Materials Science · Cross-listed from cond-mat.supr-con
Growth and Interface Engineering of Superconducting TiN on Sapphire by Thermal-Laser Epitaxy
Anthony Hyatt, Anand Ithepalli, Eegene Clara Chung et al. (+6 more)
Thermal-
laser epitaxy (TLE) extends the accessible pressure, temperature, and growth-rate regimes of conventional molecular-beam epitaxy, enabling in situ
laser annealing, high-purity buffer-layer growth, and efficient evaporation of refractory elements. These capabilities make TLE a promising platform for engineering low-loss superconducting resonators and Josephson-junction
heterostructures. Here, we study the TLE growth of TiN on sapphire and observe improved transport properties with increasing growth temperature up to 1150 C. Unfortunately, we observe that the high ammonia pressure and elevated substrate temperatures required for optimal TiN properties promote reactions at the sapphire surface, resulting in voids at the TiN-substrate interface. These defects increase interfacial surface area, introduce dangling bonds, and could compromise tunnel-barrier
heterostructures. We mitigate this degradation using an initial TiN seed layer grown at 850 C. With the seed layer, we achieve a superconducting transition temperature of 5.8 K, a residual resistivity ratio of 12.2, and a resistivity of 1.19 micro-ohm cm at 10 K, which, to our knowledge, is the lowest reported for TiN grown on sapphire.
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All authors Anthony Hyatt, Anand Ithepalli, Eegene Clara Chung, Yorick A. Birkholzer, Brendan Faeth, Huili Grace Xing, David A. Muller, Darrell G. Schlom, Debdeep Jena
Materials Science · Cross-listed from cond-mat.mes-hall
On Energy References when a Magnetic Field is Applied. A Practical Guide to Reference Energies with and without a Magnetic Field
Pedro Pereyra
We present a simple analysis of the energy references and of the
magnetic-field-dependent and system-dependent features of the Landau energy levels. The energy reference used in describing Landau quantization is usually taken as given, with the Fermi levels of the electron system and the coupled reservoir aligned in thermal
equilibrium. Nevertheless, the dependence of the Landau-level spectrum on
magnetic field can make the relation between these energy references less transparent. In this work, we show that when a
magnetic field is applied and the confined Landau levels $E_n$ are established, both the Fermi level $E_F$ and the bottom energy $E_0$ remain fixed when the
magnetic field varies. We show that the bottom energy $E_0$ is independent of $B$, and determined by a system-dependent field $B_1$, defined by the condition that the first Landau level equals the Fermi energy. In contrast, the minimum of the parabolic well depends explicitly on the
magnetic field. The resulting picture provides a simple way of displaying the relative motion of the Landau levels and the fixed Fermi level without introducing a
magnetic-field-dependent energy zero.
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Materials Science
Ultrafast Dissipative Localization of Electronic Energy in AuPt Superlattices
C. Walz, M. Mattern, J.-E. Pudell et al. (+15 more)
Controlling the spatial distribution of absorbed optical energy is central to nanoscale photothermal chemistry, plasmonics, and ultrafast materials control. Here, we show that a metallic AuPt superlattice concentrates electronic energy in Pt within a few hundred femtoseconds, regardless of the initial energy distribution between the two constituents. Ultrafast X-ray diffraction follows this energy redistribution through the amplitude of a coherent 570 GHz superlattice
phonon driven by the stress imbalance at the AuPt interfaces. Despite the nearly homogeneous absorption at 400 nm, the observed
lattice motion is identical to that produced by 800 nm excitation, which is absorbed predominantly in Pt. This dissipation driven localization of energy arises from the large electronic heat capacity of Pt and rapid electronic transport through the superlattice, providing a route to femtosecond control of nanoscale energy distributions.
