We present a comprehensive theoretical study of the magnetic field dependence of the near-field radiative heat transfer (NFRHT) between two parallel plates. We show that when the plates are made of doped semiconductors, the near-field thermal radiation can be severely affected by the application of a static magnetic field. We find that irrespective of its direction, the presence of a magnetic field reduces the radiative heat conductance, and dramatic reductions up to 700% can be found with fields of about 6 T at room temperature. We show that this striking behavior is due to the fact that the magnetic field radically changes the nature of the NFRHT. The field not only affects the electromagnetic surface waves (both plasmons and phonon polaritons) that normally dominate the near-field radiation in doped semiconductors, but it also induces hyperbolic modes that progressively dominate the heat transfer as the field increases. In particular, we show that when the field is perpendicular to the plates, the semiconductors become ideal hyperbolic near-field emitters. More importantly, by changing the magnetic field, the system can be continuously tuned from a situation where the surface waves dominate the heat transfer to a situation where hyperbolic modes completely govern the near-field thermal radiation. We show that this high tunability can be achieved with accessible magnetic fields and very common materials like $n$-doped InSb or Si. Our study paves the way for an active control of NFRHT and it opens the possibility to study unique hyperbolic thermal emitters without the need to resort to complicated metamaterials.

We employ a real-space pseudopotential method to compute the structural energies of a prototypical metal-semiconductor interface. Specifically, we examine a Pb(111) film overlaid on a Si(111) substrate as a function of the metal thickness. For each layer of Pb, we fully relax the atomic coordinates and determine the lowest-energy structure. Owing to the lattice mismatch between the Pb and Si crystal structures, we consider a large supercell containing up to 1505 atoms for the largest system. Systems of this size remain challenging for most current computational approaches and require algorithms specifically designed for highly parallel computational platforms. We examine the structural properties of the interface with respect to the thickness of the metal overlayer, e.g., the corrugation of the profile of the Pb overlayer. The combined influence of the Si substrate and quantum confinement results in a rich profile for a transition between a thin overlayer (less than a few monolayers), where the corrugation is strong, and the bulk region (more than a half-dozen layers), where the overlaid Pb film is atomically flat. This work proves the feasibility of handling systems with such a level of complexity.

Nitrides in many ways are fascinating since they often appear as superconductors, high-energy density, and hard materials. Though there exist a large variety of nitrides, noble gas nitrides are missing in nature. Pursuit of noble gas nitrides has therefore become the subject of topical interests, but remains as a great challenge since molecular nitrogen (${\mathrm{N}}_2$, a major form of nitrogen) and noble gases are both inert systems and do not interact at normal conditions. We show through a first-principles swarm-structure search that high pressure enables a direct interaction of ${\mathrm{N}}_2$ and xenon (Xe) above 146 GPa. The resultant Xe nitride has a peculiar stoichiometry of ${\mathrm{XeN}}_6$, possessing a high-energy density of approximately $2.4{\mathrm{kJ}\mathrm{g}}^{-1}$, rivaling that of the modern explosives. Structurally, ${\mathrm{XeN}}_6$ is intriguing with the appearance of chaired ${\mathrm{N}}_6$ hexagons and unusually high 12-coordination of Xe bonded with N. Our work opens up the possibility of achieving Xe nitride with superior high-energy density whose formation is long sought as impossible.

Highly accurate numerical simulations are employed to highlight the subtle but important differences in the mechanical stability of perfect crystalline solids versus amorphous solids. We stress the difference between strain values at which the shear modulus vanishes and strain values at which a plastic instability ensues. The temperature dependence of the yield strain is computed for the two types of solids, showing different scaling laws: ${\gamma }_{{}_Y}\simeq {\gamma }_{{}_Y}^0-C_1{T}^{1/3}$ for crystals versus ${\gamma }_{{}_Y}\simeq {\gamma }_{{}_Y}^0-C_2{T}^{2/3}$ for amorphous solids.

Recent experiments on the spin-ice material ${\text{Dy}}_2{\text{Ti}}_2{\text{O}}_7$ suggest that the Pauling “ice entropy,” characteristic of its classical Coulombic spin-liquid state, may be lost at low temperatures [Pomaranski et al., Nat. Phys. 9, 353 (2013)]. However, despite nearly two decades of intensive study, the nature of the equilibrium ground state of spin ice remains uncertain. Here we explore how long-range dipolar interactions $D$, short-range exchange interactions, and quantum fluctuations combine to determine the ground state of dipolar spin ice. We identify the organizational principle that ordered ground states are selected from a set of “chain states” in which dipolar interactions are exponentially screened. Using both quantum and classical Monte Carlo simulation, we establish phase diagrams as a function of quantum tunneling $g$ and temperature $T$, and find that only a very small $g_c\ll D$ is needed to stabilize a quantum spin liquid ground state. We discuss the implications of these results for ${\text{Dy}}_2{\text{Ti}}_2{\text{O}}_7$.

