Materials Science
Importance of Configurational Charge Ordering for Machine Learning Interatomic Potentials
Martin Hoffmann Petersen, Steen Lysgaard, Arghya Bhowmik et al. (+2 more)
Machine learning interatomic potentials (MLIPs) enable large-scale atomistic simulations but remain challenged in describing mixed-valence materials where charge ordering strongly influences thermodynamic stability. Here we investigate the role of configurational charge ordering in MLIP structural optimization of the battery cathode material NaFePO4. We show that conventional MLIPs fail to reproduce the correct stability of intermediate Na concentrations because structural optimization leads to incorrect Fe2+/Fe3+ charge assignments, resulting in incorrect energy ordering and convex-hull predictions. Analysis of the Fe2+/Fe3+ charge assignment during structural optimization reveals that MLIPs are unable to capture the configurational charge ordering, leading to energy errors that make it difficult to identify the experimentally observed phase as the true ground-state structure. To address this limitation, we introduce an approach that encodes charge-state information directly into the MLIP representation by distinguishing between Fe2+ and Fe3+ environments during training. Retraining CHGNet, MACE and MACELES with this representation enables accurate structural optimization, correct identification of charge ordering, and improved agreement with density functional theory convex hulls. Our results demonstrate that incorporating configurational charge ordering into the MLIP representation is essential for modeling charge-disordered materials. Beyond recovering the correct ground state, this encoding allows a specific charge ordering to be imposed and held fixed during optimization, a capability unconstrained DFT struggles to guarantee, and we show that it generalizes across four polyanion frameworks and four transition-metal ions (Fe, Mn, Co, Ni), establishing a broadly applicable strategy for modeling mixed-valence transition-metal systems.
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All authors Martin Hoffmann Petersen, Steen Lysgaard, Arghya Bhowmik, Kedar Hippalgaonkar, Juan Maria Garcia Lastra
Materials Science · Cross-listed from physics.optics
Efficient Picosecond-Laser Lift-Off of Copper Oxide from Copper: Modelling and Experiment
Andrius Žemaitis, Paulius Gečys and Mindaugas Gedvilas
Laser-induced lift-off of functional surface layers is a key process in micro- and nano-fabrication; however, optimization criteria for maximizing the lifted-off area remain insufficiently defined. In analogy to the well-established theory of efficient
laser ablation, where the maximum ablated volume per pulse is achieved at a peak fluence of F_0^{\mathrm{opt}} = e^{2} F_{\mathrm{th}}, we develop a theoretical framework for efficient
laser lift-off driven by Gaussian beams. By analytically describing the lift-off area as a function of peak fluence, beam radius, and focus position, we demonstrate that the maximum lifted-off area is achieved at a substantially lower optimal fluence, namely F_0^{\mathrm{opt}} = e^{1} F_{\mathrm{th}}. Closed-form expressions for the optimal beam radius, maximal lift-off area, and optimal focus position are derived and validated by numerical modeling. The theory is applied to picosecond
laser lift-off of copper
oxide from copper, showing excellent agreement between experimental observations and model predictions. The results reveal fundamental differences between ablation- and lift-off-dominated material removal and provide practical guidelines for maximizing process efficiency in
laser-assisted delamination, selective coating removal, and surface functionalization.
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Strongly Correlated Electrons
Towards a unified first-principles-based description of VO$_2$ using DFT+DMFT with bond-centered orbitals
Peter Mlkvik, Nicola A. Spaldin and Claude Ederer
We present a combined density-functional theory and dynamical mean-field theory (DFT+DMFT) study of the full structural phase space of rutile-based vanadium dioxide (VO$_2$), including also the less studied M2 and T phases, using an unconventional bond-centered orbital basis. The use of bond-centered orbitals allows us to treat all main phases of VO$_2$, and the structural transitions between them, using one consistent approach with moderate computational cost and without pre-pattering of the structure into dimerized and undimerized V–V pairs. We obtain two distinct insulating states on the two different types of vanadium chains in the M2 phase, a singlet-
insulator on the dimerized chains and a Mott-
insulator on the zigzag-distorted chains, which, however, are strongly coupled in the M2 phase and thus the metal-
insulator transition always occurs concomitantly for both types of sites. We also demonstrate that the M2 phase corresponds to a local energy minimum in the structural phase space of VO$_2$, the stability of which, apart from the internal structural distortion, depends crucially on the unit
cell strain relative to the undistorted rutile phase. Our calculations further indicate that the
symmetry-distinct triclinic T phase corresponds electronically to either an M1 or an M2-type
insulator with an abrupt transition as a function of distortion. Finally, we disentangle the effect of the dimerization and zigzag distortions by constructing hypothetical structures that contain only one site type, finding that the zigzag distortion strongly favors emergence of the Mott-insulating state, both as function of distortion and on-site interaction.
