By performing direct simulations of demixing for an equimolar mixture of hydrogen and helium, we find that reflectivity measurements performed during dynamic experiments could be used to identify this liquid-liquid phase transition under conditions encountered in giant planet interiors. These first principles simulations show that the reflectivity exhibits a distinctive signature at wavelengths currently available using VISAR (velocity interferometer system for any reflector) measurements. We further make the first estimation of the demixing timescale under such extreme conditions and find that it appears compatible with dynamic measurements.

The magnetic and electronic properties of metal phthalocyanines ($M$Pc) and fluorinated metal phthalocyanines (F${}_{16}$$M$Pc) are studied by means of spin density functional theory (SDFT). Several metals ($M$) such as Ca, all first $d$-row transition metals, and Ag are investigated. By considering different open shell transition metals it is possible to tune the electronic properties of $M$Pc, in particular the electronic molecular gap and total magnetic moment. Besides determining the structural and electronic properties of $M$Pc and F${}_{16}$$M$Pc, the vibrational modes of the ScPc-ZnPc series have been studied.

We propose a new single-step scheme for the generation of a Greenberger-Horne-Zeilinger (GHZ) entangled state of three single-electron excitations (flying qubits). We also present a method to get a generalized GHZ state. Our idea relies upon the most recent progress in the field of on-demand electron sources and mesoscopic Mach-Zehnder interferometry. We also provide the recipe for the unambiguous detection of this GHZ state via correlation measurements at the output, which imply the violation of a Bell-type inequality which is generalized to the case of three particles. We explain how such measurements can be achieved in the context of Mach-Zehnder interferometry and draw an actual prototype device which could be achieved with point contacts and metallic gates placed on a GaAs sample, in the integer quantum Hall effect regime.

We report total-energy electronic-structure calculations performed for Si nanowires along the [100] and [110] directions within the local density approximation in the density-functional theory. Energy-band structures of Si nanowires with a cross-sectional dimension in the range of 1 to 6 nm are clarified for a variety of cross-sectional morphologies. Sidewall roughness inevitably existing in real experimental situations is examined and its effects on the electron states are revealed. We find that the nanometer scale morphology of the cross-sections of the nanowires interestingly affects the energy-band structures. The implication of the obtained electronic structures is discussed in case that Si nanowires are utilized in the next-generation field-effect transistors.

Recent theoretical and numerical evidence suggests that localization can survive in disordered many-body systems with very high energy density, provided that interactions are sufficiently weak. Stronger interactions can destroy localization, leading to a so-called many-body localization transition. This dynamical phase transition is relevant to questions of thermalization in extended quantum systems far from the zero-temperature limit. It separates a many-body localized phase, in which localization prevents transport and thermalization, from a conducting (“ergodic”) phase in which the usual assumptions of quantum statistical mechanics hold. Here, we present numerical evidence that many-body localization also occurs in models without disorder but rather a quasiperiodic potential. In one dimension, these systems already have a single-particle localization transition, and we show that this transition becomes a many-body localization transition upon the introduction of interactions. We also comment on possible relevance of our results to experimental studies of many-body dynamics of cold atoms and nonlinear light in quasiperiodic potentials.

The influence of inelastic scattering mechanisms on the giant magnetoresistance (GMR) effect has been controversially discussed. This issue can be addressed, for example, by comparing charge currents with heat currents in magnetic multilayers that exhibit GMR. The interpretation of such experiments is based on the Wiedemann-Franz law. Due to the fact that this law only holds for elastic scattering, experimentally observed deviations from this law are usually attributed to the presence of inelastic scattering and vice versa. We develop two simple models to demonstrate that this interpretation can lead to wrong conclusions in the context of the GMR effect. We employ a measurement technique that is based on the so-called 3$\omega$ method to study the in-plane electrical and thermal conductivities of a Co/Cu multilayer in dependence on magnetic fields over a wide range of temperatures. Our experimental results indicate that the Wiedemann-Franz law holds over the temperature range investigated. Using the simple models developed, we conclude that the influence of electron-magnon scattering on the GMR effect in the Co/Cu multilayer is negligible. We further conclude that electron-phonon scattering is characterized by the same spin asymmetry as the predominant elastic scattering of electrons at interfaces in the Co/Cu multilayer.

