The exact optical response of quantum few-level systems depends crucially on the exact choice of the incoming pulse areas. We use four-wave mixing (FWM) spectroscopy to infer the coherent response and dynamics of single InAs quantum dots (QDs) and study their pulse area dependence. By combining atomic force microscopy with FWM hyperspectral imaging, we show that the retrieved FWM signals originate from individual QDs enclosed in natural photonic defects. The optimized light-matter coupling in these defects allows us to perform our studies in a wide range of driving field amplitudes. When varying the pulse areas of the exciting laser pulses, Rabi rotations of microscopic interband coherences can be resolved by the two-pulse FWM technique. We investigate these Rabi coherence rotations within two- and three-level systems, both theoretically and experimentally, and explain their damping by the coupling to acoustic phonons. To highlight the importance of the pulse area influence, we show that the phonon-induced dephasing of QD excitons depends on the pulse intensity.

We present the synthesis and the experimental and theoretical study of the new member of the francisite family, $\mathrm{C}{\mathrm{u}}_3\mathrm{Yb}{\left(\mathrm{Se}{\mathrm{O}}_3\right)}_2{\mathrm{O}}_2\mathrm{Cl}$. The compound reaches an antiferromagnetic order at $T_{\mathrm{N}}=36.7\mathrm{K}$ and experiences first-order spin-reorientation transition to weakly ferromagnetic phase at $T_{\mathrm{R}}=8.7\mathrm{K}$ evidenced in specific heat $C_{\mathrm{p}}$ and magnetic susceptibility χ measurements. Distinctly different magnetization loops in $T<T_{\mathrm{R}}$ and $T_{\mathrm{R}}<T<T_{\mathrm{N}}$ temperature ranges reflect the interplay of rare-earth and transition-metal subsystems. At low temperatures, the saturation magnetization $M_{\mathrm{s}}\sim 5.2{\mu }_{\mathrm{B}}$ is reached in pulsed magnetic-field measurements. The electron spin resonance data reveal the complicated character of the absorption line attributed to response from both copper and ytterbium ions. Critical broadening of the linewidth at the phase transitions points to quasi-two-dimensional character of the magnetic correlations. The spectroscopy of $\mathrm{Y}{\mathrm{b}}^{3+}$ ions evidences splitting of the lowest-energy Kramers doublet of ${}^2{F}_{5/2}$ excited multiplet at $T_{\mathrm{R}}<T<T_{\mathrm{N}}$ while the ground Kramers doublet splits only at $T<T_{\mathrm{R}}$. We describe the magnetic properties both above and below the spin-reorientation transition in the framework of a unified approach based on the mean-field approximation and crystal-field calculations.

Frustration and the associated phenomenon of “avoided criticality” have been proposed as an explanation for the dramatic relaxation slowdown in glass-forming liquids. To test this, we have undertaken a Monte Carlo study of possibly the simplest such problem, the two-dimensional $XY$ model with frustration corresponding to a small flux $f$ per plaquette. At $f=0$, there is a Berezinskii-Kosterlitz-Thouless transition at $T^{*}$, but at any small but nonzero $f$, this transition is avoided and replaced (presumably) by a vortex-ordering transition at much lower temperatures. We thus have studied the evolution of the dynamics for small and moderate $f$ as the system is cooled from above $T^{*}$ to below. Although we do find strongly temperature-dependent slowing of the dynamics as $T$ crosses $T^{*}$ and that simultaneously the dynamics becomes more complex, neither effect is anywhere nearly as dramatic as the corresponding phenomena in glass-forming liquids. At the very least, this implies that the properties of supercooled liquids must depend on more than frustration and the existence of an avoided transition.

We propose a scaling law for anomalous Hall effect in ferromagnetic thin films. Our approach distinguishes multiple scattering sources, namely, bulk impurity, phonon for Hall resistivity, and most importantly the rough surface contribution to longitudinal resistivity. In stark contrast to earlier laws that rely on temperature- and thickness-dependent fitting coefficients, this scaling law fits the recent experimental data excellently with constant parameters that are independent of temperature and film thickness, strongly indicating that this law captures the underlying physical processes. Based on a few data points, this scaling law can even fit all experimental data in full temperature and thickness range. We apply this law to interpret the experimental data for Fe, Co, and Ni and conclude that (i) the phonon-induced skew scattering is unimportant as expected; (ii) contribution from the impurity-induced skew scattering is negative; (iii) the intrinsic (extrinsic) mechanism dominates in Fe (Co), and both the extrinsic and intrinsic contributions are important in Ni.

