We present a theoretical model of spin transitions in homogeneous spin-crossover (SCO) materials, based on the description of the interactions between SCO molecules by the Lennard-Jones potential in which the equilibrium distances depend on the spin states of the linked molecules. By expanding the potential to order 3 considering a homogeneous lattice spacing, we demonstrate that the present model is isomorphic with an Ising-like model combining infinite long-range “ferromagnetic-like” interactions, competing with short-range “antiferromagnetic-like” between nearest neighbors in the case of harmonic interactions. When an anharmonic contribution is included, the short-range interaction becomes dependent on the average high-spin fraction, and changes its sign along the spin transition. The thermodynamic properties of the model were first investigated analytically in mean field approximation for both harmonic and anharmonic cases, for which we clarify the conditions for obtaining gradual and first-order transitions. Moreover, we demonstrated that within this approximation, the strength of the antiferromagnetic-like interactions are insufficient to produce multistep transitions. In contrast, the resolution of the previous emerging Ising-like model by Monte Carlo simulations demonstrated the possibility of obtaining gradual, first-order and two-step transitions as a result of the antagonism between long- and short-range interactions. Overall, the obtained results demonstrate the suitability of the present theoretical model to describe cooperative SCO materials. The key message of our paper is that our model can be used to predict complex behavior of spin-crossover crystals.

Spin spirals form inside the magnetic layers of antiferromagnetic and noncollinearly coupled magnetic multilayers in the presence of an external field. This spin structure can be modeled to extract the direct exchange stiffness of the magnetic layers and the strength of the interlayer exchange coupling across the spacer layer. In this article, we discuss three models which describe the evolution of the spin spiral with the strength of the external magnetic field in these coupled structures: discrete energy, discrete torque, and continuous torque. These models are expanded to accommodate multilayers with any number of ferromagnetic layers, any combination of material parameters, and asymmetry. We compare their performance when fitting to the measured magnetization data of a range of sputtered samples with one or multiple ferromagnetic layers on either side of the spacer. We find that the discrete models produce better fits than the continuous for asymmetric and multiferromagnetic structures and exhibit much better computational scaling with high numbers of atomic layers than the continuous model. For symmetric, single-layered structures, the continuous model produces the same fit statistics and outperforms the discrete models. Last, we demonstrate methods to use interfacial layers to measure the exchange stiffness of magnetic layers with low interlayer exchange coupling. An open-access website has been provided to allow the fitting of magnetization as a function of field in arbitrary coupled structures using the discrete energy model.

Disordered materials often exhibit complex magnetic and electrical transport properties at low temperatures. Investigation of such phenomena is of current interest and is being actively pursued. Here, we have investigated the impact of carrier localization on the physical properties of a half-Heusler alloy, $\mathrm{I}{\mathrm{r}}_{0.9}\mathrm{M}{\mathrm{n}}_{0.95}\mathrm{S}{\mathrm{n}}_{1.15}$. Our studies reveal that this alloy exhibits $Y$-type chemical disorder and undergoes a cluster glass transition around 67 K. At low temperatures, the longitudinal resistivity exhibits a robust upturn arising due to electron-electron interaction. Along with this, below 100 K, an anomalous Hall effect driven by an intrinsic mechanism is also noted. Remarkably, the effect of localization manifests concurrently in magnetization, longitudinal resistivity, and all components of the Hall effect. Our observations are not in analogy with the existing theories that consider the skew scattering and side jump mechanisms of the anomalous Hall effect.

Nonmagnetic chiral crystals host Kramers-Weyl fermions at time-inversion invariant momenta and can exhibit a topological nontrivial Fermi surface. Exploring topological superconductivity in chiral materials is thus an area of active research. In this study, we have successfully grown high-quality single crystals of ${\mathrm{TaGe}}_2$ which adopt a chiral hexagonal lattice structure. We conducted comprehensive experimental measurements on superconductivity, complemented by theoretical calculations. Magnetic property, electrical resistivity, and heat capacity data indicate that ${\mathrm{TaGe}}_2$ undergoes a type-I superconducting transition at 1.9 K. Further analysis of low-temperature heat capacity data suggest that ${\mathrm{TaGe}}_2$ is a weakly coupled single-band BCS superconductor. We constructed a phase diagram of critical field obtained by heat capacity and electrical resistivity measurements. Surface superconductivity was observed in this type-I superconductor with a κ value of 0.074, and the ratio of surface critical field $\left({\mu }_0{H}_{\mathrm{s}}\right)$ to the bulk critical field $\left({\mu }_0{H}_{\mathrm{c}}\right)$ is remarkably high. Our density functional theory calculations identified two independent Weyl points near Fermi level besides the Kramers-Weyl nodes, which may contribute to the unusual critical field behavior. These findings demonstrate that ${\mathrm{TaGe}}_2$ is a promising material for exploring the interplay among chirality, superconductivity, and nontrivial topology.

