We present results of electron paramagnetic resonance (EPR) and neutron-diffraction studies of ${\mathrm{CuCl}}_2·2{\mathrm{NC}}_5{\mathrm{H}}_5$, which is regarded as one of the best known uniform spin-1/2 Heisenberg antiferromagnet chain systems. We reveal that at $T_N=1.12$ K, ${\mathrm{CuCl}}_2·2{\mathrm{NC}}_5{\mathrm{H}}_5$ undergoes the transition into the magnetically ordered spiral state with $Q=(0.5,0.4,0.5)$ r.l.u. It was shown that the zigzag interchain interactions result in a noticeable geometrical frustration, also affecting the ordered moment per ${\mathrm{Cu}}^{2+}$ and EPR properties, including the angular dependence of the linewidth. The temperature dependencies of the EPR parameters are discussed.

Similar to nitrogen-vacancy centers in diamond and impurity atoms in silicon, interstitial gallium deep paramagnetic centers in GaAsN have been proven to have useful characteristics for the development of spintronic devices. Among other interesting properties, under circularly polarized light, gallium centers act as spin filters that dynamically polarize free and bound electrons reaching record spin polarizations (close to 100%). Furthermore, the recent observation of the amplification of the spin filtering effect under a Faraday configuration magnetic field has suggested that the hyperfine interaction that couples bound electrons and nuclei permits the optical manipulation of the nuclear spin polarization. Even though the mechanisms behind the nuclear spin polarization in gallium centers are fairly well understood, the origin of nuclear spin relaxation and the formation of an Overhauser-like magnetic field remain elusive. In this work we develop a model based on the master equation approach to describe the evolution of electronic and nuclear spin polarizations of gallium centers interacting with free electrons and holes. Our results are in good agreement with existing experimental observations. In particular, we are able to reproduce the amplification of the spin filtering effect under a circularly polarized excitation in a Faraday configuration magnetic field. In regard to the nuclear spin relaxation, the roles of nuclear dipolar and quadrupolar interactions are discussed. Our findings show that, besides the hyperfine interaction, the spin relaxation mechanisms are key to understand the amplification of the spin filtering effect and the appearance of the Overhauser-like magnetic field. To gain a deeper insight in the interplay of the hyperfine interaction and the relaxation mechanisms, we have also performed calculations in the pulsed excitation regime. Our model's results allow us to propose an experimental protocol based on time-resolved spectroscopy. It consists of a pump-probe photoluminescence scheme with the detection and the tracing of the electron-nucleus flip-flops through photoluminescence measurements.

We study the nonequilibrium exciton-polariton condensation in 1D to 0D and 1D to quasi-2D junctions by means of non-resonant spectroscopy. The shape of our potential landscape allows to probe the resonant transmission of a propagating condensate between a quasi-1D waveguide and cylindrically symmetric states. We observe a distinct mode selection by varying the position of the non-resonant pump laser. Moreover, we study the the case of propagation from a localized trapped condensate state into a waveguide channel. Here, the choice of the position of the injection laser allows us to tune the output in the waveguide. Our measurements are supported by an accurate Ginzburg-Landau modeling of the system shining light on the underlying mechanisms.

We report a large circular photogalvanic effect (CPGE) in a 2DEG created at the interface of a semiconductor/insulator homojunction at the $\left(10\overline{1}0\right)$ surface of a Li-doped ZnO microwire by low energy proton implantation. We show that the CPGE originates from the Rashba spin-orbit interaction at the interface. Our sample arrangement allows tuning the spin-orbit interaction strength by manipulating the electron spin orientation via an external magnetic field. The results of the present work obtained at 305 K indicate the experimental realization of a persistent spin helix in ZnO.

