Materials Science
Upscaling DFT-trained machine-learning interatomic potential toward Quantum Monte Carlo accuracy: Sulfur-vacancy migration in monolayer MoS$_2$ as a testbed
Adam Hložný, Ján Brndiar, Ye Luo et al. (+1 more)
We designed a procedure to train a machine learning interatomic potential (MLIP) at
benchmark-quality
quantum Monte Carlo (QMC) accuracy. To avoid the complexities of high-quality atomic force determination with the
stochastic QMC methods, we use a multi-fidelity approach wherein high-level QMC energies are used alongside suitably processed low-level DFT atomic forces to train a QMC fine-tuned MLIP which significantly improves both the energetics and atomic forces over the baseline DFT-based MLIP. Fine-tuning is only applied to the readout layers of an equivariant message-passing MACE MLIP. We used sulfur mono- and multiple vacancies in monolayer MoS$_2$ as a testbed and demonstrate a near QMC accuracy of the model in a number of in- and out-of-domain tests. We show that a fairly limited
dataset of QMC energies suffice to significantly improve the baseline DFT MLIP. The accuracy of our approach is demonstrated on energy and free energy migration barriers of mono- and multiple S-vacancy defects. The results open the window to large-scale near QMC quality simulations with large numbers of atoms and/or molecular dynamics configurations which would not be possible by a direct brute-force application of QMC methods.
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All authors Adam Hložný, Ján Brndiar, Ye Luo, Ivan Štich
Mesoscale and Nanoscale Physics
Quantum Batteries in two-dimensional material-based Josephson Junctions
V. Varrica, G. Gemme, F.M.D. Pellegrino et al. (+3 more)
We investigate the solid-state implementation of a Dicke-like
quantum battery consisting of a two-dimensional material-based Josephson junction inductively coupled to a resonator, using
graphene as a representative example. In this configuration, Andreev bound states naturally act as non-interacting, energetically non-degenerate two-level systems, and the setup allows for both single-
photon and two-
photon resonant processes. The coupling between the LC-
circuit flux and the supercurrent through the junction gives rise to peculiar longitudinal interaction terms that have no counterpart in the conventional Dicke model. These additional couplings can enhance energy storage for a proper range of parameters. The proposed architecture also enables an alternative, but equivalent, charging protocol that relies on tuning the superconducting phase difference across the junction.
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All authors V. Varrica, G. Gemme, F.M.D. Pellegrino, E. Paladino, M. Sassetti, D. Ferraro
Mesoscale and Nanoscale Physics
Entanglement Dynamics across a Monitored Quantum Point Contact
Anna Delmonte and Marco Schirò
We compute the
entanglement dynamics across a monitored
quantum point contact, where particle losses are recorded on a given site, and demonstrate how this single-site local monitoring substantially reshapes the
entanglement production. Contrary to the
unitary case, where
entanglement entropy grows logarithmically in time, here we find first a linear growth, up to a maximum value displaying volume-law scaling, and then a slow decay to zero, as the system empties out. We capture this crossover using a quasiparticle picture, where the first linear growth arises due to an emergent bias voltage established by the losses, which eventually decays away as the system depletes. We connect our results to studies of the Page curve and to experimentally relevant probes, via full counting statistics of charge transfer across a subregion, with only a single channel to unravel leading to a favorable scaling of the postselection overhead. Natural platforms for this setting include mesoscopic systems and ultracold atoms.
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Strongly Correlated Electrons
Competing incommensurability, electronic correlations, and superconductivity in a hybrid transition metal dichalcogenide
Jean C. Souza, Moshe Haim, Lorenzo Crippa et al. (+8 more)
The engineering of superlattices in two-dimensional van der Waals materials has enabled the realization of rich phase diagrams hosting topological and strongly correlated phases. While incommensurability is widespread in three-dimensional systems, the role of moiré potentials in bulk materials remains largely unexplored. Here, using scanning tunneling microscopy, we demonstrate that a bulk transition-metal dichalcogenide polytype, 4Hb-TaS$_2$, hosts an emergent incommensurate potential between its alternating 1T and 1H layers. Interplay with a concomitant incommensurate charge-density wave suppresses the long-range order of this potential, leading to intricate coupling with electronic correlations in the doped 1T surface layer. Combining density functional theory with dynamical mean-field theory, we show that the
lattice mismatch locally modulates the interlayer distance, thereby tuning both hybridization and charge transfer between the correlated 1T and metallic 1H layers. This redistribution of charge drives the system towards a doped Mott regime, in which the remaining local moments become self-screened, giving rise to a zero-bias resonance. We further find that bulk superconductivity competes with both the underlying landscape and the associated charge transfer. Our results establish incommensurate potentials as a previously overlooked ingredient in hybrid transition-metal dichalcogenides, highlighting their central role in the interplay between electronic correlations, charge-density-wave order, and unconventional superconductivity.
