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Physics Condensed Matter

cond-mat Watch Condensed Matter

Thursday 30 October

Mesoscale and Nanoscale Physics

Transverse and Unidirectional Spin Pumping

Ping Li, Chengyuan Cai and Tao Yu

Conventional spin pumping, driven by magnetization dynamics, is longitudinal since the pumped spin current flows normal to the interface between the ferromagnet and the conductor. We predict Hall-type/transverse and unidirectional spin pumping into conductors by near-field electromagnetic radiation emitted by, e.g., magnetization dynamics. The joint effect of the electric and magnetic fields results in a pure spin current flowing parallel to the interface, i.e., a Hall-type spin pumping, which is highly efficient due to the strong coupling to the electric field. Such a transverse spin current is unidirectional, with the spatial distribution controlled by the magnetization direction. Our finding reveals a robust approach for generating and manipulating spin currents in future low-dimensional spintronic and orbitronic devices.
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Superconductivity

Topological phases of the Bogoliubov de Gennes Hamiltonian

Klaus Ziegler

We investigate a two-dimensional superconducting system with a smoothly and periodically varying order parameter. The order parameter is modulated along one direction while remaining uniform in the perpendicular direction, leading to a spatially periodic superconducting phase. We show that the periodicity of the order parameter determines the winding number of the eigenfunctions, which serves as a topological characterization of the system. A topological invariant is identified that links the winding number directly with the Bloch vector. By solving the Bogoliubov-de Gennes equation, we obtain both plane-wave solutions describing bulk states and exponentially localized solutions that correspond to edge modes. The analytic bulk-edge connection is employed to identify the conditions under which the edge states emerge from the bulk spectrum. We find that the winding numbers depend on the boundary conditions, which differ between the plane-wave and exponential solutions. These results establish a direct connection between the spatial modulation of the order parameter, the topological structure of the eigenstates, and the emergence of edge modes in periodically modulated superconducting order parameters.
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Materials Science

Applications of Machine Learning in Polymer Materials: Property Prediction, Material Design, and Systematic Processes

Hongtao Guo Shuai Li Shu Li

This paper systematically reviews the research progress and application prospects of machine learning technologies in the field of polymer materials. Currently, machine learning methods are developing rapidly in polymer material research; although they have significantly accelerated material prediction and design, their complexity has also caused difficulties in understanding and application for researchers in traditional fields. In response to the above issues, this paper first analyzes the inherent challenges in the research and development of polymer materials, including structural complexity and the limitations of traditional trial-and-error methods. To address these problems, it focuses on introducing key basic technologies such as molecular descriptors and feature representation, data standardization and cleaning, and records a number of high-quality polymer databases. Subsequently, it elaborates on the key role of machine learning in polymer property prediction and material design, covering the specific applications of algorithms such as traditional machine learning, deep learning, and transfer learning; further, it deeply expounds on data-driven design strategies, such as reverse design, high-throughput virtual screening, and multi-objective optimization. The paper also systematically introduces the complete process of constructing high-reliability machine learning models and summarizes effective experimental verification, model evaluation, and optimization methods. Finally, it summarizes the current technical challenges in research, such as data quality and model generalization ability, and looks forward to future development trends including multi-scale modeling, physics-informed machine learning, standardized data sharing, and interpretable machine learning.
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Strongly Correlated Electrons

Invariants for (2+1)D bosonic crystalline topological insulators for all 17 wallpaper groups

Vladimir Calvera, Naren Manjunath and Maissam Barkeshli

We study bosonic symmetry-protected topological (SPT) phases in (2+1) dimensions with symmetry $G = G_{\text{space}}\times K$, where $G_{\text{space}}$ is a general wallpaper group and $K=\text{U}(1),\mathbb{Z}_N, \text{SO}(3)$ is an internal symmetry. In each case we propose a set of many-body invariants that can detect all the different phases predicted from real space constructions and group cohomology classifications. They are obtained by applying partial rotations and reflections to a given ground state, combined with suitable operations in $K$. The reflection symmetry invariants that we introduce include `double partial reflections', `weak partial reflections' and their `relative' or `twisted' versions which also depend on $K$. We verify our proposal through exact calculations on ground states constructed using real space constructions. We demonstrate our method in detail for the groups p4m and p4g, and in the case of p4m also derive a topological effective action involving gauge fields for orientation-reversing symmetries. Our results provide a concrete method to fully characterize (2+1)D crystalline topological invariants in bosonic SPT ground states.
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Superconductivity

Direct observation of the surface superconducting gap in the topological superconductor candidate \beta-PdBi2

Akifumi Mine, Takeshi Suzuki, Yigui Zhong et al. (+7 more)

