A low-energy fixed-point Hamiltonian is constructed for the $s$-wave odd-frequency pairing state with staggered ordering vector in the two-channel Kondo lattice. The effective model is justified because it reproduces low-energy behaviors of self-energy obtained by the dynamical mean-field theory. The retardation effect is essential for the odd-frequency pairing, which comes from the hybridization process between conduction electrons and pseudofermions originating from localized spins at low energies. Using the effective Hamiltonian, the electromagnetic response functions are microscopically calculated. The present system shows a “weak” Meissner effect, where both paramagnetic and diamagnetic parts contribute to the Meissner kernel to give a small total diamagnetic response in the superconducting state. This feature is in contrast to the ordinary $s$-wave BCS pairing where only the diamagnetic kernel is finite in the ground state. The staggered nature of the odd-frequency order parameter plays an important role for the sign of the Meissner kernel.

Spin-orbit splitting of surface states is analyzed within and beyond the Rashba model using as examples the (111) surfaces of noble metals, ${\mathrm{Ag}}_2\mathrm{Bi}$ surface alloy, and topological insulator ${\mathrm{Bi}}_2{\mathrm{Se}}_3$. The ab initio analysis of relativistic velocity proves the Rashba model to be fundamentally inapplicable to real crystals. The splitting is found to be primarily due to a spin-orbit induced in-plane modification of the wave function, namely, to its effect on the nonrelativistic Hamiltonian. The usual Rashba splitting—given by charge distribution asymmetry—is an order of magnitude smaller.

We propose an electron-phonon parametrization which is constructed to reproduce target geometry and harmonic frequencies taken from first principles calculations or experiment. With respect to standard electron-phonon models, it adds a “double-counting” correction, which takes into account the lattice deformation as the system is dressed by low-energy electron-phonon processes. We show the importance of this correction by studying potassium-doped picene (${\mathrm{K}}_3$ picene), recently claimed to be a superconductor with a $T_c$ of up to 18 K. The Hamiltonian parameters are derived from ab initio density functional theory, and the lattice model is solved by dynamical mean-field theory. Our calculations include the effects of electron-electron interactions and local electron-phonon couplings. Even with the inclusion of a strongly coupled molecular phonon, the Hubbard repulsion prevails and the system is an insulator with a small Mott gap of $\approx 0.2$ eV.

Electron wave functions in twisted bilayer graphene may have a strong single-layer character or be intrinsically delocalized between layers, with their nature often determined by how energetically close they are to the Dirac point. In this paper, we demonstrate that in magnetic fields, optical absorption (cyclotron resonance) spectra contain signatures that may be used to distinguish the nature of these wave functions at low energies, as well as to locate low-energy critical points in the zero-field energy spectrum. Optical absorption for two different configurations—electric field parallel and perpendicular to the bilayer—is calculated, and the configurations are shown to have different selection rules with respect to which states are connected by the perturbation. Interlayer bias further distinguishes transitions involving states of a single-layer nature from those with support in both layers. For doped systems, a sharp increase in intra-Landau-level absorption occurs with increasing field as the level passes through the zero-field saddle-point energy, where the states change character from single layer to bilayer.

Graphene exhibits unique material properties, and in electromagnetic wave technology it raises the prospect of devices miniaturized down to the atomic length scale. Here we study split-ring resonator metamaterials made from graphene and we compare them to gold-based metamaterials. We find that graphene's huge reactive response derived from its large kinetic inductance allows for deeply subwavelength resonances, although its resonance strength is reduced due to higher dissipative loss damping and smaller dipole coupling. Nevertheless, tightly stacked graphene rings may provide for negative permeability and the electric dipole resonance of graphene meta-atoms turns out to be surprisingly strong. Based on these findings, we present a terahertz modulator based on a metamaterial with a multilayer stack of alternating patterned graphene sheets separated by dielectric spacers. Neighboring graphene flakes are biased against each other, resulting in modulation depths of over $75\%$ at a transmission level of around $90\%$.

