We present a combined experimental and theoretical study of highly charged and excited electron-hole complexes in strain-free (111) GaAs/AlGaAs quantum dots grown by droplet epitaxy. We address the complexes with one of the charge carriers residing in the excited state, namely, the “hot” trions ${\mathrm{X}}^{-*}$ and ${\mathrm{X}}^{+*}$, and the doubly negatively charged exciton ${\mathrm{X}}^{2-}$. Our magnetophotoluminescence experiments performed on single quantum dots in the Faraday geometry uncover characteristic emission patterns for each excited electron-hole complex, which are very different from the photoluminescence spectra observed in (001)-grown quantum dots. We present a detailed theory of the fine structure and magnetophotoluminescence spectra of ${\mathrm{X}}^{-*},{\mathrm{X}}^{+*}$, and ${\mathrm{X}}^{2-}$ complexes, governed by the interplay between the electron-hole Coulomb exchange interaction and the heavy-hole mixing, characteristic for these quantum dots with a trigonal symmetry. Comparison between experiment and theory allows for precise charge state identification, as well as extraction of electron-hole exchange interaction constants and $g$ factors for the charge carriers occupying excited states.

We propose a type of edges arising due to the anisotropy inherent in the puckered structure of a honeycomb system such as in phosphorene. Skewed-zigzag and skewed-armchair nanoribbons are semiconducting and metallic, respectively, in contrast to their normal edge counterparts. Their band structures are tunable, and a metal-insulator transition is induced by an electric field. We predict a field-effect transistor based on the edge states in skewed-armchair nanoribbons, where the edge state is gapped by applying arbitrary small electric field $E_z$. A topological argument is presented, revealing the condition for the emergence of such edge states.

Topological matter is a trending topic in condensed matter: From a fundamental point of view, it has introduced new phenomena and tools and, for technological applications, it holds the promise of basic stable quantum computing. Similarly, the physics of localization by disorder, an old paradigm of obvious technological importance in the field, continues to reveal surprises when new properties of matter appear. This work deals with the localization behavior of electronic systems based on partite lattices, with special attention to the role of topology. We find an unexpected result from the point of view of localization properties: A robust topological metallic state characterized by a nonquantized Hall conductivity arises from strong disorder in topological systems based on bipartite lattices. The key issue is the nature of the disorder realization: selective disorder in only one sublattice. The generality of the result is based on the partite nature of most recent two-dimensional materials such as graphene or transition-metal dichalcogenides, and the possibility of the physical realization of the particular disorder demonstrated in Ugeda et al. [M. M. Ugeda et al., Phys. Rev. Lett. 104, 096804 (2010)] and Zhao et al. [L. Zhao et al., Science 333, 999 (2011)].

We derived and thoroughly tested a bond-order potential (BOP) for body-centered-cubic (bcc) magnetic iron that can be employed in atomistic calculations of a broad variety of crystal defects that control structural, mechanical, and thermodynamic properties of this technologically important metal. The constructed BOP reflects correctly the mixed nearly free electron and covalent bonding arising from the partially filled $d$ band as well as the ferromagnetism that is actually responsible for the stability of the bcc structure of iron at low temperatures. The covalent part of the cohesive energy is determined within the tight-binding bond model with the Green's function of the Schrödinger equation determined using the method of continued fractions terminated at a sufficient level of the moments of the density of states. This makes the BOP an $\mathrm{O}\left(N\right)$ method usable for very large numbers of particles. Only $dd$ bonds are included explicitly, but the effect of $s$ electrons on the covalent energy is included via their screening of the corresponding $dd$ bonds. The magnetic part of the cohesive energy is included using the Stoner model of itinerant magnetism. The repulsive part of the cohesive energy is represented, as in any tight-binding scheme, by an empirical formula. Its functional form is physically justified by studies of the repulsion in face-centered-cubic (fcc) solid argon under very high pressure where the repulsion originates from overlapping $s$ and $p$ closed-shell electrons just as it does from closed-shell $s$ electrons in transition metals squeezed into the ion core under the influence of the large covalent $d$ bonding. Testing of the transferability of the developed BOP to environments significantly different from those of the ideal bcc lattice was carried out by studying crystal structures and magnetic states alternative to the ferromagnetic bcc lattice, vacancies, divacancies, self-interstitial atoms (SIAs), paths continuously transforming the bcc structure to different less symmetric structures and phonons. The results of these calculations are compared with either experiments or calculations based on the density functional theory (DFT), and they all show very good agreement. Importantly, the lowest energy configuration of SIAs agrees with DFT calculations that show that it is an exception within bcc transition metals controlled by magnetism. Moreover, the migration energy of interstitials is significantly lower than that of vacancies, which is essential for correct analysis of the effects of irradiation. Finally, the core structure and glide of $½\langle 111\rangle$ screw dislocations that control the plastic flow in single crystals of bcc metals was explored. The results fully agree with available DFT based studies and with experimental observations of the slip geometry of bcc iron at low temperatures.

