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Physics Condensed Matter Mesoscale and Nanoscale Physics

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

Friday 21 August

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

Coherence-Based Identification of Carbon-Based Spin Qubits in Hexagonal Boron Nitride from First Principles

Hyeonsu Kim, Jaewook Lee, Huijin Park et al. (+1 more)

Carbon-related defects in hexagonal boron nitride are promising room-temperature single-spin qubits and quantum sensors, but their atomic structures remain largely unidentified. Here we show, using first-principles calculations of electron-spin decoherence, that the atomic structure of each defect is imprinted in its spin coherence. Mapping the Hahn-echo dynamics of seven candidate carbon defects across magnetic field and four isotope-engineered nuclear-spin baths, we find that electron-spin-echo envelope modulation emerges at defect-specific magnetic fields, at which the nearest-neighbor nuclear spins satisfy a cancellation condition set by their hyperfine and quadrupole couplings. Both the fields and the modulation frequencies follow from an analytical model using computed hyperfine and quadrupole tensors alone, and they shift or vanish upon isotope substitution. At low fields, the field dependence of the coherence time separates the defects into two classes according to the sublattice occupied by carbon. These decoherence fingerprints, directly testable in isotope-engineered samples, establish a structural identification route complementary to optical spectroscopy.
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All authors Hyeonsu Kim, Jaewook Lee, Huijin Park, Hosung Seo

Mesoscale and Nanoscale Physics

Phase control of magnon-phonon coupling via magnetic field

Yasuhiro Todaka, Motoki Asano, Isamu Yasuda et al. (+3 more)

We study the phase of the coupling between magnons and surface acoustic wave (SAW) phonons in magnetic thin films. The coupling constant changes from a real to a complex number as the external magnetic field is reduced. Below a transition field, the imaginary coupling constant allows SAW phonons to couple to overdamped magnons whose resonance frequency is close to zero and far from the SAW resonance. The strength of the imaginary coupling constant and the magnitude of the transition field both scale with magnetic damping. We find the coupling produces a broad, pronounced minimum in the SAW transmittance spectrum near zero magnetic field in a Ni/Ru/Ni synthetic antiferromagnet with large magnetic damping. These results demonstrate that the phase of the complex magnon-phonon coupling constant can be tuned via magnetic field in strongly damped magnets, offering a platform to explore novel regimes of magnon-phonon interactions.
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All authors Yasuhiro Todaka, Motoki Asano, Isamu Yasuda, Masashi Kawaguchi, Daiki Hatanaka, Masamitsu Hayashi

Mesoscale and Nanoscale Physics

Planar Nanofluidic Memristors Enabled by Surface Charge Gradient

Zhongyuan Zhao, Ziyi Yin, Chudi Qi et al. (+4 more)

Nanofluidic memristors, exploiting ion transport in nanochannels, hold promise for neuromorphic applications. A planar architecture is particularly desired for scalable integration with established micro- and nanofabrication technologies. Here, using the Poisson-Nernst-Planck framework, we theoretically propose planar nanofluidic memristors enabled by surface charge gradient, providing an alternative to the commonly used geometrically asymmetric architectures. The resulting memristive behavior is governed by a diffusion-mediated secondary enrichment effect. By systematically solving the PNP equations, we obtain the scaling of the characteristic memory time across the parameter space. We also reveal that the memory effect is related to the first-order moment of surface charge, for arbitrary charge profiles. These results provide a theoretical basis for rationally designing and optimizing planar nanofluidic memristors through spatially patterned surface charge.
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All authors Zhongyuan Zhao, Ziyi Yin, Chudi Qi, Yuheng Li, Shoushan Fan, Qunqing Li, Yang Wei

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

Spectral Anisotropy in Transition Radiation from Biaxial Media

Sven Ebel, Bibi Mary Francis, Min Seok Jang et al. (+4 more)

In anisotropic optical media, the electromagnetic response depends on the orientation of the optical field relative to the material's principal dielectric axes. While this direction dependence is well understood in conventional optics, it should also influence light-generation processes driven by free electrons. Here, we experimentally observe spectrally anisotropic transition radiation from biaxial van der Waals crystals. Using cathodoluminescence spectroscopy on germanium sulphide (GeS) and molybdenum oxydichloride (MoOCl$_2$) crystals, we show that the transition-radiation spectra differ along the principal in-plane optical axes. To describe this spectral anisotropy, we develop a thin-film transition-radiation model that reproduces the experimental observations. Our results demonstrate that transition radiation is a sensitive probe of the axis-dependent dielectric response of biaxial optical media and suggest that optical anisotropy can provide an additional degree of freedom for free electron-driven spectroscopy, radiation sources, and transition-radiation-based diagnostics.
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All authors Sven Ebel, Bibi Mary Francis, Min Seok Jang, Brian D. Gerardot, N. Asger Mortensen, Mauro Brotons-Gisbert, Sergii Morozov

