The resilience to deep traps and localized defect formation is one of the important aspects that qualify a material as a suited photoabsorber in solar cell devices. Here we investigate by ab initio calculations the fundamental physics and chemistry of a number of possible localized defects in hybrid methylammonium lead-iodide perovskites. Our analysis encompasses a number of possible molecular fragments deriving from the dissociation of methylammonium. In particular, we found that in stoichiometric conditions both ammonia and methylamine molecules present lone-pair localized levels well within the perovskite band gap, while the radical cation ${\mathrm{CH}}_2{\mathrm{NH}}_3^{+}$ observed by EPR after irradiation injects partially-occupied levels into the band gap but only in $p$-type conditions. These defects are thus potentially capable of significantly altering absorption and recombination properties. Amazingly, we found that additional interstitial Cl is capable of removing these localized states from the band gap. These results are consistent with the observed improvement of photoabsorption properties due to the Cl inclusion in the solution processing.

Graphene has extremely high mobility with unique linear band dispersions at the Fermi level, referred to as the Dirac cones, but the absence of the energy gap limits its application for switching devices. To open an energy gap, theoretical studies so far have introduced certain perturbations to graphene in the real or momentum space and checked whether they open a gap at the Dirac point. Here, as a reverse approach, we directly enforce energy splittings at the Dirac point with perturbations in the minimal Hilbert space at the Dirac point and then characterize the perturbations in the real space to obtain perturbed band structures throughout the Brillouin zone. Our approach provides refined descriptions of the sublattice symmetry breaking and the intervalley scattering, distinguishing clearly the sublattice symmetry breaking without intervalley scattering, the sublattice mixing without intervalley scattering, the intervalley scattering within each sublattice, and the intersublattice intervalley scattering. For fully gapped cases, the effective mass is obtained as a function of the energy gap. Our present method can be applied to band-gap engineering of graphene-like hexagonal layered materials, in general.

Silicon materials play a key role in many technologically relevant fields, ranging from the electronic to the photovoltaic industry. A systematic search for silicon allotropes was performed by employing a modified ab initio minima hopping crystal structure prediction method. The algorithm was optimized to specifically investigate the hitherto barely explored low-density regime of the silicon phase diagram by imitating the guest-host concept of clathrate compounds. In total, 44 metastable phases are presented, of which 11 exhibit direct or quasidirect band gaps in the range of ≈1.0–1.8 eV, close to the optimal Shockley-Queisser limit of $\approx 1.4$ eV, with a stronger overlap of the absorption spectra with the solar spectrum compared to conventional diamond silicon. Due to the structural resemblance to known clathrate compounds it is expected that the predicted phases can be synthesized.

Accurate determination of the coherent static structure factor of any disordered material containing substantial amounts of proton nuclei has proven to be rather problematic by neutron diffraction, due to the large incoherent cross section of ${}^1\text{H}$. This problem has continued to set severe obstacles to the reliable determination of liquid structures of hydrogenous materials up to this day, by introducing large uncertainties whenever a sample with a ${}^1\text{H}$ content larger than about 20% had to be investigated by neutron diffraction. Huge theoretical efforts over the past 40 years which were aimed at estimating the incoherent background of such data did not result in any practical solution to the problem. Here, we present data for the coherent and incoherent contributions to the total static structure of mixtures of light and heavy water. The measurements were done using the polarized neutron diffraction technique, which uniquely allows determination of the two contributions separately. The data covers a wide range of momentum transfer (0.8–21 ${\text{Å}}^{-1}$) and the entire composition range, i.e., light water contents between 0 and 100% at five different values. We show that the measured incoherent scattering can be approximated by a Gaussian function. The separately measured coherent intensities exhibit signs of small inelastic contributions. Out of several possible approaches, we have chosen to subtract a cubic background using the reverse Monte Carlo algorithm. This algorithm has the advantage of requiring an actual physical model with thousands of realistic water molecules at the correct density describing the corrected data. Finally, coherent static structure factors for five different compositions of liquid ${\mathrm{H}}_2\mathrm{O}$ and ${\mathrm{D}}_2\mathrm{O}$ mixtures are presented for which the huge incoherent background could actually be measured and separated, instead of being approximated as it has been done so far. These experimental results provide a strong hope that determining the structure of hydrogenous materials, including, e.g., protein solutions, may become feasible in the near future.

