Thin films of single-crystal ${\mathrm{Pb}}_{1-y}$${\mathrm{Bi}}_y$${\mathrm{Sr}}_2$${\mathrm{Y}}_{1-x}$${\mathrm{Ca}}_x$${\mathrm{Cu}}_2$${\mathrm{O}}_{7+\delta }$ (PbBi1212) were grown on ${\mathrm{SrTiO}}_3$ (100) substrates by a two-step growth technique in which an amorphous film is annealed at 970 ${}^{\circ }$C in a closed ceramic container prepared using the same material as the film. We find that PbBi1212 exhibits superconductivity when the Ca concentration $x$ exceeds 0.3. The effective number of holes per Cu atom $n_{\mathrm{eff}}$ is well described as $n_{\mathrm{eff}}=0.34x$. The highest onset temperature for the superconducting transition attained in the present study is 65 K. The resistivity measurement in a magnetic field reveals that the coherence lengths of Pb1212 ($y=0$) are approximately 25 and 2.7 Å along the $ab$ plane and the $c$ axis, respectively.

Using inelastic neutron scattering, we show that the onset of superconductivity in underdoped $\mathrm{Ba}({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x{)}_2{\mathrm{As}}_2$ coincides with a crossover from well-defined spin waves to overdamped and diffusive spin excitations. This crossover occurs despite the presence of long-range stripe antiferromagnetic order for samples in a compositional range from $x=0.04$ to 0.055, and is a consequence of the shrinking spin-density wave gap and a corresponding increase in the particle-hole (Landau) damping. The latter effect is captured by a simple itinerant model relating Co doping to changes in the hot spots of the Fermi surface. We argue that the overdamped spin fluctuations provide a pairing mechanism for superconductivity in these materials.

We investigate the ground states of $s=1/2$ Heisenberg antiferromagnets on the 11 two-dimensional (2D) Archimedean lattices by using the coupled-cluster method. Magnetic interactions and quantum fluctuations play against each other subtly in 2D quantum magnets and the magnetic ordering is thus sensitive to the features of lattice topology. Archimedean lattices are those lattices that have 2D arrangements of regular polygons and they often build the underlying magnetic lattices of insulating quasi-two-dimensional quantum magnetic materials. Hence they allow a systematic study of the relationship between lattice topology and magnetic ordering. We find that the Archimedean lattices fall into three groups: those with semiclassical magnetic ground-state long-range order, those with a magnetically disordered (cooperative quantum paramagnetic) ground state, and those with a fragile magnetic order. The most relevant parameters affecting the magnetic ordering are the coordination number and the degree of frustration present.

For the Ising model with Gaussian random coupling of average $J_0$ and unit variance, the zero-temperature spin-glass–ferromagnetic transition as a function of the control parameter $J_0$ can be studied via the size-$L$-dependent renormalized coupling defined as the domain-wall energy $J^R\left(L\right)\equiv E_{\text{GS}}^{\left(\text{AF}\right)}\left(L\right)-E_{\text{GS}}^{\left(\text{F}\right)}\left(L\right)$ (i.e., the difference between the ground-state energies corresponding to antiferromagnetic and ferromagnetic boundary conditions in one direction). We study numerically the critical exponents of this zero-temperature transition within the Migdal-Kadanoff approximation as a function of the dimension $d=2,3,4,5,6$. We then compare with the mean-field spherical model. Our main conclusion is that in low dimensions, the critical stiffness exponent ${\theta }^c$ is clearly bigger than the spin-glass stiffness exponent ${\theta }^{\text{SG}}$, but that they turn out to coincide in high enough dimension and in the mean-field spherical model. We also discuss the finite-size scaling properties of the averaged value and of the width of the distribution of the renormalized couplings.