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All authors Jean C. Souza, Moshe Haim, Lorenzo Crippa, Hyeonhu Bae, Edanel Fishbein, Jonathan Ruhman, Binghai Yan, Amit Kanigel, Roser Valentí, Nurit Avraham, Haim Beidenkopf
Statistical Mechanics
The cell fluid model with Curie-Weiss interactions: special cases and analytical results
O. A. Dobush, M. P. Kozlovskii, I. V. Pylyuk et al. (+2 more)
Inspired by previous extensive numerical studies of a
cell fluid model with Curie-Weiss interactions, we concentrate on some analytically tractable special cases in its description. The key ingredient of the model is a competition between global attraction and local repulsion interactions between particles with coupling constants $J_1$ and $J_2$, respectively. We provide analytical results in several limiting cases, including the ideal-gas limit $J_1=J_2=0$ and the strong-repulsion limit $J_2\gg J_1$. For $J_2\gg J_1$, a detailed analytical study is presented. We derive explicit expressions for the critical point parameters, the equation of state, and the binodal and spinodal curves in closed form. The equation of state is found to be in full agreement with that of the van der Waals
lattice gas, and the order parameter satisfies the standard Curie-Weiss equation. In a neighborhood of the critical point, a Landau expansion is shown to have the same form and
symmetry as that of the classical
lattice gas within the mean-field approximation. Moreover, based on the explicit knowledge of a few leading terms in the asymptotic expansion of the deformed exponential function governing the physics of the
cell model, we extend its validity range to include the marginal case of thermodynamic stability, $J_1=J_2$. In particular, this extension makes a consideration of the ideal-gas limit $J_1=J_2=0$ formally legitimate. For the generic marginal case $J_1=J_2\ne0$ systematically avoided in previous works, we present numerical data and phase diagrams that augment their findings for $J_2>J_1$.
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All authors O. A. Dobush, M. P. Kozlovskii, I. V. Pylyuk, R. V. Romanik, M. A. Shpot
Materials Science
Observation of magnetically switchable quantum geometric photocurrents
Qi Tian, Zhuoliang Ni, Matthew Cothrine et al. (+6 more)
In non-centrosymmetric materials, light can be rectified into two types of DC photocurrents, known as injection and shift currents, through the bulk photovoltaic effect. Recent theory has uncovered their deep relation with the two-state
quantum geometry of resonant transitions: In non-
magnetic crystals, where these currents have been routinely observed, the injection current responds to circular light and probes the Berry
curvature, while the shift current responds to linear light and probes the
geometric connection.
Magnetic crystals have been predicted to show a new set of hitherto unobserved magnetically switchable photocurrents, with the roles of linear and circular light interchanged: A linear injection current, which probes the
quantum metric, and a circular shift current, which probes the
geometric torsion. In this work, we demonstrate the existence of such currents for the first time, demonstrating the switching of the current by flipping the Néel vector in a van der Waals antiferromagnet. Furthermore, their specific frequency and temperature dependence confirm the assignment of circular shift and linear injection currents. Our work demonstrates a new way to control photocurrents in magnets that are directly tied to geometry and have promising applications in
antiferromagnetic spintronics and light harvesting.
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All authors Qi Tian, Zhuoliang Ni, Matthew Cothrine, David G. Mandrus, Eugene J. Mele, Andrew M. Rappe, Charles L. Kane, Fernando de Juan, Liang Wu
Strongly Correlated Electrons
Magnetoelastic mapping of the high-field phase diagram in the topological cubic helimagnet SrFeO$_{3}$
Masaki Gen, Shun Okumura, Shusaku Imajo et al. (+9 more)
The cubic perovskite SrFeO$_{3}$ is a prototypical centrosymmetric itinerant magnet that hosts a quadruple-${\mathbf Q}$ hedgehog-antihedgehog
lattice and exhibits a complex
magnetic-field-temperature phase diagram. Yet, the microscopic mechanism underlying the emergence of its versatile multiple-${\mathbf Q}$ phases remains unresolved. Here, we reveal the field-orientation dependence of the
magnetic phase diagram and establish an effective
spin Hamiltonian for SrFeO$_{3}$ that incorporates a cubic single-ion anisotropy together with bilinear and biquadratic interactions in momentum space, which originate from the
spin-charge coupling. In addition, we observe magnetoelastic signatures of a redistribution of the ligand-hole density upon entering the forced
ferromagnetic phase. These findings emphasize the pivotal importance of electronic itinerancy arising from the formation of a ligand-hole band in stabilizing multiple-${\mathbf Q}$ phases.
