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Physics Condensed Matter

cond-mat Watch Condensed Matter

Friday 29 May

Mesoscale and Nanoscale Physics

Nonperturbative renormalization of Haldane pseudopotentials from the exact two-electron spectrum

G.-Q. Hai, M. T. Matsubara, L. Cândido et al. (+1 more)

Haldane pseudopotentials $V_{|m|}$ provide the effective interaction parameters governing correlated states in the fractional quantum Hall regime. In conventional formulations, these quantities are obtained by projecting the Coulomb interaction onto relative-angular-momentum states within the lowest Landau level, thereby neglecting virtual transitions to higher Landau levels. Here, we formulate a nonperturbative description of effective interactions directly from the exact two-electron spectrum in a magnetic field. By solving the relative-motion problem beyond the lowest-Landau-level approximation, we define renormalized pseudopotentials $V^*_{|m|}$ from the exact eigenenergies and introduce dynamical corrections $\Delta_{|m|}=V^*_{|m|}-V_{|m|}$. The corrections remain systematically negative and depend strongly on both interaction strength and relative angular momentum, reflecting dynamical correlation effects associated with higher-state virtual admixture. The exact results reproduce the perturbative Landau-level-mixing limit at weak coupling while exhibiting substantial deviations in the strong-mixing regime, signaling the breakdown of low-order perturbative expansions. In particular, the short-range interaction channels relevant to Laughlin-type correlations undergo strong renormalization, leading to substantial modification of the effective interaction hierarchy in strongly interacting systems such as ZnO/MgZnO heterostructures. The present formulation establishes a microscopic framework for incorporating nonperturbative Landau-level-mixing effects into effective interaction theories of quantum Hall systems.
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All authors G.-Q. Hai, M. T. Matsubara, L. Cândido, B. G. A. Brito

Soft Condensed Matter

Discovering Thermodynamically Admissible Dissipation Potentials via Grammar-Based Symbolic Regression

Federico Califano and Jacopo Ciambella

Constitutive laws for inelastic materials must satisfy strict thermodynamic admissibility requirements, yet current data-driven approaches sacrifice interpretability, even when formal guarantees are provided by physics-encoded architectures. We propose a symbolic regression framework for the data-driven discovery of dissipation potentials governing the evolution of internal variables within the Generalized Standard Materials (GSM) formalism. Starting from the Clausius–Duhem inequality, we enforce the thermodynamic requirements, convexity and non-negativity, that the dual dissipation potential must satisfy to guarantee non-negative mechanical dissipation. These requirements are formulated in the general subdifferential setting, encompassing rate-dependent (viscoelastic) and viscoplastic dissipative mechanisms, including potentials with genuine elastic domains, within a unified framework. Candidate potentials are generated by a composition-extended convexity-preserving grammar that guarantees thermodynamic admissibility by construction. The framework is validated on synthetic datasets spanning Newtonian, power-law, and Bingham viscoplastic ground truths under process and measurement noise, and on experimental oscillatory shear measurements of a synthetic elastomer across multiple strain amplitudes and frequencies, where the discovered potentials reproduce the amplitude-dependent softening of the dynamic moduli and outperform a calibrated linear Zener baseline.
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Materials Science

Multiplet-Selective Photoelectron Diffraction from an Altermagnet

L. Plucinski

Direct real-space probes of altermagnetic order remain scarce. Here we introduce multiplet-selective photoelectron diffraction (PED), a methodology in which different regions of a transition-metal core-level multiplet act as distinct photoemission source waves. Using multiple-scattering calculations for the metallic altermagnet candidate CrSb, we show that selected Cr $3p$ multiplet features with predominantly $Y_1^{+1}$ and $Y_1^{-1}$ character generate robust diffraction asymmetries sensitive to altermagnetic domains. We demonstrate that both circularly and linearly polarized light provide access to the effect, while suitable combinations of domains, light polarizations, and multiplet-energy windows suppress nonmagnetic diffraction backgrounds. The proposed approach can be implemented using standard momentum-resolved photoemission instrumentation and establishes core-level PED as a practical route toward domain-resolved studies of altermagnets.
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Superconductivity

Kohn-Luttinger Superconductivity of Weyl Fermi Arcs in PtBi$_2$

Reuel Dsouza, Nikolaos Parthenios, Brian M. Andersen et al. (+1 more)

Recent experimental observations in the noncentrosymmetric Weyl semimetal PtBi$_2$ indicate unconventional superconductivity hosted by topological surface states – Weyl Fermi arcs – with a node at the center of each arc. Focusing on these Fermi arcs, we calculate the electronically mediated pairing interaction using a Kohn-Luttinger approach and find that, in a large region of the phase diagram, the leading superconducting instability has an $i$-wave symmetry featuring precisely such an intra-arc node. We study the dependence of the leading superconducting instabilities on electronic interaction parameters and chemical potential and show that the $i$-wave state is robust to changes in the model parameters. Our results provide a possible mechanism for the observation of topological $i$-wave superconductivity on the surface of PtBi$_2$ and may have implications for the broader landscape of superconducting instabilities arising from repulsive interactions on the surfaces of Weyl semimetals.
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All authors Reuel Dsouza, Nikolaos Parthenios, Brian M. Andersen, Morten H. Christensen

