A study of a force detected single-spin magnetic resonance measurement concept with atomic spatial resolution is presented. The method is based upon electrostatic force detection of spin-selection rule controlled single-electron tunneling between two electrically isolated paramagnetic states. Single-spin magnetic resonance detection is possible by measuring the force detected tunneling charge noise on and off spin resonance. Simulation results of this charge noise, based upon physical models of the tunneling and spin physics, are directly compared to measured atomic force microscopy system noise. The results show that the approach could provide single-spin measurement of electrically isolated qubit states with atomic spatial resolution at room temperature.

We present experiments on the dynamics of a two-state parametric fluctuator in a superconducting flux qubit. In spectroscopic measurements, the fluctuator manifests itself as a doublet line. When the qubit is excited in resonance with one of the two doublet lines, the correlation of readout results exhibits an exponential time decay which provides a measure of the fluctuator transition rate. The rate increases with temperature in the interval 40 to 158 mK. Based on the magnitude of the transition rate and the doublet line splitting, we conclude that the fluctuation is induced by quasiparticle tunneling. These results demonstrate the importance of considering quasiparticles as a source of decoherence in flux qubits.

We investigated polarization-resolved electronic Raman scattering in different graphitic structures, including bulk graphite and single-layer and few-layer graphene. For all investigated samples, the broad continua of interband electronic transitions were detected at an energy $\sim 0.35$ eV, while they were expected to be at $\sim 6$ eV [Phys. Rev. B 88, 085416 (2013)]. The symmetry of the observed excitations corresponds to the $A_{2g}$ irreducible representation. A quasilinear behavior of the Raman response is observed at low energies in all cases at room temperature, in agreement with performed tight-binding calculations. High-energy features at $\sim 0.8$ eV are detected in the spectra of graphite and few-layer graphene. They are attributed to interband transitions in the vicinity of the $K$ point, which involve electronic bands split by interlayer interaction. The effects of the substrate type, defect amount, and doping on the continuum line shape and symmetry are discovered. The silent layer-breathing mode is observed in a single-layer graphene. The results evidence that the electronic light scattering in graphitic structures without an external magnetic field is a powerful tool, which provides a variety of data on the structure and symmetry of low-energy electronic excitations.

We consider the pseudogap (PG) state of high-$T_c$ superconductors in the form of a composite order parameter fluctuating between $2{\mathbf{p}}_{\text{F}}$-charge ordering and superconducting (SC) pairing. In the limit of linear dispersion and at the hot spots, both order parameters are related by a SU(2) symmetry, and the eight-hot-spot model of Efetov et al. [Nat. Phys. 9, 442 (2013)] is recovered. In the general case, however, curvature terms of the dispersion will break this symmetry, and the degeneracy between both states is lifted. Taking the full momentum dependence of the order parameter into account, we measure the strength of this SU(2) symmetry breaking over the full Brillouin zone. For realistic dispersion relations including curvature we find generically that the SU(2) symmetry breaking is small and robust to the fermiology and that the symmetric situation is restored in the large paramagnon mass and coupling limit. Comparing the level splitting for different materials, we propose a scenario that could account for the competition between the PG and SC states in the phase diagram of high-$T_c$ superconductors.

We have investigated correlation between structural and electronic anisotropies in a parent compound of Fe-chalcogenide superconductor ${\mathrm{Fe}}_{1+\delta }\mathrm{Te}$ with $\delta =0.09$ by means of synchrotron x-ray diffraction and in situ in-plane resistivity anisotropy measurements with uniaxial stress applied along a tetragonal $a$ axis. This system is known to exhibit a tetragonal-to-monoclinic structural transition at $T_{\mathrm{S}}\sim 60$ K. We have confirmed that the in-plane resistivity anisotropy in the low-temperature monoclinic phase is attributed to the asymmetry in volume fractions of the monoclinic domains, as was suggested in a previous study [Jiang et al., Phys. Rev. B 88 115130 (2013)]. On the other hand, we found another in-plane resistivity anisotropy above $T_{\mathrm{S}}$. The present x-ray diffraction and resistivity anisotropy measurements have revealed that this anisotropy is not due to an onset of the low-temperature monoclinic phase but to the lattice softening enhanced toward $T_{\mathrm{S}}$. As one of the possibilities, we suggest that the orbital fluctuation contributes to the lattice softening and the resistivity anisotropy above $T_{\mathrm{S}}$.

