Magnetic moments strongly coupled to the spins of conduction electrons in a nanostructure can confine the conduction-electron motion due to scattering at almost localized Kondo singlets. We study the resulting local-moment formation in the conduction-electron system and the magnetic exchange coupling mediated by the Kondo singlets. Its distance dependence is oscillatory and induces robust ferro- or antiferromagnetic order in multi-impurity systems.

Entanglement renormalization is a real-space renormalization group (RG) transformation for quantum many-body systems. It generates the multiscale entanglement renormalization ansatz (MERA), a tensor network capable of efficiently describing a large class of many-body ground states, including those of systems at a quantum critical point or with topological order. The MERA has also been proposed to be a discrete realization of the holographic principle of string theory. Here we propose the use of symmetric tensors as a mechanism to build a symmetry-protected RG flow, and discuss two important applications of this construction. First, we argue that symmetry-protected entanglement renormalization produces the proper structure of RG fixed points, namely, a fixed-point for each symmetry-protected phase. Second, in the context of holography, we show that by using symmetric tensors, a global symmetry at the boundary becomes a local symmetry in the bulk, thus explicitly realizing in the MERA a characteristic feature of the AdS/CFT correspondence.

We study transport properties of the helical edge states of two-dimensional integer and fractional topological insulators via double constrictions. Such constrictions couple the upper and lower edges of the sample and can be made and tuned by adding side gates to the system. Using renormalization group and duality mapping, we analyze phase diagrams and transport properties in each of these cases. Most interesting is the case of two constrictions tuned to resonance, where we obtain Kondo behavior, with a tunable Kondo temperature. Moving away from resonance gives the possibility of a metal-insulator transition at some finite detuning. For integer topological insulators, this physics is predicted to occur for realistic interaction strengths and gives a conductance $G$ with two temperature $T$ scales where the sign of $dG/dT$ changes, one being related to the Kondo temperature while the other is related to the detuning.

We theoretically study the effect of polar substrates on the magneto-optical conductivity of doped monolayer graphene, where we particularly focus on the role played by surface polar phonons (SPPs). Our calculations suggest that polaronic shifts of the intra- and interband absorption peaks can be significantly larger for substrates with strong electron-SPP coupling than those in graphene on nonpolar substrates, where only intrinsic graphene optical phonons with much higher energies contribute. Electron-phonon scattering and phonon-assisted transitions are, moreover, found to result in a loss of spectral weight at the absorption peaks. The strength of these processes is strongly temperature dependent and with increasing temperatures the magneto-optical conductivity becomes increasingly affected by polar substrates, most noticeably in polar substrates with small SPP energies such as HfO${}_2$. The inclusion of a Landau level-dependent scattering rate to account for Coulomb impurity scattering does not alter this qualitative picture, but can play an important role in determining the line shape of the absorption peaks, especially at low temperatures, where impurity scattering dominates.

Hydrogenated graphene edges are assumed to be either armchair, zigzag, or a combination of the two. We show that the zigzag is not the most stable fully hydrogenated edge structure along the $\langle 2\overline{1}\overline{1}0\rangle$ direction. Instead hydrogenated Klein and reconstructed Klein based edges are found to be energetically more favorable, with stabilities approaching that of armchair edges. These new structures “unify” graphene edge topology, the most stable flat hydrogenated graphene edges always consisting of pairwise bonded C${}_2$H${}_4$ edge groups, irrespective of the edge orientation. When edge rippling is included, CH${}_3$ edge groups are most stable. These new fundamental hydrogen-terminated edges have important implications for graphene edge imaging and spectroscopy, as well as mechanisms for graphene growth, nanotube cutting, and nanoribbon formation and behavior.

We study the nonequilibrium transport for the asymmetric and negative differential magnon tunneling driven by temperature bias. We demonstrate that the many-body magnon interaction that makes the magnonic spectrum temperature-dependent is the crucial factor for the emergence of rectification and negative differential spin Seebeck effects in magnon tunneling junctions. When magnonic junctions have temperature-dependent density of states, reversing the temperature bias is able to give asymmetric spin currents and increasing temperature bias could give an anomalously decreasing magnonic spin current. We show that these properties are relevant for building spin Seebeck diodes and transistors, which could play important roles in controlling information and energy in magnonics and spin caloritronics.

