Quantum Gases
Machine Learning Phase Field Reconstruction in a Bose-Einstein Condensate
Jackson Lee and Andrew J Millis
A basic challenge in experimental physics is the extraction of information related to variables that are not directly measured. The challenge is particularly severe in
quantum systems where one may be interested in correlations of operators that are not diagonal in the measurement basis. In this paper we take a step towards addressing this issue in the context of
Boson superfluids, where standard in-situ imaging yields only the spatially resolved density, leaving the phase field - crucial for identifying topological defects such as
vortices and confirming superfluidity - indirectly encoded. Previous work has shown that the location of
vortices in the phase field may be detected, but has not solved the problems of fully reconstructing the phase or identifying the charge (
vortex vs. antivortex). This paper shows that a combination of a deep machine learning (ML) model and classical computer
vision post-processing steps can address this gap. We use realistic snapshots of the thermal state of a two-dimensional BEC in a harmonic trap using synthetic data obtained from projected Gross-Pitaevskii equation simulations to train a U-Net-based architecture to infer the absolute values of the phase field gradients from an observed density field, and then employ a separate ML model to locate the positions of the
vortex cores and a post-processing graphical analysis to determine with high accuracy the phase field, including the quantized charge of each
vortex.
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Superconductivity
High-temperature superconductivity in Nd$_{0.85}$Sr$_{0.15}$NiO$_2$ membranes under pressure
Yonghun Lee, Mengnan Wang, Xin Wei et al. (+4 more)
Lattice compression has emerged as a fundamental tuning parameter for nickelate superconductivity. Pressure acts as a trigger to induce superconductivity in bulk Ruddlesden-Popper nickelates. For infinite-layer nickelate thin films, compressive epitaxial strain and rare-earth ion chemical pressure have been used to substantially enhance the superconducting transition temperature ($T_c$). Efforts to go further have been constrained by the limits of epitaxial stability or the challenges of measuring thin films in high-pressure environments. Here, we overcome this limitation by developing a technique to incorporate freestanding infinite-layer $\mathrm{Nd_{0.85}Sr_{0.15}NiO_2}$ membranes into a diamond anvil
cell. Using this platform, we observe a strong increase in $T_c$ up to our highest measurement pressure of $\sim$90 GPa, where a superconducting downturn can be observed near
liquid nitrogen temperatures. Strikingly, we find a simple linear enhancement of $T_c$ at a rate of 0.65 K GPa$^{-1}$, with no signs of saturation. This suggests that the pairing strength in infinite-layer nickelates can be raised to a surprisingly high scale, using an approach that can be broadly applied to many two-dimensional materials.
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All authors Yonghun Lee, Mengnan Wang, Xin Wei, Yijun Yu, Wendy L. Mao, Yu Lin, Harold Y. Hwang
Soft Condensed Matter
Emergent Active Clusters through Wave-Mediated Interactions
Yuanmei Li and Rahil Valani
Collective behaviour in active matter usually emerges from interactions between individually stable units. Here we demonstrate a different route to collective organization using droplets on a vibrated fluid bath. Large droplets that are unstable in isolation become persistent active clusters through a shared Faraday wavefield. The same wave-mediated interactions that stabilize these clusters also govern their collective dynamics, producing stationary, rotating, and translating states as droplet size is varied. Our results show how field-mediated interactions can transform individually unstable particles into coherent active entities with emergent structure and function.
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Superconductivity
Superconducting orbital diode effect in SN bilayers
Yuriy A. Dmitrievtsev and Yakov V. Fominov
We study the superconducting diode effect (SDE) in a diffusive
superconductor - normal metal (SN)
bilayer subjected to an in-plane
magnetic field. The supercurrent flows along the layers, perpendicular to the field. The SDE, manifested as an asymmetry in the critical (depairing) currents and kinetic inductance for opposite current directions, arises from an orbital mechanism due to the inhomogeneous distribution of the Meissner currents caused by a spatially varying superfluid density. Recently, Levichev et al. [Phys. Rev. B 108, 094517 (2023)] demonstrated the realization of this effect in such a structure, supporting numerical calculations for an ideal interface with an experiment. In this work, we investigate the influence of a nonideal interface with finite resistance on the SDE. Employing an analytical approach, we focus on limiting cases corresponding to weak intralayer inhomogeneities. We find that the strength of the SDE depends nonmonotonically on the interface resistance when the
bilayer thickness is small compared to the coherence length. Remarkably, a nonideal interface can enhance the SDE compared to the ideal case. We also demonstrate that, in the vicinity of the phase transition, a regime of unidirectional superconductivity can be realized, in which superconductivity exists only within an interval of finite currents flowing in the same direction.