Materials Science

General-purpose LLMs as Constrained Crystal Composition Generators

Hedda Oschinski, Maximilian L. Ach, Konstantin S. Jakob et al. (+2 more)

The targeted discovery of inorganic materials remains challenging due to the vastness of compositional design spaces and the high cost of exhaustive screening. Task-specific generative artificial intelligence represents a particularly efficient alternative to screening, yet demands tedious collection of training data before providing real benefit. General-purpose large language models (LLMs) have recently shown tremendous potential for the targeted generation of single, optimal materials compositions without the need for task-specific fine-tuning. However, it is unclear whether LLMs generally pose an advantage compared to specialized generative models, in particular in large design spaces. Here, we demonstrate that such models are capable of covering entire regions of the targeted property space effectively and systematically. Using Elpasolite materials as an established benchmark for generative tasks in large chemical spaces, we find that an iterative prompt-and-response framework is able to recover on average 96% of all low-energy Elpasolites in the target region. This performance, driven mainly by iterative in-context learning, surpasses the generative abilities of previous, task-specific models. Our results establish general-purpose LLMs as flexible and accessible components for inverse materials design workflows.
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All authors Hedda Oschinski, Maximilian L. Ach, Konstantin S. Jakob, Christian Carbogno, Karsten Reuter

Soft Condensed Matter

Nonequilibrium scaling of drag forces in counterdriven fluid mixtures

Jonas Köglmayr, Florian Sammüller and Matthias Schmidt

We address the effective nonequilibrium drag force field that emerges from the microscopic interparticle interactions in steady states of counterdriven binary fluid mixtures. Using power functional scaling arguments for adaptive Brownian dynamics computer simulation results, we establish quantitatively the crossover between near-equilibrium linear response and far-nonequilibrium square root asymptotics. An algebraic expression captures both limiting cases and remains applicable in the crossover regime. Using simulation results as benchmarks, we verify that a local power functional approximation based on the scaling law reproduces the spatial nonequilibrium structure formation in inhomogenously driven systems. The crossover scenario transcends dynamical density functional theory and it sheds light on general nonequilibrium scaling of driven fluids.
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Statistical Mechanics · Cross-listed from quant-ph

(Non-)Traversable Quantum Phase Transitions

Federico Balducci, Paul M. Schindler, Andrea Solfanelli et al. (+1 more)

Quantum phase transitions manifest as an abrupt change in the ground state of a many-body system; yet it is an open question whether this sudden change necessarily precludes a continuous dynamical connection between the two phases. We introduce a classification of quantum phase transitions based on this geometric aspect of the ground-state manifold, that differs from known classifications. By leveraging the framework of counterdiabatic driving, we explicitly construct schedules that dynamically connect one phase to another. This strategy allows us to uncover a large class of quantum phase transitions, where the states on both sides are separated only by a finite geometric distance in the thermodynamic limit. We term such transitions traversable, since exact counterdiabatic driving links the two phases via a finite dynamical protocol in the thermodynamic limit. We show that multiple known transitions fall into this class – e.g., symmetry-breaking transitions obeying hyperscaling and discontinuous transitions with an enhanced continuous symmetry. We further show the existence of quantum phase transitions that cannot be crossed dynamically even with the help of nonlocal counterdiabatic driving, as they would require divergent amplitudes and frequencies. Geometrically, these nontraversable transitions correspond to an infinite distance separating the two phases of matter; we show that the class comprises continuous transitions exhibiting mean-field universality, and discontinuous transitions arising from the competition between metastable minima. Our geometric classification goes beyond the known taxonomy, is independent of local order parameters and renormalization group fixed points, and has direct implications for the complexity of state preparation and adiabatic quantum computation.
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All authors Federico Balducci, Paul M. Schindler, Andrea Solfanelli, Marin Bukov

Materials Science

Ab Initio Spinor Kadanoff-Baym Approach to Nonequilibrium Electron, Phonon and Magnon Dynamics in Itinerant Ferromagnets

Giovanni Marini

This work introduces a theoretical framework based on the Kadanoff-Baym equations in spinor space to study ultrafast magnetization dynamics in itinerant ferromagnetic systems from first principles. By incorporating spin-orbit coupling into the ab initio Hamiltonian and generalizing the self-energies to include terms beyond the charge sector, I derive scattering integrals within the Markov approximation and quasiparticle renormalizations for electrons and phonons in the presence of spin-dependent effective interactions within a many-body perturbation theory approach. I explicitly discuss how magnons emerge in this framework, and derive, through suitable approximations, a closed and tractable set of equations for coupled electron, phonon and magnon dynamics that can be solved from first principles. I discuss the equations of motion within a minimal ferromagnetic model, where the coupled equations are solved numerically. This approach allows one to treat coherent and incoherent magnetic dynamics on an equal footing, paving the way for a first principles understanding of ultrafast magnetic dynamics and demagnetization, and opening to a truly predictive theory of femtomagnetism in periodic systems.
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Soft Condensed Matter

