We examine the low-energy effective theory of phase oscillations in a two-dimensional granular superconducting sheet where the grains are arranged in a honeycomb lattice structure. Using the example of graphene, we present evidence for the engineered Dirac nodes in the bosonic excitations: the spectra of the collective bosonic modes cross at the $K$ and $K^{\prime }$ points in the Brillouin zone and form Dirac nodes. We show how two different types of collective phase oscillations are obtained and that they are analogous to the Leggett and the Bogoliubov-Anderson-Gorkov modes in a two-band superconductor. We show that the Dirac node is preserved in the presence of an intergrain interaction, despite induced changes of the qualitative features of the two collective modes. Finally, breaking the sublattice symmetry by choosing different on-site potentials for the two sublattices leads to a gap opening near the Dirac node, in analogy with fermionic Dirac materials. The Dirac node dispersion of bosonic excitations is thus expanding the discussion of the conventional Dirac cone excitations to the case of bosons. We call this case as a representative of bosonic Dirac materials (BDM), similar to the case of Fermionic Dirac materials extensively discussed in the literature.

We report a combined neutron scattering and magnetization study on the multiferroic ${\mathrm{DyFeO}}_3$, which shows a very strong magnetoelectric effect. Applying magnetic field along the $c$ axis, the weak ferromagnetic order of the Fe ions is quickly recovered from a spin reorientation transition, and the long-range antiferromagnetic order of Dy becomes a short-range one. We found that the short-range order concurs with the multiferroic phase and is responsible for its sizable hysteresis. Our $H\text{−}T$ phase diagram suggests that the strong magnetoelectric effect in ${\mathrm{DyFeO}}_3$ has to be understood with not only the weak ferromagnetism of Fe but also the short-range antiferromagnetic order of Dy.

The ${\mathrm{Li}}_{13}{\mathrm{Si}}_4$ phase is one out of several crystalline lithium silicide phases, which is a potential electrode material for lithium ion batteries and contains a high theoretical specific capacity. By means of ab initio methods like density functional theory (DFT) many properties such as heat capacity or heat of formation can be calculated. These properties are based on the calculation of phonon frequencies, which contain information about the thermodynamical stability. The current unit cell of “${\mathrm{Li}}_{13}{\mathrm{Si}}_4$” given in the ICSD database is unstable with respect to DFT calculations. We propose a modified unit cell that is stable in the calculations. The evolutionary algorithm EVO found a structure very similar to the ICSD one with both of them containing metastable lithium positions. Molecular dynamic simulations show a phase transition between both structures where these metastable lithium atoms move. This phase transition is achieved by a very fast one-dimensional lithium diffusion and stabilizes this phase.

The potential-energy landscape of a model binary Lennard-Jones structural glass is investigated as a function of applied external strain, in terms of how local structural excitations (LSEs) respond to the load. Using the activation relaxation technique and nudged elastic band methods, the evolving structure and barrier energy of such LSEs are studied in detail. For the case of a tensile/compressive strain, the LSE barrier energies generally decrease/increase, whereas under pure shear, it may either increase or decrease resulting in a broadening of the barrier energy distribution. It is found that how a particular LSE responds to an applied strain is strongly controlled by the LSE's far-field internal stress signature prior to loading.

The exchange bias effect is usually defined as horizontal shift of the field-cooled magnetization loop when an antiferromagnet is directly coupled to a ferromagnet. Uncompensated spins at the interface between the two layers are believed to cause this phenomenon. The presence of such, on the other hand, would infer a vertical, i.e., a magnetization-like shift stemming from the antiferromagnet. Observations of this effect are sparse, especially in the absence of a ferromagnet. We present a model system based on extremely Co doped ZnO in which the uncompensated spins of antiferromagnetic Co-O-Co… configurations lead to this vertical shift and therefore to a field-resistant magnetization. A simple Stoner-Wohlfarth-like model based on configurations of different sizes is used to explain the occurrence of this exchange-bias-like shift and a narrow opening of the magnetization curves.

Recent experimental results have emphasized two aspects of ${\mathrm{Tb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$, which have not been taken into account in previous attempts to construct theories of ${\mathrm{Tb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$: the role of small levels of structural disorder, which appears to control the formation of a long-range ordered state of as yet unknown nature, and the importance of strong coupling between spin and lattice degrees of freedom, which results in the hybridization of crystal-field excitons and transverse acoustic phonons. In this work, we examine the juncture of these two phenomena and show that samples with strongly contrasting behavior vis-a-vis the structural disorder (i.e., with and without the transition to the ordered state), develop identical magnetoelastic coupling. We also show that the comparison between single-crystal and powder samples is more complicated than previously thought—the correlation between the lattice parameter (as a measure of superstoichiometric ${\mathrm{Tb}}^{3+}$) and the existence of a specific heat peak, as observed in powder samples, does not hold for single crystals.

