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arXiv2022

Brittle fracture studied by ultra-high speed synchrotron X-ray diffraction imaging

Ever since the very first human-made knapped tools, the control of fracture propagation in brittle materials has been a vector of technological development. Nowadays, a broad range of applications relies on crack propagation control, from the mitigation of damages, e.g., from impacts in glass screens or windshields, to industrial processes harnessing fracture to achieve clean cuts over large distances. Yet, studying the fracture in real time is a challenging task, since cracks can propagate up to a few km/s in materials that are often opaque. Here, we report on the in situ investigation of cracks propagating at up to 2.5 km/s along a (001) plane of a silicon single crystal, using X-ray diffraction megahertz imaging with intense and time-structured synchrotron radiation. The studied system is based on the Smart Cut process, where a buried layer in a material (typically Si) is weakened by micro-cracks and then used to drive a macroscopic crack (0.1 m) in a plane parallel to the surface with minimal deviation (1 nm). The results we report here provide the first direct confirmation that the shape of the crack front is not affected by the distribution of the micro-cracks, which had been a postulate for previous studies based on post-fracture results. We further measured instantaneous crack velocities over the centimeter-wide field-of-view, which had only been previously inferred from sparse point measurements, and evidence the effect of local heating by the X-ray beam. Finally, we also observed the post-crack movements of the separated wafer parts, which can be explained using pneumatics and elasticity. Thus, this study provides a comprehensive view of controlled fracture propagation in a crystalline material, paving the way for the in situ measurement of ultra-fast strain field propagation.

cond-mat.mtrl-sci↗

arXiv2025

Integrating Predictive and Generative Capabilities by Latent Space Design via the DKL-VAE Model

We introduce a Deep Kernel Learning Variational Autoencoder (VAE-DKL) framework that integrates the generative power of a Variational Autoencoder (VAE) with the predictive nature of Deep Kernel Learning (DKL). The VAE learns a latent representation of high-dimensional data, enabling the generation of novel structures, while DKL refines this latent space by structuring it in alignment with target properties through Gaussian Process (GP) regression. This approach preserves the generative capabilities of the VAE while enhancing its latent space for GP-based property prediction. We evaluate the framework on two datasets: a structured card dataset with predefined variational factors and the QM9 molecular dataset, where enthalpy serves as the target function for optimization. The model demonstrates high-precision property prediction and enables the generation of novel out-of-training subset structures with desired characteristics. The VAE-DKL framework offers a promising approach for high-throughput material discovery and molecular design, balancing structured latent space organization with generative flexibility.

cs.LG↗

arXiv2013

Correlation between Internal States and Strength in Bulk Metallic Glass

The internal states or local structures of bulk metallic glass (BMGs) can be well reflected from the changes of density, structural relaxation as well as the elastic constants. With the increasing free volume (FV) content, more local atomic clusters are capable of simultaneous plastic shear at different sites in the metallic glasses, inducing large plasticity. In this work, we report a close correlation between the internal states and strength in a BMG and discover that the yield strength can be changed by varying of the casting current, revealing that the yielding strength of BMGs is not only intrinsically associated with the glass transition, but also with the internal states, such as free volume and elastic properties. Such results may have some implications for understanding the correlations between the internal states and mechanical properties of BMGs.

cond-mat.mtrl-sci↗

arXiv2013

Topotactic phase transformation of the brownmillerite SrCoO2.5 to the perovskite SrCoO3-δ

Oxygen stoichiometry is one of the most important elements in determining the physical properties of transition metal oxides (TMOs). A small fractional change in the oxygen content, resulting in the variation of valence state of the transition metal, can drastically modify the materials functionalities. In particular, TMOs with mixed valences have attracted attention for many energy applications. Previous studies also showed that the ability to control the number of d-band electron populations and detailed spin configurations is critical for improved catalytic performance of TMOs. In this context, SrCoO$_{x}$ (2.5 < x < 3.0) is an ideal class of materials due to the existence of two structurally distinct topotatic phases, i.e. the brownmillerite SrCoO$_{2.5}$(BM-SCO) and the perovskite SrCoO$_{3}$. Especially, BM-SCO has atomically-ordered one-dimensional vacancy channels, which can accommodate additional oxygen. Moreover, SrCoO$_{x}$ exhibits a wide spectrum of physical properties depending on the oxygen stoichiometry. Since SrCoOx has only a single knob to control the Co valence state without cation doping, it is an attractive material for studying the valence state dependent physical properties. However, so far, the growth of high quality single crystalline materials has not been much studied due to difficulty in controlling the right oxidation state. In this work, we report on the epitaxial growth of BM-SCO single crystalline films on SrTiO3 by pulsed laser epitaxy. In order to examine the topotactic phase transformation to the perovskite SrCoO$_{3-δ}$, some of samples were subsequently in-situ annealed at various oxygen pressure P(O$_{2}$). We found that post-annealing in high P(O$_{2}$) (> several hundreds of Torr) could fill some of oxygen vacancies accompanying systematic evolution in electronic, magnetic, and thermoelectric properties.