\beta-PdBi2 is one of the candidates for topological superconductors with a superconducting (SC) transition temperature (Tc) of 5.3 K, in which parity mixing of spin singlet and spin triplet has been anticipated, being crucial for the further understanding of relationship with inversion symmetry and parity mixing in the superconductivity. In this work, we measured the SC gap in high-quality single crystal of \beta-PdBi2 by using high-resolution laser angle-resolved photoemission spectroscopy below Tc. We found the isotropic SC gaps in momentum space for multiple bands, and observed that the difference between the SC gap of the topological surface bands and the bulk bands is about 0.1 meV, consistent with other experimental results. These direct and quantitative experimental results support the possibility of \beta-PdBi2 as a topological superconductor, characterized by unique crystal and electronic band structures.
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All authors Akifumi Mine, Takeshi Suzuki, Yigui Zhong, Sahand Najafzadeh, Kenjiro Okawa, Masato Sakano, Kyoko Ishizaka, Shik Shin, Takao Sasagawa, Kozo Okazaki

Materials Science

A Review of AI-Driven Approaches for Nanoscale Heat Conduction and Radiation

Ziqi Guo, Daniel Carne, Krutarth Khot et al. (+3 more)

Heat conduction and radiation are two of the three fundamental modes of heat transfer, playing a critical role in a wide range of scientific and engineering applications ranging from energy systems to materials science. However, traditional physics-based simulation methods for modeling these processes often suffer from prohibitive computational costs. In recent years, the rapid advancements in Artificial Intelligence (AI) and machine learning (ML) have demonstrated remarkable potential in the modeling of nanoscale heat conduction and radiation. This review presents a comprehensive overview of recent AI-driven developments in modeling heat conduction and radiation at the nanoscale. We first discuss the ML techniques for predicting phonon properties, including phonon dispersion and scattering rates, which are foundational for determining material thermal properties. Next, we explore the role of machine-learning interatomic potentials (MLIPs) in molecular dynamics simulations and their applications to bulk materials, low-dimensional systems, and interfacial transport. We then review the ML approaches for solving radiative heat transfer problems, focusing on data-driven solutions to Maxwell's equations and the radiative transfer equation. We further discuss the ML-accelerated inverse design of radiative energy devices, including optimization-based and generative model-based methods. Finally, we discuss open challenges and future directions, including data availability, model generalization, uncertainty quantification, and interpretability. Through this survey, we aim to provide a foundational understanding of how AI techniques are reshaping thermal science and guiding future research in nanoscale heat transfer.
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All authors Ziqi Guo, Daniel Carne, Krutarth Khot, Dudong Feng, Guang Lin, Xiulin Ruan

Materials Science

Theoretical design of the large topological magnetoelectric effect in the Co-intercalated NbS$_2$ structure

Hyowon Park and Ivar Martin

A triangular Co-ion lattice intercalated between 1-H NbS$_2$ layers can exhibit a large anomalous Hall effect (AHE) due to the finite scalar spin chirality originating from the non-coplanar $3q$ ordering of Co spins. This large AHE occurs when the scalar spin chirality is uniform in all Co layers, as indeed found in the Co$_{1/3}$NbS$_2$ case [Phys. Rev. Mater. 6, 024201 (2022)]. However, if the spin chirality were staggered with the opposite signs in the adjacent Co layers, the net AHE would disappear, yielding instead the topological magneto-electric effect. Here, we theoretically verify that a transverse electric field generates a finite orbital magnetization under such conditions, consistent with the axion-like coupling. Using first-principles calculations, we show that the resulting magneto-electric coupling, $\alpha^{zz}$ can be as large as 0.9 $e^2/2h$. We also demonstrate that the inter-layer magnetic coupling in these materials can be tuned by strain, enabling the switching between the AHE and the axionic states.
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Strongly Correlated Electrons

From Ferromagnet to Antiferromagnet: Dimensional Crossover in (111) SrRuO3 Ultrathin Films

Zhaoqing Ding, Xuejiao Chen, Lei Liao et al. (+11 more)

SrRuO3 is a canonical itinerant ferromagnet, yet its properties in the extreme two-dimensional limit on a (111) crystal plane remain largely unexplored. Here, we demonstrate a complete transformation of its ground state driven by dimensional reduction. As the thickness of (111)-oriented SrRuO3 films is reduced to a few unit cells, the system transitions from a metallic ferromagnet to a semiconducting antiferromagnet. This emergent antiferromagnetism is evidenced by a vanishing magnetic remanence and most strikingly, by the appearance of an unconventional twelve-fold anisotropic magnetoresistance. First-principles calculations confirm that an A-type antiferromagnetic order is the stable ground state in the ultrathin limit. Our findings establish (111) dimensional engineering as a powerful route to manipulate correlated electron states and uncover novel functionalities for antiferromagnetic spintronics.
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All authors Zhaoqing Ding, Xuejiao Chen, Lei Liao, Zhen Wang, Zeguo Lin, Yuelong Xiong, Junzhou Wang, Fang Yang, Jiade Li, Peng Gao, Lifen Wang, Xuedong Bai, Xiaoran Liu, Jiandong Guo

