The π-plasmon dispersion in graphene was scrutinized by momentum-resolved electron energy-loss spectroscopy with an improved momentum $q$ resolution and was found to display the square root of the $q$ dispersion characteristic of the collective excitation of two-dimensional electron systems, in contrast to previous experimental and theoretical studies which reported a linear $q$ dispersion. Our theoretical elaborations on the $q$-dependent spectra affirm this square root of $q$ relation and further unveil an in-plane electronic anisotropy. The physical property of the π plasmon is thoroughly compared to that of the two-dimensional plasmon due to carriers of the Dirac fermions. A clear distinction between the π plasmon and the two-dimensional Dirac plasmon is demonstrated, clarifying the common notion about correlating the linearly dispersed Dirac cones with the linear dispersion of the π plasmon previously reported.

Soft x-ray magnetic circular dichroism (XMCD) have been measured on the single crystal of the charge-ordered type multiferroelectric material ${\mathrm{LuFe}}_2{\mathrm{O}}_4$. By comparing the XMCD results with the bulk magnetizations, the valence-specific magnetizations (VSMs) of ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Fe}}^{3+}$ ions are obtained. Both ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Fe}}^{3+}$ ions show positive and negative remanent VSMs below the spin ordering temperature $T_{\mathrm{SO}}\sim 250$ K, showing the ferrimagnetic long-range ordering directly. Both of the remanent VSMs disappear above the $T_{\mathrm{SO}}$ and the field dependencies of the VSMs behave in paramagnetic fashions with convex and concave curvatures. However, the signs of the VSMs of ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Fe}}^{3+}$ remain opposite, nevertheless the system is in a paramagnetic phase. This is direct evidence of the robust antiferromagnetic interaction between ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Fe}}^{3+}$ ions even above the $T_{\mathrm{SO}}$. As temperature increases, furthermore, the opposite VSM of ${\mathrm{Fe}}^{3+}$ is reversed around the three-dimensional charge ordering temperature $T_{\mathrm{CO}}\sim 330$ K showing that both net magnetic moments of ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Fe}}^{3+}$ become parallel to the applied magnetic field. Possible correlation between these unusual magnetic behaviors and the ferroelectric properties of ${\mathrm{LuFe}}_2{\mathrm{O}}_4$ is discussed.

In previous ${}^{29}\mathrm{Si}$ NMR basal-plane field rotation measurements, a modest breaking of the fourfold symmetry was suggested in the hidden-order state of ${\mathrm{URu}}_2{\mathrm{Si}}_2$. Here, the angular dependence of the ${}^{29}\mathrm{Si}$ NMR linewidth is analyzed in detail, indicating the occurrence of a distribution of twofold local fields at the Si sites even in a high-quality single crystal. The relation between sample quality and the occurrence of effects having twofold symmetry is discussed.

Low temperature conductance fluctuations in individual InAs nanowires are studied as a function of gate potential, magnetic field, and the angle between the field and the nanowire axis. The fluctuation pattern as a function of magnetic field shows a reduced flux pickup as the angle is varied from perpendicular to parallel. Statistically independent traces for each angle were measured and the angle dependence of the average correlation field is compared to theory. The results are consistent with the predictions for a geometry where electrons occupy the entire sample volume rather than a surface electron gas as has been speculated.

Ultrafast scattering processes of hot electrons photoinjected into the conduction band of InSb have been studied using time- and angle-resolved photoemission spectroscopy. The nascent distributions of hot-electron packets are captured directly in energy and momentum spaces, and their ultrafast scattering processes are traced at femtosecond temporal resolution on a state-resolved basis. Hot electrons injected in the Γ valley with excess energies above the minimum of the $L$ valley show ultrafast intervalley scattering, with transition times of the order of 40 fs. The relaxation processes in the $L$ valley are resolved in energy and momentum spaces, including their backscattering into the Γ valley during relaxation. In contrast, relaxation of hot electrons with excess energy below the minimum of the $L$ valley is governed by the direct impact ionization (IMP). We reveal state-selective features of the IMP process, and we have determined the direct IMP rate to be $7\times {10}^{12}{\mathrm{s}}^{-1}$ for hot electrons with excess energy in the range of 0.35 to 0.6 eV. The direct IMP process results in a rapid increase, within 300 fs after excitation, of the electron density at the conduction band minimum (CBM), and phonon-assisted IMP by hot electrons scattered in the $L$ valley and those backscattered into the Γ valley persistently enhances the electron density up to 8 ps after excitation. By analyzing correlations between the IMP rates of hot electrons and the electron densities near the CBM, an important role of a transient Auger recombination is proposed to quantify the yield of low-energy electrons generated in the IMP process.