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All authors C. Walz, M. Mattern, J.-E. Pudell, J. Jarecki, L. Mehner, F.-C. Weber, F. Baltrusch, S. P. Zeuschner, M. Herzog, B. Ahn, J. C. Ekström, D. Kroon, A. Jurgilaitis, J. Larsson, M. Kronseder, D. Schick, A. von Reppert, M. Bargheer
Materials Science · Cross-listed from cond-mat.soft
Self-limited stacking of non-Euclidean colloidal shells: From saddles to caps
Kyle T. Sullivan, Mark J. Stevens and Gregory M. Grason
The presence of
geometric frustration in self-assemblies, stemming from a shape misfit between identical subunits, can lead to the spontaneous formation of finite-sized structures known as self-limitation. Recently proposed models of curved, colloidal shells, dubbed "curvamers", that form one-dimensional stacks have shown
curvature-induced frustration to be a promising class of particle designs for engineering self-limiting assembly. However, existing models of curvamer assembly considered only cylindrically curved shells where elastic costs are derived purely from bending. Here, we study the self-limiting behavior of a more general class of
curvature-frustrated colloidal shells, extending shapes to include non-Euclidean geometries such as spherical caps and saddles whose deformations can also incur stretching costs. We introduce continuum mechanical and discrete, coarse-grained models of shell stacks to study how qualitatively different intra-assembly strain gradients required for each geometry affect the buildup of super-extensive elastic costs. We find the accumulation of elastic energies in non-Euclidean shell stacks is sensitive to shell thickness via a dimensionless quantity related to the Foppl-von Karman number that characterizes the relative strength of stretching to bending. In particular, thin shells penalize stretching more causing faster elastic energy growth resulting in the suppression of self-limitation with sizes generally largest for cylindrical shells, smallest for spherical caps and intermediate for saddles. We additionally find that while thin spherical caps experience the strongest elastic penalties, the rapidly increasing rate of elastic penalties in stacks of saddles lead to a robust size-selection as inter-particle binding strength grows as well as relatively smaller dispersity in self-limiting stack size in comparison to cylindrical and spherical curvamers.
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Materials Science
Nanoscale Sr$_2$IrO$_4$ Freestanding Thin-Films for Flexible Electronics
Sujan Shrestha, Matthew Coile, Menglin Zhu et al. (+7 more)
We report the structural and optical properties of nanoscale Sr2IrO4 freestanding thin-films fabricated using a water-soluble Sr3Al2O6 layer. The coherent
lattice structure,
phonon modes, two-
magnon Raman scattering, and optical absorption spectra of the Sr2IrO4 nanomembrane are analogous to those of the layered iridate epitaxial thin-films and single
crystals. Remarkably, the formation of 3-unit-
cell-thick SrIrO3 and interfacial composite layers alleviates antiphase boundaries at the Sr2IrO4/Sr3Al2O6 interface, resulting in structurally-robust nanomembranes. Our experimental results show that this freestanding thin-film approach of layered oxides can provide techniques for tuning or realizing unprecedented states beyond conventional thin-film methods, suggesting a pathway in achieving flexible layered-oxide electronics.
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All authors Sujan Shrestha, Matthew Coile, Menglin Zhu, Maryam Souri, Jiwoong Kim, Rina Pandey, Joseph Brill, Jinwoo Hwang, Jong-Woo Kim, Ambrose Seo
Materials Science
Impact of Chemical Clustering on the Structural, Topological, and Functional Properties of Ba(Zr$_x$Ti$_{1-x}$)O$_3$: An Atomistic Simulation Study
Matias Baldassin, Rodrigo Machado, Marcelo Sepliarsky et al. (+1 more)
Barium zirconate titanate, Ba(Zr$_x$Ti$_{1-x}$)O$_3$ (BZT), is a leading lead-free candidate for high-performance electronic components due to its highly tunable ferroelectric-to-relaxor transition. In this study, we employ molecular dynamics simulations to investigate the effects of local chemical clustering on the structural and functional properties of BZT in both ferroelectric (x = 0.2) and relaxor (x = 0.4) regimes. Crucially, we demonstrate that Zr clustering induces a significant enhancement of local polarization across both compositions. Rather than a purely volume-driven effect, this enhancement is fundamentally governed by a spatial redistribution of local structural phases (rhombohedral, orthorhombic, and tetragonal); Zr segregation forms larger, continuous Ti-rich regions that expand highly polar rhombohedral domains and foster cooperative dipolar
alignment. This cooperative coupling thermally stabilizes the local polar order, systematically elevating the Curie, maximum permittivity, and Burns temperatures. Focusing on the relaxor composition (x = 0.4), chemical segregation promotes the formation of resilient, swirling polar topological textures,categorized into
vortices circulating around Zr-rich clusters and interstitial
vortices localized within the Ti-rich
matrix. These topological structures act as effective pinning barriers against polarization reversal, driving pronounced electrical hardening with elevated coercive fields. Ultimately, these findings establish a quantitative link between nanoscale compositional heterogeneity, polar domain
topology, and macroscopic performance, providing a robust framework for engineering lead-free perovskites via local chemical order control.
