We study the quantum beats in the polarization of the two-pulse photon echo from donor-bound exciton ensembles in semiconductor quantum wells. To induce these quantum beats, a sequence composed of a circularly polarized and a linearly polarized picosecond laser pulse in combination with an external transverse magnetic field is used. This results in an oscillatory behavior of the photon echo amplitude, detected in the ${\sigma }^{+}$ and ${\sigma }^{-}$ circular polarizations, occurring with opposite phases relative to each other. The beating frequency is the sum of the Larmor frequencies of the resident electron and the heavy hole when the second pulse is polarized along the magnetic field. The beating frequency is, on the other hand, the difference of these Larmor frequencies when the second pulse is polarized orthogonal to the magnetic field. The measurement of both beating frequencies serves as a method to determine precisely the in-plane hole $g$ factor, including its sign. We apply this technique to observe the quantum beats in the polarization of the photon echo from the donor-bound excitons in a 20-nm-thick CdTe/${\mathrm{Cd}}_{0.76}{\mathrm{Mg}}_{0.24}\mathrm{Te}$ quantum well. From these quantum beats we obtain the in-plane heavy-hole $g$ factor $g_h=-0.143\pm 0.005$.

The magnetic structure of the antiferromagnetic square cupola compound ${\mathrm{Ba}\left(\mathrm{TiO}\right)\mathrm{Cu}}_4{(\mathrm{PO}}_4{)}_4$ with the tetragonal structure is studied with ${}^{31}\mathrm{P}$ and ${}^{63,65}\mathrm{Cu}$ nuclear magnetic resonance techniques. The ${}^{31}\mathrm{P}$ magnetic hyperfine shift $K$ shows a clear splitting at the Néel temperature $T_N=9.5$ K, where the resonance breaks down into two lines when an external magnetic field is oriented along the $c$ axis and into four lines when the field is along the $a$ axis. In the paramagnetic region $K\left(T\right)$ follows the temperature dependence of the magnetic susceptibility $\chi \left(T\right)$. From the $K$ vs $\chi$ plot we determined nearly equal hyperfine field values $H{}_{\text{hf}}^a=765$ mT/${\mu }_B$ and $H{}_{\text{hf}}^c=740$ mT/${\mu }_B$ for the magnetic field oriented along $a$ and $c$, respectively. From the rotation of the single crystal in the external magnetic field we determined eight different orientations of the $K$ tensor in the paramagnetic region. In the antiferromagnetic state at $T=6$ K, the rotation of the single crystal shows at phosphorus eight different orientations of the local field $B_{\text{int}}=35.6$ mT. The NMR spectrum of copper at zero external field shows local field at copper nucleus $B_{\text{loc}}=14.77$ T, directed perpendicular to the plane of the ${\mathrm{CuO}}_4$ plaquette, confirming a previously determined quadrupolar configuration of magnetic moments ${\mathrm{\Gamma }}_3\left(2\right)$ [K. Kimura et al., Nat. Commun. 7, 13039 (2016); P. Babkevich et al., Phys. Rev. B 96, 214436 (2017)].

Nanoscale mechanical resonators are widely utilized to provide high sensitivity force detectors. Here we demonstrate that such high-quality-factor resonators immersed in superfluid ${}^4\mathrm{He}$ can be excited by a modulated flux of phonons. A nanosized heater immersed in superfluid ${}^4\mathrm{He}$ acts as a source of ballistic phonons in the liquid—“phonon wind”. When the modulation frequency of the phonon flux matches the resonance frequency of the mechanical resonator, the motion of the latter can be excited. This ballistic thermomechanical effect can potentially open up new types of experiments in quantum fluids.

We derive an effective classical model to describe the Mott transition of the half-filled one-band Hubbard model in the framework of the dynamical mean-field theory with hybridization expansion of the continuous time quantum Monte Carlo. We find a simple two-body interaction of exponential form and reveal a classical correspondence of the Mott transition driven by a logarithmically divergent interaction length. Our paper provides an alternative angle to view the Mott physics and suggests a renewed possibility to extend the application of the quantum-to-classical mapping in understanding condensed-matter physics.

