Rectangular assemblies with different aspect ratios were prepared with spherical magnetite nanoparticles (diameter $d=20\mathrm{nm}$) on lithographically patterned substrates using a variant of the meniscus force deposition method. The aspect ratio (width:length) of the rectangular assemblies was varied from very low (1:1) to very high (1:1000) values. Angle-dependent ferromagnetic resonance measurements were performed to study the influence of the shape anisotropy on the magnetic properties. Using an analytical model based on the Smit-Suhl formalism for single-domain magnets, the demagnetizing factors were determined. The analysis of the resonance signals shows that, for small aspect ratios, the ratio of the in-plane demagnetization factors is inversely proportional to the corresponding ratio of width:length; that is, the assemblies behave like a single ferromagnet due to dipolar magnetic coupling between the particles. At larger aspect ratios a more complicated behavior is observed which indicates the formation of a multidomain-like structure inside the assemblies caused by geometrical inhomogeneities in the filling of the assemblies. Micromagnetic simulations of the magnetic properties of the assemblies support this assumption qualitatively and suggest that the formation of these inhomogeneities can be controlled by the fabrication process. The results provide insights into the collective magnetic behavior of nanoparticle assemblies, highlighting the high degree of tunability of the magnetic properties of such assemblies, which makes them promising building blocks for future magnetic devices.

We theoretically study the equilibrium spin current fluctuations and the corresponding charge noise generated by the inverse spin Hall effect (ISHE) in a metal with strong spin-orbit coupling deposited on top of a quantum paramagnet. It is shown that the charge noise power spectra measured along different spatial axes can directly probe the different spin components of the boundary dynamic spin correlations of the quantum paramagnet. We report the utility of this ISHE-facilitated spin noise probe as a tool to unambiguously detect topological phase transitions in an $S=1/2$ quantum spin ladder that hosts a trivial ground state of singlet product states, but topologically protected fractional spin excitations localized at its ends. Our work demonstrates the general usefulness of ISHE-mediated spin noise spectroscopy for the detection of topological phases in quantum paramagnets.

${\mathrm{Yb}}_2{\mathrm{Si}}_2\mathrm{Al}$ may be a prototype for exploring different aspects of the Shastry-Sutherland lattice, formed by planes of orthogonally coupled Yb ions. Measurements of the magnetic susceptibility find incoherently fluctuating ${\mathrm{Yb}}^{3+}$ moments coexisting with a weakly correlated metallic state that is confirmed by measurements of the electrical resistivity. Increasing signs of Kondo coherence are found with decreasing temperature, including an enhanced Sommerfeld coefficient and Kadowaki-Woods ratio that signal that the metallic state found at the lowest temperatures is a Fermi liquid where correlations have become significantly stronger. A pronounced peak in the electronic and magnetic specific heat indicates that the coupling of the Yb moments to the conduction electrons leads to an effective Kondo temperature that is approximately 30 K. The valence of ${\mathrm{Yb}}_2{\mathrm{Si}}_2\mathrm{Al}$ has been investigated with electron spectroscopy methods. ${\mathrm{Yb}}_2{\mathrm{Si}}_2\mathrm{Al}$ is found to be strongly intermediate valent $[v_F=2.68\left(2\right)$ at 80 K]. Taken together, these experimental data are consistent with a scenario where a coherent Kondo lattice forms in ${\mathrm{Yb}}_2{\mathrm{Si}}_2\mathrm{Al}$ from an incoherently fluctuating ensemble of Yb moments with incomplete Kondo compensation, and strong intermediate valence character.

