We implement the concept of complete local symmetry in lossy acoustic waveguides. Despite the presence of losses, the existence of a spatially invariant current is shown theoretically and observed experimentally. We demonstrate how this invariant current leads to the generalization of the Bloch and parity theorems for lossy systems defining a mapping of the pressure field between symmetry-related spatial domains. Using experimental data, we verify this mapping with remarkable accuracy. For the performed experiment, we employ a construction technique based on local symmetries that allows the design of setups with prescribed perfect transmission resonances in the lossless case. Our results reveal the fundamental role of symmetries in restricted spatial domains, and they clearly indicate that completely locally symmetric devices constitute a promising class of setups with regard to the manipulation of wave propagation.

A phenomenological theory is developed for the strain-driven magnetization reorientations occurring in ultrathin ferromagnetic films coupled to ferroelectric substrates experiencing electric-field-induced piezoelectric deformations. The theory takes into account the surface/interface magnetic anisotropy playing an important role in the energetics of such films and first describes the thickness-driven spin reorientation transitions emerging in the presence of substrate-induced lattice strains. Then the threshold and critical intensities of the electric field created in a ferroelectric substrate are calculated, at which different magnetic states acquire the same energy or become unstable in a strained ferromagnetic overlayer. To demonstrate stability ranges of various possible magnetization orientations, we introduce magnetoelectric orientational diagrams, where the electric-field intensity and film thickness are employed as two variables. Such diagrams are constructed for ultrathin Ni, Fe, and $\mathrm{F}{\mathrm{e}}_{60}\mathrm{C}{\mathrm{o}}_{40}$ films coupled to single crystals of classical and relaxor ferroelectrics. The inspection of these diagrams shows that the use of multiferroic hybrids comprising ultrathin ferromagnetic films significantly enlarges the range of ferroic materials suitable for experimental observations of the strain-mediated converse magnetoelectric effect.

Using a self-consistent Bogoliubov–de Gennes approach, we theoretically study the proximity-induced density of states (DOS) in clean $\mathit{SFF}$ spin valves with noncollinear exchange fields. Our results clearly demonstrate a direct correlation between the presence of a zero-energy peak (ZEP) in the DOS spectrum and the persistence of spin-1 triplet pair correlations. By systematically varying the geometrical and material parameters governing the spin valve, we point out experimentally optimal system configurations where the ZEPs are most pronounced, and which can be effectively probed via scanning tunneling microscopy. We complement these findings in the ballistic regime by employing the Usadel formalism in the full proximity limit to investigate their diffusive $\mathit{SFF}$ counterparts. We determine the optimal normalized ferromagnetic layer thicknesses which result in the largest ZEPs. Our results can serve as guidelines in designing samples for future experiments.

We use series expansions to investigate the zero-temperature phase diagram of the Heisenberg-Kitaev model. Our work confirms the presence of four distinct magnetically ordered phases, identified in recent work, and provides accurate values of the ground state energy and order parameter throughout. We call into question the previous estimates of the location of phase boundaries, and provide an analytic argument for their location.

The magnetic critical scattering in ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ has been characterized using x-ray resonant magnetic scattering (XRMS) both below and above the three-dimensional antiferromagnetic ordering temperature $T_{\text{N}}$. The order parameter critical exponent below $T_{\text{N}}$ is found to be $\beta =0.195\left(4\right)$, in the range of the two-dimensional (2D) $XY{h}_4$ universality class. Over an extended temperature range above $T_{\text{N}}$, the amplitude and correlation length of the intrinsic critical fluctuations are well described by the 2D Heisenberg model with $XY$ anisotropy. This contrasts with an earlier study of the critical scattering over a more limited range of temperature, which found agreement with the theory of the isotropic 2D Heisenberg quantum antiferromagnet, developed to describe the critical fluctuations of the conventional Mott insulator ${\mathrm{La}}_2{\mathrm{CuO}}_4$ and related systems. Our study therefore establishes the importance of $XY$ anisotropy in the low-energy effective Hamiltonian of ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$, the prototypical spin-orbit Mott insulator.

Longitudinal and transverse transport coefficients of the $\mathrm{Ba}{\left(\mathrm{F}{\mathrm{e}}_{1-x}\mathrm{C}{\mathrm{o}}_x\right)}_2\mathrm{A}{\mathrm{s}}_2$ single crystals with $x=0,0.045,0.06$, and $0.244$ were measured in the temperature range $1.4-300\mathrm{K}$ and in magnetic fields up to 12.5 T. The resulting data were used to determine the temperature dependence of the Hall Lorenz number $\left(L_{xy}\right)$ and its evolution with doping. $L_{xy}$ is defined by the electronic contributions to the thermal and electrical conductivities and it is found to differ from its canonical behavior. This shows the emergence of a pseudogap in samples at intermediate doping.

