We study the nonlinear response of nonintegrable one-dimensional (1D) spin models using infinite matrix-product state techniques. As a benchmark and demonstration of the method, we first calculate the two-dimensional (2D) coherent spectroscopy for the exactly soluble ferromagnetic transverse field Ising model where excitations are freely moving domain walls. We then investigate the distinct signatures of confined bound states by introducing a longitudinal field and observe the emergence of strong nonrephasinglike signals. To interpret the observed phenomena, we use a two-kink approximation to perturbatively compute the 2D spectra. We find good agreement in comparison with the exact results of the infinite matrix-product state method in the strongly confined regime. We discuss the relevance of our results for quasi-$1\mathrm{D}$ Ising spin chain materials, such as ${\mathrm{CoNb}}_2{\mathrm{O}}_6$.

We show the presence of magnetic rare regions consistent with the quantum Griffiths phase in Fe-doped MnSi using detailed heat capacity, magnetization, and muon spin relaxation (μSR) measurements down to millikelvin temperatures. The slow dynamics of these rare regions at low temperatures leads to the non-Fermi-liquid behavior in heat capacity and magnetization. The μSR and magnetization results further indicate that the dynamics freezes into a cluster-glass state below $T_f\sim 1.25$ K. The results are in agreement with theoretical models proposed in the literature for metallic systems with Heisenberg symmetry that exhibit the quantum Griffiths phase in the presence of strong disorder.

The exploration of two-dimensional (2D) ferroic materials and the investigation of ferroic couplings are highly desired in view of the design of next-generation functional devices. Herein, we report through first-principles calculations that the single-layer γ-AlOOH exhibits intrinsic ferroelectric (FE), antiferroelectric (AFE), and ferroelastic properties. The polarization and antipolarization originate from the orientational displacements of hydrogen atoms actuated by two soft phonon modes of the centrosymmetric phase. We studied a possible FE switching process and the FE-AFE transformation in the cases of monolayer and bulk, resulting in the prediction of a stable bulk AFE phase with similar energy to the FE ground state. Moreover, the γ-AlOOH monolayer shows a giant ferroelastic phase transition capable of efficiently tuning the ferroelectricity, which is able to accomplish a 90 ° switching of polarization and leads to an FE/AFE quadruple state. Our work adds a unique candidate to the family of 2D ferroics, broadening the platform for the design of ferroic-based devices.

We report a theoretical investigation of a charged ${180}^{\circ }$ domain wall in ferroelectric ${\mathrm{PbTiO}}_3$, compensated by randomly distributed immobile charge defects. For this we utilize atomistic shell-model simulations and continuous phase-field simulations in the framework of the Ginzburg-Landau-Devonshire model. We predict that domain walls form a zigzag pattern and we discuss their properties in a broad interval of compensation-region widths, ranging from a couple to over 100 nm. The zigzag is accompanied by a local polarization rotation which we explain to provide an efficient mechanism for charge compensation.

Predicting atomic diffusion in concentrated magnetic systems is challenging due to thermal magnetic effects and complex magnetochemical interplay. We propose an efficient approach via kinetic Monte Carlo using ab initio parametrized models. We demonstrate its accuracy in the case of Fe-Ni alloys, where we successfully predict and explain the weak composition dependence of diffusion coefficients due to a compensation of distinct contributions of their constituents. The diffusion-behavior difference between the paramagnetic and the magnetic ground states is elucidated, evidencing the role of magnetic disorder.

Microwave absorption experiments employing a phase-sensitive external resistive detection are performed for a topical van der Waals antiferromagnet CrSBr. The magnetic-field dependence of two resonance modes is measured in an applied field parallel to the three principal crystallographic directions, revealing anisotropies, magnetic transitions, and a strong magnon-magnon coupling in this material. To account for the observed results, we formulate a microscopic spin model with a bi-axial single-ion anisotropy and interplane exchange. Theoretical calculations give an excellent description of full magnon spectra enabling us to precisely determine microscopic interaction parameters for CrSBr.

We report the fabrication of a van der Waals tunneling device hosting a defect-bound quantum dot coupled to ${\mathrm{NbSe}}_2$. We find that upon application of a magnetic field, the device exhibits a zero-bias conductance peak. The peak, which splits at higher fields, is associated with a Kondo effect. At the same time, the junction retains conventional quasiparticle tunneling features at finite bias. Such coexistence of a superconducting gap and a Kondo effect are unusual, and are explained by noting the two-gap nature of the superconducting state of ${\mathrm{NbSe}}_2$, where a magnetic field suppresses the low-energy gap associated with the Se band. Our data shows that van der Waals architectures, and defect-bound dots in them, can serve as an effective platform for investigating the interplay of Kondo screening and superconducting pairing in unconventional superconductors.

