We discuss a platform for the synthetic realization of key physical properties of helical Tomonaga Luttinger liquids (HTLLs) with ultracold fermionic atoms in one-dimensional optical lattices. The HTLL is a strongly correlated metallic state where spin polarization and propagation direction of the itinerant particles are locked to each other. We propose an unconventional one-dimensional Fermi-Hubbard model which, at quarter filling, resembles the HTLL in the long wavelength limit, as we demonstrate with a combination of analytical (bosonization) and numerical (density matrix renormalization group) methods. An experimentally feasible scheme is provided for the realization of this model with ultracold fermionic atoms in optical lattices. Finally, we discuss how the robustness of the HTLL against backscattering and imperfections, well known from its realization at the edge of two-dimensional topological insulators, is reflected in the synthetic one-dimensional scenario proposed here.

We study two-dimensional bosonic symmetry-protected topological (SPT) phases that are protected by reflection symmetry and local symmetry [$Z_N⋊R, Z_N\times R$, U(1)$⋊R$, or U(1)$\times R$], in the search for two-dimensional bosonic analogs of topological crystalline insulators in integer-$S$ spin systems with reflection and spin-rotation symmetries. To classify them, we employ a Chern-Simons approach and examine the stability of edge states against perturbations that preserve the assumed symmetries. We find that SPT phases protected by $Z_N⋊R$ symmetry are classified as ${\mathbb{Z}}_2\times {\mathbb{Z}}_2$ for even $N$ and 0 (no SPT phase) for odd $N$, while those protected by U(1)$⋊R$ symmetry are ${\mathbb{Z}}_2$. We point out that the two-dimensional Affleck-Kennedy-Lieb-Tasaki state of $S=2$ spins on the square lattice is a ${\mathbb{Z}}_2$ SPT phase protected by reflection and $\pi$-rotation symmetries.

The spin-resolved Andreev reflection processes in a hybrid ferromagnet-quantum-dot-superconductor device are theoretically studied. In particular, the transport coefficients, such as the Andreev transmission as well as the linear-response Andreev conductance, are calculated by means of the numerical renormalization group method. It is shown that, generally, transport properties are conditioned by the interplay of correlations leading to the Kondo effect, superconducting proximity effect, and ferromagnetic-contact-induced exchange field. The exchange field is shown to greatly affect the low-energy behavior of the Andreev transmission by splitting the Kondo resonance. Moreover, it leads to a nonmonotonic dependence of the Andreev conductance on the dot level position. At low temperatures, the conductance has a peak at the particle-hole symmetry point, which however becomes quickly suppressed with increasing the temperature. The mechanisms responsible for those effects are thoroughly discussed.

We study the statistical properties of charge and energy transport in electron conducting junctions with electron-phonon interactions, specifically, the thermoelectric efficiency and its fluctuations. The system comprises donor and acceptor electronic states, representing a two-site molecule or a double-quantum-dot system. Electron transfer between metals through the two molecular sites is coupled to a particular vibrational mode which is taken to be either harmonic or anharmonic, a truncated (two-state) spectrum. Considering these models we derive the cumulant generating function in steady state for charge and energy transfer, correct to second order in the electron-phonon interaction, but exact to all orders in the metal-molecule coupling strength. This is achieved by using the nonequilibrium Green's function approach (harmonic mode) and a kinetic quantum master-equation method (anharmonic mode). From the cumulant generating function we calculate the charge current and its noise and the large-deviation function for the thermoelectric efficiency. We demonstrate that at large bias the charge current, differential conductance, and the current noise can identify energetic and structural properties of the junction. We further examine the operation of the junction as a thermoelectric engine and show that while the macroscopic thermoelectric efficiency is indifferent to the nature of the mode (harmonic or anharmonic), efficiency fluctuations do reflect this property.

We consider scattering of surface plasmon polaritons (SPPs) and light by individual high refractive index dielectric nanoparticles (NPs) located on a metal (gold) substrate and supporting electric and magnetic dipole resonances in the visible spectral range. Numerical calculations are carried out by making use of the discrete dipole approximation including the multipole decomposition procedure. Extinction and scattering cross-section spectra of spheroid silicon NPs in visible and near infrared are presented and discussed. The roles of the in-plane and out-of-plane components of electric and magnetic dipoles in the scattering processes are clarified and demonstrated. It is revealed that, owing to the NP interaction with electromagnetic fields reflected from the substrate (that leads to bianisotropy), the in-plane electric and magnetic dipoles can resonantly be excited at the same wavelength. Due to this effect, the resonant unidirectional (forward) and elastic (in-plane) scattering of SPPs by oblate spheroid NPs can be realized within a narrow spectral range. In the case of normal light incidence, the bianisotropy effect can provide significant suppression of the SPP excitation because of the destructive interference between the SPP waves generated by induced electric and magnetic dipole moments. The results obtained open new possibilities for the development of SPP-based photonic components and metasurfaces, whose operation involves resonant excitations of dielectric NPs.

