Results of optical spectroscopy studies of the face-centered icosahedral (fci) single-grain Zn-Mg-Y, Zn-Mg-Ho, and Zn-Mg-Er quasicrystals (QCs) are presented. The dielectric function of the QCs was measured in the 0.01–6 eV spectral range by IR-UV spectroscopic ellipsometry and far infrared reflection spectroscopy techniques. A theoretical scheme of optical conductivity calculations is extended to account for the Fermi level positions within and below a pseudogap. The model of the QC electron energy spectrum, based on a band structure hypothesis, is suggested, which treats the electronic subsystem as a nearly free electron gas affected by intersections of the Fermi surface with several families of Bragg planes. The experimental optical spectra are reproduced in detail by theoretical calculations carried out within the framework of the model. The parameters of the electron energy spectrum deduced from an analysis of optical data are close to those previously determined in an analysis of fci Zn-Mg-$R$ valence band photoemission spectra.

Using first principles density functional calculations, we study the electronic structure of the low-dimensional multiferroic compound FeTe${}_2$O${}_5$Br to investigate the origin of the magnetoelectric (ME) effect and the role of Te ions in this system. We find that without magnetism, even in the presence of Te 5$s$ lone pairs, the system remains centrosymmetric due to the antipolar orientation of the lone pairs. Our study shows that the exchange striction within the Fe tetramers as well as between them is responsible for the ME effect in FeTe${}_2$O${}_5$Br. We also find that the Te${}^{4+}$ ions play an important role in the intertetramer exchange striction as well as contributing to the electric polarization in FeTe${}_2$O${}_5$Br, once the polarization is triggered by the magnetic ordering.

We calculate the phonon, electron, and magnon temperature profiles in yttrium iron garnet/platinum bilayers by diffusive theory with appropriate boundary conditions, in particular taking into account interfacial thermal resistances. Our calculations show that in thin film hybrids, the interface magnetic heat conductance qualitatively affects the magnon temperature. Based on published material parameters we assess the degree of nonequilibrium at the yttrium iron garnet/platinum interface. The magnitude of the spin Seebeck effect derived from this approach compares well with experimental results for the longitudinal spin Seebeck effect. Additionally, we address the temperature profiles in the transverse spin Seebeck effect.

The zero-field excitation spectrum of the strong-leg spin ladder (C${}_7$H${}_{10}$N)${}_2$CuBr${}_4$ is studied with a neutron time-of-flight technique. The spectrum is decomposed into its symmetric and asymmetric parts with respect to the rung momentum and compared with theoretical results obtained by the density matrix renormalization group method. Additionally, the calculated dynamical correlations are shown for a wide range of rung and leg coupling ratios in order to point out the evolution of arising excitations, as, e.g., of the two-magnon bound state from the strong to the weak coupling limit.

We consider a lattice model of two complex scalar matter fields $z_a,a=1,2$, under a ${\mathbb{CP}}^1$ constraint $|z_1{|}^2+{\left|z_2\right|}^2=1$, minimally coupled to a compact gauge field, with an additional Berry-phase term. This model has been the origin of a large body of works addressing novel paradigms for quantum criticality, in particular “spin-quark” (spinon) deconfinement in $S=1/2$ quantum antiferromagnets. We map the model exactly onto a link-current model, which permits the use of classical worm algorithms to study the model in large-scale Monte Carlo simulations on lattices of size $L^3$, up to $L=512$. We show that the addition of a Berry-phase term to the lattice ${\mathbb{CP}}^1$ model completely suppresses the phase transition in the $\mathrm{O}\left(3\right)$ universality class of the ${\mathbb{CP}}^1$ model, such that the original spin system described by the compact gauge theory is always in the ordered phase. The link-current formulation of the model is useful in identifying the mechanism by which the phase transition from an ordered to a disordered state is suppressed.

