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Physics Condensed Matter

cond-mat Watch Condensed Matter

Saturday 25 October

Materials Science

Reinforcement learning-guided optimization of critical current in high-temperature superconductors

Mouyang Cheng, Qiwei Wan, Bowen Yu et al. (+3 more)

High-temperature superconductors are essential for next-generation energy and quantum technologies, yet their performance is often limited by the critical current density ($J_c$), which is strongly influenced by microstructural defects. Optimizing $J_c$ through defect engineering is challenging due to the complex interplay of defect type, density, and spatial correlation. Here we present an integrated workflow that combines reinforcement learning (RL) with time-dependent Ginzburg-Landau (TDGL) simulations to autonomously identify optimal defect configurations that maximize $J_c$. In our framework, TDGL simulations generate current-voltage characteristics to evaluate $J_c$, which serves as the reward signal that guides the RL agent to iteratively refine defect configurations. We find that the agent discovers optimal defect densities and correlations in two-dimensional thin-film geometries, enhancing vortex pinning and $J_c$ relative to the pristine thin-film, approaching 60% of theoretical depairing limit with up to 15-fold enhancement compared to random initialization. This RL-driven approach provides a scalable strategy for defect engineering, with broad implications for advancing HTS applications in fusion magnets, particle accelerators, and other high-field technologies.
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All authors Mouyang Cheng, Qiwei Wan, Bowen Yu, Eunbi Rha, Michael J Landry, Mingda Li

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

Making the Virtual Real: Measurement-Powered Tunneling Engines

Rafael Sánchez, Alok Nath Singh, Andrew N. Jordan et al. (+1 more)

Quantum tunneling allows electrons to be transferred between two regions separated by an energetically forbidden barrier. Performing a position measurement that finds a particle in the barrier forces the tunneling electrons to transition from having a classically forbidden energy to an energy above the barrier height. We exploit this effect to define quantum tunneling engines that can use the unconditioned detection of virtually occupied states as a resource for power generation and cooling. Leveraging energy exchange with the detector, we show that the device can operate in a hybrid regime, enabling simultaneous cooling and power generation. Furthermore, we demonstrate measurement-assisted autonomous refrigeration and "checkpoint" cooling driven purely by a thermal bias, without the need for an applied potential. We also find a "purification-by-noise" effect when the measurement drives the system into a stationary dark state. These results underscore the intriguing dual role of measurement as a thermodynamic resource and a dark state generator.
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All authors Rafael Sánchez, Alok Nath Singh, Andrew N. Jordan, Bibek Bhandari

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

Nonlinear optical behavior of confined electrons under torsion and magnetic fields

Carlos Magno O. Pereira and Edilberto O. Silva

In this work, we investigate the influence of torsion, Aharonov-Bohm flux, and external magnetic fields on the linear and nonlinear optical properties of a confined quantum system. The confinement potential is not assumed a priori, but emerges as a radial effective potential, analogous to a quantum dot, geometrically induced by the torsion of the material. Starting from an effective radial equation derived in a nontrivial geometric background, we analytically solve for the energy spectrum and wave functions. These solutions are then employed to evaluate the optical absorption coefficients and refractive index changes, including both linear and third-order nonlinear contributions. The formalism incorporates the electric dipole approximation and accounts for intensity-dependent effects such as saturation and spectral shifts. Our results reveal that torsion and topological parameters significantly modify the optical response, leading to tunable resonances and nontrivial dispersive behavior. This work highlights the potential of geometric and topological engineering in low-dimensional systems to control and enhance nonlinear optical phenomena.
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Superconductivity

Electromagnetic Responses of Vortex Lattices in Unconventional Superconductors

Ryusuke Ikeda and Yuto Yokota

The electro-magnetic responses of ordered vortex lattices in unconventional superconductors are studied in a high field approximation. In the cases with a vortex lattice formed within the lowest Landau level of the superconducting order parameter (OP) such as a conventional s-wave paired system with a single OP and a nonchiral spin triplet paired one with multiple components of OPs, the vanishing of the superfluid stiffness for a gauge field disturbance perpendicular to the applied uniform magnetic field is found to be ensured only for the vortex lattice structures minimizing the free energy. The notion of the vanishing superfluid stiff ness ensured by minimization of the free energy is found to be satisfied in a more complex d-wave pairing case where the vortex lattice in lower fields has an anisotropic structure de viated from the six-fold hexagonal symmetry. Interestingly, such an anisotropy in the vortex lattice structure of a d-wave paired superconductor is reflected not in the resulting vortex flow conductivities obtained after minimizing the free energy but in the elastic energy describing the harmonic fluctuation around the vortex lattice state. Relevance of the obtained results to the vortex pinning effects are discussed.
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Statistical Mechanics · Cross-listed from quant-ph

