The combination of auxetic property, ferroelasticity, and ferroelectricity in two-dimensional materials offers new avenues for next-generation multifunctional devices. However, two-dimensional materials that simultaneously exhibit those properties are rarely reported. Here, we present a class of two-dimensional Janus-like structures $\mathrm{ScLa}{X}_2$ ($X=\mathrm{P}$, As, and Sb) with a rectangular lattice based on first-principles calculations. We predict that those $\mathrm{ScLa}{X}_2$ monolayers are stable semiconductors with both intrinsic in-plane and out-of-plane auxetic properties, showing a bidirectional negative Poisson's ratio effect. The value of the out-of-plane negative Poisson's ratio effect can reach −2.28/−3.06/−3.89. By applying uniaxial strain engineering, two transition paths can be found, including the VA main group element path and the rare-earth metal element path, corresponding to the ferroelastic and the multiferroic (ferroelastic and ferroelectric) phase transition, respectively. For the $\mathrm{ScLaS}{\mathrm{b}}_2$ monolayer, the external force field can not only control the ferroelastic phase transition, but it can also lead to the reversal of the out-of-plane polarization, exhibiting potential multiferroicity. The coupling between the bidirectional negative Poisson's ratio effect and multiferroicity makes the $\mathrm{ScLa}{X}_2$ monolayers promising for future device applications.

Understanding the superconductivity in relation to chemical bonding is essential for the development of superconductors. We propose that pressure-reduced ionic bonding strength is beneficial for improving superconductivity in hydrides (negative correlation between bonding strength and critical temperature). We model ionic hydrides using a prototypical ionic lattice (CsCl-type) with simple-valence metal Li/Rb and hydrogen and control the bonding strength via external pressure. First-principles calculations reveal that the ionic bonding strength in LiH increases with pressure while its critical temperature ($T_{\mathrm{c}}$) simultaneously decreases. A higher $T_{\mathrm{c}}$ at lower pressures is attributed to stronger electron-phonon coupling (EPC) induced by weaker ionic bonds and significant EPC contributions from mid-frequency phonons. RbH's pressure dependences of bonding strength and $T_{\mathrm{c}}$ are the reverse of those of LiH, and the EPC primarily results from high-frequency phonons. The distinct interorbital electron transition mechanism and amounts of charge transfer are responsible for the opposite trend of changes in bonding strength and superconductivity in LiH and RbH. The proposed correlation is further validated by the other six ionic hydrides. Substantial $T_{\mathrm{c}}$ change (e.g., 126.2 K at 100 GPa and 5.7 K at 300 GPa in LiH) in response to bonding strength variation reveals a key factor for designing new superconductors.

Training materials through periodic drive allows us to endow materials and structures with complex elastic functions. As a result of the driving, the system explores the high-dimensional space of structures, ultimately converging to a structure with the desired response. However, increasing the complexity of the desired response results in ultraslow convergence and degradation. Here, we show that by constraining the search space, we are able to increase robustness, extend the maximal capacity, train responses that previously did not converge, and in some cases accelerate convergence by many orders of magnitude. We identify the geometrical constraints that prevent the formation of spurious low-frequency modes, which are responsible for failure. We argue that these constraints are analogous to regularization used in machine learning. We propose a unified relationship between complexity, degradation, convergence, and robustness.

Ammonia is the most stable compound of nitrogen and hydrogen at ambient pressure. However, the chemical reaction of nitrogen and hydrogen is more complex and difficult to explore at high pressures. Here, we have performed extensively structural searches of ammonia-hydrogen compounds based on particle swarm optimization algorithms and first principles calculations. The calculated results show that the main reaction products of nitrogen and hydrogen under high pressure can be divided into two categories: high-energy density material (HEDM) and hydrogen storage material (HSM). Three different phases of ${\mathrm{NH}}_4$ are potential HEDMs, which are found to be stable or metastable at 40 GPa to 300 GPa, and metastable at ambient pressure with energy density of about $2.15\mathrm{kJ}/\mathrm{g}\sim 3.86\mathrm{kJ}/\mathrm{g}$. The $Pm$ phase of ${\mathrm{NH}}_{10}$ is an outstanding HSM with ultrahigh hydrogen storage (41.7 wt%) and release (29.2 wt%) capacities. These findings offer significant insights into the structural arrangements and chemical bonding patterns of ammonia-hydrogen compounds at high pressure, and suggest potential experimental avenues for elucidating how diverse metastable structures with distinct properties might be existed in planetary interiors.