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All authors Matias Baldassin, Rodrigo Machado, Marcelo Sepliarsky, Marcelo G. Stachiotti
Materials Science
Correlations of Spectroscopic and Dielectric Properties of Hafnia-Zirconia Nanoparticles
Yuriy O. Zagorodniy, Eugene A. Eliseev, Petr Jiricek et al. (+11 more)
In this work we analyze correlations of X-ray photoelectron and diffraction spectra, and dielectric properties of hafnia-zirconia nanoparticles (the chemical compositions Hf0.4Zr0.6O2 and Hf0.6Zr0.4O2, and the average size of 7.5 nm) prepared by the solid-state organo-nitrate synthesis and annealed in air. The phase state of the nanoparticles, determined by the X-ray diffraction spectroscopy, is the coexistence of the nonpolar monoclinic (42 - 76 mass %) and orthorhombic (56 - 24 mass %) phases. Concentration of the oxygen vacancies was estimated from the X-ray photoelectron spectroscopy. The increase in the intensity of the dielectric permittivity maximum observed near 350 - 450 K in a PVDF
matrix with embedded Hf0.4Zr0.6O2 nanoparticles can be related with an increase in oxygen vacancy concentration. Theoretical calculations, based on Landau-Ginzburg-Devonshire theory, explain the increase of the dielectric permittivity in Hf0.4Zr0.6O2 nanoparticles compared to Hf0.6Zr0.4O2 nanoparticles.
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All authors Yuriy O. Zagorodniy, Eugene A. Eliseev, Petr Jiricek, Jana Houdkova, Lesya Demchenko, Oksana V. Leshchenko, Victor N. Pavlikov, Ihor V. Pleskach, Anna O. Diachenko, Mikhail D. Volnyanskii, Oleksandr S. Pylypchuk, Myroslav V. Karpets, Mikhail P. Trubitsyn, Anna N. Morozovska
Materials Science · Cross-listed from physics.flu-dyn
Silk-templated Nanostrips as Superprotonic Fibre Sensors
Jianhui Zhang, Ahmed Salem, Robert Tidswell et al. (+4 more)
Transforming textile fibres into sensors can facilitate continuous physiological monitoring for improving human healthcare. Coating fibres with electronic conductors can help detect physiological stimuli but their sensitivity scales with thickness and often lowers the fibre mechanical flexibility.
Proton conductors are potential alternatives; however, they suffer from fragile physical interfaces, and sluggish kinetics. Here, we report a bio-templated, scalable strategy to create superprotonic interfaces using natural silk fibres. We exploit ordered nanofibrils on silk fibres to selectively grow continuous metal-organic framework (MOF) nanostrips. This patterned, ca. $10\,\mu\mathrm{m}$-diameter fibre-structure provides low-defect interfacial pathways for ultrafast
proton transport, yielding conductivities up to 40 $S\,cm^{-1}$ - two orders of magnitude above the state of the art - with millisecond response and high mechanical robustness. The individual micro-thread sensors facilitate respiratory monitoring with a mean bias of 0.01 breaths $min^{-1}$ in volunteer testing, unaffected by human movement and readily capture breathing signature in high-flow oxygen therapy devices. We integrate arrays of micro-thread sensors in textiles and exploit deep learning to demonstrate high-resolution spatiotemporal humidity mapping in dynamic flow-fields and resolve asymmetric respiratory patterns that elude conventional single-point detectors. Our results should have broad-ranging implications, e.g. in wearable health monitoring, bioelectronics, and human-machine interfaces.