We have analyzed magnetic-field-dependent small-angle neutron scattering (SANS) data of soft magnetic two-phase nanocomposite ferromagnets in terms of a recent micromagnetic theory for the magnetic SANS cross section [Honecker and Michels, Phys. Rev. B 87, 224426 (2013)]. The approach yields a value for the average exchange-stiffness constant and provides the Fourier coefficients of the magnetic anisotropy field and the magnetostatic field, which is related to jumps of the magnetization at internal interfaces.

We develop the formalism of the macroscopic monopolization—that is, the magnetoelectric monopole moment per unit volume—in periodic solids, and discuss its relationship to the magnetoelectric effect. For the series of lithium transition metal phosphate compounds, we use first-principles density functional theory to calculate the contributions to the macroscopic monopolization from the global distribution of magnetic moments within the unit cell, as well as from the distribution of magnetization around the atomic sites. We find one example within the series (LiMnPO${}_4$) that shows a macroscopic monopolization corresponding to a ferromonopolar ordering consistent with its diagonal magnetoelectric response. The other members of the series (Li$M$PO${}_4$, with $M=\text{Co}$, Fe, and Ni) have zero net monopolization but have antiferromonopolar orderings that should lead to $q$-dependent diagonal magnetoelectric effects.

The ferrimagnetic spinel MnV${}_2$O${}_4$ undergoes an orbital-induced cubic-to-tetragonal distortion at $T_{\mathrm{YK}}=58$ K, below which noncollinear commensurate ferrimagnetic ordering occurs. Using inelastic neutron scattering, we investigated low-energy dynamics in its cubic phase above $T_{\mathrm{YK}}$. We observed two types of coexisting short-range magnetic excitations: the dispersive spin waves centered around the Brillouin-zone centers, ${\mathbf{k}}_0$ $=$ (0, 0, 0), and quasielastic spin fluctuations centered at incommensurate wave vectors, ${\mathbf{k}}_{\mathrm{incom}}$ $=$ ($\xi$, $\xi$, 0). The coexistence of the two distinct features can be understood as a dynamic realization of the conical spiral order observed in cubic spinels such as CoCr${}_2$O${}_4$.

Antiferromagnetic (AFM) order and a spatial order peculiar to Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states, previously indicated in the quasi-two-dimensional $d$-wave superconductors CeCoIn${}_5$ with strong paramagnetic pair breaking (PPB) in a magnetic field parallel to the basal plane, are considered in the field configurations tilted from the basal plane within an approach assuming that the wavelength of the FFLO modulation is relatively long. It is demonstrated that, with increasing the tilt angle, both the AFM and FFLO orders are gradually suppressed, and that disappearance of the AFM order in the zero-temperature limit occurs at a lower angle than that of the FFLO state. Consequently, a nonmagnetic FFLO-ordered high-field superconducting phase is realized in an intermediate range of the tilt angle even at low enough temperatures. As the perpendicular field configuration ($\mathbf{H}\parallel c$) is approached by the field tilt, the nonvanishing AFM order in real space tends to occur only close to the FFLO nodal planes in contrast to the high-field behavior in the $\mathbf{H}\perp c$ case. Further, in the field versus temperature ($H$-$T$) phase diagram, the AFM order reduces, at a higher angle, to an AFM quantum critical point lying at a lower field than $H_{c2}\left(0\right)$ as a consequence of competition between the field dependencies of the nesting condition and of PPB. These features of the AFM order and the resulting $H$-$T$ phase diagram strikingly coincide with those seen in a recent NMR measurement on CeCoIn${}_5$ in tilted field configurations.

The quasiparticle interference (QPI) in Sr${}_2$RuO${}_4$ is theoretically studied based on two different pairing models in order to propose an experimental method to test them. For a recently proposed two-dimensional model with pairing primarily from the $\gamma$ band, we found clear QPI peaks evolving with energy, and their locations can be determined from the tips of the constant-energy contour (CEC). On the other hand, for a former quasi-one-dimensional model with pairing on the $\alpha$ and $\beta$ bands, the QPI spectra are almost dispersionless and may involve off-shell contributions to the scatterings beyond the CEC. The different behaviors of the QPI in these two models may help to resolve the controversy of active/passive bands and whether Sr${}_2$RuO${}_4$ is a topological superconductor.

