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Physics Condensed Matter

cond-mat Watch Condensed Matter

Friday 26 June

Materials Science

Vacancy-mediated nitrogen diffusion and aggregation via high-fluence electron beam irradiation in HPHT synthesized diamond crystal

Chikara Shinei, Yuta Masuyama, Jun Chen et al. (+5 more)

The negatively charged nitrogen vacancy (NV-) center in diamond is a promising point defect for highly sensitive quantum sensing. The formation of high-density NV- centers is essential for improving sensitivity. We performed room-temperature electron beam irradiation (EBI) and annealing on nitrogen-doped high-pressure high-temperature diamond crystals, aiming to convert all substitutional nitrogen into NV structures by increasing EBI fluence. While the Ns0 to NV0 and NV- conversion process dominated at low EBI fluences, in a high-fluence region, NV0 and NV- center and Ns0 and Ns+ concentrations decreased with increasing EBI fluence, indicating the formation of unknown nitrogen-related defects such as the H3 center, which is a nitrogen and vacancy aggregation defect. Although H3 centers were observed at high EBI fluence, their annealing temperature of 1375 +- 25 {\deg}C was lower than the typically reported temperatures over 1600 {\deg}C. We attribute this low-temperature formation to vacancy-mediated nitrogen diffusion and aggregation.
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All authors Chikara Shinei, Yuta Masuyama, Jun Chen, Hiroshi. Abe, Masashi Miyakawa, Takashi Taniguchi, Takeshi Ohshima, Tokuyuki Teraji

Materials Science

Amorphous Fe-Sn nanofilms for anomalous-Nernst heat-flux sensing

Kenji Tanabe, Ko Mibu, Atsushi Tsukazaki et al. (+1 more)

Amorphous magnetic films are promising for anomalous-Nernst heat-flux sensing because their low thermal conductivity can enhance the temperature gradient generated by an applied heat flux. However, amorphization often degrades electronic transport and thermoelectric properties, making it challenging to obtain a large anomalous Nernst response in structurally disordered films. Here, we demonstrate nanometer-thick amorphous Fe-Sn films as high-sensitivity anomalous-Nernst heat-flux sensing materials. By systematically controlling composition and thickness, we find that amorphous Fe-Sn nanofilms combine a large anomalous Nernst response with low thermal conductivity, resulting in a heat-flux sensitivity of 0.37 um/A. This value exceeds the sensitivities reported for both amorphous magnetic thin films and representative crystalline topological magnets. X-ray diffraction and Mossbauer spectroscopy show that the optimized films lack long-range crystallinity while retaining local Fe-Sn environments, suggesting that short-range atomic order contributes to the anomalous Nernst response in the amorphous matrix. The sensitivity is also reproduced on flexible polymer substrates, indicating compatibility with mechanically compliant device architectures. These results establish amorphous Fe-Sn nanofilms as a platform for anomalous-Nernst heat-flux sensing and provide a materials design route based on local-structure control and thermal-conductivity reduction.
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All authors Kenji Tanabe, Ko Mibu, Atsushi Tsukazaki, Kohei Fujiwara

Materials Science

Defying Diamagnetism: Anomalous Redshift and Robust Chiral Spin Mixing in 2D Perovskites under High Magnetic Fields up to 50 T

D. Y. Park, S. Park, Yongmin Kim et al. (+4 more)

We report circularly polarized magneto-photoluminescence (MPL) measurements on the enantiomeric pair of two-dimensional chiral perovskites R- and S-(C4H9NH3)2PbI4 in pulsed magnetic fields up to 50 T at 4.2 K. The R-form exhibits strong preferential emission into the RC channel (I_RC/I_LC = 10:1), with the S-form showing the opposite sense, robust against the applied field. Three key observations are reported: (i) an anomalous redshift of all exciton transition energies with increasing field-contrasting with the normal diamagnetic blueshift of achiral perovskites-with a non-monotonic initial blueshift to 7.6 T in the dominant RC channel of the R-form; (ii) complete invariance of PL peak positions upon reversal of the field direction, consistent with Rashba-induced mixing of the bright exciton with a nearby m_x = 0 dark state; and (iii) field-induced increase in PL intensity and narrowing of the linewidth, attributed to suppression of disorder scattering as the cyclotron orbit contracts. These results are interpreted in the framework of Rashba spin-splitting and polaron formation in the chiral two-dimensional perovskite lattice.
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All authors D. Y. Park, S. Park, Yongmin Kim, N. Myoung, Z. Yang, Y. Kohama, Y. H. Matsuda

Superconductivity

Thickness dependence of diode efficiency in superconducting Fe(Se,Te)/FeTe thin-film heterostructure devices

Kaito Arizono, Kenshin Inamura, Kouta Kondou et al. (+4 more)

