Layered semimetallic $1\mathit{T}-{\mathrm{TiTe}}_2$ has been reported to exhibit pressure-induced superconductivity, and some nontrivial topological states have also been suggested to appear under pressure. Here we investigate the optical properties of this compound by infrared reflectance measurements over a broad energy range under quasihydrostatic pressures up to 22 GPa. The observed conspicuous changes of infrared phonon spectra, in combination with x-ray diffraction data, provide clear evidence of an irreversible structural transition into a noncentrosymmetric space group ($P\overline{3}m1\to P3m1$) at a low pressure ($\sim 2.3$ GPa), before undergoing a subsequent transition to a monoclinic phase ($C2/m$) above 12 GPa. The pressure-induced irreversible gain of the low-frequency spectral weight of optical conductivity further supports the above crystal structure modification. The irreversible structural modification and associated change in electronic structure have been argued to be responsible for the emergence of superconductivity with enhanced $T_c$ and its persistence upon decompression. The low-pressure structural instability in $1\mathit{T}\text{−}{\mathrm{TiTe}}_2$ makes it necessary to revisit the theoretical investigation of its topological nature.

We study the optical properties of semiconducting transition metal dichalcogenide monolayers under the influence of strong out-of-plane magnetic fields, using the effective massive Dirac model. We pay attention to the role of spin-orbit-coupling effects, doping level, and electron-electron interactions, treated at the Hartree-Fock level. We find that optically induced valley and spin imbalance, commonly attained with circularly polarized light, can also be obtained with linearly polarized light in the doped regime. Additionally, we explore an exchange-driven mechanism to enhance the spin-orbit splitting of the conduction band, in $n$-doped systems, controlling both the carrier density and the intensity of the applied magnetic field.

We theoretically investigate transport signatures of quantum interference in highly symmetric double quantum dots in a parallel geometry and demonstrate that extremely weak symmetry-breaking effects can have a dramatic influence on the current. Our calculations are based on a master equation in which quantum interference enters as nondiagonal elements of the density matrix of the double quantum dots. We also show that many results have a physically intuitive meaning when recasting our equations as Bloch-like equations for a pseudospin associated with the dot occupation. In the perfectly symmetric configuration with equal tunnel couplings and orbital energies of both dots, there is no unique stationary-state density matrix. Interestingly, however, adding arbitrarily small symmetry-breaking terms to the tunnel couplings or orbital energies stabilizes a stationary state either with or without quantum interference, depending on the competition between these two perturbations. The different solutions can correspond to very different current levels. Therefore, if the orbital energies and/or tunnel couplings are controlled by, e.g., electrostatic gating, the double quantum dot can act as an exceptionally sensitive electric switch.

The nitrogen-vacancy (NV) center is a well utilized system for quantum technology, in particular quantum sensing and microscopy. Fully employing the NV center's capabilities for metrology requires a strong understanding of the behavior of the NV center with respect to changing temperature. Here, we probe the NV electronic spin density as the surrounding crystal temperature changes from 10 K to 700 K by examining the hyperfine interactions with a nearest-neighbor ${}^{13}\mathrm{C}$. These results are corroborated with ab initio calculations and demonstrate that the change in hyperfine interaction is small and dominated by a change in the hybridization of the orbitals constituting the spin density, thus indicating that the defect and local crystal geometry is returning towards an undistorted structure at higher temperature.

We investigate the nonequilibrium behavior of a fully connected (or all-to-all coupled) Bose-Hubbard model after a Mott to superfluid quench, in the limit of large boson densities and for an arbitrary number $V$ of lattice sites, with potential relevance in experiments ranging from cold atoms to superconducting qubits. By means of the truncated Wigner approximation, we predict that crossing a critical quench strength the system undergoes a dynamical phase transition between two regimes that are characterized at long times either by an inhomogeneous population of the lattice (i.e., macroscopical self-trapping) or by the tendency of the mean-field bosonic variables to split into two groups with phase difference $\pi$, that we refer to as $\pi$-synchronization. We show the latter process to be intimately connected to the presence, only for $V\ge 4$, of a manifold of infinitely many fixed points of the dynamical equations. Finally, we show that no fine-tuning of the model parameters is needed for the emergence of such $\pi$-synchronization, that is in fact found to vanish smoothly in presence of an increasing site-dependent disorder, in what we call a synchronization crossover.

Phononic crystals made of piezoceramic materials allow for frequency band structure tunability due to the ease of electric command use. In this paper, we develop a full elastodynamic model, taking into account piezoelectric coupling effects for the band-structure calculation of Lamb eigenmodes of a phononic crystal consisting of a piezoceramic plate with a one-dimensional periodic array of electrodes set on both sides. The dispersion-relation characteristics of these eigenmodes are nondestructively tuned via external circuit impedance loads coupled to the phononic plate, e.g., through inductance loads inducing tunable resonant flat bands that hybridize with the Lamb modes of the elastic plate, thus opening up hybridization gaps. It is shown that additional control of the shape of these resonant bands can be achieved by incorporating an impedance interconnecting two adjacent electrodes of the same side, the whole structure forming an electric quadripole. This behavior can be easily predicted with the help of our formalism combined with a simplified electric line model of the phononic crystal plate.

