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Physics Condensed Matter Mesoscale and Nanoscale Physics

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

Tuesday 15 September

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

Initial-Slip Dynamics Enables the Quantum Mpemba Effect

M. Crotti, F. Cavaliere, D. Ferraro et al. (+2 more)

We investigate the quantum Mpemba effect, whereby a system initially farther from equilibrium relaxes faster than one initially closer to equilibrium, emerging in a quantum harmonic oscillator linearly coupled to a bosonic environment. Starting from factorized thermal states at different temperatures, we derive exact analytical criteria for the occurrence of the quantum Mpemba effect valid also beyond the typically considered weak-coupling and Markovian approximations. Most importantly, we show that the effect is enabled by the initial-slip dynamics generated by the sudden switch-on of the system-bath interaction. Neglecting this contribution completely suppresses the anomalous relaxation, identifying transient system-bath correlations as the key ingredient underlying the quantum Mpemba effect.
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All authors M. Crotti, F. Cavaliere, D. Ferraro, G. Benenti, M. Sassetti

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

Interplay between Interlayer Shift and Twist: Twisted van der Waals Nanowires Driven by Rotational Twinning

Dong-gyu Kim, Kisung Kang, Hani Kang et al. (+8 more)

In van der Waals(vdW) layered materials, interlayer shift and twist have enabled the control of material properties through polytype and moire engineering. Various approaches, including bottom-up synthesis and manual layer-by-layer stacking, have been utilized to engineer targeted stacking configurations. However, the interplay between interlayer shift and twist, as well as reliable mechanisms for fine-tuning these parameters, remains largely unexplored. Here, we report a previously unrecognized twisting mechanism arising from preferred tilted stacking and twinning in vdW crystals. Electron diffraction and atomic-resolution scanning transmission electron microscopy(STEM) imaging reveal that the lattice planes of group-IV chalcogenide GeSe_{2-x}Te_x rotate continuously along the nanowire growth axis, with twist rates depending systematically on nanowire radius. Atomic-scale imaging further identifies a continuous rotational twin boundary extending along the central region of the nanowire. First-principles calculations and structural relaxation simulations confirm that the twisting deformation originates from energetic competition between the preferred interlayer stacking registry and the strain cost imposed by rotational twinning. These findings establish rotational twinning as an intrinsic route to spontaneous twist formation and provide a design principle for realizing twist-engineered vdW crystals with compatible crystal symmetries and stacking motifs.
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All authors Dong-gyu Kim, Kisung Kang, Hani Kang, Kihyun Lee, Yangjin Lee, Jinsub Park, Joong-Eon Jung, Min Kim, Myeongjin Jang, Aloysius Soon, Kwanpyo Kim

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

Electrical Probing of Sub-Néel Spin Dynamics in Two-Dimensional Antiferromagnets Using Graphene Heterostructures

Adrián García-Martín, Sara Gullace, Nicolás Montenegro-Pohlhammer et al. (+4 more)

Low-dimensional antiferromagnetic van der Waals materials have emerged as a versatile platform for exploring exotic spin dynamics and magnetic phases, yet electrically accessing these phenomena remains a major challenge because of their vanishing net magnetization and highly insulating nature. Here, we demonstrate that graphene can act as an ultrasensitive electrical transducer of hidden spin dynamics in two-dimensional antiferromagnets. By integrating graphene with the van der Waals antiferromagnet FePS$_3$, low-temperature transport measurements combined with magnetic-field and gate-voltage control reveal multiple resistance anomalies well below the Néel temperature. Their distinct temperature, magnetic-field, and carrier-density dependences enable us to associate these anomalies with magnon excitations and with a low-temperature magnetic reconfiguration of the antiferromagnetic state, both of which remain largely inaccessible to conventional magnetometry. Density functional theory calculations further show that competing antiferromagnetic spin configurations in FePS$_3$ are nearly degenerate in energy while producing markedly different electronic responses in the adjacent graphene layer. The pronounced electrostatic tunability of these signatures demonstrates that graphene directly transduces interfacial magnetic dynamics into an electrical signal. Our work establishes graphene/antiferromagnetic van der Waals heterostructures as a versatile platform for the electrical readout of spin dynamics in two-dimensional magnets.
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All authors Adrián García-Martín, Sara Gullace, Nicolás Montenegro-Pohlhammer, María José Martínez-Pérez, Rocío Sánchez-de-Armas, Carmen J. Calzado, Enrique Burzurí

Mesoscale and Nanoscale Physics

1/f frequency noise in mechanical resonators scales inversely with volume, not with quality factor

M.L. Roukes

Mechanical frequency-shift sensors have never reached their fundamental thermomechanical or quantum limits. Every material, size, and transduction scheme yet examined yields a $1/f$ floor 1-3 orders higher, with a flicker-noise coefficient $h_{-1} \propto 1/V_{\varepsilon}$. Surveying the published record, I find the invariant $h_{-1}\,V_{\varepsilon} \equiv \sigma_A ^2\, V_{\varepsilon}/(2\ln 2)$ independent of device size across 13 decades; volume-independent noise is excluded by 9$\sigma$, site-averaging by 3.8$\sigma$. No $Q$ dependence appears: reported $Q^{-n}$ laws scale equivalently when $Q$ and volume co-vary; only volume scaling survives when they do not. Mass resolution follows as $|\delta m| \propto \sqrt{h_{-1}\,V_{\varepsilon} \cdot V}$.
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Mesoscale and Nanoscale Physics