We investigated the crystal structure and lattice excitations of the ternary intermetallic stannide ${\mathrm{Ca}}_3{\mathrm{Ir}}_4{\mathrm{Sn}}_{13}$ using neutron and x-ray scattering techniques. For $T>T^{*}\approx 38$ K, the x-ray diffraction data can be satisfactorily refined using the space group $Pm\overline{3}n$. Below $T^{*}$, the crystal structure is modulated with a propagation vector of $\vec{q}=(1/2,1/2,0)$. This may arise from a merohedral twinning in which three tetragonal domains overlap to mimic a higher symmetry, or from a doubling of the cubic unit cell. Neutron diffraction and neutron spectroscopy results show that the structural transition at $T^{*}$ is of a second-order, and that it is well described by mean-field theory. Inelastic neutron scattering data point towards a displacive structural transition at $T^{*}$ arising from the softening of a low-energy phonon mode with an energy gap of $\mathrm{\Delta }\left(120\text{K}\right)=1.05$ meV. Using density functional theory, the soft phonon mode is identified as a “breathing” mode of the ${\mathrm{Sn}}_{12}$ icosahedra and is consistent with the thermal ellipsoids of the Sn2 atoms found by single-crystal diffraction data.

Subsystems of strongly disordered, interacting quantum systems can fail to thermalize because of the phenomenon of many-body localization (MBL). In this paper, we explore a tensor network description of the eigenspectra of such systems. Specifically, we will argue that the presence of a complete set of local integrals of motion in MBL implies an efficient representation of the entire spectrum of energy eigenstates with a single tensor network, a spectral tensor network. Our results are rigorous for a class of idealized systems related to MBL with integrals of motion of finite support. In one spatial dimension, the spectral tensor network allows for the efficient computation of expectation values of a large class of operators (including local operators and string operators) in individual energy eigenstates and in ensembles.

We report appearance of polar order with a ordering temperature $\left(T_C\right)$ at 28 K, which is well below the ferrimagnetic order at 225 K $\left(T_N\right)$ for ${\mathrm{CoCr}}_2{\mathrm{S}}_4$. Intriguingly, the value of spontaneous electric polarization $\left(P\right)$ is $\sim 122 \mu \mathrm{C}/{\mathrm{m}}^2$ at 15 K, which is the second largest value in Cr octahedra-based spinels after ${\mathrm{CdCr}}_2{\mathrm{S}}_4$. Incidentally, the $P$ value is $\sim 60$ times larger than the value of $P$ for the oxide counterpart ${\mathrm{CoCr}}_2{\mathrm{O}}_4$. The significant magnetoelectric coupling is verified from the magnetodielectric response and magnetic field dependent enhancement of $P$. We note that the field-dependent dielectric permittivity scales linearly to the squared magnetization in the low field regime below $\sim 10$ kOe as described by the Ginzburg-Landau theory. Synchrotron diffraction studies over a wide temperature range, $15\text{–}300$ K, illustrate strong magnetoelastic coupling at $T_N$ and isostructural distortion at $T_C$. Analyses of the diffraction patterns reveal that the occurrence of polar order involves expansion of Co tetrahedra and contraction of Cr octahedra of the spinel structure and these distortions are further enhanced driven by the magnetic field. The delicate interplay between magnetoelastic, magnetoelectric, and electroelastic couplings in ${\mathrm{CoCr}}_2{\mathrm{S}}_4$ proposes the system as a potential candidate in multiferroics.

In diffusive Josephson junctions the phase difference $\varphi$ between the superconductors strongly influences the spectroscopic features of the layer separating them. The observation of a uniform minigap and its phase modulation were only recently experimentally reported, demonstrating agreement with theoretical predictions up to now—a vanishing minigap at $\varphi =\pi$. Remarkably, we find that in the presence of intrinsic spin-orbit coupling a giant proximity effect due to spin-triplet Cooper pairs can develop at $\varphi =\pi$, in complete contrast to the suppressed proximity effect without spin-orbit coupling. We here report a combined numerical and analytical study of this effect, proving its presence solely based on symmetry arguments, which makes it independent of the specific parameters used in experiments. We show that the spectroscopic signature of the triplets is present throughout the entire ferromagnetic layer. Our finding offers a way to artificially create, control, and isolate spin-triplet superconductivity.

We study, using quantum Monte Carlo simulations, the bosonic Kondo-Hubbard model in a two-dimensional square lattice. We explore the phase diagram and analyze the mobility of particles and magnetic properties. At unit filling, the transition from a paramagnetic Mott insulator to a ferromagnetic superfluid appears continuous, contrary to what was predicted with mean field. For double occupation per site, both the Mott insulating and superfluid phases are ferromagnetic and the transition is still continuous. Multiband tight-binding Hamiltonians can be realized in optical lattice experiments, which offer not only the possibility of tuning the different energy scales over wide ranges, but also the option of loading the system with either fermionic or bosonic atoms.