Two centuries of research on phase transitions have repeatedly highlighted the importance of critical fluctuations that abound in the vicinity of a critical point. They are at the origin of scaling laws obeyed by thermodynamic observables close to second-order phase transitions resulting in the concept of universality classes, that is of paramount importance for the study of organizational principles of matter. Strikingly, in case such soft fluctuations are too abundant they may alter the nature of the phase transition profoundly; the system might evade the critical state altogether by undergoing a discontinuous first-order transition into the ordered phase. Fluctuation-induced first-order transitions have been discussed broadly and are germane for superconductors, liquid crystals, or phase transitions in the early universe, but clear experimental confirmations remain scarce. Our results from neutron scattering and thermodynamics on the model Dzyaloshinskii-Moriya (DM) helimagnet (HM) MnSi show that such a fluctuation-induced first-order transition is realized between its paramagnetic and HM state with remarkable agreement between experiment and a theory put forward by Brazovskii. While our study clarifies the nature of the HM phase transition in MnSi that has puzzled scientists for several decades, more importantly, our conclusions entirely based on symmetry arguments are also relevant for other DM-HMs with only weak cubic magnetic anisotropies. This is in particular noteworthy in light of a wide range of recent discoveries that show that DM helimagnetism is at the heart of problems such as topological magnetic order, multiferroics, and spintronics.

The present qubit technology, in particular, in Josephson qubits, allows an unprecedented control of discrete energy levels. This motivates a new study of the old pump-probe problem, where a discrete quantum system is driven by a strong drive and simultaneously probed by a weaker one. The strong drive is included by the Floquet method and the resulting quasienergy states are then studied with the probe. We study a qubit where the harmonic drive has a significant longitudinal component relative to the static equilibrium state of the qubit. Both analytical and numerical methods are used to solve the problem. We present calculations with realistic parameters and compare the results with recent experimental results. A short introduction to the Floquet method and the probe absorption is given.

The magnetoelectrostatic tailoring of the supercurrent in quantum point contact ballistic Josephson junctions is demonstrated. An etched InAs-based heterostructure is laterally contacted to superconducting niobium leads, and the existence of two etched side gates permits, in combination with the application of a perpendicular magnetic field, continuous modification of the magnetic interference pattern by depleting the weak link. For wider junctions the supercurrent presents a Fraunhofer-like interference pattern with periodicity $h/2e$, whereas by shrinking electrostatically the weak link, the periodicity evolves continuously to a monotonic decay. These devices represent tunable structures that might lead to the study of the elusive Majorana fermions.

Using first-principles calculations, we study theoretically the stable $2H$ hexagonal structure of BaMnO${}_3$. We show that from the stable high-temperature $P{6}_3/mmc$ structure, the compound should exhibit an improper ferroelectric structural phase transition to a $P{6}_{3}cm$ ground state. Combined with its antiferromagnetic properties, $2H$-BaMnO${}_3$ is therefore expected to be multiferroic at low temperature. The phase transition mechanism in ${\mathrm{BaMnO}}_3$ appears similar to what was reported in YMnO${}_3$ in spite of totally different atomic arrangement, cation sizes, and Mn valence state.

In the ferromagnetic insulator with the Dzyaloshinskii-Moriya interaction, we theoretically predict and numerically verify a topological magnon insulator, where the charge-free magnon is topologically protected for transporting along the edge/surface while it is insulating in the bulk. The chiral edge states form a connected loop as a $4\pi$- or $8\pi$-period Möbius strip in the spin-wave vector space, showing the nontrivial topology of magnonic bands. Using the nonequilibrium Green's function method, we explicitly demonstrate that the one-way chiral edge transport is indeed topologically protected from defects or disorders. Moreover, we show that the topological edge state mainly localizes around edges and leaks into the bulk with oscillatory decay. Although the chiral edge magnons and energy current prefer to travel along one edge from the hot region to the cold one, the anomalous transports are identified in the opposite edge, which reversely flow from the cold region to the hot one. Our findings could be validated within wide energy ranges in various magnonic crystals, such as Lu${}_2$V${}_2$O${}_7$.

We investigate the fundamentals of compositional patterning induced by energetic particle irradiation in model $A-B$ substitutional binary alloys using kinetic Monte Carlo simulations. The study focuses on a type of nanostructure that was recently observed in dilute Cu-Fe and Cu-V alloys, where precipitates form within precipitates, a morphology that we term “cherry-pit” structures. The simulations show that the domain of stability of these cherry-pit structures depends on the thermodynamic and kinetic asymmetry between the $A$ and $B$ elements. In particular, both lower solubilities and diffusivities of $A$ in $B$ compared to those of $B$ in $A$ favor the stabilization of these cherry-pit structures for $A$-rich average compositions. The simulation results are rationalized by extending the analytic model introduced by Frost and Russell for irradiation-induced compositional patterning so as to include the possible formation of pits within precipitates. The simulations indicate also that the pits are dynamical structures that undergo nearly periodic cycles of nucleation, growth, and absorption by the matrix.

A unified approach is used to study vibrational properties of periodic systems with first-principles methods and including anharmonic effects. Our approach provides a theoretical basis for the determination of phonon-dependent quantities at finite temperatures. The low-energy portion of the Born-Oppenheimer energy surface is mapped and used to calculate the total vibrational energy including anharmonic effects, electron-phonon coupling, and the vibrational contribution to the stress tensor. We report results for the temperature dependence of the electronic band gap and the linear coefficient of thermal expansion of diamond, lithium hydride, and lithium deuteride.