Introducing both the Berry curvature and the chiral anomaly into Landau's Fermi-liquid theory, we construct a topological Fermi-liquid theory, applicable to interacting Weyl metals in the absence of time reversal symmetry. Following the Landau's Fermi-liquid theory, we obtain an effective free-energy functional in terms of the density field of chiral fermions, where the band structure is modified, involved with an emergent magnetic dipole moment due to the Berry curvature. The density field of chiral fermions is determined by a self-consistent equation, minimizing the effective free-energy functional with respect to the order-parameter field. Beyond these thermodynamic properties, we construct a Boltzmann transport theory to encode both the Berry curvature and the chiral anomaly in the presence of forward scattering of a Fermi-liquid state, essential for understanding dynamic correlations in interacting Weyl metals. This generalizes the Boltzmann transport theory for the Landau's Fermi-liquid state in the respect of incorporating the topological structure and extends that for noninteracting Weyl metals in the sense of introducing the forward scattering. Finally, we justify this topological Fermi-liquid theory, generalizing the first-quantization description for noninteracting Weyl metals into the second-quantization representation for interacting Weyl metals. First, we introduce a topological Fermi-gas theory, integrating over high-energy electronic degrees of freedom deep inside a pair of chiral Fermi surfaces. As a result, we reproduce a topologically modified Drude model with both the Berry curvature and the chiral anomaly, given by the first-quantization description. Second, we take into account interactions between such low-energy chiral fermions on the pair of chiral Fermi surfaces. Following the Landau's Fermi-liquid theory, we perform the renormalization group analysis. We find that only forward scattering turns out to be marginal above possible superconducting transition temperatures, justifying the topological Fermi-liquid theory of interacting Weyl metals with time reversal symmetry breaking. The topological Fermi-liquid theory serves a theoretical platform for us to investigate the role of Fermi-liquid interactions in anomalous transport phenomena of interacting Weyl metals such as anomalous Hall effects, chiral magnetic and vortical effects, and negative longitudinal magnetoresistivity properties. In addition, it allows us to study how thermodynamic properties such as the Wilson's ratio and spectra of collective excitations such as zero sound modes in the Landau's Fermi-liquid state are modified due to the Berry curvature and the chiral anomaly.

Various matrix representations are used to analyze the propagation of electromagnetic waves through stratified (layered) media or cascaded circuit networks. These include ABCD matrices, scattering matrices, impedance matrices, and hybrid matrices. A less known network representation is the wave matrix. In this paper, a brief review of wave matrices is presented and their relation to other network representations derived. Wave matrices are found for common interfaces such as boundaries between dielectric media, dielectric slabs, as well as electric, magnetic, and magneto-electric sheet boundaries (generalized sheet transition conditions). These results are then used to develop an analytical synthesis approach for cascaded metasurfaces: metasurfaces consisting of a cascade of sheets separated by dielectric spacers. This is in contrast to earlier works which relied on numerical solvers or optimization methods to design such structures. A few design examples are presented to demonstrate the utility of the synthesis approach.

We theoretically investigate the magnetic properties of ${\mathrm{Sr}}_2{\mathrm{TcO}}_4$, a $4d$ transition-metal layered perovskite of the ${\mathrm{K}}_2{\mathrm{NiF}}_4$ type with half-filled $t_{2g}$ states. The effect of local Coulomb repulsion between the $t_{2g}$ orbitals is included within the density-functional theory (DFT) + $U$ and DFT + dynamical mean-field theory (DMFT) methods. The DFT + DMFT predicts paramagnetic ${\mathrm{Sr}}_2{\mathrm{TcO}}_4$ to be close to the Mott insulator-to-metal transition, similarly to the cubic compound ${\mathrm{SrTcO}}_3$. The intersite exchange interactions computed within the DFT + DMFT framework point to a strong antiferromagnetic coupling between the neighboring Tc sites within the layer. We then evaluate the Néel temperature $T_N$ within a classical Monte Carlo approach including dipolar interactions, which stabilize the magnetic order in the frustrated ${\mathrm{K}}_2{\mathrm{NiF}}_4$ lattice structure. Our approach is validated by applying it to a set of layered and cubic perovskites, for which we obtain $T_N$ in fair agreement with experiment. Within the same approach we obtain the $T_N$ of ${\mathrm{Sr}}_2{\mathrm{TcO}}_4$ to be about 450 K. We explore also the effect of anisotropy in exchange interactions due to spin-orbit coupling. These lead to a somewhat higher transition temperature, 550 K.