Accurate noise spectroscopy using quantum sensing ensembles

Dhilan T Vallury, Nikolai Dontschuk, Alexander M Jakob et al. (+2 more)

Noise spectroscopy with quantum sensors is a powerful tool for characterising the properties of the sensor environment and detecting critical magnetic phenomena. While replacing single sensors with large ensembles promises substantial gains in measurement sensitivity and field of view, the output becomes difficult to interpret when there is a variable coupling to targets across the sensing ensemble. In this work we present an analysis procedure for accessing the true source noise spectrum using ensemble coherence from dynamical decoupling measurements. We show that sequence timing plays a dual role in ensemble noise spectroscopy, setting both the frequency selectivity and the weighted collection of signal across the ensemble, and demonstrate how incorporation of the coupling distribution allows accurate parameter extraction of the noise. We apply this procedure to a model system, shallow nitrogen-vacancy centres in diamond, and show that consistent information about the spectral character of MHz-range surface noise can be obtained from two sensing ensembles featuring distinct depth distributions. Our results pave the way for using large quantum sensor ensembles for quantitative, broadband noise spectroscopy, with straightforward extension to other solid-state and molecular sensing platforms.
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All authors Dhilan T Vallury, Nikolai Dontschuk, Alexander M Jakob, Alexander J Healey, David A Simpson

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

Origin and Reduction of Coercive Fields in ZnO-based Wurtzite Ferroelectrics

Abhijeet Dhakane, Alireza Sepehrinezhad, William Prudnick et al. (+5 more)

Wurtzite ferroelectrics offer a route to semiconductor-compatible non-volatile devices, but their large coercive fields (E$_c$) constrain the operating window between polarization reversal, leakage and dielectric breakdown. Atomically localized and non-classical switching fronts have recently been identified in wurtzite nitrides, yet the microscopic feature that sets the coercive field remains unresolved. Here we use large-scale reactive molecular dynamics, parameterized against first-principles data, to follow field-driven reversal in pristine, Mg-modified and heterostructured ZnO. Reversal proceeds through rugged inversion-boundary filaments in agreement with recent observations in other wurtzite ferroelectrics. We find that the field required for switching is controlled by the advancing filament head, which is a under-coordinated reconstructed inversion-boundary region during the transient switching dynamics with a large field-aligned local polar-order amplitude that is only partially compensated by an oppositely oriented surrounding shell. Controlled strain-only and charge-doping-only perturbations show that charge redistribution lowers the coercive field more effectively than strain because it suppresses this field-setting unscreened local polar order at the filament head, with the combined effect lowering E$_c$ by $\sim$ 50 %. Layered ZnO/ZnMgO architectures further create buried nucleation sites and shorten filament propagation lengths. These results connect atomistic switching topology to a materials-design principle for substantially reducing coercive fields in ZnO-based wurtzite ferroelectrics.
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All authors Abhijeet Dhakane, Alireza Sepehrinezhad, William Prudnick, Jon-Paul Maria, Bobby Sumpter, Adri C. T. van Duin, Kyle P. Kelley, P. Ganesh

Mesoscale and Nanoscale Physics

Entropy-driven transitions between extended integer and fractional quantum Hall regimes

Kyung-Su Kim and Steven A. Kivelson

Electronic states with coexisting Wigner-crystal order can sometimes exhibit a quantum Hall effect over a finite range of electron densities—i.e., exhibit “extended” quantum Hall (QH) plateaus in the absence of disorder. Such an extended quantum Hall state can then compete with other QH states over the same density range, allowing a first-order thermal transition between them. Here, we analyze several settings in which the entropy associated with Goldstone modes (e.g., magnons or phonons) or soft gapped modes (e.g., magnetoroton) drives a finite-temperature transition between competing QH regimes. Applying this framework to moiré rhombohedral graphene, we argue that a soft magnetoroton in a fractional quantum anomalous Hall state provides a plausible bulk mechanism for the observed thermal evolution from an extended integer QH to a fractional QH regime.
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Monday 14 September

Mesoscale and Nanoscale Physics

Probing proximity-induced superconductivity in bilayer graphene using gate-defined quantum dots

Philipp Schmidt, Mathies Funke, Till Tenger et al. (+8 more)

