An interacting spinless fermion wire coupled to a three-dimensional (3D) semiconducting substrate is approximated by a narrow ladder model (NLM) with varying number of legs. We compute density distributions, gaps, charge density wave (CDW) order parameters, correlation functions, and the central charge using the density-matrix renormalization group method. Three ground-state phases are observed: a one-component Luttinger liquid, a quasi-one-dimensional (1D) CDW insulator, and a band insulator. We investigated the convergence of the NLM properties with increasing number of legs systematically and confirm that the NLM is a good approximation for the quasi-1D phases (Luttinger liquid and CDW) of the 3D wire-substrate model. The quantum phase transitions between these phases are investigated as function of the coupling between wire and substrate. The critical nearest-neighbor interaction increases with increasing coupling between wire and substrate and thus the substrate stabilizes the Luttinger liquid in the wire. Our study confirms that a Luttinger liquid or CDW insulator phase could occur in the low-energy properties of atomic wires deposited on semiconducting substrates.

We investigate the regime of strong coupling of an ensemble of two-dimensional electrons to a single-mode cavity resonator. In particular, we realize such a regime of light-matter interaction by coupling the cyclotron motion of a collection of electrons on the surface of liquid helium to the microwave field in a semiconfocal Fabry-Pérot resonator. For the corotating component of the microwave field, the strong coupling is pronouncedly manifested by the normal-mode splitting in the spectrum of coupled field-particle motion. We present a complete description of this phenomenon based on classical electrodynamics, as well as show that the full quantum treatment of this problem results in mean-value equations of motion that are equivalent to our classical result. For the counterrotating component of the microwave field, we observe a strong resonance when the microwave frequency is close to both the cyclotron and cavity frequencies. We show that this surprising effect, which is not expected to occur under the rotating-wave approximation, results from the mixing between two polarization components of the microwave field in our cavity.

The valleytronic properties in monolayer $\mathrm{Mo}{\mathrm{S}}_2$ induced by the magnetic proximity effect of an antiferromagnetic CoO(111) substrate have been investigated using density functional theory and Berry curvature calculations. The results show that monolayer $\mathrm{Mo}{\mathrm{S}}_2$ can achieve a large valley splitting of 103 meV when coupled to an antiferromagnetic insulating CoO(111) substrate, which is in a robust type-II antiferromagnetic state at room temperature. The substrate provides a Zeeman field of 152 T. Breaking of the time-reversal symmetry of monolayer $\mathrm{Mo}{\mathrm{S}}_2$ leads to a prominent anomalous Hall conductivity that has a quantized character in the range of the band edge at the $K$ and $K^{\prime }$ valleys. Based on the result that the valley-contrasting Berry curvature is opposite at the $K$ and $K^{\prime }$ valleys, a valleytronic device that can be used as a filter for both the valley and spin is proposed.

Combining scanning tunneling microscopy and atomic force spectroscopy the dependence of the lateral force to move single adsorbed atoms on surfaces on the temperature is explored. A decrease of the force with increasing temperature is observed for all atoms and surfaces investigated and may be captured in a simple model. Crossing the critical temperature as well as surmounting the critical magnetic field of a superconductor does not lead to discriminable changes in the lateral force.

We study Bi-adsorbed In atomic chains on Si(111) in order to design a one-dimensional (1D) Rashba system using first-principles calculations. From the band dispersions and spin textures, we find that this system shows 1D giant Rashba splittings. The Rashba parameters of several structures in this system are comparable with other Rashba systems. Depending on the adsorption structure, this system also shows remarkable features such as a large out-of-plane spin polarization, the reversal of spin polarization in the Rashba bands, and a metal-insulator transition. We propose a mechanism to generate a nondissipative spin current by the gap opening due to an avoided crossing of Rashba bands. This mechanism is suitable for spintronic applications without requiring an external magnetic field.