We investigate the topological properties of dynamical states evolving on periodic oriented graphs. This evolution, which encodes the scattering processes occurring at the nodes of the graph, is described by a single-step global operator, in the spirit of the Ho-Chalker model. When the successive scattering events follow a cyclic sequence, the corresponding scattering network can be equivalently described by a discrete time-periodic unitary evolution, in line with Floquet systems. Such systems may present anomalous topological phases where all the first Chern numbers are vanishing, but where protected edge states appear in a finite geometry. To investigate the origin of such anomalous phases, we introduce the phase rotation symmetry, a generalization of usual symmetries which only occurs in unitary systems (as opposed to Hamiltonian systems). Equipped with this new tool, we explore a possible explanation of the pervasiveness of anomalous phases in scattering network models, and we define bulk topological invariants suited to both equivalent descriptions of the network model, which fully capture the topology of the system. We finally show that the two invariants coincide, again through a phase rotation symmetry arising from the particular structure of the network model.

We investigate the evolution of large number of ${\delta }^{\prime }$ coherent precipitates from a supersaturated Al-8 at.% Li alloy using large-scale phase-field simulations. A chemomechanical cross-coupling between mechanical relaxation and diffusion is taken into account by considering the dependence of elastic constants of the matrix phase onto the local concentration of solute atoms. The elastic constants as a function of solute concentration have been obtained using density functional theory calculations. As a result of the coupling, inverse ripening has been observed where the smaller precipitates grow at the expense of the larger ones. This is due to size-dependent concentration gradients existing around the precipitates. At the same time, precipitates rearrange themselves as a consequence of minimization of the total elastic energy of the system. It is found that the anisotropy of the chemomechanical coupling leads to the formation of new patterns of elasticity in the matrix thereby resulting in new alignments of the precipitates.

Magnetoelectric materials have generated wide technological and scientific interest because of the rich phenomena these materials exhibit, including the coexistence of magnetic and ferroelectric orders, magnetodielectric behavior, and exotic hybrid excitations such as electromagnons. The multiferroic spinel material ${\text{CoCr}}_2{\text{O}}_4$ is a particularly interesting example of a multiferroic material, because evidence for magnetoelectric behavior in the ferrimagnetic phase seems to conflict with traditional noncollinear-spin-driven mechanisms for inducing a macroscopic polarization. With the overall goal of clarifying the magnetodielectric behavior previously reported below $T_C$ in ${\text{CoCr}}_2{\text{O}}_4$, in this paper we report an inelastic light scattering study of the magnon and phonon spectrum of ${\text{CoCr}}_2{\text{O}}_4$ as simultaneous functions of temperature, pressure, and magnetic field. Below the Curie temperature $\left(T_C=94\text{K}\right)$ of ${\text{CoCr}}_2{\text{O}}_4$ we observe a $\omega \sim 16{\text{cm}}^{-1}$ $\mathit{q}=0$ magnon having ${\text{T}}_{1g}$-symmetry, which has the transformation properties of an axial vector. The anomalously large Raman intensity of the ${\text{T}}_{1g}$-symmetry magnon is characteristic of materials with a large magneto-optical response and likely arises from large magnetic fluctuations that strongly modulate the dielectric response in ${\text{CoCr}}_2{\text{O}}_4$. The Raman susceptibility of the ${\text{T}}_{1g}$-symmetry magnon exhibits a strong magnetic-field dependence that is consistent with the magnetodielectric response observed in ${\text{CoCr}}_2{\text{O}}_4$, suggesting that magnetodielectric behavior in ${\text{CoCr}}_2{\text{O}}_4$ primarily arises from the field-dependent suppression of magnetic fluctuations that are strongly coupled to long-wavelength phonons. Increasing the magnetic anisotropy in ${\text{CoCr}}_2{\text{O}}_4$ with applied pressure decreases the magnetic-field dependence of the ${\text{T}}_{1g}$-symmetry magnon Raman susceptibility, suggesting that strain can be used to control the magnetodielectric response in ${\text{CoCr}}_2{\text{O}}_4$.

