We report the discovery of a spin one diamond lattice in ${\mathrm{NiRh}}_2{\mathrm{O}}_4$. This spinel undergoes a cubic to tetragonal phase transition at $T=440$ K that leaves all nearest neighbor interactions equivalent. In the tetragonal phase, magnetization measurements show a ${\mathrm{Ni}}^{2+}$ effective moment of $p_{\mathrm{eff}}=3.3\left(1\right)$ and dominant antiferromagnetic interactions with ${\mathrm{\Theta }}_{\mathrm{CW}}=-11.3\left(7\right)$ K. No phase transition to a long-range magnetically ordered state is observed by specific heat measurements down to $T=0.1$ K. Inelastic neutron scattering measurements on substoichiometric ${\mathrm{NiRh}}_2{\mathrm{O}}_4$ reveal possible valence-bond behavior and show no visible signs of magnetic ordering. ${\mathrm{NiRh}}_2{\mathrm{O}}_4$ provides a platform on which to explore the previously unknown and potentially rich physics of spin one interacting on the diamond lattice, including the realization of theoretically predicted quantum spin liquid and topological paramagnet states.

We report on an enhancement of the superconducting transition temperature $\left(T_{\mathrm{c}}\right)$ of the FeSe-based electric-double-layer transistor (FeSe-EDLT) by applying the multivalent oligomeric ionic liquids (ILs). The IL composed of dimeric cation (divalent IL) enables a large amount of charge accumulation on the surface of the FeSe ultrathin film, resulting in inducing electron-rich conduction even in a rather thick 10 nm FeSe channel. The onset $T_{\mathrm{c}}$ in FeSe-EDLT with the divalent IL is enhanced to be approaching about 50 K at the thin limit, which is about 7 K higher than that in EDLT with conventional monovalent ILs. The enhancement of $T_{\mathrm{c}}$ is a pronounced effect of the application of the divalent IL, in addition to the large capacitance, supposing preferable interface formation of ILs driven by geometric and/or Coulombic effect. The present finding strongly indicates that multivalent ILs are powerful tools for controlling and improving physical properties of materials.

In 2012, Björkman et al. posed the question “Are we van der Waals ready?” [T. Björkman et al., J. Phys.: Condens. Matter 24, 424218 (2012)] about the ability of ab initio modeling to reproduce van der Waals (vdW) dispersion forces in layered materials. The answer at that time was no, however. Here we report on a new generation of vdW dispersion models and show that one, i.e., the fractionally ionic atom theory with many-body dispersions, offers close to quantitative predictions for layered structures. Furthermore, it does so from a qualitatively correct picture of dispersion forces. Other methods, such as D3 and optB88vdW, also work well, albeit with some exceptions. We thus argue that we are nearly vdW ready and that some modern dispersion methods are accurate enough to be used for nanomaterial prediction, albeit with some caution required.

We have grown and characterized 30-nm-thick ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ (YBCO) films, deposited by pulsed laser deposition on both MgO (110) and ${\mathrm{SrTiO}}_3$ (001) substrates, which induce opposite strain to the superconducting layer. By carefully tuning the in situ post-annealing oxygen pressure, we achieved, in a reproducible way, films at different oxygen doping, spanning from the slightly overdoped down to the strongly underdoped region of the phase diagram. The transport properties of the films, investigated through resistance versus temperature measurements, are in perfect qualitative agreement with single crystals. Starting from these films, we have also successfully fabricated nanowires with widths down to 65 nm, at different oxygen doping. The nanostructures exhibit characteristic temperatures (as the critical temperature $T_{\mathrm{c}}$ and the pseudogap temperature $T^{*})$ similar to those of the as-grown films and carry critical current densities $J_{\mathrm{c}}$ close to the critical depairing value, limited by vortex entry. This implies that the superconducting and the normal state properties of underdoped YBCO are preserved in our films, and they can be studied as a function of the dimensionality of the system, down to the nanoscale.

