Organic charge-transfer salts show a variety of complex phases ranging from antiferromagnetic long-range order, spin liquid, bad metal, or even superconductivity. A powerful method to investigate magnetism is spin-polarized inelastic neutron scattering. However, such measurements have often been hindered in the past by the small size of available crystals as well as by the fact that the spin in these materials is distributed over molecular rather than atomic orbitals, and good estimates for the magnetic form factors are missing. By considering Wannier functions obtained from density-functional theory calculations, we derive magnetic form factors for a number of representative organic molecules. Compared to ${\mathrm{Cu}}^{2+}$, the form factors ${\left|F\left(\mathbf{q}\right)\right|}^2$ fall off more rapidly as function of $q$, reflecting the fact that the spin density is very extended in real space. Form factors ${\left|F\left(\mathbf{q}\right)\right|}^2$ for TMTTF, BEDT-TTF, and ${\text{(BEDT-TTF)}}_2$ have anisotropic and nonmonotonic structures.

We study collective excitations of a two-dimensional electron system consisting of two kinds of charge carriers: relativistic or Dirac electrons with linear dispersion and nonrelativistic electrons with parabolic dispersion. We find that the strength of the interaction between the different charge carriers, i.e., the interspecies interaction, plays a significant role in determining the number of plasmon modes as well as their dispersions in certain parameter regimes.

We marry tensor network states (TNS) and projector quantum Monte Carlo (PMC) to overcome the high computational scaling of TNS and the sign problem of PMC. Using TNS as trial wave functions provides a route to systematically improve the sign structure and to eliminate the bias in fixed-node and constrained-path PMC. As a specific example, we describe phaseless auxiliary-field quantum Monte Carlo with matrix product states (MPS-AFQMC). MPS-AFQMC improves significantly on the density-matrix renormalization group (DMRG) ground-state energy. For the $J_1-J_2$ model on two-dimensional square lattices, we observe with MPS-AFQMC an order of magnitude reduction in the error for all couplings, compared to DMRG. The improvement is independent of walker bond dimension, and we therefore use bond dimension 1 for the walkers. The computational cost of MPS-AFQMC is then quadratic in the bond dimension of the trial wave function, which is lower than the cubic scaling of DMRG. The error due to the constrained-path bias is proportional to the variational error of the trial wave function. We show that for the $J_1-J_2$ model on two-dimensional square lattices, a linear extrapolation of the MPS-AFQMC energy with the discarded weight from the DMRG calculation allows us to remove the constrained-path bias. Extensions to other tensor networks are briefly discussed.

Bulk modes (BM) are basis solutions to the Schrödinger equation, and they are useful in a number of physical problems. In the present paper, we establish a complete set of BMs for graphene ribbons at arbitrary energy. We derive analytical expressions for these modes and systematically classify them into propagating or evanescent mode. We also demonstrate their uses in efficient electronic transport simulations of graphene-based electronic devices within both the mode-matching method and the Green's function framework. Explicit constructions of Green's functions for infinite and semi-infinite graphene ribbons are presented.

From high-resolution neutron-diffraction experiments we present equation-of-state (EOS) data of elemental lead as functions of both temperature (80–298 K) and applied pressure (up to 8.9 GPa), with the pressure values derived from the NaCl pressure gauge. Based on Brown's 1999 NaCl EOS we find the bulk modulus of lead of $B_0=\mathrm{41.2(2)}$ GPa at 298 K to increase by $14\%$ to $B_0$ = 47.0(5) GPa by 80 K. The ambient temperature value coincides within 0.35 GPa (1.7%) of the value determined from published ultrasonic data. The good agreement between the neutron and ultrasonic data deteriorates if pressure values are derived from Decker's 1971 scale. Our results hence lend significant support to Brown's revised NaCl pressure standard. Furthermore, the experimental results are compared with $T$ = 0 K first-principles calculations.

We report a spatially resolved femtosecond pump-probe measurement of the lattice dynamics in bismuth made at helium temperature over a wide range of excitation levels. We demonstrate that the collapse and revival of chirped coherent $A_{1g}$ phonons arises due to laterally inhomogeneous excitation conditions, whereas their enhanced decay can be due to potential damping.

