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Physics Condensed Matter

cond-mat Watch Condensed Matter

Wednesday 21 January

Superconductivity

Divergent Pressure Response of Superconductivity in Sc$_{6}$MTe$_{2}$ ($M$ = Fe, Ru and Ir)

J.N. Graham, S.S. Islam, K. Yuchi et al. (+7 more)

Identifying and understanding non-BCS superconductivity remains a central challenge in condensed-matter physics. Here we focus on the Sc$_{6}$MTe$_{2}$ family (M =d-electron metal), which provides a unique platform of isostructural compounds exhibiting superconductivity across 3d, 4d, and 5d systems. Using hydrostatic pressure as an additional tuning parameter, muon-spin rotation (${\mu}$SR) and AC susceptibility measurements uncover strongly contrasting pressure responses of superconductivity across the Sc$_{6}$MTe$_{2}$ series. The superconducting transition temperature, $T_{\rm C}$ decreases under pressure in the 3d Fe-based compound but increases for the 4d Ru- and 5d Ir-based systems, with the Ru compound showing the largest enhancement of nearly 50% within 2 GPa. The superfluid density exhibits similarly distinct pressure dependences, remaining nearly pressure independent for Fe while decreasing with increasing pressure for Ru and Ir. This suggests fundamentally different correlations between $T_{\rm C}$ and the superfluid density. Together, these results indicate that superconductivity emerging from strongly correlated and spin-orbit-dominated regimes in Sc$_{6}$MTe$_{2}$ is likely governed by different microscopic mechanisms and offer a useful experimental basis for future microscopic theoretical studies.
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All authors J.N. Graham, S.S. Islam, K. Yuchi, P. Král, O. Gerguri, S. Huber, J. Chang, R. Khasanov, Y. Okamoto, Z. Guguchia

Strongly Correlated Electrons

Theory of Next-Generation Even-Denominator States

Misha Yutushui and David F. Mross

Even-denominator quantum Hall states are leading candidates for realizing non-Abelian topological orders, with the $\nu=\frac{5}{2}$ plateau in GaAs the first and most-studied example. Recent experiments in GaAs and bilayer graphene (BLG) have observed many `next-generation' even-denominator states at filling factors such as $\nu=\frac{3}{4}$, $\frac{3}{8}$, and $\frac{3}{10}$. We develop the theory of these states, including analyses of their bulk quasiparticles, of methods for distinguishing between pairing channels in edge transport measurements, and of their trial wavefunctions. As part of this study, we derive general relations of how flux attachment affects many universal properties of states. In particular, we prove that the topological stability of interface modes is invariant under flux attachment. We compare next-generation paired states to Bonderson-Slingerland states at the same filling factors, and demonstrate that their quasiparticles carry identical charges and obey the same exchange statistics. The next-generation and Bonderson-Slingerland states still describe distinct phases, and we find that the former are energetically favored in the lowest Landau level, while the latter are favored in the first excited level.
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Strongly Correlated Electrons

Exactly Solvable Topological Phase Transition in a Quantum Dimer Model

Laura Shou, Jeet Shah, Matthew Lerner-Brecher et al. (+3 more)

We consider a family of generalized Rokhsar-Kivelson (RK) Hamiltonians, which are reverse-engineered to have an arbitrary edge-weighted superposition of dimer coverings as their exact ground state at the RK point. We focus on a quantum dimer model on the triangular lattice, with doubly periodic edge weights. For simplicity we consider a $2\times1$ periodic model in which all weights are set to one except for a tunable horizontal edge weight labeled $\alpha$. We analytically show that the model exhibits a continuous quantum phase transition at $\alpha=3$, changing from a topological $\mathbb{Z}_2$ quantum spin liquid ($\alpha<3$) to a columnar ordered state ($\alpha>3$). The dimer-dimer correlator decays exponentially on both sides of the transition with the correlation length $\xi\propto1/|\alpha-3|$ and as a power-law at criticality. The vison correlator exhibits an exponential decay in the spin liquid phase, but becomes a constant in the ordered phase, which we explain in terms of loop statistics of the double-dimer model. Using finite-size scaling of the vison correlator, we extract critical exponents consistent with the 2D Ising universality class. Additionally, we analytically show that the topological Rényi entropy of order $\infty$ (topological min-entropy) changes from $\log2$ for the quantum spin liquid phase $\alpha<3$, to $0$ for the ordered phase $\alpha>3$, thereby analytically confirming the topological nature of the phase transition.
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All authors Laura Shou, Jeet Shah, Matthew Lerner-Brecher, Amol Aggarwal, Alexei Borodin, Victor Galitski

Mesoscale and Nanoscale Physics

In-Substrate Imaging of Diamond hBN FET Current via Widefield Quantum Diamond Microscopy