Epitaxial films of ${\mathrm{SrMnO}}_3$ and bilayers of ${\mathrm{SrMnO}}_3/{\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_3$ have been deposited by pulsed laser deposition on different substrates, namely, ${\mathrm{LaAlO}}_3$ (001), $\left({\mathrm{LaAlO}}_3{)}_{0.3}\right({\mathrm{Sr}}_2{\mathrm{AlTaO}}_6{)}_{0.7}$ (001), and ${\mathrm{SrTiO}}_3$ (001), allowing us to perform an exhaustive study of the dependence of antiferromagnetic order and exchange bias field on epitaxial strain. The Néel temperatures ($T_N$) of the ${\mathrm{SrMnO}}_3$ films have been determined by low-energy muon spin spectroscopy. In agreement with theoretical predictions, $T_N$ is reduced as the epitaxial strain increases. From the comparison with first-principles calculations, a crossover from G-type to C-type antiferromagnetic orders is proposed at a critical tensile strain of around $1.6\pm 0.1\%$. The exchange bias (coercive) field, obtained for the bilayers, increases (decreases) by increasing the epitaxial strain in the ${\mathrm{SrMnO}}_3$ layer, following an exponential dependence with temperature. Our experimental results can be explained by the existence of a spin-glass (SG) state at the interface between the ${\mathrm{SrMnO}}_3$ and ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_3$ films. This SG state is due to the competition between the different exchange interactions present in the bilayer and favored by increasing the strain in the ${\mathrm{SrMnO}}_3$ layer.

Neutron powder diffraction experiments were performed on the intermediate-valence ${\mathrm{Ce}}_2{\mathrm{RuZn}}_4$ intermetallic compound and combined with magnetic bulk measurements including high magnetic field experiments up to 58 T. Previous theoretical studies suggest that only one (here Ce1) out of two inequivalent Ce sites is magnetically active. ${\mathrm{Ce}}_2{\mathrm{RuZn}}_4$ orders antiferromagnetically at $T_N=2.3\mathrm{K}$. The magnetic structure is characterized by an incommensurate propagation vector $q_m=(0.384,0.384,\frac{1}{2})$. Assuming that the Ce2 site does not carry any substantial moment, Ce1 magnetic moments are confined to the (110)-type planes and transversely modulated with an amplitude of $1.77\left(3\right){\mu }_B$.

We study unconventional superconductivity induced by weak repulsive interactions in 2D electronic systems at van Hove singularity (VHS) where density of states is logarithmically divergent. We define two types of VHS. For systems at type-I VHS, weak repulsive interactions generically induce unconventional singlet pairing. However, and more interestingly, for type-II VHS renormalization group analysis shows that weak repulsive interactions favor triplet pairing (e.g., $p$ wave) when the Fermi surface is not sufficiently nested. For type-II VHS systems respecting tetragonal symmetry, topological superconductivity (either chiral $p+ip$ pairing or time-reversal invariant ${\mathbb{Z}}_{2}p+ip$ pairing) occurs generally. We shall also discuss relevance of our study to materials including recently discovered superconductors ${\mathrm{LaO}}_{1-x}{\mathrm{F}}_x{\mathrm{BiS}}_2$, which can be tuned to type-II VHS by doping.

We demonstrate with the finite-difference time-domain method that radiative heat transfer between two parallel gold plates can be significantly enhanced by engraving periodic grooves with a subwavelength width on the plate surfaces. The enhancement increases with a decrease in the separation distance at near-field regime and it can be further efficiently improved by having a supercell with multiple grooves with different depths. We attribute this near-field enhancement to coupling of thermally excited spoof surface plasmon polaritons, a type of artificial surface wave inherent to structured metal surfaces [J. B. Pendry, L. Martín-Moreno, and F. J. Garcia-Vidal, Science 305, 847 (2004)]. The frequency-dependent contribution to the heat transfer, or transmission-factor spectrum, is confirmed by calculating the dispersion relation of guided modes by the two parallel corrugated plates through a finite-element method. Especially, the photonic density of states derived from the dispersion relation is found to have excellent agreement to the transmission-factor spectrum.