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

Restoring heat and particle flow in strongly coupled non-equilibrium devices

Noa Ludwin, Ohad Cremerman, Milan Šindelka et al. (+1 more)

Under non-equilibrium conditions, energy/particle currents flow through a quantum system coupled to multiple baths. Although the fluxes increase in the weak coupling regime as the coupling strengthens, the reason why they decrease to zero for large coupling remains unknown. This counterintuitive behavior of energy exchange or transport phenomena is called the turnover effect. It has been predicted in several quantum systems such as photosynthetic complexes, mesoscopic junctions, quantum heat machines, and chemical networks without a single counterexample, and it results in constrained performance due to limited currents. Here, we use scattering theory to study the turnover effect produced by low-zdensity reservoirs of free particles scattered by a quantum system through localized potentials. We find that the turnover effect is a consequence of total reflection that impedes the reservoir particle from reaching the interaction region and exchanging energy with the quantum system. Moreover, we design a protocol based on quantum tunneling to avoid the turnover and to increase the maximum current. This strategy could be used to unleash the full potential of devices based on temperature/chemical potential gradients.
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All authors Noa Ludwin, Ohad Cremerman, Milan Šindelka, David Gelbwaser-Klimovsky

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

Orbital-Selective Spin Splitting in the Altermagnetic Ti$_2$XX' Monolayers

Xin Zhang and Shihao Zhang

Altermagnets have attracted intensive attention recently, owing to their unique combination of zero net magnetic moment of antiferromagnets and momentum space spin-splitting of ferromagnets. Herein, we systematically explore monolayer altermagnetic Ti$_2$XX' (X/X' = F, Cl, Br, I) materials for novel electronic properties. Especifically, spin-splitting exists in the valence bands of all studied materials, whereas several compositions exhibit nearly spin-degenerate conduction bands near the Fermi level, which is markedly different from conventional altermagnets. This phenomenon can be attributed to the bond-angle-sensitive super-exchange occurring in the $d_{xz}$ or $d_{yz}$ orbitals, while the super-exchange of the $d_{x^2-y^2}$ orbital is not sensitive to the Ti-X-Ti bond angle. Motivated by this intriguing phenomenon, Fermi level modulation via doping or heterostructure construction enables the transition between two electronic states, endowing these materials great application potential in information storage devices.
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Thursday 20 August

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

Nonlinear Diamagnetic Interactions in Ultrastrongly Coupled 2D Electrons

Dasom Kim, Kiran M. Kulkarni, Vaibhav Sharma et al. (+15 more)

The quantum Hopfield model is widely used to describe ultrastrong light–matter coupling between cavity photons and collective bosonic excitations in solids, where the diamagnetic interaction is conventionally assumed to be a constant. We experimentally demonstrate that the diamagnetic response of Landau polaritons is reduced under strong terahertz field excitation. We show that this behavior originates from field-driven redistribution of electrons into the nonparabolic regime of the conduction band of GaAs, which reduces the plasma frequency and consequently the diamagnetic interaction strength. A microscopic hot-electron model reproduces the observed nonlinear response. Motivated by this microscopic picture, we propose a nonlinear extension of the Hopfield model with a Kerr-like interaction. Our results establish a route toward nonlinear cavity quantum electrodynamics and driven ultrastrong light–matter coupling beyond the conventional linear Hopfield description, which is capable of creating uniquely quantum optical effects such as squeezed light generation.
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All authors Dasom Kim, Kiran M. Kulkarni, Vaibhav Sharma, Dukhyung Lee, Geon Lee, Sunghwan Kim, Jonas Grumm, Shuang Liang, Hongjing Xu, Fuyang Tay, Andrey Baydin, Motoaki Bamba, Andreas Knorr, Christopher J. Stanton, Michael J. Manfra, Minah Seo, Stephen Hughes, Junichiro Kono

Mesoscale and Nanoscale Physics

Interband plasmons due to Mexican hat dispersion in two-dimensional materials with inverted bands

Vladimir A. Sablikov

Two-dimensional topological materials with Mexican-hat dispersion exhibit numerous nontrivial features, such as double-valued isoenergetic contours in k-space, strong mixing of electron and hole states, a van Hove singularity in the density of states, and specific quantum geometric properties. We find that, due to these features, Mexican-hat dispersion significantly strengthens interband plasmons, reduces their damping, and gives rise to an additional branch of the plasmon spectrum when electron-hole symmetry is broken. The plasmon spectra are limited to a finite range of the wave vector, the boundaries of which depend on the strength of the electron-electron interaction. We also reveal a relationship between the features of the plasmon spectrum and singularities of the joint density of states with a finite plasmon wave vector q, as well as features of the interband quantum metric, which also depend on q.
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Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.mtrl-sci