Characterizing Pauli Propagation via Operator Complexity

Yuguo Shao, Song Cheng and Zhengwei Liu

Pauli-propagation simulation represents observables in the Pauli basis and evolves their coefficients in the Heisenberg picture. Its efficiency depends on whether the evolving operator can be accurately compressed by retaining only a limited number of Pauli terms. In this work, we bridge operator complexity and the resource cost of Pauli-propagation methods by proving that the truncation error is governed by the Operator Stabilizer Rényi entropy (OSE) $\mathcal{S}^\alpha(O)$. Our a priori bounds quantify how OSE controls the compressibility of the evolving operator and give explicit prescriptions for the Top-$K$ budget required to achieve a target accuracy. As an analytic test case, we prove that for the 1D Heisenberg model at $J_z=0$, the number of non-zero Pauli coefficients generated from a local operator grows at most quadratically with the number of Trotter steps. We then benchmark the Top-$K$ Pauli propagation on XXZ Heisenberg chains. The numerical results show high accuracy with a small truncation number $K$ in the free regime ($J_z=0$) and competitive performance against tensor-network methods, such as TDVP, in the interacting case ($J_z=0.5$). These results position OSE as a resource measure for Pauli-propagation methods.
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Materials Science

Enhanced magnetic and optical properties of oxygen deficient TiO$_{2-\delta}$ nanoparticles synthesized by environment-friendly green route using whole plant extract of Phyllanthus niruri

Latika Mishra, Vinod Kumar Dwivedi, Vishal Kumar Chakradhary et al. (+2 more)

We report magnetic, optical and oxidation states of oxygen deficient TiO$_{2-\delta}$ nanoparticles (NPs) synthesized by environment-friendly green route using Phyllanthus niruri (PN) whole plant extract instead of leaf extract. Rietveld refinement of room temperature XRD pattern confirms the formation of pure phase anatase TiO$_2$ crystals in a tetragonal structure with space group I41/amd. TEM and SEM microstructure shows agglomerated spherical shape NPs exhibiting average particle size $\sim$ 35~nm. FTIR result confirms the presence of biomolecules and functional group attached to the surface of TiO$_2$ NPs. The core level XPS of O-1s and Ti-2p confirms the presence of oxygen vacancies that leads to the mixed oxidation states of Ti (Ti$^{4+}$ and Ti$^{3+}$). UV-vis result shows a strong absorption peak ($\sim$ 250~nm) along with reduced optical band gap energy E$_g$ $\sim$ 2.75~eV, possibly arises due to the surface plasmon resonance (SPR) caused by lower band gap energy emerging from oxygen vacancies. Magnetization as a function of applied magnetic field shows ferromagnetic nature at room temperature [M$_S$ $\sim$ 0.029~emu/g and H$_C$ $\sim$ 0.0143~T]. The observed ferromagnetic behaviour can be understood by virtual hopping of electrons from Ti$^{3+}$(3d$^1$) to Ti$^{4+}$(3d$^0$)-sites, however, vice versa is prohibited.
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All authors Latika Mishra, Vinod Kumar Dwivedi, Vishal Kumar Chakradhary, Akila G. Prabhudessai, Shamshun Nehar

Materials Science · Cross-listed from physics.optics

Beam shaping techniques for pulsed laser ablation in liquids: Unlocking tunable control of nanoparticle synthesis in liquids

S. Molina-Prados, N. M. Bulgakova, A. V. Bulgakov et al. (+3 more)

Nanoparticle synthesis via pulsed laser ablation in liquids has gained prominence as a versatile and environmentally friendly approach for producing ligand-free colloids with controlled composition, size, and morphology. While pulsed laser ablation in liquids offers unparalleled advantages in terms of nanoparticle purity and material versatility, enhancing the size control and productivity require modifications of the standard pulsed laser ablation in liquid technique such as the incorporation of beam shaping techniques. Recent developments in spatial and temporal beam shaping have demonstrated their potential to revolutionize pulsed laser ablation in liquids by enabling more precise energy deposition and modified nanoparticle production dynamics. This review highlights the critical role of beam shaping, encompassing spatial shaping of the beam to influence laser-material interaction, and temporal modification to optimize pulse duration and energy delivery. The current advancements in beam shaping techniques, their impact on the nanoparticle characteristics, and their broader implications for scaling pulsed laser ablation in liquids to meet industrial demands are highlighted, offering a comprehensive perspective on the future of this dynamic field.
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All authors S. Molina-Prados, N. M. Bulgakova, A. V. Bulgakov, J. Lancis, G. Mínguez Vega, C. Doñate-Buendia

Superconductivity

Effects of successive annealing on high-field electrical transport and the upper critical field in S-substituted FeTe