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Strongly Correlated Electrons · Cross-listed from hep-th
Classification of 2D Fermionic Systems with a $\mathbb Z_2$ Flavor Symmetry
Chi-Ming Chang, Jin Chen and Fengjun Xu
We classify superfusion categories describing two-dimensional fermionic systems equipped with the universal
fermion-parity
symmetry, implemented by a topological defect line (TDL) $Z$, and an additional $\mathbb{Z}_2$
flavor symmetry generated by a $W$ TDL. Depending on whether $W$ is m-type or q-type, its fusion rules lead to three distinct classes, and solving the super-pentagon equations yields 16 consistent superfusion categories. These are labeled by invariants $(\nu_W,\nu_Z,\nu_{WZ})$, which determine the $\mathbb{Z}_8$ anomaly classes of the
symmetries generated by $W$, $Z$, and $WZ$. We also provide explicit realizations using multiple Majorana
fermions and comment on implications for fermionic CFTs and gapped phases.
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Soft Condensed Matter
A three-dimensional morphoelastic model for self-oscillations in polyelectrolyte hydrogel filaments
Ariel Surya Boiardi, Roberto Marchello, Pietro Maria Santucci et al. (+2 more)
We introduce a three-dimensional model for polyelectrolyte hydrogel filaments operating in a fluid environment under an electric field. The formulation builds on a morphoelastic framework for inextensible and unshearable rods, such that the filament's activity is encoded in electric-field-induced spontaneous
curvatures, while hydrodynamic interactions are captured via a local approximation of Stokes flows. We employ this framework to investigate the prototypical case of a filament with
elliptic cross-section clamped at its base. Under a constant and uniform electric field aligned with its axis, the filament undergoes flutter instability beyond a critical field strength, as revealed by a linear stability analysis. Depending on the model parameters, the instability is characterized by either two- or three-dimensional self-sustained oscillations. We further examine this behaviour through numerical simulations in the post-critical regime, showing that flutter may develop into large amplitude planar oscillations or more complex three-dimensional motions, through a secondary bifurcation. Although the study represents a first step towards extending state-of-the-art models for polyelectrolyte hydrogel filaments to three dimensions, the richness of the resulting dynamics achievable under time-independent forcing underscores the potential of the proposed actuation mechanism for the design of biomimetic cilia and soft robotic systems.
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All authors Ariel Surya Boiardi, Roberto Marchello, Pietro Maria Santucci, Davide Riccobelli, Giovanni Noselli
Strongly Correlated Electrons · Cross-listed from quant-ph
Quantum Randomized Subspace Iteration
Stefano Scali, Brian Coyle, Giuseppe Buonaiuto et al. (+1 more)
Resolving degenerate
quantum eigenspaces - including topologically ordered ground states and frustrated magnets - requires preparing high-fidelity states that span every direction of the target
manifold. Existing variational and projective
algorithms do not naturally cover a multi-dimensional degenerate subspace without sequential orthogonality constraints. We introduce the
quantum randomized subspace iteration (QRSI), a fully parallel construction that conjugates the
Hamiltonian by independent random
unitaries across as many branches as the degeneracy g, then invokes any chosen eigenstate-preparation primitive on each branch. The target subspace is identified from the resulting ensemble via standard subspace estimation, either classically through the coefficient
matrix or on hardware through Gram-
matrix measurements. We prove that the construction spans the full eigenspace almost surely and preserves the spectral gap exactly on every branch. For practical use, we show that these guarantees hold whenever the random rotations satisfy an anti-concentration condition over the degenerate
manifold, substantially weaker than full Haar randomness. We demonstrate QRSI on the toric code, recovering all four topological ground states, and on random
Hamiltonians with planted degeneracies.
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All authors Stefano Scali, Brian Coyle, Giuseppe Buonaiuto, Michal Krompiec
Mesoscale and Nanoscale Physics
Probing Electrostatic Disorder via g-Tensor Geometry
Edmondo Valvo, Christian Ventura-Meinersen, Michele Jakob et al. (+3 more)
Low-frequency charge noise induced by fluctuating electrostatic disorder is a major limitation for
semiconductor hole
spin qubits. Here, we analyze the quasistatic response of a hole
spin qubit to individual two-level fluctuators (TLFs). We show that, due to the anisotropy of the g-tensor, the
qubit response depends on the geometry of the fluctuator-induced dipolar perturbation. We then propose a readout protocol that isolates selected g-tensor components through an accumulated Berry phase and estimate, within our readout model, an order-unity signal-to-noise ratio with a total protocol time in the tens of microseconds. Finally, using microscopic simulations, we compute the
quantum Fisher information (QFI) to identify
magnetic field directions and confinement regimes in which the
qubit is most sensitive to disorder-induced variations of selected g-tensor components.
