Motivated by an important recent experiment [Deng et al., Science 354, 1557 (2016)], we theoretically consider the interplay between Andreev and Majorana bound states in disorder-free quantum dot-nanowire semiconductor systems with proximity-induced superconductivity in the presence of spin-orbit coupling and Zeeman spin splitting (induced by an external magnetic field). The quantum dot induces Andreev bound states in the superconducting nanowire, which show complex behavior as a function of magnetic field and chemical potential, and the specific question is whether two such Andreev bound states can come together forming a robust zero-energy topological Majorana bound state. We find generically that the Andreev bound states indeed have a high probability of coalescing together producing near-zero-energy midgap states as Zeeman splitting and/or chemical potential are increased, but this mostly happens in the nontopological regime below the topological quantum phase transition, although there are situations where the Andreev bound states could indeed come together to form a zero-energy topological Majorana bound state. The two scenarios (two Andreev bound states coming together to form a nontopological almost-zero-energy Andreev bound state or to form a topological zero-energy Majorana bound state) are difficult to distinguish just by tunneling conductance spectroscopy, since they produce essentially the same tunneling transport signatures. We find that the “sticking together” propensity of Andreev bound states to produce an apparent stable zero-energy midgap state is generic in class D systems in the presence of superconductivity, spin-orbit coupling, and magnetic field, even in the absence of any disorder. We also find that the conductance associated with the coalesced zero-energy nontopological Andreev bound state is nonuniversal and could easily be $2e^2/h$, mimicking the quantized topological Majorana zero-bias conductance value. We suggest experimental techniques for distinguishing between trivial and topological zero-bias conductance peaks arising from the coalescence of Andreev bound states.

We have investigated the disorder of epitaxial graphene close to the charge neutrality point (CNP) by various methods: (i) at room temperature, by analyzing the dependence of the resistivity on the Hall coefficient; (ii) by fitting the temperature dependence of the Hall coefficient down to liquid helium temperature; (iii) by fitting the magnetoresistances at low temperature. All methods converge to give a disorder amplitude of $(20\pm 10)$ meV. Because of this relatively low disorder, close to the CNP, at low temperature, the sample resistivity does not exhibit the standard value $\simeq h/4e^2$ but diverges. Moreover, the magnetoresistance curves have a unique ambipolar behavior, which has been systematically observed for all studied samples. This is a signature of both asymmetry in the density of states and in-plane charge transfer. The microscopic origin of this behavior cannot be unambiguously determined. However, we propose a model in which the SiC substrate steps qualitatively explain the ambipolar behavior.

We present comprehensive analytical and numerical studies on the pure dephasing of a single Mn spin in a semiconductor quantum dot due to (i) its sp-d exchange interaction with an electronic environment, and (ii) its hyperfine interaction with the nuclear spin environment. For (i), by modeling the electronic environment by an open two-level system, we provide exact analytical expressions and present detailed analysis for the Mn spin pure dephasing in both the Markovian and non-Markovian regimes. This provides a clear physical picture and a general theoretical framework based on which we estimate the Mn spin pure dephasing due to various fluctuations (such as thermal excitation, optical pumping, tunneling, or electron/hole spin relaxation) of the electronic environment and reveals a series of interesting behaviors, such as thermal, optical, and electrical control of the crossover between the Markov and non-Markov regimes. In particular, we find rapid Mn spin pure dephasing on a nanosecond time scale by the thermal fluctuation and optical pumping, but these mechanisms can be strongly suppressed by shifting the electron envelope function relative to the Mn atom with an external electric field through the quantum-confined Stark effect. The thermal fluctuation mechanism is also exponentially suppressed at low temperature. For (ii), we find that the Mn spin dephasing time is limited by the thermal fluctuation of the nuclear spins to a few microseconds even at low temperature and its value varies from sample to sample, depending on the distribution of spinful isotopes on the nearest-neighbor sites surrounding the substitutional Mn atom. Our findings may be useful to understand and suppress the Mn spin pure dephasing for its applications in quantum information processing.

