Materials Science
Philipp Rieger, Markus Weißenhofer, Sergiy Mankovsky et al. (+1 more)
Chiral phonons are attractive for spintronics applications, however, their zero-field generation in conventional antiferromagnets is forbidden by combined parity and time-reversal ($\mathcal{PT}$)
symmetry. Here we demonstrate the emergence of
chiral phonons in $\mathcal{PT}$-breaking altermagnetic systems at zero field arising from relativistic
magnon-
phonon coupling. Focusing on the prototypical altermagnet CrSb, we utilize first-principles methods to calculate the hybridized
magnon-polarons across the complete Brillouin zone. We show that this coupling imprints an altermagnetic $g$-wave
symmetry directly onto the
phonon angular momentum. Furthermore, we demonstrate anomalous
spin and
phonon angular momentum Nernst responses arising from finite Berry
curvatures. These findings establish that
chiral lattice dynamics can arise in compensated
magnetic ground states without requiring external fields, positioning bulk altermagnets as material candidates for zero-field
spin caloritronics and
chiral phononics.
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All authors Philipp Rieger, Markus Weißenhofer, Sergiy Mankovsky, Peter M. Oppeneer
Materials Science
Topological charge, helicity and vorticity conservation and the reverse spin-current model in the II-nd type multifferoics
Pavel A. Andreev
The topological charge and its density are related to the
spin vorticity, while the
spin vorticity is a part of full vorticity of the medium. The full vorticity is related to an integral of motion called the hydrodynamic helicity. Relation between two integrals of motion (the topological charge and the hydrodynamic helicity) is traced. The role of the
spin-current model of the electric polarization formation due to the specific
spin distribution in the helicity conservation is demonstrated. The reverse
spin-current model is suggested to demonstrate the contribution of the polarization in the
spin evolution equation, its importance for the helicity conservation is shown as well.
Spin-field model of the electric polarization is the second model for the electric polarization (the deformation, firstly) formation due to the specific
spin distribution, which appears from the same principles, but it has nonstationary origin. Moreover, the reverse
spin-current model gives a mechanism for the
spin structure formation due to the electric polarization in the system. It also appears as the requirement for the conservation of the helicity and the topological charge.
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Materials Science
First-Principles Investigation of the Al-V Phase Diagram
AKM Sadman Mahmud, Hassan Albuhairan, Marek Mihalkovic et al. (+1 more)
The Al-V
alloy system contains a number of phases including several with complex structures and at least two exhibiting sites of partial occupation. Through electronic density functional theory-based total energy calculations combined with methods of statistical mechanics, we examine the relative stability of phases at finite temperatures. We construct composition-continuous free energy models for the V-rich solid solution and for one of the complex intermetallic phases. In the V-rich region, we identify three ground states that transform to the solid solution at elevated temperatures. We also suggest that \phase{Al}{V_3} takes the Al15 structure as an intermediate-temperature phase stabilized by anharmonic vibrational free energy.
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All authors AKM Sadman Mahmud, Hassan Albuhairan, Marek Mihalkovic, Michael Widom
Materials Science
Spin-orbit magnetism in altermagnets
Ruojia Wang, Yuntian Liu, Renzheng Xiong et al. (+2 more)
While the anomalous
Hall effect was proposed as a transport fingerprint of altermagnets, it originates from the
spin-orbit coupling-induced net magnetization, corresponding to a distinct
magnetic phase termed
spin-orbit magnetism. However, the microscopic mechanism enabling
spin-orbit magnets to yield a prominent anomalous
Hall response despite a vanishingly small net magnetization remains elusive. Here, by employing oriented
spin group theory and
spin-orbit-coupling tensor expansion, we systematically disentangle the perturbative behaviors of orbital and
spin magnetizations with respect to
spin-orbit coupling in altermagnets. Remarkably, we find that only if the opposite-
spin sublattices are connected through a fourfold rotation, the orbital and
spin magnetizations exhibit distinct perturbative orders. In these altermagnets, we further discover a coaxial
Hall effect characterized by the induced
spin and orbital magnetizations aligning parallel to the Néel vector, which we further demonstrate by first-principles calculations in the altermagnet KV$_{2}$Se$_{2}$O. This effect holds great promise for achieving deterministic switching of the Néel order under weak external fields. Our work provides a systematic
symmetry approach to identify potential altermagnetic candidates combining a large anomalous
Hall effect with minimal net magnetization, paving the way for high-performance, stray-field-free spintronic applications.
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All authors Ruojia Wang, Yuntian Liu, Renzheng Xiong, Xiaobing Chen, Qihang Liu
Materials Science
Physical exact conditions as regularizers for exchange-correlation in solids and surface chemistry
Johannes Voss
Density functional theory (DFT) often is the method of choice for simulating the electronic properties of extended solids and surfaces from first principles due to a favorable compromise between accuracy and computational cost. In the field of heterogeneous catalysis, DFT is indispensable for deriving mechanistic insights and understanding trends in surface reactivity. The accuracy of DFT for surface reaction energetics depends strongly on the exchange-correlation (XC) approximation. We show here that optimization of such XC functionals for surface binding energies can lead to a worse description of surface reaction barriers, unless important physical exact conditions are fulfilled.
