We theoretically investigate the effect of intermolecular Coulomb interactions on transport through self-assembled molecular monolayers (or other devices based on a large number of nanoscale conductors connected in parallel). Due to the interactions, the current through different molecules become correlated, resulting in distinct features in the nonlinear current-voltage characteristics, as we show by deriving and solving a type of modified master equation, suitable for describing transport through an infinite number of interacting conductors. Furthermore, if some of the molecules fail to bond to both electrodes, charge traps can be induced at high voltages and block transport through neighboring molecules, resulting in negative differential resistance.

We propose a strained graphene double-layer (SGDL) system for detecting pseudomagnetoexcitons (PME) in the absence of external magnetic fields. The carriers in each graphene layer experience different strong pseudomagnetic fields (PMFs) due to strain engineering, which give rise to Landau quantization. The pseudo-Landau levels of electron-hole pairs under inhomogeneous PMFs in the SGDL are obtained analytically in the absence of Coulomb interactions. Based on the derived optical absorption selection rule for PMEs, we interpret the optical absorption spectra as indicating the formation of Dirac-type PMEs. We also predict that in the presence of inhomogeneous PMFs, the superfluidity-normal phase-transition temperature of PMEs is greater than that under homogeneous PMFs.

The $H-T$ phase diagrams of single crystalline electron-doped ${\text{K}}_{0.83}{\text{Fe}}_{1.83}{\text{Se}}_2$ (KFS1), ${\text{K}}_{0.8}{\text{Fe}}_2{\text{Se}}_2$ (KFS2) and hole-doped ${\text{Eu}}_{0.5}{\text{K}}_{0.5}{\text{Fe}}_2{\text{As}}_2$ (EKFA) have been deduced from tunnel diode oscillator-based contactless measurements in pulsed magnetic fields up to $57\text{T}$ for the interplane ($H\parallel c$) and in-plane ($H\parallel ab$) directions. The temperature dependence of the upper critical magnetic field $H_{c2}\left(T\right)$ relevant to EFKA is accounted for by Pauli model including an anisotropic Pauli paramagnetic contribution (${\mu }_B{H}_p=114\text{T}$ for $H\parallel ab$ and $86\text{T}$ for $H\parallel c$). This is also the case of KFS1 and KFS2 for $H\parallel ab$ whereas a significant upward curvature, accounted for by a two-gap model, is observed for $H\parallel c$. Despite the presence of antiferromagnetic lattice order within the superconducting state of the studied compounds, no influence of magnetic ordering on the temperature dependence of $H_{c2}\left(T\right)$ is observed.

Using a simple model of multiband superconductors, which can be applied in particular to Fe-based pnictides, we calculate the Josephson current $I_{\text{J}}$ in a tunnel junction composed by such superconductors. We employ the tunneling Hamiltonian method and quasiclassical Green's functions. We study both the case of coexistence of the superconducting ($\Delta$) and magnetic (SDW—spin density wave) order parameters and the case when only the superconducting order parameter exists. We show that the current $I_{\text{J}}$ depends on the mutual orientation of magnetization of the SDW in the case of nonideal nesting when the coexistence of superconducting and magnetic order parameters is possible as it takes place in Fe-based pnictides. It is found that the realization of the $\pi$ junction is possible in both cases. We compare our results for multiband superconductors without the SDW with those obtained earlier and find that they coincide if the tunneling matrix elements are real. If these elements are complex, a new term appears in the formula for the Josephson critical current.

We present a combined Raman spectroscopy and density functional perturbation theory (DFPT) study of phonon variation with composition $x$ in the mixed crystals SnS${}_x$Se${}_{2-x}$. The experimentally observed two-mode behavior of the $A_{1g}$ and $E_g$ vibrations involving Se(S) atoms is shown to arise from the lack of overlapping of the corresponding phonon dispersion bands in SnS${}_2$ and SnSe${}_2$. This offers a unique opportunity to assess local distortions of the trigonal Sn${}_3$Se pyramids in SnS${}_x$Se${}_{2-x}$ as no Se and S mode mixing is involved. The dependence of local height and base length of Sn${}_3$Se pyramids with $x$ is derived by a procedure that uses the measured $A_{1g}$ (Se) and $E_g$ (Se) phonons in SnS${}_x$Se${}_{2-x}$, those calculated by DFPT for SnSe${}_2$ at different hydrostatic pressure, DFPT phonon dispersion, and the contribution from mass-disorder induced phonon self-energy.

