In this paper, we provide a theoretical analysis of strongly interacting quantum systems confined by a time-dependent external potential in one spatial dimension. We show that such systems can be used to simulate spin chains described by Heisenberg Hamiltonians in which the exchange coupling constants can be manipulated by time-dependent driving of the shape of the external confinement. As illustrative examples, we consider a harmonic trapping potential with a variable frequency and an infinite square-well potential with a time-dependent barrier in the middle.

We present time-of-flight neutron scattering measurements on single crystals of ${\text{La}}_{2-x}{\text{Ba}}_x{\text{CuO}}_4$ (LBCO) with $0\le x\le 0.095$ and ${\text{La}}_{2-x}{\text{Sr}}_x{\text{CuO}}_4$ (LSCO) with $x=0.08$ and 0.11. This range of dopings spans much of the phase diagram relevant to high-temperature cuprate superconductivity, ranging from insulating, three-dimensional commensurate long-range antiferromagnetic order, for $x\le 0.02$, to two-dimensional (2D) incommensurate antiferromagnetism coexisting with superconductivity for $x\ge 0.05$. Previous work on lightly doped LBCO with $x=0.035$ showed a clear enhancement of the inelastic scattering coincident with the low-energy crossings of the highly dispersive spin excitations and quasi-2D optic phonons. The present work extends these measurements across the phase diagram and shows this enhancement to be a common feature to this family of layered quantum magnets. Furthermore, we show that the low-temperature, low-energy magnetic spectral weight is substantially larger for samples with nonsuperconducting ground states relative to any of the samples with superconducting ground states. Spin gaps, suppression of low-energy magnetic spectral weight as a function of decreasing temperature, are observed in both superconducting LBCO and LSCO samples, consistent with previous observations for superconducting LSCO.

We investigate theoretically how the dynamical Casimir effect can entangle quantum systems in different coupling regimes of circuit quantum electrodynamics, and show the robustness of such entanglement generation against dissipative effects, considering experimental parameters of current technology. We consider two qubit-resonator systems, which are coupled by a SQUID driven with an external magnetic field, and explore the entire range of coupling regimes between each qubit and its resonator. In this scheme, we derive a semianalytic explanation for the entanglement generation between both superconducting qubits when they are coupled to their resonators in the strong coupling regime. For the ultrastrong and deep strong coupling regimes, we design experimentally feasible theoretical protocols to generate maximally entangled polaritonic states.

We present the results of site-selective ${}^{13}\mathrm{C}$ NMR spectroscopy on an organic superconductor $\kappa$-(BEDT-TTF)${}_2{\mathrm{Cu}\left[\mathrm{N}\right(\mathrm{CN})}_2]\mathrm{Br}$ ($\kappa$-Br) doped with BEDT-STF molecules. We reveal microscopically the modulation of the local electronic state caused by the BEDT-STF doping from the ${}^{13}\mathrm{C}$ NMR measurement on two types of samples, which are ${}^{13}\mathrm{C}$ enriched $\kappa$-Br doped with naturally abundant BEDT-STF molecules, and natural $\kappa$-Br doped with ${}^{13}\mathrm{C}$ enriched BEDT-STF molecules. The results of the nuclear spin-lattice relaxation rate $1/T_1$ measured both in the normal and superconducting state suggest that the potential disorder at the BEDT-STF sites scatters antiferromagnetic interaction and superconducting Cooper pairs.