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Materials Science · Cross-listed from cond-mat.mes-hall
Moiré Phonon Condensation in Magic-Angle Twisted Bilayer Graphene
Zhanghao Zhouyin, Jyun-jie Jiang, Xianghua Kong et al. (+1 more)
Twisted
bilayer graphene reconstructs from weak breathing corrugation to large common bending near the magic angle, but the origin of this collective crossover has remained unclear. Here we show that the crossover is a soft-mode condensation of layer-symmetric $A_1$ moiré flexural
phonons: these modes soften on the breathing branch, lose stiffness near the magic angle, and freeze into the bending morphology. We call this mechanism Moiré
Phonon Condensation (MPC). At $\theta=1.08^\circ$, it is extremely surprising that displacements of all 11164 atoms in the moiré supercell, with a maximum atomic position shift of 2.30 Angstrom, is captured by only two $A_1$
phonon modes at more than $99.5\%$ spectral weight. A first-harmonic continuum theory identifies a dimensionless control parameter of the phenomenon, showing that as the twist approaches the magic angle, the growing moiré length scale amplifies a smooth stress-bending competition until the flexural stiffness changes sign. Mode-resolved tight-binding calculations further show that the condensed
phonon coordinates are electronically active. This work identifies MPC as a twist-controlled structural order parameter for moiré reconstruction.
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All authors Zhanghao Zhouyin, Jyun-jie Jiang, Xianghua Kong, Hong Guo
Materials Science
Fluctuation-Driven Enhancement of Spin-Orbit Torque near the Curie Temperature of Ultrathin Ferromagnets
Mateusz Szurek, Sergei Ivanov and Sergei Urazhdin
We investigate how
magnetic fluctuations influence
spin-orbit torque in ultrathin-film
magnetic heterostructures whose Curie temperature $T_C$ is suppressed by confinement. Above $T_C$, the damping-like contribution to
spin-orbit field is significantly enhanced while the field-like contribution is suppressed, with the two contributions exhibiting opposite field dependencies. We show that these behaviors are consistent with fluctuation driven mixing between the longitudinal and transverse interfacial
spin conductances, which enhances absorption of transversely polarized
spin current by the ferromagnet. This mechanism can be activated below $T_C$ by engineering the microscopic
magnetic state and by harnessing
spin current-generated short-wavelength
magnons, suggesting a spintronic analog of heat-assisted
magnetic recording.
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Materials Science
Origin of the reaction temperature in solid-state materials synthesis
Shibo Tan, Gabrielle E. Kamm, Alex Stangel et al. (+12 more)
Temperature plays a crucial role in solid-state materials synthesis, but there is currently no mechanistic theory to explain or predict which temperature is best to conduct a solid-state reaction. Reactions between powder precursors are conventionally assumed to be slow
diffusion-limited processes; however, recent in situ experiments show that solid-state reactions can complete in minutes above a critical onset temperature. Here, we present evidence that a transient
liquid phase forms above the metastable eutectic temperature, and that this non-
equilibrium liquid serves as a fast
diffusion medium to intermix precursors and initiate a solid-state reaction. This thermodynamic principle is agnostic to the structure or chemistry of the reactants, and can be applied towards the synthesis and manufacturing of a wide range of complex materials.
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All authors Shibo Tan, Gabrielle E. Kamm, Alex Stangel, Paul Chao, Alexander Mensah, Varun Srinivas Venkatesh, Eymana Maria, Nishkarsh Agarwal, Woohyeon Baek, John Ferrari, Katsuyo Thornton, Ashwin Shahani, Robert Hovden, Karena W. Chapman, Wenhao Sun
Materials Science
BatteryMat: a hierarchical machine-learning and DFT framework for average-voltage screening of lithium-ion cathode materials
Jaehyung Lee, Charles Rhys Campbell, Kent Zhang et al. (+1 more)
Density functional theory (DFT) predicts cathode voltages accurately but does not scale to the combinatorial chemical spaces of modern materials databases, while pure machine-learning surrogates are fast but cannot guarantee thermodynamic consistency. We introduce BatteryMat, a three-tier framework that promotes single-pass average-voltage prediction with the Atomistic Line Graph Neural Network (ALIGNN) as the primary screening signal across JARVIS-DFT, then validates survivors with ALIGNN-FF force-field delithiation profiles and automated PBE+U or optB88-vdW+U supercell DFT. The exchange-correlation functional is selected automatically by spacegroup, and the lithium metal reference is recomputed in the same plane-wave basis as the cathode runs, removing a systematic offset of about 1 V present in tabulated values. Trained on 7,610 ALIGNN-FF delithiation voltages, the ALIGNN predictor reproduces the force-field labels with a mean absolute error of 0.17 V and a coefficient of determination of 0.94; this measures
distillation fidelity to the force-field protocol, not agreement with DFT or experiment. On four commercial chemistries (LiFePO4, LiMnPO4, LiMn2O4, LiCoO2) the DFT tier reproduces the experimental average voltage to within 0.3 V and the theoretical volumetric capacity to within 5%; a fifth, non-stoichiometric layered entry is carried as an edge case. The pipeline prioritises, rather than generates, existing structures: it ranks the lithium-containing JARVIS-DFT pool into 71 candidates and a scan of about 4.49 million Alexandria structures into 213, all surrogate-level leads awaiting DFT validation rather than confirmed cathodes. BatteryMat is available at https://github.com/atomgptlab/batterymat with a demo at https://atomgpt.org/battery.