Here we report muon spin rotation ($\mu \mathrm{SR}$) experiments on the temperature and field dependence of the effective magnetic penetration depth $\lambda \left(T\right)$ in the $\eta$-carbide-type suboxide ${\mathrm{Ti}}_4{\mathrm{Ir}}_2\mathrm{O}$, a superconductor with a considerably high upper critical field. The temperature dependence of $\lambda \left(T\right)$, obtained from transverse-field (TF)-$\mu \mathrm{SR}$ measurements, is in perfect agreement with an isotropic fully gaped superconducting state. Furthermore, our ZF $\mu \mathrm{SR}$ results confirm that the time-reversal symmetry is preserved in the superconducting state. We find, however, a remarkably small ratio of $T_{\mathrm{c}}/{\lambda }_0^{-2}\sim 1.22$. This value is close to most unconventional superconductors, showing that a very small superfluid density is present in the superconducting state of ${\mathrm{Ti}}_4{\mathrm{Ir}}_2\mathrm{O}$. The presented results will pave the way for further theoretical and experimental investigations to obtain a microscopic understanding of the origin of such a high upper critical field in an isotropic single-gap superconducting system.

Various perovskites, such as ${\mathrm{BaTiO}}_3$, exhibit ferroelectric distortion driven by long-range Coulomb interactions and d-p orbital hybridization. Antiperovskites, with a structure resembling perovskites but with inverted cations and anions, display significantly distinct physical properties in terms of ferroelectricity and piezoelectricity despite their structural similarities. However, the literature on these materials is scarce. Among them, ${\mathrm{Ag}}_3\mathrm{SI}$, a three-dimensional antiperovskite superionic conductor, demonstrates exceptional ionic conduction in nonpolar phases (α and β) but shows reduced conductivity in the polar phase (γ). Our study uncovers the unconventional mechanism of ferroelectricity in ${\mathrm{Ag}}_3\mathrm{SI}$: an anomalous s-d orbital self-mixing in the polar γ phase, originating from $\mathrm{A}{\mathrm{g}}^{+}$ short-range interactions. Additionally, this antiperovskite exhibits two rare phenomena in three-dimensional solid state: a triple-well potential and negative longitudinal piezoelectric effect. Our investigation of ${\mathrm{Ag}}_3\mathrm{SI}$, establishes a theoretical framework to grasp ferroelectricity and negative piezoelectricity in antiperovskites, enriching our understanding of these captivating materials.

Detailed crystal structure, electronic and magnetic properties of ${\mathrm{PbMnO}}_3$, a member of $\mathrm{Pb}M{\mathrm{O}}_3$ ($M=3d$ transition metal ion) family of perovskites exhibiting a variety of physical properties are still elusive. We present an in-depth first-principles theoretical analysis of crystal structure, magnetic order, and lattice instabilities of its low energy states, and identify pathways connecting them using symmetry principles. We show that the ground state of ${\mathrm{PbMnO}}_3$ is a half-metallic ferromagnet with orthorhombic $Pnma$ crystal structure, which involves condensation of ${\mathrm{MnO}}_6$ octahedral rotational modes at $\frac{\pi }{a}\left[110\right]$ and $\frac{\pi }{a}\left[111\right] k$ points in the cubic perovskite structure. To develop a polar ferromagnetic half-metal, we explore the mechanism of hybrid improper ferroelectricity through its superlattice with ${\mathrm{SrMnO}}_3$. We achieve an unusual combination of polar crystal structure and ferromagnetic half-metallic state in ${\left({\mathrm{PbMnO}}_3\right)}_1/{\left({\mathrm{SrMnO}}_3\right)}_1$ superlattice. Polar structural distortion involves uncompensated antipolar displacements of Pb and Sr atoms with ${\mathrm{\Gamma }}_5^{-}$ symmetry, while its half-metallicity arises from strongly hybridized Mn $3d$ and O $2p$ states.

