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Physics Condensed Matter

cond-mat Watch Condensed Matter

Tuesday 8 September

Strongly Correlated Electrons

Kane-Lubensky phonons in Maxwell lattice frustrated Mott insulators

Hong-Hao Song and Gang v. Chen

We show that zero-energy gapless Weyl-line phonons of the Kane-Lubensky's type can arise in the three-dimensional Maxwell lattice frustrated Mott insulators through the magnetopological mechanics. In a pyrochlore antiferromagnet with the spin-lattice coupling, a magnetic field selects the spin state whose lattice distortion generates a $P4_3 32$ topological lattice. More crucially, the spin-lattice coupling and the spin configuration cause the bending of the neighbouring bonds, and converts the system into the topological Maxwell lattice. Remarkably, the resulting system is found to host the bulk zero-frequency Weyl-line phonons protected topologically, and these gapless phonons are not Goldstone modes. Unlike the conventional ${C_{\rm ph}\sim T^3}$ for the Goldstone phonons, these one-dimensional zero-mode manifolds yield a characteristic low-temperature phonon specific heat ${C_{\rm ph}\sim T^2}$. Our results could find applications in the Cr-based spinel systems, and moreover, we establish a low-energy platform where an extensive number of topological zero-frequency phonons can strongly couple to other degrees of freedom, opening a route to exotic phonon-mediated phenomena.
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Superconductivity

Magnetochiral anisotropy of thermally activated phase slips in noncentrosymmetric superconductors

Alex Levchenko

Resistive tails of narrow superconducting wires below the critical temperature are governed by phase slips, thermally activated near the transition and quantum at lower temperatures. We develop the theory of thermally activated phase slips in wires lacking inversion symmetry, modeled microscopically as a diffusive wire formed from a two-dimensional electron gas with Rashba spin-orbit coupling in an in-plane magnetic field. The phase-slip resistance acquires magnetochiral anisotropy: at fixed current it changes upon reversal of the field, and at fixed field upon reversal of the current, while the linear-response resistance remains field symmetric as required by Onsager reciprocity. The nonreciprocity originates from an asymmetry of the phase-slip activation barrier that is odd in both current and field. It is generated by the Lifshitz invariants of the free energy, the cubic gradient term and the momentum-odd quartic vertex, together with even gradient terms promoted to odd ones by the helical ground state, precisely the combination that determines the superconducting diode effect. The barrier asymmetry is therefore fixed by the diode efficiency of the same wire through a universal numerical ratio, with no adjustable parameters. The kinetic Lifshitz invariant, which makes relaxation and noise of the order parameter nonreciprocal, affects only the fluctuation prefactor at a parametrically subleading level. Because the asymmetry resides in the activation exponent, the magnetochiral signal is amplified relative to the diode asymmetry by the ratio of barrier height to temperature, and appears as a field-antisymmetric splitting of the exponential slopes of differential-resistance traces routinely used to identify thermally activated phase slips.
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Soft Condensed Matter

A unified description of flow-induced scission of wormlike micelles under shear and extensional flows

Yusuke Koide, Takato Ishida, Takashi Uneyama et al. (+1 more)

We investigate flow-induced scission of wormlike micelles under different flow kinematics using dissipative particle dynamics simulations of surfactant solutions in shear, uniaxial extensional, planar extensional, and biaxial extensional flows. The average lifetime of wormlike micelles is used to quantify the degree of scission. While flow-induced scission occurs in all flow types when the deformation rate is sufficiently high, the dependence of the average lifetime on the deformation rate differs among flow types. To provide a unified description, we introduce an effective extension rate determined by the velocity gradient tensor and micellar orientation. When expressed in terms of this effective extension rate, the average lifetimes obtained under all flow types considered collapse onto a single curve. These results demonstrate that a unified description of flow-induced scission requires not only the strength and kinematics of the imposed flow, but also the micellar orientation relative to the extensional direction.
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All authors Yusuke Koide, Takato Ishida, Takashi Uneyama, Yuichi Masubuchi

Quantum Gases

Large-Chern-number flat bands, anomalous Dirac cones, and unconventional superfluidity in square-lattice systems with SU(N) non-Abelian gauge fields