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All authors Jaehyung Lee, Charles Rhys Campbell, Kent Zhang, Kamal Choudhary
Materials Science · Cross-listed from cond-mat.mes-hall
2D Transport in an in-plane magnetic field
Aryaman Babbar and Sankar Das Sarma
A parallel in-plane
magnetic field could, in principle, distinguish between two competing physical scenarios for the experimentally observed density-tuned 2D metal-
insulator transition (where decreasing the carrier density leads to a crossover from an effective metal to an effective
insulator): Wigner crystallization or Anderson localization. Since the main scattering mechanism in 2D doped
semiconductors arises from screened random charged
impurities and screening in turn depends on the electronic density of states, the in-plane
magnetic field could distinguish between the two by decreasing screening through
spin polarization and this enhances the effective critical density for Anderson localization compared with Wigner crystallization. We give the general theory and provide results for the quantitative magnitudes of the
spin polarization effect on the transition density by focusing on two recent experiments [Z. Ge, et al, arXiv:2510.12009, T. Han, et al, arXiv:2604.00113], noting that the critical density may actually decrease if the dominant scattering is by short-ranged defects instead of long-ranged charged
impurities. The difference between the two cases arises from whether
spin polarization dominates screening (enhanced critical density) or the
Fermi surface (suppressed critical density).
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Materials Science
Phonon-Mediated Thermal Transport in Nanocrystalline Silicon Using Machine-Learning Interatomic Potentials
Houssem Rezgui, Catalina Coll Benejam, Clivia M. Sotomayor Torres et al. (+1 more)
Understanding
phonon-mediated heat transport in structurally complex materials remains a central challenge for next-generation electronic and nanomechanical devices, where grain boundaries and interfacial disorder strongly limit thermal dissipation. Although classical interatomic potentials enable large-scale simulations, their limited transferability can lead to inaccuracies in vibrational properties and interfacial
phonon scattering. In this work, we develop a machine learning-based framework for modeling thermal transport in bulk and nanocrystalline silicon by combining Gaussian approximation potential and multi-atomic cluster expansion models with
lattice-dynamical calculations and molecular dynamics. Harmonic and anharmonic force constants derived from machine-learning interatomic potentials (MLIPs) are used within a unified Phonopy/Phono3py workflow to compute
phonon dispersions, lifetimes, and
lattice thermal conductivity, providing an internally consistent description of vibrational properties. In nanocrystalline silicon, non-
equilibrium molecular dynamics simulations directly quantify the thermal boundary resistance associated with grain boundaries and reveal its sensitivity to interfacial roughness and the underlying interatomic description. Compared with the Stillinger-Weber and Tersoff potentials, the MLIPs provide a quantitatively accurate and internally consistent description of bulk and interfacial
phonon transport, enabling better predictive modeling of nanoscale thermal transport in low-dimensional materials.
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All authors Houssem Rezgui, Catalina Coll Benejam, Clivia M. Sotomayor Torres, Miguel Pruneda
Materials Science · Cross-listed from physics.optics
Terahertz-driven four-wave mixing at glass surfaces: Probing vibrational resonances and structural regimes
Mathias Hedegaard Kristensen, Jérôme Degert, Theo Guillaume et al. (+3 more)
Disordered materials such as glasses exhibit complex structural dynamics that are challenging to probe with conventional spectroscopies. We demonstrate that terahertz-driven four-wave mixing (FWM) at glass surfaces provides direct access to low-frequency vibrational modes and structural
evolution in amorphous solids. Applied to a compositional series of PbO-silicate glasses (20-54 mol% PbO), this technique resolves distinct contributions from collective
Boson-peak excitations and Pb-O / Si-O network stretching modes, and tracks their systematic
evolution across structurally distinct compositional regimes. The dominant vibrational frequency blueshifts with PbO content, reflecting the progressive
evolution of the Pb$^{2+}$ network role from silicate-modifier to ward network-former. A pronounced enhancement of the FWM signal near 44 mol% PbO coincides with the emergence of medium-range Pb-Pb correlations, while in-plane-to-out-of-plane FWM intensity ratio ($I_{\rm SS}/I_{\rm PS}$) tracks $\chi^{(3)}$ tensor anisotropy tied to Pb$^{2+}$ lone-pair spatial correlations. The non-monotonic peak in both observables at 44 mol% PbO - a composition where NMR finds no change in local Pb-O coordination and Pb-O-Pb free-
oxide linkages are negligible - provides direct evidence that a collective lone-pair reorganization occurs in the medium-range structure independently of nearest-neighbor bonding. These results establish terahertz-driven FWM as a bulk-sensitive, near-surface depth-confined ($\sim$50 nm) nonlinear spectroscopy sensitive to vibrational and electronic structural fingerprints inaccessible to linear infrared, Raman, and terahertz time-domain probes.