We study laser generation in one-dimensional distributed feedback lasers with amplifying and lossy layers. We show that when the lasing frequency differs from the transition frequencies of the amplifying medium, loss induced lasing may occur due to the broadening of the resonator mode with increasing loss in the absorbing layers. This broadening leads to a shift in the lasing frequency towards the transition frequency. As a result, the cavity mode interaction with the amplifying medium is enhanced, and the lasing threshold is lowered.

We derived a formula to describe the stimulated Brillouin spectral shape in crystals for various temperatures ranging from room temperature to liquid-helium temperature. We modeled a sample as a one-dimensional system with a finite thickness in which the optically induced phonon propagates, partly interacting with the pump and probe laser beams. When the sample length is shorter than the propagation distance (i.e., the mean free path) of phonons, the spectral shape becomes multipeaked due to the multiple phonon reflections in the sample. Such a situation can be realized in a thin film or a bulk sample at low temperatures. We experimentally measured the Brillouin gain spectra with a multipeak structure in ${\mathrm{TeO}}_2$ and ${\mathrm{PbMoO}}_4$ crystals at low temperatures. We found that these spectra were reproduced by our formula for both the coaxial and off-axis phonon propagations with respect to the laser beams. It was revealed that our formula is very useful in estimating the phonon attenuation coefficient from the observed spectra, which gradually change from Lorentzian shape to a multipeak spectrum with decreasing temperature.

The lithium-carbon binary system possesses a broad range of chemical compounds, which exhibit fascinating chemical bonding characteristics, which give rise to diverse and technologically important properties. While lithium carbides with various compositions have been studied or suggested previously, the crystal structures of these compounds are far from well understood. In this work, we present the first comprehensive survey of all ground state (GS) structures of lithium carbides over a broad range of thermodynamic conditions, using ab initio density functional theory (DFT) crystal structure searching methods. Thorough searches were performed for 29 stoichiometries ranging from ${\mathrm{Li}}_{12}\mathrm{C}$ to ${\mathrm{LiC}}_{12}$ at 0 and 40 GPa. Based on formation enthalpies from optimized van der Waals density functional calculations, three thermodynamically stable phases $({\mathrm{Li}}_4{\mathrm{C}}_3$, ${\mathrm{Li}}_2{\mathrm{C}}_2$, and ${\mathrm{LiC}}_{12})$ were identified at 0 GPa, and seven thermodynamically stable phases $({\mathrm{Li}}_8\mathrm{C}$, ${\mathrm{Li}}_6\mathrm{C}$, ${\mathrm{Li}}_4\mathrm{C}$, ${\mathrm{Li}}_8{\mathrm{C}}_3$, ${\mathrm{Li}}_2\mathrm{C}$, ${\mathrm{Li}}_3{\mathrm{C}}_4$, and ${\mathrm{Li}}_2{\mathrm{C}}_3)$ were predicted at 40 GPa. A rich diversity of carbon bonding, including monomers, dimers, trimers, nanoribbons, sheets, and frameworks, was found within these structures, and the dimensionality of carbon connectivity existing within each phase increases with increasing carbon concentration. We find that the well-known composition ${\mathrm{LiC}}_6$ is actually a metastable one. We also find a unique coexistence of carbon monomers and dimers within the predicted thermodynamically stable phase ${\mathrm{Li}}_8{\mathrm{C}}_3$, and different widths of carbon nanoribbons coexist in a metastable phase of ${\mathrm{Li}}_2{\mathrm{C}}_2$ (Imm2). Interesting mixed ${\mathit{sp}}^2\text{−}{\mathit{sp}}^3$ carbon frameworks are predicted in metastable phases with composition ${\mathrm{LiC}}_6$.

Dynamic correlations of quantum magnets provide valuable information. However, especially at finite temperature, many observations, e.g., by inelastic neutron scattering experiments, are not yet quantitatively understood. Generically, the elementary excitations of gapped quantum magnets are hardcore bosons because no two of them can occupy the same site. The previously introduced diagrammatic Brückner approach dealt with hardcore bosons of a single flavor at finite temperature. Here, this approach is extended to hardcore bosons of several kinds (flavors). The approach based on ladder diagrams is gauged with so far unknown rigorous results for the thermal occupation function of multiflavored hardcore bosons in one dimension with nearest-neighbor hopping. For low temperatures, it works very well. Furthermore, we study to which extent the approach is a conserving approximation. Empirical evidence shows that this is true only in the single-flavor case.

