We theoretically investigate a spinor polariton condensate under nonresonant pumping, based on driven-dissipative Gross-Pitaevskii equations coupled to the rate equation of a spin-unpolarized reservoir. We find the homogeneous polariton condensate can transit from the spin-unpolarized phase, where it is linearly polarized, to the spin-polarized phase, where it is elliptically polarized, depending on the cross-spin versus same-spin interactions and the linear polarization splitting. In both phases, we study elementary excitations using Bogoliubov approach, in a regime where the decay rate of total exciton density in reservoir crosses over from the slow to the fast limit. Depending on reservoir parameters, the global-phase mode can be either diffusive or gapped. By contrast, the relative-phase mode always possesses a gapped energy, undamped in the spin-unpolarized phase but weakly damped in the spin-polarized phase. In the spin-unpolarized phase, both modes are linearly polarized despite pumping and decay. However, in the spin-polarized phase, the mode polarization can be significantly affected by the reservoir and depends strongly on the circular polarization degree of the condensate. Interestingly, we demonstrate that the “ghost” branch of the Bogoliubov spectrum of the relative-phase mode can be visualized in the photoluminescence emission, distinguishable from that of the global-phase mode and thus allowing for experimental observation, when the spinor polariton condensate is elliptically polarized.

We study the Andreev reflections (ARs) at the interface of the type-II Weyl semimetal-superconductor junctions and find double ARs when the superconductor is put in the Weyl semimetal band tilting direction, which is similar to the double reflections of light in anisotropic crystals. The directions of the double (retro and specular) ARs are symmetric about the normal due to the hyperboloidal Fermi surface near the Weyl nodes, but with different AR amplitudes depending on the incident electron. When the normal direction of the Weyl semimetal-superconductor interface is changed from parallel to perpendicular with the band tilt, the double ARs gradually evolve into one retro-AR and one normal reflection, resulting in an anisotropic differential conductance, which is a unique observable signature for the double ARs in experiments.

We have studied the absorption of monochromatic microwave radiation in high-quality two-dimensional electron systems for the frequency span from 10 to 380 GHz using a bolometric method. For frequencies above 100 GHz the absorption exhibits an anomalous magnetic field dependence. Minima form at harmonics of the cyclotron resonance frequency. The results contrast previously reported data for other frequency ranges. Quasiclassical memory effects originating from the non-Markovian dynamics of electrons in a disorder potential containing short-range scatterers on top of a smooth potential background favorably account for the observed behavior.

In this work we present a detailed analysis of the second-order piezoelectric effect in wurtzite III-N heterostructures, such as quantum wells and quantum dots, grown on different substrate orientations. Our analysis is based on a continuum model using a here derived analytic expression for the second-order piezoelectric polarization vector field as a function of the incline angle $\theta$ to the wurtzite $c$ axis. This expression allows for a straightforward implementation in existing quantum well and quantum dot codes. Our calculations on III-N quantum well systems reveal that especially for semipolar structures with high incline angle values (${55}^{\circ }\le \theta \le {80}^{\circ }$ and ${105}^{\circ }\le \theta \le {120}^{\circ }$), second-order piezoelectricity noticeably contributes to the overall electric built-in field. For instance, in an InGaN/GaN multiple quantum well system with 22% In, the electric field increases by approximately 20% due to second-order piezoelectricity. Overall, when including second-order piezoelectric effects in the calculation of electric fields in GaN/AlN and InGaN/GaN quantum well systems an improved agreement between our theory and experimental literature data is observed. When studying quantum dots, at least for the here considered model geometry and growth planes, we observe that for GaN/AlN structures second-order effects are of secondary importance. The situation is different for non-$c$-plane ${\mathrm{In}}_{0.2}{\mathrm{Ga}}_{0.8}\mathrm{N}$/GaN quantum dots. For example, inside a nonpolar ${\mathrm{In}}_{0.2}{\mathrm{Ga}}_{0.8}\mathrm{N}$/GaN dot the built-in potential arising from second-order piezoelectricity is comparable in magnitude to the built-in potential originating from spontaneous and first-order piezoelectric polarization, but opposite in sign. This feature leads to a change in the built-in potential profile both in and around the ${\mathrm{In}}_{0.2}{\mathrm{Ga}}_{0.8}\mathrm{N}$/GaN quantum dot structure, which in general is relevant for electronic and optical properties of these systems.