Andreev reflection in graphene is special since it can be of two types, retro or specular. Specular Andreev reflection (SAR) dominates when the position of the Fermi energy in graphene is comparable to or smaller than the superconducting gap. Bilayer graphene (BLG) is an ideal candidate to observe the crossover from retro to specular since the Fermi energy broadening near the Dirac point is much weaker compared to monolayer graphene. Recently, the observation of signatures of SAR in BLG have been reported experimentally by looking at the enhancement of conductance at finite bias near the Dirac point. However, the signatures were not very pronounced possibly due to the participation of normal quasiparticles at bias energies close to the superconducting gap. Here, we propose a scheme to observe the features of enhanced SAR even at zero bias at a normal metal (NM)–superconductor (SC) junction on BLG. Our scheme involves applying a Zeeman field to the NM side of the NM-SC junction on BLG (making the NM ferromagnetic), which energetically separates the Dirac points for up spin and down spin. We calculate the conductance as a function of chemical potential and bias within the superconducting gap and show that well-defined regions of specular- and retro-type Andreev reflection exist. We compare the results with and without superconductivity. We also investigate the possibility of the formation of a p-n junction at the interface between the NM and SC due to a work function mismatch.

We explore the nature of the electroluminescence (EL) emission of purely $n$-doped GaAs/AlGaAs resonant tunneling diodes (RTDs) and the EL evolution with voltage. A singular feature of such a device is unveiled when the electrical output current changes from higher to lower values and the EL on-off ratio is enhanced by two orders of magnitude compared to the current on-off ratio. By combining the EL and the current properties, we are able to identify two independent impact ionization channels associated with the coherent resonant tunneling current and the incoherent valley current. We also perform the same investigation with an associated series resistance, which induces a bistable electrical output in the system. By simulating a resistance variation for the current voltage and the EL, we are able to tune the EL on-off ratio by up to six orders of magnitude. We further observe that the EL on and off states can be either direct or inverted compared to the tunneling current of the on and off states. This electroluminescence, combined with the unique RTD properties, such as the negative differential resistance and high-frequency operation, enables the development of high-speed functional optoelectronic devices and optical switches.

Fractional quantum Hall (FQH) states are known to possess an internal metric degree of freedom that allows them to minimize their energy when contrasting geometries are present in the problem (e.g., electron band mass and dielectric tensor). We investigate the internal metric of several incompressible FQH states by probing their response to band mass anisotropy using infinite DMRG simulations on a cylinder geometry. We test and apply a method to extract the internal metric of an FQH state from its guiding center structure factor. We find that the response to band mass anisotropy is approximately the same for states in the same Jain sequence, but changes substantially between different sequences. We provide a theoretical explanation of the observed behavior of primary states at filling $\nu =1/m$ in terms of a minimal microscopic model of flux attachment.

We show that a circular periodic array of silicon dielectric cylinders supports nearly bound states in the continuum (BICs) propagating along the cylinders. These propagating nearly BICs with extremely large-$Q$ factors are surrounded by resonant modes weakly leaking into the radiation continuum. We present leaky zones in the form of dispersion curves for complex eigenfrequencies dependent on propagation constant $k_z$, with the wave vector directed along the cylinders in the vicinity of different types of BICs. Symmetry-protected nearly BICs have the resonant width proportional to squared propagation constant $\mathrm{\Gamma }\sim k_z^2$; the widths of non-symmetry-protected nearly BICs behave as $\mathrm{\Gamma }\sim {(k_z-k_c)}^2$, where $k_c$ and non-symmetry-protected nearly BICs have the resonant width proportional to $k_z^4$. The latter propagating nearly BICs can serve for the transmission of a electromagnetic signal paving a way to a different type of optical fiber. We also demonstrate weakly leaking resonant modes which carry orbital angular momentum.

Each electronic band of a nanotube is associated with a specific crystal orbital angular momentum, in analogy to crystal momentum. We develop the semiclassical equations of motion of electrons in such twisted Bloch bands perturbed by electric and magnetic fields. These equations demonstrate that the absorption of twisted light generates valley-dependent current along the nanotube in the presence of an external inhomogeneous magnetic field. We show that the valley-dependent current provides a venue to detect valley polarization in twisted Bloch bands.