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All authors Edmondo Valvo, Christian Ventura-Meinersen, Michele Jakob, Stefano Bosco, Tereza Vakhtel, Maximilian Rimbach-Russ
Mesoscale and Nanoscale Physics
Detecting crossed Andreev reflection in a quantum Hall interferometer with a superconducting beam splitter
Maxime Jamotte, Tom Menei, Manohar Kumar et al. (+2 more)
We study time-domain electron interferometry in a Hong-Ou-Mandel (HOM) geometry, where a thin
superconductor between two
quantum Hall systems acts as the beam splitter. By comparing the measurable current cross correlations at the interferometer outputs with those of a normal-conducting electronic HOM setup, we show that Andreev processes strongly affect the HOM dip. Using a combination of scattering theory and numerical tight-binding simulations for a
graphene quantum Hall bar, we show that the change of charge cross correlations can be used to experimentally detect and characterize local and crossed Andreev processes.
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All authors Maxime Jamotte, Tom Menei, Manohar Kumar, Alexander Zyuzin, Thomas L. Schmidt
Superconductivity
Pressure-Induced Superconducting-like Transition in the $\it d$-wave Altermagnet Candidate CsV$_2$Se$_2$O
Yuanzhe Li, Yilin Han, Liu Yang et al. (+29 more)
Altermagnetism generates exchange-type
spin splitting without net magnetization and, in its $\it d$-wave form, resembles the angular
symmetry of unconventional $\it d$-wave superconductivity. Whether this correspondence bears directly on superconducting instabilities in real correlated materials remains open. Here we study the quasi-two-dimensional vanadium oxychalcogenide CsV$_2$Se$_2$O (CVSO), a square-net $\it d$-wave altermagnet candidate, through combined experimental and theoretical investigation of its
lattice structure, electronic structure and transport properties. At ambient pressure, CVSO is a weakly insulating parent state with a density-wave-like anomaly near 100 K, and its bulk properties are most consistent with a G-type compensated
antiferromagnetic background. Under compression, the density-wave-like feature is suppressed, the magnetoresistance evolves from predominantly negative to positive, and a superconducting-like resistive downturn emerges below about 3 K. This low-temperature anomaly is reproducible across samples and pressure media, and is suppressed by
magnetic field. Room-temperature X-ray diffraction reveals no
symmetry lowering, whereas does show a pronounced compressibility anomaly over the same pressure range. CVSO thus reveals a pressure-tuned phase diagram in which a reconstructed weakly insulating parent state gives way to strange-metal-like transport and superconducting-like behavior, echoing broader phenomenology associated with unconventional
superconductors, including cuprates and nickelates.
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All authors Yuanzhe Li, Yilin Han, Liu Yang, Wanli He, Pengda Ye, Wencheng Huang, Jiabin Qiao, Yuemei Li, Xiaodong Sun, Tingli He, Jiayi Han, Yuxiang Chen, Ruifeng Tian, Hao Sun, Yuwei Liu, Feng Wu, Baoshan Song, Zhengtai Liu, Mao Ye, Yaobo Huang, Kenichi Ozawa, Ji Dai, Massimo Tallarida, Shengtao Cui, Jie Chen, Meiling Jin, Wayne Zheng, Chaoyu Chen, Zhiwei Wang, Zhi-Ming Yu, Xiang Li, Yugui Yao
Statistical Mechanics
Multiscale perturbative approach to active matter with motility regulation
Alberto Dinelli and Pietro Luigi Muzzeddu
We present a coarse-graining method applicable to dry scalar active matter with motility regulation. Our approach, based on a multiscale perturbative expansion of the backward Kolmogorov equation, does not rely on any specific microscopic dynamics for the particles' orientations. Its generality allows us to address different forms of motility regulation, from space-dependent self-propulsion speed to taxis, and to extend the analysis to a class of non-Markovian orientational dynamics. Furthermore, we identify general conditions on the microscopic dynamics that ensure the existence of an effective large-scale
equilibrium regime. When the latter are violated, our theoretical framework is able to quantitatively capture the emergence of large-scale particle currents. We directly apply our coarse-grained theory to several models of self-propelled
agents, ranging from single particles to active
polymers, and test our analytical predictions with numerical simulations. Finally, we show that our theory naturally extends to active matter with density-mediated interactions, such as quorum sensing, with potential applications to self-organizing soft materials.