Anuj Bathla, Subrat Kumar Pradhan, Ajit Kumar Dash et al. (+7 more)

We demonstrate widefield magnetic imaging of current flow in hydrogen terminated diamond field effect transistors (FETs) through in-substrate nitrogen vacancy (NV) centers. Hydrogen termination of the diamond surface induces a two dimensional hole gas (2DHG), while an ensemble of near surface NV centers located $ \sim 1~\mu m$ below the surface enables noninvasive magnetic imaging of current flow with micrometer scale spatial resolution. The FETs were electrically characterized over a range of drain source biases $V_{ds}= 0$ to $-15V$ and gate voltages,$V_{gs}= +3$ to $-9V$ followed by in situ widefield NV magnetometry during device operation. Magnetic field maps and reconstructed current density distributions directly visualize current injection at the source drain contacts and transport beneath the hBN gated channel. Magnetic field maps reveal current density variations in the channel region owing to non-uniformities or defects in the gate dielectric. In addition, we observe a pronounced enhancement of the drain current ($\sim 600-900 \mu A$) and a shift in the apparent threshold voltage during laser illumination, reflecting photo induced changes in channel electrostatics. By correlating gate dependent magnetic images with simultaneous electrical measurements, we directly link spatial current distributions to FET transfer characteristics, providing new insight into buried interface transport and non-uniform gating effects in the transistor channel. As the methodology is compatible with top gated FETs, it can be used to map channel current distributions with micrometer resolution in emerging channel materials, such as 2D materials and wide bandgap channels, and establish widefield NV magnetometry as a powerful platform for probing charge transport in transistors and Van der Waals dielectric heterostructures.
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All authors Anuj Bathla, Subrat Kumar Pradhan, Ajit Kumar Dash, Prabhat Anand, M. Girish Chandra, Kenji Watanabe, Takashi Taniguchi, Akshay Singh, Veeresh Deshpande, Kasturi Saha

Mesoscale and Nanoscale Physics

Direct Observation of Antimagnons with Inverted Dispersion

Hanchen Wang, Junfeng Hu, Wenjie Song et al. (+14 more)

We report direct spectroscopic evidence of antimagnons, i.e., negative-energy spin waves identified by their signature inverted dispersion and left-handed precession with Brillouin light scattering (BLS) spectroscopy. We investigate an ultrathin BiYIG film with a perpendicular magnetized anisotropy that compensates the demagnetizing field. By injecting a spin-orbit torque, the magnetization is driven into auto-oscillation and eventually into a non-equilibrium reversed state above a secondary current threshold ($\sim$1.2$\times$10$^7$~A/cm$^2$). The dispersion is measured by wavevector-resolved BLS and exhibits a sharp change from an upward dispersion to a downward one, in agreement with theoretical predictions and micromagnetic simulations. Around the threshold current, we observe the coexistence of conventional magnons and antimagnons. Our work establishes antimagnons with inverted dispersion and is a first step towards exploring novel phenomena and applications due to magnon-antimagnon coupling, such as magnon amplification and magnon-antimagnon entanglement, which are part of the emerging field of antimagnonics.
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All authors Hanchen Wang, Junfeng Hu, Wenjie Song, Artim L. Bassant, Jinlong Wang, Haishen Peng, Emir Karadža, Paul Noël, William Legrand, Richard Schlitz, Jilei Chen, Song Liu, Dapeng Yu, Jean-Philippe Ansermet, Rembert A. Duine, Pietro Gambardella, Haiming Yu

Statistical Mechanics · Cross-listed from math.PR

Entropy of Soft Random Geometric Graphs in General Geometries

Oliver Baker and Carl P. Dettmann

We study the effect of the choice of embedding geometry on the entropy of random geometric graph ensembles with soft connection functions. First we show that when the connection range is small, the entropy is dependent only on the dimension of the geometry and not the shape, but for large connection ranges the boundaries of the domain matter. Next, we formulate the problem of estimating entropy as a problem of estimating the average degree of a graph with the binary entropy function as its connection function. We use this formulation to study the effect of boundaries on the entropy, and to estimate the entropy of soft random geometric graphs in complicated geometries where a closed form pair distance density is not available.
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Other Condensed Matter

Ultrafast switching of antiferromagnetic order by field-derivative torque

Pratyay Mukherjee and Ritwik Mondal

Control of magnetic order in antiferromagnets is a central challenge in the development of next-generation spintronic devices. Here, we propose and analyze magnetization switching driven by the field-derivative torque, a torque that originates from the time-derivative of an applied THz pulse acting on the staggered order parameter. Using atomistic spin simulations, we show that the field-derivative torque couples efficiently to the Néel vector, enabling deterministic switching without net spin accumulation. Further, we show that using the circularly polarised THz pulse, the FDT-induced magnetization switching reduces the required THz magnetic field by two-fold. To this end, we compute the switching and non-switching areas as a function of THz pulse width, THz magnetic field, and damping of the antiferromagnetic material. We find that the switching and non-switching areas are completely deterministic in antiferromagnets. Moreover, the switching area increases by about 55% when the FDT is considered.
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Strongly Correlated Electrons