We have studied the coupling of electronic and magnetic properties in Fe${}_{1+y}$(Te${}_{1-x}$Se${}_x$) via systematic specific heat, magnetoresistivity (MR), and Hall coefficient measurements on two groups of samples with $y=0.02$ and 0.1. In the $y=0.02$ series, we find that the $0.09<x<0.3$ composition region, where superconductivity is suppressed, has a large Sommerfeld coefficient γ (∼55–65 mJ/mol K${}^2$), positive Hall coefficient $R_H$, and negative MR at low temperature, in sharp contrast with the $x$ $=$ 0.4–0.5 region, where γ drops to ∼26 mJ/mol K${}^2$ and $R_H$ and MR become negative and positive, respectively, at low temperature. Dramatic changes of γ, as well as sign reversal in low-temperature $R_H$ and MR, are also observed across the $x\sim 0.1$ boundary, where the long-range antiferromagnetic order is suppressed. However, for the system with rich interstitial excess Fe ($y=0.1$), where bulk superconductivity is suppressed even for $x$ $=$ 0.4–0.5, the variations of γ, $R_H$, and MR with $x$ are distinct from those seen in $y=0.02$ system: γ is ∼40 mJ/mol K${}^2$ for $0.1<x<0.3$ and drops to ∼34 mJ/mol K${}^2$ for $x$ $=$ 0.4–0.5; $R_H$ and MR do not show any sign reversal as $x$ is increased above 0.3. We will show that all these results can be understood in light of the evolution of the incoherent magnetic scattering by ($\pi ,0$) magnetic fluctuations with Se concentration. In addition, with the suppression of magnetic scattering by the magnetic field, we observed the surprising effect of a remarkable increase in the superconducting volume fraction under moderate magnetic fields for $x$ $=$ 0.3–0.4 samples in the $y=0.02$ system.

We theoretically investigate heat transport in temperature-biased Josephson tunnel junctions in the presence of an in-plane magnetic field. In full analogy with the Josephson critical current, the phase-dependent component of the heat flux through the junction displays coherent diffraction. Thermal transport is analyzed in three prototypical junction geometries, highlighting their main differences. Notably, minimization of the Josephson coupling energy requires the quantum phase difference across the junction to undergo $\pi$ slips in suitable intervals of magnetic flux. An experimental setup suited to detect thermal diffraction is proposed and analyzed.

This work reports a computer modeling comparison of two solid electrolyte materials, Li${}_3$PO${}_4$ and Li${}_3$PS${}_4$, in terms of the bulk structures, Li ion mobilities, and their interface properties with vacuum and with Li metal. The simulations show that for some forms of Li${}_3$PS${}_4$, Li ions can move from a host lattice site to an interstitial site with negligible net energy change. This favorable process for the formation of vacancy-interstitial pairs is undoubtedly important for the structural and ion migration properties of $\beta$-Li${}_3$PS${}_4$. Our model results for the idealized interfaces between the electrolytes and Li metal show that Li${}_3$PO${}_4$ /Li interfaces are stable, while Li${}_3$PS${}_4$ /Li interfaces are not. We find that a Li${}_3$PS${}_4$ surface in the presence of a small amount of Li relaxes to a complicated structure involving broken P-S bonds. On the other hand, the computer models show that a very thin film of Li${}_2$S on the Li${}_3$PS${}_4$ surface can provide a protective buffer layer to stabilize the interface between the Li${}_3$PS${}_4$ electrolytes and the Li metal anode.

The high-pressure behavior of elemental Sn has been studied by angle-dispersive synchrotron x-ray diffraction up to 138 GPa under quasihydrostatic conditions at room temperature. The data confirm the occurrence of a first-order phase transition at 10.8 GPa between $\beta$-Sn (Sn-II) ($I{4}_1/amd$) and a further body-centered-tetragonal polymorph ($\gamma$-Sn or Sn-III) ($I4/mmm$). Above 32 GPa, this phase exhibits a distortion into a new body-centered-orthorhombic (bco) modification ($Immm$). Beyond 70 GPa, the structure becomes body-centered cubic (bcc) ($Im-3m$). There is a region of coexistence where the bcc reflections are observed to appear superimposed on the bco pattern above 40 GPa and the two diffraction signatures coexist until 70 GPa. We examined this possible existence of a kinetically hindered first-order phase transition between the two polymorphs by performing density functional theory (DFT) calculations with an emphasis on the potential energy in response to axial ($c/a,b/a$) distortions at constant volume. The DFT results suggest a slightly different interpretation of the structural transformations. At low pressure, the global minimum energy is always centered around $b/a=1$, and there is no indication of transformation to a bco structure. However, any small strains in the $c/a$ ratio in the system would provide an orthorhombic distortion of the observed magnitude. Such strains could be induced due to slight deviations from hydrostatic conditions in the experimental study. Concerning the possible bco-bcc phase transitions, the DFT calculations reveal an energy surface with a barrier developed between solutions with different $c/a$ values over the pressure range of interest. Crucially, the calculated barrier heights are low, and they disappear in the region of the observed phase transformation. The DFT results indicate a mechanically softened material that may exhibit localized domain structures in response to even slightly nonhydrostatic stress conditions.