Reflectance anisotropy spectroscopy (RAS) has been used to measure the optical anisotropies of bulk and thin-film ${\mathrm{Fe}}_3$${\mathrm{O}}_4$(110) surfaces. The spectra indicate that small shifts in energy of the optical transitions, associated with anisotropic strain or electric field gradients caused by the (110) surface termination or a native oxide layer, are responsible for the strong signal observed. The RAS response was then measured as a function of temperature. A distinct change in the RAS line-shape amplitude was observed in the spectral range from 0.8 to 1.6 eV for temperatures below the Verwey transition of the crystal. Finally, thin-film magnetite was grown by molecular beam epitaxy on MgO(110) substrates. Changes in the RAS spectra were found for different film thickness, suggesting that RAS can be used to monitor the growth of magnetite (110) films in situ. The thickness dependence of the RAS is discussed in terms of various models for the origin of the RAS signal.

We model short-period superlattices of $\mathrm{W}{\mathrm{O}}_3$ and $\mathrm{Re}{\mathrm{O}}_3$ with first-principles calculations. In fully relaxed superlattices we observe that octahedral tilts about an axis in the planes of the superlattices do not propagate from one material, despite the presence of the corner-shared oxygen atoms. However, we find that octahedral rotation is enhanced within $\mathrm{W}{\mathrm{O}}_3$ layers in cases in which strain couples with native antiferroelectric displacements of tungsten within their octahedral cages. Resulting structures remain antiferroelectric with low net global polarization. Thermodynamic analysis reveals that superlattices with sufficiently thick $\mathrm{Re}{\mathrm{O}}_3$ layers, the absolute number being three or more layers and the Re fraction $\ge 50\%$, tend to be more stable than the separated material phases and also show enhanced octahedral rotations in the $\mathrm{W}{\mathrm{O}}_3$ layers.

The growth of spin-crossover Fe(1,10-phenanthroline)${}_2$(NCS)${}_2$ molecules on Cu(100) surfaces in the coverage range from 0.1 to 1.8 molecular layers was studied using a scanning tunneling microscope (STM) operated in ultrahigh vacuum at low temperature ($\approx$4 K). STM imaging allowed us to extract the molecular adsorption geometry. While the first-layer molecules point their NCS groups toward the surface and their phenanthroline groups upwards, the adsorption geometry is reversed for the molecules in the second layer. For submonolayer coverages, a coexistence of molecules in the high- and low-spin states was found that is not correlated with the coverage. This coexistence is reduced for second-layer molecules, leading to a dominant spin state at low temperatures. Differential conductance spectra acquired at negative bias voltage on first- and second-layer molecules suggest that second-layer molecules are in the high-spin state and are partially electronically decoupled from the substrate. Furthermore, increasing the tip-to-sample voltage reduces the distance between the two lobes of the molecule. The current dependence of this effect suggests that a smooth spin crossover from a high- to a low-spin state occurs with increasing sample voltage. This analog spin-state switching is well described within a simple transition-state model involving modifications to the energy barriers between low- and high-spin states due to a tip-induced electric field through the Stark effect.

Topological properties of energy spectra of general one-dimensional quasiperiodic systems, describing also Bloch electrons in magnetic fields, are studied for an infinity of irrational modulation frequencies corresponding to irrational numbers of flux quanta per unit cell. These frequencies include well-known ones considered in works on Fibonacci quasicrystals. It is shown that the spectrum for any such frequency exhibits a self-similar hierarchy of clusters characterized by universal (system-independent) values of Chern topological integers which are exactly determined. The cluster hierarchy provides a simple and systematic organization of all the spectral gaps, labeled by universal topological numbers which are exactly determinable, thus avoiding their numerical evaluation using rational approximants of the irrational frequency. These numbers give both the quantum Hall conductance of the system and the winding number of the edge-state energy traversing a gap as a Bloch quasimomentum is varied.

The optical gyrotropy noise of a high-finesse semiconductor Bragg microcavity with an embedded quantum well (QW) is studied at different detunings of the photon mode with respect to the QW exciton resonances. A strong suppression of the noise magnitude for the photon mode frequencies lying above exciton resonances is found. We show that such a critical behavior of the observed optical noise power is specific to asymmetric Fabry-Perot resonators. As follows from our analysis, at a certain level of intracavity loss, the reflectivity of the asymmetric resonator vanishes, while the polarimetric sensitivity to the gyrotropy changes dramatically when moving across the critical point. The results of model calculations are in a good agreement with our experimental data on the spin noise in a single-quantum-well microcavity and are confirmed also by the spectra of the photoinduced Kerr rotation in the pump-probe experiments.