We present a systematic study of the magnetic properties of $\mathrm{L}1{}_0$ binary alloys FeNi, CoNi, MnAl, and MnGa via two different density functional theory approaches. Our calculations show large magnetocrystalline anisotropies in the order $1{\mathrm{MJ}/\mathrm{m}}^3$ or higher for CoNi, MnAl, and MnGa, while FeNi shows a somewhat lower value in the range $0.48–0.77{\mathrm{MJ}/\mathrm{m}}^3$. Saturation magnetization values of $1.3\mathrm{MA}/\mathrm{m}$, $1.0\mathrm{MA}/\mathrm{m}$, $0.8\mathrm{MA}/\mathrm{m}$, and $0.9\mathrm{MA}/\mathrm{m}$ are obtained for FeNi, CoNi, MnAl, and MnGa, respectively. Curie temperatures are evaluated via Monte Carlo simulations and show $T_{\mathrm{C}}=916\mathrm{K}$ and $T_{\mathrm{C}}=1130\mathrm{K}$ for FeNi and CoNi, respectively. For Mn-based compounds Mn-rich off-stoichiometric compositions are found to be important for the stability of a ferro- or ferrimagnetic ground state with $T_{\mathrm{C}}$ greater than $600\mathrm{K}$. The effect of substitutional disorder is studied and found to decrease both magnetocrystalline anisotropies and Curie temperatures in FeNi and CoNi.

We explore the potential of single molecules for thermodynamic cycles. To this end we propose two molecular heat engines based on the $\mathrm{Ni}{}_2$ dimer in the presence of a static magnetic field: (a) a quantum Otto engine and (b) a modified quantum Otto engine for which optical excitations induced by a laser pulse substitute for one of the heat-exchange points. For reliable predictions and to inspect the role of spin and electronic correlations we perform fully correlated ab initio calculations of the molecular electronic structure including spin-orbital effects. We analyze the efficiency of the engines in dependence of the electronic level scheme and the entanglement and find a significant possible enhancement connected to the quantum nature and the heat capacity of the dimer, as well as to the zero-field triplet states splitting.

Crystal-field effects in hexagonal closed-packed molecular $\mathrm{H}{}_2$ and $\mathrm{D}{}_2$ are examined over a broad range of pressures. The lattice distortion parameter (the deviation of the $c/a$ ratio from the ideal value 1.633), orientational order parameter, and crystal-field parameter in hexagonal close-packed (hcp) structures of $p\text{−}\mathrm{H}{}_2$, $o\text{−}\mathrm{D}{}_2$, and $n\text{−}\mathrm{H}{}_2$ are calculated using the semiempirical lattice-dynamic approach. It is shown that the lattice distortion in the $J$-even species is two order of magnitude smaller compared with that found in $n\text{−}\mathrm{H}{}_2$ and $n\text{−}\mathrm{D}{}_2$. The difference is due to splitting of the $J$-odd rotational levels in the $J$-even/$J$-odd mixtures.

In recent years, there has been a renewed interest in the proximity effect due to its role in the realization of topological superconductivity. Here, we study a superconductor–normal metal proximity system with repulsive electron-electron interactions in the normal layer. It is known that in the absence of disorder or normal reflection at the superconductor–normal metal interface, a zero-energy bound state forms and is localized to the interface [Fauchère et al., Phys. Rev. Lett. 82, 3336 (1999).]. Using the quasiclassical theory of superconductivity, we investigate the low-energy behavior of the density of states in the presence of finite disorder and an interfacial barrier. We find that as the mean free path is decreased, the zero-energy peak in the density of states is broadened and reduced. In the quasiballistic limit, the bound state eliminates the minigap pertinent to a noninteracting normal layer and a distinct peak is observed. When the mean free path becomes comparable to the normal layer width, the zero-energy peak is strongly suppressed and the minigap begins to develop. In the diffusive limit, the minigap is fully restored and all signatures of the bound state are eliminated. We find that an interfacial potential barrier does not change the functional form of the density of states peak but does shift this peak away from zero energy.

${\mathrm{La}}_{1.8-x}{\mathrm{Eu}}_{0.2}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ (LESCO) is the member of the 214 family which exhibits the largest intervals among the structural, charge ordering (CO), magnetic, and superconducting transition temperatures. By using new dc transport measurements and data in the literature, we construct the phase diagram of LESCO between $x=0.8$ and 0.20. This phase diagram has been further probed in ac, by measuring the optical conductivity ${\sigma }_1\left(\omega \right)$ of three single crystals with $x=0.11$, 0.125, and 0.16 between 10 and 300 K in order to associate the extra-Drude peaks often observed in the 214 family with a given phase. The far-infrared peak we detect in underdoped LESCO is the hardest among them, survives up to room temperature and is associated with charge localization rather than with ordering. At the CO transition for the commensurate doping $x=0.125$ instead the extra-Drude peak hardens and a pseudogap opens in ${\sigma }_1\left(\omega \right)$, approximately as wide as the maximum superconducting gap of LSCO.