The discovery of superconductivity in the infinite layer nickelates introduced a materials system analogous to the cuprates for the study of unconventional superconductivity. The synthesis of infinite layer nickelates, (${\mathrm{RNiO}}_2$, R = lanthanide) often uses calcium hydride (${\mathrm{CaH}}_2$) to facilitate the deintercalation of apical site oxygen atoms from a precursor perovskite (${\mathrm{RNiO}}_3$) phase via topotactic reduction. However, it remains uncertain whether the use of ${\mathrm{CaH}}_2$ results in the insertion of hydrogen into the infinite layer structure, and if it does, what the implications are for superconductivity. To quantify the hydrogen composition of highly crystalline infinite layer nickelates, we synthesized ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x{\mathrm{NiO}}_2$ thin films on LSAT substrates and conducted time-of-flight secondary ion mass spectroscopy measurements to generate hydrogen depth profiles. We compare the hydrogen density of nickelates prepared with and without a ${\mathrm{SrTiO}}_3$ capping layer. Additionally, we measure the hydrogen content in nickelate samples at various doping levels spanning the superconducting phase space, including the underdoped, optimally doped, and overdoped regime. We report no significant increase in hydrogen density between the perovskite and infinite layer phases in any of the measured samples. Furthermore, we put an upperbound on the hydrogen concentration of our nickelate samples to ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x{\mathrm{NiO}}_2{\mathrm{H}}_{0.05}$. Our results imply that hydrogen is not responsible for the emergence of superconductivity in the infinite layer nickelates.

We report on the discovery of a boundary-induced body-centered tetragonal iron phase in thin films deposited on ${\mathrm{MgAl}}_2{\mathrm{O}}_4$ (001) substrates. We present evidence for this phase using detailed x-ray analysis and ab initio density functional theory calculations. A lower magnetic moment and a rotation of the easy magnetization direction are observed, as compared with body-centered cubic iron. Our findings expand the range of known crystal and magnetic phases of iron, providing valuable insights for the development of heterostructure devices using ultrathin iron layers.

In orthoferrites the rare-earth ($R$) ion has a big impact on structural and magnetic properties; in particular, the ionic size influences the octahedral tilt and the $R^{3+}\text{−}\mathrm{F}{\mathrm{e}}^{3+}$ interaction modifies properties like the spin reorientation. Growth-induced strain in thin films is another means to modify materials properties since the sign of strain affects the bond length and therefore directly the orbital interaction. Our study focuses on epitaxially grown (010)-oriented $\mathrm{DyFe}{\mathrm{O}}_3$ and $\mathrm{LuFe}{\mathrm{O}}_3$ thin films, thereby investigating the impact of compressive lattice strain on the magnetically active $\mathrm{D}{\mathrm{y}}^{3+}$ and magnetically inactive $\mathrm{L}{\mathrm{u}}^{3+}$ compared to uniaxially strained single-crystal $\mathrm{DyFe}{\mathrm{O}}_3$. The $\mathrm{DyFe}{\mathrm{O}}_3$ films exhibits a shift of more than 20 K in spin-reorientation temperatures, maintain the antiferromagnetic ${\mathrm{\Gamma }}_4$ phase of the Fe lattice below the spin reorientation, and show double-step hysteresis loops for both in-plane directions between 5 and 390 K. This is the signature of an Fe-spin-induced ferromagnetic $\mathrm{D}{\mathrm{y}}^{3+}$ lattice above the Néel temperature of the Dy. The observed shift in the film spin reorientation temperatures vs lattice strain is in good agreement with isostatic single-crystal neutron diffraction experiments with a rate of 2 K/kbar.

The correlation between static magnetoelectric coupling and magnetic structure was investigated in $\mathrm{TbM}{\mathrm{n}}_{0.98}\mathrm{F}{\mathrm{e}}_{0.02}{\mathrm{O}}_3$ with magnetic field up to 8 T and down to 2 K. Single-crystal neutron diffraction experiments reveal a substantial increase in the temperature dependence of the incommensurate modulation wave vector of the antiferromagnetic phase as the magnetic field strength increases. Magnetic field-dependent pyroelectric current measurements revealed significantly higher magnetoelectric coupling at magnetic fields below 4 T than in pure $\mathrm{TbMn}{\mathrm{O}}_3$. This is due to the higher sensitivity of the incommensurably modulated cycloid structure to weak magnetic fields. Detailed analysis of our data confirmed that the ferroelectric polarization is induced by inverse Dzyaloshinskii-Moriya interaction for magnetic field strength up to 4 T, but at higher fields a departure from theoretical predictions is ascertained, giving evidence for an additional, as yet misunderstood, contribution to magnetoelectric coupling. It shows that a small 2% substitution of $\mathrm{M}{\mathrm{n}}^{3+}$ by $\mathrm{F}{\mathrm{e}}^{3+}$ has a strong impact on the magnetic structure, promoting the destabilization of the incommensurably modulated magnetic cycloidal structure of $\mathrm{TbMn}{\mathrm{O}}_3$ in a magnetic field above 5 T. We demonstrate that the magnetoelectric coupling magnitude can be tuned through suitable substitutional elements, even at low level, inducing local lattice distortions with different electronic and magnetic properties.

