Anomalous diffraction of Bijvoet pairs (BPs) is a fundamental method of structure analyses in crystallography. We demonstrate that the diffraction intensity ratio of a BP always intrinsically equals the two structure factors’ squared modulus ratio instead of the modulus ratio that has been believed to govern dynamical diffraction. High-resolution experiments of perfect quartz precisely proved this principle in pure dynamical diffraction. The underlying mechanism is that the elementary diffraction functions of BPs always obey the squared modulus ratio rule, which is universally valid for both kinematical and dynamical diffraction in any diffraction geometry. This fundamental mechanism clarifies one of the long-lasting ambiguities in x-ray crystallography and paves the way for various applications of high-resolution diffraction of BPs, particularly for large high-quality crystals.

We report on time-resolved mid-infrared-pump terahertz-transmission-probe studies of the topological-insulator materials ${\left({\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x\right)}_2{\mathrm{Te}}_3$, in which by varying $x$ charge carriers are chemically tuned to be of $n$-type or $p$-type. Relaxation dynamics is found to be different in various aspects for transitions below or above the band gap, which are selectively excited by changing the pump-pulse energy. For the below-band-gap excitation, an exponential decay of the pump-probe signals is observed, which exhibits linear dependence on the pump-pulse fluence. In contrast, the relaxation dynamics for the above-band-gap excitation is characterized by a compressed exponential decay and nonlinear fluence dependence at high pump flunences, which reflects interaction of the excited nonequilibrium states.

The inverse magnetocaloric effect in Ni-Mn based Heusler compounds occurs during the magnetostructural transition between low-temperature, low-magnetization martensite and high-temperature, high-magnetization austenite. In this study, we analyze the metamagnetic transformation of a ${\mathrm{Ni}}_{49.8}{\mathrm{Mn}}_{35}{\mathrm{In}}_{15.2}$ compound by simultaneous adiabatic temperature change $\mathrm{\Delta }{T}_{ad}$ and strain $\mathrm{\Delta }l/l_0$ measurements in pulsed magnetic fields up to 10 T. We observe a $\mathrm{\Delta }{T}_{ad}$ of −10 K and a $\mathrm{\Delta }l/l_0$ of −0.22% when the reverse martensitic transition is fully induced at a starting temperature of 285 K. By a variation of the magnetic field-sweep rates between 316, 865, and $1850\mathrm{T}{\mathrm{s}}^{-1}$, the transitional dynamics of the reverse martensitic transformation have been investigated. Our experiments reveal an apparent delay upon the end of the reverse martensitic transformation at field rates exceeding $865\mathrm{T}{\mathrm{s}}^{-1}$ which is related to the annihilation of retained martensite. As a consequence, the field hysteresis increases and higher fields are required to saturate the transition. In contrast, no time-dependent effects on the onset of the reverse martensitic transformation were observed in the studied field-sweep range. Our results demonstrate that kinetic effects in Heusler compounds strongly affect the magnetic cooling cycle, especially when utilizing a multicaloric “exploiting-hysteresis cycle” where high magnetic field-sweep rates are employed.

An electronic-driven liquid-liquid phase transition in rubidium is revealed by picosecond acoustic measurements combined with ab initio calculations. Picosecond acoustics were used to measure the melting line up to 10 GPa, finding the maximum in the melting curve at 7 GPa and 555 K. We observe the onset of a continuous liquid-liquid phase transition beginning around the melting maxima through until 16 GPa. Sound velocity shows a softening similar to that reported for liquid caesium, caused by a change in the bulk modulus during a crossover from the low-density to the high-density liquid. Guided by the ab initio calculations, we relate the changes in the thermoelastic properties to the progressive localization of the valence electrons in the pressure range of 6–16 GPa. At high pressure rubidium forms an electride liquid quantified by the appearance of interstitial quasiatoms localized in the valence electron density.