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Mesoscale and Nanoscale Physics · Cross-listed from physics.optics
Nanoscale Surface Analysis of High Entropy Alloy
Hsin-Hui Huang, Meguya Ryu, Yoshiaki Nishijima et al. (+10 more)
Nanoscale surface analysis of 1 micrometer thick high
entropy alloys (HEAs) was carried out using nano-IR for hyperspectral imaging and single point spectroscopy in the 700-1700 1/cm spectral range. Nano-IR is based on the detection of scattered light from an oscillating metal coated nano-tip in one of the arms of the
Fourier transform infrared spectrometer and has a resolution defined by the tip radius of the probe, 20 nm, regardless of the excitation wavelength. HEA CuPdAgPtAu showed an absorption and reflection increase at 900-1100 1/cm band, which is consistent with Drude-Lorenz modeling of permittivity, however, could also signify
oxide formation as tested by X-ray photoelectron spectroscopy of CuPdAgPtAu and CrFeCoNiCuMo. Realization of polarization analysis for nano-IR nano-spectroscopy in the plane perpendicular to the sample's surface is discussed and modeled. The currently available modality of surface analysis with the excitation-detection mode of the p-pol.
antenna can be extended to full 3D analysis of the orientational dependencies of local absorbance and refractive index.
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All authors Hsin-Hui Huang, Meguya Ryu, Yoshiaki Nishijima, Haoran Mu, Mohit Kumar, Nguyen Hoai An Le, Adrian Cernescu, Jitraporn Vongsvivut, Andrew Siao Ming Ang, Lina Grineviciute, Challapalli Subrahmanyam, Junko Morikawa, Saulius Juodkazis
Mesoscale and Nanoscale Physics
Anomalous Nonlinear Magnetoconductivity in van der Waals Magnet CrSBr
Junhyeon Jo, Manuel Suárez-Rodríguez, Samuel Mañas-Valero et al. (+6 more)
Nonlinear magnetoconductivity (NLMC) is a nonreciprocal transport response arising in non-centrosymmetric materials. However, this ordinary NLMC signal vanishes at zero
magnetic field, limiting its potential for applications. Here, we report the observation of an anomalous NLMC controlled by internal order parameters such as the magnetization or Néel vectors. We achieve this response by breaking both inversion and time-reversal
symmetry in artificial van der Waals
heterostructures based on the
magnetic CrSBr and insulating hBN. The nonreciprocal signal can be tuned between two different states in
ferromagnetic monolayer CrSBr and among four different states in
antiferromagnetic bilayer CrSBr, thanks to its metamagnetic transition. Remarkably, this output signal in the
ferromagnetic (
antiferromagnetic) state of CrSBr is three (one) orders of magnitude higher than those previously measured. A conductivity scaling analysis reveals the Berry connection polarizability as the origin of the anomalous NLMC. Our results pave the way for high-frequency rectifiers with magnetically switchable output polarity as well as for an efficient electrical readout of the
magnetic state of
antiferromagnetic materials.
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All authors Junhyeon Jo, Manuel Suárez-Rodríguez, Samuel Mañas-Valero, Eugenio Coronado, Ivo Souza, Fernando de Juan, Fèlix Casanova, Marco Gobbi, Luis E. Hueso
Mesoscale and Nanoscale Physics · Cross-listed from quant-ph
Reconstructing Quantum Dot Charge Stability Diagrams with Diffusion Models
Vinicius Hernandes, Joseph Rogers, Rouven Koch et al. (+5 more)
Efficiently characterizing
quantum dot (QD) devices is a critical bottleneck when scaling
quantum processors based on confined
spins. Measuring high-resolution charge stability diagrams (or CSDs, data maps which crucially define the occupation of QDs) is time-consuming, particularly in emerging architectures where CSDs must be acquired with remote sensors that cannot probe the charge of the relevant dots directly. In this work, we present a generative approach to accelerate acquisition by reconstructing full CSDs from sparse measurements, using a conditional
diffusion model. We evaluate our approach using two experimentally motivated masking strategies: uniform grid-based sampling, and line-cut sweeps. Our lightweight architecture, trained on approximately 9,000 examples, successfully reconstructs CSDs, maintaining key physically important features such as charge transition lines, from as little as 4% of the total measured data. We compare the approach to interpolation methods, which fail when the task involves reconstructing large unmeasured regions. Our results demonstrate that generative models can significantly reduce the characterization overhead for
quantum devices, and provides a robust path towards an experimental implementation.
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All authors Vinicius Hernandes, Joseph Rogers, Rouven Koch, Thomas Spriggs, Brennan Undseth, Anasua Chatterjee, Lieven M. K. Vandersypen, Eliska Greplova
Materials Science
Emergence of ferromagnetic state due to structural disorder in pseudo-binary Ce(Fe0.9Co0.1)2 compound
Andrzej Musiał, Maria Pugaczowa-Michalska, Natalia Lindner et al. (+1 more)
The changes in
magnetic properties of Ce(Fe0.9Co0.1)2 compound with increasing disorder are discussed in the paper. Homogeneous
alloys are known to undergo the phase transition from
ferromagnetic to
antiferromagnetic state accompanied by the structural distortion of the cubic Laves C15 phase into the rhombohedral one. Various stimuli, like the structural disorder, or applied
magnetic field, can force the emergence of ferromagnetism at low temperatures. We initially introduced the structural disorder using rapid quenching. Further changes were made by severe plastic deformation. The presence of a
ferromagnetic phase in a low-temperature region is reported here and accompanies the deterioration of a first-order phase transition. We show, based on electronic calculations, that the structural motifs arising from various distortions of the initial MgCu2-type structure, caused by the partial replacement of Fe with Co atoms, are characterized by stable
antiferromagnetic order. This neglects simple structural distortions as the source of ferromagnetism. The presence of a strongly defective structure understood as a topologically disordered volume, reduced the fraction transformed from a
ferromagnetic to an
antiferromagnetic state. Therefore, a strong reduction of isothermal
entropy changes was also observed, as it decreased from 1.94 Jkg-1K-1 and -1.43 Jkg-1K-1 ({\Delta}{\mu}0H = 4 T) to 0.30 Jkg-1K-1 and -0.96 Jkg-1K-1 for
antiferromagnetic-
ferromagnetic and
ferromagnetic-paramagnetic transition, respectively.