Topological defects, such as magnetic-flux-carrying quantum vortices, determine the magnetic response of superconductors and hence are of fundamental importance. Here, we show that stable $\mathbb{C}{P}^2$ skyrmions exist in three-band $s+is$ superconductors as fully self-consistent solutions to a microscopic Bogoliubov–de Gennes model. This allows us to calculate microscopically the magnetic signatures of $\mathbb{C}{P}^2$ skyrmions and their footprint in the local density of states.

The superconducting transition of ${\mathrm{SrTiO}}_3$ can be influenced by tuning its ferroelectric transition, but the underlying reasons remain poorly understood. Here, we investigate compressively strained, Sm-doped films of ${\mathrm{Eu}}_x{\mathrm{Sr}}_{1\text{–}x}{\mathrm{TiO}}_3$ that were grown by molecular beam epitaxy to determine the effect of alloying with Eu on both superconductivity and ferroelectricity, both of which are present in strained ${\mathrm{SrTiO}}_3$ films. Remarkably, superconductivity survives up to $x=0.14$. Films at the lowest alloy concentration studied here, $x=0.09$, exhibit no suppression of their superconducting transition temperature, but there is a strong reduction of the upper critical field $\left(H_{c2}\right)$, compared to nonalloyed, strained ${\mathrm{SrTiO}}_3$ films. In addition, these films lack the sharp ferroelectric transition that appears in films without Eu in second harmonic generation measurements. We postulate that Eu alloying causes a crossover from a globally ordered ferroelectric state to one with only short-range polar order. We discuss the connection between the loss of global polar order and the change in the superconducting properties.

We study the voltage-biased S-QD-S-QD-S Josephson junction, composed of three superconductors (S) and two quantum dots (QDs). In the absence of an applied voltage, the Andreev bound states on each quantum dot hybridize, forming an “Andreev molecule.” However, understanding of this system in a nonequilibrium setup is lacking. Applying commensurate dc voltages on the bijunction makes the system time periodic, and the equilibrium Andreev bound states evolve into a ladder of resonances with a finite lifetime due to multiple Andreev reflections (MARs). Starting from the time-periodic Bogoliubov–de Gennes equations we map the problem to a tight-binding chain in Floquet space. The resolvent of this non-Hermitian block matrix is obtained via a continued fraction method. We numerically calculate the Floquet-Andreev spectra which could be probed by local tunneling spectroscopy on the dots. We also consider the subgap current, and show that the Floquet resonances determine the position of the MAR steps. Proximity of the two dots causes splitting of the steps, while at large distances we observe interference effects which cause oscillations in the current-voltage curves. The latter effect should persist at very long distances.

Crystal structure, thermodynamic properties, and adiabatic demagnetization refrigeration (ADR) effect in the spin-$\frac{7}{2}$ triangular antiferromagnet $\mathrm{KBaGd}{\left({\mathrm{BO}}_3\right)}_2$ are reported. With the average nearest-neighbor exchange coupling of 44 mK, this compound shows magnetic order below $T_N=263$ mK in zero field. The ADR tests reach the temperature of $T_{min}=122\mathrm{mK}$, more than twice lower than $T_N$, along with the entropy storage capacity of 192 mJ ${\mathrm{K}}^{-1}$ ${\mathrm{cm}}^{-3}$ and the hold time of more than 8 h in the PPMS setup, both significantly improved compared to the spin-$\frac{1}{2}{\mathrm{Yb}}^{3+}$ analog. We argue that $\mathrm{KBaGd}{\left({\mathrm{BO}}_3\right)}_2$ shows a balanced interplay of exchange and dipolar couplings that together with structural randomness and geometrical frustration shift $T_{min}$ to well below the ordering temperature $T_N$, therefore facilitating the cooling.