The superconducting diode effect (SDE) is a nonreciprocal transport phenomenon, in which the superconducting critical current density depends on the polarity of the current. It has attracted recent attention because of its potential applications to a rectifier without energy dissipation. While SDE has been observed in a wide range of superconducting materials with broken inversion symmetry as well as thin-film heterostructures, the microscopic origin linking structural inversion asymmetry of electronic band, spin-orbit interaction, and vortex pinning remains to be clarified. In this study, we investigate SDE in Fe(Se,Te)/FeTe heterostructure devices as a function of the superconducting Fe(Se,Te) layer thickness tFST to elucidate the role of structural inversion asymmetry on the vortex-induced SDE. We find that the SDE efficiency monotonically increases with increasing tFST, which can be understood by considering that the band bending in the bulk Fe(Se,Te) layer induces the structural inversion asymmetry and thus, the Rashba spin-orbit interaction. In addition, we demonstrate almost 100% rectification for the Fe(Se,Te)/FeTe heterostructure devices in half- and full-wave oscillation configurations. Our findings point out the importance of structural architecture for realization of highly efficient SDE devices based on superconducting thin-film heterostructures.
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All authors Kaito Arizono, Kenshin Inamura, Kouta Kondou, Yusuke Kobayashi, Tsutomu Nojima, Jobu Matsuno, Junichi Shiogai

Materials Science

CoTAR: Topology and Atomic State Reconstruction in Condensed Phases

Hodaka Mori, Yu Miyazaki and Takechika Kikkawa

Universal machine learning interatomic potentials (uMLIPs) enable condensed-phase molecular dynamics (MD) simulations with near-first-principles accuracy, but their lack of explicit molecular topology limits bond-aware analysis and reconnection to classical force fields. Here, we present CoTAR, a hybrid graph neural network (GNN)–hidden Markov model (HMM) framework that reconstructs molecular topology, formal charges, and unpaired electrons from atomic species, coordinates, and total charge by combining message passing on a proximity graph with a van der Waals prior, chemical constraints, and temporal smoothing. Across 128 nonreactive, topology-preserving condensed-phase systems, CoTAR achieved a bond-order-weighted F1 score of 0.906 on classical-MD data; for uMLIP trajectories, few-shot fine-tuning improved the valid-snapshot rate from 38.6% to 84.7%. The reconstructed topologies also supported downstream classical MD simulations, and HMM smoothing improved system-level MD simulation feasibility from 83.6% to 85.9%, indicating that CoTAR provides a practical framework for bond-aware analysis of condensed-phase uMLIP trajectories.
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Superconductivity

The Second Law, Symmetry of Time Reversal and Thermodynamic Equilibrium in Superconductors

Vladimir Kozhevnikov

The Second law, universally applicable to all states of all sorts of matter and radiation, is undeniably the brightest jewel in the tiara of laws of thermodynamics; recall that the achievements of this law include the principle of least action, the atomistic structure of matter, the quantization of radiant heat, and much more. However, there is a significant gap in understanding the application of the Second law to magnetizing materials, especially to superconductors. This communication is targeted to fill the gap. The concepts of equilibrium and reversibility in thermodynamic processes are considered with particular attention to specifics of superconductivity. These include (1) the primary role of the field strength $\textbf{H}$ in forming magnetization; (2) the zero entropy of samples in a state of thermodynamic equilibrium; and (3) two kinds of superconducting currents in samples out of equilibrium. As shown, time reversal symmetry in superconductors is a consequence of the Second law and, therefore, is a mandatory property of these materials in a state of equilibrium. Necessary conditions for achieving the latter are framed out. Awareness of these aspects is important for advancing research on superconductivity in all kinds of superconducting materials.
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Statistical Mechanics · Cross-listed from physics.bio-ph

Self-organized robustness in mean-field interacting systems

Emmy Blumenthal and Gautam Reddy

Self-organization is a defining feature of living systems, with order often maintained through interactions between constituent units rather than centralized feedback. We introduce a tractable mean-field model of self-organized robustness, formulated as meta-optimization over the system's response to perturbations. The resulting interaction structure has an intuitive picture as a dynamically modulated landscape (“seascape”) whose shape is determined self-consistently to accelerate relaxation back to equilibrium. The collective dynamics follows an optimized Wasserstein gradient flow toward an attractor in the space of collective states. When communication is limited, interactions preferentially encode slowly relaxing modes and modes that are frequently perturbed. The model further shows that robust collective states are associated with flatter equilibrium landscapes and predicts a continuum of intermediate “reservoir states” in such systems. The model offers a perspective of self-organization as a hierarchical associative memory that operates on the scale of a collective of interacting computational units.
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Materials Science

Entropy engineering of BF-BT-based high-entropy ceramics for ultra-high energy storage performance

Yitao Jiao, Zhenhao Fan, Dawei Wang et al. (+1 more)