cond-mat.mtrl-sci↗

arXiv2013

Universal electric-field-driven resistive transition in narrow-gap Mott insulators

One of today's most exciting research frontier and challenge in condensed matter physics is known as Mottronics, whose goal is to incorporate strong correlation effects into the realm of electronics. In fact, taming the Mott insulator-to-metal transition (IMT), which is driven by strong electronic correlation effects, holds the promise of a commutation speed set by a quantum transition, and with negligible power dissipation. In this context, one possible route to control the Mott transition is to electrostatically dope the systems using strong dielectrics, in FET-like devices. Another possibility is through resistive switching, that is, to induce the insulator-to-metal transition by strong electric pulsing. This action brings the correlated system far from equilibrium, rendering the exact treatment of the problem a difficult challenge. Here, we show that existing theoretical predictions of the off-equilibrium manybody problem err by orders of magnitudes, when compared to experiments that we performed on three prototypical narrow gap Mott systems V2-xCrxO3, NiS2-xSex and GaTa4Se8, and which also demonstrate a striking universality of this Mott resistive transition (MRT). We then introduce and numerically study a model based on key theoretically known physical features of the Mott phenomenon in the Hubbard model. We find that our model predictions are in very good agreement with the observed universal MRT and with a non-trivial timedelay electric pulsing experiment, which we also report. Our study demonstrates that the MRT can be associated to a dynamically directed avalanche.

cond-mat.mtrl-sci↗

arXiv2012

Magnetic cryocooling with Gd3+ centers in a light and compact framework

The magnetocaloric effect of gadolinium formate, Gd(OOCH)3, is experimentally determined down to sub-Kelvin temperatures by direct and indirect methods. This 3D metal-organic framework material is characterized by a relatively compact crystal lattice of weakly interacting Gd3+ spin centers interconnected via light formate ligands, overall providing a remarkably large magnetic:non-magnetic elemental weight ratio. The resulting volumetric magnetic entropy change is decidedly superior in Gd(OOCH)3 than in the best known magnetic refrigerant materials for liquid-helium temperatures and low-moderate applied fields.

cond-mat.mtrl-sci↗

arXiv2013

Reduced graphene oxide thin films as ultrabarriers for organic electronics

Encapsulation of electronic devices based on organic materials that are prone to degradation even under normal atmospheric conditions with hermetic barriers is crucial for increasing their lifetime. A challenge is to develop 'ultrabarriers' that are impermeable, flexible, and preferably transparent. Another important requirement is that they must be compatible with organic electronics fabrication schemes (i.e. must be solution processable, deposited at room temperature and be chemically inert). Here, we report lifetime increase of 1,300 hours for poly(3-hexylthiophene) (P3HT) films encapsulated by uniform and continuous thin (~10 nm) films of reduced graphene oxide (rGO). This level of protection against oxygen and water vapor diffusion is substantially better than conventional polymeric barriers such as CytopTM, which degrades after only 350 hours despite being 400 nm thick. Analysis using atomic force microscopy, x-ray photoelectron spectroscopy and high resolution transmission electron microscopy suggest that the superior oxygen gas/moisture barrier property of rGO is due to the close interlayer distance packing and absence of pinholes within the impermeable sheets. These material properties can be correlated to the enhanced lag time of 500 hours. Our results provide new insight for the design of high performance and solution processable transparent 'ultrabarriers' for a wide range of encapsulation applications.

cond-mat.mtrl-sci↗

arXiv2012

Bistability loss as key feature in azobenzene (non-)switching on metal surfaces

Coinage metal adsorbed azobenzene is investigated as prototypical molecular switch. It is shown that switching capabilities are not just lost due to excited state quenching, but already due to changes in the ground state energetics. Electron demanding coadsorbates are suggested as strategy to regain the switching function.

cond-mat.mtrl-sci↗

arXiv2013

Bottom-up Engineering of Diamond Nanostructures

Engineering nanostructures from the bottom up enables the creation of carefully engineered complex structures that are not accessible via top down fabrication techniques, in particular, complex periodic structures for applications in photonics and sensing. In this work, we propose and demonstrate a bottom up approach that can be adopted and utilized to controllably build diamond nanostructures. A realization of periodic structures and optical wave-guiding is achieved by growing nanoscale single crystal diamond through a defined pattern.