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All authors Mathias Hedegaard Kristensen, Jérôme Degert, Theo Guillaume, Emmanuel Abraham, Laetitia Dalstein, Eric Freysz
Materials Science · Cross-listed from physics.chem-ph
Economised path integrals
Zezhu Zeng and David E. Manolopoulos
The Hessian of the ring
polymer spring potential in the standard Trotter path integral is a $P\times P$ symmetric circulant
matrix with a centroid
eigenvalue of zero. All such
matrices commute and are diagonalised by the same bead to normal mode transformation
matrix, and their
eigenvalues contain $\lceil P/2\rceil-1$ degenerate pairs by
symmetry. However, this still leaves some freedom to improve on the Trotter approximation: one can optimise the remaining $\lfloor P/2\rfloor$ independent non-zero normal mode frequencies to fit the exact
quantum mechanical radii of gyration of harmonic ring
polymers with frequencies in the range $0\le\omega\le\omega_{\rm max}$, where $\omega_{\rm max}$ is the maximum physical frequency in the problem of interest. The optimisation involves solving a simple least squares problem for the optimum (economised or "Eco") internal mode frequencies. The remainder of the calculation then proceeds in the same way as a Trotter path integral calculation. An example application to hexagonal ice shows that the convergence of the Eco path integral is comparable to that of the 4th order Suzuki-Chin path integral, but with purely 2nd order Trotter effort. There is no need to calculate the projected Hessians that arise in the Suzuki-Chin method by finite differences, there is no need to develop any new
estimators for observables, and once the Eco frequencies have been calculated the implementation of the Eco path integral involves changing just a few lines of a Trotter path integral code. To provide a more impressive example we have implemented the Eco method in GPUMD and used it to converge the (negative) thermal expansion coefficient and the constant pressure heat capacity of MOF-5 with a machine-learned neuroevolution potential.
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Materials Science
Piezoaxial coupling for strain-selected ferroaxial domain control
Rikuto Oiwa and Satoru Hayami
We formulate a
symmetry-based hierarchy of strain-derived conjugate fields for ferroaxial order, and demonstrate strain-selected ferroaxial domain control using first-principles calculations. Since ferroaxial order is even under both spatial inversion and time reversal, ordinary electric and
magnetic fields cannot serve as universal linear conjugate fields. Homogeneous strain, however, can generate
symmetry-allowed piezoaxial fields whose leading order is determined by the parent point group and by the chosen ferroaxial-axis component. For basal-plane strain, the leading field is linear in orthorhombic systems, quadratic in tetragonal systems, and cubic in trigonal and hexagonal systems. Cubic parent groups further split into two classes: cubic-I groups, $23$ and $m\bar{3}$, allow linear full-strain fields for selected axes, whereas cubic-II groups, $432$, $\bar{4}3m$, and $m\bar{3}m$, forbid linear fields and require quadratic or cubic strain combinations depending on the selected axis. In trigonal systems, the basal-plane deviatoric strain with signed amplitude $\varepsilon_{\rm u}$ and principal-axis angle $\theta$ gives the single-axis field $h\propto\varepsilon_{\rm u}^3\sin6\theta$. First-principles calculations for the trigonal ferroaxial compound Na$_2$BaMg(PO$_4$)$_2$ verify both the predicted angular dependence and cubic strain scaling of the ferroaxial domain splitting, and fixed-strain atomic relaxations show strain-selected
evolution from the para-axial structure. These results establish static homogeneous strain as a
symmetry-allowed conjugate field for ferroaxial order and suggest a route to ferroaxial domain control through strain-field cooling.
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Materials Science
On the capacitance gradient description in Heterodyne Kelvin Probe Force Microscopy
Hugo Valloire, Sylvain Clair, Christian Loppacher et al. (+2 more)
Kelvin probe force microscopy (KPFM) probes local surface-potential variations through the electrostatic force between a conductive tip and a surface, which depends on the potential difference and the tip-surface capacitance gradient (CG). In heterodyne KPFM, the oscillating tip is usually treated by combining a bias-modulated electric field with a first-order truncated Taylor-series expansion of the CG. Although convenient, this treatment is limited to a poorly defined small-amplitude regime and leaves the convergence of the series unresolved. Here, we establish a rigorous spectral description of the CG dynamics and of the resulting electrostatic force beyond this approximation. We formulate a non-truncated Taylor-series description of the CG and prove its convergence for a realistic Hudlet-based capacitance model in both monomodal and bimodal motion. In the monomodal case, we show the equivalence between
Fourier-series and Taylor-series descriptions, derive explicit expressions for the dominant
Fourier coefficients, and introduce order-truncation criteria that replace the usual qualitative notion of a small-amplitude regime. We then extend the formalism to bimodal motion and derive the effective CG coefficients governing the static, first-eigenmode, and second-eigenmode components of the electrostatic interaction. Numerical simulations confirm the convergence of the Taylor-based coefficients toward the
Fourier coefficients and support the truncation-regime hierarchy in both configurations. This work establishes the formal basis for describing electrostatic force components and AFM observables in open-loop heterodyne experiments and provides a general framework for CG dynamics in multimode force microscopy involving nonlinear electromechanical coupling and frequency conversion.