We study Ti $1s$ near-edge spectroscopy in ${\mathrm{PbTiO}}_3$ at various temperatures above and below its tetragonal-to-cubic phase transition, and in ${\mathrm{SrTiO}}_3$ at room temperature. Ab initio molecular dynamics (AIMD) runs on 80-atom supercells are used to determine the average internal coordinates and their fluctuations. We determine that one vector local order parameter is the dominant contributor to changes in spectral features: the displacement of the Ti ion with respect to its axial O neighbors in each Cartesian direction, as these displacements enhance the cross section for transitions to ${\mathrm{E}}_{\mathrm{g}}$-derived core-hole exciton levels. Using periodic five-atom structures whose relative Ti-O displacements match the root-mean-square values from the AIMD simulations, and core-hole Bethe-Salpeter equation calculations, we quantitatively predict the respective Ti $1s$ near-edge spectra. Properly accounting for atomic fluctuations greatly improves the agreement between theoretical and experimental spectra. The evolution of relative strengths of spectral features vs temperature and electric field polarization vector are captured in considerable detail. This work shows that local structure can be characterized from first-principles sufficiently well to aid both the prediction and the interpretation of near-edge spectra.

To better understand the underlying fundamental physical processes in nitrogen-vacancy centers in diamond, and to realize improvements in the use of this system for technological applications, it is imperative to gain new insight into the origins of the apparent inhomogeneous broadening. In this work we make use of a type of spectroscopy developed specifically for this task. The pump-probe spectroscopy closely follows Doppler-free spectroscopy used in atomic vapor. We show that the origin of inhomogeneous broadening comes from local magnetic field variations in the diamond lattice.

Magnetic artificial spin ice provides examples of how competing interactions between magnetic nanoelements can lead to a range of fascinating and unusual phenomena. We examine theoretically a class of spin ice tilings, called the pinwheel, for which near degeneracy of the spin configuration energies can be achieved. Pinwheel tiling is a simple but crucial variant on the square ice geometry, in which each nanoelement of square ice is rotated some angle about its midpoint. Surprisingly, this rotation leads to an intriguing phase transition; and even though the spins are not parallel to one another, a ferromagnetic phase is found for rotation angles near ${45}^{\circ }$. Here, magnetic domains and domain walls are found when viewed in terms of net magnetization. Moreover, the ferromagnetic behavior of the system depends on its anisotropy which we can control by array shape and size.

Bloch oscillations in a single Josephson junction in the phase-slip regime relate current to frequency. They can be measured by applying a periodic drive to a dc-biased, small Josephson junction. Phase locking between the periodic drive and the Bloch oscillations then gives rise to steps at constant current in the $I\text{−}V$ curves, also known as dual Shapiro steps. Unlike conventional Shapiro steps, a measurement of these dual Shapiro steps is impeded by the presence of a parasitic capacitance. This capacitance shunts the junction, resulting in a suppression of the amplitude of the Bloch oscillations. This detrimental effect of the parasitic capacitance can be remedied by an on-chip superinductance. Additionally, we introduce a large off-chip resistance to provide the necessary dissipation. We investigate the resulting system using a set of analytical and numerical methods. In particular, we obtain an explicit analytical expression for the height of dual Shapiro steps as a function of the ratio of the parasitic capacitance to the superinductance. Using this result, we provide a quantitative estimate of the dual Shapiro step height. Our calculations reveal that even in the presence of a parasitic capacitance, it should be possible to observe Bloch oscillations with realistic experimental parameters.

We study the influence of spinless impurities on a frustrated magnet featuring a spin-density wave (stripe) phase by means of Monte Carlo simulations. We demonstrate that the interplay between the impurities and an order parameter that breaks a real-space symmetry triggers the emergence of a random-field mechanism which destroys the stripe-ordered phase. Importantly, the strength of the emerging random fields can be tuned by the repulsion between the impurity atoms; they vanish for perfect anticorrelations between neighboring impurities. This provides a way of controlling the phase diagram of a many-particle system. In addition, we also investigate the effects of the impurities on the character of the phase transitions between the stripe-ordered, ferromagnetic, and paramagnetic phases.