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All authors Masaki Gen, Shun Okumura, Shusaku Imajo, Taro Nakajima, Karel Prokes, Shunsuke Kitou, Yusuke Tokunaga, Yoichi Nii, Koichi Kindo, Yoshimitsu Kohama, Shintaro Ishiwata, Taka-hisa Arima
Materials Science
A sulfonitride transparent conductive thin film with ultra-high refractive index
Eugène Bertin, Shima Kadkhodazadeh, José María Castillo-Robles et al. (+9 more)
With the rise of AI-assisted materials screening, extraordinary properties are now frequently predicted in experimentally uncharted material systems, highlighting the need to develop new synthesis methods for unconventional materials beyond the classic bulk powder form. Here, we establish the first thin-film growth route for any metal sulfonitride compound by realizing Zr2SN2 films with a rare and compelling combination of optical and electrical properties. Zr2SN2 is transparent across most of the visible range while exhibiting a very high average refractive index of 2.95 in the visible, exceeding expectations based on conventional refractive index-bandgap scaling. Importantly, the same Zr2SN2 film shows degenerate n-type conductivity with carrier density above 10^20 cm-3 and intragrain mobility above 8 cm2V-1s-1, approaching those of established transparent conductive
oxides. Zr2SN2 thus demonstrates that strong light-matter interaction, optical transparency and electrical conductivity can be reconciled within a single material platform, revealing a new class of high-refractive-index transparent conductors.
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All authors Eugène Bertin, Shima Kadkhodazadeh, José María Castillo-Robles, Finja Tadge, Alba Pérez Millan, Anat Itzhak, Javier Sanz Rodrigo, Manuel Dillenz, Juan Maria García Lastra, Søren Raza, Ivano E. Castelli, Andrea Crovetto
Superconductivity
Current-driven reduction of topological protection in multichannel superconductors
Alfonso Maiellaro
We investigate the robustness of topological phases in a Kitaev ladder composed of two coupled superconducting chains under the perturbing influence of a finite charge current. By introducing an effective
Hamiltonian depending on the quasiparticle momentum induced by the current, we show that the two-mode topological phase, present in the isolated ladder, is fragile against a finite current flux. To characterize this behavior, we combine bulk topological invariants with real-space diagnostics, including the edge–edge
quantum conditional mutual information $I_{ee}$, which provides an
entanglement-based signature of topological order. We further distinguish the gapped class-D sectors from a current-induced gapless superconducting regime, which emerges when the upper BdG branch crosses zero at generic momenta and the standard gapped topological invariant is no longer applicable. Our results provide an effective framework to describe how current injection and measurement processes can affect topological protection in superconducting nanostructures.
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Materials Science
Harnessing Linear and Nonlinear Optical Responses in Ferroelectric LaMoN$_3$ for Enhanced Photovoltaic Efficiency
Surajit Adhikari, Sanika S. Padelkar, Jacek J. Jasieniak et al. (+2 more)
Nitride perovskites are an emerging class of materials predicted to exhibit diverse functional properties, yet remain underexplored due to synthesis challenges of oxygen-free nitrides. Recently, LaMoN$_3$ has been reported as an oxygen-free nitride perovskite with polar
symmetry, exhibiting excellent dynamic stability and ferroelectric properties under moderate pressure. However, its phase stability, linear and non-linear optical response, excitonic and polaronic behavior, and efficiency under high pressure remain unexplored. Applying pressure enables systematic tuning of the electronic structure properties, thereby facilitating the identification of phases optimized for either linear or nonlinear optical responses. Therefore, in this work, we systematically investigate these properties of LaMoN$_3$ up to 40 GPa using first-principles methods, including density functional theory, density functional perturbation theory, many-body perturbation theory (namely G$_0$W$_0$ and BSE), and tight binding approximation model. Our study shows that LaMoN$_3$ remains dynamically stable and retains its single-phase structure up to 40 GPa. The compound exhibits an indirect bandgap that decreases from 2.17 eV (0 GPa) to 1.45 eV (40 GPa) at the G$_0$W$_0$@PBE level. Using the BSE, we find that pressure enhances the SLME while lowering the
exciton binding energy, both favorable for photovoltaic applications. The bulk photovoltaic efficiency trend with pressure mimics the behavior of the shift current density J$_SC$ , peaking near 15 GPa before declining at higher pressures due to a diminished nonlinear shift current response. These results highlight pressure-tuned regimes to enhance photovoltaic performance. We thereby propose multi-junction device, combining absorber layers optimized for linear and nonlinear optical currents, together boosting
solar energy conversion through complementary mechanisms.
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All authors Surajit Adhikari, Sanika S. Padelkar, Jacek J. Jasieniak, Alexandr N. Simonov, Aftab Alam
Mesoscale and Nanoscale Physics
Transport Enhancement and In Situ Control of Electronic Correlation via Photoinduced Modulation Doping of van der Waals Heterostructures
Collin R. Sanborn, Son T. Le, Thuc T. Mai et al. (+7 more)
Modulation doping, a well-established technique for traditional
semiconductor heterostructures, is a promising approach for tailoring carrier concentration in 2D materials devices. In this letter we report on photoinduced modulation doping in hBN-
graphene-hBN-SiO2
heterostructures utilizing standard white light sources and no additional fabrication complexity. We establish the use of this technique to both dope the channel material and to photoanneal devices, providing control over electronic doping and disorder in the
graphene channel. We analyze the transport properties by employing Drude and Landauer transport models, highlighting the ability to reversibly tune the mobility and mean scattering length of the
graphene with a high degree of accuracy. This tunability allows us to switch our device between the diffusive and quasi-ballistic transport regimes in situ. We utilize the exceptional control our technique provides over local disorder to realize
quantum Hall isospin
ferromagnetic states in a device whose initial quality would otherwise leave such states unobservable. These results demonstrate precise manipulation of carrier density and charge disorder in van der Waals
heterostructures, providing a highly accessible approach to creating high-quality devices capable of realizing correlated electronic states.