The field-induced ordering transition in the quantum spin system ${\mathrm{NiCl}}_2·4\mathrm{SC}{\left({\mathrm{NH}}_2\right)}_2$ is studied by means of neutron diffraction, ac magnetometry, and relaxation calorimetry. The interpretation of the data is strongly influenced by a finite distribution of transition fields in the samples, which was present but disregarded in previous studies. Taking this effect into account, we find that the order-parameter critical exponent is inconsistent with the BEC universality class even at temperatures below 100 mK. All results are discussed in comparison with previous measurements and with recent similar studies of disordered ${\mathrm{Ni}(\mathrm{Cl}}_{1-x}{\mathrm{Br}}_x{{)}_2·{4\mathrm{SC}(\mathrm{NH}}_2)}_2$.

A bubble metascreen, i.e., a single layer of gas inclusions in a soft solid, can be modeled as an acoustic open resonator, whose behavior is well captured by a simple analytical expression. We show that by tuning the parameters of the metascreen, acoustic superabsorption can be achieved over a broad frequency range, which is confirmed by finite element simulations and experiments. Bubble metascreens can thus be used as ultrathin coatings for turning acoustic reflectors into perfect absorbers.

A remarkable decrease of the structural transition temperature of MnNiSi from 1200 to < 300 K by chemically alloying it with MnFeGe results in a coupling of the magnetic and structural transitions, leading to a large magnetocaloric effect near room temperature. Application of relatively low hydrostatic pressures (∼2.4 kbar) lead to an extraordinary enhancement of the isothermal entropy change from $-\Delta S=44$ to 89 $\mathrm{J}/\mathrm{kg}$ K at ambient and 2.4 kbar applied pressures, respectively, for a field change of $\Delta B=5\mathrm{T}$, and is associated with a large relative volume change of about 7% with $P=2.4$ kbar.

Phase-change memory (PCM), a promising candidate for next-generation nonvolatile memories, exploits quenched glassy and thermodynamically stable crystalline states as reversibly switchable state variables. We demonstrate PCM functions emerging from a charge-configuration degree of freedom in strongly correlated electron systems. Nonvolatile reversible switching between a high-resistivity charge-crystalline (or charge-ordered) state and a low-resistivity quenched state, charge glass, is achieved experimentally via heat pulses supplied by optical or electrical means in organic conductors $\theta$-(BEDT-TTF)${}_{2}X$. Switching that is one order of magnitude faster is observed in another isostructural material that requires faster cooling to kinetically avoid charge crystallization, indicating that the material's critical cooling rate can be useful guidelines for pursuing a faster correlated-electron PCM function.

We present a study of the structural, conducting, magnetic, and optical properties of YbN thin films. Magnetic measurements reveal an antiferromagnetic Curie-Weiss temperature dependence. We find the temperature-dependent resistivity and carrier concentration to be indicative of YbN being semiconducting in nature. Along with this we observe an absorption onset at 1.5 eV, found from optical transmission and reflection measurements. This apparent combination of antiferromagnetism and semiconductivity present in YbN makes it unique among the rare earth nitrides, a series dominated by ferromagnetic semiconductors.

Recent experiments have provided evidence that one-dimensional (1D) topological superconductivity can be realized experimentally by placing transition-metal atoms that form a ferromagnetic chain on a superconducting substrate. We address some properties of this type of system by using a Slater-Koster tight-binding model to account for important features of the electronic structure of the transition-metal chains on the superconducting substrate. We predict that topological superconductivity is nearly universal when ferromagnetic transition-metal chains form straight lines on superconducting substrates and that it is possible for more complex chain structures. When the chain is weakly coupled to the substrate and is longer than superconducting coherence lengths, its proximity-induced superconducting gap is $\sim \Delta E_{\text{SO}}/J$ where $\Delta$ is the $s$-wave pair potential on the chain, $E_{\text{SO}}$ is the spin-orbit splitting energy induced in the normal chain state bands by hybridization with the superconducting substrate, and $J$ is the exchange splitting of the ferromagnetic chain $d$ bands. Because of the topological character of the 1D superconducting state, Majorana end modes appear within the gaps of finite length chains. We find, in agreement with the experiment, that when the chain and substrate orbitals are strongly hybridized, Majorana end modes are substantially reduced in amplitude when separated from the chain end by less than the coherence length defined by the $p$-wave superconducting gap. We conclude that Pb is a particularly favorable substrate material for ferromagnetic chain topological superconductivity because it provides both strong $s$-wave pairing and strong Rashba spin-orbit coupling, but that there is an opportunity to optimize properties by varying the atomic composition and structure of the chain. Finally, we note that in the absence of disorder, a new chain magnetic symmetry, one that is also present in the crystalline topological insulators, can stabilize multiple Majorana modes at the end of a single chain.