Wednesday 29 October

Mesoscale and Nanoscale Physics

Path-integral Monte Carlo estimator for the dipole polarizability of quantum plasma

Juha Tiihonen, David Trejo-Garcia, Tapio T. Rantala et al. (+1 more)

We present a path-integral Monte Carlo estimator for calculating the dipole polarizability of interacting Coulomb plasma in the long-wavelength limit, i.e., the optical region. We present comprehensive details and method validation studies for our approach based on both collective and one-particle dipole autocorrelation functions in the imaginary time. The simulation of thermal equilibrium in imaginary time has exact Coulomb interactions and Boltzmann quantum statistics. For reference, we use analytically continued Drude model as the long-wavelength limit of the Lindhard response. Our collective response shows perfect match to the analytical reference. The one-particle response is used in systematic studies of physical and numerical parameters, and to discuss the phenomenological Drude scattering model.
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All authors Juha Tiihonen, David Trejo-Garcia, Tapio T. Rantala, Marco Ornigotti

Materials Science

Effective-Hamiltonian reconstruction through Bloch-wave interferometry in bulk GaAs driven by strong THz fields

Qile Wu, Seamus D. O Hara, Joseph B. Costello et al. (+3 more)

Reconstructing effective Hamiltonians of condensed matter systems directly from experimental data is challenging because of the intricate relationship between Hamiltonian parameters and observables. Here, we reconstruct an effective three-band electron-hole (e-h) Hamiltonian in bulk GaAs based on high-order sideband generation (HSG) induced by quasi-continuous NIR and THz lasers. We perform polarimetry of high-order sidebands while varying the wavelength and polarization of the NIR laser, as well as the strength of the THz field. An analytic model is derived to incorporate the effects of both dephasing and quantum fluctuations around the semiclassical e-h recollision pathways. Surprisingly, the contribution of quantum fluctuations to the decay of sideband intensity with increasing sideband order is comparable to the contribution of dephasing. We simultaneously and unambiguously determine through Bloch-wave interferometry the effective Hamiltonian parameter that determines the e-h reduced masses, the bandgap of GaAs, and two dephasing constants associated with two e-h species. We demonstrate that full Hamiltonian reconstruction can be achieved by combining HSG measurements with absorbance spectroscopy. Unexpectedly, we find that the extracted bandgap of GaAs is about 10 meV larger than the value inferred from previous absorbance measurements. Quantum-kinetic analysis suggests that, in the HSG experiments, the e-h energy may be renormalized through Fröhlich interaction that is modulated by the strong THz fields. We also show that the energy threshold for optical-phonon emission can be suppressed by applying a strong THz field, leading to nearly constant dephasing rates.
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All authors Qile Wu, Seamus D. O Hara, Joseph B. Costello, Loren N. Pfeiffer, Ken W. West, Mark S. Sherwin

Mesoscale and Nanoscale Physics

Photoinduced Electronic Band Dynamics and Defect-mediated Surface Potential Evolution in PdSe$_2$

Omar Abdul-Aziz, Manuel Tuniz, Wibke Bronsch et al. (+9 more)

We use time- and angle-resolved photoemission spectroscopy (TR-ARPES) combined with density functional theory to investigate ultrafast carrier dynamics in low-symmetry layered semiconducting PdSe$_2$. The indirect bandgap is determined to be 0.55~eV. Following photoexcitation above this gap, we resolve a valence band shift and broadening, both lasting less than a picosecond, consistent with bandgap renormalization and carrier scattering, indicative of strong many-body interactions. Subsequently, hot carriers populate the conduction band minimum and are captured by defect states. A surface photovoltage (SPV) of $\sim$ 67~meV emerges, persisting for over 50~ps, driven by defect-assisted charge separation. The formation of native vacancies, promoted by the low-symmetry lattice, likely gives rise to the mid-gap states responsible for this long-lived SPV response. Detailed analysis of TR-ARPES spectra disentangles the contributions of bandgap renormalization, carrier scattering, defect states, and SPV. These findings establish PdSe$_2$ as a prototypical layered quantum material exhibiting exotic photoresponses on ultrafast timescales.
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All authors Omar Abdul-Aziz, Manuel Tuniz, Wibke Bronsch, Fulvio Parmigiani, Federico Cilento, Daniel Wolverson, Charles J. Sayers, Giulio Cerullo, Claudia Dallera, Ettore Carpene, Paul H. M. van Loosdrecht, Hamoon Hedayat