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All authors Hugo Valloire, Sylvain Clair, Christian Loppacher, Laurent Nony, Benjamin Grévin
Materials Science
Inverse heterodyne effect in bimodal Kelvin probe force microscopy
Hugo Valloire, Sylvain Clair, Christian Loppacher et al. (+2 more)
Heterodyne Kelvin probe force microscopy (He-KPFM) enables high-sensitivity electrostatic measurements by converting a bias-modulated interaction into a resonant response at a higher cantilever eigenmode. While the "direct" heterodyne actuation of the second eigenmode is well established, the dynamical back-action of this heterodyne-driven motion on the fundamental eigenmode has remained largely unexplored, particularly in open-loop operation where the second mode is excited to a finite amplitude. Here, we demonstrate an inverse heterodyne effect: a force component generated by heterodyne frequency conversion acts back on the first eigenmode and produces measurable inter-mode energy exchange. The analysis combines a bimodal virial and power-balance framework with a non-truncated description of the tip-surface capacitance-gradient dynamics developed and validated in a companion manuscript submitted concurrently to the same journal. On this basis, we derive closed-form expressions linking inverse heterodyne coupling to the experimentally accessible observables of non-contact AFM open-loop amplitude-modulated He-KPFM. The theory predicts that inverse heterodyne coupling appears predominantly in the dissipation channel, with a sharply resonant dependence on the demodulation frequency near the second-eigenmode resonance, while its conservative contribution to the frequency shift remains comparatively weaker under typical conditions. Ultrahigh-
vacuum experiments validate these predictions and isolate the inverse heterodyne signature through frequency- and voltage-dependent measurements. Beyond KPFM, this work connects heterodyne force microscopy to a broader class of driven multimode systems in which nonlinear coupling and frequency conversion produce inter-mode energy transfer, back-action, and dissipation-based observables.
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All authors Hugo Valloire, Sylvain Clair, Christian Loppacher, Laurent Nony, Benjamin Grévin
Materials Science
Multiscale modelling of diffusion and retention of hydrogen in multi-occupancy traps in irradiated bcc metals
Daniel Mason, Sanjeet Kaur, Samanyu Tirumala et al. (+3 more)
We use molecular dynamics simulations to directly compute the effective diffusivity of hydrogen gas atoms in homogeneous distributions of monovacancies in tungsten and vanadium, and voids in tungsten. Rather than fitting the results to an Arrhenius law, we compare to an analytic approximation for the effective diffusivity recently derived for multi-occupancy traps [Kaur et al (2025), Phys. Rev. Mater. 9:125404]. We find good agreement between full atomistic simulation and our theory, validating the analytic model for diffusivity for materials containing nanoscale defects characteristic of radiation damage. There are no parameters fitted, only physically motivated quantities that can be computed with static density functional or atomistic potential calculations. In this study we prove rapid convergence of hydrogen trap occupation to the steady state using
lattice kinetic Monte Carlo, the spontaneous emergence of voids in tungsten using atomistic simulation with empirical potentials, and molecular hydrogen formation in voids using molecular dynamics. We conclude with a prediction for
diffusion and retention of hydrogen in voids in tungsten starting from first principles. This work shows that not only is the analytic form for diffusivity and retention in multi-occupancy traps a practical scheme for making predictive simulations of hydrogen isotope
diffusion and retention in irradiated microstructures, derived and parameterized from first principles, it is superior to existing single-occupancy trap formalisms.
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All authors Daniel Mason, Sanjeet Kaur, Samanyu Tirumala, Prashanth Srinivasan, Ville Jantunen, Max Boleininger
Materials Science
Stabilization of Stone-Wales Defects in Metal-supported Graphene
Rob H. Mason, Manuka M. S. Sinharage, Hansika I. Sirikumara et al. (+2 more)
The characteristics of
graphene-metal interfaces play a decisive role in their electronic, optoelectronic, and mechanical applications. Properties such as charge transfer across the interface become particularly significant in the presence of topological defects. The stability of Stone Wales (SW) defects in
graphene is governed by the balance between three energy descriptors, the activation energy, formation energy, and restoration energy. By comparing the energy parameters obtained from first-principles density functional theory calculations, we show that SW defect formation is energetically more favorable on metal-supported
graphene. Our calculations for SW defects in
graphene/Cu(111) and
graphene/Al(111) systems indicate only a little dependence of energy profile on the type of metal. The presence of the metal substrate leads to a $\sim$ 12% increase in the formation energy and a $\sim$ 20% reduction in the activation energy, which together favor the formation of Stone Wales defects. Although the restoration energy decreases by about $\sim$ 35% in metal-supported
graphene, it remains significantly higher to prevent self-healing. As a result, once formed, the Stone Wales defects are likely to remain stable, suggesting the possibility of terminal SW defect formation in metal-supported
graphene.