We report the magnetic and electronic structures of the inverse-trirutile compound $\mathrm{C}{\mathrm{r}}_2\mathrm{Mo}{\mathrm{O}}_6$. Despite the same crystal symmetry and similar bond lengths and bond angles as $\mathrm{C}{\mathrm{r}}_2\mathrm{Te}{\mathrm{O}}_6,\mathrm{C}{\mathrm{r}}_2\mathrm{Mo}{\mathrm{O}}_6$ possesses a magnetic structure different from that in $\mathrm{C}{\mathrm{r}}_2\mathrm{Te}{\mathrm{O}}_6$. ab initio electronic structure calculations show that the sign and strength of the Cr-O-Cr superexchange coupling is strongly influenced by the hybridization between the filled $\mathrm{O}2p$ and empty $\mathrm{Mo}4d$ orbitals: the virtual transfer of an $\mathrm{O}2p$ electron to the empty $\mathrm{Mo}4d$ orbitals leaves the $\mathrm{O}2p$ partially occupied, which leads to ferromagnetic exchange between Cr moments. This result further substantiates our recently proposed mechanism for tuning the exchange interaction between two magnetic atoms by modifying the electronic states of nonmagnetic atoms in the exchange path through orbital hybridization. This approach is fundamentally different from the conventional methods of controlling the exchange interaction by either carrier injection or through structural distortions.

We report a study on the transition temperature $T_{\mathrm{C}}$ of Cr-doped topological insulator thin films, where an increase in the ferromagnetic onset can provide a pathway towards low-power spintronics in the future. Arrott plots, measured by surface-sensitive x-ray magnetic circular dichroism at the Cr $L_{2,3}$ edge as a function of field at various low temperatures, give a $T_{\mathrm{C}} \approx 7\mathrm{K}$ for the pristine surface. This is comparable to the bulk value of the film, which means that there is no indication that the spontaneous magnetization is different near the surface. Evaporation of a thin layer of Co onto the pristine surface of the in-situ cleaved sample increases the ordering temperature near the surface to $\sim 19\mathrm{K}$, while in the bulk it rises to $\sim 10\mathrm{K}$. X-ray absorption spectroscopy shows that Cr enters the ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ host matrix in a divalent state, and is unchanged by the Co deposition. These results demonstrate a straightforward procedure to increase the transition temperature of doped topological insulators.

We present thermodynamic phase diagrams showing magnetic analog of “three states of matter,” namely, spin liquid, paramagnetic, and magnetically ordered phases, obtained by unbiased quantum Monte Carlo simulations. Our simulations are carried out for Kitaev's toric codes in two and three dimensions, i.e., the simplest realizations of gapped topological $Z_2$ spin liquids, extended by a nearest-neighbor ferromagnetic Ising coupling. We find that the ordered phase borders on the spin liquid by a discontinuous transition line in three dimensions, while it grows continuously from the quantum critical point in two dimensions. In both cases, our results elucidate peculiar proximity effects to the nearby spin liquids in the high-temperature paramagnetic phase, even when the ground state is magnetically ordered. The thorough study of magnetic three states of matter is achieved by introducing the “fictitious vertex” method into the directed loop algorithm. This provides a generic prescription to simulate models with off-diagonal multispin interactions, in which the conventional scheme may suffer from intrinsic ergodicity breakdown as in the present case.

We study the Josephson effect of $d$-wave superconductor (DS)/ferromagnet insulator (FI)/DS junctions on a surface of topological insulator (TI). We calculate Josephson current $I\left(\phi \right)$ for various orientations of the junctions where $\phi$ is the macroscopic phase difference between two DSs. In certain configurations, we find anomalous current-phase relation $I\left(\phi \right)=-I\left(-\phi +\pi \right)$ with $2\pi$ periodicity. In the case where the first-order Josephson coupling is absent without magnetization in FI, $I\left(\phi \right)$ can be proportional to $cos\phi$. In symmetric junctions, the magnitude of the obtained Josephson current is enhanced due to the zero-energy states on the edge of DS on TI. Even if we introduce an $s$-wave component of pair potential in DS, we can still expect the anomalous current-phase relation in asymmetric DS junctions with $I\left(\phi =0\right)\ne 0$. This can be used to probe the induced $d$-wave component of pair potential on a TI surface in high-$T_c$ cuprate/TI hybrid structures.