Strongly Correlated Electrons

High Resolution Polar Kerr Effect Studies of Cs3Sb5 and ScV6Sn6 Below the Charge Order Transition

David R. Saykin, Qianni Jiang, Zhaoyu Liu et al. (+4 more)

We report high resolution polar Kerr effect measurements on CsV3Sb5 and ScV6Sn6 single crystals in search for signatures of spontaneous polar Kerr effect (PKE) below the charge order transitions of these materials. Utilizing two separate zero-area loop Sagnac interferometers operating at 1550 nm and 830 nm wavelengths, we studied the temperature dependence of possible PKE after training with magnetic field. While a finite field Kerr measurement yielded optical rotation expected from the Pauli susceptibility of the itinerant carriers, no signal was detected at zero-field to within the noise floor limit of the apparatus of below $\sim$100 nanoradians. Simultaneous coherent reflection measurements confirm the sharpness of the charge order transition in the same optical volume as the Kerr measurements. Application of strain to reveal a hidden flux-ordered magnetic state did not result in a finite Kerr effect.
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All authors David R. Saykin, Qianni Jiang, Zhaoyu Liu, Chandra Shekhar, Claudia Felser, Jiun-Haw Chu, Aharon Kapitulnik

Superconductivity · Cross-listed from quant-ph

Overcoming disorder in superconducting globally driven quantum computing

Riccardo Aiudi, Julien Despres, Roberto Menta et al. (+5 more)

We study the impact of static disorder on a globally-controlled superconducting quantum computing architecture based on a quasi-two-dimensional ladder geometry [R. Menta et al., Phys. Rev. Research 7, L012065 (2025)]. Specifically, we examine how fabrication-induced inhomogeneities in qubit resonant frequencies and coupling strengths affect quantum state propagation and the fidelity of fundamental quantum operations. Using numerical simulations, we quantify the degradation in performance due to disorder and identify single-qubit rotations, two-qubit entangling gates, and quantum information transport as particularly susceptible. To address this challenge, we rely on pulse optimization schemes, and, in particular, on the GRAPE (Gradient Ascent Pulse Engineering) algorithm. Our results demonstrate that, even for realistic levels of disorder, optimized pulse sequences can achieve high-fidelity operations, exceeding 99.9% for the three quantum operations, restoring reliable universal quantum logic and robust information flow. These findings highlight pulse optimization as a powerful strategy to enhance the resilience to disorder of solid-state globally-driven quantum computing platforms.
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All authors Riccardo Aiudi, Julien Despres, Roberto Menta, Ashkan Abedi, Guido Menichetti, Vittorio Giovannetti, Marco Polini, Francesco Caravelli

Strongly Correlated Electrons

Hyperbolic Fracton Model, Subsystem Symmetry and Holography III: Extension to Generic Tessellations

Yosef Shokeeb, Ludovic D.C. Jaubert and Han Yan

We generalize the Hyperbolic Fracton Model from the $\{5,4\}$ tessellation to generic tessellations, and investigate its core properties: subsystem symmetries, fracton mobility, and holographic correspondence. While the model on the original tessellation has features reminiscent of the flat-space lattice cases, the generalized tessellations exhibit a far richer and more intricate structure. The ground-state degeneracy and subsystem symmetries are generated recursively layer-by-layer, through the inflation rule, but without a simple, uniform pattern. The fracton excitations follow exponential-in-distance and algebraic-in-lattice-size growing patterns when moving outward, and depend sensitively to the tessellation geometry, differing qualitatively from both type-I or type-II fracton model on flat lattices. Despite this increased complexity, the hallmark holographic features – subregion duality via Rindler reconstruction, the Ryu-Takayanagi formula for mutual information, and effective black hole entropy scaling with horizon area – remain valid. These results demonstrate that the holographic correspondence in fracton models persists in generic tessellations, and provide a natural platform to explore more intricate subsystem symmetries and fracton physics.
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Superconductivity

Superconductor discovery in the emerging paradigm of Materials Informatics

Huan Tran, Hieu-Chi Dam, Christopher Kuenneth et al. (+2 more)