cond-mat.mtrl-sci↗

arXiv2013

12-band $\textbf{k}\cdot\textbf{p}$ model for dilute bismide alloys of (In)GaAs derived from supercell calculations

Incorporation of bismuth (Bi) in dilute quantities in (In)GaAs has been shown to lead to unique electronic properties that can in principle be exploited for the design of high efficiency telecomm lasers. This motivates the development of simple models of the electronic structure of these dilute bismide alloys, which can be used to evaluate their potential as a candidate material system for optical applications. Here, we begin by using detailed calculations based on an $sp^{3}s^{*}$ tight-binding model of (In)GaBi$_{x}$As$_{1-x}$ to verify the presence of a valence band-anticrossing interaction in these alloys. Based on the tight-binding model the derivation of a 12-band $\textbf{k}\cdot\textbf{p}$ Hamiltonian for dilute bismide alloys is outlined. We show that the band structure obtained from the 12-band model is in excellent agreement with full tight-binding supercell calculations. Finally, we apply the 12-band model to In$_{0.53}$Ga$_{0.47}$Bi$_{x}$As$_{1-x}$ and compare the calculated variation of the band gap and spin-orbit-splitting to a variety of spectroscopic measurements performed on a series of MBE-grown In$_{0.53}$Ga$_{0.47}$Bi$_{x}$As$_{1-x}$/InP layers.

cond-mat.mtrl-sci↗

arXiv2013

Quadratic-in-magnetization permittivity and conductivity tensor in cubic crystals

We present diagonal and off-diagonal elements of the permittivity and conductivity tensor up to the second order in magnetization for cubic crystals. We express all tensor elements as a function of a general sample orientation, for arbitrary magnetization direction and for (001), (011) and (111) surface orientations. Finally, we discuss, how to extract values of quadratic elements G of the second order permittivity tensors for different sample surface orientations from both experiment and ab-initio calculations.

cond-mat.mtrl-sci↗

arXiv2013

Hubbard-corrected DFT energy functionals: the LDA+U description of correlated systems

The aim of this review article is to assess the descriptive capabilities of the Hubbard-rooted LDA+U method and to clarify the conditions under which it can be expected to be most predictive. The paper illustrates the theoretical foundation of LDA+U and prototypical applications to the study of correlated materials, discusses the most relevant approximations used in its formulation, and makes a comparison with other approaches also developed for similar purposes. Open "issues" of the method are also discussed, including the calculation of the electronic couplings (the Hubbard U), the precise expression of the corrective functional and the possibility to use LDA+U for other classes of materials. The second part of the article presents recent extensions to the method and illustrates the significant improvements they have obtained in the description of several classes of different systems. The conclusive section finally discusses possible future developments of LDA+U to further enlarge its predictive power and its range of applicability.

cond-mat.mtrl-sci↗

arXiv2013

Self-Assembly of Semiconducting Single-Walled Carbon Nanotubes into Dense, Aligned Rafts

Single-walled carbon nanotubes are promising nanoelectronic materials but face long-standing challenges including production of pure semiconducting SWNTs and integration into ordered structures. Here, highly pure semiconducting single-walled carbon nanotubes are separated from bulk materials and self-assembled into densely aligned rafts driven by depletion attraction forces. Microscopy and spectroscopy revealed a high degree of alignment and a high packing density of ~100 tubes/micron within SWNT rafts. Field-effect transistors made from aligned SWNT rafts afforded short channel (~150 nm long) devices comprised of tens of purely semiconducting SWNTs derived from chemical separation within a < 1 micron channel width, achieving unprecedented high on-currents (up to ~120 microamperes per device) with high on/off ratios. The average on-current was ~ 3-4 microamperes per tube. The results demonstrated densely aligned high quality semiconducting SWNTs for integration into high performance nanoelectronics.

physics.chem-ph↗

arXiv2018

Application of compact TiO$_2$ layer fabricated by pulsed laser deposition in organometal trihalide perovskite solar cells

