Soft phonon modes describe a collective movement of atoms that transform a higher-symmetry crystal structure into a lower-symmetry crystal structure. Such structural transformations occur at finite temperatures, where the phonons (i.e., the low-temperature vibrational modes) and the static perfect crystal structures provide an incomplete picture of the dynamics. Here, principal vibrational modes (PVMs) are introduced as descriptors of the dynamics of a material system with $N$ atoms. The PVMs represent the independent collective movements of the atoms at a given temperature. Molecular dynamics (MD) simulations, here in the form of quantum MD using density functional theory calculations, provide both the data describing the atomic motion and the data used to construct the PVMs. The leading mode, ${\mathrm{PVM}}_0$, represents the $3N$-dimensional direction in which the system moves with greatest amplitude. For structural phase transitions, ${\mathrm{PVM}}_0$ serves as a generalization of soft phonon modes. At low temperatures, ${\mathrm{PVM}}_0$ reproduces the soft phonon mode in systems where one phonon dominates the phase transformation. In general, multiple phonon modes combine to describe a transformation, in which case ${\mathrm{PVM}}_0$ culls these phonon modes. Moreover, while soft phonon modes arise in the higher-symmetry crystal structure, ${\mathrm{PVM}}_0$ can be equally well calculated on either side of the structural phase transition. Two applications demonstrate these properties: first, transitions into and out of bcc titanium, and, second, the two crystal structures proposed for the $\beta$ phase of uranium, the higher-symmetry structure of which stabilizes with temperature.

Using Landau-Ginzburg-Devonshire (LGD) theory for $\mathrm{BiFe}{\mathrm{O}}_3$ dense fine-grained ceramics with quasispherical grains and nanosized intergrain spaces enriched by elastic defects, we calculated a surprisingly strong size-induced increase in the antiferromagnetic transition temperature caused by the joint action of rotomagnetic and magnetostrictive coupling. Notably, all parameters included in the LGD functional have been extracted from experiments, not assumed. Complementarily, we performed experiments for dense $\mathrm{BiFe}{\mathrm{O}}_3$ ceramics, which revealed that the shift of the antiferromagnetic transition is to $T_N\sim 690\mathrm{K}$ instead of $T_N\sim 645\mathrm{K}$ for a single crystal. To explain the result theoretically, we consider the possibility of controlling the antiferromagnetic state of multiferroic $\mathrm{BiFe}{\mathrm{O}}_3$ via biquadratic antiferrodistortive rotomagnetic, rotoelectric, magnetoelectric, and magnetostrictive couplings. According to our calculations, the highest contribution is the rotostriction contribution, while the magnetostrictive and electrostriction contributions appear smaller.

In the field of antiferromagnetic (AFM) spintronics, information about the Néel vector, AFM domain sizes, and spin-flop fields is a prerequisite for device applications but is not available easily. We have investigated AFM domains and spin-flop-induced changes of domain patterns in ${\mathrm{Mn}}_2\mathrm{Au}$(001) epitaxial thin films by x-ray magnetic linear dichroism photoemission electron microscopy (PEEM) using magnetic fields up to 70 T. As-prepared ${\mathrm{Mn}}_2\mathrm{Au}$ films exhibit AFM domains with an average size $\le 1\mu \mathrm{m}$. Application of a 30 T field, exceeding the spin-flop field, along a magnetocrystalline easy axis dramatically increases the AFM domain size with Néel vectors perpendicular to the applied field direction. The width of Néel-type domain walls (DW) is below the spatial resolution of the PEEM and therefore can only be estimated from an analysis of the DW profile to be smaller than 80 nm. Furthermore, using the values for the DW width and the spin-flop field, we evaluate an in-plane anisotropy constant ranging between 1 and $17\mathrm{eV}/\mathrm{f}.\mathrm{u}.$.

We study the resistive switching triggered by an applied electrical bias in the antiferromagnetic Mott insulator $\mathrm{S}{\mathrm{r}}_3\mathrm{I}{\mathrm{r}}_2{\mathrm{O}}_7$. The switching was previously associated with an electric-field-driven structural transition. Here we use time-resolved measurements to probe the thermal activation behavior of the switching process and acquire information about the energy barrier associated with the transition. We quantify the changes in the energy-barrier height with respect to the applied bias and find a linear decrease of the barrier with increasing bias. Our observations support the potential of antiferromagnetic transition-metal oxides for spintronic applications.