The ytterbium pyrochlore magnets, ${\mathrm{Yb}}_2{B}_2{\mathrm{O}}_7$ ($B=\mathrm{Sn},\mathrm{Ti},\mathrm{Ge}$) are well described by $S_{\text{eff}}=1/2$ quantum spins decorating a network of corner-sharing tetrahedra and interacting via anisotropic exchange. Structurally, only the nonmagnetic $B$-site cation, and hence primarily the lattice parameter, changes across the series. Nonetheless, a range of magnetic behaviors is observed: the low-temperature magnetism in ${\mathrm{Yb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ and ${\mathrm{Yb}}_2{\mathrm{Sn}}_2{\mathrm{O}}_7$ has a ferromagnetic character, while ${\mathrm{Yb}}_2{\mathrm{Ge}}_2{\mathrm{O}}_7$ displays an antiferromagnetically ordered Néel state at low temperatures. While the static properties of the ytterbium pyrochlores are distinct, inelastic neutron scattering measurements reveal a common character to their exotic spin dynamics. All three ytterbium pyrochlores show a gapless continuum of spin excitations, resembling overdamped ferromagnetic spin waves at low $Q$. Furthermore, the specific heat of the series also follows a common form, with a broad, high-temperature anomaly followed by a sharp low-temperature anomaly at $T_C$ or $T_N$. The novel spin dynamics we report correlate strongly with the broad specific heat anomaly only, remaining unchanged across the sharp anomaly. This result suggests that the primary order parameter in the ytterbium pyrochlores associated with the sharp anomaly is “hidden” and not simple magnetic dipole order.

Iron magnetic moment enhancement is observed following the irradiation of iron films with 490 keV $\mathrm{F}{\mathrm{e}}^{+}$ at room temperature. The iron magnetic moment enhancement increases to saturation with irradiation dose. The enhanced magnetic moment decays exponentially to its value before the irradiation with a time constant of 5.2 months. The iron magnetic moment enhancement is attributed to the creation of vacancy clusters having a concentration of about 20%, whereas the relaxation effects is attributed to the dissociation of these clusters.

Local structures in cubic perovskite-type $\left(\mathrm{B}{\mathrm{a}}_{0.6}\mathrm{B}{\mathrm{i}}_{0.4}\right)\left(\mathrm{T}{\mathrm{i}}_{0.6}\mathrm{S}{\mathrm{c}}_{0.4}\right){\mathrm{O}}_3$ solid solutions that exhibit reentrant dipole glass behavior have been studied with variable-temperature x-ray/neutron total scattering, extended x-ray absorption fine structure, and electron diffraction methods. Simultaneous fitting of these data using a reverse Monte Carlo algorithm provided instantaneous atomic configurations, which have been used to extract local displacements of the constituent species. The smaller Bi and Ti atoms exhibit probability density distributions that consist of 14 and 8 split sites, respectively. In contrast, Ba and Sc feature single-site distributions. The multisite distributions arise from large and strongly anisotropic off-center displacements of Bi and Ti. The cation displacements are correlated over a short range, with a correlation length limited by chemical disorder. The magnitudes of these displacements and their anisotropy, which are largely determined by local chemistry, change relatively insignificantly on cooling from room temperature. The structure features a nonrandom distribution of local polarization with low-dimensional polar clusters that are several unit cells in size. In situ measurements of atomic pair-distribution function under applied electric field were used to study field-induced changes in the local structure; however, no significant effects besides lattice expansion in the direction of the field could be observed up to electric-field values of $4\mathrm{kV}\mathrm{m}{\mathrm{m}}^{-1}$.

The material copper pyrimidine dinitrate (Cu-PM) is a quasi-one-dimensional spin system described by the spin-1/2 $XXZ$ Heisenberg antiferromagnet with Dzyaloshinskii-Moriya interactions. Based on numerical results obtained by the density-matrix renormalization group, exact diagonalization, and accompanying electron spin resonance (ESR) experiments we revisit the spin dynamics of this compound in an applied magnetic field. Our calculations for momentum and frequency-resolved dynamical quantities give direct access to the intensity of the elementary excitations at both zero and finite temperature. This allows us to study the system beyond the low-energy description by the quantum sine-Gordon model. We find a deviation from the Lorentz invariant dispersion for the single-soliton resonance. Furthermore, our calculations only confirm the presence of the strongest boundary bound state previously derived from a boundary sine-Gordon field theory, while composite boundary-bulk excitations have too low intensities to be observable. Upon increasing the temperature, we find a temperature-induced crossover of the soliton and the emergence of new features, such as interbreather transitions. The latter observation is confirmed by our ESR experiments on Cu-PM over a wide range of the applied field.

