We combined pulsed field magnetization and first-principles spin-density calculations to reveal the magnetic field-temperature phase diagram and spin state character in multiferroic $\left[{\left({\mathrm{CH}}_3\right)}_2{\mathrm{NH}}_2\right]\mathrm{Mn}{\left(\mathrm{HCOO}\right)}_3$. Despite similarities with the rare earth manganites, the phase diagram is analogous to other Mn-based quantum magnets with a 0.31 T spin flop, a 15.3 T transition to the fully polarized state, and short-range correlations that persist above the ordering temperature. The experimentally accessible saturation field opens the door to exploration of the high-field phase.

By carrying out magnetotransport measurements up to 30 T, we succeeded in detecting and analyzing quantitatively Shubnikov–de Haas oscillations in two crystals of the $RE\mathrm{FeAsO}$ ($RE=\mathrm{rare}\mathrm{earth}$) family of iron pnictides, namely, the undoped SmFeAsO and the superconducting fluorine underdoped SmFeAs(O,F). We combine experimental outcomes and ab initio calculations of electronic structure to get the following direct evidences: (i) the Shubnikov–de Haas oscillations are determined by a two-dimensional hole band, whose effective mass is $m^{*}=\left(0.50\pm 0.1\right)m_e$ in the undoped sample and $m^{*}=\left(0.89\pm 0.1\right)m_e$ in the F-doped sample, while classical magnetotransport is dominated by an electron band; (ii) in the superconducting compound we give a quantitative account of the low effectiveness of electron doping associated with fluorine substitution.

Recent experiments show that an atomic step on the surface of atomically thin metallic films can strongly affect electronic transport. Here we reveal multiple and versatile effects that such a surface step can have on superconductivity in ultrathin films. By solving the Bogoliubov-de Gennes equations self-consistently in this regime, where quantum confinement dominates the emergent physics, we show that the electronic structure is profoundly modified on the two sides of the step, as is the spatial distribution of the superconducting order parameter and its dependence on temperature and electronic gating. Furthermore, the surface step changes nontrivially the transport properties both in the proximity-induced superconducting pair correlations and the Josephson effect, depending on the step height. These results offer a new route to tailor superconducting circuits and design atomically thin heterojunctions made of one same material.

The effect of spin-orbit coupling on the electronic properties of monolayer (ML) ${\mathrm{PtSe}}_2$ is dictated by the presence of the crystal inversion symmetry to exhibit a spin-polarized band without the characteristic of spin splitting. Through fully relativistic density-functional theory calculations, we show that large spin-orbit splitting can be induced by introducing point defects. We calculate the stability of native point defects such as a Se vacancy (${\mathrm{V}}_{\text{Se}}$), a Se interstitial (${\mathrm{Se}}_i$), a Pt vacancy (${\mathrm{V}}_{\text{Pt}}$), and a Pt interstitial (${\mathrm{Pt}}_i$) and find that both the ${\mathrm{V}}_{\text{Se}}$ and ${\mathrm{Se}}_i$ have the lowest formation energy. We also find that, in contrast to the ${\mathrm{Se}}_i$ case exhibiting spin degeneracy in the defect states, the large spin-orbit splitting up to 152 meV is observed in the defect states of the ${\mathrm{V}}_{\text{Se}}$. Our analyses of orbital contributions to the defect states show that the large spin splitting is originated from the strong hybridization between Pt-$d_{x{}^2+y{}^2}+d_{xy}$ and Se-$p_x+p_y$ orbitals. Our study clarifies that the defects play an important role in the spin-splitting properties of the ${\mathrm{PtSe}}_2$ ML, which is important for designing future spintronic devices.

The technique of continuous unitary transformations has recently been used to provide physical insight into a diverse array of quantum mechanical systems. However, the question of how to best numerically implement the flow equations has received little attention. The most immediately apparent approach, using standard Runge-Kutta numerical integration algorithms, suffers from both severe inefficiency due to stiffness and the loss of unitarity. After reviewing the formalism of continuous unitary transformations and Wegner's original choice for the infinitesimal generator of the flow, we present a number of approaches to resolving these issues including a choice of generator which induces what we call the “uniform tangent decay flow” and three numerical integrators specifically designed to perform continuous unitary transformations efficiently while preserving the unitarity of flow. We conclude by applying one of the flow algorithms to a simple calculation that visually demonstrates the many-body localization transition.

In nodal-line semimetals, Coulomb interactions and short-range correlated disorder are both marginal perturbations to the clean noninteracting Hamiltonian. We analyze their interplay using a weak-coupling renormalization group approach. In the clean case, the Coulomb interaction has been found to be marginally irrelevant, leading to Fermi liquid behavior. We extend the analysis to incorporate the effects of disorder. The nodal line structure gives rise to kinematical constraints similar to that for a two-dimensional Fermi surface, which plays a crucial role in the one-loop renormalization of the disorder couplings. For a twofold degenerate nodal loop (Weyl loop), we show that disorder flows to strong coupling along a unique fixed trajectory in the space of symmetry inequivalent disorder couplings. Along this fixed trajectory, all symmetry inequivalent disorder strengths become equal. For a fourfold degenerate nodal loop (Dirac loop), disorder also flows to strong coupling, however, the strengths of symmetry inequivalent disorder couplings remain different. We show that feedback from disorder reverses the sign of the beta function for the Coulomb interaction, causing the Coulomb interaction to flow to strong coupling as well. However, the Coulomb interaction flows to strong coupling asymptotically more slowly than disorder. Extrapolating our results to strong coupling, we conjecture that at low energies nodal line semimetals should be described by a noninteracting nonlinear sigma model. We discuss the relation of our results with possible many-body localization at zero temperatures in such materials.

