We investigate the thermodynamic behavior of laser-induced spin dynamics of a perfect and a distorted ${\mathrm{Ni}}_4$ square in combination with an external thermal bath, by using the Lindblad-superoperator formalism. The energies of the planar molecules are determined with highly correlated ab initio quantum-chemistry calculations. When the distorted structure couples to the thermal bath a unique spin dynamics, i.e., a spin flip, emerges, due to the interplay of optically and thermally induced electronic transitions. The charge and spin relaxation times in dependence on the coupling strength and the bath temperature are determined and compared.

Gallium (Ga) displays several metastable phases. Superconductivity is strongly enhanced in the metastable $\beta$-Ga with a critical temperature $T_{\mathrm{c}}=6.04\left(5\right)\mathrm{K}$, while stable $\alpha$-Ga has a much lower $T_{\mathrm{c}}<1.2\mathrm{K}$. Here we use a membrane-based nanocalorimeter to initiate the transition from $\alpha$-Ga to $\beta$-Ga on demand, as well as study the specific heat of the two phases on one and the same sample. The in situ transformation is initiated by bringing the temperature to about $10\mathrm{K}$ above the melting temperature of $\alpha$-Ga. After such treatment, the liquid supercools down to $232\mathrm{K}$, where $\beta$-Ga solidifies. We find that $\beta$-Ga is a strong-coupling type-I superconductor with $\mathrm{\Delta }\left(0\right)/k_{\mathrm{B}}{T}_{\mathrm{c}}=2.00\left(5\right)$ and a Sommerfeld coefficient ${\gamma }_{\mathrm{n}}=1.53\left(4\right)\mathrm{mJ}/{\mathrm{molK}}^2$, 2.55 times higher than that in the $\alpha$ phase. The results allow a detailed comparison of fundamental thermodynamic properties between the two phases.

There are two commonly discussed forms of quantum turbulence in superfluid ${}^4\mathrm{He}$ above 1 K: in one there is a random tangle of quantized vortex lines, existing in the presence of a nonturbulent normal fluid; in the second there is a coupled turbulent motion of the two fluids, often exhibiting quasiclassical characteristics on scales larger than the separation between the quantized vortex lines in the superfluid component. The decay of vortex line density, $L$, in the former case is often described by the equation $dL/dt=-{\chi }_2(\kappa /2\pi )L^2$, where $\kappa$ is the quantum of circulation and ${\chi }_2$ is a dimensionless parameter of order unity. The decay of total turbulent energy, $E$, in the second case is often characterized by an effective kinematic viscosity, ${\nu }^{\prime }$, such that $dE/dt=-{\nu }^{\prime }{\kappa }^2{L}^2$. We present values of ${\chi }_2$ derived from numerical simulations and from experiment, which we compare with those derived from a theory developed by Vinen and Niemela. We summarize what is presently known about the values of ${\nu }^{\prime }$ from experiment, and we present a brief introductory discussion of the relationship between ${\chi }_2$ and ${\nu }^{\prime }$, leaving a more detailed discussion to a later paper.

Motivated by the recent developments in fermionic cold atoms and in nanostructured systems, we propose the model of a peristaltic quantum pump. Differently from the Thouless paradigm, a peristaltic pump is a quantum device that generates a particle flux as the effect of a sliding finite-size microlattice. A one-dimensional tight-binding Hamiltonian model of this quantum machine is formulated and analyzed within a lattice Green's function formalism on the Keldysh contour. The pump observables, as, e.g., the pumped particles per cycle, are studied as a function of the pumping frequency, the width of the pumping potential, the particles mean free path, and system temperature. The proposed analysis applies to arbitrary peristaltic potentials acting on fermionic quantum fluids confined to one dimension. These confinement conditions can be realized in nanostructured systems or, in a more controllable way, in cold atoms experiments. In view of the validation of the theoretical results, we describe the outcomes of the model considering a fermionic cold atoms system as a paradigmatic example.

