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Physics Condensed Matter

cond-mat Watch Condensed Matter

Wednesday 19 August

Soft Condensed Matter

Particle-Wall Alignment Interaction and Active Brownian Diffusion Through Narrow Channels

Poulami Bag, Shubhadip Nayak and Pulak Kumar Ghosh

We numerically examine the impacts of particle-wall alignment interactions on active species diffusion through a structureless narrow two-dimensional channel. We consider particle-wall interaction to depend on the self-propulsion velocity direction whereby some specific particle's alignments with respect to the boundary walls are stabilized most. Further, the alignment interaction is meaningful as long as particles are close to the confining boundaries. Unbiased diffusion of active particles for various possible stable velocity alignments against the walls has been examined. We show that for the most stable configuration leading to self-propulsion velocity direction perpendicular to the wall, diffusivity becomes inversely proportional to the square of alignment interaction torque. On the other hand, when the self-propulsion velocity direction making an acute angle to the channel walls is the most stable configuration, diffusion exponentially grows with strengthening alignment interaction. Hence, particle-wall interaction plays a pivotal role in the transport control of active particles through narrow channels. Moreover, the impacts of the alignment interactions on diffusion largely depend on the particle's self-propulsion properties and its chirality. Our simulation results can potentially be used to understand unbiased diffusion of artificial or living micro/nano-objects (such as virus, bacteria, Janus particles, etc.) though narrow confined structures.
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Materials Science

Operando Raman probing of mode selective electron phonon coupling in two dimensional halide perovskites

Tufan Paul, Helena Boom, Lilian Skokan et al. (+8 more)

Electron phonon coupling governs charge transport, carrier relaxation, and polaron formation in halide perovskites, yet its microscopic origin in low dimensional systems remains poorly understood. Here, we combine operando, bias dependent Raman spectroscopy with density functional theory (DFT) calculations to directly probe carrier lattice interactions in two-dimensional Ruddlesden Popper perovskites,(PEA)$_2$PbI$_4$ and its fluorinated analogue, (PEA-F)$_2$PbI$_4$. Under applied electric fields, both systems exhibit mode-selective Raman linewidth broadening predominantly near 100 cm$^{-1}$, whereas other phonon modes remain largely unaffected, revealing highly selective coupling between injected carriers and specific lattice vibrations. DFT calculations identify these modes as hybrid organic inorganic vibrations involving coupled motion of the organic spacer and symmetric Pb I equatorial stretching, rather than purely inorganic phonons. Fluorination fundamentally reconstructs the vibrational landscape by modifying molecular packing, crystal symmetry, and organic inorganic coupling, resulting in changes to the phonon density of states, longer phonon lifetimes, and an enhanced carrier mediated lattice response. Notably, electrical bias produces opposite phonon lifetime evolution in thin films and single crystals: the phonon lifetime decreases by approximately 17 to 22% in thin films but increases 20 to 26% in single crystals. These contrasting responses demonstrate that structural order plays a fundamental role in determining carrier phonon interactions and phonon relaxation pathways in two-dimensional halide perovskites.
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All authors Tufan Paul, Helena Boom, Lilian Skokan, Viren Tyagi, Mostafa Shagar, Aditi Sahoo, Silvia Colella, Geert Brocks, Andreas Ruediger, Shuxia Tao, Emanuele Orgiu

Statistical Mechanics · Cross-listed from quant-ph

Integer Linear Programming Decoder for Abelian and Non-Abelian Topological Codes

Dian Jing, Aubrey Zhang, Liang Jiang et al. (+1 more)