We discuss a strong relationship between Majorana fermions and odd-frequency Cooper pairs which appear at a disordered normal (N) nanowire attached to a topologically nontrivial superconducting (S) one. The transport properties in superconducting nanowire junctions show universal behaviors irrespective of the degree of disorder: the quantized zero-bias differential conductance at $2e^2/h$ in NS junction and the fractional current-phase ($J-\phi$) relationship of the Josephson effect in SNS junction $J\propto \sin (\phi /2)$. Such behaviors are exactly the same as those found in the anomalous proximity effect of odd-parity spin-triplet superconductors. We show that the odd-frequency pairs exist wherever the Majorana fermions stay.

We introduce a calorimetric approach to probe persistent currents in normal metal rings. The heat capacity of a large ensemble of silver rings is measured by nanocalorimetry under a varying magnetic field at different temperatures (60, 100, and 150 mK). Periodic oscillations versus magnetic field are detected in the phase signal of the temperature oscillations, though not in the amplitude (both of them directly linked to the heat capacity). The period of these oscillations (${\Phi }_0/2$, with ${\Phi }_0=h/e$ the magnetic flux quantum) and their evolution with temperature are in agreement with theoretical predictions. In contrast, the amplitude of the corresponding heat capacity oscillations (several $k_{\mathrm{B}}$) is two orders of magnitude larger than predicted by theory.

Vanadium dioxide (VO${}_2$) undergoes a phase transition at a temperature of 340 K between an insulating monoclinic $M_1$ phase and a conducting rutile phase. Accurate measurements of possible anisotropy of the electronic properties and phonon features of VO${}_2$ in the insulating monoclinic $M_1$ and metallic rutile phases are a prerequisite for understanding the phase transition in this correlated system. Recently, it has become possible to grow single domain untwinned VO${}_2$ microcrystals, which makes it possible to investigate the true anisotropy of VO${}_2$. We performed polarized transmission infrared micro-spectroscopy on these untwinned microcrystals in the spectral range between 200 cm${}^{-1}$ and 6000 cm${}^{-1}$ and have obtained the anisotropic phonon parameters and low frequency electronic properties in the insulating monoclinic $M_1$ and metallic rutile phases. We have also performed ab initio GGA+$U$ total energy calculations of phonon frequencies for both phases. We find our measurements and calculations to be in good agreement.

We theoretically investigate spin decoherence of a single nitrogen-vacancy (NV) center in diamond. Using the spin coherent-state $P$-representation method, we simulate coherence evolution of the NV center coupled to surrounding nitrogen electron (N) spins. In the system, the strength of N-N coupling is the same order as that of NV-N coupling (the strong intrabath coupling regime). We find that spin decoherence time as well as free-induction decay of the NV center depend on the spatial configuration of N spins. Both the spin decoherence rate ($1/T_2$) and dephasing rate ($1/T_2^{*})$ of the NV center increase linearly with the concentration of the N spins. Using the $P$-representation method, we also demonstrate extracting the noise spectrum of the N spin bath. The capability to calculate the noise spectrum will provide promising pathways to suppress decoherence of spin systems in the strong intrabath coupling regime.

We experimentally observe toroidal dipolar response in a planar metamaterial comprised of asymmetric split-ring resonators (ASRRs) at microwave frequency. It is shown that a toroidal molecule can be constructed through rational arrangement of planar ASRRs as meta-atoms via manipulating structural symmetry and thus coupling of the meta-atoms. We find that the toroidal resonance provides a subwavelength-scale electromagnetic localization style, and that confining the electromagnetic field inside a dielectric medium with toroidal geometry is beneficial for low-loss metamaterials. The planar scheme of manipulating the coupling among the ASRRs may stimulate research in optical regions involving toroidal multipoles. The toroidal geometry together with the Fano resonance of ASRR-induced high-$Q$ response will have enormous potential applications in enhancing light-matter interactions, e.g., for low-threshold lasing, low-power nonlinear processing, and sensitive biosensing.