We have investigated ultrafast quenching of the Tb $4f$ magnetic order in $\mathrm{C}{\mathrm{o}}_{74}\mathrm{T}{\mathrm{b}}_{26}$ alloys, induced by femtosecond hot-electron pulses. The hot-electron pulses were produced in specific nonmagnetic capping layers by infrared femtosecond laser pulses. Our experimental results show that subpicosecond dynamics of Tb $4f$ magnetic moments can be induced by nonthermal and thermal hot-electron. We further demonstrate that the demagnetization efficiencies of nonthermal and thermal hot-electron are similar. However, the characteristic demagnetization times show values of 0.35 ps for nonthermal hot-electron excitations and 1.2 ps for thermal hot-electron excitations. We explain this temporal elongation by the propagation time of thermal hot-electron through the 15-nm-thick CoTb film.

To investigate an influence of the quadrupole interaction to an antiferromagnetic phase transition at $T_{\mathrm{N}}=5$ K in ${\mathrm{HoRu}}_2{\mathrm{Al}}_{10}$ under magnetic fields $H$, we measured the specific heat, magnetization, and elastic moduli. From these measurements, we clarified the magnetic phase diagram for the three principal axes. Under $H$ along the $b$ axis, $T_{\mathrm{N}}$ decreases monotonically with increasing $H$. Although $T_{\mathrm{N}}$ in $H$ along the $a$ and $c$ axes is also reduced with increasing $H$, the phase boundary shows an anomalous inflection point around 5 T. With increasing $H$, we found an enhancement of elastic softening toward $T_{\mathrm{N}}$ due to the quadrupole interaction in the longitudinal and transverse moduli under $H\left|\right|c$. We proposed the $4f$-level scheme in ${\mathrm{HoRu}}_2{\mathrm{Al}}_{10}$, of which the ground state singlet and the first excited singlet at 0.5 K form a quasidoublet, from orthorhombic crystal electric field analyses. An interlevel quadrupole transition exists between the ground and exited singlets under fields. These suggest a possibility that the quadrupole interaction plays a central role for the inflection point in the phase diagram.

We apply the first-principles density functional theory + dynamical mean-field theory framework to evaluate the crystal-field splitting on rare-earth sites in hard magnetic intermetallics. An atomic (Hubbard-I) approximation is employed for local correlations on the rare-earth $4f$ shell and self-consistency in the charge density is implemented. We reduce the density functional theory self-interaction contribution to the crystal-field splitting by properly averaging the $4f$ charge density before recalculating the one-electron Kohn-Sham potential. Our approach is shown to reproduce the experimental crystal-field splitting in the prototypical rare-earth hard magnet ${\mathrm{SmCo}}_5$. Applying it to $R{\mathrm{Fe}}_{12}$ and $R{\mathrm{Fe}}_{12}X$ hard magnets ($R=\mathrm{Nd}$, Sm and $X=\mathrm{N}$, Li), we obtain in particular a large positive value of the crystal-field parameter $A_2^0\langle r^2\rangle$ in ${\mathrm{NdFe}}_{12}\mathrm{N}$ resulting in a strong out-of-plane anisotropy observed experimentally. The sign of $A_2^0\langle r^2\rangle$ is predicted to be reversed by substituting N with Li, leading to a strong out-of-plane anisotropy in ${\mathrm{SmFe}}_{12}\mathrm{Li}$. We discuss the origin of this strong impact of N and Li interstitials on the crystal-field splitting on rare-earth sites.

We propose that ultracold alkaline-earth-like atoms confined in one-dimensional optical lattice can realize a Kondo lattice model which hosts a symmetry-protected topological (SPT) phase and an associated quantum phase transition in a controllable manner. The symmetry protection of the phase transition is discussed from two different viewpoints: topological properties related to spatial patterns of Kondo singlets and symmetry eigenvalues of the spin states. We uncover the role of various symmetries in the phase diagram of this system by combining a weak-coupling approach by Abelian bosonization and strong-coupling pictures of ground states. Furthermore, we show that the bosonization approach correctly describes a crossover from a fermionic SPT phase to a bosonic SPT phase and an associated change of protecting symmetries as the charge degrees of freedom are frozen by the Hubbard repulsion.