New chiral de Vries smectic liquid-crystalline compounds are designed, synthesized, and investigated for perspective applications in defect-free bistable surface-stabilized ferroelectric liquid-crystal displays. In these compounds, a 5-phenyl-pyrimidine benzoate core is terminated on one side by a tri- or tetra-carbosilane group linked through an alkoxy group and an alkyl spacer and on the opposite side terminated by a chiral 2-octanol group. The stereogenic center contains either a methyl or perfluoromethyl functional group. These compounds exhibit Iso-$\mathrm{Sm}{A}^{*}-\mathrm{Sm}{C}^{*}-\mathrm{Sm}X$-Cr phases under cooling from the isotropic state. Measurements of the temperature-dependent smectic layer spacing by x-ray diffraction experiments combined with the measured apparent optical tilt angle and the birefringence reveal that $\mathrm{Sm}{A}^{*}$ phase in these compounds is of the de Vries type. In addition, the chiral compound with a tetra-carbosilane backbone, DR277, exhibits good de Vries properties with the $\mathrm{Sm}{C}^{*}$ phase exhibited over a wide temperature range. By varying the carbosilane end group, the de Vries properties are enhanced, that is, the layer shrinkage of $\sim 1.9\%$ for the tri-carbosilane DR276 is reduced to $\sim 0.9\%$ for tetra-carbosilane DR277 at $10{}^{\circ }\mathrm{C}$ below $\mathrm{Sm}{A}^{*}$ to $\mathrm{Sm}{C}^{*}$ transition temperature, $T_{\mathrm{AC}}$. For DR277, the reduction factor $R\approx 0.22$ for $T=\left(T_{\mathrm{AC}}-10\right){}^{\circ }\mathrm{C}$ is reasonably low and the apparent optical tilt angle ${\theta }_{\mathrm{app}}=35.1^{\circ }$, hence this compound is a “good de Vries smectic” LC. Therefore, synthesis of the chiral mesogen with an even higher number of carbosilane groups may lead to a further reduction or even zero-layer shrinkage exhibited at $T_{\mathrm{AC}}$ with $\mathrm{Sm}{C}^{*}$ phase extending over a wide temperature range close to the room temperature for perspective suitability in device applications. Our results for 5-phenyl-pyrimidine benzoate core-based compounds support a recently drawn conclusion by Schubert et al. [J. Mater. Chem. C 4, 8483 (2016)] from a different compound, namely that a carbosilane backbone in chiral mesogens strongly influences the de Vries properties.

Recent studies have highlighted the importance of organic ligands in the field of molecular spintronics, via which delocalized electron-spin density can mediate magnetic coupling to otherwise localized $4f$ moments of lanthanide ions, which show tremendous potential for single-molecule device applications. To this end, high-field/high-frequency electron paramagnetic resonance (EPR) spectroscopy is employed to study a neutral terbium bis-phthalocyaninato metalorganic complex, ${\left[\mathrm{TbP}{\mathrm{c}}_2\right]}^0$, with the aim of understanding the magnetic interaction between the Ising-like moment of the lanthanide ion and the unpaired spin density on the coordinating organic radical ligand. The measurements were performed on a previously unknown ${\left[\mathrm{TbP}{\mathrm{c}}_2\right]}^0$ structural phase crystallizing in the Pnma space group. EPR measurements on powder samples of ${\left[\mathrm{TbP}{\mathrm{c}}_2\right]}^0$ reveal an anisotropic spectrum, which is attributed to the spin-$\frac{1}{2}$ radical coupled weakly to the EPR-silent $\mathrm{T}{\mathrm{b}}^{\mathrm{III}}$ ion. Extensive double-axis rotation studies on a single crystal reveal two independent spin-$\frac{1}{2}$ signals with differently oriented (albeit identical) uniaxial $g$-tensors, in complete agreement with x-ray structural studies that indicate two molecular orientations within the unit cell. The easy-axis nature of the radical EPR spectra thus reflects the coupling to the Ising-like $\mathrm{T}{\mathrm{b}}^{\mathrm{III}}$ moment. This is corroborated by studies of the isostructural ${\left[\mathrm{YP}{\mathrm{c}}_2\right]}^0$ analog (where Y is nonmagnetic yttrium), which gives a completely isotropic radical EPR signal. The experimental results for the terbium complex are well explained on the basis of an effective model that introduces a weak ferromagnetic Heisenberg coupling between an isotropic spin-$\frac{1}{2}$ and an anisotropic spin-orbital moment, $J=6$, that mimics the known, strong easy-axis $\mathrm{Tb}\cdots \mathrm{P}{\mathrm{c}}_2$ crystal-field interaction.

We report on the systematic study of infrared/terahertz spectra of photocurrents in (Bi,Sb)Te based three-dimensional topological insulators. We demonstrate that in a wide range of frequencies, ranging from fractions up to tens of terahertz, the photocurrent is caused by the linear photogalvanic effect (LPGE) excited in the surface states. The photocurrent spectra reveal that at low frequencies the LPGE emerges due to free carrier Drude-like absorption. The spectra allow us to determine the room temperature carrier mobilities in the surface states despite the presence of thermally activated residual impurities in the material bulk. In a number of samples we observed an enhancement of the linear photogalvanic effect at frequencies between 30 and 60 THz, which is attributed to the excitation of electrons from helical surface to bulk conduction band states. Under this condition and applying oblique incidence we also observed the circular photogalvanic effect driven by the radiation helicity.