Wentao Zhong and Zhongbo Yan

We study topological band structures and superfluid phases in two-dimensional square-lattice systems with homogeneous SU($N$) non-Abelian gauge fields. Starting from an SU(4) gauge-field model related to the Hofstadter model with flux $\alpha=1/4$, we show that the lowest and highest bands are isolated Chern bands that carry Chern numbers $C=-4$ and give rise to four chiral edge modes in a strip geometry. Remarkably, although the two middle bands touch and form $16$ gapless Dirac cones, their combined Chern number is $C=8$. We then generalize the construction to SU($N$) systems and reveal an even–odd structure of the band topology: when $N$ is odd, all bands are isolated and carry nonzero Chern numbers; when $N$ is even, the two middle bands touch at $N^{2}$ Dirac points, while all other bands remain isolated and topologically nontrivial. We find that the uppermost and lowermost bands become increasingly flat and their Berry curvature becomes more uniform as $N$ increases, providing a promising platform for realizing fractional Chern insulating phases. We further examine the spin-$3/2$ SU(4) model with on-site attractive Hubbard interactions, exploring its superfluid phases at partial filling. We find that the non-Abelian gauge field breaks the hidden SO(5) degeneracy of the quintet pairing and selects distinct nematic superfluid states. For fillings in the middle-band regime, the resulting spectrum can host topologically protected Bogoliubov Fermi surfaces. Our results provide a starting point for exploring both topological band physics and unconventional superfluidity in synthetic SU($N$) cold-atom systems.
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Statistical Mechanics

Speed Limit for Information Acquisition in Stochastic Learning Dynamics

Shuta Kobayashi and Andreas Dechant

Neural networks acquire internal representations through learning. In this work, we formulate stochastic gradient descent (SGD) as a Markovian stochastic process and derive a Fisher-information flow speed limit that bounds the rate at which trainable parameters can acquire information about latent variables in the data-generating process. The resulting inequality decomposes the information flow into drift and noise contributions, thereby quantifying the roles of deterministic learning forces and SGD-induced fluctuations from an information-theoretic perspective. We verify the bound in analytically tractable basis-function linear regression, where the information budget predicted by the bound reproduces the ordering and characteristic time scales with which different latent variables are encoded in the learned parameters. These results establish Fisher-information speed limits as a quantitative framework for diagnosing when and how different aspects of the data-generating mechanism are acquired during stochastic learning.
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Quantum Gases

Increase of Magnetic Trap Loading Efficiency for $^{39}\mathrm{K}$ Bose–Einstein Condensation Experiments

Yihao Cheng, Xiaoyang Shen, Weixin Li et al. (+3 more)

During the experimental sequence, a large atom number is important for the creation of high-quality Bose–Einstein condensates. We report an experimental increase of about 15% in magnetic-trap (MT) loading efficiency by optimizing the MT loading process. The loading time is much shorter than the lifetime of the atoms in the MT, which means atom loss during loading due to the finite trap lifetime can be neglected. We obtain an almost pure $^{39}\mathrm{K}$ condensate with $1.2\times10^{5}$ atoms after evaporative cooling in our optical trap.
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All authors Yihao Cheng, Xiaoyang Shen, Weixin Li, Hao Du, Hongwu Zhao, Lin Xia

Soft Condensed Matter

Viscoelasticity reshapes the frequency response of a rotating magnetic particle

Zhiyuan Zhao, Tingting Sun, Han Gao et al. (+3 more)

A magnetic particle driven by a rotating magnetic field undergoes a transition from synchronous to asynchronous rotation at a critical driving frequency. The asynchronous dynamics is well understood in Newtonian fluids but remains unclear in viscoelastic media. Here, we develop a theoretical description of the asynchronous rotation of a magnetic particle in a Jeffreys-type viscoelastic fluid. The particle's time-averaged angular velocity exhibits a nontrivial frequency dependence that changes from non-monotonic to monotonic as the polymer relaxation time increases. This behavior is explained by the interplay among magnetic driving, viscoelastic relaxation, and frequency-dependent viscous dissipation. We further derive an asymptotic expression that captures the non-monotonic dependence. These results clarify how solvent and polymer contributions jointly control asynchronous rotation and provide a physical basis for guiding relevant applications in complex fluids.
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All authors Zhiyuan Zhao, Tingting Sun, Han Gao, Ye Xu, Mingcheng Yang, Masao Doi

Materials Science

Pressure Evolution of Atomic Volume Systematics in Transition Metals

Masaaki Geshi and Yuichi Akahama

We investigated the evolution of the well-known parabolic dependence of atomic volume on atomic number in transition metals under extreme compression at pressures up to 400 GPa using density functional theory calculations. Our results reveal that the ambient-pressure parabolic trend transforms into a characteristic cubic-like behavior at high pressures. This evolution is attributed to the higher compressibility of bcc transition metals associated with comparatively large increases in the total energy. The present findings are discussed in relation to previous experimental observations and first-principles calculations.
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Other Condensed Matter

Transport properties and topological phase transitions for a Creutz-Su-Schrieffer-Heeger ladder