Topological orders (TOs) are widely used as quantum error-correcting codes, with anyon excitations serving as error syndromes. For certain Abelian TOs, decoding can be performed by independently matching particle-antiparticle pairs of each species. However, matching-based decoders cannot handle more general fusion rules in either Abelian or non-Abelian TOs, nor account for noise that correlates different anyon species. While clustering decoders are more broadly applicable, they typically neglect anyon data and fusion properties, leading to poor performance in practice. In this work, we introduce a fundamentally different decoder for arbitrary TOs based on integer linear programming (ILP). The ILP formulation linearizes the error-correction problem through the introduction of auxiliary variables and encodes fusion rules as linear constraints. Classical optimization then identifies the minimum-weight error configuration. As concrete examples, we determine error-correction thresholds for three TOs: the Abelian $\mathbb{Z}_2$ TO under depolarizing noise, where charge and flux errors are correlated; the Abelian $\mathbb{Z}_3$ TO, which does not admit a pairwise matching decoder; and the non-Abelian $D_4$ TO under noise channels that generate all anyon species. We demonstrate the versatility of the ILP decoder by showing a clear performance advantage over most existing decoders in all three cases. We further extend the method to incorporate noisy syndrome measurements and propose a just-in-time variant for continuous error correction. Our results establish ILP as a natural framework for handling correlated errors and general anyon fusion rules, and as a powerful and flexible general-purpose decoder for incoherent anyon noise in arbitrary TOs, with applications to fault-tolerant quantum computation.
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All authors Dian Jing, Aubrey Zhang, Liang Jiang, Ruben Verresen

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

Materials Science

Room-temperature ferroelectrically switchable quantum geometry in few-layer WTe2 for complementary in-memory computing

Ruihan Wang, Pengfei Wang, Haoyun Chen et al. (+10 more)

Quantum geometry, describing the inherent geometric structure of electron wavefunctions in momentum space, transcends the traditional charge degree of freedom and provides a novel physical basis for information encoding and processing. The key to such new computing paradigms is the non-volatile electrical programming of quantum geometric states at room temperature, which, however, has not been established. Here, we demonstrate ferroelectrically switchable quantum geometry in few-layer WTe2, which uniquely enables complementary convolutional processing. By employing the intrinsic coupling between ferroelectric polarization and quantum geometry in few-layer WTe2, we show that the second- and third-order nonlinear anomalous Hall effects (NLAHE) can be deterministically and electrically switched in a nonvolatile and correlated manner. The switching is robust at room temperature for ~104 cycles and retention of ~105 s. Furthermore, leveraging the opposite switching behaviors of second- and third-order NLAHE at room temperature, we demonstrate complementary in-memory computing and implement a hardware-level complementary convolution kernel. This kernel overcomes the inherent directional specificity of conventional convolutional networks and achieves a texture recognition accuracy of 98%, thereby illustrating a viable pathway towards physics-native computing through exploiting exotic physics in quantum materials.
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All authors Ruihan Wang, Pengfei Wang, Haoyun Chen, Yunze Peng, Bingyan Liu, Junlin Xiong, Xueyuan Zhang, Chen Pan, Xin Chen, Shengyuan A. Yang, Shi-Jun Liang, Feng Miao, Peng Song

Other Condensed Matter

Efficient calculation of real-space lattice propagators in the presence of a Fermi sea

Oliver Tong and Mona Berciu

We present an efficient method for computing the real-space propagators (lattice Green's functions) of any tight-binding Hamiltonian in the presence of a Fermi sea with carrier concentration $x$. The method is valid for any lattice, dispersion, and dimension, provided the corresponding $x=0$ propagators are known. We show that suitable combinations of the finite-$x$ particle-addition propagators have a real or imaginary part trivially related to their $x=0$ counterpart, while the remaining part follows from a Kramers-Kronig relation that can be evaluated for all energies at once using the fast Fourier transforms. The computational cost is therefore independent of the dimensionality, unlike that of direct Brillouin-zone integration. The particle-removal propagators follow from general identities. We validate the method against direct integration for hypercubic lattices in one, two, and three dimensions, and use the 2D square lattice to illustrate how the shape of the Fermi surface is imprinted on the spatial structure of the propagators. In particular, at energies far outside the band, the propagator maps converge to the Fraunhofer diffraction pattern whose aperture is the unoccupied part of the Brillouin zone.
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Materials Science

Room-Temperature Polarity Control of the Anomalous Nernst Effect in a High-Magnetic-Anisotropy Topological Nodal-Line MnAlGe

Nanhe Kumar Gupta, Keisuke Masudaa, Masaaki Kakoki et al. (+7 more)

