We construct and analyze a microscopic model for insulating rocksalt ordered double perovskites, with the chemical formula $A_{2}B{B}^{\prime }{\text{O}}_6$, where the $B^{\prime }$ atom has a $4d^1$ or $5d^1$ electronic configuration and forms a face-centered-cubic lattice. The combination of the triply degenerate $t_{2g}$ orbital and strong spin-orbit coupling forms local quadruplets with an effective spin moment $j=3/2$. Moreover, due to strongly orbital-dependent exchange, the effective spins have substantial biquadratic and bicubic interactions (fourth and sixth order in the spins, respectively). This leads, at the mean-field level, to three main phases: an unusual antiferromagnet with dominant octupolar order, a ferromagnetic phase with magnetization along the [110] direction, and a nonmagnetic but quadrupolar ordered phase, which is stabilized by thermal fluctuations and intermediate temperatures. All these phases have a two-sublattice structure described by the ordering wave vector $\mathit{Q}=2\pi \left(001\right)$. We consider quantum fluctuations and argue that in the regime of dominant antiferromagnetic exchange, a nonmagnetic valence-bond solid or quantum-spin-liquid state may be favored instead. Candidate quantum-spin-liquid states and their basic properties are described. We also address the effect of single-site anisotropy driven by lattice distortions. Existing and possible future experiments are discussed in light of these results.

Recent proposals using heterostructures of superconducting and either topologically insulating or semiconducting layers have been put forth as possible platforms for topological quantum computation. These systems are predicted to contain Ising anyons and share the feature of having only neutral edge excitations. In this note, we show that these proposals can be combined with the recently proposed “sack geometry” for implementation of a phase gate in order to conduct robust universal quantum computation. In addition, we propose a general method for adjusting edge tunneling rates in such systems, which is necessary for the control of interferometric devices. The error rate for the phase gate in neutral Ising systems is parametrically smaller than for a similar geometry in which the edge modes carry charge: it goes as $T^3$ rather than $T$ at low temperatures. At zero temperature, the phase variance becomes constant at long times rather than carrying a logarithmic divergence.

We report the superconductivity at above 30 K in a FeSe-layer compound ${\text{K}}_{0.8}{\text{Fe}}_2{\text{Se}}_2$ (nominal composition) achieved by metal K intercalating in between FeSe layers. It is isostructural to ${\text{BaFe}}_2{\text{As}}_2$ and possesses the highest $T_c$ for FeSe-layer materials so far under ambient pressure. Hall effect indicates the carriers are dominated by electron in this superconductor. We confirm that the observed superconductivity at above 30 K is due to this FeSe-based 122 phase. Our results demonstrate that FeSe-layer materials are really remarkable superconductors via structure and carrier modulation.

We develop an analytical theory for generic disorder-driven quantum phase transitions. We apply this formalism to the superconductor-insulator transition and we briefly discuss the applications to the order-disorder transition in quantum magnets. The effective spin-$\frac{1}{2}$ models for these transitions are solved in the cavity approximation which becomes exact on a Bethe lattice with large branching number $K⪢1$ and weak dimensionless coupling $g⪡1$. The characteristic feature of the low-temperature phase is a large self-formed inhomogeneity of the order-parameter distribution near the critical point $K\ge K_c\left(g\right)$, where the critical temperature $T_c$ of the ordering transition vanishes. We find that the local probability distribution $P\left(B\right)$ of the order parameter $B$ has a long power-law tail in the region where $B$ is much larger than its typical value $B_0$. Near the quantum-critical point, at $K\to K_c\left(g\right)$, the typical value of the order parameter vanishes exponentially, $B_0\propto e^{-C/\left[K-K_c\left(g\right)\right]}$ while the spatial scale $N_{inh}$ of the order parameter inhomogeneities diverges as ${\left[K-K_c\left(g\right)\right]}^{-2}$. In the disordered regime, realized at $K<K_c\left(g\right)$ we find actually two distinct phases characterized by different behavior of relaxation rates. The first phase exists in an intermediate range of $K^{\ast }\left(g\right)<K<K_c\left(g\right)$. It has two regimes of energies: at low excitation energies, $\omega <{\omega }_d\left(K,g\right)$, the many-body spectrum of the model is discrete, with zero-level widths, while at $\omega >{\omega }_d$ the level acquire a nonzero width which is self-generated by the many-body interactions. In this phase the spin model provides by itself an intrinsic thermal bath. Another phase is obtained at smaller $K<K^{\ast }\left(g\right)$, where all the eigenstates are discrete, corresponding to full many-body localization. These results provide an explanation for the activated behavior of the resistivity in amorphous materials on the insulating side near the superconductor-insulator transition and a semiquantitative description of the scanning tunneling data on its superconductive side.

