram and analyze the dependence of $T_c$ on the chirality index $J$ and $\lambda$. We also evaluate the superfluid stiffness in the Kekulè superconducting state, thereby determining the Berezinskii-Kosterlitz-Thouless (BKT) transition temperature. Our results show that orbital-dependent interactions in the presence of band chirality favor finite-momentum pairing in time-reversal and inversion symmetric Dirac materials.
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Mesoscale and Nanoscale Physics

On the electronic and vibrational dimensionality of nanometer-scale silicon structures

Massimo V. Fischetti, Dallin O. Nielsen and Edward Chen

We discuss the problem of assessing the electronic and vibrational dimensionality of a semiconductor nanostructure: How thin and/or wide must a nanostructure be in order to induce electron and phonon confinement? Clarifying the physical justification for common criteria found in the literature, we view the electron coherence length (defined as the electron and phonon inelastic mean free path) as their `field of view' and argue (or, better yet, `speculate') that this sets the important length scale. Considering the example of Si nanosheets at room temperature, and drawing from results found in the literature, we estimate that the critical length below which electrons are subject to quantum confinement is of the order of (or smaller than) 8 nm, when their coherence length is determined by energy losses to phonons and remote phonons in gated structures. On the contrary, no single length-scale can be given for phonons: Taking their coherence length as determined by scattering with electrons and anharmonic three-phonon processes, short wavelength acoustic and optical phonons may be confined only by structures as small as 10 nm. Long-wavelength acoustic phonons, instead, may exhibit a coherence length of the order of 1 micrometer, so that they may be confined over much larger distances.
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Mesoscale and Nanoscale Physics · Cross-listed from hep-ph

Towards Quantum-Dot Detectors as Barcodes for Dark Matter Interactions

Marek Matas, Andrea Gallo Rosso, Antonio Cammarata et al. (+6 more)

Quantum dots are tunable semiconductor nanocrystals that can be produced at industrial scales. We present the first ab initio calculation of the scattering of dark matter on electrons bound in quantum dots. The momentum-dependence of a quantum dot's electronic response depends on its morphology and on the dark matter mass, interaction operator, mediator coupling, and mediator mass. Therefore, the relative rates across an array of distinct quantum dot targets form a “barcode” that carries information about the nature of the dark matter interaction. We project the sensitivity of a detector concept in which a collection of independent target subunits, each loaded with silicon quantum dots of a particular morphology, are read out by Skipper CCDs. Given a future signal, this barcode could discriminate between interaction operators and mediator types. We quantify the discrimination power for a benchmark pair of models as a function of readout noise and exposure.
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All authors Marek Matas, Andrea Gallo Rosso, Antonio Cammarata, Nora Hoch, Carlos Blanco, Jan Conrad, Rouven Essig, Tim Linden, Lindley Winslow

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

Chern insulator boundary criticality

Benjamin Moy and Eduardo Fradkin

We investigate signatures of chirality at Chern insulator transitions in the presence of a boundary. The transition between a trivial insulator and a Chern insulator with Chern number $C=1$ is described by a massless Dirac fermion whose parity anomaly gives a critical Hall conductivity $\sigma_{xy}=\frac{1}{2}\frac{e^2}{h}$. Using a Dirac mass domain wall construction, we show that the chiral edge mode delocalizes into the bulk at criticality, but the boundary fermion correlation function retains a chiral structure and acquires the scaling dimension of the bulk fermion. We compute current correlation functions and demonstrate that the anomaly of the bulk Hall response is matched by delocalized chiral modes near the boundary. Using only the residual conformal symmetry of a (2+1)d conformal field theory (CFT) in a half-space, we identify parity-odd terms in current and energy-momentum tensor correlation functions that encode these modes and determine their electromagnetic and gravitational anomaly coefficients. Our analysis therefore applies to general time-reversal breaking (2+1)d CFTs, beyond the free Dirac transition. In particular, the analysis of the gravitational anomaly is also applicable to the free Majorana CFT governing the transition between a trivial superconductor and a topological superconductor. We also extend our results to more general Chern number changing transitions and to the transition between a (3+1)d topological insulator and a trivial insulator.
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Mesoscale and Nanoscale Physics

Electronic Reconstruction at the Quasicrystal-Moiré Crossover in Twisted Bilayer Graphene

