By means of molecular dynamics simulations, we have studied heat transport in nanoporous silicon, finding that the Eucken model, widely adopted in the description of thermal transport in macroporous systems, breaks down when pores are nanometer-sized. Present atomistic results are used to inform an extension to this model, effectively describing the relationship between thermal conductivity and interface density, here identified as the key structural characteristic of a porous sample. Our model, validated against a range of pore sizes and distributions, provides a robust framework for the interpretation of the atomistic results, as well as suggesting how to estimate the average pore size through thermal transport measurements.

Diffuse magnetic scattering in the highly frustrated double perovskite ${\text{Ba}}_2{\text{YRuO}}_6$ was investigated using polarized neutrons. Consistent with previous reports, the material shows two apparent transitions at 47 and 36 K to an eventual type I face-centered-cubic magnetic ground state. The (100) magnetic reflection shows different behavior from the five other observed reflections upon heating from 1.8 K, with the former broadening well beyond the resolution limit near 36 K. Closer examination of the latter group reveals a small, but clear, increase in peak widths between 36 and 47 K, indicating that this regime is dominated by short-range spin correlations. Diffuse magnetic scattering persists above 47 K near the position of (100) to at least 200 K, consistent with strong frustration. Reverse Monte Carlo (RMC) modeling of the diffuse scattering from 45 to 200 K finds that the spin-spin correlations between nearest and next-nearest neighbors are antiferromagnetic and ferromagnetic, respectively, at temperatures near the upper ordering temperature, but both become antiferromagnetic and of similar magnitude above 100 K. The significance of this unusual crossover is discussed in light of the super-superexchange interactions between nearest and next-nearest neighbors in this material and the demands of type I order. The dimensionality of the correlations is addressed by reconstructing the scattering in the $\left(hk0\right)$ plane using the RMC spin configurations. This indicates that one-dimensional spin correlations dominate at temperatures close to the first transition. In addition, a comparison between mean-field calculations and $\left(hk0\right)$ scattering implies that further neighbor couplings play a significant role in the selection of the ground state. The results and interpretation are compared with those recently published for monoclinic ${\text{Sr}}_2{\text{YRuO}}_6$, and similarities and differences are emphasized.

The evolution of a magnetic domain structure upon excitation by an intense, femtosecond infrared (IR) laser pulse has been investigated using single-shot based time-resolved resonant x-ray scattering at the x-ray free electron laser LCLS. A well-ordered stripe domain pattern as present in a thin CoPd alloy film has been used as a prototype magnetic domain structure for this study. The fluence of the IR laser pump pulse was sufficient to lead to an almost complete quenching of the magnetization within the ultrafast demagnetization process taking place within the first few hundreds of femtoseconds following the IR laser pump pulse excitation. On longer time scales this excitation gave rise to subsequent irreversible transformations of the magnetic domain structure. Under our specific experimental conditions, it took about 2 ns before the magnetization started to recover. After about 5 ns the previously ordered stripe domain structure had evolved into a disordered labyrinth domain structure. Surprisingly, we observe after about 7 ns the occurrence of a partially ordered stripe domain structure reoriented into a novel direction. It is this domain structure in which the sample's magnetization stabilizes as revealed by scattering patterns recorded long after the initial pump-probe cycle. Using micromagnetic simulations we can explain this observation based on changes of the magnetic anisotropy going along with heat dissipation in the film.

We have performed a measurement of optical absorption in multiferroic ${\mathrm{MnWO}}_4$ in high magnetic fields. This material undergoes magnetic-field-induced successive phase transitions at low temperatures. The spectrum of the optical absorption originating from spin-forbidden $d\text{−}d$ excitations at ${\mathrm{Mn}}^{2+}$ ions was found to change with the phase transitions. The observed magnetochromism suggests that the magnetic moments align approximately along the $b$ axis in the high-field (HF) paraelectric phase above 12 T.

The Bose-Hubbard model (BHM) of interacting bosons in a lattice has been a paradigm in many-body physics, and it exhibits a Mott insulator-superfluid (MI-SF) transition at integer filling. Here a quantum simulator of the BHM using a superconducting circuit is proposed. Specifically, a superconducting transmission line resonator supporting microwave photons is coupled to a charge qubit to form one site of the BHM, and adjacent sites are connected by a tunable coupler. To obtain a mapping from the superconducting circuit to the BHM, we focus on the dispersive regime where the excitations remain photonlike. Standard perturbation theory is implemented to locate the parameter range where the MI-SF transition may be simulated. This simulator allows single-site manipulations, and we illustrate this feature by considering two scenarios where a single-site manipulation can drive a MI-SF transition. The transition can be analyzed by mean-field analyses, and the exact diagonalization was implemented to provide accurate results. The variance of the photon density and the fidelity metric clearly show signatures of the transition. Experimental realizations and other possible applications of this simulator are also discussed.

