We theoretically examine anisotropy of in-plane resistivity in the striped antiferromagnetic phase of an iron arsenide superconductor by applying a memory function approach to the ordered phase with isotropic nonmagnetic impurity. We find that the anisotropy of the scattering rate is independent of carrier density when the topology of the Fermi surface is changed after the introduction of holes. On the other hand, the anisotropy of the Drude weight monotonically decreases reflecting the distortion of the Dirac Fermi surface and eventually leads to the reverse of anisotropy of resistivity, being consistent with experiment. The origin of the anisotropy is thus attributed to the interplay of impurity scattering and anisotropic electronic states.

Using a degenerate double-exchange model, we investigate the spin excitation spectra of iron pnictides. The model consists of local spin moments on each Fe site, as well as itinerant electrons from the degenerate $d_{xz}$ and $d_{yz}$ orbitals. The local moments interact with each other through antiferromagnetic $J_1\text{−}{J}_2$ Heisenberg interactions, and they couple to the itinerant electrons through a ferromagnetic Hund coupling. We employ the fermionic spinon representation for the local moments and perform a generalized random-phase approximation calculation on both spinons and itinerant electrons. We find that in the $(\pi ,0)$ magnetically ordered state, the spin-wave excitation at $(\pi ,\pi )$ is pushed to a higher energy due to the presence of itinerant electrons, which is consistent with a previous study using the Holstein-Primakoff transformation. In the paramagnetic state, the particle-hole continuum keeps the collective spin excitation near $(\pi ,\pi )$ at a higher energy even without any $C_4$ symmetry breaking. The implications for recent high-temperature neutron scattering measurements will be discussed.

The electronic structure of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ covered by noble metals is investigated by first-principles calculations. The Dirac surface state of the topological insulator ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ hybridizes with the $sp$ states of the noble metal, which gives rise to strong reorganization of the surface electronic structure. Striking features of the modified Dirac surface state are (i) the introduction of new Dirac points within the fundamental band gap of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$, (ii) a very weak dispersion, and (iii) a multisheeted Fermi surface which results in an orientation-dependent number of conducting channels in the fundamental band gap of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$. Our findings have impact for spin-dependent surface transport.

We show that impurities and vacancies affect the thermal conductivity much more strongly than what is predicted by widely accepted models. When local distortions around point defects are strong, standard perturbative approaches fail, and phonon scattering can only be accounted for by an exact Green's function calculation. We apply the theory to the study, from first-principles, of nitrogen and vacancy defects in diamond. The thermal conductivity is computed by solving the linearized Boltzmann transport equation. The Born approximation underestimates the phonon scattering cross sections of nitrogen and vacancies by factors of 3 and 10, respectively. Thermal conductivity calculations are in good agreement with experiment.

While a Taylor-type yield stress, proportional to the square-root of the dislocation density, may appear at a macroscopic scale, it can be shown with discrete dislocation simulations that it does not accurately describe dislocation motion on the scale of individual dislocations. In this article, we first demonstrate that the Taylor term fails to capture a number of features of dislocation dynamics by comparing the results of a continuum formulation using this yield stress term to discrete dislocation dynamics simulations. We then present an alternate model, based on a mean free path formulation, and demonstrate that this model effectively reproduces the results of the discrete simulation. This mean free path model is proposed as an extension to an existing continuum theory, making use of the key fact that the velocity of dislocations in a realistic system is not single valued, but distributed over several values. The velocity distribution may also change with time. It is demonstrated that this formulation predicts features of both pileups and homogenous distributions which are in agreement with discrete dislocation simulations, but are not reproducible by traditional statistical continuum theories. Finally, possible extensions to this model are discussed, which may enhance the ability to reproduce key features of dislocation yielding.