Long-time tails, or algebraic decay of time-correlation functions, have long been known to exist both in many-body systems and in models of noninteracting particles in the presence of quenched disorder that are often referred to as Lorentz models. In classical Lorentz models, one manifestation is the long-time tail of the velocity autocorrelation function. In quantum systems, the best-known example is what is known as weak-localization effects in disordered systems of noninteracting electrons. A wide variety of techniques has been used to study these phenomena, including kinetic theory and mode-coupling theories for classical systems, and many-body theory and field-theoretic techniques for quantum mechanical ones. This paper provides a unifying, and very simple, approach to these effects. We show that simple modifications of the diffusion equation due to either a random diffusion coefficient or a random scattering potential account for both the decay exponents and the prefactors of the leading long-time tails in the velocity autocorrelation functions of both classical and quantum Lorentz models.

We present the results of a spin-polarized scanning tunneling microscopy study performed on epitaxial Tb(0001) films grown on W(110). The magnetic contrast was obtained by dipping the tip directly into the Tb film, thereby creating magnetic in-plane sensitive tips. Our differential conductance $\mathrm{d}I/\mathrm{d}U$ data reveal contrast levels which are in agreement with the tunneling magnetoresistance effect between an in-plane sensitive scanning tunneling microscopy tip and surface domains magnetized along basal $\langle 10\overline{1}0\rangle$ directions. Film-thickness-dependent studies reveal a close correlation between the structural and magnetic properties of the film. Four types of crystal defects are identified which contribute to the pinning of magnetic domains: (i) surface step edges, (ii) step dislocations, (iii) structural boundaries between differently stacked terbium (Tb) patches, and (iv) screw dislocations resulting from glide processes. As the thickness of the Tb film increases, the defect density diminishes, leading to the formation of larger domains. A detailed analysis of the domain walls leads to the conclusion that they are of Néel type and exhibit a width of $w_{60}\approx 1.4$ nm, $w_{120}\approx 2.5$ nm, and $w_{180}\approx 3$ nm for ${60}^{\circ }, {120}^{\circ }$, and ${180}^{\circ }$ domain walls, respectively.

We present a computational study to explore the potential of different experimental approaches to tune the magnetic interactions in two-dimensional van der Waals magnets. We selected ${\mathrm{CrGeSe}}_3, {\mathrm{CrGeTe}}_3$, and Janus ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{(\mathrm{Se},\mathrm{Te})}_3$ monolayers as case studies and calculated the full exchange tensors among all relevant atomic pairs and analyze their dependence on different external parameters, such as biaxial and uniaxial strain, as well as gate voltage. We focus particularly on interactions that emerge or vanish due to changes in the system's symmetry, especially under uniaxial strain and gate voltage. We find that biaxial and uniaxial strains significantly modify isotropic exchange couplings, which can lead to a transition from a ferromagnetic to an antiferromagnetic phase, while a gate voltage induces Dzyaloshinskii–Moriya interactions, forming a vortex pattern whose chirality is determined by the sign of the electric field. The electric dipole moment of the Janus material is large, raising the possibility of multiferroic behavior. The polarizability is similar for the three compounds.

Extensive investigation has recently been conducted into a new class of antiferromagnetic order known as magnetic octupole order. However, the high rank of octupoles makes it difficult to detect and manipulate them by using conventional methods such as the anomalous Hall effect. In this paper, we propose the nonlinear magnetoelectric effect (NMEE), a second-order response to an electric field that induces a spontaneous magnetization, as a finite response under magnetic octupole order. First, we classify the magnetic point groups to identify antiferromagnets with such order, and derive the NMEE tensor using quantum kinetic theory. Then, we confirm the effectiveness of the NMEE through model calculations for two specific examples: a $d$-wave altermagnet and a pyrochlore lattice with all-in/all-out magnetic order. In particular, the intrinsic NMEE exhibits a large response in a magnetic Weyl semimetal phase of the pyrochlore lattice. This enhanced response is explained by the fact that the response tensor involves the quantum metric, which is enhanced near Weyl points. Furthermore, our results show that the NMEE has a sizable value that can be detected by the magneto-optical Kerr effect.