In this study, we investigate the influence of Cr-Cr distances on the magnetic properties of triangular lattice antiferromagnets through the lens of the recently synthesized Cr compounds ${\mathrm{LiCrSe}}_2$, ${\mathrm{LiCrTe}}_2$, and ${\mathrm{NaCrTe}}_2$. Our comprehensive analysis integrates existing magnetic structure data and new insights from muon spin rotation measurements, revealing a striking mutual influence between strongly correlated electrons and structural degrees of freedom in systems possessing very different magnetic properties despite having the same crystal symmetry. In particular, we delineate how Cr-Cr distances specifically dictate the magnetic behaviors of the triangular lattice antiferromagnets ${\mathrm{LiCrSe}}_2$, ${\mathrm{LiCrTe}}_2$, and ${\mathrm{NaCrTe}}_2$. By crafting phase diagrams based on these distances, we establish a clear correlation between the structural parameters and the magnetic ground states of these materials together with a wide variety of trivalent Cr triangular lattice layered magnets. Our analysis uncovers a transition range for in-plane and out-of-plane Cr-Cr distances that demarcates distinct magnetic behaviors, highlighting the nuanced role of lattice geometry in the spin-lattice interaction and electron correlation dynamics.

Oxide interfaces feature unique two-dimensional (2D) electronic systems with diverse electronic properties such as tunable spin-orbit interaction and superconductivity. Conductivity emerges in these interfaces when the thickness of an epitaxial polar layer surpasses a critical value, leading to charge transfer to the interface. Here, we show that depositing amorphous ${\mathrm{Al}}_2{\mathrm{O}}_3$ on top of the polar oxide can reduce the critical thickness and enhance the superconducting properties for ${\mathrm{SrTiO}}_3$-based interfaces. A detailed transmission electron microscopy analysis reveals that the enhancement of the superconducting properties is linked to the expansion of the ${\mathrm{SrTiO}}_3$ lattice in a direction perpendicular to the interface. We propose that the increase in the superconducting critical temperature, $T_c$, is a result of epitaxial strain.

Thermal transport coefficient $\kappa$ is an important property that often dictates broad applications of a polymeric material, while at the same time its computation remains challenging. In particular, classical simulations overestimate the measurements of $\kappa$ in comparison to those of the experiments and thus hinder their meaningful comparison. This is even when very careful simulations are performed using the most accurate empirical potentials. A key reason for such a discrepancy is because polymers have quantum-mechanical, nuclear degrees of freedom whose contribution to the heat balance is nontrivial. In this work, two semianalytical approaches are considered to accurately compute $\kappa$ by using the exact vibrational density of states $g\left(\nu \right)$. The first approach is based within the framework of the minimum thermal conductivity model, while the second uses computed quantum heat capacity to scale $\kappa$. The computed $\kappa$ of a set of commodity polymers compares quantitatively with ${\kappa }^{\mathrm{expt}}$.

We demonstrate a close connection between the quantum critical point (QCP) and superconducting upper critical field $H_{c2}$ in the Ni-doped heavy-fermion superconductor ${\mathrm{CeCoIn}}_5$. Temperature variations of electrical resistivity $\rho \left(T\right)$ exhibit a crossover between the non-Fermi liquid and the Fermi liquid states, whose boundary for $T\to 0$, regarded as the QCP, coincides with $H_{c2}$, while $H_{c2}$ decreases to zero with increasing Ni concentrations up to 25%. Furthermore, the $A$ coefficient of the $T^2$ term in $\rho \left(T\right)$ estimated in the Fermi liquid region shows the diverging behavior with decreasing the magnetic field $H$ toward $H_{c2}$. These experimental results suggest that the emergence of the QCP is always accompanied by the breakdown of the superconducting state by $H$ in Ni-doped ${\mathrm{CeCoIn}}_5$.