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All authors Andrzej Musiał, Maria Pugaczowa-Michalska, Natalia Lindner, Zbigniew Śniadecki
Strongly Correlated Electrons
Interstitial-Electron Altermagnetism in Two Dimensions
Xia Cheng, Yang Wu, Zhenzhou Guo et al. (+5 more)
Altermagnetism has so far been associated with compensated
magnetic moments carried by atoms. Here we introduce Stoner instability induced interstitial-electron altermagnetism, a distinct mechanism in which altermagnetic order is carried instead by interstitial anionic electrons in electrides. We show that, owing to the quasi-
nucleus-free nature of interstitial electrons, the Stoner instability in electrides hosting two interstitial electrons can naturally stabilize an altermagnetic state rather than the conventional
ferromagnetic one. This mechanism leads to a practical design principle for two-dimensional materials, from which we identify monolayers Zr2N and Ti2N as representative candidates. The strong sensitivity of interstitial electrons to cavity size enables efficient strain control of the altermagnetic order and a pronounced piezo-altermagnetic effect. Moreover, we investigate the
evolution of the magnetism in Zr2N under ultrafast
laser excitation, which exhibits dynamics distinct from those in all previously reported
magnetic materials where magnetism is carried by real atoms. Our work not only offers a novel pathway to realize altermagnetism but also reveals an efficient non-
magnetic route for its control.
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All authors Xia Cheng, Yang Wu, Zhenzhou Guo, Tie Yang, Weizhen Meng, Zhenxiang Cheng, Zhi-Ming Yu, Xiaotian Wang
Materials Science
Coherent Ultrafast Excitonic Oscillations in Monolayer WS$_2$
Jorge Cervantes-Villanueva, Alberto García-Cristóbal, Davide Sangalli et al. (+1 more)
Monolayer transition metal dichalcogenides are a suitable platform for studying excitonic coherence in the light-matter coupling regime. We present an ab initio time-dependent GW-Bethe-Salpeter equation (GW-BSE) investigation of coherent excitonic dynamics in monolayer WS$_2$. By solving the coherent coupling between the A, A$^{*}$, and B
excitons under linearly polarized pump fields, we identify the microscopic origin of the resulting oscillatory dynamics and rationalize it using an effective theoretical model. Our results provide the interpretation of recently reported coherent excitonic phenomena in monolayer WS$_2$ (Nano Lett. 24, 8117 (2024)). Building on this first-principles time-resolved framework, we propose a tailored pump-probe scheme that enables the controlled generation and regeneration of coherent oscillations between excitonic states. These findings establish a predictive route for controlling excitonic coherence in two-dimensional materials, with direct relevance for ultrafast optoelectronic switches and solid-state
quantum logic devices.
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All authors Jorge Cervantes-Villanueva, Alberto García-Cristóbal, Davide Sangalli, Alejandro Molina-Sánchez
Statistical Mechanics · Cross-listed from quant-ph
Conjugate measurements, equilibration and emergent classicality
S. Adarsh, P.N. Bala Subramanian and Sreeraj T. P
Simultaneous decoherence of conjugate observables of an open
quantum system leads to a classical statistical mechanical description with constant phase space probability density in terms of a uniform ensemble. We investigate a scenario where this may be realized by measurement of basic conjugate observables of a
quantum system by the environment.
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Materials Science
Strain-released epitaxy of GaN enabled by compliant single-crystalline metal foils
Yaqing Ma, Junwei Cao, Huaze Zhu et al. (+16 more)
Heteroepitaxy conventionally relies on rigid crystalline substrates, implicitly assuming that
lattice and thermal mismatch must be accommodated within the epitaxial layer, leading to residual strain and defects that worsen with increasing substrate size. Here we demonstrate a substrate-mediated strain-partitioning regime in which
lattice and thermal mismatch are preferentially partitioned into the substrate rather than stored in the epitaxial layer. We report the epitaxial growth of single-crystalline GaN on mechanically compliant yet crystallographically ordered single-crystalline copper foils. Atomic-resolution microscopy,
geometric phase analysis and density functional theory reveal that mismatch-induced stress is primarily screened by elastic deformation of the Cu
lattice, accompanied by localized interfacial slip confined to a few atomic layers, leaving the AlN and GaN epilayers nearly strain-free despite large nominal mismatch. Leveraging this strain-released epitaxial platform, we further demonstrate dense GaN micro-light-emitting diode arrays that benefit from efficient vertical electrical conduction and thermal dissipation enabled by the metallic substrate. By establishing compliant single-
crystal metal foils as a new substrate class, this work identifies mechanical contrast as an underexplored governing parameter in heteroepitaxial design, with implications extending beyond GaN.