We discuss an efficient approach to excited electronic states within ab initio many-body perturbation theory (MBPT). Quasiparticle corrections to density-functional theory result from the difference between metallic and nonmetallic dielectric screening. They are evaluated as a small perturbation to the density-functional theory (within the local-density approximation) band structure rather than fully calculating the self-energy and evaluating its difference from the exchange-correlation potential. The dielectric screening is described by a model, which applies to bulk crystals, as well as, to systems of reduced dimension, like molecules, surfaces, interfaces, and more. The approach also describes electron-hole interaction. The resulting electronic and optical spectra are slightly less accurate but much faster to calculate than a full MBPT calculation. We discuss results for bulk silicon and argon, for the $\text{Si}\left(111\right)\text{−}\left(2\times 1\right)$ surface, the ${\text{SiH}}_4$ molecule, an argon-aluminum interface, and liquid argon.

It is shown that the bond-decorated Ising model is a realistic model for certain real magnetic compounds containing lanthanide ions. The lanthanide ion plays the role of Ising spin. The required conditions on the crystal-field spectrum of the lanthanide ion for the model to be valid are discussed and found to be in agreement with several recent ab initio calculations on ${\text{Dy}}^{3+}$ centers. Similarities and differences between the spectra of the simple Ising chain and the decorated Ising chain are discussed and illustrated, with attention to level crossings in a magnetic field. The magnetic properties of two actual examples (a ${\left[\text{DyCuMoCu}\right]}_{\infty }$ chain and a ${\text{Dy}}_4{\text{Cr}}_4$ ring) are obtained by a transfer-matrix solution of the decorated Ising model. $g$-factors of the metal ions are directly imported from ab initio results, while exchange coupling constants are fitted to experiment. Agreement with experiment is found to be satisfactory, provided one includes a correction (from ab initio results) for susceptibility and magnetization to account for the presence of excited Kramers doublets on ${\text{Dy}}^{3+}$.

We report on strong coupling of the charge-carrier plasmon ${\omega }_{PL}$ in graphene with the surface-optical phonon ${\omega }_{SO}$ of the underlying SiC(0001) substrate with low-electron concentration $\left(n=1.2\times {10}^{15}{\text{cm}}^{-3}\right)$ in the long-wavelength limit $\left(q_{\parallel }\to 0\right)$. Energy-dependent energy-loss spectra give clear evidence of two coupled phonon-plasmon modes ${\omega }_{\pm }$ separated by a gap between ${\omega }_{SO}\left(q_{\parallel }\to 0\right)$ and ${\omega }_{\text{TO}}\left(q_{\parallel }⪢0\right)$, the transverse-optical-phonon mode, in particular, for higher primary electron energies $\left(E_0\ge 20\text{eV}\right)$. A simplified model based on dielectric theory is able to simulate our energy-loss spectra as well as the dispersion of the two coupled phonon-plasmon modes ${\omega }_{\pm }$. In contrast, Liu and Willis [Phys. Rev. B 81, 081406(R) (2010)] postulate in their recent publication no gap and a discontinuous dispersion curve with a one-peak structure from their energy-loss data.

The observation of the quantum critical point in a series double quantum dot system depends on the distinct separation of two scales, $T_K⪢T^{\ast }$, where $T_K$ is the Kondo temperature and $T^{\ast }$ is the scale at which the system renormalizes away from the quantum critical point to a stable Fermi-liquid fixed point. Using the two-impurity Kondo model, we provide a derivation of $T^{\ast }$ based on the renormalization group (RG) to lowest order. This result is confirmed by a numerical RG (NRG) analysis which supplements the analytic derivation with additional quantitative precision. The form of the low-energy Fermi-liquid fixed point is derived and subsequently confirmed by the NRG. From this analysis, we conclude that the aforementioned separation of scales is satisfied, allowing the possibility that the quantum critical point may be measured in a future experiment on such double quantum dot systems.