Disorder-induced melting where the increase in positional entropy created by random pinning sites drives the order-disorder transition in a periodic solid provides an alternate route to the more conventional thermal melting. Here, using real-space imaging of the vortex lattice through scanning tunneling spectroscopy, we show that, in the presence of weak pinning, the vortex lattice in a type-II superconductor disorders through two distinct topological transitions. Across each transition, we separately identify metastable states formed through superheating of the low-temperature state or supercooling of the high-temperature state. Comparing crystals with different levels of pinning we conclude that the two-step melting is fundamentally associated with the presence of random pinning which generates topological defects in the ordered vortex lattice.

We propose an experimental setup using 3D Dirac semimetals to access a novel phenomenon induced by the chiral anomaly. We show that the combination of a magnetic field and a circularly polarized laser induces a finite charge density with an accompanying axial current. This is because the circularly polarized laser breaks time-reversal symmetry and the Dirac point splits into two Weyl points, which results in an axial-vector field. We elucidate the appearance of the axial-vector field with the help of the Floquet theory by deriving an effective Hamiltonian for high-frequency electric fields. This anomalous charge density, i.e., the chiral pumping effect, is a phenomenon reminiscent of the chiral magnetic effect with a chiral chemical potential. We explicitly compute the pumped density and the axial-current expectation value. We also take account of coupling to the chiral magnetic effect to calculate a balanced distribution of charge and chirality in a material that behaves as a chiral battery.

We theoretically investigate the development of unconventional superconductivity in the Sb(111) thin film when its Fermi level is tuned to near type-II Van Hove singularities (VHS), which locate at non-time-reversal invariant momenta. Via patch renormalization group analysis, we show that the leading instability is a chiral $p+ip$-wave superconducting order. The origin of such pairing relies on the hexagonal structure of the VHS and strong spin-orbit coupling, resulting in the anisotropy of the electron-electron scattering to provide an attractive channel. Our study hence suggests that superconducting Sb thin films originated from VHS physics may host Majorana zero modes in the magnetic vortices and provides another application perspective to such material.

We present a method for rapidly generating efficient $k$-point grids for Brillouin zone integration by using a database of precalculated grids. Benchmark results on 102 randomly selected materials indicate that for well-converged calculations, the grids generated by our method have fewer than half as many irreducible $k$-points as Monkhorst-Pack grids generated using a more conventional method, significantly accelerating the calculation of properties of crystalline materials.

We consider a strongly correlated quantum dot, tunnel coupled to two superconducting leads and capacitively coupled to a single mode microwave cavity. When the superconducting gap is the largest energy scale, multiple Shiba states are formed inside the gap. The competition of these states for the ground state signals a quantum phase transition. We demonstrate that photonic measurements can be used to probe such localized Shiba states. Moreover, the quantum phase transition can be pinpointed exactly from the sudden change in the transmission signal. Calculations were performed using the numerical renormalization-group approach.

We study from first principles two lanthanum silicate nitride compounds, ${\mathrm{LaSi}}_3{\mathrm{N}}_5$ and ${\mathrm{La}}_3{\mathrm{Si}}_6{\mathrm{N}}_{11}$, pristine as well as doped with ${\mathrm{Ce}}^{3+}$ ion, in view of explaining their different emission color, and characterizing the luminescent center. The electronic structures of the two undoped hosts are similar, and do not give a hint to quantitatively describe such difference. The $4f\to 5d$ neutral excitation of the ${\mathrm{Ce}}^{3+}$ ions is simulated through a constrained density functional theory method coupled with a $\mathrm{\Delta }\mathrm{SCF}$ analysis of total energies, yielding absorption energies. Afterwards, atomic positions in the excited state are relaxed, yielding the emission energies and Stokes shifts. Based on these results, the luminescent centers in ${\mathrm{LaSi}}_3{\mathrm{N}}_5$:Ce and ${\mathrm{La}}_3{\mathrm{Si}}_6{\mathrm{N}}_{11}$:Ce are identified. The agreement with the experimental data for the computed quantities is quite reasonable and explains the different color of the emitted light. Also, the Stokes shifts are obtained within 20% difference relative to experimental data.

The phonon Boltzmann transport equation (BTE) is a powerful tool for studying nondiffusive thermal transport. Here, we develop a new universal variational approach to solving the BTE that enables extraction of phonon mean free path (MFP) distributions from experiments exploring nondiffusive transport. By utilizing the known Fourier heat conduction solution as a trial function, we present a direct approach to calculating the effective thermal conductivity from the BTE. We demonstrate this technique on the transient thermal grating experiment, which is a useful tool for studying nondiffusive thermal transport and probing the MFP distribution of materials. We obtain a closed form expression for a suppression function that is materials dependent, successfully addressing the nonuniversality of the suppression function used in the past, while providing a general approach to studying thermal properties in the nondiffusive regime.