Cooperative Conformational Transitions in Macromolecules under Mechanical Stretching. An Exactly Solved Model for Single Molecule Experiments

Javier Orradre, Pablo M. Blanco, Sergio Madurga et al. (+3 more)

The stretching behavior of linear macromolecules undergoing conformational transitions is investigated. An exact solution is provided for a two-state system within the elastic freely jointed chain model. This minimal framework contains the smallest set of parameters required to describe such transitions: two Kuhn lengths, two elastic force constants, a free energy difference between both states and a nearest-neighbor interaction energy accounting for cooperativity. Explicit analytical expressions are derived for the chain extension and the probabilities of each state as functions of the applied force.The approach accurately reproduces the experimental force-extension curves of poly(ethylene-glycol) (PEG) and hyaluronic acid (HA), revealing no cooperativity for PEG and negative cooperativity for HA. It also describes the B-DNA to S-DNA conformational transition, a process that exhibits positive cooperativity.We analyze the mathematical conditions required for a transition and identify two fundamental driving mechanisms: differences in Kuhn lengths and differences in force constants.Extensions of the model to systems with more than two conformational states per Kuhn segment are also discussed. The results presented here apply equally to transitions that are intrinsic to the macromolecular structure or induced by ligand-receptor interactions, unifying both cases within a single thermodynamically consistent framework.
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All authors Javier Orradre, Pablo M. Blanco, Sergio Madurga, Marina I. Giannotti, Francesc Mas, Josep L. Garcés

Superconductivity

What controls the superconducting dome of electron-doped FeSe?

Paul T. Malinowski, Chad J. Mowers, Yaoju Tarn et al. (+3 more)

Superconducting domes are conspicuous features of the phase diagrams of most unconventional and high-temperature superconductors. The superconducting transition temperature ($T_{c}$) of FeSe can be dramatically enhanced with electron doping, but unlike all other high-temperature and unconventional superconductors, its full phase diagram and superconducting dome have yet to be fully explored. Here, we employ a combination of molecular beam epitaxy synthesis, alkali surface doping, in-vacuum electrical transport, and angle-resolved photoemission spectroscopy to investigate the entire superconducting dome of electron-doped FeSe, achieving a fully metallic state where superconductivity is suppressed in the heavily overdoped regime. We discover a robust scaling between $T_{c}$ and the residual resistivity ($\rho_{0}$) which holds across the entire superconducting dome, suggesting that the evolution of $T_{c}$ is heavily influenced by the evolution of the elastic scattering rate in the high-$T_{c}$ electron-doped phase. This in turn suggests that the superconducting dome in electron-doped FeSe appears to be fundamentally different than that of other unconventional superconductors where doping plays the primary role, and may be driven primarily by the sensitivity of the superconductivity to disorder.
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All authors Paul T. Malinowski, Chad J. Mowers, Yaoju Tarn, Darrell G. Schlom, Brendan D. Faeth, Kyle M. Shen

Statistical Mechanics

Solving models with generalized free fermions II: Path-product expansion and conserved charges

Kohei Fukai, Balázs Pozsgay and István Vona

Free-fermion solvability in quantum spin systems is increasingly understood to be governed by a graph Clifford algebra defined from the frustration graph of the Hamiltonian. When the frustration graph belongs to certain classes, such as the even-hole-free and claw-free (ECF) class, the Hamiltonian is solvable by hidden free fermions: it admits a free-fermion solution although it does not reduce to a Majorana bilinear under the Jordan-Wigner transformation. However, unlike in the Jordan-Wigner case, where each mode is a linear combination of single Majorana fermions, the explicit operator structure of the hidden free-fermion modes—and that of the local conserved charges—has remained obscure. In this work, we derive a path-product expansion that expresses each free-fermion mode as a linear combination of products along induced paths in the extended frustration graph. The expansion follows from the Krylov generating function and yields the modes directly, without using the transfer matrix or nonlocal conserved charges; the resulting mode decomposition also computes infinite-temperature dynamical correlation functions. We further obtain explicit expressions for local conserved charges as linear combinations of path products along induced paths; these charges apply beyond the free-fermion (ECF) class to more general claw-free frustration graphs. We also identify a family of generalized conserved charges containing both the known nonlocal conserved charges and these local charges. For the homogeneous periodic Fendley model, the local conserved charges exhibit the same Catalan-tree pattern as those of the spin-$1/2$ XXX chain.
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Materials Science

Strain-Engineered s-C$_3$N$_6$ Monolayer for Efficient Water Splitting: A first-principles study