Ryosuke Kurihara, Satoshi Hakamada, Masaki Kondo et al. (+3 more)

Since iron-based superconductors have been discovered, many scientists have focused on their characteristic properties, such as an unconventional mechanism and a high upper critical field. Sulphur-substituted FeTe compounds are one of the members of the iron-based superconductors; however, chemical processes, such as O$_2$ annealing, are needed to induce superconductivity because of the existence of excess iron in as-grown crystals. Thus, the removal of excess iron and the obtaining of clean sulphur-substituted FeTe can play a key role in the understanding of the superconducting properties and the application to the superconducting devices. In this study, we present the successive annealing effects on sulphur-substituted FeTe compounds to investigate the electrical transport properties under high magnetic fields. Our measurements show that successive annealing processes improve the electrical transport properties in the superconducting states under magnetic fields. The removal of excess iron acting as magnetic impurities is indicated by the improvement of the upper critical field and its analysis.
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All authors Ryosuke Kurihara, Satoshi Hakamada, Masaki Kondo, Ryuji Okazaki, Masashi Tokunaga, Hiroshi Yaguchi

Statistical Mechanics

Revealing Liquid-Gas Transitions with Finite-Size Scaling in Confined Systems

Chong Zha, Yanshuang Chen, Cheng-Ran Du et al. (+2 more)

The application of an external field often renders empirical criteria for identifying liquid-gas phase transitions ambiguous. Here, we demonstrate that the finite-size scaling of the density profile provides a definitive criterion to distinguish liquid-gas coexistence from a single fluid phase in field-confined systems. Our scaling method collapses the density profiles of different system sizes onto a single master curve for a one-phase system, while causing the profiles to intersect at the interface in a two-phase system. We validate this theoretical proposal through experiments and simulations of two model systems: colloidal suspensions under gravity and/or two-dimensional complex plasmas confined by a central potential. Our method is broadly applicable for detecting liquid-gas phase transitions in laboratory systems where external fields are inherent.
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All authors Chong Zha, Yanshuang Chen, Cheng-Ran Du, Peng Tan, Yuliang Jin

Mesoscale and Nanoscale Physics

Exact time-evolving resonant states for open double quantum-dot systems with spin degrees of freedom

Akinori Nishino and Naomichi Hatano

We study time-evolving resonant states in an open double quantum-dot system, taking into account spin degrees of freedom as well as both on-dot and interdot Coulomb interactions. We exactly derived a non-Hermite effective Hamiltonian acting on the subspace of two quantum dots, where the non-Hermiticity arises from an effect of infinite external leads connected to the quantum dots. By diagonalizing the effective Hamiltonian, we identify four types of two-body resonant states. For the initial states of localized two electrons with opposite spins on the quantum dots, we exactly solve the time-dependent Schroedinger equation and obtain time-evolving two-body resonant states. The time-evolving resonant states are normalizable since their wave function grows exponentially only inside a finite space interval that expands in time with electron velocity. By using the exact solution, we analyze the survival and transition probabilities of localized two electrons on the quantum dots.
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Materials Science

Suppression of Thin-Film Thermal Conductivity due to Surface Roughness

Michimasa Morita and Junichiro Shiomi

Understanding thermal transport in silicon nanostructures is crucial for effective thermal management in semiconductor devices. In such nanostructures, boundary scattering can significantly reduce thermal conductivity. Diffusive boundary scattering explains the experimentally observed thickness dependence of thermal conductivity in thin films with thicknesses of tens of nanometers; however, introducing surface roughness further reduces the thermal conductivity, which falls far below the theoretical lower limit. In this study, we calculated the thermal conductivity and phonon transport properties of rough thin films with thicknesses of up to 25 nm using anharmonic lattice dynamics and investigated the mechanisms underlying the suppression of thermal conductivity arising from surface roughness. We found that in ultrathin films with rough surfaces, thermal conductivity was suppressed by a reduction in group velocity caused by hybridization with surface-localized modes, as well as a reduction in relaxation time due to the modulation of the anharmonic interatomic force constants of surface atoms. The reduction in group velocity significantly suppressed thermal conductivity across a wide range of thicknesses and surface-roughness values. In contrast, the reduction in relaxation time exhibited strong thickness dependence. Thus, this relaxation-time reduction should be considered in ultrathin films with roughness of approximately 0.1 nm and thicknesses below 5 nm. These thermal-conductivity suppression mechanisms due to surface roughness were not considered in the boundary-scattering model, resulting in an overestimation of the thermal conductivity of the roughened thin films by up to approximately 100%.
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Mesoscale and Nanoscale Physics · Cross-listed from physics.optics

Strong Coupling beyond the High-Q Limit and Linewidth Narrowing in a Multi-Exciton Planar Microcavity

