Pressure-induced stacking sequence variations of gold are predicted from first-principles calculations. At the static condition, the ABC stacking in the face-centered-cubic (fcc) phase changes into ABCACB above at least 200 GPa, and then into ABAC corresponding to the double hexagonal-close-packed structure. By further compression, the hexagonal-close-packed (hcp) structure with the AB stacking emerges as the most stable structure of all the stacking structures. This sequence of the transitions shows a step-by-step increase of the hexagonality over a wide pressure range. The calculations of phonon free energy indicate that the stable region of the intermediate stacking structures between fcc and hcp such as ABCACB or ABAC gets to be narrow with increase of temperature and fcc directly transforms into hcp above 400 K. This direct transition from fcc to hcp is qualitatively consistent with the earlier experimental observations.

In bilayer systems consisting of an ultrathin ferromagnetic layer adjacent to a metal with strong spin-orbit coupling, an applied in-plane current induces torques on the magnetization. The torques that arise from spin-orbit coupling are of particular interest. Here we use first-principles methods to calculate the current-induced torque in a Pt-Co bilayer to help determine the underlying mechanism. We focus exclusively on the analog to the Rashba torque, and do not consider the spin Hall effect. The details of the torque depend strongly on the layer thicknesses and the interface structure, providing an explanation for the wide variation in results found by different groups. The torque depends on the magnetization direction in a way similar to that found for a simple Rashba model. Artificially turning off the exchange spin splitting and separately the spin-orbit coupling potential in the Pt shows that the primary source of the “fieldlike” torque is a proximate spin-orbit effect on the Co layer induced by the strong spin-orbit coupling in the Pt.

Using the coupled cluster method (CCM) we study the full (zero-temperature) ground-state (GS) phase diagram of a spin-half ($s=\frac{1}{2}$) $J_1$-$J_2$ Heisenberg model on a cross-striped square lattice. Each site of the square lattice has four nearest-neighbor exchange bonds of strength $J_1$ and two next-nearest-neighbor (diagonal) bonds of strength $J_2$. The $J_2$ bonds are arranged so that the basic square plaquettes in alternating columns have either both or no $J_2$ bonds included. The classical ($s\to \infty$) version of the model has four collinear phases when $J_1$ and $J_2$ can take either sign. Three phases are antiferromagnetic (AFM), showing so-called Néel, double Néel, and double columnar striped order, respectively, while the fourth is ferromagnetic. For the quantum $s=\frac{1}{2}$ model we use the three classical AFM phases as CCM reference states, on top of which the multispin-flip configurations arising from quantum fluctuations are incorporated in a systematic truncation hierarchy. Calculations of the corresponding GS energy, magnetic order parameter, and the susceptibilities of the states to various forms of valence-bond crystalline (VBC) order are thus carried out numerically to high orders of approximation and then extrapolated to the (exact) physical limit. We find that the $s=\frac{1}{2}$ model has five phases, which correspond to the four classical phases plus a new quantum phase with plaquette VBC order. The positions of the five quantum critical points are determined with high accuracy. While all four phase transitions in the classical model are first order, we find strong evidence that three of the five quantum phase transitions in the $s=\frac{1}{2}$ model are of continuous deconfined type.

Inelastic neutron scattering experiments were performed to investigate the crystalline electric field (CEF) excitations of ${\mathrm{Nd}}^{3+}$ (J = 9/2) in the iron pnictide NdFeAsO. The crystal field level structures for both the high-temperature paramagnetic phase and the low-temperature antiferromagnetic phase of NdFeAsO are constructed. The variation of CEF excitations of ${\mathrm{Nd}}^{3+}$ reflects not only the change of local symmetry but also the change of magnetic ordered state of the Fe sublattice. By analyzing the crystal field interaction with a crystal field Hamiltonian, the crystal field parameters are obtained. It was found that the sign of the fourth- and sixth-order crystal field parameters change upon the magnetic phase transition at $\sim$140 K, which may be due to the variation of exchange interactions between the 4f and conduction electrons.

