We study the Gaffnian trial wave function proposed to describe fractional quantum Hall correlations at Bose filling factor $\nu =2/3$ and Fermi filling $\nu =2/5$. A family of Hamiltonians interpolating between a hard-core interaction for which the physics is known and a projector whose ground state is the Gaffnian is studied in detail. We give evidence for the absence of a gap by using large-scale exact diagonalizations in the spherical geometry. This is in agreement with recent arguments based on the fact that this wave function is constructed from a nonunitary conformal field theory. By using the cylinder geometry, we discuss in detail the nature of the underlying minimal model and we show the appearance of heterotic conformal towers in the edge energy spectrum where left and right movers are generated by distinct primary operators.

We study the effect of spatially nonlocal correlations on the nonequilibrium dynamics of interacting fermions by constructing the nonequilibrium dynamical cluster theory, a cluster generalization of the nonequilibrium dynamical mean-field theory (DMFT). The formalism is applied to interaction quenches in the Hubbard model in one and two dimensions, and the results are compared with data from single-site DMFT, the time-dependent density matrix renormalization group, and lattice perturbation theory. Both in one and two dimensions the double occupancy quickly thermalizes, while the momentum distribution relaxes only on much longer time scales. For the two-dimensional square lattice we find a strongly momentum-dependent evolution of the momentum distribution around the Fermi energy, with a much faster relaxation near the momenta $(0,\pi )$ and $(\pi ,0)$ than near $(\pi /2,\pi /2)$. This result is interpreted as reflecting the momentum-anisotropic quasiparticle lifetime of the marginal Fermi liquid. The method is further applied to the two-dimensional Hubbard model driven by a dc electric field, where the damping of the Bloch oscillation of the current is found to be less effective than predicted by DMFT and lattice perturbation theory.

We report the finding of a series of distinct-symmetry spin-liquid (SL) states and a rich phase diagram in a half-filled honeycomb lattice Hubbard model with spin-dependent hopping amplitude $t^{\prime }$. We first study the magnetic instability of the system and find two antiferromagnetic orders beyond a critical Hubbard $U$ which increases with the ratio $t^{\prime }/t$. For $t^{\prime }$ approaching $t$, the semimetal transforms to a $U\left(1\right)$ SL and then to the Kitaev $Z_2$ SL as $U$ increases. In a wide middle range of $t^{\prime }/t$, the latter is replaced by a $U\left(1\right)$ SL to $SU\left(2\right)$ SL transition. The physical properties of the stable SL phases are discussed.

We report powder and single-crystal neutron diffraction measurements of the magnetic order in $A{\mathrm{MnBi}}_2$ ($A=\mathrm{Sr}$ and Ca), two layered manganese pnictides with anisotropic Dirac fermions on a Bi square net. Both materials are found to order at $T_{\mathrm{N}}\approx 300$ K in $\mathbf{k}=0$ antiferromagnetic structures, with ordered Mn moments at $T=10$ K of approximately $3.8 {\mu }_{\mathrm{B}}$ aligned along the $c$ axis. The magnetic structures are Néel type within the Mn-Bi layers, but the interlayer ordering is different, being antiferromagnetic in ${\mathrm{SrMnBi}}_2$ and ferromagnetic in ${\mathrm{CaMnBi}}_2$. This allows a mean-field coupling of the magnetic order to Bi electrons in ${\mathrm{CaMnBi}}_2$ but not in ${\mathrm{SrMnBi}}_2$. We find clear evidence that magnetic order influences electrical transport. First-principles calculations explain the experimental observations and suggest that the mechanism for different interlayer ordering in the two compounds is the competition between the antiferromagnetic superexchange and ferromagnetic double exchange carried by itinerant Bi electrons.

The short-time evolutions of two distinct systems, the pump and probe experiments with a semiconductor and the sudden quench of cold atoms in an optical lattice, are found to be described by the same universal response function. This analytic formula at short time scales is derived from the quantum kinetic-theory approach observing that correlations need time to form. The demand of density conservation leads to a reduction of the relaxation time by a factor of 4 in quench setups. The influence of the finite-trapping potential is derived and discussed along with Singwi-Sjølander local-field corrections including the proof of sum rules.

The influence of multiple vibrational modes on current fluctuations in electron transport through single-molecule junctions is investigated. Our analysis is based on a generic model of a molecular junction, which comprises a single electronic state on the molecular bridge coupled to multiple vibrational modes and fermionic leads, and employs a master equation approach. The results reveal that in molecular junctions with multiple vibrational modes, already weak to moderate electronic-vibrational coupling may result in high noise levels, especially at the onset of resonant transport, in accordance with the experimental findings of Secker et al. [Phys. Rev. Lett. 106, 136807 (2011)]. The underlying mechanisms are analyzed in some detail.

