We study the electron transport in a magnetically doped three-dimensional topological insulator (TI) by taking the effects of impurity-impurity exchange interactions into account. The interactions between magnetic impurities give rise to the formation of magnetic clusters with temperature-dependent mean sizes, randomly distributed on the surface of the TI. Instead of dealing with single magnetic impurities, we consider surface Dirac electrons to be scattered off magnetic clusters, and define the scattering potential in terms of clusters mean sizes. Within the semiclassical Boltzmann approach, employing a generalized relaxation-time approximation, we obtain the surface conductivity of the TI by solving four sets of recursive relations and demonstrate that the system is highly anisotropic and the surface conductivities possess nonmonotonic behaviors, i.e., they strongly depends on the direction, the mean size, and the number of magnetic clusters. We demonstrate that the dependence of the anisotropic magnetoresistance (AMR) to the spin direction of the magnetic clusters is inconsistent with the angular dependence of the TI doped with noninteracting magnetic impurities. Our results are consistent with the recent experiment on the AMR of the Cr-doped ${(\mathrm{Bi},\mathrm{Sb})}_2{\mathrm{Te}}_3$ TI.

We study the lifetime of topological qubits based on Majorana bound states hosted in a one-dimensional Rashba nanowire (NW) with proximity-induced superconductivity and nonuniform chemical potential needed for manipulation and readout. If nearby gates tune the chemical potential locally so that part of the NW is in the trivial phase, Andreev bound states (ABSs) can emerge which are localized at the interface between topological and trivial phases with energies significantly less than the gap. The emergence of such subgap states decreases the Majorana qubit lifetime at finite temperatures due to local perturbations that can excite the system into these ABSs. Using the Keldysh formalism, we study such excitations caused by fluctuating charges in capacitively coupled gates and calculate the corresponding Majorana lifetimes due to thermal noise, which are shown to be reduced in comparison with those in NWs with uniform chemical potential.

We employ active feedback to stabilize the frequency of single photons emitted by two separate quantum dots to an atomic standard. The transmission of a rubidium-based Faraday filter serves as the error signal for frequency stabilization. We achieve a residual frequency deviation of $<30$ MHz, which is less than 1.5% of the quantum dot linewidth. Long-term stability is demonstrated by Hong-Ou-Mandel interference between photons from the two quantum dots. Their internal dephasing limits the expected visibility to $V=40\%$. We observe $V_{\mathrm{lock}}=(41\pm 5)\%$ for frequency-stabilized dots as opposed to $V_{\mathrm{free}}=(31\pm 7)\%$ for free-running emission. Our technique reaches the maximally expected visibility for the given system and therefore facilitates quantum networks with indistinguishable photons from distributed sources.

Electrons confined in Si quantum dots possess orbital, spin, and valley degrees of freedom (DOF). We perform Landau-Zener-Stückelberg-Majorana (LZSM) interferometry on a Si double quantum dot that is strongly coupled to a microwave cavity to probe the valley DOF. The resulting LZSM interference pattern is asymmetric as a function of level detuning and persists for drive periods that are much longer than typical charge decoherence times. By correlating the LZSM interference pattern with charge-noise measurements, we show that valley-orbit hybridization provides some protection from the deleterious effects of charge noise. Our work opens the possibility of harnessing the valley DOF to engineer charge-noise-insensitive qubits in Si.

Recently, Wang et al. (arXiv:1704.00995) reported quantum oscillation in magnetoresistance with the periodicity in the logarithm of a magnetic field (B) for $p$-type $\mathrm{ZrT}{\mathrm{e}}_5$. They have ascribed this type of behavior to the discrete scale invariance resulting from Efimov bound states. We have prepared high-quality stoichiometric ($p$-type) $\mathrm{ZrT}{\mathrm{e}}_5$ polycrystals and observed magnetoresistance oscillations, which are periodic in B. These oscillations are in contrast to usual SdH oscillations or log B dependent oscillations as observed for tellurium-deficient and stoichiometric $\mathrm{ZrT}{\mathrm{e}}_5$, respectively. We obtained small cyclotron effective mass ($m^{*}\sim 0.05m_e$), very high mobility of $\sim 2.2\times {10}^4\mathrm{c}{\mathrm{m}}^2/\mathrm{Vs}$, and the signature of topological protected surface states in the compound. The magnetic data show zero cusp paramagnetic susceptibility, which supports the existence of topological surface states in $\mathrm{ZrT}{\mathrm{e}}_5$.