Kuo-En Chang, Aitor Garcia-Ruiz, Ta-Lei Chou et al. (+11 more)

Large twist angles in twisted bilayer graphene are widely expected to be electronically trivial, with negligible interlayer coupling and no electronic reconstruction, in contrast to the rich moiré-driven band reconstruction and correlated physics that emerge at small twist angles. Here, we show that this paradigm breaks down near a twist angle of 29{\deg}, where the system crosses over between quasicrystalline and commensurate order. Atomic-resolution transmission electron microscopy directly reveals the coexistence of near-dodecagonal quasicrystalline symmetry and emerging moiré periodicity, indicating an intermediate, nonperiodic structural regime. Magnetotransport measurements uncover strong interlayer hybridization mediated by Umklapp scattering, manifested by magneto-intersubband oscillations and a highly unconventional Landau-level spectrum. Remarkably, the Landau-level degeneracy evolves from 4- to 12-fold with increasing temperature, a behavior incompatible with two decoupled graphene monolayers. These findings establish large-angle twisted bilayer graphene as a platform where quasiperiodic symmetry fundamentally reshapes low-energy electronic states beyond the conventional moiré framework.
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All authors Kuo-En Chang, Aitor Garcia-Ruiz, Ta-Lei Chou, Yen-Ting Liu, Sheng-Chin Ho, Yu-Chiang Hsieh, Ching-Hua Kao, Chiu-Hua Huang, Ying-Mei Yang, Kenji Watanabe, Takashi Taniguchi, Ming-Wen Chu, Ming-Hao Liu, Tse-Ming Chen

Mesoscale and Nanoscale Physics

Long-time fermionic quantum transport with controlled full-state error using an adaptive reservoir-mode window

Mikhail Umanskii, Nataliya Arefyeva, Georgy Sultanov et al. (+2 more)

Real-time simulations of interacting nanostructures coupled to fermionic reservoirs can require a growing number of environmental degrees of freedom to retain long-lived correlations. We introduce tape-recorder coarse graining, which reorganizes each noninteracting lead into incoming, active, and outgoing modes. The device is propagated with the active modes, while outgoing modes are stochastically sampled and removed once their remaining integrated coupling falls below a prescribed threshold. For each outgoing-mode truncation, we derive a nonperturbative upper bound on the infidelity between the exact and truncated full device-reservoir states over any prescribed finite interval. The bound depends on the mode's remaining coupling weight and finite-interval response factors. Numerically, the active-mode count saturates in time at fixed relative threshold and grows logarithmically as the threshold is reduced. We benchmark the method on a two-site quantum point contact at zero temperature and maximal bias. For Lorentzian reservoirs, the dynamics agrees with converged HEOM calculations and the steady-state current with the Landauer-Büttiker result. For flat-band reservoirs with algebraically decaying correlations, it agrees with direct Schrödinger evolution before finite-size recurrences and reproduces the Landauer-Büttiker stationary current, while finite exponential HEOM decompositions remain unconverged. For interacting contacts, the method yields Coulomb-blockade peak splitting. In the noninteracting driven limit, it agrees with an exact Floquet Green-function calculation and reproduces coherent current suppression under periodic driving, which persists at finite Coulomb repulsion. Together, these benchmarks show that tape-recorder coarse graining enables practical long-time simulations of the full device-reservoir state in interacting fermionic transport.
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All authors Mikhail Umanskii, Nataliya Arefyeva, Georgy Sultanov, Alexey Rubtsov, Evgeny Polyakov

Mesoscale and Nanoscale Physics

Dynamics of Majorana tetron qubits under quasiparticle poisoning

Sauri Bhattacharyya and Bernard van Heck

We study the dissipative dynamics of a Majorana tetron qubit in the presence of extrinsic quasiparticle poisoning due to the coupling to external leads. From the Bloch-Redfield equation describing a finite-size topological superconductor hosting four Majorana zero modes and tunnel-coupled to fermionic reservoirs, we recover analytical expressions for the decoherence rate of a Majorana qubit at arbitrary values of the charging energy. The analysis shows that the exponential suppression of the decoherence rate is gradually removed by the energy splitting of the qubit states. These results can be useful to understand time-domain experimental data in Majorana qubit prototypes.
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Mesoscale and Nanoscale Physics