In three-dimensional critical percolation we study numerically the number of clusters $N_{\Gamma }$ which intersect a given subset of bonds $\Gamma$. If $\Gamma$ represents the interface between a subsystem and the environment, then $N_{\Gamma }$ is related to the entanglement entropy of the critical diluted quantum Ising model. Due to corners in $\Gamma$ there are singular corrections to $N_{\Gamma }$, which scale as $b_{\Gamma }ln{L}_{\Gamma }$, with $L_{\Gamma }$ being the linear size of $\Gamma$ and the prefactor $b_{\Gamma }$ is found to be universal. This result indicates that logarithmic finite-size corrections exist in the free energy of three-dimensional critical systems.

Ab initio molecular dynamics simulations are used to describe the diffusion of hydrogen in liquid aluminum at different temperatures. Quasi-instantaneous jumps separating periods of localized vibrations around a mean position are found to characterize the hydrogen motion at the microscopic scale. The hydrogen motion is furthermore analyzed using the van Hove function. We highlight a non-Fickian behavior for the hydrogen diffusion due to a large spatial distribution of hydrogen jumps. We show that a generalized continuous time random walk (CTRW) model describes the experimental diffusion coefficients in a satisfactory manner. Finally, the impact of impurities and alloying elements on hydrogen diffusion in aluminum is discussed.

We report ${}^{51}$V nuclear magnetic resonance (NMR) studies on single crystals of the multiferroic material ${\text{FeVO}}_4$. The high-temperature Knight shift shows Curie-Weiss behavior ${}^{51}K=a/(T+\theta )$, with a large Weiss constant $\theta \approx$ 116 K. However, the ${}^{51}$V spectrum shows no ordering near these temperatures, splitting instead into two peaks below 65 K, which suggests only short-ranged magnetic order on the NMR time scale. Two magnetic transitions are identified from peaks in the spin-lattice relaxation rate $1{/}^{51}{T}_1$ at temperatures $T_{N1}\approx$ 19 K and $T_{N2}\approx$ 13 K, which are lower than the estimates obtained from polycrystalline samples. In the low-temperature incommensurate spiral state, the maximum ordered moment is estimated as 1.95${\mu }_B$/Fe, or $\frac{1}{3}$ of the local moment. Strong low-energy spin fluctuations are also indicated by the unconventional power-law temperature dependence $1{/}^{51}{T}_1\propto T^2$. The large Weiss constant, short-range magnetic correlations far above $T_{N1}$, small ordered moment, significant low-energy spin fluctuations, and incommensurate ordered phases all provide explicit evidence for strong magnetic frustration in ${\text{FeVO}}_4$.

The exchange-driven contribution to the magnetoelectric susceptibility $\widehat{\alpha }$ is formulated using a microscopic model Hamiltonian coupling the spin degrees of freedom to lattice displacements and electric field, which may be constructed from first-principles data. Electronic and ionic contributions are sorted out, and the latter is resolved into a sum of contributions from different normal modes. If intrasublattice spin correlations can be neglected, the longitudinal component ${\alpha }_{\parallel }$ becomes proportional to the product of magnetic susceptibility and sublattice magnetization in accordance with Rado's phenomenological model. As an illustration, the method is applied to analyze the temperature dependence of the longitudinal magnetoelectric susceptibility of ${\text{Cr}}_2$${\text{O}}_3$ using first-principles calculations and the quantum pair cluster approximation for magnetic thermodynamics. A substantial electronic contribution is found, which is opposite to the ionic part. The sensitivity of the results to the Hubbard $U$ parameter and the sources of error are studied. It is also found that non-Heisenberg interactions are too weak to account for the sign change of ${\alpha }_{\parallel }$ in ${\text{Cr}}_2$${\text{O}}_3$.

