Using a four-band Hamiltonian, we study the phase boundary of spin-polarized-current state (SPCS) of interacting electrons in bilayer graphene. The model of spin-polarized-current state has previously been shown to resolve a number of experimental puzzles in bilayer graphene. The phase boundaries of the SPCS with and without the external voltage between the two layers are obtained in this work. An unusual phase boundary where there are two transition temperatures for a given carrier concentration is found at finite external voltage. The physics of this phenomenon is explained.

Time-resolved intensity cross-correlation measurements of radiative cascades are used for studying nonradiative relaxation processes of excited carriers confined in semiconductor quantum dots. We spectrally identify indirect radiative cascades which include intermediate phonon-assisted relaxations. The energy of the first photon reveals the multicarrier configuration prior to the nonradiative relaxation, while the energy of the second photon reveals the configuration after the relaxation. The intensity cross-correlation measurements thus provide quantitative measures of the nonradiative processes and their selection rules. We construct a model which accurately describes the experimental observations in terms of the electron-phonon and electron-hole exchange interactions. Our measurements and model provide a tool for engineering relaxation processes in semiconductor nanostructures.

Pressure-dependent powder x-ray diffraction, Raman spectroscopy, and dielectric constant measurements are performed on 0.$65{\mathrm{BiFeO}}_3\text{−}0.35{\mathrm{PbTiO}}_3$. Four structural transitions are reported in 0–34 GPa range. The transitions are observed at 0.7(1), 0.8(1), 3.4(1), and 6.1(1) GPa, where the system transformed from ambient tetragonal $P4mm$ to a mixed phase of tetragonal and rhombohedral $(P4mm+R3c)$, followed by transition to rhombohedral $R3c$, cubic Pm$3m$ and again to rhombohedral $R3c$ structure, respectively. The transformation to the cubic structure coincides with the ferroelectric-paraelectric transition. Higher-pressure cubic-to-rhombohedral transition shows a slight softening of Raman modes along with an enhancement in spontaneous strain. The high-pressure rhombohedral phase shows signatures of another ferroic transition.

We present an analytical description of localized spin-wave modes that form in a parabolic field minimum in a thin ferromagnetic film. Mode profiles proportional to Hermite functions are eigenfuctions of the applied field and exchange parts of the equations of motion, and also provide a basis for numerical approximation of magnetostatic interactions. We find that the spin-wave modes are roughly equally spaced in frequency and have roughly equal coupling to a uniform driving field. The calculated mode frequencies and corresponding profiles of localized spin-wave modes are in good agreement with micromagnetic modeling and previously published experimental results on multiple resonances from a series of localized modes detected by ferromagnetic resonance force microscopy.

We study the zero-temperature quantum phase diagram of a spin-1 Heisenberg antiferromagnet on the honeycomb lattice with both nearest-neighbor exchange coupling $J_1>0$ and frustrating next-nearest-neighbor coupling $J_2\equiv \kappa J_1>0$, using the coupled cluster method implemented to high orders of approximation, and based on model states with different forms of classical magnetic order. For each we calculate directly in the bulk thermodynamic limit both ground-state low-energy parameters (including the energy per spin, magnetic order parameter, spin stiffness coefficient, and zero-field uniform transverse magnetic susceptibility) and their generalized susceptibilities to various forms of valence-bond crystalline (VBC) order, as well as the energy gap to the lowest-lying spin-triplet excitation. In the range $0<\kappa <1$ we find evidence for four distinct phases. Two of these are quasiclassical phases with antiferromagnetic long-range order, one with two-sublattice Néel order for $\kappa <{\kappa }_{c_1}=0.250\left(5\right)$, and another with four-sublattice Néel-II order for $\kappa >{\kappa }_{c_2}=0.340\left(5\right)$. Two different paramagnetic phases are found to exist in the intermediate region. Over the range ${\kappa }_{c_1}<\kappa <{\kappa }_c^i=0.305\left(5\right)$ we find a gapless phase with no discernible magnetic order, which is a strong candidate for being a quantum spin liquid, while over the range ${\kappa }_c^i<\kappa <{\kappa }_{c_2}$ we find a gapped phase, which is most likely a lattice nematic with staggered dimer VBC order that breaks the lattice rotational symmetry.