We report a complete characterization of the crystal structure between 400 and 80 K for $R{\mathrm{Fe}}_2{\mathrm{O}}_4$ ($R$ = Rm, Yb, and Lu) compounds using high resolution x-ray synchrotron powder diffraction. The three samples have a hexagonal structure (space group $R\overline{3}m$) characterized by a sequence of double layers of mixed valence iron and oxygen atoms forming two-dimensional triangular layers separated by a single $R$-O layer along the $c$ axis. This structure is stable down to 80 K for ${\mathrm{TmFe}}_2{\mathrm{O}}_4$ and ${\mathrm{YbFe}}_2{\mathrm{O}}_4$ though a sudden expansion in the $c$ axis is observed at around 300 K coupled to a variation in the electrical properties. However, ${\mathrm{LuFe}}_2{\mathrm{O}}_4$ exhibits two structural transitions upon cooling. The splitting of some reflections and the occurrence of superstructure peaks below 320 K reveal a structural phase transition. The unit cell is monoclinic (space group $C2/m$), and there are four nonequivalent Fe sites with a maximum charge disproportionation of 0.5 $e$. The hexagonal to monoclinic transition is characterized by a sudden expansion of the $c$ axis on cooling, and it seems to be driven by the condensation of ${\mathrm{Y}}_2$ modes. At lower temperatures (∼170 K) additional splitting of several peaks indicate that the unit cell is no longer monoclinic but triclinic (space group $P\overline{1}$). This transition is characterized by a contraction of the monoclinic $ab$ plane, while the $c$ axis remains almost unchanged. There are six nonequivalent Fe sites in the triclinic cell, and the charge disproportionation magnitude is little affected.

We examine the magnetic field dependent excitations of the dimerized spin-1/2 chain, copper nitrate, with antiferromagnetic intradimer exchange $J_1=0.44\left(1\right)\mathrm{meV}$ and exchange alternation $\alpha =J_2/J_1=0.26\left(2\right)$. Magnetic excitations in three distinct regimes of magnetization are probed through inelastic neutron scattering at low temperatures. At low and high fields there are three and two long-lived magnonlike modes, respectively. The number of modes and the antiphase relationship between the wave-vector dependent energy and intensity of magnon scattering reflect the distinct ground states: A singlet ground state at low fields ${\mu }_{0}H<{\mu }_0{H}_{c1}=2.8\mathrm{T}$ and an $S_z=1/2$ product state at high fields ${\mu }_{0}H>{\mu }_0{H}_{c2}=4.2\mathrm{T}$. In the intermediate-field regime, a continuum of scattering for $\hslash \omega \approx J_1$ is indicative of a strongly correlated gapless quantum state without coherent magnons.

Magnetostriction experiments have been performed at temperatures in and above the antiferromagnetic phase of ${\mathrm{EuTiO}}_3$. For magnetic fields below the spin-flop transition the magnetostriction is large and negative and changes to small and positive for fields exceeding this threshold. At the antiferromagnetic phase transition temperature a steep anomaly in the thermal expansion is observed in zero field which broadens and moves to lower temperature with increasing field strength. A small but well resolved anisotropy in the longitudinal and transverse field configurations of the magnetostriction is observed at low temperatures, which rapidly vanishes with increasing temperature. The crossover field where the magnetostriction changes sign coincides with the field strength where the dielectric constant adopts normal quantum paraelectric behavior and signifies strong magnetoelectric coupling to be its origin.

A study of the magnetic properties of the frustrated rare-earth oxide ${\mathrm{SrGd}}_2{\mathrm{O}}_4$ has been completed using bulk property measurements of magnetization, susceptibility, and specific heat on single-crystal samples. Two zero-field phase transitions have been identified at 2.73 and 0.48 K. For the field $H$, applied along the $a$ and $b$ axes, a single boundary is identified that delineates the transition from a low-field, low-temperature magnetically ordered regime to a high-field, high-temperature paramagnetic phase. Several field-induced transitions, however, have been observed with $H\parallel c$. The measurements have been used to map out the magnetic phase diagram of ${\mathrm{SrGd}}_2{\mathrm{O}}_4$, suggesting that it is a complex system with several competing magnetic interactions. The low-temperature magnetic behavior of ${\mathrm{SrGd}}_2{\mathrm{O}}_4$ is very different compared to the other $\mathrm{Sr}{L}_2{\mathrm{O}}_4$ ($L$ = Lanthanide) compounds studied so far, even though all of the $\mathrm{Sr}{L}_2{\mathrm{O}}_4$ compounds are isostructural, with the magnetic ions forming a low-dimensional lattice of zigzag chains that run along the $c$ axis. The differences are likely to be due to the fact that in the ground state ${\mathrm{Gd}}^{3+}$ has zero orbital angular momentum and therefore the spin-orbit interactions, which are crucial for other $\mathrm{Sr}{L}_2{\mathrm{O}}_4$ compounds, can largely be neglected. Instead, given the relatively short ${\mathrm{Gd}}^{3+}–{\mathrm{Gd}}^{3+}$ distances in ${\mathrm{SrGd}}_2{\mathrm{O}}_4$, dipolar interactions must be taken into account for this antiferromagnet alongside the Heisenberg exchange terms.