Van der Waals heterostructures offer a direct way of combining two-dimensional (2D) materials with different electronic properties, such as 2D semiconductors, metals, and superconductors, in a single device. Bilayer graphene (BLG) is particularly attractive in this context, as its electrically tunable band gap enables local control of tunnel barriers and quantum dots. Here, we realize an all-2D hybrid platform based on BLG proximitized by superconducting NbSe$_2$. Using local electrostatic gates, we define tunnel barriers and quantum dots at different distances from the lateral superconductor-semiconductor interface. The quantum dots serve as local spectroscopic probes of the proximitized BLG channel segment, forming tunable superconductor-quantum dot-normal conductor junction devices. Coulomb blockade and finite-bias spectroscopy reveal a proximity-induced superconducting gap of up to $80\,\mathrm{\mu eV}$ and allow to track its evolution with increasing distance from the NbSe$_2$ contact. We find that the local density of states remains suppressed over distances exceeding 1 $\mu$m, consistent with superconducting proximity through a highly ballistic BLG channel. Our results show that BLG-superconductor hybrids offer a controllable platform where quantum dots and quantum point contacts can be well combined with superconductivity.
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All authors Philipp Schmidt, Mathies Funke, Till Tenger, Katarina Stanojević, Hubert Dulisch, Kenji Watanabe, Takashi Taniguchi, Fabian Hassler, Christian Volk, Szabolcs Csonka, Christoph Stampfer

Mesoscale and Nanoscale Physics

An improved auxiliary boson method for quantum dots with finite Coulomb interaction U

Juan Pablo Ramos-Andrade, Patricia Almeida, Ana María Calle et al. (+1 more)

In this work, we present an improved version of the formulation of the four auxiliary bosons originally proposed by Kotliar and Ruckenstein, adapted to describe a quantum dot in the Kondo regime with finite electron-electron interaction U. The proposed refinement provides a more accurate treatment of strong electronic correlations while preserving the analytical tractability of the original approach, thereby enabling a reliable description of transport properties in nanoscale systems. Benchmark calculations show good agreement with the results of the numerical renormalization group, supporting the validity of the improved formulation. Our approach therefore provides an efficient and robust framework for investigating transport through strongly correlated quantum dots.
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All authors Juan Pablo Ramos-Andrade, Patricia Almeida, Ana María Calle, Gustavo Lara

Mesoscale and Nanoscale Physics

Anderson localization in topological metals

A.A. Burkov

We address the question of whether topological metals (TM) retain their nontrivial characteristics in the presence of disorder. While in the clean case, the issue is what protects the gaplessness of the spectrum, i.e. the band-touching nodes in TM, this question needs to be reformulated in the presence of disorder. The concept of gaplessness loses its sharp meaning in this case, since spectral gaps can be filled in by disorder-induced states. What is still meaningful is entanglement: one may ask whether the long-range entanglement of the clean gapless TM survives in the presence of disorder or, in other words, whether nontrivial topology protects the TM from Anderson localization. We demonstrate that such a protection exists in the case of three-dimensional (3D) Weyl semimetals, but not in 2D and 3D (type-I) Dirac or 3D nodal line semimetals. We establish this by combining the idea of unquantized anomaly as the topological response of TM with the decorated domain wall construction, used previously to discuss average symmetry-protected topological insulating phases.
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Mesoscale and Nanoscale Physics · Cross-listed from quant-ph

Direct Observation of Dipolar-Driven Anisotropic Quantum Projection Noise in a Solid-State Spin Ensemble

Tasuku Ono, Weijie Wu, Haopu Yang et al. (+9 more)

The nitrogen-vacancy (NV) center in diamond is a prominent quantum-sensing platform. Combining readout at the quantum projection noise limit with strong dipolar interactions promises substantial gains in sensitivity. However, experimentally accessing this regime has remained a longstanding challenge. In this work, we demonstrate quantum-projection-noise-resolved readout of a strongly-interacting, two-dimensional ensemble of NV centers. Our approach leverages repetitive readout via the NV's intrinsic $^{15}$N nuclear memory at a moderate magnetic field ($\sim 0.3$ T), improving the readout fidelities by nearly an order of magnitude. This enables us to directly resolve the quantum projection noise of a coherent spin state and to watch the ensemble's intrinsic dipolar interactions shear this noise into an anisotropic profile. Our results open the door to direct measurements of spin squeezing and entanglement-enhanced sensing in the solid state.
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All authors Tasuku Ono, Weijie Wu, Haopu Yang, Lillian B. Hughes Wyatt, Benjamin Brenner, Che Liu, Collin Fan, Chris R. Laumann, Jonathan N. Hallén, Emily J. Davis, Ania C. Bleszynski Jayich, Norman Y. Yao

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

Topological superconductors and Majorana fermions based on $X$-wave magnets with $X=p,d,f,g,i $

Motohiko Ezawa

We study superconductors coupled with $X$-wave magnets with $X=p,d,f,g,i$. They are essentially different between altermagnets with $X=d,g,i$ and odd-parity magnets $X=p,f$ due to the compatibility of the spin-singlet formation of the $s$-wave superconductor. The altermagnets and odd-parity magnets show rich superconducting phase diagrams depending on the strength of the magnetic order, the chemical potential and the ratio of the on-site and nearest-neighbor interactions. Especially, we find that the altermagnetic order stabilizes the chiral ($p_{x}+ip_{y}$)-wave superconductor, which is a class D topological superconductor hosting Majorana chiral edge states. It is characterized by the Chern number. Especially, there emerge two chiral edge modes in the $i$-wave altermagnet, which are characterized by the Chern number 2. On the other hand, the $p$ ($f$)-wave odd-parity magnetic order stabilizes the $p$ ($f$)-wave topological superconductor, which is a class DIII topological superconductor hosting Majorana flat bands. It is characterized by the winding number. Majorana flat bands are robust in the presence of the mixing with the $s$-wave superconductivity, which is inevitable for odd-parity magnets although the magnitude of the mixing is tiny.
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Mesoscale and Nanoscale Physics