We analyze an open many-body system that is strongly coupled at its boundaries to interacting quantum baths. We show that the two-body interactions inside the baths induce emergent phenomena in the spin transport. The system and baths are modeled as independent spin chains resulting in a global nonhomogeneous $XXZ$ model. The evolution of the system-bath state is simulated using matrix-product-states methods. We present two phase transitions induced by bath interactions. For weak bath interactions we observe ballistic and insulating phases. However, for strong bath interactions a diffusive phase emerges with a distinct power-law decay of the time-dependent spin current $Q\propto t^{-\alpha }$. Furthermore, we investigate long-lasting current oscillations arising from the non-Markovian dynamics in the homogeneous case and find a sharp change in their frequency scaling coinciding with the triple point of the phase diagram.

We generalize the twisted quantum double model of topological orders in two dimensions to the case with boundaries by systematically constructing the boundary Hamiltonians. Given the bulk Hamiltonian defined by a gauge group $G$ and a 3-cocycle in the third cohomology group of $G$ over $U\left(1\right)$, a boundary Hamiltonian can be defined by a subgroup $K$ of $G$ and a 2-cochain in the second cochain group of $K$ over $U\left(1\right)$. The consistency between the bulk and boundary Hamiltonians is dictated by what we call the Frobenius condition that constrains the 2-cochain given the 3-cocyle. We offer a closed-form formula computing the ground-state degeneracy of the model on a cylinder in terms of the input data only, which can be naturally generalized to surfaces with more boundaries. We also explicitly write down the ground-state wave function of the model on a disk also in terms of the input data only.

We measure and analyze the statistics of nanoindentation pop-ins in a bulk metallic glass that has been annealed after being subjected to high pressure torsion. We argue that these pop-ins are avalanches of slip events. Larger slip avalanches are observed after annealing at higher temperatures. The slip avalanche size distribution shows scaling behavior and suggests the existence of a critical annealing temperature. Results are shown to be consistent with the predictions of a simple mean field theory. This implies that the statistics of slip avalanches in nanoindentation can be used to extract information about the structure, history, density, and free volume of the material. The methods employed here are expected to be applicable to a wide range of materials.

With large specific and volumetric capacity, lithiated silicon is an excellent anode for lithium-ion batteries. Its application is challenged today, however, by the formation of an amorphous $a-{\text{Li}}_x\text{Si}$ phase associated with a large volume change that occurs at relatively low Li concentration and remains only very partly understood at the microscopic level. In this paper, we characterize the full energy landscape associated with the onset of Li insertion in crystalline Si as a first step for understanding the lithiation process. We identify the diffusion mechanisms and migration energies for one to ten Li atoms in a Si crystal as well as the average lifetime of small lithium aggregates, using the kinetic activation-relaxation technique (kART), an off-lattice kinetic Monte-Carlo method with on-the-fly catalog building capabilities coupled to a newly developed force field (ReaxFF) used as potential based on ab initio results. We show that the short lifetimes of the bound states (from meV to ten meV) mean that Li atoms move in the interstitial sublattice with little interactions, explaining how high Li concentration in Si can be reached.

The stability of the four known stationary points of the cubic helimagnet energy functional: the ferromagnetic state, the conical helix, the conical helicoid, and the skyrmion lattice, is studied by solving the corresponding spectral problem. The only stable points are the ferromagnetic state at high magnetic field and the conical helix at low field, and there is no metastable state. Thermal fluctuations around the stationary point, included to quadratic order in the saddle point expansion, destabilize the conical helix in a region where the ferromagnetic state is unstable. Thus, a new intermediate phase appears which, in a region of the phase diagram, is a skyrmion lattice stabilized by thermal fluctuations. The skyrmion lattice lost the stability by lowering temperature, and a new intermediate phase of unknown type, presumably with three-dimensional modulations, appears in the lower temperature region.