E. A. Cerda-Méndez, Y. G. Rubo, K. Biermann et al. (+3 more)

We systematically study the linewidths of multilevel exciton-polariton modes as a function of the detuning in a planar hybrid microcavity (MC) with low quality factor (Q~300) operating in the linear optical response regime. Using optical reflectivity, we observe that, counterintuitively, the linewidths of the polariton modes undergo a pronounced spectral narrowing as detuning is reduced. Using optical reflectivity, we observe that, counterintuitively, the linewidths of the polariton modes undergo a pronounced spectral narrowing as detuning is reduced. By benchmarking the experimental results against three commonly used constant-loss theoretical descriptions, we find that this behavior is only partially reproduced, highlighting limitations of conventional strong-coupling models when applied to non-standard MC architectures. Our results suggest that frequency-dependent self-energy effects or correlated dissipation mechanisms, typically neglected in simplified treatments, may play an important role.
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All authors E. A. Cerda-Méndez, Y. G. Rubo, K. Biermann, A. Camacho-Guardian, A. S. Kuznetsov, P. V. Santos

Materials Science

Electric-Field-Tunable Luttinger compensated antiferromagnetism in double CrCl2 chains

Deping Guo, Weihan Zhang, Canbo Zong et al. (+2 more)

Luttinger compensated antiferromagnets (LcAFMs), combining spin polarization with vanishing net magnetization, offering distinct advantages for next-generation spintronic applications. Using first-principles calculations, we demonstrate that conventional antiferromagnetic CrCl2 double chains can be transformed into one-dimensional LcAFMs under an external electric field, exhibiting pronounced isotropic spin splitting. The magnitude of the splitting, as well as the band gap, can be effectively tuned by both in-plane and out-of-plane fields, thereby providing greater controllability than in two-dimensional counterparts. To further enhance the tunability, we design a nearly lattice-matched CrCl2/MoTe2 heterostructure and uncover that interfacial charge transfer generates a built-in electric field, inducing spin splitting comparable to that driven by external fields. These results establish interfacial engineering as a highly efficient route to realize and manipulate LcAFM states in low-dimensional magnets, expanding the design principles for spintronic functionalities at the nanoscale.
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All authors Deping Guo, Weihan Zhang, Canbo Zong, Cong Wang, Wei Ji

Disordered Systems and Neural Networks

Berezinskii-Kosterlitz-Thouless Transition and Multifractal Critical Phase in Two-Dimensional Quantum Percolation

W. S. Oliveira, J. Pimentel de Lima, F. A. Pinheiro et al. (+1 more)

We present a numerical study of the two-dimensional quantum percolation model, revealing that a critical region with multifractal eigenstates mediates the transition from localized to delocalized states. By analyzing the mean level ratio and participation entropy, we identify two distinct transitions: a Berezinskii-Kosterlitz-Thouless (BKT) transition at the classical percolation threshold, separating the localized and critical phases, and a power-law-type transition at a larger concentration, marking the onset of full delocalization. The critical phase is characterized by multifractal eigenstates, as evidenced by the generalized fractal dimension and multifractal spectrum. Altogether, our results establish that in the marginal two-dimensional case, the Anderson impurity model and the quantum percolation model belong to different universality classes.
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All authors W. S. Oliveira, J. Pimentel de Lima, F. A. Pinheiro, R. R. dos Santos

Materials Science

Machine Learning Enables Optimization of Diamond for Quantum Applications

Dane W. deQuilettes, Eden Price, Linh M. Pham et al. (+11 more)

Spins in solid-state materials, molecules, and other chemical systems have the potential to impact the fields of quantum sensing, communication, simulation, and computing. In particular, color centers in diamond, such as negatively charged nitrogen vacancy (NV$^-$) and silicon vacancy centers (SiV$^-$), are emerging as quantum platforms poised for transition to commercial devices. A key enabler stems from the semiconductor-like platform that can be tailored at the time of growth. The large growth parameter space makes it challenging to use intuition to optimize growth conditions for quantum performance. In this paper, we use supervised machine learning to train regression models using different synthesis parameters in over 100 quantum diamond samples. We train models to optimize NV$^-$ defects in diamond for high sensitivity magnetometry. Importantly, we utilize a magnetic-field sensitivity figure of merit (FOM) for NV magnetometry and use Bayesian optimization to identify critical growth parameters that lead to a 300% improvement over an average sample and a 55% improvement over the previous champion sample. Furthermore, using Shapley importance rankings, we gain new physical insights into the most impactful growth and post-processing parameters, namely electron irradiation dose, diamond seed depth relative to the plasma, seed miscut angle, and reactor nitrogen concentration. As various quantum devices can have significantly different material requirements, advanced growth techniques such as plasma-enhanced chemical vapor deposition (PE-CVD) can provide the ability to tailor material development specifically for quantum applications.
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All authors Dane W. deQuilettes, Eden Price, Linh M. Pham, Arthur Kurlej, Swaroop Vattam, Alexander Melville, Tom Osadchy, Boning Li, Guoqing Wang, Collin N. Muniz, Paola Cappellaro, Jennifer M. Schloss, Justin L. Mallek, Danielle A. Braje