We analyze the low-temperature photoluminescence spectra from two-dimensional electron systems (2DESs) confined at ${\mathrm{Mg}}_x{\mathrm{Zn}}_{1-x}\mathrm{O}$/ZnO heterojunctions as the electron density is decreased from $2.3\times {10}^{12}$ to $3.5\times {10}^{11}$  ${\mathrm{cm}}^{-2}$. The value of the quasiparticle optical density-of-states mass is directly extracted from the width of the 2DES photoluminescence band and is shown to renormalize and double from the value close to that of bulk ZnO material, $0.3m_0$ ($m_0$ is the bare electron mass), to $0.6m_0$ due to electron-electron interactions as the interaction parameter $r_s$ increases from 2.4 to 6.5. The experimentally probed quasiparticle energies far exceed the limits of Landau's Fermi-liquid theory.

The rhenium-based transition metal dichalcogenides (TMDs) are atypical of the TMD family due to their highly anisotropic crystalline structure and are recognized as promising materials for two-dimensional heterostructure devices. The nature of the band gap (direct or indirect) for bulk, few-, and single-layer forms of ${\mathrm{ReS}}_2$ is of particular interest, due to its comparatively weak interplanar interaction. However, the degree of interlayer interaction and the question of whether a transition from indirect to direct gap is observed on reducing thickness (as in other TMDs) are controversial. We present a direct determination of the valence band structure of bulk ${\mathrm{ReS}}_2$ using high-resolution angle-resolved photoemission spectroscopy. We find a clear in-plane anisotropy due to the presence of chains of Re atoms, with a strongly directional effective mass which is larger in the direction orthogonal to the Re chains ($2.2m_e$) than along them ($1.6m_e$). An appreciable interplane interaction results in an experimentally measured difference of $\approx 100-200$ meV between the valence band maxima at the $Z$ point (0,0,$\frac{1}{2}$) and the $\mathrm{\Gamma }$ point (0,0,0) of the three-dimensional Brillouin zone. This leads to a direct gap at $Z$ and a close-lying but larger gap at $\mathrm{\Gamma }$, implying that bulk ${\mathrm{ReS}}_2$ is marginally indirect. This may account for recent conflicting transport and photoluminescence measurements and the resulting uncertainty about the nature of the band gap in this material.

We propose a nanodevice based on a typical planar semiconductor heterostructure with lateral confinement potential created by voltages applied to local gates. We show how to obtain near parabolical confinement along the nanodevice, and how to use coherent states of the harmonic oscillator for spatial separation of electron densities corresponding to opposite spin directions. In such a way, an entangled state of Schrödinger's cat type is created. We have performed simulations of a realistic nanodevice model by numerically solving the time-dependent Schrödinger equation together with simultaneous tracking of the controllable confinement potential via solution of the Poisson's equation at every time step.

Using the tight-binding approach coupled with mean-field Hubbard model, we theoretically study the effect of mechanical deformations on the magnetic properties of bilayer graphene (BLG) quantum dots (QDs). Results are obtained for AA- and AB(Bernal)-stacked BLG QDs, considering different geometries (hexagonal, triangular and square shapes) and edge types (armchair and zigzag edges). In the absence of strain, our results show that (i) the magnetization is affected by taking different dot sizes only for hexagonal BLG QDs with zigzag edges, exhibiting different critical Hubbard interactions, and (ii) the magnetization does not depend on the interlayer hopping energies, except for the geometries with zigzag edges and AA stacking. In the presence of in-plane and uniaxial strain, for all geometries we obtain two different magnetization regimes depending on the applied strain amplitude. The appearance of such different regimes is due to the breaking of layer and sublattice symmetries in BLG QDs.

We present two hybrid solvation models for the calculation of the solvation structure with model 1 in a confined nanospace in bulk materials and model 2 at solid/liquid interfaces where an electrode is in contact with an electrolyte and a membrane is immersed into a solution. The hybrid theory is based on the reference interaction site method (RISM) for the solvent region. The electronic structure of a bulk material, an electrode, and a membrane is treated by density functional theory with the plane-wave basis and pseudopotentials technique. For model 1, we use the three-dimensional RISM (3D-RISM) by imposing a 3D periodic boundary condition on the system. However, for model 2, we reformulate the RISM by means of a two-dimensional boundary condition parallel to the surface and an open boundary condition normal to the surface. Four benchmark calculations are performed for the formaldehyde-water system, water packed into a zeolite framework, a NaCl solution in contact with an Al electrode, and an Al thin film immersed in a NaCl solution with different concentrations. The calculations are shown to be efficient and stable. Because of the flexibility of the RISM theory, the models are considered to be applicable to a wide range of solid/liquid interfaces.