Dielectric capacitors are promising for pulsed power applications, but the energy storage performance of lead-free bulk ceramics is often limited by low breakdown strength and large ferroelectric hysteresis. Herein, a high entropy perovskite oxide BaTi0.2Zr0.2Sn0.2Hf0.2Nb0.1Sc0.1O3 was introduced into the BF-BT matrix to develop lead free high entropy ferroelectric ceramics. Multicomponent B-site substitution induces lattice distortion, enhanced pseudocubic characteristics, relaxor behavior, and grain refinement. These effects suppress polarization hysteresis and electrical conduction, resulting in a significant increase in breakdown strength. A maximum breakdown strength of 840 kV/cm1 and a recoverable energy density of 10.55 J/cm3 were achieved. Entropy-induced microstructural heterogeneity promotes a more uniform electric-field distribution and delays dielectric breakdown. This work demonstrates entropy engineering as an effective route to achieving high breakdown strength and superior energy-storage performance in lead-free ferroelectric ceramics.
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All authors Yitao Jiao, Zhenhao Fan, Dawei Wang, Hai-Feng Li

Materials Science

Emergence of millimeter-wave resonances in self-assembled ferroelectric metamaterials

Florian Bergmann, Peter Meisenheimer, Aiden Ross et al. (+26 more)

Resonators are a key component in modern communications and computing. As demand and technological advances push component requirements into the terahertz regime, there is significant research devoted to the search for resonances at these frequencies. While uniform solid-state materials usually do not intrinsically feature resonances in this frequency range, self-assembled periodic arrays of ferroelectric nanodomains may provide an engineering route to design millimeter-wave properties. Here, we utilize prototypical dielectric-ferroelectric SrTiO3/PbTiO3 superlattices to robustly design periodic ferroelectric nano-scale domains. Phase field simulations predict an emergent domain breathing mode in complex polar textures and state-of-the-art millimeter-wave characterization shows evidence for such emergent resonances up to hundreds of GHz. Complex polar textures in these superlattices lead to emergent piezoelectric properties that also result in millimeter-wave resonances, which are predicted by second principles methods and confirmed by direct measurement. The principles investigated in this work suggest a new modality for ferroelectrics in the design of millimeter-wave electronics.
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All authors Florian Bergmann, Peter Meisenheimer, Aiden Ross, Marvin Schewe, Fernando Gómez-Ortiz, Kaiwen Yang, Xinyan Li, Thomas J. Lee, Pushpendra Gupta, Liam G.Connolloy, Tzu-Hsuan Hsu, Jack Kramer, Bryan T. Bosworth, Nicholas R. Jungwirth, Eric J. Marksz, Aaron Hagerstrom, Tomasz Karpisz, Arundhati Ghosal, Lane W. Martin, Yimo Han, Angela C. Stelson, Christian J. Long, Ruochen Lu, Lucas Caretta, Javier Junquera, Jason J. Gorman, Long-Qing Chen, Ramamoorthy Ramesh, Nathan D. Orloff

Thursday 25 June

Statistical Mechanics

Comparison of different exact generalized Langevin equations with a non-linear potential of mean force and an observable-dependent mass and friction

Benjamin J. A. Héry, Lucas Tepper and Roland R. Netz

The Mori-Zwanzig projection formalism constitutes a powerful and robust framework for deriving equations of motion in terms of generalized Langevin equations (GLEs) for an arbitrary observable using evolution and projection operators. Based on this framework, we analyze the properties of four distinct GLEs for a scalar observable including a Markovian force derived from a generally non-linear potential, a non-Markovian friction force, and an orthogonal force, commonly interpreted as a random force. While all four GLEs are exact, they differ in the memory friction kernel, which may either be dependent or independent of the observable, and by the potential, which may either include or exclude the effective kinetic energy of the observable. Inclusion of the kinetic energy in the potential is advantageous for observables whose velocity satisfies Wick's theorem, since this reproduces the correct distribution of the observable and its velocity even without contributions from the friction force and the orthogonal force.
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Materials Science

Thermal Corrections and Analysis on the Phase Stability of CsPbCl3 and Cs2AgSbCl6 during In-Situ Thermal Treatment

Ethan R. Cronk, Wenjun Xiang, Rachel Fister et al. (+4 more)

Cesium lead chloride (CsPbCl3) is a well known and principal model for inorganic perovskite halide optoelectronic research. The many available techniques including high temperature stability testing have been used to investigate the increasing interest in inorganic perovskites as primary layers in solar cell applications. Due to the nature of high temperature testing, the characterization technique, reproducibility, and the true sample temperature are vital in determining relative stability. By choosing CsPbCl3 in the investigation of the structural stability of perovskites at high temperatures, it acts as a baseline to create and verify a methodology that accurately probes sample temperature, phase transitions, and decomposition onsets. Therefore, we present a methodological approach to investigate the thermal interactions and stability of CsPbCl3 as a parent single perovskite halide based on ligand-assisted re-precipitation synthesis techniques. Where we use our approach to inform and probe thermal interactions in other cesium/chlorine compounds like Cs2AgSbCl6. By analyzing the stoichiometry and initial phases through investigations of the crystalline structure and particle morphology, we calculated temperature conversions using a control substrate and refinements to best estimate changing structure parameters. Using in-situ temperature dependent X-ray diffraction, we were able to effectively probe the phase transitions and decomposition temperatures of the investigated halide powders. Creating a process that can confirm known high temperature structural phenomena of model perovskite halides while verifying our true sample temperature. Which allowed for further testing on the thermal kinetics of on the double perovskite structure Cs2AgSbCl6 and will continue to allow us to test other perovskites and perovskite families of interest in modern high temperature perovskite halide research.
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All authors Ethan R. Cronk, Wenjun Xiang, Rachel Fister, Biswajit Ball, Feng Yan, Liping Yu, Nicholas S. Bingham