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All authors Yaqing Ma, Junwei Cao, Huaze Zhu, Yijian Song, Huicong Chen, Menglin He, Jun Yang, Ping Jiang, Tong Jiang, Han Chen, Xiang Xu, Yuqiao Zheng, Hao Wang, Muhong Wu, Yu Zou, Xiaochuan Chen, Tongbo Wei, Kaihui Liu, Wei Kong
Mesoscale and Nanoscale Physics · Cross-listed from quant-ph
Majorana-XYZ subsystem code
Tobias Busse and Lauri Toikka
Building fault-tolerant
quantum computers is a current priority for the
quantum computing industry and academic counterparts. We present a
quantum error correction code encoding a single logical
qubit into $n=L^2$ physical
qubits with distance $d = L$ as a subsystem code, and $n-O(\sqrt{n})$ logical
qubits with distance $d=3$ as a standard stabiliser code. The logical single-
qubit gates in the subsystem code are transversal. Under independent $X,Z$ noise we find a threshold of $1.4\% \pm 0.1\%$ for odd distance and $1.8\% \pm 0.2\%$ for even distance for the subsystem variant. The code detects every 1- and 2-
qubit error, and additionally every error of weight 3 and higher (constrained by the distance) that is not a product of the 3-
qubit check operations. These operations act on the gauge
qubits leaving the subsystem code space invariant. The code can be implemented by
qubits in a triangular configuration with the plaquettes as 3-
qubit check operations.
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Strongly Correlated Electrons
Antiferromagnetic stripe phase and large-gap insulating ground state of the correlated $\sqrt{3}\times\sqrt{3}$~R30$^{\circ}$-Sn/Si(111) single atomic layer
Mohammadmehdi Torkzadeh, Mattia Iannetti, Mathieu Lizée et al. (+10 more)
The one-third monolayer Sn layer on Si(111) has long been considered a
benchmark system for exploring two-dimensional Mott physics, owing to its narrow bandwidth and sizable on-site Coulomb repulsion. Previous experiments suggested the emergence of a low-temperature Mott insulating phase with an energy gap of only a few tens of meV, while theory predicted a possible
antiferromagnetic ordering that remained experimentally elusive. Here, by combining low-temperature scanning tunneling microscopy/spectroscopy with first-principles calculations, we reveal that the $\sqrt{3}\times\sqrt{3}$~R30$^{\circ}$-Sn/Si(111) surface undergoes a transition below 30K into a robust insulating state characterized by a remarkably large gap of about 440 $\pm$ 120 meV at 4K, five to ten times larger than previously reported. Quasiparticle interference imaging uncovers a well-defined $2\sqrt{3}\times\sqrt{3}$~R30$^{\circ}$-Sn/Si(111) superstructure, providing direct evidence for a two-dimensional stripe-like
antiferromagnetic order. Ab initio calculations reveal that the silicon substrate stabilizes this phase through strong nonlocal tin-tin interactions, highlighting the decisive role of substrate-driven correlations in the $\sqrt{3}\times\sqrt{3}$~R30$^{\circ}$-Sn/Si(111) system.
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All authors Mohammadmehdi Torkzadeh, Mattia Iannetti, Mathieu Lizée, Amitayhush Thakur, Maris Hervé, Francois Debontridder, Pascal David, Michele Casula, Gianni Profeta, Tristan Cren, Matteo Calandra, Cesare Tresca, Christophe Brun
Materials Science · Cross-listed from physics.chem-ph
Microscopic Structure and Dynamics of Interfacial Water at Fluorinated vs Nonfluorinated Surfaces – Insights from Ab-Initio Simulations and IR Spectroscopy
Maximilian R. Becker, Ruben Cruz, Kenichi Ataka et al. (+2 more)
Per- and polyfluoroalkyl substances are a class of synthetic chemical compounds widely used as coatings to lower surface energies. Yet the microscopic mechanisms of their weak interaction with water and organic compounds remain poorly understood. Here, we perform large-scale density-functional-theory molecular dynamics simulations to investigate water at self-assembled monolayers (SAMs) of fluorinated and non-fluorinated hydrocarbons. We analyze the interfacial water structure and compare it to the prototypical hydrophobic air-water interface. The interfacial water structure at both SAMs closely resembles that at the air-water interface, featuring a distinct depletion layer and a two-dimensional hydrogen-bond network parallel to the surface. Computed anisotropic infrared spectra reproduce key experimental signatures observed in surface-enhanced infrared absorption spectroscopy (SEIRAS), including the presence of free OH vibrations directly probing the local surface-water interactions. Notably, while the free OH stretch at the hydrocarbon SAM-water interface exhibits a red shift relative to the air-water interface, indicative of weak binding, the fluorinated SAM-water interface displays a weakly blue-shifted free OH mode, in agreement with experiment. This frequency behavior defies common interpretations based on the vibrational Stark effect. Further, we show that the reorientation dynamics of water molecules are significantly slower near the fluorinated surface, which we correlate with spectral line shapes, an effect rather expected at hydrophilic surfaces. This indicates that fluorinated SAMs, despite being macroscopically more hydrophobic than their unfluorinated counterparts, exhibit spectroscopic characteristics that neither qualify it as hydrophobic nor hydrophilic.