Friday 24 October

Soft Condensed Matter

Reciprocal swimming in granular media: the role of jamming and swimmer inertia

Amir Nazemi and Hongyi Xiao

We use particle simulations to reveal two distinct propulsion mechanisms for a scallop-like swimmer to propel itself in granular media by reciprocally flapping its wings. Based on the discrete element method, we examine the structure, kinematics, and contact forces of particles near the swimmer to quantify how jamming manifests as stagnant zones near the swimmer in a frictional granular medium, which are less intense during the opening stroke than the closing. This broken symmetry is quantified by the difference in the number of strong particle contact forces formed during opening and closing, which shows a linear relation with the swimmer's net displacement across various swimmer and medium configurations, all favoring the opening stroke. We identify a secondary propulsion mechanism in a dynamic regime with significant swimmer inertia, as the flapping period approaches the coasting time for a moving swimmer to come to rest under the medium resistance. In this case, the swimmer's net displacement is correlated to the ratio between these two time scales, and the swimming direction favors the closing stroke due to the smaller medium resistance as the swimmer coasts with closed wings.
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Soft Condensed Matter · Cross-listed from physics.flu-dyn

Mirror states enable lower viscosity lattice gases

Noah Seekins and Alexander J. Wagner

We developed a method for significantly lowering the viscosity achievable for a hydrodynamic lattice gas method. The key advance is the derivation of a mirror state that allows for a reduction of viscosity by more than an order of magnitude over existing lattice gas methods.

Materials Science

Dynamics and formation of antiferromagnetic textures in MnBi$_2$Te$_4$ single crystal

M. G. Kim, S. Boney, L. Burgard et al. (+2 more)

We report coherent X-ray imaging of antiferromagnetic (AFM) domains and domain walls in MnBi$_2$Te$_4$, an intrinsic AFM topological insulator. This technique enables direct visualization of domain morphology without reconstruction algorithms, allowing us to resolve antiphase domain walls as distinct dark lines arising from the A-type AFM structure. The wall width is determined to be 550(30) nm, in good agreement with earlier magnetic force microscopy results. The temperature dependence of the AFM order parameter extracted from our images closely follows previous neutron scattering data. Remarkably, however, we find a pronounced hysteresis in the evolution of domains and domain walls: upon cooling, dynamic reorganizations occur within a narrow $\sim$1 K interval below $T_N$, whereas upon warming, the domain configuration remains largely unchanged until AFM order disappears. These findings reveal a complex energy landscape in MnBi$_2$Te$_4$, governed by the interplay of exchange, anisotropy, and domain-wall energies, and underscore the critical role of AFM domain-wall dynamics in shaping its physical properties.
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All authors M. G. Kim, S. Boney, L. Burgard, L. Rutowski, C. Mazzoli

Materials Science

Highly Efficient Functionalization of hBN with Lithium Oxalate: A Multifunctional Platform for Composites, Ion Transport, and Spin Labeling

Bence G. Márkus, Anna Nyáry, Dávid Beke et al. (+18 more)

Multifunctional solid-state materials are crucial for enabling safer and simpler lithium battery architectures. Here we report a scalable, solvent-free mechanochemical strategy to overcome the chemical inertness of hexagonal boron nitride and produce Li$_2$(BN)$_6$C$_2$O$_4$ (LBNCO), a lithium-rich boron nitride nanostructure. High-energy milling simultaneously exfoliates hBN and induces robust covalent oxalate functionalization, yielding a nanocrystalline lamellar compound with intrinsically incorporated lithium. LBNCO exhibits Li$^+$ ionic conductivity, electronic insulation, and thermal stability exceeding $350~^{\circ}$C. Additional mechanochemical processing enables further lithium enrichment without compromising chemical stability, allowing tunable ionic conductivity. By integrating ion transport, mechanical robustness, and thermal stability within a single material, LBNCO establishes a new platform for solid-state lithium batteries and related technologies.
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All authors Bence G. Márkus, Anna Nyáry, Dávid Beke, Lili Vajtai, Sivaviswa Radhakrishnan, Vignyatha R. Tatagari, Bradlee J. McIntosh, Adil Saleem, Changlong Chen, Balázs Zsirka, Mandefro Y. Teferi, Jens Niklas, Oleg G. Poluektov, Ira D. Bloom, Fulya Dogan, Margit Kovács, Ferenc Simon, Gábor Szalontai, Leon Shaw, László Forró, Károly Németh