The altermagnetism caused by alternating crystal environment provides a unique opportunity for designing new type of valley polarization. Here, we propose a way to realize valley polarization in two-dimensional (2D) tetragonal altermagnetism by regulating the direction of magnetization. The valley polarization along with spin polarization will arise when the orientation of magnetization breaks the $C_{4z}$ lattice rotational symmetry, particularly in the conventional in-plane $x$ or $y$ directions. When the direction of magnetization switches between the $x$ and $y$ direction, the valley and spin polarizations will be reversed. This is different from the widely studied valley polarization, which occurs in out-of-plane hexagonal magnetic materials with valley physics at the -K/K point. Followed by first-principles calculations, our proposal is demonstrated in a 2D Janus tetragonal altermagnetic ${\mathrm{Fe}}_2{\mathrm{MoS}}_2{\mathrm{Se}}_2$ monolayer with good stability but very small valley splitting of 1.6 meV. To clearly see the feasibility of our proposal, an unrealistic material ${\mathrm{Ru}}_2{\mathrm{MoS}}_2{\mathrm{Se}}_2$ is used to show large valley splitting of 90 meV. In fact, our proposal can be readily extended to 2D tetragonal ferromagnetic (FM) materials, for example FM ${\mathrm{Fe}}_2{\mathrm{I}}_2$ monolayer. Our findings can enrich the valley physics, and provide new type of valley materials.

We identify a particular mechanism for the intrinsic superconducting diode effect (SDE) in multiphase superconductors. Using a Ginzburg-Landau and a microscopic two-band model, we find phase transitions into a mixed phase with finite-momentum Cooper pairs and SDE with high (including maximal) diode efficiencies, despite the individual phases exhibiting no SDE and equal inversion parity. We thus show that parity mixing—invoked in previous proposals—is not a crucial ingredient for SDE. The new mechanism may be relevant in a multitude of known multiphase superconductors like ${\mathrm{UTe}}_2$.

While the search for high-$T_c$ superconductivity was driven mostly by trial and error methodology searching for novel materials, here we provide a quasi-first-principles quantum theory for engineering superconductivity in artificial high-$T_c$ superlattices (AHTSs) with period $d$, ranging from 5.28 down to 3 nanometers, made of superconducting quantum wells of variable thickness $L$. An important feature of our quantum design is the key role of the interface internal electric field giving Rashba spin-orbit coupling in the nanoscale quantum superconducting building blocks. By tuning the geometrical conformational parameter $L/d$ around its magic ratio 2/3, we predict the superconducting dome of $T_c$ versus doping characteristic of unconventional superconductors. Quantum size effects, controlled by $L/d$, change the energy width and splitting of two quantum subbands formed by the electronic space charge confined in superconducting nanolayers. The theoretical superconducting dome $T_c$ versus charge density is shown to be controlled by the Fano-Feshbach resonance between two superconducting gaps. We have been able to predict experimental results on cuprate AHTSs by tuning the geometry of superlattices of quantum wells made of superconducting layers of thickness $L$ of modulation-doped stoichiometric Mott insulator ${\text{La}}_2{\text{CuO}}_4$ with no chemical dopants, confined within normal metal overdoped cuprate layers.

Kagome lattice systems host topologically nontrivial properties due to their unique crystal structure. Thermoelectric and magneto-thermal effects are great tools to probe the topological nature of these materials. In this study, we performed Seebeck and Nernst measurements on a kagome magnet ${\mathrm{YMn}}_6{\mathrm{Sn}}_4{\mathrm{Ge}}_2$. Anomalous Hall and anomalous Nernst effect, arising from nontrivial momentum-space Berry curvature were observed. Furthermore, we identified an additional component of the Nernst signal appearing as a large bump feature before entering the forced ferromagnetic phase, more distinguishable than in the electric Hall signals and absent in the field dependence of the magnetization. This feature indicates the geometrical Nernst effect (GNE), which is not observed in the parent compound ${\mathrm{YMn}}_6{\mathrm{Sn}}_6$. This observation suggests the existence of unusual spin textures in the compound which possibly generates finite spin chirality, leading to the GNE. Our results reveal the nontrivial topological nature in the electronic and magnetic structures in ${\mathrm{YMn}}_6{\mathrm{Sn}}_4{\mathrm{Ge}}_2$ and their sensitivity to the Ge substitution.