The hardcore extended Bose Hubbard model on a bilayer square lattice with attractive (repulsive) interplane (intraplane) interactions has been investigated numerically. Focusing on the strong interplane hopping parameter regime, triplet valence bonds of dimers are stabilized to form valence bond checkerboard solid at quarter filling. Increasing the particle number we confirm the presence of the exotic valence bond supersolid phase, where the valence bond solid ordering and the boson superfluidity coexist. Rising further the particle number will lead to a checkerboard solid of dimer pair at half filling for strong intraplane repulsion, or a valence bond Mott insulator for weak repulsive interactions. We analyze the rich ground state phase diagrams of this model, which can be experimentally realized in optical lattices of cold atoms.

We investigate the ground-state properties of a spin-1 kagome antiferromagnetic Heisenberg model using tensor-network (TN) methods. We obtain the energy per site $e_0=-1.41090\left(2\right)$, with $D^{*}=8$ multiplets retained (i.e., a bond dimension of $D=24)$, and $e_0=-1.4116\left(4\right)$ from large-$D$ extrapolation, by accurate TN calculations directly in the thermodynamic limit. The symmetry between the two kinds of triangles is spontaneously broken, with a relative energy difference of $\delta \approx 19\%$, i.e, there is a trimerization (simplex) valence-bond order in the ground state. The spin-spin, dimer-dimer, and chirality-chirality correlation functions are found to decay exponentially with a rather short correlation length, showing that the ground state is gapped. We thus identify the ground state to be a simplex valence-bond crystal. We also discuss the spin-1 bilinear-biquadratic Heisenberg model on a kagome lattice, and determine its ground-state phase diagram. Moreover, we implement non-Abelian symmetries, here spin SU(2), in the TN algorithm, which improves the efficiency greatly and provides insight into the tensor structures.

We present an investigation of the ferroelectric and magnetic domain walls in multiferroics ${\mathrm{BiFeO}}_3$ combining first-principles structural data with a phenomenological modeling of the spin structure. We show that the structural distortions at the ferroelectric walls may lead to peculiar spin arrangements with nontrivial topologies. Further, we find that the magnetic walls depend strongly on the nature of the structural discontinuity, as the three possible cases considered—which correspond, respectively, to angles of ${71}^{\circ },{109}^{\circ }$, and ${180}^{\circ }$ between the polarizations of the neighboring domains—present distinctive features.

Experimental evidence of wave properties of dendritic flux avalanches in superconducting films is reported. Using magneto-optical imaging the propagation of dendrites across boundaries between a bare NbN film and areas coated by a Cu layer was visualized, and it was found that the propagation is refracted in full quantitative agreement with Snell's law. For the studied film of 170 nm thickness and a $0.9\mu \mathrm{m}$ thick metal layer, the refractive index was close to $n=1.4$. The origin of the refraction is believed to be caused by the dendrites propagating as an electromagnetic shock wave, similar to damped modes considered previously for normal metals. The analogy is justified by the large dissipation during the avalanches raising the local temperature significantly. Additional time-resolved measurements of voltage pulses generated by segments of the dendrites traversing an electrode confirm the consistency of the adopted physical picture.

The atomic structure of metallic glasses (MGs) plays an important role in their properties. Numerous molecular dynamics (MD) simulations have revealed icosahedral short-range order (ISRO) as a dominant motif in Cu-Zr metallic glasses. However, the cooling rates utilized in most of the MD simulations (usually on the order of ${10}^{10–13}\mathrm{K}/\mathrm{s}$) can be too high to allow the structure to relax into the actual structures. By performing a long sub-$T_g$ annealing of the $\mathrm{C}{\mathrm{u}}_{64.5}\mathrm{Z}{\mathrm{r}}_{35.5}$ alloy model at 700 K up to $2.0\mu \mathrm{s}$ using MD simulations, we systematically address the evolution of medium-range order (MRO) as the cooling rates in MD simulations approach the experimental cooling rates (usually ${10}^{3–6}\mathrm{K}/\mathrm{s}$). By reducing the effective cooling rates to as low as $2.8\times {10}^7\mathrm{K}/\mathrm{s}$, we found a significant enhancement of the ISRO and Bergman-type MRO. Comparing to the widely used face-, edge-, or vertex-sharing icosahedra, we propose that the Bergman-type MRO is a much more unambiguous metric to characterize the MRO in Cu-Zr MGs. By analyzing the network formed by interpenetrating icosahedra using the graphical theory, we show that the degree of interpenetration of the icosahedra centers increases with decreasing cooling rates. The network becomes aggressively assortative, indicating that higher degree nodes tend to cluster and form backbones in the MG. All these results show that the networks in the models prepared using lower cooling rates strongly deviate from a stringlike morphology.