Assessing Orbital Optimization in Variational and Diffusion Monte Carlo

Cody A. Melton and Jaron T. Krogel

In this work, we investigate the fidelity of orbital optimization in variational Monte Carlo to improve diffusion Monte Carlo results on correlated magnetic systems, using CrSBr as a model system. We compare the performance of different optimization methods, showing that stochastic reconfiguration is a robust and reliable optimizer. We show that short range Jastrow factors are important for improving diffusion Monte Carlo, regardless of the quality of orbitals. Large active spaces are required to converge the variational energy, but ulitmately orbital optimization produces worse diffusion Monte Carlo energies when compared to standard orbitals from density functional theory. We show that this increased bias is due to larger locality errors from the use of pseudopotentials, while the fixed-node error is actually improved by using orbital optimization. Additionally, for observables other than energy, orbital optimization produces a systematically smaller mixed-estimator bias. Ultimately, we believe orbital optimization provides a reliable method to improve variational and pure fixed-node energies as well as lower mixed-estimator bias.
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Quantum Gases

Trimer Dynamics in Floquet-driven arrays of Rydberg Atoms

Edoardo Tiburzi, Lorenzo Maffi, Luca Dell'Anna et al. (+1 more)

We analyze the WAHUHA Floquet protocol recently applied to arrays of Rydberg atoms and derive beyond-leading-order corrections in the high-frequency expansion of the effective spin theory. We find that an appropriate choice of the pulses times can enforce an approximate symmetry corresponding to the conservation of the total magnetization. The interaction channels emerging from higher-order Floquet terms affect three-body bound states (trimers), which gain a significant mobility. We estimate the corresponding enhancement in 1D spin chains and conclude that their dynamics is within experimental reach. Detrimental effects due to the proliferation of particles outside of the trimer magnetization sector are found to occur and spread on time-scales slower than the trimer propagation. We further show that long-range interactions enhance trimer propagation and that two-dimensional triangular geometries can host energetically isolated trimer bands, providing a possible route to reduce resonant mixing with higher-magnetization sectors. Our results establish a concrete route to realizing mobile multiparticle bound states in Floquet-engineered Rydberg platforms.
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All authors Edoardo Tiburzi, Lorenzo Maffi, Luca Dell'Anna, Marco Di Liberto

Quantum Gases

Increasing the stability of a superfluid in a rotating necklace potential

Giulio Nesti and Luca Pezzè

Recent experiments have probed the stability of ring superfluids in the presence of Josephson barriers or Gaussian impurities. Here we present a theoretical analysis that extends beyond the regimes explored so far. We study the onset of dynamical instabilities in a ring superfluid, addressing both tunneling and hydrodynamic regimes. The stability of the system is controlled by the effective rotation frequency $\omega$, given by the difference between the initial quantized circulation and the frequency of barrier rotation. The instability occurs when $\omega$ overcomes a critical value $\omega_c$. We show that $\omega_c$ increases approximately linearly with the number of barriers, with a slope set by the barrier height and width. When the system is quenched into the dynamically unstable regime, it emits multiple solitons, which can switch or even reverse the direction of circulation. The stabilization mechanism is robust against imperfections of the potential and does not require a perfectly periodic array of barriers. In particular, we find that adding a disordered speckle potential to an ordered array of barriers can further increase $\omega_c$: disorder can therefore make a ring superfluid more resilient to dynamical instabilities.
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Materials Science

Competition between clustering and dispersion of cobalt atoms on perovskite surfaces: SrTiO3(001) and KTaO3(001)

Aji Alexander, Pankaj Kumar Samal, Llorenc Albons et al. (+8 more)

Perovskite oxides are attractive for reactions in photo/electrocatalytic schemes, and extrinsic doping is a common strategy for tuning their properties. It is widely known that extrinsic dopants impact the structure and stability of perovskite surfaces, but an atomic-scale view is missing. Here, noncontact atomic force microscopy (ncAFM) and photoelectron spectroscopy (XPS/PES) are used to combine microscopic and spectroscopic evidence of cobalt adsorption, incorporation, and clustering at surfaces of two prototypical perovskites SrTiO3 and KTaO3. A number of different sub-ML coverages and temperatures of annealing were investigated. Several common features are observed: cobalt shows a strong preference for ionic nature (+2 and +3 charge states), and remains dispersed as single atoms to a certain extent in both perovskites. Two competing mechanisms are observed upon annealing: coalescence into clusters with a mixed metallic/ionic character, and incorporation into the surface and subsurface regions. The latter is more pronounced in SrTiO3, where a cobalt-stabilized surface reconstruction is identified, whereas for KTaO3 cobalt likely incorporates in the near-surface region.
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All authors Aji Alexander, Pankaj Kumar Samal, Llorenc Albons, Jesus Redondo, Jan Skvara, Igor Pis, Lukas Fusek, Josef Myslivecek, Viktor Johanek, Dominik Wrana, Martin Setvin