The phenomenon of negative thermal expansion (NTE) deals with the increase of the lattice parameters and the volume of the unit cell when the material is thermally cooled. The NTE is typically associated with thermal phonons and anomalous spin-lattice coupling at low temperatures. However, the underlying mechanisms in the presence of strong electron correlations in multiorbital systems are not yet fully established. Here, we investigate the role of Coulomb interaction in the presence of lattice distortions in setting out the NTE effect, by focusing on the physical case of layered ${\mathrm{Ca}}_2{\mathrm{RuO}}_4$ with the $d^4$ configuration at each Ru ion site. We employ the Slater-Koster parametrization to describe the electron-lattice coupling through the dependence of the d-p hybridization on the Ru-O-Ru bond angle. The evaluation of the minimum of the free energy at finite temperature by fully solving the multiorbital many-body problem on a finite-size cluster allows us to identify the regime for which the system is prone to exhibit NTE effects. The analysis shows that the nature of the spin-orbital correlations is relevant to drive the reduction of the bond angle by cooling, and in turn the tendency toward a NTE. This is confirmed by the fact that a changeover of the electronic and orbital configuration from $d^4$ to $d^3$ by transition metal substitution is shown to favor the occurrence of a NTE in ${\mathrm{Ca}}_2{\mathrm{RuO}}_4$. This finding is in agreement with the experimental observations of a NTE effect which is significantly dependent on the transition metal substitution in the ${\mathrm{Ca}}_2{\mathrm{RuO}}_4$ compound.

Magnetic hopfions are string-like three-dimensional topological solitons, characterised by the Hopf number. They serve as a fundamental prototype for three-dimensional magnetic quasiparticles and are an inspiration for novel device concepts in the field of spintronics. Based on a micromagnetic model and without considering temperature, the existence of such hopfions has been predicted in certain magnets with competing exchange interactions. However, physical realisation of freely moving hopfions in bulk magnets have so far been elusive. Here, we consider an effective Heisenberg model with competing exchange interactions and study the stability of small toroidal hopfions with Hopf number $Q_{\text{H}}=1$ by finding first-order saddle points on the energy surface representing the transition state for the decay of hopfions via the formation of two coupled Bloch points. We combine the geodesic nudged elastic band method and an adapted implementation of the dimer method to resolve the sharp energy profile of the reaction path near the saddle point. Our analysis reveals that the energy barrier can reach substantial height and is largely determined by the size of the hopfion relative to the lattice constant.

The lifetimes of magnetic hopfions on a discrete lattice with competing exchange interactions are calculated within the framework of the transition state theory for magnetic degrees of freedom. Three sets of discrete model parameters corresponding to the same continuous micromagnetic model are considered. Minimal energy paths for hopfion collapses were found on the multidimensional energy surface of the system. The activation energies of the collapse processes have been calculated. It turned out that the activation energy differs significantly for the three considered values of the parameters, which indicates the importance of lattice effects, when the hopfion radius equals several lattice constants. Along with the collapse, the hopfion escape process through the sample boundary is studied. It is shown that this process does not require an activation energy. The lifetimes of hopfions are found and it is shown that they can exist only at temperatures of a few kelvins and practically cannot be generated due to thermal fluctuations.

In quantum electrodynamics, the zero-charge effect originates from the logarithmic dependence of the coupling constant in the action of the electromagnetic field on the ratio of the ultraviolet and infrared energy cutoffs. An analog of this effect in chiral Weyl superfluid ${}^3\mathrm{He}\text{−}A$ is the logarithmic divergence of the bending energy of the orbital anisotropy axis at low temperatures, where temperature plays the role of the infrared cutoff, and the vector of the orbital anisotropy plays the role of the vector potential of the synthetic electromagnetic field for Weyl fermions. We calculate numerically the spatial distribution of the order parameter in rotating ${}^3\mathrm{He}\text{−}A$ as a function of temperature. At temperatures close to the superfluid transition, we observe formation of vortex skyrmions known as the double-quantum vortex and the vortex sheet. These structures include alternating circular and hyperbolic merons as a bound pair or a chain, respectively. As temperature lowers towards absolute zero, we find a continuous transition in the vortex structures towards a state where the vorticity is distributed in thin tubes around the circular merons. For the vortex sheet, we present a phase diagram of the transition in the temperature–angular velocity plane and calculations of the nuclear magnetic resonance response.