Here we show that a twisted graphene trilayer made by misoriented stacking of a graphene monolayer on top of a Bernal graphene bilayer can exhibit rich and tailored electronic properties. For the case that the graphene monolayer and bilayer are strongly coupled, both the massless Dirac fermions and massive chiral fermions coexist in the twisted trilayer, leading to unique in-plane chiral tunneling. For a weak coupling between the two graphene systems, the distinct chiralities and pseudospin textures of quasiparticles in monolayer and bilayer enable vertical valley-polarized quantum tunneling between them. Intriguingly, the polarity of the valley polarization can be inverted simply by either controlling the rotational angles between the two systems or tuning the Fermi levels of the two systems. Our result implies that layered van der Waals structures assembled from individual atomic planes can create materials that harbor unusual properties and alternative functionalities depending on the stacking configuration of the crystalline layers.

Motivated by emergent phenomena at oxide surfaces and interfaces, particularly those involving transition metal oxides with perovskite crystal structure such as ${\mathrm{LaTiO}}_3/{\mathrm{SrTiO}}_3$, we examine the ferromagnetic Kondo lattice model (FKLM) in the presence of a Rashba spin-orbit coupling (RSOC). Using numerical techniques, under the assumption that the electrons on localized orbitals may be treated as classical continuum spins, we compute various charge, spin, and transport properties on square clusters at zero temperature. We find that the main effect of the RSOC is the destruction of the ferromagnetic state present in the FKLM at low electron fillings, with the consequent suppression of conductivity. In addition, near half filling the RSOC leads to a departure of the antiferromagnetic state of the FKLM with a consequent reduction to the intrinsic tendency to electronic phase separation. The interplay between phase separation on one side, and magnetic and transport properties on the other, is carefully analyzed as a function of the RSOC/hopping ratio.

We investigate the stability of “magnetic” ordering against band-filling changes and Anderson-like disorder in the Falicov-Kimball model, within dynamical mean-field theory (DMFT). The one-particle density of states is obtained by both arithmetic and geometric averages over disorder, allowing us to detect the localization transition. Varying the Coulomb interaction and disorder strength, we construct phase diagrams where we identify metallic and insulating regions, with or without magnetic ordering, and determine how these phases are affected by band filling.

A triangular lattice antiferromagnet ${\mathrm{CuFe}}_{1\text{−}x}{\mathrm{Ga}}_x{\mathrm{O}}_2$ has been known to exhibit an elliptic screw-type magnetic ordering with spontaneous electric polarization “perpendicular” to the $c$ axis, specifically along the [110] direction of the crystal. In the present study, we have performed pyroelectric measurements on ${\mathrm{CuFe}}_{1\text{−}x}{\mathrm{Ga}}_x{\mathrm{O}}_2$ $\left(x=0.035\right)$, revealing that the ferroelectric phase also exhibits spontaneous electric polarization “parallel” to the $c$ axis $\left(P_c\right)$. While the ferroelectric polarization along the [110] direction is naturally led from the symmetry of the trigonal (or monoclinic) crystal structure with the screw-type magnetic ordering, the emergence of $P_c$ is not trivial. To understand the origin of $P_c$, we have refined the crystal structure by means of resonant x-ray diffraction measurements near the $K$-absorption edges of ${\mathrm{Fe}}^{3+}$ and ${\mathrm{Cu}}^{+}$ ions. We have also reanalyzed the magnetic structure in the ferroelectric phase using the results of the previous spherical neutron polarimetry [Nakajima et al., Phys. Rev. B 79, 214423 (2009)]. On the basis of the refined magnetic and crystal structures, we have discussed the microscopic mechanism for the emergence of $P_c$.

We investigate the role of correlations in the tetragonal and collapsed tetragonal phases of ${\mathrm{CaFe}}_2{\mathrm{As}}_2$ by performing charge self-consistent DFT+DMFT (density functional theory combined with dynamical mean-field theory) calculations. While the topology of the Fermi surface is basically unaffected by the inclusion of correlation effects, we find important orbital-dependent mass renormalizations which show good agreement with recent angle-resolved photoemission experiments. Moreover, we observe a markedly different behavior of these quantities between the low-pressure tetragonal and the high-pressure collapsed tetragonal phase. We attribute these effects to the increased hybridization between the iron and arsenic orbitals as one enters the collapsed tetragonal phase.

We discuss the magnetic properties of a ${\mathrm{Sm}}_2{\mathrm{Mo}}_2{\mathrm{O}}_7$ single crystal as investigated by means of different experimental techniques. In the literature, a conventional itinerant ferromagnetic state is reported for the $\mathrm{Mo}{}^{4+}$ sublattice below $\sim 78$ K. However, our results of dc magnetometry, muon spin spectroscopy (${\mu }^{+}$SR), and high-harmonics magnetic ac susceptibility unambiguously evidence highly disordered conditions in this phase, in spite of the crystalline and chemical order. This disordered magnetic state shares several common features with amorphous ferromagnetic alloys. This scenario for ${\mathrm{Sm}}_2{\mathrm{Mo}}_2{\mathrm{O}}_7$ is supported by the anomalously high values of the critical exponents, as mainly deduced by a scaling analysis of our dc magnetization data and confirmed by the other techniques. Moreover, ${\mu }^{+}$SR detects a significant static magnetic disorder at the microscopic scale. At the same time, the critical divergence of the third-harmonic component of the ac magnetic susceptibility around $\sim 78$ K leads to additional evidence towards the glassy nature of this magnetic phase. Finally, the longitudinal relaxation of the ${\mu }^{+}$ spin polarization (also supported by results of ac susceptibility) evidences re-entrant glassy features similar to amorphous ferromagnets.