Controlling the polarity of anomalous Nernst thermopower is a promising strategy for enhancing the performances of thermoelectric applications. However, realizing such control at room temperature (RT) in topological ferromagnets with high magnetic anisotropy (Ku) remains challenging. Here, we report RT polarity control of the anomalous Nernst effect (ANE) in quasi-two-dimensional nodal-line MnAlGe epitaxial thin films through Al/Ge compositional tuning while preserving robust high Ku. This polarity reversal originates from intrinsic Berry curvature contributions modulated by sublattice-selective carrier doping, as supported by spin-resolved electronic band structure analysis and hard X-ray photoemission spectroscopy. To demonstrate the practical feasibility, we also fabricated a meander-structured device combining MnAlGe with positive and negative polarity enhanced the thermoelectric output. Our results demonstrate that tuning the Fermi level relative to the nodal-line electronic structure while preserving high Ku enables controllable ANE polarity reversal within a single material, providing a route toward RT transverse thermoelectric devices.
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All authors Nanhe Kumar Gupta, Keisuke Masudaa, Masaaki Kakoki, Benugopal Bairagya, Satoki Tazawaa, Weinan Zhoua, Hirofumi Sutoa, Ryo Toyama, Akio Kimura, Yuya Sakuraba

Materials Science

Optical poling of a quantum ferroelectric metal across the order-disorder phase transition

Mohamed Kandil, Yasuhide Tomioka, Yafei Ren et al. (+3 more)

Electron-doped strontium titanate has emerged as a prototypical quantum ferroelectric metal. It provides a fertile ground to explore how ferroelectric instability intertwined with itinerant electrons creates quantum phenomena, including unconventional superconductivity. Despite extensive studies, the microscopic origin of the ferroelectric transition remains unsettled, with distinct interpretations based on displacive mechanism driven by soft mode and order-disorder alignment of local dipoles. In particular, the local dipoles form nanoscale, spatially heterogeneous clusters, termed polar nanoregions, posing a significant challenge for probing or manipulating them. Here, using rotational anisotropy second harmonic generation, a symmetry-resolved probe, we quantify the orientational statistics of polar nanoregions in dilute electron-doped Sr$_{0.95}$Ba$_{0.05}$Ti$_{1-x}$Nb$_x$O$_3$. By tracking the alignment and meltdown of polar nanoregions in thermal cycles, we unambiguously demonstrate the order-disorder nature of the ferroelectric transition. We further show that, above transition temperature, femtosecond optical fields enable deterministic control of otherwise disordered polar nanoregions, realizing reversible write and readout of polar textures on ultrafast timescales. Our findings provide new insight into ferroelectric instability and establish an all-optical route of controlling polar metal systems where conventional electrical approaches are not feasible.
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All authors Mohamed Kandil, Yasuhide Tomioka, Yafei Ren, Ryan Comes, Isao H. Inoue, Wencan Jin

Soft Condensed Matter

Vesicle-surface-templated catalytic polymers drive differential growth in synthetic minimal cell variants

Minoru Kurisu, Taro Suzuki, Ryosuke Katayama et al. (+6 more)

Understanding how life-like behaviors can emerge from simple molecular assemblies and primitive compartments remains a central challenge in origins-of-life research. Synthetic minimal cells provide a bottom-up platform for investigating, from scratch, the minimal physicochemical principles underlying compartment growth, reproduction, and evolution. Previously, we developed a vesicle/polymer-based compartment system in which the vesicle membranes template the formation of a catalytic polymer. This polymer promotes selective incorporation of amphiphiles into the vesicle membrane, driving vesicle growth while maintaining the compositional identity and enabling spontaneous deformation and division over several generations. Here, we report about experiments in which we advanced this system beyond reproduction by systematically constructing eight synthetic minimal cell variants from combinations of two template vesicles, two catalytic polymers, and two supplied amphiphiles. The variants exhibited distinct, composition-dependent vesicle growth responses, ranging from pronounced growth to suppressed growth or vesicle shrinkage. These growth responses were described by the Hill kinetics and characterized by three parameters, revealing a multi-dimensional fitness landscape shaped by environmental conditions, in which the relative advantage of each variant depends on both composition and amphiphile availability. This framework links molecular recognition, compositional inheritance, and differential growth, providing a physicochemical route toward evolvable synthetic minimal cells.
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All authors Minoru Kurisu, Taro Suzuki, Ryosuke Katayama, Kazuki Maruyama, Daisuke Unabara, Tasuku Hamaguchi, Koji Yonekura, Peter Walde, Masayuki Imai