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All authors Rob H. Mason, Manuka M. S. Sinharage, Hansika I. Sirikumara, Sabrina Nilufar, Thushari Jayasekera
Materials Science
Deep-learning Hamiltonian reveals twist-tunable flat bands and nonlinear photocurrents in SrTiO3 moire bilayers
Meiyang Yu, Chen Shen, Ruiwen Xie et al. (+3 more)
The extension of moire physics to complex
oxides offers new ways to manipulate electronic states, but the large
oxide moire supercells make systematic first-principles calculations demanding. Here, we combine density functional theory with the E(3)-equivariant deep-learning
Hamiltonian framework DeepH-E3 to investigate the twist-angle-dependent electronic structure and optical responses of twisted
bilayer SrTiO3. The model is trained on untwisted
bilayers with different interlayer-sliding configurations and then applied to commensurate twisted
bilayers with twist angles from 8.80 degrees to 53.13 degrees. Compared with the untwisted
bilayer, decreasing twist angle systematically flattens the valence bands and leads to nearly dispersionless bands at the smallest angles studied. Based on the predicted
Hamiltonians, we evaluate the dielectric response, second-harmonic generation (SHG), shift current, and
spin Hall conductivity. The dielectric response and
spin Hall conductivity remain close to those of the untwisted
bilayer, whereas the nonlinear optical responses are more strongly affected by twisting. SHG is strongly enhanced relative to the weak untwisted response, and the shift current shows a clear twist-angle dependence within the response-calculation range (53.13 degrees-22.62 degrees). These results show that twist engineering can control electronic and optoelectronic responses in
oxide moire systems.
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All authors Meiyang Yu, Chen Shen, Ruiwen Xie, Jingwei Tao, Lijun Zhang, Hongbin Zhang
Materials Science · Cross-listed from cond-mat.str-el
Bose Einstein Condensation of Magnons in BaCuSi$_{2}$O$_{6}$: An experimental perspective
Marcelo Jaime and Franziska Weickert
Han Purple, a pigment first obtained in ancient China, is one of the earliest known synthetic pigments. Also naturally occurring, as a mineral, it is known as colinowensite (Cwn). Its chemical formula is BaCuSi$_{2}$O$_{6}$, and its structure is a layered cyclosilicate in which
magnetic Cu$^{2+}$ ions (S = 1/2) form dimers arranged on a square
lattice, making it also the first known synthetic metal dimer compound. Most interesting
magnetic properties arise from a strong intradimer
spin coupling, accompanied by weaker interdimer interactions within and between Cu-dimer layers. In zero or small
magnetic fields, BaCuSi$_{2}$O$_{6}$ remains a
quantum paramagnet. However, under high
magnetic fields between 23 and 49 Tesla – about a million times stronger than Earth's
magnetic field – it undergoes
magnetic ordering at subliquid Helium temperatures into an almost ideal easy-plane (XY)
antiferromagnetic state regarded as a realization of a Bose-Einstein condensate of
magnons. Within this experimentally accessible field range, BaCuSi$_{2}$O$_{6}$ serves as an extraordinary playground for testing predictions of
quantum many-body physics.
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Materials Science · Cross-listed from physics.optics
Engineering Disordered Many-particle Plasmonic Nanoclusters for Wafer-scale Uniform and Giant Electromagnetic Field Enhancement
Minjun Kim, Vasanthan Devaraj, Hyeon-Seok Seo et al. (+6 more)
Scalable plasmonic technologies face a critical trade-off: few-body architectures offer high enhancement but are sensitive to fabrication flaws, while scalable methods like solid-state dewetting yield large, low-enhancement gaps. We introduce a paradigm shift using a many-body plasmonic architecture inspired by statistical mechanics. By moving toward the continuum limit (N>>1), local
geometric variations are statistically averaged out, effectively decoupling optical performance from microscopic disorder. We implement this concept via a lithography- and etching-free, multi-step dewetting strategy, creating wafer-scale nanoclusters. This process strategically forms a robust many-body system by introducing numerous small satellite nanoparticles between larger particles. Crucially, this design achieves a high collective enhancement that surpasses even optimized few-body systems, despite having larger individual gaps. Under optimized conditions, these substrates exhibit a surface-enhanced Raman scattering enhancement factor approaching 4 x 108 with unprecedented reproducibility (RSD of ~10%). This scalable, low-cost concept establishes a practical route toward reproducible wafer-scale nanophotonic platforms for sensing, spectroscopy, and
quantum technologies.
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All authors Minjun Kim, Vasanthan Devaraj, Hyeon-Seok Seo, Seongjae Eom, Jeong-Su Lee, Donghan Lee, Min Yong Jeon, Thomas Zentgraf, Jong-Min Lee
Materials Science
High temperature ferromagnetism in epitaxial monolayers of Co-doped Fe5GeTe2
Jules Courtin, Fatima Ibrahim, Davide Benettin et al. (+14 more)
Magnetic van der Waals materials have mainly been investigated in their bulk form or as few-layers flakes. Due to the challenges in producing atomically thin films, only a few have been isolated as monolayers, which typically exhibit long-range
magnetic order below 150 K. In this work, we use molecular beam epitaxy to synthesize Co-doped Fe5GeTe2, achieving precise control over both thickness and composition. We demonstrate ferromagnetism well above room temperature in multilayer samples and present clear evidence of
ferromagnetic ordering in monolayers up to $\sim$200 K. The changes in Curie temperature and
magnetic anisotropy with composition exhibit similar trends in both monolayers and thicker films, indicating that the
magnetic properties are primarily governed by intralayer
magnetic interactions. Through element-specific X-ray
magnetic circular dichroism and density functional theory, we identify the substitution site of Co dopants and reveal the mechanism behind the Curie temperature enhancement induced by Co doping. Our findings suggest that, despite their weak
magnetic moment, Co dopants strengthen the
magnetic moments on neighboring Fe atoms and enhance the intralayer
ferromagnetic exchange interactions.