The last two decades have witnessed a tremendous number of computational predictions of hydride-based (phonon-mediated) superconductors, mostly at extremely high pressures, i.e., hundreds of GPa. These discoveries were heavily driven by Migdal-Éliashberg theory (and its first-principles computational implementations) for electron-phonon interactions, the key concept of phonon-mediated superconductivity. Dozens of predictions were experimentally synthesized and characterized, triggering not only enormous excitement in the community but also some debates. In this Article, we review the computational-driven discoveries and the recent developments in the field from various essential aspects, including the theoretical, computational, and, specifically, artificial intelligence (AI)/machine learning (ML) based approaches emerging within the paradigm of materials informatics. While challenges and critical gaps can be found in all of these approaches, AI/ML efforts specifically remain in its infant stage for good reasons. However, opportunities exist when these approaches can be further developed and integrated in concerted efforts, in which AI/ML approaches could play more important roles.
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All authors Huan Tran, Hieu-Chi Dam, Christopher Kuenneth, Tuoc N. Vu, Hiori Kino

Disordered Systems and Neural Networks

Grokking in the Ising Model

Karolina Hutchison and David Yevick

Delayed generalization, termed grokking, in a machine learning calculation occurs when the increase in test accuracy is delayed relative to the training accuracy. This paper examines grokking in the context of a dense neural network trained to classify 2D Ising model configurations into 4 equally spaced energy regions in the presence of weight decay. Partially with the aid of novel PCA-based network layer analysis techniques, the observed behavior is interpreted as a transition from a connected network to a group of sparse subnetworks in which the number of active weights in each layer decreases monotonically with depth. This architecture reduces classification errors resulting from a multiplicity of paths. The final network layers, as in a convolutional neural network, sequentially identify global features of the input classes, which enables generalization to previously unseen patterns.
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Superconductivity · Cross-listed from hep-th

Equivalence class of Emergent Single Weyl fermion lattice models in 3 dimensions: gapless superconductors and superfluids versus chiral fermions

Gabriel Meyniel and Fei Zhou

In this article, we put forward a practical but generic approach towards constructing a large family of $(3+1)$ dimension lattice models which can naturally lead to a single Weyl cone in the infrared (IR) limit. Our proposal relies on spontaneous charge $U(1)$ symmetry breaking to evade the usual no-go theorem of a single Weyl cone in a 3d lattice. We have explored three concrete paths in this approach, all involving fermionic topological symmetry protected states (SPTs). Path a) is to push a gapped SPT in a 3d lattice with time-reversal symmetry (or $T$-symmetry) to a gapless topological quantum critical point (tQCP) which involves a minimum change of topologies,i.e. $\delta N_w=2$ where $\delta N_w$ is the change of winding numbers across the tQCP. Path b) is to peal off excessive degrees of freedom in the gapped SPT via applying $T$-symmetry breaking fields which naturally result in a pair of gapless nodal points of real fermions. Path c) is a hybrid of a) and b) where tQCPs, with $\delta N_w \geq 2$, are further subject to time-reversal-symmetry breaking actions. In the infrared limit, all the lattice models with single Weyl fermions studied here are isomorphic to either a tQCP in a DIII class topological superconductor with a protecting $T$-symmetry, or its dual, a $T$-symmetry breaking superconducting nodal point phase, and therefore form an equivalent class. For a generic $T$-symmetric tQCP along Path a), the conserved-charge operators span a six-dimensional linear space while for a $T$-symmetry breaking gapless state along Path b), c), charge operators typically span a two-dimensional linear space instead. Finally, we pinpoint connections between three spatial dimensional lattice chiral fermion models and gapless real fermions that can naturally appear in superfluids or superconductors studied previously.
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Materials Science

Revisiting and Accelerating the Basin Hopping Algorithm for Lennard-Jones Clusters: Adaptive and Parallel Implementation in Python

Oliver Carmona, Peter Ludwig Rodríguez-Kessler, Sebastián Salazar-Colores et al. (+1 more)

We present an adaptive and parallel implementation of the Basin Hopping (BH) algorithm for the global optimization of atomic clusters interacting via the Lennard-Jones (LJ) potential. The method integrates local energy minimization with adaptive step-size Monte Carlo moves and simultaneous evaluation of multiple trial structures, enabling efficient exploration of complex potential energy landscapes while maintaining a balance between exploration and refinement. Parallel evaluation of candidate structures significantly reduces wall-clock time, achieving nearly linear speedup for up to eight concurrent local minimizations. This framework provides a practical and scalable strategy to accelerate Basin Hopping searches, directly extendable to ab initio calculations such as density functional theory (DFT) on high-performance computing architectures.
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All authors Oliver Carmona, Peter Ludwig Rodríguez-Kessler, Sebastián Salazar-Colores, Alvaro Muñoz-Castro