Two-state systems may exhibit mechanical forces of quantum origin that have no counterpart in classical physics. We show that such forces must exist in molecular magnets due to quantum tunneling between classically degenerate magnetic states. They can be observed in the presence of a microwave field when the magnet is placed in a static magnetic field with a gradient. At low temperature, the force is proportional to the tunnel splitting and it disappears on raising the temperature.

We address quantum oscillation experiments in high-$T_c$ superconductors and the evidence from these experiments for a pseudogap versus a Fermi liquid phase at high magnetic fields. As a concrete alternative to a Fermi liquid phase, the pseudogap state we consider derives from earlier work within a Gor'kov-based Landau level approach. Here the normal state pairing gap in the presence of high fields is spatially nonuniform, incorporating small gap values. These, in addition to $d$-wave gap nodes, are responsible for the persistence of quantum oscillations. Important here are methodologies for distinguishing different scenarios. To this end we examine the temperature dependence of the oscillations. Detailed quantitative analysis of this temperature dependence demonstrates that a high-field pseudogap state in the cuprates may well “masquerade” as a Fermi liquid.

We performed a high-energy resolution angle-resolved photoemission spectroscopy investigation of overdoped Ba${}_{0.1}$K${}_{0.9}$Fe${}_2$As${}_2$ with $T_c=9$ K. The Fermi surface topology of this material is similar to that of KFe${}_2$As${}_2$ and differs from that of slightly less doped Ba${}_{0.3}$K${}_{0.7}$Fe${}_2$As${}_2$, implying that a Lifshitz transition occurred between $x=0.7$ and $x=0.9$. Albeit for a vertical node found at the tip of the emerging off-$M$-centered Fermi surface pocket lobes, the superconducting gap structure is similar to that of Ba${}_{0.3}$K${}_{0.7}$Fe${}_2$As${}_2$, suggesting that the pairing interaction is not driven by the Fermi surface topology, but nevertheless reveals the possible importance of the Fermi surface topology in shaping the superconducting gap structure.

We proved the existence of a universal flexoantiferrodistortive coupling as a necessary complement to the well-known flexoelectric coupling. The coupling is universal for all antiferrodistortive systems and can lead to the formation of incommensurate, spatially modulated phases in multiferroics. Our analysis can provide a self-consistent mesoscopic explanation for a broad range of modulated domain structures observed experimentally in multiferroics.

We study the critical behavior of Anderson localized modes near intersecting flat and dispersive bands in the quasi-one-dimensional diamond ladder with weak diagonal disorder $W$. The localization length $\xi$ of the flat band states scales with disorder as $\xi \sim W^{-\gamma }$, with $\gamma \approx 1.3$, in contrast to the dispersive bands with $\gamma =2$. A small fraction of dispersive modes mixed with the flat band states is responsible for the unusual scaling. Anderson localization is therefore controlled by two different length scales. Nonlinearity can produce qualitatively different wave spreading regimes, from enhanced expansion to resonant tunneling and self-trapping.

The one-dimensional Ising model in a random field and with power-law decaying ferromagnetic bonds is studied at zero temperature. Comparing the scaling of the energy contributions of the ferromagnetic domain wall flip and of the random field à la Imry-Ma, a threshold value for the power $\rho$ of the long-range interaction can be determined, beyond which no critical behavior occurs. The critical threshold value is ${\rho }_c=3/2$, at a difference with the zero field model in which ${\rho }_c=2$. This prediction is analyzed by numerical computation of the ground states below, at, and above this threshold value. The analogy between the critical behavior of long-range one-dimensional systems and in short-range $D$-dimensional systems is investigated. At the critical threshold value of $\rho$, corresponding to the lower critical dimension, numerical evidence is found for a zero temperature transition at a finite critical field. Possible finite size crossover effects are discussed in this case. Some implications for the critical behavior of spin glasses in a field are conjectured.

