Motivated by the recent report of superconductivity in Fe-based ladder materials, we study the pairing state of a multiorbital $t$-$J$ model defined on two-leg ladders using the standard mean-field theory. We find that the superconducting order parameters change sign between the $d_{xz}$ and $d_{yz}$ orbitals in most of the phase diagram. By analogy with the two-dimensional Fe planes, we conclude that the leading pairing channel of this state belongs to the $B_{1g}$ symmetry class, which is distinct from the common $s_{\pm }$ gap with $A_{1g}$ symmetry. By smoothly interpolating from planes into ladders, we show that a first-order transition occurs between these two competing phases when the dimension of the system is reduced.

At low temperatures, the thermal wavelength of acoustic phonons in a metallic thin film on a substrate can widely exceed the film thickness. It is thus generally believed that a mesoscopic device operating at low temperature does not carry an individual phonon population. In this work, we provide direct experimental evidence for the thermal decoupling of phonons in a mesoscopic quantum device from its substrate phonon heat bath at a sub-Kelvin temperature. A simple heat balance model assuming an independent phonon bath following the usual electron-phonon and Kapitza coupling laws can account for all experimental observations.

The excited-state structure of impurity-trapped excitons are measured in a multisite system. We use a two-color (UV-IR) pulsed photoluminescence enhancement technique, which probes the interlevel transitions and dynamics of impurity-trapped excitons in doped insulating phosphor materials. The technique is applied to NaMgF${}_3$:Yb${}^{2+}$, which exhibits emission from two charge-compensation centers with peaks at 22300 cm${}^{-1}$ (448 nm) and 24000 cm${}^{-1}$ (417 nm). The observed photoluminescence enhancement is caused by a combination of intraexcitonic excitation and electron trap liberation. The electron traps are inferred to have a depth of approximately 800 cm${}^{-1}$.

We examine the magnetic excitation spectrum in the $S=2$ Heisenberg antiferromagnet CrCl${}_2$. Inelastic neutron scattering measurements on powder samples are able to determine the significant exchange interactions in this system. A large anisotropy gap is observed in the spectrum below the Néel temperature and the ratio of the two largest exchange constants is $J_c/J_b=9.1\pm 2.2$. However, no sign of a gapped quantum spin liquid excitation was found in the paramagnetic phase.

We study the nonadiabatic dynamics of a two-dimensional $p+ip$ superfluid following an instantaneous quantum quench of the BCS coupling constant. The model describes a topological superconductor with a nontrivial BCS (trivial BEC) phase appearing at weak- (strong-) coupling strengths. We extract the exact long-time asymptotics of the order parameter $\Delta \left(t\right)$ by exploiting the integrability of the classical $p$-wave Hamiltonian, which we establish via a Lax construction. Three different types of asymptotic behavior can occur depending upon the strength and direction of the interaction quench. We refer to these as the nonequilibrium phases {I, II, III}, characterized as follows. In phase I, the order parameter asymptotes to zero due to dephasing. In phase II, $\Delta \to {\Delta }_{\infty }$, a nonzero constant. Phase III is characterized by persistent oscillations of $\Delta \left(t\right)$. For quenches within phases I and II, we determine the topological character of the asymptotic states. We show that two different formulations of the bulk topological winding number, although equivalent in the BCS or BEC ground states, must be regarded as independent out of equilibrium. The first winding number $Q$ characterizes the Anderson pseudospin texture of the initial state; we show that $Q$ is generically conserved. For $Q\ne 0$, this leads to the prediction of a “gapless topological” state when $\Delta$ asymptotes to zero. The presence or absence of Majorana edge modes in a sample with a boundary is encoded in the second winding number $W$, which is formulated in terms of the retarded Green's function. We establish that $W$ can change following a quench across the quantum critical point. When the order parameter asymptotes to a nonzero constant, the final value of $W$ is well defined and quantized. We discuss the implications for the (dis)appearance of Majorana edge modes. Finally, we show that the parity of zeros in the bulk out-of-equilibrium Cooper-pair distribution function constitutes a ${\mathbb{Z}}_2$-valued quantum number, which is nonzero whenever $W\ne Q$. The pair distribution can in principle be measured using rf spectroscopy in an ultracold-atom realization, allowing direct experimental detection of the ${\mathbb{Z}}_2$ number. This has the following interesting implication: topological information that is experimentally inaccessible in the bulk ground state can be transferred to an observable distribution function when the system is driven far from equilibrium.