Motivated by the concept of Möbius aromatics in organic chemistry, we extend the recently introduced concept of fragile Mott insulators (FMI) to ring-shaped molecules with repulsive Hubbard interactions threaded by a half-quantum of magnetic flux $(hc/2e)$. In this context, an FMI is the insulating ground state of a finite-size molecule that cannot be adiabatically connected to a single Slater determinant, i.e., to a band insulator, provided that time-reversal and lattice translation symmetries are preserved. Based on exact numerical diagonalization for finite Hubbard interaction strength $U$ and existing Bethe-ansatz studies of the one-dimensional Hubbard model in the large-$U$ limit, we establish a duality between Hubbard molecules with $4n$ and $4n+2$ sites, with $n$ integer. A molecule with $4n$ sites is an FMI in the absence of flux but becomes a band insulator in the presence of a half-quantum of flux, while a molecule with $4n+2$ sites is a band insulator in the absence of flux but becomes an FMI in the presence of a half-quantum of flux. Including next-nearest-neighbor hoppings gives rise to new FMI states that belong to multidimensional irreducible representations of the molecular point group, giving rise to a rich phase diagram.

Motivated by ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$, edge quasiparticle states are analyzed based on the self-consistent solution of the Bogolyubov–de Gennes equations for a topological chiral $p$-wave superconductor. Using a tight-binding model of a square lattice for the dominant $\gamma$ band, we explore the nontrivial geometry and band structure dependence of the edge states and currents. As a peculiar finding, we show that, for high band fillings, currents flow in a reversed direction when comparing straight and zigzag edges. We give a simple explanation in terms of the positions of the zero-energy bound states using a quasiclassical picture. We also show that a Ginzburg-Landau approach can reproduce these results. Moreover, the band filling dependence of the most stable domain wall structure is discussed.

The dual boson approach [Rubtsov, Katsnelson, and Lichtenstein, Ann. Phys. 327, 1320 (2012)] provides a means to construct a diagrammatic expansion around the extended dynamical mean-field theory (EDMFT). In this paper, we present the numerical implementation of the approach and apply it to the extended Hubbard model with nearest-neighbor interaction $V$. We calculate the EDMFT phase diagram and study the effect of diagrams beyond EDMFT on the transition to the charge-ordered phase. Including diagrammatic corrections to the EDMFT polarization shifts the EDMFT phase boundary to lower values of $V$. The approach interpolates between the random phase approximation in the weak coupling limit and EDMFT for strong coupling. Neglecting vertex corrections leads to results reminiscent of the $\text{EDMFT}+GW$ approximation. We, however, find significant deviations from the dual boson results already for small values of the interaction, emphasizing the crucial importance of fermion-boson vertex corrections.

It is well known that $\left(1+1\right)$-dimensional $\left[\right(1+1\left)\text{-D}\right]$ bosonic symmetry-protected topological (SPT) phases with symmetry group $G$ can be identified by the projective representation of the symmetry at the edge. Here we generalize this result to higher dimensions. We assume that the representation of the symmetry on the spatial edge of a $\left(d+1\right)$-D SPT is local but not necessarily on site, such that there is an obstruction to its implementation on a region with a boundary. We show that such obstructions are classified by the cohomology group $H^{d+1}(G,\text{U}\left(1\right))$, in agreement with the classification of bosonic SPT phases proposed in Chen et al. [Science 338, 1604 (2012)]. Our analysis allows for a straightforward calculation of the element of $H^{d+1}(G,\text{U}\left(1\right))$ corresponding to physically meaningful models such as nonlinear $\sigma$ models with a $\theta$ term in the action. SPT phases outside the classification of Chen et al. are those in which the symmetry cannot be represented locally on the edge. With some modifications, our framework can also be applied to fermionic systems in $\left(2+1\right)$-D.