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All authors Maximilian R. Becker, Ruben Cruz, Kenichi Ataka, Joachim Heberle, Roland R. Netz
Materials Science
Hunting Structural Demons in Digital Reticular Chemistry: Lessons from Metal-Organic Frameworks
Yongchul G. Chung and Myoung Soo Lah
Digital reticular chemistry relies on accurate
crystal structures to power computational screening, data-driven discovery, and structure-property analysis, yet recent studies reveal that more than half of the top-performing candidates in major computational screening campaigns are chemically invalid. In experimental MOF databases, structural errors arise when disordered or incomplete structural models are incorrectly converted into fully specified simulation inputs. In hypothetical MOF database, structures are complete by construction but may encode chemically implausible oxidation states, coordination environments, or charge distributions. We term these erroneous structural models "structural demons." This mini-review asks three questions: where these errors enter, how we find them, and how we prevent them. On the prevention side, the key steps are keeping diffraction data and synthesis details together from the start, using consistent curation when structures enter a database, and filtering
topology choices before structure generation. Connecting these steps can keep many bad structures out of downstream databases and reduce the need to fix them later.
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Materials Science
Ultrafast Formation and Annihilation of Strongly Bound, Anisotropic Excitons
Lawson T. Lloyd, Tommaso Pincelli, Mohamed Amine Wahada et al. (+12 more)
Van der Waals (vdW) layered materials with long-range
magnetic order have the potential to enable novel optoelectronic and spintronic applications. Among these, CrSBr is an air-stable, direct band gap
semiconductor that hosts interlayer
antiferromagnetic order, a highly anisotropic electronic structure, and strongly bound
excitons. In particular,
excitons in CrSBr have been shown to inherit the quasi-one-dimensional nature of the material and also couple to the underlying spinorder. However, mechanisms of
exciton formation, dissociation, and interaction with free carriers remain largely unexplored, despite being crucial for spintronic and optoelectronic applications. Here, we employ time- and angle-resolved photoemission spectroscopy to map the electronic structure and excited state dynamics in CrSBr. We directly resolve an exceptionally large
exciton binding energy (~800 meV) and a highly anisotropic momentum space distribution of the
exciton, revealing its quasi-1D real-space character. We observe an excitation-density-dependent interconversion between bound
excitons and quasi-free carriers on sub- to few-picosecond timescales, indicating that many-body effects govern the excited-state dynamics and optical properties during the initial stages of relaxation. Our work highlights the strongly bound, anisotropic character of
excitons in CrSBr, as well as the microscopic interactions steering relaxation pathways after photoexcitation in elevated density regimes relevant for future device applications.
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All authors Lawson T. Lloyd, Tommaso Pincelli, Mohamed Amine Wahada, Alessandro De Vita, Ferdinand Menzel, Kseniia Mosina, Túlio H. L. G. Castro, Alexander Neef, Andreas V. Stier, Nathan P. Wilson, Zdeněk Sofer, Jonathan J. Finley, Martin Wolf, Laurenz Rettig, Ralph Ernstorfer
Materials Science
Emergence of multiple quasi-ferromagnetic magnon modes induced by strong magnetoelastic coupling in $TmFeO_3$ single crystal
Sourabh Manna, Felix Fuhrmann, Sergio Rodríguez Fernández et al. (+8 more)
We investigate the magnetization dynamics of $TmFeO_3$ single
crystals across the
spin-reorientation phase transition using broadband microwave absorption spectroscopy up to 87.5 GHz. Temperature- and
magnetic-field-dependent
antiferromagnetic resonance measurements reveal the characteristic softening of the quasi-
ferromagnetic (q-FM) resonance mode at the $\Gamma_2\rightarrow\Gamma_{24}$ and $\Gamma_{24}\rightarrow\Gamma_4$ transition points. The finite
magnon gap observed at the transition points reflects the strong magnetoelastic coupling. In addition to the uniform q-FM mode, multiple
magnon modes appear in the intermediate $\Gamma_{24}$ phase, separated by approximately 0.5–2 GHz and exhibiting similar field and temperature dependence. These additional modes are attributed to nonuniform
spin-wave excitations arising from the periodic
magnetic domain structure present in the intermediate phase and their hybridization with acoustic
phonons mediated by strong magnetoelastic coupling. Our results demonstrate that the
spin-reorientation transition in $TmFeO_3$ provides a natural platform for generating multiple hybridized
magnon modes, offering new opportunities for tunable magnonic excitations in rare-earth orthoferrites.
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All authors Sourabh Manna, Felix Fuhrmann, Sergio Rodríguez Fernández, Xiaoxuan Ma, Haiyang Chen, Luca M. Carrella, Edgar Galindez-Ruales, Gerhard Jakob, Olena Gomonay, Shixun Cao, Mathias Kläui
Materials Science
Crystalline b-Ga2O3 thin films deposited via reactive magnetron sputtering of a liquid Ga target
Petr Novak, Jan Koloros, Stanislav Haviar et al. (+2 more)
Ga2O3 thin films were deposited by reactive magnetron sputtering from a
liquid gallium target. The influence of deposition temperature, substrate type, and discharge parameters on the structural and electrical properties was systematically investigated. Films deposited on silicon and quartz glass exhibit polycrystalline growth, whereas sapphire substrates enable highly oriented growth of b-Ga2O3 with a preferred (-201) orientation. The lowest electrical resistivity of 7x10_3 ohm.cm was obtained for films deposited on sapphire at a temperature of 585C. At this temperature, the films reach sufficient crystalline quality to enable efficient charge carrier transport and thus the manifestation of unintentional conductivity. At higher deposition temperatures, pronounced crystallization occurs; however, it is not homogeneous throughout the entire film thickness, which leads to a deterioration of the electrical properties. These results demonstrate that, despite intrinsic limitations, reactive magnetron sputtering can be successfully employed for the preparation of Ga2O3 thin films with optimized electrical properties when appropriate substrates and deposition temperatures are selected.