Atomistic simulations of the thermodynamic properties of magnetic materials rely on an accurate modeling of magnetic interactions and an efficient sampling of the high-dimensional spin space. Recent years have seen significant progress with a clear trend from model systems to material-specific simulations that are usually based on electronic-structure methods. Here we develop a Hamiltonian Monte Carlo framework that makes use of auxiliary spin dynamics and an auxiliary effective model, the temperature-dependent spin-cluster expansion, in order to efficiently sample the spin space. Our method does not require a specific form of the model and is suitable for simulations based on electronic-structure methods. We demonstrate fast warm-up and a reasonably small dynamical critical exponent of our sampler for the classical Heisenberg model. We further present an application of our method to the magnetic phase transition in bcc iron using magnetic bond-order potentials.

Potassium metal was loaded into K-form zeolite low-silica $\mathrm{X}$ $\left(\mathrm{LSX}\right)$ under the loading pressure $P$ up to 1.5 GPa. A ferromagnetism is observed at $P\approx$ 0.9 GPa below 13 K. The loading density of K atom $n$ is estimated to be $\approx 15$ per cage at the ferromagnetic loading pressure, and the pressure-derivative loading density $dn/dP$ has a remarkable peak $\approx 11{\mathrm{GPa}}^{-1}$ at $P\approx 0.95$ GPa. The density of states at the Fermi energy is estimated to be $\approx 46$ states/eV per cage from the value of $dn/dP$ on a simple model. The observed ferromagnetism is discussed in terms of the Stoner condition and the theory of the revised Rhodes-Wohlfarth plot.

The diagrammatic $t$-matrix approximation has often been adopted to describe a dilute Fermi gas. This approximation, originally considered by V. M. Galitskii (Zh. Eksp. Teor. Fiz. 34, 151 (1958) [Sov. Phys. JETP 7, 104 (1958)]) for a repulsive interparticle interaction, was later widely utilized for an attractive Fermi gas to describe the BCS-BEC crossover from strongly overlapping Cooper pairs in weak coupling to nonoverlapping composite bosons in strong coupling. Several variants of the $t$-matrix approximation have been considered in the literature, which are distinguished by the degree of self-consistency allowed in the building blocks of the diagrammatic structure. Here, we perform a systematic and comparative study of all possible variants on the degree of self-consistency for the $t$-matrix approximation in an attractive Fermi gas, which enables us to confront their outcomes for thermodynamic and dynamical quantities on the same footing in an unbiased way. For definiteness, only the normal phase above the superfluid critical temperature is considered. The dispute that can be raised in this context, about the adequateness of introducing progressive degrees of self-consistency over and above the non-self-consistent $t$-matrix approximation for an attractive Fermi gas, parallels the recent interest in the literature in assessing the importance of various degrees of self-consistency in the context of semiconductors and insulators.

Here we report on the structural and electronic phase diagram of lithium and ethylenediamine intercalated FeSe in a wide range of dopant concentration $\left(x=0\sim 0.8\right)$. Undoped ${\left({\mathrm{C}}_2{\mathrm{H}}_8{\mathrm{N}}_2\right)}_y\mathrm{F}{\mathrm{e}}_2\mathrm{S}{\mathrm{e}}_2$ crystallizes in an orthorhombic phase. With increasing lithium doping, an orthorhombic to tetragonal phase transition occurs at $x=0.35$, and the superconducting tetragonal phase persists until $x=0.5$. Meanwhile, the $T_c$ is found dependent strongly on dopant concentration, raising rapidly from 30 K at $x=0.35$ to 45 K at $x=0.5$. The crystal structures of $\mathrm{L}{\mathrm{i}}_{0.31\left(3\right)}{\left({\mathrm{C}}_2{\mathrm{H}}_8{\mathrm{N}}_2\right)}_{0.52\left(7\right)}\mathrm{F}{\mathrm{e}}_{2.03\left(2\right)}\mathrm{S}{\mathrm{e}}_2$ are determined by using high-resolution neutron diffraction data at 5, 60, 150, and 295 K, respectively. The distortion of the FeSe tetrahedron is enhanced significantly from 150 to 295 K, meanwhile, the normal-state Hall resistivity changes sign from negative to positive in the same temperature range. The dominant hole carrier in electron-doped $\mathrm{L}{\mathrm{i}}_{0.5}{\left({\mathrm{C}}_2{\mathrm{H}}_8{\mathrm{N}}_2\right)}_y\mathrm{F}{\mathrm{e}}_2\mathrm{S}{\mathrm{e}}_2$ above 230 K suggests that the temperature-induced structure distortion may lead to a reconstruction of the Fermi surface topology and the appearance of hole pockets.