In this article, we investigate the kinetics of the iron $\alpha -\varepsilon$ transition under laser-driven ramp compression for deformation rates ranging from 3 to $9\times {10}^7{\mathrm{s}}^{-1}$. As in previous work, we observe a plateau in the rear surface velocity profile at the transition. With increasing deformation rate the transition onset pressure raises from 11 to 25 GPa, while the plateau duration decreases. These kinetic effects are well reproduced by an Avrami-type kinetics model of nucleation and growth with a constant, nanosecond scale completion time, which suggests an isokinetic regime over the explored range of strain rates.

A path method is implemented in order to precisely and generally describe the collapse of isolated skyrmions in a Co/Pt(111) monolayer, on the basis of atomic scale simulations. Two collapse mechanisms with different energy barriers are found. The most obvious path, featuring a homogeneous shrinking, gives the largest energy, whereas the lowest energy barrier is shown to comply with the outcome of Langevin dynamics under a destabilizing field of 0.25 T, with a lifetime of 20 ns at around 80 K. For this lowest energy barrier path, skyrmion destabilization occurs much before any topology change, suggesting that topology plays a minor role in the skyrmion stability. On the contrary, an important role appears devoted to the Dzyaloshinskii-Moriya interaction, establishing a route towards improved skyrmion stability.

Electrical resistivity of nanostructured granular alloys ${\mathrm{Co}}_x{\mathrm{Cu}}_{1-x}$ ($x\sim 0.01$–$0.76$) prepared by the chemical reduction method is investigated in the temperature range 2–300 K. The samples with a low cobalt content of $x\le 0.1$ show a metallic resistivity behavior. For samples with a higher cobalt content, $x\ge 0.17$, the resistivity shows a minimum. The minimum becomes more pronounced as Co content ($x$) increases and also as the temperature of minimum resistivity, $T_{\text{min}}$, increases with $x$. The resistivity minimum is obtained in this magnetic alloy system even for a cobalt concentration as high as $\sim 76\%$. Application of an external magnetic field has a negligible effect on the resistivity behavior. Detailed analysis suggests that the low-temperature upturn in resistivity most probably arises due to elastic electron-electron interaction (the quantum-interference effect). Magnetic measurements at 4 K on the same samples show the absence of long-range magnetic interaction and evidence of increasing magnetic disorder as $x$ increases beyond $\sim 10\%$. Combining the results of the two types of measurements, a model of formation of these alloy particles involving random clusters of Co atoms within the Cu matrix has been proposed.

The low-temperature softening of the TO1 phonon of ${\mathrm{SrTiO}}_3$ (STO), which determines its incipient ferroelectricity, is known to be partially hindered either in the bulk under strong electric fields, or in thin STO films. Here we show, by terahertz (THz) reflectivity measurements, that a similar effect is produced in bulk STO and at zero static field by ultrathin metallic films on its surface, like a 10-nm-thick film of ${\mathrm{SrRuO}}_3$ (SRO), or the two-dimensional electron system (2DES) present at the interface with ${\mathrm{LaAlO}}_3$. In SRO/STO, the observed hardening is well explained by the depolarizing action of the SRO free electrons which follow adiabatically the ion motion. In LAO/STO, a weaker TO1 hardening could be detected by patterning the 2DES in the form of microstripes and using a polarized THz field parallel $\left({\vec{E}}_{\parallel }\right)$ or orthogonal $\left({\vec{E}}_{\perp }\right)$ to the stripes. At 10 K, when TO1 is excited together with the free electrons by ${\vec{E}}_{\parallel }$, its absorbance is harder by about $7 {\mathrm{cm}}^{-1}$ than that measured when TO1 is coupled to the plasmon-polariton confined within the stripes, being excited by ${\vec{E}}_{\perp }$.

To study the effect of spin-orbit coupling (SOC) on spin-transfer torque in magnetic materials, we have implemented two theoretical formalisms that can accommodate SOC. Using the “charge-pumping” formalism, we find two contributions to the out-of-plane spin-transfer torque parameter $\beta$ in ballistic Ni domain walls (DWs). For short DWs, the nonadiabatic reflection of conduction electrons caused by the rapid spatial variation of the exchange potential results in an out-of-plane torque that increases rapidly with decreasing DW length. For long DWs, the Fermi level conduction channel anisotropy that gives rise to an intrinsic DW resistance in the presence of SOC leads to a linear dependence of $\beta$ on the DW length. To understand this counterintuitive divergence of $\beta$ in the long DW limit, we use the “nonequilibrium magnetization” formalism to examine the spatially resolved spin-transfer torque. The SOC-induced out-of-plane torque in ballistic DWs is found to be quantitatively consistent with the values obtained using the charge-pumping calculations, indicating the equivalence of the two theoretical methods.