The low-energy excitations of topological Weyl semimetals are composed of linearly dispersing Weyl fermions that act as monopoles of Berry curvature in the bulk momentum space. Furthermore, on the surface there exist topologically protected Fermi arcs at the projections of these Weyl points. We propose a pathway for entropy transport involving Fermi arcs on one surface connecting to Fermi arcs on the other surface via the bulk Weyl monopoles. We present results for the temperature and magnetic field dependence of the magnetothermal conductance of this conveyor belt channel. The circulating currents result in a net entropy transport without any net charge transport. We provide results for the Fermi arc mediated magnetothermal conductivity in the low-field semiclassical limit as well as in the high-field ultraquantum limit, where only chiral Landau levels are involved. Our work provides a proposed signature of Fermi arc mediated magnetothermal transport and sets the stage for utilizing and manipulating the topological Fermi arcs in thermal applications.

Understanding the pseudogap phase in hole-doped high-temperature cuprate superconductors remains a central challenge in condensed-matter physics. From a host of recent experiments there is now compelling evidence of translational-symmetry-breaking charge density wave (CDW) order in a wide range of doping inside this phase. Two distinct types of incommensurate charge order, bidirectional at zero or low magnetic fields and unidirectional at high magnetic fields close to the upper critical field $H_{c2}$, have been reported so far in approximately the same doping range between $p\simeq 0.08$ and $p\simeq 0.16$. In concurrent developments, recent high-field Hall experiments have also revealed two indirect but striking signatures of Fermi surface reconstruction in the pseudogap phase, namely, a sign change of the Hall coefficient to negative values at low temperatures in the intermediate range of hole doping and a rapid suppression of the positive Hall number without a change in sign near optimal doping $p\sim 0.19$. We show that the assumption of a unidirectional incommensurate CDW (with or without a coexisting weak bidirectional order) at high magnetic fields near optimal doping and the coexistence of both types of orders of approximately equal magnitude at high magnetic fields in the intermediate range of doping may help explain the striking behavior of the low-temperature Hall effect in the entire pseudogap phase.

The half-metallic ferromagnetic compound ${\mathrm{Sr}}_2{\mathrm{FeMoO}}_6$ is considered a fundamental material to understand the role of electronic parameters controlling the half-metallic ground state and high Curie temperature in double perovskite. We present an electronic approach using the Green's function technique and the renormalization perturbation expansion method to study the thermodynamical properties of double perovskites. The model is based on a correlated electron picture with localized Fe spins and conduction electrons interacting with the local spins via a double-exchange-type mechanism. Electron correlations within the conduction band are also included in order to study the Curie temperature $T_C$. Our results show an increases of $T_C$ by increasing the carrier density in La-doped ${\mathrm{Sr}}_2{\mathrm{FeMoO}}_6$ compounds in contrast to the case of uncorrelated itinerant electrons.

We study fractional quantum Hall states at filling fractions in the Jain sequences using the framework of composite Dirac fermions. Synthesizing previous work, we write an effective field theory consistent with all symmetry requirements, including Galilean invariance and particle-hole symmetry. Employing a Fermi-liquid description, we demonstrate the appearance of the Girvin-Macdonald-Platzman algebra and compute the dispersion relation of neutral excitations and various response functions. Our results satisfy requirements of particle-hole symmetry. We show that while the dispersion relation obtained from the modified random-phase approximation (MRPA) of the Halperin-Lee-Read (HLR) theory is particle-hole symmetric, correlation functions obtained from this scheme are not. The results of the Dirac theory are shown to be consistent with the Haldane bound on the projected structure factor, while those of the MPRA of the HLR theory violate it.

Structural and emission properties of few-electron In(Ga)P/GaInP quantum dots (QDs) representing natural Wigner molecules (WM) and whispering gallery mode (WGM) electron ($e$) cavities have been investigated. QD structures were grown using self-organized metal-organic vapor phase epitaxy and deposition from $\sim 3$ to 7 monolayers of InP at $700{}^{\circ }\mathrm{C}$. Using atomic force microscopy, transmission electron microscopy, near-field scanning optical microscopy (NSOM), and $\mu$-photoluminescence ($\mu$-PL) spectra we obtained In(Ga)P/GaInP QDs having lateral size 80–180 nm, height 5–30 nm, Ga content 0.0–0.4, density $2-10\mu {\mathrm{m}}^{-2}$, and electron population up to 20 and demonstrated control of their density and size distribution. Using high-spatial-resolution low-temperature PL spectra, NSOM imaging, and calculations of charge density distributions we observed Wigner localization and $e$-cavity effects for a series of dots having quantum confinement $\hslash {\omega }_0=0.5-6\mathrm{meV}$. We used these data together with time-resolved PL measurements to clarify the effect of Coulomb interaction and WM formation on emission spectra of few-electron QDs. We present direct observation of $2e$, $6e$, and $9e$ WMs; $2e$ and $4e$ WGMs; and Fabry-Perot $e$ modes and establish conditions of $e$-WGM-cavity formation in these QDs.