Materials Science · Cross-listed from cs.CV

Generative Image Restoration and Super-Resolution using Physics-Informed Synthetic Data for Scanning Tunneling Microscopy

Nikola L. Kolev, Tommaso Rodani, Neil J. Curson et al. (+2 more)

Scanning tunnelling microscopy (STM) enables atomic-resolution imaging and atom manipulation, but its utility is often limited by tip degradation and slow serial data acquisition. Fabrication adds another layer of complexity since the tip is often subjected to large voltages, which may alter the shape of its apex, requiring it to be conditioned. Here, we propose a machine learning (ML) approach for image repair and super-resolution to alleviate both challenges. Using a dataset of only 36 pristine experimental images of Si(001):H, we demonstrate that a physics-informed synthetic data generation pipeline can be used to train several state-of-the-art flow-matching and diffusion models. Quantitative evaluation with metrics such as the CLIP Maximum Mean Discrepancy (CMMD) score and structural similarity demonstrates that our models are able to effectively restore images and offer a two- to fourfold reduction in image acquisition time by accurately reconstructing images from sparsely sampled data. Our framework has the potential to significantly increase STM experimental throughput by offering a route to reducing the frequency of tip-conditioning procedures and to enhancing frame rates in existing high-speed STM systems.
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All authors Nikola L. Kolev, Tommaso Rodani, Neil J. Curson, Taylor J.Z. Stock, Alberto Cazzaniga

Materials Science

Liquid anomalies and Fragility of Supercooled Antimony

Flavio Giuliani, Francesco Guidarelli Mattioli, Yuhan Chen et al. (+5 more)

Phase-change materials (PCMs) based on group IV, V, and VI elements, such as Ge, Sb, and Te, exhibit distinctive liquid-state features, including thermodynamic anomalies and unusual dynamical properties, which are believed to play a key role in their fast and reversible crystallization behavior. Antimony (Sb), a monoatomic PCM with ultrafast switching capabilities, stands out as the only elemental member of this group for which the properties of the liquid and supercooled states have so far remained unknown. In this work, we use large-scale molecular dynamics simulations with a neural network potential trained on first-principles data to investigate the liquid, supercooled, and amorphous phases of Sb across a broad pressure-temperature range. We uncover clear signatures of anomalous behavior, including a density maximum and non-monotonic thermodynamic response functions, and introduce a novel octahedral order parameter that captures the structural evolution of the liquid. Moreover, extrapolation of the viscosity to the glass transition, based on configurational and excess entropies, indicates that Sb is a highly fragile material. Our results present a compelling new case for the connection between the liquid-state properties of phase-change materials and their unique ability to combine high amorphous-phase stability with ultrafast crystallization.
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All authors Flavio Giuliani, Francesco Guidarelli Mattioli, Yuhan Chen, Daniele Dragoni, Marco Bernasconi, John Russo, Lilia Boeri, Riccardo Mazzarello

Strongly Correlated Electrons

Geometric and Orbital Control of Correlated States in Small Hubbard Clusters

Shivanshu Dwivedi and Kalum Palandage

Arrays of semiconductor quantum dots provide a powerful platform to design correlated quantum matter from the bottom up. We establish a predictive framework for engineering local electron pairing in these artificial molecules by systematically deploying three control levers: lattice geometry, orbital hybridization, and external electric fields. Using Hartree-Fock simulations on canonical 3D clusters from the tetrahedron (Z = 3) to the FCC lattice (Z = 12), at and near half-filling, we uncover three fundamental design principles. (i) Geometric Hierarchy: The resilience to Coulomb repulsion U is dictated by the coordination number Z, which controls kinetic delocalization. (ii) Orbital Hybridization: Counter-intuitively, inter-orbital hopping t_orb acts not as a simple suppressor of pairing, but as a sophisticated control knob that enhances double occupancy at moderate U by engineering the on-site energy landscape. (iii) Field Squeezing: An electric field robustly induces pairing by forcing charge localization, an effect most potent in low-connectivity clusters. These principles form a blueprint for deterministically targeting charge and spin correlations in quantum-dot-based quantum hardware.
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Materials Science

Evaluation of Structural Properties and Defect Energetics in Al$_x$Ga$_{1-x}$N Alloys