We investigate the low-temperature properties of two-dimensional Lennard-Jones glass films, prepared in silico both by liquid cooling and by physical vapor deposition. We identify deep in the solid phase a crossover temperature $T^{*}$, at which slow dynamics and enhanced heterogeneity emerge. Around $T^{*}$, localized defects become visible, leading to vibrational anomalies as compared to standard solids. We find that, on average, $T^{*}$ decreases in samples with lower inherent structure energy, suggesting that such anomalies will be suppressed in ultrastable glass films, prepared both by very slow liquid cooling and vapor deposition.

We report on magnetotransport properties associated with the all-in-all-out magnetic order for ${\mathrm{Tb}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ single-crystalline thin films. Two degenerate all-in-all-out domains of an Ir sublattice can be switched to each other by magnetic field along the [111] direction as detected by a characteristic magnetoresistivity with diplike hysteresis. For a given single-domain structure, the linear magnetoresistivity, reflecting the effective magnetic field at ${\mathrm{Ir}}^{4+}$, changes its sign at around 70 K. These observations are consistently explained by the molecular fields from ${\mathrm{Tb}}^{3+}$ on ${\mathrm{Ir}}^{4+}$ moments. The molecular fields from ${\mathrm{Tb}}^{3+}$ moments compete with the external fields and dominate the magnetoresponse of ${\mathrm{Ir}}^{4+}$ moments at lower temperature, whereas the external field does at higher temperature.

We report the discovery of a long-sought-after phase of titanium nitride with stoichiometry ${\mathrm{Ti}}_3{\mathrm{N}}_4$ using diamond anvil cell experiments combined with in situ high-resolution x-ray diffraction and Raman spectroscopy techniques, supported by ab initio calculations. ${\mathrm{Ti}}_3{\mathrm{N}}_4$ crystallizes in the cubic ${\mathrm{Th}}_3{\mathrm{P}}_4$ structure [space group $I\overline{4}3d$ (220)] from a mixture of TiN and ${\mathrm{N}}_2$ above $\approx 75$ GPa and $\approx 2400$ K. The density ($\approx 5.22$ g/cc) and bulk modulus ($K_0=290$ GPa) of cubic-${\mathrm{Ti}}_3{\mathrm{N}}_4$ ($c-{\mathrm{Ti}}_3{\mathrm{N}}_4$) at 1 atm, estimated from the pressure-volume equation of state, are comparable to rocksalt TiN. Ab initio calculations based on the GW approximation and using hybrid functionals indicate that $c-{\mathrm{Ti}}_3{\mathrm{N}}_4$ is a semiconductor with a direct band gap between 0.8 and 0.9 eV, which is larger than the previously predicted values. The $c-{\mathrm{Ti}}_3{\mathrm{N}}_4$ phase is not recoverable to ambient pressure due to dynamic instabilities, but recovery of ${\mathrm{Ti}}_3{\mathrm{N}}_4$ in the defect rocksalt (or related) structure may be feasible.

We observe the magnetic proximity effect (MPE) in $\mathrm{Pt}/{\mathrm{CoFe}}_2{\mathrm{O}}_4$ bilayers grown by molecular beam epitaxy. This is revealed through angle-dependent magnetoresistance measurements at 5 K, which isolate the contributions of induced ferromagnetism (i.e., anisotropic magnetoresistance) and the spin Hall effect (i.e., the spin Hall magnetoresistance) in the Pt layer. The strong evidence for induced ferromagnetism in Pt via the anisotropic magnetoresistance is supported further by density functional theory calculations and various control measurements including the insertion of a Cu spacer layer to suppress the induced ferromagnetism. In addition, anomalous Hall effect measurements show an out-of-plane magnetic hysteresis loop of the induced ferromagnetic phase with larger coercivity and larger remanence than the bulk ${\mathrm{CoFe}}_2{\mathrm{O}}_4$. By demonstrating the MPE in $\mathrm{Pt}/{\mathrm{CoFe}}_2{\mathrm{O}}_4$, these results establish the spinel ferrite family as a promising material for the MPE and spin manipulation via proximity exchange fields.