Spin Seebeck effects (SSEs) have been investigated in highly crystalline magnetic multilayer ${[{\mathrm{Fe}}_3{\mathrm{O}}_4/\mathrm{Pt}]}_n$ films. Voltage as well as power generated by the SSE were found to be significantly enhanced with increasing the number of layers $n$. This voltage enhancement defies the simple understanding of the SSE and suggests that spin current flowing between the magnetic layers in the thickness direction plays an important role in multilayer SSE systems and the observed voltage enhancement.

The spin diffusion length for thermally excited magnon spins is measured by utilizing a nonlocal spin-Seebeck effect measurement. In a bulk single crystal of yttrium iron garnet (YIG) a focused laser thermally excites magnon spins. The spins diffuse laterally and are sampled using a Pt inverse spin Hall effect detector. Thermal transport modeling and temperature dependent measurements demonstrate the absence of spurious temperature gradients beneath the Pt detector and confirm the nonlocal nature of the experimental geometry. Remarkably, we find that thermally excited magnon spins in YIG travel over 120 µm at 23 K, indicating that they are robust against inelastic scattering. The spin diffusion length is found to be at least 47 µm and as high as 73 µm at 23 K in YIG, while at room temperature it drops to less than 10 µm. Based on this long spin diffusion length, we envision the development of thermally powered spintronic devices based on electrically insulating, but spin conducting materials.

Here we report on the effect of rare-earth Gd doping on the magnetic properties and magnetotransport of GaN two-dimensional electron gasses (2DEGs). Samples are grown by plasma-assisted molecular-beam epitaxy and consist of AlN/GaN heterostructures where Gd is $\delta$ doped within a polarization-induced 2DEG. Ferromagnetism is observed in these Gd-doped 2DEGs with a Curie temperature above room temperature and an anisotropic spontaneous magnetization preferring an out-of-plane ($c$-axis) orientation. At magnetic fields up to 50 kOe, the magnetization remains smaller for the in-plane configuration than for the out-of-plane configuration, which is indicative of exchange-coupled spins locked along the polar $c$ axis. The sample with the lowest Gd concentration ($2.3\times {10}^{14}\mathrm{c}{\mathrm{m}}^{-2}$) exhibits a saturation magnetization of $41.1{\mu }_B/\mathrm{G}{\mathrm{d}}^{3+}$ at 5 K revealing that the Gd ion spins ($7{\mu }_B$) alone do not account for the magnetization. Surprisingly, control samples grown without any Gd display inconsistent magnetic properties; in some control samples weak ferromagnetism is observed, and in others paramagnetism is observed. The ferromagnetic 2DEGs do not exhibit the anomalous Hall effect; the Hall resistance varies nonlinearly with the magnetic field but does not track the magnetization, indicating the lack of coupling between the ferromagnetic phase and the conduction-band electrons within the 2DEG.

The bulk photovoltaic effect requires a low electronic band gap (i.e., $\approx 1$–2 eV) and large electronic polarization, which is not common in known materials. Here we use first-principles calculations to design layered double perovskite oxides ${\mathrm{AA}}^{\prime }{\mathrm{BB}}^{\prime }{\mathrm{O}}_6$ which achieve the aforementioned properties in the context of Mott insulators. In our design rules, the gap is dictated by B/${\mathrm{B}}^{\prime }$ electronegativity difference in a Mott state, while the polarization is obtained via nominal $d_0$ filling on the B-site, A-type cations bearing lone-pair electrons, and $\mathrm{A}\ne {\mathrm{A}}^{\prime }$ size mismatch. Successful execution is demonstrated in ${\mathrm{BaBiCuVO}}_6, {\mathrm{BaBiNiVO}}_6, {\mathrm{BaLaCuVO}}_6$, and ${\mathrm{PbLaCuVO}}_6$.

Despite the importance of Si:P $\delta$-doped wires for modern nanoelectronics, there are currently no computational models of electron transport in these devices. In this paper we present a nonequilibrium Green's function model for electronic transport in a $\delta$-doped wire, which is described by a tight-binding Hamiltonian matrix within a single-band effective-mass approximation. We use this transport model to calculate the current-voltage characteristics of a number of $\delta$-doped wires, achieving good agreement with experiment. To motivate our transport model we have performed density-functional calculations for a variety of $\delta$-doped wires, each with different donor configurations. These calculations also allow us to accurately define the electronic extent of a $\delta$-doped wire, which we find to be at least 4.6 nm.

We present a local representation of the electronic dielectric response function, based on a spatial partition of the dielectric response into contributions from each occupied Wannier orbital using a generalized density functional perturbation theory. This procedure is fully ab initio, and therefore allows us to rigorously define local metrics, such as “bond polarizability,” on Wannier centers. We show that the locality of the bare response function is determined by the locality of three quantities: Wannier functions of the occupied manifold, the density matrix, and the Hamiltonian matrix. In systems with a gap, the bare dielectric response is exponentially localized, which supports the physical picture of the dielectric response function as a collection of interacting local responses that can be captured by a tight-binding model.