The cumulant expansion is a powerful approach for including correlation effects in electronic structure calculations beyond the $GW$ approximation. However, the expansion is not generally valid, as current implementations ignore terms that mix particle and hole states and lead to partial occupation numbers of one-electron states. These limitations are corrected here using a cumulant expansion of the retarded one-electron Green's function that includes both particle and hole contributions. The approach provides a consistent framework that improves on the $GW$ approximation to the spectral function without additional computational effort. The method is illustrated with results for the homogeneous electron gas and comparisons to experiment and other methods.

We investigate a nonmagnetic metal-insulator transition in the 1/5-depleted square lattice Hubbard model at half filling within the eight-site cellular dynamical mean field theory. We find that a metal-insulator transition without any signatures of a first-order transition, a continuous Mott transition, takes place in a certain range of parameters. The nature of the continuous Mott transition is merely a Lifshitz transition driven by the on-site Coulomb interaction. The renormalized matrix elements of hoppings and the spin-spin correlation functions reveal that the physics of this transition is a strong enhancement of the dimerization due to the nonlocal effects of the electron-electron interaction.

A theory of spin fluctuations of excitons in quantum wells in the presence of nonresonant excitation has been developed. Both bright and dark excitonic states have been taken into account. The effect of a magnetic field applied in a quantum-well plane has been analyzed in detail. We demonstrate that in relatively small fields, the spin-noise spectrum consists of a single peak centered at a zero frequency, while an increase of magnetic field results in the formation of the second peak in the spectrum, owing to an interplay of the Larmor effect of the magnetic field and the exchange interaction between electrons and holes forming excitons. Experimental possibilities to observe the exciton spin noise are discussed, particularly by means of ultrafast spin-noise spectroscopy. We show that the fluctuation spectra contain, in addition to individual contributions of electrons and holes, information about the correlation of their spins.

In this work, we identify a class of topological phases protected by nonsymmorphic crystalline symmetry dubbed “topological nonsymmorphic crystalline insulators.” We construct a concrete tight-binding model for a lattice with nonsymmorphic symmetry and confirm its topological nature by directly calculating topological surface states. Analogous to “Kramers' pairs” originating from time-reversal symmetry, we introduce “doublet pairs” originating from nonsymmorphic symmetry to define the corresponding $Z_2$ topological invariant for this phase. Based on projective representation theory, we extend our discussion to other nonsymmorphic symmetry groups that can host this topological phase which will provide guidance for the systematic search for new topological materials.

We experimentally demonstrate single beam directional perfect absorption (to within experimental accuracy) of $p$-polarized light in the near-infrared using unpatterned, deep subwavelength films of indium tin oxide (ITO) on Ag. The experimental perfect absorption occurs slightly above the epsilon-near-zero (ENZ) frequency of ITO, where the permittivity is less than 1 in magnitude. Remarkably, we obtain perfect absorption for films whose thickness is as low as ∼1/50th of the operating free-space wavelength and whose single pass attenuation is only ∼5%. We further derive simple analytical conditions for perfect absorption in the subwavelength-film regime that reveal the constraints that the thin layer permittivity must satisfy if perfect absorption is to be achieved. Then, to get a physical insight on the perfect absorption properties, we analyze the eigenmodes of the layered structure by computing both the real-frequency/complex-wavenumber and the complex-frequency/real-wavenumber modal dispersion diagrams. These analyses allow us to attribute the experimental perfect absorption condition to the crossover between bound and leaky behavior of one eigenmode of the layered structure. Both modal methods show that perfect absorption occurs at a frequency slightly larger than the ENZ frequency, in agreement with experimental results, and both methods predict a second perfect absorption condition at higher frequencies, attributed to another crossover between bound and leaky behavior of the same eigenmode. Our results greatly expand the list of materials that can be considered for use as ultrathin perfect absorbers and provide a methodology for the design of absorbing systems at any desired frequency.

The influence of magnetic impurities on the transport properties of graphene is investigated in the regime of strong applied electric fields. As a result of electron-hole pair creation, the response becomes nonlinear and dependent on the magnetic polarization. In the paramagnetic phase, time reversal symmetry is statistically preserved, and transport properties are similar to the clean case. At variance, in the antiferromagnetic phase, the system undergoes a transition between a superdiffusive to a subdiffusive spreading of a wave packet, signaling the development of localized states. This critical regime is characterized by the appearance of electronic states with a multifractal geometry near the gap. The local density of states concentrates in large patches having a definite charge-spin correlation. In this state, the conductivity tends to half the minimum conductivity of clean graphene.