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All authors Jianhui Zhang, Ahmed Salem, Robert Tidswell, Haowei Wang, Vikramjeet Singh, Laurence B. Lovat, Manish K. Tiwari
Materials Science · Cross-listed from math-ph
Don't mind the gap: Absolutely Continuous Edge Spectrum in a Mobility Gap
Simon Becker and Mengxuan Yang
We study the edge spectrum of two-dimensional
lattice Hamiltonians and
magnetic Schrödinger operators in a mobility gap, an energy interval in which the bulk spectrum is potentially localized rather than necessarily absent. Under suitable localization and boundary assumptions, we prove that a constant nonzero bulk topological index implies that the entire interval belongs to the absolutely continuous spectrum of the corresponding half-space operator. The absolutely continuous spectral multiplicity is bounded below by the absolute value of the index at almost every energy in the interval. The first version of the auxiliary-space cut construction in Section 2.3 was generated using ChatGPT 6.
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Materials Science · Cross-listed from cond-mat.mes-hall
Collective Coordinate Dynamics of Antiferromagnet: Gravity by Quantum Metric
Kuangyin Deng and Ran Cheng
The collective coordinate dynamics of antiferromagnetic and ferrimagnetic textures emerges as a geodesic equation governed by the
quantum geometry of the CP$^1$ state associated with the Néel vector, rendering an effective Riemannian
gravity and a Lorentz force in the texture motion. This unveils a physical scenario fundamentally distinct from the Thiele equation widely used for ferromagnetic textures, which is inertialess without any
gravity analogy. Applying our framework to an optically driven ultrafast antiferromagnetic domain wall, we can explain the helicity-dependent asymmetric motion observed experimentally and predict a vibrantly breathing wall width.
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Materials Science · Cross-listed from cond-mat.str-el
Staged emergence of anomalous Hall transport in a correlated uranium Weyl semimetal
Sabin Regmi, Shuxiang Zhou, Chandan K. Singh et al. (+9 more)
Understanding how electronic correlations reshape topological states remains a central challenge in
quantum materials. Here we investigate the uranium ferromagnet UPS using magnetotransport, angle-resolved photoemission spectroscopy, thermodynamic measurements, and first principles calculations. Resonant photoemission reveals narrow U-$5f$ spectral weight at the Fermi level coexisting with broad incoherent states, consistent with the itinerant/localized duality characteristic of uranium $5f$ electrons. The anomalous Hall conductivity reaches approximately $4.5\times10^{2}\,\Omega^{-1}\mathrm{cm}^{-1}$, yet does not simply follow the ordered
magnetic moment. Temperature dependent photoemission reveals relatively little change across $T_C=118$~K, followed by a pronounced redistribution of low-energy $5f$ spectral weight below approximately 90~K. First-principles calculations identify a
symmetry protected Weyl crossing with pronounced Berry
curvature and yield an intrinsic anomalous Hall conductivity of approximately $9.6\times10^{2}\,\Omega^{-1}\mathrm{cm}^{-1}$. These results demonstrate that
magnetic order, correlated electronic reconstruction, and anomalous Hall transport develop over distinct but overlapping temperature ranges, revealing how strong correlations reshape topological transport in a uranium Weyl semimetal.