We consider the general Landau-Lifshitz-Gilbert theory underlying the magnetization dynamics of a macrospin magnet subject to spin-torque effects and thermal fluctuations. Thermally activated dynamical properties are analyzed by averaging the full magnetization equations over constant-energy orbits. After averaging, all the relevant dynamical scenarios are a function of the ratio between hard and easy axis anisotropies. We derive analytically the range of currents for which limit cycles exist and discuss the regimes in which the constant energy orbit averaging technique is applicable.

We study the low-lying excitations in 5$d$ transition-metal oxide Sr${}_2$IrO${}_4$ on the localized electron picture. We find that Hund's coupling together with the spin-orbit interaction leads to exchange anisotropy which causes the spin wave gap. Introducing the isospin operators acting on Kramers' doublet in Ir atoms, we derive the effective spin Hamiltonian from a multiorbital Hubbard model with the $t_{2g}$ orbitals in the square lattice. We introduce the Green's functions including the anomalous type for the boson operators by expanding the spin operators in terms of boson operators in the lowest order of $1/S$, and solve the coupled equations of motion for those functions. Two modes are found to emerge with slightly different energies, in contrast to the spin waves in the isotropic Heisenberg model. At the $\Gamma$ point, one mode has the zero excitation energy while another has a finite energy. They have the same excitation energy at the M point, but still have different energies at the X point.

$W$-band (ν ≅ 94 GHz) electron paramagnetic resonance (EPR) spectroscopy was used for a single-crystal study of a star-shaped Fe${}_3$Cr single-molecule magnet (SMM) with crystallographically imposed trigonal symmetry. The high resolution and sensitivity accessible with $W$-band EPR allowed us to determine accurately the axial zero-field splitting terms for the ground ($S$ $=$ 6) and first two excited states ($S$ $=$ 5 and $S$ $=$ 4). Furthermore, spectra recorded by applying the magnetic field perpendicular to the trigonal axis showed a π/6 angular modulation. This behavior is a signature of the presence of trigonal transverse magnetic anisotropy terms whose values had not been spectroscopically determined in any SMM prior to this work. Such in-plane anisotropy could only be justified by dropping the so-called “giant spin approach” and by considering a complete multispin approach. From a detailed analysis of experimental data with the two models, it emerged that the observed trigonal anisotropy directly reflects the structural features of the cluster, i.e., the relative orientation of single-ion anisotropy tensors and the angular modulation of single-ion anisotropy components in the hard plane of the cluster. Finally, since high-order transverse anisotropy is pivotal in determining the spin dynamics in the quantum tunneling regime, we have compared the angular dependence of the tunnel splitting predicted by the two models upon application of a transverse field (Berry-phase interference).

We show that the intrinsic chiral ($d_{x^2-y^2}\pm i{d}_{xy}$)-wave superconducting pairing in doped graphene is significantly strengthened in the core region of a doubly quantized $s$-wave superconducting vortex produced in a graphene-superconductor hybrid structure. The chiral $d$-wave state is induced by the proximity effect, which transfers the center-of-mass angular momentum of the $s$-wave vortex to the orbital angular momentum of the chiral $d$-wave Cooper pairs. The proximity effect is enhanced by the circular geometry of the vortex and we find a ${[1+{(T-T_{c,J})}^2]}^{-1}$ temperature dependence for the chiral $d$-wave core amplitude, where $T_{c,J}$ is its intrinsic bulk transition temperature. We further propose to detect the chiral $d$-wave state by studying the temperature dependence of the low-energy local density of states in the vortex core, which displays a sudden radial change as a function of the strength of the $d$-wave core state.

We study the isovalent substitution effect by partially introducing Se on the S site in the newly discovered superconductor Nb${}_2$PdS${}_5$ ($T_c\sim$ 6 K) whose upper critical field is found to be far above its Pauli paramagnetic limit. In this Nb${}_2$Pd(S${}_{1-x}$Se${}_x$)${}_5$ (0 $\le$ $x$ $\le$ 0.8) system, superconductivity is systematically suppressed by the Se concentration and ultimately disappears when $x\ge$ 0.5, after which a semiconductinglike ground state emerges. In spite of the considerably reduced $T_c$ with Se doping, the ratio of the upper critical field $H_{c2}$ to $T_c$ remains unaffected. Moreover, the size of the heat capacity jump at $T_c$ is smaller than that expected for a BCS superconductor, implying that a strong-coupling theory cannot be the origin of this large upper critical field. In addition, the low-lying quasiparticle excitations are consistent with a nodeless gap opening over the Fermi surface. These results combined impose severe constraints on any theory of exotic superconductivity in this system.