Materials Science

PtyRANNOSAUR: Ptychography with Robust Artificial Neural Networks Optimized for Sub-Angstrom Accuracy and Ultrafast Reconstruction

Kieran Loehr, Rahim Raja, Xiaochuan Ding et al. (+5 more)

We present PtyRANNOSAUR, a data-driven neural network code that reconstructs atomic resolution electron ptychography data in seconds, 10-100x faster than standard methods. PtyRANNOSAUR uses convolutional autoencoders to map 4D-scanning transmission electron microscopy data to 2D phase images. Each model is trained on a large database of crystal structures and is tailored for a range of experimental parameters, such as accelerating voltage, convergence angle, defocus, and sample thickness. This approach yields high quality reconstructions without requiring any fine-tuning of hyperparameters. In addition, the code handles spatial partial coherence, multiple scattering, and scan position errors, which are critical for state-of-the-art electron ptychography reconstructions. By testing PtyRANNOSAUR on experimental and simulated data, we show that the neural networks accurately reconstruct atomic structures of a broad range of materials systems and can achieve high resolutions of $<0.5$ Å, comparable to the best iterative reconstructions of the same data. These advances enable near-live, state-of-the-art electron ptychography reconstructions.
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All authors Kieran Loehr, Rahim Raja, Xiaochuan Ding, Jeffrey Huang, Gillian Nolan, Sang hyun Bae, Bryan K. Clark, Pinshane Y. Huang

Mesoscale and Nanoscale Physics

Multiphoton Fingerprints of Altermagnetic Spin Splittings

Sayed Ali Akbar Ghorashi and Andrew M. Rappe

We systematically investigate multiphoton absorption as a polarization-resolved nonlinear optical probe of planar altermagnets (ALMs). We show that the angular harmonic of the altermagnetic spin splitting fixes the lowest optical absorption at which a symmetry-selective response appears: two-photon absorption for $d$-wave order, four-photon absorption for $g$-wave order, and six-photon absorption for $i$-wave order. In each case, there exists a polarization channel locked to the symmetry harmonic of the altermagnetic texture in which the direct $n$-photon contribution to the transition matrix element is absent. This changes the frequency scaling of the absorption rate relative to other polarization channels and provides a direct optical fingerprint of the underlying altermagnetic harmonic. Our results establish a hierarchy of nonlinear spectroscopic signatures that distinguishes $d$-, $g$-, and $i$-wave altermagnetic spin splittings beyond linear response.
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Materials Science

A BRAVE Alloy Design Campaign (Bayesian Risk-aware Alloy discoVery and Exploration)

Mrinalini Mulukutla, Danial Khatamsaz, Trevor Hastings et al. (+23 more)

In constrained alloy optimization, the compositions with the highest performance potential often reside at the boundary of phase stability – where the risk of experimental failure is also highest. This work demonstrates this principle through a risk-aware Bayesian optimization campaign on single-phase FCC high-entropy alloys in the Al-V-Cr-Mn-Fe-Co-Ni-Cu system. A learned feasibility classifier, integrated directly into the multi-objective acquisition function, probabilistically penalizes candidates likely to produce failed experiments while preserving access to high-performing boundary compositions. From approximately 27,000 CALPHAD-screened candidates, 48 alloys were synthesized over three closed-loop iterations targeting five objectives (yield strength, UTS/YS ratio, strain at UTS, dynamic-to-quasi-static hardness ratio, and simulated depth of penetration), exploring 0.12% of the feasible space. Two compositional regimes emerged: a V-rich, Ni-rich high-strength regime (UTS up to ${\sim}1480$~MPa at 50% elongation) and a Mn-containing high-ductility regime (UTS/YS up to 4.20 at $>$50% elongation). Among feasible alloys, vanadium simultaneously drives yield strength ($r = 0.84$) and sigma-phase formation ($r = 0.54$ with infeasibility); at V = 24~at.%, the three strongest alloys and three sigma failures share the same compositional point. Additionally, the strongest performing alloys cluster in a narrow region of compositional space (V $\geq$ 20 at.%, Ni $\geq$ 36 at.%), representing ${\sim}100$ of $27,074$ feasible candidates – a probability of $P \approx 6.5 \times 10^{-6}$ under random sampling. This dual role – consistent with the KKT prediction that constrained optima lie on active constraint boundaries – required feasibility-aware acquisition to access; hard filtering would have excluded this region entirely.
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All authors Mrinalini Mulukutla, Danial Khatamsaz, Trevor Hastings, Wenle Xu, Daniel Salas, Joydeep Kundu, Vasanth C. Shunmugasamy, Daniel Lewis, Jacob Hempel, Clinton Strosser, Alexandra Salinas, Brent Vela, Nicolas Flores, Sina Hossein Zadeh, Ali Rachidi, David Elbert, Brady Butler, James Paramore, Dimitris Lagoudas, Justin Wilkerson, George Pharr, Douglas Allaire, Vahid Attari, Ibrahim Karaman, Ankit Srivastava, Raymundo Arróyave

