We consider the spin-$1/2$ Hamiltonians proposed by Cano and Fendley [Phys. Rev. Lett. 105, 067205 (2010)], which were built to promote the well-known Rokshar-Kivelson (RK) point of quantum dimer models to spin-$1/2$ wave functions. We first show that these models, besides the exact degeneracy of RK point, support gapless spinless excitations as well as a spin gap in the thermodynamic limit, signatures of an unusual spin liquid. We then extend the original construction to create a continuous family of SU(2) invariant spin models that reproduces the phase diagram of the quantum dimer model and, in particular, show explicit evidences for existence of columnar and staggered phases. The original models thus appear as multicritical points in an extended phase diagram. Our results are based on the use of a combination of numerical exact simulations and analytical mapping to effective generalized quantum dimer models.

We study a proximity junction between a conventional $s$-wave superconductor and a conductor with Rashba spin-orbit coupling, with a specific focus on the spin structure of the induced pairing amplitude. We find that spin-triplet pairing correlations are induced by spin-orbit coupling in both one- and two-dimensional systems due to the lifted spin degeneracy. Additionally, this induced triplet pairing has a component with an odd frequency dependence that is robust to disorder. Our predictions are based on the solutions of the exact Gor'kov equations and are beyond the quasiclassical approximation.

The performance of niobium-based superconducting radio frequency (SRF) particle-accelerator cavities can be sensitive to localized defects that give rise to quenches at high accelerating gradients. In order to identify these material defects on bulk Nb surfaces at their operating frequency and temperature, a wide-bandwidth microwave microscope with localized and strong RF magnetic fields is developed by integrating a magnetic write head into the near-field microwave microscope to enable mapping of the local electrodynamic response in the multi-GHz frequency regime at cryogenic temperatures. This magnetic writer demonstrates a localized and strong RF magnetic field on bulk Nb surface with $B_{\mathrm{surface}}>{10}^2$ mT and submicron resolution. By measuring the nonlinear response of the superconductor, nonlinearity coming from the nanoscale weak-link Josephson junctions due to the contaminated surface in the cavity-fabrication process is demonstrated.

We theoretically studied spin-transfer-torque switching of a spin-valve nanopillar with a conically magnetized free layer in a thermally activated regime. Starting from the Fokker-Planck equation, analytical expressions of the switching probability $P_{\mathrm{sw}}$ are derived. Under zero external magnetic field, the thermal stability factor is well approximated as ${\mathrm{\Delta }}_0{(1-J/J_{\mathrm{sw}})}^{1.53}$, where $J/J_{\mathrm{sw}}$ represents the applied current density normalized by the switching current density and ${\mathrm{\Delta }}_0$ represents the thermal stability factor in the absence of current. The results enable us to evaluate ${\mathrm{\Delta }}_0$ of conically magnetized nanomagnet from the observed current dependence of $P_{\mathrm{sw}}$ at ambient temperature.

We find a mixed chirality $d$-wave superconducting state in the coexistence region between antiferromagnetism and interaction-driven superconductivity in lightly doped honeycomb materials. This state has a topological chiral $d+id$-wave symmetry in one Dirac valley but $d-id$-wave symmetry in the other valley and hosts two counterpropagating edge states, always protected in the absence of intervalley scattering. A first-order topological phase transition, with no bulk gap closing, separates the chiral $d$-wave state at small magnetic moments from the mixed chirality $d$-wave phase.

The hybrid halide perovskite ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$ exhibits a complex structural behavior, with successive transitions between orthorhombic, tetragonal, and cubic polymorphs around 165 and 327 K. Herein we report first-principles lattice dynamics (phonon spectrum) for each phase of ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$. The equilibrium structures compare well to solutions of temperature-dependent powder neutron diffraction. By following the normal modes, we calculate infrared and Raman intensities of the vibrations, and compare them to the measurement of a single crystal where the Raman laser is controlled to avoid degradation of the sample. Despite a clear separation in energy between low-frequency modes associated with the inorganic (${\mathrm{PbI}}_3{}^{-}{)}_n$ network and high-frequency modes of the organic ${\mathrm{CH}}_3{\mathrm{NH}}_3{}^{+}$ cation, significant coupling between them is found, which emphasizes the interplay between molecular orientation and the corner-sharing octahedral networks in the structural transformations. Soft modes are found at the boundary of the Brillouin zone of the cubic phase, consistent with displacive instabilities and anharmonicity involving tilting of the ${\mathrm{PbI}}_6$ octahedra around room temperature.

Time-dependent density functional theory and Ehrenfest dynamics are used to calculate the electronic excitations produced by a moving Ni ion in a Ni crystal in the case of energetic MeV range (electronic stopping power regime), as well as thermal energy meV range (electron-phonon interaction regime). Results at high energy compare well to experimental databases of stopping power, and at low energy the electron-phonon interaction strength determined in this way is very similar to the linear response calculation and experimental measurements. This approach to electron-phonon interaction as an electronic stopping process provides the basis for a unified framework to perform classical molecular dynamics of ion-solid interaction with ab initio type nonadiabatic terms in a wide range of energies.