We theoretically demonstrate the strong Purcell effect in $\epsilon$-near-zero ultra-anisotropic uniaxial metamaterials with an elliptic isofrequency surface. Contrary to the hyperbolic metamaterials, the effect does not rely on the diverging density of states and evanescent waves. As a result, both the radiative decay rate and the far-field emission power are enhanced. The effect can be realized in the periodic layered metal-dielectric nanostructures with a complex unit cell containing two different metallic layers.

Generalized Lorentz-Lorenz formulas are developed for the effective parameters of binary lattice metamaterials composed of a periodic arrangement of electric and/or magnetic inclusions. The proposed homogenization approach is based on a dual dipole approximation for the induced currents. The obtained formulas for the metamaterial effective electric and magnetic characteristics duly consider both electric and magnetic polarizabilities of the inclusions and completely describe the effects of frequency and spatial dispersion. Several numerical examples are provided to demonstrate the general applicability of the proposed formulas to different types of binary lattices and inclusions. It is shown that the proposed effective parameters have the capability of providing a physically sound and accurate description of wave propagation in the metamaterials in an extended range of frequencies in contrast to the equivalent parameters that can be defined in the absence of impressed sources and assuming a local anisotropic constitutive model, which hides inherent spatial dispersion effects and nonphysical features. To gain further insight into the metamaterial response and the physical meaningfulness of calculated effective parameters, the power flow of metamaterial supported modes is analyzed and its homogenized representation is compared to the complete description. A correspondence between the power flow due to the microscopic field and the effect of spatial dispersion in the homogenized parameters is established.

Design principles and predictions of new three-dimensional (3D) Dirac semimetals are presented and placed in the context of currently known materials. Three different design principles are presented (cases I, II, and III), each of which yields predictions for new candidates. For case I, 3D Dirac semimetals based on charge-balanced compounds BaAgBi, SrAgBi, YbAuSb, ${\mathrm{PtBi}}_2$, and ${\mathrm{SrSn}}_2{\mathrm{As}}_2$ are identified as candidates. For case II, 3D Dirac semimetals in analogy to graphene, ${\mathrm{BaGa}}_2$ is identified as a candidate, and BaPt and ${\mathrm{Li}}_2\mathrm{Pt}$ are discussed. For case III, 3D Dirac semimetals based on glide planes and screw axes, ${\mathrm{TlMo}}_3{\mathrm{Te}}_3$ and the $A{\mathrm{Mo}}_3{X}_3$ family, in general ($A=\text{K}$, Na, In, Tl; $X=\text{Se}$,Te), as well as the Group IVb trihalides such as ${\mathrm{HfI}}_3$, are identified as candidates. Finally, we discuss conventional intermetallic compounds with Dirac cones and identify ${\mathrm{Cr}}_2\mathrm{B}$ as a potentially interesting material.