Materials Science

Biphasic Meniscus Coating for Scalable and Material Efficient Quantum Dot Films

Shlok Joseph Paul, Letian Li, Zheng Li et al. (+4 more)

Colloidal quantum dots (cQDs) have emerged as a cornerstone of next-generation optoelectronics, offering unparalleled spectral tunability and solution-processability. However, the transition from laboratory-scale devices to sustainable industrial manufacturing is fundamentally hindered by spin-coating workflows, which are intrinsically wasteful and restricted to planar geometries. These limitations are particularly acute for high-performance cQDs containing regulated elements such as lead, cadmium, or mercury, where poor material utilization exacerbates both environmental burden and cost. Here we report a biphasic dip-coating strategy that redefines the material efficiency of nanocrystal film fabrication. By utilizing an immiscible underlayer to displace ~88% of the active reservoir volume, we demonstrate a deposition geometry that decouples material consumption from total precursor volume. Infrared PbS photodetectors fabricated via this approach maintain their performance against spin-coated benchmarks while reducing ink consumption by up to 20-fold. Our technoeconomic analysis reveals that this biphasic architecture achieves cost parity at film thicknesses an order of magnitude lower than conventional monophasic dip-coating. Our results establish a low-waste framework for solution-processed materials, providing a viable pathway for the resource-efficient manufacturing of optoelectronic devices.
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All authors Shlok Joseph Paul, Letian Li, Zheng Li, Andrew Kim, Mia Klopfestein, Stephanie S. Lee, Ayaskanta Sahu

Quantum Gases

Cooperative stabilization of persistent currents in superfluid ring networks

Marzena Ciszak, Nicola Grani, Diego Hernandez-Rajkov et al. (+3 more)

Cooperative effects in oscillator networks are often associated with enhanced stability of phase-locked solutions, which increases with system size. We show that the stabilization of persistent currents in annular atomic superfluids with periodic barriers is a concrete manifestation of this phenomenon. Under the simplifying assumption of continuity of atomic flow across identical barriers, the system reduces to a ring of locally coupled Kuramoto-like oscillators. We analytically derive the stability diagram of phase-locked configurations and quantify their robustness to noise and small random initial imperfections, finding excellent agreement with experimental observations. These results are inherent to the ring topology and independent of the specific physical platform.
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All authors Marzena Ciszak, Nicola Grani, Diego Hernandez-Rajkov, Giulia Del Pace, Giacomo Roati, Francesco Marino

Quantum Gases

The exact dynamical structure factor of one-dimensional hard rods and its universal random matrix behavior

Oleksandr Gamayun and Miłosz Panfil

We obtain an exact analytic expression for the dynamical structure factor of one-dimensional quantum gas of hard rods. Our result is valid for an arbitrary many-body state of the system, with finite temperature states and the ground state being important special cases that we analyse in detail. We demonstrate that the expression obeys fundamental relations such like the f-sum rule and the detailed balance. We also reveal the hidden fermionic structure behind the correlator. In the static limit we show that it can be written in terms of universal functions which, at zero temperature, coincide with the level spacing distribution function of the Gaussian Unitary Ensemble. Our work provides a full and exact characterisation of a dynamic correlation function in a strongly correlated interacting quantum many-body system.
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Statistical Mechanics

Stiffness induced structures and morphological transitions in semiflexible polymers

Biman Bagchi

Semiflexible polymers in poor solvents exhibit a rich variety of collapsed morphologies, including globules, toroids, and rodlike bundles, arising from the competition between attractive interactions and chain stiffness. Computer simulations and experiments on stiff and conjugated polymers have revealed complex morphological crossovers, yet a unified theoretical description remains incomplete. Here we develop a coarse-grained, field-theoretic free-energy framework for linear polymers with variable stiffness that captures these morphologies and their transitions within a common description. The theory is built on three key ingredients: a density field describing monomer attraction and excluded-volume effects, a nematic order parameter accounting for orientational ordering in dense regions, and the bending rigidity of a worm-like chain. Using simple variational ansatzes for competing morphologies, we derive analytic expressions for their free energies and identify the boundaries separating coil, globule, toroidal, and rodlike conformational regimes as functions of the reduced attraction strength and the effective persistence length. The resulting phase-diagram topology provides a transparent free-energy-based framework for interpreting morphology diagrams observed in simulations and experiments on semiflexible polymers in poor solvents. We find the possibility of the existence of a triple point involving globules, rods and toroids.
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Statistical Mechanics · Cross-listed from hep-th