Interplay of nonrelativistic and relativistic spin splittings in altermagnets

Daegeun Jo, Peter M. Oppeneer and Mohsen Yarmohammadi

Despite their compensated magnetic moments, altermagnets (AMs) exhibit nonrelativistic spin splitting (NRSS) that offers routes to spintronic functionality without relying on relativistic spin-orbit coupling. However, how NRSS influences relativistic phenomena remains largely unexplored. Here we show that NRSS plays a crucial role in reshaping the Rashba effect in AMs with broken inversion symmetry. Using a tight-binding model, we demonstrate that Rashba spin splitting, whose magnitude is typically limited by the strength of spin-orbit coupling, is governed by NRSS energy scales. The resulting Rashba bands combined with NRSS generate anomalously large charge-to-spin conversion with a Néel-vector-tunable spin polarization. First-principles calculations for the noncentrosymmetric AM GdAlSi corroborate the emergence of these effects in a real system. Our results reveal the interplay between nonrelativistic and relativistic effects in AMs and identify them as fertile platforms for spin-orbitronic applications.
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Mesoscale and Nanoscale Physics

$\mathcal{PT}$ and anti-$\mathcal{PT}$ phase transitions in a trimerized Su–Schrieffer–Heeger chain with nonreciprocal Rashba spin-orbit coupling

Milad Jangjan, Linhu Li, Longwen Zhou et al. (+1 more)

We theoretically investigate a one-dimensional trimerized Su–Schrieffer–Heeger chain with three sublattices per unit cell subjected to a nonreciprocal Rashba spin-orbit coupling. Invoking a spin-flip symmetry, the non-Hermitian Hamiltonian decomposes into two independent spin sectors, enabling a spin-resolved analysis of non-Hermitian skin effects and system symmetries. We identify a rich phase diagram consisting of four bulk phases and two edge-state phases. The bulk phases include fully $\mathcal{PT}$-unbroken (real-spectrum) and fully anti-$\mathcal{PT}$-unbroken (imaginary-spectrum) regimes, as well as two mixed phases where one band remains on the real or imaginary axis while the other two form complex-conjugate pairs. The two edge-state phases correspond to topological edge modes with either $\mathcal{PT}$-unbroken (real) or anti-$\mathcal{PT}$-unbroken (imaginary) energies. Using non-Bloch band theory and Cardano's method, we derive closed-form expressions for phase boundaries and establish the bulk-edge correspondence for each spin sector. Calculations of Berry phase and directional inverse participation ratios confirm our analytical predictions. Our results provide a minimal platform for realizing spin-resolved non-Hermitian topology and edge-selective symmetry preservation, in which the bulk and edge states belong to distinct symmetry classes.
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All authors Milad Jangjan, Linhu Li, Longwen Zhou, Mir Vahid Hosseini

Mesoscale and Nanoscale Physics

Quantum critical behavior in chains of hindered dipolar planar rotors

Wenxue Zhang, Muhammad Shaeer Moeed, Estevao De Oliveira et al. (+2 more)

We study the ground-state properties of linear chains of dipolar planar rotors hindered by a six-fold on-site potential, a model motivated by water molecules confined in the hexagonal cavities of beryl. Using density matrix renormalization group (DMRG) calculations, we locate the quantum phase transition between the disordered and ferroelectrically ordered phases using the von Neumann entanglement entropy and the Binder ratio of the polarization. A sweep of the six-fold pinning strength shows that increasing hindrance shifts the critical dipolar coupling gc to smaller values. These results suggest that crystal-field hindrance can promote, rather than suppress, dipolar ordering. This work has implications for ferroelectricity and quantum-device tuning in confined molecular rotors
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All authors Wenxue Zhang, Muhammad Shaeer Moeed, Estevao De Oliveira, Hui Li, Pierre-Nicholas Roy

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

Spin-torque microwave detectors of positive rectangular pulse signals

V. Prokopenko, O. Shtanko, I. Sotnyk et al. (+1 more)

We analyze the performance of a spin-torque microwave detector (STMD) driven by positive rectangular current pulses $I(t)$ of various amplitudes $I_0$, durations $\tau$, and repetition periods $T$ and reveal two distinct regimes of STMD operation. In the first (linear) regime, the time-averaged voltage across the detector, $U_{\rm dc}$, changes linearly with the pulse amplitude $I_0$ and depends on the ratio $\tau/T$: $U_{\rm dc} \sim I_0 (\tau/T)$. This regime is observed for a wide range of pulse amplitudes $I_0$ in the case of in-plane (IP) magnetization dynamics and for rather small pulse amplitudes $I_0 \le I_{\rm th}$ in an STMD with out-of-plane (OOP) magnetization dynamics. The other (nonlinear) regime is characterized by voltage jumps and drops and is observed only in a structure with OOP magnetization dynamics for input pulses with short repetition periods and large amplitudes $I_0 \ge I_{\rm th}$. We believe that the linear regime of STMD operation can be used to unambiguously detect input pulse parameters, which could be important for the development and optimization of spintronic devices capable of detecting and processing non-harmonic (e.g., digital) microwave signals.
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All authors V. Prokopenko, O. Shtanko, I. Sotnyk, O. Prokopenko