We use quantum Monte Carlo simulations to calculate the phase diagram and the correlation functions for the quantum phase transitions in the two-dimensional dissipative quantum XY model with and without fourfold anisotropy. Without anisotropy, the model describes the superconductor-to-insulator transition in two-dimensional dirty superconductors. With anisotropy, the model represents the loop-current order observed in the underdoped cuprates and its fluctuations, as well as the fluctuations near the ordering vector in simple models of two-dimensional itinerant ferromagnets and itinerant antiferromagnets. These calculations test an analytic solution of the model which reexpressed it in terms of topological excitations: the vortices with interactions only in space but none in time, and warps with leading interactions only in time but none in space, as well as subleading interactions which are both space and time dependent. For parameters where the proliferation of warps dominates the phase transition, the critical fluctuations as functions of the deviation of the dissipation parameter $\alpha$ on the disordered side from its critical value ${\alpha }_c$ are scale invariant in imaginary time $\tau$ as the correlation length in time ${\xi }_{\tau }={\tau }_c{e}^{{[{\alpha }_c/({\alpha }_c-\alpha )]}^{1/2}}$ diverges, where ${\tau }_c$ is a short-time cutoff. On the other hand, the spatial correlations develop with a correlation length ${\xi }_x\approx {\xi }_{0}ln\left({\xi }_{\tau }\right)$, with ${\xi }_0$ of the order of a lattice constant. The dynamic correlation exponent $z$ is therefore $\infty$. The Monte Carlo calculations also directly show warps and vortices. Their densities and correlations across the various transitions in the model are calculated and related to those of the order-parameter correlations in the dissipative quantum XY model.

Phonon transmission across interfaces of dissimilar materials has been studied intensively in recent years by using atomistic simulation tools due to its importance in determining the effective thermal conductivity of nanostructured materials. The atomistic Green's function method with interatomic force constants from the first-principles calculations has evolved to be a promising approach to study phonon transmission in many material systems that have not been well studied. However, the direct first-principles calculation for interatomic force constants becomes infeasible when the system involves atomic disorder. Mass approximation is usually used, but its validity has not been tested. In this paper, we employ the higher-order force constant model to extract harmonic force constants from the first-principles calculations, which originates from the virtual crystal approximation but considers the local force-field difference. As a feasibility demonstration of the proposed method that integrates higher-order force constant model from the first-principles calculations with the atomistic Green's function, we study the phonon transmission in the ${\mathrm{Mg}}_2\mathrm{Si}/{\mathrm{Mg}}_2{\mathrm{Si}}_{1-x}{\mathrm{Sn}}_x$ systems. When integrated with the atomistic Green's function, the widely used mass approximation is found to overpredict phonon transmission across the ${\mathrm{Mg}}_2{\mathrm{Si}/\mathrm{Mg}}_2\mathrm{Sn}$ interface. The difference can be attributed to the absence of local strain field-induced scattering in the mass approximation, which makes the high-frequency phonons less scattered. The frequency-dependent phonon transmission across an interface between a crystal and an alloy, which often appears in high-efficiency “nanoparticle in alloy” thermoelectric materials, is studied. The interfacial thermal resistance across ${\mathrm{Mg}}_2\mathrm{Si}/{\mathrm{Mg}}_2{\mathrm{Si}}_{1-x}{\mathrm{Sn}}_x$ interface is found to be weakly dependent on the composition of Sn when the composition of $\mathrm{x}$ is less than 40% but increases rapidly when it is larger than 40% due to the transition of high-frequency phonon DOS in ${\mathrm{Mg}}_2{\mathrm{Si}}_{1-x}{\mathrm{Sn}}_x$ alloys. This work could have a great impact on the design of novel nanostructures with tunable thermal properties.

Thermodynamic Ginzburg-Landau potential for temperature- and stress-induced phase transformations (PTs) between $n$ phases is developed. It describes each of the PTs with a single order parameter without an explicit constraint equation, which allows one to use an analytical solution to calibrate each interface energy, width, and mobility; reproduces the desired PT criteria via instability conditions; introduces interface stresses; and allows for a controlling presence of the third phase at the interface between the two other phases. A finite-element approach is developed and utilized to solve the problem of nanostructure formation for multivariant martensitic PTs. Results are in a quantitative agreement with the experiment. The developed approach is applicable to various PTs between multiple solid and liquid phases and grain evolution and can be extended for diffusive, electric, and magnetic PTs.