We studied the thickness-dependent shift of quantum well state (QWS) peaks and their interaction with Stark-shifted image potential states (IPSs) in the $dz/dV$ spectrum of ultrathin Ag(111) islands on a Si(111)-$\sqrt{3}\times \sqrt{3}\text{−}B$ substrate. The transmission resonance peaks for the QWSs were shifted to lower energy as the Ag island thickness increased. The shift was quantitatively explained by the quantization of the electrons in the island with the specific $\mathrm{\Gamma }\text{−}L$ dispersion of Ag. In addition, two split peaks appeared around the original IPS position when the QWS and IPS had the same energy. The split peaks were attributed to the avoidance of the QWS and IPS crossing.

We report a time-resolved study of the relaxation dynamics of Al films excited by ultrashort intense free-electron laser (FEL) extreme ultraviolet pulses. The system response was measured through a pump-probe detection scheme, in which an intense FEL pulse tuned around the Al $L_{2,3}$ edge (72.5 eV) acted as the pump, while a time-delayed ultrafast pulse probed the near-infrared (NIR) reflectivity of the Al film. Remarkably, following the intense FEL excitation, the reflectivity of the film exhibited no detectable variation for hundreds of femtoseconds. Following this latency time, sizable reflectivity changes were observed. Exploiting recent theoretical calculations of the EUV-excited electron dynamics [N. Medvedev et al., Phys. Rev. Lett. 107, 165003 (2011)], the delayed NIR-reflectivity evolution is interpreted invoking the formation of very-long-living nonthermal hot electron distributions in Al after exposure to intense EUV pulses. Our data represent the first evidence in the time domain of such an intriguing behavior.

Fermi arc surface states, a hallmark of topological Dirac semimetals, can host carriers that exhibit unusual dynamics distinct from that of their parent bulk. Here we find that Fermi arc carriers in intrinsic Dirac semimetals possess a strong and anisotropic light-matter interaction. This is characterized by a large Fermi arc optical conductivity when light is polarized transverse to the Fermi arc; when light is polarized along the Fermi arc, Fermi arc optical conductivity is significantly muted. The large surface spectral weight is locked to the wide separation between Dirac nodes and persists as a large Drude weight of Fermi arc carriers when the system is doped. As a result, large and anisotropic Fermi arc conductivity provides a novel means of optically interrogating the topological surfaces states of Dirac semimetals.

Motivated by systems that can be seen as composed of two frustrated sublattices combined into a less frustrated total lattice, we study the double-exchange model with nearest-neighbor (NN) and next-nearest-neighbor (NNN) couplings on the honeycomb lattice. When adding NN hopping and its resulting double exchange to the antiferromagnetic (AFM) Heisenberg coupling, the resulting phase diagram is quite different from that of purely Heisenberg-like magnetic models and strongly depends on electron filling. For half filling, patterns of AFM dimers dominate, where the effective electronic bands remain graphenelike with Dirac cones in all phases, from the FM to the ${120}^{\circ }$ limit. When the density of states at the Fermi level is sizable, we find noncoplanar incommensurate states as well as a small-vortex phase. Finally, a noncoplanar commensurate pattern realizes a Chern insulator at quarter filling. In the case of both NN and NNN hopping, the noncoplanar spin pattern inducing Chern insulators in triangular lattices is found to be quite stable under coupling into a honeycomb system. The resulting total phases are topologically nontrivial and either a Chern insulator with $C=2$ or a magnetic topological crystalline insulator protected by a combination or mirror-reflection and time-reversal symmetries arise.