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Statistical Mechanics
Unifying hydrodynamic theory for motility-regulated active matter: from single particles to interacting polymers
Alberto Dinelli and Pietro Luigi Muzzeddu
Understanding how microscopic motility shapes emergent collective behaviors is a challenging task in active matter, especially when self-propulsion is regulated by external cues or via quorum-sensing interactions. To address this problem, we derive a closed hydrodynamics for scalar active matter with spatially-regulated motility, under general hypotheses for the microscopic dynamics of the particles' orientations. We show that, at large scales, the contribution of the latter is entirely captured by the autocorrelation tensor of the orientations. This allows us to establish a macroscopic equivalence within a broad class of motility-regulated active systems, from single particles to active
polymers. Our formalism allows us to reveal a new form of motility-induced phase separation for quorum-sensing active
polymers, which we term anti-MIPS, where dense phases exhibit enhanced activity relative to dilute regions. Our theory shows that anti-MIPS generically arises for motility-regulated
agents with internal structure, uncovering the existence of several distinct transition pathways.
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Superconductivity
Aurélien Balzli, Louk Rademaker and Giulia Venditti
We analyze
magnetic field profiles of
vortices in thin-film
superconductors, shedding new light on this old and presumed settled problem. In sufficiently thin films with realistic Ginzburg-Landau parameter $\kappa = 1/\sqrt{2}$, the
magnetic screening around a
vortex core is neither exponential – as is expected in bulk – nor the power-law that was predicted by Pearl. Instead, a universal curve for the
magnetic field variation appears that scales with the sample thickness. The thickness dependence is consistent with the seminal Pearl length, and serves as an indication of the reduced
magnetic field screening present in two-dimensional
superconductor. Finally, we quantify the crossover from bulk-like to thin
superconductors, and establish different screening length-scales relevant for the analysis of experimental data.
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Mesoscale and Nanoscale Physics
Unidirectional information flow in a nanomagnetic metamaterial
Johannes H. Jensen, Ida Breivik, Arthur Penty et al. (+10 more)
Artificial
spin ice (ASI) are metamaterials composed of interacting nanomagnets. Although ASI hold promise for low-power computing, the ability to transmit information through these two-dimensional systems has been limited. Inspired by non-reciprocal transport in nature, we develop a framework for non-reciprocal influence between nanomagnets. Using the framework we discover a family of ASI geometries with inherent directionality. Directional ASI have the property that, when driven by an external field protocol, domains grow and reverse in the same direction, illustrating an emergent non-reciprocity of the system. Combining growth and reversal results in unidirectional domain movement through the metamaterial. We focus on one member of the directional ASI family, and demonstrate unidirectional domain growth experimentally. Furthermore, we show that the direction of growth is reconfigurable by tuning the external field strengths. Finally, we demonstrate how the directionality of the system significantly improves
memory capabilities in a reservoir computing framework. Our work is the first demonstration of an ASI with inherent directionality, offering a
magnetic computing platform that combines
memory and computation within a single neuromorphic substrate.
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All authors Johannes H. Jensen, Ida Breivik, Arthur Penty, Anders Strømberg, Henrik Tidemann Kaarbø, Dheerendra S. Bhandari, Thea M. Dale, Michael Foerster, Miguel Angel Niño, Deepak Dagur, Magnus Själander, Gunnar Tufte, Erik Folven
Disordered Systems and Neural Networks · Cross-listed from stat.ML
Sharp description of local minima in the loss landscape of high-dimensional two-layer ReLU neural networks
Jie Huang, Bruno Loureiro and Stefano Sarao Mannelli
We study the population loss landscape of two-layer ReLU networks of the form $\sum_{k=1}^K \mathrm{ReLU}(w_k^\top x)$ in a realisable teacher-student setting with Gaussian
covariates. We show that local minima admit an exact low-dimensional representation in terms of summary statistics, yielding a sharp and interpretable characterisation of the landscape. We further establish a direct link with one-pass SGD: local minima correspond to attractive fixed points of the dynamics in summary statistics space. This perspective reveals a hierarchical organisation of minima into discrete families and shows how overparameterisation changes their stability and reachability under gradient-based dynamics. In this overparameterised regime, global minima become increasingly accessible, attracting the dynamics and reducing convergence to spurious solutions. Overall, our results reveal intrinsic limitations of common simplifying assumptions, which may miss essential features of the loss landscape even in minimal neural network models.