Khushboo Dange and Alok Shukla

Photocatalytic water splitting offers a sustainable route for solar-to-hydrogen energy conversion, yet identifying stable, metal-free semiconductors with suitable electronic, optical, and band-alignment properties remains challenging. Here, we investigate the structural, mechanical, electronic, optical, and photocatalytic properties of the two-dimensional s-C$_3$N$_6$ monolayer using first-principles calculations. Ab initio molecular dynamics and elastic constant analysis confirm its thermal and mechanical stability. Hybrid HSE06 calculations reveal pristine s-C$_3$N$_6$ is a direct-band-gap semiconductor (2.62 eV). However, its conduction-band minimum lies below the hydrogen reduction potential, preventing spontaneous hydrogen evolution. To overcome this limitation, we employ biaxial and uniaxial strains (-10% to +10%) to modulate its electronic structure. We find that compressive biaxial strains of -8% and -10% uniquely tune the band edges to straddle the redox potentials, enabling spontaneous overall water splitting. Crucially, these photocatalytically active states remain mechanically and thermally stable. Optical properties calculations show the fundamental gap in both pristine and strained structures is optically dark, with the primary absorption peak in the UV region. Furthermore, a strain-induced mobility mismatch between electrons and holes facilitates efficient charge separation. However, thermodynamic modeling of surface kinetics reveals that the s-C$_3$N$_6$ surface binds intermediates strongly, necessitating a co-catalyst to overcome kinetic barriers. Our results establish strain engineering as an effective strategy to tailor band-edge alignment, carrier dynamics, and optical transitions in s-C$_3$N$_6$, highlighting its potential for stable 2D photocatalytic water splitting.
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Statistical Mechanics

Optimization of multisite reactions in complex compartmentalized media

T. V. Mendes and T. Guérin

In complex media, transport and geometric properties deeply influence the kinetics of random encounters between reactants. Here, we consider the situation where a random walker, moving in a regularly diffusing medium, has to reach and activate a target located inside a compartment characterized by fractal (obstructed) sub-diffusion. We focus on dual-site reactions, which end when two activation events occur within a given time window. Each activation event happens with a finite probability whenever the random walker visits the target. For weakly reactive targets, we demonstrate that the reaction time can be minimized for an optimal compartment size and can even be accelerated when compared to the same system without compartment. Our analytical predictions are validated through simulations of a random walker on a cubic lattice, where some sites inside the compartment are obstructed at the critical percolation threshold. Our theory illustrates the fact that adding a crowded compartment around a target, even if it slows down the motion in its vicinity, can accelerate the kinetics of complex reactions, especially for weakly reactive targets.
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Materials Science

Odd-Parity Magnons

Pu Zhang, Sun-Bo Xie, Junxi Yu et al. (+2 more)

Magnons, as charge-neutral spin excitations, can transport spin information without Joule heating and therefore offer a promising platform for low-power spintronics. However, in collinear magnets, the effective time-reversal symmetry forbids odd-parity magnon band splitting. Here we propose odd-parity magnons and establish a general mechanism for realizing them in collinear antiferromagnets. We provide a complete spin-point-group classification of odd-parity magnon splitting in two-dimensional collinear antiferromagnets by identifying the leading splitting types and their symmetry-allowed basis functions. This classification serves as a practical guide for searching for odd-parity magnons. We show that breaking effective time-reversal symmetry, for example by circularly polarized light or loop currents, can induce highly tunable $p$- and $f$-wave magnon splitting. In bilayer systems, the dynamical modulation can drive a topological magnon phase transition, accompanied by chiral edge modes and an abrupt jump in the magnon thermal Hall conductivity. Material-specific first-principles calculations further demonstrate the feasibility of this mechanism in real van der Waals antiferromagnets. Our study identifies the odd-parity magnons as a new class of spin excitations and provides a theoretical foundation for odd-parity magnons and ultrafast optically controlled topological magnonic devices.
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All authors Pu Zhang, Sun-Bo Xie, Junxi Yu, Yichen Liu, Cheng-Cheng Liu

Materials Science

Crystallisation kinetics of supercooled liquid palladium

Zuzanna Kostera, Przemyslaw Dziegielewski, Christian Bressler et al. (+13 more)

In this study, we employ classical molecular dynamics (MD) simulations to investigate the crystallisation kinetics of supercooled liquid palladium and relate the results to time-resolved X-ray diffraction measurements on rapidly quenched Pd thin films. Crystal nucleation and growth rates are determined over the temperature range $700$–$1150~\mathrm{K}$ ($0.38$–$0.65 T_{\mathrm{m}}$) by analysing the evolution of the microstructure during the liquid-to-crystal transition. The self-diffusion coefficient of Pd, obtained from the atomic mean-squared displacement, follows Arrhenius behaviour over the investigated temperature range, with an activation energy of $467(6)~\mathrm{meV/atom}$, consistent with available data for supercooled liquid metals. The steady-state homogeneous nucleation rate exhibits a maximum of approximately $4 \times 10^{35}~\mathrm{m^{-3} s^{-1}}$ near $0.5 T_{\mathrm{m}}$. Crystal growth occurs at velocities of the order of metres per second, with a temperature dependence consistent with diffusion-limited Wilson-Frenkel kinetics rather than the collision-limited regime. Based on multiple statistically independent simulations, a time-temperature-transformation (TTT) diagram for crystallisation onset is constructed. The TTT curve exhibits a nose near $0.5 T_{\mathrm{m}}$ and $100~\mathrm{ps}$, corresponding to a critical cooling rate for vitrification on the order of $10^{13}~\mathrm{K s^{-1}}.$ The simulations reproduce the crystallisation onset time and temperature observed in time-resolved X-ray diffraction experiments on optically molten Pd thin films quenched at $5 \times 10^{11}~\mathrm{K s^{-1}}.$ These results indicate that homogeneous, rather than heterogeneous, nucleation governs the achievable supercooling in the experimentally studied films.
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All authors Zuzanna Kostera, Przemyslaw Dziegielewski, Christian Bressler, Wojciech Gawelda, Konstantinos Georgarakis, Dmitry Khakhulin, Oleksii I. Liubchenko, Adam Olczak, Angel Rodriguez-Fernandez, Ryszard Sobierajski, Klaus Sokolowski-Tinten, Peihao Sun, Robert W.E. van de Kruijs, Hazem Yousef, Peter Zalden, Jerzy Antonowicz