Critical and multicritical Lee-Yang fixed points in the local potential approximation

Dario Benedetti, Fanny Eustachon and Omar Zanusso

The multicritical generalizations of the Lee-Yang universality class arise as renormalization-group fixed points of scalar field theories with complex $i\varphi^{2n+1}$ interaction, $n\in\mathbb{N}$, just below their upper critical dimension. It has been recently conjectured that their continuation to two dimensions corresponds to the non-unitary conformal minimal models $\mathcal{M}(2,2n+3)$. Motivated by that, we revisit the functional renormalization group approach to complex $\mathcal{P}\mathcal{T}$-symmetric scalar field theories in the Local Potential Approximation, without or with wavefunction renormalization (LPA and LPA' respectively), aiming to explore the fate of the $i\varphi^{2n+1}$ theories from their upper critical dimension to two dimensions. The $i\varphi^{2n+1}$ fixed points are identified using a perturbative expansion of the functional fixed-point equation near their upper critical dimensions, and they are followed to lower dimensions by numerical integration of the full equation. A peculiar feature of the complex $\mathcal{P}\mathcal{T}$-symmetric potentials is that the fixed points are characterized by real but negative anomalous dimensions $\eta$, and in low dimension $d$, this can lead to a change of sign of the scaling dimensions $\Delta=(d-2+\eta)/2$, thus requiring a novel analysis of the analytical properties of the functional fixed-point equations. We are able to follow the Lee-Yang universality class ($n=1$) down to two dimensions, and numerically determine the scaling dimension of the fundamental field as a function of $d$. On the other hand, within the LPA', multicritical Lee-Yang fixed points with $n>1$ cannot be continued to $d=2$ due to the existence of unexpected non-perturbative fixed points that annihilate with the $i\varphi^{2n+1}$ fixed points.
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Quantum Gases

Stimulated cooling in non-equilibrium Bose-Einstein condensate

Ka Kit Kelvin Ho, Vladislav Yu. Shishkov, Mohammad Amini et al. (+10 more)

We report on the experimental observation of stimulated cooling in the non-equilibrium Bose-Einstein condensate (BEC) of weakly interacting exciton-polaritons from approximately room temperature down to 20K. By resolving the condensate in energy-momentum space and performing interferometric measurements, we distinguish the condensate from thermalized particles yet occupying excited states macroscopically. In contrast to the analytical quantum theories of non-equilibrium BEC [Shishkov et al., Phys. Rev. Lett. 128, 065301 (2022)], we observe segmentation of the particle density along the excited states into two fractions both following Bose-Einstein distribution, albeit with different effective temperatures and chemical potentials. Our results indicate that the temperature of the weakly interacting Bose gas is universally set by the density-dependent chemical potential, revealing a defining property of non-equilibrium BECs. Finally, we demonstrate that the stimulated nature of the cooling process directly governs the emergence of quantum coherence of the condensate and shapes the dissipative properties of the excited states.
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All authors Ka Kit Kelvin Ho, Vladislav Yu. Shishkov, Mohammad Amini, Leonie Teresa Wrathall, Evgeny Mamonov, Darius Urbonas, Ioannis Georgakilas, Tobias Herkenrath, Michael Forster, Ullrich Scherf, Tapio Niemi, Päivi Törmä, Anton V. Zasedatelev

Materials Science

Exceptionally high carrier mobility in hexagonal diamond

Zirui He, Shang-Peng Gao and Meng Chen

Hexagonal diamond (h-diamond), or Lonsdaleite, is a promising wide-bandgap semiconductor known for its high thermal conductivity and hardness. Based on ab initio calculations, we demonstrate its exceptionally high carrier mobilities. At room temperature, the hole mobilities along the $\perp c$ and $\parallel c$ directions are 6000 and 6024 cm$^{2}$V$^{-1}$s$^{-1}$, respectively, while the corresponding electron mobilities reach 12339 and 28473 cm$^{2}$V$^{-1}$s$^{-1}$. These values are significantly superior to those of most known semiconductors, including cubic diamond. The small effective masses in h-diamond are comparable to those in the cubic phase, which cannot explain its substantially higher mobilities. Instead, two underlying mechanisms are uncovered. First, selection rules enforced by the symmetry of h-diamond significantly suppress scattering, particularly for transverse acoustic phonons, which predominate in the cubic phase around room temperature. Secondly, the spatial mismatch between the electronic wavefunctions and phonon-induced scattering potentials leads to real-space electron-phonon decoupling, which manifests as the suppression of out-of-plane polarised longitudinal acoustic scattering for holes, and a systematic weakening of acoustic scattering for electrons.
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Mesoscale and Nanoscale Physics

