Ab initio calculations often do not straightforwardly yield the thermal properties of a material yet. It requires considerable computational efforts, for example, to predict the volumetric thermal expansion coefficient ${\alpha }_V$ or the melting temperature $T_m$ from first principles. An alternative is to use semiempirical approaches. They relate the experimental values to first-principles predictors via fits or approximative models. Before applying such methods, however, it is of paramount importance to be aware of the expected errors. We therefore quantify these errors at the density-functional theory level using the Perdew-Burke-Ernzerhof functional for several semiempirical approximations of ${\alpha }_V$ and $T_m$, and compare them to the errors from fully ab initio methods, which are computationally more intensive. We base our conclusions on a benchmark set of 71 ground-state elemental crystals. For the thermal expansion coefficient, it appears that simple quasiharmonic theory, in combination with different approximations to the Grüneisen parameter, provides a similar overall accuracy as exhaustive first-principles phonon calculations. For the melting temperature, expensive ab initio molecular-dynamics simulations still outperform semiempirical methods.

The magnetic properties of NiTa${}_2$O${}_6$ were investigated by magnetic susceptibility, specific heat, electron paramagnetic resonance, neutron powder diffraction, and pulse field magnetization measurements. Accompanying ab initio DFT calculations of the spin-exchange constants complemented and supported our experimental findings that NiTa${}_2$O${}_6$ must be described as a quasi-1D Heisenberg $S=1$ spin chain system with a nearest-neighbor only antiferromagnetic spin-exchange interaction of 18.92(2) K. Interchain coupling is by about two orders of magnitude smaller. Electron paramagnetic resonance measurements on Mg${}_{1-x}$Ni${}_x$Ta${}_2$O${}_6$ ($x\approx 1\%$) polycrystalline samples enabled us to estimate the single-ion zero-field splitting of the $S=1$ states which amounts to less than 4% of the nearest-neighbor spin-exchange interaction. At 0 T NiTa${}_2$O${}_6$ undergoes long-range antiferromagnetic ordering at 10.3(1) K evidenced by a $\lambda$-type anomaly in the specific heat capacity. On application of a magnetic field the specific heat anomaly is smeared out. We confirmed the magnetic structure by neutron powder diffraction measurements and at 2.00(1) K refined a magnetic moment of 1.93(5) ${\mu }_{\mathrm{B}}$ per Ni${}^{2+}$ ion. Additionally, we followed the magnetic order parameter as a function of temperature. Last, we found saturation of the magnetic moment at 55.5(5) T with a $g$ factor of 2.14(1), with an additional high field phase above 12.8(1) T. The onset of the new high field phase is not greatly affected by temperature, but rather smears out as one approaches the long-range ordering temperature.

We study the finite-temperature phase diagram of the Heisenberg-Kitaev model on a three-dimensional hyperhoneycomb lattice. Using semiclassical analysis and classical Monte Carlo simulations, we investigate quantum and thermal order-by-disorder, as well as the magnetic-ordering temperature. We find the parameter regime where quantum and thermal fluctuations favor different magnetic orders, which leads to an additional finite-temperature phase transition within the ordered phase. This transition, however, occurs at a relatively low temperature and the entropic effects may dominate most of the finite-temperature region below the ordering temperature. In addition, we explore the magnetization process in the presence of a magnetic field and discover spin-flop transitions which are sensitive to the applied-field direction. We discuss implications of our results for future experiments on a hyperhoneycomb lattice system such as the recently discovered $\beta$-Li${}_2$IrO${}_3$.

We report two-dimensionality and effective thickness of the superconductivity of a SrTiO single-crystal surface induced by electric double-layer gating. The carrier density was tuned from 3 × 10 to 1.1 × 10 cm by gating, where superconductivity appears with $T_c$ of around 0.4 K. Typical two-dimensional behavior perfectly described by the Ginzburg–Landau equation was observed in the angular and temperature dependence of the upper critical magnetic field. The effective thickness of the superconducting layer remains nearly invariant, ranging from 11 to 13 nm, with increasing charge carrier density. This invariance contradicts the expected reduction in the thickness of the accumulation layer in a triangular quantum well model. This unexpected invariance of the superconducting layer thickness is probably a unique nature for a two-dimensional electron system in the incipient ferroelectric SrTiO.