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All authors Collin R. Sanborn, Son T. Le, Thuc T. Mai, Maria F. Munoz, Riccardo Torsi, Angela R. Hight Walker, Curt A. Richter, Samuel W. LaGasse, Aubrey T. Hanbicki, Adam L. Friedman
Quantum Gases
Interaction-enabled metal-insulator phase transition in a driven quantum gas
Camilo Cantillano, Karthick Ramanathan, Zekai Chen et al. (+6 more)
Particle transport and energy flow are central to a wide range of phenomena in the natural sciences. While interactions generically promote ergodicity and
diffusion,
quantum interference can arrest transport, defying classical expectations. Here, we experimentally investigate their interplay in a periodically driven 3D
quantum gas with tunable interactions. Strikingly, we find a sharp dynamical boundary separating localization from diffusive energy absorption. By tuning the driving amplitude and interaction strength, we map the localization-delocalization phase diagram and characterize this boundary via finite-time scaling. On the insulating side, we observe many-body dynamical localization (MBDL) featuring arrested momentum-space transport. Transport becomes subdiffusive near the boundary and diffusive in the delocalized regime, yielding a metal-
insulator transition that we interpret as localization in many-body
Hilbert space. Our results exemplify an interaction-enabled dynamical phase transition in a closed Floquet many-body system, and clarify how coherence and interactions jointly govern the
quantum-to-classical transition.
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All authors Camilo Cantillano, Karthick Ramanathan, Zekai Chen, Ang Yang, Emilio Aguilera-Valdes, Lei Ying, Manuele Landini, Hanns-Christoph Nägerl, Yanliang Guo
Statistical Mechanics · Cross-listed from quant-ph
Long-range nonstabilizerness of topologically encoded states from mutual information
David Aram Korbany, Tyler D. Ellison, David T. Stephen et al. (+1 more)
We study long-range nonstabilizerness (LRN), namely the obstruction to remove nonstabilizerness with shallow-depth local
quantum circuits. In one-dimensional settings, the mutual information between disconnected spatial regions has proven to be a powerful tool to diagnose LRN. In this work, we focus on encoded states of two-dimensional topologically-ordered systems, and explore the ability of the mutual information to serve as a diagnostic of LRN. Focusing on the concrete setting of
lattice models defined on a
torus, we show that information about LRN can be gained from the analysis of the mutual information between non-overlapping regions containing non-contractible loops, and of the change of such mutual information under modular real-space transformations. We exemplify this idea in the toric code and the non-abelian string-net model with doubled Fibonacci topological order. In the former case, we show that the mutual information provides a full classification, certifying LRN for all encoded non-stabilizer states. In the latter case, instead, our approach does not lead to a full classification, as it detects LRN for all states except from a finite subset with special transformation properties under the modular group. Finally, we discuss how our results on LRN constrain the logical gates that can be implemented fault-tolerantly on the
torus.
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All authors David Aram Korbany, Tyler D. Ellison, David T. Stephen, Lorenzo Piroli
Strongly Correlated Electrons
Emergent magnetic and charge ordered phases in freestanding ultrathin \ce{LaVO3}
Ashutosh Anand and Mukul Kabir
Transition metal
oxide perovskites are an ideal platform for exploring the interplay between
spin, orbital, charge and
lattice degrees of freedom. Among them, \ce{LaVO3} has been extensively studied in
heterostructures and superlattices, where exotic phases have been reported. Motivated by the advances in freestanding
oxide membranes, we investigate the intrinsic properties of freestanding ultrathin \ce{LaVO3} films using density functional theory. Our calculations reveal a sequence of
magnetic phase transitions with thickness, starting from stripe-AFM in monolayer until the bulk like C-AFM is recovered. Beyond four layers, polar catastrophe driven charge transfer dopes the surface layers giving rise to stripe-AFM and
ferromagnetic surface states while the central layers remain bulk like. We further explore this fact by studying charge doped monolayer, discovering that hole doping drives the system into
ferromagnetic state. Doping also induced charge ordering in the system. A striped charge ordering pattern is observed at 0.5 h/fu, while a 3:1 stripe pattern emerges at 0.25 h/fu, indicating that the periodicity of the superstructure changes with doping concentration.