The momentum spectrum of positively and negatively charged carriers created in intrinsic graphene submitted to a time-dependent external electric field is evaluated for many external field configurations. Owing to the formal analogy between relativistic quantum mechanics and the description of graphene quasiparticles in terms of the massless Dirac equation, the electron momentum density is evaluated within two-dimensional massless quantum electrodynamics coupled to a strong classical field. This allows the treatment of dynamical effects in electron-hole creation and gives a physical description in terms of the time-dependent Schwinger mechanism. At zero transverse momentum, it is shown that the Fermi bound in the electron-hole momentum spectrum is saturated in a certain momentum window and the pair density depends only on the potential difference between asymptotic potentials before and after the interaction. The pair density for nonzero transverse momenta is evaluated using numerical calculations. The numerical results demonstrate that an important number of pairs can be created by an external field through both tunneling and multiphoton processes. It is argued that these features of the dynamical pair production may facilitate the detection of the Schwinger mechanism using graphene as a condensed matter analog to quantum electrodynamics.

Path-integral Monte Carlo calculations have been performed to study the ${}^4\mathrm{He}$ adsorption on both sides of a single $\alpha$-graphyne sheet. For investigation of the interlayer correlation between the upper and the lower monolayers of ${}^4\mathrm{He}$ adatoms, the ${}^4\mathrm{He}$-substrate interaction is described by the sum of the ${}^4\mathrm{He}\text{−}\mathrm{C}$ interatomic pair potentials for which we use both Lennard-Jones and Yukawa-6 anisotropic potentials. When the lower ${}^4\mathrm{He}$ layer is a ${\mathrm{C}}_{4/3}$ commensurate solid, the upper-layer ${}^4\mathrm{He}$ atoms are found to form a kagome lattice structure at a Mott-insulating density of $0.0706{\AA }^{-2}$ and a commensurate solid at an areal density of $0.0941{\AA }^{-2}$ for both substrate potentials. The correlation between upper- and lower-layer pseudospins, which were introduced in Kwon et al. [Phys. Rev. B 88, 201403(R) (2013)] for two degenerate configurations of three ${}^4\mathrm{He}$ atoms in a hexagonal cell, depends on the substrate potential used; with the substrate potential based on the anisotropic Yukawa-6 pair potentials, the Ising pseudospins of both ${}^4\mathrm{He}$ layers are found to be antiparallel to each other whereas the parallel and antiparallel pseudospin alignments between the two ${}^4\mathrm{He}$ layers are nearly degenerate with the Lennard-Jones potentials. This is attributed to the difference in the interlayer distance, which is $\sim 4\AA$ with the Yukawa-6 substrate potential but as large as $\sim 4.8\AA$ with the Lennard-Jones potential.

Studies of spin degrees of freedom near odd-integer filling $\nu =3$ in the second Landau level have engendered conflicting accounts of the spin properties in this regime. Using resistively detected NMR as a probe of local spin density, we explore the nature of the ground state in the quantum Hall regime near $\nu =3$. Our Knight shift measurements reveal anomalies in NMR spectral line shapes near $\nu =3$, which demonstrate the presence of solid phases formed from charged quasiparticles, with maximal spin polarization (i.e., no additional spin flips) at each $\nu$. The long nuclear spin relaxation times demonstrate the absence of Skyrmions, or spin textures, in this phase. On the basis of these observations, we attempt to reconcile conflicting reports on the quasiparticle properties and spin wave excitations at $\nu \sim 3$, which may engender new paradigms for the understanding of spin excitations in a collinear ferromagnet with broken translational symmetry.

We report the observation of the intrinsic magnetic susceptibility of highly purified SWCNT samples prepared by a combination of acid treatment and density gradient ultracentrifugation (DGU). We observed that the diamagnetic susceptibility of SWCNTs increases linearly with increasing nanotube diameter. We found that the magnetic susceptibility divided by the diameter is a universal function of the scaled temperature. Furthermore, the estimated magnetic susceptibilities of pure semiconducting and pure metallic SWCNT samples suggest that they respond differently to changes in carrier density, which is consistent with theory. These findings provide experimental verification of the theoretically predicted diameter, temperature, and metallicity dependence of the magnetic susceptibility.