In the regime of strong coupling between molecular excitons and confined optical modes, the intramolecular degrees of freedom are profoundly affected, leading to a reduced vibrational dressing of polaritons compared to bare electronically excited states. However, existing models only describe a single vibrational mode in each molecule, while actual molecules possess a large number of vibrational degrees of freedom and additionally interact with a continuous bath of phononic modes in the host medium in typical experiments. In this work, we investigate a small ensemble of molecules with an arbitrary number of vibrational degrees of freedom under strong coupling to a microcavity mode. We demonstrate that reduced vibrational dressing is still present in this case, and show that the influence of the phononic environment on most electronic and photonic observables in the lowest excited state can be predicted from just two collective parameters of the vibrational modes. Besides, we explore vibrational features that can be addressed exclusively by our extended model and could be experimentally tested. Our findings indicate that vibronic coupling is more efficiently suppressed for environments characterized by low-frequency (sub-Ohmic) modes.

We have performed AFM nanomanipulation experiments on triangular Au islands (with typical linear size of 25–80 nm) previously grown on a ${\mathrm{MoS}}_2$ surface. These islands are found to move along preferential directions, independently of the angle of attack of the scanning probe. A comparison between molecular-dynamics simulations and atomically resolved STM images prove that these directions correspond to the zigzag alignments of the Mo and S atoms on the substrate. This is related to the observed systematic orientation of the islands, which is in turn a consequence of a sharp energy minimum as a function of each island's angular orientation. This directional-locked motion is entirely different from nanomanipulation involving disordered contact interfaces, where the direction of motion is determined by the island geometry and the scan pattern, and roto-translational motion is observed in arbitrary directions. Besides shedding light on the fundamental mechanisms of friction in the considered class of materials, our results could find important applications in the controlled positioning of metal nanoislands as electrodes for molecular electronics.

Control of the polarization state of light is essential for many technologies, but is often limited by weak light-matter interactions that necessitate long device path lengths or significantly reduce the signal intensity. Here, we investigate a nanoscale plasmonic aperture capable of modifying the polarization state of far-field transmitted light without loss in the probe signal. The aperture is a coaxial resonator consisting of a dielectric ring embedded within a metallic film; parity-time ($\mathcal{P}\mathcal{T}$) -symmetric inclusions of loss and gain within the dielectric ring enable polarization control. Since the coaxial aperture enables near-thresholdless $\mathcal{P}\mathcal{T}$ symmetry breaking, polarization control is achieved with realistic levels of loss and gain. Exploiting this sensitivity, we show that the aperture can function as a tunable waveplate, with the transmitted ellipticity of circularly polarized incident light changing continuously with the dissipation coefficient from $\pi$/2 to 0 (i.e., linear polarization). Rotation of linearly polarized light with unity efficiency is also possible, with a continuously tunable degree of rotation. This compact, low-threshold, and reconfigurable polarizer may enable next-generation, high-efficiency displays, routers, modulators, and metasurfaces.

This paper studies the relationship between the atomistic representation of crystalline dislocations as Kanzaki forces and the continuum representation of dislocations as Burridge-Knopoff (BK) force distributions. We first derive a complete theory of the BK force representation of dislocations in an anisotropic linear elastic continuum, showcasing a number of fundamental features found when dislocations are represented as distributions of body forces in defect-free continuum media. We then build, within the harmonic approximation, the Kanzaki force representation of dislocations in atomistic lattice models. We rigorously show that in the long-wave limit, the Kanzaki force representation converges to the continuum BK representation. We therefore justify employing the Kanzaki forces as source terms in continuum theories of dislocations. We do this by establishing a methodology to compute the Kanzaki forces of dislocations via the force constant matrix of the material's perfect lattice. We use it to study a model of a screw dislocation in bcc tungsten, where we show the existence of two distinct Kanzaki force terms: the slip Kanzaki forces, which we show directly correspond with the BK forces implied by a Volterra dislocation; and the core Kanzaki forces, which are computed from the relaxed dislocation structure, and serve to model all core effects not captured by the Volterra dislocation. We build a multipolar field expansion of both the core and the slip Kanzaki forces, showing that the dislocation core is agreeable to correction via the multipolar field expansion of the core Kanzaki forces.