A metastable perovskite ${\text{BiFe}}_{0.5}$${\text{Sc}}_{0.5}$${\text{O}}_3$ synthesized under high-pressure (6 GPa) and high-temperature (1500 K) conditions was obtained in two different polymorphs, antipolar $Pnma$ and polar $Ima2$, through an irreversible behavior under a heating/cooling thermal cycling. The $Ima2$ phase represents an original type of a canted ferroelectric structure where Bi${}^{3+}$ cations exhibit both polar and antipolar displacements along the orthogonal ${\left[110\right]}_p$ and ${\left[1\overline{1}0\right]}_p$ pseudocubic directions, respectively, and are combined with antiphase octahedral tilting about the polar axis. Both the $Pnma$ and the $Ima2$ structural modifications exhibit a long-range antiferromagnetic ordering with a weak ferromagnetic component below $T_N\sim 220$ K. Analysis of the coupling between the dipole, magnetic, and elastic order parameters based on a general phenomenological approach revealed that the weak ferromagnetism in both phases is mainly caused by the presence of the antiphase octahedral tilting whose axial nature directly represents the relevant part of Dzyaloshinskii vector. The magnetoelectric contribution to the spontaneous magnetization allowed in the polar $Ima2$ phase is described by a fifth-degree free-energy invariant and is expected to be small.

We calculate the Andreev bound states and the corresponding Josephson current for an asymmetric two-dimensional Josephson junction by solving Bogoliubov–de Gennes equations. The junction consists of a noncentrosymmetric superconductor (NCS) separated by a tunneling barrier with a variable height to a conventional $s$-wave superconductor. In addition to the antisymmetric spin-orbit coupling in the NCS on the one side, this asymmetric junction gives rise to a Rashba spin-orbit coupling at the interface. We explore the rich parameter space and recover various limiting cases such as $s$-wave/$p$-wave junction and the asymmetric $s$-wave junctions. In addition, we report a transition from a 0 junction to a $\pi /2$ junction with increasing triplet-singlet pairing ratio of the NCS, which serves as a mechanism to determine the unknown ratio in a variety of NCS's.

According to the Goldstone theorem the breaking of a continuous U(1) symmetry comes along with the existence of low-energy collective modes. In the context of superconductivity these excitations are related to the phase of the superconducting (SC) order parameter and for clean systems are optically inactive; that is, single-mode excitations do not directly couple to light. Here we show that for strongly disordered superconductors phase modes acquire a dipole moment and appear as a subgap spectral feature in the optical conductivity. This finding is obtained with both a gauge-invariant random-phase approximation scheme based on a fermionic Bogoliubov–de Gennes state and a prototypical bosonic model for disordered superconductors. In the strongly disordered regime, where the system displays an effective granularity of the SC properties, the optically active dipoles are linked to the isolated SC islands, offering a new perspective for realizing microwave optical devices.

We construct, to leading orders in the momentum expansion, an effective theory of a chiral ($p_x+i{p}_y$) two-dimensional fermionic superfluid at zero temperature that is consistent with nonrelativistic general coordinate invariance. This theory naturally incorporates the parity and time-reversal violating effects such as the Hall viscosity and the edge current. The particle number current and stress tensor are computed and their linear response to electromagnetic and gravitational sources is calculated. We also consider an isolated vortex in a chiral superfluid and identify the leading chirality effect in the density depletion profile.

Topological protection is an efficient way of warranting the integrity of quantum and nanosized systems. In magnetism, one example is the Bloch point, a peculiar three-dimensional object implying the local vanishing of magnetization within a ferromagnet. By combining surface and transmission x-ray magnetic circular dichroism photoemission electron microscopy, we experimentally confirm the existence of the simplest magnetization texture holding a Bloch point at rest: the Bloch-point domain wall in cylindrical magnetic nanowires. This opens the way to the experimental search for peculiar phenomena predicted during the motion of these protected Bloch-point-based domain walls.