Statistical Mechanics · Cross-listed from quant-ph

Entropic Rigidity in Quantum Memories: How Geometry and Algebra Control the Onset of Degeneracy Corrections

Yixin Zhao and Fei Yan

Maximum-probability (MP) decoding selects the most probable microscopic error, whereas degenerate maximum-likelihood (MLD) decoding includes the configurational entropy of an entire logical sector. Using code-capacity Pauli noise to isolate rigidity intrinsic to the code, we determine the first physical error weight $m$ at which their logical winner sets become disjoint, even under optimal MP tie resolution. Code distance imposes the universal bound $m\geq h=\lceil d/2\rceil$. We define the entropic rigidity depth $r$ through $m=h+r$ and certify a three-level hierarchy: $r=0$ for planar surface codes and two concatenated families, $r=1$ for odd-distance square toric codes and the Gross $[[144,12,12]]$ quantum low-density-parity-check code, and $r=2$ for a separable family with hypergraph product and bivariate bicycle descriptions. The onset fixes the leading operational failure gap, proportional to the $m$th power of the physical noise strength. Geometry and algebra therefore provide quantifiable controls of configurational entropy and an exact benchmark for low-noise decoder selection.
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Materials Science

Solidification-cell confinement of domain-wall pinning in additively manufactured ferromagnets

Dennis Boakye, Eric K.K. Abavare and Chuang Deng

As-built printed ferromagnets typically exhibit higher coercivity than optimized wrought materials, yet existing explanations rely on empirical fits or costly simulations. Herein, we provide a missing analytical theory that links print parameters directly to cooling rates, cellular spacing, dislocation density, and domain-wall pinning coercivity. Informed by metallographic grain data and using a single fitted constant, our model predicts six experimental datasets for pure Fe, Fe-6.9Si, and a multicomponent alloy within a factor of 1.9. We demonstrate that configurational lattice distortion is negligible, implying that single-phase printed alloys follow dilute-pinning laws. Critically, we introduce a confinement factor, $E=\sqrt{\lambda_{c}/2\delta_{w}}$, proving that solidification-induced dislocation packing makes cellular microstructures harder than conventionally cold-worked metals. The framework enables an alloy-sensitivity map to screen and rank compositions before manufacturing.
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Tuesday 18 August

Materials Science

Self-assembly and Electronic Properties of Graphyne and Graphdiyne Molecular Wires on Metallic Surfaces

Victor M. S. da Conceição, Dominike Pacine, Juliana M. Morbec et al. (+1 more)

Molecular self-assembly on solid surfaces has been the subject of extensive research, motivated by both fundamental and technological interests. On the fundamental side, these studies seek to elucidate the mechanisms governing molecular self-assembly and the resulting surface structures. From an applied perspective, they provide a route toward controlling surface reactions and engineering molecular electronic devices. Here, based on first-principles density functional theory calculations, we present a comprehensive study of self-assembled molecular wires (MWs), composed of graphyne (GY(1D)) and graphdiyne (GYD(1D))-like structures, adsorbed on Au(111), Ag(111), and Al(111) surfaces. Our total-energy calculations reveal that non-aligned MW arrays are energetically preferred on all three metal substrates. The GY(1D) and GYD(1D) molecular wires interact with the metal surfaces through van der Waals (vdW) forces, while their molecular orbitals do not contribute to the formation of metallic interface states. Simulated X-ray photoelectron spectroscopy (XPS) spectra reveal that the C 1s spectral features of the molecular wires are largely preserved upon adsorption, while the absolute binding energies undergo a substantial downshift that is nearly independent of the metal substrate, indicating that metallic screening effects dominate the adsorption-induced core-level shifts. Electronic band-structure calculations further show that the semiconducting character of the molecular wires is retained, resulting in vdW metal-semiconductor heterostructures in which the semiconducting component consists of one-dimensional semiconducting channels. These findings demonstrate that self-assembled graphyne- and graphdiyne-based molecular wires on metal surfaces provide a promising platform for the realization of low-dimensional molecular electronic devices.
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All authors Victor M. S. da Conceição, Dominike Pacine, Juliana M. Morbec, Roberto H. Miwa