Motivated by recent experiments with a Penning ion trap quantum simulator, we perform numerically exact Stochastic Series Expansion quantum Monte Carlo simulations of long-range transverse-field Ising models on a triangular lattice for different decay powers $\alpha$ of the interactions. The phase boundary for the ferromagnet is obtained as a function of $\alpha$. For antiferromagnetic interactions, there is strong indication that the transverse field stabilizes a clock ordered phase with sublattice magnetization $(M,-\frac{M}{2},-\frac{M}{2})$ with unsaturated $M<1$ in a process known as “order by disorder” similar to the nearest-neighbor antiferromagnet on the triangular lattice. Connecting the known limiting cases of nearest-neighbor and infinite-range interactions, a semiquantitative phase diagram is obtained. Magnetization curves for the ferromagnet for experimentally relevant system sizes and with open boundary conditions are presented.

Considerations of the observed bad-metal behavior in Fe-based superconductors led to an early proposal for quantum criticality induced by isoelectronic P for As doping in iron arsenides, which has since been experimentally confirmed. We study here an effective model for the isoelectronically tuned pnictides using a large-$N$ approach. The model contains antiferromagnetic and Ising-nematic order parameters appropriate for $J_1-J_2$ exchange-coupled local moments on an Fe square lattice, and a damping caused by coupling to itinerant electrons. The zero-temperature magnetic and Ising transitions are concurrent and essentially continuous. The order-parameter jumps are very small, and are further reduced by the interplane coupling; consequently, quantum criticality occurs over a wide dynamical range. Our results reconcile recent seemingly contradictory experimental observations concerning the quantum phase transition in the P-doped iron arsenides.

The hole doped Fe-based superconductors $\mathrm{B}{{\mathrm{a}}_1}_{-x}{A}_x\mathrm{F}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$ (where $A=\mathrm{Na}$ or K) show a particularly rich phase diagram. It was observed that an intermediate reentrant tetragonal phase, in which the $C_4$ fourfold rotational symmetry is restored, forms within the orthorhombic antiferromagnetically ordered stripe-type spin density wave state above the superconducting transition [S. Avci et al., Nat. Commun. 5, 3845 (2014); A. E. Böhmer et al., Nat. Commun. 6, 7911 (2015)]. A similar intermediate phase was reported to appear if pressure is applied to underdoped $\mathrm{B}{{\mathrm{a}}_1}_{-x}{\mathrm{K}}_x\mathrm{F}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$ [E. Hassinger et al., Phys. Rev. B 86, 140502(R) (2012)]. Here we report data of the electric resistivity, Hall effect, specific heat, and the thermoelectric Nernst and Seebeck coefficients measured on a $\mathrm{B}{\mathrm{a}}_{0.85}{\mathrm{K}}_{0.15}\mathrm{F}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$ single crystal under pressure up to 5.5 GPa. The data reveal a coexistence of the intermediate phase with filamentary superconductivity. The Nernst coefficient shows a large signature of nematic order that coincides with the stripe-type spin density wave state up to optimal pressure. In the pressure-induced intermediate phase the nematic order is removed, thus confirming that its nature is a reentrant tetragonal phase.

It has recently been demonstrated that dynamical magnetic correlations measured by neutron scattering in iron chalcogenides can be described with models of short-range correlations characterized by particular choices of four-spin plaquettes, where the appropriate choice changes as the parent material is doped towards superconductivity. Here we apply such models to describe measured maps of magnetic scattering as a function of two-dimensional wave vectors obtained for optimally superconducting crystals of ${\mathrm{FeTe}}_{1-x}{\mathrm{Se}}_x$. We show that the characteristic antiferromagnetic wave vector evolves from that of the bicollinear structure found in underdoped chalcogenides (at high temperature) to that associated with the stripe structure of antiferromagnetic iron arsenides (at low temperature); these can both be described with the same local plaquette, but with different interplaquette correlations. While the magnitude of the low-energy magnetic spectral weight is substantial at all temperatures, it actually weakens somewhat at low temperature, where the charge carriers become more itinerant. The observed change in spin correlations is correlated with the dramatic drop in the electronic scattering rate and the growth of the bulk nematic response upon cooling. Finally, we also present powder neutron diffraction results for lattice parameters in ${\mathrm{FeTe}}_{1-x}{\mathrm{Se}}_x$ indicating that the tetrahedral bond angle tends to increase towards the ideal value upon cooling, in agreement with the increased screening of the crystal field by more itinerant electrons and the correspondingly smaller splitting of the Fe $3d$ orbitals.