The electron-phonon interaction and related transport properties are investigated in monolayer silicene and MoS${}_2$ by using a density functional theory calculation combined with a full-band Monte Carlo analysis. In the case of silicene, the results illustrate that the out-of-plane acoustic phonon mode may play the dominant role unlike its close relative, graphene. The small energy of this phonon mode, originating from the weak $sp$${}^2$ $\pi$ bonding between Si atoms, contributes to the high scattering rate and significant degradation in electron transport. In MoS${}_2$, the longitudinal acoustic phonons show the strongest interaction with electrons. The key factor in this material appears to be the $Q$ valleys located between the $\Gamma$ and $K$ points in the first Brillouin zone as they introduce additional intervalley scattering. The analysis also reveals the potential impact of extrinsic screening by other carriers and/or adjacent materials. Finally, the effective deformation potential constants are extracted for all relevant intrinsic electron-phonon scattering processes in both materials.

We present a quantum theory for the interaction of a two-level emitter with surface plasmon polaritons confined in single-mode waveguide resonators. Based on the Green's function approach, we develop the conditions for the weak and strong coupling regimes by taking into account the sources of dissipation and decoherence: radiative and nonradiative decays, internal loss processes in the emitter, as well as propagation and leakage losses of the plasmons in the resonator. The theory is supported by numerical calculations for several quantum emitters, GaAs and CdSe quantum dots, and nitrogen vacancy (NV) centers together with different types of resonators constructed of hybrid, cylindrical, or wedge waveguides. We further study the role of temperature and resonator length. Assuming realistic leakage rates, we find the existence of an optimal length at which strong coupling is possible. Our calculations show that the strong coupling regime in plasmonic resonators is accessible within current technology when working at very low temperatures ($\lesssim 4$ K). In the weak coupling regime, our theory accounts for recent experimental results. By further optimization we find highly enhanced spontaneous emission with Purcell factors over 1000 at room temperature for NV centers. We finally discuss more applications for quantum nonlinear optics and plasmon-plasmon interactions.

Dielectric spectroscopy is used to check for the onset of polar order in the quasi-one-dimensional quantum spin system Sul-Cu${}_2$Cl${}_4$ when passing from the spin-liquid state into the ordered spiral phase in an external magnetic field. We find clear evidence for multiferroicity in this material and treat in detail its $H$-$T$ phase diagram close to the quantum critical regime.

We report pronounced modifications of the electronic and vibrational properties induced in FeO by cation vacancies, obtained within density functional theory incorporating strong local Coulomb interactions at Fe atoms. The insulating gap of FeO is reduced by about 50$\%$ due to unoccupied electronic bands introduced by trivalent Fe ions stabilized by cation vacancies. The changes in the electronic structure along with atomic displacements induced by cation vacancies affect strongly the phonon dispersions via modified force constants, including those at atoms beyond the nearest neighbors of defects. We demonstrate that theoretical phonon dispersions and their densities of states reproduce the results of inelastic neutron and nuclear resonant x-ray scattering experiments only when Fe vacancies and Coulomb interaction $U$ are both included explicitly in ab initio simulations, which also suggests that the electron-phonon coupling in FeO is strong.

We formulate a quantum theory of electron waiting time distributions for charge transport in nanostructures described by non-Markovian generalized master equations. We illustrate our method by calculating the waiting time distribution of electron transport through a dissipative double quantum dot, where memory effects are present due to a strongly coupled heat bath. We consider the influence of non-Markovian dephasing on the distribution of electron waiting times and discuss how spectral properties of the heat bath may be detected through measurements of the electron waiting time.

