Photoinduced magnetic force between nanostructures, at optical frequencies, is investigated theoretically. Till now optical magnetic effects were not used in scanning probe microscopy because of the vanishing natural magnetism with increasing frequency. On the other hand, artificial magnetism in engineered nanostructures led to the development of measurable optical magnetism. Here two examples of nanoprobes that are able to generate strong magnetic dipolar fields at optical frequency are investigated: first, an ideal magnetically polarizable nanosphere and then a circular cluster of silver nanospheres that has a looplike collective plasmonic resonance equivalent to a magnetic dipole. Magnetic forces are evaluated based on nanostructure polarizabilities, i.e., induced magnetic dipoles, and magnetic-near field evaluations. As an initial assessment on the possibility of a magnetic nanoprobe to detect magnetic forces, we consider two identical magnetically polarizable nanoprobes and observe magnetic forces on the order of piconewtons, thereby bringing it within detection limits of conventional atomic force microscopes at ambient pressure and temperature. The detection of magnetic force is a promising method in studying optical magnetic transitions that can be the basis of innovative spectroscopy applications.

The effect of epitaxial strain on the structural and electronic properties of ${\mathrm{LaVO}}_3$ is investigated through density functional theory (DFT) and dynamical mean-field theory (DMFT). Two different growth orientations of the crystal are considered, one preserving the bulk $Pbnm$ space-group symmetry and another giving rise to a symmetry lowering to $P{2}_1/m$. In the nonmagnetic DFT structures, the two growth orientations are equally favored for all tensile strains considered here, as well as for compressive strains weaker than $-3\%$. For stronger compressive strains, the $P{2}_1/m$ orientation is favored and shows a complete suppression of octahedral tilts along the out-of-plane direction. Magnetically ordered structures do not show a complete tilt suppression, but the trend points to a similar reduction of the out-of-plane V–O–V bond angles under compressive strain. Our DMFT calculations show that, in accord with room-temperature experiments, the bulk paramagnetic Mott-insulating state of ${\mathrm{LaVO}}_3$ is robust against epitaxial strains attainable in thin films, since the suppression of orbital fluctuations counteracts the effect of bandwidth increase with compressive strain. Under stronger compressive strains, the straightening of the V–O–V bonds in the $P{2}_1/m$ geometry interferes with the suppression of orbital fluctuations and hence perturbs the Mott phase more strongly, albeit not enough to achieve a metallic phase.

We present scanning tunneling microscopy and spectroscopy measurements on ${\mathrm{FeSe}}_{1-x}{\mathrm{S}}_x$ single crystals with $x=0$, $0.04$, and $0.09$. The S substitution into the Se site is equivalent to a positive chemical pressure, since S and Se have the same valence and S has a smaller ionic radius than Se. The subsequent changes in the electronic structure of FeSe induce a decrease of the structural transition temperature and a small increase in the superconducting critical temperature. The evolution of the gaps with increasing S concentration suggests an increase of the hole Fermi surface. Moreover, the vortex core anisotropy, that likely reflects the Fermi surface anisotropy, is strongly suppressed by the S substitution.

Dynamical scattering theory describes the dominant scattering process of beam electrons at targets in the transmission electron microscope (TEM). Hence, practically every quantitative TEM study has to consider its ramifications, typically by some approximate modeling. Here, we elaborate on a hierarchy within the various approximations focusing on the two principal approaches used in practice, Bloch wave and multislice. We reveal characteristic differences in the capability of these methods to reproduce the correct local propagation of the wave function, while convergent results are obtained over larger propagation distances. We investigate the dependency of local variations of the wave function on the atomic number of the atomic scatterers and discuss their significance for, e.g., inelastic scattering.