We propose a framework to study the onset of Anderson localization in disordered systems. The idea is to expose waves propagating in a random scattering environment to a sequence of short dephasing pulses. The system responds through coherence peaks forming at specific echo times, each echo representing a particular process of quantum interference. We suggest a concrete realization for cold gases, where quantum interferences are observed in the momentum distribution of matter waves in a laser speckle potential, and discuss in detail corresponding echoes in momentum space for sequences of one and two dephasing pulses. Our proposal defines a challenging but arguably realistic framework promising to yield unprecedented insight into the mechanisms of Anderson localization.

High harmonic generation (HHG) in solids is investigated. We find that interband emission is dominant for the midinfrared laser driver frequencies, whereas intraband emission dominates the far-infrared range. Interband HHG is similar to atomic HHG and therewith opens the possibility to apply atomic attosecond technology to the condensed matter phase. Interband emission is investigated with a quasiclassical method, by which HHG can be modeled based on the classical trajectory analysis of electron-hole pairs. This analysis yields a simple approximate cutoff law for HHG in solids. Differences between HHG in atoms and solids are identified that are important for adapting atomic attosecond technology to make it applicable to condensed matter.

We investigate possible realizations of exotic $\mathrm{SU}\left(N\right)$ symmetry-protected topological (SPT) phases with alkaline-earth cold fermionic atoms loaded into one-dimensional optical lattices. A thorough study of two-orbital generalizations of the standard $\mathrm{SU}\left(N\right)$ Fermi-Hubbard model, directly relevant to recent experiments, is performed. Using state-of-the-art analytical and numerical techniques, we map out the zero-temperature phase diagrams at half-filling and identify several Mott-insulating phases. While some of them are rather conventional (nondegenerate, charge-density wave, or spin-Peierls-like), we also identify, for even $N$, two distinct types of SPT phases: an orbital Haldane phase, analogous to a spin-$N/2$ Haldane phase, and a topological $\mathrm{SU}\left(N\right)$ phase, which we fully characterize by its entanglement properties. We also propose sets of nonlocal order parameters that characterize the $\mathrm{SU}\left(N\right)$ topological phases found here.

We present a relativistic tight-binding (TB) approximation method that is applicable to actual crystalline materials immersed in a uniform magnetic field. The magnetic Bloch theorem is used to make the dimensions of the Hamiltonian matrix finite. In addition, by means of the perturbation theory, the magnetic hopping integral that appears in the Hamiltonian matrix is reasonably approximated as the relativistic hopping integral multiplied by the magnetic-field-dependent phase factor. In order to calculate the relativistic hopping integral, the relativistic version of the so-called Slater-Koster table is also given in an explicit form. We apply the present method to crystalline silicon immersed in a uniform magnetic field, and reveal its energy-band structure that is defined in the magnetic first Brillouin zone. It is found that the widths of energy-bands increase with increasing the magnetic field, which indicates the magnetic-field dependence of the appropriateness of the effective mass approximation. The recursive energy spectrum, which is the so-called butterfly diagram, can also be seen in the $\mathit{k}$-space plane perpendicular to the magnetic field.

We investigate theoretically the possible low temperature structural distortion in the pyrochlore based ruthenate compound ${\mathrm{Hg}}_2{\mathrm{Ru}}_2{\mathrm{O}}_7$. Our study reveals a signature of structural distortion leading to charge disproportionation between the Ru ions in the unit cell. The charge disproportion is found to be driven by the energy gain due to large covalency originating from the overlap of extended Ru $5d$ and O 2p orbitals combined with Hg $d$-Ru $d$ hybridization. The exchange interaction operative between the charge disproportionated Ru ions turns out to be of antiferromagnetic nature, which drives the insulating state and stabilizes the noncollinear magnetic structure. Our study sheds light on the microscopic mechanism of the low temperature structural transition in this compound.