The magnetic properties of a ${\mathrm{CaCo}}_2{\mathrm{As}}_2$ single crystal are systematically studied by using dc magnetization and magnetic torque measurements. A paramagnetic to antiferromagnetic transition occurs at $T_N=74$ K, with Co spins being aligned parallel to the $c$ axis. For $H\parallel c$, a field-induced spin-flop transition was observed below $T_N$ and a magnetic transition from antiferromagnetic to paramagnetic was inferred from the detailed analysis of magnetization and magnetic torque. Finally, we summarize the magnetic phase diagram of ${\mathrm{CaCo}}_2{\mathrm{As}}_2$ based on our results in the H-T plane.

We calculate the optical conductivity by using several models for unparticle or scale-invariant matter. Within a Gaussian action for unparticles that is gauged with Wilson lines, we find that the conductivity computed from the Kubo formalism with vertex corrections yields no nontrivial deviation from the free-theory result. This result obtains because, at the Gaussian level, unparticles are just a superposition of particle fields and hence any transport property must be consistent with free theory. Beyond the Gaussian approach, we adopt the continuous-mass formulation of unparticles and calculate the Drude conductivity directly. We show that unparticles in this context can be tailored to yield an algebraic conductivity that scales as ${\omega }^{-2/3}$ with the associated phase angle between the imaginary and real parts of $arctan\frac{{\sigma }_2}{{\sigma }_1}={60}^{\circ }$, as is seen in the cuprates. Given the recent results [J. High Energy Phys. 4, 40 (2014); J. High Energy Phys. 7, 24 (2015); arXiv:1506.06769] that gravitational crystals lack a power-law optical conductivity, this constitutes the first consistent account of the ${\omega }^{-2/3}$ conductivity and the phase angle seen in optimally doped cuprates. Our results indicate that, at each frequency in the scaling regime, excitations on all energy scales contribute. Hence, incoherence is at the heart of the power law in the optical conductivity in strongly correlated systems such as the cuprates.

Spin susceptibility of Anderson impurities is a key quantity in understanding the physics of Kondo screening. Traditional numerical renormalization group (NRG) calculation of the impurity contribution ${\chi }_{\mathrm{imp}}$ to susceptibility, defined originally by Wilson in a flat wide band, has been generalized before to structured conduction bands. The results brought about non-Fermi-liquid and diamagnetic Kondo behaviors in ${\chi }_{\mathrm{imp}}$, even when the bands are not gapped at the Fermi energy. Here, we use the full density-matrix (FDM) NRG to present high-quality data for the local susceptibility ${\chi }_{\mathrm{loc}}$ and to compare them with ${\chi }_{\mathrm{imp}}$ obtained by the traditional NRG. Our results indicate that those exotic behaviors observed in ${\chi }_{\mathrm{imp}}$ are unphysical. Instead, the low-energy excitations of the impurity in arbitrary bands only without gap at the Fermi energy are still a Fermi liquid and paramagnetic. We also demonstrate that unlike the traditional NRG yielding ${\chi }_{\mathrm{loc}}$ less accurate than ${\chi }_{\mathrm{imp}}$, the FDM method allows a high-precision dynamical calculation of ${\chi }_{\mathrm{loc}}$ at much reduced computational cost, with an accuracy at least one order higher than ${\chi }_{\mathrm{imp}}$. Moreover, artifacts in the FDM algorithm to ${\chi }_{\mathrm{imp}}$ and origins of the spurious non-Fermi-liquid and diamagnetic features are clarified. Our work provides an efficient high-precision algorithm to calculate the spin susceptibility of impurity for arbitrary structured bands, while negating the applicability of Wilson's definition to such cases.

The edge channels of two-dimensional topological systems are protected from elastic reflection and are noiseless at low temperature. Yet, noise and cross correlations can be induced when electron waves partly transmit to the opposite edge via tunneling through a constriction. In particular, in a quantum spin Hall (QSH) system tunneling occurs via both spin-preserving $\left(p\right)$ and spin-flipping $\left(f\right)$ processes, each fulfilling time-reversal symmetry. We investigate the current correlations of a four-terminal QSH setup in the presence of a tunneling region, both at equilibrium and out of equilibrium. We find that, although $p$ and $f$ processes do not commute and the generic current correlation depends on both, under appropriate conditions a direct detection of two types of partition noise is possible. In particular, while the spin-preserving partitioning can be probed for any arbitrary tunnel junction with a specific configuration of terminal biases, the spin-flipping partitioning can be directly detected only under suitably designed setups and conditions. We describe two setups where these conditions can be fulfilled, and both types of partitioning can be detected and controlled.

The effects of a collection of classical two-level charge fluctuators on the coherence of a dynamically decoupled qubit are studied. Distinct dynamics is found at different qubit working positions. Exact analytical formulas are derived at pure dephasing and approximate solutions are found at the general working position, for weakly and strongly coupled fluctuators. Analysis of these solutions, combined with numerical simulations of the multiple random telegraph processes, reveal the scaling of the noise with the number of fluctuators and the number of control pulses, as well as dependence on other parameters of the qubit-fluctuators system. These results can be used to determine potential microscopic models for the charge environment by performing noise spectroscopy.