Shot noise signatures of Majorana-assisted transport in an Aharonov-Bohm interferometer

Ignacio Román-Pérez, Juan Pablo Ramos-Andrade, Pedro Orellana et al. (+1 more)

In this work, we investigate the finite-bias charge current, shot noise, and Fano factor in an Aharonov-Bohm interferometer composed of two parallel quantum dots, each side-coupled to a topological superconducting nanowire represented by an effective Kitaev chain hosting Majorana bound states at its ends. The transport properties are calculated using the nonequilibrium Green's function formalism within the equation-of-motion approach. Our results show that coupling to the Majorana bound states significantly modifies the interference pattern, producing distinctive signatures in the current, shot noise, and Fano factor. These signatures are particularly pronounced at the half-flux condition, where destructive interference suppresses the conventional transport background and enhances the visibility of Majorana-assisted transport. These findings demonstrate that finite-bias shot-noise spectroscopy provides a sensitive indirect probe of Majorana bound states in hybrid superconducting-nanowire interferometers.
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All authors Ignacio Román-Pérez, Juan Pablo Ramos-Andrade, Pedro Orellana, Luis Rosales

Mesoscale and Nanoscale Physics

Up/down-conversion of infrared light by few-layer graphene polytypes

Patrick Johansen Sarsfield, Takaaki V. Joya, Takuto Kawakami et al. (+2 more)

Optical nonlinearity of materials with broken inversion symmetry enables two-photon processes where an incoming pair of photons can generate an up-converted photon with the combined frequency, or a high-energy photon can be split into a correlated pair of down-converted photons. Here, we identify few-layer graphene films that offer up/down-conversion capability in the infrared spectral range. For mixed stacking tetralayers (ABCB), which are non-centrosymmetric 2D crystals, we find highly efficient nearly resonant up/down-conversion of [$\omega_1,\omega_2$] photon pairs into/from a photon with $0.7$eV$<\Omega=\omega_1+\omega_2<1.1$eV. We also note that a pronounced second-order nonlinearity in the spectral range of $0.7$eV$<\Omega<0.9$eV can be promoted in rhombohedral tri- and tetralayers by asymmetrical encapsulation. Potentially, for fibers coated with few-layer graphene, this opens the door for “in fiber" production of photon pairs with correlated polarizations.
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All authors Patrick Johansen Sarsfield, Takaaki V. Joya, Takuto Kawakami, Mikito Koshino, Vladimir Falko

Mesoscale and Nanoscale Physics

Induced superconductivity in selective-area grown SnTe devices

Maarten J.G. Kamphuis, Yoran F.S. Starmans, Pim J.H. Lueb et al. (+9 more)

Topological superconductors are of high interest for applications in topological quantum computation. The required topologically superconducting state can be engineered by proximityinducing superconductivity in a topological insulator. Here, we explore the induced superconductivity in selective-area grown nanowires of the topological crystalline insulator SnTe on a InP substrate, through TEM/EDX and low-temperature electronic transport studies. The observed superconducting behavior likely originates from indium in the substrate diffusing upwards into the SnTe nanowire, forming a thin layer of InxSn1-xTe at the interface between the nanowire and the substrate. InxSn1-xTe is intrinsically superconducting for indium concentrations above 2%, resulting in superconductivity within the heterostructure. Little-Parks oscillations are observed in loop-shaped nanowire networks in an out-of-plane magnetic field. The half-period shift indicative of a topological superconducting state is absent, which is explained by dominant trivial transport channels obscuring any topological signatures.
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All authors Maarten J.G. Kamphuis, Yoran F.S. Starmans, Pim J.H. Lueb, Femke J. Witmans, Marvin M. Jansen-Zilles, Marcel A. Verheijen, Reinoud Lavrijsen, Joost Ridderbos, Fabrizio Nichele, Floris A. Zwanenburg, Erik P.A.M. Bakkers, Alexander Brinkman

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

Proximity-induced charge density waves in a moiré heterobilayer

Christopher T. S. Cheung, Arash A. Mostofi and Johannes Lischner

Twisted heterobilayers of two-dimensional materials have emerged as a platform for studying emergent phases of matter. In this work, we investigate charge density waves (CDW) in a twisted NbSe$_2$/MoSe$_2$ bilayer using first-principles calculations. We observe CDW formation in both layers, even though MoSe$_2$ does not feature a CDW in its monolayer form. Moreover, we find that the CDW is highly non-uniform with filled-center, hollow-center and hexagonal CDWs coexisting in the moiré unit cells of both layers. We assess different mechanisms of CDW formation in the MoSe$_2$ layer and conclude that the dominant one is the steric repulsion between Se atoms across the van der Waals gap. The strength of this effect is highly sensitive to the interlayer separation, which explains why the CDW amplitude in the MoSe$_2$ layer depends strongly on the local stacking arrangement. Our work demonstrates that novel broken-symmetry phases can be induced in twisted heterobilayers through proximity effects.
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Mesoscale and Nanoscale Physics