Good metals are characterized by diffusive transport of coherent quasiparticle states and the resistivity is much less than the Mott-Ioffe-Regel (MIR) limit, $\frac{ha}{e^2}$, where $a$ is the lattice constant. In bad metals, such as many strongly correlated electron materials, the resistivity exceeds the Mott-Ioffe-Regel limit and the transport is incoherent in nature. Hartnoll, loosely motivated by holographic duality (AdS/CFT correspondence) in string theory, recently proposed a lower bound to the charge-diffusion constant, $D\gtrsim \hslash v_F^2/\left(k_{B}T\right)$, in the incoherent regime of transport, where $v_F$ is the Fermi velocity and $T$ the temperature. Using dynamical mean-field theory (DMFT) we calculate the charge-diffusion constant in a single band Hubbard model at half filling. We show that in the strongly correlated regime the Hartnoll's bound is violated in the crossover region between the coherent Fermi-liquid region and the incoherent (bad metal) local moment region. The violation occurs even when the bare Fermi velocity $v_F$ is replaced by its low-temperature renormalized value $v_F^{*}$. The bound is satisfied at all temperatures in the weakly and moderately correlated systems as well as in strongly correlated systems in the high-temperature region where the resistivity is close to linear in temperature. Our calculated charge-diffusion constant, in the incoherent regime of transport, also strongly violates a proposed quantum limit of spin diffusion, $D_s\sim 1.3\hslash /m$, where $m$ is the fermion mass, experimentally observed and theoretically calculated in a cold degenerate Fermi gas in the unitary limit of scattering.

Doped ${\mathrm{CuCoO}}_2$ is a candidate oxide material for thermoelectric power generation. The evolution of the band structure and thermoelectric properties of ${\mathrm{CuCoO}}_2$ upon hole and electron doping in the ${\mathrm{CoO}}_2$ layer and hole doping at the Cu site were calculated by the local-density approximation (LDA) and $\mathrm{LDA}+U$ methods and using standard Boltzmann theory. The doping was simulated by the virtual atom approximation and the supercell approach and the results were compared with previous calculations using the rigid band approximation. The calculated thermopowers are comparable for the virtual atom and rigid band approximations, but the thermopower obtained from the supercell calculation is significantly lower. The reason is the similar energy of Co and Cu $d$ orbitals and the hybridization of symmetrically related Co $a_{1g}$ and Cu $d_{z^2}$ orbitals. As a consequence, both cations contribute to the bands around the Fermi level and hence a substitution at any of the cation sites alters the band structure at $E_F$ and affects the thermoelectric properties. Our results show that in the case of hole doping, higher thermopower is obtained for substitution at the Cu site than in the ${\mathrm{CoO}}_2$ layer.

Relying on HSE03, ${\mathrm{G}}_0{\mathrm{W}}_0$, and random phase $\mathrm{approximation}+{\mathrm{G}}_0{\mathrm{W}}_0$ approaches, this study provides a complete characterization of the electronic properties of six ZnO bulk polymorphs, including quasiparticle spectra, absorption, and electron energy-loss spectra. It focuses on the series zinc blende, wurtzite, body-centered tetragonal, sodalite, and cubane structures, along which atoms have similar local tetrahedral environments and decreasing atomic density, to which the hexagonal boron nitride structure was added. All trends in the electronic properties—gap opening, blueshift of low-energy interband transitions, redshift of the main absorption peak, the frequency at which the real part of the dielectric tensor vanishes, and the plasmon energy—were interpreted in terms of the two main effects resulting from the decrease of the atomic density along the series: a bandwidth narrowing close to the gap edge and a decrease (in absolute value) of the electrostatic potential acting on the zinc and oxygen ions. This work gives firm grounds to help and foster spectroscopic experiments on ZnO polymorphs. It represents a preliminary step towards the study of thin-film properties.

We propose a strategy to create materials displaying robust and tunable magnetoelectric multiferroic properties at room temperature. The key idea is to construct heterostructures that combine two different constituents: (1) compound ${\mathrm{BiFeO}}_3$, which presents strong ferroelectric and antiferromagnetic orders well above room temperature, but displays a small magnetic moment, and (2) a ferromagnetic insulator (e.g., ${\mathrm{BiMnO}}_3$) that is only required to couple magnetically with ${\mathrm{BiFeO}}_3$. Our simulations show that it is possible to combine such materials to create superlattices that present (i) a room-temperature multiferroic state with relatively large magnetization (up to $0.3{\mu }_{\mathrm{B}}$ per transition metal atom, with the possibility to improve by finding a suitable replacement for ${\mathrm{BiMnO}}_3$), (ii) an amply customizable magnetic behavior, and (iii) a strong magnetoelectric coupling. Thus, the design strategy successfully addresses a great challenge in the area of magnetoelectric multiferroics, exploiting interfacial couplings and size (layer-thickness) effects to produce materials apt for applications.

The region of filling factors $1/3<\nu <2/5$ is predicted to support new types of fractional quantum Hall states with topological order different from that of the Laughlin-Jain or the Moore-Read states. Incompressibility is a necessary condition for the formation of such novel topological states. We find that at 6.9 mK incompressibility develops only at $\nu =4/11$ and $5/13$, while the states at $\nu =6/17$ and $3/8$ remain compressible. Our observations at $\nu =4/11$ and $5/13$ are first steps towards understanding emergent topological order in these fractional quantum Hall states.