Weak Electron-Phonon Coupling Is Insufficient to Generate Significant CISS in Two-Terminal Transport

Vipul Upadhyay and Amikam Levy

A central open question in chiral-induced spin selectivity (CISS) is whether weak electron-phonon coupling in a helical molecular junction can generate a sizable spin polarization in two-terminal transport without invoking additional strong symmetry-breaking ingredients. We address this question by implementing a self-consistent nonequilibrium Green's function (NEGF) calculation for a helical tight-binding model with spin-orbit coupling and electron-phonon interactions. The electron-phonon self-energies are evaluated self-consistently, and the transport signal is extracted using the standard magnetization-reversal protocol with a spin-polarized analyzer lead. We benchmark a fully self-consistent NEGF within the self-consistent Born approximation (SCBA) treatment for both global and local electron-phonon couplings against commonly used approximations, including diagonal self-energy schemes. We quantify how the resulting transport regime and spin polarization depend on phonon frequency, coupling strength, bias, temperature, and system size. In contrast to large polarizations and anomalous size trends reported under approximate treatments, the fully self-consistent calculation yields negligible spin polarization, additionally the electron-phonon coupling mainly renormalizes the spectrum, and transport remains quasi-ballistic across the explored parameter range.
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Materials Science

Resolving the band alignment of InAs/InAsSb mid-wave-infrared type-II superlattices

Michał Rygała, Julian Zanon, Andreas Bader et al. (+5 more)

In this work, three InAs/InAs$_{0.65}$Sb$_{0.35}$ superlattices with different periods were investigated using photoluminescence and photoreflectance measurements and their band structure was simulated using a 14 bulk-band kp model. The structures were studied by analyzing the evolution of the spectral features in temperature and excitation power to determine the origin of optical transitions. After identifying which of these are related to the superlattice mini-bands, a rich collection of observed higher-order optical transitions was compared with refractive-index calculations. This procedure was used to adjust the parameters of the theoretical model, namely the bowing parameters of the InAsSb valence band offset and bandgap. It was also shown that the spectroscopy of the higher-order states combined with numerical modeling of the refractive index is a powerful tool for improvement of the material parameters, presenting a new approach to material studies of advanced semiconductor heterostructures.
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All authors Michał Rygała, Julian Zanon, Andreas Bader, Tristan Smołka, Fabian Hartmann, Sven Höfling, Michael Flatté, Marcin Motyka

Soft Condensed Matter

Theoretical relationship between the macro-texture and micro-structure in dairy processing revealed by the multi-scale simulation of coupled map lattice

Erika Nozawa

The theoretical relationship between the macroscopic textural quality and microscopic structural quality appearing in the phase inversion processes from fresh cream via whipped cream to butter is revealed by the multi-scale simulation of coupled map lattice (CML) based on the mesoscopic elementary processes of the emulsion interfaces. Using the Young-Laplace equation, we derive the microscopic particle quantities of the size and density of air bubbles and butter grains in an emulsion from the macroscopic rheological quantities of the overrun and viscosity of the emulsion. In doing so, we focus on the size determined by the "tug-of-war" between air bubbles and butter grains via their cohesion pressures, and on the density determined by the "costume change" of the emulsion molecular complexes (clad particles, e.g., butter grain-clad air bubbles) to their suitable size. Using the obtained microscopic particle quantities, we now propose a microscopic state diagram, the size-density plane, in addition to the previously proposed macroscopic state diagram, the viscosity-overrun plane. These state diagrams reveal that while the two well-known different phase inversion processes at high and low whipping temperatures appear as the two parallel processes of viscosity dominance and overrun dominance in the viscosity-overrun plane, they appear as the two orthogonal processes of isodensity/size dominance and isosize/density dominance in the size-density plane. This theoretical simulation result is significant for the quality design of butter because it demonstrates that differences in macroscopic textural quality can be easily controlled by differences in microscopic structural quality.
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Soft Condensed Matter