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All authors Petr Novak, Jan Koloros, Stanislav Haviar, Jiri Rezek, Pavel Baroch
Quantum Gases
Simulation of single-qubit gates in spin-orbit coupled Bose-Einstein condensate with cubic-quintic nonlinearity by nonlinear perturbations
Prithwish Ghosh, Kajal Krishna Dey and Golam Ali Sekh
We consider
spin-orbit coupled Bose-Einstein condensates with cubic-quintic nonlinear interaction within the framework of second quantization formulation and find eigen states using numerical simulation and mean-field approximation. We show that two low-lying Schrodinger cat states remain degenerate up to a certain value of Raman coupling strength and these states can serve as
qubit basis. We take three different nonlinear perturbations and find that the perturbations can result in different rotations of
qubit state on Bloch sphere. We calculate the
unitary operator corresponding to each perturbation and suggest the possibilities for obtaining various gates in ultra-cold atomic system.
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Mesoscale and Nanoscale Physics
Sign control of photocurrents by spin-group-symmetry breaking in altermagnetic insulators
Gastón Blatter, Xiao Zhang, Jeroen van den Brink et al. (+2 more)
Controlling physical responses through
symmetry breaking is a central paradigm in
quantum materials, enabling novel functionalities. Here we determine the effects of
spin-group-
symmetry breaking on nonlinear optical responses of collinear altermagnetic
insulators. Using shear strain as an example, we show that the direction of
symmetry-breaking induced components of charge and
spin photocurrents are locked to the sign of the strain. In the absence of
spin-orbit coupling, this effect is intuitively captured by the
spin-gap asymmetry–an imbalance between
spin-up and
spin-down direct band gaps which couples trilinearly with the Néel order and the strain. We demonstrate this mechanism with density functional theory calculations on the recently proposed altermagnet CuWP$_2$S$_6$. Having
symmetry-guided control of both charge and
spin photocurrents allows, vice versa, to reveal and investigate altermagnetism in insulating materials by exploration of their optical responses.
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All authors Gastón Blatter, Xiao Zhang, Jeroen van den Brink, Mengli Hu, Shu Zhang
Statistical Mechanics · Cross-listed from nlin.CD
Nonequilibrium ensemble averages using nonlinear response relations
Manuel Santos-Gutierrez, Valerio Lucarini, John Moroney et al. (+1 more)
The transient time correlation function (TTCF) method is widely used in molecular fluids to compute non-
equilibrium transport quantities, providing improved signal-to-noise ratios in ensemble averages without requiring prohibitively large sample sizes. In spite of its success in molecular and turbulent fluid systems, the method has not been systematically explored for more general non-
equilibrium dynamical systems, including geophysical applications where the invariant measure is typically unknown. In this work, we present an analytical and numerical investigation of the TTCF method for computing nonlinear response functions in systems far from
equilibrium. We discuss its relation to the spectral theory of
stochastic systems, highlighting regimes where linear theory is insufficient and the advantages of TTCF. The aim of this work is to provide a framework for studying transient and steady-state responses using the TTCF approach in a broad class of nonequilibrium systems.
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All authors Manuel Santos-Gutierrez, Valerio Lucarini, John Moroney, Niccolo Zagli
Mesoscale and Nanoscale Physics · Cross-listed from physics.optics
Linear Arrays of Metal-Coated Microspheres: a Polarization-Sensitive Hybrid Colloidal Plasmonic-Photonic Crystal
Cosmin Farcău
Colloidal plasmonic-photonic
crystals represent a class of hybrid materials composed of a dielectric colloidal spheres photonic
lattice and a metal plasmonic film. In this work, the optical properties of a linear array colloidal plasmonic-photonic
crystal consisting of silver films deposited over linear arrays of polystyrene microspheres are analysed in detail. Experimental and simulated optical transmittance and reflectance spectra both with unpolarized and polarized light are used to investigate the optical response of the linear plasmonic-photonic
crystal. Among the various photonic/plasmonic modes observed, the existence of both propagative plasmonic-photonic hybrid mode and localized surface plasmon mode can be mentioned. The spectral tunability of these structures is highlighted by studying the dependence of the optical response on geometrical parameters such as sphere diameter and grating period. Finally, the linear plasmonic-photonic
crystal exhibits a polarization-selective surface-enhanced Raman scattering effect, making them of interest for both fundamental studies and development of applications based on surface-enhanced Raman spectroscopy or surface-enhanced fluorescence.
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Mesoscale and Nanoscale Physics
Hinge modes of three-dimensional Euler insulators
Yutaro Tanaka and Shingo Kobayashi
In two-dimensional systems with space-time inversion
symmetry, such as $C_{2z}T$, the reality condition on wave functions gives rise to real band
topology characterized by the Euler class, a $\mathbb{Z}$-valued topological invariant for a pair of real bands in the Brillouin zone. In this paper, we study three-dimensional $C_{2z}T$-symmetric
insulators characterized by $\bar{e}_2$, defined as the difference in the Euler classes between two $C_{2z}T$-invariant planes in the three-dimensional Brillouin zone. By deriving effective surface
Hamiltonians from generic low-energy continuum
Hamiltonians characterized by the topological invariant $\bar{e}_2$, we reveal that multiple gapless boundary states exist at the domain walls of the surface mass, which give rise to the multiple
chiral hinge modes. We also show that three-dimensional
insulators characterized by $\bar{e}_2=N$ support $N$
chiral hinge modes. Notably, due to the constraint of two occupied bands in our system, these phases are distinct from stacked Chern
insulators composed of $N$ layers. Furthermore, we construct tight-binding models for $\bar{e}_2=2$ and $3$ and numerically demonstrate the emergence of two and three
chiral hinge modes, respectively. These results are consistent with those obtained from the surface theory.