K. A. González, S. Bravo, L. Rosales et al. (+1 more)

In this work, we investigate the electronic, topological, and transport properties of a Creutz-Su-Schrieffer-Heeger (CSSH) ladder. Using a tight-binding model within the Green's function formalism, we calculate the energy spectrum, local density of states (LDOS), and electronic transmission. We first determine the energy spectrum of the CSSH ladder and analyze the different topological phases present in the system, identifying one trivial phase and three distinct nontrivial regions. We then study electronic transport and show that the transmission reproduces the different topological phases through characteristic transport signatures. Finally, we derive the conditions for the emergence of non-topological flat bands and demonstrate that these bands also provide the necessary conditions for the formation of bound states in the continuum (BICs). Our results establish a direct connection between the topological properties, flat-band formation, and electronic transport in the CSSH ladder.
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All authors K. A. González, S. Bravo, L. Rosales, P.A. Orellana

Strongly Correlated Electrons

Successive Phase Transitions from a Composite Fermion Liquid to a Fractional Quantum Hall State at {\nu}=3/2 Driven by In-Plane Magnetic Field

Xinghao Wang, L. N. Pfeiffer, A. Gupta et al. (+3 more)

We report an even-denominator fractional quantum Hall state at {\nu} = 3/2 induced entirely by in-plane magnetic field B_|| in an ultra-high-mobility GaAs quantum well. As B_|| increases, the system undergoes two successive transitions: from a composite fermion liquid to a soft-gap FQH state (B_||~12.2 T), then via a topological phase transition to a hard-gap robust FQH state (B_||~14.7 T), accompanied by a daughter state at {\nu} = 19/13. We present systematic data, and discuss a possible scenario in interpreting these findings. Our work demonstrates that topological order may be engineered through k-space Fermi contour splitting under an in-plane magnetic field.
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All authors Xinghao Wang, L. N. Pfeiffer, A. Gupta, K. W. Baldwin, K. W. West, Rui-Rui Du

Strongly Correlated Electrons

Observation of multipartite spin entanglement in a cuprate chain

S. F. R. TenHuisen, Z. Shen, V. Bhartiya et al. (+17 more)

Quantum materials are believed to host highly entangled states of matter, but probing and quantifying such entanglement has long remained experimentally elusive. Here, we use resonant inelastic X-ray scattering (RIXS) as a probe of multipartite spin entanglement. By projecting the RIXS cross section onto the appropriate spin excitation channel, we show that the Quantum Fisher Information can be extracted directly from the measured spin fluctuation spectrum. We detect at least 7-partite spin entanglement in the model one-dimensional cuprate Sr$_2$CuO$_3$, revealing one of the largest entanglement depths yet reported in a solid. The observed entanglement depth agrees with Hubbard-model expectations and persists to elevated temperatures. Our results establish RIXS as an experimental probe of many-body spin entanglement and provide a platform for extensions to charge and orbital sectors and nonequilibrium settings.
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All authors S. F. R. TenHuisen, Z. Shen, V. Bhartiya, V. Menon, P. Sharma, H. Padma, Z. Guan, W. He, M. K. Lajer, J. Li, D. Banerjee, J. Pelliciari, I. A. Zaliznyak, G. D. Gu, M. D. Lukin, S. Johnston, M. P. M. Dean, V. Bisogni, Y. Wang, M. Mitrano

Strongly Correlated Electrons

Textures as a phase-transition probe for quantum spin chains

Heitor P. Casagrande, Isaac M. Carvalho, William J. Munro et al. (+1 more)

The idea of quantum texture has been recently proposed and used as a tool for quantifying coherences and for quantum gate identification. In this work we offer a study on its usage to quantum phase transitions, demonstrating the rugosity metric as a simple tool for effective phase-transition probing. We establish the link between rugosity in the computational basis and the hierarchy of spin correlators, and analyze rugosities defined in the global ground-state and in ground-states belonging to different magnetization sectors (to which we refer to as global vs symmetry-resolved rugosities) to study the phase diagram of the Heisenberg XXZ model. We find distinct rugosity signatures at both transition points. In particular, a sharp feature appears at $\Delta=1$ already for small systems, revealing a pronounced sensitivity of the correlation hierarchy encoded by the texture to this point. Since the BKT transition coincides with the isotropic $SU(2)$ point of the XXZ model, this behavior may reflect a particular sensitivity of rugosity to the structure of the spin-correlation hierarchy at isotropy.
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All authors Heitor P. Casagrande, Isaac M. Carvalho, William J. Munro, Krissia Zawadzki

Superconductivity

Signatures of nodal superconductivity in stoichiometric FeTe

Cequn Li, Zi-Jie Yan, Yang Ge et al. (+13 more)