The orthorhombic boride family $XM{\mathrm{B}}_{14}$, where $X$ and $M$ are metal atoms, have been of great interest in hard coating applications because of such novel properties as high thermal stability, low density, chemical stability, and a low friction coefficient. However, the brittle failure of orthorhombic borides limits their mechanical stability under working environments and prevents their extended engineering applications. To provide guidelines of improving their stability, we employed density functional theory (DFT) to examine the bonding character and mechanical response of $XM{\mathrm{B}}_{14}$ under pure shear, biaxial shear, and tensile loading conditions. Two typical $XM{\mathrm{B}}_{14}$ compounds, ${\mathrm{AlLiB}}_{14}$ and $\mathrm{A}{\mathrm{l}}_{0.75}\mathrm{M}{\mathrm{g}}_{0.78}{\mathrm{B}}_{14}$, were examined to illustrate the effects of intrinsic metal vacancies. We find that the ideal strength for ${\mathrm{AlLiB}}_{14}$ is higher than that for $\mathrm{A}{\mathrm{l}}_{0.75}\mathrm{M}{\mathrm{g}}_{0.75}{\mathrm{B}}_{14}$, suggesting that ${\mathrm{AlLiB}}_{14}$ is intrinsically stronger than $\mathrm{A}{\mathrm{l}}_{0.75}\mathrm{M}{\mathrm{g}}_{0.75}{\mathrm{B}}_{14}$. The failure mechanism of both ${\mathrm{AlLiB}}_{14}$ and $\mathrm{A}{\mathrm{l}}_{0.75}\mathrm{M}{\mathrm{g}}_{0.78}{\mathrm{B}}_{14}$ arises from deconstructing ${\mathrm{B}}_{12}$ icosahedra under pure shear and biaxial shear conditions, while the structural failure under tensile deformation arises from breaking interlayer bonds between icosahedral layers.

Flatbands feature the distortion-free storage of compact localized states of tailorable shape. Their reliable storage sojourn is, however, limited by disorder potentials, which generically cause uncontrolled coupling into dispersive bands. We find that, while detuning flatband states from band intersections suppresses their direct decay into dispersive bands, disorder-induced state distortion causes a delayed, dephasing-mediated decay, lifting the static nature of flatband states and setting a finite lifetime for the reliable storage sojourn. We exemplify this generic, disorder-induced decay mechanism at the cross-stitch lattice. Our analysis, which applies platform independently, relies on the time-resolved treatment of disorder-averaged quantum systems with quantum master equations.

Using particle-based simulations, we examine the collective dynamics of skyrmions interacting with periodic pinning arrays, focusing on the impact of the Magnus force on the sliding phases. As a function of increasing pinning strength, we find a series of distinct dynamical phases, including an interstitial flow phase, a moving disordered state, a moving crystal, and a segregated cluster state. The transitions between these states produce signatures in the skyrmion lattice structure, the skyrmion Hall angle, the velocity fluctuation distributions, and the velocity-force curves. The moving clustered state is similar to the segregated state recently observed in continuum-based simulations with strong quenched disorder. The segregation arises from the drive dependence of the skyrmion Hall angle, and appears in the strong pinning limit when the skyrmions have nonuniform velocities, causing different portions of the sample to have different effective skyrmion Hall angles. We map the evolution of the dynamic phases as a function of the system density, the ratio of the Magnus force to the dissipative term, and the ratio of the number of skyrmions to the number of pinning sites.