Three band crossings can arise in three-dimensional quantum materials with certain space group symmetries. The low energy Hamiltonian supports spin one fermions and a flat band. We study the pairing problem in this setting. We write down a minimal BCS Hamiltonian and decompose it into spin-orbit coupled irreducible pairing channels. We then solve the resulting gap equations in channels with zero total angular momentum. We find that in the s-wave spin singlet channel (and also in an unusual d-wave ‘spin quintet’ channel), superconductivity is enormously enhanced, with a possibility for the critical temperature to be linear in interaction strength. Meanwhile, in the p-wave spin triplet channel, the superconductivity exhibits features of conventional BCS theory due to the absence of flat band pairing. Three band crossings thus represent an exciting new platform for realizing exotic superconducting states with enhanced energy scales. We also discuss the effects of doping, nonzero temperature, and of retaining additional terms in the $\mathbf{k}·\mathbf{p}$ expansion of the Hamiltonian.

Understanding the interplay between different orders in a solid is a key challenge in highly correlated electronic systems. In real systems this is even more difficult since disorder can have strong influence on the subtle balance between these orders and thus can obscure the interpretation of the observed physical properties. Here we present a study on $\delta$-doped ${\mathrm{La}}_2{\mathrm{CuO}}_4$ ($\delta -{\mathrm{LCO}}_N$) superlattices. By means of molecular beam epitaxy whole ${\mathrm{LaO}}_2$ layers were periodically replaced by ${\mathrm{SrO}}_2$ layers, providing a charge reservoir yet reducing the level of disorder typically present in doped cuprates to an absolute minimum. The induced superconductivity and its interplay with the antiferromagnetic order is studied by means of low-energy muon spin rotation. We find a quasi-two-dimensional superconducting state which couples to the antiferromagnetic order in a nontrivial way. Below the superconducting transition temperature, the magnetic volume fraction increases strongly. The reason could be a charge redistribution of the free carriers due to the opening of the superconducting gap which is possible due to the close proximity and low disorder between the different ordered regions.

Using spin-wave tomography (SWaT), we have investigated the excitation and the propagation dynamics of optically excited magnetoelastic waves, i.e., hybridized modes of spin waves and elastic waves, in a garnet film. By using time-resolved SWaT, we reveal the excitation dynamics of magnetoelastic waves through coherent-energy transfer between optically excited pure-elastic waves and spin waves via magnetoelastic coupling. This process realizes frequency and wavenumber selective excitation of spin waves at the crossing of the dispersion relations of spin waves and elastic waves. Finally, we demonstrate that the excitation mechanism of the optically excited pure-elastic waves, which are the source of the observed magnetoelastic waves, is dissipative in nature.

Controlled pointlike disorder introduced by 2.5-MeV electron irradiation was used to probe the superconducting state of single crystals of $\mathrm{CaK}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Ni}}_x\right)}_4{\mathrm{As}}_4$ superconductor at $x=0$ and 0.05 doping levels. Both compositions show an increase of the residual resistivity and a decrease of the superconducting transition temperature, $T_c$, at the rate of $d{T}_c/d\rho \left(T_c\right)\approx 0.19$ K/($\mu \mathrm{\Omega }\mathrm{cm}$) for $x=0$ and 0.38 K/($\mu \mathrm{\Omega }\mathrm{cm}$) for $x=0.05$, respectively. In the Ni-doped compound ($x=0.05$), the coexisting spin-vortex crystal (SVC) magnetic phase is suppressed at the rate of $d{T}_N/d\rho \left(T_N\right)\approx$ 0.16 K/($\mu \mathrm{\Omega }\mathrm{cm}$). The low-temperature variation of London penetration depth is well approximated by the power-law function, $\mathrm{\Delta }\lambda \left(T\right)=A{T}^n$, with $n\approx 2.5$ for $x=0$ and $n\approx 1.9$ for $x=0.05$ in the pristine state. Detailed analysis of $\lambda \left(T\right)$ and $T_c$ evolution with disorder is consistent with two effective nodeless energy gaps in the density of states due to robust $s_{\pm }$ pairing. Overall the behavior of $\mathrm{CaK}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Ni}}_x\right)}_4{\mathrm{As}}_4$ at $x=0$ is similar to a slightly overdoped ${\mathrm{Ba}}_{1-y}{\mathrm{K}}_y{\mathrm{Fe}}_2{\mathrm{As}}_2$ at $y\approx$ 0.5, and at $x=0.05$ to an underdoped composition at $y\approx$ 0.2.