The $J_1\text{−}{J}_2$ Heisenberg model is a “canonical” model in the field of quantum magnetism in order to study the interplay between frustration and quantum fluctuations as well as quantum phase transitions driven by frustration. Here we apply the coupled cluster method (CCM) to study the spin-half $J_1\text{−}{J}_2$ model with antiferromagnetic nearest-neighbor bonds $J_1>0$ and next-nearest-neighbor bonds $J_2>0$ for the simple cubic (sc) and body-centered cubic (bcc) lattices. In particular, we wish to study the ground-state ordering of these systems as a function of the frustration parameter $p=z_2{J}_2/z_1{J}_1$, where $z_1 \left(z_2\right)$ is the number of nearest (next-nearest) neighbors. We wish to determine the positions of the phase transitions using the CCM and we aim to resolve the nature of the phase transition points. We consider the ground-state energy, order parameters, spin-spin correlation functions, as well as the spin stiffness in order to determine the ground-state phase diagrams of these models. We find a direct first-order phase transition at a value of $p=0.528$ from a state of nearest-neighbor Néel order to next-nearest-neighbor Néel order for the bcc lattice. For the sc lattice the situation is more subtle. CCM results for the energy, the order parameter, the spin-spin correlation functions, and the spin stiffness indicate that there is no direct first-order transition between ground-state phases with magnetic long-range order, rather it is more likely that two phases with antiferromagnetic long range are separated by a narrow region of a spin-liquid-like quantum phase around $p=0.55$. Thus the strong frustration present in the $J_1\text{−}{J}_2$ Heisenberg model on the sc lattice may open a window for an unconventional quantum ground state in this three-dimensional spin model.

We show that a strong infrared laser beam obliquely incident on graphene can experience a parametric instability with respect to decay into lower-frequency (idler) photons and THz surface plasmons. The instability is due to a strong in-plane second-order nonlinear response of graphene which originates from its spatial dispersion. The parametric decay leads to efficient generation of THz plasmons and gives rise to quantum entanglement of idler photons and surface plasmon states.

Thermal properties of graphene have attracted much attention, culminating in a recent measurement of its length dependence in ribbons up to 9 $\mu \mathrm{m}$ long. In this paper, we use the improved Callaway model to solve the phonon Boltzmann transport equation while capturing both the resistive (umklapp, isotope, and edge roughness) and nonresistive (normal) contributions. We show that for lengths smaller than 100 $\mu \mathrm{m}$, scaling the ribbon length while keeping the width constant leads to a logarithmic divergence of thermal conductivity. The length dependence is driven primarily by a ballistic-to-diffusive transition in the in-plane (LA and TA) branches, while in the hydrodynamic regime when $10\mu \mathrm{m}<L<100$ $\mu \mathrm{m}$, the contribution from the in-plane branches saturates and the out-of-plane (ZA) branch shows a clear logarithmic trend, driven by the nonresistive normal contribution. We find that thermal conductivity converges beyond $L>100$ $\mu \mathrm{m}$ due to the coupling between in-plane and flexural modes. This coupling leads to renormalization of ZA phonon dispersion in the long-wavelength range, preventing further divergence of thermal conductivity. We also uncover a strong dependence on sample width, which we attribute to the interplay between nonresistive normal and diffusive edge scattering in the Poisseuille flow regime. We conclude that normal processes play a crucial role in the length and width dependence of thermal transport in graphene in the hydrodynamic regime and dictate the relative in-plane (LA+TA) to out-of-plane (ZA) contribution to transport.

The ability of graphene/polymer heterostructures to absorb GHz electromagnetic radiation was recently evidenced both theoretically and experimentally [Batrakov et al., Sci. Rep. 4, 7191 (2014) and Lobet et al., Nanotechnology 26, 285702 (2015)]. Maximum absorption was shown to depend solely on refractive indices of incident and emergence media once impedance matching conditions are fulfilled. In this paper, analytical models and numerical simulations are performed for both semi-infinite and finite slab substrate. We evidenced that only three graphene layers separated by a dielectric spacer and an epsilon-near-zero metamaterial as emergence medium allow a perfect absorption for normal incidence. The use of lossless epsilon-near-zero metamaterial prevents radiations to go through the device, because of infinite impedance, and forces them to be totally absorbed in the dissipative medium (graphene). The device is proved to be robust regarding angular incidence up to 45 deg for a semi-infinite epsilon-near-zero metamaterial. The proposed strategy is universal and can be applied to any kind of two-dimensional dissipative materials lying on epsilon-near-zero metamaterial. The proposed absorber does not rely on surface patterning or texturing and hence is more appealing for device applications.