Magnetic flux patterns are known to strongly differ in the intermediate state of type-I and type-II superconductors. Using a type-I/type-II bilayer we demonstrate hybridization of these flux phases into a plethora of unique new ones. Owing to a complicated multibody interaction between individual fluxoids, many different intriguing patterns are possible under applied magnetic field, such as few-vortex clusters, vortex chains, mazes, or labyrinthal structures resembling the phenomena readily encountered in soft-matter physics. However, in our system the patterns are tunable by sample parameters, magnetic field, current, and temperature, which reveals transitions from short-range clustering to long-range ordered phases such as parallel chains, gels, glasses, and crystalline vortex lattices, or phases where lamellar type-I flux domains in one layer serve as a bedding potential for type-II vortices in the other, configurations clearly beyond the soft-matter analogy.

The phase-field-crystal (PFC) modeling paradigm is rapidly emerging as the model of choice when investigating materials phenomena with atomistic scale effects over diffusive time scales. Recent variants of the PFC model, so-called structural PFC (XPFC) models introduced by Greenwood et al., have further increased the capability of the method by allowing for easy access to various structural transformations in pure materials [Greenwood, Provatas, and Rottler, Phys. Rev. Lett. 105, 045702 (2010)] and binary alloys [Greenwood, Ofori-Opoku, Rottler, and Provatas, Phys. Rev. B 84, 064104 (2011)]. We present an amplitude expansion of these XPFC models, leading to a mesoscale complex order parameter, i.e., phase-field, model for two-dimensional square-triangular structures. Amplitude models retain the salient atomic scale features of the underlying PFC models, while resolving microstructures on mesoscales as traditional phase-field models. The applicability and capability of these complex-order parameter models is demonstrated with simulations of peritectic solidification and grain growth exhibiting the emergence of secondary phase structures.

We present a study of the structural phase transition and the mechanical and thermodynamic properties of UO${}_2$ by means of the local density approximation (LDA)+$U$ approach. A phase transition pressure of 40 GPa is obtained from theory at 0 K, and agrees well with the experimental value of 42 GPa. The pressure-induced enhancements of elastic constants, elastic moduli, elastic wave velocities, and Debye temperature of the ground-state fluorite phase are predicted. The phonon spectra of both the ground state fluorite structure and high-pressure cotunnite structure calculated by the supercell approach show that the cotunnite structure is dynamically unstable under ambient pressure. Based on the imaginary mode along the $\Gamma$$-$$X$ direction and soft phonon mode along the $\Gamma$$-$$Z$ direction, a transition path from cotunnite to fluorite has been identified. We calculate the lattice vibrational energy in the quasiharmonic approximation using both first-principles phonon density of state and the Debye model. The calculated temperature dependence of lattice parameter, entropy, and specific heat agrees well with experimental observations in the low temperature domain. The difference of the Gibbs free energy between the two phases of UO${}_2$ has predicted a boundary in the pressure-temperature phase diagram. The solid-liquid boundary is approximated by an empirical equation using our calculated elastic constants.

Pressure and temperature dependence of the negative thermal expansion in Zn(CN)${}_2$ is fully investigated using molecular dynamics simulations with a built potential model. The advantage of this study is that it allows us to reproduce the exotic behaviors of the material, including the negative thermal expansion (NTE), the reduction of NTE with elevated temperature, the pressure enhancement of NTE, and the pressure-induced softening. Results of the study provide us detailed data to link the properties in the energy space and the real space, giving us insights to understand the properties and the connections between them.

Using x-ray magnetic circular dichroism, we show that Co porphyrin molecules adsorbed on graphene-covered Ni surfaces possess a huge in-plane (IP) orbital magnetic moment of the Co ions, despite their fourfold coordination, even comparable in size to the spin moment. Both are antiferromagnetically coupled to the Ni spins, leading to IP magnetization at zero external magnetic field. Adsorption of carbon monoxide on top of the molecules reduces the Co orbital moment by ($77\pm 6$)%. This is attributed to the change of the crystal field by the chemical stimulus which determines, together with the spin-orbit coupling (SOC), the orbital moment. Thermal desorption of the CO fully restores the initial electronic and magnetic properties.

Neutron time-of-flight spectroscopy has been employed to study the crystal-field splitting of Pr${}^{3+}$ in the pyrochlore stannate Pr${}_2$Sn${}_2$O${}_7$. The crystal field has been parameterized from a profile fit to the observed neutron spectrum. The single-ion ground state is a well-isolated non-Kramers doublet of ${\Gamma }_3^{+}$ symmetry with a large Ising-like anisotropy, ${\chi }_{zz}/{\chi }_{\perp }\approx 60$ at 10 K, but with a significant admixture of terms $|M_J\ne \pm J\rangle$, which can give rise to quantum zero-point fluctuations. This magnetic state satisfies the requirements for quantum spin-ice behavior.