Recent muon spin-rotation ($\mu$SR) measurements on SrPtAs revealed time-reversal-symmetry breaking with the onset of superconductivity [Biswas et al., Phys. Rev. B 87, 180503(R) (2013)], suggesting an unconventional superconducting state. We investigate this possibility via the functional renormalization group and find a chiral $(d+id)$-wave order parameter favored by the multiband fermiology and hexagonal symmetry of SrPtAs. This $(d+id)$-wave state exhibits significant gap anisotropies as well as gap differences on the different bands, but only has point nodes on one of the bands at the Brillouin zone corners. We study the topological characteristics of this superconducting phase, which features Majorana-Weyl nodes in the bulk, protected surface states, and an associated thermal Hall response. The lack of extended nodes and the spontaneously broken time-reversal symmetry of the $(d+id)$-wave state are in agreement with the $\mu$SR experiments. Our theoretical findings, together with the experimental evidence, thus suggest that SrPtAs is an example of chiral $d$-wave pairing and a Weyl superconductor.

Adding Au nanoparticles to YBa${}_2$Cu${}_3$O${}_{7-\delta }$ thin films leads to an increase of the superconducting transition temperature $T_{\mathrm{c}}$ and the critical current density $j_{\mathrm{c}}$. While the higher $j_{\mathrm{c}}$ can be understood in terms of a stronger pinning of the flux vortices at the Au nanoparticles, the enhanced $T_{\mathrm{c}}$ is still puzzling. In the present study, we determined the microscopic magnetic penetration profiles and the corresponding London penetration depths ${\lambda }_{\mathrm{L}}$ in the Meissner state of optimally doped YBa${}_2$Cu${}_3$O${}_{7-\delta }$ thin films with and without Au nanoparticles by low-energy muon spin rotation. By Rutherford backscattering spectrometry, we show that the Au nanoparitcles are distributed over the whole thickness of the thin-film samples. The superfluid density $n_{\mathrm{s}}\propto 1/{\lambda }_{\mathrm{L}}^2$ was found to increase in the films containing Au nanoparticles. We attribute this increase of $n_{\mathrm{s}}$ to a reduction of the defect density possibly due to defect condensation at the Au nanoparticles.

We propose here a new theoretical treatment of the spatiotemporal properties in spin-crosser solids, based on the expansion of the free energy taking into account the spatial fluctuations of the high-spin (HS) fraction. This leads to an equation of motion on the HS fraction following a reaction diffusion equation (RDE), in which most of the parameters can be derived from the experiments. This equation involves the true temporal and spatial scales at variance from the previous stochastic microscopic models, which were based on a homogeneous treatment of the crystal's properties. We have illustrated this new treatment for a two-dimensional rectangularly shaped system with a square symmetry and we could reproduce quantitatively the process of nucleation, growth, and propagation of the HS fraction inside the thermal hysteresis loop, accompanying a first-order transition. The computed spatiotemporal evolution of the system allowed one to follow the propagation of a well-defined macroscopic HS:LS interface, which was found in excellent quantitative agreement with the experimental observations of optical microscopy on the switchable spin crossover crystal [{Fe(NCSe)(py)${}_2$}${}_2$($m$-bpypz)]. The RDE treatment should generate predictive models for novel spatiotemporal effects in spin crossover solids and more generally for all kinds of switchable molecular solids.

In molecular devices, electronic degrees of freedom are coupled to vibrational modes of the molecule, offering an opportunity to study fundamental aspects of this coupling at the nanoscale. To this end, we consider the nonequilibrium heat exchange between a conduction band and a bosonic bath mediated by a single molecule. For molecules large enough so that onsite Coulomb repulsion can be dropped, we carry out an asymptotically exact calculation of the heat current, governed by the smallness of the electron-phonon coupling, and obtain the steady-state heat current driven by a finite-temperature drop. At low temperatures, the heat current is found to have a power-law behavior with respect to the temperature difference with the power depending on the nature of the bosonic bath. At high temperatures, on the other hand, the current is linear in the temperature difference for all types of bosonic baths. The crossover between these behaviors is described. Some of the results are given a physical explanation by comparing to a perturbative master-equation calculation (the limitation of which we examine).