Superconductivity in stoichiometric FeTe opens access to the FeTe endpoint of the Fe(Se,Te) phase diagram, yet the nature of its superconducting pairing state remains unresolved. In this work, we combine scanning superconducting quantum interference device (SQUID) microscopy, electrical transport, scanning tunneling microscopy and spectroscopy (STM/S), and mean-field calculations to investigate the local superfluid response and pairing state of FeTe thin films with tunable stoichiometry. Even in stoichiometric FeTe, we observe micrometer-scale spatial variations in both superfluid stiffness and superconducting transition temperature $T_c$, while the London penetration depth remains non-saturating down to 0.02$T_c$ and follows a power-law temperature dependence with an exponent of approximately 1-1.5. Together with a V-shaped low-energy density of states and two-gap modeling, these results indicate a superconducting state with gap nodes or deep minima, consistent with either a $d$-wave or nodal $s$-wave superconducting state. Our findings establish stoichiometric FeTe as a distinct superconducting regime that departs from the trend toward more isotropic gaps at intermediate Se/Te compositions, providing a new benchmark for modern microscopic theories of iron-chalcogenide superconductivity. Our work also reveals a crossover from weak to rapid suppression of $T_c$ as superfluid stiffness decreases, connecting FeTe to the broader phenomenology observed in unconventional superconductors.
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All authors Cequn Li, Zi-Jie Yan, Yang Ge, Zihao Wang, Bing Xia, Stephen Paolini, Pu Xiao, Lok-Kan Lai, Jiatao Song, Austin R. Kaczmarek, Lujin Min, Kenji Yasuda, Peter J. Hirschfeld, Jiabin Yu, Cui-Zu Chang, Katja C. Nowack

Materials Science

Atomistic modeling of molecular beam epitaxy growth of SrTiO3 and Sr2TiO4 thin films

Guangfu Luo and Dane Morgan

Molecular beam epitaxy (MBE) is renowned for its potential for atomic layer control, but unexpected growth mechanisms can potentially compromise this level of precision. In this study, we employ first-principles calculations to investigate the atomistic processes governing the MBE growth of perovskite SrTiO3 and Ruddlesden-Popper Sr2TiO4 films on a SrTiO3 substrate. We systematically explore the potential molecular species in the gas phase and their reactions and diffusion dynamics on the film surfaces and layer edges. Our analyses uncover three mechanisms of importance for understanding this type of growth. First, oxygen vacancies can be dynamically induced during surface diffusion on defect-free substrate and noticeably accelerate the diffusion processes. Second, while the SrO layer is expected to grow in a single-layer growth mode, the presence of potential TiSr defects may promote the formation of SrO islands. Lastly, adsorbed Ti atoms and TiO2 molecules on the SrO bilayer can insert into SrO bilayers, resulting in an unexpected growth sequence. These findings may have broader implications for the MBE growth of metal oxide films and provide guidance for achieving improved control over their growth processes.
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Mesoscale and Nanoscale Physics

Tunable topological narrow bands in twisted bilayer-trilayer graphene

Dong Wang, Federico Escudero, Zhen Zhan et al. (+1 more)

We investigate the low-energy band structure and topology of twisted bilayer–trilayer graphene with four stacking configurations: AB–ABC, BA–ABC, AB–ABA, and BA–ABA. Using both tight-binding and continuum models, we first establish that the two approaches show good agreement in the band structure in low-energy regime. We then study the evolution of the flat bands and their valley Chern numbers as functions of twist angle, perpendicular electric field, and the self-consistent Hartree potential. At relatively large twist angles and under electric field, we find a topological transition between the narrow bands, with the total Chern number of the flat bands following the Chern number sum rules derived from the chiral-limit description. We also observe another type of topological transition when the flat bands hybridize with adjacent remote bands, where gap closing and reopening processes lead to Chern number and charge density transfer. By constructing topological phase diagrams in the space of twist angle and electric field, we show that the perpendicular electric field provides an efficient tuning knob for controlling the stability and transitions of the Chern bands. Finally, we find that the Hartree potential mainly induce weak band shifts and reshaping in the narrow bands. However, with a combination of Hartree potential and the electric fields, the narrow bands show rich topological phase diagram. Our results clarify the interplay between the stacking, twist angle and electric field in manipulating the narrow bands and their topology in twisted bilayer–trilayer graphene, and provide guidance for engineering topological narrow bands with tunable Chern numbers in realistic twisted multilayer graphene systems.
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All authors Dong Wang, Federico Escudero, Zhen Zhan, Shengjun Yuan

Materials Science · Cross-listed from cs.LG

Learning Metamaterial Eigenmodes with Wavelet-Encoded Fourier Neural Operators

Han Zhang, Alexander Ogren, Cynthia Rudin et al. (+2 more)

