Inelastic neutron scattering is employed to study transverse spin correlations of a Heisenberg $S=1/2$ chain compound in a magnetic field of 7.5 T. The target compound is the antiferromagnetic Heisenberg $S=1/2$ chain material 2(1,4-dioxane)$·2\left({\mathrm{H}}_2\mathrm{O}\right)·{\mathrm{CuCl}}_2$, or CuDCl for short. The validity and the limitations of the scaling relation for the transverse dynamic structure factor are tested, discussed, and compared to the Tomonaga-Luttinger spin liquid theory and to Bethe-ansatz results for the Heisenberg model.

We experimentally demonstrate that arrays of interacting nanoscale ferromagnetic islands, known as artificial spin ice, develop reproducible microstates upon cycling an applied magnetic field. The onset of this memory effect is determined by the strength of the applied field relative to the array coercivity. Specifically, when the applied field strength is almost exactly equal to the array coercivity, several training cycles are required before the array achieves a nearly completely repeatable microstate, whereas when the applied field strength is stronger or weaker than the array coercivity, a repeatable microstate is achieved after the first minor loop. We show through experiment and simulation that this memory exhibited by artificial spin ice is due to a ratchet effect on interacting, magnetically charged defects in the island moment configuration and to the complexity of the network of strings of reversed moments that forms during magnetization reversal.

We present the first direct-numerical-simulation study of the statistical properties of two-dimensional superfluid turbulence in the simplified, Hall-Vinen-Bekharevich-Khalatnikov two-fluid model. We show that both normal-fluid and superfluid energy spectra can exhibit two power-law regimes, the first associated with an inverse cascade of energy and the second with the forward cascade of enstrophy. We quantify the mutual-friction-induced alignment of normal and superfluid velocities by obtaining probability distribution functions of the angle between them and the ratio of their moduli.

We consider a lattice model of itinerant electrons coupled to an array of localized classical Heisenberg spins. The nature of the ground-state-ordered magnetic phases that result from the indirect spin-spin coupling mediated by the electrons is determined as a function of density and the spin-fermion coupling $J$. At a fixed chemical potential, spiral phases exist only up to values of $J$ which are less than roughly half the electronic bandwidth. At a fixed electron density and near half filling, the system phase-separates into a half-filled antiferromagnetic phase and a spiral phase. The ferromagnetic phases are shown to be fully polarized, while the spiral phases have equal admixture of up and down spins. Phase separation survives in the presence of weak pairing field $\mathrm{\Delta }$ but disappears when $\mathrm{\Delta }$ exceeds a critical value ${\mathrm{\Delta }}_c$. If pairing fields are large enough, an additional spiral state arises at strong coupling $J$. The relevance of this study, especially the phase separation, to artificially engineered systems of adjacent itinerant electrons and localized spins is discussed. In particular, we propose a method which might allow for the braiding of Majorana fermions by changing the density and moving their location as they are pulled along by a phase separation boundary.

We study the scaling of entanglement in low-energy states of quantum many-body models on lattices of arbitrary dimensions. We allow for unbounded Hamiltonians such that systems with bosonic degrees of freedom are included. We show that, if at low enough temperatures the specific heat capacity of the model decays exponentially with inverse temperature, the entanglement in every low-energy state satisfies an area law (with a logarithmic correction). This behavior of the heat capacity is typically observed in gapped systems. Assuming merely that the low-temperature specific heat decays polynomially with temperature, we find a subvolume scaling of entanglement. Our results give experimentally verifiable conditions for area laws, show that they are a generic property of low-energy states of matter, and constitute proof of an area law for unbounded Hamiltonians beyond those that are integrable.

The numerical study of anyonic systems is known to be highly challenging due to their nonbosonic, nonfermionic particle-exchange statistics, and with the exception of certain models for which analytical solutions exist, very little is known about their collective behavior as a result. Meanwhile, the density matrix renormalization group (DMRG) algorithm is an exceptionally powerful numerical technique for calculating the ground state of a low-dimensional lattice Hamiltonian, and has been applied to the study of bosonic, fermionic, and group-symmetric systems. The recent development of a tensor network formulation for anyonic systems opened up the possibility of studying these systems using algorithms such as DMRG, though this has proved challenging in terms of both programming complexity and computational cost. This paper presents the implementation of DMRG for finite anyonic systems, including a detailed scheme for the implementation of anyonic tensors with optimal scaling of computational cost. The anyonic DMRG algorithm is demonstrated by calculating the ground-state energy of the “golden chain,” which has become the benchmark system for the numerical study of anyons and is shown to produce results comparable to those of the anyonic time-evolving block decimation algorithm and superior to the variationally optimized anyonic multiscale renormalization ansatz, at far lesser computational cost.