Clarifying the chiral domain structure of superconducting ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ has been a long-standing issue in identifying its peculiar topological superconducting state. We evaluated the critical current $I_c$ versus the magnetic field $H$ of Nb/${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ Josephson junctions, changing the junction dimension in expectation of that the number of domains in the junction is controlled. $I_c\left(H\right)$ exhibits a recovery from inversion symmetry breaking to invariance when the dimension is reduced to several microns. This inversion invariant behavior indicates the disappearance of domain walls; thus, the size of a single domain is estimated at approximately several microns.

The properties of a new class of phosphorene nanotubes (PNT) are investigated by performing first-principles calculations. We demonstrate that it is advantageous to use blue phosphorene in order to make small nanotubes and propose a way to create low-energy PNTs by the inclusion of defect lines. Five different types of defect lines are investigated and incorporated in various combinations. The resulting defect-induced faceted PNTs have negligible bending stresses which leads to a reduction in the formation energy with respect to round PNTs. Our armchair faceted PNTs have similar formation energies than the recently proposed multiphase faceted PNTs, but they have a larger variety of possible structures. Our zigzag faceted PNTs have lower formation energies than round tubes and multiphase faceted nanotubes. The electronic properties of the defect-induced faceted PNTs are determined by the defect lines which control the band gap and the shape of the electronic states at the band edges. These band gaps increase with the radius of the nanotubes and converge to those of isolated defect lines.

We argue that the ac conductivity $\sigma \left(\omega \right)$ in the many-body localized phase is a power law of frequency $\omega$ at low frequency: specifically, $\sigma \left(\omega \right)\sim {\omega }^{\alpha }$ with the exponent $\alpha$ approaching 1 at the phase transition to the thermal phase, and asymptoting to 2 deep in the localized phase. We identify two separate mechanisms giving rise to this power law: deep in the localized phase, the conductivity is dominated by rare resonant pairs of configurations; close to the transition, the dominant contributions are rare regions that are locally critical or in the thermal phase. We present numerical evidence supporting these claims, and discuss how these power laws can also be seen through polarization-decay measurements in ultracold atomic systems.

In this study we present ab initio density-functional theory calculations on stoichiometric, cation-doped, and strained $\mathrm{GaFeO}{}_3$. We start with a detailed discussion of the origin of the antiferromagnetic (AFM) superexchange in stoichiometric $\mathrm{GaFeO}{}_3$ and give a molecular orbital description of the exchange mechanism derived from our calculations. In addition, we study the properties of the Fe-O-Fe bonds for different geometries to underline the angle and distance dependence of the AFM coupling as formulated in the Goodenough-Kanamori rules. We describe the AFM ground state of $\mathrm{GaFeO}{}_3$ as a result of two intrinsic Fe-O-Fe chains that meander through the crystal along the $c$ direction. The magnetocrystalline anisotropy energies are calculated for the stoichiometric phase with and without inner cationic site disorder, and the presence of a sublattice-dependent anisotropy is examined. Furthermore, we perform our studies of ${\mathrm{Ga}}_{2-x}\mathrm{Fe}{}_x{\mathrm{O}}_3$ for varying Fe concentrations $\mathit{x}(0.0\le \mathit{x}\le 2.0)$ where at a value of $\mathit{x}=0.0$ and $\mathit{x}=2.0$ it transforms into the isomorphic $\varepsilon \text{−}\mathrm{Ga}{}_2{\mathrm{O}}_3$ and $\varepsilon \text{−}\mathrm{Fe}{}_2{\mathrm{O}}_3$ phases, respectively. The effect of strain was also studied. Incorporating dopants and applying strain to the simulation cell changes the intrinsic geometry and thus the magnetic properties of gallium ferrite.

In a previous work [Phys. Rev. B 84, 085120 (2011)], we presented the orbital-separation approach for simulating metal/insulator/metal capacitors under bias voltage within the Kohn-Sham framework of density functional theory. A limitation mentioned in that work was that it is not straightforward to calculate the amount of free charge transferred from one electrode to the other, and thus the application of the method was limited to the closed-circuit (constant-voltage) condition. Here, we show that it is actually possible in practice to calculate the free charge by examining the change in the occupation of the Kohn-Sham eigenstates near the Fermi level. Thus it is also possible to perform open-circuit (constant-charge) simulations, and we demonstrate this on a metal/ferroelectric/metal capacitor.