We propose an exactly solvable lattice Hamiltonian model of topological phases in $3+1$ dimensions, based on a generic finite group $G$ and a 4-cocycle $\omega$ over $G$. We show that our model has topologically protected degenerate ground states and obtain the formula of its ground state degeneracy on the 3-torus. In particular, the ground state spectrum implies the existence of purely three-dimensional looplike quasiexcitations specified by two nontrivial flux indices and one charge index. We also construct other nontrivial topological observables of the model, namely the $SL(3,\mathbb{Z})$ generators as the modular $S$ and $T$ matrices of the ground states, which yield a set of topological quantum numbers classified by $\omega$ and quantities derived from $\omega$. Our model fulfills a Hamiltonian extension of the $\left(3+1\right)$-dimensional Dijkgraaf-Witten topological gauge theory with a gauge group $G$. This work is presented to be accessible for a wide range of physicists and mathematicians.

Some Kondo insulators are expected to possess topologically protected surface states with linear Dirac spectrum: the topological Kondo insulators. Because the bulk states of these systems typically have heavy effective electron masses, the surface states may exhibit extraordinarily small Fermi velocities that could force the effective fine structure constant of the surface states into the strong coupling regime. Using a tight-binding model, we study the many-body instabilities of these systems and identify regions of parameter space in which the system exhibits spin density wave and charge density wave order.

An atomistic numerical scheme is developed and used to study the prototype of relaxor ferroelectrics, that is ${\mathrm{PbMg}}_{1/3}{\mathrm{Nb}}_{2/3}{\mathrm{O}}_3$ (PMN), at finite temperatures. This scheme not only reproduces known complex macroscopic properties of PMN, but also provides a deep microscopic insight into this puzzling system. In particular, relaxor properties of PMN are found to originate from the competition between (1) random electric fields arising from the alloying of Mg and Nb ions belonging to different columns of the Periodic Table within the same sublattice; (2) the simultaneous condensation of several off-center $\mathbf{k}$ points as a result of a specific short-range, antiferroelectriclike interaction between lead-centered dipoles; and (3) ferroelectriclike interactions. Such origins contrast with those recently proposed for the homovalent Ba(Zr,Ti)${\mathrm{O}}_3$ solid solution, despite the fact that these two materials have similar macroscopic properties—which therefore leads to a comprehensive understanding of relaxor ferroelectrics.

Thermodynamic properties of ZrC are calculated up to the melting point ($T^{\mathrm{melt}}\approx 3700\text{K}$), using density functional theory (DFT) to obtain the fully anharmonic vibrational contribution, and including electronic excitations. A significant improvement is found in comparison to results calculated within the quasiharmonic approximation. The calculated thermal expansion is in better agreement with experiment and the heat capacity reproduces rather closely a CALPHAD estimate. The calculations are presented as an application of a development of the upsampled thermodynamic integration using Langevin dynamics (UP-TILD) approach. This development, referred to here as two-stage upsampled thermodynamic integration using Langevin dynamics (TU-TILD), is the inclusion of tailored interatomic potentials to characterize an intermediate reference state of anharmonic vibrations on a two-stage path of thermodynamic integration between the original DFT quasiharmonic free energy and the fully anharmonic DFT free energy. This approach greatly accelerates the convergence of the calculation, giving a factor of improvement in efficiency of $\sim 50$ in the present case compared to the original UP-TILD approach, and it can be applied to a wide range of materials.

A fully quantum-mechanical calculation of the Gilbert damping constant $\alpha$ in magnetic trilayers is done by employing the torque-correlation formula within a realistic tight-binding model. A remarkable enhancement of $\alpha$ in Co/${\mathrm{NM}}_1$/${\mathrm{NM}}_2$ trilayers is obtained due to adding the caps ${\mathrm{NM}}_2$=Pd, Pt, and it decays with the thickness of the spacers ${\mathrm{NM}}_1$=Cu, Ag, Au in agreement with experiment. Nonlocal origin of the Gilbert damping is visualized with its atomic layer contributions. It is shown that magnetization in Co is damped remotely by strong spin-orbit coupling in ${\mathrm{NM}}_2$ via quantum states with large amplitude in both Co and ${\mathrm{NM}}_2$.

We report Raman scattering measurements on ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ single crystals with a different degree of out-of-plane disorder to examine the effect of $T_c$ change on the the electronic Raman response at the optimal doping level. The $B_{1g}$ peak energies for lower $T_c$ disordered samples are essentially independent of $T_c$. However, a further increase of $T_c$ by minimizing the degree of out-of-plane disorder leads to a high-energy shift of the $B_{1g}$ peak. Interestingly, abrupt change of the $B_{1g}$ peak energy occurs when the the superconducting gap energy exceeds the pseudogap energy, which results in the recovery of superconductivity-dominated Raman response in $B_{1g}$ symmetry. It suggests that these anomalous properties of the antinodal electrons are a consequence of the unconventional superconducting state of the cuprates where superconductivity coexists with the pseudogap in the ground state.