Preparing and detecting spatially extended quantum superpositions of a massive object comprises an important fundamental test of quantum theory. These quantum states are extremely fragile and tend to quickly decay into incoherent mixtures due to the environmental decoherence. Experimental setups considered up to date address this threat in a conceptually straightforward way—by eliminating the environment, i.e., by isolating an object in a sufficiently high vacuum. We show that another option exists: decoherence is suppressed in the presence of a strongly interacting environment if this environment is superfluid. Indeed, as long as an object immersed in a pure superfluid at zero temperature moves with a velocity below the critical one, it does not create, absorb, or scatter any excitations of the superfluid. Hence, in this idealized situation the decoherence is absent. In reality the decoherence will be present due to thermal excitations of the superfluid and impurities contaminating the superfluid. We examine various decoherence channels in the superfluid ${}^4\text{He}$. It is shown that the total decoherence time can be as large as tens of seconds for a ${10}^6$ amu nanoparticle delocalized over 300 nm in helium at 1 mK. Performing interference experiments in superfluid helium can provide certain practical advantages compared to conventional schemes, e.g., compensation of gravity by the buoyancy force and effective cooling.

In recent years, material-induced noise arising from defects has emerged as an impediment to quantum-limited measurement in systems ranging from microwave qubits to gravity-wave interferometers. As experimental systems push to ever smaller dimensions, extrinsic system properties can affect its internal material dynamics. In this paper, we identify intriguing regimes of material physics (defect-phonon and defect-defect dynamics) that are produced by dimensional confinement. Our models show that a range of tell-tale signatures, encoded in the characteristics of defect-induced noise, dissipation, and nonlinearity, are profoundly altered by geometry. Building on this insight, we demonstrate that the magnitude and character of this material-induced noise is transformed in microscale systems, providing an opportunity to improve the fidelity of quantum measurements. Moreover, we show that many emerging nanoelectromechanical, cavity optomechanical, and superconducting resonator systems are poised to probe these regimes of dynamics, in both high- and low-field limits, providing a way to explore the fundamental tenets of glass physics.

We experimentally study the transient voltage response of yttrium iron garnet/platinum bilayer samples subject to periodic heating up to gigahertz frequencies. We observe an intrinsic cutoff frequency of the induced thermopower voltage, which characteristically depends on the thickness of the yttrium iron garnet film. The cutoff frequency reaches values of up to 350 MHz in a 50-nm-thick yttrium iron garnet film, but drops to below 1 MHz for several-micrometer-thick films. These data corroborate the notion that the magnon spectrum responsible for the spin current emission in the spin Seebeck effect can be shaped by tuning the thickness of the ferromagnetic layer.

We show clear evidence of a quadratic speedup of a quantum annealing (QA) Schrödinger dynamics over a Glauber master equation simulated annealing (SA) for a random Ising model in one dimension, via an equal-footing exact deterministic dynamics of the Jordan-Wigner fermionized problems. This is remarkable, in view of the arguments of H. G. Katzgraber et al. [Phys. Rev. X 4, 021008 (2014)], since SA does not encounter any phase transition, while QA does. We also find a second remarkable result: that a “quantum-inspired” imaginary-time Schrödinger QA provides a further exponential speedup, i.e., an asymptotic residual error decreasing as a power law ${\tau }^{-\mu }$ of the annealing time $\tau$.