Simulation of warm dense matter requires computational methods that capture both quantum and classical behavior efficiently under high-temperature, high-density conditions. Currently, density functional theory molecular dynamics is used to model electrons and ions, but this method's computational cost skyrockets as temperatures and densities increase. We propose finite-temperature potential functional theory as an in-principle-exact alternative that suffers no such drawback. We derive an orbital-free free energy approximation through a coupling-constant formalism. Our density approximation and its associated free energy approximation demonstrate the method's accuracy and efficiency.

We report a system in which the rotational, vibrational, electronic, and structural properties of condensed molecular ${\mathrm{H}}_2$ can be measured with subnanometer precision using scanning tunneling microscopy. ${\mathrm{H}}_2$ physisorbs around $\mathrm{Ni}$ nanoparticles on $\mathrm{Au}\left(111\right)$ and displays many nonclassical characteristics, including unique disappearance upon heating that is due to changes in the time-averaged phonon ground state population. This collective phenomenon also gives rise to the appearance of submolecular features and constructive overlap at points where neighboring ${\mathrm{H}}_2$ ensembles meet. A model based on the spatial distribution of collective excitations is proposed to explain these properties.

From scanning tunneling microscopy of the misfit-layered cobaltate $\left[{\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{O}}_4\right]{}_{0.51}{\mathrm{CoO}}_2$ single crystals, we observed structural short corrugations oriented in the direction inclined with respect to the incommensurate axis. Results show that the spatial distribution of the electronic charge density is also modified in relation to the atomic displacement. Our data strongly suggest that the rocksalt structure flexibly mounted on the ${\mathrm{CoO}}_2$ layer affects the electronic properties of this system.

Robustness of edge states and non-Abelian excitations of topological states of matter promises quantum memory and quantum processing, which are naturally immune to microscopic imperfections such as static disorder. However, topological properties will not in general protect quantum systems from time-dependent disorder or noise. Here we take the example of a network of Kitaev wires with Majorana edge modes storing qubits to investigate the effects of classical noise in the crossover from the quasistatic to the fast fluctuation regime. We present detailed results for the Majorana edge correlations, and fidelity of braiding operations for both global and local noise sources preserving parity symmetry, such as random chemical potentials and phase fluctuations. While in general noise will induce heating and dephasing, we identify examples of long-lived quantum correlations in the presence of fast noise due to motional narrowing, where external noise drives the system rapidly between the topological and nontopological phases.

A thorough investigation of the effect of exchange and correlation on the electronic structure of wide-band-gap insulators ${\mathrm{ScF}}_3,{\mathrm{YF}}_3$, and ${\mathrm{AlF}}_3$ is carried out using local, semilocal, and hybrid functionals in the density functional theory framework and the $GW$ approximation with four current plasmon-pole models. It is shown that the hybrid functionals, which attribute more weight to electron exchange, lead to a decent agreement with the state of the art $GW$ results, whereas the Tran-Blaha semilocal functional does not improve significantly the local density approximation results of the insulating transition-metal trifluorides, because of the high localization of the conduction band minimum states which are mainly of $d$ character, and underestimate considerably the valence-band width of the studied materials.

The wave function of Dirac fermions is a two-component spinor. In graphene, a one-atom-thick film showing two-dimensional Dirac-like electronic excitations, the two-component representation, reflects the amplitude of the electron wave function on the $A$ and $B$ sublattices. This unique property provides unprecedented opportunities to image the two components of Dirac fermions spatially. Here, we report atomic resolution imaging of two-component Dirac-Landau levels in gapped graphene monolayers by scanning tunneling microscopy and spectroscopy. A gap of about 20 meV, driven by inversion symmetry breaking by the substrate potential, is observed in the graphene sheets on both SiC and graphite substrates. Such a gap splits the $n=0$ Landau level (LL) into two levels, $0_{+}$ and $0_{-}$. We demonstrate that the amplitude of the wave function of the $0_{+}$ LL is mainly on the $A$ sites and that of the $0_{-}$ LL is mainly on the $B$ sites of graphene, characterizing the internal structure of the spinor of the $n=0$ LL. This provides direct evidence of the two-component nature of Dirac fermions.

We studied the square-octagonal lattice of the transition metal dichalcogenide $M{X}_2$ (with $M=\mathrm{Mo}$, W; $X=\mathrm{S}$, Se, and Te), as an isomer of the normal hexagonal compound of $M{X}_2$. By band-structure calculations, we observe the graphene-like Dirac band structure in a rectangular lattice of $M{X}_2$ with nonsymmorphic space group symmetry. Two bands with van Hove singularity points cross each at the Fermi energy, leading to two Dirac cones that locate at opposite momenta. Spin-orbit coupling can open a gap at these Dirac points, inside which gapless topological edge states exists as the quantum spin Hall (QSH) effect, the 2D topological insulator.