We present an extensive study on the effect of substrate orientation, strain, stoichiometry, and defects on spin-ice physics in ${\mathrm{Ho}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ thin films grown onto yttria-stabilized-zirconia substrates. We find that growth in different orientations produces different strain states in the films. All films exhibit similar $c$-axis lattice parameters for their relaxed portions, which are consistently larger than the bulk value of 10.1 Å. Transmission electron microscopy reveals antisite disorder and growth defects to be present in the films, but evidence of stuffing is not observed. The amount of disorder depends on the growth orientation, with the (110) film showing the least. Magnetization measurements at 1.8 K show the expected magnetic anisotropy and saturation magnetization values associated with a spin ice for all orientations; shape anisotropy is apparent when comparing in- and out-of-plane directions. Significantly, only the (110)-oriented films display the hallmark spin-ice plateau state in magnetization, albeit less well defined compared to the plateau observed in a single crystal. Neutron-scattering maps on the more disordered (111)-oriented films show the $Q$ = 0 phase previously observed in bulk materials, but the $Q$ = $X$ phase giving the plateau state remains elusive. We conclude that the spin-ice physics in thin films is modified by defects and strain, leading to a reduction in the temperature at which correlations drive the system into the spin-ice state.

Dilute impurities and growth conditions can drastically affect the transport properties of ${\mathrm{TiSe}}_2$, especially below the charge-density-wave transition. In this paper, we discuss the effects of cooling rate, annealing time, and annealing temperature on the transport properties of ${\mathrm{TiSe}}_2$: slow cooling polycrystalline ${\mathrm{TiSe}}_2$ postsynthesis drastically increases the low-temperature resistivity, which is in contrast to the metallic behavior of single-crystalline ${\mathrm{TiSe}}_2$ attributed to charge doping from the residual iodine transport agent. A logarithmic increase of resistivity upon cooling and negative magnetoresistance with a sharp cusp around zero field are observed for the first time for the polycrystalline ${\mathrm{TiSe}}_2$ samples, pointing to weak-localization effects due to low dimensionality. Annealing at low temperatures has a similar, but less drastic effect. Furthermore, rapid quenching of the polycrystalline samples from high temperatures freezes in disorder, leading to a decrease in the low-temperature resistivity.

Atomic defects have a significant impact on the low-energy properties of graphene systems. By means of first-principles calculations and tight-binding models we provide evidence that chemical impurities modify both the normal and the superconducting states of twisted bilayer graphene. A single hydrogen atom attached to the bilayer surface yields a triple-point crossing, whereas self-doping and threefold symmetry breaking are created by a vacant site. Both types of defects lead to time-reversal symmetry breaking and the creation of local magnetic moments. Hydrogen-induced magnetism is found to exist also at the doping levels where superconductivity appears in magic-angle graphene superlattices. As a result, the coexistence of superconducting order and defect-induced magnetism yields in-gap Yu-Shiba-Rusinov excitations in magic-angle twisted bilayer graphene.

First-principles calculations are used to probe the effects of mechanical strain on the magnetic and optical properties of monolayer (ML) $2\mathrm{H}\text{−}\mathrm{TaS}{\mathrm{e}}_2$. A complex dependence of these physical properties on strain results in unexpected spin behavior, such as ferromagnetism under uniaxial, in-plane, tensile strain and a lifting of the Raman-active $E^{\prime }$ phonon degeneracy. We predict the noncollinear magnetic phase below 6% strain and a ferromagnet above 6% strain for some types of strain. While ferromagnetism is observed under compression along $\widehat{x}$ and expansion along $\widehat{y}$, no magnetic order occurs when interchanging the strain direction. The calculations show that the magnetic behavior of the system depends on the exchange within the $5d$ orbitals of the Ta atoms. The magnetic moment per Ta atom persists even when an additional compressive strain along the $\widehat{z}$ axis is added to a biaxially strained ML, which suggests stability of the magnetic order. Exploring the effects of this mechanical strain on the Raman-active phonon modes, we find that the $E^{″}$ and $E^{\prime }$ modes are red shifted due to Ta-Se bond elongation, and that strain lifts the $E^{\prime }$ mode degeneracy, except for the symmetrical biaxial tensile case. The electron-phonon interactions depend on both the amount of applied strain and the direction. Additionally, we note that the charge density wave (CDW) phase weakens the magnetism due to symmetry breaking and atomic displacements (Ta atoms) depending on the type and amount of applied strain. Phonon dispersion calculations confirm that using mechanical strain we can manipulate the unique CDW phase of ML-$2\mathrm{H}\text{−}\mathrm{TaS}{\mathrm{e}}_2$. Our results support the possibility of tuning the properties of two-dimensional transition metal dichalcogenide materials for nanoelectronic applications through strain.