Coupled gas and bubble dynamics at the solidification front

Bastien Isabella, Cécile Monteux and Sylvain Deville

The formation and entrapment of gas bubbles during solidification significantly influence the microstructure and mechanical properties of materials, from metallic alloys to ice. While gas segregation at the solidification front is well-documented, the real-time dynamics of bubble nucleation, growth, and engulfment-and their dependence on solidification velocity-remain poorly understood. In this study, we use in situ cryo-confocal fluorescence microscopy to investigate the coupled gas-bubble dynamics at the solidification front of carbonated water, systematically varying the solidification velocity ($V = 1-20 \mu m/s$) while maintaining a constant thermal gradient ($G = 15 K/mm$). Our experiments reveal that bubble nucleation is governed by a characteristic nucleation time, which emerges from the interplay between gas diffusion ahead of the front, nucleation kinetics, and bubble growth, all competing with the advancing solidification front. These results allow us to estimate the critical gas concentration for bubbles nucleation in carbonated water. These results offer a detailed understanding of the mechanisms controlling bubble nucleation and entrapment during solidification at constant thermal gradient. They contribute to the development of strategies to control bubble formation in industrial processes where the presence of bubbles can either be detrimental or intentionally harnessed.
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Materials Science

A General Theory of Chiral Splitting of Magnons in Two-Dimensional Magnets

Yu Xie, Dinghui Wang, Chao Li et al. (+2 more)

Magnons in antiferromagnets exhibit two chiral modes, providing an intrinsic degree of freedom for magnon-based computing architectures and spintronic devices. Electrical control of chiral splitting is crucial for applications, but remains challenging. Here, we propose the concept of extrinsic chiral splitting, involving alternating and ferrimagnet-like types, which can be induced and controlled by an electric field. A symmetry framework based on 464 collinear spin layer groups is established to classify chiral splitting characteristics and electric field responses in two-dimensional magnets. We further elucidate how the spin layer group determines the type of alternating chiral splitting and the dominant lowest-order magnetic exchange interaction. We demonstrate electric-field control over the magnitude and sign of the chiral splitting, enabling control of the spin Seebeck and Nernst effects related to thermal spin transport. This work provides a general theory for electric field manipulation of magnon chirality, paving the way for low-power magnonic logic devices.
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All authors Yu Xie, Dinghui Wang, Chao Li, Xiaofan Shen, Junting Zhang

Statistical Mechanics · Cross-listed from quant-ph

Two-Qubit Spin-Boson Model in the Strong Coupling Regime: Coherence, Non-Markovianity, and Quantum Thermodynamics

Hasan Mehdi Rizvi, Devvrat Tiwari and Subhashish Banerjee

We investigate the dynamics of a two-qubit open quantum system, in particular the two-qubit spin-boson model in the strong coupling regime, coupled to two thermal bosonic baths under non-Markovian and non-equilibrium conditions. Two complementary approaches, the Hierarchical Equations of Motion (HEOM) and Reaction Coordinate Mapping (RCM), are employed to examine various coupling regimes between the qubits and their respective baths. The dynamical features of the model and the impact of the tunneling amplitude on quantum coherence of the system are probed using the $l_1$-norm of coherence. The model is further shown to have non-Markovian evolution. The nontrivial task of calculating entropy production in the strong-coupling regime is performed using auxiliary density operators in HEOM. Motivated by the realization of a quantum thermal device in the strong-coupling regime, the non-equilibrium steady-state behavior of the system is investigated. Furthermore, the relationship between the heat and spin currents and the tunneling amplitude is probed.
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Materials Science

Quasisymmetry Enriched Gapless Criticality at Chern Insulator Transitions

Jiayu Li, Feng-Ren Fan and Wang Yao

In continuous topological phase transitions (CTPTs), the low-energy physics is governed by gap-closing subspaces, where approximate "higher" symmetries, termed quasisymmetries, may emerge. Here, we introduce the notion of quasisymmetry enrichment of these transitions. Focusing on paradigmatic normal-to-Chern insulator transitions, we identify quasisymmetries in the gapless subspaces, which subdivide CTPTs of the same universality class according to quasisymmetry charges. Gapless criticalities with nontrivial charges exhibit regulated phenomena, including intrinsic correlations between charge and pseudospin currents and continuous generalized Hall conductivities governed by the generalized Středa formula, both conventionally exclusive to gapped phases. These features arise as quasisymmetry forbids certain matrix elements, rendering the generalized Berry curvature integrable. By establishing quasisymmetry as a fundamental classifying ingredient, our work adds a new dimension for understanding the rich landscape of quantum phase transitions.
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Strongly Correlated Electrons

Anomalous Quantum Criticality at a Continuous Metal-Insulator Transition

M. S. Laad and Prosenjit Haldar

The Falicov-Kimball model (FKM) is long known to be the simplest model of correlated fermions exhibiting a novel Mott-like quantum critical point (QCP) assocaited with a continuous MIT in dimensions $D \geq 3$. It is also known to be isomorphic to an annealed binary-alloy disorder model. Notwithstanding extensive numerical studies for the FKM, analytic insight into the microscopic processes spawning novel Mott-like quantum criticality is scarce. Here, we develop a fully analytic theory for the Mott-like quantum criticality in the FKM on a hierarchical Cayley tree (Bethe lattice) by utilizing a single input from a 2-site cluster-dynamical mean-field theory (CDMFT). We find that density fluctuation modes acquire anomalous dimensions, originating from infra-red power-law singular cluster self-energies. Interestingly, we uncover, at $T=0$, that this sub-diffusive metal with glassy dynamics separating a weakly ergodic metal from a non-ergodic insulator shrinks to a single point, namely the Mott-like QCP, at least on the Bethe lattice. We detail the consequences of this anomalous quantum criticality for a range of thermal and dynamical responses in a variety of physical systems that can be effectively modelled by the FKM.
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Soft Condensed Matter