We discuss electron spin resonance (ESR) shifts in spin-1 Heisenberg antiferromagnetic chains with a weak single-ion anisotropy, based on several effective field theories: the O(3) nonlinear sigma model (NLSM) in the Haldane phase, free-fermion theories around the lower and the upper critical fields. In the O(3) NLSM, the single-ion anisotropy corresponds to a composite operator which creates two magnons at the same time and position. Therefore, even inside a parameter range where free magnon approximation is valid for thermodynamics, we have to take interactions among magnons into account in order to include the single-ion anisotropy as a perturbation. Although the O(3) NLSM is only valid in the Haldane phase, an appropriate translation of Faddeev-Zamolodchikov operators of the O(3) NLSM to fermion operators enables one to treat ESR shifts near the lower critical field in a similar manner to discussions in the Haldane phase. Our theory gives quantitative agreements with a numerical evaluation using quantum Monte Carlo simulation, and also with recent ESR experimental results on a spin-1 chain compound $\mathrm{Ni}{\left({\mathrm{C}}_5{\mathrm{H}}_{14}{\mathrm{N}}_2\right)}_2{\mathrm{N}}_3\left({\mathrm{PF}}_6\right)$.

In a scattering experiment, the induced dipole moments of a magnetoelectric point scatterer in response to driving fields are given by its polarizability tensor $\mathit{\alpha }$. Its linewidth will be dictated by the local density of optical states (LDOS) at the scatterer's position. To retrieve the magnetoelectric cross-coupling components of $\mathit{\alpha }$ for an archetypical magnetoelectric scatterer—a split ring resonator—we study the frequency-dependent extinction cross section ${\sigma }_{\mathrm{ext}}$ as a function of distance to an interface. Rather than following a purely electric or purely magnetic LDOS, we find a dependence which reflects the interplay of both dipole moments in a “mixed” magnetoelectric LDOS. For a strongly magnetoelectric cross-coupled microwave scatterer, we compare analytical point dipole with finite element method calculations.

We present the detailed formalism of the extremely correlated Fermi liquid theory, developed for treating the physics of the $t-J$ model. We start from the exact Schwinger equation of motion for the Green's function for projected electrons, and develop a systematic expansion in a parameter $\lambda$, relating to the double occupancy. The resulting Green's function has a canonical part arising from an effective Hamiltonian of the auxiliary electrons, and a caparison part playing the role of a frequency-dependent adaptive spectral weight. This adaptive weight balances the requirement at low $\omega$ of the invariance of the Fermi volume, and at high $\omega$ of decaying as $\frac{c_0}{i\omega }$, with a correlation-depleted $c_0<1$. The effective Hamiltonian $H_{\mathrm{eff}}$ describing the auxiliary fermions is given a natural interpretation with an effective interaction $V_{\mathrm{eff}}$ containing both the exchange $J_{ij}$ and the hopping parameters $t_{ij}$. It is made Hermitian by adding suitable terms that ultimately vanish, in the symmetrized theory developed in this paper. Simple but important shift invariances of the $t-J$ model are noted with respect to translating its parameters uniformly. These play a crucial role in constraining the form of $V_{\mathrm{eff}}$ and also provide checks for further approximations. The auxiliary and physical Green's function satisfy two sum rules, and the Lagrange multipliers for these are identified. A complete set of expressions for the Green's functions to second order in $\lambda$ is given, satisfying various invariances. A systematic iterative procedure for higher order approximations is detailed. A superconducting instability of the theory is noted at the simplest level with a high transition temperature.

We use photoluminescence spectroscopy to investigate the ground state of single self-assembled InGaAs lateral quantum dot molecules. We apply a voltage along the growth direction that allows us to control the total charge occupancy of the quantum dot molecule. Using a combination of computational modeling and experimental analysis, we assign the observed discrete spectral lines to specific charge distributions. We explain the dynamic processes that lead to these charge configurations through electrical injection and optical generation. Our systemic analysis provides evidence of interdot tunneling of electrons as predicted in previous theoretical work.

We report an experimental observation of high frequency magnetic excitations in terbium gallium garnet crystals using terahertz time-domain spectroscopy. We show that precessional modes of terbium magnetic sublattices can be excited by a magnetic field of a terahertz broadband pulse. We study and discuss the dependence of the observed resonances upon the temperature and the strength and orientation of the bias magnetic field. The behavior of the observed magnetic modes is in agreement with the theory of paramagnetic resonance in the multisublattice system. We also show that the illumination of the crystal with intense optical pulses destroys the magnetic ordering. Our results demonstrate that the time-domain terahertz spectroscopy can be a powerful tool by which to study high frequency properties of dielectric magnetic materials, with perceived extensions to studies in femtomagnetism and magnonics.