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All authors Sabin Regmi, Shuxiang Zhou, Chandan K. Singh, Alexei Fedorov, Jonathan Denlinger, Zeyu Ma, Yidi Wang, Jennifer E. Hoffman, Peter M. Oppeneer, Dariusz Kaczorowski, Tomasz Durakiewicz, Krzysztof Gofryk
Materials Science
Face-to-face anneal temperature controls lattice parameter in Ta(C,N) virtual substrates for AlGaN power electronics
Noah Zahn, Julia L. Martin, Michelle A. Smeaton et al. (+3 more)
Tantalum carbide (TaC) thin film ”virtual” substrates are a highly desirable material for $\text{Al}_{0.5}\text{Ga}_{0.5}\text{N}$ vertical power electronics devices due to
lattice matching, thermal expansion matching, and metallic conductivity. However, the material has not been demonstrated to support $\text{Al}_{x}\text{Ga}_{1-x}\text{N}$ layers of variable composition $x$, limiting the range of device applications. We present a method to achieve tunable rock salt $\text{Ta}\text{C}_{x}\text{N}_{1-x}$ virtual substrates via a face-to-face annealing of TaC thin films in an $\text{N}_2$ atmosphere. The results suggest that annealing temperatures below $1600\,^{\circ}\text{C}$ promote partial uptake of nitrogen onto carbon and anion vacancy sites to form rock salt $\text{Ta(C,N)}$ with intermediate anion compositions. At temperatures $\ge 1600\,^{\circ}\text{C}$, nitrogen primarily occupies the anion sublattice and the crystalline quality and surface morphology simultaneously degrade coincident with the formation of secondary phases. This study demonstrates the growth and processing parameters necessary to make tunable
lattice constant virtual substrates for $\text{Al}_{x}\text{Ga}_{1-x}\text{N}$ from $x = 0.5\text{-}1$, enabling vertically conducting power electronic devices with reduced defect density at high Al-content.
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All authors Noah Zahn, Julia L. Martin, Michelle A. Smeaton, Renae Gannon, Henry Garland, M. Brooks Tellekamp
Materials Science
Scandium diboride: a semi-metallic, lattice, thermally matched substrate for vertical AlGaN power electronics
MVS Chandrashekhar, Daniel Joel Harrison, Ahamed Raihan et al. (+22 more)
We report the properties of hexagonal (space group P6/mmm) scandium diboride ($\mathrm{ScB}_2$) single
crystals grown by a
laser diode floating zone method at growth rates of ~1mm/hr under B-rich conditions with (002) rocking curve widths $\Delta\omega$=38” approaching the quality of commercial SiC/GaN substrates.
Lattice expansion measurements reveal matching to $\mathrm{Al_{0.55}Ga_{0.45}N}$ with a coefficient of thermal expansion ~5ppm/K at typical AlGaN growth temperatures, enabling thick AlGaN layers for ultra-wide bandgap (UWBG) power electronics >1kV. We measure semi-metallic room temperature resistivity ~15$\mu\Omega$ cm, climbing to ~93$\mu\Omega$ cm at 773K with a $T^2$ dependence effectively eliminating substrate parasitic resistance, the limiting factor in exploiting the full potential of UWBG. The Debye temperature $\theta_{D,ScB_2}$ from heat capacity and
lattice expansion is ~850K well matched to $\theta_{D,ScB_2}$, but lower than the 1100K measured for Sc-rich growth conditions. We discuss Debye matching as a key substrate codesign criterion providing significant overlap in
phonon modes for heat removal and thermal matching during AlGaN growth. The competitive thermal conductivity at room temperature 53W/mK is half that from full first principles calculations, a discrepancy we attribute to the presence of Sc-vacancies generated by B-rich growth. while the resistivity is ~2x the theoretical value, indicating that both electrons and
phonons play equal role in thermal transport. The smooth ~2.5nm rms roughness surface enables advanced heat removal modalities through engineered
phonon bridges and
phonon polaritons in $\mathrm{ScB}_2$/AlGaN interfacial
heterostructures, potentially allowing ~10-100x increase in power handling over state-of-the-art GaN/SiC.