We develop a method for calculating the fundamental electronic gap of semiconductors and insulators using grand canonical quantum Monte Carlo simulations. We discuss the origin of the bias introduced by supercell calculations of finite size and show how to correct the leading and subleading finite size errors either based on observables accessible in the finite-sized simulations or from density-functional theory calculations. Our procedure is applied to carbon, silicon, and molecular hydrogen crystals, and compared to experiment for carbon and silicon.

We modify an antiferromagnetic-based helimagnet ${\mathrm{Ni}}_2{\mathrm{InSbO}}_6$ to a ferrimagnetic-based helimagnet by partial substitution of Cr for In. Upon applying a magnetic field, uncompensated moments bring about the collinear ferrimagnetic structure featured by the plateaulike magnetization and the disappearance of the magnetic superstructure. Furthermore, we have found an incommensurate magnetic phase distinct from the zero-field helix phase just below the Néel temperature as magnetic skyrmion crystal phases in some other noncentrosymmetric magnets. These findings provide a methodology to design non-coplanar magnetic states based on the material database.

We present a general coupled electron-phonon Boltzmann transport equations (BTEs) scheme designed to capture the mutual drag of the two interacting systems. By combining density functional theory based first-principles calculations of anharmonic phonon-phonon interactions with physical models of electron-phonon interactions, we apply our implementation of the coupled BTEs to calculate the thermal conductivity, mobility, Seebeck and Peltier coefficients of $n$-doped gallium arsenide. The measured low-temperature enhancement in the Seebeck coefficient is captured using the solution of the fully coupled electron-phonon BTEs, while the uncoupled electron BTE fails to do so. This work gives insights into the fundamental nature of charge and heat transport in semiconductors and advances predictive ab initio computational approaches. We discuss possible extensions of our work.

Spin-transfer torques (STT), Gilbert damping (GD), and effective spin renormalization (ESR) are investigated microscopically in a two-dimensional Rashba ferromagnet with spin-independent Gaussian white-noise disorder. Rashba spin-orbit coupling-induced anisotropy of these phenomena is thoroughly analyzed. For the case of two partly filled spin subbands, a remarkable relation between the anisotropic STT, GD, and ESR is established. In the absence of magnetic field and other torques on magnetization, this relation corresponds to a current-induced motion of a magnetic texture with the classical drift velocity of conduction electrons. Finally, we compute spin susceptibility of the system and generalize the notion of spin-polarized current.

The identification of general principles for stabilizing magnetic skyrmion phases in bulk materials over wide ranges of temperatures is a prerequisite to the development of skyrmion-based spintronic devices. Lacunar spinels with the formula ${\mathrm{GaM}}_4{\mathrm{X}}_8$, with $\mathrm{M}=\mathrm{V},\mathrm{Mo}$ and $\mathrm{X}=\mathrm{S},\mathrm{Se},$ are a convenient case study towards this goal, as they are some of the first bulk systems suggested to host equilibrium chiral skyrmions far from the paramagnetic transition. We derive the magnetic phase diagrams likely to be observed in these materials, accounting for all possible magnetic interactions, and prove that skyrmion stability in the lacunar spinels is a general consequence of their crystal symmetry rather than the details of the material chemistry. Our results are consistent with all experimental reports in this space and demonstrate that the differences in the phase diagrams of particular spinel chemistries are determined by the magnetocrystalline anisotropy, up to a normalization factor. We conclude that skyrmion formation over wide ranges of temperatures can be expected in all lacunar spinels, as well as in a wide range of uniaxial systems with low magnetocrystalline anisotropy.