Ni-Mn-$X$ ($X=\mathrm{In}$, Sn, and Sb) based Heusler alloys show a strong potential for magnetic refrigeration owing to their large magnetocaloric effect (MCE) associated with first-order magnetostructural transition. However, the irreversibility of the MCE under low field change of 0–1 T directly hinders its application as an efficient magnetic coolant. In this work, we systematically investigate thermal and magnetic properties, crystalline structure and magnetocaloric performance in $\mathrm{N}{\mathrm{i}}_{51-x}\mathrm{M}{\mathrm{n}}_{33.4}\mathrm{I}{\mathrm{n}}_{15.6}{\mathrm{V}}_x$ alloys. With the introduction of V, a stable magnetostructural transition near room temperature is observed between martensite and austenite. An extremely small hysteresis of 2.3 K is achieved for the composition $x=0.3$. Due to this optimization, the magnetic-field induced structural transition is partially reversible under 0–1 T cycles, resulting in a reversible MCE. Both magnetic and calorimetric measurements consistently show that the largest value for the reversible magnetic entropy change can reach about $5.1\mathrm{J}\mathrm{k}{\mathrm{g}}^{-1}{\mathrm{K}}^{-1}$ in a field change of 0–1 T. A considerable and reversible adiabatic temperature change of −1.2 K by the direct measurement is also observed under a field change of 0–1.1 T. Furthermore, the origin of this small hysteresis is discussed. Based on the lattice parameters, the transformation stretch tensor is calculated, which indicates an improved geometric compatibility between the two phases. Our work greatly improves the MCE performance of Ni-Mn-$X$-based alloys and make them suitable as realistic magnetic refrigeration materials.

Hot-deformed anisotropic $\mathrm{Nd}-\mathrm{Fe}-\mathrm{B}$ nanocrystalline magnets have been subjected to the grain-boundary diffusion process (GBDP) using a ${\mathrm{Pr}}_{70}{\mathrm{Cu}}_{30}$ eutectic alloy. The resulting grain microstructure, consisting of shape-anisotropic $\mathrm{Nd}-\mathrm{Fe}-\mathrm{B}$ nanocrystals surrounded by a $\mathrm{Pr}-\mathrm{Cu}$-rich intergranular grain-boundary phase, has been investigated using unpolarized small-angle neutron scattering and very small-angle neutron scattering. The neutron data have been analyzed using the generalized Guinier-Porod model and by computing, model independently, the distance distribution function. We find that the GBDP results in a change of the geometry of the scattering particles: In the small-$q$ regime, the scattering from the as-prepared sample exhibits a slope of about 2, which is characteristic for the scattering from two-dimensional platelet-shaped objects, while the GBDP sample manifests a slope of about 1, which is the scattering signature of one-dimensional elongated objects. The evolution of the Porod exponent indicates the smoothing of the grain surfaces due to the GBDP, which is accompanied by an increase of the coercivity.

Manganites are an important class of materials that exhibit a large magnetocaloric effect (MCE). The large MCE is often accompanied by complex magnetic phase transitions associated with electronic phase separation (EPS) in manganites. In previous studies, the effect of EPS on MCE has been examined in chemically synthesized manganite nanoparticles, in which the EPS vanishes when the particle size is smaller than a few tens of nanometers. However, the surface area to volume ratio is high in these small nanoparticles and surface spin disordering can lead to controversial results for MCE measurements. In this study, we used $\mathrm{L}{\mathrm{a}}_{0.425}\mathrm{P}{\mathrm{r}}_{0.2}\mathrm{C}{\mathrm{a}}_{0.375}\mathrm{Mn}{\mathrm{O}}_3$ nanodisk arrays fabricated from epitaxially grown thin films as model systems to investigate how MCE changes during a clean transition from the EPS state in thin film and 2-μm-sized disk arrays to a single ferromagnetic state in 500-nm-sized disk arrays. The MCE of the 500-nm-sized disk array is almost twice that of its parent film. Our results present an approach to improve the MCE in manganite systems for future magnetic refrigeration applications.