The spin-orbit coupling (SOC) interactions, electron-correlation effects, and Hund coupling cooperate and compete with each other, leading to novel properties, quantum phase, and nontrivial topological electronic behavior in iridium oxides. Because of the well-separated ${\mathrm{IrO}}_6$ octahedra approaching cubic crystal-field limit, the hexagonal iridates $\mathrm{S}{\mathrm{r}}_{3}M\mathrm{Ir}{\mathrm{O}}_6$ ($M=\mathrm{Sr}$, Na, and Li) serve as canonical model systems to investigate the underlying physical properties that arise from the novel $j_{\mathrm{eff}}$ state. Based on density-functional theory calculations complemented by Green's function methods, we systematically explore the critical role of SOC on the electronic structure and magnetic properties of $\mathrm{S}{\mathrm{r}}_{3}M\mathrm{Ir}{\mathrm{O}}_6$. The crystal-field splitting combined with correlation effects are insufficient to account for the insulating nature, but the SOC interaction is the intrinsic source to trigger the insulating ground states in these hexagonal iridates. The decreasing geometry connectivity of ${\mathrm{IrO}}_6$ octahedra gives rise to the increase of effective electronic correlations and SOC interactions, tuning the hexagonal iridates from low-spin $j_{\mathrm{eff}}=1/2$ states with large local magnetic moments for the $\mathrm{I}{\mathrm{r}}^{4+}\left(5d^5\right)$ ions in $\mathrm{S}{\mathrm{r}}_4\mathrm{Ir}{\mathrm{O}}_6$ to nonmagnetic singlet $J_{\mathrm{eff}}=0$ states without magnetic moments for the ${\mathrm{Ir}}^{5+}$ (${5d}^4$) ions in ${\mathrm{Sr}}_3{\mathrm{NaIrO}}_6$ and ${\mathrm{Sr}}_3{\mathrm{LiIrO}}_6$. The theoretical calculated results are in good agreement with available experimental data, and explain the magnetic properties of $\mathrm{S}{\mathrm{r}}_{3}M\mathrm{Ir}{\mathrm{O}}_6$ well.

We propose a unified scaling theory of entanglement entropy in the confinements of finite bond dimensions, dynamics, and system sizes. In the theory, the recently introduced finite-entanglement scaling is generalized to the dynamics subjected to a linear driving along with a finite system size. Competition among the three scales as well as the correlation length of the system is analyzed in detail. Interesting regimes and their complicated crossovers together with their characteristics follow naturally. The theory is verified by time-evolving-block-decimation-based numerical simulations in the one-dimensional transverse-field Ising model under a linear driving.

Experiments on resonances of conduction electrons in InGaAs/InAlAs double quantum wells at megagauss magnetic fields in the Faraday geometry are reported. We observe four magneto-optical transitions in 2D systems: two cyclotron resonances assisted by emissions of InAs-like and GaAs-like optic phonons, a combined (cyclotron-spin) resonance, and the combined resonance assisted by an emission of InAs-like phonon. The observations are very well described for three laser frequencies with the use of an eight-band (three level) $\mathbf{k}·\mathbf{p}$ model, taking into account position- and energy-dependent effective masses and spin $g$ factors. The observation of combined resonances in the Faraday configuration indicates that the energy subbands in the investigated quantum wells are not only markedly nonparabolic but also slightly nonspherical. It is indicated that the observations are possible due to the application of very high magnetic fields.

We study the compatibility between the PBEsol exchange-correlation energy functional of Phys. Rev. Lett. 100, 136406 (2008) and the rVV10 van der Waals nonlocal correlation functional of Phys. Rev. B 87, 041108 (2013). By applying a density-gradient dependence in the expression of rVV10, we develop a new functional: The PBEsol + rVV10s functional, which is remarkably accurate for layered solids, rare-gas crystals, densely packed bulk solids, and the adsorption of noble-gas atoms on metal surfaces, and is also competitive for noncovalent interactions between molecular complexes as well as for potential-energy curves of noble-gas dimers. The capacity of this dispersion-corrected functional to describe various interactions in solids makes it useful for electronic-structure calculations in solid-state physics and materials science.

We report here high-pressure–high-temperature Raman experiments performed on ${\text{BiVO}}_4$. We characterized the fergusonite and scheelite phases (powder and single crystal samples) and the zircon polymorph (nanopowder). The experimental results are supported by ab initio calculations, which, in addition, provide the vibrational patterns. The temperature and pressure behavior of the fergusonite lattice modes reflects the distortions associated with the ferroelastic instability. The linear coefficients of the zircon phase are in sharp contrast to the behavior observed in the fergusonite phase. The boundary of the fergusonite-to-scheelite second-order phase transition is given by $T_{F-\mathrm{Sch}}\left(\text{K}\right)=-166\left(8\right)P\left(\text{GPa}\right)+528\left(5\right)$. The zircon-to-scheelite, irreversible, first-order phase transition takes place at $T_{Z-\mathrm{Sch}}\left(\text{K}\right)=-107\left(8\right)P\left(\text{GPa}\right)+690\left(10\right)$. We found evidence of additional structural changes around 15.7 GPa, which in the downstroke were found to be not reversible. We analyzed the anharmonic contribution to the wave-number shift in fergusonite using an order parameter. The introduction of a critical temperature depending both on temperature and pressure allows for a description of the results of all the experiments in a unified way.