Materials Science

Mapping the Growth of Two-Dimensional $\pi$-Conjugated Polymers on Au(111): Organometallic Intermediates and Edge Terminations

Simon W. Briesenick, Wyatt A. Behn, Manuel González Lastre et al. (+6 more)

Kagome lattices provide an exciting space for the exploration of graphene-like $\pi$-conjugated molecular systems with flat bands. Using heterotriangulene-derived precursors, along with an on-surface Ullmann coupling process, makes growing polymers with Kagome lattices accessible and straightforward. Here, we use scanning tunneling microscopy alongside high-resolution atomic force microscopy to examine the evolution of tribromotrioxaazatriangulene on Au(111) into ordered, covalent films. Using density functional theory and scanning probe methods, we find previously unreported organometallic intermediate states involving Au adatoms incorporated within the growing polymer lattice. We also find that a majority of polymer edges remain brominated up to 250 $^{\circ}$C and a large number of edges bonded to Au adatoms coordinated to an adjacent bromine atom. These observations suggest that residual bromine could play a role in stabilizing the polymer edges to Au adatoms and thereby influence the growth pathways that lead to ordered Kagome polymer lattices.
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All authors Simon W. Briesenick, Wyatt A. Behn, Manuel González Lastre, Chang Wan Kang, Pablo Pou, Ekaterina D. Ulyanov, Rubén Pérez, Dmytro F. Perepichka, Peter Grutter

Statistical Mechanics · Cross-listed from hep-th

Soft QED as Open Quantum System: Infrared Cancellation and Soft-Shell Coarse Graining

Soo-Jong Rey

I formulate unresolved soft-photon sector of QED as open quantum system. Resolved charged particles and hard photons form the system, unresolved soft photons form the environment, and basic object is the reduced density matrix. A resolved outcome $f$ of multiple hard particles and photons has probability $P_{f}(i) =\sum_n|\langle f,n|S|i,0\rangle|^2=\langle i|F_f|i\rangle$, with Kraus operators $K_n= {}_{\rm soft} \langle n|S|0 \rangle_{\rm soft}$ and effect $F_f=\sum_nK_n^\dagger\Pi_fK_n$. The SK formulation places unresolved virtual and real terms in one doubled-contour expansion. At one loop they carry the same on-shell eikonal kernel with opposite signs. This elegantly organizes the QED probability: for the same observable, perturbative order, diagrams, and phase space, the OQS gives the same infrared-finite terms as the full-QED. The soft-photon evolution is a unitary coherent-state displacement driven by the scattering current. The equal-history identity of influence functional exactly normalizes this soft evolution; together with the soft-photon theorem it removes the IR divergent leading-soft factor from inclusive probability. I also derive explicit leading-soft QED realization of scale-parametrized Lindblad evolution on a fixed hard-branch space. Tracing an infinitesimal soft-photon shell produces diagonal jump operators whose entries are fixed by the corresponding eikonal emission amplitudes. The finite-shell map is a completely positive unital Schur channel and, in the sharp scale-invariant leading-soft regime, a dephasing semigroup of a completely-positive-divisible scale flow. The resulting logarithmic visibility slope and monotonic purity loss are off-diagonal predictions of the reduced-state description. The same controlled-displacement dilation gives the Sudakov probability, Poisson soft-photon multiplicities, and the bremsstrahlung number spectrum.
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Materials Science

Ultrafast non-thermal suppression of ferroelectricity by carrier screening in LiNbO3

Man Tou Wong, Zhuquan Zhang, Zi-Jie Liu et al. (+1 more)

Ferroelectric materials are key to energy-efficient electronics, memory, and optical applications. While charge carriers typically screen and suppress ferroelectricity, their role under nonequilibrium conditions remains elusive. Here, we use femtosecond laser pulses to liberate trapped carriers in LiNbO3 and track the response using time-resolved second-harmonic generation and stimulated Raman scattering. Even dilute photoexcited carriers induce a rapid yet enduring suppression of polarization and Raman susceptibility. Fluence- and temperature-dependent analyses confirm the suppression is non-thermal and arises from transient carrier screening. These findings reveal an efficient, reversible, and symmetry-preserving mechanism to modulate ferroelectricity on ultrafast timescales, offering a new route to control ferroic and competing quantum phases.
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All authors Man Tou Wong, Zhuquan Zhang, Zi-Jie Liu, Keith A. Nelson

Strongly Correlated Electrons

SU(4) Heisenberg model on the hyperhoneycomb lattice

I. V. Lukin and A. G. Sotnikov

We study the ground state of the SU(4) Heisenberg model on the hyperhoneycomb lattice using three-dimensional projected entangled pair states. We show that it is possible to compute physical observables for the ground states using loop expansions, which converge quickly on tree-like lattices. Our extrapolations to the limit of infinite bond dimensions point toward gapless spin-liquid ground state.
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Statistical Mechanics