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Soft Condensed Matter
Self-organization and memory formation in two-dimensional jammed deformable matter under cyclic compression
Rahul Nayak, Satyavani Vemparala and Pinaki Chaudhuri
We study the athermal mechanical response of deformable ring assemblies to quasistatic compression. Beyond jamming, further densification induces buckling of rings, resulting in macroscopic mechanical softening. Under cyclic compression, monodisperse systems anneal toward a nearly reversible path passing through an ordered state, whereas polydisperse systems converge to stable, hysteretic limit cycles. These limit cycles encode a robust
memory of the training history that is retained even under subsequent overdriving. We show that macroscopic hysteresis in the disordered packings originates from directionally asymmetric non-affine deformations at the microscale while keeping contact network largely intact. Our findings demonstrate how particle deformability governs collective self-organization and
memory formation in jammed soft matter.
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Mesoscale and Nanoscale Physics · Cross-listed from quant-ph
Factorization rule for multitime correlations in non-Markovian open quantum systems
Thomas K. Bracht and Moritz Cygorek
Experiments performed on
quantum systems often measure multitime correlation functions. When
quantum systems are weakly coupled to their environment, the time
evolution of such correlation functions can be reduced to that of the reduced density
matrix by the
quantum regression
theorem (QRT). While no QRT is available for general non-Markovian open
quantum systems, we show that for time-independent
Hamiltonians and finite
memory times $\tau_c$, an exact factorization rule exists that relates higher-order multitime correlations to products of lower-order correlations. Consequently, all information needed to reconstruct $n$-time correlations is contained in a temporal volume of $\mathcal{O}(\tau_c^n)$. On the example of
quantum dots coupled to
phonons, we demonstrate that this factorization makes numerical calculations of multitime correlations extremely efficient and even enables semianalytical solutions in systems where the standard QRT breaks down.
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Statistical Mechanics
Aggregation-Fragmentation Processes with Broken Detailed Balance
P. L. Krapivsky
We study aggregation-fragmentation processes in which pairs of clusters can aggregate, and each cluster can break into two fragments. If the rates of aggregation and fragmentation do not depend on the masses, detailed balance does not hold, but nonequilibrium steady states can still be deduced from an exact solution for the Laplace transform. For models in which aggregation rates remain constant but fragmentation rates scale as $(\text{mass})^\beta$, detailed balance holds only when $\beta=1$. Away from this solvable case, we employ asymptotic techniques and show that when $\beta\geq 0$, the steady states share similarities with those from the mass-independent ($\beta=0$) model. An instantaneous shattering transition with continuous mass loss occurs when $\beta<0$.
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Materials Science
Donghang Xie, Haozhe Wang, Zhe Zhang et al. (+13 more)
Antiferromagnets (AFMs) hold significant potential for spintronic devices owing to their insensitivity to external
magnetic fields and the absence of stray fields. Beyond these inherent advantages, an AFM can manipulate the
magnetic dynamics of a ferromagnet (FM) layer in AFM/FM
bilayers, whereas the mechanism of such manipulation remains controversial. Here, we investigate the
magnetic dynamics of AFM/FM Mn2Au/CoFeB
bilayers via
Ferromagnetic Resonance (FMR). It is found that the Néel temperature of 2-nm-thick Mn2Au is as low as ~40 K, in sharp contrast to that of bulk Mn2Au, which exceeds 1000 K. In the Mn2Au(2 nm)/CoFeB(4 nm)
bilayer, the
magnetic damping ${\alpha}$ of the CoFeB layer increases from 0.013 to 0.047 as temperature decreases from 160 K to 10 K, accompanied by a synchronous increase in the exchange coupling field H_rot. Such an increase in ${\alpha}$ is attributed to the enhanced
spin angular momentum transfer from CoFeB to Mn2Au, mediated through AFM-FM exchange coupling between Mn2Au and CoFeB, which is enhanced by the Mn2Au
antiferromagnetic ordering as the temperature decreases. Our study provides deeper insights into AFM/FM dynamics and spintronic storage technology.
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All authors Donghang Xie, Haozhe Wang, Zhe Zhang, Zishuang Li, Jiahua Lu, Ronghua Liu, Jun Du, Bo Liu, Yu Yan, Liang He, Jing Wu, Rong Zhang, Bo Liu, Tiejun Zhou, Yongbing Xu, Xuezhong Ruan
Quantum Gases
Multipolar exchange in a many-body homonuclear mixture of atoms in different internal states
M. Bulakhov, A.S. Peletminskii and Yu.V. Slyusarenko
We develop a general method for constructing the many-body
Hamiltonian of pairwise interactions describing homonuclear mixtures of atoms occupying states with different total angular momenta or other
quantum numbers. The advantage of the irreducible spherical tensor operator formalism is demonstrated: these operators give the
Hamiltonian an explicit physical structure, account for all scattering channels, and include multipolar exchange interactions. The latter correspond to the exchange of both angular-momentum projections and the total angular momentum. Particular realizations of the general
Hamiltonian, widely used in the physics of ultracold gases, are also analyzed. The resulting
Hamiltonian provides a universal framework for investigating a broad range of
quantum many-body phenomena in bosonic and fermionic atomic gases.