In tunnel junctions between ferromagnets and heavy elements with strong spin orbit coupling the magnetoresistance is usually dominated by tunneling anisotropic magnetoresistance (TAMR). This makes conventional DC spin injection techniques impractical for determining the spin relaxation time ${\tau }_{\mathrm{s}}$. Here, we show that this obstacle for measurements of ${\tau }_{\mathrm{s}}$ can be overcome by second-harmonic spin-injection magnetoresistance (SIMR). In the second-harmonic signal the SIMR is comparable in magnitude to TAMR, thus enabling Hanle-induced SIMR as a powerful tool to directly determine ${\tau }_{\mathrm{s}}$. Using this approach we determined the spin relaxation time of Pt and Ta and their temperature dependences. The spin relaxation in Pt seems to be governed by the Elliott-Yafet mechanism due to a constant resistivity $\times \mathrm{spin}$ relaxation time product over a wide temperature range.

We report a systematic study on Edelstein magnetoresistance (Edelstein MR) in $\mathrm{C}{\mathrm{o}}_{25}\mathrm{F}{\mathrm{e}}_{75}\text{/}\mathrm{Cu}\text{/}\mathrm{B}{\mathrm{i}}_2{\mathrm{O}}_3$ heterostructures with a strong spin-orbit interaction at the $\mathrm{Cu}\text{/}\mathrm{B}{\mathrm{i}}_2{\mathrm{O}}_3$ interface. We succeed in observing a significant dependence of the Edelstein MR on both Cu layer thickness and temperature, and also develop a general analytical model considering distinct bulk and interface contributions on spin relaxation. Our analysis, based on the above model, quantitatively illustrates a unique property of the spin transport near the Rashba interface, revealing a prominent role of the spin relaxation process by determining the ratios of the spin relaxation inside and outside the interface. We further find the characteristic spin transport is unaffected by temperature. Our results provide an essential tool for exploring the transport in a system with spin-momentum-locked two-dimensional states.

Activating multiple symmetry modes and promoting a strong coupling between different modes by strain are indispensable to stabilize a polar ferroelectric (FE) phase from a nonpolar perovskite. Herein, through first-principles calculations, we propose an undiscovered and general avenue to engineering ferroelectricity in photovoltaic perovskites with a Ruddlesden-Popper (RP) structure. It is demonstrated that an experimentally accessible compressive strain can induce an in-plane polarization in RP perovskite halides thin films, resulting in an unusual paraelectric to FE phase transition. The detailed analysis on structure and energy reveals that the unusual FE phase transition in the perovskite halides stems from the strong coupling between strain and antiferrodistortive (AFD) mode. Further calculations show that the strain-AFD coupling-induced ferroelectricity is not only exhibited by perovskite halides but also observed in perovskite sulfides such as ${\mathrm{Ba}}_3{\mathrm{Zr}}_2{\mathrm{S}}_7$. Moreover, it is found that the strained FE thin film possesses a suitable band gap of 1.6 eV for photovoltaic application. These findings not only unfold a general way to engineering nonpolar-to-polar transition, but also open an avenue to design optimal FE semiconductors for solar cell applications.

We study the phase diagram of an elastic interaction model for spin crossover (SC) materials with antiferromagnetic-like short-range interactions. In this model, the interplay between the short-range interaction and the long-range interaction of elastic origin causes complex phase transitions. For relatively weak elastic interactions, the phase diagram is characterized by tricritical points, at which antiferromagnetic (AF) -like and ferromagnetic (F) -like spinodal lines and a critical line merge. On the other hand, for relatively strong elastic interactions, unusual “horn structures,” which are surrounded by the F-like spinodal lines, disorder (D) spinodal lines, and the critical line, are realized at higher temperatures. These structures are similar to those obtained in our previous study [Phys. Rev. B 93, 064109 (2016)] of an Ising antiferromagnet with infinite-range ferromagnetic interactions, and we find universal features caused by the interplay between the competing short-range and long-range interactions. The long-range interaction of elastic origin is irrelevant (inessential) for the critical line. In contrast, the AF-like, F-like, and D spinodal lines result from the long-range interaction of elastic origin. This difference causes qualitatively different features of domain formation or nucleation of the new phase: clustering occurs in the former case, while clustering is absent in the latter. Based on the phase diagrams, we discuss the patterns and clustering features of two-step SC transitions, in which the AF-like phase is realized in the intermediate temperature region.