Materials Science · Cross-listed from physics.chem-ph

Polyaniline as a conductive polymer and its role in improving the efficiency and conductivity of perovskite solar cells

Mehran Hosseinzadeh Dizaj

This article investigates the role of polyaniline as a conductive polymer in the active layer of perovskite solar cells. Samples were created by incorporating polyaniline into the transport layers to assess its impact on enhancing efficiency and conductivity. The application of this polymer across various layers of the cell structure led to improved stability and performance. Given its high doping capability, polyaniline was examined in detail, particularly focusing on two types of oxidation doping and its integration into the hole transport layer. Graphene oxide and reduced graphene oxide were chosen as comparative models, and their performance was evaluated against the standard polyaniline configuration. Laboratory results revealed that power conversion efficiency increased by 17.5% with graphene oxide and by 36.8% with reduced graphene oxide. Furthermore, short-circuit current density improved by 9.8% and 23.1%, respectively. These findings are consistent with existing studies in the field and support the validity of the approach.
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Materials Science · Cross-listed from physics.chem-ph

Multi-Species Multi-Reaction Model Parametrization of a Commercial NCA Positive Electrode from Half-Cell Open-Circuit Potential Data

Michele Galasso, Petr Čech, Ivo Horstkötter et al. (+3 more)

Physics-based models of lithium-ion batteries require the open-circuit potential (OCP) of each electrode as a function of its lithiation. The multi-species multi-reaction (MSMR) model provides a compact, thermodynamically grounded description of this relationship, representing each insertion reaction by a standard potential, a site fraction and a disorder parameter. Here, we parametrize the MSMR model for nickel-cobalt-aluminum oxide (NCA), a cathode chemistry widespread in automotive cells, using the positive electrode of a commercial silicon-graphite/NCA cell. Cathode material was harvested from a pristine cell and from cells cycle-aged to the end of life under two state-of-charge windows, and its half-cell OCP was measured by slow galvanostatic cycling. Smooth differential-capacity curves were obtained by histogram counting, and the charge and discharge branches were fitted with a constrained, scale-invariant optimization that keeps every reaction anchored to a visible peak. We report the first MSMR parameter set for an NCA cathode and show, through a differential-capacity peak analysis across aging states, that its thermodynamic signature is essentially unchanged by aging. This finding supports the common modeling practice of holding the electrode's intrinsic parameters fixed and ascribing aging to electrode-level capacity and alignment changes.
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All authors Michele Galasso, Petr Čech, Ivo Horstkötter, Simon Schwunk, Zuzana Vlčková Živcová, Václav Knap

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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Soft Condensed Matter

Modelling the onset and evolution of immiscible viscous fingering in porous media

Paulo L. K. Caetano Chang, Kundan Kumar, Arne Skauge et al. (+1 more)

The simulation of viscous fingering in porous media is of direct relevance to displacement processes in petroleum engineering and hydrogeology. Building on recent work proposing a modelling approach for well-defined fingers at very adverse viscosity ratios, we investigate the physical mechanisms behind viscous fingering and the modelling requirements for capturing the finger scales and saturation patterns observed in experiments. We simulate and match a viscous fingering experiment at a viscosity ratio of $\mu_{o}/\mu_{w}{=}2000$, discussing the physical significance of each modelling step. Linear stability analysis is used to characterize the early-stage instability of the displacement. Subsequent numerical simulations show that, for the simulated finger scales to match the experiment, the most unstable wavelength at onset must be several times smaller than the desired finger width—so that, after accounting for shielding and merging in the nonlinear regime, the fingers remain thin. Small-scale channelling effects are also required to disrupt the trailing stable region commonly observed in simulations of viscous fingering in nearly homogeneous media. Finally, we show that including a weakly oil-wet capillary pressure function enables our model to capture the bypassed oil observed in the experiment.
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All authors Paulo L. K. Caetano Chang, Kundan Kumar, Arne Skauge, Kenneth S. Sorbie

Statistical Mechanics

Exact expressions of correlation functions between two spins in the boundary row of the two-dimensional rectangular Ising model with periodic-free boundary conditions and finite size