Machine learning surrogates based on neural operators have shown broad applicability in solving forward PDE problems. However, eigenvalue problems, in which an eigenparameter and one of several valid eigenmodes must be simultaneously solved, remain difficult because standard operator learning formulations assume a unique input-output map. This work demonstrates that Fourier Neural Operators (FNOs), combined with wavelet-based encodings of PDE inputs, can learn and predict multiple eigenmodes of the elastic wave equation, corresponding to deformation modes of acoustic waves propagating through arbitrary metamaterial geometries. We provide a mechanistic explanation and experimental evidence for why wavelet encodings are well matched to the dual spatial-spectral structure of the FNO, enabling deterministic mode selection on both continuous-valued and binary-valued geometries within a single model, and for why prediction accuracy varies with geometric discontinuities. For metamaterial design, the resulting surrogate accelerates the simulation stage of the design cycle by three orders of magnitude relative to finite element analysis on a consumer-grade CPU, while preserving high fidelity. These results also carry broader implications for designing input encodings in other multi-mode PDE solvers based on spectral neural operators.
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All authors Han Zhang, Alexander Ogren, Cynthia Rudin, Johann Guilleminot, L. Catherine Brinson

Mesoscale and Nanoscale Physics

Electronic Orbital Angular Momentum Driven by Finite-Momentum Phonons: Beyond Chiral and Axial Phonons

Yongho Park, Jeonghun Sohn, Sejoong Kim et al. (+1 more)

We show that finite-momentum phonons generate electronic orbital angular momentum (OAM) even without phonon axiality or chirality, with the response controlled by the phonon wave vector $q$ and frequency $\omega$. We develop a general gauge-field theory in which a unitary transformation absorbs the phonon displacement into emergent vector and scalar potentials acting on the electrons, providing a perturbative classification of the OAM response in $q$ and $\omega$. We derive $q$- and $\omega$-scaling laws for AC and DC responses. Notably, AC OAM arises even for linearly polarized phonons, with its magnitude and sign tunable by $q$. Its generation is governed by the matching between the phonon geometry and the electronic orbital texture rather than solely by the phonon angular momentum. The connection of the DC response to the electronic Berry curvature further supports that phonon angular momentum is not the only degree of freedom governing electronic OAM generation. Time-dependent tight-binding simulations under acoustic-phonon driving independently confirm the core predictions. Our results extend phonon-driven OAM beyond chiral and axial phonons and establish a wave-vector-tunable route to orbitronics, accessible with surface acoustic waves.
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All authors Yongho Park, Jeonghun Sohn, Sejoong Kim, Kyoung-Whan Kim

Strongly Correlated Electrons

Selective coupling of high-order phonons in La2-xSrxCuO4

Ke-Jun Xu, Paulina Majchrzak, Nathan Giles-Donovan et al. (+14 more)

In crystals and molecules with strong coupling between the charge carriers and atomic vibrations, high order harmonics of the vibrational excitations can be observed with intensity following the Franck-Condon envelope. Here, we uncover a new regime of electron-phonon coupling in La2-xSrxCuO4 using resonant inelastic X-ray scattering. In the undoped compound, we find sharp peaks at approximately 85$\pm$5 meV, 180$\pm$7 meV, and 330$\pm$5 meV with no observable dispersion, and an absence of appreciable intensity from other modes observed in Raman spectroscopy in this energy range. The 180 meV and 330 meV excitations have energies approximately consistent with two-phonon and four-phonon in-plane Cu-O bond stretching mode, suggesting that the resonantly excited valence electrons strongly couple to this mode. These excitations exhibit significant doping dependence, eventually becoming unresolvable at 10% hole doping, where the low energy charge excitations are dominated by dispersive plasmons. The observation of selective coupling to even-ordered phonons and stark contrast with the Raman spectra indicate anomalous electron-phonon coupling beyond the Franck-Condon picture. One intriguing possibility is the existence of locally paired quasiparticles in insulating cuprates.
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All authors Ke-Jun Xu, Paulina Majchrzak, Nathan Giles-Donovan, Sangheon Kim, Jaewon Choi, Jun Okamoto, Ganesha Channagowdra, Hsiao-Yu Huang, Di-Jing Huang, Seiki Komiya, Takao Sasagawa, Shuichi Wakimoto, Wei-Sheng Lee, Zhi-Xun Shen, Thomas P. Devereaux, Dung-Hai Lee, Robert J. Birgeneau

Mesoscale and Nanoscale Physics

Edge energy fluctuation: A unified theory of friction scaling laws in twisted layered material interfaces