We investigate the classical phase diagram of the stuffed honeycomb Heisenberg lattice, which consists of a honeycomb lattice with a superimposed triangular lattice formed by sites at the center of each hexagon. This lattice encompasses and interpolates between the honeycomb, triangular, and dice lattices, preserving the hexagonal symmetry while expanding the phase space for potential spin liquids. We use a combination of iterative minimization, classical Monte Carlo, and analytical techniques to determine the complete ground state phase diagram. It is quite rich, with a variety of noncoplanar and noncollinear phases not found in the previously studied limits. In particular, our analysis reveals the triangular lattice critical point to be a multicritical point with two new phases vanishing via second order transitions at the critical point. We analyze these phases within linear spin wave theory and discuss consequences for the $S=1/2$ spin liquid.

We studied the effects of the external magnetic field and photon flux on timing jitter in photon detection by straight superconducting NbN nanowires. At two wavelengths 800 and 1560 nm, statistical distribution in the appearance times of photon counts exhibits Gaussian shape at small times and an exponential tail at large times. The characteristic exponential time is larger for photons with smaller energy and increases with external magnetic field while variations in the Gaussian part of the distribution are less pronounced. Increasing photon flux drives the nanowire from the discrete quantum detection regime to the uniform bolometric regime that averages out fluctuations of the total number of nonequilibrium electrons created by the photon and drastically reduces jitter. The difference between standard deviations of Gaussian parts of distributions for these two regimes provides the measure for the strength of electron-number fluctuations; it increases with the photon energy. We show that the two-dimensional hot-spot detection model explains qualitatively the effect of magnetic field.

In situ high-pressure high-temperature synchrotron x-ray diffraction studies of the nickel-hydrogen system reveals the synthesis of a nickel polyhydride, ${\mathrm{Ni}}_2{\mathrm{H}}_3$. We observe the formation of NiH at pressures above 1 GPa, which remains stable to 52 GPa at room temperature. Laser heating to above 1000 K at this pressure initiates a transition to a phase which we determine as ${\mathrm{Ni}}_2{\mathrm{H}}_3$, crystallizing in a body-centered monoclinic unit cell. The ${\mathrm{Ni}}_2{\mathrm{H}}_3$ phase was observed to convert back to NiH below 25 GPa, and upon further decompression to atmospheric conditions, NiH slowly releases hydrogen with time.

The compression behavior of crystalline and amorphous germania holds considerable interest as an analog for silica and for understanding the structural response of $\mathrm{A}{\mathrm{X}}_2$ compounds generally. In this paper, the $\alpha \text{−}\mathrm{Pb}{\mathrm{O}}_2$-type and $Pa\overline{3}$-type polymorphs of $\mathrm{Ge}{\mathrm{O}}_2$ were investigated under high pressure using angle-dispersive synchrotron x-ray diffraction in the laser-heated diamond anvil cell. Theoretical calculations based on density functional theory were also performed. The experimental pressure-volume data were fitted to third-order Birch-Murnaghan equations of state. The fit parameters for the $\alpha \text{−}\mathrm{Pb}{\mathrm{O}}_2$ type are $V_0=53.8\left(2\right){\AA }^3,K_{0T}=293\left(7\right)\mathrm{GPa}$ with fixed $K_{0T}^{\prime }=4$, where $V,K_T$, and $K_T^{\prime }$ are the volume, isothermal bulk modulus, and pressure derivative of the bulk modulus and the subscript zero refers to ambient conditions. The corresponding parameters for the $Pa\overline{3}$-type phase are $V_0=50.3\left(3\right){\AA }^3,K_{0T}=342\left(12\right)\mathrm{GPa}$ with fixed $K_{0T}^{\prime }=4$. The theoretical calculations are in good agreement with the experimental results with slight underestimation and overestimation of $V_0$ and $K_{0T}$, respectively. A theoretical Hugoniot was calculated from our data and compared to shock equation of state data for vitreous and rutile-type $\mathrm{Ge}{\mathrm{O}}_2$. The high-pressure phase observed on the Hugoniot is most consistent with either the $\alpha \text{−}\mathrm{Pb}{\mathrm{O}}_2$-type or $\mathrm{CaC}{\mathrm{l}}_2$-type phase. Finally, we have compared our data on crystalline germania with existing studies on the corresponding phases of $\mathrm{Si}{\mathrm{O}}_2$ to better understand the effects of cation substitution on phase transformations and equations of state in group 14 dioxides.