By systematic chemical substitution of Pt and Ni in the newly discovered superconductor ${\mathrm{Nb}}_2{\mathrm{PdS}}_5$ ($T_c\sim 6$ K), we study the evolution of its superconducting properties with doping, focusing on the behavior of the upper critical field $H_{c2}$. In contrast to the previous results of Se doping on S sites, superconductivity is found to be rather robust against the Pt and Ni dopants on the one-dimensional Pd chains. Most strikingly, the reduced $H_{c2}$, i.e., the ratio of $H_{c2}/T_c$, is seen to be significantly enhanced by the heavier Pt doping but suppressed in the Ni-doped counterparts, distinct from the nearly constant value in the Se-doped samples. Our findings therefore suggest that the upper critical field of this system can be modified in a tunable fashion by chemical doping on the Pd chains with elements of varying mass numbers. The spin-orbit coupling on the Pd sites, by inference, should play an important role in the observed superconductivity and on the large upper critical field beyond the Pauli pair-breaking field.

Tunneling two-level systems (TLSs), present in dielectrics at low temperatures, have been recently studied for fundamental understanding and superconducting device development. According to a recent theory by Burin et al. [Phys. Rev. Lett. 110, 157002 (2013)], the TLS bath of any amorphous dielectric experiences a distribution of Landau-Zener transitions if exposed to simultaneous fields. In this experiment we measure amorphous insulating films at millikelvin temperatures with a microwave field and a swept electric field bias using a superconducting resonator. We find that the maximum dielectric loss per microwave photon with the simultaneous fields is approximately the same as that in the equilibrium state, in agreement with the generic material theory. In addition, we find that the loss depends on the fields in a way which allows for the separate extraction of the TLS bath dipole moment and density of states. This method allows for the study of the TLS dipole moment in a diverse set of disordered films, and provides a technique for continuously inverting their population.

An effective medium theory for resonant and nonresonant metamaterials for flexural waves in thin plates is presented. The theory provides closed-form expressions for the effective mass density, rigidity, and Poisson's ratio of arrangements of isotropic scatterers in thin plates, valid for low frequencies and moderate filling fractions. It is found that the effective Young's modulus and Poisson's ratio are induced by a combination of the monopolar and quadrupolar scattering coefficient, as it happens for bulk elastic waves, while the effective mass density is induced by the monopolar coefficient, contrarily as it happens for bulk elastic waves, where the effective mass density is induced by the dipolar coefficient. It is shown that resonant positive or negative effective elastic parameters are possible, being therefore the monopolar resonance responsible for creating an effective medium with negative mass density. Several examples are given for both nonresonant and resonant effective parameters and the results are numerically verified by multiple scattering theory and finite element analysis.

High-field magnetization and high-frequency electron spin resonance (ESR) are employed to differentiate magnetism between an orthorhombic and a hexagonal majority phase of ${\mathrm{Ba}}_3{\mathrm{CuSb}}_2{\mathrm{O}}_9$. For the orthorhombic sample, an ESR signal changes its temperature dependence at $T_S\sim 200$ K, suggesting a static Jahn-Teller (JT) ordering. A magnetization curve follows a power-law behavior $M\sim H^{{\alpha }_m}$ with the exponent ${\alpha }_m=0.72\pm 0.06$ for $8<H<26$ T and ${\alpha }_m=1.06\pm 0.04$ for $H>26$ T. The ESR linewidth exhibits a critical-like divergence, $\Delta H_{pp}\left(T\right)\propto T^{-\alpha }$ with the exponents of $\alpha =0.22\pm 0.07$ and $0.32\pm 0.04$. The sublinear magnetization and the critical ESR line broadening are taken as evidence of a random singlet state. For the hexagonal sample, both $\Delta H_{pp}\left(T\right)$ and $g$ factor are described by the same thermally activated process with the energy barrier of 300 K. This evidences intrinsic coupling of spins to orbital degrees of freedom and thereby gives support for a dynamic spin-orbital entangled state. Our results demonstrate that magnetism in the spin-orbital coupled compound ${\mathrm{Ba}}_3{\mathrm{CuSb}}_2{\mathrm{O}}_9$ is dictated by a spatiotemporal structure of the JT distortions.