Ze Liu

Interlayer twisting in van der Waals (vdW) layered materials generates moire superlattices, unlocking unique opportunities for engineering their electronic and mechanical functionalities. Here, building on two key insights - the invariance of the potential energy of complete moire tiles during sliding, and the symmetry-breaking effect induced by the slider edge - we propose the concept of edge energy fluctuation (EEF) and demonstrate that the EEF governs the sliding energy barrier of finite-sized sliders. Interestingly, we found that even at a fixed twist angle and sliding direction, significant frictional anisotropy emerges when the slider edge is cut along different directions, resulting in distinct friction scaling laws. Furthermore, we present a friction engineering paradigm for twisted layered material interfaces through the deliberate introduction of hierarchical edges, offering a promising route for friction control in superlubric devices.
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Monday 7 September

Soft Condensed Matter

A thermomechanical framework for strongly nonlocal continua

Matthew R. Kuhn

The paper presents a consistent thermomechanical framework for strongly nonlocal continua, a type of generalized media in which a material's response at a point depends on deformation and temperature gradients within its neighborhood. Such dependence is responsible for localization phenomena having an intrinsic size, and nonlocal modeling also serves as a practical regularization tool to prevent mesh dependence. Nonlocality is of the integral type, using a kernel function that gives the relative influence of neighboring points on a central point. The paper develops three sources of nonlocality: (1) nonlocal momentum balance equations; (2) nonlocal conservation laws (including the first and second laws of thermodynamics); and (3) a material's nonlocal constitutive structure. Balance equations are derived using the principle of virtual power, permitting non-smooth force and displacement fields and non-smooth boundary surfaces with indistinct normal directions. The balance equations differ from those of classical local continua. Conservation laws also differ from those of local continua, with the mechanical power and heating at a point being averaged over its neighborhood. These laws are developed for processes that are sufficiently slow to minimize additional meso-scale kinetic energy due to internal turbulence. Stress, entropy, and dissipative forces are obtained as averaged derivatives of the free energy function with respect to strain, temperature, and internal variables. An example is presented of a stretched elastoplastic bar with a small defect, and nonlocality is shown to impart a characteristic size to the deformation pattern.
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Statistical Mechanics

Diagonal Pair Coulomb Density Matrix on a Sphere: a numerical study

Riccardo Fantoni

We tabulate, on the diagonal, the $n$th power of the primitive approximation for the pair Coulomb density matrix on a sphere, with up to $n=5$, and we discuss its behavior at particles contact as $n$ grows. Our results show that even on a curved surface quantum statistical physics is fundamentally different from its classical counterpart. If in the latter one needs to artificially introduce a regularization of the divergent attractive Coulomb potential preventing particles contact, for example with a hard core, in the former this is not necessary.
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Strongly Correlated Electrons

Hamiltonian fingerprints of a collective mode across the BCS-BEC crossover

Yogeshwar Prasad

Collective excitations can survive profound changes in the microscopic degrees of freedom that generate them. We track a layer-odd collective response from an exact weak-coupling hybridisation anchor into the paired regime of an attractive Hubbard bilayer and find that its Hamiltonian sensitivities evolve in a manner consistent with a crossover from single-particle interlayer hybridisation towards collective pair superexchange. At weak coupling, an exact identity for identical tunnel-coupled layers fixes a relative-phase Gaussian kernel zero at $2t_h$, independent of the intralayer hopping. Sign-free determinant quantum Monte Carlo reveals an interaction-driven softening of the many-body spectral scale without analytic continuation, while Gaussian response theory supplies the continuous pole interpretation and shows a shift from layer-density to relative-pair-phase character. In the paired regime, the characteristic response scale acquires a growing fractional sensitivity to the intralayer hopping and an interaction dependence partway towards the $1/U$ law. These results show how a collective excitation can be tracked across a fermion-to-composite-boson crossover while constraining changes in its microscopic drive.
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Statistical Mechanics

Leveraged Learning: entropy cleared per bit received

Daniel Chernowitz

An expository essay containing some new results. A learner holds a prior belief over Boolean maps that answer a finite set of $Q$ questions, and receives answers one by one. Each answer carries surprisal and can clear predictive uncertainty about both the question asked and questions still unasked. We quantify this effect by the leverage: table entropy cleared per bit of surprisal received. Taking expectations over the truth prior and the question order, define the aggregate leverage as the ratio of expected uncertainty cleared to expected surprisal received. It equals one for independent answers and can exceed one for correlated answers. At finite size, this ratio is determined exactly by a single sequence: the mean entropy $G_\ell$ of the answers to $\ell$ questions. As the number of input bits grows at fixed asked fraction $t=\ell/Q$, a limiting increment profile $\gamma(t)$ determines the macroscopic learning curve. With $\eta_0$ the limiting initial entropy per question, the leverage becomes $L(t)= \frac{\eta_0-(1-t)\gamma(t)} {\int_0^t\gamma(x),dx}$. For exchangeable priors, de Finetti's representation gives a constant bulk profile $\gamma(t)$: deduction is confined to a boundary layer at $t=0$, and the leverage is forced to a hyperbolic form, as surprisal grows linearly. By contrast, we construct a simplicity prior with nontrivial bulk learning by grading Boolean maps by their polynomial degree over $\mathbb{F}_2$ and allocating weight across degree classes through a CDF $F$. Reed-Muller capacity then yields $\gamma(t)=1-F(t)$. This realizes any nonincreasing profile taking values in $[0,1]$, together with the corresponding macroscopic leverage curve.
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Materials Science · Cross-listed from physics.app-ph