We study the quantum geometric heat flux in the nonequilibrium spin-boson model. By adopting the noninteracting-blip approximation that is able to accommodate the strong system-bath coupling, we show that there exists a nonzero geometric heat flux only when the two-level system is nondegenerate. Moreover, the pumping, no pumping, and dynamic control of geometric heat flux are discussed in detail, compared to the results with Redfield weak-coupling approximation. In particular, the geometric energy transfer induced by modulation of two system-bath couplings is identified, which is exclusive to quantum transport in the strong system-bath coupling regime.

Neutron diffraction measurements exploring the magnetic and structural phase behaviors of the candidate $J_{\mathrm{eff}}=1/2$ Mott insulating iridate Sr${}_2$IrO${}_4$ are presented. Comparisons are drawn between the correlated magnetism in this single-layer system and its bilayer analog Sr${}_3$Ir${}_2$O${}_7$, where both materials exhibit magnetic domains originating from crystallographic twinning and comparable moment sizes. Weakly temperature-dependent superlattice peaks violating the reported tetragonal space group of Sr${}_2$IrO${}_4$ are observed, supporting the notion of a lower structural symmetry arising from a high-temperature lattice distortion, and we use this to argue that moments orient along a unique in-plane axis demonstrating an orthorhombic symmetry in the resulting spin structure. Our results demonstrate that the correlated spin order and structural phase behaviors in both single-layer and bilayer Sr${}_{n+1}$Ir${}_n$O${}_{3n+1}$ systems are remarkably similar and suggest comparable correlation strengths in each system.

The complexity of competing orders in cuprates has recently been multiplied by a number of bulk evidences of charge ordering with wave vector that connects the antinodal region of the Fermi surface. This result contradicts many spectroscopic results of the nodal nesting. To resolve this issue, we carry out a unified study of the resulting electronic fingerprints of both nodal and antinodal nestings (NNs/ANs) and compare with angle-resolved photoemission, scanning tunneling spectroscopic data, as well as bulk-sensitive Hall-effect measurements. Our result makes several definitive distinctions between them in that while both nestings gap out the antinodal region, AN induces an additional quasiparticle gap below the Fermi level along the nodal direction, which is so far uncharted in spectroscopic data. Furthermore, we show that the Hall coefficient in the AN state obtains a discontinuous jump at the phase transition from an electronlike nodal pocket (negative value) to a large holelike Fermi surface (positive value), in contrast to a continuous transition in the available data. We conclude that individual NNs and ANs have difficulties in explaining all of the data. In this spirit, we study a possibility of coexisting NN and AN phases within a Ginsburg-Landau functional formalism. An interesting possibility of disorder pinned “chiral” charge ordering is finally discussed.

Multilayer graphene systems with a rhombohedral stacking order harbor nearly flat bands in their single-particle spectrum. We propose ansatz states to describe the surface-localized states of flat band electrons. The absence of kinetic dispersion near the Fermi level leaves the interaction as a dominate mechanism to govern the low-energy physics of a low-density electron system. We build up an effective lattice model in two interacting low-energy bands, where the full terms of the Coulomb interaction, including those long-range and off-diagonal parts, have been considered. The interaction matrix coefficients in the many-body Hamiltonian model are directly calculated for a trilayer system using orthonormal Wannier basis. We then present a flat band projection to yield an interaction-only lattice model for flat band electrons. We find that this limited model might energetically favor a ferromagnetic quantum crystal under certain conditions.

We report that V${}_{\mathrm{O}}$ cohesion-isolation transition caused by carrier injection/removal is a generalized resistance switching mechanism of binary-oxide-based resistive random-access memories (ReRAMs). We propose universal guiding principles by which ReRAM with unipolar and bipolar operations can be designed by controlling electrode work functions. We found by first-principles calculations that structural phase transition with V${}_{\mathrm{O}}$ cohesion-isolation is the physical origin of the resistance switching mechanism of binary-oxide-based ReRAM. Based on our theory, we can propose a guiding principle toward bipolar switching ReRAM with stable high work function metal electrodes.

We propose using an optical cavity to enhance the sensitivity of a magnetometer relying on the detection of the spin state of a high-density nitrogen-vacancy ensemble in diamond using infrared optical absorption. The role of the cavity is to obtain a contrast in the absorption-detected magnetic resonance approaching unity at room temperature. We project an increase in the photon shot-noise limited sensitivity of two orders of magnitude in comparison with a single-pass approach. Optical losses can limit the enhancement to one order of magnitude, which could still enable room-temperature operation. Finally, the optical cavity also allows us to use less pumping power when the cavity is resonant at both the pump and the infrared probe wavelength.