Spatiotemporal programming via asymmetric dielectric engineering for nonvolatile 2D optoelectronics

Xiaoguang Luo, Xiaolong Zhang, Honglei Chen et al. (+11 more)

Ambipolar two dimensional (2D) semiconductors integrated with floating-gate architectures offer a promising platform for nonvolatile, reconfigurable electronics. However, the switching between p-n and n-p junction polarities has conventionally required complex multi-gate designs, hindering the scalability and integration density. Here, we demonstrate a spatiotemporal programming strategy using a dual-floating-gate architecture with a symmetry broken tunneling dielectric. An asymmetric dielectric stack creates distinct tunneling thresholds for two floating gates, enabling a single input gate to encode spatial doping profiles in the 2D channel via defined voltage pulse sequences. We achieve on demand, nonvolatile, and reversible switching between p-n and n-p configurations with excellent retention and endurance. The reconfigurable homojunction serves as a multifunctional platform for logic encoding, rectification, photodetection, and in sensor computing. This work establishes a design paradigm that replaces spatial input complexity with spatiotemporal programming, paving the way for high-density, multifunctional intelligent hardware.
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All authors Xiaoguang Luo, Xiaolong Zhang, Honglei Chen, Jiaming Wang, Fan Liu, Jiongtao Zhang, Junqiang Zhang, Yihan Yin, Jinpeng Xu, Lei Ying, Renjing Xu, Yingchun Cheng, Xuetao Gan, Wei Huang

Mesoscale and Nanoscale Physics

Effects of residual exchange coupling on simultaneously driven spin qubits

Heun Mo Yoo, Tanner M. Janda, Victor Yu et al. (+3 more)

Exchange coupling and microwave drives are widely used control mechanisms for spin qubits. However, the influence of exchange coupling on microwave-driven spin dynamics is not fully understood. We report simultaneous drive measurements of two spin qubits as a function of exchange coupling and drive power. When the Rabi frequency exceeds exchange, we observe a beating pattern in the Rabi oscillations. In the opposite limit, two exchange-split patterns emerge in the Rabi chevrons. We find that these exchange-induced effects are suppressed when the difference in the Rabi frequencies exceeds the exchange coupling. A theoretical model is developed that reproduces the main features in our data.
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All authors Heun Mo Yoo, Tanner M. Janda, Victor Yu, Michael J. Gullans, Adam R. Mills, Jason R. Petta

Mesoscale and Nanoscale Physics

A self-compliant selector-free memory with functional bismuth ferrite

Suman Roy, Subhabrata Das, Priyanka Sahu et al. (+5 more)

In-memory computing requires selector-free operation, low variability, and stable multilevel switching, all of which remain challenging to achieve in a single system. Here, we demonstrate a robust selector-free memory based on a multiferroic BiFeO3 that combines self-compliance with stable bipolar switching, high yield, long endurance, and extended retention. We observe that BiFeO3 memristive crossbar arrays become self-compliant through a naturally formed TiOx interfacial layer, enabling forming-free bipolar switching without external selector devices, addressing major bottlenecks that have limited the practical deployment of resistive memories. The array delivers stable bipolar switching with a 97% device yield, endurance beyond 105 cycles, and data retention exceeding 105 s with low cycle-to-cycle and device-to-device variability. In addition, the devices exhibit 16 well-separated conductance states, enabling 4-bit multilevel operation via precise pulse-controlled programming for digit visualization in edge applications. Noise measurements reveal predominantly 1/f-type behavior with minimal random telegraph noise, indicating robust conductance stability across the programmable window. These results establish BiFeO3 as a practical materials platform for multibit, selector-free crossbar memories offering high reliability, making it well-suited for scalable computing architectures based on functional materials.
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All authors Suman Roy, Subhabrata Das, Priyanka Sahu, Majid Ahmadi, Sameer Kumar Mallik, Kaushik Ghosh, Saroj Prasad Dash, Satyaprakash Sahoo

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

Wrinkles and Magnetic Flux Trapping in Graphite Nanoflakes: A Possible Source and Manifestation of Room-Temperature Superconductivity

Muhammad Saad, Sergey I. Nikitin, Dmitry A. Tayurskii et al. (+1 more)