Epitaxial thin films of the substitutionally alloyed half-Heusler series $\mathrm{CoT}{\mathrm{i}}_{1-x}\mathrm{F}{\mathrm{e}}_x\mathrm{Sb}$ were grown by molecular beam epitaxy on InAlAs/InP(001) substrates for concentrations $0.0\le x\le 1.0$. The influence of Fe on the structural, electronic, and magnetic properties was studied and compared to that expected from density functional theory. The films are epitaxial and single crystalline, as measured by reflection high-energy electron diffraction and x-ray diffraction. Using in situ x-ray photoelectron spectroscopy, only small changes in the valence band are detected for $x\le 0.5$. For films with $x\ge 0.05$, ferromagnetism is observed in SQUID magnetometry with a saturation magnetization that scales linearly with Fe content. A dramatic decrease in the magnetic moment per formula unit occurs when the Fe is substitutionally alloyed on the Co site indicating a strong dependence on the magnetic moment with site occupancy. A crossover from both in-plane and out-of-plane magnetic moments to only in-plane moment occurs for higher concentrations of Fe. Ferromagnetic resonance indicates a transition from weak to strong interaction with a reduction in inhomogeneous broadening as Fe content is increased. Temperature-dependent transport reveals a semiconductor to metal transition with thermally activated behavior for $x\le 0.5$. Anomalous Hall effect and large negative magnetoresistance (up to -18.5% at 100 kOe for $x=0.3)$ are observed for higher Fe content films. Evidence of superparamagnetism for $x=0.3$ and 0.2 suggests, for moderate levels of Fe, that demixing of the $\mathrm{CoT}{\mathrm{i}}_{1-x}\mathrm{F}{\mathrm{e}}_x\mathrm{Sb}$ films into Fe-rich and Fe-deficient regions may be present. Atom probe tomography is used to examine the Fe distribution in an $x=0.3$ film. Statistical analysis reveals a nonhomogeneous distribution of Fe atoms throughout the film, which is used to explain the observed magnetic and electrical behavior.

Dynamic mechanical analysis (DMA) measurements as a function of temperature, frequency, and dynamic force amplitude are used to perform a detailed study of the domain wall motion in ${\mathrm{LaAlO}}_3$. In previous DMA measurements Harrison et al. [Phys. Rev. B 69, 144101 (2004)] found evidence for dynamic phase transitions of ferroelastic domain walls in ${\mathrm{LaAlO}}_3$. In the present work we focus on the creep-to-relaxation region of domain wall motion using two complementary methods. We determine, in addition to dynamic susceptibility data, waiting time distributions of strain jerks during slowly increasing stress. These strain jerks, which result from self-similar avalanches close to the depinning threshold, follow a power-law behavior with an energy exponent $\varepsilon =1.7\pm 0.1$. Also, the distribution of waiting times between events follows a power law $N\left(t_w\right)\propto t_w^{-(n+1)}$ with an exponent $n=0.9$, which transforms to a power law of susceptibility $S\left(\omega \right)\propto {\omega }^{-n}$. The present dynamic susceptibility data can be well fitted with a power law, with the same exponent ($n=0.9$) up to a characteristic frequency $\omega \approx {\omega }^{*}$, where a crossover from stochastic DW motion to the pinned regime is well described using the scaling function of Fedorenko et al. [Phys. Rev. B 70, 224104 (2004)].

Nominal InN monolayers grown by molecular beam epitaxy on GaN(0001) are investigated combining in situ reflection high-energy electron diffraction (RHEED), transmission electron microscopy (TEM), and density functional theory (DFT). TEM reveals a chemical intraplane ordering never observed before. Employing DFT, we identify a novel surface stabilization mechanism elastically frustrated rehybridization, which is responsible for the observed chemical ordering. The mechanism also sets an incorporation barrier for indium concentrations above 25% and thus fundamentally limits the indium content in coherently strained layers.

We present time-of-flight neutron total scattering and polarized neutron scattering measurements of the magnetically frustrated compounds ${\mathrm{NaCaCo}}_2{\mathrm{F}}_7$ and ${\mathrm{NaSrCo}}_2{\mathrm{F}}_7$, which belong to a class of recently discovered pyrochlore compounds based on transition metals and fluorine. The magnetic pair distribution function (mPDF) technique is used to analyze and model the total scattering data in real space. We find that a previously proposed model of short-range XY-like correlations with a length scale of 10–15 Å, combined with nearest-neighbor collinear antiferromagnetic correlations, accurately describes the mPDF data at low temperature, confirming the magnetic ground state in these materials. This model is further verified by the polarized neutron scattering data. From an analysis of the temperature dependence of the mPDF and polarized neutron scattering data, we find that short-range correlations persist on the nearest-neighbor length scale up to 200 K, approximately two orders of magnitude higher than the spin freezing temperatures of these compounds. These results highlight the opportunity presented by these new pyrochlore compounds to study the effects of geometric frustration at relatively high temperatures, while also advancing the mPDF technique and providing an opportunity to investigate a genuinely short-range-ordered magnetic ground state directly in real space.