In first-order Raman scattering, helicity of circularly polarized incident light is either conserved or changed depending on the Raman modes. When the helicity of incident light changes in the scattered light, the angular momentum of a photon is transferred to the material. Here, we present the conservation law of pseudoangular momentum in the helicity-dependent Raman scattering for a $N$-fold ($N=1–4,6$) rotational symmetry of a crystal. Furthermore, the conservation law of electron-phonon interaction is discussed by considering the vibration direction of a phonon that has the same or lower symmetry than the symmetry of the crystal, which is essential to allow the helicity change in Raman scattering in a highly symmetric material, such as graphene. We also discuss the conservation law of pseudoangular momentum in Rayleigh scattering and show that the helicity change is allowed only in the crystal with one- or twofold rotational symmetry.

We present a theoretical analysis of dynamic spin injection due to spin-dependent tunneling between a quantum well (QW) and a bound state split in spin projection due to an exchange interaction or external magnetic field. We focus on the impact of Coulomb correlations at the bound state on spin polarization and sheet density kinetics of the charge carriers in the QW. The theoretical approach is based on kinetic equations for the electron occupation numbers taking into account high order correlation functions for the bound state electrons. It is shown that the on-site Coulomb repulsion leads to an enhanced dynamic spin polarization of the electrons in the QW and a delay in the carriers tunneling into the bound state. The interplay of these two effects leads to nontrivial dependence of the spin polarization degree, which can be probed experimentally using time-resolved photoluminescence experiments. It is demonstrated that the influence of the Coulomb interactions can be controlled by adjusting the relaxation rates. These findings open a new way of studying the Hubbard-like electron interactions experimentally.

In this article we propose a system capable of terahertz (THz) radiation with quantum yield above unity. The system consists of nanoparticles where the material composition varies along the radial direction of each nanoparticle in such a way that a ladder of equidistant energy levels emerges. By then exciting the highest level of this ladder we produce multiple photons of the same frequency in the THz range. We demonstrate how we can calculate a continuous material composition profile that achieves a high quantum yield and then show that a more experimentally friendly design of a multishell nanoparticle can still result in a high quantum yield.

This paper is devoted to photocarrier transport across a two-dimensional graphene-semiconductor Schottky junction. We study linear response to monochromatic light with excitation energy well below the semiconductor band gap. The operation mechanism relies on both photoelectric and thermionic emission from graphene to a two-dimensional semiconductor under continuous illumination and zero bias. Due to the thermalization bottleneck for low-energy carriers in graphene, the photoelectric contribution is found to dominate the photoresponse at near-infrared excitation frequencies and below. The extended thermalization time provides an interesting opportunity to facilitate the interlayer photocarrier transport bypassing the thermalization stage. As a result, the total photoresponsivity rapidly increases with excitation wavelength making graphene-semiconductor junctions attractive for photodetection at the telecommunication frequency.