Farshid Reza, Beihan Chen and Miaomiao Jin

Al$_x$Ga$_{1-x}$N alloys are essential for high-performance optoelectronic and power devices, yet the role of composition on defect energetics remains underexplored, largely due to the limitations of first-principles methods in modeling disordered alloys. To address this, we employ a machine learning interatomic potential (MLIP) to investigate the structural and defect-related physical properties in Al$_x$Ga$_{1-x}$N. The MLIP is first validated by reproducing the equation of state, lattice constants, and elastic constants of the binary endpoints, GaN and AlN, as well as known defect formation and migration energies from density functional theory and empirical potentials. We then apply the MLIP to evaluate elastic constants of AlGaN alloys, which reveals a non-linear relation with alloying effect. Our results reveal that nitrogen Frenkel pair formation energies and the migration barriers for nitrogen point defects are highly sensitive to the local chemical environment and migration path. In contrast, Ga and Al vacancy migration energies remain relatively insensitive to alloy composition, whereas their interstitial migration energies exhibit stronger compositional dependence. These results provide quantitative insight into how alloying influences defect energetics in AlGaN, informing defect engineering strategies for improved material performance.
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Other Condensed Matter · Cross-listed from quant-ph

From Divergent Series to Geometry: Resurgence of the Quantum Metric

Marcos J. Hernández, Bogar Díaz and J. David Vergara

In this work, we analyze perturbative expansions of the quantum metric tensor (QMT) in anharmonic oscillators, focusing on quartic, sextic, and $d$-dimensional models. Using high-order perturbation theory, we show that the divergent QMT series exhibit factorial growth. Our analysis identifies universal non-perturbative scales, with coefficients displaying large-order behavior consistent with resurgence theory. Then, we apply resurgence and Borel–Padé resummation to the QMT. Comparisons with exact diagonalization confirm that Borel–Padé resummations yield accurate results, especially for the ground state. For completeness, we also present the analysis of the energy eigenvalues in the examples. Our findings extend resurgent techniques from energies to the QMT, highlighting the interplay between quantum geometry and non-perturbative physics.
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Mesoscale and Nanoscale Physics

Selective Parametric Amplification of Degenerate Modes in Electrostatically Transduced Coupled Beam Resonators

Vishnu Kumar, Nishta Arora, Bhargavi B.A. et al. (+2 more)

Parametric excitation in coupled mechanical systems has enabled advances in sensing, computation, and phonon control. The function of distinct phase modes using parametric driving remains insufficiently explored. Here, we investigate the nonlinear and parametric response of degenerate phase modes in a Double Ended Tuning Fork (DETF) resonator. Our measurements reveal pronounced nonlinearity and parametric amplification in the out-phase mode, attributed to the dominant contribution of the coupling beam, while the in-phase mode remains predominantly linear. Uniquely, we demonstrate parametric excitation through the coupling spring, enabling selective amplification and de-amplification controlled via the relative phase between harmonic and parametric drives. A parametric gain of $\sim$13 dB is achieved in the out-phase mode, with phase-dependent modulation of amplification, indicating its suitability for signal processing, logic operations, and memory elements based on degenerate modes. These results establish a new approach to exploiting mode-specific nonlinear dynamics in coupled resonators for emerging applications in sensing and phononic control.
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All authors Vishnu Kumar, Nishta Arora, Bhargavi B.A., Akshay Naik, Saurabh A. Chandorkar

Superconductivity

Nonadiabatic and anharmonic effects in high-pressure H3S and D3S superconductors

Shashi B. Mishra and Elena R. Margine

Superconductivity in compressed H3S arises from the interplay between high-frequency phonons and a pronounced van Hove singularity near the Fermi level. Using first-principles calculations, we investigate the superconducting properties of H3S and D3S at 160 and 200 GPa, explicitly incorporating anharmonic lattice dynamics and first-order vertex corrections to electron-phonon (e-ph) interactions, thereby going beyond the Migdal approximation underlying conventional Migdal-Eliashberg theory. We find that both anharmonicity and nonadiabatic vertex corrections suppress the effective e-ph coupling and reduce the superconducting critical temperature (Tc). Calculations performed within the energy-dependent full-bandwidth Eliashberg formalism, including both anharmonic and vertex effects, yield Tc values in close agreement with experimental measurements for D3S at both pressures and for H3S at 200 GPa.
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Materials Science

Mutual enhancement of altermagnetism and ferroelectricity

Chi Sun and Jacob Linder

We consider theoretically the possibility of coexisting ferroelectric and metallic altermagnetic order, which has recently been predicted in insulating and semiconducting systems via ab initio calculations. Solving self-consistently a mean-field Hubbard model, accounting also for the energy cost of distorting the lattice to produce an electric polarization, our results show that metallic altermagnetism and ferroelectricity suppress or enhance each other depending on the doping level of the system. Close to half-filling, the system can lower its energy by becoming altermagnetic, but at the expense of losing the electric polarization. Away from half-filling, the coexistence of ferroelectricity and altermagnetism is much more robust toward an increase in the energy cost associated with the deformation of the lattice. Therefore, our results suggest that filling fractions corresponding to doping relatively far away from half-filling constitute the most promising regime to look for coexistent ferroelectricity and metallic altermagnetism with mutual enhancement. Moreover, we propose a way to electrically tune altermagnetism between nodal and nodeless phases as well as achieving coexistence of a nodal and nodeless phase for the two spin species.
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Statistical Mechanics