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Materials Science
Generation of an anomalous linearly dispersing spin-polarized band in Bi-based topological insulators
Matthias Kronseder, Thomas Mayer, Jan Minár et al. (+11 more)
We report the generation of an anomalous linearly dispersing,
spin-polarized band in Bi-based topological
insulator (TI) thin films, induced by soft Ar-ion bombardment followed by annealing. This extra band – which we call the anomalous linearly dispersing state (ALS) – is superimposed on the regular band structure including the topological surface state (TSS), spans an unusually large energetic range of up to ${\sim}\,\SI{650}{\milli\electronvolt}$ at the $\Gamma$-point, and appears near the
Fermi energy.
Spin-resolved measurements indicate
spin-momentum locking with a helicity opposite to that of the regular TSS. The
Fermi velocity of the ALS, $v_\mathrm{F} = (5.1\pm 0.4)\times 10^{5}\,\frac m s$, is indistinguishable from that of the regular TSS, $(5.3\pm 0.5)\times 10^{5}\,\frac m s$. The observation is reproducible across samples of varying thickness and was confirmed at two independent synchrotron radiation facilities. We discuss different mechanisms for the physical origin of the observed ALS including sputtering-induced TSS relocation, bi-layer formation by,e.g., chalcogen removal, and high-index surface relocation.
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All authors Matthias Kronseder, Thomas Mayer, Jan Minár, Magdalena Marganska, Hedwig Werner, Florian Schmid, Rebeca Diaz-Pardo, Ivana Vobornik, Jun Fuji, Cornelia Streeck, Alexander Gottwald, Hendrik Kaser, Bernd Kästner, Christian H. Back
Materials Science
Improvement of piezocatalytic performance of Na0.5Bi0.5TiO3 perovskite using K doping for efficient Rhodamine B degradation
Salma Ayadh, Salma Touili, Mbarek Amjoud et al. (+9 more)
Piezocatalysis, based on the piezoelectric properties of catalysts, breaks down the barrier between mechanical energy and chemical energy. It describes the use of charges induced by piezoelectricity to assist typical chemical processes while harvesting various forms of mechanical green energy. The performance of piezocatalysis is predominantly governed by the piezoelectric properties of materials. The main aim of this work is to evaluate and analyze the potential of potassium doped sodium bismuth titanate Na0.5-xKxBi0.5TiO3 abbreviated as NKxBT (0, 0.15, 0.20, and 0.25), as a piezocatalyst in the degradation of the organic dye Rhodamine B RhB under ultrasonic vibration. The synthesis of NKxBT nanopowders was conducted using the sol-gel autocombustion method. Coupled structural analysis reveals the presence of an intermediate Morphotropic Phase Boundary (MPB, where two phases coexist) in the optimal NK15BT composition. The piezocatalytic degradation results showed a total piezo-degradation in only 90 min and a rate constant 8 times higher than the undoped NK0BT. The enhanced piezocatalytic activity results from a synergistic effect of MPB presence, reduced particle size, optimal bandgap and high
lattice strain. The NK15BT sample also demonstrated good reusability and good mineralization.
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All authors Salma Ayadh, Salma Touili, Mbarek Amjoud, Daoud Mezzane, Mohamed Goune, Jaafar Ghanbaja, Manal Benyoussef, Hana Ursic, Nejc Suban, Mustapha Raihane, Zdravko Kutnjak, Mimoun El Marssi
Disordered Systems and Neural Networks · Cross-listed from cs.LG
A Boundary-Layer Mechanism for One-Third Scaling in Online Softmax Classification
Marcel Kühn, Yoon Thelge and Bernd Rosenow
Hard-label classification is usually trained with smooth surrogate losses such as softmax cross-
entropy. We isolate a mechanism by which this mismatch between smooth surrogate and discrete labels produces power-law learning curves for a one-layer softmax student trained by online SGD on Gaussian inputs with argmax teacher labels. In the thermodynamic limit, the dynamics reduce to a growing student-teacher
alignment and a residual student variance sustained by SGD noise. At late times, only shrinking layers of examples around teacher decision boundaries remain active. This yields an $\alpha^{-1/3}$ law in training time $\alpha$ not only for the test loss but also for the generalization error, one minus test accuracy. The error law is an online-noise effect. The loss law persists without gradient noise. Learning-rate annealing separates the two exponents, improving the error exponent from 1/3 toward 1/2 while slowing the loss. Simulations support the predictions. Correlated Gaussian inputs and whitened pretrained features change transients, not the late-time decay. Experiments with deep neural networks can show compatible error scaling. The mechanism is asymptotic and complementary to spectral explanations and data-statistics explanations of neural scaling laws.