Materials Science

Direct Observation of Chemical Short-Range Order in CoCrNi Alloy Using Neutron Diffraction

Vinícius P. Bacurau, Camilo Salvador, Guilherme C. Stumpfa et al. (+8 more)

This study provides experimental evidence of chemical short-range order (CSRO) in the equiatomic CoCrNi alloy, identified through neutron diffraction. The phenomenon manifests as a distinct diffuse peak at Q = 1.85 A-1, the intensity increases under thermodynamically favorable conditions for CSRO development such as prolonged aging (100 h and 240 h) at 748 K or shorter aging (24 h) at slightly higher temperature (798 K). The degree of ordering was measured by integrating the diffuse scattering intensity, revealing that the gas-atomized sample, i.e. the sample with the least amount of CSRO, still displays approximately 70% of the CSRO level observed in the sample subsequently aged for 240 h at 748 K, i.e. the sample with the highest amount of CSRO produced in this study. Predictive atomistic simulations reproduced both the presence and position of the diffuse peak, while two-dimensional fast Fourier transform (FT-2D) analyses indicated that reflections at (1 1/2 0) within the <001> zone axis originate from some structural projections associated with like D022, Pt2Mo and D1a motifs. Complementary small-angle neutron scattering (SANS) measurements identified Ni-rich, disk-shaped domains with radii of approximately 11 A and thicknesses of about 1 A, consistent with nanoscale CSRO characteristic length scale. These findings demonstrate that CSRO is an intrinsic and energetically favorable feature of the CoCrNi system, remaining stable even under rapid solidification and further enhanced by low-temperature aging. Combined use of neutron diffraction and atomistic modeling provides a framework for probing local ordering phenomena in multi-principal element alloys (MPEAs).
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All authors Vinícius P. Bacurau, Camilo Salvador, Guilherme C. Stumpfa, Angelo F. Andreolli, Caroline B. Stocoa, Eric M. Mazzer, Lewis Owen, Yifan Cao, Rodrigo Freitas, Daniel Miracle, Francisco G. Coury

Statistical Mechanics · Cross-listed from math.PR

Multi-point functions of a full-plane two-state fuzzy $4$-Potts model

Hong-Bin Chen and Jiaming Xia

We study the full-plane two-state fuzzy $4$-Potts model, obtained by assigning independent balanced $\{\pm1\}$ spins to the open clusters of a critical $q=4$ random-cluster configuration. Its spin field has the same law as a single-spin projection and as the polarization field of the isotropic Ashkin–Teller model at its Potts point. We prove that, after proper normalization, all even multi-point spin correlation functions converge to explicit conformally covariant Coulomb-gas type neutral charge sums. As a consequence, we prove convergence in law of the rescaled magnetization field and identify its limit in law with $2\mathopen{:}\cos(X/2)\mathclose{:}$ for a full-plane Gaussian free field $X$. The proof combines the Baxter–Kelland–Wu coupling, convergence of the six-vertex height function to the Gaussian free field, and a charge-completion mechanism: an enlarged discrete sum over charge assignments with total charge in $4\mathbb Z$ produces a combinatorial cancellation of connection patterns, while only the neutral charge sector survives in the scaling limit.
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Soft Condensed Matter

Spontaneous flows and interfacial instabilities in oxygen-sensitive living active matter

Azam Gholami, Sangram Gore, Sai V. R. Ambadipudi et al. (+2 more)

Active fluids generate motion and stress internally, but in living systems their activity is often regulated by environmental fields that organisms consume or produce. How such fields localise active stresses and create flow-generating interfaces remains unclear. Here we show that oxygen organises suspensions of the flagellated microswimmer Euglena gracilis into an annular living interface. In circular chambers with an air-exposed periphery, a labyrinthine bioconvective pattern and an annular cellular accumulation emerge nearly simultaneously, whereas sealing the periphery suppresses the annulus. The accumulation then sharpens, develops finite-wavelength protrusions and forms a long-lived, collectively rotating corona. An oxygen-coupled polar active-fluid model qualitatively recapitulates this progression: oxygen transport, cellular consumption and oxygen-regulated swimming and reorientation assemble and position the annulus, whereas dipolar active stresses destabilise it and generate collective flow. These results show how a metabolically shaped chemical field can create and activate a living interface, linking taxis, bioconvection and active interfacial hydrodynamics.
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All authors Azam Gholami, Sangram Gore, Sai V. R. Ambadipudi, Iraj Gholami, Albert J. Bae