By developing an algorithm for evaluating the basis states for the composite fermions with negative effective magnetic field, we perform the composite-fermion-diagonalization study for the fully spin-polarized fractional quantum Hall states at the filling factors $\nu =3/10$, 4/13, and 5/17 in the range $2/7<\nu <1/3$. These observed states correspond to a partially filled second effective Landau level, for the composite fermions carrying four vortices, with filling factor $\overline{\nu }=1/2$, 1/3, and 2/3, respectively, analogous to the previously studied states of composite fermions with two attached vortices in the range $1/3<\nu <2/5$. We show that the character of these states in the range $2/7<\nu <1/3$ replicates the same for the states in the range $1/3<\nu <2/5$ having identical $\overline{\nu }$: chiral $p$-wave pairing with an anti-Pfaffian correlation of composite fermions carrying six quantized vortices produces an incompressible state at $\nu =3/10$; an unconventional interaction between composite fermions, resulting from the suppression of fermion pairs with relative angular momentum three and producing the fractional quantum Hall effect of composite fermions in the second effective Landau level with $\overline{\nu }=1/3$ and its particle-hole conjugate filling factor 2/3, reproduces incompressible states at $4/13$ and $5/17$ filling factors. We further estimate the thermodynamic limit of the ground-state energies and calculate the lowest energy gap for the neutral collective excitations of these states.

In this work, we have systematically studied structural, electronic, and magnetic properties of atomic-scale defects in 2D transition metal dichalcogenides $M{X}_2$ ($M\hspace{0.17em}=\hspace{0.17em}\mathrm{Mo}$ and W; $X\hspace{0.17em}=\hspace{0.17em}\mathrm{S}$, Se, and Te) by density functional theory. Various types of defects, e.g., $X$ vacancy, $X$ interstitial, $M$ vacancy, $M$ interstitial, and $MX$ and $XX$ double vacancies, have been considered. It has been found that the $X$ interstitial has the lowest formation energy ($\sim 1$ eV) for all the systems in the $X$-rich condition, whereas for the $M$-rich condition, $X$ vacancy has the lowest formation energy except for $M{\text{Te}}_2$ systems. Both these defects have very high equilibrium defect concentrations at growth temperatures (1000–1200 K) reported in literature. A pair of defects, e.g., two $X$ vacancies or one $M$ and one $X$ vacancies, tend to occupy the nearest possible distance. No trace of magnetism has been found for any one of the defects considered. Apart from $X$ interstitial, all other defects have defect states appearing in the band gap, which can greatly affect the electronic and optical properties of the pristine systems. Our calculated optical properties show that the defect states cause optical transitions at $\sim 1.0$ eV, which can be beneficial for light emitting devices. The results of our systematic study are expected to guide the experimental nanoengineering of defects to achieve suitable properties related to band gap modifications and characterization of defect fingerprints via optical absorption measurements.

We present a theoretical model to describe the double-resonant scattering process in arbitrary carbon nanotubes (CNTs). We use this approach to investigate the defect-induced $D$ mode in CNTs and unravel the dependence of the $D$-mode frequency on the CNT diameter and on the energy of the resonant optical transition. Our approach is based on the symmetry of the hexagonal lattice and geometric considerations; hence the method is independent of the exact model that is chosen to describe the electronic band structure or the phonon dispersion. We finally clarify the diameter dependence of this Raman mode that has been under discussion in the past and demonstrate that, depending on the experimental conditions, in general two different dependencies can be measured. We also prove that CNTs with an arbitrary chiral index can exhibit a $D$ mode in their Raman spectrum, in contrast to previous symmetry-based arguments. Furthermore, we give a direct quantification of the curvature-induced phonon frequency corrections of the $D$ mode in CNTs with respect to graphite.

Nitrogen-vacancy (NV) color centers in diamond have emerged as promising quantum solid-state systems, with applications ranging from quantum information processing to magnetic sensing. One of the most useful properties of NVs is the ability to read their ground-state spin projection optically at room temperature. Using theoretical analysis of Purcell-enhanced NV optical coupling, we identify parameters for a significantly enhanced single-to-noise ratio (SNR) of the optical spin-state readout. We then demonstrate that a combined increase in spontaneous emission (through Purcell enhancement) and in optical excitation could significantly increase the readout SNR. Our combined analytical and numerical analysis, which is relevant for various optically active solid-state systems, differentiates between state-mixing processes and provides insights into the optimal light-matter coupling design.