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Materials Science
Oxygen-Mediated Phase Evolution in Sputtered Cu-W-O: Insights into Surface Chemistry Variability
José Montero-Amenedo
Thin films of Cu-W-O ternary compounds were fabricated via DC magnetron co-sputtering from Cu and W metallic targets under controlled oxygen partial pressures, followed by thermal annealing. Low-oxygen conditions favored the formation of a single CuWO4 phase, whereas higher oxygen levels produced a mixture of CuWO4 and Cu3WO6. Structural and optical properties were investigated by X-ray diffraction (XRD) and spectrophotometry, revealing phase coexistence and changes in preferential orientation depending on the deposition conditions. A detailed and carefully validated X-ray photoelectron spectroscopy (XPS) analysis provides insight into the surface chemical environment of Cu and W, indicating the presence of compositional inhomogeneities and surface-bulk differences associated with Cu migration and segregation. While the W 4f core levels remain remarkably stable across all tested oxygen partial pressures, a systematic shift is observed in the Cu 2p3/2 binding energy. Wagner plot analysis confirms that this displacement is dominated by initial-state effects, reflecting modifications of the Cu ground-state electronic structure and Cu-O-W hybridization rather than changes in final-state screening. Our findings demonstrate that sputtered Cu-W-O films, even when nominally identified as CuWO4, can exhibit substantially different structural and electronic states depending on synthesis conditions, highlighting the need for rigorous characterization to ensure reproducibility in ternary
oxide research.
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Statistical Mechanics
Complex paths for real stochastic processes
D. A. Baldwin, A.J. McKane and S.P. Fitzgerald
The calculation of the decay rate of a metastable state in the path-integral formulation of
stochastic processes is revisited. Previous derivations of this rate were achieved at the cost of a step that is difficult to justify mathematically. We show that this difficulty can be resolved by working with a complex extremal solution that arises naturally in the Itô formulation of the path integral. To make the analysis as transparent as possible, we choose a simple potential for which the extremal solution can be written in terms of elementary functions. The mechanism identified here, however, is not restricted to this example and holds more generally.
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Statistical Mechanics
Steady-state phonon heat currents and differential thermal conductance across a junction of two harmonic phonon reservoirs
Eduardo C. Cuansing and Juan Rafael K. Bautista
We study
phonon transport in junctions of two harmonic reservoirs coupled together by a spring. The exact steady-state heat currents and thermal conductance are calculated using nonequilibrium Green's functions. We find that the heat currents follow
Fourier's law and the thermal conductance has a peak whenever the
phonon spectra match. At lower temperatures, however, the thermal conductance maximum may not coincide with the spectra-matching peak due to the exclusion of higher-frequency
phonons, whose spectra may match, from participating in the transport. Furthermore, we find that increasing the coupling spring constant increases the thermal conductance. Lastly, the magnitude of the steady-state heat currents and thermal conductance are the same whether the direction of
phonon flow is from left to right or vice versa, even with mass and spring constant asymmetry. The properties of this basic model can serve as a reference for more complicated setups of
phonon transport in molecular junctions.
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Soft Condensed Matter
Physical Properties of Dextran Solutions as Model Crowding Media
Giuliano Migliorini, Josipa Cecic Vidos, Josef Hamacek et al. (+2 more)
The role of macromolecular crowding in living systems is widely appreciated, but artificial crowders used to model these effects in vitro are often inadequately characterized. In this work, we examine density, viscosity,
polymer self-
diffusion and water
diffusion in crowded dextran systems. Dextran viscosity and self-
diffusion follow size-dependent trends, collectively described by universal functions of the overlap concentration corresponding to a Flory exponent of 0.44, characteristic of branched
polymers. Viscosity increases with concentration as a power law, with a crossover from dilute to semi-dilute behaviors. Dextran self-
diffusion decays exponentially: this can be interpreted in light of Rosenfeld's excess
entropy scaling hypothesis. Water self-diffusivity and specific volume decrease with concentration, but show no dependence on
polymer size. We show how these results can be used to construct the true volume fraction of crowders, which takes into account bound water. Overall, our findings showcase the power of
polymer physics concepts in macromolecular crowding studies in vitro.
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All authors Giuliano Migliorini, Josipa Cecic Vidos, Josef Hamacek, Anand Yethiraj, Francesco Piazza
Materials Science
Challenges and mitigation pathways in coating silver nanowire networks with metallic oxides by RF magnetron sputtering
Amaury Baret, Ambreen Khan, Sude Akin et al. (+4 more)
As silver nanowire (AgNW) networks reach increasing technological maturity, research efforts are progressively shifting toward their integration into functional devices. In this context, it is essential to assess how thin film coating processes affect the structural and functional integrity of these transparent conducting networks.
Radio Frequency (RF) magnetron sputtering is among the most widely used and industrially scalable deposition techniques, making a detailed understanding of its impact on AgNW networks particularly critical. In this work, we experimentally investigate the degradation of AgNW networks observed under specific RF magnetron sputtering regimes. By varying deposition time, oxygen partial pressure, target material, buffer layers and
plasma power, we analyze how sputtering conditions influence the electrical, morphological, and structural properties of the networks. Based on these observations, we identify viable strategies to mitigate or suppress network degradation, thereby enabling safer and more reliable coating protocols. These results provide practical guidelines for the integration of AgNW networks into multilayer device architectures.