Research on the physical properties of materials at the nanoscale is crucial for the development of breakthrough nanotechnologies. One of the key properties to consider is the ability to conduct heat, i.e., its thermal conductivity. Graphene is a remarkable nanostructure with exceptional physical properties, including one of the highest thermal conductivities (TCs) ever measured. Graphene nanoribbons (GNRs) share most fundamental properties with graphene, with the added benefit of having a controllable electronic bandgap. One method to achieve such control is by twisting the GNR, which can tailor its electronic properties, as well as change their TCs. Here, we revisit the dependence of the TC of twisted GNRs (TGNRs) on the number of applied turns to the GNR by calculating more precise and mathematically well defined geometric parameters related to the TGNR shape, namely, its twist and writhe. We show that the dependence of the TC on twist is not a simple function of the number of turns initially applied to a straight GNR. In fact, we show that the TC of TGNRs requires at least two parameters to be properly described. Our conclusions are supported by atomistic molecular dynamics simulations to obtain the TC of suspended TGNRs prepared under different values of initially applied turns and different sizes of their suspended part. Among possible choices of parameter pairs, we show that TC can be appropriately described by the initial number of turns and the initial twist density of the TGNRs.

Doping of two-dimensional layered semiconducting materials is becoming pivotal in tailoring their electronic properties, enabling the development of advanced electronic and optoelectronic devices, where the selection of dopant is important. Here, we demonstrate the potential of substitutional vanadium (V) doping in monolayer molybdenum disulfide ($\mathrm{Mo}{\mathrm{S}}_2$) lattice in different extents leading to tunable electronic and optoelectronic properties. We found that low-level V doping (∼1 $\text{at.}\%$) induces $p$-type characteristics in otherwise $n$-type monolayer $\mathrm{Mo}{\mathrm{S}}_2$, whereas medium-level doping (∼5 $\text{at.}\%$) leads to an ambipolar semiconductor. Degenerately doped $\mathrm{Mo}{\mathrm{S}}_2$ (∼9 $\text{at.}\%$) facilitates a transition from semiconducting towards metallic (metal-like) with reduced electrical resistivity (∼4.5 $\mathrm{\Omega }\text{m}$ of $\mathrm{Mo}{\mathrm{S}}_2$ to $\sim 2.2\times {10}^{-5}\mathrm{\Omega }\text{m}$), low activation energy for transport (∼11 meV), and electric field independent drain current in field effect transistor–based transfer characteristics. A detailed temperature- and power-dependent photoluminescence study along with density functional theory–based calculations in support unravels the emergence of an excitonic transition at ∼850 nm with its intensity dependent on the amount of vanadium. This study shows the potential of V doping in $\mathrm{Mo}{\mathrm{S}}_2$ for generating multifunctional two-dimensional materials for next generation electronics, optoelectronics, and interconnects with systematic control over its electronic structure in a wide range.

A copper boride phase found on Cu(110) exhibits dimensional crossover from 1D to 2D structures. The discovered surface phase, $7\times ‘1\text{’}\text{−}\mathrm{B}/\mathrm{Cu}\left(110\right)$, is composed of alternating two-atom and three-atom rows that are separated by trenches. The narrower rows behave as a 1D template for atomic boron chains, and the wider rows provide a new dimensional degree of freedom that facilitates the generation of 2D clusters with chirality. The $7\times ‘1\text{’}$ phase is nonperiodic in the chain direction and exhibits intermediate boron coverage, between that of the quasiperiodic 1D and periodic 2D phases of copper boride on Cu(110), demonstrating a unique feature of phase transitions at the surface.