Competing interactions produce finite-size textures in myriad condensed matter systems, typically forming elongated stripe or round bubble domains. Transitions between stripe and bubble phases, driven by field or temperature, are expected to be reversible in nature. Here we report on the distinct character of the analogous transition for nanoscale spin textures in chiral Co/Pt-based multilayer films, known to host Néel skyrmions, using microscopy, magnetometry, and micromagnetic simulations. Upon increasing field, individual stripes fission into multiple skyrmions, and this transition exhibits a macroscopic signature of irreversibility. Crucially, upon field reversal, the skyrmions do not fuse back into stripes, with many skyrmions retaining their morphology down to zero field. Both the macroscopic irreversibility and the microscopic zero-field skyrmion density are governed by the thermodynamic material parameter determining chiral domain stability. These results establish the thermodynamic and microscopic framework underlying ambient skyrmion generation and stability in chiral multilayer films and provide immediate directions for their functionalization in devices.

A detailed electronic phase diagram of perovskite-type oxides ${\mathrm{Sr}}_{1–x}{\mathrm{La}}_x\mathrm{Fe}{\mathrm{O}}_3$ ($0\leqq x\leqq 0.5$) was established by synchrotron x-ray diffraction, magnetization, and transport measurements for polycrystalline samples synthesized by a high-pressure technique. Among three kinds of helimagnetic phases in $\mathrm{SrFe}{\mathrm{O}}_3$ at zero field, two of them showing multiple-$\mathit{q}$ helimagnetic spin textures tend to rapidly disappear in cubic symmetry upon the La substitution with $x$ less than 0.1, which accompanies the loss of metallic nature. On the other hand, the third helimagnetic phase apparently remains robustly in ${\mathrm{Sr}}_{1–x}{\mathrm{La}}_x\mathrm{Fe}{\mathrm{O}}_3$ with $x$ higher than 0.1, followed by merging to the spin/charge ordered phase with $x\sim 1/3$. We propose an important role of itinerant ligand holes on the emergence of multiple-$\mathit{q}$ states and a possible link between the third (putative single-$\mathit{q}$) helimagnetic phase in $\mathrm{SrFe}{\mathrm{O}}_3$ and the spin/charge ordered phase in ${\mathrm{Sr}}_{2/3}{\mathrm{La}}_{1/3}\mathrm{Fe}{\mathrm{O}}_3$.

A combined experimental and computational study is reported on a hitherto unrecognised single lanthanide catalyst for the breaking of molecular nitrogen and formation of ammonia at ambient temperature and low pressure. We combine in situ electrical conductance and electron diffraction measurements to track the conversion from the lanthanide metals to the insulating lanthanide nitrides. The efficiency of the conversion is then interpreted using $\mathrm{DFT}+U$ calculations, suggesting a molecular nitrogen dissociation pathway separate from that well established for transition metals. Finally, we show that exposure of the lanthanide surfaces to both molecular nitrogen and hydrogen results in the formation of ammonia.

We report active control of the friction force at the contact between a nanoscale asperity and a ${\mathrm{La}}_{0.55}{\mathrm{Ca}}_{0.45}\mathrm{Mn}{\mathrm{O}}_3$ (LCMO) thin film using electric fields. We use friction force microscopy under ultrahigh vacuum conditions to measure the friction force as we change the film resistive state by electric-field-induced resistive switching. Friction forces are high in the insulating state and clearly change to lower values when the probed local region is switched to the conducting state. Upon switching back to an insulating state, the friction forces increase again. Thus we demonstrate active control of friction without having to change the contact temperature or pressure. By comparing with measurements of friction at the metal-to-insulator transition and with the effect of applied voltage on adhesion, we rule out electronic excitations, electrostatic forces, and changes in contact area as the reasons for the effect of resistive switching on friction. Instead, we argue that friction is limited by phonon relaxation times, which are strongly coupled to the electronic degrees of freedom through distortions of the $\mathrm{Mn}{\mathrm{O}}_6$ octahedra. The concept of controlling friction forces by electric fields should be applicable to any materials where the field produces strong changes in phonon lifetimes.