Materials Science

Nonlinear phononic slidetronics

Pooja Rani and Dominik M. Juraschek

Van der Waals ferroelectrics are conventionally switched by sliding the different layers between stacking orders with opposing electric polarizations. Ultrashort laser pulses have been proposed to launch shear modes and induce switching, with often unfeasible large pulse energies however. Here, we demonstrate switching of ferroelectricity in bilayer hexagonal boron nitride through nonlinearly excited phonons. We show that the efficiencies of conventional coherent phonon excitation mechanisms, including infrared absorption and Raman scattering techniques, are too low to overcome the energy barrier separating the two ferroelectric states. We demonstrate instead that excitation of high-frequency intralayer modes leads to a tilting of the interlayer potential-energy landscape that enables changing the stacking order. Our results provide an avenue towards efficient phononic slidetronics, enabling ultrafast control of the stacking order in van der Waals materials.
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Materials Science

Highly Tunable Phonon Polaritons via Metal Intercalation

Mariia Stepanova, Minh Ngo, Mashnoon Alam Sakib et al. (+5 more)

Phonon polaritons in van der Waals crystals offer mid-infrared light confinement deep below the diffraction limit, making them promising for nanophotonics applications. However, the practical use of phonon polaritons remains limited, in part due to the lack of precise control over the phonon polariton dispersion, as crystal lattice vibrations are often inert to external stimuli. Here, we address this challenge by zerovalent metal intercalation of $\alpha$-MoO$_3$. Photo-induced force microscopy shows that introducing tin into the van der Waals gap modulates the phonon polariton dispersion by up to $38.5\pm0.5\%$, which is the highest amount of tunability among non-mechanical modulation approaches, to the best of our knowledge. Intercalation with various metal species preserves the phonon polariton lifetimes, while modulating the dielectric permittivity in agreement with the density functional theory and analytical calculations. Our results establish metal intercalation as a practical route to reconfigurable mid-infrared nanophotonics.
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All authors Mariia Stepanova, Minh Ngo, Mashnoon Alam Sakib, Wills Harris, Joshua Bocanegra, Ruqian Wu, Kristie J. Koski, Maxim R. Shcherbakov

Materials Science

Unravelling the oxygen influence in cubic bixbyite In$_2$O$_3$ on Raman active phonon modes by isotope studies

Johannes Feldl, Roland Gillen, Janina Maultzsch et al. (+5 more)

In this study, we performed comprehensive investigations on the Raman active phonon modes in cubic bixbyite In$_2$O$_3$, an important oxide based, wide-bandgap semiconductor. Fundamental insights into the lattice dynamics are revealed, by determining the atomistic contribution to all modes and their frequencies by density functional perturbation theory calculations. Those simulations were performed for different compositions of $^{16}$O and $^{18}$O isotope ratios, including their pure states. An increasing red-shift of the mode frequencies with increasing $^{18}$O content for all modes, due to the increased atomic mass, is revealed. For the seven lowest energy modes, this relative shift is below 1%, whereas for the remaining 15 higher energetic modes a shift of about 5.5% was identified. All modes have energy contributions of both indium and oxygen lattice sites, except for one, which corresponds to a pure oxygen vibrational state. Applying Raman spectroscopy, those results could be verified experimentally with excellent agreement. Investigated samples consisted of a bulk single crystal with $^{16}$O isotopes and a MBE grown thin film as the $^{18}$O sample. Time-of-flight secondary ion mass spectrometry confirms the purity of the oxygen isotope in the sample. These isotopologue studies allow for a direct experimental access to fundamental material properties in cubic In$_2$O$_3$ by means of Raman spectroscopy. For example, we speculate, that the presence of oxygen vacancies in In$_2$O$_3$ would result in a shift of modes that are dominated by O-vibrations, e. g., $E_{g}^{(4)}$ or $A_{g}^{(4)}$, towards lower frequencies.
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All authors Johannes Feldl, Roland Gillen, Janina Maultzsch, Alexandra Papadogianni, Joe Kler, Zbigniew Galazka, Oliver Bierwagen, Manfred Ramsteiner

Materials Science

First-principles study of phase stability and magnetic properties of B2 AlCr, AlMn, AlFe, AlCo and AlNi aluminides

Haireguli Aihemaiti, Esmat Dastanpour, Anders Bergman et al. (+1 more)