We studied solid solution effects on the mechanical properties of nanocrystalline (NC) ${\mathrm{Pd}}_{100-\mathrm{x}}{\mathrm{Au}}_{\mathrm{x}}$ alloys $(0\le x<50\mathrm{at}.\%)$ at the low end of the nanoscale. Concentration has been used as control parameter to tune material properties (elastic moduli, Burgers vector, stacking fault energies) at basically unaltered microstructure (grain size $D\approx 10\mathrm{nm}$). In stark contrast to coarse grained fcc alloys, we observe solid solution softening for increasing Au content. The available predictions from models and theories taking explicitly into account the effect of the nanoscale microstructure on the concentration-dependent shear strength have been disproved without exception. As a consequence, it is implied that dislocation activity contributes only marginally to strength. In fact, we find a linear correlation between shear strength and shear modulus, which quantitatively agrees with the universal behavior of metallic glasses discovered by Johnson and Samwer [Phys. Rev. Lett. 95, 195501 (2005)].

Motivated by recent experiments on liquid ${}^3\mathrm{He}$ reporting emergence of novel superfluid phases in globally anisotropic aerogels, our previous theory on superfluid ${}^3\mathrm{He}$ in globally anisotropic aerogels is extended to incorporate the effects of anisotropy of the quasiparticle scattering cross section on the strong-coupling (SC) contributions to the Ginzburg-Landau (GL) free energy on the basis of the spin-fluctuation (paramagnon) approach to the SC contributions developed by Brinkman et al. [Phys. Rev. A 10, 2386 (1974)]. In the globally isotropic case, impurity effects on the SC correction destabilize the A phase even at higher pressures of about 30 bar and make the B phase the only state in equilibrium, while SC contributions accompanied by a global stretched anisotropy to the GL quartic terms generally tend to broaden the stability region of the A phase compared with that of the B phase. In particular, in contrast to the cases in bulk and in the isotropic aerogel, the SC corrections to the GL quadratic terms are not negligible in the globally anisotropic case but may change the sign of the apparent anisotropy depending on the magnitude of the frequency cutoff of the normal paramagnon propagator. Based on this sign change of the apparent anisotropy, we discuss different strange observations on superfluid ${}^3\mathrm{He}$ in porous media such as the disappearance of the polar superfluid phase at higher pressures seen in nematically ordered aerogels and the absence of B and A phases with planar $\widehat{\mathbf{l}}$ vector in a stretched aerogel.

We present a scaling theory for the entanglement spectrum under an external driving. Based on the static scaling of the Schmidt gap and the theory of finite-time scaling, we show that the Schmidt gap can signal the critical point and be used to estimate the critical exponents no matter in the finite-size scaling region or in the finite-time scaling region. Crossover between the two regions is also demonstrated. We verify our theory using both the one-dimensional transverse-field Ising model and the one-dimensional quantum Potts model. Our results confirm that the Schmidt gap can be regarded as a supplement to the local order parameter.

The crystal structure of a disordered form of ${\mathrm{Cr}}_{1/3}{\mathrm{NbS}}_2$ has been characterized using diffraction and inelastic scattering of synchrotron radiation. In contrast to the previously reported symmetry $\left(P{6}_{3}22\right)$, the crystal can be described by a regular twinning of an average $P{6}_3$ structure with three disordered positions of the Cr ions. Short-range correlations of the occupational disorder result in a quite intense and structured diffuse scattering; a static nature of the disorder was unambiguously attributed by the inelastic x-ray scattering. The diffuse scattering has been modeled using a reverse Monte-Carlo algorithm assuming a disorder of the Cr sublattice only. The observed correlated disorder of the Cr sublattice reduces the temperature of the magnetic ordering from 130 to 88 K and drastically modifies the field dependence of the magnetization as it is evidenced by the SQUID magnetometery. We conclude that, in contrast to the magnetic properties assumed for a helicoidal spin structure, the compound under study (which has the same chemical composition and an average structure very close to previous reports) shows the response of a ferromagnetically ordered anisotropic material.

