Congruent lithium niobate and lithium tantalate mixed crystals have been grown over the complete compositional range with the Czochralski method. The structural and vibrational properties of the mixed crystals are studied extensively by x-ray diffraction measurements, Raman spectroscopy, and density functional theory. The measured lattice parameters and vibrational frequencies are in good agreement with our theoretical predictions. The observed dependence of the Raman frequencies on the crystal composition is discussed on the basis of the calculated phonon displacement patterns. The phononic contribution to the static dielectric tensor is calculated by means of the generalized Lyddane-Sachs-Teller relation. Due to the pronounced dependence of the optical response on the Ta concentration, lithium niobate tantalate mixed crystals represent a perfect model system to study the properties of uniaxial mixed ferroelectric materials for application in integrated optics.

We report a study on the complete spin-wave spectrum inside a vortex-state nanodisk. Transformation of this spectrum is continuously monitored as the nanodisk becomes gradually magnetized by a perpendicular magnetic field and encounters a second-order phase transition to the uniformly magnetized state. This reveals the bijective relationship that exists between the eigenmodes in the vortex state and the ones in the saturated state. It is found that the gyrotropic mode can be continuously viewed as a uniform phase precession, which uniquely softens (its frequency vanishes) at the saturation field to transform above into the Kittel mode. By contrast, the other spin-wave modes remain finite as a function of the applied field, while their character is altered by level anticrossing.

The isotope effect in superconductivity (SC) is used to make a concrete connection to a quantum critical point (QCP) that is tunable by isotopic mass substitution. We find a distinct contribution to the isotope exponent in SC and derive an explicit relation to the critical exponent of a QCP. The relation between the two exponents is general and can be used as an experimental signature for the connection between SC and a QCP. We demonstrate it in a scenario where the SC pairing is due to modes related to a structural instability. Within this model the isotope exponent is derived in terms of microscopic parameters.

We perform a numerical simulation of the dynamics of quantized vortices produced by coflow in a square channel using the vortex filament model. Unlike the situation in thermal counterflow, where the superfluid velocity ${\mathit{v}}_s$ and normal-fluid velocity ${\mathit{v}}_n$ flow in opposite directions, in coflow, ${\mathit{v}}_s$ and ${\mathit{v}}_n$ flow in the same direction. Quantum turbulence in thermal counterflow has been long studied theoretically and experimentally, and its various features have been revealed. In recent years, an experiment on quantum turbulence in coflow has been performed to observe different features of thermal counterflow. By supposing that ${\mathit{v}}_s$ is uniform and ${\mathit{v}}_n$ takes the Hagen-Poiseuille profile, which is different from the experiment where ${\mathit{v}}_n$ is thought to be turbulent, we calculate the coflow turbulence. Vortices preferentially accumulate on the surface of a cylinder for ${\mathit{v}}_s\simeq {\mathit{v}}_n$ by mutual friction; namely, the coflow turbulence has an attractor. How strongly the vortices are attracted depends on the temperature and velocity. The length of the vortices increases as the vortices protruding from the cylindrical attractor continue to wrap around it. As the vortices become dense on the attractor, they spread toward its interior by their repulsive interaction. Then, the superfluid velocity profile induced by the vortices gradually mimics the normal-fluid velocity profile. This is an indication of velocity matching, which is an important feature of coflow turbulence.

Perpendicular-to-the-plane magnetic field tuned reentrant superconductivity in out-of-equilibrium, quasi-one-dimensional (quasi-1D) planar nanowires is a novel, counterintuitive phenomenon. It was not until recently that a microscopic mechanism explaining the phenomenon as arising from the coexistence of superconductivity with phase-slip driven dissipation was developed. Here we present results on reentrance phenomena in quasi-1D aluminum nanowires with in-plane magnetic fields, transverse and longitudinal to the nanowire axis. The response to in-plane transverse magnetic fields in this geometry is qualitatively different from that previously reported for perpendicular-to-the-plane field experiments and for in-plane longitudinal field studies. The different feature in the data is an abrupt return to the superconducting state with increasing field at values of field corresponding to a single flux quantum for a short wire and a fractional flux quantum for a long wire. Since these findings are dramatically different from those involving perpendicular-to-the-plane magnetic fields, a different mechanism, as yet unidentified, may be at work.