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

Optimisation and Precision Tuning of Localised Surface Plasmon Resonance in AuFON Systems

Luis Alfonso Guerra Hernández, Osmar Gil Salas, Jorge Enrique Rueda Parada et al. (+2 more)

Metal film on nanosphere (MFON) plasmonic systems have emerged as nanostructures with useful properties for molecular detection. This work presents the optimisation and discussion of the characterisation results of Au-film on nanosphere (AuFON) systems. Through experiments and numerical simulations, the resonance energies of localised surface plasmon modes and the spatial distribution of light on the nanostructured surface were identified. The results highlight the dependence of these modes on changes in nanostructure dimensions and the optical conditions of the incident radiation, thereby optimising the amplification of optical signals for applications in molecular detection.
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All authors Luis Alfonso Guerra Hernández, Osmar Gil Salas, Jorge Enrique Rueda Parada, Alejandro Fainstein, Andrés Alejandro Reynoso

Materials Science

Co-optimization of spin coherence and valley splitting in Si/SiGe heterostructures

Peihong Zhang, Xuedong Hu, Saif Ullah et al. (+1 more)

Single electron spins can be used to encode and process information in semiconductor quantum devices. Progress has been hindered by materials challenges, such as the small energy splitting between low-lying valley states and hyperfine coupling to nuclear spins. Here we use density functional theory to optimize the valley splitting and spin dephasing time in realistic Si/SiGe heterostructures. Reductions in the Si quantum well width generally increase the valley splitting. However, in narrow quantum wells, a larger fraction of the electronic wavefunction resides in the SiGe buffer layers, which increases the hyperfine coupling with spinful $^{73}$Ge. Our work shows that Si/SiGe heterostructures with 3~–~4~nm wide quantum wells and $^{73}$Ge and $^{29}$Si concentrations of 50 ppm should support average valley splittings $E_{v}$~$>$~500~$\mu$eV and spin dephasing times $T_2^*$ exceeding 15~$\mu$s assuming an effective quantum dot area of 700 nm$^2$. In addition, sharper Si/SiGe interfaces in general result in larger valley splittings and longer spin dephasing times.
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All authors Peihong Zhang, Xuedong Hu, Saif Ullah, Jason R. Petta

Mesoscale and Nanoscale Physics

Feasibility study of continuous electronic Pomeranchuk cooling with a flavor-degenerate Wigner crystal

Robin J. Dolleman, Ammon Fischer, Lennart Klebl et al. (+5 more)

Achieving sub-millikelvin electron temperatures in nanoelectronic devices could unveil new transport phenomena, extend quantum coherence times, and enhance the precision of quantum metrology. However, maintaining such low temperatures continuously remains a long-standing challenge. Here, we propose and simulate an on-chip cooling cycle that harnesses the entropy difference between an electron liquid (EL) and a Wigner crystal (WC) in flavor-degenerate flat-band materials. Cooling is driven by a current through a device with a locally gated region. Within this region, the charge carrier density is tuned such that a WC forms beneath the gate. As carriers transition from an EL to WC phase, their entropy increases, extracting heat and the sliding WC advects this heat along the device. The heat is then released when carriers transition back to the EL phase, which establishes distinct hot and cold regions and a steady temperature gradient over the device. Simulations show net cooling for sufficiently low current densities, typically below $1~\mathrm{nA}/\mu\mathrm{m}$, whereas Joule heating dominates at higher currents. Within the gated region, we estimate cooling powers of up to $8.4~\mathrm{aW}/\mu\mathrm{m}$ at a bath temperature of $4~\mathrm{mK}$. Our approach can achieve electron temperatures well below $1~\mathrm{mK}$ under suitable conditions, promising a route towards continuous on-chip cooling in this temperature regime. Our approach applies to any flat-band material with low-energy flavor degeneracy (valley and/or orbital) and low disorder, including gapped Bernal-stacked bilayer graphene, rhombohedral-stacked multilayer graphene, and magic-angle twisted bilayer graphene.
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All authors Robin J. Dolleman, Ammon Fischer, Lennart Klebl, Alexander Rothstein, Dante M. Kennes, Bernd Beschoten, Florian Libisch, Christoph Stampfer

Materials Science

A Practical Guide for Diagnosing Imaginary Phonon Modes in Metal–Organic Frameworks: The Case of MOF-5

Julia Santana-Andreo and Caterina Cocchi

Assessing the dynamical stability of computationally predicted metal–organic frameworks (MOFs) is essential to distinguish synthetically feasible structures from dynamically unstable ones. However, reliable first-principles phonon calculations on these systems remain challenging: their large, flexible unit cells and soft collective modes make the vibrational spectrum highly sensitive to the numerical settings. Using MOF-5 as a representative case study, we establish a finite-displacement workflow to identify and isolate the origins of imaginary phonon modes. We demonstrate how numerical force convergence thresholds, real-space grid resolutions, symmetry-standardization protocols, and alternative unit-cell representations can qualitatively and spuriously alter the predicted lattice stability. Once numerical noise is confidently excluded, the remaining imaginary modes can be analyzed through mode mapping, with stochastic Monte Carlo symmetry-breaking distortions outlined as a complementary strategy for more complex landscapes. This protocol provides a robust, transferable strategy for the reliable assessment of dynamical stability and lattice vibrations in flexible porous frameworks.
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Soft Condensed Matter