Mesoscale and Nanoscale Physics

Signatures of a light-induced exciton condensate exhibiting BEC-BCS crossover

Khanh Duy Nguyen, Gabriele Berruto, Yunhe Bai et al. (+7 more)

Exciton condensates provide a platform to study quasiparticle pairing, Bose-Einstein condensation-Bardeen-Cooper-Schrieffer (BEC-BCS) crossover, and excitonic topological phenomena. Achieving a nonequilibrium exciton condensate allows the ultimate tunability of these emergent phenomena. Yet, evidence of a light-induced, nonequilibrium exciton condensate and its BEC-BCS crossover remains elusive. Here, we use time- and angle-resolved photoemission spectroscopy to demonstrate signatures of a non-equilibrium exciton condensate and its BEC-BCS crossover in monolayer MnBi2Te4. Following optical excitation, a distinctive hole-like dispersion representing excitons emerges and persists for >20 ps. Strikingly, energy-domain sharpening in the valence band occurs 2 ps after time zero and exhibits a sharp onset at a threshold pump fluence of 0.84 mJ/cm2. The delayed and strongly nonlinear response is difficult to reconcile with transient field effects or conventional carrier-induced band shifts but is consistent with a model of exciton condensation governed by a Berezinskii-Kosterlitz-Thouless transition. The estimated threshold exciton density agrees quantitatively with the Nelson-Kosterlitz critical density. At higher fluences, the exciton feature develops a camel-back-shaped dispersion, consistent with the BEC-BCS crossover in the condensate framework. Our work establishes ultrathin MnBi2Te4 as a model system for studying nonequilibrium exciton condensates with a connection to superconductivity and exciton-driven topological phases.
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All authors Khanh Duy Nguyen, Gabriele Berruto, Yunhe Bai, Thomas Marchese, Woojoo Lee, Haoran Lin, Jiangang Yang, Chong Liu, Y. Shirley Meng, Shuolong Yang

Mesoscale and Nanoscale Physics

Coulomb blockade-like transport and enhanced memory in organic transistors embedded with sub-nm Pt nanoparticles for neuromorphic computing

Arash Ghobadi, Thomas B. Kallaos, Abhi Abhijeet et al. (+5 more)

Organic transistors are playing an increasingly important role for neuromorphic applications. However, devices that rely solely on ferroelectric switching or on interface traps for their multi-conductance states exhibit limited memory windows. Here, we introduce an ultrathin oxide layer with a uniform distribution of sub-nm platinum nanoparticles (PtNPs) at the interface of a polymer semiconducting and a ferroelectric dielectric in a thin film transistor architecture. The interfacial stack, Al$_2$O$_3$/PtNP/Al$_2$O$_3$, provides a viable route for localized charge trapping and de-trapping in a region where it can most effectively influence the channel conductance. The organic transistors display a large memory window (> 20 V) in their current-voltage characteristics. The sub-nm PtNPs give rise to features that are consistent with room temperature Coulomb blockade-like transport, supporting discrete and well-separated levels within the memory window. The devices support multimodal programming using electrical and optical stimuli with both long-term plasticity and enhanced short-term plasticity (STP) phenomena. These results open new directions for implementing STP in the development of neuromorphic computing.
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All authors Arash Ghobadi, Thomas B. Kallaos, Abhi Abhijeet, Stephen C. Klue, Joseph C. Mathai, Carsten A. Ullrich, Shubhra Gangopadhyay, Suchismita Guha

Mesoscale and Nanoscale Physics

Floquet Theory for Light-Driven Rotation of Dipolar and Multipolar Particles

Amane Takano, Minoru Kanega and Masahiro Sato

Nano- or micro-particle rotation driven by light has been well known in the fields of optical manipulation and optical physics since the end of the last century. It is viewed as a sort of angular-momentum transfer from light to material, but its microscopic analysis based on the Hamiltonian or the equation of motion has been less developed. We model this rotation with a simple setup of an electrically dipolar or multipolar particle irradiated by circularly polarized laser and comprehensively analyze the Langevin-type equation of motion by using the Floquet theory for dissipative classical systems and the mode separation method. Furthermore, we numerically compute the time evolution of the particle. As a result, we accurately estimate the dependence of the laser-frequency, laser-intensity, particle mass, temperature, and friction (dissipation) on the laser-driven rotation. We determine the “nonequilibrium phase diagram” of the laser-driven rotation in a broad parameter regime, which consists of three regimes: the rotation frequency $\Omega\propto\omega^{-1}$, $\Omega\propto\omega^{-3}$, or $\Omega=\omega$ ($\omega$ is the laser frequency). Comparing our theoretical result with some experiments, we show that the result of the overdamped Langevin equation is qualitatively consistent with the experiments.
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Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.mtrl-sci