We have studied cubic boron arsenide (BAs) by Raman spectroscopy and first-principles simulations. The Raman $T_2$ phonon involving B vibrations in ${}^{\mathrm{nat}}$BAs shows a complex isotope two-mode behavior, not seen before in isotopically disordered materials. The majority isotope ${}^{11}$B phonon at 704 ${\mathrm{cm}}^{-1}$ obeys average isotopic mass dependence and mass disorder renormalization, whereas a band at 723 ${\mathrm{cm}}^{-1}$ stems from predominately ${}^{10}$B vibrations in ${}^{\mathrm{nat}}$BAs. The observed lack of $T_2$ LO–TO splitting in both ${}^{\mathrm{nat}}$BAs and ${}^{11}$BAs and third- and fourth-order anharmonicity contributions compensating each other in the temperature dependence of the $T_2$ phonon are among the other curious vibrational properties of BAs.

We study deterministic magnetic reversal of a perpendicularly magnetized Co layer in a Co/MgO/Ta nanosquare driven by spin Hall torque from an in-plane current flowing in an underlying Pt layer. The rate-limiting step of the switching process is domain wall (DW) depinning by spin Hall torque via a thermally assisted mechanism that eventually produces full reversal by domain expansion. An in-plane applied magnetic field collinear with the current is required, with the necessary field scale set by the need to overcome DW chirality imposed by the Dzyaloshinskii-Moriya interaction. Once Joule heating is taken into account the switching current density is quantitatively consistent with a spin Hall angle ${\theta }_{SH}\approx 0.07$ for 4 nm of Pt.

Coherent metallic screening structures shown in core-level photoemission spectra for strongly correlated oxide materials were studied by hard x-ray photoemission spectroscopy (HAXPES) and the configuration interaction (CI) theory based on the cluster model, including the coherent metallic screening process. For the La${}_{1-x}$Ba${}_x$MnO${}_3$ thin film, the normalized intensity of the coherent metallic screening structure ($I_S$) seen in the Mn 2$p$ core-level spectra was proportional to the square of the hybridization strength, ($V$*), between the transition metal 3$d$ and coherent metallic states. In contrast, the normalized $I_S$ seen in the V 2$p$ core-level spectra for the V${}_{1-x}$W${}_x$O${}_2$ thin film was not proportional to ($V$*). The different behaviors of the normalized $I_S$ for the La${}_{1-x}$Ba${}_x$MnO${}_3$ and V${}_{1-x}$W${}_x$O${}_2$ thin films as a function of $V$* were understood by the series of the CI cluster model calculation. From the CI cluster model calculation, we found that the charge transfer energy (Δ*) between the transition metal 3$d$ states and the coherent metallic states strongly affect the normalized $I_S$ in the 2$p$ core-level photoemission final states. Therefore, the behavior of the normalized $I_S$ in the 2$p$ core-level HAXPES is thus not simply explained by the change of $V$*. The electronic structure parameters such as Δ* and $V$* relating to the coherent metallic states strongly contribute to the variation of the normalized $I_S$ in the photoemission final states. In contrast, we found that the detailed $I_S$ evaluation in the core-level HAXPES experiments for materials, in which the coherent metallic screening structures appear, allows us to evaluate the value of $V$* when we have the experimental core-level spectra and the electronic structure parameters set for a reference material. We also found that the intensity at the Fermi level is proportional to ($V$*), as expected from the Anderson impurity model.

Cherenkov second-harmonic generation (CSHG) is a powerful tool for three-dimensional domain wall profiling in ferroic bulk crystals. Here, we apply this noninvasive technique for tracking head-to-head charged domain walls (CDWs) across millimeter-thick ferroelectric single-crystalline lithium niobate. CSHG sensitively reveals the inclination $\alpha >0$ of any such CDW with a superb optical resolution. Moreover, we deduce fully charged head-to-head CDWs ($\alpha$ = 90${}^{\circ }$) to be much rougher and to show protrusions, domain inclusions, and novel topologies. Our findings provide insight into the mechanisms of electron transport and charge trapping in CDWs, as is mandatory for their use in prospective nanoelectronic devices.