Using ab initio Density Functional Theory (DFT) calculations, we investigate the electronic structure, phase stability, and magnetic properties of equiatomic binary alloys between Al and 3d magnetic transition elements (Cr, Mn, Fe, Co, and Ni). Thermodynamically, all five binary aluminides are more stable in the ordered B2 phase than in the disordered body centered cubic phase, and Co is found to be the strongest B2 forming element with Al. The AlCo and AlNi compounds with B2 structure are verified to be non-magnetic, whereas AlFe turns out to be weakly magnetic, which is consistent with other DFT calculations employing similar exchange-correlation approximations. Magnetic simulations based on the Heisenberg Hamiltonian predict an antiferromagnetic ground state for the hypothetical B2 AlCr, which is also confirmed by direct DFT calculations. Doping AlCr with Co leads to an antiferromagnetic to ferromagnetic transition, where ferromagnetism is to a large extent attributed to Cr atoms. The phase stability and magnetic trends are explained using electronic structure arguments. The present findings contribute to a deeper understanding of the phase stability and magnetic properties of AlX binary alloys, providing insights into the formation mechanisms of the B2 structure with 3d magnetic transition metals.
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All authors Haireguli Aihemaiti, Esmat Dastanpour, Anders Bergman, Levente Vitos

Materials Science

Magnetic transition in B2 Al-Cr-Co alloys

Haireguli Aihemaiti, Esmat Dastanpour, Shashank Chaturvedi et al. (+3 more)

Using Density Functional Theory (DFT) calculations and Monte-Carlo (MC) simulations, we investigate the recently reported magnetic transition in B2 Al-Cr-Co alloys. The Cr sublattice is alloyed with different amounts of Co in the antiferromagnetic (AFM) B2 AlCr binary alloy and the resulting exchange interactions are analyzed within the Heisenberg Hamiltonian framework. DFT results reveal that at low Co concentrations the system favors the AFM order, while at high Co contents a transition to the ferromagnetic (FM) state is observed. Within the FM stability field, the Curie temperature (TC), obtained within the mean-field approximation, is below ~160 K and decreases with Co concentration. The calculated exchange parameters evolve systematically with Co content, and the trends are consistent with the DFT total energies. The magnetic configurations obtained from MC simulations follow the DFT results at low Cr levels but predict a spin-glass behavior for alloys containing more than 40 at.% Co on Cr sublattice. These findings provide a fundamental understanding of how the chemistry-driven changes in exchange interactions affect magnetism in the B2 Al-Cr-Co alloys.
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All authors Haireguli Aihemaiti, Esmat Dastanpour, Shashank Chaturvedi, Shuo Huang, Anders Bergman, Levente Vitos

Statistical Mechanics

Emerging correlations between diffusing particles evolving via simultaneous resetting with memory

Denis Boyer and Satya N. Majumdar

We study the emergence of correlations between $N$ components of the position of a diffusive walker in $N$ dimensions that starts at the origin and resets to previously visited sites with certain probabilities. This is equivalent to $N$ independent one-dimensional diffusive processes starting from the origin and being subject to simultaneous resetting to positions visited in the past. Resetting follows a memory kernel that interpolates between resetting to the origin only, and the preferential relocation model, a path-dependent process which is highly non-Markov. For weak memory, the correlation coefficient between two components of the $N$-dimensional process grows monotonously with time and tends at late times to a constant bounded by $1/5$, the value corresponding to the non-equilibrium steady state of resetting to the origin. When memory is sufficiently long-ranged, the correlation is non-monotonous and reaches a maximum at a finite time before converging to its asymptotic value. These two regimes are separated by a critical memory parameter value. In the limiting case of the preferential relocation model, the components become uncorrelated at both short and long times, but the correlation vanishes logarithmically slowly at late times. The emergence of correlations through resetting can be described in a unified way in all cases by noticing that the processes are conditionally independent and identically distributed, even in the presence of memory. In the non-Markovian case, the conditioning parameter is the duration of a Brownian path composed of several parts of the full trajectory of a fixed duration $t$.
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Materials Science

Altermagnetism, Kagome Flat Band, and Weyl Fermion States in Magnetically Intercalated Transition Metal Dichalcogenides

Avinash Sah, Ting-Yong Lim, Clayton Conner et al. (+5 more)

Altermagnetic (AM) compounds have recently emerged as a promising platform for realizing unconventional quantum phases, enabled by their unique spin-split band structure at zero net magnetization. Here, we present a first-principles investigation of magnetically intercalated transition metal dichalcogenides (TMDs) of the form XY$_4$Z$_8$ (X $=$ Mn, Fe, Co, Ni, Cr, or V; Y $=$ Nb or Ta; and Z $=$ Se or S), identifying a subset of new versatile AM candidates. Our results establish a systematic correlation between interatomic geometry, quantified by the ratio of interlayer to intralayer spacing, and the magnetic ground states. Systems with A-type antiferromagnetic order exhibit momentum-dependent spin splitting consistent with AM behavior. The combination of the AM spin-splitting and the spin-orbit coupling leads to the emergence of Weyl nodes together with the corresponding topological Fermi arc surface states. Moreover, we identify flat bands near the Fermi level that originate from the intercalant-induced formation of an effective kagome-like sublattice in the TMD layer. These results collectively establish magnetically intercalated TMDs as a promising platform for engineering altermagnetism, flat bands, and Weyl fermions within a single material family, facilitating the development of topological and spintronic applications.
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All authors Avinash Sah, Ting-Yong Lim, Clayton Conner, Amarnath Chakraborty, Giovanni Vignale, Tay-Rong Chang, Pavlo Sukhachov, Guang Bian