Transition metal dichalcogenides (TMDCs) have emerged as a new two-dimensional material's field since the monolayer and few-layer limits show different properties when compared to each other and to their respective bulk materials. For example, in some cases when the bulk material is exfoliated down to a monolayer, an indirect-to-direct band gap in the visible range is observed. The number of layers $N$ ($N$ even or odd) drives changes in space-group symmetry that are reflected in the optical properties. The understanding of the space-group symmetry as a function of the number of layers is therefore important for the correct interpretation of the experimental data. Here we present a thorough group theory study of the symmetry aspects relevant to optical and spectroscopic analysis, for the most common polytypes of TMDCs, i.e., $2Ha$, $2Hc$ and $1T$, as a function of the number of layers. Real space symmetries, the group of the wave vectors, the relevance of inversion symmetry, irreducible representations of the vibrational modes, optical activity, and Raman tensors are discussed.

Density functional theory (DFT) based molecular dynamics simulations have been performed of a 1,4-benzenedithiol molecule attached to two gold electrodes. To model the mechanical manipulation in typical break junction and atomic force microscopy experiments, the distance between two electrodes was incrementally increased up to the rupture point. For each pulling distance, the electric conductance was calculated using the DFT nonequilibrium Green's-function approach for a statistically relevant sample of configurations extracted from the simulation. With increasing mechanical strain, the formation of monoatomic gold wires is observed. The conductance decreases by three orders of magnitude as the initial twofold coordination of the thiol sulfur to the gold is reduced to a single S–Au bond at each electrode and the order in the electrodes is destroyed. Independent of the pulling distance, the conductance was found to fluctuate by at least two orders of magnitude depending on the instantaneous junction geometry.

We generalize the thermal pure quantum (TPQ) formulation of statistical mechanics, in such a way that it is applicable to systems whose Hilbert space is infinite dimensional. Assuming particle systems, we construct the grand-canonical TPQ (gTPQ) state, which is the counterpart of the grand-canonical Gibbs state of the ensemble formulation. A single realization of the gTPQ state gives all quantities of statistical-mechanical interest, with exponentially small probability of error. This formulation not only sheds new light on quantum statistical mechanics but also is useful for practical computations. As an illustration, we apply it to the Hubbard model, on a one-dimensional (1D) chain and on a two-dimensional (2D) triangular lattice. For the 1D chain, our results agree well with the exact solutions over wide ranges of temperature, chemical potential, and the on-site interaction. For the 2D triangular lattice, for which exact results are unknown, we obtain reliable results over a wide range of temperature. We also find that finite-size effects are much smaller in the gTPQ state than in the canonical TPQ state. This also shows that in the ensemble formulation the grand-canonical Gibbs state of a finite-size system simulates an infinite system much better than the canonical Gibbs state.

We study superconducting FeSe $(T_{\mathrm{c}}=9\mathrm{K})$ exhibiting the tetragonal-orthorhombic structural transition $(T_{\mathrm{s}}\sim 90\mathrm{K})$ without any antiferromagnetic ordering, by utilizing angle-resolved photoemission spectroscopy. In the detwinned orthorhombic state, the energy position of the $d_{yz}$ orbital band at the Brillouin zone corner is $50\mathrm{meV}$ higher than that of $d_{xz}$, indicating the orbital order similar to the NaFeAs and ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ families. Evidence of orbital order also appears in the hole bands at the Brillouin zone center. Precisely measured temperature dependence using strain-free samples shows that the onset of the orbital ordering ($T_{\mathrm{o}}$) occurs very close to $T_{\mathrm{s}}$, thus suggesting that the electronic nematicity above $T_{\mathrm{s}}$ is considerably weaker in FeSe compared to ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ family.

From density functional theory within the generalized gradient approximation we predict a structure of stanene with dumbbell units (DBs), and show that it is a two-dimensional topological insulator with an inverted band gap which can be tuned by compressive strain. Furthermore, we propose that the boron nitride sheet and reconstructed $\left(2\times 2\right)$ InSb(111) surfaces are ideal substrates for the experimental realization of DB stanene, maintaining its nontrivial topology. Combined with standard semiconductor technologies, such as magnetic doping and electrical gating, the quantum anomalous Hall effect, Chern half metallicity, and topological superconductivity can be realized in DB stanene on those substrates. These properties make the two-dimensional supported stanene a good platform for the study of quantum spin Hall insulators as well as other exotic quantum states of matter.