We devise a scheme to characterize tunneling of an excess electron shared by a pair of tunnel-coupled dangling bonds on a silicon surface—effectively a two-level system. Theoretical estimates show that the tunneling should be highly coherent but too fast to be measured by any conventional techniques. Our approach is instead to measure the time-averaged charge distribution of our dangling-bond pair by a capacitively coupled atomic-force-microscope tip in the presence of both a surface-parallel electrostatic potential bias between the two dangling bonds and a tunable midinfrared laser capable of inducing Rabi oscillations in the system. With a nonresonant laser, the time-averaged charge distribution in the dangling-bond pair is asymmetric as imposed by the bias. However, as the laser becomes resonant with the coherent electron tunneling in the biased pair the theory predicts that the time-averaged charge distribution becomes symmetric. This resonant symmetry effect should not only reveal the tunneling rate, but also the nature and rate of decoherence of single-electron dynamics in our system.

The doubly connected polygonal geometry of planar graphene rings is found to bring forth topological configurations for accessing nontrivial relativistic quantum field (RQF) theory models that carry beyond the constant-mass Dirac-fermion theory. These include the generation of sign-alternating masses, solitonic excitations, and charge fractionization. The work integrates a RQF Lagrangian formulation with numerical tight-binding Aharonov-Bohm electronic spectra and the generalized position-dependent-mass Dirac equation. In contrast to armchair graphene rings (aGRGs) with pure metallic arms, certain classes of aGRGs with semiconducting arms, as well as with mixed metallic-semiconducting ones, are shown to exhibit properties of one-dimensional nontrivial topological insulators. This further reveals an alternative direction for realizing a graphene-based nontrivial topological insulator through the manipulation of the honeycomb lattice geometry, without a spin-orbit contribution.

We present a theoretical approach for the simulation of the optical response and light propagation in aggregates and in ordered arrays of small noble-metal clusters with discrete electronic structure. We construct the Hamiltonian for the aggregate system based on the time-dependent density functional theory electronic states of the individual subunits and describe the interaction between them using the dipole approximation. The time evolution of the aggregate under the influence of the external electric field is obtained from the numerical solution of the time-dependent Schrödinger equation with the coupled excitonic Hamiltonian. For each subunit, the time-dependent dipole moment is calculated using the reduced density matrix formalism. Such quantum-mechanically determined dipole moments are used to simulate the spatiotemporal distribution of the electric field produced by the array. Additionally, we introduce an approximate self-consistent iterative approach to treat arrays consisting of many subunits which are of interest in the context of nanoplasmonics, nano-optical applications, and development of light-harvesting materials. The developed methodology is illustrated first on the example of Ag${}_2$ and Ag${}_8$ cluster pairs. Subsequently, light propagation in a triangular-shaped array consisting of six Ag${}_8$ clusters is simulated.

The mysterious second-order quantum phase transition, commonly attributed to the “hidden-order” (HO) state, in heavy-fermion metal URu${}_2$Si${}_2$ exhibits a number of paradoxical electronic and magnetic properties which cannot be associated with any conventional order parameter. We characterize and reconcile these exotic properties of the HO state based on a spin-orbit density wave order (SODW), constructed on the basis of a realistic density functional theory band structure. We quantify the nature of the gapped electronic and magnetic excitation spectrum, in agreement with measurements, while the magnetic moment is calculated to be zero owing to the spin-orbit coupling induced time-reversal invariance. Furthermore, a new collective mode in the spin-1 excitation spectrum is predicted to localize at zero momentum transfer in the HO state which can be visualized, for example, by electron spin resonance at zero magnetic field or polarized inelastic neutron scattering measurements. The results demonstrate that the concomitant broken and invariant symmetries protected SODW order not only provides insights into numerous nontrivial hidden-order phenomena, but also offers a parallel laboratory to the formation of a topologically protected quantum state beyond the quantum spin Hall state and Weyl semimetals.