Mesoscale and Nanoscale Physics

Emergent spatiotemporal order and nonreciprocity in driven-dissipative nonlinear magnetic systems

Vincent Flynn and Benedetta Flebus

The identification of platforms with independently tunable nonlinearity and non-Hermiticity promises a quantitative route to far-from-equilibrium universality across many-body systems. Here we show that a conventional ferromagnetic multilayer realizes this paradigm: balancing a dc drive against Gilbert damping stabilizes a self-organized, current-carrying nonequilibrium condensate that spontaneously breaks spacetime-translation symmetry. The chirality of this spin superfluid limit cycle state generates an inherently nonreciprocal flow: long-wavelength magnons of opposite chirality acquire asymmetric dispersions and propagate direction-selectively, realizing a spin superfluid diode. This asymmetry is flow-borne - it reflects broken Galilean invariance and requires neither structural asymmetry nor finely tuned gain-loss balance. Linearized dynamics in the comoving superfluid frame are intrinsically pseudo-Hermitian and, in the long-wavelength sector, can be mapped to a (1+1)D wave equation on curved spacetime. Spatial modulation of the drive enables the generation of sonic horizons that parametrically amplify magnon pairs and produce Hawking-like particle-hole emission. Our results establish a tabletop route from nonlinear dissipative-driven magnetization dynamics to nonreciprocal transport, nonequilibrium phase transitions, and analogue-gravity kinematics.
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Disordered Systems and Neural Networks

Phase diagram of amorphous quantum spin Hall insulators

Ranadeep Roy and Yuan-Ming Lu

In light of recent progress in the study of amorphous topological phases, we investigate the effects of structural disorder on the topological properties of a two-dimensional quantum spin Hall insulator modeled by the Bernevig-Hughes-Zhang Hamiltonian. Using a real-space formulation of the Z2 invariant for Dirac-type Hamiltonian, we map out the phase diagram as a function of disorder strength and the mass parameter. Our results reveal that under the influence of structural disorder, a system can either undergo a phase transition from a topologically non-trivial to a topologically trivial phase or from a trivial to non-trivial phase. Remarkably, in certain parameter regimes, the system exhibits a re-entrant behaviour: a topologically non-trivial phase in the perfect lattice undergoes a transition to a trivial state under the influence of weak disorder but re-emerges as the disorder strength is further increased. We corroborate these findings through analysis of the bulk-boundary correspondence and transport calculations.
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Statistical Mechanics

Machine-learning-derived protocols for information-based work extraction from active particles

Grzegorz Szamel

We propose and analyze a process that extracts useful work from a single active particle maintained at constant temperature in a harmonic potential by measuring the relative sign of the self-propulsion and the confining force and then adjusting the stiffness of the potential. First, we show analytically that useful work can be extracted by stepwise changes of the stiffness. Then, we use a machine learning procedure to find time-dependent stiffness change protocols. We find that these protocols involve discontinuous initial changes of the stiffness opposite to the expected direction, which resemble initial jumps analytically found by Garcia-Millan et al. [Phys. Rev. Lett. 135, 088301 (2025)] in a different information-based work extraction process. The learned protocols allow to extract significantly larger amounts of useful work. The work extracted exceeds that allowed by the conventional second law for feedback-controlled processes, which can be rationalized by the non-equilibrium character of the system considered.
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Statistical Mechanics · Cross-listed from quant-ph

The evolution of Liouville von Neumann master equations in the Pechukas-Yukawa framework

Mumnuna Aziz Qureshi

This paper presents a novel formalism for the out of equilibrium dynamics of the density matrix, capable of describing highly entangled many-body interactions. The evolution of quantum states is evaluated via eigenvalue dynamics of a general Hamiltonian system, perturbed by a parametrically evolving variable $\lambda(t)$ that carries the time-dependence. This is achieved using the Pechukas-Yukawa mapping of the evolution of the energy levels governed by their initial conditions on a generalised Calogero-Sutherland model of a 1D classical gas. As such, quantum systems can be described exactly in their entirety from eigenvalue dynamics. Under this description, we provide an improved understanding of the relationship between nonequilibrium quantum phase transitions and decoherence which has significant impacts to a wide range of applications.
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