Mei Tao

We present exact expressions of correlation functions between two spins in the boundary row of the two-dimensional rectangular Ising model with periodic-free boundary conditions and finite size. Some properties of exact expressions obtained are discussed. The fundamental property of the Ising model that the long range order emerge as the temperature decrease is shown clearly; expressions of the correlation functions in the thermodynamic limit varies depending on the order of taking limits of two parameters L and N; the impact of different sizes on correlation functions is illustrated with the aid of diagrams. Expressions of the correlation function discussed in this paper in the thermodynamic limit has been presented by previous researchers, and we prove that expressions obtained in this article is identical in form to expressions provided by previous researchers in the thermodynamic limit.
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Materials Science

Localized Excitons and Exciton-Phonon Coupling in Antiferromagnetic AgCrP2S6

Jessica McDivitt, Dimitar Pashov, Mark van Schilfgaarde et al. (+4 more)

AgCrP2S6 combines a low-symmetry thiophosphate framework with an antiferromagnetic Cr sublattice and nonmagnetic Ag sites, providing a setting in which covalency and magnetic localization compete in the low-energy optical response. We combine single-crystal x-ray diffraction, Raman spectroscopy, lattice-dynamical calculations, and self-consistent vertex corrected Feynman diagrammatic many-body approaches to determine the structural, electronic, and excitonic properties of bulk AgCrP2S6. The material remains monoclinic between 100 and 300 K, with additional Ag-site disorder resolved at low temperature, and the optimized structure is dynamically stable. The many-body calculations yield a reduced quasiparticle gap relative to Cr trihalides, but the lowest excitons remain predominantly local: the calculated low-energy (~1.4 eV) excitation cluster is best described as a weakly bright, strongly anisotropic Frenkel exciton with dominant onsite d-d character and substantial ligand-assisted d-p admixture. Symmetry analysis in the C2h setting identifies this state as predominantly Bu and constrains its leading exciton-phonon coupling channels to Ag diagonal renormalization and Bg-mediated bright-dark mixing. These results place AgCrP2S6 in a localized excitonic regime in which the stronger p-d hybridization, relative to the more ionic Cr trihalides, narrows the quasiparticle gap, while the antiferromagnetic exchange and Ag-site dilution disfavor the intersite coherence associated with strongly delocalized low-energy excitons.
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All authors Jessica McDivitt, Dimitar Pashov, Mark van Schilfgaarde, Justin C. Johnson, Jeffrey L. Blackburn, Anna A. Berseneva, Swagata Acharya

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

Materials Science

Magnetic interactions and origin of high Curie temperatures in high Mn content (MnSb2Te4)x(Sb2Te3)1-x quantum materials

Candice R. Forrester, Christophe Testelin, Kaushini Wickramasinghe et al. (+3 more)

Understanding the magnetic interactions that promote high TC in topological quantum materials is essential to effectively design materials whose magnetic configuration persists at high temperatures and are thus practical to integrate into commercial spintronic devices. Here we provide evidence of the origin of the high TC observed in mixed Mn1+ySb2-yTe4 septuple layers and Sb2-yMnyTe3 quintuple layer structures. Analysis of the quintuple layer/septuple layer structures explored their magnetic behavior through different models of Mn-incorporation in the crystal, evidenced as Mn spin S = 5/2, and provided an explanation for the low magnetization per Mn atom observed, signature of competing ferromagnetic and antiferromagnetic interactions between Mn2+ ions. Our studies provide insight to better understand and control the Mn incorporation in our samples to optimize their properties.
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All authors Candice R. Forrester, Christophe Testelin, Kaushini Wickramasinghe, David Hrabovsky, Lia Krusin-Elbaum, Maria C. Tamargo

Materials Science

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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Materials Science · Cross-listed from physics.chem-ph

Coupled-cluster molecular properties across the main group that extrapolate beyond training size

Wenhao He, Xu Chen, Noah Song et al. (+10 more)