The near-critical unitary dynamics of quantum Ising spin chains in transversal and longitudinal magnetic fields is studied using an artificial neural network representation of the wave function. A focus is set on strong spatial correlations which build up in the system following a quench into the vicinity of the quantum critical point. We compare correlations obtained by optimizing the parameters of the network states with analytical solutions in integrable cases and time-dependent density matrix renormalization group (tDMRG) simulations, as well as with predictions from a semiclassical discrete truncated Wigner analysis. While the semiclassical approach yields quantitatively correct results only at very short times and near zero transverse fields, the neural-network representation is applicable in a much wider regime. However, for quenches close to the quantum critical point the representation becomes inefficient. For nonintegrable models we show that in regimes where tDMRG is limited to short times due to extensive entanglement growth, also the neural-network parametrization converges only at short times.

We present a complete calculation of spin waves in magnetic stacks, including the interfacial Dzyaloshinskii-Moriya interaction (iDMI) as a boundary condition, and discuss the influence of interlayer coupling and magnetic anisotropy on the spin-wave nonreciprocity. We show that the usual simple approach relating the iDMI strength to the slope of the spin-wave nonreciprocity versus the spin-wave number, in the case of a thin ferromagnetic film in contact with a heavy metal exhibiting a strong-orbit coupling, is not valid in many more complex structures. In the case of stacks made of identical magnetic layers, an analytical method allowed us to check that the effect of iDMI on the entire structure is captured by including the total thickness of the ferromagnetic layers. The experimental data obtained for some systems by means of Brillouin light scattering are analyzed through the presented models.

We report a neutron scattering study of the metallic triangular lattice antiferromagnet ${\mathrm{PdCrO}}_2$. Powder neutron diffraction measurements confirm that the crystalline space group symmetry remains $R\overline{3}m$ below $T_N$. This implies that magnetic interactions consistent with the crystal symmetry do not stabilize the noncoplanar magnetic structure, which was one of two structures previously proposed on the basis of single crystal neutron diffraction measurements. Inelastic neutron scattering measurements find two gaps at low energies, which can be explained as arising from a dipolar-type exchange interaction. This symmetric anisotropic interaction also stabilizes a magnetic structure very similar to the coplanar magnetic structure, which was also suggested by the single crystal diffraction study. The higher-energy magnon dispersion can be modelled by linear spin-wave theory with exchange interactions up to sixth nearest neighbors, but discrepancies remain which hint at additional effects unexplained by the linear theory.

We report Co $L_3$-edge resonant inelastic x-ray scattering on ${\mathrm{LaCoO}}_3$ at 20 K. We observe excitations with sizable dispersion that we identify as intermediate-spin (IS) states. Theoretical calculations that treat the IS states as mobile excitons propagating on the low-spin background support the interpretation. The present result shows that mobility substantially reduces the energy of IS excitations in part of the Brillouin zone, which makes them important players in the low-energy physics of ${\mathrm{LaCoO}}_3$ together with immobile high-spin excitations.

The electric field-induced dissociation is studied for excited states of the yellow exciton series of ${\mathrm{Cu}}_2\mathrm{O}$. With increasing principal quantum number $n$, corresponding to rising exciton energy, the field strength for dissociation decreases as expected. Surprisingly, within a manifold belonging to a particular $n$ this trend is reversed as the required dissociation field increases with rising energy. In agreement with calculations we attribute this finding to the distribution of the exciton wave functions in the potential landscape. While the low energy states in the multiplet are shifted towards the side where the potential is lowered by the electric field, thereby facilitating dissociation, the high energy states are moved to the other side stabilizing them up to higher fields.