While noncentrosymmetric ${\mathrm{MoS}}_2$ monolayer is known to exhibit efficient second-harmonic generation (SHG), there is currently no agreement on its absolute nonlinear susceptibility ${\chi }^{\left(2\right)}$, varying over three orders of magnitude according to recent experiments. In order to resolve this conflicting issue, we have studied the nonlinear optical properties of ${\mathrm{MoS}}_2$ monolayer grown by chemical vapor deposition. The polycrystalline nature of the monolayer was directly probed by the SHG polarization dependence across the grain boundaries using femtosecond pulses. Broadband wavelength-dependent SHG response $\left(\lambda =1.1–2.0\mu \mathrm{m}\right)$ using picosecond pulses was studied by comparing the relative SHG counts of ${\mathrm{MoS}}_2$ to quartz and incorporating the structural and optical characteristics of the monolayer. Significant nonlinear optical dispersion gives rise to ${\chi }^{\left(2\right)}\sim 430$ pm/V at 580 nm, where SHG is neither affected by any excitonic absorption/resonance nor by fundamental absorption. We also show that ${\chi }^{\left(2\right)}$ must be derived from a thin bulk (sheet) optical nonlinearity and that the previous measurements are in fact all consistent, together with our measurements and first-principle calculations.

We present a study of spin-1/2 Kondo singlets in single electron transistors under a microwave frequency bias excitation. We compare time-averaged conductance $G$ to predicted universal response with respect to microwave frequency, oscillation amplitude, and the Kondo temperature and find a nonadiabatic response when the microwave photon energy $hf$ is comparable to the Kondo temperature $k_B{T}_K$. We show that our measurements are qualitatively consistent with the predictions for the radiation-induced decoherence rate of the Kondo spin.

We study the effect of Zn alloying on the hydrogen desorption properties of ${\mathrm{Mg}(\mathrm{BH}}_4{)}_2$ using ab initio simulations. In particular, we investigate formation/reaction enthalpies/entropies for a number of compounds and reactions at a wide range of temperatures and Zn concentrations in ${\mathrm{Mg}}_{1-x}{\mathrm{Zn}}_x{(\mathrm{BH}}_4{)}_2$. Our results show that the thermodynamic stability of the resulting material can be significantly lowered through Zn alloying. We find, e.g., that the solid solution ${\mathrm{Mg}}_{2/3}{\mathrm{Zn}}_{1/3}{(\mathrm{BH}}_4{)}_2$ has a reaction enthalpy for the complete hydrogen desorption of only 25.3 kJ/mol ${\mathrm{H}}_2$—a lowering of 15 kJ/mol ${\mathrm{H}}_2$ compared to the pure phase and a corresponding lowering in critical temperature of 123 K. In addition, we find that the enthalpy of mixing is rather small and show that the decrease in reaction enthalpy with Zn concentration is approximately linear.

The anomalous Hall conductivity of “dirty” ferromagnetic metals is dominated by a Berry-phase contribution which is usually interpreted as an intrinsic property of the Bloch electrons in the pristine crystal. In this work we evaluate the geometric Hall current directly from the electronic ground state with disorder and then recast it as an integral over the crystalline Brillouin zone. The integrand is an effective $k$-space Berry curvature, obtained by unfolding the Berry curvature from the small Brillouin zone of a large supercell. Therein, disorder yields a net extrinsic Hall contribution, which we argue is related to the elusive side-jump effect. As an example, we unfold the first-principles Berry curvature of an ordered ${\mathrm{Fe}}_3\mathrm{Co}$ alloy from the original fcc-lattice Brillouin zone onto a bcc-lattice zone with four times the volume. Comparison with the virtual-crystal Berry curvature clearly reveals the symmetry-breaking effects of the substitutional Co atoms.