The paper reports on a possible manifestation of local high-temperature superconductivity in graphite nanoflakes obtained from the bulk pyrolytic graphite by an extended grinding and subsequent annealing at 670 K in the air. Analysis of transmission electron microscopy (TEM) images shows that such treatment leads to a formation of high-density wrinkle-type defects at the basal-plane surface. The sample reveals the magnetic flux trapping - one of the key signatures of superconductivity - that persists up to 390 K and above. At the same time, both the as-ground (unannealed) and annealed in the vacuum samples manifest negligible flux trapping, and no wrinkles in their TEM images were found. The direct correlation between the appearance of high-density wrinkle arrays and the detection of the trapped magnetic flux above the room temperature strongly suggests that wrinkled regions serve a source of local high-temperature superconductivity in graphite.
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All authors Muhammad Saad, Sergey I. Nikitin, Dmitry A. Tayurskii, Roman V. Yusupov

Mesoscale and Nanoscale Physics

Opto-Electrical Detection of Donor Bound Excitons in Silicon-on-Insulator Substrate

A. Kanniainen, A. S. Kumar, A. Sammak et al. (+2 more)

Spin of a donor bound electron in silicon has been shown to be a very coherent qubit system but lacks a coherent optical interface. There does, however, exist a donor bound exciton transition that can be excited optically but decays dominantly via an Auger recombination producing an electrical signal. This provides an opto-electrical pathway for spin readout. Scaling this readout to single-spin level will require interfacing the spins with silicon photonics for efficient guiding of photons, which in turn will require moving to silicon-on-insulator (SOI) substrates. Here we demonstrate first ensemble opto-electronic measurements of donor bound excitons in SOI material, including isotopically purified 28-Si device layers. The experiments show pronounced shifts in the resonance wavelengths and broadenings of the transition linewidths compared to the bulk experiments.
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All authors A. Kanniainen, A. S. Kumar, A. Sammak, G. Scappucci, J. T. Muhonen

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

Extracting dipole orientations from asymmetric plasmonic nanostructures towards machine-learning-assisted spectropolarimetry

P. Christian Simo, Michaela Zbytovska, Annika Mildner et al. (+4 more)

In this work, nanoparticles with various asymmetries are analyzed for their azimuthal orientations using polarimetric dark-field spectroscopy at different analyzing angles of a linear polarizer. This approach reveals their spectral behavior in terms of electric far-field dipole intensities when modeled with an analytical dipole model. By simultaneously fitting the spectra from a set of analyzer angles, the respective dipole orientations are extracted. In a statistical approach, all non-repeating permutations are further studied with a machine learning algorithm. The resulting azimuthal distribution of dipole orientations coincides well with the geometric orientations derived from simulations and electron microscope images. A histogram gradient boosting regressor evaluates the impact of the measurement setup on the simultaneously fitted sets, linking the weights of the analyzer angles to the asymmetry in the plasmonic systems. This comprehensive spectroscopic method improves the accuracy of dipole orientation measurements and enables modern machine learning models to interpret potentially complex features of nanostructures.
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All authors P. Christian Simo, Michaela Zbytovska, Annika Mildner, Lukas Lang, Melanie Sommer, Dieter P. Kern, Monika Fleischer

Mesoscale and Nanoscale Physics

Percolation threshold in cluster-assembled nanogranular palladium films and related strategies for hydrogen detection

A. Debot, D. Balakrishnan, A.M. Philippe et al. (+1 more)

Electrical transport in cluster-assembled nanogranular palladium films was investigated in-situ during growth by Supersonic Cluster Beam Deposition. The percolation onset is observed at a thickness of 1.5 nm, while the percolation completion at 2.5 nm. Films at the percolation thickness (2 nm) and beyond the percolation end (5 nm) were subsequently exposed ex-situ to 0.25% hydrogen in air to investigate how hydrogen-induced structural changes in Pd clusters modulate electrical transport and can be exploited as mechanism for selective hydrogen detection. Implantation of palladium clusters in soft polymer styrene-ethylene-butylene-styrene was also explored to promote clusters separation and enhance quantum tunnelling phenomena within electrical transport. The observed electrical transport pattern features stable baselines in air, suggesting material stability, fast response time (T50% < 10 s), and the coexistence of reversible and irreversible components in conduction changes induced by hydrogen exposure. The irreversible component is tentatively attributed to hydrogen-promoted partial coalescence of Pd clusters. The behaviour of the polymer implanted samples suggests an alternative gas detection mechanism where hydrogen-induced polymer swelling dominates over palladium structural modifications. These results disclose novel possible transducing routes for miniaturized hydrogen detection and multimodal sensing.
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All authors A. Debot, D. Balakrishnan, A.M. Philippe, E. Barborini

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

Beyond binary at submicron dimensions: crossed-ellipse MTJ free layers as multi-state cells for spintronic crossbars

Arup Ghosh, Ariel Zaig, Colin Ducarme et al. (+3 more)