We propose and analyze a quantum electromechanical system composed of a monolithic quartz bulk acoustic wave oscillator coupled to a superconducting transmon qubit via an intermediate $LC$ electrical circuit. Monolithic quartz oscillators offer unprecedentedly high effective masses and quality factors for the investigation of mechanical oscillators in the quantum regime. Ground-state cooling of such mechanical modes via resonant piezoelectric coupling to an $LC$ circuit, which is itself sideband cooled via coupling to a transmon qubit, is shown to be feasible. The fluorescence spectrum of the qubit, containing motional sideband contributions due to the couplings to the oscillator modes, is obtained and the imprint of the electromechanical steady state on the spectrum is determined. This allows the qubit to function both as a cooling resource for, and transducer of, the mechanical oscillator. The results described are relevant to any hybrid quantum system composed of a qubit coupled to two (coupled or uncoupled) thermal oscillator modes.

A semiclassical quantization condition is derived for Landau levels in general spin-orbit coupled systems. This generalizes the Onsager quantization condition via a matrix-valued phase which describes spin dynamics along the classical cyclotron trajectory. We discuss measurement of the matrix phase via magnetic oscillations and electron spin resonance, which may be used to probe the spin structure of the precessing wave function. We compare the resulting semiclassical spectrum with exact results which are obtained for a variety of spin-orbit interactions in two-dimensional systems.

The impact of the electron-electron Coulomb interaction on the optical conductivity of graphene has led to a controversy that calls into question the universality of collisionless transport in this and other Dirac materials. Using a lattice calculation that avoids divergences present in previous nodal Dirac approaches, our work settles this controversy and obtains results in quantitative agreement with experiment over a wide frequency range. We also demonstrate that dimensional regularization methods agree, if the regularization of the theory in modified dimensions is correctly implemented. Tight-binding lattice and nodal Dirac theory calculations are shown to coincide at low energies even when the nonzero size of the atomic orbital wave function is included, conclusively demonstrating the universality of the optical conductivity of graphene.

We investigate the possibility of creating X waves, or localized wave packets, in resonantly excited exciton-polariton superfluids. We demonstrate the existence of X-wave traveling solutions in the coupled exciton-photon system past the inflection point, where the effective mass of lower polaritons is negative in the direction perpendicular to the wave vector of the pumping beam. Contrary to the case of bright solitons, X waves do not require nonlinearity for sustaining their shape. Nevertheless, we show that nonlinearity is important for their dynamics, as it allows for their spontaneous formation from an initial Gaussian wave packet. Unique properties of exciton-polaritons may lead to applications of their X waves in long-distance signal propagation inside novel integrated optoelectronic circuits based on excitons.

The Peltier effect and the Ettingshausen effect are investigated in graphene nanoribbons, where charge current produces heat current along the longitudinal direction in the former case, and longitudinal charge current generates transverse heat current in the latter case. With the aid of the nonequilibrium Green's function and the Landauer-Büttiker formalism, the Peltier coefficient ${\mathrm{\Pi }}_c$ and the Ettingshausen coefficient ${\mathrm{E}}_c$ are obtained. We found that the Kelvin relation is always valid for the longitudinal thermoelectric transport, i.e., ${\mathrm{\Pi }}_c=\mathcal{T}{S}_c$, with $\mathcal{T}$ the temperature and $S_c$ the Seebeck coefficient. In contrast, for transverse magnetothermoelectric transport, the Kelvin relation breaks down and ${\mathrm{E}}_c\ne \mathcal{T}{N}_c$ usually, with $N_c$ the Nernst coefficient. In the region of weak magnetic field, the Ettingshausen effect depends strongly on device parameters. When the Fermi energy $E_F$ is close to the Dirac point, the Ettingshausen effect of the semiconducting armchair graphene nanoribbon is much stronger than that of the metallic one. When $E_F$ is far away from the Dirac point, the Ettingshausen coefficient ${\mathrm{E}}_c$ oscillates around zero. When under a strong magnetic field, ${\mathrm{E}}_c$ is independent of the device parameters and swells only near the Dirac point. Further, the dependence of ${\mathrm{E}}_c$ on $E_F$ can be scaled by $E_F/k_B\mathcal{T}$, with a peak value of $(2ln2)k_B\mathcal{T}/e$ for the three-terminal system and $(\frac{4}{3}ln2)k_B\mathcal{T}/e$ for the four-terminal system. We also study the impact of disorder on the Ettingshausen effect. Regardless of the magnetic field strength, ${\mathrm{E}}_c$ is robust against moderate disorder scattering. In addition, in the strong magnetic field, ${\mathrm{E}}_c$ with additional regular oscillating structure can be caused by disorder.