The London penetration depth is evaluated for isotropic materials for any transport and pair-breaking Born scattering rates. Besides known results, a number of features are found. The slope $|d\rho /d\theta |$ of the normalized superfluid density $\rho ={\lambda }^2\left(0\right)/{\lambda }^2\left(\theta \right)$ at the transition $\theta =T/T_c=1$ has a minimum near the value of the pair-breaking parameter separating gapped and gapless states. The low-$T$ exponentially flat part of $\rho$ for the $s$-wave materials is suppressed by increasing pair breaking. For strong $T_c$ suppression by magnetic impurities the “Homes scaling” ${\lambda }^{-2}\left(0\right)\propto \sigma T_c$ with $\sigma$ being the normal conductivity gives way to ${\lambda }^{-2}\left(0\right)\propto \sigma T_c^2$. For the $d$-wave order parameter, the transport and spin-flip Born scattering rates enter the theory only as a sum; in particular, they affect the $T_c$ depression in the same manner. We confirm that the linear low-temperature behavior of $\rho$ in a broad range of the combined scattering parameter turns to the $T^2$ behavior only when the critical temperature is suppressed at least by a factor of 3 relative to the clean limit $T_{c0}$. Moreover, in this range, $\rho \left(\theta \right)$ is only weakly dependent on the scattering parameter, i.e., it is nearly universal.

Unlike time-reversal topological insulators, surface metallic states with Dirac cone dispersion in the recently discovered topological crystalline insulators (TCIs) are protected by crystal symmetry. To date, TCI behaviors have been observed in SnTe and the related alloys Pb${}_{1-x}$Sn${}_x$Se/Te, which incorporate heavy elements with large spin-orbit coupling (SOC). Here, by combining first-principles and ab initio tight-binding calculations, we report the formation of a TCI in relatively lighter rocksalt SnS and SnSe. This TCI is characterized by an even number of Dirac cones at the high-symmetry (001), (110), and (111) surfaces, which are protected by the reflection symmetry with respect to the ($\overline{1}$10) mirror plane. We find that both SnS and SnSe have an intrinsically inverted band structure even without the SOC and the SOC is necessary only to open the bulk band gap. The bulk band gap evolution upon volume expansion reveals a topological transition from an ambient pressure TCI to a topologically trivial insulator. Our results indicate that the SOC alone is not sufficient to drive the topological transition.

We present a unified Boltzmann transport theory for the drag resistivity ${\rho }_{\mathrm{D}}$ in two-component systems close to a second-order phase transition. We find general expressions for ${\rho }_{\mathrm{D}}$ in two and three spatial dimensions, for arbitrary population and mass imbalance, for particle- and holelike bands, and show how to incorporate, at the Gaussian level, the effect of fluctuations close to a phase transition. We find that the proximity to the phase transition enhances the drag resistivity upon approaching the critical temperature from above, and we qualitatively derive the temperature dependence of this enhancement for various cases. In addition, we present numerical results for two concrete experimental systems: (i) three-dimensional cold atomic Fermi gases close to a Stoner transition and (ii) two-dimensional spatially separated electron and hole systems in semiconductor double quantum wells.

A novel type of spaser with the net amplification of surface plasmons (SPs) in a doped graphene nanoribbon is proposed. The plasmons in the THz region can be generated in a doped graphene nanoribbon due to nonradiative excitation by emitters like two level quantum dots located along a graphene nanoribbon. The minimal population inversion per unit area, needed for the net amplification of SPs in a doped graphene nanoribbon, is obtained. The dependence of the minimal population inversion on the surface plasmon wave vector, graphene nanoribbon width, doping, and damping parameters necessary for the amplification of surface plasmons in the armchair graphene nanoribbon is studied.