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All authors Amaury Baret, Ambreen Khan, Sude Akin, Lionel Teulé-Gay, Daniel Bellet, Aline Rougier, Ngoc Duy Nguyen
Mesoscale and Nanoscale Physics
Experimental Verification of a Universal Operator Growth Hypothesis
M. Engelsberg and Wilson Barros Jr
F$^{19}$
nuclear magnetic resonance free induction decay (FID) data are used to verify the predictions of a universal growth hypothesis for the Lanczos coefficients proposed by Parker et al. Our results strongly support this hypothesis and permit to calculate values of the growth parameter $\alpha$ for three
crystal orientations. For the
magnetic field parallel the [100]
crystal axis, we found $\alpha =3.161 \times 10^{4} sec^{-1}$. The special experimental conditions required for the observability of a singularity in the analytic continuation of the FID, which from the experimental data was found to be of branch-point type, are discussed.
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Materials Science
On the origin of superlattice stacking faults nucleation via climb of Frank partial in CoNi-based superalloys
Zhida Liang, Yinan Cui, Li Wang et al. (+4 more)
High-temperature deformation in superalloys is governed by the cooperative glide-climb motion of dislocations. Superlattice stacking faults (SFs) in the gamma
prime phase are predominantly interpreted as nucleating via conservative Shockley partial glide. Here, we demonstrate that non-conservative climb of a/3<111> Frank partials constitutes a general and kinetically viable pathway for both superlattice intrinsic (SISFs) and extrinsic stacking faults (SESFs) formation in the L12 structure of CoNi-based superalloys during compression at 850 Celsius. High-resolution transmission electron microscopy reveals that Frank partials form at gamma/gamma
prime interface can climb into the gamma
prime phase, generating SISFs via positive climb and SESFs via negative climb. Importantly, the negative climb-assisted nucleation of SESFs is experimentally confirmed for the first time, and the observed positive climb-assisted SISF configuration differs fundamentally from previously reported mechanisms. We show that these Frank partials originate from the reaction between a leading 30 degree Shockley partial and a 60 degree mixed dislocation on conjugate {111} planes, producing energetically stable configurations that promote subsequent climb. Energetic and kinetic analyses demonstrate that solute segregation induced reduction of SF energy provides a dominant contribution to Frank partial climb, enabling sustained climb and consequent SF expansion. Quantitative comparisons further indicate that, at elevated temperatures, solute drag-controlled Shockley glide can achieve mobilities comparable to vacancy
diffusion-controlled Frank climb. These findings establish climb-assisted SF formation as a unified deformation mechanism in gamma
prime phase, and that both SISF and SESF expansion can proceed through Frank partial climb.
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All authors Zhida Liang, Yinan Cui, Li Wang, Xin Liu, Bin Liu, Yong Liu, Fengxian Liu
Mesoscale and Nanoscale Physics · Cross-listed from quant-ph
Coherent Control of Nanoscale Nuclear Spin Ensembles in the Spin Noise Regime
Ana Martin, Roberto Rizzato, Carlos Munuera-Javaloy et al. (+3 more)
Spin defects in solids, such as the nitrogen-vacancy (NV) center in diamond, have emerged as a key tool for detecting
nuclear spins at the nanoscale. While active
nuclear spin control via
radio-frequency (RF) irradiation is often unnecessary for standard
spin-noise detection, it becomes essential for advanced protocols like multidimensional nanoscale NMR. In this work, we investigate
nuclear spin control using correlation spectroscopy techniques. We demonstrate, both theoretically and experimentally, that the resulting
nuclear spin dynamics depend critically on the initial RF phase and its orientation relative to the NV crystalline axis. Depending on these parameters, identical
nuclear rotations can yield full, partial, or even vanishing contrast in the NV readout. These findings highlight a previously underappreciated aspect of
spin manipulation in the
spin-noise regime: the link between the phase and direction of the applied RF field and its direct impact on correlation-based experiments. Consequently, imperfect calibration of these parameters can lead to ambiguous signal contrasts and misinterpretation of the underlying
nuclear spin dynamics. Our results provide deeper insight into nanoscale
spin control and pave the way toward reliable multidimensional
spin resonance experiments.