A numerical method to calculate optical conductivity based on a pump-probe setup is presented. Its validity and limits are tested and demonstrated via concrete numerical simulations on the half-filled one-dimensional extended Hubbard model both in and out of equilibrium. By employing either a steplike or a Gaussian-like probing vector potential, it is found that in nonequilibrium, the method in the narrow-probe-pulse limit can be identified with variant types of linear-response theory, which, in equilibrium, produce identical results. The observation reveals the underlying probe-pulse dependence of the optical conductivity calculations in nonequilibrium, which may have applications in the theoretical analysis of ultrafast spectroscopy measurements.

Anisotropic dipole-dipole interactions between ultracold dipolar fermions break the symmetry of the Fermi surface and thereby deform it. Here we demonstrate that such a Fermi surface deformation induces a topological phase transition—the so-called Lifshitz transition—in the regime accessible to present-day experiments. We describe the impact of the Lifshitz transition on observable quantities such as the Fermi surface topology, the density-density correlation function, and the excitation spectrum of the system. The Lifshitz transition in ultracold atoms can be controlled by tuning the dipole orientation and, in contrast to the transition studied in crystalline solids, is completely interaction driven.

Spontaneous symmetry breakings, metal-insulator transitions, and transport properties of magnetic-domain-wall states in pyrochlore iridium oxides are studied by employing a symmetry-adapted effective Hamiltonian with a slab perpendicular to the (111) direction of the pyrochlore structure. Emergent metallic domain wall, which has an unconventional topological nature with a controllable and mobile metallic layer, is shown to host Fermi surfaces with modulated helical spin textures resembling Rashba metals. The helical nature of the domain-wall Fermi surfaces is experimentally detectable by anomalous Hall conductivity, circular dichroism, and optical Hall conductivity under external magnetic fields. Possible applications of the domain-wall metals to spin-current generation and “half-metallic” conduction are also discussed.

We study the behavior of magnetic superconductors which involve a local attractive interaction between electrons, and a coupling between local moments and the electrons. We solve this Hubbard-Kondo model through a variational minimization at zero temperature and validate the results via a Monte Carlo based on static auxiliary field decomposition of the Hubbard interaction. Over a magnetic coupling window that widens with increasing attractive interaction, the ground state supports simultaneous magnetic and superconducting order. The pairing amplitude remains $s$-wave like, without significant spatial modulation, while the magnetic phase evolves from a ferromagnet, through noncollinear “spiral” states, to a Néel state with increasing density and magnetic coupling. We find that at intermediate magnetic coupling, the antiferromagnetic-superconducting state is gapless, except for the regime of Néel order. We map out the phase diagram in terms of density, magnetic coupling, and attractive interaction, establish the electron dispersion and effective “Fermi surface” in the ground state, provide an estimate of the magnetic and superconducting temperature scales via Monte Carlo, and compare our results to available data on the borocarbides.

The electronic and magnetic properties of the hydride superconductor CaFeAsH, which superconducts up to 47 K when electron doped with La, and the isovalent alloy system ${\mathrm{CaFeAsH}}_{1-x}{\mathrm{F}}_x$ are investigated using density functional based methods. The $\vec{Q}=(\pi ,\pi ,0)$ peak of the nesting function $\xi \left(\vec{q}\right)$ is found to be extremely strong and sharp, and the additional structure in $\xi \left(\vec{q}\right)$ associated with the near-circular Fermi surfaces (FSs) that may impact low energy excitations is quantified. The unusual band introduced by H, which shows strong dispersion perpendicular to the FeAs layers, is shown to be connected to a peculiar van Hove singularity just below the Fermi level. This band provides a three-dimensional electron ellipsoid Fermi surface not present in other Fe-based superconducting materials nor in CaFeAsF. Electron doping by 25% La or Co has a minor effect on this ellipsoid Fermi surface, but suppresses FS nesting strongly, consistent with the viewpoint that eliminating strong nesting and the associated magnetic order allows high $T_c$ superconductivity to emerge. Various aspects of the isovalent alloy system ${\mathrm{CaFeAsH}}_{1-x}{\mathrm{F}}_x$ and means of electron doping are discussed in terms of influence of incipient bands.