At finite temperature and in presence of disorder, a one-dimensional elastic interface displays different scaling regimes at small and large lengthscales. Using a replica approach and a Gaussian variational method (GVM), we explore the consequences of a finite interface width $\xi$ on the small-lengthscale fluctuations. We compute analytically the static roughness $B\left(r\right)$ of the interface as a function of the distance $r$ between two points on the interface. We focus on the case of short-range elasticity and random-bond disorder. We show that for a finite width $\xi$ two temperature regimes exist. At low temperature, the expected thermal and random-manifold regimes, respectively, for small and large scales, connect via an intermediate “modified” Larkin regime, that we determine. This regime ends at a temperature-independent characteristic “Larkin” length. Above a certain characteristic temperature that we identify, this intermediate regime disappears. The thermal and random-manifold regimes connect at a single crossover lengthscale, that we compute. This is also the expected behavior for zero width. Using a directed polymer description, we also study via a second GVM procedure and generic scaling arguments, a modified toy model that provides further insights on this crossover. We discuss the relevance of the two GVM procedures for the roughness at large lengthscale in those regimes. In particular, we analyze the scaling of the temperature-dependent prefactor in the roughness $B\left(r\right)\sim T^{2þ}{r}^{2\zeta }$ and its corresponding thorn exponent $þ$. We briefly discuss the consequences of those results for the quasistatic creep law of a driven interface, in connection with previous experimental and numerical studies.

We report measurements of the magnetic excitation spectrum of the layered antiferromagnet ${\text{La}}_2{\text{CoO}}_4$ by time-of-flight neutron inelastic scattering. In the energy range probed in our experiments (0–250 meV) the magnetic spectrum consists of spin-wave modes with strong in-plane dispersion extending up to 60 meV, and a nearly dispersionless peak at 190 meV. The spin-wave modes exhibit a small $\left(\sim 1\text{meV}\right)$ dispersion along the magnetic zone boundary. We show that the magnetic spectrum can be described very well by a model of a Heisenberg antiferromagnet that includes the full spin and orbital degrees of freedom of ${\text{Co}}^{2+}$ in an axially distorted crystal field. The collective magnetic dynamics are found to be controlled by dominant nearest-neighbor exchange interactions, strong XY-like single-ion anisotropy and a substantial unquenched orbital angular momentum.

A logical foundation of equilibrium state density functional theory in a Kohn-Sham-type formulation is presented on the basis of Mermin’s treatment of the grand canonical state by exploiting functional Legendre transforms. It is simpler and more satisfactory compared to the usual derivation of the ground-state theory and free of most remaining open points of the latter. The existence of the functional derivative of the corresponding density functional $F\left[n\right]$ at all densities of grand canonical equilibrium states is proved even in the spin-density matrix version of the theory. It may, in particular, be relevant with respect to cases of spontaneous symmetry breaking such as noncollinear magnetism and orbital order.

Self-trapping of an electron due to its interaction with bending fluctuations in a flexible crystalline membrane is considered. Due to the dependence of the electron energy on the corrugations of the membrane, the electron can create around itself an anomalously flat (or anomalously corrugated, depending on the sign of the interaction constant) region and be confined there. Using the Feynman path integral approach, the autolocalization energy and the size of the self-trapped state (flexuron) are estimated. It is shown that typically the size of the flexuron is of the order of the wavelength of fluctuations at the border between harmonic and anharmonic regimes. The flexuron states are connected with the fluctuation tail of the electron density of states, the asymptotic behavior of this tail being determined by the exponent of the renormalized bending rigidity.

We measured the electrical conductivity $\left(\sigma \right)$ of ${\text{Fe}}_{0.96}\text{O}$ (FeO hereafter) at high pressures and temperatures up to 169 GPa and 2050 K, simultaneously with the synchrotron x-ray diffraction measurements. The results showed that a B8-structured FeO (B8 FeO) synthesized above 140 GPa adopts a normal NiAs-type structure and has an electrical conductivity of about $3\times {10}^5\text{S}/\text{m}$ at 150 GPa and 300 K. The conductivity of B8 FeO decreased with increasing temperature, which is a metallic behavior, in contrast to the semiconducting behaviors of the B1 and rhombohedrally distorted B1 (rB1) phases. However, the conductivities of the B1 and rB1 phases are not much different from that of the metallic B8 phase at equivalent pressures.