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Materials Science
Two-dimensional alternating ferrimagnetism with strain-controlled half-metallic state and valley polarization
W. Z. Zhuo, Z. H. Guan, Z. L. Peng et al. (+4 more)
The discovery of altermagnetism offers new opportunities for exploring novel
quantum states and developing spintronic devices for enabling momentum dependent
spin splitting in compensated systems, while zero net magnetization limit its manipulability using conventional
magnetic method. Here, we propose 2D alternating ferrimagnetism,a phase merging alternating momentum dependent
spin splitting with a finite net magnetization. A tight binding model reveals that alternating ferrimagnetism originates from uncompensated magnetization in altermagnets, facilitating concurrent net magnetization and alternating
spin splitting. First principles calculations and Monte Carlo simulations demonstrate stable alternating ferrimagnetism in strained and Cr substiting V2Te2O, which exhibit strain tunable net magnetization, reversable half
metallicity and valley polarization, accompanied by long range
magnetic order above room temperature. By combining altermagnetic and
ferromagnetic properties, alternating ferrimagnetism expand the 2D magnetism landscape and offer pathways for energy efficient spintronic applications.
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All authors W. Z. Zhuo, Z. H. Guan, Z. L. Peng, Y. N. Pan, J. Chen, Y. Yang, M. H. Qin
Disordered Systems and Neural Networks
Possible Topological Decoherence Transition in Relativistic Electron Beams Propagating through Coulomb-Disordered Media
Yury A. Budkov
We show that the mutual coherence of a relativistic electron beam in a Coulomb-disordered medium is governed by an effective two-dimensional compact phase field with a logarithmic correlation function. The corresponding Gaussian free-field action exhibits a stiffness inversely proportional to the propagation length. When the compact nature of the phase is taken into account, the system supports
vortex excitations that interact as a two-dimensional Coulomb gas. Renormalization-group analysis of this gas indicates the existence of a critical sample thickness $L_c$ at which a Berezinskii–Kosterlitz–Thouless (BKT) transition may occur, separating a regime of algebraic decoherence from one where free
vortices proliferate and coherence is destroyed exponentially. The critical thickness is expressed through fundamental microscopic parameters and could be observed in transmission electron microscopy of
liquid cells or cryogenic samples.
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Materials Science
Photoemission intermittency via stochastic gating in rubrene nanowires coupled to plasmonic silver nanoparticles
Moha Naeimi, Waqas Pervez, Frithjof Harmsen et al. (+2 more)
In this work, we report a new nanoscale phenomenon observed as photoemission intermittency (On-Off electron emission), manifested as
stochastic bursts in electron yield at quasi-one-dimensional organic wires and silver nanoparticles interface. Energy-resolved measurements reveal that the emitted electrons carry out hybrid information, containing photoelectron yield enhancement associated with the nanoparticles and kinetic energies determined by the organic
semiconductor. The intermittency results in a dynamic shift of the electron spectra correlating with the photoelectron yield. We attribute the observed behaviour to the photo-hole accumulation and
stochastic gating of charge due to electron-hole separation at the nano interface. These findings introduces the photoemission intermittency as a nanoscale phenomenon indicating a new dynamic regime of charge assisted emission at organic-plasmonic interfaces. Keywords: rubrene, nanoparticle, PEEM,
exciton, charge
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All authors Moha Naeimi, Waqas Pervez, Frithjof Harmsen, Ingo Barke, Sylvia Speller
Superconductivity
Development and characterization of YBa$_2$Cu$_3$O$_{7-x}$/SrTiO$_3$ nanomembrane platform for silicon photonics integration
J.S. Madhira, G. Potemkin, B. Kardynal et al. (+3 more)
Integration of high-temperature superconducting devices with photonic
circuits requires a material platform that preserves superconducting properties, provides interfacial heat transport control and optical accessibility. A YBa$_2$Cu$_3$O$_{7-x}$/SrTiO$_3$ (YBCO/STO) nanomembrane platform, enabling integration with SiO$_2$/Si substrates, is developed and characterized. Millimeter-scale ultra-thin YBCO films are grown on a STO/Sr$_{1.5}$Ca$_{1.5}$Al$_2$O$_6$
bilayer, released on a STO nanomembrane, and transferred onto SiO$_2$/Si substrates. X-ray diffraction confirms that crystallinity of STO and YBCO is preserved after transfer. Microbridges patterned from transferred YBCO films exhibit a critical temperature of 88.8 K and a critical current density of 6.8 MA/cm$^2$ at 77 K, comparable with those of YBCO films on STO substrates. Statistical measurements show a device fabrication yield of 93%, a critical-temperature variation of only 0.05 K, and 10% variation in critical current density, demonstrating excellent reproducibility. The thermal boundary conductance across the van der Waals interface between the YBCO/STO platform and the SiO$_2$/Si substrate is strongly reduced compared to that of epitaxial YBCO film on STO substrate. The reduced thermal coupling prolongs nonequilibrium states in superconducting nanostructures, which is advantageous for detector applications. These results establish ultra-thin YBCO films on optically transparent STO nanomembranes as a platform for integrating high-T$_c$ superconducting devices with silicon photonics.