Limits of the Non-Linear Generalized Langevin Equation: Cross-Correlations, Irreversibility and Desynchronization

Bernd Jung and Gerhard Jung

The generalized Langevin equation (GLE) is widely used to model complex soft-matter systems, including biomolecular dynamics, by incorporating memory effects and colored noise into coarse-grained descriptions. However, recent results suggest that combining memory with non-linear forces, ubiquitous in soft matter, introduces fundamental analytical inconsistencies. Here, using a simplified model, we investigate the practical numerical consequences of these analytical results. We show that non-linear forces generate cross-correlations with the noise, modifying the fluctuation-dissipation theorem and rendering the noise position-dependent and irreversible. This implies that the commonly assumed reversible Gaussian noise in GLE simulations fails to capture essential features of the microscopic fluctuations. For weak non-linearities, these issues can be partially resolved either by using an iterative optimization of memory or by using microscopically consistent noise, which unexpectedly synchronizes GLE trajectories with the underlying microscopic dynamics. For stronger non-linearities like high barriers or shoulders in the external potential, however, iterative reconstruction fails and we observe desynchronization, indicating that the non-linear GLE no longer correctly reproduces the microscopic dynamics. Our results show in which situations non-linear GLEs can be accurately applied and when they fail, thus providing practical guidance for their application to coarse-grain soft-matter systems.
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Mesoscale and Nanoscale Physics

Valley-polarized Orbital and Spin Magnetism Induced by Femtosecond Optical Pulses in Two-Dimensional Semiconductors

M. S. Mrudul and Peter M. Oppeneer

We theoretically investigate the ultrafast generation of spin and orbital magnetism in a two-dimensional gapped Dirac system with spin-orbit coupling. This system is representative of two-dimensional hexagonal semiconductors, such as transition-metal dichalcogenides that exhibit valley-selective optical selection rules arising from the valley-contrasting magnetic texture of their band structure. Using a time-dependent density-matrix formalism, we demonstrate that circularly polarized laser pulses generate nonequilibrium magnetization under both resonant and multiphoton resonant conditions. We show that the induced spin and orbital magnetic moments can be distinctly controlled via the photon energy and polarization of the driving field. Furthermore, spin and orbital dynamics originate from fundamentally different light-matter coupling mechanisms, leading to qualitatively dissimilar temporal behaviors. The orbital magnetic moment couples directly to the external electric field, resulting in faster dynamics and pronounced Rabi-like oscillations, whereas the spin response develops gradually through spin-orbit coupling. Consequently, orbital dynamics is significantly more sensitive to electron-hole dephasing than the spin response. Our results highlight the importance of properly accounting for orbital contributions in future technologies that utilize femtosecond control of magnetism.
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Soft Condensed Matter

Droplets sitting on thin elastic sheets: A study with the boundary element method

Salik Sultan, Josua Grawitter, Gonçalo C. Antunes et al. (+1 more)

Elasto-capillarity of a droplet wetting an elastic sheet provides an interesting system, both for fundamental and applied research. The droplet sinks into the sheet and assumes the shape of a lens. To determine the equilibrium shape in simulations, we formulate a boundary element method (BEM) extending our earlier approaches, and apply the BEM to three specific protocols for the boundary conditions of the sheet. For a clamped elastic sheet, we use various morphological metrics to demonstrate that the lens shape crucially depends on the sheet thickness. Stretching the sheet isotropically, allows for an additional control parameter to influence the droplet shape and the tension in the sheet, which we quantify by radial profiles of the azimuthal and radial elastic stresses. We further demonstrate how the focal length of a liquid lens can be tuned by varying the applied tension. Finally, stretching the sheet along one direction, elongates the droplet, and the sheet shows folds and dimples.
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All authors Salik Sultan, Josua Grawitter, Gonçalo C. Antunes, Holger Stark

Quantum Gases

Charged Bose polarons at finite momentum

Grover Andrade-Sánchez and Arturo Camacho-Guardian

Charged impurities in quantum fluids have unveiled new classes of strongly correlated many-body states across condensed matter, ultracold gases, and hybrid atom-ion platforms. While previous studies have primarily focused on their ground-state and static properties, much less is known about their finite-momentum behavior, which governs transport, dissipation, and quasiparticle stability. Here, we investigate the momentum-dependent properties of a charged Bose polaron using a diagrammatic approach within second-order perturbation theory, explicitly accounting for the finite-range nature of the ion-atom interaction. We show that the interaction range introduces a characteristic momentum scale at which many-body dressing and dissipation are maximized, leading to a non-monotonic behavior of the damping rate and quasiparticle energy. In the high-momentum regime, we uncover a scaling law $\Gamma_p \sim 1/p$, signaling the suppression of many-body dressing and the recovery of quasi-free impurity dynamics, in stark contrast to the divergent behavior predicted by contact-interaction perturbative treatments.
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Materials Science