A ferromagnetic (FM) thin film deposited on a substrate of $\mathrm{Pb}\left({\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}\right){\mathrm{O}}_3\text{−}{\mathrm{PbTiO}}_3$ (PMN-PT) is an appealing heterostructure for the electrical control of magnetism, which would enable nonvolatile memories with ultralow-power consumption. Reversible and electrically controlled morphological changes at the surface of PMN-PT suggest that the magnetoelectric effects are more complex than the commonly used “strain-mediated” description. Here we show that changes in substrate morphology intervene in magnetoelectric coupling as a key parameter interplaying with strain. Magnetic-sensitive microscopy techniques are used to study magnetoelectric coupling in Fe/PMN-PT at different length scales, and compare different substrate cuts. The observed rotation of the magnetic anisotropy is connected to the changes in morphology, and mapped in the crack pattern at the mesoscopic scale. Ferroelectric polarization switching induces a magnetic field-free rotation of the magnetic domains at micrometer scale, with a wide distribution of rotation angles. Our results show that the relationship between the rotation of the magnetic easy axis and the rotation of the in-plane component of the electric polarization is not straightforward, as well as the relationship between ferroelectric domains and crack pattern. The understanding and control of this phenomenon is crucial to develop functional devices based on FM/PMN-PT heterostructures.

We report detailed magnetometry and high-frequency electron spin resonance (HF-ESR) measurements which allow detailed investigation on Li-Fe antisite disorder in single-crystalline $\mathrm{Li}\mathrm{Fe}\mathrm{P}{\mathrm{O}}_4$, i.e., exchange of ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Li}}^{+}$ ions. The data imply that magnetic moments of ${\mathrm{Fe}}^{2+}$ ions at Li positions do not participate in long-range antiferromagnetic order in $\mathrm{Li}\mathrm{Fe}\mathrm{P}{\mathrm{O}}_4$ but form quasifree moments. Anisotropy axes of the magnetic moments at antisite defects are attached to the main crystallographic directions. The local character of these moments is confirmed by associated linear resonance branches detected by HF-ESR studies. Magnetic anisotropy shows up in significant zero-field splittings of $\mathrm{\Delta }=220\left(3\right)$ GHz, ${\mathrm{\Delta }}^{\prime }\sim 50$ GHz, and a highly anisotropic $g$ factor, i.e., $g_a=1.4, g_b=2.0$, and $g_c=6.3$. We demonstrate a general method to precisely determine Fe-antisite disorder in $\mathrm{Li}\mathrm{Fe}\mathrm{P}{\mathrm{O}}_4$ from magnetic studies which implies a density of paramagnetic ${\mathrm{Fe}}^{2+}$ ions at Li positions of 0.53%.

The possibility of modifying the work function of electrodes is important for optimizing the energy barriers for charge injection (extraction) at the interface to an organic material. In this paper, we perform density-functional theory calculations to investigate the impact of dithiol-terminated polyethylene glycol [PEG(thiol)] based self-assembled monolayers with different numbers of PEG repeat units on the work function of the Au(111) surface. We find that a monolayer of PEG(thiol) decreases the work function of the Au(111) surface, where the magnitude of this reduction strongly depends on the length of the PEG backbone. The main contribution arises from the dipole due to the adsorption-induced charge rearrangement at the interface. Our work reveals a pronounced odd-even effect that can be traced back to the dipole moment of the PEG(thiol) layer.