Coupled-cluster theory defines the accuracy standard for molecular electronic-structure properties but scales too steeply for routine application, whereas density-functional theory is affordable yet systematically biased. We resolve this trade-off with a single equivariant network, HARP (Hamiltonian Read-out for Properties), that predicts an effective one-electron Hamiltonian from one inexpensive B3LYP/def2-SVP calculation and derives a broad suite of properties from it (energy, optical gap, dipole, quadrupole, polarizability, Mulliken atomic charges, and Mayer bond orders) at coupled-cluster accuracy across nine main-group elements, including the under-served phosphorus, sulfur, and chlorine chemistries. The model is trained on a new in-house dataset of multi-property labels computed at the CCSD(T) level for all nine elements. On a held-out test set, it reduces the error of every property by a factor of 3.8 to 270 relative to semi-local, hybrid, and double-hybrid DFT (referenced to composite CCSD(T)/cc-pVTZ), while adding only ~0.1 s wall time per molecule, delivering coupled-cluster-quality predictions at the cost of a single DFT calculation. Critically, deriving every property from a predicted Hamiltonian rather than pooling per-atom features builds the correct size-scaling into the model architecture: on pi-conjugated oligothiophenes it matches finite-field CCSD polarizability to ~1% and the EOM-CCSD optical gap to ~3% at the largest sizes where those references remain affordable (44 and 37 atoms, where a single CCSD field point already costs ~500x the model's entire inference) and extrapolates the corrected trends to 58-atom chains, a regime where pooling-based architectures fail by construction. Accurate extrapolation is therefore set by the model's inductive bias rather than by the training data.
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All authors Wenhao He, Xu Chen, Noah Song, Haowei Xu, Tim S. Hindges, Bohan Li, Zihan Lin, Yu Yao, Avetik R. Harutyunyan, Fang Liu, Yao Wang, Hao Tang, Ju Li

Superconductivity

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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Materials Science · Cross-listed from physics.optics

Bayesian Optimization for Practical H2 Sensors: Inverse Design of Pd-based Plasmonic Metasurfaces

Pernilla Ekborg-Tanner, Athanasios Theodoridis, Joachim Fritzsche et al. (+3 more)

Hydrogen detection is becoming increasingly important as its use grows across energy and industrial systems. Optical sensing platforms based on palladium (Pd) nanoparticles are attractive for this task because hydrogen uptake directly alters their plasmonic response. Organizing such nanoparticles into periodic two-dimensional arrays, known as metasurfaces, further enhances their optical response through collective resonances. However, the large design space presented by chemical composition, nanoparticle geometry, and array structure calls for systematic approaches for optimizing complex nanoalloy metasurface geometries. Here, we develop an inverse-design framework based on Bayesian optimization that couples first-principles dielectric functions with electromagnetic simulations to identify high-performance PdAu nanodisk arrays for hydrogen sensing in the 1 to 100 mbar range where the flammability of H2 becomes a concern. We use our approach to search a five-dimensional design space, comprising nanodisk height and radius, array pitch, polymer coating thickness, and Au fraction in order to maximize the H-induced change in extinction at a single wavelength of choice. The results show that integrating first-principles optical models with data-efficient optimization yields experimentally feasible nanoparticle metasurfaces tailored for targeted hydrogen pressures, while providing a pathway to future multiobjective sensor design. They also reveal remaining gaps in the modeling methodologies that still limit the quantitative reliability of the approach.
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All authors Pernilla Ekborg-Tanner, Athanasios Theodoridis, Joachim Fritzsche, Christoph Langhammer, Andrea Baldi, Paul Erhart

Strongly Correlated Electrons

Rare-earth spin textures and a route to electronic inhomogeneity in $Pr_2Ir_2O_7$