By applying ultrafast optical spectroscopy to electron-doped ${\mathrm{La}}_{1.9}{\mathrm{Ce}}_{0.1}{\mathrm{CuO}}_{4\pm \delta }$, we discern a bosonic mode of electronic origin and provide the evolution of its coupling with the charge carriers as a function of temperature. Our results show that it has the strongest coupling strength near $T_c$ and can fully account for the superconducting pairing. This mode can be associated with the two-dimensional antiferromagnetic spin correlations emerging below a critical temperature $T^{†}$ larger than $T_c$. Our work may help to establish a quantitative relation between bosonic excitations and superconducting pairing in electron-doped cuprates.

We report high-pressure Raman scattering measurements on the tetragonal phase of InTe corroborated with the first-principles density functional theory and synchrotron x-ray diffraction measurements. Anomalous pressure-dependent linewidths of the $A_{1\mathrm{g}}$ and $E_{\mathrm{g}}$ phonon modes provide evidence of an isostructural electronic transition at $\sim 3.6\mathrm{GPa}$. The first-principles theoretical analysis reveals that it is associated with a semiconductor-to-metal transition due to increased density of states near the Fermi level. Further, this pressure induced metallization acts as a precursor for structural phase transition to a face centered cubic phase ($Fm\overline{3}m$) at $\sim 6.0\mathrm{GPa}$. Interestingly, theoretical results reveal a pressure induced band inversion at the $Z$ and $M$ points of the Brillouin zone corresponding to pressures $\sim 1.0$ and $\sim 1.4\mathrm{GPa}$, respectively. As the parity of bands undergoing inversions is the same, the topology of the electronic state remains unchanged, and hence InTe retains its trivial band topology $({\mathbb{Z}}_2=0)$. The pressure dependent behavior of the $A_{1\mathrm{g}}$ and $E_{\mathrm{g}}$ modes can be understood based on the results from the synchrotron x-ray diffraction, which shows anisotropic compressibility of the lattice in the $a$ and $c$ directions. Our Raman measurements up to $\sim 19\mathrm{GPa}$ further confirms the pressure induced structural phase transition from a face-centered to primitive cubic ($Fm\overline{3}m$ to $Pm\overline{3}m$) at $P\sim 15\mathrm{GPa}$.

Hexagonal boron nitride ($h$-BN) is a layered crystal that is attracting a great deal of attention as a promising material for nanophotonic applications. The strong optical anisotropy of this crystal is key to exploit polaritonic modes for manipulating light-matter interactions in 2D materials. $h$-BN has also great potential for solid-state neutron detection and neutron imaging devices, given the exceptionally high thermal neutron capture cross section of the boron-10 isotope. A good knowledge of phonons in layered crystals is essential for harnessing long-lived phonon-polariton modes for nanophotonic applications and may prove valuable for developing solid-state ${}^{10}\mathrm{BN}$ neutron detectors with improved device architectures and higher detection efficiencies. Although phonons in graphene and isoelectronic materials with a similar hexagonal layer structure have been studied, the effect of isotopic substitution on the phonons of such lamellar compounds has not been addressed yet. Here we present a Raman scattering study of the in-plane high-energy Raman active mode on isotopically enriched single-crystal $h$-BN. Phonon frequency and lifetime are measured in the 80–600-K temperature range for ${}^{10}\mathrm{B}$-enriched, ${}^{11}\mathrm{B}$-enriched, and natural composition high quality crystals. Their temperature dependence is explained in the light of perturbation theory calculations of the phonon self-energy. The effects of crystal anisotropy, isotopic disorder, and anharmonic phonon-decay channels are investigated in detail. The isotopic-induced changes in the phonon density of states are shown to enhance three-phonon anharmonic decay channels in ${}^{10}\mathrm{B}$-enriched crystals, opening the possibility of isotope tuning of the anharmonic phonon decay processes.

In the equation of motion approach to the two-time Green's functions, conventional Tyablikov-type truncation of the chain of equations is rather arbitrary and apt to violate the analytical structure of Green's functions. Here, we propose a practical way to truncate the equations of motion using operator projection. The partial projection approximation is introduced to evaluate the Liouville matrix. It guarantees the causality of Green's functions, fulfills the time translation invariance and the particle-hole symmetry, and is easy to implement in a computer. To benchmark this method, we study the Anderson impurity model using the operator basis at the level of Lacroix approximation. Improvement over conventional Lacroix approximation is observed. The distribution of Kondo screening in the energy space is studied using this method.

Reservoir engineering is a powerful tool for the robust generation of quantum states or transport properties. Using both a weak-coupling quantum master equation and the exact solution, we show that directional transport of electrons through a double quantum dot can be achieved through an appropriately designed electronic environment. Directionality is attained through the interference of coherent and dissipative coupling. The relative phase is tuned with an external magnetic field, such that directionality can be reversed, as well as turned on and off dynamically. Our work introduces fermionic-reservoir engineering, paving the way to a new class of nanoelectronic devices.