In the present work we report on the synthesis, crystal structure, and physical properties (resistivity, magnetization, heat capacity) in combination with density functional theory (DFT) calculations of the electronic structure and phonon properties for the intermetallic compound LaPtSi. LaPtSi crystallizes in its own noncentrosymmetric structure type $($space group $I{4}_{1}md$; $a=0.42502\left(1\right)$ nm and $c=1.4525\left(5\right)$ nm$)$, which is an ordered ternary derivative of the centrosymmetric $\alpha$-ThSi${}_2$-structure. The weakly correlated compound LaPtSi (Sommerfeld value $\gamma =6.5$ mJ/molK${}^2)$ exhibits superconductivity below $T_c=3.35$ K and appears to be a fully gapped, weakly coupled $s$-wave BCS superconductor. The experimental observations are supported by DFT calculations which show that, despite a substantial spin-orbit splitting of the Fermi surfaces, a spin-singlet pairing is prevalent.

We present an approach which is based on the one-particle irreducible (1PI) generating functional formalism and includes electronic correlations on all length scales beyond the local correlations of dynamical mean-field theory (DMFT). This formalism allows us to unify aspects of the dynamical vertex approximation (D$\Gamma$A) and the dual fermion (DF) scheme, yielding a consistent formulation of nonlocal correlations at the one- and two-particle level beyond DMFT within the functional integral formalism. In particular, the considered approach includes one-particle reducible contributions from the three- and more-particle vertices in the dual fermion approach, as well as some diagrams not included in the ladder version of D$\Gamma$A. To demonstrate the applicability and physical content of the 1PI approach, we compare the diagrammatics of 1PI, DF, and D$\Gamma$A, as well as the numerical results of these approaches for the half-filled Hubbard model in two dimensions.

We report a study of a single-crystal La${}_3$Co${}_4$Sn${}_{13}$ by means of the specific heat and ${}^{59}$Co nuclear magnetic resonance (NMR) spectroscopy. A first-order phase transition with a marked peak at $T^{*}\simeq 152$ K has been discerned by the specific heat measurement. The observed transition has been connected to a structural change from a simple cubic to a body-centered-cubic superstructure with crystallographic cell doubling, accompanied by the Fermi surface reconstruction. Indeed, NMR observations clearly indicate a significant change in the local electronic characteristics across this phase transition. The spin-lattice-relaxation rate measurement further provides an estimate of Co 3$d$ Fermi-level density of states $N_d$($E_F$), revealing a visible reduction in $N_d$($E_F$) in the low-temperature phase. This finding essentially associated with the partially gapped Fermi surfaces would be appropriate for the isostructural analog of Sr${}_3$Ir${}_4$ Sn${}_{13}$, which has been claimed to possess charge-density-wave (CDW) behavior with a three-dimensional crystallographic structure.

We study the entanglement spectrum of noninteracting band insulators, which can be computed from the two-point correlation function, when restricted to one part of the system. In particular, we analyze a type of partitioning of the system that maintains its full translational symmetry, by tracing over a subset of local degrees of freedom, such as sublattice sites or spin orientations. The corresponding single-particle entanglement spectrum is the band structure of an entanglement Hamiltonian in the Brillouin zone. We find that the hallmark of a nontrivial topological phase is a gapless entanglement spectrum with an “entanglement Fermi surface.” Furthermore, we derive a relation between the entanglement spectrum and the quantum geometry of Bloch states contained in the Fubini-Study metric. The results are illustrated with lattice models of Chern insulators and ${\mathbb{Z}}_2$ topological insulators.

Understanding the grain size effect on diffusion in nanocrystals has been hampered by the difficulty of measuring diffusion directly in experiments. Here large-scale atomistic modeling is applied to understand the diffusion kinetics in nanocrystals. Enhanced short-circuit diffusivity is revealed to be controlled by the rule of mixtures for grain-boundary diffusion and lattice diffusion, which can be accurately described by the Maxwell-Garnett equation instead of the commonly thought Hart equation, and the thermodynamics of pure grain-boundary self-diffusion is not remarkably affected by varying grain size. Experimentally comparable Arrhenius parameters with atomic detail validate our results. We also propose a free-volume diffusion mechanism considering negative activation entropy and small activation volume. These help provide a fundamental understanding of how the activation parameters depend on size and the structure-property relationship of nanostructured materials from a physical viewpoint.