The trends in electronic band structure are studied in the cubic $AB{X}_3$ halide perovskites for $A=\text{Cs}$; $B=\text{Pb}$, Sn, Ge, Si; and $X=\text{I}$, Br, Cl. The gaps are found to decrease from Pb to Sn and from Ge to Si, but increase from Sn to Ge. The trend is explained in terms of the atom $s$ levels of the group-IV element and the atomic sizes which changes the amount of hybridization with $X\text{−}p$ and hence the valence bandwidth. Along the same series spin-orbit coupling also decreases and this tends to increase the gap because of the smaller splitting of the conduction band minimum. Both effects compensate each other to a certain degree. The trend with halogens is to reduce the gap from Cl to I, i.e., with decreasing electronegativity. The role of the tolerance factor in avoiding octahedron rotations and octahedron edge sharing is discussed. The Ge containing compounds have tolerance factor $t>1$ and hence do not show the series of octahedral rotation distortions and the existence of edge-sharing octahedral phases known for Pb and Sn-based compounds, but rather a rhombohedral distortion. ${\mathrm{CsGeI}}_3$ is found to have a suitable gap for photovoltaics both in its cubic (high-temperature) and rhombohedral (low-temperature) phases. The structural stability of the materials in the different phases is also discussed. We find the rhombohedral phase to have lower total energy and slightly larger gaps but to present a less significant distortion of the band structure than the edge-sharing octahedral phases, such as the yellow phase in ${\mathrm{CsSnI}}_3$. The corresponding silicon based compounds have not yet been synthesized and therefore our estimates are less certain but indicate a small gap for cubic ${\mathrm{CsSiI}}_3$ and ${\mathrm{CsSiBr}}_3$ of about $0.2\pm 0.2$ eV and $0.8\pm 0.6$ eV for ${\mathrm{CsSiCl}}_3$. The intrinsic stability of the Si compounds is discussed.

The exchange-correlation potential experienced by an electron in the free space adjacent to a solid surface or to a low-dimensional system defines the fundamental image states and is generally important in surface and nanoscience. Here we determine the potential near the two- and one-dimensional electron gases (EG), doing this analytically at the level of the exact exchange of the density-functional theory (DFT). We find that, at $r_{\perp }\gg k_F^{-1}$, where $r_{\perp }$ is the distance from the EG and $k_F$ is the Fermi radius, the potential obeys the already known asymptotic $-e^2/r_{\perp }$, while at $r_{\perp }\lesssim k_F^{-1}$, but still in vacuum, qualitative and quantitative deviations of the exchange potential from the asymptotic law occur. The playground of the excitations to the low-lying image states falls into the latter regime, causing significant departure from the Rydberg series. In general, our analytical exchange potentials establish benchmarks for numerical approaches in the low-dimensional science, where DFT is by far the most common tool.

In a recent paper by S. Moroz, C. Hoyos, and L. Radzihovsky [Phys. Rev. B 91, 195409 (2015)], it is claimed that the conductivity at low frequency $\omega$ and small wave vector $q$ along the edge of a quantum Hall system (that possesses Galilean invariance along the edge) contains a universal contribution of order $q^2$ that is determined by the orbital spin per particle in the bulk of the system, or alternatively by the shift of the ground state. (These quantities are known to be related to the Hall viscosity of the bulk.) In this Comment we calculate the real part of the conductivity, integrated over $\omega$, in this regime for the edge of a system of noninteracting electrons filling either the lowest, or the lowest $\nu (\nu =1,2,...)$, Landau level(s), and show that the $q^2$ term is nonuniversal and depends on details of the confining potential at the edge. In the special case of a linear potential, a form similar to the prediction is obtained; it is possible that this corrected form of the prediction may also hold for fractional quantum Hall states in systems with special forms of interactions between electrons.

The nature of the fractional quantum Hall state at Landau level filling factor 5/2 remains elusive despite intensive experimental and theoretical work. While the leading theoretical candidates are Moore-Read Pfaffian (Pf) and its particle-hole conjugate anti-Pfaffian (APf), neither received unambiguous experimental support. We show that a state that is intermediate between them, made of alternating stripes of Pf and APf in the bulk, is a viable candidate. Such a state is shown to be incompressible and thus a charge insulator in the bulk, but a heat conductor due to the presence of gapless neutral bulk modes. We argue that the properties of such a state are consistent with existing numerical and experimental work, and discuss possible experimental probes of its presence.

By decreasing the transversal confinement potential in interacting one-dimensional spinless electrons and populating the second energetically lowest subband for not too strong interactions system transitions into a quasi-one-dimensional state with dominant superconducting correlations and one gapless mode. By combining effective field theory approach and numerical density matrix renormalization group simulations, we show that this quasi-one-dimensional state is a topological state that hosts zero-energy edge modes. We also study the single-particle correlations across the interface between these quasi-one-dimensional and single-channel states.