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All authors MVS Chandrashekhar, Daniel Joel Harrison, Ahamed Raihan, Astrid D. Kengne, R. Shipra, Han Xie, Tasnia Jabin, Monte Hendrix, Ethan Scott, Roshan S. Annam, Sharad Mahatara, Evan N. Crites, Allana G. Iwanicki, Luke J. Meiler, Maxime Siegler, Renae N. Gannon, Steven R Spurgeon, Ashutosh Giri, Rajeswari Kolagani, Joshua A. Burrow, Stephan Lany, Patrick Hopkins, Tyrel M. McQueen, Michael Spencer, Satya Khushwaha
Materials Science · Cross-listed from cond-mat.supr-con
Electronic Reconstruction Towards Topological Superconductivity in FeTe
Hongtao Rong, Yang Ge, Zi-Jie Yan et al. (+13 more)
The recent discovery of intrinsic superconductivity in stoichiometric FeTe films has renewed interest in the Te-rich end member of the iron chalcogenides for studies of unconventional and topological superconductivity, yet its intrinsic electronic structure remains unresolved. In this work, we combine molecular beam epitaxy, angle-resolved photoemission spectroscopy (ARPES), electrical transport measurements, density functional theory, and embedded dynamical mean-field theory to track the electronic reconstruction of 20-unit-
cell FeTe films as Te annealing progressively removes excess interstitial Fe and drives the system from an antiferromagnetic metal to a
superconductor. We find that this
evolution is accompanied by recovered quasiparticle coherence, reduced electronic correlations, a Lifshitz transition, and a topological phase transition, yielding dxy-dominated hole and electron pockets that favor inter-pocket scattering. In addition, a shallow dxz/dyz-derived hole band located about 2 meV below the Fermi level may provide an incipient-band pairing channel, while scattering between the two electron pockets at M may offer additional pairing channels. High-resolution polarization-dependent
laser ARPES measurements further reveal a topological surface state whose circular dichroism is consistent with the expected orbital-angular-momentum texture of stoichiometric FeTe. These results establish the intrinsic low-energy electronic structure of superconducting FeTe and identify the electronic states most relevant to superconductivity. The coexistence of intrinsic superconductivity and a topological surface state establishes stoichiometric FeTe as a promising platform for exploring topological superconductivity and possible Majorana bound states.
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All authors Hongtao Rong, Yang Ge, Zi-Jie Yan, Haoran Lin, Bing Xia, Xiaoda Liu, Zihao Wang, Pu Xiao, Lok-Kan Lai, Stephen Paplini, Jiatao Song, Jiangang Yang, Peter J. Hirschfeld, Shuolong Yang, Jiabin Yu, Cui-Zu Chang
Materials Science
III-V antiphase boundaries are not generated by Si or Ge substrate step edges
Charles Cornet, Sreejith Pallikkara Chandrasekharan, Audrey Gilbert et al. (+7 more)
We critically review recent experimental and theoretical advances on the valence- and structure-mismatched heteroepitaxy of III-V
semiconductors on group-IV substrates. We examine key aspects including wetting behavior, atomic configurations at the hetero-interface, substrate passivation, and the stability of antiphase boundaries (APBs). By synthesizing these findings with pioneering studies, we propose a refined description of antiphase boundaries formation. Our analysis shows that hetero-interface formation is dictated by substrate terrace reconstruction, demonstrating that APBs are not generated at monoatomic step edges. This generalized III-V/IV growth framework provides new insights for the integration of III-V
semiconductors on group-IV platforms and, more broadly, for valence-mismatched heteroepitaxy.