Resonant elastic x-ray scattering is a powerful technique for measuring multipolar order parameters. In this paper, we theoretically and experimentally study the possibility of using this technique to detect the proposed multipolar order parameters in ${\mathrm{URu}}_2{\mathrm{Si}}_2$ at the U-$L_3$ edge with the electric quadrupolar transition. Based on an atomic model, we calculate the azimuthal dependence of the quadrupolar transition at the U-$L_3$ edge. The results illustrate the potential of this technique for distinguishing different multipolar order parameters. We then perform experiments on ultraclean single crystals of ${\mathrm{URu}}_2{\mathrm{Si}}_2$ at the U-$L_3$ edge to search for the predicted signal, but do not detect any indications of multipolar moments within the experimental uncertainty. We theoretically estimate the orders of magnitude of the cross section and the expected count rate of the quadrupolar transition and compare them to the dipolar transitions at the U-$M_4$ and U-$L_3$ edges, clarifying the difficulty in detecting higher order multipolar order parameters in ${\mathrm{URu}}_2{\mathrm{Si}}_2$ in the current experimental setup.

Nearly aligned graphene on hexagonal boron nitride (G/BN) can be accurately modeled by a Dirac Hamiltonian perturbed by smoothly varying moiré pattern pseudospin fields. Here, we present the moiré-band model of G/BN for arbitrary small twist angles under a framework that combines symmetry considerations with input from ab initio calculations. Our analysis of the band gaps at the primary and secondary Dirac points highlights the role of inversion symmetry breaking contributions of the moiré patterns, leading to primary Dirac point gaps when the moiré strains give rise to a finite average mass, and to secondary gaps when the moiré pseudospin components are mixed appropriately. The pseudomagnetic strain fields, which can reach values of up to $\sim 40$ T near symmetry points in the moiré cell stem almost entirely from virtual hopping and dominate over the contributions arising from bond length distortions due to the moiré strains.

The hallmark of topological phases is their robust boundary signature whose intriguing properties—such as the one-way transport on the chiral edge of a Chern insulator and the sudden disappearance of surface states forming open Fermi arcs on the surfaces of Weyl semimetals—are impossible to realize on the surface alone. Yet, despite the glaring simplicity of noninteracting topological bulk Hamiltonians and their concomitant energy spectrum, the detailed study of the corresponding surface states has essentially been restricted to numerical simulation. In this work, however, we show that exact analytical solutions of both topological and trivial surface states can be obtained for generic tight-binding models on a large class of geometrically frustrated lattices in any dimension without the need for fine-tuning of hopping amplitudes. Our solutions derive from local constraints tantamount to destructive interference between neighboring layer lattices perpendicular to the surface and provide microscopic insights into the structure of the surface states that enable analytical calculation of many desired properties including correlation functions, surface dispersion, Berry curvature, and the system size dependent gap closing, which necessarily occurs when the spatial localization switches surface. This further provides a deepened understanding of the bulk-boundary correspondence. We illustrate our general findings on a large number of examples in two and three spatial dimensions. Notably, we derive exact chiral Chern insulator edge states on the spin-orbit-coupled kagome lattice, and Fermi arcs relevant for recently synthesized slabs of pyrochlore-based ${\mathrm{Eu}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ and ${\mathrm{Nd}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$, which realize an all-in-all-out spin configuration, as well as for spin-ice-like two-in-two-out and one-in-three-out configurations, which are both relevant for ${\mathrm{Pr}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$. Remarkably, each of the pyrochlore examples exhibit clearly resolved Fermi arcs although only the one-in-three-out configuration features bulk Weyl nodes in realistic parameter regimes. Our approach generalizes to symmetry protected phases, e.g., quantum spin Hall systems and Dirac semimetals with time-reversal symmetry, and can furthermore signal the absence of topological surface states, which we illustrate for a class of models akin to the trivial surface of Hourglass materials $\mathrm{KHg}X$ where the exact solutions apply but, independently of Hamiltonian details, yield eigenstates delocalized over the entire sample.