The coexistence of phases, characterized by different electronic degrees of freedom, commonly occurs in layered superconductors. Among them, alkaline intercalated chalcogenides are model systems showing the microscale coexistence of paramagnetic (PAR) and antiferromagnetic (AFM) phases, however, the temporal behavior of different phases is still unknown. Here, we report a visualization of the atomic motion in the granular phase of ${\mathrm{K}}_x{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_2$ using x-ray photon correlation spectroscopy. Unlike the PAR phase, the AFM texture reveals an intermittent dynamics with avalanches as in martensites. When cooled down across the superconducting transition temperature $T_c$, the AFM phase goes through an anomalous slowing behavior, suggesting a direct relationship between the atomic motions in the AFM phase and the superconductivity. In addition to providing compelling evidence of avalanchelike dynamics in a layered superconductor, the results provide a basis for different theoretical models to describe the quantum states in inhomogeneous solids.

A combination of x-ray-diffraction, x-ray absorption, x-ray magnetic circular dichroism, macroscopic magnetization, and muon-spin relaxation measurements is used to investigate the interplay between structure (both crystallography and microstructure) and magnetism in ${\mathrm{Sr}}_2{\mathrm{YIrO}}_6$ as a function of both chemical (Ca-doping) and physical (hydrostatic) pressure. X-ray absorption spectroscopy clearly shows that the physical pressure is more effective in modifying the structure. On the other hand, the dichroic measurements evidence a constant magnetic signal with physical pressure and strong differences with Ca-doping. Muon-spin relaxation reveals the presence of magnetic order, even when this is hidden in the magnetization data. From the combined analysis, the magnetic results are explained in terms of the presence of ${\mathrm{Ir}}^{6+}$ or ${\mathrm{Ir}}^{4+}$ ions in magnetic clusters, most likely located at and triggered by antisite disorder. The measurements under high physical pressure indicate that the magnetic state is independent of the crystallographic details such as Ir-O distances and Ir-O-Y angles.

We analyze the influence of optically generated nonequilibrium phonons on the spin relaxation and effective spin temperature of an individual Cr atom inserted in a quantum dot. Using a three-pulse pump-probe technique, we show that the spin relaxation measured in resonant optical pumping experiments strongly depends on the optical excitation conditions. We observe for an isolated Cr in the dark a heating time shorter than a few hundred nanoseconds after an initial high-power nonresonant excitation pulse. A cooling time larger than a few tens of microseconds, independent of the excitation, is obtained in the same experimental conditions. We show that a tunable spin-lattice coupling dependent on the density of nonequilibrium phonons can explain the observed dynamics. Low-energy excitation conditions are found where the Cr spin states $S_z=\pm 1$ can be efficiently populated by a nonresonant optical excitation, prepared and read out by resonant optical pumping and conserved in the dark for a few microseconds.

Comprehension of electrical and mechanical properties of atomic-sized metal contacts are of central importance for a number of different fields, e.g., the development of interconnects in nanoelectronics, the description of friction on the nanoscale, and break-junction experiments. Au-Au nanocontacts are experimentally particularly suited because of the chemical inertness and serve as a valuable testbed for charge transport in the quantum regime. Here we use the tip of a low-temperature scanning tunneling microscope to form atomic contacts with a reconstructed Au(111) surface. As both electrodes are perfectly stable throughout 19 000 individual measurements, the atomic configuration of the point contact can be precisely controlled by variation of the tip position with respect to the substrate atoms and the reconstruction domains with different stacking. This allows us to reveal the influence of the two last atomic layers of each electrode on the conductance and the stiffness of the junction. Four different conductance regimes can be distinguished and explained by two atomic conductance channels. The stiffness of the junction can be inferred from the adhesion hysteresis that is reduced at threefold hollow sites and closed-packed domains of the reconstructed surface. Our experimental results will allow profound tests of atomic-scale theoretical simulations in the field.