We present a theoretical study for the scanning tunneling microscopy (STM) spectra of surface-supported magnetic nanostructures, incorporating strong correlation effects. As concrete examples, we study Co and Mn adatoms on the Cu(111) surface, which are expected to represent the opposite limits of Kondo physics and local moment behavior, using a combination of density functional theory and both quantum Monte Carlo and exact diagonalization impurity solvers. We examine in detail the effects of temperature $T$, correlation strength $U$, and impurity $d$ electron occupancy $N_d$ on the local density of states. We also study the effective coherence energy scale, i.e., the Kondo temperature $T_K$, which can be extracted from the STM spectra. Theoretical STM spectra are computed as a function of STM tip position relative to each adatom. Because of the multiorbital nature of the adatoms, the STM spectra are shown to consist of a complicated superposition of orbital contributions, with different orbital symmetries, self-energies, and Kondo temperatures. For a Mn adatom, which is close to half-filling, the STM spectra are featureless near the Fermi level. On the other hand, the quasiparticle peak for a Co adatom gives rise to strongly position-dependent Fano line shapes.

Many-body perturbation theory in the $GW$ approximation is currently the most accurate and robust first-principles approach to determine the electronic band structure of weakly correlated insulating materials without any empirical input. Recent $GW$ results for ZnO with more careful investigation of the convergence with respect to the number of unoccupied states have led to heated debates regarding the numerical accuracy of previously reported $GW$ results using either pseudopotential plane waves or all-electron linearized augmented plane waves (LAPWs). The latter has been arguably regarded as the most accurate scheme for electronic-structure theory for solids. This work aims to solve the ZnO puzzle via a systematic investigation of the effects of including high-energy local orbitals (HLOs) in the LAPW-based $GW$ calculations of semiconductors. Using ZnO as the prototypical example, it is shown that the inclusion of HLOs has two main effects: it improves the description of high-lying unoccupied states by reducing the linearization errors of the standard LAPW basis, and in addition it provides an efficient way to achieve the completeness in the summation of states in $GW$ calculations. By investigating the convergence of $GW$ band gaps with respect to the number of HLOs for several other typical examples, it was found that the effects of HLOs are highly system-dependent, and in most cases the inclusion of HLOs changes the band gap by less than 0.2 eV. Compared to its effects on the band gap, the consideration of HLOs has even stronger effects on the $GW$ correction to the valence-band maximum, which is of great significance for the $GW$ prediction of the ionization potentials of semiconductors. By considering an extended set of semiconductors with relatively well-established experimental band gaps, it was found that in general using a HLO-enhanced LAPW basis significantly improves the agreement with experiment for both the band gap and the ionization potential, and overall the partially self-consistent $G{W}_0$ approach based on the generalized gradient approximation gives an optimal performance.

We study the photon-statistical behavior of resonance fluorescence from self-assembled InAs quantum dots (QDs) as a function of the density of free charge carriers introduced by an above band-gap laser. Second-order correlation measurements show bunching behavior that changes with above-band laser power and is absent in purely above-band excited emission. Resonant photoluminescence excitation spectra indicate that the QD experiences discrete spectral shifts and continuous drift due to changes in the local charge environment. These spectral changes, combined with the tunneling of charges from the environment to the QD, provide an explanation of the bunching observed in the correlations.