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All authors Ana Martin, Roberto Rizzato, Carlos Munuera-Javaloy, Dileep Singh, Dominik B. Bucher, Jorge Casanova
Materials Science
Synergistic Interplay between Surface Polarons and Adsorbates for Photocatalytic Nitrogen Reduction on TiO$_2$(110)
Manoj Dey, Ritesh Kumar and Abhishek Kumar Singh
Photocatalytic nitrogen reduction under ambient conditions represents a promising pathway toward sustainable ammonia production. However, the fundamental mechanisms, particularly the role of photogenerated charge carriers and their interactions with surface defects and adsorbates, remain elusive. Here, we employ density functional theory with Hubbard U corrections and hybrid functionals to demonstrate that the synergistic interactions between photogenerated electron polarons and point defects are essential for enabling nitrogen reduction on TiO$_2$(110). We reveal that water adsorption promotes polaron migration from subsurface to surface sites, while subsequent water dissociation stabilizes polarons near oxygen vacancies through
proton coupled electron polaron transfer (PCEpT). This surface localization of polarons is critical for effective N$_2$ adsorption and activation. Our findings are consistent with previous experimental reports utilizing EPR that confirm the presence of reduced Ti species and STM, which shows the presence of water dimers on the surface. Moreover, the simultaneous interaction between polarons and reaction intermediates facilitates polaron transfer, thereby driving the completion of the nitrogen reduction reaction. Our findings elucidate the pivotal role of surface polarons in photocatalytic nitrogen fixation and provide mechanistic insights applicable to a broad range of
oxide surfaces and interfaces capable of hosting small polarons, offering new design principles for efficient photocatalysts operating under ambient conditions.
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Soft Condensed Matter
DWS-based microrheology of triblock copolymers
René Tammen, Xiaoying Tang, Ren Liu et al. (+2 more)
The thermally reversible phase transitions in aqueous solutions of the triblock copolymers known as Pluronic and their related textures are well-researched. However, their corresponding rheological properties are less studied. In particular, their high-temperature behavior is difficult to access with classical rheology. Here we demonstrated that Diffusing Wave Spectroscopy (DWS)-based microrheology allows us to study the phase transition and the associated viscoelastic properties of Pluronic F127 solutions for temperatures from 5 C to 80 C. From the measured intensity-autocorrelation functions we can extract effective viscosities and determine the critical micellization temperature and concentration. Moreover,the high EO/PO (arm-to-core) ratio of F127 and its polydispersity play a critical role in the high-temperature re-entrant
liquid phase, due to decreasing solubility of PEO along with the dehydration of the PPO core. The microscopic viscoelastic
moduli G'({\omega}) and G”({\omega}) help to determine these phase transitions and provide mechanical properties in the solid phase that are not readily accessible with standard multi-particle tracking techniques due to limited Brownian motion.
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All authors René Tammen, Xiaoying Tang, Ren Liu, Iliya D. Stoev, Erika Eiser
Materials Science
Nonlinear electron-phonon coupling drives light-induced symmetry switching in charge-density waves
Christoph Emeis and Fabio Caruso
Ultrafast optical excitation in charge-density wave (CDW)
crystals can transiently suppress long-range order, driving the
lattice toward higher
symmetry on femtosecond timescales. Here, we formulate and implement a first-principles theory of light-induced melting of CDW order. The approach is based on the structural dynamics in the Heisenberg picture, and it explicitly accounts for quartic
lattice anharmonicities, nonlinear electron-
phonon interactions, and photoexcitation-induced modifications of the potential energy surface. We illustrate these concepts through first-principles calculations of the ultrafast melting of CDW order in monolayer TiSe$_2$ - a prototypical CDW
crystal with a 2$\times$2 structural reconstruction. The simulations are in good agreement with existing experiments, and they capture the defining features of CDW melting, such as the damped coherent structural motion, the transient renormalization of the soft mode, and the restoration of CDW order over timescales of a few picoseconds. Besides identifying nonlinear electron-
phonon interactions as the primary mechanism driving
symmetry switching in CDW systems, our work establishes a generally applicable theoretical framework to treat quartic anharmonicities and light-induced phase transitions in first-principles ultrafast dynamics simulations.
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Materials Science
Competing thermalization pathways of photoexcited hot electrons
Christopher Seibel, Tobias Held, Markus Uehlein et al. (+1 more)
Photoexcited hot carriers in solids can drive processes, such as photocatalytic reactions on the surface, beyond those available in thermal
equilibrium. Hot-electron-mediated reaction pathways are limited by the thermalization of the nonequilibrium electron distribution through microscopic scattering events. Commonly, thermalization is exclusively attributed to electron-electron scattering, whereas electron-
phonon scattering is considered relevant mainly for the energy equilibration with the
lattice. With a kinetic model based on full Boltzmann collision integrals, we demonstrate that each scattering mechanism alone can thermalize the electron distribution, albeit along different trajectories in phase space. We find an opposite dependence on the excitation strength of the respective thermalization times and show that both processes can become comparable for weak excitations, corresponding to a sample temperature increase of a few Kelvin. Our results unravel the contributions of electron-electron and electron-
phonon scattering to the thermalization across the full range of experimental excitation strengths up to the melting regime, thus facilitating the prediction of thermalization times for hot-carrier-based applications.