A continuum/atomistic approach for predicting lattice instability during crystal-crystal phase transformations (PTs) is developed for the general loading with an arbitrary stress tensor and large strains. It is based on a synergistic combination of the generalized Landau-type theory for PTs and molecular dynamics (MD) simulations. The continuum approach describes the entire dissipative transformation process in terms of an order parameter, and the general form of the instability criterion is derived utilizing the second law of thermodynamics. The feedback from MD allowed us to present the instability criterion for both direct and reverse PTs in terms of the critical value of the modified transformation work, which is linear in components of the true stress tensor. It was calibrated by MD simulations for direct and reverse PTs between semiconducting silicon Si i and metallic Si ii phases under just two different stress states. Then, it describes hundreds of MD simulations under various combinations of three normal and three shear stresses. In particular, the atomistic simulations show that the effects of all three shear stresses along cubic axes on lattice instability of Si i are negligible, which is in agreement with our criterion.

The phonon anharmonicity in Se-doped $\mathrm{S}{\mathrm{b}}_2\mathrm{T}{\mathrm{e}}_3$ system is probed both macroscopically and microscopically using temperature-dependent synchrotron powder diffraction, Raman spectroscopic studies, and heat-capacity measurements. Gruneisen parameter (${\gamma }_{\mathrm{G}}$) is calculated to explain anharmonicity in the polycrystalline $\mathrm{S}{\mathrm{b}}_2\mathrm{T}{\mathrm{e}}_{3-x}\mathrm{S}{\mathrm{e}}_x$ samples. The thermal variation of structural parameters, structural anisotropy, Debye temperature, velocity of sound, and isothermal compressibility of $\mathrm{S}{\mathrm{b}}_2\mathrm{T}{\mathrm{e}}_{3-x}\mathrm{S}{\mathrm{e}}_x$ at room temperature are estimated. Analysis revealed that structural anisotropy and phonon anharmonicity are correlated. Further, lattice thermal conductivities (${\kappa }_L$), evaluated in the Umklapp scattering limit in terms of ${\gamma }_{\mathrm{G}}$, indicate that the phonon anharmonicity increasing with Se($x$) content reduces ${\kappa }_L$. A plausible explanation is provided on the basis of Se-doped softening of transverse optical phonon mode.

The perovskite $\mathrm{C}{\mathrm{a}}_{0.5}\mathrm{B}{\mathrm{i}}_{0.5}\mathrm{Fe}{\mathrm{O}}_3$ undergoes a remarkable sequence of charge-disproportionation (CD) and charge-transfer (CT) transitions on cooling due to competing electronic instabilities: $\mathrm{C}{\mathrm{a}}_{0.5}{\mathrm{Bi}}^{3+}{}_{0.5}\mathrm{F}{\mathrm{e}}^{3.5+}{\mathrm{O}}_3\to \mathrm{C}{\mathrm{a}}_{0.5}{\mathrm{Bi}}^{3+}{}_{0.5}{\mathrm{Fe}}^{3+}{}_{0.67}{\mathrm{Fe}}^{4.5+}{}_{0.33}{\mathrm{O}}_3\left(\mathrm{CD}\mathrm{phase}\right)\to \mathrm{C}{\mathrm{a}}_{0.5}{\mathrm{Bi}}^{3+}{}_{0.25}{\mathrm{Bi}}^{5+}{}_{0.25}\mathrm{F}{\mathrm{e}}^{3+}{\mathrm{O}}_3\left(\mathrm{CT}\mathrm{phase}\right)$. The accompanying changes in charge and spin ordering have been determined from neutron diffraction and physical property measurements. The CT phase adopts a simple $G$-type antiferromagnetic structure of $\mathrm{F}{\mathrm{e}}^{3+}$ spins but the CD phase adopts an unusual charge and magnetic arrangement in which $\mathrm{F}{\mathrm{e}}^{3+}$ spins are antiferromagnetically ordered but the $\mathrm{F}{\mathrm{e}}^{4.5+}$ moments have no long-range order due to magnetic frustration and form a spin glass at low temperatures.