Monolayer graphene is an example of materials with multivalley electronic structure. In such materials, the valley index is being considered as an information carrier. Consequently, relaxation mechanisms leading to loss of valley information are of interest. Here, we calculate the rate of valley relaxation induced by charged impurities in graphene. A special model of graphene is applied, where the $p_z$ orbitals are two-dimensional Gaussian functions, with a spatial extension characterized by an effective Bohr radius $a_{\text{eB}}$. We obtain the valley relaxation rate by solving the Boltzmann equation, for the case of noninteracting electrons, as well as for the case when the impurity potential is screened due to electron-electron interaction. For the latter case, we take into account local-field effects and evaluate the dielectric matrix in the random phase approximation. Our main findings are as follows: (i) The valley relaxation rate is proportional to the electronic density of states at the Fermi energy. (ii) Charged impurities located in the close vicinity of the graphene plane, at distance $d\lesssim 0.3\text{Å}$, are much more efficient in inducing valley relaxation than those farther away, the effect of the latter being suppressed exponentially with increasing graphene-impurity distance $d$. (iii) Both in the absence and in the presence of electron-electron interaction, the valley relaxation rate shows pronounced dependence on the effective Bohr radius $a_{\text{eB}}$. The trends are different in the two cases: In the absence (presence) of screening, the valley relaxation rate decreases (increases) for increasing effective Bohr radius. This last result highlights that a quantitative calculation of the valley relaxation rate should incorporate electron-electron interactions as well as an accurate knowledge of the electronic wave functions on the atomic length scale.

We present an extensive study of the structure, composition, electronic, and magnetic properties of Ce-Pt surface intermetallic phases on Pt(111) as a function of their thickness. The sequence of structural phases appearing in low-energy electron diffraction may invariably be attributed to a single underlying intermetallic atomic lattice. Findings from both microscopic and spectroscopic methods prove compatible with ${\mathrm{CePt}}_5$ formation when their characteristic probing depth is adequately taken into account. The intermetallic film thickness serves as an effective tuning parameter which brings about characteristic variations of the cerium valence and related properties. Soft x-ray absorption (XAS) and magnetic circular dichroism (XMCD) prove well suited to trace the changing Ce valence and to assess relevant aspects of Kondo physics in the ${\mathrm{CePt}}_5$ surface intermetallic. We find characteristic Kondo scales of the order of ${10}^2$ K and evidence for considerable magnetic Kondo screening of the local Ce $4f$ moments. ${\mathrm{CePt}}_5/\mathrm{Pt}\left(111\right)$ and related systems therefore appear to be promising candidates for further studies of low-dimensional Kondo lattices at surfaces.

A Hg(Cd)Te insulator heterostructure hosts a two-dimensional electron system that can simulate the physics of Dirac fermions with only a single valley. We investigate the magnetotransport properties of this structure and show that, unlike most two-dimensional crystals with spin and valley coupled levels, the Shubnikov–de Haas oscillations exhibit a high spin polarization in the absence of any valley degree of freedom. This effect can be observed using magnetospectroscopy measurements for quantum well thicknesses corresponding to either the topologically trivial or quantum spin Hall phases. The pseudospin texture of the electrons near the Fermi level is also studied and we show that a tunable pseudospin-polarized quantum Hall liquid can only be observed for thicknesses corresponding to the inverted regime.

The thermal expansion of polycrystalline ${\mathrm{ReO}}_3$ is studied in vacuum by x-ray diffraction from 300 to 600 K and by extended x-ray absorption fine structure (EXAFS) at the Re $L_3$ edge from 30 to 600 K. A detailed EXAFS analysis is presented up to the sixth coordination shell around Re. The crystal thermal expansion is weak throughout the explored range, negative below 100 K, positive from 150 to 500 K, and negative again above 500 K. The analysis of mean square relative displacements obtained by EXAFS and of mean square displacements available from x-ray and neutron diffraction measurements confirms the presence of significant static disorder in the investigated samples. EXAFS results suggest that below 500 K in ${\mathrm{ReO}}_3$ at least some of the octahedra are slightly and disorderly rotated by an average angle $\langle \theta \rangle$, whose value decreases when the temperature increases. This explains why ${\mathrm{ReO}}_3$, in spite of the possibility of supporting rigid unit modes, shows a weak positive expansion at increasing temperatures, from 150 up to 500 K. Results are discussed and compared with recent experimental and theoretical work on materials characterized by negative thermal expansion.