We have fabricated a series of superlattices composed of ${\text{V}}_{1-x}{\text{W}}_x{\text{O}}_2$ ($x=0$ or 0.08 with $1+2x$ $d$ electron per V atom) and ${\text{TiO}}_2$ (no $d$ electron) to investigate the interface and carrier-confinement effects of the metal-insulator phenomena of ${\text{VO}}_2$. This study was also motivated by the prediction of a half-metallic state with a semi-Dirac point at the ${\text{TiO}}_2/{\text{VO}}_2$ interface [V. Pardo and W. E. Pickett, Phys. Rev. Lett. 102, 166803 (2009)]. The growth conditions of the superlattices were optimized so that we could reproduce the known electronic states for the constituent compounds in case of the single-layer films, namely, ${\text{VO}}_2$ exhibiting a paramagnetic metal to spin-singlet insulator transition at around room temperature, ${\text{V}}_{0.92}{\text{W}}_{0.08}{\text{O}}_2$ $\left(\text{W}:{\text{VO}}_2\right)$ being metallic down to the lowest temperature, and ${\text{TiO}}_2$ being a wide-gap band insulator. We found no metallic ground state in these superlattices in contradiction with the theoretical prediction. The ${\text{TiO}}_2/{\text{VO}}_2$ superlattices always show a resistive transition, corresponding to the metal-insulator transition in the ${\text{VO}}_2$ single layer, at around 300 K for the ${\text{VO}}_2$-layer thickness varying from 15 monolayers (ML) down to 3 ML. The resistive transition is accompanied with the structural change, consistent with the V-V dimerization in ${\text{VO}}_2$ along $c$ axis, suggesting the robust spin-singlet bond formation as well as the dimeric lattice distortion persistent even at the ${\text{TiO}}_2/{\text{VO}}_2$ interface. In the case of ${\text{TiO}}_2/\text{W}:{\text{VO}}_2$ superlattices, the insulating ground state revives while the metal-insulator transition temperature decreases from 230 to 95 K as the $\text{W}:{\text{VO}}_2$ thickness is increased from 10 to 40 ML. These results indicate the persistent competition between the spin-singlet bond formation and the kinetic energy of correlated electrons regulated by the $\text{W}:{\text{VO}}_2$ thickness.

We introduce a model for amorphous grain boundaries in graphene and find that stable structures can exist along the boundary that are responsible for local density of states enhancements both at zero and finite $\left(\sim 0.5\text{eV}\right)$ energies. Such zero-energy peaks, in particular, were identified in STS measurements [J. Červenka, M. I. Katsnelson, and C. F. J. Flipse, Nat. Phys. 5, 840 (2009)] but are not present in the simplest pentagon-heptagon dislocation array model [O. V. Yazyev and S. G. Louie, Phys. Rev. B 81, 195420 (2010)]. We consider the low-energy continuum theory of arrays of dislocations in graphene and show that it predicts localized zero-energy states. Since the continuum theory is based on an idealized lattice scale physics it is a priori not literally applicable. However, we identify stable dislocation cores, different from the pentagon-heptagon pairs that do carry zero-energy states. These might be responsible for the enhanced magnetism seen experimentally at graphite grain boundaries.

The Meissner effect has been studied in $\text{Ba}{\left({\text{Fe}}_{0.926}{\text{Co}}_{0.074}\right)}_2{\text{As}}_2$ and ${\text{Ba}}_{0.6}{\text{K}}_{0.4}{\text{Fe}}_2{\text{As}}_2$ single crystals and compared to well known, type-II superconductors ${\text{LuNi}}_2{\text{B}}_2\text{C}$ and ${\text{V}}_3\text{Si}$. Whereas flux penetration is mostly determined by the bulk pinning (and, perhaps, surface barrier) resulting in a large negative magnetization, the flux expulsion upon cooling in a magnetic field is very small, which could also be due to pinning and/or surface-barrier effects. However, in stark contrast with the expected behavior, the amount of the expelled flux increases almost linearly with the applied magnetic field, at least up to our maximum field of 5.5 T, which far exceeds the upper limit for the surface barrier. One interpretation of the observed behavior is that there is a field-driven suppression of magnetic pair breaking.