Self-organized flows break morphological symmetry in active/passive systems

Rainer Backofen and Axel Voigt

We consider a phase-separating mixture of active and passive fluids and explore morphological asymmetries of the emerging dominantly bicontinous dynamic emulsion. Two-dimensional numerical simulations reveal that the geometric and topological asymmetries can solely be explained by self-organized flows in the active region. As in inertial turbulence an inverse energy cascade in the active region leads to the formation of condensates. The size of these mesocales vortices is determined by the locally available space in the emulsion. As these condensates accumulate energy they impact the fluctuation of the surrounding interface and thus form a tight coupling between the flow field and the dynamic morphology. While explored for active/passive systems the symmetry-breaking mechanism can be generalized to heterogeneous active systems and proposes a way to control the morphology of various functional soft materials.
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Quantum Gases

Ab initio path-integral Monte Carlo results for the one-particle spectral function of the warm dense electron gas

Paul Hamann, Michael Bonitz, Jan Vorberger et al. (+1 more)

We present quasi-exact ab initio path-integral Monte Carlo results for the Matsubara Green's function of the uniform electron gas (UEG) at finite temperature over a broad range of coupling strengths ($r_s=1,\dots,10)$. We further extract the static self-energy $\Sigma_\infty(p)$ and perform an analytic continuation for spectral function $A(p,\omega)$, conclusively ruling out the possibility of distinct satellite features at these conditions. In addition, our work opens up intriguing avenues to study the single-particle spectrum and density of states of real warm dense matter systems based on first principles.
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All authors Paul Hamann, Michael Bonitz, Jan Vorberger, Tobias Dornheim

Strongly Correlated Electrons

Dielectric formalism of the 2D uniform electron gas at finite temperatures

Fotios Kalkavouras, Tobias Dornheim, Paul Hamann et al. (+1 more)

We present a comprehensive analysis of the two-dimensional uniform electron gas (2D-UEG or more commonly 2DEG) at finite temperature, spanning a broad range of densities / coupling strengths ($0.01\le{r}_s\le20$) and temperatures / degeneracy parameters ($0.01\le\Theta= k_B T/E_F \le 10$). Within the self-consistent dielectric formalism, we construct two-dimensional versions of the Singwi-Tosi-Land-Sjölander (STLS) and hypernetted-chain (HNC) approximation based schemes. We benchmark the accuracy of the STLS and the HNC schemes against new state-of-the-art path-integral Monte Carlo data. We also report structural and thermodynamic properties across the full $(r_s,\Theta)$ phase diagram domain studied, identify regimes in which these schemes remain quantitatively reliable, and provide an accurate parametrization of the exchange–correlation free energy of the finite-temperature 2DEG.
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All authors Fotios Kalkavouras, Tobias Dornheim, Paul Hamann, Panagiotis Tolias

Soft Condensed Matter

Energy-efficient time series processing in real-time with fluidic iontronic memristor circuits

T. M. Kamsma, Y. Gu, C. Spitoni et al. (+3 more)

Iontronic neuromorphic computing has emerged as a rapidly expanding paradigm. The arrival of angstrom-confined iontronic devices enables ultra-low power consumption with dynamics and memory timescales that intrinsically align well with signals of natural origin, a challenging combination for conventional (solid-state) neuromorphic materials. However, comparisons to earlier conventional substrates and evaluations of concrete application domains remain a challenge for iontronics. Here we propose a pathway toward iontronic circuits that can address established time series benchmark tasks, enabling performance comparisons and highlighting possible application domains for efficient real-time time series processing. We model a Kirchhoff-governed circuit with iontronic memristors as edges, while the dynamic internal voltages serve as output vector for a linear readout function, during which energy consumption is also logged. All these aspects are integrated into the open-source pyontronics package. Without requiring input encoding or virtual timing mechanisms, our simulations demonstrate prediction performance comparable to various earlier solid-state reservoirs, notably with an exceptionally low energy consumption of over 5 orders of magnitude lower. These results suggest a pathway of iontronic technologies for ultra-low-power real-time neuromorphic computation.
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All authors T. M. Kamsma, Y. Gu, C. Spitoni, M. Dijkstra, Y. Xie, R. van Roij