The electron momentum density (EMD) in liquid silicon (Si) has been measured by synchrotron-based Compton scattering. The observed variation in the valence EMD upon melting, reflecting a semiconductor-metal transition of Si, is well explained by the collapse of the Jones zone of crystalline Si. However, the shape of the EMD of liquid Si is considerably broad and retains fairly solid ($\alpha$-Si)-like characteristics. The analysis of the Fourier-transformed Compton profiles reveals that the valence electronic state in liquid Si exhibits a marked deviation from the free-electron gas features.

Many low-dimensional materials are well described by integrable one-dimensional models such as the Hubbard model of electrons or the Heisenberg model of spins. However, the small perturbations to these models required to describe real materials are expected to have singular effects on transport quantities: integrable models often support dissipationless transport, while weak nonintegrable terms lead to finite conductivities. We use matrix-product-state methods to obtain quantitative values of spin/electrical and thermal conductivities in an almost integrable gapless ($XXZ$-like) spin chain. At low temperatures, we observe power laws whose exponents are solely determined by the Luttinger liquid parameter. This indicates that our results are independent of the actual model under consideration.

A systematic diagrammatic expansion for Gutzwiller-wave functions (DE-GWF) is formulated and used for the description of superconducting (SC) ground state in the two-dimensional Hubbard model with electron-transfer amplitudes $t$ (and $t^{\prime }$) between nearest (and next-nearest) neighbors. The method is numerically very efficient and allows for a detailed analysis of the phase diagram as a function of all relevant parameters ($U$, $\delta$, $t^{\prime }$) and a determination of the kinetic-energy driven pairing region. SC states appear only for substantial interactions, $U/t\gtrsim 3$, and for not too large hole doping, $\delta \lesssim 0.32$ for $t^{\prime }=0.25t$; this upper critical doping value agrees well with experiment for the cuprate high-temperature superconductors. We also obtain other important features of the SC state: (i) the SC gap at the Fermi surface resembles $d_{x^2-y^2}$-wave only around the optimal doping and the corrections to this state are shown to arise from the longer range of the pairing; (ii) the nodal Fermi velocity is almost constant as a function of doping and agrees quantitatively with the experimental results; (iii) the SC transition is driven by the kinetic-energy lowering for low doping and strong interactions.

We study the thermoelectric effect of two-dimensional metals on a square lattice within semiclassical Boltzmann transport theory with particular focus on electron-electron scattering. We compute the electrical conductivity and the Seebeck coefficient as a function of band filling and temperature for generically chosen hopping parameters in a two-dimensional tight-binding model. The Boltzmann equation is solved numerically after computing the full collision integral, taking the angular and radial degrees of freedom into account. These degrees of freedom of the collision integral, neglected in the standard single-relaxation-time approximation, play an important role if the transport coefficients show unconventional features. Within our detailed numerical simulation, we show that the widely used Mott formula to compute the Seebeck effect is not sufficient to describe the thermoelectric effect in the presence of strong electron-electron scattering. Furthermore, we study the Seebeck coefficient and its temperature dependence in the vicinity of a Lifshitz transition and demonstrate that it shows remarkable parallels to transport features near a quantum critical point.