Knowing a quantum system's environment is critical for its practical use as a quantum device. Qubit sensors can reconstruct the noise spectral density of a classical bath, provided long enough coherence time. Here, we present a protocol that can unravel the characteristics of a more complex environment, comprising both unknown coherently coupled quantum systems, and a larger quantum bath that can be modeled as a classical stochastic field. We exploit the rich environment of a nitrogen-vacancy center in diamond, tuning the environment behavior with a bias magnetic field, to experimentally demonstrate our method. We show how to reconstruct the noise spectral density even when limited by relatively short coherence times, and identify the local spin environment. Importantly, we demonstrate that the reconstructed model can have predictive power, describing the spin qubit dynamics under control sequences not used for noise spectroscopy, a feature critical for building robust quantum devices. At lower bias fields, where the effects of the quantum nature of the bath are more pronounced, we find that more than a single classical noise model are needed to properly describe the spin coherence under different controls, due to the back action of the qubit onto the bath.

Spin torques play a crucial role in operative properties of modern spintronic devices. To study current-driven magnetization dynamics, spin-torque terms providing the action of spin-polarized currents have previously often been added in a phenomenological way to the Landau-Lifshitz-Gilbert equation describing the local spin dynamics, yet without derivation from fundamental principles. Here, starting from the Dirac-Kohn-Sham theory and incorporating nonlocal spin transport we rigorously derive the various spin-torque terms that appear in current-driven magnetization dynamics. In particular we obtain an extended magnetization dynamics equation that precisely contains the nonrelativistic adiabatic and relativistic nonadiabatic spin-transfer torques of the Berger and Zhang-Li forms as well as relativistic spin-orbit torques. We derive in addition a previously unnoticed relativistic spin-torque term and moreover show that the various obtained spin-torque terms do not appear in the same mathematical form in both the Landau-Lifshitz and Landau-Lifshitz-Gilbert equations of spin dynamics.

Temperature-induced transition of thin superconductor/ferromagnet/normal-metal (S/F/N) hybrid structure to an in-plane Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state is accompanied by vanishing of effective inverse magnetic field penetration depth ${\mathrm{\Lambda }}^{-1}$ [S. V. Mironov et al., Phys. Rev. Lett. 121, 077002 (2018)]. Here we show that ${\mathrm{\Lambda }}^{-1}$ goes to zero only in the limit of zero magnetic field $H\to 0$ and at any finite parallel $H$ or in-plane current $I$ it is finite and positive in the FFLO state, which implies a diamagnetic response. We demonstrate that ${\mathrm{\Lambda }}^{-1}$ has a nonmonotonic dependence on $H$ and $I$ not only in the parameter range corresponding to the FFLO phase domain but also in its vicinity. We find that for S/F/N/F/S structures with certain thicknesses of F layers there is a temperature-, current-, and magnetic-field-driven transition to and out of the FFLO phase with a simultaneous jump of ${\mathrm{\Lambda }}^{-1}$.

A new phenomenological model is proposed to describe the evolution of the Fermi surface (FS) in a wide range of dopings. It reproduces the key features of the cuprates in the underdoped phase above the superconducting temperature $T_c$. It is shown that the explicit accounting of strong electron correlation in the framework of the $t-J$ model taken in complementary to the translational symmetry breaking induced by the charge density wave (CDW) gives rise to the Fermi surface reconstruction (FSR) into small electron pockets. While the strong Coulomb repulsion leads to an emergence of the arclike Fermi surface in the pseudogap (PG) phase, the Bragg reflection on the boundaries of the reduced Brillouin zone (BZ) opens up a possibility to close the quasiparticle orbits. Direct calculation of the FS properties allows us to unveil the scenario of the experimentally observed transition to the CDW phase that sets in at the doping level $\delta \approx 0.08$ and is accompanied by a divergence of the carriers effective mass and the sign reversals of the Hall and Seebeck coefficients.