It is shown that the presence of matrix dipole moments induced by external electric fields can modify the Hall response in two-dimensional topological insulators. In the case of the quantum anomalous Hall effect the induced transverse currents acquire an extra term, being proportional to the Hall conductance and the time derivative of the applied electric field. In the case of the quantum spin Hall effect both spin and charge transverse currents appear simultaneously. In virtue of the bulk-edge correspondence the coupling between the chiral edge channels and the electromagnetic field changes, allowing for an extra nonminimal coupling term. Both effects can be measured through transport and tunneling experiments.

Regarding light absorption/emission efficiency, silicon presents the fundamental drawback of its indirect band gap. It is long known, though, that optical properties are greatly enhanced in materials which comprise different kinds of nanocrystalline Si covered by or embedded in Si oxide layers. Conversely, their amorphous counterparts have received far less attention, such that no general consensus about the emission mechanism prevails. We report here on an efficiently luminescent material based on amorphous Si nanoparticles (a-Si NPs) embedded in a nonstoichiometric Si oxide matrix, which exhibits intense, broadband emission from the a-Si NPs, spectrally separable from the defect luminescence of the suboxide matrix. Apart from the brightness of the emitted light, the nanometer-size a-Si inclusions present the technological advantage of needing very moderate annealing temperatures (450${}^{\circ }$C–700${}^{\circ }$C) for their production. The combined use of high pressure, experimentally as well as theoretically, allowed us to trace back the microscopic origin of the photoluminescence to radiative recombination processes between confined states of the a-Si NPs. The signature of quantum confinement is found in the magnitude and sign of the pressure coefficient of different optical transition energies. The pressure derivatives exhibit a universal dependence on particle size, determined solely by the confinement energy of the discrete electron state involved in the radiative recombination process.

The pressure dependence of the boson peak in glasses within the framework of the extended soft potential model is reconsidered, taking effects at small pressures into account. One of these is the pressure dependence of the elastic constants, changing the interaction between the soft localized modes and thus changing the quasilocalized vibrations (QLVs) of the boson peak. This and other effects require the introduction of additional parameters to describe all the different influences of the pressure in detail. As in the first treatment of the problem, the dominating high-pressure influence remains the creation of pressure forces, which have to be added to the random forces responsible for the boson peak formation. The pressure forces consist of a correlated and an uncorrelated part (correlated with respect to the already existing random forces). Both lead to the $P^{1/3}$ dependence observed in high-pressure experiments, but the uncorrelated part vanishes at small pressure $P$. The comparison to experiment is divided into a small pressure part, accessible through low-temperature heat capacity and thermal expansion measurements, and the high-pressure part, mostly Raman scattering measurements of the boson peak under pressure. The results suggest that the latter are dominated by the uncorrelated part of the forces, probably due to pressure-induced relaxations.

Diffusion, a ubiquitous phenomenon in nature, is a consequence of particle number conservation and locality in systems with sufficient damping. In this paper, we consider diffusive processes in the bulk of Weyl semimetals, which are exotic quantum materials, recently of considerable interest. In order to do this, we first explicitly implement the analytical scheme by which disorder with anisotropic scattering amplitude is incorporated into the diagrammatic response-function formalism for calculating the “diffuson”. The result, thus, obtained is consistent with transport coefficients evaluated from the Boltzmann transport equation or the renormalized uniform current vertex calculation as it should be. We, thus, demonstrate that the computation of the diffusion coefficient should involve the transport lifetime and not the quasiparticle lifetime. Using this method, we then calculate the density response function in Weyl semimetals and discover an unconventional diffusion process that is significantly slower than conventional diffusion. This gives rise to relaxation processes that exhibit stretched exponential decay instead of the usual exponential diffusive relaxation. This result is then explained using a model of thermally excited quasiparticles diffusing with diffusion coefficients which are strongly dependent on their energies. We elucidate the roles of the various energy and time scales involved in this novel process and propose an experiment by which this process may be observed.