Hyperferroelectricity is an interesting phenomenon. The hexagonal ABC-type semiconductor LiZnAs was discovered to be hyperferroelectric (HFE) [Garrity et al., Phys. Rev. Lett. 112, 127601 (2014)]. ZnO is a technologically important semiconductor and possesses a wurtzite crystal structure similar to LiZnAs. It raises an intriguing question of whether ZnO is HFE. Here we use various approaches to address this important question by determining the electric equation of state, the free energy of ZnO under an open-circuit boundary condition (OCBC), and the vibration properties of a LO phonon. We find the following: (i) The $D\sim \lambda$ curve of ZnO, where $D$ is electric displacement and $\lambda =P/0.844$ is a parameter directly proportional to polarization $P$, exhibits one and only one root at $\lambda =0$. (ii) Under OCBC, the free energy of ZnO does not produce a minimum at the structural phase of nonzero polarization. (iii) The LO phonon with computed frequency ${\omega }_{\mathrm{LO}}=255{\mathrm{cm}}^{-1}$ in centrosymmetric ZnO is not soft and does not have an imaginary frequency. These results corroborate the others and consistently lead to the conclusion that, although ZnO is interestingly on the edge of becoming a HFE, it is not yet a HFE. We further provide a physical origin explaining why ZnO is not HFE and reveal a possibility that may turn ZnO into a HFE.

We use temperature-dependent Hall measurements to identify contributions of spin Hall, magnetic proximity, and sublattice effects to the anomalous Hall signal in heavy metal/ferrimagnetic insulator heterostructures with perpendicular magnetic anisotropy. This approach enables detection of both the magnetic proximity effect onset temperature and the magnetization compensation temperature and provides essential information regarding the interfacial exchange coupling. Onset of a magnetic proximity effect yields a local extremum in the temperature-dependent anomalous Hall signal, which occurs at higher temperature as magnetic insulator thickness increases. This magnetic proximity effect onset occurs at much higher temperature in Pt than W. The magnetization compensation point is identified by a sharp anomalous Hall sign change and divergent coercive field. We directly probe the magnetic proximity effect using x-ray magnetic circular dichroism and polarized neutron reflectometry, which reveal an antiferromagnetic coupling between W and the magnetic insulator. Finally, we summarize the exchange-coupling configurations and the anomalous Hall-effect sign of the magnetized heavy metal in various heavy metal/magnetic insulator heterostructures.

Magnetic skyrmions are topological spin textures that have been observed in bulk magnets and magnetic multilayers. For bulk magnetic materials, their noncollinear spin profiles have often been studied by using Lorentz transmission electron microscopy (TEM). We experimentally utilized Lorentz TEM imaging to study an inversion asymmetric ${\left[\mathrm{Pt}\left(1.5\mathrm{nm}\right)/\mathrm{Co}\left(1\mathrm{nm}\right)/\mathrm{W}\left(1\mathrm{nm}\right)\right]}_8$ heterostructure that exhibits Néel-type skyrmions at zero field. By tracking the evolution of skyrmion diameters as a function of magnetic fields, we determined the strength of the interfacial Dzyaloshinskii-Moriya interaction (DMI). Our results suggest that in situ Lorentz TEM imaging combined with simulations can provide valuable quantitative information about the interfacial DMI strengths, which can be helpful for optimizing skyrmion materials. Furthermore, we show that in theory, Lorentz TEM can identify the spin chirality of Néel-type skyrmions, although an experimental verification is challenging due to the relatively low signal-to-noise ratio.

Hexagonal ferrites ($h\text{−}R{\mathrm{FeO}}_3$, $R$ = Sc, Y, Ho-Lu) have recently been identified as a new family of multiferroic complex oxides. We study electronic and magnetic properties of $h\text{−}{\mathrm{YbFeO}}_3$ ferrite within the density-functional theory using the generalized gradient approximation (GGA) with consideration of strong Coulomb correlations $\left(\mathrm{GGA}+U\right)$ in the framework of the fully relativistic spin-polarized Dirac linear muffin-tin orbital band-structure method. The $4f$ electrons of Yb are explicitly treated as valence electrons. The x-ray absorption spectra and x-ray magnetic circular dichroism (XMCD) at the Yb $M_{4,5}$, Fe $L_{2,3}$, and O $K$ edges were investigated theoretically. The calculated results are in good agreement with experimental data. We found that the $\mathrm{GGA}+U$ approach with Hubbard $U_{\mathrm{eff}}=6.1\mathrm{eV}$ and 3.3 eV for Yb and Fe, respectively, well describes the XMCD spectra at the Yb $M_{4,5}$, Fe $L_{2,3}$, and O $K$ edges.