We study the electronic transport properties of a superconductor (S) with a mixed $s+p$-wave pairing attached to a ferromagnetic metal (F) and a normal electrode (N) in an SFN configuration. Using the quasiclassical Green's function method, we compute the differential conductance $\sigma$ of the junction and demonstrate its dependence on the direction of the exchange field relative to the direction of the $d$-vector of the pair potential. If the $p$-wave triplet dominates the pairing, the zero bias conductance depends on the relative direction between the triplet $d$-vector and the exchange field. In contrast, if the $s$-wave singlet dominates the pairing, the zero bias conductance is isotropic with respect to the field direction. Furthermore, at zero temperature, the zero bias conductance height can only take two values as a function of $r$, the parameter quantifying the relative amount of $s$- and $p$-wave pairing, with an abrupt change at $r=1$ when the superconductor goes from a singlet to triplet dominated ground state. Moreover, we show that the relative amount of $s$- and $p$-wave pairing can be estimated from the dependence of the finite bias conductance on the exchange field direction. Our results provide a way to characterize parity-mixed superconductors performing electrical measurements.

Density functional theory (DFT) is one of the main tools for studying the electronic structure of solids and molecules. Nevertheless, one of the main drawbacks of the implementation of DFT is the so-called self-interaction error (SIE) that can yield undesired delocalization errors and ultimately results in the prediction of metals instead of experimentally observed insulators. These SIEs can be amended by using more evolved exchange-correlation functionals than standard local density approximation such as the recent meta-generalized gradient approximation strongly constrained and appropriately normalized (SCAN) functional that is successful in describing electronic properties of $3d$ transition metal oxides. Nevertheless, the ability of such a functional to describe electronic properties of materials involving more localized states such as $4f$ orbitals is rather elusive. Here, we show that, even though SCAN can sometimes predict the insulating character of some compounds, it often fails in predicting the correct band edge orbital character of insulators. By comparing our SCAN results with benchmark simulations obtained with more accurate hybrid DFT calculations, we ascribe this failure to insufficiently amended SIEs by SCAN that results in an underestimation of Hund's splitting associated with $4f$ states. Thus, although appropriate for $3d$ transition metal elements, the SCAN functional is not yet a sufficient platform for studying electronic properties of materials involving rare-earth elements where $4f$ states play a key role in the properties.

We derive macroscopic expressions for the polarization and orbital magnetization of a Chern insulator in its zero-temperature ground state using a previously developed formalism for treating microscopic polarization and magnetization fields in extended media. In the limit of a topologically trivial insulator, our results reduce to those of the “modern theories of polarization and magnetization.” In a Chern insulator, however, we find a generically nonvanishing microscopic free current density, the macroscopic average of which vanishes. Moreover, the expression that we obtain for the polarization is qualitatively similar to that of the “modern theory,” while the expressions for the orbital magnetization fundamentally differ; the manner in which they differ elucidates the distinct philosophies of these theoretical frameworks.

We study the dissipative dynamics of a one-dimensional bosonic system described in terms of the bipartite Bose-Hubbard model with alternating gain and loss. This model exhibits the $\mathcal{P}\mathcal{T}$ symmetry under some specific conditions and features a $\mathcal{P}\mathcal{T}$-symmetry phase transition. It is characterized by an order parameter corresponding to the population imbalance between even and odd sites,similar to the continuous phase transitions in the Hermitian realm. In the noninteracting limit, we solve the problem exactly and compute the parameter dependence of the order parameter. The interacting limit is addressed at the mean-field level, which allows us to construct the phase diagram for the model. We find that both the interaction and dissipation rates induce $\mathcal{PT}$-symmetry breaking. On the other hand, periodic modulation of the dissipative coupling in time stabilizes the $\mathcal{PT}$-symmetric regime. Our findings are corroborated numerically on a tight-binding chain with gain and loss.

In the ultrathin films of three-dimensional topological insulators (TIs), modification of the Berry phase caused by surface hybridization was predicted to allow for a crossover between weak antilocalization (WAL) and weak localization (WL). Here we report on a systematic study of the magnetotransport properties of ${\left({\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x\right)}_2{\mathrm{Te}}_3$ (BST) thin films with well-controlled thickness, doping level, and chemical potential. We found that the sign of magnetoconductance in perpendicular magnetic field remains negative in all circumstances, suggesting absence of global WL in the ultrathin BST films. This is attributed to long-range disorder, as well as conductance corrections by quantum interference and electron-electron interaction. Our work shows that the electron transport in ultrathin TIs is distinctively different from the WAL regime in the systems with symplectic symmetry, and the WL or weak insulator regime associated with the orthogonal symmetry class.