We present perturbative analytical results of the Landau level quasienergy spectrum, autocorrelation function, and out-of-plane pseudospin polarization for a single graphene sheet subject to intense circularly polarized Terahertz radiation. For the quasienergy spectrum, we find a striking nontrivial level-dependent dynamically induced gap structure. This photoinduced modulation of the energy band structure gives rise to shifts of the revival times in the autocorrelation function and it also leads to modulation of the oscillations in the dynamical evolution of the out-of-plane pseudospin polarization, which measures the angular momentum transfer between light and graphene electrons. For a coherent state, chosen as an initial pseudospin configuration, the dynamics induces additional quantum revivals of the wave function that manifest as shifts of the maxima and minima of the autocorrelation function, with additional partial revivals and beating patterns. These additional maxima and beating patterns stem from the effective dynamical coupling of the static eigenstates. We discuss the possible experimental detection schemes of our theoretical results and their relevance in new practical implementation of radiation fields in graphene physics.

We experimentally investigate charge carrier transport in a graphene $p-n$ junction device by using independent $p$-type and $n$-type electrostatic gating which allow full characterization of the junction interface in the quantum Hall regime covering a wide range of filling factors $[-10\le ({\nu }_1,{\nu }_2)\le 10]$. Recent charge transport measurements across a graphene $p-n$ junction in this quantized regime presume that equilibration of all of the Landauer-Büttiker edge states occurs across the $p-n$ junction interface. Here we show that, in our devices, only the edge state associated with the lowest Landau level fully equilibrate across the $p-n$ junction, while none of the other edge states equilibrate to transmit current across the junction.

Strongly hybridized plasmon-photon modes are observed in a microtubular cavity coated with a gold nanocap. Polarization-resolved measurements reveal that the transverse magnetic polarized light predominantly excites the hybrid resonant modes in these optoplasmonic microcavities. Remarkably, the exterior field of the hybrid mode is enhanced by more than an order of magnitude over previously reported designs and is caused by the light confinement in an extremely thin cavity wall. Both finite element method calculations and an effective potential approach confirm our experimental observations. Our work reveals the basic physical mechanisms responsible for exciting hybrid modes in optoplasmonic microcavities and as such is relevant for both fundamental and applied studies in photonics and plasmonics.

Bilayer graphene hosts valley-chiral one-dimensional modes at domain walls between regions of different interlayer potential or stacking order. When such a channel is close to a superconductor, the two electrons of a Cooper pair, which tunnel into it, move in opposite directions because they belong to different valleys related by the time-reversal symmetry. This kinetic variant of Cooper pair splitting requires neither Coulomb repulsion nor energy filtering but is enforced by the robustness of the valley isospin in the absence of atomic-scale defects. We derive an effective normal/superconducting/normal (NSN) model of the channel in proximity to an $s$-wave superconductor, calculate the conductance of split and spin-entangled pairs, and interpret it as a result of local Andreev reflection, in contrast to the widespread identification of Cooper pair splitting with crossed Andreev reflection in an NSN geometry.

Magnetic and electronic states in $\left({\mathrm{LaMnO}}_3\right){}_2\left({\mathrm{SrMnO}}_3\right){}_2$ superlattices fabricated on an $\left({\mathrm{LaAlO}}_3\right){}_{0.3}\left({\mathrm{SrAl}}_{0.5}{\mathrm{Ta}}_{0.5}{\mathrm{O}}_3\right){}_{0.7}$ substrate, which exhibit a large nontrivial negative magnetoresistance (MR) effect, have been investigated. The crystal structure and the Mn valence state were determined using x-ray scattering measurements near the Mn $K$ edge. These measurements revealed that the Mn valences in the ${\mathrm{LaMnO}}_3$ and ${\mathrm{SrMnO}}_3$ layers are 3+ and 4+, respectively; that is, valence modulation coincides with the La/Sr stacking structure. The Mn spin structure was studied by means of resonant soft x-ray scattering at the Mn $L_{2,3}$ edge and neutron magnetic scattering measurements. We succeeded in detecting a magnetic signal indicating ferromagnetism at the interface. Finally, we suggest that the origin of the MR is the competition between ferromagnetism at the interface and underlying antiferromagnetism.

This paper is devoted to the development of perturbation theory for studying the properties of graphene sheets of finite size, at nonzero temperature and chemical potential. The perturbation theory is based on the tight-binding Hamiltonian and arbitrary interaction potential between electrons, which is considered as a perturbation. One-loop corrections to the electron propagator and to the interaction potential at nonzero temperature and chemical potential are calculated. One-loop formulas for the energy spectrum of electrons in graphene, for the renormalized Fermi velocity and also for the dielectric permittivity, are derived.