We report resonant ultrasound spectroscopy (RUS), dilatometry/magnetostriction, magnetotransport, magnetization, specific-heat, and ${}^{119}\text{S}\text{n}$ Mössbauer spectroscopy measurements on SnTe and ${\text{Sn}}_{0.995}{\text{Cr}}_{0.005}\text{Te}$. Hall measurements at $T=77\text{K}$ indicate that our Bridgman-grown single crystals have a $p$-type carrier concentration of $3.4\times {10}^{19}{\text{cm}}^{-3}$ and that our Cr-doped crystals have an $n$-type concentration of $5.8\times {10}^{22}{\text{cm}}^{-3}$. Although our SnTe crystals are diamagnetic over the temperature range $2\le T\le 1100\text{K}$, the Cr-doped crystals are room-temperature ferromagnets with a Curie temperature of 294 K. For each sample type, three-terminal capacitive dilatometry measurements detect a subtle $0.5\mu \text{m}$ distortion at $T_c\approx 85\text{K}$. Whereas our RUS measurements on SnTe show elastic hardening near the structural transition, pointing to co-elastic behavior, similar measurements on ${\text{Sn}}_{0.995}{\text{Cr}}_{0.005}\text{Te}$ show a pronounced softening, pointing to ferroelastic behavior. Effective Debye temperature, ${\theta }_D$, values of SnTe obtained from ${}^{119}\text{S}\text{n}$ Mössbauer studies show a hardening of phonons in the range 60–115 K $\left({\theta }_D=162\text{K}\right)$ as compared with the 100–300 K range $\left({\theta }_D=150\text{K}\right)$. In addition, a precursor softening extending over approximately 100 K anticipates this collapse at the critical temperature and quantitative analysis over three decades of its reduced modulus finds $\Delta C_{44}/C_{44}=A{\left|\left(T-T_0\right)/T_0\right|}^{-\kappa }$ with $\kappa =0.50\pm 0.02$, a value indicating a three-dimensional softening of phonon branches at a temperature $T_0\sim 75\text{K}$, considerably below $T_c$. We suggest that the differences in these two types of elastic behaviors lie in the absence of elastic domain-wall motion in the one case and their nucleation in the other.

We develop an iterative, numerically exact approach for the treatment of nonequilibrium quantum transport and dissipation problems that avoids the real-time sign problem associated with standard Monte Carlo techniques. The method requires a well-defined decorrelation time of the nonlocal influence functional for proper convergence to the exact limit. Since finite decorrelation times may arise either from temperature or from a voltage drop at zero temperature, the approach is well suited for the description of the real-time dynamics of single-molecule devices and quantum dots driven to a steady state via interaction with two or more electron leads. We numerically investigate two nontrivial models: the evolution of the nonequilibrium population of a two-level system coupled to two electronic reservoirs, and quantum transport in the nonequilibrium Anderson model. For the latter case, two distinct formulations are described. Results are compared to those obtained by other techniques.

Compounds of ${\text{Sr}}_{1-x}{\text{La}}_x{\text{Fe}}_2{\text{As}}_2$ were synthesized by solid-state reaction at 1273 K, under pressures of 2 or 3 GPa. The ${\text{Sr}}_{1-x}{\text{La}}_x{\text{Fe}}_2{\text{As}}_2$ phase was dominant up to $x=0.5$ and superconductivity was observed at $x\ge 0.2$. A maximum critical temperature $T_c$ of 22 K and a maximum shielding volume fraction of $\sim 70\mathrm{\%}$ were obtained at $x=0.4$. This is the first experimental demonstration of electron-doped superconductivity in 122-type iron pnictides, where electrons were doped through aliovalent substitution at the site of the alkaline-earth metal. The optimal $T_c$ was slightly higher than that for the Co-doped (directly electron-doped) samples (19 K) and much lower than that for the hole-doped case (37 K). The bulk superconductivity range was narrower than that for the Co-substituted case and both ranges were much narrower than that for the hole-doped case. These observations revealed that differences in the electron-doping mode (direct or indirect) did not have a prominent effect on the optimal $T_c$ or superconductivity range, compared with differences in carrier polarity.

We derive the renormalization-group equations describing all the short-range interactions in bilayer graphene allowed by symmetry, the long-range Coulomb interaction, and band-structure parameters. For certain range of parameters, we predict the first-order phase transition to the uniaxially deformed gapless state accompanied by the change in the topology of the electron spectrum.