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All authors Jules Courtin, Fatima Ibrahim, Davide Benettin, Djordje Dosenovic, Roberto Sant, Pâmella Vasconcelos Borges Pinho, Vincent Polewczyk, Alain Marty, Isabelle G de Moraes, Matthieu Jamet, Denis Jalabert, Fadi Choueikani, Philippe Ohresser, N. B. Brookes, Hanako Okuno, Mairbek Chshiev, Frédéric Bonell
Materials Science
Modelling the mean inner potential of alloyed and strained materials
Marco Schowalter, Peer Kruse and Andreas Rosenauer
In this publication, we study the influence of strain and alloying on the mean inner potential (MIP) using density functional theory (DFT) within an augmented plane waves plus local orbitals basis set. Two major effects have been identified allowing to model the influence of strain and alloying on the mean inner potential with a reasonable accuracy. First, alloying for constant volume results in a linear relationship between the MIP and the concentration. Second, the MIP scales with changes in volume as we already pointed out in an earlier publication (M. Schowalter, D. Lamoen, A. Rosenauer, P. Kruse, and D. Gerthsen, Appl. Phys. Lett. 85, 4938-4940 (2004)). Specifically, a linear relationship between MIP and concentration x was found for AlGaAs (nearly no change in
lattice parameter), whereas InGaP and GeSi (volume changes with concentration x) exhibits a clear bowing. The bowing can be modeled by taking the rescaling of the MIP with the varying volume additionally into account. The rescaling could be also used to model the dependence of the MIP on strained binary
cells and the density dependence of e.g. amorphous materials.
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Materials Science
Orientation-resolved ultrafast spin reorientation dynamics in ferrimagnetic DyCo$_5$
Johanna Richter, Martin Hennecke, Martin Schmidbauer et al. (+3 more)
Under quasi-static conditions, ferrimagnetic DyCo$_5$ thin films exhibit a thermally induced
spin-reorientation transition, in which the
equilibrium magnetization changes from an out-of-plane to an in-plane orientation, mostly as a result of the competing
magnetic anisotropy contributions of the Dy $4f$ rare-earth and Co $3d$ transition-metal sublattices. While this
equilibrium transition has been studied, it remains an open question whether such a reorientation can be triggered on ultrafast timescales using femtosecond
laser excitation. In this work, we investigate the ultrafast
spin-reorientation dynamics in DyCo$_5$. The time-dependent orientation of the magnetization vector following femtosecond
laser excitation is quantified by combining polar with transverse magneto-optical Kerr effect (MOKE) measurements in the extreme ultraviolet spectral range, which are sensitive to the out-of-plane and in-plane magnetization component, respectively. Both techniques are implemented at the Co M$_{3,2}$ resonance and are complemented by visible-light MOKE measurements. This combined approach allows us to resolve the canting of the magnetization from an out-of-plane toward an in-plane orientation and to determine the characteristic timescales of the transient
spin-reorientation process.
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All authors Johanna Richter, Martin Hennecke, Martin Schmidbauer, Ilie Radu, Clemens von Korff Schmising, Stefan Eisebitt
Materials Science · Cross-listed from cond-mat.mes-hall
Interfacial Noncollinear Filtering of Spin Hall Currents
Dan-Yang Han, Zi-An Wang, Hong-Liang Chen et al. (+6 more)
Spin Hall currents generated in nonmagnetic materials are conventionally regarded as bulk responses whose polarization is fixed by
crystal symmetry. This view has motivated the search for intrinsically low-
symmetry spin sources when unconventional
spin polarizations are required. Here we point out that, in realistic
heterostructures, the device-relevant quantity is not the fully
symmetry-averaged bulk
spin Hall current, but the emitted
spin current transmitted across the interface. We therefore establish emitted
spin currents as bulk-interface hybrid responses and propose interfacial noncollinear filtering as a mechanism to bypass the bulk-
symmetry constraint. A low-
symmetry interfacial
spin-orbit field, generally noncollinear with the momentum-resolved
spin polarization of the incident
spin Hall current, imposes
spin-dependent transmission and converts hidden momentum-resolved
spin-polarization components into an observable unconventional emitted
spin current. Using both a rotationally symmetric minimal model and a realistic high-
symmetry Dirac-semimetal model, we show that conventional
spin Hall sources can emit sizable out-of-plane
spin currents when their hidden bulk
spin Hall textures are selectively transmitted by the interfacial
spin-orbit field. Our results reveal that
spin-current polarization emerges from the cooperative action of bulk and interfacial responses, providing a strategy for reprogramming
spin-current polarization in high-efficiency, CMOS-compatible
spin Hall materials without relying on intrinsically low-
symmetry bulk
crystals or external
symmetry-breaking schemes.