We address polarization instability in a freely decaying polariton condensate created by 2-ps-long linearly polarized laser pulses in the upper sublevel of the lower-polariton (LP) branch in a GaAs-based microcavity with reduced symmetry. The generated linearly polarized condensate is found to lose its stability at excitation densities above the threshold value: it passes into the regime of inner Josephson oscillations with strongly oscillating circular and diagonal linear polarization degrees, as well as monotonically decreased oscillations in linear polarization accompanied by a gradual increase in the condensate of the low-sublevel component. These phenomena occur with a relatively small decrease in the total polarization and spatial coherence of the spinor condensate. At high LP densities, the LP-LP interaction leads to the nonlinear Josephson effect. All effects are found to be well reproduced by the model based on spinor Gross-Pitaevskii equations. The cause of the instability was clarified by considering a simplified model of the spinor 0D oscillator: it is spin anisotropy of the LP-LP interaction. The threshold density was shown to increase with decreasing difference $\mathrm{\Delta }\alpha$ of the constants of the interparticle interaction of LPs with identical and opposite spins as ${\delta }_l/\mathrm{\Delta }\alpha$, where ${\delta }_l$ is the LP level splitting. The reduction in the linear polarization is connected with the fact that the LPs escaping from the condensate with oscillating circular polarization carry off more energy than from the linearly polarized one.

We investigate the properties of the magnetocapacitance and dissipation factor of epitaxial graphene Hall bars with different electrode configurations to gain insight into the underlying physical mechanisms. The dependence of magnetocapacitance and dissipation factor on the magnetic field shows how the screening ability of the two-dimensional electron gas (2DEG) changes at the transition from the nonquantized to the quantized state. Both magnetocapacitance and dissipation factor exhibit a characteristic and correlated voltage dependence, which is attributed to the alternating contraction and expansion of the nonscreening 2DEG regions due to the alternating local electric field. Two regimes with seemingly different voltage dependencies are explained as the limiting cases of weak and strong electric fields of the same general voltage dependence. Electric fields in the plane of the 2DEG are found to cause about three orders of magnitude more ac dissipation than perpendicular electric fields. This strong directionality is attributed to the fact that the electrons are mobile in the plane of the 2DEG but are confined in the third dimension. In the quantized state, not only the screening edge of the 2DEG but also compressible puddles embedded in the bulk cause ac dissipation, as follows from the measured frequency dependence. Finally, characteristic parameters like the width of the screening edge, the threshold voltage, and the charging time of the compressible puddles are determined.

The strong electron-lattice interactions in correlated electron systems provide unique opportunities for altering the material properties with relative ease and flexibility. In this Rapid Communication, we use localized strain control via a focused-ion-beam patterning of $\mathrm{Ti}{\mathrm{O}}_2$ substrates to demonstrate that one can selectively engineer the insulator-to-metal transition temperature, the fractional component of the insulating and metallic phases, and the degree of optical anisotropy down to the length scales of the intrinsic phase separation in $\mathrm{V}{\mathrm{O}}_2$ thin films without altering the quality of the films. The effects of localized strain control on the strongly correlated electron system are directly visualized by state-of-the-art IR near-field imaging and spectroscopy techniques and x-ray microdiffraction measurements.

We numerically study the behavior of spin-$1/2$ fermions on a two-dimensional square lattice subject to a uniform magnetic field, where opposite spins interact via an on-site attractive interaction. Starting from the noninteracting case where each spin population is prepared in a quantum Hall state with unity filling, we follow the evolution of the system as the interaction strength is increased. Above a critical value and for sufficiently low flux density, we observe the emergence of a twofold quasidegeneracy accompanied by the opening of an energy gap to the third level. Analysis of the entanglement spectra shows that the gapped ground state is the bosonic $1/2$ Laughlin state. Our work therefore provides compelling evidence of a topological phase transition from the fermionic quantum Hall state at unity filling to the bosonic Laughlin state at a critical attraction strength of the order of the one-body spectrum linewidth.

We propose characterization of the three-dimensional topological insulator by using the Chern number for the entanglement Hamiltonian (entanglement Chern number). Here we take the extensive spin partition of the system, that pulls out the quantum entanglement between up spin and down spin of the many-body ground state. In three dimensions, the topological insulator phase is described by the section entanglement Chern number, which is the entanglement Chern number for a periodic plane in the Brillouin zone. The section entanglement Chern number serves as an interpolation of the ${\mathbb{Z}}_2$ invariants defined on time-reversal invariant planes. We find that the change of the section entanglement Chern number protects the Weyl point of the entanglement Hamiltonian, and the parity of the number of Weyl points distinguishes the strong topological insulator phase from the weak topological insulator phase.