We present numerical results for the reduction of the lattice thermal conductivity tensor components $\left\{{\kappa }_{\alpha ,\beta }\right\}$ in ultrashort period (Si)${}_n$(Ge)${}_n$[001] superlattices, with $1\le n\le 8$, where $n$ represents the number of atomic bilayers. The calculations are made within the single-mode relaxation-time approximation, accounting for interatomic bond length relaxation and employing phonon dispersion relations obtained from density functional perturbation theory, a model anharmonic Hamiltonian to deal with three-phonon interactions involving acoustic as well as optical phonons in a two-material superlattice structure, and an improved scheme for phonon scattering due to mass smudging at interfaces. The cross-plane component of the conductivity is around 4.1 times smaller than the in-plane component for the $n=8$ case at room temperature and an interface mass mixing (IMS) scattering strength of 0.05. Both the in-plane and cross-plane components of the conductivity decrease sharply with the superlattice period when the strength of the IMS scattering is kept constant. Incorporating physical considerations into the behavior of the IMS scattering, we predict a minimum of the thermal conductivity for $n\approx 4$. We estimate that a small amount of interface mass smudging results in a reduction of around 3%–14% in the $zz$ conductivity component for temperatures of 100 to 700 K when boundary scattering is relatively weak. We estimate relevant phonon scattering rates to explain available experimental conductivity measurements on a system comparable in size to the (8,8) superlattice.

A solid state electronic nanodevice is an intrinsically open quantum system, exchanging both energy with the host material and carriers with connected reservoirs. Its out-of-equilibrium behavior is determined by a nontrivial interplay between electronic dissipation and decoherence induced by inelastic processes within the device, and the coupling of the latter to metallic electrodes. We propose a unified description, based on the density matrix formalism, that accounts for both these aspects, enabling us to predict various steady-state as well as ultrafast nonequilibrium phenomena, nowadays experimentally accessible. More specifically, we derive a generalized density-matrix equation, particularly suitable for the design and optimization of a wide class of electronic and optoelectronic quantum devices. The power and flexibility of this approach is demonstrated with the application to a photoexcited triple-barrier nanodevice.

The microscopic structure of the polar (000$\overline{1}$) and (0001) surfaces of lithium niobate is investigated by atomic-resolution frequency modulation atomic force microscopy and first-principles calculations. It is found that the surface reconstructs at annealing temperatures sufficiently high to drive off external adsorbates. In particular a ($\sqrt{7}\times \sqrt{7}$)$R$19.1${}^{\circ }$ reconstruction is found for the (000$\overline{1}$) surface. Density-functional theory calculations show that—apart from the $(\sqrt{7}\times \sqrt{7})$—a series of adatom-induced surface reconstructions exist that lower the surface energy and reduce the surface charge.

Strong spin-orbit coupling in topological insulators results in the ubiquitously observed weak antilocalization feature in their magnetoresistance. Here we present magnetoresistance measurements in ultrathin films of the topological insulator Bi${}_2$Se${}_3$ and show that in the two-dimensional quantum limit, in which the topological insulator bulk becomes quantized, an additional negative magnetoresistance feature appears. Detailed analysis associates this feature with weak localization of the quantized bulk channels, thus providing evidence for this quantization. Examination of the dephasing fields at different temperatures indicates different scattering mechanism in the bulk vs the surface states.

Cation displacements, oxygen octahedral tilts, and magnetism of epitaxial, ferrimagnetic, insulating GdTiO${}_3$ films sandwiched between cubic SrTiO${}_3$ layers are studied using scanning transmission electron microscopy and magnetization measurements. With decreasing GdTiO${}_3$ film thickness, structural (GdFeO${}_3$-type) distortions are reduced, concomitant with a reduction in the Curie temperature. Ferromagnetism persists to smaller deviations from the cubic perovskite structure than is the case for the bulk rare-earth titanates. The results indicate that the ferromagnetic ground state is controlled by the narrow bandwidth, exchange and orbital ordering, and only to second order depends on the amount of the GdFeO${}_3$-type distortion.