A spin liquid is a new state of matter with topological order where the spin moments continue to fluctuate coherently down to the lowest temperatures rather than develop static long-range magnetic order as found in conventional magnets. For spin liquid behavior to arise in a material the magnetic Hamiltonian must be “frustrated”, where the combination of lattice geometry, interactions, and anisotropies gives rise to competing spin arrangements in the ground state. Theoretical Hamiltonians which produce spin liquids are spin ice, the Kitaev honeycomb model, and the kagome antiferromagnet. Spin liquid behavior, however, in real materials is rare because they can only approximate these Hamiltonians and often have weak higher-order terms that destroy the spin liquid state. ${\mathrm{Ca}}_{10}{\mathrm{Cr}}_7{\mathrm{O}}_{28}$ is a new quantum spin liquid candidate with magnetic ${\mathrm{Cr}}^{5+}$ ions that possess quantum spin number $S=\mathrm{½}$. The spins are entirely dynamic in the ground state and the excitation spectrum is broad and diffuse, as is typical of spinons which are the excitations of a spin liquid. In this paper we determine the Hamiltonian of ${\mathrm{Ca}}_{10}{\mathrm{Cr}}_7{\mathrm{O}}_{28}$ using inelastic neutron scattering under high magnetic field to induce a field-polarized paramagnetic ground state and spin-wave excitations that can be fitted to extract the interactions. We further explore the phase diagram by using inelastic neutron scattering and heat capacity measurements and establish the boundaries of the spin liquid phase as a function of magnetic field and temperature. Our results show that ${\mathrm{Ca}}_{10}{\mathrm{Cr}}_7{\mathrm{O}}_{28}$ consists of distorted kagome bilayers with several isotropic ferromagnetic and antiferromagnetic interactions where, unexpectedly, the ferromagnetic interactions are stronger than the antiferromagnetic ones. This complex Hamiltonian does not correspond to any known spin liquid model and points to new directions in the search for quantum spin liquid behavior.

A detailed understanding of the relation between microscopic structure and phonon propagation at the nanoscale is essential to design materials with desired phononic and thermal properties. Here we uncover a new mechanism of phonon interaction in surface oxidized membranes, i.e., native oxide layers interact with phonons in ultrathin silicon membranes through local resonances. The local resonances reduce the low frequency phonon group velocities and shorten their mean free path. This effect opens up a new strategy for ultralow thermal conductivity design as it complements the scattering mechanism which scatters higher frequency modes effectively. The combination of native oxide layer and alloying with germanium in concentration as small as 5% reduces the thermal conductivity of silicon membranes to 100 times lower than the bulk. In addition, the resonance mechanism produced by native oxide surface layers is particularly effective for thermal conductivity reduction even at very low temperatures, at which only low frequency modes are populated.