We study spin-resolved resonant tunneling in a system of two quantum dots sandwiched between doped quantum wells. In the coherent (Dicke) regime, i.e., when quantum dot separation is smaller than the Fermi wavelength in a two-dimensional electron gas in quantum wells, application of an in-plane magnetic field leads to a pronounced spin-resolved structure of conductance peak line shape even for very small Zeeman splitting of the quantum dots' resonant levels. In the presence of electron-gas spin-orbit coupling, this spin-resolved structure is washed out due to Fermi surface deformation in the momentum space. We also show that Aharonov-Bohm flux penetrating the area enclosed by tunneling electron pathways completely destroys the conductance spin structure.

We theoretically examine the role of Kerr nonlinearities for graphene plasmonics in nanostructures, specifically in nanoribbons. The nonlinear Kerr interaction is included semiclassically in the intraband approximation. The resulting electromagnetic problem is solved numerically by self-consistent iteration with linear steps using a real-space discretization. We derive a simple approximation for the resonance shifts in general graphene nanostructures, and obtain excellent agreement with numerics for moderately high field strengths. Near plasmonic resonances the nonlinearities are strongly enhanced due to field enhancement, and the total nonlinearity is significantly affected by the field inhomogeneity of the plasmonic excitation. Finally, we discuss the emergence of a plasmonic bistability which exists for energies red-shifted relative to the linear resonance. Our results offer insights into the role of nonlinear interaction in nanostructured graphene and pave the way for experimental investigation.

Tetracene and pentacene molecules attached to the surface of neon clusters have been spectroscopically investigated. The fluorescence spectra indicate that the molecules are immobilized on the surface and, to a large extent, do not form complexes. By varying the number of attached molecules, laser power, or neon cluster size, we find a systematic fluorescence lifetime reduction up to a factor of 20 indicating a cooperative coupling in our system. For averaged intermolecular distances greater than 33 Å, we attribute the reduction in fluorescence lifetime to Dicke superradiance, while for smaller intermolecular distances, nonradiative decay mechanisms cause additional lifetime reduction.

A simple and efficient approximation scheme to study electronic transport characteristics of strongly correlated nanodevices, molecular junctions, or heterostructures out of equilibrium is provided by steady-state cluster perturbation theory. In this work, we improve the starting point of this perturbative, nonequilibrium Green's function based method. Specifically, we employ an improved unperturbed (so-called reference) state ${\widehat{\rho }}^S$, constructed as the steady state of a quantum master equation within the Born-Markov approximation. This resulting hybrid method inherits beneficial aspects of both the quantum master equation as well as the nonequilibrium Green's function technique. We benchmark this scheme on two experimentally relevant systems in the single-electron transistor regime: an electron-electron interaction based quantum diode and a triple quantum dot ring junction, which both feature negative differential conductance. The results of this method improve significantly with respect to the plain quantum master equation treatment at modest additional computational cost.

We develop a general theory for one-way optical guiding in magnetized periodic particle arrays. Necessary conditions for noneven dispersion curves are derived and presented in the context of Frieze symmetry groups. It is shown, for example, that one-way guiding can be supported in particle strips consisting of geometrically isotropic particles arranged in transversely asymmetric arrays. A specific example is the case of two parallel isotropic particle chains with different periods. The previously studied one-way effect based on the two-type rotation principle is shown to be a special case. In the latter the exclusion of the appropriate Frieze symmetries is achieved in a single linear chain by associating a geometric rotation to each particle, thus providing the narrowest possible one-way waveguides. It is also shown that nearly any randomly created period may result in uneven dispersion and one-way guiding.