We study the low-temperature magnetotransport properties of several highly disordered amorphous indium oxide (a:InO) samples. Simultaneously fabricated devices comprising a two-dimensional (2D) film and $10\text{−}\mu \mathrm{m}$-long wires of different widths were measured to investigate the effect of size as we approach the 1D limit, which is around 4 times the correlation length, and happens to be around 100 nm for a:InO. The film and the wires showed magnetic field ($B$)-induced superconductor to insulator transition (SIT). In the superconducting side, the resistance increased with decrease in wire width, whereas an opposite trend is observed in the insulating side. We find that this effect can be explained in light of charge-vortex duality picture of the SIT. Resistance of the 2D film follows an activated behavior over the temperature ($T$), whereas, the wires show a crossover from the high-$T$-activated to a $T$-independent behavior. At high-temperature regime the wires' resistance follow the film's until they deviate and became independent of $T$. We find that the temperature at which this deviation occurs evolves with the magnetic field and the width of the wire, which show the effect of finite size on the transport.

We have analyzed the crucial role a proper form of the Coulomb interaction plays on the even and odd denominator fractional quantum Hall effects in a two-dimensional electron gas (2DEG) in the ZnO heterointerface. In this system, the Landau level gaps are much smaller than those in conventional GaAs systems. The Coulomb interaction is also very large compared to the Landau level gap even in very high magnetic fields. We therefore consider the influence of higher Landau levels by considering the screened Coulomb potential in the random-phase approximation. Interestingly, our exact diagonalization studies of the collective modes with this screened potential successfully explain recent experiments of even and odd denominator fractional quantum Hall effects, in particular, the unexpected absence of the 5/2 state and the presence of the 9/2 state in ZnO. Additionally, our study also reveals a strong presence of spin-reversed excitations in the 7/2 state in accordance with the experimental observation.

We study ultrafast scattering dynamics of hot electrons photoinjected with high excess energies in the Γ valley of the conduction band of GaAs, using time- and angle-resolved photoemission spectroscopy and ab initio calculations. At ultrafast rates of the order of 10 fs, the packets in the Γ valley are transformed into hot-electron ensembles (HEEs) quasiequilibrated in momentum space but not in energy space. The energy relaxation of the HEEs takes place as a whole on a longer time scale with rates dependent only on the excess energy, irrespective of the momenta of hot electrons. Both momentum scattering and energy relaxation are ruled by the electron-phonon interaction.

We present the valence electron energy-loss spectrum and the dielectric function of monoclinic hafnia ($\mathrm{m}-{\mathrm{HfO}}_2$) obtained from time-dependent density-functional theory (TDDFT) predictions and compared to energy-filtered spectroscopic imaging measurements in a high-resolution transmission-electron microscope. Fermi's golden rule density-functional theory (DFT) calculations can capture the qualitative features of the energy-loss spectrum, but we find that TDDFT, which accounts for local-field effects, provides nearly quantitative agreement with experiment. Using the DFT density of states and TDDFT dielectric functions, we characterize the excitations that result in the $\mathrm{m}-{\mathrm{HfO}}_2$ energy-loss spectrum. The sole plasmon occurs between 13 and 16 eV, although the peaks $\sim 28$ and above 40 eV are also due to collective excitations. We furthermore elaborate on the first-principles techniques used, their accuracy, and remaining discrepancies among spectra. More specifically, we assess the influence of Hf semicore electrons ($5p$ and $4f$) on the energy-loss spectrum, and find that the inclusion of transitions from the $4f$ band damps the energy-loss intensity in the region above 13 eV. We study the impact of many-body effects in a DFT framework using the adiabatic local-density approximation (ALDA) exchange-correlation kernel, as well as from a many-body perspective using “scissors operators” matched to an ab initio $GW$ calculation to account for self-energy corrections. These results demonstrate some cancellation of errors between self-energy and excitonic effects, even for excitations from the Hf $4f$ shell. We also simulate the dispersion with increasing momentum transfer for plasmon and collective excitation peaks.

We present an embedding scheme for periodic systems that facilitates the treatment of the physically important part (here a unit cell or a supercell) with advanced electronic structure methods, that are computationally too expensive for periodic systems. The rest of the periodic system is treated with computationally less demanding approaches, e.g., Kohn-Sham density-functional theory, in a self-consistent manner. Our scheme is based on the concept of dynamical mean-field theory formulated in terms of Green's functions. Our real-space dynamical mean-field embedding scheme features two nested Dyson equations, one for the embedded cluster and another for the periodic surrounding. The total energy is computed from the resulting Green's functions. The performance of our scheme is demonstrated by treating the embedded region with hybrid functionals and many-body perturbation theory in the GW approach for simple bulk systems. The total energy and the density of states converge rapidly with respect to the computational parameters and approach their bulk limit with increasing cluster (i.e., computational supercell) size.

Soft x-ray linear and circular dichroism (XLD, XMCD) experiments at the Ce ${\mathrm{M}}_{4,5}$ edges are being used to determine the energy scales characterizing the Ce $4f$ degrees of freedom in the ultrathin ordered surface intermetallic ${\mathrm{CeAg}}_x$/Ag(111). We find that all relevant interactions, i.e., Kondo scattering, crystal field splitting and magnetic exchange coupling occur on small scales. Our study demonstrates the usefulness of combining x-ray absorption experiments probing the temperature dependence of both linear and circular dichroism owing to their strong sensitivity for anisotropies in both charge distribution and paramagnetic response, respectively.