Magnetic skyrmions are nanosized magnetization whirls that exhibit topological robustness and nontrivial magnetoelectrical properties, such as emergent electromagnetism and intriguing spin dynamics in the microwave-frequency region. In chiral magnets, skyrmions are usually found at a pocket in the phase diagram in the vicinity of the ordering temperature, wherein they order in the form of a hexagonal skyrmion lattice (SkL). It is generally believed that this equilibrium SkL phase is a uniform, long-range-ordered magnetic structure with a well-defined lattice constant. Here, using high-resolution small-angle resonant elastic x-ray scattering, we study the field and temperature dependence of the skyrmion lattice in FeGe and ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$ membranes. Indeed, ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$ shows the expected rigid skyrmion lattice, known from bulk samples, that is unaffected by tuning field and temperature within the phase pocket. In stark contrast, the lattice constant and skyrmion size in FeGe membranes undergo a continuous evolution within the skyrmion phase pocket, whereby the lattice constant changes by up to 15% and the magnetic scattering intensity varies significantly. Using micromagnetic modeling, it is found that for FeGe the competing energy terms contributing to the formation of the skyrmion lattice fully explain this breathing behavior. In contrast, for ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$ this stabilizing energy balance is less affected by the smaller field variation across the skyrmion pocket, leading to the observed rigid lattice structure.

High-resolution laser-based angle-resolved photoemission measurements have been carried out on ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ (Bi2212) and ${\mathrm{Bi}}_2{\mathrm{Sr}}_{2-x}{\mathrm{La}}_x{\mathrm{CuO}}_{6+\delta }$ (Bi2201) superconductors. Unexpected hybridization between the main band and the superstructure band in Bi2212 is clearly revealed. In the momentum space where one main Fermi surface intersects with one superstructure Fermi surface, four bands are observed instead of two. The hybridization exists in both the superconducting state and the normal state, and in Bi2212 samples with different doping levels. Such a hybridization is not observed in Bi2201. This phenomenon can be understood by considering the bilayer splitting in Bi2212, the selective hybridization of two bands with peculiar combinations, and the altered matrix element effects of the hybridized bands. These observations indicate that the origin of the superstructure bands is intrinsic to the ${\mathrm{CuO}}_2$ planes. Therefore, understanding physical properties and superconductivity in Bi2212 should consider the complete Fermi surface topology which involves the main bands, the superstructure bands, and their interactions.

We construct a set of exact, highly excited eigenstates for a nonintegrable spin-1/2 model in one dimension that is relevant to experiments on Rydberg atoms in the antiblockade regime. These states provide a new solvable example of quantum many-body scars: their sub-volume-law entanglement and equal energy spacing allow for infinitely long-lived coherent oscillations of local observables following a suitable quantum quench. While previous works on scars have interpreted such oscillations in terms of the precession of an emergent macroscopic SU(2) spin, the present model evades this description due to a set of emergent kinetic constraints in the scarred eigenstates that are absent in the underlying Hamiltonian. We also analyze the set of initial states that give rise to periodic revivals, which persist as approximate revivals on a finite timescale when the underlying model is perturbed. Remarkably, a subset of these initial states coincides with the family of area-law entangled Rokhsar-Kivelson states shown by Lesanovsky to be exact ground states for a class of models relevant to experiments on Rydberg-blockaded atomic lattices.

Noncentrosymmetric superconductors with line nodes are expected to possess topologically protected flat zero-energy bands of surface states, which can be described as Majorana modes. We here investigate their fate if residual interactions beyond BCS theory are included. For a minimal square-lattice model with a plaquette interaction, we find stringlike integrals of motion that form Clifford algebras and lead to exact degeneracies. These degeneracies strongly depend on whether the numbers of sites in the $x$ and $y$ directions are even or odd and are robust against disorder in the interactions. We show that the mapping of the Majorana model onto two decoupled spin compass models [Y. Kamiya et al., Phys. Rev. B 98, 161409(R) (2018)] and extra spectator degrees of freedom only works for open boundary conditions. The mapping shows that the three-leg and four-leg Majorana ladders are integrable, while systems of larger width are not. In addition, the mapping maximally reduces the effort for exact diagonalization, which is utilized to obtain the gap above the ground states. We find that this gap remains open if one dimension is kept constant and even, while the other is sent to infinity, at least if that dimension is odd. Moreover, we compare the topological properties of the interacting Majorana model to those of the toric-code model. The Majorana model has long-range entangled ground states that differ by ${\mathbb{Z}}_2$ fluxes through the system on a torus. The ground states exhibit string condensation similar to the toric code but the topological order is not robust. While the spectrum is gapped—due to spontaneous symmetry breaking inherited from the compass models—states with different values of the ${\mathbb{Z}}_2$ fluxes end up in the ground-state sector in the thermodynamic limit. Hence, the gap does not protect these fluxes against weak perturbations.