The electronic and magnetic properties of epitaxial ${\mathrm{LaNiO}}_3/{\mathrm{LaAlO}}_3$ superlattices can be tuned by layer thickness and substrate-induced strain. Here, we report on direct measurements of the $k$-space-resolved electronic structure of buried nickelate layers in superlattices under compressive strain by soft-x-ray photoemission. After disentangling strong extrinsic contributions to the angle-dependent signal caused by photoelectron diffraction, we are able to extract Fermi surface information from our data. We find that with decreasing ${\mathrm{LaNiO}}_3$ thickness down to two unit cells (2 uc) quasiparticle coherence becomes strongly reduced, in accord with the dimension-induced metal-to-insulator transition seen in transport measurements. Nonetheless, on top of a strongly incoherent background a residual Fermi surface can be identified in the 2 uc superlattice whose nesting properties are consistent with the spin-density-wave (SDW) instability recently reported. The overall behavior of the Ni $3d$ spectra and the absence of a complete gap opening indicate that the SDW phase is dominated by strong order parameter fluctuations.

We theoretically study bound states generated by magnetic impurities within conventional $s$-wave superconductors, both analytically and numerically. In determining the effect of the hybridization of two such bound states on the energy spectrum as a function of magnetic exchange coupling, relative angle of magnetization, and distance between impurities, we find that quantum phase transitions can be modulated by each of these parameters. Accompanying such transitions, there is a change in the preferred spin configuration of the impurities. Although the interaction between the impurity spins is overwhelmingly dominated by the quasiparticle contribution, the ground state of the system is determined by the bound-state energies. Self-consistently calculating the superconducting order parameter, we find a discontinuity when the system undergoes a quantum phase transition as indicated by the bound-state energies.

The mechanism responsible for the extraordinary interface conductivity of ${\mathrm{LaAlO}}_3$ on ${\mathrm{SrTiO}}_3$ and its insulator-metal transition remains controversial. Here, using density functional theory calculations, we establish a comprehensive and coherent picture that the interplay of electronic reconstructions, lattice distortions, and surface oxygen vacancies fully compensates the polarization potential divergence in ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$, explaining naturally the experimental observations under different conditions. While lattice distortions and a charge redistribution between LaO and ${\mathrm{AlO}}_2$ sublayers play a dominant role in the insulating state, a spontaneous appearance of 1/4 oxygen vacancies per ${\mathrm{AlO}}_2$ sublayer at the ${\mathrm{LaAlO}}_3$ surface accompanied by $0.5e^{-}$ charge transfer into the interface is responsible for interface conductivity and the discontinuous transition in ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$. Our model also explains properties of ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ prepared with different growth conditions.

The theoretical formulation developed in Phys. Rev. B 83, 205424 (2011) is applied to explore how surface electronic structure modifies the noncontact friction force between two silver slabs. We find that the friction shows three distinct regimes for different thicknesses. For very thin slabs $N=1–7$ monolayers (ML) quantum friction vs velocity shows oscillations which are due to the quantum size effect. At about $10ML$ friction rapidly increases because the surface state energies fall below $E_F$, which opens a new intraband electron-hole excitation channel. And for $N>15$ the friction is strongly enhanced due to the excitation of acoustic surface plasmons.

Magnetic frustration in three dimensions (3D) manifests itself in the $\text{spin}-\frac{1}{2}$ insulator ${\mathrm{Li}}_2{\mathrm{CuW}}_2{\mathrm{O}}_8$. Density-functional band-structure calculations reveal a peculiar spin lattice built of triangular planes with frustrated interplane couplings. The saturation field of 29 T contrasts with the susceptibility maximum at 8.5 K and a relatively low Néel temperature $T_N\simeq 3.9\mathrm{K}$. Magnetic order below $T_N$ is collinear with the propagation vector $(0,\frac{1}{2},0)$ and an ordered moment of 0.65(4) ${\mu }_B$ according to neutron diffraction data. This reduced ordered moment together with the low maximum of the magnetic specific heat ($C^{max}/R\simeq 0.35$) pinpoint strong magnetic frustration in 3D. Collinear magnetic order suggests that quantum fluctuations play a crucial role in this system, where a noncollinear spiral state would be stabilized classically.