The Deformed Image Vortex Ansatz: A Perturbation-Aware Description of Magnetic Vortices in In-Plane Fields

Thomas G. Coppée, Colin Ducarme, Simon de Wergifosse et al. (+1 more)

Thiele-based descriptions of magnetic vortex dynamics in thin ferromagnetic nanodots rely on magnetization ansätze that describe the equilibrium texture but cannot represent perturbation-induced deformations. We introduce the Deformed Image Vortex Ansatz (DIVA): a perturbation-aware ansatz in which the response to an external perturbation is built into the magnetization profile itself, rather than appended to the dynamics as a correction. Here, we demonstrate the concept for a uniform, stationary in-plane field applied to a Permalloy nanodot, for which the deformation is analytically tractable. A symmetry-based perturbative expansion identifies the leading deformation as a single $m = 1$ harmonic around the disk, while energy minimization and a dominant-balance analysis yield a closed-form interpolant for the radial profile. Benchmarked against micromagnetic simulations on Permalloy disks of aspect ratio $t/R = 0.1$ and $0.0125$, this realization reduces the disk-averaged angular deviation by a factor of 3 to 6 relative to the two-vortex ansatz, depending on geometry and field, and reduces the total-energy deviation by about a factor of six in the thicker disk.
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All authors Thomas G. Coppée, Colin Ducarme, Simon de Wergifosse, Flavio Abreu Araujo

Mesoscale and Nanoscale Physics

Observation of magnetic quantum phase crossovers in a semiconductor spin ladder

Elizaveta Morozova, Xin Zhang, Utso Bhattacharya et al. (+9 more)

Understanding collective phases of strongly correlated quantum magnets relies on theoretically tractable model systems with precise microscopic control. Antiferromagnetic spin ladders provide such a setting, hosting field-tunable gapped and gapless phases at half filling and unconventional pairing tendencies upon doping. Here, we realize a programmable Heisenberg spin ladder in a half-filled germanium quantum dot array featuring site-resolved, continuously tunable exchange interactions. Under a fixed magnetic field, we vary the rung and leg coupling to map the rung-singlet, canted antiferromagnetic, and fully polarized phases. Hamiltonian-learning protocols combining equilibrium and dynamical measurements quantitatively characterize the ladder, incorporating spin-orbit interactions to reproduce the observed crossover behavior. Measurements of higher-order spin correlators – including four-point correlations inaccessible to conventional bulk probes – reveal signatures of the underlying phase structure despite the finite size. Our results establish germanium quantum dot arrays as a controllable platform for quantum magnetism, opening routes to investigate unconventional superconductivity in doped ladders.
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All authors Elizaveta Morozova, Xin Zhang, Utso Bhattacharya, Pablo Cova Fariña, Daniel Jirovec, Alexander Nico-Katz, Stefan D. Oosterhout, Sougato Bose, Giordano Scappucci, Menno Veldhorst, Eugene Demler, Lieven M. K. Vandersypen

Mesoscale and Nanoscale Physics

Robust stabilization of high-frequency magnetic droplets in W-CoFeB-MgO nanoconstriction spin Hall nano-oscillators

Hind Prakash, Arunima TM, Roman Khymyn et al. (+1 more)

Magnetic droplets are highly nonlinear spin-wave solitons that can be excited in nanoscale spintronic devices with strong perpendicular magnetic anisotropy. Although extensively studied in nanocontact-based spin-torque oscillators, their stabilization in pure spin current-driven devices such as spin Hall nano-oscillators (SHNOs) has remained elusive. Here, we micromagnetically demonstrate the robust stabilization of non-propagating high-frequency droplets in CMOS-compatible W/CoFeB/MgO nanoconstriction SHNOs under oblique magnetic fields. While the constriction geometry gives rise to noncircular droplet shapes in an inhomogeneous effective field landscape, stable droplet modes exhibiting complete core magnetization reversal and pronounced hysteresis are observed. At lower current densities, droplets display breathing oscillations with periodic expansion and contraction, whereas higher drive currents lead to drift, deformation, and the emergence of sidebands around the fundamental frequency. Tuning the strength and orientation of the applied magnetic field alters the effective field landscape, allowing droplets to escape confinement and propagate over distances exceeding 2$\mu$m.
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All authors Hind Prakash, Arunima TM, Roman Khymyn, Himanshu Fulara