We introduce a variational algorithm to simulate quantum many-body states based on a tree tensor network ansatz which releases the isometry constraint usually imposed by the real-space renormalization coarse graining. This additional numerical freedom, combined with the loop-free topology of the tree network, allows one to maximally exploit the internal gauge invariance of tensor networks, ultimately leading to a computationally flexible and efficient algorithm able to treat open and periodic boundary conditions on the same footing. We benchmark the novel approach against the 1D Ising model in transverse field with periodic boundary conditions and discuss the strategy to cope with the broken translational invariance generated by the network structure. We then perform investigations on a state-of-the-art problem, namely, the bilinear-biquadratic model in the transition between dimer and ferromagnetic phases. Our results clearly display an exponentially diverging correlation length and thus support the most recent guesses on the peculiarity of the transition.

Ferroelectric field-effect devices based on perovskite oxide materials offer a new possibility to exploit emergent interfacial effects such as the electrostatic modification of the transport and magnetic properties of strongly correlated materials and to prove the magneto-electric coupling at the interface between the two different ferroic materials. Here we report on the reversible modulation of the interfacial magnetic and magnetotransport properties of ${\mathrm{La}}_{0.825}{\mathrm{Sr}}_{0.175}{\mathrm{MnO}}_3$ thin films induced by switching the ferroelectric polarization of a top ${\mathrm{PbZr}}_{0.2}{\mathrm{Ti}}_{0.8}{\mathrm{O}}_3$ layer. Anisotropic magnetoresistance (AMR) measurements were performed applying a magnetic field $H$ in a plane perpendicular to the current density. By rotating $H$ from the out-of-plane towards the in-plane direction, upon the ferroelectric polarization switching, a modulation of the normalized AMR amplitude was achieved. The dynamical electrostatic coupling at the interface of the two oxides is responsible for a reconstruction of the $\mathrm{Mn}3d{e}_g$ orbitals which in turn affects the surface magnetic anisotropy of the magneto-electric system. The present work might have a broader impact, including in the field of multiferroic tunnel junctions, due to a better understanding of the coupling at the interface of the two ferroic oxides where the influence of the polarization on the magnetic degree of freedom is accomplished.

Several metallic ${\mathrm{SrTiO}}_3$-based systems, including reduced ${\mathrm{SrTiO}}_{3\text{−}\delta }$, Nb-doped ${\mathrm{SrTiO}}_3$, and two-dimensional electron gas at the ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3\left(001\right)$ interface, have been epitaxially fabricated on various substrates inducing different strain, from −2.98% (compressive) to +0.99% (tensile). For all the ${\mathrm{SrTiO}}_3$-based systems, strain reduces conductivity. Tensile strain, however, is much more effective at reducing conductivity compared to compressive strain. Further, carrier mobility is found to be more sensitive to strain than carrier density. Calculations based on density functional theory show that strain can break the cubic symmetry of ${\mathrm{TiO}}_6$ octahedron, lift the degeneracy of $\mathrm{Ti}3d$ orbitals, and reduce the number of available states at the bottom of the conduction band to cause low carrier mobility. Our results show the critical features of strain effect on the conducting ${\mathrm{SrTiO}}_3$-based systems, and shed some light on strain engineering of these functional materials.

Coherent electron-spin dynamics in ${\text{Ga}}_{1-x}{\text{Mn}}_x\text{N}$ has been investigated by time-resolved Kerr rotation spectroscopy. The effective electron $g$ factor shows a linear increase with the Mn concentration due to the s-d exchange coupling between the conduction electrons and the $d$-shell electrons of ${\mathrm{Mn}}^{3+}$ impurities. The magnitude and sign of the s-d exchange constant are determined precisely to be $N_0\alpha =+0.23\pm 0.02\mathrm{eV}$, indicative of a ferromagnetic s-d exchange coupling in GaMnN. The determined $N_0\alpha$ is consistent with the typical value found in most diluted magnetic semiconductors and reveals that GaMnN is indeed not an exception.