Spatially Structured Entanglement from Nonequilibrium Thermal Pure States

Chen Bai, Mao Tian Tan, Bastien Lapierre et al. (+1 more)

We study quantum quench dynamics in (1+1)-dimensional critical systems, starting from thermal pure states called crosscap states, and evolving them under spatially inhomogeneous Hamiltonians. The spatial inhomogeneity is introduced through a deformation of the Hamiltonian, expressed as linear combinations of the generators of the $SL^{(q)}(2,\mathbb{R})$ subalgebra of the Virasoro algebra. We analyze the free massless Dirac fermion theory and holographic conformal field theory as prototypical examples of integrable and non-integrable dynamics. Consistent with general expectations, "Möbius-type" deformations lead to thermalization in the non-integrable case, and to periodic revivals in the integrable one. In contrast, "sine-square-type" and "displacement-type" deformations prevent both thermalization and scrambling, instead producing late-time, graph-like entanglement patterns. These patterns emerge from the interplay between the deformed Hamiltonian and the crosscap initial state and appear to be universal: they are determined solely by the deformation profile while remaining largely insensitive to microscopic details. Finally, we perform a holographic calculation in three-dimensional gravity using AdS$_3$/CFT$_2$, which reproduces the main features of our (1+1)-dimensional study.
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All authors Chen Bai, Mao Tian Tan, Bastien Lapierre, Shinsei Ryu

Superconductivity

Theories of Superconducting Diode Effects

Daniel Shaffer and Alex Levchenko

Superconducting diode effects (SDE), both in bulk superconductors and in Josephson junctions, have garnered a lot of attention due to potential applications in classical and quantum computing, as well as superconducting sensors. Here we review various mechanisms that have been theoretically proposed for their realization. We first provide a brief historical overview and discuss the basic but subtle phenomenological Ginzburg-Landau theory of SDE, emphasizing the need to the simultaneous breaking of time-reversal and inversion symmetries. We then proceed to more microscopic treatments, focusing especially on implementations in noncentrosymmetric materials described by the Rashba-Zeeman model. Finally, we review proposals based on other condensed matter systems such as altermagnets, valley polarized and topological materials, and systems out of equilibrium.
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Statistical Mechanics

Beyond the Arcsine Law: Exact Two-Time Statistics of the Occupation Time in Jump Processes

Arthur Plaud and Olivier Bénichou

Occupation times quantify how long a stochastic process remains in a region, and their single-time statistics are famously given by the arcsine law for Brownian and Lévy processes. By contrast, two-time occupation statistics, which directly probe temporal correlations and aging, have resisted exact characterization beyond renewal processes. In this Letter we derive exact results for generic one-dimensional jump processes, a central framework for intermittent and discretely sampled dynamics. Using generalized Wiener-Hopf methods, we obtain the joint distribution of occupation time and position, the aged occupation-time law, and the autocorrelation function. In the continuous-time scaling limit, universal features emerge that depend only on the tail of the jump distribution, providing a starting point for exploring aging transport in complex environments.
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Mesoscale and Nanoscale Physics

Sweet-spot protection of hole spins in sparse arrays via spin-dependent magnetotunneling

Esteban A. Rodríguez-Mena, Biel Martínez, Ahmad Fouad Kalo et al. (+2 more)

Recent advances in the scaling of spin qubits have led to the development of sparse architectures where spin qubits are distributed across multiple quantum dots. This distributed approach allows for qubit manipulation through hopping and flopping modes and may enable spin shuttling protocols to entangle spins beyond nearest neighbors. Here, we develop a microscopic theory of a minimal sparse array formed by a hole in a double quantum dot. We show the existence of spin-dependent magnetic corrections to the tunnel couplings that help preserve existing sweet spots, even for quantum dots with different $g$-factors, and introduce new ones that are not accounted for in the simplest models. Our analytical and numerical results explain observed sweet spots in state-of-the-art shuttling and cQED experiments, are relevant to hopping and flopping modes, and apply broadly as corrections to each interdot tunnel link in sparse array encodings of any size.
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All authors Esteban A. Rodríguez-Mena, Biel Martínez, Ahmad Fouad Kalo, Yann-Michel Niquet, José C. Abadillo-Uriel