We present a comparative study of the optical properties—reflectance, transmission, and optical conductivity—and Raman spectra of two layered bismuth-tellurohalides BiTeBr and BiTeCl at 300 and 5 K, for light polarized in the a-b planes. Despite different space groups, the optical properties of the two compounds are very similar. Both materials are doped semiconductors, with the absorption edge above the optical gap which is lower in BiTeBr (0.62 eV) than in BiTeCl (0.77 eV). The same Rashba splitting is observed in the two materials. A non-Drude free carrier contribution in the optical conductivity, as well as three Raman and two infrared phonon modes, are observed in each compound. There is a dramatic difference in the highest infrared phonon intensity for the two compounds, and a difference in the doping levels. Aspects of the strong electron-phonon interaction are identified. Several interband transitions are assigned, among them the low-lying absorption $\beta$ which has the same value 0.25 eV in both compounds, and is caused by the Rashba spin splitting of the conduction band. An additional weak transition is found in BiTeCl, caused by the lower crystal symmetry.

Electron flow through a quantum point contact in the presence of spin-orbit coupling is investigated theoretically in the context of the scanning gate microscopy (SGM) conductance mapping. Although in the absence of the floating gate the spin-orbit coupling does not significantly alter the conductance, we find that the angular dependence of the SGM images of the electron flow at the conductance plateaus is substantially altered as the spin-orbit interaction mixes the orbital modes that enter the quantum point contact. The radial interference fringes that are obtained in the SGM maps at conductance steps are essentially preserved by the spin-orbit interaction as backscattering by the tip preserves the electron spin although the effects of the mode mixing are visible.

We report measurements consistent with the valley Kondo effect in Si/SiGe quantum dots, evidenced by peaks in the conductance versus source-drain voltage that show strong temperature dependence. The Kondo peaks show unusual behavior in a magnetic field that we interpret as arising from the valley degree of freedom. The interplay of valley and Zeeman splittings is suggested by the presence of side peaks, revealing a zero-field valley splitting between 0.28 to 0.34 meV. A zero-bias conductance peak for nonzero magnetic field, a phenomenon consistent with valley nonconservation in tunneling, is observed in two samples.

We measure direct multiphoton photoemission of the Au(111) surface state with 800-nm laser pulses. We observe the parabolic dispersion in the angular distribution of photoelectrons having absorbed between four and seven photons. The ${\mathbf{k}}_{\parallel }$ dispersion we measure can be explained in terms of Shockley-state replicas, with a nascent hot electrons distribution at ${\mathbf{k}}_{\parallel }$ above the Fermi level. Moderate laser power densities, of the order of $100\mathrm{GW}/\mathrm{cm}{}^2$, resulted in large electron yields, indicating the importance of multiphoton excitations to define the electronic and magnetic properties of matter in the first hundred femtoseconds after laser excitation.

We demonstrate giant magnetoresistance in $\mathrm{Fe}/\mathrm{MoS}{}_2/\mathrm{Fe}$ junctions by means of ab initio transport calculations. We show that junctions incorporating either a monolayer or a bilayer of $\mathrm{MoS}{}_2$ are metallic and that Fe acts as an efficient spin injector into $\mathrm{MoS}{}_2$ with an efficiency of about 45%. This is the result of the strong coupling between the Fe and S atoms at the interface. For junctions of greater thickness, a maximum magnetoresistance of $\sim$300% is obtained, which remains robust with the applied bias as long as transport is in the tunneling limit. A general recipe for improving the magnetoresistance in spin valves incorporating layered transition metal dichalcogenides is proposed.

Imperfections in the crystal structure, such as point defects, can strongly modify the optical and transport properties of materials. Here, we study the effect of point defects on the optical and dc conductivities of single layers of semiconducting transition metal dichalcogenides with the form $M{S}_2$, where $M=\mathrm{Mo}$ or W. The electronic structure is considered within a six band tight-binding model, which accounts for the relevant combination of $d$ orbitals of the metal $M$ and $p$ orbitals of the chalcogen S. We use the Kubo formula for the calculation of the conductivity in samples with different distributions of disorder. We find that $M$ and/or S defects create midgap states that localize charge carriers around the defects and which modify the optical and transport properties of the material, in agreement with recent experiments. Furthermore, our results indicate a much higher mobility for $p$-doped ${\mathrm{WS}}_2$ in comparison to ${\mathrm{MoS}}_2$.