arXiv subjects

Tomáš Grabec

Publications and source records attributed to Tomáš Grabec.

6 recordsLinked to original sources

arXiv2026

Ultra-transient grating spectroscopy for visualization of surface acoustics

Ultrasonic wave propagation across material surfaces reveals essential information about the materials' elastic behavior. The elastodynamic response of the surface is characterized by the Green's function that fully captures all its direction-dependent and frequency-dependent features. Here we present the first direct experimental visualization of the frequency-domain angular-resolved Green's function, including all its complex details resulting from elastic anisotropy. We achieve this visualization using transient grating spectroscopy (TGS), which is a method otherwise well established for measuring Rayleigh-type surface acoustic waves (SAWs). But here we focus on early-time thermoacoustic phenomena in the TGS experiment, revealing that, along with the transient standing-wave patterns of SAWs, there also emerge oscillations with at least an order of magnitude shorter lifetimes. These oscillations superpose into dynamic displacement patterns that are transient with respect to the classical transient timescales in TGS; the optical diffraction signal from these 'ultra-transient' gratings enables capturing the surface acoustic response with exceptional detail, and the resulting experimental angular dispersion maps strikingly replicate the theoretical frequency-domain Green's functions. By utilizing this feature, ultra-transient grating spectroscopy (UTGS) becomes a powerful tool for detailed contactless characterization of anisotropic solids, opening new pathways for studying single-crystalline or nanostructured materials.

cond-mat.mtrl-sci↗

arXiv2026

On the proposed concept of mechanical phasons in Ni-Mn-Ga modulated martensite

We discuss modulation phasons as a possible source of unusual elastic behavior of five-layer modulated (10\,M) martensite of the Ni-Mn-Ga shape memory alloy. This material exhibits anomalous macroscopic shear compliance along specific planes perpendicular to the modulation vector, and this compliance disappears when the modulations become incommensurate. Using a simple mechanical model, we show that modulation phasons in Ni-Mn-Ga can have macroscopic mechanical manifestations, and that the resulting 'mechanical phasons' can relax external shear loadings for commensurate and weakly incommensurate modulations, but not for strongly incommensurate modulations. The model merges ideas from the adaptive martensite theory and electronic-structure considerations, and enables straightforward explanations of several properties of the 10\,M lattice, such as spontaneous monoclinic distortion or easy formation and propagation of $a/b$ twins.

cond-mat.mtrl-sci↗

arXiv2026

Anomalous thermal and elastic properties of an epitaxial NiTi film exhibiting R-phase

Shape memory alloys like NiTi are at the core of emerging thermal management applications, including elastocaloric refrigeration, thermoelastic harvesting, and latent heat storage. Most of these applications benefit from a small scale due to the accelerated heat exchange, but obtaining precise functional properties of films is challenging. Here we demonstrate that transient grating spectroscopy (TGS) enables characterization of elastic coefficients and thermal diffusivity of a 3 $μ$m thick epitaxial NiTi film during a thermally induced phase transformation. The in-situ measurement of a complete austenite$\rightarrow$R-phase$\rightarrow$martensite$\rightarrow$austenite temperature cycle reveals that the elastic properties exhibit a crossover of the shear moduli (from $c^\prime < c_{44}$ in austenite to $c^\prime > c_{44}$ in martensite) and that the thermal diffusivity changes by 450 $\%$ between the R-phase and austenite. This dramatic change, together with the absence of hysteresis between the R-phase and austenite, makes NiTi a promising material candidate for thermal switches. The results indicate that the change in thermal diffusivity originates from an anomalous heat capacity of the R-phase. Furthermore, our TGS study provides temperature-dependent thermal and elastic properties required for simulating thermal management microsystems using this material.

cond-mat.mtrl-sci↗

arXiv2026

Study of the acoustic and thermal response of an elastically anisotropic solid to a sub-nanosecond laser pulse in transient grating spectroscopy

Transient grating spectroscopy (TGS) is a material characterization technique based on laser-induced thermoelastic excitation of thermal and acoustic gratings. On opaque samples, these gratings are dynamic surface displacements that reflect the sample's elastic and thermal properties, enabling both types of parameters to be determined from a single experiment. Here, we develop a detailed finite element model (FEM) of the TGS experiment that fully captures the coupling between the thermal and mechanical fields, as well as the optical detection of surface displacement using a heterodyning approach. Using custom-designed two-dimensional elements, the model is particularly suitable for analyzing TGS measurements on anisotropic media, for which analytical theory is insufficient. The simulation captures not only the anisotropic relaxation of the thermoelastic field but also several acoustic features that arise at very short (ultra-transient) timescales and provide additional information about the elastic properties of the examined material. The model offers new opportunities for the in silico testing of various modifications of TGS experiments and their applications to a broad class of materials.

cond-mat.mtrl-sci↗

arXiv2026

Detecting the Onset and Progression of Spinodal Decomposition using Transient Grating Spectroscopy

Spinodal decomposition can degrade corrosion resistance and embrittle materials. The ability to quickly, conclusively, and non-destructively detect the onset of spinodal decomposition before catastrophic materials degradation would represent a significant advance in materials testing. We demonstrate that spinodal decomposition can be detected in binary Fe-Cr alloys via modulus stiffening using in situ and ex situ transient grating spectroscopy (TGS). The key mechanistic insight is the non-linearity in elastic moduli as function of Cr content renders a spinodally decomposed Fe-Cr alloy stiffer than an equivalent solid solution for a certain range of initial chromium compositions. We confirm the presence of spinodal decomposition in the 36 at.% chromium alloy using differential scanning calorimetry (DSC), linked to known spinodal decomposition energetics, and show via atomistic simulations that elastic modulus stiffening is expected after spinodal decomposition in the 36 at.% chromium alloy. The results of this study suggest the potential use of TGS as a practical tool for non-destructive evaluation of key materials susceptible to such degradation.

cond-mat.mtrl-sci↗

arXiv2024

Compliant Lattice Modulations Enable Anomalous Elasticity in Ni-Mn-Ga Martensite

High mobility of twin boundaries in modulated martensites of Ni-Mn-Ga-based ferromagnetic shape memory alloys holds a promise for unique magnetomechanical applications. This feature has not been fully understood so far, and in particular it has yet not been unveiled what makes the lattice mechanics of modulated Ni-Mn-Ga specifically different from other martensitic alloys. Here, results of dedicated laser-ultrasonic measurements on hierarchically twinned five-layer modulated (10 M) crystals fill this gap. Using a combination of transient grating spectroscopy and laser-baser resonant ultrasound spectroscopy, it is confirmed that there is a shear elastic instability in the lattice, being significantly stronger than in any other martensitic material and also than what the first-principles calculations for Ni-Mn-Ga predict. The experimental results reveal that the instability is directly related to the lattice modulations. A lattice-scale mechanism of dynamic faulting of the modulation sequence that explains this behavior is proposed; this mechanism can explain the extraordinary mobility of twin boundaries in 10 M.

cond-mat.mtrl-sci↗