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Mesoscale and Nanoscale Physics
On the interpretation of Hahn echo measurements in electron spin resonance scanning tunneling microscopy
Paul Greule, Wantong Huang, Máté Stark et al. (+5 more)
Electron
spin resonance scanning tunneling microscopy (ESR-STM) has become a powerful tool for probing
spin dynamics and coherence of individual atoms and molecules on surfaces. In this work, we perform Rabi oscillation and Hahn echo pulse protocols on individual iron phthalocyanine (FePc) molecules on MgO/Ag(001) using ESR-STM. While Hahn echo protocols are widely used to extract
spin coherence times, we show that in ESR-STM they are particularly susceptible to misinterpretation due to tunneling electrons generated by the applied
radio-frequency (RF) voltage. The RF voltage not only drives the
spin, but simultaneously probes and relaxes it, which consequently leads to an exponential decay that reflects
spin relaxation rather than intrinsic phase coherence. We moreover show that varying both delay times in the refocusing pulse sequence is a reliable way to ensure a coherent nature of the echo signal. The extracted decay for the latter protocol suggests that T2 is approximately 30 ns and is thus closer to the decoherence time observed in Rabi oscillation measurements. This is significantly shorter than values reported in previous echo measurements. Our findings underscore the need for caution in interpreting T2 times from Hahn echo and Carr-Purcell protocols in ESR-STM and provide practical criteria for distinguishing true
spin echoes from tunneling-induced relaxometry signals.
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All authors Paul Greule, Wantong Huang, Máté Stark, Kwan Ho Au-Yeung, Christoph Wolf, Soo-hyon Phark, Andreas J. Heinrich, Philip Willke
Materials Science
Probing deuterium-induced magnetic phase transitions in TbCo alloys with in-situ polarized neutron reflectometry
Robbie G. Hunt, Gunnar K. Pálsson, Matías P. Grassi et al. (+3 more)
Hydrogen-based magneto-ionics is a promising approach for rapid magnetoelectric control of spintronic devices. Most investigations so far into the magneto-ionic manipulation of rare-earth transition-metal
alloys have used electrochemical methods for evaluating the magnetoelectric properties, but this technique makes it difficult to discriminate between the effects of competing ionic species. In this work, we use atmospheric loading to evaluate the effect of an isotope of hydrogen, deuterium, on the
magnetic properties of TbCo films using in-situ polarized
neutron reflectometry. With this approach, we are able to simultaneously measure the magnetization, thickness expansion and deuterium concentration of TbCo films. We quantitatively observe the deuterium concentrations at which the paramagnetic phase transition occurs for a Tb-rich film, and the weakening of out-of-plane
magnetic anisotropy for a Co-rich film. For the Tb-rich film the expansion of the film thickness is the primary mechanism identified for the paramagnetic phase transition, while for the Co-rich film no thickness expansion is observed. We also find that an oxidized interface is insensitive to deuterium loading, but remains exchange coupled to the rest of the film and can be indirectly manipulated by loading of deuterium in the
alloy. We expect these results to be directly translatable to that of hydrogen.
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All authors Robbie G. Hunt, Gunnar K. Pálsson, Matías P. Grassi, Victoria Kabanova, Alexey Vorobiev, Gabriella Andersson
Mesoscale and Nanoscale Physics · Cross-listed from physics.optics
Metal-coated microsphere monolayers as surface plasmon resonance sensors operating in both transmission and reflection modes
Cosmin Farcău
Metal-coated microsphere monolayers (MCM) are a class of plasmonic
crystals consisting of noble metal films over arrays of self-assembled colloidal microspheres. Despite their ease of fabrication and tunable plasmonic response, their optical sensing potential has been scarcely explored. Here, silver coated polystyrene sphere monolayers are proposed as surface plasmon resonance sensors capable of functioning in both transmission (T) and reflection (R) readout modes. An original and key point is the use of ~200 nm colloids, smaller than in MCM studied before. It allowed us to reveal a previously unobserved, additional/secondary Enhanced Optical Transmission band, which can be exploited in sensing, with higher sensitivity than the better-known main transmission band. The reflection configuration however, is almost an order of magnitude more efficient for sensing than the transmission one. We also evidenced a strong impact of the adsorbate location on the metal surface on the sensing efficiency. Electric field distribution analysis is performed to explain these results. Proof-of-concept experiments on the detection of 11-MUA molecular monolayers, performed in both readout modes, confirm the behaviors observed through FDTD simulations. Results in this paper can serve as guidelines for designing optimized sensors based on metal-coated colloidal monolayers, and more generally for plasmonic sensors based on metal nanostructured films.