Recent progress toward the fabrication of Majorana-based qubits has sparked the need for systematic approaches to optimize experimentally relevant parameters for the realization of robust Majorana bound states. Here, we introduce an efficient numerical method for the real-space optimization of tunable parameters, such as electrostatic potential profiles and magnetic field textures, in Majorana wires. Combining ideas from quantum control and quantum transport, our algorithm, applicable to any noninteracting tight-binding model, operates on a largely unexplored parameter space and opens new routes for Majorana bound states with enhanced robustness. Contrary to common belief, we find that spatial inhomogeneities of parameters can be a resource for the engineering of Majorana bound states.

We address the hydrodynamics of operator spreading in interacting integrable lattice models. In these models, operators spread through the ballistic propagation of quasiparticles, with an operator front whose velocity is locally set by the fastest quasiparticle velocity. In interacting integrable systems, this velocity depends on the density of the other quasiparticles, so equilibrium density fluctuations cause the front to follow a biased random walk, and therefore to broaden diffusively. Ballistic front propagation and diffusive front broadening are also generically present in nonintegrable systems in one dimension; thus, although the mechanisms for operator spreading are distinct in the two cases, these coarse-grained measures of the operator front do not distinguish between the two cases. We present an expression for the front-broadening rate; we explicitly derive this for a particular integrable model (the “Floquet-Fredrickson-Andersen” model), and argue on kinetic grounds that it should apply generally. Our results elucidate the microscopic mechanism for diffusive corrections to ballistic transport in interacting integrable models.

I demonstrate the potential of reinforcement learning (RL) to prepare quantum states of strongly periodically driven nonlinear single-particle models. The ability of Q-learning to control systems far away from equilibrium is exhibited by steering the quantum Kapitza oscillator to the Floquet-engineered stable inverted position in the presence of a strong periodic drive within several shaking cycles. The study reveals the potential of the intraperiod (micromotion) dynamics, often neglected in Floquet engineering, to take advantage over pure stroboscopic control at moderate drive frequencies. Without any knowledge about the underlying physical system, the algorithm is capable of learning solely from tried protocols and directly from simulated noisy quantum measurement data, and is stable to noise in the initial state and sources of random failure events in the control sequence. Model-free RL can provide new insights into automating experimental setups for out-of-equilibrium systems undergoing complex dynamics, with potential applications in quantum information, quantum optics, ultracold atoms, trapped ions, and condensed matter.

The understanding of the dynamical properties of skyrmions is a fundamental aspect for the realization of a competitive skyrmion based technology beyond complementary metal-oxide semiconductors. Most of the theoretical approaches are based on the approximation of a rigid skyrmion. However, thermal fluctuations can drive a continuous change of the skyrmion size via the excitation of thermal modes. Here, by taking advantage of the Hilbert-Huang transform, we demonstrate that at least two thermal modes can be excited which are nonstationary in time. In addition, one limit of the rigid skyrmion approximation is that this hypothesis does not allow for correctly describing the recent experimental evidence of skyrmion Hall angle dependence on the amplitude of the driving force, which is proportional to the injected current. In this paper, we show that, in an ideal sample, the combined effect of fieldlike and dampinglike torques on a breathing skyrmion can indeed give rise to such a current dependent skyrmion Hall angle. While here we design and control the breathing mode of the skyrmion, our results can be linked to the experiments by considering that the thermal fluctuations and/or disorder can excite the breathing mode. We also develop a generalized Thiele equation and propose an experiment to validate our findings.

A microscopic calculation and symmetry argument reveal superconductivity in the vicinity of parity-violating magnetic order. In a crystal structure lacking local space inversion symmetry, an augmented cluster magnetic multipole order may break global inversion symmetry, and therefore it is classified into an odd-parity multipole order. We investigate unconventional superconductivity induced by an odd-parity magnetic multipole fluctuation in a two-dimensional two-sublattice Hubbard model motivated by ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$. We find that even-parity superconductivity is more significantly suppressed by spin-orbit coupling than that in a globally noncentrosymmetric system. Consequently, two odd-parity superconducting states are stabilized by magnetic multipole fluctuations in a large spin-orbit coupling region. Both of them are identified as $Z_2$ topological superconducting states. The obtained gap function of intersublattice pairing shows a gapped or nodal structure protected by nonsymmorphic symmetry. Our finding implies a family of odd-parity topological superconductors. Candidate materials are discussed.