Effects of the surface on double surface Fermi arcs in a realistic tight-binding model of Na$_3$Bi (100)

Vasilios K. Passias and Lucas K. Wagner

Na$_3$Bi is a topological Dirac semimetal (TDSM) that can support double surface Fermi arcs (DSFAs), which are important for its material classification and their potential use in spintronic devices. However, it is unclear how the surfaces affect realistic Na$_3$Bi (100) systems. To investigate the effects of diverse surfaces on DSFAs, we consider first principles derived tight-binding models of two Na$_3$Bi (100) terminations. On the stoichiometric termination, we find DSFAs as expected. On the non-stoichiometric structure there are DSFAs on one termination and two loops that retain many of the properties of DSFAs. Thus, the local properties such as spin momentum locking and hybridization with the Dirac points appear to be more robust than their global connectivity. That is, the surface can reshape the arc fingerprint without destroying the arc-like physics.
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Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.supr-con

Josephson transport in YBa${}_{2}$Cu${}_{3}$O${}_{7}$ weak links created by focused-helium-ion-beam irradiation: Analysis based on diffusive-SNS-junction model

Tetsuro Misawa, Shigeyuki Ishida, Hiroshi Eisaki et al. (+3 more)

Fabrication of YBCO weak links by focused helium ion beam irradiation is a promising approach for realizing high-temperature superconducting Josephson junction devices. Although empirical dose-characteristic relationships have been established, the underlying transport mechanisms remain unclear. In this study, we perform a detailed investigation of the transport properties of YBCO weak links fabricated using a helium ion microscope (HIM) and provide a unified phenomenological description of the observed behavior based on the theory of SNS junctions with a diffusive metallic interlayer. We demonstrate that the temperature dependence of the critical current $I_{\mathrm{c}}$ and the $I_{\mathrm{c}}R_{\mathrm{n}}$ product are well described by diffusive SNS junction models over a wide temperature range. Analyses show that the observed dose dependences of $I_{\mathrm{c}}$ and $I_{\mathrm{c}}R_{\mathrm{n}}$ cannot be explained solely by variations in the effective Thouless energy $E_{\mathrm{T}}$. The discrepancy suggests reduced interface transparency and a reduction in the density of states, leading to a decrease in the effective number of conducting channels contributing to transport. This interpretation is also consistent with the observed exponential increase in $R_{\mathrm{n}}$ with irradiation dose. These results provide a diffusion-based framework for understanding Josephson transport and guiding junction design in helium-ion-irradiated YBCO weak links.
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All authors Tetsuro Misawa, Shigeyuki Ishida, Hiroshi Eisaki, Yukinori Morita, Shinichi Ogawa, Chiharu Urano

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

Amorphous and Nanocrystalline Topological Semimetal YPtBi/W/CoFeB Heterostructures for BEOL-Compatible Spin-Orbit Torque Devices

Quang Le, Brian R. York, Cherngye Hwang et al. (+17 more)

Spin-orbit torque (SOT) devices require spin-source materials that combine efficient charge-to-spin conversion with back-end-of-line (BEOL) thermal compatibility. Here, we show that YPtBi/W/CoFeB heterostructures deposited directly on Si/SiOx remain predominantly amorphous or weakly nanocrystalline from room temperature to 400 {\deg}C while preserving a large effective damping-like SOT response. Anomalous Hall and harmonic Hall measurements, together with X-ray diffraction, cross-sectional transmission electron microscopy, X-ray reflectivity, and electron energy-loss spectroscopy, show that the response does not correlate with bulk crystallization of YPtBi. Instead, the interfacial analysis indicates that the strongest trend of the spin Hall angle is associated with the chemistry of the upper YPtBi/W boundary: the effective SOT response tracks the integrated W concentration at that YPtBi surface. Meanwhile, a two-spin source analysis shows that the Pt-W-rich interlayer provides only a small positive correction, insufficient to explain the large negative effective spin Hall angle by itself. The dominant control variable is therefore inferred to be the incorporation of W into the upper YPtBi interface, which plausibly modifies the local electronic structure of YPtBi and amplifies the stack-level response. These results provide a more physically constrained interpretation of the stack behavior and identify a BEOL-compatible route to disordered topological spin-source layers for scaled SOT memory and compute-in-memory hardware.
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All authors Quang Le, Brian R. York, Cherngye Hwang, Xiaoyong Liu, Tsann Lin, Xiaoyu Xu, Yudi Wang, Jia Li, Mazin Osman, Katherine Le, Maher Osman, Son Le, Lei Xu, Maki Maeda, Tuo Fan, Yu Tao, Hisashi Takano, Sho Kagami, Ohiro Fujie, Pham Nam Hai