Interlayer exchange coupling in strong/weak/strong ferromagnetic multilayers is investigated as a function of external magnetic field and temperature, with the focus on the magnetization switching near the Curie transition in the spacer composed of a diluted ferromagnet of concentration paramagnetic in the bulk. The effect of an externally applied reversing magnetic field on the width of the thermomagnetic transition is studied experimentally and explained theoretically as a result of the interplay between the proximity-induced exchange and the Zeeman effects in the system. Of high potential for applications should be the ability to switch one of the ferromagnetic outer layers using magnetic field, temperature, or a combination of the two.

The detailed derivation of the quantum Landau-Lifshitz-Bloch (qLLB) equation for simple spin-flip scattering mechanisms based on spin-phonon and spin-electron interactions is presented and the approximations are discussed. The qLLB equation is written in a form suitable for comparison with its classical counterpart. The temperature dependence of the macroscopic relaxation rates is discussed for both mechanisms. It is demonstrated that the magnetization dynamics is slower in the quantum case than in the classical one.

${}^{59}\mathrm{Co}$ NMR experiments have been performed on single crystals of the layered cobaltate ${\mathrm{Na}}_x{\mathrm{CoO}}_2$ with $x=0.77$, which is an antiferromagnet with Néel temperature $T_N=22$ K. In this metallic phase, six Co sites are resolved in the NMR spectra, with distinct quadrupole frequencies ${\nu }_Q$, magnetic shifts $K_{ZZ}$ and nuclear spin lattice relaxation rates $1/T_1$. Contrary to the $x=1/2$ or $x=2/3$ phases, the 3D stacking of the Na planes is not perfect for $x=0.77$ but this does not influence markedly the electronic properties. We evidence that the magnetic and charge properties of the Co sites are highly correlated with each other as $K_{ZZ}$ and ${(1/T_1)}^{1/2}$ scale linearly with ${\nu }_Q$. The data analysis allows us to separate the contribution ${\nu }_Q^{\mathrm{latt}}$ of the ionic charges to ${\nu }_Q$ from that ${\nu }_Q^{\mathrm{el}}$ due to the hole orbitals on the Co sites. We could extend coherently this analysis to all the known phases in the Na cobaltate phase diagram. The variation with $x$ of ${\nu }_Q^{\mathrm{latt}}$ is found to fit rather well numerical computations done in a point charge model. The second term ${\nu }_Q^{\mathrm{el}}$ allowed us to deduce the hole concentration on the cobalts. These detailed experimental results should stimulate theoretical calculations of the electronic structure involving both the Co orbital configurations and DMFT approaches to take into account the electronic correlations.

Hexagonal boron nitride substrates have been shown to dramatically improve the electric properties of graphene. Recently, it has been observed that when the two honeycomb crystals are close to perfect alignment, strong lattice distortions develop in graphene due to the moiré adhesion landscape. Simultaneously, a gap opens at the Dirac point. Here, we derive a simple low-energy electronic model for graphene aligned with the substrate, taking into account spontaneous strains at equilibrium and pseudogauge fields. We carry out a detailed characterization of the modified band structure, gap, local and global density of states, and band topology in terms of physical parameters. We show that the overall electronic structure is strongly modified by the spontaneous strains.

The complex perovskite oxide ${\mathrm{SrRuO}}_3$ shows intriguing transport properties at low temperatures due to the interplay of spin, charge, and orbital degrees of freedom. One of the open questions in this system is regarding the origin and nature of the low-temperature glassy state. In this paper we report on measurements of higher-order statistics of resistance fluctuations performed in epitaxial thin films of ${\mathrm{SrRuO}}_3$ to probe this issue. We observe large low-frequency non-Gaussian resistance fluctuations over a certain temperature range. Our observations are compatible with that of a spin-glass system with properties described by hierarchical dynamics rather than with that of a simple ferromagnet with a large coercivity.