Kyle G Sherman and Michael J Lawler

The pyrochlore iridate $Pr_2Ir_2O_7$ remains metallic at low temperatures where its family members insulate. Recently, scanning tunneling spectroscopy experiments have revealed an inhomogeneous mixture of Kondo-screened and Kondo-destroyed regions when its surface is produced by cleaving at room temperature while it remains uniform when cleaved at low temperatures. We ask whether the frustrated praseodymium spin texture seeds this inhomogeneity. To study this, we produce Monte-Carlo-sampled Pr spin-ice configurations, presumed to evolve slowly in time, and couple through a local Kondo exchange $J_K$ to an eight-band Hartree-Fock model of the Ir electrons. We further find the charge gap of the all-in-all-out state common in the other family members closes smoothly with the coupling $J_K$, and the recently proposed monopole-rich “jellyfish” textures further suppress the insulating behavior by $\Delta J_{Kc}\approx 0.03\,t$. Additionally, scanning a small cluster across a large Pr surface yields a synthetic tunneling map that fractures into islands within a sea. Although our classical simulable theory omits the Kondo singlet, the spatial modulation of the Fermi-level density of states it produces is, through the exponential Doniach sensitivity of the Kondo temperature, sufficient to tip the local balance between screening and magnetic order; a frustration-driven route to the observed Kondo/Kondo-destroyed inhomogeneity.
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Materials Science

Writing and erasing skyrmions by single ultrafast laser pulses in monolayer Janus 2D magnets

Guangyao Miao, Yonglong Ga, Chang Liu et al. (+21 more)

Skyrmions in 2D magnets are promising candidates for nonvolatile, low-power, and high-density spintronic memories. However, their experimental realization at the 2D limit remains challenging, owing to the difficulty in engineering the required chiral magnetic interactions. Here, we report the creation and direct imaging of Néel-type skyrmions in Janus 2D chromium chalcogenides using synchrotron X-ray photoemission electron microscopy, and scanning nitrogen-vacancy magnetometry, which exhibit field-free stability, nonvolatility, and size tunability. First-principles calculations and micromagnetic simulations reveal that Janus-surface-induced inversion-symmetry breaking enhances the Dzyaloshinskii-Moriya interaction, providing the microscopic mechanism for skyrmion stabilization and tunability. We further achieve reversible skyrmion writing and erasing using a single ultrafast laser pulse in a magnetic field as low as 300 Oe, demonstrating the excellent manipulability of this 2D magnetic system. These results establish Janus engineering as a route to creating and manipulating nonvolatile skyrmions in atomically thin magnets, with implications for skyrmion-based low-power spintronic devices.
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All authors Guangyao Miao, Yonglong Ga, Chang Liu, Pan Chen, Yichen Jin, Florian Kronast, Wenxin Cheng, Zhaoqing Ding, Kai Hu, Zongnan Zhang, Nikolai Severin, Chenxi Meng, Patil Shubhada, Sergio Valencia, Meng Meng, Qinlin Guo, Xiaoran Liu, Jiandi Zhang, Yangmu Li, Carlos-Andres Palma, Jürgen P. Rabe, Hongxin Yang, Weihua Wang, Jiandong Guo

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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Strongly Correlated Electrons

Finite-temperature Green's function cluster expansion from thermofield doubles: Breakdown of the polaron picture

M. R. Carbone, S. Fomichev, B. Kloss et al. (+4 more)

We introduce a method, numerically exact in principle, for computing the momentum- and frequency-resolved single-particle Green's function of a polaron at finite temperature. The method, which we refer to as the finite-temperature Green's function cluster expansion, combines two ingredients: the generalized Green's function cluster expansion, a numerically exact extension of the momentum average family of methods that solves the polaron problem at zero temperature through a hierarchy of equations of motion for restricted phonon cloud configurations; and the thermofield double formalism, which maps the thermal trace onto a pure-state expectation value over a doubled Hilbert space. The resulting equations of motion have the same algebraic structure as those of the multi-boson zero-temperature theory, with the temperature entering through a Bogoliubov-type mixing angle that controls the coupling to a set of fictitious bath bosons. We implement the method in our open-source software package and benchmark it on the one-dimensional Holstein polaron, computing spectral functions, dispersions, lifetimes, and effective masses across coupling regimes and temperatures up to $T/\Omega \sim 1$. Where finite-temperature density matrix renormalization group results are available, we find quantitative agreement at affordable computational cost. The method recovers momentum-resolved spectra directly in frequency space, with no time evolution or analytic continuation. We also discuss the practical costs of the approach. In particular, since the doubled phonon Hilbert space has a non-trivial configuration structure in which real and fictitious clouds compete, convergence in the corresponding cloud parameters requires care.
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All authors M. R. Carbone, S. Fomichev, B. Kloss, A. J. Millis, M. Berciu, D. R. Reichman, J. Sous