We demonstrate the simultaneous dressing of both vacuum-to-exciton and exciton-to-biexciton transitions of a single semiconductor quantum dot in a high-Q micropillar cavity, using photoluminescence spectroscopy. Resonant two-photon excitation of the biexciton is achieved by spectrally tuning the quantum dot emission with respect to the cavity mode. The cavity couples to both transitions and amplifies the Rabi frequency of the likewise resonant continuous wave laser, driving the transitions. We observe strong-field splitting of the emission lines, which depend on the driving Rabi field amplitude and the cavity-laser detuning. A dressed state theory of a driven 4-level atom correctly predicts the distinct spectral transitions observed in the emission spectrum, and a detailed description of the emission spectra is further provided through a polaron master equation approach which accounts for cavity coupling and acoustic phonon interactions of the semiconductor medium.

The magnetotransport of highly mobile two-dimensional electrons in wide GaAs single quantum wells with three populated subbands placed in tilted magnetic fields is studied. The bottoms of the lower two subbands have nearly the same energy while the bottom of the third subband has a much higher energy $(E_1\approx E_2\ll E_3)$. At zero in-plane magnetic fields, magnetointersubband oscillations (MISO) between the $i\text{th}$ and $j\text{th}$ subbands are observed and obey the relation ${\mathrm{\Delta }}_{ij}=E_j-E_i=k\hslash {\omega }_c$, where ${\omega }_c$ is the cyclotron frequency and $k$ is an integer. An application of in-plane magnetic field produces dramatic changes in MISO and the corresponding electron spectrum. Three regimes are identified. At $\hslash {\omega }_c\ll {\mathrm{\Delta }}_{12}$, the in-plane magnetic field increases considerably the gap ${\mathrm{\Delta }}_{12}$, which is consistent with the semiclassical regime of electron propagation. In contrast, at strong magnetic fields $\hslash {\omega }_c\gg {\mathrm{\Delta }}_{12}$ relatively weak oscillating variations of the electron spectrum with the in-plane magnetic field are observed. At $\hslash {\omega }_c\approx {\mathrm{\Delta }}_{12}$, the electron spectrum undergoes a transition between these two regimes through magnetic breakdown. In this transition regime MISO with odd quantum number $k$ terminate, while MISO corresponding to even $k$ evolve $continuously$ into the high-field regime corresponding to $\hslash {\omega }_c\gg {\mathrm{\Delta }}_{12}$.

Motivated by recent experiments observing spin-orbit torque (SOT) acting on the magnetization $\vec{m}$ of a ferromagnetic (F) overlayer on the surface of a three-dimensional topological insulator (TI), we investigate the origin of the SOT and the magnetization dynamics in such systems. We predict that lateral F/TI bilayers of finite length, sandwiched between two normal metal leads, will generate a large anti-damping-like SOT per very low charge current injected parallel to the interface. The large values of anti-damping-like SOT are spatially localized around the transverse edges of the F overlayer. Our analysis is based on adiabatic expansion (to first order in $\partial \vec{m}/\partial t$) of time-dependent nonequilibrium Green functions (NEGFs), describing electrons pushed out of equilibrium both by the applied bias voltage and by the slow variation of a classical degree of freedom [such as $\vec{m}\left(t\right)$]. From it we extract formulas for spin torque and charge pumping, which show that they are reciprocal effects to each other, as well as Gilbert damping in the presence of SO coupling. The NEGF-based formula for SOT naturally splits into four components, determined by their behavior (even or odd) under the time and bias voltage reversal. Their complex angular dependence is delineated and employed within Landau-Lifshitz-Gilbert simulations of magnetization dynamics in order to demonstrate capability of the predicted SOT to efficiently switch $\vec{m}$ of a perpendicularly magnetized F overlayer.

Solving the Schrödinger problem for electrons in graphene subjected to both a stationary magnetic field and a strong high-frequency electromagnetic wave (dressing field), we found that the dressing field drastically changes the structure of Landau levels in graphene. As a consequence, the magnetoelectronic properties of graphene are very sensitive to the dressing field. Particularly, it is demonstrated theoretically that the dressing field strongly changes the optical spectra and the Shubnikov–de Haas oscillations. As a result, the developed theory opens a way for controlling magnetoelectronic properties of graphene with light.