The full comprehension of magnetic phenomena at the femtosecond (fs) time scale is of high demand for current material science and technology. Here we report the observation of coherent collective modes in the antiferromagnetic insulator nickel oxide (NiO) identified by a frequency of 0.86 THz, which matches the expected out-of-plane single-mode magnon resonance. Such collective excitations are inelastically stimulated by extreme ultraviolet (EUV) pulses delivered by a seeded free-electron laser (FEL) and subsequently revealed probing the transient optical activity of NiO looking at the Faraday effect. Moreover, the unique capability of the employed FEL source to deliver circularly polarized pulses allows us to demonstrate optomagnetic control of such collective modes at EUV photon energies. These results may set a starting point for future investigations of magnetic materials at time scales comparable or faster than those typical of exchange interactions.

We report the fabrication of epitaxial ${\mathrm{Y}}_3\mathrm{F}{\mathrm{e}}_5{\mathrm{O}}_{12}$ (YIG) thin films on $\mathrm{G}{\mathrm{d}}_3\mathrm{G}{\mathrm{a}}_5{\mathrm{O}}_{12}$ (111) using a chemical solution method. Cubic YIG is a ferrimagnetic material at room temperature, with excellent magneto-optical properties, high electrical resistivity, and a very narrow ferromagnetic resonance, which makes it particularly suitable for applications in filters and resonators at microwave frequencies. But these properties depend on the precise stoichiometry and distribution of $\mathrm{F}{\mathrm{e}}^{3+}$ ions among the octahedral/tetrahedral sites of a complex structure, which hampered the production of high-quality YIG thin films by affordable chemical methods. Here we report the chemical solution synthesis of YIG thin films, with excellent chemical, crystalline, and magnetic homogeneity. The films show a very narrow ferromagnetic resonance (long spin relaxation time), comparable to that obtained from high-vacuum physical deposition methods. These results demonstrate that chemical methods can compete to develop nanometer-thick YIG films with the quality required for spintronic devices and other high-frequency applications.

Ionic-liquid/gel-based transistors have emerged as a potentially ideal means to accumulate high charge-carrier densities at the surfaces of materials such as oxides, enabling control over electronic phase transitions. Substantial gaps remain in the understanding of gating mechanisms, however, particularly with respect to charge carrier vs oxygen defect creation, one contributing factor being the dearth of experimental probes beyond electronic transport. Here we demonstrate the use of synchrotron hard x-ray diffraction and polarized neutron reflectometry as in operando probes of ion-gel transistors based on ferromagnetic $\mathrm{L}{\mathrm{a}}_{0.5}\mathrm{S}{\mathrm{r}}_{0.5}\mathrm{Co}{\mathrm{O}}_{3-\delta }$. An asymmetric gate-bias response is confirmed to derive from electrostatic hole accumulation at negative gate bias vs oxygen vacancy formation at positive bias. The latter is detected via a large gate-induced lattice expansion (up to 1%), complementary bulk measurements and density functional calculations enabling quantification of the bias-dependent oxygen vacancy density. Remarkably, the gate-induced oxygen vacancies proliferate through the entire thickness of 30–40-unit-cell-thick films, quantitatively accounting for changes in the magnetization depth profile. These results directly elucidate the issue of electrostatic vs redox-based response in electrolyte-gated oxides, also demonstrating powerful approaches to their in operando investigation.

Recent experimental work has revealed that the unusually strong, isotropic structural negative thermal expansion in cubic perovskite ionic insulator ${\mathrm{ScF}}_3$ occurs in excited states above a ground state tuned very near a structural quantum phase transition, posing a question of fundamental interest as to whether this special circumstance is related to the anomalous behavior. To test this hypothesis, we report an elastic and inelastic x-ray scattering study of a second system ${\mathrm{Hg}}_2{\mathrm{I}}_2$ also tuned near a structural quantum phase transition while retaining stoichiometric composition and high crystallinity. We find similar behavior and significant negative thermal expansion below 100 K for dimensions along the body-centered-tetragonal $c$ axis, bolstering the connection between negative thermal expansion and zero-temperature structural transitions. We identify the common traits between these systems and propose a set of materials design principles that can guide discovery of new materials exhibiting negative thermal expansion.