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All authors Charles Cornet, Sreejith Pallikkara Chandrasekharan, Audrey Gilbert, Milan Silvestre, Rozenn Bernard, Pascal Turban, Gilles Patriarche, Eric Tournie, Laurent Pedesseau, Jean-Baptiste Rodriguez
Materials Science
On the dual character of Zn impurity in SnTe: Tuning thermoelectric and topological properties
Kacper Pryga and Bartlomiej Wiendlocha
We present a first-principles study of the electronic structure and thermoelectric properties of Zn-doped SnTe using the Korringa-Kohn-Rostoker method within the coherent potential approximation, complemented by pseudopotential calculations. SnTe is a lead-free analogue of PbTe and a candidate thermoelectric material in which Zn doping has been experimentally reported to enhance the performance of $p$-type samples. We show that Zn introduces a resonant-like impurity state, located within the conduction band, which evolves strongly depending on the Zn concentration. This feature leads to a significant enhancement of the thermopower in $n$-type SnTe. In the valence band, Zn doping induces L-$\Sigma$ band convergence, also resulting in an increased $p$-type Seebeck coefficient over a broad concentration range and delaying the onset of the bipolar effect. We further demonstrate that the Zn-induced band-structure modifications drive a transition from an inverted to a trivial band ordering, indicating a controllable topological phase transition. Our results clarify the microscopic role of Zn in SnTe and identify doping as a mechanism for simultaneously tuning thermoelectric and topological properties.
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Materials Science
Revealing tilt-driven structural heterogeneity in hybrid improper ferroelectrics by spatially-resolved crystallography
Evie Ladbrook, Jon P. Wright and Mark S. Senn
Domain walls in ferroic oxides can host emergent functionalities that differ fundamentally from those of the surrounding bulk, making their microscopic structure central to understanding and controlling macroscopic behaviour. We use scanning three-dimensional X-ray diffraction to map the spatial
evolution of coupled octahedral distortion modes across ferroelastic domain walls in the hybrid improper ferroelectric Ca$_{2.15}$Sr$_{0.85}$Ti$_{2}$O$_{7}$. By combining tomographic reconstruction with
symmetry-adapted analysis of the octahedral rotation and tilt and associated strain modes, we map their spatial
evolution across individual walls in the bulk
crystal. We find that the walls are intrinsically extended, with widths far exceeding those expected for conventional Ising-like ferroic interfaces. The wall structure is characterised by a continuous rotation of the tilt order parameter through a tetragonal intermediate. These results provide new microscopic insight into the structural mechanisms governing ferroelastic wall formation, ferroelectric switching, and emergent functionality at domain walls.
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Materials Science
Axial thermal expansion from free-energy minimization with temperature-dependent force constants in phonopy: a technical report, with $\alpha$- and $\omega$-Ti as the worked example
Atsushi Togo
In a hexagonal
crystal the $a$ and $c$ axes change their lengths at different rates with increasing temperature, and the thermal expansion is described by two coefficients, one for each axis. The
equilibrium lattice parameters at each temperature are found by minimizing the Helmholtz free energy over the
lattice parameters, and the coefficients are their logarithmic derivatives with respect to temperature. This report describes a procedure for this minimization as implemented in phonopy. The free energy is computed with harmonic force constants at a finite set of values of $a$ and $c$, and a free-energy surface over $a$ and $c$ is fitted to these values. In a variant, the harmonic force constants are replaced by temperature-dependent force constants from a self-consistent harmonic approximation, with the forces computed by a machine-learning potential fitted at each of these values. The $\alpha$ and $\omega$ phases of titanium are the worked example. Both calculations give negative thermal expansion of the $c$ axis of $\alpha$-Ti at low temperature and none in $\omega$-Ti, and they differ most for the $c$ axis of $\alpha$-Ti. The axial coefficients are harder to compute than the volumetric one, because the two axes are coupled in the free energy. In both phases of titanium the thermal effect that lengthens $a$ also shortens $c$. The coefficient of the $c$ axis is then the sum of two terms of opposite sign that partly cancel, and a small error in either term gives a large relative error in the coefficient. The report measures how much each setting of the procedure changes the thermal expansion coefficients, with the $c$ axis of $\alpha$-Ti as the most sensitive case.