We develop a time-dependent variational Monte Carlo (t-VMC) method for quantum dynamics of strongly correlated electrons. The t-VMC method has been recently applied to bosonic systems and quantum spin systems. Here we propose a time-dependent trial wave function with many variational parameters, which is suitable for nonequilibrium strongly correlated electron systems. As the trial state, we adopt the generalized pair-product wave function with correlation factors and quantum-number projections. This trial wave function has been proven to accurately describe ground states of strongly correlated electron systems. To show the accuracy and efficiency of our trial wave function in nonequilibrium states as well, we present our benchmark results for relaxation dynamics during and after interaction quench protocols of fermionic Hubbard models. We find that our trial wave function well reproduces the exact results for the time evolution of physical quantities such as energy, momentum distribution, spin structure factor, and superconducting correlations. These results show that the t-VMC with our trial wave function offers an efficient and accurate way to study challenging problems of nonequilibrium dynamics in strongly correlated electron systems.

Graphene hosting a pair of collinear adatoms in the phantom atom configuration has density of states vanishing in the vicinity of the Dirac point which can be described in terms of the pseudogap scaling as cube of the energy, $\mathrm{\Delta }\propto {\left|\varepsilon \right|}^3$, which leads to the appearance of spin-degenerate bound states in the continuum (BICs) [Phys. Rev. B 92, 045409 (2015)]. In the case when adatoms are locally coupled to a single carbon atom the pseudogap scales linearly with energy, which prevents the formation of BICs. Here, we explore the effects of nonlocal coupling characterized by the Fano factor of interference $q_0$, tunable by changing the slope of the Dirac cones in the graphene band structure. We demonstrate that three distinct regimes can be identified: (i) for $q_0<q_{c1}$ (critical point) a mixed pseudogap $\mathrm{\Delta }\propto {\left|\varepsilon \right|,\left|\varepsilon \right|}^2$ appears yielding a phase with spin-degenerate BICs; (ii) near $q_0=q_{c1}$ when $\mathrm{\Delta }\propto {\left|\varepsilon \right|}^2$ the system undergoes a quantum phase transition (QPT) in which the new phase is characterized by magnetic BICs, and (iii) at a second critical value $q_0>q_{c2}$ the cubic scaling of the pseudogap with energy $\mathrm{\Delta }\propto {\left|\varepsilon \right|}^3$ characteristic to the phantom atom configuration is restored and the phase with nonmagnetic BICs is recovered. The phase with magnetic BICs can be described in terms of an effective intrinsic exchange field of ferromagnetic nature between the adatoms mediated by graphene monolayer. We thus propose a new type of QPT resulting from the competition between two ground states, respectively characterized by spin-degenerate and magnetic BICs.

In strained graphene, lattice deformation can create pseudomagnetic fields affecting the behavior of massless Dirac fermions and result in zero-field Landau level-like quantization. In the presence of an external magnetic field, valley-polarized Landau levels are predicted to be observed because the strain-induced pseudomagnetic fields are of opposite directions in the $K$ and $K^{\prime }$ valleys of graphene. However, an experimental verification of such a unique valley-polarized Landau quantization has not been reported so far. Here, we present experimental spectroscopic measurements in strained graphene on Rh foil by scanning tunneling microscopy. We directly observed the splitting of the Landau level in the quantum Hall regime and we interpreted the experimental result as the valley-polarized Landau level induced by the coexistence of the pseudomagnetic fields and external magnetic fields. The observed result paves the way to exploit novel electronic properties in graphene through the combination of the spatially varying strain (or the pseudomagnetic fields) and the external magnetic fields.

The control of nuclear spins in quantum dots is essential to explore their many-body dynamics and exploit their prospects for quantum information processing. We present a unique combination of dynamic nuclear spin polarization and electric-dipole-induced spin resonance in an electrostatically defined double quantum dot (DQD) exposed to the strongly inhomogeneous field of two on-chip nanomagnets. Our experiments provide direct and unrivaled access to the nuclear spin polarization distribution and allow us to establish and characterize multiple fixed points. Further, we demonstrate polarization of the DQD environment by nuclear spin diffusion which significantly stabilizes the nuclear spins inside the DQD.