We report the observation of low-lying collective charge and spin excitations in the second Landau level at $\nu =2+1/3$ and also for the very fragile states at $\nu =2+2/5$ and $2+3/8$ in inelastic light scattering experiments. These modes exhibit a clear dependence on filling factor and temperature substantiating the unique access to the characteristic neutral excitation spectra of the incompressible fractional quantum Hall effect (FQHE) states. A detailed mode analysis reveals low-energy modes at around 70 $\mu \mathrm{eV}$ and a sharp mode slightly below the Zeeman energy interpreted as gap and spin-wave excitation, respectively. The lowest-energy collective charge excitation spectrum at $\nu =2+1/3$ exhibits significant qualitative similarities with its cousin state in the lowest Landau level at $\nu =1/3$ suggesting similar magnetoroton minima in the neutral excitations. The mode energies differ by a scaling of 0.15 indicating different interaction physics in the $N=0$ and $N=1$ Landau levels. The striking polarization dependence in elastic and inelastic light scattering is discussed in the framework of anisotropic electron phases that allow for the stabilization of nematic FQHE states. The observed excitation spectra provide new insights by accessing quantum phases in the bulk of electron systems and facilitate comparison with different theoretical descriptions of those enigmatic FQHE states.

We consider a question motivated by the third law of thermodynamics: Can there be a local temperature arbitrarily close to absolute zero in a nonequilibrium quantum system? We consider nanoscale quantum conductors with the source reservoir held at finite temperature and the drain held at or near absolute zero, a problem outside the scope of linear response theory. We obtain local temperatures close to absolute zero when electrons originating from the finite temperature reservoir undergo destructive quantum interference. The local temperature is computed by numerically solving a nonlinear system of equations describing equilibration of a scanning thermoelectric probe with the system, and we obtain excellent agreement with analytic results derived using the Sommerfeld expansion. A local entropy for a nonequilibrium quantum system is introduced and used as a metric quantifying the departure from local equilibrium. It is shown that the local entropy of the system tends to zero when the probe temperature tends to zero, consistent with the third law of thermodynamics.

A detailed microscopic and quantitative description of the electronic and magnetic properties of ${\mathrm{Gd}}^{3+}$-doped ${\mathrm{YCo}}_2{\mathrm{Zn}}_{20}$ single crystals $({\mathrm{Y}}_{1-x}{\mathrm{Gd}}_x{\mathrm{Co}}_2{\mathrm{Zn}}_{20},0.002\lesssim x\le 1.00)$ is reported through a combination of temperature-dependent electron spin resonance (ESR), heat capacity, and $dc$ magnetic susceptibility experiments, plus first-principles density functional theory (DFT) calculations. The ESR results indicate that this system features an exchange bottleneck scenario wherein various channels for the spin-lattice relaxation mechanism of the ${\mathrm{Gd}}^{3+}$ ions can be identified via exchange interactions with different types of conduction electrons at the Fermi level. Quantitative support from the other techniques allows us to extract the exchange interaction parameters between the localized magnetic moments of the ${\mathrm{Gd}}^{3+}$ ions and the different types of conduction electrons present at the Fermi level ($J_{fs},J_{fp}$, and $J_{fd}$). Despite the complexity of the crystal structure, our combination of experimental and electronic structure data establish ${\mathrm{GdCo}}_2{\mathrm{Zn}}_{20}$ as a model RKKY system by predicting a Curie-Weiss temperature ${\theta }_C=-1.2\left(2\right)$ K directly from microscopic parameters, in very good agreement with the bulk value from magnetization data. The successful microscopic understanding of the electronic structure and behavior for the two end compounds ${\mathrm{YCo}}_2{\mathrm{Zn}}_{20}$ and ${\mathrm{GdCo}}_2{\mathrm{Zn}}_{20}$ means they can be used as references to help describe the more complex electronic properties of related materials.

We quantify the stability of the formation of multipolar states against always present interchain couplings in quasi-one-dimensional spin-$\frac{1}{2}$ chain systems with a frustrating in-chain $J_1-J_2$ exchange, including parameter regimes that are of direct relevance to many edge-shared cuprate spin-chain compounds. Three representative types of antiferromagnetic interchain coupling and the presence of uniaxial exchange anisotropy are considered. The magnetic phase diagrams are determined by density matrix renormalization group calculations and completed by very accurate analytic and numerical results for the nematic and the dipolar phases employing the hard-core-boson approach. We establish that a sizable interchain coupling has a strong influence on the possible instability of multipolar phases at high magnetic fields in the vicinity of the saturation fields in favor of the usual dipolar one-magnon phase. Moreover, skew interchain couplings strongly affect the pitch of spiral states. Our theoretical results bring to the fore candidate materials close to quantum nematic/triatic ordering.