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Materials Science
Amorphous insulating MoS$_x$ films deposited by magnetron sputtering as a low-index optical coating
S.G. Martanov, E.V. Tarkaeva, A.V. Muratov et al. (+4 more)
While much effort are now concentrated on exploration of crystalline form of MoS$_2$ as a two-dimensional material, we show that room-temperature DC magnetron sputtering from pressurized MoS$_2$ powder target produces amorphous MoS$_x$ films with optical properties fundamentally different from those of crystalline MoS$_2$. The films were found to have a sulfur content of approximately $x=1.8$, indicating sulfur deficiency. They are optically isotropic and exhibit rather low optical losses, with a refractive index of about 2 over the visible and near-infrared spectral range. Their compatibility with lift-off lithography and dry etching makes amorphous MoS$_x$ a promising material for integrated photonic applications.
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All authors S.G. Martanov, E.V. Tarkaeva, A.V. Muratov, A.V. Lubenchenko, O.N. Pavlov, A.G. Vitukhnovsky, A.Yu. Kuntsevich
Materials Science
Vacancy aggregation enhances NV- spin coherence in diamond: a cluster-correlation-expansion study of multi-vacancy spin baths in semiconductors
Chikara Shinei
Annealing a
semiconductor makes its vacancies mobile; they aggregate into multi-vacancy complexes that often carry
spin. Such centres are a
magnetic-noise source for any
spin qubit among them, in silicon and silicon carbide as well as in diamond, and they are accordingly blamed for NV-decoherence in irradiated and implanted diamond, with two coherence records credited to removing them. Cluster-correlation-expansion simulations driven by published electron-paramagnetic-resonance parameters invert that attribution. At a fixed paramagnetic
spin density a multi-vacancy bath gives a Hahn-echo coherence time 3.1-7.1 times longer than a bath of isolated negative vacancies, over three decades of concentration. A bath dephases the
qubit because its
spins exchange
spin projections with one another, and two can exchange only if their transition frequencies match. A fine-structure splitting shifts those frequencies. Were every defect on the same crystallographic site, all would shift alike and still match: worth only a factor 1.15. Real defects occupy
symmetry-equivalent sites pointing in different directions, so neighbours land at different frequencies and stop exchanging: a further 2.7. What governs the coherence time is therefore the fraction of bath pairs sharing a transition frequency, not the zero-field splitting. Vacancy aggregation extends NV-
spin coherence rather than shortening it.
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Materials Science
Small-supercell and Small-dataset Training Strategy of Machine Learning Interatomic Potentials for Point Defects
Zhenxing Dai, Mingjue Ni, Xinpeng Li et al. (+3 more)
Machine learning interatomic potentials (MLIPs) can treat large-scale material systems with near first-principles accuracy and have been widely used to accelerate point-defect simulations. However, the training of MLIPs usually relies on large amounts of DFT data. This issue is particularly pronounced for charged defects, for which DFT calculations of large supercells are required to avoid long-range Coulomb interactions and finite-size effects, making the construction of
datasets computationally expensive. In this work, we propose an efficient MLIP training scheme for neutral and lowly charged point defects based on small supercells (less than 100 atoms) and limited number of DFT calculations. The scheme requires only four DFT structural relaxations to construct the training
dataset and the trained MLIPs dedicated for the defect can predict the defect formation energies in larger supercells (over 200 atoms) with small errors (mostly smaller than 0.3 eV). Using defects in GaN, SiO$_2$, and $\mathrm{Cu}_2\mathrm{ZnSnS}_4$ as representative examples, we evaluate the extrapolation capability of this scheme for predicting defect total energies and structural relaxations across different supercell sizes. The results show that an MLIP trained only on single small defect supercell produces severe errors when treating large supercells. Incorporating defect data from multiple supercell sizes improves the predictive performance of the MLIP, while adding pristine defect-free bulk supercells further enhances the accuracy. These results provide practical guidance for training MLIP models of defect systems with low computational costs.
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All authors Zhenxing Dai, Mingjue Ni, Xinpeng Li, Menglin Huang, Anderson Janotti, Shiyou Chen