We study the circularly polarized photoluminescence of negatively charged (${\mathrm{NV}}^{-}$) and neutral (${\mathrm{NV}}^0$) nitrogen-vacancy ensembles and neutral vacancies (${\mathrm{V}}^0$) in diamond crystals exposed to magnetic fields of up to 10 T. We determine the orbital and spin Zeeman splitting as well as the energetic ordering of their ground and first-excited states. The spin-triplet and -singlet states of the ${\mathrm{NV}}^{-}$ are described by an orbital Zeeman splitting of about 9 $\mu \mathrm{eV}/\mathrm{T}$, which corresponds to a positive orbital $g$-factor of $g_L=0.164$ under application of the magnetic field along the (001) and (111) crystallographic directions, respectively. The zero-phonon line (ZPL) of the ${\mathrm{NV}}^{-}$ singlet is defined as a transition from the ${}^1{E}^{\prime }$ states, which are split by $g_L{\mu }_{\text{B}}B$, to the ${}^1{A}_1$ state. The energies of the zero-phonon triplet transitions show a quadratic dependence on intermediate magnetic field strengths, which we attribute to a mixing of excited states with nonzero orbital angular momentum. Moreover, we identify slightly different spin Zeeman splittings in the ground (gs) and excited (es) triplet states, which can be expressed by a deviation between their spin $g$-factors: $g_{S,\text{es}}=g_{S,\text{gs}}+\mathrm{\Delta }g$ with values of $\mathrm{\Delta }g=0.014$ and 0.029 in the (001) and (111) geometries, respectively. The degree of circular polarization of the ${\mathrm{NV}}^{-}$ ZPLs depends significantly on the temperature, which is explained by an efficient spin-orbit coupling of the excited states mediated through acoustic phonons. We further demonstrate that the sign of the circular polarization degree is switched under rotation of the diamond crystal. A weak Zeeman splitting similar to $\mathrm{\Delta }g{\mu }_{\text{B}}B$ measured for the ${\mathrm{NV}}^{-}$ ZPLs is also obtained for the ${\mathrm{NV}}^0$ zero-phonon lines, from which we conclude that the ground state is composed of two optically active states with compensated orbital contributions and opposite spin-1/2 momentum projections. The zero-phonon lines of the ${\mathrm{V}}^0$ show Zeeman splittings and degrees of the circular polarization with opposite signs. The magnetophotoluminescence data indicate that the electron transition from the ${}^{1}T_2$ states to the ${}^{1}A$ ground state defines the zero-phonon emission at 1.674 eV, while the ${}^{1}T_2\to {}^{1}E$ transition is responsible for the zero-phonon line at 1.666 eV. The ${}^{1}T_2$ (${}^{1}E$) states are characterized by an orbital Zeeman splitting with $g_L=0.071$ (0.128).

We propose an efficient computational method for evaluating the self-energy matrices of electrodes to study ballistic electron transport properties in nanoscale systems. To reduce the high computational cost incurred in large systems, a contour integral eigensolver based on the Sakurai-Sugiura method combined with the shifted biconjugate gradient method is developed to solve an exponential-type eigenvalue problem for complex wave vectors. A remarkable feature of the proposed algorithm is that the numerical procedure is very similar to that of conventional band structure calculations. We implement the developed method in the framework of the real-space higher-order finite-difference scheme with nonlocal pseudopotentials. Numerical tests for a wide variety of materials validate the robustness, accuracy, and efficiency of the proposed method. As an illustration of the method, we present the electron transport property of the freestanding silicene with the line defect originating from the reversed buckled phases.

We synthesize single crystals of ${\mathrm{PuB}}_4$ using an Al-flux technique. Single-crystal diffraction data provide structural parameters for first-principles density functional theory (DFT) calculations. By computing the density of states, the $Z_2$ topological invariant using the Wilson loop method, and the surface electronic structure from slab calculations, we find that ${\mathrm{PuB}}_4$ is a nonmagnetic strong topological insulator with a band gap of 254 meV. Our magnetic susceptibility, heat capacity, and resistivity measurements are consistent with this analysis, albeit with a smaller gap of 35 meV. DFT plus dynamical mean-field theory calculations show that electronic correlations reduce the size of the band gap, and provide better agreement with the value determined by resistivity. These results demonstrate that ${\mathrm{PuB}}_4$ is a promising actinide material to investigate the interplay of electronic correlations and nontrivial topology.

We analyze the static and dynamical properties of a one-dimensional topological lattice, the fermionic Su-Schrieffer-Heeger model, in the presence of on-site interactions. Based on a study of charge and spin correlation functions, we elucidate the nature of the topological edge modes, which, depending on the sign of the interactions, either display particles of opposite spin on opposite edges, or a pair and a holon. This study of correlation functions also highlights the strong entanglement that exists between the opposite edges of the system. This last feature has remarkable consequences upon subjecting the system to a quench, where an instantaneous edge-to-edge signal appears in the correlation functions characterizing the edge modes. Besides, other correlation functions are shown to propagate in the bulk according to the light cone imposed by the Lieb-Robinson bound. Our study reveals how one-dimensional lattices exhibiting entangled topological edge modes allow for a nontrivial correlation spreading, while providing an accessible platform to detect spin-charge separation using state-of-the-art experimental techniques.