Mesoscale and Nanoscale Physics

Search for Majorana Bound States in Short Chains of Proxmitised Quantum Dots

Bogdan R. Bułka and Karol I. Wysokiński

Majorana zero modes (MZM) appearing at the ends of artificially created one-dimensional p-wave superconductors have been intensively studied recently both theoretically and experimentally. Among possible platforms, proximitised semiconducting wires, and short chains of quantum dots with a superconductor in between were investigated. Here, we propose a different platform consisting of a chain of quantum dots (QDs) sandwiched between an s-wave superconductor and a strong spin-orbit semiconductor, subject to a Zeeman magnetic field. Neglecting spin-conserving hopping processes between QDs and local on-dot superconducting correlations induced by the superconducting proximity effect, reduces the Hamiltonian to the sum of two equivalent Hamiltonians with two independent Hilbert spaces. The resulting model has a staggered structure due to spin-flipping processes $t_{so}$ and cross-Andreev reflections $\Delta_{CAR}$ between neighbouring dots. Our central result is the phase diagram of a short chain consisting of four QDs and coupled to two external reservoirs, obtained by means of the Green function in chiral Majorana representation. The modulus of the retarded Green function, probing the whole chain and calculated for zero energy, is shown to contain information on topology and spatial character of Majorana zero modes. The features observed in the Green function nicely agree with those obtained from the transfer matrix approach. In particular the region in parameter space in which Majorana zero modes display oscillatory wave functions are well reproduced. Likewise the borders of the fermion parity changes obtained by the Green function agree with those obtained by other means.
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Mesoscale and Nanoscale Physics · Cross-listed from quant-ph

Gradient flow towards quantum states with ideal quantum geometry

T. Shiga and T. Fukui

We propose a gradient-flow method for quantum states in lattice models, generated by an action consisting of the quantum-metric and the square of the Berry curvature. These two terms drive the spectral projector toward Bogomolny saturation and uniform Berry curvature, respectively. We show that, due to a no-go theorem for finite-dimensional projectors, the two conditions cannot in general be satisfied simultaneously in lattice models. Hence the flow is expected to approach a nontrivial fixed point that balances the two geometric requirements. For the Wilson-Dirac model, we demonstrate that the flowed projector exhibits almost uniform Berry curvature while remaining close to the Bogomolny bound. We further construct a short-range truncated flattened Hamiltonian from the flowed projector and obtain a lattice model with nearly flat bands and nearly uniform Berry curvature. We also apply the method to the Hofstadter model and confirm the roles of the metric and Berry-curvature terms in a Chern band with higher Chern number.
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Mesoscale and Nanoscale Physics · Cross-listed from quant-ph

Krylov Tomography and Finite-Uncertainty Certification of Exceptional-Point Dynamics

Aritra Ghosh and M. Bhattacharya

Exceptional points (EPs) can produce striking responses, but locating one does not reveal how much of its dynamics an excitation accesses, which responses a detector distinguishes, or whether a missing signal is absent or undetected. We introduce Krylov tomography, a preparation- and measurement-aware framework that uses time-resolved data and models with stated uncertainty limits to answer these questions, while also bounding offset-induced departures from exact-EP behavior. As an explicit illustration of the general framework, we present finite-precision simulations of a red-sideband optomechanical model, whose second-moment coherence sector contains a third-order EP, certifying preparation-sensitive access to two and three directions. Krylov tomography thus bridges EP structure and finite-precision measurements, providing a general framework for non-Hermitian systems.
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Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.supr-con

Internal cyclotron probe of the vertical polarizability of a surface-state electron on helium

Kirill Shulga

We study a surface-state electron on liquid helium in a tilted magnetic field. The tilt couples the cyclotron ladder to the vertical Rydberg-like subbands of the same electron and thereby realizes a self-coupled Landau-subband Hamiltonian. In the far-detuned regime, the cyclotron line acquires a state-dependent shift described by standard second-order perturbation theory. The resulting expression identifies the cyclotron resonance as a direct probe of the dynamical polarizability of the vertical ladder. The theory is intrinsically multilevel. Exact diagonalization confirms a broad microwave window in which sizable state-dependent cyclotron shifts are expected while the system remains dispersive.
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Mesoscale and Nanoscale Physics

Coherence protection of a silicon hole spin qubit with phase-modulated microwave driving

Sayyid I. Ibad, Yusuke Sato, Takuma Kuno et al. (+8 more)

Hole spins in silicon quantum dots are a promising platform for quantum computing due to their strong intrinsic spin-orbit coupling (SOC), which enables fast, all-electrical control. However, this coupling also increases their susceptibility to charge noise, thereby limiting coherence times. Moreover, holes in silicon are also affected by hyperfine interactions with residual nuclear spins in the silicon substrate, introducing a non-negligible source of low-frequency noise. Here, we implement a phase-modulated concatenated continuous driving (CCD) technique for hole spin qubits to suppress low-frequency noise through microwave phase modulation. This approach stabilizes Rabi oscillations and extends the oscillation decay time compared to the conventional method. Furthermore, by defining a qubit in the CCD frame, we achieve coherent control while simultaneously protecting the qubit from noise, confirming coherence protection during gate operations. These results demonstrate a viable route toward noise-robust hole spin qubits.
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All authors Sayyid I. Ibad, Yusuke Sato, Takuma Kuno, Itaru Yanagi, Toshiyuki Mine, Ryuta Tsuchiya, Digh Hisamoto, Hiroyuki Mizuno, Raisei Mizokuchi, Jun Yoneda, Tetsuo Kodera