The nature of the Dirac quasiparticles in topological insulators calls for a direct investigation of the electron-phonon scattering at the surface. By comparing time-resolved ARPES measurements of the topological insulator ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ with different probing depths, we show that the relaxation dynamics of the electronic temperature of the conduction band is much slower at the surface than in the bulk. This observation suggests that surface phonons are less effective in cooling the electron gas in the conduction band.

Recent scattering-type scanning near-field optical spectroscopy (s-SNOM) experiments on single-layer graphene have reported Dirac plasmon lifetimes that are substantially shorter than the dc transport scattering time ${\tau }_{\mathrm{tr}}$. We argue that this discrepancy arises from the fact that the plasmon lifetime is fundamentally different from ${\tau }_{\mathrm{tr}}$ since it is controlled by the imaginary part of the current-current linear response function at finite momentum and frequency. Taking this into account, we show that a minimal theory of the extrinsic lifetime of Dirac plasmons due to scattering against charged impurities yields a plasmon damping rate which is in good agreement with s-SNOM experimental results.

A combined analysis of x-ray absorption and resonant reflectivity data was used to obtain the orbital polarization profiles of superlattices composed of four-unit-cell-thick layers of metallic LaNiO${}_3$ and layers of insulating $RX$O${}_3$ ($R=\text{La}$, Gd, Dy and $X=\text{Al}$, Ga, Sc), grown on substrates that impose either compressive or tensile strain. This superlattice geometry allowed us to partly separate the influence of epitaxial strain from interfacial effects controlled by the chemical composition of the insulating blocking layers. Our quantitative analysis reveals orbital polarizations up to 25$\%$. We further show that strain is the most effective control parameter, whereas the influence of the chemical composition of the blocking layers is comparatively small.

A transport-based formulation is used to model three-dimensional objects made of a uniaxial wire medium. The resulting drift-diffusion equation is used to calculate the extinction/scattering cross section of various uniaxial wire medium objects. A comparison is made with measurement and full wave simulation results. This method is faster than full wave simulation and provides physical insight into electrodynamic processes within the homogenized medium. The effect of wire period and thickness on the homogenized model is investigated, and a Debye length parameter is used to access the effect of wire length.

We study the Holstein-Hubbard model at half filling to explore ordered phases including superconductivity (SC), antiferromagnetism (AF), and charge order (CO) in situations where the electron-electron and electron-phonon interactions are strong (comparable to the electronic bandwidth). The model is solved in the dynamical mean-field approximation with a continuous-time quantum Monte Carlo impurity solver. We determine the superconducting transition temperature $T_c$ and the SC order parameter and show that the phonon-induced retardation or the strong Coulomb interaction leads to a significant reduction and shift of the $T_c$ dome against the effective electron-electron interaction $U_{\text{eff}}$ given by the Hubbard $U$ reduced by the phonon-mediated attraction in the static limit. This behavior is analyzed by comparison to an effective static model in the polaron representation with a renormalized bandwidth. In addition, we discuss the superconducting gap $\Delta$ and $2\Delta /T_c$ to reveal the effect of the retardation and the Coulomb interaction. We also determine the finite-temperature phase diagram including AF and CO. In the moderate-coupling regime, there is a hysteretic region of AF and CO around $U_{\mathrm{eff}}=0$, while the two phases are separated by a paramagnetic metal in the weak-coupling regime and a paramagnetic insulator in the strong-coupling regime.

A pseudomorphic monolayer of Au on Ir(001) is easily preparable and has a long lifetime in ultrahigh vacuum. Its geometrical and electronic structure was investigated by ab initio calculations within density functional theory. For a wide range of energies (below 100 eV) and angles of incidence of spin-polarized primary electrons we measured and calculated the intensity and spin asymmetry of the specularly reflected beam. In the resulting energy-angular maps, which show good agreement between experiment and theory, we identify several regions of high asymmetry and sizable intensity. These regions, together with the long lifetime in ultrahigh vacuum, make pseudomorphic Au on Ir(001) very suitable as a two-dimensional spin-polarizing mirror in momentum microscopes and behind dispersive electron energy analyzers.