We present a Josephson junction based on a Ge-Si core-shell nanowire with transparent superconducting Al contacts, a building block which could be of considerable interest for investigating Majorana bound states, superconducting qubits, and Andreev (spin) qubits. We demonstrate the dc Josephson effect in the form of a finite supercurrent through the junction and establish the ac Josephson effect by showing up to 23 Shapiro steps. We observe multiple Andreev reflections up to the sixth order, indicating that charges can scatter elastically many times inside our junction and that our interfaces between superconductor and semiconductor are transparent and have low disorder.

The design of multifunctional bulk metallic glasses is limited by the lack of a quantitative understanding of the variables that control the glass-forming ability of alloys. Both geometric frustration (e.g., differences in atomic radii) and energetic frustration (e.g., differences in the cohesive energies of the atomic species) contribute to the glass-forming ability. We perform molecular dynamics simulations of binary Lennard-Jones mixtures with only energetic frustration. We show that there is little correlation between the heat of mixing $\mathrm{\Delta }{H}_{\mathrm{mix}}$ and critical cooling rate $R_c$, below which the system crystallizes, except that $\mathrm{\Delta }{H}_{\mathrm{mix}}<0$. By removing the effects of geometric frustration, we show strong correlations between $R_c$ and the variables ${\epsilon }_{-}=({\epsilon }_{BB}-{\epsilon }_{AA})/({\epsilon }_{AA}+{\epsilon }_{BB})$ and ${\overline{\epsilon }}_{AB}=2{\epsilon }_{AB}/({\epsilon }_{AA}+{\epsilon }_{BB})$, where ${\epsilon }_{AA}$ and ${\epsilon }_{BB}$ are the cohesive energies of atoms $A$ and $B$ and ${\epsilon }_{AB}$ is the pair interaction between $A$ and $B$ atoms. We identify a particular composition-dependent combination of ${\epsilon }_{-}$ and ${\overline{\epsilon }}_{AB}$ that collapses the data for $R_c$ over nearly 4 orders of magnitude in cooling rate. By performing local structural analyses, we find that energetic frustration, even in the absence of geometric frustration, can induce short-range fivefold symmetric order that impedes crystallization. This result emphasizes that energetic frustration plays an important role in determining the glass-forming ability, and thus it should be taken into account in the design of new metallic glass formers.

Hybrid heterostructures, made of organic molecules adsorbed on two-dimensional metal monochalcogenide, generally unveil interfacial effects that improve the electronic properties of the single constitutive layers. Here, we investigate the interfacial electronic characteristics of the F4-TCNQ/single-layer GaSe heterostructure. A sharp F4-TCNQ/GaSe interface has been obtained and characterized by x-ray photoemission spectroscopy. We demonstrate that a high electron transfer from 1TL GaSe into the adsorbed F4-TCNQ molecules takes place, thereby yielding a reduction in the excess negative charge density of GaSe. Additionally, the direct band-structure determination of the heterostructure has been carried out using angle-resolved photoemission spectroscopy, shedding light on essential features such as doping and band offset at the interface. Our results indicate that the buried 1TL GaSe below the F4-TCNQ layer exhibits a robust inversion of the valence dispersion at the Γ point, forming a Mexican-hat-shaped dispersion with 120 ± 10 meV of depth. Our experiments also reveal that F4-TCNQ can significantly tune the electronic properties of 1TL GaSe by shifting the band offset of about 0.16 eV toward lower binding energies with respect to the Fermi level, which is a key feature for envisioning its applications in nanoelectronics.

Recent developments in electrical transportation and renewable energies have significantly increased the demand of hard magnetic materials with a reduced critical rare-earth content, but with properties comparable to (Nd,Dy)-Fe-B permanent magnets. Though promising alternative compositions have been identified in high-throughput screenings, the thermodynamic stability of these phases against decomposition into structures with much less favorable magnetic properties is often unclear. In the case of Ce-Fe-Ti alloys, we have used finite temperature ab initio methods to provide this missing information. Employing state-of-the-art approaches for vibrational, electronic, and magnetic entropy contributions, the Helmholtz free energy, $F(T,V)$, is calculated for the desired hard magnetic ${\mathrm{CeFe}}_{11}\mathrm{Ti}$ phase and all relevant competing phases. The latter have been confirmed experimentally by employing reactive crucible melting (RCM) and energy-dispersive x-ray spectroscopy (EDS). Our ab initio based free energy calculations reveal that the presence of the ${\mathrm{CeFe}}_2$ Laves phase suppresses the formation of ${\mathrm{CeFe}}_{11}\mathrm{Ti}$ up to 700 K. The result is in agreement with RCM, in which ${\mathrm{CeFe}}_{11}\mathrm{Ti}$ is only observed above 1000 K, while the ${\mathrm{CeFe}}_2$ and ${\mathrm{Ce}}_2{\mathrm{Fe}}_{17}$ phases are stable at lower temperatures.