Quasi-two-dimensional (2D) systems, such as an electron gas confined in a quantum well, are important model systems for many-body theories. Earlier studies of the crossover from 3D to 2D in ground-state density-functional theory showed that local and semilocal exchange-correlation functionals which are based on the 3D electron gas are appropriate for wide quantum wells, but eventually break down as the 2D limit is approached. We now consider the dynamical case and study the performance of various linear-response exchange kernels in time-dependent density-functional theory. We compare approximate local, semilocal, and orbital-dependent exchange kernels, and analyze their performance for inter- and intrasubband plasmons as the quantum wells approach the 2D limit. 3D (semi)local exchange functionals are found to fail for quantum well widths comparable to the 2D Wigner-Seitz radius $r_s^{2\mathrm{D}}$, which implies in practice that 3D local exchange remains valid in the quasi-2D dynamical regime for typical quantum well parameters, except for very low densities.

We carry out an analytical study of quantum spin ice, a $\mathrm{U}\left(1\right)$ quantum spin liquid close to the classical spin-ice solution for an effective spin-$\frac{1}{2}$ model with anisotropic exchange couplings $J_{zz}$, $J_{\pm }$, and $J_{z\pm }$ on the pyrochlore lattice. Starting from the quantum rotor model introduced by Savary and Balents [Phys. Rev. Lett. 108, 037202 (2012)], we retain the dynamics of both the spinons and gauge field sectors in our treatment. The spinons are described by a bosonic representation of quantum XY rotors, while the dynamics of the gauge field is captured by a phenomenological Hamiltonian. By calculating the one-loop spinon self-energy, which is proportional to $J_{z\pm }^2$, we determine the stability region of the $\mathrm{U}\left(1\right)$ quantum spin-liquid phase in the $J_{\pm }/J_{zz}$ versus $J_{z\pm }/J_{zz}$ zero-temperature phase diagram. From these results, we estimate the location of the boundaries between the spin-liquid phase and classical long-range ordered phases.

Temperature-dependent transport measurements on ultrathin antiferromagnetic Mn films reveal a heretofore unknown nonuniversal weak-localization correction to the conductivity which extends to disorder strengths greater than $100\mathrm{k}\Omega$ per square. The inelastic scattering of electrons off of gapped antiferromagnetic spin waves gives rise to an inelastic scattering length which is short enough to place the system in the three-dimensional regime. The extracted fitting parameters provide estimates of the energy gap ($\Delta \approx 16\mathrm{K}$) and Heisenberg exchange constant ($J\approx 1000\mathrm{K}$).

We experimentally investigate magnetic frustration effects in thermally active artificial kagome spin ice. Starting from a paramagnetic state, the system is cooled down below the Curie temperature of the constituent material. The resulting magnetic configurations show that our arrays are locally brought into the so-called spin ice 2 phase, predicted by at-equilibrium Monte Carlo simulations and characterized by a magnetic charge crystal embedded in a disordered kagome spin lattice. However, by studying our arrays on a larger scale, we find the unambiguous signature of an out-of-equilibrium physics. Comparing our findings with numerical simulations, we interpret the efficiency of our thermalization procedure in terms of kinetic pathways that the system follows upon cooling and which drive the arrays into degenerate low-energy manifolds that are hardly accessible otherwise.

We analyze theoretically the interaction between chiral molecules and localized surface plasmons in subwavelength metallic structures of arbitrary shape. The chiral molecule is modeled using Condon's classical description of the molecular dipole moment that depends on the magnetic field [E. U. Condon, Rev. Mod. Phys. 9, 432 (1937)]. This model is included in an eigenmode theory of coupled plasmonic systems. In the limit of dipole-dipole interactions, the theory predicts there is no change in the resonance frequency of the surface plasmon in the presence of a chiral molecule and there is no change in the amplitude of the resonance in the presence of a uniform distribution of chiral molecules. This implies that to observe the effects of the chirality of molecules it may be necessary to form localized surface plasmons with more complex charge distributions. We also examine the absorption of light of the combined system of the metal structure and chiral molecule. The theory predicts that the chirality-dependent absorption in the metal structure averages to zero for a uniform distribution of molecules, with the observed absorption occurring entirely within the molecule due to excitation by the incident light and the fields from the surface plasmon. Apart from the expected circular dichroism, the theory also predicts a chirality-dependent absorption arising from a metal nanostructure illuminated with linearly polarized light and an absorption arising from those chiral properties of the molecule usually associated with optical activity.