${}^{75}\mathrm{As}$ nuclear magnetic (NMR) and quadrupolar (NQR) resonance were used, together with Mössbauer spectroscopy, to investigate the magnetic state induced by Mn for Fe substitutions in F-doped ${\mathrm{LaFe}}_{1-x}{\mathrm{Mn}}_x\mathrm{AsO}$ superconductors. The results show that 0.5% of Mn doping is enough to suppress the superconducting transition temperature $T_c$ from 27 K to zero and to recover the magnetic structure observed in the parent undoped LaFeAsO. Also the tetragonal to orthorhombic transition of the parent compound is recovered by introducing Mn, as evidenced by a sharp drop of the NQR frequency. The NQR spectra also show that a charge localization process is at play in the system. Theoretical calculations using a realistic five-band model show that correlation-enhanced RKKY exchange interactions between nearby Mn ions stabilize the observed stripe magnetic order. These results give compelling evidence that F-doped LaFeAsO is a strongly correlated electron system at the verge of an electronic instability.

In this work, we propose a method for calculating the free energy of anisotropic quantum spin systems. We use the Hubbard-Stratonovich transformation to express the partition function of a generic bilinear superexchange Hamiltonian in terms of a functional integral over classical time-dependent fields. In the general case the result is presented as a product of traces over single spins subject to a time-dependent field. The traces may be evaluated in closed form in the case of systems with large Ising anisotropy. In the general case we derive a compact expression for the contribution of Gaussian spin fluctuations to the free energy. We show how anisotropic spin interactions lead to anisotropies in the free energy, giving rise to pinning of the spontaneous magnetization along preferred directions

Experimental measurements performed on MgO-based perpendicular magnetic tunnel junctions show a strong dependence of the stability voltage-field diagrams as a function of the direction of the magnetic field with respect to the plane of the sample. When the magnetic field is applied in-plane, systematic nonlinear phase boundaries are observed for various lateral sizes. The simulation results based on the phenomenological Landau-Lifshitz-Gilbert equation including the in-plane and out-of-plane spin transfer torques are consistent with the measurements if a second-order anisotropy contribution is considered. Furthermore, performing the stability analysis in linear approximation allowed us to analytically extract the critical switching voltage at zero temperature in the presence of an in-plane field. This study indicates that in the noncollinear geometry investigations are suitable to detect the presence of the second-order term in the anisotropy. Such higher order anisotropy term can yield an easy-cone anisotropy which reduces the thermal stability factor but allows for more reproducible spin transfer torque switching due to a reduced stochasticity of the switching. As a result, the energy per write event decreases much faster than the thermal stability factor as the second-order anisotropy becomes more negative. Easy-cone anisotropy can be useful for fast-switching spin transfer torque magnetic random access memories provided the thermal stability can be maintained above the required value for a given memory specification.

${\mathrm{SiO}}_2$ and ${\mathrm{HfO}}_2$ are both high-k, wide-gap semiconductors, currently used in the microelectronic industry as gate barriers. Here we investigate whether the same materials can be employed to make magnetic tunnel junctions, which in principle can be amenable for integration in conventional Si technology. By using a combination of density functional theory and the nonequilibrium Green's functions method for quantum transport we have studied the transport properties of $\mathrm{Co}\left[0001\right]/{\mathrm{SiO}}_2\left[001\right]/\mathrm{Co}\left[0001\right]$ and $\mathrm{Fe}\left[001\right]/{\mathrm{HfO}}_2\left[001\right]/\mathrm{Fe}\left[001\right]$ junctions. In both cases we found a quite large magnetoresistance, which is explained through the analysis of the real band structure of the magnets and the complex one of the insulator. We find that there is no symmetry spin filtering for the Co-based junction since the high transmission ${\mathrm{\Delta }}_{2^{\prime }}$ band crosses the Fermi level, $E_{\mathrm{F}}$, for both spin directions. However, the fact that Co is a strong ferromagnet makes the orbital contribution to the two ${\mathrm{\Delta }}_{2^{\prime }}$ spin subbands different, yielding magnetoresistance. In contrast for the Fe-based junction symmetry filtering is active for an energy window spanning between the Fermi level and 1 eV below $E_{\mathrm{F}}$, with ${\mathrm{\Delta }}_1$ symmetry contributing to the transmission.