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

Cavity-Enhanced Activation of Radiatively Suppressed Light-Hole Exciton Emission in Colloidal Nanoplatelets

Komal Sharma, Riya Dutta, Prathmesh Deshmukh et al. (+2 more)

Light-hole (LH) excitons provide access to well-defined polarization and spin degrees of freedom that are central to quantum photonics and chiral light-matter interactions. Achieving LH emission is challenging because LH states are energetically unfavoured and typically relax non-radiatively. Existing strategies to access LH excitons rely on modifying the electronic band structure through strain, shape anisotropy, or piezoelectric fields, approaches that are material-specific and offer limited post-synthesis tunability. Here we demonstrate an all-photonic route to activate LH exciton emission in colloidal CdSe-CdS nanoplatelets (NPLs) using a distributed Bragg reflector (DBR) cavity, without altering the underlying band structure. In the absence of a cavity mode, the system exhibits amplified spontaneous emission from heavy-hole (HH) states without detectable LH emission at low excitation powers. By spectrally matching a cavity resonance to the LH exciton, cavity-coupled LH emission emerges at significantly lower excitation powers. Temperature-dependent spectroscopy reveals reversible switching between LH- and HH-coupled emission through exciton-cavity detuning, while polarization-resolved and spectrally resolved time-resolved photoluminescence measurements provide independent evidence distinguishing the cavity-coupled LH and HH emission channels. These findings establish cavity engineering as a general materials-level approach for accessing radiatively suppressed optical states.
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All authors Komal Sharma, Riya Dutta, Prathmesh Deshmukh, Vinod M.Menon, Jaydeep K. Basu

Mesoscale and Nanoscale Physics

Giant orbital Hall effect from cubic Dresselhaus orbital coupling

Gwen Sevilen and Kyoung-Min Kim

The orbital Berry curvature (OBC) governs the intrinsic orbital Hall conductivity (OHC), a central quantity in orbitronics. Previous approaches for enhancing the OHC have primarily relied on a linearin-momentum, Rashba-type orbital coupling. Here, we show that cubic Dresselhaus orbital coupling offers a new route to enhancing the OHC. Using an effective two-orbital band model, we show that the cubic coupling generates momentum-space hot spots, absent in the purely linear case, at which the OBC is strongly enhanced. This local enhancement, together with the multiplicity of the hot spots, boosts the OHC by more than an order of magnitude relative to the linear-coupling value. We further find that the OHC diverges inversely with the level splitting in the small-splitting limit, with a divergence coefficient universally seventeen times larger than that of the linear case. These results establish cubic Dresselhaus coupling as a route to giant orbital Hall responses, opening new avenues for orbitronic device applications.
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Mesoscale and Nanoscale Physics

Twisted magnon frequency combs in ferromagnetic nanorings

Liu Xuejuan, Zheng Xingen, Li Zhengyi et al. (+5 more)

We systematically investigate the emergence of twisted magnon frequency combs (tMFCs) and their higher-order modes arising from strong nonlinear coupling between vortex-core gyration and azimuthal spin-wave modes in ferromagnetic nanorings. The comb spacing is set by the gyrotropic frequency, which is controlled by both the size of the central hole and external magnetic fields. Remarkably, for the larger hole diameter (50 nm), an additional magnon mode emerges, leading to additional tMFC families. We also demonstrate that the selection rules still hold for different nanorings. In addition, the external in-plane magnetic field provides an effective means to tune the tMFC, while the response strongly depends on the nanostructure geometry. The nanodisk shows an approximately symmetric response under field reversal, whereas the response of nanorings depends strongly on the size of the central hole. For a small hole diameter (5 nm), the low-field response becomes asymmetric, and the tMFC spacing increases with field magnitude over the higher field branches. A larger hole diameter (50 nm) raises the gyrotropic frequency, yielding a sparser sideband structure near the drive frequency. Our results show that ferromagnetic nanorings support geometrically and magnetically tunable tMFCs.
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All authors Liu Xuejuan, Zheng Xingen, Li Zhengyi, Zhang Zhizhi, Li Xiaoguang, Sun Hai-Peng, Li Hui, Zhou Cangtao

Wednesday 19 August

Mesoscale and Nanoscale Physics

Electronic and chemical phase identification in photoemission experiments using unsupervised machine learning