Spintronic crossbars are promising hardware platforms for energy-efficient neuromorphic computing, but conventional magnetic tunnel junctions (MTJs) are binary, limiting the information density and synaptic precision of each crosspoint. Here, MuMax3 micromagnetic simulations are used to investigate how size and aspect ratio control the switching and remanent-state landscape of crossed-ellipse MTJ free layers, with permalloy as the main model system and CoFeB checks for transferability. At fixed 8:1 aspect ratio, shrinking the device from 16 micron x 2 micron to 80 nm x 10 nm lowers the absolute switching current but raises the switching field from about 11 Oe to about 301 Oe and increases the SOT current density. At fixed major axis 1.6 micron, aspect-ratio tuning produces a low-field four-state regime, while lower aspect ratios stabilize additional remanent states with lower switching fields and current densities. A 1.6 micron x 0.8 micron device exhibits twelve accessible remanent plateaus in its angle-resolved planar Hall response, with the resolved state count depending on both aspect ratio and absolute size. Projected MTJ readout gives multiple electrical levels, while minimum-energy-path calculations show that the twelve configurations are not all thermally independent. Independent MuMax+ calculations reproduce the multistate topology and reveal lower-barrier multistep escape pathways between nominally distant states. These results define a geometry-dependent design window for scalable multistate spintronic crossbar cells.
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All authors Arup Ghosh, Ariel Zaig, Colin Ducarme, Thomas Coppée, Flavio Abreu Araujo, Lior Klein

Mesoscale and Nanoscale Physics

Two Strategies to Measure Spin-Orbit-Torque Efficiency Acting on the Insulating Magnet Li$_{0.5}$Al$_{0.7}$Fe$_{1.8}$O$_4$

Orion Smedley, Thow Min Jerald Cham, Daisy O'Mahoney et al. (+10 more)

For insulating magnets, widely-used electrically-based techniques to measure spin-orbit-torque efficiency can suffer from poor signal-to-noise ratios and unwanted artifacts, while optical measurements based on conventional magneto-optical Kerr effect (MOKE) imaging often lack sufficient sensitivity. Here we analyze two strategies for measuring the spin-torque efficiency acting on an insulating magnet, focusing on the torque from Pt acting on thin films of the low-damping magnet lithium aluminum ferrite (LAFO). First, lateral + longitudinal spin-torque ferromagnetic resonance (ST-FMR) allows separation of the spin-torque signal from artifacts due to spin-pumping and resonant heating. With this technique, we measure a spin-torque efficiency $\xi^\text{ST-FMR}_\text{DL}\sim\ 0.07$, comparable to but a bit less than for Pt acting on metallic ferromagnets. Second, we attempted optical measurements of spin-orbit-torque-induced magnetic deflection using Sagnac interferometry. We find that Sagnac interferometry provides sufficient sensitivity to measure signals from 14.4 nm LAFO thin films, but the inferred torque efficiency is much weaker than the result from lateral + longitudinal ST-FMR. We speculate that there may be another contribution to the Sagnac signal other than current-induced magnetic deflection, perhaps an out-of-plane component of itinerant electron spins. This becomes relevant for insulating magnets in which the magneto-optic coupling coefficient is very weak.
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All authors Orion Smedley, Thow Min Jerald Cham, Daisy O'Mahoney, Sanyum Channa, Xin Yu Zheng, Anna Janni, Lauren J. Riddiford, Yuhan Liang, Bozo Vareskic, Zbigniew Galazka, Yunqiu Kelly Luo, Yuri Suzuki, Daniel C. Ralph

Mesoscale and Nanoscale Physics

Optical activation of nonlinear Hall effect in topological insulators with warped Fermi surface

Mohammad Shafiei, Farhad Fazileh and Milorad V. Milošević

Topological insulators (TIs) with hexagonally warped Fermi surface are natural platforms for the nonlinear Hall effect, as warping breaks inversion symmetry while preserving time-reversal symmetry (TRS). Here we show that this inversion breaking alone is insufficient: although warping generates a strongly anisotropic Berry curvature, the preserved threefold rotational symmetry forces the equilibrium Berry curvature dipole (BCD) to vanish identically. We demonstrate that linearly polarized light removes this symmetry obstruction: in the off-resonant Floquet regime, it lowers the rotational symmetry while preserving TRS, thereby generating a finite BCD whose magnitude, orientation, and sign are continuously tunable by the light intensity and polarization. For realistic Bi$_2$Te$_3$ parameters, we show that the induced BCD reaches $\sim$0.03~nm, yielding microampere-scale nonlinear Hall currents under experimentally accessible conditions. Our results therefore establish Floquet symmetry engineering as a route to activating the symmetry-forbidden nonlinear transport on TI surfaces without breaking the TRS.
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Mesoscale and Nanoscale Physics

Unified contact-free formulation of linear-response transport theory

Ziqian Wang and Ji Feng

Thermal Hall transport is conventionally formulated as a current–current Kubo response supplemented by an energy-magnetization correction obtained through an auxiliary pseudogravitational magnetic field. Motivated by this structure, we develop a unified contact-free moment formulation of linear response to scalar sources. For channels whose source contributions vanish in the dc limit, the transport coefficient is the low-frequency residue of a retarded moment–moment correlator, with equal-time endpoint contributions incorporated before the dc limit is taken. The formulation uses only scalar sources, requires no separate magnetization subtraction, is independent of the local-current gauge, and is invariant under admissible redistributions of bond or interaction energy. It organizes particle, grand-energy, and specified spin-density channels within a common source–moment response matrix. Quadratic Landau–Lifshitz spin waves illustrate the method: the secular growth of polarization-moment correlations isolates the finite Hall residue and yields the magnon thermal Hall and spin Nernst coefficients from the same paraunitary band geometry. The formulation provides a unified operator framework that simplifies the organization of particle, thermal, and spin transport and can streamline calculations by eliminating separate contact and magnetization corrections.
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Mesoscale and Nanoscale Physics · Cross-listed from hep-th