We calculate the collective-mode spectrum in models of superconductors with attractive interactions in $s$ and $d$ channels as a function of their relative strength, across a phase diagram that includes transition between $s$-, ($s+id$)-, and $d$-wave ground states. For one-band systems, we recover the largely known results for the phase, amplitude, and Bardasis-Schrieffer modes of pure $s$ or $d$ states, and show how the well-defined Bardasis-Schrieffer mode softens near the $s+id$ phase boundary and evolves in a characteristic manner through the $s+id$ phase as a mixed-symmetry mode. For two-band systems, we consider a model of hole-doped Fe-based superconductors, and find in the case of an $s$-wave ground state a well-defined Bardasis-Schrieffer mode below the lowest gap edge, as well as a second, damped mode of this type between the two gap energies. Both modes soften as the $s+id$ phase is approached, and only a single “mixed-symmetry Bardasis-Schrieffer mode” below the pair-breaking continuum propagates in the $s+id$ phase itself. These modes coexist with a damped Leggett mode with collective frequency between the two gap scales. In the pure $d$ state, no Bardasis-Schrieffer type $s$ excitonic mode exists at low $T$. We briefly discuss Raman scattering experiments and how they can be used to identify an $s+id$ state, and to track the evolution of competing $s$ and $d$ interactions in these systems.

Electrical resistivity measurements on ${\mathrm{La}}_{1-x}{\mathrm{Sm}}_x{\mathrm{O}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiS}}_2(x=0.1,0.3,0.6,0.8)$ have been performed under applied pressures up to 2.6 GPa from 2 K to room temperature. The superconducting transition temperature $T_c$ of each sample significantly increases at a Sm-concentration-dependent pressure $P_{\mathrm{t}}$, indicating a pressure-induced phase transition from a low-$T_c$ to a high-$T_c$ phase. At ambient pressure, $T_c$ increases dramatically from 2.8 K at $x=0.1$ to 5.4 K at $x=0.8$; however, the $T_c$ values at $P>P_{\mathrm{t}}$ decrease slightly with $x$, and $P_{\mathrm{t}}$ shifts to higher pressures with Sm substitution. In the normal state, semiconducting-like behavior is suppressed, and metallic conduction is induced with increasing pressure in all of the samples. These results suggest that the pressure dependence of $T_c$ for the ${\mathrm{BiS}}_2$-based superconductors is related to the lattice parameters at ambient pressure and enable us to estimate the evolution of $T_c$ for ${\mathrm{SmO}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiS}}_2$ under pressure.

The nature of superconducting fluctuation effects in the isovalently substituted iron pnictide ${\mathrm{BaFe}}_2\left({\mathrm{As}}_{1-x}{\mathrm{P}}_x{)}_2\right(x\approx 0.35)$ is probed through measurements of the magnetization and magnetoconductivity around the superconducting transition. The results, obtained with magnetic fields up to 9 T applied in the two main crystal directions, are consistent with anisotropic Ginzburg-Landau (GL) approaches for finite applied magnetic fields. The analysis allowed us to determine with accuracy the out-of-plane, ${\xi }_c\left(0\right)$, and in-plane, ${\xi }_{ab}\left(0\right)$, GL coherence lengths. Significant differences are found between the ${\xi }_c\left(0\right)$ values resulting from electrical transport and magnetization data. According to recent theoretical approaches, these differences could be interpreted in terms of the multiband nature of this material. The analysis of data in the low-field region around the transition temperature also suggests that phase fluctuations, although possibly relevant in other Fe-based superconductors, may play a negligible role in this compound.

The extraction of exchange parameters from measured spin-wave dispersion relations has severe limitations particularly for magnetic compounds such as the transition-metal perovskites, where the nearest-neighbor exchange parameter usually dominates the couplings between the further-distant-neighbor spins. Very precise exchange parameters beyond the nearest-neighbor spins can be obtained by neutron spectroscopic investigations of the magnetic excitation spectra of isolated multimers in magnetically diluted compounds. This is exemplified for manganese trimers in the mixed three- and two-dimensional perovskite compounds $\mathrm{KM}{\mathrm{n}}_x\mathrm{Z}{\mathrm{n}}_{1-x}{\mathrm{F}}_3$ and ${\mathrm{K}}_2\mathrm{M}{\mathrm{n}}_x\mathrm{Z}{\mathrm{n}}_{1-x}{\mathrm{F}}_4$, respectively. It is shown that the small exchange couplings between the second-nearest-neighbor and the third-nearest-neighbor spins can be determined unambiguously and with equal precision as the dominating nearest-neighbor exchange coupling.