Matthew Staab, Joseph Pandur, Eli Rotenberg et al. (+3 more)

Vacuum ultraviolet photoemission spectroscopies are very information-rich experiments, but due to their surface sensitivity, data are often collected on an initially uncharacterized surface. Traditional raster-grid approaches for locating optimal measurement regions can be time-consuming. In this work, we introduce AARDVARK, a generalizable framework for sample exploration that leverages dimensionality reduction and Gaussian process regression to guide initial sample searches in spatially-resolved photoemission experiments. By utilizing UMAP as a target for a Gaussian process, the algorithm efficiently identifies boundaries of spectroscopically distinct regions, dynamically adapting to variations in sample characteristics. The algorithm enables real-time decision making in measurement selection, optimizes data acquisition, and presents a robust framework for future autonomous sample exploration in photoemission experiments.
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All authors Matthew Staab, Joseph Pandur, Eli Rotenberg, Chris Jozwiak, Aaron Bostwick, Inna Vishik

Mesoscale and Nanoscale Physics

Altermagnetic memcapacitors

Martin Latorre and Alvaro S. Nunez

We propose a spintronic memcapacitance effect based upon altermagnetic multiferroic materials. We identify the rare-earth vanadates RVO$_3$ as a concrete platform, with all key parameters tied to measured properties. Under an oscillating electric field, the resulting charge and spin currents trace pinched hysteresis loops that close tangentially at zero field – the hallmark of memcapacitive, type-2 memdevice behavior – with charge current densities exceeding, by a factor of about 3.6, the lowest deterministic switching current density reported for optimized spin-transfer-torque magnetic tunnel junctions. We model the system theoretically as a dimerized two-orbital $d$-wave altermagnetic lattice via a Su–Schrieffer–Heeger-type bond modulation, in the spirit of the spin-dependent Rice–Mele model, thereby coupling the altermagnetic order to field-switchable charge and spin polarizations. The associated polarization loops close tangentially at zero field and yield a sign-changing, history-dependent “butterfly” differential capacitance, identifying the device as a genuine memcapacitor. Both responses are protected by the same inversion symmetry, so charge and spin channels switch simultaneously with no separate control needed. These results establish altermagnetic multiferroics, realized concretely in RVO$_3$, as an efficient, non-volatile platform for combined electric and spintronic memory.
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Mesoscale and Nanoscale Physics · Cross-listed from quant-ph

Rethinking Quantum Circuits

Steven Rayan

These notes develop four interconnected ways of reading a quantum circuit. A circuit for us begins as an operational composition of gates; then, it becomes a diagram whose local equalities may be used as calculations; next, it becomes a protected process once errors, syndromes, and logical degrees of freedom are separated; and finally, it becomes geometric when its connectivity, topology, and boundary data are treated as physical design parameters. The development begins at the level of bits and qubits before appealing to Deutsch's and Grover's algorithms as basic examples of quantum circuits. With the basics in hand, we interpret quantum circuits diagramatically, leading us to compact closed string diagrams and the ZX-calculus. After that, we consider how to correct quantum circuits by introducing the Knill–Laflamme condition, homological surface codes, and related concepts with a view towards thinking of these as operations on diagrams. The lectures eventually arrive at the properties of hyperbolic quantum codes and the prospect of physical superconducting circuits emulating the negatively-curved lattices needed to support those codes. These mathematical ideas and physical experiments, taken together, represent one way to impart a geometric layer onto quantum circuits. By the very end, we bring the ideas nearly full circle by assessing the extent to which these device physics experiments operationalize the basic ZX diagrams encountered much earlier in the story. While the later material reports on original research, and while the discussion becomes increasingly mathematical as the sections progress, no prior knowledge of quantum information, quantum computing, or quantum error correction is actually assumed.
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Mesoscale and Nanoscale Physics

Channel-selective magnetic filtering in a nodal-line semimetal

Hironmoy Pratihar, Alessandro De Martino and Arijit Kundu

We study quantum transport through a magnetic barrier in a nodal-line semimetal. When the Fermi energy lies near the nodal ring, the Fermi surface has toroidal geometry. Each cross-section in a plane parallel to the nodal ring consists of two concentric contours, inner and outer, carrying distinct transport channels. We show that a magnetic barrier resolves these two channels: because the contours enclose different momentum-space areas, they accommodate the field-induced transverse-momentum shift unequally, and the inner channel is cut off at a weaker barrier strength than the outer. Using a two-band effective Hamiltonian and a wave-function matching approach, we obtain closed-form, channel-resolved transmission amplitudes. Over a finite window of barrier strength the inner contour is fully blocked while the outer still transmits, so the barrier acts as a channel-selective filter. This sequential quenching shapes the two-terminal conductance, which decreases with barrier strength as the two channels close in turn and terminates once the outer channel is cut off, providing experimentally accessible fingerprints of the toroidal Fermi surface of a nodal-line semimetal.
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