Inelastic light scattering experiments on low-energy intrasubband spin-density excitations (SDEs) are performed in (001)-grown modulation-doped GaAs-AlGaAs single quantum wells in in-plane external magnetic fields. The investigated samples possess balanced linear Bychkov-Rashba ($\alpha$) and Dresselhaus ($\beta$) spin-orbit strengths in two different configurations, $\alpha =\beta$ and $\alpha =-\beta$. Both configurations lead to an extreme anisotropy of the spin splitting of the conduction band, where the in-plane directions of maximum spin splitting for both configurations are perpendicular to each other. The spin splitting asymmetry can be directly detected via the SDE by breaking of the time-reversal symmetry due to transfer of a momentum $\mathbf{q}$ in the quantum-well plane. In addition, the application of an in-plane magnetic field ${\mathbf{B}}_{\mathrm{ext}}\perp \mathbf{q}$ allows us to modulate the effective magnetic field. Via a numerical line-shape analysis of the experimental SDE spectra, we determine the relevant parameters of the samples. We find that the linear spin-orbit strength $\left|\alpha \right|=\beta$ is comparable for both samples, while the electron $g$ factors are markedly different. Furthermore, we experimentally quantify the values of the maximum internal spin-orbit fields, which are as high as $B_{\mathrm{so}}\sim 18\mathrm{T}$ for both samples.

Interacting and tunable quantum dots (QDs) have been extensively exploited in condensed matter physics and quantum information science. Using a low-temperature scanning tunneling microscope (STM), we both create and directly image a new type of coupled QD system in graphene, a highly interacting quantum relativistic system with tunable density. Using detailed scanning tunneling spectroscopy (STS) measurements, we show that Landau quantization inside a potential well enables novel electron confinement via the incompressible strips between partially filled Landau levels (LLs), forming isolated and concentric LL QDs. By changing the charge density and the magnetic field we can tune continuously between single- and double-concentric LL QD systems within the same potential well. In the concentric QD regime, single-electron charging peaks of the two dots intersect, displaying a characteristic avoidance pattern. At moderate fields, we observe an unconventional avoidance pattern that differs significantly from that observed in capacitively coupled double-QD systems. We find that we can reproduce in detail this anomalous avoidance pattern within the framework of the electrostatic double-QD model by replacing the capacitive interdot coupling with a phenomenological charge-counting system in which charges in the inner concentric dot are counted in the total charge of both islands. The emergence of such strange forms of interdot coupling in a single potential well, together with the ease of producing such charge pockets in graphene and other two-dimensional (2D) materials, reveals an intriguing testbed for the confinement of 2D electrons in customizable potentials.

We introduce a simple determinant diagrammatic Monte Carlo algorithm to compute the ground-state properties of a particle interacting with a Fermi sea through a zero-range interaction. The fermionic sign does not cause any fundamental problem when going to high diagram orders, and we reach order $N=30$. The data reveal that the diagrammatic series diverges exponentially as ${(-1/R)}^N$ with a radius of convergence $R<1$. Furthermore, on the polaron side of the polaron-dimeron transition, the value of $R$ is determined by a special class of three-body diagrams, corresponding to repeated scattering of the impurity between two particles of the Fermi sea. A power-counting argument explains why finite $R$ is possible for zero-range interactions in three dimensions. Resumming the divergent series through a conformal mapping yields the polaron energy with record accuracy.