The average density of states of two- and three-dimensional Weyl fermions is studied in the self-consistent Born approximation (SCBA) and its corrections. The latter have been organized in terms of a $1/N$ expansion. It turns out that an expansion in terms of the disorder strength is not applicable, as previously mentioned by other authors. Nevertheless, the $1/N$ expansion provides a justification of the SCBA as the large $N$ limit of Weyl fermions.

We use Schwinger-boson mean-field theory (SBMFT) to study the ground state of the spin-$S$ triangular-lattice Heisenberg model with nearest ($J_1$) and next-nearest ($J_2$) neighbor antiferromagnetic interactions. Previous work on the $S=1/2$ model leads us to consider two spin-liquid Ansätze, one symmetric and one nematic, which upon spinon condensation give magnetically ordered states with ${120}^{\circ }$ order and collinear stripe order, respectively. The SBMFT contains the parameter $\kappa$, the expectation value of the number of bosons per site, which in the exact theory equals $2S$. For $\kappa =1$ there is a direct, first-order transition between the ordered states as $J_2/J_1$ increases. Motivated by arguments that in SBMFT, smaller $\kappa$ may be more appropriate for describing the $S=1/2$ case qualitatively, we find that in a $\kappa$ window around 0.6, a region with the (gapped $Z_2$) symmetric spin liquid opens up between the ordered states. As a consequence, the static structure factor has the same peak locations in the spin liquid as in the ${120}^{\circ }$ ordered state, and the phase transitions into the ${120}^{\circ }$ and collinear stripe-ordered states are continuous and first order, respectively.

Boron nitride quantum dots (BNQDs) functionalized with chemical ligands exhibit intriguing optoelectronic properties due to the quantum confinement effect. This paper presents peculiar insights on the effect of side defects on the electronic structure and optical properties of BNQDs functionalized with different chemical bonds including hydrogen (H), nitrogen (N), hydroxyl (OH), amine (${\mathrm{NH}}_2$), and thiol groups (inspired by experimental reports of functionalized BN nanosheets and nanotubes) Weng et al., Chem. Soc. Rev. 45, 3989 (2016). Hybrid density functional simulations and Green's function calculations indicate an intriguing coexistence of two different Peierls-like distortions in the functionalized low-dimensional material. The presence of side defects increases the side strain and creates interband electronic states. As a result, the band gap of BNQDs could vary in a wide range depending on the type of chemical bonds and surface disorders. Enhanced edge states also improve the photoluminescence emission of the quantum dots. These side-defect enriched states in BNQDs create optical and electrical responses which could offer unprecedented potential for large scale nanophotonics such as photovoltaic, bioimaging, and quantum communication.

We study theoretically the spin-induced and photon-induced fluctuations of optical signals from a singly-charged quantum dot-microcavity structure. We identify the respective contributions of the photon-polariton interactions, in the strong light-matter coupling regime, and of the quantum back action induced by photon detection on the spin system. Strong spin projection by a single photon is shown to be achievable, allowing the initialization and measurement of a fully-polarized Larmor precession. The spectrum of second-order correlations is deduced, displaying information on both spin and quantum dot-cavity dynamics. The presented theory thus bridges the gap between the fields of spin noise spectroscopy and quantum optics.

In recent years there has been increasing excitement regarding nanomotors and particularly current-driven nanomotors. Despite the broad variety of stimulating results found, the regime of strong Coulomb interactions has not been fully explored for this application. Here we consider nanoelectromechanical devices composed of a set of coupled quantum dots interacting with mechanical degrees of freedom taken in the adiabatic limit and weakly coupled to electronic reservoirs. We use a real-time diagrammatic approach to derive general expressions for the current-induced forces, friction coefficients, and zero-frequency force noise in the Coulomb blockade regime of transport. We prove our expressions obey Onsager's reciprocity relations and the fluctuation-dissipation theorem for the energy dissipation of the mechanical modes. The obtained results are illustrated with a nanomotor consisting of a double quantum dot capacitively coupled to rotating charges. We analyze the dynamics and performance of the motor as a function of the applied voltage and loading force for trajectories encircling different triple points in the charge stability diagram.

Finite-length one-dimensional topological superconductor wires host localized Majorana zero modes at their ends. In realistic models, these appear only after a topological quantum critical point is crossed by external tuning of parameters. Thus, there is a universal finite-size scaling, governed by the critical point, that dictates the evolution of the energy of the Majorana modes near the transition. We first describe this scaling, then apply it in detail to an explicit synthetic topological superconductor model. Our work not only connects Ising quantum criticality with realistic nanowires in the presence of spin-orbit coupling, Zeeman splitting, and superconductivity, but also provides a viable experimental route for discerning the existence of the topological quantum critical point.