Ferroelectric field-effect transistors offer a potential for its important role in integrated memory and computing systems, and research on them is actively ongoing. In this study, we investigated the ferroelectric properties of $\mathrm{Pb}\left(\mathrm{Z}{\mathrm{r}}_{0.7}\mathrm{T}{\mathrm{i}}_{0.3}\right){\mathrm{O}}_3$, which is lattice matched with the La-doped high mobility perovskite oxide semiconductor $\mathrm{B}{\mathrm{a}}_{1-x}\mathrm{L}{\mathrm{a}}_x\mathrm{Sn}{\mathrm{O}}_3$. Growth of the $r$ phase in epitaxial $\mathrm{Pb}\left(\mathrm{Z}{\mathrm{r}}_{0.7}\mathrm{T}{\mathrm{i}}_{0.3}\right){\mathrm{O}}_3$ on $\mathrm{B}{\mathrm{a}}_{1-x}\mathrm{L}{\mathrm{a}}_x\mathrm{Sn}{\mathrm{O}}_3$ was confirmed and its basic ferroelectric and dielectric properties were studied by polarization-electric and capacitance-voltage measurement. We then studied the field effect of $\mathrm{Pb}\left(\mathrm{Z}{\mathrm{r}}_{0.7}\mathrm{T}{\mathrm{i}}_{0.3}\right){\mathrm{O}}_3$ on the electrical properties of $\mathrm{B}{\mathrm{a}}_{1-x}\mathrm{L}{\mathrm{a}}_x\mathrm{Sn}{\mathrm{O}}_3$ as we vary the La doping concentration. We find that the field effect of $\mathrm{Pb}\left(\mathrm{Z}{\mathrm{r}}_{0.7}\mathrm{T}{\mathrm{i}}_{0.3}\right){\mathrm{O}}_3$ on $\mathrm{B}{\mathrm{a}}_{1-x}\mathrm{L}{\mathrm{a}}_x\mathrm{Sn}{\mathrm{O}}_3$ is determined by competition between its ferroelectric and dielectric properties, depending on the La doping concentration. In high La doping rates, the field effect is dominated by the ferroelectric switching while in lower La doping rates, the effect is mainly by dielectric response as the depolarization field in the depleted layer weakens the ferroelectric effect. As per the width and direction of the hysteresis, it is also controlled by the competition between the counterclockwise ferroelectric response and the clockwise dielectric response due to the trapped charges near the interface. This study offers insights into optimizing the field effect by understanding the complex interplay between ferroelectric materials and low carrier density semiconductors.

Thermal expansion of Co-containing invar alloys of GX1Ni29-Co17 ${\mathrm{Fe}}_{54}{\mathrm{Co}}_{17}{\mathrm{Ni}}_{29}$ and stainless invar ${\mathrm{Fe}}_{39}{\mathrm{Co}}_{50}{\mathrm{Cr}}_9{\mathrm{Ni}}_2$ was investigated from the viewpoint of local structure by analyzing temperature-dependent extended x-ray absorption fine-structure (EXAFS) spectra combined with the computational simulations based on the path-integral effective classical potential (PIECP) method. For detailed comparative discussion, FeNi invar alloys of 36invar, 42invar, and 45invar were also examined. It is found by EXAFS that in stainless invar, Co exhibits a noticeable invar effect, while the invar effect on Co in GX1Ni29-Co17 is negligibly small. The PIECP simulations provide qualitative agreement with this finding, exemplifying that the Co magnetization is more effectively suppressed in stainless invar with a temperature rise, because of a smaller lattice constant and shorter corresponding interatomic distances. The present study clearly demonstrates the importance of the local structural point of view to understand the detailed low thermal expansion mechanism, in which microscopic local thermal expansion often meaningfully differs from macroscopic lattice thermal expansion.

Transition metal carbides, nitrides, and carbonitrides, known as MXenes, exhibit exceptional conductivity, stability, and large specific surface area, rendering them promising candidates for anode materials in rechargeable batteries. Herein, we investigate the electrochemical characteristics of the Janus MXene ${\mathrm{V}}_2\mathrm{COS}$ monolayer, as an anode material of alkali metal ion batteries by using first-principles calculations. The phonon band structure and ab initio molecular dynamics simulations confirm the stability of the Janus ${\mathrm{V}}_2\mathrm{COS}$ monolayer. The mechanical and electrical properties of the Janus ${\mathrm{V}}_2\mathrm{COS}$ monolayer are explored and proved to have good  mechanical stability and electrical conductivity. The surface of the Janus ${\mathrm{V}}_2\mathrm{COS}$ monolayer demonstrates the facile adsorption of alkali metal ions and low diffusion barriers. As an anode material, the recyclability of the Janus ${\mathrm{V}}_2\mathrm{COS}$ has been verified in the ion intercalation/deintercalation processes. Furthermore, the theoretical specific capacities and the open-circuit voltages of the Janus ${\mathrm{V}}_2\mathrm{COS}$ monolayer are calculated to be $165.54\mathrm{mA}\mathrm{h}\text{/}\mathrm{g}$ and 2.62 V for Li, $662.18\mathrm{mA}\mathrm{h}\text{/}\mathrm{g}$ and 0.76 V for Na, and $294.43\mathrm{mA}\mathrm{h}\text{/}\mathrm{g}$ and 0.4 V for K, respectively. It presents that the Janus ${\mathrm{V}}_2\mathrm{COS}$ monolayer is a potential anode material of sodium-ion batteries and potassium-ion batteries.