We study the quantum Hall effect of two-dimensional electron gas in black phosphorus in the presence of perpendicular electric and magnetic fields. In the absence of a bias voltage, the external magnetic field leads to a quantization of the energy spectrum into equidistant Landau levels, with different cyclotron frequencies for the electron and hole bands. The applied voltage reduces the band gap, and eventually a semiconductor-to-semimetal transition takes place. This nontrivial phase is characterized by the emergence of a pair of Dirac points in the spectrum. As a consequence, the Landau levels are not equidistant anymore but follow the ${\varepsilon }_n\propto \sqrt{nB}$ characteristic of Dirac crystals as graphene. By using the Kubo-Bastin formula in the context of the kernel polynomial method, we compute the Hall conductivity of the system. We obtain a ${\sigma }_{xy}\propto 2n$ quantization of the Hall conductivity in the gapped phase (standard quantum Hall effect regime) and a ${\sigma }_{xy}\propto 4(n+1/2)$ quantization in the semimetallic phase, characteristic of Dirac systems with nontrivial topology.

Recent studies have focused on laser-induced gaps in graphene which have been shown to have a topological origin, thereby hosting robust states at the sample edges. While the focus has remained mainly on these topological chiral edge states, the Floquet bound states around defects lack a detailed study. In this paper we present such a study covering large defects of different shape and also vacancy-like defects and adatoms at the dynamical gap at $\hslash \mathrm{\Omega }/2$ $(\hslash \mathrm{\Omega }$ being the photon energy). Our results, based on analytical calculations as well as numerics for full tight-binding models, show that the bound states are chiral and appear in a number which grows with the defect size. Furthermore, while the bound states exist regardless of the type of the defect's edge termination (zigzag, armchair, mixed), the spectrum is strongly dependent on it. In the case of top adatoms, the bound state quasienergies depend on the adatoms energy. The appearance of such bound states might open the door to the presence of topological effects on the bulk transport properties of dirty graphene.

Ab initio molecular dynamics with electronic friction (AIMDEF) is a valuable methodology to study the interaction of atomic particles with metal surfaces. This method, in which the effect of low-energy electron-hole (e-h) pair excitations is treated within the local density friction approximation (LDFA) [Juaristi et al., Phys. Rev. Lett. 100, 116102 (2008)], can provide an accurate description of both e-h pair and phonon excitations. In practice, its applicability becomes a complicated task in those situations of substantial surface atoms displacements because the LDFA requires the knowledge at each integration step of the bare surface electron density. In this work, we propose three different methods of calculating on-the-fly the electron density of the distorted surface and we discuss their suitability under typical surface distortions. The investigated methods are used in AIMDEF simulations for three illustrative adsorption cases, namely, dissociated ${\mathrm{H}}_2$ on Pd(100), N on Ag(111), and ${\mathrm{N}}_2$ on Fe(110). Our AIMDEF calculations performed with the three approaches highlight the importance of going beyond the frozen surface density to accurately describe the energy released into e-h pair excitations in case of large surface atom displacements.

The effect of dc electric field on transport of highly mobile two-dimensional electrons is studied in wide GaAs single quantum wells placed in titled magnetic fields. The study shows that in perpendicular magnetic field resistance oscillates due to electric-field induced Landau-Zener transitions between quantum levels that correspond to geometric resonances between cyclotron orbits and periodic modulation of electron density of states. Magnetic field tilt inverts these oscillations. Surprisingly the strongest inverted oscillations are observed at a tilt corresponding to nearly absent modulation of the electron density of states in regime of magnetic breakdown of semiclassical electron orbits. This phenomenon establishes an example of quantum resistance oscillations due to Landau quantization, which occur in electron systems with a ${\displaystyle constant}$ density of states.