We extend the quasiclassical formalism for diffusive superconductors by deriving anisotropic (gradient) corrections to the Usadel equation. We demonstrate that in a number of physical situations such corrections may play a crucial role, being responsible for the effects which cannot be recovered within the standard Usadel approximation. One of them is the so-called photoelectric effect in superconductors and superconducting-normal (SN) hybrid structures. Provided a superconducting part of the system is irradiated by an external ac electromagnetic field, the charge imbalance develops and a nonvanishing dc voltage is induced across the SN interface. In the presence of magnetic impurities in a superconductor the magnitude of this effect becomes large and can easily be detected in modern experiments.

We theoretically study the crossover between spin Hall effect and spin swapping, a recently predicted phenomenon that consists of the interchange between the current flow and its spin polarization directions [M. B. Lifshits and M. I. Dyakonov, Phys. Rev. Lett. 103, 186601 (2009)]. Using a tight-binding model with spin-orbit coupled disorder, spin Hall effect, spin relaxation, and spin swapping are treated on equal footing. We demonstrate that spin swapping and spin Hall effect present very different dependencies as a function of the spin-orbit coupling and disorder strengths and confirm that the former exceeds the latter in the parameter range considered. Three setups are proposed for the experimental observation of the spin swapping effect.

We report anomalous Hall effects exhibiting a hysteresis loop as large as about 10 T in a ferromagnetic superlattice comprising ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_3$ and ${\mathrm{SrRuO}}_3$ layers. The superlattices grown by pulsed laser deposition exhibit a strong antiferromagnetic interlayer coupling below 110 K, where both ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_3$ and ${\mathrm{SrRuO}}_3$ layers show anomalous Hall effects. With increasing magnetic-field strength, the anomalous Hall resistivity in the superlattices changes its sign depending on the magnetization directions of the ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_3$ and ${\mathrm{SrRuO}}_3$ layers. As a consequence of competition among the antiferromagnetic interlayer coupling, the Zeeman effect, and magnetic anisotropies, the width of the hysteresis loop in the anomalous Hall resistivity in the superlattices becomes larger than 8 T at 10 K, clearly greater than those observed in ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_3$ and ${\mathrm{SrRuO}}_3$ single layer films.

The mechanism of the tunnel electroresistance effect of a $\mathrm{Co}\text{/}\mathrm{PbTi}{\mathrm{O}}_3\text{/}\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3$ multiferroic tunnel junction is studied in detail using experimental and theoretical methods. Based on experimental data, we present a model that explains the correlation between the polarization of the ferroelectric material and the observed resistance state based on the effective change of the tunnel barrier thickness. We show that the observed thickness variation can neither be completely attributed to the asymmetric inverse piezoelectric effect in the classical sense, nor to asymmetric screening of the polarization charge. The analysis of detailed ab initio calculations quantitatively demonstrates that a mixture of electronic and structural phenomena is responsible for the change in effective tunnel barrier thickness upon polarization reversal. On the one hand, the ferroelectric material exhibits a reversible metallization at one of the interfaces, which shifts the boundary between the ferroelectric material and the electrode. On the other hand, a piezoelectric effect that stems from different terminations of the ferroelectric ultrathin film towards the electrodes magnifies this effect. Combined, the electrically switchable effective change in thickness is as large as 0.15 nm, which dominates the resistive switching effect in the presented junction that involves a 3.2 nm thin $\mathrm{PbTi}{\mathrm{O}}_3$ film. This work contributes to the deeper understanding of fundamental mechanisms that lead to tunnel electroresistance and imposes new ways for tailoring the characteristics of electroresistive tunnel junctions.

Direct (nongradient) coupling between a gapless bosonic field and a Fermi surface results in the destruction of Landau quasiparticles and a breakdown of Fermi liquid theory. Such a non-Fermi-liquid phase arises in spin-orbit coupled ferromagnets with spontaneously broken continuous symmetries due to strong coupling between rotational Goldstone modes and itinerant electrons. These systems provide an experimentally accessible context for studying non-Fermi-liquid physics. Possible examples include low-density Rashba coupled electron gases, which have a natural tendency towards spontaneous ferromagnetism, or topological insulator surface states with proximity-induced ferromagnetism. Crucially, unlike the related case of a spontaneous nematic distortion of the Fermi surface, for which controlled field theory calculations predict that the non-Fermi-liquid regime will be masked by a superconducting dome, we show that the non-Fermi-liquid phase in spin-orbit coupled ferromagnets is stable.