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All authors Christopher Seibel, Tobias Held, Markus Uehlein, Baerbel Rethfeld
Materials Science
Balancing Thermodynamics, Kinetics, and Reversibility in Ti-Doped MgB2H8: A First-Principles Assessment of a Practical Solid-State Hydrogen Storage Material
Sikander Azam and Wilayat Khan
Hydrogen storage remains a key challenge for the development of a sustainable hydrogen energy system, where materials must satisfy requirements on storage capacity, thermodynamics, kinetics, and reversibility. Complex borohydrides are attractive due to their high hydrogen density, but their practical use is limited by slow hydrogen
diffusion and unfavorable desorption thermodynamics. In this work, we present a first-principles study of pristine and Ti-doped MgB2H8 as a solid-state hydrogen storage material. Density functional theory calculations show that pristine MgB2H8 has a high gravimetric hydrogen capacity of about 14.9 wt percent, but also a relatively high hydrogen desorption enthalpy of about 42 kJ per mol H2 and
diffusion barriers around 0.5 eV, which limit its performance at moderate temperatures. Substitutional doping with Ti at the Mg site improves these properties while maintaining structural stability. The doped system retains a high hydrogen capacity of about 10.4 wt percent and shows a reduced desorption enthalpy of about 36 kJ per mol H2, placing it within a favorable thermodynamic range for hydrogen release. Nudged elastic band calculations show a reduction in hydrogen migration barriers to about 0.38 eV, indicating improved
diffusion kinetics.
Phonon and elastic analyses confirm that Ti doping preserves stability. Electronic structure analysis shows that Ti 3d states near the
Fermi level weaken B-H bonding and stabilize intermediate hydrogen configurations, explaining the improved behavior. These results identify Ti-doped MgB2H8 as a promising hydrogen storage material.
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Quantum Gases
Many-body dynamical localization in Fock space
Nathan Dupont, Bruno Peaudecerf, David Guéry-Odelin et al. (+4 more)
We investigate the emergence of many-body dynamical localization (MBDL) in the Fock space of an interacting two-mode bosonic system subject to periodic driving. Using a mapping to the paradigmatic kicked-top model, we analyze the interplay between classical chaotic
diffusion and
quantum interference effects. While the mean-field (classical) dynamics exhibits bounded ergodic
diffusion along the population imbalance axis, the
quantum dynamics reveals strong suppression of transport in Fock space, in close analogy with Anderson localization in disordered
lattices. We characterize the localization length, its scaling with particle number and driving parameters, and reveal the spectral crossover from random-
matrix to Poisson statistics as the many-body ensemble localizes. We highlight the connection between MBDL and discrete time
crystals. Our findings offer a promising avenue to study the Anderson transition in Fock space.
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All authors Nathan Dupont, Bruno Peaudecerf, David Guéry-Odelin, Gabriel Lemarié, Bertrand Georgeot, Christian Miniatura, Nathan Goldman
Materials Science
Favorable half-Heusler structure of synthesized TiCoSb alloy: a theoretical and experimental study
Pallabi Sardar, Suman Mahaka, Soumyadipta Pal et al. (+5 more)
The most favorable structure of the synthesized TiCoSb half-Heusler
alloy is explored theoretically and experimentally, and the best structure for thermoelectric conversion is reported. Rietveld refinement of the X-ray diffraction data employing four probable structures of the HH
alloy is performed to obtain the best fit and identify the crystallized structure. However, microstructural characterization is performed using the energy dispersive X-ray spectroscopy and transmission electron microscopy to reveal the stoichiometry and Bragg reflection planes of the synthesized polycrystalline
lattice structure of TiCoSb HH
alloy. Theoretical investigation is performed by implementing the first principle calculation using the Full Potential Linearized Augmented Plane Wave method in the
Quantum Espresso software package. The most probable structure is explored by estimating the minimum energy at
equilibrium volume and electronic structure of the TiCoSb half-Heusler
alloy of the four probable structures considered. The theoretical and experimental data are corroborated, and the most probable structure is identified for the crystallized TiCoSb HH
alloy. The thermoelectric properties of the most probable structure are estimated.
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All authors Pallabi Sardar, Suman Mahaka, Soumyadipta Pal, Shamima Hussain, Vinayak B. Kamble, Pintu Singha, Diptasikha Das, Kartick Malik