Quantum geometry of gravitons

M. Mehraeen

We uncover the quantum geometric structure of graviton Wigner functions and stress-energy tensors via quantum field theory in curved spacetime, revealing the phase-space geometry of spacetime and relativistic quantum-state manifolds. We show that the polarization mode expansion of the underlying graviton field operator is fully captured by quantum geometry, as encoded in the spinor-helicity formalism, thereby establishing the geometric framework from the outset. Applying this to a weak gravitational background, we demonstrate the roles of the quantum metric and connection in describing graviton transport beyond the chiral vortical effect. We also clarify the role of the quantum metric in normalizing the perfect-fluid graviton stress tensor within this approach. This work paves the path for explorations of multistate Hilbert-space geometry in high-energy and gravitational physics. In addition, this framework naturally encompasses lower-spin excitations, allowing for a unified quantum geometric treatment of bosonic and fermionic many-body systems in condensed matter and particle physics.
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Mesoscale and Nanoscale Physics

Quasilocal transport in the integer quantised Hall effect: adiabatic separation, local Ohm's law, and Joule heating of the incompressible strips

A. Siddiki

We reassess the semiclassical route to integer quantum Hall transport around one idea: the separation of fast, quantised cyclotron motion from a slow, self-consistently screened guiding-centre landscape. Bohr–Sommerfeld quantisation of the canonical action fixes the Landau spectrum; smooth confinement turns it into Born–Oppenheimer surfaces with adiabatic parameter $\ell|\nabla V|/\hbar\omega_c$; screening supplies the self-consistent field. Transport is quasilocal, $R_H=\int\rho_Hj_y\,dx/\int j_y\,dx$ with $j_y\propto1/\rho_l$, so within the regularised local Ohm model the incompressible strips attract the current by weight and the computed Joule-power profile follows the current profile. In linear response the intrinsic electrostatic AC correction of the frozen-profile model is reactive, $\mathrm{Im}\,R_H\propto\omega$, with $\mathrm{Re}\,R_H-R_H^{\rm DC}\propto\omega^{2}$ below $10^{-9}$ up to $2$~MHz. With the current fed back into the electrostatics at $1$–$5\,\mu$A, where the Hall drop is one to five cyclotron gaps, the driven boundary condition breaks the reflection symmetry of the bar, the Hall drop concentrates in one strip (interchanged to $10^{-9}$ under current reversal), and the plateau narrows from its low-field flank while, for the GaAs parameters studied, its high-field edge stays at $\nu(0)=2$. A local transverse thermal closure inspired by Akera, evaluated as a post-process on the frozen profile with two phonon-loss laws, indicates that the narrow current-carrying strips of the low-field flank may heat by of order the lattice temperature; with the activated-heat-capacity closure the cold strip loses its stationary solution at model-dependent currents of $0.3$–$2\,\mu$A. The thermal conclusions are bounded by the loss law and by the frozen profile, and are presented as estimates.
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Mesoscale and Nanoscale Physics

1D Interface-Induced Real-Space Berry Curvature Gradient Encoded in SpinValley-Coupled Nonlinear Photocurrent in Lateral Heterostructure

Suman Kumar Chakraborty, Jinu James, Biswajeet Nayak et al. (+6 more)

Symmetry breaking enables helicity-dependent optoelectronic responses in quantum materials. In two-dimensional transition-metal dichalcogenides (TMDs), circular photocurrent (CPC) is typically observed under oblique illumination via the circular photon drag effect, while realization of the intrinsic circular photogalvanic effect (CPGE) under normal incidence remains challenging. Here, we report a clear signature of CPGE in chemical vapor deposition-grown monolayer 2D lateral heterostructure (LHS). By probing the nonlocal photocurrent across the 1D hetero-interface, we observe a helicity-dependent photocurrent consistent with a spin-valley-coupled CPGE-dominated mechanism driven by interface-induced Berry-curvature asymmetry, i.e., Berry curvature dipole. Spatially and spectrally resolved measurements under selective excitation of the MoSe2, WSe2, and the interface regions exhibit distinct magnitudes and polarities of the CPG responses. These contrasting behaviors highlight the opposite spin-valley dichroism for MoSe2 and WSe2, as well as enhanced valley mixing at the 1D interface. Our findings establish LHS as a robust platform for realizing a hetero-interface-induced CPGE-like response and highlight its potential for valleytronics and optospintronics.
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All authors Suman Kumar Chakraborty, Jinu James, Biswajeet Nayak, David Tebbe, Purbasha Ray, Christoph Stampfer, Bernd Beschoten, Lutz Waldecker, Prasana Kumar Sahoo