Continuing uncertainty in the high-pressure melt curves of bcc transition metals has spawned renewed research interest in the phase diagrams of these materials, with tantalum becoming an important prototype. The present paper extends the quantum-based investigation of high-$T, P$ polymorphism and melt in Ta that was begun in Paper I [Haskins et al., Phys. Rev. B 86, 224104 (2012)] on five candidate cubic and hexagonal structures (bcc, A15, fcc, hcp, and hex-ω) to here treat four promising orthorhombic structures (Pnma, Fddd, Pmma, and α-U). Using DFT-based MGPT multi-ion potentials that allow accurate MD simulations of large systems, we showed in Paper I that the mechanically unstable fcc, hcp, and hex-ω structures can only be stabilized at high-$T, P$ by large anharmonic vibrational effects, requiring systems of $\sim 500$ atoms to produce size-independent melt curves and reliable calculations of thermodynamic stability. This reversed a previous small-cell quantum-simulation prediction of a high-$T, P$ hex-ω phase. Subsequent DFT calculations have now suggested a more energetically favorable and mechanically stable Pnma structure, which again small-cell quantum simulations predict could be a high-$T, P$ phase. Our present MGPT total-energy and phonon calculations show that not only Pnma, but all four orthorhombic structures considered here, are similarly energetically favorable, and that Fddd in addition to Pnma is mechanically stable at $T=0$ up to 420 GPa. MGPT-MD simulations further reveal spontaneous temperature-induced Pnma$\to$ bcc and Fddd$\to$ bcc transformations at modest temperatures, peaking at $\sim 1450\mathrm{K}$ near 100 GPa. At high temperatures near melt, we find $T$-dependent $c/a$ and $b/a$ axial ratios and large stabilizing anharmonicity present in all four orthorhombic structures. The anharmonicity drives significantly larger melt size effects, requiring systems of $\sim 1000–4000$ atoms to produce converged melt curves for reliable predictions of relative thermodynamic stability. In the large-cell limit, with $\sim 40000$ solid-phase atoms and accurate two-phase MGPT-MD melt simulation, we find that Pnma, Fddd and α-U have melt temperatures that are equal to bcc over small pressure ranges in the vicinity of 100 GPa, but that the orthorhombic melt temperatures never exceed bcc up to 420 GPa. This finding suggests that Pnma, Fddd, and α-U remain highly competitive metastable phases that could coexist with bcc and possibly be observed experimentally. Finally, to add additional insight into our results we have constructed global Helmholtz free energies for the A15, Pnma, and Fddd phases of Ta, complementing previous free energies obtained for the bcc, fcc, and liquid phases.

An important experimental signature of localization is the low-frequency ac conductivity, which typically vanishes as ${\omega }^{\varphi }$. The exponent $\varphi =2$ for Anderson insulators, whereas for many body-localized insulators $\varphi$ is a continuously varying exponent, $1\le \varphi \le 2$. In this work, we study the low-frequency ac conductivity of localized superconductors, in which disorder is strong enough to localize all quasiparticles, while remaining weak enough to leave superconductivity intact. We find that, while the ac conductivity still follows the general form $\sigma \left(\omega \right)\sim {\omega }^{\varphi }$, the exponent $\varphi$ can be markedly different from the characteristic value for localized insulators. This difference occurs due to singularities in the low-energy density of states, permitted by the effective particle-hole symmetry around the Fermi level. In particular, in certain symmetry classes at zero temperature, we obtain $\varphi >2$. We further identify an interesting temperature-dependent crossover in the scaling form of the ac conductivity, which could be useful for the experimental characterization of localized superconductors.