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Materials Science
Photoinduced strain and polarization switching in barium titanate in the far-infrared spectral range
Maarten Kwaaitaal, Daniel Lourens, Carl S. Davies et al. (+1 more)
Short mid-infrared
laser pulses efficiently facilitate ultrafast manipulation of ferroic order parameters, including full reversal of magnetization or ferroelectric polarization, with the invoked mechanisms relating to the properties of polar
phonons in ionic
crystals. Much less is known, however, about the behaviour of such order parameters in response to an excitation in the far-infrared range, where
phonons are more collective and less polar. Here we investigate transient crystallographic strains and polarization switching in ferroelectric barium titanate (BaTiO3) driven by an excitation in the frequency range of 5-8 THz, or wavelengths of 35-60 um. We find that switching persists in a large part of this range, but is governed primarily by optical absorption rather than by the longitudinal optical
phonons or epsilon-near-zero conditions that dominate in the mid-infrared regime.
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All authors Maarten Kwaaitaal, Daniel Lourens, Carl S. Davies, Andrei Kirilyuk
Materials Science
The Unreconstructed {\alpha}-Al$_{2}$O$_{3}$(0001) Surface is Inhomogeneous and Rough
Johanna I. Hütner-Reisch, Andrea Conti, David Kugler et al. (+4 more)
Alumina (Al$_{2}$O$_{3}$) is a key material for thin-film growth and heterogeneous catalysis, where the atomic surface structure critically impacts performance. Using noncontact atomic force microscopy (nc-AFM) combined with density functional theory (DFT) calculations, we challenge the common assumption that the unreconstructed $\alpha$-Al$_{2}$O$_{3}$(0001) surface is atomically flat and uniformly Al-terminated. This widely accepted bulk termination satisfies polarity compensation requirements but results in highly undercoordinated surface Al cations at the surface. Despite substantial inward relaxation of these Al cations, we find that the (1 ${\times}$ 1) surface remains inherently metastable, relative to the thermodynamically stable $(\sqrt{31} \times \sqrt{31})R\pm9\deg$ surface reconstruction that forms at high temperatures above 1000 {\deg}C. Nc-AFM imaging of the unreconstructed surface reveals a rough and disordered morphology, with only nanometer-scale regions exhibiting the ordered Al-terminated (1 $\times$ 1) structure. Our results show that the unreconstructed Al$_{2}$O$_{3}$(0001) surface is intrinsically inhomogeneous, reconciling conflicting experimental observations and challenging the validity of commonly used atomistic models.
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All authors Johanna I. Hütner-Reisch, Andrea Conti, David Kugler, Florian Mittendorfer, Michael Schmid, Ulrike Diebold, Jan Balajka
Materials Science
A Sc2C2@C88 cluster based ultra-compact multi-level probabilistic bit for matrix multiplication
Haoran Qi, Guohao Xi, Yuan-Biao Zhou et al. (+19 more)
Information units are progressively approaching the fundamental physical limits of the integration density, including in terms of extremely small sizes, multistates and probabilistic traversal. However, simultaneously encompassing all of these characteristics in a unit remains elusive. Here, via real-time in situ electrical monitoring, we clearly observed
stochastic alterations of multiple conductance states in Sc2C2@C88. The true random bit sequence generated exhibited an autocorrelation function whose confidence interval fell within \pm 0.02, demonstrating high-quality randomness. The alterations of multiple conductance states are controllable, that is, whose probability distributions could traverse from 0 to 1, enabling us to factorize 551 into its
prime factors. Furthermore, we proposed a
matrix-chain multiplication scheme and experimentally verified the multiplication of two 4 \times 4 state-transition
matrices with a small maximum error < 0.05. Combined with theoretical calculations, the
stochastic but controllable multistates are probably attributed to the rich energy landscape, which could be stepwise changed by the electric field. Our findings reveal extremely small multi-level probabilistic bit for
matrix multiplication, which pave the way for ultracompact intelligent electronic devices.
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All authors Haoran Qi, Guohao Xi, Yuan-Biao Zhou, Xinrong Liu, Yifu Mao, Jian Yang, Jun Chen, Kuojuei Hu, Weiwei Gao, Shuai Zhang, Xiaoqin Gao, Jianguo Wan, Da-Wei Zhou, Junhong An, Xuefeng Wang, De-Chuan Zhan, Minhao Zhang, Cong Wang, Wei ji, Yuan-Zhi Tan, Su-Yuan Xie, Fengqi Song
Disordered Systems and Neural Networks
Multifractal Analysis of the Non-Hermitian Skin Effect: From Many-Body to Tree Models
Shu Hamanaka
The non-Hermitian skin effect is an anomalous localization phenomenon induced by nonreciprocal dissipation and has attracted considerable
attention in recent years both theoretically and experimentally. In this article, we review the multifractal aspects of the non-Hermitian skin effect. In particular, we discuss how the many-body skin effect exhibits multifractality in many-body
Hilbert space, unlike the trivial
Hilbert-space occupation of the single-particle skin effect on crystalline
lattices. We further highlight that the many-body skin effect can coexist with random-
matrix spectral statistics, in contrast to the multifractality associated with many-body localization, which typically accompanies the absence of ergodicity. We also introduce a solvable model on a Cayley tree as an effective description of the many-body
Hilbert space, in which the multifractal dimensions can be obtained analytically. This review provides a unified perspective on multifractal structures in the non-Hermitian skin effect across single-particle, many-body, and tree models, and clarifies their distinctive relation to ergodicity in open
quantum systems.
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