Capacitance of Undoped Thin-Film Diodes

Oskar J. Sandberg, Mathias Nyman, Stefan Zeiske et al. (+3 more)

The capacitance of thin-film diodes based on undoped semiconductors are dominated by injected charge carriers rather than doping-induced carriers. However, an analytical framework of the capacitance in these devices has remained elusive. Here, we derive an analytical description of the capacitance, fully accounting for injected charge carrier and electrode charge effects. Based on these findings, a method to extract the built-in voltage in these devices is presented. The theoretical framework is substantiated numerically by drift-diffusion simulations and experimentally on organic solar cells.
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All authors Oskar J. Sandberg, Mathias Nyman, Stefan Zeiske, Staffan Dahlstrom, Johannes Benduhn, Ardalan Armin

Mesoscale and Nanoscale Physics

Nonlinear Topological Orbital Responses of Antiferromagnetic Skyrmions

Amir N. Zarezad, Arne Brataas and Alireza Qaiumzadeh

Antiferromagnetic skyrmions evade the skyrmion Hall effect, but compensation suppresses their conventional topological charge Hall signal. We predict a semiclassical nonlinear topological orbital response of an isolated skyrmion in a $\mathcal{PT}$-symmetric hexagonal antiferromagnet without spin-orbit coupling. In the diffusive, weak-emergent-field regime, the spin-dependent emergent Lorentz force reshapes the carrier distribution, generating a local orbital Hall-current correction and a local orbital accumulation, both quadratic in the applied electric field. The current correction requires spin-asymmetric longitudinal scattering, whereas the accumulation survives spin-symmetric scattering. When the spin-diffusion length greatly exceeds the strip width, the current correction persists while the accumulation approaches zero. Both signals are even under electric-field reversal, odd under reversal of the skyrmion topological charge $Q$, and helicity independent within this model. These symmetries enable rectified detection and distinguish this mechanism from the $Q$-even quantum-regime topological orbital Hall effect.
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Statistical Mechanics · Cross-listed from physics.bio-ph

Transcription rate dynamics and RNA copy number noise: General relations and data-driven predictions

Amara McCune, Ido Golding, Shlomi Reuveni et al. (+1 more)

Stochastic models of gene expression typically begin with a microscopic model of transcription and propagate its statistics to the RNA distribution. Here we develop a doubly stochastic framework in which RNA production is Poisson conditional on a time-varying transcription rate $\lambda(t)$. For RNA lifetimes drawn independently from an arbitrary finite-mean distribution, we derive the RNA Fano factor in terms of the lifetime survival function and the autocovariance of $\lambda(t)$. For a Poisson degradation process with rate $\mu$, the survival kernel provides an exponential temporal filter, and the result depends only on the mean, variance, and normalized autocorrelation of $\lambda(t)$. These quantities may be calculated from an explicit rate model or, under ergodicity and adequate sampling, estimated from a sufficiently long rate trajectory. We validate the data-driven estimator using simulated transcription-rate trajectories, without supplying the known autocorrelation to the estimator. We also obtain exact analytical results for transcription-rate dynamics modeled by a discrete M/M/1 process and by drift-diffusion with reflecting, periodic, or first-passage-reset boundaries, and verify each result by direct simulation of the coupled transcription-rate and RNA copy-number processes.
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All authors Amara McCune, Ido Golding, Shlomi Reuveni, Sarah Kostinski

Soft Condensed Matter

Substrate-induced crystallisation in polymers: Exceptional impact of prefreezing versus heterogeneous nucleation on kinetics and morphology