The rigorous finite-temperature QED formalism of the polarization tensor is used to study the combined effect of nonzero mass gap $m$ and chemical potential $\mu$ on the Casimir force and its thermal correction in the experimentally relevant configuration of a Au sphere interacting with a real graphene sheet or with graphene-coated dielectric substrates made of different materials. It is shown that for both a free-standing graphene sheet and for graphene-coated substrates the magnitude of the Casimir force decreases as $m$ is increased, while it increases as $\mu$ is increased, indicating that these parameters act in opposite directions. According to our results, the impact of $m$ and/or $\mu$ on the Casimir force for graphene-coated plates is much smaller than for a free-standing graphene sheet. Furthermore, computations show that the Casimir force is much stronger for graphene-coated substrates than for a free-standing graphene sample, but the thermal correction and its fractional weight in the total force are smaller in the former case. These results are applied to a differential setup that was recently proposed to observe the giant thermal effect in the Casimir force for graphene. We show that this experiment remains feasible even after taking into account the influence of the nonzero mass-gap and chemical potential of real graphene samples. Possible further applications of the obtained results are discussed.

Phosphorene is a bidimensional material that has properties useful for semiconductor devices. In this work we studied the electronic and optical properties of this material using the $GW$ approximation and the Bethe-Salpeter equation (BSE) methods. We stress the importance of a careful convergence study of the most relevant parameters, and we show how they affect the result of the calculations. A comparison with previous results is given. The QP band gap obtained was 2.06 eV and it is in good agreement with experimental results. BSE calculations were performed on top of $G_0{W}_0$ to include excitonic effects. The absorption spectrum was analyzed and an optical gap of 1.22 eV was obtained. The calculated excitonic binding energy is 0.84 eV, also in good agreement with experimental results.

The properties of graphene quantum dots can be manipulated via lateral electric fields. Treating electrons in such structures as confined massless Dirac fermions, we derive an analytical expression for the quadratic Stark shift valid for arbitrary angular momentum and quantum dot size. Moreover, we determine the perturbative regime, beyond which higher-order field effects are observed. The Dirac approach is validated by comparison with atomistic tight-binding simulations. Finally, we study the influence on the Stark effect of band gaps produced by, e.g., interaction with the substrate.

Exploring the properties of strongly correlated systems through quantum simulation with photons, cold atoms, or polaritons represents an active area of research. In fact, the latter sheds light on the behavior of complex systems that are difficult to address in the laboratory or to tackle numerically. In this study, we discuss an analog of graphene formed by exciton-polariton spin vortices arranged into a hexagonal lattice. We show how graphene-type dispersion at different energy scales arises for several types of exciton-polariton spin vortices. In contrast to previous studies of exciton polaritons in artificial lattices, the use of exciton-polariton spin vortex modes offers a richer playground for quantum simulations. In particular, we demonstrate that the sign of the nearest-neighbor coupling strength can be inverted.

In a recent Rapid Communication [A. Stan, Phys. Rev. B 93, 041103(R) (2016)], the reliability of the Keldysh-Kadanoff-Baym equations (KBE) using correlated self-energy approximations applied to linear and nonlinear response has been questioned. In particular, the existence of a universal attractor has been predicted that would drive the dynamics of any correlated system towards an unphysical homogeneous density distribution regardless of the system type, the interaction, and the many-body approximation. Moreover, it was conjectured that even the mean-field dynamics would be damped. Here, by performing accurate solutions of the KBE for situations studied in that paper, we prove these claims wrong, being caused by numerical inaccuracies.

We have investigated the band structure at the $\mathrm{\Gamma }$ point of the three-dimensional topological insulator ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ using magnetospectroscopy over a wide range of energies $(0.55–2.2$ eV) and in ultrahigh magnetic fields up to 150 T. At high energies $(E>0.6$ eV) the parabolic approximation for the massive Dirac fermions breaks down and the Landau-level dispersion becomes nonlinear. At higher energies around 0.99 and 1.6 eV, additional strong absorptions are observed with temperature and magnetic field dependences which suggest that they originate from higher band gaps. Spin-orbit splittings for the further lying conduction and valence bands are found to be 0.196 and 0.264 eV.

At ambient pressure, FeSe undergoes a structural, tetragonal-to-orthorhombic, phase transition at $T_s\simeq 90$ K without any magnetic ordering on further cooling. FeSe thus provides an arena for examining the nematic phase without the complications following the reconstruction of the Fermi surface due to the antiferromagnetic order within the orthorhombic state. We perform an optical-reflectivity investigation across the structural transition, as a function of uniaxial stress in order to detwin the specimen. These measurements reveal a hysteretic behavior of the anisotropic optical response to uniaxial stress for $T\le T_s$, which extends to energy scales of about 0.5 eV. The sign changes of the optical anisotropy between distinct energy intervals suggest a complex evolution of the polarized electronic structure in the nematic phase. The temperature dependence of the optical anisotropy for the fully detwinned specimen is furthermore acting as a proxy for the order parameter of nematicity.