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All authors Dan-Yang Han, Zi-An Wang, Hong-Liang Chen, Bo Li, Wen-Jian Lu, Yu-Ping Sun, Liang Liu, Shu-Hui Zhang, Ding-Fu Shao
Materials Science · Cross-listed from cond-mat.str-el
Local moment magnon spectrum in conduction electron tunnelling
Peter Thalmeier and Alireza Akbari
The surface tunnelling spectrum of a dual system consisting of localised moments with
antiferromagnetic order coupled to conduction electrons by on-site exchange interaction is investigated. In the static approximation of the local moment order it is known that
magnetic band reconstruction leads to an anomalous tunnelling spectrum at the
magnetic ordering vector of local moments although the latter cannot contribute directly. In this work we consider dynamic effects by including the scattering of conduction electrons from the local moment
magnon excitations. They lead to self energies and renormalisation of conduction states which in turn appreciably modify the tunnelling spectrum beyond the influence of static order, interpreted as the appearance of
magnon sidebands.
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Materials Science
Anomalous Piezoelectricity from Polarization-Dependent Electrostriction in Wurtzites
Guo-Dong Zhao, Aiden Ross, Yueze Tan et al. (+3 more)
The piezoelectric coefficient is a third-rank tensor connecting the strain or stress with the electric field or polarization, whereas the electrostriction coefficient is a fourth-rank tensor relating the strain to the square of electric polarization. The electrostriction tensor components in the current literature are often treated as constants independent of polarization, resulting in piezoelectric tensor components that are linearly proportional to polarization and the dielectric susceptibility tensor. Here, we study the electrostriction and piezoelectricity in strongly polar wurtzites, including AlN, Al$_{1-x}$Sc$_x$N, Al$_{1-x}$B$_x$N, GaN, and ZnO. We discover that electrostriction and the elastic modulus in wurtzites are both strongly polarization-dependent, and the piezoelectric coefficient is highly nonlinear with respect to polarization, including the anomalous possibility that decreasing polarization increases the electromechanical strain response. These unusual dependencies of electrostriction and piezoelectric effects on polarization arise from the
evolution of a layered reference nonpolar structure toward a tetrahedrally coordinated wurtzite network structure as the polarization increases. The findings have important implications in understanding the thermodynamics of the general class of wurtzite ferroelectrics and in manipulating their piezoelectric and ferroelectric behaviors.
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All authors Guo-Dong Zhao, Aiden Ross, Yueze Tan, Yujie Zhu, Jia-Mian Hu, Long-Qing Chen
Materials Science
Efficient Bethe-Salpeter Equation Calculations Based on Numerical Atomic Orbitals and Norm-Conserving Pseudopotentials: Dual-${\boldsymbol k}$-Mesh Strategy
Ziqing Guan, Yu Cao, Min-Ye Zhang et al. (+3 more)
We present an efficient implementation of the Bethe–Salpeter equation (BSE) based on numerical atomic orbitals (NAOs) and norm-conserving pseudopotentials within the ABACUS+LibRPA framework. By exploiting the localized resolution-of-identity (LRI) technique, the screened Coulomb interaction is cast into a real-space, unit-
cell-indexed form $W_{\mu\nu}(\boldsymbol R)$ that is inherently short-ranged and well localized. This spatial locality enables an efficient
Fourier interpolation of the BSE kernel from the coarse $\boldsymbol k$-mesh used in the preceding $GW$ calculation to an arbitrarily dense $\boldsymbol k$-mesh on which the BSE
Hamiltonian is assembled and diagonalized, thereby giving rise naturally to a dual-$\boldsymbol k$-mesh workflow. Building on this scheme, we systematically examine the convergence of the absorption spectra with respect to the NAO basis set, the auxiliary basis set, and the $\boldsymbol k$-point sampling.
Benchmark calculations for both molecular and periodic systems collectively validate the accuracy of the present implementation and establish the dual-$\boldsymbol k$-mesh strategy as a practical and reliable approach for $GW$+BSE calculations.
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All authors Ziqing Guan, Yu Cao, Min-Ye Zhang, Peize Lin, Ruiyi Zhou, Xinguo Ren
Materials Science
From Closed-Loop Optimization to Open Decision Making: Coupled Digital Twins for Predictive and Autonomous Microscopy
Yu Liu, Boris Slautin, Ian Mercer et al. (+2 more)
Automated experimentation is moving from closed-loop optimization toward open decision-making, where human or AI planners must forecast the consequences of candidate actions before executing them. Such forecasts require a model of both sides of the experiment: how the sample is likely to respond and what the instrument is likely to detect. We therefore introduce a coupled digital-twin framework that separates these roles and then links them. In this framework, the sample twin encodes material state inferred from prior knowledge and measurements till the moment. The instrument twin captures signal formation, feedback dynamics, and operating constraints based on prior knowledge. When coupled, the two twins estimate expected outcomes, uncertainty, and risk for candidate microscope operations. For amplitude-modulation scanning probe microscopy, we realize this framework with a physics-informed encoder of force-distance curves, a deterministic scanner model of cantilever and feedback dynamics, and sparse learned residual corrections. The encoder first recovers scanner-driving descriptors with sub-nanometer accuracy. The calibrated scanner then reproduces typical traces within a few nanometers and identifies operating-point noise amplification as the main source of mismatch. Supplementary phase analysis localizes residual error to the phase channel, which clarifies where added physics is needed. Together, these results establish coupled sample and instrument twins as a practical foundation for predictive microscope operation and autonomous experimental planning.
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All authors Yu Liu, Boris Slautin, Ian Mercer, Jon-Paul Maria, Sergei V. Kalinin