We study a simple model of three-dimensional fermions close to a quadratic band touching point, built from the celebrated Luttinger single-particle Hamiltonian and an attractive contact interaction between the particles. Such a system displays a quantum critical point between the semimetallic and an $s$-wave superconducting phase at which the low-energy “Luttinger fermions” are inextricably coupled to the order parameter fluctuations. The quantum critical point is perturbatively accessible near four spatial dimensions, where it features nontrivial scaling with dynamical exponent $1<z<2$ and emergent rotational and particle-hole symmetries. Some features of the criticality, such as oscillatory corrections to scaling and its enhanced symmetry, are discussed.

The adiabatic connection formula of ground-state density functional theory relates the correlation energy to a coupling-constant integral over a purely potential contribution, and is widely used to understand and improve approximations. The corresponding formula for thermal density functional theory is cast as an integral over temperatures instead, ranging upward from the system's physical temperature. We also show how to relate different correlation components to each other, either in terms of temperature or coupling-constant integrations. We illustrate our results on the uniform electron gas.

We report the synthesis, crystal structure, and characterization by means of single-crystal x-ray diffraction, neutron powder diffraction, and magnetic, thermal, and transport measurements of the new heavy-fermion compounds ${\mathrm{Ce}}_2{M\mathrm{Al}}_7{\mathrm{Ge}}_4$ $(M=\text{Co},\text{Ir},\text{Ni},\text{Pd})$. These compounds crystallize in a noncentrosymmetric tetragonal space group $\mathit{P}\overline{4}{2}_{1}m$, consisting of layers of square nets of Ce atoms separated by Ge-Al and M-Al-Ge blocks. ${\mathrm{Ce}}_2{\mathrm{CoAl}}_7{\mathrm{Ge}}_4,{\mathrm{Ce}}_2{\mathrm{IrAl}}_7{\mathrm{Ge}}_4$, and ${\mathrm{Ce}}_2{\mathrm{NiAl}}_7{\mathrm{Ge}}_4$ order magnetically below $T_M=1.8$, 1.6, and 0.8 K, respectively. There is no evidence of magnetic ordering in ${\mathrm{Ce}}_2{\mathrm{PdAl}}_7{\mathrm{Ge}}_4$ down to 0.4 K. The small amount of entropy released in the magnetic state of ${\mathrm{Ce}}_{2}M{\mathrm{Al}}_7{\mathrm{Ge}}_4$ $(M$ = Co, Ir, Ni) and the reduced specific heat jump at $T_M$ suggest a strong Kondo interaction in these materials. ${\mathrm{Ce}}_2{\mathrm{PdAl}}_7{\mathrm{Ge}}_4$ shows non-Fermi liquid behavior, possibly due to the presence of a nearby quantum critical point.

The detectability lemma is a useful tool for probing the structure of gapped ground states of frustration-free Hamiltonians of lattice spin models. The lemma provides an estimate on the error incurred by approximating the ground space projector with a product of local projectors. We provide a simpler proof for the detectability lemma which applies to an arbitrary ordering of the local projectors, and show that it is tight up to a constant factor. As an application, we show how the lemma can be combined with a strong converse by Gao to obtain local spectral gap amplification: We show that by coarse graining a local frustration-free Hamiltonian with a spectral gap $\gamma >0$ to a length scale $O\left({\gamma }^{-1/2}\right)$, one gets a Hamiltonian with an $\mathrm{\Omega }\left(1\right)$ spectral gap.

Understanding how the complex intermolecular configurations and nanostructure present in organic semiconductor donor-acceptor blends impacts charge carrier motion, interactions, and recombination behavior is a critical fundamental issue with a particularly major impact on organic photovoltaic applications. In this study, kinetic Monte Carlo (KMC) simulations are used to numerically quantify the complex bimolecular charge carrier recombination behavior in idealized phase-separated blends. Recent KMC simulations have identified how the encounter-limited bimolecular recombination rate in these blends deviates from the often used Langevin model and have been used to construct the new power mean mobility model. Here, we make a challenging but crucial expansion to this work by determining the charge carrier concentration dependence of the encounter-limited bimolecular recombination coefficient. In doing so, we find that an accurate treatment of the long-range electrostatic interactions between charge carriers is critical, and we further argue that many previous KMC simulation studies have used a Coulomb cutoff radius that is too small, which causes a significant overestimation of the recombination rate. To shed more light on this issue, we determine the minimum cutoff radius required to reach an accuracy of less than $\pm 10\%$ as a function of the domain size and the charge carrier concentration and then use this knowledge to accurately quantify the charge carrier concentration dependence of the recombination rate. Using these rigorous methods, we finally show that the parameters of the power mean mobility model are determined by a newly identified dimensionless ratio of the domain size to the average charge carrier separation distance.