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All authors J.S. Madhira, G. Potemkin, B. Kardynal, D. Grützmacher, T. Schäpers, M. Lyatti
Materials Science
Quanchao Du, Zhenlong Zhang, Yuanjun Jin et al. (+7 more)
Odd-parity magnetism represents an intriguing frontier in unconventional magnets; however, its realization has traditionally relied on noncollinear
magnetic orders accompanied by broken
spin conservation, which inevitably causes
spin relaxation and dissipative transport of
spin-encoded information. Here, we uncover a distinct route toward
spin-conserving odd-parity
magnon splitting enabled by antichiral Haldane-like flux (AHF) in collinear
antiferromagnetic multiferroics.
Symmetry analysis reveals that such AHF driven by intra-sublattice Dzyaloshinskii-Moriya interactions (DMI) gives rise to
symmetry-dependent odd-parity
magnon splittings, ranging from $p$-wave and $f$-wave to nodeless forms. Moreover, the coupling between ferroelectric modes and DMI provides an electric-field knob for manipulating
chiral band splitting and
magnon transport. Combining density functional theory (DFT) calculations, we identify a series of promising candidate materials in both 2D and bulk
antiferromagnetic multiferroics. Our work provides new insights into the realization of odd-parity
chiral magnons in collinear antiferromagnets and magnetoelectric coupling mechanisms in multiferroics.
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All authors Quanchao Du, Zhenlong Zhang, Yuanjun Jin, Rui Li, Haibo Xie, Zhe Wang, Zhijun Jiang, Lei Zhang, Hongjian Zhao, Jinyang Ni
Strongly Correlated Electrons
Equilibrium Stabilization of a Hidden Phase Like Metallic State in 1T-TaS2
Anil Rajapitamahuni, Suji Park, Houk Jang et al. (+3 more)
Electronic phases that lie outside the
equilibrium ground state offer a route to explore competing configurations in correlated materials. In 1T-TaS2, ultrafast excitation accesses a metallic hidden phase that is distinct from the commensurate insulating ground state. Here we use angle-resolved photoemission spectroscopy to show that an equivalent electronic configuration is stabilized in exfoliated intermediate-thickness 1T-TaS2 flakes, where it persists up to room temperature before evolving through a different sequence of electronic transitions. This
equilibrium hidden-phase-like state hosts a metallic band with finite
Fermi-level spectral weight while retaining the characteristic hybridization gaps associated with the
star-of-David band folding. These results establish a platform for controlling competing electronic states in layered materials, with implications for both
quantum science and phase change technologies.
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All authors Anil Rajapitamahuni, Suji Park, Houk Jang, Asish K. Kundu, Elio Vescovo, Turgut Yilmaz
Materials Science
Enrichment of rhombohedral stacking by mechanical exfoliation of graphite
Krisztián Márity, Konrád Kandrai, Gergely Dobrik et al. (+5 more)
Rhombohedral (ABC) graphite hosts a surface-localized flat band that supports correlated and topological electronic phases, but its experimental study is limited by the scarcity of ABC stacking in natural graphite, which is dominated by Bernal (AB) stacking. Here we show that the routine mechanical exfoliation step itself enriches the rhombohedral content of graphite flakes, and that a simple blade-assisted exfoliation step, which introduces additional shear, amplifies the effect further. Using large-area Raman 2D-band skewness mapping we measure ABC content at area fractions of 3% in the pristine source
crystal, 16% after conventional exfoliation, and 26% after blade-assisted exfoliation for thick flakes. In thin flakes ($<20$ layers) the per-flake area fraction reaches 75% in the upper tail of the distribution. Tracking individual flakes before and after blade-assisted exfoliation shows that wrinkles seed AB-ABC domain walls, and uniaxial strain can move these walls. Blade-assisted mechanical exfoliation therefore removes one of the bottlenecks to the preparation of ABC-rich graphite samples for studies of correlated and topological phases in rhombohedral graphite.
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All authors Krisztián Márity, Konrád Kandrai, Gergely Dobrik, Zsolt E. Horváth, Kristóf Németh Dániel, György Kálvin, Levente Tapasztó, Péter Nemes-Incze
Materials Science · Cross-listed from physics.app-ph
Simultaneously monitoring Ga adsorption and desorption kinetics on GaN(0001) using four in situ techniques
Huaide Zhang, Philipp John, Jingxuan Kang et al. (+3 more)
We present a systematic investigation of Ga adsorption and desorption kinetics on the wurtzite GaN(0001) surface using four in situ techniques operated simultaneously: reflection high-energy electron diffraction,
laser reflectometry, line-of-sight quadrupole mass spectrometry, and optical pyrometry. Flux- and temperature-dependent experiments are performed for Ga coverages ranging from the submonolayer to the droplet regime. Despite their distinct transient responses, the signals from all four techniques and their trends with surface coverage are quantitatively reproduced by a unified kinetic model of Ga adsorption,
diffusion, and desorption. An Arrhenius analysis of the Ga adlayer desorption yields an activation energy of (2.87 $\pm$ 0.04) eV.
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All authors Huaide Zhang, Philipp John, Jingxuan Kang, Lutz Geelhaar, Yongjin Cho, Oliver Brandt