Jarzynski equality for counterwork under reversed memory-filtered driving

Pierre Nazé

We introduce a counterwork functional generated by a sign-inverting memory-filtered effective protocol. Given an imposed protocol $\lambda(t)$, the effective protocol $\Lambda(t)$ is obtained by applying an active protocol-memory kernel to $\dot{\lambda}(t)$, rather than by invoking a passive bath response. The counterwork is then the ordinary Hamiltonian work associated with $H(\Gamma,\Lambda(t))$, so that Jarzynski's equality applies directly to it. When $\Lambda(t)$ reverses the endpoints of $\lambda(t)$, the corresponding free-energy difference satisfies $\Delta F_C=-\Delta F_W$, and the exponential average of the counterwork is the reciprocal of that of the original work. We derive the kernel normalization realizing this reversed displacement and show, by Jensen's inequality, that the product of the exponentials of the average work and counterwork is bounded by unity, implying $\langle C\rangle+\langle W\rangle\geq0$. Thus negative average counterwork is possible only when compensated by the average work of the original operation. We further discuss the counteroperation under incomplete thermodynamic information, showing that the robust strategy is to enforce endpoint reversal while minimizing dissipated counterwork.
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Strongly Correlated Electrons · Cross-listed from hep-lat

Sampling the Schwinger Model with Gauge-Equivariant Diffusion

Octavio Vega and Aida X. El-Khadra

We present a first study of a diffusion-based approach to accelerated sampling of the $N_f = 2$ lattice Schwinger model. Our work is inspired by recent and growing successes in developing such generative models for ensemble generation in LFT to overcome the well-known critical slowing down problem. We train a U(1)-equivariant score-based generative model to sample gauge link configurations from the marginal Schwinger model. By computing model likelihoods, we obtain unbiased estimates for observables that closely match those produced by MCMC simulations. We also demonstrate improvement over HMC as measured qualitatively by a reduction in topological freezing near critical parameters.
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Mesoscale and Nanoscale Physics

Collection, characterization, and precision measurement of levitated charged nanoparticles

B. E. Kane, Joyce Coppock, Sunghyun Kim et al. (+1 more)

We describe apparatus and experimental procedures for high stability precision measurements of levitated nanoscale particles confined in an ion trap in high vacuum. We discuss methods for particle generation and collection using electrospray emission, for rapid characterization by direct imaging of thermal motion, and for transfer of the particle from the trap where it is collected to a separate analysis trap in order to achieve better vacuum and lower noise. In the analysis trap at high vacuum (pressure $p\simeq10^{-8}$ Torr), we employ thermostatic control of the trapped particle oscillation amplitudes, allowing long-term, precision measurements of oscillation frequencies, from which the charge to mass ratio ($Q/M$) can be deduced. Under these conditions, we achieve $Q/M$ measurement precision approaching $10^{-5}$. This sensitivity will enable, for example, investigations of the surface chemistry of $\mu$m-scale levitated materials in ultra-high vacuum environments.
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All authors B. E. Kane, Joyce Coppock, Sunghyun Kim, Sarah Westgate

Strongly Correlated Electrons

Magnetic long-range order at finite temperature in two-dimensional hyperbolic lattices

Alexander Hickey and Joseph Maciejko

Infrared singularities of gapless Goldstone modes preclude magnetic long-range order at finite temperature in conventional two-dimensional systems. By studying the spin-$S$ Heisenberg model on regular tilings of the hyperbolic plane, we show that this obstruction is absent in negatively curved space. Using spin-wave theory, we find that the zero-energy collective modes required by symmetry carry vanishing local spectral weight and are separated from the thermodynamic bulk magnon continuum by a finite gap in the bulk local spectral density. As a result, local transverse correlations remain short ranged, with a finite correlation length, despite the presence of Goldstone modes associated with the broken SO(3) spin-rotation symmetry. Stronger negative curvature is found to suppress quantum fluctuations in bulk thermodynamic quantities, pushing the ordered state toward "mean-field-like" behavior. We further estimate the ordering temperature from the thermal spin-wave correction to the ordered moment. These results establish hyperbolic geometry as a route to finite-temperature magnetic order that circumvents the Mermin-Wagner obstruction without breaking or modifying the continuous symmetry.
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Superconductivity

Elimination of Flux Trapping in Superconducting Circuits in Ambient Magnetic Fields

Rohan T. Kapur, Alex Wynn, Sergey K. Tolpygo et al. (+10 more)