Element-specific magnetic spin and orbital magnetic moments of $3d\text{−}4f$ double-decker metallacrown molecules have been investigated using x-ray magnetic circular dichroism. The double-decker metallacrowns comprise one rare-earth Gd(III) or Tb(III) ion embedded between two squared scaffolds of four Ni(II) ions. We observe a strong increase of the Ni(II) moments if the molecules are dissolved in methanol, indicating a spin crossover from a low-spin to a high-spin state. In contrast, dichloromethane does not change the spin state. This result is explained by a change of the coordination environment of nickel. The comparison of charge-transfer multiplet calculations with the experimental absorption spectra confirm the different ligand fields.

Quantum anomalous Hall (QAH) phases in magnetic topological insulators are characterized by the scattering-free chiral edge currents protected by their nontrivial bulk band topology. To fully explore these intriguing phenomena and application of topological insulators, high-temperature material realization of QAH phases is crucial. In this paper, based on extensive first-principles density functional theory calculations, we predict that perovskite bilayers ${\left({\mathrm{La}X\text{O}}_3\right)}_2$ ($X$ = Pd, Pt) imbedded in the (111) ${\left({\mathrm{La}X\text{O}}_3\right)}_2/{\left({\mathrm{LaAlO}}_3\right)}_{10}$ superlattices are high-Curie-temperature ferromagnets that host both QAH and Dirac nodal ring semimetal phases, depending on the biaxial strain and onsite electron correlation. In particular, both the direction (the Chern number sign) and spin polarization of the chiral edge currents are tunable by either onsite electron correlation or biaxial in-plane strain. Furthermore, the nontrivial band gap can be enhanced up to 92 meV in the ${\mathrm{LaPdO}}_3$ bilayer by the compressive in-plane strain and can go up to as large as 242 meV when the Pd atoms are replaced by the heavier Pt atoms. Finally, the microscopic mechanisms of the ferromagnetic coupling and other interesting properties of the bilayers are uncovered by analyzing their underlying electronic band structures.

Using the dynamical mean-field theory (DMFT) as a “booster rocket”, the functional renormalization group (fRG) can be upgraded from a weak-coupling method to a powerful computation tool for strongly interacting fermion systems. The strong local correlations are treated nonperturbatively by the DMFT, while the fRG flow can be formulated such that it is driven exclusively by nonlocal correlations, which are more amenable to approximations. We show that the full frequency dependence of the two-particle vertex needs to be taken into account in this approach, and demonstrate that this is actually possible—in spite of the singular frequency dependence of the vertex at strong coupling. We are thus able to present results obtained from the DMFT-boosted fRG for the two-dimensional Hubbard model in the strongly interacting regime. We find strong antiferromagnetic correlations from half filling to 18% hole doping and at the lowest temperature we can access, a sizable $d$-wave pairing interaction driven by magnetic correlations at the edge of the antiferromagnetic regime.

A topological superconductor nanowire bears a Majorana bound state at each of its ends, leading to unique transport properties. As a way to probe these, we study the finite frequency noise of a biased junction between a normal metal and a topological superconductor nanowire. We use the nonequilibrium Keldysh formalism to compute the finite frequency emission and absorption noise to all order in the tunneling amplitude, for bias voltages below and above the superconducting gap. We observe noticeable structures in the absorption and emission noise, which we can relate to simple transport processes. The presence of the Majorana bound state is directly related to a characteristic behavior of the noise spectrum at low frequency. We further compute the noise measurable with a realistic setup, based on the inductive coupling to a resonant LC circuit, and discuss the impact of the detector temperature. We have also computed the emission noise for a nontopological system with a resonant level, exhibiting a zero-energy Andreev bound state, to show the specificities of the topological case. Our results offer an original tool for the further characterization of the presence of Majorana bound states in condensed matter systems.

We address the possible emergence of spin triplet superconductivity in ${\mathrm{CrO}}_2$ bilayers, which are half-metals with fully spin-polarized conducting bands. Starting from a lattice model, we show that chiral $p+ip$ states compete with nonchiral $p$-wave ones. At large doping, the $p+ip$ channel has a sequence of topological phase transitions that can be tuned by gating effects and interaction strength. Among several phases, we find chiral topological phases having a single Majorana mode at the edge. We show that different topological superconducting phases could spontaneously emerge in the vicinity of the van Hove singularities of the band.

We study the equilibrium properties of a ferromagnetic insulator/superconductor structure near a magnetic domain wall. We show how the domain wall size is affected by the superconductivity in such structures. Moreover, we calculate several physical quantities altered due to the magnetic domain wall, such as the spin current density and local density of states, as well as the resulting tunneling conductance into a structure with a magnetic domain wall.