We show that the radiative heat flux between two nanoparticles can be significantly amplified when they are placed in proximity of a planar substrate supporting a surface resonance. The amplification factor goes beyond two orders of magnitude in the case of dielectric nanoparticles, whereas it is lower in the case of metallic nanoparticles. We analyze how this effect depends on the frequency and on the particle-surface distance by clearly identifying the signature of the surface mode producing the amplification. Finally, we show how the presence of a graphene sheet on top of the substrate can modify the effect by making an amplification of two orders of magnitude possible also in the case of metallic nanoparticles. This long-range amplification effect should play an important role in the thermal relaxation dynamics of nanoparticle networks.

In this paper we report the detailed study of magnetic phase diagrams, low-temperature magnetic structures, and the magnetic field effect on the electrical resistivity of the binary intermetallic compound ${\mathrm{Tb}}_3\mathrm{Ni}$. The incommensurate magnetic structure of the spin-density-wave type described with magnetic superspace group $P{112}_1/a{1}^{\prime }\left(ab0\right)0ss$ and propagation vector ${\mathbf{k}}_{\mathrm{IC}}=\left[0.506,0.299,0\right]$ was found to emerge just below Néel temperature $T_{\mathrm{N}}=61$ K. Further cooling below 58 K results in the appearance of multicomponent magnetic states: (i) a combination of ${\mathbf{k}}_1=\left[\frac{1}{2},\frac{1}{2},0\right]$ and ${\mathbf{k}}_{\mathrm{IC}}$ in the temperature range $51<T<58$ K; (ii) a mixed magnetic state of ${\mathbf{k}}_{\mathrm{IC}}, {\mathbf{k}}_1$, and ${\mathbf{k}}_2=\left[\frac{1}{2},\frac{1}{4},0\right]$ with the partially locked-in incommensurate component in the temperature range $48<T<51$ K; and (iii) a low-temperature magnetic structure that is described by the intersection of two isotropy subgroups associated with the irreducible representations of two coupled primary order parameters (OPs) ${\mathbf{k}}_2=\left[\frac{1}{2},\frac{1}{4},0\right]$ and ${\mathbf{k}}_3=\left[\frac{1}{2},\frac{1}{3},0\right]$ and involves irreducible representations of the secondary OPs ${\mathbf{k}}_1=\left[\frac{1}{2},\frac{1}{2},0\right]$ and ${\mathbf{k}}_4=\left[\frac{1}{2},0,0\right]$ below 48 K. An external magnetic field suppresses the complex low-temperature antiferromagnetic states and induces metamagnetic transitions towards a forced ferromagnetic state that are accompanied by a substantial magnetoresistance effect due to the magnetic superzone effect. The forced ferromagnetic state induced after application of an external magnetic field along the $b$ and $c$ crystallographic axes was found to be irreversible below 3 and 8 K, respectively.

Magnetic deflection experiments on isolated Co-doped Nb clusters demonstrate a strong size dependence of magnetic properties, with large magnetic moments in certain cluster sizes and fully nonmagnetic behavior of others. There are in principle two explanations for this behavior. Either the local moment at the Co site is absent or it is screened by the delocalized electrons of the cluster, i.e., the Kondo effect. In order to reveal the physical origin, first, we established the ground state geometry of the clusters by experimentally obtaining their vibrational spectra and comparing them with a density functional theory study. Then, we performed an analysis based on the Anderson impurity model. It appears that the nonmagnetic clusters are due to the absence of the local Co moment and not due to the Kondo effect. In addition, the magnetic behavior of the clusters can be understood from an inspection of their electronic structure. Here magnetism is favored when the effective hybridization around the chemical potential is small, while the absence of magnetism is signaled by a large effective hybridization around the chemical potential.

Reprogrammability of magnonic band structure in layered permalloy/Cu/permalloy nanowires is demonstrated to depend on the relative orientation of the two layers magnetization. By using Brillouin light spectroscopy, we show that when the layers are aligned parallel two dispersive modes, with positive and negative group velocity, are observed while when the magnetic layers are aligned antiparallel, only one dispersive mode, with positive group velocity, is detected. Our findings are successfully compared and interpreted in terms of a microscopic (Hamiltonian-based) method. An explanation for the observed behavior can be attributed to mode-mixing (or hybridization) effect when the two magnetic layers are aligned antiparallel. This work opens the path to magnetic field-controlled reconfigurable magnonic crystals with multimodal frequency transmission characteristics.