Superconductor digital electronics and quantum computing with superconducting qubits are promising next-generation computing technologies. When cooled down or operated in the presence of a nonzero background magnetic field $B_r$, superconducting thin films comprising the circuits can trap magnetic vortices that can degrade circuit or qubit performance. In this work, we report a practical solution for eliminating flux trapped during cooldown in ambient magnetic fields, $B_r\leq 60$ $\upmu$T, based on controlled local thermal gradients and moats, etched holes in the superconducting films of the circuit. Thermal gradients created by integrated on-chip resistive heaters move vortices towards the moats, where they become trapped away from circuitry regions and pinning sites. Using magnetic imaging and electrical circuit readout, we demonstrate that this approach is capable of removing magnetic flux trapped during field cooling and magnetic flux nucleated by circuit operation. If used in an environment with basic magnetic shielding, this solution is capable of suppressing all magnetic flux in a large-scale circuit, overcoming one of the long-standing challenges preventing high-performance scalable computing using superconductors.
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All authors Rohan T. Kapur, Alex Wynn, Sergey K. Tolpygo, Neel Parmar, Anil Mankame, Adam A. Libson, Rabindra Das, Michele Kelley, Pauli Kehayias, Nathaniel J. O'Connor, Collin N. Muniz, Justin L. Mallek, Jennifer M. Schloss

Mesoscale and Nanoscale Physics

Dirac fermions in non-Hermitian magnetic fields: Zero modes and index theorem

Christopher A. Leong and Bitan Roy

In a Lorentz symmetric non-Hermitian (NH) Dirac theory, containing the canonical relativistic Hamiltonian accompanied by a masslike anti-Hermitian Dirac operator, when the associated NH parameter becomes spatially modulated it couples massless Dirac fermions as NH gauge fields. With specific choices of such resulting NH gauge potential, the system experiences NH magnetic fields. When a planar Dirac system encloses a finite flux of such NH magnetic fields, a manifold of spatially localized right or left zero-energy eigenmodes appear in the spectrum, which we numerically anchor from microscopic realizations of NH magnetic fields on graphene's honeycomb lattice. Potential experimental platforms to test these predictions are discussed. Altogether, zero-energy NH flat bands of right or left modes promise fascinating future realizations of NH magnetic catalysis, strongly-coupled NH fractional topological phases, and NH chiral anomaly, to name a few.
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Strongly Correlated Electrons

Hidden-ordered Dirac fermions

Bitan Roy

I propose a Hermitian extension of the Lorentz-symmetric Dirac theory by complementing the associated Hamiltonian with another masslike anticommuting Dirac operator. The resulting theory manifests the iconic linear energy-momentum relationship in any dimension ($d$) and hence the emergent nodal quasiparticle excitations are named hidden-ordered Dirac fermions, which are symmetry protected and their responses are analogous to those in original Dirac systems, however, in terms of a renormalized (due to the hidden ordering) Fermi velocity. Typically, such a hidden ordering pushes any quantum phase transition into an insulation toward even stronger coupling in any $d>1$. However, depending on the internal algebra between the candidate insulating order parameter and masslike Dirac operator, the hidden-ordering may survive or disappear near the corresponding itinerant quantum critical point. I construct lattice models for such hidden-ordered massless Dirac fermions and outline promising platforms (numerical and experimental) to test these predictions.
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Mesoscale and Nanoscale Physics · Cross-listed from physics.optics

Accessing both electrochemical SEIRA and SERS with ultrasensitive metamaterials for enhanced molecular identification

Nicolas Spiesshofer, Tabitha Jones, Sarah May Sibug Torres et al. (+11 more)

Surface-enhanced IR absorption (SEIRA) and surface-enhanced Raman spectroscopy (SERS) are complementary techniques that allow for ultrasensitive chemical fingerprinting. Non-invasive optical sensing would be significantly improved by a robust implementation of a reusable substrate that combines these techniques. Here, we present an electrochemically-cleanable metamaterial that enables combined real-time SEIRA and SERS in flow. This metamaterial facilitates the study of surface-adsorbed species and diffusion layers, elicits spectral shifts from changes in nanogap refractive index of 1400 nm/RIU, and delivers ultrasensitive analyte detection. Combining SERS and SEIRA clarifies molecular (electro)chemical transformations and tracks changes in selection rules and symmetry breaking at the analyte-electrode interface. This development in enhanced multimodal spectro-electrochemistry is suited for multiple domains, including understanding charge transport mechanisms and interfacial dynamics at electrodes, and is capable of real-time flow monitoring for a wide range of molecular processes.
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All authors Nicolas Spiesshofer, Tabitha Jones, Sarah May Sibug Torres, Zoltan Sztranyovszky, Caleb Todd, Shijie Zhu, Yeeun Roh, Rakesh Arul, Alexander Squires, Ivana Qianqi Lin, Angela Demtriadou, David O. Scanlon, Viv Lindo, Jeremy J. Baumberg