Graphene is a fascinating two-dimensional material that is being widely investigated for use in electronic devices due to its unique electronic and materials properties. Also, because of its high thermal stability and inertness, it is considered a promising candidate for use as a protection layer for metal substrates. Here, graphene films grown on Ru(0001) are held at 600 K while exposed to oxygen (${\mathrm{O}}_2$) and then investigated with helium low-energy ion scattering (LEIS). LEIS spectra collected at different scattering angles confirm that oxygen does not adsorb to graphene but instead intercalates between the graphene and the substrate. The intercalated ${\mathrm{O}}_2$ desorbs when the sample is annealed to 800 K. It is shown that this is a much lower temperature than is needed to remove chemisorbed atomic oxygen from Ru, thus inferring that the intercalated oxygen is molecular. During the desorption process, some of the graphene is etched away via a chemical reaction with the oxygen, with the proportion desorbing as ${\mathrm{O}}_2$ or reacting to etch the graphene being dependent on the amount of intercalated ${\mathrm{O}}_2$.

Using first-principles density functional theory calculations, we systematically investigate the structural, electronic, and vibrational properties of bulk and potential single-layer structures of perovskitelike ${\mathrm{CsPb}}_2{\mathrm{Br}}_5$ crystal. It is found that while Cs atoms have no effect on the electronic structure, their presence is essential for the formation of stable ${\mathrm{CsPb}}_2{\mathrm{Br}}_5$ crystals. The calculated vibrational spectra of the crystal reveal that not only the bulk form but also the single-layer forms of ${\mathrm{CsPb}}_2{\mathrm{Br}}_5$ are dynamically stable. Predicted single-layer forms can exhibit either semiconducting or metallic character. Moreover, the modification of the structural, electronic, and magnetic properties of single-layer ${\mathrm{CsPb}}_2{\mathrm{Br}}_5$ upon formation of vacancy defects is investigated. It is found that the formation of Br vacancy (i) has the lowest formation energy, (ii) significantly changes the electronic structure, and (iii) leads to ferromagnetic ground state in the single-layer ${\mathrm{CsPb}}_2{\mathrm{Br}}_5$. However, the formation of Pb and Cs vacancies leads to $p$-type doping of the single-layer structure. Results reported herein reveal that the single-layer ${\mathrm{CsPb}}_2{\mathrm{Br}}_5$ crystal is a novel stable perovskite with enhanced functionality and a promising candidate for nanodevice applications.

We investigate the relation between crystal stacking faults in individual wurtzite InAs and GaAs nanowires and the intensity of the forbidden longitudinal optical (LO) phonon mode in the Raman spectra. Micro-Raman spectroscopy and transmission electron microscopy are combined on the same individual nanowires to evaluate the LO mode intensity as a function of the stacking fault density. A clear increase in the LO mode intensity was observed when the stacking fault density was increased. Our results confirm the utility of Raman spectroscopy as a powerful tool for detecting crystal defects in nanowires.

We report on experiments addressing the nonlinear interaction between a nanomechanical mode and position fluctuations. The Duffing nonlinearity transduces the Brownian motion of the mode and of other nonlinearly coupled ones into frequency noise. This mechanism, ubiquitous to all weakly nonlinear resonators thermalized to a bath, results in a phase diffusion process altering the motion: two limit behaviors appear, analogous to motional narrowing and inhomogeneous broadening in NMR. Their crossover is found to depend nontrivially on the ratio of the frequency noise correlation time to its magnitude. Our measurements obtained over an unprecedented range covering the two limits match the theory of Y. Zhang and M. I. Dykman [Phys. Rev. B 92, 165419 (2015)], with no free parameters. We finally discuss the fundamental bound on frequency resolution set by this mechanism, which is not marginal for bottom-up nanostructures.

A definition of the Grüneisen parameters for anisotropic materials is derived based on the response of phonon frequencies to uniaxial stress perturbations. This Grüneisen model relates the thermal expansion in a given direction (${\alpha }_{ii}$) to one element of the elastic compliance tensor, which corresponds to the Young's modulus in that direction ($Y_{ii}$). The model is tested through ab initio prediction of thermal expansion in zinc, graphite, and calcite using density functional perturbation theory, indicating that it could lead to increased accuracy for structurally complex systems. The direct dependence of ${\alpha }_{ii}$ on $Y_{ii}$ suggests that materials which are flexible along their principal axes but rigid in other directions will generally display both positive and negative thermal expansion.