Magnetic domains reconfiguration on the Fe3O4(110) surface across the Verwey transition by Spin-Polarized Low-Energy Electron Microscopy

C. Gutiérrez-Cuesta, A. Mandziak, J.E. Prieto et al. (+6 more)

We have studied the (110) surface of Fe$_3$O$_4$ single crystals by means of spin-polarized low-energy electron microscopy (SPLEEM). After preparation by sputtering and annealing a well defined reconstructed surface was achieved, composed of rows aligned in the [010] direction. By acquiring SPLEEM images along different spin directions the vector magnetization was mapped on the surface, both at room temperature and at a temperature well below the Verwey transition. At room temperature, domains were observed with their magnetization aligned along the two <111> bulk easy axes which are in the (110) surface plane. They presented 180$^\circ$, 71$^\circ$ and 109$^\circ$ Néel-type domain walls. Below the Verwey transition, the magnetization directions changed to regions where the magnetization was oriented along the in-plane [100] and [001] directions. Those observations can be interpreted as the presence of magnetized regions on the surface where the monoclinic $c$ axis is in-plane in the former, and regions where the $c$ is out-of-plane in an oblique direction in the latter. However, the magnetization was at all times within the surface plane, with no out-of-plane component detected.
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All authors C. Gutiérrez-Cuesta, A. Mandziak, J.E. Prieto, P. Nita, A. Mascaraque, U. Choudhry, J. Turner, A. Stibor, J. de la Figuera

Strongly Correlated Electrons

Emergence of spin entanglement with the pseudogap onset in the Fermi-Hubbard model

Frederic Bippus, Thomas Chalopin, Gabriele Bellomia et al. (+6 more)

Despite decades of intense theoretical and experimental investigation, the two-dimensional Fermi-Hubbard model still resists a complete microscopic understanding. Conventional approaches typically probe global observables and locally resolved correlation functions. Here, we develop a complementary perspective based on the measurement of entanglement. Using both an ultracold-atom quantum simulator and numerical simulations based on the dynamical vertex approximation, we find that entanglement is closely tied to the onset of the enigmatic pseudogap regime: spin-singlet entanglement emerges only as the pseudogap sets in and, in contrast to classical correlations, remains confined to nearest-neighbour sites in this regime. Our results, therefore, disfavour purely classical-fluctuation theories of the pseudogap and constrain microscopic models to those that develop nearest-neighbour spin-singlet entanglement at the pseudogap onset.
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All authors Frederic Bippus, Thomas Chalopin, Gabriele Bellomia, Gergő Roósz, Titus Franz, Antoine Georges, Anna Kauch, Immanuel Bloch, Karsten Held

Statistical Mechanics

Global thermodynamics for heat-conducting fluids under weak gravity

Naoko Nakagawa and Shin-ichi Sasa

We study liquid-gas coexistence under gravity and heat conduction from the viewpoint of global thermodynamics. We construct a variational free-energy function for the fixed-global-temperature description and decompose it into two parts. The first has the same configurational form as the equilibrium weak-gravity free energy with gravity replaced by the effective gravity, and it determines the first-order configurational transition between the two separated liquid-gas arrangements. The second is a residual excess-latent-heat contribution that vanishes without heat conduction. Although it does not decide which separated liquid-gas arrangement is thermodynamically favored, this residual part is needed to derive the fundamental relation in the laboratory variables and to recover thermodynamic observables such as the spatially averaged pressure. The same residual contribution reshapes the barrier geometry, ridge/valley structure, and interfacial anomalies of the fixed-global-temperature free-energy landscape. Numerical examples based on the van der Waals model illustrate the resulting landscape structure, and estimates of experimental scales suggest a setup for detecting the effective-gravity inversion.
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Disordered Systems and Neural Networks · Cross-listed from physics.soc-ph

q-Exponential Random Graphs: higher-order networks from simple constraints

David Dobáš, Diego Garlaschelli and Petr Jizba

Exponential Random Graphs (ERGs) are among the most widely used network models, derived as principled least-bias graph ensembles that maximize Shannon entropy under constraints on the expected values of given structural properties. However, it has been recently (re)discovered that, in the absence of additional information privileging Shannon entropy, the most agnostic inferential construction should maximize the broader class of Uffink entropies. The resulting entropy-maximizing distribution changes from the exponential (Boltzmann-Gibbs) to the so-called q-exponential one. Since maximizing Shannon entropy may produce an unjustified independence between degrees of freedom, here we investigate how the most popular ERGs with independent edges (namely, the Erdos-Renyi and configuration models) generalize to higher-order q-Exponential Random Graphs with dependent edges in the non-Shannon case, while keeping their defining constraints (number of links and degree sequence, respectively) unchanged. We find features, such as a phase transition between sparse and dense regimes, that are absent in the original ERGs but typical of higher-order networks, plus novel phenomena such as richer assortativity and clustering profiles, which allow for the coexistence of link sparsity and triadic closure. These results show that higher-order networks do not necessarily require higher-order constraints, as they naturally arise from simpler ones in a framework that is even more agnostic than Shannon's.
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