Materials Science

3D Imaging of Complex Skyrmion and Hopf Topologies in an Extended Sample

I. Binnie, H. Fang, B. Shearer et al. (+16 more)

Spin textures are key for emergent magnetic phenomena such as topological protection and underpin novel spintronic device paradigms based on racetrack memory, logic gates, and neuromorphic computing. Using a coherent diffractive imaging technique called vector ptycho-tomography, in combination with algorithms that are robust to noise, we image the 3D magnetic texture of skyrmion and Hopf topologies with no prior assumptions about the sample. This directly reveals experimentally for the first time an extended 3D skyrmion lattice, including the domain wall shape, topological charge, helicity, and Hopf index. Our findings demonstrate experimentally that dipole stabilized skyrmions in Fe/Gd multilayers exhibit barrel-shaped skyrmion tubes with a twisted helicity, transitioning from N$\'e$el-type winding at the surfaces to both clockwise and counterclockwise Bloch-type winding in the bulk, that can also be described as fractional hopfions. We image a lattice of 24 skyrmions with topological charge 1, average depth-dependent domain wall width of 23 to 40 nm, depth-dependent twisted helicity from $\pm$155$\deg$ to $\pm$30$\deg$, and fractional Hopf index of $\pm$0.3. Over 10 TB of data were analyzed to yield a fully-resolved 3D reconstruction over a >0.4 $\mu$m$^3$ volume, with high fidelity down to the Nyquist limit of 8 nm. This method fills a key gap in the current landscape of magnetic imaging by enabling high-resolution, element-specific 3D reconstructions of full-field extended spin textures - offering a new route for exploring the topological complexity of magnetic materials in three dimensions.
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All authors I. Binnie, H. Fang, B. Shearer, A. Grafov, N. Jenkins, Y. Shao, C. O'Leary, Y. Liao, T. Feggeler, A. Oh, S. Yazdi, J. Zou, B. Wang, E-E. Cating, S. A. Montoya, D. Shapiro, J. Miao, H. C. Kapteyn, M. M. Murnane

Mesoscale and Nanoscale Physics

Specific absorption rate of uniaxial single-domain nanomagnets: stochastic spin dynamics versus linear response theory

H. Kachkachi

We compute the specific absorption rate of a uniaxial single-domain nanomagnet driven by an alternating magnetic field by two methods: i) direct numerical integration of the stochastic (Langevin) Landau–Lifshitz–Gilbert equation (the LLL approach), and ii) linear response theory (LRT) based on the Debye susceptibility with the Néel relaxation time $\tau_\mathrm{N}$. We first analytically show that both methods are equivalent for small magnetic field amplitude, and then compute their deviation $\Lambda\equiv \mathrm{SAR}_{\mathrm{LLL}}/\mathrm{SAR}_{\mathrm{LRT}}-1$ as a function of the magnetic field amplitude for two temperatures chosen on opposite sides of the Debye resonance. One of the main results is that the sign and magnitude of $\Lambda$ are governed by the dimensionless product $\omega\tau_\mathrm{N}$, in addition to the linearity parameter $\xi=\mu_{s}B_{0}/k_{B}T$ for the easy-axis geometry considered here. Indeed, below resonance ($\omega\tau_\mathrm{N}<1$), linear response theory overestimates the specific absorption rate. In contrast, above resonance ($\omega\tau_\mathrm{N}>1$, the regime typical of blocked nanoparticles), linear response theory can underestimate the specific absorption rate by up to $\sim70\%$ at $\xi\sim2$. We expect this work to provide quantitative guidance for the use of linear response theory in magnetic hyperthermia and related nanoscale heat-transport problems, and to serve as a single-particle benchmark for extensions to many-spin and interacting systems.
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Statistical Mechanics

Exact subsystem dynamics in the deterministic Floquet-PXP model

Katja Klobas

The dynamics of local subsystems in a thermodynamically large quantum many-body system can be understood as effectively open as the system produces its own effective bath. The action of this bath can be characterised in terms of the so-called influence matrices. In generic situations, the complexity of these objects grows unfavourably with time, however, there exist solvable cases where influence matrices can be characterised exactly even in the presence of non-trivial interactions. Here we show that Rule 201, a deterministic version of the Floquet-PXP model, is one of these solvable instances. Indeed, it admits influence matrices given by a finite-dimensional matrix-product operator (MPO) that solves a finite set of algebraic conditions. We provide the solution, and characterise multi-time autocorrelation functions.
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Statistical Mechanics

Proactivity and pinning in the non-reciprocal XY model with vision anisotropy

Gabriele Bandini, Asja Jelic and Andrea Gambassi

We study a non-reciprocal XY model on a square lattice, in which spins interact with their nearest neighbors through vision-induced anisotropic interaction. Such anisotropy breaks rotational symmetry and leads to the pinning of the spin orientation along preferred lattice directions. We systematically characterize this phenomenon for different interaction kernels, including modulated, sinusoidal, von Mises, and hard vision-cone couplings, and for two classes of microscopic update rules: Glauber and Langevin dynamics. A central result of this work is the identification and detailed analysis of two distinct contributions that naturally arise in the Langevin formulation, which we refer to as the reactive and the proactive term. We derive the corresponding equations governing both local fluctuations and the global orientation, and use them to characterize the mechanisms responsible for directional pinning. We show that both reactive and proactive contributions can generate global pinning, whereas their role in determining local pinning depends on the specific interaction kernel and may differ qualitatively. Our analysis clarifies the distinction between local and global pinning, explains the emergence of preferred lattice directions in the different models considered, and reconciles apparent discrepancies reported in previous studies. More generally, it provides a microscopic framework for understanding lattice-induced orientational selection in non-reciprocal XY models.
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