Ferroelectrics are a technologically important class of materials that are used in actuators, sensors, transducers, and memory devices. Introducing porosity into these materials offers a method of tuning functional properties for certain applications, such as piezo- and pyroelectric sensors and energy harvesters. However, the effect of porosity on the polarization switching behavior of ferroelectrics, which is the fundamental physical process determining their functional properties, remains poorly understood. In part, this is due to the complex effects of porous structure on the local electric field distributions within these materials. To this end, freeze-cast porous lead zirconate titanate (PZT) ceramics were fabricated with highly oriented, anisometric pores and an overall porosity of 34 vol.%. Samples were sectioned at different angles relative to the freezing direction, and the effect of pore angle on the switching behavior was tracked by measuring simultaneously the temporal polarization and strain responses of the materials to high-voltage pulses. Finite-element modeling was used to assess the effect of the pore structure on the local electric field distributions within the material, providing insight into the experimental observations. It is shown that increasing the pore angle relative to the applied electric field direction decreases the local electric field, resulting in a reduced domain-wall dynamic and a broadening of the distribution of switching times. Excellent longitudinal piezoelectric ($d_{33}=630\mathrm{pm}/\mathrm{V}$) and strain responses ($S_{\mathrm{bip}}=0.25\%$ and $S_{\mathrm{neg}}=0.13\%$, respectively), comparable to the dense material ($d_{33}=648\mathrm{pm}/\mathrm{V}$, $S_{\mathrm{bip}}=0.31\%$, and $S_{\mathrm{neg}}=0.16\%$), were found in the PZT with anisometric pores aligned with the poling axis. Orienting the pores perpendicular to the poling axis resulted in the largest reductions in the effective permittivity (${\varepsilon }_{33}^{\sigma }=200$ compared to ${\varepsilon }_{33}^{\sigma }=4100$ for the dense PZT at 1 kHz), yielding the highest piezoelectric voltage coefficient ($g_{33}=216\times {10}^{-3}\mathrm{Vm}/\mathrm{N}$) and energy-harvesting figure of merit ($d_{33}{g}_{33}=73\times {10}^{-12}{\mathrm{m}}^2/\mathrm{N}$). These results demonstrate that a wide range of application-specific properties can be achieved by careful control of the porous microstructure. This work provides an understanding of the interplay between the local electric field distribution and polarization reversal in porous ferroelectrics, which is an important step towards further improving the properties of this promising class of materials for sensing, energy harvesting, and low-force actuators.

Solid-state technology based on thermoelectric (TE) materials enables the conversion of heat into electricity offering an environmentally friendly solution to energy conservation. Here, we use density functional theory calculations to show that ${\mathrm{BaAgYTe}}_3$, a layered semiconductor, exhibits low lattice thermal conductivity (${\kappa }_l$) and a high thermoelectric figure of merit. Our calculations reveal that the presence of bonding inhomogeneity, resulting from the coexisting rigid and fluctuating sublattices, favorably helps in both the electronic and phonon transports. We show that low ${\kappa }_l$ in this compound mainly originates from (a) the small group velocities of the acoustic modes, (b) quasilocalized low-frequency optical phonons that give rise to multiple scattering channels, and (c) strong lattice anharmonicity. While the calculations of the atomic displacement parameters and bonding analysis reveal relatively weaker bonding of Ag atoms and establish the heterogeneity in the chemical bonding, the strong anharmonicity is manifested in the large mode Gruneisen parameters. Thus, our work provides a theoretical prediction that warrants experimental verification and should encourage further exploration of potential TE materials in the same crystal family.

There is enormous recent interest in weak, van der Waals-type (vdW) interactions due to their fundamental relevance for two-dimensional materials and the so-called vdW heterostructures. Tackling this problem using computer simulation is very challenging due to the nontrivial, nonlocal nature of these interactions. We benchmark different treatments of London dispersion forces within the density functional theory (DFT) framework on hexagonal boron nitride or graphene monolayers on Ir(111) by comparing the calculated geometries to a comprehensive set of experimental data. The geometry of these systems crucially depends on the interplay between vdW interactions and wave function hybridization, making them excellent test cases for vdW-treated DFT. Our results show strong variations in the calculated atomic geometry. While some of the approximations reproduce the experimental structure, this is rather based on a posteriori comparison with the “target results.” General predictive power in vdW-treated DFT is not achieved yet and might require new approaches.