Organometal trihalide perovskite solar cells have been rapidly developed and attracted much attention in recent years due to their high photoelectric conversion efficiency and low cost. Pulsed laser deposition (PLD) is a widely adopted technology which is used in the preparation of thin films, especially oxide thin films. With this technology, the thickness and composition of films can be conveniently and accurately controlled. In the structure of perovskite solar cells, TiO$_2$ layer working as the n-type semiconductor is used to block holes and transport electrons into electrode, which is crucial for the performance of whole devices. We introduced the PLD technique into preparation of TiO$_2$ layer. In comparison with common spin coating method, TiO$_2$ layer prepared by this technique is ultrathin and more compact. Compact TiO$_2$ (c-TiO$_2$) layers with optimized thickness of 32 nm have been prepared by the PLD method and the highest efficiency of 13.95 % for the MAPbI$_3$-based solar cell devices has been achieved.

physics.app-ph↗

arXiv2012

Thermoelectricity in ternary rare-earth systems

Crystallographic data, Seebeck coefficient, electrical resistance and thermal conductivity are reported for a large number of rare-earth compounds, manifestations of the Kondo effect being discussed. In more detail, thermoelectric properties of Yb3Co4Ge13 and Yb3Co4Sn13 compounds and Yb2CeCo4Ge13 and Yb2.3La0.7Co4Ge13 solid solutions are presented.

cond-mat.mtrl-sci↗

arXiv2013

The $\mathbf Γ$-limit of the two-dimensional Ohta-Kawasaki energy. I. Droplet density

This is the first in a series of two papers in which we derive a $Γ$-expansion for a two-dimensional non-local Ginzburg-Landau energy with Coulomb repulsion, also known as the Ohta-Kawasaki model in connection with diblock copolymer systems. In that model, two phases appear, which interact via a nonlocal Coulomb type energy. We focus on the regime where one of the phases has very small volume fraction, thus creating small "droplets" of the minority phase in a "sea" of the majority phase. In this paper we show that an appropriate setting for $Γ$-convergence in the considered parameter regime is via weak convergence of the suitably normalized charge density in the sense of measures. We prove that, after a suitable rescaling, the Ohta-Kawasaki energy functional $Γ$-converges to a quadratic energy functional of the limit charge density generated by the screened Coulomb kernel. A consequence of our results is that minimizers (or almost minimizers) of the energy have droplets which are almost all asymptotically round, have the same radius and are uniformly distributed in the domain. The proof relies mainly on the analysis of the sharp interface version of the energy, with the connection to the original diffuse interface model obtained via matching upper and lower bounds for the energy. We thus also obtain a characterization of the limit charge density for the energy minimizers in the diffuse interface model.

math-ph↗

arXiv2012

Hard X-ray photoelectron spectroscopy on buried, off-stoichiometric CoxMnyGez (x : z = 2 : 0.38) Heusler thin films

Fully epitaxial magnetic tunnel junctions (MTJs) with off-stoichiometric Co2-based Heusler alloy shows a intense dependency of the tunnel magnetoresistance (TMR) on the Mn composition, demonstrating giant TMR ratios of up to 1995% at 4.2 K for 1. This work reports on the electronic structure of non-stoichiometric CoxMnyGez thin films with a fixed Co/Ge ratio of x : z = 2 : 0.38. The electronic structure was investigated by high energy, hard X-ray photoelectron spectroscopy combined with first-principles calculations. The high-resolution measurements of the valence band of the non-stoichiometric CoxMnyGez films close to the Fermi energy indicate a shift of the spectral weight compared to bulk Co2MnGe. This is in agreement with the changes in the density of states predicted by the calculations. Furthermore it is shown that the co-sputtering of Co2MnGe together with additional Mn is an appropriate technique to adjust the stoichiometry of the CoxMnyGez film composition. The resulting changes of the electronic structure within the valence band will allow to tune the magnetoresistive characteristics of CoxMnyGez based tunnel junctions as verified by the calculations and photoemission experiments.

cond-mat.mtrl-sci↗

arXiv2013

The influence of ultrafast temporal energy regulation on the morphology of Si surfaces through femtosecond double pulse laser irradiation

The effect of ultrashort laser-induced morphological changes upon irradiation of silicon with double pulse sequences is investigated under conditions that lead to mass removal. The temporal delay between twelve double and equal-energy pulses (Ep=0.24J/cm2 each, with pulse duration tp=430fs, 800nm laser wavelength) was varied between 0 and 14ps and a decrease of the damaged area, crater depth size and periodicity of the induced subwavelength ripples (by 3-4%) was observed with increasing pulse delay. The proposed underlying mechanism is based on the combination of carrier excitation and energy thermalization and capillary wave solidification and aims to provide an alternative explanation to the control of ripple periodicity by temporal pulse tailoring. This work demonstrates the potential of pulse shaping technology to improve nano/micro processing.

cond-mat.mtrl-sci↗