We show that the equations underlying the $GW$ approximation have a large number of solutions. This raises the question, how can we find the physical solution? We provide two theorems which explain why the methods currently in use usually find the correct solution. These theorems are general enough to cover a large class of similar methods. An efficient algorithm for including self-consistent vertex corrections well beyond $GW$ is described and used in a numerical validation of the two theorems. The effect of a simple mixing scheme on solutions obtained iteratively is also investigated.

Using a modified spin-wave theory which artificially restores zero sublattice magnetization on finite lattices, we investigate the entanglement properties of the Néel ordered $J_1-J_2$ Heisenberg antiferromagnet on the square lattice. Different kinds of subsystem geometries are studied, either corner-free (line, strip) or with sharp corners (square). Contributions from the $n_G=2$ Nambu-Goldstone modes give additive logarithmic corrections with a prefactor $n_G/2$ independent of the Rényi index. On the other hand, $\pi /2$ corners lead to additional (negative) logarithmic corrections with a prefactor $l_q^c$ which does depend on both $n_G$ and the Rényi index $q$, in good agreement with scalar field theory predictions. By varying the second neighbor coupling $J_2$ we also explore universality across the Néel ordered side of the phase diagram of the $J_1-J_2$ antiferromagnet, from the frustrated side $0<J_2/J_1<1/2$ where the area law term is maximal, to the strongly ferromagnetic regime $-J_2/J_1\gg 1$ with a purely logarithmic growth $S_q=\frac{n_G}{2}lnN$, thus recovering the mean-field limit for a subsystem of $N$ sites. Finally, a universal subleading constant term ${\gamma }_q^{\mathrm{ord}}$ is extracted in the case of strip subsystems, and a direct relation is found (in the large-$S$ limit) with the same constant extracted from free lattice systems. The singular limit of vanishing aspect ratios is also explored, where we identify for ${\gamma }_q^{\text{ord}}$ a regular part and a singular component, explaining the discrepancy of the linear scaling term for fixed width vs fixed aspect ratio subsystems.

We report on the electronic and magnetic properties of Co atoms adsorbed on topological insulator (TI) surfaces, such as ${\mathrm{Bi}}_2{\mathrm{Se}}_3, {\mathrm{Bi}}_2{\mathrm{Te}}_3$, and on magnetically doped ${\mathrm{Bi}}_{1.9}{\mathrm{Mn}}_{0.1}{\mathrm{Te}}_3$ and ${\mathrm{Bi}}_{1.98}{\mathrm{Fe}}_{0.02}{\mathrm{Se}}_3$ in comparison to a nontopological insulator surface, ${\mathrm{Bi}}_2{\mathrm{S}}_3$. Co atoms are used as probes of impurity affected surface electronic properties of TI and non-TI tetradymite semiconductors. X-ray spectroscopy, x-ray magnetic circular dichroism, and scanning tunneling microscopy and spectroscopy were employed. We found that the electronic properties of Co adatoms do not depend much on the substrate. In particular, no TI specific phenomenon was found. Magnetic dichroism measurements reveals an in-plane easy axis of adsorbed Co atoms on all topological insulators and ${\mathrm{Bi}}_2{\mathrm{S}}_3$, but not on the ferromagnetic substrate ${\mathrm{Bi}}_{1.9}{\mathrm{Mn}}_{0.1}{\mathrm{Te}}_3$, for which an out-of-plane easy axis was found.

Density functional theory and density functional tight binding are applied to model electron transport in copper nanowires of approximately 1- and 3-nm diameters with varying crystal orientation and surface termination. The copper nanowires studied are found to be metallic irrespective of diameter, crystal orientation, and/or surface termination. Electron transmission is highly dependent on crystal orientation and surface termination. Nanowires oriented along the [110] crystallographic axis consistently exhibit the highest electron transmission while surface oxidized nanowires show significantly reduced electron transmission compared to unterminated nanowires. Transmission per unit area is calculated in each case; for a given crystal orientation we find that this value decreases with diameter for unterminated nanowires but is largely unaffected by diameter in surface oxidized nanowires for the size regime considered. Transmission pathway plots show that transmission is larger at the surface of unterminated nanowires than inside the nanowire and that transmission at the nanowire surface is significantly reduced by surface oxidation. Finally, we present a simple model which explains the transport per unit area dependence on diameter based on transmission pathways results.