Clarification of the electronic structure near the Fermi level is important in understanding the origin of ferromagnetism in the prototypical ferromagnetic semiconductor GaMnAs. Here, we perform ultrafast transient reflectivity spectra measurement, which is a powerful tool for selective detection of absorption edges in GaMnAs. The results show that the Fermi level of GaMnAs exists in the band gap. By using the Kramers-Kronig relation, we find the Mn-induced electronic states around the Fermi level, confirming that the ferromagnetism is stabilized by spin-polarized impurity-band holes.

Recent ab initio calculations and experiments reported insulating-semimetallic phase transitions in multilayer phosphorene under a perpendicular dc field, pressure, or doping, as a possible route to realize topological phases. In this work, we show that even a monolayer phosphorene may undergo Lifshitz transitions toward semimetallic and topological insulating phases, provided it is rapidly driven by in-plane time-periodic laser fields. Based on a four-orbital tight-binding description, we give an inversion-symmetry-based prescription in order to apprehend the topology of the photon-renormalized band structure, up to the second order in the high-frequency limit. Apart from the initial band insulating behavior, two additional phases are thus identified. A semimetallic phase with massless Dirac electrons may be induced by linear polarized fields, whereas elliptic polarized fields are likely to drive the material into an anomalous quantum Hall phase.

We study the properties of excited states in one-dimensional many-body localized (MBL) systems using a matrix product state algorithm. First, the method is tested for a large disordered noninteracting system, where for comparison we compute a quasiexact reference solution via a Monte Carlo sampling of the single-particle levels. Thereafter, we present extensive data obtained for large interacting systems of $L\sim 100$ sites and large bond dimensions $\chi \sim 1700$, which allows us to quantitatively analyze the scaling behavior of the entanglement $S$ in the system. The MBL phase is characterized by a logarithmic growth $S\left(L\right)\sim log\left(L\right)$ over a large scale separating the regimes where volume and area laws hold. We check the validity of the eigenstate thermalization hypothesis. Our results are consistent with the existence of a mobility edge.

Superconductivity in topological band structures is a platform for realizing Majorana bound states and other exotic physical phenomena such as emergent supersymmetry. This potential nourishes the search for topological materials with intrinsic superconducting instabilities, in which Cooper pairing is introduced to electrons with helical spin texture such as the Dirac surface states of topological insulators, forming a time-reversal symmetric topological superconductor on the surface. We employ first-principles calculations and angle-resolved photoemission spectroscopy experiments to reveal that ${\mathrm{PbTaSe}}_2$, a noncentrosymmetric superconductor, possesses a nonzero ${\mathbb{Z}}_2$ topological invariant and fully spin-polarized Dirac surface states. Moreover, we analyze the phonon spectrum of ${\mathrm{PbTaSe}}_2$ to show how superconductivity emerges in this compound due to a stiffening of phonons by the Pb intercalation, which diminishes a competing charge-density-wave instability. By combining our findings on the topological band structure and the superconducting electron pairing, our work establishes ${\mathrm{PbTaSe}}_2$ as a stoichiometric superconductor with topological Dirac surface states. This type of intrinsic topological Dirac superconductors holds great promise for studying aspects of topological superconductors such as Majorana zero modes.

Thermal density functional theory calculations often use the Mermin-Kohn-Sham scheme, but employ ground-state approximations to the exchange-correlation (XC) free energy. In the simplest solvable nontrivial model, an asymmetric Hubbard dimer, we calculate the exact many-body energies and the exact Mermin-Kohn-Sham functionals for this system and extract the exact XC free energy. For moderate temperatures and weak correlation, we find this approximation to be excellent. We extract various exact free-energy correlation components and the exact adiabatic connection formula.

We report high-resolution angle-resolved photoemission spectroscopy measurements on the honeycomb iridate ${\mathrm{Na}}_2{\mathrm{IrO}}_3$. Our measurements reveal the existence of a metallic surface band feature crossing the Fermi level with nearly linear dispersion and an estimated surface carrier density of $3.2\times {10}^{13}{\mathrm{cm}}^{-2}$, which has not been theoretically predicted or experimentally observed, and provides the first evidence for metallic behavior on the boundary of this material, whereas the bulk bands exhibit a robust insulating gap. We further show the lack of theoretically predicted Dirac cones at the $\overline{M}$ points of the surface Brillouin zone, which confirms the absence of a stacked quantum spin Hall phase in this material. Our data indicates that the surface ground state of this material is exotic and metallic, unlike as predicted in theory, and establishes ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ as a rare example of a strongly correlated spin-orbit insulator with surface metallicity.