We have performed neutron inelastic scattering and resonant inelastic x-ray scattering (RIXS) at the Cu-$L_3$ edge to study high-energy magnetic excitations at energy transfers of more than 100 meV for overdoped ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ with $x=0.25$ $(T_c=15$ K) and $x=0.30$ (nonsuperconducting) using identical single-crystal samples for the two techniques. From constant-energy slices of neutron-scattering cross sections, we have identified magnetic excitations up to $\sim 250$ meV for $x=0.25$. Although the width in the momentum direction is large, the peak positions along the $(\pi ,\pi )$ direction agree with the dispersion relation of the spin wave in the nondoped ${\mathrm{La}}_2{\mathrm{CuO}}_4$ (LCO), which is consistent with the previous RIXS results of cuprate superconductors. Using RIXS at the Cu-$L_3$ edge, we have measured the dispersion relations of the so-called paramagnon mode along both $(\pi ,\pi )$ and $(\pi ,0)$ directions. Although in both directions the neutron and RIXS data connect with each other and the paramagnon along $(\pi ,0)$ agrees well with the LCO spin-wave dispersion, the paramagnon in the $(\pi ,\pi )$ direction probed by RIXS appears to be less dispersive and the excitation energy is lower than the spin wave of LCO near $(\pi /2,\pi /2)$. Thus, our results indicate consistency between neutron inelastic scattering and RIXS, and elucidate the entire magnetic excitation in the $(\pi ,\pi )$ direction by the complementary use of two probes. The polarization dependence of the RIXS profiles indicates that appreciable charge excitations exist in the same energy range of magnetic excitations, reflecting the itinerant character of the overdoped sample. A possible anisotropy in the charge excitation intensity might explain the apparent differences in the paramagnon dispersion in the $(\pi ,\pi )$ direction as detected by the x-ray scattering.

We formulate the three-dimensional $\text{SU}\left(2\right)$ Landau level problem in cubic lattices with time-reversal invariance. By taking a Landau-type $\text{SU}\left(2\right)$ gauge, the system can be reduced into one dimension, as characterized by the $\text{SU}\left(2\right)$ generalization of the usual Harper equations with a periodic spin-dependent gauge potential. The surface spectra indicate the spatial separation of helical states with opposite eigenvalues of a lattice helicity operator. The band topology is investigated from both the analysis of the boundary helical Fermi surfaces and the calculation of the ${\mathbb{Z}}_2$ index based on the bulk wave functions. The transition between a three-dimensional weak topological insulator to a strong one is studied as varying the anisotropy of hopping parameters.

Recent measurements conducted over a large range of temperature and carrier density have found that the Seebeck coefficient exhibits an approaching disorder-free transport feature in high-mobility conjugated polymers [D. Venkateshvaran et al., Nature 515, 384 (2014)]. It is difficult for the current Seebeck coefficient model to interpret the feature of the charge transport approaching disorder-free transport. We present a general analytical model to describe the Seebeck effect for organic semiconductors based on the hopping transport and percolation theory. The proposed model can well explain the Seebeck feature of the polymers with approaching disorder-free transport, as well as that of the organic semiconductors with the general disorder. The simulated results imply that the Seebeck coefficient in the organic semiconductors would happen to transfer from temperature dependence to temperature independence with the decrease of the energetic disorder.

The recently suggested mechanism [Y. Song and H. Dery, Phys. Rev. Lett. 113, 047205 (2014)] of the three-terminal spin transport is based on the resonant tunneling of electrons between ferromagnetic and normal electrodes via an impurity. The sensitivity of current to a weak external magnetic field stems from a spin blockade, which, in turn, is enabled by strong on-site repulsion. We demonstrate that this sensitivity exists even in the absence of repulsion when a single-particle description applies. Within this description, we calculate exactly the resonant-tunneling current between the electrodes. The mechanism of magnetoresistance, completely different from the spin blocking, has its origin in the interference of virtual tunneling amplitudes. Spin imbalance in ferromagnetic electrodes is responsible for this interference and the resulting coupling of the Zeeman levels. This coupling also affects the current in the correlated regime.