As a prototypical Mott insulator with ferromagnetic ordering, ${\mathrm{YTiO}}_3$ (YTO) is of great interest in the study of strong electron correlation effects and orbital ordering. Here we report the first molecular beam epitaxy (MBE) growth of YTO films, combined with theoretical and experimental characterizations of the electronic structure and charge transport properties. The obstacles of YTO MBE growth are discussed and potential routes to overcome them are proposed. DC transport and Seebeck measurements on thin films and bulk single crystals identify $p$-type Arrhenius transport behavior with an activation energy of $\sim 0.17\mathrm{eV}$ in thin films, consistent with the energy barrier for small hole polaron migration from hybrid density functional theory calculations. Hard x-ray photoelectron spectroscopy measurements show the lower Hubbard band at 1.1 eV below the Fermi level, whereas a Mott-Hubbard band gap of $\sim 1.5\mathrm{eV}$ is determined from photoluminescence measurements. These findings provide critical insight into the electronic band structure of YTO and related materials.

Amorphous solids lack long-range order. Therefore identifying structural defects—akin to dislocations in crystalline solids—that carry plastic flow in these systems remains a daunting challenge. By comparing many different structural indicators in computational models of glasses, under a variety of conditions we carefully assess which of these indicators are able to robustly identify the structural defects responsible for plastic flow in amorphous solids. We further demonstrate that the density of defects changes as a function of material preparation and strain in a manner that is highly correlated with the macroscopic material response. Our work represents an important step towards predicting how and when an amorphous solid will fail from its microscopic structure.

The stilbazolium-based organic salt, DAST, is probably the most well-known commercial organic non-linear optical (NLO) material. Chemical derivatives of DAST have therefore been designed to engender even better performing NLO materials; the DAST-derivative, BP3, is one success in this regard. This paper provides a proof-of-principle in the use of structure factors, $F$, to determine the effects of the crystalline environment on intramolecular charge transfer (ICT), and thus the second-order NLO response of such molecules. A concerted experimental and computational approach is adopted. In particular, the application of the relatively new x-ray wave function refinement method, which tempers theoretical calculations with experimentally derived structure factors to yield an isolated molecule influenced by crystal-field forces, enables the impact of those crystal-field forces on ICT to be established. This study employs high-resolution x-ray diffraction data for its experimental component, this resolution being necessary given the marked challenges that are associated with the complicated interionic interactions within an organic salt. The results presented pinpoint the molecular-scale features that afford BP3 better second-order NLO prospects than DAST, laying the foundations for the molecular engineering of DAST derivatives that are better tailored to optimize NLO function.

We investigate the multidomain structures in the tetragonal and orthorhombic phases of ${\mathrm{BaTiO}}_3$ and the impact of the presence of domain walls on the intermediary phase transition. We focus on the change in the transition temperatures resulting from various types of domain walls and their coupling with an external electric field. We employ molecular dynamics simulations of an ab initio effective Hamiltonian in this study. After confirming that this model is applicable to multidomain configurations, we show that the phase-transition temperatures strongly depend on the presence of domains walls. Notably we show that elastic ${90}^{\circ }$ walls can strongly reduce thermal hysteresis. Further analysis shows that the change in transition temperatures can be attributed to two main factors: long-range monoclinic distortions induced by walls within domains and domain wall widths. We also show that the coupling with the field further facilitates the reduction of thermal hysteresis for orthorhombic ${90}^{\circ }$ walls, making this configuration attractive for future applications.

The characterization of simple elemental systems is key to benchmarking first-principles modeling of electronic and vibrational behaviors of materials. A large body of literature has been built for most elemental systems in the periodic table. However, surprisingly little neutron work has been performed to probe the vibrational properties of iridium, likely due to its large neutron absorption cross section. Nonetheless, iridium is of significant scientific and technological interest due to large relativistic electron effects and electron-phonon coupling, particularly in strongly correlated iridate compounds. Here, we report temperature-dependent inelastic neutron scattering measurements of the vibrational properties of iridium, from which we extract key thermodynamic properties. To overcome the challenge of the large neutron absorption of iridium, we developed a simple postprocessing correction procedure. The measured densities of phonon states compare well with quasiharmonic density functional theory calculations, although the obtained experimental phonon Grüneisen parameters are much larger than expected, reaching as high as $\gamma =4.5$, indicating substantial anharmonicity.