Marthinus van Niekerk, Muhammad Tariq and Oleksandr Dolynchuk

Crystallisation of liquids often occurs at solid surfaces, via heterogeneous nucleation or prefreezing. Understanding this process is crucial for controlling crystal morphology in thin films and developing efficient nucleating agents - especially for polymers, typically processed in a molten state. However, the efficiency of nucleation and prefreezing with respect to crystallisation kinetics and semicrystalline polymer morphology has not been well studied and remains unclear. Here, we address this challenge by conducting a systematic study of substrate-induced crystallisation in dewetted droplets of the model polymer poly($\varepsilon$-caprolactone) (PCL) on various crystalline and amorphous substrates. Using polarized light optical microscopy, droplet crystallisation temperature was monitored and varied across the substrates, revealing three scenarios: close-to-homogeneous nucleation on glass and silicon; heterogeneous nucleation on ordered and disordered carbon-rich substrates; and prefreezing on graphene. The temperature-dependent nucleation rate was analysed and found to obey classical nucleation theory for all systems except PCL on graphene, where it diverges as all the droplets crystallise simultaneously. The highest in- and out-of-plane PCL crystal orientation is found on graphene, where the epitaxially grown lamellae extend vertically over several hundred nanometres. These results demonstrate that prefreezing has a superior influence on crystallisation kinetics and crystal morphology, making prefreezing substrates perfect nucleating agents.
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Quantum Gases

Limit Cycles in a Photonic Dimer with Tuneable Non-Hermitian Interactions

Kevin J. H. Peters, Peter Schnorrenberg, Daniel Ehrmanntraut et al. (+2 more)

Interactions govern the emergence of collective behaviour in classical and quantum many-body systems. While conservative interactions are well known to generate nonlinear phenomena ranging from self-trapping to pattern formation, it remains largely unexplored whether purely dissipative – i.e., non-Hermitian – interactions can give rise to similarly rich dynamics and nontrivial system states. Here, we experimentally realise tuneable non-Hermitian interactions in two coupled condensates of light confined within a dye-filled double-well microcavity. Local coupling to molecular reservoirs generates the effective dissipative photon interactions. We show that the interplay between coherent tunnelling and interactions stabilises limit-cycle oscillations, a hallmark of nonlinear dynamics traditionally associated with Hermitian nonlinearities. By tuning the reservoir coupling, we map out the dynamical phase diagram comprising stable fixed points and limit cycles, thereby demonstrating direct control over the interaction strength. Our experimentally validated model reveals both supercritical and subcritical Hopf bifurcations, giving rise to hysteresis, bistability and excitability. These results validate dissipative interactions as a mechanism for organising collective nonlinear dynamics in driven-dissipative systems and pave the way towards exploring nonequilibrium many-body physics through controlled dissipation.
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All authors Kevin J. H. Peters, Peter Schnorrenberg, Daniel Ehrmanntraut, Nikolas Longen, Julian Schmitt

Mesoscale and Nanoscale Physics

Steady reservoir-mediated self-trapping of polariton condensates

Pavel N. Kozhevin, Oleg I. Utesov, Igor S. Aranson et al. (+2 more)

Nonequilibrium bosonic condensates of exciton-polaritons generated by tightly focused incoherent optical beams are typically expected to form a stationary ballistically expanding state with the notable exception of experimentally observed compact self-trapped states [Phys. Rev. Lett. 123, 047401 (2019)]. We show that the back-action of the condensate on the optically pumped excitonic reservoir via bosonic stimulated scattering can cause local reservoir depletion, producing a steady condensate confined in a self-induced potential trap. Notably, this self-trapped state is formed by persistent converging rather than radially expanding polariton currents and exists even below the conventionally defined condensation threshold. This state remains steady over nanosecond-scale times and is eventually ejected from the self-induced trap due to a bullet-type instability, enabling spiking neuromorphic dynamics in networks of polariton condensates.
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All authors Pavel N. Kozhevin, Oleg I. Utesov, Igor S. Aranson, Sergey V. Koniakhin, Anton V. Nalitov

Strongly Correlated Electrons

Vertical quantum confinement in bulk MoS$_2$

Jairo Obando-Guevara, Álvaro González-García, Marcin Rosmus et al. (+5 more)

We experimentally observe quantum confinement states in bulk MoS$_2$ using Angle-Resolved Photoemission Spectroscopy (ARPES). The band structure at the $\overline{\Gamma}$ point reveals quantum well states (QWSs) linked to vertical quantum confinement of the electrons, confirmed by the absence of dispersion in k$_z$ and a strong intensity modulation with photon energy. Notably, the binding energy dependence of the QWSs vs $n$ does not follow the quadratic dependence of a two-dimensional electron gas. Instead, a linear behaviour is observed that is consistent with a parabolic-like quantum well. This confinement arises from the mechanical exfoliation preparation method, which leads to the detachment of a multilayer stack from the underlying bulk. This is confirmed by Density Functional Theory (DFT) calculations. The quantum confinement in bulk-like MoS$_2$ not only offers the opportunity to explore intersubband transitions to exploit optical properties but also provides a means to study fundamental quantum phenomena in multilayer stacks of different thicknesses.
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All authors Jairo Obando-Guevara, Álvaro González-García, Marcin Rosmus, Natalia Olszowska, César González, Miguel Ángel González-Barrio, Antonio Tejeda, Arantzazu Mascaraque