A major challenge for applications of graphene is the creation of a tunable electronic band gap. Hexagonal boron nitride has a lattice very similar to that of graphene and a much larger band gap, but B-N and C do not alloy: B-C-N materials tend to phase separate into $h$-BN and C domains. Quantum confinement within the finite-sized C domains of a mixed B-C-N system can create a band gap, albeit within an inhomogeneous system. Here we investigate the properties of hybrid $h$-BN/C sheets with real-space pseudopotential density functional theory. We find that the electronic properties are determined not just by geometrical confinement, but also by the bonding character at the $h$-BN/C interface. B-C terminated carbon regions tend to have larger gaps than N-C terminated regions, suggesting that boron-carbon bonds are more stable. We examine two series of symmetric structures that represent different kinds of confinement: a graphene dot within a $h$-BN background and a $h$-BN antidot within a graphene background. The gaps in both cases vary inversely with the size of the graphenic region, as expected, and can be fit by simple empirical expressions.

We report on the scanning tunneling spectroscopy experiments on single crystals of IrTe${}_2$. A structural supermodulation and a local density-of-states (LDOS) modulation with a wave vector of $q=1/5\times 2\pi /a_0$ ($a_0$ is the lattice constant in the $ab$ plane) have been observed at 4.2 K, where the sample is in the monoclinic phase. As synchronized with the supermodulation, the LDOS spatially modulates within two energy ranges (below $-$200 meV and around $-$100 meV). We further investigated the effect of the local perturbations including the antiphase boundaries and the twin boundaries on the LDOS. These perturbations also modify the LDOS below $-$200 meV and around $-$100 meV, even though the lattice distortions induced by these perturbations appear to be different from those by the supermodulation. Our results indicating several microscopic structural effects on the LDOS seem to offer crucial keys for the establishment of the microscopic model describing the parent state.

We report on the modulation of two-dimensional (2D) bands of Cu(111) surface-state electrons by three isostructural supramolecular honeycomb architectures with different periodicity or constituent molecules. Using Fourier-transformed scanning tunneling spectroscopy and model calculations, we resolved the 2D band structures and found that the intrinsic surface-state band is split into discrete bands. The band characteristics including band gap, band bottom, and bandwidth are controlled by the network unit cell size and the nature of the molecule-surface interaction. In particular, Dirac cones emerge where the second and third bands meet at the $K$ points of the Brillouin zone of the supramolecular lattice.

We have performed a search for stable compounds in the hafnium-carbon (Hf-C) system at ambient pressure using a variable-composition ab initio evolutionary algorithm implemented in the uspex code. In addition to the well-known HfC, we predicted two additional thermodynamically stable compounds ${\mathrm{Hf}}_3{\mathrm{C}}_2$ and ${\mathrm{Hf}}_6{\mathrm{C}}_5$. The structure of ${\mathrm{Hf}}_6{\mathrm{C}}_5$ with space group $C2/m$ contains 22 atoms in the conventional cell, and this prediction revives the earlier proposal by Gusev and Rempel [Phys. Status Solidi A 135, 15 (1993)]. The stable structure of ${\mathrm{Hf}}_3{\mathrm{C}}_2$ also has space group $C2/m$ and is more energetically favorable than the $Immm,P\overline{3}m1,P2$, and $C{222}_1$ structures put forward by Gusev and Rempel [Phys. Status Solidi A 135, 15 (1993)]. The dynamical and mechanical stabilities of the newly predicted structures have been verified by calculations of their phonons and elastic constants. Structural vacancies are found in the ordered defective rock-salt-type HfC. Chemical bonding, band structure, and Bader charges are presented and are discussed. All three compounds are weak metals with increasing metallicity as the vacancy concentration increases. The mechanical properties of the hafnium carbides nonlinearly decrease with increasing vacancy concentration, indicating the defect tolerance of this refractory compound. It is, therefore, possible to tune the hardness, ductility, and electrical conductivity by varying the stoichiometry of the hafnium carbides.