Recently, a test for a sign-changing gap function in a candidate multiband unconventional superconductor involving quasiparticle interference data was proposed. The test was based on the antisymmetric, Fourier transformed conductance integrated over a range of momenta $\mathbf{q}$ corresponding to interband processes, which was argued to display a particular resonant form, provided the gaps changed sign between the Fermi-surface sheets connected by $\mathbf{q}$. The calculation was performed for a single impurity, raising the question of how robust this measure is as a test of sign-changing pairing in a realistic system with many impurities. Here we reproduce the results of the previous work within a model with two distinct Fermi-surface sheets, and we show explicitly that the previous result, while exact for a single nonmagnetic scatterer and also in the limit of a dense set of random impurities, can be difficult to implement for a few dilute impurities. In this case, however, appropriate isolation of a single impurity is sufficient to recover the expected result, allowing a robust statement about the gap signs to be made.

We have studied the electronic structure of ${\mathrm{Ba}}_{1-x}{\mathrm{Sr}}_x{\mathrm{V}}_{13}{\mathrm{O}}_{18}$ $\left(x=0,0.2,1\right)$ at different temperatures across the trimerization and charge-order transitions using hard x-ray photoemission spectroscopy (HAXPES). The V $2p$ HAXPES indicates ${\mathrm{V}}^{2+}/{\mathrm{V}}^{3+}$ charge order and fluctuation in the high-temperature tetramer phase, low-temperature trimer phase, and intermediate-temperature charge-order phase in the series of $x=0,0.2,1$. In the valence-band HAXPES, although the spectral weight at the Fermi level tends to be suppressed in all the phases due to strong charge order or fluctuation, it increases in the trimer phase at $x=0.2$, in agreement with the decrease of resistivity and the increase of itinerant electrons in the trimer phase. Interestingly, in the most conducting $x=1$ without the charge-order phase, the spectral weight at the Fermi level is strongly suppressed even in the trimer phase, indicating that charge fluctuation in the trimer phase is different between $x=0.2$ and 1.

We analyze the charge- and spin-response functions of rare-earth nickelates $R{\mathrm{NiO}}_3$ and their heterostructures using random-phase approximation in a two-band Hubbard model. The interorbital charge fluctuation is found to be the driving mechanism for the rock-salt-type bond order in bulk $R{\mathrm{NiO}}_3$, and good agreement of the ordering temperature with experimental values is achieved for all $R{\mathrm{NiO}}_3$ using realistic crystal structures and interaction parameters. We further show that magnetic ordering in bulk is not driven by the spin fluctuation and should be instead explained as ordering of localized moments. This picture changes for low-dimensional heterostructures, where the charge fluctuation is suppressed and overtaken by the enhanced spin instability, which results in a spin-density-wave ground state observed in recent experiments. Predictions for spectroscopy allow for further experimental testing of our claims.

Motivated by the normal state of the cuprates in which the integrated spectral weight of the optical conductivity or optical sum increases faster than a linear function of the particle density, we derive an $f$-sum rule for a system in which the kinetic-energy operator in the Hamiltonian is a general function of the momentum squared. Such a kinetic energy arises in scale invariant theories and can be derived within the context of holography. Our derivation of the $f$-sum rule is based on the gauge couplings of a nonlocal Lagrangian in which the kinetic operator is a fractional Laplacian of order $\alpha$. We find that the $f$-sum rule in this case deviates from the standard linear dependence on the particle density. We find two regimes. At high temperatures and low densities, the optical sum is proportional to $n{T}^{\frac{\alpha -1}{\alpha }}$ where $T$ is the temperature. At low temperatures and high densities, the optical sum is proportional to $n^{1+\frac{2(\alpha -1)}{d}}$ with $d$ being the number of spatial dimensions. The result in the low-temperature and high-density limit, when $\alpha <1$, can be used to qualitatively explain the behavior of the effective number of charge carriers in the cuprates at various doping concentrations.