We studied the Mn ion dissolution mechanism from the $\left(0\overline{1}0\right)$ surface of ${\mathrm{Li}}_2{\mathrm{MnO}}_3$ using the reference interaction site method coupled with the effective screening medium method in density functional theory. We found that the solvation of ${\mathrm{F}}^{-}$ ions in the solvent may play a role in the Mn dissolution process through Coulombic interactions by depleting the Mn $3d$ and O $2p$ electrons, weakening the Mn–O bonds. Delithiation of ${\mathrm{Li}}_2{\mathrm{MnO}}_3$ lowers the activation energy of Mn dissolution even without ${\mathrm{F}}^{-}$ ions, and this energy is further reduced in the presence of ${\mathrm{F}}^{-}$ ions.

The two-dimensional molecular conductor $\kappa$-(${\mathrm{BETS})}_2\mathrm{Mn}{\left[\mathrm{N}{\left(\mathrm{CN}\right)}_2\right]}_3$ undergoes a sharp metal-to-insulator phase transition at $T_{\mathrm{MI}}\approx 21$ K, which has been under scrutiny for many years. We performed comprehensive infrared investigations along the three crystallographic directions as a function of temperature down to 10 K, complemented by electron spin resonance and dc-transport studies. The in-plane anisotropy of the optical conductivity is more pronounced than in any other $\kappa$-type BEDT-TTF or related compound. The metal-insulator transitions affect the molecular vibrations due to the coupling to the electronic system; in addition we observe a clear splitting of the charge-sensitive vibrational modes below $T_{\mathrm{MI}}$ that evidences the presence of two distinct BETS dimers in this compound. The $\mathrm{Mn}[\mathrm{N}{\left(\mathrm{CN}\right)}_2{]}_3^{-}$ layers are determined by the chain structure of the anions resulting in a rather anisotropic behavior and remarkable temperature dependence of the vibronic features. At low temperatures the electron spin resonance properties are affected by the ${\mathrm{Mn}}^{2+}$ ions via $\pi \text{−}d$ coupling and antiferromagnetic ordering within the $\pi$ spins: The $g$ factor shifts enormously with a pronounced in-plane anisotropy that flips as the temperature decreases; the lines broaden significantly, and the spin susceptibility increases upon cooling, with a kink at the phase transition.

There is an ongoing quest to replace indium-tin-oxide (ITO), the currently best $n$-type conductive transparent oxide (TCO), with a more readily available and cheaper material. The most common alternative is fluorine-doped tin-oxide, but it cannot match the high carrier concentration of ITO. Based on electronic structure calculations, we propose titanium-tin-oxide as an alternative. In its anatase form, $\mathrm{Ti}{\mathrm{O}}_2$ was proposed as a TCO itself, and high carrier concentrations could be achieved by cation-site doping. Unfortunately, anatase is not the ground state of $\mathrm{Ti}{\mathrm{O}}_2$, and its stable rutile form cannot effectively be doped $n$-type, because of electron self-trapping in small polaron states. It is shown here that $\mathrm{T}{\mathrm{i}}_x\mathrm{S}{\mathrm{n}}_{1-x}{\mathrm{O}}_2$ alloys, which are known to preserve the rutile structure, are free from polaronic trapping and preserve the $\mathrm{Sn}{\mathrm{O}}_2$-like conduction band edge up to $x<0.2$, with an effective electron mass smaller than 0.4 $m_0$. While cation-site doping is not favored in the mostly covalently bonded $\mathrm{Sn}{\mathrm{O}}_2$, it is shown here that alloying with Ti increases the ionicity, making cation-site doping favorable. Nb donors on Sn sites have a higher solubility and lower activation energy than F on O sites, making the achievement of higher carrier concentrations possible.