Based on multiscale calculations combining ab initio methods with spin dynamics simulations, we perform a detailed study of the magnetic behavior of ${\mathrm{Ni}}_2\mathrm{MnAl}/\mathrm{Fe}$ bilayers. Our simulations show that such a bilayer exhibits a small exchange bias effect when the ${\mathrm{Ni}}_2\mathrm{MnAl}$ Heusler alloy is in a disordered B2 phase. Additionally, we present an effective way to control the magnetic structure of the ${\mathrm{Ni}}_2\mathrm{MnAl}$ antiferromagnet, in the pseudo-ordered B2-I as well as the disordered B2 phases, via a spin-flop coupling to the Fe layer.

We report an unusual magnetic ground state in single-crystal, double-perovskite $\mathrm{B}{\mathrm{a}}_2\mathrm{YIr}{\mathrm{O}}_6$ and Sr-doped $\mathrm{B}{\mathrm{a}}_2\mathrm{YIr}{\mathrm{O}}_6$ with $\mathrm{I}{\mathrm{r}}^{5+}\left(5d^4\right)$ ions. Long-range magnetic order below 1.7 K is confirmed by dc magnetization, ac magnetic susceptibility, and heat-capacity measurements. The observed magnetic order is extraordinarily delicate and cannot be explained in terms of either a low-spin $S=1$ state, or a singlet $J_{\mathrm{eff}}=0$ state imposed by the spin-orbit interactions (SOI). Alternatively, the magnetic ground state appears consistent with a SOI that competes with comparable Hund's rule coupling and inherently large electron hopping, which cannot stabilize the singlet $J_{\mathrm{eff}}=0$ ground state. However, this picture is controversial, and conflicting magnetic behavior for these materials is reported in both experimental and theoretical studies, which highlights the intricate interplay of interactions that determine the ground state of materials with strong SOI.

Electron-hole ($e-h$) compensation is a hallmark of multiband semimetals with extremely large magnetoresistance (XMR) and has been considered to be the basis for XMR. Recent spectroscopic experiments, however, reveal that YSb with nonsaturating magnetoresistance is uncompensated, questioning the $e-h$ compensation scenario for XMR. Here we demonstrate with magnetoresistivity and angle-dependent Shubnikov–de Haas (SdH) quantum oscillation measurements that YSb does have nearly perfect $e-h$ compensation, with a density ratio of $\sim 0.95$ for electrons and holes. The density and mobility anisotropy of the charge carriers revealed in the SdH experiments allow us to quantitatively describe the magnetoresistance with an anisotropic multiband model that includes contributions from all Fermi pockets. We elucidate the role of compensated multibands in the occurrence of XMR by demonstrating the evolution of calculated magnetoresistances for a single band and for various combinations of electron and hole Fermi pockets.

An equilibrium distribution function which corresponds to thermalization of electrons in contacts due to scattering processes is introduced for the purpose of reformulation of the inflow boundary conditions for the Wigner kinetic equation. The importance of the proposed concept for more realistic descriptions of transport phenomena in nanodevices is illustrated by determination of characteristics of the nanowire in cases of dissipative and dissipationless states of its active region. Quantitative results show how transport characteristics of the nanodevice are modified by the contacts.

We investigate a theoretical model for effective carrier mobility to comprehensively describe the behavior of the perpendicular carrier transport across multiple quantum well (MQW) structures under applied electric field. The analytical expressions of effective mobilities for thermionic emission, direct tunneling, and thermally assisted tunneling are derived based on the quasithermal equilibrium approximation and the semiclassical approach. Effective electron and hole mobilities in InGaAs/GaAsP MQWs predicted from our model are in good agreement with the experimental results obtained from the carrier time-of-flight measurement near room temperature. With this concept, the complicated carrier dynamics inside MQWs can be simplified to an effective mobility, an equivalent parameter that is more straightforward to handle and can be easily incorporated in the conventional drift-diffusion model.