X-ray pair distribution function (pdf) and U L${}_3$ extended x-ray absorption fine structure (EXAFS) and neutron pdf measurements that give identical results for UO${}_2$ show U(VI)-oxo moieties with x rays for mixed valence U${}_4$O${}_9$ and U${}_3$O${}_7$, in contrast to the neutron data that indicate only U(V) sites with no short U-O bonds as well as other differences. In addition, although the EXAFS spectra of UO${}_2$ are essentially identical at 30, 100, and 200 K, those of the UO${}_{2+x}$ compounds exhibit different nearest-neighbor U-O distributions at each temperature. Further tunneling polaron-type behavior is found in the broadening of the features of the O K-edge x-ray absorption spectra (XAS) of the UO${}_{2+x}$ compounds. Raman spectra of powders also show a large increase in scattering cross section with increasing O content that would originate in a change in the electronic structure that increases the overall polarizability. The XAS and Raman also show that U${}_4$O${}_9$ does not behave as a linear combination of the UO${}_2$ and U${}_3$O${}_7$ fluorite endpoints. The properties induced by mobile rather than static charged quasiparticles were explored by optical pumping of the metal-to-metal charge-transfer transition. The temperature dependence of 4.71 eV pump–1.57 eV probe reflectivity on UO${}_2$ that initially populates the U 6$d$-dominated portion of the upper Hubbard band (UHB) shows a sharp 28-μsec lifetime peak at 25 K that may be associated with the fluctuations of its antiferromagnetic transition. Pumping at 3.14 eV into the 5$f$-dominated portion of the UHB shows an analogous 2.8-μsec peak but also a plateau bracketing this peak that ends in a cusp at 50–60 K and an abrupt change in the hardening rate of a novel 12–15 GHz phonon that is the signature for the quasiparticle quantum phase. The different results for the different excitation channels indicate a highly specific nonthermal relaxation mechanism. These results constitute the first observation of a distinct phase of photoinduced quasiparticles that is sufficiently coupled to the lattice to undergo a gap-opening transition. When the intragap state is probed with a terahertz time domain spectroscopy (TTDS) measurement 33 psec after a 3.14 excitation pulse, it shows increased absorption in the 0.5–1.1 THz range with a decrease in temperature from ∼30 to 10 K instead of the expected decrease, a result consistent with the presence of a condensate. These results are too extreme to originate in the dynamical, nonadiabatic, coupled charge-transfer–phonon/tunneling polaron scenario previously used for doped Mott-Hubbard insulators with intermediate electron-phonon coupling and therefore indicate novel physics. One possibility that could cause all of these behaviors is that a collective, dynamical, charge transfer-coupled Peierls distortion involving the 2 U(V) ↔ U(IV)+U(VI)-oxo excitation occurs coherently over an entire domain to cause the atoms in this domain to condense into a system with Bose-Einstein or Bose-Einstein-Hubbard properties.

We use a two-orbital double-exchange model including superexchange interactions, Jahn-Teller lattice distortions, and long-range Coulomb interactions to investigate the origin of magnetically disordered interfaces between ferromagnetic metallic (FM) and antiferromagnetic insulating (AFI) manganites in FM/AFI superlattices. The induced magnetic moment in the AFI layer varies nonmonotonically with increasing AFI layer width as seen in the experiment. We provide a framework for understanding this nonmonotonic behavior which has a one-to-one correspondence with the magnetization of the FM interface. The obtained insights provide a basis for improving the tunneling magnetoresistance in FM/AFI manganite superlattices by avoiding a magnetic dead layer in the FM manganite.

It is well known that the three-dimensional (3D) electronic topological insulator (TI) with charge-conservation and time-reversal symmetry cannot have a trivial insulating surface that preserves symmetry. It is often implicitly assumed that if the TI surface preserves both symmetries then it must be gapless. Here we show that it is possible for the TI surface to be both gapped and symmetry preserving, at the expense of having surface-topological order. In contrast to analogous bosonic topological insulators, this symmetric surface topological order is intrinsically non-Abelian. We show that the surface-topological order provides a complete nonperturbative definition of the electron TI that transcends a free-particle band-structure picture, and could provide a useful perspective for studying strongly correlated topological Mott insulators.